Dynamic Surface Reconstruction Governs Hydrogen Evolution Activity of Mo2C Electrocatalysts in Alkaline Media

Abstract

Molybdenum carbide (Mo₂C) has emerged as earth-abundant candidates for hydrogen-evolution reaction (HER) catalysts, yet the impact of surface oxidized species on their performance remains unsettled. Here, we compare the activity of pristine Mo₂C with a Mo/Mo₂C heterostructure synthesised by carbothermal reduction and follow their structural evolution under working conditions by in situ Mo K-edge X-ray absorption spectroscopy and Raman spectroscopy complemented by density-functional theory (DFT). Despite its metallic component, Mo/Mo₂C (204 mV@10 mA cm⁻²) delivers lower HER activity than Mo₂C (117 mV@10 mA cm⁻²). Spectro-electrochemical studies reveal that both catalysts oxidise toward tetra-oxo (MoO₄)²⁻ motifs during operation, but the transformation is faster and more extensive in Mo/Mo₂C. EXAFS reveals that Mo₂C stabilises a defect-rich MoOₓ layer resembling MoO₂, contributing to the enhanced HER activity while Mo/Mo₂C undergoes pronounced oxidative transformation that depletes the active sites. The in situ formed and regenerable active species from surface reconstructed Mo₂C@MoO_2−x bestows the catalyst with high activity. DFT calculations indicate that the reconstructed Mo₂C@MoO_2−x optimises the Gibbs free energy of hydrogen adsorption by preserving moderate Mo-H binding, whereas excessive oxidation attenuates binding and retards the Volmer-Heyrovsky steps. Thus, we identify controllable, self-limited surface reconstruction, rather than a metallic Mo constituent as the key performance descriptor, guiding the design of stable carbide-based catalysts for alkaline water electrolyser technologies.