Molecular insights into electrical double layers of room-temperature ionic liquids near functionalized graphene surfaces

Electrical double layer capacitors (EDLCs) with electrolytes of room-temperature ionic liquids (RTILs) are attracting substantial interest as a new class of electrical energy storage devices. Carbide-derived carbons (CDCs), which provide high specific surface area, are considered as promising candidates for electrodes in EDLCs. CDCs are usually synthesized by halogen etching of various carbides, and hence contain certain surface chemistries. However, the influence of these surface functional groups on the EDL structure has not been fully understood. Since the performance of EDLCs is governed by EDLs based on RTIL electrolytes at the solid-liquid interface, a comprehensive investigation is needed to understand the effects of surface functional groups on the capacitive behavior. In this work, we perform molecular dynamics (MD) simulations to model RTILs near functionalized graphene surface as well as pristine graphene surface. Different functional groups are studied, including hydrogen, oxygen, hydroxyl and nitrogen groups. The differential capacitance of each system is hence obtained and compared with each other as well as experimental data from cyclic voltammetry and galvanostatic cycling. Interactions of ions with surface are characterized and compared for different surfaces. Changes in ion distribution and orientation of RTIL electrolytes in the EDL are observed. With all these analyses, we are then able to establish a better understanding of the effects of surface functional groups on the EDL structure and capacitance.