Cyclic Voltammetry (CV)

Student authors: Zichen Zheng 2018 & Zhengzhou Qiu 2019

How CV works

Cyclic voltammetry (CV) is a powerful and popular electrochemical technique commonly employed to investigate the reduction and oxidation processes of molecular species. Figure 1.1 shows an example of cyclic voltammetry.

Figure 1. An example of cyclic voltammetry.

3. How do we set up the experimental instruments to get CV data?

Reference Electrode (RE) has a well-defined and stable equilibrium potential. It is used as a reference point against which the potential of other electrodes can be measured in an electrochemical cell (Figure. 2)

Figure 2. A brief picture of Cu-Cu(II) reference electrode.

Working Electrode (WE) carries out the electrochemical event of interest.

4. Steps of collecting the data (optional)

1. Recording background scan.
2. Measure the open circuit potential (OCP). When the electrochemical cell is assembled, and the analyte has been added, a potential develops at the electrodes. The potential can be observed when no current is flowing is called OCP. OCP gives information about the redox state of the materials in solution as well as the concentration of different species when the solution contains a mixture.1
3. RE and WE create undesired resistance, i.e. ohmic drop. So we need to minimize it.
4. Recording the cyclic voltammogram. The electrodes are connected to the potentiostat, and the experimental parameter will be set up through the potentiostat software.

How to interpret the data

• Clear up the confusion: there are two ways of making the CV graph, one is US Convention and the other is IUPAC Convention.

Basic elements of the CV profile

• • X-axis: the applied potential (E) that is imposed to the system
  • Y-axis: the resulting current (i) that is passed.
  • The arrow indicates the beginning and sweeps direction of the first segment (or “forward scan”)
  • In the caption of Figure 1, there was a value of “υ = 100 mV/s.” υ is the scan rate. It means that during the experiment the potential was varied linearly at the speed (scan rate) of 100 mV per second.

• • From A to D, the potential is scanned negatively (cathodically); it switches at point D, and from D to G the potential is scanned positively (anodically). So the full cycle goes as A -> D -> G.
  • From A to D, the [Fc+] is reduced to Fc. At point C, the peak cathodic current (ip,c) is observed. The current indicates the diffusion of Fc+ from the surface of electrode.
  • The volume of solution at the surface of the electrode containing the reduced Fc, called the diffusion layer, continues to grow through the scan. So Fc+ transport to the electrode slower. So as scanning to more negative potentials, the rate of diffusion of Fc+ from the bulk solution to the electrode surface becomes slower, resulting in a decreased current.
  • At point D, the scan direction reversed. As the [Fc+] decrease and [Fc] increase, Fc is oxidized back to Fc+.
  • At B and E, [Fc+] = [Fc]
  • At C and F, we have the estimated E0
• some CV figures with abnormal shapes.
• • When there are multiple peaks, it means that there are probably multifold redox reaction happening
  • The curves are not smooth: depends on how the molecules in the electrolyte move to/out of the electrode’s surface to do the reaction/after finished reacting with the electrodes. Since the electrolyte is not stirred, there might be some delay in the molecule’s motion
• Another ~potential~ CV graph
• This is not as “duck-shaped” as the other examples
• This is of potential electrode material, so once a certain potential is applied to the system it jumps up to a near-maximum current (current comes from the electrolytes adsorbing to the electrode surface, so reaching a high current quickly means there are high charge/discharge rates). When the scan is reversed it drops back down to a near minimum current. For supercapacitor electrodes that depend on fast charge/discharge rates a more rectangular graph is ideal
• This also shows cyclability, so having a similar shape at high scan rates (blue vs orange) means that it has high cyclability

Importance of the scan rate

• It controls how fast the applied potential is scanned.
• Faster scan rate → smaller diffusion layer → more electrons coming → higher currents

Works cited

1. Elgrishi, N; Rountree, K. J; McCarthy, B. D.; Rountree, E. S; Eisenhart, T. T; Dempsey, J. L. Journal of Chemical Education 2018 95 (2), 197-206

Further reading and practice

1. The supporting information of Dempsey’s paper1 is a good source for practicing.
2. You can also visualize the scan in CV cycle on this website.