Recently, water-free, proton conducting, polymer electrolytes are attracting attention due to their possible application as fuel cell membranes at intermediate temperatures (100 ºC to 200 ºC). Phosphonic acid groups are considered feasible anhydrous proton conducting moieties due to the high degree of proton self-dissociation arising from their intrinsic amphoteric character and high mobility of protonic charge carriers. In this work, we have synthesized and characterized model, phosphonic acid functionalized proton conducting polymers, poly(vinylbenzyloxy-alkyl-phosphonic acid)s, for the purpose of exploring the relationship between molecular architecture and performance characteristics. These novel proton conducting materials were characterized for their thermal stability, nanostructure, and performance properties. Thermogravimetric analysis (TGA) indicates that the polymers are thermally stable up to 140 °C, where the condensation of phosphonic acid groups starts to occur. Results from small angle X-ray scattering (SAXS) show a peak corresponding to a Bragg spacing of approximately 21 Å to 24 Å, which is attributed to layer-like structure formation of the phosphonic acid containing conducting channels. The proton conductivity increases with temperature, reaching a value on the order of 3 x 10-4 S/cm at 140 °C under nominally anhydrous conditions.
Citation: Chemistry of Materials
Pub Type: Journals
fuel cells, polymers, dynamics, conductivity