Synthesis and characterization of tetra-imidazolium hydroxides poly(fluorenylene ether sulfone) anion exchange membranes

Anion exchange membranes of poly(fluorenylene ether sulfone) containing imidazolium hydroxide-functionalized fluorenyl groups were synthesized by sequential polycondensation, chloromethylation, substitution with imidazolium and ion exchange. They showed elevated molecular weight, high solubility in polar aprotic solvents and strong chemically and thermally stability in comparison to alkyl quaternary ammonium-functionalized polymers. Different levels of substitution and ion exchange were tested; the resulting ionomer membranes showed high ion exchange capacities (IECs) of up to 1.96 mmol g−1. The imidazolium-functionalized copolymer membranes showed lower water affinity and excellent chemical stability in alkaline conditions. They exhibited hydroxide conductivity above 10−2 S cm−1 at room temperature and outstanding chemical stability for up to 7 days without significant losses of ion conductivity.