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Electrochemical
impedance spectroscopy
is a powerful technique used
to characterize materials
based on how they impede
the flow of electricity
in applications as diverse as microbiology
and corrosion resistance.
The electrical conductivity of a sample
is based on the make up
of all of the components
of the sample.
Because of this, EIS can also be used
to detect changes in the
quantity or structure
of each component.
EIS is performed by applying
a small sinusoidal electrical load
across electrodes connected to a sample
at a wide range of frequencies.
Based on the measured response,
impedance is computed at
each of the frequencies.
Computer software is then
used to plot the results
and build an equivalent circuit model
that is representative
of the observed data.
The typical goal of using EIS
is breaking down the sample's
total electrical impedance
into contributions from
mechanisms such as resistance,
capacitance, or induction.
This video will illustrate
the principles and procedures
involved in EIS to determine
the impedance of a material.
It will also demonstrate how to create
equivalent circuit models of the sample.
Electrical resistance is the
ability of a circuit element
to resist the flow of electricity.
And Ohm's Law defines resistance
as voltage divided by current.
When dealing with AC currents, however,
electrical impedance is a more accurate
and general measure of the ability
to resist the flow of electricity.
This is because,
in addition to the
resistance of the material,
it accounts for the
contribution of mechanisms,
such as capacitance and induction.
If an applied AC signal is sinusoidal
and the response is linear,
the current produced
will also be sinusoidal,
but shifted in phase.
To account for the
frequency and phase shift
we can build impedance equations
for components of a circuit
using Euler's Relationship
and complex numbers.
These models are used to interpret data
showing impedance to be
independent of frequency for resistors,
inversely related to
frequency for capacitors,
and directly related to
frequency for inductors.
During EIS testing, the instrument applies
an alternating field voltage to a sample
and measures the current response.
The real and imaginary
components of impedance
are calculated by
determining the phase shift
and change in amplitude
at different frequencies.
A Nyquist plot is generated
by plotting the imaginary
component on the Y axis
and the real component on the X axis.
One of the simplest Nyquist
plots is a semi circle.
The plot is then used
to build a circuit model
that best represents the
impedance of the sample.
During modeling, physical processes
correspond to elements of a circuit.
For example, an electrical double layer
corresponds to a capacitor.
The equivalent circuit model for this plot
is represented by a resistor in series
with a resistor and capacitor in parallel.
This is a common starting place
for the interpretation of a Nyquist plot.
The software will present you
with equivalent circuit models
based on your Nyquist plot
for you to choose from.
If these models do not fit your data
you can manually model a
circuit to fit the data,
a complicated task.
In the next section,
we will show you how to
test a control sample
and an experimental sample with EIS
and then build an equivalent circuit
to represent the observed impedance data.
Gather EIS instruments and a test module.
Hook the test module up
to the EIS instruments
via two electrodes to model
a simple known circuit.
Open the ZPlot software on the computer
to set the parameters for the test module.
Set DC potential to zero,
AC amplitude to 10 millivolts,
and the drop down arrow
to versus open circuit.
Set initial frequency to one
times 10 to power six hertz,
final frequency to 100 hertz,
and interval to 10.
Select logarithmic and steps per decade.
Measure, then sweep, to
start a new recording,
and begin collecting data.
Compare measured values to expected values
found on the front of the test module.
If the values do not match,
check wiring and equipment,
and retest.
Obtain the sample of beta alumina
and put it in the assembly.
Working in the fume hood,
insert the assembly into the tube furnace
and attach the electrodes.
Open the ZPlot software
keeping the same parameters
used for the test module
and press measure, then sweep.
Open the ZView software
to view the results
as you did for the test module.
Save the plots.
Choose two points to fit the semi circle.
Then press the instant fit button
to choose the best
equivalent circuit model.
For simplification,
we ran this experiment
at room temperature.
EIS tests are usually run by
varying amplitude or voltage
as well as temperature.
Let's now take a look at our results.
Results of the EIS are
presented in a Nyquist plot
showing real impedance
versus complex impedance
at each frequency tested.
Multiple options of circuits
to model your data are provided,
it is best to choose the simplest model
that still accurately reflects the data.
Next, choose an equivalent circuit,
and using the resulting data,
let's calculate the
conductivity of the sample.
Data can also be fitted to a linear line
using the equation for conductivity.
Using the values found
through repeated testing for this sample,
a conductivity of 1.67
millisiemens per centimeter
is calculated,
as compared to the
reported conductivity value
of approximately 4.1
millisiemens per centimeter.
This indicates that the
model we chose was a good,
though not perfect fit.
Now that you appreciate the methods
of measuring and modeling impedance
using electrochemical
impedance spectroscopy,
let's take a look at some of
the applications for this tool.
EIS can be used to look at
microorganisms in a sample.
When bacteria grow on a sample
it can change the electrical
conductivity of the sample.
Because of this, EIS can be
used to measure impedance
to determine population growth.
This technique is known
as impedance microbiology.
EIS is also used in
the paint and corrosion
prevention industries.
Materials that show an
electrical resistance
of less than 10 to the power
six Ohms per centimeter square
may not protect against the
electrical chemical processes
that attack surfaces everyday.
EIS testing predicts the
corrosion resistance properties
of materials to be used
in harsh environments,
saving billions of dollars
in repairs every year
in the United States alone.
You've just watched JoVE's introduction
to electrochemical impedance spectroscopy.
You should now understand
how to test and model the
impedance characteristics
of materials.
Thanks for watching.
