Stepwise Generation of Hydrogen and Oxygen in Water Electrolysis via an Iodine-Based Redox Reaction

Water electrolysis is a promising method for producing hydrogen using clean energy. However, in conventional reactions, hydrogen and oxygen are generated simultaneously, requiring a robust structural design to prevent gas mixing and limiting the flexibility of the system. In this work, we aimed to produce these two gases separately by introducing redox species so that the system would become more flexible. Because the reaction potential of the redox species should lie between the hydrogen and oxygen evolution reaction potentials, iodine redox species were selected. In the first step, in which the evolution of oxygen is restrained, iodide ions are added to the anolyte to undergo an oxidation reaction, producing iodate ions. At a potential of +1.54 V (vs RHE), the faradaic efficiencies for the production of iodate ions and hydrogen were 98 and 98%, respectively. In the second step, when hydrogen evolution is suppressed, iodate ions are added to the catholyte to undergo a reduction reaction, producing iodide ions. At the potentials of −0.16 and −0.065 V (vs RHE), the maximum faradaic efficiency to produce iodide ions was achieved. Thus, hydrogen and oxygen were successfully produced individually with high efficiencies.