What is a redox reaction?

A redox reaction, also referred to as an oxidation-reduction reaction, involves the loss or gain of electrons. This typically happens in an electrochemical cell with working, reference and counter electrodes. Anodic and cathodic working electrodes are the site of electron transfer.

A + B ---> A⁺ + B^-

• Oxidation happens on the working electrode if it is driven to a positive potential compared to the reference electrode. When the electrons in the electrode are at a higher energy than the lowest unoccupied molecular orbital (LUMO) of the redox active species in the system the electron is transferred to the redox species. (A ---> A⁺ + e^-)
• Reduction reaction will happen when a negative potential is applied to the working electrode. (B + e^- ---> B^- )

Faradaic current: electrical current generated by the transfer of charges as a result of redox reactions.

What happens during a redox reaction?

In an electrochemical cell, when the potential (voltage) reaches the point where an oxidation or reduction is induced, current starts to flow. The point at which the current starts is the onset oxidation or reduction potential (E_onset). This is shown below where E_pa is the anodic peak potential, E_pc is the cathodic peak potential and E_1/2 is the half-wave potential:

E_1/2 = (E_pa + E_pc) / 2

Oxidation: When the applied potential rises past the half-wave potential, oxidation becomes thermodynamically favourable. The current also continues to increase. The oxidation process, however, becomes limited by rate of diffusion process to the electrode surface. The redox active species within the electrochemical cell struggles to reach the electrode fast enough. This is characterised by a drop of the current (resembling a duck beak) as fewer species are oxidised per second.

For a chemically reversible charge transfer process, the electrode potential sweep is then reversed and scanned back to the initial value. Up to this point oxidation can still take place.

Reduction: When the potential becomes sufficiently negative the reversal results in the appearance of the cathodic peak. It exceeds the half-wave potential due to the reduction of the electrochemically generated species generated during the first sweep. The reversed potential sweep is mostly associated with a reductive (negative) current.

As more oxidized species become reduced the current increases, characterised by the duck tail. As this happens a cyclic voltammogram becomes complete.

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References

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1. Sandford, C. et al. (2019). A synthetic chemist's guide to electroanalytical tools for studying reaction mechanisms, Chem. Sci, 10. doi:10.1039/C9SC01545K
2. Elgrishi, N. et al. (2018). A Practical Beginner’s Guide to Cyclic Voltammetry, J. Chem. Educ., 95. doi:10.1021/acs.jchemed.7b00361