Biselective microporous Trӧger's base membrane for effective ion separation

Much remains unknown for the selective translocation of ions across microporous polymeric membranes and it maintains a daunting challenge to pursue a polymeric membrane that can efficiently separate both anions and cations of comparable size. Here we explore the ion transport across hydrophilic microporous membrane derived from polymers of intrinsic microporosity, specifically the Trӧger's Base (TB) polymer DMBP-TB, through protonation and quaternization. We find the ion conduction through the resultant DMBP-TB + membrane exhibits an obvious confining effect and a comprehensive study on the dependence of charge selectivity on ion concentrations reveals two distinct regimes, with an inverse from anion-selective to non-selective. The concentration-driven and electric-driven transport of anions or cations show a strong size sieving effect, with a cutoff size of around ~8.2 Å. The confining effect and the size effect transform to the best combination of a Li + flux of 190.8 mmol m −2 h −1 and a Li + /Mg 2+ selectivity of 21, which may benefit the extraction of Li resources. Our work provides new facts about the confined ion transport and potential applications of hydrophilic microporous polymeric membranes that feature easy fabrication, simple processing, and low cost. The highly rigid and contorted structure of Trӧger's Base polymer, specifically DMBP-TB, frustrate the packing of polymer chains, affording solution-processable microporous materials. When protonated or quaternized, the micropores serve as ion-conducting channels. The resulting DMBP-TB + membrane behaves differently from conventional ion exchange membranes, demonstrating an inverse in charge selectivity. Distinct confining effect and strong size sieving effect are observed for cross-membrane ion transport. The transport of anions or cations with sizes bigger than 8.2 Å across the membrane is restricted. • Ion conductive membranes were prepared from hydrophilic microporous polymers. • Ion selectivity of the hydrophilic microporous membranes was examined. • Charge effect, confining effect, and size effect were observed for ion permeation. • Hydrophilic microporous membranes can discriminate both anions and cations. • Ion sieving based on size effect was reported.