Technologies and prospects for decoupled and membraneless water electrolysis

Water electrolysis produces hydrogen and oxygen using electricity. Conventional electrolysers couple the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) that occur simultaneously in cells divided by separators into cathodic and anodic compartments. This division prevents hazardous hydrogen and oxygen mixing, but increases the cost of electrolysers and limits their efficiency and dynamic operation under variable load conditions that characterize solar and wind energies. In this Review, we discuss strategies and technologies that decouple the HER and OER in time and/or place. Decoupled water electrolysis utilizes redox mediators such as the polyoxometalate phosphomolybdic acid (H₃PMo₁₂O₄₀) or nickel (oxy)hydroxide (Ni(OH)₂/NiOOH) to mediate ion (H⁺ or OH^-) exchange between the cathode and the anode that generate hydrogen and oxygen in different stages or separate cells. Consequently, the risk of hydrogen and oxygen mixing is reduced, in comparison to conventional electrolysis, especially at low currents, enabling safe operation under partial load conditions, which is important for operation with solar and wind energies. Alternatively, redox couples such as silicotungstic acid (H₄SiW₁₂O₄₀) or bromide/bromate (Br^-/BrO₃^-) can store and release hydrogen or oxygen by turn, dividing the HER or OER into electrochemical and chemical subreactions that can yield high efficiency decoupled water electrolysis.