Mussel-inspired tannic acid/polyethyleneimine assembling positively-charged membranes with excellent cation permselectivity

The extraction of valuable target ions through monovalent cation exchange membranes (MCEMs) has been increasingly attracting in modern energy and environmental fields. However, the separation performance of MCEMs in terms of the permselectivity and cation fluxes, is typically restricted by membrane architecture and applied materials. Recently, mussel-inspired surface modification methods have been deployed in new membrane fabrications with special surface characteristics and functions. Herein, a facile layer-by-layer assembly method was designed to construct a series of de novo positively-charged tannic acid/polyethyleneimine (TA/PEI) membranes containing a negatively-charged support membrane and a TA/PEI selective layer. Notably, the peculiar support membrane with a much dense structure and abundant cation exchange groups can enable our TA/PEI membranes to possess high total cation fluxes. The selective layer with vast positive charges ensures mussel-inspired TA/PEI assembled positively-charged membranes to have a high permselectivity. Most importantly, compared with the separation performance of the state-of-the-art MCEMs, the superior separation performance of our developed new MCEMs at 5 mA·cm −2 and 10 mA·cm −2 is beyond the current “Upper Bound” plot between Na + flux and the permselectivity (Na + /Mg 2+ ), which opens new avenues for the construction of MCEMs. Furthermore, high purity of Li + (95.37%) can be obtained through deploying mussel-inspired TA/PEI assembled positively-charged membranes with high permselectivity of Li + /Mg 2+ (13.72), proving its great potentials in the field of resource recovery towards sustainability. • TA/PEI membranes possess high Na + flux and the permselectivity. • The surface positive charges are significantly enhanced after crosslinking. • The current density has a critical influence on the permselectivity of MCEMs. • TA/PEI membranes can be effectively used in Na + /Mg 2+ and Li + /Mg 2+ separations. • High purity of Li + (95.37%) can be obtained after 2 times of running ED process.