Series-Connected Tetracation Partially Cross-Linked Anion Exchange Membranes: Insight towards Consequences of Alkyl Spacer Length

To systematically study how the length of alkyl spacers between cationic head groups in a cross-link impacts the performance of a multication anion exchange membrane (AEM), a series of cross-linkers with varied chain lengths of alkyl spacers between cations from 3-methylene (−CH2−)3 to 10-methylene (−CH2−)10 groups are prepared. Specifically, a series of partially cross-linked poly(phenylene oxide)-based AEMs with series-connected tetracations along with a monocation pristine membrane with a pendent hexyl chain is synthesized. The highest ionic conductivity of 17.33 mS cm–1 at 30 °C in water is realized for a partially cross-linked membrane having series-connected tetracations with butyl spacer between cations (AEM-Q5-C4). On the contrary, decreased water uptake and ionic conductivities are observed when the alkyl spacer chain is lengthened from the butyl to decyl group between cations while keeping the same degree of functionalization. Comparatively, high limiting current density (5.29 mA cm–2) for AEM-Q5-C4 membrane offers a broad window to achieve fast brackish water electrodialysis (ED) at higher applied potential/current. When being applied to electrodialysis, relatively higher NaCl removal percentage of 94.13% and energy efficiency of 82.01% and comparatively low energy consumption of 1.11 kWh kg–1 are obtained for AEM-Q5-C4 membrane among all prepared membranes tested for ED application. This work thus highlights the suitability of a partially cross-linked motif (AEM-Q5-C4) for brackish water electrodialysis.