Study On Metal Hydroxide/activated Carbon Hybrid Supercapacitor With High Capacity

Electrochemical capacitors, also called supercapacitors, which combine the advantages of both dielectric capacitors that can deliver high power within a very short time and rechargeable batteries that store high energy, have found an increasing important role in power source applications such as hybrid electric vehicles, short-term power source for mobile electronic devices, etc. In this thesis, the basic principle of electrochemical capacitors, the research developments of electrode materials were reviewed, and prepared relevant electrode materials. The microstructures and morphologies of these materials were investigated by TGA,XRD, SEM,TEM and BET measurements. The electrochemical performance has been evaluated by galvanostatic charge-discharge, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS). The main content is the following:Cobalt hydroxide nanoflakes are successfully synthesized by a facile chemical precipitation method. An asymmetric supercapacitor has been constructed withα-Co(OH)2 as the positive electrode and activated carbon as the negative electrode, and was fabricated in the 2 M KOH electrolyte. The results show that the material have a network-like structure, which consists of interconnected nanoflakes, shows anisotropic morphology characteristics and the formation of a loosely packed microstructure in the nanometer scale, and it corresponds to the layeredα-Co(OH)2 structure with low crystallinity. The BET specific surface area of the Co(OH)2 materials was 85.4 m2/g, and possess a narrow mesoporous distribution at around 4-20 nm. The unique structure plays a basic role in the morphology requirement for electrochemical accessibility of electrolyte OH- to Co(OH)2 active material and a fast diffusion rate within the redox phase. Electrochemical test indicated that the specific capacitance was 735F/g and demonstrated its potential application in electrochemical capacitors. Theα-Co(OH)2/AC hybrid supercapacitor possess a very high potential window, high energy density and high power density. The maximum specific capacitance and energy density of the asymmetric supercapacitor for a cell voltage between 0 and 1.6 V are 72.4 F/g and 25.8 Wh/kg,respectively. Moreover, the hybrid supercapacitor also exhibited a good electrochemical stability with 93.2% of the initial capacitance over consecutive 1000 cycle numbers.Stabilized Al-substitutedα-Ni(OH)2 materials were successfully synthesized by a chemical coprecipitation method. An asymmetric supercapacitor has been constructed withα-Ni(OH)2 as the positive electrode and activated carbon as the negative electrode, and was fabricated in the 2 M KOH electrolyte. The experimental results showed that that the presence of dissolved Al cations can suppresses theα→β-nickel hydroxide transformation in alkaline electrolyte. The 7.5% Al-substitutedα-Ni(OH)2 materials exhibited high specific capacitance (2080 F/g) and excellent rate capability due to the high stability of Al-substitutedα-Ni(OH)2 structures in alkaline media, suggesting its potential application in electrode material for supercapacitors. Theα-Ni(OH)2/AC hybrid supercapacitor possess a very high potential window, high energy density and high power density. The maximum specific capacitance and energy density of the asymmetric supercapacitor for a cell voltage between 0.4 and 1.6 V are 127 F/g and 42 Wh/kg, respectively. Moreover, the hybrid supercapacitor also exhibited a good electrochemical stability with 82% of the initial capacitance over consecutive 1000 cycle numbers.