Hydrogen evolution catalysis by molybdenum sulfides (MoSx): are thiomolybdate clusters like [Mo3S13]2- suitable active site models?

Molybdenum sulfides are highly active hydrogen evolution reaction (HER) catalysts based on earth-abundant elements. In their most active forms, amorphous solid-state MoSx show HER activity comparable to noble metals. Due to the amorphous structure of the catalysts, insights into the reaction mechanism, the nature of the catalytic site(s) and catalyst deactivation are difficult to obtain. This perspective summarizes recent developments in our understanding of molybdenum sulfide HER catalysis and explores whether molecular molybdenum sulfido clusters - so-called thiomolybdates - are suitable models to study catalytic processes of Mo-S compounds. Further, the perspective raises fundamental questions relating to the reactivity, degradation and repair of thiomolybdate HER catalysts and discusses whether lessons can be learned from related moieties, in particular the iron molybdenum cofactor (FeMoco) in the enzyme nitrogenase.