A facile tuning of polyelectrolyte cross-linking chemistry for improved permselectivity and stability of a monovalent cation exchange membrane

This work describes the preparation of a monovalent selective cation-exchange membrane (MSCEM) with high selectivity, stability, and low resistance compared to state-of-the-art membranes. These key properties were successfully improved by structural tuning of cross-linking chemistry between the selected polyelectrolyte, polyethyleneimine (PEI), and cation exchange membrane (CEM) in a controlled manner. A facile three-step single-side modification approach was implemented, where the bio-inspired polymer polydopamine (PDA) strongly binds PEI to the CEM, followed by Glutaraldehyde (GA) cross-linking via Schiff base reaction. The change in surface morphology, physico-chemical, and electrochemical properties of the modified membranes was investigated. Selectivity of the prepared membranes was tested at varying current densities (i.e., 30 %, 50 %, and 70 % limiting current density) with two different synthetic solutions mimicking the cation composition of brackish groundwater and reverse osmosis concentrate. The observed permselectivities at different current densities and compositions ranged between 2–3.5 (Na⁺ over Ca²⁺) and 4–16 (Na⁺ over Mg²⁺) – comparable to best-in-class commercial MSCEM. The electric resistance was towards the lower range of the commercial MSCEM. The modified membranes were stable after 8 operation cycles with almost constant selectivity.