Abstract
Among several types of redox flow batteries (RFBs) under development, non-aqueous redox flow batteries (NRFBs) have the potential to approach the energy density of lithium-ion batteries, while maintaining the advantages of flow systems, including ability to decouple power and energy ratings, and thermal stability. Despite their promise, NRFBs suffer from low energy densities because the solubility limitation of redox species in non-aqueous solvents remains relatively lower compared to water. One promising concept for drastically improving the energy density of NRFBs is the utilization of solid charge storage materials, which are reversibly oxidized or reduced in the electrolyte tanks upon interaction with the redox active species (mediators) dissolved in electrolyte (i.e., redox-targeting flow battery (RTFB)). Herein, we demonstrate a RTFB using a highly stable, bio-inspired mediator, vanadium(IV/V)bis-hydroxyiminodiacetate (VBH), coupled with cobalt hexacyanoferrate (CoHCF) as the solid charge storage material. Based on the charge/discharge cycling experiments, the energy capacity was found to be enhanced by similar to 5x when CoHCF pellets were added into the tank compared to the case without CoHCF. With the pellet approach, up to similar to 70% of the theoretical capacity of CoHCF were utilized at 10 mA cm(-2) current density. Sufficient evidence has indicated that this concept utilizing redox-targeting reactions makes it possible to surpass the solubility limitations of the active material, allowing for unprecedented improvements to the energy density of RFBs.