Amorphous Nickel Hydroxide Shell Encloses the Nickel Sulfide for Electrochemical Water Oxidation at Low Overpotential.

Despite the remarkable hydrogen evolution reaction (HER) performance demonstrated by transition metal sulfides, their practical application in overall water splitting remains constrained by insufficient oxygen evolution reaction (OER) activity, particularly under alkaline conditions. This limitation primarily arises from unstable active sites and sluggish reaction kinetics at high oxidation potentials. To address these challenges, we present a novel heterostructure engineering strategy through gradient phase control. The core innovation of this work lies in the construction of a unique amorphous/crystalline heterointerface through precise phase modulation. Our hierarchical architecture features phase-mixed nickel sulfides of NiS and NiS (use NiS abbreviation) encapsulated by an amorphous Ni(OH) shell. In which the NiS provides rapid electron transport channels, and the amorphous shell serves as both protective and activation functions through dynamically reconfigurable surface coordination. As expected, the synthesized NiS/Ni(OH) catalyst achieved excellent performance, it achieves a current density of 100 mA cm⁻ at only 160 mV overpotential, significantly outperforming its precursor materials, Ni(OH)₂ (446 mV) and NiS (212 mV). More significantly, the catalyst demonstrates exceptional operational stability with < 2% activity decay during prolonged 200-h continuous electrolysis. This provides a new way to commercialize non-precious metal electrocatalysts instead of precious metal catalysts.