Microemulsions: Breakthrough Electrolytes for Redox Flow Batteries

Abstract

Aqueous and non-aqueous redox flow batteries (RFBs) have limited energy and current densities, respectively, due to the nature of the electrolytes. New approaches to electrolyte design are needed to improve the performance of RFBs. In this work, we combined a highly conductive aqueous phase and an organic redox-active phase in a microemulsion to formulate a novel RFB electrolyte. As a proof-of-concept, we demonstrate an RFB using this microemulsion electrolyte with maximum current density of 17.5 mA·cm−2 with a 0.19 M posolyte and 0.09 M negolyte at a flow rate of only ∼2.5 ml·min−1, comparable to early vanadium electrolyte RFBs at similar flow rates on a per molar basis. The novel active negolyte component is an inexpensive oil-soluble vitamin (K3). By combining aqueous and organic phases, the solvent potential window and energy density may be increased without sacrificing current density and new redox couples may be accessed. Microemulsion electrolytes show great promise for improved performance and increased energy densities in aqueous RFBs but the path forward is complex. We end with discussion of areas that need work to achieve the potential of these electrolytes.

Redox Flow Batteries: Electrolyte Chemistries Unlock the Thermodynamic Limits

“…Recently, micellar solubilization has been proposed as a new approach to designing aqueous electrolytes for RFBs (Figure 11). [147] Microemulsion electrolytes have merits of decoupled electrolyte conductivity in aqueous phase and redox reaction in non‐aqueous phase. As such, the selection of organic solvents is no longer limited by their solubility in supporting electrolytes.…”

“…Thus, the original inner hydration sheath is not Illustration of a RFB with microemulsion electrolytes comprised of water (blue) and oil (yellow) phases. [147] affected. Such design strategy allows decoupling the ion-pairing and ion conduction in electrolytes and may find promising applications in RFBs.…”

The Cluster Design and Redox Behavior Characterization of Polyoxometalates for Redox Flow Batteries

“…POMs are widely used in various energy storage fields due to its unique molecular size and corresponding charge density [88–91] . Different types of POMs have various structural characteristics.…”

Flow batteries with solid energy boosters

“…11. Due to their chemical decoupling of energy and power, SEB-RFBs routinely achieve much larger area-specific power (ASP) than other high-specific energy flow batteries, i.e., slurry [103,104] and microemulsions [134,135] RFBs. 12.…”