Enhanced Photoelectrochemical Water Splitting Performance of BiVO₄ Photoanode by Integrating Electron-Rich Polyoxometalate into Metal–Organic Framework

Bismuth vanadate (BiVO4) photoanodes face challenges such as rapid electron–hole recombination, slow water oxidation kinetics, and poor surface stability. To tackle these problems, this study incorporates a Keggin-type polyoxometalate (POM) anion, characterized by a high negative charge and the absence of precious metals, into the MIL-101(Cr)/BiVO4 composite photoanode. The POM anion effectively enhances charge separation and injection within the BiVO4 photoanode under minimal optimization conditions by separating photogenerated holes from the MIL-101 framework. The NiPOM/MIL-101(Cr)/BiVO4 photoanode achieves a photocurrent density of 4.5 mA cm–2 at 1.23 V vs reversible hydrogen electrode (RHE), approximately 4 times higher than that of pristine BiVO4. Furthermore, the oxygen evolution reaction yield of the photoanode reaches 36.2 μmol after 90 min of reaction, with faradic efficiencies consistently exceeding 84%, indicating the predominant involvement of photogenerated charge carriers in photoelectrochemical (PEC) water splitting. At an applied potential of 1.23 V vs RHE, the applied bias photon-to-current efficiency (ABPE) reaches 1.16%. The charge transfer mechanism of the photochemical process reveals that the uniform distribution of the NiPOM cocatalyst within the hydrophilic zeolite cage of MIL-101(Cr) promotes the formation of additional chemically active sites and facilitates electron transfer from its Ni site to the BiVO4 surface, significantly boosting photocurrent density. This mechanism effectively suppresses the dissolution of V5+ ions and enhances the oxidation stability during photoelectrochemical water decomposition. This work presents a novel strategy for designing POM-based molecular synergistic catalysts to enhance the efficiency and stability of PEC water splitting, thereby facilitating the efficient conversion of solar energy to clean hydrogen fuel.