Regulating the Helmholtz layer through nickel-doped molybdenum dioxide surface fixation of hydrated sodium ions

One of the challenges in hydrogen electrocatalysis is the decrease in reaction kinetics when moving from acidic electrolytes to basic ones. Herein, we demonstrate that molybdenum dioxide (MoO₂) can serve as a pH-universal catalyst towards the hydrogen evolution reaction (HER) when doped with Ni ions (Ni_0.34Mo_0.66O₂). Ni, as the most electronegative transition metal, effectively activates surface O ions by increasing their proton affinity once Ni displaces Mo at lattice sites. This activation effect is more remarkable in alkaline electrolytes due to the higher surface electron density, which leads to a higher hydrogen coverage. As a result, a lower barrier for hydrogen desorption is found under alkaline condition. The larger proton affinity of surface O sites under alkaline conditions also induces chemical adsorption of hydrated Na⁺ ions, which enhances water connectivity and accelerates proton transfer kinetics. Ni_0.34Mo_0.66O₂ exhibits a remarkable enhancement in HER kinetics under alkaline conditions.