(709a) Enhancement of Self-Diffusion of Ionic Liquids Near Electrodes By an Electric Field

Ionic liquids are promising electrolytes for electrochemical systems because of their generally wide electrochemical window, non-volatility, and non-flammability. One of the key challenges in bringing ionic liquids to industry is to improve their low ion transport rates. In this study, we use molecular dynamics simulations to examine the transport properties near surfaces, including electrode surfaces. The results show the emergence of ordered structure near the surfaces, which are fundamentally changed when an electric field is applied. These structural changes are accompanied by changes in the transport properties of the ionic liquid. We simulate nanoconfined [EMIM][TFSI], where the surfaces are modeled as featureless, solid walls in order to focus on self-assembly arising from liquid interactions only (rather than interactions with particular features of surfaces). Under no electric field, the ionic liquid forms neutral layers near the surfaces, where ions are ordered to maximize local charge neutrality in each layer. When we apply an electric field across the electrodes, a different structure arises, where now there are alternating layers of cations and anions. Diffusion properties in the directions parallel and perpendicular to the face of the electrodes vary significantly in these two cases, and can be understood in terms of the change in structure. Based on these results, we hypothesize that transport properties of the ionic liquid can be enhanced in certain directions under electric field. The results of this study are relevant to electrochemical systems because a critical step in forming the electric double layer and driving electrochemical reactions is the transport of ions near the interface.