Asymmetric Coupling Induces Spin-Flipping in NiFe-MOF Cocatalyst for Boosting BiVO4 Photoanode Water Oxidation

Water splitting via photoelectrochemical (PEC) system is a promising approach for sustainable hydrogen production. Yet, photogenerated charge accumulation and recombination impede it by water oxidation kinetics due to not only four-electron transfer process but also the oxygen spin state change. Here, a modulation of spin state in NiFe-based metal-organic framework (NiFe-MOF) cocatalyst is employed on BiVO4 photoanodes to accelerate interface kinetics. X-ray magnetic circular dichroism combined with spin-polarized density functional theory calculations reveal that Fe incorporation induces a spin-state transition of the Ni dz2 orbital (dz2↓ to dz2↑) through asymmetric Fe-O-Ni bond coupling within the MOF nodes. The resulting downward shift of the Ni dz2 energy level weakens the adsorption of oxygenated intermediates (*OH/*OOH) on Ni sites, thereby facilitating *OOH desorption and O2 release during oxygen evolution. Consequently, the NiFe-MOF/BiVO4 photoanodes exhibit a significantly enhanced PEC performance, delivering a photocurrent density of 6.0 mA cm-2 at 1.23 V versus Reversible Hydrogen Electrode (RHE) under simulated sunlight. This work highlights asymmetric metal–oxygen bond-driven electron spin engineering as an effective strategy to boost PEC activity.