Structural and Electrochemical Relationships of Bicontinuous Polysorbate-20 Microemulsions

Abstract

Research and discovery of new electrolytes is crucial for advancing the fundamental understanding and development of new opportunities in electrochemical systems. Through an Energy Frontier Research Center (EFRC) for Breakthrough Electrolytes for Energy Storage (BEES), the approach we are taking is to establish a system for decoupling the nature and solubility of electroactive material from the conductivity and transport of ions in the surrounding solution. We accomplish this with microemulsions as a technique to solubilize hydrophobic redox active materials in an aqueous solvent. Microemulsions are isotropic, nanoscale dispersions of two immiscible liquids, stabilized by a surfactant, and cosurfactant. Microemulsions are an interesting electrolyte because they possess several properties that make them useful as an energy storage medium – they are thermodynamically stable, have fast dynamics, possess large interfacial areas, and can solubilize large concentrations of hydrophobic redox-active compounds in an aqueous electrolyte solution. Cyclic voltammetry experiments have shown reversible behavior in microemulsions up to 22 V/s with a double layer capacitance less than expected with such high scan rates. In order to understand the electrochemical behavior, we have begun with understanding the nanoscale structure of the microemulsion. Presented here are the small angle neutron scattering results on microemulsion systems with increasing surfactant loading and how the nanostructure correlates with the electrochemical response. That is as the strength of the amphiphile increases the maximum current density peak decreases and the half-wave potentials shift to lower potentials suggesting the local environment of the redox active species changes as the composition of the microemulsion is varied. Figure 1