Uptake of protic electrolytes by polybenzimidazole-type polymers – Model for the absorption isotherm and electrolyte/polymer interactions

Phosphoric acid doped polybenzimidazol (PBI) membranes are commonly used as proton conducting membrane in high temperature polymer electrolyte fuel cells (HT-PEFC). In this study we want to present a general (thermodynamic) model for the absorption process of protic electrolytes by PBI-type polymers, which is able to describe the whole accessible doping range. We have analysed literature and own data on the uptake of phosphoric, sulphuric and perchloric acid by non-cross-linked m-PBI [1, 2] and AB-PBI [3] and by a commercial cross-linked PBI derivative (Fumapem AM-55©). The uptake of protic electrolytes by PBI-type polymers can be described satisfactorily by a BET-like adsorption isotherm (GAB model), assuming a multilayer-like adsorption process. In addition to the thermodynamic data spectroscopic data from Raman studies are taken into account [4]. It is possible to correlate domains in the adsorption isotherms to the protonation of the polymer chains, the formation of H-bonds directly to the chains and to still absorbed electrolyte molecules.