Design Considerations for Solid Charge Storage Materials in Redox-Mediated Flow Batteries

A promising solution to increase the energy density of redox flow batteries (RFBs) is the incorporation of solid charge storage material (the “booster”) into the electrolyte reservoir, which undergoes reversible redox-mediation reactions with the active material (the “mediator”), effectively decoupling solubility and energy density while maintaining the scalability of the traditional RFB architecture. While this approach has attracted growing attention, there has not been a systematic study of how booster composition and morphology affect performance. Herein, we explore the effects of the conductive additive, the porosity of the pellet, and the loading ratio of booster to electrolyte. Prussian blue and potassium ferro/ferricyanide were selected as the booster and mediator, respectively. The maximum booster utilization was ∼46% with Super C65 carbon black, 73% porosity, and 100–120 mg booster ml −1 electrolyte. Pellets with bimodal pore size distribution, whose microstructure included a significant volume of submicron pores which enable intra-pellet diffusion, performed significantly better than pellets with primarily large pores. Measuring the solid-mediator reaction throughout cycling shows that it is limited to a small window of mediator SOC during discharging, necessitating improved kinetics and mass transport to keep pace with the reaction at the electrode.