The Microstructure Of Ionic Liquids At Electrified Interfaces

Supercapacitor is a novel kind of energy storage device known to store energy in the electrical double layer?EDL?,and then it is especially important for improving the performance of supercapacitor to study the EDL structure and understand its effects.Ionic liquids with high operating voltage are considered to be good electrolytes used for supercapacitors.Carbon is currently the main electrode material,however when studying EDL structure,mica is an advantage material both in experiment and simulation because of its high mechanical strength as well as good atomic flatness,and the nature of negatively charged.In this thesis,molecular dynamic?MD?simulation was employed to research the structure of ionic liquids at electrified interfaces such as carbon and mica,and forecast the variation when applied to the supercapacitor.The main research contents include as follows:1)In the simulation of the interface between dicationic ionic liquids and carbon,it was found that the symmetric dication in IL[C₆?mim?₂]?Tf₂N?₂ is denser at and closer to the graphene electrode than the asymmetric one in IL[C₆?tma??mim?]?Tf₂N?₂ and the ordering of dication in[C₆?mim?₂]?Tf₂N?₂ at the interface is greatly increased after the carbon surface is charged,which is advantageous to enhance the capability of supercapacitor.2)During the progress of study the EDL structure,the simulation results of interfacial structure of three ionic liquids[C₂mim][Tf₂N],[C₂mim][BF₄]and[C₄mim][PF₆],at mica surface were compared with the results of atomic force microscope?AFM?experiment,and showed that the tip of AFM has ion selectivity,which means the tip only responses the position of ions with larger volume and mass,and is more sensitive with the volume.The concept of electrical neutral coefficient was introduced to explain that the selectivity is due to the tendency of the system between probe and surface to maintain electrical neutrality.3)A series of ionic liquids?[Cnmim][Tf2N],[Cnmpyr][Tf2N],[Cn?mim?2]?Tf2N?2,[Cn?mpyr?2]?Tf2N?2,n=4,6,8,10?were employed in this thesis to explore the interfacial structure at mica surface with different surface charge.The result shows that the change of monocation's structure caused by surface charge derives from the variation of the ion orientation.Two ion states of dications were observed at mica surface,one state with two electric groups on surface and the other with only one group on surface.For the latter state,the position of group far away from surface is independent of the length of carbon chain connecting the two groups in a dication but is relate to the surface charge density.