ELECTROCHEMICAL METHODS Fundamentals and Applications - Allen.J.Bard

22 I* Chapter 1. Introduction and Overview of Electrode Processes
Interpreting the rate of an electrode reaction is often more complex than doing the same
for a reaction occurring in solution or in the gas phase. The latter is called a homogeneous
reaction, because it occurs everywhere within the medium at a uniform rate. In contrast, an
electrode process is a heterogeneous reaction occurring only at the electrode-electrolyte interface.
Its rate depends on mass transfer to the electrode and various surface effects, in addition
to the usual kinetic variables. Since electrode reactions are heterogeneous, their
reaction rates are usually described in units of mol/s per unit area; that is,
(1.3.4)
where у is the current density (A/cm 2 ).
Information about an electrode reaction is often gained by determining current as a
function of potential (by obtaining i-E curves). Certain terms are sometimes associated
with features of the curves. 8 If a cell has a defined equilibrium potential (Section 1.1.1),
that potential is an important reference point of the system. The departure of the electrode
potential (or cell potential) from the equilibrium value upon passage of faradaic current is
termed polarization. The extent of polarization is measured by the overpotential, rj,
rj = E - E {
eq
(1.3.5)
Current-potential curves, particularly those obtained under steady-state conditions, are
sometimes called polarization curves. We have seen that an ideal polarized electrode
(Section 1.2.1) shows a very large change in potential upon the passage of an infinitesimal
current; thus ideal polarizability is characterized by a horizontal region of an i-E curve
(Figure 1.3.5a). A substance that tends to cause the potential of an electrode to be nearer
to its equilibrium value by virtue of being oxidized or reduced is called a depolarizer? An
{a) Ideal polarizable electrode
(b) Ideal nonpolarizable electrode
Figure 1.3.5 Current-potential curves for ideal (a) polarizable and (b) nonpolarizable electrodes.
Dashed lines show behavior of actual electrodes that approach the ideal behavior over limited
ranges of current or potential.
8
These terms are carryovers from older electrochemical studies and models and, indeed, do not always represent
the best possible terminology. However, their use is so ingrained in electrochemical jargon that it seems wisest
to keep them and to define them as precisely as possible.
9
The term depolarizer is also frequently used to denote a substance that is preferentially oxidized or reduced, to
prevent an undesirable electrode reaction. Sometimes it is simply another name for an electroactive substance.