Discovery and design of novel electrolytes is crucial for improvement in electrochemical energy storage systems such as redox flow batteries (RFBs). Aqueous RFBs typically use metal salts solubilized in highly conductive acidic or basic solutions, enabling production of large current densities during operation. However, aqueous RFBs suffer from limited energy densities imposed by the electrolyte solvent stability window. Nonaqueous RFBs demonstrate increased energy densities with respect to aqueous electrolytes by taking advantage of nonpolar solvents with much larger voltage windows. However, the increased energy density is often accompanied by a loss in current density due to increased resistance through the cell. Microemulsions, thermodynamically stable mixtures of oil, water, and emulsifiers, are being studied as electrolytes due to the potential for combining beneficial properties from both aqueous and nonaqueous electrolytes. They facilitate incorporation of water-insoluble, nonpolar redox species in aqueous electrochemical systems through dispersed oil domains. Microemulsions have nanoscopic heterogeneity, where nonpolar and polar regions are separated at the nanoscale by an amphiphile but are macroscopically homogeneous. Nonpolar redox species partition into the stable but dynamic nonpolar regions of the fluid, while salt dissolved in the aqueous region serves as a supporting electrolyte to maintain solution conductivity. The inherent complexity of these solutions raises fundamental questions about their behavior in electrochemical systems. Transport of redox species and aqueous ions, as well as electrochemical kinetics are dependent on microemulsion composition which influences oil and water domain structures. An understanding of the interrelationship between composition, microstructure, and electrochemical behavior, is needed if these structured fluids are to be utilized as breakthrough electrolytes for energy storage. These relationships are probed through phase diagram behavior studies, spectroscopic analyses, and electrochemical methods. It has been shown that composition and the related structure greatly influence the resulting electrochemical behavior of these systems. Furthermore, it is demonstrated that microemulsion composition alters electrochemical behavior, regardless of redox couple chemistry. Finally, the first oil-soluble redox species used as the primary redox active components in microemulsion redox flow batteries will be presented (Figure). Polarization curve analysis, areal specific resistance measurements, and cycling identify key questions that will be motivate second generation improvements in this technology.

This work was supported as part of the Breakthrough Electrolytes for Energy Storage (BEES), an Energy Frontier Research Center funded by the United States. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0019409.