Ab Initio Studies of Ultrathin Ionic Liquid Films on Au (111) Surface

Abstract

Room-temperature ionic liquids (RTILs) as alternative electrolytes are of great interest to electrochemical energy storage devices, such as supercapacitors. A key to understanding the performance of these systems is to know the ion arrangement near the electrode surface and the ion-electrode interaction. In this work, we will present our ab initio studies of a few layers of 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide (EMMIM TFSI) ionic liquid on gold (111) surface, a system that was recently studied in a novel experiment using low-energy electron and photoemission electron microscopy (LEEM/PEEM). [1] Our work probes different ion arrangements on Au (111) and the corresponding work function change. We further investigate the most stable ion arrangement at different electrode charge densities and identify their out-of-plane layering structure. The dynamics of the ions with reordering and drifting under electric field can be inferred as well. We then use our findings to explain the observed changes in electron reflectivity/photoemission yield in LEEM/PEEM. Together, our results provide important understanding towards the structures and responses of ionic liquids on an electrified metal surface. This research was carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under contract DE-SC0012704. [1] Wattaka Sitaputra, Dario Stacchiola, James F. Wishart, Feng Wang, and Jerzy T. Sadowsk, In situ Probing of Ion Ordering at Electrified Ionic Liquid/Au Interface, Adv. Mater. DOI:10.1002/adma.201606357