Crystalline-Amorphous Ni₂P₄O₁₂/NiMoO_x 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_x nanoarrays supported on nickel foam (c-Ni₂P₄O₁₂/a-NiMoO_x/NF) for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution is reported. The c-Ni₂P₄O₁₂/a-NiMoO_x/NF can deliver a current density of 10 mA cm^–2 at a low potential of 78 mV for HER, and a current density of 20 mA cm^–2 at an overpotential of 250 mV for OER. Moreover, it only requires a small cell voltage of 1.55 V at 10 mA cm^–2 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-Ni₂P₄O₁₂/a-NiMoO_x is transformed into c-Ni₂P₄O₁₂/a-NiMoO₄ during the HER process, and into c-Ni₂P₄O₁₂/a-NiOOH in the OER process. This work may provide a new strategy for uncovering the catalytic mechanism of crystalline-amorphous catalysts.