Performance and stability of ether-free high temperature proton exchange membranes with tunable pendent imidazolium groups

Abstract

(Benz)imidazoliums are demonstrated to be effective for the retention of phosphoric acid (PA) in PA doped proton exchange membranes (PEMs) due to the formation of (benz)imidazolium–biphosphate ion-pairs. However, there is less research on the stability of the polymer materials with pendent (benz)imidazoliums. Herein, novel ether-free poly(biphenyl isatin-co-imidazolium)s (PIBI-Qx) were prepared as PA doped PEMs via a one-step superacid catalysed polycondensation and methylation process. The dissolution of PA/PIBI-Qx membranes at high temperature can be inhibited effectively via tuning the methylation degree of the pendent imidazole. Moreover, the PA retention and conductivity retention of the PA/PIBI-Q70 (70% imidazolium content) membrane are over 20% and 30% higher than those of the PA/mPBI membrane, respectively, after 100 h at 80 °C/40%RH. Thus, the corresponding cell is stable for at least 60 h at 160 °C with 8% RH, while a significant drop of 27% in the peak power density is observed for the PA/mPBI membrane-based cell. Furthermore, the thermal/acidic stability of PIBI-Qx membranes with pendent imidazolium groups was studied, and two dominating degradation mechanisms e.g. “de-methylation” of imidazolium and “retro-Friedel–Crafts reaction” in the polymer backbone are verified via XPS spectral analysis.