Enhanced monovalent anion selectivity of poly(2,6-dimethyl-1,4-phenylene oxide)-based amphoteric ion exchange membranes having rough surface

Abstract

Amphoteric polyelectrolytes have been used as the materials for the high-performance ion-exchange membranes applied in redox flow battery and electrodialysis (ED) etc. In this work, four amphoteric ion-exchange membranes (AIEMs) based on sodium 1-propanesulfonate-3-methylimidazole (MIm-PS) and 1-butylimidazole (BIm) decorated poly(2,6-dimethyl-1,4-phenylene oxide) have been prepared for mono-/bi-valent anion separation by using the electrodialyzer. The optimized AIEM has a low surface area resistance of 5.79 Ω⋅cm2. During the ED testing, at 2.5 mA⋅cm−2, the perm-selectivity (PSO42−Cl−) tested in a binary mixture of 0.05 M NaCl and 0.05 M Na2SO4, achieves the value of 16.12 at 30th min, which is significantly higher than that of anion-exchange membrane (1.68) as the reference. This superior separation performance has been further verified by the perm-selectivity of 55.5, tested in a single solution of 0.05 M NaCl or 0.05 M Na2SO4. The high perm-selectivity results from the electrostatic repulsion difference of mono-/bi-valent ions against the negatively-charged –SO3– groups, accompanying with pore-size sieving effect resulting from the smaller ion cluster space of 0.261 nm. It is interesting to find that AIEMs show the increased ion flux values of Cl−/SO42−, though the presence of the negatively-charged –SO3− groups, which is possibly related to the convex particle-like surface that increasing the effective surface area for ions passing. This work may provide a reference to guide the architectural design of advanced monovalent anion-selective IEMs.