# Unravelling the Stability Stressors of Atomically Dispersed Fe–N–C Oxygen Reduction Catalysts

Xiaohong Xie, Boyang Li, Pan Xu, Moulay Tahar Sougrati, Ricardo Garcia-Serres, David Cullen, A. Jeremy Kropf, Fan Xia, Miao Song, Sulay Saha, et al.

► To cite this version:

Xiaohong Xie, Boyang Li, Pan Xu, Moulay Tahar Sougrati, Ricardo Garcia-Serres, et al. Unravelling the Stability Stressors of Atomically Dispersed Fe–N–C Oxygen Reduction Catalysts. Journal of the American Chemical Society, 2025, 147 (52), pp.48117-48126. 10.1021/jacs.5c15451. hal-05441772

HAL Id: hal-05441772

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# Unravelling the Stability Stressors of Atomically Dispersed Fe-N-C Oxygen Reduction Catalyst

Xiaohong Xie,§ Boyang Li,§ Pan Xu,§ Moulay Tahar Sougrati, Ricardo Garcia-Serres, David A. Cullen, A. Jeremy Kropf, Fan Xia, Miao Song, Sulay Saha, Yachao Zeng, Mark H. Engelhard, Mark E. Bowden, Hanguang Zhang, Litao Yan, Teresa Lemmon, Xiaohong S. Li, Ulises Martinez, Yingwen Cheng, Gang Wu, Piotr Zelenay, Vijay Ramani, Deborah J. Myers, Frédéric Jaouen,* Lijun Yang,* Guofeng Wang,* and Yuyan Shao*

## ABSTRACT

Enhancing catalytic stability of Fe-N-C catalysts for cathodic oxygen reduction in proton exchange membrane fuel cells necessitates an in-depth understanding of their degradation mechanisms. This study identifies key stressors affecting Fe-N-C catalysts stability, specifically acidic-environment, oxygen ($O_2$), and reactive oxygen species (ROS). Through ex situ/operando experiments, we show that the oxidation of local carbon by acidic-environment + $O_2$ + ROS, along with the demetallation of catalytic $FeN_xC_y$ sites by $O_2$ or $O_2$ + ROS, are the primary factors responsible for the initial fast degradation of Fe-N-C catalysts. The demetallation of $FeN_xC_y$ sites, influenced by $O_2$, in particular by $O_2$ + ROS, leads to the subsequent gradual degradation of Fe-N-C. Notably, $FeN_4C_{12}$ type active sites are more susceptible to demetallation than $FeN_4C_{10}$ type sites in $O_2$ or $O_2$ + ROS. Our findings indicate that, besides constructing more stable $FeN_xC_y$ sites, preventing local carbon oxidation and scavenging of ROS are all critical for maintaining stability of Fe-N-C catalysts.

## 1. INTRODUCTION

Electrocatalysis of the oxygen reduction reaction (ORR) is a cornerstone of proton exchange membrane fuel cells (PEMFCs).¹ The harsh operating environment in PEMFCs demands catalysts with both high activity and stability and, due to these requirements, platinum group metals (PGMs) have been the predominant catalysts of choice.²,³ Nevertheless, the high costs and low reserves of PGMs preclude their large-scale application.⁴ PGM-free Fe-N-C, which are typically synthesized through pyrolysis of Fe-containing polymers⁵,⁶ or metal-organic frameworks (e.g., ZIF-8),⁷⁻⁹ have achieved decent beginning-of-life activity and are among the most promising alternatives to PGM catalysts. However, Fe-N-C suffers significantly from insufficient stability when catalyzing ORR in acidic conditions.¹⁰

The performance of PEMFCs with Fe-N-C cathodes often degrades by ~40-80% within only tens of hours of operation at practical cell voltages exceeding 0.6 V.¹¹,¹² This degradation typically features a rapid initial loss in the first few hours, followed by a slower, more gradual decline over the remaining operational time.¹³⁻¹⁵ The chemical and structural degradation of Fe-N-C is primarily attributed to catalyst carbon corrosion,¹⁶⁻¹⁸ $FeN_xC_y$ active sites demetallation,¹⁹⁻²³ and $FeN_xC_y$ sites deactivation by surface N/O functionalities.²⁴⁻²⁶ Additionally, reactive oxygen species (ROS), such as ´O, ´OH, and ´OOH radicals (the dot denotes an unpaired electron), contribute to the degradation process.²⁶,²⁷ The attack of Fe-N-C by ROS triggers an increased production of hydrogen peroxide ($H_2O_2$) during the ORR by $FeN_xC_y$ active sites,²¹,²⁶ which subsequently increases the ROS levels,²⁸ creating a positive feedback loop that deteriorates the Fe-N-C performance and ultimately leads to partial or complete destruction of $FeN_xC_y$ sites. These degradation processes are believed to occur concurrently or be
interdependent, making it challenging to identify the key factors that dictate Fe-N-C degradation and spanning their entire lifetime.

One recent breakthrough has quantitatively identified Fe dissolution as the primary cause of Fe-N-C activity decay under temperature/gas-controlled gas-diffusion electrode conditions.²⁹ End-of-test analyses of Fe-N-C showed that different types of FeNₓCᵧ active sites suffer demetallation with different rates and the rapid demetallation of the FeN₄C₁₂ sites (denoted as S1) is responsible for the fast degradation of Fe-N-C.²⁰ This finding suggests that the rational catalyst design should focus on increasing the density of more durable FeNₓCᵧ active sites, specifically the FeN₄C₁₀ type (denoted as S2)).³⁰, ³¹ However, controversy arises since a number of studies have shown that Fe-N-C still degrades rapidly even when stable S2 sites constitute relatively a high proportion of >40% (Table S1). Moreover, the mechanism for S1 sites exhibit much easier demetallation than S2 sites also remains unclear. Therefore, enhancing the stability of Fe-N-C catalysts requires more in-depth understanding of the underlying degradation mechanisms. Here, we delineate the degradation of a highly active atomically dispersed Fe-N-C catalyst and its dependence on the key stability stressors associated with the ORR.

## 2. RESULTS AND DISCUSSION

### 2.1. Atomically Dispersed Fe-N-C Catalyst

<figure><img src="image_2.png" /><figcaption>**Figure 1** (A) Bright-field STEM image of Fe-N-C-600 catalyst. (B) Dark-field STEM images and the corresponding energy dispersive X-ray spectroscopy line scanning profiles of Fe-N-C-600 and counterpart Fe-N-C-Δ. (C) HAADF-STEM image and the corresponding EELS spectrum to verify the Fe, N, and C co-existence at the atomic level in Fe-N-C-600. (D) and (E) Fe K-edge XANES (D) and Fourier-transformed EXAFS (E) spectra of Fe-N-C-600, Fe foil, Fe(II)Pc, O₂-Fe(III)Pc, and with the O₂-Fe(III)Pc structure given in the inset of (D).</figcaption></figure>

We first developed a single atomic Fe-N-C catalyst characterized by a high density of FeNₓCᵧ site. The synthesis involved the pyrolysis of a Fe(acac)₃@ZIF-8 (acac: acetylacetonate) precursor²¹ (Figures S1 and S2) under a novel continuous two-step pyrolysis protocol: first in ammonia (NH₃) (no gas flow) at temperatures below 600 °C and then in argon (Ar) (with gas flow) at temperatures above 600 °C. This resultant material is designated as Fe-N-C-600 and 600
is the optimal NH₃-to-Ar switching temperature (see Methods in the Supporting Information for details, Figure S2). The pre-treatment in NH₃ led to not only the formation of a porous hollow structure but also a relative increase in Fe content of Fe-N-C-600 compared to a catalyst (Fe-N-C-Δ) that was synthesized with the same method but with only Ar (Figures 1A,B, S2, S3, and Table S2). Atomic-scale High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization of Fe-N-C-600 reveals a high density of atomically dispersed Fe sites (Figures 1C and S4); X-ray diffraction verifies the absence of metallic/oxide/carbide phase of Fe at macroscopic scale (Figure S1). The electron energy loss point spectrum (EELS) (Figure 1C), obtained from a single Fe atom within Fe-N-C-600 (cycled in green in Figure 1C), shows the co-location of Fe, N, and C, affirming the formation of FeNₓCᵧ coordination (Figure S4).⁵ In contrast, the counterpart Fe-N-C-Δ catalyst retained the dense rhombic dodecahedron structure of ZIF-8 precursor (Figure S3) and with lower density of single Fe atoms (Figure S5). This highlights the significant role of the NH₃ pre-treatment in synthesizing the Fe-N-C-600 catalyst.

More evidence of FeNₓCᵧ and insights into the local coordination environment were obtained from X-ray absorption spectroscopy (XAS).³² As shown in Figure 1D, the X-ray absorption near-edge structure (XANES) spectrum indicates that the adsorption threshold position of Fe-N-C-600 located between Fe phthalocyanine (Fe(II)Pc) and Fe phthalocyanine-4,4',4",4'''-tetrasulfonic acid (compound with an O₂ axial ligand atop the Fe atom) (O₂-Fe(III)Pc, inset of Figure 1D), implying that the valence of Fe in Fe-N-C-600 is situated between +2 and +3. This further suggests that Fe-N-C-600 contains a mixture of Fe(II)NₓCᵧ and Fe(III)NₓCᵧ sites.¹⁴,³³ Comparative analysis of our experimental XANES spectrum with the XANES simulations of Zitolo et al.³⁴ suggests that Fe-N-C-600 comprised mainly the Fe(III)N₄C₁₂ (S1) and Fe(II)N₄C₁₀ (S2) sites (Figure S6). The Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectrum of Fe-N-C-600 further shows solely the Fe-N/Fe-O interactions with a bond length (~1.49 Å) typical of FeNₓCᵧ sites and no evidence of Fe-Fe scattering (Figures 1E and S7). This indicates that Fe is atomically dispersed, which, combined with the XANES signature, confirms that FeN₄(Oₓ)Cᵧ is the dominant Fe species in Fe-N-C-600. The presence of S1 (~56%) and S2 (~30%) sites in the catalyst is further confirmed by ⁵⁷Fe Mössbauer spectroscopic characterization (Figure S8 and Table S3). Notably, the structure and chemistry (in terms of porosity (Figure S9), size, Fe content/dispersion, type/ratio of FeNₓCᵧ sites, etc.) of our Fe-N-C-600 catalyst are similar to that of majority single atomic Fe-N-C catalysts in the literature,⁷, ²⁰, ³⁰, ³⁴⁻³⁸ making it a representative catalyst for studying the degradation of Fe-N-C catalysts with the ORR.

The ORR activity of catalysts was first evaluated using a rotating ring-disk electrode (RRDE) in O₂-saturated 0.5 M H₂SO₄. Fe-N-C-600 exhibits high activity with an onset potential (E_on, E at a current density of –0.1 mA cm⁻²) of about 1.0 V (versus the reversible hydrogen electrode (RHE)) and a half wave-potential (E₁/₂) of approximately 0.87 V (Figure 2A). This ORR activity is among the best reported for PGM-free ORR catalysts in acidic conditions.⁸, ¹⁴, ¹⁵, ³¹ In contrast, Fe-N-C-Δ exhibited a lower performance, with an E_on of about 0.91 V and an E₁/₂ around 0.82 V, significantly trailing behind Fe-N-C-600. The NH₃ treatment at 600 °C was found to maximize the ORR activity, correlating with an increase in atomically dispersed Fe content (Figures S3 and S10). RRDE results also showed that the H₂O₂ yield for Fe-N-C-600 was below 2% at a loading of 600 µg cm⁻², rising only slightly to 3-5% at an ultra-low loading of 37.5 µg cm⁻² (Figure S10). This confirms that Fe-N-C-600 predominantly follows a four-electron (4 e⁻) ORR pathway.³⁹, ⁴⁰
for 2 hours ($C_2$), and acid with $O_2$ and $H_2O_2$ at 80 °C for 2 hours ($C_3$) (see Methods in the Supporting Information for details). These conditions were specifically designed to mimic those of the aforementioned in operando conditions of acidic-environment, $O_2$, and $O_2 + ROS$, respectively. The loss of the ORR activity (kinetic current density at 0.85 V ($\Delta j_k$, 0.85 V)) and turnover frequency (TOF), as well as the change in Fe content ($\Delta m$) of the treated catalysts (i.e., $C_1$, $C_2$, and $C_3$) versus the pristine catalyst, were determined with RDE and inductively coupled plasma optical emission spectroscopy (ICP-OES), respectively. $C_2$ and $C_3$ were further characterized by $^{57}$Fe Mössbauer spectroscopy.

RDE test reveals that the $\Delta j_k$ for $C_1$ is -20% (Figures 3A and S23). This activity decay aligns well with the results from in situ RDE and PEMFC stability tests and is likely attributed to the loss of very limited amount of acid-unstable $FeN_xC_y$ sites (resulting from Fe dissolution in $H_2SO_4$),$^{16,20}$ decrease of TOF of remaining $FeN_xC_y$ sites due to mild (i.e., reversible) oxidation of local carbon of $FeN_xC_y$ sites,$^{26}$ or a combination of both. For $C_2$, $\Delta j_k$ increases to -59%, and for $C_3$, it reaches -90%. These results confirm that the exposure to $O_2$ and in particular to $O_2$ with $ROS$ during the ex situ treatments significantly exacerbate catalyst degradation.

<figure><img src="image_3.png" /><figcaption>**Figure 3** (A-C) relative loss of catalytic activity (i.e., $\Delta j_k$, 0.85 V) (A), Fe (B), and TOF (C) of Fe-N-C-600 after the ex situ treatments. The error bars represent the S.D. for three independent experiments performed under the given conditions. (D) $^{57}$Fe Mössbauer spectra of the pristine, $C_2$, and $C_3$ catalysts (measured with an EMF of 7 T) and absolute intensity of each fitted spectral component. (E) $H_2$/air PEMFC stability of Fe-N-C-600 at 0.7 V. Cathode: 2.5 mg cm$^{-2}$ $Fe-N-C-600$, anode: 0.1 mg cm$^{-2}$ $Pt$, 5.0 cm$^2$ MEA, NR211 membrane, 1.0 bar $H_2$/air, 80 °C, and 100% RH.</figcaption></figure>

ICP-OES analysis reveals that the $\Delta m$ for $C_1$ is only -2.5% (Figure 3B), confirming $FeN_xC_y$ sites in Fe-N-C-600 remain largely stable in Ar-saturated $H_2SO_4$. An analysis of the Fe site density (SD) of catalysts confirms that the Fe SD for $C_1$ is nearly unchanged (Figures S24 and S25), which, in combination with the fact that the TOF of $C_1$ decreased (Figures 3C and S25), indicates that the activity loss of $C_1$ is primarily due to the decrease in catalytic ORR TOF. For $C_2$, the $\Delta m$ increases to -24.2%, and for $C_3$, it further rises to -47.5%. These findings show that $O_2$ exposure contributes to the demetallation of $FeN_xC_y$ sites that are otherwise stable under inert
(i.e., acidic-environment) condition and ROS further demetallizes FeNₓCᵧ sites that are stable under O₂.

To distinguish the effects of carbon oxidation and demetallation on the ORR catalytic activity in acidic-environment, O₂, and O₂ + ROS, we performed in situ activity recovery RDE tests on C₁, C₂, and C₃. This process electrochemically removes the surface O functionalities (Figures S23 and S25),²⁶,⁴⁷ and the recovered electrodes are designated as C₁R, C₂R, and C₃R, respectively. As shown in Figure 3A, the activity (jₖ, 0.85 V) recovery is substantial for C₁R, confirming that the activity loss under acidic-environment is caused mainly by decrease of TOF,²⁶ rather than by the loss of Fe. In contrast, the amount of activity recovery is minimal for C₂R and negligible for C₃R, respectively. This is due to the severe destruction, i.e., demetallation, of the FeNₓCᵧ sites under O₂ or O₂ + ROS, preventing recovery of activity despite some improvements in TOF for the remaining FeNₓCᵧ sites (Figure 3C). Therefore, although carbon oxidation plays a role in Fe-N-C degradation under these conditions, FeNₓCᵧ demetallation emerges as the primary mechanism for Fe-N-C degradation with the existence of O₂ or O₂ + ROS. This is further supported by the well-matched trend observed with Δjₖ and Δm for catalysts C₁-C₃.

Catalysts C₂ and C₃ were further characterized using cryogenic ⁵⁷Fe Mössbauer spectroscopy. Five spectral components were needed to appropriately fit their spectra with an external magnetic field (EMF) of 7 T, including the three spectral components D1, D2, and D3 adopted for the fitting of Fe-N-C-600 and two additional components D4 and D5 (Figures 3D and S8). Components D1 (Fe S1 sites) and D2 (Fe S2 sites) are identical across all catalysts, while D3, assigned to high-spin Fe²⁺ species, has slightly different parameters for C₂ and C₃ compared to fresh Fe-N-C-600 catalyst, and it is therefore labelled as D3' in C₂ and C₃ (Table S3). Components D4 and D5 can be assigned to atomically dispersed low-spin (LS) Fe²⁺ and high-spin (HS) Fe³⁺ species, respectively, based on their isomer shifts and hyperfine field splitting values. They may be assigned to modified FeNₓCᵧ environment relative to D1 and D2, respectively, possibly resulting from formation of functional groups next to FeN₄ sites. Similar D4 and D5 components have also been identified by cryo ⁵⁷Fe Mössbauer spectroscopy under EMF for a single atom Fe-N-C catalyst prepared by Mg-Fe metal-exchange.⁴⁸ A comparison of the absolute signal intensities for the spectral components across all the catalysts identifies that the intensities of S1 (i.e., D1) and S2 (i.e., D2) decrease gradually, with S1 losing more intensity than S2 (see column figure in Figure 3D) (note that the amount of catalyst used for the Mössbauer analyses was consistent across catalysts (within ±4% experimental error)). Notably, the absolute intensities of D4 and D5 did not increase from C₂ to C₃, suggesting that the presence of O₂ + ROS enhanced the destruction of S1 and S2 sites, resulting in Fe dissolution from catalysts. In contrast, exposure to O₂ only led to modification of S1 and S2 without their complete destruction, resulting in the formation of new components D4 and D5. The total signal intensity of all components decreases from Fe-N-C-600 to C₂ and C₃, in line with the reduction in Fe content determined with ICP-OES and nitrite stripping (Table S5).

Upon further inspection of Figure 2D, it is now possible to distinguish the degradation of Fe-N-C and its dependence on the key stability stressors at different stages of decay (Figure S26). As shown in Figures 3E and S27, in the initial rapid activity loss phase (Zone I), a recovery voltage (e.g., 0.05 V versus RHE, 5 minutes) applied to the Fe-N-C-600 cathode led to certain recovery of the activity. Similar recoveries have also been reported in the literature for atomic Fe-N-C catalysts in RDE and PEMFC measurements (Figure S28). This observation shows that Fe-N-C degradation involves a complex interplay between local carbon oxidation induced by the acidic-environment, O₂, and ROS, as well as demetallation driven by O₂ + ROS. We note that the
specific degradation mechanisms for different Fe-N-C catalysts — whether local carbon oxidation or Fe demetallation — depend on the chemistry and structure of the Fe-N-C catalysts. In the subsequent degradation phase (i.e., Zone II), while carbon oxidation continues, albeit at relatively slower rate due to prior oxidation in Zone I, the primary degradation mechanism shifts to the demetallation of the FeNₓCᵧ active sites, driven by O₂ + ROS. Our in situ CO₂ emission tests also excluded irreversible carbon oxidation (i.e., direct oxidation to CO₂) as a significant contributor to Fe-N-C catalyst degradation under given conditions (Figure S29).

## 2.3. DFT Calculations

To simulate local carbon oxidation, we constructed various computational structures with an O or OH attached to a carbon atom adjacent to S1 or S2 site and designated these structures as S1-O, S1-OH, S2-O, and S2-OH (Figures 4A, S30, and S31). We predicted the thermodynamic free-energy evolution through 4e⁻ associative ORR pathways on S1, S1-O, S1-OH, S2, S2-O, and S2-OH using the computational hydrogen electrode method.⁴⁹ The results reveal that all the FeNₓ sites can bind the oxygen intermediates appropriately to complete the ORR (Table S6). Figure 4B and 4C shows that the free-energy change for the elementary steps of the ORR with S1, S1-O, S1-OH, S2, S2-O, and S2-OH are negative (i.e., exergonic) when the electrode potential (U) is lower than the corresponding limiting potential⁵⁰ of 0.71, 0.58, 0.7, 0.63, 0.85, and 0.51 V, respectively. Thus, all the FeN₄ sites are predicted to be thermodynamically capable of electrocatalyzing ORR through 4e⁻ pathway. In addition, the activation energy (Eₐ) required for OOH dissociation (Figure S32), are predicted to be 0.71, 0.72, and 0.59 eV on S1, S1-OH, and S2, respectively. In contrast, S1-O, S2-O, and S2-OH, require higher Eₐ of 0.98, 1.09, and 1.01 eV, respectively, to dissociate OOH. The above results reveal that the presence of the OH group has minimal effect on the ORR activity (i.e., TOF) of S1, but it decreases the activity of S2. Meanwhile, O adsorbate reduces the activity of both S1 and S2.

<figure><img src="image_4.png" /><figcaption>**Figure 4** (A) Atomistic structures of S1 and S2, S1- or S2-O/OH and Oads@S1- or Oads@S2-O/OH (ads: O₂, O, or OH). In the figure, the grey, blue, orange, red, and white represent C, N, Fe, O, and H atoms, respectively. (B) and (C) Predicted free-energy evolution for the ORR via 4e⁻ associative pathway on (B) S1, S1-O, and S1-OH sites under 0.71 V electrode potential (U) and (C) S2, S2-O, and S2-OH sites under 0.63 V electrode potential. The asterisk denotes the adsorption sites. (D) and (E) Predicted ΔG for demetallation of S1 (D) and S2 (E) sites without, with O or OH, and with O₂ or O₂-O/OH adsorbates. Higher positive ΔG value indicates higher resistance against the demetallation.</figcaption></figure>
To better understand the stability of the Fe-N-C catalysts and the influence of the key stability stressors, we then performed the spin-polarized density functional theory (SP-DFT) simulations to assess the tendency for demetallation, focusing on the role of O₂ and ROS. We simulated the Gibbs free energy change (ΔG) for demetallation of S1 and S2 sites under some representative scenarios: (1) S1 and S2 without adsorbates, (2) adsorption of O or OH (O or OH from H₂O₂ and *O or *OH from ORR) on C adjacent to FeN₄ (e.g., S1-O, S1-OH, S2-O, and S2-OH), (3) adsorption of O₂ on Fe (e.g., O₂@S1 and O₂@S2), and (4) coadsorption of O₂-O and O₂-OH on Fe-C (e.g., O₂@S1-O, O₂@S1-OH, O₂@S2-O, and O₂@S2-OH) (Figures 4A and S33). The simulations indicate that S1 and S2, in the absence of adsorbates, require a ΔG larger than 3.0 eV for the demetallation (Figure 4D and 4E), which shows that they are structurally stable under these conditions. This is consistent with our ex situ experiment results under deaerated condition. When an O or OH is added onto the C near FeN₄ site, the ΔG values for demetallation of S1 and S2 decrease but remain above 1.7 eV. For the sites where O₂ is directly adsorbed on Fe, the ΔG for the demetallation process is calculated to be only 0.31 eV for O₂@S1 but nearly 1.25 eV for O₂@S2, implying that, from a thermodynamic perspective,⁵¹ O₂ promote the demetallation of S1 but not S2 site. Moreover, we calculated the ΔG values for the coadsorption of O₂-O/OH on S1 and S2: 0.27, 0.5, 0.78, and 0.93 eV for O₂@S1-O, O₂@S1-OH, O₂@S2-O, and O₂@S2-OH, respectively. This suggests that both S1 and S2 sites are unstable under such a coadsorption condition. We note that although neither S1 nor S2 are stable with O₂ or O₂-O or O₂-OH adsorptions, the tendency for demetallation of S1 is higher than that of S2 under O₂ or O₂-O/O₂-OH (i.e., O₂ + ROS) conditions. The ΔG for O₂@S2-OH further decreases to 0.40 eV when adding another OH onto the C adjacent to S2, indicating that S2 becomes even more unstable with a high concentration of OH (i.e., ROS). Our computational findings align with the ⁵⁷Fe Mössbauer spectral results, which reveal that although S2 is more stable than S1 with key ORR stability stressors, it is not stable enough under high ROS concentration conditions.

### 3. CONCLUSION

In this work, we investigated the complex degradation mechanisms behind single atomic Fe-N-C catalyst. We identified that the oxidation of local carbon, driven by key stability stressors, i.e., the acidic-environment, O₂, and ROS, along with the demetallation of FeNₓCᵧ active sites by O₂ + ROS, are the primary causes of the initial fast degradation of Fe-N-C. Demetallation of the FeNₓCᵧ active sites by O₂ or O₂ + ROS is primarily responsible for the subsequent slow degradation of Fe-N-C. Similar degradation behavior can be observed for the Fe-N-C-Δ and our previously reported Mn-N-C⁵² catalysts (Figure S34). These findings show that the degradation of Fe-N-C for the ORR involves a complex interplay between mainly FeNₓCᵧ local carbon oxidation (Figure S35) and Fe demetallation. This is different from earlier research results that the degradation of Fe-N-C can be primarily ascribed to either carbon oxidation¹⁶⁻¹⁸, ²⁵, ²⁶, ⁵³ or FeNₓCᵧ sites demetallation¹⁹, ²¹⁻²³, ²⁹, ⁵⁴⁻⁵⁶. Therefore, besides constructing more stable FeNₓCᵧ sites (e.g., S2³⁰, ³¹, Pt-FeNₓCᵧ²⁹), regulating FeNₓ surrounding environment to prevent local carbon oxidation, that is, to alleviate or circumvent the decreasing of FeNₓCᵧ sites ORR TOF, is crucial for improving the stability of Fe-N-C catalysts. Moreover, we observed that S1 active sites are more prone to demetallation than S2 sites under O₂ or O₂ + ROS conditions. These findings show the primary reason for why S1 sites typically exhibit the much easier demetallation than S2 sites, which is a more in-depth understanding of degradation mechanisms of the FeNₓCᵧ active sites compared with previous studies.²⁰, ²⁹, ³⁰, ⁵⁵ This suggest that although an increasing in the content of relatively stable S2 sites enhanced the stability of Fe-N-C
catalysts,²⁰, ³⁰, ³¹, ⁵⁷ mitigating S2 demetallation by ROS is also critically important for maintaining stability of Fe-N-C catalysts.

# 4. EXPERIMENTAL SECTION

The experimental details — including catalyst synthesis, ex situ demetallation experiments, catalyst characterizations, Mössbauer analysis, electrochemical tests, and DFT simulations — are described in the Supporting Information.

## ■ ASSOCIATED CONTENT

The Supporting Information is available free of charge at: xxx xxx

General methods; additional XRD, TEM, HAADF-STEM, XANES, FT-EXAFS, XPS, Mössbauer spectroscopy, nitrogen sorption isotherms and pore size distribution, and electrochemical results; additional simulation results (PDF)

## ■ AUTHOR INFORMATION

### Corresponding Authors

**Frédéric Jaouen** – ICGM, University of Montpellier, CNRS, ENSCM, 34293 Montpellier, France; Email: frederic.jaouen@umontpellier.fr

**Lijun Yang** – Ballard Power Systems Inc., Burnaby, BC V5J 5J8, Canada; Email: lijun.yang@ballard.com

**Guofeng Wang** – Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States; Email: guw8@pitt.edu

**Yuyan Shao** – Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States; Email: yuyan.shao@pnnl.gov

### Authors

**Xiaohong Xie** – Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States; Present Address: State Key Laboratory of Advanced Chemical Power Sources, School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China

**Boyang Li** – Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States

**Pan Xu** – Ballard Power Systems Inc., Burnaby, BC V5J 5J8, Canada

**Moulay Tahar Sougrati** – ICGM, University of Montpellier, CNRS, ENSCM, 34293 Montpellier, France

**Ricardo Garcia-Serres** – Université Grenoble Alpes, CNRS, CEA, IRIG, Laboratoire de Chimie et Biologie des Métaux, 17 Rue Des Martyrs, 38000, Grenoble, France

**David A. Cullen** – Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

**A. Jeremy Kropf** – Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States

**Fan Xia** – Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States

**Miao Song** – Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States
**Sulay Saha** – Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States

**Yachao Zeng** – Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States

**Mark H. Engelhard** – Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States

**Mark E. Bowden** – Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States

**Hanguang Zhang** – Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States

**Litao Yan** – Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States

**Teresa Lemmon** – Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States

**Xiaohong S. Li** – Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States

**Ulises Martinez** – Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States

**Yingwen Cheng** – Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States

**Gang Wu** – Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States

**Piotr Zelenay** – Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States

**Vijay Ramani** – Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States

**Deborah J. Myers** – Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States

Complete contact information is available at: xxx xxx

## Author Contributions

§X.X., B.L., and P.X. contributed equally to this work.

## Notes

The authors declare no competing financial interest.

## ■ ACKNOWLEDGEMENTS

The authors acknowledge support from the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office through the Electrocatalysis consortium (ElectroCat) and the DOE program managers, Gregory Kleen, David Peterson, Simon Thompson, Dimitrios Papageorgopoulos. The XPS measurement was conducted at the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility at Pacific Northwest National Laboratory (PNNL) which is sponsored by DOE's Office of Biological and Environmental Research (BER). PNNL is operated by Battelle for the US DOE under Contract DE-AC05-76RLO1830. X-ray spectroscopy analysis were
carried out at MRCAT at the Advanced Photon Source (APS), an Office of Science user facility
operated for the DOE's Office of Science by Argonne National Laboratory (ANL) under
Contract DE-AC02-06CH11357. The operation of MRCAT is supported both by DOE and the
MRCAT member institutions. ANL is operated for the US DOE by UChicago Argonne LLC
under Contract DE-AC02-06CH11357. Electron microscopy research of material was conducted
as a part of a user project at the Center for Nanophase Materials Sciences (CNMS), a DOE's
Office of Science user facility at Oak Ridge National Laboratory. The Spin-polarized DFT
calculations were performed on the computers of the University of Pittsburgh Center for
Research Computing (CRC) as well as the Extreme Science and Engineering Discovery
Environment (XSEDE), which is funded by the National Science Foundation (NSF) grant
number ACI-1053575. Cryogenic Mössbauer characterization received funding from the French
National Research Agency (Labex ARCANE, CBH-EUR-GS, ANR-17-EURE-0003).

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