Doping-Induced Enhancement of Hydrogen Evolution at MoS₂ Electrodes

Rate theory and DFT calculations of hydrogen evolution reaction (HER) on MoS₂ with Co, Ni and Pt impurities show the significance of dihydrogen (H₂*) complex where both hydrogen atoms are interacting with the surface. Stabilization of such a complex affects the competing Volmer-Heyrovsky (direct H₂ release) and Volmer-Tafel (H₂* intermediate) pathways. The resulting evolution proceeds with a very small overpotential for all dopants ( $$ =0.1 to 0.2 V) at 25 % edge substitution, significantly reduced from the already low $$ =0.27 V for the undoped edge. At full edge substitution, Co-MoS₂ remains highly active ( $$ =0.18 V) while Ni- and Pt-MoS₂ are deactivated ( $$ =0.4 to 0.5 V) due to unfavorable interaction with H₂*. Instead of the single S-vacancy, the site of intrinsic activity in the basal plane was found to be the undercoordinated central Mo-atom in threefold S-vacancy configurations, enabling hydrogen evolution with $$ =0.52 V via a H₂* intermediate. The impurity atoms interact favorably with the intrinsic sulfur vacancies on the basal plane, stabilizing but simultaneously deactivating the triple vacancy configuration. The calculated shifts in overpotential are consistent with reported measurements, and the dependence on doping level may explain variations in experimental observations.