Permselectivity of ionene-based, Aemion® anion exchange membranes

Abstract

This work reports the permselectivity of an ionene-based anion exchange membrane – ionenes are a class of polymer in which cationic charged groups are located directly on the polymer backbone, as opposed to being pendant. The ionone studied, in the form of commercially available Aemion® anion exchange membranes, is based on hexamethyl-p-terphenyl poly (bibenzimidazolium) (HMT-PMBI), which has been explored in several electrochemical devices, yet their transport numbers, permselectivity, and through-plane conductivity, have not been reported. Measurement of transport properties was carried out on membranes (AF1-HNN8-25, AF1-HNN8-50, AF1-HNN5-25, AF1-HNN5-50) of varying ion exchange capacity and thickness, using the static method in which inherent experimental errors due to liquid junction potentials were elucidated and circumvented. Permselectivity, area specific conductance, and conductivity of Aemion® AEMs are correlated using their fixed charge group concentration. Due to its low water content and high fixed group concentration, the permselectivity of AF1-HNN5-X is higher than AF1-HNN8-X AEMs, despite the latter's higher IEC, but its ionic resistance is lower. Thicker AEMs were confirmed to be more permselective, which can be explained by their relatively lower water content and consequently higher fixed charge group concentration. The permselectivity, area specific conductance, and permselectivity to resistance ratio (α/R) of Aemion® AF1-HNN5-50 were found to be comparable to Selemion AMV, a common anion exchange membrane. The permselectivity/resistance (α/R) value of AF1-HNN8-25 was three times higher than Selemion AMV by virtue of its high ionic conductance.