Improving the fixed charge density of sustainably produced saloplastic anion exchange membranes

Abstract

Recent studies have shown that sustainable ion exchange membranes can be fabricated by hot-pressing polyelectrolyte complexes (PECs), resulting in saloplastic membranes. Among these, the anion exchange membrane (AEM) formed from the strongly charged polyelectrolyte pair, poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyl-dimethylammonium chloride) (PDADMAC) stands out due to its excellent chemical stability. However, the performance of this membrane is limited by its comparatively low fixed charge density. To address this limitation, we aimed to enhance the fixed charge density through incremental PDADMAC overcharging during the complexation step, followed by optimisation of hot-pressing conditions to produce dense, freestanding films. This approach allows for precise control over membrane charge and improves the reproducibility of films, thereby overcoming challenges in the processing and handling of non-stoichiometric PECs. NMR spectroscopy was used to quantify the fixed charge of the saloplastic AEMs before and after testing, providing a reliable and time-efficient method for assessing stability. Our results showed that a PDADMAC overcompensation of ∼30 mol% optimised the fixed charge density without compromising membrane stability. The enhanced membrane exhibited an 84% improvement in ionic conductivity (4.3 ± 0.3 mS cm⁻¹ in 0.5 M KCl) compared to the original membrane. Notably, all membranes displayed excellent permselectivity (>90%) in 0.1 M KCl, and at higher electrolyte concentrations, a moderate improvement in permselectivity was observed with the increase in fixed charge density. Overall, this study presents a simple yet effective methodology for quantifying and optimising the fixed charge density of saloplastic membranes, resulting in significantly improved performance.