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

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Electrochemistry - Building a Potentiostat, CV, EIS Chemistry 243 - Experiment 3 Winter 2019 Electrochemistry - Building a Potentiostat, CV, EIS Chemistry 243 - Experiment 3 Winter 2019 Reference An excellent introduction/refresher on cyclic voltammetry can be found in the following ​ ​ article: Elgrishi et. al., “A Practical Beginner’s Guide to Cyclic Voltammetry”. 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 voltage 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 working electrode. 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.
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