Download Gamry Potensiostat (FAS2) Instructions and other useful information

Gamry Potensiostat (FAS2)
Instructions and other useful
information
General Info
An electrochemical cell must consist of at least two electrodes and one electrolyte. An electrode
may be considered to be an interface at which the mechanism of charge transfer changes between
electronic (movement of electrons) and ionic movement of ions. An electrolyte is a medium
through which charge transfer can take place by the movement of ions.
In a cell used for electroanalytical measurements there are always three electrode functions (see
below). The first of the three electrodes is the indicating electrode also known as the test or
working (GREEN) electrode. This is the electrode at which the electrochemical phenomena
being investigated takes place.
The second functional electrode is the reference (WHITE) electrode. This is the electrode whose
potential is constant enough that it can be taken as the reference standard against which the
potentials of the other electrodes present in the cell can be measured.
The final functional electrode is the auxiliary or counter (RED) electrode which serves as a
source or sink for electrons so that current can be passed from the external circuit through the cell.
In general, neither its true potential nor current is ever measured or known.
Indicator electrodes
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(Noble metal indicator electrodes)
There are a number of noble metal electrodes currently available for voltammetric studies. In
order of frequency of use, they are platinum, gold and silver followed occasionally by palladium,
rhodium and iridium. Various polycrystalline forms including sheets, rods and wires are
commercially available in high purity and the materials are readily machined into useful shapes.
All of the noble metals have an over potential for hydrogen evolution. All of the noble metals
adsorb hydrogen on their surfaces although gold does so to a lesser extent. Palladium adsorbs
hydrogen into the bulk metal in appreciable quantities and is not recommended for use as a
cathode in protic solvents.
(Carbon indicator electrodes)
As an inert electrode material, carbon is useful for both oxidation and reduction in both aqueous
and non aqueous solutions. Only graphitic forms of carbon conduct and are therefore useful as
electrode materials. Ordinary spectroscopic grade graphite rods can be used for work in which
the surface area of the electrode does not need to be well defined. Other types of carbon
electrode include the vitreous (glassy) carbon electrode and the carbon paste electrode.
Reference electrodes
The ideal reference electrode should posses the following properties;
•
•
•
•
•
it should be reversible and obey the Nernst equation with respect to some species in the
electrolyte
its potential should be stable with time
its potential should return to the equilibrium potential after small currents are passed
through the electrode
if it is an electrode like the Ag/AgCl reference electrode, the solid phase must not be
appreciably soluble in the electrolyte
it should show low hysteresis with temperature cycling
One of the commonest reference electrodes is the KCl saturated calomel half cell (SCE). A
simple form of this electrode can be assembled by adding to a tube, mercury metal, a small
amount of solid mercury (II) chloride, several grams of solid KCl and some distilled water.
Connection to the external measuring circuit can be made by using a fine platinum wire dipping
into the mercury pool. The potential of the SCE can be obtained from:
3
The principal shortcoming of the SCE as a reference is that the solubility of KCl
changes substantially with temperature and therefore the cell potential has a
relatively large temperature coefficient.
Instrumentation
(Photographs courtesy of Windsor Scientific, Slough, UK and BAS inc., USA)
Modern electroanalytical measurements are normally performed with software driven
potentiostats, two examples of which are shown above. Further details about the instruments are
available from the manufacturers.
Electrochemical Cell
The normal material for cell construction is pyrex glass for reasons both of visibility and general
chemical inertness. The size of the cell is variable and depends upon the volume, cost and
degree of dilution of the sample being studied. If material availability and cost poses no
problems, then 25-50cm3 cells can be used. With enzymes, cells with volumes of approximately
1cm3 are preferable because of the costs involved. Some electrochemical measurements can be
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run in cells under an atmosphere of air. Oxygen however is electrochemically active and its
solubility in water is sufficiently great that oxygen reduction can be a problem. As a
consequence, most measurements are carried out under an inert atmosphere of either nitrogen or
helium.
The photograph shows a conventional three electrode cell as used in the authors laboratory
showing the working electrode, reference electrode and auxiliary electrode. The cell lid is made
from resistant PTFE plastic. A gas line for ebulliating the solution with nitrogen or helium is
also evident. (Photograph courtesy of BAS inc., USA)
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User Instructions
Logon to “Potensiostat” on the logon computer in the center corridor
Software Available:
PHE200
EIS300
PV220
VFP600
Cyclic Voltammetry
Click on the “Gamry Framework” icon
Click on “Experiment”
Move mouse over “A Physical Electrochemistry”
Move mouse over and click on “5 Cyclic Voltammetry”
Pstat: Leave FAS2 selected
Test Identifier: Automatic label showing the type of test selected
Output File: Pick your directory (or create one) under C:\Users\
NOTE: You MUST add a "*..DTA" filename extension for your data filenames
Electrode Area: In cm2
Notes…: A place to identify your sample in the output file
Initial E (V): Starting Potential (see description below for vs Eref and vs Eoc)
Scan Limit 1 (V): First peak or maximum voltage
Scan Limit 2 (V): First valley or minimum voltage
Final E (V): Ending Potential
Scan rate (mV/S): Potential ramping rate
Step size (mV): Potential difference per step
Cycles (#):
I/E Range Mode: Auto or Fixed
Max Current (mA):
IRComp: None or PF or CI
PF Corr (ohm):
Equil.Time (s):
Init Delay: Off or On Time(s) Stab. (mV/s
Conditioning: Off or On Time(s) E(V)
Advanced Pstat Setup: Off or On
Electrode Setup: Off or On
Then click “OK” button
“Hardware Settings” box appears
I/E Stability: Fast or Norm or Slow
CA Speed: Fast or Norm or Med or Slow
Vch Range: 30mV or 300mV or 3V or 30V
Vch Filter: 300kHz or 1kHz or 5Hz or Auto
Ich Filter: 300kHz or 1kHz or 5Hz or Auto
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Then click “OK” button
“Electrode Settings” box appears
Electrode Type: None or Solid or DME or SMDE or HMDE or Rotating
Purge/Stir Cell: Off or On Time(s) Quiet(s)
Click on Chart icon (“Start Analysis”) to Analyze Data
“File”
“Open” Find your file
Test Identifier
The Identifier parameter is a string that is used as a name. It is written to the data file, so it can be
used to identify the data in database or data manipulation programs.
The Identifier string defaults a name derived from the technique's name. While this makes an
acceptable curve label, it does not generate a unique descriptive label for a data set.
The Identifier string is limited to 80 characters. It can include almost any normally printable
character. Numbers, upper and lower case letters, and most normal punctuation characters
including spaces are valid.
Output File
The default value of the Output File parameter is an abbreviation of the technique name with a
".DTA" filename extension. We recommend that you use a ".DTA" filename extension for your data
filenames. The data analysis package assumes that all data files have ".DTA" extensions.
NOTE: The software does not automatically append the ".DTA" filename extension. You must add
it yourself.
If the script is unable to open the file, an error message box, "Unable to Open File," is generated.
Common causes for this type of problem include:
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•
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An invalid filename.
The file is already open under a different Windows application.
The disk is full.
After you select OK in the error box, the script returns to the Setup box where a new filename can
be entered.
Electrode Area
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The Electrode Area parameter is the surface area of the electrode (in cm²) exposed to the sample
solution. If you do not wish to enter an area, leave this parameter at its default value of 1.0 cm².
Notes: Describe your sample and number in series if applicable
The Notes string is limited to 400 characters. It can include all printable characters including
numbers, upper and lower case letters, and the most normal punctuation including spaces. TAB
characters are not allowed in the Notes string.
You can divide your Notes into lines using ENTER.
Initial E
The Initial E parameter is the starting potential of the scan segment. This potential can be
selected in a versus Eoc or versus Eref. This potential is entered in Volts.
Scan Limit 1
The Scan Limit 1 parameter is the first apex potential in a Cyclic voltammetry scan. This potential
can be selected in a versus Eoc or versus Eref. This potential is entered in Volts.
Final E
The Final E parameter is the ending potential of the scan segment. This potential can be selected
in a versus Eoc or versus Eref. This potential is entered in Volts.
Scan Rate
The Scan Rate parameter defines the speed of the potential sweep during data acquisition.
The Scan Rate is entered in units of mV/sec. A practical bound on the Scan Rate is 1000 mV/sec.
Higher Scan Rates may run, but can yield inaccurate data due to the inability of the software to
acquire data points fast enough.
The Scan Rate parameter when combined with the Step Size parameter determines time between
data points and thus the data acquisition rate used in the experiment.
Time (seconds/point) = [ Step Size (mV/point) ] / [ Scan Rate (mV/second) ]
The maximum data acquisition rate is dependent on the speed of the computer, the configuration
of Windows and the other software currently executing. As a guideline, you should avoid sample
times below 100 µs. Note that for scans faster than 1 ms that the acquired data will only be
displayed once the experiment has completed. This reduces the chance that the computer will
limit the acquisition speed.
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Step Size
The Step Size parameter determines the spacing between the data points in mV. A typical Step
Size setting is between 1 and 5 mV.
The Step Size parameter combines with the scan range on any given cycle to determine the
number of data points.
# Points = [ Scan Range (mV) ] / [ Step Size (mV) ]
The total number of data points must be less than 64000 for all cycles.
The Step Size parameter also combines with the Scan Rate parameter to determine the time
interval between the data points.
Cycles
The Cycles parameter controls the number of times the potential scan will be repeated during the
experiment. Conceptually it is the number of times the potential will cycle from the Initial E setting
to Scan Limit 1 to Scan Limit 2 to the Final E setting.
I/E Range Mode
The I/E Range Mode parameter controls the autorange state of the I/E converter. If Auto is
selected, the I/E Range will be able to freely adjust based on measured currents. If Fixed is
selected, the I/E Range will be fixed on a range which is able to measure the current entered in the
Max Current parameter.
For fast experiments, it is recommended that Fixed be used for the I/E Range Mode. This setting
will prevent glitches in the current measurement as the I/E Range resistor is switched.
Max Current
The Max Current parameter controls the current measurement range when the I/E Range Mode is
Fixed. When the I/E Range Mode is Auto, the Max Current parameter specifies the maximum
expected starting current.
You enter a Max Current value that is the largest current that you expect to see during the scans.
From this information the software sets the current range used in the experiment. In order to use
the most sensitive range that will not overload, the software will chose the current range based on
a value that is 89% of the full scale current range. For example, when using a PC4/750, if a Max
Current of 66 mA is input, the current range will be 75 mA. On the other hand, if a Max Current
of 67 mA is entered, the 750 mA current range will be selected.
NOTE: The Max Current parameter is a current not a current density. The electrode area is not
used calculation of the current range to use.
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If your current data looks very choppy and steppy, the problem could be a poorly selected current
range. If you enter a Max Current value of 10 mA and the maximum current in your sweeps is only
100
A,
. Theyou
result
a reisonly us ing 1/100th
significant quantization error. Rerun the test entering a smaller Max Current in Setup.
If your current data shows perfectly flat, horizontal regions, the current has most likely overloaded
the potentiostat's current measurement circuits. Check that the value that you entered for the Max
Current parameter is larger that the actual measured cell current. Try rerunning the test with a
larger value for the Max Current.
IR Comp
The IR Comp parameter specifies the type of IR Compensation to be used during data acquisition.
There are three possible settings for this parameter.
None
No IR Compensation is performed.
PF
Positive Feedback IR Compensation takes a user entered correction value (PF Corr) to correct for
the uncompensated resistance. Because this value is entered at the beginning of the experiment,
and is not measured after every point, this compensation method is suitable for fast experiments.
CI
Current Interrupt IR Compensation will be used. In this technique, a current interrupt measurement
is after each point, and a determination of the uncompensated resistance is made based on the
drop in the voltage measurement. This technique is not suitable for fast experiments, and Positive
Feedback IR Compensation should be used instead.
PF Corr
The Positive Feedback Correction parameter is used during Positive Feedback IR Compensation.
This parameter is where the value for the uncompensated resistance is entered in ohms. This
resistance may already be known, or may need to be measured first. The Ru Estimation
experiment gives a measured value for the uncompensated resistance.
Equilibration Time
The Equilibration Time corresponds to the amount of time the cell spends at the Initial E setting
with the cell turned on. This allows the electrode and the solution time to equilibrate if needed. The
current is not monitored during the equilibration time.
The time is entered in seconds and must be an integer value. If it is not an integer value, it will be
rounded to an integer value when the experiment executes. If you do not wish to have the cell
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equilibrate at the Initial E setting, set the Equilibration Time to zero. The maximum setting for the
Equilibration Time is longer than practically needed (>10^9 seconds).
Initial Delay
The Initial Delay phase of the experiment is the first step to occur in the experimental sequence.
This phase of the experiment is used to stabilize the open circuit voltage of the sample prior to any
applied signal and to measure that open circuit potential.
The Initial Delay is turned On or Off with the Initial Delay parameter check box in the Setup
dialog. The Initial Delay Time parameter is the time that the sample will be held at the open circuit
prior to the scan. The delay may stop prior to the Initial Delay Time if the Stability criterion for Eoc is
met.
The units for Time are seconds. The minimum time is one second. The maximum time is 400,000 s
(more than 4 days). Below 1000 seconds, the time resolution is 1 s. Between 1000 and 10,000 s,
the resolution is 10 s and above 10,000 s it is 100 s.
In many cases, you really do not want to delay for a fixed time. What you really want is to delay
until Eoc stops drifting. The Stability parameter allows you to set a drift rate that you feel represents
a stable Eoc. If the absolute value of the drift rate falls below the Stability parameter, the Initial
Delay phase of the experiment ends immediately, disregarding the programmed Initial Delay
Time. Enter a Stability setting of zero to assure that the delay will last for the full Time.
The units of Stability are mV/s. A typical value is 0.05 mV/s. The upper limit in this parameter is 8
V/s, well above the range of practical stabilities with real cells. The lower limit of the Stability
parameter is set by your patience. A stability of 0.01 mV/s means that a 1 mV drift takes 100s. The
software will always take data long enough to resolve a 1 mV change in the potential at the
requested drift rate.
No open circuit voltage measurement will take place if the initial delay is turned off. In this case,
the open circuit voltage is defaulted to 0.0 volts.
Conditioning
One of the first steps in the experimental sequence is the optional conditioning of the electrode.
Conditioning is used to insure that the electrode has a known surface state at the start of the
electrode. You may condition the electrode to remove an oxide film or to grow one. Conditioning
can be turned On or Off with the Conditioning check box on the Setup menu. Conditioning is done
potentiostatically at the Conditioning E for a known time, Conditioning Time.
E is the potential applied during the conditioning phase of the experimental sequence. The
conditioning potential has an allowed range of ±8 V. E is always specified versus the reference
electrode.
Time is the length of time that the sample is potentiostatically controlled at the Conditioning E.
The units for Time are seconds. The minimum time is one second. The maximum time is 400,000 s
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(more than 4 days). Below 1000 seconds, the time resolution is 1 s. Between 1000 and 10,000 s,
the resolution is 10 s and above 10,000 s it is 100 s.
Advanced Pstat Setup
The Advanced Pstat Setup checkbox, if checked, will bring up the Hardware Settings dialog. This
dialog is used to control specific aspects about your hardware. If you are not an advanced user, or
simply wish to use the default hardware settings as specified in the scripts, just un-check this box.
If however, you wish to specifically set some hardware items, check this box and you will be
presented with further options upon pressing Ok.
The Hardware Settings dialog will look similar to the picture depicted below.
Electrode Setup
The Electrode Setup checkbox, if checked, will bring up the Electrode Setup dialog. This dialog is
used to control specific aspects about your electrode. If you are not an advanced user, or simply
wish to use the default electrode settings, just un-check this box. If however, you need to
specifically set the electrode type or stir/purge conditions, some hardware items, check this box
and you will be presented with further options upon pressing Ok.
The Electrode Setup dialog will look similar to the picture depicted below.
The electrode types are:
None
Solid
DME
SMDE
HMDE
Rotating
No special electrode
Solid Type Electrode
Dropping Mercury Electrode
Static Mercury Drop Electrode
Hanging Mercury Drop Electrode
Rotating Disk Electrode
If you select a Rotating electrode, you will be shown an additional setup dialog shown below.
In this setup dialog you specify the rotation speed of the electrode. This speed is entered in
Revolutions Per Minute (RPM). If you wish to have the rotation stop at the end of the experiment,
select the checkbox to turn off rotation after the experiment.
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Cyclic Voltammetry Experimental Sequence
1. A Runner window is created by the Framework and the "Cyclic Voltammetry.EXP" script is
run in this window.
2. The script creates the Setup dialog box which becomes the active window and accepts
changes in the experimental parameters. This Setup box remembers the experimental
settings from the last time this script was run. To restore the parameters to the values
defined in the script, select the Default button. If the Advanced Pstat Setup is toggled to the
on position a second Setup dialog box contain hardware configuration details will become
the active window allowing the user to modify the hardware configuration used during the
experiment.
3. The script next obtains the use of the potentiostat specified during Setup and opens the
data file using the Output File name. If the potentiostat is in use or the file cannot be
opened, the script returns you to the Setup dialog box.
4. The file header information is written to the data file.
This information is written to the file prior to data acquisition. If the experiment is aborted,
the output file contains only this information.
This header information includes:
a. Tags identifying possible analyses
b. The current time and date
c. A list of the Setup parameters and hardware configuration
5.
If Initial Delay is on, then the cell is turned off and the specimen's Eoc is measured
for the time specified as the Initial Delay time or until the potential stabilizes to a value less
than the stability setting. A plot of potential versus time is always displayed. The last
measured potential is recorded as Eoc. If Initial Delay is off, this step is skipped and Eoc is
assumed to be 0.0 V vs. Ref.
6.
The script conditions the electrode if Conditioning was specified in the Setup.
Conditioning is done by applying a fixed potential for a defined time. A plot of current
versus time is displayed during Conditioning.
7.
Finally an actual scan occurs. The potential of the sample is set to Initial E and is
held at that value for the Equil. Time. The potential is then swept from the Initial E to Scan
Limit 1, then to Scan limit 2 and then to Final E. If Scan limit 2 equals Final E then the scan
stops at that value. Current readings at fixed voltage intervals are taken during the sweep.
The voltage interval between steps is defined as the Resolution in the Setup dialog box.
If the number of Cycles exceeds one, the potential will repeat the sweep for n cycles. Note
that if Initial E does not equal Final E the potential will jump from Final E to Initial E as each
cycle is repeated.
The sweep is actually a staircase ramp. The sample is potentiostatted at the Initial E, a
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delay of one sample period occurs (sample period = 1/Scan Rate * Resolution), and a
reading of the current is taken. The potential is then stepped by a few mV as defined by
Resolution, a delay of one Sample Period occurs, and the next current reading is taken.
Stepping the potential, delaying and acquiring data points continues until the potential
equals the Final E.
If Autoranging was selected in Setup at each point the current range is automatically
switched to the optimal range for the measured cell current. If Positive Feedback IR
Compensation has been selected all data is continuously corrected for IR drop. If Current
Interrupt IR Compensation has been selected, each potential is corrected for the measured
IR drop of the preceding point.
A plot of I vs. E is displayed during the scan.
8.
The data is written to the output file and the script cleans up and halts.
Once the scan is over, the cell is turned off. The acquired data is written to the output file.
The script then waits for you to select Skip. Once you do so, the script closes everything
that's open, including the Runner window.
*
*
Current and Voltage Definitions
A current value of +1.2 mA can mean different things to workers in different areas of
electrochemistry. To an analytical electrochemist it represents 1.2 mA of cathodic current. To a
corrosion scientist it represents 1.2 mA of anodic current. In the PHE200's standard techniques we
follow the analytical convention for current. Positive currents are cathodic, arising from a reduction
at the electrode under test. This convention is the opposite of the current convention used in other
Gamry application software packages such as the DC105, and EIS300.
Potentials can also be a source of confusion. Throughout Gamry's software the equilibrium
potential assumed by the electrode in the absence of electrical connections to the electrode is
called the Open Circuit Potential, Eoc.
In the PHE200, all potentials are specified or reported as the potential of the working electrode with
respect to either the reference electrode or this open circuit potential. The former is always labeled
as "vs Eref" and the later is labeled as "vs Eoc". The equations used to convert from one form of
potential to the other are:
E vs Eoc = ( E vs Eref) - Eoc
E vs Eref = ( E vs Eoc) + Eoc
Regardless off whether potentials are versus Eref or versus Eoc, one sign convention is used. The
more positive a potential, the more anodic it is.
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Introduction
Welcome to the Gamry Instruments, Inc. PHE200 Physical Electrochemistry package. This
package is meant for researchers who are performing studies in the area of electrochemistry.
Included in the package are techniques for performing linear sweep and cyclic voltammetry
experiments, as well as chronopotentiometry, chronoamperometry, chronocoulometry, and
controlled potential coulometry techniques. An experiment which determines uncompensated
resistance (Ru) is also included.
*
References
The following are references are useful for learning more about the techniques that are available in
the PHE200. Certain parts of the text will refer you to specific references.
Cyclic Voltammetry
R. S. Nicholson, Anal. Chem., 37, 1351 (1965).
Electrochemical Methods: Fundamental and Applications, Allen J. Bard and Larry R.
Faulkner, John Wiley & Sons, New York (2000) pp. 226ff. ISBN 0-471-04372-9.
R. S. Nicholson and I Shain, Anal. Chem., 36, 706 (1964), and Anal. Chem., 37, 178 (1965).
Chronocoulometry
Electrochemical Methods: Fundamental and Applications, Allen J. Bard and Larry R. Faulkner,
John Wiley & Sons, New York (2000) pp. 210ff. ISBN 0-471-04372-9.
Chronopotentiometry
Electrochemical Methods: Fundamental and Applications, Allen J. Bard and Larry R.
Faulkner, John Wiley & Sons, New York (2000) pp. 305ff. ISBN 0-471-04372-9.
*
Purpose
Cyclic Voltammetry is used to study the mechanism, kinetics, and thermodynamics of chemical
reactions. Both heterogeneous reactions occurring at the electrode surface, and homogeneous
reactions in solution can be studied.
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In the classical Cyclic Voltammetry triangle waveform, the potential is swept from an Initial E, to
vertex E, and back to Final E, where Final E equals Initial E. An example of this applied waveform
is shown below. Repeating this waveform for N times will perform N cycles of Cyclic Voltammetry.
In the PHE200 we use the more generic double vertex triangular waveform shown below. This
applied waveform allows the user to set a second vertex potential (Scan Limit 2 in the software)
which could be more positive than the initial potential. Setting ScanLimit2 and Final E to equal the
Initial E can perform the classically defined triangle waveform for cyclic voltammetry.
Lets look at a simple example of a Cyclic Voltammetry experiment, using the classically defined
triangular waveform. For the case of a simple one-electron transfer reaction, the resulting current
vs. voltage plot will give the familiar "duck shape" waveform shown below. In these cases, the
reversible potential for the electron transfer can be evaluated from the half-wave potential for the
redox process.
Electron transfer kinetics can also be studied by varying the scan rate of the applied potential and
observing the increase in
Ep (
Nicholson).
An overall review of potential sweep voltammetry
methods is covered in Chapter 6 Bard and Faulkner.
In cases where the chemistry of the system is more complicated, cyclic voltammetry can be used
to determine the mechanisms and kinetics involved. In their work in the 1960's, Nicholson and
Shain published a series of articles that discussed the use of cyclic voltammetry to study chemical
systems which included chemical reactions either proceeding or following the electron transfer
seen in the cyclic voltammogram (Nicholson and Shain). The user is encouraged to review these
works for a better understanding of the versatility of the cyclic voltammetry experiment.
*
Experiment Menu Commands
A typical Experiment pull down menu is shown in the figure below. The items found on this menu
are not the same in all systems. They depend on:
•
•
•
Which Gamry Instruments Windows based software systems you have installed.
The order in which you installed your software systems.
The most recently run experiments.
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•
Custom modifications that have been done to the menu.
All of the commands in the Experiment menu directly or indirectly open a Runner window and
then compile and execute an experimental script in that window.
Typical Experiment Pull Down Menu
The Experiment pull down menu is always divided into 3 sections. You use
The bottom section of the menu to access standard techniques.
The middle section of the menu to rerun a recently used script.
The upper section of the menu to run a Named script.
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Analysis Menu Commands
A typical Analysis pull down menu is shown in in the figure below.
The Analysis Pull Down Menu
The Analysis menu is used to access the Gamry Echem Analyst. The menu is divided into two
sections. The top section allows the user to start the Gamry Echem Analyst. The bottom section
is a list of the most recently used (MRU) data files.
The Start Analysis command is used to start the Gamry Echem Analyst. No data file will be
opened.
Clicking on the MRU list will start the Gamry Echem Analyst and open the data file selected from
the MRU.
*
Gamry Framework: Library Routines
Math functions
Abs() Calculate the absolute value of a number
Exp() Calculate ex of a number
Index() Convert a quantity to an INDEX
LineOpt() Find a line noise rejecting frequency
Log() Calculate natural logarithm of a number
Log10() Calculate base-10 log of a number
Modulus Convert Real, Imaginary to Modulus
Phase Convert Real, Imaginary to Phase
Pow() Calculate number raised to a power
Rand() Generate a random number
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Real() Convert quantity to floating point
Round() Round off a REAL to N decimal places
Sqrt() Calculate square root of a number
Trigonometry Functions
ArcCos() Arc Cosine
ArcSin() Arc Sine
ArcTan() Arc Tangent
Cos() Cosine
Cosh() Hyperbolic Cosine
DtoR() Convert Degrees to Radians
NormalD() Normalize an angle to +/- 360 Degrees
NormalR() Normalize an angle to +/- 2
RtoD() Convert Radians to Degrees
Sin() Sine
Sinh() Hyperbolic Sine
Tan() Tangent
Tanh() Hyperbolic Tangent
Mathematical Classes
class COMPLEX Complex number class
String Functions
Ascii() Convert a character to an Ascii number
Char() Convert an Ascii number to a character
IsAlnum() Check if character is alphanumeric
IsAlpha() Check if character is letter
IsDigit() Check if character is numeric
IsLower() Check if character is lowercase letter
IsPunct() Check if character is punctuation
IsSpace() Check if character is white-space
IsUpper() Check if character is uppercase letter
IsXdigit() Check if character is hexadecimal digit
StrCmp() Compare two strings
StrGet() Get ith character in a string
StrLen() Calculate length of string
StrLwr() Convert string to lowercase
StrSet() Set ith character in a string
StrUpr() Convert string to uppercase
Time functions
DateStamp() Generate a string containing a date
Time() Set a variable to the current time
TimeStamp() Generate a string containing a time
Statistical functions
Mean() Calculate Mean of a data set
StatsOne() Calculate Statistics on one data set
StatsTwo() Calculate Statistics between two data sets
Delay and Program Control functions
Abort() Immediately abort a script
Dawdle() Halt script until SKIP selected
Execute() Launch a child script
ExecWait() Launch a child script and wait for completion
Pause() Pause script for a set number of seconds
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Sleep() Tell a script to sleep until a specified time
Suspend() Allows a user to continue or abort
WinExec() Start up a Windows application
Yield() Let other Windows programs gain control
Output functions
Print() Output to the current output file
Printl() Output to file with automatic newline
Sprint() Output an item to a STRING
Video Display functions
Stdout() Sends output to the STDOUT window
StdoutActivate() Brings STDOUT window to foreground
Error() Report a fatal error and terminate the run
Headline() Update the headline above the data curve
Notify() Update runner window status display
Query() Ask operator a multiple choice question
Setup() Open a dialog box for parameter input
Warning() Warn operator - wait for OK to proceed
Class and Object Manipulation functions
ClassIndex() Numerical sorting of classes
ClassName() Place class name in a STRING
ClassNew() Dynamically create a class
ClassAddISel() Dynamically add an instance variable
ClassAddCSel() Dynamically add a class variable
FindClassByName() Return a class given a STRING
ObjectNew() Dynamically create a new object
Setup Disk File functions
SetupRestore() Recover an old setup on disk
SetupSave() Save a setup on disk
Miscellaneous functions
Callin() Dynamically acquire DLL library function
Config() Read an INI file value
LoadLibrary() Dynamically load a DLL library
SetConfig() Set an INI file value
VectorCount() Determine size of a Vector
VectorNew() Generate a new VECTOR
Parameter Classes having Dialog license suitable for Setup
class CHANNEL Used to setup multiplexed experiments
class DLGSPACE Used to generate white space in Setup()
class IQUANT An integer parameter for general use
class LABEL A short string for general use
class NOTES A multiline string too long to SAVE/RECALL
class ONEPARAM Complex class- 1 TOGGLE, 1 QUANT
class OUTPUT An output file
class POTEN A potential with vs Eref & vs Eoc info.
class QUANT A real parameter for general use
class SELECTOR Radio button selection
class STATIC String used for descriptive purposes
class TOGGLE An on/off parameter for general use
class TWOPARAM Complex class -1 TOGGLE, 2 QUANTS
19
Instrument Driver Classes
class PSTAT A potentiostat
class MUX An ECM8 Multiplexer
Classes having Signal licenses used by curve functions
class ICONST A constant current
class VCONST A constant voltage signal
Curve Classes
class CGEN Generic curve class
class CURVE A generic name for all this group's
classes
class IVT Generic IVT curve used for conditioning
class OCV Curve to measure open circuit potential
Miscellaneous Classes
class DATACOL Hold data extracted from CURVE object
class ARRAY Array class which can hold any object
*
Cyclic Voltammetry Experimental Sequence
1. A Runner window is created by the Framework and the "Cyclic Voltammetry.EXP" script is
run in this window.
2. The script creates the Setup dialog box which becomes the active window and accepts
changes in the experimental parameters. This Setup box remembers the experimental
settings from the last time this script was run. To restore the parameters to the values
defined in the script, select the Default button. If the Advanced Pstat Setup is toggled to the
on position a second Setup dialog box contain hardware configuration details will become
the active window allowing the user to modify the hardware configuration used during the
experiment.
3. The script next obtains the use of the potentiostat specified during Setup and opens the
data file using the Output File name. If the potentiostat is in use or the file cannot be
opened, the script returns you to the Setup dialog box.
4. The file header information is written to the data file.
This information is written to the file prior to data acquisition. If the experiment is aborted,
the output file contains only this information.
This header information includes:
a. Tags identifying possible analyses
b. The current time and date
c. A list of the Setup parameters and hardware configuration
5.
If Initial Delay is on, then the cell is turned off and the specimen's Eoc is measured
for the time specified as the Initial Delay time or until the potential stabilizes to a value less
than the stability setting. A plot of potential versus time is always displayed. The last
measured potential is recorded as Eoc. If Initial Delay is off, this step is skipped and Eoc is
assumed to be 0.0 V vs. Ref.
20
6.
The script conditions the electrode if Conditioning was specified in the Setup.
Conditioning is done by applying a fixed potential for a defined time. A plot of current
versus time is displayed during Conditioning.
7.
Finally an actual scan occurs. The potential of the sample is set to Initial E and is
held at that value for the Equil. Time. The potential is then swept from the Initial E to Scan
Limit 1, then to Scan limit 2 and then to Final E. If Scan limit 2 equals Final E then the scan
stops at that value. Current readings at fixed voltage intervals are taken during the sweep.
The voltage interval between steps is defined as the Resolution in the Setup dialog box.
If the number of Cycles exceeds one, the potential will repeat the sweep for n cycles. Note
that if Initial E does not equal Final E the potential will jump from Final E to Initial E as each
cycle is repeated.
The sweep is actually a staircase ramp. The sample is potentiostatted at the Initial E, a
delay of one sample period occurs (sample period = 1/Scan Rate * Resolution), and a
reading of the current is taken. The potential is then stepped by a few mV as defined by
Resolution, a delay of one Sample Period occurs, and the next current reading is taken.
Stepping the potential, delaying and acquiring data points continues until the potential
equals the Final E.
If Autoranging was selected in Setup at each point the current range is automatically
switched to the optimal range for the measured cell current. If Positive Feedback IR
Compensation has been selected all data is continuously corrected for IR drop. If Current
Interrupt IR Compensation has been selected, each potential is corrected for the measured
IR drop of the preceding point.
A plot of I vs. E is displayed during the scan.
8.
The data is written to the output file and the script cleans up and halts.
Once the scan is over, the cell is turned off. The acquired data is written to the output file.
The script then waits for you to select Skip. Once you do so, the script closes everything
that's open, including the Runner window.
21
*
PHE200 Analysis Overview
The PHE200 uses the Gamry Echem Analyst to display, analyze, and print acquired data files.
The Gamry Echem Analyst also allows the user to use experiment specific analysis commands,
such as normalize current by scan rate in Cyclic Voltammetry experiments, by selecting those
analysis commands from experiment specific pull down menus.
The Gamry Echem Analyst uses Visual Basic for Applications (VBA) as a scripting language to
control the graphing and manipulation of the experimental data. Gamry's standard VBA analysis
scripts are accessible by the user, so that they can be modified to include special analysis routines
that are customized to your laboratories needs. Please refer to the Gamry Echem Analyst section
of Help for information on customizing analysis scripts.
*
Cyclic Voltammetry Analysis Overview
The Cyclic Voltammetry analysis script allows the user to open Cyclic Voltammetry data files by
selecting the data file from the File Open window. The data file will open, and a series of tabbed
pages are created in a new window. These tabs will contain a graph of the data, experimental
setup information, experimental notes, and hardware settings.
The standard Cyclic Voltammetry analysis script shows a plot of current (I) vs. voltage (E) on the
first page. Common analyses performed from this graph include background subtractions, peak
find, and normalize by area or scan rate. Descriptions of the analysis commands in the Cyclic
Voltammetry menu can be found in the Commands section of the PHE200 Data Analysis Help.
*
Cyclic Voltammetry Analysis Pages
22
The standard cyclic voltammetry analysis contains the following pages at startup:
Chart
The Chart page is the heart of the Cyclic Voltammetry analysis. It contains the data plotted in a
Voltage versus Current format.
Experimental Setup
The Experimental Setup page contains a list of the parameters used to acquire the data presented
on the Chart page.
Experimental Notes
The Experimental Notes are the notes entered by the user prior to the data acquisition. The Notes
can be edited in the analysis, and saved with the changes to an analysis data file.
Hardware Settings
The Hardware Settings Page contains all of the potentiostat settings used during the run of the
experiment.
Additional Pages Which May Become Visible
Integration
This page contains a grid of integrated regions which were defined using the Integrate command.
Quick Integration
This page contains a grid of integration values based on different sections in visible traces based
on the most recent use of the Quick Integrate command.
Min/Max
This page contains a grid of minimum and maximum values of each visible trace based on the
most recent use of the Min/Max command.
Peak Locations
This page contains a grid of Peak locations and magnitudes based on the most recent use of the
Peak Find command.
*
Integrate
23
The Integrate command is used to integrate the current to achieve a total charge value. This
command requires that an portion of a specific curve be selected. The command will operate on
only the active trace, and the results will be placed on a new Integrate page, assuming one does
not already exist. In the case where this page already exists, the new information will append the
old information.
1. The steps to use this command are as follows:
1. Make the trace on which to integrate a region the Active Trace. This can be done by
right clicking on the trace and pressing Activate Trace.
2. Select a region on this trace using the Select Portion of Curve tool
. To use this
tool, left click with the mouse close to one bounding point of the region. Next, left
click again near the second bounding point of the region. The region will become
highlighted. If you select an improper range, no region may be displayed. To reselect a range, simply toggle the Select Portion of Curve tool, and try again.
3. Once a range is selected, press the Integrate command on the document menu.
The algorithm will now begin determine the area of the region.
4. A new page will be added to the document, with a grid containing the integration
information. If regions had been integrated previously, the new region information
will simply append the existing information on the Integration page. The region
information will also be displayed in the QuickView window at the bottom of the
graph. You can hide the QuickView window by right clicking on the handle of the
toolbar, and un-checking QuickView.
5. The baseline is either zero, or the Line object which is selected to be the baseline for
the region. The Region Baselines command allows you to specify which line goes
with which region.
6. Additional regions can be found by repeating steps 1 through 3. To clear regions you
can use the Clear Regions command.
Note that the results are presented both on their own page, and in the QuickView area at the
bottom of the chart. You can hide the QuickView window by right clicking on the handle of the
toolbar, and un-checking QuickView.
*
Quick Integrate
The Quick Integrate command is used to integrate the current to achieve a total charge value.
This command requires that an X Region be selected. The command will operate on all visible
traces, and the results will be placed on a new Quick Integrate page, assuming one does not
already exist. In the case where this page already exists, the new information will overwrite the old
information.
24
The steps to use this command are as follows:
1. Select an X region using the Select X Region tool
. To select a region using this tool, left
click in the main chart body at one bounding edge of the region. Holding the mouse button
down, drag the mouse to the second bounding edge, and release the left button. You will
then have a highlighted region on your chart.
2. Once the region is selected, you may select the Quick Integrate command from the
document menu. As soon as you select this command, the computer will begin to calculate
the charge for the different sections of the curve. There can be up to three sections in a
cyclic voltammetry curve. These sections are defined by the vertices of the applied voltage
signal. In the case where you have a Vinit, a VLimit 1, and a VLimit2, you would have a 2
section curve. In the case where you have a Vinit, a VLimit1, a VLimit2, and a Vfinal, you
would have a 3 section curve. The integrated current is listed by section.
Note that the results are presented both on their own page, and in the QuickView area at
the bottom of the chart. You can hide the QuickView window by right clicking on the handle
of the toolbar, and un-checking QuickView.
*
Min/Max
The Min/Max command is used to find the minimum and maximum values of the data plotted on
the Y-Axis. This command requires that an X Region be selected. If you do not select a region,
the search will be performed over the entire range of the data set. The command will operate on
all visible traces, and the results will be placed on a new Min/Max page, assuming one does not
already exist. In the case where this page already exists, the new information will overwrite the old
information.
The steps to use this command are as follows:
1. Select an X region using the Select X Region tool
. To select a region using this tool, left
click in the main chart body at one bounding edge of the region. Holding the mouse button
down, drag the mouse to the second bounding edge, and release the left button. You will
then have a highlighted region on your chart.
2. Once the region is selected, you may select the Min/Max command from the document
menu. As soon as you select this command, the computer will begin to locate the minimum
and maximum values for each visible trace.
Note that the results are presented both on their own page, and in the QuickView area at
25
the bottom of the chart. You can hide the QuickView window by clicking the X on the sidebar of the QuickView.
*
Peak Find
The Peak Find command is used to have the computer search a region of data for a localized peak
or valley. The algorithm looks for a change in the slope of the specified region, which indicates a
peak or a valley. The algorithm then searches in this area for the maximum or minimum value
which it then takes as its peak. This algorithm is written in VBA and may be viewed or modified by
the advanced user as necessary.
The steps to use this command are as follows:
1. Make the trace on which to search for peaks the Active Trace. This can be done by right
clicking on the trace and pressing Activate Trace.
2. Select a region on this trace using the Select Portion of Curve tool
. To use this tool, left
click with the mouse close to one bounding point of the region. Next, left click again near
the second bounding point of the region. The region will become highlighted. If you select
an improper region, no peaks may be found. To re-select a region, simply toggle the Select
Portion of Curve tool, and try again.
3. Once a region is selected, press the Peak Find command on the document menu. The
algorithm will now begin to search for peaks in the selected region. If no peaks are found,
you will see a message box stating no peaks have been found. Try selecting another
region.
4. If peaks are found, a new page will be added to the document, with a grid containing the
peak information. If peaks had been found previously, the peak information will simply
append the existing peak information on the peak page. The peak information will also be
displayed in the QuickView window at the bottom of the graph. You can hide the QuickView
window by clicking the X on the side-bar of the QuickView.
5. You can now display the peak on the chart if you wish by using the Mark Peaks tool
.
Selecting this tool will display the peak location, and the distance from the peak to the
baseline. The baseline is either zero, or the Line object which is selected to be the baseline
for the peak. The Peak Baselines command allows you to specify which line goes with
which peak.
6. Additional peaks can be found by repeating steps 1 through 4. To clear peaks you can use
the Clear Peaks command.
*
*
26
http://www.pineinst.com/echem/files/LMCBP-PROF1.pdf
*
http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l5html/cvr.html
*
http://employees.oneonta.edu/viningwj/
*
http://www-biol.paisley.ac.uk/marco/Enzyme_Electrode/Chapter1/Ferrocene_animated_CV1.htm
Animated Cyclic Voltammetry experiment
This shows the cyclic voltammogram for the reversible oxidation (forward sweep) and reduction
(reverse sweep) for hydroxy-ferrocene. In aqueous buffered electrolyte, hydroxy-ferrocene
undergoes a simple outer sphere one-electron redox process according to the following scheme:
The heterogeneous kinetics of this reaction are rapid so that over a wide range of sweep rates, the
reaction is reversible. The important points to note with the above cyclic voltammogram are as
27
follows;
•
The ferrocene compound is initially present in the reduced state. The voltage scan starts
at a potential negative of the Eo value for this couple and hence, there is no flow of
current.
•
As the voltage approaches the Eo value (320mV) a positive current begins to flow
indicating that the ferrocene molecule is being oxidised. The current continues to rise
(exponentially). This is known as the kinetic region of the voltammogram.
•
As the voltage increases, the rate of reaction also increases until a point is reached when
the process becomes limited by the mass transfer of ferrocene from the bulk to the
electrode surface. The current then begins to fall and a peak is produced. The decay
profile follows a t-1/2 temporal relationship.
•
As the voltage sweep is reversed (when the switching potential is reached), the oxidised
material that is in the vicinity of the electrode is reduced resulting in a reduction peak of
similar magnitude.
•
Because this is a reversible system, the peak separation is 57mV.
*
http://www.cartage.org.lb/en/themes/sciences/Chemistry/Electrochemis/Electrochemical/CyclicVoltammetry
/CyclicVoltammetry.htm
Cyclic Voltammetry
Cyclic voltammetry (CV) is very similar to LSV. In this case the voltage is swept
between two values (see below) at afixed rate, however now when the voltage
reaches V2 the scan is reversed and the voltage is swept back to V1
28
A typical cyclic voltammogram recorded for a reversible single electrode transfer
reaction is shown in below. Again the solution contains only a single electrochemical
reactant
The forward sweep produces an identical repsonse to that seen for the LSV
experiment. When the scan is reversed we simply move back through the
2+
equilibrium positions gradually converting electrolysis product (F back to reactant
3+
(Fe ). The current flow is now from the solution species back to the electrode and
so occurs in the opposite sense to the forward seep but otherwise the behaviour can
be explained in an identical manner. For a reversible electrochemical reaction the
CV recorded has certain well defined characteristics.
I) The voltage separation between the current peaks is
29
II) The positions of peak voltage do not alter as a function of voltage scan rate
III) The ratio of the peak currents is equal to one
IV) The peak currents are proportional to the square root of the scan rate
The influence of the voltage scan rate on the current for a reversible electron
transfer can be seen below
As with LSV the influence of scan rate is explained for a reversible electron transfer
reaction in terms of the diffusion layer thickness.
The CV for cases where the electron transfer is not reversible show considerably
different behaviour from their reversible counterparts. The figure below shows the
voltammogram for a quasi-reversible reaction for different values of the reduction
and oxidation rate constants.
30
The first curve shows the case where both the oxidation and reduction rate
constants are still fast, however, as the rate constants are lowered the curves shift to
more reductive potentials. Again this may be rationalised interms of the equilibrium
at the surface is no longer establishing so rapidly. In these cases the peak
separation is no longer fixed but varies as a function of the scan rate. Similarly the
peak current nolonger varies as a function of the square root of the scan rate.
By analysing the variation of peak position as a function of scan rate it is possible to
gain an estimate for the electron transfer rate constants.
Information provided by: http://www.bath.ac.uk
31
Software Features Available
Physical Electrochemistry
1.
2.
3.
4.
5.
6.
7.
Chronoamperometry
Chronocoulometry
Chronopotentiometry
Controlled Potential Coulometry
Cyclic Voltammetry
GetRu
Linear Sweep Voltammetry
Pulse Voltammetry
1.
2.
3.
4.
5.
6.
7.
8.
9.
A.
B.
C.
D.
Sampled DC Voltammetry
Differential Pulse Voltammetry
Square Wave Voltammetry
Normal Pulse Voltammetry
Reverse Normal Pulse Voltammetry
Sampled DC Stripping Voltammetry
Differential Pulse Stripping Voltammetry
Square Wave Stripping Voltammetry
Normal Pulse Stripping Voltammetry
Reverse Normal Pulse Stripping Voltammetry
Potentiostatic Generic Pulse
Galvanostatic Generic Pulse
GetRu.exp
Electrochemical Impedance
1.
2.
3.
4.
5.
6.
7.
8.
Galvanostatic EIS
Hybrid EIS
Mott Schottky
Multiplexed Potentiostatic EIS
Potentiostatic EIS
Single Frequency EIS
Multiplexed Potentiostatic EIS Repeating
Multiplexed Potentiostatic EIS Repeating Sequential
32