Mechanistic Role of Two-State Reactivity in a Molecular MoS2 Edge-Site Analogue for Hydrogen Evolution Electrocatalysis.

We report a first-principles quantum chemical study of the mechanistic pathways for the hydrogen evolution reaction (HER) by the molecular electrocatalyst [(PY5Me2)Mo(S2)]2+. By determining the relative thermodynamics of many possible species, we propose a mechanism fully consistent with all experimental observations. We also show the presence of two close-lying spin surfaces with the high spin state having a slightly less favorable reactivity profile than the low spin state. The energy of the high spin state is related to the ease of reduction of the S2 moiety and can be disrupted by interaction between S2 and a Lewis base. From this understanding, an explanation for the nearly 400 000-fold increase in turnover frequency on Hg drop electrode compared to glassy carbon is demonstrated. A next-generation catalyst based on the same motif has been designed to stabilize the more reactive low spin state and improve catalytic function without the need of Hg. Calculations indicate that this new species would have greatly improved HER reactivity and operate at a similar overpotential as the original system.