Optimizing functional layer of cation exchange membrane by three-dimensional cross-linking quaternization for enhancing monovalent selectivity

ABSTRACT Monovalent cation perm-selective membrane (MCPMs) allow fast and selective transport of monovalent cations, and they are promisingly required for extraction of special ions, such as lithium extraction, acid recovery and sea salt production. Herein, we report a novel strategy to design the critical functional layers of MCPMs with both space charge repulsion and cross-linked dense screenability. The in-situ deposition polymerization of pyrrole was carried out on the surface of sulfonated polyphenyl sulfone (SPPSU) substrate membrane followed by cross-linking quaternization of the polypyrrole (PPy) layer with diiodinated functional molecules, thus, the membrane obtained more excellent selective permeability and stable transport properties of monovalent cations. It confirms that the designed PPy layers with charged surface and cross-linking structure improved the hydrophilicity, facilitated cation transport and increased ion flux. Meanwhile, for the dense PPy layer, the charged cross-linked structure endowed the functional layer with the synergistic characteristics of Donnan exclusion and pore size sieving for positively charged ions, which improved the monovalent cation perm-selectivity of the membranes. At a constant current density of 5.1 mA/cm2, the optimal membrane exhibited superior perm-selectivity (PNaMg = 2.07) and monovalent cation flux (JNa+ = 2.80 * 10 −8 (mol cm−2 s−1)) during electrodialysis.