Introduction

Modern society proposed higher requirements on energy to protect the environment. In the past decades, the exponential growth of fossil fuel consumption significantly increased greenhouse gas (GHG) emissions, leading to global concern about climate change. To substantially reduce global GHG emissions, renewable energies, such as wind, solar, and tide should play a crucial role. However, due to the intrinsically unstable power generation of these renewable energies, it is necessary to convert them with an intermediate carrier that can generate power stably and sustainably. Hydrogen (H2) is a promising candidate as an energy carrier for wide applications in the near future, especially when produced from renewable electricity by water electrolyzers, stored for long periods, and reconverted on demand to electric power in fuel cells without any GHG emission. H2/air proton exchange membrane fuel cell (PEMFC) is one of the most promising fuel cell types, especially for transport and portable power applications. However, due to the sluggish oxygen reduction reaction (ORR) on the cathode and the acidic environment of PEMFC, large quantities of platinum (Pt)-based electrocatalysts are necessary to reach the acceptable power performance of PEMFCs, which impedes their broader commercialization and threatens their sustainability. While advances have been made in reducing Pt usage in PEMFCs, such as using Pt-alloys to increase the intrinsic activity of Pt, the cost of automotive fuel cell system is still prohibitively high. Therefore, developing platinum-group metal-free catalysts for ORR in acidic media to ultimately replace Pt-based catalysts is highly desirable. Nowadays, the leading platinum-group metal-free catalysts for ORR in PEMFCs are single transition metal atoms (M=Fe, Mn, or Co) embedded in a nitrogen-doped carbon matrix (M-N-C). Nevertheless, this subclass of catalysts is insufficient for automotive PEMFCs. It is necessary to further improve the performance by increasing the turnover frequency and/or density of active sites in M-N-C materials. Both require rationalizing synthesis methods. Besides the intrinsic activity, the mass transport properties are also important to improve the fuel cell performance of M-N-C catalysts in PEMFC, because a thicker layer on the cathode is required with M-N-C compared to Ptbased catalysts, to compensate for their lower mass activity. In this case, the design of micromeso or micro-meso-macroporous textures is important to improve the mass transport properties of M-N-C cathodes, facilitating O2 diffusion to the active sites. In this thesis, aerogelbased Fe-N-C catalysts were successfully synthesized by pyrolyzing Fe-doped Resorcinol-Formaldehyde-Melamine (RMF) aerogel. The synthesis conditions were carefully investigated to elucidate the variation of their electrochemical performance with the porosity of catalysts.

Chapter 1 State-of-the-art on Platinum-group metalfree catalyst for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cell

Ré sumé Franç ais L'é nergie est la pierre angulaire de la civilisation moderne et la produire de faç on durable est une condition pré alable né cessaire. L'augmentation des é missions de gaz à effet de serre causé e par la consommation de combustibles fossiles suscite des pré occupations mondiales concernant le changement climatique. Dé velopper une é conomie de l'hydrogè ne vert basé e sur les é nergies renouvelables est largement considé ré comme une solution prometteuse pour l'avenir de l'é nergie. La pile à combustible à membrane é changeuse de protons (PEMFC) est un dispositif propre et performant permettant l'utilisation de l'hydrogè ne dans le domaine des transports et du portable. Cependant, en raison de la lenteur de la ré action de ré duction de l'oxygè ne (ORR) à la cathode et de l'environnement acide des PEMFC, de grandes quantité s d'é lectrocatalyseurs à base de platine (Pt) sont né cessaires pour atteindre les performances de puissance acceptables, ce qui entrave leur commercialisation plus large é chelle. Bien que des progrè s aient é té ré alisé s pour ré duire les quantité s de Pt utilisé es dans les PEMFC, comme l'utilisation d'alliages de Pt pour augmenter l'activité intrinsè que du Pt, le coût du systè me de pile à combustible reste prohibitif. Par consé quent, le dé veloppement de catalyseurs à base de mé taux non nobles pour l'ORR en milieu acide pour remplacer à terme les catalyseurs à base de Pt est hautement souhaitable. De nos jours, ces principaux catalyseurs pour l'ORR dans les PEMFC sont à base de mé taux de transition (M = Fe, Mn ou Co) inté gré s dans une matrice de carbone dopé e à l'azote (M-N-C). Né anmoins, l'activité de ce catalyseur est insuffisante pour alimenter les PEMFC dans le secteur automobile. Il est né cessaire d'amé liorer encore les performances en augmentant la fré quence de rotation (TOF) et/ou la densité des sites actifs (SD) dans les catalyseurs M-N-C.

Ce chapitre passe en revue les progrè s ré alisé s sur les é lectrocatalyseurs Fe-N-C. Il commence par pré senter le mé canisme ORR et de la structure du site actif dans les catalyseurs Fe-N-C. Diffé rent du catalyseur à base de Pt, l'é tape qui dé finit la ciné tique du catalyseur Fe-N-C implique un transfert d'é lectrons couplé aux protons dans la ré gion contrôlé e par le transport de masse, conduisant ainsi à une activité intrinsè que infé rieure à une surtension é levé e. Au contraire, ce n'est pas le facteur limitant dans la ré gion contrôlé e par la ciné tique. Par consé quent, grâ ce à une optimisation minutieuse de leur structure, les catalyseurs Fe-N-C pourraient concurrencer avec succè s les catalyseurs à base de Pt. Le site actif du catalyseur Fe-N-C est gé nér alement considé ré comme une structure de coordination Fe-N4. Sur la base des analyses faites par [START_REF] Flyagina | Identifying the Catalytic Active Sites in Heteroatom-Doped Graphene for the Oxygen Reduction Reaction[END_REF] Fe Mössbauer et par spectroscopie d'absorption des rayons X, trois configurations Fe-N4 diffé rentes ont é té distingué es, le spin bas (Fe(II)-N4/C, D1), le spin intermé diaire (Fe(II)-N2+2 /C, D2) et le spin haut (N-Fe-N2+2/C, D3). Parmi eux, les sites D1 et D3 ont une orbitale 3dz² inoccupé e ou occupé e par un seul spin, ils se lient donc directement à l'extré mité de l'oxygè ne accompagnant l'oxydation de Fe (II) en Fe (III). D'autre part, puisque le site D2 a entiè rement occupé l'orbite 3dz 2 , cela suggè re qu'il n'est pas accessible à O2 ou qu'il se lie faiblement à O2.

Les facteurs affectant l'activité du catalyseur Fe-N-C sont ensuite discuté s. Les diffé rents centres mé talliques pré sentent une activité intrinsè que diffé rente. Fe est le plus actif parmi les diffé rents centres de mé taux de transition et certains catalyseurs bimé talliques peuvent amé liorer la durabilité du catalyseur. Les proprié té s de transport de masse sont é galement importantes pour amé liorer les performances des catalyseurs Fe-N-C dans les PEMFC, car une couche plus é paisse à la cathode est né cessaire pour compenser une activité massique infé rieure du catalyseur sans Pt. Dans ce cas, un support carboné hautement poreux est important pour amé liorer les proprié té s de transport de masse des cathodes Fe-N-C, facilitant la diffusion de l'O2 vers les sites actifs et l'évacuation de H2O produite. De nos jours, les catalyseurs Fe-N-C les plus performants sont des catalyseurs à base de mé sopores, soit pré paré s à partir d'une mé thode de « hard templating », soit d'un catalyseur à base de structure organomé tallique (MOF). Les deux types de catalyseurs contiennent un volume de micropores é levé pour hé berger des sites actifs et un volume de mé so/macropores pour amé liorer les proprié té s de transport de matiè re. Ré cemment, certains catalyseurs Fe-N-C à base d'aé rogel ont é té synthé tisé s avec succè s en pyrolysant un aé rogel de ré sorcinol-formaldé hyde-mé lamine (RMF) dopé au Fe. Les aé rogels sont des candidats idé aux comme support de carbone mé soporeux en raison de leur surface spé cifique é levé e et de leurs larges volumes de pores. De plus, la porosité et la surface spé cifique des aé rogels de carbone sont ajustables en modifiant les conditions de synthè se et de procé dé s. Le protocole de synthè se de l'aé rogel RMF é vite l'utilisation de « hard templating» avec de l'acide fluorhydrique, et est donc plus respectueux de l'environnement et adapté à une production à grande é chelle.

La durabilité du catalyseur Fe-N-C est un autre facteur important qui doit ê tre pris en compte avant qu'il ne soit largement utilisé dans les PEMFC. La perte d'activité des catalyseurs Fe-N-C est principalement attribué e à l'oxydation/attaque du site actif par H2O2 en milieu acide. L'augmentation du degré de graphitisation pourrait ê tre une mé thode prometteuse pour ré duire la corrosion du carbone au cours du processus ORR, pré servant ainsi davantage de sites actifs. En outre, diffé rentes configurations de sites actifs pré sentent é galement une stabilité diffé rente. Le site D1 n'est pas durable durant le fonctionnement de la PEMFC, se transformant rapidement en oxydes, car il est exposé en surface et se lie directement à l'oxygè ne. En revanche, le site D2 est plus durable en raison de l'absence de changement d'é tat d'oxydation pendant le processus ORR. Par consé quent, la durabilité des catalyseurs Fe-N-C pourrait ê tre amé lioré e en augmentant la densité des sites D2 et en inté grant les sites D1 dans un support de carbone plus graphitique.

General context

Energy is the cornerstone of modern civilization and the prerequisite for sustainable development. Over the past few decades, global primary energy consumption increased enormously with the step of population growth and industrialization of developing countries. Since fossil fuels are the primary energy source for almost all countries, greenhouse gas (GHG) emission, which mainly indicates carbon dioxide and methane, significantly increased and resulted in environmental pollution and the global warming effect cannot be ignored [START_REF] Hua | Development of renewable energy in Australia and China: A comparison of policies and status[END_REF] . According to the statistic of the International Energy Agency (IEA), the average annual growth of GHG emission is 2.6% per year between 1950 and 2009, as seen in Figure 1.1. In recent years, many countries have experienced an unprecedented number of climate events, with extreme weather leading to flooding, drought, and wildfires, and causing food shortages, health problems, and major damage to ecosystems and human habitats. How best to combat climate change and global warming while satisfying the world's energy consumption without impairing the global economy is an urgent problem for every country. Fortunately, since the 21 st United Nations Climate Change Conference (COP 21) in Paris, Parties to the United Nations Framework Convention on Climate Change (UNFCCC) reached a landmark agreement to combat climate change and to accelerate and intensify the actions and investments needed for a sustainable low carbon future. In 2018 the Intergovernmental Panel on Climate Change (IPCC) found that global net anthropogenic CO2 emission would need to reach net zero around 2050 in a pathway consistent with limited global temperature increases to 1.5º C 3 . The European Union (EU) is considering zero emissions as an objective for 2050, and others are likely to do the same. As a result, the average annual growth of GHG emission rate in the past decade (2010-2019) has been slowed to 1.1% per year, and the total CO2 emission reached a high of 36.1 gigatonnes (Gt) in 2019. Reasons for this decadal slowdown include a global reduction in new coal capacity additions, the steady substitution of coal for gas in the power sectors of developed countries, and the increasing pace of renewable energy deployments worldwide 2 . However, global GHG emissions are still far from reaching a plateau, not to mention a reversal of this trend. Although the total GHG emission was reduced by 5.1% in 2020 due to the impacts of the Covid-19 pandemic, this value quickly rebounded to reach its highest-ever annual level, accompanied by the recovery of energy demand from the Covid-19 pandemic in 2021. A 6% increase from 2020 pushed emissions to 36.3 Gt. The EU's energy policy aims to ensure a competitive, sustainable, and secure energy supply [START_REF] Witt | Combining scenario planning, energy system analysis, and multi-criteria analysis to develop and evaluate energy scenarios[END_REF] . To achieve this energy policy triangle, a serious impetus exists to diversify our energy sources, reducing our reliance on fossil fuels by turning to renewable energy such as solar, wind, and hydroelectric power [START_REF] Chu | Opportunities and challenges for a sustainable energy future[END_REF][START_REF] Seh | Combining theory and experiment in electrocatalysis: Insights into materials design[END_REF] . However, due to the uncontrollable change of weather conditions, the stability of their output poses a challenge when their share of the future primary energy mix rises. To get over this hurdle, a hydrogen (H2) economy based on renewable energy, including H2 production, storage, and conversion, is widely considered a promising solution for the future of energy. H2 is an excellent energy carrier to alternate fossil fuels that gear to the requirements of renewable, zero emission, and high quantity. Its potential role has similarities with that of electricity, but the crucial difference is that H2 is a chemical energy carrier composed of molecules and not only electrons. This distinction underpins all the reasons H2 might outcompete electricity in some situations [START_REF]The Future of Hydrogen[END_REF][START_REF] Nazir | Is the H2 economy realizable in the foreseeable future? Part I: H2 production methods[END_REF] . As a low-carbon chemical energy carrier, hydrogen is a leading option for reducing those hard-to-abate emissions, such as transport, heating, and industrial raw materials, because it can be stored, combusted, and combined in chemical reactions similar to natural gas, oil, and coal. H2 can be obtained from several resources, such as water electrolytic splitting or hydrocarbon reforming. Since the latter method still has a high CO2 intensity upstream, the water-splitting method has a better prospect. Nowadays, the electricity cost for water splitting is one of the significant barriers to a sustainable energy system and economy. However, there are opportunities to produce low-carbon H2 on a large scale if renewable power generation becomes sufficiently cheap and widespread. This offers a means of escaping the constraint of burning carbon-based fossil fuels [START_REF] Nazir | Is the H2 economy realizable in the foreseeable future? Part I: H2 production methods[END_REF] .

In the hydrogen economy, fuel cells are regarded as the energy conversion devices of the future due to their excellent compatibility with H2 energy carrier. They enable secure and suitable energy conversion in an environmental and relatively cost-effective way [START_REF] Sharaf | An overview of fuel cell technology: Fundamentals and applications[END_REF][START_REF] Nazir | Is the H2 economy realizable in the foreseeable future? Part III: H2 usage technologies, applications, and challenges and opportunities[END_REF] . A fuel cell is an electrochemical device that converts the chemical energy of a fuel directly into electrical energy, accompanied by some thermal energy and H2O as byproducts. While in the case of heat engines, producing electricity is a multi-step process that involves combustion to produce thermal energy from the internal chemical energy of the fuel. Then this thermal energy is converted into mechanical energy, and finally, this mechanical energy is converted into electrical energy using a generator. Generally, as the number of energy conversion processes increases in a certain device, the overall system efficiency of the device decreases. The onestep nature of fuel cell provides an efficient and clean mechanism for energy conversion compared with the traditional combustion-based processes [START_REF] Sharaf | An overview of fuel cell technology: Fundamentals and applications[END_REF] . On the other hand, heat engines are limited by the Carnot efficiency between their low and high working temperatures. Additionally, the conception without moving parts of fuel cells also means quiet operation without the noise of vibration. At the same time, their inherent modularity allows for simple construction and a diverse range of applications in portable, stationary, and transportation power generation. The following introductory section provides a brief overview of the electrochemical technologies targeting fuel cells, especially proton exchange membrane fuel cells.

Proton Exchange Membrane Fuel Cell Fundamentals of fuel cell

A fuel cell stack consists of a stack of membrane electrode assemblies (MEA) and bipolar plates. As the core of a single cell, the MEA consists of a membrane, two dispersed catalyst layers, and two gas diffusion layers (GDL) [START_REF] Thompson | Direct hydrogen fuel cell electric vehicle cost analysis: System and high-volume manufacturing description, validation, and outlook[END_REF] . The electrodes consist of a porous material covered with a layer of catalyst. In a fuel cell stack, each bipolar plate supports two adjacent cells. The bipolar plates typically have four functions: (1) to distribute the fuel and oxidant within the cell, (2) to facilitate water management within the cell, (3) to separate the individual cells in the stack, and (4) to carry current away from the cell. Additionally, in the absence of cooling plates, they also facilitate heat management.

Figure 1.2 illustrates the elemental composition of a single cell of proton exchange membrane fuel cell (PEMFC). H2 fuel is delivered from a gas-flow stream to the anode and then oxidized to protons by the half-cell reaction (1-1), called hydrogen oxidation reaction (HOR). The protons migrate through the proton exchange membrane (PEM) to the cathode, while the electrons are forced through an external circuit to the cathode. At the cathode, the electrons and protons react with the oxygen (O2) supplied from an external gas-flow stream to form water, as the half-cell reaction (1-2), which is called oxygen reduction reaction (ORR). Thus the overall reaction in the fuel cell can be written as (1-3), with every stack unit offering a maximum potential of 1.229 V without consideration of any dissipation.

Anode: 2H2 → 4H + + 4e -E 0 = 0.000 V vs. SHE (1-1) Cathode: O2 + 4H + + 4e -→ 2H2O E 0 = 1.229 V vs. SHE (1-2) Total: H2 + ½ O2 → H2O + Welectrical + Qheat E 0 = 1.229 V (1-3) The efficiency can then be calculated from the ratio between the electrical power output and the fuel input, as showed in equation (1-4) [START_REF] Barbir | Efficiency and economics of proton exchange membrane (PEM) fuel cells[END_REF] :

𝜂 𝐹𝐶 = electrical power output (𝑃 𝐹𝐶 ) fuel input (𝐹 𝑖𝑛 )

.

(1-4)

PFC is simply a product of potential (V) and current (I):

𝑃 𝐹𝐶 = 𝑉 • 𝐼 (1-5)
Fin is a product of fuel consumption rate and its energy content, usually given as enthalpy (ΔH). Hydrogen's heating value is 142 kJ• g -1 . According to the Faraday's Law and reaction (1-1), the quantity of hydrogen consumption (𝑞 H 2 ) can be expressed as equation (1-6):

𝑞 H 2 = 𝑚𝐼 𝑛𝐹 (1-6)
where m is hydrogen molecular weight (2.016), n is number of electrons transferred for every hydrogen molecule (2), and F is Faraday constant (96,485 C mol -1 ). The Fin is therefore:

𝐹 𝑖𝑛 = 𝑞 H 2 • Δ𝐻 = 𝑚Δ𝐻 𝑛𝐹 𝐼 (1-7)
For the electrochemical reaction (1-1), the expression 𝑚Δ𝐻/𝑛𝐹 has a value of 1.482 V. This is also called the thermoneutral or reversible potential, which corresponds to the maximum possible energy (both electrical and thermal) resulting from the chemical reaction. The 𝜂 𝐹𝐶 can then be simplified by elimination current and expressed as the ratio of operating voltage and 1.482V:

𝜂 𝐹𝐶 = V 1.
482 (1-8) Therefore, the maximum theoretical hydrogen fuel cell efficiency at 25°C is 1.229/1.482 = 0.83. 𝑉 ohm = 𝑗𝑅 𝑗 (1-11) where, α is the transfer coefficient; j is current density (A cm -2 ); j0 is exchange current density (A cm -2 ); jL is limiting current density (A cm -2 ); and Rj is cell resistance (Ω cm 2 ).

Activation and mass transport losses can exist at both the cathode and anode, while the ohmic losses are mainly due to resistance in electrolyte. The cell potential can be then present as:

𝑉 cell = 𝑉 0 -𝑉 act,a -𝑉 act,c -𝑉 mt,a -𝑉 mt,c -𝑉 ohm (1-12) When no load is connected to the fuel cell, i.e., at open circuit, the potential of PEMFC is typically around 1 V. When the load is connected to the fuel cell, the cell potential decreases further as more current is generated, leading to lower efficiency. Since the power density of fuel cells increases with decreasing cell voltage, the efficiency at the maximum power density of fuel cells is much lower than that at partial loads, which makes the fuel cells very attractive and efficient for applications with highly variable loads where most of the time fuel cells are operated at low load and high efficiency. Therefore, the fuel cell nominal efficiency is an arbitrary value, ranging anywhere between 0.3 and 0.6, depending on the operation potential. This result implies the efficiency of a fuel cell may be increased by adding more cells to achieve the expected output power density. However, due to the economic factors limit, the individual cells should be controlled at an adapted number. Reducing the potential loss in the single cell is crucial in increasing the efficiency of fuel cell. The ohmic losses are generally minimal among overall potential losses if an appropriate coating method is employed [START_REF] Akula | Mesoporous textured Fe-N-C electrocatalysts as highly efficient cathodes for proton exchange membrane fuel cells[END_REF] . Therefore, the efforts should be focused on reducing the activation and mass transport losses.

The activation losses are determined by the property of the electrode, which contributes the most to the total loss of voltage [START_REF] Akula | Mesoporous textured Fe-N-C electrocatalysts as highly efficient cathodes for proton exchange membrane fuel cells[END_REF] . If the electrode is metallic, it has strongly overlapping orbitals and energy levels as a continuum. Therefore, the energy of the electrons can be increased by increasing the applied voltage, which can affect the Fermi-Level of the metal. Suppose the electrode is in contact with a solution containing reducible species, the Fermi-Level of the electrode may be increased enough to help the electron overcome the barrier of reaction and reduce them, such as ORR [START_REF] Pletcher | A First Course in Electrode Processes[END_REF] . The electron is transferred from the metal to the lowest unoccupied molecular orbital (LUMO) of the solution species. The reverse may also occur: Fermi-Level can be reduced to oxidize solution species by accepting the electron from their highest occupied molecular orbital (HOMO) in the metal (such as HOR). The difference between the energy levels of the metal and the solution species determines whether the electron transfer is thermodynamically favorable but not its kinetics. Electrode kinetics describes how the rate of electron transfer at an electrode surface is affected by the applied voltage.

The reduction reaction can be written as the reaction:

O + e -↔ R (1-13) where O represents the oxidized form of the solution species and R represents the reduced form. The rates of the forward, reduction Rred, and backward, oxidation Rox, reactions can be expressed as -14) where kred and kox are the rate constants of reduction and oxidation and [O] and [R] are the concentrations of the species O and R respectively. As well as the expression of concentration, the reaction rate can also be expressed by the electrical current at the electrode surface, i, as this is the rate of electron transfer. The reductive current (ired) and oxidative current (iox) are then written as ired = nFAkred [O]s iox = nFAkox[R]s (1-15) where n, F, and A are the number of electrons transferred during the reaction, Faraday's constant, and the electrode area, respectively. [O]s and [R]s are the concentration of species O and R at the electrode surface, where the corner mark indicates they are different from the concentration in bulk solution. The net rate of reaction (Rnet) is the difference between forward and backward reaction rates. Therefore, the net current (inet) can be expressed as

Rred = kred[O] Rox = kox[R] ( 1
inet = ired -iox = nFAkred[O]s -nFAkred[R]s (1-16)
Rate constants vary with temperature according to the Arrhenius rate equation:

k = A exp(-Ea/RT) (1-17)
where Ea is the activation energy of the reaction, R is the gas constant, T is the temperature in Kelvin, and A is the pre exponential factor. Because the rate constants kred and kox are also affected by the applied potential at the electrode surface, the kinetic equation must describe the effect of potential (E) upon rate, as the Arrhenius equation does with temperature. To achieve this, the Butler-Volmer equation of electrochemical kinetics has been proposed, which can be written as (1-18) for the single electron transfer reaction [START_REF] Bard | Electrochemical Methods: Fundamentals and Applications[END_REF] :

𝑖 𝑛𝑒𝑡 = 𝑖 0 (𝑒 𝛼𝐹(𝐸-𝐸 𝑒𝑞 ) 𝑅𝑇 -𝑒 - (1-𝛼)𝐹(𝐸-𝐸 𝑒𝑞 ) 𝑅𝑇
) (1-18) where Eeq is the equilibrium potential for the reaction and E is the potential applied to the electrode surface. i0 is the exchange current, representing the current flowing both ways at Eeq (ired or iox, where there is zero net current). i0 has equation: 1-𝛼 (1-19) where k 0 is the rate constant for the reaction at Eeq. α is called the symmetry factor. The physical meaning of the terms α and 1-α in equation (1-18) and (1-19) are to do with the position of the activation energy peak for the reaction along the reaction coordinate. α here is the position of the activation energy peak along the reaction coordinate of the oxidation reaction. Therefore 1-α represents the peak position along the reaction coordinate of the reverse reaction (reduction reaction). A value for α of 0.5 (1-α = 0.5) signifies that the peak for the activation energy barrier of both the reduction and oxidation reaction is halfway along the reaction coordinate. Thus, half of the energy from the potential drop at the electrode-electrolyte interface is used to push the reaction, as the reaction coordinate is downhill after that. Values of α different from 0.5 imply an asymmetry in the reaction coordinate, which means the peak of the activation energy barrier lying closer to reactants or products. In these cases, the proportion of the energy from the potential drop is different for the oxidation and reduction reactions.

𝑖 0 = 𝐹𝐴𝑘 0 [𝑅] 𝛼 [𝑂]
As E-Eeq above is the difference between the applied and equilibrium potential, it is commonly replaced with the symbol η, which is known as the overpotential. Thus, the equation (1-18) can also be written as the most common form:

𝑗 𝑛𝑒𝑡 = 𝑗 0 (𝑒 𝛼𝐹𝜂 𝑅𝑇 -𝑒 -(1-𝛼)𝐹𝜂 𝑅𝑇 ) (1-20)
where j0 is the exchange current density and jnet is the net current density. The effect of exchange current density on the current-potential response is shown in Figure 1.3 [START_REF] Pletcher | A First Course in Electrode Processes[END_REF] . High values of j0 are observed for fast, well-catalyzed reactions such as the HOR in PEMFCs. The complimentary ORR has a j0 several orders of magnitude lower than this due to its slow kinetics. This is in line with the equation(1-10) that a higher j0 close to the Eeq leads to a lower activation loss. To overcome this low j0, excess overpotential has to be paid for the ORR, which reduces the voltage of every unit cell. In order to increase the efficiency of PEMFCs, a high quantity of catalysts on the cathode is necessary, thus significantly increasing the cost. In this case, j0 is crucial in electrocatalysis as it describes the ability of the catalyst to promote the reaction. Mass transport losses are essential factors that should be considered as well. In order to maintain the continuous isothermal operation of fuel cells, the heat and water byproducts must be continuously removed from the cells. Otherwise, the accumulated water in the MEA would mute the O2 to access the catalyst. Figure 1.4 exhibits a structure of MEA in the single cell [START_REF] Reshetenko | Design of PGM-free cathodic catalyst layers for advanced PEM fuel cells[END_REF] . The thickness of catalyst layer on the cathode (~90 µm) is much higher than that on the anode (< 10µm) due to huge quantity of catalysts was employed to promote the sluggish ORR, leading to higher mass transport resistance. In general, mass transport losses in the platinum-group metal-free catalyst MEA are much higher than that in the platinum-contained cell due to their significantly thicker catalyst layer (more than 50-70 µm for platinum-group metal-free catalyst compared to 4-8 µm for Pt-based catalyst). As a result, a highly porous structure is usually necessary for platinum-group metal-free catalysts to reduce the mass transport resistance [START_REF] Akula | Mesoporous textured Fe-N-C electrocatalysts as highly efficient cathodes for proton exchange membrane fuel cells[END_REF][START_REF] Reshetenko | Impedance Spectroscopy Characterization of PEM Fuel Cells with Fe-N-C-Based Cathodes[END_REF][START_REF] Reshetenko | Systematic study of back pressure and anode stoichiometry effects on spatial PEMFC performance distribution[END_REF] . In PEMFC, the proton exchange membrane also contributes partial mass transport losses. The membrane separates two half-reactions and allows the protons to pass through to balance the proton concentration of two sides. An ideal membrane for high-power PEMFCs should possess high proton conductivity under low humidity conditions and good electrochemical and mechanical stability [START_REF] Sharaf | An overview of fuel cell technology: Fundamentals and applications[END_REF] . Nowadays, perfluorosulfonic acid (PESA) is the most commonly used membrane material for PEMFCs [START_REF] Jiao | Designing the next generation of proton-exchange membrane fuel cells[END_REF] , which benefits from the stability of the backbone and high ionic conductivities [START_REF] Gittleman | Durability of Perfluorosulfonic Acid Membranes for PEM Fuel Cells[END_REF] . PESA consists of three regions: (1) a polytetrafluoroethylene acts as the backbone, ( 2) side chains of -O-CF2-CF2-O-CF2-CF2-which connect the molecular backbone to the third region, and (3) ion clusters consisting of sulfonic acid ions [START_REF] Kraytsberg | Review of Advanced Materials for Proton Exchange Membrane Fuel Cells[END_REF] . When the membrane becomes hydrated, the protons in the third region become mobile and moving between sulfonic acid sites with the help of hydrogen bond [START_REF] Gittleman | Durability of Perfluorosulfonic Acid Membranes for PEM Fuel Cells[END_REF][START_REF] Kraytsberg | Review of Advanced Materials for Proton Exchange Membrane Fuel Cells[END_REF] . The most commonly used PFSA membrane is Dupont's Nafion ® , a copolymer of tetrafluoroethylene backbone and sulfonic acid-terminated perfluoro vinyl ether pendant, which offers a proton conductivity of 0.13 S cm -1 (75 º C and 100% relative humidity (RH)) and durability above 60,000 h 22 . To improve the power density of PEMFCs, the primary strategy is to reduce the thickness of the membrane. However, thin membranes also face challenges of mechanical damage or electrochemical degradation [START_REF] Jiao | Designing the next generation of proton-exchange membrane fuel cells[END_REF] . Besides, the operation temperature of PESA membranes is below 100 °C because the waterfilled channels determine the proton conductivity. Unfortunately, heat is necessary for an excellent kinetic reaction, but too much heat will cause the membrane to dry up and increase the mass transport resistance. Therefore, some new membranes used in high-temperature PEMFC, such as sulfonated hydrocarbon polymers, have been considered promising alternatives owing to their high proton conductivity and thermal stability at high temperatures and humidities [START_REF] Jiao | Designing the next generation of proton-exchange membrane fuel cells[END_REF] . [START_REF] Reshetenko | Design of PGM-free cathodic catalyst layers for advanced PEM fuel cells[END_REF] . Anode: Pt/C catalyst, Cathode: Fe-Mn-N-C catalyst.

Different types of fuel cell

There are several major types of fuel cells, as showed in Table 1.1 [START_REF] Aminudin | An overview: Current progress on hydrogen fuel cell vehicles[END_REF][START_REF] Mekhilef | Comparative study of different fuel cell technologies[END_REF] , which are mainly classified by different electrolytes, charge carriers, and operating temperatures. Phosphoric Acid Fuel Cells (PAFCs), High/Low Temperature Proton Exchange Membrane Fuel Cells (HT/LT-PEMFCs), Alkaline Fuel Cells (AFCs), and Alkaline anion exchange membrane fuel cells (AAEMFCs) work at relatively low temperatures, which essentially require pure hydrogen to be supplied to the anode. Accordingly, using hydrocarbon or alcohol fuels requires an external fuel processor that converts the fossil fuel into hydrogen to be incorporated into the system. In contrast, the Solid Oxide Fuel Cells (SOFCs) and Molten-Carbonate Fuel Cells (MCFC) operating at higher temperatures have the advantage that both CO and H2 can be electroactive at the anode, thus more suitable for hydrocarbon fuel. Air can be employed directly as the reactant at the cathode of almost all of these types of fuel cell except AFC because the electrolyte risks the degradation by the CO2 in air. Among these different fuel cells, PEMFCs have many advantages such as low operating temperature, sustained operation at a high current density, low weight, compactness, the potential for low cost and volume, long stack life, fast start-ups, and suitability for discontinuous operation [START_REF] Kraytsberg | Review of Advanced Materials for Proton Exchange Membrane Fuel Cells[END_REF] . These features make PEMFCs the most promising candidate for wide applications ranging from portable power and transportation to large-scale stationary power systems for buildings and distributed generation. Fuel cell vehicle (FCV) is one of the major applications of PEMFC. Compared with battery electric vehicles (BEVs), H2 has inherent advantages over lithium-ion batteries, including higher energy density and shorter refueling time for FCVs [START_REF] Gröger | Review-Electromobility: Batteries or Fuel Cells?[END_REF] . The FCV also outperforms the BEV under sub-zero temperature conditions because the BEV often exhibits considerably reduced discharge capacity [START_REF] Jiao | Designing the next generation of proton-exchange membrane fuel cells[END_REF] . The cost of FCVs is currently higher than that of BEVs for shortrange vehicles (below 322 km); however, the sequence is inversed with high annual production rates, particularly for long-range vehicles (above 483 km) [START_REF] Gröger | Review-Electromobility: Batteries or Fuel Cells?[END_REF] . As a significant milestone in PEMFC commercialization, Toyota introduced their first FCV, Mirai, in 2017. The fuel cell system adopts advanced design and materials to achieve a Pt-loading of 0.365 mg cm -2 , 2.0 kW kg -1 , and 3.1 kW L -1 for 114 kW generations [START_REF] Wang | Materials, technological status, and fundamentals of PEM fuel cells -A review[END_REF] . At the end of 2020, the stack power density of the second generation reached 4.4 kW L -1 , an increase of 42% compared with the previous generation. The European Union Fuel Cells and Hydrogen 2 Joint Undertaking (EU-FCH2JU) recently demonstrated a PEMFC stack with a power density of 5.38 kW L -1 at a current density of 2.67 A cm -2 and a single-cell voltage of 0.6 V, and the goal is 9.3 kW L -1 by 2024 [START_REF] Jiao | Designing the next generation of proton-exchange membrane fuel cells[END_REF] .

Currently, there are two major barriers to the wide application of PEMFC: cost and durability. The total cost of a PEMFC system includes all the major components: the stack and the balance of plant (BOP), the latter including the air loop, humidifier and water recovery loop, high-and low-temperature coolant loops, the fuel loop, and the fuel cell system controller. Each of the two parts accounts for about 50% of the total system cost. The most significant contributions to the stack cost are the MEAs and bipolar plates, and the air loop is by far the costliest subsystem within the BOP [START_REF] Thompson | Direct hydrogen fuel cell electric vehicle cost analysis: System and high-volume manufacturing description, validation, and outlook[END_REF] . The current state is 40 $• kW -1 for 500,000 per year product volume, reduced by 60% in the past decades. The ultimate DOE target is 30 $• kW -1 for fuel cells in FCVs and 600,000 $ per bus for FC bus [START_REF] Wang | Materials, technological status, and fundamentals of PEM fuel cells -A review[END_REF] . Among all the crucial technologies, catalysts are the fundament of fuel cells that decide the cost and conversion efficiency of the devices. However, even if the manufacturing cost has been reduced as much as possible by mass manufacture, the cost of catalysts is less dependent on manufacturing volume. As seen in Figure 1.5, among the components, the catalysts contribute 41% of the total cost of PEMFC at high-volume production, primarily due to the use of noble metal catalysts to promote the sluggish ORR on the cathode [START_REF] Thompson | Direct hydrogen fuel cell electric vehicle cost analysis: System and high-volume manufacturing description, validation, and outlook[END_REF] . It is thus obvious that the use of Pt affects significantly the PEMFC cost and consequently its competitiveness on the market. Furthermore, Pt is considered as a critical raw material by European Union due to its irreplaceable character in several highly important industrial applications, lack of interior resources and general scarcity [START_REF] Duclos | Closing the loop: life cycle assessment and optimization of a PEMFC platinum-based catalyst recycling process[END_REF] . On the other hand, the old catalysts cannot be directly reused in a new MEA owing to the chemical and structural modifications of the MEA components during the operation of PEMFC. Therefore, the application of Pt-based catalysts requires a mature recycling system to manage the Pt in a closed loop [START_REF] Duclos | Closing the loop: life cycle assessment and optimization of a PEMFC platinum-based catalyst recycling process[END_REF] . The well-established methods are based on a pyrometallurgical process, which is not economically and environmentally viable due to a huge emission of toxic hydrofluoric acid (HF) [START_REF] Xu | Recycling of membrane electrode assembly of PEMFC by acid processing[END_REF][START_REF] Zhao | Reclaim/recycle of Pt/C catalysts for PEMFC[END_REF] . For example, Xu et al. [START_REF] Xu | Recycling of membrane electrode assembly of PEMFC by acid processing[END_REF] recovered Pt by dipping waste MEA in hot sulfuric acid, where the polymeric membrane and carbon support were oxidized and dissolved. The Pt catalyst was collected as metallic particles with an average diameter of 35 nm. The novel recovery processes focus on selective dissolution of Pt to reduce the HF release. Duclos et al. [START_REF] Duclos | Process development and optimization for platinum recovery from PEM fuel cell catalyst[END_REF] leached the Pt from MEA by a mixture an oxidant (H2O2 or HNO3) and hydrochloric acid. Since no incineration is carried-out within this pure hydrometallurgical process, no HF is released. Recently, Sharma et al. [START_REF] Sharma | Pt/C Electrocatalyst Synthesis from Recycling of the Spent PEMFC Membrane Electrode Assembly: A Closed Loop Circular Economy[END_REF] recovered Pt in MEA by electrochemical dissolution in a dilute acidic bath. The Pt was then precipitated in form of (NH4)2PtCl6, a precursor for preparing Pt/C electrocatalyst. This method shows high overall recovery efficiency of >90% and is suitable for industrial upscaling, which is possible to be the last piece for closing the loop of Pt.

To date, the DOE 2025 target for Pt-based catalysts has been fulfilled (activity of 1.08 A mgPt -1 at 0.9 V with 0.1 mgPt cm -2 ) [START_REF] Chong | Ultralow-loading platinum-cobalt fuel cell catalysts derived from imidazolate frameworks[END_REF] , the long-term, large-scale deployment of PEMFCs in the field of automotive production still requires the use of elements that are far less expensive and more abundant than Pt. Therefore, developing highly efficient platinum-group metal-free catalysts is crucial to make the PEMFC more economical.

Bipolar plates 28%

Membrane 8%

Catalyst 41%

GDLs 6%

MEA frame 7%

Balance of stack 10%

The lifespan is another critical issue that restricts the large-scale commercialization of PEMFC. Currently, the lifetime of PEMFCs is only 2500-3000 h when it is used as a power plant for passenger vehicles, which is far from the near-term durability target of DoE (5000 h). Compared with the lifetime of vehicle PEMFCs, their life as a fixed power source can reach 30,000 h. The main reason for this huge disparity is that the operating conditions of the vehicle fuel cell are very complex, requiring frequent switches between start-stop, idling, high power, and load changing [START_REF] Chen | The reactant starvation of the proton exchange membrane fuel cells for vehicular applications: A review[END_REF] . Studies have shown that gas starvation during load change is the leading cause of fuel cell decay, which causes severe consequences of interaction: corrosion of the carbon support, cell reversal, and uneven current distribution. These consequences will further lead to catalyst agglomeration, loss, detachment, increased diameter of catalyst particles, reduced active surface area, and decreased catalyst layer thickness. Therefore, to ensure the fuel cell system can perform sufficiently long to serve the whole vehicle lifetime (5000 h or 150,000 miles on average), efforts to enhance the durability of catalysts are emergency.

Electrocatalysts of Oxygen Reduction Reaction (ORR)

In the PEMFC system, the ORR pathway always requires more energy input than the HOR pathway, which results in a more significant part of polarization resistance due to its sluggish kinetics at a low temperature (80-100°C) [START_REF] Zheng | Nanostructured Metal-Free Electrochemical Catalysts for Highly Efficient Oxygen Reduction[END_REF] . As presented in Table 1.2, ORR is a multi-step reaction processing via either a four-electron (4e -) path to directly produce H2O as final products or a two-step two-electron (2e -) path with H2O2 as the intermediate species, depending on the inherently electrocatalytic properties of catalysts. A higher selectivity of 4e -reduction indicates better electrocatalytic ORR efficiency of a given catalyst. In conventional fuel cell technology, precious Pt supported on a conductive carbon composite (Pt/C) is the most commonly used 4e - ORR catalyst due to the excellent activity of Pt [START_REF] Banham | Current Status and Future Development of Catalyst Materials and Catalyst Layers for Proton Exchange Membrane Fuel Cells: An Industrial Perspective[END_REF][START_REF] Wu | Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction[END_REF] . The kinetic barriers require a higher Pt loading (~0.4 mg• cm -2 ) at the cathode to achieve a desirable fuel cell performance [START_REF] Shao | Recent Advances in Electrocatalysts for Oxygen Reduction Reaction[END_REF] , which dedicates to the high cost of FCVs. Therefore, reducing its loading by nano-structuring and alloying, or even completely replacing it with low-cost platinum-group metal-free catalysts of ORR, is necessary for PEMFC. In order to well design the high-performance catalysts for ORR, the mechanism of the reaction on the electrode should be investigated and presented below.

Noble metal electrocatalysts

The detailed mechanism of ORR in acidic media can be divided into three pathways, as shown in [START_REF] Zhang | Are Fe-N-C Electrocatalysts an Alternative to Pt-Based Electrocatalysts for the Next Generation of Proton Exchange Membrane Fuel Cells?[END_REF] . For Pt-based and Fe-N-C catalysts, the associative-dissociative pathway is considered dominant [START_REF] Wang | A Review of Precious-Metal-Free Bifunctional Oxygen Electrocatalysts: Rational Design and Applications in Zn-Air Batteries[END_REF][START_REF] Kulkarni | Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction[END_REF] , containing three different intermediates OOH*, O*, and OH*. However, their binding energies are strongly correlated and change monotonically for different catalysts because all the adsorbates bind to the surface through an O atom, i.e., ΔGOH* =1/2 ΔGO* and ΔGOH* = ΔGOOH* -3.2 eV. In this case, the catalyst surface reactivity can be quantified as the function of the binding energy of OH*. An ideal ORR electrocatalyst should bind intermediates neither too strongly nor too weakly [START_REF] Zhang | Are Fe-N-C Electrocatalysts an Alternative to Pt-Based Electrocatalysts for the Next Generation of Proton Exchange Membrane Fuel Cells?[END_REF] . This results in a volcano-plot-like correlation between the activity and the binding energy of the intermediates. Among all investigated elements, Pt exhibits the closest-to-optimum OH* binding energy, being only 0.1-0.2 eV too strong [START_REF] Kulkarni | Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction[END_REF] . The ORR selectivity (2e -or 4e -pathway) mainly depends on the dissociation energy barrier of absorbed O2 molecules on the surface of active sites. The O2 dissociation energy barrier on the surface of Pt is very low, thus greatly favoring the 4e -pathway [START_REF] Banham | Current Status and Future Development of Catalyst Materials and Catalyst Layers for Proton Exchange Membrane Fuel Cells: An Industrial Perspective[END_REF][START_REF] Shao | Recent Advances in Electrocatalysts for Oxygen Reduction Reaction[END_REF][START_REF] Marković | Oxygen Reduction Reaction on Pt and Pt Bimetallic Surfaces: A Selective Review[END_REF] . The ORR behaviors of low-index Pt surfaces follow the order of Pt(100) < Pt(111) < Pt(110) in the weakly adsorbed electrolyte, such as HClO4 solution [START_REF] Zhang | Are Fe-N-C Electrocatalysts an Alternative to Pt-Based Electrocatalysts for the Next Generation of Proton Exchange Membrane Fuel Cells?[END_REF][START_REF] Shao | Recent Advances in Electrocatalysts for Oxygen Reduction Reaction[END_REF][START_REF] Nie | Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction[END_REF] . As a result, octahedral Pt nanoparticles bound by (111) facets were found to be more active in ORR than cubic ones bound by (100) facets, consistent with the bulk single crystal regular [START_REF] Vojislav | Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability[END_REF] . The particle size effect on ORR has also been a long-standing problem that has yet to be solved. Smaller particle sizes can reach a larger specific surface, which suggests an enhancement of mass activity. The optimized mass activity for Pt nanoparticles is observed in the size of 2-3 nm [START_REF] Perez-Alonso | The Effect of Size on the Oxygen Electroreduction Activity of Mass-Selected Platinum Nanoparticles[END_REF][START_REF] Shao | Electrocatalysis on Platinum Nanoparticles: Particle Size Effect on Oxygen Reduction Reaction Activity[END_REF] . Further downsizing of the Pt nanoparticles results in inferior intrinsic specific activity due to dominating low coordinated steps and kinks leading to too strong oxygen binding [START_REF] Shao | Electrocatalysis on Platinum Nanoparticles: Particle Size Effect on Oxygen Reduction Reaction Activity[END_REF] .

Because of the limitation of monometallic Pt, alloy catalysts with a lower content of expensive Pt become a better choice to reduce the cost. In past years, many Pt-based alloy systems with a wide range of compositions, such as PtPd, PtAu, PtAg, PtCu, PtFe, PtNi, PtCo, and PtW, have been studied and accordingly, improved ORR activities of these alloy systems have been reported [START_REF] Nie | Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction[END_REF] . Several hypotheses have been proposed to draw correlations between the enhanced activity of Pt alloys over Pt and their properties, such as structural factors and electronic factors. According to the mechanism of ORR on Pt surface discussed before, the shortening of Pt-Pt interatomic distance, which is favorable for the dissociative adsorption of O2, the increasing catalytic activity of Pt alloys shows a strong correlation with the decrease in interatomic distance between Pt atoms. It is found the base-metal elements smaller than Pt, when alloyed with Pt, enter the crystal structure through substitution and cause a lattice contraction resulting in more favorable sites that enhance the dissociative adsorption of oxygen. From the view of the surface electronic structure of Pt, the electron density in the 5d orbitals of Pt can be modified by base-metal elements [START_REF] Kulkarni | Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction[END_REF] . Some DFT calculations have shown that modified chemisorption properties for adsorbed oxygen species on a Pt alloy surface are largely caused by short-range electronic charge-transfer effects and long-range geometric lattice strain. These electronic and geometric effects result in a shift of the energetic center of Pt-projected d-states affecting surface adsorbate bond strengths and thus leading to altered chemisorption of reactants, intermediates, and products [START_REF] Shao | Recent Advances in Electrocatalysts for Oxygen Reduction Reaction[END_REF][START_REF] Nie | Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction[END_REF] .

Stamenkovic et al. investigated the polycrystalline alloy films of Pt3M catalysts (M = Ni, Co, Fe, V, and Ti) to understand the role of 3d metals in the catalytic activity of Pt alloys [START_REF] Stamenkovic | Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure[END_REF] . They found that the catalytic activity for the ORR on such alloys was dependent on the nature of the 3d metal. The relationship exhibited a 'volcano-type' behavior, as shown in Figure 1.7 [START_REF] Stamenkovic | Trends in electrocatalysis on extended and nanoscale Ptbimetallic alloy surfaces[END_REF] . The maximum catalytic activity was controlled by a balance between adsorption energies of reactive intermediates and surface coverage by spectator species. In this aspect, Ni, Co, and Fe were found to be the most effective alloying elements. The nanoparticles of Pt3M (M = Fe, Ni, and Co) were synthesized and supported on high specific area carbon to investigate their ORR performance. The alloys showed more than 2-fold specific activity versus Pt at 0.9 V. Among them, Pt3Co/C had the highest activity, reaching 3.2 mA cm -2 . Chong et al. further composed Pt3Co alloy with a platinum group metal-free catalyst (Co-N-C) to overcome the shortages of ultralow Pt catalysts through a synergistic interaction, reaching both high mass activity of 1.08 A mgPt -1 at 0.9 V in PEMFC and good durability [START_REF] Chong | Ultralow-loading platinum-cobalt fuel cell catalysts derived from imidazolate frameworks[END_REF] .

M-N-C electrocatalysts

Non-noble metal electrocatalysts are the most promising candidates for the large-scale applied catalysts of ORR. In the past decades, numerous materials have been widely investigated for ORR in acid and/or alkaline media, such as metal oxides, [START_REF] Liang | Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction[END_REF][START_REF] Kumar | Effect of the Oxide-Carbon Heterointerface on the Activity of Co 3 O 4 /NRGO Nanocomposites toward ORR and OER[END_REF][START_REF] Shao | Recent Advances in Electrocatalysts for Oxygen Reduction Reaction[END_REF] metal chalcogenides, [START_REF] Zhao | Effect of Se in Co-based selenides towards oxygen reduction electrocatalytic activity[END_REF][START_REF] Gao | Nanostructured metal chalcogenides: synthesis, modification, and applications in energy conversion and storage devices[END_REF][START_REF] Zhao | Tungsten doped Co-Se nanocomposites as an efficient non precious metal catalyst for oxygen reduction[END_REF] metal-free nanostructured carbon materials, [START_REF] Flyagina | Identifying the Catalytic Active Sites in Heteroatom-Doped Graphene for the Oxygen Reduction Reaction[END_REF][START_REF] Nagy | Nitrogen doped mesoporous carbon aerogels and implications for electrocatalytic oxygen reduction reactions[END_REF] and especially metalnitrogen-carbon (M-N-C) catalysts [START_REF] Jaouen | Oxygen Reduction Catalysts for Polymer Electrolyte Fuel Cells from the Pyrolysis of Iron Acetate Adsorbed on Various Carbon Supports[END_REF][START_REF] Lefè Vre | Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells[END_REF][START_REF] Yang | Boosting Oxygen Reduction Catalysis with Fe-N 4 Sites Decorated Porous Carbons toward Fuel Cells[END_REF][START_REF] Santori | Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeN x Moieties in Fe-N-C Catalysts[END_REF][START_REF] Tan | Rational design and construction of nanoporous iron-and nitrogen-doped carbon electrocatalysts for oxygen reduction reaction[END_REF][START_REF] Xiao | Microporous Framework Induced Synthesis of Single-Atom Dispersed Fe-N-C Acidic ORR Catalyst and Its in Situ Reduced Fe-N 4 Active Site Identification Revealed by X-ray Absorption Spectroscopy[END_REF] . These catalysts have shown promising activity towards ORR, but their performances are still relatively low compared with Pt-based catalysts. Therefore, intensive research has been carried out in order to improve their reactions. The low cost of platinum-group metal-free catalysts allows a higher catalyst load on the cathode to offset their lower catalytic performance compared with Pt-based catalysts. However, the mass transport problems become more considerable due to a thicker catalyst layer [START_REF] Proietti | Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells[END_REF] .

Among various non-precious metal ORR catalysts, metal-nitrogen-carbon (M-N-C) materials (M = Co, Fe, Ni, Mn) are the most promising platinum-group metal-free catalysts for ORR, which have gained increasing attention due to their promising catalytic activity displayed towards ORR and the utilization of inexpensive precursor materials. Research into M-N-C complexes for ORR started in 1964 when Jasinski discovered that cobalt phthalocyanine showed ORR activity [START_REF] Jasinski | A New Fuel Cell Cathode Catalyst[END_REF] . Later, Yeager reported the first M-N-C composites for ORR by the pyrolysis of non-N4-macrocycles precursors [START_REF] Wiesener | N4-chelates as electrocatalyst for cathodic oxygen reduction[END_REF] . After that, numerous efficient ORR catalysts based on different M-N-C sources have been developed. Some recently reported Fe-N-C catalysts present an initial ORR activity approaching that of platinum-based catalysts in rotating disk electrode (RDE) setup and in PEMFC [START_REF] Akula | Mesoporous textured Fe-N-C electrocatalysts as highly efficient cathodes for proton exchange membrane fuel cells[END_REF][START_REF] Liu | Atomically dispersed iron sites with a nitrogen-carbon coating as highly active and durable oxygen reduction catalysts for fuel cells[END_REF][START_REF] Mehmood | High loading of single atomic iron sites in Fe-NC oxygen reduction catalysts for proton exchange membrane fuel cells[END_REF][START_REF] Jiao | Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites[END_REF][START_REF] Kisand | Templated Nitrogen-, Iron-, and Cobalt-Doped Mesoporous Nanocarbon Derived from an Alkylresorcinol Mixture for Anion-Exchange Membrane Fuel Cell Application[END_REF] . To prepare a M-N-C catalyst, the synthesis usually starts from a metal precursor, a nitrogen precursor, and a carbon support (which can also be replaced by a carbon precursor). A high-temperature pyrolysis of the mix of precursors is necessary to obtain the catalytic sites for ORR in the acid medium of PEMFC. As a result, various preparation conditions can affect the performance of the final catalysts, mainly including the metal precursor and ligand, metal loading, carbon support/precursor, and heat treatment conditions. However, the ORR mechanism on M-N-C catalysts and the properties of active sites should be clarified first to enhance the trial and error efficiency.

According to previous research, the associative-dissociative pathway is considered dominant in both Pt-based and M-N-C catalysts [START_REF] Wang | A Review of Precious-Metal-Free Bifunctional Oxygen Electrocatalysts: Rational Design and Applications in Zn-Air Batteries[END_REF][START_REF] Kulkarni | Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction[END_REF] . The slowest step of this pathway is the first charge transfer, i.e., the rate-determining step (RDS). A similar volcano plot is also established for several M-N-C catalysts by Sun et al., that Fe-N4 active site presents a closest-to-optimum OH* binding energy (ΔGOH*), similar to the Pt 72 . However, the Fe-N-C catalysts are far less active than Pt-based catalysts. Tse et al. proposed the low activity of Fe-N-C catalysts might be due to a different mechanism in acidic media [START_REF] Tse | Elucidating Proton Involvement in the Rate-Determining Step for Pt/Pd-Based and Non-Precious-Metal Oxygen Reduction Reaction Catalysts Using the Kinetic Isotope Effect[END_REF] . Unlike the Pt-based catalyst, which does not involve a proton transfer step (i.e., Pt + O2 + e - Pt-OO* -), the RDS of Fe-N-C involves a protoncoupled electron transfer (PCET), i.e., Nx-Fe + O2 + H + + e - Nx-Fe-OOH*) [START_REF] Tse | Elucidating Proton Involvement in the Rate-Determining Step for Pt/Pd-Based and Non-Precious-Metal Oxygen Reduction Reaction Catalysts Using the Kinetic Isotope Effect[END_REF] . They investigated the change of kinetic rate of Pt-based and Fe-N-C catalysts by replacing proton with deuteron (i.e., kinetic isotope effect KIE). The Pt-based catalyst shows a KIE of 1, indicating its RDS is a proton-independent electron transfer process. However, for the Fe-N-C catalyst, a higher KIE of 2 was observed, demonstrating the involvement of protons in the ORR mechanism at or before the RDS [START_REF] Tse | Elucidating Proton Involvement in the Rate-Determining Step for Pt/Pd-Based and Non-Precious-Metal Oxygen Reduction Reaction Catalysts Using the Kinetic Isotope Effect[END_REF] . This would suggest that the ORR performance of the Fe-N-C catalyst in acidic media is intrinsically inferior to that of the Pt-based catalyst due to the increased complexity of the PCET on the Fe-N-C catalyst. Chen et al. further elucidated that the PCET arises from the 2e -pathway with the generation of H2O2 [START_REF] Chen | Kinetic Isotopic Effect Studies of Iron-Nitrogen-Carbon Electrocatalysts for Oxygen Reduction Reaction[END_REF] . They investigated the KIE of a Fe-N-C catalyst for ORR extrapolated from the mass-transport-controlled region (at 0.425 V vs. reversible hydrogen electrode RHE) and the kinetic-controlled region (0.7-0.9 V vs. RHE). In the mass-transport-controlled region, the protons are involved in the RDS due to the change of mechanism, leading to a higher peroxide yield. On the other hand, the RDS at low overpotential is proton-independent in the kinetic-controlled region. Therefore, the proton dependency is not the limiting factor for the ORR performances of Fe-N-C catalysts [START_REF] Chen | Kinetic Isotopic Effect Studies of Iron-Nitrogen-Carbon Electrocatalysts for Oxygen Reduction Reaction[END_REF] . These materials could successfully compete with Pt-based catalysts, through careful optimization of their structure. Similar to Pt-based catalysts, the Fe-N-C catalysts derived from separated iron and carbon/nitrogen precursors present a wide diversity of active sites. Hitherto, three different types of active sites have been explored to display ORR activity in acid electrolyte: (i) atomically dispersed iron moiety (Fe-NxCy) [START_REF] Kramm | Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells[END_REF][START_REF] Bouwkamp-Wijnoltz | On Active-Site Heterogeneity in Pyrolyzed Carbon-Supported Iron Porphyrin Catalysts for the Electrochemical Reduction of Oxygen: An In Situ Mössbauer Study[END_REF][START_REF] Ramaswamy | Activity Descriptor Identification for Oxygen Reduction on Nonprecious Electrocatalysts: Linking Surface Science to Coordination Chemistry[END_REF][START_REF] Lefè Vre | Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells[END_REF][START_REF] Koslowski | Nature of the Catalytic Centers of Porphyrin-Based Electrocatalysts for the ORR: A Correlation of Kinetic Current Density with the Site Density of Fe-N4 Centers[END_REF][START_REF] Jia | Experimental Observation of Redox-Induced Fe-N Switching Behavior as a Determinant Role for Oxygen Reduction Activity[END_REF] , (ii) N-doped carbon moiety (C-N) [START_REF] Zheng | Nanostructured Metal-Free Electrochemical Catalysts for Highly Efficient Oxygen Reduction[END_REF][START_REF] Liu | Metal-Free Nitrogen-Doped Carbon Foam Electrocatalysts for the Oxygen Reduction Reaction in Acid Solution[END_REF][START_REF] Guo | Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts[END_REF] , and (iii) iron-based nanoparticles encapsulated by N-doped carbon [START_REF] Olson | Bifunctional Oxygen Reduction Reaction Mechanism on Non-Platinum Catalysts Derived from Pyrolyzed Porphyrins[END_REF][START_REF] Strickland | Highly active oxygen reduction non-platinum group metal electrocatalyst without direct metal-nitrogen coordination[END_REF][START_REF] Wiggins-Camacho | Mechanistic Discussion of the Oxygen Reduction Reaction at Nitrogen-Doped Carbon Nanotubes[END_REF][START_REF] Deng | Iron Encapsulated within Pod-like Carbon Nanotubes for Oxygen Reduction Reaction[END_REF][START_REF] Hu | Hollow Spheres of Iron Carbide Nanoparticles Encased in Graphitic Layers as Oxygen Reduction Catalysts[END_REF] . Generally, Fe-NxCy moieties are considered to be the most effective ORR active sites of Fe-N-C catalysts, in which Fe plays a major role as the core of the active site, [START_REF] Lefè Vre | Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells[END_REF][START_REF] Koslowski | Nature of the Catalytic Centers of Porphyrin-Based Electrocatalysts for the ORR: A Correlation of Kinetic Current Density with the Site Density of Fe-N4 Centers[END_REF][START_REF] Kramm | Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells[END_REF] and the turnover frequency of Fe-NxCy moieties is controlled by the structure of macrocycle (x, y values) [START_REF] Xiao | Microporous Framework Induced Synthesis of Single-Atom Dispersed Fe-N-C Acidic ORR Catalyst and Its in Situ Reduced Fe-N 4 Active Site Identification Revealed by X-ray Absorption Spectroscopy[END_REF][START_REF] Wu | Synthesis-structure-performance correlation for polyaniline-Me-C nonprecious metal cathode catalysts for oxygen reduction in fuel cells[END_REF] . The precise identification of x and y values is complicated due to the multiple chemical environments of Fe in catalysts, which will be further discussed later. On the contrary, some researchers also reported that Ndoped carbons with 'metal-free' active sites (C-N) catalyze the ORR [START_REF] Zheng | Nanostructured Metal-Free Electrochemical Catalysts for Highly Efficient Oxygen Reduction[END_REF] . They claimed that metals only help to develop more active sites and promote the formation of a high degree of sp 2 -hybridized carbon required for high electrical conductivity [START_REF] Liu | Metal-Free Nitrogen-Doped Carbon Foam Electrocatalysts for the Oxygen Reduction Reaction in Acid Solution[END_REF][START_REF] Guo | Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts[END_REF] . The Fe-rich nanoparticles can be observed in highly loaded Fe-N-C catalysts, but assessing their reactivity towards ORR remains challenging because of the simultaneous presence of Fe-NxCy moieties [START_REF] Zhang | Are Fe-N-C Electrocatalysts an Alternative to Pt-Based Electrocatalysts for the Next Generation of Proton Exchange Membrane Fuel Cells?[END_REF] . Specifically, a synergetic effect between Fe-rich nanoparticles and Fe-NxCy moieties has been widely proposed [START_REF] Jiang | Understanding the High Activity of Fe-N-C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-Nx[END_REF][START_REF] Sun | Atomic-Level Fe-N-C Coupled with Fe3C-Fe Nanocomposites in Carbon Matrixes as High-Efficiency Bifunctional Oxygen Catalysts[END_REF][START_REF] Ao | Unraveling the high-activity nature of Fe-N-C electrocatalysts for the oxygen reduction reaction: the extraordinary synergy between Fe-N4 and Fe4N[END_REF][START_REF] Ao | Markedly Enhanced Oxygen Reduction Activity of Single-Atom Fe Catalysts via Integration with Fe Nanoclusters[END_REF] proposed that Fe/Fe3C nanoparticles could dramatically improve the activity of the adjacent Fe-Nx by modifying its electronic structure, favoring the O2 adsorption in alkaline media. In acidic media, the Fe-rich nanoparticles encapsulated into a thin graphitic N-doped carbon layer (Fe@N-C) usually present excellent ORR activity and durability [START_REF] Choi | Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe-N-C Catalysts[END_REF][START_REF] Li | Fe3O4-Encapsulating N-doped porous carbon materials as efficient oxygen reduction reaction electrocatalysts for Zn-air batteries[END_REF][START_REF] Varnell | Identification of carbon-encapsulated iron nanoparticles as active species in non-precious metal oxygen reduction catalysts[END_REF][START_REF] Wu | High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt[END_REF] . In this case, the encapsulated Fe-rich nanoparticles are not involved in the ORR process, but they exert a positive electronic effect on the outer surface of the graphitic carbon layer that favors the stabilization of the peroxide intermediates, leading to 4e -pathway [START_REF] Choi | Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe-N-C Catalysts[END_REF] .

The precise structure of Fe-NxCy moieties is conventionally extrapolated from Mössbauer and X-ray adsorption spectroscopy [START_REF] Kramm | Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells[END_REF][START_REF] Zitolo | Identification of catalytic sites for oxygen reduction in iron-and nitrogendoped graphene materials[END_REF] . The possible Fe-Nx coordination number in the first shell ranges from 1-4, and the different coordination structures of the Fe-Nx configuration will result in different catalytic performances [START_REF] Ramaswamy | Activity Descriptor Identification for Oxygen Reduction on Nonprecious Electrocatalysts: Linking Surface Science to Coordination Chemistry[END_REF][START_REF] Artyushkova | Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal-Nitrogen-Carbon Electrocatalysts[END_REF]100 . The iron atom with two pyridinic N atoms on the edge of the carbon layer (Fe-N2/C) was initially proposed as an active species [START_REF] Koslowski | Nature of the Catalytic Centers of Porphyrin-Based Electrocatalysts for the ORR: A Correlation of Kinetic Current Density with the Site Density of Fe-N4 Centers[END_REF] . The inplane Fe-N3 and Fe*-NxC4-x (C is located at the first coordination shell) moieties are also observed 101,102 . In the research of Li et al., three Fe-NxC4-x (x=1, 3, 4) were prepared by pyrolysis of N-doped carbon with Fe salts at increasing temperature and observed that the ORR activity decreased in the order of Fe-N4 > Fe-N3C > Fe-N1C3 100 . Consequently, Fe-N4 sites were considered the best moieties for ORR, including Fe-N4 and Fe-N2+2 103 , and the investigation of the second coordination shell is established on this coordination number. Recent works also proposed higher coordination numbers, including a nitrogen atom below the plane, leading to a N-FeN4 structure 104,105 . However, these higher coordination numbers can still be regarded as a deviation from Fe-N4 sites. On the basis of Mössbauer Spectroscopy, Kramm et al. originally distinguished different Fe-N4 moieties in the catalysts prepared using Fe acetate, Fe porphyrin, and NH3 as the precursor [START_REF] Kramm | Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells[END_REF] . Fe-containing moieties consisted of low spin (Fe(II)-N4/C moiety, D1), intermediate spin (Fe(II)-N2+2/C, D2), and high spin (N-Fe-N2+2/C, D3), as shown in Figure 1.8. D1 is structurally similar to the Fe-N4 center in FePc. These species are generally obtained after iron porphyrin complexes pyrolysis and are moderately active in acid medium 106 . The Fe in both D2 and D3 sites are coordinated with four pyridinic N atoms, which are attached to the edges of opposite graphene planes and restricted to the micropores in the carbon structure. Different from D2 site, D3 has an extra fifth graphitic nitrogen atom in the plane underneath the Fe atom. Among them, only D1 and D3 sites are active for ORR because of the electronic population of the 3d orbitals in the Fe(II) ion, among which the 3dz 2 orbital is reputed to bind oxygen end-on. This energy level is unoccupied for D1 (S = 0) or singly occupied for D3 (S = 2), while it is fully occupied for D2 (S=1). Therefore, D2 is unable to accept one electron from oxygen to form an adduct to the Fe-N2+2/C site. D3 is proposed to be the most active site for ORR due to the presence of Nprot in the vicinity of D3 [START_REF] Kramm | Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells[END_REF]107 . In later research, Mukerjee's group provided further insights on the D1 moiety and its variations as a function of potential [108][109][110] . The active sites undergo the Fe 2+ /Fe 3+ redox transition, forming high spin "in-plane" OH-Fe(III)-N4 sites at high potential and "out-of-plane" Fe(II)-N4 sites at low potential as the D1 site described in Figure 1.9 109 . The latter is considered the active site, while the site-blocking effect resulting from water activation was viewed as detrimental 111 . On the basis of in-situ Mössbauer spectra, Li et al. further confirmed this reversible switch from Fe(III)N4C12 to Fe(II)N4C12 with the decrease of potential from 0.8 V to 0.2 V, also triggering the removal of an axial OH adsorbate 105 . The site-blocking effect plays a prominent role in limiting ORR, describing the phenomenon of active sites being blocked by reaction intermediates. Since the metal-oxygen binding energy (ΔGOH*) for the Fe-N4 center is slightly stronger than the optimum value and the active sites for ORR are generated from the reversible redox reaction (1-21), increasing the redox potential can significantly enhance the intrinsic activity due to an increased turnover frequency 111 . This result is in line with the conclusion that the D2 site is less active. In the research of Li et al. 111 , the D2 site with pyridinic N coordinated Fe exhibits a Fe 2+ /Fe 3+ redox peak around 0.64 V. In contrast, the Fe-MOF derived catalyst exhibits a redox peak at 0.78 V, which is coupled to the D1 moiety with pyrrolic N coordinated Fe. The higher redox potential of D1 as compared to D2 is related to the longer Fe-N-C bond length that enriches the charge of the Fe center by suppressing the Fe-to-ligand back-donation 109 .

Fe 3+ -OH* + H + + e -↔ Fe 2+ + H2O

(1-21) Zitolo et al. investigated the detailed structures of the Fe-NxCy sites using X-ray absorption near-edge spectroscopy (XANES) [START_REF] Zitolo | Identification of catalytic sites for oxygen reduction in iron-and nitrogendoped graphene materials[END_REF] . XANES is more sensitive to the arrangement of atoms around the photo-absorber, which helps investigate the chemical environment of the nitrogen atoms 112 . The simulation results show that the Fe-N4C12 structure (Figure 1.10c) fits best with the experimental data up to 90 eV above the absorption edge, but a discrepancy is still present at higher energies. When the analysis is refined by including a second oxygen molecule along the axial direction, two curves are perfectly coincident as shown in Figure 1.10e. This result further demonstrated the ability of O2 to bind on the D1 site along with a reversible switch of Fe(II)/(III) 113 . On the contrary, the oxide state of Fe in the D2 site is independent of the change of potential, which indicates that it is either not accessible to O2 or binds O2 weakly 105 . Newly emerging evidence suggests that the Fe-N4 might not be exactly the active site: the chemical environment around this center could also significantly impact the ORR activity. It was demonstrated that the porosity in the pristine carbon black during the pyrolysis is important for achieving high catalytic activity 114 . This implies that the active site forms at/near the micropores of the nitrogenized graphite. Besides, they also found that a supply of nitrogen atoms is necessary to activate the Fe-Nx complexes during the pyrolysis, which accords with the theory of Doublet 3 structure.

Parameters that impact performance of catalysts

According to the mechanism of ORR on the M-N-C catalysts discussed before, the M-N4 center is the active site of the catalyst. Therefore, all experimental conditions that affect the state of the M-N4 center can simultaneously affect the catalytic performance. The metal precursor and ligand structure decide the properties of the M-N4 center. The different metal centers exhibit different activity towards ORR and different oxidation states of metal can also impact the formation of M-N4 center 115 . The ligand added during the impregnation procedure assists the formation of active sites and becomes a part of the nitrogen source in the following heat treatment step 116 . Metal loading also plays an important role in achieving high catalytic activity because it decides the concentration of active site in the catalyst. However, the optimum loading cannot be discussed independently. The nature of the precursors used, surface properties of carbon support, and the heat-treatment conditions can significantly affect the state and distribution of the M-N4 center, thus impacting the best metal loading 117 . The porosity of carbon support can deeply affect the mass transfer property during the oxygen reduction reaction and may also contain some nitrogen atoms. Generally, a larger specific surface and higher nitrogen content can lead to higher ORR activity 118 . Heat treatment is the most complicated procedure during the preparation because a series of reactions happen during pyrolysis. Both the temperature and atmosphere can significantly affect the formation of M-N4 moieties, thus having different ORR activity 119 . The influence of these conditions has been widely investigated, and some of the results are summarized in Table 1.3. 115 . They prepared catalysts with different metal centers by a heattreatment process using MSO4 (M=Mn, Fe, Co, Ni, Cu) and ZnCl2 as metal precursors and polyacrylonitrile (PAN) as the nitrogen source. 115 . The results indicate that the presence of metal ions is essential to obtain materials of high electrocatalytic activity towards ORR. The different metal ions influence the electrocatalytic behavior of the materials formed to a different extent. Fe is considered to be the best at E=0.8 V, followed by Co and Mn. As indicated by the authors, a Fe-containing catalyst can catalyze the ORR through a quasi-4e -reduction. However, in an alkaline electrolyte, different metal centers show similar performance due to the activity of carbon base in an alkaline solution. 115 Chu and Jiang found that high ORR activity can also be achieved from binary transition metal macrocycles. 132 They synthesized Fe and Co tetraphenylporphyrin (FeTPP/CoTPP) catalyst, which shows the optimum catalytic activity towards ORR compared with the single metalloporphyrin (FeTPP or CoTPP), as seen in Figure 1.12. Generally, CoTPP catalyzes 2e - reduction of oxygen to hydrogen peroxide at more positive potentials, and FeTPP catalyzes 4e - reduction of oxygen to water at less positive potentials. However, iron porphyrin can also catalyze hydrogen peroxide reduction to water. Adding iron porphyrin to cobalt porphyrin makes it possible to obtain a more positive onset potential and achieve 4e -reduction of oxygen to water by removing the intermediate product of peroxide. On the other hand, Co species can facilitate the formation of graphitized carbon during pyrolysis, potentially benefiting ORR catalysts by enhancing the stability of the catalysts 133 . Therefore, it is an efficient approach to generating excellent catalysts with the combination of Fe and Co. Most recently, Tatyana et al. developed a set of bi-metallic Fe-Mn-N-C electrocatalysts by hard templating method [START_REF] Reshetenko | Design of PGM-free cathodic catalyst layers for advanced PEM fuel cells[END_REF] . Adding Mn decreases the amount of hydrogen peroxide formation, which should improve the durability of the catalyst. According to the mechanism established by Dodelet et al. 134 and now widely confirmed by experiments, nitrogen is a necessary component of the catalyst in order to form active sites. The various resources of nitrogen precursors can affect the morphology of catalysts and the structure of active sites. Therefore, the catalytic performance can be improved by selecting different nitrogen precursors. At present, the typical nitrogen precursors are tetraphenylporphyrin (TPP) [120][121][122] , polyaniline (PANI) 128 , polyacrylonitrile (PAN) 115,125,128 , phthalocyanine (Pc) 123 , phenanthroline (Phen) 116,118,127,128 , 2-aminobenzimidazole (2-ABI) 129 , and polyethyleneimine (PEI) 130 et al., as listed in Table 1.3. Alexey et al. synthesized a series of Fe-PEI-based catalysts using the sacrificial support method to investigate the impact of the iron-to-PEI ratio and molecular weight of PET 130 . The Fe-N-C materials obtained at 800 °C and the sacrificial support method ensure the surface area for all materials is over 1000 m 2 g -1 as well as the quantity of mesoporous. In this case, the catalytic activity of ORR was not significantly affected by the variation of molecular weight but highly depended on the mass ratio of Fe to PEI. The iron-free material possesses extremely low activity, and the maximum catalytic activity is found for samples with the mass ratio of Fe-PEI of 1:3. The higher PEI content decreases the catalytic activity due to the dilution of the active sites when excess PEI is used. Another research conducted by Fu et al. investigated the effect of dual nitrogen sources, PANI and Phen, which becomes an efficient strategy to produce Fe-N-C catalyst with higher ORR activity 128 . As seen in Figure 1.13d, the dual-N source product exhibits the best ORR performance compared with the other catalysts prepared by using only one single N source, and its yield of H2O2 is also controlled under a poor level. The excellent ORR performance could be attributed to the unique structure of the catalyst. Its high electrochemical surface area can host a large number of active sites and control meso/macropores sizes and their distribution to promote the mass transfer property on the surface and within bulk of the catalyst. Here, Phen acts as a pore-forming agent that can expand the external PANI shell during the decomposition. Simultaneously, the PANI shell converted to graphene-like structures through graphitization in the presence of iron species during pyrolysis processes 135 . Therefore, the obtained graphene-like morphology can serve as a high surface area carbon matrix hosting active sites to catalyze the ORR. Meantime, the generated abundant mesopores inside the graphene framework can facilitate mass transfers. The membrane electrode assembly using this catalyst at the cathode delivered exceptional Pmax values of 1.06 and 0.38 W cm -2 in the H2-O2 and H2-air fuel cell, respectively. This progress indicates that the N precursor directly affects the structure of the final catalyst, including its morphology and concentration of active sites, thus impacting the ORR activity of the catalyst.

Besides different precursors of Fe and N used, Li et al. reveal that the preexisting Fe-N coordination structure can also help form Fe-N4 active sites in the nitrogen-doped carbon with a high degree of graphitization 136 . They synthesized Fe-N-C materials from iron precursors with preexisting Fe-N coordination (FePc or FePhen3) and compared their performance with the catalyst synthesized from individual iron and nitrogen precursors (FeAc and Phen). The ionothermal carbonization method (ICM) was used as an efficient strategy to synthesize Ndoped carbons with tailored porous structures and high specific surface areas. The addition of eutectic ZnCl2/NaCl to organic carbon and nitrogen precursors can modulate the porous structure. ZnCl2 with a low melting point acted as a porogen due to its early evaporation during pyrolysis. On the other hand, NaCl salt with a high melting point remained in the carbon matrix during the entire pyrolysis process, being removed only after pyrolysis via acid washing, generating a specific porous structure through a templating effect. The RDE results of the different catalysts showed that using individual iron and nitrogen precursors leads to poor ORR activity due to the low density of FeNx moieties. A significant fraction of Fe was present in the form of iron carbide and nanoparticles of oxides. Their research suggests that compared with the highly disordered N-C matrix (e.g., ZIF-8 derived Fe-N-C catalysts), it is more challenging to integrate a high density of FeNx moieties in a highly graphitic carbon matrix.

Metal loading in the catalysts

Besides the nature of the iron and nitrogen precursors used, metal loading also plays an important role in achieving high catalytic activity. The optimum metal loading is determined by the nature of the precursors used (both metal and nitrogen), the carbon support, and the heattreatment conditions 117,[137][138][139][140] . In general, when metal loading is far less than the optimum concentration, the activity of the catalysts increases linearly with metal content. When the metal loading far exceeds the optimum concentration, the clusters of metal begin to appear and lead to degradation of activity. The activity of these catalysts improves drastically as soon as the Fe content is increased to a few tens of ppm and saturates at the same potential for both Fe precursors. However, the activity of catalysts prepared with FeTMPP precursor saturates at higher Fe contents than FeAc due to a better dispersion of the FeTMPP on carbon base. The macrocyclic structure of FeTMPP guarantees the isolation of the atomic Fe moieties during the pyrolysis process, preventing the formation of Fe-containing particles 121 . The higher saturated content of FeTMPP also corresponds to better performance than FeAc in the fuel cell, as shown in Figure 1.14b. Their TEM results confirm the appearance of Fe clusters when Fe loading is more than saturation, which interprets the degradation of catalytic activity for high Fe content. Different metal centers also have different optimum concentrations. Jaouen and Dodelet synthesized a series of Fe-N-C and Co-N-C catalysts with metal loading spanned from 0.005 to 5 wt.% on the same carbon support to investigate the impact of metal content 117 . The metal saturation of the catalysts was determined by the kinetic current, which relates to the average turn-over frequency of the active sites directly. As seen in Figure 1.15a, at low metal content (≤0.2 wt.% for Fe and ≤1 wt.% for Co), the activity increases with increasing metal content because only the metal content limits the activity. Whereas at the threshold metal content, the activity levels off and, upon larger metal contents (>1 wt.% for Fe and Co), even decreases dramatically by one to two decades. The explanation for this is the drop in the micropore area, as seen in Figure 1.15b, which suggests reducing active sites concentration. A more interesting result is the different behaviors of the catalysts with different metal centers. The linear increase in activity of Fe catalysts in the region 0-0.2 wt.% can be explained by all Fe atoms forming active sites during the heat treatment, which is in accordance with previous research 113 . However, for the Co catalysts, the ORR activity increases about as the square root of the metal content (region 0-1 wt.%). One possible explanation is Co can form several species, among which only one catalyzes the ORR. If the fraction of the Co species active for the ORR to the total Co species decreases with increasing Co content, then this could increase the ORR activity with Co content being less than proportional. Some inactive Co species can also agglomerate onto the active Co species, which leads to higher threshold metal content than Fe ones (1 wt.% against 0.2). This hypothesis is reinforced by the results of time-of-flight secondary-ion mass spectroscopy, in which none of the four families of ions (CoNCy + , CoN2Cy + , CoN3Cy + , CoN4Cy + ) gives a particularly strong signal compared with others 141 . The properties of carbon support also affect optimum metal loading. Wang et al. proposed the nitrogen content limited the formation of active centers 137 . In the case of low nitrogen content, the best activity reaches a lower mass loading than the carbon support with high nitrogen content. The texture and specific surface of carbon support are the main factors that impact the optimum metal loading. Large specific surface area and tailored porous structure carbon can generally contain more active sites compared with amorphous carbon support. For example, using rho-ZIF, which possesses interior cavities to host metal precursor, Wei et al. 121 found that 3.8 wt.% of Fe is the optimum value for the highest activity. Wang et al. use a carbon black KJ-600, which possesses a high BET surface area (~1400 m 2 g -1 ), reach to 10 wt.% as the optimum mass loading 139 . However, as for the catalysts using carbonized polymer, such as PANI, which possesses neither a porous structure nor a high specific surface, their optimum values are usually lower than 1 wt.% 108,140 .

Carbon supports

Carbon supports are one of the components of M-N-C materials to endow the catalysts with a high density of exposed active sites and excellent mass transfer properties owing to their high specific surface area and porous structure. In addition, the nature of the carbon surface can also influence the structure of the active sites in the final catalyst 133,[142][143][144][145] . In the early research of M-N-C materials, carbon blacks were almost exclusively used as catalyst support. New carbon materials have been tested to improve the electrochemical activity and stability of the catalysts, such as carbon nanotubes 146 , mesoporous carbons 118 , and MOF materials 116,142 . With respect to carbon blacks, these new carbon materials are different both in their structural conformation and porous structure. 145 to investigate the influence of specific surface area, porous structure, and surface property on catalytic activity. In their synthetic procedure, iron (II) acetate was firstly adsorbed on the carbon black, followed by pyrolysis of the mixture at 900 °C in NH3/H2/Ar to introduce N into the catalysts, thus the N content in the catalysts is limited. They found that the activity of the catalysts varies significantly from one carbon support to another, but neither the specific surface area of the catalysts nor the distribution of their macro/mesopores is a determining factor for the catalytic activity. The most crucial factor is the N content of the materials; the higher it is, the higher the density of the catalytic sites on their surface and the better the electrocatalyst, as seen in Figure 1.16a. This result is because the low N content limits the concentration of active sites. As the pyridinic nitrogen is the most crucial component of the active sites, if the proportion of the pyridinic nitrogen keeps constant for all catalysts, the variation in total nitrogen content accounts for the ORR activity. The vertical line at very little nitrogen content in Figure 1.16a demonstrates the catalytic effect caused by other factors. As seen in Figure 1.16b, the oxygen concentration accounts for the variation because iron oxide sites can also present catalytic activity even if they are much weaker than the Fe-N4 center. The larger specific surface area is supposed to lead to a higher concentration of the catalytic sites, resulting in better catalytic activity. However, their research shows little relation between the activity and total specific surface area, as seen in Figure 1.16c. On the other hand, it seems to be a weak correlation between the catalytic activity and the micropore area, as shown in Figure 1.16d. This relationship corresponds to the active sites hosted in micropores of catalysts 119 . Some chemical treatments can activate the carbon blacks to increase the catalytic activity. Popov et al. prepared a Fe-N-C catalyst using the HNO3-treated carbon black as the carbon support. The catalyst presented higher ORR performance on RDE than one prepared with as-received carbon thanks to more hydrophilic oxygen groups introduced by HNO3 treatment 143 . Generally, carbon blacks with a high specific surface area are mainly contributed by micropores less than 1 nm. This feature can host a large quantity of active sites but disadvantages the carbon when used as catalyst support. When the average diameter of the pores is less than 2 nm, the fuel supply may not occur fluently, and the activity of the catalyst may be limited 133 . Compared with carbon blacks, mesoporous carbons present a higher specific surface area and mesopore volume, thus possessing better mass transfer properties. The ordered mesoporous carbons have recently received great attention due to their controllable pore sizes, high specific surface areas, and large pore volumes. Nanoporous carbons with a 3D-ordered pore structure have shown abilities to improve the mass transport of reactants and products during fuel cell operation 118 . Osmieri et al. synthesized mesoporous carbon supported Fe-N-C catalyst (Fe-N/MPC) 130 . The carbon support was synthesized via the hard-templating method. The carbon source was impregnated into commercial silica mesoporous nanoparticles, followed by a heat treatment at 830 °C under a nitrogen atmosphere to complete the carbonization. Finally, the silica template was removed by washing it with HF solution.

As seen in

As seen in Figure 1.17a 133 , among all the catalysts prepared with different carbon support, the most active is Fe-N/MPC, which has 100 mV higher Eon (onset potential is defined as the potential required to generate a current density of 0.1 mA cm -2 ) in comparison with the less active catalyst, acetylene black supported catalyst (Fe-N/AB). The reason of better performance with mesoporous carbon is revealed in Figure 1.17b. The pore size distribution analysis shows the majority of the pores in the mesoporous carbon supported catalysts (Fe-N/MPC1 and Fe-N/MPC2) catalysts are between 2 and 4 nm, whereas for the AB and multi-walled carbon nanotubes (MWCNT) supported catalysts mainly contain macropores and mesopores with diameter >20 nm. Using carbon support with a high specific surface area and an ordered porous structure seems helpful to allow the formation of more active ensembles and favor their accessibility to the reactants. Another interesting phenomenon is MWCNT support shows a higher diffusion-limited current density. MWCNTs tend to form agglomerates which are very difficult to disperse in the ink solution and consequently have a bad dispersion onto the glassy carbon. Besides ordered mesoporous carbons prepared by hard templates, carbon gels have recently attracted attention as a new form of mesoporous carbon. Their surface area, pore volume, and pore-size distribution are tunable surface properties related to the synthesis and processing conditions, which can produce a wide spectrum of materials with unique properties 147 . In addition, no need to remove the silica template during synthesis makes them more environmentally friendly than ordered mesoporous carbons prepared by the hard template method. Generally, carbon gels are obtained from the sol-gel polycondensation of specific organic monomers, typically resorcinol (R) and formaldehyde (F). Two monomers are dissolved in solution with catalyst to conduct polycondensation, followed by drying and pyrolysis in an inert atmosphere. According to the drying method, there are three types of carbon gels: carbon xerogels, carbon aerogels, and carbon cryogels 133 . The xerogels are produced by conventional drying methods such as evaporation under heating conditions. The evaporation of the solvent by heating the wet gel leads to a collapse of the pore structure due to changes in the surface tension of the solvent upon the formation of the vapor-liquid interface. This collapse results in a significant gel shrinkage, giving a dense polymer. Job et al. found the shrinkage of porous structure in xerogel was in evitable, even though the evaporation condition was carefully controlled 148 . To maintain the entire porous structure of the wet gels, they must be dried in supercritical conditions of the solvent. In this case, the solvent can be removed in the absence of a vapor-liquid interface, and the surface tension can be eliminated 149 . The resulting materials are called aerogels. According to the solvent, the pressure and/or the temperature can be elevated. By exchanging the solvent contained within the voids of a wet gel (for example, water) with an alternative solvent (for example, acetone) miscible in supercritical CO2, conditions can be softened. Ideally, this supercritical drying process leaves the gel structure unchanged without shrinkage of the internal voids or pores, thus having higher specific surface area and pore volumes than the corresponding xerogels. Zanto et al. synthesized a series of carbon aerogels and xerogels with different parameters to compare their surface properties 150 . On average, the carbon aerogels exhibited higher specific surface areas and pore volumes than the carbon xerogels. The highest specific surface area and pore volume for carbon aerogels were 929 m 2 g -1 and 1.42 cm 3 g -1 , respectively. The corresponding values for the carbon xerogels were 591 m 2 g -1 and 0.44 cm 3 g -1 . However, the supercritical drying process also makes carbon aerogels quite expensive. Hence, the freeze-drying method is proposed as a less expensive and safer procedure for obtaining mesoporous carbons called cryogels. Suzuki et al. 151 compared the porous properties of the RF cryogels with aerogels. They prepared organic RF hydrogels with different ratios of resorcinol to catalyst (R/Cat) and different ratios of resorcinol to water (R/W). Meantime, the pH of the solution and R/F ratio were kept constant. They found for the same organic gel, the total pore volume of cryogels only reached 60% of aerogel because the shrinkage during freeze drying is more significant than during supercritical drying. Fe-N-C aerogel catalysts for ORR were synthesized recently from RF aerogel 152,153 and will be further discussed in section 1.4.

Metal-organic frameworks (MOFs) are crystalline porous hybrid materials comprising coupling units coordinated by electron-donating organic ligands 154 . MOFs have the highest specific surface areas reported for porous materials (up to 6200 m 2 g -1 ) 155 and large internal pore volumes with tailored pore sizes and apertures. These features make them become ideal candidates for carbon support of Fe-N-C catalysts. MOFs can be obtained using inexpensive and high-yield synthesis methods at low temperatures (~250 °C). Synthesis under hydrothermal conditions is the most frequent method. To date most commonly used MOFs are zeolitic imidazolate frameworks (ZIFs) which possess the topology of zeolites but are composed of synthetically versatile imidazolate groups coordinating either Zn(II) or Co(II). ZIFs own large interior cages and relatively small windows, which make them extremely attractive as host matrice for encapsulating and immobilizing large guest molecules 121 . Based on the features of ZIFs, ball milling is a fast, energy-efficient method for promoting reactions between iron precursors and ZIFs in the solid state. A typical synthesis procedure is shown in Figure 1.18. Wei et al. synthesized rho-ZIF-supported Fe-N-C catalysts. Iron precursor (Fe-TPP) acted as a pore filler that filled into the large interior cages of rho-ZIF by ball milling to form the hostguest Fe-TPP ⊂ rho-ZIF complexes. The complexes were then conducted pyrolysis to get the final Fe-N-C catalysts. In their research, the onset potential (current density reaches 0.1 mA cm -2 ) of the best catalysts is 1.04 V in 0.1 M KOH solution at a rotation rate of 1600 rpm, which is very close to that of commercial Pt/C (1.02V). Jaouen et al. compared the electrocatalytic activity of ZIF-supported Fe-N-C catalysts with carbon black 127 . The ball milling method was used to prepare the mixtures of iron precursor and carbon support. Black Pearls 2000 was selected due to its large specific surface area (1379 m 2 g -1 ), close to that of ZIF used (1700 m 2 g -1 ). Therefore, the only difference between the two supports was the porous structure. According to their results, the catalysts prepared with ZIF support exhibit better electrochemical performance both in RDE and in PEMFC, where the current density can reach 4.4 A g -1 in RDE and 4.33 A g -1 in PEMFC at 0.9 V vs. RHE. However, the corresponding values of the Black Pearl 2000 supported catalyst can only reach 0.41 A g -1 in RDE and 2.07 A g -1 in PEMFC. Currently, the most state-of-the-art Fe-N-C catalyst was prepared by a ZIF-based precursor, which was reported by Jiao et al. [START_REF] Jiao | Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites[END_REF] . A Zn-N-C material was first prepared from ZIF-8 precursor, followed by a chemical vapor deposition of FeCl3 vapor on the Zn-N-C bed. The Zn in Zn-N4 can be atomically replaced by FeCl3 vapor to form highly accessible Fe-N4 active sites at a relatively low temperature (750 º C in this method), thus retaining higher N content necessary for high site density. The performance of the final catalyst in H2/O2 PEMFC reached a current density of 44 mA cm -2 at 0.89 V, only 0.01 V lower than the DOE 2025 target 156 .

In summary, ideal carbon support should simultaneously feature a large surface area, appropriately porous structure, and high conductivity, facilitating reactant diffusion and electron transfer during the electrocatalytic reactions.

Heat-treatment conditions

A high-temperature treatment condition also significantly affects the active-site formation for the Fe-N-C catalysts and properties of carbon supports, thus impacts on both ORR activity and stability of the catalysts 119,130,157 . As we mentioned before, although the pyrolysis temperature of Fe-N-C materials can range from 500 to 2500 °C, the catalysts with the best electrochemical performance are obtained under 1000 °C generally, as seen in Table 1.3. The ultrahigh temperature (>2000 °C) is only used for some phenolic-based carbons because they are not very graphitizable 157 . The pyrolysis can be carried in inert environments (Ar or N2) or a reactive environment (H2 or NH3). In inert environments, the temperature decides the concentration of active sites formed and the properties of the carbon support. Serov et al. investigated the impacts of pyrolysis temperature (from 700 to 900 °C) in an inert atmosphere on the activity of Fe-N-C catalysts 130 . For all the catalysts prepared with different carbon supports, the products pyrolyzed at 800 °C always have the best electrochemical performance. According to the analysis of XPS, the products at 800 °C have the largest amounts of pyridinic N and N associated with Fe, which are the moieties that have been suggested to be one of the possible active sites. At higher temperatures of pyrolysis increase in quaternary, graphitic, and pyrrolic N is observed, which reduces the quantity of active site, thus leading to the decrease of electrocatalytic activity. The impacts of pyrolysis temperature on the surface properties of RF carbon aerogels were investigated by Pekala et al. 157 . The specific surface area decreases as the pyrolysis temperature increases, reaching a limiting value of 900 °C. However, the situation in the NH3 atmosphere is different. The ammonia treatment can activate carbon support to increase the electrochemical activity at high pyrolysis temperatures (typically 950 °C). Jaouen et al. proposed three consecutive steps to illustrate the effect of NH3 on carbon support 119 . The first step is the incorporation of nitrogen atoms in the carbon. During the ammonia treatment, ammonia might react with carbon matrix or surface groups by the reaction 3.6.1 158 . This reaction increases the surface area with the formation of micropores and basic N functionalities are integrated into the carbon matrix. The final step is completing active sites in the micropores by reacting iron with ammonia. According to their research, the micropores created during ammonia treatment are crucial for catalytic activity, which means the active sites are hosted in the micropores. However, a different opinion was proposed by Kramm et al. that the improved ORR activity by NH3-induced iron nitride formation cannot be clarified 158 . They found the hydrogen peroxide formation is increased for the NH3-treated catalysts, as seen in Figure 1.19, which indicates a variation of the reduction mechanism from 4e -to 2e -. However, the Fe-N4 active sites prefer a 4e -mechanism. On the basis of these results, the increased turnover frequency may be due to the iron nitride or other ORR active surface groups on the carbon base formed during the NH3 treatment.

Csolid + NH3 →HCN ↑ +H2 ↑ According to the pyrolysis conditions mentioned before, high temperature pyrolysis in inert atmosphere will lose the active nitrogen (pyrrolic and pyridinic N) on the surface, whereas NH3 can introduce new nitrogen groups and active sites. Therefore, recent reports proposed a second pyrolysis procedure on synthesized M-N-C catalysts. Koslowski et al. prepared Fe-N-C catalysts by a secondary heat-treatment method 159 . The mixture of Fe precursor and carbon support was first pyrolyzed at 800 °C in N2. The obtained catalyst was then conducted ammonia treatment at 800 °C in NH3 or N2 as a contrast. The results of XPS measurements show that the treatment in NH3 leads to incorporation of nitrogen. Meantime, the increasing percentage of pyrrolic N suggests an increased concentration of catalytic centers (Fe-N4), which can explain the remarkable improvement of catalytic activity after the NH3 treatment.

Besides the effect on catalytic activity, the pyrolysis temperature also deeply impacts the stability of the catalysts. Improved stability has been observed for catalysts synthesized at high pyrolysis temperatures in N2 or Ar because high pyrolysis temperature can lead to generating more quaternary nitrogen functionalities, which have less prone to degradation by the protonation reaction. At the same time, the graphitized carbon would be easily formed at high temperatures, which can enhance the corrosion resistance of the catalysts to an acidic medium 130 . However, these catalysts displayed an initial ORR activity inferior to that resulting from lower temperature heat treatment procedures due to low active site concentration 160 .

In summary, the ORR performance of the Fe-N-C catalysts highly depends on the pyrolysis condition, both the temperature and atmosphere. The target of pyrolysis is to obtain the maximum concentration of active sites and retain the porous structure of the carbon base. In general, the first pyrolysis of organic gels prefers the temperature of 800 °C in N2 or Ar to maximize the density of active sites. It is also an efficient method to create active sites by second pyrolysis in NH3 at higher temperatures, typically 950 °C. The higher temperature also plays a positive role in helping the graphitization of the carbon support to improve the catalytic stability.

Carbon aerogels

Carbon aerogels are ideal candidates for mesoporous carbon support due to their high specific surface and pore volumes. In addition, the porosity and specific surface of carbon aerogels are tunable according to synthesis and processing conditions. Ritter et al. summarized the influence of synthesis conditions on the final properties of the RF organic and carbon aerogels 161 . Overall, RF organic gels undergo two main stages during synthesis. The first stage is associated with the preparation of the sol mixture and the subsequent gelation and curing of the gel. The second stage is associated with the drying of the wet gel, which has been discussed before. The most important factors that affect the properties of the organic gel during the first stage are the catalyst concentration, the initial gel pH, and the concentration of the solids in the sol. Na2CO3 is the most commonly used alkaline catalyst for the polymerization reaction. This catalyst activates a small portion of R to act as the site for the growth of the monomer particles. Typically, the molar R/Cat ratio ranges between 50 and 300. Low R/Cat ratios result in small polymer particles (3-5 nm) that are interconnected with large necks; this produces higherdensity gels 162 . In contrast, high R/C ratios result in large polymer particles that are connected by narrow necks in a "string-of-pearls" fashion 161 . The initial gel pH depends on the ratio of catalyst to water (Cat/W). The impact of pH is schematically shown in Figure 1.20. If the concentration of the catalyst is high (Cat/W) is large), the starting points of the growth of the particles exist densely. Therefore, a small amount of the reactant is consumed per particle, and consequently, the size of the particles becomes small. On the other hand, if the concentration of the catalyst is low (Cat/W is small), the starting points exist sparsely. Hence, a large amount of the reactant is consumed per particle, and consequently, the particles can grow large. Accordingly, the pore size becomes relatively small if Cat/W is large, and vice versa 163 . As for the solid content (R/W), low R/W gives a large pore volume in carbon aerogel because the pores are the gel voids formed by the interconnected particles, and the voids depend on the amount of water used as a diluent in the synthesis of the gels 151 .

The corresponding carbon aerogels are produced by the carbonization of the dried gel and sometimes the activation of the carbonized derivative. The detailed conditions will be further discussed in the section. Here, some phenomena are summarized. The carbonization procedure is carried out by heating the sample in flowing N2 or Ar at a temperature ranging from 500 to 2500°C . During carbonization, the aerogel loses oxygen and hydrogen functionalities and there is an enrichment in carbon, giving a highly pure carbon structure. At low carbonization temperatures, the macropore volume decreases, and the mesopore volume increases due to the shrinkage of the material, whereas the micropore volume and surface area are enlarged due to the evolution of gases during carbonization. At higher temperatures, all these parameters tend to decrease due to the graphitization of the carbon aerogel 164 . Besides the specific surface area and porosity of the carbon aerogel, nitrogen functionalities on the surface of the porous carbons also play an important role either in catalytic reactions themselves or in increasing the dispersion of active components and, thus, the activity. Ncontaining can be introduced into carbon aerogels in several ways. The most common method is adding nitrogen-containing compounds, such as melamine (M) [165][166][167][168][169][170][171][172][173] , urea 167 , and aminophenol 166 , into the initial polycondensation mixture of RF. Another approach is the use of ammonia-assisted pyrolysis of dry organic gel 174 . Besides, the physical methods are also useful to yield carbons with high nitrogen content, such as N2 or NH3 plasma treatment 175,176 . However, the porous structure of carbon aerogels can hardly retain under the conditions of plasma. Among all the nitrogen-containing compounds, M is the most widely used monomer in RF gels. Therefore, the melamine-containing RF gels is also noted as RMF gels. Although M has outstandingly high nitrogen content, the polymerization reaction between M and F leads to polymers with only limited specific surface area. The area can be improved by adding R to the reaction mixture. As shown in Figure 1.21, a condensation reaction occurs among the hydroxymethyl groups between MF and RF to form methylene and methylene ether bridged polymers, which can co-condense to form small clusters consisting of branched polymeric species. The small clusters act as nucleation sites with plenty of unreacted particles capable of reaction, which can continuously grow to form a 3D polymer, and thus may result in a homogeneous RMF network 165 . According to this mechanism of polymerization, the final surface properties and porosity of aerogels are sensitive to the ratio of M to R (M/R) 177,178 . Nagy et al. prepared a series of mesoporous RMF carbon aerogels with 0-4 wt.% nitrogen content by tuning the M/R ratio during the polymerization process 179 . The M/R ratio ranges from 0.07 to 1. At low melamine content, where M/R <0.4, the samples display the familiar structure of quasi-uniform spherical units, which become more compact with increasing initial melamine content. Although they have similar specific surface area and total pore volume (~700 m 2 g -1 ), their pore size distribution reveals that increasing the M/R ratio shifts the peak position to lower values and, at the same time, reduces the area, i.e., both the size and the volume of the mesopores decrease. Whereas the sample with high melamine content (M/R=1) has a different morphology, which contains irregular interconnected open cells of size around 500 nm, thus leading to a low specific surface area (80 m 2 g -1 ). Although the advantages possessed by carbon aerogel and the wide application as the support of metal nanoparticles 133 , little research has been reported using carbon aerogel as the support of Fe-N-C catalysts. Wang et al. prepared a Fe-N-C catalyst with carbon aerogel as the support 180 . The ferrocene and melamine were used as the precursors of iron and nitrogen, respectively. The carbon aerogel was firstly activated by the KOH solution, and then the iron and nitrogen precursors were impregnated into the support. The final products were obtained by pyrolysis under 700 (Fe-N-CA-700), 800 (Fe-N-CA-800), and 900 °C (Fe-N-CA-900). As seen in

Stability of M-N-C catalysts in acidic media and PEMFC

Even if a lot of research on M-N-C catalysts has focused on increasing the initial activity and power performance in PEMFC, the stability of the catalysts is another important factor that should be considered in the development of new catalysts. If the catalyst particles cannot maintain their structure over the lifetime of the fuel cell, changes in the morphology of the catalyst layer from the initial state will result in a loss of electrochemical activity. The nearterm durability target of DoE is 5000 h (equivalent to 150,000 miles on the road) for fuel cell vehicles and 50,000 h for stationary devices 181,182 . Some works have reported that detrimental H2O2 byproducts may result in the degradation of active site structures. Lefevre and Dodelet treated pyrolyzed Fe-N-C catalysts with a 5% H2O2 in H2SO4 183 . They attributed the activity loss to the oxidation/attack of the active site structures by H2O2 itself or radical species formed by the simultaneous presence of H2O2 and Fe ions. Jaouen et al. investigated the degradation of a highly active Fe-N-C catalyst in an acidic medium by identical-location electron microscopy and Mössbauer spectroscopy to give an insuit view of the degradation process 184 . They found the degradation effect of H2O2 mainly aims at carbon corrosion, whereas the Fe-N4 is more resistant to H2O2. The high environmental temperature and work potential can accelerate this degradation. The carbon corrosion obviously destroys the porous structure of the carbon support, thus leading to the decay of mass transfer property and reducing the electrocatalytic activity of the catalysts. Meantime, the carbon corrosion also slightly increases the charge-transport resistance of the cycled cathode, which is another factor that leads to high Tafel slopes of the polarization curves. On the other hand, the corrosion of carbon destroys the active sites indirectly. As seen in Figure 1.23, the Fe-N-C catalyst initially contains two types of FeNx sites assigned to micropore-hosted and graphenedefect sites. Since the carbon oxidation rate is faster on zigzag and armchair edges than on the basal plane, micropore-hosted FeNx sites, the main active sites for ORR, are eliminated faster than graphene-defect FeNx sites. Loss of the ligand environment of the Fe ions leads to their solvation and removal from the cathode with PEMFC exhaust water. In summary, pyrolyzed M-N-C materials are promising platinum-group metal-free catalysts for the ORR in fuel cell applications. The metal center, metal content, nitrogen source content, carbon support, and heat treatment conditions show significantly impacts on the catalytic performance of M-N-C materials. Among them, Fe is generally considered as the most active metal center in M-N-C materials, whereas the impacts of other factors should be further determined based on different synthesis methods. Therefore, this thesis concentrated on Fe-N-C materials and selected carbon aerogel with high specific surface area and pore volume as the carbon support. The impacts of Fe precursor, melamine (nitrogen source) content, and heat treatment conditions on the catalytic performance will be discussed.

On the other hand, it is well-accepted that a high degree of graphitization is important for catalyst durability, as we mentioned in sections 1.3.3.3 and 1.3.3.4. Although pyrolysis at a high temperature can efficiently increase the degree of graphitization, thus enhancing the durability, the adverse impacts on the active sites and porosity of carbon support should also be taken into account.

In the end, it seems to have a trade-off between activity and stability in Fe-N-C catalysts, and the heat treatment conditions, specifically the pyrolysis temperature employed, will result in performance variations, with the optimal conditions being material dependent and determined by the systematic investigation.

Chapter 2 Preliminary Investigation to Establish the Synthesis Protocol and Characterization Methods

Ré sumé Franç ais

Ce chapitre pré sente la mé thode de synthè se gé né rale des catalyseurs d'aé rogel Fe-N-C basé s sur l'aé rogel RMF et dé crit les techniques principales de caracté risation des é chantillons. La synthè se a commencé à partir de la pré paration de l'hydrogel RMF en mé langeant les pré curseurs qui sont le ré sorcinol (R), le formaldé hyde (F), la mé lamine (M) et le pré curseur de Fe, dans de l'eau. Une solution de Na2CO3 a é té utilisé e comme catalyseur de polycondensation. L'hydrogel obtenu a é té vieilli à 70 ° C pendant 120 h, puis soumis à une procé dure d'é change eau-acé tone avant le sé chage supercritique au CO2. Ensuite, le gel sé ché a é té pyrolysé à 800 ° C dans un flux de N2 pendant 1 h pour obtenir l'aé rogel de carbone. Les aé rogels ont é té broyé s, suivis par un lavage à l'acide pour é liminer les nanoparticules de Fe non souhaité es. Enfin, un deuxiè me traitement thermique dans un flux de NH3/N2 (10/90) à 950 °C a é té effectué pour activer la poudre de catalyseur.

Les conditions de broyage par ball milling et le second traitement thermique ont d'abord é té é tudié es. Afin de pré parer une couche de catalyseur de haute qualité , le broyage à billes est une procé dure importante pour obtenir des particules de catalyseurs de taille approprié e et uniforme. Les impacts de la taille des billes et du temps de broyage ont é té considé ré s comme les principaux facteurs. Nous avons comparé la distribution granulomé trique de la poudre aprè s diffé rents temps de broyage à l'aide d'une analyse de diffusion dynamique de la lumiè re. La condition finale choisie est un pré -broyage de l'aé rogel carbonisé avec une combinaison de billes de ZrO2 de 6×10 mm+3×20 mm à 400 rpm/min pendant 15 min pour obtenir une poudre de carbone grossiè re, puis broyé avec des billes de ZrO2 de 5 mm à 400 rpm/min pendant 60 min. Le traitement à l'ammoniac cré e des micropores hé bergeant des sites Fe-N4 et affecte la nature de l'azote dans le carbone. Ces caracté ristiques en ont fait une mé thode efficace pour augmenter l'activité catalytique des catalyseurs Fe-N-C. Nous avons étudié l'impact de ce traitement en regardant l'effet de la dilution du NH3 sur la modification du catalyseur. Une atmosphè re dilué e de NH3 dans du N2 permet d'uniformiser le traitement dans notre four tubulaire par rapport à un traitement sous NH3 pur. Les tests é lectrochimiques montrent que tous les é chantillons à diffé rentes positions dans le four pré sentent des performances ORR similaires avec du NH3 dilué , alors qu'elles sont significativement diffé rentes pour le catalyseur traité dans l'atmosphè re de NH3 pur.

Les caracté risations physico-chimiques utilisé es dans ce manuscrit ont é té briè vement introduites et les conditions de tests é lectrochimiques en configuration d'é lectrode tournante (RDE) et en monocellule de PEMFC ont é té discuté es en dé tail. Un rapport ionomè re/catalyseur (I/C) approprié est crucial pour la mesure correcte de l'activité catalytique. Il est important de maintenir une couche d'encre composée de ionomère (I, classiquement du Nafion) et de catalyseur (C) sur la surface du carbone vitreux de l'électrode tournante, mais trop d'ionomè re augmente é galement la ré sistance du transfert d'é lectrons à l'interface entre le catalyseur et l'oxygè ne. Le rapport I/C=0.44 a é té utilisé pour notre test RDE. L'augmentation supplé mentaire de la teneur en ionomè re augmente la ré sistance de la couche catalytique et est dé favorable. La situation est diffé rente dans les tests en PEMFC car seuls les sites actifs couverts de Nafion peuvent pré senter une activité ORR. Par consé quent, une valeur I/C plus é levé e est né cessaire pour maximiser l'activité massique des catalyseurs. La valeur I/C optimale est de 2.0 pour nos catalyseurs à base d'aé rogel de carbone en raison de leur surface BET et de leur volume de micro-mé sopores trè s é levé s. This chapter presents the general synthesis method of the Fe-N-C aerogel catalysts and describes the crucial techniques that were utilized in the characterization of samples.

Synthesis Fe-N-C aerogel catalysts

Fe-N-C aerogel catalysts

The carbon aerogels have a high specific surface area and a porous structure. These features allow them both to host a large number of active sites on the surface and to facilitate water evacuation and oxygen diffusion in MEA. Therefore, they are ideal candidates to prepare Fe-N-C catalysts with high activity and good mass transport properties 185 . Herein, The Fe-N-C aerogel catalysts were synthesized based on a one-pot sol-gel method, which was developed by our group 186,187 . Overall, the synthesis undergoes three main stages, as the protocol shown in The first stage is the preparation of the hydrogel containing carbon, nitrogen, and iron precursors (sol, gelation, aging). Then drying is conducted, leading to an organic aerogel (exchange of solvent, supercritical drying). The final step is the thermal treatments, obtaining the Fe-N doped carbon aerogel (pyrolysis in inert flow, grinding, acid washing, and ammonia treatment). A detailed protocol is described below.

All chemicals were utilized as received. The resorcinol (99%) and NaOH (98%) were purchased from Alfa Aesar. Melamine (99%) and formaldehyde (37 wt% sol. in water, stabilized with 5-15% methanol) were obtained from Acros Organics. Na2CO3 (99.99%) was produced by Honeywell Fluka. All the iron precursors were obtained from Sigma Aldrich.

The first is the preparation of the hydrogel, including the gelation and aging step. In this stage, the hydrogels were prepared based on polycondensation of resorcinol (R), formaldehyde (F), and melamine (M), where R and M are the carbon and nitrogen source, respectively, and F is the cross-linker which helps form the network. R-F and M-F pre-polymers were prepared separately. When they were mixed, the co-condensation reaction occurred among the hydroxymethyl groups between M-F and R-F pre-polymers to form methylene and methylene ether bridged polymers, which build up the skeleton of the hydrogel. In this stage, the iron precursor is added, and the pH adjusted. Therefore, the recipe of hydrogel is crucial to control the texture of the gel, including the mesopore volume in the final catalyst. The second stage involves water-acetone solvent exchange and drying of the wet gel. Supercritical drying is employed to maintain the texture of the hydrogel in the dried gel (aerogel). The last step is the pyrolysis of the iron-organic aerogel into carbonaceous aerogel under the inert flow (carbonization), which is the crucial step for the formation of active sites 188 .

Here is a typical recipe for Fe-N-C catalysts. The absolute quantity of reagents can be varied according to the expected quantity of the final product.

First, solution A (R-F pre-polymer) contained 3.6 g of R, 5 mL of F solution, and 22 ml of water. 0.75 ml of 0.38 M aqueous Na2CO3 (Cat) solution was then added as the catalyst of polycondensation and was stirred for 45 min at 40 o C in a water bath, with R:F: Cat = 1:2:0.008. The solid content in solution A is 22%, estimated according to the formula (2-1):

𝑆𝑜𝑙𝑖𝑑 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 = 𝑡𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑣𝑒𝑛𝑡 (2-1)
Separately, solution B (M-F pre-polymer) was prepared from 2.0 g of M, 3.5 mL of F, 0.7 mL of Cat solution, and 12 mL water with the mole ratio of M:F:Cat = 1:3:0.016. The mixture was stirred for 30 min at 70 °C. The solid content of solution B was also kept at 22%. Subsequently, solution A and B were mixed and stirred for 5 min at room temperature. The mole ratio of R to M in the mixture solution is 2/1 in this recipe. Afterward, Fe precursor was added and stirred for 5 min, followed by the pH adjustment to 8 (or 6 according to the nature of the Fe precursor) by adding NaOH solution. The quantity of Fe precursor (𝑚 𝐹𝑒 𝑝𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟 ) in this step can only be roughly estimated because we can neither predict the exact mass loss of organic aerogel after two heat treatments process nor know the loss of Fe during acid leaching. According to our previous experiments, the total mass loss of organic aerogel is around 70%. Assuming Fe won't be removed during acid leaching, 𝑚 𝐹𝑒 𝑝𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟 can be estimated by formula (2-2):

𝑚 𝐹𝑒 𝑝𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟 = 𝑡𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑 × 𝑤 × 30% × 𝑀 𝐹𝑒 𝑝𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟 𝑀 𝐹𝑒 (2-2)
where 𝑀 𝐹𝑒 𝑝𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟 and 𝑀 𝐹𝑒 are the molecular weight of Fe precursor and Fe, respectively, and w is the expected weight percentage of Fe in the final catalyst. For example, when the Fe precursor is FeCl3 (Mw 162.20 g mol -1 ), and the expected Fe content is 1 wt.%, 0.08 g of FeCl3 is required. It must be noted that this value cannot ensure the expected Fe content due to the constraints we mentioned before. The real value must be determined by other characterization methods, such as inductively coupled plasma mass spectrometry (ICP-MS).

The phenomenon worth taking care of in this step is that red rust precipitation occurred once the iron precursor was added, which is caused by the coagulation of low-weight RF and MF pre-polymers in the mixture solution. This phenomenon was significant when Fe salts were used because free Fe ions could act as the coagulation center. In general, around 3 mL NaOH solution was consumed to dissolve the precipitation and adjust pH to 8. The increased pH made these pre-polymers negatively charged, thus being separated. In order to obtain a homogenous texture, the sol was kept stirring at 70 °C until gelation occurred. 10 mL of distilled water was covered on the surface of the gel to avoid unexpected evaporation of solvent during the aging process, which probably crashed the porous structure of hydrogel. The hydrogel was then conserved in a water bath at 70 °C for 120 h to complete polymerization.

The aged hydrogel was immersed in 300 mL of acetone for a water-acetone solvent exchange procedure. The solvent was refreshed 3 times a day for 3 days to remove traces of water. This procedure is necessary because acetone is miscible in supercritical CO2. Therefore, the conditions of supercritical drying will be softened compared to water ones. After all these exchanges, the hydrogel was subjected to the supercritical drying to get the RMF aerogel.

The carbon aerogel was obtained by pyrolyzing RMF aerogels at 800 °C in N2 flow for 1 hour. Before heating, the atmosphere in the tube furnace was purged by N2 flow. The heating program was set as follows: 2 °C min -1 from room temperature to 350 °C and dwelling for 30 min. Most of the decomposition reactions started at this stage with the loss of volatile species. The temperature was then increased to 800 °C with a ramp of 4 °C min -1 and stabilized for 1 h to complete the carbonization.

To obtain the powders that satisfy the application on the membrane electrode assembly, the aerogel blocks were then ball milled to fine powders. The conditions will be detailed in section 2.1.2. The obtained powder was then subjected to acid leaching with 0.5 M H2SO4 at 80 °C for 7 h in a refluxing device to remove the iron nanoparticles that were possibly generated during the pyrolysis 187 . The suspension was separated by centrifugation to get the powder and washed several times with distilled water until the pH of the solution was close to neutral. Finally, the sample was dried in an oven at 100 °C overnight.

In order to create more active sites, a second pyrolysis was conducted on the sample at 950 °C in NH3 flow. According to our previous research, when pure NH3 was used, the degree of treatment was uneven for different positions of the tube due to the dilution of NH3, which led to different catalytic activity for the same batch of samples. Therefore, diluted NH3 was used to improve the uniformity of treatment regardless of the position in the oven. The investigation will be discussed in section 2.1.3.

Since the recipe of hydrogel plays a decisive role in the texture and the catalytic activity of the final catalyst, most of my research has focused on the first stage by modifying the synthesis parameters to obtain a panel of catalysts with various characteristics in order to select the best conditions leading to the performing catalyst, eventually. The investigation of this stage can be divided into two aspects:

1. Impact of Fe precursor: different Fe precursors, Fe contents, and states of Fe in the precursor lead to different Fe states in the organic aerogel, such as hydroxide or coordinated Fe, which will further impact the active sites in the final catalysts. 2. Composition of RMF hydrogel: since the M is the main nitrogen source in this stage, the increasing M content could introduce more active N, which can coordinate with Fe.

Ball milling conditions

The cathode catalyst layer consists of a mixture of catalyst particles, ionomer membrane, and void space 189 . For Pt-based catalysts, a typical thickness of this layer is 5-10 µm due to their ultrahigh activity to ORR that only a small quantity (0.1-0.4 mg cm -2 ) is enough to get appropriate power density [START_REF] Reshetenko | Design of PGM-free cathodic catalyst layers for advanced PEM fuel cells[END_REF]190 . On the contrary, the loading of Fe-N-C catalysts (1-5 mg cm -2 ) is ca. 10 times of Pt-based catalysts to compensate for their lower intrinsic activity, which leads to a thicker catalyst layer (∼4 mg• cm -2 , resulting in 70-100 μm thick layer) [START_REF] Reshetenko | Design of PGM-free cathodic catalyst layers for advanced PEM fuel cells[END_REF][START_REF] Proietti | Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells[END_REF]191,192 . In order to prepare high-quality catalyst layer, ball milling is an important procedure to get catalyst particles with suitable and uniform sizes. Besides, previous research of Ratso et al. also demonstrated that the Fe-N-C catalyst prepared with modified ball milling conditions could increase the diffusion-limited current density values because the decreased particle size leads to an increase in the active surface on the electrode 193 . The impacts of ball size and the time of grinding were considered the main factors. In general, longer grinding time can lead to smaller particles size before reaching the limit of ball milling. Our target was to find the minimum time to reach the limit. We also investigated the impacts of pre-grinding before or after the first pyrolysis. The pre-grinding of large dried gel blocks is also important to make sure the initial particle size satisfies the machine specifications. Organic aerogel is more fragile than carbon aerogel, which is easier to be smashed. On the other hand, the melt of MF gel during pyrolysis will lead to agglomeration of pre-ground powder, which is possible to increase the grinding time.

The ZrO2 pot and balls are shown in Figure 2.2, where 3 sizes of the ball (20 mm, 10 mm, and 5 mm diameter) were prepared. In the pre-grind step, a mixed size of 20 mm and 10 mm balls were used to ensure the large blocks were completely smashed to a coarse powder. The 5 mm balls were used in the following grinding step to get fine powder. The organic aerogel sample pre-ground before pyrolysis was named AN1-1. In contrast, the sample pre-ground after pyrolysis was named AN1-2. The flow diagram of the two methods is shown in Figure 2.3. For the AN1-1, 3 g of dried gel was poured into a 50 ml ZrO2 pot with 90 g ZrO2 balls combination of 6×10 mm + 3×20 mm. The ball-to-powder mass ratio was kept at 30:1 for all grinds. The pot was then sealed under air and ball-milled for 15 min at 400 rpm milling speed to smash the large gel blocks. The powder of organic aerogel was then pyrolyzed in N2 at 800 °C. 1.5 g of the obtained carbon aerogel was ground with 45 g zirconium-oxide balls of 5 mm diameter at 400 rpm for 1 h. The samples were collected every 15 min in order to investigate the impact of the grind time. As for the AN1-2, the organic aerogel block was first pyrolyzed in N2 at 800 °C to get carbon aerogel. 3.0 g of material were smashed with 10 mm + 20 mm ZrO2 balls (pre-grind step) and ground with 5 mm ZrO2 balls. The distribution of the particle sizes was measured in ethanol using a dynamic light scattering (DLS) analyzer (HPPS, Malvern instruments Inc, Worcestershire, UK) at ICGM/CNRS (seen in annex A.2.1). The results were shown in Figure 2.4. For both methods, the particle sizes are smaller than 5 µm, which satisfies the minimum powder size in the catalyst layer. Initially, DLS curves present two peaks at small (around 200 nm) and large (>1 µm) particle sizes. As time went on, the peak area at large size gradually reduced. For the AN1-1, which is pre-ground before pyrolysis, large particles at ca. 500 nm were found after the first 15 min of grind. This result conformed to the agglomeration of powder during pyrolysis. As a result, the peak at large particle size didn't disappear even after 1 h grind. In contrast, the particle sizes of AN1-2 were reduced to less than 1 µm after grinding for 60 min and got the uniform distribution of the particle size. The carbonized blocks can be well smashed in the pre-grind stage, thus more efficient.

In conclusion, we choose method 2 as the ball milling condition: First, carbonization at 800 °C, then pre-grind with ZrO2 balls combination of 6×10 mm + 3×20 mm at 400 rpm for 15 min to get the coarse carbon powder, and finally, the product was ground with 5 mm ZrO2 balls at 400 rpm for 60 min.

Conditions of NH3 treatment

The ammonia treatment creates micropores hosting Fe-N4 sites and affects the nature of nitrogen in the carbon 194 . These features made it an effective method to increase the catalytic activity of Fe-N-C catalysts. However, in our tubular furnace, the first trail shows that pure NH3 leads to an uneven treatment degree due to the decreasing concentration gradient from inlet to outlet. Here, a diluted NH3 (10% NH3 + 90% N2) was employed to improve the diffusion of the active component, thus increasing the homogeneity of treatment. To investigate the impact of different positions in the furnace, 4 ceramic boats containing 300 mg of carbon aerogel powder spread in the center of the boat, were placed one by one, as seen in Figure 2.5a. The heating program was set up as follows: increase the temperature from room temperature to 950 °C with a ramp 4 °C min -1 and stabilize for 15 min under 0.5 L min -1 N2 flow, then switch the gas to 0.6 L min -1 NH3/N2 mixture flow dwelling for 30 min. Finally, the system is cooled down under N2 flow to room temperature. Due to the different NH3 concentrations and positions in the furnace, a series of products with different mass losses were obtained, and their electrochemical performances were investigated. The mass loss results of electrochemical tests are listed in Table 2.1, and their polarization curves are shown in Figure 2.5b and c. It can be seen clearly that the pure NH3-treated samples exhibit large differences in mass loss and activity between the different positions in the furnace. The sample located at the entrance (position 4) is almost completely etched by NH3, thus presenting the worst catalytic activity. On the other hand, the samples treated with 10% NH3 present closer mass loss, and most importantly, their polarization curves are almost identical. This result proved that decreasing the NH3 concentration could efficiently improve the diffusion of NH3 in the tube. Therefore, the samples at different positions in the furnace can be treated evenly. However, samples treated with 10% NH3 concentration are less active than samples treated in pure NH3 at position 1 due to insufficient treatment degree. In order to approach the mass loss of the best sample, the treatment time should be prolonged, and we choose 45 min as the appropriate duration. In conclusion, the 10% NH3 can efficiently improve the consistency of treatment degree, thus providing the catalysts with identical activity. Additionally, the decreased concentration also requires longer treatment time to get appropriate activity. Therefore, the final treatment condition was determined as follows: About 1 g of carbon aerogel powder was spread uniformly in two ceramic boats placed in the center of the tube side by side. Raise from room temperature to 950 °C with a ramp 4 °C min -1 and stabilize for 15 min under 0.5 L min -1 N2 flow, then switch the gas to 0.6 L min -1 diluted NH3 flow, dwelling for 45 min. Finally, the system is cooled down under N2 flow to room temperature.

Physicochemical characterizations

The morphologies of catalysts were investigated by scanning electron microscopy (SEM) with a MAIA operated at 3.0 kV. The porosity and Brunauer-Emmett-Teller (BET) area were determined by N2 sorption method at -196 º C (seen in annex A.2.2). Transmission electron microscopy (TEM) was used to detect the presence of Fe-based nanoparticles. A JEOL 2010 TEM operated at 200 kV with a resolution of 0.19 nm was used. X-ray diffraction (XRD) of catalyst powder was also performed to verify the existence of crystalline solid (seen in annex A.2.3). The dried organic aerogel was investigated by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy to determine the integration of RF and MF prepolymers. The measurements were performed in the range of 4000-600 cm -1 on a Bruker Tensor 27 machine using OPUS 7.8 software. The graphitic degree of carbon aerogel was evaluated by Raman spectroscopy (seen in annex A.2.4) on a Renishaw InVia instrument in a backscattering configuration (with 532 nm as excitation). The global Fe content in the catalyst was determined by inductively coupled plasma mass spectrometry (ICP-MS) on a PerkinElmer NexION 2000c instrument. X-ray photoelectron spectroscopy (XPS) was used to identify the surface elemental composition of catalysts (seen in annex A.2.5). Among them, TEM, ICP-MS, and Raman analysis were carried out by LEPMI/CNRS in university Grenoble, France. Other characterizations (XRD, N2 sorption, and XPS) were performed at Mines Paris in Sophia Antipolis, France. The chemical environment of Fe in catalysts was investigated by [START_REF] Flyagina | Identifying the Catalytic Active Sites in Heteroatom-Doped Graphene for the Oxygen Reduction Reaction[END_REF] Fe Mössbauer spectra (seen in annex A.2.6) and X-ray absorption spectroscopy (XAS) (seen in annex A.2.7). The measurements were carried out by ICGM/CNRS in university Montpellier, France and Synchrotron SOLEIL in Saint Aubin, France, respectively.

Electrochemical characterizations Rotating Disk Electrode (RDE) tests and ink formulation

The electrochemical tests were carried out in a three-electrode cell with a rotating disk electrode (RDE, OrigaTrod instrument) at room temperature with a potentiostat/galvanostat from Bio-Logic (HCP-803). The setup is shown in

Description of the cell

The glassware was cleaned by 12 h immersion in Piranha Solution (seen in annex A.1) to clean organic residues off substrates before the first-time use and then thoroughly rinsed with milliQ H2O (Millipore, 18.2 MΩ cm, 1-3 ppm TOC).

The Reversible Hydrogen Electrode (RHE) is composed of a platinum wire in contact with both gaseous hydrogen (H2) and electrolyte solution (0.05 mol L -1 H2SO4). Graphite electrode is used as the counter electrode. RDE glassy carbon (GC) tips are polished with the diamond paste of 1 μm on a micro-cloth polishing pad following a small "8" pattern for 5 min. The tip was then subjected to washes with acetone, ethanol, and distilled water. For each wash, the electrodes are passed to ultrasound (Elma S40, 140 W) for 15 min. Finally, the electrode is dried by N2 purge.

The preparation of catalyst ink

Prior to the ink preparation, the Fe-N-C catalysts have been well ground by ball milling. The ink was then prepared with obtained powder according to the following steps:

(1) 10 mg Fe-N-C catalyst, 95 µL of 5 wt% Nafion D-520, 752 µL of ethanol, and 46 µL of distilled water were mixed in a microcentrifuge tube of 2 mL. (2) 30 min of sonication with ultrasound (Elma S40, 120 W) and 10 min of agitation with a vortex mixer was employed to help separate the powder in the suspension, then repeated for more time.

(3) 7 µL of prepared ink was dropped onto the polished GC electrode (5 mm diameter with an area of 0.196 cm 2 ), which obtained a catalyst loading of 400 µg cm -2

The ratio of ionomer to catalyst (I/C) can be calculated according to formula (2-3):

I/C = 𝑉 × 𝜌 × 5% 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 (2-3)
where V and ρ are the volume and density of the Nafion solution, respectively. Here, V = 95 μL, ρ = 0.93 g cm -3 , and the mass of catalyst is 10 mg, thus leading to I/C = 0.44.

An appropriate I/C ratio is crucial for the correct measurement of the catalytic activity. It's important to maintain the catalyst film on the GC surface of RDE, but too much ionomer also increases the resistance of electron transfer on the interface between the catalyst and oxygen. 195 The different formulas of ink and the results of their electrochemical test are listed in Table 2.2. According to their polarization curves shown in Figure 2.7a, the current density at 0.8 V decreased with increasing the I/C ratio. The increasing Tafel slopes in Figure 2.7b indicate an increasing electron transfer resistance due to a thicker Nafion membrane. Therefore, I/C = 0.44 is the appropriate formula for ink.

Electrochemical tests

Cyclic voltammetry (CV) and linear scan voltammetry (LSV) tests are done on the Fe-N-C catalyst powders. The obtained data was ohmic-drop (iR-drop) corrected, where ohmic-drop was defined by the solution resistance between the working electrode and the reference electrode. The ohmic drop is determined by performing an impedance measurement at high frequencies before running other experiments (CV or LSV). The magnitude of uncompensated resistance (Ru) is dependent on the concentration of electrolyte, e.g., Ru is approximately 30 Ω in 0.05 M H2SO4.

All tests were carried out in 0.05 M H2SO4 aqueous solution. The electrolyte solution was previously saturated with O2 for 15 min. The ORR activity of catalysts was measured by LSV at 1600 rpm, with a scan rate of 10 mV s -1 from 1.10 to 0.00 V vs. RHE. Oxygen was kept bubbling during the measurement to maintain saturation in the electrolyte. To calculate the electron transfer number during the ORR process, LSV at another 3 different angular velocities (1225, 900, and 625 rpm) were performed under the same condition.

Subsequently, the electrolyte solution was purged with N2 for 15 min in order to eliminate dissolved O2 in the electrolyte. Then CV between 0 and 1.0 V vs. RHE at 50 mV s -1 was carried out in the stagnant (unstirred) solution until stable conditions were obtained, followed by 2 cycles at 10 mV s -1 at the same electrochemical window. The N2 was kept bubbling in the solution during the test but at a slower rate to maintain the atmosphere on the surface of the electrolyte. After CV tests, a LSV with the same condition of oxygen-saturated solution was conducted to determine the background current of the catalysts.

All LSV curves were corrected by subtracting the background current recorded in the N2saturated electrolyte. The kinetic current density of the catalyst controlled only by the ORR kinetics was calculated by the Koutecky-Levich equation (2-4):

1 𝑗 𝑘 = 1 𝑗 𝐹 - 1 𝑗 𝑙𝑖𝑚 (2-4)
𝑗 𝑘 , 𝑗 𝐹 𝑎𝑛𝑑 𝑗 𝑙𝑖𝑚 are the kinetic current density (in A cm -2 ), faradic current density (in A cm -2 ), and limiting current density (in A cm -2 ), respectively.

The mass activity (in A g -1 ) is defined by the equation (2-5):

𝑗 𝑀 = - 𝑖 𝑘 𝑚 (2-5)
where 𝑗 𝑀 > 0 and m is the catalyst loading (0.4 mg cm -2 ) on the GC. 𝑗 𝑀 at 0.8 V was reported and compared.

Durability tests

All samples were conducted a primary durability test after LSV measurements. The electrolyte was N2-saturated before and kept N2-bubbling during the test. Two CV cycles between 0 and 1.0 V vs. RHE at 50 mV s -1 were carried out, followed by two cycles under the same electrochemical window at 10 mV s -1 . Then 500 CV cycles were conducted between 0.6 and 1.0 V vs. RHE with a scan rate of 50 mV s -1 . Finally, a CV cycle between 0 and 1.0 V vs. RHE at 10 mV s -1 was recorded to compare the variation of the electrochemical available area. After the durability test, ORR polarization curves (LSV curve) were recorded with the same electrode in the O2-saturated electrolyte to compare the degradation of mass activity.

In Chapter 5, the effect of NH3 treatment on the catalyst stability was investigated by a stronger accelerated stress test (AST) protocol composed of 2000 CV cycles between 0.6 and 1.0 V vs. RHE with a scan rate of 50 mV s -1 . A CV cycle between 0 and 1.0 V vs. RHE at 10 mV s -1 was recorded every 500 cycles to investigate the variation of curves. The AST protocol was conducted both in N2 and O2-saturated electrolyte to investigate the degradation effects under different atmospheres. During the AST in N2 saturated electrolyte, the RDE is stationary, whereas, in O2-saturated electrolyte, the RDE kept a rotation rate of 1600 rpm. The catalysts performed by AST in N2 or O2 saturated electrolyte were labeled by AST-N2 and AST-O2, respectively. After the AST protocol, LSV curves were recorded in O2-saturated electrolyte as well to compare the degradation of mass activity.

Single cell tests and ink formulation

The activity of catalysts was further investigated in the single cell of PEMFC to compare the difference with the RDE setup. The experiments were completed in the ICGM/CNRS facilities.

Preparation of membrane electrode assembly (MEA)

The measurements were performed on a 5 cm² MEA. For the preparation of the cathode, 20 mg of catalyst, 915 µL 5 wt. % Nafion (1100W) solution, 300 µL of 1-propanol, and 272 µL of deionized water were mixed and subjected to sonication for 1 hour. Different from RDE, a high I/C ratio is necessary for the MEA since only the active sites attached to the electrolyte network can present activity 195 . The ink was deposited in several batches on a 5 cm 2 gas diffusion layer (GDL S29BC) to make the catalyst layer as even as possible, resulting in a catalyst loading of 4.1 mg cm -2 . Then the deposited ink was dried at 80 °C overnight to get the cathode, as seen in Figure 2.8a. The dried cathode was hot-pressed at 135 °C with a Pt/C anode (S28BC, with a Pt load of 0.5 mg cm -2 ) and a Nafion membrane (NR211, 25 m) at a pressure of 2 tons for 2 min to get the MEA.

Electrochemical tests of single cell

The obtained MEA was installed in a fuel cell fixture of Electrochem. Inc, and the experiments are run with a Biologic potentiostat (Figure 2.8b). The temperature of the humidifiers and cell was raised to 80 °C . When the temperature was stable, O2 and H2 flows were switched on (150 mL min -1 ), and gauge pressures were set to 1 bar, leading to the 1.5 bar absolute pressures of O2 and H2 since the water vapor saturation pressure is 0.5 bar at 80 °C . All the pipelines and connectors were wrapped up in thermal insulators to prevent the formation of low-temperature zone, leading to the condensation of water.

The cell was left at an open-circuit voltage (OCV) for about 5 min to record an average value. The CV curve was recorded between 0.9 V and 0.3 V at a scan rate of 1 mV s -1 . After, an electrochemical impedance spectroscopy measurement was conducted at OCV, with frequencies from 1kHz -1Hz. The durability of the catalyst was investigated by Chronoamperometry (CA) spectrum at 0.5 V, which presents the decline of current density with operation hours.

Impact of ink formula on single cell test

In the PEMFC environment, the O2 molecules in direct contact with the catalytic sites are dissolved in the hydrophilic phase of Nafion 195 . Therefore, it is important to cover all the micropore-hosted active sites by the Nafion network to maximize the mass activity of catalysts. On the other hand, the electron transfer resistance can be increased by a too-thick electrolyte layer. An appropriate I/C value should be found to measure the activity correctly. Similar research has been done by Jaouen et al. 195 . The optimum I/C value for their catalysts in PEMFC was found between 1.5 and 2, which is higher than the value of carbon-supported Pt catalyst due to a higher BET area (650-1000 m 2 g -1 ). Our catalysts also present a high BET area after ammonia treatment, typically larger than 1000 m 2 g -1 . Therefore, a high I/C value is also expected to get the best activity in PEMFC.

Table 2.3 shows the 4 different formulas of ink and the corresponding current density at 0.8 and 0.5 V from their polarization curves (Figure 2.9a). Figure 2.9a shows the effect of the Nafion content in the cathode ink formulation, with both the activity and power performance dramatically increasing from I/C =1.0 to I/C =2.0 and thereafter only slightly decreasing from I/C =2.0 to 2.5. Nyquist plots of electrochemical impedance spectroscopy showed little change with I/C (Figure 2.9b), while cathode CVs recorded in H2/N2 PEMFC mode showed dramatically increasing capacitive currents from I/C =1.0 to I/C =2.0 and only very slightly increasing from I/C =2 to 2.5 (Figure 2.9c). Therefore, the data indicate that the optimum I/C ratio is 2.0 for this catalyst, necessary for covering most of the catalyst surface with ionomer (maximizing the activity) without leading to excessive ionomer thickness (minimizing O2 diffusion issues through the film and the filling of meso-macropores). This optimum I/C ratio for the Fe-N-C aerogel catalyst in PEMFC is comparable to the one optimized for two other high SBET Fe-N-C catalysts (with similar PEMFC cathode preparation methods) 195 . For the two catalysts labeled as ball milling Fe-N-C catalysts 1 and 2 in that work, the optimum I/C ratio was found to be 1.75 and 2, respectively, while their SBET values were 730 and 1000 m 2 g -1 , respectively. The BET area of all catalysts in our work is ca. 1100-1600 m 2 g -1 , and the optimum ratio I/C of 2.0 is thus in line with previous results. The other catalysts were then investigated with a fixed I/C value of 2.0.

Chapter 3 Impact of Fe Precursors on the Performance of Fe-N-C Catalysts towards Oxygen Reduction Reaction in Acidic Medium

Ré sumé Franç ais

Fe est le centre des sites actifs Fe-N4 dans le catalyseur carboné . La nature de Fe dans la solution de pré curseurs joue un rôle crucial lors de la formation de la structure Fe-N4, affectant ainsi profondé ment l'activité des catalyseurs. Les facteurs influant sur la dispersion des ions ferreux et ferriques dans la solution de pré curseurs doivent ê tre examiné s, y compris les é tats d'oxydation du Fe, le contre-ion des sels de Fe et les ligands attaché s. Dans ce chapitre, des catalyseurs ont é té synthé tisé s à partir de diffé rents pré curseurs de fer et d'additifs à base de ligands azoté s pour é tablir la relation entre le pré curseur de Fe et les performances ORR des catalyseurs. De plus, les impacts des diffé rentes valeurs de pH dans la solution de pré curseur et la quantité de pré curseur de Fe ont é galement é té discuté s.

Dans la section 3.1, la porosité et la performance ORR des catalyseurs Fe-N-C pré paré s avec diffé rents pré curseurs de Fe, y compris le chlorure de fer (Fe(III)Cl3), le nitrate de fer (Fe(III)(NO3)3), le sel de Mohr ((NH4 )2Fe(II)(SO4)2) et la phtalocyanine de fer (II) (Fe(II)Pc), ont é té comparé es. Les catalyseurs pré paré s à pH=8 avec FeCl3 et Fe(NO3)3 (Fe 3+ ) pré sentent la meilleure activité é lectrochimique à 0,8 V à la fois dans la configuration RDE et PEMFC, lé gè rement plus é levé e que pour (NH4)2Fe(SO4)2 (Fe 2+ ) en raison à leur excellente solubilité dans la solution de pré curseur conduisant à un faible rendement de nanoparticules riches en Fe et à une porosité similaire. Les é tats d'oxydation de Fe et des contre-ions montrent peu d'effet sur les performances catalytiques finales si le pré curseur de Fe est soluble dans l'eau. La phtalocyanine insoluble (FePc, Fe 2+ ) ne semble pas ê tre un bon choix à pH=8 car elle conduit à une faible porosité de l'aé rogel de carbone. La faible teneur en Fe dans le catalyseur pré paré avec FePc a mis en é vidence que le pré curseur de Fe insoluble ne peut pas ê tre bien dispersé dans la solution de pré curseur, conduisant ainsi à un rendement plus é levé de nanoparticules riches en Fe, qui sont majoritairement é liminé es par le lavage à l'acide. La structure Fe-N4 pré existante dans le pré curseur FePc ne peut pas augmenter la densité de sites actifs dans le catalyseur final car cette structure est dé composé e au cours de la pyrolyse, avant d'atteindre la tempé rature qui cré e les sites actifs.

Dans la section 3.2, l'impact d'un faible pH de polycondensation avec un catalyseur FePc a é té é tudié . Cette recherche vise à augmenter la porosité du catalyseur pré paré avec FePc car une concentration plus faible en Na2CO3 facilite un volume mé soporeux plus é levé . Les ré sultats sur FePc avec pH=6 indiquent que la ré duction du pH (c'est-à -dire la ré duction de la teneur en base en solution) est un moyen efficace d'obtenir un catalyseur avec un volume de mé sopores plus é levé . Bien qu'il ne montre qu'une lé gè re amé lioration de l'activité é lectrochimique par rapport à un catalyseur ré alisé à pH=8, une grande amé lioration des proprié té s de transport de matiè re est observé e. Il est à noter que cette modification de pH n'est possible que pour le pré curseur de fer FePc car l'ajustement du pH est obligatoire pour les autres pré curseurs à base de sel de fer pour dissoudre la pré cipitation, phé nomè ne pré judiciable à la formation d'un gel tridimensionnel.

Dans les sections 3.3 et 3.4, la modification des pré curseurs de Fe, y compris l'introduction d'additifs sous forme de ligand et l'augmentation de la quantité de pré curseur de Fe ont é té é tudié es. Le ligand EDTA augmente la solubilité du pré curseur de Fe mais entraî ne une perte de Fe lors de l'é tape d'é change de solvant. En consé quence, tous les catalyseurs pré paré s avec un ligand EDTA pré sentent des teneurs en Fe ultra faibles. La phé nentroline (Phen) gé nè re des pré cipitations insolubles dans la solution mixte de dé part, ce qui conduit à une texture diffé rente par rapport au catalyseur sans ligand. Par contre, une teneur é levé e en Fe peut ê tre obtenue avec le catalyseur Phen sans aucune nanoparticule observable, indiquant une teneur é levé e en sites actifs. Cependant, le faible volume des micropores limite l'amé lioration de l'activité é lectrochimique. En conclusion, en raison de l'environnement chimique compliqué de la solution mixte de dé part, les additifs sous forme de ligands pré sentent peu d'amé lioration de l'activité é lectrochimique des catalyseurs. Le point crucial parmi ces catalyseurs est que leur activité est é troitement lié e à leur teneur en Fe, en particulier la teneur en Fe de surface caracté risé e à partir de l'analyse XPS. L'étude menée sur des catalyseurs à base de FePc avec des supports carboné s contenant ou pas de la mé lamine dans la solution de dé part, indique que la simple augmentation de la quantité de pré curseur de Fe ne conduit qu'à des nanoparticules non dé siré es dans le catalyseur final et plus largement dans les catalyseurs sans mé lamine. La structure de coordination pré existante dans FePc pré sente peu d'influence sur la formation des centres actifs car les macrocycles Fe-N4 contenus dans le pré curseur FePc sont dé composé s lors de l'étape de pyrolyse.

Fe is the center of Fe-N4 active sites in the carbonaceous catalyst. The nature of Fe in the precursor solution plays a crucial role during the formation of the Fe-N4 structure, thus deeply affecting the activity of catalysts. The factors showing impacts on the dispersion of ferrous and ferric ions in the precursor solution should be concerned, including oxide states of Fe, the counter ion of Fe salts, and the ligands attached. In this chapter, catalysts were synthesized from different iron precursors and nitrogenous ligand additives to find the relationship between the Fe precursor and the ORR performance of catalysts. In addition, the impacts of different pH values in the precursor solution and quantity of Fe precursor were also discussed.

Impact of different iron precursors

The impact of different Fe precursors will be discussed in this section. Fe precursors with different oxide states or counter ions were selected: iron chloride (Fe(III)Cl3), iron nitrate (Fe(III)(NO3)3), Mohr's salt ((NH4)2Fe(II)(SO4)2) and iron (II) phthalocyanine (Fe(II)Pc). Other synthesis conditions were kept constant: the theoretical iron content was 1 wt.%, the mole ratio of resorcinol (R)/melamine (M) was 2/1, and the pH value of polymerization was adjusted to 8 after the addition of Fe precursors. The phenomena during the synthesis, morphologies of carbon aerogel, and electrochemical performance of catalysts were investigated.

Synthesis of catalysts

FeCl3 precursor is a benchmark Fe(III) precursor, which has been used in the previous research of our group 186,187 . Fe(NO3)3 is another widely applied Fe(III) precursor for preparing Fe-N-C catalysts 196,197 . It is also soluble in water but possesses a different counter ion with FeCl3, which might affect the polymerization process in the precursor solution. The oxide state of Fe is interesting to be investigated because of the considerable difference in solubility between Fe 3+ and Fe 2+ at high pH values in the solution. According to the solubility product constant (Ksp) of Fe(OH)3 2.79×10 -39 and Fe(OH)2 4.87×10 -17 , the saturated solubility of Fe 3+ and Fe 2+ are 2.79×10 -21 mol L -1 and 4.87×10 -5 mol L -1 at pH=8, respectively 198 . The formation of hydroxide precipitation probably affects the dispersion of Fe in the hydrogel. (NH4)2Fe(SO4)2 was selected as the Fe(II) precursor for its ability to resist oxidation in solution 199 . The precursor FePc is quite special from the other precursors for its preexisting Fe-N4 structure, which is in favor of synthesizing catalysts with high ORR activity 136 .

The phenomena during the preparation of precursor solution with FeCl3 have been described in section 2.1.1. Similar phenomena were observed with Fe(NO3)3 precursor: red rust precipitation was generated when Fe(NO3)3 was added and then dissolved after adjusting pH to 8. As for the (NH4)2Fe(SO4)2, dark green precipitation formed initially, then quickly turned to rust red when NaOH solution was added, and finally disappeared at pH=8. Since the FePc is insoluble in water, the precipitation won't form. In this case, the pH adjustment of the mixture solution isn't necessary (but done in order to keep all the synthesis with the same comparable pH), which may lead to different porosity of carbon aerogel. This will be further discussed in section 3.1.2.

The total mass loss was calculated according to the two pyrolysis steps, as shown in Table 3.1. The first pyrolysis at 800 º C under N2 flow, also called the carbonization step, transformed the organic aerogel into a relatively pure carbon structure by removing most of the oxide, nitrogen, and hydrogen groups 200 . In this step, the mass loss of all the aerogels prepared with different iron precursors was around 55%. No significant changes were found because the decomposition of counter ions in the iron precursors was negligible compared to the loss quantity of the organic part. The second thermal treatment at 950 º C under N2/NH3 flow, which is the activation step, led to different mass loss range from 40% to 50% due to the different etching degree of the carbon aerogels.

The iron contents obtained from ICP-MS of all catalysts are listed in Table 3.1. The catalyst prepared with Fe(NO3)3 precursor presents a slightly higher Fe content than the theoretical value (1 wt.%), whereas that of others is lower. The difference in Fe content between catalysts is more than the variation of total mass loss, indicating a portion of Fe was removed by the acid washing process, such as Fe-rich nanoparticles. Since the Fe-N4 active sites were considered to be resistant to acid washing and preserved in the final catalyst 186 , the Fe precursor leading to high Fe content indicates a low yield of Fe-rich nanoparticles. The possible reason is the ferric ions of two precursors (Fe(NO3)3 and FeCl3) can be better dispersed in the aerogel. Therefore, the Fe atoms are less agglomerated during the pyrolysis. On the contrary, the catalysts prepared with (NH4)2Fe(SO4)2 and FePc precursors possess lower Fe content than the catalyst with Fe(III) precursors, indicating higher yields of Fe-rich nanoparticles of Fe(II) precursors.

Texture and structure of catalysts

The SEM images displayed in Figure 3.1a-d provide a basic view of the morphology of catalysts. According to the figures, the pore size distribution is quite different depending on the solubility of Fe precursors. The catalysts prepared with Fe salts (FeCl3, Fe(NO3)3, and (NH4)2Fe(SO4)2) show similar porosity on the surface of particles, where the mesopores can be observed clearly. However, the particle surface of the catalyst prepared with FePc seems more compact. The specific surface area and the pore size distribution of the catalysts were then further investigated by the nitrogen adsorption method. 3.1, which confirms the observation under SEM. In the case of pH = 8 and R/M=2/1, the catalyst with insoluble FePc as iron precursor displays the lowest specific surface area and mesopore volume, which differs from those prepared with soluble Fe salts of similar texture. Besides, the mesopore size peak of the FePc catalyst in Figure 3.1f is around 5 nm, which is smaller than that of the Fe salts catalysts (around 7 nm). The solubility of iron precursors seems to be determinant for allowing an adequate development of the mesopore structure but exhibits little impact on micropore volume. According to the model of gelation progress proposed by Al-Muhtaseb et al. 200,201 , the condensation step is realized by the interconnection of prepolymer particles. Therefore, a small condensation unit will reduce the pore size of the hydrogel. The insoluble FePc particles at the micron level can disturb and increase the condensation nucleus to accelerate the interconnection between the prepolymer particles, which limits the growth of condensation units and leads to small mesopore size. On the contrary, the carbon aerogels prepared with soluble iron salts could be coordinated by the amino group of melamine to slow down the interconnection between the polymer particles, which promotes the growth of condensation units and, consequently, a higher porosity. The soluble iron salts, including FeCl3, Fe(NO3)3, and (NH4)2Fe(SO4)2, give similar mesopore volume in the final catalysts, indicating different oxide states of iron or counter ions do not significantly affect the polycondensation step.

The distribution of micropores is mainly related to the two different thermal treatments. The micropores are produced by the pyrolysis of the organic aerogel and increased in quantity by the anisotropic NH3 etching of the carbon matrix 153,194 . The micropore sizes are around 0.75 nm for all catalysts, which perfectly corresponds to the size of the Fe-N4 macro-cycle proposed by Dodelet et al. [START_REF] Kramm | Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells[END_REF] The micropore volumes of all catalysts are almost identical due to the same heat treatment conditions. The existence of iron nanoparticles in catalysts was determined by transmission electron microscope (TEM), as shown in Figure 3.2a-d. None of the nanoparticles are observed in the images, indicating that all unexpected Fe species can be removed by acid washing. Besides, the carbon substrates are almost amorphous, indicating the aerogel structure can be conserved after heat treatments. The powder X-ray diffraction (PXRD) patterns shown in Figure 3.2e confirm the TEM observation. No significant diffraction peak can be indexed to Fe-contained species in all catalysts. The large bump at around 2θ=26° and small bump at 2θ=45° correspond to the low crystallization carbon, which is observed under TEM. Three weak peaks at 2θ=26.4°, 34.3°, and 54.6° can be found in some catalysts, but they cannot be assigned to any Fe-contained species.

Chemical composition of catalysts

In order to understand the influence of the ion precursor on the activity of the catalysts, it is crucial to investigate the chemical environment on their surface because the active sites exposed on the surface contribute the most to the ORR process. X-ray photoelectron spectroscopy (XPS) studies were performed on the catalysts before electrochemical tests. The analysis is unable to be carried out on the catalysts after electrochemical tests because the quantity on the rotating disk electrode is far from filling up the sample holder of XPS. Besides, the ionomer added in ink can change the element composition as well. The element composition was obtained according to the method in annex A.2.5 and listed in Table 3.2. The main substrate C contributes most in the surface component (more than 90 at.%) as expected. All catalysts exhibit a significant amount of N (around 3 at.%), which is the most interesting because it's one of the main elements of active sites. Trace of Fe can also be identified, and the values are close to that of ICP-MS. In general, the XPS is considered to be a surface characterization technology and, ICP-MS presents the global element content. The similar Fe content from the two methods implies a uniform distribution of Fe on the surface and the in bulk of particles. It should also be kept in mind that the measurements made with XPS on Fe are obtained within the detection limits of the equipment and on porous granular materials. In order to figure out the chemical environment of Fe, the deconvolution of XPS spectra was supposed to be conducted for the spectra of N and Fe. However, because the Fe content is close to the detection limit of XPS, the spectra of Fe are too noisy to be analyzed. Figure 3.3b gives an example of the Fe 2p spectrum of the catalyst prepared with FeCl3. Only the double peaks of the split Fe 2p spectrum at binding energies (BEs) of 711.1 and 724.3 eV can be identified, which reveals the existence of trivalent iron 202 . Besides, the binding energies are also close to the value of the Fe-N bond identified by Wang et al. 203 As an alternative option, the N 1s spectrum is taken into consideration since the states of nitrogen directly relate to the formation of Fe-Nx centers. The deconvolution was followed with the method of Atanassov et al. [START_REF] Artyushkova | Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal-Nitrogen-Carbon Electrocatalysts[END_REF] For consistency among all spectra of catalysts, the full width at half-maximum (FWHM) of fitting peaks was constrained between 1.2-1.4 eV as described in annex A.2.5. Figure 3.3a shows an example of N 1s spectrum fitted with six peaks. The peak at the lowest BE of 398.0 eV comes from either imine or cyano groups. The peak at 398.8 eV is accepted to be pyridinic N with one electron contributed to the π system, and the pyrrolic N with two π electrons is present at 400.9 eV. These two nitrogen species are most important because they can coordinate with Fe to form active sites. The peaks that appeared at 402.0 eV and 403.3 eV were attributed to quaternary and graphitic nitrogen, respectively. In addition, according to the different test conditions and properties of samples, different deconvolution methods could be used. For example, some research didn't take nitrile species into consideration, thus the BE of pyridinic N moving to 398.3 eV [START_REF] Artyushkova | Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal-Nitrogen-Carbon Electrocatalysts[END_REF]204 . In our analysis, if the peak of nitrile was eliminated, the FWHM of pyrrolic and Fe-Nx fitting peaks should be enlarged to 1.5 eV to get an acceptable residual. Therefore, it is better to conserve the nitrile peak to maintain the consistency of analysis results. When a fixed width of 1.2 eV was used during peak decomposition, another peak at 399.9 eV had to be included in the fit. This peak was confirmed to be the superposition peak of Fe-Nx species by Atanassov's group with density functional theory calculations of binding energy shifts for Fe-N-C graphene-based materials [START_REF] Artyushkova | Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal-Nitrogen-Carbon Electrocatalysts[END_REF]204,205 .

The content of different N species obtained from the deconvolution of XPS plots for all catalysts is listed in Table 3.2. A significant peak of Fe-Nx at 399.9 eV can be assigned for all catalysts, and the quantity varies with different Fe precursors. The catalysts with FeCl3 and FePc possess higher Fe-Nx content than the others, even though their Fe contents from XPS are similar. One possible reason could be the attenuation lengths of Fe 2p, and N 1s photoelectrons are remarkably different. The atomic percent of nitrogen detected comes from 10 to 15 nm of the top surface of the sample, while the iron is only detected from the top 2-4 nm of the surface 206 . Therefore, the peak at 399.9 eV assigned from the XPS spectrum could be considered as identification of the existence of Fe-Nx active sites. However, for the quantitative study of active site content, more direct characterization of Fe, such as Mossbauer spectroscopy, should be employed. On the other hand, pyridinic and pyrrolic nitrogens also play a crucial role in improving electrochemical activity. As reported in the papers, the graphene enriched with pyrrolic N or the carbon nano-tubes with high pyridinic N displayed better catalytic activity than the carbon supports without N dopant 207,208 . The catalyst with (NH4)2Fe(SO4)2 precursor presents high content of pyridinic and pyrrolic N, which is also possible to present high activity.

Electrochemical characterization of catalysts

The activity of catalysts to ORR was evaluated in a three-electrode setup as presented in section 2.3.1, where electrochemical measurements were performed using a rotating disk electrode. The Fe precursor will be selected for further improvement according to the electrochemical activity. The CV curves in N2 saturated electrolyte and the polarization curves in O2 saturated electrolyte at a rotating speed of 1600 rpm of all catalysts are shown in Figure 3.4a and b, respectively. The electrolyte is 0.05 mol L -1 H2SO4, and the scanning rate is 10 mV s -1 . The current density values at 0.8 V vs. RHE are also listed in Table 3.3. These values were extracted from the ORR polarization curves after correction of the Ohmic drop and subtracting the background current measured in N2 saturated electrolyte, which is accepted to identify the ORR activity 209 . The catalysts prepared with FeCl3 and Fe(NO3)3 display the highest mass activity at 0.8 V (around 2.0 A g -1 ) among all, whereas the two others are slightly lower. This result is consistent with the sequence of their Fe content obtained from ICP-MS. The micropore volume exhibits little relationship with activity tendency, where the micropore volumes are almost the same (around 0.44 cm 3 g -1 ) for all catalysts with mass activities ranging from 1.72 and 2.05 A g -1 . Theoretically, the surface composition is more important to determine the activity because the electrochemical reactions mainly conduct on the interface between catalysts and the electrolyte. However, due to the large uncertainty of Fe content from the XPS method, the variation of Fe content cannot be clearly presented. As a result, the ICP-MS results are used instead of the Fe atoms are uniformly distributed on the surface and in bulk. On the contrary, the atomic percentage of Fe-Nx doesn't show an obvious relationship with the electrochemical activities of catalysts. The possible reason is the different attenuation lengths between Fe and N mentioned before that all Fe-Nx nitrogen is not on the surface of the particle to present their electrochemical activity 206 . Besides, the peak at around 400 eV can also be attributed to amine, which also confuses the accurate active site content [START_REF] Artyushkova | Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal-Nitrogen-Carbon Electrocatalysts[END_REF] . The impact of N will be further discussed in chapter 4, where the role of different N species will be clarified.

The limiting current density (obtained from Figure 3.4b at 0.2 V) is increased with mesopore volume. Two catalysts (FeCl3 and (NH4)2Fe(SO4)2) which have the largest mesopore volume, display the highest limiting current density, whereas the FePc catalyst with the lowest mesopore volume shows the lowest current. The same tendency has also been discovered by Carmelo et al., that the carbon support does not influence the onset potential of the catalysts, but its contribution becomes relevant when the rate-determining step is the mass transfer, i.e., at high overpotential 196 . The large mesopore volume contributes to a large electrochemically available surface area for a similar micropore volume, which is consistent with the increase in cyclic voltammetry area in Figure 3.4a. Figure 3.4c shows the Tafel plots derived from the polarization curves. It seems the Tafel slopes are closely related to the mesopore volume of catalysts, where the catalyst with the lowest mesopore volume (prepared with FePc) corresponds to the highest value (66 mV dec -1 ) while the catalyst with the highest mesopore volume (prepared with FeCl3) corresponds to the lowest value (52 mV dec -1 ). With respect to reaction micro-kinetics, lower Tafel slope values correspond to faster reaction rates at the electrode surface 210 because Tafel plots represent the measured current density increase as a function of the overpotential. Therefore, the relationship between the Tafel slope and mesopore volume supports that a high mesopore volume of Fe-N-C aerogel catalyst can facilitate the mass transport in the RDE setup. It should be mentioned that Tafel slopes for all catalysts are essentially the same in acid media (~ 60 mV dec -1 ), suggesting that the rate-determining step of the ORR mechanism is the same. According to the simulated behavior of the Tafel relation for the oxygen reduction reaction by assuming different elementary steps as the rate-determining step, the Tafel slope around 60 mV dec -1 may be associated with the reaction mechanism in which the rate-determining step is affected by the proton-coupled electron transfer (PCET) instead of a single electron transfer reaction, and thus the reaction rate depends on the applied potential 210 . In this case, the different ORR activities are mainly related to the different catalyst-specific active areas for the catalysts prepared with different iron precursors in acid media, thus giving us the activity tendency that coincides with the variation of CV area 211 .

The durability tests were carried out for all catalysts with 500 cycles between 0.6-1.0 V in N2 saturated electrolyte. The activity was then determined by linear scan voltammetry in O2 saturated electrolyte. The results are shown in Figure 3.4d, and the loss ratios are listed in Table 3.3. The catalysts display different degrees of degradation after the test. The catalyst prepared with FePc shows the best durability (22% loss of activity after 500 cycles of CV), whereas the others are similar (~ 45% loss). The degradation of Fe-N-C catalysts in an acidic medium is caused by the etching of the carbon matrix and dissolution of Fe, which leads to the loss of active sites 184 . During the CV cycles, the degradation of the Fe-N4 active sites and of the supporting carbon aerogel occurs in parallel at high potential. Less current density means electrochemical oxidation on carbon, thus less etching. Therefore, it becomes reasonable that the best durability is obtained by FePc, which possesses the lowest electrochemical available area. Their activities were further investigated in a 5 cm 2 single cell of PEMFC in the conditions described in section 2.3.2, with a scanning rate of 1 mV s -1 . As the polarization curves shown in Figure 3.5, catalysts show the same trend with RDE tests at 0.8 V. The catalyst prepared with Fe(NO3)3 displays the best performance, and the catalyst prepared with FePc is less active than the others in the single cell. Besides pure charge-transfer limitations, other characteristics, such as the accessibility of O2 and proton to the active sites, limit the performance in the single cell. The enlarged difference between catalysts prepared with FePc and the others is probably due to its poor mesopore volume restricting the contact between O2 and active sites. For all the catalysts, it is important to notice that the polarization curves do not collapse at high current densities (j > 0.5 A cm -2 geo), demonstrating the excellent mass transport properties of carbon aerogel catalysts.

Conclusion

The impact of four different iron precursors on the porosity and the electrochemical activity of Fe-N-C aerogel catalysts were investigated. The iron salts with different oxide states and counter ions can affect the solubility and the reactivity of iron ions in precursor solution, thus changing the polycondensation reaction during the gelation step, which leads to different porosity and electrochemical activity. We found that the catalysts prepared with Fe(III), FeCl3, and Fe(NO3)3 display the best electrochemical activity, which corresponds to high mesopore volume and electrochemical available area. The catalyst prepared with Fe(NO3)3 exhibits the higher current density at 0.8 V both in the RDE setup and single cell associated with the higher Fe content. The catalyst prepared with (NH4)2Fe(SO4)2 possesses an electrochemical activity and porosity slightly lower than those of FeCl3. The insoluble FePc seems not to be a good choice since it leads to a carbon aerogel with a low porosity at pH=8 which seems to limit the performance of the catalyst. However, its insolubility also made the modification of pH possible, which will be discussed in section 3.2

Impact of different pH with Fe phthalocyanine precursor

The impacts of pH value on porosity and activity will be detailed in this part. The research was focused on the precursor FePc, which is insoluble in water and therefore does not cause the coagulation of prepolymers in the mixed solution. According to the mechanism of gelation, the carbon aerogels prepared at pH=6 show larger development of porosity than their corresponding counterparts obtained at pH 8 212 . Therefore, low pH is in favor of the development of mesoporous structure and obtaining the catalyst with better mass transport properties. To reach pH=6, no extra NaOH was added into the mixture solution A+B, which corresponded to a ratio of (R+M)/C ~ 100 for pH=6, whereas the value for pH=8 is 10. The phenomena during the synthesis, the morphology of carbon aerogel, and electrochemical performance were investigated.

Synthesis of catalysts

The FePc is a promising iron precursor to prepare the Fe-N-C catalyst with high electrochemical activity due to its pre-existing Fe-N4 structure 213 . However, rapid polycondensation of the RMF gel happened due to a high amount of the catalyst in the solution at a high pH value. Numerous nuclei form a gel with small construction units. On the contrary, by using a low concentration of catalyst, the particles have enough time to grow up, and the pores between the particles become larger by geometrical effect 200 . As expected, the catalyst prepared in pH=6 took 1 h to form the gel, which is much longer than that in pH=8. The other phenomena during synthesis were essentially the same, as well as the total mass loss after the two pyrolyses (N2 and N2/NH3). 3.4. Catalyst in pH=6 exhibits a larger mesopore size centered around 10 nm (5 nm for pH=8) and mesopore volume (three times higher than in pH=8), indicating that a low pH value favors the development of the mesoporous structure. The Fe content obtained from ICP-MS of the catalyst in pH=6 is lower than the pH=8, which should be further confirmed with the XPS method.

Texture and structure of catalysts

SEM images displayed in

Chemical composition of catalysts

The XPS characterization was employed to obtain the chemical environment on the surface of catalysts. The results of elemental composition are listed in Table 3.5. The catalysts prepared in pH=6 and 8 possess different iron content on the surface and in bulk, which implies an uneven distribution of Fe. One possible reason for this phenomenon is that the catalyst with a larger mesopore volume is also deeply etched during NH3 thermal treatment. Therefore, more active sites are exposed on the surface and detected by XPS. In addition, the nitrogen contents of the two catalysts are identical, which implies the limitation of the quantity of active sites and activity by the quantity of Fe. The deconvolution was conducted to figure out the composition of different N species, and the results were listed in Table 3.5, along with the results of pH=8 as a comparison. The Fe-Nx content of pH=6 decreased, whereas the other two active N species (pyridinic and pyrrolic N) contents slightly increase.

Electrochemical characterization of catalysts

The activity of the two catalysts towards ORR was evaluated in the RDE setup. The CV and polarization curves are shown in Figure 3.8a and b, respectively. Their current densities at 0.8 V are listed in Table 3.6. The catalyst prepared in pH=6 exhibits a larger electrochemical available area according to the CV curves, which is in accord with its larger specific surface area and mesopore volume. According to the results obtained with different Fe precursors, the quantity of Fe-Nx cannot be directly related to the activity. However, their mass activities at 0.8 V are almost the same, indicating that different pH conditions have little influence on the formation of active sites. This result is reasonable because FePc is insoluble in water, and the dispersion of the precursor is only decided by its initial particle size. The larger mesopore volume of pH=6 makes the reactants accessible to the active sites, thus giving a higher limited current. Therefore, both micro-and mesopore volumes are important to obtain catalysts with high electrochemical activity. The Tafel slopes derived from the polarization curves, as shown in Figure 3.8c, are consistent with the trend of activity of samples, indicating a faster reaction rate is obtained with the catalyst prepared in pH=6. Besides, the durability tests in N2 saturated electrolyte also give similar results, that about 20% loss of activity after 500 cycles. Their activity in 5 cm 2 single cell of PEMFC was also investigated. The polarization curves are shown in Figure 3.9. Their activities at 0.8 V are similar, which is consistent with the results in the RDE setup. The activity of FePc-pH=6 shows more enhancement at higher overpotential due to a 3 times higher mesopore volume in pH=6 than pH=8.

Conclusion

The results on FePc with pH=6 indicate that keeping the mixed solution in pH=6 (i.e., reducing the base content in the solution) is an efficient way to get a catalyst with higher mesopore volume. Although it only shows a slight enhancement in electrochemical activity, a large improvement in mass transport properties is observed. It should be noted that this condition is only possible for the iron precursor FePc because adjusting pH is mandatory for other iron precursors to dissolve the precipitation, a phenomenon that is detrimental to the formation of a three-dimensional gel.

Impact of additive ligands

The impacts of ligand additives in the precursor solution were investigated. According to the literature, the iron precursors with preexisting Fe-N coordination (such as FePc and iron phenanthroline FePhen) could lead to Fe-N-C catalysts with better activity 214 . Besides, the introduced ligand can also improve the solubility of Fe ions in solution and modify the dispersion of Fe atoms in the carbon matrix. Two ligands containing N as the coordination site were selected: ethylenediaminetetraacetic (EDTA) and phenanthroline (Phen). EDTA is a classic chelating ligand that can coordinate with Fe ions at a molar ratio of 1:1 by forming two Fe-N bonds and four Fe-O bonds. The formation constants (Kf) of Fe(II)EDTA and Fe(III) EDTA are 2.0×10 14 and 1.3×10 [START_REF] Pan | Advances and challenges in alkaline anion exchange membrane fuel cells[END_REF] , respectively 215 . The chelate with Fe(III) is more stable than Fe(II), which is expected to have better dispersion in the precursor solution. Vecchio et al. employed metals (Fe or Co) chelated with EDTA to synthesize M-N-C catalysts with improved activity and stability. The EDTA was used as a low-cost and bio-compound nitrogen precursor to encapsulate the metal center. In their results, the complex ion with high Kf tends to present the highest relative percentage of N-metal interaction 196 . Their impact during the synthesis on the morphology, the surface chemical composition, and the electrochemical performance were investigated.

Synthesis of catalysts

For preparing the complexes, the ligands and Fe precursor were mixed in 2 mL distilled water and heated at 70 °C for 10 min before being added to the resorcinol-melamine-formaldehyde solution. The EDTA can form a light-yellow solution with both Fe precursors, and the precipitation is generated when the precursor solutions have been mixed. After adjusting the pH value to 8 by adding around 3 mL of NaOH solution, the precipitation disappeared, and a maroon solution was obtained. Another noteworthy phenomenon was observed during the solvent exchange (between water and acetone for CO2 supercritical drying), and the solvent exchanged became light yellow colored. The possible reason is that Fe complexes are more soluble in water than acetone, which may lead to the loss of Fe precursor. The Fe content obtained from ICP-MS demonstrated this negative effect of EDTA. As seen in Table 3.7, for both Fe precursors, the Fe contents of the catalysts prepared with EDTA ligand are significantly lower than the catalysts prepared without ligand. This result indicates that the soluble chelated Fe ions are hardly conducive to high Fe content in the catalysts due to the loss of iron during the solvent exchange. The best existing state of Fe in the mixed solution is well-dispersed colloid particles, which can be adsorbed on the skeleton of the hydrogel.

In the case of the Phen ligand, a dark red solution formed with both Fe salts and the orangered precipitations were generated when complexes were introduced into the mixed solution. However, the precipitation cannot be dissolved by adjusting the pH value. Finally, the suspension took more than 1 hour to finish the gelation. The phenomena during solvent exchange were also different from catalysts prepared with EDTA ligand. The exchanged solvent was colorless, which meant that the [Fe(II)Phen3] 2+ and [Fe(III)Phen3] 3+ were conserved in the hydrogel. The difference between the two ligands can be attributed to their microstructure. The Phen ligand possesses a conjugation π-system to adsorb onto the organic matrix skeleton by π-π stacking interaction. Therefore, the complexes can be well attached to hydrogel to avoid the loss of Fe during the solvent exchange. The ICP-MS results also demonstrated that the catalysts prepared with Phen ligand have higher Fe content than the catalysts without ligand. However, since Phen led to quite different phenomena during the polycondensation step compared with EDTA, the morphology was supposed to be different from others. Interesting differences were noticed in the textural variation of the aerogels depending on the ligand additives. The catalysts prepared with ligands seem to have larger mesopore sizes than the catalyst without ligand. Figure 3.10d shows the nitrogen adsorption isotherms at -196 °C of the catalysts, alongside the pore sizes distribution in Figure 3.10e. The pore volumes of catalysts obtained by DFT analysis are listed in Table 3.7. It can be clearly seen that both catalysts prepared with ligand additive display larger mesopore volume than catalysts without ligand and centered at a larger mesopore size. However, this result should be attributed to different reasons for the two ligands according to their different phenomena observed during synthesis. A similar phenomenon was observed in the catalysts prepared with EDTA and without ligand. The maroon precipitation was generated when Fe precursor was introduced, and then it was dissolved after adjusting pH. On the basis of the polycondensation mechanism of RMF hydrogel, EDTA may increase the solubility of Fe ion, thus reducing the nuclei number in the mixed solution and leading to larger mesopore volume 218 . As for the catalyst prepared with Phen, since precipitation cannot be dissolved by adjusting pH, the final catalyst is more likely to be a stack of large particles instead of an integrated network of aerogel. The mesopore volume may be contributed by the space between these large particles.

Texture and structure of catalysts

In contrast to the variation of mesopore volume, the micropore volumes and BET surface were significantly decreased for the catalysts prepared with ligand, although the same heat treatment conditions were used (both in N2 and N2/NH3). Since micropores are widely considered to be the containers of Fe-N4 active sites, decreased micropore volume may lead to weak ORR activity. The SEM images in Figure 3.11a-c give a general view of the porosity of the catalysts prepared with (NH4)2Fe(SO4)2 precursor and different ligands alongside the catalyst without ligand as the comparison. The EDTA ligand leads to a similar morphology with the catalyst without ligand under SEM, whereas the Phen ligand changes the texture of aerogel due to undissolved precipitation in the mixed solution. Their nitrogen adsorption isotherms at -196 °C and distribution of pore sizes are shown in Figure 3.11d and e, respectively. The catalyst prepared with EDTA exhibits a similar mesopore volume to the catalyst without ligand (mesopore size around 6-10 nm), which may be attributed to the lower formation constant of Fe(II)EDTA complex than Fe(III)EDTA. Phen ligand leads to a larger mesopore size (peak at around 20 nm) and mesopore volume, which is similar to the results of Fe(NO3)3 precursor. SEM image Figure 3.11b exhibits its texture seems like a stack of large particles instead of a porous network. The micropore volume of catalysts prepared with the ligand is decreased, which is similar to the situation of Fe(NO3)3 precursor. This result implies a reduced ORR activity, which will be discussed later. The TEM and XRD were employed to determine the existence of Fe nanoparticles. As for the catalysts prepared with Phen ligand, the same peaks with at 2θ=26.4°, 34.3°, and 54.6° can be found, but they cannot be identified to any Fe-contained species.

Chemical composition of catalysts

The surface chemical environment of catalysts was investigated by XPS. The results of catalysts prepared with Fe(NO3)3 and ligands are listed in Table 3.8, along with the catalyst without ligand as a comparison. The Fe contents of the catalyst prepared with the ligand are close to the results of ICP-MS, indicating a uniform distribution of Fe in bulk and on the surface.

The details on the chemical bonding states of the surface N-groups are provided by deconvolution of N 1s spectra with the method of Atanassov et al., as discussed in section 3.1 [START_REF] Artyushkova | Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal-Nitrogen-Carbon Electrocatalysts[END_REF] . The atomic percentages of different N groups on the surface are listed in Table 3.8. The peak at 399.9 eV can be assigned for all catalysts, whereas, for the catalyst with EDTA ligand, the atomic percentage ratio of Fe-Nx/Fe is larger than the maximum coordination number of Fe (Fe-N4). As we mentioned before, this peak may be attributed to other N species, such as amines 206 . Therefore, this peak cannot be simply attributed to N coordinated with Fe due to the different synthesis methods of catalysts. The pyridinic N of the EDTA catalyst is decreased compared to a non-ligand catalyst, but that of the Phen catalyst slightly increases. The pyrrolic N contents for all catalysts are almost identical. Their impacts on electrochemical activity will be discussed in the next section. The XPS analysis results of catalysts prepared with (NH4)2Fe(SO4)2 and different ligand additives are listed in Table 3.9, as well as the deconvolution results of N 1s spectra. Similar to the Fe content obtained from ICP-MS, XPS analysis also reveals low Fe content on the surface. The Fe atoms are uniformly dispersed in the carbon aerogel, but the low Fe content of the catalysts prepared with EDTA ligand suggests a low concentration of active sites. For the catalyst prepared with EDTA ligand, Fe-Nx content obtained from deconvolution of N 1s spectra is much higher than the maximum value (Fe-Nx/Fe > 4), which also proves that this peak may be attributed to other N species. The N content of catalyst prepared with Phen ligand is decreased compared to non-ligand catalyst. The impacts on their activity will be discussed later.

Electrochemical characterization of catalysts

The impacts of ligand additives on the electrochemical activity of two series catalysts were evaluated in a three-electrode RDE setup. The CV and polarization curves of Fe(NO3)3-based catalysts are shown in Figure 3.14a and b, respectively. The curves of catalyst without ligand are also plotted as a comparison. The corresponding current density values at 0.8 V vs. RHE are listed in Table 3.10. The catalyst prepared without ligand displays the best electrochemical activity and the stability after 500 cycles in N2 saturated electrolyte. The CV areas are correlated to the specific surface area of the catalysts of the samples, which shows that the surface is accessible to the electrolyte. The non-ligand catalyst shows the largest CV area, corresponding to its largest BET area. The catalysts prepared with ligand show a lower BET area (Table 3.7), thus presenting a lower CV area.

The mass activity at 0.8 V of catalysts increases with their Fe content, indicating an increased density of active sites. This result confirmed the guess before that low micropore volume of catalysts prepared with ligand show weak ORR activity. The limit current at 0.2 V of the catalysts prepared with the ligand is slightly higher than the non-ligand catalyst due to higher mesopore volume improving the mass transport property in the RDE setup. The Tafel slopes shown in Figure 3.14c are similar for all catalysts, indicating that they have the same ORR mechanism. Therefore, ligand additives do not affect the properties of Fe-N4 active sites. Figure 3.14d shows the normalized current density at 0.8 V after the durability test. The EDTA catalyst lost most of the activity because its ultra-low Fe loading and high mesopore volume made the demetallation effect more significant than the others. The same tendency can be found when (NH4)2Fe(SO4)2 is used as the Fe precursor. As the CV and polarization curves shown in Figure 3.15a and b, the catalyst prepared without ligand additive displays the best mass activity according to the current density at 0.8 V. The electrochemical available areas obtained from CV curves exhibit the same tendency with the electrochemical activity and Fe content of catalysts. EDTA leads to low Fe content and micropore volume in the final catalyst, thus weak ORR activity. Non-ligand and phen catalysts possess similar Fe content, thus presenting similar mass activity. Their Tafel slopes are shown in Figure 3.15c also indicate that all catalysts have the same ORR mechanism, which is not affected by different ligands or Fe precursors. Figure 3.15d shows that catalysts prepared with ligands exhibit slightly higher durability than that without ligand after 500 CV cycles, which is opposite to Fe(NO3)3 precursor.

It is interesting to remark on the different effects between the two ligands. The EDTA ligand leads to a decreased Fe content because the [Fe(II)EDTA] 2-and [Fe(III)EDTA] -complexes can be removed during the solvent exchange step. On the contrary, phen shows much less effect on the Fe content than EDTA but significantly changes the morphology of aerogel under SEM.

Comparing their different effects on electrochemical activity, the Fe content seems more critical than morphology, which is always proportional to the mass activity of catalysts regardless of the variation of mesopore volume. Besides, since the electrochemical reaction occurs on the interface between catalyst and electrolyte, the surface Fe content obtained from XPS should be a relatively accurate parameter to determine the activity. However, due to the high uncertainty of XPS analysis, the ICP-MS can be used if the Fe distribution is uniform. The XPS value becomes more important when the difference between ICP-MS and XPS cannot be ignored, i.e., an uneven distribution of Fe occurs. The activity in the 5 cm 2 single cell PEMFC of catalysts prepared with different ligand additives was investigated (method described in section 2.3.2). The polarization curves of the catalysts prepared with Fe(NO3)3 or (NH4)2Fe(SO4)2 precursor are shown in Figure 3.16a and b, respectively. For the Fe(NO3)3 precursor, the activities of the catalyst prepared with EDTA or phen ligand are almost identical, whereas both significantly declined compared with the nonligand catalyst. Their difference in micropore volume can be the explanation. The non-ligand catalyst has the largest micropore volume thus, the most active. Two catalysts prepared with ligands show identical polarization curves due to their similar micropore volume. The catalysts prepared with (NH4)2Fe(SO4)2 precursor present the same tendency, which is also in accord with the variation of their micropore volume.

Conclusion

The impacts of two classic N-containing ligands on the porosity and activity of catalysts have been investigated. The impacts of ligands are similar for the two Fe precursors with different oxide states. The possible reason is that Fe 2+ is easy to be oxidized in the solution at pH=8. Therefore, the oxide states show little influence during synthesis. The EDTA can greatly increase the solubility of Fe ions, which leads to loss of iron during solvent exchange step. Therefore, all catalysts prepared with EDTA display poor electrochemical activity. Phen generates indissoluble precipitation in the mixed solution, which leads to a different texture from the non-ligand catalyst. High Fe content can be obtained in Phen catalyst without any observable nanoparticles, indicating a high content of active sites. However, the low micropore volume limits the improvement of electrochemical activity. In conclusion, due to the complicated chemical environment of mixed solution, the ligand additives exhibit slight improvement in the electrochemical activity of catalysts.

The crucial point among these catalysts is that their activity is closely related to their Fe content, especially the surface Fe content obtained from XPS analysis. However, due to the Fe content in the Fe-N-C catalyst approaching the detection limit of XPS (less than 1 wt.%), the uncertainty is too large to analyze quantity. Therefore, if ICP-MS and XPS values are close, the bulk Fe content obtained from ICP-MS could be used as a more precise value to be analyzed. Suppose the difference between two values cannot be ignored due to uneven distribution of Fe. In that case, the XPS value seems to be more reliable, even though a larger uncertainty should be tolerated. Besides, since the Fe content is the only important factor we found here, the impacts of increasing Fe content will be further investigated in the next section.

Impact of different Fe content

As discussed in section 3.3, the Fe contents in the catalyst significantly influence the electrochemical activity and thus should be further investigated. If all the Fe are atomically dispersed in the carbon aerogel, the activity is supposed to show a linear relationship with Fe content. However, in order to form more Fe-N4 active sites, the N content should be increased simultaneously, otherwise, the nanoparticles of Fe may be found in the catalyst 219 . The FePc precursor possesses a pre-existing coordination structure, which can ensure the supplementation of N while increasing the Fe content. Therefore, the melamine content can remain constant. The catalysts with different theoretical Fe content were synthesized with FePc precursor, as shown in Table 3.11. The pH value of the mixed solution was controlled at around 6 according to the results in section 3.2. The solid content of hydrogel is 20 wt.%. Two different R/M ratios were selected to investigate the impact of the N source content for the Fe precursor with pre-existing coordination structure. If the pre-existing coordination structure can be preserved in the final catalyst, the existence of an extra N source is unnecessary. Therefore, we removed the N source in the carbon aerogel to compare the difference between two series of catalysts, with and without melamine.

Texture and structure of catalysts

The phenomena during synthesis were essentially the same as discussed in section 3.2. The black suspension was obtained when FePc precursor was added, then the hydrogel formed after around 40 minutes. Their iron contents were determined by the ICP-MS method, as shown in Table 3.11. For the composition R/M=2/1, the catalyst with low theoretical Fe content (0.5 wt.%) presents an identical value, whereas, for the catalysts with more Fe precursor, the ICP-MS results are lower than theoretical values. It is possible that more nanoparticles were generated with increasing of FePc quantity, which was then removed by acid leaching. A similar tendency is observed with the catalysts with composition R/M=2/0, in which no melamine is introduced. The melamine exhibits little impact on the total Fe content of final catalysts (atomically dispersed and nanoparticles) that we have 0.61 wt.% vs. 0.56 wt.% for the 1 wt.% theoretical content and 1.51 wt.% vs. 1.86 wt.% for the 2 wt.% theoretical content. However, these results cannot lead to the conclusion that the pre-existing structure can be preserved before the chemical environments of Fe are determined. The SEM images shown in Figure 3.17a-c exhibit that all the catalysts prepared with composition R/M=2/1 display similar mesopore size. The N2 adsorption isotherm curves at -196 °C and corresponding pore size distribution plots are shown in Figure 3.17d and Figure 3.17e, respectively. The corresponding pore volumes are listed in Table 3.11. The mesopore volumes are almost identical and the distribution of mesopore size is centered around 10 nm for all catalyst, which is consistent with the observation from SEM images.

The micropore volumes of the catalyst also decrease with the increase of FePc quantity. If we suppose that all Fe ions are atomically dispersed in the catalyst, since the Fe-N4 active sites are hosted in the micropores, the micropore volume should increase with Fe content rather than the other way around. This inversed tendency suggests the existence of unexpected Fe contained species in the catalysts. Except for small amounts of iron (0.5wt.%), the Fe contents of catalysts prepared with high Fe precursor quantity are less than the theoretical values. For larger amounts of Fe precursor, the iron atoms can be quite close and coalesce with the heat treatments. In fact, nanoparticles larger than 2 nm can block access to micropores and small mesopores, which explains the variation of micropore volume. The SEM images of catalysts prepared without melamine (R/M=2/0) are shown in Figure 3.18a-c. The morphology of catalysts with 1 wt.% and 2 wt.% Fe content is similar, whereas the catalyst with 3 wt.% seems to be constituted by larger compact particles. 3.11, which proves the observation from SEM images. The mesopore volume decreases with the increase of FePc quantity. The phthalocyanine (Pc) in the precursor is another carbon source during the synthesis, whereas it is non-porous. One thing should be noted for the catalysts prepared with the same quantity of FePc but different R/M, the catalyst with low M content always has a larger mesopore volume, which is in accord with the results in the literature [START_REF] Nagy | Nitrogen doped mesoporous carbon aerogels and implications for electrocatalytic oxygen reduction reactions[END_REF] . In this series, the micropore volume also decreases while increasing Fe content due to the formation of Fe-rich nanoparticles. The TEM images of catalysts prepared with different quantities of FePc at R/M=2/1 are shown in Figure 3.19a-c, respectively. No iron nanoparticles are detectable (>1-2 nm) for all catalysts, which means the Fe atoms can be well dispersed in the carbon matrix even if the Fe content is increased to 1.5 wt%. For the catalyst with 2wt% Fe theoretical, the presence of some Fe nanoparticles not detectable by TEM because they are very small. Meanwhile, no significant diffraction peak can be identified from their XRD patterns (Figure 3.19d) also demonstrate no crystalline detectable iron phases in the catalysts. The atomically dispersed Fe suggests their Fe contents directly relate to their activity.

The situation is different with the catalyst prepared at R/M=2/0, as shown in Figure 3.20a-c. Nanoparticles can be observed by TEM in all catalysts, as seen in the white frame of figures. As the quantity of FePc increases, more and more nanoparticles can be observed. Meanwhile, from the XRD patterns of the catalysts, as seen in Figure 3.20d, a small peak at around 2θ=44º gradually appears with an increasing amount of FePc, which can be indexed to the (111) diffraction of γ-Fe. The same acid washing treatment was conducted on all catalysts, but the dissolution of the larger nanoparticles was not fully achieved. Therefore, these Fe nanoparticles are encapsulated in the carbon sheets, protecting the larger one eliminated from acid washing. In this case, the quantity of atomically dispersed Fe in the catalysts cannot be counted by their total Fe content, suggesting the activity show a weak relationship with ICP-MS results. The variation in micropore volume can be explained by their low quantity of active sites.

The difference between the two series concerning the presence or not of visible Fe nanoparticles can be linked to the presence of nitrogen (brought by melamine) which can attract the iron and avoid its coalescence into large nanoparticles during the heat treatments.

Chemical composition of catalysts

The chemical compositions of catalyst surface were investigated by XPS method. The results of catalysts prepared with different quantities of FePc at R/M=2/1 are listed in Table 3.12. The Fe content on the surface of catalysts increases with the increase of the FePc quantity introduced in the sol from 0.5 wt.% to 1 wt.%, then keeps constant from 1 wt.% to 2 wt.%. In fact, both 0.5 wt.% and 1 wt.% show uneven distribution of Fe on the surface and in bulk. The Fe in the 0.5 and 1 wt.% catalysts is more concentrated on the surface (ex: 0.13 at.% in bulk vs. 0.26 at.% on the surface for 1wt%), whereas the 2 wt.% catalyst is more homogeneous (0.33 at.% in bulk vs. 0.27 at.% on the surface). The former situation can be attributed to the etching of carbon during NH3 treatment, which removes the carbon but preserves the Fe to form active sites. The 2 wt.% catalyst has similar surface Fe content to 1 wt.% catalyst but significantly increased in bulk. The possible reason is a small amount of Fe nanoparticles are sparsely dispersed in the carbon, which are hard to find under TEM. The nanoparticles are encapsulated by carbon sheets that cannot be detected by XPS, whereas ICP-MS can give the whole Fe content regardless of if they are on the surface or in bulk.

The N content decreases with increasing Fe content, as shown in Table 3.12. The deconvolution results of their N 1s spectra show more than 40% loss of pyridinic N and almost 30% loss of pyrrolic N. The Fe-Nx composition decreases the least among three active N species. The impacts on their activity will be investigated. The XPS results of catalysts prepared with different quantities of FePc at R/M=2/0 are listed in Table 3.13. The Fe content of all catalysts is almost identical regardless of their quantity of FePc. This result demonstrates the Fe nanoparticles observed under TEM are encapsulated because they cannot be detected by XPS. Therefore, these nanoparticles are not formed during NH3 treatment but originally after the first pyrolysis. It should be noted that even though the catalysts with the same FePc content possess similar Fe content in ICP-MS results for two R/M series, the series without melamine (R/M=2/0) always show lower Fe content in XPS results due to the formation of Fe nanoparticles. This result emphasizes the importance of melamine during the formation of active sites. Although all Fe atoms are located in the Fe-N4 coordination structure in FePc precursor, the Fe nanoparticles can still form during the carbonization step, which means the pre-existing active site structure cannot be preserved. In this case, melamine becomes the true N source of active sites, which provides abundant active N to coordinate with Fe during pyrolysis. The surface N contents of series R/M=2/0 don't show a significant relationship with Fe content, whereas they are always lower than the series R/M=2/1 due to the lack of melamine. According to the deconvolution of their N 1s spectra, the pyrrolic N component of series R/M=2/0 is only half the content of series R/M=2/1, but the pyridinic N almost keeps constant. The different variation of N components implies that melamine is a source of pyrrolic N. In summary, the melamine content seems to be the crucial factor that affects the formation of Fe-N4 active sites. Besides, melamine tends to form pyrrolic N in the final catalyst, which is possible to propose that the Fe atoms are coordinated with pyrrolic N to form the active sites. The impacts of melamine content will be further investigated in chapter 4.

Electrochemical characterization of catalysts

The CV and polarization curves tested in the RDE setup of catalysts at R/M=2/1 are shown in Figure 3.21a and b, respectively. Their current densities at 0.8 V are listed in Table 3.14. As expected, the current density increases with Fe content but exhibits a nonlinear relationship. The MA0.8 is almost doubled when theoretical Fe content is increased from 0.5 wt.% to 1.0 wt.%, but only a 30% increase when it is further increased to 2 wt.%. The outstanding enhancement of activity can be explained by the uneven distribution of Fe in the 1 wt.% catalyst, as discussed before, that the XPS result presents 0.26 at.% (~1.19 wt.%) of Fe on the surface, which is twice the 0.5 wt.% catalyst, corresponding to the increase of MA0.8. When the FePc is further increased to 2 wt.%, the surface Fe content remains constant, implying the formation of less active Fe species. Therefore, the current density doesn't show a linear relationship with Fe content.

Their electrochemical available area obtained from CV curves decreases with increasing Fe content, which is different from the tendency observed with different ligands in section 3.3. However, the variation of CV areas closely relates to the N content here, that the electrochemical available area decreases with the total N content of catalysts. The CV area in N2 saturated electrolyte contains both the redox of active N species (pyridinic and pyrrolic N) as well as Fe-N4 active sites. For the catalysts prepared with different FePc quantities, the redox of N species dominates surface reactions because there are more than ten times more abundant than Fe, and all catalysts show similar morphology. The catalysts prepared with different ligands cannot show the same tendency because their different morphologies lead to different activities of surface N groups.

The Tafel slopes derived from polarization curves are shown in Figure 3.21c. The values of all catalysts are in the range of 60-70 mV dec -1 , corresponding to the classic atomically dispersed Fe-N-C catalyst. For the catalysts based on nanoparticles, the Tafel slopes are usually in the range of 90-100 mV dec -1 . 219 Therefore, the less active nanoparticles found in 2 wt.% catalyst have little contribution to the activity. The normalized current densities after 500 cycles in N2 saturated electrolyte are shown in Figure 3.21d. The catalysts with 0.5 wt.% and 1 wt.% FePc exhibited a similar loss ratio after 500 cycles, around 33%, while the 2 wt.% catalyst showed almost double the loss. The possible reason for the huge decline is that the less active nanoparticles can be dissolved into electrolyte during CV test. The CV and polarization curves tested in the RDE setup of catalysts at R/M=2/0 are shown in Figure 3.22a and b, respectively. Their current densities at 0.8 V are also listed in Table 3.14. The CV areas also increase with surface N content obtained from XPS spectra. Their current densities at 0.8 V are almost identical, corresponding to their similar surface Fe content (0.18 at.%, 0.21 at.%, and 0.22 at.%). The Tafel slopes derived from their polarization curves are shown in Figure 3.22c. The values are around 60 mV des -1 , corresponding to the behavior of atomically dispersed active sites. Despite their Fe content in bulk ranging from 0.56 wt.% to 2.33 wt.%, the nanoparticles encapsulated in the carbon sheets are non-active to ORR. All the catalysts prepared at R/M=2/0 are less active than the series R/M=2/1, in agreement with the tendency of their surface Fe content. As we have mentioned in the section of chemical composition, melamine plays a crucial role during the formation of active sites, which cannot be replaced by pre-existing Fe-N4 structure in the precursor. The normalized current densities after 500 cycles in N2 saturated electrolyte are shown in Figure 3.22d. All the catalysts show a similar loss ratio (around 50%), which is greater than the catalyst without metallic Fe (R/M=2/1, 0.5 wt.%, and 1 wt.%). The same explanation of R/M=2/1, 2 wt.% catalysts can be employed, that the Fe particles are dissolved into electrolyte during CV test, which leads to the loss of active center.

Conclusion

The catalysts with different quantities of FePc precursor were prepared to investigate the impacts on activity. Two series with different R/M ratios are prepared to investigate the effect of pre-existing coordination structure. For the series prepared at R/M=2/1, the activity increases with Fe content but shows a non-linear relationship due to the formation of Fe nanoparticles at high Fe content. The Fe nanoparticles can be found in all the catalysts prepared at R/M=2/0. Their activities are independent of their Fe contents and are almost identical. In fact, the activity of all the catalysts closely relates to their Fe content on the surface instead of in bulk. In the series R/M=2/0, no matter how much the quantity of FePc increases, the limit of surface Fe content is around 0.22 at.%, and for the series R/M=2/1, the limit seems to be increased to 0.27 at.%. Otherwise, unexpected nanoparticles appear in the catalyst.

In conclusion, increasing the quantity of Fe precursor increases the activity, but too much is not beneficial. Simply increasing Fe content only leads to nanoparticles instead of atomic iron in the final catalyst. On the other hand, the melamine content seems very important because the limit of atomic surface Fe content can be increased by melamine, which is directly related to the activity. Besides, the pre-existing coordination structure presents little impact on forming active centers. Even though all Fe atoms are contained in the Fe-N4 macrocycles in the FePc precursor, the microstructure will be decomposed during the carbonization step. Therefore, the undissolved FePc powders are more likely to form nanoparticles because they are easier to agglomerate than the soluble Fe salts. In the next chapter, the impact of melamine content will be further investigated, promising to improve the activity of catalysts.

Chapter 4 Impact of Melamine Content on the Performance of Fe-N-C Catalysts towards Oxygen Reduction Reaction Activity

Ré sumé Franç ais

Dans ce chapitre, les impacts de la teneur en mé lamine sur l'activité ORR et les proprié té s de transport de masse des catalyseurs à base d'aé rogel Fe-N-C ont é té systé matiquement é tudié s.

Dans la section 4.1, une sé rie de catalyseurs Fe-N-C ont é té synthé tisé s en modulant la teneur en mé lamine (M), afin de dé finir les limites des paramè tres de gé lification et d'explorer les relations entre les paramè tres de gé lification, la texture des maté riaux, et l'activité ORR. Avec FeCl3 comme pré curseur de fer, avec l'augmentation de la teneur en M, le volume des mé sopores varie non liné airement avec un maximum à R/M=2/1. Il diminue ensuite rapidement à haute teneur en M en raison de la ré organisation du squelette carboné avec un dé part massif d'azote au cours de la pyrolyse. La quantité absolue de Fe augmente avec la teneur en M, ainsi que la teneur en N. Les spectres XAS et Mössbauer du [START_REF] Flyagina | Identifying the Catalytic Active Sites in Heteroatom-Doped Graphene for the Oxygen Reduction Reaction[END_REF] Fe montrent la pré sence de rares nanoparticules riches en Fe et que le site D1 est la configuration majoritaire des sites actifs Fe-N4. D'aprè s l'analyse des spectres XPS, la teneur en N pyrrolique est positivement corré lé e avec l'augmentation de la teneur en M, ce qui confirme que la plupart des atomes de Fe actifs sont coordonné s par l'azote pyrrolique, en accord avec les spectres Mössbauer. L'azote atomiquement dispersé dans la mé lamine peut stabiliser le Fe pendant la pyrolyse, é vitant ou minimisant la formation de nanoparticules de fer. Les tests é lectrochimiques en RDE montrent une relation liné aire entre l'activité massique et la teneur en Fe jusqu'à 0.3 at.% (FeCl3-R/M=2/2), avec l'activité massique (MA0.8) la plus é levé e de 3.0 A g -1 à 0.8 V. Cette tendance est conforme à la caracté risation XPS indiquant que la plupart du Fe est pré sent sous forme de sites Fe-Nx dispersé s atomiquement. Une augmentation supplé mentaire de la teneur en mé lamine a ré duit l'activité massique car le faible volume de mé sopores limite les proprié té s de transport de matiè re. Leurs performances ORR en PEMFC pré sentent une tendance similaire qu'en configuration RDE, à savoir que le FeCl3-R/M = 2/2 pré sente les meilleures performances. Toutes les courbes de polarisation sous H2-air en PEMFC ne s'effondrent pas, ce qui indique d'excellentes proprié té s de transport de masse des catalyseurs à base d'aé rogel.

Dans la section 4.2, les catalyseurs Fe-N-C sont synthé tisé s avec diffé rents pré curseurs de Fe (FePc et Fe(NO3)3) et une teneur en M é levé e pour confirmer si la tendance pré cé dente convient à d'autres pré curseurs de Fe. A haute teneur en M, pour le pré curseur Fe(NO3)3, l'activité ORR du catalyseur est significativement augmenté e, alors que pour le pré curseur FePc, elle n'est que lé gè rement augmenté e car ce pré curseur, insoluble ne peut pas ê tre bien dispersé dans l'aé rogel. En combinant le meilleur pré curseur (Fe(NO3)3) et une haute teneur en M (R/M=2/2), le meilleur catalyseur a é té obtenu avec une MA0.8=3.58 A g -1 en configuration RDE qui est proche de la poudre de Pajarito commerciale (PMF D1240). En PEMFC, sous H2/O2, Fe(NO3)3-R/M=2/2 montre é galement la densité de courant la plus é levé e, de 113 mA cm -2 à 0,8 V, et de 44 mA cm -2 à 0.85 V. Cette valeur n'est que de 0.05 V infé rieur à l'objectif DOE 2025 dé fini pour des catalyseurs sans mé tal du groupe du platine. En outre, la courbe de polarisation ne s'effondre pas à haute densité de courant, indiquant ses bonnes proprié té s de transport de masse. Ces catalyseurs sont donc trè s prometteurs.

The discussion in Chapter 3 reveals a positive relationship between atomically dispersed Fe content and ORR activity of Fe-N-C catalyst. The mass activity of Fe-N-C catalysts relates to the number of accessible Fe-N4 sites per mass of catalyst (site density, SD) and the average ORR turnover frequency (TOF) of all accessible sites [START_REF] Jiao | Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites[END_REF] . In the absence of pathways to increase the TOF, increasing the absolute amount of atomically dispersed Fe is an efficient way to increase the SD, thus the mass activity of the Fe-N-C catalysts [START_REF] Jiao | Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites[END_REF] . However, simply increasing the quantity of Fe precursor in the preparation of aerogel precursor only increases the yield of less active Fe-rich nanoparticles instead of atomically dispersed Fe, as discussed in section 3.4. On the contrary, the melamine content affects significantly the ORR activity of Fe-N-C aerogel catalysts. The presence of well-dispersed N and Fe sources in the aerogel precursor can promote the formation of Fe-N4 active sites and reduce the yield of Fe-rich nanoparticles during pyrolysis 220,221 . As seen in section 3.4, all catalysts prepared with R/M=2/1 show a clear view of carbon aerogel substrate under TEM, whereas the nanoparticles can be observed for those prepared without melamine (R/M=2/0). However, excessive M content can decrease the porosity of the aerogel and even impede the gelation process, which may lead to the problem of mass transport [START_REF] Nagy | Nitrogen doped mesoporous carbon aerogels and implications for electrocatalytic oxygen reduction reactions[END_REF]222 .

In this chapter, a series of Fe-N-C catalysts were synthesized by modulating the M content, with FeCl3 as benchmark Fe precursor, to define the limits of the gelation parameters and to identify the relationships between the gelation parameters, materials' structure, porosity, activity, and mass-transport properties. Subsequently, the impacts of melamine content (M) on the other Fe precursors, including FePc and Fe(NO3)3, were investigated as well.

Impact of melamine content with FeCl3 precursor

Synthesis of catalysts

The Fe-N-C catalysts were prepared according to the protocol described in Chapter 2. The M content was modulated according to the molar ratio of R/M=2/x at the aerogel preparation stage and the molar ratio of reactants is 2:(4+3x):x:(0.017+0.016x):60 for R:F:M:NaC:W. The final catalysts were named by FeCl3-R/M=2/x, with the M content x ranging from 0 to 2.5. The molar ratio (R+M):Fe was kept constant with a value of 1:0.01, targeting ca. 1 wt. % Fe in the final catalysts by assuming no iron was removed during the acid leaching step and around 70% mass loss of (R+M+F) after pyrolysis. The specific quantities of products were listed in Table 4.1. The composition of Fe-doped organic aerogels before pyrolysis was characterized by Fourier transform infrared (FTIR) spectroscopy (Figure 4.1) to follow the variation of their composition. The assignments of absorption bands are listed in Table 4.2. The spectra of RF and MF aerogel without Fe were also plotted in the same figure as the reference. For the catalyst FeCl3-R/M=2/0 (no melamine), the FTIR spectrum is similar to that of RF aerogel. The absorption bands at 1479 cm -1 belong to the CH2 scissor vibrations, which derive from the condensation of formaldehyde. 223 The absorption band at 1614 cm -1 corresponds to stretching vibrations of benzene ring in resorcinol. 223,224 The two absorption bands at 1222 and 1092 cm - 1 are associated with C-O-C asymmetric and symmetric stretching vibrations of methylene ether bridges, respectively, which have been previously identified in the FTIR spectra of RF aerogels. 223 This bond demonstrates the existence of benzylether structures, which are typically derived from the polycondensation between methylol and resorcinol in basic conditions. 200,201 With increasing melamine content, the absorptions derived from MF aerogel become more intense. Two sharp absorption bands at 1544 and 810 cm -1 can be unambiguously assigned to vibration modes specific to the melamine triazine ring. 225 The relatively broad absorption band at 1350 cm -1 can be assigned to the stretching vibration of C-N bond between aromatic carbon and amine function. 225 An additional absorption band is visible at around 1000 cm -1 , which cannot be assigned to RF or MF aerogel. It can instead be assigned to a N-CH2-O stretching mode, indicative of crosslinking between resorcinol and melamine with methylene derived from formaldehyde as the linker. 226 The peak intensity increases with melamine content, indicating the proper integration of nitrogen atoms in the RMF aerogel backbone.

Texture and structure of catalysts

Further characterizations were then conducted on the final catalysts after the two thermal treatments (N2 and N2/NH3). Their total mass loss after two pyrolysis steps almost keeps constant with increasing melamine content (around 80% in Table 4.3), whereas their morphologies are dramatically changed. As seen by SEM in TEM shows a similar evolution. For the catalysts prepared with low M content, R/M ranging from 2/0 to 2/1, their TEM images (Figure 4.3) show a highly porous texture, which is similar to those reported for RMF aerogel without Fe. [START_REF] Nagy | Nitrogen doped mesoporous carbon aerogels and implications for electrocatalytic oxygen reduction reactions[END_REF]228 When R/M reaches 2/1.5, some pore-less platelets can be observed and finally become the dominant morphology at R/M=2/2.5. This result implies the decrease in the porosity of carbon aerogel as M content increases. 4.3. The catalysts with low melamine content (FeCl3-R/M=2/0-2/1) present relatively higher BET surface and mesopore volume than those prepared with high melamine content (FeCl3-R/M=2/1.5-2/2.5). This trend is consistent with the observation by SEM. It should be noted that the mesopore volume does not vary linearly with melamine content. It slightly increases at R/M=2/0 to 2/1, from 0.83 to 1.67 cm 3 g -1 , then sharply decreases to only 0.02 cm 3 g -1 at R/M=2/2.5 (Table 4.3). The explanation of this tendency should be found in the microstructure of the aerogel. The skeleton of RMF hydrogel was constructed by linking the RF and MF units in the precursor solution, and a part of the interspace between these units became the mesopores after drying and pyrolysis. 200,201 Different from thermally stable benzene rings in R, the triazine rings in M were decomposed during pyrolysis. At low M content, the venting of M decomposition products helps create mesopores without destroying the skeleton of the aerogel. However, when this decomposition becomes dominant in RMF aerogel, the skeleton collapses, thus presenting a more compact morphology and lower porosity. [228][229][230] In contrast to the mesopore volume, the micropore volume is almost constant with the R/M ratio at 0.42-0.45 cm 3 g -1 . The micropores are mainly formed during pyrolysis and mainly by the reaction between NH3 and carbon matrix, indicating a similar etching degree of carbon aerogel substrate. The minimum crystallite size of catalysts can be obtained from XRD and Raman spectroscopy, as shown in Figure 4.5a and b, respectively. The XRD patterns and Raman spectra show that the carbon matrix is poorly organized for all R/M values, as indicated by their large peak width at 2θ=25.6º in XRD patterns, a peak assigned to the (002) diffraction of graphite, and as indicated by a high ratio of the D1 to G band intensities (ID1/IG around 4.7) from Raman spectra. 231,232 The average values of graphite crystallite thickness (Lc) and the in-plane graphite crystallite size (La) were derived from the full width at half-maximum (fwhm) of the peak at 2θ=25.6º in XRD patterns (corresponding to d002=0.36 nm) and ID/IG of Raman spectra, respectively (Figure 4. 5c andd). The results listed in Table 4. 4 show that both La and Lc values are very small. The changes in La and Lc are however moderate, with the Lc values ranging from 1.7 to 1.1 nm and La ranging from 1.11 to 0.89 nm. The global trend is a slight decrease of both La and Lc when R/M increase. This indicates a moderately increasing average structural disorder of the graphite matrix in the final catalysts with increasing melamine content at the aerogel preparation stage. Since the carbon atoms situated on the edge of graphite particles are preferentially etched by ammonia compared to carbon atoms located in the plane of the graphite crystallites, 184,194 the etching process trend to start at the margin between two minimum crystallite particles and the melamine content does not affects the phenomena. The catalysts prepared with different melamine content present a similar etching degree by NH3 and micropore volume. The Fe content of catalysts was determined by ICP-MS, as listed in Table 4.3. A similar tendency with FePc precursor in section 3.4 is observed that Fe content increases with M content, even though the initial quantity of FeCl3 is targeting to around 1 wt.% Fe content in the final catalysts. Two main hypotheses can explain this. In the first one, adding melamine helps coordinate and disperse iron in the aerogel before pyrolysis, decreasing the amount of metallic iron or iron oxide particles formed during or after pyrolysis, and therefore minimizing the amount of iron leached during the acid washing step. In the second hypothesis, the increasing amount of melamine increases mass loss of C and N as volatile products during pyrolysis, indirectly increasing the iron content in final catalysts. However, the very low Fe amount measured with R/M = 2/0 supports that our first hypothesis is the main factor accounting for the Fe content in the final catalysts. Moreover, no Fe-rich particles can be observed from TEM images or identified from XRD patterns, suggesting that Fe exists as atomically dispersed Fe-N4 sites. However, nanometric particles of iron or iron oxides are difficult to track with TEM or XRD. Therefore, and Fe coordination was characterized by dedicated spectroscopies, as discussed in the next section.

In summary, all the catalysts in this series present a high specific surface area (>1000 m² g -1 ) and high micropore volume, which is usually considered necessary to reach a high ORR activity with Fe-N-C catalysts. However, their very different iron contents are possible to dominate their activities instead. The relationship between micropore volume, Fe content and activity will be clarified later.

Chemical composition of catalysts and investigation of active site structure

The chemical composition of catalysts was investigated by XPS (Table 4.5). The Fe content measured by XPS is similar to that measured with ICP-MS, indicating a uniform distribution of Fe in bulk and on the surface of catalysts. The N content also shows an increasing trend with increasing M, although the relative increase is less than that of Fe content. These results indicate only a portion of nitrogen atoms in catalysts participate in the forming of Fe-N4 active sites.

The N 1s narrow scan spectra identify a dramatic change in the type of N-groups present, the catalysts prepared with R/M = 2/0 showing a very weak signal at ca. 401 eV while the relative intensity of this signal increases with increasing melamine content (Figure 4.6a). The spectra were then analyzed by the deconvolution method used in Chapter 3. As the results in Table 4.5, the increasing peak at 401 eV corresponds to the component of pyrrolic N. Insights into the type of Fe species present were obtained from Fe K-edge XAS and 57 Fe Mössbauer spectroscopies (from Synchrotron Soleil facilities and ICGM, respectively). Figure 4.7a shows selected XANES spectra for three catalysts in the series and the XANES spectrum of a reference Fe-N-C catalyst derived from ZIF-8 (labeled Fe0.5 from ICGM laboratory) for which Fe is known to be present as Fe-Nx active sites only [START_REF] Zitolo | Identification of catalytic sites for oxygen reduction in iron-and nitrogendoped graphene materials[END_REF] . The spectra show high similarities, suggesting Fe is mainly present as Fe-Nx coordination structures [START_REF] Zitolo | Identification of catalytic sites for oxygen reduction in iron-and nitrogendoped graphene materials[END_REF]233 . However, the presence of nanometric iron oxides cannot be excluded due to similar XANES spectra with Fe-N4 sites. 105 The Fourier transform of the EXAFS signals of down-selected catalysts (Figure 4.7b) are also similar to that of Fe0.5, with a main signal at ca. 1.5 Å, assigned to Fe-N or Fe-O bonds. [START_REF] Zitolo | Identification of catalytic sites for oxygen reduction in iron-and nitrogendoped graphene materials[END_REF]187,233,234 As for XANES, the presence of nanometric iron oxide cannot be excluded since the signal from Fe-N bonds in Fe-Nx sites and Fe-O from oxides cannot be distinguished. The signal from Fe-Fe backscattering is not always discernible in nanometric oxides due to the size and/or amorphous character of nano-oxides. 105 The iron coordination was further investigated with [START_REF] Flyagina | Identifying the Catalytic Active Sites in Heteroatom-Doped Graphene for the Oxygen Reduction Reaction[END_REF] Fe Mössbauer spectra collected at -268 º C. Low-temperature acquisition enables distinguishing in particular nanometric iron oxides, contributing with a sextet component from atomically dispersed Fe-Nx sites contributing with quadrupole doublets. 105 The spectra of selected catalysts (Figure 4.7c-e) could be well fitted with appropriate combinations of doublet and sextet components. The main signal in all catalysts is a quadrupole doublet with quadrupole splitting (QS) values of 0.98-1.13 mm s -1 , and isomer shift (IS) value of 0.46-0.47 mm s -1 , which is assigned to a high-spin O-Fe(III)N4C12 configuration (D1), with O representing an oxygenated species adsorbed on the metal center (Table 4.6). [START_REF] Zitolo | Identification of catalytic sites for oxygen reduction in iron-and nitrogendoped graphene materials[END_REF]105,235 A second quadrupole doublet with a high IS and QS is identified, and labeled as D3. On the basis of its distinct IS values (0.97-1.20 mm s -1 ), this component can be unambiguously assigned to highspin ferrous centers. While the existence of high spin Fe(II)-Nx sites cannot be excluded, the recent observation that the presence of this D3 component in Fe-N-C materials coincides with the use of Fe-chloride salts for their synthesis suggests that D3 is due to the presence of ferrouschloride, with the quadrupole doublet signal of FeCl2• 4H2O having Mössbauer parameters (IS and QS values) matching those of D3 [START_REF] Jiao | Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites[END_REF] . As the present series of catalysts is prepared with FeCl3, we assign D3 in the current work to FeCl2• 4H2O [START_REF] Jiao | Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites[END_REF] . Independent experimental evidence for the presence of FeCl2 after FeCl3 chemical vapor deposition on Zn-N-C also supported the assignment of D3 [START_REF] Jiao | Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites[END_REF] . The relative contribution of D3 to the absorption signal in the present work is however small, 5-8% in FeCl3-R/M=2/0.5 and FeCl3-R/M=2/1 and is not observed in FeCl3-R/M=2/2 or FeCl3-R/M=2/2.5. For the latter two, another quadrupole doublet with IS comparable to D1 but with higher QS (2.85 mm s -1 ) is observed, labeled D2, accounting for 16% of the signal. This doublet is commonly co-observed with the D1 component in Mössbauer spectra of Fe-N-C catalysts and assigned to a low or medium spin Fe(II)N4 configuration 235 . The ferrous state of D2 may be explained by its location in the bulk of the carbon matrix or by a low affinity with O2. The presence of D2 in the catalyst prepared with the highest melamine content coincides with a decreasing BET area (Table 4.3), supporting that the minor D2 contribution may result from the non-accessibility of O2 to some Fe-Nx sites. Mössbauer spectroscopy also identifies an important contribution from a broad sextet in FeCl3-R/M=2/0.5, which is assigned to high-spin ferric oxide 187 . The high linewidth (3.19 mm s -1 , Table 4.6) supports ultra-small clusters that are not fully magnetically ordered at -268 º C. No sextet is detected in the spectra of FeCl3-R/M=2/1 and FeCl3-R/M=2/2, while two well-defined sextets (LW of only 0.27 mm s -1 ) are identified in the spectrum of FeCl3-R/M=2/2.5, assigned to ferric oxides, but accounting for only 8% of the signal (Table 4.6). Therefore, Mössbauer spectroscopy confirms the XANES and EXAFS results but also provides more quantitative insights, especially identifying an important contribution from ultra-small iron oxide clusters for samples prepared at low R/M value. This shows that the addition of melamine is critical to forming Fe-Nx sites during the first pyrolysis. Without melamine or with too little melamine, iron probably mainly forms metallic or metal carbide species during pyrolysis. Upon exposure to air or to acid washing, these particles can remain in zerovalent state if they are perfectly covered by a graphite shell, or they can be converted partially or fully to iron oxides, depending on oxygen/proton accessibility to the surface of these particles and depending on the particle size. The iron oxide signal in FeCl3-R/M=2/0.5 must thus be a consequence of the presence of iron zerovalent particles after the first pyrolysis stage. Fe cations in an acid solution can indeed partially reprecipitate, depending on the acid nature, temperature, and local iron concentration. Together with the ICP-MS data, we can conclude that nitrogen introduced by M can bind the iron atoms and stabilize them to avoid the formation of iron nanoparticles with optimized M content. Last but not least, the signal/noise ratio and absolute signal intensity of the Mössbauer absorption spectra also reflect the Fe content in the sample, with low signal and high noise for the sample with low melamine content (R/M=2/0.5). The major contribution of D1 in this series of catalysts (increasing absolute absorption intensity of D1 with the melamine content) also suggests that the nitrogen species involved in coordinating iron in D1 sites is pyrrolic nitrogen. Pyrrolic nitrogen was indeed shown by XPS to be absent in the sample prepared with R/M =2/0 and to increase with increasing M content continuously. Moreover, the presence of D2 sites at high M content can explain decreased pyrrolic N/Fe value because Fe atoms in D2 sites are coordinated by pyridinic N. After correcting the Fe content by percentage of D1 in Table 4.6, the pyrrolic N/Fe values of FeCl3-R/M=2/2 and FeCl3-R/M=2/2.5 in Table 4.5 are 4.2 and 4.0, respectively, which correspond well to the stoichiometry in Fe-N4 sites.

Electrochemical characterization of catalysts

The CV curves of catalysts prepared with different M content are shown in Figure 4.8a. FeCl3-R/M=2/2.5 has the lowest electrochemical active area, whereas FeCl3-R/M=2/1 has the largest surface area. This agrees with tendency that the larger specific surface area and mesopore volume lead to a larger electrochemical available area (Table 4.3) 233 . However, the variation of their CV areas is moderate, even though their BET areas are reduced by ca. 1/3 and mesopore volume almost disappeared when M content increased from R/M=2/1 to 2/2.5 (from 1.67 to 0.02 cm 3 g -1 ). The possible reason is the catalysts with higher M content also possess higher Fe content, which contributes more electrochemical active surface that compensates for their lower specific surface area.

The polarization curves show increasing half-wave potential with increasing M content up to R/M = 2/2, while at R/M = 2/2.5, the activity seems to achieve a plateau or even to decrease slightly (Figure 4.8b). All polarization curves except those of FeCl3-R/M=2/0 and FeCl3-R/M=2/2.5 present a sharp kinetic-to-diffusion transition region, indicative of adequate masstransport properties, at least in RDE setup 187 . The catalysts exhibit similar Tafel slopes around 60 mV dec -1 (Figure 4.8c), consistent with the literature and demonstrating that the ratedetermining step of the ORR mechanism is the same in this series of catalysts. [START_REF] Xiao | Microporous Framework Induced Synthesis of Single-Atom Dispersed Fe-N-C Acidic ORR Catalyst and Its in Situ Reduced Fe-N 4 Active Site Identification Revealed by X-ray Absorption Spectroscopy[END_REF]210,233,236 This is in line with the dominant signal from the component D1 in the Mössbauer spectra, supporting that the change in ORR activity is mainly due to a change in the number of sites, while the fundamental ORR mechanism on the active sites is unmodified. The mass activity increases by ca 4-fold from R/M = 2/0 to 2/1 (0.47 to 2.04 A g -1 ), then increases slowly from 2/1 to 2/2 (2.04 to 3.00 A g -1 ), and decreases slightly at R/M = 2/2.5 (2.22 A g -1 ). The mass activity for the optimized material, FeCl3-R/M=2/2, is only slightly lower than that of the commercial Fe-N-C catalyst (Pajarito powder PMF D14401, 4.04 A g -1 as seen in Figure 4.8b). In the search for structure-property relationships, Figure 4.8e plots the mass activity of catalysts as a function of their Fe content obtained from ICP-MS. A linear trend can be observed up to the Fe content of 0.3 at.% (1.36 wt.%), corresponding to the catalyst FeCl3-R/M=2/2, with the highest mass activity 3.0 A g -1 for this series. This trend is explained by Mössbauer spectroscopy, showing that most of Fe atoms are present as atomically dispersed Fe-Nx sites, except at low melamine content. Therefore, the site density also increases with the bulk Fe content. Further increasing the M content (and the Fe content) results in a slight decline of the activity due to the restricted mass transfer. The very low mesopore volume in FeCl3-R/M=2/2.5 prevents reactants in electrolyte from accessing the active sites. This can be demonstrated by its higher Tafel slope (79 mV dec -1 ), representing a slow kinetic-to-diffusion transition region 187 . The mass activity also shows a positive correlation with the pyrrolic N content obtained from XPS analysis (Figure 4.8f), which further demonstrates the Fe-N4 active sites are constructed by pyrrolic N. It should be noticed that the activity of our catalysts shows little relationship with micropore volume, which is generally considered an index of the number of active sites [START_REF] Yang | Boosting Oxygen Reduction Catalysis with Fe-N 4 Sites Decorated Porous Carbons toward Fuel Cells[END_REF][START_REF] Xiao | Microporous Framework Induced Synthesis of Single-Atom Dispersed Fe-N-C Acidic ORR Catalyst and Its in Situ Reduced Fe-N 4 Active Site Identification Revealed by X-ray Absorption Spectroscopy[END_REF]105,194 . This lack of correlation is due to the limitation of active Fe at low M content, not by micropores, in the present synthetic approach. Figure 4.9 provides a further description of the formation pathway of Fe-N4 active sites. The organic aerogel was first carbonized to form N-doped carbon layers at around 500 °C, and Fe precursor was transformed to nanoparticles simultaneously. When further increasing the temperature, the Fe atoms are released from Ferich nanoparticles. In the rich melamine aerogel base, the free Fe atoms have more chance to be captured into a neighbor NC defect during the pyrolysis, thus forming more Fe-N4 active sites 213 . On the contrary, in the poor melamine aerogel base, few Fe atoms can be hosted, thus leading to a low density of active sites. Figure 4.8d shows the normalized ORR mass activities after 500 cycles of CV between 0.6 and 1.0 V vs. RHE in N2 saturated 0.05 M H2SO4. The MA values of catalysts prepared with low M content (R/M=2/0-2/1) drop around 50%, whereas activity loss is less than 30% for the catalysts prepared with high M content (R/M=2/1.5-2/2.5). Better durability is accompanied by a sharp decrease of their mesopore volume when M content increases from R/M=2/1 to 2/2.5 associated with a high Fe amount. This coincidence implies a potential relationship between durability and mesopore volume and Fe content of the catalyst. The carbon oxidation reaction occurs at high potential during CV cycles and amorphous carbon domains that are preferentially corroded 231 . The characterizations of XRD and Raman spectra show that average minimum crystallite sizes are almost identical for all catalysts, indicating that the degree of graphitization of the carbon matrix is similar. In this case, the reason is linked to the presence of Fe. The degradation of catalysts may be explained because the catalysts with low M content have Ferich nanoparticles that are unstable during CV cycles in an acidic medium. The existence of Fe oxides has been demonstrated by the Mössbauer spectrum in the sample FeCl3-R/M=2/0.5 (Figure 4.7c), and more Fe oxides can be expected in FeCl3-R/M=2/0 due to its lower M content. As discussed before, these oxides are converted from the Fe nanoparticles perfectly covered by a graphite shell to survive from acid leaching. During the CV cycles, the protective layers of graphite are corroded to expose Fe oxide nanoparticles so that they can be eliminated by acidic electrolyte. As a result, not only the activity from micropore-hosted Fe-N4 sites but the contribution of Fe oxides is eliminated compared with catalysts prepared with high M content (M more than R/M=2/1.5). Therefore, the catalysts containing oxide nanoparticles lose more activity than those with pure Fe-N4 active sites. However, this explanation does not adapt to the sample FeCl3-R/M=2/1, in which no Fe oxide is detected by the Mössbauer spectrum (Figure 4.7d). Because the carbon oxidation rate is faster on the edge of carbon sheets than on the basal plane, the etching of carbon starts from the boundary between crystallite, where the micropores created by NH3 treatment situate as well 184 . Therefore, it can be inferred that the Fe-N4 sites hosted between boundaries are eliminated faster than graphene-defect sites 184 . The possible reason for the relatively low durability of FeCl3-R/M=2/1 is its high mesopore volume helps increase the accessibility of electrolyte to the edge of crystallite, thus leading to more electrochemical oxidation.

To gain further insights into the fundamental reasons for the evolution of the mass activity with the melamine content, the SD and the TOF (Table 4.7) were evaluated using the nitrite adsorption/reduction electrochemical method 231 . The SD value increases with M content from R/M=2/0 to 2/1, reaching the maximum value of 8.04×10 18 sites g -1 , whereas it declines when further increasing M content. These results are not in agreement with the previous conclusion that the increasing mass activity is because the increasing Fe content leads to higher SD values instead of TOF. To explain this discrepancy, note that the decline of SD when M content increased from R/M=2/1 to 2/1.5 coincides with a sharp decrease of mesopore volume, which may limit the accessibility of reductive NO2 -to active sites 231 . Therefore, the SD values at high M content are possibly underestimated. This agrees with the research of Strasser et al. that the SD (CO) values derived from gaseous CO cryo chemisorption were 2-8 folds greater than the SD (NO2 -) values 237 . This is largely attributed to the fact that Fe surface sites located inside micropores are less accessible for reductive NO2 -adsorption, whereas gaseous CO has facile accessibility to those sites. Hence, these sites are not probed by the nitrite stripping technique, and the SD values obtained from nitrite stripping should be regarded as the low bound of the surface Fe site density of the catalyst. The precaution of the SD result of nitrite stripping was also proposed in previous research on ammonia treated Fe-N-C catalyst 231 . Two main issues related to the nitrite stripping method were proposed to explain the discrepancy: (i) the measurement of NO2 -adsorption/desorption needs to be performed at pH=5.2, which is less acidic than the normal conditions of RDE tests; (ii) even if most of the ORR activity is lost during the NO2 -adsorption, a non-negligible activity remains, suggesting that NO2 -could not be adsorbed into all exposed sites because of their very high chemical specificity 231,237 . Thus, this method cannot be applied for all Fe-N-C catalysts, in particular for those treated by NH3based second pyrolysis, as the chemisorption of NO2 -might be impacted by the basicity of Ngroups. This NH3 treatment can enormously increase the basicity of the N-C matrix and consequently reduce the interaction strength between the NO2 -and the Fe-N4 active sites because of the electron-donating properties of the neighboring nitrogen groups. It should be mentioned that even though our SD values are underestimated, they are still comparable with the catalysts measured under the same conditions. In the work of Kumar et al. 231 , their catalyst labeled as Fe-HT2 reaches the SD value of 2.6×10 18 sites g -1 (MA0.8= 4.2 A g -1 ), which is only half of FeCl3-R/M=2/2 with similar mass activity (MA0.8=3.0 A g -1 ).

On the contrary, the SD value measured at low M content is possible to be relatively overestimated because the increase of SD value is much lower than mass activity from R/M = 2/0 to 2/1 (ca.2 folds of SD vs. ca. 4 folds of mass activity). The unexpectedly high SD measured at low R/M can be explained by the lack of selectivity of the nitrite stripping method, which counts not only Fe-Nx sites on the surface but also Fe oxides. This caveat of the nitrite stripping method had been unveiled in the frame of a study on the degradation mechanisms of Fe-N-C in O2 saturated acidic solution 238 . The significant presence of Fe oxides is detected by [START_REF] Flyagina | Identifying the Catalytic Active Sites in Heteroatom-Doped Graphene for the Oxygen Reduction Reaction[END_REF] Fe Mössbauer spectroscopy in FeCl3-R/M=2/0.5 (accounting for 41 % of the signal intensity), and it can be hypothesized that even more Fe oxides would be present in FeCl3-R/M=2/0. Combining the two factors, the variation of SD with M content becomes moderate and seems uncorrelated with mass activity. The TOF values are calculated by using the SD values measured at pH=5.2 and the mass activity measured in 0.05 M H2SO4. The TOF values seem consistent with the mass activity variation, which keeps increasing from R/M=2/0 to 2/2 then declining at R/M=2/2.5. However, this coincidence cannot be the explanation of mass activity because [START_REF] Flyagina | Identifying the Catalytic Active Sites in Heteroatom-Doped Graphene for the Oxygen Reduction Reaction[END_REF] Fe Mössbauer spectroscopy has demonstrated the nature of the main active sites are D1 when M content is more than R/M=2/1. Hence the TOF values should keep constant. The variation of SD values is too moderate to affect the tendency of calculation results of TOF values.

The ORR activity and power performance of these catalysts were then investigated in a 5 cm 2 single cell PEMFC. Figure 4.10a shows the H2/O2 polarization curves of all catalysts, with trends in activity at 0.8 V and overall cell performance in line with the trends observed in RDE. For a quantitative comparison of ORR activity trend with the melamine content, Figure 4.10b reports the current density at 0.8 V in PEMFC (a voltage at which the current density is mostly controlled by ORR kinetics) as a function of the melamine content. It can be seen that the trend and proportional change of activity with M is similar to that observed in RDE (Figure 4.8b), increasing by a factor 4 from R/M = 2/0 to 2/2 and thereafter decreasing by about 35% from R/M = 2/2 to 2/2.5. The similar activity trends observed in RDE and PEMFC support that the I/C ratio of 2 for the PEMFC cathode is appropriate for all catalysts of the present series. Otherwise, lower-than-expected activity would have been observed for some materials in PEMFC. For assessing the oxygen-transport properties of the different Fe-N-C cathodes (different R/M values for aerogel preparation), we compare the current densities measured at 0.3 V (voltage at which mass-transport plays a key role on the current density achieved) with pure O2 or with air (Figure 4.10d). The figure shows that the magnitude of the decrease in current density when switching from O2 to air is comparable for all catalysts, despite huge differences in the mesopore volume of this series of catalysts. Therefore, mesopores are not a necessary criterion to reach high PEMFC performance of Fe-N-C catalysts. For the current synthesis approach, the addition of melamine is therefore critical to increase the Fe content, SD, and ORR activity. In contrast, the impact is less significant on the mass-transport properties of the cathode active layers.

Conclusion

In conclusion, the impacts of melamine content on the ORR activity and mass transport properties of Fe-N-C aerogel catalysts were systematically investigated. The mesopore volume of aerogel catalysts slightly increased at low M content (from R/M=2/0 to 2/1) due to the decomposition of M during pyrolysis, then sharply decreased at higher M content because too many bond ruptures collapse the skeleton of aerogel. In contrast, the average graphite crystallite size and the micropore volume were almost unaffected by the change in M content. The absolute amount of Fe increased with M content, and the absolute amount of Fe-N4 sites also increased with M content. From the analysis of XPS spectra, the pyrrolic N content is positively correlated with the increasing M content, supporting that most active Fe atoms are ligated by pyrrolic nitrogen. This is in line with Mössbauer spectra, identifying that most of the Fe sites are D1 sites, assigned to porphyrinic type FeN4C12 structures. The atomically dispersed nitrogen in melamine can stabilize the Fe during pyrolysis, avoiding or minimizing the formation of iron nanoparticles. The electrochemical tests in RDE show a linear relationship between mass activity and Fe content up to 0.3 at.% (FeCl3-R/M=2/2), with the highest mass activity of 3.0 A g -1 at 0.8 V. This trend is in line with the physico-chemical characterization that most of Fe atoms exist as atomically dispersed Fe-Nx sites. Further increase of melamine content reduces the mass activity due to low accessibility to the active sites. Their ORR performance in PEMFC exhibits a similar trend with RDE setup using adapted ink formula. All polarization curves in H2-air PEMFC do not collapse, indicating excellent mass transport properties of aerogel-based catalysts.

Impact of melamine content with different Fe precursors

In this section, the Fe-N-C catalysts were synthesized with modulated M content and different Fe precursors (FePc and Fe(NO3)3) in order to compare the results obtained with FeCl3 and to see if the previous conclusion can be generalized to another salt or iron precursor with preexisting active sites (section 3.1). Therefore, it is expected to obtain the best catalyst with adapted M content and Fe(NO3)3 precursor. The two catalysts were prepared following the same gel composition with FeCl3-R/M=2/2, and the quantity of Fe precursor was decided by molar ratio (R+M):Fe=1:0.01 to gain ca.1 wt.% Fe in the final catalysts as well. As for the FePc precursor, the pH value of polycondensation process was controlled at around 6 to preserve as many mesopores as possible, according to the discussion in section 3.2. The pH value of polycondensation process with Fe(NO3)3 precursor was adjusted to 8 to dissolve the precipitation generated after introducing iron salt as for FeCl3. The two catalysts are noted by FePc-R/M=2/2 and Fe(NO3)3-R/M=2/2, respectively. After two pyrolysis steps, their total mass loss is almost constant for different Fe precursors (around 80% in Table 4.8), which is consistent with other catalysts. The Fe contents determined by ICP-MS are similar to FeCl3-R/M=2/2, which are higher than the value of catalyst prepared with the same iron precursor at low M content (R/M=2/1). These results further confirm the stabilization effect of M on the atomically dispersed Fe in the carbon aerogel. Meanwhile, their high Fe content also implies a higher activity.

Texture and structure of catalysts

The morphologies of two catalysts are dramatically changed under SEM. As seen in Figure 4.11a and b, the catalysts prepared with FePc precursor display a porous structure. In contrast, the particle surface of Fe(NO3)3-R/M=2/2 is more compact, and no significant mesopore can be observed. The N2 adsorption isotherm curves at -196 °C and corresponding pore size distribution plots are shown in Figure 4.11c and d, respectively, and the corresponding pore volumes are listed in Table 4.8. In according with the observation under SEM, even though the R/M value was constant, the different Fe precursor and pH conditions still affect the porosity of the final catalyst. Compared with the catalysts prepared with iron salts (FeCl3 and Fe(NO3)3), FePc-R/M=2/2 has the highest mesopore volume (0.73 cm 3 g -1 ), which is far over the others. The mesopore volume in FeCl3-R/M=2/2 (0.25 cm 3 g -1 ) declines to one-third of FePc-R/M=2/2 and almost disappears in Fe(NO3)3-R/M=2/2 (0.04 cm 3 g -1 ). The negative effect on mesopore volume of increasing M content is confirmed for all catalysts, whereas the degree of decline is different. The possible reason for this discrepancy is their different pH values during polycondensation. According to the discussion in section 3.2, the mesopore volume of the catalyst prepared at pH=6 is much higher than at pH=8 with FePc as the precursor. NaOH solution is not only a reagent of pH adjustment for the solubilization of the precipitates but also a catalyst for polycondensation of RMF hydrogel. Using a high concentration of catalyst causes the formation of many small network clusters during gelation step. The pores between clusters are also small as a result 200 . Therefore, the catalyst FePc-R/M=2/2 prepared at pH=6 gives a much higher mesopore volume than the catalyst prepared at pH=8. Moreover, this explanation is also appropriate for the difference between FeCl3-R/M=2/2 and Fe(NO3)3-R/M=2/2. Since the target of pH adjustment is to dissolve the precipitation after introducing the iron salt precursors, the quantity of NaOH is decided by the point that the precipitation disappears and the transparent solution is obtained. The catalyst Fe(NO3)3-R/M=2/2 consumed more NaOH to dissolve the precipitation thus the final pH value is slightly higher than FeCl3-R/M=2/2. On the contrary, even though the mesopore volume of catalyst Fe(NO3)3-R/M=2/2 is small, it possesses a similar BET surface area with FePc-R/M=2/2 due to higher micropore volume. This result indicates the mesopore volume does not affect the accessibility of NH3 to the carbon aerogel. The TEM images of two catalysts (Figure 4.12) show a similar trend with SEM on mesopore volume with N2 sorption results. Figure 4.12a presents a high porous carbon layers for FePc (as FeCl3), whereas the carbon in Figure 4.12b is more compact for Fe(NO3)3). No nanoparticles can be observed in both figures, indicating most of Fe is atomically dispersed in carbon aerogel. Figure 4.12c shows the XRD patterns of two catalysts. Similar to our previous discussion, no significant peak belonging to iron oxide or metallic iron can be indexed. Two bumps at around 2θ=26° and 45° correspond to the diffraction of low crystalized carbon. Three weak peaks at 2θ=26.4°, 34.3°, and 54.6° that are not able to index can be found in Fe(NO3)3-R/M=2/2, which may relate to the by-products of iron salt.

Chemical composition of catalysts

The chemical composition of catalysts was investigated by XPS (Table 4.9). The surface Fe content of FePc-R/M=2/2 measured by XPS is close to that measured with ICP-MS, indicating a uniform distribution of Fe in bulk and on the surface of catalysts, whereas the bulk Fe contents of FePc-R/M=2/0 and FePc-R/M=2/1 are much lower than the surface Fe content. In section 3.4, higher Fe content on the surface of later catalysts is attributed to more Fe atoms being exposed by NH3 treatment. This phenomenon can be explained by two possible hypotheses: (i) with the etching of carbon during NH3 treatment, the Fe atoms detached from edge of carbon layers are left and reform Fe-N4 centers on the surface to realize the enrichment of Fe; (ii) some encapsulated Fe-rich nanoparticles are exposed after the above carbon layers are removed by NH3. The first hypothesis can be verified by the variation of Fe content before and after pyrolysis, which will be further discussed in Chapter 5. The similar Fe contents of FePc-R/M=2/2 obtained from ICP-MS and XPS maybe support the second hypothesis. As discussed in section 4.1, the addition of melamine is crucial for the formation of atomically dispersed Fe. Therefore, high melamine content in FePc-R/M=2/2 can reduce the yield of Fe-rich nanoparticles, thus promoting the uniform distribution of Fe, whereas FePc-R/M=2/0 and FePc-R/M=2/1 with relatively low M contents are more likely to exist Fe-rich nanoparticles, which leads to uneven distribution of Fe. However, this should be further discussed with the results of electrochemical tests. The XPS value of Fe(NO3)3-R/M=2/2 is slightly higher than the result of ICP-MS, indicating a higher concentration of Fe on the surface (as for FeCl3). It should be noted that the FePc precursor has a stronger trend to form Fe-rich nanoparticles than the other Fe salt precursors. The possible reason is that insoluble FePc powder is less dispersed than soluble Fe salts, thus tending to agglomerate during pyrolysis. This result implies that the preexisting active site structure in Fe precursor cannot be conserved during the first carbonization step. by The research of Li et al. can support this guess, that no Fe-N4 sites can be found if the pyrolysis temperature is lower than 600 º C, much higher than the decomposition temperature of FePc 213 . The coordination structure in the FePc decomposed at high temperature and released free Fe atoms, which may form Fe-N4 active sites if they were captured by the N-C defect nearby or sintered and aggregated into Fe clusters in the absence of nitrogen defects in the carbon aerogel. The N content also shows an increasing trend with increasing M content, especially the pyrrolic N. The molar ratios of pyrrolic N/Fe are close to 4 for FePc catalysts, indicating a high proportion of D1 site, whereas the Fe(NO3)3 catalyst has a similar ratio to the FeCl3 catalyst, implying the formation of D2 site. These results suggest that the mesopores are still important in exposing more D1 sites on the catalyst surface, although they exhibit little effect on mass transport properties.

Electrochemical characterization of catalysts

The CV curves of catalysts prepared with different Fe precursors at R/M=2/2 are shown in Figure 4.13a, along with two samples prepared at low M content as the comparison. It is interesting to see that among three catalysts prepared with FePc precursor and different R/M values, the FePc-R/M=2/1 has the largest electrochemical available area, corresponding well to its largest BET area and micropore volume (refer to Table 3.11). Although the FePc-R/M=2/0 has the largest mesopore volume, it contributes less to both BET and electrochemical available areas than micropore. The BET area of Fe(NO3)3-R/M=2/2 is also slightly higher than that of FePc-R/M=2/2, thus giving a slightly larger CV area (Table 4.9). The polarization curves of catalysts prepared with FePc precursor in Figure 4.13b show an increasing tendency of half-wave potential with increasing M content up to R/M = 2/2. They all present a sharp kinetic-to-diffusion transition region, indicating their adequate masstransport properties in RDE setup 187 . The catalysts exhibit similar Tafel slopes around 60 mV dec -1 (Figure 4.13c), which means their ORR mechanism is similar to FeCl3-R/M=2/2 [START_REF] Xiao | Microporous Framework Induced Synthesis of Single-Atom Dispersed Fe-N-C Acidic ORR Catalyst and Its in Situ Reduced Fe-N 4 Active Site Identification Revealed by X-ray Absorption Spectroscopy[END_REF]210,233,236 . This aligns with the increasing pyridinic N content in XPS, suggesting the primary active sites are D1 configuration as well. The mass activities of the catalysts at 0.8 V vs. RHE are derived from their polarization curves, as listed in Table 4.10. Different from the series with FeCl3 precursor, the mass activity of the FePc series shows a linear relationship with Fe content determined by XPS instead of the Fe content determined by ICP-MS, as seen in Figure 4.13e. The mass activity sharply increases by ca. 2.5-fold from R/M = 2/0 to 2/1 (0.67 to 1.67 A g -1 vs.0.61 refer in Table 3.14), then only slightly increases from 2/1 to 2/2 (1.67 to 2.12 A g -1 ). This discrepancy is due to the uneven distribution of Fe in bulk and on the surface for the catalysts prepared with FePc precursor. According to the discussion before, the catalytic activity directly relates to the accessible SD and TOF of the active site, which relates to the number of active Fe atoms exposed on the surface and the property of active sites. Therefore, the mass activity should be more closely associated with surface Fe content, which is obtained from XPS instead of the ICP-MS method. In section 4.1, the ICP-MS results are used to plot the relationship between mass activity and Fe content because the uncertainty of the ICP-MS method is lower than that of the XPS method at the detection limit of XPS. However, a linear relationship can still be found due to the uniform distribution of Fe in the catalysts prepared with FeCl3 precursor. In contrast, the catalysts prepared with FePc precursor have more Fe atoms on the surface. Therefore, the mass activity only presents a linear relationship with the XPS value. According to the results in section 4.1, increasing Fe content directly relates to increasing SD instead of TOF. Therefore, the surface Fe atoms should possess similar properties. It should be clarified that this inference does not mean the second hypothesis proposed in section 4.2.2 should be denied because the iron oxide nanoparticles exposed to NH3 can be further transformed to Fe-N4 active sites 194 . In Figure 4.13f, a linear relationship between mass activity and pyrrolic N content can also be found, which demonstrates that Fe-N4 active sites are constructed by pyrrolic N. The mass activity for the optimized material, Fe(NO3)3-R/M=2/2, reaches 3.58 A g -1 , the highest among all catalysts. The polarization curve of Fe(NO3)3-R/M=2/2 also shows a sharp kinetic-to-diffusion transition and a flatter plateau at high overpotential, indicating the catalyst has good mass transport properties even though the mesopores are quasi-absent.

Figure 4.13d shows the normalized ORR mass activities after 500 cycles of CV between 0.6 and 1.0 V vs. RHE in N2 saturated 0.05 M H2SO4. The MA values dropped by 43% and 23% after the durability test for FePc-R/M=2/2 and Fe(NO3)3-R/M=2/2, respectively. This discrepancy may be due to the higher mesopore volume of FePc-R/M=2/2, which increases the proportion of D1 site. Since D1 sites exposed on the surface of catalysts are considered less stable than D2 covered by few layers of carbon, Fe(NO3)3-R/M=2/2 with a small portion of D2 sites preserved more activity 239 . ). All catalysts show good mass-transport properties which is verified by their sharp kinetic-to-diffusion regions. Among all, Fe(NO3)3-R/M=2/2 presents the highest half-wave potential and mass activity at 0.8 V, consistent with the result in section 3.1 that Fe(NO3)3 precursor shows the best activity among the catalysts prepared with different precursors at R/M=2/1. FePc-R/M=2/2 has the highest limited current density at the diffusioncontrolled region, corresponding to its highest mesopore volume. Figure 4.14b shows the polarization curves of these catalysts in H2/O2 PEMFC, with the ink formulation keeping the I/C ratio of 2. The trend of mass activity is in line with the trends observed in RDE that the Fe(NO3)3-R/M=2/2 shows the highest current density of 113 mA cm -2 at 0.8 V and the current density of 44 mA cm -2 is reached at 0.85 V, which is only 0.05 V lower than the DOE 2025 target for platinum group metal-free electrocatalysts 156 . Meanwhile, no polarization curve is collapsed even for the current density reaches ca. 1700 mA cm -2 at 0.3 V, indicating the excellent mass transport properties of Fe(NO3)3-R/M=2/2. Similar to the observation in the series of FeCl3 precursor, the quasi-absence of mesopores in Fe(NO3)3-R/M=2/2 (0.02 cm 3 g -1 ) does not affect the accessibility of gaseous oxygen and evacuation of water, indicating the mesopores are not necessary for the catalyst to reach high H2/O2 PEMFC performance. In summary, this section further expands the conclusion in section 4.1 that high melamine content promotes the formation of Fe-N4 active sites regardless of the properties of Fe precursor. The preexisting Fe-N4 coordination structure in the precursor cannot reduce the yield of Fe-rich nanoparticles because it cannot be directly transformed to Fe-N4 active sites anchored on the carbon substrate. The evolution of Fe atoms in FePc is similar to those in iron salt precursors in which high temperature decomposes the initial structure and release free Fe atoms. They are captured by the N-C defect nearby to form Fe-N4 active sites or sintered into aggregated Fe clusters. This evolution pathway is in line with our results. In the end, by combining the best precursor (Fe(NO3)3) and high M content (R/M=2/2), the best catalyst was obtained with a MA0.8=3.58 A g -1 in RDE setup, which is close to the commercial Pajarito powder (PMF D14401). In H2/O2 PEMFC, Fe(NO3)3-R/M=2/2 also shows the highest current density of 113 mA cm -2 at 0.8 V, and the current density of 44 mA cm -2 is reached at 0.85 V, which is only 0.05 V lower than the DOE 2025 target for platinum group metal-free catalyst 156 . Besides, the polarization curve does not collapse at high current density, indicating its good mass transport properties, promising to be widely used in the future.

Chapter 5 Impact of Ammonia Treatment on the Active Sites of Fe-N-C Catalysts

Ré sumé Franç ais

Dans ce chapitre, les impacts des diffé rentes é tapes de synthè se ont é té é tudié s sur l'activité catalytique et la durabilité des catalyseurs. La discussion porte principalement sur les effets du traitement au NH3 sur les aé rogels de carbone en comparant les caracté ristiques du catalyseur aprè s deux conditions diffé rentes de second traitement thermique, soit sous atmosphè re de N2 soit sous NH3 dilué . Outre la mé lamine, le traitement NH3 est une autre source importante d'azote dans le catalyseur final. Par consé quent, é tudier la variation des espè ces N aprè s traitement dans NH3 aiderait à comprendre le mé canisme d'activation des catalyseurs.

Aprè s le processus de lavage à l'acide, la teneur en Fe a diminué en raison de l'é limination des nanoparticules riches en Fe, puis a montré diffé rents degré s d'augmentation aprè s le deuxiè me traitement thermique en fonction du flux gazeux. Le catalyseur traité dans NH3 dilué a une teneur en Fe plus faible que celle du catalyseur traité dans N2 car une partie de Fe est perdue avec l'attaque du carbone.

En ce qui concerne la variation des espè ces N, leurs spectres XPS ré vè lent que le traitement NH3 pré sente des effets diffé rents de ceux dus à un accroissement de la teneur en mé lamine, où le pourcentage atomique de N pyridinique a é té significativement augmenté avec NH3 au lieu de N pyrrolique pour une augmentation de M. Puisque la configuration D1 est le principal site actif dans nos catalyseurs à base d'aé rogel, où le N pyrrolique est impliqué dans la premiè re couche de coordination, le traitement NH3 peut ne pas introduire de nouveaux sites actifs mais augmenter l'activité des sites existants, c'est-à -dire augmenter la valeur du TOF. Une premiè re é tude a é té mené e pour é lucider l'effet du traitement NH3 sur l'augmentation de l'activité massique du catalyseur Fe-N-C. En consé quence, deux fonctions principales du traitement au NH3 peuvent ê tre dé duites de leurs diffé rents spectres XPS et de leurs courbes de polarisation: (i) Lorsque les couches de carbone sont corrodé es par le NH3, davantage de sites actifs sont exposé s. Pendant ce temps, certains atomes de Fe libé ré s ré agissent avec le N pyrrolique et sont ancré s à la surface du carbone. La ré action entre NH3 et Fe est exclue car le N pyridinique n'est pas impliqué dans la formation de sites actifs Fe-N4. (ii) Le N pyridinique introduit par le traitement NH3 augmente la basicité à la surface des catalyseurs, ce qui peut amé liorer considé rablement le TOF des sites actifs. Il serait né cessaire de caracté riser la variation du centre Fe avant et aprè s traitement par des spectres Mössbauer pour confirmer les ré sultats de l'XPS.

Le catalyseur traité au NH3 pré sente une durabilité ré duite lors des tests accé lé ré s de durabilité (AST) dans un é lectrolyte saturé en O2, probablement en raison des groupes azoté s hautement basiques introduits par le NH3 augmentant le taux de dé mé tallisation des catalyseurs Fe-N-C en milieu acide. Il serait né cessaire de caracté riser l'é volution de la SD et de la valeur TOF des catalyseurs par une mé thode adapté e avant et aprè s les diffé rents AST.

In this chapter, the impacts of different synthesis steps will be investigated. The discussion focuses on the effects of NH3 treatment by comparing the characteristics of the catalyst after two different second heat treatment conditions, under N2 or diluted NH3 (NH3/N2 = 10/90) atmosphere, on the catalytic active sites and the durability. Throughout the synthesis protocol of Fe-N-C aerogel catalysts, there are mainly two nitrogen sources for the formation of Fe-N4 active sites, including melamine in precursor solution and NH3 during the second heat treatment. The effects of melamine content have been discussed in Chapter 4, that the content of Fe-N4 active site increase with melamine content. Both XPS and [START_REF] Flyagina | Identifying the Catalytic Active Sites in Heteroatom-Doped Graphene for the Oxygen Reduction Reaction[END_REF] Fe Mössbauer spectra demonstrated that melamine mainly incorporated pyrrolic N into the carbon matrix, which constructed the high-spin O-Fe(III)N4C12 configuration. This conclusion raises a question: do the N atoms incorporated by NH3 treatment participate in the formation of active sites? Previous research has revealed that a second heat treatment under NH3 atmosphere usually results in a 3-fold increase in ORR mass activity versus a N2 second heat treatment under the same conditions 194,231,240 .

Thereafter, efforts have been made to elucidate the microscopic mechanism of ammonia treatment. Jaouen et al. proposed an activation process comprising three consecutive steps: (i) incorporation of nitrogen atoms in the carbon, (ii) micropore formation through the reaction between carbon and NH3, and (iii) completion of active sites in the micropores by reaction of Fe with NH3 194 . The first two steps are the preconditions for the formation of new active sites, which can be demonstrated by increasing N content and micropore volume of catalysts after NH3 treatment, respectively 194,240 . The last step was used to be considered as the main reason of increasing mass activity because additional active sites were generated at the carbon surface 240 . However, this hypothesis is insufficient to be compatible with the previous conclusion in Chapter 4 that the active sites are mainly generated during the first pyrolysis by the reaction between released Fe atoms and N-C defects nearby. Since most Fe-rich nanoparticles were removed by acid washing, it was impossible to generate a large number of active site during the second NH3 treatment. Recent research of Kumar et al. proposed different insights on the reason of increasing mass activity based on the site density values determined by nitrite adsorption/reduction method 231 . In their research, although a 1.8-fold increase in mass activity was assessed after NH3 treatment, the SD values of the catalysts are almost unchanged, indicating no significant increase of active sites at the carbon surface. On the contrary, they found a 1.3-fold increase of TOF value, indicating the possible effects of neighbor nitrogen atoms incorporated by NH3 treatment. Herein, we try to elucidate the variation of Fe and N contents after the second heat treatment by XPS method and further investigate their electrochemical performance. The effect of acid washing will be discussed as well.

Synthesis of catalysts

Figure 5.1 schematically illustrates the samples extracted at different steps of synthesis. The aerogel was prepared with FePc as the Fe precursor, R/M=2/1, and pH=6. The carbon aerogel blocks were obtained by pyrolyzing RMF aerogels at 800 °C in N2 flow for 1 hour. They were then ball milled to fine powder labeled as FePc BM. The conditions were detailed in section 2.1.2. The obtained powder was then subjected to acid leaching with 0.5 M H2SO4 at 80 °C for 7 h in a refluxing device to remove the iron nanoparticles that were possibly generated during the pyrolysis 187 . The obtained powder was labeled as FePc AW. In order to investigate the effect of NH3 treatment, the powder was separated into two parts and then subjected to a second pyrolysis under different atmospheres. The sample labeled as FePc NH3 was treated at 950 °C in NH3 diluted flow, as the condition described in section 2.1.3. The sample labeled as FePc N2 was prepared as the comparison following the same heating program under N2 flow. As seen in Table 5.1, the aerogel lost around 56% of mass after the first pyrolysis due to the decomposition of melamine and ether groups. After the second pyrolysis under N2 (FePc N2), the carbon aerogel lost another 16% of weight, reaching a total mass loss of 63%, due to further organization of the carbon matrix at higher temperature. As for the sample treated with diluted NH3 (FePc NH3), the mass loss during the second pyrolysis was 47%, reaching a total mass loss of 77%. This discrepancy demonstrated the chemical etching of NH3 at high temperature. The carbon gasification process comprises the main reaction (C+NH3  HCN+H2) and minor reaction (C+2H2  CH4) 187 . The difference in their morphologies and chemical compositions will be discussed later. Both catalysts exhibit a highly porous structure and similar mesopore size, indicating that mesopores have been generated during the first pyrolysis. The specific surface area and the pore size distribution of catalysts were then further investigated by nitrogen adsorption method. Figure 5.2c shows the isotherm curves measured at -196 ºC . If we ignore the adsorption quantity, the catalysts treated under different atmospheres show similar isotherm curves, including features associated with micro/mesoporous materials. Both curves present a steep rise at low relative pressures, indicating the presence of micropores. At higher relative pressures, a hysteresis loop appears due to the capillary condensation of mesopores. The difference between two isothermal curves is that FePc NH3 has a much higher adsorption quantity than FePc N2, especially in the region of low relative pressures, indicating a larger micropore volume. Thus, FePc NH3 also has a larger BET area than FePc N2 (Table 5.1). The pore size distribution curves analyzed using the 2D-NLDFT-Heterogenerous surface calculation method are shown in Figure 5.2d, and the corresponding textural characteristics are listed in Table 5.1. In line with the observation from isothermal curves and SEM images, the micropore volume of FePc NH3 is more than 2-fold that of FePc N2 (0.43 vs. 0.18 cm 3 g -1 ), while the mesopore volume only increases ca. 1.5-fold (1.51 vs. 1.01 cm 3 g -1 ). Since N2 is inert to carbon at 950 º C, it can be considered that the mesopores in FePc N2 are mainly inherited from gel formation and particle organization. The micropores in FePc N2 come from the reorganization of carbon atoms during pyrolysis. The difference in pore size distribution between FePc NH3 and FePc N2 can be attributed to the pure effect of NH3 etching. According to Figure 5.2d, the peak of micropore size increases from 0.5 nm to 1.0 nm after NH3 treatment due to the gasification of amorphous carbon. On the contrary, the mesopore size is almost identical (around 10 nm), and the volume of mesopores is less impacted because the mesopore walls are constructed by small graphitic domains with more sluggish kinetic on the reaction with NH3. Since the amorphous carbon can be located between graphitic domains on the walls, the mesopore volumes can be slightly increased by removing these areas, and micropores are then created on the mesopore walls 187,194,231 . etching of NH3 but contains less Fe than FePc N2. We can then recall hypothesis (i) in section 4.2.2 and further discuss that with the etching of carbon during NH3 treatment, the Fe atoms detached from edge of carbon layers are left and reform Fe-N4 centers on the surface to realize the enrichment of Fe. The variation of Fe in FePc N2 can be considered the benchmark without any extra loss of carbon except further graphitization at 950 º C. The Fe content in FePc NH3 is lower than FePc N2 and slightly higher than FePc AW, indicating that although most of the Fe was lost with the etching of carbon, a small portion of Fe atoms remained and accumulated on the surface to form new active sites with NH3. Therefore, hypothesis (i) is correct in explaining the uneven distribution of Fe in the catalysts prepared with FePc.

Texture and structure of catalysts

The TEM images of all samples extracted at different steps are shown in Figure 5.3a-d. Although the existence of acid-soluble Fe compounds can be inferred from the reduced Fe content after acid washing, the quantity of Fe-rich nanoparticles is not enough to be directly observed under TEM. Meanwhile, no significant diffraction peak of Fe-contained species can be identified from their XRD patterns (Figure 5.3e) because the quantity of nanoparticles is very low and the particle size is small. In this case, the intensity of diffraction peaks is too weak to be identified, and the width of peaks is enlarged and easily hidden in the background.

Chemical composition of catalysts

The surface chemical compositions of samples were investigated by the XPS method, as listed in Table 5.2. The Fe content of FePc BM determined by XPS (0.13 at.%) is lower than that of ICP-MS (0.20 at.%), indicating an uneven distribution of Fe on the surface and in bulk. The possible reason is Fe-rich nanoparticles filled in mesopores or encapsulated by carbon layers cannot be detected by XPS. After acid washing, FePc AW shows uniform distribution of Fe, suggesting most of the nanoparticles have been removed. In contrast, the Fe-N4 sites are stable to acid washing and uniformly distributed in the whole carbon matrix. This result also confirms that most active sites have been generated during the first pyrolysis. After the second heat treatment, the Fe content on the surface of FePc N2 increased by 2 folds and even more for FePc NH3. The N2 heat treatment enhances the graphitic degree of carbon layers while removing heteroatoms on the margins. Thus, the relative content of Fe increases. Since N2 is inert during heat treatment, the carbon etching process is inhibited. No free Fe atoms are released in this step to form new active sites on the surface, thus the distribution of Fe is still uniform. In contrast, the catalyst treated in NH3 exhibited an uneven distribution of Fe due to the release of free Fe atoms accumulated on the surface by etching amorphous carbon.

The Fe K-edge XAS spectra were used to investigate the evolution of Fe at different steps. The XANES data (Figure 5.4) for all samples are similar to the reference Fe-N-C catalyst (Fe0.5) with almost pure Fe-Nx active sites shown in Chapter 4, indicating Fe is mainly present as Fe(II)/(III) oxidation states 219 . Their Fourier transform of the EXAFS signals are also similar to that of the reference Fe0.5, with a main signal at ca. 1.5 Å, assigned to Fe-N or Fe-O bonds, [START_REF] Zitolo | Identification of catalytic sites for oxygen reduction in iron-and nitrogendoped graphene materials[END_REF]187,233,234 and a minor signal at ca. 2.4 Å, assigned to Fe-C backscattering from the second coordination sphere 219 . The absence of signals at ca. 2.2 Å and 2.7 Å, corresponding to the Fe-Fe bond length in metallic Fe and oxides, respectively, indicates all samples are free of metallic Fe particles and well-ordered Fe oxides, which is consistent with the observation in XANES and XRD patterns [START_REF] Santori | Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeN x Moieties in Fe-N-C Catalysts[END_REF]187 . The Fe-N4 sites thus can be considered as the main contributor to the signal at 1.5 Å. However, the presence of nanometric Fe oxides cannot be excluded in FePc BM since the signal at ca. 2.7 Å is indiscernible in nanometric oxides due to the size and/or amorphous character of nanoparticles 105 . The intensities of XANES spectra are slightly stronger for FePc NH3 than FePc N2. The research of Santori et al. interpreted this by a higher coordination number in the first coordination sphere surrounding Fe (either N or O atoms), relating to the interaction between Fe-N4 moieties and axial O2 [START_REF] Santori | Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeN x Moieties in Fe-N-C Catalysts[END_REF] . However, the relative intensity in their research shows an inverse trend to our results, that the sample treated in NH3 showed a lower intensity than that treated in argon. This discrepancy might be elucidated by the uneven distribution of Fe in FePc NH3. In general, the samples treated in argon/N2 are considered to have a higher average oxidation state of Fe, thus showing a stronger FeN4-O2 interaction and higher coordination number. The lower oxidation state of Fe in the NH3 treated sample is due to the presence of highly basic nitrogen groups that are directly involved in Fe cation ligation [START_REF] Santori | Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeN x Moieties in Fe-N-C Catalysts[END_REF] . On the contrary, the more concentrated Fe atoms on the surface in FePc NH3 are oxidized easily to a higher oxidation state during the conservation under air, thus exhibiting a higher coordination number than FePc N2. The N 1s narrow scan spectra of all samples are shown in Figure 5.5a. FePc BM and FePc AW show similar spectra, whereas FePc NH3 shows a more intense signal at ca. 398.8 eV instead of 400.9 eV. The spectra were then analyzed by the deconvolution method used in Chapter 3 to further elucidated the variation of N species during the second heat treatment. As the results listed in Table 5.2, the content of all N species was almost kept constant after acid washing, whereas it significantly changed after the second heat treatment regardless of N2 or diluted NH3 atmosphere. The content of thermally unstable nitrile N was greatly decreased for both FePc N2 and FePc NH3, which confirms our previous description of increasing graphitic degree during second heat treatment by losing thermally unstable ingredients. The variation of three active N species content was plotted in Figure 5.5b. Comparing two samples treated under different atmospheres, FePc NH3 shows a lower pyrrolic N content (0.86 at.% vs. 1.05 at.%) but a higher pyridinic N content (0.51 at.% vs. 0.36 at.%), which exhibits a different effect from melamine. This trend is consistent with the research of Kumar et al. that the pyridinic N content increased after NH3 treatment 231 . The variation of Fe-Nx content is explained by the contribution of the amine peak at 399.9 eV is predominant instead of Fe-Nx. It should be noted that in Chapter 4, we mentioned that all N species in catalyst FeCl3-R/M=2/0 were introduced by ammonia treatment. The absolute contents of all N species are almost unchanged with melamine content, except pyrrolic N. Herein, we can further conclude that the pyridinic N species content is not affected by different Fe precursors by comparing the deconvolution results in Table 5.2 and Table 4.5. According to these phenomena, it can be deduced that a dynamic equilibrium exists between gaseous NH3 and different N species anchored on carbon under certain conditions, regardless of the Fe and melamine content. Since the research in Chapter 4 has demonstrated that major active sites are constructed by pyrrolic N, whereas NH3 treatment mainly introduces pyridinic N, it is possible that NH3 can only modify the N species on the carbon instead of reacting with Fe to form new active sites.

Moreover, the molar ratio of pyrrolic N/Fe is close to 5 in FePc N2, which is over the stoichiometric number in Fe-N4 sites, indicating the existence of uncoordinated pyrrolic N on the surface. This value decreases to 3.3 in FePc NH3, indicating excessive Fe appears on the surface because the pyrrolic N is insufficient to coordinate all free Fe atoms released by the etching of carbon. This ratio is similar to the FeCl3-R/M=2/2, suggesting a small part of Fe atoms are coordinated by pyridinic N. Besides the number of active sites, many publications reported a positive effect on the TOF of active sites due to highly basic N groups introduced by NH3 pyrolysis 105,194,231 . This may be demonstrated by increasing pyridinic N content after NH3 treatment since pyridinic N is a stronger base than pyrrolic N. The basic character of pyridinic N is attributed to the remaining two nonbonded electrons in the sp 2 orbital, which do not participate in the 6π-electron conjugated system 241 . In summary, NH3 treatment draws our attention back to the pyridinic N, which is not considered involved in the Fe cation ligation in Chapter 4. This section deduced the same conclusion according to the invariable pyridinic N content with Fe content. Therefore, the reaction between free Fe atoms and pyrrolic N on the surface should generate new active sites. However, our inference related to the evolution of active sites requires more evidence on the chemical environment of Fe, such as the [START_REF] Flyagina | Identifying the Catalytic Active Sites in Heteroatom-Doped Graphene for the Oxygen Reduction Reaction[END_REF] Fe Mössbauer spectra. The increasing basicity on the surface is demonstrated by increasing pyridinic N content, which is related to the increasing TOF value of Fe-N4 sites.

Electrochemical characterization of catalysts

The CV curves obtained in the RDE setup of samples extracted from different steps are shown in Figure 5.6a. All the CV curves exhibit a typical profile characteristic of carbonaceous materials. FePc NH3 presents the largest electrochemical available area, in agreement with its highest BET area. Figure 5.6b exhibits the corrected ORR polarization curves recorded in O2 saturated electrolyte. The corresponding normalized mass activities at 0.8 V vs. RHE are listed in Table 5.3. As can be seen, the samples at different steps present different activities. Although the active sites are supposed to be created during the first heat treatment, FePc BM exhibits a low ORR activity. According to its low Fe content from XPS, it is possible that these active sites are hidden under carbon layer. In addition, Wu et al. also proposed another explanation for highly porous catalysts: the Fe-rich nanoparticles on the catalyst surface impeded the accessibility of oxygen to active sites, [START_REF] Wu | Synthesis-structure-performance correlation for polyaniline-Me-C nonprecious metal cathode catalysts for oxygen reduction in fuel cells[END_REF] even if, in our case, no Fe based nanoparticles are detected by XDR or TEM. After acid washing, the mass activity of FePc AW is slightly reduced, which can be attributed to two factors: (i) the contribution of less active Fe-rich nanoparticles is removed by acid washing; (ii) the acid washing results in a partly oxidized catalyst surface, which could disrupt ORR active sites. The reduced Fe content from ICP-MS after acid washing (from 0.20 to 0.12 at.% seems to support the hypothesis (i) Moreover, the removal of unstable Fe-rich nanoparticles from the catalyst surface presumably exposes more Fe-N4 active sites, which should result in a higher ORR activity [START_REF] Wu | Synthesis-structure-performance correlation for polyaniline-Me-C nonprecious metal cathode catalysts for oxygen reduction in fuel cells[END_REF] . This inversed variation of mass activity suggests a considerable contribution of mass activity by Fe oxides. The increasing mass activity of FePc N2 demonstrates hypothesis (ii) because the second heat treatment in N2 can be envisioned to "repair" the partially oxidized surface by removing some heteroatoms. The mass activity of FePc N2 is ca. 2.5 times higher than FePc BM, indicative of new active sites exposed after removing Fe oxides on the catalyst surface. The NH3 treated catalyst (FePc NH3) exhibits a 3-fold increase in mass activity than FePc N2. According to the discussion in the previous section, the NH3 treatment caused a burn-off of the catalyst material, which leads to a higher specific surface area and exposal of active sites buried by low crystalized carbon. Meantime, the Fe atoms released from destroyed active sites accumulate on the surface and react with pyrrolic N to form new catalytic centers. However, the high mass activity of FePc NH3 cannot be simply explained by the increase of the active sites (0.26 at.% for FePc NH3 vs. 0.20 at.% for FePc N2), even though we assume all Fe atoms on the surface are atomically dispersed. This result indicates the active sites of FePc NH3 present a higher TOF value than those of FePc N2. The Tafel plots derived from polarization curves are shown in Figure 5.6c. The FePc NH3 shows a much lower Tafel slope than the others (ca. 60 mV dec -1 for FePc NH3 vs. ca. 90 mV dec -1 for the others), indicating a different property of active sites. This discrepancy might be evidence that NH3 treatment increases the TOF value of Fe-N4 sites by introducing highly basic nitrogen groups 105 .

The NH3 treatment is known to lead to less durable catalyst 231 . Here, the effect of NH3 treatment on the catalyst stability was investigated by a stronger accelerated stress test (AST) protocol composed of 2000 CV cycles between 0.6 and 1.0 V vs. RHE with a scan rate of 50 mV s -1 . The catalysts performed by AST in N2 or O2 saturated 0.05 M H2SO4 were labeled by AST-N2 and AST-O2, respectively. The polarization curves of catalysts after AST are shown in Figure 5.6d and e. Figure 5.6f shows the normalized ORR mass activities at 0.8 V of catalysts extracted from their polarization curves after AST. MA values dropped by 50% after AST in N2-saturated electrolyte for FePc N2 and % for FePc NH3, whereas the difference in O2saturated electrolyte was significantly increased (-70% for FePc N2 vs. -77% for FePc NH3). The loss of activity in N2-saturated is mainly caused by cleaving the Fe-N bonds at high potentials, thus promoting the displacement of Fe atoms onto the basal planes. Eventually, these Fe atoms agglomerate nanoclusters after AST 231 . A similar loss ratio after AST N2 for FePc N2 and FePc NH3 suggests that their active sites have the same property. Since the major active sites in our aerogel catalysts are D1 configuration, which has been identified to be a non-durable FeNx site to O2 in Fe-N-C materials during the electrochemical tests 105 , higher loss ratios of mass activity for both FePc N2 and FePc NH3 thus can be expected, in agreement with the results shown in Table 5.3. Fe-N4 sites can also be transformed to Fe oxides by oxygen during AST O2. The different loss ratio after AST O2 between two catalysts was recently explained by higher demetallation rates for NH3 treated Fe-N-C catalysts in an acid medium. The involvement of highly basic nitrogen groups neighbor to Fe-N4 sites leads to higher TOF but also fast protonation of N groups in acid medium, thus leading to demetallation of the most active sites 105 . In order to further demonstrate our inference, the active SD and TOF before and after different AST should be determined for both catalysts. The demetallation of catalysts is supposed to be validated by a decreased SD value. Figure 5.7 shows the H2/O2 polarization curves in a single cell of catalysts treated in different atmospheres. FePc NH3 presents a higher activity than FePc N2 in PEMFC, indicating that NH3 treatment is an efficient way to increase the activity of Fe-N-C catalysts in RDE setup and PEMFC. The measurements were performed with Tcell = 80°C, 100 % RH, Pcathode = Panode = 2 barabs, the H2, and O2 flow rates were 150 mL min -1 while the air flow rate was 480 mL min -1 .

In summary, two main functions of NH3 treatment on increasing the mass activity of Fe-N-C catalyst can be inferred from their different XPS spectra and polarization curves:

(i)
As NH3 corrodes the carbon layers, more active sites are exposed, as seen in Figure 5.8. Meantime, some released free Fe atoms are reacted with pyrrolic N and anchored on the carbon surface. The reaction between NH3 and Fe is excluded because pyridinic N is not involved in the formation of Fe-N4 active sites. However, this conclusion should be further demonstrated by the characterization of the chemical environment of Fe, such as the Mössbauer spectrum. (ii) (ii) The pyridinic N introduced by NH3 treatment increases the basicity on the surface of catalyst, which can significantly enhance the TOF of the sites. The NH3 treated catalyst exhibits a decreased durability during AST in O2-saturated electrolyte due to the highly basic nitrogen groups introduced by NH3 increasing the demetallation rate of Fe-N-C catalysts in an acid medium. An adapted method for aerogel catalysts should be performed before and after the different AST of both FePc N2 and FePc NH3 catalysts to observe the evolution of the SD and the TOF values.

Chapter 6 Impact of Cerium Dioxide on the Durability of Fe-N-C Catalysts

Ré sumé Franç ais

La principale source de dé sactivation des catalyseurs Fe-N-C en é lectrolyte saturé en O2 est principalement causé e par le H2O2 produit pendant l'ORR impliquant la voie à 2e -. Diffé rent des catalyseurs à base de Pt, les catalyseurs Fe-N-C ont une activité extrê mement faible pour la ré duction de H2O2. Par consé quent, un pié geur de H2O2 supplé mentaire pourrait ê tre judicieux pour aider à amé liorer la durabilité des catalyseurs Fe-N-C. Dans ce chapitre, les catalyseurs à base d'aé rogel Fe-N-C chargé s de CeO2 avec diffé rentes teneurs en CeO2 ont é té synthé tisé s pour é tudier les impacts sur la durabilité des catalyseurs Fe-N-C. L'é chantillon FeCl3-R/M=2/2 a é té sé lectionné comme catalyseur Fe-N-C de ré fé rence. Deux pré curseurs de Ce ont é té choisis, CeCl3• 7H2O et la nanopoudre de CeO2, pour é tudier les impacts de la solubilité des pré curseurs et de leur inté gration au sein du catalyseur final.

Dans la section 6.1, CeCl3 a é té utilisé comme pré curseur de CeO2 soluble dans l'eau et introduit dans la solution mixte de dé part simultané ment avec le pré curseur de Fe. Le volume mé soporeux du catalyseur Fe-N-C dopé au CeO2 a é té significativement diminué car la pré sence de CeCl3 dans la solution de pré curseur né cessite plus de NaOH pour dissoudre la pré cipitation modifiant la formation du gel. Les teneurs finales en Ce de tous les é chantillons sont infé rieures aux valeurs thé oriques de Fe/Ce en raison de la perte de pré curseur de Ce lors de l'é tape d'é change de solvant. Lorsque la charge de Ce est infé rieure à 0.11 at.% (~1 wt.%), des impacts né gligeables sur les performances de l'ORR peuvent ê tre observé s. Cependant, lors de l'augmentation supplé mentaire de la teneur en CeO2, un excè s de CeO2 peut entraver la formation et l'exposition de sites actifs Fe-N4, entraî nant une diminution du volume des micropores et des performances ORR. Bien que la capacité particuliè re du CeO2 à pié ger les peroxydes ait é té rapporté e dans des publications pré cé dentes, les catalyseurs Fe-N-C dopé s au CeO2 ne pré sentent pas une meilleure durabilité par rapport au catalyseur de ré fé rence dans notre test de durabilité . La raison possible est que les nanoparticules de CeO2 ne sont dé posé es qu'à la surface du carbone, ce qui entraî ne une faible interaction avec les sites actifs Fe-N4.

Dans la section 6.2, des nanoparticules de CeO2 ont é té utilisé es comme pré curseur de Ce pour obtenir une distribution plus uniforme de la taille des particules de CeO2. Cependant, en raison d'une faible interaction entre les nanoparticules de CeO2 et le squelette de l'hydrogel, la plupart d'entre elles ont é té perdues lors de l'é tape d'é change de solvant et une trè s faible teneur en Ce a é té obtenue dans le catalyseur final. En comparaison, l'introduction de nanoparticules de CeO2 par broyage à billes des nanoparticules de CeO2 avec notre catalyseur FeCl3-R/M=2/2 aprè s l'é tape de lavage à l'acide et avant l'étape de traitement sous NH3 dilué , permet de bien pré server le pré curseur de Ce. Semblable au pré curseur CeCl3, une teneur é levé e en CeO2 entraî ne é galement un volume de micropores et une activité ORR infé rieurs, et pré sente des impacts né gligeables sur la durabilité .

Par consé quent, la mé thode de dopage CeO2 doit ê tre encore modifié e pour obtenir une meilleure synergie entre les sites actifs CeO2 et Fe-N4, par exemple en pré parant du CeO2 encapsulé par quelques couches de carbone pour combiner é troitement deux types de sites.

Besides the investigation of the ORR performance of Fe-N-C catalyst, the durability during the catalytic processes is also an important indicator for evaluating the catalysts. However, the inherent stability of most Fe-N-C catalysts is significantly attenuated with the increased operating time of tests. According to the 2025 DOE technical targets of PEMFC, the loss in initial catalytic mass activity should be less than 40% after their accelerated stress test (AST) in a H2/N2 single cell of 25-50 cm 2 . The DOE's AST protocol contains 30,000 square wave cycles between 0.6 V and 0.95 V 156,242 . In contrast, most of our aerogel-based Fe-N-C catalysts reach the DOE-target limit of mass activity loss after only 500 cycles in RDE setup under similar test conditions. As a result, It is urgent to find new strategies that can effectively prevent the loss of mass activity.

As discussed in Chapter 5, the main source of deactivation of Fe-N-C catalysts in N2 saturated electrolyte is the demetallation of Fe-N4 active sites 184 , mainly caused by carbon corrosion when subjected to potentials above ca. 1.0 V 184,243,244 . Since the electrochemical oxidation is fast on the edges of carbon layer, in which micropore-hosted Fe-N4 sites are located, the loss of ligand environment of Fe ions leads to their solvation and elimination of active sites on the surface simultaneously 184,245 . In the presence of O2, the loss of mass activity of Fe-N-C catalysts becomes more severe due to the H2O2 or radical oxygen species (ROS) produced during ORR via 2e -pathway 246 . Although the percentages of H2O2 formation for atomically dispersed Fe-N-C catalysts is not significantly higher than that of Pt-based catalyst 233 , the residence time of H2O2 on the electrode may be longer with Fe-N-C catalysts due to their extremely poor activity for the electro-reduction of H2O2 to H2O and the chemical disproportionation of H2O2 246 . In contrast, platinum is an excellent catalyst for both 4e -reduction of O2 to H2O and 2e -reduction of H2O2 to H2O 247 . As a result, H2O2 and ROS are pointed as the key degradation source for Fe-N-C catalysts during the ORR. Choi et al. found the ORR activity decay by H2O2 origins from reduced TOF value of active sites 248 , which is different from the destruction of Fe-N4 active sites resulting from the electrochemical oxidation of the surrounding carbon support, inducing a decreased number of active sites. Goellner et al. found when H2O2 reacts with Fe 2+ species, such as those in Fe-N4 moieties, ROS are generated via Fenton-like reactions, promoting the formation of oxygen functionalities on the carbon matrix, especially epoxy groups. The formation of such oxidized groups decreases the electron density around the Fe center, weakening the O2-binding energy in Fe-N4 ensembles and decreasing the TOF of the ORR 243,246,248 . A more careful investigation with [START_REF] Flyagina | Identifying the Catalytic Active Sites in Heteroatom-Doped Graphene for the Oxygen Reduction Reaction[END_REF] Fe Mössbauer spectroscopy was carried out by Li et al., in which the durable and non-durable Fe-N4 sites were identified 105 . The high-spin D1 site is not durable in operating PEMFC, quickly transforming to ferric oxides due to the localized carbon surface oxidation or protonation of highly basic N atoms involved in active sites. In contrast, the D2 site is shown to be more durable due to its lower amount of ROS produced during ORR or its subsurface location. This discrepancy was also demonstrated by the variation of N1s spectra from XPS after AST. The signal for pyrrolic N, including Fe-N4 in pyrrolic environment (corresponding to D1 site), is almost absent in the spectrum collected after the AST in acid electrolyte. On the contrary, the signal of pyridinic N, including Fe-N4 in pyridinic environment (corresponding to D2 site), still exists after AST, indicating a higher durability 243 . Considering the major active sites in our Fe-N-C aerogel catalysts are D1 configuration, the durability of catalysts might be increased by adding cocatalysts to scavenge H2O2 or ROS generated during ORR in acid medium.

As a widely-recognized rare earth oxide, cerium oxide (CeO2) has a unique oxygen storage and release capacity, prominent reversible redox property (Ce 4+ /Ce 3+ ), and possesses a large number of oxygen vacancies, which are beneficial to ORR catalytic process 249,250 . Wei et al. first reported that eliminating H2O2 by the cocatalyst of CeO2 in Fe-N-C catalysts could improve their durability 251 . They found that Ce 3+ can reduce H2O2 quickly by following reactions, protecting the catalyst from being attacked by H2O2:

O2 + 2H + + 2e - H2O2 2CeO2 + 2H + + 2e - Ce2O3 + H2O Ce2O3 + H2O2  2CeO2 + H2O
As the CeO2 mass ratio increases, the yield of H2O2 decreases and the durability increases. However, the initial mass activity of Fe-N-C catalysts decreases at high CeO2 content. As reported by Lv et al., the electronic interaction between CeO2 and metal in the co-synthesized catalyst may enhance the oxygen absorption on the catalyst to improve ORR activity 252 . In contrast, the excessive loading amount (>1 wt.% ) may lead to the aggregation of CeO2 nanoparticles and hinder the exposure of Fe-N4 active sites, resulting in a decrease of the catalytic performance on the contrary 253 . Therefore, the CeO2 loading should be carefully controlled to preserve the ORR activity with increased durability.

Herein, we synthesized CeO2-loaded Fe-N-C aerogel catalysts with varying CeO2 contents. FeCl3-R/M=2/2 was selected as the pristine Fe-N-C catalyst and two Ce precursors were tested, CeCl3• 7H2O (99.9% from Sigma-Aldrich) and CeO2 nanopowder (<25 nm particle size from Sigma-Aldrich), to investigate the impact of precursor solubility. The Ce precursors were introduced in the solution with Fe precursor, the solubility of Ce precursor thus affecting the polycondensation process of hydrogel. The catalysts prepared with different Ce precursors and Ce content were labeled as "FeCl3/Ce precursor = molar ratio of Fe/Ce". For example, FeCl3/CeCl3=1/1 representing CeCl3 as the precursor and the targeted molar ratio of Fe/Ce=1/1.

CeCl3 precursor

CeCl3 is a commonly used water soluble Ce 3+ precursor. In this section, the content of CeO2 cocatalyst was adjusted by different molar ratios of FeCl3/CeCl3, including 1/0.5, 1/1, and 1/1.5. In the presence of CeCl3, more NaOH solution is required to dissolve the rust-red precipitation. The final pH of the polycondensation step is slightly higher than 8, which may result in different porosity of carbon aerogel compared to the pristine sample. The electrochemical performances and durability of catalysts in RDE setup and single cell were investigated.

Texture and structure of catalysts

The SEM images shown in Figure 6.1a-c provide a basic view of the morphology of catalysts prepared with different CeCl3 contents. All samples present a more compacted texture than the pristine catalyst FeCl3-R/M=2/2 (Figure 4.2e) and are similar to the catalyst prepared with higher melamine content (FeCl3-R/M=2/2.5, Figure 4.2f). This result is consistent with our previous research on the effect of pH value (section 3.2) that a higher pH value leads to smaller condensation units during the condensation step, thus reducing the pore size of aerogel.

Their N2 sorption results confirmed the observation under SEM. Figure 6.1d and e exhibit the N2 adsorption-desorption isotherms and corresponding pore size distribution curves, respectively. All samples present a steep rise at low relative pressures, indicating the presence of micropores. In contrast, at higher relative pressures, all curves show a platform instead of a hysteresis loop, suggesting the absence of mesopores in catalysts. The textural properties calculated from the nitrogen sorption isotherms are listed in Table 6.1. The mesopores are absent in all catalysts due to their high melamine content and pH value. Their BET areas and total pore volumes present decreasing trends as the Ce content increases, mainly caused by the decrease in micropore volume. In previous research on Fe precursor and melamine content, the micropore volumes are almost unchanged due to the same NH3 treatment condition. However, the micropore volume significantly decreases in the presence of Ce, suggesting the existence of CeO2 can prevent the gasification reaction between NH3 and the carbon matrix. The NH3 is possible to be consumed by reducing Ce 4+ to Ce 3+ , which protects the carbon layer from being attacked 251 . The Fe and Ce contents were determined by ICP-MS method, and the results are listed in Table 6.1. Two samples with relatively low Ce content, FeCl3/CeCl3=1/0.5 and 1/1, show similar Fe content, whereas that of the sample with higher Ce content (FeCl3/CeCl3=1/1.5) is slightly decreased. Moreover, the Fe contents of all samples are slightly lower than that of pristine catalyst (FeCl3-R/M=2/2 with 0.30 at.% of Fe in Table 4.3). Since the total mass loss of FeCl3/CeCl3=1/0.5 and 1/1 are close to the pristine catalyst, the discrepancy in Fe content might be explained by their higher yield of Fe-rich nanoparticles during the first pyrolysis in N2, which were then removed by acid washing. In the presence of CeCl3 in the precursor solution, more NaOH was required to dissolve the precipitation. However, a higher pH value is possible to promote the formation of larger precipitation particles of iron hydroxide, thereby slowing the release of single free Fe atoms during the pyrolysis. Another possible reason for the higher yield of Fe-rich nanoparticles is that CeO2 impedes the transformation from the free Fe atom to the Fe-N4 site. The reactive N-C defect nearby the Fe might be hidden by CeO2 nanoparticles, thus reducing the chance of free Fe atoms being captured by N-C defects. Since the Fe content decreases with increasing Ce content, whereas the consumption of NaOH solution is kept constant, the second hypothesis could be the main reason for the high yield of Fe-rich nanoparticles. This hypothesis might be demonstrated by preparing Fe-N-C catalyst without CeO2 precursor at the same pH value, i.e., using the same quantity of NaOH solution, and comparing their Fe content in the final catalysts.

Although the Ce content in the final catalyst presents an increasing tendency with the quantity of CeCl3, the absolute content is far less than the expected value. In FeCl3/CeCl3=1/0.5, only 0.03 at.% was obtained, only reaching a Fe/Ce value of 8/1. This result indicates a considerable loss of Ce precursor during the solvent exchange and/or acid-washing steps. FeCl3/CeCl3=1/1 and FeCl3/CeCl3=1/1.5 also present lower Ce contents than expected values, reaching Fe/Ce values of 1/0.5 and 1/1, respectively. It seems the absolute loss of Ce for all samples is around 0.1 at.% regardless of the initial quantity of CeCl3. This result suggests the Ce precursor is mainly lost in the solvent exchange step, and the unprecipitated CeCl3 is removed with water. The loss quantity is decided by pH value and its impact on the solubility of cerium hydroxide, thus similar pH value leading to a similar loss of Ce.

On the other hand, if CeO2 could be removed by acid leaching, the catalyst with higher Ce content shall exhibit a higher loss of Ce because more CeO2 nanoparticles are exposed to acid, which is not the case. It should be noticed that the effect of CeO2 on the texture of the catalyst, including BET area and micropore volume, is more significant when Ce content is higher than 0.11 at.% (corresponding to 1.28 wt.%). The micropore volumes of FeCl3/CeCl3=1/0.5 and 1/1 are still close to the pristine catalyst, whereas the sharply decreased to only half the value in FeCl3/CeCl3=1/1.5. Therefore, the FeCl3/CeCl3=1/1 is possible to present similar activity with the pristine catalyst with enhanced durability. The CeO2 nanoparticles can be observed clearly under TEM images. As seen in Figure 6.2ac, more and more CeO2 nanoclusters were deposited onto the catalyst surface with increasing quantities of CeCl3. The distribution of CeO2 particle size is uneven, with particle size varying from a few nanometers to tens of nanometers. Their carbon layers are similar to the pristine catalyst, which shows a pore-less morphology. X-ray diffraction patterns of the catalysts are shown in Figure 6.2d. Two lumps at around 2θ=26.0° and 43.6° correspond to the low crystalized carbon. It can be observed that two lumps become narrower at high Ce content, suggesting CeO2 can increase the graphitic degree of carbon matrix, probably by limiting etching during the diluted NH3 thermal treatment to the low crystallized carbon parts. Meanwhile, two weak peaks at 2θ=28.5°and 47.3° can be observed in the pattern of FeCl3/CeCl3=1/1.5, which can be assigned to ( 111) and ( 220) crystal planes of CeO2, respectively 253 . However, the diffraction peaks of CeO2 in FeCl3/CeCl3=1/0.5 and 1/1 are hidden in the background due to their low Ce content.

Chemical composition of catalysts

The XPS characterization was employed to obtain the chemical environment on the surface of catalysts. The chemical compositions of all elements identified are listed in Table 6.2. The Fe contents of XPS are slightly higher than that of ICP-MS for all catalysts, indicating Fe atoms are more concentrated on the surface. Moreover, the Ce content exhibits a similar tendency with Fe, that all Ce contents on the surface is higher than that in bulk. In this case, the surface oxygen content also increases with Ce due to the contribution of metal oxide. The total N contents of FeCl3/CeCl3=1/0.5 and 1/1 are slightly higher than the pristine catalyst (3.76 at.% in Table 4.5). This phenomenon was mentioned by Bai et al. that the addition of CeO2 could facilitate the fixation of N during pyrolysis of the precursors 253 . However, this does not mean more Fe-N4 active sites can be obtained because these extra N atoms should preferentially be located around CeO2. On the contrary, FeCl3/CeCl3=1/1.5 exhibits a lower total N content than the pristine catalyst because high CeO2 content limits the reaction between carbon layers and NH3, as confirmed by its low micropore volume. The deconvolution results of their N1s spectra are shown in Table 6.2. The pyrrolic N, which is considered to coordinate with iron in Fe-N4 active sites, gradually decreases with the increase of Ce content. This result implies a decreasing trend of active site number on the surface, which may lead to low catalytic performance.

Electrochemical characterization of catalysts

In order to investigate the effects of Ce doping on ORR activity and stability of the pristine catalyst, the catalysts with different Ce loading amounts were measured in 0.05 M H2SO4 electrolyte in RDE setup. Figure 6.3a shows the CV tests of catalysts in N2 saturated electrolyte, along with the pristine catalyst as the comparison. Comparing all CV curves of the samples, two catalysts with low Ce content (FeCl3/CeCl3=1/0.5 and 1/1) present similar electrochemical available areas with the pristine, indicating low CeO2 doping content does not affect the exposure of active sites. In contrast, the catalyst with high Ce content (FeCl3/CeCl3=1/1.5) exhibits a much lower CV area due to its lower BET area and micropore volume. This discrepancy indicates high CeO2 content may hinder the formation and exposure of Fe-N4 active sites. Figure 6.3b shows the ORR polarization curves of catalysts measured in O2 saturated electrolyte. As expected, FeCl3/CeCl3=1/0.5 and 1/1 show similar mass activity with the pristine catalyst, indicating a weak impact on the properties of Fe-N4 active sites at low Ce content. The possible explanation of these results is these CeO2 nanoparticles, obtained by co-precipitation with Fe precursor, are only deposited on the carbon surface instead of encapsulated or embedded into carbon layers. In this case, the distance between CeO2 particles and Fe cores is too long to affect the electron density of Fe-N4 moieties 253 . Moreover, the large particle size of CeO2 also limits the unveiling of oxygen defects in CeO2, and thus they cannot well integrate with Fe-N4 active sites. At high CeO2 content, the mass activity of FeCl3/CeCl3=1/1.5 is sharply decreased, indicating a loading limit of CeO2 is around 0.11 at.% (1.28 wt.%). When the content of CeO2 is further increased, excessive CeO2 hinders the Fe-N4 active sites. However, it should be noted that this limit is only appropriate for the weakly attached CeO2 particles. It is possible to enhance the loading limit by adjusting the doping method of Ce to realize a stronger integration between CeO2 and Fe-N4 moieties. The Tafel plots drawn from the corrected polarization curves are shown in Figure 6.3c. It is clear that FeCl3/CeCl3=1/0.5 and 1/1 show similar Tafel slopes around 60 mV dec -1 , which is consistent with the literature and demonstrates that the rate-determining step of the ORR mechanism is the same [START_REF] Xiao | Microporous Framework Induced Synthesis of Single-Atom Dispersed Fe-N-C Acidic ORR Catalyst and Its in Situ Reduced Fe-N 4 Active Site Identification Revealed by X-ray Absorption Spectroscopy[END_REF]210,233,236 . However, the value of FeCl3/CeCl3=1/1.5 is increased to 70 mV dec -1 , indicating a different ORR mechanism which may be caused by the presence of CeO2 nanoparticles.

The durability tests of CeO2 doped catalysts were conducted in N2 saturated electrolyte in the RDE setup. Figure 6.3d shows the mass activity after 500 CV cycles between 0.6 and 1.0 V. No significant difference between CeO2-doped catalysts and the pristine catalyst can be observed, with mass activity loss for all catalysts ranging from 30% to 40%. As discussed before, the loss of mass activity in the N2 saturated electrolyte is mainly caused by the electrochemical oxidation of carbon at high potential, leading to the Fe solvation. Therefore, similar durability of CeO2 doped catalysts indicates CeO2 cannot help reducing the electrochemical oxidation of carbon. Since no H2O2 or ROS was produced in our durability test condition, the effect of CeO2 as a scavenger of H2O2 should be further investigated. The ORR activity and durability of these catalysts were then investigated in a 5 cm 2 single cell PEMFC. The ink formulation was determined in section 2.3, with N/C=2. Figure 6.4a shows the H2/O2 polarization curves of all catalysts. It can be seen that the trend of activity with Ce content is similar to that observed in RDE. Two catalysts with low Ce content (FeCl3/CeCl3=1/0.5 and 1/1) show slightly lower activities than the pristine catalyst, whereas that of the catalyst with high Ce content (FeCl3/CeCl3=1/1.5) is much lower than the pristine catalyst. To get a view of the effect of CeO2 in H2/O2 PEMFC, the corresponding cathodes were operated at 0.5 V in the cell for about 20 h. All catalysts present a quick decline in the first 2 hours, and then the slope becomes flatter. The CeO2 exhibits a poor effect on scavenging peroxides due to a weak interaction between Fe-N4 sites and CeO2. It should be noted that different decay rates may suggest different deactivation mechanisms of active sites in PEMFC. The flatter decay after 2 hours is supposed to be caused by H2O2 produced during the ORR, which is considered to be a non-recoverable activity decay, whereas the reason for rapid decay in the first 2 hours is still controversial. Jaouen et al. initially proposed a mechanism relating to the nitrogen-protonation and anion-binding effects in Fe-N-C catalysts 107,195 . Figure 6.6 shows the initial spatial relations between Fe-N4 active site and highly basic N group were shown in the as-prepared catalyst. The active site is assumed to possess a high TOF value for the ORR as long as nearby basic N-groups are protonated but not bound to an anion. However, when these protonated NH + groups are bound by anions, the TOF of active sites becomes "normal" again. The HSO4 -anion adsorption is fast in RDE since the anions can diffuse freely, while it is probably much slower in PEMFC since the sulfonate anions are bound to the polymeric backbone of the Nafion surrounding the catalyst in the PEMFC electrode.

In a fully humidified PEMFC cathode, the protons can swiftly access the basic N-groups to get high TOF active sites. However, they would later switch to the low TOF state when polymeric chains slowly approach the NH + groups, resulting in anion binding. This effect can explain the sharp activity decay in the first 2 hours, and this part is recoverable by removal of counter anions by chemical or heat treatment 107 . If this hypothesis is correct, the rate of decay of the current density should relate to the size of pendent chains of ionomer. However, when Nafion is replaced by other ionomers bearing shorter pendant chains, such as Flemion, or those with pendant chains too big to enter into micropores, such as tetrasulfonate hydrogen porphyrins, no effect on the initial decay rate can be observed 254 . In this case, a new hypothesis was proposed that the first decay of the current density is related to water flooding of the micropores, and the rate at which water flooding occurs depends on the hydrophilicity of the micropore walls, the latter varying with the heteroatom content on the catalyst surface 254 . For the hydrophobic catalyst layer, the fast decay rate can be slowed down, but with elapsed time, the carbonaceous cathode is slowly oxidizing and becomes more and more hydrophilic. However, this hypothesis was latterly argued by Choi et al. that the majority of the micropores were wetted at the beginning of life. Although some degree of additional catalyst layer wetting occurred during the stability test, such a small increase could not explain the significant performance loss that was observed 255,256 . Therefore, the reason for the rapid decay at the beginning of the durability test should be further explored. In summary, CeO2 doped Fe-N-C samples with different CeO2 content were prepared with CeCl3 precursor. The composition of FeCl3-R/M=2/2 was used as the pristine catalyst. The final Ce contents of all samples are lower than the theoretical Fe/Ce values due to the loss of Ce precursor during the solvent exchange step. The mesopore volume of CeO2 doped Fe-N-C catalyst was significantly decreased because the presence of CeCl3 in the precursor solution requires more NaOH to dissolve the precipitation modifying the gel formation. Appropriate low Ce loading (0.11 at.%) does not affect the micropore volume and ORR activity. However, when the content of CeO2 is further increased, excessive CeO2 limits the effect of the NH3 thermal treatment. It may hinder the formation and exposure of Fe-N4 active sites, resulting in a decline in the micropore volume and ORR performance. Although the special ability of CeO2 to scavenge peroxides has been reported in previous publications, the CeO2 doped Fe-N-C catalysts based on the insertion of Ce precursor in the gel do not exhibit better durability than the pristine catalyst in our durability test. The possible reason is CeO2 nanoparticles are only attached to the carbon surface, resulting in a weak interaction with Fe-N4 active sites. Therefore, the synthesis method should be modified to realize a better integration between CeO2 and Fe center.

CeO2 nanoparticle precursor

Texture and structure of catalysts

In this section, CeO2 nanoparticles were used as the Ce precursor. Compared with CeCl3, it is supposed to obtain a more uniform particle size of CeO2 because the size is given by the supplier as less than 25 nm. For comparison, we also tried to introduce the CeO2 nanoparticles into pristine catalyst by ball milling method, after the acid washing step but before NH3 treatment, which is supposed to reduce the loss of Ce. The CeO2 nanopowder and FeCl3-R/M=2/2 were ground with ZrO2 balls of 5 mm diameter at 400 rpm for 2 h. The samples were collected, and then conducted to NH3 treatment. The initial molar ratio of FeCl3/CeO2 is 1/1, and the sample is labeled as FeCl3/CeO2=1/1 BM.

The SEM images shown in Figure 6.6a-d show a basic view of the morphology of catalysts prepared with different CeO2 contents or introduction methods. Similar to the catalysts prepared with CeCl3 precursor, all samples present a more compacted texture than the pristine catalyst FeCl3-R/M=2/2 (Figure 4.2e). Some small holes can be observed on FeCl3/CeO2=1/1 BM, which may be caused by mechanical milling with CeO2 nanoparticles. Their N2 sorption results confirmed the observation under SEM. Figure 6.6d and e exhibit the N2 adsorption-desorption isotherms and pore size distribution curves, respectively. Similar to the series of CeCl3, the isothermal curves of all samples exhibit a quasi-absence of mesopore volume. Their texture characteristics calculated from nitrogen sorption isotherms are listed in Table 6.4. It seems the CeO2 nanoparticles introduced in the precursor solution do not affect the morphology of catalysts. All samples (FeCl3/CeO2=1/0.5, 1/1, and 1/1.5) show similar specific surface area and micropore volume. On the contrary, if the CeO2 is introduced by ball milling after acid washing of pristine catalyst, the specific surface area and micropore volume are halved. To gain further insight into the effects of the introduction method on the catalyst morphology, the Fe and Ce contents were determined by the ICP-MS method, and the results are listed in Table 6.4. Samples with CeO2 introduced in precursor solution (FeCl3/CeO2=1/0.5, 1/1, and 1/1.5) show very low Ce content (less than 0.1 at.%), far less than the designed value. The possible reason is electrically neutral CeO2 nanoparticles cannot be well adsorbed onto the RMF hydrogel. In contrast, the CeCl3 precursor can reach a higher Ce content in the final catalyst because the fresh cerium hydroxide precipitation can form hydrogen bonds to anchor on the skeleton of hydrogel. In this case, CeO2 nanoparticles are easier to remove during solvent exchange. Since the Ce content is less than 0.1 at.%, the negative effects of CeO2 on micropore volume and BET area are negligible. In contrast, FeCl3/CeO2=1/1 BM shows a high Ce content in the final catalyst due to no Ce loss during NH3 treatment. The final Ce content is higher than the designed value (molar ratio of Fe/Ce=1/1.5) due to the loss of Fe during the acid-washing step. Moreover, the micropore volume and BET area of FeCl3/CeO2=1/1 BM is halved due to its high Ce content. This result is in agreement with the previous result with CeCl3, that the limit of Ce content should be lower than 0.11 at.%. Otherwise, the presence of excessive CeO2 may prevent the gasification reaction between carbon and NH3. The CeO2 nanoparticles can also be observed under TEM images (Figure 6.7a-c). However, due to their low Ce content, only some small CeO2 nanoparticles are sparsely dotted on the carbon. More and larger CeO2 particles can be found in FeCl3/CeO2=1/1 BM (Figure 6.7d). Their carbons also show a pore-less morphology, in agreement with their poor mesopore volume. Figure 6.7e shows the X-ray diffraction patterns of catalysts. FeCl3/CeO2=1/0.5, 1/1, and 1/1.5 show two lumps at around 2θ=26.0° and 43.6°, corresponding to the low graphitized carbon. These two peaks become more intense for FeCl3/CeO2=1/0.5 BM, confirming that high CeO2 content can promote the graphitization of carbon at high temperature. Therefore, the micropore volume of FeCl3/CeO2=1/0.5 BM is only half the others. Two peaks of CeO2 at 2θ=28.5°and 47.3° can be observed in the patterns of FeCl3/CeO2=1/1 and 1/1.5 253 . The diffraction peaks of CeO2 are more intense than those in the CeCl3 series because commercial CeO2 nanoparticles have larger and well crystallized particles. In the pattern of FeCl3/CeO2=1/1 BM, two more peaks at 2θ=33.0°and 56.3° can be identified, which can be assigned to ( 200) and ( 311) crystal planes of CeO2.

Chemical composition of catalysts

The surface chemical compositions of catalysts were investigated by XPS characterization. As the results listed in Table 6.5, the Fe and N contents of CeO2 doped catalysts (FeCl3/CeO2=1/0.5, 1/1, and 1/1.5) obtained from XPS are similar to the pristine catalyst, indicating the structural composition of Fe-N4 sites are still well preserved in the existence of low content CeO2. However, the total N content of FeCl3/CeO2=1/1 BM is significantly decreased, which is in agreement with the previous conclusion that high CeO2 content prevents the formation of Fe-N4 active sites. The deconvolution results of N1s spectra are listed in Table 6.5. Catalysts with low Ce content show similar pyrrolic N content with pristine catalyst, whereas the value decreases in FeCl3/CeO2=1/1 BM because CeO2 impedes the formation of Fe-N4 sites. The Ce contents obtained from XPS of FeCl3/CeO2=1/0.5, 1/1, and 1/1.5 are similar to the results of ICP-MS. In contrast, the value in FeCl3/CeO2=1/1 BM is much higher than ICP-MS method because the CeO2 introduced by BM can only deposit on the surface of carbon aerogel particles.

Electrochemical characterization of catalysts

In order to test and verify the effect of CeO2 on ORR activity and stability, the catalysts with different Ce loading amounts were measured in 0.05 M H2SO4 electrolyte in the RDE setup. Figure 6.8a shows the CV curves of catalysts recorded in N2 saturated electrolyte, along with the pristine catalyst as the comparison. As expected, the catalysts with low Ce content (FeCl3/CeO2=1/0.5, 1/1, and 1/1.5) present similar electrochemical available area with the pristine catalyst, whereas FeCl3/CeO2=1/1 BM shows very low CV area due to its low BET area at high CeO2 content. Their polarization curves in the O2 saturated electrolyte are shown in Figure 6.8b, and corresponding mass activities at 0.8 V are listed in Table 6.6. The mass activity of the catalyst is comparable with the pristine catalyst at low Ce content, whereas it dramatically drops at high Ce content (FeCl3/CeO2=1/1 BM with Ce more than 0.11 at.%). This result is consistent with previous research on CeCl3 precursor. Therefore, it might be concluded that different Ce precursors and introduction methods of Ce do not change the property of CeO2 nanoparticles and Fe-N4 sites in the final catalyst but only affect the loading of CeO2, which presents a crucial effect on the mass activity of the catalyst. The Tafel plots of catalysts derived from the polarization curves are shown in Figure 6.8c. The Tafel slopes of samples with low CeO2 content (FeCl3/CeO2=1/0.5, 1/1, and 1/1.5) are slightly higher than that of pristine catalyst, suggesting the oxygen is reduced by a different mechanism in the presence of CeO2 187,210 . The value is further increased at higher CeO2 content in FeCl3/CeO2=1/1 BM, indicating the contribution of CeO2 to ORR activity is also increased. The durability tests were carried out for all catalysts with 500 CV cycles between 0.6-1.0 V in N2 saturated electrolyte. The mass activities after the durability test and the loss ratio are listed in Table 6.6. Catalysts with low CeO2 content (FeCl3/CeO2=1/0.5, 1/1, and 1/1.5) exhibit loss ratios in the range of 25%-29%. These results are similar to those of pristine catalyst and CeCl3 series, indicating the change of Ce precursor to CeO2 cannot enhance the interaction between Ce and Fe-N4 sites. The loss ratio of FeCl3/CeO2=1/1 BM with higher Ce content is even increased, indicating that excessive CeO2 can lead to more severe carbon corrode by electrochemical oxidation. The activity and durability of catalysts with low Ce content (FeCl3/CeO2=1/0.5, 1/1, and 1/1.5) were further investigated in a 5 cm 2 single cell PEMFC. The tests were not conducted on FeCl3/CeO2=1/1 BM due to poor ORR activity. Figure 6.9a shows their H2/O2 polarization curves, along with the pristine as the comparison. Their curves present a similar trend with the result in RDE setup that the activity of FeCl3/CeO2=1/1 is slightly higher than the other two. However, the difference is too small to say FeCl3/CeO2=1/1 is the best composition. To further investigate their durability in H2/O2 PEMFC, the corresponding cathodes were operated at 0.5 V in the cell for about 20 h. As seen in Figure 6.9b, all catalysts present similar decline behavior with CeCl3 series, that a sharp decrease at the beginning followed by a flatter decline. According to the discussion in section 6.1, the latter is non-recoverable decay due to the loss of Fe sites. Since the H2O2 scavenger mainly affects the slope of non-recoverable decay, the similar decline slope indicates the presence of CeO2 in the pristine catalyst cannot inverse the attenuation trend of Fe-N4 sites with increasing operation time. The poor effect of CeO2 on scavenging peroxides can also be attributed to a weak interaction between Fe-N4 sites and CeO2. In summary, an alternative Ce precursor (CeO2 nanopowder) was used to prepare CeO2 doped Fe-N-C samples to achieve a more uniform particle size distribution. Different from the CeCl3 precursor, which can be transformed to cerium hydroxide to anchor on the RMF hydrogel, CeO2 nanoparticles are only deposited on the hydrogel surface, thus leading to a high loss ratio CeO2 during the solvent exchange step and very low Ce content in the final catalyst. In comparison, introducing CeO2 nanoparticles by ball milling after the acid washing step can preserve the Ce precursor well. Similar to the CeCl3 series, low Ce loading does not affect the micropore volume and ORR activity. In contrast, these are significantly decreased at high Ce content because the presence of excessive CeO2 may hinder the formation and exposure of Fe-N4 active sites. The series CeO2 nanopowder does not exhibit better durability than the pristine catalyst in our test conditions, indicating different Ce precursor does not affect the properties of CeO2 in the final catalyst. Therefore, the interaction between CeO2 and Fe-N4 active sites is still too weak to scavenge the peroxide nearby. The function of CeO2 is possible to be highlighted by approaching the CeO2 to Fe-N4 sites. To achieve this, the particle size of CeO2 should be further reduced to have more chance to meet Fe-N4 sites. It is also possible to directly graft carbon layers around CeO2 nanoparticles with new synthesis methods, which is supposed to improve both activity and durability of the Fe-N-C catalyst.

Conclusion and Perspective

Proton exchange membrane fuel cells (PEMFCs) are considered promising technology for stationary and mobile applications because of their high energy conversion efficiency and power density. These devices promote the possibility of establishing a clean and sustainable society. Due to sluggish oxygen reduction reaction (ORR) on the cathode and the acidic environment of PEMFC, large quantities of Pt-based electrocatalysts are necessary to reach the acceptable power performance of PEMFCs, which impedes their broader commercialization and threatens their sustainability. Therefore, developing platinum-group metal-free catalysts for the ORR in acid media is highly desirable. In this manuscript, a series of state-of-art Fe-N-C aerogel catalysts were synthesized and characterized.

The impacts of different iron precursors on the porosity and electrochemical activity of Fe-N-C catalysts were investigated. The iron salts with different oxide states and counter ions were selected to investigate the impacts on porosity and electrochemical activity. The catalysts prepared at pH=8 with FeCl3 and Fe(NO3)3 (Fe 3+ ) display the best electrochemical activity at 0.8 V both in the RDE setup and PEMFC, slightly higher than for (NH4)2Fe(SO4)2 (Fe 2+ ) due to their excellent solubility in precursor solution leading to a low yield of Fe-rich nanoparticles and similar porosity. The oxide states of Fe and counter ions show little effect on final catalytic performance if the Fe precursor is soluble in water. The insoluble phthalocyanine (FePc, Fe 2+ ) is not a good choice at pH=8 since it led to low porosity of carbon aerogel. Although the porosity could be increased at a lower pH value, the current density at 0.8 V was only slightly increased. The low Fe content in the catalyst prepared with FePc evidenced that insoluble Fe precursor cannot be well dispersed in the precursor solution, thus leading to a higher yield of Fe-rich nanoparticles, which can be eliminated by acid washing. The pre-existing Fe-N4 structure in the FePc precursor cannot increase the density of active sites in the final catalyst because this structure is decomposed before reaching the temperature that creates active sites.

The modification of Fe precursors, including the introduction of ligand additives and increase of Fe precursor quantity, did not receive expected results. EDTA ligand increases the solubility of Fe precursor but leads to a loss of Fe during the solvent exchange step. As a result, all catalysts prepared with EDTA ligand exhibit ultra-low Fe contents. Phenentroline (Phen) generates insoluble precipitations in the mixed solution, which leads to a different texture from the non-ligand catalyst. High Fe content can be obtained in Phen catalyst without any observable nanoparticles, indicating a high content of active sites. However, the low micropore volume limits the improvement of electrochemical activity. In conclusion, due to the complicated chemical environment of mixed solution, the ligand additives exhibit little improvement in the electrochemical activity of catalysts. The crucial point among these catalysts is their activity is closely related to their Fe content, especially the surface Fe content obtained from XPS analysis. However, simply increasing the quantity of Fe precursor only leads to more unexpected nanoparticles in the final catalyst. The pre-existing coordination structure presents little impact on forming active centers. Even though all Fe atoms are contained in the Fe-N4 macrocycle in the FePc precursor, the microstructure will be decomposed during the carbonization step.

The impacts of melamine content on the ORR activity and mass transport properties of Fe-N-C aerogel catalysts were systematically investigated. The absolute amount of Fe was found to increase with M content, and the absolute amount of Fe-N4 sites also increased with M content. The atomically dispersed nitrogen in melamine can stabilize the Fe during pyrolysis, avoiding or minimizing the formation of iron nanoparticles. From the analysis of XPS spectra, the pyrrolic N content is positively correlated with the increase of the M content, supporting that most of the active Fe atoms are coordinated by pyrrolic nitrogen, in agreement with Mössbauer spectra. Electrochemical tests in RDE show a linear relationship between mass activity up to a Fe content of 0.3 at.%, corresponding to the catalyst FeCl3-R/M=2/2, with the highest mass activity of 3.0 A g -1 . This trend is in line with the physicochemical characterization, indicating that most Fe is present as atomically dispersed Fe-Nx sites. This trend is also suitable for other Fe precursors, such as Fe(NO3)3 and FePc. By combining the best precursor (Fe(NO3)3) and high M content (R/M=2/2), the best catalyst was obtained with a MA0.8=3.58 A g -1 (at 0.8 V vs. RHE and in 0.05 M H2SO4) in RDE setup which is close to the commercial Pajarito powder (PMF D14401). In H2/O2 PEMFC, Fe(NO3)3-R/M=2/2 also shows the highest current density of 113 mA cm -2 at 0.8 V, and the current density of 44 mA cm -2 is reached at 0.85 V, which is only 0.05 V lower than the DOE 2025 target for platinum group metal-free catalyst 156 . Besides, the polarization curve does not collapse at high current density, indicating its good mass transport properties, which is promising to be widely used in the future.

This study highlights is the successful preparation of Fe-N-C catalysts with adjustable and high-purity Fe-N4 active sites embedded in a carbon aerogel substrate. We show that Fe-N-C aerogel can lead to a highly efficient ORR catalyst by controlling the synthesis conditions: (i) a soluble Fe(III) precursor should be selected, such as FeCl3 and Fe(NO3)3, to ensure that the Fe and N precursors are atomically mixed; (ii) high melamine (nitrogen source) content is necessary for RMF aerogel to facilitate the formation of active sites. The major Fe-N4 active site was determined to be D1 configuration by mutual confirmation of the XPS spectrum of nitrogen (N 1s) and [START_REF] Flyagina | Identifying the Catalytic Active Sites in Heteroatom-Doped Graphene for the Oxygen Reduction Reaction[END_REF] Fe Mössbauer spectrum. Compared to the traditional preparation method of Fe-N-C catalysts, i.e., mixing a microporous support (carbon black, such as BP2000 or metalorganic framework, such as ZIF-8), a pore filler (nitrogen precursor, such as phenanthroline), and Fe precursor by ball-milling method, followed by pyrolysis, our one-pot synthesis method can atomically mix Fe and N precursors in the solution, thus reducing the yield of Fe-rich nanoparticles. Comparing to the ordered mesopore carbon-based Fe-N-C catalysts prepared from the sacrificial support method (i.e., Pajarito powders), the synthesis of aerogel-based catalysts is more environmentally friendly since there is no need to remove silica support by hydrofluoric acid, suggesting this method is more suitable for scaling up production in future.

However, the durability of Fe-N-C aerogel catalysts still needs to be improved before their application in the practical PEMFC. All aerogel-based catalysts lost more than 90% of their initial activity after a strong AST process (conducted by LEPMI, between 0.6 and 1 V for 10,000 cycles at 80°C in O2 saturated 0.1 M H2SO4), which is far from reaching the DOE 2025 target. Two aspects should be considered:

The first is to further clarify the mechanism of the ammonia treatment process on the activation of catalysts because although this treatment can significantly increase the initial ORR activity of Fe-N-C catalyst, their long-term stability is usually decreased. In this manuscript, we made a primary investigation to elucidate the effect of NH3 treatment on increasing the mass activity of Fe-N-C catalyst. Two main functions of NH3 treatment can be inferred from their different XPS spectra and polarization curves: (i) As NH3 corrodes the carbon layers, more active sites are exposed. Meanwhile, some released free Fe atoms react with pyrrolic N and are anchored on the carbon surface. The reaction between NH3 and Fe is excluded because pyridinic N is not involved in the formation of Fe-N4 active sites. (ii) The pyridinic N introduced by NH3 treatment increases the basicity on the surface of catalyst, which can significantly enhance the TOF of active sites. The variation of Fe center before and after treatment should be further characterized by the Mössbauer spectrum to confirm the results of XPS. The NH3 treated catalyst exhibits a decreased durability during AST in O2-saturated electrolyte, possibly due to the highly basic nitrogen groups introduced by NH3 that increase the demetallation rate of Fe-N-C catalysts in acid medium. The nitrite adsorption/reduction method should be performed before and after the different AST to observe the evolution of the SD and the TOF value of catalysts.

The second is to introduce a scavenger of H2O2 to improve the durability of Fe-N-C catalysts. Primary research on CeO2 scavenger was conducted in this manuscript. The CeO2 doping in Fe-N-C aerogel catalyst does not affect its durability regardless of Ce precursor and Ce content. The final Ce contents of all samples are lower than the theoretical Fe/Ce values due to the loss of Ce precursor during the solvent exchange step. The catalyst with the best activity is the one with 0.11 at.% Ce loading. When the content of CeO2 is further increased, excessive CeO2 may hinder the formation and exposure of Fe-N4 active sites, resulting in a decline in the micropore volume and ORR performance. Although the special ability of CeO2 to scavenge peroxides has been reported in previous publications, the CeO2 doped Fe-N-C catalysts do not exhibit better durability than the pristine catalyst in our durability test. The possible reason is CeO2 nanoparticles are only deposited on the carbon surface, resulting in a weak interaction with Fe-N4 active sites. Therefore, the CeO2 doping method should be further modified to obtain a better synergy between CeO2 and Fe-N4 active sites, such as preparing few-layer carbon encapsulated CeO2 to closely combine two types of sites. Besides, CeO2 can also be replaced by some Ptbased nanoparticles as a highly efficient peroxide scavenger. As reported in the literature, an ultra-low introduced Pt content (1 wt.%) is expected to lead to Pt nanoparticles that can exhibit a synergetic effect with Fe-N4 sites and scavenge H2O2 intermediate to protect the carbon layer. This method is promising to get low-cost Fe-N-C catalysts with high ORR performance and durability to reach DOE 2025 target. Studies are in progress on our best catalyst by ICGM.

Quantitative calculations were performed using the XPS peak areas. The surface ratio of all elements of the samples was calculated by using the area of the core level peaks, normalized to the photoemission cross section by assuming a homogeneous distribution arrangement model. The calculation was conducted on five spots and the final element content was determined by the average of five values. Further deconvolution of N1s spectra was conducted based on the method of Atanassov et al., containing six components: nitrile, pyridinic, Fe-Nx or amine, pyrrolic, quaternary, and graphitic. The binding energy of pyridinic N was fixed at 398.8 eV as the benchmark, and the position of other peak was determined by their binding energy difference with pyrinidic N. For the major components (pyrrolic, Fe-Nx, and nitrile), the uncertainty of binding energy position was limited in ±0.2 eV. The value is enlarged to ±0.4 eV for the minor components (quaternary and graphitic) due to low intensity of signal at high binding energy. For the consistency among all other spectra from electrocatalysts, the peaks during spectral curve fitting were constrained to have a full width at half maximum (fwhm) between 1.2 and 1.3 eV.

A.2.6 57 Fe Mössbauer spectroscopy

The 57 Fe Mössbauer spectra (Wissel, Germany) were acquired in transmission mode. The source was 57 Co: Rh. The velocity driver was operated in constant acceleration mode with a triangular velocity waveform, and the velocity scale was calibrated with the magnetically split sextet of a high-purity α-Fe foil at room temperature. A mass of ca. 200 mg of catalyst was mounted in a 2 cm 2 holder. Mössbauer measurements were performed at -268°C with a helium flow cryostat (SHI-850 Series from Janis, USA). The spectra were analyzed by fitting the experimental spectrum with an appropriate combination of quadrupole doublets and sextets with Lorentzian profiles.

A.2.7 X-ray absorption spectroscopy

Fe K-edge XAS were collected at room temperature at SAMBA beamline (Synchrotron SOLEIL, Saint-Aubin, France). The beamline is equipped with a sagittally focusing Si 220 monochromator, and X-ray harmonics are removed by two Pd-coated mirrors. The catalyst powders were pelletized as disks of 10 mm diameter with 1 mm thickness, using Teflon powder (1 μm particle size) as a binder. Measurements were carried out in transmission or fluorescence mode, depending on the iron content.

A.2.8 Nitrite stripping

The nitrite adsorption method was used to determine the active SD and TOF of catalysts. The measurement was realized through the method first introduced by Malko et al using 0.125 M NaNO2 in acetate buffer 0.5 M pH 5.2 and a loading of 0.3 mg cm -2 of catalysts prepared from different R/M ratio 231 . ORR polarization curves were carried out at 5 mV s -1 and 1600 rpm of rotation for unpoisoned, poisoned and recovered catalysts surface. The nitrite stripping was recorded in a potential range of 0.2 to -0.2 V vs. RHE. The SD and TOF were determined using the following equations: 𝑆𝐷(𝑔 -1 ) = 𝑄 𝑠𝑡𝑟𝑖𝑝 (𝐶 𝑔 -1 ) 𝑁 𝐴 (𝑚𝑜𝑙 -1 ) 𝑛 𝑠𝑡𝑟𝑖𝑝 𝐹 (𝐶 𝑚𝑜𝑙 -1 ) (6-1)

𝑇𝑂𝐹(𝑠 -1 ) = 𝑀𝐴 0.8 (𝐴 𝑔 -1 ) 𝑀𝑆𝐷 (𝑚𝑜𝑙 𝑔 -1 ) × 𝐹 (𝐶 𝑚𝑜𝑙 -1 ) (6-2)

Where Qstrip is the excess coulometric charge associated with the reduction of one adsorbed nitrite ligand per site and and nstrip is the number of electrons associated with this stripping. F and NA are the Faraday and Avogadro constants, respectively. MA0.8 is the mass activity measured at 0.8 V vs. RHE at pH = 1 and MSD is the SD expressed in mol g -1 . In this work, the SD values were calculated considering a 3 electrons process for the reduction reaction with the formation of NH3OH + 258 .

A.2.9 Inductively coupled plasma mass spectrometry (ICP-MS)

A PerkinElmer NexION 2000c instrument was used. Daily prepared iron ( 56 Fe) solutions were used for the calibration curves. Five solutions were prepared from commercial standard mono-element ICP-MS solution (Carl Roth GmbH & co. KG, 1000 mg L -1 ) in order to obtain five-point calibration curves at 0, 2, 5, 10, and 20 µg L -1 . Rhodium ( 103 Rh) was used as the internal standard. To avoid any polyatomic interferences, the helium collision mode was used.

Figure 1 . 1

 11 Figure 1.1 Total CO2 emissions from energy combustion and industrial processes from 1900-2021 2 .

Figure 1 . 2

 12 Figure 1.2 Schematic of a single cell of PEMFC.

Figure 1 . 3

 13 Figure 1.3 Influence of the exchange current density, j0, on the j-η response when n=1 and α=0.5. (a) j0 = 0.1, (b) j0 = 0.01, and (c) j0 = 0.001 A cm -2 14 .

Figure 1 . 4

 14 Figure 1.4 Schematic of the MEA in a single cell of PEMFC 16 . Anode: Pt/C catalyst, Cathode: Fe-Mn-N-C catalyst.

Figure 1 . 5

 15 Figure 1.5 Diagram of the cost of different part in proton exchange membrane fuel cell 10 .

Figure 1 . 6

 16 [START_REF] Zhang | Are Fe-N-C Electrocatalysts an Alternative to Pt-Based Electrocatalysts for the Next Generation of Proton Exchange Membrane Fuel Cells?[END_REF] . The O2 molecule is first adsorbed and activated on the surface of the catalyst to form O2* (I), where * represents the active site of the catalyst. The O-O bond is then cleaved in the following three different ways: (i) Dissociative pathway: the O-O bond is cleaved directly into two O* intermediates (II), which are then successively reduced to OH* (III and IV) and H2O* and eventually end up with desorption of H2O*(V); (ii) Associative-dissociative pathway: OOH* is first generated from O2* (VI), and then the O-O bond is cleaved into O* and OH* (VII), followed by their successive reductions to form H2O (IV and V); (iii) Associative pathway: the O2* is sequentially reduced to OOH* (VI) and HOOH* (VIII), then 2 OH* can be generated and finally reduced to H2O* (IX)

Figure 1 . 6

 16 Figure 1.6 Reaction schemes of ORR process in acidic media 39 .

Figure 1 . 7

 17 Figure 1.7 Relationships between experimentally measured specific activity for the ORR on Pt3M surfaces in 0.1 M HClO4 versus the d-band center position 50 .

Figure 1 . 8

 18 Figure 1.8 Side views and top views of the proposed structures of Fe-Nx species: (Doublet 1): the Fe-N4/C catalytic site in heat-treated, macrocycle-based catalysts. (Doublet 2): the Fe-N2+2 like micropore-hosted site found in the catalyst prepared with Fe acetate and heat-treated in ammonia 88 .

FeFigure 1 . 9

 19 Figure 1.9 Schematic of "out-of-plane" Fe(II)N4C12 D1 site and "in-plane" Fe(II)N4C10 D2 site .

Figure 1 . 10

 110 Figure 1.10 Comparison between the K-edge XANES experimental spectrum of Fe (black dashed lines) and the theoretical spectrum calculated with the depicted structures (solid red lines). (a) FeN4C10 moiety; (b) FeN2+2C4+4 moiety; (c) FeN4C12 moiety; (d) FeN4C12 moiety with one O2 molecule adsorbed in end-on mode; (e) FeN4C12 moiety with two O2 molecules adsorbed in end-on mode; (f) FeN4C10 moiety with one O2 molecule adsorbed in side-on mode. The brown, blue, grey and red spheres represent iron, nitrogen, carbon and oxygen atoms respectively 98 .

Figure 1 .

 1 11 shows the polarization curves of synthetic catalysts as the function of the metal precursor in acidic media (Figure 1.11 a) and alkaline media (Figure 1.11 b)

Figure 1 . 11

 111 Figure 1.11 Polarization curves of oxygen electrodes made from different catalysts in (a) 2.25 M H2SO4; (b) 7 M KOH at 25º C. 115

Figure 1 . 12

 112 Figure 1.12 Effect of metal atoms on the polarization behavior for ORR at various heat-treated metalloporphyrins coated RDE in oxygen saturated 0.5 M H2SO4 solution 132 .

Figure 1 .

 1 Figure 1.13 (a-c) Schematic illustration of the synthesis of Fe-N-C catalysts using PANI and Phen as nitrogen sources; (d) ORR polarization plots of different Fe-N-C catalysts (0.5 M H2SO4; electrode rotation speed: 900 rpm; loading: 0.6 mg cm - 2 ; scan rate: 10 mV s -1 ); (e) electron transfer number and H2O2 yield of prepared catalysts. 128

Dodelet

  et al. investigated the catalytic activity of Fe-N-C materials prepared with various Fe contents 138 , as seen in Figure 1.14a. Two curves show different tendencies of two catalysts prepared with different Fe precursors, FeAc or Fe tetramethoxyphenyl porphyrin (FeTMPP).

Figure 1 .

 1 Figure 1.14 (a) Changes in RDE of the catalytic activity for ORR, expressed as Vpr (Voltage at peak reduction for 0 rpm) with the Fe content for materials prepared with FeAc or FeTMPP as precursors; (b) polarization curves obtained at 80 °C in H2-O2 fuel cell using the catalysts prepared with FeAc and FeTMPP as precursors 138 .

Figure 1 .

 1 Figure 1.15 (a) Activity for ORR at room temperature and (b) micropore specific area against metal content for Fe (■) and Co (▲)-based catalysts 117 .

Figure 1 . 16

 116 Figure 1.16 Change in catalytic activity (from Vpr) with (a) total N surface concentration of the catalysts from XPS results; (b) total O surface concentration of the catalysts from XPS results; (c) total specific surface area of the catalysts from BET; (d) micropore surface area of the catalysts from BET 145 .

Figure 1 .

 1 Figure 1.17 (a) Polarization curves for the five Fe-N-C catalysts, the two I/C catalysts, and a commercial 20 wt.% Pt/C catalyst recorded in RDE at 900 rpm in O2-saturated 0.5 M H2SO4; (b) BJH desorption pore-size distribution of the Fe-N-C catalysts 133 .

Figure 1 . 18

 118 Figure 1.18 The schematic illustration for the non-solution fabrication process of Fe-N-C catalyst 121 .

Figure 1 . 19

 119 Figure 1.19 Relative hydrogen peroxide formation in dependence of the potential for the original catalyst (OC) and samples #E (NH3-treated catalyst) and #F (acid-leached product of #E) 158 .

Figure 1 . 20

 120 Figure 1.20 Images of gelation (a) C/W is large; (b) C/W is small 163 .

Figure 1 . 21

 121 Figure 1.21 Schematic representation of the preparation of RMF frameworks and their optimized network fragments 165 .

Figure 1 .

 1 22 a, trace Fe plays a crucial role in ORR activity. The uniform Fe-N4 active sites account for excellent ORR activity of Fe-N-CA catalysts. The most active catalyst (Fe-N-CA-800) has a positive Eonset of 0.92 V, which is only slightly lower than that of Pt/C. The excellent mass transfer property of carbon aerogel support suggests a promising application in Fe-N-C catalysts.

Figure 1 .

 1 Figure 1.22 (a) Polarization curves of different samples at 1600 rpm in O2-saturated 0.1 M KOH solution; (b) kinetic current at 0.7 V of different samples 180 .

Figure 1 . 23

 123 Figure 1.23 Schematic illustration of carbon corrosion and the ensuring loss of micropore-hosted and graphene-defect Fe-Nx sites. N: blue, Fe: red 184 .

Figure 2

 2

Figure 2 . 1

 21 Figure 2.1 Schematic illustration of synthesis procedure of Fe-N-C aerogel catalysts.

Figure 2 .

 2 Figure 2.2 (a) ZrO2 pot and (b) balls for planetary ball milling method.

Figure 2 . 3

 23 Figure 2.3 Flow diagram of two grinding methods.

Figure 2 . 4

 24 Figure 2.4 The distribution of the ball milled particles sizes for (a) AN1-1 and (b) AN 1-2.

Figure 2 .

 2 Figure 2.5 (a) Positions of samples in tube furnace and direction of NH3 flow; Polarization curves of samples at different positions treated in (b) pure NH3 and (c) 10% NH3.

  Figure 2.6.

Figure 2 .

 2 Figure 2.6 (a) Three-electrode cell with the rotating disk electrode and (b) parameters of ZIR.

Figure 2 .

 2 Figure 2.7 (a) Polarization curves and (b) Tafel slopes of the catalyst prepared with different ink formula.

Figure 2 . 8

 28 Figure 2.8 (a) 5 cm 2 dried cathode with catalyst (Fe-N-C) loading of 4.1 mg cm -2 ; (b) Instrument of single cell tests.

Figure 2 . 9

 29 Figure 2.9 Investigation of ink formulations for PEMFC (a) H2-O2 polarization curves, (b) Nyquist plots, and (c) CV curves of catalyst FeCl3-R/M=2/1 with different I/C values.

Figure 3 .

 3 1e shows the isotherm curves at -196°C of the catalysts, and Figure 3.1f gives the curves of pore size distribution, analyzed by using the 2D-NLDFT-Heterogenerous surface calculation method. The results are listed in Table

Figure 3 .

 3 Figure 3.1 (a)-(d) SEM and (e) N2 adsorption isotherms at -196 °C of the catalysts prepared with different iron precursors;(f) pore size distribution plots obtained by using the 2D-NLDFT-Heterogenerous Surface method.

Figure 3 .

 3 Figure 3.2 (a)-(d) TEM images and (e) normalized XRD patterns of catalysts prepared from different Fe precursor.

Figure 3 .

 3 Figure 3.3 (a) N 1s and (b) Fe 2p XPS spectra of the catalyst prepared with FeCl3 precursor.

Figure 3 .

 3 Figure 3.4 (a) Cyclic voltammetry curves in N2-saturated 0.05 mol L -1 H2SO4; (b) Oxygen reduction reaction polarization curves in O2-saturated 0.05 mol L -1 H2SO4; (c) Tafel plots derived from polarization curves of the catalysts prepared with different iron precursors; and (d) normalized current after 500 CV cycles between 0.6-1.0 V in N2 saturated electrolyte.

Figure 3 . 5

 35 Figure 3.5 Polarization curves of the catalysts prepared with different iron precursors in the single cell. The measurements were performed with Tcell = 80°C, 100 % RH, Pcathode = Panode = 2 barabs, the H2, and O2 flow rates were 150 mL min -1 while the air flow rate was 480 mL min -1 .

Figure 3 .

 3 6a and b provide a basic view of the porosity of catalysts prepared at different pH. It can be seen clearly that catalyst prepared in pH=6 displays a more open porous structure. The BET area and pore size distribution obtained from nitrogen adsorption analysis demonstrate this observation, as shown in Figure 3.6c and d. The pore volumes are listed in Table

Figure 3 .

 3 Figure 3.6 (a), (b) SEM images of the catalysts prepared in different pH with FePc precursor; (c) N2 adsorption isotherms at -196 °C and (d) pore size distribution plots obtained by using the 2D-NLDFT-Heterogenerous Surface method.

Figure 3 .

 3 Figure 3.7 (a), (b) TEM images and (c) normalized XRD patterns of catalysts prepared in different pH with FePc precursor.

Figure 3 . 8 (

 38 Figure 3.8 (a) Cyclic voltammetry curves in N2 saturated 0.05 mol L -1 H2SO4; (b) Oxygen reduction reaction polarization curves; (c) Tafel plots derived from polarization curves of the catalysts prepared in different pH with FePc precursor; and (d) normalized current after 500 CV cycles between 0.6-1.0 V in N2 saturated electrolyte.

Figure 3 . 9

 39 Figure 3.9 Polarization curves of the catalysts prepared in different pH with FePc precursor in the single cell. The measurements were performed with Tcell = 80°C, 100 % RH, Pcathode = Panode = 2 barabs, the H2, and O2 flow rates were 150 mL min -1 while the air flow rate was 480 mL min -1 .

  2+ has been widely used as an indicator in analytical chemistry due to its excellent stability and solubility in water. It is interesting because the formation constant of [Fe(II)Phen3] 2+ is larger than [Fe(III)Phen3] 3+ , 10 21 , and 10 14 , respectively, which is contrary to most of the ligands 216 . Abundant Fe-N connections make Phen a promising ligand to prepare the catalysts with better electrochemical activity. Ziloto et al. synthesized Co-N-C with improved activity by introducing Phen into Co precursor 217 .Li et al. mixed iron acetate (FeAc) with Phen by ball milling method to get the precursor with Fe-N6 coordination. The obtained catalyst also exhibited better electrochemical activity than the catalyst prepared without Phen 136 . Here, we synthesized the catalysts with two different ligands (EDTA and Phen) and two Fe precursors with different oxide states (Fe(NO3)3 and (NH4)2Fe(SO4)2), as shown in Table3.7.

Figure 3 .

 3 Figure 3.10a and b show the SEM images of the catalysts prepared with the Fe(NO3)3 precursor and different ligands alongside the catalyst without ligand as a comparison.

Figure 3 .

 3 Figure 3.10 (a), (b) SEM images of the catalysts prepared with Fe(NO3)3 precursor and different ligand additives; (c) N2 adsorption isotherms at -196 °C and (d) pore size distribution plots obtained by using the 2D-NLDFT-Heterogenerous Surface method.

Figure 3 .

 3 Figure 3.11 (a)-(d) SEM images of the catalysts prepared with (NH4)2Fe(SO4)2 precursor and different ligand additives; (e) N2 adsorption isotherms at -196 °C and (f) pore size distribution plots obtained by using the 2D-NLDFT-Heterogenerous Surface method.

Figure 3 .

 3 12a-c and Figure 3.13a-c summarize the TEM images for catalysts synthesized with ligand additives. As for the catalysts prepared without ligand, no Fe-rich nanoparticle can be observed under the view of TEM, indicating that Fe atoms are well dispersed in the carbon matrix. Besides, no significant peak of Fe-containing species can be identified from XRD patterns of catalysts, as shown in Figure 3.12d and Figure 3.13d, confirming the observation of TEM.

Figure 3 .

 3 Figure 3.12 (a)-(c) TEM images and (d) normalized XRD patterns of catalysts prepared with Fe(NO3)3 and different ligand additives.

Figure 3 .

 3 Figure 3.13 (a)-(c) TEM images and (d) normalized XRD patterns of catalysts prepared with (NH4)2Fe(SO4)2 and different ligand additives.

Figure 3 .

 3 Figure 3.14 (a) Cyclic voltammetry curves in N2 saturated 0.05 mol L -1 H2SO4; (b) Oxygen reduction reaction polarization curves; (c) Tafel plots derived from polarization curves of the catalysts prepared with Fe(NO3)3 and different ligand additives; and (d) normalized current after 500 CV cycles between 0.6-1.0 V in N2 saturated electrolyte.

Figure 3 .

 3 Figure 3.15 (a) Cyclic voltammetry curves in N2 saturated 0.05 mol L -1 H2SO4; (b) Oxygen reduction reaction polarization curves; (c) Tafel plots derived from polarization curves of the catalysts prepared with (NH4)2Fe(SO4)2 and different ligand additives; and (d) normalized current after 500 CV cycles between 0.6-1.0 V in N2 saturated electrolyte.

Figure 3 . 16

 316 Figure 3.16 Polarization curves of the catalysts prepared with (a) Fe(NO3)3 or (b) (NH4)2Fe(SO4)2 and different ligand additives in the single cell. The measurements were performed with Tcell = 80°C, 100 % RH, Pcathode = Panode = 2 barabs, the H2, and O2 flow rates were 150 mL min -1 while the air flow rate was 480 mL min -1 .

Figure 3 . 17

 317 Figure 3.17 (a), (b) SEM images of the catalysts prepared with different quantities of Fe precursors at R/M=2/1; (c) N2

  Figure 3.17(a), (b) SEM images of the catalysts prepared with different quantities of Fe precursors at R/M=2/1; (c) N2 adsorption isotherms at -196 °C and (d) pore size distribution plots obtained by using the 2D-NLDFT-Heterogenerous Surface method.

Figure 3 .

 3 18d and e show the N2 adsorption isotherm curves at -196 °C and corresponding pore size distribution plots. The pore volumes are listed in the Table

Figure 3 .

 3 Figure 3.18 (a), (b) SEM images of the catalysts prepared with different quantities of Fe precursors at R/M=2/0; (c) N2 adsorption isotherms at -196 °C and (d) pore size distribution plots obtained by using the 2D-NLDFT-Heterogenerous Surface method.

Figure 3 .

 3 Figure 3.19 (a)-(d) TEM images and (e) normalized XRD patterns of catalysts prepared with different quantities of Fe precursors at R/M=2/1.

Figure 3 .

 3 Figure 3.20 (a)-(d) TEM images and (e) normalized XRD patterns of catalysts prepared with different quantities of Fe precursors at R/M=2/0.

Figure 3 .

 3 Figure 3.21 (a) Cyclic voltammetry curves in N2 saturated 0.05 mol L -1 H2SO4; (b) Oxygen reduction reaction polarization curves; (c) Tafel plots derived from polarization curves of the catalysts prepared with different quantities of Fe precursors at R/M=2/1; and (d) normalized current after 500 CV cycles between 0.6-1.0 V in N2 saturated electrolyte.

Figure 3 .

 3 Figure 3.22 (a) Cyclic voltammetry curves in N2 saturated 0.05 mol L -1 H2SO4; (b) Oxygen reduction reaction polarization curves; (c) Tafel plots derived from polarization curves of the catalysts prepared with different quantities of Fe precursors at R/M=2/0; and (d) normalized current after 500 CV cycles between 0.6-1.0 V in N2 saturated electrolyte.

  -C asymmetric vibration of methylene ether bridges between resorcinols 223,227 1092 C-O-C symmetric vibration of methylene ether bridges between resorcinols 223,227 1000 N-CH2-O stretching vibration of methylene ether bridges between melamine and resorcinol 225,226 810 Triazine ring 225

Figure 4 . 1

 41 Figure 4.1 FTIR spectrum of organic aerogel prepared with different R/M ratios and FeCl3 precursor. The spectra were normalized by the absorption peak at 1614 cm -1 .

Figure 4 . 2 ,

 42 the catalysts prepared with low M content (x<1, Figure 4.2a-c) display the familiar stacking appearance of covalently linked spherical units, and porous texture on the surface of particles can be observed. However, with the increasing of M content, a more compact texture appears at x=1.5 (Figure 4.2d), then completely occupies the view at x=2.5 (Figure 4.2f).

Figure 4 . 2 (

 42 Figure 4.2 (a)-(f) SEM images of the catalysts prepared with different R/M ratios and FeCl3 precursor.

Figure 4 . 3 (

 43 Figure 4.3 (a)-(f) TEM images of the catalysts prepared with different R/M ratios and FeCl3 precursor.

Figure 4 .

 4 Figure 4.4 (a) N2 adsorption isotherms at -196 °C and (b) pore size distribution plots obtained by using the 2D-NLDFT-Heterogenerous Surface method of the catalysts prepared with different R/M ratios and FeCl3 precursor.

Figure 4 . 5

 45 Figure 4.5 Structural and textural characterization of the catalysts prepared with different R/M ratios and FeCl3 precursor.

( a )

 a XRD patterns and (b) Raman spectra; (c) Graphite crystalline thickness in the direction perpendicular to the graphene layers obtained from XRD (Lc) and (d) in-plane graphite crystallite size obtained from Raman spectroscopy (La).

Figure 4 .

 4 6b presents the variation of different N species with increasing M content. It shows that only the pyrrolic N content is positively correlated with the M content. In contrast, the other N-species are almost constant or only increase slightly with increasing M content. Moreover, the molar ratio of pyrrolic N/Fe is close to 5.4 at R/M=2/0, then decreases with increasing M content, and finally, the values are stable at around 4 until FeCl3-R/M=2/1.5. This number corresponds well to the stoichiometry in Fe-N4 sites, suggesting all nitrogen atoms involved in the ligation of Fe are pyrrolic N. The values are decreased to around 3 for FeCl3-R/M=2/2 and FeCl3-R/M=2/2.5, suggesting the existence of other Fe species. The possible reason of relatively high pyrrolic N/Fe values at low M content is the Fe content is too low to full fill all N4C12 macrocycles. It should be noted that all N atoms in catalyst FeCl3-R/M=2/0 were introduced by ammonia treatment, whereas the absolute contents of all N species are almost unchanged, except pyrrolic N. These results imply M content mainly affects the pyrrolic N content, which relates to the Fe content, whereas the condition of NH3 treatment modulates the other N components.

Figure 4 . 6 (

 46 Figure 4.6 (a) N 1s XPS spetra of the catalysts prepared with different R/M ratios and FeCl3 precursor; (b) Evolution of different N species content with R/M.

Figure 4 .

 4 Figure 4.7 (a) XANES and (b) Fourier transforms of the EXAFS spectra measured at Fe K-edge. Mössbauer spectroscopy characterization of selected catalysts. (c) FeCl3-R/M = 2/0.5, (d) FeCl3-R/M = 2/1, (e) FeCl3-R/M = 2/2, and (f) FeCl3-R/M = 2/2.5. The spectra were acquired at -268 º C.

Figure 4 . 8

 48 Figure 4.8 Electrochemical characterization of catalysts prepared from different R/M ratio and FeCl3 precursor recorded in 0.05 M H2SO4. (a) CV curves in N2-saturated electrolyte; (b) Polarization curves in O2-saturated electrolyte at  = 1600 rpm and v = 10 mV s -1 ; (c) Tafel plots of the O2-transport and background-current corrected kinetic current obtained from the steady-state Ohmic drop-corrected I-E curves at = 1600 rpm; (d) normalized current after 500 CV cycles between 0.6-1.0 V in N2 saturated electrolyte; relationships between (e) mass activity in RDE setup and Fe content, (f) mass activity in RDE setup and pyrrolic N content.

Figure 4

 4 Figure 4.9 Evaluation pathway of Fe-rich nanoparticles to Fe-N 4 active site during the pyrolysis in poor/rich melamine content aerogel.

Figure 4 . 10

 410 Figure 4.10 Investigation of the set of aerogel Fe-N-C catalysts as PEMFC cathodes. (a) H2/O2 PEMFC polarization curves. (b) Current density at 0.8 V as a function of the melamine content at the aerogel preparation stage. (c) H2-air PEMFC polarization curves. (d) Comparison of the current density at 0.3 V measured with O2 or air, for each catalyst. The measurements were performed with Tcell = 80°C, 100 % RH, Pcathode = Panode = 2 barabs, the H2, and O2 flow rates were 150 mL min -1 while the air flow rate was 480 mL min -1 .

Figure 4 .

 4 Figure 4.11 (a)-(c) SEM images of the catalysts prepared with different Fe precursors (d) N2 adsorption isotherms at -196 °C and (e) pore size distribution plots obtained by using the 2D-NLDFT-Heterogenerous Surface method.

Figure 4 .

 4 Figure 4.12 (a)-(c) TEM images and (d) normalized XRD patterns of the catalysts prepared with different Fe precursors at R/M=2/2.

Figure 4 . 13

 413 Figure 4.13 Electrochemical characterization of catalysts prepared with different Fe precursors with R/M=2/2. The catalysts prepared with low M content and FePc precursor were also presented as the comparison. (a) CV curves in N2-saturated electrolyte; (b) polarization curves in O2-saturated electrolyte at  = 1600 rpm and v = 10 mV s -1 ; (c) Tafel plots derived from polarization curves; and (d) normalized mass activity after 500 CV cycles between 0.6-1.0 V in N2 saturated electrolyte; (e) relationships between mass activity in RDE setup and Fe content determined by XPS or ICP-MS respectively; (f) relationships between mass activity in RDE setup and pyrrolic N content.

Figure 4 .

 4 Figure 4.14a compares the polarization curves in the RDE setup of all catalysts prepared with different Fe precursors at R/M=2/2, along with the curve of commercial Pajarito catalyst (Pajarito powder PMF D14401). All catalysts show good mass-transport properties which is verified by their sharp kinetic-to-diffusion regions. Among all, Fe(NO3)3-R/M=2/2 presents the highest half-wave potential and mass activity at 0.8 V, consistent with the result in section 3.1 that Fe(NO3)3 precursor shows the best activity among the catalysts prepared with different precursors at R/M=2/1. FePc-R/M=2/2 has the highest limited current density at the diffusioncontrolled region, corresponding to its highest mesopore volume. Figure4.14b shows the polarization curves of these catalysts in H2/O2 PEMFC, with the ink formulation keeping the I/C ratio of 2. The trend of mass activity is in line with the trends observed in RDE that the Fe(NO3)3-R/M=2/2 shows the highest current density of 113 mA cm -2 at 0.8 V and the current density of 44 mA cm -2 is reached at 0.85 V, which is only 0.05 V lower than the DOE 2025 target for platinum group metal-free electrocatalysts 156 . Meanwhile, no polarization curve is collapsed even for the current density reaches ca. 1700 mA cm -2 at 0.3 V, indicating the excellent mass transport properties of Fe(NO3)3-R/M=2/2. Similar to the observation in the series of FeCl3 precursor, the quasi-absence of mesopores in Fe(NO3)3-R/M=2/2 (0.02 cm 3 g -1 ) does not affect the accessibility of gaseous oxygen and evacuation of water, indicating the mesopores are not necessary for the catalyst to reach high H2/O2 PEMFC performance.

Figure 4 . 14

 414 Figure 4.14 Polarization curves in (a) RDE setup and (b) H2/O2 PEMFC of catalysts prepared with different Fe precursors at R/M=2/2. The commercial catalyst Pajarito powder is presented as the comparison.

Figure 5 . 1

 51 Figure 5.1 Schematic illustration of samples extracted at different synthesis procedure.

Figure 5 .

 5 Figure 5.2 a and b show the SEM images of catalysts prepared by different second heat treatment conditions.Both catalysts exhibit a highly porous structure and similar mesopore size, indicating that mesopores have been generated during the first pyrolysis. The specific surface area and the pore size distribution of catalysts were then further investigated by nitrogen adsorption method. Figure5.2c shows the isotherm curves measured at -196 ºC . If we ignore the adsorption quantity, the catalysts treated under different atmospheres show similar isotherm curves, including features associated with micro/mesoporous materials. Both curves present a steep rise at low relative pressures, indicating the presence of micropores. At higher relative pressures, a hysteresis loop appears due to the capillary condensation of mesopores. The difference between two isothermal curves is that FePc NH3 has a much higher adsorption quantity than FePc N2, especially in the region of low relative pressures, indicating a larger micropore volume. Thus, FePc NH3 also has a larger BET area than FePc N2 (Table5.1). The pore size distribution curves analyzed using the 2D-NLDFT-Heterogenerous surface calculation method are shown in Figure5.2d, and the corresponding textural characteristics are listed in Table5.1. In line with the observation from isothermal curves and SEM images, the micropore volume of FePc NH3 is more than 2-fold that of FePc N2 (0.43 vs. 0.18 cm 3 g -1 ), while the mesopore volume only increases ca. 1.5-fold (1.51 vs. 1.01 cm 3 g -1 ). Since N2 is inert to carbon at 950 º C, it can be considered that the mesopores in FePc N2 are mainly inherited from gel formation and particle organization. The micropores in FePc N2 come from the reorganization of carbon atoms during pyrolysis. The difference in pore size distribution between FePc NH3 and FePc N2 can be attributed to the pure effect of NH3 etching. According to Figure5.2d, the peak of micropore size increases from 0.5 nm to 1.0 nm after NH3 treatment due to the gasification of amorphous carbon. On the contrary, the mesopore size is almost identical (around 10 nm), and the volume of mesopores is less impacted because the mesopore walls are constructed by small graphitic domains with more sluggish kinetic on the reaction with NH3. Since the amorphous carbon can be located between graphitic domains on the walls, the mesopore volumes can be slightly increased by removing these areas, and micropores are then created on the mesopore walls187,194,231 .

Figure 5 .

 5 Figure 5.3 (a)-(d) TEM images of the samples extracted at different steps and (e) normalized XRD patterns.

Figure 5 . 4 (

 54 Figure 5.4 (a) XANES and (b) Fourier transforms of the EXAFS spectra measured at Fe K-edge of the samples extracted at different steps.

Figure 5 .

 5 Figure 5.5 a) N 1s XPS spetra of the samples extracted at different steps; (b) Evolution of different N species content with the different synthesis steps.

Figure 5 . 6

 56 Figure 5.6 Electrochemical characterization of the samples extracted at different steps recorded in 0.05 M H2SO4. (a) CV curves in N2-saturated electrolyte; (b) Polarization curves in O2-saturated electrolyte at  = 1600 rpm and v = 10 mV s -1 ; (c) Tafel plots of the O2-transport and background-current corrected kinetic current obtained from the steady-state Ohmic drop-corrected I-E curves at = 1600 rpm; Polarization curves of catalysts before/after 2000 CV cycles between 0.6-1.0 V in N2 and O2-saturated electrolyte respectively (d) FePc N2 and (e) FePc NH3; (f) normalized mass activity at 0.8 V after durability test.

Figure 5 . 7

 57 Figure 5.7 H2/O2 PEMFC polarization curves of the catalysts prepared with different second heat treatment conditions.

Figure 5 . 8

 58 Figure 5.8 Comparison between carbon substrates before and after NH3 treatment.

Figure 6 .

 6 Figure 6.1 (a)-(c) SEM images of the catalysts doped by different molar ratios of Ce; (d) N2 adsorption isotherms at -196 °C and (e) pore size distribution plots obtained by using the 2D-NLDFT-Heterogenerous Surface.

Figure 6 .

 6 Figure 6.2 (a)-(c) TEM images of the catalysts doped by different molar ratios of Ce and (d) normalized XRD patterns.

Figure 6 . 3

 63 Figure 6.3 Electrochemical characterization of the catalysts doped by different molar ratios of Ce recorded in 0.05 M H2SO4. (a) CV curves in N2-saturated electrolyte; (b) Polarization curves in O2-saturated electrolyte at  = 1600 rpm and v = 10 mV s -1 ; (c) Tafel plots of the O2-transport and background-current corrected kinetic current obtained from the steady-state Ohmic drop-corrected I-E curves at = 1600 rpm; (d) normalized mass activity at 0.8 V after 500 CV cycles between 0.6-1.0 V in N2-saturated electrolyte.

Figure 6 .

 6 Figure 6.4 (a) Polarization curves and (b) durability tests in H2/O2 PEMFC of catalysts doped by different molar ratios of Ce. The measurements were performed with Tcell = 80°C, 100 % RH, Pcathode = Panode = 2 barabs, the H2, and O2 flow rates were 150 mL min -1 while the air flow rate was 480 mL min -1 .

Figure 6 . 5

 65 Figure 6.5 Change of the chemical state of protonable N-groups and simultaneous change of the TOF for the ORR of Fe-N4 active sites when the Fe-N-C catalyst is in H2SO4 electrolyte 107 .

Figure 6 .

 6 Figure 6.6 (a)-(d) SEM images of the catalysts doped by different molar ratios of Ce; (e) N2 adsorption isotherms at -196 °C and (f) pore size distribution plots obtained by using the 2D-NLDFT-Heterogenerous Surface.

Figure 6 .

 6 Figure 6.7 (a)-(d) TEM images of the catalysts doped by different molar ratios of Ce and (e) normalized XRD patterns.

Figure 6 . 8

 68 Figure 6.8 Electrochemical characterization of the catalysts doped by different molar ratios of Ce recorded in 0.05 M H2SO4. (a) CV curves in N2-saturated electrolyte; (b) Polarization curves in O2-saturated electrolyte at  = 1600 rpm and v = 10 mV s -1 ; (c) Tafel plots of the O2-transport and background-current corrected kinetic current obtained from the steady-state Ohmic drop-corrected I-E curves at = 1600 rpm; (d) normalized mass activity at 0.8 V after 500 CV cycles between 0.6-1.0 V in N2-saturated electrolyte.

Figure 6 .

 6 Figure 6.9 (a) Polarization curves and (b) durability tests in H2/O2 PEMFC of catalysts doped by different molar ratios of Ce. The measurements were performed with Tcell = 80°C, 100 % RH, Pcathode = Panode = 2 barabs, the H2, and O2 flow rates were 150 mL min -1 while the air flow rate was 480 mL min -1 .

Table 1 . 1

 11 Summary of different types of fuel cell.

	Types	Electrolyte	Charge carriers	Operating temperature	Application
	SOFC	Metallic oxide solid ceramic	O 2-	500-1000 °C	Stationary power generation system 24
					Electric utility for
					natural gas and
	MCFC	Molten carbonate salt mixture	CO3 2-	500-1000 °C	coal-based power plants, industrial,
					and military
					applications 24
	PAFC	Phosphoric acid	H +	150-220 °C	High capacity power station 24
					Electric power in
					submarines,
	AFC	Potassium hydroxide	OH -	60-90 °C	boats, forklift trucks and niche
					transportation
					applications 24
	AAEMFC	Alkaline anion exchange membrane	OH -	60-100 °C	Energy storage device for grid applications 25
		Proton conductor			Portable and
	HT	polymer membrane	H +	120-180 °C	stationary power
	PEMFC	for high temperature			system, especially
	LT	Proton exchange membrane	H +	60-100 °C	in transportation 24,26

Table 1 . 2

 12 The possible reaction pathways during ORR in acidic media and their standard potentials.

Reaction E 0 /V vs SHE

	Direct 4e -pathway	O2 + 4H + +4e -→2H2O	1.23
	Peroxide (2e -) pathway	O2 + 2H + +2e -→H2O2 2H2O2+ 2H + +2e -→2H2O	0.67 1.77

Table 1 . 3

 13 Preparation precursors, support and pyrolysis condition of the Fe (or Co)/N/C catalysts.

	Metal precursor	Support	Heat treatment (°C; time in h; atmosphere)
	FeTPP-Cl 120	Vulcan XC-72R	700 ; 2 ; Ar
	FeTPP 121	rho-ZIF	950 ; 3 ; N2
	FeTPP 122	Vulcan XC-72R	1000 ; 2 ; Ar
	FePc 123	Vulcan XC-72R	300-600 ; 2 ; N2
	Ferrocene 122	Vulcan XC-72R	1000 ; 2 ; Ar
	Fe(Co)Ac 124	PTCDA	600-1000 ; 1 ; Ar/H2/NH3
	FeSO4-PAN 125,115	P33	850 ; 5 ; Ar
	Fe(NO3)3-PAN 126	-	800 ; 2 ; Ar
	FeAc-phenanthroline(phen) 127	Black Pearls 2000	1050 ; 1 ; Ar 950 ; 0.8 ; NH3
	FeAc-phen/PANI 128	KL 600 carbon black
	Fe(Phen)3 116	adenine	900 ; 1 ; Ar
	FeCl3-Phen 118	Mesoporous carbon	800 ; 1 ; N2
	FeCl3-2-ABI 129	KL 600 carbon black	950 ; 3 ; Ar
	Fe(NO3)3-PEI 130	-	700-900 ; 1-3 ; N2
	Co(NO3)2-PEI 131	Graphene oxide	850 ; 0.5 ; NH3
	1.3.3.1 Metal precursor and ligands

Table 1 .

 1 4.1 shows the details of the preparation of typical Fe(Co)/N/C electrocatalysts. The metal precursors are either organometallic (ferrocene), metal chelates (iron phenanthroline FePhen, iron phthalocyanine FePc), or simple Fe salts such as acetate or chloride. The nature of the metallic center in the precursor plays a crucial role in activity improvement after heat treatment. All research clearly show that iron and cobalt exhibit the highest activity towards ORR among all metal centers. Ohms et al. studied the influence of different transition metal ions on oxygen reduction

Table 1 .3, the

 1 most popular carbon supports are Vulcan XC 72 and Black Pearls. Jaouen et al. measured the ORR activity of several Fe-N-C catalysts prepared with different carbon black supports

Table 2 . 1

 21 Mass loss and catalytic activity of products in the different position of tube furnace treated with pure or 10% NH3.

	NH3 concentration	Position	Mass loss	Current density at 0.8 V (A/g)
		1	64%	3.15
	100%	2 3	72% 80%	2.56 1.78
		4	97%	1.70
		1	45%	2.09
	10%	2 3	49% 57%	1.91 1.82
		4	58%	2.06

Table 2 . 2

 22 Catalytic activity of a catalyst with different ink formula in RDE set up

	I/C	Nafion 5 wt% (μL)	Ethanol (μL)	H2O (μL)	Current density at 0.8 V (A g -1 )
	0.44	95	752	46	2.20
	1.4	332	515	46	1.80
	2.0	475	372	46	1.18
	2.5	594	253	46	0.78

Table 2 . 3

 23 Catalytic activity of a catalyst with different ink formula in MEA.

	I/C	Catalyst (mg)	Nafion 5 wt% (μL)	propanol (μL)	H2O (μL)

Current density at 0.8 V (mA cm -2 ) Current density at 0.5 V (mA cm -2 )

	1.0	20	458	300	272	2.39	103.15
	1.4	20	641	300	272	11.23	179.26
	2.0	20	915	300	272	36.62	580.17
	2.5	20	1145	300	272	26.20	472.49

Table 3 . 1

 31 Physicochemical characterization of the catalysts with different Fe precursors.

	Fe Precursor	Total mass loss wt.%	ICP Fe	BET surface (m 2

g -1 ) Vtotal (cm 3 g -1 ) Vmicro (cm 3 g -1 ) Vmeso (cm 3 g -1 ) wt.% at.%

	FeCl3	78	0.85	0.18	1596	2.20	0.44	1.67
	FePc	78	0.75	0.16	1318	0.94	0.44	0.44
	Fe(NO3)3	75	1.13	0.24	1498	1.62	0.42	1.09
	(NH4)2Fe(SO4)2	81	0.70	0.15	1665	1.92	0.46	1.34

Table 3 . 2

 32 Surface chemical composition of catalysts prepared with different Fe precursor and deconvolution of their N 1s spectra.

						Nitrile	Pyridinic	Fe-Nx	Pyrrolic	Quaternary	Graphitic
	Fe Precursor	C at.%	O at.%	Fe at.%	N at.%	at.% 398.0 eV	at.% 398.8 eV	at.% 399.9 eV	at.% 400.9 eV	at.% 402.0 eV	at.% 403.3 eV
	FeCl3	93.69	2.89	0.22	3.21	0.84	0.52	0.52	0.78	0.38	0.18
	Fe(II)Pc	93.01	3.78	0.22	3.04	0.87	0.42	0.55	0.72	0.33	0.15
	Fe(NO3)3	95.50	1.62	0.22	2.67	0.63	0.43	0.37	0.70	0.38	0.16
	(NH4)2Fe(SO4)2 95.37	1.38	0.18	3.08	0.85	0.50	0.43	0.82	0.35	0.13

Table 3 . 3

 33 Electrochemical characteristics of the catalysts with different Fe precursors in RDE setup.

	Fe Precursor	MA0.8 A g -1	MA0.8 after 500 cycles A g -1	Loss ratio	Tafel slope mV dec -1
	FeCl3	2.04	1.08	-47%	52
	FePc	1.72	1.34	-22%	66
	Fe(NO3)3	2.05	1.24	-40%	60
	(NH4)2Fe(SO4)2	1.89	0.84	-56%	55

Table 3 . 4

 34 Physicochemical characteristics of the catalysts in different pH with FePc precursor.

pH Total mass loss wt.% ICP Fe BET surface (m 2 g -1 ) Vtotal (cm 3 g -1 ) Vmicro (cm 3 g -1 ) Vmeso (cm 3 g -1 ) wt.% at.%

	6	77	0.61	0.13	1424	2.00	0.43	1.51
	8	78	0.75	0.16	1318	0.94	0.44	0.44

Table 3 . 5

 35 Chemical composition of catalyst surface prepared in different pH values with FePc precursor and deconvolution of their N 1s spectra.

	Fe

Precursor C at.% O at.% Fe at.% N at.% Nitrile at.% Pyridinic at.% Fe-Nx at.% Pyrrolic at.% Quaternary at.% Graphitic at.% 398.0 eV 398.8 eV 399.9 eV 400.9 eV 402.0 eV 403.3 eV

	6	94.77	1.89	0.26	3.09	0.73	0.55	0.46	0.86	0.36	0.14
	8	93.01	3.78	0.22	3.04	0.87	0.42	0.55	0.72	0.33	0.15

Table 3 . 6

 36 Electrochemical characteristics of the catalysts prepared in different pH with FePc precursor in RDE setup.

	Fe Precursor	MA0.8 A g -1	MA0.8 after 500 cycles A g -1	Loss ratio	Tafel slope mV dec -1
	6	1.67	1.38	-17%	60
	8	1.72	1.34	-22%	68

Table 3 . 7

 37 Physicochemical characterization of the catalysts prepared with Fe(NO3)3 or (NH4)2Fe(SO4)2 and different ligand additives.

	Fe Precursor	Ligand	Total mass loss wt.%	ICP Fe wt.% at.%	BET surface (m 2 g -1 )	Vtotal (cm 3 g -1 )	Vmicro (cm 3 g -1 )	Vmeso (cm 3 g -1 )
		EDTA	73	0.36	0.08	1263	2.59	0.33	2.22
	Fe(NO3)3	Phen	74	0.86	0.18	1226	2.62	0.34	2.23
		-	75	1.13	0.25	1498	1.62	0.42	1.09
		EDTA	72	0.16	0.03	1274	1.63	0.37	1.21
	(NH4)2Fe(SO4)2	Phen	74	0.73	0.16	1455	2.32	0.43	1.83
		-	81	0.70	0.15	1665	1.92	0.46	1.34

Table 3 . 8

 38 Chemical composition of catalyst surface prepared with Fe(NO3)3 and different ligand additives and deconvolution of their N 1s spectra.

						Nitrile	Pyridinic	Fe-Nx	Pyrrolic	Quaternary	Graphitic
	Ligand	C at.%	O at.%	Fe at.%	N at.%	at.% 398.0 eV	at.% 398.8 eV	at.% 399.9 eV	at.% 400.9 eV	at.% 402.0 eV	at.% 403.3 eV
	EDTA	94.07	3.19	0.11	2.63	0.73	0.35	0.48	0.66	0.31	0.10
	Phen	92.66	4.40	0.23	2.72	0.66	0.47	0.57	0.67	0.24	0.10
	-	95.50	1.62	0.22	2.67	0.63	0.43	0.37	0.70	0.38	0.16

Table 3 . 9

 39 Chemical composition of catalyst surface prepared with (NH4)2Fe(SO4)2 and different ligand additives and deconvolution of their N 1s spectra.

						Nitrile	Pyridinic	Fe-Nx	Pyrrolic	Quaternary	Graphitic
	Ligand	C at.%	O at.%	Fe at.%	N at.%	at.% 398.0 eV	at.% 398.8 eV	at.% 399.9 eV	at.% 400.9 eV	at.% 402.0 eV	at.% 403.3 eV
	EDTA	93.92	2.72	0.06	3.30	0.95	0.45	0.51	0.80	0.40	0.19
	Phen	93.20	3.85	0.20	2.76	0.74	0.40	0.45	0.66	0.36	0.15
	-	95.37	1.38	0.18	3.08	0.85	0.50	0.43	0.82	0.35	0.13

Table 3 .

 3 10 Electrochemical characteristics of the catalysts with Fe(NO3)3 or (NH4)2Fe(SO4)2 and different ligand additives in RDE setup.

	Fe Precursor	Ligand	MA0.8 A g -1	MA0.8 after 500 cycles A g -1	Loss ratio	Tafel slope mV dec -1
		EDTA	0.80	0.27	-67%	67
	Fe(NO3)3	Phen	1.32	0.73	-48%	63
		-	2.05	1.24	-40%	60
		EDTA	1.01	0.64	-39%	68
	(NH4)2Fe(SO4)2	Phen	1.74	1.08	-40%	63
		-	1.89	0.84	-56%	55

Table 3 .11 Physicochemical

 3 characteristics of the catalysts with different quantities of Fe precursors and different R/M ratio.

	Name	R/M

Theoretical Fe content wt.% Total mass loss wt.% ICP Fe BET surface m 2 g -1 Vtotal cm 3 g -1 Vmicro cm 3 g -1 Vmeso cm 3 g -1 wt.% at.%

	FePc 0.5wt.% R/M=2/1		0.5	80%	0.50	0.11	1574	2.06	0.47	1.51
	FePc 1wt.% R/M=2/1	2/1	1	77%	0.61	0.13	1424	2.00	0.43	1.51
	FePc 1.5wt.% R/M=2/1		2	74%	1.51	0.33	1282	2.39	0.37	1.97
	FePc 0.5wt.% R/M=2/0		1	73%	0.56	0.12	1533	2.78	0.41	2.28
	FePc 1wt.% R/M=2/0	2/0	2	66%	1.86	0.41	1300	2.53	0.35	2.13
	FePc 1.5wt.% R/M=2/0		3	68%	2.33	0.51	1147	2.13	0.31	1.77

Table 3 .

 3 12 Chemical composition of catalysts surface prepared with different quantities of Fe precursors at R/M=2/1 and deconvolution of their N 1s spectra.

	Theoretical Fe content wt.%	C at.%	O at.%	Fe at.%	N at.%	Nitrile at.% 398.0 eV	Pyridinic at.% 398.8 eV	Fe-Nx at.% 399.9 eV	Pyrrolic at.% 400.9 eV	Quaternary at.% 402.0 eV	Graphitic at.% 403.3 eV
	0.5	94.38	1.98	0.20	3.45	0.86	0.60	0.56	0.92	0.36	0.15
	1	94.77	1.89	0.26	3.09	0.73	0.55	0.46	0.86	0.36	0.14
	2	93.33	3.85	0.27	2.55	0.62	0.35	0.43	0.66	0.32	0.16

Table 3 .

 3 13 Surface chemical composition of catalysts prepared with different quantities of Fe precursors at R/M=2/0 and deconvolution of their N 1s spectra.

	Theoretical Fe content wt.%	C at.%	O at.%	Fe at.%	N at.%	Nitrile at.% 398.0 eV	Pyridinic at.% 398.8 eV	Fe-Nx at.% 399.9 eV	Pyrrolic at.% 400.9 eV	Quaternary at.% 402.0 eV	Graphitic at.% 403.3 eV
	1	94.87	2.04	0.18	2.72	0.84	0.55	0.51	0.48	0.24	0.10
	2	96.22	1.69	0.21	1.87	0.54	0.34	0.30	0.41	0.21	0.08
	3	96.09	1.74	0.22	1.95	0.56	0.36	0.32	0.41	0.21	0.09

Table 3 .

 3 14 RDE tests of the catalysts with different quantities of Fe precursors at different R/M ratio.

	R/M

Theoretical Fe content wt.% MA0.8 A g -1 MA0.8 after 500 cycles A g -1 Loss ratio Tafel slope mV dec -1

		0.5	0.85	0.56	-34%	65
	2/1	1	1.67	1.09	-35%	61
		2	2.16	0.85	-66%	60
		1	0.67	0.32	-52%	55
	2/0	2	0.51	0.26	-49%	66
		3	0.89	0.49	-45%	62

Table 4 . 1

 41 Specific quantities of reagents for preparing Fe-N-C aerogel catalysts with different R/M ratios and FeCl3 precursor.

	R/M	Resorcinol	Melamine	Formaldehyde	FeCl3	Na2CO3
		g	g	mL	g	g
	2/0	3.6	0	5.00	0.066	0.030
	2/0.5	3.6	1	6.75	0.075	0.044
	2/1	3.6	2	8.50	0.080	0.058
	2/1.5	3.6	3	10.25	0.095	0.072
	2/2	3.6	4	12.00	0.110	0.086
	2/2.5	3.6	5	13.75	0.124	0.100

Table 4 .

 4 2 FTIR band assignment of Fe doped RMF aerogel spectra.

Table 4 . 3

 43 Physicochemical characterization of the catalysts with different R/M ratios and FeCl3 precursor.

	R/M	Total mass loss wt.%	ICP Fe wt.%	at.%	BET surface (m 2 g -1 )	Vtotal (cm 3 g -1 )	Vmicro (cm 3 g -1 )	Vmeso (cm 3 g -1 )
	2/0	74%	0.33	0.07	1481	1.37	0.44	0.83
	2/0.5	78%	0.49	0.10	1595	2.13	0.43	1.60
	2/1	78%	0.85	0.18	1596	2.20	0.44	1.67
	2/1.5	77%	1.17	0.25	1256	0.87	0.42	0.38
	2/2	81%	1.36	0.30	1271	0.78	0.45	0.25
	2/2.5	80%	1.57	0.34	1099	0.46	0.42	0.02

Table 4 . 4

 44 La and Lc values of the catalysts prepared with different R/M ratios and FeCl3 precursor.

	R/M	La nm	Lc nm
	2/0	1.11	1.7
	2/0.5	1.06	1.5
	2/1	0.93	1.2
	2/1.5	0.95	1.2
	2/2	0.93	1.2
	2/2.5	0.89	1.1

Table 4 . 5

 45 Surface chemical composition of catalysts prepared with different R/M ratios and deconvolution of their N 1s spectra.

	R/M	C at.%

O at.% Fe at.% N at.% Nitrile at.% Pyridinic at.% Fe-Nx at.% Pyrrolic at.% Quaternary at.% Graphitic at.% Pyrrolic/Fe 398.0 eV 398.8 eV 399.9 eV 400.9 eV 402.0 eV 403.3 eV

	2/0	95.57	1.39	0.10	2.94	1.05	0.50	0.45	0.54	0.28	0.12	5.4
	2/0.5 95.22	1.63	0.16	3.00	0.94	0.55	0.43	0.61	0.35	0.13	3.8
	2/1	93.69	2.89	0.22	3.21	0.84	0.52	0.52	0.78	0.38	0.18	3.5
	2/1.5 94.32	1.97	0.27	3.44	0.79	0.52	0.45	1.00	0.46	0.22	3.7
	2/2	93.54	2.32	0.36	3.76	0.94	0.57	0.50	1.14	0.43	0.18	3.2
	2/2.5 92.91	2.49	0.43	4.17	1.01	0.66	0.53	1.30	0.48	0.19	3.0

Table 4 . 6

 46 Spectral parameters derived from the fitting of Mössbauer spectra. Isomer shift (IS), quadrupole splitting (QS), Linewidth (LW), Hyperfine Field (H) and relative signal area (%).

	R/M	component	IS mm s -1	QS mm s -1	LW mm s -1	H T	Relative area%
		D1	0.47	0.98	1.26	-	54
	2/0.5	D3	1.20	2.72	0.50	-	5
		Sextet 1	1.19	-	3.19	43	41
	2/1	D1	0.47	1.10	1.25	-	92
		D3	0.97	2.90	0.69	-	8
	2/2	D1	0.32	1.07	0.89	-	76
		D2	0.56	2.62	1.44	-	24
		D1	0.46	1.13	1.04	-	76
	2/2.5	D2	0.45	2.85	2.41	-	16
		Sextet 1	0.69	-0.1	0.27	43.5	3
		Sextet 2	0.65	+0.1	0.27	53.7	5

Table 4 .

 4 7 RDE tests of the catalysts prepared with different R/M ratios and FeCl3 precursor. The mass activity was measured in O2-saturated 0.05 M H2SO4 at 1600 rpm and scan rate of 10 mV s -1 . The SD and the TOF values were determined by nitrite stripping method considering a 3 electrons process(A.2.8)

	R/M	MA0.8 A g -1	MA0.8 after 500 cycles A g -1	Loss ratio	Tafel slope mV dec -1	SD×10 18 g -1	TOF e s -1 site -1
	2/0	0.47	0.26	-47%	59	4.52	0.65
	2/0.5	0.79	0.39	-51%	60	6.02	0.82
	2/1	2.04	1.08	-47%	52	8.04	1.58
	2/1.5	2.34	1.73	-26%	60	6.30	2.32
	2/2	3.00	2.22	-26%	60	5.66	3.31
	2/2.5	2.22	1.59	-28%	69	5.95	2.33

Table 4 . 8

 48 Physicochemical characterization of the catalysts with different Fe precursors at R/M=2/2.

	Fe precursor	Total mass loss wt.%	ICP Fe wt.% at.%	BET surface (m 2 g -1 )	Vtotal (cm 3 g -1 )	Vmicro (cm 3 g -1 )	Vmeso (cm 3 g -1 )
	FePc	78%	1.42	0.31	1180	1.22.	0.40	0.73
	Fe(NO3)3	79%	1.31	0.28	1221	0.55	0.47	0.04
	FeCl3	81%	1.36	0.30	1271	0.78	0.45	0.25

Table 4 . 9

 49 Surface chemical composition of catalysts prepared with different Fe precursors and deconvolution of their N 1s spectra.

	Fe

precursor C at.% O at.% Fe at.% N at.% Nitrile at.% Pyridinic at.% Fe-Nx at.% Pyrrolic at.% Quaternary at.% Graphitic at.% Pyrrolic/Fe 398.0 eV 398.8 eV 399.9 eV 400.9 eV 402.0 eV 403.3 eV

	FePc	93.90 2.33	0.30	3.47	0.80	0.51	0.45	1.12	0.39	0.20	3.7
	Fe(NO3)3	93.24 2.44	0.39	3.94	0.93	0.64	0.48	1.25	0.46	0.18	3.2
	FeCl3	93.54	2.32	0.36	3.76	0.94	0.57	0.50	1.14	0.43	0.18	3.2

Table 4 .

 4 10 RDE tests of the catalysts prepared with different Fe precursors at R/M=2/2.

	Fe precursor	R/M	MA0.8 A g -1	MA0.8 after 500 cycles A g -1	Loss ratio	Tafel slope mV dec -1
	FePc	2/2	2.12	1.21	-43%	66
	Fe(NO3)3	2/2	3.58	2.74	-23%	61
	FeCl3	2/2	3.00	2.22	-26%	60

Table 5 . 2

 52 Surface chemical composition of the samples extracted at different synthesis steps.

	Sample	C at.%

O at.% Fe at.% N at.% Nitrile at.% Pyridinic at.% Fe-Nx at.% Pyrrolic at.% Quaternary at.% Graphitic at.% 398.0 eV 398.8 eV 399.9 eV 400.9 eV 402.0 eV 403.3 eV

	FePc BM 95.57	1.39	0.13	3.72	1.10	0.48	0.55	1.12	0.34	0.13
	FePc AW 95.22	1.63	0.11	3.73	1.05	0.36	0.65	1.10	0.42	0.15
	FePc N2	93.69	2.89	0.20	2.99	0.65	0.36	0.37	1.05	0.39	0.16
	FePc NH3 92.91	2.49	0.26	3.09	0.73	0.51	0.45	0.86	0.36	0.14

Table 5 .

 5 3 RDE tests of the samples extracted at different steps.

	Sample	MA0.8 A g -1	MA0.8 after N2 AST A g -1	Loss ratio (N2)	MA0.8 after O2 AST A g -1	Loss ratio (O2)	Tafel slope mV dec -1
	FePc BM	0.20	-	-	-	-	89
	FePc AW	0.07	-	-	-	-	94
	FePc N2	0.65	0.32	-50%	0.20	-70%	85
	FePc NH3	1.67	1.05	-37%	0.38	-77%	64

Table 6 . 1

 61 Physicochemical characterization of catalysts doped by different molar ratios of Ce. CeCl3 was used as the Ce precursor.

	Fe/Ce	Total mass loss wt.%	ICP Fe wt.% at.%	ICP Ce wt.% at.%	BET surface (m 2 g -1 )	Vtotal (cm 3 g -1 )	Vmicro (cm 3 g -1 )	Vmeso (cm 3 g -1 )
	pristine	81%	1.36	0.30	-	-	1271	0.78	0.45	0.25
	1/0.5	79%	1.09	0.24	0.32	0.03	1155	0.51	0.45	0.03
	1/1	81%	1.10	0.24	1.28	0.11	1034	0.46	0.40	0.03
	1/1.5	74%	0.96	0.21	2.75	0.25	564	0.32	0.20	0.10

Table 6 . 2

 62 Surface chemical composition of catalysts doped by different molar ratios of Ce.

							Nitrile	Pyridinic	Fe-Nx	Pyrrolic	Quaternary	Graphitic
	Fe/Ce	C at.%	O at.%	Fe at.%	Ce at.%	N at.%	at.% 398.0 eV	at.% 398.8 eV	at.% 399.9 eV	at.% 400.9 eV	at.% 402.0 eV	at.% 403.3 eV
	pristine	93.54	2.32	0.36	-	3.76	0.94	0.57	0.50	1.14	0.43	0.18
	1/0.5	92.55	2.65	0.38	0.10	4.32	1.07	0.70	0.59	1.32	0.44	0.20
	1/1	91.79	3.43	0.35	0.20	4.22	1.13	0.60	0.64	1.24	0.45	0.18
	1/1.5	91.59	4.09	0.33	0.45	3.56	0.84	0.61	0.53	1.05	0.37	0.15

Table 6 .

 6 3 RDE tests of the catalysts doped by different molar ratios of Ce.

	Fe/Ce	MA0.8 A g -1	MA0.8 after 500 cycles A g -1	Loss ratio	Tafel slope mV dec -1
	1/0.5	2.35	1.65	-30%	59
	1/1	2.57	1.60	-38%	63
	1/1.5	0.69	0.44	-36%	70
	Pristine	3.00	2.22	-26%	60

Table 6 . 4

 64 Physicochemical characterization of catalysts doped by different molar ratios of Ce. CeO2 nanoparticles were used as the Ce precursor.

	Fe/Ce	Total mass loss wt.%	ICP Fe wt.% at.%	ICP Ce wt.% at.%	BET surface (m 2 g -1 )	Vtotal (cm 3 g -1 )	Vmicro (cm 3 g -1 )	Vmeso (cm 3 g -1 )
	pristine	81%	1.36	0.30	-	-	1271	0.78	0.45	0.25
	1/0.5	79%	1.19	0.26	0.12	0.01	1185	0.51	0.46	0.02
	1/1	84%	1.20	0.26	0.77	0.07	1151	0.51	0.44	0.04
	1/1.5	80%	1.22	0.27	0.60	0.07	1089	0.50	0.42	0.05
	1/1 BM	74%	1.00	0.22	3.71	0.33	578	0.32	0.21	0.09

Table 6 . 5

 65 Surface chemical composition of catalysts doped by different molar ratios of Ce.

							Nitrile	Pyridinic	Fe-Nx	Pyrrolic	Quaternary	Graphitic
	Fe/Ce	C at.%	O at.%	Fe at.%	Ce at.%	N at.%	at.% 398.0 eV	at.% 398.8 eV	at.% 399.9 eV	at.% 400.9 eV	at.% 402.0 eV	at.% 403.3 eV
	pristine	93.54	2.32	0.36	-	3.76	0.94	0.57	0.50	1.14	0.43	0.18
	1/0.5	92.25	3.35	0.35	0.10	3.96	0.93	0.65	0.62	1.21	0.40	0.15
	1/1	91.92	3.40	0.36	0.16	4.15	1.02	0.67	0.61	1.22	0.44	0.19
	1/1.5	92.33	3.56	0.34	0.12	3.65	0.87	0.59	0.58	1.07	0.39	0.15
	1/1 BM	91.09	4.52	0.33	0.79	3.28	0.84	0.47	0.48	0.93	0.41	0.16

Table 6 .

 6 6 RDE tests of the catalysts doped by different molar ratios of Ce.

	Fe/Ce	MA0.8 A g

-1 MA0.8 after 500 cycles A g -1 Loss ratio Tafel slope mV dec -1

	1/0.5	2.60	1.85	-29%	68
	1/1	3.16	2.36	-25%	66
	1/1.5	2.70	1.95	-28%	68
	1/1 BM	1.21	0.79	-35%	75
	pristine	3.00	2.22	-26%	60

Acknowledgements v

Acknowledgements

The Fe contents of all samples extracted at different stages were determined by ICP-MS method, as listed in Table 5.1. FePc BM shows the highest Fe content, then decreases after the acid washing due to the removal of acid-soluble Fe compounds. This result explains the lower Fe content than the theoretical value in the catalysts with low melamine content. It confirms the conclusion of Chapter 4 that melamine can reduce the yield of Fe-rich nanoparticles during the first pyrolysis. After the second heat treatment, Fe contents in FePc N2 and FePc NH3 increase again with the loss of materials. However, the increasing degrees are inconsistent with their mass loss during the second heat treatment, that the FePc NH3 loses more carbon due to the

Annexes

A.1 The preparation of Piranha Solution H2O2 (35%) solution was purchased from ACROS Organics TM , and H2SO4 (96%) was purchased from Fisher Scientific. All chemicals were used directly without any treatment.

1. 500 mL of 35% H2O2 solution was poured into a flask which was placed in an ice bath.

The solution was under vigorous magnetic stirring. 2. 500 mL of 96% H2SO4 was introduced by successive addition of 15 mL. During this step, ice should be supplied regularly to ensure the temperature at 0 °C. 3. When H2SO4 was totally added, the mixture is kept stirring at room temperature for 24 h.

A.2 Characterization techniques and operating conditions A.2.1 Dynamic light scattering analysis

Particle size measurements were performed using a dynamic light scattering analyser (HPPS, Malvern Instruments Inc, Worcestershire, UK) incorporating Non-Invasive Back-Scattered optics. Catalyst suspensions were prepared using 1 mg of catalyst in 20 mL of absolute ethanol. After 60 minutes of sonication at 20 º C, the catalyst suspensions were placed in a disposable PMMA cuvette and loaded in the instrument sample holder. The temperature was allowed to equilibrate for 10 minutes at 25 º C. Samples were then analyzed using a collection time of 20 minutes.

A.2.2 Nitrogen sorption analysis at -196 °C

Nitrogen sorption isotherms were measured with a Micromeritics ASAP 2020 instrument. The weight of empty sample tube was recorded at the beginning of the preparation. A mass of ca.150 mg of catalyst was then weighed and sealed in the sample tube. Prior to the measurement, the sample was degassed under 10 mmHg pressure at 200 °C for 8.3 hours with a ramp rate of 10 °C min -1 . The mass of catalyst was calculated by mass of degassed tube minus empty tube. The N2 adsorption/desorption measurement was carried out at -196 °C in a liquid nitrogen Dewar bottle. BET area was determined via the Brunauer-Emmett-Teller equation while the total pore volume (Vt) was estimated from the adsorption isotherm at the relative pressure of 0.995. The pore size distribution (PSD) of micropore volume (Vmicro) and mesopore volume (Vmeso) was calculated with Solution of the Adsorption Integral Equation Using Splines (SAIEUS) program based on the adsorption isotherms. 2D-NLDFT-Heterogeneous Surface model was used which incorporates energy heterogeneity and geometric corrugation on the surface of carbon pores.

A.2.3 X-ray diffraction

X-ray diffraction (XRD) analysis was performed using an X"pert Pro Philips diffractometer using filtered Cu Kα radiation (λ=0.15418 nm) generated at 40 kV and 30 mA. The signal was recorded from 2θ=10-60º with a 0.05º step and 5 sec hold per step in order to obtain high quality XRD signals. The diffraction angles of the peaks were determined with High score software. The interlayer spacing of carbon layer (d002) was calculated by Bragg's law:

The average values of perpendicular graphene layers (Lc) were calculated from the Debye-Scherrer equation:

where β is the line broadening at half the maximum intensity in radians after subtracting the background.

A.2.4 Raman spectroscopy

Raman spectroscopy was used to differentiate the carbon structure of the different electrocatalysts. A Renishaw InVia instrument in a backscattering configuration (with 532 nm as excitation) was used for each measurement. An objective focus of X50 was used to collect the scattered radiation. In order to prevent the excess heating or probable degradation of the samples under the laser beam, the spectrometer was used in a 'line mode' with low incident power densities (~50 µW µm -2 ). LabSpec 5.0 software was used for fitting the Raman spectra and five bands were considered, as our previous work. 244 The average values of in-plane graphite crystallite size (La) was calculated from the Knight and White equation: 257 𝐿 𝑎 (𝑛𝑚) = 4.4 ( 𝐼 𝐷1 𝐼 𝐺 ) -1 (A-3)

A.2.5 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to identify the elemental composition on the surface of catalysts. To prepare the sample for analysis, appropriate amount of powder was taken by a straw and poured on the support. The powder was then pressed by a spoon with the protection of aluminum foil to get a plat and dense surface. The support was put in an ultrahigh vacuum chamber and then close the entrance to start the vacuum pump until the pressure is lower than 5×10 -8 mbar.

The measurements were performed using Thermo ScientificTM K-AlphaTM spectrometer equipped with a RESOLVE 120MCD5 hemispherical electron analyzer and an Al Kα1,2 monochromatic source (1486.6 eV). The constant pass energy mode was used to record both survey and high-resolution spectra, with pass energies 100 and 20 eV respectively. All samples were analyzed with a spot size of 200 μm and repetitive tests were conducted on four different spots of all samples to ensure the accuracy of the measurements. A survey spectrum and higher resolution multiplex scan spectra (C 1s, O 1s, N 1s, and Fe 2p core levels) were obtained and analyzed by Avantage. The intensity of the Fe 2p peak in the high-resolution scan is very low due to low Fe content in the catalysts, we increase the scan number to 64 to increase the signal/noise ratio of final spectrum.

ABSTRACT

The proton exchange membrane fuel cell (PEMFC) is a crucial device for the development of the hydrogen energy economy. A large amount of platinum-based catalyst is required at the cathode to accelerate the oxygen reduction reaction, which limits the commercialization of the fuel cell on a large scale. Today, Fe-N-C material is regarded as a promising nonnoble metal catalyst to replace platinum, but its lower activity and material transport with this type of catalyst remain problematic. In this manuscript, a series of Fe-N-C aerogel catalysts were synthesized by pyrolysis of resorcinol-melamine-formaldehyde (RMF) aerogels containing an Fe precursor. The highly porous structure of the aerogel facilitates the transport of material and its synthesis method allows to control its composition. The impacts of the nature of the Fe precursor and the melamine content on the properties of the active centers and the ORR activity of the catalysts have been studied and links between synthesis parameters and electrochemical performances have been established, in RDE as well as in single cell. Concerning the durability, a study of the addition of CeO2 as a peroxide scavenger has also been carried out.

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MOTS CLÉS

Catalyseur Fe-N-C, aérogel de carbone, réaction de réduction de l'oxygène, pile à combustible à membrane échangeuse de protons RÉSUMÉ La pile à combustible à membrane échangeuse de protons (PEMFC) est un dispositif crucial pour le développement de l'économie basée sur l'hydrogène comme vecteur énergétique. Une grande quantité de catalyseur à base de platine est nécessaire à la cathode pour accélérer la réaction de réduction de l'oxygène, ce qui limite la commercialisation de la pile à combustible à grande échelle. Aujourd'hui, le matériau Fe-N-C est considéré comme un catalyseur prometteur à base de métal non noble pour remplacer le platine, mais son activité plus faible et le transport de matière avec ce type de catalyseur restent problématiques. Dans ce manuscrit, une série de catalyseurs à base d'aérogels de Fe-N-C a été synthétisée par pyrolyse d'aérogels de résorcinol-mélamineformaldéhyde (RMF) contenant un précurseur de Fe. La structure très poreuse de l'aérogel facilite le transport de matière et sa méthode de synthèse permet de contrôler sa composition. Les impacts de la nature du précurseur de Fe et de la teneur en mélamine sur les propriétés des centres actifs et l'activité ORR des catalyseurs ont été étudiés et des liens entre paramètres de synthèse et performances électrochimiques ont été établi, en RDE comme en monocellule. Concernant la durabilité, une étude de l'ajout de CeO2 comme piégeur de peroxyde a également été menée. KEYWORDS Fe-N-C catalyst, carbon aerogel, oxygen reduction reaction, proton exchange membrane fuel cell