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

# Highly active Ptₓ(SnO₂)₁₀₀-x/C catalysts towards the hydrogen oxidation reaction and tolerant to the presence of carbon monoxide

Franck Koffi Bouho<sup>a</sup>, Weliton Silva Fonseca<sup>a, ID</sup>, Sirine Ben Latifa<sup>a</sup>, Thibault Rafaïdeen<sup>a, ID</sup>, Frédéric Pailloux<sup>b</sup>, Julie Rousseau<sup>a</sup>, Christine Canaff<sup>a</sup>, Sophie Morisset<sup>a</sup>, Tèko W. Napporn<sup>a, c, ID</sup>, Christophe Coutanceau<sup>a, c, * ID</sup>

## ARTICLE INFO

### Keywords:

CO

EMSI

HOR

PEMFC

Platinum

Tin oxide

Tolerance

## ABSTRACT

Ptₓ(SnO₂)₁₀₀-x/C materials consist of 1.5 to 2.5 nm nanoparticles deposited on a carbon support and decorated by SnO₂. Pt and Sn are slightly alloyed (5 % to 8 %) in nanoparticles. From XPS study the Sn 3d peaks present a shift towards lower binding energy values, indicating clearly an electron transfer from Pt(PtSn) towards SnO₂ (electronic metal support interaction – EMSI). The incorporation of SnO₂ into Pt enhances the catalytic activity towards the HOR at 0.02 V vs. RHE. The mass activity doubles, increasing from 147 mA mgₚt⁻¹ for Pt/C to 286 mA mgₚt⁻¹ for Pt₅₀(SnO₂)₅₀/C, while the specific activity increases fourfold, from 0.33 mA cmₚt² to 1.33 mA cmₚt². These enhancements are attributed to EMSI. Ptₓ(SnO₂)₁₀₀-x/C catalysts exhibit lower onset potentials for COads removal compared to Pt/C, both in the absence and presence of hydrogen. The high rate of CO removal is attributed to the bifunctional mechanism, while the shift of the COads oxidation onset potential towards lower values from a ligand effect. At 60 °C, all catalysts show activity for the HOR from 0.05 V vs. RHE in the presence of CO. Among them, Pt₅₀(SnO₂)₅₀/C demonstrates the highest HOR activity and the greatest CO tolerance.

## 1. Introduction

Three decades after the Rio conference, the world experiences the effects of climate change disruption caused by the accumulation of greenhouse gases in the atmosphere. To limit the global warning below 2 °C at the end of the century, the European Union has signed the Green Deal, aiming for carbon neutrality by 2050 [1–3]. Therefore, a transition toward more environmentally friendly energy sources is necessary. In this context, proton exchange membrane fuel cell (PEMFC) technology fed with green hydrogen (produced from renewable energy sources) can be a good alternative as energy supplier. Indeed, PEMFC can produce electrical energy without carbon dioxide (CO₂) emissions [4]. It is a mature technology that offers numerous advantages in terms of efficiency and power density, facilitating its use in automotive, stationary and aerospace applications. The main reactions taking place within a PEMFC are the hydrogen oxidation reaction (HOR) at the anode and the oxygen reduction reaction (ORR) at the cathode [5]. The overall reaction produces water, electricity and heat according to equation (1):

$$H_2 + O_2 \rightarrow H_2O + \text{Electricity} + \text{Heat} \quad (1)$$

The democratisation of the PEMFC technology requires the reduction of electrode manufacturing costs, particularly because they contain platinum (Pt). Indeed, Pt is the most active metal for both the HOR and ORR in acidic environment. Gasteiger et al. [6] showed that reducing the Pt loading in anode down to 0.05 mg cm⁻² did not result in any voltage loss when using pure hydrogen (H₂). However, the presence of impurities in hydrogen, such as carbon monoxide (CO), CO₂, hydrogen sulphide (H₂S), etc., even as trace amounts, leads to decrease dramatically the performance of PEMFCs [7–10]. Currently, H₂ is mainly produced from steam reforming of natural gas, and contains ca. 1 % CO after the water gas shift reactions. Papageorgopoulos and De Bruijn [11] showed that 1 % CO in H₂ reformate poisons 98 % of the active surface sites of a Pt catalyst used at the anode of the fuel cell. As contamination by CO is a major problem in PEMFC technology, new anodic CO-tolerant Pt-based catalysts modified by metals with oxyphilic properties have emerged [12–16], amongst them platinum-tin (PtSn) materials because Sn is less expensive than ruthenium (Ru), and more efficient in term of CO-

<footnote><sup>a</sup> Université de Poitiers, IC2MP, UMR CNRS 7285, 86073 Cedex 9 Poitiers, France</footnote>

<footnote><sup>b</sup> Institut Pprime, Université de Poitiers-CNRS-ENSMA, UPR 3346, France</footnote>

<footnote><sup>c</sup> French Research Network on Hydrogen (FRH2), Research Federation No. 2044 CNRS, France</footnote>

<footnote>* Corresponding author at: Université de Poitiers, IC2MP, UMR CNRS 7285, 86073 Cedex 9 Poitiers, France.
E-mail address: christophe.coutanceau@univ-poitiers.fr (C. Coutanceau).</footnote>
tolerance [17,18].

If metallic Sn possesses a good electronic conductivity, it is not very stable in acidic media; it suffers from corrosion and oxidizes easily in air. Tin oxide (SnO₂) is a n-type semiconductor [19], which offers interesting properties for PEMFC applications. Zhang et al. [20] synthesized SnO₂ with a high surface area (205 m² g⁻¹) and used it as a support for Pt nanoparticles (NPs). They showed that SnO₂ promoted good dispersion of the Pt NPs, and they obtained equivalent activity and better stability for the HOR than with classical Pt/C catalysts. Dou et al. [21] drawn the same conclusions using tin oxide nanocluster (SnO₂) with parallel nanorods as anode catalyst support for PEMFCs. The possibility of a strong metal support interaction (SMSI) or electronic metal support interaction (EMSI) was recently proposed to explain the improved electrochemical properties of Pt-SnO₂ composites [22]. SMSI/EMSI has been evidenced through the improvement of durability and stability of Pt NPs decorating WO₃ nanoplatelets, as well as through the enhancement of the HOR activity. These improvements have been indeed explained in terms of synergetic effects and H spill over [23]. In both these cases, the SMSI/EMSI effect was explained in terms of shift of the Pt d-band centre due to electron transfer from the oxide support towards Pt nanoparticles. The second interest of using SnO₂ as catalyst support is related to its oxidizing properties, which could promote the electro-oxidation of CO to CO₂. [24] Metal oxides have been shown to improve, for example, alcohol oxidation by decreasing the development of adsorbed carbon monoxide (COads) on the catalyst surface. [25,26].

Despite extensive studies, the mechanism underlying the high CO tolerance of Ptₓ(SnO₂)₁₀₀-x/C catalysts remains debated. Some authors attribute it to the electronic (ligand) effect, where SnO₂ incorporation modifies the Pt electronic structure (ESMI), weakening the Pt-COads bond. Others emphasize the bifunctional mechanism, in which tin oxides dissociate water (H₂O) and generate adsorbed hydroxyl species (OHads) at lower potentials than Pt, thereby promoting COads oxidation [24,27–29].

This study aims to clarify which mechanism predominates for both hydrogen oxidation and COads oxidation on Ptₓ(SnO₂)₁₀₀-x/C catalysts. For this purpose, catalytic materials of different atomic compositions, Ptₓ(SnO₂)₁₀₀-x/C, are synthesized by a polyol method and comprehensively characterised by different physicochemical techniques, including thermogravimetry analysis (TGA), inductive coupled plasma – optical emission spectroscopy (ICP-OES), high resolution transmission electron microscopy (HR-TEM), energy dispersive x-ray spectroscopy (EDS), energy filtered selected area electron diffraction (EF SAED), x-ray diffraction (XRD) and x-ray photoemission spectroscopy (XPS). Then, the electrochemical behaviours of the catalysts towards the HOR in the absence and in the presence of CO are assessed by cyclic voltammetry (CV), linear scan voltammetry (LSV) and chronoamperometry (CA), and results compared with the objective to further propose a structure/ reactivity relationship and reaction mechanisms.

## 2. Experimental

### 2.1. Catalysts preparation

The synthesis of catalytic materials is split into two distinct steps. The first step consists in synthesising the SnO₂ colloid. It involves dissolving a given mass of tin chloride salt (SnCl₂·2H₂O, Sigma Aldrich, purity > 98.9 %) in ethylene glycol (puriss. p.a., purity > 99.5 % from Fluka) for 2 h under magnetic stirring and adjusting the pH of the medium to 7–8 using a 1 M sodium hydroxide (NaOH, semiconductor grade, 99.99 % purity, Sigma-Aldrich) in ethylene glycol (EG) solution. Then the mixture is heated at reflux (ca. 190 °C) for 2 h with oxygen (O₂) bubbling (U Quality from Air Liquide). The light-yellow coloration confirms the formation of tin oxide NPs.

The second step involves the dissolution of hexachloroplatinic acid (H₂PtCl₆·6H₂O, 99.9 % purity from Alfa Aesar) in EG for half an hour under magnetic stirring, the adjustment of the pH to 11 with a 1 M NaOH

in EG solution, and the addition of a given volume of the previous SnO₂ colloidal solution to achieve nominal molar ratios of Pt₁₀₀, Pt₉₀(SnO₂)₁₀, Pt₆₅(SnO₂)₃₅ and Pt₅₀(SnO₂)₅₀. The mixture is then heated at 160 °C for 2 h. Once the temperature has decreased to room temperature, a mass of carbon Vulcan XC 72 (Cabot Corp.) pre-treated at 400 °C under nitrogen (N₂, U quality from Air Liquid) flow for 4 h is added to reach a metal loading of 40 wt%. Finally, the mixture is washed with ultra-pure (UP) water (MilliQ, Millipore, 18.2 MΩ cm) and acetone (> 99 %, WWR Chemicals), then dried at 60 °C for 2 h.

### 2.2. Physicochemical characterisation

TGA are performed to determine the material metal loadings with a TA Instrument model SDT Q 600. Measurements were carried out under air flow at 100 mL min⁻¹ between 25 °C and 800 °C with a temperature ramp of 10 °C min⁻¹.

XRD analyses are carried out using a PANalytical diffractometer in Bragg-Bentano configuration, equipped with a Cu anode powered at 45 kV and 40 mA. Measurements are recorded between 20° and 80° with steps of 0.083° and an acquisition time of 2 s.

ICP-OES analyses are carried out using a 5110 Agilent VDV apparatus to determine the bulk atomic ratio of Pt and Sn.

TEM was carried out using a JEOL (200 keV) model JEM 2100 with a LaB₆ gun (maximum resolution of 0.19 nm). HRTEM, EDS mapping and energy filtered selected area electron diffraction (EF-SAED) analysis were performed using a JEM 220FS TEM/STEM equipped with a shottky-FEG operated at 200 kV and fitted with an in-column omega-filter. EF-SAED diffractograms were recorded on a GATAN-OneView CMOS camera; the width of the slit for energy-filtering was set to 10 eV around the zero-loss peak to get rid of the strong inelastic scattering coming from the thick carbon support. EDS spectra and maps were acquired with a 60 mm² Bruker X-FLASH SDD; a single-tilt Be-holder was used to minimize the background contribution and spurious signals coming from the TEM. For elemental mapping, the electron probe size was set to 1.5 nm. High angle annular dark-field scanning transmission electron micrographs (HAADF-STEM) were obtained simultaneously.

XPS analysis were carried out with a Kratos Axis Ultra DLD spectrometer using a monochromatic Al Kα source (1486.6 eV) operating at 180 W (12 mA, 15 kV). Pressure in analytic chamber was 9 × 10⁻⁸ Pascal. High-resolution spectra were recorded using an analysis area of 300 µm × 700 µm and 40 eV pass energy. This pass energy corresponds to Ag 3d5/2 FWHM of 0.55 eV. Data were acquired with 0.1 eV steps. The measurements were performed without using the charge neutralizer because all samples were conductive. The binding energy scale and spectrometer work function were calibrated previously with pure gold foil. The following binding energies regions were recorded: C 1 s, O 1 s, Sn 3d and Pt 4f. Atomic ratios were calculated using sensitivity factors provided by the manufacturer. Peak fitting was achieved using Casa XPS software (version 2.3.26) applying Gaussian-Lorentzian profiles (Lorentzian 30 % and 60 %). Prior to quantification, the background was subtracted with a Shirley function.

### 2.3. Electrochemical characterisation

All electrochemical measurements are performed at the controlled temperature of 25 °C in a 0.1 M HClO₄ (Suprapur, Merck) in UP water supporting electrolyte using an interfaced Autolab PGSTAT302N potentiostat (Metrohm). The standard three-electrode electrochemical cell is fitted with a reversible hydrogen electrode (RHE, HydroFlex from Gaskatel) as the reference electrode and a glassy carbon plate (2 cm² geometric surface area) as the counter electrode. The working electrode consists of a glassy carbon rotating disc of 0.196 cm² geometric surface area (RDE GC50.S, Metrohm) on which the catalytic layer is deposited. The catalytic ink is composed of 5 mg of a catalytic powder dissolved in 750 µL of UP water, 250 µL of 2-propanol (anhydrous 99.5 %, Sigma-Aldrich) and 100 µL of Nafion® solution (5 %wt. in aliphatic alcohol
from Aldrich). The mixture is homogenised in an ultrasonic bath (Fisherbrand 15,051 ultrasonic bath, 2.75 L, 37 kHz, 320 W) for 30 min, and 5 µL of the catalytic ink are deposited from a syringe onto a freshly polished glassy carbon substrate. The solvent is evaporated in a N₂ flow at the electrode rotation rate of 400 rpm (revolution per minute).

Cyclic voltammograms (CVs), linear scan voltammograms (LSVs) and CO (N47 quality from Air Liquide) stripping measurements were recorded over a potential range from 0.05 to 1.00 or 1.20 V vs. RHE at a potential scan rate of 0.02 V s⁻¹. CO stripping measurements are recorded after 30 min-chronoamperometry (CA) at 0.10 V vs. RHE in a CO saturated electrolyte (5 min) followed by N₂-bubbling for 25 min to remove dissolved CO. CO stripping measurements in presence of H₂ (H2 HG quality from Air Liquide) are recorded at a rotation rate of 1,600 rpm after a 30 min-chronoamperometry at 0.10 V vs. RHE in a CO saturated electrolyte (5 min) followed by N₂-bubbling for 15 min to remove dissolved CO, and H₂-bubbling for 10 min to saturate the electrolyte. HOR polarization curves are recorded in a H₂-saturated electrolyte at a potential scan rate of 0.005 V s⁻¹ and different electrode rotation rates.

# 3. Results and discussion

## 3.1. Physicochemical characterisation of the materials

First, TGA analyses were performed to check the metal loadings on the carbon support; values ranging from 37 wt% to 39 wt%, i.e., very close to the targeted one (40 wt%), were obtained. ICP-OES analyses indicated that the bulk compositions of materials were also very close to the nominal ones (Table 1). Both these results put together indicate that the synthesis reactions have occurred quantitatively. Further, mass activity of catalysts was determined taking into account these characterisations.

X-ray diffractograms were recorded to obtain information on the phases composing the catalysts (Fig. 1). All XRD patterns display a peak at 2θ ≈ 25° attributed to the (002) planes of turbostratic graphite in Vulcan XC 72 carbon [30] and peaks at 2θ of *ca.* 39.8°, 46.4°, 67.6° and 81.4° assigned to (111), (200), (220) and (311) planes of the face-centred cubic (fcc) structure of Pt (JCPDS card n°. 04–0802) [31], respectively. For tin containing catalysts, additional peaks appear and grow in intensity as the tin content increases. These peaks located at *ca.* 26.4°, 34.1° and 51.8° correspond to the (110), (101) and (211) planes of a SnO₂ phase having a tetragonal rutile structure (JCPDS card n°. 41–1445) [32,33], respectively. Moreover, the diffraction peaks assigned to Pt are shifted towards lower 2θ values, which can be due to the formation of an alloy between Pt and Sn. Indeed, the incorporation of Sn into the Pt lattice should result in an increase of the interplanar spacing and therefore in a shift of the diffraction peaks towards lower 2θ values [34]. The crystallite sizes and the corresponding lattice parameters have been calculated from the Pt (111) diffraction peak using the Debye-Scherrer formula and the Bragg equation (Table 1), respectively. The lattice parameter values are higher for the bimetallic materials than for the monometallic Pt/C catalyst, leading to low degrees of alloying from *ca.* 5% to *ca.* 8% as the tin content in catalysts increases.

TEM images of catalysts in Fig. 2 A, B, C and D show metal particles with small sizes well dispersed on the carbon support. However, according to the difference of contrast, it seems that two morphologies are

<caption>**Table 1**</caption>

Physicochemical characterisation of the Pt-based catalysts: metal weight loadings by TGA, bulk atomic composition by ICP-OES, crystallite size (*L*<sub>v</sub>) and cell parameter (*a*) by XRD and particle size (*d*<sub>TEM</sub>) by TEM.

<table><thead><tr><th></th><th>Metal wt.%</th><th>Comp. at%</th><th>L<sub>v</sub> (nm)</th><th>d<sub>TEM</sub></th><th>a<sub>cell</sub> (nm)</th></tr></thead><tbody><tr><td>Pt/C</td><td>37</td><td>100</td><td>2.5</td><td>2.1</td><td>3.92</td></tr><tr><td>Pt<sub>90</sub>(SnO<sub>2</sub>)<sub>10</sub>/C</td><td>39</td><td>91/9</td><td>1.9</td><td>2.0</td><td>3.97</td></tr><tr><td>Pt<sub>65</sub>(SnO<sub>2</sub>)<sub>35</sub>/C</td><td>39</td><td>61/39</td><td>2.5</td><td>2.6</td><td>3.97</td></tr><tr><td>Pt<sub>50</sub>(SnO<sub>2</sub>)<sub>50</sub>/C</td><td>38</td><td>51/49</td><td>1.8</td><td>1.7</td><td>3.98</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** XRD patterns of the Pt-based catalysts and peak assignation for the face centred cubic and hexagonal structures of Pt and SnO₂ according to the JCPDS 04–0802 and JCPDS 41–1445 cards, respectively.</figcaption></figure>

deposited on the carbon support for tin-containing catalysts: isolated spherical nanoparticles with diameter close to 2 nm as determined from the histograms in Fig. 2 E, F, G and H, and agglomerate of 4–6 nm in size. EDS mappings (Fig. 2I, J and K) show that Pt and Sn are homogeneously distributed on the support. Their signals are superimposed, indicating that both metals are close each to the other. Energy filtered selected area electron diffraction patterns are displayed in Fig. 2 L. For the monometallic Pt/C catalysts, the ring sequence is characteristic of the fcc (Fm3m) structure of Pt. For the Pt<sub>50</sub>(SnO<sub>2</sub>)<sub>50</sub>/C, additional rings appear, which could be assigned to the P4/4mnm tetragonal crystal structure of SnO₂.

XPS measurements were performed to determine the oxidation states of surface Pt and Sn atoms and possible electronic interactions between both metals. Considering the binding energy (BE) values in the Pt 4f orbital core spectra (Fig. 3 A, B, C and D), the fits of the spectra indicate that Pt is in all cases mainly present at the zero valent state (Pt⁰ at BEs of *ca.* 71.2 and *ca.* 74.5 eV, for 4f 7/2 and 4f 5/2, respectively) [35,36], then at the +2 valent state (PtO or Pt(OH)₂ at BEs of *ca.* 72.0 and 75.3 eV) and as trace amount at the four valent state (PtO₂ at BEs of *ca.* 75.2 eV and 78.5 eV) [37]. The ratio of Pt species varies between the catalysts, but no trend is observed, and the variation could be only due to exposure of catalyst to air (ex situ XPS analyses). It is also worth to note that the peak position seems to not shift with the tin atomic ratio in the catalysts (Fig. 3 E), as if no interaction between both Pt(PtSn) and SnO₂ occurred.

Now considering the Sn 3d orbital core spectra (Fig. 4), the fit of the signal indicates that for Pt<sub>90</sub>(SnO<sub>2</sub>)<sub>10</sub>/C (Fig. 4A) and Pt<sub>65</sub>(SnO<sub>2</sub>)/C (Fig. 4B), Sn is present at different oxidation states leading to 3d 5/2 and 3d 3/2 peaks at BEs of *ca.* 485.4 eV and 493.8 eV, and *ca.* 487.2 eV and 495.6 eV, respectively. The BE value of the Sn 3d5/2 at *ca.* 485.4 eV is higher than that recorded for the pure Sn metal under the same conditions on our XPS apparatus (485.0 eV). It is close to that obtained by Wang et al. [38] on PtSn bimetallic nanoparticles (485.6 eV). These authors noted that such a value was higher than those usually reported in the literature for metallic Sn (485.0 eV) [35,39] but lower than values typically reported for oxidic Sn species (485.8 eV to 487.0 eV) [38]. Based on studies on surface alloying at the Sn-Pt(111) interface, Galeotti et al. [40] have shown that alloying induced a shift by *ca.* 0.3 eV towards higher BE with respect to the same peaks for bulk Sn. A BE value of 485.3 eV was also assigned to the formation of a PtSn alloy by Virnovskaja et al. [41]. The formation of an alloy, even at a low degree, has been evidenced by XRD measurements. But the partial oxidation of Sn at
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** (A, B, C, and D) TEM images of the Pt-based catalysts, (E, F, G, H) corresponding size distribution histograms for isolated NPs, (I, J, K) EDS mapping of the Pt₅₀(SnO₂)₅₀/C catalyst and (L) EF-SAED analyses of the carbon support, Pt/C and Pt₅₀(SnO₂)₅₀/C catalysts.</figcaption></figure>

the surface of Pt nanoparticles can also occurs and the BE value recorded can represent the average of at least both these processes [38]. The Sn 3d5/2 peak at higher BE (between 486.9 eV and 487.4 eV) is certainly due to the presence of a highly oxidized phase [37], and a SnO₂ structure has been identified by both XRD and EF-SAED measurements. The position of these peaks is shifted towards lower BE values compared to that for the pure SnO₂/C material (Fig. 4 D), and the delta increases with the ratio of SnO₂ in the catalyst (Fig. 4 E), which could again translate into an electron transfer from Pt and/or PtSn towards SnO₂ [42,43]. This electron transfer does not produce any observable BE shift for Pt (Fig. 3E). Such phenomenon was already observed by Ma et al. for PtₓSeₓ [44] and by Babu et al. [45] for RuₓSeᵧ materials, where the BE values for Se 3d shifted towards lower values while those of Pt 4f or Ru 3d remained unchanged, which was proposed to be due to the open d-shell configuration of Pt or Ru. This electron transfer could explain the higher BE values for Sn⁰ than that generally obtained from the literature and the lower one for SnO₂ in Sn-containing catalysts than in the pure SnO₂/C material, as well as the decrease of the intensity of the Sn⁰ peak with

respect to that at higher BE (between 486.9 eV and 487.4 eV), to completely disappear for the Pt₅₀Sn₅₀/C catalyst (Fig. 4C); indeed, as the amount of Sn increases in the catalysts, the alloying degree remains low but the formation of highly oxidized tin species increases).

From this comprehensive characterisation of the catalysts, Pt(SnO₂)/C materials correspond to slightly alloyed PtSn (5 % to 8 %) nano-particles of ca. 2 nm mean diameter in strong interaction with a SnO₂ structure, allowing an electronic metal support interaction (EMSI) [43,46]. Indeed, physicochemical characterization of the catalysts indicates clearly the existence of metallic Pt and of SnO₂, and also that Pt and SnO₂ are localized at the same place, which will favour interactions between those species.

### 3.2. Electrochemical characterisation of catalysts

CVs were recorded in a N₂-purged 0.1 M HClO₄ aqueous electrolyte at 0.02 V s⁻¹ and 25 °C (Fig. 5A). The CVs show a decrease of current in the HUPD/desorption (hydrogen underpotential deposition/desorption)
<figure><img src="image_4.png" /><figcaption>Fig. 3. XPS core level spectra of Pt 4f recorded on (A) Pt/C, (B) Pt<sub>90</sub>(SnO<sub>2</sub>)<sub>10</sub>/C, (C) Pt<sub>65</sub>(SnO<sub>2</sub>)<sub>35</sub>/C, (D) Pt<sub>50</sub>(SnO<sub>2</sub>)<sub>50</sub>/C catalysts (each component used to fit the spectra are presented with the corresponding legend) and (E) comparison of the XPS core level spectra of Pt 4f recorded for all the catalysts.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. XPS core level spectra of Sn 3d recorded on (A) SnO<sub>2</sub>/C, (B) Pt<sub>90</sub>(SnO<sub>2</sub>)<sub>10</sub>/C, (C) Pt<sub>65</sub>(SnO<sub>2</sub>)<sub>35</sub>/C, (D) Pt<sub>50</sub>(SnO<sub>2</sub>)<sub>50</sub>/C catalysts (each component used to fit the spectra are presented with the corresponding legend) and (E) comparison of the XPS core level spectra of Sn 3d recorded for all the catalysts.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 5. (A) Cyclic voltammograms of Pt-based catalysts in N₂-purged 0.1 mol L⁻¹ HClO₄ electrolyte, (B) CO stripping voltammograms on the Pt-based catalysts in N₂-purged 0.1 mol L⁻¹ HClO₄ electrolyte after CO adsorption for 5 min at 0.10 V vs. RHE (scan rate = 0.02 V s⁻¹, T = 25 °C and electrode rotation rates, ω = 1600 rpm).</figcaption></figure>

potential region on Pt (between 0.05 V and 0.40 V vs. RHE) and an increase of the capacitive current between 0.40 V and 0.50 V vs. RHE with the increasing SnO₂ content. These behaviours indicate that the surface of platinum is covered by tin, most likely under the form of a SnO₂ phase, as it is known to be a good capacitive material [47]. The electroactive surface area of the catalysts has been assessed from the HUPD region (EASA HUPD) after correction of the capacitive double layer current density [48]. EASA HUPD values in Table 2 confirm the decrease of surface Pt availability with the increase of tin content in catalysts.

The CO stripping profile recorded on Pt/C (Fig. 5B) is typical of a material synthesized by the polyol method [49–51], displaying an apparent onset potential of oxidation at ca. 0.65 V vs. RHE, followed by a shoulder and the main peak centred at ca. 0.80 V vs. RHE. The assignment of the different features can be found in our previous work [51]. The CO stripping profiles of tin-containing materials show two peaks, one with a maximum close to 0.50 V vs. RHE and the other one with a maximum in the 0.70 V – 0.80 V vs. RHE range (depending on the tin ratio), in agreement with previous studies on PtSn catalysts [36,52]. The charge involved for the removal of CO decreases from 50.5 C for Pt/C to 37.7 C for Pt₅₀(SnO₂)₅₀/C, confirming the lower availability of Pt surface as the ratio of tin in the catalyst increases. Also, the onset potential dramatically shifted toward lower values. From *in situ* infrared spectroscopy measurements of CO oxidation on Pt₃Sn(hkl) surfaces, Stamenković et al. [53] proposed two different forms of adsorbed CO on Pt₃Sn(111), with different reactivity, but did not observe the same behaviour on Pt₃Sn(100). Moreover, other authors studying CO oxidation on PtSn nanoparticles supported on carbon [54] did not obtain any evidence of different adsorbed CO by *in situ* infrared spectroscopy, nor of a ligand effect. They proposed then that the bifunctional mechanism was the main oxidation process at low potential. However, here the oxidation onset potential decreases from 0.28 V vs. RHE for the Pt₉₀(SnO₂)₃₀/C catalyst to 0.22 V vs. RHE for the Pt₅₀(SnO₂)₅₀/C one. This is an important observation, because Huang et al. [52] have shown using *in situ* XAS (x-ray absorption spectroscopy) measurements that in addition to a bifunctional mechanism involving the Pt-Sn/Pt-Sn-OH redox couple at low potentials, the change of the onset potential for the CO oxidation

with the Sn coverage indicated that the CO oxidation could be promoted by weakening Pt-CO bond through a Ligand effect involving both strong Pt-Sn bonds and interaction between Pt-CO and Sn-OH co-adsorbed species.

The decrease in Pt surface availability with the increase of the tin ratio in catalysts could be detrimental for their electrocatalytic activity towards the HOR. Then HOR polarization curves were recorded at different rotation rates in a H₂-saturated 0.1 M HClO₄ aqueous electrolyte at 0.005 V s⁻¹ and 298 K (Fig. 6 A, D, G and J), from which the corresponding Koutecky-Levich plots were drawn (Figures B, E, H and K).

Kinetic parameters such as the kinetic current density ($j_k$), exchange current density ($j^*$) and Tafel slope (b) are then determined by using the generalized Koutecky-Levich equation [12,49]. Tafel plots are given in Fig. 6 C, F, I and L. Table 2 gives the values of the kinetic parameters obtained for all the materials.

$$ \frac{1}{j} = \frac{1}{j_1^{\text{diff, elec}}} + \frac{1}{j_1^{\text{film}}} + \frac{1}{j_k} = \frac{1}{j_1^{\text{diff, elec}}} + \frac{1}{j_1^{\text{film}}} + \frac{1}{j^*}e^{\eta/b} \quad (2) $$

where j is the current density at a given electrode potential, $j_1^{\text{diff, elec}}$ the diffusion limiting current density of H₂ in the electrolyte, $j_1^{\text{film}}$ the diffusion limiting current density in the catalytic film, and η is the HOR overpotential.

The j- and j_k values are all very close to that determined for the pure Pt/C catalyst, although the EASA are much lower, particularly in the case of the Pt₅₀(SnO₂)₅₀/C catalyst. Indeed, the catalyst mass activity (MA) and specific activity (SA) dramatically increase with the increase of tin content, by factors two and four, respectively. It is well known that SnO₂ is not active for the HOR, but its interaction with Pt or PtSn seems to induce a modification of the electronic state of the Pt or PtSn surface active sites (EMSI), which accelerates the HOR kinetics. It is generally proposed an electron transfer from the metal oxide toward Pt to explain the SMSI effect [22,23]. It is worth noting that the same electron transfer, i.e., from the metal oxide toward Pt, is evoked for the enhancement of both the ORR [22] and the HOR [55]. Shen et al. [55]

<caption>**Table 2**</caption>

Electroactive surface area (EASA HUPD) and kinetic data for the HOR assessed from rotating disc electrode measurements in Fig. 6 at Pt-based catalysts: kinetic current density ($j_k$), specific activity (SA) and mass activity (MA) at 0.02 V vs. RHE, and exchange current density ($j_0$) and Tafel slope (b).

<table><thead><tr><th></th><th>Pt/C</th><th>Pt<sub>90</sub>(SnO<sub>2</sub>)<sub>10</sub>/C</th><th>Pt<sub>65</sub>(SnO<sub>2</sub>)<sub>35</sub>/C</th><th>Pt<sub>50</sub>(SnO<sub>2</sub>)<sub>50</sub>/C</th><th>Pt/C<br>Tanaka</th><th>Pt<sub>60</sub>Ru<sub>40</sub>/C<br>Tanaka</th></tr></thead><tbody><tr><td>EASA<sub>HUPD</sub> (m<sup>2</sup> g<sub>pi</sub><sup>-1</sup>)</td><td>50.1</td><td>49.7</td><td>27.6</td><td>20.5</td><td>99.9</td><td>48.3</td></tr><tr><td>j<sub>k</sub> (mA cm<sup>-2</sup>)</td><td>6.3</td><td>6.2</td><td>5.6</td><td>5.8</td><td>4.4</td><td>2.3</td></tr><tr><td>SA (mA cm<sup>2</sup>)</td><td>0.33</td><td>0.30</td><td>0.63</td><td>1.33</td><td>0.10</td><td>0.18</td></tr><tr><td>MA (mA mg<sub>pi</sub><sup>-1</sup>)</td><td>147</td><td>151</td><td>211</td><td>286</td><td>101</td><td>88</td></tr><tr><td>j<sup>*</sup> (mA cm<sup>-2</sup>)</td><td>1.37</td><td>1.40</td><td>1.25</td><td>1.30</td><td>1.15</td><td>1.24</td></tr><tr><td>b (mV dec<sup>-1</sup>)</td><td>28</td><td>30</td><td>29</td><td>28</td><td>31</td><td>34</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>Fig. 6. (A, D, G, J) Hydrodynamic polarization curves recorded at a scan rate = 0.005 V s⁻¹ and T = 25 °C in H₂-saturated 0.1 mol L⁻¹ HClO₄ electrolyte on the Pt-based catalysts at different electrode rotation rates, (B, E, H, K) corresponding Koutecky-Levich straight lines determined at different potentials and derived Tafel plots (C, F, I, L).</figcaption></figure>

explained that “the collaboration between tungsten oxide and tungsten carbide species could reduce the reliance on Pt while enhancing both the HOR activity and the anti-CO poisoning capability, by proposing an electron transfer from the metal oxide towards Pt, and cited Vayssilov et al. [56] who showed in their article the contrary, i.e. an electron transfer from Pt toward the tungsten oxide. Moreover, some authors identified electron transfer from the Pt nanoparticles toward the oxide support on Pt-ceria catalysts, and oxygen transfer from ceria to Pt [56],

or from Ru toward SnO₂ for Ru-SnOₓ/Al₂O₃ catalyst [57]. Recently, Chen et al. [58] have pointed out the role of the electronegativity of different catalytic materials (Fe, Ni, Ru, and Pt, with electronegativity values on the Pauling scale of 1.83, 1.91, 2.20 and 2.28, respectively) on the adsorption/activation of H₂: the higher is the electronegativity with respect to that of H (2.20 on the Pauling scale), the higher the adsorption of H under H⁺ form.

Several mechanisms were proposed in the literature for the HOR,
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** (A) Cyclic voltammograms of the Ptₓ(SnO₂)₁₀₀-x/C catalysts recorded at 25 °C in H₂-purged 0.1 mol L⁻¹ HClO₄ electrolyte after CO adsorption for 5 min at 0.10 V vs. RHE; (B) Cyclic voltammograms of Pt/C catalysts recorded at 25 °C and 60 °C in N₂-purged 0.1 mol L⁻¹ HClO₄ electrolyte after CO adsorption for 5 min at 0.10 V vs. RHE; (C) Cyclic voltammograms recorded of the Ptₓ(SnO₂)₁₀₀-x/C catalysts recorded at 60 °C in H₂-purged 0.1 mol L⁻¹ HClO₄ electrolyte after CO adsorption for 5 min at 0.10 V vs. RHE; (D) Cyclic voltammograms of Ptₓ(SnO₂)₁₀₀-x/C catalysts recorded at 25 °C in N₂-purged 0.1 mol L⁻¹ HClO₄ electrolyte after CO adsorption for 5 min at 0.10 V vs. RHE. (Scan rate = 0.020 V s⁻¹, ω = 1600 rpm).</figcaption></figure>

it displays a high electrocatalytic activity for HOR, (ii) hence, it helps reducing the atomic quantity of Pt by a half in the catalyst, which allows for reducing the production costs of fuel cell anode materials, (iii), it allows removing COads from the low potentials of ca. 0.2 V and 0.05 V vs. RHE at 20 °C and 60 °C, respectively, (iv) hence, it possesses an activity for the HOR from 0.2 V and 0.05 V vs. RHE in the presence of CO at 20 °C and 60 °C, respectively, and (v) it allows feeding the fuel cell with less pure (and then less costly) hydrogen.

### CRediT authorship contribution statement

**Franck Koffi Bouho:** Writing – review & editing, Writing – original draft, Investigation.
**Weliton Silva Fonseca:** Writing – review & editing, Writing – original draft, Investigation.
**Sirine Ben Latifa:** Writing – review & editing, Writing – original draft, Investigation.
**Thibault Rafaïdeen:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization.
**Frédéric Pailloux:** **Julie Rousseau:** Writing – review & editing, Writing – original draft, Formal analysis.
**Christine Canaff:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis.
**Sophie Morisset:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis.
**Téko W. Napporn:** Writing – review & editing, Writing – original draft, Validation, Resources, Methodology, Investigation, Conceptualization.
**Christophe Coutanceau:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Formal analysis, Conceptualization.

### Declaration of competing interest

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

### Acknowledgements

This work was performed in the framework of the PEMFC95 project funded by the "France 2030" government investment plan managed by the French Research Agency, under the reference "ANR-22-PEHY-0005". The authors also acknowledge financial support from the European Union (ERDF) and Région Nouvelle Aquitaine. This work pertains to the French government program "Investissements d'Avenir" (EUR INTREE, reference ANR-18-EURE-0010).

### Data availability

Data will be made available on request.

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