A highly active and durable hybrid Ni/NiOOH catalyst by synergistic high-temperature deposition and electrochemical...

Constructing a heterointerface has become a preferred strategy for the hydrogen evolution reaction (HER) due to the synergistical H₂O dissociation and *H adsorption. Ni/Ni(OH)₂ hybrid catalysts with an isogenous heterointerface have exhibited great potential in the alkaline HER. However, designing high performance Ni/Ni(OH)₂ and understanding the catalytic mechanisms still remains challenging. Herein, we demonstrate that the HER performance of Ni/Ni(OH)₂ depends significantly on the interface density and deprotonation. Experimentally, Ni and Ni(OH)₂ grains are refined to enlarge the interface density at elevated temperature, and the activity and stability are rationally tuned by delicately regulating deprotonation at various oxidization potentials. Theoretical calculations reveal that the deprotonation energy decreases with grain refinement, which promotes the interface electron redistribution. The deprotonation lowers the H₂O dissociation energy and alleviates *H adsorption, but the excessive deprotonation leads to strong *OH adsorption, retarding H₂O dissociation, whereas the stability is enhanced. The optimum Ni/Ni(OH)₂ hybrid catalyst reaches an outstanding HER performance with an overpotential of 30 mV@10 mA cm^-2 and stable activity for over 300 hours at an extremely large current density (2.0 A cm^-2), surpassing most of the reported HER catalysts. This work initiates a new pathway to improving catalytic performance by regulating the interface density and valence state.