Manipulating interlayer morphology in electrospun bipolar membranes: A key to overcome the trade-off between perm-selectivity and resistance

Abstract

Bipolar membranes (BPMs) face a fundamental performance trade-off between low resistance and high perm-selectivity. Here, we report a breakthrough strategy using scalable electrospinning and precisely controlled hot-pressing to fabricate electrospun BPMs that simultaneously achieve both. A phytic acid and Fe3+ complex was utilized for the first time as a water dissociation catalyst. By controlling the number of electrospun mats in hot-pressing, we successfully developed electrospun BPMs with tunable thicknesses ranging from 40 to 90 μm. Critically, we demonstrate that the perm-selectivity and conductance both increase as membrane thickness decreases. Electrochemical impedance spectroscopy reveals that while ohmic resistance decreases with thickness, water dissociation resistance dominates the overall resistance by two orders of magnitude. Mechanistic insight gained by systematically varying hot-pressing force and applying selective pre-hot-pressing to the cation-exchange layer (CEL), interfacial layer (IL), or anion-exchange layer (AEL). This shows that the penetration of the AEL into the IL impedes proton crossover, thereby increasing the perm-selectivity of the BPM. Significantly, while a lower number of electrospun mats in hot-pressing results in a higher pressure exerted on the penetration of the AEL into the IL, the thinner electrospun BPM exhibited both lower resistance and higher perm-selectivity. Guided by this mechanism, we fabricated a BPM by hot-pressing only one AEL and one highly selective and catalytic CEL, exhibiting a voltage drop value of 1.0 V at 100 mA/cm2 and a perm-selectivity of 95 %. This work offers valuable insights into the structure design of electrospun BPM, overcoming the trade-off between resistance and perm-selectivity.