Design And Preparation Of Meso-structural Carbon-based Nanomaterials And Their Supercapacitive Performances

Carbon materials are the most widely investigated and used electrode materials for electrical double layer capacitors (EDLC), which feature the advantages of abundant, low cost, variety of structures and morphologies, good conductivity, electrochemical stability, etc. The design and exploration of advanced carbon-based electrode materials is the frontier topic in the field of supercapacitors today. This dissertation focuses on the design, preparation, supercapacitive performances and the relative mechanism of novel mesostructural carbon-based nanomaterials. By applying the basic idea for preparing carbon nanocages by in situ MgO template method developed in our group, this study effectively regulated the surface wettability, pore structure and conductivity of the carbon-based nanomaterials by doping, pore size expansion or using new template. Three mesostructured carbon-based nanomaterials were obtained. The correlations between structures, physicochemical properties and EDLC performances were well investigated. The main progresses are summarized as followed.1. Hierarchical nitrogen-doped carbon nanocages. In our previous study, we obtained the novel hierarchical carbon nanocages (hCNCs) with large specific surface area, high conductivity and multiscale porous structure by in situ MgO template method, which demonstrate quite good EDLC performances in aqueous electrolyte. Due to the hydrophobic characteristic of pure carbon material, the ion-accessible surface area of the carbon nanocages is not high enough yet. In this study, by using pyridine as precursor, the hierarchical nitrogen-doped carbon nanocages (hNCNCs) were controllably prepared by in situ MgO template method, which also feature the large specific surface area, high conductivity and multiscale porous structure. In addition, the introduction of thed polar C-N bonds much improved the wettability of hNCNCs and reduced the charge transfer resistance and equivalent series resistance of the corresponding surpercapacitor. hNCNCs exhibits a large area-normalized capacitance of 17.4 μF cm-2, much superior to 11.8 μF cm-2 for the undoped one, and an ultrahigh specific capacitance up to 313 F g-1 at 1 A g-1 in 6 mol L-1 KOH aqueous electrolyte. The capacitance still maintain 234 F g-1 at 100 A g-1, demonstrating the excellent rate capability. The corresponding supercapacitor delivers the high energy density (10.90 Wh kg-1) and power density (22.22 kW kg-1). The supercapacitor displays an excellent cycling stability of～98% capacitance retention after 20000 cycles at the high current density of 10 A g-1.2.3D porous few-layer graphene (3DG). Nitrogen doping improved the wettability for hNCNCs but reduced the conductivity to certain degree. Increasing the wettability while maintaining or even enhancing the conductivity of carbon nanomaterials is an even better choice for acquiring higher rate capability and power density. Based on the fact that graphene grown on copper substrate has high conductivity, an in situ porous copper template method was developed to prepare 3DG with PMMA as solid carbon source. The obtained 3DG integrates the advantages of large specific surface area, hierarchical micro-meso-macropore open structure, and high conductivity. The oxygen-containing functionalities introduced during the template-etching process make 3DG exhibits quite good wettability. As EDLC electrode material,3DG presents the excellent supercapacitive performances in both aqueous and ionic liquid electrolytes. In aqueous electrolyte,3DG exhibits the specific capacitance of 231 Fg-1@1Ag-1 and 129 Fg-1@2000 Ag-1, demonstrating the ultrahigh rate capability. The corresponding supercapacitor delivers the high energy density (8.0 Wh kg-1) and power density (199.7 kW kg-1). The capacitance retention is-99% after 20000 cycles at the high current density of 100 Ag-1, presenting the excellent cycling stability. In ionic liquid of EMIMBF4,3DG exhibits the specific capacitance of 226 Fg-1@1 Ag-1 and 135 Fg-1@200 Ag-1, demonstrating the excellent rate capability. The corresponding supercapacitor delivers the high energy density of 125.5 Wh kg-1 approaching to the level for Li-ion batteries and high power density of 152.9 kW kg-1. The capacitance retention is-91% after 20000 cycles at the high current density of 100 Ag-1, also exhibiting an excellent cycling stability.3. Pore-size-expanded carbon nanocages by CO2 activation. Ionic liquid electrolyte has a wide operating voltage window favorable to high energy density, but usually has a large ion size and high viscosity. Carbon-based nanocages are full of the micropores in shells with size of ca.0.6 nm. This small pore size is not beneficial to the transfer of ionic liquid with large ion size (especially at high operating rates). In this study, CO2 activation was employed to expand the pore size of carbon nanocages, with the product noted as a-hCNCs, through Boudouard reaction (C+CO2=2CO). a-hCNCs has the larger specific surface area and micropore size than the un-activated hCNCs, with enhanced conductivity for the products prepared at 800 and 900℃. With EMIMBF4 as electrolyte, a-hCNCs exhibits an ultrahigh specific capacitance of 278 Fg-1@1 Ag-1 and 173 Fg-1@200 Ag-1, demonstrating the high rate capability. The corresponding supercapacitor delivers the ultrahigh power density and energy density of 480.1 kW kg-1 and 153.8 Wh kg-1, respectively. The capacitance retention is ～93.8% after 20000 cycles at a high current density of 100 Ag-1, demonstrating an excellent cycling stability. Without sacrificing the high rate capability, power density and cycling stability of EDLC, the energy density of 153.8 Wh kg-1 reachs the current level of Li-ion batteries. As compared with hCNCs, the much improved supercapacitive performances of a-hCNCs are ascribed to the larger specific surface area which provides more space for charge storage, the higher conductivity and the bigger micropore size which elevate the transfer rates of electron and ion.The three novel mesostructured carbon-based nanomaterials in this study could be conveniently prepared with low cost, which ensures them with broad potential applications as high-performance EDLC electrode materials.