Enhancing Solid-State Conductivity through Acid or Base Doping of Protic Imidazolium and Imidazolinium Triflate Salts

Protic salts, including those showing plastic crystal behavior, are an interesting family of materials being rediscovered for use as solid conducting materials. This work compares two such compounds, 2-methylimidazolium triflate, [2-MeHIm][TfO], and 2-methylimidazolinium triflate, [2-MeHImn][TfO]. This subtle change in structure resulted in substantial differences in thermal behavior; [2-MeHIm][TfO] shows two solid-solid phase transitions while none were observed for [2-MeHImn][TfO]. However, both compounds are in a disordered solid state (ΔS_f < 40 JK^-1 mol^-1) resulting in relatively high room temperature conductivities. Neat [2-MeHImn][TfO] had a conductivity above 10^-5 S cm^-1, but the addition of either the component acid or base as dopants resulted in a substantial increase in conductivity, to over 10^-4 S cm^-1 at 30 °C. Solid-state ¹H and ¹⁹F NMR studies suggest that the dopants enhance the conduction primarily by increasing the proportion of mobile ions in the solid matrix. However, [2-MeHIm][TfO] also displays evidence of proton hopping via the cation.