Electrochemical impedance spectroscopy and cyclic voltammetry of ferrocene in acetonitrile/acetone system

Abstract

The oxidation of ferrocene (FeCp₂) to ferrocenium cation (FeCp ⁺₂ ) (where Cp: cyclopentadienyl anion, C₅H₅ ⁻) was investigated by means of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) on either platinum (Pt) or glassy carbon (G-C) electrodes in acetonitrile (ACN), acetone (ACE), and acetonitrile (ACN)/acetone (ACE) binary mixtures with n-tetrabutylammonium hexafluorophosphate (TBAPF₆) as background electrolyte at T = 294.15 K. The half-wave potentials (E _1/2), the diffusion coefficients (D), and the heterogeneous electron-transfer rate constants (k _s) were derived. The activation free energies for electron transfer (ΔG ^≠_exp ) were experimentally determined and compared with the theoretical values (ΔG ^≠_cal ). The electron-transfer process was reversible and diffusion-controlled in all investigated solvent mixtures. The changes on the metal–ligand bond lengths upon electron transfer were almost insignificant. The E _1/2 values were shifted to less positive potentials with the increase of the ACN content. The k _s values obtained on Pt electrode were slightly larger compared to k _s measured on G-C electrode, while in both cases the k _s values were diminished with the enrichment of the mixtures in ACN. The EIS spectra confirmed that the rate-determining step in the whole process is the diffusion of the FeCp₂ species and thus the process can be properly characterized as diffusion-controlled.