Chemically robust ether-free polymer-based bipolar membrane with a 3D junction for high-performance water splitting and efficient acid/base production

In this study, ether-free poly(5-Bromoisatin-co-p-Terphenyl) was synthesized to prepare two ion-exchange polymers derived from a single polymer backbone, with the aim of optimizing the synthesis process of ionomers and leveraging the chemical stability of the ether-free polymer to develop high-performance bipolar membranes (BPMs) with high chemical stability. The anion-exchange (AEM) and cation-exchange (CEM) membranes prepared using the two ion-exchange polymers demonstrated an ion exchange capacity (IEC) of 1.8 meq g⁻¹, swelling ratios ranging from 9.4%–16.6% and 5.0%–6.5%, and water uptake values of 21.3%–49.8% and 15.2%–17.6% from 30 °C to 80 °C, respectively. More importantly, the ether-free AEM and CEM effectively prevent mass loss over four weeks in 4 M NaOH and H₂SO₄ at a level of 4%–5%. To additionally endow an ether-free BPM with structural stability and high water-splitting efficiency, dual electrospinning (DE) was used to prepare BPMs (3D-BPM) with a 3D junction, which can provide an interlocking effect to prevent delamination, a large interfacial area available for water splitting, and better pathways for the diffusion of water molecules and water dissociation product ions. The optimized BPM (i.e., DE1), featuring a 3D junction layer constituting 25% of the total membrane thickness, achieved a water-splitting voltage of 1.38 V and transmembrane voltage (TMV) of 2.2 V at 100 mA/cm², corresponding to almost half of those of a 2D-BPM (i.e., DE0) prepared using the same ion-exchange polymers. Also, DE1 exhibited exceptional acid-base production efficiency, with a current efficiency of 85.2%, which was about 6%p higher than DE0, while decreasing the energy consumption (4.32 kWh/kg) by 8.5% compared to DE0. This work highlights that the ether-free 3D-BPM can achieve chemical/structural stability and high water-splitting capability simultaneously.