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In situ NMR metrology reveals reaction mechanisms in redox flow batteries

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.

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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.

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.

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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).

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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.

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Correspondence to Clare P. Grey.

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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.

ad, Spectra of 100 mM (a) and 1 mM (bd) 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. bc, 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.

ad, 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. eh, The FWHM of the proton signals A, B and C of DHAQ (eg) 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.

ac, 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). cd, 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.

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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

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