Thin and defect-free ZIF-8 layer assisted enhancement of the monovalent perm-selectivity for cation exchange membrane

Zeolitic imidazolate framework-8 (ZIF-8) has the potential to enhance the monovalent perm-selectivity of cation exchange membranes based on the size sieving effect resulting from its regular structure and tunable pore size. However, constructing a dense and defect-free ZIF-8 layer with controllable thickness on the membrane surface is still a challenge. In this study, a method of interfacial synthesis based on the cation exchange membrane (CEM) was proposed. When the zinc ions adsorbed in the basal CEM contact with the dimethylimidazole (Hmim) outside the membrane via slow and continuous counter diffusion, an interfacial reaction occurs and forms a thin and defect-free ZIF-8 layer on the basal CEM surface. The modified CEM presented high Na⁺/Mg²⁺ perm-selectivity of 2.90 and improved Na⁺ flux (7.58 × 10⁻⁸ mol∙cm⁻²∙s⁻¹), which were 3.82 and 1.69 times higher than those of the original CEM, respectively. Besides, the ZIF-8 layer of interfacial reaction could further grow in the conventional ZIF-8 precursor solution to obtain higher monovalent perm-selectivity by sacrificing certain separation efficiency. The further modified CEM presented higher Na⁺/Mg²⁺ perm-selectivity (7.96) and Na⁺ flux (8.52 × 10⁻⁸ mol∙cm⁻²∙s⁻¹).