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Large-scale energy storage is becoming increasingly critical to balance the intermittency between renewable energy production and consumption. Redox Flow Batteries (RFBs), based on inexpensive and sustainable redox-active materials, are promising storage technologies. A RFB (figure on the left) consists of two tanks of redox-active electrolytes, one catholyte and one anolyte, and its capacity can be scaled up just by increasing the volume of the tanks. The electrolytes flow through an electrochemical cell where redox reactions happen. Due to this design, one of the distinct features of RFBs is the decoupling of their energy storage and power generation, which provides different opportunities for in situ monitoring.

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We have developed in situ NMR metrologies to probe the electrolyte in the flow path (on-line detection), or in the battery cell (operando detection). A wide range of redox processes can be readily studied. The in situ NMR spectroscopy unravelled the decomposition of redox-active electrolytes and guided the regeneration of these species, prolonging the battery lifetime by seventeen times.

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Coupling the in situ NMR techniques to other (flow) characterisation methods, including EPR, mass spectrometry and/or UV-Vis, we have demonstrated the possibility of multi-modal on-line characterisations. The figure on the right presents the animated in situ 1H NMR and EPR spectra of 10 mM DHAQ as a function of electrochemical cycling. Probing the electron and nuclear spins simultaneously allows reaction mechanisms to be determined and quantified.

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Representative publications:

1. Zhao, E. W., Liu, T., Jónsson, E., Lee, J., Temprano, I., Jethwa, B. J., Wang, A., Smith, H., Carretero-González, J., Song, Q., Grey, C. P. “In situ NMR metrology reveals reaction mechanisms in redox flow batteries” Nature 2020, 579, 224-228.

2. Zhao, E. W., Jónsson, E., Jethwa, B. J., Hey, D., Lyu, D., Brookfield, A., Klusener, P. A. A., Collison, D., Grey, C. P. “Coupled in situ NMR and EPR studies reveal the electron transfer rate and electrolyte decomposition in redox flow batteries” J. Am. Chem. Soc. 2021, 143, 1885-1895.

3. Jing, Y.,# Zhao, E. W.,# Goulet, M. A.,# Bahari, M., Fell, E., Jin, S., Davoodi, A., Jónsson, E., Wu, M., Grey. C. P., Gordon, R., D., Aziz, M. “In situ electrochemical recomposition of decomposed redox-active species in aqueous organic flow batteries” Nature Chemistry 2022, 14, 1103-1109. (#equal contribution)

4. Wu B, Aspers L E G R, Kentgens A P M, Zhao E W “Operando benchtop NMR reveals reaction intermediates and crossover in redox flow batteries” J. Magn. Reson. 2023, 351, 107448 (invited).