Proton conductivity of phosphoric acid doped polybenzimidazole and its composites with inorganic proton conductors

Abstract

Phosphoric acid doped polybenzimidazole (PBI) and PBI composite membranes have been prepared in the present work. The PBI composites contain inorganic proton conductors including zirconium phosphate (ZrP), (Zr(HPO 4) 2· nH 2O), phosphotungstic acid (PWA), (H 3PW 12O 40· nH 2O) and silicotungstic acid (SiWA), (H 4SiW 12O 40· nH 2O). The conductivity of phosphoric acid doped PBI and PBI composite membranes was found to be dependent on the acid doping level, relative humidity (RH) and temperature. A conductivity of 6.8×10 −2 S cm −1 was observed for PBI membranes with a H 3PO 4 doping level of 5.6 (mole number of H 3PO 4 per repeat unit of PBI) at 200 °C and 5% RH. A higher conductivity of 9.6×10 −2 S cm −1 was obtained by composite of 15 wt.% of ZrP in a PBI membrane under the same conditions. Homogeneous membranes with good mechanical strength were prepared by introducing PWA (20–30 wt.%) and SiWA (20–30 wt.%) into PBI, and their conductivity were found to be higher than or comparable with that of the PBI membrane at temperatures up to 110 °C.