12
2 Electrochemistry and Electrodeposition
contact each other, an interface occurs where electrochemical reactions may take
place. When one of the conducting phases in contact with each other is an electron
conductor and the other is an ionic conductor, an electrode is formed. The concept
of electrode always involves more than one conducting phases with different charge
carrier mechanisms. A detailed analysis of the scientific approach to the electrode
definition can be found elsewhere [11]. What is important for us is that the region
where the transition from a particular conduction mechanism to another one takes
place has the properties that give rise to the functionality of the electrode, should
this transition zone be a single well-defined interface or a thick layer of gradually
varying composition. The charge (Q) passing through an electrode is related to the
amount of material (n, measured in mol) undergoing an electrochemical reaction,
the proportionality factor being the Faraday constant F (96,485 C mol
−1 ), and the
number of electrons taking part in the electrode reaction, z. If a single electrode
reaction takes place, we get the simple form of Faraday’s law:
Q = zFn
(2.1)
Carrying out a derivation with respect of the time and dividing both sides of the
equation with the electrode surface area, the relationship of the surface-normalized
reaction rate (v) and the current density (j) is obtained:
j = zFv
(2.2)
2.3 Electrodes and Electrochemical Cells
If two electrodes share an ionic conducting phase (or, these ionically conducting
phases with possibly different composition at least contact each other without a
change in the conduction mechanism), an electrochemical cell is made. Since the
study of an electrode always involves the possibility of charge transfer between the
electron-conducting and ion-conducting phases and charge accumulation within any
of these phases is impossible, the study of one particular electrode requires another
electrode, too; i.e., electrochemical cells are required for any measurement.
The potential difference between two electrodes of a cell is the so-called cell
potential difference (E cell ), regardless of the nature of the electrodes. In the description
of an electrochemical cell, three features of the cell are inherently connected with each
other: (i) the cell diagram, (ii) the cell reaction and (iii) the cell potential difference.
In accord with the convention accepted nowadays, the cell reaction should be written
in a way so that the flow of the positive charge within the ion-conducting phase points
from the electrode shown in the left-hand side of the cell diagram to the right one.
This also means that cell reaction has to be written so that an oxidation process takes
place at the electrode at left. The cell potential difference is obtained as the potential
of the electrode shown at the right minus the potential of the electrode shown at left.
2 Electrochemistry and Electrodeposition
contact each other, an interface occurs where electrochemical reactions may take
place. When one of the conducting phases in contact with each other is an electron
conductor and the other is an ionic conductor, an electrode is formed. The concept
of electrode always involves more than one conducting phases with different charge
carrier mechanisms. A detailed analysis of the scientific approach to the electrode
definition can be found elsewhere [11]. What is important for us is that the region
where the transition from a particular conduction mechanism to another one takes
place has the properties that give rise to the functionality of the electrode, should
this transition zone be a single well-defined interface or a thick layer of gradually
varying composition. The charge (Q) passing through an electrode is related to the
amount of material (n, measured in mol) undergoing an electrochemical reaction,
the proportionality factor being the Faraday constant F (96,485 C mol
−1 ), and the
number of electrons taking part in the electrode reaction, z. If a single electrode
reaction takes place, we get the simple form of Faraday’s law:
Q = zFn
(2.1)
Carrying out a derivation with respect of the time and dividing both sides of the
equation with the electrode surface area, the relationship of the surface-normalized
reaction rate (v) and the current density (j) is obtained:
j = zFv
(2.2)
2.3 Electrodes and Electrochemical Cells
If two electrodes share an ionic conducting phase (or, these ionically conducting
phases with possibly different composition at least contact each other without a
change in the conduction mechanism), an electrochemical cell is made. Since the
study of an electrode always involves the possibility of charge transfer between the
electron-conducting and ion-conducting phases and charge accumulation within any
of these phases is impossible, the study of one particular electrode requires another
electrode, too; i.e., electrochemical cells are required for any measurement.
The potential difference between two electrodes of a cell is the so-called cell
potential difference (E cell ), regardless of the nature of the electrodes. In the description
of an electrochemical cell, three features of the cell are inherently connected with each
other: (i) the cell diagram, (ii) the cell reaction and (iii) the cell potential difference.
In accord with the convention accepted nowadays, the cell reaction should be written
in a way so that the flow of the positive charge within the ion-conducting phase points
from the electrode shown in the left-hand side of the cell diagram to the right one.
This also means that cell reaction has to be written so that an oxidation process takes
place at the electrode at left. The cell potential difference is obtained as the potential
of the electrode shown at the right minus the potential of the electrode shown at left.