Proton dynamics in sulfonated ionic salt composites: Alternative membrane materials for proton exchange membrane fuel cells

Hydrated Nafion, the most prevalent proton exchange membrane utilizes a vehicular mechanism for proton conduction. However, there is an increasing need for such membranes to perform under anhydrous conditions, at high temperatures, which would employ a structural transport mechanism for proton conductivity. Here, several solid-acids are characterized, both as pristine salts, and as polymer composites. Materials of interest include benzimidazolium methanesulfonate (BMSA), imidazolium methanesulfonate (IMSA), and imidazolium trifluoromethanesulfate (IFMS). The proton dynamics of these solid acids are characterized as pure salts, and as composites, embedded into porous Teflon, by solid state NMR. It was determined that spin lattice (T1) relaxation of the composites are systematically lower than that of the pure salt, indicating that local dynamics are enhanced in the composites. Spin–spin relaxation (T2∗) was measured as a function of temperature to determine the activation energy for local mobility for each salt and composite. The activation energy for local proton mobility in each salt decreased after being inserted into porous Teflon. Finally, the long-range ion transport was characterized using impedance spectroscopy. The IFMS–Teflon composite possessed the lowest activation energy for local proton mobility, the highest thermal stability, and the most favorable proton conductivity, among the investigated materials.