Molecular Dynamics Simulation Study On The Structure And Properties Of The Interface Of Imidazolium Ionic Liquid/gold Electrod

Ionic liquids,as a high-performance and designable green electrolyte,have been widely used in energy devices and electrochemical systems.Since the ionic liquids/electrode interface is the site where electrochemical reactions occur in these systems,so the interfacial structure and properties have a significant effect on the interfacial chemical reactions.Different anions and cations can be combined to form up to 10¹⁸ kinds of ionic liquids.The changes in anions and cations will inevitably alter the double-layer structure of the interface,thereby affecting the electrochemical reaction of the system.Therefore,it is of great significance to systematically reveal the influence of anions and cations on the double layer structure of the ionic liquids/electrode interface and its impact on electrochemical processes.In this thesis,a combination of molecular dynamics simulation and quantum chemical calculations was used to study the effects of substitution number and alkyl chain length of the cation ring,as well as different anions on the double layer structure and electrochemical properties of the imidazole ionic liquids/gold electrode interface system at the atomic（molecular）scale.The results will provide theoretical basis and assistance for the application of ionic liquids in the field of electrochemistry.The electrical double-layer structures and property of the 1-ethyl-3-methylimidazolium bis（trifluoromethylsulfonyl）imide（[Emim]TFSI）ionic liquids/gold electrode system have been studied by quantum chemical and molecular dynamics simulation.Three surfaces of gold electrode such as Au（100）,Au（110）and Au（111）were considered.It was demonstrated that without applying an electric field,higher the surface energy of the gold electrode surface is,the lower interaction energy between the cations or the anions with the electrode surface is.Among the three surfaces,the interaction between[Emim]⁺and the electrode surface is slightly greater than that between TFSI^-and the electrode surface.The worm-like patterns on the electrode surface were found for[Emim]⁺and TFSI.When an electric field was applied,the higher the surface energy of the electrode,the higher the interaction energy between cations and the electrode surface is,but the interaction energy between anions and the electrode surface is also related to the orientation of cations.Regardless of whether an electric field is applied or not,cations exhibit parallel distribution on Au（110）and inclined distribution on Au（100）and Au（111）.The stronger interaction between anions or cations and the charged electrode is,the more aggregation of anions and cations on the electrode surface is,resulting in a thinner double-layer and a larger differential capacitance.The quantum chemical calculations and molecular dynamics simulations are used to reveal the interfacial electrical double-layer structures and property of different substituted imidazole ionic liquids on the Au（100）surface.Three ionic liquids of 1-methylimidazolium bis（trifluoromethyl-sulfonyl）imide（[Mim]TFSI）,1-ethyl-3-methylimidazolium bis（trifluorometh-ylsulfonyl）imide（[Emim]TFSI）,1-ethyl-2,3-dimethylimidazolium bis（trifluoromethylsulfonyl）-imide（[Emmim]TFSI）were studied.It was shown that the number of cation substitutions increased,the charge density on the imidazole ring increased,and the interaction between the cation and the electrode enhanced,resulting in an increase in the interaction energy.It was found that ions are distributed inclined on Au（100）surface presenting a worm-like pattern.However,when the interaction between one ion and Au（100）is about twice or more than that of other ions,the ions are arranged in parallel on the surface showing a linear pattern.In the[Mim]TFSI system,ETFSI^-/Au（100）≈2E_[Mim]⁺/Au（100）results in a linear pattern of anions.E_[Emmim]⁺/Au（100）≈2ETFSI^-/Au（100）was found in[Emmim]TFSI system,which exhibits a linear pattern of cations on Au（100）.At the same time,the conformation of TFSI^-changes from trans to cis due to the stronger interaction between[Emmim]⁺and Au（100）which increase its orderliness and presenting a linear pattern.When applying an electric field,the interaction and orientation change trend between anions,cations,and electrodes are almost same as that no electric field is applied.However,due to the influence of the electric field,[Emmim]⁺changes from parallel orientation to slightly incline.Due to the stronger interaction between anions or cations and Au（100）in the[Emmim]TFSI system,the distribution of anions and cations are tight,and the double-layer on Au（100）is thin,resulting in a better electrochemical performance.The interfacial electrical double-layer structures and property of different alkyl chain length of imidazole ionic liquids on the Au（100）surface have been investigated by quantum chemical calculations and molecular dynamics simulations.Three ionic liquids like 1-ethyl-3-methy-limidazolium bis（trifluoromethylsulfonyl）imide（[Emim]TFSI）,1-butyl-3-methylimidazolium bis（trifluoromethylsulfonyl）imide（[Bmim]TFSI）and 1-hexyl-3-methylimidazolium bis（trifluoro-methylsulfonyl）imide（[Hmim]TFSI）were investigated.It was observed that as the alkyl chain length increased,the charge density on the imidazole ring increased,and the interaction between the cation and electrode enhanced,resulting in an increase in the interaction energy.It can be seen that without applying an electric field,the interaction energy between cations and Au（100）in[Bmim]TFSI and[Hmim]TFSI systems is approximately twice that of anions.Therefore,[Bmim]⁺and[Hmim]⁺exhibit linear patterns on Au（100）surface,while anions display worm-like patterns.When applying an electric field,the interaction between cations and Au（100）surface decreased as the alkyl chain length increased.The cations become tilted and sparsely arrange on the surface,and the double-layer on Au（100）is thick,resulting in a worse electrochemical performance.The quantum chemical calculations and molecular dynamics simulations are used to reveal the interfacial electrical double-layer structures and property of imidazole ionic liquid composed of anions with different structure on the Au（100）surface.Three ionic liquids,1-butyl-3-methylimidazolium tetrafluoroborate（[Bmim]BF₄）,1-butyl-3-methylimidazoliumhexafluoro-phosphate（[Bmim]PF₆）and 1-butyl-3-methylimidazolium bis（trifluoromethylsulfonyl）imide（[Bmim]TFSI）,were selected.Regardless of whether an electric field is applied or not,the bottom three fluorine atoms of the tetrahedral（wedge-shaped）tetrafluoroborate anion（BF₄^-）and the bottom fluorine atoms of the octahedral（spherical）hexafluorophosphate anion（PF₆^-）faced towards the Au（100）electrode.The TFSI^-stood upright on Au（100）surface with one trifluoromethyl（CF₃）facing towards the surface and another CF₃ group facing towards bulk.It was demonstrated that without applying an electric field,the higher electronegativity of anion central element,the lower and higher interaction energy between the cation and anion with the electrode surface,respectively.Meanwhile,since the interaction energy between the cations and Au（100）are more than twice than that of anions,the cations exhibited linear arrangements,while the anions form the worm-like patterns.When applying an electric field,the higher electronegativity of anion central element is,the lower interaction energy between the[Bmim]⁺or the anion with the Au（100）electrode is.In addition,a higher volume ratio of[Bmim]⁺to BF₄^-in the[Bmim]BF₄ system leads to the formation of a thinner double-layer on the Au（100）surface.Conversely,a smaller volume ratio of[Bmim]⁺to TFSI^-in the[Bmim]TFSI system results in a thicker double-layer on the electrode.Therefore,the system with larger volume ratio of cations and anions has better electrochemical performance.