Redox-Mediated Water Splitting for Decoupled H2 Production

Green H2 production from renewable energy sources by water splitting is desired to reduce the use of fossil fuels and CO2 emissions. The past decade has witnessed the fast development of electrolytic water splitting for H2 generation, with most of the studies focusing on the development of superior electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Equally important is the innovation of water splitting system design to tackle the critical issues confronted by conventional water electrolysis. Some interesting developments have recently emerged with additional redox-mediated process to separate the HER and OER in time and space. In this Review, the various strategies for decoupled H2 production are critically reviewed. Based on the additional process, which is introduced in conjunction with the electrochemical process for decoupled electrochemical or chemical H2 production, the distinct operating principle is highlighted for each strategy, and the underlying connections to other strategies are delineated. Lastly, the implications of the decoupled operations in addressing the formidable issues of conventional water electrolysis, their inherent constraints for practical implementation, and potential solutions are broadly discussed.