Investigating Indirect Redox-Targeting Reactions for High Energy Density Redox Flow Batteries

Redox flow batteries (RFBs) are considered to be one of the key technologies for addressing the intermittency problem in renewable energy sources, due to their unique architecture that allows for unprecedented scalability and flexibility, and cost-effectiveness in long-duration storage [1]. Despite their promise, typical RFBs suffer from low energy density because of the limit imposed by the solubility of the redox active species in the electrolyte [2]. One promising approach to address this limitation is the utilization of solid charge storage materials in the tanks to reversibly reduce or oxidize dissolved species in the electrolyte via indirect redox-targeting reactions without being connected to an external circuit [3-4]. In this way, the capacity is no longer dependent on the concentration of soluble species but rather the quantity of solids in the tanks. This concept [5] demonstrates the promise for exploiting the unique benefits of solid- and liquid-phase redox chemistry, combining the high-energy-density of Li-ion batteries, and the scalability and safety of RFBs.<br/><br/>In this presentation, we demonstrate a high energy density redox-targeting flow battery (RTFB) using a highly stable bio-inspired active material (vanadium(IV/V)bis-hydroxyiminodiacetate (VBH)) and a suitable solid storage material [5]. VBH [2,5] provides an ideal scaffold to investigate the complex electrochemical processes that occur during RTFB operation, which remain virtually unexplored [6]. For the solid storage material, a Prussian Blue Analogue (PBA) is synthesized and coupled with VBH mediators. Our recent efforts on understanding the interplay between two kinetic processes: electrochemical reaction in the flow cell and an indirect redox-targeting reaction in the tank are discussed. Cycling experiments are performed with and without the solid energy booster material to provide evidence that the addition of a compatible solid material greatly improves the energy density. Optimization of the critical parameters such as the amount of solid storage material, the operating current density, and the electrolyte flow rate is presented. Furthermore, an empirical approach to matching RTFB mediators with solid charge storage materials is introduced.<br/><br/><b>References:</b><br/>[1] Z. Li, M. S. Pan, L. Su, P.-C. Tsai, A. F. Badel, J. M. Valle, S. L. Eiler, K. Xiang, F. R. Brushett, Y.-M. Chiang, <i>Joule</i>, <b>1</b>, 306-327 (2017).<br/>[2] 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, <i>RSC Adv., </i>11, 5432-5443 (2021).<br/>[3] R. Yan, Q. Wang, <i>Adv. Mater.</i>, <b>30</b>, 1802406 (2018)<br/>[4] X. Wang, M. Zhou, F. Zhang, H. Zhang, Q. Wang, <i>Curr. Opin. Electrochem., </i><b>29</b>, 100743 (2021).<br/>[5] J. Egitto, T. C. Gokoglan, S. K. Pahari, J. N. Bolibok, S. R. Aravamuthan, F. Liu, X. Jin, P. J. Cappillino, E. Agar, <i>ASME. J. Electrochem. En. Conv. Stor., </i>(2022) https://doi.org/10.1115/1.4054697<br/>[6] M. Moghaddam, S. Sepp, C. Wiberg, A. Bertei, A. Rucci. P. Peljo, <i>Molecules</i>, <b>26</b>, 2111 (2021).