A subscription to JoVE is required to view this content. Sign in or start your free trial.

In This Article

  • Summary
  • Abstract
  • Introduction
  • Protocol
  • Results
  • Discussion
  • Disclosures
  • Acknowledgements
  • Materials
  • References
  • Reprints and Permissions

Summary

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

Abstract

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.

Introduction

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.

Protocol

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

  1. CV Test
    1. Polish a glassy carbon electrode with 0.05 µm 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.
    2. 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.
    3. 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.
    4. 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.
    5. 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.
  2. Flow Cell Assembly
    1. 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.
    2. Dry the flow battery parts (cell compartments, tubing, 5 mL glass vials, graphite felts, and a porous separator) in a vacuum oven at 70 °C for overnight, move them into the glove box, and cool down to the environmental temperature.
    3. 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.
  3. Demonstration of Symmetric Electrochemistry
    1. 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.
    2. 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.
    3. 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.
    4. Measure electron spin resonance (ESR) spectrum of the above four electrolytes.25
      1. With tube sealant, seal a small amount (~10 µL) of the positive and negative in a PTFE tubing (1/16" OD and 1/32" ID) at both ends, and then seal it in a quartz ESR tube (4 mm diameter).
      2. Mount the ESR tubing to an ESR spectrometer fitted with an SHQE resonator with microwave frequency ~9.85 GHz (X band).
      3. Collect the ESR spectrum for the four electrolytes in Section 1.3.3.
  4. Flow Cell Test
    1. Assemble a flow cell following Section 1.2.
    2. 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.
    3. 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.
    4. 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.

2. FTIR-based SOC Determination

  1. FTIR Feasibility Validation
    1. 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.
    2. Measure FTIR for the three electrolyte solutions.
      1. 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.
      2. Put the FTIR cell into a storage container and transfer it out of the glove box.
      3. Quickly mount the FTIR cell to a spectrometer and collect the FTIR spectrum.
    3. Assemble a flow cell following Section 1.2.
    4. 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.
    5. Fully charge the flow cell until the voltage reaches 2.2 V. Stop the charging and the pump.
    6. Measure the FTIR spectra for both the positive and negative electrolytes, respectively, following the procedure in Section 2.1.2.
    7. 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.
    8. Measure the FTIR spectrum for each of the solutions in Section 2.1.6, following the procedure in Section 2.1.2.
  2. FTIR Measurement of SOC
    1. Assemble a flow cell following Section 1.2.
    2. 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.
    3. Charge the flow cell at a constant current of 10 mA/cm2 at a flow rate of 20 mL/min.
    4. 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.
    5. Measure the FTIR spectrum for the above five sample aliquots, following the procedure in Section 2.1.2.
    6. Measure the ESR spectrum for the above five sample aliquots, following the procedure in Section 1.3.4.

Results

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 bat...

Discussion

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...

Disclosures

The authors have nothing to disclose.

Acknowledgements

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 (http://pubs.rsc.org/en/content/articlehtml/2016/ta/c6ta01177b). PNNL is a multi-program national laboratory operated by Battelle for DOE under Contract DE-AC05-76RL01830.

Materials

NameCompanyCatalog NumberComments
PTIOTCI AmericaA5440>98.0%
Tetrabutylammonium hexafluorophosphateSigma-Aldrich86879electrochemical grade, ≥99.0%
MeCNBASF50325685Battery grade
Silver nitrateSigma-Aldrich20439099.9999% trace metals basis
Gamma alumina powderCH InstrumentsCHI120
Graphite feltSGLGFD3Vacuum-dry at 70°C for 24 h
Porous separatorDaramicAA800Vacuum-dry at 70°C for 24 h
Battery TesterWuhan LAND electronics Co., Ltd.Lanhe1A current range
Electrochemical WorkstationSolartron AnalyticalModuLab
glove boxMBRAUNLabmaster SPoxygen and water levels <1 ppm
ESR spectrometerBruker Elexsys 580 Equipped with an SHQE resonator with microwave frequency ~9.85 GHz (X band) at 2 mW power, with 100 kHz field modulation

References

  1. Dunn, B., Kamath, H., Tarascon, J. M. Electrical Energy Storage for the Grid: A Battery of Choices. Science. 334 (6058), 928-935 (2011).
  2. Yang, Z. G., et al. Electrochemical Energy Storage for Green Grid. Chem. Rev. 111 (5), 3577-3613 (2011).
  3. Wang, W., Luo, Q., Li, B., Wei, X., Li, L., Yang, Z. Recent Progress in Redox Flow Battery Research and Development. Adv. Funct. Mater. 23 (8), 970-986 (2013).
  4. Skyllas-Kazacos, M., Chakrabarti, M. H., Hajimolana, S. A., Mjalli, F. S., Saleem, M. Progress in Flow Battery Research and Development. J. Electrochem. Soc. 158 (5), 55-79 (2011).
  5. Weber, A. Z., et al. Redox Flow Batteries: A Review. J. Appl. Electrochem. 41 (10), 1137-1164 (2011).
  6. Noack, J., Roznyatovskaya, N., Herr, T., Fischer, P. The Chemistry of Redox-Flow Batteries. Angew. Chem. Int. Ed. 54 (34), 9775-9808 (2015).
  7. Soloveichik, G. L. Flow Batteries: Current Status and Trends. Chem. Rev. 115 (20), 11533-11558 (2015).
  8. Leung, P., Li, X., de Leon, C. P., Berlouis, L., Low, C. T. J., Walsh, F. C. Progress in Redox Flow Batteries, Remaining Challenges and Their Applications in Energy Storage. RSC Adv. 2 (27), 10125-10156 (2012).
  9. Gong, K., Fang, Q., Gu, S., Li, S., Yan, Y. Nonaqueous Redox-Flow Batteries: Organic Solvents, Supporting Electrolytes, and Redox Pairs. Energy Environ. Sci. 8 (12), 3515-3530 (2015).
  10. Shin, S. H., Yun, S. H., Moon, S. H. A Review of Current Developments in Non-aqueous Redox Flow Batteries: Characterization of Their Membranes for Design Perspective. RSC Adv. 3 (24), 9095-9116 (2013).
  11. Cappillino, P. J., et al. Application of Redox Non-Innocent Ligands to Non-Aqueous Flow Battery Electrolytes. Adv. Energy Mater. 4 (1), 1300566 (2014).
  12. Suttil, J. A., et al. Metal Acetylacetonate Complexes for High Energy Density Non-aqueous Redox Flow Batteries. J. Mater. Chem. A. 3 (15), 7929-7938 (2015).
  13. Brushett, F. R., Vaughey, J. T., Jansen, A. N. An All-Organic Non-aqueous Lithium-Ion Redox Flow Battery. Adv. Energy Mater. 2 (11), 1390-1396 (2012).
  14. Wei, X., et al. Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All-Organic Redox Flow Battery. Angew. Chem. Int. Ed. 54 (30), 8684-8687 (2015).
  15. Nagarjuna, G., et al. Impact of Redox-Active Polymer Molecular Weight on the Electrochemical Properties and Transport Across Porous Separators in Nonaqueous Solvents. J. Am. Chem. Soc. 136 (46), 16309-16316 (2014).
  16. Wei, X., et al. TEMPO-Based Catholyte for High-Energy Density Nonaqueous Redox Flow Batteries. Adv. Mater. 26 (45), 7649-7653 (2014).
  17. Wei, X., et al. Towards High-Performance Nonaqueous Redox Flow Electrolyte Via Ionic Modification of Active Species. Adv. Energy Mater. 5 (1), 1400678 (2015).
  18. Fan, F. Y., et al. Polysulfide Flow Batteries Enabled by Percolating Nanoscale Conductor Networks. Nano Lett. 14 (4), 2210-2218 (2014).
  19. Pan, H., et al. On the Way Toward Understanding Solution Chemistry of Lithium Polysulfides for High Energy Li-S Redox Flow Batteries. Adv. Energy Mater. 5 (16), 1500113 (2015).
  20. Escalante-Garcia, I. L., Wainright, J. S., Thompson, L. T., Savinell, R. F. Performance of a Non-Aqueous Vanadium Acetylacetonate Prototype Redox Flow Battery: Examination of Separators and Capacity Decay. J. Electrochem. Soc. 162 (3), 363-372 (2015).
  21. Wei, X., et al. Microporous Separators for Fe/V Redox Flow Batteries. J. Power Sources. 218, 39-45 (2012).
  22. Skyllas-Kazacos, M., Kazacos, M. State of Charge Monitoring Methods for Vanadium Redox Flow Battery Control. J. Power Sources. 196 (20), 8822-8827 (2011).
  23. Brooker, R. P., Bell, C. J., Bonville, L. J., Kunz, H. R., Fenton, J. M. Determining Vanadium Concentrations Using the UV-Vis Response Method. J. Electrochem. Soc. 162 (4), 608-613 (2015).
  24. Petchsingh, C., et al. Spectroscopic Measurement of State of Charge in Vanadium Flow Batteries with an Analytical Model of VIV-VV Absorbance. J. Electrochem. Soc. 163 (1), 5068-5083 (2016).
  25. Duan, W., et al. A Symmetric Organic-Based Nonaqueous Redox Flow Battery and Its State of Charge Diagnostics by FTIR. J. Mater. Chem. A. 4 (15), 5448-5456 (2016).
  26. Potash, R. A., McKone, J. R., Conte, S., Abruña, H. D. On the Benefits of a Symmetric Redox Flow Battery. J. Electrochem. Soc. 163 (3), 338-344 (2016).
  27. Kim, H. S., et al. A Tetradentate Ni(II) Complex Cation as a Single Redox Couple for Non-aqueous Flow Batteries. J. Power Sources. 283, 300-304 (2015).
  28. Shinkle, A. A., Sleightholme, A. E. S., Griffith, L. D., Thompson, L. T., Monroe, C. W. Degradation Mechanisms in The Non-aqueous Vanadium Acetylacetonate Redox Flow Battery. J. Power Sources. 206, 490-496 (2012).
  29. Li, Z., et al. Electrochemical Properties of an All-Organic Redox Flow Battery Using 2,2,6,6-Tetramethyl-1-Piperidinyloxy and N-Methylphthalimide. Electrochem. Solid-State Lett. 14 (12), 171-173 (2011).
  30. Schaltin, S., et al. Towards an All-Copper Redox Flow Battery Based on a Copper-Containing Ionic Liquid. Chem. Commun. 52, 414-417 (2016).
  31. Luo, Q., et al. Capacity Decay and Remediation of Nafion-based All-Vanadium Redox Flow Batteries. ChemSusChem. 6 (2), 268-274 (2013).

Reprints and Permissions

Request permission to reuse the text or figures of this JoVE article

Request Permission

Explore More Articles

Electrochemical EvaluationState Of ChargeSymmetric Organic Redox Flow BatteryPTIOFourier Transform Infrared SpectroscopyCyclic VoltammetryFlow Cell AssemblyGraphite FeltPorous SeparatorGasket

This article has been published

Video Coming Soon

JoVE Logo

Privacy

Terms of Use

Policies

Contact Us

Recommend to library

JoVE NEWSLETTERS

Research

Education

Authors

Librarians

ABOUT JoVE

Copyright © 2025 MyJoVE Corporation. All rights reserved