【作者】 张晶；

【导师】 康龙； 孔令斌；

【作者基本信息】 兰州理工大学 ， 材料学， 2010， 博士

【摘要】 超级电容器是一种新型的绿色储能器件,其储能性能介于二次电池和传统电容器之间。超级电容器的主要优点是充放电速率快、效率高;其主要缺点是能量密度低。如何提高超级电容器的能量密度已迫在眉睫。根据E=1/2CV2可知,提高超级电容器的能量密度可以通过两种有效的途径:一个是提高电极材料的电容值（C）。已进行的研究表明,高性能的超级电容器需要高性能的电极材料,电极材料不仅要求具有一般材料所具备的固体氧化还原性能,而且要求孔结构与比表面积的匹配性。基于此,本论文将具有多孔结构、大比表面积的介孔碳CMK-3作为载体,成功制备出Ni（OH）₂/CMK-3、Co（OH）₂/CMK-3和聚吡咯/CMK-3三种新型复合材料。三种新型复合材料拥有独特的多孔结构和大的有效比表面积,得到了高的比电容值;另一个提高超级电容器的能量密度的途径是增大电容器的工作电压（V）。为了进一步优化三种新型复合材料的电位窗口以提高能量密度,将复合材料与介孔碳组成混合型电容器。具体开展的研究内容如下:（1）将成功制备出的介孔碳CMK-3作为载体,利用简单的液相沉淀方法合成Ni（OH）2/CMK-3和Co（OH）2/CMK-3复合电极材料。利用XRD、SEM、TEM和BET等技术对电极材料的微观结构和形貌进行了分析。研究表明:Ni（OH）₂/CMK-3和Co（OH）2/CMK-3复合电极材料均具有大比表面积和分级多孔结构。复合材料的分级多孔结构分别存在微孔、介孔、大孔,三种不同尺度的孔结构。其中,大孔结构是由活性材料的纳米片堆砌而成,形成“离子缓冲水池”,为活性离子提供了快速进出电极表面的扩散通道;介孔结构是来自于CMK-3本身的孔结构,为活性离子扩散到电极的体相提供了导电通路,有利于降低离子扩散阻抗,使得离子扩散速率加快;此外,CMK-3介孔壁间的微孔结构可提供更大的双电层电容。复合材料中纳米片状结构具有较大的比表面积,有利于充分利用电极材料的电活性位发生氧化还原反应,使得该系列复合材料具有非常高的比电容值。在5mA/cm²的电流密度下,Ni（OH）₂/CMK-3 （15wt%CMK-3）和Co（OH）2/CMK-3 （20wt%CMK-3）复合物分别具有2570和753F/g的超高比电容值。（2）在（1）的基础上,将制备出的CMK-3在不同浓度的HNO₃溶液中进行表面修饰,通过化学氧化聚合的方法,将修饰后的CMK-3（m-CMK-3）作为载体与导电聚合物聚吡咯（PPy）结合,制备得到PPy/m-CMK-3复合材料。SEM研究结果表明,PPy薄层在载体m-CMK-3的碳纤维束上包覆,该复合物结构疏松,呈三维多孔结构,孔隙率增加,渗透性改善,有利于促使电解液中的活性离子扩散到电极表面和体相当中,发生氧化还原反应,产生大的法拉第赝电容。m-CMK-3的含量为18 wt%时,PPy/m-CMK-3复合材料比容量高达427 F/g。m-CMK-3的三维多孔结构、大比表面积和表面活性在优化PPy/m-CMK-3复合材料的结构上起了重要作用,使活性物质PPy更分散,提高了PPy的利用率。此外,良导体m-CMK-3载体会使得PPy/m-CMK-3复合电极材料的电阻系数减小,进一步提高了电极的大功率特性和电化学循环稳定性。（3）介孔碳CMK-3在浓HNO₃溶液中进行表面修饰,表面含氧官能团对CMK-3的比电容有明显提升作用,由145F/g增加到200 F/g。引入含氧官能团的CMK-3更适合于高功率超级电容器应用。考虑到金属氧化物和导电聚合物电位窗口均较窄,其功率特性尚需进一步提高。结合CMK-3材料自身优良的导电特性,提出以介孔碳基纳米复合物为正极,表面修饰过的介孔碳为负极组装混合型超级电容器。经过测试,基于Ni（OH）2/CMK-3、Co（OH）2/CMK-3和PPy/m-CMK-3复合电极的混合电容器的电位窗口得到大幅度的提高,三种电容器在5mA的充放电电流下,其比容量分别为92.5F/g、122F/g和57F/g。电化学性能的改善得益于以较大比表面积和适当孔径分布的CMK-3为负极,可以促使纳米复合物在较为宽的电位窗口内通畅地进行法拉第反应,维持其优异的电容性能。另一方面,电位窗口的提高,极大程度上改善电容器的功率密度和能量密度,尤其在较大的电流密度下更有优势。此外,三种混合电容器的都有具有优越的循环稳定性,1000次循环后比电容量均保持在90%以上。更多还原

【Abstract】 Supercapacitors have been recognized as unique energy storage devices which can fill the gap between conventional dielectric capacitors and batteries. It has the advantages such as quick charge-discharge ability and high charge-discharge efficiency. Meanwhile its defects such as low energy density can not be ignored. Therefore it is urgent to study the way of improving the energy density of supercapacitors. According to E=1/2CV², there are two methods that can improve the energy density effectively: One is to improve the specific capacitance of electrode materials. Studies on supercapacitors are mainly focused on the preparation of high performance electrode material. The electrode material must show not only the redox characteristic that is not possessed by general bulk material, but also the characteristic matching of the pore structure and the surface area. In this dissertation, the mesoporous carbon CMK-3 is used as the support materials, which yields mesoporous structure and large specific surface area. We have proposed a new strategy to synthesize a series of CMK-3-based nanocomposite, such as Ni（OH）₂/CMK-3, Co（OH）₂/CMK-3 and PPy/CMK-3 nanocomposite. The active materials with porous structure and large surface area on the surface of CMK-3 support can increase the utilization of active materials greatly, which could be attributed to its structure that allows the active material to be readily accessible for electrochemical reactions. Furthermore, the nano-size reduces the distance within the active materials over which the electrolyte must transport ions. The other of enhancing the energy density is to improve the cell potential of hydrid capacitor. In order to improve the power and energy performances, three new composite electrodes are made to be applied in hydrid supercapacitor. The main studies are as follows:（1） Mesoporous carbon CMK-3s are successfully synthesized by using hard template method. We have proposed a new strategy to synthesize a series of CMK-3-based metal hydroxides nanocomposite （Ni（OH）2/CMK-3 and Co（OH）₂/CMK-3）, which uses the CMK-3 as the support, and have applied these materials in the fields of supercapacitors. The microstructures and morphologies of these materials were investigated by XRD、SEM、TEM and BET measurements. Results have shown both Ni（OH）₂/CMK-3 and Co（OH）₂/CMK-3 nanocomposite yield large specific surface area and hierarchically porous structure. There are three types of pores with different nanoscales in both of composites. They are micropore, mesopore and macropore. In the composite, macropore formed by interconnected nickel hydroxide nanoflakes would provide a fast diffusion channel for electrolyte and act as the ion-buffering reservoirs to reduce the diffusion distances to the interior surfaces. Meanwhile, the mesoporous structure, mainly originated from CMK-3 mesopore wall, can provide low-resistant pathways for the ions through the porous structure, as well as a shorter diffusion route because of the ordered mesoporous channels. In addition, the micropores located within the mesopore wall are sopposed to be most efficient in a double-layered formation. The large specific surface area of the composite is very helpful for making full use of the electroactive sites for the faradic reactions, which can also contribute to the excellent capacitive characteristics. For example, the nanocomposite of Ni（OH）₂/CMK-3 （15wt% CMK-3） and Co（OH）₂/CMK-3（20wt% CMK-3） shows the specific capacitance of 2570 F/g and 753 F/g under a current density of 5mA/cm², respectively. The strategy has revealed great potential of the CMK-3-based nanocomposite materials, and obtained the best electrochemical capacitive values of supercapacitors.（2） Based on the results （1）, chemically modified CMK-3 is prepared by wet-oxidative method in HNO₃ solution with different concentrations. A novel PPy/m-CMK-3 nanocomposite is successfully prepared by in-situ chemical oxidative polymerization. The thin layer of PPy on m-CMK-3 with large space between ordered nanowires can be effective to obtain fully reversible and fast redox behavior, which contribute to the pseudo-capacititance. Electrochemical tests show that m-CMK-3/PPy nanocomposite with 82wt% PPy loading electrode reaches the maximum SC of 427 F/g under a current density of 5mA/cm². As the support of m-CMK-3, its unique porous structure, large specific surface area and surface activity have played an important role in optimizing the structure of PPy/m-CMK-3 nanocomposite, making active materials more dispersed as well as improving the availability of PPy. In addition, the introduction of m-CMK-3 makes the composite have higher conductivity, lower charge-transfer resistance, more active sites for faradaic reaction better rate capability, and better cycle performance.（3） After chemically modified CMK-3 in HNO₃ solution, the surface oxide groups could enhance the specific capacitance of CMK-3, whose specific capacitance is up to 200 F/g from 145 F/g. Mesoporous carbons introduced oxygen functional groups by oxidation treatment were more suitable for the application of high power density supercapacitor. In view of the low window potential of active material （Ni（OH）₂, Co（OH）₂, PPy）, the power density is hard to increase, which may limit the applications in the energy storage fields. Combined with the mesoporous carbon CMK-3 of good characteristics, a hybrid capacitor has been designed, using CMK-3-based nanocomposite and modified-CMK-3 （m-CMK-3） as positive and negative electrode, respectively. For example, as the Ni（OH）₂/CMK-3, Co（OH）₂/CMK-3 nanocomposite and PPy/m-CMK-3 composite electrode, results have shown that the asymmetric supercapacitor has excellent capacitance of 92.5F/g, 122F/g and 57F/g, respectively. The corresponding potential window has increased from 0.4 to 1.6V. All these profit from using the CMK-3 as the negative electrode materials with large surface and proper pore distribution, which ensures that the CMK-3-based nanocomposite proceed with the faradic reaction in the larger applied potential range. The hybrid supercapacitor exhibits improved power and energy performances by the increased potential window, particularly in the larger current density. Furthermore, the CMK-3-based nanocomposite and m-CMK-3 hybrid capacitor could be produced quickly and it possessed high charge-discharge efficiency and good cycle performance.更多还原