Impact of confinement and polarizability on dynamics of ionic liquids.

Johannes Gäding,Gabriele Tocci,Patrick Huber,Mark Busch,Robert H Meißner

doi:10.1063/5.0077408

Abstract

Polarizability is a key factor when it comes to an accurate description of different ionic systems. The general importance of including polarizability into molecular dynamics simulations was shown in various recent studies for a wide range of materials, ranging from proteins to water to complex ionic liquids and for solid-liquid interfaces. While most previous studies focused on bulk properties or static structure factors, this study investigates in more detail the importance of polarizable surfaces on the dynamics of a confined ionic liquid in graphitic slit pores, as evident in modern electrochemical capacitors or in catalytic processes. A recently developed polarizable force field using Drude oscillators is modified in order to describe a particular room temperature ionic liquid accurately and in agreement with recently published experimental results. Using the modified parameters, various confinements are investigated and differences between non-polarizable and polarizable surfaces are discussed. Upon introduction of surface polarizability, changes in the dipole orientation and in the density distribution of the anions and cations at the interface are observed and are also accompanied with a dramatic increase in the molecular diffusivity in the contact layer. Our results thus clearly underline the importance of considering not only the polarizability of the ionic liquid but also that of the surface.

Highlights

Room temperature ionic liquids (RTILs) are considered as promising materials for a wide range of chemical and electrical applications
While most previous studies focused on bulk properties or static structure factors, this study investigates in more detail the importance of polarizable surfaces on the dynamics of a confined ionic liquid in graphitic slit pores, as, e.g., evident in modern electrochemical capacitors or in catalytic processes
A recently developed polarizable force field using Drude oscillators is modified in order to describe a particular room temperature ionic liquid (RTIL) accurately and in agreement with recently published experimental results

Summary

Introduction

Room temperature ionic liquids (RTILs) are considered as promising materials for a wide range of chemical and electrical applications. The growing attention to these materials rises mainly from their distinct properties: as a room temperature molten salt they have almost no vapor pressure, a high thermal stability and a large liquidus and electrochemical window.[1,2,3,4,5] Ionic liquids are used as solvents in the processing of aerogels, as a silica supported catalyst in water and in other homogeneous and heterogeneous catalytic processes.[6,7,8,9,10,11,12] Concerning electrochemistry, they attract great attention as promising electrolytes in energy storage applications such as batteries and supercapacitors[13,14,15,16] and their associated charging dynamics have been extensively studied with molecular simulations[17,18,19]. The conductivity of ionic liquids is generally rather low[20] and can be increased by the addition of solvents that effectively reduce their viscosity[21,22]. With the ever increasing emergence of nanoporous carbons as electrodes or as support materials for catalytic reactions in combination with RTILs, the interactions between those materials and the resulting dynamics are of great interest as RTILs often continue to exceed expectations,for example, in terms of high voltage charging[15,23] and increasing catalytic effectiveness.[24,25]

Methods

Results

Conclusion