Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

In situ NMR metrology reveals reaction mechanisms in redox flow batteries

Nature volume 579, pages 224–228 (2020)Cite this article

Subjects

Abstract

Large-scale energy storage is becoming increasingly critical to balancing renewable energy production and consumption1. Organic redox flow batteries, made from inexpensive and sustainable redox-active materials, are promising storage technologies that are cheaper and less environmentally hazardous than vanadium-based batteries, but they have shorter lifetimes and lower energy density2,3. Thus, fundamental insight at the molecular level is required to improve performance4,5. Here we report two in situ nuclear magnetic resonance (NMR) methods of studying redox flow batteries, which are applied to two redox-active electrolytes: 2,6-dihydroxyanthraquinone (DHAQ) and 4,4′-((9,10-anthraquinone-2,6-diyl)dioxy) dibutyrate (DBEAQ). In the first method, we monitor the changes in the 1H NMR shift of the liquid electrolyte as it flows out of the electrochemical cell. In the second method, we observe the changes that occur simultaneously in the positive and negative electrodes in the full electrochemical cell. Using the bulk magnetization changes (observed via the 1H NMR shift of the water resonance) and the line broadening of the 1H shifts of the quinone resonances as a function of the state of charge, we measure the potential differences of the two single-electron couples, identify and quantify the rate of electron transfer between the reduced and oxidized species, and determine the extent of electron delocalization of the unpaired spins over the radical anions. These NMR techniques enable electrolyte decomposition and battery self-discharge to be explored in real time, and show that DHAQ is decomposed electrochemically via a reaction that can be minimized by limiting the voltage used on charging. We foresee applications of these NMR methods in understanding a wide range of redox processes in flow and other electrochemical systems.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Schematics of the two in situ NMR setups.
Fig. 2: In situ pseudo-2D 1H NMR spectra acquired during electrochemical cycling.
Fig. 3: NMR analyses of self-exchange electron-transfer reactions.
Fig. 4: In situ NMR and mass spectrometry analyses of electrolyte decomposition and battery self-discharge.

Similar content being viewed by others

Data availability

The data that support the findings of this study are available from https://doi.org/10.17863/CAM.46274 and the corresponding author upon reasonable request.

References

  1. US Energy Information Administration. Annual Energy Outlook 2018 (US Department of Energy, 2018).

  2. Wei, X. et al. Materials and systems for organic redox flow batteries: status and challenges. ACS Energy Lett. 2, 2187–2204 (2017).

    Article  CAS  Google Scholar 

  3. Leung, P. et al. Recent developments in organic redox flow batteries: a critical review. J. Power Sources 360, 243–283 (2017).

    Article  ADS  CAS  Google Scholar 

  4. Yang, B., Hoober-Burkhardt, L., Wang, F., Prakash, G. K. S. & Narayanan, S. R. An inexpensive aqueous flow battery for large-scale electrical energy storage based on water-soluble organic redox couples. J. Electrochem. Soc. 161, A1371–A1380 (2014).

    Article  CAS  Google Scholar 

  5. Lin, K. et al. Alkaline quinone flow battery. Science 349, 1529–1532 (2015).

    Article  ADS  CAS  PubMed  Google Scholar 

  6. Tong, L. et al. UV–vis spectrophotometry of quinone flow battery electrolyte for in situ monitoring and improved electrochemical modeling of potential and quinhydrone formation. Phys. Chem. Chem. Phys. 19, 31684–31691 (2017).

    Article  CAS  PubMed  Google Scholar 

  7. Lawton, J. S., Aaron, D. S., Tang, Z. & Zawodzinski, T. A. Electron spin resonance investigation of the effects of vanadium ions in ion exchange membranes for uses in vanadium redox flow batteries. ECS Trans. 41, 53–56 (2012).

    Article  CAS  Google Scholar 

  8. Webster, R. D. In situ electrochemical-NMR spectroscopy. Reduction of aromatic halides. Anal. Chem. 76, 1603–1610 (2004).

    Article  CAS  PubMed  Google Scholar 

  9. Friedl, J. et al. Asymmetric polyoxometalate electrolytes for advanced redox flow batteries. Energy Environ. Sci. 11, 3010–3018 (2018).

    Article  CAS  Google Scholar 

  10. Cao, S.-H. et al. In situ monitoring potential-dependent electrochemical process by liquid NMR spectroelectrochemical determination: a proof-of-concept study. Anal. Chem. 89, 3810–3813 (2017).

    Article  CAS  PubMed  Google Scholar 

  11. Pecher, O., Carretero-González, J., Griffith, K. J. & Grey, C. P. Materials’ methods: NMR in battery research. Chem. Mater. 29, 213–242 (2017).

    Article  CAS  Google Scholar 

  12. Wang, H. et al. In situ NMR spectroscopy of supercapacitors: insight into the charge storage mechanism. J. Am. Chem. Soc. 135, 18968–18980 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  13. Kwabi, D. G. et al. Alkaline quinone flow battery with long lifetime at pH 12. Joule 2, 1894–1906 (2018); correction 2, 1907–1908 (2018).

    Article  CAS  Google Scholar 

  14. de Boer, E. & MacLean, C. NMR study of electron transfer rates and spin densities in p-xylene and p-diethylbenzene anions. J. Chem. Phys. 44, 1334–1342 (1966); erratum 47, 3102 (1967).

    Article  ADS  Google Scholar 

  15. Pell, A. J., Pintacuda, G. & Grey, C. P. Paramagnetic NMR in solution and the solid state. Prog. Nucl. Magn. Reson. Spectrosc. 111, 1–271 (2019).

    Article  CAS  PubMed  Google Scholar 

  16. Evans, D. F. The determination of the paramagnetic susceptibility of substances in solution by nuclear magnetic resonance. J. Chem. Soc. 1959, 2003–2005 (1959).

    Article  Google Scholar 

  17. Johnson, C. S. Jr. Nuclear transverse relaxation in electron-transfer reactions. J. Chem. Phys. 39, 2111–2114 (1963).

    Article  ADS  CAS  Google Scholar 

  18. de Boer, E. & MacLean, C. Spin densities in the alkyl groups of alkyl-substituted naphthalene negative ions, determined by NMR. Mol. Phys. 9, 191–193 (1965).

    Article  ADS  Google Scholar 

  19. Bertini, I., Luchinat, C., Parigi, G. & Ravera, E. NMR of Paramagnetic Molecules 2nd edn (Elsevier, 2017).

  20. Rosso, K. M., Smith, D. M. A., Wang, Z., Ainsworth, C. C. & Fredrickson, J. K. Self-exchange electron transfer kinetics and reduction potentials for anthraquinone disulfonate. J. Phys. Chem. A 108, 3292–3303 (2004).

    Article  CAS  Google Scholar 

  21. Grampp, G., Galán, M. & Sacher, M. Kinetics of photoinduced electron transfer reactions of some anthraquinone radical anions with various inorganic ions: comparison with Marcus cross-relation. Ber. Bunsenges. Phys. Chem 99, 111–117 (1995).

    Article  CAS  Google Scholar 

  22. Rosokha, S. V. & Kochi, J. K. Continuum of outer- and inner-sphere mechanisms for organic electron transfer. Steric modulation of the precursor complex in paramagnetic (ion-radical) self-exchanges. J. Am. Chem. Soc. 129, 3683–3697 (2007).

    Article  CAS  PubMed  Google Scholar 

  23. Meisel, D. & Fessenden, R. W. Electron exchange and electron transfer of semiquinones in aqueous solutions. J. Am. Chem. Soc. 98, 7505–7510 (1976).

    Article  CAS  Google Scholar 

  24. Goulet, M.-A. et al. Extending the lifetime of organic flow batteries via redox state management. J. Am. Chem. Soc. 141, 8014–8019 (2019).

    CAS  PubMed  Google Scholar 

  25. Donovan, P. M. & Scott, L. T. 4,11-bisanthenequinone and 10,10′-bianthrone: simple one-step syntheses from anthrone. Polycycl. Aromat. Compd. 28, 128–135 (2008).

    Article  CAS  Google Scholar 

  26. Filipescu, N., Avram, E. & Welk, K. D. Photooxidative transformations of anthrone, bianthronyl, and bianthrone in acid solution. J. Org. Chem. 42, 507–512 (1977).

    Article  CAS  Google Scholar 

  27. Dalitz, F., Cudaj, M., Maiwald, M. & Guthausen, G. Process and reaction monitoring by low-field NMR spectroscopy. Prog. Nucl. Magn. Reson. Spectrosc. 60, 52–70 (2012).

    Article  CAS  PubMed  Google Scholar 

  28. Hogben, H. J., Krzystyniak, M., Charnock, G. T. P., Hore, P. J. & Kuprov, I. Spinach – a software library for simulation of spin dynamics in large spin systems. J. Magn. Reson. 208, 179–194 (2011).

    Article  ADS  CAS  PubMed  Google Scholar 

  29. Johnson, C. S., Jr. Diffusion ordered nuclear magnetic resonance spectroscopy: principles and applications. Prog. Nucl. Magn. Reson. Spectrosc. 34, 203–256 (1999).

    Article  ADS  CAS  Google Scholar 

  30. Shaw, A. A., Salaun, C., Dauphin, J.-F. & Ancian, B. Artifact-free PFG-enhanced double-quantum-filtered COSY experiments. J. Magn. Reson. A 120, 110–115 (1996).

    Article  ADS  CAS  Google Scholar 

  31. Willker, W., Leibfritz, D., Kerssebaum, R. & Bermel, W. Gradient selection in inverse heteronuclear correlation spectroscopy. Magn. Reson. Chem. 31, 287–292 (1993).

    Article  CAS  Google Scholar 

  32. Zwahlen, C. et al. Methods for measurement of intermolecular NOEs by multinuclear NMR spectroscopy: application to a bacteriophage λ N-peptide/boxB RNA complex. J. Am. Chem. Soc. 119, 6711–6721 (1997).

    Article  CAS  Google Scholar 

  33. Boyer, R. D., Johnson, R. & Krishnamurthy, K. Compensation of refocusing inefficiency with synchronized inversion sweep (CRISIS) in multiplicity-edited HSQC. J. Magn. Reson. 165, 253–259 (2003).

    Article  ADS  CAS  PubMed  Google Scholar 

  34. Stoll, S. & Schweiger, A. EasySpin, a comprehensive software package for spectral simulation and analysis in EPR. J. Magn. Reson. 178, 42–55 (2006).

    Article  ADS  CAS  PubMed  Google Scholar 

  35. Dieterich, V. et al. Estimating the cost of organic battery active materials: a case study on anthraquinone disulfonic acid. Transl. Mater. Res. 5, 034001 (2018).

    Article  CAS  Google Scholar 

  36. Yang, Z. et al. Alkaline benzoquinone aqueous flow battery for large-scale storage of electrical energy. Adv. Energy Mater. 8, 1702056 (2018).

    Article  CAS  Google Scholar 

  37. Gerhardt, M. R. et al. Anthraquinone derivatives in aqueous flow batteries. Adv. Energy Mater. 7, 1601488 (2017).

    Article  CAS  Google Scholar 

  38. Orita, A., Verde, M. G., Sakai, M. & Meng, Y. S. A biomimetic redox flow battery based on flavin mononucleotide. Nat. Commun. 7, 13230 (2016).

    Article  ADS  CAS  PubMed  PubMed Central  Google Scholar 

  39. Wei, X. et al. TEMPO-based catholyte for high-energy-density nonaqueous redox flow batteries. Adv. Mater. 26, 7649–7653 (2014).

    Article  CAS  PubMed  Google Scholar 

  40. Janoschka, T. et al. An aqueous, polymer-based redox-flow battery using non-corrosive, safe and low-cost materials. Nature 527, 78–81 (2015).

    Article  ADS  CAS  PubMed  Google Scholar 

  41. Chen, J.-J., Symes, M. D. & Cronin, L. Highly reduced and protonated aqueous solutions of [P2W18O62]6− for on-demand hydrogen generation and energy storage. Nat. Chem. 10, 1042–1047 (2018).

    Article  CAS  PubMed  Google Scholar 

  42. Winsberg, J., Hagemann, T., Janoschka, T., Hager, M. D. & Schubert, U. S. Redox-flow batteries: from metals to organic redox-active materials. Angew. Chem. Int. Ed. 56, 686–711 (2017).

    Article  CAS  Google Scholar 

  43. Kowalski, J. A., Su, L., Milshtein, J. D. & Brushett, F. R. Recent advances in molecular engineering of redox active organic molecules for nonaqueous flow batteries. Curr. Opin. Chem. Eng. 13, 45–52 (2016).

    Article  Google Scholar 

  44. Kwon, G. et al. Multi-redox molecule for high-energy redox flow batteries. Joule 2, 1771–1782 (2018).

    Article  CAS  Google Scholar 

  45. DeBruler, C. et al. Designer two-electron storage viologen anolyte materials for neutral aqueous organic redox flow batteries. Chem 3, 961–978 (2017).

    Article  CAS  Google Scholar 

  46. Service, R. F. Advances in flow batteries promise cheap backup power. Science 362, 508–509 (2018).

    Article  ADS  CAS  PubMed  Google Scholar 

  47. Ji, Y. et al. A phosphonate-functionalized quinone redox flow battery at near-neutral pH with record capacity retention rate. Adv. Energy Mater. 9, 1900039 (2019).

    Article  CAS  Google Scholar 

  48. Huskinson, B. et al. A metal-free organic–inorganic aqueous flow battery. Nature 505, 195–198 (2014).

    Article  ADS  CAS  PubMed  Google Scholar 

  49. Ding, Y., Zhang, C., Zhang, L., Zhou, Y. & Yu, G. Molecular engineering of organic electroactive materials for redox flow batteries. Chem. Soc. Rev. 47, 69–103 (2018).

    Article  CAS  PubMed  Google Scholar 

  50. Holland-Cunz, M. V., Cording, F., Friedl, J. & Stimming, U. Redox flow batteries—concepts and chemistries for cost-effective energy storage. Front. Energy 12, 198–224 (2018).

    Article  Google Scholar 

  51. Ding, Y., Li, Y. & Yu, G. Exploring bio-inspired quinone-based organic redox flow batteries: a combined experimental and computational study. Chem 1, 790–801 (2016).

    Article  CAS  Google Scholar 

  52. Ross, R. T. Dipolar broadening of EPR spectra due to solute segregation in frozen aqueous solutions. J. Chem. Phys. 42, 3919–3922 (1965).

    Article  ADS  CAS  Google Scholar 

Download references

Acknowledgements

E.W.Z. and C.P.G. acknowledge support from Centre of Advanced Materials for Integrated Energy Systems (CAM-IES), via EPSRC grant number EP/P007767/1. E.W.Z., R.J. and C.P.G. acknowledge support from Shell. E.W.Z. acknowledges support from the Manifest exchange programme via EPSRC grant number EP/N032888/1. T.L. acknowledges support from the Schlumberger Fellowship, Darwin College. E.J. acknowledges support from the Swedish Research Council. We thank A. Brookfield for assistance with the EPR measurement; P. A. A. Klusener from Shell, H. Bronstein, I. Fleming, D. S. Wright, K. Märker, C. Xu, P. C. M. M. Magusin from the University of Cambridge and E. Castillo-Martínez from Universidad Complutense de Madrid for discussions; R. Tan from Imperial College London and D. Lyu, Y. Kim, Y. Jin, and J. Lu from the University of Cambridge for assistance setting up the redox flow battery. A.W. and Q.S. acknowledge Imperial College start-up funding and CAM-IES seed funding. J.C.G. acknowledges support from the Spanish Ministry of Science, Innovation and Universities through a Ramon y Cajal Fellowship (RYC-2015-17722) and the Retos Project (MAT2017-86796-R).

Author information

Author notes

  1. Tao Liu

    Present address: Shanghai Key Laboratory of Chemical Assessment and Sustainability, Department of Chemistry, Tongji University, Shanghai, China

  2. Jeongjae Lee

    Present address: School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea

Authors and Affiliations

  1. Department of Chemistry, University of Cambridge, Cambridge, UK

    Evan Wenbo Zhao, Tao Liu, Erlendur Jónsson, Jeongjae Lee, Israel Temprano, Rajesh B. Jethwa, Holly Smith & Clare P. Grey

  2. Department of Physics, Chalmers University of Technology, Gothenburg, Sweden

    Erlendur Jónsson

  3. Barrer Centre, Department of Chemical Engineering, Imperial College London, London, UK

    Anqi Wang & Qilei Song

  4. Institute of Polymer Science and Technology, ICTP-CSIC, Madrid, Spain

    Javier Carretero-González

Authors
  1. Evan Wenbo Zhao
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Tao Liu
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Erlendur Jónsson
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Jeongjae Lee
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. Israel Temprano
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. Rajesh B. Jethwa
    View author publications

    You can also search for this author in PubMed Google Scholar

  7. Anqi Wang
    View author publications

    You can also search for this author in PubMed Google Scholar

  8. Holly Smith
    View author publications

    You can also search for this author in PubMed Google Scholar

  9. Javier Carretero-González
    View author publications

    You can also search for this author in PubMed Google Scholar

  10. Qilei Song
    View author publications

    You can also search for this author in PubMed Google Scholar

  11. Clare P. Grey
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

C.P.G. supervised the project. E.W.Z and C.P.G. conceived the idea. E.W.Z. designed the in situ setups and performed the NMR and EPR experiments and analysis. T.L. and E.W.Z. performed the infrared experiments. E.J. performed the calculations of reaction equilibrium, CV fittings and NMR chemical shift. J.L. performed the DFT calculations of the radical species and their proton hyperfine couplings. I.T. and E.W.Z. designed and performed the in situ mass spectrometry with assistance from T.L. R.J. performed the CV experiments. A.W. synthesized DBEAQ under the supervision of Q.S. H.S. and J.C.G. assisted in performing the in situ NMR experiments. All authors contributed to the discussion of the project. E.W.Z. and C.P.G. wrote the manuscript with input from all co-authors.

Corresponding author

Correspondence to Clare P. Grey.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 The in situ NMR probe setups and pulse sequence.

a, Image of the flow-through (on-line) sampling tube positioned inside a micro-imaging probe (Bruker 2.5). b, The operando cell assembly positioned inside the NMR probe. Inset, magnification of the components of the cell assembly.c, Each spectrum is acquired by collecting ns free induction decays (FIDs; that is, scans) with a recycle delay of d1. A second time delay, d2, is introduced between each spectrum. The pulse width, p1, for a 90° pulse is 27 μs. The acquisition time for each FID (aq) is 1.5 s. d, Measured flow rates as a function of rotary speed of the pump.

Extended Data Fig. 2 The in situ pseudo-2D and stacked 1H NMR spectra versus battery voltage and current.

a, Operando NMR spectra of 10 cm3 of 100 mM DBEAQ, against 20 cm3 of 150 mM potassium ferrocyanide. The FWHM of the water signal acquired via on-line detection is 6 Hz, compared to the FWHM of 59 Hz acquired in the operando setup. b, Shift of the DHAQ-HB and water resonances (Δδ) as a function of SOC measured via on-line and operando detection. The shifts of the water resonances were only included in the plot after there was clear peak separation between the water signals arising from the catholyte and anolyte electrolytes. c, Labelling of the protons, and DFT-derived volumetric plots of the singly occupied molecular orbital of DBEAQ3•−. The values of the isotropic Fermi contact hyperfine coupling constants (in MHz) were derived from the DFT calculation. d, On-line NMR spectra of 20 cm3 of 100 mM DBEAQ (anolyte), against 20 cm3 of 300 mM potassium ferrocyanide (catholyte).

Extended Data Fig. 3 EPR spectra of DHAQ3•− radical anions.

a–d, Spectra of 100 mM (a) and 1 mM (b–d) DHAQ solution reduced to 50% SOC. The 1 mM solution spectra were fitted with a single component comprising non-deuterated DHAQ3•− (b), two components comprising non-deuterated and singly deuterated DHAQ3•− (c), and two components comprising non-deuterated and doubly deuterated DHAQ3•− (d). The labels used to describe the three proton positions are given in the structure of the radical shown in b. The residual is the difference between the experimental and fitted spectra. At 100 mM, an intense EPR signal of DHAQ3•− was observed at 3,518 G (g-factor, 2.0036). However, the signal is strongly broadened by a combination of the electron dipolar interactions at such high radical concentrations and the rapid electron-transfer reactions. This leads to the loss of the hyperfine coupling features52. At 1 mM, hyperfine coupling features were resolved. e, Table showing DFT- and EPR-derived isotropic hyperfine coupling constants (Aiso) and contact shifts (δFC) of the proton resonances. The source of the difference between the DFT- and EPR-derived hyperfine coupling constants is probably due to solvent (water) coordination, H–D exchange, and/or electron hopping.

Extended Data Fig. 4 CV and concentration of radical anions.

a, CV of 5 mM DHAQ and 5 mM DBEAQ in D2O with 1 M KOH. The potential was scanned at 20 mV s−1, the scan starting by sweeping from positive to negative potentials. b, Fraction of DHAQ3•− radicals calculated on the basis of the Evans method as a function of SOC for DHAQ (100 mM) under a charging current of 100 mA and a flow rate of 13.6 cm3 min−1 (measured twice; red and blue), and a charging current of 150 mA and a flow rate of 33.3 cm3 min−1 (green). The figures on the right and left are the same data plotted without and with error bars. The errors are ± half the FWHM of the water signal. c, Concentration of DHAQ3•− radicals as a function of SOC for 100 mM and 200 mM DHAQ. d, Fraction of radicals as a function of SOC for 100 mM DHAQ and 100 mM DBEAQ. e, Relative concentrations of AQ2−, AQ3•− and AQ4− as a function of SOC of the system and the equilibrium constant, Kc. AQ2−, AQ3•− and AQ4− are represented by dashed, dotted and solid lines, respectively. Kc = 1.26 and Kc = 10.35 correspond to values derived for DBEAQ and DHAQ, respectively (corresponding to E1 – E2 values of 6 mV and 60 mV)5,13. Curves corresponding to Kc = 0.1 and Kc = 20 are also shown, to illustrate the effect using the smaller and larger values E1 – E2 = −58.3 mV and 76.6 mV, respectively. The former negative value illustrates the case in which the radical is strongly disfavoured. f, The AQ3•− and AQ4− concentration at 50% SOC, as a function of Kc. AQ3•− and AQ4− are the black dotted and blue solid lines, respectively. g, Experimentally derived radical concentrations as a function of SOC. Supplementary Information equations S6 and S7 were used to fit the experimental data, along with an SOC lag parameter ranging from 8% to 12%, where x is the fraction of AQ3•− for a given number of electrons n (discussed, as is the derivation of the equations, in the Supplementary Information). The fit to the data was done in SOC steps of 1%. h, The CV of a 100 mM DHAQ. ‘2e-Rev’ and ‘2e-QRev’ refer to a two electron reversible and two-electron quasi-reversible model, respectively, and the ‘1e+1e’ curves (with and without fitting constraints) refer to a two-step, single-electron, quasi-reversible process.

Extended Data Fig. 5 On-line 1H NMR spectra and battery electrochemical profile during galvanostatic cycling.

a, 20 cm3 of 100 mM DHAQ against 40 cm3 of 150 mM potassium hexacyanoferrate(ii)/50 mM potassium hexacyanoferrate(iii). The flow rate is 33.3 cm3 min−1 and the current is 150 mA. With d1 = 20 s, an acquisition time (aq) = 1.5 s and ns = 4, each spectrum is a snapshot of the electrochemical processes averaged over 2.5% of the SOC. b, 20 cm3 of 200 mM DHAQ against 40 cm3 of 250 mM potassium hexacyanoferrate(ii)/50 mM potassium hexacyanoferrate(iii). The flow rate is 13.6 cm3 min−1 and the current is 150 mA. With d1 = 15 s, aq = 1.5 s and ns = 8, each spectrum is a snapshot of the electrochemical processes averaged over 2.9% of the SOC. c, 20 cm3 of 100 mM DBEAQ against 40 cm3 of 150 mM potassium hexacyanoferrate(ii)/50 mM potassium hexacyanoferrate(iii). The flow rate is 13.6 cm3 min−1 and the current is 100 mA. With d1 = 7 s, aq = 1.5 s and ns = 8, each spectrum is a snapshot of the electrochemical processes averaged over 2.1% of the SOC.

Extended Data Fig. 6 Variable-temperature NMR of DHAQ2−/DHAQ3•− radical anion solution.

a, Variable-temperature NMR spectra acquired from a solution containing a radical concentration of 5 mM in 100 mM DHAQ. Black, experimental spectra; navy, deconvoluted spectra; magenta: fitted-sum spectra. b, c, Variable-temperature NMR spectra of peak B with 5 mM (b), and 9 mM (c) of radicals. As the experiments were performed by locking the magnetic field on the shift of the water resonance (which is temperature-dependent), the shift of peak A was manually set to 7.3 ppm so as to more readily compare the different spectra.

Extended Data Fig. 7 On-line 1H NMR spectra performed to quantify the electron-transfer rates.

a–d, 30 mM (a), 50 mM (b), 100 mM 2,6-DHAQ (c), and 100 mM 2,6-DBEAQ (d) during charge/reduction at 1 mA. e–h, The FWHM of the proton signals A, B and C of DHAQ (e–g) and D, E, F, G, H and I of DBEAQ (h) as a function of the radical concentration. i, The electron-transfer rate constant, kex, calculated for 30 mM, 50 mM and 100 mM DHAQ, and 100 mM DBEAQ, at different radical concentrations and SOC, under the slow-exchange approximation.

Extended Data Fig. 8 NMR and DFT calculations to identify the decomposition products.

a–c, DOSY (a), COSY (b) and heteronuclear single quantum coherence (HSQC) (c) spectra of the aliquot extracted from the H cell at 470 h (see Methods section ‘H-cell experiments in an argon glovebox’ for details) in an airtight NMR tube. The lines above the COSY and HSQC spectra indicate the connectivity of the H and C atoms. d, DFT-derived (calculated) proton chemical shift of DHA and DHAL. e, In situ NMR during the galvanostatic cycling at 100 mA after the potential-hold (1.7 V) step. Green arrows, DHA3− or DHAL3−. The signal at 8.4 ppm arises from an impurity.

Extended Data Fig. 9 In situ 1H NMR study of the decomposition reaction under different cycling conditions.

a, b, Proposed H–D exchange (top) and decomposition (bottom) reaction and products of DHAQ4− (a) and DBEAQ4− (b). c, d, NMR spectra of DHAQ during potential hold at 1.4 V (c) and 1.7 V (d). Green arrows, DHA3− or DHAL3−. e, f, NMR spectra of DBEAQ during potential hold at 1.4 V (e) and 1.7 V (f). The signal at 8.4 ppm arises from an impurity.

Extended Data Fig. 10 Effect of radicals and flow on longitudinal and transverse relaxation.

a, The voltage profile during intermittent charging at 100 mA of 20 cm3 of 100 mM DHAQ against 40 cm3 of 200 mM K4Fe(CN)6. The red arrows indicate the times when the flow and electrochemical cycling were paused, and the relaxation measurements commenced. After the measurements finished, the flow and electrochemical cycling were resumed. b, T1 (blue) and T2 (green) relaxation times of the HOD signal as a function of SOC. c, T1 relaxation time of the DHAQ proton B/B″ signal as a function of SOC. The error bars represent the 95% confidence level from the fit. d, e, Proton NMR spectra of DHAQ (HA, HB and HC; d) and HOD (e) acquired at different flow rates via on-line detection. f, g, Signal integral of HOD (f) and DHAQ (g) as a function of flow rate and residence time in the detection region of the NMR probe. h, The FWHM of the HOD and DHAQ signals as a function of flow rate and residence time, and the labelling of protons.

Supplementary information

Supplementary Information

This file contains calculations of electron spin density distribution, NMR chemical shifts, reaction equilibrium constants, bulk magnetic susceptibility, radical concentrations and electron transfer rate constants, four supplementary figures of NMR, IR and MS data and two Supplementary Tables.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Zhao, E.W., Liu, T., Jónsson, E. et al. In situ NMR metrology reveals reaction mechanisms in redox flow batteries. Nature 579, 224–228 (2020). https://doi.org/10.1038/s41586-020-2081-7

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41586-020-2081-7

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing