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Amorphous MoS₃ (a-MoS₃) 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 Mo₃S_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 Mo₃S₉ cluster can be removed as H₂S under HER conditions. This partial desulfurization leads to a stable working state of Mo₃S₈ or Mo₃S₇ with HER catalytic activity at moderate thermodynamic overpotentials. The desulfurization process simultaneously induces water adsorption on undercoordinated molybdenum sites. The so-formed hydrated Mo₃S_9–x-clusters can exhibit two distinct active sites. On Mo₃S₈(H₂O)₂, the top SH* species are active for …