Ionic Processes in Water Electrolysis: The Role of Ion-Selective Membranes

Ionic separation and recombination processes in water electrolysis and fuel cell devices are of equal importance as the electron transfer processes that occur at the electrode surfaces. We illustrate the basic thermodynamic concepts governing the flow of ions in electrolyzer and fuel cell systems and the effects of pH gradients on the electrochemical phenomena. Particularly, we focus on the use of bipolar membranes, which are composed of anion- and cation-selective membranes enabling operation with different pH environments at the anode versus the cathode. The use of bipolar membranes thus broadens the materials' availability and could enable low-cost electrolysis systems that operate at very high efficiency. We end by discussing different materials and highlighting key gaps needed to realize such a system.