Electrochemical Potential-Dependent Stability and Activity of MoS3 during the Hydrogen Evolution Reaction

Amorphous MoS 3 ( a -MoS 3 ) is an appealing low-cost catalyst for the hydrogen evolution reaction (HER), which is a promising process for electrocatalytic hydrogen generation. In this study, we scrutinize the stability and HER catalytic activity of carbon-supported Mo 3 S 9– x -clusters under electrochemical conditions by using grand-canonical density functional theory (GC-DFT) coupled with a cluster-continuum solvation strategy. We show that some sulfur atoms of the Mo 3 S 9 cluster can be removed as H 2 S under HER conditions. This partial desulfurization leads to a stable working state of Mo 3 S 8 or Mo 3 S 7 with HER catalytic activity at moderate thermodynamic overpotentials. The desulfurization process simultaneously induces water adsorption on undercoordinated molybdenum sites. The so-formed hydrated Mo 3 S 9– x -clusters can exhibit two distinct active sites. On Mo 3 S 8 (H 2 O) 2, the top SH* species are active for the HER, whereas OH* species are involved in the HER on Mo 3 S 7 (H 2 O) 3 . By comparison with a previous study of the HER catalyzed by 2H–MoS 2 edge sites, we demonstrate that S-defective a -MoS 3 is an efficient HER electrocatalyst. Moreover, in contrast to active sites on 2H–MoS 2, the HER mechanism on Mo 3 S 9– x -clusters involves a protonation step instead of the common proton-coupled electron transfer, an elementary reaction step that required GC-DFT to be identified.