Role of Hyper-Reduced States in Hydrogen Evolution Reaction at Sulfur Vacancy in MoS₂

Using the multiscale simulation combining ab initio calculations and kinetic Monte Carlo (KMC) simulations, we theoretically investigate the hydrogen evolution reaction (HER) on the sulfur vacancy of a MoS2 monolayer. Unlike metal catalysts, the protonation step and the charging step proceed independently in semiconducting MoS2. Interestingly, the barrier for hydrogen evolution decreases when the vacancy site is hyper-reduced with extra electrons. The turnover frequency and polarization curve obtained from the KMC simulation agree well with extant experimental data, and the major HER paths underscore the role of hyper-reduced states, particularly when the overpotential is applied. The strain effect is also simulated, and it is found that the tensile strain enhances HER by reducing the energy cost of hyper-reduced states. The estimated reduction in the overpotential agrees favorably with the experiment while the hydrogen binding energy alone cannot account for it, suggesting that the full-blown KMC simulation should be used to accurately predict the variation of HER performance under various conditions. By uncovering the nature of the catalytic reaction at the sulfur vacancy of MoS2 and revealing a design principle in which the facile formation of hyper-reduced states plays an important role, the present work will pave the way for developing HER catalysts that may replace Pt.