Joel Olsson

N-Spirocyclic Quaternary Ammonium Ionenes for Anion-Exchange Membranes

The development of cationic polymers for anion-exchange membranes (AEMs) with high alkaline stability and conductivity is a considerable challenge in materials chemistry. In response, we here present the synthesis and properties of N-spirocyclic quaternary ammonium ionenes (spiro-ionenes) containing 5- and 6-membered rings fused by central nitrogen cations. High-molecular weight and film-forming spiro-ionenes are successfully synthesized in cyclo-polycondensations of tetrakis(bromomethyl)benzene and dipiperidines under mild conditions. These polyelectrolytes show excellent thermal and alkaline stability with no degradation detected by NMR spectroscopy after more than 1800 h in 1 M KOD/D2O at 80 °C. Even at 120 °C, the spiro-ionenes display reasonable alkaline stability. Transparent and mechanically robust AEMs based on ionically cross-linked blends of spiro-ionene and polybenzimidazole reach OH- conductivities up to 0.12 S cm-1 at 90 °C. The current findings demonstrate that spiro-ionenes constitute a new class of alkali-stable anion-exchange polymers and membranes.

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

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.