A Protocol for Electrochemical Evaluations and State of Charge Diagnostics of a Symmetric Organic Redox Flow Battery

The overall goal of this video protocol is to experimentally demonstrate the electrochemical performance and Fourier transform infrared based state of charge diagnostics of a symmetric, non-aqueous, PTIO based, redox flow battery. So this method can answer key questions in the grid energy storage field, such as how people can make sure the flow battery are running safely and reliably, especially in the long term operations. The main advantage for this technique is that the battery's state of charge can be determined through simple cost-effective Fourier transform infrared spectroscopy that is also integrable for real-time monitoring.

Generally, individuals new to this technique will struggle because in flow batteries the solvent and supporting electrolytes will have very strong interferences during the quantification of the electroactive material. We first had this idea when we discover that PTIO has an absorption peak distinguishable from the solvent and the salt and is among the other oxidation states. All electrochemical tests, including cyclic voltammetry in flow cells, are performed in an argon filled glove box with water and oxygen levels of less than one ppm.

Assemble the cyclic voltammetry setup in a 25 milliliter, three-neck, pear-shaped flask, with a polished glassy carbon working electrode, a graphite felt strip counter electrode, and a silver, silver nitrate 10 millimolar reference electrode. Next, dissolve 52 milligrams of PTIO and 0.87 grams of tetrabutylammonium hexafluorophosphate in 1.1 grams of acetonitrile. Add three milliliters of this solution to the flask to submerge the tips of the three electrodes.

Connect the electrodes to an electrochemical work station. Measure the cyclic voltammetry curves in the voltage range of minus 1.75 to 0.75 volts at a scan rate of 100 millivolts per second. The potential gap between the two redox pairs is the theoretical cell voltage of the PTIO flow battery.

To assemble the flow cell, first cut the graphite felt to an area of one by 10 square centimeters using a razor blade. Similarly, cut a porous separator to an area of three by 12 square centimeters and two gaskets, each with a hole of one by 10 square centimeters to accommodate the graphite felts. Dry the flow battery parts in a vacuum oven at 70 degrees Celsius overnight.

The next day, move the parts into the glove box and allow them to cool down to the environmental temperature. Assemble the flow cell using a torque wrench, preset at 125 inch pounds. Then connect the electrolyte flow tubings to the flow cell.

To perform the flow cell test, dissolve 1.05 grams of PTIO and 3.60 grams TBAPF6 in 3.60 grams of acetonitrile in the glove box. Then add four milliliters of the solution to each glass vial. Flow the electrolytes at 20 milliliters per minute.

Connect the positive and negative current collectors of the flow cell to an electrochemical work station. Measure the electrochemical impedance spectroscopy of the flow cell in the frequency range from 100 kiloHertz to one Hertz at the open circuit potential. The voltage cutoffs of these flow cells are relatively sensitive to the cell by cell variation of impedance and need to be adjusted at the very first cycle of each cell to avoid overcharge.

Connect the positive and negative current collectors of the flow cell to the battery tester. Set up voltage cutoffs of 0.8 and 2.2 volts in a constant current of 20 milliamps per square centimeter in the battery operation software. Repeatedly charge and discharge the PTIO flow cell.

Prepare an acetonitrile solvent sample, a 1.0 molar TBAPF6 electrolyte solution, and a 0.5 molar PTIO, 1.0 molar TBAPF6 electrolyte solution in the glove box. Because the charge of PTIO species are air sensitive, sealable FTIR cells should be used to avoid air contact. An air tight container is recommended when transporting the FTIR cell out of the glove box to the FTIR spectrometer.

Add a small volume of each solution to a sealable FTIR cell with potassium bromide windows and a path length of 0.2 millimeters. Seal the FTIR cell and put the FTIR cell in a storage container and transfer it out of the glove box. Mount the FTIR cell to a spectrometer and proceed to collect the FTIR spectrum.

Then compare the acquired FTIR spectra of acetonitrile, 1.0 molar TBAPF6 and 0.5 molar PTIO, 1.0 molar TBAPF6 electrolyte. Next add 4.0 milliliters of the 0.5 molar PTIO, 1.0 molar TBAPF6 solution to each glass vial. Flow the electrolytes at 20 milliliters per minute.

Fully charge the flow cell until the voltage reaches 2.2 volts. Then stop the charging in the pump and collect the positive and negative electrolytes. Measure the FTIR spectra for both the positive and negative electrolytes as before.

Compare their FTIR spectra with that of the original PTIO solution. Working in a glove box, prepare a series of PTIO solutions in 1.0 molar TBAPF6 in acetonitrile. Measure and compare the FTIR spectra of each of the solutions to obtain the calibration curve.

To measure the state of charge, first assemble another flow cell. Add 11 milliliters of the 0.5 molar PTIO, 1.0 molar TBAPF6 solution to each glass vial. Flow the electrolytes at 20 milliliters per minute.

Charge the flow cell at a constant current of 10 milliamps per square centimeter. At various charge times, stop the cell charge and electrolyte flow and take small aliquots of the electrolytes from anolyte and catholyte side glass vials before resuming the cell. Finally, measure and compare the FTIR spectra of the five sample aliquots.

The electrochemical performance of the PTIO flow cell is shown here. The PTIO cell shows a theoretical cell voltage of 1.73 volts from cyclic voltammetry. Cycling at 20 milliamps per square center for the 0.5 molar cell led to an average coulombic efficiency of 90%a voltaic efficiency of 67%and an energy efficiency of 60%However, the flow cell exhibited a capacity fading.

The feasibility for using FTIR to determine the SOC is validated by the FTIR peaK at 1218 inverse centimeters. Here, the solvents and salt have minimal interference to PTIO. The three oxidation states of PTIO can be distinguished from each other.

The logarithm of the peak intensity exhibits a linear dependence on the PTIO concentration and be used as the calibration curve. The PTIO concentration and state of charge of a PTIO flow cell at several time intervals during charging were determined by FTIR measurements. The accuracy of FTIR based SOC determination is cross-validated by electron spin resonance measurements.

Once mastered, this technique can be done in eight hours if it is performed properly. While attempting this procedure it's important to remember to avoid oxygen or moisture exposure to the samples. Following this procedure, other methods, like measuring FTIR as function of cycle number can be performed in order to answer additional questions, like state of health and material's degradation.

After watching the video, you should have a good understanding of how to evaluate the electrochemical performance of a given redox flow battery chemistry and how to measure the SOC of redox flow batteries that use FTIR sensitive redox materials.