Redox Mediated and Decoupled Water Electrolysis Using Soluble Inorganic Mediators: A Comprehensive Review

The worldwide trend toward the green energy systems has enhanced academic research into the biological synthesis of green hydrogen through the electrolysis of water. Conventional electrolytic systems require simultaneous Hydrogen Evolution Reaction and Oxygen Evolution Reactions that are separated by ion-exchange membranes; such systems are prohibitively expensive, complex to operate, and their lifetime is limited. Decoupled and redox-, redox-, and electrolysis Redox can now be utilized as a promising paradigm, with redox couples being used as electron shuttles to either spatially or temporarily separate Hydrogen Evolution Reaction and Oxygen Evolution Reaction to allow membrane-free systems or scalable multi-reactor systems. This paper critically reviews the current developments in the field of mediated and decoupled water electrolysis with a focus on Ferri/ferrocyanide systems used in alkaline water and chromium-based mediators used in acidic water. Systematic analyses are made of rationalizations of reaction mechanisms, thermodynamic viabilities, kinetic profiles, material choices, performance metrics, safety considerations, and scalability issues. Comparative evaluation with other possible mediator systems including iodine, cerium, and manganese couples is made, and the merits and limitations of each of the systems are brought to light. The discussion highlights the opportunities to achieve cost-efficient hydrogen production on a laboratory scale and outlines the relevant research gaps that should be filled to take these technologies to the next stage of practical and industry-deliverable application.