Polybenzimidazoles with pendant quaternary ammonium groups as potential anion exchange membranes for fuel cells

Abstract

A series of novel polybenzimidazoles with pendant quaternary ammonium groups (QPBIs) have been synthesized by grafting 4-methyl-4-glycidylmorpholin-4-ium chloride (MGMC) onto the polybenzimidazole with pendant amino groups (H 2N-PBI) at 80 °C for 10 h. The grafting yields are estimated to be 104, 84 and 50% by elemental analysis, respectively at the feed ratio of MGMC/H 2N-PBI = 2:1, 3:2 and 1:1. The remaining unreacted pendant amino hydrogen atoms of the H 2N-PBI were further used to react with the epoxy groups of bisphenol A diglycidyl ether (BADGE) for cross-linking. The hydroxide conductivity of the cross-linked QPBI membranes increases with an increase in ion exchange capacity (IEC) and temperature. The highest hydroxide conductivity of 0.056 S cm −1 has been achieved with the QPBI-2/1 (here, the ‘2/1’ refers to the molar ratio of MGMC to H 2N-PBI in feed) membrane at 80 °C under fully hydrated state. All the QPBI main chains show extremely good chemical stability in strong alkaline solution (6.0 M NaOH) at 60 °C. However, the quaternary ammonium groups of the QPBI membranes except the QPBI-1/1 are unstable (degradable) in 6.0 M NaOH at 60 °C leading to large reduction in hydroxide conductivity.