Charge Transfer Mediators in Photoelectrochemical Water Splitting: From Interfacial Regulation to Rational Design.

Photoelectrochemical (PEC) water splitting is a promising route for solar-to-chemical energy conversion and storage, yet its efficiency remains limited by interfacial charge recombination, inefficient carrier extraction, and sluggish reaction kinetics. Charge transfer mediators have emerged as critical interfacial components bridging photoelectrodes, cocatalysts, and conductive substrates, enabling directional carrier transport, suppressed recombination, and enhanced catalytic activity. However, their roles, design principles, and mechanisms in PEC systems remain insufficiently understood. This review systematically defines charge transfer mediators in PEC water splitting, clarifies their multifunctional roles in charge transfer, charge storage, and interfacial protection, and summarizes representative material platforms, including carbon-based materials, MXenes, metal oxides, molecular materials, nonmetal nitrides, and other emerging mediators. Particular emphasis is placed on the relationship between mediator structures, interfacial configurations, electronic properties, and PEC performance. Advanced characterization techniques and theoretical approaches for elucidating charge transfer pathways and reaction mechanisms are also discussed to establish structure-property-performance correlations. Finally, key challenges and future perspectives are highlighted, positioning charge transfer mediators as an emerging paradigm for constructing efficient, stable, and practical PEC systems.