Development of Highly Alkaline Stable OH–-Conductors Based on Imidazolium Cations with Various Substituents for Anion Exchange Membrane-Based Alkaline Fuel Cells

Abstract

We synthesized various imidazolium-based cations (IM+-1 to IM+-5) such as OH–-conducting groups, and the substitution effect of imidazoliums with substituents at their C2, C4, and C5 positions on their alkaline stability is investigated through 1H NMR spectroscopy and molecular simulation. The effect of the methoxy group at the C2 position on the imidazolium shows increased lowest unoccupied molecular orbital (LUMO) values, suggesting enhanced alkaline stability for the methoxy-substituted ones. However, the LUMO isosurface analysis, together with Mulliken charge and Fukui nucleophilic function values, indicate that the methoxy group makes the imidazolium ring more unstable. In addition, the alkyl chain length effect at the C4/C5 position reveals similar behaviors by showing that the longer alkyl group enhances the electrophilicity of the imidazolium ring, making it unstable against OH– ions. This investigation reveals an important information that density functional theory calculation-based comparisons of the stability of the conductors should be performed by considering not only the LUMO energy but also other parameters such as the LUMO isosurface of the developed conductors. Overall, the IM+-1 and IM+-2 with methyl and ethyl groups at C4/C5 and the 2,6-dimethylphenyl group at the C2 position reveal much higher alkaline stability than the typically used conductors in anion exchange membranes.