Elucidating the Time Scale and Geometry of Phosphate and Phosphonate Rotation in Solid Acid Electrolytes Using Multinuclear NMR

The dynamics of the anions in a series of solid acid electrolyte materials have been determined using solid-state 31P CODEX NMR. The time scale and activation energy of the phosphate reorientation were quantified; however, the characterization proved to be nontrivial due to interactions with the quadrupolar 133Cs nuclei. The series of compounds studied included other known proton conductors, where the alkali cation was replaced systematically with Rb+, Tl+, or K+. The nuclear properties of the cations, together with the relative proton conductivities of these electrolyte analogues, allowed for a complete analysis of the interactions (both structure reorientations and spin dynamics) giving rise to the observed CODEX buildup curves. The high-temperature super-protonic transition, and the narrow range of thermal stability, limits the usefulness of cesium dihydrogen phosphate (CDP) in fuel cell applications. With the goal of achieving a broader range of thermal stability and enhanced proton dynamics, the phosphate anion was substituted for methyl phosphonate. This altered the type of motion available to the anion and significantly reduced the energy barrier for anion rotation in this electrolyte.