Quantitative Electroanalysis in Ionic Liquids Using Scanning Electrochemical Cell Microscopy

Steady-state voltammetry of the redox mediator ferrocene in the room temperature ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate was carried out using scanning electrochemical cell microscopy (SECCM) on a glassy carbon substrate. The measured linear sweep voltammograms (LSVs) were analyzed using recently developed analytical expressions, and validated using numerical modeling, to extract the diffusion coefficient, D, and heterogeneous rate constant, k⁰, where only the pipet half angle and pipet radius, measured using electron microscopy, were needed to determine the kinetic and transport parameters. Sufficient data were collected to quantify these parameters using statistical analysis providing values for D of 4.1 × 10^-7 cm² s^-1, and k⁰ of 1.0 × 10^-2 cm s^-1, which were in good agreement with literature values. Through the use of several different pipet diameters, it was possible to adjust the rate of mass transport of ferrocene, and it was shown that Koutecký-Levich analysis could be used to extract the kinetic current using SECCM, providing a value of k⁰ of 1.0 × 10^-2 cm s^-1, matching the results obtained using the analytical expression. Finally, individual LSVs exhibiting ultralow currents in the femtoampere and picoampere range were successfully fit using the analytical expression to map k⁰ across a carbon surface.