Manipulating interlayer morphology in electrospun bipolar membranes: a key to overcoming 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 was gained by systematically varying the 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 an enhanced 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 cm⁻² and a perm-selectivity of 95%. This work offers valuable insights into the structural design of electrospun BPMs, overcoming the trade-off between resistance and perm-selectivity.