Alkaline Stability of Poly(phenylene oxide) Based Anion Exchange Membranes Containing Imidazolium Cations

Abstract

There is a growing interest in using anion exchange membranes (AEMs) as separators in alkaline membrane fuel cell (AMFCs) and in other energy conversion and storage systems such as redox flow batteries (RFBs), alkaline water electrolyzers (AWEs) and reverse electrodialysis (RED) cells. The most commonly used cation group in AEMs is the benzyl trimethylammonium cation. However, it had been shown that quaternary ammonium-based AEMs are sensitive towards Hofmann elimination (1) and direct nucleophilic elimination reactions (2) that result in loss of ion exchange capacity (IEC) and ionic conductivity. To solve the alkaline stability issue inherent to quaternary-ammonium-group-containing AEMs, several alternative cations, including imidazolium, benzimidazolium, guanidinium, phosphonium and metal cations have been proposed and investigated. Imidazolium-based AEMs have drawn researchers’ interests mainly because of their high hydroxide ion conductivities. In addition to 1-methylimidazole and 1,2-dimethylimidazole, a series of modified imidazole bases, namely 1-heptyl-2-methyl-1H-imidazole and 1-dodecyl-2-methyl-1H-imidazole, were synthesized, wherein these modified bases contained a long alkyl chain at the N-3 position . Anion exchange membranes (AEMs) were prepared with derived imidazolium cations either grafted onto the benzyl position of poly(phenylene oxide)(PPO) or affixed using a hexyl (6-carbon) spacer chain. AEMs with cations affixed to the benzyl position were synthesized by bromination of PPO followed by reaction with the imidazole bases. AEMs with the hexyl spacer were synthesized by Friedel-Crafts acylation of 6-bromo-1-hexanoyl chloride on PPO, followed by reduction of the ketone group and reaction with the imidazole bases. The alkaline stability of the various imidazolium-cation-containing AEMs was evaluated by: 1) measuring the change in IEC after immersion in 1M KOH at 60°C for up to 14 hours; and 2) using 1-D and 2-D NMR spectroscopy to investigate any changes in chemical structure. Both NMR and IEC experiments showed that PPO functionalized with modified imidazolium cations (with long alkyl chain at the N-3 position) did not exhibit better alkaline stability than benchmark AEMs made with 1-methylimidazole. However, the use of a six-carbon spacer did improve AEM alkaline stability when the AEM was prepared directly using 1-methylimidazole and 1,2-dimethylimidazole, but not when the AEM was prepared with the modified imidazolium cations (with a long alkyl chain at the N-3 position).