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

Abstract

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 5M NaOH aqueous solution at 80°C for 168h. 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 1M NaOH aqueous solution at 60°C for over 48h, demonstrating that the alkaline stabilities of modular cation and the corresponding AEM seems less correlated. A single H2-O2 fuel cell using PPO-AdIm-34 membrane showed a peak power density of 46.0mWcm−2 and an open circuit voltage (OCV) of 0.90V at 45°C without any optimization of the electrodes, demonstrating that the as-prepared membranes have potential application in AEMFCs.