Activity Enhancement of Molybdenum Carbide in Alkaline Hydrogen Evolution Reaction through Oxidation-Gradient Modulation

Abstract

Featured by their Pt-like electronic structure, molybdenum carbides have been widely developed for efficiently catalyzing the hydrogen evolution reaction (HER). It is noteworthy that the oxophilicity of transition-metal atoms can give rise to the inevitable surface oxidation of molybdenum carbides, which has a noticeable impact on their HER activities. However, such a significant detail was usually documented in theory simulations and rarely explored by well-controlled experiments. Herein, advanced surface-science techniques using vacuum-connected setups are performed to deliberately prepare oxidation-gradient molybdenum carbide-oxide model electrocatalysts and evaluate the corresponding alkaline HER performance. The performance evaluations demonstrate that the minimal oxygen-modified Mo₂C exhibits the best alkaline HER activity among all model electrocatalysts. In situ XPS combined with quasi in situ XPS under different applied negative potentials reveals that tailoring the Mo₂C surface decorated with oxygen-containing species can facilitate the desorption of produced OH* intermediates from water activation, thus avoiding the deep oxidation issue of the catalyst surface and accelerating the regeneration of active sites in the alkaline HER process. Moreover, a comparable trend of HER performance is also observed on the synthetically practical Mo₂C powder catalysts, which further proves our hypothesis deduced from the model system. Our strategy of oxygen-terminated Mo₂C model electrocatalysts and the utilization of advanced spectroscopy characterizations may pave an interesting route for the rational design of low-cost but highly efficient oxygen-modified molybdenum carbide catalysts for practical water electrolysis.