Cyclic voltammetry

In electrochemistry, cyclic voltammetry (CV) is a type of voltammetric measurement where the potential of the working electrode is ramped linearly versus time.

Cyclic voltammetry is generally used to study the electrochemical properties of an analyte in solution[2][3][4][1] or of a molecule that is adsorbed onto the electrode.

The spike in anodic (positive) current observed between t0 and t1 is due to the oxidation of the analyte in the solution when the correct potential is reached.

The current decreases after the initial spike as the concentration of oxidable analyte is depleted near the surface of the working electrode due to mass transport limitations.

In this situation, the CV experiment only samples a small portion of the solution, i.e., the diffusion layer at the electrode surface.

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).

in the figure shows the proportionality of the peak currents to the square root of the scan rate when additionally

This leads to the so called Randles–Sevcik equation and the rate determining step of this electrochemical redox reaction can be assigned to diffusion.

The solvent, electrolyte, and material composition of the working electrode will determine the potential range that can be accessed during the experiment.

Stirring the solution between cyclic voltammetry traces is important in order to supply the electrode surface with fresh analyte for each new experiment.

To run cyclic voltammetry experiments at very high scan rates a regular working electrode is insufficient.

High scan rates create peaks with large currents and increased resistances, which result in distortions.

Reactions occurring at the counter electrode surface are unimportant as long as it continues to conduct current well.

To maintain the observed current the counter electrode will often oxidize or reduce the solvent or bulk electrolyte.

[8] Potentiodynamic techniques also exist that add low-amplitude AC perturbations to a potential ramp and measure variable response in a single frequency (AC voltammetry) or in many frequencies simultaneously (potentiodynamic electrochemical impedance spectroscopy).

Such techniques target steady state conditions and produce waveforms that appear the same when scanned in either the positive or negative directions, thus limiting them to linear sweep voltammetry.

[17][18] Low molecular weight antioxidants, molecules that prevent other molecules from being oxidized by acting as reducing agents, are important in living cells because they inhibit cell damage or death caused by oxidation reactions that produce radicals.

[20] Because traditional methods to determine antioxidant capacity involve tedious steps, techniques to increase the rate of the experiment are continually being researched.

[23] It is important to note that whenever cyclic voltammetry is utilized, it is usually compared to spectrophotometry or high-performance liquid chromatography (HPLC).

[24] Applications of the technique extend to food chemistry, where it is used to determine the antioxidant activity of red wine, chocolate, and hops.

The technique being evaluated uses voltammetric sensors combined in an electronic tongue (ET) to observe the antioxidant capacity in red wines.

These electronic tongues (ETs) consist of multiple sensing units like voltammetric sensors, which will have unique responses to certain compounds.

While cyclic voltammetry was successfully used to generate currents using the wine samples, the signals were complex and needed an additional extraction stage.

[25] Additionally, the time was reduced, the sample did not have to be pretreated, and other reagents were unnecessary, all of which diminished the popularity of traditional methods.

[22] Using the graph produced by cyclic voltammetry, the total phenolic and flavonoid content can be deduced in each of the three samples.

It was observed that cocoa powder and dark chocolate had the highest antioxidant capacity since they had high total phenolic and flavonoid content.

Hops, the flowers used in making beer, contain antioxidant properties due to the presence of flavonoids and other polyphenolic compounds.

Figure 1. Typical cyclic voltammogram where j pc and j pa show the peak cathodic and anodic current densities respectively for a reversible reaction with a 5 mM Fe redox couple reacting with a graphite electrode in 1M potassium nitrate solution. E PA and E PC denote the corresponding electrode potentials (vs. Ag/AgCl) of maximal reaction rates. For an ideal reversible (Nernstian) reaction the theoretical peak separation (E PA - E PC ) is 57 mV [ 1 ] .
Figure 2. Cyclic voltammetry: Potential waveform in blue (left y-axis), current answer in red (right y-axis). Electrolyte as in figure 1. Potential vs. Ag/AgCl in both figures. A comparison of this experiment with and without 5 mM Fe species can be found here .
Potential against time, current against time and voltammogram (current against potential) for a one-electron, reversible redox couple diffusing freely in solution. The current density is normalised by 0.446 F C sqrt(D F nu / R T). Reductive current counted as negative.
Maximum anodic and cathodic peak currents expressed as and as function of scan rate for a graphite electrode in the above mentioned electrolyte 5mM Fe analyte in 1 M . Dotted lines are fits with in the power fit. Data on GitHub