Ion transport in a class of imidazole-based liquid/solid protic ionics

Abstract

A class of protic ionic-compounds were prepared by Brønsted acid–base reaction of imidazole or benzimidazole with one of the following acids: trifluoromethanesulfonic, nonafluorobutanesulfonic, para-toluenesulfonic and trifluoroacetic. Except those based on trifluoroacetic acid, all prepared compounds are thermally stable up to at least 270 °C. They are solid up to temperatures between 134 and 220 °C, depending on their constituent acid and base. A simple physico-mathematical model of ion motion in the lattice was developed and implemented to correctly interpret frequency-dependent electrical response of these materials, particularly in the solid state, and investigate their ion-conducting behavior as a function of temperature. These ionic compounds display sensible ionic conductivity up to ca. 5 × 10⁻⁴ and 5 × 10⁻² S cm⁻¹ in the solid and molten state, respectively, under fully anhydrous conditions. The presence of absorbed water, after brief exposure to an ambient atmosphere, enhances conduction properties remarkably. Conductivity values up to 10⁻³ and 10⁻¹ S cm⁻¹ were registered, respectively, in the solid and molten state, after short exposure to (humid) ambient air. It is argued how absorbed water molecules may remove protons from (ImH)⁺ or (BImH)⁺ groups, thereby enabling a chain mechanism of proton-hopping through non-protonated Im or BIm sites. It is discussed how these results and methods may inspire designing protic ionic-materials at the solid-state, with enhanced proton conduction even under fully-anhydrous conditions.