Trends in Alkaline Hydrogen Evolution Activity on Cobalt Phosphide Electrocatalysts Doped with Transition Metals

Understanding the mechanism of the alkaline hydrogen evolution reaction (HER) and finding a design principle to enhance the catalytic performance of electrocatalysts are of great significance in paving the way of alkaline water electrolysis. Here, taking cobalt phosphide (CoP) as a model material, we establish the alkaline HER activity trends as a function of a fundamental property, the proportion of the unoccupied 3d orbital (P-un). By doping CoP with a range of 3d and 4d transition metals with different P-un, we show that the intrinsic alkaline HER kinetics increases with P-un. DFT calculations indicate that the transition metals with higher P-un not only act as the oxophilic sites for enhanced water activation but also modulate the electronic structure of CoP to endow an optimized H adsorption (Delta G*(H)), Moreover, by further comparing the correlation between water adsorption and dissociation or Delta G*(H) and experimental alkaline HER activities, we find that water dissociation is the rate-determining step for alkaline HER.