Ethan Self1 Frank Delnick1 Rose Ruther2 Jagjit Nanda1

1, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
2, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States

Redox flow batteries (RFBs) are promising energy storage devices for grid-level applications due to their extraordinarily long cycle life and the ability to independently scale their energy and power densities. The energy density of conventional RFBs is dictated by their capacity (which is directly related to the solubility of the redox species in the electrolyte) and operating potential. Numerous RFB chemistries utilizing both aqueous and non-aqueous electrolytes have been explored.[1-3] In general, aqueous RFBs have low operating potentials ca. 1.5 V, resulting in poor energy densities (25 – 30 Wh/kg for an all vanadium RFB), whereas systems containing organic electrolytes with wider electrochemical windows have moderately higher energy densities.

The present study describes a revolutionary new approach[4-7] which uses mediated electrochemical reactions involving anion radical species to utilize a high capacity anode in a redox flow configuration. Rather than using the solvated anion radicals as the primary energy storage medium, herein the anion radicals are transferred to a plug flow reactor containing an active material powder which is charged/discharged through mediated electrochemical reactions. In this configuration, the anion radical species can be recycled several times throughout the cell stack during a single charge/discharge cycle, effectively decoupling the RFB’s energy density from the redox species’ solubility in the electrolyte. This approach can conceivably be used to achieve energy densities exceeding 200 Wh/kg which is ~10 times greater than that of conventional RFBs.

This presentation will describe the mechanism of a mediated RFB and discuss important design considerations (e.g., selection of appropriate electrolyte, mediators, active materials, etc.). Results showing electrochemically mediated sodiation and desodiation of an anode in a redox flow configuration will be presented. The structural evolution of the active material during charge/discharge as determined from Raman spectroscopy and X-ray diffraction (XRD) will also be discussed.

Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy.

[1] X. Wei et al., ACS Energy Lett. 2017, 2, 2187-2204.
[2] D. S. Aaron et al., J. Power Sources 2012, 206, 450-453.
[3] C. N. Sun et al., J. Electrochem. Soc. 2014, 161, A981-A988.
[4] Q. Huang, H. Li, M. Grätzel, Q. Wang, Phys. Chem. Chem. Phys. 2013, 15, 1793-1797.
[5] Q. Wang, Q. Huang, US Patent Application, Pub. No. US 2014/0178735 A1 2014.
[6] J. Yu, et al., Nat. Commun. 2017, 8, 14629.
[7] T. M. Anderson, et al., US Patent No. 9,548,509 B2 2017.