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Performing Cyclic Voltammetry Measurements Using Model 2450-EC Or 2460-EC Electrochemistry Lab System

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Performing Cyclic Voltammetry Measurements Using Model 2450-EC Or 2460-EC Electrochemistry Lab System Performing Cyclic Voltammetry Measurements Using Model 2450-EC or 2460-EC Electrochemistry Lab System Application Note Chemical engineers, chemists, and other scientists use electri- This application note outlines using the 2450-EC or 2460-EC cal measurement techniques to study chemical reactions and Electrochemistry Lab System to perform cyclic voltammetry dynamics. Cyclic voltammetry (CV), a type of potential sweep using a built-in test script and electrochemistry translation method, is the most commonly used measurement technique. cable accessory kit. The electrochemistry translation cable en- CV involves sweeping an electrode potential linearly as a ables the user to easily make connections from the 4-terminal function of time and measuring the resulting current that flows instrument to a 2, 3 or 4-terminal electrochemical cell. These through the circuit, which is typically a 3-electrode electro- systems also include a USB drive that contains six preloaded chemical cell. The resulting I-V data provides important elec- test scripts for performing electrochemical test methods. trochemical properties about the analyte under investigation. The CV test script has adjustable parameter settings and Cyclic voltammetry measurements are typically performed supports real-time plotting of a voltammogram on the Source- using a potentiostat, an often-used electrochemical measure- Meter’s display. The user inputs the test parameters through ment instrument. A Keithley Model 2450-EC or Model 2460-EC a series of pop-up menus that appear on the display. After the Electrochemistry Lab System can be used as an alternative test is executed, the data is stored to a USB drive inserted in for performing cyclic voltammetry and other electrochemistry the instrument’s front panel. Figure 1 shows a voltammogram tests, including use as a general-purpose lab tool for basic plotted on the display of a 2450 instrument using the cyclic sourcing and measurement functions. These instruments are voltammetry test script. programmable and can source and measure both current and voltage. They also allow users to plot the I-V results and save data without an external computer controller. Application Note The following procedure can be used to perform a measurement for each point in Electrochemical Cell the scan, E(i): 1. Select a potential (E) for RE with Working respect to WE. Electrode Reference 2. Adjust the voltage across the en- Electrode tire cell (CE to WE) to get desired E (closed loop control). Counter Electrode 3. Measure i. 4. Select (step) a new E and repeat pro- Figure 1. 2450-EC system plotting a voltammogram. cedure until the scan is finished. The procedure can be a single sweep between two potentials. If this is the case, this is called linear sweep voltammetry. The Basics of Cyclic Voltammetry The procedure may also be configured that when a certain potential is reached, the sweep is inverted. This is called cy- Figure 2 illustrates a typical electrochemical measurement clic voltammetry. This cycle may be repeated multiple times circuit made up of an electrochemical cell, an adjustable volt- during an experiment. age source (VS), an ammeter (AM), and a voltmeter (VM). The three electrodes of the electrochemical cell are the working 5. Plot the results and derive parameters of interest electrode (WE), reference electrode (RE), and the counter from the data. (or auxiliary) electrode (CE). The voltage source (V ) for the S Once the experiment is complete, the measured current is potential scan is applied between the working electrode and plotted as a function of the potential in a graph known as the counter electrode. The potential (E) between the reference a voltammogram. The example voltammogram in Figure 3 electrode and the working electrode is measured with the volt- shows four voltage vertices: E1 (initial potential), E2 (second, meter, and the overall voltage (V ) is adjusted to maintain the s switching potential), E3 (third, switching potential), and E4 (final desired potential at the working electrode with respect to the potential). The voltage peaks in the waveform are the anodic reference electrode. The resulting current (i) flowing to or from (Epa) and the cathodic (Epc) peak potentials. In this example, the working electrode is measured with the ammeter (A ). This M the scan begins at E1 and the potential becomes increasing process is usually repeated for a range of E. more positive causing the anodic current to rise rapidly and Simplified 3-Electrode peak at the anodic peak potential (Epa). At E2, the scan direc- Circuit Electrochemical Cell tion switches to negative for the reverse scan. As the current CE becomes more negative, cathodic current will flow as the elec- trode process is reduced. A cathodic peak potential occurs at RE Adjustable Epc. At the third potential, E3, the direction is reversed again V and the voltage is swept until it reaches E4. From the potential S sweep, important information about the experiment can be WE VM derived and analyzed. 1.E-03 E Epa 8.E-04 6.E-04 Oxidation 4.E-04 E1, E4 E2 ) 2.E-04 A ( A t M n e 0.E+00 i r r u C -2.E-04 E3 -4.E-04 Figure 2. Simplified measurement circuit for performing cyclic voltammetry. -6.E-04 Reduction -8.E-04 Epc -1.E-03 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Potential (V) Figure 3. Example voltammogram generated using Model 2450-EC system. 2 www.keithley.com Performing Cyclic Voltammetry Measurements Using 2450-EC or 2460-EC Electrochemistry Lab System Using the 2450-EC or 2460-EC When the instrument is programmed to source voltage in the remote sense configuration, internal sensing provides Systems to Perform Cyclic Voltammetry a feedback voltage that is measured and compared to the Follow these steps to perform cyclic voltammetry testing using programmed voltage level. If the feedback voltage is less than the 2450-EC or 2460-EC Systems and the CyclicVoltammetry the programmed voltage level, then the voltage source is test script. increased until the feedback voltage equals the programmed voltage level. Remote sensing compensates for the voltage Make Connections from the 4-Terminal drop in the test leads and analyte, ensuring that the pro- SourceMeter to the 3-Electrode Cell grammed voltage level is delivered to the working electrode. To perform cyclic voltammetry the instrument is set up to force Download and Run the Test Script voltage and measure current in a 4-wire (remote sense) con- figuration. The four terminals of the instrument are connected Once the connections from the instrument to the cell are to the 3-electrode electrochemical cell, as shown in Figure 4. made, the test script can be executed. The cyclic voltammetry test script was created using TSP® (Test Script Processing) SourceMeter code. TSP technology can embed complete test programs Force LO Counter inside the instrument. Each instrument has an embedded Electrode test script processor, which allows executing test programs Sense LO 3-Electrode (scripts), without the use of an external computer. Any editor, Electrochemical V Reference such as Notepad or Keithley’s Test Script Builder software, Electrode Cell - S can be used to edit a script. V Working M Electrode The CyclicVoltammetry script has been preloaded into the + instrument. The test script is executed by pressing the Active Script Indicator at the top of the instrument’s home screen, Sense HI then touching the CyclicVoltammetry test script, as shown in Figure 5. A M Force HI i+ Execute the Test Script Figure 4. Connecting an electrochemistry lab system to an electrochemical Once the test script begins to execute, the user must respond cell for cyclic voltammetry. to prompts on the instrument’s touchscreen to define the The Force HI and Sense HI terminals are connected to the parameters of the test. working electrode. At this terminal, the voltage is forced and Acquire the Open Circuit Potential: During execution, the the current is measured from the working electrode to the measured open circuit potential (Eoc) is displayed and the counter electrode. The Sense LO terminal is connected to the user must respond if the value is acceptable. The open circuit reference electrode. The Force LO terminal is connected to potential of the electrochemical cell is measured while the the counter electrode. The instrument measures the voltage instrument is in the voltmeter mode. When the open circuit difference between the working and reference electrodes potential is measured, the current source is set to output 0A. (between the Sense HI and Sense LO terminals) and ensures the voltage is Electrochemical Cell kept constant. Working Electrode Reference Electrode Counter Electrode The CyclicVoltammetry script executes when it is pressed on the touchscreen display Figure 5. Executing the CyclicVoltammetry test script from the home screen of the 2460 instrument. www.tektronix.com 3 Application Note The CyclicVoltammetry script This high impedance voltage measurement is made using the Select the Scan Rate: Next, the scan rate is specified from executes when it is pressed on instrument in a 4-wire configuration as shown inFigure 6. It’s a range of 1.0mV/s to 3500 mV/s. The scan rate defines the the touchscreen display unnecessary to change any test leads manually. rate at which the potential will be linearly swept during the experiment. Model 2450 or 2460 SourceMeter The instrument does not output a truly linear voltage; instead, Force LO CE it outputs very small digitized steps from 0.1mV to 10mV, de- pending upon the scan rate. These voltage step sizes are used Sense LO 3-Electrode during scanning based on the user-set scan rate: RE Electrochemical Cell 0A • 100µV Step Size: 1.0mV/s < scan rate < 35mV/s WE Eoc VM • 1mV Step Size: 35mV/s < scan rate < 350mV/s • 10mV Step Size: 350mV/s < scan rate < 3500mV/s Sense HI Select the Number of Cycles: The number of cycles selected (from 1 to 100) determines how many times each scan will be repeated.
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