High-temperature fuel cell membranes based on mechanically stable para-ordered polybenzimidazole prepared by direct casting

Poly[2,2′-(p-phenylene)-5,5′-bibenzimidazole] (pPBI) is synthesized by mixing 3,3′-diaminobenzidine tetrahydrochloride and terephthalic acid in polyphosphoric acid (PPA) at 240 °C for 12 h. pPBI can be cast in membrane form directly from a polymerization solution (direct casting) or from a methanesulfonic acid solution (MSA casting). Membranes obtained by direct casting are almost amorphous and have much higher acid doping levels and larger water uptake values than those prepared by MSA casting. As a result, the pPBI membrane from direct casting has a high proton conductivity of 0.15 S cm−1 (at 180 °C and 1% relative humidity) at an acid doping level of 8.78 mol H3PO4 per mole of polymer repeat unit. Moreover, this membrane has good mechanical stability (tensile strength of 57 MPa at 22 °C and 24% relative humidity) even at this high doping level.