Synthesis And Supercapacitor Performances Of Three-dimensional Nitrogen-doped Graphene Hydrogels

Graphene,a novel two-dimensional sheet of sp2-hybrized carbon,is composed of free-standing carbon atoms which are densely stacked into a six-angle type honeycomb crystal structure.A mount of ?-conjugate system existed in graphene skeleton,which are contributed to good electrical conductivity,thermal stability and the excellent mechanical.Graphene can be considered as an ideal candidate for electric double-layer capacitor(EDLC)electrode materials.However,restacking between the lamellae limits its industrial application.Because of ?-? interaction and Van der Waals force existed in graphene sheets.Doping graphene with nitrogen elements can tailor its band structure,modulate its physical and chemical properties and facilitate its practical applications.In recent years,graphene has been proposed to be used in field effect transistors,supercapacitors,lithium ion batteries,fuel cells,sensors and so on.Supercapacitors have been recognized as a new energy storage device,which exhibit high energy density,power density,long cyclic life and fast charging/discharging rate.However,supercapacitors have the lower energy density and power density than that of lithium ion batteries.In addition,supercapacitors of liquid electrolyte are easy to leak,even blast.High cost and high internal resistance are also the constraints.The drawbacks limited their further applications.Recently,most researchers in this area have been focused on the preparation of different electrode materials.Graphene hydrogels haVe attracted widespread attention for both theoretical studies and applications due to its excellent mechanical strength,electrical conductivity,thermal stability,adsorption capability and lightweight nature.Presently,researchers reported kinds of strategies to prepare 3D graphene architectures,including hydrothermal methods,chemical reduction,polymerization,and cross-linking reactions.Therefore,it's still a great challenge of the research on supercapacitor to look for the green and efficient electrode materials.Meanwhile,it is worthwhile to explore another new technology to assemble 3D architecture of graphene-based.In this thesis,a simple one-pot way for preparation of nitrogen-doped graphene hydrogel is reported and systematically studied their structures and capacitiVe properties.Based on the above experience results,three dimensional nitrogen-doped graphene hydrogel/a-Ni(OH)2(NGHs/a-Ni(OH)2)nanocomposite was successfully achieved by hemogenous precipitation method with NGHs as raw materials and Ni(NO3)2·6H2O as nickel source,urea aqueous solution as hydrolysis condition.The structures and capacitive properties of this nanocomposite were also systematically studied.The dissertation is mainly focused on:(1)Synthesis and performances of NGHs.In this thesis,we choose graphene oxide(GO)as precursors and urea as reducing-doping agents to prepare three dimensional nitrogen-doped graphene hydrogel through one-step hydrothermal route.The as-generated materials were characterized by Fourier transform infrared spectroscopy(FTIR),Raman spectroscopy,X-ray powder diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),nitrogen adsorption-desorption analysis and electrochemical measurements.All the results revealed that nitrogen elements were doped into the graphene plane by three styles(pyridinic N,pyrrolic N,and graphitic/quaternary N)at the same time GO sheets were reduced,and the content of nitrogen in the range of 6.04-6.84 at.%was obtained by adjusting the weight ratio of GO to urea.In addition,the NGHs have lightweight nature(the density of hydrogel is ca.0.81?0.94 g/cm3,and the density of aerogel is ca.0.021?0.028 g/cm3)and numerous interconnected three dimensional porous networks.Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)displayed NGHs with a mount of three-dimensional porous structure and highly wrinkled and overlapping structures.Electrochemical characterizations demonstrated good capacitive behaVior(230.6 F/g,at 0.5 A/g)and superior cycling stability(capacitance retention?90%after 1200 cycles)of NGHs.At last,the possible mechanism of assembly process of NGHs and the possible N insertion pathway in the hydrothermal route were also proposed.During the hydrothermal process,urea continuously release NH3,which reacts with the oxygen functional groups covered on GO sheets to produce intermediates(e.g.amide,amine).Once the intermediates undergo dehydration(pyridine,pyridone)or decarbonylation(e.g.pyrrole)reaction,relatively stable structures were formed by amidation process and reorganization of unsaturated carbon and N atom incorporation.Finally,N atom was successfully doped into graphene framework by three styles(2)Synthesis and performances of NGHs/?-Ni(OH)2 nanocomposite.A series of NGHs/?-Ni(OH)2 nanocomposites with different electrochemical activities were prepared via two-step method by controlling the mass ratio of NGHs to Ni(OH)2 on NGHs as raw materials and Ni(NO3)2·6H2O as nickel source,urea aqueous solution as hydrolysis condition.In more detail,firstly,we prepared NGHs according to the aboVe method.Then,we explored a serial of experiment conditions,such as the optimum precipitant,the optimum reaction temperature and time,the optimum mass ratio of NGHs to Ni(OH)2.The results prove that the optimum nanocomposite was obtained with the reaction temperature was 85 ? and the reaction time was 6 h.Furthermore,it was found that stirring was contributed to Ni(OH)2 nanosheets deposited NGHs nanosheets.Electrochemical properties were characterized by cyclic voltammetry,electrochemical impedance spectroscopy and galvanostatic charge/dis-charge.Electrochemical tests showed that the optimum mass ratio of NGHs to Ni(OH)2 was 1:30,which exhibit excellent capacitive behavior.The specific capacitiance of the NGHs/a-Ni(OH)2 nanocomposite is 1542.1 F/g at 50 mV/s.SEM results revealed that with the increase of mass ratio the nanocomposites had more wrinkles and Ni(OH)2 nanosheets agglomerated into spherical structures.The results proVe that the Ni(OH)2 with this spherical structures have high specific surface area and electrochemical utilization rate.Additionally,the possible mechanism of nanocomposites was also proposed,which was hemogenous precipitation method.In a word,urea was disintegrated at about 70 ?:(NH2)2CO+3H2O(?)2NH4OH+CO2 ?.Then,NH4OH uniformly distributed the mixture solution,which lead to Ni(OH)2 spherical structures deposited into NGHs.