Synthesis of Poly (2,2′‐(1,4‐phenylene) 5,5′‐bibenzimidazole) (<i>para</i>‐PBI) and Phosphoric Acid Doped Membrane for Fuel Cells

Abstract A detailed investigation of the polymerisation conditions for poly(2,2′‐(1,4‐phenylene)5,5′‐bibenzimidazole) ( para ‐PBI) in polyphosphoric acid (PPA) led to the preparation of high molecular weight polymer. The polymer solutions could be used directly to produce phosphoric acid (PA)‐doped PBI membranes by the PPA process. These membranes showed very high PA doping levels, i.e., up to ∼30 moles of PA per mole of PBI repeat unit. The ionic conductivity of the membranes was 0.24 S cm –1 at 160 °C. Fuel cell performance was measured at various operating conditions by changing temperature, pressure and gases and verified the high carbon monoxide (CO) tolerance of fuel cells operating at this temperature.