Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells

Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells

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Perspective

Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells Tristan Asset1 and Plamen Atanassov1,*

Fuel cell technology is on its verge of deployment as one of the solutions for decarbonization of transportation. It currently uses platinum-based catalysts, being the largest materials cost factor, subject to market volatility, limited availability, and unfavorable geopolitical source location. Hence, Earth-abundant elements-based materials, and among those, platinum-free catalysts, could be an ultimate solution. Among several such catalysts, the transition-metal nitrogen-carbon ones have shown adequate activity and promise in durability, the latest being the most vulnerable treat. Recent years have seen initial successes in incorporation of iron-nitrogencarbon catalysts in fuel cells and their evaluation under automotive relevant conditions. The catalysts can be described as N-doped, graphene-like carbonaceous materials, with transition metal atomically dispersed and associated with the pyridinic nitrogen-containing in-plane or edge defects in graphene. Here, we provide a view on these materials’ chemical composition and morphology that provide for the reactivity and stability of transition-metal-containing active sites.

INTRODUCTION One of the main obstacles for global introduction of carbon-neutral hydrogen-based energy technologies is the lack of efficient, inexpensive, and durable cathode catalysts for the oxygen reduction reaction (ORR).1 The most effective catalytic materials are Pd- or Pt-based (for alkaline and proton exchange membrane technologies, respectively).2–6 During the last decade, their specific activity has been tremendously improved through the use of alloying elements (3d transition metals6 or rare-Earth elements7) or modification of the electrocatalyst morphology (preferential orientation8,9 or highly defective surfaces5,10). However, these platinum group metals electrocatalysts still substantially contribute to the cost of most electrochemical energy technologies (fuel cells, electrolyzers, etc.) because of their scarcity and limited global availability. Thus, the search for less expensive materials is crucial and such pursuit has been very active for many years. While the earliest works used molecular catalysts, such as transition metal nitrogen (M-N4 and, more specifically, Fe-N4 macrocyclic complexes11–13), substantial practical progress has been achieved by introducing pyrolytic materials obtained at high temperatures (here referred to as iron-nitrogen-carbon (Fe-N-C)).14–17 Over the last decade, a widespread opinion emerged that such materials, viewed as graphene derivatives with nitrogen and transition metal moieties, including close structural analogs of Fe-N4 centers, which can be claimed as responsible for the ORR activity.

Context & Scale Our current energy production model strongly relies on fossil fuels, thus being flawed at different levels: (1) the fossil fuels’ transformation generates CO2, the main protagonist of global warming, and (2) their reserves are finite. Thus, shifting toward renewable energies has become a necessity. When focusing on electrification of mobility as one of the main de-carbonization strategies, proton exchange membrane fuel cells (PEMFCs) are an option for medium and heavyduty vehicles and a suitable replacement in train and ship engines, thanks to their ability to generate power from H2 and O2. The oxygen reduction reaction is currently catalyzed by Pt-based nanomaterials thanks to their exceptional activity and adequate stability, provided by the intrinsic properties of platinum metal (i.e., ‘‘close-to-optimal’’ oxygen intermediate binding and water release). However, platinum is scarce, unevenly distributed, and expensive. This raises the need to investigate other materials, prone to replace it in the upcoming decades and ensure broader introduction of fuel cell technology. While metalnitrogen-carbon appears as the most advanced of this new

While these pyrolyzed materials exhibit activity and stability higher than intact Fe-N4 complexes, the nature of the active sites is still a field of heated debate along with

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the morphological and chemical parameters controlling their activity and stability, e.g., (1) the exact role played by the metal-free moieties (N-pyridinic, N-pyrrolic, etc.) in the ORR and their interest as active sites in Fe-N-C electrocatalysts18, (2) the importance of carbon basal plane graphitization19 (i.e., by impacting the p-electron delocalization and nanostructure stability, etc.), and (3) the nature of the different aging processes for the Fe-N-C electrocatalysts.20 This Perspective aims to discuss these different aspects (e.g., active sites, morphology) and to provide a set of realistic, non-formal, structure-based descriptors for activity and durability to help design the next generation of Fe-N-C electrocatalysts for PEMFCs. The Plurality of Active Sites for Fe-N-C Electrocatalysts To establish descriptors, one must first understand the plurality of active sites observed for the M-N-C electrocatalysts. In opposition to materials synthesized from macromolecular complexes (e.g., porphyrin, phthalocyanine) that only present Fe-N4 active sites, such carbonaceous materials exhibit various moieties that contribute to the ORR, which can be divided in two categories, i.e., (1) the Fe-containing moieties and (2) the Fe-free moieties. The Fe-containing moieties exist under various forms, observed or hypothesized, including (1) Fe-N421,22 and Fe-N2+223–25 (bridging between two carbon crystallites), (2) N-Fe-N2+223,24 (bridging between two carbon crystallites with N-graphitic in the plane underneath the Fe atom), and (3) Fe-N4+1 (Fe-N4 with an in-plane N-graphitic underneath the Fe atom). Figure 1 provides a detailed representation of the listed moieties. Depending on its position in the carbon crystallite, iron coordination with nitrogen also changes, ranging from Fe-N2 (on the crystallite edges) to out-of-the-plane Fe-N326,27 and Fe-NxC4 x.27 Additionally, the formation of Fe-containing nanoparticles (NPs, e.g., magnetic a- and g-Fe and Fe-carbides and nitrides [Figure 1] encapsulated in the carbon matrix to maintain stability during operation) is often observed, especially for electrocatalysts exhibiting a high metal loading.28,29

platinum group metal-free (PGMfree) electrocatalysts generation, they exhibit lower activity than the Pt-based materials and substantially suffer in durability. Closing this activity gap and providing avenues to reach a sensible stability is a critical challenge for PEMFCs improvement and commercialization.

These species can be identified by several methods, namely (1) X-ray photoelectron spectroscopy (XPS), using the nitrogen core-level shifts (i.e., the change in binding energies due to the modification of an element chemical environment, e.g., from Fe-N4 to Fe-Nx-C4 x or Fe-N3),26,27 see Figure 1A; (2) X-ray adsorption spectroscopy, and, more specifically, extended X-ray adsorption fine structure (EXAFS) at the Fe K edge, which describes the coordination number and Fe distance with its closer neighbors,14,28 see Figure 1D; (3) Mossbauer spectroscopy, which allows the assessment of the Fe-containing moieties (i.e., FeN4, FeN2+2, N-FeN2+2, or FeN4+1 that corresponds to the D1, D2, and D3 doublets observed by Mossbauer spectroscopy; see Figure 1E and Kramm et al.,23,24 and Zitolo et al.28) and Fe-containing nanoparticles (that correspond to the singlet or sextets evidenced in Figure 1E); and (4) aberration corrected scanning transmission electron microscopy (AC-STEM) and energy electron loss spectroscopy (EELS), see Figures 1B and 1C. Direct imaging of the Fe-N-C sites (i.e., Fe-Nx and metal-free) is a critical challenge, as the methods mentioned above rely on fitting and thus indirect evidence of the active site nature. However, although already achieved for model Fe-doped graphene sheets (see Chung et al.30), the imaging of real, active, Fe-N-C electrocatalysts is yet to be achieved since carbon layer thickness and active site density render the imaging and identification of a specific moiety extremely difficult. The Fe-Nx moieties (e.g., Fe-N4, Fe-N2+2, N-Fe-N2+2) differ by electronic structure, coordination, environment, and therefore activity for the ORR. The electronic orbital filling differs when comparing Fe-N4, Fe-N2+2, and N-Fe-N2+2 (i.e., emptied dz2, filled dz2, or half-filled dz2, respectively24). This affects the O2 binding onto Fe-Nx

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& Biomolecular Engineering and National Fuel Cell Research Center, University of California, Irvine, Irvine, CA 92697, USA *Correspondence: [email protected] https://doi.org/10.1016/j.joule.2019.12.002

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Figure 1. The Plurality of Active Sites Observed in Fe-N-C Electrocatalysts and the Analysis Methods Used to Identify Them (A) X-ray photoelectron spectroscopy patterns in the N1s region for a Fe-N-C electrocatalyst. (B) Electron energy loss spectrographs of an Fe-N-C electrocatalyst. (C) Focus on the highlighted section of (B). (D) Fourier transform of extended X-ray adsorption fine structure. (E) 57 Fe Mo¨ ssbauer pattern and their fittings. (F) Side view of the main graphitic plane presented in the figure to evidence the difference between in-plane and edge active sites. Analyses in (A)–(E) were performed on a state-of-the-art electrocatalyst synthesized by the sacrificial support method, using nicarbazin as a precursor.

moieties: emptied and half-filled dz2 orbitals allow the formation of s-bonds with O2 (and subsequent p-bonds, with the filled and half-filled dxz and dyz orbitals). However, a filled dz2 orbital (e.g., Fe-N2+2) cannot bind O2 in a non-bent configuration (i.e., bending modifies the symmetry and therefore the orbitals involved in the s-bonding). This renders Fe-N2+2 inactive for the ORR. Furthermore, the Fe-N2+2 moieties bridge between two crystallites (see Figure 1), i.e., their preferential location is in the micropores (d < 2 nm).23,24 The micropores are (1) impregnated only with difficulty by the ionomer in alkaline and PEMFCs31 and (2) easily flooded during operation.32 Thus, they should present depreciated performance in membrane electrode assemblies (MEAs), despite being the Fe-N2+2 and N-Fe-N2+2 active sites preferential location.24,33 The nature of the N-Fe-N2+2 site, i.e., the D3 doublet, shows the Mo¨ssbauer spectroscopy limitations. Kramm et al.23,24 identified this doublet as N-Fe-N2+2 (with a neighboring protonated nitrogen), i.e., a complex structure found in micropores and connected to a graphitic nitrogen underneath. Such a structure could also correspond to an in-plane FeN4+1 moiety (see Figure 1F), with a protonated nitrogen in the vicinity. Fe-based nanoparticles were sometimes considered

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as an ORR active site (e.g., Tylus et al.,16 Strickland et al.,34 or Wu et al.35). However, it has been recently evidenced that their preferential oxidation during Fe-N-C electrocatalysts aging resulted in no decay in activity in an acid environment, hence suggesting that such nanoparticles are inactive for the ORR at low pHs,36–38 in opposition to alkaline environments.39 The configuration of the carbons surrounding the Fe-N4 (and Co-N4) moieties has been investigated by density functional theory (DFT) and assumed to be Fe-N4C10 (i.e., bound to pyridinic nitrogen, see Figure 1).27,40 However, Fe-N4C12 (i.e., bound to pyrrolic nitrogen, see Figure 1) was observed by EXAFS in a metal organic framework (MOF)-pyrolyzed electrocatalyst28 and has also exhibited good performance in alkaline environments.41 DFT studies suggested that the formation of clusters (i.e., FexNy, e.g., Fe2N5, etc.; see Holby et al.42,43 for more details) is more thermodynamically favorable than the formation of their mono-atomic counterparts. Although hinted by experimental results,44 such nanostructures require better characterization methods (e.g., AC-STEM on thin carbonaceous materials) to see their existence experimentally confirmed. In substance, Fe-N4 and Fe-N4+1 are believed to be the main Fe-containing active sites for the ORR. Their surface abundance can be tuned using pyrolysis (e.g., pyrolysis at T = 950 C for t = 5 to 60 min to increase the surface availability of said moieties, and hence the overall electrocatalytic activity of the pyrolyzed materials23). Interestingly, the Fe-Nx site coordination observed by EXAFS is sometimes x < 4. Although sometimes ascribed to the contribution of small Fe-C NPs to the overall coordination number,14 it also implies that Fe-Nx moieties with x = 2, 3 exist in the Fe-N-C electrocatalysts, as represented in Figure 1. Fe-N2 is an edge moiety, whereas Fe-N3 and Fe-N4 are in-plane moieties. Thus, their reactivity is dependent on the carbon plane orientation versus the mesopores (2 nm < d < 50 nm). A ‘‘plane-exposed’’ carbon basal plane (i.e., a carbonaceous material with its carbon basal plane facing the mesopores) would result in Fe-N3, Fe-N4, and Fe-N4+1 active sites, whereas ‘‘edge exposed’’ would result in Fe-N2 and edge nitrogen moieties as the principal active sites.18,27 Nitrogen is observed in M-N-C electrocatalysts as (1) part of the Fe-Nx moieties or (2) in metal-free moieties. Its presence is assessed by XPS in the N1s region (i.e., 396 to 408 eV, see Figure 1A). Among the different metal-free moieties, N-pyridinic, N-pyrrolic, N-graphitic, N+, and N-protonated are of great interest.18,27,45,46 Whereas FeNx moieties are S or S* sites (i.e., the ORR occurs through a 4e or 2 3 2e O2 / H2O, see Figure 2), the metal-free moieties are S1 (i.e., 2e for O2 / H2O2, e.g., N-pyrrolic, N-graphitic, N+, or N-protonated) or S2 (i.e., 2e for H2O2 / H2O, e.g., N-pyridinic).47–50 The reactivity of these moieties is dependent on the solution pH. For example, the N-pyridinic pKa is 6.5. Hence, a shift from acidic to alkaline media would result in a change from a protonated pyridinic N-H to N , a Lewis base and preferential adsorption site for the O2 species.45 The pH also impacts the overall ORR mechanism on M-N-C electrocatalysts, i.e., the specific adsorption of OHads in a strong alkaline environment (e.g., pH R 14) blocks the Fe-containing moieties and results in an outer-sphere electron transfer mechanism. This facilitates H2O2 formation as a side product in acidic environments (e.g., pH % 0), where an inner-sphere electron transfer mechanism is favored. This increases the importance of the nature of the underlying electrode metal and orientates the ORR toward a 4e process19,51 (i.e., at pH R 14, a non-specificity of the electrode nature is observed). Hence, a pH R 14 environment allows a wide range of electrocatalysts for the ORR, but the reduction is mainly limited to H2O2 (with rare exceptions, e.g., Ru-based electrocatalysts51), whereas an acid environment results in an increased complexity in the electrocatalyst design but also enables a 4e

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Figure 2. Oxygen Reduction Reaction Mechanism on the Different Active Sites (S, S*, S1, and S2) Commonly Observed in Fe-N-C Electrocatalysts (A) O2 in solution or surface diffusion to the active sites. (B) O 2 adsorption on the different active sites. (C) O 2 reduction in H 2 O (on S sites) or H 2 O2 (on S* or S 1 sites) followed by diffusion in solution. (D) Re-adsorption on S* or S 2 sites. (E) H 2 O2 reduction in H2 O, followed by diffusion in solution.

ORR pathway. It is important to note that electrocatalyst selectivity toward H2O2 greatly decreases at pH = 13, as little or no H2O2 production (and an exchanged number of electrons 4) was evidenced for Fe-N-C and metal-free N-C electrocatalysts in 0.1 M KOH.52–56 Thus, Fe-N-C materials are promising for alkaline membrane fuel cells (AMFCs) as well. However, to the authors appreciation, the greatest limitation of AMFCs does not arise from the electrocatalysts reactivity (e.g., Fe-N-C electrocatalysts in an alkaline environment often exhibit similar, or higher, performances than Pt NPs supported on carbon57–60) but from the anion exchange membrane, in opposition to PEMFCs. Further insights into the metal-free moieties ORR mechanisms can be extrapolated from the phenomena observed on metal-free electrocatalysts (i.e., N-doped carboneous

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structures). In these, (1) the carbon next to the N-pyridinic moieties is believed to be the main active site for the ORR in an acidic environment, as it behaves as a Lewis base prone to adsorb the O2 species50; and (2) N-graphitic involvement in the ORR is difficult to assess, i.e., Kabir et al.48 reported that, in both alkaline and acidic environments, an increase in N-graphitic content occurred simultaneously to an ORR activity depreciation and an increased H2O2 yield, whereas Lai et al.47 reported a positive role of N-graphitic in alkaline media. A ‘‘plane-exposed’’ electrocatalyst implies that, among the metal-free moieties, N-graphitic would be the ORR main contributor (but not the sole contributor because of the intrinsic defectivity of the carbon plane, which could result in the presence of in-plane N-pyridinic). An ‘‘edge-exposed’’ electrocatalyst would favor the N-pyridinic, N-pyrrolic, and their N-protonated counterparts. As a result of the inner-sphere electron transfer mechanism and the resulting surface sensitivity, the activity of the metal-free electrocatalysts in an acidic environment is far below the M-N-C activity, hence implying that the Fe-Nx moieties (and therefore, a ‘‘plane-exposed’’ electrocatalyst) should be favored. Additionally, (1) carbon edges are preferentially corroded, hence making ‘‘edge-exposed’’ electrocatalyst and edge moieties more sensitive to corrosion than their ‘‘plane-exposed’’ counterpart; and (2) Fe-N2 (i.e., the edge Fe-Nx) is more favorable to the OH-Fe-N2 formation than its Fe-N4 counterpart. Therefore, favoring plane-exposed electrocatalysts, with Fe-N4 moieties and high stability, is critical. The Fe-N326 reactivity has been little investigated, thus limiting the statements on its eventual importance as an ORR moiety. Identifying those specific moieties can be performed by XPS, EXAFS, and Mo¨ssbauer (i.e., the D1 and D3 doublets) spectroscopy (see Figure 1). Here, the challenges lie in (1) increasing the resolution of the microscopy techniques (e.g., high resolution transmission electron microscopy [TEM], AC-STEM) to unambiguously assess the active site chemistry on state-of-the-art electrocatalysts (see Figures 1B and 1C) and (2) developing tools to identify which moieties between the carbon edges and basal planes are exposed to the mesopores. The Carbon Support Defectivity and p-Electron Delocalization The reactivity of the Fe-N4 moieties also depends on the carbon basal plane electronic properties.16,19,61,62 Indeed, in pyrolyzed materials, Fe-N4 is integrated to the carbon basal plane: its electronic properties impact the Fe(III)/Fe(II) redox transition potential, i.e., the shift from an out-of-plane Fe(II)-N4 to an in-plane HO-Fe(III)N4.16,61 This out-of-plane to in-plane transition is opposite to the observations in non-pyrolyzed Fe macrocycles (i.e., there, in-plane Fe(II)-N4 transforms into out-ofplane HO-Fe(III)-N4) and is believed to be partly responsible for the enhanced reactivity of the pyrolyzed electrocatalysts, and the out-of-plane Fe(II)-N4 is believed to be the main ORR catalytic site.16,61,62 Thus, the Fe(III)/Fe(II) redox transition potential shift toward higher potentials results in the Fe(II)-N4 stabilization and therefore increased performance for the ORR at low overpotentials.16,19,62 The redox transition potential is dependent on the p-electron delocalization in the carbon basal plane, e.g., pyrolyzing a Fe macrocycle at T = 800 C results in a potential shift from ca. 0.15 V versus RHE (in 0.1 M HClO4) to ca. 0.80 V versus RHE, as the Fe(II)-N4 surroundings shift from a p-electron-rich macrocycle to a p-electron-deficient graphitic system.19 For M-N-C electrocatalysts, the p-electron delocalization is impacted by the carbon defectivity. The latter is linked to the XPS C1s full width at half maximum (FWHM), i.e., a broader FWHM indicates a high carbon disruption, e.g., edge sites, holes, heteroatoms (see Figure 3A). Increasing the carbon basal plane defectivity disrupts the p-electron delocalization, hence increasing its electron withdrawing nature. In opposition, a low-defect carbon basal plane exhibits

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Figure 3. Qualitative Representation of the Carbon Basal Plane Defectivity on the O2 Binding Energy (A–D) (A) Highly defective carbon basal plane, (B) low-defectivity carbon basal plane, (C) ordered carbon basal plane, and (D) turnover frequency for the ORR versus the O2 binding energy, with three qualitative examples corresponding to (A)–(C).

a non-disrupted p-electron delocalization and an electron donating nature, hence increasing the electron density in the Fe(II)-N4.19 Ramaswamy et al.19 inferred that a ‘‘high-electron density’’ Fe(II)-N4 would result in a too-strong binding of the oxygen intermediates, induced by an up-shifting of the dz2 metal orbital and a subsequent increase in the molecular hardness (i.e., the gap between dz2 and the molecular oxygen energy level). However, a strong p-electron withdrawing nature would result in Fe(II)-N4 weakly binding the oxygen intermediates. Similar to the Sabatier principle, it is important to optimize the structural defects’ density in the carbon basal plane to achieve an optimal binding energy on the Fe(II)-N4 moieties, schematically represented in Figure 3. The volcano plot presented in Figure 3D is built on a similar logic to the work presented by Zagal and Koper63 for M-N4 chelates, although qualitatively because of the multiplicity of the pyrolyzed M-N-C actives sites (see previous section and Zagal and Koper63). In Figure 3D, a Fe-N4 site in an ordered carbon plane (e.g., Figure 3C) will bind the oxygen too strongly. Increasing the carbon support defectivity induces a decrease of the O2 binding energy, hence bringing the electrocatalyst toward the volcano plot apex (e.g., Figure 3B). A highly defective carbon basal plane will then result in an electrocatalyst that binds O2 too weakly (e.g., Figure 3A). The carbon basal plane defectivity should also be considered in terms of durability: defective carbon (i.e., small crystallites, edges, and heteroatoms) exhibits numerous preferential corrosion sites, reducing its overall durability. The Stability of M-N-C Electrocatalysts Carbonaceous materials stability has been widely investigated in the frame of their utilization as supports for Pt-based ORR electrocatalysts, where the carbon corrosion

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Figure 4. Changes in Chemistry and Morphology during the Electrocatalyst Aging (A–C) (A) Fresh electrocatalyst, (B) aged electrocatalyst, and (C) schematic representation of the changes in reactivity for the FeN 4 moiety as a result of the modification of its surroundings (i.e., increase in the neighboring site defectivity and oxidation state).

(C / COsurf / CO2) is catalyzed by the Pt nanoparticles, hence inhibiting the advantages of a graphitic carbon at E versus RHE < 1.0 V.64 This does not hold true for MN-C electrocatalysts as the carbon corrosion, despite being thermodynamically favorable at E versus RHE > 0.2 V, is not directly catalyzed by the active sites. Thus, M-N-C electrocatalyst durability is enhanced by (1) increasing the carbon graphitization and (2) diminishing the density of exposed carbon edges. Such a result is achieved by increasing the pyrolysis temperature (e.g., T = 1,150 C versus T = 1,050 C under NH3 atmosphere) but can be detrimental to the electrocatalyst initial reactivity.32 Therefore, surrounding the out-of-plane Fe(II)-N4 with a graphitized carbon basal plane would result in a slightly too-strong oxygen binding site (due to the carbon p-electron donating nature) and decreased activity but higher stability. Interestingly, this points toward M-N-C with a low atomic percent in active sites (e.g., Fe % 0.2 atom %), as these would disrupt the carbonaceous structure, thus enhancing the p-electron withdrawing behavior of the carbon along with diminishing its stability. Choi et al.65 evidenced that carbon is partially oxidized (i.e., C / COsurf) during the ORR, thus modifying the p-electron withdrawing properties of the carbon basal plane and weakening the oxygen binding onto the Fe-N4 moiety (i.e., from 0.59 to +0.33 eV, when shifting from a clean Fe-N4 moiety to a Fe-N4 moieties with epoxy oxides, e.g., C–O, on the neighboring atoms, as evidenced in Figure 4B—see ‘‘i’’ and ‘‘ii’’). Hence, one should consider synthesizing a catalyst with a slightly too-strong binding strength to achieve an ideal electrocatalyst, as the binding strength will decrease after the COsurf formation (as illustrated in the volcano plot shown in Figure 4C). Several reports indicate that the Fe-Nx moieties and metal-free moieties stability is directly linked to the stability of the carbon, i.e., there is no preferential degradation

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of the moieties.37,66 However, the Fe nanoparticles (e.g., a- and g-Fe) dissolve (as Fe2+) during operation (i.e., at E versus RHE < 0.70 V versus RHE), leading to membrane poisoning and performance degradation.36–38 Additionally, H2O production during operation results in the filling of micropores with water, hence rendering inactive the active sites inside the micropores.32,67 The exact mechanism of this deactivation (i.e., fast transport of water in the micropores or flooding, is still widely discussed in the literature32,67–69). Finally, the formation of Fe nanoclusters has been recently observed (see Chen et al.70), indicating that, during aging, iron also migrates on (or in between) the graphene planes and re-inserts itself in a nitrogenrich environment. This ultimately results in the formation of Fe clusters in nitrogenrich regions. Unpublished works indicate that they can ultimately evolve into Fe nanoparticles that are prone to dissolution. Thus, the rate of formation of the Fe nanoclusters, and of their transformation in nanoparticles, appear to be good indicators of an Fe-N-C electrocatalyst stability. The different degradation mechanisms faced by the Fe-N-C electrocatalysts (presented in Figures 4A and 4B) indicate that achieving high stability is through (1) a highly graphitized carbon support, (2) an absence of Fe-based nanoparticles, and (3) a low density of active sites to avoid further disruption of the p-electron delocalization. It is important to note that the understanding of the Fe-N-C aging mechanism mainly arises from aging at E % 1.0 V versus RHE, with the notable exception of Kumar et al.71 Interestingly, they reported demetallation of an M-N-C electrocatalyst synthesized from a metal organic framework during a start-up/shutdown aging (i.e., 1.0 – 1.5 V versus RHE cycling). Thus, investigating how various Fe-N-C electrocatalysts withstand start-up/shutdown procedures is critical. Conclusions In this manuscript, we thoroughly discussed the structural properties of the Fe-N-C electrocatalysts (i.e., their active site nature, the types of defects and surface oxides in carbonaceous matrixes, etc.) and correlated them with the catalytic activity. Fe-N-C electrocatalysts exhibit a wide range of different moieties, e.g., Fe, N-containing moieties (Fe-N4, Fe-N2+2, etc.); metal-free, N-containing moieties (N-pyridinic, N-pyrrolic, etc.); and Fe-containing nanoparticles (metallic iron and iron carbides). Because of the Fe-N-C complex morphology, those moieties are found in different morphology structures, e.g., in the micro-, meso-, and macropores. These chemical moieties are displayed in the carbon basal planes: graphene sheets, sometimes referred to as ‘‘bulk graphene defects,’’ or at the carbon edges. The morphological relationship between the graphene structures with respect to morphology brings natural correlation of edge defects with micropores and in-plane defects with mesopores. This environment and defectivity strongly impacts the moieties and nanoparticles reactivity in the ORR. For example, Fe-N2+2 moieties combine filled-dz2 electron orbitals and, therefore, a different binding preference for O2 than Fe-N4 or N-Fe-N2+2 moieties. In-plane moieties (Fe-N4, Fe-N4+1, etc.) should be considered advantageous over carbon-edge moieties (i.e., N-pyridinic, Fe-N2, Fe-N2+2, etc.). Indeed, the carbon edges are prone to corrosion and are likely found in the micropores, which, in addition to being susceptible to flooding, are difficult for the ionic species and ionomer to access. Increasing the carbon plane defectivity results, through an increased p-electron withdrawing nature, in an enhanced activity of the Fe-N4 moieties. However, this has its limits, as (1) a highly defective carbon plane would result in a Fe-N4 moiety binding the oxygen too weakly and (2) decreased carbon durability is manifested. Additionally, the carbon basal plane partial oxidation during operation further diminishes the oxygen intermediate binding strength. Hence, an

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ideal Fe-N-C electrocatalyst should (1) expose carbon basal planes in its mesopores, thus favoring Fe-N4 and Fe-N4+1 reactivity and (2) exhibit a low density of structural defects. Although such electrocatalysts should bind O2 too strongly ex situ, the inoperando conditions imply modifications of the carbon chemistry (i.e., its partial oxidation) that decrease the O2 binding strength. Fe-based nanoparticles should be absent in the next-generation Fe-N-C, as, in addition to being electrochemically inactive, they are preferentially oxidized, resulting in poisoning of the ionomer. This catalyst integration in MEAs for fuel cell design is strongly dependent on the distribution of the active sites between those morphological classes and hierarchical porosity. Fe-N-C catalyst durability or at least their degradation mechanisms are also strongly influenced by the ‘‘display’’ of the catalytically active moieties as edge defects in easy-to-flood micropores or in-plane defects in mesopores. By considering the environment, the electronic and physico-chemical properties of the catalytically active moieties (active sites), and the morphological properties of the carbonaceous materials structure, we suggest that an ‘‘ideal’’ Fe-N-C electrocatalyst is to (1) achieve a close-to-optimal turn-over frequency for the ORR in centers displayed in accessible pores and (2) to maintain its activity during aging by oxidation resistance and water repulsion. We also strongly suggest the main morphological descriptors (pore size distribution, size of graphitic crystalline domains, and stacking number) can be correlated with the electrocatalyst activity.

ACKNOWLEDGMENTS The authors acknowledge Bob Buckingham (University of California Irvine) for thorough proofreading of the manuscript.

AUTHOR CONTRIBUTIONS T.A. and P.A. contributed equally to the manuscript. Both authors discussed the manuscript, designed the figures, and wrote the manuscript, along with approving its final version.

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