Hydrothermal Preparation And Electrochemical Performance Of Nonmetal Heteroatom-Doped Graphene Aerogels

In recent years,supercapacitors have become a research and application hotspot in the field of new energy.Although traditional supercapacitors can maintain high specific capacitance and energy density,however,they still have poor charge-discharge rate performance and short cycle life,which are mainly due to the structure and specific surface area of the electrode material.Carbon is the basic element that constitutes the living organism and has always played a leading role in the history of human development.Graphene is a planar connected structure composed of a single layer of carbon atoms with sp~2 bonding,and the perfect crystal structure leads to excellent thermal conductivity,electrical conductivity,large specific surface area and easy-to-modify surface chemistry,which can be widely used in energy fields.The three-dimensional porous graphene aerogel(GA)is formed by two-dimensional graphene sheets through chemical cross-linking.It not only has the excellent electrical conductivity,thermal conductivity and mechanical strength of graphene,but also has extremely high porosity.Compared with traditional high specific surface area materials,the effective surface area of GA as electrode materials for supercapacitors does not depend on the distribution of pores in the solid state.The electrode material with three-dimensional connected porous structure has endless potential in new green energy fields such as supercapacitors,and it is a weathervane in the future energy storage field.However,graphene is a semi-metal/semiconductor material with zero band gap,low switching ratio and large leakage current,which leading to the impossibility to be applied in the field of electronic devices on a large scale.In order to improve the electrochemical performance of graphene supercapacitors,the band gap of graphene can be opened by non-metallic heteroatom chemical doping methods,and the graphene energy band structure can be adjusted to increase its specific capacitance.In this thesis,GA with diverse non-metallic heteroatom chemical doping were synthesized rapidly with graphene oxide(GO)as precursors.The electrode materials were physically characterized and their electrochemical properties were analyzed to obtain the electrode materials with the best performance.The main research contents are as follows:(1)The silicon-doped graphene aerogels(Si-GA-5%,Si-GA-10%,Si-GA-15%,Si-GA-20%)have been prepared with a doping concentration of 5%,10%,15%,and 20%by means of one-step hydrothermal synthesis,in which GO and silicon dioxide are adopted as carbon and silicon source,respectively.The hydrothermal method not only achieves the self-assembly of GA,but also successfully doped Si atoms into the graphene lattice,thereby improving its electrochemical performance.Characterizations show that the Si-GA has a porous structure.The specific capacitance of Si-GA-15%reaches 464.4 F/g at 1 A/g current density,meanwhile,the specific capacitance can still reach 300 F/g at 50 A/g,and the capacitance remains to 64.6%,and the capacity retention rate is 95.94%after 1000 cycles(10 A/g).Therefore,Si-GA prepared in this experiment has great potential as a supercapacitor electrode.(2)We also prepared boron-doped graphene aerogels(B-GA-5%,B-GA-10%,B-GA-15%,B-GA-20%)with different doping concentration by means of one-step hydrothermal synthesis,in which boric acid is adopted as boron source.In the hydrothermal process,GO is effectively reduced and stably doped with boron.The content of C and B elements in B-GA-15%is 75.16%and 4.02%,and the thermal stability is excellent.In the three-electrode system,the specific capacitance of B-GA-15%at current density of 1 A/g can reach 254.4 F/g,and the capacitance retention rates at current density of 50 A/g are 70.75%.The capacitance retention rate of 1000 cycles under 10 A/g current density reach 95.99%.The results confirm that B-GA has great advantages as a supercapacitor electrode.(3)The nitrogen-doped graphene aerogels(N-GA-5%,N-GA-10%,N-GA-15%,N-GA-20%)have also been prepared with different doping concentration,in which melamine with high nitrogen content is adopted as nitrogen source.Physical characterization confirmed that N-GA has a rich pore structure and good thermal stability.Among them,N-GA-15%is mainly dominated by pyrrole nitrogen and the nitrogen content is as high as 15.69%.The specific capacitance of N-GA-15%at a current density of 1 A/g can reach 243.2 F/g,and the capacitance retention rate at a current density of 50 A/g reaches 67.85%,meanwhile,the capacitance retention rate of 1000 cycles at a current density of 10 A/g reaches 96.35%.Therefore,N-GA has rich application value in the field of energy storage.