Cyclic voltammetry

Cyclic voltammetry is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment, a voltage is applied to a working electrode in solution and current flowing at the working electrode is plotted versus the applied voltage to give the cyclic voltammogram. Cyclic voltammetry can be used to study the electrochemical properties of species in solution as well as at the electrode/electrolyte interface.

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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 a 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., whether or not 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. This relationship is described by the Cottrell equation.

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 include glassy carbon, platinum, and 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

Potentiodynamic techniques also exist 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.

See also

• Voltammetry
• Electroanalytical methods