Hypercrosslinked poly(oxindole biphenylene) anion exchange membranes with microporosity boosting acid/alkali recovery

Membrane processes are vital to acid/alkali recovery in an energy-saving and environmentally friendly way, and their performance hinges largely on the ion exchange membranes (IEMs) that are preferentially permeated by the small ions of H⁺ and OH⁻ over other larger ions. However, it remains challenging to design IEMs with both high H⁺/OH⁻ permeation rates and high ion selectivity. We here report anion exchange membranes with microporosity (HC–P3P) fabricated by a simple hypercrosslinking of side-chain type poly(oxindole biphenylene) membrane (P3P). This hypercrosslinking reaction turns linear backbones into hypercrosslinked networks and thus enhances the anti-swelling property. Benefitting from their micropore confinement imposed by micropore channels, they can efficiently separate H⁺ and OH⁻ from other larger ions. The excellent performance of acid recovery of these hypercrosslinked membranes is demonstrated by a diffusion dialysis process, with an extremely high H⁺/Fe²⁺ selectivity of 5160 (278.92 times for the commercial membrane DF-120, 7.22 times of the original P3P membrane) and a comparable H⁺ dialysis coefficient of 4.53 × 10⁻³ m h⁻¹. Besides, the HC-P3P membranes also exhibit superiority in electrodialysis base recovery, showing a very high OH⁻/WO₄²⁻ selectivity of 491.41 (41.19 times for the commercial membrane Neosepta® ACS) and a competitive OH⁻ flux of 7.14 mol m⁻² h⁻¹. Importantly, HC-P3P membranes perform much better than typically reported membranes in terms of striking an excellent balance between H⁺/OH⁻ transport rate and ion selectivity. This work suggests the great potential of hypercrosslinked IEMs for resource recovery beyond acid/alkali recovery.