Effects of Substituents and Substitution Positions on Alkaline Stability of Imidazolium Cations and Their Corresponding Anion-Exchange Membranes

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 ¹H 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.