2.6 Electrodes in Experimental Cells
19
2.6 Electrodes in Experimental Cells
When electrochemical cells are used in practice, the number of electrodes is often
larger than two, and the role of the electrode may be various. Therefore, it is worthwhile looking over the terms occurring in connection with practical electrochemical
cells where the electrode names are related to their functionality, in contrast to the
categorization presented earlier concerning the nature of the electrode processes
(Sect. 2.4).
The working electrode is the one at which the processes taking place are in the
focus of interest. It is generally desired that both the potential of this electrode and the
current passed are monitored. However, the potential reference and the current lead
for the cell are usually not the same. For potential reference, or reference electrode,
the best choice is to use an equilibrium electrode whose potential is fixed and stable.
Alternatively, if the system does not make it possible to use an equilibrium electrode
for potential reference, a non-equilibrium electrode with a sufficient stability at the
time scale of the experiment can also be used, which is often termed as a quasireference electrode. Regardless of the realization of the potential reference, the stability of
the system requires that the reference electrode should not carry any current, which
makes it necessary to use a counter or auxiliary electrode that can be loaded with
the same current as the working electrode. The potential of the counter electrode is
seldom monitored but its potential is important because the total cell voltage (i.e.,
the potential difference between the working and the counter electrode) determines
the power needed to operate a cell. Therefore, a counter electrode usually exhibits
a large surface area as compared to the working electrode for decreasing the total
cell voltage. Another important requirement towards the counter electrode is that its
reaction product should not contaminate the working electrode environment. This
condition is often fulfilled by constructing cells with several compartments in which
the product of the reaction taking place on one electrode cannot reach the other one.
A peculiar feature of electrochemistry is that an electrochemical cell can be operated in various modes. If the total current is controlled, we encounter the galvanostatic control of the cell. In contrast, when the potential of the working electrode is
controlled, one arrives at the potentiostatic operation. Both operation modes can be
carried out in a more advanced manner, by prescribing sweeps or pulses. However,
whichever signal shape is used for the controlled parameter, a current control is
equivalent to the regulation of the reaction rate, while the potential control is a tool
of the regulation of the driving force of the processes.
Finally, it is important to mention that electrodes may get various names based on
the sign of the current passing through them. For the anode, the current is positive that
is associated with the flux of positively charged species from the electron-conducting
phase towards the ionically conducting phase, which is equivalent to that an oxidation
reaction takes place on this electrode. The cathode is an electrode where reduction
takes place and the charge flux is the opposite than at the anode. These categories
are not an inherent feature of the electrodes but change as the current direction is
reversed. Nevertheless, it is common (though formally incorrect) that some electrode
19
2.6 Electrodes in Experimental Cells
When electrochemical cells are used in practice, the number of electrodes is often
larger than two, and the role of the electrode may be various. Therefore, it is worthwhile looking over the terms occurring in connection with practical electrochemical
cells where the electrode names are related to their functionality, in contrast to the
categorization presented earlier concerning the nature of the electrode processes
(Sect. 2.4).
The working electrode is the one at which the processes taking place are in the
focus of interest. It is generally desired that both the potential of this electrode and the
current passed are monitored. However, the potential reference and the current lead
for the cell are usually not the same. For potential reference, or reference electrode,
the best choice is to use an equilibrium electrode whose potential is fixed and stable.
Alternatively, if the system does not make it possible to use an equilibrium electrode
for potential reference, a non-equilibrium electrode with a sufficient stability at the
time scale of the experiment can also be used, which is often termed as a quasireference electrode. Regardless of the realization of the potential reference, the stability of
the system requires that the reference electrode should not carry any current, which
makes it necessary to use a counter or auxiliary electrode that can be loaded with
the same current as the working electrode. The potential of the counter electrode is
seldom monitored but its potential is important because the total cell voltage (i.e.,
the potential difference between the working and the counter electrode) determines
the power needed to operate a cell. Therefore, a counter electrode usually exhibits
a large surface area as compared to the working electrode for decreasing the total
cell voltage. Another important requirement towards the counter electrode is that its
reaction product should not contaminate the working electrode environment. This
condition is often fulfilled by constructing cells with several compartments in which
the product of the reaction taking place on one electrode cannot reach the other one.
A peculiar feature of electrochemistry is that an electrochemical cell can be operated in various modes. If the total current is controlled, we encounter the galvanostatic control of the cell. In contrast, when the potential of the working electrode is
controlled, one arrives at the potentiostatic operation. Both operation modes can be
carried out in a more advanced manner, by prescribing sweeps or pulses. However,
whichever signal shape is used for the controlled parameter, a current control is
equivalent to the regulation of the reaction rate, while the potential control is a tool
of the regulation of the driving force of the processes.
Finally, it is important to mention that electrodes may get various names based on
the sign of the current passing through them. For the anode, the current is positive that
is associated with the flux of positively charged species from the electron-conducting
phase towards the ionically conducting phase, which is equivalent to that an oxidation
reaction takes place on this electrode. The cathode is an electrode where reduction
takes place and the charge flux is the opposite than at the anode. These categories
are not an inherent feature of the electrodes but change as the current direction is
reversed. Nevertheless, it is common (though formally incorrect) that some electrode