Page 240 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN005B-205 June 15, 2001 20:24

Electrochemical Engineering 147

Many of the concepts from ordinary chemical kinetics
have counterparts in electrode kinetics. In both types of
systems, energy barriers must be surmounted by the re-
actants to form products, and increasing the temperature
increases the probability that this barrier can be overcome.
The important difference in electrochemical systems is
that the reaction rate can be increased by increasing the
potential difference at the electrode surface. In fact, a sig-
nificant advantage of electrochemical processes is that an
increase in overpotential of 1 V can increase the reaction
8
rate by a factor of 10 . In an ordinary chemical reaction
a temperature increase of several hundred degrees centi-
FIGURE 2 Electrolyte conductivity as a function of concentration grade is required to produce an equivalent change.
◦
for common aqueous electrolytes at 25 C. Electrode kinetics are influenced by the potential dif-
ference established across a layer immediately adjacent
carriers. Typically, the conductivity of an aqueous elec- to the electrode surface. As the electrode is polarized,
−1
trolyte is between 0.001 and 1 ohm −1 cm . By contrast charges build up on the surface of the electrode, and a
the electrical conductivity of a metal is of the order of corresponding charge distribution of opposite sign builds
˚
−1
100,000 ohm −1 cm . Most ionic solutions increase in up in the solution about 10 A from the electrode surface.
conductivity with increasing ionic concentration and with These two separated regions of charge are referred to as
increasing temperature. Many solutions exhibit a conduc- the double layer. The original model for the double layer,
tivity maximum that is due to incomplete dissociation of proposed by Helmholtz in 1879, was a parallel-plate ca-
the solute molecules. Salt solutions typically increase in pacitor. Since the distance between the parallel layers of
conductivity by about 2% per C. The conductivities of charge is so small, even a modest potential difference of
◦
several common electrolytes are shown in Fig. 2. 100 mV across the double layer leads to an enormous
6
It is usually desirable to use high-conductivity elec- electric field strength, more than 10 V/cm. More detailed
trolyte in an electrochemical process. Ohmic losses, which models of the double layer have subsequently been devel-
are inversely proportional to conductivity, result in in- oped, but the general concept of electrode kinetics being
creased energy consumption. Because the additional en- influenced by the strong field adjacent to the electrode
ergy is converted to heat, low-conductivity electrolytes surface is still valid.
may require increased thermal management. In industrial The passage of a net current through an electrode im-
practice both the temperature of the electrolyte and the plies that the electrode is no longer at equilibrium and
concentration of reacting ions are maintained at relatively that a certain amount of overpotential is present at the
high levels. Production of hydrogen by the electrolysis of electrode–electrolyte interface. Since the overpotential
water is carried out at 85 Cin6 N KOH solution. An- represents a loss of energy and a source of heat production,
◦
other common technique for increasing conductivity is to a quantitative model of the relationship between current
increase the concentration of charge carriers by adding density and overpotential is required in design calcula-
compounds that dissociate in the solvent but do not par- tions. A fundamental model of the current–overpotential
ticipate in the electrode reactions. Such compounds are relationship would proceed from a detailed knowledge of
called supporting or indifferent electrolytes. For instance, the electrode reaction mechanism; however, mechanistic
adding sufficient sulfuric acid to a copper sulfate solution studies are complicated even for the simplest reactions.
can increase the conductivity by an order of magnitude. In addition, kinetic measurements are strongly influenced
In the electrodeposition of copper, sulfuric acid does not by electrode surface preparation, microstructure, contam-
react, but it is frequently added as a supporting electrolyte. ination, and other factors. As a consequence, a current–
overpotential relation is usually determined experimen-
tally, and the data are often fitted to standard models.
G. Electrode Kinetics
A somewhat general model is that represented by the
The rate at which an electrochemical process proceeds is Butler-Volmer equation,
governed by the intrinsic electrode kinetics or by mass-
α a F α c F
transport processes. If reactants are readily available at an i = i 0 exp η s − exp − η s , (11)
RT RT
electrode surface, then mass-transport limitations do not
govern the overall rate; in this section we shall consider where i 0 is the exchange-current density, α a the an-
this case, in which sluggish kinetics govern the rate. odic transfer coefficient, and α c the cathodic transfer