Cyclic voltammetry (CV) is an electrochemical technique used to investigate the reduction and oxidation (redox) behavior of chemical species. This method involves applying a varying electrical potential to a working electrode and simultaneously measuring the resulting electrical current. The output, known as a cyclic voltammogram, shows the current response as a function of the applied potential. Interpreting this plot provides insight into reaction mechanisms, thermodynamic properties, and the kinetics of electron transfer.
Graph Fundamentals: Axes and Scan Direction
A cyclic voltammogram is a two-dimensional graph where the horizontal axis represents the applied potential, and the vertical axis represents the measured current. The potential, or voltage, is typically measured in Volts (V) against a stable reference electrode. The current, which represents the flow of electrons, is usually measured in Amperes (A) or microamperes (uA).
The experiment begins at an initial potential, and the potential is then swept linearly toward a switching potential, which defines one boundary of the scan window. This sweep is known as the forward scan. Once the switching potential is reached, the scan direction is reversed, and the potential sweeps back toward the initial potential, completing the cycle.
This triangular potential waveform gives the technique its name and characteristic shape. The rate at which the potential changes over time is called the scan rate, measured in Volts per second (V/s). This rate influences the shape and height of the resulting curve.
Identifying Anodic and Cathodic Reactions
The peaks that appear on the cyclic voltammogram correspond directly to chemical reactions occurring at the electrode surface. These peaks reveal the two fundamental processes of electrochemistry: oxidation and reduction. The direction of the current distinguishes between these two reactions.
A positive current indicates an anodic reaction, which is oxidation where the chemical species loses electrons. The potential at which this oxidation occurs most rapidly is called the Anodic Peak Potential (\(E_{pa}\)), and the maximum current reached is the anodic peak current (\(i_{pa}\)).
Conversely, a negative current signifies a cathodic reaction, which is reduction where the species gains electrons. The potential at which the reduction rate is highest is the Cathodic Peak Potential (\(E_{pc}\)), with the corresponding current being the cathodic peak current (\(i_{pc}\)). The presence of a reduction peak on the reverse scan indicates that the product formed during the initial reaction is stable enough to be converted back.
Determining Key Electrochemical Properties
The positions and shapes of the anodic and cathodic peaks provide quantitative information about the underlying electrochemical process. One important value is the formal potential (\(E^0\)), which approximates the thermodynamic equilibrium potential for the redox reaction. For a straightforward, reversible reaction, \(E^0\) is calculated by taking the average of the anodic (\(E_{pa}\)) and cathodic (\(E_{pc}\)) peak potentials: \(E^0 = (E_{pa} + E_{pc}) / 2\).
The separation between the two peaks, known as the peak separation (\(\Delta E_p = E_{pa} – E_{pc}\)), is a diagnostic tool for assessing the reversibility of the electron transfer. For a perfectly reversible reaction involving a single electron transfer at 25 °C, the theoretical separation is approximately 59 millivolts (mV). Larger values of \(\Delta E_p\) suggest slower electron transfer kinetics, classifying the reaction as quasi-reversible or irreversible.
Another relationship links the peak current (\(i_p\)) to the concentration of the electroactive species and the scan rate. For reactions controlled by diffusion—meaning the rate is limited by how quickly the species can move to the electrode surface—the peak current is directly proportional to the concentration of the analyte. This proportionality allows the technique to be used for quantitative analysis.
In a diffusion-controlled process, the peak current is proportional to the square root of the scan rate. By plotting the peak current against the square root of the scan rate, a linear relationship confirms that the reaction is limited by the movement of the species in the solution. Deviations from this linearity or a peak current proportional to the scan rate itself can indicate that the species is instead adsorbed onto the electrode surface.