With a rising global population, increasing energy demands, and impending climate change, major concerns have been raised over the security of our energy future. Developing sustainable, fossil-free pathways to produce fuels and chemicals of global importance could play a major role in reducing carbon dioxide emissions while providing the feedstocks needed to make the products, we use daily.

The share of renewable energy worldwide is expected to increase by 38% in the period between 2011 and 2035. Consequently, the share of energy from fluctuating energy sources, such as wind, sun and ocean current, will escalate and new alternatives for energy conversion, energy storage and load management will be needed. One prospective goal is to develop electrochemical conversion processes that can convert molecules in the atmosphere (e.g., water, carbon dioxide, and nitrogen) into highervalue products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia) by coupling to renewable energy. Hydrogen is one of the most valuable energy carriers, which can contribute to various forms of energy, such as, production of electricity via fuel cells, fuel for internal combustion engines or gas turbines, or as a raw material to produce synthetic fuels via Sabatier or Fischer-Tropsch process. In some situations, it may be suitable to simply inject hydrogen into the existing natural gas-based infrastructure.

Producing hydrogen via water electrolysis technologies using surplus, low cost power from renewables offers the possibility of increased production capacity and load management with no greenhouse emissions. Alkaline electrolysis cell (AEC) is one of the most promising methods for producing high purity hydrogen to be used as a clean and renewable energy carrier, meanwhile AEC is also become the current standard (state-of-the-art) for industrial large-scale water electrolysis systems.

A basic AEC needs two electrodes: the anode and cathode supported with their respective oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysts. Of the two half-reaction, OER is more energy-consuming due to the high thermodynamic potential required for water oxidation (1.23 V vs RHE (reversible hydrogen electrode)) and the sluggish kinetic hindrance of the multi-electron evolving charge-transfer reactions. Currently, noble transition metal oxides, such as RuO2 and IrO2, have been regarded as the state-of-the-art oxygen evolution reaction electrocatalysts (OEEs), while their large-scale applications were impeded by the scarcity, high cost, and poor durability.

Accordingly, in order to replace these noble metal-based electrocatalysts, extensive efforts have been devoted to developing alternative OER electrocatalysts based on the inexpensive and earthabundant first-row transition-metal (e.g. Fe, Co, Ni, Cu), which are highly active and stable under OER conditions, thus making water splitting more practically feasible. Although great progress has been made in designing these types of non-noble metal based catalysts, the suboptimal intrinsic activity, the limited active sites exposure and the inherently poor electronic conductivity are key problem to hamper further catalytic performance enhancement of these catalysts. On the other side, complex preparation methods and expensive conductive polymers as binder for electrode fabrication, minimize their potential for practical applications. The in situ electrochemical activation (ECA) method has been mostly applied for OER in alkaline condition, which is a novel electrochemical tuning access to enhance catalytic performances in a facile, mild, green and controllable way. Such tuning strategy opens up a fresh new pretreatment technique for electrocatalysts, which can not only obtain comparable excellent OEEs General introduction II without adopting complex chemical synthesis method, but also largely avoid use of expensive binder and easy access to promising self-supporting electrocatalysts or even bifunctional electrocatalysts for overall water splitting.

Moreover, OER is a process involved electrocatalytic reactions at the boundary of gas-liquid-solid phases. Catalytic reactions take place at the interface of the catalyst and the electrolyte, catalytic activity can also be restricted due to the lack of bubble transport channels. In practice, rational design of the OER catalyst architecture can dramatically promote activity in electrocatalytic water splitting. Therefore, ingenious designing the morphology, structure, and composition of the electrocatalysts is pivotal for achieving high catalytic activity towards OER. To achieve effective ways of further promoting OER performance at the three-phase boundary, the design of hierarchical porous supports with facile charge/electrolyte/bubble migration and higher catalysts loading contributions to the catalytic OER activity has to be further explored.

Currently, OER catalyst needs to meet the most stringent requirements in industrial applications such as delivery of very high current density (≥ 500 mA cm -2 ) at low overpotentials (≤ 300 mV), and certain mechanical robustness and prolonged durability during harsh oxidizing conditions and the complicated mechanism of the OER. On the other hand, for large-scale industrial applications, AECs also require the commodious scale production of large areal type electrodes (e.g. where the geometric surface area of the electrode can reach several square meters). Currently, electrolytic codeposition and high-velocity oxy-fuel spraying are two effective and potential avenues of fabricating large-sized oxygen evolution electrode that meet industrial needs. Additionally, electrochemical activation (ECA) strategy as an efficient technique for activating the integrated electrodes (precatalysts) to enhance electrocatalytic performance and preparing self-supported OEEs, has been mostly applied.

Enlightened by the above discussion, engineering highly active and stable earth-abundant transition metal-based OEEs with 3D porous architectures via electrolytic codeposition and high-velocity oxy-fuel spraying in combination with in situ ECA, respectively, is the main objective of the present PhD study.

Furthermore, this dissertation gives more in-depth analysis of their catalytic activities, microstructure evolution and reaction mechanism. In addition, inspired by the process of developing OEE, we initially develop HER electrocatalysts (HEEs) by dynamic self-optimization of the same precatalysts as OEEs, coupled with the obtained OEEs to form a two-electrode configuration of AEC, providing new dimensions in the design of newly highly active, cost effective and stable electrocatalysts for overall water splitting. This thesis is divided into six chapters, as follows:

Chapter I provides a brief introduction about fundamentals of water electrolysis and water electrolysis technologies, synthetic strategies of OER electrocatalytic anode material, including reaction mechanism of OER electrocatalytic anode material, key performance parameters of OER electrocatalytic anode material, AEC technology, industrial application parameters in AEC developers and synthetic strategies of OER electrocatalytic anode material for AEC. Finally, the objectives of the thesis, the originality and novelty of PhD study and thesis organization are presented.

Chapter II introduces physical characterizations and electrochemical characterizations, respectively. Physical characterizations including Scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Transmission electron microscope (TEM) are employed to test the morphologies, structures and chemical valences of the fabricated anode General introduction III catalytic layers. Brunauer-Emmett-Teller (BET, ASAP 2020 V4.00) analysis is conducted to study the surface area and the porosity distribution of all the catalysts. Electrochemical characterizations, including Electrochemical workstation (Metrohm Autolab PGSTAT-128N) equipped home-made threeelectrode air-tight alkaline electrolytic cell and Gas-chromatograph are used to characterize the catalytic activity of the prepared anode material. Polarization curves and Cyclic voltammetry (CV) curves, Tafel slopes, Electrochemical active surface area (ECSA) and double-layer capacitance (Cdl), electrochemical impedance spectroscopy (EIS) and Chronopotentiometry are measured in electrochemical tests. Chapter III includes a fundamental study of using one-step electrolytic codeposition as a novel method for preparation of the hybrid electrocatalysts for OER by fabrication of high-efficient Co3O4 and Graphene oxide (GO) on the porous framework. The commercial nickel foam (NF) with typical micropores are researched as the framework during electrolytic codeposition process. The morphology and microstructure of the hybrid catalysts deposited onto the surface of the NF are optimized by adjusting additives and electrolytic codeposition parameters to produce OEEs with similar catalytic performance and long-term stability as the noble metal-based catalysts (e.g. IrO2 and RuO2). Subsequently, these pre-catalysts are activated upon in situ ECA. The morphological, structural and chemical evolution of OEEs before and after activated via in situ ECA are investigated by SEM, XRD, TEM, BET, Raman and XPS. Electrochemical performances of the finally obtained OEEs are evaluated by Polarization curves and CV curves, Tafel slopes, Cdl, EIS and Chrono potentiometric methods. Finally, the relationship between the OER performance and the morphological, structural and chemical evolution of OEEs are discussed in detail.

Chapter IV mainly presents how high-velocity oxy-fuel (HVOF) spraying followed by chemical activation (CA), combines with in situ ECA, as a versatile synthesis approach to producing hierarchical porous core-shell structure with Jamborite (Ni(OH)2/NiOOH hybrid) nanosheets in situ generating. This typical nanostructure owns highly intrinsic catalytic activity, large active surface areas and facile charge/electrolyte/bubble migration, thus ensuring excellent OER performance comparable to the stateof-the-arts noble metal-based electrocatalysts. This chapter introduces a novel process of preparing OEEs, HVOF followed by CA, combining with in situ ECA, especially focuses on morphological, structural and chemical evolution of the OEEs upon CA and in situ ECA by SEM, XRD, TEM, BET, Raman and XPS. Finally, the OER catalytic mechanism of the core@shell/NiAl@ Jamborite (Ni(OH)2/NiOOH hybrid) structure is revealed by combining the oxygen evolution activity tests, which include Polarization curves and CV, Tafel slopes, Cdl, EIS and Chrono potentiometric methods.

Chapter V considers preparing promising HEEs (H-NF@CGO, H-HNA-CA ) once verified a way possessing the potential to obtain excellent OEEs (O-NF@CGO, O-HNA-CA), coupling with which form a two-electrode configuration (O-NF@CGO (+) ||H-NF@CGO (-) , O-HNA-CA (+) ||H-HNA-CA (-) ), rivalling the integrated state-of-the-art IrO2/NF (+) ||Pt foil (-) electrode in alkaline overall water splitting.

We hope our work can promote advanced development toward large-scale H2 production using excess electrical power whenever and wherever available.

Chapter VI summarizes the conclusions and contributions of this research and provides recommendations for the future work; put forward new exploration directions for future fabrication of OEEs and HEEs, such as self-supported bifunctional transition metal compounds (TMC) via 3D printing or Melting process in combination with in situ ECA, and developing the effective of magnetic field on electrode fabrication and applications.

Chapter I Introduction of water electrolysis

Background

Hydrogen, as an energy carrier, has become increasingly important, mainly in the last two decades.

Presently, the technologies that dominate hydrogen production include reforming of natural gas, gasification of coal and petroleum coke, as well as gasification and reforming of heavy oil. Although water electrolysis has been known for around 200 years, it still contributes only a minor fraction of the total hydrogen production (4 % of the worldwide hydrogen production). [1][2][3][4] When compared to other available methods, water electrolysis has the advantage of producing extremely pure hydrogen (> 99.9 %), ideal for some high value-added processes such as manufacture of electronic components. 5 Simultaneously, exploiting safe, clean, and sustainable energy sources is a major societal and technological challenge in the 21st century. [6][7][8] Solar, wind and hydro energy, are in principle able to meet a large portion of the global energy demand. However, they are intermittent and require efficient and economic storage solutions. Among various storage solutions, the water splitting reaction is an attractive solution as it enables the sustainable production of hydrogen, a desirable energy carrier. [8][9][10][11] The overall concept of hydrogen produced by water electrolysis for distributed power generation routes is shown in Although offering an effective way to make high-purity hydrogen, the practical use of electrochemical water splitting for mass hydrogen production is limited because it is a strongly uphill reaction with a large overpotential (commercial electrolyzers typically operate at a cell voltage of 1.8-2.0 V, much higher than the theoretical minimum value of 1.23 V). 12 Many innovative processes have been subject to intense research and some have already reached high efficiencies. Implementation of electrocatalysts for OER and HER is an efficient way to overcome the large water-splitting overpotentials, which makes the whole process more energy-efficient. 11 Figure 1-1 The overall concept of hydrogen produced by water electrolysis for distributed power generation routes. 13 A variety of water electrolysis systems have been proposed developed and constructed over the years. Among different methods of hydrogen production, AEC is considered to be the current standard for large-scale water electrolysis systems, because of the highest production rate and the lowest production cost. 14 AECs hold many advantages in comparison to other systems. As cheaper, non-noble metals are stable in alkaline media, the relatively low cost of the electrode materials is one of the main advantages of alkaline systems over electrolyzers using proton-conducting electrolytes. Therefore, the main part of published literature on alkaline electrolysis describes electrodes based on low cost materials such as nickel for both anode and cathode. However, finding low cost electrode materials that are both efficient and have long term stability is one of the remaining challenges with the field of AEC.

Fundamentals of water electrolysis

The principle

Water electrolysis is a process where electricity is used to decompose water into its componentsgaseous hydrogen and oxygen according to:

H 2 O(l) → H 2 (g) + 1 2 O 2 (g) (1-1)
In detail, water electrolysis consists of two half-cell reactions: hydrogen evolution reaction (HER) at the cathode (2H + (aq) + 2e -→ H 2 (g)) and oxygen evolution reaction (OER) (2H 2 O(l) → O 2 (g) + 4H + (aq) + 4e -)) at the anode. Theoretically, the applied potential of electrolysis cell voltage is 1.23 V under the standard condition (298 K, 1 atm = 1.013×10 5 Pa). In practice, the real water electrolysis needs a much larger voltage to overcome unfavorable drawbacks such as high overpotentials and sluggish kinetics, especially for OER. Thus, it is essential to gain insight into electrochemical reaction process for providing guidelines of designing efficient catalysts.

Generally, the HER process is a two-electron transfer process in individual Volmer-Heyrovsky or Volmer-Tafel mechanism, where the absorbed hydrogen (H * ) as intermediate on the electrode surface participates in the reaction pathways. The HER process highly depends on pH value of electrolytes. In acid solution, the HER proceeds according to the following steps:

(i) Proton discharging to form an adsorbed hydrogen atom:

H + + e -→ H * (Volmer reaction) (1-2)
Subsequently, molecular H2 can be formed via the following two alternative steps depending on the surface of electrodes.

(ii) The quick coupling of two adsorbed hydrogen atoms to generate a hydrogen molecule:

H * + H * → H 2 (Tafel reaction) (1-3)
(iii) Combination of one adsorbed hydrogen atom with a proton to generate a hydrogen molecule:

H * + H + → H 2 (Heyrovsky reaction) (1-4)
In alkaline condition, molecular H2O participates in Volmer and Heyrovsky reactions due to the low concentration of H + :

H 2 O + e -→ H * + OH -(Volmer reaction) (1-5)
H * + H 2 O → H 2 + OH -(Heyrovsky reaction) (1-6)
The free energy of hydrogen adsorption (ΔG H * ) is a crucial descriptor for the HER catalyst. [15][16][17] The negative ΔG H * represents favorable combination of H * with the surface of electrode and Volmer step, whereas too negative value may cause slow proceeding of subsequent Tafel or Heyrovsky steps. In contrast, the positive value of ΔG H * leads to weaker interactions between protons and electrode surface and thus an inefficiency of the entire process.

On the anode of OER process, it is a more complicated reaction containing four-electron transfer and more surface-adsorbed intermediates. It is also a pH-dependent process (in alkaline or acid), where the alkaline OER is mostly researched as shown in the following pathway [18][19][20] :

OH -+ * → OH ads + e - (1-7
)

OH ads + OH -→ O ads + H 2 O + e - (1-8)
The ''*" means active site of the catalyst and ''ads" means the adsorption on the catalyst. The generation of molecular O2 can be proceeded by two possible pathways as follows:

(i) The combination of two Oads intermediates to form O2:

O ads + O ads → O 2 (1-9)
(ii) Oads coupling with OH -to form OOHads and subsequent combination with OH -to form O2:

O ads + OH -→ OOH ads + e - (1-10)

OOH ads + OH -→ O 2 + H 2 O + e - (1-11)
In theory, the thermodynamic potential value of OER is 1.23 V at 298 K (vs. reversible hydrogen electrode (RHE)). Actually, a much higher potential is required for OER because of activation barriers and solution resistances, which will cause excessive energy consumption and decrease conversion efficiency. In screening proper electrocatalysts for OER with low cost and high efficiency, first low transition metals such as Ni, Co and Fe and their derivatives are demonstrated to be comparable to stateof-the-art noble electrocatalyst of Ir/Ru species. [21][22][23] A typical water electrolysis cell consists of two electrodes, a diaphragm, an electrolyte and a power supply. The most common type of water electrolysis is AEC, as illustrated in Figure 12.

Figure 1-2 Conceptual set-up of an AEC. 14 When current is applied to the system, hydrogen gas is formed at the cathode and oxygen gas at the anode. The core of an electrolysis unit is an electrochemical cell, which is filled with electrolyte and has two electrodes connected with an external power supply. At a certain voltage, which is called critical voltage, between both electrodes, the electrodes start to produce hydrogen gas at the negatively biased electrode and oxygen gas at the positively biased electrode. The amount of gases produced per unit time is directly related to the current that passes through the electrochemical cell. The diaphragm is to hinder the mixing of gasses developed at the two electrodes and allows the ions to pass. The gas produced can be captured in their pure form when drifting up from the electrolyte. The reactions taking place at the electrodes, i.e. the half-cell reactions, differ between the types of water electrolysis techniques applied.

Reaction mechanism of OER catalysts in alkaline medium

A general mechanism of OER on metal oxides in alkaline medium (red line) is described in Figure 1-3. 9,24,25 Here the active site is simply drawn as "M". In the first step, a hydroxyl radical is adsorbed on the active site to give M-OH by 1 e -oxidation of hydroxide. Coupled proton and electron removals from M-OH then gives M-O. In one pathway, nucleophilic attack of hydroxyl anion on M-O coupled with 1 e -oxidation yields the hydroperoxide intermediate M-OOH. A further proton-coupled electron transfer resulted in the release of O2 and the regeneration of the free active site. In another pathway, combination of two M-O species gives directly O2 and M. The mechanism depicted here forms the blueprint for the majority of proposed mechanisms, with the main variation being the number of electron or proton transfer in individual steps. In some cases, proton and electron transfers are proposed to be decoupled. 26 Figure 123The OER mechanism for alkaline conditions (the blue line refers to the acid conditions). The black line indicates that the oxygen evolution involves the formation of a peroxide (M-OOH) intermediate (black line)

while another route for direct reaction of two adjacent oxo (M-O) intermediates (green) to produce oxygen is possible as well. 9 The relationship of current density (j) and overpotential (η) in an electrochemical reaction can be described by the Butler-Volmer Eq. (1-12). Here 𝑗 0 is change current density, n is the number of transferred electrons, F is the Faraday's constant, R is the universal gas constant, T is the thermodynamic temperature, 𝛼 𝑎 and 𝛼 𝑐 is the transfer coefficient of anodic reaction and cathodic reaction, respectively. 𝛼 𝑎 and 𝛼 𝑐 are normally assumed to be 0.5. When the anodic overpotential is sufficiently large, the cathodic current is negligible. Thus, Eq. (1-12) can be simplified into Eq. (1-13), the logarithm form of which is the Tafel equation (Eq. (1-14)). 9,27 For a multi-step electrochemical reaction, the Tafel slope (Eq. (1-15)) may provide information of the reaction mechanism. Assuming that a ratedetermining step (RDS) exists, the Tafel slope can be described as Eq. (1-16). 𝛼 𝑎 = 𝑛 𝑟 𝛽 + 𝑛 𝑏 𝑣 (1-16) Where 𝑛 𝑏 is the number of electrons transferred before the RDS, ν is the number of ratedetermining steps, 𝑛 𝑟 is the number of electrons that participate in the rate-determining step, and β is the transfer coefficient of the RDS (typically assumed to be 0.5). Eq. (1-16) provides a simple yet rapid means to identify the RDS of OER. This simple treatment, however, is not sufficient to account for the real kinetic behaviors of OER catalysts. The same Tafel values may originate from different reaction pathways. Moreover, the treatment assumes a low coverage of the surface intermediate before the RDS, which does not always hold especially at high overpotentials. The transfer coefficient can deviate from 0.5 when the reorganization energy is comparable to overpotential, or when there is a significant barrier in the electron transfer. 28,29 Due to these limitations, a full kinetic analysis, including rate order determinations, in addition to Tafel analysis, is necessary to establish the detailed reaction mechanism. 26- 28 Another important tool is first-principle DFT simulations of Tafel kinetics, which gives fundamental information about the reaction mechanism. 27

Performance parameters of OER catalysts in alkaline medium

Electrode and electrolyte

The working electrodes/substrates play a critical role in determining the performance and greatly influence the reaction rate due to their different structure, conductivity, degree of wettability and access of catalyst to an electrolyte. On the basis of the structure and degree of electrolyte movement, electrode supports are divided into two categories: flat surface electrode and 3D electrode. 30 The flat surface electrodes, like glassy carbon (GC), Cu/Ti foil and indium doped tin oxide (ITO) substrate, allow single-way penetration of electrolyte that limits the catalysis only on surface of catalyst, while the 3D substrates, e.g. carbon cloth/paper (CC and CP) and Ni foam, allow multiple pathways for electrolyte penetration from all sides of catalyst and involve all the material in catalytic reaction. [31][32][33] All these electrode substrates have their own advantages and limitations such as, GC electrode is easy to handle and widely used in literature but offers a limited loading of catalyst (~1 mg cm -2 ). 34 In addition, GC electrode requires a binder to stabilize the catalysts, resulting in poor wettability, unwanted and unescapable powder agglomeration. Furthermore, these binders also increase the resistance, block the active sites and prevent diffusion of ions. 35 Thus, to get rid of nonconductive binders, various research groups make efforts to grow catalyst directly on the conductive substrates like Ni foam, CC, CP, which also brings strong electrical connection among the catalyst and current collector. Importantly, 3D texture comes from the Ni foam or graphene foam brings high gas diffusivity and easy penetration of electrolyte and results in better catalysis. However, direct fabrication of catalyst on supportive substrate may lead to the pealing of catalyst due to mismatched growth, resulting in poor stability, which may be resolved by surface modification of substrate for better connection. Further, the control over the mass loading of active materials through direct growth is difficult that may bring challenges for comparative performance analysis. Recently Zhang et al. used three different electrodes i.e. Ni foam, Au (111) electrode and gold-plated Ni foam to analyze their effect on the electrochemical performance of different catalyst. 18 The results show that gold plated Ni foam results much better electrocatalytic performance compared to other two electrodes due to the higher conductivity. 30 Different electrolytes such as alkaline or acid extremely influence the performance of electrode material. The OER electrocatalysis in alkaline solution is more favorable but difficult in neutral electrolytes and reaches at very low performance in acidic solution. Currently, most of the research is focused on finding the OER electrocatalysts which are stable in alkaline conditions like carbon-based materials, oxides or oxyhydroxides of transition metals, hybrids, complex ternary (spinel and perovskites). 30

Onset overpotential and overpotential

The onset overpotential is the applied potential with apparent anodic currents. However, it is difficult to determine the precise potential values. Usually, the onset overpotential is calculated at current density of 0. Much modern electrochemical equipment frequently includes circuitry for electronic compensation of the iR. iR-compensation can exclude the effect of the resistance from the solution and electrode.

However, the current density obtained without iR compensation can give the integral catalytic system and actual HER performance of the electrodes. Furthermore, the resistance between the tip of the Luggin capillary and the surface of the working electrode can be effectively reduced by placing the electrode surface close to the tip of the Luggin capillary.

In order to quantitatively compare the performance of the catalysts, the overpotential value for the current density of 10 mA cm -2 (i.e., the current density expected for a 12.3% efficient solar watersplitting device) is regarded as an important reference. An ideal electrocatalyst can deliver higher current density with lower overpotential. As for carbon-based composites, due to their high electrochemical surface area, it tends to cause high capacitive current during the test process. An effective method to reduce the capacitive current is to set up a low sweep rate in linear-sweep voltammetry (LSV) experiment (such as 1 or 2 mV s -1 ).

All the potentials are referenced to RHE in current studies. While, potentials are often measured and quoted with respect to reference electrodes other than the RHE, which is not very convenient from an experimental standpoint. The common references are the saturated calomel electrode (SCE) or silversilver chloride electrode. In order to unify the standard of evaluation, the potentials in the final graphs should be converted to the potentials versus the RHE according to the calculation:

E (vs RHE) = E (vs SCE) + E 0 SCE +0.0592* pH (1-17) (E 0 SCE = 0.242 V) E (vs RHE) = E (vs Ag/AgCl, saturated KCl) + E 0 (Ag/AgCl, saturated KCl) + 0.0592* pH (1-18)
E 0 (Ag/AgCl, saturated KCl) = 0.197 V However, if the Hydrogen Reference Electrode (HRE, i.e., HydroFlex, which is produced by Gaskatel GmbH from Germany) is used as the reference electrode. The potential measured using this HRE as the reference electrode don't require any complex conversion to RHE, as in the case allowing any test temperature and pH of the electrolyte, namely ERHE = EHRE. Furthermore, HRE is suitable for the full pH-range from -2 to 16 and temperature-range from -303 K to 473 K.

Electrochemical impedance spectrum analysis

Electrochemical impedance spectrum (EIS) analysis under operating conditions is conducted to probe the OER kinetics and the electrode/electrolyte interface reactions. The charge transfer resistance (Rct) is related to the interface charge-transfer process of the electrode, which can be obtained from the diameter of the semicircles in the high frequency zone. The lower Rct value indicates a faster reaction rate. Notably, the larger the overpotential is, the smaller Rct will be. In addition, the semicircles in the low frequency zones stand for the adsorption of oxidized species on the electrode surface, namely adsorption resistance (Rad). Because the onset potential is associated with Rad, the smaller Rad is, the more positive of the onset potential will be. Before the impedance spectrum test, the open circuit voltage should be tested. After being stable in the open circuit voltage, impedance spectrum should be conducted under oxygen evolution voltage, which always corresponds to the potential at 10 mA cm -2 . EIS measurements are carried out in the configuration from the frequency range of 10 5 -0.01 Hz. 13

Tafel slope and exchange current density

The reaction kinetics for OER can be revealed by Tafel slopes, which can be obtained by the following three methods: 1) measure directly by electrochemical workstation; 2) convert from Polarization curves; 3) obtain from the impedance data. In the first way, the applied voltage range should cover the potential of the water reduction reaction. Normally, the voltage range remains consistent with that of the LSV test. The second method is the most common way to achieve the Tafel slope value. In brief, the Polarization curves are replotted as overpotential vs log (current density). Owing to the large amounts of O2 bubbles produced at high overpotential, the log (current density) usually deviates from linear relation at such potential range. So, the Tafel slope is determined by the linear portions at low overpotential fitted to the calculation:

η = blog j + a (Tafel equation) (1-17)
Where η is overpotential, j is the current density, and b is the Tafel slope). As for the last method, the Tafel slopes are calculated from the linear fitting plots of log (Rct) vs overpotential, in which Rct represents the charge transfer resistance in the equivalent circuit fitted in impedance spectroscopy. 36 Compared with the first two methods, Tafel slopes gained by this approach can purely reflect the charge transfer kinetic of the electrode reaction. Regardless of which method you use, the linearity of the linear part of the fitting curve reaches at least 99.9%, at which point the data is accurate. 13 Exchange current density (J0) serves as a key descriptor to appraise the catalytic efficiency, which depends on electrode materials, electrode surface state, electrolyte composition and experimental temperature. It can be calculated by extrapolating the Tafel plots to the x-axis. According to Eq. (1-17), the J0 can be obtained assuming η is zero. It should be mentioned that J0 is considered to be proportional to the catalytically active surface area. The large J0 indicates the large surface area, fast electron transfer rate, and favorable OER kinetics.

Electrochemical active surface area

The electrochemical double-layer capacitance (Cdl) is measured to investigate the electrochemical active surface area (ECSA). An efficacious way to evaluate the Cdl is to measure the capacitance of the double layer at the solid-liquid interface employing CV. CV is performed in a certain potential window without the faradaic processes. 37 First, the CV experiments are operated under a series of sweep speeds (10-200 mV s -1 ), and usually 5-10 successive points are selected. With the increase in scanning speed, the obtained CV curves may deviate from the rectangle. In order to ensure the linear relationship of the final data, it requires a suitable scanning speed to be chosen. Next, the differences in current density variation (ΔJ = J a -J c ) at overpotential (intermediate value of the applied potential range) are to be plotted against scan rate. The plotted lines can be fitted by linear regression enabling the estimation of Cdl, where the slope is twice Cdl. The large Cdl manifests more exposed surface reactive sites and much higher current density. Noticeably, for carbon composites, the Cdl mainly comes from carbon supports.

For this reason, Cdl does not accurately assess the activity of this kind of material, and some other factors need to be emphasized.

Stability

Long-term stability is one of the most important parameters for OER catalysts. Chronopotentiometry test is more popular among the researchers, and the potential is usually related to 10 mA cm -2 , while much larger current densities, such as 50, 100, 500, 800 and 1000 mA cm -2 are employed as a criterion for evaluating stability based on industrial application recently. Normally, 10 hours or much longer is regard as a benchmark for comparison. 31,[38][39][40][41][42] In addition, along with the O2 bubbles from accumulation to release, the measured i-t curves will present in a typical serrate shape.

The generated O2 bubbles not only cover the catalytic sites for oxygen production, but also may lead the electrode materials falling off from the electrode surface in the process of bubble release, although the negative influence of O2 bubbles can be partially weakened by stirring. More interestingly, the anode material is inevitably oxidized 31,43 and slightly strained during the OER process from oxygen generation to release.

Turnover frequency

Up to now, the evaluation of catalytic activity mainly depends on comparison of the overpotential under the same current density. However, different tests are always with different electrocatalysts loading mass, which makes it difficult to judge the catalytic performance. In order to address this issue, turnover frequency (TOF) gradually becomes a crucial detector to evaluate the catalytic ability. TOF is defined by the transformation of the number of reactant molecules on per active site in unit time. The high TOF value shows the outstanding catalytic activity. For TOF calculations, the number of active sites is determined by CVs data, which gathered from 1.2 to 1.7 V vs RHE in 0.1 or 1.0 M KOH aqueous solution with a scan rate of 10-80 mV s -1 . The oxidation peak current of redox species shows linear dependence on the scan rate. 44 The slope of the line can be calculated based on the formula:

𝑠𝑙𝑜𝑝𝑒 = 𝑛 2 𝐹 2 𝐴Γ 0 /4𝑅𝑇 (1-18)
Where n is number of electrons transferred, F is Faraday's constant; A is surface area of electrode;

Γ0 is surface concentration of active sites (mol cm -2 ), R and T are ideal gas constant and absolute temperature, respectively. 45 TOF values can be calculated from the formula:

TOF = JA 4Fm (1-19)
Where J is the current density at certain an overpotential, A is the area of electrode, indicates the mole of electrons consumed for evolving one mole of O2 from water, F is Faraday's constant (96485 C mol -1 ) and m is the number of the moles of active sites. The factor 1/4 in the equation represents that four electrons are required to form one oxygen molecule from four protons. 46

Faradic efficiency

Faradaic efficiency (FE) describes the transfer efficiency of electric charge in an electrochemical reaction system, and it is the ratio between the utilized charge by reactants and the amount of total charge of the external circuits. FE plays a vital role in detecting the complete decomposition of water. FE losses may occur when heat or by-products formed during the reaction. The FE of the OER process is calculated by comparing the amount of experimentally quantified O2 (gas chromatography or a watergas displacing method) with theoretically calculated O2 (galvanostatic or potentiostatic electrolysis by integration).

Water electrolysis technologies

The while SOEC is operated at gas phase conditions at temperatures in the range of 800-1,000°C. The advantage of the high operation temperature is a significant reduction in the electrical energy demand for hydrogen production. Consequently, developers of water electrolysis systems are increasingly looking in the direction of higher operation temperatures. The drawbacks of working at elevated temperatures are however greater challenges regarding the decomposition of materials, which often causes corrosion of metals and the degradation of polymers used for sealing and etc., which in fact are the main reasons for the SOEC still being at the R&D stage.

For the commercially available electrolyzers, PEMEC electrolyzers display the best efficiencies at higher current densities. The investment cost for PEMEC is much larger than for their alkaline counterparties. The capital cost for commercial electrolyzers are estimated to be 1000-1200 € kWel -1 for the largest alkaline systems compared to 1860-2320 € kWel -1 for small PEMEC electrolyzers.

Additionally, the durability of the materials for PEMEC is much less than for alkaline electrolysis. The lifetime of commercial alkaline electrolyzers is said to be about 60,000-90,000 hours compared to 20,000-60,000 hours for PEMEC electrolyzers. Table 1 summarizes component materials as well as performance and cost parameters. There are other two methods of hydrogen production, steam electrolysis and the chlor-alkali production, from which hydrogen is the byproduct.

As seen from the Cell area (m 2 ) <4 14 <0.3 14 <0.01 14 Operating Temp. (℃) 60-80 47 50-80 47 650-1000 48 Operating Pressure (bar) <30 14 <200 14 <25 14 Production Rate C (m 3 H2h -1

) <760 14 <40 14 <40 14 Gas purity (%) >99.5 5 99.99 5 99.9 a

Stack Lifetime (h) 60,000-90,000 5 20,000-60,000 a Where no reference is provided, data were derived during expert elicitations.

b Perovskite-type lanthanum strontium manganese (La0.8Sr0.2MnO3).

c Refers to norm cubic meter of hydrogen (at standard conditions) and respective electrical energy consumption (kWhel) if applicable.

d N.A: information not available

Alkaline electrolysis cell technology

Alkaline electrolysis cell (AEC) is the incumbent water electrolysis technology and widely used for large-scale industrial applications since 1920. 50 AEC systems are readily available, durable and exhibit relatively low capital cost due to the avoidance of noble metals and relatively mature stack components. 5,47 The principle layout of AEC is shown in Figure 1-5. The electrodes are immersed in a liquid electrolyte separated by a diaphragm. The electrolyte is usually a 25-30 % KOH aqueous solution.

It is circulated for the removal of product gas bubbles and heat either by pumps or by natural circulation due to temperature gradients and buoyancy of the gas bubbles. The electrolyte is stored in two separated drums for each product gas (O2 and H2) which serve also as gas-liquid-separator. The product gas quality after drying is typically in the range of 99.5-99.9 % for H2 and 99-99.8 % for O2 which can be increased to above 99.999 % by catalytic gas purification (deoxidizer). 51 The partial reaction at the electrodes is given by:

2H 2 O + 2e -= H 2 + 2OH - Cathode (1.3.1.1) 2OH -= 1 2 O 2 + H 2 O + 2e - Anode (1.3.1.2)
As seen, water is consumed on the cathode side while water is produced on the anode side. As a result, the lye streams of both sides must be mixed before entering the electrolyzer to prevent a respective dilution or concentration of the electrolyte streams. This results in a contamination of product gases due to the electrolyte streams carrying dissolved gases, as the separators can only remove the gas bubbles.

A critical contamination (flammable mixture) in part load has to be avoided by a proper control of the lye circulation, which is automatically the case for natural circulation. 51,52 However, low current density and operating pressure negatively impact system size and hydrogen production costs. Also, dynamic operation (frequent start-ups and varying power input) is limited and can negatively affect system efficiency and gas purity. Therefore, development is focused on increasing current density and operating pressure, as well as system design for dynamic operation, to allow operation with intermittent renewable sources, for example. 5,14,47 Figure 1-5 Layout of AEC system. 51

Other methods of hydrogen production

PEMEC systems are based on the solid polymer electrolyte (SPE) concept for water electrolysis that was first introduced in the 1960s by General Electric to overcome the drawbacks of AECs. 50 The technology is therefore less mature than AEC and mostly used for small-scale applications. Key advantages are high power density and cell efficiency, provision of highly compressed and pure hydrogen, and flexible operation. 47 Disadvantages include expensive platinum catalyst and fluorinated membrane materials, high system complexity due to high pressure operation and water purity requirements, and shorter lifetime than AEC at present. Current development efforts are therefore targeted at reducing system complexity to enable system scale-up and reducing capital costs through less expensive materials and more sophisticated stack manufacturing processes. the operation temperature to 773-973 K (from 923 to 1273 K) to enable the commercialization of this technology. 14,48 Steam electrolysis is a technology that reaches higher total energy efficiency compared to AEC and PEMEC. From the thermodynamic viewpoint of water decomposition, it is more advantageous to electrolyze water at high temperature (1073-1273 K) because the energy is supplied in mixed form of electricity and heat. The main advantage is that a substantial part of the energy needed for the electrolysis process is added as heat, which is much cheaper than electric energy. 53 Hydrogen, as a byproduct, also can be produced from Chlor-alkali industry. The main product of the chlor-alkali electrolysis process, which uses NaCl as a raw material, is chlorine. The industrially used electrolysis cells are the mercury, the diaphragm and the membrane cell. 53 Only Electrolyzer Corp. Ltd. uses monopolar cell type, while all others are using bipolar cell type. The PTFE bonded noble metals are cathodes and anodes in the cells of General Electric (operating at 500 mA cm -2 with the cell voltage of 1.7 V), while others are using either Ni-coated steel, or activated Nicoated steel. The operation current density varies from 134 mA cm -2 to 200 mA cm -2 with the operating cell voltage varying from 1.75 V to 1.9 V. 5,54 Hydrogen purity (%) 99.9 98.9-99.9 99.9 99.99 >99.0

Industrial application parameters in alkaline electrolysis cell developers

Synthetic strategies of OER catalysts for alkaline medium

Oxygen evolution reaction (OER) has been historically one of the most important reactions in the realm of electrochemistry due to the fundamental and industrial importance of the electrolysis of water.

To date, some noble metals and their oxides, such as Ru, Ir, RuO2, and IrO2, are recognized to be the best OER electrocatalysts in aqueous acidic and alkaline solutions since their lowest overpotential for the OER at practical current densities. 10,55 However, their widespread commercial applications are limited significantly because of their low abundance and high cost. Therefore, some researchers also try to build Ru-or Ir-based ternary metal oxides to reduce the amount of the noble metals required while gaining their excellent performance in reducing the OER overpotential. However, commercial electrolyzers operate in highly conductive medium, that is, either in acidic or alkaline conditions.

Unfortunately, RuOx, IrOx, and their composites exhibit substantial stability in only acidic medium, that is, they are not stable in alkaline media, due to forming unstable film. 56,57 Consequently, there is an active quest for finding inexpensive electrocatalysts to efficiently conduct OER with a minimum overpotential. Accordingly, extensive efforts have been devoted to the design, synthesis, and characterization of alternative OER electrocatalysts based on first-row transition metal compounds (TMC) that are low cost, highly active, and stable long-term under oxidizing conditions, thus making the overall water splitting more practically feasible. [58][59][60][61][62][63][64][65][66][67][68] In fact, we should not only focus on the development of the best performing catalysts. In addition, developing catalyst assembly and subsequent activation processes, investigating the evolution of the structure and OER performance for the integrated electrode in commercial AEC during long-term service, are the keys to determine whether the newly developed catalytic materials have the potential to replace the existing commercial electrodes, for the reason that these outstanding catalysts are almost synthesized with unsupported structure, such as nanoparticle, nanosheet, core-shell nanostructure, and other ingenious nanostructures. However, they must assemble with their supports to form an integrated electrode in actual industrial service. It's hard to fully play the inherent performance for the original catalysts once they are assembled as integrated electrode for service in commercial AEC, often with the loss of activity and stability.

Meanwhile, OER is a surface reaction (or more precisely within an ultrathin film), rational design of the electrode architecture can dramatically promote activity in the electrochemical performance. [69][70][71][72] Therefore, in order to achieve effective ways of further improving electrocatalytic reactions at the three phase boundary, the design of 3D electrodes/supports with facile charge/electrolyte/bubble migration and higher catalyst loading contributions to the catalytic activity are widely explored. 73,74 In situ ECA strategy as an efficient activation technique for the integrated electrodes to enhance OER performance has been mostly applied. [75][76][77][78][79][80][81][82] The electrode surfaces are modified from bulk phase upon ECA treatment, which can tune local electronic structures, create more active species, enlarge surface area and thus improve the OER performances. 77,83,84 Meanwhile, this technique can couple the atomic, electronic structures with electrocatalysis mechanisms for water splitting. 75,76,82 Compared to traditional chemical treatment, the in situ electrochemical activation has superior advantages such as facile operation, mild environment, variable control, high efficiency and flexibility. 85

Noble metals and their oxides as OER catalysts

Noble metals as well as their oxides are the most studied catalysts due to their excellent performance towards OER. It is experimentally proved that iridium (Ir) and ruthenium (Ru) materials are more active towards OER as compared to platinum (Pt) and palladium (Pd) (Pt < Pd < Ir < Ru). 56,86,87 Thus, researchers mainly focus on Ir and Ru based catalysts. Ir and Ru are still considered as the stateof-the-art OER electrocatalysts with relatively low overpotential and Tafel slope and particularly superior stability. [88][89][90] Additionally, alloying noble metals together can result in a synergetic effect while the ordered uniform structure of metallic lattice is somewhat altered by the arrangement of two or more atoms with different sizes. This approach has shown potential results for improving the electrocatalytic activities of noble metal for various electrochemical reactions including OER. [91][START_REF] Koper | Thermodynamic theory of multi-electron transfer reactions: Implications for electrocatalysis[END_REF][START_REF] St | Rutheniumalloy electrocatalysts with tunable hydrogen oxidation kinetics in alkaline electrolyte[END_REF][START_REF] Wang | Pt-Ru catalyzed hydrogen oxidation in alkaline media: oxophilic effect or electronic effect?[END_REF][START_REF] Li | Low-temperature, seed-mediated synthesis of monodispersed hyperbranched PtRu nanoparticles and their electrocatalytic activity in methanol oxidation[END_REF][START_REF] Liu | Synthesis and activation of PtRu alloyed nanoparticles with controlled size and composition[END_REF][START_REF] Neyerlin | Combinatorial study of high-surface-area binary and ternary electrocatalysts for the oxygen evolution reaction[END_REF][START_REF] Pei | Ir-Cu nanoframes: one-pot synthesis and efficient electrocatalysts for oxygen evolution reaction[END_REF][START_REF] Cullen | Impact of IrRu oxygen evolution reaction catalysts on Pt nanostructured thin films under start-up/shutdown cycling[END_REF][100] For oxides, iridium oxide (IrO2) and ruthenium oxide (RuO2) catalysts are commonly considered as benchmark electrocatalysts for OER owing to their high electrocatalytic activities toward OER in both acidic and alkaline solution. [101][102][103][104][105][106][107] As be reported earlier, RuO2 is unstable under high anodic potential and is going to deteriorate (dissolve) during OER, while IrO2 is a more stable catalyst than RuO2, but less active than RuO2 for OER. 108 To further optimize the activity and stability of RuO2 and IrO2, Ru-Ir based oxides have been synthesized with the high surface area by co-precipitation method in ethanol medium. 109 It was found that the Ru0.9Ir0.1O2 presents activity close to that of RuO2, importantly with the significant improvement of the stability. 30

First-row transition metal compounds as OER catalysts

As mentioned earlier, noble metal as well as their oxides are well-known OER electrocatalysts with high activity but suffer from high cost and low abundance. Hence considerable efforts have been devoted

to developing cost-effective OER catalysts derived from earth-abundance elements. First-row TMC mainly including Co-based-, [110][111][112][113][114][115][116] Fe-based-, 113,114,117,118 and Ni-based-, 111, 117-121 are the mostly studied noble-free metal OER catalysts. Among these compounds, the metal oxides, 110, 117 (oxy) hydroxides, 111,118 phosphates, 112 phosphides, 113,114,119 sulfides, 120 selenides, 115,121 and nitrides 116 display some of the most outstanding OER performance. Noteworthy, recent studies reveal that the essential active sites of the metal phosphides, sulfides, and selenides for OER are likely the surface oxides, (oxy) hydroxides species formed during the electrochemical oxidation process. 76,77,81,122 With this in mind, the synthetic and structural tuning of metal oxides, (oxy) hydroxides toward enhanced activity and stability has been a focus in OER electrocatalysis.

Co-based transition metal compounds as OER catalysts

Co-based TMC have emerged as a promising candidate for investigating the OER in an alkaline medium owing to their environmental friendliness, abundant reserves, thermal stability, and low cost. 82, Hence, synthesis and fine structural optimization of cobalt-based catalysts and a deep understanding of their structure-property relationships in the OER have been actively pursued. Generally, these cobalt-based TMC OER catalysts can be classified into oxides/(oxy) hydroxides, phosphides and sulfides. All these three kinds of OER catalysts will be discussed in detail below and are summarized in

Fe-Based transition metal compounds as OER catalysts

Iron offers significant advantages in comparison with precious metals and even other transition metals. Firstly, iron is the second most abundant metal in the Earth's crust, and thus is far more inexpensive than other metals often used for constructing water-splitting electrocatalysts. 170 Secondly, iron is relatively less toxic and is present as a key element in various biological systems. 171,172 Thirdly, iron-based catalysts are known to effectively mediate some important biological processes in nature.

Meanwhile, iron also has multiple valence state, and has become a highly visible material for OER. 173,174 Compared with other transition metals such as Co and Ni, Fe based OER electrocatalysts have not been as broadly investigated, especially for pure Fe (oxy)hydroxides. Generally, Fe is particularly well known to be incorporated as a dopant/impurity into Ni-/Co-based OER catalysts, dramatically improving their OER performances. Interestingly, some recent studies proposed that Fe is the active site and that the Ni or Co (oxy)hydroxide primarily provides an electrically conductive, chemically stabilizing, and high-surface area host. Some related work has been summarized in Table 1-4.

In situ electrochemical activation based OER catalysts

The in situ ECA method has been mostly applied for OER in alkaline condition, which is a novel electrochemical tuning access to enhance catalytic performances in a facile, mild, green and controllable way. It has been first explored by Cui's group to synthesize transition metal (e.g., Co, Ni and Fe) oxides (TMOs) from sulfides (TMS). The electrodeposited TMOs on carbon fiber cloth were first transferred to TMS followed by in situ ECA back to TMOs (referred as ECT-TMOs). Different from traditional electrodeposited TMO nanosheets, ECT-TMOs became porous and composed of small grains with decreased crystalline degree. 226 Such small grains created more defects and surface catalytic sites for OER, which can be verified by increased current densities, lower onset potentials and larger ECSA. 227- 230 Furthermore, the enhanced OER activities of binary, ternary or quaternary ECT-TMOs can also be observed under identical electrochemical tuning. One explanation for enhanced OER properties is the higher oxidation state of metals under anodic potentials, which can facilitate the multi-electron transfer in water oxidation. 231,232 In addition, the low-crystalline, porous nanostructures containing abundant defects and surface sites contribute to high OER performance. 75 Recently, induced lattice distortion upon in situ ECA for efficiently triggering water splitting by enhancing intrinsic activity were gradually realized by researchers. 233,234 Such tuning strategy opens up a fresh new pretreatment technique for electrocatalysts, which is quite distinguished from traditional chemical treatment always requiring harsh and polluted conditions.

These works (as shown in Table 1-6) greatly motivated peer workers to investigate more available transition metal materials under electrochemical activation and gain deeper insight into activation mechanism.

Objectives

In order to obtain promising OEEs withstand extremely high current density (≥ 500 mA cm -2 ) which is required practically for commercial industrial electrolyzers, the OEEs should fulfill multiple criteria simultaneously: (i) high intrinsic activity for OER, (ii) huge ECSA, (iii) facile charge/electrolyte/bubble migration at the same time, and (iv) high tolerance against corrosion under strong oxidizing conditions.

Additionally, the catalytically active phase must also have a strong adhesion to its framework, and thereby it can be prevented from peeling off during long-term electrocatalysis. In this case, a long-time steady catalytic output at large current densities is generally unrealizable due to the presence of the common problem of catalyst peeling during electrocatalysis as well as the large interfacial resistance between the catalyst and its framework. Meanwhile, in order to facilitate the OEEs commercialization, it is not negligible to discover a manufacturing method capable of realizing large-size (≤ 4 m 2 ) and high production efficiency of the electrode. However, to date, it is still quite difficult to design versatile OEEs that can meet all the above standards.

Thus, designing hierarchical porous OEEs with cost effective, high catalytic reactivity, high structural stability, and high electronic conductivity via suitable synthesis approaches, eventually become objectives of my PhD work and deserve to be strongly pursued, toward meeting our goal of making efficient OEEs at large current densities. Additionally, we also consider preparing promising HEEs at the same time while discovering a way to have the potential to obtain excellent OEEs, coupling with which form a two-electrode configuration for the commercial AEC, in order to achieve efficient overall water splitting.

In details:

(1) Engineering hybrid pre-catalysts by fabrication of additives (Co3O4 and GO) with high efficient OER activity on the typical commercial nickel foam (NF) via electrolytic codeposition is the first step of our goals to obtain promising OEEs. The second step is to activate these pre-catalysts upon in situ ECA by in situ oxidation of Ni crystals to wrinkled Ni(OH)2/NiOOH nanosheets, thus, obtaining a versatile OEE (denoted as the activated NF@CGO).

(2) High-velocity oxy-fuel (HVOF, a common thermal spray process) followed by chemical activation (CA) is employed to design a promising hierarchical porous OEE, to achieve huge ECSA, and facile charge/electrolyte/bubble migration. Subsequently, Jamborite (Ni(OH)2/NiOOH hybrid)

nanosheets are in situ generate on the surface of the pre-catalysts upon in situ ECA, eventually formation of a promising OEE with high intrinsic activity (denoted as HNA-CA-ECA).

(3) Preparing HEEs with the same approach while OEEs are made, coupling with the obtained OEEs and forming a two-electrode configuration, rivalling the integrated state-of-the-art IrO2 (+) ||Pt (-) electrode in alkaline overall water splitting. We hope our work can promote advanced development toward large-scale H2 production using excess electrical power whenever and wherever available.

The originality and novelty of PhD study

(1) Both of the methods, electrolytic codeposition and HVOF followed by CA, are simple and are two of the few methods that are conducive to commercial production of OEEs based on the fact both of these have the potential to achieve large-scale, high efficient production of OEEs with large dimensions for AEC.

(2) The application of the in situ activation process makes the two kinds of OEEs abovementioned self-supported and binder-free architectures, which is not only easy to operate, but also can generate robust and highly active nanostructures on the surface of the pre-catalysts, avoiding the additional resistance between the catalyst powders and current collector (e.g., glassy carbon) within the powderbased electrodes, and long-lasting OER performance gradually deteriorated when bearing extremely large current density, if electrical insulating binding agents (e.g., Nafion) are utilized.

(3) For the activated NF@CGO, such a remarkable performance can benefit from the self-assembly of the introduced OER catalysts deposited on NF, in situ-generated Ni(OH)2/NiOOH nanoflakes, and their synergistic effects. A catalytic layer with an arbitrary thickness, excellent electrical conductivity, and highly efficient catalytic activity for OER can be obtained. While maintaining the sufficiently high performance, this uniform catalytic layer also allows us to maintain long-term durability of OEE without significant deterioration during harsh and oxidative OER operation conditions, which is one of the drastic requirements necessary for industrial applications.

(4) For HNA-CA-ECA, the micro-voids retained in resulting coatings by HVOF can not only increase the ECSA, but also facilitate the appearance of macropores with de-alloying of the precursor coatings upon CA process. The micro-voids and macropores combine with mesopores upon subsequent in situ ECA, forming the hierarchical porous architecture, which is not only conducive to electron/ion transport and the formation of oxygen, but also facilitates the oxygen bubble detachment, consequently, promoting reaction kinetics of OER. The increasement of intrinsic activity is mainly achieved by latticedistorted Jamborite nanosheets.

Chapter II Experimental details

Synthesis and fabrication approaches

Electrolytic codeposition process

Electrodeposition is a well-known conventional surface modification method to improve the surface characteristics, decorative and functional, of a wide variety of materials, 1 the schematic of which is shown in Figure 2-1a. Two electrodes, power source and electrolyte form the main components of electrodeposition.

Electrolytic codeposition is an electrodeposition method of incorporation of additive-particles into metallic coatings obtained from electrolytes containing the particles in a suspended state (as shown in Figure 2-1b),which is a novel method for preparation of the hybrid electrocatalysts towards water splitting, with the mechanism of two successive adsorption steps. It was proposed by N. Guglielmi in 1972, and is a versatile and scalable industrial process that only requires simple equipment. 2,3 For this study, electrolytic codeposition was carried out with a simple two-electrode system using a field-assisted mechanism consisting of two steps. In the first step, which is of a physical nature, nanoparticles in the electrolyte were surrounded by a cloud of adsorbed species, mainly metal ions, which approached the cathode and finally produced a layer of loosely adsorbed particles on the cathodic surface with a high coverage, driven by fractional irregular Brownian motion. Second, these nanoparticles released an adsorbed ion cloud and strongly adsorbed on the electrode surface due to the development of a higher electricfield force under more negative potentials.

Materials

Modified Watts Ni baths and electrolytic codeposition process conditions are shown in Table 2-1.

The solutions were prepared from analytic-grade chemicals (procured from Sigma) and double-distilled water. The pH value of the bath was measured using the the HM-20E pH-meter and was adjusted to appropriate values with H3BO3. Co3O4 powders with a particle size of 50 nm (Figure 2-1) were also procured from Sigma. Graphene oxide (GO) nanoplatelets (Figure 2-3) were procured from Suzhou Tanfeng Graphene Technology Co., Ltd., China. Nickel foam (NF), (Figure 234) was used as a substrate, and a high-purity (99.99%) electrolytic nickel plate or wire mesh were used as the soluble anodes.

Surface area was ca. ten times greater than that of the cathode to ensure that there no problems would arise from anode polarization, particularly at high current densities. The substrate was chemically degreased, rinsed with distilled water, and then scrubbed with alcohol and acetone. Next, it was acid cleaned and activated and rinsed with distilled water, then placed vertically in the electroplating cell with magnetic stirring, parallel to the anode, with an anode-cathode distance of 5 cm.

Equipment of electrolytic codeposition process

The electrolytic codeposition process was carried out in the simplest two-electrode system (Figure 2345). The final suspension for electrolytic codeposition was prepared from the Ni bath solution, containing 0.05g L -1 lauryl sodium sulfate (SDS), 0.02g L -1 Co3O4, and 0.01 g L -1 GO. In order to obtain a stable nanoparticle suspension, it was essential to sequentially prepare the Ni bath solution, SDS solution and the two types of suspensions of nanoparticles or nanosheets, and subsequently, to add the latter three solutions or suspensions to the Ni bath solution, respectively. The hybrid NF@CGO was obtained with the plating conditions after 45 min. Similarly, NF@N (blank), NF@GO (containing 0.05g L -1 SDS and 0.01 g L -1 GO) and NF@C (containing 0.05g L -1 SDS and 0.02g L -1 Co3O4) were obtained with the plating conditions by adding different additives to Ni bath solution.

Figure 2-5 Optical photo of the applied two-electrode plating system in electrolytic codeposition process.

High-velocity oxy-fuel spraying process

As an effective process of catalyst deposition, high-velocity oxy-fuel (HVOF) was employed to obtain a well-bonded precursor coating due to supersonic particle velocity. This technique can not only obtain well-coated coatings at a lower temperature than other thermal spraying processes, thereby minimizing the adverse effects of oxidation, but also instantly break up and remove the oxide layer from the surface of the newly obtained coating. 5, 6

Materials

Equipment of high-velocity oxy-fuel spraying

HVOF is a novel means for catalyst deposition, where the particles of the catalyst precursor powder injected into the flame of HVOF guns are accelerated to high velocities, roughly 650 m/s on average.

An FST-JP5000 HVOF gun (Figure 23456789) was employed for this purpose. For all the experiments, oxygen and kerosene were used as the oxy-fuel mixture, with flow rates of 1600 and 0.433 SLPM, respectively.

The coatings were deposited at a powder-feeding rate of 5 kg/h and a constant spray distance of 380 mm (Table 2-

In situ electrochemical activation

The in situ electrochemical activation (ECA) method has mostly been applied for engineering OEEs, which is a novel electrochemical reconstruction access to enhance catalytic performances in a facile, mild, green, and controllable manner. [7][8][9][10][11][12][13][14][15][16][17][18] For this study, we set an extremely large current density (800 mA cm -2 ), 6 M KOH, and 353K as the conditions of in situ ECA, considering the strict industrial criteria of AEC that OER process should be energetically catalyzed over a long period at extremely high current densities (≥ 500 mA cm -2 ) with overpotentials ≤ 300 mV, which hinders the widespread implementation of these available electrocatalysts. [19][20][21] We aim to develop cost-effective, highly active, and durable OEEs and reveal the inherent relationship between the morphological, structural, and chemical evolution and OER catalytic activity of said OEEs using the in situ ECA approach.

Physical characterizations

XRD was performed with Bruker AXS D8 focus, equipped with a cobalt anticathode (λ = 1.78897 Å) and operated at 35 kV, 40 mA. SEM were carried out using a JEOL-JSM-7800F system equipped with an EDS analysis system (BRUCKER SDDX-Flash 6130). Transmission electron microscopy (TEM) measurements were done using a FEI Tecnai G 2 F20 S-TWIN TEM operated at 200 kV in the TEM/STEM mode. Samples were prepared by ultrasonic dispersion in ethanol and then a droplet of the dispersion allowed to dry on a holey-carbon coated copper grid. Bright-field and high-angle annular dark field (HAADF) images were recorded. Brunauer-Emmett-Teller (BET, ASAP 2020 V4.00) analysis was conducted to study the surface area and the porosity distribution of the samples. Raman spectroscopy tests were performed by using SENTERRA R200-L with the 532 nm line of an 𝐴𝑟 + laser as the excitation source to validate the structure of CNTs and the formation of Al4C3 phase in the spectral range from 400 to 2000 cm -1 . XPS measurements were carried out by Escalab 250Xi (Thermo Fisher Ltd), equipped with an Al Kα (λ=1486.6 eV) X-ray source. The spectra were measured with an energy step of 0.05 eV and normalized to the binding energy of the C1s peak (284.8 eV). Distilled water and ethanol were used to thoroughly clean all the samples before testing commenced.

Electrochemical characterizations

All the as-prepared OEEs were activated by CV scanning 50 times, from 1.2 V-1.7 V vs RHE, with the scan rate of 100 mV s -1 at the conditions of 6 M KOH at 353 K in the three-electrode electrochemical test system. In order to ensure the effective comparison of data, all the control OEEs would also be activated under the same condition. All electrochemical measurements were carried out with PGSTAT-128N (Metrohm Autolab, Switzerland) electrochemical workstation. Hydrogen Reference Electrode HydroFlex (Gaskatel GmbH, Germany) was used as the reference electrode. The potential measured using this Hydrogen Reference Electrode (HRE) as the reference electrode didn't require any complex conversion to Reversible Hydrogen Electrode (RHE), as in the case allowing any test temperature and pH of the electrolyte, namely ERHE = EHRE. Furthermore, HRE is suitable for the full pH-range from -2 to 16 and temperature-range from -303 K to 473 K. Even if we tested at temperatures above room temperature (353 K) and high concentrations (6 M KOH), the data remained stable and reliable. Pt foil (10 mm×10 mm×0.2 mm) was used as the counter electrode. Bare NS, bare NF, HNA-CA and the HNA-CA-ECA catalysts were used directly as the working electrode, without further treatments. In order to better illustrate the catalytic activity of the materials, we tested all the samples under two conditions-1 M KOH at room temperature and 6 M KOH at 353 K (which refer to industrial applications). The overpotential (η) was calculated by using the formula: η (V) = ERHE-1.23. Polarization and CV curves were recorded at a scan rate of 2, 5 mV s -1 , respectively, and each measurement was repeated at least three times in order to avoid any incidental error. In order to provide reliable electrochemical data and avoid the overlap between Ni 2+ /Ni 3+ oxidation and OER, polarization curves were recorded from high initial potentials to low final potentials. Tafel slopes were derived from OER polarization curves obtained by plotting overpotential against log(current density) at a scan rate of 1 mV s -1 and iR correction (EiR=E-i×RS) in all the above test conditions. The resistance of the electrochemical system was estimated from a simulated equivalent circuit of the electrochemical impedance spectroscopy (EIS), which was performed over a scanning frequency ranging from 100 kHz to 0.1 Hz with a 10 mV sinusoidal AC dither under a fixed bias of 1.53 V vs RHE (η = 300 mV). The ECSA was determined by measuring the capacitive current associated with double layer charging from the scan rate CV-dependence. Here, the CV potential window was 1.07 V-1.17 V vs RHE. The scan rates were 10, 20, 40, 60, 80, 100, and 120 mV s -1 . The double-layer capacitance (Cdl) was estimated Δj = (jcharge -joff charge) at 1.12 V vs RHE against the scan rate. The linear slope is twice of the double-layer capacitance Cdl.

Chapter III Development of OER electrocatalysts by combining electrolytic codeposition and in situ electrochemical activation

Introduction

Total increase in energy consumption and urgent environmental concern intensively render an urgent demand for the use of renewable energy sources (such as solar and wind energy) as viable alternatives to nonrenewable fossil fuels. [1][2][3] However, these sustainable energy sources are usually intermittent and fluctuant in nature due to regional or seasonal factors, and thus, efficient energy storage solutions are highly demanded. 4,5 As a carbon-free energy carrier, hydrogen has long been hailed as an ideal alternative to fossil fuels apart from its high conversion efficiency and gravimetric energy density. 5- 7 To date, electrochemical water splitting to produce hydrogen is turning out to be a promising technology, of which alkaline water electrolysis (AWE) compared with the proton exchange membrane (PEM) electrolyzers exhibit inherent low-cost characteristic due to possibility of use of non-precious catalysts rather the noble ones. [8][9][10][11] Generally, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are two halfcell reactions of electrochemical water splitting. Compared with HER, OER, with a complex fourelectron transfer process, is recognized as the main bottleneck in water splitting because it is thermodynamically unfavorable and kinetically sluggish. [8][9][10][11][12][13] In addition, this half-cell reaction induces structural changes into electrode materials that cause performance degradation and ultimately lead to electrode breakdown. 14 Therefore, developing highly effective and durable electrocatalysts are required for the OER to afford high current density at low overpotential. 11,15 In general, noble metal-based electrocatalysts such as IrO2 and RuO2 are recognized as the state-of-the-art catalysts for the OER. 2,11,16,17 Despite their superior activity, the large-scale use of these noble-metal based catalysts is hampered by the high cost, scarcity and the poor stability over long-term operations. 12,[18][19][20][21] Accordingly, in order to replace these expensive electrocatalysts, extensive efforts have been devoted to developing alternative OER electrocatalysts based on the inexpensive and earth-abundant first row transition-metal (e.g. Fe,

Co, Ni, Cu), which are highly active and stable under OER conditions, thus making water splitting more practically feasible. [22][23][24][25][26][27][28][29][30] Although great progress has been made in designing these type of non-noble metal based catalysts, the majority of them are in the form of nanostructures including nanoparticles, nanosheets and nanoribbons, which are required complex preparation methods and also expensive conductive polymers as binder for electrode fabrication, minimizing their potential for practical applications. 5,21,[31][32][33][34] From an industrial perspective, nickel is typically used as the promising electrode material in commercial electrocatalytic electrolysis units in highly alkaline electrolytes owing to its relatively high stability and favourable activity. 35,36 Particularly, nickel foam (NF) with a long-range ordered porous framework, which not only exposes more reaction active sites but also accelerate fast ion and electron transport, is the most widely used conductive substrate for the construction of three-dimensional (3D) catalysts. 37,38 Unfortunately, as an oxygen evolution electrode (OEE), Ni metals possess low efficiency for water splitting. Doping of other transition metal oxides or non-metal elements at Ni site drastically improves its OER catalytic performance and stability in alkaline solution. [39][40][41] Among various nonprecious metal-based catalysts, nanostructured cobalt tetroxide (Co3O4) has received particular attention due to its high abundance, low cost, relatively high OER activity and durability in alkaline electrolytes. [42][43][44][45][46] In order to further ameliorate the catalytic activity by enhancing charge transport capacity and increasing the exposure of active sites, the rational construction and design of an advanced electrode configuration with active metal or metal oxides, a hybridized nanocarbon material (e.g.

reduced graphene oxide-r-GO or GO) is an appealing strategy. [47][48][49][50] Currently, OEE needs to meet the most stringent requirements in industrial applications such as delivery of very high current density (≥ 500 mA cm -2 ) at low overpotentials (≤ 300 mV), and certain mechanical robustness and prolonged durability during harsh oxidizing conditions and the complicated mechanism of the OER. 51,52 On the other hand, for large-scale industrial applications, AWEs also require the commodious scale production of large areal type electrodes (e.g. where the geometric surface area of the electrode can reach several square meters). At the same time, since the developed OEE needs to be stable in strong alkaline media (pH >14) and at an intermediate temperature (> 353K), a practical solution to ensure greater efficiency of operations during the hard anodic condition 53 cannot be avoided, especially for a metal element. Electrolytic codeposition is a process largely used for the reinforcement of submicron and nanosized metallic particles, polymer additives or nonmetallic compounds in the metal matrix. The co-deposition process is an auspicious alternative to deposit layers with appropriate microstructure and properties preventing the rise of residual stress. This cost-effective process not only involves a simple and controllable setup, but it also can be performed at approximately room temperature.

This process also is a widely used and effective methodology to apply coatings in a large scale and to create a layer uniformly covering the surface of components even with complex and irregular shapes.

Here, we report a co-doped 3D NF by Ni crystals, Co3O4 nanoparticles, GO nanosheets and in situ generated Ni(OH)2/NiOOH nanoflakes via electrolytic codeposition in combination with in situ electrochemical activation as a promising electrocatalyst (the activated NF@CGO) for OER in AWE.

The 3D interconnected bi-continuous macroscopic porous structure across the whole NF substrate offers a very high specific surface area, electrical conductivity and integrity. 52 The Ni crystals in the activated NF@CGO not only possesses the properties of good electrical conductivity, high specific surface area, but also the close contact of Co3O4 nanoparticles and GO nanosheets on the Ni foam, which waives the need for adding auxiliary conducting material or binders, allows for efficiently deliver large current densities at low OER over potential.

Interestingly, Ni(OH)2/NiOOH nanoflakes with ultra-high OER activity, which are formed by oxidative etching of fine micron-sized nickel grains in the subsequent electrochemical activation, will grow in situ and cover the surface of the electrode. Therefore, a catalytic layer with an arbitrary thickness, excellent electrical conductivity and high-efficient catalytic activity for OER can be obtained. While maintaining the sufficiently high performance, this uniform catalytic layer also allows maintain longterm durability of OEE without significant deterioration during harsh and oxidative OER operation condition, which is one of the drastic requirements necessary for industrial application. The development of such hierarchical nanostructures is an important step in advanced nanotechnology to effectively perceive better OER performance by intelligent combination of Multiplex components.

Effect of additives in electrolytic codeposition on OER performance

In order to reasonably reveal the synergistic effect of all active species on OER, polarization curves for NF, NF@N (blank), NF@GO (containing 0.05g L -1 SDS and 0.01 g L -1 GO), NF@C (containing 0.05g L -1 SDS and 0.02g L -1 Co3O4) and NF@CGO (containing 0.05g L -1 SDS, 0.01 g L -1 GO and 0.02g L -1 Co3O4) catalysts in 1 M KOH at RT with a scan rate of 1 mV s -1 are obtained. As shown in Figure 3-1a, NF@CGO generates the OER current density of 10 mA cm -2 at the lowest potential and the smallest overpotential of 140 mV and 1.37 V vs. RHE, respectively, as compared with other counterparts, NF (349 mV and 1.579 V vs. RHE), NF@N (339 mV and 1.569 V vs. RHE.), NF@GO (301 mV and 1.531

V vs. RHE) and NF@C (272 mV and 1.502 V vs. RHE). By comparing the overpotentials of different catalysts, obviously, Ni crystals, Co3O4 or GO alone also has contributions to OER, which is not more impressive. However, when all active species are reasonably combined to form NF@CGO, the most promising catalytic activity is produced, which eloquently clarifies their synergistic effect on OER.

Furthermore, as can be concluded from Nyquist plots (Figure 3-1b), the smallest interfacial Rct of the hybrid NF@CGO than the other catalysts clearly indicate its better electronic transport capability and better reactant diffusivity towards electrode surface, which clearly justify the superior OER performance of the hybrid NF@CGO and synergistic effect of all active species.

Effect of current density in electrolytic codeposition on OER performance

The current density of electrolytic codeposition may have effect on the OER catalytic performance of NF@CGO, because the current density may not only affect the deposition efficiency, but also determine the maximum particle diameter of the active additive that can be deposited on the NF surface.

Figure 3-2 showed OER performance of NF@CGO with different current densities in electrolytic codeposition (NF@CGO-0.5, NF@CGO-1.5 and NF@CGO-2.5 stands for electrolytic codeposition current density of 0.5, 1.5 and 2.5 A dm -2 ). Figure 3-2a showed the CVs appeared for the three kinds of NF@CGO electrodes. The area enclosed by the CV positively related the ECSA of the electrode, so the ECSA became larger along with the increase of the current density of electrolytic codeposition.

Meanwhile, the large slope (like Tafel slope from Polarisation curve) of the linear part of CV curve predicted more favorable and fast-catalytic OER kinetics, suggesting worsened OER kinetics with increasing current density of electrolytic codeposition. Hence, the NF@CGO-1.5 had better OER performance owning its larger ECSA and faster OER kinetics. Furthermore, as could be concluded from Nyquist plots (Figure 3-2b), the smallest interfacial Rct of the NF@CGO-1.5 than the other catalysts clearly indicated its better electronic transport capability and better reactant diffusivity towards electrode surface, which clearly justify the superior OER performance of the NF@CGO-1.5.

3.4

In situ electrochemical activation of a codoped heterogeneous system as a highly efficient OER electrocatalyst

Synthesis strategy of the NF@CGO

The hybrid NF@CGO electrode was prepared by co-doping the catalyst nanoparticles onto the 3D NF (see Experimental details). Figure 3-3 shows the schematic illustration of electrolytic codeposition, which is currently one of the most widely used electrochemical synthesis methods to fabricate the most active composite materials. 54 NF@CGO matrix composite coatings were electrodeposited in a modified Watt's bath by using electrolytic codeposition techniques. Electrolytic codeposition, with the mechanism of two successive adsorption steps, which was proposed by N. Guglielmi in 1972, is a versatile and scalable industrial process that only requires simple equipment. 55,56 A two-step electrolytic codeposition was carried using a two-electrode system through a field-assisted mechanism consisting of two steps. In the first step, which is of a physical nature, nanoparticles in the electrolyte were surrounded by a cloud of adsorbed species mainly metal ions approach the cathode and finally produce a layer of loosely adsorbed particles on the cathodic surface with a high coverage driven by fractional irregular Brownian motion (indicated by the random red arrows in Figure 3-3). Secondly, these nanoparticles released an adsorbed ion clouds and strongly adsorb on the electrode surface (indicated by the red arrows in the direction of the vertical electrode in Figure 3-3) due to the development of a higher electric field force under more negative potentials. 59 Finally this self-assembly 3D electrode (NF@CGO) has been fabricated using NF as the skeleton and also Ni crystals, Co3O4 nanoparticles and GO nanosheets as OER composite catalysts.

Physical characterization of the activated NF@CGO

Physical characterizations of the activated NF@CGO catalysts would be showed as below. It is found that, the deposition is uniform on the macroscopic 3D skeleton of NF resulting in high catalyst loadings and large amounts of accessible active sites (Figure 3-5a and Figure 3-4b). The mesoporous feature of the activated hybrid nanosheets observed by SEM and TEM was confirmed by Brunauer-Emmett-Teller (BET) measurement. The N2 adsorption/desorption isotherm curve (Figure 3-12a) possesses the typical feature of mesoporous materials, with a BET surface area of 150.6 m 2 g -1 , the total pore volume of 0.04 cm 3 g -1 and the average size of the mesopores is estimated to be about 4.8 nm (Figure 3-12a, inset). The large active surface area as well as the mesoporous structure, not only greatly increases the amount of exposed electroactive sites, but also promotes mass transfer and OH - diffusion, which is expected to improve the electrocatalytic performance. 42,58 The hybrid matrix of metallic nickel and GO nanosheets like a torus closely links the petals (Figure 3-5b), so that electrons can be transmitted to each petal as quickly as possible while exciting each active site for OER reaction.

The introduction of the heterogeneous catalytic phases in electrolytic codeposition changes the nickel nucleation mechanism, from homogeneous nucleation to heterogeneous nucleation, and refine nickel crystals which promote the oxidative etching of nickel in the process of in situ electrochemical activation due to the larger surface energy. This causes Co3O4 nanoparticles and GO nanoflakes originally covered by the nickel layer to be exposed to the electrolyte, with more active sites appearing. Moreover, nickel crystals with mesoporous caused by oxidation etching mentioned above are in situ conversion of nickel into Ni(OH)2/NiOOH nanoflakes with ultra-high OER activity. They form a complete catalytic system, which will eventually lead to a highly active and stable OEE. As physical parameters have a huge effect on the catalytic properties of OER catalysts, various analyses have been carried out on the activated hybrid NF@CGO. As shown in Figure 3-11 and Figure 3-13, the XRD patterns of the NF@CGO show sharp and well-defined peaks at 2θ = 36.5⁰, 43.1⁰, 45.1⁰, 70.2⁰ and 77.5⁰, which correspond to Co3O4 (ICSD no. 28158), which is in agreement with SEM and TEM results. 3,59 Figure 3-14 shows the comparative Raman spectra of commercial Co3O4, GO and the hybrid NF@CGO. Four distinct peaks in the range of 450-750 cm -1 for the hybrid NF@CGO are observed corresponding to E1g, F2g 2 , F2g 1 , and A1g vibrational modes of spinel Co3O4. Noticeably, these peaks are in a same position compared to those for the commercial Co3O4. 42 Additionally, hybrid NF@CGO also shows a pair of peaks at 475 and 554 cm -1 ascribable to the Ni-O vibration in NiOOH, and a broad peak at 800-1100 cm -1 , characteristic of "active oxygen" on the NiOOH surface, which is in agreement with TEM results. 57,60,61 On the other hand, peaks at 1350 cm -1 , 1600 cm -1 , 2700 cm -1 and 2900 cm -1 are identified for the hybrid NF@CGO corresponding to the D, G, 2D, and (D+G) bands of carbonaceous material, which is comparable with those in the GO Raman spectrum, further demonstrating the coexistence of GO in the hybrid NF@CGO illustrated by the elemental mapping images of SEM and TEM nanosheets ( along with their weak shake-up satellites at high binding energy side (Figure 3-16a), suggesting coexistence of Co 2+ and Co 3+ in the hybrid activated NF@CGO. The Co 2p peaks of hybrid activated NF@CGO match well with those of commercial Co3O4 except slight shift to higher binding energy side, which is probably ascribed to the fact that the species of Ni and C near Co3O4 in hybrid activated NF@CGO change coordination environments of Co ions. 63 The application of mixed metal oxides containing Co ions of different valence states usually results in the decrement of OER overpotential. 5 As illustrated in Figure 3-16b, the characteristic peaks of Ni from the hybrid activated NF@CGO is detected. In the Ni 2p region, the fitting peaks at 874. There are two kinds of distinct peaks with varying detection points in the hybrid activated NF@CGO. In the spectra of some sites, the peek still retained the characteristics of GO, but the peak intensities associated with oxygenated groups are much weaker than those in the spectrum of GO for other sites. Small amount of oxygen-containing functional groups of GO nanosheets in the hybrid activated NF@CGO was probably reduced to electrochemically reduce graphene oxide (ERGO) by gaining electrons from the cathode during the electrolytic codeposition process, eventually formed GO/ERGO, which further strengthens the OER electrocatalytic activity of the hybrid activated NF@CGO. 62,[64][65][66][67] These results further confirmed the formation of hierarchically structured activated NF@CGO, which are composed of the deposited Co3O4, GO/ERGO and in situ generated Ni(OH)2/NiOOH, onto the 3D NF. XPS spectra of the hybrid activated NF@CGO.

Electrocatalytic oxygen evolution at the activated NF@CGO

To better characterize electrochemical performance, besides the evaluation of OER performance of electrode in 1M KOH at room temperature (RT), electrochemical tests is also conducted on half-cell in 6 M KOH solution and 353 K in three electrode system to meet rigorous industry requirements.

Electrochemical characterizations of NP, NF and the hybrid activated NF@CGO in 1 M KOH at RT are shown in Figure 3-17. In order to quantitatively compare the performance of all Ni-based catalysts, polarization curves for NP, NF and the hybrid activated NF@CGO catalysts in 1 M KOH at RT with a scan rate of 1 mVs -1 are also obtained as shown in Figure 3-17a. The potential required for water oxidation to obtain the current density of 10 mA cm -2 is commonly used to judge the OER activity. 70 As can be concluded from Figure 3-17a, interestingly, the activated NF@CGO generates the OER current density of 10 mA cm -2 at a much lower potential and smaller overpotential of 212 mV and 1.422V vs. RHE, respectively, as compared with other counterparts, NF (349 mV and 1.579 V vs. RHE) and NP (434 mV and 1.664 V vs. RHE.), which is considerably much closer or even more superior than the earlier reports (as shown in Table 3-1). Therefore, as based on the above observation, the activated NF@CGO showed an excellent OER activity with a very low overpotential, whereas bare NP and NF show hardly any OER activity. CoxFe1-xN0.5 (Glass carbon) 71 1 M KOH 10 266 30 2019

Ni QD@NC@rGO (Glass carbon) 72 1 M KOH 10 265 65 2019

CoZn-Se (Glass carbon) 73 1 M KOH 10 320 66 2019

CoOx-NeC/TiO2C (Glass carbon) 74 1 M KOH 10 220 75 2019

S|NiNx-PC/EG (Glass carbon) 75 1 M KOH 10 280 45 2019

NiFeMo IOS@ (NF) 76 1 M KOH 10 198 36 2019

Ni-Co3O4 nanosheets (NF) 77 1 M KOH 10 170 76.9 2018

OESSC (Stainless Steel) 78 1 M KOH 10 290 38 2019

Co3O4/MoS2 (NF) 79 1 M KOH 10 205 45 2019

Co3-xAlxO4 (Glass carbon) 80 1 M KOH 10 248 80.6 2019

Ni-BDC/Ni(OH)2 (Glass carbon) 81 1 M KOH 10 320 41 2019

Ni-Co-hydroxide/Ni2P2O7 (NF) 82 1 M KOH 10 197 63 2019

Fe-Co3O4 H-NSs (NF) 83 1 M KOH 10 204 38 2018

Ni3S2 (NF) 84 1 M KOH 10 312 111 2018 NF@Ni/C (NF) 85 1 M KOH 10 256 62 2018

Fe(II)-MOF-74 (NF) 86 1 M KOH 10 207 41.1 2019 IP/NP (NF) 87 1 M KOH 10 185 47 2019

NiCoSe2@NiO@CoNi2S4@CoS2(NF) 88 1 M KOH 10 353 159. 1 2018 Ni3(S1-xSex)2@NiOOH (NF) 89 1 M KOH 10 320 48 2018

Moreover, as shown in Figure 3-17b, the hybrid activated NF@CGO catalyst exhibits much smaller Tafel slope (43 mV dec -1 ) as compared with that of NP (126 mV dec -1 ) and NF (98 mV dec -1 ) catalysts suggesting more favorable and fast-catalytic OER kinetics (OER intermediate adsorption and desorption)

for the activated NF@CGO hybrid catalyst in an alkaline environment compared with other counterparts.

Moreover, the electrochemical double-layer capacitance (Cdl), which is assumed to be directly proportional to the electrochemical active surface area (ECSA), is ascertained from CV in a potential scan range of 1.07-1.17 V vs RHE where the Faradic current is negligible (Figure 345678910111213141516171819). The improved Cdl of the activated NF@CGO hybrid catalyst (12.39 mF cm -2 ) compared to NP (0.44 mF cm -2 ) and NF (0.96 mF cm -2 ) suggests a higher ECSA of the activated NF@CGO. The much superior ECSA of the activated NF@CGO compared with other catalysts would be likely a key role for its high OER performance. As can be seen in Figure 3-17c, the much superior Cdl and consequently, ECSA of the activated NF@CGO along with hierarchal structure and more catalytically active sites compared with other catalysts would be likely key factors make the activated NF@CGO a catalyst shows the much higher overall OER performance in terms of lower overpotential and higher current density, than other

Ni-based catalysts. The smaller Cdl value of NP compared with that of NF possessing a 3D porous morphology with a large surface area can be attributed to the its two-dimensional-planar morphology with low surface area. Figure 3-17d shows the Nyquist plots based on EIS analysis. The Nyquist plots fitted well with the equivalent circuit model presented in the inset image, where Rs represents the solution resistance (i.e. the crossover point at high frequency with the real axis) and Rct is ascribed to the interfacial charge transfer resistance at the catalyst/electrolyte interface, which is directly related to the catalytic kinetics. 21,90,91 The smaller interfacial charge transfer resistance of the hybrid activated NF@CGO than the NP and NF catalysts clearly indicates its better electronic transport capability and better reactant diffusivity towards electrode surface, which clearly justify the superior OER performance of the hybrid activated NF@CGO. Furthermore, we tested the catalytic performances of the hybrid activated NF@CGO electrode under extreme conditions in concentrated KOH solution, intermediate temperature and very high current, which are quite often encountered in commercial alkaline water electrolyzers. As shown in Figure 3-20a, long-term durability OER performance of the hybrid activated NF@CGO was investigated using a chrono potentiometric method by applying a current density of 800 mA cm -2 in 6 M KOH at 353 K. The hybrid electrode exhibited exceptional stability, with almost constant overpotential of 350 mV (230 mV with iR correction) over 100 h, indicating that the hybrid activated NF@CGO electrode was catalytically stable and mechanically robust during the harsh anodic condition. The polarization curve of the activated NF@CGO hybrid after 100 h shows no deterioration compared with the original one, suggesting that the fast-catalytic kinetic is still maintained after the 100 h operation (Figure 3-20b). In addition, the activated NF@CGO hybrid maintained its morphologies after long term electrocatalytic performance indicative of its stable and good structural properties (Figure 3-21). The hybrid activated NF@CGO has the potential to meet the long-term stability of catalytic performance for future commercial AWE at higher current densities with the lowest overpotential. The water electrolyzer with H2 and O2 bubbles generated show in Figure 3-20c.

Discussion

The nickel nanolayer covering on the surface of the hybrid activated NF@CGO during electrolytic codeposition would be slowly transformed to Ni(OH)2/NiOOH nanoflakes upon in situ electrochemical activation. 57 Etching will inevitably occur under harsh conditions (1. The aforementioned results reveal the high catalytic activity and excellent stability of the hybrid activated NF@CGO when used as an electrocatalyst for OER. Such an excellent OER performance could be explained as follows. Firstly, compared to the bare NP and NF, the hybrid activated NF@CGO possessed more exposed active sites, which were favourable for the proton-coupled electron transfer process during the water oxidation reaction. Secondly, the unique structural features of the activated hybrid NF@CGO (i.e. a large surface area and hierarchical structures and interconnected electrically conductive networks) facilitated the access of the electrolytes to the catalysts. Thirdly, the electrolytic codeposition process with self-assembly was an effective method for the formation of uniform catalysts' layer on 3D NF, resulting from the intact interaction between them without any binders. This could facilitate the rapid electron/charge transfer in the OER, accelerate the reaction kinetics of electrocatalysis and guarantee prolonged durability.

Summary

In summary, a highly efficient, freestanding hybrid activated NF@CGO with hierarchical structures was prepared by the self-assembly of Ni crystals, Co3O4 nanorods, GO/ERGO nanoflakes and in situ generated Ni(OH)2/NiOOH nanoflakes onto 3D NF via electrolytic codeposition process followed by in situ electrochemical activation process. Based on our experiment, the obtained hybrid NF@CGO exhibits excellent catalytic activity and stability for OER in both medium and harsh alkaline solution, which is comparable or even better than the state-of-the-art related OER electrocatalysts, making the hybrid activated NF@CGO a promising candidate for a next-generation OER electrocatalyst.

Remarkably, the synergistic effects of the hierarchical structure with uniform distribution of the interconnected active species, which offer a large active surface area, fast mass transport and fast electron transport in the electrode led to the promising catalytic performance. Furthermore, the activated NF@CGO showed pronounced long-term operational stability (in 6 M KOH at 353 K) with a very lower overpotential (350 mV) at the constant current density of 800 mA cm -2 over 100 h with no deterioration.

We ensured excellent and durable performance for industrial applications in the harsh oxidative anodic condition by the design of a thick enough catalyst layer with a uniform distribution over the 3D NF. It is anticipated that the electrolytic codeposition approach can be easily adapted in industry for large-scale production. This work might provide a strategy to design highly active, stable and universally available 3D porous electrocatalysts for water splitting and inspire the development of next-generation electrocatalysts for renewable energy applications.

Chapter IV Development of OER electrocatalysts by combining high-velocity oxy-fuel spraying and in situ electrochemical activation

Introduction

As mentioned before, OER is more energy-consuming due to the high thermodynamic potential required for water oxidation (1.23 V vs NHE at pH 0) and the sluggish kinetic hindrance of the multielectron evolving charge-transfer reactions. 1,2 Recently, earth-abundant transition metal-based oxides, (oxy) hydroxides, and their derivatives have been extensively used as catalysts for OER due to their tunability, abundance, and potential stability in alkaline electrolyte. [3][4][5] Nevertheless, the suboptimal intrinsic activity, the limited active sites exposure and the inherently poor electronic conductivity are key problem to hamper further catalytic performance enhancement of these catalysts. [6][7][8] Focusing on these issues, several rational strategies were employed to further develop OER catalysts with abundant active sites exposure, fast charge transfer ability and super electronic conductivity. Han et. al increased the density of the crystalline-amorphous phase boundary to enhance the intrinsic activity and electronic conductivity of the catalyst. 1 Shinagawa et.al induced "active oxygen" to improve the intrinsic activity of the NiOOH nanosheets. 5 Chen et. al systematically demonstrated transition metal chalcogenides derived nano porous oxides as excellent water oxidation catalysts by increasing active sites exposure. 2 Meanwhile, induced lattice distortion for efficiently triggering hydrogen evolution by enhancing intrinsic activity were explored extensively by researchers. 9, 10 These mysterious structure defects, which can greatly increase the catalytic activity, still need to be further explored and utilized.

Moreover, OER is a process involved electrocatalytic reactions at the boundary of gas-liquid-solid phases. Catalytic reactions take place at the interface of the catalyst and the electrolyte, catalytic activity can also be restricted due to the lack of bubble transport channels. 11,12 In practice, rational design of the OER catalyst architecture can dramatically promote activity in electrocatalytic water splitting. 13,14 Therefore, ingenious designing the morphology, structure, and composition of the electrocatalysts is pivotal for achieving high catalytic activity towards OER. To achieve effective ways of further promoting OER performance at the three-phase boundary, the design of 3D supports with facile charge/electrolyte/bubble migration and higher catalysts loading contributions to the catalytic OER activity has to be further explored. Recently, it has been recognized that the catalytic performance of electrocatalysts would be extremely optimized due to the improvement of intrinsic activity or active sites exposure upon in situ electrochemical oxidation tuning process, in which the electrocatalysts experienced morphology, structural or compositional changes with nanopore and nanostructure defects, emerging amorphous phase, oxygen vacancy and lattice distortion. [15][16][17][18][19] Thermal spraying processes which are cost-effective and rapid manufacturing, have shown their capability to deposit electrocatalytically active electrode coatings for water splitting mainly by increasing large surface areas. [20][21][22][23][24][25][26] Among different thermal spraying techniques, high velocity oxy-fuel (HVOF) is expected to be a promising method for producing active electrode coatings for water splitting for the reason that HVOF characterized by high particle velocity and relatively low temperatures allows deposition of surface coatings with low-oxide contents. 24 However, it is far from satisfying industrial needs to obtain excellent catalytic electrodes for water splitting simply by increasing the active surface area. Therefore, it is urgent to use coatings obtained by thermal spraying process as precatalysts, combined with some post-treatment methods to greatly improve the intrinsic activity of the catalytic electrodes.

Enlightened by the above discussion, herein, we report an effective approach for phase, surface, and morphology of elaborated 3D hierarchically porous architecture with lattice-dislocated nickel (oxy) hydroxide nanosheets in situ growth, namely HNA-CA-ECA, for OER electrocatalyst application. The 3D hierarchically porous architecture is properly designed by chemical dealloying activation (CA) of the high-velocity oxy-fuel (HVOF) sprayed Ni-Al alloy coating (HNA) with micro-voids randomly distributed. In the CA process of HNA, the aluminum element in the coating is selectively etched away, leaving a large number of nanopores and capillary channels, which constitute a hierarchically interlaced 3D network (HNA-CA) together with micro-voids remaining in precursor HNA. The HAN-CA not only structurally prepares for the formation of the lattice-dislocated nickel (oxy) hydroxide nanosheets carrying optimal intrinsic activity, which are subsequently in situ generated by electrochemical activation (ECA), but also delicately provides a huge surface for in situ growth of these highly active nanosheets and a typical 3D hierarchical porous network ensuring efficient and rapid charge/electrolyte/bubble migration.

HNA-CA-ECA as a highly efficient OER electrocatalyst

Synthesis of the HNA-CA-ECA

Firstly, the precursor HNA coating was prepared by depositing Ni-Al alloy powders on the nickel plate (NP) via HVOF. The porous structure was designed by leaving some micro-voids in the coating with specific HVOF parameters (Figure 4-1a). As a novel process of catalyst deposition, HVOF was employed to obtain a well-bonded precursor coating to meet the long-term catalytic stability used as a catalyst because the supersonic Ni-Al alloy particles ejected from the HVOF gun can instantly break up and remove the oxide layer from the surface of the newly obtained coating. 27 The crystal structures of the obtained HNA coating were maintained compared with those of the feedstock powders, both of which were composed of nickel-rich phase (NiAl phase) and aluminum-rich phase (Ni2Al3 phase) by an examination of the X-ray diffraction (XRD, Figure 4-2). Subsequently, the 3D hierarchically porous architecture, HNA-CA, was elaborated by CA of the HNA in the 6 M KOH aqueous solution (contained 10 wt.% KNa tartrate-tetrahydrate) at 353 K for 24 h. As shown in Figure 4-2b, a large number of microchannels/microcracks appeared resulting from the leaching of aluminum atoms out of the aluminum-rich phase. Moreover, the zone near the micro-voids left in the precursor HNA coating was more severely etched. To avoid precipitation of aluminum hydroxide in the microchannels/microcracks of the obtained sponge-like structure -HNA-CA, KNa tartrate-tetrahydrate (Rochelle salt) as a complexing agent was added into the strong alkali aqueous solution to formation of water-soluble complexes by complexation of aluminum hydroxide. 28 Together with micro-voids left in the precursor HNA coating, a hierarchically interlaced 3D architecture was constituted, giving rise to the strikingly enhanced active surface area, accelerated electron/ion transport, and further promoted reaction kinetics of OER. The morphology of the 3D architecture could be inherited into the as de-alloyed products, and it was called as DIE or corrosion inheritance effect. 29 Notably, these microchannels/microcracks emerged by etching under the alkali aqueous solution, resulting that all fresh surface would be completely wetted by the almost identical alkali electrolyte and entirely participated in the OER process. Simultaneously, a large number of mesopores were mainly distributed at the boundary of the core-shell nanostructure. It could be concluded that aluminum was selectively removed during CA, and the retained mesoporous Ni crystallites was spontaneously oxidized, which was an ideal way to grow NiO outside the Ni crystallites core. 29 Benefiting from the unique architecture of mesoporous Ni/NiO-core/shell nanostructures, the OER performance of HNA-CA catalysts would be promoted significantly. Notably, the Jamborite-(Ni(OH)2/NiOOH hybrid) nanosheets had huger active specific surface areas due to the genetic structure of the porous precursor HNA-CA, and more importantly, ECA also triggered more mesoporous defects, which were assessed by N2 adsorption-desorption measurements.

In addition to the HNA sample, the N2 adsorption-desorption isotherms of the HNA-CA and HNA-CA-ECA samples (Figure 4-11a) could be classified as type IV with a hysteresis loop, indicating that the obtained samples were mesoporous materials. The Brunauer-Emmett-Teller (BET) surface areas were calculated to be 9.1, 33.0, and 77.7 m 2 g -1 for HNA, HNA-CA, and HNA-CA-ECA, respectively.

Additionally, the mesoporous feature of the samples was confirmed by the pore size distribution (Figure 4-11b). The average pore diameters are in the mesoporous region. To further reveal structural defects at lower latitudes, high-resolution transmission electron microscopy (HR-TEM) was employed. To eliminate the interference of intermetallic compounds in XRD detection of nickel oxides, (oxy) hydroxides during the CA and ECA, Raman spectroscopy was further utilized to investigate the oxide phases in the samples, as shown in Figure 4-14As expected, no apparent peaks were observed on HNA, consistent with its metallic nature. Meanwhile, the Raman peaks in the HNA-CA sample cantered at 450 and 560 cm -1 can be assigned to the first-order transverse optical mode (TO) and one-phonon longitudinal optical mode (LO) of NiO, respectively. 18,[38][39][40] However, the broadening of Raman peaks suggested the high degree of disorder 29 The Ni 2p high-resolution XPS spectra of the samples were shown in Figure 4-15a. In the Ni 2p region from HNA sample, the binding energies (BEs) at 852.9 and 870.3 eV could be assigned to metallic Ni (Ni 2p1/2 and Ni 2p3/2, respectively, identified as "Ni 0 ") from the Ni-Al alloy coating, while the BE at 855.8 eV was corresponded to shakeup satellite (identified as "Sat.") of Ni 0 . Notably, after the CA process, the Ni 0 peaks weakened, and two new pairs of peaks (873.2 and 855.6 eV) appeared, which corresponded to Ni 2+ from NiO present in the HNA-CA sample, indicating that spontaneous oxidation occurs during CA. 17,18 Impressively, a comparison of the spectra of the HNA-CA-ECA sample with that of HNA-CA clearly showed that the Ni 3+ (the BEs at 875.6 and 857.9 eV) appeared with the Ni 0 peaks vanishing and the Ni 2+ weakening, manifesting the acceleration of Ni 0 being oxidized to NiO and Ni(OH)2 (identified as "Ni 2+ ") and further oxidized NiOOH (identified as "Ni 3+ ") due to the large amount of oxygen released during the ECA process in the HNA-CA-ECA catalysts. 31,33,44 Note that the incorporation of Ni 3+ into the primitive lattice of Ni/NiO nanoparticles upon ECA, generating a local lopsided Coulomb force, would cause a subtle atomic rearrangement, providing a large number of electrochemical reaction active sites. 10 The HR-TEM image from Figure 4-12a advantageously demonstrated this phenomenon. In the O 1s region from the HNA-CA-ECA sample (Figure 4-15b), the BEs at 531.8 and 530.7 eV can be indexed to oxygen-hydrogen and nickel-oxygen bonds, respectively.

The peak at 533.8 eV can be attributed to adsorbed water or possibly adsorbed O2. 44 As indicated in A commercial IrO2 catalyst loaded on NF (marked as IrO2/NF) and bare NP, bare NF and HNA-CA samples were also measured as controls. To prepare the IrO2/NF electrodes, 100 mg IrO2 (Alfa Aesar), 200 μL Nafion (Alfa Aesar), 1 mL ethanol and 1 mL deionized water were ultrasonicated for 60 min to obtain a homogeneous dispersion. Then, three pieces of clean NFs were dipped into the dispersion, which were then dried in air at 333 K for 6 h. The mass loading of IrO2 catalyst on nickel foam was controlled to be ca. 3 mg cm -2 . In order to quantitatively compare the performance of all Ni-based catalysts, polarization curves for NP, NF, HNA-CA and the HNA-CA-ECA catalysts in 1 M KOH at RT with a scan rate of 1 mV s -1 are also obtained, as shown in Figure 4-17a. The potential required for water oxidation to obtain the current density of 10 mA cm -2 was used to judge the OER activity. As can be concluded from the crossover point at a high frequency with the real axis) and Rct was ascribed to the interfacial charge transfer resistance at the catalyst/electrolyte interface, which was directly related to the catalytic kinetics.

The smaller interfacial charge transfer resistance of the HNA-CA-ECA compared to the HNA-CA clearly indicated its better electronic transport capability and better reactant diffusivity towards the electrode surface, which clearly justified the superior OER performance of the HNA-CA-ECA catalysts.

The unfavorable catalytic OER kinetics and electronic transport capability of HNA-CA might be restricted by the intrinsically poor electronic conductivity of the amorphous phase (Figure 4-5) commonly discovered between Ni and NiO in HNA-CA. 1 Additionally, the electrochemical double-layer capacitance (Cdl, as shown in Figure 4-17d), which was assumed to be directly proportional to the electrochemical active surface area (ECSA, Figure 4-18), was ascertained from CV in a potential scan range of 1.07-1.17 V vs. RHE where the Faradic current was negligible. The improved Cdl of HNA-CA-ECA catalysts (71.7 mF cm -2 ) compared to NP (0.3 mF cm -2 ), NF (1.4 mF cm -2 ), and HNA-CA (56.3 mF cm -2 ) suggested a higher ECSA of HNA-CA-ECA catalysts. The much superior ECSA of HNA-CA and HNA-CA-ECA catalysts compared to other catalysts would likely be one of key reasons for their high OER performance. Consequently, the hierarchically interlaced 3D network HNA-CA-ECA led to the exposure of a large number of electroactive sites to electrolyte ions for OER, while the coexistence of mesopores (2-50 nm, between inter-nanosheets) and macropores (> 50 nm, between inter-microspheres and micro-voids) effectively shortened ions transports paths and efficiently promoted ions diffusion of the electrolyte. By virtue of its large specific surface area and hierarchical porous structure, a much superior Cdl was obtained, resulting in a promising OER performance in terms of lower overpotential and higher current density than other Ni-based catalysts. Despite being a 3D structure, the Cdl value of HNA-CA was nearly 50 times larger than NF because of the unique hierarchically interlaced 3D mesoporous network upon CA.

The smallest Cdl value of NP compared to that of NF possessing a 3D porous morphology with a large surface area could be attributed to its two-dimensional-planar morphology with low surface area (Figure 4-19). These results were considerably much closer to or even more superior than the earlier reports (Table 4-1).

Discussion

The above data confirmed that the promising OER catalyst, HNA-CA-ECA, was obtained upon Finally, although HNA-CA did not obtain a bad OER performance mainly because of the appearance of the Ni/NiO-core/shell nanostructures and a large amount of amorphous boundaries between them, which unfortunately ineluctably attached the obstacle of poor interparticle electron migration during the OER process, resulting in lower OER catalytic kinetics. 9 In this regard, the concept that introducing subtle distortion of atomic arrangement into the ultrathin nanosheets was emerged in response to the above-mentioned drawback, the benefits of which could maintain the original electron conjugated system along the two-dimensional plane and be conducive to quick electron transfer between the domain boundaries.

Summary

In conclusion, an effective strategy has been developed to induce defects in the HNA-CA-ECA catalysts, including nanostructure defects (lattice distortions) for enhancing intrinsic catalytic activity and morphology defects (micro-voids, microchannels/microcracks, and mesopores) for increasing active surface area and accelerating electron/ion transport, which finally, resulted in an active and stable OER electrocatalyst. The factors of inducing lattice distortion in HNA-CA-ECA catalysts mainly involved two aspects: one was due to the change of the valence state of nickel, causing local lopsided Coulomb force and the emergent micro-mechanic filed derivate disturbance; the other was thanks to the severe strain of nanosheets from the mechanical stress generated by the formation and release of a large number of oxygen molecules in OER and the thermal stress induced by the circumstance temperature (353 K).

Notably, the novel methodology of combining conventional thermal spray process and twoconsecutive-activation has the potential to produce large-size-electrode for AWE, which can broaden our horizons and provide new dimensions in design of highly active and stable self-supporting OEEs.

Meanwhile, it is anticipated that a series of self-supporting electrodes can be developed using this effective and environmentally friendly strategy.

Chapter V Electrochemical characterizations of dynamic self-optimized catalysts in hydrogen evolution reaction and overall water splitting

Introduction

Efficient development of low-cost bifunctional electrocatalysts with superior activity for both the sluggish anodic OER and the fairly facile cathodic hydrogen evolution reaction (HER) is of great importance for the widespread application of the water-splitting technique. [1][2][3][4][5][6] Unfortunately, active OER catalysts generally work well in a neutral or basic medium, whereas most HER catalysts perform better in an acidic medium, which complicates the water-splitting system due to requirements for different accessories and preparation and optimization procedures. [7][8][9][10][11][12] Thus, the search for cost-efficient bifunctional catalysts with high activity for a full water-splitting cell in the same electrolyte is of key importance. [13][14][15][16] The evolution of bifunctional electrocatalysts has diminished the complexity of catalytic setup and further reduced the overall cost because a single electrolyte, single electrocatalyst, and single container are utilized in this system, which could greatly facilitate the advancement of hydrogen technologies. 1,[17][18][19][20] In situ ECA, as a new pretreating technique for enhanced catalytic performance for water splitting, mainly comprises electrochemical processes such as OER 17, and HER 17, 23-25, 27, 47-49 . The in situ ECA strategy utilizes as-prepared materials as precatalysts to create active species on surfaces in situ, which yields a stronger binding force between scaffolds and active films. Such a unique hybrid structure may be beneficial for faster electron transportation and structural stability. Additionally, during in situ ECA treatments, the catalyst surfaces are modified from the bulk phase, which can allow for tuning local electronic structures, creating more active species, increasing surface area and thus improving catalytic performance. This technique can also couple atomic, electronic structures with electrocatalysis mechanisms for water splitting. Therefore, in situ ECA techniques show advantages over traditional optimization treatments, such as facile operation, variable control, high efficiency, flexibility, and being eco-friendly. 50 Notably, in situ ECA is a dynamically self-optimizing (DSO) mechanism. It has gradually come to be used as a strategy for developing bifunctional electrocatalysts 17,[23][24][25][26][27]51 for efficient and durable overall water splitting because these special precatalysts can be dynamically self-optimized under both the anodic and cathodic environments, inducing high-active substance catalyzing water splitting grown in situ on the surface of the precatalysts.

In the work presented in this chapter, we developed HEEs (marked as H-NF@CGO, H-HNA-CA) by DSO of the precatalysts (NF@CGO and HNA-CA), and coupled them with previously discussed Additionally, the electrode kinetics for HER were further measured by electrochemical impedance spectroscopy (EIS), as shown in Figure 5-2. The H-NF@CGO exhibits much lower charge transfer resistance than H-NF@GO, state-of-the-art Pt foil, H-NF@N, and H-NF confirming the more favorable HER kinetics of the H-NF@CGO. The incorporation of GO and Co3O4 into Ni crystals possibly induces significantly enhanced HER activity in comparison to the individual H-NF@N, suggesting the synergistic effect of Ni, GO, and Co3O4 for HER. Moreover, the synergy between the restructuring NF@CGO skeletons and their supported newly constructed active phases may also promote the catalytic activity of the H-NF@CGO. To further understand the superior HER catalytic activity of H-NF@CGO, the electrochemically active surface areas (ECSAs) of the prepared catalysts were measured, as shown in Figure 5-3(a-c).

Briefly, the ECSAs were determined through capacitance measurements using a simple cyclic voltammetry (CV) method in the non-Faradaic region of 0-0.10 V vs RHE. Based on the measured capacitances (1/2 slope, Figure 5-3d), which are proportional to the effective active surface areas, the Cdl value of H-NF@CGO (18.09 mF cm -2 ) is significantly higher than that of H-NF (1.36 mF cm -2 ) and

Pt foil (4.82 mF cm -2 ), suggesting that H-NF@CGO has a much larger ECSA. This result demonstrates that the large ECSA resulting from the constructional features of the H-NF@CGO contributes to its high catalytic activity. The stability of the catalyst is another important indicator in practical devices. Hence, a Chronoamperometry test (Figure 5-4) was carried out at a constant current density of -100 mA cm -2 in 1 M KOH solution to compare the operational stability of the H-NF@CGO catalyst with that of the state-of-the-art commercial Pt foil. As can be seen, the H-NF@CGO catalyst exhibited a remarkable stability and completely maintained its high catalytic activity at 8 hours of continuous operation, whereas the Pt foil displayed obvious attenuation (overpotential decay to 16.1%) after the stability test for 8 hours. Notably, the self-supported H-NF@CGO catalyst is mainly obtained upon simple and effective in situ ECA based on HER. The sharp contrasts confirm the outstanding durability of H-NF@CGO catalyst, and further demonstrate that this kind of self-supporting electrode based on in situ ECA is an effective development for improving the stability of catalysts. This is due to in situ growth of active substances on the surface of the 3D porous NF@CGO framework, which avoids the disadvantages of using traditional conductive adhesives. promoting effect of the restructuring NF@CGO skeletons also contributes to the enhanced HER catalytic activity and high durability of the porous H-NF@CGO electrode.

O-NF@CGO (+) ||H-NF@CGO (-) electrocatalytic overall water splitting

Encouraged by the exceptional activity and stability of the DSO NF@CGO for OER and HER, an alkaline water electrolyzer was constructed by applying O-NF@CGO and H-NF@CGO as anode and cathode materials. Notably, an overall water splitting current density of 10 mA cm -2 was attained at only 1.68 V vs. RHE cell potential (polarization curve) in the two-electrode configuration, while NP (NP (+) ||NP (-) ), NF (NF (+) ||NF (-) ), and state-of-the-art commercial (IrO2/NF (+) ||Pt (-) ) required cell potentials of 1.88 V vs. RHE, 1.82 V vs. RHE, and 1.72 V vs. RHE, respectively, suggesting the superior electrocatalytic O2-H2-evolving activity of the DSO NF@CGO (Figure 5-5 a). To the best of our knowledge, DSO NF@CGO is comparable with other noble-free metal-based bifunctional catalysts reported to date. 24,25,27 Additionally, the two-electrode configuration kinetics for overall water splitting were further measured by EIS, as shown in Figure 5-5 b. The DSO NF@CGO (O-NF@CGO (+) ||H-NF@CGO (-) )

exhibits much lower charge transfer resistance than NP (NP (+) ||NP (-) ) and NF (NF (+) ||NF (-) ), and is comparable with state-of-the-art commercial (IrO2/NF (+) ||Pt (-) ), confirming the favorable overall water splitting kinetics of the DSO NF@CGO (O-NF@CGO (+) ||H-NF@CGO (-) ). The synergy between the restructuring NF@CGO skeletons and their supported newly constructed active phases upon DSO may promote the catalytic activity of the DSO NF@CGO (O-NF@CGO (+) ||H-NF@CGO (-) ). Moreover, when a constant current density of 10 mA cm -2 was applied across the anode and cathode to monitor the durability of the DSO NF@CGO (O-NF@CGO (+) ||H-NF@CGO (-) ), almost no activity loss was observed over a period of 26 hours with a lower overpotential ca. 460 mV (Figure 56), which indicates the excellent catalytic activity and exceptional robustness of the DSO NF@CGO electrodes in comparison with state-of-the-art commercial (IrO2/NF (+) ||Pt (-) ) (with overpotential ca. 610 mV only over a period of 12 hours), and NF (+) ||NF (-) (with overpotential ca. 700 mV over a period of 6 hours). In summary, H-NF@CGO was prepared by dynamic self-optimization of the fresh NF@CGO to effectively catalyze HER. The resultant H-NF@CGO catalyst exhibited exceptional HER performance.

In addition, the DSO NF@CGO (O-NF@CGO (+) ||H-NF@CGO (-) ) delivered excellent activity and stability when applied as the cathode and anode for HER and OER, and the attained activity levels surpass those of well-known state-of-the-art noble metal-based catalysts.

Electrochemical characterization of the activated HNA-CA

electrocatalyst in HER and overall water splitting

Electrocatalytic hydrogen evolution of the H-HNA-CA

The HER electrocatalytic performance of the H-HNA-CA was investigated using a typical three- To further understand the superior HER catalytic activity of H-HNA-CA, the ECSAs of the prepared catalysts were measured, as shown in Figure 5-9. Briefly, the ECSA was determined through capacitance measurements using the CV method in the non-Faradaic region of -0.09-0.03 V vs RHE.

Based on the measured capacitances (1/2 slope, Figure 5-10), which are proportional to the effective ECSA, the Cdl value of H-HNA-CA (30.88 mF cm -2 ) is significantly higher than that of HNA-CA (8.18 mF cm -2 ), suggesting that H-HNA-CA has a much larger ECSA. This result demonstrates that the large ECSA resulting from the constructional features of the H-HNA-CA contributes to its high catalytic activity. (-) ), NF (NF (+) ||NF (-) ), and state-of-the-art commercial (IrO2/NF (+) ||Pt (-) )

required cell potential of 1.88 V vs. RHE, 1.82 V vs. RHE, and 1.72 V vs. RHE, respectively, suggesting the superior electrocatalytic O2-H2-evolving activity of the DSO HNA-CA (Figure 5-12a). To the best of our knowledge, DSO HNA-CA is comparable with other noble-free metal based bifunctional catalysts reported to date. 24,25,27 Additionally, the two-electrode configuration kinetics for overall water splitting were further measured by EIS, as shown in In addition, when a constant current density of 10 mA cm -2 was applied across the anode and cathode to monitor the durability of the DSO HNA-CA (O-HNA-CA (+) ||H-HNA-CA (-) ), almost no activity loss was observed over a period of 69 hours with a lower overpotential ca. 350 mV (Figure 5- 6), which renders the excellent catalytic activity and exceptional robustness of the DSO HNA-CA electrodes in comparison to state-of-the-art commercial (IrO2/NF (+) ||Pt (-) ) (with a overpotential ca. 610 mV only over a period of 12 hours), and NF (+) ||NF (-) (with a overpotential ca. 700 mV over a period of 6 hours). In summary, H-HNA-CA was prepared by DSO of the fresh HNA-CA to effectively catalyze HER.

The resultant H-HNA-CA catalyst exhibited exceptional performance towards HER. In addition, the DSO HNA-CA (O-HNA-CA (+) ||H-HNA-CA (-) ) delivered excellent activity and stability when applied as the cathode and anode for HER and OER, and the attained activity levels surpass those of well-known state-of-the-art noble metal-based catalysts.

Chapter VI Conclusions and perspectives nanosheets as the highly active substance of OER, the extremely high specific surface, and the accelerated electron/ion transport associated with the elaborated 3D hierarchically porous architecture.

Notably, the novel methodology has the potential to produce large-sized electrodes for AWE, which can broaden the horizons of the field and provide new dimensions in the design of highly active and stable self-supported OER electrocatalysts.

Furthermore, we have also explored initial development of effective HEEs upon in situ ECA based on HER, coupled with the obtained superior OEEs forming two-electrode configurations, respectively, both of which rival the integrated state-of-the-art IrO2 (+) /Pt (-) electrode in alkaline overall water splitting.

In summary, a methodology of fabricating easy-to-commercialize, high-performance catalytic electrodes by combining electrolytic codeposition or HVOF followed by CA, with in situ ECA, respectively, has been realized and well developed. The in situ ECA based on OER or HER mentioned above exhibits DSO behavior, which is facile, flexible, effective, and eco-friendly as a strategy of activating self-supported electrodes for efficient and durable overall water splitting. We hope our work can promote advanced development toward large-scale H2 production using excess electrical power whenever and wherever available.

Résumé

La décomposition électrolytique de l'eau en hydrogène et oxygène à l'aide d'électricité renouvelable générée par les courants marins ou à partir d'énergie éolienne ou solaire, constitue l'une des voies les plus propres et directes pour produire de l'hydrogène. Toutefois, la production de grands volumes d'hydrogène par décomposition électrolytique de l'eau comporte un verrou technologique qui réside dans la forte surtension à vaincre à l'anode où de l'oxygène est dégagé. Ce travail de thèse s'est attaché donc à mettre au point des matériaux d'électrodes capables de catalyser de l'eau en oxygène de

Figure 1

 1 Figure 1-1.

  water electrolysis technique is not at all a new invention. The technique was developed over two centuries ago by the two companions Nicholson and Carlisle. Up to now, the main water electrolysis technologies available are AEC, which represents the most mature technology and has been applied for large-scale hydrogen production in the MW-scale already in the beginning of the 20th century. The other main water electrolysis technologies are proton exchange membrane electrolysis cell (PEMECC, also named solid polymer electrolysis (SPE)) and solid oxide electrolysis cell (SOEC).Figure 1-4 depicts the technology set-up of PEMEC and SOEC.

Figure 1 - 4

 14 Figure 1-4 Conceptual set-up of PEMEC and SOEC. 14

4

 4

Figure 2 - 1

 21 Figure 2-1 Schematics of electrodeposition (a) and electrolytic codeposition (b).

Figure 2 -

 2 Figure 2-2 SEM image of Co3O4 powders.

Figure 2 - 3

 23 Figure 2-3 SEM image of GO nanoplatelets.

Figure 2 - 4

 24 Figure 2-4 SEM image of NF.

Figure 2 - 7 )

 27 Figure 2-7) were prepared to use as the feedstock powders to deposit on the nickel plate (NP). The XRD patterns of Ni-Al alloy powders confirm the coexistence of Ni2Al3 (JCPDS No. 14-0648) and NiAl (JCPDS No. 44-1188). The NSs (15 × 10 × 0.5 mm 3 ) with evenly distributed of 2-mm diameter apertures (Figure 2-8) were ultrasonically cleaned by acetone, deionized water, and absolute ethyl alcohol in sequence for 25 min, and then dried in the air for 30 min.

Figure 2 - 6

 26 Figure 2-6 SEM images of Ni-Al alloy powders. (a, b) Top view; (c, d) Cross section view.

Figure 2 - 7

 27 Figure 2-7 Particle size distribution and XRD patterns of Ni-Al alloy powders. (a) Particle size distribution; (b) XRD patterns.

Figure 2 - 8

 28 Figure 2-8 Images of NP. (a) Optical photo; (b) SEM image.

  2). These spraying parameters were selected to reduce the temperature of the spray particles for deposition of solid-state particles, to minimize oxidation, and to create rough surfaces and appropriate micro-voids. Then the porous Ni-Al alloy coatings with a thickness of ca. 50 um were obtained after 10 passes of depositing on NP. The Ni-Al alloy coating deposited on NP via HVOF is marked as HNA.

Figure 2 - 9

 29 Figure 2-9 HVOF equipment: FST-JP5000. (a) Optical photo; (b) Schematic.

Figure 3 - 1

 31 Figure 3-1 Electrochemical characterizations of NF, NF@N, NF@GO, NF@C and NF@CGO in 1 M KOH at RT. (a) Polarization curves with the scan rate of 1 mV s -1 ; (b) Nyquist plots.

Figure 3 - 2

 32 Figure 3-2 The effect of different current densities of electrolytic codeposition on the OER performance of NF@CGO. (a) CVs with the scan rate of 3 mV s -1 ; (b) Nyquist plots.

Figure 3 - 3

 33 Figure 3-3 Schematic illustration of the synthesis of the hybrid NF@CGO electrode.

Figure 3 -

 3 Figure 3-4 (a) SEM image of bare NF; (b) SEM image of hybrid NF@CGO electrode; (c) Crosssection of the hybrid NF@CGO electrode, inset illustration: high magnification morphology.

Figure 3 -

 3 5b displays a high-resolution SEM (HR-SEM) of the hybrid NF@CGO marked in Figure3-5a. The 3D NF surface was completely covered by nanoflowers made of Co3O4 , GO, and clusters of Ni species. Each cluster shows highly rippled nanosheet structures, which are significantly different from the morphologies of activated bare NF and bare NP (Nickel plate with the size of 15×10×0.3 mm, Figure3-6 and Figure3-7). The lateral extension of the nanosheets ranged from dozens of nanometers to 100 nm and the thickness is generally ca. 10 nm and the rippled nanosheets are interconnected, forming mesoporous and microporous nanostructures with the pore size from ca. 30 nm to ca. 100 nm (Figure3-8). These microporous or mesoporous in the activated NF@CGO are exposed to the electrolyte for the access of ions to form electrochemical double layers. Elemental mapping analysis of Figure3-5c shows that Co, O and C are homogeneously distributed in the nanoflower nanostructures with Ni surrounding them.The above results suggest that mesoporous nanostructures with Co, O and C elements were formed on the macro-porous NF, forming a hierarchically structured electrode.

Figure 3 -

 3 Figure 3-5 (a) SEM image of the activated NF@CGO; (b) HR-SEM image of the nanoflowers in Figure 3-5a; (c) Corresponding elemental mappings of the single nanoflower in Figure 3-5b.

Figure 3 -

 3 Figure 3-6 (a) SEM image of the activated bare NP; (b) High resolution SEM image of the area squared in Figure 3-6a.

Figure 3 -

 3 Figure 3-7 (a) SEM image of the activated bare NF; (b) High resolution SEM image of the area squared in Figure 3-7a; (c) High resolution SEM image of the area squared in Figure 3-7b.

Figure 3 - 8 57 (

 3857 Figure 3-8 High resolution SEM image of Ni cluster.

Figure 3 -

 3 Figure 3-9 (a) TEM image of the hybrid nanoflakes scratched off from the hybrid activated NF@CGO; (b) Corresponding SAED pattern of Co3O4 and Ni(OH)2/NiOOH, respectively; (c) Corresponding HR-TEM image of Co3O4; (d) Corresponding HR-TEM image of Ni(OH)2/NiOOH.

Figure 3 -

 3 Figure 3-10 TEM images of the hybrid nanoflakes scratched off from the hybrid activated NF@CGO.

Figure 3 -

 3 Figure 3-11 The typical XRD of commercial GO, bare NF and the hybrid activated NF@CGO.

Figure 3 -

 3 Figure 3-12 N2 adsorption-desorption isotherms of the hybrid NF@CGO, Inset: Pore size distribution. (a) after activated; (b) before activated. Base on the N2 adsorption-desorption isotherms, the BET surface of NF@CGO before and after activation were measured to be 40.5 and 150.6 m 2 g -1 , respectively.

Figure 3 -

 3 5b, Figure3-5c, Figure3-9a) and Figure3-10 ).42, 62

Figure 3 -

 3 Figure 3-13 The typical XRD spectra of the hybrid NF@CGO and bare NF.

Figure 3 -

 3 Figure 3-14 Raman spectra of commercial Co3O4 nanoparticles, commercial GO nanosheets and the hybrid NF@CGO.

  7 and 857.1 eV represent Ni 3+ from NiOOH, whereas the fitting peaks at 873.1 and 855.2 eV correspond to the small amount of Ni 2+ from Ni(OH)2 and the fitting peaks at 870.1 and 852.7 eV correspond to a minimum amount of metallic Ni present in the hybrid activated NF@CGO. It is clear that the main nickel-based species on the surface of the hybrid activated NF@CGO can be confirmed as NiOOH, with a minority of Ni(OH)2 and metallic Ni. The C1s XPS spectra of the hybrid activated NF@CGO indicated the presence of three types of carbon bonds: C=C-C (284.7 eV), C-O (286.8 eV), C=O (288.3 eV) (Figure 3-16c).

Figure 3 -

 3 Figure 3-15 XPS survey spectra of the activated hybrid activated NF@CGO electrode.

Figure 3 -

 3 Figure 3-16 (a) High-resolution Co 2p XPS spectra of commercial Co3O4 and the hybrid activated NF@CGO; (b) High-resolution Ni 2p XPS spectra of the hybrid activated NF@CGO; (c) High-resolution C 1s

Figure 3 -

 3 18 showed the CVs appeared for the Ni-based electrodes. A couple of oxidation-reduction peaks marked as pa1 and pc1 appeared at the potentials 1.48 V vs. RHE and 1.24 V vs. RHE. The part of the voltammogram having these peaks is analogous to that observed for the NP and NF electrode and the peak potential values coincided with the reported values.68, 69

Figure 3 -

 3 Figure 3-17 Electrochemical characterizations of the activated NP, NF and hybrid activated NF@CGO in 1 M KOH at RT. (a) Polarization curves with the scan rate of 1 mV s -1 ; (b) Corresponding Tafel plots; (c) Cdl curves; (d) Nyquist plots.

Figure 3 -

 3 Figure 3-18 CV curves of bare NP, bare NF and hybrid activated NF@CGO in 1M KOH at RT.

Figure 3 -

 3 Figure 3-19 CV curves of bare NP (a), bare NF (b) and hybrid activated NF@CGO (c) at various scan rates (10 mV s -1 , 20 mV s -1 , 40 mV s -1 , 60 mV s -1 , 80 mV s -1 , 100 mV s -1 and 120 mV s -1 ) in the potential range of 1.07-1.17 V vs RHE and the current measured at 1.12 V vs RHE plotted as a function of scan rate.

Figure 3 -

 3 Figure 3-20 The long-term durability of the activated hybrid NF@CGO electrode in 6 M KOH at 353K. (a) Chronopotentiometry curve; (b) Comparison of polarization curves before and after CA-100 h; (c) An optical photo showing the water electrolyzer with H2 and O2 bubbles generated. Scale bare in (c) is 10 mm.

Figure 3 -

 3 Figure 3-21 (a) SEM image of hybrid activated NF@CGO after CA-100 h; (b) High magnification of a

2 - 1 . 7 V, 6 M

 2176 KOH at 353 K) until Co3O4 and GO/ERGO are encountered and exposed to the electrolyte. The above conclusions would be confirmed by comparing surface morphology (Figure 3-22) and Raman spectra (Figure 3-23) of the NF@CGO before and after in situ ECA. More interestingly, the BET surface area of the activated NF@CGO increased from 40.5 to 150.6 m 2 g -1 , an increase of nearly 4 times (Figure 3-12). This would greatly increase the catalytic active sites since most of the electrode surface would be covered by Ni(OH)2/NiOOH nanoflakes, Co3O4 nanorods and GO/ERGO nanosheets, referring to the Pourbaix diagrams of Ni and Co under 353 K (Figure 3-24). 40 In addition, the Co3O4 nanoparticles used in the electrolytic codeposition were mostly agglomerated into microporous spheres with different sizes. The pore sizes were found to be about 30 nm (Figure 3-25). These pores would be filled with nickel nucleation and growth during electrolytic codeposition (Figure 3-4c inset illustration), which are be slowly oxidized and finally etched out during in situ electrochemical activation. This process led to the formation of Co3O4 with porous spherical shape coated with GO nanosheets on NF skeleton (Figure 3-5b, Figure 3-5c and Figure 3-9a)).

Figure 3 -

 3 Figure 3-22 SEM image of the hybrid NF@CGO. (a) before activated; (b) after activated.

Figure 3 -

 3 Figure 3-23 Raman spectra of the before activated NF@CGO and after activated NF@CGO.

Figure 3 -

 3 Figure 3-24 The Pourbaix diagrams for Ni (a), and Co (b) in water under 353 K.

Figure 3 -

 3 Figure 3-25 (a) SEM image of commercial Co3O4; (b) High resolution SEM image of the area squared inFigure 3-25a; (c) High resolution SEM image of the area squared in Figure 3-25b.

Finally, the

  nickel (oxy) hydroxide-Jamborite (Ni(OH)2/NiOOH hybrid) nanosheets are formed upon in situ electrochemical oxidation of the HNA-CA with 3D hierarchically porous architecture (Figure 4-1c and Figure 4-3c). In a summary, the synthesis was very effective and could be easily scaled up for practical use.

Figure 4 - 1

 41 Figure 4-1 Cross-section views of typical SEM images. (a) HNA; (b) HNA-CA; (c) HNA-CA-ECA.

Figure 4 - 2

 42 Figure 4-2 The typical XRD spectra of Ni-Al alloy powder and the HNA coating.

Figure 4 - 3

 43 Figure 4-3 Top views of typical SEM images. (a) HNA; (b) HNA-CA; (c) HNA-CA-ECA.

4. 2 . 2

 22 Physical characterization of the HNA-CA-ECA Physical characterizations of the HNA-CA-ECA catalysts would be showed as below. The crystal structures of the HNA, HNA-CA, and HNA-CA-ECA samples were studied by the X-ray diffraction (XRD) spectra, as shown in Figure 4-4. The XRD patterns of HNA sample confirmed the coexistence of Ni2Al3 (JCPDS No. 14-0648) and NiAl (JCPDS No. 44-1188). Obviously, the peaks of the Ni2Al3 (JCPDS No. 14-0648) in the HNA sample disappeared in that of the HNA-CA sample, while the new peaks located at 51.99°, 60.81°, and 91.70° could be indexed as crystal planes (111), (200), and (220) of metallic Ni (JCPDS No. 89-7128), respectively. More interestingly, the (111) peak located at 51.99° of metallic Ni was asymmetrical, with a bulge on its left side, suggesting a superposition of the peak from another new phase. It could be assigned to the (012) peak located at 50.72° of NiO (JCPDS No. 44-1159), together with other weak broad peaks located at 43.54°, 74.60°, 90.50°, and 95.73°, which might be attributed to nanosized or low crystallinity.

Figure 4 - 4

 44 Figure 4-4 XRD patterns of the as-achieved HNA, HNA-CA and HNA-CA-ECA catalysts

Figure 4 -

 4 Figure 4-5 HR-TEM image of the unique architecture of mesoporous Ni/NiO-core/shell in the HNA-CA.

Figure 4 - 6

 46 Figure 4-6 XRD patterns of the as-achieved HNA, HNA-CA and HNA-CA-ECA with different ECA time.

Figure 4 -

 4 Figure 4-7 (a) Typical SEM image of the HNA-CA-ECA; (b) HR-SEM image of the wrinkled nanosheets.

Figure 4 - 8

 48 Figure 4-8 TEM image of the ribbon-like nanosheets scratched off from the HNA-CA-ECA catalysts (insert: corresponding EDS).

Figure 4 -

 4 Figure 4-9 (a) Elemental mappings in Figure 4-8 purple square area; (b) Colours in elemental mapping images: blue for Ni, purple for O, green for Al, and red for K; K is due to the residual electrolyte (6 M KOH).

Figure 4 -

 4 Figure 4-10 (a) TEM image of the nanoparticle scratched off from the HNA-CA sample; (b) Corresponding EDS.

Figure 4 -

 4 Figure 4-11 (a) N2 adsorption-desorption isotherms of the samples; (b) Pore size distributions of HNA, HNA-CA and HNA-CA-ECA. Base on the N2 adsorption-desorption isotherms, the BET surface of the samples were measured to be 9.1, 33.0, and 77.7 m 2 g -1 , respectively.

Figure 4 -

 4 Figure 4-12 (a) HR-TEM image of the lattice-dislocated Jamborite nanosheet in Figure 4-8 yellow square area; (b) SAED pattern of the HNA-CA-ECA catalysts in Figure 4-8 whole area.

43 Figure 4 -

 434 Figure 4-13 Raman spectra of the as-achieved HNA, HNA-CA and HNA-CA-ECA catalysts.

Figure 4 -

 4 Figure 4-14 XPS survey spectra. (a) HNA; (b) HNA-CA; (c) HNA-CA-ECA.

Figure 4 - 47 Figure 4 -

 4474 Figure 4-15c, the aluminum species (metal and oxide) in the aluminum-rich phase on the surface of the HNA sample was gradually etched, while the Ni crystallites were gradually exposed to the electrolyte and spontaneously oxidized into Ni/NiO core/shell nanostructures.[45][46][47]

Figure 4 - 4 . 2 . 3

 4423 Figure 4-16 Schematic illustration of the synthesis procedure of HNA-CA-ECA catalysts.

Figure 4 -

 4 Figure 4-17 Electrochemical characterizations of the activated NP, NF and the as-achieved HNA-CA and HNA-CA-ECA catalysts in 1 M KOH at RT. (a) Polarization curves with the scan rate of 1 mV s -1 ; (b) Corresponding Tafel plots; (c) Nyquist plots; (d) Cdl curves; (e) A Chronopotentiometry test carried out at a constant current density of 10 mA cm -2 in 1 m KOH solution.

Figure 4 -

 4 17a, interestingly, HNA-CA-ECA catalysts generated the OER current density of 10 mA cm -2 at a smaller overpotential of 193 mV (with the potential of 1.423 V vs. RHE), compared to other counterparts -NP (433 mV and 1.663 V vs. RHE.), NF (348 mV and 1.578 V vs. RHE), IrO2/NF (216 mV and 1.446 V vs. RHE) and HNA-CA (226 mV and 1.456 V vs. RHE). Therefore, based on the above observation, the HNA-CA, especially the HAN-CA-ECA catalysts, showed an excellent OER activity with the lowest overpotential, whereas bare NP and NF show hardly any OER activity.Accordingly, as shown in Figure4-17b, the HAN-CA-ECA catalysts exhibited a much smaller Tafel slope (39 mV dec -1 ) compared to that of NP (128 mV dec -1 ), NF (95 mV dec -1 ), IrO2/NF (65 mV dec -1 ) and HNA-CA (58 mV dec -1 ) catalysts, suggesting more favorable and fast-catalytic OER kinetics (OER intermediate adsorption and desorption) for HNA-CA-ECA catalysts in an alkaline environment compared to other counterparts due to in situ generation of well-crystallized Jamborite-(Ni(OH)2/NiOOH hybrid) with subtle lattice distortions (Figure 4-8, Figure 4-9 and Figure 4-12a).

Figure 4 -

 4 Figure 4-17c showed the Nyquist plots based on EIS analysis. The Nyquist plots fit well with the equivalent circuit model presented in the inset image, where RS represented the solution resistance (i.e.

Figure 4 -2020

 4 Figure 4-18 CV curves of bare NP (a), bare NF (b), HNA-CA (c), HNA-CA-ECA catalysts at various scan rates (10 mV s -1 , 20 mV s -1 , 40 mV s -1 , 60 mV s -1 , 80 mV s -1 , 100 mV s -1 and 120 mV s -1 ) in the potential range of 1.07-1.17 V vs RHE and the anodic charging current measured at 1.12 V vs RHE plotted as a function of scan rate.

Figure 4 -

 4 Figure 4-20 The long-term durability of the HNA-CA-ECA catalysts in 6 M KOH at 353 K. (a) Chronopotentiometry curve; (b) An optical photo showing the water electrolyzer with H2 and O2 bubbles generated.

Figure 4 -

 4 Figure 4-21 HR-SEM image of the HNA-CA-ECA catalysts after chronoamperometric method 168 h by applying a current density of 800 mA cm -2 in 6 M KOH at 353 K.

Figure 4 -

 4 Figure 4-24), which were 192.7, 227.1, 228.2, and 233.1 mF cm -2 . The results demonstrated that the Cdl of the catalyst only by CA (HNA-CA-ECA0h) was very ultra-huge, revealing that the process of CA could effectively tune the active specific surface area of the catalysts due to the 3D hierarchically porous architecture. Moreover, with the extension of ECA time, the active specific surface area gradually increased, but not significantly, manifesting that the architecture was not destroyed during the harsh ECA; therefore, the HNA-CA-ECA catalyst showed excellent long-lasting performance.

Figure 4 -Figure 4 -

 44 Figure 4-22 Electrochemical performance of the as-prepared catalysts with different ECA times in 6 M KOH at

Figure 4 -

 4 Figure 4-24 Cdl curves of the as-prepared catalysts with different ECA time in 6 M KOH at 353K.

Figure 4 -

 4 Figure 4-25 (a) Polarization curves of the as-prepared catalysts with different ECA time in 6 M KOH at 353K; (b) Corresponding Tafel plots.

Fortunately, by in

  situ electrochemical transition simply upon ECA, we obtained lattice-distorted Jamborite-(Ni(OH)2/NiOOH hybrid) nanosheets, which made HNA-CA-ECA become a promising electrocatalyst towards OER. Regarding the causes of lattice distortion, on the one hand, as previously-mentioned, changes in the valence state of nickel would cause local lopsided Coulomb force, and the emergent micro-mechanic filed derivate disturbance would consequentially induce subtle distortion of the atomic arrangement. On the other hand, the mechanical stress generated by the formation and release of a large number of oxygen molecules in OER, together with the thermal stress induced by the circumstance temperature (353 K), advantageously caused strain of the nanosheets, thereby contributing to the formation of lattice distortions, thus finally engineering additional exposed active sites of the HAN-CA-ECA catalysts for the OER. This conclusion could be fully verified by the curved ribbon-like Jamborite-(Ni(OH)2/NiOOH hybrid) nanosheets appearing in the TEM image (Figure 4-7a) of the HNA-CA-ECA catalysts.

Figure 5 -

 5 Figure 5-1 (a) Polarization curves for NF, NF@N, NF@GO, Pt foil and H-NF@CGO at a scan rate of 2 mV s -1 (iR-corrected). (b) The corresponding Tafel slope plots derived from (a).

Figure 5 - 2

 52 Figure 5-2 Nyquist plots of NF, NF@N, NF@GO, Pt foil and H-NF@CGO in1 M KOH at an overpotential of -150 mV.

Figure 5 - 3

 53 Figure 5-3 CV curves of H-NF (a); Pt foil (b); H-NF@CGO (c); Corresponding Cdl curves. CV curves at various scan rates (10 mV s -1 , 20 mV s -1 , 40 mV s -1 , 60 mV s -1 , 80 mV s -1 , 100 mV s -1 and 120 mV s -1 ) in the potential range of 0-0.10 V vs RHE and the anodic charging current measured at 0.05 V vs RHE plotted as a function of scan rate.

Figure 5 - 4 A

 54 Figure 5-4 A Chronopotentiometry test of H-NF@CGO and Pt foil carried out at a constant current density of -100 mA cm -2 in 1 m KOH solution.

Figure 5 - 5

 55 Figure 5-5 Overall water splitting bifunctional activities of the DSO NF@CGO (O-NF@CGO (+) ||H-NF@CGO (-) ) in a two-electrode configuration in1 M KOH. (a) Polarization curves at a scan rate of 2 mV s -1 ; (b)Nyquist plots at an overpotential of 300 mV. NP (NP (+) ||NP(-) ), NF (NF (+) ||NF(-) ) and state-of-the-art commercial (IrO2/NF (+) ||Pt(-) ) for comparison.

Figure 5 - 6 A

 56 Figure 5-6 A Chronopotentiometry test of the DSO NF@CGO (O-NF@CGO (+) ||H-NF@CGO(-) ) carried out at a constant current density of 10 mA cm -2 in 1 M KOH solution. NF (NF (+) ||NF(-) ) and state-of-the-art commercial (IrO2/NF (+) ||Pt(-) ) for comparison.

  electrode configuration with Hydrogen Reference Electrode HydroFlex (Gaskatel GmbH, Germany) and commercial IrO2/NF used as reference electrode and counter electrode, respectively. The H-HNA-CA was activated by a one-hour Chronopotentiometry test at a current density of -800 mA cm -2 with the conditions of 6 M KOH at 353 K in the three-electrode electrochemical test system. In order to ensure effective comparison of data, NP, NF, HNA-CA and Pt foil also were activated under the same condition. H-HNA-CA was performed in 1 M KOH by polarization curve with NP, NF, HNA-CA, and Pt foil as a comparison. The corresponding iR-corrected polarization curves on the five surfaces are shown in Figure5-7a. As expected, the NP and NF both exhibit fairly poor activity toward HER with -100 mA cm -2 current response up to ca. -302 mV and -290 mV vs. RHE, respectively, while H-HNA-CA shows high-performance HER activity with a low potential of -87 mV vs. RHE, which is much more positive than HNA-CA (-106 mV vs. RHE). Notably, the potential required for H-HNA-CA is less than that of Pt foil (-94 mV vs. RHE), to deliver current density of -100 mA cm -2 . Taking the catalytic current density of 10 mA cm -2 for direct comparison, HNA-CA and H-HNA-CA both require an ultralow overpotential of ca. -20 mV, while NP and NF require very high overpotentials of -195 mV and -168 mV, respectively, and that of Pt foil is 51.9 mV, suggesting the superior electrocatalytic H2evolving activity of H-HNA-CA. This overpotential contrasts favorably with the commercial Pt and most values previously reported for HER catalysts.[66][67][68][69][70][71][72][73] Accordingly, HNA-CA and H-HNA-CA exhibit very small Tafel slopes of 25.7 mV dec -1 and 28.2 mV dec -1 (Figure5-7b), which are much lower than those of NP (251.2 mV dec -1 ), NF (194.3 mV dec -1 ), and state-of-the-art Pt foil (57.5 mV dec -1 ), demonstrating the faster HER kinetics of HNA-CA, especially for H-HNA-CA.

Figure 5 -

 5 Figure 5-7 (a) Polarization curves for NP, NF, HNA-CA, Pt foil and H-HNA-CA at a scan rate of 2 mV s -1 (iRcorrected). (b) The corresponding Tafel slope plots derived from (a).

Figure 5 - 8

 58 Figure 5-8 Nyquist plots of NP, NF, HNA-CA, Pt foil and H-HNA-CA in1 M KOH at an overpotential of -150 mV.

Figure 5 - 9

 59 Figure 5-9 CV curves (a) HNA-CA; (b) H-HNA-CA. CV curves at various scan rates (10 mV s -1 , 20 mV s -1 , 40 mV s -1 , 60 mV s -1 , 80 mV s -1 , 100 mV s -1 and 120 mV s -1 ) in the potential range of -0.09-0.03 V vs RHE and the anodic charging current measured at -0.05 V vs RHE plotted as a function of scan rate.

Figure 5 -

 5 Figure 5-10 The corresponding Cdl curves derived from Figure 5-9.

Figure 5 - 11 A 5 . 3 . 2 O

 511532 Figure 5-11 A Chronopotentiometry test of H-HNA-CA and Pt foil carried out at a constant current density of -10 mA cm -2 in 1 m KOH solution. All of the above results reveal the highly efficient and robust stability of the self-supported H-HNA-CA cathode. Such an enhanced HER catalytic performance compared with unmodified materials (bare NP) could be rationalized as follows. (1) The self-supported design with good substrate adhesion removes the extra electrical resistance raised by binders, leads to a better electrical connection, and promotes electron transport. (2) A mild evolution of H2 gas bubbles upon DSO based on HER drives in situ structural rearrangement and results in a highly porous structure with exposed active edges possessing sites with fairly high activity. (3) The 3D hierarchical H-HNA-CA facilitates fast mass transport and electrolyte penetration and allows efficient accessibility of more surface area as well as active sites for electrocatalytic reactions. (4) The active substance generated in situ over the restructuring

Figure 5 -

 5 12b. The DSO HNA-CA (O-HNA-CA(+) ||H-HNA-CA(-) ) exhibits much lower charge transfer resistance than NP (NP (+) ||NP(-) ) and NF (NF (+) ||NF(-) ), and is comparable with state-of-the-art commercial (IrO2/NF (+) ||Pt(-) ), confirming the favorable overall water splitting kinetics of the DSO HNA-CA (O-HNA-CA (+) ||H-HNA-CA(-) ). The synergy between the restructuring NiAl skeletons and their supported newly constructed active phases upon DSO may promote the catalytic activity of the DSO HNA-CA (O-HNA-CA (+) ||H-HNA-CA(-) ).

Figure 5 -

 5 Figure 5-12 Overall water splitting bifunctional activities of the DSO HNA-CA (O-HNA-CA (+) ||H-HNA-CA (-) ) in a two-electrode configuration in1 M KOH. (a) Polarization curves at a scan rate of 2 mV s -1 ; (b) Nyquist plots at an overpotential of 300 mV. NP (NP (+) ||NP(-) ), NF (NF (+) ||NF(-) ) and state-of-the-art commercial (IrO2/NF (+) ||Pt(-) ) for comparison.

Figure 5 - 13 A

 513 Figure 5-13 A Chronopotentiometry test of the DSO HNA-CA (O-HNA-CA (+) ||H-HNA-CA(-) ) carried out at a constant current density of 10 mA cm -2 in 1 m KOH solution. NF (NF (+) ||NF(-) ) and state-of-the-art commercial (IrO2/NF (+) ||Pt(-) ) for comparison.

6. 1

 1 ConclusionsTwo kinds of high performance OER electrocatalysts were fabricated through two conventional coating processes combined with in situ ECA, guided by the present PhD study objective of development of highly active and stable earth-abundant OER electrocatalysts with 3D hierarchical porous architecture via facile, effective and potentially commercial means.For the first OER electrocatalyst, a codoped 3D NF by Ni crystals, Co3O4 nanoparticles, GO nanosheets, and in situ-generated Ni(OH)2/NiOOH nanoflakes via electrolytic codeposition, in combination with in situ ECA based on OER, was reported as a promising OER electrocatalyst in alkaline electrolyzer. Notably, this hybrid catalyst shows good electrocatalytic performance of OER in 1.0 M KOH with 212 mV overpotentials to achieve a catalytic current density of 10 mA cm -2 , which is comparable with the state-of-the-art noble catalysts. The hybrid catalyst, as an electrocatalytically active and robust OEE, also exhibits strong long-term electrochemical durability, releasing a large current density of 800 mA cm -2 at 350 mV without degradation after a 100-hour test. Such a remarkable performance can be benefiting from the introduced OER catalyst materials deposited on NF, in situgenerated Ni(OH)2/NiOOH nanoflakes and their synergistic effects. Collectively, so far, the hybrid catalyst developed in this work is one of the most efficient OEEs in alkaline electrolytes, and could potentially be implemented in large-scale water electrolysis systems.For the second OER electrocatalyst, an efficient method of combining high-velocity oxy-fuel (HVOF) spraying followed by chemical activation (CA), and in situ ECA based on OER, was developed to obtain a promising self-supported OER electrocatalyst with lattice-distorted Jamborite (Ni(OH)2/NiOOH hybrid) nanosheets generated in situ on the elaborated 3D hierarchically porous NiAl framework. The catalyst developed in this work exhibits not only exceptionally low overpotential (10 mA cm -2 at an overpotential of 193 mV) and a low Tafel slope (39 mV dec -1 ) toward oxygen evolution reaction (OER), but also remarkable stability, completely maintaining its high catalytic activity over 40 hours of continuous operation in 1 M KOH, both of which surpass the state-of-the-art noble commercial catalysts. Moreover, it also exhibits strong long-term electrochemical durability, releasing a large current density of 800 mA cm -2 at an overpotential of 229 mV without degradation after a 168-hour test in 6 M KOH at 353 K. The remarkable features of this catalyst lie in the synergistic effect of the high intrinsic activity arising from the in situ growth of lattice-dislocated Jamborite (Ni(OH)2/NiOOH hybrid)

  façon efficace et stable, en utilisant des éléments chimiques suffisamment abondants sur terre. Pour cela nous avons exploré des composés présentant des porosités à structures hiérarchiques et des procédés de préparation efficaces, aisées à mettre en oeuvre et susceptibles d'un usage à l'échelle industrielle. Nous avons développé deux types d'électrocatalyseurs d'oxydation de l'eau en oxygène en mettant au point deux voies de préparation impliquant chacune une phase d'activation in situ. Le premier type est une mousse de nickel dopée à la fois avec des cristaux de nickel, des nanoparticules de tétroxyde de tricobalt et des nanofeuilles d'oxyde de graphène via nickelage électrolytique, suivi d'une activation électrochimique in situ pour former de l'hydroxyde de nickel et des nano-plaques d'oxy-hydroxyde du même métal. Ce catalyseur hybride s'est avéré avoir des performances électrocatalytiques de bon niveau, comparables à celles des électrodes à base de métaux nobles qui sont disponibles dans l'état actuel de la technique ; il a en outre fait preuve d'une excellente stabilité en fonctionnement. Ces propriétés remarquables semblent liées à la fois aux dépôts formés sur la mousse de nickel par les différentes phases actives citées, aux nanoparticules d'oxy-hydroxyde de nickel, ainsi qu'aux effets de synergie qu'elles y induisent. Le second type d'électrocatalyseurs a été obtenu en combinant la projection thermique (HVOF) et un processus d'activation chimique puis électrochimique. Le matériau résultant possède de nanocouches du type jamborite formée in situ, sur la matrice poreuse à structure hiérarchique. Le catalyseur développé dans ce travail présente non seulement une surtension et une pente de Tafel exceptionnellement faibles, mais également une stabilité remarquable. Ces performances sont dues à un puissant effet de synergie dans laquelle interviennent la grande activité intrinsèque des nanofeuilles de jamborite et la grande rapidité des transports d'électrons et d'ions assurée par l'architecture poreuse hiérarchique. Il convient de noter que cette nouvelle méthodologie a le potentiel de produire des électrodes de grandes tailles apte à l'électrolyse alcaline de l'eau et crée ainsi de nouvelles perspectives dans le cadre de la conception d'électrocatalyseurs à la fois très actifs et stables. Nous avons également développé, initialement, des électrocatalyseurs destinés à la réduction de l'eau en hydrogène, qui impliquent également une activation électrochimique in situ. Ces électrodes peuvent être ainsi couplées aux électrodes précitées d'oxydation de l'eau en oxygène pour former des cellules électrochimiques complètes à deux électrodes, dont les performances rivalisent avec celles développées par le couple dioxyde de ruthénium/platine qui représente le meilleur état de la technique dans le cadre de la production d'hydrogène et d'oxygène par électrolyse de l'eau. En résumé, en combinant des techniques conventionnelles de revêtement et d'activation électrochimique in situ, ce travail a permis de développer une méthodologie de préparation d'électrodes catalytiques offrant de hautes performances et susceptibles de commercialisation. La technique d'activation électrochimique in situ exploite un comportement

Table 1

 1

	-1, AECs have by far the largest power capacity and are available up to
	the 6 MW range, compared to the maximum of 2 MW for PEMEC. The low power consumption and,
	thus, low production rate for PEMEC makes AEC the current standard for large-scale hydrogen
	production techniques.

Table 1

 1

	-1 Main characteristics of AEC, PEMEC and SOEC systems. 14
	Parameters	AEC	PEMEC	SOEC
	Electrolyte	Aq. potassium hydroxide (20-	Polymer membrane (e.g.	Yttria stabilized Zirconia
		40 wt.% KOH) 5	Nafion) 47	(YSZ) 48
	Cathode	Ni, Ni-Mo alloys 5	Pt, Pt-Pd 47	Ni/YSZ 48
	Anode	Ni, Ni-Co alloys 5	RuO2, IrO2 47	LSM b /YSZ 48
	Current density (A cm -2 )	0.2-0.4 47	0.6-2.0 47	0.3-2.0 48
	Cell voltage (V)	1.8-2.4 47	1.8-2.2 47	0.7-1.5 48

Table 1

 1

	-2 compares cell operating conditions for the alkaline water electrolysis in five companies:
	De Nora S.A.P., Norsk Hydro, Electrolyzer Corp. Ltd., Teledyne Energy systems and General Electric.

Table 1 -

 1 2 Water electrolyzer developers and cell operating conditions.5

	Parameter	De Nora S.A.P	Norsk Hydro	Electrolyzer	Teledyne Energy	General Electric
				Corp. Ltd.	systems
	Cell type	B-FP	B-FP	M-T	B-FP	B-FP
	Anode	Expanded Ni-plated	Activated Ni-	Ni-coated Steel	Ni screen	PTFE-bonded
		Mild steel	coated Steel			Noble metal
	Cathode	Activated Ni-plated	Activated Ni-	Steel	Ni screen	PTFE-bonded
		Steel	coated Steel			Noble metal
	Pressure (MPa)	Ambient	Ambient	Ambient	0.2	0.4
	Temperature (℃)	80	80	70	82	80
	Electrolyte	29 % KOH	25 % KOH	28 % KOH	35 % KOH	Nafion
	Current density (A m -2 )	1500	1750	1340	2000	5000
	Cell voltage (V)	1.85	1.75	1.9	1.9	1.7
	Current efficiency (%)	98.5	98.5	>99	NR	NR
	Oxygen purity (%)	99.6	99.3-99.7	99.3-99.7	>98.0	>98.0

Table 1 -

 1

	Catalysts	Methods	Electrolyte	𝜂 10𝑚𝐴/𝑐𝑚 2 (mV	Tafel (mV/dec)
				)
	O-vacancy-rich Co3O4 146	Mild solvothermal reduction	1 M KOH	NA	49.1
	Co3O4-250 147	Anodization	1 M KOH	275	NA
	Co3O4/rGO nanocomposites 148	Electrostatic interaction/induced growth	1 M KOH	346	47
	Pd@PdO-Co3O4 Nano-cubes 149	Thermal decomposition	1 M KOH	310	70
	3D urchin-like Co3O4 150	Hydrothermal reaction	1 M KOH	410	65
	Nano-porous Zn-doped Co3O4 151	Hydrothermal method	0.1 M KOH 172	34
	NiO/Co3O4@ NC 152	Thermal annealing	1 M KOH	240	73
	Co3O4/Co-Fe DSNBs 153	Thermal annealing	1 M KOH	297	61
	Co-Nx/C NRA 154	Decomposition	1 M KOH	300	62.3
	Co/Co9S8@NSOC-800 155	One-step pyrolysis	1 M KOH	373	80
	Ni-Co-S/NF 156	Hydrothermal and calcination synthesis	1 M KOH	NA	133.8

3. Table 1-3 The electrocatalytic activity of Co-based TMC OER catalysts.

Table 1 -

 1 4 The electrocatalytic activity of Fe-based TMC OER catalysts.

	Catalysts	Methods	Electrolyte	𝜂 10𝑚𝐴/𝑐𝑚 2 (mV	Tafel (mV/dec)
				)
	FeOOH2nm/LDH 175	Hydrothermal method	1 M KOH	174	27
	Co2-Fe-B 176	Chemical method	1 M KOH	298	62.6
	Fe7S8 nanosheet 177	Modified liquid exfoliation	1 M KOH	270	43
	NiFeS/NF 178	Hydrothermal sulfidation process	1 M KOH	65	119.4
	Ni1.5Fe1.5-O 179	Chemical dealloying	1 M KOH	244	39
	Fe3O4/Co3S4 180	Hydrothermal and calcination	1 M KOH	270	56
	Iron-rich V-Co-Fe 181	Chemical method	1 M KOH	307	36
	Ni-Fe(OxHy) 182	Physically mixing	1 M KOH	298	37
	Fe-N-CIG 183	Heat treatment	0.1 M KOH 390

.2.3 Ni-Based transition metal compounds as OER catalysts

	Ni0.75Fe0.25-P/PO3@fCNT 214	Decoration and phosphorization	1 M KOH	239	34.4
	γ-NiOOH/NiNPS 215	In situ activation	1 M KOH	-	46
	NiO@MoO3/VC 216	Hydrothermal treatment	1 M KOH	280	64.5
	holey Ni(OH)2 217	Etching method	1 M KOH	293	65
	NiCoO2/CoO/Ni3N 209	Hydrothermal method	1 M KOH	247	35
	PANiO 218	Phosphorized method	1 M KOH	138	36
	RuO2/NiO/NF 219	Hydrothermal and calcination	1 M KOH	250	50.5
	Fe-Ni(OH)2/NF 220	Hydrothermal	1 M KOH	235	51.5
	Ni/NiO@G-SH 221	Reduction method	0.1 M KOH	270	46
	Ni@NiO 222	Mild etching	1 M KOH	382	103
	NiP120 223	Electroless deposition and robocasting	1 M KOH	286	75.1
	NiCo2O4/Ni2P 224	Deposition and phosphorization	1 M KOH	250	58
	N + -5e15 225	Irradiation	0.1 M KOH	400	136
	NiOOH-NiCr2O4/NF 84	In situ electrochemical conversion	1 M KOH	-	104
	Catalysts	Methods	Electrolyte	𝜂 10𝑚𝐴/𝑐𝑚 2 (mV	Tafel (mV/dec)
				)
	Fe11%-NiO/NF 192	Solvothermal synthesis	1 M KOH	206	49.4
	NiO-Ni3S2/NF 203	Sulfidation treatment	1 M KOH	340	75
	Elox_H2S_NF 204	Thermal sulfurization	1 M KOH	256	41
	Ni0.82Co0.18O@C/NF 205	Pyrolysis	1 M KOH	190	65
	NiO/Ni 206	Laser ablation	1 M KOH	294	41
	NiB0.45-250/Cu 207	Electroless plating method	1 M KOH	296	58
	Ni3N 208	Hydrothermal method	1 M KOH	277	118
	NiCoON NSAs/NF 209	Hydrothermal method	1 M KOH	247	35
	(Zn)Ni-LDH/N-rGO 210	Solvothermal process	1 M KOH	290	44
	Ni-Fe LDH 211	Hydrothermal method	1 M KOH	174	27
	Ce-NiO-E 212	Sol-gel method and annealing	1 M KOH	382	118.7
	NiSxPy 213	Hydrothermal reaction	1 M KOH	192	95

Currently, Ni-based OER catalysts are arousing increasing research interest due to their earthabundant nature and theoretically high catalytic activity. Generally, such catalysts mainly include Nibased oxides, and Ni-based hydroxides/oxyhydroxide (see Table

1

-5Table 1-5). From an industrial perspective, nickel is typically used as the promising electrode material in commercial electrocatalytic electrolysis units in highly alkaline electrolytes owing to its relatively high stability and favorable activity. Particularly, nickel foam (NF) with a long range ordered porous framework, which not only exposes more reaction active sites but also accelerate fast ion and electron transport, is the most widely used conductive substrate for the construction of three-dimensional (3D) catalysts.

[198][199][200][201][202]

Table

1

-5 The electrocatalytic activity of Ni-based TMC OER catalysts.

Table 1 -

 1 6 The electrocatalytic activity of In situ ECA based OER electrocatalysts.

	Catalysts	Methods	Electrolyte	𝜂 10𝑚𝐴/𝑐𝑚 2 (mV)	Tafel (mV/dec)
	ECT-S-Co0.37Ni0.26Fe0.37O 75	In situ ECA	1 M KOH	232	37.6
	Fe0.33Co0.67OOHPNSAs/CFC 83	In situ ECA	1 M KOH	266	30
	CoTeNR/NF 82	In situ ECA	1 M KOH	202	75
	NiOOH-NiCr2O4/NF 84	In situ ECA	1 M KOH	-	104
	CoP/C 122	In situ ECA	0.1 M KOH	360	66
	NixFe1-xSe2-DO 81	In situ ECA	1 M KOH	195	28
	Mn-Co-P@MnCo2O4/Ti 157	In situ ECA	1 M KOH	269	102
	E-CoOx/CF 235	In situ ECA	1 M KOH	249	50
	MnGa4/NF 236	In situ ECA	1 M KOH	291	98
	Fe1Ni1 LDH 237	In situ ECA	1 M KOH	253	34
	MoOx@N3-doped MoS2-x 238	In situ ECA	1 M KOH	270	61

Table 2 -

 2 1 Bath composition and plating conditions for nickel electrolytic codeposition.

	Composition and condition (units)	Magnitude
	NiSO4• 6H2O (g/L)	250
	NiCl2• 6H2O (g/L)	40
	H3BO3 (g/L)	40
	Lauryl sodium sulfate (g/L)	0.05
	Tricobalt tetroxide (g/L)	0.02
	Graphene oxide (g/L)	0.01
	Temperature (°C)	50
	pH	4-5
	Time (min)	45
	Current density(A/dm 2 )	0.5-2.5
	Stirring rate (rpm)	300

Table 2 -

 2 2 HVOF spraying parameters.

	Parameters	Value
	Torch	JP-5000
	Oxidizer: Fuel	O2 (1600L/min): Kerosene (26 L/h)
	Substrate	NS
	Powder	Ni-Al alloy
	Powder flow	5 kg/h
	Projection distance	380 mm
	External cooling	Three air jets
	2.1.2.3 Chemical activation

In a subsequent chemical activation (CA) process, the as-obtained precursor coatings (the porous HNA) were chemically activated in 6 M KOH (containing 10 wt.% KNa tartrate-tetrahydrate) at 353 K for 24 h to form HNA-CA with a hierarchically interlaced 3D network. To avoid a precipitation of aluminum hydroxide in the mesopores/microchannels of the obtained sponge-like structure, the solution contained 10 wt.% KNa tartrate-tetrahydrate (Rochelle salt, procured from Sigma) as a complexing agent. The resulting HNA-CA samples were easily susceptible to oxidation because of extremely high activity, so they were kept under 6 M KOH solution after activation.

Table 3 -

 3 1 Comparison of the OER performance of hybrid activated NF@CGO with the recently reported state-of-the-art catalysts.

	Materials (substrate)	Electrolyte I/mA cm -2	η/mV Tafel slope mV dec -1	Year
	NF@CGO (NF)-This work	1 M KOH	10	212	43	2019

Abbreviation

Abbreviation Explanations AEC Alkaline electrolysis cell PEMEC Proton exchange membrane electrolysis cell SOEC Solid oxide electrolysis cell CoxFe1-xN0.5 (Glass carbon) 48 1 M KOH 10 266 30

Ni QD@NC@rGO (Glass carbon) 49 1 M KOH 10 265 65

CoZn-Se (Glass carbon) 50 1 M KOH 10 320 66

CoOx-NeC/TiO2C (Glass carbon) 51 1 M KOH 10 220 75

S|NiNx-PC/EG (Glass carbon) 52 1 M KOH 10 280 45

NiFeMo IOS@ (NF) 53 1 M KOH 10 198 36

Ni-Co3O4 nanosheets (NF) 54 1 M KOH 10 170 76.9

OESSC (Stainless Steel) 55 1 M KOH 10 290 38

Co3O4/MoS2 (NF) 56 1 M KOH 10 205 45

Co3-xAlxO4 (Glass carbon) 57 1 M KOH 10 248 80.6

Ni-BDC/Ni(OH)2 (Glass carbon) 58 1 M KOH 10 320 41

Ni-Co-hydroxide/Ni2P2O7 (NF) 59 1 M KOH 10 197 63

Fe-Co3O4 H-NSs (NF) 60 1 M KOH 10 204 38

Ni3S2 (NF) 61 1 M KOH 10 312 111 NF@Ni/C (NF) 62 1 M KOH 10 256 62

Fe(II)-MOF-74 (NF) 63 1 M KOH 10 207 41.1 IP/NP (NF) 64 1 M KOH 10 185 47

NiCoSe2@NiO@CoNi2S4@CoS2(NF) 65 1 M KOH 10 353 159.1

Ni3(S1-xSex)2@NiOOH (NF) 66 1 M KOH 10 320 48 OEEs (marked as O-NF@CGO and O-HNA-CA, respectively) to form two-electrode configurations (O-NF@CGO (+) ||H-NF@CGO (-) , O-HNA-CA (+) ||H-HNA-CA (-) ). These two-electrode configurations were both compared with the integrated state-of-the-art IrO2/NF (+) ||Pt foil (-) electrode by electrochemical characterizations in alkaline overall water splitting.

Electrochemical characterization of the activated NF@CGO

electrocatalyst in HER and overall water splitting

Electrocatalytic hydrogen evolution of the H-NF@CGO

The HER electrocatalytic performance of the H-NF@CGO was investigated using a typical threeelectrode configuration with Hydrogen Reference Electrode HydroFlex (Gaskatel GmbH, Germany)

and with the commercial NF as reference electrode and counter electrode, respectively. The H-NF@CGO was activated by a one-hour Chronopotentiometry test at a current density of -800 mA cm -2 with the conditions of 6 M KOH at 353 K in the three-electrode electrochemical test system. In order to ensure the effective comparison of data, H-NF, H-NF@N, H-NF@GO and Pt foil were also activated under the same conditions. H-NF@CGO was performed in 1 M KOH by polarization curve with H-NF, H-NF@N, H-NF@GO, and Pt foil as comparisons. The corresponding iR-corrected polarization curves for the five samples are shown in Figure 5-1a. As expected, the NF exhibits fairly poor activity toward HER with -100 mA cm -2 current response up to ca. -365 mV vs. RHE, while H-NF@CGO shows highperformance HER activity with a low potential of -265 mV vs. RHE, which is much more positive than H-NF@N (-339 mV vs. RHE) and H-NF@GO (-306 mV vs. RHE). In particular, Pt foil reached a very high potential -354 mV vs. RHE, yielding a current density of -100 mA cm -2 . Taking the catalytic current density of 10 mA cm -2 for direct comparison, H-NF@CGO requires only -138 mV overpotential, while H-NF, H-NF@N, H-NF@GO and Pt foil require overpotential (ƞ) of -237, -210, -181, and -170 mV, respectively, suggesting the superior electrocatalytic H2-evolving activity of H-NF@CGO. This overpotential contrasts favorably with that of the commercial Pt and those previously reported for nonprecious metal-based HER catalysts in alkaline media. 48,[52][53][54][55][56][57][58][59][60][61][62][63][64][65] Accordingly, H-NF@CGO exhibits a small Tafel slope of 42.6 mV dec -1 (Figure 5-1b), which is much lower than those for H-NF (216.2 mV dec - 1

), H-NF@N (75.8 mV dec -1 ), H-NF@GO (51.9 mV dec -1 ) and state-of-the-art Pt foil (82.1 mV dec -1 ), demonstrating the faster HER kinetics of H-NF@GO.

Perspectives

In order to further develop our work on hydrogen production by water splitting, we hope to continue investigating on the following aspects in the future.

For the catalytic mechanism, we will further improve the physical and electrocatalytic characterizations to fully reveal the essential connections between catalytic performance and the evolution of phase, surface, and morphology on the surface of the electrocatalysts, especially for HER electrocatalysts. If possible, we will work with researchers in the density functional theory field to better explain the catalytic mechanisms of the resultant electrocatalysts.

For the preparation process, we will further optimize the fabrication processes which have been employed, with the aim of improving catalytic activity and durable performance. In addition, we will try to prepare more large electrodes close to the needs of industry and assemble them into a full cell, to directly serve the industrial production of electrolyzed water.

In addition, we will put forward new exploration directions for future fabrication of bifunctional electrocatalysts for overall water splitting, such as 3D-printing or melting processes, in combination with in situ ECA. Based on the characteristics of these processes, they could be used to design selfsupported catalytic electrode materials.

After initial exploration, we find that electrocatalytic reactions relevant for water splitting can be improved by employing magnetic heating of the prepared nickel-based catalysts. The combination of the conventional manufacturing process and the magnetic field to produce highly active electrocatalysts is also in its infancy and achieving remarkable results. Hence, developing the effective use of magnetic fields in electrode fabrication and applications will represent another interesting attempt to obtain promising electrocatalysts.

Abstract

Splitting water into hydrogen and oxygen by electrolysis using electricity from intermittent ocean current, wind, or solar energies is one of the easiest and cleanest routes for high-purity hydrogen production and an effective way to store the excess electrical power without leaving any carbon footprints. The key dilemma for efficient large-scale production of hydrogen by splitting of water via the hydrogen and oxygen evolution reactions is the high overpotential required, especially for the oxygen evolution reaction. Hence, engineering highly active and stable earth-abundant oxygen evolution electrocatalysts with three-dimensional hierarchical porous architecture via facile, effective and commercial means is the main objective of the present PhD study. Finally, we developed two kinds of good performance oxygen evolution electrocatalysts through two different way combined with in situ electrochemical activation.

For the first oxygen evolution electrocatalyst, we report a codoped nickel foam by nickel crystals, tricobalt tetroxide nanoparticles, graphene oxide nanosheets, and in situ generated nickel hydroxide and nickel oxyhydroxide nanoflakes via electrolytic codeposition in combination with in situ electrochemical activation as a promising electrocatalyst for oxygen evolution reaction. Notably, this hybrid catalyst shows good electrocatalytic performance, which is comparable to the state-of-the-art noble catalysts. The hybrid catalyst as an electrocatalytically-active and robust oxygen evolution electrocatalyst also exhibits strong long-term electrochemical durability. Such a remarkable performance can be benefiting from the introduced active materials deposited on nickel foam, in situ generated nickel oxyhydroxide nanoflakes and their synergistic effects. It could potentially be implemented in large-scale water electrolysis systems.

For the second oxygen evolution electrocatalyst, an efficient means of combining high-velocity oxy-fuel spraying followed by chemical activation, and in situ electrochemical activation based on oxygen evolution reaction has been developed to obtain a promising self-supported oxygen evolution electrocatalyst with lattice-distorted Jamborite nanosheets in situ generated on the three-dimensional hierarchical porous framework. The catalyst developed in this work exhibits not only exceptionally low overpotential and Tafel slope, but also remarkable stability. Such a remarkable feature of this catalyst lies in the synergistic effect of the high intrinsic activity arising from the lattice-dislocated Jamborite nanosheets as the highly active substance, and the accelerated electron/ion transport associated with the hierarchical porous architecture. Notably, this novel methodology has the potential to produce largesize-electrode for alkaline water electrolyzer, which can provide new dimensions in design of highly active and stable self-supported electrocatalysts.

Furthermore, we have also initially developed good hydrogen evolution electrocatalysts upon in situ electrochemical activation, coupled with the obtained superior oxygen evolution electrocatalysts forming two-electrode configurations, respectively, both of which rivalled the integrated state-of-the-art ruthenium dioxide-platinum electrode in alkaline overall water splitting.

In summary, a methodology of fabricating easy-to-commercial, high performance catalytic electrodes by combining traditional coating processes with in situ electrochemical activation has been realized and well developed. The in situ electrochemical activation mentioned above is a dynamic self-

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