| 超级电容器电极材料结构设计、合成及电化学储能机制研究 | |
| Alternative Title | Structure Design, Fabrication and Electrochemical Energy Storage Mechanism of Electrode Materials for Supercapacitors |
| 王大伟 | |
| Subtype | 博士 |
| Thesis Advisor | 成会明 |
| 2009-01-21 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 金属研究所 |
| Degree Discipline | 材料学 |
| Keyword | 超级电容器 电极材料 离子输运 氧化还原反应 |
| Abstract | 超级电容器是非常有前景的功率型电源器件，其研发重点为电极材料研究，包括储能机制、结构设计、材料合成和性能提升。然而，针对电极材料储能机制的研究非常匮乏，并因此制约了高性能电极材料的结构设计与可控合成。本论文主要通过多孔电极材料的电化学储能机制包括电极过程动力学和电极界面反应机理等方面研究，来发展高性能多孔电极材料的结构设计原则与可控合成技术。 首先提出了电场驱动离子输运是多孔电极材料储能的动力学控制步骤，受孔内电解液离子输运阻力和输运距离的影响。该机理基于三个因素(孔道长径比、孔道规整度和表面含氧官能团浓度)，全面考虑了多孔材料的孔道几何结构和表面化学性质，突破了孔径决定电极材料动力学性能这一传统观点。孔道长径比是几何结构判据，综合考虑孔长与离子输运距离以及孔径与离子输运阻力之间的相互关系，能够直观、可靠地比较不同多孔电极材料的动力学性能差异，可为电极材料的结构设计与合成提供指导。孔道规整度用于评价孔道结构的完整性，存在结构缺陷则规整度低；孔道结构缺陷可增强离子散射并降低电极材料的动力学性能，规整度高则动力学性能好。表面含氧官能团为极性基团，可增加极性电解液在电极材料表面的润湿性，并降低孔内电解液离子的输运阻力。选择具有可控孔道几何结构和可调表面化学性质(硼修饰、硝酸氧化)的有序中孔炭进行实验，并证明了上述机理。 在上述机制研究的基础上，首次提出了基于电解液在不同尺度孔内独特的物理化学性质的电极材料层次孔结构设计原则。大孔内电解液保持了体相电解液性质，可用于降低电解液离子在多孔电极材料颗粒内部的输运距离；中孔内电解液离子与孔壁碰撞概率低，可用于降低电解液离子在多孔结构中的输运阻力；大孔－中孔协同作用，可显著减小孔道长径比；微孔内的强电势可束缚离子，提高电极材料电荷存储密度。将大孔－中孔－微孔有机集成，可获得离子输运距离短、输运阻力小，并存储高密度电荷的高性能电极材料。采用无机多模板方法合成出层次孔炭电极材料。该方法具有原料廉价易得、设备简单、可规模化生产等特点。通过改变模板浓度和种类、炭化温度及气氛可控制层次孔炭材料的中孔孔径分布、石墨片层结构、孔道表面化学性质和宏观形态(粉末或块体)。研究发现含高导电性局域石墨片层结构的层次孔炭材料具有快速的离子输运过程和较低的等效串联电阻(80 mΩ)。该材料的功率密度指标超过PNGV标准(15 kWkg−1)，达到25 kWkg−1，同时能量密度可通过使用高电压电解液得到进一步提升(1 V电解液10 Whkg−1，2.3 V电解液18 Whkg−1，4 V电解液69 Whkg−1)。 炭材料表面电荷存储机制与表面化学性质和电子结构密切相关。采用氨气炭化方法制备含氮层次孔炭材料，含氮官能团主要为N-5和N-6结构，含有少量N-Q和N-X结构，并且含氮量可通过改变合成条件加以调节。实验发现根据含氮官能团在氧化还原反应前后的结构转化，其氧化还原反应存在两种主要机制：氮原子的直接氧化还原反应和氮原子改性的羟基氧化还原反应，并提出了三个具体反应式。通过理论计算发现氧化态官能团带相对正电荷，还原态官能团带相对负电荷，从电荷密度角度解释了上述反应机理。含氮官能团氧化还原反应动力学受含氮量影响。氮原子含有五个电子，其中三个参与成键，其余两个为孤对电子，不参与形成大π键，因而降低了载流子浓度和电子转移速率。随着含氮量增加，载流子浓度减小，电子转移速率降低，从而氧化还原反应动力学变差，增加氧化还原峰电势差。 氧化镍电极材料的主要不足是孔隙率低和导电率差，前者限制离子输运过程，后者限制电子传导过程，均不利于改善电极过程动力学。利用嵌段共聚物导向共沉淀方法合成出层次孔氧化镍电极材料，发现改变烧结温度可调节其大孔形貌、中孔尺寸和结晶度。提高烧结温度，可减小孔道长径比，增加结晶度和导电率，促进电极过程动力学。该材料容量可达225 Fg−1，高于球状中孔氧化镍。 根据锂离子体相储能包含液相孔内离子输运和固相孔壁内离子扩散双重动力学控制步骤的特点，提出调控多孔纳米材料(如纳米管)的孔道长径比和孔壁尺寸以分别促进孔内液相及壁内固相离子输运过程，实现提高电极过程动力学的目标。采用阳极氧化方法合成了结构可控的二氧化钛纳米管阵列，通过改变溶剂、氧化电压及时间可调节其管长、管内径及管壁厚。减小管腔长径比和管壁厚可有效促进电极过程动力学。在此基础上，组装出基于二氧化钛纳米管阵列的锂离子超级电容器，获得了基于电极材料质量计算的高能量密度(25~40 Whkg−1)和较高功率密度(3000 Wkg−1)，性能优于其他基于二氧化钛负极和多孔炭正极的锂离子超级电容器(等同计算方式)。 |
| Other Abstract | 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. |
| Pages | 164 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/17151 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 王大伟. 超级电容器电极材料结构设计、合成及电化学储能机制研究[D]. 金属研究所. 中国科学院金属研究所,2009. |
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