Simultaneous Manipulation of Membrane Enthalpy and Entropy Barriers towards Superior Ion Separations

Sub-nanoporous membranes with ion selective transport functions are important for energy utilization, environmental remediation, and fundamental bioinspired engineering. Although mono/multivalent ions can be separated by monovalent ion selective membranes (MISMs), the current theory fails to inspire rapid advances in MISMs. Here, we apply transition state theory (TST) by regulating the enthalpy barrier (ΔH) and entropy barrier (ΔS) for designing next-generation monovalent cation exchange membranes (MCEMs) with great improvement in ion selective separation. Using a molecule-absorbed porous material as an interlayer to construct a denser selective layer can achieve a greater absolute value of ΔS for Li⁺ and Mg²⁺ transport, greater ΔH for Mg²⁺ transport and lower ΔH for Li⁺ transport. This recorded performance with a Li⁺/Mg²⁺ perm-selectivity of 25.50 and a Li⁺ flux of 1.86 mol ⋅ m^-2 ⋅ h^-1 surpasses the contemporary "upper bound" plot for Li⁺/Mg²⁺ separations. Most importantly, our synthesized MCEM also demonstrates excellent operational stability during the selective electrodialysis (S-ED) processes for realizing scalability in practical applications.