Poly(arylene alkylene piperidinium)s show greatly improved alkaline stability and ion conductivity in comparison to current state of the art poly(arylene piperidinium)s.

Alicyclic quaternary ammonium cations having all the β-protons in a strain-free ring structure are in general highly base-resistant and are thus very attractive to employ for anion exchange membrane (AEM) applications. However, tethering cations such as N,N-dimethylpiperidinium (DMP) to polymer backbones without introducing any weak links is quite challenging. In the present study, we have attached pairs of piperidine rings in their 4-position to fluorene and 2,7-diphenylfluorene via methylene bridges using straightforward S N 2 reactions. These fluorenes were subsequently utilized as monomers in polyhydroxyalkylations to prepare poly(fluorene alkylene)s with different contents of the piperidine groups. AEMs were cast after quaternizing the piperidine groups to introduce DMP and spirocyclic 6-azonia-spiro-[5,5]undecane-6-ium (ASU) cations, respectively. The AEMs reached very high hydroxide ion conductivities, 100−156 mS cm −1 at 80 °C, in the ion exchange capacity range 1.8−2.4 mequiv g −1 . X-ray scattering showed ionomer peaks indicating ionic clustering with a characteristic distance d = 2.0−2.9 nm depending on the ion exchange capacity. The AEMs displayed high thermal stability, up to ∼250 °C, and 1 H NMR data indicated no degradation after storage in 5 M aq NaOH during 168 h at 90 °C. However, degradation started under very severe conditions (10 M, 90 °C) with ∼75% of the total ionic loss in all the AEMs assigned to Hofmann β-elimination. The overall results show that fluorene-based AEMs carrying DMP and ASU cations via methylene bridges display an attractive combination of ionic phase separation, thermal and chemical stability, and hydroxide conductivity, making them viable alternatives for use in alkaline fuel cells and water electrolyzers..

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A series of ether-free poly­(p-terphenyl piperidinium)­s carrying both cationic and zwitterionic groups were prepared and evaluated as anion-exchange membranes (AEMs). First, a zwitterionic monomer (zwPip) was synthesized by sulfoalkylation of N-methyl-4-piperidone (Pip) using 1,4-butane sultone. Pip and zwPip were then employed in superacid-mediated polyhydroxyalkylations with p-terphenyl to prepare precursor copolymers containing 20, 30, and 40 mol % of zwPip units. Next, the Pip units were fully quaternized in a Menshutkin reaction with iodomethane. AEMs cast from these copolymers with ion-exchange capacities between 1.8 and 2.2 m equiv g–1 displayed excellent hydroxide ion conductivities, reaching up to 171 mS cm–1 at 80 °C. At lower temperatures, 20 and 40 °C, the water uptake decreased with increasing zwitterionic content, most probably because of the increased ionic cross-linking through piperidinium–sulfonate complexes. AEMs containing 30 and 40% zwPip units showed only a minor ionic loss (<10%) when stored in 1 M aq NaOH solution at 80 °C over a period of 720 h. The overall results show that the co-introduction of zwitterionic and cationic groups is an efficient strategy to initiate ionic cross-linking and improve the performance of hydroxide-conducting AEMs.