| 其他摘要 | Supercapacitors are promising for power applications. The key of supercapacitor technology is research and development of advanced electrode materials, which concerns about energy storage mechanism, structure design, materials synthesis and performance promotion. However, the specific investigation targeting at understanding energy storage mechanism is quite rare, and hence restricts the structure design and controllable fabrication of high-performance electrode materials. This dissertation focuses on the energy storage mechanism (including electrode kinetics and interfacial reaction), structure design principles and controllable synthesis techniques of high-performance porous electrode materials.
Ion transport driven by electric field, which is the kinetically controlling stage of energy storage process in porous electrode materials, depends on inner-pore electrolyte ion transport resistance and distance. The ion transport process is influenced by three criteria including pore aspect ratio, pore regularity and surface oxygen functional group population. This ion transport mechanism comprehensively considers the effects of geometrical structure and surface chemical property of porous electrode materials, representing a major breakthrough over the traditional concept that pore diameter determines electrode kinetics. The pore aspect ratio is a geometrical criterion, combining the synergistic effects of pore length and pore diameter, which enables the comparison of different porous electrode materials and guides the design of high-performance electrode materials. The pore regularity reflects the content of pore defects, the more the defects the lower the regularity. Since pore defects significantly scatter ions, the higher the pore regularity the better the electrode kinetics. Surface oxygen functional groups can enhance the polarity of pore surfaces and then reduce ion transport resistance. The above mechanism was rationalized by using ordered mesoporous carbon with tailorable pore structures and surface chemical properties (boron modification and nitric acid oxidation).
Hierarchical porous structure design is based on the different behaviors of electrolyte in differently-sized pores. Electrolyte in macropores, which maintains its bulk phase behavior, can reduce the transport distance of ions inside porous particles. Electrolyte ions have small probability to crash against pore walls of mesopores, and hence reduce ion transport resistance. Macropore and mesopore can synergistically minimize pore aspect ratio. The strong electric potential in micropores can trap ions and enhance charge storage density. Combination of macro-meso-micropores results in high-performance electrode materials with short ion transport distance, low resistance and large charge storage density. Hierarchical porous carbon was prepared by inorganic multi-template method, which is cheap, simple and scalable. The mesopore size distribution, presence of localized graphitic structures, surface chemical property and macroscopic form of hierarchical porous carbon can be controlled by adjusting template types, concentrations, carbonization temperatures and atmospheres. Hierarchical porous carbon with conductive localized graphitic structures has fast ion transport and small equivalent series resistance (80 mΩ). The power density can exceed PNGV target (15 kWkg−1) and reach 25 kWkg−1, and the energy density can be increased by using high-voltage electrolytes (1 V electrolyte 10 Whkg−1, 2.3 V electrolyte 18 Whkg−1, 4 V electrolyte 69 Whkg−1).
Surface charge storage mechanism on carbon surface is related to the surface chemical environment and electronic structure. Nitrogen-modified hierarchical porous carbon is obtained by ammonia-assisted carbonization. The nitrogen functional groups mainly comprise N-5 and N-6 groups, with a small amount of N-Q and N-X. The atomic concentration of nitrogen can be tailored by changing synthesis conditions. By tracing the structure evolution of nitrogen functional groups before and after redox reactions, we notice two major redox mechanisms, involving the direct redox reaction of nitrogen heteroatoms and the redox reaction of nitrogen-modified hydroxyl group. Theoretical calculation shows that oxidative groups are positively charged and reductive groups are negatively charged, which confirms the above redox mechanisms. A nitrogen atom has five electrons, three of which participate in bonding, while the rest of them stay in isolated electron couple. This decreases charge carrier density and hence electron transfer rate. The increment of nitrogen will reduce electron transfer rate, impede redox reaction kinetics and hence increase the potential separation of redox peaks.
Nickel oxide, typical of transitional metal oxide electrode materials for supercapacitor, has major drawbacks of low porosity and poor conductivity, which hinders the improvement of electrode kinetics. Nickel oxide with hierarchical porous structure was fabricated using block copolymer directed co-precipitation method. The macropore morphology, mesopore size and crystallinity can be adjusted depending on calcination temperature. Raising temperature reduces pore aspect ratio, increases crystallinity and conductivity, and hence improves electrode kinetics.
Inner-pore ion transport in liquid phase and inner-wall ion diffusion in solid phase are two kinetically controlling stages in bulk storage of lithium ion. The kinetics of this process can be tailored by adjusting pore aspect ratio and pore wall thickness. Titania nanotube arrays with different tube length, inner tube diameter and tube wall thickness were prepared by changing solvent, anodic oxidation voltage and period. Minimizing tube aspect ratio and tube wall thickness can improve electrode kinetics. A lithium ion supercapacitor based on titania nanotube array was constructed with large energy density (25~40 Whkg−1) and moderate power density (3000 Wkg−1), which are better than other type lithium ion supercapacitors. |
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