Ex-situ NMR and Raman studies of H3PO4 and H2O uptake of polybenzimidazole membranes for PEMFC

Polybenzimidazole (PBI) doped with H3PO4 is the most commonly used membrane material for high-temperature polymer fuel cells. Despite this fact, only less is known on the chemical equilibria of all species inside the membrane as a function of the composition of the ternary system PBI - H3PO4 - H2O. This includes details on the proton transfer processes, on the dominant conduction mechanisms and on the condensation equilibria, leading to the formation of diphosphoric acid and higher homologues species. In this study Raman and NMR spectroscopy is used to investigate the chemical interactions between H3PO4, H2O and PBI vs. the H3PO4 doping level. We could obtain information on the H-bond formation between H3PO4 and the polymer chains, on tautomeric processes as well as on the presence of not directly bounded H3PO4 at high doping levels. Investigations were performed with uncrosslinked and crosslinked m-PBI and AB-PBI.