Synthesis And Application Of Graphene Based Nanomaterials In Ionic Liquid

Graphene has been widely applied in renewable energy storage,catalysis and electronics ascribed to its high specific surface area,good thermal stability,high conductivity and excellent mechanical properties.Room temperature ionic liquids have been widely used in catalytic and electrochemical applications due to their negligible vapor pressure,wide electrochemical window,high conductivity and thermal stability.Compared with water and traditional organic solvents,ionic liquids have obvious advantages in composition structure and property,therefore ionic liquids are promising candidates as supercapacitor electrolytes and green media for organic synthesis.This thesis focuses on preparations of a series of graphene-based composite materials by using chemical methods.The morphologies and structures of the as-prepared composite materials are well characterized,and the supercapacitive,electrochemical and interphase interface catalytic performance of the as-prepared materials are studied using ionic liquids as electrolyte.The main contents in this thesis are summarized as follows:?1?Three-dimensional nitrogen-doped reduced graphene oxide hydrogel?N-RGOH?was successfully synthesized by simple one-step hydrothermal reduction of graphene oxide aqueous solution in the present of ammonium bicarbonate.The results showed that the as-prepared N-RGOH exhibited 3D porous structure with a large specific surface area(335.6 m² g^-1)and a high nitrogen content?10.8 at%?.The electrochemical performance of the N-RGOH was investigated in 1-butyl-3-methylimidazolium hexafluorophosphate[BMIM]PF6 ionic liquid by cyclic voltammetry and galvanostatic charge-discharge.Based on the synergistic effects of porous structure and nitrogen-doping,the N-RGOH based supercapacitor delivered a specific capacitance of 48.6 F g^-1 at 0.5 A g^-1,which equaled to the specific capacitances of 194.4 F g^-1 of the N-RGOH electrodes.Simultaneously,the N-RGOH based supercapacitor exhibited good electrochemical stability.On account of a wide working voltage of 3.2 V for the ionic liquid electrolyte,the energy density of the N-RGOH supercapacitor can reach up to 94.5 W h kg^-1.?2?Porous nitrogen-doped graphene?PNG?has been prepared via simple thermal treatment of graphene oxide and urea,and the morphology and structure of the PNG have been characterized by using a range of electron microscopy,X-ray photoelectron spectroscopy and other techniques.The electrochemical performances of the PNG have been investigated in an ionic liquid electrolyte by cyclic voltammetry and galvanostatic charge-discharge.The PNG electrode delivers a specific capacitance of 310 F g^-1 at 1 A g^-1 with good cycling stability over 4000 cycles.Meanwhile,the ionic liquid electrolyte enables a very wide working voltage of 3 V.leading to a high energy density up to 163.8 W h kg^-1.?3?Three-dimensional graphene hydrogel?RGOH?was successfully prepared by simple one-step hydrothermal reduction of grapheme oxide aqueous solution with poly dimethyldiallylammonium chloride?PDDA?used as promoter.SEM images show that the as-prepared RGOH exhibited interconnected porous structure,XRD and Raman results show that the RGOH was successfully reduced but there were still some defects on the surface of RGOH after the hydrothermal process.The capacitive performances of the RGOH electrodes were studied in benzoquinone?C₆H₄O₂?/1-butyl-3-methyl imidazole four fluorine boric acid salt?[BMIM]BF4?ionic liquids as a redox electrolyte.The results show that C₆H₄O₂/BMIMBF4 used as electrolyte can significantly increase the capacitance performance of RGOH-based supercapacitor(specific capacitance of 49.5 F g^-1)and energy density(114.2 W h kg^-1).?4?Three-dimensional nitrogen doped graphene?N-RGO?was firstly prepared by thermal treatment,and RuO₂ was decorated on the N-RGO to obtain RuO₂/N-RGO by a hydrothermal method.SEM,XRD and XPS results show that RuO₂ nanoparticles were uniformly dispersed on the surface of nitrogen doped reduced grapheme oxide,and the porous structure is formed by the nitrogen doped reduced graphene oxide sheets and nitrogen atoms.The capacitive performance of the RuO₂/N-RGO have been investigated in H₂O/[BMIM]BF4 ionic liquid electrolyte by cyclic voltammetry and galvanostatic charge-discharge.The RuO₂/N-RGO exhibited good capacitance performance with a specific capacitance of 305 F g^-1 at a current density of 0.5 A g^-1,the specific capacitance only decreased 9%after 2000 cycles.The results demonstrated that the addition of water had negligible influence on the working voltage of[BMIM]BF4 ionic liquid,and the electrochemical window can still reach 2.8 V.Simultaneously,the addition of water could effectively promote the redox reaction of RuO₂ in the electrolyte,greatly improving the pseudocapacitance of the electrode material.Hence.the energy density of RuO₂/N-RGO electrode can reach 160.6 kg W h^-1.?5?Silver?Ag?nanoparticles supported on nitrogen doped reduced graphene oxide hydrogel was successfully prepared by the hydrothermal reduction using ethylene glycol and ammonium bicarbonate as reducing agent and nitrogen source,respectively.SEM results show that the Ag/N-RGOH exhibited interconnected porous structure and Ag nanoparticles were evenly dispersed on the surface and porous framework of N-RGOH.The catalytic properties of Ag/N-RGOH on electrochemical reduction of benzyl chloride were investigated by cyclic voltammetry and chronoamperometry.The electrochemical testing results show that the Ag/N-RGOH possessed positive potential and high peak current density on electrochemical reduction of benzyl chloride,indicating good catalytic activity of the Ag/N-RGOH.?6?Palladium?Pd?nanoparticles supported on reduced graphene oxide?Pd/RGO?was successfully in-situ prepared in 1-butyl-3-methylimidazolium hexafluorophosphate?[BMIM]PF6?ionic liquid?Pd/RGO-IL?by phase transfer method from water.The prepared Pd/RGO catalyst was characterized by transmission electron microscopy,X-ray photoelectron spectroscopy,X-ray diffraction,Fourier transform infrared spectroscopy and thermogravimetric analysis.In situ synthesis of Pd nanoparticles were uniformly dispersed on the surface of RGO with an average size distribution of 2 nm.The as-prepared Pd/RGO-IL exhibits good catalytic activity and long-term catalytic stability for Heck reactions in IL.The first product conversion is 95%and still keeps 91%after being cycled for ten times.The TEM images of the Pd/RGO after the tenth run show that Pd nanoparticles are still stably suspended in the IL with an average size of approximately 2.0 nm and there is almost no agglomeration during the long-term reaction.This may due to the double protection of the organic cation[BMIM]+ of IL and the evenly dispersed RGO in the IL.Hence,Pd nanoparticles supported on reduced graphene oxide show remarkable long-term catalytic stability in IL.