Boosting Z-scheme water splitting via increasing electron transport by manipulating multiple redox-active sites and potentials in metal hexacyanoferrate modifiers

Abstract

Metal hexacyanoferrates (MHCFs) are attractive cocatalysts for Z-scheme water splitting owing to their tunable redox properties at the C-coordinated Fe^III/Fe^II sites, enabling efficient electron transfer from shuttle redox mediators. MHCFs can incorporate multiple transition metal centers through ambidentate cyanide ligands; however, rational design principles for utilizing redox-active metals at the N-coordinated sites remain unclear. Herein, we show that photocatalytic H₂ evolution is governed by the relative alignment of redox potentials of MHCF modifiers and aqueous electron donors. A series of MHCFs comprising redox-active metals (i.e., Mn, Fe, Co, Cu) and redox-inactive metals (i.e., In, Ni) were loaded on Rh-Cr mixed-oxide-modified TaON, and their electrochemical properties were correlated with photocatalytic H₂ evolution using electron donors with different electron-donating ability. MHCFs containing redox-active metal species with redox potential significantly more negative than those of the electron donors (e.g., the N-coordinated Fe^III/Fe^II in FeHCF, and Cu^II/I in CuHCF) suppressed photocatalytic activity due to backward electron transfer (reduction via photoexcited electrons), whereas MnHCF, with both Mn^III/II and Fe^III/Fe^II potentials more positive than the donors, exhibited the highest activity. Furthermore, the incorporation of multiple metals in a Mn-based high-entropy MHCF (K₂Mn_0.4Fe_0.15Co_0.15Ni_0.15Cu_0.15[Fe(CN)₆]) improved durability while maintaining appropriate redox potentials, yielding a higher amount of gas evolution compared with MnHCF in Z-scheme water splitting. These findings provide a design strategy for multi-redox-active MHCF cocatalysts, highlighting that the optimal choice and tuning of metal sites can achieve both high efficiency and durability in water splitting systems.