Influence of the pH Gradient on Bipolar Membrane Operation

Bipolar membranes (BPMs) provide a key framework for integration of earth abundant catalysts in energy conversion systems and development of water purification systems. Efficient BPM operation requires water dissociation (WD) catalysts in the BPM, however, understanding of the catalyzed WD process and the impact of operating conditions has remained limited. Here, Nafion-Aemion BPMs employing known WD catalysts (graphene oxide, aluminum hydroxide, titanium oxide, iridium oxide) were investigated using electrochemical analyses as a function of catalyst loading and pH gradient conditions up to 50 mA cm-2. Altered catalyst loadings allowed the balance between field strength and catalyst utilization to be observed, while control of the pH gradient provided insight to catalyst layer operation and the limiting WD process. These results were then related to a current-voltage expression for BPM operation, where the number of ionizable catalyst sites available for proton transfer processes, space charge region thickness at the limiting WD interface, and catalyst dielectric constant are key factors. Graphene oxide was limited by hydroxide formation near the anion exchange layer, titanium oxide and iridium oxide were limited by proton formation near the cation exchange layer, while aluminum hydroxide was limited by both processes. Graphene oxide and aluminum hydroxide exhibited high field utilization and WD activity, attributed to low dielectric constants and catalyst structure. These results indicate key areas for improving BPM operation and provide methods to determine the limiting WD process.