(309c) Ionic Liquid-Based Supercapacitors: Insight From Molecular Simulations

Electrical double layer capacitors (EDLCs), or supercapacitors, continue to gain an increasing attention as promising electrical energy storage devices that can be complementary to batteries, particularly when high power density and fast power delivery/uptake and long cycling life are required.  The EDLCs store energy via the accumulation of ions in a nanometer-thick layer at electrochemically-stable high specific surface area active electrodes. A notable improvement in EDLC performance has been achieved due to recent advances in understanding charge storage mechanisms, development of advanced nanostructured electrodes and electrochemically stable electrolytes such as room temperature ionic liquids. Despite the extensive experimental work on these systems, the specifics of what happens inside and on the surfaces of most promising nanostructured electrodes are still not well understood. In this presentation, we will show how molecular dynamics simulations using state-of-the-art methodologies offer crucial insight into understanding the molecular level phenomena that influence supercapacitor performance and provide guidance for the design of novel materials.   Specifically, we systematically investigate performance of various ionic liquids in nanoconfined geometries and on various nanoparticle surfaces. Our simulations show that depending on complex interplay of anion/cation asymmetry in shape, size, details of charge distribution, and specifics of ion-surface interactions, the capacitance on positive and negative electrodes can be very asymmetric. We discuss which structural characteristics of electrode (nanopore size, surface roughness and curvature) are beneficial for the capacitance enhancement as well as examine mechanisms of ion mobility in the nanoconfined geometries during charging/discharging processes. Using the obtained molecular level insight we discuss how to optimize the design and selection of electrolyte/electrode combinations to achieve increased capacitance in ELDC.