Functionalizing polystyrene with N-alicyclic piperidine-based cations via Friedel-Crafts alkylation for highly alkali-stable anion-exchange membranes

Different anion-exchange membranes (AEMs) based on polystyrene (PS) carrying benzyltrimethyl ammonium cations are currently developed for use in alkaline fuel cells and water electrolyzers. However, the stability of these state-of-the-art cations needs to be further improved. Here, we introduce highly alkali-stable mono- and spirocyclic piperidine-based cations onto PS by first performing a superacid mediated Friedel-Crafts alkylation using 2-(piperidine-4-yl)propane-2-ol. This is followed by quaternization of the piperidine rings either using iodomethane to produce N,N-dimethylpiperidinium cations or cyclo-quaternizations using 1,5-dibromopentane and 1,4-dibromobutane, respectively, to obtain N-spirocyclic quaternary ammonium cations. Thus, it is possible to functionalize up to 27% of the styrene units with piperidine rings, and subsequently achieve complete quaternization. The synthetic approach ensures that all the sensitive β-hydrogens of the cations are present in ring structures to provide high stability. AEMs based on these polymers show high alkaline stability and less than 5% ionic loss was observed by ¹H NMR spectroscopy after 30 days in 2 M aq. NaOH at 90 °C. AEMs functionalized with N,N-dimethylpiperidinium cations show a higher stability than the ones carrying N-spirocyclic quaternary ammonium. Careful analysis of the latter revealed that the rings formed in the cyclo-quaternization are more prone to degrade via Hofmann elimination than the rings introduced in the Friedel-Crafts reaction. AEMs with an ion exchange capacity of 1.5 meq g^-1 reach a hydroxide conductivity of 106 mS cm^-1 at 80 °C under fully hydrated conditions. The AEMs are further tuned and improved by blending with polybenzimidazole (PBI). For example, an AEM containing 2 wt% PBI shows reduced water uptake and much improved robustness during handling, and reaches 71 mS cm^-1 at 80 °C. The study demonstrates that the critical alkaline stability of PS-containing AEMs can be significantly enhanced by replacing the benchmark benzyltrimethyl ammonium cations with N-alicyclic piperidine-based cations.