Investigation of the conduction properties of phosphoric and sulfuric acid doped polybenzimidazole
When complexed with a strong inorganic acid such as sulfuric or phosphoric acid, films of polybenzimidazole (PBI) show two types of behaviour, depending on the time of immersion in the corresponding acid bath. The first type, prepared at shorter doping times, has conductivity in the range 10–5–10–4 S cm–1, whilst the second, of conductivity >10–3 S cm–1 is formed after more prolonged immersion. The ‘switch-over' from one state to the other is at 10–11 h in H3PO4, and 2–3 h in H2SO4. PBI has a remarkable capacity to concentrate H3PO4 and, even in an acid bath of concentration 3 mol dm–3, the acid concentration within a PBI membrane is ca. 14.5 mol dm–3. IR spectroscopy performed on hydrated PBI membranes as a function of temperature, and on acid-complexed membranes as a function of the amount of sorbed acid confirms proton transfer from H3PO4 to the imino groups of PBI and, at high doping levels, the presence of undissociated H3PO4.