Coupled <i>In Situ</i> NMR and EPR Studies Reveal the Electron Transfer Rate and Electrolyte Decomposition in Redox Flow Batteries

We report the development of in situ (online) EPR and coupled EPR/NMR methods to study redox flow batteries, which are applied here investigate redox-active electrolyte, 2,6-dihydroxyanthraquinone (DHAQ). The radical anion, DHAQ3–•, formed as a reaction intermediate during reduction DHAQ2–, was detected its concentration quantified electrochemical cycling. fraction anions found be concentration-dependent, decreasing total DHAQ increases, we interpret terms competing dimer formation mechanism. Coupling two techniques—EPR NMR—enables rate constant for electron transfer between DHAQ3–• DHAQ4– determined. quantify changes “high-voltage” hold by NMR spectroscopy correlate it quantitatively capacity fade battery. decomposition products, 2,6-dihydroxyanthrone 2,6-dihydroxyanthranol, were identified this hold; they shown undergo subsequent irreversible oxidation at 0.7 V, so that no longer participate electrochemistry battery when operated standard voltage window cell. with faster being observed higher concentrations. Taking advantage inherent properties system, work demonstrates possibility multi-modal characterizations characterization techniques applicable range systems.