Study On Composite Electrode Materials Of Supercapacitors With Ultrahigh Energy Densities Based On Mesoporous Carbon Support Materials

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.