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 poly-benzimidazoles (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, (HPO4-)-P-2 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 degrees C/0% RH and 76.6 at 120 h under 80 degrees 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 degrees C) than that of pristine 6FPBI (0.039 cm(-1) at 170 degrees 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. (C) 2020 Elsevier Ltd. All rights reserved.