Mechanism of Hydrogen Evolution Reaction on 1T-MoS2 from First Principles

The IT phase of transition-metal dichalcogenides (TMDs) has been demonstrated in recent experiments to display excellent catalytic activity for hydrogen evolution reaction (HER), but the catalytic mechanism has not been elucidated so far. Herein, using IT MoS2 as the prototypical TMD material, we studied the HER activity on its basal plane from periodic density functional theory (DFT) calculations. Compared to the nonreactive basal plane of 2H phase MoS2, the catalytic activity of the basal plane of IT phase MoS2 mainly arises from its affinity for binding H at the surface S sites. Using the binding free energy (Delta G(H)) of H as the descriptor, we found that the optimum evolution of H-2 will proceed at surface H coverage of 12.5% similar to 25%. Within this coverage, we examined the reaction energy and barrier for the three elementary steps of the HER process. The Volmer step was found to be facile, whereas the subsequent Heyrovsky reaction is kinetically more favorable than the Tafel reaction. Our results suggest that at low overpotential, HER can take place readily on the basal plane of IT MoS2 via the Volmer Heyrovsky mechanism. We further screened the dopants for the HER activity and found that substitutional doping of the Mo atom by metals such as Mn, Cr, Cu, Ni, and Fe can make IT MoS2 a better HER catalyst.