DEMS study of hydrogen evolution reaction at Ni/reduced graphene oxides/Mo2C composites

Abstract

In the present paper, Ni nanoparticles on graphene-based materials (Ni-GMs) and Mo₂C composites (Ni-Mo₂C) were tested as electrocatalysts towards the hydrogen evolution reaction (HER) using differential electrochemical mass spectrometry (DEMS) in alkaline medium. Precise determination of the Tafel slope and rate-determining step (RDS), along with the reaction mechanism, was achieved. The findings show that Ni deposition and the addition of Mo₂C particles significantly enhance the catalytic activity, leading to low overpotentials towards the HER. Among the tested composites, Ni/rGO/Mo₂C demonstrated the best onset potential (-54 mV_RHE), followed by Ni/N-rGO/Mo₂C (-63 mV_RHE) and Ni/SNrGO/Mo₂C (-100 mV_RHE), with ionic current measurements (m/z = 2) confirming a Volmer-Heyrovsky pathway as the RDS. The addition of Mo₂C, while not altering the reaction mechanism, significantly enhanced onset potentials and kinetics, with Tafel slopes for Ni-GMs ranging from 54 to 70 mV·dec⁻¹, underscoring improved HER efficiency. Furthermore, the stability of these composites, particularly Ni/N-rGO/Mo₂C, was tested overextended HER operation, showing sustained current densities and reduced degradation. Electrochemical impedance spectroscopy (EIS) revealed that Ni/N-rGO/Mo₂C exhibited the lowest charge-transfer resistance, facilitating efficient electron transfer, which likely contributes to its superior durability. The incorporation of heteroatoms such as N and S further enhanced performance by preventing the adverse conductivity effects associated with Mo₂C, thereby maintaining high catalytic activity. Collectively, these results indicate that Ni-Mo₂C composites could be considered as promising catalysts to be used as cathodes in electrolysers.