Polyoxometalate hole reservoir unlocks visible-light-driven overall water splitting with a single photocatalyst via...

Visible-light-driven overall water splitting is a promising route to solar hydrogen production, but achieving it with a single photocatalyst remains difficult. Many visible-light-responsive photocatalysts can drive either H₂ or O₂ evolution in the presence of suitable electron donors or acceptors, yet fail to drive overall water splitting. Rhodium-doped SrTiO₃ (SrTiO₃ : Rh) is a representative visible-light-responsive photocatalyst for H₂ evolution. However, it does not achieve overall water splitting because of its limited ability to oxidize water to O₂, although its conduction-band and Rh-donor levels are thermodynamically suitable for H₂ and O₂ evolution, respectively. Here, we introduce the concept of a "hole reservoir", defined as an aqueous redox couple that captures photogenerated holes, stores them at potentials more positive than the water oxidation potential, and subsequently delivers them to a water oxidation catalyst to drive O₂ evolution. Unlike conventional Z-scheme systems, which couple H₂- and O₂-evolution photocatalysts through a redox mediator, this design eliminates the need for photoexciting an O₂-evolution photocatalyst; instead, water oxidation is catalytically driven by the oxidizing equivalents accumulated within the hole reservoir. We identified [SiW₁₁O₃₉Co^III/II(H₂O)]^5-/6- (SiCo^III/IIW₁₁) polyoxometalate as an effective hole reservoir that couples H₂ evolution on SrTiO₃ : Rh with RuO₂-catalyzed water oxidation. This strategy enables steady and stoichiometric overall water splitting in a CrO _x /Rh/SrTiO₃ : Rh-RuO₂ suspension as well as simultaneous H₂ and O₂ evolution over RuO₂/Ru/SrTiO₃ : Rh, thereby establishing a new framework to convert visible-light-responsive photocatalysts that intrinsically drive only a single half-reaction into overall water-splitting systems.