Teixeira Keyla

Kavita Teixeira Santos

Laetitia Kumar

Hongxin Dubau

Sandrine Ge

Carlos Berthon-Fabry

Ana Sant

Fabio Henrique Vasconcellos

Tristan Lima

Plamen Asset

Viktoriia A Atanassov

Saveleva

∥ K Kumar

∥ L Dubau

H Ge

S Berthon-Fabry

C S A Vasconcellos

F H B Lima

T Asset

P Atanassov

V A Saveleva

P Glatzel

X Li

F Jaouen

F Maillard

Spontaneous aerobic ageing of Fe-N-C materials and consequences on oxygen reduction reaction kinetics

Keywords: Oxygen reduction reaction, Fe-N-C electrocatalysts, Carbon corrosion, Fe demetalation, Stability, Mitigation strategy

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Human activities release massive amounts of greenhouse gas in the atmosphere. Protonexchange membrane fuel cell (PEMFC) running on air and renewably-produced hydrogen could efficiently mitigate global warming; however, this technology uses scarce and expensive platinum-group metals (PGMs) to electrocatalyze the cathodic oxygen reduction reaction (ORR).

To replace PGMs at the PEMFC cathode, research focuses on transition metal atoms embedded into nitrogen-doped carbon matrix (Metal-N-C) catalysts, and Fe-N-C has been identified as the most active Metal-N-C catalyst for the ORR [START_REF] Wan | Fe-N-C electrocatalyst with dense active sites and efficient mass transport for high-performance proton exchange membrane fuel cells[END_REF][START_REF] Asset | Iron-nitrogen-carbon catalysts for proton exchange membrane fuel cells[END_REF][START_REF] Asset | Electrocatalysis with single-metal atom sites in doped carbon matrices[END_REF]. Fe-N-C catalysts can be prepared by: i) pyrolysis of Fe macrocycles (phthalocyanines, porphyrins, tetraazaannulene, etc), carbon (C) and nitrogen (N) precursors [START_REF] Gewirth | Nonprecious metal catalysts for oxygen reduction in heterogeneous aqueous systems[END_REF][START_REF] Yang | A universal ligand mediated method for large scale synthesis of transition metal single atom catalysts[END_REF] ii) pyrolysis of Fe-N-C precursors prepared from Fe salts, C and N organic precursors and securing mesoporosity via a hard templating with silica, [START_REF] Pylypenko | Non-platinum oxygen reduction electrocatalysts based on pyrolyzed transition metal macrocycles[END_REF] iii) pyrolysis of Fe-N-C precursors prepared from iron salt and metal-organic frameworks (MOF) (soft templating), [START_REF] Proietti | Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells[END_REF][START_REF] Ma | Cobalt imidazolate framework as precursor for oxygen reduction reaction electrocatalysts[END_REF][START_REF] Armel | Effect of ZIF-8 crystal size on the O 2 electro-reduction performance of pyrolyzed Fe-N-C catalysts[END_REF] iv) pyrolysis of Fe-N-C precursors prepared from Fe salts and polymers [START_REF] Jiang | Recent advance on polyaniline or polypyrrole-derived electrocatalysts for oxygen reduction reaction[END_REF] or v) vapor deposition of Fe onto a preexisting N-C support [START_REF] Jiao | Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N 4 sites[END_REF][START_REF] Mehmood | High loading of single atomic iron sites in Fe-NC oxygen reduction catalysts for proton exchange membrane fuel cells[END_REF][START_REF] Barrio | FeNC oxygen reduction electrocatalyst with high utilisation penta-coordinated sites[END_REF]. Each of these synthesis methods can be fine-tuned to achieve single Fe atoms coordinated to four nitrogen atoms (FeN 4 ), identified as the most active sites toward the ORR. A rapid drop in ORR performance is frequently reported within the first hours of operation in thin-film rotating disk electrode (RDE, liquid electrolyte) or in PEMFC systems (solid electrolyte) [START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF][START_REF] Osmieri | Durability evaluation of a Fe-N-C catalyst in polymer electrolyte fuel cell environment via accelerated stress tests[END_REF], followed by a slower but continuous performance decay [START_REF] Chenitz | A specific demetalation of Fe-N4 catalytic sites in the micropores of NC_Ar + NH 3 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][START_REF] Yin | Kinetic models for the degradation mechanisms of PGM-free ORR catalysts[END_REF]. Hence, the identification of degradation mechanisms of Metal-N-C catalysts and the development of mitigation strategies has recently become the focus of several research groups worldwide [START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF][START_REF] Choi | Stability of Fe-N-C catalysts in acidic medium studied by operando spectroscopy[END_REF][START_REF] Choi | Minimizing operando demetallation of Fe-N-C electrocatalysts in acidic medium[END_REF][START_REF] Choi | The Achilles' heel of iron-based catalysts during oxygen reduction in an acidic medium[END_REF][START_REF] Kumar | Physical and chemical considerations for improving catalytic activity and stability of non-precious-metal oxygen reduction reaction catalysts[END_REF][START_REF] Kumar | Fe-N-C electrocatalysts' durability: Effects of single atoms' mobility and clustering[END_REF][START_REF] Li | Identification of durable and non-durable FeN x sites in Fe-N-C materials for proton exchange membrane fuel cells[END_REF]. For the most mature Fe-N-C catalysts, it has long been thought that demetallation of FeN x sites was the primary degradation mechanism in acidic media [START_REF] Choi | Stability of Fe-N-C catalysts in acidic medium studied by operando spectroscopy[END_REF][START_REF] Choi | Minimizing operando demetallation of Fe-N-C electrocatalysts in acidic medium[END_REF][START_REF] Kumar | Physical and chemical considerations for improving catalytic activity and stability of non-precious-metal oxygen reduction reaction catalysts[END_REF]. However, experimental and theoretical studies have reported that FeN x sites are relatively stable in the potential range of 0 -1 V vs. the reversible hydrogen electrode (RHE) in O 2 -free acidic electrolyte [START_REF] Choi | Minimizing operando demetallation of Fe-N-C electrocatalysts in acidic medium[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] Santori | Effect of pyrolysis atmosphere and electrolyte pH on the oxygen reduction activity, stability and spectroscopic signature of FeN x moieties in Fe-N-C catalysts[END_REF][START_REF] Holby | Acid stability and demetalation of PGM-free ORR electrocatalyst structures from density functional theory: A model for "single-atom catalyst" dissolution[END_REF]. Recent findings from Kumar et al. in RDE [START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF] and Osmieri et al. in PEMFC device [START_REF] Osmieri | Durability evaluation of a Fe-N-C catalyst in polymer electrolyte fuel cell environment via accelerated stress tests[END_REF] have established that the combination of high electrochemical potential and presence of O 2 is a major stressor of Fe-N-C catalysts. Indeed, free Fe cations (and even FeN x sites) react with hydrogen peroxide (H 2 O 2 ), a well-known ORR intermediate, yielding reactive oxygen species (ROS) such as superoxide (  O 2 -), hydroxyl (  OH)

and hydroperoxyl (  OOH) radicals. These radicals readily corrode the carbon matrix, leading in fine to the destruction of FeN x sites, and their release as free Fe cations with the exhaust water or their reprecipitation into ORR-inactive Fe nano-oxides (FeO x ). Carbon oxidation reaction (COR) also partially accounts for the drop in ORR performance of Fe-N-C catalysts. In acidic electrolyte, bulk COR takes place at electrode potentials higher than 1.0 V vs. RHE and its rate is strongly accelerated by temperature [START_REF] Choi | The Achilles' heel of iron-based catalysts during oxygen reduction in an acidic medium[END_REF][START_REF] Castanheira | Carbon corrosion in proton-exchange membrane fuel cells: From model experiments to real-life operation in membrane electrode assemblies[END_REF][START_REF] Castanheira | Carbon corrosion in proton-exchange membrane fuel cells: Effect of the carbon structure, the degradation protocol, and the gas atmosphere[END_REF]. Surface COR is, by definition, restricted to the formation of oxygen-containing surface groups, without irreversible release of C atoms as carbon monoxide (CO) or carbon dioxide (CO 2 ) gas [START_REF] Choi | The Achilles' heel of iron-based catalysts during oxygen reduction in an acidic medium[END_REF]. Despite the mild surface oxidation, the oxygen-surface groups modify the π-electron delocalization around the FeN x sites, leading to a drop of their turnover frequency (TOF) [START_REF] Asset | Iron-nitrogen-carbon catalysts for proton exchange membrane fuel cells[END_REF][START_REF] Choi | The Achilles' heel of iron-based catalysts during oxygen reduction in an acidic medium[END_REF]. Similarly, the protonation of highly basic N groups (typically, occurring during their immersion in acidic electrolyte or solid proton conductors) alters the electronic structure of FeN x sites and depreciates their TOF. The latter degradation mechanism is more marked for catalysts that are (re)pyrolised under ammonia (NH 3 ) in the last stage of their synthesis [START_REF] Kumar | Fe-N-C electrocatalysts' durability: Effects of single atoms' mobility and clustering[END_REF][START_REF] Herranz | Unveiling N-protonation and anion-binding effects on Fe/N/C-catalysts for O 2 reduction in PEM fuel cells[END_REF]. In parallel to the fast degradation of the ORR performance observed at the beginning of life, Boldrin et al. [START_REF] Boldrin | Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O 2[END_REF] recently reported a drop in ORR activity of Fe-N-C catalysts after one year of atmospheric storage. The authors argued that the adsorption of water or oxygen species onto the carbon surface leads to the formation of oxygen-containing surface groups (such as epoxides, then aldehydes over time), which modify the electronic environment of the FeN x sites in a way similar to surface COR [START_REF] Choi | The Achilles' heel of iron-based catalysts during oxygen reduction in an acidic medium[END_REF]. Partial recovery of the ORR performance loss observed after one-year storage by heat treatment (600°C, argon (Ar), 1h) or by reductive treatment (-0.3 V vs. RHE, 30 s) supported the author's claim.

Similarly to what was shown by Boldrin et al. [START_REF] Boldrin | Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O 2[END_REF], in this study, we report a drop of the ORR activity over time for six Fe-N-C catalysts prepared using different synthetic approaches (hard templating with silica, carbonization of metal organic framework, aerogel chemistry, wet impregnation of high surface area carbon black), and thus featuring distinct textural, structural and chemical properties. However, in contrast to Boldrin et al. [START_REF] Boldrin | Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O 2[END_REF], we show that the loss of ORR activity could not be recovered by electrochemical reductive treatment, but only by the 600° C treatment in Ar, likely due to the longer aging time investigated here, up to 4 years.

Experimental

Electrocatalysts

The six Fe-N-C catalysts investigated in this study were synthesized by different academic laboratories and analyzed in their pristine state and after different ambient storage durations. The ZIF-8-derived Fe-N-C catalysts from CNRS -University of Montpellier were produced either by ramp pyrolysis from room temperature to 1050°C at 5°C min -1 , then 1 h dwell time at 1050°C

under Ar (CNRS-RP) or flash pyrolysis by inserting directly the catalyst precursor in the oven pre-equilibrated at 1050°C, 1 h dwell time at 1050°C under Ar (CNRS-FP). They were formerly labeled as Fe 0.5 RP and Fe 0.5 FP in Ref. [START_REF] Kumar | Physical and chemical considerations for improving catalytic activity and stability of non-precious-metal oxygen reduction reaction catalysts[END_REF]. Detailed physicochemical characterization can be found in Ref. [START_REF] Proietti | Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells[END_REF]. Two other Fe-N-C catalysts, derived from nicarbazin, were prepared by University of California Irvine using hard silica template, and single (UCI-1) or double (UCI-2) pyrolysis steps. The first pyrolysis was performed at T = 975°C (ramp pyrolysis, 31.25 °C min -1 from 525°C to 900°C, 9.4 °C min -1 from 900°C to 975°C, hold for 45 min at 975°C) in N 2 :H 2 gas mix 93:7, and the second pyrolysis at T = 950°C (flash, 30 min) in N 2 :NH 3 gas mix 90:10.

They were previously referred to as Fe-HT1 and Fe-HT2, respectively and have been thoroughly characterized in Ref. [START_REF] Kumar | Fe-N-C electrocatalysts' durability: Effects of single atoms' mobility and clustering[END_REF]. A modified pre-polycondensation method was used to prepare a Fe-N-C aerogel catalyst (ARMINES) at MINES Paristech using a one-pot synthesis protocol first introduced in Ref. [START_REF] Wang | Influence of the synthesis parameters on the proton exchange membrane fuel cells performance of Fe-N-C aerogel catalysts[END_REF]. The Fe, N and C precursors were all introduced into the sol simultaneously. The resulting gels were dried and pyrolised at 800 • C in N 2 for 1 h, followed by a second pyrolysis at 950 •C under flowing N 2 :NH 3 gas mixture 90:10 for 45 min. To remove Fe nanoparticles, acid washing was performed in 0.5 M H 2 SO 4 at 80°C for 7 h between the two pyrolysis steps. Finally, a Fe-N-C material was synthesized by Universidade de São Paulo (IQSC) using the strategy previously introduced by Zhang et al. [START_REF] Yang | A universal ligand mediated method for large scale synthesis of transition metal single atom catalysts[END_REF]. In brief, 1,10phenanthroline (Sigma-Aldrich) was dissolved in ethanol, followed by the addition of Fe(NO 3 ) 3 .9H 2 O (Merck, 99.95%), and the system was maintained under reflux. Black Pearls ® 2000 (Cabot Corp.) carbon powder was added to the as formed tris(1,10phenanthroline)iron(II) complex, and the system was opened for solvent evaporation. The resulting powder was heat treated in a tubular oven under Ar atmosphere, at 1050 °C, for 1 h. All electrocatalysts were stored in closed flasks (Qorpak™ Clear Borosilicate Sample Vials with Caps type) over a period of 47 months in total. For the sake of understanding and mitigating ORR activity losses, a given amount of the ARMINES catalyst was also stored in a desiccator for 24 months and another one in an Ar box for the same amount of time. A flask containing CNRS-FP catalyst was also kept in an Ar box for 24 months. The same ageing was conducted on a Co-N-C material synthesized using the same synthesis protocol than CNRS-RP (Co-CNRS-RP), and first introduced in Ref. [START_REF] Kumar | Physical and chemical considerations for improving catalytic activity and stability of non-precious-metal oxygen reduction reaction catalysts[END_REF] and on a benchmark 20 wt. % Pt/Vulcan XC72 catalyst (TEC10V20E from Tanaka Kikinzoku Kogyo, TKK).

Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (X-EDS)

The Fe-N-C materials were observed by TEM and analyzed via X-EDS in their pristine form and after storage in closed flasks for different amount of time. We used a TEM JEOL 2010 with a resolution of 0.19 nm and operated it at 200 kV. X-EDS analyses were performed in five different Fe-based particle-free regions of the TEM grid. Quantification of the Fe content was performed using the Fe K line and the K-factors provided by the software.

High-angle annular dark-field scanning transmission electron microscopy imaging (HAADF-STEM)

HAADF-STEM images were obtained using a Nion UltraSTEM 200, equipped with C3/C5 aberration corrector. The instrument was operated at 60 kV to mitigate the beam damage. The convergence semi-angle was 33 mrad, while the inner and outer collection semi-angle of HAADF detector were 70 and 210 mrad, respectively.

Scanning transmission electron microscopy (STEM)

The STEM/X-EDS elemental maps were acquired using a JEOL 2100F microscope operated at 200 kV equipped with a retractable large angle Centurio Silicon Drift detector. We used the Fe K, C K and O K lines and the K-factors provided by the JEOL software.

X-ray absorption spectroscopy (XAS) and X-ray emission (XES) spectroscopy

Fe Kβ high energy-resolution fluorescence-detected X-ray absorption near edge structure (HERFD-XANES) and Kβ XES spectra were recorded at beamline ID26 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Si (111) double-crystal monochromator was used for the measurements. The maximum of Fe Kβ fluorescence line (7.058 keV) was selected with an emission spectrometer in Rowland geometry with five Ge (620) analyser crystals (Bragg angle, 79°). The incident beam absolute energy calibration was performed using a reference metallic Fe foil by setting the first inflection point of the Fe K edge at 7112 eV. The incident energy was varied from 7100 to 7200 eV over Fe K edge. HERFD-XANES spectra were recorded in continuous mode during 20 sec with an energy step of 0.1 eV, where at least 15 spectra collected at different spots on the sample were averaged. The XES spectra were collected with a step size of 0.1 eV and an incident energy of 7800 eV. The analysed catalyst powders were sealed in the Kapton tubes (external diameter -3.0 mm, thickness -0.03 mm, GoodFellow), the spectroscopic measurements were performed at room temperature and under air. HERFD-XANES and XES spectra were normalized in area using the ranges 7100 -7200 eV and 7030 -7070 eV, respectively.

X-ray photoelectron spectroscopy (XPS)

The X-ray photoelectron spectra were obtained using a Kratos Axis DLD Ultra XPS, with an Al K α monochromatic source operating at 150 W. The operating pressure was 2 × 10 -9 Torr. The high-resolution spectra in the C1s and N1s region were obtained with a pass energy of 20 eV, 3 min of acquisition time for C1s and 40 min of acquisition time for N1s. The data was acquired on three different regions. CasaXPS was used for the data analysis. A linear background subtraction was used and a 70 % Gaussian/30 % Lorentzian shape was used for the curve fitting.

N1s was fitted with an established set of components based on previous study [START_REF] Artyushkova | Oxygen binding to active sites of Fe-N-C ORR electrocatalysts observed by ambient-pressure XPS[END_REF].

Deconvolution of the C1s narrow scan spectrum into different functional groups considered C-C (sp2 and sp3-hybridized carbon atoms, 284.7 and 287.9 eV, respectively), C-N (carbon atoms near N-containing moieties at ca. 286 eV), and oxygen-containing surface groups (hydroxyl, epoxy, ethers, carbonyl and/or quinone-like, carboxylic acid, lactone, anhydride ranging from ca.

287 to 293 eV).

Electrochemical measurements

Catalyst's suspensions were prepared by mixing of 10 mg of each material, 50 µL of a Nafion solution (5 wt.% in a mixture of lower aliphatic alcohols and water), 854 µL of 2-propanol and 372 µL of ultrapure water (Millipore, 18.2MΩ cm, total organic compounds < 3 ppb). An aliquot was then drop-casted onto a glassy carbon disk (5 mm diameter, Sigradur G from Hochtemperatur-Werkstoffe GmbH) embedded in polychlorotrifluoroethylene cylinder and used as working electrode. The catalyst's loadings were 800 and 300 µg powder cm -2 for electrochemical characterizations and electrochemical quantification of the active sites, respectively. For IQSC, a catalyst's loading of 600 µg powder cm -2 was used for the electrochemical characterizations.

To avoid organic contamination, all glassware used in this study was previously cleaned using Caro's acid (50 % v/v of H 2 SO 4 /H 2 O 2 ) and then boiled in ultrapure water (Millipore, 18.2 MΩ cm, 1-3 ppm total organic compounds) before being used. We used a three-electrode cell composed of: (i) a glassy carbon plate as counter-electrode; (ii) a commercial reversible hydrogen electrode (RHE, Hydroflex, Gasktel GmbH) and (iii) a thin-film rotating disk electrode as working electrode. The electrodes were electrically connected using a bi-potentiostat equipped with a potentiostat with an analog linear scan generator (Autolab PGSTAT302N) and NOVA 2.0 software was used to record the electrochemical measurements. All electrode potentials are referred to the reversible hydrogen electrode (RHE).

Prior to the RDE measurements, the electrolyte was deaerated with Ar (> 99.999 %, Messer), and 50 cyclic voltammograms (CVs) between 0.0 and 1.0 V were performed at 100 mV s -1 in Arpurged 0.1 mol L -1 H 2 SO 4 electrolyte ('break-in cycles') followed by 3 CVs at 10 mV s -1 and 1 CV at 2 mV s -1 ('characterization cycles'). The 2 mV s -1 cycle served to correct the ORR polarization curves for pseudocapacitive currents. The ORR polarization curves were performed between 0.0 and 1.0 V vs. RHE, at 1600 rpm,

T = 25 °C in O 2 -saturated 0.1 M H 2 SO 4 electrolyte.
For the IQSC catalyst, the electrochemical measurements were performed in 0.5 M H 2 SO 4 and v = 50 mV s -1 for the CVs and v = 5 mV s -1 for ORR polarization curves.

The quantification of the active sites was performed using the in situ electrochemical method, firstly reported by Malko et al. [START_REF] Malko | In situ electrochemical quantification of active sites in Fe-N/C non-precious metal catalysts[END_REF] The thin-film RDE was firstly cleaned by performing different CVs in Ar-and O 2 -saturated acetate buffer solution (pH 5.2). The catalyst was then poisoned using a nitrite solution (0.125 M of NaNO 2 ). After one CV between 1.0 and 0.3 V vs.

RHE, nitrite stripping was recorded. The excess coulometric charge measured before and after nitrite stripping was used to calculate the number of catalytic sites located at the surface of a catalyst (site density, SD, sites g -1 powder ) assuming 5 electrons exchanged per NO, and one NO adsorbed per active site:

(Equation 1)

Where Q strip (C g -1 powder ) and n strip refer to the excess coulometric charge and the number of electrons, respectively, N A (mol -1 ) and F (C mol -1 ) represent the Avogadro number and Faraday constant, respectively. The Fe-N-C loading for all in situ nitrite stripping experiments was 0.3 mg cm -2 .

From SD and MA (the mass activity (A g -1 powder ) measured at E = 0.8 V vs. RHE in 0.1 M H 2 SO 4

for CNRS, UCI, ARMINES, or 0.5 M H 2 SO 4 for IQSC), the turnover frequency (TOF, s -1 ) for the ORR could be determined:

(Equation 2)

Results

Physicochemical properties of the fresh catalysts

Insights into the carbon morphology and the average Fe content of the six Fe-N-C catalysts were gained using TEM images and point X-EDS analyses. As most of the catalysts (except the IQSC and the ARMINES catalysts) used in this study have been extensively characterized in our former works [START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF][START_REF] Kumar | Physical and chemical considerations for improving catalytic activity and stability of non-precious-metal oxygen reduction reaction catalysts[END_REF][START_REF] Kumar | Fe-N-C electrocatalysts' durability: Effects of single atoms' mobility and clustering[END_REF], we here briefly summarize the main findings. As shown in Figure 1 (a-b), the CNRS-RP and CNRS-FP catalysts are composed of stacked carbon sheet-like layers.

The Fe content was 0.79 ± 0.34 and 0.99 ± 0.55 at. % for CNRS-RP and CNRS-FP, respectively (Figure 1g) and former extended X-ray absorption fine structure (EXAFS) and 57 Fe Mössbauer spectroscopy characterizations established that CNRS catalysts contain only or mostly FeN x moieties [START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF][START_REF] Kumar | Physical and chemical considerations for improving catalytic activity and stability of non-precious-metal oxygen reduction reaction catalysts[END_REF]. The same holds true for the UCI catalysts, composed of atomically dispersed single Fe atoms embedded into stacked graphitic sheets (Figure 1c and Figure 1d) [START_REF] Kumar | Fe-N-C electrocatalysts' durability: Effects of single atoms' mobility and clustering[END_REF]. X-EDS analyses indicated a lower Fe content for this group of catalysts: 0.08 at. % and 0.14 at. % for the UCI-1 and UCI-2, respectively (Figure 1g). For the IQSC catalyst (Figure 1e), the X-EDS analyses indicated a lower Fe content, 0.09 at. %, and the TEM image shows the absence of any Fe particle or cluster in agreement with the aberration-corrected HAADF-STEM and XAS analyses of Ref. [START_REF] Yang | A universal ligand mediated method for large scale synthesis of transition metal single atom catalysts[END_REF]. Finally, TEM images showed that the aerogel Fe-N-C catalyst synthesized by ARMINES is composed of spherical nanoparticles connected to each other, with inter-particle pore size ranging from few nm to > 100 nm. In this synthesis, inspired from the work described in Ref. [START_REF] Wang | Influence of the synthesis parameters on the proton exchange membrane fuel cells performance of Fe-N-C aerogel catalysts[END_REF], an acid washing step is performed between the two pyrolysis (first one under N 2 and second one under N 2 :NH 3 mix) to remove metallic Fe, Fe oxide and Fe carbide nanoparticles, and this was verified using EXAFS and 57 Fe Mössbauer spectroscopy [START_REF] Wang | Influence of the synthesis parameters on the proton exchange membrane fuel cells performance of Fe-N-C aerogel catalysts[END_REF][START_REF] Ge | Fe-N-Carbon aerogel catalysts with high content of Fe-N 4 active sites for high performance oxygen reduction reaction[END_REF]. Summing up, all synthesized materials thus feature atomic dispersion of Fe atoms but had distinct textural, structural and chemical properties.

Electrochemical properties of the fresh catalysts

The ORR mass activity (MA) of five out of the six Fe-N-C catalysts investigated in this study was assessed in 0.1 M H 2 SO 4 with conventional RDE set-up at LEPMI while that of the IQSC catalyst was determined in 0.5 M H 2 SO 4 at IQSC. As-measured ORR curves and CVs can be found in Figure S1, from which the MA values were derived after corrections for iR (Ohmic) drop, pseudocapacitive current, and O 2 diffusion limitation. Because MA depends on the number of catalytic sites located at the surface of a catalyst (SD) and on the turnover frequency at a given E, the SD values were determined using the in situ nitrite stripping method that was first introduced by Malko et al. [START_REF] Malko | In situ electrochemical quantification of active sites in Fe-N/C non-precious metal catalysts[END_REF]. The TOF was then calculated by combining MA values measured in 0.1 (or 0.5) M H 2 SO 4 and SD values (Equation 2). It is noteworthy that, despite their similar Fe content (Figure 1g), the two CNRS catalysts perform differently towards the ORR, with CNRS-RP being much more active than CNRS-FP (Figure 2a). As revealed by Figure 2b and Figure 2c, better ORR performance of CNRS-RP compared to CNRS-FP results from its 5-fold higher TOF, despite a ca. 2-fold lower SD. For the UCI catalysts group, the double-pyrolysed UCI-2 catalyst was more active than the UCI-1 catalyst, in line with the conclusion of Koslowski et al. that a second pyrolysis under ammonia introduces more N moieties on the carbon surface and reveals FeN x active sites initially hidden in the bulk of the carbon matrix due to its etching (HCN and CH 4 production) [35]. The higher SD and TOF values for the UCI-2 catalyst compared to the UCI-1 catalyst support this conclusion (Figure 2b and

Figure 2c

). A high SD but a low TOF value were measured for the ARMINES catalyst. High SD is in line with its highly porous texture (see BET surface area (S BET ) in Table S1 and Figure S2)

but its lower TOF value compared to those of CNRS, UCI and IQSC catalysts was surprising.

Indeed, the ARMINES catalyst experienced a dual pyrolysis (first pyrolysis at 800 •C in N 2 for 1 h, second pyrolysis at 950 •C under flowing N 2 :NH 3 gas mixture 90:10 for 45 min), which should have led to high TOF value as discussed in the above. The poor degree of graphitization of this catalyst in comparison to others (see La, Lc and interlayer spacing values in Table S1) may account for such low TOF. Indeed, in turbostratic carbons, high electrical conductivity is usually found parallel to the quasi-graphitic layers due to the delocalized π-electrons, whereas low conductivity prevails perpendicular to the quasi-graphitic layers. Last, the IQSC catalyst featured similar TOF as the UCI-1 and CNRS-RP catalysts, however the conventional ion complexation/adsorption/pyrolysis approach used for its synthesis led to a lower SD value.

Spontaneous decrease of the ORR mass activity upon storage in closed flasks in atmospheric conditions

Figure 3 illustrates how the ORR activity changes upon storage under atmospheric conditions in closed flasks over a period of 47 months (note that the flasks were not pumped before storage).

A marked decrease of MA was observed for all catalysts independently from their synthesis protocols. Also obvious from Figure 3a is the fact that the rate at which the MA decreased is catalyst-dependent, and seems related to the initial MA value. Indeed, Figure 3b displays a correlation between the MA remaining after t months (with t = 48 months for IQSC and 47 months for CNRS-RP, CNRS-FP, UCI-1 and UCI-2 catalysts) and the initial MA, except for CNRS-FP. The latter is the only material that was prepared with a flash pyrolysis method, possibly explaining why it is an outlier. (-15 %) catalysts. The trend was opposite on the UCI-1 catalyst (+200 % in SD), indicating that a higher number of Fe atoms is exposed at the surface, most likely as a result of the corrosion of the N-C matrix (which will be discussed later in this manuscript). However, as nitrite stripping not only senses surface Fe-N x sites but also Fe-oxides (Fe-O x ), [START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF] the coordination of the additional Fe atoms (Fe-N x or Fe-O x sites) measured by nitrite stripping for the aged UCI-1 relative to the pristine UCI-1 remains ambiguous. The SD of Fe-N x sites may even have decreased from the pristine UCI-1 to the aged UCI-1, and be overcompensated by a strong increase in surface FeO x sites. Regardless of the synthesis protocol and of the pyrolysis atmosphere, the use of MOF as precursors and the resulting high microporosity of the Fe-N-C seem to limit the drop in the TOF value, as the intrinsic activity of the FeN x moieties is mostly maintained on CNRS-RP (-12 %), whereas the other catalysts exhibit a relative decrease in TOF ranging from -68 % and -71 % (IQSC and UCI-2, respectively) to -90 % (UCI-1). The strongest decrease in TOF observed for UCI-1 further supports that the increase in SD observed during aging of UCI-1 is related to carbon corrosion, forming FeO x atoms/clusters on the surface from initially bulk FeN x sites. As FeO x has very low ORR activity (low TOF) but is sensed by nitrite stripping, this would contribute to further lowering the average TOF measured for the aged UCI-1 material (as the TOF is obtained by dividing the overall mass activity by the overall SD).

Changes in structure and chemistry after 47 months of storage in closed flasks

TEM images and point X-EDS analyses of the fresh and aged catalyst powders confirmed the trends derived from the in situ nitrite stripping method. Indeed, Figure 3 1.05±0.21 10 18 sites g -1, see Table S1). The higher Fe at. % is believed to facilitate the aggregation of single Fe atoms over time, and to account for the most pronounced drop in local Fe content measured by X-EDS for CNRS-RP vs. IQSC. For the UCI-1 catalyst, the Fe content in particle-free zones slightly increased after 47 months (0.08  0.02 at. % and 0.12  0.00 at. % for the pristine and the 47-month aged UCI-1 catalyst), in qualitative agreement with the increased SD. In contrast, no change of the local Fe content in particle-free zones was observed for the UCI-2 electrocatalyst, in line with its slight decrease in SD after 47 months storage (Figure 3c). As the overall Fe content should be identical in a fresh and an aged catalyst for ambient storage (no possible gain or loss in Fe) and as analyses were performed in particle-free areas, slightly increasing Fe content monitored by local X-EDS analyses in particle-free areas confirms that the carbon matrix was slightly corroded i.e. that Fe atoms initially positioned in sublayer(s) of the carbon matrix became exposed at the surface during ageing. Nevertheless, COR was probably restricted to the outermost surface layers as Raman spectra (not shown)

showed no change in the bulk carbon structure (Raman spectroscopy is a bulk technique).

To gain atomic-scale insights, we used Cs-corrected atom-resolved high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The HAADF-STEM images of the pristine, 2-year and 4-year aged UCI-2 catalysts displayed in Figure 4a confirmed atomic dispersion of Fe atoms for the fresh UCI-2 catalyst but also revealed that small Fe aggregates with sizes comprised between 3 and 5 nm are present after 2 years of ageing under atmospheric conditions (Figure 4b). The fraction of aggregated nanoparticles increased for the 4 year-aged catalyst (Figure 4c and Figure 4d). To give a statistical value to our measurements, HAADF-20 STEM images were also taken in other areas of the fresh and aged UCI-2 catalyst: they show the same phenomena, in the same proportions. The HAADF-STEM images therefore indicate that aggregation occurred only to a minor extent and could not account for the 15 % decrease in SD observed for this catalyst over a period of time of 47 months. The changes in the spectra after storage of the CNRS-RP catalyst provide evidence for an increase of the average oxidation state of Fe via increase of the Fe K edge white line intensity (Figure 6a) and slight growth in the average spin state of Fe atoms based on the shift of Kβ mainline towards higher energies (Figure 6b) coupled with increase of Kβ' shoulder intensity (inset of Figure 6b). Changes in Fe electronic structure are typically accompanied by the reorganization of the local coordination sphere around Fe. The latter can be tracked by the analysis of the Fe pre-edge features resulting from 1s  3d transitions (inset of Figure 6a),

where the shrink of the pre-edge intensity observed for the aged CNRS-RP catalyst can be attributed to a decrease in the ratio between 4-fold and 5-/6-fold coordinated sites [START_REF] Westre | A multiplet analysis of Fe K-edge 1s → 3d pre-edge features of iron complexes[END_REF]. This supports that an increased fraction of Fe cations was exposed to the surface or became axially coordinated. A significant part of the signal of the Mössbauer spectrum of the fresh CNRS-RP catalyst is related to the quadrupole doublet D2, assigned to a Fe(II)-N 4 site without any axial ligand [START_REF] Kumar | On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts[END_REF]. Aging in ambient conditions therefore likely modified these sites into L-Fe(III)-N 4 sites, possibly as a result of carbon surface corrosion. The additional axial ligand (L) could be H 2 O (from water vapor in ambient air) or axial ligand produced during carbon corrosion (in particular, nitrogen containing fragments as byproducts of N-C corrosion could axially bind a Fe-N 4 site).

Discussion

Combining the results of chemical, physical and electrochemical techniques, we now seek at highlighting the similarities but also the differences of ageing between the catalysts. We first emphasize that the deactivation phenomenon reported in this study is not specific to a given type of Fe-N-C catalyst but seems to be universal for this class of materials. For the UCI-2 and IQSC materials, a slight decrease in SD and a strong decrease in the TOF were noticed (Figure 3). In contrast, the CNRS-RP catalyst experienced a pronounced decrease in SD but almost no change in TOF. Local X-EDS analyses confirmed these trends, and elemental mappings as well as HERFD-XANES and XES analyses indicated that the oxidation state of Fe atoms increased in the aged CNRS-RP catalyst. The UCI-1 material is an exception, with an apparent increase in SD (likely associated with surface carbon corrosion and the formation of FeO x on the surface from Fe atoms initially present as bulk FeN x sites) and a marked decrease of the TOF.

As no electrochemical potential (no electrolyte) was applied during storage under atmospheric conditions, and as all tested Fe-N-C materials were stored in closed flasks, we hypothesized that oxygen or water molecules contained in the air played a major role on the reported chemical and physical changes, and the associated irreversible loss in ORR performance. Indeed, atmospheric conditions place the Fe-N-C catalysts in a corrosion situation, where oxygen act as an oxidant (towards the carbon matrix and the single Fe atoms) and the small amount of water adsorbed on surface act as electrolyte (although of limited ionic conductivity). To verify our hypothesis, we compare in Figure 7 the X-ray photoelectron spectra measured on the pristine and 24-month aged UCI-2 and CNRS-FP catalysts. To further confirm the key role of oxygen and/or water contained in air, the CNRS-FP was stored for 24 months in an Ar-purged glovebox, while the ARMINES catalyst was stored for 24 months in a desiccator. No loss in ORR activity was observed for the CNRS-FP catalyst (Figure S5), confirming that O 2 is required to promote the deactivation phenomenon reported here, which we named spontaneous aerobic ageing. This experiment also indicated that storage under Ar glovebox is an efficient way to mitigate the adverse effects of this phenomenon. In contrast, we noticed that the loss in ORR mass activity of the ARMINES catalyst was ca 25 % lower when stored in an Ar glovebox in comparison to storage for the same period of time in the desiccator (Figure S5). This result makes sense as desiccator protects samples from moisture not from oxygen. We also stress that the decrease in MA upon storage under atmospheric conditions was irreversible and independent on the metal. Indeed, the ORR activity could not be restored, even partially, using the electrochemical reduction protocol of Boldrin et al. [START_REF] Boldrin | Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O 2[END_REF] for 24-month or 47-month aged Fe-N-C catalysts (aged UCI-1 and aged ARMINES, 30, 60 or 300 s of reductive polarization at -0.3 V vs. RHE in 0.1 M H 2 SO 4 , see Figure S6). In contrast, the 53-month aged UCI-1 material could be fully reactivated using the pyrolysis conditions (600°C, Ar atmosphere, 1h) proposed by Boldrin et al. (Figure S7). The differences between reductive polarization and thermal treatment are not surprising in view of the different energy supplied to the aged material to restructure during these reactivation processes. Let us also point that, even if the initial mass activity could be recovered and even slightly exceeded with the thermal reactivation process, the atomic-scale structure of the active sites is likely to be different in the pristine and thermallyreactivated UCI-1 catalysts. Different SD and TOF values measured on the pristine and thermally-treated UCI-1 catalyst support this hypothesis: the results provided in Table S3 indicate that a small number of active sites with high TOF controls the ORR activity for the pristine UCI-1. In contrast, the ORR activity of the thermally-reactivated UCI-1 catalyst is controlled by the density of (poorly-active) catalytic surface sites. Thermal reactivation of the UCI-1 catalyst, however, is an important result that confirms the outcomes presented in the seminal study by Boldrin et al. [START_REF] Boldrin | Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O 2[END_REF]. Discussing no reactivation of the aged UCI-1 and ARMINES catalysts during reductive polarization, we would like to stress the important difference between the study of Boldrin et al. [START_REF] Boldrin | Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O 2[END_REF] and this study regarding the time scale:

 Either only 24 h cumulative exposure to O 2 (and in fact only 1 h continuous exposure, repeated 24 times) or one year storage under atmospheric conditions in Boldrin et al.

[30],

 The present study: up to 47 months exposure to atmospheric conditions

The complete reactivation of the Imperial College London material [START_REF] Boldrin | Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O 2[END_REF] after one-year ambient air storage therefore suggests that oxygen-containing groups formed during one year of storage under atmospheric conditions were "reversible" i.e. they can be electrochemically reduced or desorbed and the ORR activity could be restored. Conversely, the loss of ORR activity assessed in our study was "irreversible", at least for the UCI-1 and ARMINES catalysts, suggesting that the corrosion of the carbon matrix during four years led to irreversible changes in structure, chemistry and electrocatalytic activity. We can only point at the longer duration of air exposure in the present study, or different susceptibility of Fe-N-C catalysts to surface COR, to explain the difference between the present study and the study of Boldrin et al. [START_REF] Boldrin | Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O 2[END_REF].

Finally, in an effort to generalize our findings to different types of Metal-N-C catalysts but also to benchmark carbon-supported Pt nanoparticles used in the field of PEMFC, the ageing in ambient condition was studied on a Co-N-C material synthesized using the same synthesis protocol as CNRS-RP (Co-CNRS-RP), and first introduced in Ref. [START_REF] Kumar | Physical and chemical considerations for improving catalytic activity and stability of non-precious-metal oxygen reduction reaction catalysts[END_REF], and on a benchmark Pt/Vulcan XC72 catalyst (TEC10V20E from Tanaka Kikinzoku Kogyo, TKK). We found that nearly 60 % of the ORR mass activity measured at E = 0.8V vs. RHE was lost after 58 months of storage under atmospheric conditions for the Co-CNRS-RP catalyst (see Figure S8). In contrast, no loss of ORR activity was observed for the benchmark Pt/Vulcan XC72 catalyst stored for more than 6 years in the same conditions (see Figure S9). However, the nature of the active sites in Pt/C and Fe-N-C catalysts is intrinsically different: Fe-N-C rely on single Fe cations embedded into a N-doped carbon matrix with specific surface area exceeding 500-600 m 2 g -1 hence any change in structure or chemistry of these active sites affects their SD and TOF values.

In contrast, active sites are located onto the surface of Pt nanoparticles (ca. 3 nm in size i.e., containing several thousands of atoms) and Vulcan XC72 is a carbon black produced by partial oxidation of hydrocarbons, with specific surface area ca. 250 m 2 g -1 . Movements of Pt nanoparticles onto the Vulcan XC72 support, such as those caused by surface COR, are thus unlikely to irreversibly damage the active sites of Pt/C nanoparticles, and it was no surprise to note that the ORR specific activity did not change over time.

CONCLUSIONS

We report here that Fe-N-C materials comprising single iron atom sites, the most broadly used non-platinum-group metal catalysts for the oxygen reduction reaction in acidic electrolyte, loose mass activity when stored under atmospheric conditions for long period of time. Six Fe-N-C materials prepared by different methods were studied. Electron microscopy identified spontaneous aggregation of single Fe atoms into Fe or Fe-oxide aggregates for some catalysts.

The magnitude of the ORR activity loss however depends much on the preparation method of the Fe-N-C material. Even more complex, the deconvolution of the ORR mass activity into the site density, measured by in situ nitrite stripping method, and the turnover frequency reveals that some Fe-N-C materials show a strong decrease in turnover frequency but a mild decrease in site density, while other Fe-N-C materials show the opposite behavior. The catalytic de-activation after 4 years could not be restored by reductive electrochemical treatment, but could be restored by a thermal treatment under argon at 600°C. A similar aging phenomenon was also observed on a single-metal-atom Co-N-C catalyst synthesized using the same synthesis protocol as CNRS-RP catalyst, except for the nature of the transition metal. In contrast, no loss of the mass or specific activity towards the oxygen reduction reaction was observed for a benchmark Pt/C catalyst. Oxygen and water proved responsible for the deactivation process, and storage under dry argon helped mitigating the adverse effects of spontaneous aerobic ageing, while packaging under vacuum would very likely yield similar result.

Figure 1 .

 1 Figure 1. a-f) Representative TEM images and g) Fe content measured by local X-EDS analyses

Figure 2 .

 2 Figure 2. Electrochemical characterizations of the six Fe-N-C catalysts used in this study: (a)

Figure 3 .

 3 Figure 3. a) Time-dependency of the ORR mass activity for five Fe-N-C catalysts; b)

  e-n indicate little to no change of the carbon matrix morphology for the catalysts pyrolysed under Ar atmosphere (CNRS-RP, CNRS-FP and IQSC). However, X-EDS analyses performed on five different particle-free zones indicated a decrease of 61 and 33 % in the Fe content after 47 months of storage for CNRS-RP and after 48 months of storage for IQSC electrocatalysts, respectively (Figure3d). Despite the incertitude related to each characterization technique, these values are in excellent agreement with the 78 % and 30 % decrease of the SD value reported for these two catalysts in Figure3c. Discussing the different Fe content losses between CNRS-RP and IQSC catalysts, we underline that the IQSC synthesis method uses a pre-existing conductive carbon powder, while CNRS-RP uses as a carbon source the carbon atoms present in the methylimidazole ligand of the MOF ZIF-8 and also the carbon atoms present in phenanthroline (no existing conductive carbon phase in CNRS-RP synthesis before the pyrolysis step). The two methods result in nearly equivalent BET surface area but higher Fe content and SD value for CNRS-RP in comparison to IQSC catalyst (0.79±0.34 vs. 0.09±0.02 at. % and 4.45±0.45 10 18 vs.

Figure 4 .

 4 Figure 4. HAADF-STEM images of the UCI-2 catalyst a) before or after b) 2 years or c-d) 4

Figure 5 .Figure 6 .

 56 Figure 5. Scanning transmission electron microscopy images and associated X-EDS elemental

Figure 6 .

 6 Figure 6. Kβ HERFD-XANES (a) and XES (b) spectra recorded on a fresh and after storage

Figure 7 .

 7 Figure 7. a) Variation of the content in oxygen, carbon and nitrogen monitored from O1s, C1s

Acknowledgements

Science is also about sharing passion, exchanging ideas and enjoying life: we dedicate this work to the 60 th birthday of our friend and colleague, Prof. Plamen Atanassov. This work was performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials "CEMAM" n• ANR-10-LABX-44-01. The authors thank the European Synchrotron Radiation facility for provision of synchrotron radiation facilities at beamline ID06, and gratefully acknowledge financial support from the French National Research Agency through the ANIMA (grant number ANR-19-CE05-0039) project. K.T.S. acknowledges Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Brazil (grant number 88887.598551/2021-00) and CAPES/COFECUB program (grant numbers: 88887-187755/2018-00 and Ph-C 914/18). C.V. and F.H.B.L. are thankful to the São Paulo State Research Foundation (FAPESP -grant number: 2019/22183-6) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq -grant number: #309465/2019-2). We thank Nicolas Bibent (ICGM) for performing the reactivation pyrolysis. Dr. K. Teixeira Santos and Dr. K. Kumar contributed equally to this work.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at DOI.


Viktoriia Saveleva

Kavita Kumar

Pascal Theis

Nicole Segura Salas

Ulrike Kramm

Frederic Jaouen

Frédéric Maillard

Pieter Glatzel

Fe-based Single Atom Electrocatalyst Structure Probed by X-ray Emission Spectroscopy

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Fe-based Single Atom Electrocatalyst Structure Probed by X-ray Emission Spectroscopy

Viktoriia A. Saveleva 1 , Kavita Kumar 2 , Pascal Theis 3 , Nicole Segura Salas 3 , Ulrike I. Kramm 3 , Frédéric Jaouen [START_REF] Paul | Influence of the metal center in M-N-C catalysts on the CO2 reduction reaction on gas diffusion electrodes[END_REF] , Frédéric Maillard 2 , Pieter Glatzel [START_REF] Asset | Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells[END_REF] Evaluation of the electrocatalyst performance data includes an electrode preparation step. In this work, we compare the structural composition of several Fe-N-C materials prepared via different synthesis routes and used to electrocatalyze the oxygen reduction reaction in proton-exchange membrane fuel cells, before and after catalyst layer preparation using high energy resolution fluorescence detected X-ray absorption (HERFD XANES) and X-ray emission spectroscopies (XES). The measurements were performed at beamline ID26 of the European synchrotron (ESRF).

Acknowledgment

We acknowledge the European Synchrotron Radiation Facility for provision of beamtime at ID26 beamline (DOI: 10.15151/ESRF-ES-799089889). P.T. and U.I.K. acknowledge financial support by the BMBF young researcher group FeNC-StRedO (funding number 03XP0092). F.M., F.J. and K.K. gratefully acknowledge financial support from the French National Research Agency through the ANIMA (grant number ANR-19-CE05-0039) and the DEEP (grant number ANR-21-CE05-0021) projects.

ESRF -The European Synchrotron -71 Avenue des Martyrs, Grenoble, FRANCE -Tel +33 (0)4 76 88 20 00 This aspect should be taken into account when discussing the catalyst structureactivitystability/durability correlation(s) as well as analyzing in situ/operando measurements performed on these materials, where one should consider the electronic state of Fe in the prepared electrode and not in the catalyst powder.

X-ray Absorption & Emission Spectroscopies

Electrode preparation

Fe-N-C-based electrodes

Taken from [1] Catalyst Ink

• Water • i-PrOH • Nafion Kβ 2,5
Kβ"

Valence-to-core x100

  1 ESRF, The European Synchrotron, 71 Avenue des Martyrs, CS40220, 38043 Grenoble Cedex 9, France 2 Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, Grenoble INP, LEPMI, 38000 Grenoble, France 3 TU Darmstadt, Department of Chemistry, Catalysts and Electrocatalysts Group, Technical University of Darmstadt, Otto-Berndt-Str. 3, 64287 Darmstadt, Germany 4 ICGM, Univ. Montpellier, CNRS, ENSCM, 34293 Montpellier, France viktoriia.saveleva@esrf.fr

•

  FeN x sites structure partially evolves from square planar (or square pyramidal) to octahedral by adding -O 2 /-OH bond(s) • This change is universal for Fe-N-C materials, the magnitude of change varies with catalyst initial composition • Nafion presence enhances the formation of oxygenated ligands in the ink.

  Credits photo: S. CANDE / ESRF Beamline ID26, ESRF DCM: 2 x Si 111 Analyzer Crystals: 5 x Ge 620 Analyzed features: Pre-edge intensity and position Fe K edge 1 st inflection point White line intensity Kβ 1,3 -Kβ' intensity and position *NCR = Nafion-to-Catalyst Ratio


Clément Atlan

Corentin Chatelier

Isaac Martens

Maxime Dupraz

Steven Leake
email: leake@esrf.fr

Marie-Ingrid Richard
email: mrichard@esrf.fr

Arnaud Viola

Frédéric Maillard
email: frederic.maillard@grenoble-inp.fr

M.-I Richard

Breathing Pt nanoparticle imaged using in situ BCDI

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The relationship between surface strain and the rate of a (electro-)catalytic reaction was unveiled by Hammer and Nørskov using density functional theory (DFT) calculations [1,2] . They proposed that the rate of the sluggish oxygen reduction reaction (ORR) would be enhanced on Pt-based catalysts binding *OH species ca. 0.10 -0.15 eV more weakly than Pt(111), [1,2] later experimentally verified using a Pt3Ni(111)-skin surface. [3] Nevertheless these predictions did not translate to Pt-based nanocatalysts, in part because these feature present multiple catalytic sites with a range of binding energies. Also, DFT calculations consider catalytic surfaces in vacuum, i.e. in the absence of water molecules and electric field. Hence, an in situ picture of how strain is distributed on Pt-based surfaces is still lacking.

In this contribution, we took benefit of recent advances in Bragg Coherent Diffraction Imaging (BCDI) [4,5] and of the fourth generation Extremely Brilliant Source of the European Synchrotron (ESRF-EBS, Grenoble, France) to map strain over Pt nanoparticles in electrochemical environment. Our results reveal that strain is heterogeneously distributed between highly-and weakly-coordinated surface atoms, and propagates from the surface to the bulk of the Pt nanoparticle as (bi)sulphates anions adsorb on the surface.

Figure 1 :

 1 Figure 1: Representation of the strain distribution observed on a Pt nanoparticle under potential control. 0.05 M H 2 SO 4 .