Highly conductive and permselective anion exchange membranes for electrodialysis desalination with series-connected dications appending flexible hydrophobic tails

Abstract

A series of series-connected dication ionic ionomers with flexible hydrophobic tails (QPTs) designed for electrodialysis desalination (ED) anion exchange membranes have been synthesized successfully from a monoionic quaternization agent N-(2-(dimethylamino)ethyl)-N,N-dimethylhexan-1-aminium bromide and brominated poly(2,6-dimethyl-1,4-phenylene oxide). The properties of QPTs membranes were evaluated in terms of ion exchange capacity (IEC), water uptake (WU), swelling ratio (SR), thermal stability, membrane area resistance (Rm), permselectivity (P) and ED experiments. The results confirm that the coaction of the series-connected dications linking by a short chain of two methylene groups and the middle length aliphatic tails offers QPTs excellent conductivity and permselectivity simultaneously. Especially, QPT-4 membrane has an excellent permselectivity of 94.0% and the lowest Rm of 1.60 Ω cm2 at the highest IEC of 2.33 mmol g−1. Moreover, QPT-4 membrane shows the best ED performances with current efficiency of 89.14%, salt flux of 75.57 mg m−2 s−1 and energy consumption of 2.16 kWh kg−1 in the mostly reported NaCl ED system, which are much higher than those of commercial TWEDA1 (the corresponding values of 83.96%, 71.19 mg m−2 s−1 and 2.67 kWh kg−1). The excellent ED stability in the repeated tests further qualifies QPT-4 to be a very promising candidate for ED application.