Uptake of Protic Electrolytes by Polybenzimidazole Type Polymers: Adsorption Isotherm and Electrolyte/Polymer Interactions

Polybenzimidazol (PBI)membranes doped with phosphoric acid are used as protonconducting membrane in high-temperature polymer electrolyte fuel cells (HTPEFCs). There is no general model for the thermodynamics of the adsorption process over the whole accessible doping range for all published data. The interactions between H₃PO₄ and polymer chains, the polycondensation equilibria of H₃PO₄ and the implications on proton conductivity are still largely unknown. In this study, we demonstrate that the uptake of a protic electrolyte by a polymer with basic moieties, i.e. a PBI-type polymer, can be described satisfactorily with a BET-like adsorption isotherm. The adsorption equilibria of the uptake process are analysed using literature data on the H₃PO₄, H₂SO₄ and HClO₄ uptake of non-crosslinked m-PBI and AB-PBI as well as using our own investigations on the H₃PO₄ uptake of a commercial crosslinked PBI derivative, Fumapem AM-55. In addition to the thermodynamic data of the adsorption process, Raman data on m-PBI taken from the literature and our own Raman investigations on Fumapem AM-55 are taken into account. It is possible to correlate domains in the adsorption isotherms with specific features in the Raman spectra. Three stages for the uptake of a protic electrolyte can be distinguished: i) the protonation of the polymer chains, ii) the formation of H bonds directly with the chains and iii) the formation of H bonds with electrolyte molecules that are still adsorbed.