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Green H2 production fromrenewable energy sources bywater splitting is desired to reduce the use of fossil fuels and CO2 emissions. The past decade has witnessed the fast developmentof electrolytic water splitting for H2 generation, withmost of the studies focusing on the development of superior electrocatalystsfor hydrogen evolution reaction （HER） and oxygen evolution reaction（OER）. Equally important is the innovation of water splitting systemdesign to tackle the critical issues confronted by conventional waterelectrolysis. Some interesting developments have recently emergedwith additional redox-mediated process to separate the HER and OERin time and space. In this Review, the various strategies for decoupledH2 production are critically reviewed. Based on the additionalprocess, which is introduced in conjunction with the electrochemicalprocess 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 addressingthe formidable issues of conventional water electrolysis, their inherentconstraints for practical implementation, and potential solutionsare broadly discussed.
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