Insight into the adsorption of Imidazolium-based ionic liquids on graphene by first principles simulation

The ionic liquid (ILs)-graphene interface is important in understanding the behaviors of ILs electrolyte in carbon-based supercapacitors and Li-ion batteries. Molecular simulation is one of the excellent ways to investigate the mechanism. However, due to the multiple interacting sites between ILs and graphene, it demands a lot of computational time to scan. In this work, an easy method was obtained, and it is for predict the adsorption energy and screen the proper ILs on graphene. The behavior of five kinds of ILs, composed by 1-ethyl-3-methylimidazolium cation ([Emim]+) and anions ([BF4]-, [PF6]-, [Cl]-, [Br]-, and [OAc]-) on the graphene surface were studied by using density functional theory calculations. The stable adsorbed structures were optimized and the influence of the anions was further evaluated. Moreover, charge density difference and noncovalent interaction analysis were performed to figure out the adsorption energies difference on ILs. The results illustrated the correlations among cation–anion interaction, charge distribution, charge variation and adsorption energies. The weaker cation–anion interaction, the bigger charge variation of the ILs and the stronger adsorption energy for [Emim][PF6] and [Emim][BF4], while other three ILs are in the opposite law. Based on this, we can quickly screen suitable ionic liquids as electrolytes and apply them to graphene-based supercapacitors. Apart from this, in order to investigate the distribution of the ions, two pairs of ILs on the graphene were studied and different arrangement configurations were compared, respectively. The crossover-like structures are more stable when compared with the line-like structures. The above results could provide a novel sight on the view of the Graphene-ILs (G-ILs) microstructure, which could be helpful to design new electrolyte systems in electrochemistry applications.