Interfacial Interactions as an Electrochemical Tool To Understand Mo-Based Catalysts for the Hydrogen Evolution Reaction

Crystalline and amorphous transition-metal chalcogenides such as MoS2 are currently recognized as state of the art non-precious transition metal catalysts for the hydrogen evolution reaction (HER). Nevertheless, despite numerous studies dedicated to their electrocatalytic activities, the exact nature of the active sites and their interaction with interfacial water remain largely elusive. In this work, amorphous and crystalline MoS2 catalysts were prepared by electrodeposition and chemical exfoliation, respectively, and compared with other Mo-based compounds. Herein, we show that all of these compounds exhibit two reduction mechanisms in low proton concentration: proton reduction occurs at low overpotential followed by water reduction at higher overpotential. We show that both the chemical composition and the structure of the catalyst influence the activity of the proton reduction but that none of those materials efficiently catalyze water reduction. Finally, we demonstrate by using different cations (Li+, Na+, and K+) or using deuterated electrolytes that the active sites for the proton reduction mechanism are probably different for amorphous and exfoliated crystalline MoS2.