Voltage-dependent interfacial structure and properties of room temperature ionic liquids: operando X-ray studies

Northwestern University, Evanston IL

Investigators

Abstract

In this project funded by the Chemical Structure Dynamics and Mechanisms (CSDM-A) program of the Chemistry Division, Prof. Pulak Dutta of Northwestern University is using X-rays from synchrotron radiation sources to study liquids composed purely of molecular ions (known as room-temperature ionic liquids, or RTILs). These novel liquids have been proposed for a variety of applications such as electrolytes in batteries and supercapacitors, solvents, etc., and they have a number of properties that are quite different from those of more commonly known water-based liquid electrolytes. The nanoscale origins of these unusual properties are investigated by studying how the ions arrange themselves near electrodes when variable voltages are applied, and how these arrangements develop and change with time. These studies will develop the knowledge necessary to not just understand the observed properties of RTILs, but ultimately to design and control these behaviors. This would potentially allow improved batteries and capacitors, easy ways to manipulate small droplets in microfluidic systems, etc. This project focuses on room-temperature ionic liquids (RTILs), which are molten salts with molecular anions and/or molecular cations The objectives are to study experimentally how the static and dynamic arrangements of RTIL ions near electrode interfaces respond to electric fields; to correlate this behavior with the molecular structures of the ions present in the RTIL; and to seek to understand how the structures determine or affect the observed electrochemical and electrowetting properties. The primary experimental probe is in situ X-ray reflectivity and grazing incidence X-ray diffraction from solid-RTIL interfaces, using national synchrotron facilities. The dependence of the interfacial structure and its dynamics on the ions present, the nature of the electrode surface, and on applied direct and alternating voltages, are being studied and compared to existing theoretical predictions. The anticipated outcome is a better fundamental understanding of the nanoscale interfacial behavior of RTILs, and thus improved knowledge of how to select or design ionic liquids in order to optimize desired properties for practical applications.