Effects of Surface Complexity and Moisture on Graphene Oxide–Ionic Liquid Interfaces

Supercapacitors based on the interface between graphene oxide (GO) and ionic liquids (ILs) are an appealing and potentially safe energy storage solution due to both the low volatility of IL electrolytes compared to many traditional electrolytes and the cost effectiveness of using GO as electrodes. Molecular dynamics (MD) simulations can provide key insights into the atomic-scale structuring of these electrolyte–electrode interfaces. However, prior simulation studies of GO–IL interfaces have typically modeled GO as an idealized single layer of intact graphene with a near-homogeneous distribution of hydroxyl and epoxy groups. Such models neglect known GO structural complexities such as turbostratic sheet stacking, the presence of lateral sheet gaps and porosity, nonhomogeneous oxygen group distributions, and topological defects that induce roughness. Here, MD simulations of a typical IL electrolyte, ethyl-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI], interfaced with three structurally complex multilayered GO stacks are reported: one with heterogeneous oxygen distribution and defects, one as an “aged” variant with reduced oxygen content, and a traditional model with uniform oxygen distribution, flat surface, and no defects. The influence of the presence of water on interfacial structuring is also explored. The outcomes indicate that heterogeneous oxygen distribution and subnanoscale surface roughness do not greatly alter the IL structuring at the interface compared to a smoother GO surface with homogeneous oxygen distribution, provided that the total average surface oxygen content is similar. However, the presence of a small amount of water greatly increases ion mobility in both the interface and the bulk liquid and was found to enhance ion mobility at lateral GO sheet edges. These findings contribute to our understanding of how to design GO–IL-based energy storage materials.