Electrochemical Impedance Spectroscopy Using the BAS

Electrochemical Impedance Spectroscopy Using the BAS
Electrochemical Impedance
Spectroscopy Using the
BAS-Zahner IM6 and IM6e
Impedance Analyzers
Adrian W. Bott, Ph.D.
Bioanalytical Systems, Inc.
2701 Kent Avenue
West Lafayette, IN
47906-1382
This article discusses the IM6 and SIM modules of the Thales software
system that is used for the operation of the BAS-Zahner IM6 and IM6e
impedance analyzers. The IM6 module is used for data acquisition,
whereas the SIM module is used for data analysis, equivalent circuit
synthesis, and fitting of experimental data.
E-mail:
[email protected]
Electrochemical impedance
spectroscopy is becoming increasingly popular as an analytical technique for a number of applications,
including characterization of batteries, fuel cells, coatings, ceramics,
semiconductors, sensors, and corrosion studies. The BAS-Zahner IM6
and IM6e Impedance Analyzers
combine high precision hardware
and powerful, user-friendly software
to simplify the acquisition and analysis of high quality, reliable impedance data.
Although the hardware specifications of the IM6 and IM6e differ
(T1), the operating system for both
is the Thales software. This is a
modular system, with separate programs for impedance data acquisition and data analysis, as well as
programs for techniques such as cyclic voltammetry, linear polarization, and electrochemical noise data
acquisition and analysis.1 The pro-
53
grams for impedance data acquisition (IM6) and data analysis (SIM)
are the subject of this article.
IM6 — Impedance Data
Acquisition
Activating the IM6 icon in the
Thales menu leads to the screen
shown in F1. The first item to examine is the check cell connections
screen (when this icon is activated,
the potentiostat is automatically
switched off, which allows the connections to be changed safely). This
shows the cell lead BNC connections, and the order of connection for
two, three, and four electrode operation (F2). Note that there are four
cell leads, two for the working (test)
electrodes, and one for each of the
reference and counter (auxiliary)
electrodes. The two connections for
the test electrode are for the currentmeasuring circuit (power connec-
tion), and for the potential-control
circuit (sense connection). A buffer
amplifier is also available, if a higher
input impedance is required for the
reference electrode (this amplifier is
plugged into the Probe E connection,
and substitutes for the test electrode
sense and reference electrode BNC
connections).
Once the cell connections have
been made, the experimental parameters can be entered. The DC potential and the AC amplitude are
adjusted using the test-sampling and
potentiostat control screen (F3).
When this screen is first entered,
the potentiostat is switched off and
the rest potential is displayed in the
DC POTENTIAL display box. This
allows the rest potential to be monitored as a check for system stability.
Once the rest potential has stabilized, the potentiostat can be
switched on2 (the required value is
entered in the POTENTIAL dialog
Current Separations 17:2 (1998)
T1
Comparison of IM6
and IM6e.
IM6e
IM6
Upper
frequency limit
1 MHz
8 MHz
Currrent range
±1A
±3A
Rise time
300 ns
100 ns
Expansion slots
1
4
Floppy disk drive
no
yes
IEEE-488 bus
no
yes
F1
Menu for IM6 program.
F2
Connection scheme for
three electrode system
without buffer amplifier.
F3
Test-sampling and
potentiostat control
screen.
Current Separations 17:2 (1998)
box). The background DC current is
now displayed in the DC CURRENT
display box. Once the DC parameters have been set, the AC parameters
can be optimized by measuring the
impedance at selected single frequencies. The required parameters
are the FREQUENCY, the AMPLITUDE, and the number of COUNTS.
While these single frequency meas-
urements are being made, the AC
potential and current wave forms
and spectra are displayed in the upper left corner. These provide an indication of the quality of the
experimental data and the linearity
of the system response, and the AC
amplitude and number of counts can
be adjusted if necessary. This provides an efficient method for optimizing the experimental parameters
(the alternative method of running a
full frequency spectrum for each set
of parameters is considerably more
time consuming).
Once the AC amplitude and the
number of counts have been optimized, the frequency range, the
number of data points, and the
number of cycles are entered using
the recording parameters screen (F4).
Three parameters can be used to
define the Frequency range: a Start
at value, an Upper limit, and a Lower
limit. The default measurement starts
at an intermediate value, sweeps to
the high frequency limit, and then
sweeps to the low frequency limit.
The measurement at the same high
frequency points on the forward and
reverse sweep provides an indication
of system stability without adding
significantly to the experimental
time, as any change in the system
over this time scale will alter the
impedance and phase angle at a
given frequency. A single sweep
measurement from high to low frequency can be obtained by setting
the Start at and Upper limit frequencies to the same value. Changing the
Sweep Mode changes the initial direction of the frequency sweep.
One problem inherent in impedance experiments is the balance between the improvement in the data
quality obtained by increasing the
number of frequencies used and the
number of cycles collected at each
frequency and the increase in the
time required for the experiment,
which can lead to time-dependent
artifacts. This problem is particularly
pronounced when low frequencies
are used. The IM6 program offers a
partial solution by dividing the frequency range into two (greater than
54
and less than 66 Hz) when setting the
number of frequencies (Steps per
decade) and the number of cycles at
each frequency (Number of measure
F4
Recording parameters
screen.
periods).3 This allows the number of
frequencies and cycles to be increased at high frequencies without
significantly increasing the time re-
quired for the experiment (changes
in the time are shown in the Expected
Time display box).
Once the parameters have been
set, the experiment can be started, by
clicking either the START Recording
icon in the recording parameters
screen or the record impedance spectrum icon in the main IM6 menu.
During the experiment, the data is
displayed in real time in either a
Nyquist or a Bode plot (other formats are available), as well as the AC
wave forms and spectra, and the
measured numerical values (F5).
When the experiment is complete, the data can be displayed using
display spectrum. However, if more
comprehensive data analysis is required, the data must be saved, and
then loaded into the SIM program.
SIM Impedance data analysis
F5
Real time data display
during experiment.
F6
Menu for SIM program.
55
The SIM program is used for
data analysis, including equivalent
circuit synthesis, and fitting of experimental and simulated data.
Clicking the SIM icon in the main
Thales menu produces the menu
shown in F6.
The first step is to load one or
more experimental data files (multiple
data files can be displayed simultaneously, either overlayed or in 3-D).
These can then be viewed in the
Edit/Show Measure Data menu (F7).
The impedance data can be displayed using either a Bode plot (F8)
or a Nyquist plot (F9). Selection of
the display mode, together with the
axis scales, is made in the define
diagram type screen. The data can be
smoothed, edited (i.e., the phase angle and impedance values can be altered), printed, exported to the
Thales CAD graphics program, or
saved in an ASCII text format (for
export to external data analysis and
plotting programs).
The other menu items are select
measure samples for fitting and Kramers-Kronig rule check. As noted
above, the data density is typically
higher at high frequencies than at
low frequencies. Hence, if all the
data points in the spectrum were
Current Separations 17:2 (1998)
F7
The basis of the Kramers-Kronig
rule is that the phase angle and impedance are related; that is, the impedance values can be calculated
from the phase angle values, and vice
versa. However, the KramersKronig rule only applies if the following four conditions are met by
the system under investigation:
Edit/Show Measure
Data menu.
Causality The measured response
of the system is due
only to the applied perturbation, and there are
no contributions from
other sources.
Linearity The relationship
between the perturbation and the response
should be linear.
F8
Bode plot.
Stability The system must return
to its original state when
the perturbation is
removed.
Finiteness The impedance must be
finite at all frequencies.
F9
Nyquist plot.
used when optimizing the fit of
simulated data to experimental data,
there would be a bias towards high
frequencies. It is therefore advisable
to select data points equally distribCurrent Separations 17:2 (1998)
uted on the logarithmic frequency
scale when fitting simulated data to
experimental data, and this is
achieved using select measure samples for fitting.4
In the Kramer-Kronig rule
check, the impedance is calculated
from the phase angle, and the calculated impedance is compared with
the measured impedance. Any deviation between the calculated and experimental values shows that one of
the above conditions has not been
maintained during the experiment,
and hence the measured data are not
reliable. In practice, the most common cause of deviation is system
instability, particularly during measurements at low frequencies, which
may require several hours. This is
illustrated in F10, which shows deviations at low frequencies.
Interpretation of impedance data is
often based on the principle of equivalent circuits, which states that it is possible to build a circuit consisting of
electronic components that gives an
impedance spectrum identical to that
obtained experimentally. This principle also requires that there is a correlation between the components of the
equivalent circuit and the components
of the experimental system.
The SIM program can be used to
build an equivalent circuit, calculate
56
the theoretical impedance spectrum
for this circuit, and optimize the values of the circuit components to provide the best fit with the experimental
data. Equivalent circuits can be built
using a graphical editor (F11),
which is accessed by clicking the
Edit/Create Model icon.
The available circuit elements
are displayed on the right side of the
model editor. These include a resistor, an inductance, a capacitor, a constant phase element, the Warburg
impedance, two finite diffusion elements (hyperbolic cotangent and hyF10
Kramers-Kronig
rule check.
F11
Graphical editor for
constructing equivalent
circuits.
57
perbolic tangent functions), and elements for spherical diffusion and homogeneous reactions. There are also
specific models for surface relaxation, a passive layer (Young surface
layer impedance), and a porous electrode. In addition, user-defined elements can be written, and used in
equivalent circuits.
The method for building an
equivalent circuit is as follows. The
individual elements are selected
from the table, the parameter
value(s) are entered, and the connection of this element is then defined.
As each element is specified, it is
displayed in the graphics box on the
left of the screen. In the example
shown in F11 and F12 (the Randles
circuit), a resistor was first selected
(F12a) (charge-transfer resistance).
The next element was the Warburg
impedance (semi-infinite diffusion),
which was connected in series to the
resistor (F12b). A capacitor was
then selected (double-layer capacitance), and was connected in parallel
with the first two elements (F12c).
The final element was a resistor (solution resistance), which was connected in series (F12d).
This method can be used to build
a wide range of circuits. Once constructed, the equivalent circuits can
be edited, either to change a parameter value, or to change an element
(e.g., a capacitor to a constant phase
element). Elements can also be inserted or deleted, although this may
not be advisable for more complicated circuits, since the linear order
of the model may be affected.
Once the equivalent circuit has
been constructed, the theoretical impedance spectrum for this circuit
can be viewed using plot diagram in
the Calculate Transfer Function
menu (the plot diagram and define
diagram type functions in this menu
are identical to the analogous functions in the Edit/Show Measure
Data menu). The parameter values
of the circuit can also be optimized
to provide the best fit to a selected
experimental impedance spectrum
using the CNRLS (Complex Nonlinear Regression Least-Squares)
Fitting routine.
Before the fitting routine can be
used, both an experimental data file
and an equivalent circuit (model) must
be specified. In addition, the number
of data points used in the fitting routine
must be specified (see above).
Before the fitting operation is
started, the screen displays the
equivalent circuit being used, and the
initial parameter values. While the
fitting operation is in progress, the
parameters values used for each iteration are displayed, as well as the
difference in the experimental and
Current Separations 17:2 (1998)
F12
Assembly of the Randles
circuit using the SIM
graphical editor.
F13
Progress of the
fitting operation.
simulated impedance and phase angle values for each of the selected
frequencies (F13). Iterations will
continue until either the absolute error or the improvement in the error
on consecutive iterations is less than
0.1%. The optimized parameter values will then be displayed, together
with the maximum and mean errors
in the impedance and phase angle
(F14).
The significance and error are
also listed for each parameter. The
significance is the maximum value
of dZ/dP (P = parameter value) (i.e.,
it is a measure of the effect of that
parameter on the impedance; it is
important to note that this is frequency dependent). The error is related to the difference between the
experimental and calculated impedances at the frequency for which the
significance is quoted.
The experimental data and the
optimized calculated data can be
compared using plot diagram in the
Calculate Transfer Function menu
(F15). The line represents the calculated data, and the crosses show the
selected experimental data.
Automatic series
measurement
F14
Results of fitting
operation.
A function that is standard on the
IM6, but optional on the IM6e, is the
ability to run series of measurements.
This enables the user to record a series of impedance spectra either as a
function of internal variables, such as
time, potential, and current, or as a
function of external variables, such as
temperature. There are a number of
features of SIM that can be used for
such measurement series.
3-D plots
The data from series measurements can be plotted on the same set
of axes, either as a 2-D plot, or as a
3-D plot, where the third axis is the
parameters varied in the series measurement. The 3-D plot in F16 shows
how the impedance vs. frequency
plot varies as a function of the applied DC potential.
Current Separations 17:2 (1998)
58
F15
Comparison of
experimental and
simulated data for a
stainless steel electrode.
Series fitting
The series fitting operation will
fit all selected data sets of the series
measurement to one selected equivalent circuit. The optimized parameter
values for each data set are saved in
separate files.
Single frequency analysis
F16
3-D plot of impedance
spectra as a function of
applied DC potential.
F17
Variation of impedance
and phase angle as a
function of potential at
100 Hz.
The 3-D plot displays the variation of impedance and/or phase angle as a function of the varied
parameter for the entire frequency
spectrum. The variation of either
impedance and/or phase angle as a
function of the varied parameter at a
single frequency can also be displayed (F17) (i.e., a cross-section
of the 3-D plot). This display is
available for both the experimental
spectra and those calculated based
on the equivalent circuits. The variation of the parameter values of the
components of the equivalent circuits can also be plotted for the calculated spectra.
It has been shown in the above
discussion how the Thales software
used for the IM6 and IM6e impedance
analyzers facilitates the acquisition
and analysis of electrochemical impedance data. The capabilities of these
instruments can be further extended by
the addition of other modules of the
Thales system, which use the same
graphical approach for cyclic voltammetry, linear polarization, electrochemical noise analysis, and
capacitance/potential measurements.
Endnotes
1.
2.
3.
4.
59
The additional modules are
standard on the IM6 and optional
on the IM6e.
In this discussion, the system will
be under potentiostatic control.
However, galvanostatic control is
also available (the switch in the
lower right corner is used to select
the required mode).
Note that measurements are made
at single frequencies over the
whole frequency range.
The select measure samples for
fitting also allows user-defined
selection of data points to be used
for fitting.
Current Separations 17:2 (1998)
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement