Role of Oxidized Mo Species on the Active Surface of Ni–Mo Electrocatalysts for Hydrogen Evolution under Alkaline Conditions

A Ni−Mo composite functions as a promising non-noble metal electrocatalyst for \nthe hydrogen evolution reaction (HER) in alkaline water. Despite its industrial relevance, the \nkinetic origin of the high catalytic activity remains under debate. The present report discusses a \nreaction mechanism of HER on Ni−Mo catalysts by combining experimental and theoretical \nstudies. In contrast to a Ni catalyst, a Ni−Mo catalyst is insensitive to CO gas introduced during \nHER. In situ spectroscopic measurements including Raman spectroscopy and electron \nparamagnetic resonance (EPR) show that Mo3+ prevails during HER catalysis. Density functional \ntheory (DFT) simulations corroborate the thermodynamic stability and HER activity of Mo3+- \ncontaining centers on Ni(111) at HER potentials. Notably, Ni is demonstrated to play no direct role as a catalytic site but to \neffectively disperse and activate the oxidized catalytic Mo species. The results illustrate how to improve the electrocatalytic activity \nfor alkaline HER.