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

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Redox flow batteries
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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.

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. For example, using the bulk magnetization changes (observed via the proton NMR shift of the water resonance) and the line broadening of the proton shifts of the anthraquinone resonances as a function of the state of charge, we measure the concentration of paramagnetic species, determine the extent of electron delocalization of the unpaired spins over the radical anions, identify and quantify the rate of electron transfer between the reduced and oxidized species. The in situ NMR spectroscopy allows for the decomposition of redox-active electrolytes to be followed and for the degradation to be quantitatively correlated to the capacity fade of the flow battery.

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.

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.

  

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