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 bypass the solubility limitation and drastically improve the energy density is the addition of insoluble charge storage materials into the electrolyte tanks. These redox active materials are reversibly oxidized/reduced in the tanks upon interaction with the redox active species dissolved in the electrolyte. Thus, the capacity is no longer dependent on the concentration of soluble species but rather the quantity of solids in the tanks [3-4]. This new concept is called redox-mediated flow battery (RMFB).

In this presentation, we demonstrate a high energy density RMFB using a highly stable bio-inspired active material (vanadium(IV/V)bis-hydroxyiminodiacetate (VBH)) and a suitable solid storage material. VBH, as a high-stability active material [5], provides an ideal scaffold to investigate the complex electrochemical processes that occur during RMFB operation, which remain virtually unexplored [6]. For the solid storage material, a Prussian Blue Analogue (PBA) is synthesized and coupled with VBH mediators. Symmetric cell 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. Compatibility between the PBA and VBH is investigated using cyclic voltammetry (CV) experiments. Optimization of the critical parameters such as the amount of solid storage material, the operating current density, and the electrolyte flow rate is presented.

References:

[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, Joule, 1, 306-327 (2017).

[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, RSC Adv., 11, 5432-5443 (2021).

[3] R. Yan, Q. Wang, Adv. Mater., 30, 1802406 (2018)

[4] X. Wang, M. Zhou, F. Zhang, H. Zhang, Q. Wang, Curr. Opin. Electrochem., 29, 100743 (2021).

[5] T. C. Gokoglan, S. K. Pahari, A. Hamel, R. Howland, P. J. Cappillino, E. Agar, J. Electrochem. Soc., 166, A1745-A1751 (2019).

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