Phase Diagram Approach to Study Acid and Water Uptake of Polybenzimidazole-Type Membranes for Fuel Cells

Polybenzimidazole (PBI) membranes doped with phosphoric acid are the most commonly used materials for high-temperature polymer electrolyte fuel cells. Despite this fact, only little is known on the chemical equilibria of the species inside the membrane as a function of the composition of the ternary system PBI - H3PO4 - H2O. This includes details on the condensation equilibria, leading to the formation of diphosphoric acid and higher homologues species and on the proton transfer processes, i.e. on the dominant conduction mechanisms. As a first step to understand the thermodynamic equilibria in the ternary system PBI - H3PO4 - H2O the phase diagrams for two different polymers, poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole) (m-PBI) and the recent commercially available Fumapem AM-55, were evaluated from own and from experimental data found in literature. Miscibility gaps are identified and equilibrium lines separating one- and two-phase fields in the diagrams are estimated.