(232u) Molecular Dynamics Study of the Capacitive Performance of Oxidized Graphene

Electrochemical capacitors, which are commonly referred to as supercapacitors, offer relatively high energy and power densities and extended cycle lifetimes. Subsequently, they are attracting substantial interest as a new class of electrical energy storage devices. Graphene layers improve the specific surface areas and electrical conductivity of electrodes and have been implemented in novel material composites in these systems. Although recent efforts have broadened the understanding of electrosorption of charges on heterogeneous features of these materials, most modeling studies approximate the graphene electrodes as pristine surfaces without functional groups or defects. To reconcile these efforts with non-idealized graphene allotropes that are more common in supercapacitors, we present a comprehensive study of the effects of oxygen groups on the structure of the electric double layer (EDL) and the resulting capacitance.

We conducted molecular dynamics (MD) simulations of different electrolytes on pristine/functionalized graphene, and the electrolytes included both neat ionic liquids and ionic liquids in acetonitrile (ACN) solution. We complemented computational results with experimental cyclic voltammetry data to further clarify the effects of the functional groups. The findings show that ions have higher interaction energies with functionalized surfaces due to the increased contribution of electrostatic interactions. Higher electrode-electrolyte interaction energies attract co-ions and/or expel counter-ions, thus decreasing the capacitance of neat ionic liquids on oxidized surfaces. The functional groups have lower influence on the interaction energy of the electrode with organic ACN solvents. However, solvated electrolytes exhibit different electrochemical behaviors than their solvent-free counterparts. Since the molecular size of the cation in ionic liquids is much larger than ACN, inner-most layers at the cathode are primarily occupied by the solvent. Subsequently, capacitance of cathode was similar for both functionalized and pristine graphene. However, since the anions aggregate at the innermost layer of the anode, functional groups decreased the capacitance of the anode. These results corresponded well with the electrochemical tests, and improved our fundamental understanding of the effects of surface chemistry on the performance of different electrolytes in supercapacitors.