1H–1H Double Quantum NMR Investigation of Proton Dynamics in Solid Acids

Abstract

Currently, the most popular proton exchange membrane (PEM) for fuel cell applications is Nafion. However, Nafion does not retain its high conductivity at high temperatures due to its dependence on water for proton transport. Because operational temperatures higher than the evaporation point of water are desirable, a family of solid acids was investigated. Cations known to transport protons were paired with anions to make acidic salts. Solid acids discussed here include imidazole paired with trifluoromethanesulfuric acid as well as imidazole, benzimidazole and adenine paired with methanesulfonic acid. Solid-state NMR was utilized to show the relative mobility of protons through double-quantum filter (DQF) experiments. The POST-C7 homonuclear dipolar-recoupling scheme was paired with DUMBO homonuclear decoupling to produce 1H double-quantum coherence buildup curves for the hydrogen-bonded protons of interest. Experimental buildup curves, which reflect both local structure as well as dynamics, are compared to theoretical curves of the static system. The SPINEVOLUTION-simulated curves utilized up to eight pairs of homonuclear dipolar couplings within a sphere of 7 Å diameter centered on the proton of interest. Steep buildup of the DQ curve and maxima at short recoupling times in the buildup curves indicate strong dipole–dipole coupling and are interpreted to indicate limited dynamics of the H-bonded protons. In contrast, shallower buildup curves and maxima at longer recoupling times imply that H-bonded protons (in an otherwise similar structure) are associated with local mobility, which reduces their local dipolar coupling and may facilitate proton transport. Bulk proton conductivities measured via electrochemical impedance spectroscopy were compared to DQF measurements to understand proton conduction within these materials.