A thermodynamic approach to proton conductivity in acid-doped polybenzimidazole

Conductivity measurement on films of acid-doped polybenzimidazole (PBI) was performed by impedance spectroscopy. A complete set of data for PBI–H3PO4 and PBI–H2SO4 complexes is reported as a function of temperature (30–90°C) and isostatic pressure (1–4000 bars). Experimental results indicate an activated mechanism for the proton migration. Isobar and isothermal conductivity data allow the determination of the characteristic thermodynamic parameters ΔV* (4–10 cm3 mol−1), ΔH* (0.6–1.1 eV) and ΔS* (40–190 J mol−1 K−1). The dependence of these parameters on temperature and pressure is consistent with thermodynamic predictions. Based on a previous IR spectroscopic study of PBI–acid complexes, a microscopic model is developed, suggesting proton transfer from one imide site to another in which the anionic species participate by a Grotthus mechanism.