The Difference in the Effects of <i>IR</i>-Drop from the Negative Capacitance of Fast Cyclic Voltammograms

Abstract

Diffusion-controlled cyclic voltammograms at fast scan rates show peak shifts, as well as decreases in the peak currents from predicted diffusion-controlled currents, especially when the currents are large in a low concentration of supporting electrolytes. This has been conventionally recognized as an IR-drop effect due to solution resistance on the peaks, as well as a heterogeneously kinetic effect. It is also brought about by the negatively capacitive currents associated with charge transfer reactions. The reaction product generates dipoles with counterions to yield a capacitance, the current of which flows oppositely to that of the double-layer capacitance. The three effects are specified here in the oxidation of a ferrocenyl derivative using fast scan voltammetry. The expression for voltammograms complicated with IR-drop is derived analytically and yields deformed voltammograms. The peak shift is approximately linear with the IR-voltage, but exhibits a convex variation. The dependence of some parameters on the peaks due to the IR-drop is compared with those due to the negative capacitance. The latter is more conspicuous than the former under conventional conditions. The two effects cannot be distinguished specifically except for variations in the conductance of the solution.

Keywords

• <i>IR</i>-drop
• negative capacitance
• double-layer capacitance
• fast scan cyclic voltammograms
• microelectrode reactions