Rational Design of NiFeO_X and Cobalt-Integrated Hydroxyapatite Nanoarchitecture on BiVO₄ Photoanode for Enhanced and Durable Solar to Water Oxidation

Solar-driven photoelectrochemical (PEC) water splitting offers a sustainable route to solar-to-fuel conversion; however, its practical application is hindered by sluggish oxygen evolution reaction (OER) kinetics and severe surface charge recombination. BiVO₄ (BVO) is a promising photoanode material but suffers from inefficient charge separation and limited operational stability. Herein, we report a dual-catalyst strategy in which cobalt-incorporated hydroxyapatite (Co-HAP) is synergistically integrated with NiFeO_X nanosheets to simultaneously enhance OER activity and durability. The optimized BVO/NiFeO_X/Co-HAP photoanode achieves a photocurrent density of 6.36 mA·cm⁻² at 1.23 V vs reversible hydrogen electrode (RHE) under AM 1.5G illumination in neutral phosphate electrolyte, nearly sevenfold higher than pristine BVO. Comprehensive analyses reveal that the NiFeO_X/Co-HAP nanosheet framework promotes efficient interfacial charge extraction, establishes a favorable surface electric field, and suppresses hole-electron recombination. The 2D nanoarchitecture provides abundant Co active sites, while interfacial NiFeO_X accelerates hole extraction and facilitates electron transfer from Ni to V sites, thereby mitigating V⁵⁺ dissolution. Notably, the photoanode demonstrates extended operational stability of ≈120 h in a phosphate electrolyte. This work highlights a robust design strategy that leverages the synergistic ion-exchange capacity of HAP and the charge-extraction ability of NiFe catalysts to advance efficient and durable PEC water-splitting systems.