Influence\nof the pH Gradient on Bipolar Membrane Operation

Bipolar\nmembranes (BPMs) provide a key framework for integration\nof earth abundant catalysts in energy conversion systems and development\nof water purification systems. Efficient BPM operation requires water\ndissociation (WD) catalysts in the BPM; however, understanding of\nthe catalyzed WD process and the impact of operating conditions have\nremained limited. Here, Nafion-Aemion BPMs employing known WD catalysts\n(graphene oxide, aluminum hydroxide, titanium oxide, iridium oxide)\nwere investigated using electrochemical analyses as a function of\ncatalyst loading and pH gradient conditions up to 50 mA cm<sup>–2</sup>. Altered catalyst loadings allowed the balance between field strength\nand catalyst utilization to be observed, while control of the pH gradient\nprovided insight into catalyst layer operation and the limiting WD\nprocess. These results were then related to a current–voltage\nexpression for BPM operation, where the number of ionizable catalyst\nsites available for proton transfer processes, space charge region\nthickness at the limiting WD interface, and catalyst dielectric constant\nare key factors. Graphene oxide was limited by hydroxide formation\nnear the anion exchange layer; titanium oxide and iridium oxide were\nlimited by proton formation near the cation exchange layer, while\naluminum hydroxide was limited by both processes. Graphene oxide and\naluminum hydroxide exhibited high field utilization and WD activity,\nattributed to low dielectric constants and catalyst structure. These\nresults indicate key areas for improving BPM operation and provide\nmethods to determine the limiting WD process.