In situ NMR metrology reveals reaction mechanisms in redox flow batteries

Evan Wenbo Zhao, Tao Liu, Erlendur Jónsson, Jeongjae Lee, Israel Temprano, Rajesh B. Jethwa, Anqi Wang, Holly Smith, Javier Carretero-González, Qilei Song and Clare P. Grey ()
Additional contact information
Evan Wenbo Zhao: University of Cambridge
Tao Liu: University of Cambridge
Erlendur Jónsson: University of Cambridge
Jeongjae Lee: University of Cambridge
Israel Temprano: University of Cambridge
Rajesh B. Jethwa: University of Cambridge
Anqi Wang: Imperial College London
Holly Smith: University of Cambridge
Javier Carretero-González: Institute of Polymer Science and Technology, ICTP-CSIC
Qilei Song: Imperial College London
Clare P. Grey: University of Cambridge

Nature, 2020, vol. 579, issue 7798, 224-228

Abstract: 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.

Date: 2020
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DOI: 10.1038/s41586-020-2081-7

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