Synthesis and characterization of main-chain type polyimidazolium-based alkaline anion exchange membranes

We report the synthesis of main-chain type cationic polyimidazoliums with various C2,4,5-substituents (including hydrogen, methyl, and phenyl groups) using progressive poly-Radziszewski reaction. The effect of C2,4,5-substituent on the chemical stability of polyimidazoliums was studied by characterizing small-molecule imidazolium cation under alkaline condition. The alkaline stability of imidazolium derivatives including 1,3-dihexyl-imidazolium ([DHIm]+), 1,3-dihexyl-2-methyl-imidazolium ([DHMIm]+), 1,3-dihexyl-2,4,5-trimethyl-imidazolium ([DHTMIm]+), 1,3-dihexyl-4,5-dimethyl-2-phenyl-imidazolium ([DHPDMIm]+), and 1,3-dihexyl-2,4,5-triphenyl-imidazolium ([DHTPIm]+) was investigated by proton nuclear magnetic resonance (1H NMR) and validated by density functional theory calculations (GGA-BLYP parameters). [DHDMPIm]+ cation with the highest reaction barrier energy value exhibited the highest chemical stability in CD3OD/D2O KOH solution at elevated temperatures. The synthesized anion-exchange membranes (AEMs) based on poly(1-dodecyl-2-phenyl-4,5-dimethyl-imidazolium) exhibited promising alkaline stability in strong base solution at 80 °C. These results potentially boost the practical application of main-chain type AEMs in alkaline fuel cells.