# Modulating the Fe–N 4 Active Site Content by Nitrogen Source in Fe–N–C Aerogel Catalysts for Proton Exchange Membrane Fuel Cell

Hongxin Ge, Nicolas Bibent, Keyla Teixeira Santos, Kavita Kumar, Julien Jaxel, Moulay-Tahar Sougrati, Andrea Zitolo, Marc Dupont, Frédéric Lecoeur, Michel Mermoux, et al.

► To cite this version:

Hongxin Ge, Nicolas Bibent, Keyla Teixeira Santos, Kavita Kumar, Julien Jaxel, et al.. Modulating the Fe–N 4 Active Site Content by Nitrogen Source in Fe–N–C Aerogel Catalysts for Proton Exchange Membrane Fuel Cell. ACS Catalysis, 2023, 13 (2), pp.1149-1163. 10.1021/acscatal.2c05394. hal-03940872

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Submitted on 16 Jan 2023

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# Modulating the Fe-N<sub>4</sub> active sites content by nitrogen source in Fe-N-C aerogel catalysts for Proton Exchange Membrane Fuel Cell

Hongxin Ge (a), Nicolas Bibent (b), Keyla Teixeira Santos (c), Kavita Kumar (c), Julien Jaxel (a), Moulay-Tahar Sougrati (b), Andrea Zitolo (d), Marc Dupont (b), Frédéric Lecoeur (b), Michel Mermoux (c), Vincent Martin (c), Laetitia Dubau (c), Frédéric Jaouen (b), Frédéric Maillard (c), Sandrine Berthon-Fabry (a*)

(a) MINES ParisTech, PSL University PERSEE - Centre procédés, énergies renouvelables et systèmes énergétiques, CS 10207 rue Claude Daunesse F-06904 Sophia Antipolis Cedex, France.

(b) ICGM, Univ. Montpellier, CNRS, ENSCM, 1919 route de Mende, 34293 Montpellier, France

(c) Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, Grenoble INP, LEPMI, 38000 Grenoble, France

(d) Synchrotron SOLEIL, L'orme des Merisiers, BP 48 Saint Aubin, 91192 Gif-sur-Yvette, France.
*Corresponding author.

E-mail address: [sandrine.berthon-fabry@mines-paristech.fr](mailto:sandrine.berthon-fabry@mines-paristech.fr). Tel: +33 493957547

Fax: +33 493957535

## Abstract

Fe-N-C material is regarded a promising non-precious-metal catalyst for oxygen reduction reaction (ORR) to replace Pt-based catalysts, but its activity and mass transport remain problematic before a large scale application in proton exchange membrane fuel cells (PEMFCs). Our previous research developed an Fe-N-C aerogel catalyst by pyrolyzing resorcinol-melamine-formaldehyde (RMF) aerogel containing iron precursors. The abundance of micro and mesopores in aerogel is known to improve the mass-transport properties of Fe-N-C cathodes in PEMFC, facilitating the diffusion of O₂ to the Fe-N₄ sites. Herein, to further improve the ORR activity while maintaining good mass transport properties, a series of Fe-N-C aerogel catalysts was synthesized by modulating the nitrogen source (melamine) content and the texture in the RMF aerogel precursor. The Fe content in catalysts presents a positive relationship with melamine content in the aerogel, with adequate texture, indicating the important function of nitrogen source in stabilizing Fe atoms during pyrolysis to form Fe-N₄ active sites. ⁵⁷Fe Mössbauer spectroscopy revealed a majority of O-Fe(III)N₄C₁₂ configuration of the active sites, which is consistent with the variation of pyrrolic N content with Fe derived from X-ray photoelectron spectroscopy. As a result, the mass activity of the series of catalysts exhibits a linear relationship with Fe content and reaches to 3.0 A g⁻¹ at 0.8 V vs RHE in 0.05 M H₂SO₄ and rotating disk electrode (RDE) setup. Their performance in PEMFC exhibits same tendency with the RDE setup. In addition, the H₂/air PEMFC polarization curves do not show any diffusion-limited current density effects, even at 0.7 A cm⁻² with a cathode based on a Fe-N-C catalyst prepared with high melamine content. This work reveals the importance of nitrogen source to reach high atomically dispersed Fe content in Fe-N-C catalysts with low yield of Fe nanoparticles and mass transport properties in PEMFC are not affected by low mesopore volume for aerogel-based catalysts.
**Key words:** Non-precious metal catalyst, Fe-N-C catalyst, carbon aerogel, oxygen reduction reaction, PEMFCs

**Graphical abstract**

<figure><img src="image_2.png" /></figure>

# 1 Introduction

Climate change is a global emergency that goes beyond national borders. To substantially reduce global greenhouse gas emissions, renewable energies, such as wind, solar, and tide should play a key role. However, due to the intrinsically intermittent power generation of these renewable energies, it is necessary to convert them with an intermediate carrier that can generate power stably and sustainably. Herein, hydrogen ($H_2$) is a promising candidate as an energy vector, because it can be produced from renewable electricity by water electrolyzers,¹ stored for long periods,² and reconverted on demand to electric power in fuel cells, without any greenhouse gas emission.³

$H_2$/air proton exchange membrane fuel cell (PEMFC) is one of the most promising fuel cell types, especially for transport and portable power applications. However, due to the sluggish oxygen reduction reaction (ORR) on the cathode and the acidic environment of PEMFC, large quantities of platinum (Pt)-based electrocatalysts are necessary to reach the acceptable power performance of PEMFCs, which impedes their broader commercialization and threatens their sustainability.⁴ Therefore, developing non-precious metal catalysts for the ORR in acid media is highly desirable. In the past decades, numerous materials have been widely investigated for ORR both in acid and/or alkaline media, such as metal oxides,⁵⁻⁷ metal chalcogenides,⁸⁻¹⁰ metal-free nanostructured carbon materials,¹¹,¹² and especially transition metal-nitrogen-carbon (Me-N-C) catalysts.¹³⁻¹⁸ Some of recently reported iron (Fe)-N-C catalysts present an initial ORR activity approaching that of platinum-based catalysts in rotating disk electrode (RDE) setup and in PEMFC.¹⁹⁻²³
Me-N-C catalysts are typically synthesized by pyrolyzing precursors of metal, nitrogen, and carbon, and among them, Fe-N-C catalysts were found to be the most active.<sup>14,15,24-26</sup> Atomically dispersed Fe centers coordinated by pyridinic and/or pyrrolic nitrogen atoms (usually, four nitrogen atoms, Fe-N<sub>4</sub> site) are considered as the most active sites in Fe-N-C catalysts, such sites having likely different site-specific activity and stability depending on their detailed configuration, the local carbon matrix structure, and/or the type of pores that connects them to the electrolyte.<sup>24,27-32</sup> Therefore, the mass activity of Fe-N-C catalysts relates to the number of accessible Fe-N<sub>4</sub> sites per mass of catalyst (site density, SD) and the average ORR turnover frequency (TOF) of all accessible sites.<sup>21</sup> The TOF may change depending on the detailed structure of the Fe-N<sub>x</sub>C<sub>y</sub> sites,<sup>18,26,27,30</sup> the delocalization of electrons on the carbon surface, and the presence of highly basic N-groups on the surface.<sup>33-35</sup> Since the pathways to increase the TOF are limited, another way to enhance the mass activity of Fe-N-C catalysts is to increase the SD. This can be achieved either by increasing the absolute amount of atomically dispersed Fe or by increasing the fraction of Fe-N<sub>4</sub> sites that are accessible to O<sub>2</sub>.<sup>21</sup> However, a straightforward increase of Fe content, without precaution or synthesis optimization, usually increases the yield of Fe clusters rather than Fe-N<sub>4</sub> sites. A large specific surface area (S<sub>BET</sub>) of the carbon substrate can help to promote the atomic dispersion of Fe at high Fe contents.<sup>16,18,36</sup> To our knowledge, the most active Fe-N-C catalyst might be considered to be the one reported by Jiao *et al.*,<sup>21</sup> which was synthesized by chemical vapor deposition of Fe on a Zn-N-C substrate containing Zn-N<sub>4</sub> sites, itself derived from a sacrificial Zn-based metal organic framework (MOF). The interconnected alveolar nanostructure of the MOF-derived carbon leads to a large surface area and micropore volume while FeCl<sub>3</sub> vapor leads to a transmetallation event from Zn-N<sub>4</sub> to Fe-N<sub>4</sub> active sites, achieving an unprecedented high SD of 1.0×10<sup>20</sup> sites g<sup>-1</sup> (measured by nitrite stripping assuming 3 electron mechanism). On the other hand, mass transport properties are also important to improve fuel cell performance with Fe-N-C cathodes, because a thicker catalyst layer on the cathode is required with Fe-N-C compared to platinum-based catalysts, to compensate for their lower mass activity.<sup>37</sup> The design of micro-meso or micro-meso-macroporous structures can improve the mass transport properties of Fe-N-C cathodes, facilitating the diffusion of O<sub>2</sub> to the Fe-N<sub>4</sub> sites, the latter being often located in micropores.<sup>38</sup> For example, Atanassov *et al.* have developed a family of ordered micro-mesoporous Fe-N-C catalysts with the silica templating approach, that offers interesting mass-transport properties in H<sub>2</sub>/air PEMFCs.<sup>39-42</sup> However, hydrofluoric acid is required to remove the silica template, which is extremely hazardous and environmentally unfriendly. The commercial Pajarito Powder non-precious-metal catalysts are also prepared in this way.
Carbon aerogels are promising candidates of carbonaceous substrate for large-scale preparation of Fe-N-C catalysts, owing to their high specific surface area, tunable pore size distribution, and high electrical conductivity.<sup>12,43-47</sup> Other types of carbon gels, including xerogels and cryogels, have comparatively low specific surface area and low pore volume due to the collapse of the gel skeleton during gel drying.<sup>48,49</sup> Supercritical drying is a desirable method to remove organic solvents from hydrogels without causing excessive stress on the solid network, preserving the highly porous structure of the obtained aerogel.<sup>46</sup> However, no highly active carbon aerogel-derived Fe-N-C catalysts in acidic media has been reported hitherto due to the lack of Fe-N<sub>4</sub> active sites.<sup>50-54</sup> Therefore, it is still a challenge to develop a practical method to prepare the Fe-N-C aerogel catalyst with both high content of Fe-N<sub>4</sub> active site and excellent mass transport properties. Elbaz *et al.* combined the advantages of the three-dimensional covalent organic framework and carbon aerogel in heat-treated iron porphyrin aerogels, which achieve both high surface area and density of active sites.<sup>55-57</sup>

Our group has previously synthesized Fe-N-C aerogels by pyrolyzing resorcinol-formaldehyde-melamine organic aerogel, where the impacts of iron precursor, process parameters, and heat treatment condition on the microstructure, composition, and ORR performance were carefully investigated.<sup>58,59</sup> Resorcinol (acronym R) and melamine (acronym M) are the sources of C and N respectively, and the Fe precursor was introduced during the polymerization step. Different from the post-introduction of N and Fe sources onto a pre-existing carbon substrate, the presence of well-dispersed N and Fe sources in the aerogel precursor can promote the formation of Fe-N<sub>4</sub> active sites and reduce the yield of Fe-rich nanoparticles during pyrolysis.<sup>51,60</sup> However, excessive M content can decrease the porosity of the aerogel and even impede the gelation process.<sup>12,61</sup>

Herein, a series of Fe-N-C catalysts were obtained and characterized by modulating the M content, in order to define the limits of the gelation parameters and to identify the relationships between the gelation parameters, materials' structure, porosity, activity, and mass-transport properties. The porosity and specific surface area of the catalysts were determined by nitrogen (N<sub>2</sub>) sorption. The nitrogen composition was studied by X-ray photoelectron spectroscopy (XPS), and the Fe content was determined by inductively coupled plasma mass spectrometry (ICP-MS). The coordination environment of Fe was investigated by X-ray absorption spectroscopy (XAS) and <sup>57</sup>Fe Mössbauer spectroscopy. With the present synthesis approach, we found that increasing the amount of M is crucial for increasing both the N and Fe content, promoting the formation of Fe-N<sub>4</sub> sites and therefore the activity of the catalysts, as measured in RDE and in PEMFC. The M content also modifies in a
non-linear way the mesopore volume, leading to a series of materials with various pore size distributions. No positive correlation was however found between mesopore volume and mass-transport properties, as examined with H₂/air PEMFC tests. The optimized catalyst in this work was obtained with a R/M molar ratio of 2/2, featuring Fe mainly as Fe-N₄ sites with a D1 quadrupole doublet Mössbauer signature, assigned to high spin Fe(III)-N₄ site. The optimized catalyst outperforms a commercial Fe-N-C catalyst from Pajarito Powder, selected as a comparison, in H₂/air PEMFC test.

## 2 Experimental methods

### Catalyst synthesis

The Fe-N-C catalysts were prepared according to a protocol adapted from earlier works.⁵⁸,⁵⁹ Resorcinol (99%, Alfa Aesar), formaldehyde (acronym Fo, 37wt% solution, Acros Organics), melamine (99%, Acros Organics), and anhydrous FeCl₃ (98%, Acros Organics) were used as received. All these precursors were mixed with defined ratios (see below) in distilled water, together with anhydrous sodium carbonate (acronym NaC, 99.9999%, Fluka) as the catalyst to promote polymerization. The M content was modulated according to the molar ratio of R/M at the aerogel preparation stage. The reactants and solvent were stirred at 70 °C with the molar ratio of 2:(4+3x):x for R:Fo:M, where x is the relative molar content of M. The catalyst content was kept at a molar ratio of R:NaC=2:0.033, and the solid content of (R+M+Fo) was controlled to 20% by adapting the content of water. FeCl₃ was added after 30 min polymerization. The molar ratio (R+M):Fe was kept constant with a value of 1:0.01, targeting ca. 1 wt. % Fe in the final catalysts by assuming no iron was removed during the acid leaching step and around 70% mass loss of (R+M+F) after pyrolysis based on our pre-experimental work.⁵⁸,⁵⁹ The specific quantities of products were listed in **Table S1**. Subsequently, the pH of solution was adjusted to 8 by adding NaOH solution (2 M) under continuous stirring until gel formation. The hydrogel was then cured in a water bath at 70 °C for 120 h. The obtained hydrogels were subjected to a water-acetone exchange 3 times per day, for 3 days, followed by CO₂ supercritical drying to preserve the nanostructured porosity. Subsequently, the organic aerogels were cut into small pieces and then pyrolyzed at 800 °C under N₂ flow for 1 h. The tube furnace was heated with a ramp rate of 4 °C min⁻¹ and maintained for 1 h at 800°C. Afterward, the furnace was turned off to cool down to room temperature. The N₂ flux was kept on during the whole process.

The carbonized aerogel pieces were sealed in a zirconium-oxide pot of 50 mL with balls of the same material, and subjected to ball-milling with machine Retsch S100. In detail, the mass ratio of balls to sample was kept at 30:1. The large pieces were pre-ground with 10 mm diameter balls at 400 rpm for
15 min, and further ground with 5 mm diameter balls at 400 rpm for 60 min. The obtained powders were washed at 80°C with 0.5 M H₂SO₄ for 7 h in a refluxing device to remove Fe particles or other acid-unstable Fe species generated during pyrolysis. The acid-washed powder was then subjected to a second heat treatment at 950°C under a diluted NH₃ (10% NH₃ + 90% N₂) atmosphere. The furnace was heated with a ramp rate of 4 °C min⁻¹ until 950°C under N₂ and then switched to diluted NH₃. After holding at 950 °C for 45 min under diluted NH₃, the gas was switched back to pure N₂ and turned off the furnace. The samples were let to cool down to room temperature under N₂ flux. The final catalysts are labeled as FeCl₃-R/M=2/x, where x is the relative molar amount of M, as described previously in the synthesis protocol.

Physicochemical characterization

The morphologies of the catalysts were determined by scanning electron microscopy (SEM) (MAIA) operated at 3.0 kV. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (X-EDS) were used to differentiate the carbon morphologies and the local Fe content of the different materials, respectively. A JEOL 2010 TEM operated at 200 kV with a resolution of 0.19 nm was used. At least five different catalyst regions of the TEM grids were analyzed by X-EDS. Nitrogen sorption isotherms were measured at -196°C with a Micromeritics ASAP 2020 instrument, after degassing the samples under 10 mmHg at 200 °C for 8.3 hours. S<sub>BET</sub> was determined via the Brunauer–Emmett–Teller (BET) equation while the total pore volume (V<sub>t</sub>) was estimated from the adsorption isotherm at the relative pressure of 0.995. The pore size distribution (PSD) was calculated using the 2D-NLDFT-Heterogeneous Surface method based on the adsorption isotherms. The micropore volume (V<sub>micro</sub>) and mesopore volume (V<sub>meso</sub>) were obtained from the PSD analysis. The cross-linking degree of the aerogel before pyrolysis was investigated by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. The measurements were performed in the range of 4000-600 cm<sup>-1</sup> on a Bruker Tensor 27 machine using OPUS 7.8 software. X-ray photoelectron spectroscopy (XPS) was used to identify the elemental composition of the surface of the catalysts. The measurements were performed using Thermo ScientificTM K-AlphaTM spectrometer equipped with an Al Kα1,2 monochromatic source (1486.6 eV). All samples were analyzed with a spot size of 200 µm and repetitive tests were conducted on four different spots of all samples to ensure the accuracy of the measurements. A survey spectrum and higher resolution multiplex scan spectra (C 1s, O 1s, N 1s, and Fe 2p core levels) were obtained and analyzed by Avantage. For the fitting, the peaks were constrained to have a full width at half-maximum between 1.2 and 1.4 eV, and the peak positions were identified by fixing their difference of binding energy to pyridinic N at 398.8 eV.<sup>62–64</sup> The global Fe content in the catalysts was determined by inductively
coupled plasma mass spectrometry (ICP-MS). A PerkinElmer NexION 2000c instrument was used. Daily prepared iron (⁵⁶Fe) solutions were used for the calibration curves. Five solutions were prepared from commercial standard mono-element ICP-MS solution (Carl Roth GmbH & co. KG, 1000 mg L⁻¹) in order to obtain five-point calibration curves at 0, 2, 5, 10, and 20 µg L⁻¹. Rhodium (¹⁰³Rh) was used as the internal standard. To avoid any polyatomic interferences, the helium collision mode was used. X-ray diffraction (XRD) analysis was performed using an X”pert Pro Philips diffractometer using filtered Cu Kα radiation (λ=0.15418 nm) generated at 40 kV and 30 mA. The diffraction angles of the diffraction peaks were determined with High score software. The average values of perpendicular graphene layers (L_c) and their interlayer spacing (d_002) were calculated from the Debye-Scherrer equation.

Raman spectroscopy was used to differentiate the carbon structure of the different electrocatalysts. A Renishaw InVia instrument in a backscattering configuration (with 532 nm as excitation) was used for each measurement. An objective focus of X50 was used to collect the scattered radiation. In order to prevent the excess heating or probable degradation of the samples under the laser beam, the spectrometer was used in a ‘line mode’ with low incident power densities (~50 µW/µm²). LabSpec 5.0 software was used for fitting the Raman spectra and five bands were considered, as our previous work.⁶⁵ The average values of in-plane graphite crystallite size (L_a) was calculated from the Knight and White equation:⁶⁶

$$L_a(nm) = 4.4 \left( \frac{I_{D1}}{I_G} \right)^{-1} \qquad (Equation 1)$$

The ⁵⁷Fe Mössbauer spectra (Wissel, Germany) were acquired in transmission mode. The source was ⁵⁷Co: Rh. The velocity driver was operated in constant acceleration mode with a triangular velocity waveform, and the velocity scale was calibrated with the magnetically split sextet of a high-purity α-Fe foil at room temperature. A mass of ca. 200 mg of catalyst was mounted in a 2 cm² holder. Mössbauer measurements were performed at -268°C with a helium flow cryostat (SHI-850 Series from Janis, USA). The spectra were analyzed by fitting the experimental spectrum with an appropriate combination of quadrupole doublets and sextets with Lorentzian profiles.

Fe K-edge XAS were collected at room temperature at SAMBA beamline (Synchrotron SOLEIL, Saint-Aubin, France). The beamline is equipped with a sagittally focusing Si 220 monochromator, and X-ray harmonics are removed by two Pd-coated mirrors. The catalyst powders were pelletized as disks of 10 mm diameter with 1 mm thickness, using Teflon powder (1 µm particle size) as a binder. Measurements were carried out in transmission or fluorescence mode, depending on the iron content.
Electrochemical measurements

RDE measurements were performed under atmospheric pressure and room temperature in a three-electrode cell with a potentiostat/galvanostat from Bio-Logic (HCP-803). The reference electrode was a reversible hydrogen electrode (RHE) and the counter electrode was a graphite electrode. The catalyst ink was prepared by ultra-sonicating 10 mg catalyst powder, 95 µL 5 wt. % Nafion D-520, 752 µL ethanol, and 46 µL deionized water for 1 h. 7 µL of this ink was dropped on a 0.196 cm² glassy carbon disk, resulting in a catalyst loading of 0.4 mgFe-N-C/cm². The ORR activities were measured in an O₂-saturated 0.05 M H₂SO₄ aqueous solution. Linear scan voltammograms were recorded (LSV) at 10 mV s⁻¹ from 1.1 to 0 V vs. RHE under constant revolution rate (1600 rpm). The same test was repeated in N₂-saturated electrolyte to eliminate the pseudo-capacitive current ('background' current). All obtained data were corrected for Ohmic-drop (iR-drop). Cyclic voltammograms between 0 and 1.0 V vs. RHE were measured at 10 mV s⁻¹ in N₂-saturated electrolyte. The kinetic current densities of all catalysts were defined by the Koutecky-Levich equation as shown in Equation 2.

$$ \frac{1}{j_k} = \frac{1}{j_F} - \frac{1}{j_{lim}} \qquad (\text{Equation 2}) $$

where jₖ, jF and jlim represent the kinetic current density (mA cm⁻²), Faradic current density (mA cm⁻²) and limiting current density (mA cm⁻²), respectively.

The mass activity (A g⁻¹) was calculated as follows (Equation 3):

$$ j_M = - \frac{j_k}{m} \qquad (\text{Equation 3}) $$

where jM (defined > 0) is the mass activity, and m is the catalyst loading (0.4 mg cm⁻²) on the working electrode.

To determine the SD and TOF values, the nitrite adsorption and stripping analysis were realized through the method first introduced by Malko et al using 0.125 M NaNO₂ aqueous solution and a loading of 0.3 mgFe-N-C/cm². The nitrite stripping was recorded in a potential range of 0.2 to -0.2 V vs. RHE in an acetate buffer 0.5 M pH 5.2 solution.⁶⁷ ORR polarization curves were carried out at 5 mV/s and at 1600 rpm and were recorded for the unpoisoned, poisoned and recovered catalysts surface. The SD and TOF values were determined using the following equations:

$$ SD(g^{-1}) = \frac{Q_{strip}(C g^{-1}) N_A(mol^{-1})}{n_{strip} F (C mol^{-1})} \qquad (\text{Equation 4}) $$

$$ TOF(s^{-1}) = \frac{MA_{0.8} \text{ at pH 1 } (A g^{-1}) N_A(mol^{-1})}{SD(g^{-1}) \times F(C mol^{-1})} \qquad (\text{Equation 5}) $$

Where Qstrip is the excess coulometric charge associated with the reduction of one adsorbed nitrite ligand per site and nstrip is the number of electrons associated with this stripping. F and NA are the Faraday and Avogadro constants, respectively. MA₀.₈ is the mass activity measured at 0.8 V vs. RHE
at pH = 1. In this work, the SD values were calculated considering a 3 electrons process for the reduction reaction with the formation of NH₃OH⁺.⁶⁸

For PEMFC measurements, the ink formulation applied to all Fe-N-C catalysts was 20 mg of catalyst, 915 µL of 5 wt.% Nafion solution (1100 EW Nafion, Sigma Aldrich), 63 µL of 1-propanol (>99.9% purity, Sigma Aldrich), and 272 µL of deionized water. The reagents were mixed and subjected to sonication for 1 hour. The ink was deposited on a 4.84 cm² gas diffusion layer (Sigracet 29-BC from Baltic Fuel Cells), depositing 210 µL aliquot of the ink each time followed by drying, and repeating this process until the total ink volume was deposited, resulting in a catalyst loading of 4.1 mg cm⁻². This ink formulation corresponds to a mass ratio of dry Nafion to catalyst (N/C) of 2. The effect of N/C was investigated in detail for the catalyst FeCl₃-R/M=2/1, and also for the commercial Fe-N-C catalyst from Pajarito Powder. The cathode was then dried at 80 °C, and hot-pressed at 135 °C with a Pt/C anode and a Nafion membrane (Nafion NR-211). The anode used for all PEMFC tests was 0.5 mgPt cm⁻² on Sigracet 28-BC. PEMFC measurements were performed with a single-cell fuel cell with a serpentine flow field (Fuel Cell Technologies) using an in-house test bench and a Biologic potentiostat with a 50 A load and EC-Lab software. For H₂/O₂ PEMFC measurements, the temperature of the cell was 80 °C, the backpressure was 1 bar on each side, the flow rate was 150 mL min⁻¹ on each side, and the relative humidity 100% was on each side. The conditions were the same for H₂/air measurements, except for the flow rate on the cathode side, which was increased to 480 mL min⁻¹ of synthetic air (Air Liquide). Polarization curves were recorded by scanning the voltage at 1 mV s⁻¹.

## 3 Results and Discussion

### Physico-chemical characterizations of catalyst precursors and catalysts

**Figure 1** shows the FTIR spectra of the Fe-doped aerogels before pyrolysis, which constitute the precursors of the Fe-N-C catalysts, prepared with R/M ratio varying from 2/0 to 2/2.5. The assignment of absorption bands is listed in **Table S2**. For FeCl₃-R/M=2/0 (no melamine), the FTIR spectrum is similar to that of an aerogel prepared only from resorcinol and formaldehyde (RF seen in **Figure 1**), without FeCl₃. The absorption band at 1479 cm⁻¹ belongs to the CH₂ scissor vibrations (from the condensation of the hydroxymethyl derivatives created in the resorcinol formaldehyde reaction), and the absorption band at 1614 cm⁻¹ corresponds to benzene ring stretching vibrations.⁶⁹⁻⁷¹ The two absorption bands at 1222 and 1092 cm⁻¹ are associated with C-O-C asymmetric/symmetric stretching vibrations of methylene ether bridges, which have been previously identified in the FTIR spectra of
RF aerogels.<sup>69,71</sup> This bond demonstrates the existence of benzylether structure which is typically derived from the polycondensation between methylol and resorcinol in basic condition.<sup>43,46</sup> With increasing melamine content, the absorptions derived from MF aerogel become more intense. Two sharp absorption bands at 1544 and 810 cm<sup>-1</sup> can be unambiguously assigned to vibration modes that are specific to the triazine ring of melamine.<sup>72</sup> The relatively broad absorption band at 1350 cm<sup>-1</sup> can be assigned to the stretching vibration of C-N bond between aromatic carbon and amine function.<sup>72</sup> An additional absorption band is visible at around 1000 cm<sup>-1</sup>, which cannot be assigned to resorcinol-formaldehyde nor melamine-formaldehyde aerogel. It can instead be assigned to a N-CH<sub>2</sub>-O stretching mode, indicative of crosslinking between resorcinol and melamine with methylene derived from formaldehyde as the linker.<sup>73</sup> The peak intensity increases with melamine content, indicating the proper integration of nitrogen atoms in the RMF aerogel backbone.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** FTIR spectra of organic aerogel prepared with different R/M ratios.</figcaption></figure>

All other characterizations were performed on the catalysts, i.e. after all synthetic steps, including the final thermal treatment in ammonia. Representative TEM images of the catalysts prepared with different melamine content are shown in **Figure 2a-c** and **Figure S1a-c**. The images reveal a change in the carbon matrix morphology between, on one hand, samples prepared with R/M ratios from 2/0 to 2/1.5, and, on the other hand, samples prepared with R/M ratios of 2/2 or 2/2.5. For the catalysts with low M content, typically FeCl<sub>3</sub>-R/M=2/0.5, their TEM images show a highly porous texture, which is similar to those reported for RMF aerogel.<sup>12,74</sup> When R/M reaches 2/1.5, some pore-less platelets can be found, and finally, become the dominant morphology at R/M=2/2.5. A similar evolution can be observed in their SEM images as well (Figure 2d-f and Figure S1d-f). The catalysts
with low M content are formed by stacking spherical particles, indicating a high porosity as observed in TEM images. On increasing the M content, large and densely packed particles are observed, which implies a lower surface area and porosity.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Carbon morphology of the catalysts prepared with R/M=2/0.5, 2/1.5, and 2/2.5. (a)-(c) TEM images and (d)-(f) SEM images.</figcaption></figure>

The graphitic structure was further investigated with Raman spectroscopy and XRD. The XRD patterns and Raman spectra (Figure S2a and b) show that the carbon matrix is poorly organized for all R/M values, as indicated by their large peak width at 2θ=25.6° in XRD patterns, a peak assigned to the (002) diffraction of graphite, and as indicated by a high ratio of the D to G band intensities (I<sub>D</sub>/I<sub>G</sub> around 4.7) from Raman spectra.<sup>34,75</sup> The average values of in-plane graphite crystallite size (L<sub>a</sub>) and the graphite crystallite thickness (L<sub>c</sub>) were derived from I<sub>D</sub>/I<sub>G</sub> of Raman spectra and full width at half-maximum of the peak at 2θ=25.6° in XRD patterns (corresponding to d<sub>002</sub>=0.36 nm), respectively (Figure 3a and b). An example of Raman fit is shown in Figure S2c. Both L<sub>a</sub> and L<sub>c</sub> values are lower at high melamine content, the decrease occurring when switching from R/M = 2/0.5 to R/M = 2/1. This indicates an increasing average structural disorder of the graphite matrix in the final catalysts with increasing melamine content at the aerogel preparation stage. The changes in L<sub>a</sub> and L<sub>c</sub> are however moderate, and the L<sub>a</sub> values in particular are not commensurate with the size of the platelets seen by TEM for R/M values of 2/2 and 2/2.5, larger than 100 nm. These results indicate
that both the large platelets and highly porous texture are constructed by highly defective and/or nanometric units, whereas the decomposition of melamine leads to their different morphologies.¹²

Moreover, no Fe-rich particles can be observed from TEM images or identified from XRD patterns (Figure 2a-c and Figure S2b), suggesting that Fe exists as atomically dispersed Fe-N₄ sites.¹⁸ However, nanometric particles of iron or iron oxides can be difficult to track with TEM or XRD, and Fe coordination was characterized by dedicated spectroscopies, as discussed later.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Structural and textural characterization of the catalysts prepared with different R/M ratios. (a) graphite crystalline thickness in the direction perpendicular to the graphene layers obtained from XRD, (b) in-plane graphite crystallite size obtained from Raman spectroscopy, (c) isotherms adsorption/desorption and (d) pore size distribution from N₂ adsorption isotherms at -196 °C for the catalysts.</figcaption></figure>

The lower mesoporosity in samples prepared with high melamine content is confirmed by N₂ sorption isotherms (Figure 3c, d and Table 1). The mesopore volume is slightly increased at R/M=2/0 to R/M=2/1, from 0.83 to 1.67 cm³ g⁻¹, then sharply decreased to only 0.02 cm³ g⁻¹ at R/M=2/2.5 (Table 12
1). The skeleton of hydrogel was constructed by linking the RF and MF units in the precursor solution and a part of the interspace between these units became the mesopores after drying and pyrolysis.43,46 Different from thermally stable benzene rings in R, the triazine rings in M were decomposed during pyrolysis. At low M content, the evaporation of M decomposition products helps create more mesopores without destroying the skeleton of the aerogel. However, when this decomposition occurs in large numbers at high M content, it leads to a collapse of the aerogel texture, thus presenting more compact morphology and lower specific surface area and porosity.47,74,76 In contrast to the mesopore volume, the micropore volume is almost constant with the R/M ratio, at 0.42-0.45 cm³ g⁻¹. The micropores are mainly formed by NH₃ etching of the carbon matrix, and the carbon atoms located on the edge of graphite particles are preferentially etched by ammonia compared to carbon atoms located in the plane of the graphite crystallites.77 Since the melamine content weakly affects the average crystallite size of graphite after the first pyrolysis, the NH₃ etching degrees of carbon aerogels are similar and thus obtaining similar micropore volume.

In summary, all the catalysts in this series present a high specific surface area (>1000 m² g⁻¹) and high micropore volume, which are usually considered necessary criteria to reach a high ORR activity with Fe-N-C catalysts. In addition, the hugely contrasting mesopore volume in this set of catalysts is useful to investigate the mass-transport properties in PEMFC and whether mesopores are important or not for high performance (discussed later).

<table><caption>**Table 1.** Surface and pore volume characteristics of Fe-N-C aerogel catalysts.</caption><thead><tr><th rowspan="2">R/M</th><th>BET surface</th><th>V<sub>t</sub></th><th>V<sub>micro</sub></th><th>V<sub>meso</sub></th></tr><tr><th>(m² g⁻¹)</th><th>(cm³ g⁻¹)</th><th>(cm³ g⁻¹)</th><th>(cm³ g⁻¹)</th></tr></thead><tbody><tr><td>2/0</td><td>1481</td><td>1.37</td><td>0.44</td><td>0.83</td></tr><tr><td>2/0.5</td><td>1595</td><td>2.13</td><td>0.43</td><td>1.60</td></tr><tr><td>2/1</td><td>1596</td><td>2.20</td><td>0.44</td><td>1.67</td></tr><tr><td>2/1.5</td><td>1256</td><td>0.87</td><td>0.42</td><td>0.38</td></tr><tr><td>2/2</td><td>1271</td><td>0.78</td><td>0.45</td><td>0.25</td></tr><tr><td>2/2.5</td><td>1099</td><td>0.46</td><td>0.42</td><td>0.02</td></tr></tbody></table>

The chemical composition of the catalysts was investigated with ICP-MS and XPS. The bulk Fe content determined by ICP-MS increases dramatically with increasing melamine content (Table 2), even though the initial quantity of Fe precursor was kept at around 1 wt.% (with (R+M):Fe molar ratio kept constant) for all catalyst preparations. Two main hypotheses can explain this correlation. In the first one, the addition of melamine helps coordinate and disperse iron in the aerogel before pyrolysis, decreasing the amount of metallic iron or iron oxide particles formed during or after
pyrolysis, and therefore minimizing the amount of iron leached during the acid washing step. In the second hypothesis, the increasing amount of melamine leads to increasing mass loss of C and N as volatile products during pyrolysis, which indirectly increases the iron content in final catalysts. The very low Fe amount measured with R/M = 2/0 however supports that our first hypothesis is the main factor accounting for the Fe content in the final catalysts. Additionally, this hypothesis is supported by spectroscopic characterization of the iron environment in catalysts, as discussed later. The chemical composition was investigated by XPS (Table 2). The Fe content measured by XPS is similar to that measured with ICP-MS. While the N content also shows an increasing trend with increasing M, the relative increase is less than the increase in Fe content (Table 2). However, the N₁s narrow scan spectra identify a dramatic change in the type of N-groups present, the catalysts prepared with R/M = 2/0 showing a very weak signal at ca. 401 eV (pyrrolic N) while the relative intensity of this signal increases with the melamine content (Figure 4a).

To clarify the evolution of the type of N-species present as a function of the R/M synthesis parameter, the N 1s spectra were fitted with a set of six peaks assigned to different N-species, including nitrile at 398.0 eV, pyridinic N at 398.8 eV, coordinated N (Fe-Nₓ) or amine at 399.9 eV, pyrrolic N at 400.9 eV, quaternary N at 402.0 eV, and graphitic N at 403.3 eV, as listed in Table 2. An example of such a fit is given in Figure S3. Figure 4b presents the variation of the different N species with increasing M content. Notably, only the pyrrolic N content is positively correlated with the M content, whereas the other N-species are almost constant or only increase slightly with increasing M content. The molar ratio of pyrrolic N/Fe is close to 5.4 for the catalyst FeCl₃-R/M=2/0 then decreases with increasing M content to stabilize at around 4 until FeCl₃-R/M=2/1.5 (Table 2). This number corresponds well to the stoichiometry in Fe-N₄ sites, suggesting all nitrogen atoms involved in the ligation of Fe are of pyrrolic nature. The values are decreased to around 3 for FeCl₃-R/M=2/2 and FeCl₃-R/M=2/2.5, suggesting the existence of other Fe species.

<table><caption>**Table 2.** Chemical composition of the Fe-N-C aerogel catalysts.</caption><thead><tr><th rowspan="2">R/M</th><th colspan="2">ICP-MS Fe</th><th colspan="3">XPS</th><th>Nitrile at%</th><th>Pyridinic at%</th><th>Fe-N<sub>x</sub> (amine) at%</th><th>Pyrrolic at%</th><th>Quaternary at%</th><th>Graphitic at%</th><th rowspan="2">Pyrrolic/Fe</th></tr><tr><th>wt.%</th><th>at.%</th><th>C at.%</th><th>N at.%</th><th>O at.%</th><th>Fe at.%</th><th>398.0 eV</th><th>398.8 eV</th><th>399.9 eV</th><th>400.9 eV</th><th>402.0 eV</th><th>403.3 eV</th></tr></thead><tbody><tr><td>2/0</td><td>0.33</td><td>0.07</td><td>95.57</td><td>2.94</td><td>1.39</td><td>0.10</td><td>1.05</td><td>0.50</td><td>0.45</td><td>0.54</td><td>0.28</td><td>0.12</td><td>5.4</td></tr><tr><td>2/0.5</td><td>0.49</td><td>0.10</td><td>95.22</td><td>3.00</td><td>1.63</td><td>0.16</td><td>0.94</td><td>0.55</td><td>0.43</td><td>0.61</td><td>0.35</td><td>0.13</td><td>3.8</td></tr><tr><td>2/1</td><td>0.85</td><td>0.18</td><td>93.69</td><td>3.21</td><td>2.89</td><td>0.22</td><td>0.84</td><td>0.52</td><td>0.52</td><td>0.78</td><td>0.38</td><td>0.18</td><td>3.5</td></tr><tr><td>2/1.5</td><td>1.17</td><td>0.25</td><td>94.32</td><td>3.44</td><td>1.97</td><td>0.27</td><td>0.79</td><td>0.52</td><td>0.45</td><td>1.00</td><td>0.46</td><td>0.22</td><td>3.7</td></tr><tr><td>2/2</td><td>1.36</td><td>0.30</td><td>93.54</td><td>3.76</td><td>2.32</td><td>0.36</td><td>0.94</td><td>0.57</td><td>0.50</td><td>1.14</td><td>0.43</td><td>0.18</td><td>3.2</td></tr><tr><td>2/2.5</td><td>1.54</td><td>0.34</td><td>92.91</td><td>4.17</td><td>2.49</td><td>0.43</td><td>1.01</td><td>0.66</td><td>0.53</td><td>1.30</td><td>0.48</td><td>0.19</td><td>3.0</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Figure 4.** (a) N<sub>1s</sub> XPS patterns, (b) evolution of different N species content with R/M.</figcaption></figure>

Insights into the type of Fe species present were obtained from Fe K-edge XAS and <sup>57</sup>Fe Mössbauer spectroscopies. **Figure 5a** shows selected X-ray absorption near edge structure (XANES) spectra for three catalysts of the series, as well as the XANES spectrum of a reference Fe-N-C catalyst derived from ZIF-8 (labeled Fe<sub>0.5</sub>) and for which Fe is known to be present as Fe-N<sub>x</sub> active sites only.<sup>27</sup> The spectra show high similarities, suggesting Fe is mainly present as Fe-N<sub>x</sub> coordination structures,<sup>27,36</sup> although the presence of nanometric iron oxides cannot be excluded due to similar XANES spectra with Fe-N<sub>4</sub> sites.<sup>30</sup> The Fourier-transformed EXAFS signals of down-selected catalysts (**Figure 5b**) are also similar to that of Fe<sub>0.5</sub>, with the main signal at *ca* 1.5 Å, assigned to Fe-N or Fe-O bonds.<sup>27,36,59,78</sup> As for XANES, the presence of nanometric iron oxide cannot be excluded since the signal from Fe-N bonds in Fe-N<sub>x</sub> sites and Fe-O from oxides cannot be distinguished, and the signal from Fe-Fe backscattering is not always discernible in nanometric oxides due to size and/or amorphous character of nano-oxides.<sup>30</sup> The iron coordination was further investigated with <sup>57</sup>Fe Mössbauer spectra collected at -268 °C. The low-temperature acquisition enables distinguishing in particular nanometric iron oxides, contributing with a sextet component from atomically dispersed Fe-N<sub>x</sub> sites contributing with quadrupole doublets.<sup>30</sup> The spectra of selected catalysts (**Figure 5c-f**) could be well-fitted with appropriate combinations of doublet and sextet components. The main signal in all catalysts is a quadrupole doublet with quadrupole splitting (QS) values of 0.98-1.13 mm s<sup>-1</sup> and isomer shift (IS) value of 0.46-0.47 mm s<sup>-1</sup>, which is assigned to a high-spin O-Fe(III)N<sub>4</sub>C<sub>12</sub> configuration (D1), with O representing an oxygenated species adsorbed on the metal center (**Table 3**).<sup>27,30,79</sup> A second quadrupole doublet with a high IS and high QS is identified, labeled as D3. On the basis of its distinct IS values (0.97-1.20 mm s<sup>-1</sup>), this component can be unambiguously assigned
to high-spin ferrous centers. While the existence of high spin Fe(II)-Nₓ sites cannot be excluded, the recent observation that the presence of this D3 component in Fe-N-C materials coincides with the use of Fe-chloride salts for their synthesis suggests that D3 is due to the presence of ferrous-chloride, with the quadrupole doublet signal of FeCl₂·4H₂O having Mössbauer parameters (IS and QS values) matching those of D3.²¹ As the present series of catalysts is prepared with FeCl₃, we assign D3 in the present work to FeCl₂·4H₂O.²¹ Independent experimental evidence for the presence of FeCl₂ after FeCl₃ chemical vapor deposition on Zn-N-C also supported the assignment of D3.²¹ The relative contribution of D3 to the absorption signal in the present work is however small, 5-8% in FeCl₃-R/M=2/0.5 and FeCl₃-R/M=2/1, and is not observed in FeCl₃-R/M=2/2 or FeCl₃-R/M=2/2.5. For the latter two, another quadrupole doublet with IS comparable to D1 but with higher QS (2.85 mm s⁻¹) is observed, labeled D2, accounting for around 20% of the signal. This doublet is commonly co-observed with the D1 component in Mössbauer spectra of Fe-N-C catalysts and assigned to a low or medium spin Fe(II)N₄ configuration.⁷⁹ The ferrous state of D2 may be explained by its location in the bulk of the carbon matrix, or by a low affinity with O₂. The presence of D2 in the catalyst prepared with the highest melamine content coincides with a decreasing BET area (Table 1), supporting that the minor D2 contribution may result from the non-accessibility of O₂ to some Fe-Nₓ sites. Mössbauer spectroscopy identifies in addition an important contribution from a broad sextet in FeCl₃-R/M=2/0.5, which is assigned to high-spin ferric oxide.⁵⁹ The high linewidth (3.19 mm s⁻¹, Table 3) supports ultra-small clusters, that are not fully magnetically ordered at -268 °C. No sextet is detected in the spectra of FeCl₃-R/M=2/1 and FeCl₃-R/M=2/2, while two well-defined sextets (LW of only 0.27 mm s⁻¹) are identified in the spectrum of FeCl₃-R/M=2/2.5, assigned to ferric oxides, but accounting for only 8% of the signal (Table 3). Therefore, Mössbauer spectroscopy confirms the XANES and EXAFS results, but also provides more quantitative insights, especially identifying an important contribution from ultra-small iron oxide clusters for samples prepared at low R/M value. This shows that the addition of melamine is crucial for the formation of Fe-Nₓ sites during the first pyrolysis. Without melamine or with too little melamine, iron probably mainly forms metallic or metal carbide species during pyrolysis. Upon exposure to air or acid washing, these particles can remain in zerovalent state if they are perfectly covered by a graphite shell, or they can be converted partially or fully to iron oxides, depending on oxygen/proton accessibility to the surface of these particles and depending on the particle size. The iron oxide signal in FeCl₃-R/M=2/0.5 must thus be a consequence of the presence of iron zerovalent particles after the first pyrolysis stage. Fe cations in an acid solution can indeed partially reprecipitate, depending on the acid nature, temperature, and local iron concentration. Together with the ICP-MS data, we can conclude that nitrogen introduced by M can bind the iron atoms and stabilize them to avoid the formation of iron nanoparticles, with optimized
M content. Last but not least, the signal/noise ratio and absolute signal intensity of the Mössbauer absorption spectra also reflect the Fe content in the samples, with low signal and high noise for the sample with low melamine content (R/M=2/0.5). The major contribution of D1 in this series of catalysts (increasing absolute absorption intensity of D1 with the melamine content) also suggests that the nitrogen species involved in coordinating iron in D1 sites is pyrrolic nitrogen. Pyrrolic nitrogen was indeed shown by XPS to be absent in the sample prepared with R/M=2/0 and to continuously increase with increasing M content. Moreover, the presence of D2 site at high M content can explain decreased pyrrolic N/Fe value because Fe atoms in D2 site are coordinated by pyridinic N. We then corrected the Fe content by percentage of D1 in **Table 3**. In this case, the values of FeCl₃-R/M=2/2 and FeCl₃-R/M=2/2.5 in **Table 2** become 4.2 and 4.0, respectively, which correspond well to the stoichiometry in Fe-N₄ sites.

<table><caption>**Table 3.** Spectral parameters derived from the fitting of Mössbauer spectra. Isomer shift (IS), quadrupole splitting (QS), Linewidth (LW), Hyperfine Field (H) and relative signal area (%).</caption><thead><tr><th>R/M</th><th>component</th><th>IS / mm s<sup>-1</sup></th><th>QS / mm s<sup>-1</sup></th><th>LW / mm s<sup>-1</sup></th><th>H / T</th><th>Relative area %</th></tr></thead><tbody><tr><td rowspan="3">2/0.5</td><td>D1</td><td>0.47</td><td>0.98</td><td>1.26</td><td>-</td><td>54</td></tr><tr><td>D3</td><td>1.20</td><td>2.72</td><td>0.50</td><td>-</td><td>5</td></tr><tr><td>Sextet 1</td><td>1.19</td><td>-</td><td>3.19</td><td>43</td><td>41</td></tr><tr><td rowspan="2">2/1</td><td>D1</td><td>0.47</td><td>1.10</td><td>1.25</td><td>-</td><td>92</td></tr><tr><td>D3</td><td>0.97</td><td>2.90</td><td>0.69</td><td>-</td><td>8</td></tr><tr><td rowspan="2">2/2</td><td>D1</td><td>0.32</td><td>1.07</td><td>0.89</td><td>-</td><td>76</td></tr><tr><td>D2</td><td>0.56</td><td>2.62</td><td>1.44</td><td>-</td><td>24</td></tr><tr><td rowspan="5">2/2.5</td><td>D1</td><td>0.46</td><td>1.13</td><td>1.04</td><td>-</td><td>76</td></tr><tr><td>D2</td><td>0.45</td><td>2.85</td><td>2.41</td><td>-</td><td>16</td></tr><tr><td>Sextet 1</td><td>0.69</td><td>-0.1</td><td>0.27</td><td>43.5</td><td>3</td></tr><tr><td>Sextet 2</td><td>0.65</td><td>+0.1</td><td>0.27</td><td>53.7</td><td>5</td></tr></tbody></table>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 5.** (a) XANES and (b) Fourier transforms of the EXAFS spectra measured at Fe K-edge. Mössbauer spectroscopy characterization of selected catalysts. (c) FeCl₃-R/M = 2/0.5, (d) FeCl₃-R/M = 2/1, (e) FeCl₃-R/M = 2/2, and (f) FeCl₃-R/M = 2/2.5. The spectra were acquired at -268°C.</figcaption></figure>

Electrochemical characterization

The polarization curves of catalysts show increasing half-wave potential with increasing M content up to R/M = 2/2, while at R/M = 2/2.5, the activity seems to reach a plateau or even decrease slightly
**(Figure 6a).** All polarization curves except those of FeCl₃-R/M=2/0 and FeCl₃-R/M=2/2.5 present a sharp kinetic-to-diffusion transition region, indicative of adequate mass-transport properties, at least in RDE setup.⁵⁹ The polarization curves were corrected from O₂ diffusion in solution, pseudocapacitive current, and Ohmic resistance to draw Tafel plots (**Figure 6b**). The catalysts exhibit similar Tafel slopes around 60 mV dec⁻¹, which is consistent with the literature and demonstrates that the rate-determining step of the ORR mechanism is the same in this series of catalysts.¹⁸,³⁶,⁸⁰,⁸¹ This is in line with the dominant signal from the component D1 in the Mössbauer spectra, supporting that the change in ORR activity is mainly due to a change in the number of sites, while the fundamental ORR mechanism on the active sites is unmodified. The polarization curves were also used to determine the mass activity of the catalysts at 0.8 V vs. RHE (MA₀.₈) after normalization by the catalyst loading (0.4 mg cm⁻²), as listed in **Table S3**. The mass activity increases *ca.* 4-fold from R/M = 2/0 to 2/1 (0.47 to 2.04 A g⁻¹), then increases slowly from 2/1 to 2/2 (2.04 to 3.00 A g⁻¹) and decreases slightly at R/M = 2/2.5 (2.22 A g⁻¹). The mass activity for the optimized material, FeCl₃-R/M=2/2, is only slightly lower than that of the commercial Fe-N-C catalyst (Pajarito powder PMF D14401, 4.04 A g⁻¹ as seen in **Figure 6a**). In the search for structure-property relationships, **Figure 6c** plots the mass activity of catalysts as a function of their Fe content obtained from ICP-MS. A linear trend can be observed up to the Fe content of 0.3 at.% (1.36 wt.%), corresponding to the catalyst FeCl₃-R/M=2/2, with the highest mass activity 3.0 A g⁻¹ for this series. This trend is explained by Mössbauer spectroscopy, showing that most of Fe are present as atomically dispersed Fe-Nₓ sites, except at low melamine content. Thus, as the bulk Fe content increases, the site density also expectedly increases. Further increasing the M content (and the Fe content) results in a slight decline of the activity due to the restricted mass transfer. The very low mesopore volume in FeCl₃-R/M=2/2.5 prevents reactants in the electrolyte from accessing the active sites. This can be demonstrated by its higher Tafel slope (79 mV dec⁻¹), representing a slow kinetic-to-diffusion transition region.⁵⁹ The mass activity also shows a positive correlation with the pyrrolic N content obtained from XPS analysis (**Figure 6d**), which further demonstrates the Fe-N₄ active sites are constructed by pyrrolic N. Interestingly, the activity of our catalysts shows little relationship with micropore volume, which is generally considered as an indicator of the number of active sites.¹⁶,¹⁸,³⁰,⁷⁷ This lack of correlation is due to the limitation of active Fe at low M content, not by micropores, in the present synthetic approach.
<figure><img src="image_13.png" /><figcaption>**Figure 6.** Electrochemical characterization of catalysts prepared from different R/M ratios recorded in 0.05 M H₂SO₄. (a) Polarization curves in O₂-saturated electrolyte at ω = 1600 rpm and ν = 10 mV s⁻¹; (b) Tafel plots of the O₂-transport and background-current corrected kinetic current obtained from the steady-state Ohmic drop-corrected I-E curves at ω= 1600 rpm; relationships between (c) mass activity in RDE setup and Fe content, (d) mass activity in RDE setup and pyrrolic N content.</figcaption></figure>

To gain further insights into the fundamental reasons for the evolution of the mass activity with the
melamine content, the SD and the TOF (Table S3 and Figure S4-S9) were evaluated using the nitrite
adsorption/reduction electrochemical method.³⁴ The SD value increases with M content from
R/M=2/0 to 2/1, reaching the maximum value of 8.04×10¹⁸ sites g⁻¹, whereas it then declines when
further increasing M content. These results are not in agreement with our previous conclusion that
the increasing mass activity is because the increasing Fe content leads to higher SD values, instead
of TOF. To explain this discrepancy, note that the decline of SD when M content increased from
R/M=2/1 to 2/1.5 coincides with a sharp decrease of the mesopore volume, which may limit the
accessibility of reductive NO₂⁻ to active sites.³⁴ Therefore, the SD values at high M content may be
underestimated. This agrees with the research of Strasser et al. that some Fe surface sites located deep inside micropores are inaccessible to NO₂⁻. The SD measured by nitrite stripping can thus be regarded as the low bound of Fe-N-C catalysts.⁸² On the contrary, the SD value measured at low M content is likely overestimated, because the increase of SD value from R/M = 2/0 to 2/1 is much lower than the increase in mass activity from R/M = 2/0 to 2/1 (ca. 2 folds of SD vs. ca. 4 folds of mass activity). The unexpected high SD measured at low R/M can be explained by the lack of selectivity of the nitrite stripping method, which counts not only Fe-Nₓ sites on the surface but also Fe oxides. This caveat of the nitrite stripping method had been unveiled in the frame of a study on the degradation mechanisms of Fe-N-C in O₂ saturated acidic solution.⁸³ The significant presence of Fe oxides is detected by ⁵⁷Fe Mössbauer spectroscopy in FeCl₃-R/M=2/0.5 (accounting for 41 % of the signal intensity), and it can be hypothesized that even more Fe oxides would be present in FeCl₃-R/M=2/0. Combining the two factors, the variation of SD with M content becomes moderate and seems uncorrelated with mass activity. In summary, the results of nitrite stripping must be analyzed with caution according to the characteristics of the catalyst.

The ORR activity and power performance of these catalysts were then investigated in a 5 cm² single cell PEMFC. Prior to screening all the catalysts, the Nafion/Catalyst (N/C) ratio in the cathode ink formulation was first optimized with FeCl₃-R/M=2/1. The high S<sub>BET</sub> of all catalysts in this study, and high mesopore volume for some of the catalysts, may require more Nafion in the cathode active layer than Pt/C catalysts (e.g. Pt on Vulcan, with Vulcan XC72 having S<sub>BET</sub> of 200-250 m² g⁻¹ or TEC10E20E, ca 800 m² g⁻¹). Figure S10a shows the effect of the Nafion content in the cathode ink formulation, with both the activity and power performance dramatically increasing from N/C = 1 to N/C = 2, and thereafter only slightly decreasing from N/C = 2 to 2.5. Nyquist plots of electrochemical impedance spectroscopy showed little change with N/C (Figure S10b), while cathode CVs recorded in H₂/N₂ PEMFC mode showed dramatically increasing capacitive currents from N/C = 1 to N/C = 2 and only very slightly increasing from N/C = 2 to 2.5 (Figure S10c). Therefore, the data indicate that the optimum N/C ratio is 2 for this catalyst, necessary for covering most of the catalyst surface with ionomer (maximizing the activity) without leading to excessive ionomer thickness (minimizing O₂ diffusion issues through the film, and the filling of meso-macropores). This optimum N/C ratio for the Fe-N-C aerogel catalyst in PEMFC is comparable to the one optimized for two other high S<sub>BET</sub> Fe-N-C catalysts (with a similar PEMFC cathode preparation method).⁸⁴ For the two catalysts labeled as ball milling Fe-N-C catalysts 1 and 2 in that work, the optimum N/C ratio was found to be 1.75 and 2, respectively, while their S<sub>BET</sub> values were 730 and 1000 m² g⁻¹, respectively. The S<sub>BET</sub> values of the series of catalysts in the present work are ca. 1100-1600 m² g⁻¹ and the optimum ratio N/C of
2 is therefore in line with previous results. The other catalysts were then investigated with a fixed N/C value of 2. **Figure 7a** shows the H₂/O₂ polarization curves of all catalysts, with trends in activity at 0.8 V and overall cell performance in line with the trends observed in RDE. For a quantitative comparison of ORR activity trend with the melamine content, **Figure 7b** reports the current density at 0.8 V in PEMFC (a voltage at which the current density is mostly controlled by ORR kinetics) as a function of the melamine content during aerogel preparation. It can be seen that the trend and proportional change of activity with M are similar to that observed in RDE (**Figure 6c**), increasing by a factor of 4 from R/M = 2/0 to 2/2, and thereafter decreasing by about 35% from R/M = 2/2 to 2/2.5. The highest current density at 0.8 V cell voltage is achieved to 104 mA cm⁻² at R/M=2/2 and the maximum power density (Pmax) is 0.549 W cm⁻². Similar observed activity trends in RDE and PEMFC support that the N/C ratio of 2 for the PEMFC cathode is appropriate for all catalysts of the present series, otherwise lower-than-expected activity would have been observed for some materials in PEMFC. The cell performance of commercial Fe-N-C catalyst (Pajarito Powder, PMF D14401) was also tested under the same condition (**Figure S11a**). While the commercial catalyst showed slightly higher ORR activity in RDE setup than that of the most active catalyst (FeCl₃-R/M=2/2 in **Figure 6b**), its current density at 0.8 V in PEMFC is lower than that of any of the aerogel derived catalysts. This indicates unoptimized cathode preparation for the commercial catalyst. The high hydrophobicity of Fe-N-C catalysts derived from silica-templating (as is the case for this commercial catalyst) may require a totally different ink formulation than for hydrophilic Fe-N-C catalysts. The PEMFC data obtained herein on the commercial catalyst can therefore not be used for comparison with the performance of the aerogel-derived catalysts. As an alternative, Tatyana *et al.* recently reported a high-performance mesopore textured Fe-N-C catalyst (Pajarito Powder 230-002-C29) with Pmax = 0.645 W cm⁻² in H₂-O₂ PEMFC, which is only slightly higher than our best catalyst.²² H₂/air PEMFC polarization curves are shown in **Figure 7c**, resulting in similar overall trends with R/M as observed with pure O₂, except that the cathode based on the catalyst with the highest melamine content (FeCl₃-R/M=2/2.5) seems to suffer from increased mass-transport issues at high current density. This could be related to the quasi-absence of mesopores in this catalyst (0.02 cm³ g⁻¹, **Table 1**) while FeCl₃-R/M=2/2, prepared with less melamine, still shows some mesopores (0.25 cm³ g⁻¹, **Table 1**). For assessing the oxygen-transport properties of the different Fe-N-C cathodes (different R/M values for aerogel preparation), we compare the current densities measured at 0.3 V (voltage at which mass-transport plays a key role on the current density achieved) with pure O₂ or with air (**Figure 7d**). The figure shows that the magnitude of the decrease in current density when switching from O₂ to air is comparable for all catalysts, despite huge differences in the mesopore volume of this set of catalysts (**Table 1**). One can therefore conclude that mesopores are not a necessary criterion to
reach high PEMFC performance of Fe-N-C catalysts. For the present synthesis approach, the addition of melamine is therefore critical to increase the Fe content, SD, and ORR activity, while it impacts little the mass-transport properties of the cathode active layers. Macropores must exist in the cathode active layers for O₂ and water transport, but cannot be captured by N₂ sorption isotherms measured on the Fe-N-C powders nor cathode layers, the technique having an upper limit of detection of pores with size of ca. 100 nm.

In summary, Fe-N-C catalysts with adjustable and high purity Fe-N₄ active sites embedded in a carbon aerogel substrate were synthesized for the first time in this work. The key conclusion regarding the preparation is that an abundant N source is necessary in the aerogel precursor, in order to minimize the yield of Fe-rich nanoparticles. However, it is not a sufficient condition to obtain mainly or only Fe-N₄ active sites. The content of Fe-rich species (especially Fe oxides) cannot be ignored in most Fe-N-C aerogel catalysts derived from the pyrolysis of polymer, even though an acid washing was conducted after the carbonization.⁴⁸,⁸⁵ In our previous work, 47% of the absorption area in the Mössbauer spectrum of a Fe-N-C prepared with R/M = 2/1 was assigned to Fe oxide, 25% to zerovalent Fe (the remainder, 28%, to D1 sites), whereas the Fe-N-C catalyst prepared with the same R/M ratio in the present work shows only 8% of iron chloride particle (D3 signal) and the remainder (92%) is assigned to D1 sites.⁵⁹ The main differences between the synthesis methods in this and our former work are ammonia treatment and ball-milling conditions. Between them, a stronger ball-milling condition in the present work is considered to be the main reason for the absence of zerovalent Fe or oxide, i.e. 15 min with 10 mm diameter ZrO₂ balls plus 60 min with 5 mm diameter ZrO₂ balls at 400 rpm, whereas only 15 min grind with 10 mm diameter ZrO₂ balls at 400 rpm was applied previously.⁵⁸,⁵⁹ The strong ball-milling condition leads to fin powder size and exposes most of Fe nanoparticles embedded in the pores of carbon aerogel so that they can be eliminated during the acid wash. On the contrary, the Fe-rich nanoparticles embedded in the carbon particles cannot be completely removed by acid washing, as they were detected by Mössbauer spectrum.⁵⁹ Due to the advantages of high purity Fe-N₄ active sites and carbon aerogel substrate, Fe-N-C aerogel catalyst with R/M=2/2 exhibits outperformance both in mass activity and mass transport properties, which makes it a promising candidate for application.
<figure><img src="image_14.png" /><figcaption>**Figure 7.** Investigation of the set of aerogel Fe-N-C catalysts as PEMFC cathodes. (a) H₂-O₂ PEMFC polarization and power density curves. (b) Current density at 0.8 V as a function of the melamine content at the aerogel preparation stage. (c) H₂-air PEMFC polarization and power density curves. (d) Comparison of the current density at 0.3 V measured with O₂ or air, for each catalyst. The measurements were performed with Tcell = 80°C, 100 % RH, Pcathode = Panode = 2 barabs, and the H₂ and O₂ flow rates were 150 mL min⁻¹ while the air flow rate was 480 mL min⁻¹.</figcaption></figure>

Conclusions

In this study, we showed that high melamine content is crucial to obtain only atomically dispersed Fe-N₄ active sites in Fe-N-C aerogel catalysts. The absolute amount of Fe was found to increase with M content, and the absolute amount of Fe-N₄ sites also increases with M content. From the analysis of XPS spectra, the pyrrolic N content is positively correlated with the increase of the M content, supporting that most of the active Fe atoms are coordinated by pyrrolic nitrogen, in agreement with Mössbauer spectra. The atomically mixed nitrogen and Fe in aerogel precursor can stabilize the Fe during pyrolysis, avoiding or minimizing the formation of iron nanoparticles. Electrochemical tests
in RDE show a linear relationship between mass activity up to a Fe content of 0.3 at.%, corresponding
to the catalyst FeCl₃-R/M=2/2, with the highest mass activity of 3.0 A g⁻¹ at 0.8 V vs. RHE. This trend
is in line with the physicochemical characterization indicating that most Fe is present as atomically
dispersed Fe-Nₓ sites. Further increase in melamine content (R/M=2/2.5) reduces the mass activity
possibly because of the formation of less active D2 sites and low accessibility to the active sites led
by a sharp decrease of mesopore. Their ORR performance exhibits a similar trend in PEMFC. No
diffusion-limited current density effect is seen in all H₂-air PEMFC polarization curves, indicating
excellent mass transport properties of aerogel-based catalysts. Considering the low mesopore volume
of the catalyst with high melamine content, the accessibility to the active sites is obtained by the
presence of macropores in the cathode active layers.

**Supporting information**

Details for preparing Fe-N-C aerogel catalysts with different R/M values; assignment of FT-IR spectra; MA toward the ORR, SD, and TOF values in RDE setup; SEM and TEM images; Raman spectra and XRD patterns; deconvolution method of N₁s XPS spectrum; CVs and ORR polarization curves of nitrite stripping method to determine SD and TOF values; CVs, ORR polarization curves and Nyquist plots for aerogel catalysts with different ink formulations in PEMFC; ORR polarization curves and Nyquist plots of Pajarito powders with the same ink formulation of aerogel catalyst.

**Acknowledgements**

The authors thank Frédéric Georgi (MINES Paris/CEMEF) for XPS analysis. This work was
supported by the French National Research Agency in the frame of the ANIMA project (grant number
n°ANR-19-CE05-0039) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
Brazil (grant number 8887.598551/2021-00).

**CRediT authorship contribution statement**

**Hongxin Ge**: Writing - original draft, Drawing, Samples synthesis and characterization, Investigation, Formal analysis, Data curation. **Nicolas Bibent**: Investigation, Formal analysis, **Keyla Teixeira Santos**: Sample collection and analysis; Investigation; Formal analysis; Drawing; Data curation. **Kavita Kumar**: Sample collection and analysis; Investigation; Formal analysis; Drawing, Data curation; Writing - review & editing, **Julien Jaxel**: Supercritical drying and SEM analysis, **Moulay Sougrati**: Investigation, Formal analysis, **Andrea Zitolo**: Formal analysis, **Marc Dupont**:
Methodology, **Frédéric Lecoeur**: Methodology, **Vincent Martin**: Methodology; ICP-MS measurements; Formal analysis; Data curation. **Michel Mermoux**: Raman spectroscopy measurements; Formal analysis; Data curation. **Laetitia Dubau**: TEM measurements; Formal analysis; Data curation. **Frédéric Jaouen**: Conceptualization, Writing – review & editing, **Frédéric Maillard**: Conceptualization; Data curation; Funding acquisition; Project administration; Supervision; Validation; Writing – review & editing. **Sandrine Berthon-Fabry**: Conceptualization, Project administration, Formal analysis, Data curation Writing – review & editing.

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