Z-scheme water splitting with a redox mediator, which transports electrons from an H2- to an O2-evolving photocatalyst, has been widely studied to achieve efficient overall water splitting. However, backward electron transfer with the redox mediator is an intrinsic drawback of Z-scheme water splitting, because it interrupts the desired H2 and O2 evolution. Although a CrOx shell coating on metal cocatalysts on an H2-evolving photocatalyst is widely used to block backward electron transfer, this strategy relies on potentially toxic chromium species of which elution is possible under the photocatalytic operation. Herein, we demonstrate a chromium-free approach that suppresses backward electron transfer by manipulating electrostatic interactions at the photocatalyst-mediator interface. The designed charge-switchable cobalt complex serves as a superior mediator that selectively suppresses backward electron transfer from a positively charged H2-evolving photocatalyst without using a CrOx shell, leading to efficient Z-scheme water splitting with an apparent quantum yield of 7.2%.
Itagaki et al. (Mon,) studied this question.