Porous polybenzimidazole membranes doped with phosphoric acid: Highly proton-conducting solid electrolytes

New proton-conducting polymer electrolytes based on new porous films of polybenzimidazole doped with phosphoric acid have been obtained. The porous polybenzimidazole films were prepared by leaching out a low-molecular-weight compound using a selective solvent of the porogen from polymer/porogen mixtures. This method allows control over the porosity level up to porosities of 75%. The pore size and morphology strongly depend on the porogen/PBI ratio as observed by SEM. The pore size varies from < 100 nm with spherical shape for low porosities to very irregular and interconnected structures of 10-15 mum at the highest porosity levels. Different porogens lead to smaller pore sizes in the order dibutyl phthalate > dimethyl phthalate > diphenyl phthalate > triphenyl phosphate. The acid uptake of the membranes, and therefore the ionic conductivity of the films, increases with the porosity. Ionic conductivity as high as 5 x 10(-2) S/cm and mechanically stable membranes are easily obtained by soaking the highly porous films in phosphoric acid solutions.