| With the increasing consumption of fossil fuels, energy crises and environmental pollution are increasingly serious, which put forward a more severe challenge to human health, energy security and environmental protection, and so develop new clean energy is urgently need. Fuel cells and lithium-ion battery, which is expected to solve the above problems, have small environmental pollution and high energy conversion efficiency, but the development of electrode material with excellent performance is the key to improve the electrochemical performances of fuel cells and lithium-ion battery.In this thesis, a series of doped carbon-based nanomaterials were prepared by taking advantage of the confined spaces of layered hydroxides, and the relationship between the synthesis process and the structure of the materials was studied, the ORR electrocatalytic and Li strorage performances were tested and evaluated, and the relationship between material structure and electrochemical performance was investigated deeply. The main research contents are as follows:1. Selective edge-positioning of S and N in dual-doped carbon nanosheets (NSCNs) were prepared based on the interlayer confined effect of magnesium-aluminum layered double hydroxides (MgAl-LDH),and the ORR electrocatalytic and Li storage performance and mechanism were investigated. Metanilic anions were first intercalated into the layers of MgAl-LDHs by a facile hydrothermal process, and then the NSCNs were obtained after calcination and acid etching. The as-prepared NSCNs were composed of many interconnected nanosheets, showed abundant hierarchical micro-/mesopores and high specific surface area. In this method, the nanospace confinement of LDH layers not only facilitate the formation of planar pyridinic and pyrrolic N (up to 90.3% at 900 ?), but also alleviate N loss at high temperature. Edge-doping not only introduces abundant active sites at the edges but also induces the formation of hole defects with more exposed active sites, which strongly promote the ORR catalytic activity and Li storage capacity. Used as ORR electrocatalysts, the NSCNs exhibited excellent ORR activity in alkaline electrolytes, compared to a commercial Pt/C catalyst, with much better tolerance for methanol oxidation and higher stability. Used as the anode materials for Li-ion batteries, the NSCNs exhibited super-high reversible specific capacity (2240 mAh g-1 at 0.2 A g-1 after 100 cycles), excellent rate capability (983 mAh g-1 at 4 A g-1) and long-term cycling stability (950 mAh g-1 at 4 A g-1 after 500 cycles).Furthermore, XPS measurements and DFT calculatiton results show doped pyrrolic N atoms can combine with Li+ to form Li3V during charge-discharge cycling and dissociate from the carbon structure, and then more edge carbon atoms are formed for Li storage, but the pyridinic N and quaternary N atoms can not dissociate from the carbon structure.2. Hollow carbon sphere composites composed of Co9S8/N-doped carbon nanosheets (Co9S8/NHCS) were prepared based on the interlayer confined effect of CoAl-LDH, and the ORR electrocatalytic performances were investigated. Metanilic anions were first intercalated into the layers of CoAl-LDHs by a facile hydrothermal process, and then the Co9S8/NHCS were obtained after calcination and selective acid etching. The as-prepared Co9S8/NHCS show the unique hollow spherical structure composed of numerous porous carbon nanoflakes, and monodisperse Co9S8 nanoparticles are embedded within the carbon nanoflakes. The advantages of this structure are threefold: Based on nanospace confinement method, monodisperse Co9S8 nanoparticles can be obtained, which make the catalytic active sites are exposed and easily accessible resulting in high catalytic active; The carbonization of metanilic anions was accompanied by the formation of Co9S8 nanoparticles make the Co9S8 nanoparticles embedded within the carbon nanoflakes, leading to a good stability; The unique hollow structure of precursor lead to the formation of hollow structure and hierarchical pore architecture, which improve the transportation properties of the electrolyte ions, reaction intermediates, and products during the ORR process.Futhermore, the effects of calcination temperature and hollow sphere structure on the structure and catalytic activity of catalysts were systematically investigated, and the results indicated 900 ? is the optimal temperature and the hollow sphere structure have an important role in enhancing the ORR catalytic activity. Electrochemical measurements show the Co9S8/NHCS catalysts prepared at 900 ? exhibit superior oxygen reduction reaction activity, excellent durability and tolerance for methanol oxidation in alkaline and acidic media.3. Hollow carbon sphere composites composed of carbon nanosheets containing Co-Nx species (Co-N/C) were prepared based on the interlayer confined effect of CoAl-LDH, and the ORR electrocatalytic activity and its active sites in various pH medium were investigated. Metanilic anions were first intercalated into the layers of CoAl-LDHs by a facile hydrothermal process, and then the Co-N/C was obtained after calcination and acid etching.The as-prepared Co-N/C exhibited a unique hollow spherical structure composed of numerous carbon nanoflakes containing Co-Nx species, showed abundant hierarchical micro-/mesopores and high specific surface area.Electrochemical measurements show the Co-N/C catalysts prepared at 900 ?exhibit excellent ORR catalytic activity with a high half-wave potential and large diffusion-limited current in alkaline and neutral solutions, which are comparable to, even higher than that of commercial Pt/C electrocatalyst, and show excellent durability and tolerance for methanol oxidation. Furthermore,through meticulous investigating of the effects of mask ion (SCN- and F-) on the ORR activity of Co-N/C catalysts, and comparing the ORR activity before and after the destruction of Co-Nx sites in different pH medium, we propose that the Co-Nx sites directly as the ORR active sites in acidic and neutral solutions, but have a negligible effect on the ORR activity in an alkaline environment.4. Cobalt sulfide and N doped carbon nanosheets composites with flower-like morphology (Co9S8/Co1-xS@NC) were prepared based on the interlayer confined effect of Co(OH)2, and the formation mechanism and Li storage performance were investigated. Metanilic anions were first intercalated into the layers of Co(OH)2 by a facile hydrothermal process, and then the Co9S8/Co1-xS@NC were obtained after calcination of the intercalated Co(OH)2 precursor and S powder under N2 atmosphere. The as-prepared Co9S8/Co1-xS@NC show a flower-like morphology, including cobalt sulfide nanoparticles with small particle size partially embedded within the nitrogen-doped carbon nanosheets and few-layer graphene covered the external surface of cobalt sulfides. Through investigating the carbonization/vulcanization mechanism of intercalated Co(OH)2 precursor, we conclude that the decomposition and carbonization of metanilic anions occurred from 200 to 400 ? to form the carbon nanosheets, which is accompany by the sublimation of S, the decomposition of Co(OH)2 layers and the simultaneous formation of Co1-xS, therefore, the Co1-xS nanoparticles can be embedded within the carbon nanosheets. With increasing of the temperature, part Co1-xS is gradually converted to Co9S8 and the graphitized degree of the carbon nanosheets is further improved. The structure advantages of Co9S8/Co1-xS@NC are as follows: the cobalt sulfides with small particle size could shorten the diffusion length of Li+ and alleviate their volume strain during .the Li+ insertion/extraction process, which distinctly enhance the cycling stability and rate capability; The N-doped carbon matrix and few-layer graphene outside of the particle surface not only prevent the particle aggregation and reduce the particle-to-particle interfacial resistance, but also effectively accommodate the volume expansion of cobalt sulfide nanoparticles and reduce the dissolution of polysulfide intermediates in the organic electrolytes, leading to good cycling performance; The thin particle-nanoflake structure can greatly shorten the diffusion distance for both electrons and Li+,which make the cobalt sulfide nanoparticles are fully utilized, resulting in high specific capacity; The flower-like morphology with large specific surface area and hierarchical pore structure facilitated the fast transport of Li+ and electrons,and allow easy penetration by the electrolyte into the inside of electrode.Furthermore, the composition of cobalt sulfides could be easily controlled by adjusting the annealed temperature and the mass ratio of precursor and S powders. When evaluated as an anode material for LIBs, the flower-like Co9S8/Co1-xS@NC-0.75 composites obtained at 900 0C showed excellent Li strorage performances.The confined reaction method of layered hydroxides presented in this thesis can be extended to the preparation of other carbon-based functional materials for supcapacitors, solar cells, sensors, environmental protection and catalysis. |
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