# Highly dispersed Pt–Fe–Ru nanoparticles on mesoporous bio-based hydrochar for effective oxygen reduction reactions in acidic and alkaline electrolytes

El Mehdi Chatir, Thibault Rafaïdeen, Anthony Thomas, Christophe

Coutanceau

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El Mehdi Chatir, Thibault Rafaïdeen, Anthony Thomas, Christophe Coutanceau. Highly dispersed Pt–Fe–Ru nanoparticles on mesoporous bio-based hydrochar for effective oxygen reduction reactions in acidic and alkaline electrolytes. Electrochimica Acta, 2025, 529, pp.146353. 10.1016/j.electacta.2025.146353. hal-05055092

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Submitted on 3 May 2025

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# Highly dispersed Pt–Fe–Ru nanoparticles on mesoporous bio-based hydrochar for effective oxygen reduction reactions in acidic and alkaline electrolytes

El Mehdi Chatir<sup>a, ID</sup>, Thibault Rafaïdeen<sup>a ID</sup>, Anthony Thomas<sup>b,c</sup>, Christophe Coutanceau<sup>a,c,* ID</sup>

## ARTICLE INFO

### Keywords:

- Bio-based carbon
- Cathode electrocatalysts
- Oxygen reduction reaction
- Platinum-iron-ruthenium trimetallic nanoparticle

## ABSTRACT

This study aims to develop a cost-effective, high-performance electrocatalyst using mesoporous bio-derived hydrochar support. The mesoporous hydrochar was synthesized from cellulose through hydrothermal carbonization followed by $H_3PO_4$ activation, yielding a material with a high specific surface area (1137 $m^2 g^{-1}$), mesoporosity (537 $m^2 g^{-1}$), abundant defect sites ($I_D/I_G = 1.9$), and diverse surface functionalities. These properties enabled the uniform dispersion of low-load (≤ 2 wt%) platinum, iron, and ruthenium nanoparticles through wet impregnation and thermal reduction, forming synergistic trimetallic nanoparticles on the hydrochar surface. Electrochemical evaluations demonstrated highly efficient 4 $e^-$ ORR activity of a trimetallic Pt-Ru-Fe catalyst in both acidic and alkaline media. In 0.1 M $HClO_4$, the catalyst exhibited an onset potential of 0.91 V vs. RHE and a Tafel slope of 69 mV $dec^{-1}$, while in 0.1 M KOH, the onset potential and Tafel slope improved to 0.94 V vs. RHE and 47 mV $dec^{-1}$, respectively, highlighting its versatility across different electrolytes. Overall, this work provides a pathway to significantly reduce PEMFC costs while maintaining high catalytic performance, offering a sustainable and economically viable solution for the advancement of fuel cell technologies.

## 1. Introduction

The dawn of the current era has witnessed remarkable economic growth, yet it has also cast a shadow of concern over the environment and public health [1]. This concerning situation is mainly attributed to the widespread reliance on fossil resources driven by the demands of industrial and post-industrial development [2]. The excessive consumption of fossil resources has resulted in a troubling increase in greenhouse gas emissions, further exacerbating the global challenge of climate change and its associated consequences, including the escalation of global temperatures and a rise in extreme weather events [3]. In response to this existential crisis, the global community has demonstrated an unprecedented determination to transition towards a sustainable and resilient future.

In this context, biomass has emerged as a promising sustainable alternative to fossil resources. Biomass is considered as nature's $CO_2$ sink, as it is formed through the activation of molecules like $CO_2$, $O_2$, $N_2$, and others via various biochemical processes, most notably

photosynthesis [4]. However, challenges such as optimizing biomass conversion technologies must be addressed to fully harness the potential of biomass [5–9]. In this regard, Bergius accidentally discovered in 1913 a green technology for biomass conversion, called hydrothermal carbonization [10], which is presumably close to natural coalification. This technology is nowadays exploited for converting bioresources such as wet biomass, polysaccharides, and derivatives (furfural, xylose, and glucose) into a carbon-rich material known as hydrochar [11–13]. Despite its limited textural properties, hydrochar exhibits high chemical reactivity due to its abundant oxygen functional groups and low aromaticity, making it an excellent precursor for engineered carbon materials [14–16]. By tailoring its porosity, surface functionality, and defect density, hydrochar can be converted into advanced carbon materials capable of replacing fossil-derived electrode and catalyst supports in energy applications such as solid polymer fuel cells (SPEFCs) [17–20].

Proton exchange membrane fuel cells (PEMFCs) hold significant potential as a clean energy supplier, efficiently converting hydrogen and oxygen into electricity with zero emissions. However, their large-scale

<footnote><sup>a</sup> Université de Poitiers, IC2MP, CNRS, France</footnote>

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

<footnote><sup>c</sup> FRH2, CNRS, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: christophe.coutanceau@univ-poitiers.fr (C. Coutanceau).</footnote>
adoption is restricted by the high cost and limited durability of platinum-based catalysts, particularly for the oxygen reduction reaction (ORR) at the cathode [21]. Traditionally, Pt nanoparticles are supported on fossil-derived carbon blacks such as Vulcan XC-72, with high Pt mass loading [19]. To improve sustainability and cost-effectiveness, recent research has explored biomass-derived carbon supports as viable alternatives while simultaneously aiming to reduce Pt usage [17–20]. Although bio-based materials have demonstrated promising catalytic properties, achieving both high stability and catalytic efficiency with ultra-low Pt loadings remains challenging [22]. In this context, previous studies have reported the stabilization of Pt at an ultra-low loading (0.4 wt%) on nitrogen-doped carbon, achieving a half-wave potential of 0.76 V vs. RHE with excellent durability [23]. Liu et al., [23] reported that nitrogen functional groups play a crucial role in stabilizing Pt and enhancing its ORR activity. Furthermore, integrating lower-cost transition metals such as Fe, Ni, Co, and Ru to form bimetallic and trimetallic Pt alloys has proven effective in improving stability and catalytic performance while minimizing Pt consumption [22,24–29].

Building on these advancements, this work describes the synthesis of mesoporous hydrochar derived from cellulose as a sustainable support for low-Pt catalysts and evaluates the electrocatalytic activity, selectivity, and stability of such catalysts towards the ORR. The well-developed mesoporosity, surface functionality, and defect-rich structure of the hydrochar, along with the in-situ incorporation of nitrogen functionalities, contribute to the stabilization of Pt nanoparticles. Additionally, Fe and/or Ru are introduced to form bimetallic (Pt-Fe, Pt-Ru) and trimetallic (Pt-Fe-Ru) catalysts, further enhancing ORR activity and improving catalyst durability.

## 2. Experimental section

### 2.1. Synthesis of mesoporous hydrochar (MHC)

Hydrochar was prepared via the hydrothermal carbonization of cellulose. Briefly, a dispersion of cellulose (100 g L⁻¹) in Milli-Q water (Millipore, 18.2 MΩ cm resistivity, < 2 ppb total organic carbon) was transferred into a stainless-steel autoclave and heated at 200 °C for 12 h. The resulting dark brown solid was thoroughly washed with distilled water and dried at 100 °C for 6 h. Subsequently, 3 g of hydrochar and 10.6 g of phosphoric acid (H₃PO₄, 85 % purity from Merck) were mixed with 20 mL of Milli-Q water, magnetically stirred for 4 h at room temperature, and then dried overnight at 110 °C. The dried solid was subjected to thermal activation under a nitrogen (U quality, Air Liquide) atmosphere at 500 °C for 1 h (gas flow rate: 50 mL min⁻¹, heating rate of 5 °C min⁻¹).

### 2.2. Preparation of Ru-N-MHC and Fe-N-MHC and Fe-Ru-N-MHC

For the preparation of Fe-Ru-N-MHC catalysts (total catalyst mass of 200 mg), aqueous solutions of ruthenium chloride (RuCl₃·xH₂O, 99.9 % purity, 38 % Ru, from Alfa Aesar) 0.5 mg·mL⁻¹ and Fe (Fe(NO₃)₃·9H₂O, 99.9 % from Merk) 1 mg·mL⁻¹ were prepared. Subsequently, 2 mL of each solution was added to 100 mL of Milli-Q water. An appropriate amount of MHC was introduced into the solution under magnetic stirring, followed by sonication for 2 h. The resulting suspension was dried in an oven at 80 °C and then ground with 2.0 g of urea (CO(NH₂)₂, ≥ 99 % purity from Sigma Aldrich). The pyrolysis of the mixed powder was conducted at 750 °C for 1 h under a nitrogen atmosphere (flow rate of 50 mL·min⁻¹). The resulting catalysts were denoted as Fe-N-MHC, Ru-N-MHC, and Fe-Ru-N-MHC. A metal-free catalyst, prepared under identical conditions but without the wet impregnation step, was denoted as N-MHC.

### 2.3. Deposition of Pt, Pt-Ru, Pt-Fe, and Pt-Fe-Ru nanoparticles on MHC

For the preparation of catalysts with a total mass of 200 mg, aqueous

solutions of 2 mg mL⁻¹ Pt (H₂PtCl₆·6 H₂O, 99.9 % purity, 37.5 % Pt, from Sigma Aldrich), 0.5 mg·mL⁻¹ Ru, and 1 mg·mL⁻¹ Fe were prepared. Subsequently, 2 mL of each solution was added to 100 mL of Milli-Q water. An appropriate amount of MHC was introduced into the mixture under magnetic stirring, followed by sonication for 2 h. The resulting suspension was dried in an oven at 80 °C and then ground with 2.0 g of urea. To reduce Pt, the mixed powder was heated to 750 °C at a rate of 5 °C·min⁻¹ and maintained at this temperature for 1 h under a nitrogen atmosphere (flow rate of 50 mL·min⁻¹). The atmosphere was then switched to hydrogen (flow rate of 50 mL·min⁻¹), and the material was held at 750 °C for an additional hour. The resulting catalysts were denoted as Pt-N-MHC, Pt-Fe-N-MHC, Pt-Ru-N-MHC, and Pt-Fe-Ru-N-MHC. A nitrogen-free catalyst, prepared under identical conditions but without using urea, was denoted as Pt-Fe-Ru-MHC (Scheme 1).

### 2.4. Electrochemical measurements

The electrocatalytic ink for electrochemical measurements was prepared as follows: 17.7 mg of electrocatalyst was dispersed in 2.65 mL of Milli-Q water and sonicated for 30 min. Subsequently, 0.355 mL of a 5 % Nafion solution in aliphatic alcohols (Sigma Aldrich) was added, and the mixture was further sonicated for 3 min. For the fabrication of the working electrode, 8 µL of the catalyst ink was dispensed onto a glassy carbon rotating disc electrode of 5 mm diameter (RDE GC50.S, 0.196 cm² geometric surface area from Metrohm). Electrochemical experiments were conducted in an electrochemical cell connected to an interfaced PGSTAT302N potentiostat (Metrohm) with a reversible hydrogen electrode (RHE, HydroFlex from Gaskatel) as the reference electrode and a 3 cm² glassy carbon electrode as the counter electrode. The catalysts' performances were evaluated in 0.1 M HClO₄ (Perchloric acid 70 % Suprapur from Merck) and 0.1 M KOH (semiconductor grade 99.99 % purity from Sigma-Aldrich) in saturated N₂ or O₂ (U quality from Air Liquid) electrolytes at a potential scan rate of 5 mV s⁻¹. All current densities were normalized to the electrode's geometric surface area.

In this study, parameters such as the kinetic current density ($j_k$), mass activity ($M_A$), and the number of transferred electrons ($n_e$) during the oxygen reduction reaction (ORR) were determined using the Koutecký-Levich and Levich equations. The kinetic current density was calculated using the Koutecký-Levich equation:

$$ \frac{1}{j} = \frac{1}{j_k} + \frac{1}{j_d} \qquad (1) $$

where $j$ (mA cm⁻²) is the measured current density and $j_d$ (mA cm⁻²) is the diffusion-limited current density. The mass activity ($M_A$) was determined as:

$$ M_A = \frac{j_k}{m_{Pt}} \qquad (2) $$

where $m_{Pt}$ is the Pt loading density (µgPt cm⁻²). The number of transferred electrons ($n_e$) was estimated from the Levich equation

$$ j_d = 0.2n_e \cdot FD^{2/3} \nu^{-1/6} C_0 \omega^{1/2} \qquad (3) $$

where $F$ is Faraday's constant, $D$ is the oxygen diffusion coefficient (1.93 10⁻⁵ cm² s⁻¹ in 0.1 M HClO₄ and 1.90 10⁻⁵ cm² s⁻¹ in 0.1 M KOH) [30–32], $\nu$ is the kinematic viscosity of the electrolyte (0.01 cm² s⁻¹ in 0.1 M HClO₄ and 0.1 M KOH), $C_0$ is the bulk oxygen concentration (1.1 10⁻⁶ mol cm⁻³ in 0.1 M HClO₄ and 1.2 10⁻⁶ mol cm⁻³ in 0.1 M KOH) [30–32], and $\omega$ is the electrode rotation speed (revolution per minute, rpm).

### 2.5. Physicochemical characterization of catalysts

The synthesized samples were thoroughly characterized using
<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Preparation of electrocatalysts from cellulose.</figcaption></figure>

various techniques to analyze their structural, morphological, textural, and elemental properties. Raman spectroscopy was employed to investigate the structural features and defects in the carbon framework, with spectra acquired in a HORIBA Jobin Yvon Labram HR 800 UV confocal Raman microscope with an excitation wavelength of 532 nm. Elemental composition, including carbon, hydrogen, nitrogen, and sulfur contents, was determined using CHNS analysis performed on a UNICUBE CHNS analyzer. The metallic mass loading was determined via inductively coupled plasma optical emission spectroscopy (ICP-OES) employing an Agilent 5110 ICP-OES device. Transmission electron microscopy (TEM) was conducted to observe the morphology and structural features of the samples at the nanoscale, using a JEOL STEM 2100 UHR operated at 200 kV with a resolution of 0.19 nm. The surface area, pore size distribution, and pore volume of the MHC were evaluated through N₂ physisorption measurements conducted at 77 K using a Tristar 3000 device. X-ray

photoelectron spectroscopy (XPS) was utilized to determine the surface chemical states and elemental composition, with spectra collected using a Kratos Axis Ultra DLD spectrometer with an Al Mono X-ray source operating at a power of 150 W. Survey spectra were acquired with a step size of 1 eV and a pass energy of 160 eV, while high-resolution spectra were recorded with a step size of 0.1 eV. Charge correction was applied relative to the C 1s peak in high-resolution spectra.

**3. Results and discussion**

The treatment of cellulose through the two-step process resulted in the generation of activated hydrochar with a rough and irregular surface morphology, as observed through microscopy. The MHC exhibited a type VI isotherm with H3 hysteresis loops, characteristic of mesoporous materials with slit-shaped and irregular pores [33] (Fig. 1A). Specific

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (A) N₂-physorption isotherm of MHC; (B) RAMAN spectrum of MHC and Pt-X-MHCs; (C) XPS survey spectrum of MHC; (D) EIS Nyquist plots of MHC in 0.1 M HClO₄ at 25 °C; (E) RDE polarisation curves recorded on MHC at different electrode rotation rates (400, 600, 900, 1600, and 2500 rpm) in O₂-saturated 0.1 M HClO₄ at 25 °C, inset: CV of MHC in O₂- or N₂-saturated 0.1 M HClO₄ electrolyte at 25 °C without rotation of the electrode.</figcaption></figure>
surface area analysis using the BET model revealed a high surface area of 1137 m² g⁻¹, with pronounced mesoporosity (537 m² g⁻¹) contributing to the total surface area (Table 1). Elemental analysis revealed that MHC is composed of 60 % carbon, 4 % hydrogen, and 33 % oxygen. In addition to carbon and oxygen, the XPS analysis identified phosphorus on the surface of MHC (Fig. 1C). The phosphorus presence was attributed to the interaction between H₃PO₄ and the hydrochar matrix during chemical activation, which facilitated the incorporation of phosphorus species into the carbon matrix [34,35]. In the C1s region, peak corresponding to C(sp²)-C(sp²) bonds in graphitic carbon was observed at 284.32 eV, alongside peaks associated with C—O bonds in phenol, carboxyl, lactone, and phosphonate groups at a binding energy of 285.24 eV (Figure S1). Additionally, C=O bonds in carbonyl, carboxyl, and lactone groups were identified at 286.51 eV (Figure S1) [36,37]. In the O1s region, the peak at 530.60 eV was assigned to C=O bonds in carbonyl groups and P=O bonds in phosphate groups, while the peak at 532.37 eV was attributed to singly bonded oxygen in C—O groups such as carboxyl, phenol, lactone, and C—O—P (Figure S1) [36,38]. Finally, a peak at 534.58 eV corresponded to oxygen bonds in chemisorbed water and oxygen [36,37]. Raman spectroscopy showed two characteristic peaks at around 1350 cm⁻¹ and 1601 cm⁻¹, corresponding to the G and D bands [39], with a high defect degree (I<sub>D</sub>/I<sub>G</sub> = 1.9), indicating a disordered carbon structure (Fig. 1 B and Table 2).

The electrochemical properties of the MHC were evaluated using electrochemical impedance spectroscopy (EIS) in 0.1 M HClO₄ over a frequency range of 0.001 Hz to 1 kHz (Fig. 1D). The Nyquist plot showed a steep linear trend, pointing to significant resistance and diffusion-controlled processes, likely stemming from the porous structure of the MHC [40]. An inset of the plot highlighted a subtle semicircle in the low-frequency region, corresponding to charge transfer resistance at the electrode/electrolyte interface. This dual behavior reflects ion transport limitations and moderate electrochemical activity of the material. Consistent with these findings, the activity towards the ORR was assessed in 0.1 M HClO₄ using rotating disk electrode (RDE) measurements (Fig. 1E). Throughout the investigated potential range, no well-defined mass-transfer-limited region was observed, signifying negligible catalytic activity, with an onset potential of 0.3 V vs. RHE. These results align with the EIS data, indicating limited ORR activity under the tested conditions. Nevertheless, the abundant mesoporosity, high defect density, and oxygen-containing functional groups confer the MHC with strong interaction and sorption capacities for metal precursors in aqueous solutions, making it a promising candidate for immobilizing metallic active phases and enhancing catalytic functionalities [17,41–43].

To enhance the catalytic activity of MHC, the material was immobilized with 2 % Pt and subsequently reduced under hydrogen flow at 750 °C in the presence of urea as a nitrogen precursor. TEM revealed that the Pt particles uniformly dispersed on the MHC, though they exhibited a heterogeneous particle size distribution (Fig. 2), two phase appeared as well-defined nanoparticles with two distinct size populations: small

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

MHC textural properties: BET specific surface area ($S_{BET}$), micropore surface area ($S_{μ}$—t-plot), mesopore surface area ($S_{mes}$—t-plot) pore widths, pore volumes, pore surfaces, total pore volume ($V_t$) and average pore diameter ($D_p = 4 \times V_t/S_{BET}$).

<table><caption>**Table 2**</caption><thead><tr><th>S<sub>BET</sub><br>(m<sup>2</sup><br>g<sup>-1</sup>)</th><th>S<sub>μ</sub> t-<br>plot<br>(m<sup>2</sup><br>g<sup>-1</sup>)</th><th>S<sub>mes</sub><br>t-plot<br>(m<sup>2</sup><br>g<sup>-1</sup>)</th><th colspan="4">Surface and Pore volume distribution<br>by NLDFT-method</th><th>D<sub>p</sub><br>(nm)</th></tr></thead><tbody><tr><td>1137</td><td>880</td><td>257</td><td>Pore<br>width<br>Volume<br>(cm<sup>3</sup> g<sup>-1</sup>)</td><td>&lt; 2<br>nm<br>0.38</td><td>2-4<br>nm<br>0.30</td><td>4-50<br>nm<br>0.16</td><td>Total<br>0.84</td><td>2.9</td></tr><tr><td></td><td></td><td></td><td>Surface<br>(m<sup>2</sup> g<sup>-1</sup>)</td><td>600</td><td>498</td><td>39</td><td>1137</td><td></td></tr></tbody></table>

D- and G- bands position and defect degree ($I_D/I_G$) obtained from RAMAN spectra.

<table><thead><tr><th>Sample</th><th>D-bands (cm<sup>-1</sup>)</th><th>G-bands (cm<sup>-1</sup>)</th><th>I<sub>D</sub>/I<sub>G</sub></th></tr></thead><tbody><tr><td>MHC</td><td>1350</td><td>1601</td><td>1.9</td></tr><tr><td>Pt-N-MHC</td><td>1344</td><td>1605</td><td>2.9</td></tr><tr><td>Pt-Fe-N-MHC</td><td>1344</td><td>1605</td><td>2.9</td></tr><tr><td>Pt-Ru-N-MHC</td><td>1344</td><td>1605</td><td>2.8</td></tr><tr><td>Pt-Fe-Ru-N-MHC</td><td>1343</td><td>1605</td><td>2.9</td></tr></tbody></table>

particles ranging from 1 to 2 nm and larger ones between 5 and 400 nm in diameter. Moreover, this modification significantly increased the structural defects/vacancies from 1.9 to 2.9 (Table 2 and Fig. 1), while the XPS analysis revealed the introduction of new nitrogen functional groups, including pyridinic (398.1 eV), pyrrolic (399.02 eV), and graphitic nitrogen (400.53 eV) [23], which are recognized as effective metal-coordination sites [23,42,44]. X-ray photoelectron spectroscopy (XPS) deconvolution of the Pt 4f peaks further demonstrated strong interactions between the nitrogen functional groups and platinum, indicating their role in anchoring Pt nanoparticles (Fig. 3A). The Pt 4f<sub>7/2</sub> spectrum revealed three distinct peaks at binding energies of 72.15 eV, 73.69 eV, and 75.08 eV with contributions of 44.01 %, 40.02 %, and 15.97 %, respectively [23,44,45]. The peak at 71.41 eV corresponds to metallic platinum (Pt<sup>0</sup>) (Fig. 3A, B and C and Table 3). The peak at 73.7 eV is associated with Pt-N bonding, where platinum is stabilized in a Pt<sup>2+</sup> oxidation state due to interactions with nitrogen within the doped carbon structure [46]. This highlights the critical role of pyridinic, pyrrolic, and graphitic nitrogen functional groups as effective metal-coordination sites. The peak at 75.0 eV corresponds to Pt<sup>4+</sup> species, likely arising from PtO<sub>2</sub> [47].

The ORR performance was evaluated in HClO₄ electrolyte using the RDE measurements. The polarization curves showed a notable shift in the onset potential from 0.3 V to 0.78 V vs. RHE (Fig. 4). Additionally, a well-defined mass-transfer-limited region appeared between 0 and 0.3 V vs. RHE, indicating a substantial improvement in the catalytic activity. The number of exchanged electrons ($n_e$), estimated using the Koutecký-Levich equation, was found to be 3.08, suggesting that the oxygen reduction process occurred via a mixed 2e⁻ (Eqs. (4) and (5)) and 4e⁻ pathway (eq. 6) [48]. The Pt-N-MHC catalyst exhibited a Tafel slope of 85 mV dec⁻¹, indicating moderate catalytic activity for the oxygen reduction reaction (ORR) in an acidic electrolyte. Moreover, its durability was limited, retaining only 33 % of its initial catalytic activity after a 3600-second chronoamperometric test at 0.50 V vs. RHE (Fig. 4D). This significant activity decay highlights the insufficient stability of the catalyst under prolonged electrochemical conditions. These findings underscore the need for further optimization to improve both the durability and overall performance of the Pt-N-MHC catalyst in acidic environments. However, these modifications have successfully created active sites that significantly enhance the oxygen reduction reaction activity.

$$O_2 + 4 H^+ + 2 e^- \rightarrow H_2O_2 \quad (4)$$

$$H_2O_2 + 2 H^+ + 2 e^- \rightarrow 2 H_2O \quad (5)$$

$$O_2 + 4 H^+ + 4 e^- \rightarrow 2 H_2O \quad (6)$$

To further enhance the catalytic activity and durability of the Pt-N-MHC catalyst, the formation of Pt-based alloys with Fe and/or Ru was carried out. Pt-Fe and Pt-Ru alloys are known to significantly alter Pt's electronic structure, optimizing the adsorption energy of oxygenated intermediates and thereby accelerating oxygen reduction reaction (ORR) kinetics [22,24–27]. The incorporation of Fe and Ru resulted in a progressive reduction in the size of the larger particle population. In Pt-Fe-N-MHC, the larger particles ranged from 5 to 250 nm, while in the Pt-Ru-N-MHC sample, they further decreased to 5–200 nm (Fig. 2). The smaller particle population remained within 1 to 2.5 nm across both
<figure><img src="image_5.png" /><figcaption>Fig. 2. TEM images of (A,B) Pt-N-MHC, (C,D) Pt-Fe-N-MHC, (E,F) Pt-Ru-N-MHC, and (G,H) Pt-Fe-Ru-N-MHC.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. (A) XPS survey and (B) N 1s spectrum of Pt-N-MHC. (C,D, E,F) Pt4f high resolution spectrum of Pt-N-MHC, Pt-Fe-N-MHC, Pt-Ru-N-MHC, and Pt-Fe-Ru-N-MHC, respectively.</figcaption></figure>

<table><caption>**Table 3**

Deconvolution of high-resolution Pt 4f XPS spectrum of Pt-N-MHC, Pt-Fe-N-MHC, Pt Ru-N-MHC, and Pt-Fe-Ru-N-MHC.</caption><thead><tr><th rowspan="2"></th><th colspan="3">Pt-N-MHC</th><th colspan="3">Pt-Fe-N-MHC</th><th colspan="3">Pt-Ru-N-MHC</th><th colspan="3">Pt-Fe-Ru-N-MHC</th></tr><tr><th>Pt (O)</th><th>Pt (II)</th><th>Pt (IV)</th><th>Pt (O)</th><th>Pt (II)</th><th>Pt (IV)</th><th>Pt (O)</th><th>Pt (II)</th><th>Pt (IV)</th><th>Pt (O)</th><th>Pt (II)</th><th>Pt (IV)</th></tr></thead><tbody><tr><td>Binding energy (eV)</td><td>72.15</td><td>73.69</td><td>75.08</td><td>71.97</td><td>73.45</td><td>75.35</td><td>71.75</td><td>73.05</td><td>75.16</td><td>71.73</td><td>73.02</td><td>75.12</td></tr><tr><td></td><td>75.48</td><td>77.02</td><td>78.41</td><td>75.30</td><td>76.82</td><td>78.68</td><td>75.08</td><td>76.43</td><td>78.49</td><td>75.06</td><td>76.44</td><td>78.45</td></tr><tr><td>Peak area (%)</td><td>44.01</td><td>40.02</td><td>15.97</td><td>54.22</td><td>32.78</td><td>13</td><td>34.16</td><td>52.85</td><td>12.99</td><td>36.77</td><td>45.58</td><td>17.65</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (A) Cyclic voltammograms of Pt-N-MHC recorded in 0.1 M HClO₄, (B) Polarization curves of the ORR on Pt-N-MHC generated in 0.1 M HClO₄, (C) ORR Tafel plots of (a) Pt-N-MHC, (b)Pt-Fe-N-MHC, (c) Pt-Ru-N-MHC, Pt-Fe-Ru-N-MHC (d) and Pt-Fe-Ru-N-MHC (e) generated in O₂-saturated 0.1 M HClO₄ electrolyte with a rotation rate of 1600 rpm, and (D) Chronoamperometry curves recorded on the electrocatalysts in O₂-saturated 0.1 M HClO₄ at 0.5 V vs. RHE with a rotation speed of 1600 rpm. (T = 25 °C, scan rate = 5 mV s⁻¹).</figcaption></figure>

samples. The combined presence of Fe and Ru led to the most pro-
nounced reduction in the larger particle size range, which was limited to
5–80 nm, while the smaller particles remained between 1 and 2 nm. This
finding highlights the beneficial effect of combining these metals to
achieve better Pt dispersion. The EDS-TEM cartography revealed the
successful formation of bi- and trimetallic structures (Pt-Fe, Pt-Ru, and
Pt-Fe-Ru) (Fig. 5). The mapping results indicate that the metals are
uniformly dispersed, suggesting alloy formation. XPS analysis revealed a
slight shift of peaks in the Pt region toward lower binding energies
(Fig. 3C, D, and E). This shift is attributed to the strong interaction be-
tween Pt, Fe, and Ru atoms and also suggests the formation of bi- or
trimetallic alloys [24]. Interestingly, the peak at 73.7 eV, associated
with Pt-N bonding, remained largely unchanged, indicating a strong
interaction between Pt and nitrogen within the N-doped carbon struc-
ture. For the bi- and trimetallic catalysts, the Fe 2p spectrum revealed
the existence of Fe⁰, Fe²⁺, and Fe³⁺ at binding energies of 707 eV, 709.5
eV, and 712 eV, respectively [49](Figure S3). The deconvolution of Ru
3p₃/₂ high-resolution spectrum revealed two distinct peaks at 462.5 eV
and 466 eV which are ascribed to Ru⁰ and oxidized Ruₓ⁺ species,
respectively [49](Figure S4). It should be noted that Fe and Ru did not
significantly affect the carbon surface functionality (oxygen and nitro-
gen functional groups) or structural defects (Fig. 1B and Table 2 and
Figure S2).

The effect of Fe and Ru on the catalytic activity of the Pt-N-MHC
catalyst was assessed in 0.1 M HClO₄ electrolyte, and the results are
presented in (Fig. 6). Incorporating Fe and Ru significantly enhanced the
efficiency of the Pt-based catalyst, as evidenced by an improvement in
the onset potential from 0.78 V vs. RHE for Pt-N-MHC to 0.85 V vs. RHE
and 0.87 V vs. RHE for Pt-Fe-N-MHC and Pt-Ru-N-MHC, respectively

(Table 4). This modification also increased the M_A at 0.7 V vs. RHE from 1.14 A mgPt⁻¹ for Pt-N-MHC to 2.60 A mgPt⁻¹ and 4.40 A mgPt⁻¹ for Pt-Fe-N-MHC and Pt-Ru-N-MHC, respectively. Additionally, the Tafel slopes were reduced from 81 mV dec⁻¹ to 73 and 69 mV dec⁻¹ for Pt-Fe-N-MHC and Pt-Ru-N-MHC, respectively, indicating improved reaction kinetics. While Fe incorporation did not result in a tangible improvement in catalyst stability, Ru demonstrated enhanced stability, retaining 55 % of its initial catalytic activity, compared to the monometallic Pt catalyst, which retained only 33 % (Fig. 4D).

Interestingly, the trimetallic structure, Pt-Fe-Ru-N-MHC (Pt, Fe, and
Ru mass loading determined by ICP-OES were 2.20 %, 0.45 %, and 0.93
%, respectively), exhibited the best catalytic performance, achieving an
onset potential of 0.91 V vs. RHE, a Tafel slop of 68 mV dec⁻¹, and a Mₐ
of 8.34 A mgPt⁻¹ at 0.7 V vs. RHE (Fig. 6). This trimetallic catalyst also
favored oxygen reduction through a 4 e⁻ pathway, further enhancing its
activity. Moreover, its durability was significantly improved, retaining
64 % of its initial catalytic activity after a 3600-second chro-
noamperometric test at 0.50 V vs. RHE (Fig. 4D). These enhancements
can be attributed to the synergistic effects of Fe and Ru, which modify
the electronic structure of Pt, optimize oxygen intermediate adsorption
energies, and stabilize the catalyst surface during operation [22,24–27].
The catalytic activity of Pt-Fe-Ru-N-MHC was further explored by esti-
mating the electrochemical capacitance C<sub>DL</sub> in the non-faradaic region,
which is proportional to the active surface area (Fig. 6E). By plotting *j*
(mA cm<sup>-2</sup>) values obtained at 1 V vs. RHE versus scan rate (between 10
mV s<sup>-1</sup> and 50 mV s<sup>-1</sup>), the C<sub>DL</sub> of Pt-Fe-Ru-M was estimated to be 15 mF
cm<sup>-2</sup>, which exceeds the C<sub>DL</sub> values of commercial Pt (20 %)/C (12.44
mF cm<sup>-2</sup>) [50]. It should be noted that the electrochemical parameters
of Pt-Fe-Ru-N-MHC such as the E<sub>onset</sub>, E<sub>1/2</sub>, and M<sub>A</sub> are comparable to the
<figure><img src="image_8.png" /><figcaption>Fig. 5. TEM-EDS mapping, (A) Pt-Fe-N-MHC, (B) Pt-Ru-N-MHC, and (C) Pt-Fe-Ru-N-MHC.</figcaption></figure>

commercial Pt (20 %)/C, further illustrating its efficiency in ORR processes [50].

In addition to investigating the effects of Fe and Ru, the role of nitrogen on the catalytic activity was evaluated by synthesizing a nitrogen-free Pt-Fe-Ru-MHC catalyst and comparing its performance to the nitrogen-doped Pt-Fe-Ru-N-MHC (Fig. 6 and Table 4). The results revealed a significant decline in the catalytic activity for the nitrogen-free catalyst. Specifically, the onset potential dropped from 0.91 V for Pt-Fe-Ru-N-MHC to 0.78 V, while the mass activity decreased sharply from 8.34 Å $mg_{Pt}^{-1}$ to 1.14 Å $mg_{Pt}^{-1}$ (Table 4). Furthermore, the chronoamperometric stability was reduced from 64 % retention of initial activity to only 50 % after 3600 s at 0.50 V vs. RHE (Fig. 4). These findings illustrate the critical role of nitrogen in anchoring and stabilizing the active phases, particularly Pt, Fe, and Ru, within the carbonaceous support structure. Nitrogen doping not only enhances the electronic interactions between the catalyst and the support but also facilitates the formation of highly active Pt-N bonds, which contribute to the improved oxygen adsorption and activation during the ORR [23,51].

For comparison purposes, Pt-free catalysts were prepared under similar conditions. The catalysts labeled as N-MHC, Ru-N-MHC, Fe-N-MHC, and Ru-Fe-N-MHC exhibited low ORR catalytic activity (Fig. S5). In 0.1 M $HClO_4$ electrolyte, the linear sweep voltammetry (LSV) curves generated through rotating disk electrode measurements revealed no well-defined mass-transfer-limited region, signifying negligible catalytic activity. These findings signal the crucial role of Pt in ORR. Furthermore, while Fe, Ru, and nitrogen cannot independently promote efficient ORR, their combined presence serves to enhance and stabilize the activity of Pt during the ORR process. Notably, the slightly lower diffusion-limited

current densities with respect to the theoretical diffusion-limited current density for a 4-electron process (for instance, ca. 5.71 mA cm$^{-2}$ in $HClO_4$ 0.1 M at 1600 rpm) [52,53] can be due to the limited conductivity of the hydrochar, which we plan to improve in future studies. Also, the ultra-low Pt mass loading in the catalysts leads to limited numbers of active sites available for ORR per unit volume of catalytic layer, the catalytic layer is then thicker than for classical ones, limiting the oxygen/protons diffusion process towards catalytic sites and the ORR process, which results in a lower measured current density. However, the values of the Koutecky-Levich slopes were consistent with ca. 3 exchanged electrons for the Pt-N-MHC to 4 for the best catalyst Pt-Fe-Ru-N-MHC.

To further illustrate the superiority of the trimetallic Pt-Fe-Ru-N-MHC catalyst, its performance was also compared to the monometallic Pt-N-MHC catalyst in an alkaline electrolyte (Fig. 7). While the onset potential ($E_{onset}$), half-wave potential ($E_{1/2}$), and Pt $M_A$ (0.7 V vs. RHE) of Pt-N-MHC were 0.87 V vs. RHE, 0.71 V vs. RHE, and 11.8 Å $mg_{Pt}^{-1}$, respectively, the Pt-Fe-Ru-N-MHC catalyst demonstrated significantly better performance, with values of 0.94 V vs. RHE, 0.80 V vs. RHE, and 52.56 Å $mg_{Pt}^{-1}$ (Fig. 7). Additionally, the number of electrons transferred ($n_e$) during the ORR was calculated and found to be 2.89 for Pt-N-MHC, indicating an incomplete reduction of oxygen to water (Eqs. (6) and (7)). In contrast, the Pt-Fe-Ru-N-MHC exhibited a significantly higher $n_e$-value of 3.57, approaching the ideal value of 4, which corresponds to the complete four-electron reduction pathway (Eq. (8)) (Fig. 7). This improvement highlights the synergistic effect between Pt, Fe, and Ru in the trimetallic system, which not only enhances catalytic activity but also optimizes the reaction pathway by promoting the more efficient four-electron reduction of oxygen.
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** (A) CV of (a) Pt-N-MHC, (b) Pt-Fe-N-MHC, (c) Pt-Ru-N-MHC, (d) Pt-Fe-Ru-N-MHC, and (e) Pt-Fe-Ru-MHC electrocatalysts recorded in O₂-saturated 0.1 M HClO₄ electrolyte, (B) Polarization curves recorded on electrocatalysts in O₂ saturated 0.1 M HClO₄ with a rotation rate of 1600 rpm, (C) onset potential (Eonset) and mass activity @ 0.7 V vs. RHE, 1600 rpm of electrocatalysts in O₂-saturated 0.1 M HClO₄; (D) Mₐ (A mgpt⁻¹) and electron number (0.1 V vs. RHE) obtained in O₂ saturated 0.1 M HClO₄ electrolyte. (E) CVs of Pt-Fe-Ru-N-MHC recorded in a non-Faradaic region with scan rates ranging between 10 mV s⁻¹ and 50 mV s⁻¹. (F) Cathodic (grey) and anodic (black) current density measured at 1 V vs. RHE under different scan rates.</figcaption></figure>

<table><caption>**Table 4**
Electrochemical parameters for the ORR in HClO₄ electrolyte on the electrocatalysts.</caption><thead><tr><th>Sample</th><th>E<sub>onset</sub><br>(V vs. RHE)</th><th>E<sub>1/2</sub><br>(V vs. RHE)</th><th>n<sub>e.</sub><br>(at 0.1 V vs. RHE)</th><th>M<sub>A</sub> (A mgpt<sup>-1</sup>)<br>(at 0.7 V vs. RHE)</th></tr></thead><tbody><tr><td>Pt-N-MHC</td><td>0.78</td><td>0.48</td><td>3.07</td><td>1.17</td></tr><tr><td>Pt-Fe-N-MHC</td><td>0.85</td><td>0.58</td><td>4.16</td><td>2.60</td></tr><tr><td>Pt-Ru-N-MHC</td><td>0.87</td><td>0.64</td><td>4.26</td><td>4.40</td></tr><tr><td>Pt-Fe-Ru-N-MHC</td><td>0.91</td><td>0.68</td><td>3.97</td><td>8.34</td></tr><tr><td>Pt-Fe-Ru-MHC</td><td>0.81</td><td>0.54</td><td>4.00</td><td>1.14</td></tr><tr><td>Pt/C [50]</td><td>0.89</td><td>0.76</td><td>NI</td><td>~ 1.75</td></tr></tbody></table>

$$O_2 + H_2O + 2 e^- \rightarrow OH^- + HO_2^- \quad (6a)$$

$$HO_2^- + H_2O + 2 e^- \rightarrow 3 OH^- \quad (7)$$

$$O_2 + 2 H_2O + 4 e^- \rightarrow 4 OH^- \quad (8)$$

The Pt-Fe-Ru-N-MHC catalyst demonstrated superior performance in promoting the ORR in alkaline media (0.1 M KOH) compared to acidic electrolytes. In alkaline media, it exhibited an E₁/₂ of 0.8 V vs. RHE, a mass activity of 52.56 A mgpt⁻¹ at 0.7 V vs. RHE, and a Tafel slope of 47 mV dec⁻¹, significantly outperforming its performance in acidic conditions (Fig. 8). Moreover, chronoamperometric tests revealed remarkable stability in 0.1 M KOH electrolyte, with the catalyst retaining 89 % of its

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** (A) Cyclic voltammograms of Pt-N-MHC and Pt-Fe-Ru-N-MHC generated in O₂-saturated 0.1 M KOH electrolyte; (B) Polarization curves recorded on Pt-N-MHC and Pt-Fe-Ru-N-MHC in O₂-saturated 0.1 M KOH electrolyte with a rotation rate of 1600 rpm.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** (A) cyclic voltammograms recorded on Pt-Fe-Ru-N-MHC in O₂-Saturated 0.1 M KOH and 0.1 M HClO₄ electrolytes, (B) Chronoamperometry curves recorded on Pt-Fe-Ru-N-MHC in O₂-saturated 0.1 M HClO₄ (0.5 V vs. RHE) and 0.1 M KOH (0.7 V vs. RHE) electrolytes with a rotation rate of 1600 rpm; (C) Polarisation curves recorded on Pt-Fe-Ru-N-MHC in O₂-saturated 0.1 M HClO₄ and 0.1 M KOH with a rotation speed of 1600 rpm; (D) ORR Tafel plots of Pt-Fe-Ru-N-MHC generated under O₂-saturated atmosphere with a rotation speed of 1600 rpm in 0.1 M KOH and 0.1 M HClO₄.</figcaption></figure>

initial activity after 3 h of continuous operation at 0.7 V vs. RHE (Fig. 8). This represents a substantial improvement over its performance in acidic media, where the stability and durability were noticeably lower (55 % in 3 h). Overall, these results highlight the potential of the Pt-Fe-Ru trin metallic structure as a highly effective catalyst capable of delivering excellent performance in both acidic and alkaline electrolytes, making it a promising candidate for practical applications in diverse electrochemical systems.

## 4. Conclusion

In summary, this work demonstrates the potential of utilizing cellulose-derived activated hydrochar as a sustainable support for preparing high-performance electrocatalysts. The MHC exhibited a high surface area with abundant mesoporosity and defective degree, providing a suitable platform for metal deposition and interaction. The deposition of ultra-Pt mass loading (2 %) onto the MHC resulted in limited ORR activity. However, this activity was significantly enhanced by introducing Fe and Ru into the Pt-based catalyst. The trimetallic Pt-Fe-Ru-N-MHC catalyst demonstrated a highly efficient 4e⁻ ORR activity in both alkaline and acidic electrolytes, with excellent onset potentials, improved mass activity, and a reduced Tafel slopes. Overall, the successful integration of Fe and Ru with Pt on the in-situ N-doped MHC provides a promising pathway for developing efficient and durable catalysts for various electrochemical applications, including fuel cells and metal-air batteries.

## CRediT authorship contribution statement

**El Mehdi Chatir:** Writing – original draft, Methodology, Investigation. **Thibault Rafaïdeen:** Formal analysis, Investigation, Methodology, Writing – review & editing. **Anthony Thomas:** Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. **Christophe Coutanceau:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition.

## 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.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2025.146353.

## Data availability

Data will be made available on request.
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