Abstract
Redox-mediated flow batteries, which incorporate solid charge storage materials, offer the potential to increase the energy density of redox flow batteries toward that of Li-ion batteries. These systems comprise two reactions on each side of the flow cell: the redox reaction of the dissolved species (the “mediator”) at the electrode driven by the applied current, and the solid-mediator reaction in the external tank driven by the mediator’s equilibrium potential. The relative rates of these two reactions determine solid utilization and limit the achievable current density. To better understand the interplay between these reactions, operando ultramicroelectrode cyclic voltammetry was used to track the solid-mediator reaction at various current densities, which revealed that the discharge is the rate-limiting step due to asymmetric solid-mediator reaction rates indicating non-Nernstian behavior of the solid. Prussian blue and potassium ferro/ferricyanide were chosen as the solid and mediator, respectively. Solid utilization decreased from 52% to 30% as current density was increased from 10 mA cm-2 to 50 mA cm-2. Using insights derived from these results, cycling strategies were developed which maximize current density while mitigating losses in solid utilization, providing practical guidance for designing operating protocols that balance power and energy density in redox-mediated flow batteries.