Part 4: Cyclic voltammetry - IV

5. Faraday current and charging current

In cyclic voltammetry, besides of the Faraday current caused by the redox reaction, the charging current, the current flows due to charge and discharge in the electric double layer formed at the electrode interface. This is generally corrected by subtracting the measured (background) cyclic voltammetry of the electrolyte containing no redox substance.

Fig. 4-5-1 shows the cyclic voltammogram (red line) and overwrited background (brown line), and the red line subtracted the background, the (blue line).

Fig. 4-5-1 Cyclic voltammogram before correction (red line), background (brown line) and after correction (blue line)

6. Quasi-reversible and non-reversible cyclic voltammetry

An electrochemical reaction with a slow electron transfer rate at the electrode interface is called as a non-reversible reaction. This is due to the reaction rate constant k0 (n is the number of electrons transferred).

                                    Reversible: k0 > 0.3(nv)1/2
                                    Quasi-reversible: 2 x 10-5 (nv)1/2 < k0 < 0.3(nv)1/2
                                    Non-reversible: k0 < 2 x 10-5 (nv)1/2

Fig. 4-6-1 is a cyclic voltammogram of a quasi-reversible reaction, which the reaction rate k0 of cyclic voltammogram is changed using simulation software. In such cyclic voltammogram, although the peak potential difference spreads, the midpoint potential Em can be used to estimate the standard electrode potential E0 if the extent of spread is almost the same on the oxidation side and the reduction side. While, when a clear peak does not appear, it becomes difficult to estimate the standard electrode potential E0 from the midpoint potential Em.

Fig. 4-6-1 Cyclic voltammograms of varying reaction rate.

As described above, the criteria for classification of reversible and non-reversible reactions using reaction rate constants depend on the number of electrons tranferred (n) and the square root of the scan rate (v), respectively. Thus, for example, in the case of one-electron reaction of k0 = 0.05 cm/s, it is quasi-reversible at v = 0.1 V/s, but reversible at v = 0.01 V/s. Thus, even if the sample to be measured is the same, in some cases it become possible to analyze as a reversible reaction by reducing the scan rate of cyclic voltammetry.