Influence of Alkali Cations on Redox Matching and Capacity Access in Redox-Mediated Flow Batteries

Cost-effective redox flow batteries (RFBs) offer reliable energy storage for intermittent solar and wind sources; however, their energy density is inherently lower than that of lithium-ion batteries because of solubility limitations. The redox-mediated flow battery (RMFB) concept addresses this limitation by combining the operational flexibility of RFBs with the high energy density of solid-state batteries. In this system, a solid material which is immobilized inside the electrolyte (the booster) undergoes charge/discharge indirectly through electron transfer mediated by a dissolved active species (mediator). As a result, the energy density of the RMFB is ideally determined by the amount of solid material incorporated. Beyond booster engineering and material screening, the intercalating cation to the booster upon discharge is able to aid in this potential alignment. In this work, electrochemical techniques including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic charge-discharge (GCD), and operando ultramicroelectrode cyclic voltammetry (UME-CV) are employed to identify the optimal utilization window of the booster and quantify trends in redox mediation kinetics by comparing the impact of three alkali cations on redox mediation in Prussian blue (booster)/ferri/ferrocyanide (mediator) systems. The results indicate that Li⁺ and Na⁺ diffuse more slowly within the booster compared to K⁺, leading to superior intercalation dynamics with K⁺. Under the tested conditions with varying mediator concentrations and cation species, the maximum booster utilization was found to be 35% using 200 mM potassium ferri/ferrocyanide. These findings highlight that achieving higher energy densities in RMFBs requires careful optimization of mediator concentration, electrolyte composition, and redox mediation kinetics.