Scalable Thin-Film Composite Cation Exchange Membrane for Separation of Monovalent and Divalent Cations in Electrodialysis

Ion exchange membranes (IEMs) separate ions of opposite charges, enabling a variety of applications, including desalination and mineral recovery via electrodialysis (ED). ED is uniquely suited for ion separation and recovery from high-salinity waters, especially for ions of low concentrations, as ions, instead of water, are transported in an electrical field. However, the application of ED is limited by the lack of selectivity of the counterions in the existing IEMs. Here, we developed a thin-film composite cation exchange membrane (TFC-CEM) consisting of an ultrathin polyamide coating, a porous polysulfone support, and a dense cation exchange polymer layer, which provides excellent separation between divalent and monovalent cations, high mechanical strength, and effective separation between cations and anions. At a constant current of 0.24 mA cm^-2, the TFC-CEM demonstrated monovalent over divalent cation selectivity of 49.32-136.79 in mixed cation solutions with a divalent to monovalent cation molar ratio of 25. The selectivity increased with increasing divalent to monovalent cation concentration ratios and correlated strongly with the difference in the hydrated size of the cations. Furthermore, the asymmetric structure of the TFC-CEM led to unique limiting current density behaviors, revealing that ion transport through the polyamide coating was rate-limiting. Finally, TFC-CEM demonstrated great potential for lithium extraction from hypersaline brines.