Decoupled electrochemical water-splitting systems: a review and perspective

Electrochemical water splitting is a promising technology to renewably generate hydrogen fuel from water. One particular drawback of conventional water splitting is that the hydrogen-forming reduction reaction is tightly coupled, both spatially and temporally, to the oxygen-forming oxidation reaction. This coupling poses challenges in both conventional and direct-solar-powered electrolysis systems, including gas crossover and separator degradation, sometimes necessitating the use of precious metal catalysts. In decoupled water splitting, the conventional electrolysis reactions are separated spatially, temporally, or both, via coupling to an intermediate redox mediator. Decoupled water-splitting systems are flexible and modular by nature, with other proposed benefits including facile coupling to renewable power sources, utilization of earth-abundant catalysts, and intrinsically safe operation. Here we review recent advances in decoupled water splitting and related fields, mainly categorizing decoupled systems by mediator phase and standard potential. We offer insight to how decoupling may be advantageous, and which tradeoffs need to be considered for practical implementation. We conclude our review with discussion of known technological hurdles and note opportunities for future discovery.