The impact of poly (ionic liquid) on the phosphoric acid stability of polybenzimidazole-base HT-PEMs

Polybenzimidazole-based electrolyte membranes with stable proton carriers and high phosphoric acid retention continue to pose a challenge for applying high temperature polymer electrolyte membrane fuel cells. A series of highly conductive composite membranes based on fluorine-containing polybenzimidazoles (6FPBI) and poly (ionic liquid) (PIL) are prepared. The PIL contains epoxy groups, which can act as cross-linkers forming cross-linking networks by in-situ reaction to limit phosphoric acid leaking. Meanwhile, H2PO4− ions as proton carriers can be fixed to PILs by strong ionic force after base-acid doping process. The obtained composite membranes exhibited enhanced phosphoric acid stability, up to 73.1% at 400 h under 160 °C/0% RH and 76.6 at 120 h under 80 °C/40% RH. A strong correlation is found between the content of PIL and the proton conductivity. PIL containing membranes show higher proton conductivity (0.069 S cm−1 at 170 °C) than that of pristine 6FPBI (0.039 cm−1 at 170 °C) with similar PA uptake (151–171%). The composite membranes also show improved proton conductivity and mechanical properties. This work indicates that the PIL preparation strategy and the properties of the composite membranes pave the way for highly conductive stable proton-conducting membrane applications.