Density Functional Theory for the Structure and Dynamics of Electric Double Layers of Ionic Liquids

Engineering Sciences and Fundamentals

01C11 Interfacial Aspects of Electrochemical Energy Storage Systems
Dan Steingart and Holly J. Martin

Density functional theory for the structure and dynamics of electric double layers of ionic liquids

Ke Wang¹, De-En Jiang² and Jianzhong Wu¹

¹Department of Chemical and Environmental Engineering, University of California, Riverside, California 92521 and ²Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831

Abstract

Ionic liquids have been used in diverse electrochemical systems including energy storage devices such as supercapacitors, batteries and fuel cells. Because of the intrinsic complexity of organic ions and strong electrostatic correlations, the electrochemical properties of ionic liquids often defy the descriptions of conventional mean-field methods such as the Gouy-Chapman-Stern (GCS) theory for equilibrium properties and the Poisson-Nernst-Plank (PNP) equation for the dynamics. In this work, we introduce the classical density functional theory (DFT) as an alternative to conventional methods to describe equilibrium and time-dependent properties of  electric double layers unique to ionic liquids. By considering the molecular size, topology, and electrostatic correlations, we have examined the equilibrium structures of ionic liquids near electrodes and the responses of ionic profiles, electrostatic potential, surface charge density and surface power density during electrode charging. We have also examined major factors responsible for the unique features of electric-double layers in ionic-liquids including capacitance oscillation in porous electrodes and formation of long-range alternating structures of cations and anions at charged surfaces.