Influence of Structure, Interionic Interactions, Interfacial slip and Viscous-electric Coupling Phenomena on the Rheology of Nanoconfined Ionic Liquids

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The overall goal of this project is to advance the knowledge about structure-property relationships for ionic liquids, which are organic molten salts with tunable compositions. Besides high ionic conductivity, ionic liquids exhibit low volatility, good thermal stability, low flammability, and a wide electrochemical window. These properties could enable safe energy storage devices with higher energy and power density and longer-term stability than is possible with current technologies. However, the performance of these devices depends on the viscosity of the ionic liquids in extremely narrow spaces, and their high viscosity has been linked to reduced electrical conductivity and capacitance, poor wetting of porous electrodes, and lower power density. Therefore, research that addresses the flow characteristics of nanoconfined ionic liquids is of paramount importance. This project will examine the relationship between liquid composition and flow behavior in nanoscale channels. Thereby, it will enable to establish design principles of ionic liquids as electrolytes for next-generation supercapacitor and battery technologies. The proposed research will also contribute to the development of the US work force by training two graduate research assistants at the intersection between materials chemistry and engineering, interfacial science, and rheology. This multi-disciplinary approach will help broaden participation of underrepresented groups in research and benefit engineering education. The research project will address three major issues in the flow of nanoconfined ionic liquids: the relationship between liquid structure and nanorheology, the effects of surface composition (wettability) and nanoscale roughness on material behavior, and the connection between electrostatic and viscous phenomena. The experimental study will use a surface forces apparatus by confining the liquids between two surfaces with controlled characteristics. Measurements of static and hydrodynamic forces as well as dynamic shear will be carried out on a set of judiciously selected ionic liquids with a wide range of compositions. The findings of these studies will be used to develop a framework to predict the rheology of nanoconfined ionic liquids. Furthermore, the knowledge generated by this research will enable practitioners to modulate the nanorheological response of ionic liquids through the precise control of their structure, surface wettability, roughness and potential.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.