Poly(arylene alkylene)s with pendant N-spirocyclic quaternary ammonium cations for anion exchange membranes

Abstract

Aromatic polymers functionalised with cycloaliphatic quaternary ammonium (QA) cations are currently emerging as base-stable anion exchange membranes (AEMs) for use in alkaline fuel cells and water electrolyzers. In the present work, we first prepared poly(biphenyl piperidine)s by superacid-mediated polycondensations, and then introduced different N-spirocyclic QA cations via cyclo-quaternisation of the piperidine rings. The resulting polymers and AEMs were free of diaryl ether linkages and benzylic C-H bonds, and showed very high thermal stability and hydroxide ion conductivity. Alkaline testing up to 120 °C implied that the alkaline stability of the spirocyclic cations was limited by distortions of the ring conformations caused by the rigid polymer backbone. As a consequence, the ring directly attached to the backbone degraded significantly faster by Hofmann β-elimination than the pendant ring in the spirocyclic cations. These results provide valuable insights towards the molecular design of highly thermochemically stable AEMs functionalised with N-spirocyclic QA cations.