Poly(arylene piperidinium)s – a new class of anion-exchange membranes

Anion exchange membranes (AEMs) are crucial components of various electrochemical devices. To function well they require both high chemical and mechanical robustness, as well as a sufficient hydroxide conductivity. However, due to the aggressive nature of the hydroxide ion, which causes polymer degradation, the applicability of AEMs is so far limited.1-2 As a consequence, a lot of effort is presently devoted to molecular design and exploration of new synthetic strategies to prepare alkali-stable AEMs.3-5Herein, the synthesis and properties of poly(arylene piperidinium)s (PAPipQs) devoid of alkaline labile ether bonds and benzylic hydrogens are presented.6 An initial study of a model cation, 4,4-diphenyl-1,1-dimethylpiperidinium, indicated excellent alkaline stability at 120 °C. In subsequent superacid catalysed polycondensations, medium molecular weight poly(arylene piperidine)s were prepared from commercially available N-methyl-4-piperidone and biphenyl or terphenyl (Scheme 1a). The resulting polymers showed glass transition temperatures at 280 °C or higher. After quaternization using different alkyl halides, mechanically robust and transparent AEMs with excellent thermal stabilities were prepared by solution casting (Scheme 1b). Evaluation of the alkaline stability of poly(terphenyl piperidinium)s by immersion in 2 M aq. NaOH at 90 °C showed that the dimethyl derivative possessed the highest alkaline stability. A mere ~5% loss of the cations was observed, with the ionic loss increasing with increasing alkyl chain length. These AEMs reached a hydroxide conductivity of 89 mS cm-1 at 80 oC in the fully hydrated state. We are currently developing synthetic strategies towards PAPipQs with reduced IEC values for improved AEM performance