Universal Interfacial Engineering via Amorphous Inorganic Binders: Passivating Surface States and Accelerating Hole Transfer across Metal Oxide Photoanodes in Photoelectrochemical Water Oxidation

Interfacial engineering of photoanodes is essential for efficient solar water splitting; however, many existing strategies rely on complex, multistep processes or hydrothermal growth, limiting scalability and reproducibility. Here, we present a universal interfacial engineering platform based on a sub-2 min acidic redox-assisted deposition (ARD) of an amorphous inorganic binder, cobalt-manganese oxyhydroxide (CMOH). Using two-step-fabricated BiVO₄ as a literature-standard benchmark, we demonstrate that an ultrathin CMOH overlayer markedly enhances photoelectrochemical (PEC) water oxidation performance. The conformal CMOH coating (∼3.5 nm) is deposited under ambient conditions via a simple dip-coating process and is broadly applicable to metal oxide photoanodes, including BiVO₄, WO₃, and ZnO. The CMOH-modified BiVO₄ photoanode delivers a high photocurrent density of ∼6.0 mA cm⁻² and a hole-transfer efficiency of ∼82% at 1.23 VRHE, together with a remarkable applied bias photon-to-current efficiency (ABPE) of 1.68% at 0.66 VRHE. These enhancements originate from effective surface-state passivation by the CMOH overlayer, leading to over a 4-fold reduction in the surface recombination rate constant, as revealed by intensity-modulated photocurrent spectroscopy (IMPS). In addition, the reduced Tafel slope (78.5 mV dec⁻¹) suggests that the CMOH layer promotes interfacial charge transfer and enhances the water oxidation kinetics of the BiVO₄ photoanode. This work elucidates the critical role of amorphous CMOH oxygen evolution catalysts in mediating photoinduced charge-carrier dynamics and establishes a facile and scalable strategy to broadly enhance the PEC performance of metal oxide photoanodes for large-scale solar fuel production.