Date of Award

12-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Mechanical Engineering

Advisor

Miller, Richard

Committee Member

Qiao, Rui

Committee Member

Li, Gang

Committee Member

Wang, Pingshan

Abstract

Room-temperature ionic liquids (RTILs) are a promising class of electrolyte that are composed entirely of ions but are liquid at room temperature. Their remarkable properties such as wide electrochemical window make them ideal electrolytes in many electrochemical systems. Because the non-equilibrium transport of RTILs often determines the performance of these systems, a fundamental understanding of such transport is needed. Here, using molecular dynamic (MD) and continuum simulations, we investigated the non-equilibrium transport of RTILs in three scenarios relevant to the application of RTILs in electrochemical systems: the electroosmotic flow (EOF) of RTILs through nanochannels, the electrokinetic transport of RTILs through nanopores, and the charging kinetics of the double layers near planar electrodes. For EOFs of RTILs through nanochannels, we discovered that their strength greatly exceeds that predicted by the classical hydrodynamic theories. We traced the unexpected flow strength to the short-wavelength nature of the EOFs in RTILs, which requires the generalized hydrodynamics (i.e., nonlocal law for the shear stress-strain rate relation) for describing such flows. The EOF in RTILs is thus a rare example in which short-wavelength hydrodynamics profoundly affects flow measurables. For the electrokinetic transport of RTILs through nanopores, we discovered that, in pores wetted by RTILs a gradual dewetting transition occurs upon increasing the applied voltage, which is accompanied by a sharp increase in ionic current. These phenomena originate from the solvent-free nature of RTILs and are in stark contrast with the transport of conventional electrolytes through nanopores. Amplification of these phenomena is possible by controlling the properties of the pore and RTILs, and we showed that it is especially pronounced in charged nanopores. For the charging kinetics of the double layers near planar electrodes, we found that, the potential across the double layers can oscillate during charging when the charging current is large. Such oscillation originates from the sequential growth of the ionic space charge layers near the electrode surface. This allows the evolution of double layers in RTILs with time, an atomistic process difficult to visualize experimentally, to be studied by analyzing the cell potential under constant current charging conditions.

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