# Monitoring the Morphological Changes of Skeleton-PtCo Electrocatalyst during PEMFC Start-Up/Shut-Down probed by in situ WAXS and SAXS

Marek Janssen, Jakub Drnec, Isaac Martens, Jonathan Quinson, Rebecca Pittkowski, Daesung Park, Philipp Weber, Matthias Arenz, Mehtap Oezaslan

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## ► To cite this version:

Marek Janssen, Jakub Drnec, Isaac Martens, Jonathan Quinson, Rebecca Pittkowski, et al., Monitoring the Morphological Changes of Skeleton-PtCo Electrocatalyst during PEMFC Start-Up/Shut-Down probed by in situ WAXS and SAXS. ChemSusChem, 2024, pp.e202400303. 10.1002/cssc.202400303. [https://hal.science/hal-05098740v1](https://hal.science/hal-05098740v1)

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[https://hal.science/hal-05098740v1](https://hal.science/hal-05098740v1)

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# Monitoring the Morphological Changes of Skeleton-PtCo Electrocatalyst during PEMFC Start-Up/Shut-Down probed by in situ WAXS and SAXS

Marek Janssen,[a] Jakub Drnec,[b] Isaac Martens,[b] Jonathan Quinson,[c] Rebecca Pittkowski,[d] Daesung Park,[e, f] Philipp Weber,[a] Matthias Arenz,[g] and Mehtap Oezaslan*[a]

Advanced *in situ* analyses are indispensable for comprehending the catalyst aging mechanisms of Pt-based PEM fuel cell cathode materials, particularly during accelerated stress tests (ASTs). In this study, a combination of *in situ* small-angle and wide-angle X-ray scattering (SAXS & WAXS) techniques were employed to establish correlations between structural parameters (crystal phase, quantity, and size) of a highly active skeleton-PtCo (sk-PtCo) catalyst and their degradation cycles within the potential range of the start-up/shut-down (SUSD) conditions. Despite the complex case of the sk-PtCo catalyst comprising two distinct fcc alloy phases, our complementary techniques enabled *in situ* monitoring of structural changes in

each crystal phase in detail. Remarkably, the *in situ* WAXS measurements uncover two primary catalyst aging processes, namely the cobalt depletion (*regime I*) followed by the crystal-lite growth *via* Ostwald ripening and/or particle coalescence (*regime II*). Additionally, *in situ* SAXS data reveal a continuous size growth over the AST. The Pt-enriched shell thickening based on the Co depletion within the first 100 SUSD cycles and particle growth induced by additional potential cycles were also collaborated by *ex situ* STEM-EELS. Overall, our work shows a comprehensive aging model for the sk-PtCo catalyst probed by complementary *in situ* WAXS and SAXS techniques.

## 1. Introduction

Major challenges of polymer electrolyte membrane fuel cells (PEMFCs), which need to overcome for a broad commercialization, are the insufficient long-term durability and high loadings

of scarce and expensive cathode catalyst materials, such as platinum group metals (PGMs).[1-5] Over the last decades, the performance of Pt-based catalysts for the oxygen reduction reaction (ORR) has been enhanced by maximizing the number of active sites and/or by altering the binding energies of the oxygen-containing intermediate species, which are induced by the critical interplay between electronic and geometric effects.[6-10] One of the pioneering works on the field of Pt-Co catalysts showing their high ORR performance was published by Toda et al.[11] Nowadays, the most promising catalyst material classes are the acid leached and electrochemically dealloyed Pt-based nanoparticles (NPs).[10,12-16] They are prepared by the selective dissolution of the less noble metal from the alloy NPs either by acid treatment or electrochemical dealloying, resulting in the formation of a Pt-enriched surface layer/shell. The electrochemical dealloying generates core-shell NPs with several monolayers of Pt.[12-14,17] In contrast, the acid leaching of the Pt-M alloy NPs (M=transition metals such as Co, Ni, etc.) forms a rough skeleton surface consisting of only a few Pt monolayers.[10,15,16,18-21] Controlling the structural parameters such as particle size, atomic composition, crystal structure, and Pt shell thickness for core-shell or skeleton Pt-based catalysts is crucial to tune their ORR activity and long-term durability. Moreover, the utilization of the Pt-skeleton catalysts has several advantages because of flexible handling and less costly manufacturing compared to the electrochemically treated catalysts. Chen et al. reported a two-fold increase in the ORR activity of the skeleton PtCo NPs with particle size of 4.1 nm compared to pure Pt NPs with 3.6 nm size.[22] They proposed that the ORR activity enhancement is caused by the formation of a percolated structure of a Pt-rich surface and Pt-poor core

[a] M. Janssen, P. Weber, M. Oezaslan
Technical Electro catalysis Laboratory, Institute of Technical Chemistry,
Technische Universität Braunschweig, 38106 Braunschweig, Germany
E-mail: m.oezaslan@tu-braunschweig.de

[b] J. Drnec, I. Martens
European Synchrotron Radiation Facility (ESRF), 38000 Grenoble, France

[c] J. Quinson
Biological and Chemical Engineering Department, Aarhus University, 40
Åbogade, 8200 Aarhus, Denmark

[d] R. Pittkowski
Department of Chemistry, University of Copenhagen, 2100 Copenhagen,
Denmark

[e] D. Park
Physikalisch-Technische Bundesanstalt (PTB), 38116 Braunschweig, Germany

[f] D. Park
Laboratory of Emerging Nanometrology (LENA), Technische Universität
Braunschweig, 38106 Braunschweig, Germany

[g] M. Arenz
Department of Chemistry, Biochemistry and Pharmaceutical Sciences,
University of Bern, 3012 Bern, Switzerland

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Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cssc.202400303

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© 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH. This is
an open access article under the terms of the Creative Commons Attribution
Non-Commercial License, which permits use, distribution and reproduction
in any medium, provided the original work is properly cited and is not used
for commercial purposes.
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regions within the individual NPs. Moreover, Durst et al. investigated the structure reversibility of PtCo catalyst after alternating treatment with acid and heat.[18] The heat treatment leads to the formation of a Pt skin and smooth particle surface, which can be roughened again after the acid leaching.

Despite the immense efforts in the last 20 years, most highly active Pt-based cathode catalyst materials do not fulfill the required durability and cost targets proposed by the US Department of Energy (DOE).[23] The cathode catalyst materials strongly suffer from the highly corrosive conditions that occur in particular during start-up/shut-down (SUSD) events of the PEMFCs.[24] During a SUSD event, a H₂/air gas front passes through the anode, leading to a rise of the cathode voltage up to ~1.50 V<sub>RHE</sub>.[25] This high cathode voltage leads to fatal damage of the catalyst support material.[25-28] Additional degradation processes can also occur during the operation of PEMFC and are generally classified into: (electro)chemical dissolution of Pt and/or less noble metal, Ostwald ripening based on the dissolution of smaller NPs and their re-deposition onto larger NPs, coalescence *via* particle migration and particle detachment due to carbon corrosion.[29-32]

In order to optimize the lifetime of Pt-based ORR catalyst materials, a deeper understanding of the degradation mechanisms is necessary by using advanced *operando*, *in situ* and *ex situ* characterization techniques. For instance, Wang et al. showed similar electrochemical durability behavior of different 4–5 nm skeleton PtCo NPs (Pt:Co atomic ratio of 1.25–1.4) compared to Pt/C (~2 nm) during the potential cycling between 0.60 and 1.20 V<sub>RHE</sub>.[33] The skeleton PtCo NPs (3–4 nm, Pt:Co ratio 3.7:1) prepared by Kang et al. exhibited comparable performance loss to commercial Pt/C (~3.5 nm) during 10k cycles between 0.60–1.10 V<sub>RHE</sub>, which was explained by the thickening of Pt-enriched shell (loss of Co content by 20–40 at.%) and size growth to ~5 nm (+42%).[34] Therefore, *in situ* X-ray based techniques such as (anomalous) small-angle X-ray scattering ((A)SAXS), wide-angle X-ray scattering (WAXS), X-ray diffraction (XRD) or X-ray absorption spectroscopy (XAS) are very powerful tools to correlate the changes in catalyst structure with the electrochemical parameters during accelerated stress tests (ASTs).[35-49] Recently, Martens et al. have applied simultaneously *in situ* XRD and SAXS techniques to distinguish between Ostwald ripening, migration, and coalescence processes of ~2 nm Pt/C catalyst during 5k potential steps (holding time of 3 s per step) from 0.60 to 1.10 V<sub>RHE</sub>.[41] The first 400–500 cycles accelerate the particle migration due to the weak particle – support interactions followed by a growth of the particles up to ~6 nm which is controlled by Ostwald ripening and coalescence.

Furthermore, Miller et al. reported a gradual electrochemical Co dissolution of PtCo catalyst as main degradation process during potential cycling (30k cycles, 0.60–1.00 V<sub>RHE</sub>) based on the Co K edge X-ray absorption near edge spectroscopy (XANES) data.[50] Analysis of the Pt L<sub>III</sub> edge extended X-ray absorption fine structure spectroscopy (EXAFS) data also showed a decrease in the partial coordination number of Pt–Co, indicating a loss of Co atoms during the AST experiment. From the Co K edge XANES analysis, they claimed that the Co

dissolution directly occurs at the beginning of the AST. In another study, Gilbert et al. evaluated changes in particle size distributions of ~5 nm Pt<sub>3</sub>Co core-shell catalyst depending on the AST protocol using ASAXS technique.[51] They observed the dissolution of smaller NPs (below 3 nm) over the course of the AST experiments (triangular profile from 0.60–1.00 V<sub>RHE</sub>, square wave from 0.40–1.05 V<sub>RHE</sub> with 10 s hold, both for 1.5 k cycles). Furthermore, the Pt<sub>3</sub>Co core-shell NPs, ranging from 3 nm to 5 nm in size show a substantial loss of cobalt accompanied with an enhanced dissolution of Pt. A skeleton Pt<sub>73</sub>Ni<sub>27</sub> catalyst (~5 nm) reported by Wang et al. shows improved ORR performance and durability compared to those of ~6 nm Pt/C.[52] During the AST (4k cycles between 0.6–1.1 V<sub>RHE</sub> at 60 °C), the ECSA and ORR activity decrease by ~10% and 57%, respectively. *In situ* XANES and *ex situ* elemental analyses were applied to correlate the ECSA and activity losses with a strong relative Ni depletion of 57% (from Pt<sub>73</sub>Ni<sub>27</sub> to Pt<sub>87</sub>Ni<sub>13</sub>).[52]

Very recently, Schröder et al. comprehensively investigated the degradation behavior of a bimodal Pt/C (mean particle sizes of 1–2 nm and 4–5 nm) catalyst by *in situ* SAXS and WAXS techniques.[38,53] They showed that the degradation processes of the bimodal Pt/C catalyst are strongly associated with the applied AST parameters (e.g. upper vertex potential, triangular vs. square-wave, cycle number). They also pointed out that the Ostwald ripening is the dominant degradation mechanism for smaller NPs.[38] The smaller NPs (1–2 nm) are prone to grow faster particularly under these AST conditions: 25 repeats of steps between 0.66 and 1.06 V<sub>RHE</sub> (3 s holding time of each), and afterwards two cyclic voltammetry profiles between 1.06 and 1.56 V<sub>RHE</sub> (100 mV s<sup>-1</sup>).[38] In addition, they investigated the catalyst layer dependent degradation behavior of the bimodal Pt/C in another AST protocol (5k potential steps from 0.60 to 1.00 V<sub>RHE</sub>, 3 s holding time per step). They reported that Ostwald ripening is predominant at the catalyst layer-electrolyte interface for smaller Pt NPs (1–2 nm), while particle loss of the larger Pt NPs (4–5 nm) prevails within the deeper catalyst layers.[53]

In this study, we prepared highly ORR active skeleton-PtCo (sk-PtCo) catalyst consisting of two disordered face-centered cubic (fcc) alloy phases with different compositions and sizes and comprehensively investigated their aging under very harsh conditions (800 cycles, triangular profile from 0.50–1.50 V<sub>RHE</sub> with 50 mV s<sup>-1</sup>). Complementary *in situ* WAXS and SAXS techniques were applied to correlate the changes in lattice parameter, phase quantity and particle/crystallite size of the individual phase within the sk-PtCo catalyst with the cycle number of the AST protocol. Based on our findings from structural characterization combined with the electrochemical results, we develop a more descriptive degradation model of the sk-PtCo catalyst in correlation with the number of SUSD cycles during the AST protocol.
## 2. Experimental Part

### 2.1. Synthesis of sk-PtCo Alloy NPs Supported on High Surface Area Carbon (HSAC)

Precursor PtCo alloy NPs supported on HSAC with atomic ratio of 1:1 were prepared by using wet impregnation – freezing – thermal annealing under reductive atmosphere as reported previously.[54,55] 425 mg of Co(NO₃)₂·6 H₂O (Alfa Aesar, 99.999%) were dissolved in 8.5 mL of ultra-purified water (MembraPure, Astacus2, 20 MΩ cm at 25 °C) and added to 1000 mg commercial 28.5 wt.% Pt/HSAC catalyst (TEC10E30E, Tanaka Kikinzoku International Inc., TKK). The suspension was shortly ultra-sonicated and mixed (Scientific Industries, Vortex Genie2) before horn-sonication (Branson Digital Sonifier 250d, amplitude of 10%) for 30 minutes. Afterwards, the viscous black slurry was immediately frozen in liquid nitrogen and freeze-dried (Model P10-85, Dieter Piatkowski Forschungsgeräte) in vacuum (0.021 mbar) for 48 to 72 h. The impregnated and dried powder was annealed under reductive atmosphere (5 vol.% H₂/ 95 vol.% Ar, supplied by Westfalengas AG, quality of 5.0) with a constant flow rate of 160 mL min⁻¹ in a three-zone tube furnace (Model HZS, Carbolite Gero Ltd.). The temperature was firstly increased up to 250 °C with 10 °C min⁻¹ and held for 2 h. Then, the temperature was further risen (10 °C min⁻¹) up to 800 °C and held for 6 h. Finally, the skeleton-PtCo (sk-PtCo) NPs were generated by acid treatment of the precursor alloy NPs in 1 M H₂SO₄ (ROTH, Supra Quality 95%) for 2 h at 15 °C and subsequently washed several times with ultra-purified water until the washing solution was pH neutral again. For comparison, the commercial 28.5 wt.% Pt/HSAC was heat-treated using the same annealing protocol as described above, referred to as HT–Pt.

### 2.2. In Situ and Ex Situ SAXS and WAXS Measurements

In situ SAXS and WAXS investigations on the sk-PtCo catalyst were carried out quasi-simultaneously at ID31 beamline of European Synchrotron Radiation Facility (ESRF, Grenoble). The beam with 77 keV energy and 7×10¹⁰ phs⁻¹ flux was focused (3.5×9 µm², vertical x horizontal) on the sample inside a home-made electrochemical three-electrode cell.[41] 2D Dectris Pilatus CdTe 2 M detector was used for both WAXS and SAXS measurements in a grazing incident geometry. The WAXS data were recorded at an X-ray wavelength of 0.161 Å. After every 50th cycle of cyclic voltammetry (CV) of the accelerated stress test (AST) protocol, the WAXS measurements were performed at a detector distance of 451 mm followed by a move of the detector to 6435 mm distance for subsequent SAXS measurements. The time difference between both WAXS and SAXS measurements was ~450 s. For the in situ SAXS and WAXS experiments, the sk-PtCo catalyst was dispersed onto a gas diffusion layer (29 BC, SGL carbon) and used as working electrode. Freshly calibrated Ag/AgCl and Pt wire were used as the reference and counter electrodes. All experiments were performed in 0.1 M HClO₄ electrolyte solution. The catalyst

cleaning protocol consisted of eight cycles from 0.05 to 1.00 V RHE with a scan rate of 50 mV s⁻¹ until steady-state CV profiles reached. Afterwards, two CV profiles were recorded at 20 mV s⁻¹ in the same potential range to estimate the electro-chemically active Pt surface area (ECSA) via underpotentially deposited hydrogen (Hₐₗₚ₉₉) method. The start-up/shut-down (SUSD) protocol involved 800 cycles from 0.50 to 1.50 V RHE with 50 mV s⁻¹, while after every 50th cycle two CV profiles at 20 mV s⁻¹ between 0.05 to 1.00 V RHE were acquired to monitor the changes in the Hₐₗₚ₉₉ region. At every 50th SUSD cycle, in situ SAXS and WAXS data were recorded by holding the potential at 0.50 V RHE. In this step, the first 50 s at 0.50 V RHE were used for conditioning to reduce the Pt surface oxide. Moreover, ex situ SAXS and WAXS data were collected from the powder samples to avoid a strong background signal from the GDL consisting of PTFE coated carbon fibers.

The 2D diffractograms for both WAXS and SAXS were radially integrated using pyFAI software package.[56] Rietveld refinement of the WAXS data was carried out using GSAS-II software (version 5467).[57] Pure high surface area carbon material was taken for the background correction. SAXS analysis was performed as described in the previous studies with a self-written MATLAB code.[53,58] Briefly, the scattering data were fitted to the general expression:

$$I(q) = A \cdot q^{-n} + C \cdot \int P(q, R)V(R)D(R)dR \quad (1)$$

where the first term, A·q⁻ⁿ, is the power law with A and n as free parameter, C is a scaling constant, P is a sphere form factor, R is the particle radius, V is the particle volume, D is the log-normal size distribution and the scattering vector is q = 4π · sin(θ)/λ, respectively. The sphere form factor is expressed as:

$$P(q, R) = \left( 3 \frac{\sin(qR) - qR\cos(qR)}{(qR)^3} \right)^2 \quad (2)$$

and the log-normal distribution is given by:

$$D(R) = \frac{1}{R\sigma\sqrt{2\pi}} \exp\left(-\frac{[\ln(R/R_0)]^2}{2\sigma^2}\right) \quad (3)$$

### 2.3. Ex Situ Structural Characterization

JEOL JEM2100F and FEI Tecnai G2 20S-Twin transmission electron microscopes (TEM) were employed to evaluate the morphology and mean particle size of these samples. More than 300 particles from several TEM images were analyzed using the software ImageJ (version 1.50i). For detailed structural and chemical analysis of the sk-PtCo NPs at the subnanometer level, electron energy loss spectroscopy (EELS) technique was performed in the scanning transmission electron microscopy (STEM) mode using a double aberration-corrected JEOL Neoarm
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200F TEM equipped with a cold field emission gun (FEG). The energy resolution for EELS measurements was 0.5 eV. To cover the entire energy loss range for O, Co, and Pt, an accelerating voltage of 200 kV was selected. The experimentally measured convergent angle (α) was 27.4 mrad and the collection angle (β) of the EELS spectrometer was bigger than 54.8 mrad. M4 Tornado plus micro-X-ray fluorescence (μ-XRF) spectrometer (Bruker) equipped with a rhodium X-ray source operated at 50 kV and 300 μA and two SDD detectors (type: 660 PAR -LE) was employed to determine the Pt:Co ratio of the pristine and treated sk-PtCo catalyst samples. Data from 8 to 10 randomly distributed point measurements with a spot size of 20 μm and acquisition time of 300 s were analyzed using Bruker Software M4 TORNADO (version 1.6.621.0).

## 2.4. Rotating Disc Electrode (RDE) Measurements

About 5.7 mg of sk-PtCo or HT-Pt catalyst powder were added to the solution, consisting of 4.7 mL of ultra-purified water, and 0.3 mL of 2-propanol (Fisher Chemicals, HPLC), while 5.5 mg of Pt/HSAC were dispersed in 4 mL of ultra-purified water and 1 mL of 2-propanol. After hom-sonication for 30 minutes, 10 µL as an aliquot of the ink dispersion was pipetted onto a freshly prepared glassy carbon (GC) electrode (5 mm diameter, Pine Research Instrumentation) and dried at 40 °C for 20 minutes in air. The calculated Pt loadings were around $15-17 \ \mu g_{Pt} cm_{geo}^{-2}$. The GC electrodes were previously polished with 0.05 µm aluminium oxide powder (Buehler) using a Microcloth polishing cloth (Buehler) followed by a washing step in ultra-purified water, 2-propanol and ultra-purified water for 5 minutes each using an ultra-sonication bath. Electrochemical experiments were carried out in a home-made three-compartment glass cell using a potentiostat (VSP-300, Biologic) and a RDE rotator (Pine Research Instrumentation). The three-electrode configuration consisted of a gold wire (counter electrode), a mercury-mercury sulfate reference electrode and a thin-catalyst film RDE as working electrode. All reported potentials were referred to the reversible hydrogen electrode (RHE) scale. The used Ar and $O_2$ gases were supplied by Westfalengas AG with a quality of 5.0. 0.1 M $HClO_4$ (70 Vol.%, 99.999%, Sigma Aldrich) electrolyte solution was used. The cleaning and conditioning processes consisted of three cycles from 0.06 to 1.00 $V_{RHE}$ at 50 mV s$^{-1}$ followed by 200 cycles at 500 mV s$^{-1}$ in Ar-saturated 0.1 M $HClO_4$. After electrolyte exchange, three CV profiles were recorded at 50 mV s$^{-1}$ between 0.06 and 1.00 $V_{RHE}$ to determine the $H_{upd}$ [cm$^2$] by integrating the hydrogen ad-/desorption region corrected by the double layer current and assuming a pseudo-capacity of $210 \ \mu C cm_{Pt}^{-2}$ for polycrystalline Pt.[59] The AST protocol was performed in Ar-saturated 0.1 M $HClO_4$ by recording 800 cycles from 0.50 to 1.50 $V_{RHE}$ with 50 mV s$^{-1}$ and reflects the SUSD conditions. After every 50$^{th}$ SUSD cycle, two cycles from 0.05 to 1.00 $V_{RHE}$ at 20 mV s$^{-1}$ were taken to estimate the ECSA. Prior to and after the AST protocol, the ORR performance was measured by three linear sweep voltammograms from 0.06 to 1.10 $V_{RHE}$ with 5 mV s$^{-1}$ and 1600 rpm (rounds per minute) in $O_2$-saturated 0.1 M $HClO_4$.

# 3. Results

## 3.1. Morphology, Structure and Composition of the sk-PtCo Catalyst at Begin-Of-Life

Ex situ TEM and SAXS techniques were employed to evaluate the morphology, particle size and structure of all catalysts at the begin-of-life (BOL). Here, the acid treatment of the precursor PtCo alloy catalyst prepared by wet-impregnation – freeze-drying – annealing route generates in situ the active catalyst for the oxygen reduction reaction (ORR), referred to as skeleton-PtCo (sk-PtCo) catalyst. This catalyst class is distinguished by a Pt-enriched particle surface induced by the Co dissolution within the nearest surface layers, as often reported in the literature.[18,21,60,61] The fine-structure and catalytic properties of the here investigated sk-PtCo catalyst will be shown in this and the following sections.

From ex situ μ-XRF data, the atomic ratio of the sk-PtCo NPs clearly alters from $Pt_{52 \pm 4}Co_{48 \pm 4}$ to $Pt_{67 \pm 4}Co_{33 \pm 4}$ after acid leaching, signifying a significant Co depletion. Representative TEM micrographs of the skeleton-PtCo (sk-PtCo), heat-treated Pt (HT-Pt), and commercial Pt catalysts supported on high surface area carbon (HSAC) are displayed in the top panel of the Figure 1. The last two were taken as reference materials for this work. Herein, the HT-Pt catalyst was prepared from the commercial 28.5 wt.% Pt/HSAC using the same annealing protocol as for the Pt-Co catalyst. Apparently, the nanoparticles (NPs) are well dispersed and show relatively narrow particle size distributions (bottom panel of the Figure 1, dashed lines, right y-axis and Figure S1 of the supporting information (SI)). It is noted that the sk-PtCo catalyst shows a bimodal size distribution of $3.8 \pm 0.4$ nm and $6.0 \pm 0.4$ nm (Figure 1a, b and Figure S1a). The mean particle sizes for the HT-Pt and Pt NPs were determined to be $4.9 \pm 0.8$ nm and $2.3 \pm 0.4$ nm, respectively. Based on the ex situ SAXS data (see Figure S2 of the SI), the bottom panel of the Figure 1 (solid lines, left y-axis) shows the particle size distributions with mean particle sizes of $4.5 \pm 1.4$ nm, $4.2 \pm 1.1$ nm, and $2.5 \pm 0.8$ nm for the sk-PtCo, HT-Pt, and Pt catalysts, respectively. We can sum up, that the mean particle sizes of the sk-PtCo, HT-Pt, and Pt catalysts from the ex situ SAXS and TEM data are in good agreement.

Figure 2 displays the ex situ WAXS data of the as-prepared sk-PtCo, HT-Pt, and Pt catalysts. For each catalyst, the Bragg peaks at around 4.1°, 4.8°, 6.7°, and 8.2° correspond to the (111), (200), (220), and (311) lattice planes of the disordered face-centered cubic (fcc) unit cell with a space group of Fm$\bar{3}$m. It is noted that no peaks from monometallic platinum and cobalt are detected for the sk-PtCo catalyst, see Figure 2a. Additionally, for the sk-PtCo catalyst a clear shift in the Bragg peak positions to higher 2Θ values is observed. For instance, the (220) lattice plane of pure fcc Pt is denoted with a dashed dark grey line. This peak shift for the sk-PtCo, which can be seen more clearly in the enlargement in Figure 2d, is based on the Co insertion into the Pt lattice and thus the formation of disordered fcc alloy crystal phases.

Table 1 summarizes the results of the crystal structure, lattice parameter, and crystallite size for the sk-PtCo, HT-Pt, and
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<figure><img src="image_9.png" /><figcaption>**Figure 1.** TEM micrographs (top panel) and respective particle size distributions (bottom panel) of the sk-PtCo (a, b), HT-Pt (c, d), and Pt (e, f) electrocatalysts supported on HSAC obtained from analyses of TEM (dashed lines) and ex situ SAXS (solid lines) data, respectively.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 2.** Background-corrected ex situ WAXS data of the as-prepared sk-PtCo (a), HT-Pt (b), and Pt (c) catalysts collected at an X-ray wavelength of 0.161 Å. Red and black marks denote the two different disordered fcc alloy crystal phases of the sk-PtCo catalyst (a). The (220) lattice plane of pure fcc Pt is denoted with a dashed dark grey line. Enlargement of the Bragg peak which is ascribed to the (200) lattice plane for all electrocatalysts is shown in (d). For the Rietveld refinement, the reference pattern of Pt-Co (Fm3m, #1524808) and pure Pt (Fm3m, #1011113) from the crystallography open database (COD)[67] were taken.</figcaption></figure>

Pt catalysts obtained from the quantitative Rietveld refinement analysis of the WAXS data. As an example, a Rietveld refinement of the *ex situ* WAXS data for the sk-PtCo catalyst is illustrated in Figure S3. Note, Bragg peak broadening based on strain effects

was not considered. In line with the asymmetric peak shape, the quantitative Rietveld refinement analysis indicates the appearance of two disordered fcc Pt-Co phases for the sk-PtCo catalyst, denoted with black and red marks in Figure 2a and
<figure><img src="image_11.png" /></figure>

<table><caption>**Table 1.** Lattice parameter, phase quantification, and crystallite size of the sk-PtCo, HT–Pt, and Pt catalysts. The atomic composition was calculated from the lattice parameter according to Vegard's law. The R<sub>wp</sub> reflects the goodness of the fit determined by the quantitative Rietveld refinement.</caption><thead><tr><th>catalyst</th><th>space group</th><th>lattice parameter / Å</th><th>Rietveld quantification / wt.%</th><th>crystallite size / nm</th><th>composition via Vegard's law / at.%</th><th>R<sub>wp</sub></th></tr></thead><tbody><tr><td rowspan="2">sk-PtCo</td><td>Fm&#773;3m</td><td>3.772 ± 0.001</td><td>60 ± 2</td><td>5.9 ± 0.1</td><td>Pt<sub>58</sub>Co<sub>42</sub></td><td rowspan="2">2.543</td></tr><tr><td>Fm&#773;3m</td><td>3.843 ± 0.001</td><td>40 ± 2</td><td>3.7 ± 0.1</td><td>Pt<sub>78</sub>Co<sub>22</sub></td></tr><tr><td rowspan="2">HT-Pt</td><td>Fm&#773;3m</td><td>3.922 ± 0.001</td><td>96 ± 2</td><td>3.9 ± 0.1</td><td>Pt<sub>100</sub></td><td rowspan="2">2.759</td></tr><tr><td>Fm&#773;3m</td><td>3.922 ± 0.001</td><td>4 ± 2</td><td>12.6 ± 0.1</td><td>Pt<sub>100</sub></td></tr><tr><td>Pt</td><td>Fm&#773;3m</td><td>3.912 ± 0.001</td><td>100</td><td>2.3 ± 0.1</td><td>Pt<sub>100</sub></td><td>0.827</td></tr></tbody></table>

Figure 2d. The major fcc Pt–Co alloy phase (red marks) with 60±2 wt.% exhibits a lattice parameter of 3.772±0.001 Å and a crystallite size of 5.9±0.1 nm, see Table 1. According to Vegard's law, this lattice parameter corresponds to a Pt:Co atomic ratio of 58:42. For completeness, the minor alloy phase (black marks in Figure 2a and Figure 2d) with a lattice parameter of 3.843±0.001 Å and a crystallite size of 3.7±0.1 nm shows a Pt:Co atomic ratio of 78:22. It is noted that the size of the alloy phase grows with increasing intermixture of both elements in the unit cell. We strongly believe, that the different observed crystallite sizes are based on the degree of alloying during the annealing in a reductive atmosphere.[62] Due to the inhomogeneity of the alloying, the Co poor NPs remain mainly in their initial size stage (~2 nm), and are therefore smaller. Altogether, the ex situ WAXS data reveals the existence of two disordered fcc Pt–Co crystal phases for the sk-PtCo catalyst, that are strongly different in crystallite size and lattice parameter/atomic composition.

Furthermore, the heat treatment of the commercial Pt catalyst leads to narrower Bragg peaks, comparing Figure 2b with Figure 2c. In addition, the asymmetric peak shape of the HT–Pt catalyst (see Figure 2d) indicates the appearance of two fcc phases with different quantities and crystallite sizes. Based on the Rietveld refinement analysis, the mean crystallite size increases from 2.3±0.1 nm for the Pt catalyst to 3.9±0.1 nm (96±2 wt.%) and 12.6±0.1 nm (4±2 wt.%) for the HT–Pt catalyst after heat-treatment. Note, the crystallite size affects the lattice parameter (see Table 1), e.g. with increasing crystallite size a bulk behavior is observed.[63–66] Nonetheless, the variation in the lattice parameter resulting from distinct crystallite size is significantly less than that caused by the alloying of Pt and Co. Generally, the crystallite sizes obtained from Rietveld refinement analysis of all electrocatalysts are in good agreement with the mean particle sizes obtained from ex situ TEM and SAXS techniques.

## 3.2. Electrochemical ORR Activity and Durability

For the sk-PtCo, HT–Pt, and Pt catalysts, the electrochemically active Pt surface area (ECSA) and Pt surface area-based specific ORR activity (SA) were evaluated as a function of the number of cycles during the accelerated stress test (AST). The AST protocol

involves cyclic voltammetry (CV) measurements with 800 cycles and 50 mV s⁻¹ between 0.50–1.50 V<sub>RHE</sub> in Ar-saturated 0.1 M HClO₄, simulating the conditions of the start-up/shut-down (SUSD) for PEMFCs.

The CV profiles of the sk-PtCo catalyst at the begin-of-life (BOL, solid line) and end-of-life (EOL, dashed line) are displayed in Figure 3a. The initial CV profile of the sk-PtCo shows the typical electrochemical features of a clean Pt surface, indicating the underpotential deposition of hydrogen (H<sub>Upd</sub>) (0.06–0.4 V<sub>RHE</sub>), the electric double layer capacitance (0.4–0.6 V<sub>RHE</sub>), and the formation of Pt (hydro-oxide species at >0.6 V<sub>RHE</sub>. As a direct comparison, the changes in CV profiles of the Pt and HT–Pt catalysts during the AST experiments are illustrated in Figure S4. From the H<sub>Upd</sub> region, the initial ECSA values were determined to be 45±5 m² g<sub>Pt</sub>⁻¹, 52±5 m² g<sub>Pt</sub>⁻¹, and 70±11 m² g<sub>Pt</sub>⁻¹ for the sk-PtCo, HT–Pt, and Pt catalysts, respectively. The trend of the initial ECSA values can be explained by the particle size effects, i.e., the ECSA value decreases with increasing particle size.[68,69]

Figure S5 displays the evolution of the CV profiles for each catalyst during the AST protocol. All catalysts show a drastic loss of H<sub>Upd</sub> region as well as the appearance of oxidation/ reduction of the (hydro-)quinone at ~0.6 V<sub>RHE</sub> as an indicator for carbon corrosion.[70–73] Table 2 summarizes all electrochemical results (ECSA and ORR activity) combined with the mean particle sizes obtained from TEM data of the three catalysts at BOL and EOL. In addition, Figure S6 displays the ECSA values as a function of the cycle number during the AST. To better highlight the catalyst aging processes, the cycle number-resolved ECSA values were normalized by the initial ECSA, referred to as N-ECSA, see Figure 3b. It is evident that based on the quantitative analysis of the N-ECSA all catalysts underwent varying degrees of electrochemical aging during the AST experiments. It is noted that only the first 50 SUSD cycles show a gradual increase in the N-ECSA. This can be explained by an additional cleaning effect and surface roughening under these harsh conditions, as reported in the literature.[73–75] At the EOL, the ECSA values for the sk-PtCo, HT–Pt, and Pt catalysts are 30±3 m² g<sub>Pt</sub>⁻¹, 38±5 m² g<sub>Pt</sub>⁻¹, and 35±2 m² g<sub>Pt</sub>⁻¹, which corresponds to a loss of -33%, -27%, and -50%, respectively. These ECSA losses over the course of the AST experiment are associated with the growth of particle size confirmed by TEM data, namely from 3.8±0.4 nm and 6.0±0.4 nm to 5.2±0.8 nm (+37% and -13% for the smaller and larger size population)
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 3.** (a) CV profiles of the sk-PtCo catalyst at BOL (solid blue line) and EOL (dashed black line) after applying the AST protocol (800 cycles, 0.50–1.50 $V_{RHE}$). The CV profiles were recorded at 50 mV s⁻¹ and Pt loading of 15–17 µg cm⁻² geo in Ar-saturated 0.1 M HClO₄. (b) Cycle number dependent (normalized) N-ECSA for the sk-PtCo (blue), HT–Pt (grey), and Pt (black) catalysts. The normalization of the ECSA is related to the initial ECSA value (N-ECSA = ECSA/ECSA<sub>initial</sub> at given cycle number).</figcaption></figure>

for the sk-PtCo catalyst, from 4.9±0.8 nm to 5.3±0.8 nm (+8%) for the HT–Pt catalyst, and from 2.3±0.4 nm to 4.3±0.6 nm (+87%) for the Pt catalyst, see Figure 7.

The polarization curves and the resulting ORR activities for
the sk-PtCo, HT–Pt, and Pt catalysts at the BOL (solid line) and

EOL (dashed line) are displayed in Figure S8 and Figure 3c. At
the BOL, the sk-PtCo catalyst clearly exhibits the highest SA
value (665 ± 91 µA cm<sub>pt</sub><sup>-2</sup>) at 0.90 V<sub>RHE,iR-free</sub> compared to the
HT–Pt (297 ± 42 µA cm<sub>pt</sub><sup>-2</sup>) and Pt (189 ± 29 µA cm<sub>pt</sub><sup>-2</sup>) catalysts.
However, after the AST the sk-PtCo catalyst shows a significant
decay of the SA. Although the SA of the sk-PtCo catalyst
decreases to 363 ± 54 µA cm<sub>pt</sub><sup>-2</sup> (-45%), this SA value is still
exceeding those of the HT–Pt (282 ± 21 µA cm<sub>pt</sub><sup>-2</sup>, -9%) and Pt
(250 ± 10 µA cm<sub>pt</sub><sup>-2</sup>, +33%) at EOL. Only, the Pt catalyst shows
an increase of the SA after the AST, which can be explained by
the drastic size growth (+87%). It is known that the SA value
increases with increasing particle size, as reported from our and
other works.[69,74,76]

Hence, we can sum up that the sk-PtCo and HT–Pt catalysts
show improved retention in ECSA compared to the ~2 nm Pt
catalyst. The improved electrochemical durability under very
harsh AST conditions is likely related to the larger initial particle
size and higher resistance of the carbon support material after
thermal annealing.

3.3. Degradation Behavior of sk-PtCo Catalyst Probed by in situ WAXS and SAXS

In situ WAXS data were recorded to monitor the detailed
changes in lattice parameter, phase quantity, and crystallite size
of the two disordered fcc Pt–Co alloy phases during the AST
experiment. The *in situ* WAXS data of the sk-PtCo catalyst after
every 50^th^ SUSD cycle number of the AST protocol are illustrated
in Figure S9. As a guide for the eye, the enlarged region of the
WAXS data especially for the (111) and (200) lattice planes of
the sk-PtCo catalyst are shown in Figure 4. It is obvious that
with increasing cycle number of the AST experiment, the Bragg
peaks are shifted to lower 2Θ values, denoted with black
dashed lines in Figure 4. This clear shift of the Bragg peaks
signifies gradual increase in the lattice parameters. In addition,
the peaks become narrower as the number of cycles of the AST
experiment increases, which indicates a significant size growth
for the sk-PtCo catalyst.

Very remarkably, the structural changes in lattice parameter,
phase quantity, and crystallite size can be correlated with the
cycle number of the AST experiment based on the quantitative
Rietveld refinement analysis of each disordered fcc Pt–Co alloy
phase. Figure 5 and Table S1 show the detailed analysis results
for this complex case of the sk-PtCo catalyst comprising two
distinct fcc alloy phases. As an example, the Rietveld refinement
of the *in situ* WAXS data for the sk-PtCo catalyst after 50 SUSD
cycles is displayed in Figure S10.

First, we wanted to stress that the initial atomic composi-
tions and crystallite sizes for the disordered fcc Pt–Co phases
(Pt<sub>71±1</sub>Co<sub>29±1</sub> with 4.2±0.2 nm and Pt<sub>91±1</sub>Co<sub>09±1</sub> with 3.5±
0.2 nm) are different compared to that obtained from the ex
situ WAXS data (see Table 1). Before starting the AST protocol,
the applied cleaning protocol leads to additional Co dissolution,
which is accompanied by a shrinkage of particle size.[77]
Furthermore, the WAXS data is statistically significant and the
data quality is high enough to establish the lattice parameters
<figure><img src="image_14.png" /></figure>

<table><caption>**Table 2.** ECSA, Pt surface area-based specific ORR activity (SA) at 0.90 V<sub>RHE,IR-free</sub> and mean particle sizes obtained from TEM analysis of the sk-PtCo, HT–Pt, and Pt catalysts prior to and after the AST experiments (800 cycles, 0.50–1.50 V<sub>RHE</sub>).</caption><thead><tr><th rowspan="2">catalyst</th><th colspan="2">ECSA / m<sup>2</sup> g<sub>Pt</sub><sup>-1</sup></th><th colspan="2">SA / µA cm<sub>Pt</sub><sup>-2</sup></th><th rowspan="2">Δ SA / %</th><th colspan="2">particle size TEM / nm</th></tr><tr><th>BOL</th><th>EOL</th><th>BOL</th><th>EOL</th><th>BOL</th><th>EOL</th></tr></thead><tbody><tr><td>sk-PtCo</td><td>45 ± 5</td><td>30 ± 3</td><td>665 ± 91</td><td>363 ± 54</td><td>-45</td><td>3.8 ± 0.4<br>6.0 ± 0.4</td><td>5.2 ± 0.8</td></tr><tr><td>HT-Pt</td><td>52 ± 5</td><td>38 ± 5</td><td>297 ± 42</td><td>282 ± 21</td><td>-9</td><td>4.9 ± 0.8</td><td>5.3 ± 0.8</td></tr><tr><td>Pt</td><td>70 ± 11</td><td>35 ± 2</td><td>189 ± 29</td><td>250 ± 10</td><td>+33</td><td>2.3 ± 0.4</td><td>4.3 ± 0.6</td></tr></tbody></table>

<figure><img src="image_15.png" /><figcaption>**Figure 4.** Background-corrected in situ SUSD cycle number-resolved WAXS data of the sk-PtCo catalyst during the AST protocol (800 SUSD cycles, 0.50–1.50 V<sub>RHE</sub>) plotted with an y-offset. The dashed black lines denote the shift of the Bragg peaks corresponding to the (111) and (200) lattice planes.</figcaption></figure>

($\pm 0.010$ Å corresponds to the atomic variation of $\pm 1-2$ at.% according to Vegard's law) and crystallite sizes ($\pm 0.2$ nm) with reasonable accuracy using Rietveld refinement analysis.

<figure><img src="image_16.png" /><figcaption>**Figure 5.** Cycle number dependent course of the lattice parameter (top), phase quantity (middle), and crystallite size (bottom) of both initial disordered fcc Pt<sub>71±1</sub>Co<sub>29±1</sub> (red) and Pt<sub>91±1</sub>Co<sub>09±1</sub> (black) components within the sk-PtCo catalyst during the AST protocol (800 SUSD cycles, 0.50–1.50 V<sub>RHE</sub>). The gray vertical window denotes the transition of the main degradation processes (from Co depletion to size growth) of the Pt<sub>71±1</sub>Co<sub>29±1</sub> (red) phase, while the predominant aging process of Pt<sub>91±1</sub>Co<sub>09±1</sub> (black) is controlled by size growth via Ostwald ripening and particle coalescence. The compositions of the fcc Pt–Co alloy phases were established by the linear relationship of the lattice parameter of each component according to Vegard's law. It is important to note that the error bars (lattice parameter of $\pm 0.010$ Å; resulting in a composition of $\pm 1-2$ at.% according to the Vegard's law; crystallite size of $\pm 0.2$ nm) describe the accuracy of the model used to fit the WAXS data.</figcaption></figure>
<figure><img src="image_17.png" /></figure>

In Figure 5, top panel, the lattice parameter of the initial Pt<sub>71±1</sub>Co<sub>29±1</sub> phase (red) clearly increases within the first 100–150 SUSD cycles and then the change becomes less. This course of the lattice parameter points out two main regimes of the catalyst aging processes, depending on the number of SUSD cycles. The *regime I*, comprising the initial 100–150 cycles, is dominated by the dissolution of residual cobalt from the initial Pt<sub>71±1</sub>Co<sub>29±1</sub> phase. But, with increasing cycle number, the main degradation process alters and is very likely controlled by Ostwald ripening and/or particle coalescence, which is referred to as the *regime II*. The transition between the *regime I* and *II* is denoted with the gray-colored window in Figure 5. Based on the Vegard's law, the composition of this single disordered fcc Pt–Co phase alters from initial Pt<sub>71±1</sub>Co<sub>29±1</sub> (BOL) to Pt<sub>74±1</sub>Co<sub>26±1</sub> (100–150 SUSD cycles) and then to Pt<sub>76±1</sub>Co<sub>24±1</sub> (EOL), indicating a successive Co depletion over the course of 800 potential cycles (0.50–1.50 V<sub>RHE</sub>). Unlike, the second disor- dered fcc Pt<sub>91±1</sub>Co<sub>09±1</sub> phase (black) seems to be more stable in terms of lattice parameter (3.890±0.010 Å to 3.903±0.010 Å) and corresponding composition (Pt<sub>91±1</sub>Co<sub>09±1</sub> to Pt<sub>94±1</sub>Co<sub>06±1</sub>) during the entire AST experiment.

Figure 5, middle panel displays the changes in the relative quantity of both disordered fcc Pt–Co crystal phases during the potential cycling of the AST protocol. As in the case of the lattice parameter analysis, two regimes can be identified. In the *regime I* (<150 cycles), the proportion of the Pt<sub>71±1</sub>Co<sub>29±1</sub> phase drastically decreases from 62±6 wt.% to 48±5 wt.% (–23%) followed by the *regime II* with a lower decay behavior (from 48±5 wt.% to 37±4 wt.% after 800 potential cycles). The opposite tendency for the proportion of the second fcc Pt<sub>91±1</sub>Co<sub>09±1</sub> phase was found. For the fcc Pt<sub>91±1</sub>Co<sub>09±1</sub> phase, a rapid increase in their proportion from 38±4 wt.% to 52±5 wt.% is observed in the *regime I* and then a moderate growth rate up to 63±6 wt.% in the *regime II*. The overall changes in the relative quantity of both Pt–Co phases show a clear transformation of the initial Pt<sub>71±1</sub>Co<sub>29±1</sub> phase into the Pt-rich phase during the AST, which is already taking place at the beginning of the AST.

Finally, the evolution of the crystallite size of both disordered fcc Pt–Co phases during the entire AST protocol is shown in Figure 5, bottom panel. For both initial Pt<sub>71±1</sub>Co<sub>29±1</sub> and Pt<sub>91±1</sub>Co<sub>09±1</sub> phases, the crystallite size gradually grows in different degrees with increasing cycle number of the AST. More precisely, the crystallite size of the initial Pt<sub>71±1</sub>Co<sub>29±1</sub> phase slightly increases from 4.2±0.2 nm to 4.8±0.2 nm (+14%), indicating a high robustness under these harsh conditions. Unlike, the second initial Pt<sub>91±1</sub>Co<sub>09±1</sub> phase shows a stronger change in the crystallite size (3.5±0.2 nm to 4.5±0.2 nm, +29%). This observation is consistent with the result reported from the literature, showing that the stability of the particles is strongly controlled by their initial size.[74,78,79] We suggest that the potential-induced size growth is based on Ostwald ripening and/or particle coalescence, which cannot be distinguished with the techniques applied in this work.

Thus, we can sum up that the *in situ* WAXS experiment clearly uncovers the different aging processes of the relatively complex sk-PtCo catalyst consisting of two disordered fcc

phases. In other words, during the first 100–150 SUSD cycles the dominant degradation mechanism of the initial 4.2 ± 0.2 nm Pt<sub>71±1</sub>Co<sub>29±1</sub> phase is the Co depletion (*regime I*) followed by the moderate growth of the crystallite size (*regime II*). In contrast, for the second initial Pt<sub>91±1</sub>Co<sub>09±1</sub> phase, the main degradation process is controlled by the gradual crystallite size growth *via* Ostwald ripening and/or particle coalescence induced by the potential cycling up to 1.50 V<sub>RHE</sub>.

In addition to the *in situ* WAXS investigations, SAXS experiment was conducted to monitor *in situ* the changes in the particle size distribution and mean particle size of the sk-PtCo NPs during the AST protocol. The obtained *in situ* SAXS data and the corresponding fits as a function of the number of SUSD cycles for the sk-PtCo catalyst are plotted with an *y*-offset in Figure S11 of the SI. Generally, the maxima of the *in situ* SAXS data are shifted to smaller *q* values with increasing SUSD cycle number (denoted with an arrow, Figure S11), indicating a clear particle growth. Very briefly, the *ex situ* and *in situ* WAXS data show the appearance of two disordered fcc alloy crystal phases with different crystallite sizes for the sk-PtCo catalyst (compare Figure 2, Figure 5, and Table 1). As the mean sizes of both Pt–Co phases are in the same range (below <4.2 nm) at the BOL and very similar (4.5–4.8 nm) at the EOL, the *in situ* SAXS data can be fitted by a monodisperse population with a single size with a relatively large deviation. Therefore, we decided to fit all data sets with the same model of monodisperse size distribution to avoid an overfitting. This result underlines the complementary use of SAXS and WAXS data to investigate the degradation mechanisms of electrocatalysts.

Figure 6a displays the mean particle size distribution of the sk-PtCo NPs derived from the *in situ* SAXS data over the course of the AST protocol. Evidently, a clear shift to larger particle size and a broadening of the particle size distribution with increasing SUSD cycle number are observed. In Figure 6b, the mean particle size analysis over the entire SUSD cycles shows a successive size growth from around 4.5 nm to 6.5 nm (+44%) for the sk-PtCo NPs despite their initial large size at BOL. It is important to note that the error bars in Figure 6b are not derived from the Gaussian fit or particle size distribution, but describe the accuracy of the model used to fit the SAXS data (see experimental part). Altogether, the *in situ* SAXS data shows a continuous and monotonous increase in mean particle size of the sk-PtCo over the course of the AST protocol, supporting the results of the *in situ* WAXS analysis.

### 3.4. Ex Situ Post-Mortem Analysis of the sk-PtCo Catalyst During and After AST

All three electrocatalysts were also characterized using *ex situ* techniques such as STEM-EELS, TEM, and μ-XRF to correlate the changes in chemical composition and mean particle size with the SUSD cycle number of the AST protocol and support the findings of the *in situ* SAXS and WAXS study.

Figure 7 displays the annular dark field (ADF) images and the corresponding EELS elemental maps (Pt: red, Co: green) of the sk-PtCo catalyst after 0 (BOL), 100, and 800 SUSD potential
<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /><figcaption>**Figure 6.** SUSD cycle number (800 cycles, 0.50–1.50 V<sub>RHE</sub>) dependent (a) particle size distributions and (b) corresponding mean particle size of the sk-PtCo catalyst obtained from the in situ SAXS data.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Figure 7.** Annular dark field (ADF) images (a, c, e) and overlay (b, d, f) of the EELS elemental maps of the sk-PtCo catalyst at BOL (a, b), after 100 SUSD cycles (c, d) and at EOL (e, f). In the EELS maps, the platinum and cobalt are denoted with red and green, respectively. The white arrow in the EELS map (f) indicates the appearance of particle coalescence due to the formation of multiple Co-rich cores.</figcaption></figure>
<figure><img src="image_21.png" /></figure>

cycles. From the EELS elemental maps, it is confirmed that the acid-treated sk-PtCo NPs form a thin Pt-enriched shell, while the particle core still consists of both metals. A clear thickening of the Pt-enriched shell is evident after applying 100 SUSD cycles (Figure 7d). Finally, the EELS elemental maps of the sk-PtCo NPs after 800 SUSD cycles (at EOL, Figure 7f) reveal a significant growth of the particle size as well as a strong thickening of the Pt-enriched shell. The increase of particle shell thickness is associated with the successive Co depletion. On the other hand, the observed particle growth is very likely based on Ostwald ripening and/or particle coalescence induced by the SUSD cycling. In particular, the formation of elongated particles with multiple Co-rich cores (see Figure 7f, white arrow) indicates the appearance of particle coalescence due to carbon corrosion and/or weak particle-support interactions. Overall, the STEM-EELS data reveal the thickening of the Pt shell and size growth based on the Co dissolution and Ostwald ripening/particle coalescence during the AST protocol (800 SUSD cycles, 0.50–1.50 V<sub>RHE</sub>).

Based on the *ex situ* μ-XRF data, the SUSD cycle number dependent course of the overall composition for the sk-PtCo catalyst is shown in Figure S12 of the SI. The acid leaching leads to a Pt:Co atomic ratio of 67:33 (±4 at.%). It is notable that the first 100 cycles of the AST protocol show the highest relative loss of cobalt at –43% (Pt:Co ratio of 83:17 with ±3 at.%). Additional SUSD cycles do not significantly influence the composition of the sk-PtCo catalyst. Therefore, we can conclude that the Co leaching takes place mainly in the first 100 SUSD cycles of the AST protocol. This is in good agreement with the increase of the lattice parameter based on Co depletion observed from *in situ* WAXS data analysis. To confirm the particle growth with another technique, the representative TEM micrographs of the sk-PtCo catalyst and the corresponding particle size distributions prior to and after 50, 100, 300, and 800 SUSD cycle numbers are shown in Figure S13. The mean particle sizes of the sk-PtCo catalyst gradually increases initially from 3.8±0.4 nm and 6.0±0.4 nm (BOL) to 5.2±0.8 nm (EOL, +37% and –13% for the smaller and larger size population) at the end of AST experiment. Despite the difference in absolute values, the complementary *in situ* SAXS and WAXS and *ex situ* TEM data show a similar trend of a particle/crystallite growth during the AST experiments. It is noted that the SAXS and WAXS data are more statistically meaningful compared to the TEM data, where only ‘few’ NPs can be counted from the micrographs. This can be biased to describe the NPs of the catalyst sample (e.g., not under electrochemical control during AST) from the TEM data. However, the analysis of the TEM data indicates a trend of the evolution of different size proportions within the particle size distribution induced by potential cycling. For instance, the fraction of smaller NPs below 3 nm gradually decreases during the first 300 SUSD cycles and disappears completely after 800 cycles of the AST protocol, see Figure S14. The low stability of smaller NPs under these highly harsh SUSD conditions is in good agreement with the *in situ* WAXS and SAXS data.

Thus, the *post mortem* structural analyses revealed that the electrochemical stability of the NPs increases with larger particle

size during the AST protocol. Due to the initial larger size, the particle growth for the sk-PtCo and HT–Pt catalysts is moderate within the 800 cycles of the AST protocol compared to the smaller sized Pt catalyst, see Figure S7. For pure Pt catalysts, the main degradation processes are related to the particle growth *via* Ostwald ripening and/or particle coalescence within the few hundreds of SUSD cycle numbers. In contrast, the sk-PtCo catalyst shows the Co depletion and the thickening of the Pt-enriched shell within the first 100 SUSD cycles as a dominant degradation process. Afterwards, the catalyst aging is controlled by particle growth *via* Ostwald ripening and/or particle coalescence. For all NP catalysts, carbon corrosion associated with particle detachment cannot be excluded, as the CV profiles during the AST clearly show the appearance of the oxidation/reduction peak of (hydro–)quinone at 0.55 to 0.70 V<sub>RHE</sub>, see Figure S5. Other works also reported that the carbon corrosion associated with particle detachment becomes noticeable with increasing SUSD cycle numbers.[26,74,80–83]

## 4. Discussion

Based on the complementary *in situ* WAXS and SAXS techniques, we are aimed at to develop a comprehensive degradation model for the sk-PtCo catalyst consisting of two disordered fcc alloy phases with different compositions and sizes. Especially with such a complex catalyst material system, which is not very different from other technically relevant electrocatalysts, it is necessary to obtain deeper insights how the individual phases degrade under the highly corrosive conditions. Therefore, our elaborated degradation model includes the critical stages of the individual crystal phases of the sk-PtCo catalyst depending the cycle number of the AST protocol (800 cycles, 0.50–1.50 V<sub>RHE</sub>, 50 mV·s⁻¹, Ar-saturated 0.1 M HClO₄) and is visualized as a sketch in Figure 8. Notedly, the sizes given in Figure 8 were determined from the *in situ* WAXS investigations on the sk-PtCo catalyst. The appearance of two fcc disordered phases in the sk-PtCo catalyst is very likely related to annealing process, leading to an alloy formation to different extents. Definitely, the use of complementary *ex situ* and *in situ* techniques such as TEM, SAXS and WAXS is essential to obtain a more descriptive degradation model of the sk-PtCo catalyst in correlation with the number of SUSD cycles during the AST protocol. The direct comparison of *ex situ* and *in situ* techniques used in this comprehensive study is summarized in Table S2 of the SI and shows a similar change in crystallite and particle sizes for the sk-PtCo catalyst prior to and during AST protocol.

The sk-PtCo catalyst prepared by acid treatment of the precursor alloy shows very good ORR activity, although it is a complex mixed material with two disordered fcc alloy phases with different compositions and sizes, namely ~60 wt.% Pt<sub>71±1</sub>Co<sub>29±1</sub> phase with ~4 nm size and ~40 wt.% Pt<sub>91±1</sub>Co<sub>09±1</sub> with ~3 nm obtained from the *in situ* WAXS data. Based on Rietveld refinement analysis of the *in situ* WAXS data (see Figure 5 and Figure S9), we were able to uncover a variation of the predominant degradation processes of these Pt–Co phases despite their relatively high platinum content. More precisely,
<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /><figcaption>**Figure 8.** Main degradation processes for the individual disordered fcc Pt–Co phases (Pt<sub>71±1</sub>Co<sub>29±1</sub> with 4.2 nm and Pt<sub>91±1</sub>Co<sub>09±1</sub> with 3.4 nm) in the sk-PtCo catalyst as a function of the SUSD cycles during cyclic voltammetry accelerated stress test (AST) protocol (800 cycles, 0.50–1.50 V<sub>RHE</sub>, 50 mV s<sup>-1</sup>, Ar-saturated 0.1 M HClO<sub>4</sub>) obtained from the in situ WAXS data. The grey and red balls denote the Platinum and Cobalt atoms, respectively.</figcaption></figure>

for the disordered fcc Pt<sub>71±1</sub>Co<sub>29±1</sub> phase at ~4 nm size the first 100–150 SUSD cycles of the AST experiment constitute the *regime I* of the main catalyst aging process, in which a moderate Co dissolution prevails. The tendency of the dissolution of internal Co atoms in the sub-surface layers is based on the strong surface fluctuation of the Pt-enriched particle surface induced by the potential cycling between 0.50–1.50 V<sub>RHE</sub>. In particular, the Pt–O place exchange mechanism that occurs above 1 V<sub>RHE</sub> (depending on the particle size and composition) leads to strong surface roughening and therefore to an enhancement of the Co dissolution from the sub-surface layers. This Co depletion is also accompanied with the thickening of the Pt-enriched particle shell within the first 100–150 SUSD cycles, as shown from the elemental EELS maps (Figure 7) and μ-XRF data (Figure S12). Afterwards, the composition of the Pt–Co phase is not significantly influenced by additional potential cycling. Now *regime II* comes into play and the catalyst degradation is mainly controlled by the size growth due to Ostwald ripening and/or particle coalescence. Moreover, the phase quantification shows a drastic reduction of the most catalytically active Pt<sub>71±1</sub>Co<sub>29±1</sub> phase from 62 ± 6 wt.% to 37 ± 4 wt.% during the AST. This observation can be explained by the gradual transformation of the initial Pt<sub>71±1</sub>Co<sub>29±1</sub> phase into the Pt-rich phase over the course of the AST protocol. Obviously, the loss of the Pt<sub>71±1</sub>Co<sub>29±1</sub> phase and the thickening of the Pt-enriched particle shell cause a dramatic decrease of the ORR activity after the 800 SUSD cycles. Unlike to the initial Pt<sub>71±1</sub>Co<sub>29±1</sub> phase, the second disordered fcc Pt<sub>91±1</sub>Co<sub>09±1</sub> phase at smaller size (~3 nm) grows faster and continuously over the course of the AST experiment. Thus, the *in situ* WAXS data indicates that the size growth is the main degradation process for this Pt-rich phase and is based on Ostwald ripening and/or particle coalescence. These dynamics of the particle size

growth of the sk-PtCo NPs as a function of the cycle number of the AST experiment were also corroborated by the complementary *in situ* WAXS data (Figure 6), which already occur from the beginning of the AST. According to the population of different size groups for the sk-PtCo catalyst, smaller NPs are less electrochemically stable and prone to dissolve at lower anodic potentials. Even within the first 50 cycles, the size groups of <2 nm disappear in favor to the larger sizes obtained from the TEM analysis, see Figure S14. The direct comparison between the population of the size groups after 300 and 800 SUSD cycles points out a drastic shift to larger diameters, signifying the low electrochemical stability of smaller NPs. It is noted, that carbon corrosion associated with particle detachment cannot be excluded during the AST. In Figure S5, the CV profiles show the presence of the hydroquinone/quinone redox couple, indicating the degradation of carbon support material under these highly harsh conditions.

We can sum up, that complementary *in situ* WAXS and SAXS techniques are very powerful tools to uncover the main catalyst aging processes of the relatively complex sk-PtCo catalyst consisting of two disordered fcc phases with different compositions and sizes under dynamic SUSD conditions. In fact, the main degradation process of small Pt-rich NPs is the size growth *via* Ostwald ripening and/or particle coalescence from the beginning of the AST protocol, see Figure 8. If the Co content is significant (more than 10 at.%) and the particle size is larger than 4 nm, the Co depletion accompanied by the thickening of the Pt-enriched shell is dominant within the first 100–150 SUSD cycles. When the SUSD cycle number is further increased, the main degradation process alters and the size growth *via* Ostwald ripening and/or particle coalescence occurs preferentially (see Figure 8).
<figure><img src="image_24.png" /></figure>

## 5. Conclusions

A highly active sk-PtCo catalyst with a thin Pt-enriched shell for ORR was successfully synthesized via wet-impregnation – freeze-drying – thermal annealing followed by acid leaching in 1 M H₂SO₄. Thereby, the sk-PtCo catalyst NPs have a complex structure, consisting of two fcc alloy phases with different compositions and sizes (Pt<sub>71±1</sub>Co<sub>29±1</sub> with ~4 nm and Pt<sub>91±1</sub>Co<sub>09±1</sub> with ~3 nm), which undergo different aging processes. During the AST protocol (800 cycles, 0.50–1.50 V<sub>RHE</sub>), the sk-PtCo catalyst shows a loss of ECSA by –33% and SA by –45%, but still exceeds the performance of heat-treated and pristine Pt catalyst materials. Remarkable, using complementary in situ WAXS and SAXS techniques, we were able to uncover the different main degradation mechanisms of each disordered fcc Pt–Co phase within the sk-PtCo catalyst in detail as a function of the SUSD cycle number. For the smaller ~3 nm Pt<sub>91±1</sub>Co<sub>09±1</sub> phase, the potential cycling between 0.50–1.50 V<sub>RHE</sub> leads to a continuous size growth due to Ostwald ripening and/or particle coalescence as a main catalyst aging process. On the other hand, the fcc Pt<sub>71±1</sub>Co<sub>29±1</sub> phase with ~4 nm size degrades by further Co dissolution within the first 100–150 SUSD cycles (regime I). Subsequently, the size growth via Ostwald ripening and/or particle coalescence predominates with increasing SUSD cycles of the AST experiment, which is classified as regime II. Irrespective of the composition of the Pt–Co crystal phases, the in situ WAXS data shows a continuous and monotone size growth already from the beginning of the AST experiment based on the Ostwald ripening and/or particle coalescence. Therefore, a clear shift of the mean particle size towards larger particle size and a significant loss of population of smaller particles below 3 nm are observed from the TEM data. *Ex situ* μ-XRF data corroborates the results from the *in situ* WAXS data, that that the Co dissolution mainly takes place during the first 100 SUSD cycles. Additionally, *ex situ* STEM-EELS reveals that the Co dissolution is also accompanied by thickening of the Pt-enriched shell. The thickening of the Pt-enriched shell and the transformation of the Pt<sub>71±1</sub>Co<sub>29±1</sub> phase into the Pt-rich phase cause the dramatic decrease of the ORR activity.

From the complementary *in situ* WAXS and SAXS techniques, we can develop a more descriptive degradation model of the sk-PtCo catalyst consisting of two disordered fcc alloy phases with different sizes and atomic compositions in correlation with the number of SUSD cycles. Detailed knowledge of the main degradation processes will help to design highly active and robust Pt-based ORR electrocatalyst materials to be used in PEMFCs, especially under very harsh and dynamic conditions such as SUSD events.

## Notes

The authors declare no competing financial interest.

## Acknowledgements

Financial support from the Federal Ministry of Education and Research (BMBF, FKZ 03SF0539, FKZ 03SF0645 and FKZ 01DR22004) is gratefully acknowledged. We also thank the DFG for funding the JEOL JEM2100FTEM (INST 184/106-1 FUGG). The authors thanks ESRF for providing a beamtime at ID31 beamline (proposal No. CH-5695 and No. CH-1607) and Helena Isern and Florian Russello for the help with experiment preparation. J.Q. acknowledges the European Union's Horizon-2020 research and innovation program under Marie Skłodowska-Curie Grant Agreement 840523 (CoSolCat). R. P. thanks the Danish National Research Foundation (DNRF149) - Center for High Entropy Alloy Catalysis for the support. This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy - EXC-2123 QuantumFrontiers – 390837967. We thank DFG funding under grant INST 188/441-1 FUGB. We acknowledge Jan Simke from ZELMI/TU Berlin for TEM measurements. Open Access funding enabled and organized by Projekt DEAL.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords:** skeleton-PtCo nanoparticles (NPs) · oxygen reduction reaction (ORR) · start-up/shut-down (SUSD) · small-angle X-ray scattering (SAXS) · wide-angle X-ray scattering (WAXS) · accelerated stress test (AST)

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Manuscript received: February 13, 2024

Revised manuscript received: March 19, 2024

Accepted manuscript online: March 20, 2024

Version of record online: April 17, 2024