Cyclic voltammetry
Cyclic voltammetry is a type of potentiodynamic electrochemical measurement. To obtain a cyclic voltammogram, the voltage is varied in a solution and the change in current is measured with respect to the change in voltage. It is a specific type of voltammetry used for studying the redox properties of chemicals and interfacial structures.
Explanation
In a cyclic voltammetry experiment, as in other controlled potential experiments, a potential is applied to the system, and the faradaic current response is measured (a faradaic current is the current due to a redox reaction). The current response over a range of potentials (a potential window) is measured, starting at an initial value and varying the potential in a linear manner up to a pre-defined limiting value. At this potential (often referred to as a switching potential), the direction of the potential scan is reversed, and the same potential window is scanned in the opposite direction (hence the term cyclic). This means that, for example, species formed by oxidation on the first (forward) scan can be reduced on the second (reverse) scan. This technique is commonly used, since it provides an fast and simple method for initial characterization of a redox-active system. In addition to providing an estimate of the redox potential, it can also provide information about the rate of electron transfer between the electrode and the analyte, and the stability of the analyte in the electrolyzed oxidation states (e.g., do they undergo any chemical reactions)
For the majority of experiments the electroactive species is in the form of a solution. The three-electrode method is the most widely used because the electrical potential of reference does not change easily during the measurement.
The method uses a reference electrode, working electrode, and counter electrode (also called the secondary or auxiliary electrode). Electrolyte is usually added to the test solution to ensure sufficient conductivity. The combination of the solvent, electrolyte and specific working electrode material determines the range of the potential.
Cyclic voltammetry
In cyclic voltammetry, the electrode potential follows a linearly ramping potential vs. time as shown.
The potential is measured between the reference electrode and the working electrode and the current is measured between the working electrode and the counterelectrode. This data is then plotted as current (i) vs. potential (E). As the waveform shows, the forward scan produces a current peak for any analytes that can be reduced through the range of the potential scan. The current will increase as the potential reaches the reduction potential of the analyte, but then falls off as the concentration of the analyte is depleted close to the electrode surface. As the applied potential is reversed, it will reach a potential that will reoxidize the product formed in the first reduction reaction, and produce a current of reverse polarity from the forward scan. This oxidation peak will usually have a similar shape to the reduction peak. As a result, information about the redox potential and electrochemical reaction rates of the compounds are obtained.
For instance if the electronic transfer at the surface is fast and the current is limited by the diffusion of species to the electrode surface, then the current peak will be proportional to the square root of the scan rate.
Electrodes
Electrodes can be either static or rotating. With rotation, convection is achieved but diffusion as the only process (i.e., the rate-controlling step) is eliminated. Some very common voltammetry electrodes consist of glassy carbon, platinum, and/or gold.
In some more rare experiments the electroactive species is fixed to the surface, for instance in microparticle voltammetry. Also the term cyclic voltammetry is used to describe experiments in which two immiscible liquids (each one containing a reference and a counter electrode) are in contact. Such a four electrode cell uses the interface between the two liquids as the working electrode.
Potentiodynamic techniques
There are also potentiodynamic techniques that add low-amplitude ac perturbation to a potential ramp and measure variable response in a single frequency (ac voltammetry) or in many frequencies simultaneously (potentiodynamic electrochemical impedance spectroscopy). The response in alternating current is two-dimensional – it is characterised by amplitude and phase. The amplitude and phase depend differently on frequency for constituents of ac response attributed to different processes (charge transfer, diffusion, double layer charging, etc.). Frequency response analysis enables simultaneous monitoring of the various processes that contribute to the potentiodynamic ac response of electrochemical system.