Unveiling the In Situ Dissolution and Polymerization of Mo in Ni4 Mo Alloy for Promoting the Hydrogen Evolution Reaction.

NiMo alloys are efficient electrocatalysts in alkaline water electrolyzer for the hydrogen evolution reaction (HER). Metals are usually considered to be stable during the cathodic process. However, the actual behaviors of Mo in the NiMo alloys are unexplored. Here, we present the instability of Mo in the Ni4 Mo alloy as a highly efficient HER electrocatalyst in an alkaline medium. Mo in Ni4 Mo is oxidized and dissolved in the form of MoO4 2- first. The dissolved MoO4 2- will re-adsorb on the electrode surface and polymerize. Theoretical calculations indicate that the adsorption of the dimer Mo2 O7 2- can promote the HER activity of metal Ni. The addition of MoO4 2- to the electrolyte can not only repair the durability of Ni4 Mo alloy, but also facilitate the HER activity of pure metal of Ni, Fe, and Co. Our findings provide insight into the structural transformation mechanism and performance-enhanced origin of cathodic materials under the reaction conditions.