The Preparation Of Heteroatom-doped Graphene Aerogels And Their Application In Supercapacitors

As a new type of energy storage devices,supercapacitors have the advantages of large power density,short charge and discharge time,long cycling life,wide working temperature range,pollution-free,and maintenance-free,and thus was widely used in the fields of new energy sources,electric vehicles,and electronic devices.According to the difference of the storage mechanism,supercapacitors can be divided into electric double layer capacitors and Faraday capacitors.Electrode materials and electrolytes are the core of supercapacitors,and determine the performance of supercapacitors.Graphene,with a high specific surface area,good chemical stability,and excellent electronic conductivity,was widely used in the field of supercapacitors.The surface morphology,microstructure,pore structure of graphene can affect its electrochemical properties.In order to improve the specific capacitance of graphene-based supercapacitors,the chemical properties of graphene need to be modified by heteroatom.For electrolytes,the study is more concentrated in the increasing the conductivity of the electrolyte and reducing the viscosity and widening the electrochemical window.Using ionic liquids as electrolyte can increase the energy density of supercapacitor.In this thesis,different heteroatom-doped graphene aerogels were synthesized from different precursors.The surface morphology,structural composition,specific surface area of the materials were analyzed.The electrochemical properties of these materials were systematically studied,main contents are as follows:1.Using GO as precursor,hydrazine hydrate and sulfur powder as nitrogen source and sulfur source,respectively,nitrogen and sulfur co-doped graphene aerogel（NS-GA）was prepared by one-pot process.The NS-GA was characterized by X-ray diffraction,X-ray photoelectron spectroscopy,and Raman spectrometer.The results show that graphene oxide undergoes nitrogen and sulfur doping while being reduced in the hydrothermal process.The as-prepared materials were investigated as supercapacitor electrodes in an ionic liquid（1-ethyl-3-methylimidazolium tetrafluoroborate,EMIMBF4）electrolyte.Notably,the NS-GA-5 based on the supercapacitor delivered an energy density of 100.7 Wh kg^-1 at a power density of 0.94 kW kg^-1 and 45.2 Wh kg^-1 even at a large power density of 20 kW kg^-1.The power density and energy density of this supercapacitor are much higher than those of commercial supercapacitors,indicating the superiority of heteroatom co-doped graphene.2.Three nitrogen-doped graphene aerogels were prepared by using guanidine phosphate,guanidine carbonate and guanidine hydrochloride as reductants and dopants,and GO as precursor.The nitrogen contents of these three graphene aerogels are high（PRGA,CRGA and HRGA）,the main forms of nitrogen element in graphene frame are pyridine nitrogen,pyrrole nitrogen and quaternary nitrogen.The electrochemical results show that the specific capacitances of PRGA,CRGA and HRGA are 179,148 and 193 F g^-1,respectively.The PRGA,CRGA and HRGA based on the supercapacitors also have good cycling stability.After1000 cycles of charge and discharge,the capacitance retentions of PRGA,CRGA and HRGA are 85%,81%and 86%of their initial values,respectively.3.Nitrogen-doped graphene was prepared by one-step hydrothermal method using urea phosphate and GO as raw materials.Nitrogen-doped graphene aerogels（PUGA-10,PUGA-30and PUGA-5）were obtained by optimizing the ratio of urea phosphate to GO.The as-prepared materials have mesoporous structure.Electrochemical results show that PUGA-30 not only has a higher specific capacitance,but also has a higher energy density and power density based on the supercapacitor.