10-70
intermediate systems, the Nyquist plot is a semi-circle at high frequencies (kinetic
control) and changes to a straight line at low frequencies (diffusion control); that is,
the different controlling influences can be detected in one experiment.
Although the Nyquist plot is the most commonly used method of data presentation,
the other plots are useful for various systems. For example, the Bode plots have better
resolution than the Nyquist plot, so are useful if there is more than one rate-
determining process controlling the system. The Bode plot also provides explicit
illustration of the frequency dependence of the system, whereas this is only implicit
in the Nyquist plot.
Although this approach seems simple, it should be noted that electrochemical cells do
not behave like ideal RC circuits; for example, C
dl
is often potential dependent. In
addition, the impedance plots can be very complicated, and can be difficult to
interpret (although a large amount of information can often be obtained). For
example, in the Nyquist plot, there may be more than one semi-circle, and these may
not be in the positive quadrant of the plot (positive Z" values suggests inductive
behavior, which cannot easily be attributed to any component of the cell).
Nevertheless, impedance measurements have been used extensively in studies on the
rate of electron transfer; for example, corrosion and battery studies. They have also
been used to measure the rate of electron transfer through electroactive polymer films
on electrode surfaces.
Setting up IMP experiments
The BAS Impedance Module is required for
IMP
experiments. It is strongly
recommended that this Module is used in conjunction with the Faraday cage, which
shields the cell from electronic interferences.
Before any
IMP
experiments can be run, the BAS 100B/W must be calibrated. This
is done using
Measure Impedance
in the
Control
Menu. This calibration requires a
1000 ohm resistor, which is connected to the working electrode on one side and the
reference and auxiliary electrodes on the other. The
Initial E
(in the
General
Parameters
) must be set to zero. The
Reference
command calibrates the entire
frequency range, using the default parameters. The results are stored in a calibration
table in battery-backed RAM, so the
Reference
procedure does not have to be
repeated over the lifetime of the battery. This calibration requires many hours, so is
best done overnight. If the Impedance module was purchased with the BAS 100 B/W,
this calibration was done in the factory before shipping. If not, it must be done by the
user.
The
Calibration
command generates a special calibration table to be used for a given
set of user-defined conditions (e.g., frequency range, amplitude). However, this
calibration table is not stored in battery-backed RAM, and is lost when the main
power is switched off. Once the calibration tables have been set up, either may be
used (select the
Use Reference/Calibration Constants
)
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