Impact of thermal annealing on the physicochemical and electrocatalytic properties of molybdenum carbide thin films for hydrogen evolution

Presence of Mo 2 C and Mo 5+ on the surface of thin film electrocatalysts enhances HER compared to having oxides and metallic Mo on the surface. Engineering the surface to have abundant HER promoting species is the way to enhance Mo 2 C’s role in production of green hydrogen. • Ambient exposure before annealing drives beneficial Mo 2 C phase transformations. • In-situ annealing is not always best at suppressing surface oxides. • Optimized bulk-surface coupling directly improves HER performance. • Mixed-valence Mo 5+ surface states facilitate faster electron–proton coupling. Molybdenum carbide (Mo 2 C) is a promising, cost-effective alternative to platinum for scalable hydrogen production through the hydrogen evolution reaction (HER), owing to its favourable electronic structure. To fabricate Mo 2 C thin films with enhanced catalytic properties, a comprehensive understanding of the interplay between bulk and surface properties is essential. Here, we present a systematic study on the effect of magnetron sputtering deposition and post-deposition thermal annealing conditions (in-situ and ex-situ) on the crystal structure and electrocatalytic efficiency of Mo 2 C thin films for HER. By combining structural, surface, and electrochemical analyses, we identified significant variations in bulk phase evolution and surface composition that directly affect HER efficiency. Pulsed DC sputtered films that were exposed to ambient conditions prior to ex-situ annealing consistently demonstrated superior HER efficiency, achieving a low overpotential of 280 mV at a current density of 10 mA/cm 2 and a Tafel slope of 87 mV / dec. These values significantly outperform the non-annealed pulsed DC films (346 mV, 91 mV/dec) and the RF sputtered in-situ annealed films (387 mV, 117 mV / dec). The enhanced efficiency of the ex-situ annealed pulsed DC films is attributed to a more favourable synergistic combination: bulk crystallization of the catalytically active Mo 2 C and metallic Mo (enhancing conductivity) coupled with a surface enriched in beneficial mixed valence Mo 5+ states (mediating electron transport). In contrast, the in-situ annealed RF films exhibited poor bulk crystallinity and a detrimental surface composition dominated by insulating Mo 6+ . These results emphasize the crucial importance of considering both bulk crystallization pathways and the resulting surface oxidation states when optimizing carbide-based catalysts, providing practical insights for the rational design of efficient electrodes for hydrogen evolution.