# Impact of Carbon N-Doping and Pyridinic-N Content on the Fuel Cell Performance and Durability of Carbon-Supported Pt Nanoparticle Catalysts

Elisabeth Hornberger, Thomas Merzdorf, Henrike Schmies, Jessica Hübner, Malte Klingenhof, Ulrich Gernert, Matthias Kroschel, Björn Anke, Martin Lerch, Johannes Schmidt, et al.

<figure><img src="image_1.png" /></figure>

## To cite this version:

Elisabeth Hornberger, Thomas Merzdorf, Henrike Schmies, Jessica Hübner, Malte Klingenhof, et al. "Impact of Carbon N-Doping and Pyridinic-N Content on the Fuel Cell Performance and Durability of Carbon-Supported Pt Nanoparticle Catalysts." ACS Applied Materials & Interfaces, 2022, 14 (16), pp. 18420-18430. 10.1021/acsami.2c00762. http://dx.doi.org/10.1021/acsami.2c00762

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<figure><img src="image_2.png" /></figure>
Impact of Carbon N-Doping and N-Pyridinic Content on the Fuel Cell Performance and Durability of Carbon Supported Pt Nanoparticle Catalysts

Elisabeth Hornberger<sup>a,†,*</sup>, Thomas Merzdorf<sup>a,‡</sup>, Henrike Schmies<sup>a</sup>, Jessica Hübner<sup>a</sup>, Malte Klingenhof<sup>a</sup>, Ulrich Gernert<sup>a</sup>, Matthias Kroschel<sup>a</sup>, Björn Anke<sup>a</sup>, Martin Lerch<sup>a</sup>, Johannes Schmidt<sup>a</sup>, Arne Thomas<sup>a</sup>, Raphaël Chattot<sup>b</sup>, Isaac Martens<sup>b</sup>, Jakub Drnec<sup>b</sup> and Peter Strasser<sup>a,*</sup>

<sup>a</sup> Department of Chemistry, Technische Universität Berlin, 10623 Berlin, Germany

<sup>b</sup> ESRF, The European Synchrotron, 71 Avenue des Martyrs, CS40220, 38043 Grenoble Cedex 9, France

<sup>†</sup> E.H. and T.M. contributed equally to this paper.

* Corresponding authors: [elisabeth.hornberger@tu-berlin.de](mailto:elisabeth.hornberger@tu-berlin.de), [pstrasser@tu-berlin.de](mailto:pstrasser@tu-berlin.de)

E.H. and H.S. outlined the study. E.H. performed the synthesis, XRD, ZP and RDE experiments. T.M. performed the FC and NDIR tests. J.H. performed the physisorption experiments. M.Kl. and J.S. performed the XPS experiments. A.T. provided access to the XPS instrument at TU Berlin. EM images were acquired by M.Kl. (TEM) and U.G. (SEM/TSEM). M.Kr. performed the TGA experiments. B.A. and M.L. helped with the ammonolysis. R.C., I.M. and J.D. performed the HE-XRD experiments and assisted with the data analysis. P.S. supervised the study. E.H. and T.M. analyzed the data and composed the manuscript. All authors participated in the discussion.
Abstract

The cathode catalyst layer of proton exchange membrane fuel cells (PEMFC) consists generally of an active platinum catalyst, carbon support and ionomer. The ionomer distribution is of particular importance since it enables the proton transport to the active sites but can also poison those and decrease the oxygen accessibility. Here, we report the successful synthesis and fuel cell implementation of Pt electrocatalysts supported on N-doped carbon and the N-induced effect on the catalyst's durability. Pt nanoparticles deposited on N-doped carbons were prepared using a customized fluidized bed reduction reactor. RDE measurements in acidic media reveal stable and active catalyst materials for the oxygen reduction reaction, comparable in mass activity to other Pt catalysts in literature. *Operando* high energy X-ray diffraction (HE-XRD) experiments were conducted to investigate the N-doping implications on the degradation behavior of the catalysts. In addition, the catalysts were tested in membrane electrode assemblies in a PEMFC. We provide evidence that the heteroatom-doping and thus the incorporation of defects into the carbon backbone does not mitigate the carbon corrosion under high potential cycling (1–1.5 V) but can promote the performance under operating conditions (0.6–0.9 V).
Introduction

Green hydrogen generated from water electrolysis using renewable energy power sources is a clean and sustainable option to meet the fossil fuel free energy demands. The conversion from the chemical energy stored in the hydrogen bond into usable electricity finds application in the low temperature proton exchange membrane fuel cell (PEMFC), which is already utilized in automotive, heavy-duty vehicles or stationary devices. The extensive commercialization of such PEMFC devices is limited by cost, power density and durability issues when comparing with competing technologies such as traditional combustion engines or battery systems. ¹, ² Efforts have been made to reduce the amount of Pt in the catalyst material while maintaining the performance and improving the stability of the cathode catalytic layer (CCL). ³, ⁴ The cathode side of the PEMFC where the oxygen reduction reaction (ORR) takes place is one of the key concerns due to the sluggish kinetics of the ORR that cause large overpotential losses. The catalytic activity and the power density are strongly depending on the realization of the triple phase boundary: Pt catalyst, oxygen gas and ionomer electrolyte interface. The ionomer in low temperature PEMFC, such as the commonly used Nafion polymer, not only provides protons, but also free sulfonate groups that can adsorb on the active Pt surface and attenuate the performance in the low current density region. ⁵⁻⁷ Inhomogeneously distributed ionomer, however, limits the oxygen transport towards the Pt surface and reduces the performance in the high current density region. ⁸⁻¹¹ One solution for improved ionomer-catalyst interaction was proposed to be N-doped carbon materials. ⁴, ¹² Surface -NHₓ groups can be introduced to the carbon support, e.g. via ammonolysis, which develop Coulombic interactions with the -SO₃⁻ groups of the ionomer beneficially influencing the ionomer distribution towards an increased homogeneity. ¹³ Orfanidi et al. applied this concept to a solid Vulcan XC 72R (moderate surface area (SA), higher degree of graphitization) and Ott et al. used a mesoporous Ketjenblack EC-300J (internal pores, high SA) as carbon material for the ammonolysis. ⁴, ¹² Both studies showed an improved H₂/air performance in membrane electrode assembly (MEA) testing that was directly attributed to an improved ionomer distribution and thus an improved oxygen mass transport at high current densities. The latter study demonstrated an impressive performance that was further credited to highly accessible pores. The concept of accessible pores refers to porous carbon materials with preferred pore openings of 4–7 nm that can host Pt particles in the internal pores protected from the ionomer poisoning but are still accessible by protons and oxygen. ¹⁴ Such catalysts with accessible pores can facilitate both high ORR activity and reactant transport properties without adversely affecting each other.

The durability of Pt/C catalysts proves to be an issue in fuel cell application. ³, ¹⁵, ¹⁶ Degradation phenomena in relation to the Pt particles of the catalyst have been identified as a) particle growth
accompanied with decreased electrochemical active surface area (ECSA) due to coalescence/Ostwald ripening and aggregation/agglomeration processes, b) Pt dissolution and c) particle migration/detachment.¹⁷ The carbon support material is prone to degradation phenomena due to carbon corrosion via electrochemical oxidation in the presence of water following equation 1 at potentials above 0.207 V¹⁸:

$$C + H_2O \rightarrow CO_2 + 4H^+ + 4e^- \quad 1$$

However, the kinetics of the carbon oxidation reaction are poor under typical fuel cell operating conditions. Several studies reported that minor carbon corrosion develops for potentials above 0.6 V, and becomes severe at potentials above 1.4 V in situations of start-up/shutdown and/or local fuel starvation at the anode.¹⁹⁻²² Reiser et al.²² rationalized that at these conditions when the anode is partially exposed to both, hydrogen and oxygen, the electrolyte potential drops, resulting in a current flow of opposite direction at the oxygen exposed region (compared to the normal fuel cell mode) that causes a raised cathode interfacial potential with a high oxidative impact in the carbon. Furthermore, the presence of Pt particles is believed to further promote the carbon corrosion in the electrochemical environment.²³, ²⁴ ORR catalysts with durable Pt and carbon compounds are crucial for a successful application of the PEMFC.

In an earlier work, we prepared a set of N-doped Vulcan carbons via ammonolysis at elevated temperature and quantitatively investigated the degradation due to carbon corrosion with high-temperature differential electrochemical mass spectrometry (HT-DEMS) at 140 °C.²⁵ MEAs fabricated of unmodified and N-doped Vulcan carbons were subjected to increasing potentials from 0.06 V up to 1.05 V and the volatile products of the cathode were analyzed using the HT-DEMS. The direct comparison of unmodified and N-doped Vulcan revealed a sharp decrease of the corrosion current accompanied by a comparable strong decrease in mass ion current with m/z = 44 for CO₂ for the N-doped Vulcan. We concluded from this that the N-functionalization has a beneficial influence on the carbon corrosion durability of the pure Vulcan material. This and a remarkable rotating disk electrode (RDE) durability with minor ORR activity losses of the platinized N-doped carbon catalyst (Pt/N-Vulcan) suggests that N-doped carbon could act as promising support material for the PEMFC cathode with high carbon corrosion resistance durability and improved ionomer distribution. However, detailed investigation with MEA single cell measurements was missing to complete the presented results.

In the present work, unmodified and N-doped carbon were prepared and uniformly decorated with Pt nanoparticles of narrow particle size distribution using a customized fluidized bed reduction reactor.²⁶ The set of catalysts were physico-chemically characterized and investigated towards their
ORR performance and catalyst durability with focus on Pt stability and carbon corrosion using MEA-based fuel cell measurements, in addition to RDE. To further elucidate the effects of the N-doping, *operando* high energy X-ray diffraction (HE-XRD) experiments were conducted. With these investigations, we help to understand the degradation phenomena towards improved ORR catalysts for the low temperature PEMFC.

# Results and Discussion

## Physico-chemical Characterization

For this study, a set of Pt-based ORR catalysts were prepared using commercial Vulcan XC 72R (denoted as V) carbons and from-it-derived N-doped Vulcan carbons. Typically, the N-doped carbons were functionalized via pre-oxidation with $HNO_3$ and ammonolysis at the elevated temperatures of 400 °C and 800 °C as published before (denoted as N-V-XXX, with XXX representing the ammonolysis temperature, synthesis details in supporting information).¹², ¹³, ²⁵

The pre-oxidation step of the synthesis leads to an increase of O-moieties at the carbon surface in form of carboxylic, hydroxylic and $NO_x$ groups for oxidized Vulcan (O-V, see Table 1).¹² These oxygenated surface groups are then successively reduced during the ammonolysis steps to nitrogenated surface groups. The elemental bulk composition was determined using elemental analysis of carbon, hydrogen, nitrogen, and sulfur (CHNS analysis). The N-doped carbons show a low degree of N-incorporation into the C matrix of 2.7 and 1.4 at-% N for N-V-400 and N-V-800, respectively, indicating a decomposition of less stable N- and/or O-groups with higher temperature. The unmodified V carbon contains no bulk N-content. The remaining O-content was determined to be 3.7 and 2.2 at-% O for N-V-400 and N-V-800, respectively. N-V-400 therefore has the highest degree of heteroatom-doping.

<table><caption>Table 1: Elemental composition of the unmodified and N-doped carbons.</caption><thead><tr><th>Sample</th><th>at-% N</th><th>at-% C</th><th>at-% H</th><th>at-% O<br>(calculated residual<br>to 100 at-% total)</th></tr></thead><tbody><tr><td>V (unmodified)</td><td>0.0 ± 0.0</td><td>98.4 ± 0.4</td><td>0.2 ± 0.3</td><td>1.4</td></tr><tr><td>O-V</td><td>0.0 ± 0.0</td><td>89.9 ± 0.1</td><td>0.0 ± 0.0</td><td>10.1</td></tr><tr><td>N-V-400</td><td>2.7 ± 0.2</td><td>93.6 ± 0.5</td><td>0.1 ± 0.0</td><td>3.6</td></tr></tbody></table>
<table><tr><td>N-V-800</td><td>1.4 ± 0.3</td><td>96.5 ± 1.9</td><td>0.0 ± 0.0</td><td>2.1</td></tr></table>

V (Vulcan XC 72R) can be considered as a solid carbon.¹⁴ Solid carbons like V have moderate surface areas (<500 m²·g⁻¹) and a high degree of external pores. An example of a porous carbon would be Ketjenblack carbon (e.g. Ketjenblack EC-300J) with high surface areas (500-2000 m²·g⁻¹) and high degree of internal pores. It is known that the functionalization via pre-oxidation and ammonolysis has an influence on the surface area and porosity of the carbon support. To elucidate this, the carbons were characterized with nitrogen physisorption experiments (see Figure S 1 for the isotherms and details of the pore size analysis). Table 2 displays the surface area determined by the Brunauer-Emmett-Teller (BET) method and a pore size analysis using a quenched solid density functional theory (QSDFT) method. The QSDFT model is based on the carbon adsorption branch kernel at 77 K and was selected to fit the pore shape with a slit pore model (pore diameter <2 nm) and cylindrical pore model (pore diameter >2 nm). The size of micropores is defined as < 2 nm and of mesopores as 2–50 nm.²⁷ The total surface area of both methods yield in comparable values, considering that results from BET measurements are prone to overestimate the surface area while results from QSDFT often underestimate the surface area due to density differences of bulk nitrogen and nitrogen adsorbed in micropores.²⁸ The BET SA of the unmodified V carbon is 229 m²·g⁻¹. After the ammonolysis, the N-doped carbons show a decreased SA when being ammonolysed at 400 °C (199 m²·g⁻¹) and an increased SA when being treated at 800 °C (245 m²·g⁻¹). The pore size analysis reveals that both N-doped carbons have an increased amount of mesopores of ~26–29 %. This could be explained by ammonia etching for both carbon materials at the present mesopores.²⁹, ³⁰ The micropore ratio of N-V-400 drastically decreases by 52 %. This decrease of nitrogen gas accessible micropores could be explained by a blockage of these micropores by surface groups.⁴, ³¹ For N-V-800, these blocked pores might open up again due to the decomposition/incorporation of the surface groups at higher temperature. Therefore, the micropore/total ratio is like V. The solid character of the N-doped V materials is preserved after the carbon functionalization. The surface area stays at a moderate level and the pores remain external and no internal pores are created. Thermogravimetric analysis (TGA) measurements were performed to investigate the thermal stability of the carbon materials in Ar atmosphere and revealed a high thermal stability for the pure carbon supports up to >500 °C. (Figure S 2).
<table><caption>Table 2: Surface characterization of the unmodified and N-doped carbons in terms of surface area, pore structure and surface charge.</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2">BET SA<br/>/ m²/g</th><th colspan="3">QSDFT SA / m²/g</th><th rowspan="2">Ratio of<br/>micropores/total<br/>/ %</th><th rowspan="2">Surface zeta<br/>potential / mV</th></tr><tr><th>micropores</th><th>mesopores</th><th>total</th></tr></thead><tbody><tr><td>V (unmodified)</td><td>229</td><td>91</td><td>103</td><td>195</td><td>47</td><td>27.8</td></tr><tr><td>N-V-400</td><td>199</td><td>44</td><td>130</td><td>174</td><td>25</td><td>16.4</td></tr><tr><td>N-V-800</td><td>245</td><td>87</td><td>133</td><td>221</td><td>40</td><td>28.9</td></tr></tbody></table>

The dispersion behavior of the carbon materials in aqueous media was examined via the measurement of the surface zeta potential (ZP, Table 2). High absolute values of the ZP suggest increased surface charges that result in a low degree of aggregation due to strong repulsive forces within the dispersion.³² This is of interest for the ink formulation that commonly use a mixture of water and organic solvents as ethanol or isopropanol and following preparation of homogeneous electrode layers. The ZP is 27.8 mV for V, decreases for N-V-400 to 16.4 mV and increases again to 28.9 mV for N-V-800. From the results a similar good dispersion behavior is expected for V and N-V-800, whereas N-V-400 might show less stable dispersion.

The crystalline structure of the carbon materials was studied with XRD, and the collected patterns are typical for V-based materials and indicate no drastically enhanced crystallinity of the graphitic character after the ammonolysis, because of the relatively low ammonolysis temperatures (Figure S 3).
<figure><img src="image_3.png" /><figcaption>Figure 1: Structural and morphological characterization of the pristine catalysts in terms of a) XRD patterns and TEM images of b) Pt/V, c) Pt/N-V-400, d) Pt/N-V-800 and e) commercial Pt/C (HISPEC3000).</figcaption></figure>

The unmodified and N-doped carbons were decorated with Pt particles using a fluidized bed reduction reactor.²⁶ The carbon materials were impregnated in a slurry approach with a Pt salt (H₂PtCl₆), homogenized using a horn sonifier, quenched in liquid nitrogen and dried *in vacuo*. The impregnated dry powders were transferred to the fluidized bed reduction reactor and reduced at 200 °C for 2 h in hydrogen atmosphere (4 % H₂ in Ar). The collected catalyst powders have a nominal low Pt loading of 20 wt-% Pt (Table S 1) and show broad face-centered cubic (fcc) Pt phase XRD reflections suggesting small crystallite sizes (Figure 1a). The Pt particle size is extracted from analysis of TEM images (Table S 1). The Pt particles are of small size (~2 nm) and uniformly distributed on the carbon supports. The Pt deposition method using the slurry approach with an excess volume of solvents and the solid character of the V carbon led to a distribution of Pt particles on the external surface of the carbon for the majority of the particles. This can be qualitative visualized by combining scanning electron microscopy (SEM) and transmission SEM (TSEM) images. SEM imaging exposes the Pt particles deposited on the exterior surface only, while the bright field TSEM imaging displays the Pt particles on both the exterior and interior surface. Figure S 4 illustrates an exemplary set of SEM and TSEM images for Pt/V.

To further elucidate the N environment of the final catalyst, X-ray photoelectron spectroscopy (XPS) analysis of the N 1s peak was performed after the Pt deposition because the introduction of Pt nanoparticles could have an influence of the N environment compared to the pure carbon materials. Based on earlier work, the presence of five N moieties were identified and deconvoluted: pyridinic-N, pyrrolic-N, quaternary-N, graphitic-N, and N-oxide (Figure S 5 and Table S 2). XPS is very surface
sensitive, therefore it is of no surprise that Pt/V reveals a very small N 1s peak even though no bulk N content was found in CHNS analysis. The N-content could derive from the unknown industrial production processes of Vulcan XC 72R. The large percentage of N moieties for all three catalyst is pyridinic-N and pyrrolic-N. As shown before, the fraction of pyridinic-N increases with increased ammonolysis temperature while the fraction of pyrrolic-N decreases.¹², ²⁵ This is in agreement with an incorporation of N atoms into the carbon matrix while pyrrolic-N decompose at elevated temperatures under release of HCN to form pyridinic-N.³³

## Electrochemical Characterization

To investigate the initial ORR performance, the catalysts were tested using the rotating disk electrode (RDE) technique. As reference, a widely known commercial 20 wt-% Pt/C with Vulcan XC 72R as carbon support (HISPEC3000, Johnson Matthey) was investigated in parallel. Figure 1a includes the fcc-Pt phase pattern and Figure 1e the morphology of HISPEC3000. Interestingly, the Pt particle size appear somewhat larger when analyzed the TEM micrographs (~3 nm, Table S 1) than suggested by the broad reflections in the XRD pattern. This could be explained by Pt particles that are composed of crystallites with smaller domain sizes prepared in the yet unrevealed synthesis process of the manufacturer. The size distribution of the Pt particles of HISPEC3000 is less uniformly and narrow then for the homemade Pt/(N-)C samples prepared in the fluidized bed reduction reactor.

<figure><img src="image_4.png" /><figcaption>Figure 2: Electrochemical characterization during potential cycling following the carbon corrosion C-AST and the Pt stability Pt-AST protocols using RDE in terms of the evolution of a) mass-based activity, b) specific activity and c) ECSA<sub>H-upd</sub>.</figcaption></figure>

Figure 2 and Table S 3 depict the RDE results in terms of Pt mass-based activity and ECSA<sub>H-upd</sub>, and the Pt mass-free specific activity. The ORR Pt mass-based activities of all catalysts (homemade and commercial) were evaluated as ~0.2 A mg<sup>-1</sup> at 0.9 V versus reversible hydrogen electrode (RHE). This is a typical value for spherical pure Pt particles on a solid carbon and confirm that the N-doping of the
carbon support itself has no pronounced influence on the intrinsic activity in aqueous media of the active noble metal particles supported on it. However, the ORR activity values do vary slightly, because the distribution of the Pt particles on the different carbon support is unique for each catalyst. Pt/N-V-400 has the highest initial mass activity, followed by Pt/V and Pt/N-V-800, respectively. The specific activity as metric of the intrinsic Pt-mass free catalyst activity follows the same trend as the mass-based ORR activity. The values for ECSA<sub>H-upd</sub> normalized to the Pt mass of the catalysts are ranging from 69 to 84 m² gPt⁻¹.

The catalysts were subjected to RDE stability tests adopted from the accelerated stress testing (AST) protocol of the US Department of Energy (DoE) for fuel cell tests³⁴: a) The carbon corrosion protocol that aims to accelerate support corrosion while minimizing degradation of the catalyst itself (5,000 cyclic voltammograms (CV): 500 mV·s⁻¹ between 1.0 and 1.5 V<sub>RHE</sub>, denoted as C-AST). The potential range used in the C-AST protocol mimics what would occur in an unmitigated system start-up or shut-down. b) The Pt stability protocol seeks to maximize catalyst degradation while minimizing support corrosion (10,000 square wave voltammograms (SWV): steps between 0.6 V<sub>RHE</sub> (3 s) and 0.95 V<sub>RHE</sub> (3 s), denoted as Pt-AST). The Pt-AST protocol uses a potential range expected for an automotive drive cycle. Figures S 6–9 illustrate the evolution of the CV and LSV curves before and after the stability test for all catalysts. When tested with the C-AST protocol, all catalysts show remarkable retention of the mass-based activity while Pt/V has a somewhat lower but still high mass activity. Evaluation of the CV shape evolution exposes a slight increase of the overall current response in the capacitive current area (~0.4–0.6 V<sub>RHE</sub>) suggesting oxidation, roughening and/or enhanced wettability of the carbon support due to cycling in the high potential regime (Figure S 6-9). The ECSA<sub>H-upd</sub> of Pt/V and Pt/N-V-800 increases after the C-AST suggesting an additional cleaning and roughening of the Pt particles surface. For Pt/N-V-400, the changes of the ECSA<sub>H-upd</sub> are within the error. More interestingly, the C-AST reveals no signs of carbon corrosion for all three catalysts in terms of evolution of a chinon-hydrochinon redox pair (~0.5–0.75 V<sub>RHE</sub>) in the CV. This suggests a good carbon corrosion durability for this set of catalysts. Yet it must be noted, that the RDE C-AST stability test in liquid electrolyte at room temperature do not necessarily translate toward full cell measurements.³⁵ For the MEA setup, key parameters of the system change, e.g. electrode preparation, proton and oxygen transport, or water management. Therefore, a detailed analysis of the carbon corrosion in MEA environment is presented in the upcoming section.

The Pt-AST reveals a good and comparable retention of the mass-activity for Pt/V and Pt/N-V-800, while for Pt/N-V-400 it is only fair. This behavior follows the evaluation trends of the ECSA<sub>H-upd</sub>. Only small losses of the active Pt surface area during potential cycling are found for Pt/V and Pt/N-V-800. TEM images and the particle size distributions suggest that this is due to low degree of particle
growth via Ostwald ripening processes and/or agglomeration (Figure S 10), while the Pt particles
overall remain uniform distributed on the carbon support. The loss of the active Pt surface area is
more severe for Pt/N-V-400 even though TEM images exhibit no significantly pronounced particle
growth or agglomeration compared to the other samples. However, TEM imaging generates only
very local information. In summary, we prepared a set of Pt-based catalysts on unmodified and N-
doped carbon supports using the fluidized bed reduction reactor that shows good retention of mass
activity, ECSA<sub>H-upd</sub> and no carbon corrosion when tested in RDE towards durability (C-AST: 5,000 CV,
1-1.5 V<sub>RHE</sub>, 500 mV·s<sup>-1</sup>, Pt-AST: 10,000 SWV, 0.6 V<sub>RHE</sub> and 0.95 V<sub>RHE</sub>, 3 s at each potential).

Operando HE-XRD Experiments

For a more global and detailed investigation of the structural stability of the catalysts, *operando* HE-XRD experiments were performed at the Extremely Brilliant Source of the European Synchrotron Radiation Facility (ESRF-EBS, Grenoble, France) at beamline ID31. Because the availability of beam time and the total experimental time is limited, we decided to subject the catalyst to a combined Pt- and C-AST with a degradation protocol consisting of 5,000 square wave cycles (steps between 0.6 V<sub>RHE</sub> (3 s) and 1.1 V<sub>RHE</sub> (3 s), denoted as durability (D-)AST). Note that half a Pt-AST consisting of only 5,000 cycles between 0.6 and 0.95 V<sub>RHE</sub> at a scan rate of 100 mV·s<sup>-1</sup> did not trigger any measurable degradation for any of the catalysts.<sup>25</sup> Figure S 11 shows the evolution of the X-ray diffraction pattern as excerpt of the Pt[220] Bragg reflection. To further elucidate the structural evolution, a detailed analysis of the diffraction patterns using the Rietveld refinement approach was conducted.<sup>36</sup> For the acquired *operando* diffraction patterns, it was found that one fcc-Pt phase was adequate for a good fit of the crystalline material, herein the crystalline Pt particles. Figure S 12 illustrates the evolution of the CVs during potential cycling and allows to extract the ECSA<sub>H-upd</sub> during potential cycling. Figure 3 illustrated the evolution of ECSA<sub>H-upd</sub>, lattice parameter and coherence length as function of the number of cycles of the D-AST.
<figure><img src="image_5.png" /><figcaption>Figure 3: Electrochemical and structural response of operando liquid cell HE-XRD experiments (5,000 SWV between 0.6 (3 s) and 1.1 V<sub>RHE</sub> (3 s) in 0.1 M HClO<sub>4</sub>) of Pt/V (grey), Pt/N-V-400 (red), Pt/N-V-800 (blue) and comm. Pt/C (HISPEC3000, green). Evolution of a) ECSA<sub>H-upd</sub> as fraction of the maximum value b) lattice parameter *a*, c) coherence length.</figcaption></figure>

The HE-XRD patterns (Figure S 11) reveal an overall sharpening of the reflections with increased maximum intensity during the potential cycling for all catalysts. This can be understood as growth of the particle and is accompanied by the CV shape evolution (Figure S 12): Globally, the total charges of H<sub>upd</sub> region (0.05–0.4 V<sub>RHE</sub>) and Pt-O/OH region (0.7–1 V<sub>RHE</sub>) are decreasing over the course of the D-AST. Interestingly, the H<sub>upd</sub> fingerprint charge and thus the ECSA<sub>H-upd</sub> values are increasing in the first 100 cycles (Figure 3a). This behavior could be explained by a cleaning of the Pt surface as reported in literature during the first 100 cycles of the D-AST.<sup>37</sup> Prior to the D-AST, a conditioning procedure was performed for 100 cycles between 0.05 and 1 V<sub>RHE</sub> at 100 mV/s. The D-AST now has a more oxidative upper potential limit of 1.1 V<sub>RHE</sub>, therefore producing cleaner surface. The conditioning was continued and a maximum value for the ECSA<sub>H-upd</sub> was recorded for all three homemade samples the stage of 100 cycles and 10 cycles for HISPEC3000. The strongest relative increase of the ECSA<sub>H-upd</sub> is found for Pt/V, followed by HISPEC3000, Pt/N-V-800 and Pt/N-V-400, respectively. The lattice parameter *a*, however, is showing a contrary behavior (Figure 3b). The strongest relative contraction of the lattice is found for Pt/N-V-400, followed by Pt/N-V-800 and Pt/V, respectively. Previous works
on Pt nanoparticles strain dynamics revealed that lattice parameter contraction is associated to desorption of various charged species from the surface.<sup>38, 39</sup> HISPEC3000 shows no such contraction.

After the beneficial cleaning of the Pt surface over the first 100 cycles, the degradation phenomena of particle growth due to Ostwald ripening and agglomeration, Pt dissolution and/or particle detachment start to take over with increasing cycle number. The ECSA<sub>H-upd</sub> values are decreasing and in the HE-XRD patterns the Pt reflections start to sharpen up (Figure S 11). Figure 3c shows the continuously increase of the coherent length as indicator of particle growth. Interestingly, the Pt/N-V-400, Pt/N-V-800 and HISPEC3000 samples degrade much faster in ECSA<sub>H-upd</sub> and coherent length than Pt/V. Further, the shape evolution of the CVs reveals a drastic degree of carbon corrosion visible in the evolution of a chinon-hydrochinon redox pair around 0.5–0.75 V<sub>RHE</sub> for Pt/N-V-800 and somewhat lower degree of carbon corrosion for Pt/N-V-400 (Figure S 13). Pt/V and HISPEC3000 have only very little formation of a chinon-hydrochinon redox pair in the CV. Both Pt/N-V catalysts exhibit an enhanced carbon corrosion of the N-doped carbon supports. This interferes with the presumption of enhanced carbon corrosion durability as predicted from earlier work<sup>25</sup> and the preliminary RDE investigation, where none of the catalyst indicated any pronounced carbon corrosion when subjected to the C-AST (5000 cycles, 1–1.5 V<sub>RHE</sub>, 500 mV·s<sup>-1</sup>). In summary, the incorporation of N via pre-oxidation and ammonolysis in the C matrix seems to result rather in an introduction of weak spots then in a stabilization of the C matrix.

It can be concluded that the fluidized bed reactor is an effective tool for the synthesis of Pt-based catalysts. The uniform distribution of the homemade Pt particles and the carbon corrosion resistance of V support exhibit good structural stability for Pt/V, especially in the direct comparison with the Pt/V reference HISPEC3000. Even though Pt/N-V-400 and Pt/N-V-800 were platinized using the same method, the two catalysts show less carbon corrosion resistance and consequently much higher degradation in the *operando* HE-XRD experiment.

## MEA-based Single Cell Measurement

To further investigate the catalyst properties in a more realistic application, MEA-based single cell measurements were conducted with a reproduced synthesis batch of Pt/V, Pt/N-V-400, and Pt/N-V-800. Each deposition of Pt particles on the different carbon materials is subjected to fluctuations and therefore resulting in a unique Pt loading, Pt surface area and particle size distribution. The individual
ORR performances of the catalysts determined using RDE experiments reveal small variations within the error bar (Table S 4). Fraction of the N-moieties from deconvolution of N 1s XP spectra are depicted in Figure S 14 and Table S 5. MEAs for a 10 cm² single-cell were prepared for Pt/V, Pt/N-V-400, Pt/N-V-800 and HISPEC3000 with a geometric Pt mass loading of 0.15 mgPt·cm⁻² and an ionomer to carbon ratio (I/C) of 0.6 at the cathode using a long chain ionomer. The anodes were kept identical to minimize fluctuation and prepared using a commercial 50 wt-% Pt/C catalyst (Umicore) with a geometric Pt mass loading of 0.1 mgPt·cm⁻². Figure 4a illustrates the beginning of life (BOL) H₂/air performance at 100 % relative humidity (RH) (measurement conditions are described in the supporting information). Each polarization curve was corrected for the monopolar plate and conduct resistance. The measurement for this determination was conducted with an MEA without catalyst layer, therefore each part of the system and the cell, except the catalyst layers, is subtracted from the overall contribution.

The BOL H₂/air performance is similar for all three homemade catalysts. At low current densities, Pt/N-V-800 has a slightly higher intrinsic activity than Pt/V and Pt/N-V-400. However, at high current densities a larger potential drop is observed for Pt/N-V-800 indicating oxygen mass transport limitations. This corresponds well with similar observations by Ott *et al.* who reported larger potential drop at high current densities when using N-doped Ketjenblack ammonolyzed at elevated temperature (600 °C) as carbon support for the Pt/C cathode at 200 kPa<sub>abs</sub>, suggesting that further improvement of the cell components (flow field, gas diffusion layer, etc.) are necessary for optimized oxygen supply.<sup>12</sup> When comparing the BOL H₂/air performance of the homemade Pt/V and the commercial Pt/C of HISPEC3000 (Figure S 15), both catalyst reveal a comparable performance even though the homemade Pt/V has a smaller Pt particle size compared to HISPEC3000as derived from analysis of TEM images, indicating that the preparation of the catalyst materials using the fluidized bed reduction reactor results in good Pt utilization competing with commercial catalysts.
<figure><img src="image_6.png" /></figure>

<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" /><figcaption>Figure 4: Fuel cell performance of Pt/V (grey), Pt/N-V-400 (red) and Pt/N-V-800 (blue) under potential cycling. a) Beginning of life polarization curve of 10 cm² MEA. b) Evolution of voltage @ 0.8 A·cm⁻² of Pt-AST protocol (30,000 SWV between 0.6 (3 s) and 0.95 V (3 s)). c) BOL and EOL ECSA of Pt-AST. d) Evolution of voltage @ 0.6 A·cm⁻² of C-AST protocol (5,000 CV between 1.0 and 1.5 V at a scan rate of 500 mV·s⁻¹). 3) Relative fraction of carbon₈MEA loss as determined via NDIR of C-AST.</figcaption></figure>
Following the preliminary RDE investigations and the suggested DoE protocols, two AST were conducted: Pt-AST (30,000 square wave voltammograms (SWV) between 0.6 and 0.95 V with 3 s holding time at each potential) and C-AST (5,000 CV, 1–1.5 V, 500 mV·s⁻¹).³⁴ One metric to evaluate the evolution of the polarizations curves is the investigation of the voltage loss at certain current densities.

Figure 4b and Table 3 illustrate the voltage loss at 0.8 A·cm⁻² for Pt-AST. All three homemade catalyst present a comparable degradation behavior. Each catalyst has a drastic voltage loss over the first 5,000 cycles, followed by further moderate voltage losses over the next 25,000 cycles. This results in comparable total voltage losses for Pt/V and Pt/N-V-800 and slightly higher voltage losses for Pt/N-V-400. This trend is accompanied by the evolution of the ORR mass activity where Pt/N-V-400 has a lower retention than Pt/V and Pt/N-V-800 (Figure S 16). However, the overall performance of Pt/N-V-800 is better over the course of the Pt-AST. Figure 4c and Table 3 report the ECSA evolution during the Pt-AST. Pt/V exhibits with 72 ± 2 cm²·gPt⁻¹ the highest ECSA_H-upd measured in the MEA setup. Interestingly, both Pt/N-V-400 and Pt/N-V-800 expose a significantly lower ECSA_H-upd of only 48 and 45 m²·gPt⁻¹, respectively. Further research is needed to explain this behavior because several components in the CCL could be accountable.

During the Pt-AST, all catalysts suffer from severe loss of the ECSA<sub>H-upd</sub>, highest for Pt/N-V-400 with 68 % and somewhat lower for Pt/V and Pt/N-V-800 with ~60 %. These results indicate that the catalyst degradation of Pt/V and Pt/N-V-800 is comparable, with the latter having a better overall performance during the Pt-AST, thus making it an overall better catalyst for the ORR. Pt/N-V-400 reaches only a moderate performance, leading to the conclusion, that the different material properties of Pt/N-V-400 and Pt/N-V-800 have a significant influence on the performance of the catalysts. Those could be attributed to the different ammonolysis temperatures of 400 °C and 800 °C. It could be deduced that a maximized N content is no guarantor for improved MEA performance. The environment of the N species seems to be crucial. Here, Pt/N-V-800 consists of a larger fraction of pyridinic-N moieties (Figure S 14c and Table S 5). The basic character of such pyridinic-N that could in combination act with the acidic sulfonic groups of the Nafion ionomer as Brønsted acid-base pair resulting in a better distributed ionomer and thus better MEA performance. It must be noted that these effects become more prominent when measurements are performed in dry conditions (reduced RH values) but these are beyond the scope of this study. Another beneficial impact could be the increased surface area and broadened pore structure of the carbon when treated at 800 °C, resulting in an improved mass transport.⁴⁰ Altogether, the various indicators meet in a sweet spot for an optimized modified carbon support material for improved MEA performance, that will need further investigation to disentangle the various lose ends.
For the C-AST, the potential loss at 0.6 A·cm⁻² is investigated and depicted in Figure 4d and Table 3. Both Pt/N-V catalysts show better initial performance at this current density, however after the first 200 cycles a significantly higher potential loss is observed. While Pt/V shows no significant potential drop, Pt/N-V-400 and Pt/N-V-800 reveal a potential loss of 115 mV and 66 mV, respectively. Over the course of the 5,000 cycles of the C-AST, all catalysts end up with severe degradation of the catalyst layer under MEA operating conditions. This is in contrast with the assumptions drawn from TGA measurements that suggested higher thermal stability of the N-doped carbon supports (Figure S 2). The carbon corrosion was observed with a non-dispersive infrared (NDIR) analyzer located at the cathode gas exhaust, measuring the gas concentrations of CO and CO₂ *online* (Figure S 17). This method has the significant advantage over the conventional mass spectrometer measurements that the volatile carbon corrosion products can be continuously measured during the C-AST without the need to take aliquots. Though, only specific gases can be measured, in this case O₂, CO and CO₂. The measured combined concentrations of CO and CO₂ (COₓ) can then be integrated and converted into a carbon mass. Factoring in the carbon loading of the catalyst layer, the relative loss of carbon can be extracted (Figure 4e). This carbon mass originates from the whole single-cell measurement setup and all carbon-based materials must be considered when attributing the carbon loss: catalyst material of the cathode (Pt/C), ionomer (Nafion membrane and additional ionomer in the catalyst layer), gas diffusion layer, graphite flow field, gaskets. All single cell components were kept identical besides the cathode catalyst material. Therefore, the C-AST was also performed with an MEA without cathode catalyst layer, indicating no significant carbon corrosion during the 5000 cycles, leading to the conclusion, that the measured CO and CO₂ are mainly evolved carbon from the catalyst layer. The carbon corrosion of all three catalysts over the first 1,000 cycles is similar, indicating that the V carbon and its N-doped derivates have the same carbon degradation pattern independent of the synthesis procedure (Figure 4e). This contrasts with the performance trends observed as the potential loss at 0.6 A·cm⁻² over the course of the C-AST. The data suggests that while the total amount of oxidized carbon is equal for Pt/V, Pt/N-V-400 and Pt/N-V-800, the influence on the MEA performance is much more detrimental, giving rise to significative differences. A possible explanation for this could be that the integration of defects, in form of nitrogen (and oxygen) atoms, into the support matrix facilitates pore collapsing after high potential excursions by destabilizing the carbon structure, which would then lead to the decreased performance of the N-doped carbons.
<table><caption>Table 3: BOL and EOL MEA-based fuel cell performance (at 0.8 and 0.6 A cm⁻² for the Pt-AST and C-AST protocol, respectively), ECSA, catalytic (mass-based) activity evaluated at 0.9 V for the Pt-AST protocol (30,000 SWV between 0.6 (3 s) and 1.1 V (3 s)) and for the C-AST protocol (5,000 CV between 1.0 and 1.5 V at a scan rate of 500 mV·s⁻¹).</caption><thead><tr><th></th><th>BOL Performance<br>at 0.8 A·cm<sup>-2</sup> / mV</th><th>EOL Performance<br>at 0.8 A·cm<sup>-2</sup> / mV</th><th>Performance loss<br>at 0.8 A·cm<sup>-2</sup> / mV</th><th>BOL ECSA /<br>m<sup>2</sup>·g<sub>Pt</sub><sup>-1</sup></th><th>EOL ECSA /<br>m<sup>2</sup>·g<sub>Pt</sub><sup>-1</sup></th><th>loss of initial<br>ECSA / %</th><th>BOL catalytic<br>activity / A·g<sub>Pt</sub><sup>-1</sup></th><th>EOL catalytic<br>activity / A·g<sub>Pt</sub><sup>-1</sup></th><th>loss of initial<br>catalytic activity / %</th></tr></thead><tbody><tr><td><strong>Pt-AST</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Pt/V</td><td>663 ± 7</td><td>579 ± 3</td><td>85</td><td>72 ± 2</td><td>29 ± 3</td><td>59</td><td>54 ± 1</td><td>53 ± 6</td><td>2</td></tr><tr><td>Pt/N-V-400</td><td>659 ± 2</td><td>564 ± 6</td><td>95</td><td>48 ± 5</td><td>16 ± 1</td><td>68</td><td>58 ± 5</td><td>36 ± 2</td><td>37</td></tr><tr><td>Pt/N-V-800</td><td>676 ± 3</td><td>589 ± 17</td><td>86</td><td>45 ± 2</td><td>18 ± 3</td><td>60</td><td>58 ± 5</td><td>53 ± 4</td><td>9</td></tr><tr><td>HISPEC3000</td><td>676 ± 22</td><td>585 ± 39</td><td>91</td><td>60 ± 2</td><td>21 ± 10</td><td>64</td><td>64 ± 14</td><td>55 ± 7</td><td>15</td></tr><tr><td><strong>C-AST</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Pt/V</td><td>667 ± 7</td><td>468</td><td>201</td><td>72 ± 2</td><td>-</td><td>-</td><td>54 ± 1</td><td>80</td><td>-46 (gain)</td></tr><tr><td>Pt/N-V-400</td><td>683 ± 7</td><td>322 ± 11</td><td>336</td><td>48 ± 5</td><td>-</td><td>-</td><td>58 ± 5</td><td>59 ± 8</td><td>-2 (gain)</td></tr><tr><td>Pt/N-V-800</td><td>682 ± 2</td><td>321 ± 1</td><td>351</td><td>45 ± 2</td><td>-</td><td>-</td><td>58 ± 5</td><td>49 ± 7</td><td>16</td></tr><tr><td>HISPEC3000</td><td>675 ± 10</td><td>444 ± 12</td><td>236</td><td>60 ± 2</td><td>-</td><td>-</td><td>64 ± 14</td><td>71 ± 1</td><td>-11 (gain)</td></tr></tbody></table>
Conclusion

In this study, Pt electrocatalysts on three different Vulcan-based carbon supports for the ORR were prepared using a customized fluidized bed reduction reactor. One support material was a commercial Vulcan used as reference for this study (Pt/V). The other two support materials were N-doped Volcans with different ammonolysis temperatures (Pt/N-V-400 and Pt/N-V-800). The three homemade catalysts were compared to a commercial Pt/C (HISPEC3000). The physicochemical characterization of the supports disclosed the importance of the synthesis temperature: Pt/N-V-400 has the highest level of heteroatom-doping, the lowest ZP as well as the smallest fraction of micropores. The other two catalysts, Pt/V and Pt/N-V-800, have comparable fractions of micropores and a similar ZP yet a higher level of heteroatom-doping for Pt/N-V-800. In RDE measurements, all three catalysts have initially similar mass-based activity and ESCA<sub>H-upd</sub> values and maintain them after both Pt and carbon corrosion degradation protocol testing, showing no enhanced influence on the performance of the N-doping of the supports in acidic electrolyte. The catalysts were subjected to a more severe stability test and investigated using *operando* HE-XRD experiments at beamline ID31 at the ESRF to discover a link for the evolution of structural and electrochemical characteristics. The experiments revealed a superior stability of the Pt/V sample in liquid acidic media that outperformed the commercial Pt/C (HISPEC3000) reference material. However, Pt/N-V-400 and Pt/N-V-800 exhibited a proneness towards degradation phenomena, especially carbon corrosion, within the D-AST. MEA measurements were conducted to test the catalysts in a PEMFC environment. The different support materials had a minor impact on the durability during Pt-AST, Pt/N-V-800 and Pt/V revealed similar degradation behavior, whereas Pt/N-V-400 had slightly higher performance losses over the course of the Pt-AST. However, Pt/N-V-800 demonstrated a better overall performance compared to Pt/V, suggesting a higher activity under operating conditions for the N-doped carbon support. We suggested that this could be due to an improved ionomer distribution due to higher amounts of pyridinic-N moieties in the carbon matrix. For the C-AST, the heteroatom-doping of the support material seems detrimental as the performance loss is severely higher compared to the unmodified carbon, while all catalysts emit similar amounts of carbonous gases as observed with NDIR measurements. This leads us to the conclusion, that the heteroatom-doping and thereby the incorporation of defects into the carbon matrix does not benefit the carbon corrosion durability under high potential cycling (1–1.5 V) but can favor the performance under operating conditions (0.6–0.9 V). The investigation of the carbon corrosion is an important tool to gain further insight into the degradation behavior of the support material occurring while optimizing the performance of ORR catalysts in PEMFCs. N-doped carbons used as support for Pt-based ORR electrocatalysts represent a promising
option to further tweak the three-phase boundary at the active sites by improving the ionomer distribution on the support material.
Acknowledgement

This project received financial support from the German Federal Ministry of Education and Research (BMBF) through grant 03XP0251 ("KorrZellKat") and grant 03SF0531B ("HT-linked"). We thank Juana Krone for the elemental analysis and Astrid Müller-Klaucke for ICP-OES measurements. We thank the Zentraleinrichtung Elektronenmikroskopie of Technische Universität Berlin and Stefanie Kühl for their support with electron microscopy measurements. We thank Detre Teschner and the Department of Inorganic Chemistry of the Fritz-Haber-Institut of the Max-Planck-Gesellschaft for providing access to the XPS instrument. We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities.
References

1. Gröger, O.; Gasteiger, H. A.; Suchsland, J.-P., Review—Electromobility: Batteries or Fuel Cells? *Journal of The Electrochemical Society* **2015**, *162* (14), A2605-A2622.

2. Cunanan, C.; Tran, M.-K.; Lee, Y.; Kwok, S.; Leung, V.; Fowler, M., A Review of Heavy-Duty Vehicle Powertrain Technologies: Diesel Engine Vehicles, Battery Electric Vehicles, and Hydrogen Fuel Cell Electric Vehicles. *Clean Technologies* **2021**, *3* (2), 474-489.

3. Kongkanand, A.; Mathias, M. F., The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells. *J Phys Chem Lett* **2016**, *7* (7), 1127-37.

4. Orfanidi, A.; Madkikar, P.; El-Sayed, H. A.; Harzer, G. S.; Kratky, T.; Gasteiger, H. A., The Key to High Performance Low Pt Loaded Electrodes. *Journal of The Electrochemical Society* **2017**, *164* (4), F418-F426.

5. Subbaraman, R.; Strmcnik, D.; Stamenkovic, V.; Markovic, N. M., Three Phase Interfaces at Electrified Metal-Solid Electrolyte Systems 1. Study of the Pt(hkl)-Nafion Interface. *J. Phys. Chem. C*, **2010**, *144*, 8414-8422.

6. Subbaraman, R.; Strmcnik, D.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M., Oxygen reduction reaction at three-phase interfaces. *ChemPhysChem* **2010**, *11* (13), 2825-33.

7. Kodama, K.; Motobayashi, K.; Shinohara, A.; Hasegawa, N.; Kudo, K.; Jinnouchi, R.; Osawa, M.; Morimoto, Y., Effect of the Side-Chain Structure of Perfluoro-Sulfonic Acid Ionomers on the Oxygen Reduction Reaction on the Surface of Pt. *ACS Catalysis* **2017**, *8* (1), 694-700.

8. Suzuki, T.; Kudo, K.; Morimoto, Y., Model for investigation of oxygen transport limitation in a polymer electrolyte fuel cell. *Journal of Power Sources* **2013**, *222*, 379-389.

9. Weber, A. Z.; Kusoglu, A., Unexplained transport resistances for low-loaded fuel-cell catalyst layers. *J. Mater. Chem. A* **2014**, *2* (41), 17207-17211.

10. Jinnouchi, R.; Kudo, K.; Kitano, N.; Morimoto, Y., Molecular Dynamics Simulations on O 2 Permeation through Nafion Ionomer on Platinum Surface. *Electrochimica Acta* **2016**, *188*, 767-776.

11. Kudo, K.; Jinnouchi, R.; Morimoto, Y., Humidity and Temperature Dependences of Oxygen Transport Resistance of Nafion Thin Film on Platinum Electrode. *Electrochimica Acta* **2016**, *209*, 682-690.

12. Ott, S.; Orfanidi, A.; Schmies, H.; Anke, B.; Nong, H. N.; Hubner, J.; Gernert, U.; Gliech, M.; Lerch, M.; Strasser, P., Ionomer distribution control in porous carbon-supported catalyst layers for high-power and low Pt-loaded proton exchange membrane fuel cells. *Nat Mater* **2020**, *19* (1), 77-85.

13. Arrigo, R.; Hävecker, M.; Wrabetz, S.; Blume, R.; MLerch, M.; McGregor, J.; Parrott, E. P. J.; Zeitler, A.; Gladden, L. F.; Knop-Gericke, A.; Schlögl, R.; Sheng Su, D., Tuning the Acid/Base Properties of Nanocarbons by Functionalization via Amination. *Journal of the American Chemical Society* **2010**, *132*, 9616-9630.

14. Yarlagadda, V.; Carpenter, M. K.; Moylan, T. E.; Kukreja, R. S.; Koestner, R.; Gu, W.; Thompson, L.; Kongkanand, A., Boosting Fuel Cell Performance with Accessible Carbon Mesopores. *ACS Energy Letters* **2018**, *3* (3), 618-621.

15. Banham, D.; Zou, J.; Mukerjee, S.; Liu, Z.; Yang, D.; Zhang, Y.; Peng, Y.; Dong, A., Ultralow platinum loading proton exchange membrane fuel cells: Performance losses and solutions. *Journal of Power Sources* **2021**, *490*.

16. Pan, L.; Ott, S.; Dionigi, F.; Strasser, P., Current challenges related to the deployment of shape-controlled Pt alloy oxygen reduction reaction nanocatalysts into low Pt-loaded cathode layers of proton exchange membrane fuel cells. *Current Opinion in Electrochemistry* **2019**, *18*, 61-71.

17. Shao-Horn, Y.; Sheng, W. C.; Chen, S.; Ferreira, P. J.; Holby, E. F.; Morgan, D., Instability of Supported Platinum Nanoparticles in Low-Temperature Fuel Cells. *Topics in Catalysis* **2007**, *46* (3-4), 285-305.
18. Eastwood, B. J.; Christensen, P. A.; Armstrong, R. D.; Bates, N. R., Electrochemical oxidation of a carbon black loaded polymer electrode in aqueous electrolytes. *J Solid State Electrochem* **1999**, *3* (179-186).

19. Maass, S.; Finsterwalder, F.; Frank, G.; Hartmann, R.; Merten, C., Carbon support oxidation in PEM fuel cell cathodes. *Journal of Power Sources* **2008**, *176* (2), 444-451.

20. Maillard, F.; Bonnefont, A.; Micoud, F., An EC-FTIR study on the catalytic role of Pt in carbon corrosion. *Electrochemistry Communications* **2011**, *13* (10), 1109-1111.

21. Macauley, N.; Papadias, D. D.; Fairweather, J.; Spernjak, D.; Langlois, D.; Ahluwalia, R.; More, K. L.; Mukundan, R.; Borup, R. L., Carbon Corrosion in PEM Fuel Cells and the Development of Accelerated Stress Tests. *Journal of The Electrochemical Society* **2018**, *165* (6), F3148-F3160.

22. Reiser, C. A.; Bregoli, L.; Patterson, T. W.; Yi, J. S.; Yang, J. D.; Perry, M. L.; Jarvi, T. D., A Reverse-Current Decay Mechanism for Fuel Cells. *Electrochemical and Solid-State Letters* **2005**, *8* (6).

23. Willisau, J.; Heitbaum, J., The influence of Pt-activation on the corrosion of carbon in gas diffusion electrodes - A DEMS study*. *J. Electroanal. Chem* **1984**, *161*, 93-101.

24. SIROMA, Z.; TANAKA, M.; TANIMOTO, K.; YASUDA, K.; Inaba, M.; TASAKA, A., Electrochemical Corrosion of Carbon Materials in an Aqueous Acid Solution. *Electrochemistry Communications* **2006**, *7*, 258-260.

25. Schmies, H.; Hornberger, E.; Anke, B.; Jurzinsky, T.; Nong, H. N.; Dionigi, F.; Kühl, S.; Drnec, J.; Lerch, M.; Cremers, C.; Strasser, P., Impact of Carbon Support Functionalization on the Electrochemical Stability of Pt Fuel Cell Catalysts. *Chemistry of Materials* **2018**, *30* (20), 7287-7295.

26. Hornberger, E.; Schmies, H.; Paul, B.; Kühl, S.; Strasser, P., Design and Validation of a Fluidized Bed Catalyst Reduction Reactor for the Synthesis of Well-Dispersed Nanoparticle Ensembles. *Journal of The Electrochemical Society* **2020**, *167* (11), 114509.

27. McNaught, A. D.; Wilkinson, A., *IUPAC. Compendium of Chemical Terminology*. 2nd ed.; Blackwell Scientific Publications: Oxford, 1997.

28. Orfanidi, A.; Rheinländer, P. J.; Schulte, N.; Gasteiger, H. A., Ink Solvent Dependence of the Ionomer Distribution in the Catalyst Layer of a PEMFC. *Journal of The Electrochemical Society* **2018**, *165* (14), F1254-F1263.

29. Jaouen, F.; Dodelet, J.-P., Non-noble electrocatalysts for O2 reduction: how does heat treatment affect their activity and structure? Part I. Model for carbon black gasification by NH3: parametric calibration and electrochemical validation. *J. Phys. Chem. C* **2007**, *111*, 5963-5970.

30. Sherwook, T. K.; Gilliland, E. R.; Ing, S. W. J., Hydrogen cyanide: Synthesis from elements and from ammonia and carbon. *Indust. Eng. Chem.* **1960**, *52* (7).

31. Toupin, M.; Bélanger, D., Spontaneous Functionalization of Carbon Black by Reaction with 4-Nitrophenyldiazonium Cations. *Langmuir* **2008**, *24* (5), 1910-1917.

32. Zhang, Z.; Ma, M.; Chen, C.; Cai, Z.; Huang, X., The morphology, structure and electrocatalytic ability of graphene prepared with different drying methods. *RSC Advances* **2016**, *6* (33), 28005-28014.

33. Arrigo, R.; Havecker, M.; Schlogl, R.; Su, D. S., Dynamic surface rearrangement and thermal stability of nitrogen functional groups on carbon nanotubes. *Chem Commun (Camb)* **2008**, (40), 4891-3.

34. Fuel Cell technical Roadmap. https://www.energy.gov/sites/prod/files/2017/11/f46/FCTT_Roadmap_Nov_2017_FINAL.pdf (accessed 08/03/2021).

35. Qiao, Z.; Wang, C.; Zeng, Y.; Spendelow, J. S.; Wu, G., Advanced Nanocarbons for Enhanced Performance and Durability of Platinum Catalysts in Proton Exchange Membrane Fuel Cells. *Small* **2021**, e2006805.

36. Rietveld, H. M., A profile refinement method for nuclear and magnetic structures. *J. Appl. Crystallogr.* **1969**, *2*, 65-71.
37. Harzer, G. S.; Schwämmlein, J. N.; Damjanović, A. M.; Ghosh, S.; Gasteiger, H. A., Cathode Loading Impact on Voltage Cycling Induced PEMFC Degradation: A Voltage Loss Analysis. *Journal of The Electrochemical Society* **2018**, *165* (6), F3118-F3131.

38. Martens, I.; Chattot, R.; Rasola, M.; Blanco, M. V.; Honkimäki, V.; Bizzotto, D.; Wilkinson, D. P.; Drnec, J., Probing the Dynamics of Platinum Surface Oxides in Fuel Cell Catalyst Layers Using in Situ X-Ray Diffraction. *ACS Appl. Energy Mater.* **2019**, *2* (11), 7772-7780.

39. Chattot, R.; Martens, I.; Mirolo, M.; Ronovsky, M.; Russello, F.; Isern, H.; Braesch, G.; Hornberger, E.; Strasser, P.; Sibert, E.; Chatenet, M.; Honkimäki, V.; Drnec, J., Electrochemical Strain Dynamics in Noble Metal Nanocatalysts. *J. Am. Chem. Soc.* **2021**, *143* (41), 17068-17078.

40. Ott, S.; Bauer, A.; Du, F.; Dao, T. A.; Klingenhof, M.; Orfanidi, A.; Strasser, P., Impact of Carbon Support Meso-Porosity on Mass Transport and Performance of PEMFC Cathode Catalyst Layers. *ChemCatChem* **2021**.