3. BASIC ASPECTS OF ELECTROCHEMISRY
CHAPTER 1:
BASIC ASPECTS OF
ELECTROCHEMISRY

4. Fundamentals of
Electrode Reactions

5. 1. ELECTRON TRANSFER REACTIONS
Example : reduction in solution of Fe(III) ion
Through a reducing agent (redox reaction in a homogeneous phase):
[Fe(H2O)6]3+ + [V(H2O)6] [Fe(H20)6]2+ + [V(H2O)6]3+
Through an electrode (redox reaction in a heterogeneous phase):
[Fe(H2O)6]3+ + e [Fe(H2O)6]2+

6. Mechanisms 1. Diffusion of the species through the solution
2. Interaction between reagents(homogenous phase) or interactions between reagents and electrode(heterogeneous phase)
3. Formation of short- or long-lived intermediates due to variations in electronic configurations, to the eventual substitution of ligands, etc.

7. Oxidation-reduction reactions in a homogenous phase
classified as:
outer-sphere reactions
the charge transfer takes place as soon as the two reagents collide, without the occurrence of any exchange of ligands (which would imply breaking of old or formation of new bonds in the reaction intermediate).

8. inner-sphere reactions
involves a 'transition state’ in which a mutual strong penetration of the coordination spheres of the reagents occurs (therefore, strong interaction between reagents), whereas in the outer-sphere reactions there is no overlap of the coordination spheres of the reagents (therefore, there is weak interaction between reagents)

9. Mechanism:
Intermediate
III II

10. Electron transfer mediated by electrodes (heterogeneous phase)
outer-sphere mechanism
electron transfer between the electrode and the active site through the layer of solvent directly in contact with the electrode surface
chemical interaction between the electrode and electroactive specie can be considered nil (apart from their electrostatic interaction)
The majority of electrochemically induced redox processes in inorganic chemistry proceed (or are assumed to proceed) through outer-sphere mechanisms

11. inner-sphere mechanism
electron exchange occurs between the electrode and the electroactive species (the metal core or its ligand)
direct contact with the electrode surface

12. 2. Fundamentals of electron transfers at an electrode

13. Some fundamental concepts:
1. The Electrode/Solution System
Electrode reaction : a heterogeneous chemical process involves the passage of an electron from an electrode (metal or semiconductor) to a chemical species in solution, or vice versa

14. electrode/solution system partitions into four regions:3 4 1 2

15. 2. The Nature of Electrode Reactions
The simple reaction:
Ox + ne Red
in reality is a sequence of elementary processes to maintain a continuous flow of electrons:

16. first, the electrode surface must be continually supplied with reagent

17. then, the heterogeneous electron transfer process from the solid electrode to the species Ox must takes place (through an inner- or outer-sphere mechanism)

18. finally, the reaction product (Red) must be removed from the electrode surface, in order to allow the access of further amounts of Ox to the electrode surface

19. Overall rate of the process
by the slowest elementary step
the mass transport (from the bulk of the solution to the electrode surface, and vice versa)
heterogeneous electron transfer (from the electrode to the electroactive species, or vice versa)

20. coupled chemical reactions
Some phenomena complicate the electrode reactions:
coupled chemical reactions
Following chemical reactions (following, obviously, the electron transfer)
Preceding chemical reactions (preceding, naturally, the electron transfer)

21. adsorption
Electron exchange without the interaction of the species Ox and Red with the electrode surface
It is possible that the adsorption of the species Ox or Red might cause poisoning of the electrode surface, thus preventing any electron transfer process

22. formation of phases
The electrode reaction involves the formation of a new phase (e.g. electro-deposition processes used in galvanizing metals)
a multi-stage process since it requires a first nucleation step followed by crystal growth (in which atoms must diffuse through the solid phase to then become located in the appropriate site of the crystal lattice)

23. The Current as a Measurement of the Rate of an Electrode Reaction
Faraday’s law:
C mol-1
reaction rate, v (mol s-1)
faradic current
Ox + ne Red

24. The Potential as a Measurement of the Energy of the Electrons inside the Electrode
Fermi level
Threshold potential : standard potential, E°
(S /S- & S+ /S)
S + e- → S-
S- → S + e-

25. The Biunique Relationship Between
Current and Potential
Since the potential regulates the energy of the electron exchanges, it also controls the rate of such exchanges and, hence, the current
This biunique correspondence between current and potential implies that if one of the two parameters is fixed the other, consequently, also becomes fixed