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® membrane 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 resolution and sensitivity for the operando study of RFBs, and anticipate a broad application of operando benchtop NMR methods for studying flow electrochemistry targeted for different applications.