Electrochemical water splitting is a promising technology to renewably generate hydrogen fuel from water. One 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. This chapter focuses on 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. Water electrolyzers paired with electricity generated from renewable resources can provide green hydrogen, i.e. hydrogen formed with no associated carbon dioxide emissions. Conventional electrolysis proceeds by submerging two charged electrodes into an aqueous environment, simultaneously generating hydrogen at one electrode and oxygen at the other. The tight spatial and temporal coupling of the production of hydrogen and oxygen can cause issues associated with safety, longevity of device components, and system flexibility. Decoupled electrolysis separates the hydrogen- and oxygen-evolution reactions by alternately pairing them to the oxidation or reduction of an intermediate compound, known as a mediator. The physical implementation and operation of the system varies substantially depending on the choice of mediator, of which potentially hundreds exist. Decoupled electrolysis systems can by nature be intrinsically safe, modular, and flexible, making them an interesting technology for a sustainable energy future. Research in this field combines important concepts from electrolysis, batteries, reactor design, redox flow batteries, and photovoltaics, as well as other disciplines.
