Preparations Of Novel Ionic Liquids And Their Electrochemical Performances

Room Temperature Ionic Liquids(RTILs), known as room temperature molten salts, are composed of organic cations and organic or inorganic anions.They have zero liquid vapor pressure, non-flammablity, wide liquid state, high electric conductivity, wide electrochemical window, thus can be used as electrolyte in lithium-ion battery or supercapacitors. At present, ionic liquid cations of alkyl imidazoles, alkyl pyridine, alkyl piperidine, quaternary ammonium salt, phosphate salts and so on are commonly used. Meanwhile,anions such as trifluoromethyl sulfonyl imide, hexafluorophosphate, halogen,tetrafluoroboric acid salt, nitrate are used usually in ionic liquids. However, the ionic liquids composed of the above cations and anions such as alkyl imidazoles trifluoromethyl sulfonyl imide ionic liquid have high viscosity and low conductivity, having a big influence on the electrochemical properties as electrolyte. Thiocyanate has flaten structure, small size, and thus can improve the conductivity of ionic liquids, enhancing the electrochemical performance as electrolyte. In comparison withbiocaboxylate(HCO3-)based ionic liquids which are sensitive to air and water, the electrochemical performance of supercapacitor using alkyl imidazole bicarbonate ionic liquids as electrolytes can be improved.In this work, alkyl imidazole thiocyanate and alkyl imidazole bicarbonate ionic liquids are designed, fabricated, and characterized by 1H-NMR, FT-IR and TGA. Meanwhile the supercapacitors were assembled using graphene as the electrode and the as-prepared ionic liquids as electrolytes. The corresponding electrochemical properties of the supercapacitors were tested with cyclic voltammetry, AC impedance, and galvanostatic charge-discharge. The effects of organic solvents and the concentration of the ionic liquids in the electrolyte werealso studied. The main results of the work are summerized as follows:(1) It was demonstrated that the alkyl imidazole thiocyanate ionic liquids exhibit excellent thermal stability and [Emim]SCN possesses the higher ionic conductivity, which is up to 6.12 m S/cm at 25 ?C. The electrochemical window,internal resistance and the specific capacitance of graphene are strongly dependent on the alkyl chains of ionic liquids. The electrochemical window of ionic liquids increases with the increase of alkyl chain length. Specifically,[Bmim]SCN shows the widest potential window of 2.5 V, while, 1.8 V of[Pmim]SCN and 1.6 V of [Emim]SCN, respectively. The supercapacitor using graphene as electrode exhibits high specific capacitance of 250 F/g in pure[Emim]SCN electrolyte at 5 m V/s. Overall, the graphene electrode in the as-synthesized ionic liquids showss good stability at room temperature.(2) Different solvents and concentration can affect strongly the electrochemical performances of as-prepared ionic liquids as electrolytes in supercapacitor with graphene electrodes. Among several organic solvents including PC, AN, DMSO and DMF, the graphene shows the best electrochemical performance in [Emim]SCN/PC electrolyte. In addition, the electrochemical properties of graphene in [Emim]SCN/PC electrolyte are better than those in [Pmim]SCN/PC, and [Bmim]SCN/PC electrolytes. This can be attributed to that [Emim]SCN has relatively shorter alkyl chain, which results in the high conductivity of the electrolyte and thus improves the electrochemical performance of the supercapacitor. The concentration of ionic liquids exhibits a significant influence on the electrochemical performances of the supercapacitor.The supercapacitor exhibits high specific capacitance(161 F/g) at 10 m V/s in 2.0M [Emim]SCN/PC electrolyte, and has an excellent stability.(3) Compared with different organic solvents, the graphene electrode shows the best electrochemical performance in electrolyte of HCO3- based ionic liquids desolved in PC,too. In [Bmim]HCO3/PC electrolyte,graphene shows the best electrochemical properties among four kinds of ionic liquids with different alkyl chains. Moreover, the graphene electrode exhibits high specific capacitance(198.2 F/g) at 10 m V/s and a wide potential window(2 V) in 2.0 M[Bmim]HCO3/PC electrolyte. At a scan rate of 20 m V/s, the graphene electrode exhibits a specific capacitance of 148.7 F/g and slight decay of 9 % after 1000 cycles, indicating the excellent cycling stability.