(462d) Investigating Diffusivity of Solvated Ionic Liquids through Molecular Dynamics Screening

Supercapacitors are electrochemical devices that exhibit high power density but are currently limited in their role of energy storage due to low energy density. A potential solution for improving energy density is the use of room temperature ionic liquids (RTILs), a class of organic solvents that are liquid at room temperature[1]. RTILs have certain physiochemical properties, including a wide operational voltage window which theoretically allows for a higher energy density of supercapacitor devices[2],[3]. Despite this attractive property, RTILs exhibit low diffusivities which can negatively affect the charge/discharge rates of supercapacitors. To overcome this issue, RTILs are often solvated in organic solvents to improve transport properties without a substantial loss in operational voltage window[4]. Thus in order to optimize the performance of supercapacitor devices, it is desirable to fully understand the interactions between RTILs and solvents.

While RTIL interaction with a limited number of organic solvents has been explored with molecular dynamics (MD), difficulties in predicting behavior remain due to the large design space and complex interactions of these systems[5]. With this as motivation, we take a screening approach to expand our knowledge of RTIL-solvent interactions. As a result, we have implemented the necessary algorithms and force fields within MoSDeF[6] to enable screening of [BMIM+][Tf2N-] solvated in 21 distinct solvents over a range of 18 different concentrations. Self-diffusivities of RTILs in each system are obtained by mean squared displacement (MSD) calculations in order to determine trends between RTIL transport, solvent selection, and solvent concentration. Additionally, we compute the ion pairs and ion cages in each system to provide insight into how the addition of each solvent affects RTIL structure. From the ability to sample a larger portion of the RTIL-solvent design space, we find that RTIL diffusivity increases with the bulk diffusivity of the pure solvent and increases with greater solvent concentration. The trends presented can further guide the selection of solvents and concentration for electrochemical applications of RTILs.

References

[1] J. F. Brennecke and E. J. Maginn, “Ionic Liquids: Innovative Fluids for Chemical Processing”,AIChE

Journal, 2001, 47, 2384-2389.

[2] L. P. Rebelo, J. N. Lopes, J. M. Esperanca, E. Filipe, “On the Critical Temperature, Normal Boiling Point, and Vapor Pressure of Ionic Liquids”, Journal of Physical Chemistry B, 2005, 109, 6040-6043

[3] T. Sato, G. Masuda, K. Takagi, “Electrochemical Properties of Novel Ionic Liquids for Electric Double Layer Capacitor Applications”, Electrochimica Acta, 2004, 49, 3603-3611.

[4] R. Lin, P. Huang, J. Segalini, C. Largeot, P.L. Taberna, J. Chmiola, Y. Gogotsi, P. Simon, “Solvent Effect on the Ion Adsorption from Ionic Liquid Electrolyte Into Sub-nanometer Carbon Pores”, Electrochimica Acta, 2009, 54, 7025-7032.

[5] N. Osti, K. Van Aken, M. Thompson, F. Tiet, D. Jiang, P.T. Cummings, Y. Gogotsi, E. Mamontov, “Solvent Polarity Governs Ion Interactions and Transport in a Solvated Room-Temperature Ionic Liquid”, The Journal of Physical Chemistry Letters, 2016, 8, 167-171.

[6] “MoSDeF” [Online]. Available: https://github.com/mosdef-­hub.