Theoretical Study On The Interaction And Microstructure Of [Emim][AA] Ionic Liquids And Graphene

Ionic liquids（ILs）have attracted attention as a new type of electrolyte in electrochemical devices such as supercapacitors and lithium-ion batteries due to their very low vapor pressure,wide electrochemical stability window,and high conductivity.When ionic liquids are used as electrolytes,carbon-based materials are typically used as electrodes,such as graphene,doped graphene,etc.Graphene is a new material with a large specific surface area,high conductivity,and good chemical stability.Its excellent electrochemical performance makes it an ideal electrode.In recent years,graphene/ILs have become a good electrode/electrolyte choice.The energy storage mechanism of electrical double layer capacitors（EDLCs）lies in the diffusion of particles with opposite charges in the electrolytes.Ions accumulate at the electrode/electrolyte interface and form electrical double layers（EDLs）.Since charge storage in EDLCs is an interface phenomenon,in-depth research on the characteristics of electrolyte/electrode interfaces is essential.This paper aims to explore the interface interactions and microstructure of four ionic liquids,namely1-ethyl-3-methylimidazolium glycine[Emim][GLY],1-ethyl-3-methylimidazolium serine[Emim][SER],1-ethyl-3-methylimidazolium valine[Emim][VAL],and1-ethyl-3-methylimidazolium phenylalanine[Emim][PHE],with graphene electrodes,in order to deepen the understanding of the EDLs structure and energy storage mechanism of EDLCs.It is the key to designing and applying the best EDLCs.The interface interaction between ILs/graphene and ILs/N-graphene was studied by density functional theory（DFT）.The optimized geometric structures and the negative values of Gibbs free energy change(ΔG_ad)indicated that the cations of ILs were more easily adsorbed on the electrode surface,and the adsorption process of ILs was spontaneous.N doping could effectively control the local charge density distribution and improve the ion binding process,so the binding energy,adsorptionΔG_adand charge transfer between ILs and N-graphene were better than those between ILs and graphene.Since the binding energy and charge transfer value between the ILs and the electrodes were very small,we further analyzed the weak interaction between the electrolytes and the electrodes.The quantum theory of atoms in molecules（QTAIM）was a powerful tool for exploring weak interactions,and calculations indicated that the intermolecular interactions between ILs and electrodes were essentially noncovalent.Noncovalent interaction analysis（NCI）also indicated that the van der Waals interaction dominated the interface region.Finally,the energy decomposition analysis method was used to divide the energy between ILs and electrodes into electrostatic,repulsive,and dispersion interactions.The analysis showed that the contribution of dispersion force was significantly greater than that of repulsive force and electrostatic force,further verifying the noncovalent interaction between the electrolytes and the electrodes.This article revealed the effects of different electrodes and ILs on interface properties,providing theoretical guidance for the rational design of EDLCs.The microstructure of ILs electrolytes on uncharged and charged electrode surfaces was studied using molecular dynamics（MD）simulation.There was a co-adsorption of anions and cations near the uncharged graphene.ILs exhibited a layered structure,forming a dense layer near the electrodes and showing a gradual attenuation trend from the electrode surfaces to the bulk solution.Zero charge potential（PZC）provided a baseline for describing EDLs near polarized electrodes,and a positive PZC indicated that the electrode surfaces had a greater affinity for cations than anions.In addition,the mean square displacement（MSD）of ILs was calculated,indicating that the diffusion of ILs in graphene was limited,and the diffusion motion of components perpendicular to graphene was the slowest.When the charge density of the electrode increased,the strong electric field gradually reduced the density of cations around the positive electrodes and the density of anions around the negative electrodes.Through the snapshot of ILs on the surface of graphene,it was found that ILs had an obvious orientation on the graphene surface.The distribution of imidazole rings in[Emim]⁺was basically parallel to the uncharged graphene.However,in the charged state of graphene,the imidazole ring of the cation was more likely to form a higher inclination angle with the negative electrode surface.This content revealed the distribution of ILs in EDLCs,providing theoretical insights for the EDLs structure and energy storage mechanism of EDLCs.