1. Electrochemistry: Introduction Electrochemistry at your finger tips
Part 2: Voltammetry (linear sweep voltammetry & cyclic voltammetry)

3. Historical preface:
All modern electrochemistry starts from polarography technique developed by J. Heyrovsky
The dropping mercury electrode (DME) was introduced by J. Heyrovsky in 1920s.
J. Heyrovsky
M. Shikata
The first polarograph designed by J. Heyrovský and M. Shikata (1924)

4. Typical diffusional voltammogram
Linear sweep voltammetry & cyclic voltammetry:
Current
Potential
Faradaic current
Final E
A current corresponding to the reduction or
oxidation of some chemicalsubstance.
Capacitance current
Diffusionally limited Faradaic current
Initial E E0
Time
Potential
Typical diffusional voltammogram

5. E > E0 ; only capacitance current is observed
Working electrode - + + +
Oxidized redox species - -
Electrolyte cations and anions
Potential is negative shifted - + - +
Oxidized redox species do not react - +
Electrolyte cations and anions are re-organized near the electrode

6. - + - + - + - + - + - + E < E0 ; Faradaic current is observed
Working electrode - + - + - +
Potential is negative shifted
Reduced redox species are generated e- - + -
Oxidized redox species started to react + - +

7. - + - + - + - + - + - + E < E0 ; Faradaic current is observed
Reduced redox species are generated e- - + - + - +
Potential is negative shifted e- - +
Reduced species diffuse from the electrode to the bulk solution - + -
Oxidized species diffuse to the electrode from the bulk solution +
Surface concentration of the oxidized species is very low

8. Diffusion is particularly significant in an electrolysis experiment since the conversion reaction only occurs at the electrode surface. Consequently there will be a lower reactant concentration at the electrode than in bulk solution. Similarly a higher concentration of product will exist near the electrode than further out into solution.

9. Linear sweep voltammogram & the concentration profile:
Distance from the electrode surface
Before the electrochemical reaction
Concentration of the
oxidized species
Upon reaching the diffusionally limited current
Current
Potential

10. Current development upon linear potential scan

11. Concentration profile upon linear potential scan

12. Voltammograms: S-shape or peak shape?
electrode rotation
Hydrodynamic electrode:
1. Rotating disk electrode
2. Mercury dropping electrode
Current
Stationary electrode
Potential
electrode growth

13. Cyclic voltammetry is composed of two linear potential sweeps performed in the opposite directions

14. Comparison of a linear sweep voltammogram and a cyclic voltammogram
A typical linear sweep voltammogram showing the important parameters.
A typical cyclic voltammogram showing the important parameters.

15. Animated Cyclic Voltammetry experiment
This animation shows the cyclic voltammogram for the reversible oxidation (forward sweep) and reduction (reverse sweep) for hydroxy-ferrocene. In aqueous buffered electrolyte, hydroxy-ferrocene undergoes a simple outer sphere one-electron redox process according to the following scheme:

16. e-  t1/2 Ip / mA v1/2 / (mV s-1)1/2
How we can recognize diffusional electrochemical processes:
Randles-Sevcik equation (for reversible systems):
Ip=(2.69x105)n3/2ACD1/2v1/2 e-
 t1/2
diffusion
Ip / mA
I / mA
E / V
Time / sec
v1/2 / (mV s-1)1/2

17. In order to prove the diffusional character of linear sweep or cyclic voltammogram, we need to perform the measurements with different potential scan rates.

18. Experimental results should look like these plots:

19. Reversible diffusional CV-1
Reduction
Epc
Cathodic current
Cathodic current
Current / mA
Oxidation
Anodic current
Anodic current
Epa
Ep = 59/n mV +1 -1
Potential / V

20. E0
Nernst equation +
Potential / V -

21. Reversible electrochemical process:
The oxidized & reduced redox species are always at equilibrium with the electrode at any applied potential.
The electron transfer rate constants should be high enough to maintain the equilibrium at the used potential scan rate.
The reversible CV should be used to find E0, but the kinetics of the electrochemical process cannot be found. e- e-

22. How it might look like on your computer

23. Quasi-Reversible diffusional CV in the case of slow electron transfer process-1
E1/2
Epc
I / mA
The cathodic & anodic peaks might be shifted non-symmetrically:
E1/2
= E0
Epa
Ep > 59/n mV +1 -1
E / V

24. linear sweep voltammogram
Effect of the electron transfer rate constant on the shape of linear sweep or cyclic voltammograms
linear sweep voltammogram
cyclic voltammogram

25. R.S. Nicholson, I. Shain, Anal. Chem. 1964, 36, 706-723.
To calculate the rate constant we need to know:
1. E as a function of the potential scan rate
2. Number of electrons per molecule
3. Diffusion coefficient and concentration of the redox species
E / mV
log( v / mV s-1)
The most important theoretical paper about diffusionally limited cyclic voltammetry:
R.S. Nicholson, I. Shain, Anal. Chem. 1964, 36,

26. Q + e- Q- reversible electron transfer step
Irreversible electrochemical processes with the following chemical steps:
Q + e Q- reversible electron transfer step
Q- + H QH. irreversible chemical step (protonation)
Problems:
1. No information about E0
2. No information about kinetics
Useful for analytical purposes only!

27. A calibration plot can be still useful for analytical purposes.
There is no information about the redox potential or kinetics of the irreversible electrochemical process.
A calibration plot can be still useful for analytical purposes.
current
concentration

28. How to change irreversible (quasi-reversible) process to reversible:
1. If the process is limited by the slow electron transfer - make the scan rate slower.
2. If there is an irreversible chemical step after the fast electron transfer process: make the scan rate faster.

29. A multi-step electrochemical process

30. Recommended textbooks
on electrochemistry:
Electrochemical Methods : Fundamentals and Applications, by A.J. Bard and Larry R. Faulkner
Analytical Electrochemistry,
by Joseph Wang
Broadening Electrochemical Horizons: Principles and Illustration of Voltammetric and Related Techniques,
Edited by Alan Bond