Alkaline Chemical Stability of Polymerized Ionic Liquids with Various Cations
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2015 American Chemical Society. The success of long-lasting low-cost (nonplatinum) alkaline fuel cells is dependent on the development of anion exchange membranes (electrolyte separator) with high alkaline chemical stability. In this study, a series of methacrylate-based polymerized ionic liquids (PILs) were synthesized with various covalently attached cations: butylimidazolium, butylmethylimidazolium, trimethylammonium, pentamethylguanidinium, butylpyrrolidinium, and trimethylphosphonium. The alkaline chemical stability of these PILs was examined in tandem with their analogous ionic salts: 1-butyl-3-methylimidizolium chloride, 1-butyl-2,3-dimethylimidazolium chloride, tetramethylammonium chloride, benzyltrimethylammonium chloride, hexamethylguanidinium chloride, 1,1-butylmethylpyrrolidinium chloride, and tetramethylphosphonium chloride. The degradation mechanisms and extent of degradation were quantified using 1H NMR spectroscopy at various pHs (in D2O), and temperature. The PILs with imidazolium and pyrrolidinium cations showed enhanced chemical stability relative to the PILs with ammonium and phosphonium cations. Interestingly, direct correlations were not observed between the PILs and their analogous small molecule ionic salts; significant degradation was observed in imidazolium ionic salts, most notably at high temperature/high pH conditions, while the pyrrolidinium-, ammonium-, and phosphonium-based ionic salts showed no degradation under any of the conditions examined. Additionally, results on the imidazolium ionic salts showed that methyl substitution in the C2 position limited the ring-opening degradation reaction, whereas the PIL with the unsubstituted imidazolium actually showed higher chemical stability relative to its substituted PIL counterpart. Overall, the alkaline chemical stability of the PILs in this study showed no correlation to that of their analogous small molecule ionic salts, suggesting that alkaline chemical stability studies on small molecules may not provide a solid basis for evaluating alkaline stability in polymers, counter to the hypothesis in many previous studies.
