Surface-Engineered BiVO4 Photocatalyst Enables Selective Water Oxidation for Water Splitting via a Photocatalysis–Electrolysis Hybrid System

Selective water oxidation in the presence of thermodynamically favored competing oxidation reactions remains a major challenge for photocatalytic systems. BiVO4 is one of the most efficient visible-light-responsive photocatalysts for water oxidation and exhibits high activity in Fe3+-mediated water-oxidation systems. However, although accumulated Fe2+ can introduce competing oxidation pathways under strongly acidic Fe3+/Fe2+ redox conditions, effective strategies for suppressing these processes and maintaining selective water oxidation remain largely unexplored. Here, we demonstrate a surface-engineering strategy that overcomes this limitation in BiVO4. Systematic investigation revealed that the poor activity of BiVO4 under strongly acidic Fe3+/Fe2+ conditions arises not only from catalyst degradation but also from competition with Fe2+ oxidation reactions. Surface modification of BiVO4 with W species, originally intended to improve acid durability, not only suppressed catalyst dissolution but also mitigated Fe2+-related performance losses, resulting in a marked enhancement of photocatalytic water oxidation. The activity enhancement correlated with highly dispersed surface W species localized near the outermost region of BiVO4. Notably, the highest activity was obtained before or near the onset of detectable WO3 crystallization, suggesting that the surface state of W species in this temperature range is more closely associated with the enhanced activity than crystalline WO3 domains under strongly acidic Fe3+/Fe2+ conditions. The optimized photocatalyst maintained efficient water oxidation while suppressing Fe2+-related performance losses, achieving an apparent quantum yield of 63% under Fe3+-based conditions and approximately 30% even under strongly acidic and concentrated Fe(ClO4)3 conditions relevant to hybrid water splitting. The optimized photocatalyst maintained selective water oxidation even as Fe2+ accumulated to approximately 100 mM, enabling nearly quantitative Fe3+-to-Fe2+ conversion under strongly acidic conditions. When incorporated into a photocatalysis–electrolysis hybrid system, the enhanced photocatalytic activity was directly translated into approximately 1.5-fold higher water-splitting performance than that of the benchmark H-Fe-Cs-WO3 system. These findings demonstrate that selective water oxidation can be maintained even in the presence of thermodynamically favored competing oxidation reactions through appropriate photocatalyst surface engineering, providing a useful strategy for photocatalysts operating in redox-mediated reaction environments, including photocatalysis–electrolysis hybrid water-splitting systems.