Synthesis And Electrochemical Capacitive Properties Of Nitrogen-Enriched Carbon-based Materials And Their Composites

Electrode materials have always been one of the key factors that affecting and restricting the development of supercapacitors.The exploration of electrode materials with low-cost and superior electrochemical performance is a top priority for the development of supercapacitors.Although great progress has been made on carbon materials,it is still unsatisfactory.The main reason is that pristine carbon materials store electric energy only through electric double layer capacitors,which results in their relatively low capacitance.The development of nitrogen-rich carbon-based materials seems to be a great promising solution.However,usually carbon materials with high nitrogen content are not easily to be prepared,and the state-of-the-art preparation techniquestend to be complicated and costly.More importantly,there are still many problems in nitrogen-rich carbon-based materials that need to beinvestigated.In this thesis,a series of nitrogen-rich carbon materials and their composites were designed and synthesized,and their supercapacitive performance were systematically studied.(1)Ultrathin g-C₃N₄ film was grown directly on the NiCo₂O₄ nanoneedle array/carbon cloth by a simple low-temperature CVD method,and then the ultrathin g-C₃N₄ film coated NiCo₂O₄ nanoneedle array with a novel"core-shell"structure was finally obtained.Characterizations show that the g-C₃N₄ film has a nanometer-scale thickness(1¹0 nm),such ultrathin thickness ensures that the g-C₃N₄ ultrathin film will not block the penetration of electrolyte and reactions between NiCo₂O₄ nanoneedles and electrolyte,on the contrary,it improves the mechanical stability of the NiCo₂O₄ nanoneedles and finally increases the stability of the whole electrode.Electrochemical tests show that the as-prepared ultrathin g-C₃N₄ film coated NiCo₂O₄ nanoneedle array/carbon cloth integrated electrode possesses high specific areal capacitance(2.83 F cm^-2 at a current density of 1 mA cm^-2)and excellent cycling stability(15 mA cm^-2 after 10,000 cycles,corresponding to 94%of the initial capacity).(2)Firstly,g-C₃N₄ was repaired by solvothermal method combined with post multi-step thermal treatment,then carbon self-repairing g-C₃N₄ nanosheets with high specific surface area,ultrathin thickness and porous structure were synthesized.Material characterizations show that the thickness of the g-C₃N₄ nanosheets is only about 3.5 nm,the specific surface area is as high as 220.7 m² g^-1,and the pores are dominated by mesopores.Secondly,a facile hydrothermal method was developed to synthesize carbon self-repairing porous g-C₃N₄ nanosheets/NiCo₂S₄ nanoparticles hybrid composite for supercapacitor electrode.In the hybrid composite,NiCo₂S₄ nanoparticles with high conductivity are uniformly distributed on the 2D porous g-C₃N₄ nanosheets.Electrochemical tests show that the composite can reach 1000 F g^-1 even at a large current density of 20 A g^-1,and its capacitance only loses 7.4%after 10,000 cycles.(3)Two-dimensional ultra-thin carbon nanosheets with ultra-high non-graphitic nitrogen content were synthesized by one-step pyrolysis method using low-cost chemicals melamine and D-glucosamine HCl as pyrolysis precursors.In this process,g-C₃N₄ acts as both self-sacrificing template and nitrogen source.Characterizations results show that the total nitrogen content of the carbon nanosheets is as high as 20.29 wt.%,and the non-graphite nitrogen content is as high as 17.36 wt.%;also,thanks to the g-C₃N₄self-sacrificial template,the as-synthesized carbon nanosheets feature two-dimensional(2D)ultrathin thickness(3⁴ nm)and porous structure.These novel characteristics make the as-prepared carbon nanosheets an excellent supercapacitor electrode material in terms of superior specific capacitance(316.8 F g^-1 at 1 A g^-1)and excellent cycling stability(without obvious capacitance loss after 10,000 cycles at 10 A g^-1).(4)Ultrathin non-graphitic nitrogen-doped carbon nanosheets/carbon nanotubes was successfully synthesized by one-step pyrolysis process by introducing nickel source.The gas NH₃ released during the pyrolysis process reduces Ni²⁺into catalytically active metallic nickel nanoparticles,which further catalyzes the formation of carbon nanotubes on the carbon nanosheet.In addition,the nitrogen atom is stabilized in the system by forming a Ni-N bond during pyrolysis,and finally the nitrogen content of the sample is maintained at a high level.Electrochemical tests show that the composite has a specific capacitance of279 F g^-1 at a current density of 0.5 A g^-1 and a capacitance of 240 F g^-1 at 8 A g^-1,indicating that the material has a high rate capacity.In addition,there is no significant capacitance loss after cycling 10,000 times at a current density of 10 A g^-1.