(192r) Molecular Simulation of Ionic Liquid Systems: Effects of Solvation and Humidification

Room temperature ionic liquids (RTILs) are a class of molten organic salts with high electrochemical stability that drive their utility in energy-relevant applications, such as electrical energy storage, gas separation, and cellulose processing. Two potential pitfalls in many applications are water uptake and slow dynamics. Despite typically containing hydrophobic constituents, common ionic liquids are extremely hygroscopic, even taking up water vapor found in ambient air. Water content can be beneficial or detrimental to a given application, thereby motivating its study in applied systems. The slow transport properties in neat RTILs are due to the strong electrostatic interaction between counter-ions, which is often mitigated by solvation in organic solvents; however, this parameter space is not fully explored. To this end, we use molecular dynamics (MD) simulations in combination with quasi-elastic neutron scattering (QENS) experiments to examine two specific systems to better understand the effects of water and solvation. First, we examine how bulk RTIL transport properties are impacted by solvent concentration and solvation in five organic solvents of different polarity and found that increasing solvent concentration and solvent polarity both enhance the diffusivity of the cation. Second, we study the same RTIL confined in a microporous carbon and exposed to deuterated water vapor. We find humidity-dependent changes in the structure of the confined RTIL, which correlate with increased diffusivity reported by QENS and increased capacitive performance reported by electrochemistry experiments. Together, these two studies can aid in the design of RTIL-based supercapacitors.