# Behaviour of Pt$_{10}$ – $x$Ru$_x$/C catalysts towards the hydrogen oxidation reaction in acidic medium in the presence of adsorbed CO

Koffi F. Bouho, Thibault Rafaïdeen, Têko W Napporn, Christophe

Coutanceau

► To cite this version:

Koffi F. Bouho, Thibault Rafaïdeen, Têko W Napporn, Christophe Coutanceau. Behaviour of Pt$_{10}$ – $x$Ru$_x$/C catalysts towards the hydrogen oxidation reaction in acidic medium in the presence of adsorbed CO. Electrochimica Acta, 2024, 498, pp.144697. 10.1016/j.electacta.2024.144697. hal-04650780

HAL Id: hal-04650780

https://hal.science/hal-04650780v1

Submitted on 17 Jul 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

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

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Behaviour of Pt<sub>10-x</sub>Ru<sub>x</sub>/C catalysts towards the hydrogen oxidation reaction in acidic medium in the presence of adsorbed CO

Franck Koffi Bouho<sup>a</sup>, Thibault Rafaïdeen<sup>a</sup>, Têko W. Napporn<sup>a,b</sup>, Christophe Coutanceau<sup>a,b,*</sup>

## ARTICLE INFO

**Keywords:**

Activity

CO stripping

Hydrogen oxidation reaction

Platinum

Ruthenium

## ABSTRACT

Pt<sub>10-x</sub>Ru<sub>x</sub>/C materials are synthesised using a classical polyol method and comprehensively characterised by thermogravimetric analysis, inductively coupled plasma - optical emission spectroscopy, electron transmission microscopy and x-ray diffraction. In N<sub>2</sub>-saturated electrolyte, the PtRu/C catalysts display lower onset potentials for adsorbed CO (CO<sub>ads</sub>) removal than the pure Pt/C one. Although the surface coverages of Pt by Ru are high, the PtRu/C catalysts display higher activity than the Pt/C one towards the hydrogen oxidation reaction (HOR), but the same mechanism occurs on all the catalysts. However, when the surface of the catalysts is saturated by CO<sub>ads</sub>, the HOR starts at a lower potential on Pt/C than on PtRu/C; Pt/C displays higher activity toward the HOR in the presence of CO<sub>ads</sub> between 0.40 and 0.55 V vs RHE. On Pt/C, a small part of CO<sub>ads</sub> desorbs into CO<sub>2</sub> from 0.40 V through an Eley-Rideal mechanism, freeing very active Pt sites for the HOR. For potentials higher than 0.55 V vs RHE, the PtRu/C catalysts display higher activity for CO<sub>ads</sub> removal through the bifunctional mechanism. Ru atoms also block the most active Pt sites for the CO<sub>ads</sub> removal and the HOR.

## 1. Introduction

The energy transition will lean on electrification with renewables of mobility, buildings and industry [1,2]. In this context, hydrogen represents an unavoidable vector for the electrical energy storage/conversion [3,4]. Currently, hydrogen is mainly produced (95 %) worldwide from fossil resources by thermochemical processes (natural gas and oil reforming, as well as coal gasification) [5]. Water electrolysis powered by renewable energy sources is considered as a powerful mean to generate green hydrogen. However, it is unlikely that water electrolysis alone could provide the necessary hydrogen quantity for the decarbonation of both the mobility and the industry. Thermochemical processes are also envisaged for its production from biomass [5,6].

An important use of hydrogen is the production of electricity through fuel cells [7,8]. Proton exchange membrane fuel cells (PEMFCs) are the most mature systems, particularly for transportation applications [9]. PEMFC anode materials contain platinum (Pt) because of its ability to dissociate hydrogen (H<sub>2</sub>) into two hydronium ions (H<sup>+</sup>) with the release of two electrons (e<sup>-</sup>) at low temperature. But, Pt surfaces are very prone to strongly adsorb poisoning species such as carbon monoxide (CO) [10]. The adsorption of CO (CO<sub>ads</sub>) on Pt surfaces hinders the ability of Pt catalysts to dissociate H<sub>2</sub> and to oxidise adsorbed hydrogen atoms (H<sub>ads</sub>) into H<sup>+</sup>. However, the reforming of bio-sourced carbon-containing compounds produces hydrogen with significant amounts of CO [11]. The removal of CO to reach the hydrogen grade necessary for PEMFCs (< 0.2 ppm of CO) [12,13] can be performed through different purification steps [14], but at the expense of the hydrogen cost. Consequently, a better comprehension of the interactions between Pt-based catalysts and CO in the presence of hydrogen is of paramount importance for the design and fabrication of highly CO-tolerant electrode materials.

Different mechanisms were proposed for the oxidation of a saturated CO<sub>ads</sub> layer at Pt nanoparticles (Pt NPs). In the low potential region from ca. 0.30 V to ca. 0.60 V vs RHE, a prepeak of CO<sub>ads</sub> oxidation can arise on Pt catalysts displaying (100) and/or (111) surface nanodomains together with defects (corners, edges, steps, etc.). [15,16] Friedrich et al. [17] first proposed that this prepeak, which appears in a potential region where Pt is not expected to activate water [18], could be explained by an Eley-Rideal mechanism involving CO<sub>ads</sub> species and H<sub>2</sub>O molecules (Eqs. (1) and (2)). This assumption was latter evidenced experimentally by Urchaga et al. [19] on the basis of cyclic voltammetry coupled with electrochemical quartz nanobalance measurements and of Pt catalysts to dissociate H<sub>2</sub> and to oxidise adsorbed hydrogen atoms (H<sub>ads</sub>) into H<sup>+</sup>. However, the reforming of bio-sourced carbon-containing compounds produces hydrogen with significant amounts of CO [11]. The removal of CO to reach the hydrogen grade necessary for PEMFCs (< 0.2 ppm of CO) [12,13] can be performed through different purification steps [14], but at the expense of the hydrogen cost. Consequently, a better comprehension of the interactions between Pt-based catalysts and CO in the presence of hydrogen is of paramount importance for the design and fabrication of highly CO-tolerant electrode materials.

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

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

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: christophe.coutanceau@univ-poitiers.fr (C. Coutanceau).</footnote>
chronoamperometry measurements followed by their modelling.

$$Pt-CO_{ads} + H_{2}O \rightarrow Pt-COOH_{ads} + H^{+} + e^{-} \quad (1)$$

$$Pt-COOH_{ads} \rightarrow Pt + CO_{2} + H^{+} + e^{-} \quad (2)$$

For higher electrode potentials, the Pt sites become able to activate water molecules and to adsorb OH species, and it is recognised since the works of Gilman [20,21] that a Langmuir-Hinshelwood mechanism involving CO<sub>ads</sub> and OH<sub>ads</sub> for the formation of CO<sub>2</sub> (Eqs. (3) and (4)) is involved.

$$Pt + H_{2}O \rightarrow Pt-OH_{ads} + H^{+} + e^{-} \quad (3)$$

$$Pt-CO_{ads} + Pt-OH_{ads} \rightarrow 2 Pt + CO_{2} + H^{+} + e^{-} \quad (4)$$

Owing to their higher oxophilic character compared to Pt, molybdenum (Mo), tin (Sn) and moreover ruthenium (Ru) are often used to modify Pt catalysts and increase their tolerance to CO poisoning. [22-24] It is generally proposed that the removal of CO<sub>ads</sub> from Pt surfaces follows a bifunctional mechanism, where the more oxophilic metal (here considering Ru) activates water molecules and adsorbs OH species [25,26] at lower potentials than platinum (Eqs. (5), (6) and (7)):

$$Pt + CO \rightarrow Pt-CO_{ads} \quad (5)$$

$$Ru + H_{2}O \rightarrow Ru-OH_{ads} + H^{+} + e^{-} \quad (6)$$

$$Pt-CO_{ads} + Ru-OH_{ads} \rightarrow Pt + Ru + CO_{2} + H^{+} + e^{-} \quad (7)$$

In addition, interactions between Pt and Ru can modify the electronic structure of Pt surface atoms (electronic effect), leading to weaken the CO adsorption strength [27,28].

Therefore, in this contribution nanostructured Pt/C and Pt<sub>10-x</sub>Ru<sub>x</sub>/C (atomic ratios, x = 0, 3 and 5) materials are synthesised, comprehensively characterised and studied using linear scan voltammetry (LSV) and cyclic voltammetry (CV) vis à vis their electrocatalytic behaviour toward the hydrogen oxidation reaction (HOR) in the absence and in the presence of CO<sub>ads</sub> (sub)monolayers. The electrocatalytic behaviour of Pt-based catalysts (such as PtRu materials) toward the HOR and their tolerance toward CO are most often studied by performing separated experiments: oxidation of pure H<sub>2</sub>, oxidation of H<sub>2</sub>/CO mixtures and CO stripping, or directly in fuel cells. In the first cases, the influence of the presence of hydrogen during CO stripping measurement is not considered. In the second case, only the effect of the presence of CO on the cell performance can be observed. Here, the comparison of the results obtained in both the absence and the presence of hydrogen on surfaces with controlled (sub)monolayer of CO<sub>ads</sub> will provide new insights on the electrocatalytic behaviour of such catalysts. Particularly it is shown that the modification of a Pt/C catalyst by Ru doesn't improve the HOR in the presence of CO<sub>ads</sub>, as generally proposed, since a slightly lower onset potential for the HOR is recorded on Pt/C than on Pt<sub>10-x</sub>Ru<sub>x</sub>/C.

## 2. Experimental methods

### 2.1. Synthesis and characterisation of catalysts

The Pt<sub>10-x</sub>Ru<sub>x</sub>/C (atomic ratios, x = 0, 3 and 5) catalysts are synthesised using a classical polyol method [29,30]. The syntheses are carried out in two steps as described in the review published by Fievet et al. [31]. In the first step, 100 mg of metal salts (H<sub>2</sub>PtCl<sub>6</sub>·6H<sub>2</sub>O and RuCl<sub>3</sub>·3H<sub>2</sub>O, 99.9 % purity from Alfa Aesar) are mixed in 100 mL ethylene glycol (puriss. p.a., purity > 99.5 % from Fluka) in convenient ratios to produce Pt, Pt<sub>7</sub>Ru<sub>3</sub> and Pt<sub>5</sub>Ru<sub>5</sub> nanoparticles (atomic ratios). A solution of 1.0 mol L<sup>-1</sup> NaOH (semiconductor grade 99.99 % purity from Sigma-Aldrich) in ethylene glycol is then added dropwise to reach a pH value of 11. The mixture is heated at 160 °C for 3 h, and then allowed to cool down to room temperature.

A known small volume of the Pt colloidal suspension is collected and

the unsupported Pt NPs are settled down by centrifugation (Eppendorf centrifuge 5804) at 9000 rpm (revolution per minute) for 10 min. The supernatant is carefully removed and the Pt NPs are cleaned by addition of ultrapure water. The process is repeated four times. Those unsupported Pt nanoparticles will serve to determine the ratios of (100) and (111) surface domains using the Ge<sup>4+</sup> and Bi<sup>3+</sup> spontaneous adsorption/electrochemical desorption methods, respectively (Supplementary information section, SI 4).

In the second step, Vulcan XC 72 (Cabot Corp.) carbon powder previously treated at 400 °C for 4 h under N<sub>2</sub> atmosphere to remove impurities is added to the Pt<sub>10-x</sub>Ru<sub>x</sub> colloidal suspensions in order to target the nominal metal loading of 40 wt%. The pH is then decreased to 1 by addition of H<sub>2</sub>SO<sub>4</sub> (Suprapur from Merck) to allow particle deposition on the carbon support. After stirring for 24 h, the mixture is filtrated over a 0.22 µm hydrophilic PVDF membrane filter (Durapore® from Millipore) and washed several times with acetone (> 99 %, WWR Chemicals) and ultrapure water (MilliQ from Millipore, 18.2 MΩ cm resistivity). The retentate is then dried overnight in an oven at 60 °C.

The actual metal loading of catalysts is determined by thermogravimetric analysis (TGA) using a TA Instrument model SDT Q 600, the bulk composition by inductively coupled plasma – optical emission spectroscopy (ICP-OES) analysis using a 5110 Agilent VDV apparatus, the mean particle size by transmission electron microscopy (TEM) using a JEOL JEM 2100 (UHR) microscope with a resolution of 0.19 nm, and the mean crystallite size and microstructure by x-ray diffraction (XRD) using a PANalytical Empyrean X-ray diffractometer. Physicochemical characterisation data for Pt<sub>10-x</sub>Ru<sub>x</sub>/C materials are given in Table 1.

### 2.2. Electrochemical measurements

The electrochemical measurements are performed using an interfaced Autolab PGSTAT302N potentiostat (Metrohm), in a classical thermostated three-electrode cell equipped with a reversible hydrogen electrode (RHE, HydroFlex from Gaskatel) as reference electrode, a rotating glassy carbon disc working electrode (RDE GC50.S, 0.196 cm<sup>2</sup> geometric surface area, from Metrohm) as substrate for the catalysts, and a counter-electrode consisting in a 2 cm [2] surface area glassy carbon plate.

The cyclic voltammetry (CV), linear scan voltammetry (LSV) and CO stripping experiments are carried out at T = 20 °C, with a potential scan rate of 20 mV s<sup>-1</sup> and an electrode rotation rate of 1600 rpm. The CO stripping measurements are recorded after 5 min CO (CO N47 quality from Air Liquide) adsorption at a fixed potential value E<sub>ads</sub> on the catalyst surface from a CO-saturated 0.1 mol L<sup>-1</sup> HClO<sub>4</sub> (Suprapur from Merck) electrolyte followed by 20 min bubbling of N<sub>2</sub> (N<sub>2</sub> U quality from Air Liquide) under potential control to remove dissolved CO. CO-stripping measurements are also recorded in H<sub>2</sub>-saturated electrolyte after 20 min bubbling of N<sub>2</sub> to remove dissolved CO and then saturation of the electrolyte by bubbling H<sub>2</sub> (H<sub>2</sub> HG quality from Air Liquide) for 20 min under electrode potential control. The polarisation curves at different electrode rotation rates for the determination of the kinetic parameters for the HOR are carried out at a potential scan rate of 5 mV s<sup>-1</sup> and T = 20 °C in a H<sub>2</sub>-saturated 0.1 mol L<sup>-1</sup> HClO<sub>4</sub> electrolyte. Because the reference electrode is an RHE, all potentials in the present

<caption>Table 1</caption>

Physicochemical characterisation of the Pt<sub>10-x</sub>Ru<sub>x</sub>/C catalysts: metal weight loadings by TGA, bulk atomic composition by ICP-OES, crystallite size (L<sub>v</sub>) et cell parameter (a) by XRD and particle size (d<sub>TEM</sub>) by TEM.

<table><thead><tr><th></th><th>Pt/C</th><th>Pt<sub>7</sub>Ru<sub>3</sub>/C</th><th>Pt<sub>5</sub>Ru<sub>5</sub>/C</th></tr></thead><tbody><tr><td>Metal wt%</td><td>34.2</td><td>33.9</td><td>34.2</td></tr><tr><td>Comp. at%</td><td>100</td><td>Pt: 77<br>Ru: 23</td><td>Pt: 48<br>Ru: 52</td></tr><tr><td>L<sub>v</sub> (nm)</td><td>2.5</td><td>2.2</td><td>1.7</td></tr><tr><td>a<sub>cell</sub> (nm)</td><td>3.92</td><td>3.92</td><td>3.88</td></tr><tr><td>d<sub>TEM</sub></td><td>2.1</td><td>1.6</td><td>1.5</td></tr></tbody></table>
contribution are quoted to this electrode.

For the preparation of the working electrode, a catalytic ink is formulated by mixing and ultrasonically homogenising (30 s in an Fisherbrand 15,051 ultrasonic bath, 2.75 L, 37 kHz, 320 W) 5 mg of a Pt<sub>10-x</sub>Ru<sub>x</sub>/C catalytic powder with 750 µL of ultrapure water, 250 µL of isopropanol and 100 µL of Nafion suspension (5 wt % in aliphatic alcohols from Sigma Aldrich). Then, 5 µL of the ink (corresponding to m<sub>cata</sub> = 115.96 µg cm<sup>-2</sup> and, considering the targeted and actual metal loadings of 40 wt.% and 34 wt.%, respectively, to nominal m<sub>metal</sub> = 46.3 µg cm<sup>-2</sup> and actual m<sub>metal</sub> = 36.9 µg cm<sup>-2</sup>) are dispensed onto the glassy carbon disc. The ink drop on the glassy carbon electrode is uniformized and dried at room temperature under 400 rpm rotation and N<sub>2</sub> flow.

## 3. Results and discussion

### 3.1. Physicochemical characterisations of the Pt<sub>10-x</sub>Ru<sub>x</sub>/C catalysts

The catalysts are characterised using different physicochemical methods in order to assess their actual weight loading and bulk composition and to obtain information on their microstructure. The TGA measurements lead to metal loading values close to 34 wt % for all the materials, a little bit lower than the targeted one of 40 wt%. This seems to indicate that the metallic salts introduced in the synthesis reactor have not been totally reduced to form nanoparticles, and that the reaction temperature and/or time have certainly to be increased to achieve the reduction of the whole metallic salts. However, ICP-OES measurements show that the actual metallic bulk compositions of catalysts are very close to the targeted ones (Table 1).

Fig. 1 presents the XRD patterns recorded on the different materials. For the Pt/C catalyst, the diffraction peaks at 2θ > 30° correspond to the face-centred cubic (fcc) structure of Pt (JCPDS 04–0802 card). The peak at 2θ = 25° is assigned to the {002} set of planes of turbostratic graphite. [32] XRD patterns of PtRu materials show the typical diffraction peaks related to the fcc structure of Pt, but slightly shifted toward higher 2θ values for the Pt<sub>5</sub>Ru<sub>5</sub>/C material, which could be due to a small degree of alloying [33]. But, XRD patterns of PtRu catalysts show the growing of diffraction peaks at 2θ values of ca. 38° and 44°, which correspond to Ru {100} and Ru{101} sets of planes in the hexagonal compact (hcp) structure (JCPDS 6–663 card).

Fig. 2 presents typical TEM images recorded on the different catalysts with the corresponding particle size distribution histograms. For all the catalysts, the metal nanoparticles are relatively well disseminated on the carbon support, with small density of aggregates. The mean particle

<figure><img src="image_3.png" /><figcaption>Fig. 1. XRD patterns of the Pt<sub>10-x</sub>Ru<sub>x</sub>/C (x = 0, 3 and 5) catalysts; ★diffraction peaks related to fcc structure of Pt (JCPDS 04–0802 card) and ● diffraction peaks related to hcp Ru (JCPDS 6–663 card).</figcaption></figure>

sizes are in the range from 1.5 to 2.1 nm, and decrease with the ruthenium ratio.

From the above characterisation results, it is proposed that the microstructure of the PtRu catalysts consists of alloyed PtRu core (with low alloying degree) decorated by Ru clusters. Such kind of microstructure was already observed for the synthesis of PtRu/C catalysts by mixing two separate precursor colloidal suspensions of Pt and Ru before the reduction step [34], and was shown to be more active toward methanol electrooxidation (involving the formation of CO adsorbed species) than pure alloys. Here, such a microstructure can be due to a lower reduction kinetics of Ru<sup>3+</sup> cations than that of Pt<sup>4+</sup> cations [29].

### 3.2. Electrochemical characterisations of Pt<sub>10-x</sub>Ru<sub>x</sub>/C catalysts

The study starts from classical CO stripping measurements with the aim to confirm that the modification of Pt by Ru allows desorbing CO<sub>ads</sub> from lower potentials than on a pure Pt/C material. The complete CO stripping CVs recorded on the different catalysts are given in the supplementary information section, SI 1. The second cycle after CO<sub>ads</sub> removal from the surfaces always displays the typical electrochemical features of clean Pt<sub>10-x</sub>Ru<sub>x</sub>/C catalysts. Fig. 3a compares the CO stripping LSVs recorded in N<sub>2</sub>-purged electrolyte. Ru-containing catalysts allow achieving lower CO oxidation onset potentials, and the higher the Ru atomic ratio is, the lower the onset potential seems to be. However, a zoom on the potential region from 0.40 V to 0.70 V shows that a small but observable current density is also recorded on Pt/C. But, as expected, the removal of CO from the surface occurs more rapidly in the 0.40 V to 0.70 V potential range when the Pt/C catalyst is modified by ruthenium [35,36].

The second part of the study consists of recording the hydrogen oxidation polarisation curves on the different catalysts at different rotation rates of the electrode (Fig. 4) in order to assess and compare their catalytic activity toward the HOR. Whatever the catalyst composition, all sets of experiments lead to same polarisation curves with same values of diffusion limiting current densities at potentials higher than ca. 0.075 V, although the available Pt surface is lower for the Ru-modified catalysts than for the Pt/C one, as evidenced by the CVs of clean catalysts in a pure supporting electrolyte. It has been clearly shown by Gasteiger et al. [37] that Ru displayed a catalytic activity toward HOR at least 2–3 orders of magnitude lower than Pt. But, in the considered range of rotation speeds, the electrooxidation rate of H<sub>2</sub> on the remaining Pt surface sites is far higher than that of the hydrogen diffusion rate in the electrolyte [37]. The kinetic current densities (j<sub>k</sub>) at 0.02 V, the exchange current densities (j<sub>o</sub>) and the Tafel slopes (b) could be assessed from these polarisation curves using a generalised Koutecky-Levich equation [24,38] that considers the processes occurring within the catalytic layer (Eq. (8)):

$$ \frac{1}{j} = \frac{1}{j_1^{\text{diff, elec}}} + \frac{1}{j_1^{\text{film}}} + \frac{1}{j_0 e^{\eta/b}} = \frac{1}{j_1^{\text{diff, elec}}} + \frac{1}{j_1^{\text{film}}} + \frac{1}{j_k(\eta)} \quad (8) $$

where, j<sub>1</sub><sup>diff, elec</sup> is the limiting current density of hydrogen diffusion from the bulk electrolyte toward the electrode surface, j<sub>1</sub><sup>film</sup> the limiting current density related to the phenomena occurring inside the catalytic layer (hydrogen diffusion in the catalytic film toward the catalytic surface and adsorption on the catalytic sites), j<sub>o</sub> the exchange current density, b the Tafel, η the overpotential and j<sub>k</sub>(η) the kinetic current at a given overpotential.

The associated mechanism of hydrogen oxidation and the mathematical treatment of the generalised Koutecky-Levich equation are described in the supplementary information section, SI 2. The following values were considered for calculations: 8 × 10<sup>-3</sup> cm<sup>2</sup> s<sup>-1</sup> for the kinematic viscosity of the electrolyte, 4.5 × 10<sup>-5</sup> cm<sup>2</sup> s<sup>-1</sup> for the hydrogen diffusion coefficient and 7.2 × 10<sup>-7</sup> mol cm<sup>-3</sup> for the hydrogen solubility in 0.1 mol L<sup>-1</sup> HClO<sub>4</sub> [39]. Results are given in Table 2. The Tafel slope (b) value is always close to 30 mV dec<sup>-1</sup>, which indicates that the
<figure><img src="image_4.png" /><figcaption>Fig. 2. Transmission electron microscopy (TEM) images of the Pt<sub>10-x</sub>Ru<sub>x</sub>/C (x = 0, 3 and 5) catalysts.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. (a) Linear scan voltammograms recorded on the Pt<sub>10-x</sub>Ru<sub>x</sub>/C (x = 0, 3 and 5) catalysts in N<sub>2</sub>-purged 0.1 mol L<sup>-1</sup> HClO<sub>4</sub> electrolyte after CO adsorption for 5 min at 0.10 V vs RHE; (b) Linear scan voltammograms recorded on the Pt<sub>10-x</sub>Ru<sub>x</sub>/C (x = 0, 3 and 5) catalysts in H<sub>2</sub>-purged 0.1 mol L<sup>-1</sup> HClO<sub>4</sub> electrolyte after CO adsorption for 5 min at 0.10 V vs RHE (scan rate = 20 mV s<sup>-1</sup>, T = 20 °C, ω = 1600 rpm).</figcaption></figure>

HOR occurs following the Volmer-Tafel mechanism (with the Tafel step being the rate determining step) [40] for all catalysts. The values of the kinetic current density ($j_k$) at 0.02 V assessed for each catalyst are not significantly different, which indicates a similar activity toward the HOR although the available Pt surface is lower for the Ru-modified catalysts than for the Pt/C one. However, the values of the exchange current density ($j_o$) increases with the increase of the ruthenium content in the catalyst, demonstrating the following intrinsic catalytic activity enhancement: Pt/C < Pt<sub>7</sub>Ru<sub>3</sub>/C < Pt<sub>5</sub>Ru<sub>5</sub>/C. Because the HOR kinetics data are corrected from the H<sub>2</sub> diffusion limitation in both the bulk electrolyte and the catalytic layer, they could then be interpreted as a higher turnover frequency of hydrogen on Pt sites in the presence of ruthenium.

The third part of this study consists of performing CO stripping measurements in a H<sub>2</sub>-saturated electrolyte and comparing the behaviour of the different Pt<sub>10-x</sub>Ru<sub>x</sub>/C catalysts (Fig. 3b). The first surprising

observation is that, in the presence of hydrogen, the Pt/C catalyst displays an oxidation wave starting at a lower potential (ca. 0.40 V) than that recorded for CO-stripping in a pure supporting electrolyte (ca. 0.65 V). Some authors proposed that the maximum surface coverage of Pt nanoparticles by CO<sub>ads</sub> was ca. 0.85 [41,42], which could explain this observation. But, the Pt surface seems completely blocked in the low potential range from 0.05 V to 0.40 V. This fact rather suggests that some specific CO<sub>ads</sub> species are desorbed from the Pt surface from ca. 0.40 V, making some Pt sites available for the HOR at low potentials (remember that in Fig. 3a a small current was observed in the 0.40 V to 0.70 V potential range for the CO stripping measurement on Pt/C in pure electrolyte); in other words, some Pt sites are more active for the electrocatalytic CO<sub>ads</sub> removal than others. For higher potentials, a kind of oxidation plateau is observed until 0.80 V followed by the main CO<sub>ads</sub> oxidation peak. On the PtRu/C catalysts, the CO stripping measurements in H<sub>2</sub>-saturated electrolyte show that the onset potential of the oxidation
<figure><img src="image_6.png" /><figcaption>Fig. 4. (a,c,e) Hydrodynamic polarisation curves recorded at 20 °C in H₂-saturated 0.1 mol L⁻¹ HClO₄ electrolyte on the Pt₁₀ₓRuₓ/C (x = 0, 3 and 5, respectively) catalysts (rate of s = 5 mV s⁻¹, T = 20 °C, electrode rotation rates ω of 400, 900, 1600 and 2500 rpm), and (b,d,f) Koutecky-Levich straight lines determined at different potentials from the corresponding polarisation curves.</figcaption></figure>

**Table 2**

Kinetics data for the HOR assessed from rotating disc electrode measurements of the HOR in Fig. 4 at Pt₁₀ₓRuₓ/C catalysts: kinetic current density ($j_k$) at 0.02 V vs RHE, exchange current density ($j_o$) and Tafel slope (b).

<table><thead><tr><th></th><th>j<sub>k</sub> @ 0.02 V vs RHE<br>/ mA cm<sup>-2</sup></th><th>j<sub>o</sub><br>/ mA cm<sup>-2</sup></th><th>Tafel slope (b)<br>/ mV dec<sup>-1</sup></th></tr></thead><tbody><tr><td>Pt/C</td><td>3.9</td><td>0.82</td><td>29</td></tr><tr><td>Pt<sub>7</sub>Ru<sub>3</sub>/C</td><td>4.1</td><td>0.85</td><td>27</td></tr><tr><td>Pt<sub>5</sub>Ru<sub>5</sub>/C</td><td>3.9</td><td>0.90</td><td>32</td></tr></tbody></table>

wave coincides with that for CO stripping in pure supporting electrolyte (ca. 0.45 V). From this potential, Ru becomes able to activate water and adsorb OH species that participate to the bifunctional mechanism, resulting in the desorption of COₐds from the surface into CO₂. The second surprising observation is that the Pt/C catalyst displays higher activity towards the HOR in the presence of COₐds than Ru-modified ones, as higher oxidation current densities are recorded from ca. 0.40 V to 0.55 V. This suggests that in PtRu/C catalysts the most active Pt sites for CO removal are either blocked by Ru or their electronic structure is modified by the presence of Ru, hindering their activity toward the HOR.
However, for potentials between 0.55 V and 0.85 V, the PtRu/C catalysts display higher COads oxidation current density. This observation is of paramount importance as it is generally admitted that PtRu catalysts are more tolerant toward CO poisoning than Pt/C catalysts. It is then essential to better understand the interaction between Pt and CO in the presence of hydrogen in the electrolyte.

### 3.3. Effect of COads on Pt/C toward the HOR

The study is completed by examining more deeply the behaviour of the Pt/C catalyst toward the oxidation of COads in the absence and in the presence of hydrogen.

The first set of experiments consists of recording the CO stripping voltammograms as a function of the CO adsorption potential on Pt/C. Fig. 5a compares the two first voltammograms recorded for the CO stripping measurements in a N₂-saturated 0.1 mol L⁻¹ HClO₄ electrolyte after adsorption of CO at potentials of 0.10 V, 0.45 V, 0.60 V and 0.70 V, as examples (for a sake of clarity, CO stripping profiles for all studied adsorption potentials are accessible in the supplementary information section, SI 3). In each case, the second cycle displays the classical clean Pt/C electrochemical behaviour, from which the electroactive surface area is determined by integrating the hydrogen desorption region from 0.05 to 0.40 V after correction of the double layer capacitive current [43]. Data are given in Table 3. All the Pt/C electrodes display very similar electrochemical active surface area (ECSA) values close to 50 m² gPt⁻¹ (between 45 and 52 m² gPt⁻¹), evidencing the reproducibility of the electrode preparation. Such ECSA values are in line with those reported previously for Pt/C catalysts prepared by the same polyol synthesis method [29,44]. Very small changes are observed in the CO stripping features for CO adsorption potentials (Eads) of 0.10 V, 0.45 V and 0.60 V, whereas for Eads = 0.70 V, the CO oxidation peak is much smaller. Moreover, for Eads values of 0.10 V, 0.45 V and 0.60 V, no hydrogen adsorption/desorption feature is visible in the first cycle in the potential range from 0.05 V to 0.40 V, whereas hydrogen adsorption/desorption features are present for Eads = 0.70 V, with lower current densities than in the second cycle. For Eads values lower or equal to 0.60 V, a CO surface saturation is achieved whereas a partial surface coverage is obtained for higher adsorption potentials. Moreover, the higher the CO adsorption potential is, the lower the potential of the maximum of the main COads oxidation peak is. Therefore, it can be proposed that the increase of Eads from 0.10 V to 0.60 V involves a slight decrease of the Pt surface coverage by CO, hence a slight relaxation of the adsorbed CO layer and further an easier CO oxidation rate. Cuesta et al. [45] have indeed shown

<table><caption>**Table 3**
Electroactive surface area of Pt/C electrode prepared for the CO-stripping measurements in pure supporting electrolyte and in H₂-saturated electrolyte, after CO adsorption at different electrode potentials.</caption><thead><tr><th>CO adsorption potential / V vs RHE</th><th>0.1</th><th>0.3</th><th>0.45</th><th>0.5</th><th>0.6</th><th>0.7</th></tr></thead><tbody><tr><td>ECSA after CO-stripping in pure supporting electrolyte / m² gPt⁻¹</td><td>52</td><td>50</td><td>47</td><td>45</td><td>52</td><td>48</td></tr><tr><td>ECSA after CO-stripping in H₂-saturated electrolyte / m² gPt⁻¹</td><td>46</td><td>55</td><td>56</td><td>51</td><td>58</td><td>50</td></tr></tbody></table>

that the CO coverage of a Pt(Poly) surface depended on the adsorption potential and decreased with increasing the adsorption potential from 0.10 V to 0.65 V.

The second set of experiments consists of adsorbing CO on the Pt surface of Pt/C catalysts at different Eads, of removing CO from the bulk electrolyte by bubbling N₂ under potential control (same potentials as those for CO adsorption) for 20 min, of saturating the electrolyte by bubbling H₂ for 20 min still under potential control (same potentials as those for CO adsorption) and then of performing a linear potential variation at 1600 rpm from Eads to 0.05 V, followed by three cyclic voltammograms between 0.05 V and 1.20 V (Fig. 6). The second and third CVs display always the classical features of clean Pt/C, indicating that COads is completely removed from the platinum surface after the first CV between 0.05 and 1.20 V. Another interesting observation is that the second CV corresponding to clean platinum surface is always centred around the diffusion limiting current density value obtained for the electrooxidation of H₂ at 1600 rpm on a clean Pt/C catalyst (Fig. 4). The electroactive surface area is determined by integrating the hydrogen desorption region from 0.05 to 0.40 V after correction of the double layer capacitive current. Data are given in Table 3. Again, all the Pt/C catalysts display very similar ECSA values, close to 53 m² gPt⁻¹ (between 46 and 58 m² gPt⁻¹), slightly higher in average than those determined after CO-stripping in pure electrolyte.

For a sake of clarity, Fig. 5b displays and compares only the polarisation curves after CO adsorption at different potentials, Eads, recorded from Eads to 0.05 V and then from 0.05 V to 1.00 V. For Eads = 0.10 V, the catalyst surface is completely blocked from 0.05 V to 0.40 V, as no current is detected over this potential range. The HOR starts at 0.40 V and reaches a current density plateau of ca. 2.0 mA cm⁻² at 0.65 V. This indicates that a small part of CO is oxidised from this potential, but the CO coverage of the Pt surface is still too important to reach the limiting

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** (a) Two first voltammograms recorded on the Pt/C catalyst in N₂-purged 0.1 mol L⁻¹ HClO₄ electrolyte after saturation of the surface by CO for 5 min at different adsorption potentials (Eads) of 0.10 V, 0.45 V, 0.60 V and 0.70 V vs RHE; (b) Linear scan voltammograms recorded on the Pt/C catalysts in H₂-purged 0.1 mol L⁻¹ HClO₄ electrolyte after saturation of the surface by CO for 5 min at different adsorption potentials (Eads) of 0.10 V, 0.35 V, 0.40 V, 0.50 V, 0.60 V and 0.70 V vs RHE (scan rate = 20 mV s⁻¹, T = 20 °C, ω = 1600 rpm).</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 6. Cyclic voltammograms recorded on the Pt/C catalyst in H₂-saturated 0.1 mol L⁻¹ HClO₄ electrolyte after saturation of the surface by CO for 5 min at different adsorption potentials (Eads) of (a) 0.10 V, (b) 0.30 V, (c) 0.45 V, (d) 0.50 V, (e) 0.60 V and (f) 0.70 V vs RHE (scan rate = 20 mV s⁻¹, T = 20 °C, ω = 1600 rpm). The potential was first negatively varied from Eads to 0.05 V vs RHE (blue line), then from 0.05 V to 1.20 V vs RHE for CO stripping (red line), then from 1.20 V to 0.05 V vs RHE followed by two successive cyclic voltammograms recorded from 0.05 V to 1.20 V vs RHE (black line). In each graph, the blue arrow indicates the CO adsorption potential (Eads).</figcaption></figure>

current density value of ca. 2.5 mA cm⁻² at 1600 rpm for the HOR on a clean Pt/C catalyst (Fig. 4). A second oxidation wave appears from 0.65 to 0.80 V, and at last an oxidation peak arises between 0.80 V and 1.00 V, centred at 0.83 V. It is worth noting that the potential of 0.40 V corresponds to the onset of the so-called COads oxidation prepeak, which is due to the oxidation of adsorbed CO at low coordinated Pt atoms (defects, steps, edges, corners) [26,45] through an Eley-Redeal mechanism [19]. The features of COads oxidation at higher potentials display

the same oxidation shoulder from ca. 0.65 V followed by an oxidation peak centred at 0.83 V as for the CO stripping in the absence of H₂ (Fig. 5a). These features can arise from a Langmuir-Hinshelwood mechanism on aggregated Pt nanoparticles and isolated Pt nanoparticles, respectively [46,47].

For Eads = 0.30 V, increasing small current densities are recorded in the low potential range from 0.05 V to 0.35 V, indicating that more Pt sites are made available for the HOR in this potential region. From 0.35
V, same features as those recorded after CO adsorption at 0.10 V are observed, but shifted toward lower potentials by ca. 0.04 V, which indicates that the CO coverage is lower, allowing higher accessibility of hydrogen to the surface, but still high enough to avoid reaching the limiting current density value of the HOR on a clean Pt/C catalyst.

For $E_{ads} \ge 0.45$ V, a hydrogen limiting current density is reached in the low potential region. For $E_{ads} = 0.45$ V, the achieved limiting current density is lower than those for higher $E_{ads}$, and the CO oxidation waves and peak are still both visible at higher potentials. For $E_{ads} > 0.45$ V, the limiting current densities in the low potential region are the same and correspond to the value obtained for $H_2$ oxidation at 1600 rpm on a clean Pt/C catalyst (Fig. 4). Fig. 7 shows the chronoamperometry curves recorded at 1600 rpm in $H_2$-saturated electrolyte after saturation of the Pt/C catalyst by CO at $E_{ads} = 0.45$ V and 0.70 V. The limiting current density of $H_2$ oxidation increases from ca. 1.70 mA cm$^{-2}$ for $E_{ads} = 0.45$ V to ca. 2.5 mA cm$^{-2}$ for $E_{ads} = 0.70$ V, i.e. achieving the diffusion limiting current densities for $H_2$ oxidation on clean Pt/C. These results are fully in agreement with those obtained by cyclic voltammetry in Fig. 5b. Clearly, for $E_{ads}$ higher than 0.30 V, an increasing part of the Pt surface is freed from CO, which allows the HOR occurring.

However, the intensity of the CO oxidation peak recorded in the potential range from 0.60 V to 1.00 V (Fig. 5a) shows that for $E_{ads} = 0.45$ V, 0.50 V and 0.60 V the coverage of the platinum surface by CO is very high and did not decrease significantly with $E_{ads}$, whereas the activity toward the HOR highly increases in the potential range from 0.05 V to 0.45 V. The HOR diffusion limiting current density at 1600, 2500 and 3600 rpm in the 0.20 V to 0.40 V range obtained after CO saturation of the surface at 0.45 V reaches values of ca. 1.70 mA cm$^{-2}$, ca. 2.00 mA cm$^{-2}$ and ca. 2.40 mA cm$^{-2}$, respectively (Fig. 8). These values are always lower than those of the diffusion limiting current densities recorded on a clean Pt/C catalyst at the same electrode rotation rates, and the difference between values increases with the increase of the rotation rate. This observation indicates that in this potential range, the HOR occurs under a mixed diffusion and kinetic control, and that the kinetic limitation is due to the low availability of platinum surface sites. But, the HOR current density value at 1600 rpm obtained after CO adsorption at 0.45 V corresponds to 68% of the HOR current density obtained on the clean Pt/C catalyst at 1600 rpm. However, CO stripping measurements are very far to show such level of CO removal from the surface, which suggests that some Pt sites are very active for the HOR and control the whole kinetics of the reaction.

It is generally admitted that the most active Pt sites for $CO_{ads}$

<figure><img src="image_9.png" /><figcaption>Fig. 7. Chronoamperometry recorded at 0.45 V and 0.7 V vs RHE at an electrode rotation rate of 1600 rpm in $H_2$-saturated 0.1 mol L$^{-1}$ $HClO_4$ electrolyte on a Pt/C catalyst after saturation of the surface at $E_{ads} = 0.45$ V and 0.70 V vs RHE ($T = 20$ °C).</figcaption></figure>

electrooxidation at low electrode potentials involve low-coordinated atoms (corners, edges, steps, defects, etc.) [15,25,45,48-50]. Actually, the occurrence of the $CO_{ads}$ oxidation pre-peak is related to the presence of both (100) and (111) surface domains, and defects (corners, steps, edges, etc.) [16,51]. The mean ratios of oriented surface domains on the Pt nanoparticles have been estimated using the $Ge^{4+}$ and $Bi^{3+}$ probing methods [52-54], leading to ca. 13% (100) and 3% (111) surface domains (supplementary information section, SI 4). Therefore, it is reasonable to propose that such sites exist on the Pt/C catalyst, which can be first freed from $CO_{ads}$, possibly through an Eley-Rideal mechanism [19], making them available for the HOR at lower potentials than the other sites.

Because 68% of the HOR limiting current density obtained on clean Pt/C at 1600 rpm is achieved for the catalysts strongly poisoned by $CO_{ads}$ at 0.45 V, it is also reasonable to propose that those low coordinated sites are much more actives than the other ones for the HOR. Indeed, it has been shown that the recovering of Pt surface in the CO-stripping pre-peak didn't exceed 10% [19,55], and is likely far less in our case. Moreover, the modification of the Pt surface by Ru leads to improve the HOR activity in clean hydrogen environment but not it in the presence of $CO_{ads}$, and $CO_{ads}$ has even a negative effect. It is then proposed that the most active Pt sites (corners, edges, steps, defects, etc.) are blocked by Ru atoms, hindering the occurrence of the HOR on these sites. On $Pt_{10-x}Ru_x/C$ catalysts, the HOR initiates at the same potential as that for $CO_{ads}$ oxidation, thanks to the removal of $CO_{ads}$ from the catalyst surface through a bifunctional mechanism [25,26].

Therefore, the engineering of the surface structure of Pt nanoparticles could be a way to improve the activity towards HOR in the presence of CO. Shaped Pt nanoparticles such as nano-cubes, nano-oc-tahedrons and nano-cuboctahedrons display an important $CO_{ads}$ oxidation pre-peak due to the presence of both large and small (111) and (100) surface domains (in different ratios according to the structures), and corners, steps, edges, etc. [51,16]. It has been shown for a long time that the spontaneous deposition from a $RuCl_3$ acidic solution of Ru on Pt (100) [56,57] and P(111) [56,58] single crystals occurred. Fortunately, this process leads to a homogeneous deposition of Ru on the Pt(111) or Pt(100) surfaces and is not preferentially nucleated by the crystallographic defects at the surface. [57-59] This spontaneous ruthenium deposition method was also successively applied for the modification of ca. 10 nm diameter Pt NPs. [60] The use of such shaped Pt nano-structures and the preferentially modification of the large surface domains by Ru atoms could help to improve the tolerance toward CO by favouring both the Eley-Rideal mechanism on defects and the bifunctional mechanism on surface domains at low potentials, as well as the whole HOR activity.

## 4. Conclusion

To summarise, $Pt_{10-x}Ru_x/C$ ($x = 0, 3, 5$) catalysts were synthesised using a classical polyol method. Characterisation by transmission electron microscopy showed that the $Pt_{10-x}Ru_x/C$ materials consisted of well disseminated spherical-like nanoparticles (of ca. 2.0 nm diameter) on the carbon support. X-ray diffraction led to the conclusion that the microstructure of the PtRu/C materials consisted of an alloyed PtRu core (with low level of alloying) decorated by Ru clusters. Electrochemical characterisation of the catalyst in $H_2$-saturated 0.1 mol L$^{-1}$ $HClO_4$ electrolyte using the RDE method indicated that the presence of Ru at the surface of the materials didn't affect the HOR mechanism on the catalysts but improved the kinetics of the catalysts toward the HOR. CO stripping measurements in $N_2$-saturated 0.1 mol L$^{-1}$ $HClO_4$ electrolyte confirmed that the presence of Ru involved a better tolerance toward $CO_{ads}$ as the onset potential of $CO_{ads}$ oxidation decreased with the Ru/Pt ratio. However, same experiments performed in $H_2$-saturated 0.1 mol L$^{-1}$ $HClO_4$ electrolyte showed that the HOR started at a slightly lower potential on the pure Pt/C catalyst than on the PtRu/C ones. This finding is of paramount importance as it shows that in the presence of hydrogen
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Cyclic voltammograms recorded on the Pt/C catalyst in H₂-saturated 0.1 mol L⁻¹ HClO₄ electrolyte after saturation of the surface by CO for 5 min at 0.45 V. (a) 1600 rpm, (b) 2500 rpm and (c) 3600 rpm (scan rate = 20 mV s⁻¹, T = 20 °C).</figcaption></figure>

the modification of Pt by Ru will not necessarily imply a better tolerance of the catalyst toward CO.

To better understand this behaviour, a deeper study of the Pt and CO interaction in the presence of hydrogen has been performed on the Pt/C catalyst. It was shown that surface defects (corners, edges, steps, defects, etc.) were the most active for both CO<sub>ads</sub> removal and HOR at low potentials. This finding allowed proposing that the HOR activity in the presence of CO could be first improved by tuning the surface structure of Pt nanoparticles. It was also shown that Ru atoms could block the Pt surface defects, which could explain the higher activity of CO-poisoned Pt/C material for HOR than CO-poisoned PtRu/C materials in the potential range from 0.40 V to 0.55 V, but allowed removing more rapidly CO<sub>ads</sub> from 0.55 V through the bifunctional mechanism. This third finding led us to propose that preferentially modifying the (111) and (100) surface domains of shaped Pt nanoparticles (nano-cubes, nano-octahedrons and nano-cuboctahedrons) by Ru atoms and keeping the Pt surface defects free of Ru atoms could help to improve the tolerance toward CO by favouring both the Eley-Rideal mechanism on defects at low potentials and the bifunctional mechanism on surface domains at higher potentials, as well as the HOR activity over the whole studied potential range.

CRediT authorship contribution statement

**Franck Koffi Bouho:** Writing – original draft, Methodology, Investigation, Formal analysis.
**Thibault Rafaïdeen:** Writing – original draft, Methodology, Investigation, Formal analysis.
**Téko W. Napporn:**

Writing – original draft, Supervision, Methodology, Formal analysis, Conceptualization.
**Christophe Coutanceau:** Writing – original draft, Supervision, Project administration, Methodology, Funding acquisition, Formal analysis, Conceptualization.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Franck Koffi Bouho reports financial support was provided by French National Research Agency.

Data availability

Data will be made available on request.

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).
Supplementary materials

Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.electacta.2024.144697.

References

[1] Perspective for the clean energy transition – the critical role of buildings, International Energy Agency, April 2019. https://iea.blob.core.windows.net/assets/026bfb1b-821d-48bc-8a0e-7c10280c62bc/Perspectives_for_the_Clean_Energy_Technology_2019.pdf (accessed May 5th, 2023).

[2] Smart electrification with renewables: driving the transformation of energy services, IRENA (2022), ISBN: 978-92-9260-367-0. https://www.irena.org/-/media/A/Files/IRENA/Agency/Publication/2022/Feb/IRENA_Smart-Electrification_Renewables_2022.pdf?rev=f4add985f6284a7cb811977c65b281b1 (accessed May 5th, 2023).

[3] The future of hydrogen - seizing today's opportunities. Report prepared by the IEA for the G20, Japan (2019). https://iea.blob.core.windows.net/assets/9e3a3493-b9a6-4b7d-b499-7ca48e357561/The_Future_of_Hydrogen.pdf (accessed May 5th, 2023).

[4] Hydrogen roadmap for sustainable path to Euro energy transition. Fuel cells and hydrogen joint undertaking (FCH JU), Fuel Cells Bull 2019 (2019) 13, https://doi.org/10.1016/S1464-2859(19)30121-X.

[5] A. Arregi, M. Amutio, G. Lopez, J. Bilbao, M. Olazar, Evaluation of thermochemical routes for hydrogen production from biomass: a review, Energy Convers Man 165 (2018) 696-719, https://doi.org/10.1016/j.enconman.2018.03.089.

[6] M. Binder, M. Kraussler, M. Kuba, M. Luisser, Hydrogen from biomass gasification. In IEA bioenergy: Task 33. Edited by Rauch Reinhard, ISBN 978-1-910154-59-5. (2019). https://www.ieabioenergy.com/wp-content/uploads/2019/01/Wasserstofstudie_IEA-final.pdf (accessed May 6th, 2023).

[7] P. Rullo, L. Nieto Degliuomini, M. García, M. Basualdo, Model predictive control to ensure high quality hydrogen production for fuel cells, Int. J. Hydrogen Energy 39 (2014) 8635-8649, https://doi.org/10.1016/j.ijhydene.2013.12.069.

[8] J.J. Lamb, B.G. Pollet, Editorial overview: energy transformation 2022 - Electrochemical energy storage transformations, Curr. Op. Electrochem. 35 (2022) 10107, https://doi.org/10.1016/j.coec.2022.101074.

[9] T. Larriba, R. Garde, M. Santarelli, Fuel cell early markets: techno-economic feasibility study of PEMF Cbased drive trains in materials handling vehicles, Int. J. Hydrogen Energy 38 (2013) 2009-2019, https://doi.org/10.1016/j.ijhydene.2012.11.048.

[10] M. Watanabe, S. Motoo, Chemisorbed CO on a polycrystalline platinum electrode The effect of conditioning of the surface and of partial pressure of CO, J. Electroanal. Chem. 206 (1986) 197-208, https://doi.org/10.1016/0022-0728 (86)90268-8.

[11] S. Balasubramanian, C.E. Holland, J.W. Weidner, Electrochemical removal of carbon monoxide in reformate hydrogen for fuelling proton exchange membrane fuel cells, Electrochem. Solid-State Lett. 13 (2010) B5-B7, https://doi.org/10.1149/1.3267850.

[12] Hydrogen fuel quality specifications for polymer electrolyte fuel cells in road vehicles - report to the safety, codes and standards program, Fuel Cell Technologies Office, U.S. Department of Energy,. 2016. Published November 2, https://www.energy.gov/sites/prod/files/2016/11/f34/fcto_h2燃料_ fuel_ quality _specs_pem_fct_road_vehicles.pdf (accessed May 6th, 2023).

[13] Y. Matsuda, T. Shimizu, Y. Hashimasa, Effect of carbon monoxide on polymer electrolyte fuel cell performance with a hydrogen circulation system, J. Electrochem. Soc. 167 (2020) 044509, https://doi.org/10.1149/1945-7111/ab732c.

[14] C. Coutanceau, S. Baranton, Electrochemical conversion of alcohols for hydrogen production: a short overview, WIRE Energy Environ. (2016) 388-400, https://doi.org/10.1002/wene.193.

[15] P. Urchaga, S. Baranton, C. Coutanceau, G. Jerkiewicz, Electro-oxidation of CO<sub>chem</sub> Pt nanosurfaces: solution of the peak multiplicity puzzle, Langmuir 28 (2012) 3658-3663, https://doi.org/10.1021/la202913b.

[16] P. Urchaga, S. Baranton, C. Coutanceau, Changes in COchem oxidative stripping activity induced by reconstruction of Pt (1 1 1) and (1 0 0) surface nanodomains, Electrochim. Acta 92 (2013) 438-445, https://doi.org/10.1016/j.electacta.2013.01.042.

[17] K.A. Friedrich, F. Henglein, U. Stimming, W. Unkauf, Size dependence of the CO monolayer oxidation on nanosized Pt particles supported on gold, Electrochim. Acta 45 (2000) 3283-3293, https://doi.org/10.1016/S0013-4686(00)00430-8.

[18] B. Andreaus, F. Maillard, J. Kocylo, E.R. Savinova, M. Eikerling, Kinetic modeling of CO<sub>ad</sub> monolayer oxidation on carbon-supported platinum nanoparticles, J. Phys. Chem. B 110 (2006) 21028-21040, https://doi.org/10.1021/jp063856k.

[19] P. Urchaga, S. Baranton, C. Coutanceau, G. Jerkiewicz, Evidence of an Eley-Rideal mechanism in the stripping of a saturation layer of chemisorbed CO on platinum nanoparticles, Langmuir 28 (2012) 13094-13104, https://doi.org/10.1021/la302388p.

[20] S. Gilman, The mechanism of electrochemical oxidation of carbon monoxide and methanol on platinum. I. Carbon monoxide adsorption and desorption and simultaneous oxidation of the platinum surface at constant potential, J. Phys. Chem. 67 (1963) 1898-1905, https://doi.org/10.1021/j100803a041.

[21] S. Gilman, The mechanism of electrochemical oxidation of carbon monoxide and methanol on platinum. II. The "Reactant-Pair" mechanism for electrochemical oxidation of carbon monoxide and methanol, J. Phys. Chem. 68 (1964) 70-80, https://doi.org/10.1021/j100783a013.

[22] M. Gonzalez-Hernandez, E. Antolini, J. Perez, CO tolerance and stability of PtRu and PtRuMo electrocatalysts supported on N-doped graphene nanoplatelets for polymer electrolyte membrane fuel cells, Int. J. Hydrogen Energy 45 (2020) 5276e84, https://doi.org/10.1016/j.ijhydene.2019.05.208.

[23] N. Narischat, T. Takeguchi, T. Mori, S. Iwamura, I. Ogino, S. Mukai, W. Ueda, Effect of the mesopores of carbon supports on the CO tolerance of Pt<sub>2</sub>Ru<sub>3</sub> polymer electrolyte fuel cell anode catalyst, Int. J. Hydrogen Energy 41 (2016) 13697e704, https://doi.org/10.1016/j.ijhydene.2016.05.272.

[24] M.J. Croissant, T. Napporn, J.M. Léger, C. Lamy, Electrocatalytic oxidation of hydrogen at platinum-modified polyaniline electrodes, Electrochim. Acta 43 (1998) 2447-2457, https://doi.org/10.1016/0013-4686(96)00127-2.

[25] M.J. Lee, J.S. Kang, Y.S. Kan, D.Y. Chung, H. Shin, C.Y. Ahn, S. Park, M.J. Kim, S. Kim, K.S. Lee, Y.E. Sung, Understanding the bifunctional effect for removal of CO poisoning: blend of a platinum nanocatalyst and hydrous ruthenium oxide as a model system, ACS Catal 6 (2016) 2398-2407, https://doi.org/10.1021/acscatal.5b02580.

[26] M. Watanabe, S. Motoo, Electrocatalysis by ad-atoms: part III. Enhancement of the oxidation of carbon monoxide on platinum by ruthenium ad-atoms, J. Electroanal. Chem. 60 (1975) 275-283, https://doi.org/10.1016/S0022-0728(75)80262-2.

[27] N. Dimakis, M. Cowan, G. Hanson, E.S. Smotkin, Attraction-repulsion mechanism for carbon monoxide adsorption on platinum and platinum-ruthenium alloys, J. Phys. Chem. C 113 (2009) 18730-18739, https://doi.org/10.1021/jp9036809.

[28] C. Molochas, P. Tsiakaras, Carbon monoxide tolerant Pt-based electrocatalysts for H₂-PEMFC applications: current progress and challenges, Catalysts 11 (2021) 1127, https://doi.org/10.3390/catal11091127.

[29] S. Harish, S. Baranton, C. Coutanceau, James Joseph, Microwave assisted polyol method for the preparation of Pt/C, Ru/C and PtRu/C nanoparticles and its application in electro-oxidation of methanol, J. Power Sources 214 (2012) 33-39, https://doi.org/10.1016/j.jpowsour.2012.04.045.

[30] Y. Li, X. Zhu, Y. Chen, S. Zhang, J. Li, J. Liu, Rapid synthesis of highly active Pt/C catalysts with various metal loadings from single batch platinum colloid, J. Energy Chem. 47 (2020) 138-145, https://doi.org/10.1016/j.jechem.2019.12.004.

[31] F. Fiévet, S. Ammar-Merah, R. Brayner, F. Chau, M. Giraud, F. Mammeri, J. Peron, J.Y. Piquemal, L. Sicard, G. Viau, The polyol process: a unique method for easy access to metal nanoparticles with tailored sizes, shapes and compositions, Chem. Soc. Rev. 47 (2018) 5187-5233, https://doi.org/10.1039/C7CS00777A.

[32] H. Zhu, X. Li, F. Han, Z. Dong, G. Yuan, G. Ma, A. Westwood, K. He, The effect of pitch-based carbon fiber microstructure and composition on the formation and growth of SiC whiskers via reaction of such fibers with silicon sources, Carbon N Y 99 (2016) 174-185, https://doi.org/10.1016/j.carbon.2015.12.002.

[33] C. Coutanceau, A.F. Rakotodrainibe, A. Lima, E. Garnier, S. Pronier, J.M. éger, C. Lamy, Preparation of Pt-Ru bimetallic anodes by galvanostatic pulse electrodeposition: characterization and application to the direct methanol fuel cell, J. Appl. Electrochem. 34 (2004) 61-66, https://doi.org/10.1007/s10800-007-9462-6.

[34] L. Dubau, F. Hahn, C. Coutanceau, J.M. Léger, C. Lamy, On the structure effects of bimetallic PtRu electrocatalysts towards methanol oxidation, J. Electroanal. Chem. 554-555 (2003) 407-415, https://doi.org/10.1016/S0022-0728(03)00308-5.

[35] C. Coutanceau, S. Baranton, C. Lamy, Determination of reaction mechanisms occurring at electrocatalysts using electrochemical methods, spectroelectrochemical measurements and analytical techniques, in "Modern aspects of Electrochemistry, P. Balbuena, V. Subramanian, In: Theory and Experiment in Electroanalysis, 50, Springer Science + Business Media, 2010, pp. 397-501, https://doi.org/10.1007/978-1-4419-5594-4_10.

[36] C. Xu, L. Wang, X. Mu, Y. Ding. Nanoporous PtRu alloys for electrocatalysis. Langmuir 26 (2010) 7437-7443. 10.1021/la9041474.

[37] H.A. Gasteiger, N.M. Marković, P.N. Ross Jr, Hi and CO electrooxidation on well- characterized Pt, Ru, and Pt-Ru. 1. Rotating disk electrode studies of the pure gases including temperature effects, J. Phys. Chem. 99 (1995) 8290-8301, https://doi.org/10.1021/j100020a063.

[38] S. Lankiang, M. Chiwata, S. Baranton, H. Uchida, C. Coutanceau, Oxygen reduction reaction at binary and ternary nanocatalysts based on Pt, Pd and Au, Electrochim. Acta. 182 (2015) 131-142, https://doi.org/10.1016/j.electacta.2015.09.061.

[39] Z. Pu, R. Cheng, J. Zhao, Z. Hu, C. Li, W. Li, P. Wang, I. Saana Amiinu, Z. Wang, M. Wang, D. Chen, S. Mu, Anion-modulated platinum for high-performance multifunctional electrocatalysis toward HER, HOR, and ORR, iScience 23 (2020) 101793, https://doi.org/10.1016/j.isci.2020.101793.

[40] X. Tian, P. Zhao, W. Sheng, Hydrogen evolution and oxidation: mechanistic studies and material advances, Adv. Mater. 31 (2019) 1808066, https://doi.org/10.1002/adma.201808066.

[41] T.J. Schmidt, H.A. Gasteiger, G.D. Stab, P.M. Urban, D.M. Kolb, R.J. Behm, Characterization of high-surface-area electrocatalysts using a rotating disk electrode configuration, J. Electrochem. Soc. 145 (1998) 2354, https://doi.org/10.1149/1.1838642.

[42] T. Binninger, E. Fabbri, R. Kotz, T.J. Schmidt, Determination of the electrochemically active surface area of metal-oxide supported platinum catalyst, J. Electrochem. Soc. 161 (2014) H121-H128, https://doi.org/10.1149/2.055403jes.

[43] M. Łukaszewski, M. Soszko, A. Czerwiński, Electrochemical methods of real surface area determination of noble metal electrodes – an overview, Int. J. Electrochem. Sci. 11 (2016) 4442-4469, https://doi.org/10.20964/2016.06.71.

[44] E. Lebègue, S. Baranton, C. Coutanceau, Polyol synthesis of nanosized Pt/C electrocatalysts assisted by pulse microwaves activation, J. Power Sourc. 196 (2011) 920-927, https://doi.org/10.1016/j.jpowsour.2010.08.107.
[45] A. Cuesta, A. Couto, A. Rincón, M.C. Pérez, A. López-Cudero, C. Gutiérrez, Potential dependence of saturation CO coverage of Pt electrodes: the origin of the pre-peak in CO-stripping voltammograms. Part 3: pt(poly), J. Electroanal. Chem. 586 (2006) 185–195, https://doi.org/10.1016/j.jelechem.2005.10.006.

[46] F. Maillard, S. Schreier, M. Hanzlik, E.R. Savinova, S. Weinkauf, U. Stimming, Influence of particle agglomeration on the catalytic activity of carbon-supported Pt nanoparticles in CO monolayer oxidation, Phys. Chem. Chem. Phys. 7 (2005) 385–393, https://doi.org/10.1039/B411377B.

[47] E.A. Ciapina, S.F. Santos, E.R. Gonzalez, Electrochemical CO stripping on nanosized Pt surfaces in acidic media: a review of the issue of peak multiplicity, J. Electroanal. Chem. 815 (2018) 47–60, https://doi.org/10.1016/j.jelechem.2018.02.047.

[48] N.M. Markovic, B.N. Grgur, C.A. Lucas, P.N. Ross, Electrooxidation of CO and H₂/CO mixtures on Pt(111) in acid solutions, J. Phys. Chem. B 103 (1999) 487–495, https://doi.org/10.1021/jp983177c.

[49] N.M. Markovic, B.N. Grgur, C.A. Lucas, P.N. Ross, Surface electrochemistry of CO and H₂/CO mixtures at Pt(100) interface: electrode kinetics and interfacial structures, J. Phys. Chem. B 103 (1999) 9616–9623, https://doi.org/10.1021/jp991566g.

[50] N.P. Lebedeva, N.T.M. Koper, J.M. Feliu, R.A. van Santen, Role of crystalline defects in electrocatalysis: mechanism and kinetics of CO adlayer oxidation on stepped platinum electrodes, J. Phys. Chem. B 106 (2002) 12938–12947, https://doi.org/10.1021/jp0204105.

[51] J. Solla-Gullón, P. Rodríguez, E. Herrero, A. Aldaz, J.M. Feliu, Surface characterization of platinum electrodes, Phys. Chem. Chem. Phys. 10 (2008) 1359–1373, https://doi.org/10.1039/b709809j.

[52] P. Rodríguez, E. Herrero, J. Solla-Gullón, F.J. Vidal-Iglesias, A. Aldaz, J.M. Feliu, Electrochemical characterization of irreversibly adsorbed germanium on platinum

stepped surfaces vicinal to Pt(100), Electrochim. Acta 50 (2005) 3111–3121, https://doi.org/10.1016/j.electacta.2004.10.086.

[53] P. Rodríguez, E. Herrero, J. Solla-Gullón, F.J. Vidal-Iglesias, A. Aldaz, J.M. Feliu, Specific surface reactions for identification of platinum surface domains: surface characterization and electrocatalytic tests, Electrochim. Acta 50 (2005) 4308–4317, https://doi.org/10.1016/j.electacta.2005.02.087.

[54] J. Solla-Gullón, P. Rodríguez, E. Herrero, A. Aldaz, J.M. Feliu, Surface characterization of platinum electrodes, Phys. Chem. Chem. Phys. 10 (2008) 1359–1373, https://doi.org/10.1039/B709809J.

[55] M. Wakisaka, T. Ohkanda, T. Yoneyama, H. Uchida, M. Watanabe, Structure of a CO adlayer on a Pt(100) electrode in HClO₄ solution studied by in situ STM, Chem. Commun. (2005) 2710–2712, https://doi.org/10.1039/B503148F.

[56] A. Crown, C. Johnston, A. Wieckowski, Growth of ruthenium islands on Pt(hkl) electrodes obtained via repetitive spontaneous deposition, Surf. Sci. 506 (2002) L268–L274, https://doi.org/10.1016/S0039-6028(02)01317-1.

[57] V.D. Colle, M.J. Giz, G. Tremiliosi-Filho, Spontaneous deposition of Ru on Pt (100): morphological and electrochemical studies. preliminary results of ethanol oxidation at Pt(100)/Ru, J. Braz. Chem. Soc. 14 (2003) 601–609, https://doi.org/10.1590/S0103-50532003000400017.

[58] E. Herrero, J.M. Feliu, A. Wieckowski, Scanning tunneling microscopy images of ruthenium submonolayers spontaneously deposited on a Pt(111) electrode, Langmuir 15 (1999) 4944–4948, https://doi.org/10.1021/la990219v.

[59] A. Crown, I.R. Moraes, A. Wieckowski, Examination of Pt(111)/Ru and Pt(111)/Os surfaces: STM imaging and methanol oxidation activity, J. Electroanal. Chem. 500 (2001) 333–343, https://doi.org/10.1016/S0022-0728(00)00450-2.

[60] P. Waszczuk, J. Solla-Gullón, H.S. Kim, Y.Y. Tong, V. Montiel, A. Aldaz, A. Wieckowski, Methanol electrooxidation on platinum/ruthenium nanoparticle catalysts, J. Catal. 203 (2001) 1–6, https://doi.org/10.1006/jcat.2001.3389.