Dynamic surface reconstruction governs the hydrogen evolution activity of Mo₂C electrocatalysts in alkaline media

Molybdenum carbide (Mo₂C) has emerged as an earth-abundant catalyst for the hydrogen-evolution reaction (HER), yet the impact of surface-oxidized species on its performance remains unknown. Here, we compare the activity of pristine Mo₂C with a Mo/Mo₂C heterostructure synthesised by carbothermal reduction and evaluate their structural evolution under working conditions using in situ Mo K-edge X-ray absorption spectroscopy and Raman spectroscopy complemented by density functional theory (DFT). Despite its metallic component, Mo/Mo₂C delivers a lower HER activity (204 mV at 10 mA cm^-2) than Mo₂C (117 mV at 10 mA cm^-2). Spectro-electrochemical studies reveal that both catalysts oxidise to tetra-oxo (MoO₄)^2- motifs during operation, but the transformation is faster and more extensive in the case of Mo/Mo₂C. EXAFS analysis reveals that Mo₂C stabilises a defect-rich MoO_x 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 bestow 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, while excessive oxidation attenuates binding and retards the Volmer-Heyrovsky step. Thus, we identify a controllable, self-limited surface reconstruction step, rather than the metallic Mo constituent, as the key performance descriptor, guiding the design of stable carbide-based catalysts for alkaline water electrolyser technologies.