Ionic Control of Functional Zeolitic Imidazolate Framework-Based Membrane for Tailoring Selectivity toward Target Ions.

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Ion exchange membranes with strong ionic separation performance have strategic importance for resource recovery and water purification, but the current state-of-the-art membranes suffer from inadequate ion selective transport for the target ions. This work proposes a new class of zeolitic imidazolate framework (ZIF)-based anion exchange membranes (named as S@ZIF-AMX) with suppressed multivalent anion mobility and enhanced target ion transport via an ionic control strategy under alternating current driven assembly. In electrodialysis with an initial concentration of 50 mM of NaBr, NaCl, Na2SO4, and Na3PO4 (mixed feed) and a current density of 10 mA cm-2, the S@ZIF-AMX membrane demonstrated an excellent transport of the target ion (Cl-) based on the synergy between the Cl- regulated ZIF cavity and the electrostatic interaction with sulfonic groups. The separation efficiency and permselectivity of PO43-/Cl- through S@ZIF-AMX largely increased to 83% and 32, respectively, compared to 42% and 4.0 of the pristine AMX membrane (a commercial anion exchange membrane), respectively. Furthermore, the separation between SO42- and Cl- was also enhanced, the separation efficiency and permselectivity of SO42-/Cl- increased from 11% and 1.4 to 45% and 4.3, respectively. In addition, the combined strategy developed in the S@ZIF-AMX membrane was proven effective in promoting Cl- transport by shifting the separation equilibrium of the ion pair Br-/Cl-, which is known to be extremely challenging. This work provides a new design strategy toward pushing the limits of current ion exchange membranes for target ion separation in water, resource, and energy applications.