A New Class of {MoxSy}-Based Electrocatalysts: From Local Electrochemistry to Their Implementation in Proton Exchange Membrane Electrolysers
Authors: El Khoueiry Maria , Al Bacha Serge , Falaise Clement , Leclerc Nathalie , Cadot Emmanuel , Assaud Loïc 
Published: December 2024

The mass production of green H2 has become a crucial challenge to decarbonize the industry and to develop sustainable mobilities. In this context, water electrolysis is a technology of choice, provided that electricity is renewable and electrolyzer technology meets OPEX and CAPEX performance [1,2].
Among several water electrolysis processes, Proton Exchange Membrane Water Electrolysis (PEMWE), is one of the most promising technologies thanks to its ability (i) to operate at close to ambient temperature (<100°C), (ii) at high current density (>1 A/cm2) and (iii) to generate highly pure H2. Moreover, it is compatible to be coupled to variable sources of renewable energy. However, PEM water electrolysis faces some limitations due to its high cost particularly due to the use of Platinum Group Metals (PGMs) electrocatalysts.
Molecular engineering of efficient electrocatalysts for the Hydrogen Evolution Reaction (HER) is an attractive approach for controlling the spatial environment of specific building units selected for their intrinsic functionality required within the multistep HER process [3]. As the {Mo-S} cores has been identified as one as the most promising HER electrocatalysts, it has been demonstrated that the covalent association between the {Mo3S4} core and tungsten-containing redox-active polyoxometalates (POM) produces a striking synergistic effect featured by high HER performance [4]. Various experiments carried out either in homogeneous or in heterogeneous conditions showed that this synergistic effect arises from the direct connection between the {Mo3S4} cluster and the POM unit. An innovative and straightforward approachconsistsof polymerizing monomeric tungstate anions under acidic conditions in the presence of the HER-active cluster [Mo3S4(H2O)9]4+ to yield highly insoluble {Mo3S4}-containing polymers, abbreviated Mo3S4@WOx. Herein, we showed that Mo3S4@WOx inorganic polymers exhibit strong propensity to interface with carbon-based nanomaterials e.g. nanotubes, reduced graphene oxide or carbon black to be integrated as electrode active component within PEM electrolyser cells [5].
The electrocatalytic complexes have been thoroughly characterized by electron microscopies, EDS, Raman spectroscopy and FTIR.
In order to ensure a precise control of the surface nanostructuring of the electrocatalytic complexes, they were characterized by Scanning ElectroChemical Microscopy coupled to Atomic Force Microscopy (AFM-SECM) [6]. This powerful technique allowed to determine the topological orientation effect of the catalysts on their electrochemical performance towards the hydrogen evolution reaction (Figure 1). Additional local measurements including Quantitative NanoMechanical (QNM) have been performed to characterize at the catalytic site scale de topographical, mechanical and electrochemical properties of the HER electrode.
After optimizing the stoichiometry, the catalytic ink formulation and the deposition method to prepare Membrane Electrode Assemblies (MEA), the Mo3S4@WOx were implemented in PEM electrolysis cells (4 cm2) in order to characterize their performance in real operating conditions. Preliminary results showed that synergistic effect between the catalytic unit and the WOx polymer was retained leading to highly stable and efficient HER cathodes. A comparison of the performance of our electrocatalytic system with commercial MEA (Pt/C, Nafion™, IrO2). Start of life performance was evaluated by recording polarization curves and electrochemical impedance spectra (Figure 2). The durability was studied by accelerated stress tests. The outlook of this study will consist of implementing our electrocatalysts in large-scale PEM electrolysers (>250 cm2) as a proof-of-concept, in semi pilot systems.
References:
[1] M. Chatenet, et. al., Chem. Soc. Rev. 2022, 51, 4583
[2] A. C. Bhosale, et. al.,Renew. Sustain. Energy Rev. 2020, 133, 110286
[3] Y. Hou, et al., Nature Materials 2011, 10 (6), 434
[4] J. Tourneur, et al., J. Am. Chem. Soc. 2019, 141 (30), 11954
[5] J. Al Cheikh et al., Mater. Adv. 2020, 1, 430
[6] D. Polcari et al., Chem. Rev. 2016, 116, 13234

Figure 1