Environmental Engineering Reference
In-Depth Information
Dissolution of zinc will proceed only at potentials more positive than
0.76 V
(SHE).
There are two principal types of polarization, i.e., activation polarization and
concentration polarization. These are discussed below.
Activation Polarization
Activation polarization arises out of a slow step in the electrode reaction for
which an activation energy in the form of an increment in potential is required for
the reaction to proceed. This will be best exemplified by the hydrogen evolution
reaction.
The hydrogen evolution reaction consists of several steps as shown in Fig.
2.11. Either the electron transfer step (no. 2) or the formation of hydrogen mole-
cules (no. 3) is deemed to be the slowest step in the reaction sequence and the
rate of overall reaction will depend on how fast or how slow it is proceeding.
Therefore, in order to have a higher rate of reaction, expressed in terms of in-
creased current density, an increase in potential is to be effected. The relationship
between reaction rate and change in potential (overvoltage) is expressed by the
Tafel equation:
i
i 0
η a β
log
(2.39)
where
η a is overvoltage activation polarization, in volts
is a constant, called Tafel constant (also expressed in volts), and is usually
of the order of 0.1 V
A graphical representation of the above equation, as applied to hydrogen evo-
lution reaction, with a
β
slope of 0.1 V is shown in Fig. 2.12. It can be noted
from the graph that 0.1 V change in overvoltage can effect a 10-fold increase or
decrease in the reaction rate.
β
Figure 2.11
Steps involved in hydrogen reduction reaction.
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