Mono-transition-metal-substituted polyoxometalates as shuttle redox mediator for Z-scheme water splitting under visible light

Because the majority of Z-scheme water splitting systems employ a shuttle redox mediator that allows electron transfer between two photocatalyst materials, the development of an effective redox mediator is crucial for improving the overall efficiency of water splitting reaction in Z-scheme systems. We recently revealed that some polyoxometalates (POMs) such as manganese-substituted Keggin-type silicotungstate ([SiW11O39MnIII/II(H2O)]5−/6−) can function as stable redox mediators. However, the limited number of POMs available for the Z-scheme system hinders the understanding of the fundamental properties of the employed POM. Herein, a series of vanadium-substituted POMs, i.e., [XVVW11O40]n−/[XVIVW11O40](n+1)− (X = B, Si, and P), and a cobalt-substituted POM [SiW11O39CoIII(H2O)]5−/[SiW11O39CoII(H2O)]6− were synthesized. Moreover, the fundamental properties, such as redox potential and chemical stability, of the POMs and their impact on water splitting photocatalysis efficiency were evaluated to provide guidance for designing efficient POM redox mediators. The substitution of constituent elements was found to affect their redox potentials, chemical stability under varied pH conditions, light absorption, and photocatalytic activity. All V-substituted POMs were found to possess an appropriate redox potential (i.e., between the potentials of water reduction and oxidation); however, their relatively high absorption in the visible light region appeared to be a disadvantage in terms of light shielding. Among the V-substituted ones, [SiVVW11O40]5−/[SiVIVW11O40]6− exhibited stability in repeated redox cycles over a wider pH range (2.5–6.5). In contrast, the redox potential of [SiW11O39CoIII(H2O)]5−/[SiW11O39CoII(H2O)]6− was more positive than the water oxidation potential, making it unsuitable for use as a redox mediator.