Alkaline Chemical Stability and Ion Transport in Polymerized Ionic Liquids with Various Backbones and Cations
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2016 American Chemical Society. The development of anion exchange membranes (AEMs) with high alkaline chemical stability, as well as high ion conductivity, is crucial to the implementation of long-lasting, low-cost (nonplatinum) alkaline fuel cells (AFCs). In this study, 12 polymerized ionic liquids (PILs) were synthesized with various backbones and cations (backbones: ethyl methacrylate, undecyl methacrylate, undecyl acrylate, styrene; covalently attached cations: butylimidazolium, trimethylammonium, butylpyrrolidinium). 1H NMR spectroscopy was employed to determine the alkaline degradation mechanisms and extent of degradation at high pH (in D2O) at 60 C for 1 week (168 h). For the butylpyrrolidinium cation, ethyl and undecyl methacrylate backbones proved to be more stable than the undecyl acrylate backbone. Styrene (vinylbenzene) functionalized with butylpyrrolidinium cation surpassed the stability of the benchmark benzyltrimethylammonium (BTMA) cation, where the former possessed the highest overall observed chemical stability with 0% degradation after 1 week in 20 mol equiv of KOH at 60 C. This butylpyrrolidinium styrene-based PIL, poly(VBBP-Br), also had a high bromide conductivity of 14.5 mS cm-1 at 60 C and 90% relative humidity. Additionally, poly(VBBP-Br) achieved the desirable AEM properties of relatively low water uptake (hydration number = 5.6 mol H2O/mol cation) at high ion exchange capacity (IEC = 4.1 mmol g-1). The styrene/butylpyrrolidinium pairing offers several advantages over the popular styrene/BTMA pairing (e.g., higher conductivity, higher chemical stability, lower water uptake), while similarly to BTMA, allows for facile functionalization that is easily incorporated into existing AEM styrene backbones. Overall, these results show the promise and practicality of the butylpyrrolidinium styrene-based AEM chemistry for AFC applications.
