A new type of bipolar membrane (BPM) with a high interfacial area 3D water splitting junction has been fabricated and tested. BPMs were made by single and dual fiber electrospinning, followed by dimethylformamide vapor exposure and hot-pressing, with sulfonated poly(ether ether ketone) as the cation-exchange polymer and quaternized poly(phenylene oxide) as the anion-exchange material. Al(OH)₃ nanoparticles were added to the junction to facilitate water splitting. The resultant trilayer membrane had an inner bipolar junction (interpenetrating anion-exchange and cation-exchange polymer nanofibers) ∼10 μm in thickness with dense/neat ionomer outer layers. The extended bipolar reaction zone enabled water splitting at ultra-high current densities (up to 1.1 A cm⁻²) with no evidence of membrane dehydration or blistering, which is common for a planar junction BPM. The novel electrospun membrane morphology offers significant benefits for electrodialysis separations, electrochemical hydrogen generation, and self-humidifying fuel cells, where high operating current densities and membrane stability are required.

The lamination of a cation exchange layer (CEL) and an anion exchange layer (AEL) to form a hybrid bipolar membrane (BPM) can have several unique advantages over conventional monopolar ion exchange membranes in (photo-)electrolysis. Upon application of a reverse bias, the ordinarily slow water dissociation reaction at the AEL/CEL junction of the BPM is dramatically accelerated by the large electric field at the interface and by the presence of catalyst in the junction. Using electrochemical impedance spectroscopy (EIS), we have found a counterbalanced role of the electric field and the junction catalyst in accelerating water dissociation in a BPM. Experimental BPMs were prepared from a crosslinked AEL and a Nafion CEL, with a graphite oxide (GO) catalyst deposited at the junction using layer-by-layer (LBL) assembly. BPMs with an interfacial catalyst layer were found to have smaller electric fields at the interface compared to samples with no added catalyst. A comprehensive numerical simulation model showed that the damping of the electric field in BPMs with a catalyst layer is a result of a higher water dissociation product (H⁺/OH⁻) flux, which neutralizes the net charge density of the CEL and AEL. This conclusion is further substantiated by EIS studies of a high-performance 3D junction BPM that shows a low electric field due to the facile catalytic generation and transport of H⁺ and OH⁻. Numerical modeling of these effects in the BPM provides a prescription for designing membranes that function at lower overpotential. The potential drop across the synthetic BPM was lower than that of a commercial BPM by more than 200 mV at >100 mA cm⁻² reverse bias current density, with the two membranes having similar long-term stability.

Anion exchange membranes were fabricated from diamine-crosslinked poly(phenylene oxide) polyelectrolyte nanofibers that were functionalized with either benzyl trimethylammonium or 1,2-dimethylimidazolium groups and embedded in a reinforcing matrix of polyphenylsulfone (PPSU). Nanofibers of the polyelectrolyte precursor, brominated poly(phenylene oxide) with 0.95 CH₂Br groups per mer, and PPSU were simultaneously electrospun onto a common collector and the resulting dual fiber mat was converted into a dense and defect-free anion exchange membrane by sequential soaking in a diamine crosslinker solution, exposure to chloroform vapor, and immersion in a solution of either trimethylamine or 1,2-dimethylimidazole. The degree of crosslinking and the ratio of uncharged PPSU to functionalized poly(phenylene oxide) polyelectrolyte were varied to yield composite membranes with an effective ion exchange capacity (IEC) ranging from 1.2–2.8 mmol g⁻¹. A membrane with benzyl trimethylammonium-functionalized polyelectrolyte fibers (4.0 mmol g⁻¹ IEC) with 15% crosslinks and 50 wt% uncharged PPSU exhibited a high hydroxide ion conductivity in water at 23 °C (66 mS cm⁻¹), reasonable water swelling (96%), robust mechanical properties (15 MPa stress-at-break in the hydrated state), and good chemical stability in 1.0 M KOH at 60 °C. Initial hydrogen/oxygen fuel cell tests with this membrane (40 μm thick) were promising, with a peak power density of 320 mW cm⁻².

Two low equivalent weight perfluorosulfonic acid (PFSA) polymers (825 EW and 733 EW) were successfully electrospun into nanofibers by adding as little as 0.3 wt% of high molecular weight poly(ethylene oxide) as a carrier polymer. The electrospun fiber morphology transitioned from cylindrical filaments to flat ribbons as the total concentration of PFSA + carrier in solution increased from 5 wt% to 30 wt%. PFSA nanofiber mats were transformed into defect-free dense membranes using a four-step procedure: (i) annealing the PFSA polymer during which time intersecting fibers were welded to one another at cross points (ii) mechanically compacting the mats to increase the volume fraction of nanofibers to ∼75%, (iii) imbibing an inert polymer, Norland Optical Adhesive (NOA) 63, into the mats (to fill entirely the void space between nanofibers) and then crosslinking the NOA with UV light, and (iv) removing the poly(ethylene oxide) carrier polymer by boiling the membrane in 1.0 M H₂SO₄ and then in deionized water. The resulting membranes exhibited higher proton conductivities than that of commercial Nafion 212 membrane (0.16 S/cm at 80 °C and 80% relative humidity and 0.048 S/cm at 80 °C and 50% relative humidity for a membrane with 733 EW nanofibers), with low water swelling (liquid water swelling of 18% for membrane with high conductivity). The proton conductivity of both EW nanofiber composite membranes increased linearly with the PFSA nanofiber volume fraction, whereas gravimetric water swelling was less than expected, based on the volume fraction of ionomer. There was a significantly improvement in the mechanical properties of the nanofiber composite membranes, as compared to recast homogeneous PFSA films.