Theoretical Study On The Microstructure Of [C_nmim][BF₄],[C_nmim][PF₆] Ionic Liquid Systems And Their Interface With Graphene Electrodes

As a new type of green materials,ionic liquids（ILs）have become important candidates for novel solvents and electrolytes in industrial applications.The main disadvantage of ILs as a novel solvent is their high viscosity,which can be significantly reduced by mixing ILs with ethanol of similar polarity.ILs/ethanol mixtures have shown excellent properties different from traditional solvents in catalytic reactions,organic synthesis and other fields.Graphene is a novel electrode material with high specific surface area,high electrical conductivity,and excellent chemical properties,and ILs/graphene has emerged as a promising electrolyte/electrode alternative.When ILs electrolytes are applied to supercapacitors,the energy storage of supercapacitors relies on the charge separation that occurs during charging,which is caused by the layer of ions at the interface.Therefore,the performance of supercapacitors depends on the interfacial structure of ILs/graphene.In this paper,pure 1-alkyl-3-methylimidazolium tetrafluoroborate[C_nmim][BF₄]（n=2,4,6）and 1-alkyl-3-methylimidazolium hexafluorophosphate[C_nmim][PF₆]（n=4,6）and its binary mixture with ethanol and the microstructure of five ILs on the surface of graphene electrode were analyzed,in order to explore the effect of cationic alkyl side chain length and anion size on the microstructure.Molecular dynamics（MD）simulations can accurately describe the microstructure of ILs and thus predict their macroscopic properties.In this paper,the microstructures of pure ILs,ILs/ethanol mixtures,and ILs/graphene interfaces were analyzed by MD simulation method.In pure ILs,radial distribution functions（RDFs）indicate that the electrostatic attraction between the smaller size[BF₄]^–and cation is strong,and cation-anion interactions are more likely to occur.The free space of anions and cations in the five pure ILs was analyzed by Voronoi Polyhedral（VP）,and it was found that the stronger hydrogen bond interaction between the ring hydrogen atom and the anion resulted in smaller free space around the ring hydrogen atom,and the terminal hydrogen H_M of the cationic methyl side chain and anion are the farthest away,and have more free space.In all five ILs/ethanol mixtures,the density values decreased with increasing ethanol mole fraction.RDFs show that ethanol does not penetrate between cations and anions,and[C₆mim]⁺cations with longer alkyl side chains are more strongly aggregated with ethanol.When interacting with ethanol,[BF₄]^–with the smaller radius is more preferential than[PF₆]^–interacting with ethanol.The free space of anions and cations in the ILs/ethanol mixture was analyzed by VP analysis,and it was found that more free space came from[C₆mim]⁺with longer alkyl side chains and[PF₆]^–with larger radius.In this paper,the nanostructures of ILs at the interface of uncharged and charged graphene electrodes were analyzed by simulating charged electrodes,and the charging electrode is simulated by applying a charge to graphene.Through the number density distribution,it is found that there is co-adsorption of cations and anions at the graphene interface,and the cations with longer alkyl side chains and[PF₆]^–with larger radius form a lower number density at the interface.With the increase of electrode charge,the oscillation of the layered structure of ILs is more obvious,and the enlarged mutual attraction leads to the accumulation of anions（cations）at the positive electrode（negative electrode）.Through the number density analysis of the terminal hydrogen,it was found that the terminal hydrogen of[C₂mim]⁺on the negative electrode surface tends to be closer to the electrode surface with the increase of the negative electrode charge.While the terminal hydrogen of the other four ILs showed the opposite trend,the terminal hydrogen of the cation extends away from the graphene wall as the charge of the negative electrode increases.Through the calculation of the diffusion coefficient（D）,it was found that the diffusion movement of ILs confined within graphene was suppressed,and the diffusion movement of the component perpendicular to the direction of graphene was the slowest.With the increase of the cationic alkyl side chain and the increase of the anion size,the diffusion coefficient decreases.