Microemulsions are a breakthrough electrolyte for energy storage (BEES) that is emerging as an alternative to traditional RFB electrolytes because of their ability to produce thermodynamically stable, homogeneous solutions of polar and non-polar molecules. The stabilization of oil and water solutions allows microemulsion electrolytes to accommodate organic redox species with fast electron transfer rates that are not soluble in the aqueous phase while still offering the high conductivity of an aqueous salt electrolyte. Microemulsion electrolytes consist of two immiscible phases stabilized by a surfactant, a non-polar phase, solubilizing the redox material—or the redox material can be the entire oil phase—and an aqueous brine phase. Moreover, each component can be environmentally and biologically safe. For example, the aqueous phase only consists of salt and water, the surfactant used in this study, polysorbate-20, is found in many cosmetics and food products, and some of the redox materials investigated in our lab are vitamins such as menadione (vitamin K3) and tocopherol (vitamin E).

Microemulsions form many self-assembled bulk nanoscale structures, such as oil-in-water or water-in-oil droplets and bicontinuous. Furthermore, microemulsions exhibit ordering near surfaces that can be different from the bulk structure. Understanding the bulk and surface structures is critical to understanding a material's properties, such as conductivity and diffusion to an electrode surface. Understanding structural changes with composition variation and surface hydrophobicity provide a path to optimizing the balance between acceptable conductivity levels in the electrolyte and the high volume of an oil phase bearing the redox-active species in a µE. This optimization is critical to achieving high-performing electrochemical systems using µEs.

This talk describes structural studies of microemulsions prepared from water, toluene, butanol, and polysorbate-20. Small-angle neutron scattering was used to monitor the development of the bicontinuous system as a function of the water-to-surfactant mass ratio on a constant oil-to-surfactant dilution line, revealing how the domain size, correlation length, amphiphilicity factor, and bending moduli change with composition. Kratky and Porod analyses are also employed, providing further structural detail of the scattering domains. We demonstrate that controlling the water-to-surfactant ratio with a constant oil-to-surfactant dilution affects the bicontinuous phase, reveals a sizeable compositional region with structural similarities, and provides insight into the correlation of structure to physical properties. Additionally, neutron reflectivity was used to probe the surface ordering of the microemulsions with hydrophilic and amphiphilic surfaces.