(642a) 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.