Crystalline-Amorphous Ni P O /NiMoO Nanoarrays for Alkaline Water Electrolysis: Enhanced Catalytic Activity via In Situ Surface Reconstruction.

Water electrolysis affords a promising approach to large-scale hydrogen yield, but its efficiency is restrained by the sluggish water dissociation kinetics. Here, an efficient bifunctional electrocatalyst of in situ formed crystalline nickel metaphosphate on amorphous NiMoO nanoarrays supported on nickel foam (c-Ni P O /a-NiMoO /NF) for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution is reported. The c-Ni P O /a-NiMoO /NF can deliver a current density of 10 mA cm at a low potential of 78 mV for HER, and a current density of 20 mA cm at an overpotential of 250 mV for OER. Moreover, it only requires a small cell voltage of 1.55 V at 10 mA cm for robust water splitting with outstanding long-term durability over 84 h. Various spectroscopic studies reveal that in situ surface reconstruction is crucial for the enhanced catalytic activity, where c-Ni P O /a-NiMoO is transformed into c-Ni P O /a-NiMoO during the HER process, and into c-Ni P O /a-NiOOH in the OER process. This work may provide a new strategy for uncovering the catalytic mechanism of crystalline-amorphous catalysts.