(Invited) Electrospun Nanofiber Composite Bipolar Membranes

Abstract

Bipolar membranes, typically a laminate of an anion-exchange and cation-exchange membrane, have the unique capability to split water at a potential as low as 0.8 V while conventional electrolysis requires at least 1.2 V. Such membranes are employed in a number of important industrial processes, including the production of mineral acids, the recovery of organic acids from fermentation processes, pH control, and ion-exchange resin regeneration. The use of bipolar membranes in electrodialysis separations eliminates unwanted salt efflux and disposal problems and offers significant savings in electrical energy consumption. Recently, bipolar membranes have also been examined for use in self-humidifying hydrogen/air fuel cells, where the recombination of electrochemically generated hydroxide ions and protons at the bipolar junction produces water that improves membrane hydration, with higher power output at reduced feed gas humidity conditions. We report here for the first time on the design, fabrication, and testing of a new bipolar membrane that was made by polymer nanofiber electrospinning, where the anion-exchange polymer is quaternized poly(phenylene oxide) and the cation-exchange polymer is sulfonated poly(ether ether ketone). This new membrane has a 3-D nanofiber bipolar junction with a very high interfacial area, unlike the typical 2D planar junction. This extended junction morphology is important for two reasons: (1) it improves the attachment (adhesion) of the two ion-exchange layers so that ballooning/delamination is minimized or eliminated and (2) the increased bipolar junction surface area will lead to a better distribution of the water splitting reaction and lower membrane voltage drop for the same operating current density. The procedure for membrane fabrication will be described and the performance of electrospun films will be compared to bipolar membranes in the literature.