| The heterogeneous electron transfer rate constant for the redox couple ferrocene/ferrocenium (Fc/Fc+) in various bis((trifluoromethyl)sulfonyl)imide-based ionic liquids was investigated as a function of temperature and solution viscosity. In most solvents, the electron transfer rate for Fc/Fc+ is found to be extremely fast. However, this does not appear to be the case in viscous ionic liquids. A number of different electrochemical techniques have been used to study the heterogeneous electron transfer kinetics of soluble redox couples, including cyclic voltammetry, rotating disk electrode voltammetry, and electrochemical impedance spectroscopy. Only the latter technique gives good results for systems that exhibit fast kinetics. In this work, we measured the standard heterogeneous rate constant and the diffusion coefficient for ferrocene and ferrocenium in ionic liquids based on the tri-1-butylmethylammonium, 1-methyl-3-ethylimidazolium, 1-butyl-3-methylimidazolium, and 1-butylpyridinium cations and the bis((trifluoromethyl)sulfonyl)imide anion. This work has also necessitated measurement of the physical and transport properties of the 1-methyl-3-ethylimidazolium bis((trifluoromethyl)sulfonyl)imide, 1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide, and 1-butylpyridinium bis((trifluoromethyl)sulfonyl)imide ionic liquids as a function of temperature because this data does not currently exist. Additionally, viscosity and density data are needed in order to test the applicability of the Stokes-Einstein equation. |
