Operando benchtop NMR reveals reaction intermediates and crossover in redox flow batteries

Redox flow batteries (RFBs) provide a promising battery technology for grid-scale energy storage. High- field operando NMR analyses of RFBs have yielded useful insight into their working mechanisms and helped improve battery performance. Nevertheless, the high cost and large footprint of a high-field NMR system limit its implementation by a wider electrochemistry community. Here, we demonstrate an operando NMR study of an anthraquinone/ferrocyanide-based RFB on a low-cost and compact 43 MHz benchtop system. The chemical shifts induced by bulk magnetic susceptibility effects differ remarkably from those obtained in high-field NMR experiments, due to the different orientations of the sample relative to the external magnetic field. We apply Evans method to estimate the concentrations of paramagnetic anthraquinone radical and ferricyanide anions. The degradation of 2,6-dihydroxy- anthraquinone (DHAQ) to 2,6-dihydroxy-anthrone and 2,6-dihydroxy-anthranol has been quantified. We further identified the impurities commonly present in the DHAQ solution to be acetone, methanol and formamide. The crossover of DHAQ and impurity molecules through the sseparation Nafion (R) mem- brane was captured and quantified, and a negative correlation between the molecular size and crossover rate was established. We show that a benchtop NMR system has sufficient spectral and temporal resolu- tion and sensitivity for the operando study of RFBs, and anticipate a broad application of operando bench- top NMR methods for studying flow electrochemistry targeted for different applications.(c) 2023 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

By using high-field operando NMR analysis, researchers gained insights into the working mechanisms of RFBs and improved battery performance. However, the high cost and large footprint of the high-field NMR system limited its implementation. As an alternative, they demonstrated an operando NMR study of an RFB using a low-cost and compact 43 MHz benchtop system, obtaining different results from high-field NMR experiments and providing new insights into RFBs. They concluded that the benchtop NMR system has sufficient capabilities for operando studies of RFBs and could be widely applied in flow electrochemistry research.