The alkaline stability of imidazolium salts and imidazolium-based alkaline anion-exchange membranes (AEMs) was investigated in this work. C2-substituted (with methyl, isopropyl or phenyl groups) imidazolium salts, 3-ethyl-1,2-dimethyl imidazolium bromine ([EDMIm][Br]), 3-ethyl-2-isopropyl-1-methylimidazolium bromine ([EIMIm][Br]), and 3-ethyl-1-methyl-2-phenyl- imidazolium bromine ([EMPhIm][Br]), were synthesized and characterized. The effect of the C2-substitution on the alkaline stability of imidazolium salts was investigated by 1H and 13C NMR spectroscopy. Compared with the C2-unsubstituted imidazolium salt, 3-ethyl-1-methylimidazolium bromine ([EMIm][Br]), the alkaline stability of C2-substituted imidazolium salts is significantly enhanced at elevated temperatures, probably due to the steric hindrance of the substituents, which protected the imidazolium cations against the hydroxide attack. Moreover, the higher LUMO energies may also improve the alkaline stability of imidazolium salts. The alkaline stability of C2-substituted imidazolium salts was found to be in the order [EDMIm][Br] > [EIMIm][Br] > [EMPhIm][Br]. This work provides a feasible approach for enhancing the chemical stability of C2-substituted imidazolium salts, which has potential applications for alkaline anion-exchange membranes.

Imidazolium cations with various N3-substituents (including methyl, butyl, heptyl, dodecyl, isopropyl, and diphenylmethyl groups) were synthesized and investigated in terms of their alkaline stability. The effect of the N3-substituent on the alkaline stability was studied by quantitative 1H NMR spectra and density functional theory (GGA-BLYP) calculations. The isopropyl substituted imidazolium cation ([DMIIm]+) with the highest LUMO energy value exhibited the highest alkaline stability in aqueous NaOH. The [DMIIm]+ cation also exhibited higher alkaline stability than that of a quaternary ammonium cation, benzyltrimethyl-ammonium ([BTMA]+), in CD3OD/D2O NaOH solution at elevated temperatures. This observation inspired the preparation of [DMIIm]+-based alkaline anion exchange membranes (AEMs) which showed high alkaline stability in alkaline solution.

The synthesis and characterization of pyrrolidinium cation based anion exchange membranes (AEMs) are reported. Pyrrolidinium cations with various N-substituents (including methyl, ethyl, butyl, octyl, isopropyl, 2-hydroxylethyl, benzyl, and cyclohexylmethyl groups) were synthesized and investigated with respect to their chemical stability in alkaline media. The influence of substitutions on alkaline stability of pyrrolidinium cations was investigated by quantitative 1H nuclear magnetic resonance spectroscopy (NMR) and theoretical approaches. N,N-Ethylmethyl-substituted pyrrolidinium cation ([EMPy]+) exhibited the highest alkaline stability in this study. The synthesized AEMs based on [EMPy]+ show promising alkaline stability in strongly basic solution. The study of this work should provide a feasible way for improving the alkaline stability of pyrrolidinium cation based AEMs.

Imidazolium cations with butyl groups at various substitution positions (N1-, C2-, and N3-), 1-butyl-2,3-dimethylimidazolium ([N1-BDMIm](+)), 2-butyl-1,3-dimethylimidazolium ([C2-BDMIm](+)), and 3-butyl-1,2-dimethylimidazolium ([N3-BDMIm](+)), were synthesized. Quantitative (1)H NMR spectra and density functional theory calculation were applied to investigate the chemical stability of the imidazolium cations in alkaline solutions. The results suggested that the alkaline stability of the imidazolium cations was drastically affected by the C2-substitution groups. The alkaline stability of imidazolium cations with various substitution groups at the C2-position, including 2-ethyl-1-butyl-3-methylimidazolium ([C2-EBMIm](+)), 1,2-dibutyl-3-methylimidazolium ([C2-BBMIm](+)), and 2-hydroxymethyl-1-butyl-3-methylimidazolium ([C2-HMBMIm](+)), was further studied. The butyl group substituted imidazolium cation ([C2-BBMIm](+)) exhibited the highest alkaline stability at the elevated temperatures. The synthesized anion-exchange membranes based on the [C2-BBMIm](+) cation showed promising alkaline stability. These observations should pave the way to the practical application of imidazolium-based anion exchange membrane fuel cells.

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