Electrochemical Studies Of Electrical Double Layer And Charge Transfer Kinetics At Ag(111) Electrode/room-temperature Ionic Liquid Interface

Ionic liquids are a class of substance,which consist entirely of ionic species and are often fluid at or around room temperature.Compared with traditional electrolyte solutions,ionic liquids have many extraordinary properties,especially the wide electrochemical window,low vapor pressure and good ionic conductivity,which make them of much interest to electrochemists.However,unlike the extensive applications of ionic liquids in electrochemistry such as electrodeposition,electric condenser and electrocatalysis,the researches of both the double layer structure of and charge transfer kinetics at electrode/ionic liquid interface are still in their infancies.The models about the electric double layer and its potential distribution are imperfect.Especially,the surface etching and reconstruction of the gold monocrystalline electrodes mostly employed in current studies make the interfacial processes more complex.There are hardly any theoretical models of interfacial charge transfer kinetics,and most of the experiments on this subject are conducted on polycrystalline electrodes,so that the double layer structure and kinetics of charge transfer cannot be accurately correlated.Therefore,it is essential to employ monocrystalline electrodes whose surfaces would not go reconstruction or be etched in ionic liquids as substrates for further investigations of both electrical double layer and interfacial charge transfer kinetics,which not only helps obtain more information about interfacial structures and reaction kinetics but also favors the correlation between them.By utilizing in-situ STM(EC-STM),in-situ AFM(EC-AFM)force spectroscopy and electrochemical impedance spectroscopy(EIS),the interfacial structure investigations have been carried out on Ag(111)electrode in imidazolium-based ionic liquids,the Ag(111)undergoing no surface reconstruction and ionic liquids being broadly used in electrochemistry.Based on the results of the interfacial structure,focused work on the charge transfer kinetics at "Ag(111)/ionic liquids" interface have been conducted.The influence of crystallographic structure,cation alkyl side chain length as well as temperature are studied,and relevance between interfacial structures and reaction kinetics in ionic liquids is analyzed in detail.The main results are outlined as follows:1.Surface two-dimensional structures and vertical arrangement at"Ag(111)/[BMI][PF6]" and "Ag(111)/[EMI][TFSI]" interfaces are studied by EC-STM and EC-AFM force curve measurements.The electrical equivalent circuits based on the layered structures determined according to AFM force curves and combined STM characterization are put forward to fit the impedance spectroscopy,and the potential dependency of interfacial parameters are analyzed.Five molecular layers are formed in the ionic liquid side,including three charged layers and two neutral layers.In contrast,no layered structures are observed on polycrystalline Ag electrodes.These systems are favorable for the subsequent investigations of interfacial charge transfer kinetics.2.Potential step chronoamperometry,cyclic voltammetry and electrochemical impedance spectroscopy are performed to study and compare the redox kinetics of chromocene in[EMI][TFSI]on Ag(111)and polycrystalline Ag electrodes,and the kinetic parameters of the electrode reaction are determined by classical electrochemical methods.The charge transfer rate constant on Ag(111)is nearly a half of that on polycrystalline Ag electrode,which may be caused by the more stable and ordered interfacial layered structure on Ag(111).Most likely,the more compact double layer at Ag(111)/ionic liquid interface restricts the recombination of the solvent layer around chromocene so that the required energy of reaction is enhanced.3.The interfacial charge transfer process are studied by electrochemical impedance spectroscopy,and the impedance data are fitted with the same electrical equivalent circuit at all potentials.The fitted element parameters show explicit axisymmetric at formal potential,which is attributed to the restriction of EIS measurement,which cannot provide valid information in the limiting diffusion controlled region of potential.4.The influences of the alkyl side chains length of cations on chromocene redox kinetics on Ag(111)are systematically investigated using various ionic liquids with different cations(EMI+,BMI+ and OMI+)by electrochemical techniques.Both classical electrochemical and semiintegration methods are applied to determine the kinetic parameters.The results show that symmetrical factor and charge transfer rate constant become smaller as the length of alkyl side chain increases.Based on the consideration of the double layer structures in these systems,it can be related to the position and potential where the reaction occurred in the surface layer of ionic liquid.5.The electrochemical reaction of chromocene on Ag(111)in[EMI][TFSI]is studied at various temperatures.Semiintegration method and electrochemical software DigiElch are used to analyze the reaction kinetic parameters,their temperature dependencies and the diffusional and reaction activation energy.As temperature increases,the symmetrical factor remains almost free of changes,while the reorganization energy decreases.Furthermore,the difusional activation energy of chromocene is smaller than that of chromocenium,and the relationship between reaction activation energy and reorganization energy of chromocene changes with temperature.All of these are closely related to the interfacial double layer structure.