Question

I am doing an experiment involving cyclic voltammetry. What is going on at the surface of the electrode when scan rate and concentration is changed?

Answer 1

Cyclic voltammetry, provides us the opportunity to explore the electrochemical behavior of species generated at the electrode, while scanning the potential in both directions

For example: The below figure shows the cyclic voltammogram for the same redox couple at both

(a) faster scan rate

(b) slower scan rate

- At the faster scan rate we see two peaks (oxidation peak and reduction peak).

- Whereas at the slower scan rate one peak disappears. Because slow scan rate allows sufficient time for the reduced product (R) to participate in a chemical reaction whose products are not electroactive.

Concentration Change at the surface of the electrode

Let us consider at the surcae the following reaction occurs,

O R P

In cyclic voltammetry experiment, equilibrium values of O and R can only be maintained, if both O and R are stable on the experimental time scale. Suppose R formed is converted to P, then more R must be generated to compensate for the loss of R.

Therefore, the rate of reduction increases and E_pc (peak cathodic) moves to a more positive value (shown in figure). In addition, i_pa/i_pc (anodic currect / cathodic current) is less than one (since only a fraction of the molecules that were reduced on the forward scan are available for reoxidation on the reverse scan).