(Keynote) Unraveling the Interplay between Soluble Mediators and Solid Boosters in Redox-Mediated Flow Batteries

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Redox flow batteries (RFBs) offer a promising solution for long-duration, grid-scale energy storage due to their inherent decoupling of energy and power components [1]. However, their relatively low energy density, caused by the solubility limits of redox-active species, poses a challenge. This limitation can be addressed by incorporating a solid charge storage material (the “booster”) into the external electrolyte reservoir. The booster engages in an indirect redox reaction with a dissolved mediator species, effectively decoupling energy density from solubility constraints [2]. These redox-mediated flow batteries (RMFBs) operate through a combination of electrochemical reactions: a current-driven redox process at the electrode and an indirect, Nernstian-driven redox reaction within the reservoir. To design RMFBs that achieve optimal performance, a deeper understanding of the indirect redox mediation mechanism is essential, particularly regarding the balance and interplay between these two reactions [3]. In this work, we investigate the dynamics of redox mediation using operando ultramicroelectrode cyclic voltammetry to monitor the state-of-charge (SOC) of the ferri/ferrocyanide mediator during cycling with a Prussian blue booster. Our findings reveal that booster utilization is significantly influenced by the properties of the supporting salt cations [4], which affect the intercalation potential and redox matching between the mediator and booster [5]. Through systematic variation of current density, mediator concentration, electrolyte composition, and electrode design parameters, we identify critical factors governing the efficiency of the indirect mediation process. These insights provide a foundational framework for the rational design of RMFBs, enabling improved utilization of solid boosters and offering pathways to enhance energy density and efficiency. References: Caiado, A.A. et al. Binder-coated carbon cloth electrodes for all-vanadium redox flow batteries, Journal of The Electrochemical Society , 2024, 171: p. 120524. Ye, J. et al. Redox targeting-based flow batteries. Journal of Physics D: Applied Physics , 2019, 52(44): p. 443001. Matteucci, N.J. et al. Toward electrochemical design principles of redox-mediated flow batteries. Current Opinion in Electrochemistry , 2023, 42: p. 101380. Egitto, J. et al. Toward high energy density redox targeting flow batteries with a mushroom-derived electrolyte. ASME Journal of Electrochemical Energy Conversion and Storage , 2022, 19(4): p. 041005. Moghaddam, M. et al. Thermodynamics, charge transfer and practical considerations of solid boosters in redox flow batteries. Molecules , 2021, 26(8): p. 2111.

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