| Because of social and environmental problems attributed to global warming and fossil fuel shortage, the needs for renewable new energy sources have encouraged researchers to develop advanced energy storage and conversion systems, such as solar cells and lithium-ion batteries. Among them, supercapacitor, also known as electrochemical capacitor, has attracted much attention as anew mold of energy storage device with excellent power density, good rate performance and cycle life. According to the energy storage mechanism, supercapacitors can be categorized into the electrical double layer capacitor on the electrode surface and the Faraday reaction energy storage capacitor on the electrode surface respectively. By comparison, the pseudo-capacitors exhibit better energy density than the double layer capacitor, and also have a good prospect as an ideal electrochemical active materials.Recently, layered double hydroxides, which have the unique layered structure with abundant ions and transition metals providing a rich source of active sites and electronic channels, have aroused a wide range of research interests. As a supercapacitor electrode material, it can also deliver double layer capacitance and Faraday capacitance.Additionally, high oxidation resistance, low cost,natural environmental protection are potential advantages of layered double hydroxides as a good pseudo-capacitor material.However, the relatively low conductivity limits the transfer of the electrolyte ions and electrons, and the metal oxide volume expansion effect influences the electrochemical performance. This research is based on cobalt - aluminum layered double hydroxides,using acid and alkali etching to build hierarchical structure of electrode material,improve the layered double hydroxides agglomeration and stacking. Moreover, it could improve the surface area of the material significantly,therefore the active sites are fully exposed to shorten the transmission of electrons and ions,furher to ameliorate the conductivity and the stability. Specific research work is as follows:1.With one step hydrothermal preparation of hdrangea shaped cobalt - aluminum layered double hydroxides,the morphology of the precursor was controlled by the precursor concentration. And subsequently,NaOH was applied to retain its morphology nder the premise of etching part of the hole made of aluminum to increase the specific surface area of the material. The porous structure not only provides more electrochemical active sites with higher specific capacity and but also can effectively prevent the volume expansion of the material to improve the cycle stability in the charging and discharging process. The materials exhibit the optimal electrochemical properties when the etching time was 4h. Under the current density of 0.5 A g-1, the specific capacity is as high as 873 F g-1. After the material is assembled into an asymmetric capacitor, the charge and discharge rate is still 79.9%.2. Cobalt-aluminum layered double hydroxides with two-dimensional hexagonal strcture was prepared by refluxing method. Then,the pH was adjusted in NaOH aqueous solution at room temperature, to obtain poly-dopamine coated two dimensional sheet layered double hydroxides, which was followed by the filtration film-acid etching-under the protection of nitrogen calcination-acid etching to fabricate a three-dimensional multilayer hollow capsule structure of carbon paper materials. With nitrogen protection, when the temperature is 650℃, the performance is the best. Given a high current density of 10 A g-1,the specific capacity is still as high as 152 F g-1.Assembled into the device, the material delivers a high retention rate of 85.6% after cycling 5000 tests. |
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