- Building a Potentiostat, CV, EIS Chemistry 243 - Experiment 3 Winter 2019

Reference An excellent introduction/refresher on cyclic can be found in the following ​ ​ article: Elgrishi et. al., “A Practical Beginner’s Guide to ”. Journal of ​ Chemical Education, 2017, DOI: 10.1021/acs.jchemed.7b00361 ​ ​ ​

For a primer on electrochemical impedance spectroscopy, see the website of the Gamry ​ ​ Instrument company, especially this page. For a reading on using equivalent circuit models ​ ​ to fit EIS data, see this pdf. ​ ​

Pre-lab requirements and skills 1) Reading circuit diagrams; construction and use of electronic circuits. 2) General recollection of cyclic voltammetry and electrochemical impedance spectroscopy.

In-lab objectives 1) Learn how to build a three-op-amp potentiostat circuit; 2) Set up a three-electrode cyclic voltammetry measurement; 3) Detect the current from the reduction of the ferricyanide anion. 4) Collect impedance data on the reduction of the ferricyanide anion.

In this lab, you will build a simple three-op-amp potentiostat circuit that will allow you to apply a to an electrochemical cell and detect the current produced as the result of a redox process. You will use a LabView program (already written for you) to apply a series of triangular waveforms to the counter electrode of the cell and to read the current coming from the . You will also perform the same measurement with a commercial potentiostat, and also perform electrochemical impedance spectroscopy measurements with that same potentiostat. From these measurements we will try to deduce various facts about the ferro/ferricyanide redox couple.

Background

Cyclic voltammetry (CV) is a versatile analytical technique for the study of electro-active ​ species. Since it is so versatile, easy to use, and relatively cheap, it finds uses in electrochemistry (obviously), organic chemistry, inorganic chemistry, and biochemistry. CV is often the first experiment performed in the electrochemical study of new compounds. The effectiveness of CV results from its ability to rapidly observe redox behavior over a wide range of applied potentials, and to quickly change those potentials to observe fast redox and chemical reactions. When a potential is applied to a solution of a redox-active compound and a redox reaction happens, the resulting current can be read, amplified, and plotted against the applied potential at that time. A cyclic voltammogram is then a kind of ​ ​ spectrum in which the applied voltage is plotted against the resulting current.

In this experiment you will observe the redox behavior of a commonly-used electro-active species, the ferricyanide anion. The reactions that will occur in the solution as the potential goes from positive to negative and back are:

3- - 4- Fe(CN)6 ​ + e ​ → Fe(CN)6 ​ ​ ​ ​ ​

4- 3- - Fe(CN)6 →​ Fe(CN)6 ​ + e ​ ​ ​ ​ ​ in which the ion is reduced or oxidized during a one-electron process, depending on the 3- 4- potential being applied. A redox couple (in this case, Fe(CN)6 ​ and Fe(CN)6 )​ in which the ​ ​ ​ ​ ​ ​ members are rapidly reduced and oxidized at the working electrode is said to be electrochemically reversible. The formal reduction potential Eo for such a reversible couple ​ ​ ​ is the mean of Epa and Epc (the potentials of the anodic and cathodic peaks, respectively) and ​ ​ ​ ​ the ipa and ipc (the heights of those peaks) should be very close in magnitude. The number ​ ​ ​ ​ of electrons (n) involved in the redox reaction for a reversible couple is related to the difference of the peak potentials by:

Epa - Epc = 59 mV / n ​ ​ ​ ​

Therefore, the cyclic voltammogram can give information about the number of electrons transferred in a redox reaction; or, if that quantity is known (as it is in this case), the voltammogram can tell you whether the reaction is reversible or irreversible.

The reversibility of a reaction can also be investigated by determining the dependence of the peak height (the amount of current produced) on the scan rate. The peak current in reversible systems for the forward scan is given by the Randles-Sevcik equation:

8 3/2 1/2 1/2 ipc = 2.69x10 ​ n ​ AD ​ v ​ C ​ ​ ​ ​ ​ ​

2 where, ipc = peak current in Amps; n = # electrons involved; A = electrode area in m ​ ; D = ​ ​ 2 ​ diffusion coefficient in m /s;​ C = concentration in mol/L; and v = scan rate in V/s. As this ​ equation suggests, a plot of peak current vs. the square root of the scan rate should give a straight line if the reaction is reversible.

Electrochemical impedance spectroscopy (EIS) is another kind of electrochemical ​ measurement that can be performed with a potentiostat, but the way in which the measurement is taken and the information that you can get from it will be different. In EIS, the potentiostat is used to apply a series of low-voltage AC frequencies to the sample, and the impedance of the chemical system (its resistance to current flow) is measured Figure 1 shows the relationship between the input current and the system response.

Figure 1. An applied AC voltage is sent into the chemical system by the potentiostat, and the system’s response is shifted with respect to both phase and amplitude. Source: Gamry Instruments, Inc.

To make an EIS measurement, the AC signal is applied at a constant voltage (potentiostatic EIS) or current (galvanostatic EIS) and the response is measured. Then, the impedance is calculated at that frequency according to Zω = Eω/Iω, where Eω is the frequency-dependent ​ ​ ​ ​ ​ ​ ​ ​ potential and Iω is the frequency-dependent current. Next, the frequency is changed and a ​ ​ new impedance is calculated from the resulting response.

The resulting data can be plotted as either a Bode plot (a plot of log f vs. log modZ, the ​ ​ modulus of the impedance in the complex plane), or as a Nyquist plot (in which the real and imaginary parts of Z are plotted against one another). In either case, the next step in an EIS analysis is to model your results as an equivalent electrical circuit made up of passive components (resistors, capacitors, etc). Don’t worry, the software can do this for you.

The kinds of circuit elements used to reproduce the data on your Bode/Nyquist plot is dictated by the amplitude decrease and phase shift observed. An amplitude decrease without a phase shift is modeled by just a resistor; amplitude decrease with phase shifts may represent a capacitor, a series of resistors and capacitors, or some other more complicated equivalent circuit.

The circuit elements in the equivalent circuit also have physical meanings, as shown in Figure 2 below. At the electrode’s surface, the formation of a double layer (a polarized solvent layer) may act like a capacitor by preventing charge from passing between the liquid electrolyte and the surface of the electrode. This process of electron transfer also has a characteristic resistance, the charge transfer resistance. These two elements are the parallel capacitor and resistor in the circuit diagram in Figure 2. The resistor in series with those elements represents the electrolyte resistance, which depends on the geometry of the electrochemical cell and the supporting electrolyte used in the solution.

Figure 2. A redox reaction at an electrode surface modeled as a basic Randles cell (thanks, Gamry Instrument Co!).

Another process that is not shown in Figure 2, but can be seen on a Nyquist plot, is diffusion (modeled as a Warburg impedance). On the Nyquist plot, the Warburg impedance shows up as a 45° line; on a Bode plot, it has a phase shift of 45°. A circuit in which a Warburg impedance term is added to the Randles cell models a system in which polarization at the electrode’s surface is due to a combination of kinetic and diffusion processes.

Part 1: Construction and Use of the Potentiostat Circuit

The circuit diagram for the potentiostat is given below in Figure 3. You should notice the familiar technique being used to convert the current from the working electrode (WE): the transimpedance amplifier, or current-to-voltage amplifier, that we used with the photodiode last week. Also notice the buffer amplifier that keeps the reference electrode (RE) from drawing current. The other op amp “drives” the counter electrode (CE) by applying a voltage from the NI USB-6002 data acquisition device. The feedback capacitor on this op amp helps prevent unstable operation.

To output the waveform to the circuit and read the voltage from the current-to-voltage op amp attached to the working electrode, you will use a National Instruments USB-6002 analog-to-digital converter device. The analog output channel ‘ao0’ will be used to output the triangle wave voltage relative to ground. The analog input channel ‘ai0’ will be used to read this voltage, so that you can see what is actually being applied to the circuit. The analog input channel ‘ai3’ will be used to read the voltage from the current-to-voltage op amp. Take a look at the various wires connected to the input and output terminals of the USB-6002 and make sure they make sense to you.

Figure 3. The three-op-amp potentiostat setup that you’ll build on the solderless breadboard. Power supplies for the op amps should be 20V (to give ±10V when the voltage divider is used).

The LabView program “Cyclic Voltammetry USB 6002.vi” will control the potentiostat for this experiment. The program will tell the analog output channel to send a triangle waveform, with an amplitude that you specify, to the counter electrode. This voltage will result in a current between the counter and working electrodes, and that current will be amplified by the current-to-voltage amplifier and detected by the program. It will plot the applied potential on the x-axis and detected current (calculated from the voltage coming out of the op amp and the feedback resistor) on the y-axis to create the cyclic voltammogram.

Cyclic Voltammetry Measurement

A solution of 5 mM K₃[Fe(CN)₆] (potassium ferricyanide) in 0.5 M KCl (as a background ​ ​ electrolyte) has been prepared for you. Fill a 100 mL beaker up about a third of the way with the solution and arrange your three electrodes (platinum working and counter electrodes, Ag/AgCl reference electrode) so that they are not touching each other.

Open the LabView program mentioned above and run it using the default settings (scanning from 0 to -600 mV and back, scan rate of 300 mV/s). You should see a peak in the positive direction from the reduction of ferricyanide ion. You may see a downward peak as the scan sweeps back the other way, but if it’s there it will be very small. If you see neither peak, you can try changing the endpoints of the scan, or the scan rate. You can do multiple scans, but you should gently stir the solution before each one to restore equilibrium conditions.

Change the second switching potential so that it’s something greater than zero, say 100 mV. The small, sloping current you see is called capacitive current, and it’s due to the ​ ​ background electrolyte. Current due to oxidation or reduction of an analyte, by contrast, is called Faradaic current. ​ ​

Part 2: Cyclic Voltammetry

Your electrochemical cell for the voltammetric analysis will be a 100 mL beaker that you fill about halfway with the solution being investigated. Three electrodes are involved in this measurement: the working electrode (a platinum disc electrode or just a piece of platinum wire), the counter electrode (a platinum wire), and the reference electrode (an Ag/AgCl electrode). The platinum electrodes both need to be free of impurities, so they should be cleaned before each solution is analyzed by rubbing them gently with a Kimwipe dampened with DI water (platinum disks should be cleaned with a polishing cloth). The electrodes need to be arranged so that they are not touching each other or, ideally, the sides or bottom of the beaker. It’s very important that the alligator clips holding the wires not touch the solution, or the clips themselves will oxidize and you’ll have to replace the solution.

The cable from the potentiostat terminates in a variety of connectors. Hook up the following:

Table 1. Potentiostat Cable Connections Color Type Name Connection

Blue Alligator Clip Working Sense Working electrode

Green Alligator Clip Working electrode Working electrode

White Pin jack Reference Reference electrode

Red Alligator Clip Counter electrode Counter electrode

Orange Alligator Clip Counter sense Counter electrode

The two black connectors should be connected together but should not touch any other electrode. If your voltammograms look strange, you can try disconnecting the orange alligator clip and leaving it unconnected, not touching any other electrodes. If they still look strange, you may have to rely on software averaging to clean them up.

Add the solution to the beaker and add a small stir bar, making sure that the stir bar will not hit any of the electrodes when it spins. Oxygen is terrible for electrochemical measurements, so you will need to degas your sample by bubbling argon or nitrogen (whatever is available) through the sample for at least 5 minutes. When finished, arrange the gas line so it hangs just above the surface of the solution to “blanket” the solution with inert gas and keep out any trespassing oxygen molecules.

While deoxygenation is proceeding, the scan parameters can be set. In CV, a potentiostat is ​ ​ used to apply the and read the resulting currents. Our potentiostat is a card inserted into a relatively old computer, so turn on that computer (if it isn’t already on) and open the potentiostat control program “Gamry Instruments Framework”. You should see a green circle and the word “Potentiostat”; if you don’t we’re in trouble. Next click on Experiment → Named Script and find “Cyclic Voltammetry.exp”. This is a script file that tells the potentiostat how to perform this kind of measurement. When you open that script, a window will pop up that allows you to set your scan parameters. The initial scan settings for this analysis are shown below in Figure 4.

Figure 4. Scan settings for cyclic voltammetry.

Collect a voltammogram with these settings, and also with higher and lower scan rates. If the signal gets noisy at the high scan rates, you can do some software averaging when you do the data analysis. Between each reading, you can “regenerate” the initial conditions at the disc electrode’s surface by stirring the solution for about 10 seconds. Repeat this process for the two other K3Fe(CN)6 solutions if you have time, remembering to ​ ​ ​ ​ de-oxygenate the new solutions when they are poured into the cell. Also try the measurement with 1 M KCl solution containing no ferricyanide anion, to see if any current is able to flow in the absence of redox reactions.

After each scan, you can see your data by pressing F2 (at the end of the scan), then at the top of the screen clicking Analysis → (your filename). This will bring up your voltammogram, which you can get information about by clicking the “Cyclic Voltammetry” menu option at the top of the new window. The most useful option here will be “Peak Find”. To use this, first click on the “Select Portion of the Curve using the Mouse” button (which looks like a little computer mouse) and click on either side of the peak you want information about. Then go to the menu and click “Peak Find”. A table will pop up with the height of the peak (in units of current) and the position of the peak (in units of voltage, relative to the reference electrode). Once you have each peak “found”, you can also use the option “Delta Ep” in the Cyclic Voltammetry menu to find the voltage difference between the peaks (useful when solving for n, the number of electrons transferred).

Record the heights (in units of current) of the cathodic and anodic peaks and the potentials at which those peaks occur for each of the voltammograms. In each case, plot each of the peak heights against the square root of the scan rate. Also calculate the number of electrons involved in the reaction based on the potentials and the equation given in the first section of this lab manual.

Part 3: Electrochemical Impedance Spectroscopy

Introduction to the system: the simple Randles Cell

Since chemical systems are messy, it’s good to get an idea of what an ideal set of data would look like for the Randles cell mentioned above. The EIS Dummy Cell that comes with the potentiostat can be used for this purpose. Hook up the wire leads of the potentiostat according to the instructions on the dummy cell, then run a Potentiostatic EIS measurement (Experiment → Potentiostatic EIS) in Gamry Framework using the settings shown below in Figure 5.

Figure 5. Potentiostatic EIS settings for the Randles dummy cell. Press OK to run the scan. When it finishes, you can analyze your sample in the same way you did for CV. In this case, though, your data will look very different. In the Data Analysis window, you can choose to view your data as a Bode plot (x vs. x) or as a Nyquist plot (x vs. x). Both of these can tell us different things about the behavior of our system.

We will next model the data with an equivalent circuit model. In the EChem Analyst window, click on the Impedance menu, then click “Fit a Model (Simplex Method)”. Open the file “randles” and click “Auto-Fit”. This will model an ideal Randles Cell fit onto both your Bode and Nyquist plots. Check to see if the values of C and R given match what’s written on the dummy cell.

EIS of an actual chemical system

For this part of the lab we will use the same three electrodes as in Part 2, and a solution 3- 4- with equal concentrations of Fe[CN]6 ​ and Fe[CN]6 ​ (the solution has been made for you; ​ ​ ​ ​ the concentration of each ion is 10 mM and the supporting electrolyte is 1 M KCl). On the Gamry Framework panel, open the Experiment called “Potentiostatic EIS”. Set the scan parameters as shown in Figure 6, then press OK to run the scan. When it finishes, you can analyze your sample in the same way you did for the dummy cell previously.

We will next model the data with an equivalent circuit model. In the EChem Analyst window, click on the Impedance menu, then click “Model Editor”. Open the file “randles”; this will give you the Randles Cell model you used previously, which is commonly used to model reactions at an electrode surface (see Figure 2). Modify this circuit so that it includes a Warburg impedance, which allows the model to account for a combination of kinetic and charge transfer control of the reaction at the electrode’s surface (see Figure 7). Fit your data with both models and evaluate whether the reaction at the electrode’s surface is kinetically controlled or not.

Figure 6. Potentiostatic Electrochemical Impedance Spectroscopy settings.

Figure 7. The Randles cell equivalent circuit with the additional Warburg impedance element inserted.