Source: Laboratory of Dr. Kayla Green — Texas Christian University
A Cyclic Voltammetry (CV) experiment involves the scan of a range of potential voltages while measuring current. In the CV experiment, the potential of an immersed, stationary electrode is scanned from a predetermined starting potential to a final value (called the switching potential) and then the reverse scan is obtained. This gives a 'cyclic' sweep of potentials and the current vs. potential curve derived from the data is called a cyclic voltammogram. The first sweep is called the 'forward scan' and the return wave is called the 'reverse scan'. The potential extremes are termed the 'scan window'. The magnitude of reduction and oxidation currents and the shape of the voltammograms are highly dependent on analyte concentration, scan rates, and experimental conditions. By varying these factors, cyclic voltammetry can yield information regarding the stability of transition metal oxidation state in the complexed form, reversibility of electron transfer reactions, and information regarding reactivity. This video will explain the basic setup for a cyclic voltammetry experiment including analyte preparation and setting up the electrochemical cell. A simple cyclic voltammetry experiment will be presented.
In a cyclic voltammetry experiment the potential applied between the reference electrode and working electrode increases in a linear fashion with time (scan rate (V/s)). Concomitantly, the current is measured between the working and counter (or auxiliary) electrode resulting in data that is plotted as current (i) vs. potential (E). Reduction and oxidation events are observed and assigned in the resulting plots. Reduction events occur at analyte specific potential voltages where the reaction M+n + e- → M+n-1 (M = metal) is energetically favored (known as reduction potential) and measured by increasing current values. The current will increase as the voltage potential reaches the reduction potential of the analyte, but then falls off as the maximum rate of mass transfer has been reached. The current goes down only to reach equilibrium at some steady value. Oxidation reactions (M+n → M+n+1 + e-) can also be observed as a decrease in current values at potentials that energetically favor the loss of electron(s).
The resulting voltammograms are then analyzed and the potential (Ep) and current (Ip) data for both reduction and oxidation events under each setup experimental conditions are noted. This information can be utilized to evaluate the reversibility of coupled reduction and oxidation events. As noted above peak potentials (Epa and Epc) and the peak currents (ipc and ipa) are the fundamental parameters used to characterize a redox couple or event. During a reversible redox process, the oxidized and reduced forms of a compound are in equilibrium at the electrode surface. The Nernst equation describes the relationship between potential and the equilibrium ratio, ([R] / [O])x=0.
Where, is called the formal potential of the reaction and takes into account the activity coefficients and other experimental factors.
Specifically, the peak current of a reversible reaction is given by:
where, ip is peak current in amperes, n is the number of electrons involved, A is the area of the electrode in cm2, Do is the diffusion constant (cm2/s), v is the scan rate (V/s) and Co* is the bulk concentration (moles/cm3). The diffusion constant can be measured using more extensive experiments detailed elsewhere and are not the focus of this video1. However, more basic guidelines can be used for evaluating the reversibility of a system1. Criteria for a totally reversible system1:
- at various scan rates n = number of electrons
- at various scan rates
- |ipa/ipc| = 1 at various scan rates
- Ep is independent of v v = scan rate
- at potentials beyond Ep, i-2 t
Simple diagnostic tests for defining a totally irreversible system at 25 °C are:
- No reverse peak (this refers to chemical irreversibility, but not necessarily to electron transfer irreversibility)
- Epc shifts for each decade increase in v (electrochemical irreversibility)
Finally, diagnostic tests for defining a quasi-reversible system are:
- Epc shifts negatively with increasing v
The position of the reduction and/or oxidation events can be used to infer information about the electronic nature of transition metal complexes and the effects on ligands as donors. For example, the Fe+3/+2 reduction potential of ferrocene derivatives is very sensitive to the electronic environment provided by the cyclopentadienyl (Cp) ligand set. Electron donating (withdrawing) Cp substituents increase (decrease) the electron density on the iron center and shift the redox potential to negative (positive) values relative to Fc.
In this protocol ferrocene will be used as an example. Experimental conditions such as solvent, electrolyte choice, and the potential range studied (scan window) are largely dictated by analyte solubility and experimental conditions. Users are encouraged to consult relevant texts such as Bard and Faulkner1 to learn more.
1. Preparation of Electrolyte Solution
- Prepare an electrolyte stock solution (10 mL) composed of 0.1 M [Bu4N][BF4] in CH3CN.
- Place the electrolyte solution in the electrochemical vial, add a small stir bar, and place the cap onto the vial as shown in Figure 1.
- Check to ensure that the nitrogen lead is in the electrolyte solution. Stir and degas the electrolyte solution with a gentle stream of dry N2 gas (~10 min) to remove redox active molecular oxygen.
- During step 1.3, carefully insert the working electrode (e.g. glassy carbon), counter (Pt), and reference electrodes (Ag/AgNO3) into the Teflon cell top. Connect the cell stand leads to the appropriate electrode.
Figure 1. Setup of an electrochemical cell.
2. Obtaining a Background Scan
- Define the experimental conditions for the solvent. For acetonitrile, the scan window is typically +2,000 mV – -2,000 mV.
- Run and save voltammograms of the electrolyte solution at a range of scan rates (e.g. 20 mV/s, 100 mV/s, and/or 300 mV/s).
- Check the resulting scan to ensure that there are no impurities in the electrolyte solution or remaining oxygen. A clean system will have no redox events. If the setup is contaminated, the electrodes and glassware will need to be cleaned and the electrolyte solution remade using clean components.
3. Preparation of Analyte Solution
- Combine the analyte (~2-5 mM, final concentration) of interest with the electrolyte solution prepared above.
- Check to ensure that the nitrogen lead is in the electrolyte solution. Stir and degas the analyte/electrolyte solution with a gentle stream of dry N2 gas (~10 min) to remove redox active molecular oxygen.
4. Cyclic Voltammetry of Analyte
- Perform multiple cyclic voltammogram experiments at scan rates from 20 mV – 1,000 mV (dependent upon the cell stand capabilities). Begin each scan using the calculated open circuit potential.
- Methodically vary the scan direction [(+ to –) and (– to +)] and scan window to isolate redox events of interest. The voltammogram should always start from zero current (open circuit). Ferrocene (Fc) undergoes an oxidation reaction to ferrocenium (Fc+).
- Many groups standardize data to the Fc/Fc+ redox couple. In this practice, ~2 mg of Fc are added to the analyte solution and step 4.2 is repeated for referencing purposes. In data analysis, all spectra are normalized to the Fc/Fc+ couple set to 0.00 V. A table of normalized reduction potentials is available2.
5. Cleaning of Electrodes and the Electrochemical Cell
- Carefully unclamp and remove each electrode from the electrochemical cell.
- Rinse the reference electrode with acetonitrile and dry with a Kimwipe. Store in reference electrode storage solution.
- GENTLY clean the working and counter electrode according to guidelines from manufacturers (e.g. BASi: http://www.basinc.com/mans/pguide.pdf) to remove the redox reaction products that accumulate during some experiments.
Cyclic voltammetry, or CV, is a technique used to study a wide range of electrochemical properties of an analyte or system.
Voltammetry experiments are performed by applying a potential sweep to an electrochemical system, and then measuring the resulting current. The resulting plot of applied potential vs. current is called a voltammogram.
Cyclic voltammetry is executed the same way, except that after a linear potential sweep reaches the set value, it is then ramped in the opposite direction back to the initial potential. The shape of the voltammogram, specifically the peaks and peak locations, provides key insight into the properties of the analyte, such as oxidation-reduction, or redox potentials.
This video will demonstrate how to setup, run, and interpret cyclic voltammetry experiments in the laboratory.
Cyclic voltammetry is typically performed in a three-electrode cell. First, the working electrode is where the reaction of interest takes place. Working electrodes are often made of inert materials, such as gold, platinum, or carbon.
Next, the counter electrode is used to close the current circuit in the cell. It is also composed of inert materials, most frequently a platinum wire. Finally, the reference electrode is used as a point of reference for the system, as it has a stable and well-known potential. Thus, the applied potential is reported versus the reference potential.
The cell contains the analyte, which is dissolved in a solvent. The solvent cannot react with the analyte, and cannot be redox active within the desired scan window. In most experiments, a supporting electrolyte is used to minimize solution resistance. The electrolyte is often a salt solution, as it has a high ionic strength and conductivity.
To run an electrochemical test in a three-electrode cell, current flow is induced between the working and counter electrodes. Applied potential is controlled by manipulating the polarization of the counter electrode. However, the potential is measured between the working electrode and the known stable potential of the reference electrode. The potential is subsequently adjusted to maintain a specified potential difference between the working and reference electrodes.
In a CV experiment, the potential is linearly ramped to the "switching" potential, and is then reversed back to the starting potential, thereby making a "cyclical" sweep. The potential limits are termed the scan window. The resulting voltammogram shows features corresponding to redox events in the system.
For a single electron redox event, the forward potential sweep results in a cathodic peak. At this "cathodic peak potential," the analyte is reduced, meaning that electrons are gained. The reverse sweep causes an anodic peak, where oxidation occurs. At this "anodic peak potential," electrons are stripped from the products formed in the forward sweep. The shapes of these peaks are highly dependent on analyte concentration, scan rate, and experimental conditions.
Now that the basics of cyclic voltammetry have been explained, let's take a look at how to run a CV scan in the laboratory.
To begin the procedure, prepare 10 mL of an electrolyte stock solution. Add the electrolyte solution to an electrochemical cell. Add a small stir bar, place the cell on a stir plate and cap it.
Stir and degas the solution with a gentle stream of nitrogen gas. This removes oxygen, which is redox active. Prior to use, rinse the reference electrode with the solvent and dry with a Kimwipe. Then, gently clean the working and counter electrodes according to guidelines from the manufacturers.
While the solution is degassing, insert the three electrodes into the Teflon cell top. Connect the electrodes to the appropriate leads of the setup.
Obtain a background scan to verify the solution is not electrochemically active over the range of the scan window. From the resulting scan, verify there are no impurities or remaining oxygen. If redox events are present, clean the electrodes and glassware, and remake the solution.
Combine the analyte of interest with the electrolyte solution. Stir and degas the solution with a dry nitrogen stream to remove oxygen. Perform multiple cyclic voltammogram experiments at multiple scan rates, depending on the system capabilities. Begin each scan at the open circuit potential, the value where no current flows.
Methodically vary the scan window to isolate redox events of interest. Vary the scan direction to ensure it does not impact the events. Perform this step at multiple scan speeds. Once all scans have been collected, unclamp and remove each electrode from the cell. Rinse the reference electrode and dry with a Kim-wipe. Store it in the electrode storage solution. Gently clean the working and counter electrodes prior to storage, and rinse the electrode cell.
The resulting cyclic voltammograms are analyzed and the potential and current data for both reduction and oxidation events under each experimental setup are noted.
CV can be used to determine whether redox reactions are reversible, or irreversible. In a reversible system, both reduction and oxidation occurs, producing respective peaks. Additionally, the ratio of the cathodic current to anodic current should be approximately 1. Finally, in a reversible system, the mean peak potential is unaffected by the potential scan rate.
In an irreversible system, there is no reverse peak. Also, the peak current should be proportional to the square root of the scan rate.
Many fields of study that use electroactive species benefit from CV experiments.
Dopamine is a long-studied neurotransmitter, known for its importance is drug abuse, psychiatric illnesses, and degenerative disorders. The ability to examine the release of dopamine in real-time has been a goal for neuroscience. In this example, the oxidation of dopamine in the brain is measured with microelectrodes, using CV. Various pharmacological agents were applied to the brain region of interest to test their affect on dopamine release.
The capability of neural recording prosthetics decreases with time post-implantation. In this example, CV was used to monitor the effectiveness of an implant.
The electrode material and roughness, as well as surrounding tissue influenced the shape of the curve. A high charge carrying capacity, determined by the area of the curve, indicated a well functioning setup. A brief voltage pulse was used to rejuvenate the implant.
Microbial bioelectrochemical systems are a growing field of study with applications such as bioremediation.
Certain bacteria are electrochemically active, particularly when they are assembled in layers on a surface, called biofilms. These cells were grown in a bioreactor, and controlled electrochemically. As the cells grew in the bioreactor, cyclic voltammetry was used to monitor the current generated by the cells, thereby determining when the reactants were depleted.
You've just watched JoVE's introduction to cyclic voltammetry. You should now understand how to run and interpret a CV scan.
Thanks for watching!
A CV scan of ferrocene at 300 mV/s in acetonitrile was carried out and the corresponding voltammogram is shown in Figure 2.
The ΔE can be derived from the data in Figure 2 based on the difference between Epa and Epc.
The cyclic voltammograms overlaid in Figure 3 represent consecutive experiments performed on the same system at different scan rates. As noted in above, a linear plot of Ip vs. v1/2 (inset in Figure 3) shows that the reaction is diffusion controlled.
The position of the E1/2 or redox event (Epa or Epc) can be used to determine the effects that the ligand has on the redox active metal center providing the electrochemical response. Figure 4 shows a series of ferrocene-based congeners with varying substitutions on the Cp ring. As shown in Figure 5, the electron withdrawing halide results in the E1/2 value of this complex to be shifted to more positive potentials because the oxidized form is destabilized by the electron withdrawing ligand. The electron donating methyl groups of compound C result in the E1/2 to shift to more negative potentials as the oxidized species is stabilized.
Figure 2. A CV scan of ferrocene at 150 mV/s in acetonitrile. Please click here to view a larger version of this figure.
Figure 3. A cobalt-containing compound that gives rise to one reduction event. The inset shows a linear correlation between ip and v1/2. Please click here to view a larger version of this figure.
Figure 4. A series of ferrocene-based compounds. Please click here to view a larger version of this figure.
Figure 5. The resulting cyclic voltammograms of A-C (Figure 4) show a marked shift in E1/2 due to the electronic ligand effects attached to the metal center. Please click here to view a larger version of this figure.
- Bard, A. J., Faulkner, L. A. Electrochemical methods: Fundamentals and Applications. 2nd ed. New York: Wiley; 833 p. (2001).
- Geiger, W. E., Connelly, N. G. Chemical Redox Agents for Organometallic Chemistry. Chem Rev. 96 (2), 877-910, (1996).