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

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• 作者：Cameron L. Bentley ; Jiezhen Li ; Alan M. Bond ; Jie Zhang
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：August 4, 2016
• 年：2016
• 卷：120
• 期：30
• 页码：16516-16525
• 全文大小：521K
• 年卷期：0
• ISSN：1932-7455

文摘

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 (k⁰) 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(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 Fc^0/+ redox couple displays the characteristics of an “ideal” outer-sphere electron transfer process in IL media.