Boosting Z-Scheme-Type Water Splitting by Controlling FeIII/FeII Redox Potential in Metal Hexacyanoferrate Modifiers Loaded on H₂-Evolving Photocatalyst

Promoting the oxidation and reduction processes in Z-scheme-type water splitting is crucial for improving the overall efficiency. However, boosting the oxidation and reduction of the redox mediator has not been thoroughly explored. We recently showed that certain metal hexacyanoferrates (MHCFs) loaded on the surface of photocatalysts can enhance the oxidation of various redox mediators via FeIII/FeII redox cycles in MHCFs. In this study, given that the redox potentials of FeIII/FeII in MHCFs vary with different M cations, we aimed to demonstrate the effectiveness of controlling the redox potentials of MHCF modifiers and polyoxometalate (POM) redox couples in promoting redox oxidation and the resultant hydrogen (H2) evolution. The modification with MHCF significantly increased the H2 evolution rates as the redox potential difference between MHCF and POM widened, indicating that the growing driving force for electron injection from POM to MHCF is key to enhancing H2 evolution.