Mass-transport and heterogeneous electron-transfer kinetics associated with the ferrocene/ferrocenium process in ionic liquids

The ferrocene/ferrocenium (Fc0/+) redox couple is regarded as a kinetically facile process under voltammetric conditions. It also possesses a nearly "solvent independent" formal potential, and for this reason is commonly used as a "reference" redox system for electrochemical studies in nonaqueous electrolyte media. Fc0/+ has also been adopted as a "model system" in ionic liquid (IL) media, although conflicting reports on the mass-transport and kinetics have brought its "ideality" into question. In this study, the mass-transport and heterogeneous electron-transfer kinetics associated with the Fc0/+ process at a platinum electrode are reported in 14 ILs with dynamic viscosities (η) ranging from 20 to 620 cP. The diffusivity of Fc (DFc) was calculated in each of the ILs using convolution voltammetry and was found to be inversely proportional to the viscosity of the medium, as per the Stokes-Einstein relation (i.e., D ∝ 1/η). The heterogeneous electron-transfer rate constant (k0) associated with the Fc0/+ process was measured in each of the ILs using large-amplitude Fourier transformed alternating current (FTAC) voltammetry, and a plot of ln(k0) versus ln(η) was found to be linear, with a slope of -1.0, as predicted by the Marcus theory of electron transfer for an adiabatic process that involves predominantly solvent reorganization rather than inner-shell vibrations. Analysis of the ln(k0) versus ln(η) data suggests a slight dependence of k0 on the constituent anion of the IL, which is thought to arise due to electrostatic interactions between the anion and positively charged Fc+. Finally, extrapolating the D versus 1/η and ln(k0) versus ln(η) plots to η values typically encountered in acetonitrile-based electrolyte media (i.e., 0.5 cP) predicts D and k0 values of approximately 2 × 10-5 cm2 s-1 and 10 cm s-1, in excellent agreement with literature reports. Overall, the results presented in this study strongly suggest that the Fc0/+ redox couple displays the characteristics of an "ideal" outer-sphere electron transfer process in IL media.