Molecular Dynamics Study of Electric Double Layer Capacitors: Modifying Electrode and Electrolyte Composition

Abstract

Electric double layer capacitors (EDLCs) are energy storage devices that are the subject of active research due to high power density, moderate energy density, broad operating temperature windows, and long cycle lifetimes. Distinct from batteries, which convert external energy to charges through redox reactions, EDLCs store charge by electrosorption of ions from electrolyte onto an electrode surface, resulting in an electric double layer. This physical charge storage mechanism enables EDLCs to yield greater power densities over batteries, but also limits their energy density. The past few years has seen a huge increase in the performance of supercapacitors due to the discovery of novel electrode and electrolyte materials, better understanding of charging/discharging mechanisms, as well as more intelligent design of hybrid systems. In this work, molecular dynamics (MD) simulations were conducted to study the performance of carbon-based EDLCs. We applied different modifications to the carbon-based materials, including nitrogen doping, edge sites, and surface oxidization, and evaluated their corresponding influences on the performance. Nitrogen doping and edge sites were both found to be effective to increases the capacitance of graphene-based EDLCs, but both of these modifications needed careful selection of doping/edge configurations. While oxidizations on nonporous carbon electrodes decreased capacitance, oxidizations in porous confinements exhibited different influences on small pores (0.8 nm) and large pores (2.6 nm). In addition, we investigated the effects of electrolyte composition and concentration on the capacitance and ion dynamics on various carbon electrodes. It was found that the increase of cation size decreased the capacitance and ion dynamics. Moreover, solvation of ionic liquid electrolytes drastically enhanced ion dynamics, but its effects on capacitance depended on the specific carbon electrodes used.