Promising Components for High-Temperature PEM Fuel Cells: Thermally Stable and Highly Proton Conducting Heteropoly Acids

Heteropoly acids (HPA) have attracted attention of proton exchange membrane fuel cell community because of their exceptionally high proton conductivities, e.g., 0.2 sec/cm for 12-phosphotungstic acid at room temperature. TGA, DRIFTS, MAS, and PFGSE NMR techniques were used to investigate various HPA with respect to their thermal stability, hydrogen bonding, proton acidity, and proton diffusion coefficients. TGA measurements indicated the HPA in interest maintained a considerable amount of water and proton up to > 400°C. MAS ¹H chemical shifts of HPA were in the range of 8-11, compounds with main group elements in the center consistently showing lower values than those with transition metal elements as the heteroatom. Diffusion coefficients of the HPA were mostly in the order of 10_-6 sq cm/sec, much higher than those of other types of proton conducting solids.