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In previous sections the basic principles behind voltammetry have been introduced. Now we
examine the effect of introducing a small sinusodial voltage perturbation in addition to the usual
cyclic voltammetry waveform.
AC Voltammetry
AC Voltammetry typically involves the application of a sinusodially oscillating voltage to a
electrochemical cell. The AC experiment when used in conjuction with a lockin amplifier or
frequency analyser offers considerably increased sensitivity over the early described techniques
and can also reveal important mechanistic and kinetic information not easily available using more
tradition voltammetric techniques.
An AC voltammetric measurement is usually performed in a electrochemical cell where diffusion
is the dominate mode of transport. The AC voltage is often combined with either a steady DC
signal or voltage sweep, for example, the following figure shows a cyclic voltammetric signal
with an AC perturbation
The most important aspect of this figure is the magnitude of the AC perturbation which is seen
to be small in comparison to the overall change in voltage occurring during the sweep. Typically
an amplitude of 5 mV or less is employed in AC measurements. This small perturbation ensures
only slight changes in concentrations occur close to the electrode surface and allows
mathematical analysis to assume that the effect on the electrode kinetics can be calculated in a
linear manner, even though the electrode kinetics strictly have an exponential dependence on the
applied voltage. The figure below shows how the current varies when an AC component is added
to a normal CV experiment. In this case the sine wave amplitude (peak to peak) was set as 40
mV, far larger than a true experiment in order to illustrate the current variation.
However this is not the signal that is usually recorded or presented from the experiment, since
a lockin amplifier or frequency response analyser allows the component of the current which is
varying sinusodially to be separated from the dc signal. Passing the signal above through the
lockin amplifier provides the magnitude of the change over each cycle and appears as
The maximum change is seen to occur at E0 as this is the region where the electrode
kinetics are most sensitive to voltage changes, whereas at the two extremes of the voltage range
there is no variation since the electrode kinetics are insignificantly affected and so the current
doesn't vary. Next we will focus on an individual cycle, to establish the phase relationship
between the current and voltage
It is clear that the current maximum is shifted from the applied voltage maximum by 45o and this phase shift can be understood by
studying the concentration profiles close to the electrode surface during a voltage cycle. Again
for illustration purposes a large voltage amplitude has been employed and the concentration
profiles generated are shown below
In the top figure the voltage is dropping from zero to the negative minimum and the surface
concentration of the reactant is dropping as expected. However the current is dependent upon the
flux of material to the surface and the maximum of this flux occurs between the yellow and
mauve curves. The origin of the phase shift can therefore be seen to result from the diffusional
process occurring. Mathematical analysis of the phase relationship by solution of Fick's laws for
the reversible AC voltammogram shows that the phase angle seen in the above voltammogram
is exactly as predicted. The lower figure shows the concentrations as the voltage moves
from the minimum back uptowards the zero line. The gradient can be seen to change direction
and consequently a current begins to flow in the opposite direction to that in the top figure.
Also a ripple in the concentration profile is seen as the diffusion is unable to keep up with the
changes induced on the surface, hence again the phase difference between the voltage and
current. We will discover later in the impedance discussions the effect of the electron transfer
kinetics on the voltammetry.
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