Osmotic Pressure of Ionic Liquids in an Electric Double Layer

Abstract

An ionic liquid is a salt and exist in a liquid state at room temperature because its melting point lower than usual salts such as NaCl, KCl, and etc., and it is also referred to as the Room Temperature Ionic Liquids (RTILs). It is defined as liquid or glass because the size and irregularity of the anions and cations hinder the stable ion pairs and crystallization. The ionic liquids show high ionic-conductivity. Besides, they show stability in various thermal and chemical conditions that they exhibit such properties as flame retardancy, low vapor pressure. Consequently, the ionic liquids are used as electrolytes in many electrochemical systems like solar cell, fuel cell, and lithium ion battery systems. As the requirement for high capacity electrochemical systems increases, the size of unit system tends to be small and the nano-porous electrode becomes favorable. As the system size becomes nano-scale, however, peculiar physical phenomena can occur, such as Electric Double Layer (EDL) overlapping and electro-capillary phenomenon. Especially, the EDL overlapping can cause the pressure acting on the surface and it can affect the stability of electrochemical systems in turn. In this thesis, I investigated the osmotic pressure of ionic liquids based on continuum approach and atomistic approach. At first, by following the development of electric double layer modeling, I studied basic theory related to the electric double layer. And next, by using Bazant-Storey-Kornyshev (BSK) model, I found out that the osmotic pressure at the nanoslit surface and nanoslit center is susceptible to the effects of steric factor, correlation length, and nanoslit size. In the need of verification the continuum approach, atomistic approach was used collaterally. As for the atomistic approach, Molecular Dynamics (MD) simulation with Large-scale Atomic/Molecular Massively Parallel Simulation (LAMMPS) was used. By doing MD simulation, I analyzed the ion distribution in bulk and nanoslit region, and calculated the osmotic pressure at the surface. In the course of the verification of the continuum approach, BSK model was modified considering additional factor, and I developed the new continuum model for describing the discrete features, which cannot represented in previous continuum model.