Structure, Partitioning, and transport behavior of microemulsion Electrolytes: Molecular dynamics and electrochemical study

Abstract

Microemulsion electrolyte has been proposed as redox flow battery electrolyte to decouple the redox active species’ solubility and conductivity by taking advantage of stabilizing both aqueous and nonaqueous phases in nanoscale. Herein, molecular dynamic (MD) analysis was applied to characterize a family of Tween® 20/1-butanol/KNO3aq/Toluene microemulsions with different compositions, determining their microstructure that tends to adopt under standard conditions and evaluating their stability through total energy evolution. Instead of forming a densely packed surfactant layer that divides aqueous and nonaqueous phases, toluene was trapped in the nonpolar surfactant chain. Based on this finding, we proposed an alternative mathematical expression to correlate surface area-to-volume ratio with correlation length which provided a good accordance between simulation and experimental results. Moreover, the nature of the oil–water interface was revealed by quantitatively analyzing the hydrogen bonds between the surfactant/cosurfactant and the water. Electrochemical analyses are performed to examine how the structures of microemulsion electrolytes correlate to their electrochemical performance. The results indicate that absorption on the electrode surface, ionic conductivity, and connectivity of both aqueous and nonaqueous phases are of central significance in affecting the redox reaction. These findings, therefore, shed light on guiding the formulation of microemulsion electrolytes and regulating their electrochemical performance as electrolytes for redox flow battery applications. In the end, we proposed an electrochemical reaction mechanism of Fc in the microemulsion electrolyte.