Controlling the BiOI-to-BiVO ₄ Transformation for High-Performance Photoanodes and Monolithic BiVO ₄ –Cu ₂ ZnSnS ₄ Tandem Water Splitting

Photoelectrochemical (PEC) water splitting is a promising route for direct solar-to-hydrogen (STH) conversion efficiency, but its efficiency is limited by the performance of available photoanodes. Bismuth vanadate (BiVO₄), with its ideal ∼2.4 eV bandgap for visible-light absorption, is a leading candidate, yet its practical application is constrained by severe charge carrier separation and sluggish charge transfer kinetics. To this end, we engineered an optimized BiVO₄ photoanode by precisely controlling the electrodeposition pathway of the BiOI precursor. This singular parameter governed the formation of compact, uniform BiVO₄ films upon annealing, yielding superior PEC performance via a maximized charge separation yield and enhanced surface kinetics. As a result, the FTO/BiVO₄/CoPi photoanode achieved a photocurrent density of 5.51 mA cm^-2 at 1.23 V_RHE, and an applied bias photon-to-current efficiency (ABPE) of 1.58%. Harnessing this optimized photoanode, we constructed a bias-free tandem device by integrating it with a Cu₂ZnSnS₄ (CZTS) photocathode. This integrated system achieved a solar-to-hydrogen (STH) conversion efficiency of 2.10% with an operating current density of 1.71 mA cm^-2. This work provides a viable strategy for developing efficient and stable PEC systems for solar hydrogen production.