Despite the numerous advantages of redox flow batteries (RFBs), the energy density of these systems is limited by the concentration of the redox active species in the electrolyte. [1,2] One promising approach to address this limitation is the utilization of solid charge storage materials in the tanks. Similar to a typical flow battery operation, the mediator (i.e., redox active species), which is oxidized in the flow cell, is pumped to the external tanks. Upon interaction between the mediator and the solid charge storage materials in the tank (i.e., redox targeting reaction), the oxidized mediator is reduced, and charge is transferred to the solid charge storage material. [3,4] In this way, the redox-targeting reaction enables shuttling of charges between the flow cell and the solid charge storage material contained in the tanks. As such, the capacity is no longer dependent on the concentration of soluble species but rather the quantity of solids, which typically have much higher density. In spite of their promise, the fundamental aspects of redox-targeting flow battery (RTFB) systems are not fully understood.

Herein, we demonstrate a high energy density RTFB using highly stable redox active species and Prussian Blue analog solid storage materials. [5] Our recent efforts to understand the interplay between two kinetic processes: the electrochemical reaction in the flow cell and the indirect redox-targeting reaction in the tank are discussed. Cycling experiments are performed with and without the solid storage material to provide evidence that the addition of a compatible solid material greatly improves the energy density. The cation intercalation process that occurs in the redox-targeting reaction is explored as an approach to matching RTFB mediators with solid charge storage materials. Furthermore, an in-line ultramicroelectrode voltammetry set-up is utilized to obtain basic understanding of how solid charge storage materials interact with mediators and how counter cation affects the kinetics of the indirect redox targeting reaction.

References

[1] S.K Pahari, T.C. Gokoglan, B.R.B. Visayas, J. Woehl, J.A. Golen, R. Howland, M.L. Mayes, E. Agar, P.J. Cappillino RSC Advances, 2021, 11, 5432-5443.

[2] R. Yan, Q. Wang Advanced Materials, 2018, 30, 1802406.

[3] F. Zhang, M. Gao, S. Huang, H. Zhang, X. Wang, L. Liu, M. Han, Q. Wang Advanced Materials, 2022, 34, 2104562.

[4] M. Moghaddam, S. Sepp, C. Wiberg, A. Bertei, A. Rucci. P. Peljo Molecules, 2021, 26, 2111.

[5] J. Egitto, T.C. Gokoglan, S.K. Pahari, J.N. Bolibok, S.R. Aravamuthan, F. Liu, X. Jin, P.J. Cappillino, E. Agar Journal of Electrochemical Energy Conversion and Storage, 2022, 19, 041005.