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 being developed for use in alkaline fuel cells and water electrolyzers. However, the stability in relation to 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-dimethyl piperidinium cations or by 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 of the sensitive ?-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-dimethyl piperidinium cations show 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 mequiv 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.