Electrochemical Behavior Study In Room Temperature Ionic Liquid-based Systems

Room temperature ionic liquid has great potential for electrochemical application due to its wide electrochemical window and good conductivity. The composition, structure and property of ionic liquids are much different from those of water and organic solvents; therefore the electrode system constructed by ionic liquids becomes one of the major and important subjects in electrochemistry research. The work presented in this thesis involves a series of electrochemical studies in room temperature ionic liquid solutions. The electrochemical deposition and polymerization, ionic liquid as electrolyte for supercapacitor, and electrochemical characteristics in ionic liquid microemulsions have been studied. The main contents of seven original studies are presented as follows:(1) Graphene oxide (GO) and Au(Ⅲ) were electrochemically co-reduced in an ionic liquid for preparing graphene/Au nanocomposites successfully, and the electrochemical property of the as-prepared graphene/Au nanocomposites are evaluated. The I-t curves recording the electrochemical reduction processes indicate that the simultaneous reduction of GO and Au(Ⅲ) is favorable to each other. The results of the X-ray diffraction patterns, scanning electron microscopy and transmission electron microscopy display the gold particles with a diameter of about10nm are dispersed uniformly on the surface of the electrochemical reduced graphene. Furthermore, the graphene/Au nanocomposites demonstrate great electrochemical catalytic activity and stability toward the oxidation of dopamine.(2) Polythiophene (PTh) was electropolymerized onto multi-walled carbon nanotubes (MWCNT) modified glassy carbon (GC) electrode in the ionic liquid BmimPF6solution successfully. The SEM images display that the PTh films with the morphology of nanofiber are wrapped with the MWCNT. The electrochemical tests indicate the PTh/MWCNT nanocomposites have better capacitive property than both pure MWCNT and pure PTh and possess the specific capacitance of110F/g. The result of the long-term stability of the supercapacitor demonstrates that the specific capacitance still remains90%of the initial capacitance after1000consecutive charge-discharge cycles, indicating good stability.(3) A new kind of supercapacitor constructed by using graphene as electrode material and ionic liquid as electrolyte was fabricated. The graphene prepared by chemical reduction of the graphene oxide was characterized by X-ray diffraction patterns and scanning electron microscopy. The performance of the supercapacitor was evaluated by electrical impedance spectroscopy, cyclic voltammetry and galvanostatic charge-discharge. The results demonstrate that the supercapacitor with the combined advantages of both graphene and ionic liquid shows enhanced performance with the specific capacitance, energy density and specific power density of132F/g,143.7Wh/Kg and30kW/Kg respectively.(4) The trace amount of water in the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4mpyrr][NTf2]) was removed through adding zirconium tetrachloride, subsequently the electrochemistry of zirconium tetrachloride in the dry [C4mpyrr][NTf2] is studied. The result suggests that ZrCl4is a facile and general ionic liquid drying agent for use in voltammetry. The one electron reduction of Zr(Ⅳ)/Zr(Ⅲ) is proved by chronoamperometry, accompanying an oxidation process forming a monolayer on the surface of the electrode.(5) Both IL and water are typical green solvents and have good electric conductivity. Water-in-ionic liquid (W/IL) microemulsion and ionic liquid-in-water (IL/W) microemulsion were prepared by mixing hydrophobic ionic liquid1-butyl-3-methylimidazolium hexafluorophosphate, TX-100and water. The electrochemistry behavior of Ag+in the two ionic liquid microemulsions was studied. The cyclic voltammetry of Ag/Ag+obtained from IL/W microemulsion system exhibits a crossover, which is different from that obtained from W/IL microemulsion system. The electrodeposits obtained from W/IL microemulsion system are nano-granular, while those obtained from IL/W microemulsion system are planar. The difference of the electrochemistry behavior and morphologies are attributed to the different microenvironments of the microemulsions. (6) Gold nanoparticles were electrodeposited directly from a new electrolyte system:water-in-ionic liquid (W/IL) microemulsion, the electrodeposition process was discussed. The cyclic voltammetry result of Au(Ⅲ) shows that the effect of the precursor apparent concentration on the reduction peak current density is similar to that in homogeneous solution such as aqueous solution, and the effect of scan rate on the reduction peak current density is different from that in homogeneous solution. The result of linear-sweep voltammograms on a rotating disk electrode in the W/IL microemulsion suggests that the rotating can accelerate the collision of the nano-pool to the surface of the electrode, as a result of accelerating the electrodeposition of Au. The activation energy of the electrode reaction is26.7KJ/mol. The gold electrodeposits characterized by scanning electron microscopy and X-ray diffraction are face-centered cubic and nanoparticles with a diameter of about25nm. In addition, the electrochemical properties of the gold nanoparticles toward the electrooxidation of glycerol are much higher than bare gold.(7) The nonaqueous ionic liquid (IL) microemulsions composed of1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4), Triton X-100and toluene were prepared. It’s shown that the nonaqueous IL microemulsion with good electrical conductivity and wide electrochemical window can be used as electrolyte for electrochemical research. The electrochemical properties of the nonaqueous IL microemulsions were studied by using potassium ferricyanide as electroactive probe. It is found that the reversibility of the [Fe(CN)6]3-/[Fe(CN)6]4-electrode reaction in the nonaqueous IL microemulsions is better than that in pure IL. However, the electrochemical behavior of the probe in the nonaqueous IL microemulsions is influenced by different microenvironments (oil-in-IL, IL-in-oil and bicontinuous microemulsions). The electrochemical property of the probe in the oil-in-IL microemulsion is the best.