Electrocatalytic Properties of {Mo-S}-Based Compounds with Regard to the Hydrogen Evolution Reaction and Application to PEM Water Electrolysis
Authors: Assaud Loïc 
Published: December 2020

Large amounts of molecular hydrogen (H2) are used in the chemical industry for a quite diverse set of applications, including gasoline refining, the synthesis of nitrogenous fertilizers, metal reduction, and many others.
In this communication, we report on the synthesis of a family of thiomolybdic {MoS}-based electrocatalysts and their systematic electrochemical characterization for the hydrogen evolution reaction. The electrocatalysts, free of noble metals, have been further mixed with high specific surface area carbon agents (graphene and Vulcan) in view of catalytic ink formulation for implementation into high surface electrolysis proton exchange membrane cells. As a proof-of-concept, the electrocatalysts have been tested into areal electrolysis cells of 7 cm2. The results show high electrocatalytic efficiency and faradaic yields getting closer to what expected on metal Pt-based electrodes, making the thiomolybdic complexes good candidates for their implementation into practical electrolysis cells.
The importance of this study is that it provides a full study, describing the synthesis and preparation of the electrocatalytic complexes and their physico-chemical and electrochemical characterization. As a proof-of-concept, our thiomolybdic based-electrode has been characterized into areal low pH solutions in large-scale electrolysis cells (7 cm2) towards practical applications in electrolyzers.
Molecular molybdenum sulphide species have been extensively studied and considered as versatile chemical platform allowing to develop multifunctional catalytic materials, such as metal-substituted derivatives or coordination complexes, specifically designed with regard to the applied process.
In order to further investigate the HER properties of such class of compounds, we report herein on the development of {Mo3S4}-based molecular HER catalysts, taking benefit of the well-known chemical properties of the aqua cluster [Mo3S4(H2O)9]4+, used as precursor in this work.
The HER electrocatalytic performances of this hybrid catalyst have been investigated in both homogeneous and heterogenous phase, with the goal to design the first {Mo3S4}-based PEM (Proton Exchange Membrane) cathode for water electrolysis.
The electrodes modified with Mo3S4 catalysts exhibit a high HER activity with an onset potential at −407 mV/SCE and a Tafel slope of −150 mV/dec. The potential at which a current density of 30 mA/cm2 was passed in 0.1 M H2SO4 (pH = 0.7) was found to be -0.5 V/SCE on Pt, -0.6 V/SCE on {Mo3S4}-based molecular HER catalysts + Vulcan. Finally, the complexes have been implemented at the cathode of PEM water electrolysis cells in place of platinum. The polarization curves have been measured and compared to those obtained with platinum. The HER overvoltage was increased by ~250 mV with the complex compared to Pt. These results show that {MoS}-based electrocatalysts are good candidates to replace Pt for the HER in PEM water electrolysis technology but optimization is still required and will be discussed.