Xiong Zhang

Lai Truong-Phuoc

Tristan Asset

Sergey Pronkin

Cuong Pham- Huu

Are Fe-N-Cs An Alternative to Pt-based Electrocatalysts for the Next Generation of Proton Exchange Membrane Fuel Cells?

Keywords: Fe-N-C electrocatalysts, Pt-based electrocatalysts, oxygen reduction reaction, proton exchange membrane fuel cells, durability, catalytic layer

Notable progresses have been made for low Pt-based and Fe-N-C electrocatalysts for proton exchange membrane fuel cells (PEMFCs) applications. Particularly, the research and development of cost-effective Fe-N-Cs have witnessed significant improvements in the recent years, due to the need of an alternative candidate to the scare, and expensive, platinum. Numerous studies have been exploring the design and optimization of Fe-N-Cs and generally make simple performance comparison with commercial Pt/C catalysts on the basis of rotating disk electrode (RDE) and/or membrane electrode assembly (MEA), thus arguing that said materials are comparable, or could even surpass, Pt/C electrocatalysts. The resulting question, i.e. 'are Fe-N-Cs an alternative to Pt-based electrocatalysts in PEMFCs?' is the centerpiece of this review. Here, the interconnectedness and differences between Fe-N-Cs and Pt-based materials are discussed from fundamental insights to applications in PEMFCs, covering the oxygen reduction reaction (ORR) mechanism, the nature of active sites and its effect on the intrinsic activity, the rational design of catalyst layer, the electrocatalysts performances in PEMFCs and their durability in operating conditions. This review provides a global picture of the current research state in the field of PEMFCs electrocatalysts, focusing onto the two most important types of cathode materials and aiming to shed light on the potential of Fe-N-Cs as replacement for Pt-based catalysts along with pointing out the research directions toward the next generation of PEMFCs.

Introduction

Addressing the challenges related to the climate change requires the development of green and sustainable energy conversion or storage systems, [START_REF] Seh | Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design[END_REF] such as fuel cells. The latter has attracted great research interest throughout the past decades, with one of the key objectives of said research being to meet the energy requirements needed to become an alternative to fossil energy-based systems. [START_REF] Wan | Molecular Design of Single-Atom Catalysts for Oxygen Reduction Reaction[END_REF] Proton exchange membrane fuel cells (PEMFCs) operates through two half-cell reactions, namely the hydrogen oxidation reaction (HOR), and the oxygen reduction reaction (ORR), with electric energy output and pure water as products. The ORR exhibits six or more orders of magnitude slower kinetics than the HOR, hence being the limiting reaction. [START_REF] Debe | Electrocatalyst Approaches and Challenges for Automotive Fuel Cells[END_REF] Nowadays, platinum-based catalysts are generally regarded as the state-of-the-art electrocatalysts for ORR. Loadings as high as 0.4 mgPt/cm 2 are required to promote the ORR while meeting the requirements for automotive practical application. However, the high cost and scarcity of Pt significantly impede the development of PEMFCs. [START_REF] Wu | Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction[END_REF][START_REF] Banham | Current Status and Future Development of Catalyst Materials and Catalyst Layers for Proton Exchange Membrane Fuel Cells: An Industrial Perspective[END_REF] As such, a target of reducing the Pt usage down to 0.1 mgPt/cm 2 without compromising the PEMFC activity or durability was proposed by United States Department of Energy (DOE) for 2025. 6 Strategies such as alloying Pt with a second, less expensive, metal have been put forward to decrease the Pt loading. This incidentally decreases the cost of the electrocatalyst in the PEMFCs stacks while achieving the enhanced activity owing to the optimized binding energy of the ORR intermediates. [START_REF] Huang | High-Performance Transition Metal-Doped Pt3Ni Octahedra for Oxygen Reduction Reaction[END_REF][START_REF] Cui | Compositional Segregation in Shaped Pt Alloy Nanoparticles and Their Structural Behaviour During Electrocatalysis[END_REF] To date, the DOE 2025 target has been fulfilled (i.e. activity of 1.08 A/mgPt@0.9VIR-free with 0.033 mgPt/cm 2 and 36 % losses in mass activity after 30k cycles vs. DOE target being an activity of 0.44 A/mgPt@0.9VIR-free with 0.1 mgPt/cm 2 and a durability of < 40 % losses in mass activity after 30k cycles [START_REF] Chong | Ultralow-Loading Platinum-Cobalt Fuel Cell Catalysts Derived from Imidazolate Frameworks[END_REF] ), but long term, large-scale deployment of PEMFCs in the field of automotive production might require the use of elements that are far less scare than Pt. As such, earth-abundant platinum-group metal free (PGM-free) based electrocatalysts and more specifically, metal-nitrogen-carbon (M-N-C) electrocatalysts have been of growing interest, due to their low cost, CO-tolerance and high intrinsic activity toward the ORR. [START_REF] Yang | Unveiling the High-Activity Origin of Single-Atom Iron Catalysts for Oxygen Reduction Reaction[END_REF][START_REF] Chen | Atomically Dispersed Metal Catalysts for Oxygen Reduction[END_REF][START_REF] Gawande | Carbon-Based Single-Atom Catalysts for Advanced Applications[END_REF] Cobalt phthalocyanine has been proposed in 1964 by Jasinski as an active phase for the ORR. [START_REF] Jasinski | A New Fuel Cell Cathode Catalyst[END_REF] Following this pioneering work, M-N-C electrocatalysts prepared by high temperature pyrolysis of metal-macrocycle complexes adsorbed on carbon substrate were investigated and showed promising ORR activity. [START_REF] Franke | Investigation of the Influence of Thermal Treatment on the Properties of Carbon Materials Modified by N4-Chelates for the Reduction of Oxygen in Acidic Media[END_REF][START_REF] Gouerec | Oxygen Reduction in Acidic Media Catalyzed by Pyrolyzed Cobalt Macrocycles Dispersed on an Active Carbon: The Importance of the Content of Oxygen Surface Groups on the Evolution of the Chelate Structure During the Heat Treatment[END_REF] In 1989, it was reported by Yeager et al. that M-N-C electrocatalysts could be synthesized through pyrolysis of separated metal, carbon and nitrogen sources which enabled better flexibility and control of the electrocatalysts physico-chemical properties. [START_REF] Gupta | Heat-Treated Polyacrylonitrile-Based Catalysts for Oxygen Electroreduction[END_REF] To date, tremendous progresses have been achieved for the preparation of Fe-N-C electrocatalysts (iron being considered as the best 3d-transition metal for M-N-C applied to the ORR) leading to high performances in fuel cells. Very recently, the catalytic activity (0.0442 A/cm 2 @0.9 VIR-free) or durability (30 mV losses at 0.8 A/cm 2 after 30k cycles) of Fe-N-C-containing PEMFCs reached the DOE targets (0.044 A/cm 2 @0.9 VIR-free,  30 mV losses at 0.8 A/cm 2 after 30k cycles) for 2025 [START_REF] Liu | Atomically Dispersed Iron Sites with a Nitrogen-Carbon Coating as Highly Active and Durable Oxygen Reduction Catalysts for Fuel Cells[END_REF] , although not simultaneously. In the light of those exciting achievements for both Fe-N-Cs and Pt-based catalysts, getting a comparison statement between both electrocatalysts' families is of extreme interest.

Such is the aim of this review that will cover, on both Pt-based electrocatalysts and Fe-N-Cs (1) the ORR mechanism; (2) the nature of active sites and their plurality; (3) the intrinsic activity regulation; (4) the structural engineering of cathode catalyst layer in PEMFCs; (5) the degradation mechanisms. Finally, outlooks and challenges are addressed to answer the question brought forth in this review, i.e. the alternatives to Ptbased electrocatalysts provided by Fe-N-Cs.

The ORR mechanism of Pt/C vs. Fe-N-C

One must first clarify fundamental mechanistic insights for the ORR mechanism on both Fe-N-Cs and Pt based electrocatalysts to be able to establish a meaningful comparison. The ORR proceeds by either four-electron (4e -) pathway with H2O (acidic media) and OH -(alkaline media) as primary reduction products or two-electron (2e -) pathway with H2O2 (in acid) and (in alkaline) as main products. [START_REF] Iglesias | N-Doped Graphitized Carbon Nanohorns as a Forefront Electrocatalyst in Highly Selective O2 Reduction to H2O2[END_REF][START_REF] Melchionna | The Rise of Hydrogen Peroxide as the Main Product by Metal-Free Catalysis in Oxygen Reductions[END_REF] The detailed mechanism in acidic media can be divided into three pathways as shown in The association-dissociative pathway typically involves three oxygen intermediates, i.e. OOH*, O*, OH* (Figure 1A) and the theoretical electrocatalytic activity is strongly dependent of their binding free energies. Nørskov's group [START_REF] Kulkarni | Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction[END_REF][START_REF] Abild-Pedersen | Scaling Properties of Adsorption Energies for Hydrogen-Containing Molecules on Transition-Metal Surfaces[END_REF][START_REF] Viswanathan | Universality in Oxygen Reduction Electrocatalysis on Metal Surfaces[END_REF] proposed the following scaling relations between each of the aforementioned intermediates, i.e. Δ GOH* = 1/2 ΔGO* and ΔGOH* = ΔGOOH* -3.2 eV, thus implying that the catalyst surface reactivity can be quantified as the function of one binding energy, e.g. OH*. According to the Sabatier principle, the ideal ORR electrocatalyst should bind its intermediates species neither too strong nor too weak. [START_REF] Sabatier | Hydrogénations Et Déshydrogénations Par Catalyse[END_REF] This results in the existence of volcano-plot like correlation between the activity and the binding energy of the intermediates (Fig-

ure 1B and 1C

). Among all investigated elements, Pt exhibits the closest-to-optimum OH* binding energy, being only 0.1 -0.2 eV too strong. This can be further improved through modulation of its electronic structure (e.g. downshifting the 5d-band center of the Pt surface) to decrease its affinity toward OH* species. A similar volcano plot is also established for several 3d-transition metal-N-C catalysts, as shown in Figure 1C.

Fe-N-C appears as the most active M-N-C, owing to its FeN4 active site and binds OH* slightly too strongly as well, thus implying that modifying its 3d-band structure might also results in activity enhancement. Interestingly, the apex potential for extended metallic surfaces is ca. 100 mV higher than for M-N-Cs (Figure 1B and 1C), hinting toward an intrinsically inferior activity for Fe-N-Cs vs. Pt-based catalysts.

The ORR selectivity (H2O2 vs. H2O) mainly depends on the dissociation energy barrier of absorbed O2 molecule on the surface of the active site. The O2 dissociation energy barrier on pure carbon materials surface is relatively high due to the weak oxygen adsorption, thus favoring H2O2 generation. [START_REF] Kulkarni | Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction[END_REF][START_REF] Yan | First-Principles Study of the Oxygen Adsorption and Dissociation on Graphene and Nitrogen Doped Graphene for Li-Air Batteries[END_REF] On the contrary, the O2 dissociation energy barrier (dominated by associative-dissociative pathway as previously mentioned)

on metal surfaces or metal-doped nitrogen-carbon surface is much lower, thus greatly favoring the 4e -pathway for both Pt and Fe-N-C catalysts. [START_REF] Ou | First-Principle Study of the Adsorption and Dissociation of O2 on Pt (111) in Acidic Media[END_REF][START_REF] Liu | Mn-and N-Doped Carbon as Promising Catalysts for Oxygen Reduction Reaction: Theoretical Prediction and Experimental Validation[END_REF] The first charge transfer (Step VI in Figure 1A) is considered as the determining step (RDS) for both Pt and Fe-N-C electrocatalysts. [START_REF] Janik | First-Principles Analysis of the Initial Electroreduction Steps of Oxygen over Pt (111)[END_REF][START_REF] Chen | Kinetic Isotopic Effect Studies of Iron-Nitrogen-Carbon Electrocatalysts for Oxygen Reduction Reaction[END_REF][START_REF] Malko | Kinetic Isotope Effect in the Oxygen Reduction Reaction (ORR) over Fe-N/C Catalysts under Acidic and Alkaline Conditions[END_REF] Tse et al. [START_REF] Tse | Elucidating Proton Involvement in the Rate-Determining Step for Pt/Pd-Based and Non-Precious-Metal Oxygen Reduction Reaction Catalysts Using the Kinetic Isotope Effect[END_REF] evidenced the RDS of Pt does not involve a proton transfer step, i.e. Pt + O2 + e -→ Pt-OO* -, which was later confirmed by George et al. and Talarposhti et al. in acidic media, owing a high density of Pt nanoparticles onto the carbon surface. [START_REF] George | Kinetic Isotope Effect as a Tool to Investigate the Oxygen Reduction Reaction on Pt-Based Electrocatalysts-Part I: High-Loading Pt/C and Pt Extended Surface[END_REF][START_REF] Rezaei Talarposhti | Kinetic Isotope Effect as a Tool to Investigate the Oxygen Reduction Reaction on Pt-Based Electrocatalysts-Part II: Effect of Platinum Dispersion[END_REF] In opposition to Pt catalyst, the RDS of Fe-N-C involves a protoncoupled electron transfer (PCET), i.e. Nx-Fe + O2 + H + + e -→ Nx-Fe-OOH*. [START_REF] Tse | Elucidating Proton Involvement in the Rate-Determining Step for Pt/Pd-Based and Non-Precious-Metal Oxygen Reduction Reaction Catalysts Using the Kinetic Isotope Effect[END_REF] This would suggest that in acidic media, the ORR performance of Fe-N-C electrocatalyst is intrinsically inferior to that of Pt catalyst due to the increased complexity of the PCET on the Fe-N-C. Chen et al. [START_REF] Chen | Kinetic Isotopic Effect Studies of Iron-Nitrogen-Carbon Electrocatalysts for Oxygen Reduction Reaction[END_REF] proposed that the PCET was arising from the 2e -, H2O2

generating pathway, thus implying that the 4e -transfer was proton-independent. Such independence would underline that (i) minimizing the 2e -pathway selectivity on Fe-N-C is critical and (ii) owing that the proton independency is upheld, the Fe-N-Cs would not be limited by the slow PCET and therefore could potentially complete with Pt-based electrocatalysts.

The ORR process is complicated not only because of multiple electron and proton transfer steps, but also due to the pH dependence of its mechanism. Here, the ORR mechanism as a function of pH is elucidated for both Pt and Fe-N-C. To understand the mechanism, one must first understand the chemical state of the double-layer, which has been discussed in details in the pivotal works of Ramaswamy and Mukerjee: [START_REF] Ramaswamy | Influence of Inner-and Outer-Sphere Electron Transfer Mechanisms During Electrocatalysis of Oxygen Reduction in Alkaline Media[END_REF][START_REF] Ramaswamy | Fundamental Mechanistic Understanding of Electrocatalysis of Oxygen Reduction on Pt and Non-Pt Surfaces: Acid Versus Alkaline Media[END_REF] (i) in acidic media and operating ORR conditions, the inner-Helmholtz plane (IHP) consists of chemisorbed O2, OH* and H2O dipoles, whereas the outer-Helmholtz plane (OHP) consists of solvated O2, electrolyte anions and H3O + ; (ii) in alkaline media and operating ORR conditions, the double-layer chemistry is similar, the notable differences being the alkali metal ions replacing the protons from the acidic environment and the fact that the OH* now arises from direct OH -adsorption rather than H2O activation at high potentials. This directly impacts both the Pt and Fe-N-C reactivity: (i) for Pt, in acidic media, the ORR occurs through reduction of the adsorbed O2 in the IHP, in concert with proton transfers from the OHP to the IHP, leading to a 4e -reduction without desorption of the intermediate (Figure 1D); (ii) in alkaline environment, two mechanisms are observed, owing to the partial formation of an oxide layer onto the Pt surface: an inner sphere electron transfer (ISET) on oxide-free Pt-sites where O2 can chemisorb, which leads to a 4e -transfer, similarly to (i); a non-surface specific outer sphere electron transfer (OSET) on the Pt-oxide, where the solvated O2 forms an hydrogen bond with the OH* that ultimately leads to a 2e -pathway and HO 2 -generation (Figure 1D). Logically, the OSET contribution increases with the OH* coverage and, thus, dominates in strong alkaline environment on Pt (pH  14), as illustrated in Figure 1D. The domination of the 2e -pathway at pH ≥ 14 also occurs on Fe-N-C. On the latter, (i) little to no contribution of the 2e -pathway is observed in mild alkaline solution (at pH < 14), indicating that O2 adsorption on the Fe-Nx center is favored over OH* formation, leading to the formation of HO 2 -. Said reduction is (ii) not favorable in acidic environment, owing to the weak H2O2 binding on the active sites, thus leading to a mix of H2O2 and H2O being generated on Fe-N-C (see Figure 1D). [START_REF] Ramaswamy | Fundamental Mechanistic Understanding of Electrocatalysis of Oxygen Reduction on Pt and Non-Pt Surfaces: Acid Versus Alkaline Media[END_REF][START_REF] Meng | pH-Effect on Oxygen Reduction Activity of Fe-Based Electro-Catalysts[END_REF] In summary, in alkaline environment, the Pt surface gradually becomes (with the pH increases) non-specific and dominated by 2e - pathway, whereas Fe-N-C electrocatalysts mainly follow a 4e -pathway. In acidic environment, the ORR on Pt/C occurs through the 4e -pathway, whereas lower selectivity and H2O2 generation is observed on Fe-N-C. The pH variation induced by the alkaline vs. acid environment also indirectly impacts the Fe-N-Cs by modifying the chemistry of the nitrogen [START_REF] Rojas-Carbonell | Effect of pH on the Activity of Platinum Group Metal-Free Catalysts in Oxygen Reduction Reaction[END_REF][START_REF] Herranz | Unveiling N-Protonation and Anion-Binding Effects on Fe/N/C Catalysts for O2 Reduction in Proton-Exchange-Membrane Fuel Cells[END_REF] neighboring the Fe-Nx actives sites (e.g. stronger withdrawing electron effect from N-H respect to N -induced by protonation of pyridinic N from alkaline to acidic media, thus affecting the 3d-band density of iron atoms).

In this section, we underlined that (i) both Fe-N-C (through its Fe-N4 moiety) and Pt are respectively the best M-N-C and best element for the ORR, although they bind the oxygen intermediates slightly too strongly; (ii) both electrocatalysts families follow a dissociative-associative pathway (or associative for the Fe-N-Cs with low selectivity).

The low selectivity of the Fe-N-Cs in acidic media, along with a PCET being the rate determining step on the 2e -pathway, are here identified as two of the key limiting aspects of the Fe-N-Cs when being compared to Pt-based electrocatalysts. To overcome them, one direction would be to finely tune the electronic properties of the active sites, along with the chemical surface properties of the Fe-N-Cs, to slightly decrease the OH* binding energy and to increase the ORR selectivity toward a 4e -pathway. However, before proposing structural modifications, one must understand the nature, and plurality, of the Fe-N-Cs active sites, which diversity is equal to the one of Pt-based electrocatalysts. Each accessible platinum atom contributes, to a certain extent, to the ORR. For Ptbased nanoparticles, the activity strongly depends on (i) the Pt atom coordination and

(ii) its average distance with its closest neighbors. Both strongly impact the Pt electronic structure and, thus, the OH* binding strength. This was the first observed on monocrystalline surfaces, by investigating different crystalline orientations and showing that Pt (110) > Pt (111) > Pt (100) in terms of activity. [START_REF] Zhang | Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters[END_REF] This order changes when switching from Pt to Pt3Ni, i.e. Pt3Ni (111) > Pt3Ni (110) > Pt3Ni (100), hence explaining the interest for PtxNi structures with high (111) facets density, e.g. octahedra [START_REF] Cui | Octahedral PtNi Nanoparticle Catalysts: Exceptional Oxygen Reduction Activity by Tuning the Alloy Particle Surface Composition[END_REF] .

Interestingly, the activity of the basal planes is modified in presence of steps, atomic vacancy, etc. [START_REF] Maciá | On the Kinetics of Oxygen Reduction on Platinum Stepped Surfaces in Acidic Media[END_REF][START_REF] Kuzume | Oxygen Reduction on Stepped Platinum Surfaces in Acidic Media[END_REF][START_REF] Gómez-Marín | Some Reflections on the Understanding of the Oxygen Reduction Reaction at Pt (111)[END_REF][START_REF] Gómez-Marín | Oxygen Reduction on Nanostructured Platinum Surfaces in Acidic Media: Promoting Effect of Surface Steps and Ideal Response of Pt (111)[END_REF][START_REF] Bandarenka | Elucidating the Activity of Stepped Pt Single Crystals for Oxygen Reduction[END_REF][START_REF] Shao | Recent Advances in Electrocatalysts for Oxygen Reduction Reaction[END_REF] (see Figure 2A left). Specifically, it appears that the high-coordinated 'bottom-of-the-step' site is key for the ORR on Pt extended surfaces. Further investigation by Calle-Vallejo and Tymoczko et al. [START_REF] Calle-Vallejo | Finding Optimal Surface Sites on Heterogeneous Catalysts by Counting Nearest Neighbors[END_REF] evidenced that an optimal active site should exhibit a generalized coordination number (i.e. a coordination number considering the first and second row of neighbors) of 8.3, whereas Le Bacq et al. [START_REF] Bacq | Effect of Atomic Vacancies on the Structure and the Electrocatalytic Activity of Pt-Rich/C Nanoparticles: A Combined Experimental and Density Functional Theory Study[END_REF] underlined the key role played by the interatomic distance of a given active site onto its activity. Those aspects play a fundamental role in the design of Pt-based ORR electrocatalysts: (i) the optimized mass activity for Pt nanoparticles is observed in the size of 2-3 nm. [START_REF] Shao | Electrocatalysis on Platinum Nanoparticles: Particle Size Effect on Oxygen Reduction Reaction Activity[END_REF][START_REF] Perez-Alonso | The Effect of Size on the Oxygen Electroreduction Activity of Mass-Selected Platinum Nanoparticles[END_REF] Further downsize of the Pt nanoparticles results in inferior intrinsic specific activity due to dominating low-coordinated steps and kinks leading to too strong oxygen binding. [START_REF] Shao | Electrocatalysis on Platinum Nanoparticles: Particle Size Effect on Oxygen Reduction Reaction Activity[END_REF] Incidentally, this explains why single Pt atoms or Pt nanoclusters (albeit promising in terms of Pt utilization [START_REF] Liu | Carbon-Supported Divacancy-Anchored Platinum Single-Atom Electrocatalysts with Superhigh Pt Utilization for the Oxygen Reduction Reaction[END_REF][START_REF] Liu | Reconstructing the Coordination Environment of Platinum Single-Atom Active Sites for Boosting Oxygen Reduction Reaction[END_REF] ) present a low intrinsic activity for the ORR, i.e. because of their extremely low coordination and the resulting too strong binding of the intermediates species; 48 (ii) as observed in Figure 1B, Pt (111) shows a slightly too strong adsorption strength of the oxygen intermediates, which can be optimized by following three approaches, namely (a) designing PtM (M = 3d-transition metals [START_REF] Zhang | Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters[END_REF][START_REF] Greeley | Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts[END_REF] or rareearth elements [START_REF] Escudero-Escribano | Tuning the Activity of Pt Alloy Electrocatalysts by Means of the Lanthanide Contraction[END_REF][START_REF] Colic | Pt Alloy Electrocatalysts for the Oxygen Reduction Reaction: From Model Surfaces to Nanostructured Systems[END_REF] ) without modifying the coordination or nanoparticle size, leading to an activity improvement through strain [START_REF] Strasser | Lattice-Strain Control of the Activity in Dealloyed Core-Shell Fuel Cell Catalysts[END_REF][START_REF] Mavrikakis | Effect of Strain on the Reactivity of Metal Surfaces[END_REF][START_REF] Tian | Engineering Bunched Pt-Ni Alloy Nanocages for Efficient Oxygen Reduction in Practical Fuel Cells[END_REF] and ligand 60 effects (for the 3d-transition metal) along with more complex structural modifications (e.g. Kagome layer for the rare-earth elements, [START_REF] Escudero-Escribano | Tuning the Activity of Pt Alloy Electrocatalysts by Means of the Lanthanide Contraction[END_REF] grain boundary, [START_REF] Asset | A Review on Recent Developments and Prospects for the Oxygen Reduction Reaction on Hollow Pt-Alloy Nanoparticles[END_REF] structural defects, [START_REF] Asset | A Review on Recent Developments and Prospects for the Oxygen Reduction Reaction on Hollow Pt-Alloy Nanoparticles[END_REF][START_REF] Chattot | Beyond Strain and Ligand Effects: Microstrain-Induced Enhancement of the Oxygen Reduction Reaction Kinetics on Various PtNi/C Nanostructures[END_REF] surface distortion 63 etc.)

(see Figure 2A right). Indeed, those effects result in the alteration of the Pt 5d-band electronic structure and, as such, a near-optimal oxygen-based adsorbates binding energy (b) optimizing the structure through the design of shape-controlled nanostructures, aiming to expose preferential facets of Pt to maximize the catalytic activity. [START_REF] Zhang | Platinum-Based Nanocages with Subnanometer-Thick Walls and Well-Defined, Controllable Facets[END_REF][START_REF] Wang | Synthesis of Monodisperse Pt Nanocubes and Their Enhanced Catalysis for Oxygen Reduction[END_REF] (e.g.

Pt3Ni octahedra dominated by (111) facets, see Figure 2A left) (c) implementing structural defects into the Pt-based nanostructures, aiming to design active sites with an optimal generalized coordination number and optimal atomic distance with the nearest neighbors and, therefore, an optimal binding energy for the ORR intermediates. [START_REF] Calle-Vallejo | Finding Optimal Surface Sites on Heterogeneous Catalysts by Counting Nearest Neighbors[END_REF][START_REF] Asset | A Review on Recent Developments and Prospects for the Oxygen Reduction Reaction on Hollow Pt-Alloy Nanoparticles[END_REF] Those different approaches led to a wide range of highly performing, innovative Ptbased electrocatalysts, that are discussed in further extent in Ref. [START_REF] Shao | Recent Advances in Electrocatalysts for Oxygen Reduction Reaction[END_REF][START_REF] Asset | A Review on Recent Developments and Prospects for the Oxygen Reduction Reaction on Hollow Pt-Alloy Nanoparticles[END_REF][START_REF] Ly | Integrating Nanostructured Pt-Based Electrocatalysts in Proton Exchange Membrane Fuel Cells[END_REF] . The precise identification of FeNxCy coordination shells (x, y values) remains challenging and is conventionally extrapolated from Mössbauer spectroscopy 67 and X-ray adsorption spectroscopy, more specifically, extended X-ray adsorption fine structure (EXAFS). [START_REF] Zitolo | Identification of Catalytic Sites for Oxygen Reduction in Iron-and Nitrogen-Doped Graphene Materials[END_REF] The possible Fe-Nx coordination number in the first shell is proposed to be x = 1 -4, based on experimental and theoretical calculation findings [START_REF] Li | Preparation of Fe-N-C Catalysts with FeNx (X= 1, 3, 4) Active Sites and of Their Activities for the Oxygen Reduction Reaction and Performances in Proton Exchange Membrane Fuel Cells[END_REF][START_REF] Shen | Atomically FeN2 Moieties Dispersed on Mesoporous Carbon: A New Atomic Catalyst for Efficient Oxygen Reduction Catalysis[END_REF][START_REF] Lefèvre | Fe-Based Catalysts for the Reduction of Oxygen in Polymer Electrolyte Membrane Fuel Cell Conditions: Determination of the Amount of Peroxide Released During Electroreduction and Its Influence on the Stability of the Catalysts[END_REF][START_REF] Lefèvre | Molecular Oxygen Reduction in PEM Fuel Cells: Evidence for the Simultaneous Presence of Two Active Sites in Fe-Based Catalysts[END_REF][START_REF] Zhu | Hierarchically Porous M-N-C (M = Co and Fe) Single-Atom Electrocatalysts with Robust MNx Active Moieties Enable Enhanced ORR Performance[END_REF][START_REF] Kabir | Computational and Experimental Evidence for a New TM-N3/C Moiety Family in Non-PGM Electrocatalysts[END_REF][START_REF] Holby | Structure of Fe-Nx-C Defects in Oxygen Reduction Reaction Catalysts from First-Principles Modeling[END_REF][START_REF] Liu | Edge-Hosted Fe-N3 Sites on a Multiscale Porous Carbon Framework Combining High Intrinsic Activity with Efficient Mass Transport for Oxygen Reduction[END_REF] (Figure 2B).

Similarly to

Notably, on the basis of EXAFS and Mössbauer findings, recent works have been discussing higher coordination numbers, including the presence of a nitrogen atom below the plane, thus leading to a N-FeN4 [START_REF] Liang | Two Types of Single-Atom FeN4 and FeN5 Electrocatalytic Active Centers on N-Doped Carbon Driving High Performance of the SA-Fe-NC Oxygen Reduction Reaction Catalyst[END_REF][START_REF] Lai | Metal-Organic-Framework-Derived Fe-N/C Electrocatalyst with Five-Coordinated Fe-Nx Sites for Advanced Oxygen Reduction in Acid Media[END_REF][START_REF] Kramm | Structure of the Catalytic Sites in Fe/N/C-Catalysts for O2-Reduction in PEM Fuel Cells[END_REF][START_REF] Kramm | Correlations between Mass Activity and Physicochemical Properties of Fe/N/C Catalysts for the ORR in PEM Fuel Cell Via 57 Fe Mössbauer Spectroscopy and Other Techniques[END_REF] or N-FeN4-N [START_REF] Zhu | Unravelling the Structure of Electrocatalytically Active Fe-N Complexes in Carbon for the Oxygen Reduction Reaction[END_REF][START_REF] Liu | Discriminating Catalytically Active FeNx Species of Atomically Dispersed Fe-N-C Catalyst for Selective Oxidation of the C-H Bond[END_REF] structure (left inset in Figure 2B). The chemistry and relative abundancy of the Fe-containing moieties are usually controlled by two parameters, i.e. (i) the pyrolysis temperature [START_REF] Li | Preparation of Fe-N-C Catalysts with FeNx (X= 1, 3, 4) Active Sites and of Their Activities for the Oxygen Reduction Reaction and Performances in Proton Exchange Membrane Fuel Cells[END_REF][START_REF] Liu | Discriminating Catalytically Active FeNx Species of Atomically Dispersed Fe-N-C Catalyst for Selective Oxidation of the C-H Bond[END_REF] and (ii) their position in the carbon structure, i.e. FeN2 is commonly observed on the edge of the carbon crystallite [START_REF] Asset | Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells[END_REF] while FeN3 and FeNxC4-x (where C is located at first coordination shell with x  4) moieties are observed in-plane (Figure 2B). [START_REF] Kabir | Computational and Experimental Evidence for a New TM-N3/C Moiety Family in Non-PGM Electrocatalysts[END_REF][START_REF] Asset | Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells[END_REF] The first and second coordination shell of the moiety most often considered as best for the ORR, the FeN4, is widely debated, [START_REF] Liu | Metal-Nitrogen Coordination Moieties in Carbon for Effective Electrocatalytic Reduction of Oxygen[END_REF] and often established from a combination of Mössbauer and EXAFS findings combined with modelling. Hence, it is essential to look to this discussion with a critical eye, i.e. (i) the fitting of the Mössbauer/EXAFS data can often correspond to several geometries and (ii) the pyrolytic methods undergone to synthesize the Fe-N-Cs lead to the presence of a wide variety of coordination/chemistry, i.e. that there is not one type of active site in a given electrocatalyst. Unfortunately, obtaining a direct proof of the existence of a given coordination (e.g. by high resolution electron microscopy combined by elemental mapping) is extremely challenging on Fe-N-Cs synthesized by pyrolytic methods, leaving the exact nature of the active sites as hypotheses, albeit strongly backed with indirect evidences. Furthermore, inadequate characterization procedure can also affect the obtained data and, thus, the interpretation. For example, for Mössbauer spectroscopy, low temperature is required for a precise identification of the different moieties, as α-Fe2O3 nanoparticles share similar Mössbauer parameters to FeN4C12 sites at room temperature. [START_REF] Li | Identification of Durable and Non-Durable FeNx Sites in Fe-N-C Materials for Proton Exchange Membrane Fuel Cells[END_REF][START_REF] Kramm | 57 Fe Mössbauer Spectroscopy Characterization of Electrocatalysts[END_REF] On the basis of Mössbauer spectrum results, Kramm et al. [START_REF] Kramm | Structure of the Catalytic Sites in Fe/N/C-Catalysts for O2-Reduction in PEM Fuel Cells[END_REF][START_REF] Kramm | Correlations between Mass Activity and Physicochemical Properties of Fe/N/C Catalysts for the ORR in PEM Fuel Cell Via 57 Fe Mössbauer Spectroscopy and Other Techniques[END_REF] originally proposed that the Fe-containing moieties consisted of low spin (here referred as D1, and considered as a Fe(II)-N4 moiety), intermediate spin (D2, Fe(II)-N2+2, Fe situated between two carbon crystallites) and high spin (D3, N-Fe(II)-N2+2, Fe situated between two carbon crystallites with N-graphitic in the plane underneath the Fe atom or FeN4+1 with adjacent protonated nitrogen). Only D1 and D3 sites were deemed to exhibit high ORR activity while D2 was considered as inactive due to the Fe fully occupied 3d orbital failing to bind O2 in a non-bent conformation. Although the D3 sites are proposed to be most active site for ORR, [START_REF] Kramm | Structure of the Catalytic Sites in Fe/N/C-Catalysts for O2-Reduction in PEM Fuel Cells[END_REF][START_REF] Kramm | Correlations between Mass Activity and Physicochemical Properties of Fe/N/C Catalysts for the ORR in PEM Fuel Cell Via 57 Fe Mössbauer Spectroscopy and Other Techniques[END_REF] they are preferentially hosted in micropores, [START_REF] Lefèvre | Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells[END_REF] which compromises their performance in PEMFC operating conditions due to their inaccessibility to ionomer, [START_REF] Yarlagadda | Boosting Fuel Cell Performance with Accessible Carbon Mesopores[END_REF] flooding and demetallation [START_REF] Yang | Activity, Performance, and Durability for the Reduction of Oxygen in PEM Fuel Cells, of Fe/N/C Electrocatalysts Obtained from the Pyrolysis of Metal-Organic-Framework and Iron Porphyrin Precursors[END_REF][START_REF] Chenitz | A Specific Demetalation of Fe-N4 Catalytic Sites in the Micropores of NC_Ar+NH3 Is at the Origin of the Initial Activity Loss of the Highly Active Fe/N/C Catalyst Used for the Reduction of Oxygen in PEM Fuel Cells[END_REF] . In recent studies, Mukerjee's group [START_REF] Li | Structural and Mechanistic Basis for the High Activity of Fe-N-C Catalysts toward Oxygen Reduction[END_REF][START_REF] Jia | Spectroscopic Insights into the Nature of Active Sites in Iron-Nitrogen-Carbon Electrocatalysts for Oxygen Reduction in Acid[END_REF][START_REF] Jia | Experimental Observation of Redox-Induced Fe-N Switching Behavior as a Determinant Role for Oxygen Reduction Activity[END_REF] provided further insights on the D1 moiety and its variations as a function of potential, ascribing it to high spin 'in-plane' OH-Fe(III)-N4 sites (axially coordinated to oxygen from water activation) at high potential and 'out-of-the-plane' Fe(II)-N4 sites at low potential, the latter being considered as the active site [START_REF] Li | Asymmetric Volcano Trend in Oxygen Reduction Activity of Pt and Non-Pt Catalysts: In Situ Identification of the Site-Blocking Effect[END_REF] while the site-blocking effect resulted from water activation was viewed as detrimental. [START_REF] Li | Identification of Durable and Non-Durable FeNx Sites in Fe-N-C Materials for Proton Exchange Membrane Fuel Cells[END_REF][START_REF] Li | Asymmetric Volcano Trend in Oxygen Reduction Activity of Pt and Non-Pt Catalysts: In Situ Identification of the Site-Blocking Effect[END_REF][START_REF] Xiao | Microporous Framework Induced Synthesis of Single-Atom Dispersed Fe-NC Acidic ORR Catalyst and Its in Situ Reduced Fe-N4 Active Site Identification Revealed by X-Ray Absorption Spectroscopy[END_REF] In this end, intrinsic activity could be significantly improved by increasing the Fe(III)/Fe(II) redox transition potential. [START_REF] Li | Asymmetric Volcano Trend in Oxygen Reduction Activity of Pt and Non-Pt Catalysts: In Situ Identification of the Site-Blocking Effect[END_REF][START_REF] Osmieri | Elucidation of Fe-NC Electrocatalyst Active Site Functionality Via in-Situ X-Ray Absorption and Operando Determination of Oxygen Reduction Reaction Kinetics in a PEFC[END_REF] The local carbon structure in the second shell surrounding the FeN4 moiety also varies on distinct FeN4Cy sites: 'y' values are found to be eight (i.e.

FeN2+2C8, micropore sites, bridging two carbon crystallite) [START_REF] Charreteur | Fe/N/C Non-Precious Catalysts for PEM Fuel Cells: Influence of the Structural Parameters of Pristine Commercial Carbon Blacks on Their Activity for Oxygen Reduction[END_REF][START_REF] Jaouen | Cross-Laboratory Experimental Study of Non-Noble-Metal Electrocatalysts for the Oxygen Reduction Reaction[END_REF][START_REF] Liu | Role of Local Carbon Structure Surrounding FeN4 Sites in Boosting the Catalytic Activity for Oxygen Reduction[END_REF] , ten (i.e. FeN4C10, bound to pyridinic N), [START_REF] Li | Identification of Durable and Non-Durable FeNx Sites in Fe-N-C Materials for Proton Exchange Membrane Fuel Cells[END_REF] or twelve (i.e. FeN4C12, bound to pyrrolic N) [START_REF] Zitolo | Identification of Catalytic Sites for Oxygen Reduction in Iron-and Nitrogen-Doped Graphene Materials[END_REF][START_REF] Li | Identification of Durable and Non-Durable FeNx Sites in Fe-N-C Materials for Proton Exchange Membrane Fuel Cells[END_REF]100 . The FeN4C12 moieties, surface accessible, correspond to the D1 doublet [100][101] . The moiety location is also of critical importance when addressing the material stability, e.g. FeN2,

FeN2+2Cy moieties are edge sites while FeN4Cy, FeN3 and FeNxC4-x moieties are more likely 'plane-exposed' sites (i.e. situated in the carbon basal plane facing the mesopore,

Figure 2B

). [START_REF] Asset | Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells[END_REF][102][103] Hence a mesoporous carbon favors FeN4+1, FeN4Cy, FeN3, FeNxC4-

x as main active sites, whereas more 'edge-exposed' (e.g. armchair, zig-zag edge of carbon basal plane and micropores) lead to FeN2 and FeN2+2Cy as main active sites.

Noteworthy, the carbon edges are preferentially corroded, thus suggesting that, from a durability standpoint, the development of 'plane-exposed' sites should be favored.

Assessing the reactivity of Fe-based nanoparticles (metallic Fe, iron oxides, carbides and nitrides) observed in highly loaded Fe-N-Cs remains challenging, due to the simultaneous presence of FeNxCy sites. Extensive studies have revealed that they act as the main ORR active site in alkaline medium. [104][105][106][107][108][109][110][111] Specifically, a synergetic effect between Fe-nanoparticles and FeNxCy moieties has been widely proposed. [112][113][114][115][116][117][118][119][120] For instance, in alkaline media, Wei's group 121 proposed that Fe/Fe3C nanoparticles could dramatically improve the activity of the adjacent Fe-Nx by modifying the electronic structure of the latter, which favored the O2 adsorption. In acidic media, the Fe-based nanoparticles (Fe/Fe3C) encapsulated into thin graphitic N-doped carbon layer (Fe@N-C, see Figure 2B) usually present an excellent ORR activity and durability. [122][123][124][125][126][127][128][129] However, the encapsulated Fe-based nanoparticles are not involved in the ORR process, but exert a positive electronic effect on the outer surface of graphitic carbon layer that favors the stabilization of the H2O2 intermediates and enables full 4e -ORR. [124][125]128 More importantly, they are protected from direct contact with the electrolyte and H2O2

intermediates, thus exhibiting high stability. 124,128 Indeed, Choi et al. 130 recently confirmed that these carbon-encapsulated Fe nanoparticles (Fe@N-C) electrocatalysts presented a high ORR activity leading to 4e -reduction, dominated by the indirect four electron oxygen reduction (S*, see Figure 3, O2 → H2O2 and H2O2 → H2O) associative pathway. However, 'electrolyte-exposed' iron-based nanoparticles are ORR inactive in acidic environment, as revealed by the independence of the ORR activity on their presence. [130][131][132] As such, the latter should be avoided in acidic environment as it also contributes, through its corrosion during operation, to the poisoning of membrane and the formation of strong oxidative radical HO • by Fenton reaction. [131][132] Mitigation methods, such as the use of sulfur-based precursors, have been recently introduced to prevent their excessive formation. [133][134][135] Overall, the contribution to the catalytic activity of the iron-based nanoparticles in acidic environment strongly depends on the thickness of the N-doped carbon shell surrounding them, which itself depends of the synthesis procedure (e.g. pyrolysis temperature and type of carbon, nitrogen precursors). [136][137] Altogether, the improved ORR performance can be achieved through (1) increasing the sites density of atomic, D1, FeNxCy sites or rational design of Fe@N-C site with proper coated N-doped carbon layer; (2) removing the naked Fe-based nanoparticles to prevent the poisoning of membrane or the formation of high corrosive reaction radical species (ROS) induced by Fenton reaction due to the dissolution of Fe nanoparticles during ageing. [131][132] Nitrogen atoms are also incorporated into carbon matrix during pyrolysis as ironfree, nitrogen-containing functionalities. Such nitrogen species in carbon matrices have been assigned by X-ray photoelectron spectroscopy and density functional theory (DFT) calculations, including imine or nitrile groups ( 398.0 eV), 138 pyridinic N (398.3 -398.8 eV), 139 amine group (399.5 -399.8 eV), [138][139] pyrrolic N or hydrogenated N (400.6 -400.9 eV), [139][140] and a mixture of quaternary N, graphitic N and protonated pyridinic N (401.5 -403.0 eV), 138,[141][142] . The nitrogen-based moieties are highly sensitive to their chemical environment, with different binding energies observed for chemically identical moieties (e.g. pyridinic) if they are found in the basal plane or on the carbon edge. 139 The N-containing functionalities only catalyze part of the ORR: in acidic media, whereas FeNxCy and Fe@N-C sites are S or S* with direct four electron reduction or single site series 2  2 e -electron reduction processes (see Figure 3); the pyrrolic N, protonated or hydrogenated N, quaternary N (N + ) and graphitic N are S1 sites (O2 → H2O2, Figure 3) and pyridinic N is a S2 site (H2O2 → H2O, Figure 3). 130,[138][139][142][143] The exact nature of the pyridinic N site is still controversial, with Guo et al. 144 suggesting that the latter is responsible for a Lewis-basic behavior of the neighboring carbon atoms, but Kondo et al. 145 arguing that the lone pair on the pyridinic nitrogen implies a negatively charged nitrogen and thus that the surrounding carbon atoms should be positively charged, rendering them inadequate as a Lewis base. This should be also considered in the prism of the pH variations, as the phenomena changes in acidic environment, with the protonation of the pyridinic N (pKa = 6.5) [START_REF] Meng | pH-Effect on Oxygen Reduction Activity of Fe-Based Electro-Catalysts[END_REF][START_REF] Rojas-Carbonell | Effect of pH on the Activity of Platinum Group Metal-Free Catalysts in Oxygen Reduction Reaction[END_REF][START_REF] Herranz | Unveiling N-Protonation and Anion-Binding Effects on Fe/N/C Catalysts for O2 Reduction in Proton-Exchange-Membrane Fuel Cells[END_REF][146][147] that modify its charge delocalization and thus, in theory, the Lewis basicity of the adjacent carbon atoms. Notably, N-containing moieties exhibit nearly no activity or far lower ORR activity in acidic environment, with respect to FeNxCy moieties. [START_REF] Asset | Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells[END_REF]130,146 Furthermore, they act as defects in the carbon structure and, therefore, starting points for degradations during operation in acidic environment, although this defectivity can be positively used to tune the electronic structure of the iron, as explored in the following section. enhancing the ORR (see Figure 4A). On the contrary, a highly graphitized carbon surface with fewer defects results in an increased electron donating property and a stronger OH* binding, therefore compromising the ORR activity (Figure 4A). It is worth noting that, although increasing the structural defects density can potentially optimize the ORR activity, highly defective carbon planes ('overshooting') will not only result in too weak OH* binding energy (shift to the right side of volcanic plot, Figure 4A) but also reduce their durability due to large number of preferential corrosion sites. [START_REF] Liu | Atomically Dispersed Iron Sites with a Nitrogen-Carbon Coating as Highly Active and Durable Oxygen Reduction Catalysts for Fuel Cells[END_REF][START_REF] Asset | Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells[END_REF] Taken together, a trade-off between graphitization degree and structural defects should be carefully considered for optimization of both intrinsic activity and durability.

Ligand effect surrounding FeN4 site.

In addition to the effect of -electron delocalization of carbon basal plane on delectron density of Fe center, the so-called ligand effect also plays an important role in tuning the ORR activity. 166 A variety of element dopants with distinct electronegativity have been investigated, with sulfur-doping being the most commonly studied. [167][168][169][170][171][172][173] Generally, the S-doping does not affect the coordination of Fe center: the sulfur atom is either adjacent or away from the nitrogen atoms in the FeN4 site. [174][175] Two forms of S dopants with electron-withdrawing oxidized S (C-SOx) 176 and electron-donating thiophene-like S (C-S-C) [175][176][177][178] are proposed to tune the electron structure of the FeN4 moiety. Specifically, the introduction of oxidized sulfur (-C-SOx) 176 on the neighboring FeN4 sites significantly decreases the d-band center of Fe by an electron-withdrawing effect, which shifts OH* binding energy toward the apex direction of the volcano plot (Figure 4A). On the contrary, the thiophene-like C-S-C increases the electron density of the Fe center due to a strong electron-donating effect thus brings the electrocatalyst toward the stronger adsorption energy side (Figure 4A). Besides S-doping, other elements such as B, 179-180 , P, [181][182][183] and O 184 with different withdrawing/donating groups are also documented. Independently of the dopant nature, it is important to note that (1) the effect on the d-band center strongly depends on the distance between the dopant and the FeN4 moiety; 178 (2) as aforementioned for the structural defects, the content of dopants should be carefully controlled so as to obtain optimal binding energy on Fe center (e.g. excessive amount of C-SOx or electron-withdrawing groups resulting in too weak OH* binding on FeN4 sites-right side of volcano plot, Figure 4A). aiming to improve the ORR intrinsic activity.

The regulation of intrinsic activity of Pt-based catalysts

Tuning the surroundings of a given active site echoes with the recent efforts undergone to design innovative Pt-based electrocatalysts, as touched upon in Section 3, and that Sabatier-driven, volcano-shaped activity is easily applied to this family of electrocatalysts as well. Indeed, considering Pt (111) as the 'Fe(II)-N4 like' site, binding OH* 0.1 -0.2 eV too strongly, it appears that, as illustrated on Figure 4B: (1) the addition of 3d-transition metals and/or the use of clearly defined crystalline orientations (e.g. Pt3Ni(111), octahedra, nanoframes) lead to a downshifting of the Pt 5d orbital and the resulting weakening of the OH* adsorption strength, thus greatly increasing the ORR activity. [START_REF] Zhang | Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters[END_REF][START_REF] Cui | Octahedral PtNi Nanoparticle Catalysts: Exceptional Oxygen Reduction Activity by Tuning the Alloy Particle Surface Composition[END_REF]185 But such modification also impacts the nanostructure durability, as well-organized facets and 3d transition metals both exhibit low stability in highly acidic environment; [START_REF] Chattot | Surface Distortion as a Unifying Concept and Descriptor in Oxygen Reduction Reaction Electrocatalysis[END_REF]186 (2) properly decreasing the Pt nanoparticles size enables more exposure of active sites, but excessive downsizing exposes a large density of edges sites with decreased coordination number (Figure 4B). This negatively impacts the ORR activity due to too strong OH* adsorption as well as decreasing the nanoparticles durability due to the easier aggregation of small particles induced by large surface energy;

(3) the addition of localized structural defects (e.g. platinum adatoms, concavities) modifies the local environment of a given platinum atom and, through the changes in its microstrain and local coordination, thus its OH* binding strength (Figure 4B). 48-49, collapse when exposed to harsh environmental conditions. 187 Those aspects bring forth similar conclusions than those presented for the Fe(II)-N4 site, i.e. that an excess in structural defects or alloying elements would be detrimental for the ORR activity, by 'over-shooting' the optimal OH* binding strength, and that such modified nanostructures would, in addition, show depreciated electrocatalytic activity (Figure 4B).

Engineering of cathode catalyst layer (CL) for Pt-based and Fe-N-C electrocatalysts and applications in PEFMCs

Although high ORR activity is usually obtained in 3-electrode cell rotating disk electrode (RDE) measurements, the experimental conditions widely differ from those into which the electrocatalysts are meant to operate, i.e. PEMFCs, the activities obtained in the latter being far more meaningful than those obtained in liquid electrolyte.

In PEMFCs, several challenges arise from the mass transport limitations (e.g. O2 and H + transport to the interface, H2O removal). The ORR in PEFMCs occurs at the triplephase boundary (catalyst-electrolyte-gas) and involves (i) the electrocatalyst and (ii) H + , and O2 circulating through the ionomer and/or the electrode porosities to/from said electrocatalyst and (iii) the provisioning/removal of H2O. 188 Therefore, apart from the rational design of electrocatalysts with high ORR performance, the engineering of catalytic layers with a well-designed triple-phase boundary is of great importance for practical application, as illustrated in Figure 5. First, a good ionomer coverage of the carbon support must be achieved, as to ensure optimal active site accessibility to the charged species. For example, Ott et al. 189 modified carbon black surfaces by nitrogen doping, enhancing the interaction between the ionomer and the carbon surface and leading to an even ionomer coverage, thus significantly reducing the oxygen transport resistance and improving the fuel cell power density for Pt-based electrocatalysts. Likewise, Huang's group 190 also reported the chemical modification of carbon support with surface oxidation, identifying an optimal surface oxygen loading to create favorable carbon-ionomer interaction and to balance hydrophobic and hydrophilic properties on the catalytic layer, favoring the water management. The thickness of the ionomer layer, should also be considered carefully, as Harada et al. 191 showed that a dense ionomer layer (> 3 nm) on the top of the active sites resulted in limited local O2 diffusion. In the presence of Pt-based nanostructures, the interactions between the latter and the ionomer have to be considered, owing to the poisoning of the Pt surface by the side chain of perfluorosulfonated ionomers (e.g. -SO3 -). 192 A recent study to address this challenge consisted in depositing a thin layer of carbon (< 1 nm) on the Pt nanoparticles surface to prevent poisoning without compromising gas/proton permeability. 193 Another way to address the Pt poisoning is to optimize the pore structure of the carbon support, which tunes the location of the Pt-based nanoparticles: exposure of the Pt nanoparticles on the carbon support outer surface is favorable, as it eases the O2 accessibility, but increase the ionomer poisoning effect. Contrarily, the deposition of Pt particles in the carbon inner porosities can efficiently prevent the poisoning effect and particles aggregation during ageing, 194 at the cost of depreciated gas transport (see Figure 5A). To this end, Yarlagadda et al. [START_REF] Yarlagadda | Boosting Fuel Cell Performance with Accessible Carbon Mesopores[END_REF] identified a perfect trade-off through porous carbon design with a bottleneck pore size of 4 -7 nm, in which Pt particles located inside pore did not suffer from poisoning by the ionomer (contactless with ionomer) nor high gas transport resistance (Figure 5A). Inferior intrinsic activity and limited active sites density usually result in thicker catalytic layers for Fe-N-Cs with respect to that of Pt-based catalysts (60-100 m of CL for Fe-N-Cs vs. 10 m for identical performance for Pt-based catalyst). [195][196][197] However, the increased thickness of the Fe-N-Cs catalytic layer not only increases the mass transport limitation, 198 but decreases the utilization of the catalyst located within the catalytic layer and increases the possibility for its detachment and flooding during operation. 196,[199][200] In this end, two strategies are employed for the optimization of Fe-N-Cs performances in PEMFCs as shown in the Figure 5B: (1) increasing the accessibility of FeN4 sites and rationalizing the design of the Fe-N-Cs porous structure. Very recently, Jiao et al. 201 proposed an innovative approach to bring the percentage of utilized active sites near 100%, by first pyrolyzing a nanosized zeolitic imidazolate framework (ZIF-8) and then using chemical vapor deposition of FeCl3 to replace the 'vapor-accessible' ZnN4 moieties originally present in ZIF-8 by FeN4. An activity in PEMFC of 33 mA/cmgeo 2 at 0.9 V(IR-free) was reached, close to target of 2025 set by DOE. The design of an optimized, hierarchical, porous structure would also greatly increase the FeN4 accessibility, by exposing inaccessible or buried FeN4 sites, along with favoring the mass transport of reactants and products. 159,202 However, an overly complex porous network could ultimately be detrimental, by increasing the structure defectivity (thus lowering its stability and compromising the intrinsic activity of FeN4 sites, see Figure 4A) as well as rendering some sites non-accessible to the ionomer; (2) optimizing the catalytic layer structure. Similar to Pt-based systems, the ionomer content plays an important role in facilitating, or limiting, gas transportation and ionic species conductivity.

Thus, trade-off between gas transport and proton conductivity should be achieved, 203 in the form of an uniform coverage of the Fe-N-C electrocatalyst by a thin layer of ionomer (< 3 nm). 204 As far as the catalyst-ionomer interactions are concerned, Artyushkova et al. 205 revealed the pivotal role played by the nitrogen species within the carbon network: the hydrogenated N and positively charged protonated N interact strongly with negatively charged sulfonated groups of perfluorosulfonated ionomers (e.g. Nafion®), hence disrupting the ionomer network and limiting the proton transport. On the contrary, surface oxides with a negative charge were deemed responsible for an even distribution of ionomer and optimal interaction between the Fe-N-C and ionomer (Figure 5B).

To provide a clear insight into the current state of the Fe-N-Cs as PEMFCs electrocatalysts vs. Pt-based systems, a list of some of the most promising Fe-N-Cs and Ptbased systems is provided in Table 1 andTable 2. To allow for a meaningful comparison, the technical details (e.g. pressure, gas atmosphere, flow rate, etc.) are also provided. It is worth noting that the lack of unified durability test protocols for Fe-N-Cs make the data comparison on that specific aspect meaningless. a DOE: 2025 performance target of department of energy, US; b loading of catalyst. c total outlet pressure (absolute) for a fully humidified H2 and O2/air. d flow rate of H2 and O2/air. e relative humidity. f current density (IR-corrected) at applied voltage and gas atmosphere (0.9 V under O2 or 0.8 V under air). g rated power under O2 or air. * indicates that IR correction is not specified or no IR-correction is applied.

Table 2 The MEA-based PEMFC performances ranking the best among reported Pt-based electrocatalysts a DOE: 2025 performance target of department of energy, US; b loading of Pt; c total outlet pressure (absolute) fully humidified for H2 and O2/air; d flow rate of H2 and O2; e relative humidity; f current density (IR-corrected) at applied voltage under O2 (0.9 V); g Accelerated stress test (AST) at given potential range according to DOE catalyst durability evaluation protocols 6 ; h mass activity loss at 0.9 V (IR-corrected), and voltage loss at 0.8 A/cm 2 after 30000 cycles (unless other stated).

Entry a L b mgPt/cm 2 P c kPaabs T ( o C) FR d (H2/O2) sccm RH e % j0.9V f O2 A/mgPt j0.9V f O2 mA/cm 2 AST g Condition (V) AST h MA(
In the case of Pt-based catalyst, great accomplishments on both activity and durability have been achieved, even surpassing the performance target set by DOE 2025.

(Table 2). Hence, it appears that the electrocatalytic performance of Pt-based electrocatalysts is not decisive for the commercial success, the limitations rather arising from the scarcity and cost of platinum, requiring either 100% efficiency in terms of Pt recycling 222 for further utilization or greater advances in terms of catalytic improvement.

Although innovative structures far outperforming PtCo have been identified, owing to the directions discussed in Section 3 and 4, translating them from the laboratory-scale to large-scale industry level remains extremely difficult. [START_REF] Ly | Integrating Nanostructured Pt-Based Electrocatalysts in Proton Exchange Membrane Fuel Cells[END_REF] Fe-N-C electrocatalysts present lower performance with respect to Pt-based electrocatalyst in both activity and durability. The MOF-derived Fe-N-Cs are the most promising, e.g. very recently, Wu's group [START_REF] Liu | Atomically Dispersed Iron Sites with a Nitrogen-Carbon Coating as Highly Active and Durable Oxygen Reduction Catalysts for Fuel Cells[END_REF] reported a MOF-derived NH4Cl treated Fe-N-C electrocatalyst that reached the 2025 DOE target in terms of activity (Table 1), through the successful implementation of structural defects in the vicinity of the FeN4 sites. However, those materials showed poor durability (up to 94 % activity loss after 30,000 potential cycles between 0.6 V and open circuit potential). By post-deposition of a N-doped carbon layer onto the Fe-N-C surface, the durability was found to meet the U.S 2025 DOE target, while exhibiting decreased activity. indicates that both parameters are often correlated, as discussed in Section 4, and that designs that are not bound by the Volcano plot presented in Figure 4A are needed to go 'beyond' those limitations.

By opposition, Fe-N-Cs are better performing than Pt-based electrocatalysts in alkaline environment, as reported in liquid electrolyte 135,[223][224][225] , and beneficiate from the less corrosive environment. This highlights their potential application as anion exchange membrane fuel cells (AEMFCs) cathode electrocatalysts. However, the design of an AEMFC has to consider several challenges that are cathode catalytic layer independent, i.e. (i) carbonation is usually encountered in the presence of CO2 (from the air feed), which dramatically decreases the ionic conductivity of the AEM, the pH and greatly impacts the performances; 226-228 (ii) water management is required to maintain membrane hydration, as water serves as both as a reactant and as a product at the cathode and anode, respectively, while avoiding flooding; [229][230][231] (iii) the alkaline environment the HOR kinetics at the anode, thus requiring the identification of adapted electrocatalysts for said reaction. 232 As such, the research and development of AEMFCs has lagged far behind the one of PEMFCs. However, recent years have witnessed significant advancements, and high peak power density (e.g. > 1 W/cm 2 in H2/O2) using Fe-N-C catalysts at the cathode have been reported. [233][234][235][236] Specifically, Mustain' group 237 recently developed an AEMFC with a state-of-the-art Fe-N-C electrocatalyst, which achieved a peak power density of > 2 W/cm 2 under H2/O2, while > 1 W/cm 2 of power density was still obtained under H2/Air (CO2-free).

Here, two critical points were evidenced: (i) the gap between the Pt-based electrocatalysts and Fe-N-C electrocatalysts has been steadily decreasing throughout the years.

Fe-N-C are now close to achieve the 2025 DOE target in terms of durability and activity 3 times in terms of j0.9V in A/g -1 Pt and mA/cm 2 geo, see Table 2), the main objective being now to even further decrease the Pt content by implementing advanced Pt-based nanostructures in PEMFCs; (ii) hence, for both Fe-N-C and Pt-based electrocatalysts, the transition from liquid electrolyte to PEMFCs is not trivial. Notably, advanced-Pt nanostructures designed around porosity, structural defects or preferential crystallite

orientation have yet to be successfully implemented in PEMFCs. The final application should be kept in mind when designing the electrocatalyst, and the iron sites should be excluded from domains where the ionomer/gases cannot access (e.g. micropores, etc.).

The functionalization of the carbon support (or, in the Fe-N-C case, the iron-free moieties) is also pivotal, as it controls the ionomer dispersion over the catalyst layer. Drastically increasing the density of iron sites would be a direction to even further increase the Fe-N-Cs competitiveness, but, as evidenced by Wu's group, obtaining an active and stable Fe-N-C electrocatalyst is extremely challenging. Furthermore, (i) the Fe-N-Cs perform remarkably well in AEMFCs, implying that, owing that the membrane and anode limitations of those systems are addressed, they would be of extreme interest; (ii) M-N-C as supports for Pt-based electrocatalysts have recently encountered interest, [START_REF] Chong | Ultralow-Loading Platinum-Cobalt Fuel Cell Catalysts Derived from Imidazolate Frameworks[END_REF] in the light of a synergy between the Co@Pt core@shell nanoparticles and a Co-N-C electrocatalyst, the former reducing any H2O2 generated on the latter, thus increasing the selectivity, activity and durability while decreasing the Pt loading.

Degradation mechanism of Pt-based and Fe-N-C electrocatalysts

As a result of the promises hold by their activity, the durability of Pt-based and Fe-N-C electrocatalysts should be carefully addressed. For both families, at the micrometer scale, the thinning of cathode catalyst layer (the decomposition of carbon support and ionomer) is often observed during ageing, which results in cracking or detachment of the catalytic layer. 196,238 In consequence, the overall activity suffers a dramatic decrease, especially in the high current region as it is there controlled by the mass transport properties. The catalyst layer corrosion is strongly accelerated by start-up/shut-down events, during which the local heterogeneity on the feed gas concentration also leads to extreme degradation. [239][240][241][242] The changes observed at the micrometer scale are direct consequences of phenomena occurring at the atomic/nanometric level, which will be discussed in detail in this section. An overview of the mechanisms as a function of the potential at which the catalytic layer is exposed is presented in Figure 6. For Pt-based electrocatalysts, the corrosion of the carbon support occurs in a wide potential range, forming CO/CO2 or carbon surface oxygen groups. 243 It preferentially occurs on amorphous carbon or carbon-edge defects, thus highlighting the importance of carbon support nature. [244][245] The corrosion rate is significantly enhanced in the presence of Pt nanoparticules. 246 During accelerated stress tests (ASTs) at high potentials (i.e. 1.0 -1.5 V vs. RHE, simulating start-up/shutdown events on real PEMFCs operation), the collapse of the carbon structure is observed, impeding mass-transport of reactant gas and water removal from catalyst layer. [247][248][249] Furthermore, the carbon partial oxidation (i.e. generation of surface oxygen groups) also improves the surface hydrophilicity, thus leading to water management issue. [249][250] Besides the carbon support itself degradation, the support corrosion usually induces the loss of electrochemical surface area (ECSA) owing to the detachment of Pt-based nanoparticles and subsequent agglomeration on the support 247, 251 while the crystallite migration and Pt nanoparticle coalescence at high potential was proposed to be dependent on the Pt nanoparticles-support interactions and, especially, to the surface oxygen group coverage. 252 The dissolution rate of platinum is strongly dependent on the initial particle size, with the smaller ones showing faster dissolution rate compared to the larger ones (Gibbs-Thompson equation). 253 The dissolution phenomenon are usually enhanced at higher potentials. [254][255] . When exposed to ASTs, the Pt dissolution can be subdivided into anodic scan and cathodic scan dissolution, with the former being less affected by the upper/lower potential limit (UPL/LPL) and showing slower dissolution rate compared to the cathodic sweep. Topalov et al. 256 used the place exchange mechanism to justify this phenomenon, with Pt exchanging its place with O at > 1.1 V vs. RHE and being subsequently dissolved during the cathodic scan. It is noteworthy that the Pt z+ ions generated from the dissolution of platinum are not only redeposited within catalyst layer, but diffusing into the PEM where Pt z+ can be reduced by H2 crossing-over from anode side, leading to the formation of a Pt-band inside the PEM. 250,[257][258][259] Crystallite growth is also commonly observed during platinum nanoparticles ageing, and is mainly attributed to (a) the coalescence of nanoparticles and (b) the dissolution of small Pt nanoparticles into Pt 2+ , followed by redeposition on the larger size Pt nanoparticles (electrochemical Ostwald ripening). 260 The operating conditions (e.g. the UPL, 261 relative humidity, 262 temperature, 263 etc.) and physicochemical properties of electrocatalyst itself (e.g. the initial particles size and shape, 264 pore size distribution, 264 etc.) play a crucial role for the stability of Pt nanoparticles during ageing.

PtM/C (M = Co, Ni) suffers from additional ageing mechanisms due to the preferential dissolution of the transition metal during operation. 265 Asset et al. 187 emphasized that the lower UPL was crucial to prevent substantial dissolution of Ni, the growth of crystallites size and the loss of structural defects. Sneed et al. 266 evidenced that Pt-rich, small size, PtM nanoparticles are stable while larger ones, with higher content of 3d transition metal, are continuously leaching the 3d-transition metal during the operation, which leads to the formation of hollowed-out shells. This dissolution incidentally leads to the loss of the strain effect induced by the presence of the alloying element, [267][268] although it might under specific ageing conditions, lead to the formation of hollow Ptrich nanoparticles with enhanced activity. 186 Owing to their smaller initial ECSA (low Pt loading, larger nanostructures), the PtM often exhibit enhanced local O2 transport resistance, which is magnified during ASTs, thus dramatically decreasing the performance at high current density. [START_REF] Yarlagadda | Boosting Fuel Cell Performance with Accessible Carbon Mesopores[END_REF]269 The dissolved transition metal M z+ cations are not reduced in PEMFCs operation condition, thus still existing as M z+ ions in the catalytic layer, [270][271] where they can significantly decrease the proton transport property through strong interaction with the sulfonic acid groups in the perfluorosulfonated membrane and ionomer, 272-273 alongside decreasing the oxygen permeability. 248,274 The Dodelet et al. [START_REF] Yang | Activity, Performance, and Durability for the Reduction of Oxygen in PEM Fuel Cells, of Fe/N/C Electrocatalysts Obtained from the Pyrolysis of Metal-Organic-Framework and Iron Porphyrin Precursors[END_REF]275 proposed that the electro-oxidation of the carbon transformed the hydrophobic surface to hydrophilic one, which led to easier micropores water flooding where most of the active FeNx sites were hosted. As such, the micropores water filling impedes the O2 transport to the FeNx active sites, along with leading to their demetallation through Le Chatelier principle, [START_REF] Chenitz | A Specific Demetalation of Fe-N4 Catalytic Sites in the Micropores of NC_Ar+NH3 Is at the Origin of the Initial Activity Loss of the Highly Active Fe/N/C Catalyst Used for the Reduction of Oxygen in PEM Fuel Cells[END_REF] inducing dramatic performance drop.

Synthesizing the Fe-N-C at high pyrolysis temperature usually lead to an higher graphitization degree and lower heteroatoms doping content, which makes the carbonaceous surface more hydrophobic, hence preventing the flooding. with the potential range extending to 1 V vs. RHE at 80 o C. 129 The generated Fe 2+ can readily diffuse into the membrane and subsequently decrease the proton conductivity.

In addition, the presence of Fe 2+ also leads to a Fenton reaction, favoring the formation of high oxidative radical oxygen species (ROS), which will be discussed in the following paragraphs. The demetallation of Fe-based nanoparticles, e.g. low valence Fe metal or iron carbide is also observed at high potential range (i.e. above 1 V vs. RHE) accompanied with the post-formation of high valence iron compounds (e.g. Fe(OH)3). 276 Atomic FeNxCy moiety is believed to be highly stable and its destruction mainly occurs at high potential range (i.e. 1 V -1.5 V vs. RHE), probably arising from the cleavage of Fe-N bond induced by severe carbon corrosion. [START_REF] Chen | Atomically Dispersed Metal Catalysts for Oxygen Reduction[END_REF]131,276 The destruction of the FeNxCy moiety generally results in the formation of Fe-based nano-cluster or particles and their subsequent dissolution. Besides, the surface mobility and possible transformations of FeNxCy edge and in-plane sites at medium potential range (0.6 - Choi et al. 278 observed that, although the FeNxCy moiety was structurally stable at low potential range, its turnover frequency (TOF) was also dependent of partial chemical carbon oxidation induced by the ROS, which modified the electronic structure of the carbon backbone and, thus, the binding strength of oxygen-based species on the Fe atom center. In addition, CO/CO2 gas evolution is observed at low potential range (< 0.9 V vs. RHE) and the degradation extent is independent of operation temperature and potential range, implying that the chemical oxidation of the carbon surface also plays a vital role in the Fe-N cleavage. Owing to the points aforementioned, it appears that increasing the Fe-N-Cs durability goes through (i) the decrease of Febased nanoparticles (to reduce Fe 2+ -induced membrane poisoning and Fenton reaction);

(ii) a highly favored 4e -reactive pathway, to limit the H2O2 generation and (iii) an adequate graphitization of the support, to achieve an acceptable trade-off between durability and activity. Interestingly, this last point is not only critical for Fe-N-C electrocatalysts, but also for Pt-based electrocatalysts, as the degradation of the carbon support also leads to the active phase detachment, agglomeration, etc. However, going in the direction of highly graphitized supports usually results in low carbon specific surface, i.e. low dispersion of the Pt-based nanoparticles and decreased ECSA. Nevertheless, to the advantage of the Fe-N-C electrocatalysts, the carbon-related corrosion ought to be weaker on the latter, owing that a 4e -pathway is favored and Fe-based nanoparticles are removed, since the FeNx moieties do not catalyze the COR, by opposition to Ptbased nanoparticles.

Conclusion and perspectives

Over the past years, considerable progresses have been achieved in developing efficient Pt-based and Fe-N-C electrocatalysts for PEMFCs applications. Here, we compare those two families of electrocatalysts on the basis of (i) their structural prop- ing the challenges related to the implementation of the more complex Pt-based nanostructures in PEMFCs, the Fe-N-Cs hold great promises. However, further improvements in the intrinsic activity should imply new design approaches, as achieving both activity and durability objectives would be challenging by simply moving on the volcano plot branches (see Figure 4). Such new approaches are numerous, ranging from the design of bi-metallic active sites to break the scaling relationships, to functionalizing the carbon backbone with heteroatoms that would not decrease its stability (e.g.

fluorine 247 ) while adequately modifying the iron electronic structure.

Several other points of interest arise from this review, that apply for both Pt-based electrocatalysts and Fe-N-Cs: (i) electrocatalysts should always be designed with the catalytic layer in mind, as numerous promising electrocatalysts cannot be yet implemented in PEMFCs owing to poor ionomer distribution around/within the catalyst, unproper morphology for solid electrolyte applications, etc; (ii) especially for Fe-N-Cs, the use of rigorous, and standardized, characterization protocols for the activity and durability is essential, as a proper comparison of those materials stability in their own family, and with Pt-based electrocatalysts, is challenging.

21 Figure 1 .

 211 Figure 1. (A) The generalized reaction pathway toward ORR; the red lines correspond to steps involving a proton and an electron (i.e. either proton coupled electron transfer or sequential electron transfer/proton transfer); (B) the limiting potential plot for various metal catalysts as a function of OH* Gibbs free energy (GOH), adapted from Ref. 21; (C) the limiting potential plot for various M-N-Cs (M = Fe, Co, Mn, Cu, Ni) catalysts as a function of GOH, adapted

3 .Figure 2 .

 32 Figure 2. The nature of active sites (A) of Pt-based electrocatalysts (using a Pt octahedron single crystal (111) particles (left) and hollow PtM bimetallic particle (right) as example); and (B) of the various FeNx-containing, N-containing and metallic nanoparticles discussed in the literatures on Fe-N-C electrocatalysts.

  Pt-based electrocatalysts, the Fe-N-C electrocatalysts derived from separated iron, carbon/nitrogen precursors present a wide diversity of active sites. However, where the Pt-based electrocatalysts active sites diversity arises from the distance, and coordination, with other platinum atoms, the Fe-N-Cs diversity arises from the presence or absence of Fe, along with the coordination with nitrogen. Specifically, as shown in Figure 2B, three type of active sites are observed: (1) atomically dispersed iron moiety (FeNxCy), (2) N-doped carbon moiety (C-N), (3) iron-based nanoparticles.

Figure 3 . 4 .

 34 Figure 3. The ORR reaction pathway on the distinct active sites in the Fe-N-Cs electrocatalysts.

Figure 4 .

 4 Figure 4. Schematic illustration of activity regulation by the modification of the electronic structure of the active site on Fe-N-C electrocatalysts (A), and Pt-based electrocatalysts (B)

Figure 5 .

 5 Figure 5. Schematic illustration of cathode catalyst layer and triple-phase boundary engineering on Pt-based (A) and Fe-N-C (B) electrocatalysts.

Figure 6 .

 6 Figure 6. The degradation mechanism of Pt-based (above arrow) and Fe-N-Cs (below arrow) electrocatalysts as a function of potential (given vs. RHE).

  durability of Fe-N-Cs, especially in PEMFC condition, is far below the target of DOE in 2025 6 and results of numerous mechanisms. Similarly to Pt, the degradation mechanisms are (i) potential dependent and (ii) chemistry dependent (e.g. Fe-based nanoparticle, FeNxCy moiety). They include, as illustrated in Figure 6, (i) micropore flooding, (ii) demetallation, including the dissolution of Fe-based nanoparticles and disintegration of the FeNx moiety, (iii) carbon corrosion, including electrochemical oxidation and chemical oxidation. The first mechanism would lead to the "deactivation" of the active sites induced by its inaccessibility to the reactants. Such deactivation might be reversible, through water management or catalyst regeneration, while the latter two result in the "loss" of performance due to destruction of active site (i.e. irreversible oxidation of carbon surface and disintegration of FeNx sites).

  erties and ORR mechanism; (ii) their intrinsic ORR activity and (iii) performances in PEMFCs and durability, to assess the potential of Fe-N-Cs are replacement for Pt-based electrocatalysts in PEMFCs applications. Both families of electrocatalysts beneficiate from a wide variety of actives sites and of means to control the latter' activities. Indeed, Pt-based electrocatalysts activity can be finely tuned by the addition of structural defects, alloying elements and/or morphological changes to expose crystallites with preferential orientation, whereas the Fe-N-Cs active site nature, density, localization and carbon backbone defectivity play a crucial role in the electrocatalyst activity and selectivity. For both families of electrocatalysts, the design approaches are currently governed by the Sabatier principle, i.e. the reaction is occurring on a given active site (Ptatom vs. FeNx) that should bound the oxygen intermediate neither too strongly nor too weakly. Hence, they are limited by the scaling relationships between the different oxygen intermediates, which ultimately limits their improvement. Going 'beyond' the top of the volcano plot discussed in Section 4 would require a true paradigm shift in terms of electrocatalyst design, by using, for example, co-catalysis (i.e. an ORR mechanism where the different reaction steps are undergone onto different active sites of the electrocatalyst, e.g. evidenced bi-metallic center for M-N-Cs). Freeing the electrocatalyst design from the scaling relationships is especially crucial for Fe-N-C electrocatalysts, as the means of tuning their activity (i.e. by adding a controlled density of structural defects to modify the d-band of iron) impact their stability. In the current design landscape, however, the Fe-N-Cs more and more appear as a credible alternative to Ptbased electrocatalysts, as their performances are now reaching the DOE 2025 target. This point comes with several additional positive features for Fe-N-C electrocatalysts:(i) although still behind for PEMFCs applications, Fe-N-Cs are more interesting than Pt-based electrocatalysts as AEMFCs cathodes; (ii) the Fe-N-Cs, especially in the current economical, and geopolitical context, beneficiate of being only composed of earthabundant elements, thus being free of the strain that currently exists on the platinum resources; (iii) durability wise, owing that the design of Fe-N-Cs is greatly improved (limitation of the density of actives sites in micropores, removal of the exposed Febased nanoparticles, enhanced selectivity for the 4e -pathway, proper graphitization degree), the Fe-N-Cs should exhibit excellent stability in operating ORR conditions, as they should not suffer from Pt-induced carbon corrosion. In conclusion, and consider-

Table 1

 1 The MEA-based PEMFC performances of 'state-of-the-art' Fe-N-Cs

	Entry a	L b mg/cm 2	P c kPaabs	T o C	FR d (H2/O2) sccm	RH e %	j0.9V f mA/cm 2 O2	Pmax g mW/cm 2 O2	FR d sccm (H2/Air)	j0.8V f mA/cm 2 Air	Pmax g mW/cm 2 Air	Ref. Year
	DOE	4.0	150	80	-	100	44	1000	-	150	500	6
	S-doped Fe-N-C	4.0	150 (250)	80	200/200	100	-	940* (1030*)	-	-	-	206 2015
	Metal Organic Frame-
	work (MOF)-derived	3.5-4.0	150	80	-	100	-	603	-	-	-	207 2015
	Fe-N-C
	Polyamide + cyana-mide derived Fe-N-C	4.0	100	80	200/200	100	-	870	200/200	90	390*	208 2017
	Carbon black + Phe-nanthroline + polyani-	4.0	238	80	300/400	100	-	1060*	300/400	120	380*	209 2017
	line derived Fe-N-C
	MOF-derived Fe-N-C	2.8	250	80	300/400	100	35*	1141	-	-	-	210 2017
	Fe-N-C	2.0	450	80	300/400	100		823*	-	-	-	10	2018

  [START_REF] Yang | Activity, Performance, and Durability for the Reduction of Oxygen in PEM Fuel Cells, of Fe/N/C Electrocatalysts Obtained from the Pyrolysis of Metal-Organic-Framework and Iron Porphyrin Precursors[END_REF]

  Similar to the case of Pt-based electrocatalysts, carbon corrosion is also prevalent in the Fe-N-Cs during ageing, but it is not catalyzed by the presence of Fe-based moieties. The dramatic carbon support degradation during start-up/shut-down events also holds true, leading to similar consequences (i.e. loss of active sites, mass-transport lim-131 proposed the potential range for CO2 and CO formation to be above 0.9 V vs. RHE at 50 o C. The COR rate strongly increases at higher temperature/potential.277 On the other hand, the performance losses during age-

	measurement. 129
	1 V vs. RHE) during
	ageing at 80 o C have also been recently reported, resulting in the formation of 2D Fe
	cluster. 160
	ing at low potential range (i.e. 0.6 V -1 V vs. RHE) are usually governed by chemical
	oxidation of carbon surface, where the ROS formed from H2O2 by the Fenton reaction
	are responsible for the carbon corrosion. Such oxidation phenomena is particularly true
	in PEMFC operation condition (O2 feed, high current density) since the generated
	amount of H2O2 is far higher in comparison to the one detected in an RDE

itation, water management issues). The carbon corrosion is believed to exert more severe effects on the Fe-N-Cs since the active sites (i.e. FeNxCy) are embedded into carbon matrix and thus are disintegrated with the corrosion of the carbon. The carbon corrosion is also caused by carbon oxidation reaction (COR), which can be classified into two mechanisms, i.e. electrochemical oxidation (carbon electrooxidation to CO and CO2) and chemical oxidation.

Choi et al.

Acknowledgments C. P.-H. thanks the TRAINER project (Catalysts for Transition to Renewable Energy Future) of the "Make our Planet Great Again" program (Ref. ANR-17-MPGA-0017) for support and S. P. thanks the Institut Carnot MICA (COM-Gra 2020) for support. X. Z. finally would like to thank the China Scholarship Council (CSC) for the PhD grant during his stay at ICPEES.

Author Information

Corresponding Authors *Email: t.asset@unistra.fr; *Email: sergey.pronkin@unistra.fr ; *Email: cuong.pham-huu@unistra.fr