Bipolar Membranes with a High Interfacial Area 3D Electrospun Junction

Bipolar membranes (BPMs), typically laminated layers of anion-exchange and cation-exchange polymers, have the unique capability of splitting water into H⁺ and OH^- at a potential near 0.83 V. Such membranes are used in electrodialysis membrane separation processes, water electrolyzers, CO₂ electrolysis cells, and self-humidifying fuel cells. We report here on recent developments regarding BPMs with a high interfacial area, 3D nanofiber water splitting junction.

A junction layer was made by the simultaneous electrospinning of anion-exchange and cation-exchange polymers, with the addition of catalyst nanoparticles to facilitate water splitting. Solvent vapor exposure and hot-pressing closed all interfiber pores, to create a dense film of interpenetrating and interlocking nanofibers of positively and negatively charged polymers. Dense films of solution cast anion and cation exchange polymers were then hot-pressed onto the junction layer to create a tri-layer BPM. BPMs were made with a junction layer of 12-15 mm and a total membrane thickness of 50-75 mm. Membranes were prepared with sulfonated poly(ether ether ketone) or perfluorosulfonic acid as the cation-exchange polymer and either quaternized poly(phenylene oxide) or AEMION^ä (an imidazolium-based polymer sold by Ionomr Innovations, Inc.) as the anion-exchange polymer. Al(OH)₃ or graphene oxide particles were used as the junction layer catalyst. Membranes were evaluated in an H-cell (steady-state current-voltage data collection) and for constant current operation in a flow cell with an aqueous Na₂SO₄ electrolyte. The 3D junction membranes performed exceptionally well: (i) the water dissociation potential was very low and (ii) the extended bipolar reaction zone for water splitting allowed for high current density operation (up to 1 A/cm²) with no evidence of membrane degradation.

In this talk, the effect of membrane composition (the type/amount of anion and cation exchange polymers) and structure (the thickness of the layers) on short-term and long-term membrane performance will be discussed.