In electrochemistry, cyclic voltammetry (CV) is a type of potentiodynamic measurement. In a cyclic voltammetry experiment, the working electrode potential is ramped linearly versus time. Unlike in linear sweep voltammetry, after the set potential is reached in a CV experiment, the working electrode's potential is ramped in the opposite direction to return to the initial potential. These cycles of ramps in potential may be repeated as many times as needed. The current at the working electrode is plotted versus the applied voltage (that is, the working electrode's potential) to give the cyclic voltammogram trace. Cyclic voltammetry is generally used to study the electrochemical properties of an analyte in solution or of a molecule that is adsorbed onto the electrode.
Experimental method
In cyclic voltammetry (CV), the electrode potential ramps linearly versus time in cyclical phases (Figure 2). The rate of voltage change over time during each of these phases is known as the experiment's scan rate (V/s). The potential is measured between the working electrode and the reference electrode, while the current is measured between the working electrode and the counter electrode. These data are plotted as current (i) versus applied potential (E, often referred to as just 'potential'). In Figure 2, during the initial forward scan (from t0 to t1) an increasingly reducing potential is applied; thus the cathodic current will, at least initially, increase over this time period, assuming that there are reducible analytes in the system. At some point after the reduction potential of the analyte is reached, the cathodic current will decrease as the concentration of reducible analyte is depleted. If the redox couple is reversible, then during the reverse scan (from t1 to t2), the reduced analyte will start to be re-oxidized, giving rise to a current of reverse polarity (anodic current) to before. The more reversible the redox couple is, the more similar the oxidation peak will be in shape to the reduction peak. Hence, CV data can provide information about redox potentials and electrochemical reaction rates.
For instance, if the electron transfer at the working electrode surface is fast and the current is limited by the diffusion of analyte species to the electrode surface, then the peak current will be proportional to the square root of the scan rate. This relationship is described by the Randles–Sevcik equation. In this situation, the CV experiment only samples a small portion of the solution, i.e., the diffusion layer at the electrode surface.
Characterization
The utility of cyclic voltammetry is highly dependent on the analyte being studied. The analyte has to be redox active within the potential window to be scanned.
The analyte is in solution
Reversible couples
Often the analyte displays a reversible CV wave (such as that depicted in Figure 1), which is observed when all of the initial analyte can be recovered after a forward and reverse scan cycle. Although such reversible couples are simpler to analyze, they contain less information than more complex waveforms.
The waveform of even reversible couples is complex owing to the combined effects of polarization and diffusion. The difference between the two peak potentials (Ep), ΔEp, is of particular interest. ΔEp = Epa - Epc > 0
This difference mainly results from the effects of analyte diffusion rates. In the ideal case of a reversible 1e- couple, ΔEp is 57 mV and the full-width half-max of the forward scan peak is 59 mV. Typical values observed experimentally are greater, often approaching 70 or 80 mV. The waveform is also affected by the rate of electron transfer, usually discussed as the activation barrier for electron transfer. A theoretical description of polarization overpotential is in part described by the Butler–Volmer equation and Cottrell equation equations. In an ideal system the relationships reduces to for an n electron process.
Focusing on current, reversible couples are characterized by ipa/ipc = 1.
When a reversible peak is observed, thermodynamic information in the form of a half cell potential E01/2 can be determined. When waves are semi-reversible (ipa/ipc is close but not equal to 1), it may be possible to determine even more specific information (see electrochemical reaction mechanism).