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Development and Application of Operando NMR Methods for Studying Electrocatalysis and Redox Flow Batteries

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Redox flow batteries

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.

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.

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.

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).


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In situ NMR probe development

In situ NMR is a valuable tool for studying electrochemical devices, including redox flow batteries and electrocatalytic reactors, capable of detecting reaction intermediates, metastable states, time evolution of processes or monitoring stability as a function of electrochemical conditions. We have developed a parallel line detector for spatially selective in situ electrochemical NMR spectroscopy. The detector consists of 17 copper wires and is doubly tuned to 1H/19F and X nuclei ranging from 63Cu (106.1 MHz) to 7Li (155.5 MHz). The flat geometry of the parallel line detector allows its insertion into a high electrode surface-to-volume electrochemical flow reactor, enabling a detector-in-a-reactor design. This integrated device is named “eReactor NMR probe”. Inside the eReactor probehead, the working electrode also functions as a ground plate that confines the radio-frequency (B1) field between the parallel line detector and the electrode. Combined with B1-selective pulse sequences, selective detection of the nuclei at the electrode-electrolyte interface, that is within a distance of 800 μm from the electrode surface, can be achieved. The selective detection of 7Li and 19F nuclei is demonstrated using two electrolytes, LiCl and LiBF4 solutions, respectively. The eReactor NMR probe is a general method and will be applied for studying the electrode-electrolyte interfaces in a wide range of environmentally relevant electrochemical reactions.

1. Luo R, Janssen H, Kentgens A, Zhao E W, Manuscript Submitted,

2. Zhao E W, Janssen H, Luo R, Kentgens A “An NMR probe for in situ electrochemical spectroscopy” EU patent application 23206408.9, Oct. 2023.

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