Microwave‐Induced Plasma Disproportionation Enables NiO@Ni–Carbon Heterostructures With Accelerated γ‐NiOOH Formation for Efficient Overall Water Splitting

ABSTRACT NiO‐shelled Ni‐core (NiO@Ni) composites with tunable NiO/Ni ratios, in the presence or absence of conductive carbon supports, are widely recognized as low‐cost and earth‐abundant bifunctional electrocatalysts for overall water splitting. However, their practical performance remains constrained by sluggish formation of catalytically active γ‐NiOOH phase and uncontrolled distribution of active sites. Herein, we report a highly efficient bifunctional electrocatalyst consisting of NiO@Ni with an optimized bulk NiO/Ni ratio anchored on biowaste‐derived porous graphitic carbon (PGC), denoted as PGC–NiO@Ni, synthesized via a single‐step microwave (MW)‐induced N ₂ plasma‐assisted disproportionation strategy. During this process, Ni salts undergo simultaneous oxidation and reduction to NiO and metallic Ni, respectively, with the NiO/Ni ratio regulated by MW irradiation time, power, and precursor stoichiometry. The optimized NiO/Ni ratio (0.75) enhances population and spatial distribution of active sites, while the conductive PGC support functions as an efficient electronic sink that mitigates surface charge accumulation on semiconducting NiO, thereby promoting rapid and unimpeded γ‐NiOOH formation. Consequently, it delivers 195 mV at 100 mA cm ⁻² for the oxygen evolution reaction (OER) and 244 mV at 100 mA cm ⁻² for the hydrogen evolution reaction (HER), while a two‐electrode electrolyzer operates at 1.50 V with 165 h stability and 98% Faradaic efficiency.