# Deciphering the Atomic-Scale Degradation of Carbon-Supported Platinum–Yttrium Nanoalloys during the Oxygen Reduction Reaction in Acidic Medium

Carlos Campos-Roldán, Raphaël Chattot, Jean-Sébastien Filhol, Hazar Guesmi, Nuria Romero, Rémi Bacabe, Pierre-Yves Blanchard, Valentin Vinci,

Jakub Drnec, Jacques Rozière, et al.

► To cite this version:

Carlos Campos-Roldán, Raphaël Chattot, Jean-Sébastien Filhol, Hazar Guesmi, Nuria Romero, et al. Deciphering the Atomic-Scale Degradation of Carbon-Supported Platinum–Yttrium Nanoalloys during the Oxygen Reduction Reaction in Acidic Medium. ACS Catalysis, 2024, 14 (16), pp.11941-11948. 10.1021/acscatal.4c02616. hal-04672590

HAL Id: hal-04672590

https://hal.umontpellier.fr/hal-04672590v1

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Deciphering the Atomic-Scale Degradation of
Carbon-Supported Platinum-Yttrium Nanoalloys
during the Oxygen Reduction Reaction in Acidic
Medium

Carlos A. Campos-Roldán†*, Raphaël Chattot†, Jean-Sébastien Filhol†, Hazar Guesmi†, Nuria

Romero§, Rémi Bacabe†, Pierre-Yves Blanchard†, Valentin VinciΔ, Jakub DrnecΔ, Jacques

Rozière†, Deborah J. Jones†, Sara Cavaliere †*

† ICGM, Univ. Montpellier, CNRS, ENSCM, 34095 Montpellier cedex 5, France

§CNRS, Université de Toulouse, UPS, INPT, Laboratoire de Chimie de Coordination, 205 Route

de Narbonne, 31077 Toulouse, France

Δ ESRF, European Synchrotron Radiation Facility, 71 Avenue des Martyrs, CS40220, 38043

Grenoble Cedex 9, France

*Corresponding authors:

carlos-augusto.campos-roldan@umontpellier.fr

sara.cavaliere@umontpellier.fr

KEYWORDS:

21 Pt alloys, Rare Earth Metals, *In situ/operando* measurements, Oxygen Reduction Reaction,

22 Proton Exchange Membrane Fuel cells.
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ABSTRACT

Platinum-yttrium alloys are considered promising candidates to satisfy the challenging requirements for the cathodic oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Nevertheless, the practical structure-activity-stability trends of these electrocatalysts in the form of carbon-supported nanostructures are poorly understood, especially under the operating conditions. Herein, carbon-supported PtₓY nanoalloys were explored during the electrochemical ORR environment, following the atomic-scale degradation steps that the nanoalloys experience during operation. Our results reveal that PtₓY/C nanoalloys undergo considerable structural modification during the early stage of electrochemical cycling. Moreover, *operando* techniques identify that, during accelerated stress testing under O₂ atmosphere, the majority of nanoalloy degradation occurs during the initial 1,000 electrochemical cycles, and is accompanied by a diminished ORR performance. The observed *operando* structure-activity-stability trends guide further optimization routes for carbon-supported Pt-Y nanoalloys as PEMFC cathode electrocatalysts.

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# 1. INTRODUCTION

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Current decarbonization targets are priority in our society, and proton-exchange membrane fuel cells (PEMFCs) have gained attention as energy conversion devices that can contribute importantly to fulfilling these objectives.¹ Nevertheless, their massive production and commercialization are still hindered by technical barriers that need to be overcome, including performance, durability, cost and fuel efficiency,², ³ and one particular bottleneck is the electrocatalyst activity and long term durability for the sluggish oxygen reduction reaction (ORR). A well-known approach to enhance the ORR electrocatalytic activity is by alloying Pt with late transition metals (usually Ni, Co, Fe, Cu, etc), which also decreases simultaneously the amount of Pt and, therefore, the cost.⁴ The long-term stability of Pt-based nanoalloys during the ORR, however, represents a crucial issue that must be tackled.⁵⁻⁷

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Since their introduction as ORR electrocatalysts in 2009,⁸ platinum-yttrium (Pt-Y) alloys have been considered as promising candidates to satisfy the demanding ORR requirements from their predicted high activity and long-term stability. Pioneering theoretical studies⁸ concluded that the exceptionally high negative alloy formation energy (-1.02 eV/atom for Pt₃Y)⁹ represents a partial thermodynamic barrier for yttrium diffusion through the alloy lattice, which should provide high electrochemical stability. Further research on bulk sputter-cleaned Pt₃Y polycrystalline electrodes⁸, ¹⁰, ¹¹ and unsupported PtₓY nanoparticles (NPs)¹² revealed that the high electrocatalytic activity of these alloys stems from the formation of a specific structure, in which the Pt-Y alloy is protected by a thin Pt-rich overlayer, which induces a lateral compressive strain in the Pt-rich shell.¹³ Indeed, the surface specific ORR activity showed an exponential dependence on the degree of *ex situ* measured compressive strain of the Pt-rich shell.¹² In the particular case of unsupported PtₓY NPs, a particle size dependency was also identified, and an
outstanding ORR mass activity of 3.05 A $mg_{Pt}^{-1}$ was obtained for NPs of ca. 9 nm.<sup>12</sup>

Nevertheless, after the applied accelerated stress test (AST), which consisted of 9,000 potential
cycles from 0.6 to 1.0 $V_{RHE}$ (triangular potential wave at 100 mV s<sup>-1</sup>) in O<sub>2</sub>-saturated 0.1 M
$HClO<sub>4</sub>$, the unsupported $Pt_X$ NPs lost ca. 64 % of their initial mass activity.

Inspired by the contributions discussed above, further research focused on the challenging
production of carbon-supported $Pt_X$ nanoalloys,<sup>14-17</sup> the practical and desired form for an ORR
electrocatalyst. Even though several studies claimed the formation of carbon-supported Pt-Y
nanoalloys (more usually yttrium (hydr)oxide-decorated Pt NPs instead of the actual alloy),<sup>18</sup> the
first solid evidence was reported in 2016<sup>17</sup> and 2018<sup>15</sup>. However, the measured ORR mass
activity of the Pt-Y/C nanoalloys reported so far is in the range between 0.1 and 0.6 A $mg_{Pt}^{-1}$,<sup>14</sup>
<sup>15, 17, 19-21</sup> which is still far below that of the reference value of 3.05 A $mg_{Pt}^{-1}$ of unsupported 9 nm
$Pt_X$ NPs,<sup>12</sup> c.f. **Figure S1** and **Table S1**. This discouraging trend raises critical questions for the
viability of $Pt_X$/C as ORR electrocatalyst, such as 1) the discrepancy between the theoretically
predicted high stability and the experimentally observed high activity loss; 2) the reason for the
activity gap between unsupported NPs and carbon-supported NPs; 3) the electrochemical
threshold and driving force for alloy degradation under the reaction environment; 4) the
electrochemically induced metal dissolution structural transitions during operation, and so on.

Of huge potential in this context, the development of *in situ* and *operando* techniques, such as
wide-angle synchrotron X-ray scattering (WAXS),<sup>22</sup> X-ray absorption spectroscopy (XAS),<sup>23</sup>
*online* inductively coupled plasma mass spectrometry (ICP-MS),<sup>24</sup> etc., has provided
fundamental insights at the atomic scale on the degradation of Pt-based electrocatalysts in real or
simulated PEMFC electrochemical environments. Nevertheless, the studies that have used these
advanced techniques for the characterization of Pt-Y nanoalloys are scarce. For instance,
Escudero-Escribano et al. have used *in situ* grazing incident X-ray diffraction (GIXRD) to study the formation, induced strain and correlation lengths of the Pt-rich overlayer on Gd/Pt(111) single-crystal electrodes, reporting that the induced compressive strain that the Pt overlayer experiences is relaxed upon repeated electrochemical cycling in the potential range 0.6-1.0 V<sub>RHE</sub>. This strain relaxation effect is stronger as the upper potential limit increases. To the best of our knowledge, the only publication concerning the *in situ* characterization of Pt-Y nanoalloys is given by Malacrida et al.<sup>25</sup> *In situ* ambient pressure X-ray photoelectron spectroscopy (APXPS) was used to investigate the dealloying mechanism of unsupported Pt<sub>x</sub>Y NPs under near PEMFC operating conditions, when the nature of oxygenated near-surface species was observed as a function of the applied potential.<sup>25</sup> From this study, the authors concluded that a post-synthesis acidic wash is needed to form a protective Pt-rich overlayer, and to avoid the presence/leaching of Y<sup>3+</sup> cations that may compromise the conduction properties of the PEMFC membrane. Nevertheless, there is still a need for a deeper understanding of the relatively novel Pt<sub>x</sub>Y/C nanoalloys during the ORR operating conditions.

For this purpose, we have synthesized carbon-supported Pt<sub>x</sub>Y nanoalloys *via* a solid-state approach, and have used advanced techniques for their characterization, i.e., *operando* WAXS, *online* ICP-MS, atomic resolution HAADF-STEM, and have interpreted the results with the assistance of DFT calculations. We have followed, from the early electrochemical activation to the end of the applied AST, the time- and potential-resolved metal dissolution, the variations in crystal structure, the evolution of the local chemical composition and the morphology of Pt<sub>x</sub>Y/C nanoalloys as ORR electrocatalysts. The extracted results are rationalized and critically discussed, shedding light for the first time on the *operando* relationship between physico-chemical properties, ORR activity and stability of carbon-supported Pt-Y nanoalloys.
### 3. RESULTS AND DISCUSSION

PtₓY/C nanoalloys were synthesized using the carbodiimide complex route developed by Hu *et al.*¹⁴ using carbon Ketjenblack EC-300J as a support. The resulting powder was treated *via* an acidic wash in N₂-saturated 0.5 M H₂SO₄²¹, ²⁶ (see **Supplementary Information S2**). The as-prepared electrocatalyst crystalline structure, morphology and chemical properties were characterized *ex situ*, and reported in our previous contribution.²¹ Briefly, the chemical analysis, determined by ICP-MS, sets a Pt content of *ca.* 27 % *wt.* and an Y content of *ca.* 3 % *wt.* The higher atomic Pt:Y obtained by ICP-MS relative to the Pt₃Y crystalline structure stoichiometry could be related to the formation of the Pt-rich shell and the possible presence of small pure Pt NPs. Besides, the *ex situ* characterization confirms the formation of Pt₃Y nanoalloy, and the presence of a dominant population of NPs of *ca.* 5 nm was observed, as well as some agglomerates of *ca.* 12 nm dispersed over the carbon support.

The ORR electrocatalytic activity and long-term stability were investigated using the rotating disk electrode (RDE) technique in 0.1 M HClO₄ electrolyte at 25 °C (**Supplementary Information S2**). These screenings attest that PtₓY/C surpasses the initial ORR mass activity of the Pt/C reference (0.58 A mgPt⁻¹ vs. 0.23 A mgPt⁻¹ at 0.9 V<sub>RHE</sub>, respectively). The electrocatalysts' stability was evaluated using an AST inspired from the Fuel Cell Technical Team of the U.S Department of Energy (DoE/FCTT) protocol, which consists of 30,000 potential cycles from 0.60 to 0.95 V<sub>RHE</sub> (square potential wave, 3 s at each potential limit with a transition of 0.5 s in between)²⁷ in O₂-saturated 0.1 M HClO₄. After the AST, the PtₓY/C and the reference Pt/C lost, respectively, *ca.* 35 % and *ca.* 44 % of their initial mass activity (see **Figure**
S3). Even though these results clearly show an enhanced performance of PtₓY/C with respect to the Pt/C reference, the activity value and retention of PtₓY/C do not correlate with the expected trend.

Previously, we have reported that Pt-rare earth nanoalloys undergo significant structure transitions during the early surface conditioning step, namely, the electrochemical activation.²⁸,²⁹ Such transitions, often not investigated or underestimated, do induce a considerable effect on the electrocatalyst activity and its retention.³⁰, ³¹ Therefore, *online ICP-MS* and *operando WAXS* measurements were conducted to follow any possible degradation of PtₓY/C during the electrochemical activation, see **Figure 1**.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** (a) Specific dissolution profiles extracted from *online ICP-MS* measurements; (b) schematic representation of the PtₓY nanoalloy dissolution and stabilization; and (c) microstructural refined parameters extracted from *operando WAXS* measurements [where (i) indicates the electrode potential wave profile used, (ii) the phase weight fraction, (iii) scale factor, (iv) lattice constant and (v) coherent domain size of the Pt₃Y alloy and Pt phases] of PtₓY/C during the electrochemical activation. The highlighted region in (a) and (c) indicates the first polarization scan. The arrows serve as a guide to the eye.</figcaption></figure>
A sharp peak is observed in the specific dissolution profiles, c.f. **Figure 1a**, for Pt and a broad peak for Y, which decay through the potential cycling until the signals are stabilized, revealing that the first cathodic scan (first polarization from the open circuit potential, OCP, to 0.05 V<sub>RHE</sub>) induces metal dissolution. It may be noticed that Pt dissolution starts before Y dissolution, which suggests that the partial or the total dissolution of the protective Pt overlayer formed *ex situ* (after the post-synthesis acidic wash) exposes the Y atoms to the acidic electrolyte, see **Figure 1b**.

These Y atoms are quickly oxidized into $Y^{3+}$ ($E^{0}_{Y^{3+}/Y^{0}} = -2.38$ V<sub>SHE</sub>), which represents a strong segregation driving force toward the surface and, eventually, the Y atoms are dissolved. This process might proceed until Pt is sufficiently available to form a thicker protective overlayer, reconstructing the surface and stabilizing the NP structure, see **Figure 1b**. In line with the specific dissolution profiles, the refined parameters extracted from *operando* WAXS measurements, **Figure 1c**, also reveal that as soon as the electrode potential goes from the OCP to 0.05 V<sub>RHE</sub>, evolution of the structure of the $Pt_{x}Y$ nanoalloys is triggered. Quantitatively, the metallic phase weight fraction of the $Pt_{3}Y$ alloy decreases from ca. 60 % to 45 %, and a Pt face-centered cubic (*fcc*) phase increases from ca. 40 % to 55 %. This trend might be linked to the Y dissolution observed in **Figure 1a**. The scale factor intensities, however, seem to be independent from each other, namely, the steady decrease of the $Pt_{3}Y$ scale factor (related to the Y dissolution) is not correlated to the sudden growth of the *fcc* Pt scale factor. This behavior suggests that the increase of the *fcc* Pt phase might originate from the electrochemical reduction of an amorphous phase containing Pt, even at such low electrode potential.<sup>32</sup> Besides, we do not rule out the probability that the dissolved Pt species might redeposit forming pure Pt crystalline domains during the potential cycling.<sup>33</sup> Furthermore, during the electrochemical activation, the
lattice constant of both Pt₃Y and fcc Pt phases experienced a contraction of ca. -0.4 % and -0.5 %, respectively. Finally, while the average coherent domain size of the Pt₃Y phase slightly grows from ca. 7.5 nm to 8 nm, that of the fcc Pt phase seems to be almost constant (ca. 1.2 nm).

After the electrochemical activation, three potentiodynamic cycles at 5 mV s⁻¹, from 0.05 to 0.95 V$_{RHE}$, were performed to clearly resolve the transient dissolution behavior of Pt and Y. **Figure 2a** shows the specific dissolution profiles of Pt$_x$Y/C under these conditions. In agreement with the literature,²⁴, ³³⁻³⁷ the predominant Pt dissolution signal is located at the cathodic scan. This feature is related to the electrochemical reduction of the formed Pt oxides.³⁵ Besides, the specific Pt dissolution profile also reveals the presence of an additional anodic dissolution peak during the first potential cycle. This signal could be associated with the dissolution of low-coordinated sites through the surface oxidation.²⁴ Moreover, the Y dissolution follows the first anodic Pt dissolution, and low-intense broad signals appear during the cathodic Pt dissolution, suggesting the stabilization of a protective Pt-rich shell after the electrochemical activation.

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**Figure 2.** (a) Specific dissolution profiles of PtₓY/C acquired during slow potentiodynamic cycles (3 cycles at 5 mV s⁻¹); (b) OH-adsorption induced segregation energy as function of the Y position in the Pt slab. The negative values of segregation energies indicate the preference of Y to migrate toward the surface. Representative HAADF-STEM analysis after the electrochemical activation of PtₓY/C: (c) atomic resolution micrograph (the insert shows the corresponding SAED); (d) EDX elemental mapping; and (e) EDX line scan chemical analysis.

For the sake of clarity, **Figure 2b** exhibits the OH-adsorption induced segregation energy diagram as function of the Y atom position in the Pt slab, i.e., Y atoms exposed at the surface (first surface layer) and protected by *n* Pt monolayers (*n* =1, 2 or 3). This diagram unveils that Y atoms present a strong affinity for OH, which favors their segregation towards the surface and eventually their dissolution (the more negative the segregation energy, the easier the segregation), in line with previous reports on Pt₃Y alloys covered by a Pt monolayer.³⁸ Notwithstanding, this trend is suppressed once Y atoms are protected by at least 3 Pt monolayers, stabilizing a core/shell (Pt-Y alloy/Pt-rich overlayer) structure. To gain more insights, high resolution HAADF-STEM analyses were carried out after the electrochemical activation, and are presented in **Figure 2c-e** and **Supplementary Information S3**. The average particle size measured *ex situ* is not affected by the electrochemical activation (see below). Besides, the atomic-resolution micrographs and their respective EDX chemical mapping/profiles clearly confirm the presence of the Pt-Y nanoalloy after the electrochemical activation, as NPs presenting a Pt enrichment of 0.6-1.2 nm at the surface. Considering that the thickness of an atomic monolayer of Pt is ~ 0.2 nm,³⁹ the observed Pt-rich overlayer is 3-6 atom thick, in agreement with the diagram shown in **Figure 2b**.
Following the PtₓY/C nanoalloy stabilization after electrochemical activation, the effect of the square potential wave used in the AST on compositional degradation was investigated by electrochemical *online* ICP-MS. To clearly deconvolute and resolve the dissolution events during the square potential wave, we have held the lower and upper potential limits (0.60 and 0.95 V<sub>RHE</sub> respectively) for 3 min, following a slow transition between them at 5 mV s<sup>-1</sup>. **Figure 3a** shows the acquired specific dissolution profiles using three cycles under the aforementioned conditions.

Looking at the Pt specific dissolution profile, the anodic dissolution signal through the surface oxidation (yellow arrows in **Figure 3a-b**) peaks as soon as the electrode potential reaches 0.95 V<sub>RHE</sub>, followed by a slow decay. This anodic dissolution event could be associated with the formation and dissolution of metastable Pt species (e.g., amorphous surface oxides,<sup>40</sup> Pt-OₓHᵧ<sup>41</sup>), the origin of which comes from the slow Pt oxidation kinetics.<sup>40</sup> After the formation of a stable Pt oxide, signal decay is observed indicating that the surface is passivated. Meanwhile, the cathodic dissolution event through the reduction of the stabilized Pt oxides (red arrows in **Figure 3a-b**) takes place, and results in a more intense single peak. This signal has been largely attributed to the oxide place-exchange mechanism, i.e., the exchange of the original lattice sites of surface Pt atoms by O atoms. Simultaneously, the Y specific dissolution profile presents signals during the first cycle at both anodic and cathodic regimes, which are attenuated during the second and third cycles. Based on **Figure 2b**, it might be expected that once the stabilized Pt shell is partially dissolved, its thickness eventually decreases, leading to higher segregation driving force of the previously protected Y atoms, and to the subsequent dissolution of Y until Pt diffusion re-stabilizes the NP structure. This observation might suggest that the stabilization of the Pt-rich shell is critical in hampering Pt-Y segregation.
<figure><img src="image_8.png" /><figcaption>**Figure 3.** (a) Specific dissolution profiles of Pt<sub>x</sub>Y/C acquired during simulated AST potential cycles (square-like wave, 0.60 – 0.95 V<sub>RHE</sub>, 3 min at each potential limit with a transition between them at 5 mV s<sup>-1</sup>). The yellow and red arrows (oxidation and reduction, respectively) serve as a guide to the eye. (b) schematic representation of the Pt<sub>x</sub>Y/C dissolution during the AST potential cycles; (c) microstructural refined parameters extracted from *operando* WAXS measurements (metallic weight phase fraction, lattice constant and coherent domain size) of the Pt<sub>3</sub>Y alloy and Pt phases detected during the applied AST (square-like wave, 0.60 – 0.95 V<sub>RHE</sub>, 3 s at each potential limit with a transition between them of 0.5 s).</figcaption></figure>

Furthermore, the degradation of the Pt<sub>x</sub>Y/C nanoalloys during the AST (square potential wave, 3 s at each potential limit with a transition of 0.5 s between) was followed by means of *operando* WAXS and *online* ICP-MS in O<sub>2</sub>-saturated 0.1 M HClO<sub>4</sub> at 25 °C. The evolution of the WAXS refined microstructural parameters during the AST (5,000 cycles) are shown in **Figure 3c**. The metal phase weight fraction of the Pt<sub>3</sub>Y alloy lies between 43 - 48 % during the first 1,000 cycles, and reaches an almost constant value of *ca.* 45 % afterwards. Moreover, during the first 1,000 cycles, the metal phase weight fraction of the *fcc* Pt phase fluctuates between 57 - 52 %, and stabilizes *ca.* 55 % afterwards. While the lattice constant of the *fcc* Pt phases underwent an
increasing trend (lattice constant expansion), the Pt₃Y phase follows a decreasing behavior (lattice constant contraction). This trend clearly reflects the expected steady loss of the compressive strain effect, with negative effect on the electrocatalytic ORR performance. Finally, the average coherent domain size trend of the Pt₃Y phase revealed a slight increase from *ca.* 7.8 nm to 8.4 nm during the first 600 cycles, continuing to *ca.* 8 nm (1,000 cycles), *ca.* 7.7 nm (2,000 cycles) and *ca.* 8 nm thereafter. Besides, the *fcc* Pt phase slight grew during the first 1,000 cycles from *ca.* 1.20 nm to 1.35 nm, achieving an almost constant value of *ca.* 1.37 nm thenceforth. Therefore, the strongest structural degradation during the AST takes place during the first 1000 cycles, in line with previous results on Gd/Pt(111) single-crystal electrodes.

Owing to the fact that the most pronounced microstructural variations are discernible during the first 1,000 cycles, the Pt and Y dissolution were monitored by *online ICP-MS* during the first 1,000 cycles of the AST (Supplementary Information S4). The total dissolved Pt and Y under these conditions, *i.e.*, the integrated specific dissolution rates, are *ca.* 155 pg and 201 pg, respectively, which represent a loss of 0.01 % and 0.13 % of the initial electrode loading after 1,000 AST cycles.

Enhanced kinetics of the sluggish ORR in acidic medium require the binding energies (ΔE) of the key reaction intermediates, *i.e.*, O* and OH*, to be weaker than those with Pt (111) by ~0.2 eV and ~0.1 eV, respectively.⁴² DFT computations were performed to track the ΔE*O* and ΔE*OH* as a function of the Pt-Pt interatomic distance, *c.f.* **Figure 4** and **Supplementary Information S5**. The Pt-Pt interatomic distances of PtₓY/C nanoalloys during the AST were extracted from the *operando* WAXS measurements and are plotted in **Figure 4**, from which a modest weakening of ΔE*O* (~ 0.02 eV) and ΔE*OH* (~ 0.001 eV) respect to Pt (111) may be observed.
This result indicates that the induced compressive strain is attenuated by the thickening of the Pt

277 overlayer, which leads to lower ORR activity.

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Figure 4. DFT calculated binding energies, with respect to Pt (111) surface, of (a) adsorbed O, and (b) adsorbed OH as function of the Pt-Pt interatomic distance. Purple points represent the experimental average Pt-Pt interatomic distance of PtₓY/C nanoalloys during the AST (5000 square-like wave, 0.60 – 0.95 V<sub>RHE</sub>, 3 s at each potential limit with a transition between them of 0.5 s).

With this knowledge, the local morphology and chemical composition of the PtₓY/C electrocatalyst were investigated after the AST in the RDE setup (30,000 square potential wave, 3 s at each potential limit with a transition of 0.5 s between in O₂-saturated 0.1 M HClO₄ at 25 °C) using high resolution HAADF-STEM analyses, c.f. **Figure 5** and **Supplementary Information S6**. **Figure 5a** shows the evolution of the average particle size from the *ex situ* state to the end of the AST: while the electrochemical activation does not strongly affect the average particle size, the potential cycling of the AST causes a growth of the electrocatalyst particles from 5.4 nm to 6.6 nm, in line with the average coherent domain size trend in **Figure 3c**. The ECSA variations shown in **Figure S3** might be related to this particle size evolution. Although the acquired micrographs indicate the predominance of dense NPs, the presence of porous
structures after the AST was also observed (Supplementary Information S6), which are
expected due to the dealloying process occurring on potential cycling.<sup>7, 39</sup> The atomic-resolution
micrographs and their respective EDX chemical mapping/profiles, **Figure 5b-d**, clearly confirm
the presence of the Pt-Y nanoalloy after the AST, with a thicker Pt enrichment at the border of
the NPs of *ca.* 1.2-1.6 nm, being equivalent to a Pt shell of 6-8 atom thick.

<figure><img src="image_11.png" /><figcaption>**Figure 5.** Representative HAADF-STEM analysis of PtₓY/C after the AST: (a) evolution of the average particle size, from the *ex situ* state, the electrochemical activation and at the end of the AST; (b) atomic resolution micrograph (the insert shows the corresponding SAED); (c) EDX elemental mapping; and (d) EDX line scan chemical analysis.</figcaption></figure>

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4. CONCLUSION
Carbon-supported PtₓY nanoalloys were extensively studied, for the first time, under the electrochemical conditions for the ORR by means of *operando* WAXS, *online* ICP-MS, atomic resolution HAADF-STEM and DFT calculations. This allowed monitoring of the atomic-scale degradation steps undergone by the electrocatalyst from the early electrode surface conditioning (or electrochemical activation) to the end of the applied AST. Such results clearly revealed that the PtₓY/C nanoalloys underwent considerable degradation during the early operation steps (electrochemical activation), with metal dissolution and crystalline structure evolution being observed, and surface reconstruction.

Furthermore, *operando* measurements identified that the strongest nanoalloy degradation, in terms of metal dissolution and structural evolution, takes place during the first AST 1,000 cycles under O₂ atmosphere, which eventually diminished the ORR kinetic benefit from the Pt-Y alloy.

Although the expected high ORR performance of PtₓY/C was not observed, the proposed *operando* structure-activity-stability trends guides further optimization of the delicate activity/stability trade-off of this system. Besides, we believe that this work might inspire further in-depth understanding of the degradation of carbon-supported Pt-based nanoalloys during the harsh ORR electrochemical environment.

## AUTHOR INFORMATION

### Corresponding Authors

*Carlos A. Campos-Roldán

Email: carlos-augusto.campos-roldan@umontpellier.fr
329 *Sara Cavaliere

330 Email: sara.cavaliere@umontpellier.fr

331

332 ORCID

333 C.A. Campos-Roldán: 0000-0002-1517-9037

334 R. Chattot: 0000-0001-6169-530X

335 J.S. Filhol: 0000-0002-3681-9267

336 H. Guesmi: 0000-0002-9369-523X

337 N. Romero: 0000-0002-2704-7779

338 P.Y. Blanchard: 0000-0003-1659-6868

339 J. Drnec: 0000-0002-9520-1555

340 J. Rozière: 0000-0001-6211-5047

341 D. Jones: 0000-0003-3787-2462

342 S. Cavaliere: 0000-0003-0939-108X

343

344 Author Contributions

345 The manuscript was written through contributions of all authors. All authors have given approval

346 to the final version of the manuscript.

347 Author Contributions
Carlos A. Campos-Roldán: Conceptualization, writing-original draft, formal analysis, investigation, **Raphaël Chattot**: Conceptualization, writing-review & editing, formal analysis, investigation, **Jean-Sébastien Filhol**: Writing-review & editing, formal analysis, methodology, **Hazar Guesmi**: Writing-review & editing, formal analysis, methodology, **Nuria Romero**: Methodology, investigation, writing-review & editing, **Rémi Bacabe**: Investigation, **Pierre-Yves Blanchard**: Writing-review & editing, supervision, **Valentin Vinci**: Writing-review & editing, investigation, **Jakub Drnec**: Writing-review & editing, formal analysis, investigation, **Jacques Rozière**: Funding acquisition, conceptualization, writing-original draft, supervision, **Deborah J. Jones**: Funding acquisition, conceptualization, writing-original draft, supervision, **Sara Cavaliere**: Funding acquisition, conceptualization, writing-original draft, supervision.

## ASSOCIATED CONTENT

The following files are available free of charge.

**Supporting Information:** Comparison of the current state-of-the-art, experimental methodology, computational details, *ex situ* complementary characterization, complementary HAADF-STEM analyses after the electrochemical activation and after the AST, and *online* ICP-MS dissolution profile during the first AST 1,000 cycles are available in the Supporting Information.

## ACKNOWLEDGEMENTS

The research leading to these results has received funding from the IMMORTAL project, which receives funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (now Clean Hydrogen Partnership) under grant agreement No 101006641. This Joint Undertaking receives support from the European Union's Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe Research. We thankfully
372 acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of
373 synchrotron radiation facilities at the ID31 beamline (DOI 10.15151/ESRF-ES-
374 1049168140). We also thankfully acknowledge the technical support in the electron
375 microscopy from Dr. Vincent Collière from the Centre de Microcaractérisation Raimond
376 Castaing de Toulouse (UAR3623).

377
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