Abstract
Redox-mediated flow batteries (RMFBs) offer a viable pathway to combine the scalability of redox flow batteries with the high energy density of solid-state storage by enabling indirect charge transfer between a dissolved redox mediator and solid charge-storage materials housed in external tanks [1]. While this architecture decouples energy density from solubility constraints, system performance is governed by the dynamic interplay between electrochemical reactions at the electrodes and chemical redox reactions in the tanks. Mismatches between these reaction rates can limit solid utilization and constrain achievable power density, particularly under practical operating conditions [2]. In this talk, we present recent insights from our group on the design and operation of RMFBs, with a particular focus on charge/discharge asymmetry and multi-stage operating current protocols. Using Prussian blue solid boosters and ferri/ferrocyanide mediators as model systems, we demonstrate how solid microstructure, conductive additives, electrolyte composition, and intercalating cation identity collectively influence mediator–booster kinetics and accessible utilization windows [3]. Operando ultramicroelectrode voltammetry enables direct tracking of mediator state of charge during cycling, revealing pronounced asymmetry in solid–mediator reaction rates between charge and discharge and identifying regimes where tank reactions become rate-limiting [4]. Building on these insights, we show how multi-stage and asymmetric current protocols can be employed to increase operating current density while mitigating losses in solid utilization. Together, these results provide a unified framework for co-designing materials, electrolytes, and charge-discharge strategies in RMFBs, highlighting pathways to simultaneously enhance energy density and power density in next-generation flow-assisted energy storage systems. References [1] Q. Huang, H. Li, M. Grätzel, and Q. Wang, “Reversible chemical delithiation/ lithiation of LiFePO 4 : towards a redox flow lithium-ion battery.” Phys. Chem. Chem. Phys., 15, 1793 (2013). [2] N. J. Matteucci, C. T. Mallia, B. J. Neyhouse, and F. R. Brushett, “Development of a physics-informed modeling framework for redox-mediated flow batteries.” J. Electrochem. Soc., 172, 070539 (2025). [3] D. Rourke, E. Ergun, S. Chaurasia, T. M. Poudel, P. J. Cappillino, and E. Agar, “Design Considerations for Solid Charge Storage Materials in Redox-Mediated Flow Batteries,” J. Electrochem. Soc., 172, 100513 (2025) [4] E. Schröter, C. Stolze, J. Meyer, M. D. Hager, and U. S. Schubert, “Organic redox targeting flow battery utilizing a hydrophilic polymer and its in-operando characterization via state-of-charge monitoring of the redox mediator.” Chem. Sus. Chem., 16, e202300296 (2023).