Preparation And Properties Of Hydroxides, Oxides/Graphene Composites As Electrode Materials For Supercapacitor

Supercapacitors have attracted much interest as one of the energy storage systems. They can be used either by themselves as the primary power source or in combination with batteries or fuel cells. Moreover, they exhibit such high power density that they can complement the deficiency of rechargeable batteries in the energy storage field. The researeh work of supercapacitor mainly focuses on the electrode material. Transition metal oxides/hydroxides are attractive materials for electrodes of supercapacitors due to their high specific capacitance. More recently, there has been a significant interest in exploring graphene as ideal electrode materials with its theoretical surface areas of 2630 m²/g. Also, its chemical stability, high electrical and thermal conductivity, and mechanical strength and flexibility are attractive as conformal electrode materials particularly for flexible supercapacitors. This dissertation has investigated the elecortde materials preparation and capacitive property. The mains are as follows:1. ZnO/reduced graphene oxide composites were synthesized by using a two-step method in which KOH reacts with Zn(NO₃)₂ in the aqueous dispersions of graphite oxide (GO) to form Zn(OH)₂/graphite oxide precursor, followed by thermal treatment in air. It was found that the dispersion of reduced graphene oxide (rGO) sheets within composites was a key for achieving excellent capacitive performance of samples. On the other hand, the mass ratio of ZnO to rGO determined whether rGO sheets within composites were dispersed or agglomerated. The composite achieved homogenous incorporation of rGO sheets within the ZnO matrix when the mass ratio of ZnO to rGO was equal to 93.3:6.7. This composite, in which weight percent of rGO was only 6.7%, appeared in the SEM images to be almost entirely filled with the rGO sheets coated by ZnO and exhibited high specific capacitance and excellent cycling ability. Furthermore, the sheets overlapped to form a three-dimensional network structure, through which electrolyte ions easily access the surface of rGO or electrochemical active sites. The homogeneously incorporated rGO sheets were shown to provide 128% enhancement in specific capacitance as compared with 135F g^-1 for pure zinc oxide sample. Besides, the unexpected phenomena involved in the experimental processes were discussed in detail. 2. Composites of SnO₂ nanoparticles supported by reduced graphene oxide (SnO₂/RGO composites) have been fabricated through a simple chemical route in a water system. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) observation of composite with the mass ratio of 76.1:23.9 (SnO₂:RGO) reveals that SnO₂ nanoparticles (about 4-8nm in size) homogeneously locate on graphene sheets. Interestingly, the morphology features of RGO in composites are significantly influenced by the number of supported SnO₂ nanoparticles. In the composite, SnO₂ act as spacers to effectively prevent the agglomeration of graphene sheets. Furthermore, the graphene sheets with good electrical conductivity server as a conductor for fast electron transfer between the active materials and charge collector, as well as buffered spaces to accommodate the volume expansion/contraction during discharge/charge process. The composite used as electrode materials in supercapacitors was testified to exhibit a high specific capacitance of 326F/g at the specific current of 1A/g in 1M H₂SO₄ electrolyte with rather excellent cycle life along with about 97.2% specific capacitance retained after 1000 cycle tests. These encouraging results indicate that the electrochemical performance of as-prepared nanocomposites could be enhanced remarkably by the positive synergistic effect between RGO and SnO₂.3. The sheet-like Al-dopingα-Co(OH)₂ was successfully prepared using cobalt chloride and aluminum nitrate as the raw material and polyethylene glycol as the structure-directing agent by a simple chemical coprecipitation method. The structure and morphology were characterized using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), respectively. The components of products were analyzed by FT-IR and thermogravimetry (TG). The electrochemical performances were investigated by cyclic voltammetry and constant current charge/discharge techniques. It was found that the molar ratio of Co²⁺ to Al³⁺ had a significant influence on electrochemical capacitive behaviors of the resultant products. The electrochemical measurements showed that the Al-dopingα-Co(OH)₂ with optimal molar ratio up to 8:2 (Co²⁺ to Al³⁺) exhibited excellent specific capacitance (1108F/g). Furthermore, there was only 6.9% decay in the available capacity over 500 cycles.4. Al-substitutedα-Co(OH)₂/GO composites with supercapacitive properties were prepared by chemical co-precipitated method in which cobalt nitrate and aluminum nitrate were used as the raw material, and graphite oxide was employed as carrier. The as-prepared materials were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and fourier transform infrared spectroscopy (FT-IR). Cyclic voltammetry (CV) and galvanostatic charge/discharge measurements showed that the Al-substitutedα-Co(OH)₂/GO electrode material had excellent electrochemical capacitance. The specific capacitance of 1137F·g^-1 was achieved in 6 M KOH solution at a current density of 1A·g^-1 within a potential range of 0–0.5V. Moreover, only 12% losses of the initial specific capacitance were found after 500 cycles at a current density of 1A·g^-1.