Mass-Transport and Heterogeneous Electron-Transfer Kinetics Associated with the Ferrocene/Ferrocenium Process in Ionic Liquids

The ferrocene/ferrocenium (Fc 0/+ ) 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. Fc 0/+ 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 Fc 0/+ process at a platinum electrode are reported in 14 ILs with dynamic viscosities (η) ranging from 20 to 620 cP. The diffusivity of Fc ( D Fc ) 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 ( k 0 ) associated with the Fc 0/+ process was measured in each of the ILs using large-amplitude Fourier transformed alternating current (FTAC) voltammetry, and a plot of ln( k 0 ) 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( k 0 ) versus ln(η) data suggests a slight dependence of k 0 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( k 0 ) versus ln(η) plots to η values typically encountered in acetonitrile-based electrolyte media (i.e., 0.5 cP) predicts D and k 0 values of approximately 2 × 10 –5 cm 2 s –1 and 10 cm s –1, in excellent agreement with literature reports. Overall, the results presented in this study strongly suggest that the Fc 0/+ redox couple displays the characteristics of an “ideal” outer-sphere electron transfer process in IL media.