Molecular Simulation of Ionic Liquid Mixtures: Applications to Capacitive Energy Storage

Room temperature ionic liquids (RTILs) are a class of molten organic salts with remarkable physical and electrochemical properties that make them appealing candidates for electrolytes in high-performance supercapacitors. Already boasting an enormous library of binary neat ionic liquids from the billions of possible combinations of chemical moieties, properties of ionic liquid electrolytes can further be tuned by forming mixtures. Here, we present molecular dynamics (MD) contributions to combined experimental and simulation studies on ionic liquid mixtures. We begin with a relatively simple case of mixtures of the ionic liquids EMIM-Tf2N and EMIM-BF4 and examine how composition impacts electric double layer (EDL) structure and capacitance on planar graphene. Then, we study mixtures of EMIM-Tf2N and HMIM-Tf2N in a bimodally nanoporous carbon and examine how ions selectively partition themselves into different sized pores and connect this to macroscopic capacitance data from electrochemistry experiments. Together, these studies demonstrate concrete cases of unique electrochemical behavior with a mixed ionic liquid electrolytes.