Technologies and prospects for decoupled and membraneless water electrolysis

Water electrolysis is needed for producing green hydrogen using renewable energy sources. Conventional electrolyzers couple the hydrogen and oxygen evolution reactions that occur simultaneously in cells divided by separators into cathodic and anodic compartments that produce hydrogen and oxygen, respectively. This division is crucial to prevent hazardous mixing of these gasses, but it increases the electrolyzers' cost and limits their efficiency due to the separator’s electrical resistance that increases the internal cell resistance. In this Review, we present a transformative strategy to address these challenges by decoupling the hydrogen and oxygen evolution reactions in time and/or place. To this end, decoupled water electrolysis (DWE) utilizes redox mediators that mediate the ion exchange between the cathode and anode that generate hydrogen and oxygen in different stages or separate cells. We review different types of mediators, both soluble redox molecules and solid redox electrodes, discuss their advantages and disadvantages in different modes of operation and compare them to conventional electrolyzers. We highlight modes that enable safe and efficient operation without membranes, and their potential implications for cost reduction and simplified construction of modular and scalable DWE systems. Finally, we outline a development pathway to MW-scale membraneless DWE systems and discuss their challenges and opportunities.