Heterometallic FeCoNi Metal–Organic Framework Integrated BiVO ₄ for Boosted Photoelectrocatalytic Water Oxidation

Bismuth vanadate (BiVO4) is a promising photoanode material for photoelectrochemical (PEC) water splitting due to its suitable band gap, favorable band edge positions, and high theoretical efficiency. However, its practical application is limited by poor charge separation efficiency, sluggish hole transport, and slow oxygen evolution reaction (OER) kinetics. Metal–organic frameworks (MOFs), particularly heterometallic variants, have garnered attention as cocatalysts owing to their high surface area, uniform distribution of active sites, and tunable electronic structures. In this work, a trimetallic MOF cocatalyst (FeCoNi-ZIF) was synthesized using 2-methylimidazole (2-MIM) as the organic linker and directly grown in situ on BiVO4 to improve interfacial charge transfer and OER kinetics. The polymetallic synergy in FeCoNi-ZIF enhances exposure of active sites and optimizes intermediate adsorption energies. To further boost hole transport, a NiO hole transport layer (HTL) was introduced, forming a ZIF-NiO-BiVO4 composite photoanode. This dual-modified photoanode achieves an elevated photocurrent density of 4.48 mA cm–2 at 1.23 V vs RHE and sustains stable performance over 30 h. The formation of a p–n heterojunction between NiO and BiVO4 facilitates hole extraction, while the FeCoNi-ZIF cocatalyst ensures efficient surface catalysis. These results demonstrate the significant synergistic effects of the FeCoNi-ZIF and NiO HTL modifications in overcoming intrinsic limitations of BiVO4 for efficient and durable PEC water oxidation.