A Novel in-Situ Synchrotron XANES Technique to Study All Vanadium Redox Flow Battery
Authors: Liu Qi , Sun Chengjun , Yang Fan , Ren Yang , Heald Steve M , Xie Jian 
Published: February 2020

Vanadium redox flow battery (VRB) is one of the most suitable energy storage systems for large-scale intermittently renewable energy storage systems1-2. In the VRB, positive and negative electrolytes play important roles to store the energy3-4. Consequently, in order to develop new electrolyte solutions for the VRBs with higher energy and power density, as well as longer cycling life and better safety characteristics, the in-depth understanding of the relationship between the oxidation state /local structure changes of the electrolytes and electrochemical performance is critically important. Synchrotron radiation based X-ray adsorption fine structure (XAFS) could be very informative when applied to probe the environment around atoms of all states of matter5. Also the X-ray near-edge structure (XANES) could be especially powerful to determine the average oxidation state and local symmetry of the elements in all phases. With these considerations in mind, we carried out in situ XANES measurements during cycling of an all vanadium redox flow battery (VRB).
In this study, the home-made VRB single cell was charged at a constant current density up to a 1.65 V, then the cell was discharged at the constant current density until the voltage reach 0.85 V3. In situXANES characterization was performed at the K-edge of the vanadium to monitor the change of the valence state of V in both positive and negative electrolyte solutions.
To better illustrate the XANES spectra evolution, 2D contour plots of XANES spectra for both positive and negative electrolytes have been presented in Fig 1(a) and (b). In addition, the VRB voltage profile in the first charge/discharge process is displayed on the side of the XANES spectra. Obviously, the electrochemical process for both sides is well correlated with the evolution of the XANES data. For example, the inflection point at 1.65 V in the voltage profile coincides well the critical point at contour plot of XANES spectra. The spectra of vanadium XANES during the first charge process is shown in Fig 1.(c) and (d). During the first charge process, the electrolytes were charged at a constant current density until the V4+ was converted to V2+ (negative electrode electrolyte) and V5+(positive electrode electrolyte). Hence, in the positive half-cell, the K-edge of the vanadium continuously shifts toward the higher binding energy. This shift indicates an increase in the average valence state of vanadium in the electrolyte (Fig.1c), while the K-edge of vanadium will shifts back to the lower binding energy, which indicate an decrease in the average valence state of vanadium in the electrolyte (not shown in this abstract). Meanwhile, in the negative half-cell, the K-edge of the vanadium continuously shifts toward to the lower binding energy during the charge process (Fig. 1d), while it will shift back during the discharge process (not shown in this abstract). In summary, a new synchrotron based in situ XANES technique has been applied to study the average oxidation state of the electrolytes in the all vanadium redox flow battery (VRB) under realistic cycling conditions.
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