The proton dynamics of imidazole methylphosphonate: an example of cooperative ionic conductivity

Abstract

Imidazole methylphosphonate models the hydrogen bonding and dynamics of a potential anhydrous polymer electrolyte. Understanding the behavior of anhydrous electrolytes is crucial to creating polymer materials with better performance in a fuel cell environment. This model salt exhibits ionic conductivity in the solid-state and the method of ion conduction in the solid-state differs depending on the choice of anion and cation pairs. Previous investigation of a sulfonate analogue suggests fast ring dynamics, which contributes to the ionic conductivity. However, ¹³C CODEX NMR shows that rotation of the imidazole ring is somewhat slower in the methlyphosphonate compound, with a timescale for the two-site ring flip of 31 ± 9 ms at ambient temperature. ³¹P CODEX and variable temperature ¹H MAS NMR spectra confirm that ionic conductivity is facilitated by dynamics at the bifurcated hydrogen bonds between anions, with a timescale of 57 ± 4 ms at ambient temperature for rotation of the phosphonate about the C_3v axis. Increasing temperature introduces thermal motion and promotes the rotation of the imidazole ring together with the rotation of the phosphonate group. This leads to a cooperative mechanism of ion conduction between imidazole and the methylphosphonate at higher temperatures, which was unseen in a previous study of the benzimidazole methylphosphonate analogue.