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Abstract：Constructing heterointerface has been the preferential strategy for hydrogen evolution reaction (HER) due to the synergistical H 2 O dissociation and *H adsorption. Ni/Ni(OH) 2 hybrid catalyst with isogenous heterointerface have exhibited great potential in 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 interface density and deprotonation. Experimentally, Ni and Ni(OH) 2 grains are refined to enlarge the interface density at the elevated temperature, and the activity and stability are rationally tuned by delicately regulating deprotonation at varied oxidization potential. Theoretical calculations reveal that the deprotonation energy decreases with refining grains, which promotes the interface electron redistribution. The deprotonation lowers H 2 O dissociation energy and alleviates *H adsorption, but the excessive deprotonation leads to strong *OH adsorption, retarding H 2 O dissociation whereas the stability is enhanced. The optimum Ni/Ni(OH) 2 hybrid catalyst reaches to an outstanding HER performance of the overpotential of 30mV@10 mA. cm -2 and the activity stability for over 300 hours at the 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.
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