# Investigation of the electrooxidation of polyols issued from biomass resources for application as liquid organic hydrogen carriers (LOHC)

Issam Nciri, Roméo Kouamé, Thibault Rafaïdeen, Claude Lamy, Jesus Gonzalez-Cobos, Cláudia Morais, Stève Baranton, Karine de Oliveira Vigier, Christophe Coutanceau

► To cite this version:

Issam Nciri, Roméo Kouamé, Thibault Rafaïdeen, Claude Lamy, Jesus Gonzalez-Cobos, et al. Investigation of the electrooxidation of polyols issued from biomass resources for application as liquid organic hydrogen carriers (LOHC). Journal of Electroanalytical Chemistry, 2026, 1009, pp.119994. 10.1016/j.jelechem.2026.119994. hal-05530350

HAL Id: hal-05530350

https://hal.science/hal-05530350v1

Submitted on 27 Feb 2026

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 CC BY 4.0 - Attribution - International License
<figure><img src="image_2.png" /></figure>

# Investigation of the electrooxidation of polyols issued from biomass resources for application as liquid organic hydrogen carriers (LOHC)

Issam Nciri<sup>a</sup>, Roméo Kouamé<sup>a</sup>, Thibault Rafaïdeen<sup>a</sup>, Claude Lamy<sup>c,d</sup>, Jesus Gonzalez-Cobos<sup>b</sup>, Cláudia Morais<sup>a</sup>, Stève Baranton<sup>a</sup>, Karine De Oliveira Vigier<sup>a</sup>, Christophe Coutanceau<sup>a,d,*</sup>

## ARTICLE INFO

Keywords:

Ethylene glycol

Glycerol

Electrooxidation
(di)acids

LOHC

Thermodynamic

Reaction kinetics

Selectivity

## ABSTRACT

Biosourced polyols such as ethylene glycol (EG) and glycerol (GL) have very high theoretical H₂ storage capacity, of 8.15 wt% and 6.15 wt%, respectively. Moreover, both loaded and unloaded molecules display no or much lower toxicity, higher safety and are more environmentally-friendly (being also biodegradable), as well as cheaper than the classical LOHCs. Therefore, they could be excellent candidates as biosourced LOHCs, minimizing the risk during transportation. In addition, thermodynamic data indicate that these molecules can theoretically be oxidized from the very low potentials of ca. 0.11 V vs. RHE and ca. 0.27 V vs. RHE for GL and EG, respectively, with the release of hydrogen with electrical energy consumptions of ca. 20 kJ molᵢH₂¹ and ca. 53 kJ molᵢH₂², respectively, values lower or equivalent to that for classical LOHC. Then, several PtM/C and PdM/C catalysts were synthesized by a water-in-oil microemulsion method and characterized. Their behavior towards the GL and EG electrooxidation was followed by cyclic voltammetry, in situ infrared spectroscopy, and chro-noamperometry with analysis of products by HPLC. The Pt₉Bi₁/C catalysts displayed the best activity and selectivity towards the formation of C3 and C2 oxidized compounds for GL and EG oxidation, respectively. For cell voltage ≤ 0.60 V vs. RHE, GL oxidation leads mainly to glyceraldehyde, then glyceric acid, tartronic acid and a small amount of formic acid. EG electrooxidation allows the formation of glycolic acid as main product, oxalic acid and an unidentified intermediate that is consumed when increasing the electrolysis time, but no formic acid. For these reasons, EG appears as the best candidate as LOHC.

## 1. Introduction

The shift from fossil sources (petroleum, coal, natural gas) for the production of heat and electrical energy to intermittent renewable sources (solar powered photovoltaic cells, wind turbines, tidal plants, etc.) for electrical energy involves the necessity to develop electricity storage methods for its transportation and/or use where and when the demand is present. Owing to its high gravimetric energetic capacity (33 kWh kgH₂⁻¹) [1], hydrogen (H₂) has been proposed as a good candidate for the chemical storage of renewable electricity, as it can moreover be produced through water electrolysis without release of greenhouse effect gases [2]. However, its low volumetric capacity (ca. 3 kWh/Nm³H₂ at 1 bar and 15 °C) limits its possibility of transportation and use, so that hydrogen has generally to be either compressed [3] or liquified [4].

However, the conventional compression and liquefaction technologies are associated with several challenges, including low storage density, boil-off losses, relatively high costs, safety issues and transportation concerns [5].

Liquid Organic Hydrogen Carriers (LOHCs) represent a promising alternative to hydrogen storage for long-distance transport [6]. The handling of a liquid presents a similar level of complexity to that of hydrocarbons already in use worldwide, with the advantage of hydrogen as a sustainable energy vector. LOHCs are organic compounds that can be reversibly hydrogenated and dehydrogenated. Starting with an unloaded liquid (a liquid able to bind H₂), the exothermic hydrogenation reaction converts it to a loaded liquid (a liquid carrying H₂), which can be transported from the site of H₂ production to the site of its utilization. At the utilization site, the endothermic dehydrogenation

<footnote><sup>a</sup> IC2MP, UMR 7285, CNRS - Université de Poitiers, Poitiers, France</footnote>

<footnote><sup>b</sup> IRCELyon, UMR 5256, CNRS - Université de Lyon, France</footnote>

<footnote><sup>c</sup> ICGM, UMR 5253, CNRS - Université de Montpellier, Montpellier, France</footnote>

<footnote><sup>d</sup> French Research Network on Hydrogen (FRH2), FR 2044, CNRS, France.</footnote>

<footnote>* Corresponding author at: IC2MP, UMR 7285, CNRS - Université de Poitiers, Poitiers, France.</footnote>

<footnote>E-mail address: christophe.coutanceau@cnrs.fr (C. Coutanceau).</footnote>
reaction of the loaded liquid releases H₂ and regenerates the unloaded liquid. Most commonly, reversibility is achieved in LOHCs via the saturation and unsaturation of carbon-carbon bonds. The suitability of a LOHC for hydrogen storage then depends on the strength and density of these bonds, the long-term stability of the compounds during repeated hydrogenation and dehydrogenation, and the cost of synthesizing the compounds. In addition, the toxicity should be low, and it is advantageous if both forms of the LOHC have high boiling points and low freezing points: high boiling point for easier separation from produced hydrogen during dehydrogenation and low freezing point to prevent clogging of equipment and to maintain pumpability at low temperatures.

Various compounds have been proposed as LOHCs (Table 1) [7]. The most well-investigated LOHCs are (hydrogenated and dehydrogenated forms) methylcyclohexane and toluene (MCH-TOL) [8], dibenzyltoluene and perhydro-dibenzyltoluene (DBT-PDBT) [9], and N-ethylcarbazole and dodecahydro-N-ethylcarbazole (NEC-DNEC) [10], although many others have been suggested [11]. Their gravimetric hydrogen storage densities are 6.1% (wt.) for MCH-TOL, 6.2% (wt.) for DBT-PDBT, and 5.8% (wt.) for NEC-DNEC, while the volumetric hydrogen storage densities are 47 kg/m³ for MCH-TOL, 64 kg/m³ for DBT-PDBT, and 54 kg/m³ for NEC-DNEC. But all these compounds are produced from fossil carbon industries and present health and/or environmental issues (Table 1).

On the other hand, biosourced polyols, such as ethylene glycol -EG- (produced by the catalytic hydrogenolysis of lignocellulosic biomass [17]) and glycerol -GL- (by-product from the first-generation biofuel industry based on transesterification of vegetal oils [18] or from sugar fermentation to produce bioethanol [19]) can be theoretically reversibly

<table><caption>**Table 1**
Comparison of LOHCs in terms of safety pictograms, H₂ gravimetric storage and cost in Europe.</caption><thead><tr><th>LOHC (unloaded/ loaded)</th><th>Formula</th><th>Pictogram</th><th>H₂ Wt %</th><th>Cost ($ kg<sup>-1</sup>)</th></tr></thead><tbody><tr><td>MCH/TOL</td><td><img src="image_0.png" alt="Chemical structure of MCH"></td><td><img src="image_1.png" alt="Pictogram of a chemical compound with a warning symbol (Exclamation mark, Exclamation mark with a line, Exclamation mark with a triangle)"></td><td>6.1</td><td>TOL: 0.96 [12]</td></tr><tr><td>DBT/PDBT</td><td><img src="image_2.png" alt="Chemical structure of DBT"></td><td><img src="image_3.png" alt="Pictogram of a chemical compound with a warning symbol (Exclamation mark, Exclamation mark with a line, Exclamation mark with a triangle)"></td><td>6.2</td><td>DBT: 4-5 [13]</td></tr><tr><td>NEC / DNEC</td><td><img src="image_4.png" alt="Chemical structure of NEC"></td><td><img src="image_5.png" alt="Pictogram of a chemical compound with a warning symbol (Exclamation mark)"></td><td>5.8</td><td>NEC: 3 [14]</td></tr><tr><td>Oxalic acid/EG-water</td><td><img src="image_6.png" alt="Chemical structure of oxalic acid"></td><td><img src="image_7.png" alt="Pictogram of a chemical compound with a warning symbol (Exclamation mark)"></td><td>8.15</td><td>EG: 0.68 [15]</td></tr><tr><td>Tartronic acid/GL-water</td><td><img src="image_8.png" alt="Chemical structure of tartronic acid"></td><td><img src="image_9.png" alt="Pictogram of a chemical compound with a warning symbol (Exclamation mark)"></td><td>6.15</td><td>GL: 0.96 [16]</td></tr></tbody></table>

dehydrogenated into the corresponding monocarboxylic or dicarboxylic acids by electrochemical reforming without breaking the —C—C— bonds, as follows:

$$CH_2OH - R - CH_2OH + H_2O = CH_2OH - R - COOH + 2 H_2 \quad (1)$$

$$CH_2OH - R - CH_2OH + 2 H_2O = HOOC - R - COOH + 4 H_2 \quad (2)$$

and these carboxylic acids can be rehydrogenated by catalytic processes into the corresponding polyols. Table 1 compares the theoretical H₂ gravimetric capacities of some classical LOHC to those achievable by using biosourced polyols, with reference to the molar mass of the maximum loaded compounds. In addition to allow high theoretical H₂ storage capacity, 8.15 wt% and 6.15 wt% for EG and GL, respectively (considering two water molecules in the calculation), both loaded and unloaded molecules display no or much lower toxicity, higher safety and are more environmentally-friendly (being also biodegradable), as well as cheaper than the classical LOHCs. Therefore, they could be excellent candidates as biosourced LOHCs, minimizing the risk during transportation.

If LOHCs are used, dehydrogenation and hydrogenation reactions occur during the whole cycle. For practical application, in addition to the lower toxicity and cost than classical LOHC and higher or equivalent theoretical hydrogen mass storage capacity, the dehydrogenation and hydrogenation of the LOHC should be reversible, which implies here that the —C—C— bond is not broken, conversely to classical LOHCs. Therefore, before deciding, if the polyols chosen (EG and GL) are appropriate to be used as a LOHC, it is necessary to study the two reactions independently. Hydrogenation reactions are exothermal processes well mastered by the refinery industries, whereas endothermal dehydrogenation reactions are not so usual. Herein, the dehydrogenation reaction by electrochemical processes of biosourced polyols, such as EG and GL, will be deeply investigated by the selective electrooxidation of a terminal alcohol function. Indeed, for classical LOHCs, the positive enthalpy of dehydrogenation requires performing the reaction at relatively high temperatures, in the range from 250 °C to 450 °C [5,6,20]. Such high temperatures may also lead to cleavage of the —C—C— bonds with catalyst coking (catalyst deactivation) and degradation of the LOHC during multiple hydrogenation/dehydrogenation cycles [21].

Electrochemical methods are well adapted for dehydrogenation reactions, and electrochemical oxidation (ECO) has been widely studied for various compounds issued from the biomass [22]. The advantages of the dehydrogenation step under electrochemical control (electro-reforming) are the use of sustainable chemicals (water and electrons), the control and the minimization of the energy consumption (the reaction will be performed with a cell voltage below 1.00 V and ideally below 0.60 V at ambient temperature) and the high purity of the hydrogen produced without further separation steps. The total selectivity of the oxidation reaction towards mono- or diacid without —C—C— breaking is the main challenge to overcome to ensure release of high hydrogen amount and cyclability of the LOHC. Owing to the chemical structures of the biosourced polyols, their dehydrogenation will necessitate the choice of adapted catalytic materials to reach high selectivity towards the production of carboxylic acids without —C—C— bond breaking [23,24]. To reach these objectives, pgm (platinum group metals) -based catalysts are unavoidable as they allow performing the reaction at much lower potentials than non-pgm based-ones [25]. Moreover, non-pgm catalysts, although cheaper, are generally working at potential higher than 1.00 V vs. RHE and are known to favor the breaking of the —C—C— bond [26–28], which is of course incompatible with the requirements for LOHC. The catalysts should also be stable for long term applications. Therefore, in this contribution we first aim at determining the thermodynamics of the electro-dehydrogenation of EG and GL into carboxylic acids and at comparing the energy consumption with classical hydrogen storage methods (pressurized H₂, liquid H₂ and aromatic LOHCs) to evaluate their potentiality as biosourced LOHCs.
Then, the electrooxidation of glycerol (since it is largely produced from biomass resources for the biodiesel industry and has been largely studied since a long time [29]) and ethylene glycol (since it allows theoretically higher mass storage capacity) will be experimentally investigated, and results compared. The electro-dehydrogenation reactions of GL and EG will be studied by cyclic voltammetry, selectivity by in situ infrared reflectance spectroscopy and chronoamperometry coupled with analytical methods, such as chromatographic techniques, to determine the intermediates and products formed and stability of the catalysts.

## 2. Theoretical concepts

### 2.1. Thermodynamic considerations

First of all, it seems important to recall the storage density of hydrogen under its various states, pressurized gas under several pressures and liquid hydrogen, and to compare these values to those of some other compounds, such as CH₄, CH₃OH, NH₃, and biosourced polyols, etc. (Table 2).

Obviously, liquids display higher hydrogen storage capacity than gases. Ammonia and hydrazine appear to be very good candidates for hydrogen storage, but pressurization is needed for liquid ammonia and both these compounds have safety and toxicity issues [30]. Alcohols (methanol and ethanol) and biosourced polyols (ethylene glycol and glycerol) also lead to interesting hydrogen storage potentiality.

To be used as a LOHC, the alcohols/polyols must be selectively electrooxidized into dehydrogenated species on suitable electrocatalysts without breaking the —C—C— bonds, thus avoiding full dehydrogenation to CO₂, and allowing their re-hydrogenation reactions by catalytic reverse processes, without using too much external energy. Table 3 shows different reaction intermediate products which can be formed during the electrocatalytic oxidation of the alcohol.

The selective electrooxidation of one alcoholic group into a carboxylic acid without breaking the —C—C— bond, leads to the production of 2 hydrogen molecules, as shown in reaction (3) in the case of ethanol.

$$ \mathrm{CH_3 - CH_2OH + H_2O \rightarrow CH_3 - COOH + 2 H_2} \quad (3) $$

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

Physical state at 20 °C and 1 bar (except when specified), H₂ content in g L⁻¹ and wt% (values calculated from densities at 20 °C and 1 bar, except when specified) for different compounds considered for hydrogen storage comparatively to pure hydrogen under different storage methods.

<table><caption>**Table 3**</caption><thead><tr><th>Fuel</th><th>Physical state</th><th>H<sub>2</sub> density/ g L<sup>-1</sup></th><th>H<sub>2</sub> gravimetric capacity/wt%</th></tr></thead><tbody><tr><td>H<sub>2</sub> (without storage)</td><td>gas</td><td>0.089</td><td>100</td></tr><tr><td>H<sub>2</sub> in a 200-bar tank</td><td>gas</td><td>14</td><td>1.5</td></tr><tr><td>H<sub>2</sub> in a 350-bar tank</td><td>gas</td><td>24</td><td>4.7</td></tr><tr><td>H<sub>2</sub> in a 700-bar tank</td><td>gas</td><td>40</td><td>5.5</td></tr><tr><td>Liquid H<sub>2</sub></td><td>liquid</td><td>71</td><td>14</td></tr><tr><td>CH<sub>4</sub> (methane without storage, 20 °C)</td><td>gas</td><td>16.75</td><td>25</td></tr><tr><td>CH<sub>3</sub>OH (methanol)</td><td>liquid</td><td>99</td><td>12.5</td></tr><tr><td>CH<sub>3</sub>-CH<sub>2</sub>OH (ethanol)</td><td>liquid</td><td>102.6</td><td>13</td></tr><tr><td>CH<sub>3</sub>-O-CH<sub>3</sub> (dimethylether at 0 °C, 1 bar)</td><td>gas</td><td>0.275</td><td>13</td></tr><tr><td>CH<sub>3</sub>-O-CH<sub>3</sub> (at 20 °C, 4.1 bar)</td><td>liquid</td><td>87</td><td>13</td></tr><tr><td>CH<sub>2</sub>OH- CH<sub>2</sub>OH (ethylene glycol)</td><td>liquid</td><td>108</td><td>9.7</td></tr><tr><td>CH<sub>2</sub>OH-CHOH-CH<sub>2</sub>OH (glycerol)</td><td>liquid</td><td>65.3</td><td>8.7</td></tr><tr><td>NH<sub>3</sub> (ammonia without storage, 20 °C)</td><td>gas</td><td>0.124</td><td>17.6</td></tr><tr><td>NH<sub>3</sub> (20 °C, 8.5 bars)</td><td>liquid</td><td>107.4</td><td>17.6</td></tr><tr><td>NH<sub>2</sub>-NH<sub>2</sub> (without storage, 20 °C)</td><td>liquid</td><td>127.6</td><td>12.5</td></tr><tr><td>NH<sub>2</sub>-NH<sub>2</sub>, H<sub>2</sub>O (monohydrate)</td><td>liquid</td><td>123.5</td><td>12</td></tr></tbody></table>

Some intermediate compounds produced under partial oxidation of alcohols (without cutting the —C—C— bonds).

<table><thead><tr><th>Alcohol</th><th>Intermediate products formed by partial electrooxidation</th></tr></thead><tbody><tr><td>Ethanol</td><td>Acetaldehyde, acetic acid</td></tr><tr><td>Ethylene glycol</td><td>Glycolaldehyde, glycolic acid, glyoxylic acid, oxalic acid</td></tr><tr><td>Glycerol</td><td>Glyceraldehyde, dihydroxyacetone, glyceric acid, hydroxypyruvic acid, tartronic acid, mesoxalic acid</td></tr></tbody></table>

On the other hand, ethylene glycol and glycerol, which are considered in this work, can both be selectively electrooxidized into mono or diacids — see Eqs. (1 and 2), generating up to twice more hydrogen than with a mono-alcohol. Moreover, those polyols are issued from biomass resources. The selective oxidation of the alcoholic groups without breaking the —C—C— bonds to carboxylic acids, can lead to relatively high amounts of hydrogen providing interesting weight storages, in particular by the selective oxidation into diacids — see Eq. (2). In the case of glycerol, it leads either to glyceric acid (Eq. 4) or to tartronic acid (Eq. 5), as follows:

$$ \mathrm{CH_2OH - CHOH - CH_2OH + H_2O \rightarrow CH_2OH - CHOH - COOH + 2 H_2} \quad (4) $$

$$ \mathrm{CH_2OH - CHOH - CH_2OH + 2 H_2O \rightarrow HOOC - CHOH - COOH + 4 H_2} \quad (5) $$

Knowing the thermodynamic data of formation, ΔHf and ΔGf, under standard states (usually the liquid state at 25 °C and 1 bar pressure), of the different compounds involved, from Chemeo [31] and under liquid state (Ebeling et al. [32]), it is easy to calculate the different energies involved in the dehydrogenation process by partial oxidation of the alcohol function into carboxylic acid (SI 1). This allowed us to evaluate the Gibbs (ΔG<sub>dehyd</sub>) and enthalpy (ΔH<sub>dehyd</sub>) energies needed for their selective dehydrogenation into monoacids or diacids, and further the corresponding reversible cell voltage, U<sub>rev</sub> = (ΔG/4 nF), and the thermoneutral voltage, U<sub>th</sub> = (ΔH/4 nF) (with 4n the total number of Faraday for the formation 2n moles of H₂), which are the minimum cell voltages needed to initiate and to run the reaction. In the case of reaction (4), one can calculate the total energy supplied to glycerol to obtain 2 hydrogen moles per oxidized mole, together with the minimum cell voltage U<sub>th</sub> and the reversible cell voltage U<sub>cell</sub> to begin the electrochemical reforming of glycerol:

$$ \begin{aligned} \Delta H_{\text{dehyd}} &= \Delta H_{\text{glyceric}} - \Delta H_{\text{GL}} - \Delta H_{\text{H2O}} = -864 + 671 + 285.8 \\ &= 92.8 \, \text{kJ/mol} \end{aligned} \quad (6) $$

and,

$$ U_{\text{th}} = \Delta H_{\text{dehyd}}/\text{nF} = 92.8 \times 10^3/(4 \times 96485) = 0.24 \, \text{V} \quad (7) $$

$$ \begin{aligned} \Delta G_{\text{dehyd}} &= \Delta G_{\text{glyceric}} - \Delta G_{\text{GL}} - \Delta G_{\text{H2O}} = -951 + 752 + 237.2 \\ &= 38.2 \, \text{kJ/mol} \end{aligned} \quad (8) $$

and,

$$ U_{\text{cell}} = \Delta G_{\text{dehyd}}/\text{nF} = 38.2 \times 10^3/(4 \times 96485) = 0.099 \, \text{V} \quad (9) $$

Similarly, for the electrooxidation of glycerol to the dicarboxylic tartronic acid (reaction 5), one obtains:

$$ \begin{aligned} \Delta H_{\text{dehyd}} &= \Delta H_{\text{tartronic}} - \Delta H_{\text{GL}} - 2 \Delta H_{\text{H2O}} = -1062 + 671 + 2 \times 285.8 \\ &= 180.6 \, \text{kJ/mol} \end{aligned} \quad (10) $$

and,

$$ U_{\text{th}} = \Delta H_{\text{dehydo}}/\text{nF} = 180.6 \times 10^3/(8 \times 96485) = 0.23 \, \text{V} \quad (11) $$
$$ \begin{aligned} \Delta G_{\text{dehyd}} &= \Delta G_{\text{irtronic}} - \Delta G_{\text{GL}} - 2 \Delta G_{\text{H2O}} = -1146 + 752 + 2 \times 237.2 \\ &= 80.4 \, \text{kJ/mol} \end{aligned} \qquad (12) $$

and,

$$ U_{\text{cell}} = \Delta G_{\text{dehyd}}/\text{nF} = 80.4 \times 10^3/(8 \times 96485) = 0.104 \, \text{V} \qquad (13) $$

Results in Table 4 give dehydrogenation energy values of the same order or even lower than those encountered in the dehydrogenation of classical LOHCs, such as MCH (68.3 kJ mol⁻¹) [33], 18H-DBT (65 kJ mol⁻¹) [34], or 12H-NEC (50.5 kJ mol⁻¹) [35]. Considering the bio-sourced origin, low cost and low toxicity of the starting compound, a maximum cell voltage of ca. 0.5 V is targeted to achieve an acceptable dehydrogenation energy lower than 100 kJ mol⁻¹, i.e., < 13.9 kWhg⁻¹, which involves the search for the convenient catalyst. The cell voltage values, determined similarly to the case of glycerol, for the oxidation of EG either to glycolic acid or oxalic acid, are very close to those obtained for the selective oxidation of glycerol (Table 4).

Even if the thermodynamics seems to be favourable, the kinetics of the electrocatalytic oxidation of the polyols is relatively slow leading to anodic overpotentials, and thus to higher cell voltages at the relatively high current densities *j* required for high production rates of hydrogen (SI 2). A comprehensive overview of the catalysts developed for the electrooxidation of glycerol with the aim of producing selectively value-added compounds has been proposed [36]. The authors explained that non-noble metals (Ni-based catalysts) were only utilizable in alkaline media, but led to high cell voltages and breaking of the —C—C— bond, which was confirmed by Houache et al [37] In both acidic and alkaline media, Pt- and Pd-based binary and ternary electrocatalysts display high activity from low potentials and selectivity towards C3 oxidized compounds. But it is proposed that the activity of catalysts towards alcohol electrooxidation is much higher in alkaline media than in acidic ones [38]. Therefore, in the following, the dehydrogenation reactions of GL and EG will be studied in alkaline medium by cyclic voltammetry (CV) and chronoamperometry (CA) on Pt- and Pd-based catalysts, coupled with *in situ* infrared reflectance spectroscopy and analytical methods to determine the intermediates and products formed. This study will allow gaining insights into the reaction mechanisms and to propose future routes to enhance the efficiency of the dehydrogenation reaction of biosourced polyols and as a final objective to suggest their possible use as LOHCs.

### 3. Experimental

Pt/C, Pt9Bi1/C, Pt8Ag2/C, Pt7Ag3/C, Pt7Cu3/C, Pd/C, Pd9Bi1/C, Pd7Ag3/C and Pd7Cu3/C materials have been synthesized via a “water in oil” microemulsion method using H2PtCl6·6 H2O, K2PdCl4, AgNO3, BiCl3, and CuCl2·2 H2O precursors from Alfa Aesar (purity 99,9%). The protocol for the synthesis can be found elsewhere [39,40]. Briefly, it consists first to prepare a homogeneous mixture of 37 g of n-heptane (Sigma-Aldrich, HPLC grade) and 16 g of Brij (Brij L4, Sigma-Aldrich). Then the microemulsion is formed by adding the accurate volume of aqueous solution containing the metal salts at the desired metal molar ratio, and stirring. Once a translucent and stable suspension is obtained,

100 mg NaBH4 (99%, Sigma-Aldrich) is added to the mixture to reduce the metal salts and to obtain a colloidal solution of metal nanoparticles (NPs). To reach 40 wt% metal loading, a predetermined amount of carbon (Vulcan XC72, CABOT) pre-treated under N2(g) at 400 °C for 4 h. is added to the colloidal solution. After 15 min sonication of the mixture using an ultrasonic bath, it is filtered on a hydrophilic PVDF membrane (0.22 µm pore diameter, Millipore). NPs are cleaned by repetitive soaking in acetone, ethanol, and ultrahigh purity water (at least 5 times in each solvent) and dried in an oven at 60 °C overnight. At last, the catalytic powder is thermally treated in an oven at 200 °C in air for 2 h to remove any possible remaining surfactant through combustion.

Catalysts were characterized by thermogravimetric analysis (TGA using a TA Instrument SDT Q600) to determine the metal loading, inductively coupled plasma - optical emission spectroscopy (ICP-OES using a PerkinElmer Optima 2000 DV) for their atomic composition, transmission electron microscopy measurements (using a JEOL JEM 2100 (UHR) with a resolution of 0.19 nm) for determining their NP mean size and size distribution, and X-ray diffraction (XRD using a PANalytical Empyrean X-ray diffractometer; measurements were performed in a step mode, with a step interval of 0.06° and a fixed acquisition time of 10 s at each value of 2θ, for their microstructure. Data are given in Table 5.

Cyclic voltammograms (CVs.) and linear scan voltammetry measurements were performed at 20 °C in a conventional three-electrode electrochemical cell using a reversible hydrogen electrode (RHE) and a glassy carbon plate (3 cm² geometric surface area) as reference and counter electrodes, respectively. The working electrode was prepared by cast dropping a catalytic ink onto a glassy carbon disc (0.071 cm⁻² geometric surface area). The catalytic ink consisted in the dispersion of 17.7 mg of catalytic powder in 2.117 mL of ultra-pure water, 0.529 mL of 2-propanol (LC-MS CHROMASOLV®, Fluka) and 0.354 mL of Nafion® solution (5 wt% Nafion® perfluorinated resin solution in aliphatic alcohols). After homogenization in an ultrasonic bath (about 50 s), 3 µL of the catalytic ink was deposited on the freshly polished glassy carbon disc, leading to a metal loading of 100 µgmetal cm⁻². Nine cyclic voltammograms (CVs.) were performed between 0.05 V vs. RHE and 1.00 V vs. RHE at 50 mV s⁻¹ until stable CVs. were obtained. Then,

<caption>**Table 5**</caption>

Physicochemical characterization of catalysts: metal loading from TGA, atomic ratio of the modifier metal (M) from ICP-OES, mean particle size $\bar{d}_{TEM}$ from TEM, cell parameter $a_{DRX}$, Scherrer length $L_v$ from XRD and electroactive surface area (EASA) from the hydrogen desorption region determined by cyclic voltammetry.

<table><caption>**Table 4**</caption><thead><tr><th>Catalyst</th><th>Metal loading (%)</th><th>Metal M (at.%)</th><th>d̅<sub>TEM</sub> (nm)</th><th>a<sub>DRX</sub> (nm)</th><th>L<sub>v</sub> (nm)</th><th>EASA (m² g<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Pt/C</td><td>39</td><td>0</td><td>5.8</td><td>0.392</td><td>3.5</td><td>41</td></tr><tr><td>Pt<sub>8</sub>Bi<sub>1</sub>/C</td><td>37</td><td>11</td><td>5.2</td><td>0.392</td><td>4.3</td><td>–</td></tr><tr><td>Pt<sub>8</sub>Ag<sub>2</sub>/C</td><td>42</td><td>25</td><td>4.9</td><td>0.393</td><td>3.5</td><td>20.4</td></tr><tr><td>Pt<sub>7</sub>Ag<sub>3</sub>/C</td><td>37</td><td>41</td><td>4.5</td><td>0.393</td><td>3.0</td><td>13</td></tr><tr><td>Pt<sub>7</sub>Cu<sub>3</sub>/C</td><td>40</td><td>41</td><td>3.9</td><td>0.386</td><td>2.9</td><td>17</td></tr><tr><td>Pd/C</td><td>38</td><td>0</td><td>5.6</td><td>0.386</td><td>2.5</td><td>109</td></tr><tr><td>Pd<sub>9</sub>Bi<sub>1</sub>/C</td><td>40</td><td>13</td><td>6.4</td><td>0.394</td><td>3.6</td><td>–</td></tr><tr><td>Pd<sub>7</sub>Ag<sub>3</sub>/C</td><td>40</td><td>29</td><td>5.9</td><td>0.395</td><td>2.8</td><td>95</td></tr><tr><td>Pd<sub>7</sub>Cu<sub>3</sub>/C</td><td>40</td><td>39</td><td>5.0</td><td>0.386</td><td>2.9</td><td>94</td></tr></tbody></table>

Standard thermodynamic data, cell voltage and number of hydrogen moles ($n_{H2}$) produced by the electrochemical reforming of biosourced polyols.

<table><thead><tr><th>Compounds</th><th>Formula</th><th>Molecular weight M</th><th>ΔH<sub>formation</sub> /kJ/Mol</th><th>ΔG<sub>formation</sub> /kJ/Mol</th><th>n<sub>H2</sub></th><th>ΔH<sub>dehyd</sub> / kJ mol<sup>-1</sup><sub>polyol</sub></th><th>U<sub>th</sub> / V</th><th>ΔH<sub>dehyd</sub> / kJ mol<sup>-1</sup><sub>H2</sub></th><th>ΔG<sub>dehyd</sub> /kJ mol<sup>-1</sup><sub>polyol</sub></th><th>U<sub>rev</sub> /V</th><th>ΔG<sub>dehyd</sub> /kJ mol<sup>-1</sup><sub>H2</sub></th></tr></thead><tbody><tr><td>Ethylene glycol</td><td>C<sub>2</sub>H<sub>6</sub>O<sub>2</sub></td><td>62.01</td><td>-460.0</td><td>-304.7</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>Glycolic acid</td><td>C<sub>2</sub>H<sub>4</sub>O<sub>3</sub></td><td>76.05</td><td>-583.0</td><td>-436.6</td><td>2</td><td>+162.6</td><td>+0.42</td><td>+81.3</td><td>+105.3</td><td>+0.273</td><td>52.65</td></tr><tr><td>Oxalic acid</td><td>C<sub>2</sub>H<sub>2</sub>O<sub>4</sub></td><td>90.03</td><td>-792.0</td><td>-565.5</td><td>4</td><td>+239.6</td><td>+0.31</td><td>+59.9</td><td>+213.6</td><td>+0.277</td><td>53.4</td></tr><tr><td>Glycerol</td><td>C<sub>3</sub>H<sub>8</sub>O<sub>3</sub></td><td>92.09</td><td>-671.0</td><td>-752.0</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>Glyceric acid</td><td>C<sub>2</sub>H<sub>6</sub>O<sub>4</sub></td><td>106.07</td><td>-864.0</td><td>-951.0</td><td>2</td><td>+92.8</td><td>+0.24</td><td>+45.7</td><td>+38.2</td><td>+0.099</td><td>19.1</td></tr><tr><td>Tartronic acid</td><td>C<sub>3</sub>H<sub>4</sub>O<sub>5</sub></td><td>120.06</td><td>-1062.0</td><td>-1146.0</td><td>4</td><td>+180.6</td><td>+0.23</td><td>+44.8</td><td>+80.4</td><td>+0.104</td><td>20.1</td></tr><tr><td>Liquid water</td><td>H<sub>2</sub>O</td><td>18.02</td><td>-285.8</td><td>-237.2</td><td>1</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td></tr></tbody></table>
the catalytic activity was studied by recording the stable third linear scan voltammogram at 5 mV s⁻¹ in 0.1 M NaOH (Semiconductor Grade 99.99%, Aldrich) + 0.1 M polyol (Reagent Plus 99%, Sigma Aldrich) solution in ultra-pure water using a PGZ 402 potentiostat from Voltalab.

In situ infrared spectroscopy measurements were performed in single potential alteration infrared reflectance spectroscopy (SPAIR) mode [41,42], allowing following in situ the formation of reaction intermediates and products at the electrode/electrolyte interface during a linear potential variation at a scan rate of 1 mV s⁻¹. 512 scans were recorded in the wavenumber range from 1000 cm⁻¹ to 4000 cm⁻¹ with a resolution of 4 cm⁻¹ every 50 mV for 35 s (0.035 V) between 0.05 V vs. RHE and 1.15 V vs. RHE, and averaged to obtain each spectrum. Final spectra are normalized according to the following equation:

$$ \Delta R/R = (R_{Ei} - R_{Eref})/R_{Eref} \quad (14) $$

where $R_{Eref}$ is the reflectance recorded at 0.05 V vs. RHE and $R_{Ei}$ that recorded at the electrode potential $E_i$. Down going absorption bands correspond to the formation of chemical functions, whereas up going ones to the consumption of a species at the electrode surface.

Chronoamperometry (CA) measurements were performed at 293 K in a 5 cm² single filter press type electrolysis cell fitted with a Pd₉Bi₁/C

anode (2.0 mgₘetal cm⁻² and 0.5 mgₘetal cm⁻² for GL and EG electrolysis, respectively) and a Pt/C cathode (0.5 mgₚt cm⁻²), both separated by a simple blotting paper and mechanically pressed in the cell. The setup was the same as that described by Rafaïdeen et al. [43] for the electrolysis of glucose and xylose, except the anode catalysts and the reaction solution. Here, both compartments of the cell were fed under recirculation mode at 5 mL min⁻¹ with a 30 mL solution containing 2 mol L⁻¹ GL in 0.5 mol L⁻¹ NaOH or 1 mol L⁻¹ EG in 0.1 mol L⁻¹ NaOH. The current was recorded during 3 h for EG and GL electrolysis at different cell voltages applied by using a Voltalab PGZ402 potentiostat (Radiometer analytical). Reaction products were analyzed by HPLC (Varian Prostar HPLC equipped with a Transgenomic ICSep ICE-COREGEL 107H column for organic acids, aldehydes, alcohols and ketone molecules separation). The chromatography was performed with 0.007 mol L⁻¹ H₂SO₄ aqueous solution as eluent at 0.6 mL min⁻¹ flow rate with an UV detector set at λ =210 nm.

# 4. Results and discussion

## 4.1. Physicochemical characterizations of catalysts

According to data in Table 5, the metal loadings and NP

<figure><img src="image_3.png" /><figcaption>Fig. 1. TEM images of catalysts and their related size distribution histogram for (a) PtM/C and (b) PdM/C materials.</figcaption></figure>
compositions are always close to the nominal ones, indicating that the syntheses were quantitative. TEM images in Fig. 1a and b for PtM/C and PdM/C, respectively, show that these catalysts consist of NPs relatively well dispersed on the surface of the Vulcan XC72 carbon support and having mean sizes between 3.9 and 5.8 nm.

XRD patterns in Figs. 2a and 5b for PtM and PdM/C catalysts, respectively, show typical peaks related to the face centered cubic structure of both Pt and Pd metals (JCPDS 04–0802 and JCPDS 05–0681 cards, respectively). In some cases, a small shift of the peaks is interpreted by a small alloying level of Pt or Pd with the second metal. In the case of Pd-based catalysts, some peaks (indexed by a black star in Fig. 2b) related to the presence of a PdO tetragonal phase can be observed (JCPDS 41–1107 card).

At last, typical Pt/C and Pd/C cyclic voltammograms were recorded on the catalysts in an aqueous 0.1 mol L⁻¹ NaOH supporting electrolyte (Fig. 3). The modification of these catalysts by a second metal leads to a decrease of the current densities in the hydrogen region from 0.05 V vs. RHE to 0.4 V vs. RHE, which indicates that the Pt and Pd surfaces are partly recovered by the second metal, and confirms the decrease of the EASA (Table 5). The modification of Pt and Pd by Bi inhibits completely the hydrogen adsorption/desorption process. Moreover, both the reduction of Pt or Pd and Bi surface oxides give rise to a unique reduction feature. The reduction of metal oxides occurring in the same potential range, the surface oxide reduction peak cannot be used to assess the EASA of PtBi and PdBi.

## 4.2. Glycerol oxidation reaction (GOR)

As explained before, glycerol electrooxidation has been widely studied on various PGM-based catalysts in alkaline media [36]. In his PhD thesis [44], Kouamé compared the activity of carbon supported PtM and PdM nanocatalysts (M being a metal of d or p group: Bi, Cu, Ag, Co, In, Cu, Ni) with different atomic compositions. He found that only the modification of Pt and Pd by three metals, Bi, Cu and Ag, allowed shifting the onset oxidation potential towards lower values. He also showed that PtM catalysts were more active than PdM ones over the potential range from 0.20 V vs. RHE to 0.80 V vs. RHE (Fig. 4a). The

onset potential for the GOR decreases from 0.60 V vs. RHE to 0.45 V vs. RHE for the Pd/C and Pd₇Cu₃/C, respectively, whereas it decreases from 0.40 V vs. RHE for Pt/C to ca. 0.25 V vs. RHE for the Pt₇Cu₃/C. In both cases, the catalytic activity for the GOR is enhanced by the modification with the second metal, with the order of activity at low potentials being:

$$ \mathrm{Pt_7Cu_3/C} > \mathrm{Pt_8Ag_2/C} > \mathrm{Pt_9Bi_1/C} > \mathrm{Pt/C} > \mathrm{Pd_7Cu_3/C} > \mathrm{Pd_8Ag_2/C} \\ > \mathrm{Pd_9Bi_1/C} > \mathrm{Pd/C} $$

However, *in situ* infrared spectroscopy measurements (Fig. 4b to e) clearly show that different absorption bands and different relative intensities between them are obtained on the different catalysts, which therefore indicates changes in selectivity. Table 6 gives the assignment of infrared peaks to chemical functional groups or compounds [45,46]. All catalysts display the bands A at ca. 1075 cm⁻¹ and B at ca. 1110 cm⁻¹ indicating the presence of glycolate and glycerate/tartronate respectively, and the bands in the 1300–1600 cm⁻¹ spectral range are assigned to carboxylate group vibrations of the different compounds, except for the broad band at 1392 cm⁻¹ (band F) obtained with Pt/C which is attributed to carbonate. Carbonate formation at low potentials has been previously reported and associated to the glycolate and/or oxalate formation [47]. The modification of Pt by Ag or Cu leads to the breaking of the —C—C— bond, as the absorption band at ca. 1950–2000 cm⁻¹ related to the presence of adsorbed CO [48,49] is visible on the spectra. Conversely, this band is not visible for the GOR on the Pt₉Bi₁/C catalysts, and moreover, the band at 2343 cm⁻¹ assigned to the formation of interfacial CO₂ [50,51] appears at higher potentials and is much less intense on this catalyst than on the others. The Pt₉Bi₁/C catalyst appears therefore more selective towards the formation of C3 compounds (mainly glycerate and tartronate) for potentials lower than 0.80 V vs. RHE. From the spectra in Fig. 4e and the assignation of infrared absorption bands, it can be concluded that the Pt₉Bi₁/C catalyst is more selective towards the formation of C3 compounds.

The modification of TiO₂ substrate by BiO₃ clusters has been studied for the photoelectrooxidation of glycerol, and it was shown that BiO₃ favored the adsorption of the middle OH of glycerol to form selectively dihydroxyacetone [52]. However, the oxidation state of bismuth in our

<figure><img src="image_4.png" /><figcaption>Fig. 2. XRD patterns of (a) PtM/C catalysts and (b) PdM/C catalysts.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3. Cyclic voltammograms recorded in N₂-purged 0.1 mol L⁻¹ NaOH at a scan rate of 5 mV s⁻¹ and T = 20 °C on (a) PtM/C catalysts and (b) PdM/C catalysts.</figcaption></figure>

Pt₉Bi₁/C catalyst is not the same as in the previous cited article. Another study concerning the effect of the doping of a Co₃O₄ spinel by Bi has shown enhancement activity towards the glycerol electrooxidation but at potential higher than 1.00 V vs RHE and selectivity towards the formation of formate with —C—C— bond breaking [53]. A detailed explanation of the unique electrocatalytic behavior of Pt₉Bi₁/C materials towards the electrooxidation of glycerol has been proposed by Kouamé et al [54] By mixing molecular dynamic simulation, comprehensive physicochemical characterizations and electrochemical evaluations, these authors showed that the catalyst consisted to Pt NPs of ca. 5 nm diameters decorated by Bi clusters in strong interaction with platinum (that stabilized those clusters). This structure led to a single-carbon adsorption mode on Pt and an electronic effect for the desorption of low oxidized species from Pt sites driven by the early stage of the Bi⁰ to Bi¹¹ transition at low potential.

Pt₇Cu₃/C allows achieving a lower onset potential than Pt₉Bi₁/C catalyst for the GOR, which makes this catalyst the most active amongst the studied ones. Pt₇Ag₃/C leads to the same activity as Pt₉Bi₁/C. However, according to the *in situ* infrared study, Pt₇Cu₃/C and Pt₇Ag₃/C catalysts are able to break the —C—C— bond from very low potentials, conversely to Pt₉Bi₁/C catalyst. It can be seen in Fig. 3a that the modification of Pt by Bi leads to the total disappearance of the hydrogen adsorption/desorption features at low potentials, indicating that the availability of Pt sites is very low. This low availability and dilution of Pt sites explain the single-carbon adsorption mode on Pt proposed by Kouamé et al [54] In the case of PtCu and PtAg catalysts, the hydrogen adsorption/desorption features are visible in Fig. 3a of the manuscript, which evidences high availability of Pt sites, and then the possibility of two-carbon adsorption mode that can favor the -C-C- bond breaking, the formation of adsorbed CO species and their oxidation into Cl compounds through a bifunctional mechanism, where Cu [55] and Ag [56] activate water at low potentials. But, for LOHC application, the reversibility of the hydrogenation/dehydrogenation reactions for several cycles is fundamental, and this requirement implies that the —C—C— bonds of the molecule are not broken during the dehydrogenation reaction.

To get further information, chronoamperometry measurements were performed in an electrolysis cell fitted with a Pt/C cathode, either a Pt₉Bi₁/C or a Pt₇Cu₃/C anode and a blotting paper as separator (Fig. 5). The reaction products were analyzed by HPLC after 4 h electrolysis at 0.4 and 0.55 V. Results are given in Table 7. As expected, higher currents are obtained with the increase of the applied cell voltage from 0.40 V to 0.55 V for both anode catalysts. However, the observation that higher currents were recorded with the Pt₉Bi₁/C anode rather than with the Pt₇Cu₃/C one was less expected considering the polarization curves in Fig. 4a. This behavior could be due to higher poisoning of the Pt₇Cu₃/C surface by CO than that of the Pt₉Bi₁/C, as shown by *in situ* FTIRS measurements.

After 4 h of electrolysis, the Pt₉Bi₁/C catalysts leads to high selectivity in glyceraldehyde, in agreement with results from Cobos-Gonzales et al [24] The total selectivity in C3 compounds reaches 86% to 87% for ca. 10% to 15% glycerol conversion. Considering the glycerol conversion of 13.75% (0.275 mol of glycerol converted) at 0.55 V with a Pt₉Bi₁/C anode, the maximum hydrogen production if forming only the diacid should be 1.1 mol H₂ (4 H₂ molecules per glycerol molecule), with an energy consumed of 106.1 kJ molH₂. Calculation on only C3 selectivity leads to 0.38 mol H₂ per glycerol molecule, 2.9 times less, which corresponds to 36.6 kJ molH₂. The same calculation made for the Pt₇Cu₃/C catalyst shows that 1.05 mol H₂ should be ideally obtained, but only 0.372 mol are obtained, i.e., 2.8 times less. However, the Pt₇Cu₃/C catalyst leads to higher ratios of degradation products than the Pt₉Bi₁/C anode.

In addition to activity and selectivity, the problem of stability of the Pt₉Bi₁/C catalyst could be an issue for further development. In the cell voltage range considered for the application of ethylene glycol or glycerol as LOHC, i.e., Ucell < 1.00 V and ideally <0.6 V, the anode potential is lower than 1.00 V vs. RHE, therefore, carbon corrosion rate is extremely slow and will probably not limit the stability of the catalyst. In the paper of Kouamé et al. [54], the Pt₉Bi₁/C catalyst was subjected to an accelerated stress test consisting of 1000 cycles between 0.05 V and 1.00 V vs. RHE. Under these harsh conditions, the catalysts being cycled between potentials of metal reduction and of Bi dissolution, which is not the case in electrolysis cell working at constant cell voltages lower than 1.00 V, no change in the Pt₉Bi₁/C cyclic voltammogram was observed after AST, indicating the very good stability of this catalyst. Moreover, the Pt₉Bi₁/C catalyst has been shown to be very active and selective for the electrooxidation of other organic molecules, such as glucose. In previous works, we have performed long terms electrolysis measurements (4–6 h) for the electroreforming of glycerol [24] and glucose [40]. Each time, the electrolyte after the electrolysis experiment was analyzed by ICP MS and no trace of metals was detected in the solution.

### 4.3. Electrooxidation of ethylene glycol (EG)

The electrooxidation of EG has been studied on the same electrocatalysts as for GL. Fig. 6a compares the Linear Sweep Voltammograms (LSVs.) recorded on Pt/C and Pd/C. As in the case of glycerol, the Pt/C catalyst displays a lower onset potential for the EG oxidation (0.35 V vs. RHE) than the Pd/C one (0.50 V vs. RHE). The modification of Pt by Bi, Ag or Cu leads to decrease the onset potential down to 0.25 V vs. RHE, as in the case of GL, but the best catalyst is the Pt₉Bi₁/C material in terms of higher activity in the potential range from 0.25 V vs. RHE to 0.80 V vs. RHE (Fig. 6b). It can also be noted that, considering the peak current density, the maximum activity for the EG oxidation is almost twice lower
<figure><img src="image_6.png" /><figcaption>Fig. 4. a) Polarization curves recorded on different PtM/C catalysts in contact with 0.1 mol L⁻¹ glycerol in 0.1 mol L⁻¹ NaOH at T = 20 °C and a scan rate of 5 mV s⁻¹, (b, c, d and e) infrared spectra recorded for the oxidation of 0.1 mol L⁻¹ glycerol in 0.1 mol L⁻¹ NaOH electrolyte on (b) Pt/C, (c) Pt<sub>7</sub>Cu<sub>3</sub>/C, (d) Pt<sub>7</sub>Ag<sub>3</sub>/C and (e) Pt<sub>9</sub>Bi<sub>1</sub>/C at a scan rate of 1 mV s⁻¹ and 20 °C, with a resolution of 4 cm⁻¹. Vertical scale: ΔR/R.</figcaption></figure>

than that for GL oxidation.

The selectivity of PtM/C catalysts was first studied by in situ infrared spectroscopy. Fig. 6c to f show the infrared spectra recorded as a function of the electrode potential for the EG oxidation on the different catalysts. Considering the pure Pt/C catalyst (Fig. 6c), the absorption bands at ca. 1074 cm⁻¹, 1232 and 1325 cm⁻¹, put together with those located at 1357 cm⁻¹ and 1578 cm⁻¹, speak for the formation of glycolates [41,57], although other carboxylates can be formed [50]. The absorption band at 1410 cm⁻¹ could be attributed to the formation of both glycolate and carbonate [41]. But Du et al. [58] found that an infrared peak related to carbonates should appear in the 1310

cm⁻¹–1411 cm⁻¹ wavenumber region according to their coordination with the support (free or adsorbed). The rise of the absorption band at 2043 cm⁻¹ related to the formation of interfacial CO₂ from ca. 0.75 V vs. RHE seems to be linked to the previous formation of carbonates. This aspect will be discussed latter. Usually, CO₂ should not be formed in alkaline medium, but hydroxyl ions (OH⁻) are consumed during the oxidation reactions and their diffusion in the electrolyte thin film due to the configuration of the infrared set-up may be limited, leading to local pH decreases. It is also worth noticing that an absorption band located between 1900 cm⁻¹ and 2000 cm⁻¹, and shifting with the electrode potential towards higher wavenumbers appears as soon as 0.05 V vs.
<table><caption>**Table 6**

Wavenumber of the infrared absorption peaks observed for the electrooxidation of GL, indexation and assignation of the peaks [45,46,48–51].</caption><thead><tr><th>Wavenumber/cm<sup>-1</sup></th><th>Indexation</th><th>Assignment</th></tr></thead><tbody><tr><td>1076</td><td>A</td><td>Glycolate</td></tr><tr><td>1110</td><td>B</td><td>Glycerate/Tartronate</td></tr><tr><td>1220 (weak)</td><td>C</td><td>ν(C—O) of Glycolate or Tartronate</td></tr><tr><td>1310</td><td>D</td><td></td></tr><tr><td>1332</td><td>I</td><td></td></tr><tr><td>1358</td><td>E</td><td></td></tr><tr><td>1380</td><td>L</td><td>Carboxylates (symmetric and antisymmetric stretching and CH<sub>n</sub> deformations)</td></tr><tr><td>1420</td><td>J</td><td></td></tr><tr><td>1457</td><td>G</td><td></td></tr><tr><td>1590</td><td>H</td><td></td></tr><tr><td>1390 (broad)</td><td>F</td><td>Carbonate</td></tr><tr><td>1950–2000</td><td>CO<sub>L</sub></td><td>carbon monoxide linearly adsorbed on Pt</td></tr><tr><td>2343</td><td>CO<sub>2</sub></td><td>antisymmetric stretching of carbon dioxide</td></tr></tbody></table>

RHE. As explained before, this band corresponds to the formation of adsorbed CO. An additional peak at 1304 cm⁻¹ appears very soon, as

soon as Pt becomes active for the EG oxidation (at ca. 0.40 V vs. RHE), and has been proposed to come from the formation of oxalate [41]. To summarize, CO<sub>ads</sub> is formed for E ≥ 0.10 V vs. RHE, CO<sub>3</sub><sup>2-</sup> for E ≥ 0.45 V vs. RHE, then CO<sub>2</sub> for E ≥ 0.75 V vs. RHE, and glycolate, oxalate and others C1/C2 carboxylates for E ≥ 0.40 V vs. RHE. Table 8 gives the assignment of the infrared peaks to chemical functions and the associated references.

The spectra recorded for the EG electrooxidation on Pt<sub>8</sub>Ag<sub>2</sub>/C (Fig. 6d) display an intense CO<sub>ads</sub> band (in the 2000 cm<sup>-1</sup> region) for E ≥ 0.15 V vs. RHE, one band assigned to interfacial CO<sub>2</sub> (2343 cm<sup>-1</sup>) for E ≥ 0.65 V vs. RHE reaching very small intensity compared to the spectra recorded on Pt/C, the bands related to the formation of glycolate and other carboxylates from E ≥ 0.25 V vs. RHE and a shoulder at ca. 1304 cm<sup>-1</sup> not visible on the other spectra from PtM/C materials, indicating the formation of small amount of oxalate. The EG electrooxidation on Pt<sub>7</sub>Cu<sub>3</sub>/C doesn't lead to a CO<sub>ads</sub> absorption band, but CO<sub>2</sub> is formed for E ≥ 0.75 V vs. RHE, and glycolate and other carboxylates are also produced for E ≥ 0.40 V vs. RHE (Fig. 6d), but no oxalate. The Pt<sub>9</sub>Bi<sub>1</sub>/C catalyst displays about the same behavior as Pt<sub>7</sub>Cu<sub>3</sub>/C, except that the formation of glycolate and other carboxylates starts at lower potential

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Cell current versus time recorded for the electrolysis of 2 mol L⁻¹ glycerol in 0.5 mol L⁻¹ NaOH at 20 °C on a (a) Pt<sub>9</sub>Bi<sub>1</sub>/C at a cell voltage of 0.40 V, (b) Pt<sub>9</sub>Bi<sub>1</sub>/C at a cell voltage of 0.55 V, (c) Pt<sub>7</sub>Cu<sub>3</sub>/C at a cell voltage of 0.40 V and (d) Pt<sub>7</sub>Cu<sub>3</sub>/C at a cell voltage of 0.55 V.</figcaption></figure>

<table><caption>**Table 7**

Product distribution and selectivity (S) after 4-h electrolysis of 2 mol L⁻¹ glycerol in 0.5 mol L⁻¹ NaOH at 20 °C.</caption><thead><tr><th rowspan="2">Catalyst</th><th colspan="4">Pt<sub>9</sub>Bi<sub>1</sub>/C</th><th colspan="4">Pt<sub>7</sub>Cu<sub>3</sub>/C</th></tr><tr><th colspan="2">0.4</th><th colspan="2">0.55</th><th colspan="2">0.4</th><th colspan="2">0.55</th></tr><tr><th>Cell voltage</th><th>mol L<sup>-1</sup></th><th>S (%)</th><th>mol L<sup>-1</sup></th><th>S (%)</th><th>mol L<sup>-1</sup></th><th>S (%)</th><th>mol L<sup>-1</sup></th><th>S (%)</th></tr></thead><tbody><tr><td>Products</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Glyceraldehyde</td><td>0.133</td><td>72</td><td>0.179</td><td>60.2</td><td>0.085</td><td>59.1</td><td>0.161</td><td>57.5</td></tr><tr><td>Glyceric acid</td><td>0.011</td><td>6.1</td><td>0.045</td><td>15.1</td><td>0.011</td><td>7.9</td><td>0.045</td><td>15.9</td></tr><tr><td>Tartronic acid</td><td>0.014</td><td>7.7</td><td>0.035</td><td>11.8</td><td>0.011</td><td>7.8</td><td>0.024</td><td>8.4</td></tr><tr><td>Mesoxalic acid</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0.007</td><td>5.2</td><td>0.01</td><td>3.7</td></tr><tr><td>Total C3</td><td>0.158</td><td>85.8</td><td>0.259</td><td>87.1</td><td>0.114</td><td>80</td><td>0.240</td><td>85.5</td></tr><tr><td>Glycolic acid</td><td>0.015</td><td>8.3</td><td>0.016</td><td>5.5</td><td>0.019</td><td>13.1</td><td>0.023</td><td>8.1</td></tr><tr><td>Others</td><td>0.012</td><td>6.3</td><td>0.022</td><td>7.4</td><td>0.01</td><td>6.9</td><td>0.18</td><td>6.4</td></tr></tbody></table>
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Comparison of the LSVs. recorded on (a) Pt/C and Pd/C and (b) on different PtM/C and Pd<sub>9</sub>Bi<sub>1</sub>/C catalysts in 0.1 mol L<sup>-1</sup> ethylene glycol in 0.1 mol L<sup>-1</sup> NaOH at 20 °C and a scan rate of 5 mV s<sup>-1</sup>; (c, d, e, f) infrared spectra recorded for the oxidation of 1 mol L<sup>-1</sup> ethylene glycol in 0.1 mol L<sup>-1</sup> NaOH electrolyte on (c) Pt/C, (d) Pt<sub>8</sub>Ag<sub>2</sub>/C, (e) Pt<sub>7</sub>Cu<sub>3</sub>/C and (f) Pt<sub>9</sub>Bi<sub>1</sub>/C at a scan rate of 1 mV s<sup>-1</sup> and 20 °C, with a resolution of 4 cm<sup>-1</sup>. Vertical scale: ΔR/R.</figcaption></figure>

<caption>**Table 8**</caption>

Wavenumber of the infrared absorption peaks observed for the electrooxidation of EG, indexation, assignation of the peaks and related references.

<table><thead><tr><th>Wavenumber / cm<sup>-1</sup></th><th>Indexation</th><th>Assignation</th><th>References</th></tr></thead><tbody><tr><td>1074</td><td>A</td><td>Glycolate</td><td>[41]</td></tr><tr><td>1232</td><td>B</td><td>ν(C—O) of glycolate</td><td>[41,50,57]</td></tr><tr><td>1304</td><td>C</td><td>Oxalate</td><td>[41]</td></tr><tr><td>1325</td><td>D</td><td>Carboxylates</td><td>[41]</td></tr><tr><td>1354</td><td>E</td><td>Adsorbed carboxylates</td><td>[41]</td></tr><tr><td>1410</td><td>F</td><td>Carboxylate/carbonates</td><td>[41,58]</td></tr><tr><td>1578</td><td>G</td><td>Carboxylates</td><td>[41,50,57]</td></tr><tr><td>1950–2000</td><td>H</td><td>CO<sub>ads</sub></td><td>[48,49]</td></tr><tr><td>2343</td><td>I</td><td>CO<sub>2</sub></td><td>[50,51]</td></tr></tbody></table>

(E ≥ 0.25 V vs. RHE) and that more CO<sub>2</sub> is produced from 0.65 V vs. RHE (but still less than in the case of Pt/C). The disappearance of the band located at 1304 cm<sup>-1</sup> assigned to the formation of oxalate could be due to the presence of foreign metal atoms at the surface of Pt blocking a bridged adsorption of EG and favoring a linear adsorption. Now,

considering the peak F appearing very soon for all catalysts, it is symmetrical in the case of all the PtM/C catalysts over the whole potential range studied, but asymmetrical for Pt/C with some shoulders in the wavenumber range from 1410 cm<sup>-1</sup> to 1325 cm<sup>-1</sup> (see Du et al. [58] who propose that a CO<sub>3</sub><sup>2-</sup> absorption peak should appear) from the EG oxidation onset potential to 0.70 V vs. RHE, i.e., from where the CO<sub>2</sub> peaks starts to rise. It is then proposed that carbonates are formed with Pt/C and Pt<sub>8</sub>Ag<sub>2</sub>/C catalysts (which have an higher EASA according to values in Table 5 and CVs in Fig. 3, and therefore likely higher amount of adjacent Pt surface sites available) at low potentials, and that because of pH decrease due to oxidation reactions, CO<sub>2</sub> is formed at higher potentials.

It is worth noticing that carbonates are formed on catalysts that are also able to produce oxalate, confirming the possibility of bridged adsorption of EG to give oxalate without —C—C— bond breaking, or carbonate with —C—C— bond leakage. For the other PtM catalysts, at least at low potential, glycolate seems to be the main reaction product.

According to cyclic voltammetry and *in situ* infrared studies, Pt<sub>9</sub>Bi<sub>1</sub>/C appears to be the most active and selective catalysts for the EG electrooxidation without —C—C— bond breaking. Chronoamperometry
measurements were performed in an electrolysis cell fitted with a Pt₉Bi₁/C anode and a Pt/C cathode at two different cell voltages: 0.60 V and 1.00 V (Fig. 7). As in the case of GL oxidation, higher currents are obtained with the increase of the applied cell voltage from 0.60 V to 1.00 V.

After 3 h electrolysis at a cell voltage of 0.60 V (Fig. 7a), the main reaction products are glycolic acid, oxalic acid and an unidentified compound at a retention time of 7.4 min. (Fig. 7c) The intensity of this peak decreases with the electrolysis time, which indicates that the compound related to it is a reaction intermediate, considering the electrolyte recirculation mode used in the present study. A small amount of formic acid is also detected as soon as 1 h electrolysis, which decreases with the electrolysis time, indicating that it is not formed by EG electrooxidation. Several small peaks are also observed close to 14 min, which are always present, even in the chromatograms recorded for the EG electrolysis at 1.00 V (Fig. 7d). These peaks don't increase with the electrolysis time and are therefore likely due to the presence of impurities. For the electrolysis at 1.00 V (Fig. 7b), the production of oxalic acid and formic acid have drastically increased and glyoxylic acid is also formed (Fig. 7d), at the expense of the formation of the unidentified compounds and of glycolic acid (Table 9). This confirms that the unidentified compound, maybe monoaldehyde-dimer or glyoxal, is a reaction intermediate for the formation of oxalic acid and glyoxylic acid (and further formic acid at 1.0 V). Indeed, according to *in situ* infrared spectroscopy measurements for the EG oxidation on Pt₉Bi₁/C, no C1 compounds are detected for anode potential lower than 0.75 V vs. RHE; it is then very unlikely that at the low cell voltage of 0.60 V (low anode potential) the —C—C— bond breaking takes place. Therefore, it seems that at 0.6 V, the Pt₉Bi₁/C catalysts is very selective towards C2 oxidized

<table><caption>**Table 9**
Product distribution after 3-h electrolysis of 1 mol L⁻¹ EG in 0.1 mol L⁻¹ NaOH at T = 20 °C.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Cell voltage</th></tr><tr><th>0.6 V</th><th>1.0 V</th></tr><tr><th></th><th colspan="2">Product concentration (mol L<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Glycolic acid</td><td>5.60 10<sup>-3</sup></td><td>4.14 10<sup>-3</sup></td></tr><tr><td>Glyoxylic acid</td><td>0</td><td>1.73 10<sup>-3</sup></td></tr><tr><td>Oxalic acid</td><td>3.7 10<sup>-4</sup></td><td>1.94 10<sup>-3</sup></td></tr><tr><td>Formic acid</td><td>2.5 10<sup>-4</sup></td><td>2.9 0.0 10<sup>-3</sup></td></tr><tr><td>Aldehyde</td><td>present</td><td>absent</td></tr></tbody></table>

compounds, particularly glycolic acid.

The unidentified compound at a retention time of 7.4 min cannot be an acid, because all acids from ethylene glycol can be identified. Considering its retention time and the type of HPLC column used, it contains 2 or more carbon atoms. Indeed, compounds with a number of atoms between 2 and 6 have retention times lower than ca. 10 min [59], whereas those with only 1C have retention times higher than 10 min. As it was explained before, the intensity of this peak decreases with the electrolysis time, which seems to indicate that this unidentified compound is a reaction intermediate. For higher cell voltages (anode potential), the production of oxalic acid and formic acid have drastically increased and glyoxylic acid is also formed, and the unidentified compounds is no more detected, indicating that its conversion is rapid. The formation of aldehyde as a reaction intermediate has already been proposed for the oxidation of polyols by Han and Liu [60] and seems reasonable in our case. From these observations, we propose that the unidentified compound may be a dimer of monoaldehyde or glyoxal, as

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** (a,b) Cell current versus time recorded for the electrolysis of 1 mol L⁻¹ EG in 0.1 mol L⁻¹ NaOH at 20 °C on a Pt₉Bi₁/C at cell voltages of (a) 0.6 V and (b) 1.0 V, (c,d) chromatograms recorded for different electrolysis times at a cell voltage of (c) 0.6 V and (d) 1.0 V.</figcaption></figure>
it happens often with aldehydes [61,62], acting as a reaction intermediate for the formation of oxalic acid and glyoxylic acid (and further formic acid at 1.0 V).

## 4.4. Discussion

Light biosourced polyols such as ethylene glycol and glycerol possess theoretical high hydrogen gravimetric density, up to 8.15 wt% H₂, considering the two water molecules necessary for the complete conversion of EG into oxalic acid (Table 1). Moreover, thermodynamic data indicate that these molecules can theoretically be oxidized from the very low potentials of ca. 0.11 V vs. RHE and ca. 0.27 V vs. RHE for GL and EG, respectively, corresponding to an electrical energy consumption of ca. 20 kJ mol⁻¹H₂ and ca. 53 kJ mol⁻¹H₂, respectively, values lower or equivalent to that for classical LOHC.

For both polyols, the Pt₉Bi₁/C has shown the best performances in terms of activity (reaction rate), selectivity towards C3 oxidized compounds, and was shown to be very stable under harsh conditions of AST [54]. Glycerol can be oxidized at $E \le 0.60$ V vs. RHE, but glyceraldehyde is the main reaction product. Such a behavior has already been observed in an alkaline medium by Gonzalez-Cobos et al. [24] using an outstanding ternary Pt₄.₅Pd₄.₅Bi₁/C catalyst. Even in acidic medium, Kwon et al. [63] and Garcia et al. [64] showed that the modification of platinum surfaces by bismuth atoms led to activity enhancement but favored the selectivity towards the production of dihydroxyacetone (isomer of glyceraldehyde). EG can also be oxidized at $E \le 0.60$ V vs. RHE on a Pt₉Bi₁/C, leading selectively to glycolic acid, oxalic acid and an intermediate compound, likely an aldehyde or dialdehyde, that is consumed with time to form glycolic acid and/or oxalic acid. No —C—C— bond breaking and formation of formic acid has been shown, in agreement with previous results of Demarconay et al. [23] using Pt₉Bi₁/C or Pt₄.₅Pd₄.₅Bi₁/C as anode catalysts.

Therefore, EG seems to be a better candidate as LOHC than GL. Furthermore, as explained in the introduction section, hydrogenation reactions are exothermal processes well mastered by the refinery industries. The hydrogenation of oxalic acid can be performed in aqueous phase [65] limiting the problem of thermal decomposition of solid acids at relatively low temperature. Moreover, recent results in our laboratory concerning the hydrogenation of C2 acids on Ru based catalysts have shown the possibility to achieve a very high selectivity towards EG formation with a conversion yield of ca. 100% (results to be published).

On the other hand, electrochemical process for the electroconversion of organics are mastered for long times at the industrial scale [66]. Moreover, in the present case, the systems considered are close to that of water electrolysis, which is also well mastered at the industrial level [67], but assisted by biomass [68]. Moreover, if the cell voltage for the electrochemical dehydrogenation of polyol is kept lower than 0.40 V vs RHE, which is possible by tailoring the anode catalyst, then the energy of the dehydrogenation process ($\Delta H_{dehyd.}$) will lie between 60 and 70 kJ mol⁻¹H₂, which is of the same order than that for classical LOHCs [7].

But, although biosourced polyols have numerous advantages compared to classical LOHCs (very little impact on the environment and health, biodegradability, low cost, higher hydrogen gravimetric capacity, see Table 1), the hydrogen release rate is slower for the electro-reforming of polyols by electrochemical methods (0.23 NmH₂ h⁻¹ m⁻², i. e., 14.4 L h⁻¹ g⁻¹catal for glycerol electroreforming at 0.70 V [24]), with the outstanding Pt₉Bi₁/C catalyst compared with those reached with classical LOHCs through thermocatalytic method (higher than 100 L h⁻¹ g⁻¹cat in the case of NEC dehydrogenation with 0.1 mol% Pt deposited on Al₂O₃ [69]). Therefore, several aspects have to be improved before integrating this technology in industry. Although the onset potential values for GL and EG electrooxidation are in agreement with the thermoneutral voltages, the reaction rates between 0.25 V vs. RHE and 0.60 V vs. RHE should be increased, as well as their dehydrogenation into diacids. This can be performed by developing more active electro-catalysts, such as ternary materials, while keeping or improving the

selectivity towards highly oxidized (acids or diacids) C2 or C3 compounds. It has indeed been shown in the case of ethanol electrooxidation that the modification of a PtSn catalyst by Ru led to enhance the ethanol conversion rate, to decrease the CO₂ yield, and to improve the selectivity towards acetic acid [70]. Because Pt₉Bi₁/C and Pt₇Cu₃/C catalysts have displayed very close electrocatalytic behavior, in terms of activity and selectivity, as shown by in situ infrared spectroscopy measurements, PtCuBi ternary systems could also be interesting to study.

At last, it is necessary to develop very conductive and robust anion exchange membrane in order to avoid the addition of NaOH or KOH, and to benefit the advantages of acidic electrolysis cells. Such development will also allow decreasing the internal resistance of the electrochemical reactor, operating at high H₂ pressure in the cathode side, and using non-noble metals for the hydrogen evolution reaction at the cathode. In addition, such development will allow avoiding the degradation of electrolyzer materials and safety issues in strong alkaline media, and increasing the concentration of polyol in the feeding solution and hence the efficiency of the system.

## 5. Conclusion

In this contribution, we have explored the possibility of using EG or GL as LOHC. Indeed, these polyols are biosourced compounds and have very little impact on the environment and health (as well as their oxidized counterparts) and are biodegradable, conversely to classical fossil-sourced LOHCs. These polyols are cheaper and have higher hydrogen gravimetric capacity than most of the classical LOHCs. The hydrogenation reaction of acids or diacids is exothermic and well-m Mastered by industry, but dehydrogenation reactions necessitate to work at high temperatures. Electrochemical methods for dehydrogenation (oxidation) work under mild conditions (room T, 1 atm) and are adapted for such a reaction.

Pt₉Bi₁/C catalysts allows operating at low onset potentials, close to the thermoneutral voltages. We have shown that EG is preferable to GL, in terms of higher hydrogen gravimetry, higher reactivity to form acidic compounds and higher selectivity towards C2 compounds (no —C—C— bond breaking). We also have pointed out the necessity to increase the conversion rate at low potentials ($\le 0.60$ V vs. RHE), while keeping high selectivity into the formation of C2 acids, to decrease the energy consumption. It is also important to achieve high concentrations of reactants and products. Combining modeling and surface nanoengineering methods is a good way to improve catalytic activity and selectivity by developing active sites with the convenient electronic structure. Engineering of electrochemical microreactors is another way to enhance electrochemical processes in industrial devices [71].

The present contribution paves the way for a cleaner and safer hydrogen economy by the development of electrochemical processes to convert biosourced compounds, low cost and environmentally friendly LOHC into their dehydrogenated counterparts.

### CRediT authorship contribution statement

**Issam Nciri:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis. **Roméo Kouamé:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis. **Thibault Rafaïdeen:** Writing – review & editing, Writing – original draft, Supervision, Methodology, Investigation, Data curation, Conceptualization. **Claude Lamy:** Writing – review & editing, Writing – original draft, Validation, Methodology, Formal analysis, Conceptualization. **Jesus Gonzalez-Cobos:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. **Cláudia Morais:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation. **S. Baranton:** Writing – review & editing, Writing – original draft, Supervision, Methodology, Investigation, Funding acquisition, Formal analysis. **Karine De Oliveira Vigier:** Writing – review & editing, Writing –
original draft, Validation, Supervision, Resources, Investigation, Fund-
ing acquisition. **Christophe Coutanceau:** Writing – review & editing,
Writing – original draft, Validation, Supervision, Project administration,
Methodology, Investigation, Formal analysis, Data curation,
Conceptualization.

Declaration of competing interest

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

Acknowledgements

This work was performed in the framework of the BHyoLOHC project
funded by the “France 2030” government investment plan managed by
the French Research Agency, under the reference “ANR-22-PEHY-
0016”. The authors acknowledge financial support from the European
Union (ERDF) and Région Nouvelle Aquitaine. Claude Lamy acknowl-
edges Deborah Jones and Eric Clot for their kind welcome in the D4-
ICGM team in Montpellier.

**Appendix A. Supplementary data**

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.jelechem.2026.119994.

**References**

[1] C. Coutanceau, S. Baranton, T. Audichon, Hydrogen Electrochemical Production, in: B. Pollet (Ed.), Hydrogen Energy and Fuel Cells Primers, Ch. 1, Elsevier, Amsterdam, 2017, pp. 1–6. DOI: 10.1016/B978-0-12-811250-2.00001-7.

[2] M. Chatenet, B.G. Pollet, D.R. Dekel, F. Dionigi, J. Deseure, P. Millet, R.D. Braatz, M.Z. Bazant, M. Eikerling, I. Staffell, P. Balcombe, Y. Shao-Horn, H. Schäfer, Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments, Chem. Soc. Rev. 51 (2022) 4583-4762, https://doi.org/10.1039/d0cs01079k.

[3] D. Marciuš, A. Kovač, M. Firak, Electrochemical hydrogen compressor: recent progress and challenges, Int. J. Hydrog. Energy 47 (2022) 24179–24193, https://doi.org/10.1016/j.ijhydene.2022.04.134.

[4] S.Z.S. Al Ghafri, S. Munro, U. Cardella, T. Funke, W. Notardonato, J.P. Martin Trusler, J. Leachman, R. Span, S. Kamiya, G. Pearce, A. Swanger, E.D. Rodriguez, P. Bajada, F. Jiao, K. Peng, A. Siahvashi, M.L. Johns, E.F. May, Hydrogen liquefaction: a review of the fundamental physics, engineering practice and future opportunities, Energy Environ. Sci. 15 (2022) 2690–2731, https://doi.org/10.1039/d2ee00099g.

[5] J. Andersson, S. Gronkvist, Large-scale storage of hydrogen, Int. J. Hydrog. Energy 44 (2019) 11901–11919, https://doi.org/10.1016/j.ijhydene.2019.03.063.

[6] M. Hurskainen, J. Ihonen, Techno-economic feasibility of road transport of hydrogen using liquid organic hydrogen carriers, Int. J. Hydrog. Energy 45 (2020) 32098–32112, https://doi.org/10.1016/j.ijhydene.2020.08.186.

[7] P.T. Aakko-Saksaa, C. Cook, J. Kiviaho, T. Repo, Liquid organic hydrogen carriers for transportation and storing of renewable energy – review and discussion, J. Power Sources 396 (2018) 803–823, https://doi.org/10.1016/j.jpowsour.2018.04.011.

[8] K. Li, H. An, P. Yan, C. Yang, T. Xiao, J. Wang, S. Zhou, Hydrogenation of toluene to methyl cyclohexane over PtRh bimetallic nanoparticle-encaged hollow mesoporous silica catalytic nanoreactors, ACS Omega 6 (2021) 5846–5855, https://doi.org/10.1021/acsomega.0c06268.

[9] J. Li, Q. Zhang, J. Zhang, R. Gao, X. Zhang, J.-J. Zou, L. Pan, Intrinsic kinetics of Dibenzyltoluene hydrogenation over a supported Ni catalyst for green hydrogen storage, Ind. Eng. Chem. Res. 63 (2024) 220–232, https://doi.org/10.1021/acs.iecr.3c03613.

[10] Y. Huang, Y. Si, Y. Xiang, S. Yao, Y. Cheng, Integration of hydrogenation and dehydrogenation based on N-Ethylcarbazole as a liquid organic hydrogen carrier, Ind. Eng. Chem. Res. 62 (2023) 6953–6962, https://doi.org/10.1021/acs.iecr.3c00408.

[11] A. Bourane, M. Elanany, Thang V. Pham, Sai P. Katikaneni, An overview of organic liquid phase hydrogen carriers, Int. J. Hydrog. Energy 41 (2016) 23075–23091.

[12] Toluene Price, December 2024 and Outlook, https://businessanalytiq.com/pro-
curementanalytics/index/toluene-price-index/; accessed December, 14th, 2024.

[13] https://ec.europa.eu/research/participants/documents/downloadPublic?docu-
mentIds=080166e5c551f4c2&appId=PPGMS.

[14] E.M.T. Uijthof, B.S. Chavan, M.J. Sluijer, V.C. Komath, A.G.J. van der Ham, H. van den Berg, J.-P. Lange, A. Higler, S. Wijnans, Liquid organic hydrogen carriers: process design and economic analysis for manufacturing N-ethylcarbazole, J. Adv. Manuf. Process. 6 (2024) e10173, https://doi.org/10.1002/amp2.10173.

[15] Ethylene Glycol, Price December 2024 and Outlook, https://businessanalytiq.com/
procurementanalytics/index/ethylene-glycol-price-index/; accessed December,
14th, 2024.

[16] Glycerol Price, December 2024 and Outlook, https://businessanalytiq.com/procurementanalytics/index/glycerol-price-index/; accessed December, 14th, 2024.

[17] T.D.J. te Molder, S.R.A. Kersten, J.P. Lange, M.P. Ruiz, Ethylene glycol from lignocellulosic biomass: impact of lignin on catalytic hydrogenolysis, Ind. Eng. Chem. Res. 60 (2021) 7043–7049, https://doi.org/10.1021/acs.iecr.1c01063.

[18] P. Vignesh, A. Remigious, P. Kumar, N.S. Ganesh, V. Jayaseelan, K. Sudhakar, Biodiesel generation: an overview, Oil & Gas Sci. Technol. – Rev. IFP Energies Nouvelles 76 (2021) 1–15, https://doi.org/10.2516/ogst/2020088.

[19] A.S. Aldiguier, S. Alfenore, X. Cameleyre, G. Goma, J.L. Uribelarrea, S.E. Guilloult, C. Molina-Jouve, Synergetic temperature and ethanol effect on *Saccharomyces cerevisiae* dynamic behaviour in ethanol bio-fuel production, Bioprocess Biosyst. Eng. 36 (2004) 217–222, https://doi.org/10.1007/s00449-004-0352-6.

[20] J. Meng, F. Zhou, H. Ma, X. Yuan, Y. Wang, J. Zhang, A review of catalysts for methylcyclohexane dehydrogenation, Top. Catal. 64 (2021) 509–520, https://doi.org/10.1007/s11244-021-01465-6.

[21] V. Sage, J. Patel, P. Hazewinkel, Q. Ul Ain Yasin, F. Wang, Y. Yang, K. Kozielski, C. Li, Recent progress and techno-economic analysis of liquid organic hydrogen carriers for Australian renewable energy export – a critical review, in: Int. J. Hydrogen Energy 56, 2024, pp. 1419–1434, https://doi.org/10.1016/j.ijhydene.2023.12.269.

[22] C. Coutanceau, N. Neha, T. Rafaïdeen, Electrocatalytic transformation of biosourced organic molecules, Curr. Op. Electrochem. 38 (2023) 101210, https://doi.org/10.1016/j.coelec.2023.10121.

[23] L. Demarconnay, S. Brimaud, C. Coutanceau, J.-M. Léger, Ethylene glycol electrooxidation in alkaline medium at pluri-metallic Pt based catalysts, J. Electroanal. Chem. 601 (2007) 169–180, https://doi.org/10.1016/j.jelechem.2006.11.00.

[24] J. Gonzalez Cobos, S. Baranton, C. Coutanceau, Development of bi-modified PtPd Nanocatalysts for the electrochemical reforming of polyols into hydrogen and value-added chemicals, ChemElectroChem 3 (2016) 1694–1704, https://doi.org/10.1002/cec1.201600147.

[25] X. Liu, X. He, D. Xiong, G. Wang, Z. Tu, D. Wu, J. Wang, J. Gu, Z. Chen, Electro-reforming of PET plastic to C2 chemicals with concurrent generation of hydrogen and electric energy, ACS Catal. 14 (2024) 5366–5376, https://doi.org/10.1021/acscatal.4c00370.

[26] Y. Ma, H. Ge, Y. Zhang, N. Jian, J. Tang, Z. Hu, J. Yu, J. Arbiol, C. Li, L. Li, A. Cabot, J. Li, Electrooxidation of PET alkaline hydrolysate to formate and glycolate enabled by the synergistic interaction of ag and NiFe-LDH, Chin. J. Catal. 80 (2026) 282–292, https://doi.org/10.1016/j.s1872-2067(25)64819-4.

[27] J. Wang, X. Li, M. Wang, T. Zhang, X. Chai, J. Lu, T. Wang, Y. Zhao, D. Ma, Electrocatalytic valorization of poly(ethylene terephthalate) plastic and CO₂ for simultaneous production of formic acid, ACS Catal. 12 (2022) 6722–6728, https://doi.org/10.1021/acscatal.2c01128.

[28] W. Wang, X. He, Z. Tu, D. Xiong, S. Dong, T. Zhang, D. Wu, J. Wang, Z. Chen, Spontaneous electronic redistribution strategy for efficient electrocatalytic upcycling PET (polyethylene terephthalate) and nitrate wastes, ACS Catal. 15 (2025) 9574–9583, https://doi.org/10.1021/acscatal.5c01907.

[29] M. Simoes, S. Baranton, C. Coutanceau, Electrochemical valorization of glycerol, ChemSusChem 5 (2012) 2106–2124, https://doi.org/10.1002/cssc.201200335.

[30] N.V. Rees, R.G. Compton, Carbon-free energy: a review of ammonia- and hydrazine-based electrochemical fuel cells, Energy Environ. Sci. 4 (2011) 1255–1260, https://doi.org/10.1039/c0ee00809e.

[31] Cheméo, Chemical & Physical Properties by Cheméo, 2026 https://www.chemeo.
com/ (accessed February 7, 2025).

[32] K.M. Ebeling, D. Bongartz, S. Mürtz, R. Palkovits, A. Mitsos, Thermodynamic and economic potential of glycerol oxidation to replace oxygen evolution in water electrolysis, Ind. Eng. Chem. Res. 63 (2024) 8250–8260, https://doi.org/10.1021/acs.iecr.3c03615.

[33] X. Dai, R. Verma, X. Zhang, J. Bai, S.K. Verma, H. Yun, J. Wang, C.K. Dixit, S. K. Verma, Critical analysis on catalytic methylcyclohexane dehydrogenation reaction: a review, Int. J. Hydrog. Energy (2024), https://doi.org/10.1016/j.ijhydene.2024.08.148.

[34] H. Jorschick, P. Preuster, S. Dürr, A. Seidel, K. Müller, A. Bösmann, P. Wasserscheid, Hydrogen storage using a hot pressure swing reactor, Energy Environ. Sci. 10 (2017) 1652–1659, https://doi.org/10.1039/C7EE00476A.

[35] Y. Li, X. Guo, S. Zhang, Y. He, A perspective review on N-heterocycles as liquid organic hydrogen carriers and their hydrogenation/dehydrogenation catalysts, Energy Fuel 38 (2024) 12447–12471, https://doi.org/10.1021/acs.energyfuels.4c01633.

[36] C. Coutanceau, S. Baranton, R.B. Kouamé, Selective electro-oxidation of glycerol into value-added chemicals: a short overview, Front. Chem. 7 (2019) 100, https://doi.org/10.3389/fchem.2019.00010.

[37] M.S.E. Houache, R. Safari, U.O. Nwabarac, T. Rafaïdeen, G.A. Botton, P.J.A. Kenis, S. Baranton, C. Coutanceau, E.A. Baranova, Selective electrooxidation of glycerol to formic acid over carbon supported Ni₁ₓMₓ (M= bi, pd and au) Nano-catalysts and co-electrolysis of CO₂, ACS Appl. Energy Mater. 3 (2020) 8725–8738, https://doi.org/10.1021/acsaem.0c01282.

[38] D. Bayer, S. Berenger, M. Joos, C. Cremers, J. Tübke, Electrochemical oxidation of C2 alcohols at platinum electrodes in acidic and alkaline environment, Int. J. Hydrog. Energy 35 (2010) 12660–12667, https://doi.org/10.1016/j.ijhydene.2010.07.102.

[39] S.D. Lankiang, S. Baranton, C. Coutanceau, Electrocatalytic behaviour towards oxygen reduction reaction of carbon supported PtₓMₓAuₓ (M = Ni, cu, co) binary
and ternary catalysts, Electrochim. Acta 242 (2017) 287-299, https://doi.org/10.1016/j.electacta.2017.05.036.

[40] N. Neha, S.R.B. Kouamé, T. Rafaïdeen, S. Baranton, C. Coutanceau, Remarkably efficient carbon-supported nanostructured platinum-bismuth catalysts for the selective electrooxidation of glucose and methyl-glucoside, Electrocatalysis 12 (2021) 1-14, https://doi.org/10.1007/s12678-020-00586-y.

[41] P.A. Christensen, A. Hammett, The oxidation of ethylene glycol at a platinum electrode in acid and base: An in situ FTIR study, J. Electroanal. Chem. 260 (1989) 347-359, https://doi.org/10.1016/0022-0728(89)87149-9.

[42] B. Beden, M.-C. Morin, F. Hahn, C. Lamy, "In situ" analysis by infrared reflectance spectroscopy of the adsorbed species resulting from the electro sorption of ethanol on platinum in acid medium, J. Electroanal. Chem. 229 (1987) 353-366, https://doi.org/10.1016/0022-0728(87)85152-5.

[43] T. Rafaïdeen, S. Baranton, C. Coutanceau, Highly efficient and selective electrooxidation of glucose and xylose in alkaline medium at carbon supported alloyed PdAu nanocatalysts, Appl. Catal. B Environ. 243 (2019) 641-656, https://doi.org/10.1016/j.apcatb.2018.11.006.

[44] B.S.R. Kouamé, Etude de la valorisation de molécules bio-sourcées par conversion électrocatalytique, Université de Poitiers, 2020 https://theses.hal.science/tel-03123033v1/file/2020-Kouame-Bitty-Serge-Romeo-These.pdf (accessed August 6th, 2025).

[45] C.J. Pouchert, The Aldrich Library of Infrared Spectra, 3rd ed., Aldrich Chemical Company, Inc., Milwaukee, 1981.

[46] J. González-Cobos, S. Baranton, C. Coutanceau, A systematic in situ infrared study of the electrooxidation of C3 alcohols on carbon-supported Pt and Pt-bi catalysts, J. Phys. Chem. C 120 (2016) 7155-7164, https://doi.org/10.1021/acs.jpcc.b600295.

[47] T. Hu, Y. Wang, Q. Liu, L. Zhang, H. Wang, T. Tang, W. Chen, M. Zhao, J. Jia, In-situ synthesis of palladium-base binary metal oxide nanoparticles with enhanced electrocatalytic activity for ethylene glycol and glycerol oxidation, Int. J. Hydrog. Energ. 42 (2017) 25951-25959, https://doi.org/10.1016/j.ijhydene.2017.08.160.

[48] A. Couto, A. Rincón, M.C. Pérez, C. Gutierrez, Adsorption and electrooxidation of carbon monoxide on polycrystalline platinum at pH 0.3-13, Electrochim. Acta 46 (2001) 1285-1296, https://doi.org/10.1016/S0013-4686(00)00714-3.

[49] J. Gomes, C.A. Martins, J. Giz, G. Tremiliosi-Filho, G.A. Camara, Insights into the adsorption and electro-oxidation of glycerol: self-inhibition and concentration effects, J. Catal. 301 (2013) 154-161, https://doi.org/10.1016/j.jcat.2013.02.007.

[50] A. Dailey, J. Shin, C. Korzeniewski, Ethylene glycol electrochemical oxidation at platinum probed by ion chromatography and infrared spectroscopy, Electrochim. Acta 44 (1998) 1147-1152, https://doi.org/10.1016/S0013-4686(98)00217-5.

[51] 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.

[52] L. Luo, W. Chen, S.-M. Xu, J. Yang, M. Li, H. Zhou, M. Xu, M. Shao, X. Kong, Z. Li, H. Duan, Selective Photoelectro catalytic glycerol oxidation to dihydroxyacetone via enhanced middle hydroxyl adsorption over a Bi₂O₃-incorporated catalyst, J. Am. Chem. Soc. 144 (2022) 7720-7730, https://doi.org/10.1021/jacs.2c00465.

[53] Y. Wang, Y.-Q. Zhu, Z. Xie, S.-M. Xu, M. Xu, Z. Li, L. Ma, R. Ge, H. Zhou, Z. Li, X. Kong, L. Zheng, J. Zhou, H. Duan, Efficient electrocatalytic oxidation of glycerol via promoted OH⁺ generation over single-atom-bismuth-doped spinel Co₃O₄, ACS Catal. 12 (2022) 12432-12443, https://doi.org/10.1021/acscatal.2c03162.

[54] B.S.R. Kouamé, S. Baranton, P. Brault, C. Canaff, W. Chamorro-Coral, A. Caillard, K. De Oliveira Vigier, C. Coutanceau, Insights on the unique electro-catalytic behavior of PtBi/C materials, Electrochim. Acta 329 (2020) 135161, https://doi.org/10.1016/j.electacta.2019.135161.

[55] X. Li, Y. Zhou, Y. Du, J. Xu, W. Wang, Z. Chen, J. Cao, PtCu nanoframes as ultra-high performance electrocatalysts for methanol oxidation, Int. J. Hyd. Energy 44 (2019) 18050-18057, https://doi.org/10.1016/j.ijhydene.2019.05.072.

[56] B. Thi, X. Lam, M. Chiku, E. Higuchi, H. Inoue, PtAg nanoparticle electrocatalysts for the glycerol oxidation reaction in alkaline medium, Adv. Nanopart. 5 (2016) 167-175, https://doi.org/10.4236/annp.2016.53018. Published Online August 2016 in SciRes. http://www.scirp.org/journal/annp doi:10.4236/annp.2016.53018.

[57] R.B. de Lima, V. Paganin, T. Iwasita, W. Vielstich, On the electrocatalysis of ethylene glycol oxidation, Electrochim. Acta 49 (2003) 85-91, https://doi.org/10.1016/j.electacta.2003.05.004.

[58] H. Du, C.T. Williams, A.D. Ebner, J.A. Ritter, In situ FTIR spectroscopic analysis of carbonate transformations during adsorption and desorption of CO₂ in K-promoted HTlc, Chem. Mater. 22 (2010) 3519-3526, https://doi.org/10.1021/cm100703e.

[59] T. Rafaïdeen, N. Neha, B.R.S. Kouamé, S. Baranton, C. Coutanceau, Electroreforming of glucose and xylose in alkaline medium at carbon supported alloyed Pd₃Au₇ Nanocatalysts: effect of aldose concentration and electrolysis cell voltage, Clean Technol. 2 (2020) 184-203, https://doi.org/10.3390/cleantechnol2020013.

[60] S. Han, B. Liu, Robust Formate electrosynthesis from mixed biomass polyols by a direct carbon-carbon bond cleavage route, J. Am. Chem. Soc. 148 (2026) 778-789, https://doi.org/10.1021/jacs.5c16299.

[61] V.A. Yaylayan, S. Harty-Majors, A.A. Ismail, Investigation of the mechanism of dissociation of glycolaldehyde dimer (2,5-dihydroxy-1,4-dioxane) by FTIR spectroscopy, Carbohydr. Res. 309 (1998) 31-38, https://doi.org/10.1016/S0008-6215(98)00129-3.

[62] E. Bertheussen, A. Verdaguier-Casadevall, D. Ravasio, J.H. Montoya, D.B. Trimarco, C. Roy, S. Meier, J. Wendland, J.K. Nørskov, I.E.L. Stephens, I. Chorkendorff, Acetaldehyde as an intermediate in the electroreduction of carbon monoxide to ethanol on oxide-derived copper, Angew. Chem. Int. Ed. 55 (2016) 1450-1454, https://doi.org/10.1002/anie.201508851.

[63] Y. Kwon, Y. Birda, I. Spanos, P. Rodriguez, M.T.M. Koper, Highly selective electro-oxidation of glycerol to hydroxyacetone on platinum in the presence of bismuth, ACS Catal. 2 (2012) 759-764, https://doi.org/10.1021/CS200599g.

[64] A.C. Garcia, Y.Y. Birda, G. Tremiliosi-Filho, M.T.M. Koper, Glycerol electrooxidation on bismuth modified platinum single crystals, J. Catal. 346 (2017) 117-124, https://doi.org/10.1016/j.jcat.2016.12.013.

[65] L. Ben Neon, I. Nciri, M. Drobek, M. Bechelany, C. Coutanceau, K. De Oliveira Vigier, Anne Julbe, Exploring stabilized alumina phase coatings as a catalyst carrier for oxalic acid hydrogenation, Chem. Methods (2025) e202500085, https://doi.org/10.1002/cmtd.202500085.

[66] D.S.P. Cardoso, B. Sljuki, D.M.F. Santos, C.A.C. Sequeira, Organic electrosynthesis: from laboratorial practice to industrial applications, Org. Process. Res. Dev. 21 (2017) 1213-1226, https://doi.org/10.1021/acs.oprd.7b00004.

[67] M. Chatenet, B.G. Pollet, D.R. Dekel, de F. Dionigi, J. Deseure, P. Millet, R. D. Braatz, M.Z. Bazant, M. Eikerling, I. Staffell, P. Balcombe, Y. Shao-Horn, H. Schäfer, Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments, Chem. Soc. Rev. 51 (2022) 4583-4762, https://doi.org/10.1039/d0cs01079k.

[68] C. Dolle, N. Neha, Christophe Coutanceau, Electrochemical hydrogen production from biomass, Curr. Opin. Electrochem. 31 (2022) 100841, https://doi.org/10.1016/j.coeclec.2021.100841.

[69] N. Brückner, K. Obesser, A. Bçsmann, D. Teichmann, W. Arlt, J. Dungs, P. Wasserscheid, Evaluation of industrially applied heat-transfer fluids as liquid organic hydrogen carrier systems, ChemSusChem 7 (2014) 229-235, https://doi.org/10.1002/cssc.201300426.

[70] S. Rousseau, C. Coutanceau, C. Lamy, J.-M. Léger, Direct ethanol fuel cell (DEFC): electrical performances and reaction products distribution under operating conditions with different platinum-based anodes, J. Power Sources 158 (2006) 18-24, https://doi.org/10.1016/j.jpowsour.2005.08.027.

[71] Y. Cao, J. Knijff, A. Delparish, M.F.N. D'Angelo, T. Noël, A divergent paired electrochemical process for the conversion of furfural using a divided-cell flow microreactor, ChemSusChem 14 (2021) 590-594, https://doi.org/10.1002/cssc.202002833.