Electrochemical Impedance Spectroscopy as a Tool for Electrochemical Rate Constant Estimation

This technique is useful for characterizing the kinetics of oxidation and reduction of organic compounds, and predicting their behavior as an active compound of light-emitting diodes, solar cells, or batteries. The main advantage of impedance spectroscopy is that it allows separation and individual analysis of different polar processes, according to their A/C responses. To begin, dissolve 0.4 millimoles of tetrabutylammonium tetrafluoroborate and four times 10 to the negative three millimoles of the organic compound of interest in four milliliters of dichloromethane.

Pipette two milliliters of this working solution into a three milliliter electrochemical cell, such as a glass V vial and close the cell with a gasket cap. Store the remaining working solution for later measurements. Next, mount a polishing cloth on an immobile support and moisten the cloth with several drops of 0.05 micrometer alumina slurry.

Polish a one millimeter diameter platinum disk working electrode for 30 seconds using moderate pressure. Afterwards, rinse the polished working electrode with DCM three times to remove residual alumina particles. Then insert the polished electrode into the electrochemical vial through the gasket cap.

Next obtain a platinum wire counterelectrode, and ignite a butane torch. Anneal the electrode by carefully holding it in the flame, just until it starts reddening. Anneal a silver wire reference electrode in the same way, and allow both electrodes to cool.

Then mount the wire electrodes in the electrochemical cell through the gasket cap, being careful to keep the electrodes from touching each other. Connect the three electrodes to the potentiostat. Equip the electrochemical cell with an argon gas line and bubble argon through the working solution for 20 minutes.

Close the flow of argon before beginning the measurements. To begin the initial characterization, open the cyclic voltammetry program in the potentiostat software. Set the initial potential to zero volts, the minimum potential to negative two volts, the maximal scanning potential to two volts, and the scanning rate to 100 millivolts per second.

Acquire the voltammogram of the working solution. Note the potential values at the maxima of the anodic and the cathodic peaks. Calculate the average of the peak potentials of the anodic and cathodic peaks to estimate the redox potential.

Next use a spatula to add about 10 milligrams of ferrocene to the working solution in the electrochemical cell. Bubble argon through the solution for five minutes to ensure complete dissolution of the ferrocene. Then in the cyclic voltammetry program, change the minimal and maximal scanning potentials to negative one volts and one volts respectively.

Acquire another voltammogram which will show a small reversible ferrocene trace. Average the anodic and cathodic peak potentials of the ferrocene to estimate its reversible oxidation potential in the working solution. Then determine the redox potential of the organic compound with respect to ferrocene.

Lastly to clean the electrochemical cell, fill it with DCM and empty it five times. Following characterization by cyclic voltammetry, place another two milliliters of the working solution in a clean three milliliter electrochemical cell. Clean the electrodes as previously described, insert them into the cell, and reconnect them to the potentiostat.

Deaerate the working solution by bubbling argon through it for 20 minutes. Then, open the staircase EIS program in the potentiostat software. Set a potential range of 0.1 volts on either side of the redox potential of the compound of interest, for a total range of 0.2 volts.

Set the potential increment to 0.01 volts, the frequency range as 10 kilohertz to 100 hertz, the number of frequencies in the logarithmic scale to 20, the wait time to five seconds, the A/C voltage amplitude to 10 millivolts, and the measures per frequency to two. Run the experiment and wait for the set of spectra to be collected. Once the experiment has finished, open the EIS spectrum analyzer program.

The demonstrated program is universal for impedance spectrum analysis. However it's not necessary to use this exact set-up, as numerous other software options can be used. Import an automatically registered spectrum generated by the EIS experiment.

Then construct a simple equivalent electrical circuit for the spectrum. Set the initial upper and lower limits to one times 10 to the negative seven and one times 10 to the negative eight for the capacitor, 2000 and 100 for resistor one, 1000 and 100 for resistor two. Then, fit the model.

Repeat the fitting until the calculated values stop changing. If the R-squared parametric and amplitude values exceed one times 10 to the negative two, test another EEC. For more complex EECs, set the initial upper and lower limits for the Warburg element to 50, 000 and 10, 000 respectively.

If any parameter has error values exceeding 100%after fitting, remove that parameter and try another EEC. Once the spectrum has been fitted to an appropriate EEC, record the charge transfer resistance and the potential at which the spectrum was registered. Repeat this process for all registered spectra.

Cyclic voltammetry of this organic compound revealed a reversible oxidation process at 0.7 volts versus ferrocene. Impedance spectra of the redox processes on the electrode's surface were subsequently registered and analyzed. The impedance spectra were fitted with various equivalent electrical circuits to identify the best analog for the electrochemical process.

The charge transfer resistance represented here as R2 was extracted from each fitted spectrum. The inverse charge transfer resistance values were plotted with respect to electrode potential versus ferrocene, along with the theoretical dependence of the inverse charge transfer resistance on electrode potential. The standard electrochemical rate constant was then estimated by varying the equilibrium potential and the rate constant until a reasonable fit for the experimental data was achieved.

The demonstrated technique can be used jointly with other methods of investigating an electric organic compound when its redux properties are crucial. Following this procedure, other spectrochemical methods like ESR, UV-Vis-NIR can be performed to answer additional questions about changes in molecule structure caused by electrochemical processes. While attempting this procedure, remember to account for other processes that occur in real systems that may complicate the obtained results.

In case of irreversible reactions like polymerization, this technique cannot be expected to give reasonable results. After its development, this technique paved the way for researchers in the field of charge transfer kinetics in organic electronics to better predict the redux performance of molecules and materials. Once mastered, this technique can be done in two hours if it's performed properly.