Name: A Protocol for Electrochemical Evaluations and State of Charge Diagnostics of a Symmetric Organic Redox Flow Battery (Video) | JoVE Journal
Uploaded: 2017-02-13T12:30:00.000+00:00
Duration: 9 min 49 s
Description: Learn electrochemical evaluation and state of charge diagnostics for symmetric organic redox flow batteries using FTIR spectroscopy. This cost-effective method enables real-time monitoring for safe, reliable grid energy storage.

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5 minutes

This work was financially supported by Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The authors also acknowledge Journal of Materials Chemistry A (a Royal Society of Chemistry journal) for originally publishing this research (<a href="http://pubs.rsc.org/en/content/articlehtml/2016/ta/c6ta01177b">http://pubs.rsc.org/en/content/articlehtml/2016/ta/c6ta01177b</a>). PNNL is a multi-program national laboratory operated by Battelle for DOE under Contract DE-AC05-76RL01830.

Note: All the solution preparations, cyclic voltammetry (CV) tests, and flow cell assembly and tests were carried out in an argon-filled glove box with water and O2 levels less than 1 ppm.
1. Electrochemical Evaluations of PTIO Flow Cells
<ol>
	<li>CV Test
	<ol>
		<li>Polish a glassy carbon electrode with 0.05 &#181;m gamma alumina powder, flush it with deionized water, put it in under vacuum at room temperature for overnight, and transfer it into a glove box.</li>
		<li>Dissolve silver nitrate (8.5 mg) with MeCN (5 mL) in the glove box, i.e., 10 mM AgNO3. Add the solution into the glass tube of a silver/silver nitrate reference electrode.</li>
		<li>Assemble the glassy carbon working electrode, a graphite felt strip counter electrode, and the silver/silver nitrate reference electrode on a 25 mL three-neck pear-shaped flask.</li>
		<li>Dissolve PTIO (52 mg) and tetrabutylammonium hexafluorophosphate (TBAPF6, 0.87 g) in MeCN (1.10 g), i.e., 0.1 M PTIO/1.0 M TBAPF6. Add the solution to the flask to submerge the tips of the three electrodes.</li>
		<li>Connect the electrodes to an electrochemical workstation. Measure the CV curves within the voltage range of -1.75-0.75 V at a scan rate of 100 mV/s. Determine the theoretical cell voltage of the PTIO flow battery by the potential gap between the two redox pairs. 
		NOTE: It should be noted that the Ag/Ag+ reference electrode configuration is a pseudo-reference electrode by its nature. As a result, redox peaks might shift during long-term CV measurements. Nevertheless, such shift normally has a negligent influence on the voltage gap between redox couples, and would not affect the cell voltage value.</li>
	</ol>
	</li>
	<li>Flow Cell Assembly
	<ol>
		<li>Cut the graphite felts to an area of 1 x 10 cm2 using a razor blade. Similarly, cut a porous separator to an area of 3 x 12 cm2.</li>
		<li>Dry the flow battery parts (cell compartments, tubing, 5 mL glass vials, graphite felts, and a porous separator) in a vacuum oven at 70 &#176;C for overnight, move them into the glove box, and cool down to the environmental temperature.</li>
		<li>Assemble the flow cell parts in the order of an end plate, a copper plate current collector, a half cell, a graphite felt, a gasket, a porous separator, a graphite felt, a half cell, a copper plate current collector, and an end plate. Fasten the assembly with eight threaded bolts against the two end plates using a torque wrench pre-set at 125 inch pounds. Connect the electrolyte flow tubings to the flow cell. The cell assembly is shown in Figure 1.</li>
	</ol>
	</li>
	<li>Demonstration of Symmetric Electrochemistry
	<ol>
		<li>Assemble the flow cell according to Section 1.2. Dissolve PTIO (10 mg) and TBAPF6 (3.3 g) with MeCN (4.4 g) in the glove box, i.e., 5.0 mM PTIO/1.0 M TBAPF6. Add 4 mL of the solution to each of the two glass vials. Pump the electrolytes to flow using a peristaltic pump at a flow rate of 20 mL/min.</li>
		<li>Connect the positive and negative current collectors of the flow cell to the battery tester. Charge the flow cell at a constant current density of 5 mA/cm2 until the voltage reached 1.9 V. Stop the charging. Pump out the electrolytes into the glass vials.</li>
		<li>Mix a 1 mL positive electrolyte with a 1 mL negative electrolyte in a separate vial. Now there are four electrolytes: the original, the positive, the negative, and the mixed.</li>
		<li>Measure electron spin resonance (ESR) spectrum of the above four electrolytes.25
		<ol>
			<li>With tube sealant, seal a small amount (~10 &#181;L) of the positive and negative in a PTFE tubing (1/16&#34; OD and 1/32&#34; ID) at both ends, and then seal it in a quartz ESR tube (4 mm diameter).</li>
			<li>Mount the ESR tubing to an ESR spectrometer fitted with an SHQE resonator with microwave frequency ~9.85 GHz (X band).</li>
			<li>Collect the ESR spectrum for the four electrolytes in Section 1.3.3.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Flow Cell Test
	<ol>
		<li>Assemble a flow cell following Section 1.2.</li>
		<li>Dissolve PTIO (1.05 g) and TBAPF6 (3.50 g) with MeCN (3.60 g) in the glove box, i.e., 0.5 M PTIO/1.0 M TBAPF6. Add 4 mL of the solution to each glass vial. Flow the electrolytes at 20 mL/min.</li>
		<li>Connect the positive and negative current collectors of the flow cell to an electrochemical workstation. Measure the impedance of the flow cell in the frequency range from 100 kHz to 1 Hz at the open circuit potential. Calculate the area-specific resistivity (ASR) by multiplying the ohmic resistance (high-frequency impedance) by the active area of the flow cell.</li>
		<li>Connect the positive and negative current collectors of the flow cell to the battery tester. Set up the voltage cutoffs of 0.8 and 2.2 V and the constant current of 20 mA cm-2 in the battery operation software. Repeatedly charge/discharge the PTIO flow cell.</li>
	</ol>
	</li>
</ol>
2. FTIR-based SOC Determination
<ol>
	<li>FTIR Feasibility Validation
	<ol>
		<li>Prepare the following three electrolyte solutions in the glove box: (a) MeCN (0.50 g); (b) TBAPF6 (0.23 g) with MeCN (0.30 g), i.e., 1.0 M TBAPF6; (c) PTIO (75 mg) and TBAPF6 (0.25 g) with MeCN (0.26 g), i.e., 0.5 M PTIO/1.0 M TBAPF6.</li>
		<li>Measure FTIR for the three electrolyte solutions.
		<ol>
			<li>Add a small volume (~0.05 mL) of each solution to a sealable FTIR cell with KBr windows and a path length of 0.2 mm. Seal the FTIR cell.</li>
			<li>Put the FTIR cell into a storage container and transfer it out of the glove box.</li>
			<li>Quickly mount the FTIR cell to a spectrometer and collect the FTIR spectrum.</li>
		</ol>
		</li>
		<li>Assemble a flow cell following Section 1.2.</li>
		<li>Dissolve PTIO (1.05 g) and TBAPF6 (3.50 g) with MeCN (3.60 g) in the glove box, i.e., 0.5 M PTIO/1.0 M TBAPF6. Add 4 mL of the solution to each glass vial. Flow the electrolytes at 20 mL/min.</li>
		<li>Fully charge the flow cell until the voltage reaches 2.2 V. Stop the charging and the pump.</li>
		<li>Measure the FTIR spectra for both the positive and negative electrolytes, respectively, following the procedure in Section 2.1.2.</li>
		<li>Prepare a series of PTIO solutions (0.05-0.5 M) in 1.0 M TBAPF6 in MeCN in the glove box with the compositions in Table 1.</li>
		<li>Measure the FTIR spectrum for each of the solutions in Section 2.1.6, following the procedure in Section 2.1.2.</li>
	</ol>
	</li>
	<li>FTIR Measurement of SOC
	<ol>
		<li>Assemble a flow cell following Section 1.2.</li>
		<li>Dissolve PTIO (2.9 g) and TBAPF6 (9.6 g) with MeCN (9.8 g) in the glove box, i.e., 0.5 M PTIO/1.0 M TBAPF6. Add 11 mL of the solution to each of the two glass vials. Flow the electrolytes at 20 mL/min.</li>
		<li>Charge the flow cell at a constant current of 10 mA/cm2 at a flow rate of 20 mL/min.</li>
		<li>At the charge time of 0, 18, 36, 54, and 72 min, stop the cell charge and electrolyte flow, take small aliquots (0.2 mL) of the electrolytes from anolyte and catholyte side glass vials, and then resume the cell.</li>
		<li>Measure the FTIR spectrum for the above five sample aliquots, following the procedure in Section 2.1.2.</li>
		<li>Measure the ESR spectrum for the above five sample aliquots, following the procedure in Section 1.3.4.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

As we demonstrated before,25 FTIR is capable of non-invasively detecting the SOC of the PTIO flow battery. As a diagnostic tool, FTIR is particularly advantageous because of its easy accessibility, fast response, low cost, small space requirement, facility for online incorporation, no detector saturation, and the ability to correlate structural information to investigate molecular evolutions during flow battery operation. Figure 3e illustrates a proposed flow battery device integr...

Redox flow batteries store energy in liquid electrolytes that are contained in external reservoirs and are pumped to internal electrodes to complete electrochemical reactions. The stored energy and power can thus be decoupled leading to excellent design flexibility, scalability, and modularity. These advantages make flow batteries well-suited for stationary energy storage applications for integrating clean yet intermittent renewable energies, increasing grid asset utilization and efficiency, and improving energy resiliency and security.1,2,3 Traditional aqueous flow batteries suffer from limited energy density, mostly due to the narrow voltage window to avoid water electrolysis.4,5,6,7,8 In contrast, non-aqueous electrolytes based flow batteries are being widely pursued because of the potential for achieving high cell voltage and high energy density.9,10 In these efforts, a variety of flow battery chemistries have been investigated, including metal-coordination complexes,11,12 all-organic,13,14 redox active polymers,15 and lithium hybrid flow systems.16,17,18,19
However, the potential of non-aqueous flow batteries has yet to be fully demonstrated due to the major technical bottleneck of limited demonstration under flow battery-relevant conditions. This bottleneck is closely associated with a number of performance-limiting factors. First, the small solubility of most electroactive materials leads to low energy density delivery by non-aqueous flow cells. Second, the rate capability of non-aqueous flow batteries is largely limited by the high electrolyte viscosity and resistivity at relevant redox concentrations. The third factor is the lack of high-performance membranes. Nafion and ceramic membranes show low ionic conductivity with non-aqueous electrolytes. Porous separators have demonstrated decent flow cell performance, but suffer considerable self-discharge because of relatively large pore size.14,20 Typically, mixed-reactant electrolytes containing both anolyte and catholyte redox materials (1:1 ratio) are used to reduce redox materials crossover, which however sacrifices the effective redox concentrations, typically by half.14,21 Overcoming the aforementioned bottleneck requires improvements in materials discovery, battery chemistry design, and flow cell architecture to achieve battery-relevant cycling.
Battery status monitoring is essentially important for reliable operations. Off-normal conditions including overcharge, gas evolution, and material degradation can cause damages to battery performance and even battery failure. Especially for large-scale flow batteries involving large amounts of battery materials, these factors can cause serious safety issues and investment loss. State of charge (SOC) describing the depth of charge or discharge of flow batteries is one of the most important battery status parameters. Timely SOC monitoring can detect potential risks before they reach threatening levels. However, this area seems to be under-addressed so far, especially in non-aqueous flow batteries. Spectrophotoscopic methods such as ultraviolet-visible (UV-vis) spectroscopy and electrolyte conductivity measurements have been evaluated in aqueous flow battery for SOC determination.22,23,24
We have recently introduced a novel symmetric non-aqueous flow battery design based on a new ambipolar redox material, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO).25 This flow battery holds the promise to address the aforementioned challenges of non-aqueous flow batteries. First, PTIO has a high solubility (2.6 M) in the battery solvent of acetonitrile (MeCN) that is promising to enable a high energy density. Second, PTIO exhibits two reversible redox pairs that are moderately separated and thus can form a symmetric battery chemistry by itself. We have also demonstrated that a distinguishable PTIO peak in the FTIR spectra can be correlated with the concentration of unreacted PTIO in the flow cell, which leads to spectroscopic determination of the SOC, as cross-validated by ESR results.26 Here we present a protocol to elaborate procedures for electrochemical evaluations and FTIR-based SOC diagnostics of the PTIO symmetric flow battery. This work is expected to trigger more insights in maintaining the safety and reliability during long-term flow battery operations, especially in real-world grid applications.

<table><tbody><tr><td>PTIO</td><td>TCI America</td><td>A5440</td><td>&gt;98.0%</td></tr><tr><td>Tetrabutylammonium hexafluorophosphate</td><td>Sigma-Aldrich</td><td>86879</td><td>electrochemical grade, &amp;ge;99.0%</td></tr><tr><td>MeCN</td><td>BASF</td><td>50325685</td><td>Battery grade</td></tr><tr><td>Silver nitrate</td><td>Sigma-Aldrich</td><td>204390</td><td>99.9999% trace metals basis</td></tr><tr><td>Gamma alumina powder</td><td>CH Instruments</td><td>CHI120</td><td></td></tr><tr><td>Graphite felt</td><td>SGL</td><td>GFD3</td><td>Vacuum-dry at 70 &amp;deg;C for 24 h</td></tr><tr><td>Porous separator</td><td>Daramic</td><td>AA800</td><td>Vacuum-dry at 70 &amp;deg;C for 24 h</td></tr><tr><td>Battery Tester</td><td>Wuhan LAND electronics Co., Ltd.</td><td>Lanhe</td><td>1 A current range</td></tr><tr><td>Electrochemical Workstation</td><td>Solartron Analytical</td><td>ModuLab</td><td></td></tr><tr><td>glove box</td><td>MBRAUN</td><td>Labmaster SP</td><td>oxygen and water levels &amp;lt;1 ppm</td></tr><tr><td>ESR spectrometer</td><td>Bruker&amp;nbsp;</td><td>Elexsys 580&amp;nbsp;</td><td>Equipped with an SHQE resonator with microwave frequency ~9.85 GHz (X band) at 2 mW power, with 100 kHz field modulation</td></tr></tbody></table>

a protocol for electrochemical evaluations and state of charge diagnostics of a symmetric organic redox flow battery

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

Redox flow batteries have been considered as one of the most promising stationary energy storage solutions for improving the reliability of the power grid and deployment of renewable energy technologies. Among the many flow battery chemistries, non-aqueous flow batteries have the potential to achieve high energy density because of the broad voltage windows of non-aqueous electrolytes. However, significant technical hurdles exist currently limiting non-aqueous flow batteries to demonstrate their full potential, such as low redox concentrations, low operating currents, under-explored battery status monitoring, etc. In an attempt to address these limitations, we recently reported a non-aqueous flow battery based on a highly soluble, redox-active organic nitronyl nitroxide radical compound, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO). This redox material exhibits an ambipolar electrochemical property, and therefore can serve as both anolyte and catholyte redox materials to form a symmetric flow battery chemistry. Moreover, we demonstrated that Fourier transform infrared (FTIR) spectroscopy could measure the PTIO concentrations during the PTIO flow battery cycling and offer reasonably accurate detection of the battery state of charge (SOC), as cross-validated by electron spin resonance (ESR) measurements. Herein we present a video protocol for the electrochemical evaluation and SOC diagnosis of the PTIO symmetric flow battery. With a detailed description, we experimentally demonstrated the route to achieve such purposes. This protocol aims to spark more interests and insights on the safety and reliability in the field of non-aqueous redox flow batteries.

The unique advantages of the symmetric PTIO flow battery system are highly ascribed to the electrochemical properties of PTIO, an organic nitroxide radical compound. PTIO can undergo electrochemical disproportionation reactions to form PTIO+ and PTIO− (Figure 2a). These two redox pairs are moderately separated by a voltage gap of ~1.7 V (Figure 2b) and can be used as both anolyte and catholyte redox materials in a symmetric battery c...

We present the protocols for electrochemically evaluating a symmetric non-aqueous organic redox flow battery and for diagnosing its state of charge using FTIR.

Redox flow batteries have been considered as one of the most promising stationary energy storage solutions for improving the reliability of the power grid ...

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.

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Learn electrochemical evaluation and state of charge diagnostics for symmetric organic redox flow batteries using FTIR spectroscopy. This cost-effective method enables real-time monitoring for safe, reliable grid energy storage.

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

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