25.4: Cyclic Voltammetry

In linear sweep voltammetry we scan the potential in one direction, either to more positive potentials or to more negative potentials. In cyclic voltammetry we complete a scan in both directions. Figure \(\PageIndex{1}\)a shows a typical potential-excitation signal. In this example, we first scan the potential to more positive values, resulting in the following oxidation reaction for the species R.

\[R \rightleftharpoons O+n e^{-} \label{cv1} \]

When the potential reaches a predetermined switching potential, we reverse the direction of the scan toward more negative potentials. Because we generated the species O on the forward scan, during the reverse scan it reduces back to R.

\[O+n e^{-} \rightleftharpoons R \label{cv2} \]

Cyclic voltammetry is carried out in an unstirred solution, which, as shown in Figure \(\PageIndex{1}\)b, results in peak currents instead of limiting currents. The voltammogram has separate peaks for the oxidation reaction and for the reduction reaction, each characterized by a peak potential and a peak current.

Figure \(\PageIndex{1}\). Details for cyclic voltammetry. (a) One cycle of the triangular potential-excitation signal showing the initial potential and the switching potential. A cyclic voltammetry experiment can consist of one cycle or many cycles. Although the initial potential in this example is the negative switching potential, the cycle can begin with an intermediate initial potential and cycle between two limits. (b) The resulting cyclic voltammogram showing the measurement of the peak currents and peak potentials.

The peak current in cyclic voltammetry is given by the Randles-Sevcik equation

\[i_{p}=\left(2.69 \times 10^{5}\right) n^{3 / 2} A D^{1 / 2} \nu^{1 / 2} C_{A} \label{cv3} \]

where n is the number of electrons in the redox reaction, A is the area of the working electrode, D is the diffusion coefficient for the electroactive species, \(\nu\) is the scan rate, and C_A is the concentration of the electroactive species at the electrode. For a well-behaved system, the anodic and the cathodic peak currents are equal, and the ratio i_p,a/i_p,c is 1.00. The half-wave potential, E_1/2, is midway between the anodic and cathodic peak potentials.

\[E_{1 / 2}=\frac{E_{p, a}+E_{p, c}}{2} \label{cv4} \]

Scanning the potential in both directions provides an opportunity to explore the electrochemical behavior of species generated at the electrode. This is a distinct advantage of cyclic voltammetry over other voltammetric techniques. Figure \(\PageIndex{2}\) shows the cyclic voltammogram for the same redox couple at both a faster and a slower scan rate. At the faster scan rate, \(\PageIndex{2}\)a, we see two peaks. At the slower scan rate in Figure \(\PageIndex{2}\)b, however, the peak on the reverse scan disappears. One explanation for this is that the products from the reduction of R on the forward scan have sufficient time to participate in a chemical reaction whose products are not electroactive.

Figure \(\PageIndex{2}\). Cyclic voltammograms for R obtained at (a) a faster scan rate and at (b) a slower scan rate. One of the principal uses of cyclic voltammetry is to study the chemical and electrochemical behavior of compounds. See this chapter’s additional resources for further information.