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.

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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Imidazolium cations are promising candidates for preparing anion-exchange membranes because of their good alkaline stability. Substitution of imidazolium cations is an efficient way to improve their alkaline stability. By combining density functional theory calculations with experimental results, it is found that the LUMO energy correlates with the alkaline stability of imidazolium cations. The results indicate that alkyl groups are the most suitable substituents for the N3 position of imidazolium cations, and the LUMO energies of alkyl-substituted imidazolium cations depend on the electron-donating effect and the hyperconjugation effect. Comparing 1,2-dimethylimidazolium cations (1,2-DMIm+) and 1,3-dimethylimidazolium cations (1,3-DMIm+) with the same substituents reveals that the hyperconjugation effect is more significant in influencing the LUMO energy of 1,3-DMIms. This investigation reveals that LUMO energy is a helpful aid in predicting the alkaline stability of imidazolium cations.