Electrochemical Behavior of Iron-based Imidazolium Chloride Ionic Liquids

In order to study the electrochemical properties of the iron-based imidazolium chloride ionic liquids, two kinds of ionic liquids - trivalent (Fe(III)-IL) and trivalent/divalent coexisting system (Fe(III/II)-IL) - were constructed by mixing 1-butyl-3-methylimidazolium chloride (BmimCl), FeCl3·6H2O and FeCl2·4H2O. [Bmim]+, [Bmim2Cl]+, [FeCl4]− and [Fe3Cl7]− in Fe(III/II)-IL were detected by UV-vis absorption spectroscopy and liquid chromatography - mass spectrometry with an electrospray ionization source (ESI-MS). The electrochemical behaviors of [FeCl4]− and [Fe3Cl7]− on a platinum electrode were investigated by cyclic voltammetry. Platinum microelectrode was used to reduce the effect of poor conductivity of these ionic liquids. Cyclic voltammetric data indicated that the electrode reaction of FeCl4-/[Fe3Cl7]− at platinum electrode was a quasi-reversible process. The electrode reaction of FeCl4− in the ionic liquids was followed by a homogeneous reaction. Both migration and diffusion of electroactive ions affected the electrochemical behavior of Fe(III)/Fe(II) on Pt microelectrode. The values of apparent diffusion coefficient, viscosity, and conductivity of Fe(III)-IL and Fe(III/II)-IL over the temperature range from 293 K to 333 K were measured, and the corresponding activation energies for mass and charge transport were calculated by Arrhenius equation. The activation energies of Fe(III)-IL and Fe(III/II)-IL are between 20 and 35 kJ·mol−1. Moreover, the influence of co-solvents, 1,2-dichloroethane and acetonitrile, on Fe(III/II)-IL was discussed. Results showed that addition of strong polar acetonitrile has more pronounced effect than less polar 1,2-dichloroethane on viscosity, conductivity and diffusion coefficient of Fe(III/II)-IL.