N3-adamantyl imidazolium cations: Alkaline stability assessment and the corresponding comb-shaped anion exchange membranes

In the pursuit of alkaline stable cationic ions as promising candidates for high performance anion exchange membranes, two novel imidazoliums with N3-adamantyl substituent along with four imidazoliums with N3-methyl and butyl-substituents were synthesized and systematically assessed the impact of N3- and C2-substitutions on their chemical stabilities. Substituent at the N3-position prevented the degradation of imidazolium, and bulky and rigid adamantyl substituent was the most effective. The 1-adamantyl-2-methyl-3-ethylimidazolium cation (AdMEIm) exhibited the highest alkaline stability among the six cationic ions, which is stable under 5 M NaOH aqueous solution at 80 degrees C for 168 h. Combined with the highly chemical stability of N3-adamantyl substituted imidazolium and excellent chemical and thermal stability of poly(2,6-dimethyl phenylene oxide) (PPO), the PPO-based AEMs functionalized with N3-adamantyl substituted imidazolium were further synthesized and characterized. However, unlike the small-molecule imidazolium salt with exceptional alkaline stability, a considerable ring-opening degradation of imidazolium was taken place for AEM functionalized with the stable N3-adamantyl substituent when exposed to 1 M NaOH aqueous solution at 60 degrees C for over 48 h, demonstrating that the alkaline stabilities of modular cation and the corresponding AEM seems less correlated. A single H-2-O-2 fuel cell using PPO-AdIm-34 membrane showed a peak power density of 46.0 mW cm(-2) and an open circuit voltage (OCV) of 0.90 V at 45 degrees C without any optimization of the electrodes, demonstrating that the as-prepared membranes have potential application in AEMFCs.