Ion Transport in a Class of Imidazole-Based Liquid / Solid Protic-Ionics

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. Excepting 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 ion conductivity up to c.a. 510-4 and 510-2 S cm-1 in the solid and molten state, respectively, and fully anhydrous conditions. Presence of absorbed water, after brief exposure to ambit 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.