Enhancing Ion Selectivity Sieving Performance of Cation Exchange Membranes via COF Layer with Sub-1 Nm Charged Channels

Abstract

The customized design of the ion transport structure in ion exchange membranes (IEMs) is crucial for achieving efficient ion selective transport. The combination of IEMs with regularly porous materials holds promise for realizing high-performance ion-selective transport. Herein, monovalent selective cation exchange membranes (MSCEMs) are successfully fabricated by constructing a positively charged covalent organic framework (TpTG_Cl) on the surface of a cation exchange membrane (SPPSU-Cl), with its pore size precisely tailored to 0.78 nm. Benefiting from the unique pore structure of TpTG_Cl, the resulting composite membranes demonstrated excellent monovalent cation fluxes (1796.42, 1516.06, and 1329.91 mmol m⁻² h⁻¹ for K⁺, Na⁺, and Li⁺, respectively), achieved highly efficient separation of monovalent and divalent cations, and exhibited remarkably low modified layer resistance (0.32 Ω cm²). Experiments and molecular dynamics simulations further verified that counterion-mediated positively charged channels effectively enhanced the force difference between monovalent and divalent cations, thereby facilitating the rapid migration of monovalent cations. Interestingly, variations in counterion species within these positively charged channels led to distinct differences in membrane properties. This study proposes a novel channel chemistry-based design strategy for MSCEMs through structural optimization and interfacial engineering, which has significant implications for applications in water treatment, energy storage, and resource recovery.