The effect of acid doping on the conductivity of polybenzimidazole (PBI)

The protonic conductivity of polybenzimidazole (PBI) in various electrolytes was systemically studied. It was shown that blank PBI is a protonic insulator. Its conductivity measured in acid electrolyte is [similar to] 10-10 to 10-9 S- cm-1. The same conductivity was obtained for measurements taken on PBI used in a PEM fuel cell configuration. After doping in H2SO4, HCl, HNO3, HClO4 or H3PO4 it exhibited high protonic conductivity. The conductivity depends on the type of doping acid agent and its concentration. The highest conductivity obtained (i.e. 0.0601 S · cm-1 for PBI doped with 16 mol/L H2SO4) was as good as that of Nafion [registered trademark] 117. The conductivity changes in the order H2SO4 > H3PO4 > HClO4 > HNO3 > HCl for high concentrations (11-16 mol/L) of the doping acid. It was shown that the high conductivities of PBI films were obtained after doping them with H2O4, H3PO4 or HClO4. These high values make them suitable candidates for fuel cell applications. From these results, it was shown that the conductivity of blank PBI in acid media must be measured before the film is doped with acid.