Study On Metal Oxide And Its Composite Materials For Supercapacitor Electrode

Supercapacitor is a new type of energy storage equipments with high power characteristics and good cycle life; it is also one of the hotspots in new type of chemical energy sources studies. In this thesis, we have reviewed the newest development in research of electrode material of supercapacitor devices and prepared relevant electrode materials. The microstructures and morphologies of these materials were investigated by 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:Several cobalt hydroxide,nickel hydroxide and nickel oxide materials are successfully synthesized by different chemical precipitation methods. The microstructures,morphologies and electrochemical performance of these materials were investigated. The results show that the metal oxide materal with layered structure and broad space between the layers has excellent electrochemical properties. The obtained high specific capacitance for the electrode materials is mainly attributed to the effective distributions of the pore size and the effective specific surface area, and for the factors which effect to the specific capacitance of electrode material, the effective mesoporous distribution > specific surface area.In order to validate the above results and to obtain electrode materials with high specific capacitance for supercapatitor, nickel-cobalt oxide nano-flakes materials and cobalt-manganese oxide nano-flakes materials are successfully synthesized by a facile chemical coprecipitation method. The structure characterizations show that the mixed metal oxides which composed of pure metal oxide have a less crystallization. The Co0.56Ni0.44 oxides possesses a narrow mesoporous distribution at around 2-7 nm. A maximum specific capacitance of 1227 F/g can be obtained for the Co_0.56Ni_0.44 oxide electrode, which is three times greater than that of pure NiO. The Mn_0.36Co_0.64 oxides possesses a narrow mesoporous distribution at around 2-20 nm and a broad porous distribution at around 20-140 nm. A maximum specific capacitance of 370 F/g can be obtained for the Mn0.36Co0.64 oxide electrode, which is seven times greater than that of pure Mn3O4. The electrochemical tests also show that the mixed metal oxides both have good charge-discharge properties at high current density. Since the size range of the hydrated ions in the electrolyte is typically 6-7.6 , and the pore size at the range of 8-50 ? is the effective one required to increase either the pseudocapacitance or electric double-layer capacitance. The obtained high specific capacitance for the synthesized mixed metal oxides materials is mainly attributed to the effective distributions of the pore size. Moreover, the mixed metal oxides both have a network-like microstructure which builds up of many interleaving thin nano-flakes. We assume that the unique structure plays a basic role in the morphology requirement for electrochemical accessibility of electrolyte OH to the mixed metal oxide active material and a fast diffusion rate within the redox phase. It is believed that this unique structure provides an important morphological foundation for the extraordinary high specific capacitances.The NaY molecular sieve was hydrothermally synthesized by direct crystallization, then USY and DUSY was synthesized by mend zeolite NaY with NH4Cl and HCl. Finally, the Ni(OH)₂/USY and Co(OH)₂/USY composite were synthesized by a liquid-precipitated method, which used the super-stable Y zeolite as the template, and we hven applied these materials in the supercapacitor fields. In the synthesize process of NaY molecular sieve, we can obtained the needed molecular sieve by control the synthesize condition. The results show that after USY is loaded with hydroxide, a salient morphology change has taken place on the outer surface of USY, it is important to note that the whisker structure shows anisotropic morphology extending from the exterior of USY to interparticle open space, and forming a lossely packed microstructure sizing down to a few nanometers with ample space between adjacent whikers. This highly dispersed active material possesses a very high specific surface area and a loosely packed microstructure in the nanometer scale. The special microstructure can accommodate the electrochemical accessibility of electrolyte ions to not only the surface but also in the solid bulk,which is very helpful for making full use of the electroactive sites to take place the faradic reaction, and contributed to the excellent capacitive characteristics. The composite of Ni(OH)₂/USY (50 wt% loading Ni(OH)₂) shows the specific capacitance of 1670 F/g, or 3360.7 F/g after correcting for the weight percentage of the Ni(OH)₂ phase. The composite of Co(OH)₂/USY (90 wt% loading Co(OH)₂) shows the specific capacitance of 716 F/g, or 795 F/g after correcting for the weight percentage of the Co(OH)₂ phase. It is obvious that after using high-surface-area USY zeolite as a template for the directed synthesis of redox-active nanocomposite, the specific capacitance of the active material in the composite electrode increased in most degree.Al-substitutedα-Ni(OH)₂ electrode materials were successfully synthesized by a simple chemical coprecipitation method. The results show that the presence of dissolved Al cations can suppresses theα→β-nickel hydroxide transformation in alkaline electrolyte, improve the reversibility of the electrode reaction, but can't increase the specific capacitance of theα-Ni(OH)₂ electrode. The effect mechanism is that the increasing of the positive charges between the brucite-type layer can enhance the bonding strength with the brucite-type layer, ensure the anion and H2O molecule not run out and stabilize the crystal lattice constant. The results showed that the 7.5% Al-containingα-Ni(OH)₂ materials exhibited high specific capacitance (2,098 F/g),better rate capability and excellent cycling reversibility, suggesting its potential application in electrode material for supercapacitors.An asymmetric supercapacitor has been constructed with nickel-cobalt oxide nano-flakes 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 corresponding potential window has increased from 0.4 to 1.6 V. The maximum specific capacitance of the asymmetric supercapacitor is 97 F/g and show better rate capability. All these profit from using the active carbon as the negative electrode materials with large surface and proper pore distribution, which ensure the Co_0.56Ni_0.44oxides precede with the faradic reaction in the larger applied potential range. The Co_0.56Ni_0.44 oxides/AC asymmetric supercapacitor has a good specific energy and power density, For example, the specific energy was 34.5 Wh/kg at a power density of 133.3 W/kg and still keeps 22.5 Wh/kg at a power density of 1333.3 W/kg.