CoupledIn SituNMR and EPR Studies Reveal the Electron Transfer Rate and Electrolyte Decomposition in Redox Flow Batteries

Abstract

We report the development of in situ (online) EPR and coupled EPR/NMR methods to study redox flow batteries, which are applied here to investigate the redox-active electrolyte, 2,6-dihydroxyanthraquinone (DHAQ). The radical anion, DHAQ 3−• , formed as a reaction intermediate during the reduction of DHAQ 2− , was detected and its concentration quantified during electrochemical cycling. The fraction of the radical anions was found to be concentration-dependent, the fraction decreasing as the total concentration of DHAQ increases, which we interpret in terms of a competing dimer formation mechanism. Coupling the two techniquesEPR and NMRenables the rate constant for the electron transfer between DHAQ 3−• and DHAQ 4− anions to be determined. We quantify the concentration changes of DHAQ during the "high-voltage" hold by NMR spectroscopy and correlate it quantitatively to the capacity fade of the battery. The decomposition products, 2,6-dihydroxyanthrone and 2,6-dihydroxyanthranol, were identified during this hold; they were shown to undergo subsequent irreversible electrochemical oxidation reaction at 0.7 V, so that they no longer participate in the subsequent electrochemistry of the battery when operated in the standard voltage window of the cell. The decomposition reaction rate was found to be concentration-dependent, with a faster rate being observed at higher concentrations. Taking advantage of the inherent flow properties of the system, this work demonstrates the possibility of multi-modal in situ (online) characterizations of redox flow batteries, the characterization techniques being applicable to a range of electrochemical flow systems.

Electrochemical Regeneration of Anthraquinones for Lifetime Extension in Flow Batteries

“…11, 12 Goulet and Tong et al, however, reduced the reaction rate by restricting the negative electrolyte (negolyte) state of charge (SOC) and converted a substantial amount of the DHA(L) 2− back to DHAQ 2− by aeration of the negolyte, thereby extending the overall lifetime of the battery. 11 The DHAQ 4− can also be electrochemically reduced to form DHA(L) 2− , 12,13 which can be subsequently electrochemically oxidized to the dimer (DHA)2 4− . 11 Since the direct electrosynthesis of the water-soluble anthraquinones from their anthracene precursors has already been demonstrated by some of us, via a multiple-electron transfer processes, 14 The evolution of in situ 1 H NMR spectra of 100 mM DHAQ during the discharge phase of an electrochemical cycle is shown in Figure 1a.…”

Novel Li3VO4 Nanostructures Grown in Highly Efficient Microwave Irradiation Strategy and Their In‐Situ Lithium Storage Mechanism

“…In‐situ synchrotron diffraction technology, as an excellent analytical tool, can directly identify the real‐time structural variations of electrode nanomaterials during charge−discharge cycling, [ 3 , 4 , 5 ] which is critical for understanding the Li‐storage mechanism and relative structural changes in the electrode. High‐resolution in‐situ SXRD patterns were collected via the Powder Diffraction Beamline at the Australian Synchrotron.…”

“…In this scheme, however, accurate control of nanostructural growth and the electrochemical reaction mechanism face two major challenges. [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ] First, the fast preparation of nanomaterials relies on their thermodynamics (including the most stable state and lowest energy barrier) and kinetics (related to the reaction rate and reaction order) in complicated chemical reactions. As is well known, the electrochemical performances depend strictly on the available electrode materials for rechargeable batteries, such as lithium‐ion batteries (LIBs) [ 13 ] and sodium‐ion batteries (SIBs).…”

A microelectrode-based sensor for measuring operando active species concentrations in redox flow cells

“…11 There are numerous experimental methods that can be used to track SOC and SOH in RFBs, which vary depending on the electrolyte chemistry and the desired analytical information. Spectroscopic techniques, including spectrophotometry, 12,13 nuclear magnetic resonance spectroscopy, [14][15][16] electron paramagnetic resonance spectroscopy, 14,17,18 and infrared spectroscopy, 19,20 can enable quantitative measurements of the species concentration and, in some cases, elicit chemical information about the stability and decomposition products of different species within the electrolyte, but these typically require specialized hardware and infrastructure to be performed in situ or operando. 21 In lieu of more comprehensive chemical characterizations of the electrolyte, one can monitor physicochemical (e.g., density, 22 viscosity 23 ) and electrochemical descriptors (e.g., conductivity, 24,25 open-circuit potential 26 ) that in many instances correlate to electrolyte SOC.…”