Electrochemical cycling and beyond: unrevealed activation of MoO3 for electrochemical hydrogen evolution reactions.

We demonstrate electrochemical cycling-induced reduction of MoO3 to monoclinic molybdenum dioxide and molybdenum sub-oxides (MoO3-x), which exhibit excellent electrochemical hydrogen evolution reaction (HER) activity. The conversion of MoO3 during cycling was probed; after 250 cycles, the redox peaks were found to diminish with an onset potential shift and increased HER current density. At 400 cycles, the insertion/deinsertion processes observed in the initial cycles are completely absent and the HER current density is enhanced to the maximum. The effect of MoO3 morphology and size on the electrochemical reduction of MoO3 was also studied.