Electrochemical and physicochemical properties of cyclic amine-based Brønsted acidic ionic liquids

Several low-cost ionic liquids (ILs) containing pyrrolidinium, piperidinium, or azepanium units as cations, and formate, trifluoroacetate, or hydrogen sulfate groups as anions, are synthesized. The electrochemical and physicochemical properties, such as viscosity, density, conductivity, electrochemical window of the ILs, and the diffusion coefficient of ferrocene in the ILs are studied. All ILs studied here are liquid at room temperature with a high ionic conductivity (up to 30.7 mS cm−1) at room temperature. Hydrogen sulfate-based ILs exhibits a larger electrochemical window (3.98–3.99 V) than formate- and trifluoroacetate-based ILs (3.22–3.29 V). The results of this study can be used to choose the proper combination of cations and anions for the desired applications.