With the cost of renewable energy near parity with fossil fuels, energy storage is paramount. We report a breakthrough on a bioinspired NRFB active-material, with greatly improved solubility, and place it in a predictive theoretical framework.

Durable and efficient energy storage is a critical aspect of modern electrical grids, especially those comprising energy from intermittent and renewable sources. Non-aqueous redox flow batteries (NRFB) are a promising technology to meet this growing need, with the potential to greatly exceed the energy density of their aqueous counterparts while maintaining key advantages over Li-ion batteries. These advantages include decoupled power and energy ratings, thermal stability and the capability of long-duration storage. Notwithstanding these promising attributes, the development of NRFB has been severely hampered by chemical instability of active materials charged and/or discharged states. Herein we demonstrate the excellent electrochemical stability of a recently reported NRFB active material, vanadium(iv/v)bis-hydroxyiminodiacetate (VBH) using operando spectroscopic measurements. This technique shows tight coupling between changes in the concentrations of the vanadium(iv) and vanadium(v) ions and the applied current. This direct measurement of electrochemical stability is widely available, and its routine use to characterize potential redox active species during cycling, verifying a clean transition between redox states, would be of great value to the NRFB community. Further, we report a method of large-scale preparation of VBH that makes use of inexpensive chemical feedstocks, overcoming another important obstacle to its implantation in an NRFB system.

Active-material solubility is critical in determining NRFB energy density, yet a predictive model accounting for solid-state cohesion energy has remained elusive. Herein we present such, based on an empirically calibrated computational framework.

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 enhance by ~5x when CoHCF pellets were added into the tank compared to the case without CoHCF. With the pellet approach, up to the ~70% of the theoretical capacity of CoHCF were utilized at 10 mA cmȡ2 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.