Unveiling the In Situ Dissolution and Polymerization of Mo in Ni₄Mo 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.