Graphene quantum dots-mixed anion exchange membranes for much-enhanced mono-/divalent anion separation by shielding effect

Anion exchange membranes (AEMs) that enable effective mono-/divalent anion separation are highly desired for many environment-related applications. Conventional AEMs are characterized with microphase-separated microstructures and rely on the electrostatic attraction between anions and positively charged elements within hydrophilic regions to selectively transport anions. Sufficient charged groups are usually required to ensure high anion permeability, but the stronger interaction encountered by divalent anions than that by monovalent anions results in very low selectivity. We here propose a shielding effect-based strategy, that is, simply mixing piperidinium-based AEMs with carboxylated graphene quantum dots (GQD). These negatively charged GQD can electrostatically interact with the polymer matrix, thereby enhancing the connectivity of hydrophilic regions and also leading to an obvious shielding effect which causes a larger decrease in electrostatic attraction for divalent anions than that for monovalent anions. With the introduction of GQD, the resulting AEMs show obvious enhancement in both anion permeability and anion selectivity, with a Cl - transport rate of 3.2 mol m -2 h -1 and a Cl - /SO 4 2- selectivity of 21.8 (8 times of the commercial ACS, 5 times of the original AEMs without GQD), and exceed other reported microphase-separated polymer membranes. This work offers a simple and promising strategy for high-performance ion-selective membranes.