Ordered Mesoporous Metastable alpha-MoC1-x with Enhanced Water Dissociation Capability for Boosting Alkaline Hydrogen Evolution Activity

Abstract

The sluggish reaction kinetics of the alkaline hydrogen evolution reaction (HER) remains an important challenge for water-alkali electrolyzers, which originates predominantly from the additional water dissociation step required for the alkaline HER. In this work, it is demonstrated theoretically and experimentally that metastable, face-centered-cubic alpha-MoC1-x phase shows superior water dissociation capability and alkaline HER activity than stable, hexagonal-close-packed Mo2C phase. Next, high surface area ordered mesoporous alpha-MoC1-x (MMC) is designed via a nanocasting method. In MMC structure, the alpha-MoC1-x phase facilitates the water dissociation reaction, while the mesoporous structure with high surface area enables a high dispersion of metal NPs and efficient mass transport. As a result, Pt nanoparticles (NPs) supported on MMC (Pt/MMC) show substantially enhanced alkaline HER activity in terms of overpotentials, Tafel slopes, mass and specific activities, and exchange current densities, compared to commercial Pt/C and Pt NPs supported on particulate alpha-MoC1-x or beta-Mo2C. Notably, Pt/MMC shows very low Tafel slope of 30 mV dec(-1), which is the lowest value among the reported Pt-based alkaline HER catalysts, suggesting the critical role of MMC in enhancing the HER kinetics. The promotional effect of MMC support in the alkaline HER is further demonstrated with an Ir/MMC catalyst.