Cyclic Voltammetry

Cyclic voltammetry (CV) is a standard electrochemical analytical technique in which the potential applied to the working electrode is swept between two potential limits at a constant scan rate. The main advantage of this technique is its ability to easily visualize and provide a wealth of qualitative information about the redox reactions that are occurring.

Figure 5.2 presents an example input voltage and the resulting cyclic voltammogram.

The primary parameters that dictate CV are the initial potential (E0, point A), switching potential (E , point B), and the scan rate (⌫). The initial potential is the potential that is first applied between the working and reference electrodes at the beginning of the scan.

The scan rate has units of mV/s and is the rate at which the potential is changed. The applied potential is changed at this constant rate until the switching potential is reached.

The direction of the scan is then reversed, and the applied potential is changed at the same magnitude of scan rate until the initial potential is reached once more. This process can then be repeated for multiple cycles. The current response at the working electrode is measured throughout this process and current vs. potential plotted on a cyclic voltammogram.

For a three electrode cell, the initial potential is typically the positive limit of the potential window to be scanned. As the potential is scanned towards negative potentials, no redox reactions take place until the reduction potential of the analyte is reached, at which point a cathodic peak characteristic of analyte reduction appears. As the reduction peak grows, analyte immediately near the electrode surface is depleted, and a diffusion layer develops next to the electrode surface which replenishes analyte concentration from the bulk. At the point of maximum current, the rate of analyte consumption is equal to the rate of mass transfer to the electrode. As the potential is scanned further negatively, the measured current decreases as the system becomes mass transfer limited. When the switching potential is reached, the scan direction reverses and the same process occurs at the electrode for the reverse oxidation reaction. Typically, as the scan rate increases, the magnitudes of current for both redox

Figure 5.2: Left: Potential applied during CV; Right: Example cyclic voltammogram.

peaks increase due to less depletion of the diffusion layer at the electrode at higher scan rates.

Key parameters of interest from the cyclic voltammogram are the potential and current values of the cathodic (reduction) and anodic (oxidation) peaks (Ep,c, Ip,c and Ep,a, Ip,a

respectively. The separation of the peak potentials, the relative values of the peak currents, and the relationship between the peak potentials and currents with scan rate can provide insight into the reversibility and kinetic values of the redox reaction.

A Nernstian redox couple is defined as a redox couple that exhibits reversible behavior and fast electrode kinetics and therefore follows the Nernst equation (Equation 5.9). U is the cell potential at open circuit, U^⇥ is the standard reduction potential of the reaction, R is the universal gas constant, T is the temperature, n is the number of electrons involved in reaction, F is Faraday’s constant, mi is the molality of species i, and si is the stoichiometric coefficient of species i. The Nernst equation relates the equilibrium potential to the concentrations of reactants and produts. The potential difference between the reduction peak (negative) and oxidation peak (positive), Ep, should be equal to 59/n mV at 25 [8] for Nernstian couples.

The reactions occuring at the electrodes can be said to diffusion controlled as the rate at which reaction occurs depends solely on the rate of mass transport of the reduced or oxidized species to the electrode surface. While the strength of CV lies in its ability to quickly and easily gather qualitative data about a set of redox reactions, some quantitative information can be gathered as well,

as-suming a Nernstian couple. By varying the scan rate and relating it to the resulting peak currents, the diffusivity of the analyte may be calculated. This relationship is described by the Randles-Sevcik Equation (Eq. 5.10), where F is Faraday’s constant, R is the universal gas constant, T is temperature, n is the number of electrons involved in the reaction, A is the reaction area, DO is the analyte diffusivity, CO is the bulk analyte concentration, and ⌫ is the scan rate. At 25 C, this relationship reduces to Eq. 5.11.

ip = 0.4463 The key assumptions that provide validity to this relationship are fast electrode kinetics, constant area, and constant bulk concentration. Assuming a Nernstian couple, this therefore requires a robust experimental setup to ensure these criteria are satisfied. Plotting the peak current (ip) against the square root of the scan rate (⌫^1/2) results in a straight line whose slope is proportional to the diffusivity of the analyte.

5.3.3 Chronoamperometry

Chronoamperometry (CA) is another potential-controlled electrochemical analytical tech-nique. Instead of gradually changing the applied potential as in CV, a sustained potential step is applied to the electrode and the resulting current response over time is measured.

While the current response is still dependent on reaction kinetics, CA allows for the transient response to be investigated and analyzed. An example chronoamperogram is presented in Figure 5.3.

The potential steps chosen for chronoamperometry are typically first identified using a potential sweep method such as CV in order to locate the redox potentials for the system. As seen in the example, multiple potential steps may be performed in succession, corresponding to potentials of separate reduction or oxidation processes. In this instance, a cathodic po-tential is first applied (Point A), and the current quickly increases to its maximum cathodic current and decays following an inverse square root law to a steady state value. As the current decreases to steady state, the concentration of analyte at the electrode surface is depleted until the reaction rate is completely mass transfer controlled and dictated by the diffusion of species to the electrode surface. Once sufficient time has been allowed for steady state behavior, a second, anodic potential is applied (Point B), and the reverse reaction occurs. Deviations from the inverse square root decay indicate non-Nernstian behavior and can provide insight into reaction kinetics.

Chronoamperometry provides a more quantitative approach to characterizing kinetic pa-rameters. The Cottrell equation (Eq. 5.12) provides another method for calculating the diffusivity of an analyte via the transient current response. On a Cottrell plot, the current

response (i(t)) is plotted against the inverse square root of time (t ^1/2). Again, for a Nern-stian process, this results in a straight line whose slope is proportional to the diffusivity of the analyte.

Figure 5.3: Top: Potential applied during CA; Bottom: Chronoamperometric current re-sponse.

i(t) = id(t) = nF AD_O^1/2C_O^⇤

⇡^1/2t^1/2 (5.12)