Page 62 - Lindens Handbook of Batteries

ELECTROCHEMICAL PRINCIPLES AND REACTIONS 2.19

on the physical structure (such as tortuosity, pore sizes), the conductivity of the solid matrix and the
electrolyte, and the electrochemical kinetic parameters of the electrochemical processes. A detailed
treatment of such complex porous electrode systems can be found in Newman. 10

2.6 ELECTROCHEMICAL TECHNIQUES

Many steady-state and transient electrochemical techniques are available to the experimentalist to deter-
mine electrochemical parameters and assist in both improving existing battery systems and evaluating
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couples as candidates for new batteries. Some of these techniques are described in this section.

2.6.1 Cyclic Voltammetry

Of the electrochemical techniques, cyclic voltammetry (or linear sweep voltammetry) is one of
the more versatile techniques available to the electrochemist. The derivation of the various forms
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of cyclic voltammetry can be traced to the initial studies of Matheson and Nicols and Randles.
Essentially the technique applies a linearly changing voltage (ramp voltage) to an electrode. The
scan of voltage might be ±3 V from an appropriate rest potential such that most electrode reactions
would be encompassed. Commercially available instrumentation provides voltage scans as wide
as ±5 V.
To describe the principles behind cyclic voltammetry, for convenience let us restate Eq. (2.9),
which describes the reversible reduction of an oxidized species O,
O+ e  R (2.9)
n 
where O = oxidized species
R = reduced species
n = number of electrons involved in electrode process
In cyclic voltammetry, the initial potential sweep is represented by
E = E - νt (2.41)
i
where E = initial potential
i
t = time
ν = rate of potential change or sweep rate (V/s)
The reverse sweep of the cycle is defined by

E = E + ν′t (2.42)
i
where ν′ is often the same value as ν. By combining Eq. (2.42) with the appropriate form of the
Nernst equation [Eq. (2.6)] and with Fick’s laws of diffusion [Eqs. (2.34) and (2.35)], an expres-
sion can be derived that describes the flux of species to the electrode surface. This expression is a
complex differential equation and can be solved by the summation of an integral in small successive
increments. 14–16
As the applied voltage approaches that of the reversible potential for the electrode process, a
small current flows, the magnitude of which increases rapidly but later becomes limited at a poten-
tial slightly beyond the standard potential by the subsequent depletion of reactants. This depletion
of reactants establishes concentration profiles which spread out into the solution, as shown in
Fig. 2.18. As the concentration profiles extend into the solution, the rate of diffusive transport at
the electrode surface decreases and with it the observed current. The current is thus seen to pass