Heterogeneous Dynamics and Electrolyte Applications of Ionic Liquids

Abstract

Ionic liquids (ILs), or room-temperature ionic liquids (RTILs), have attracted considerable attention from both academia and industry, due to their peculiar properties and wide applicabilities as electrolytes. In this dissertation, we study both their dynamic properties as a glass-forming liquid and the applicability as electrolytes in supercapacitors via molecular dynamics simulations. In the first part, we study how dynamic heterogeneity in ionic liquids is affected by the length scale of structural relaxation and the ionic charge distribution by adopting two differently charged models of ionic liquid and their uncharged counterpart. In one model of ionic liquid, the charge distribution in the cation is asymmetric, and in the other it is symmetric, while their neutral counterpart has no charge with the ions. It is found that all the models display heterogeneous dynamics, exhibiting subdiffusive dynamics and nonexponential decay of structural relaxation. We investigate the lifetime of dynamic heterogeneity, τdh, in these systems by calculating the three-time correlation functions to find that τ_dh has in general a power-law behavior with respect to the structural relaxation time, τ_α : τ_dh ∝ (τ_α)^(ζ_dh). Although the dynamics of the asymmetric-charge model is seemingly more heterogeneous than that of the symmetric-charge model, the exponent is found to be similar, ζ_dh ≈ 1.2, for all the models studied in this work. The same scaling relation is found regardless of interactions, i.e., with or without Coulomb interaction, and it holds even when the length scale of structural relaxation is long enough to become the Fickian diffusion. This fact indicates τ_dh is a distinctive time scale from τ_α, and the dynamic heterogeneity in these systems is mainly affected by the short-range interaction and the molecular structure. In the second part, we study electric double layer capacitors (EDLCs) in the parallel plate configuration of graphene oxide (GO). Electric double layer capacitors (EDLCs), or supercapacitors, offer high power densities and moderate energy densities for energy storage applications. The oxidation range of electrode is varied from 0% (pure graphene) to 100% (fully oxidized GO) by decorating graphene surface with hydroxyl groups. Two different electrolytes, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4–) as an ionic liquid (IL) and its 1.3 M solution in acetonitrile as an organic electrolyte (OE), are considered. While the area-specific capacitance tends to decrease with increasing electrode oxidation for both electrolytes, its details show interesting differences between OE and IL, including the extent of decrease. The difference in the capacitances between them is pronounced at the cathode of low oxidation, and the anion is identified as the key species in screening ability that makes the difference in the capacitance. Hydrogen bonding between the hydroxyl groups of GO and the anions is the major factor that yields this unexpected result. For detailed insight into these differences, the screening mechanisms of electrode charges by electrolytes and their variations with electrode oxidation are analyzed with special attention paid to the aspects shared by and the contrasts between IL and OE.