| Increasing energy demand, uncertainties in energy supply and global warming necessitate the development of sustainable energy conversion and energy storage systems, such as solar cells, batteries, fuel cells and supercapacitors. Supercapacitors (SCs) are one of the most promising high performance energy storage devices and are also referred to as electrochemical capacitors or ultracapacitors. SCs exhibit significant characteristics such as, low equivalent series resistance, fast power delivery or uptake, high-power density, high cycle efficiency and long life cycle.Generally, the electrode materials of supercapacitors are of three types, carbon materials, conducting polymers and transition metal oxides. Carbon materials have outstanding electrical properties, long life-cycles and great mechanical properties, but small double layer capacitance. Conducting polymers are inexpensive and flexible, but have poor cyclability. Transition metal oxide materials have high specific capacitance but poor electronic conductivity, and in some cases they are expensive. The three kind materials have their own advantages and disadvantages. Therefore, fabrication of binary or ternary composites with a combined and balanced merit of different materials is an effective way to obtain electrode materials with high supercapacitive performance. Graphene is an intriguing two-dimensional carbon material and emerges as an excellent electrode material for EDLC because of its high electrical conductivity and theoretical high surface area (2630 m2 g-1). Among the transition metal oxides, nickel and cobalt oxides (NiCo2O4) are considered as one of the most promising pseudocapacitor electrode materials due to its abundance, low cost, environmental harmlessness and high electrochemical performance. Hence fabrication of metal oxides (NiCo2O4) and graphene oxide (GO) composites as electrode materials of supercapacitor are triggering increased interests from both academic and industrial scientists.A simple hydrothermal process for fabrication of a nanocomposite comprising NiCo2O4 nanowires anchored on graphene oxide was developed by using CoCl2·6H2O, NiCl2·6H2O and graphene oxide as precursors and urea as precipitation agent. The NiCo2O4/graphene oxide (NiCo2O4/GO) composites materials were finally obtained after high temperature calcination. The morphology and structure of the NiCo2O4/GO composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and Raman spectroscopy. A highly dispersed NiCo2O4 nanowires, which can allow rapid electrolyte ions diffusion and fast electron transport, were formed on the surfaces of graphene oxide sheets. The NiCO2O4 nanowires have a length of 3-5 um and a diameter of 100-300 nm, and the sizes of NiCo2O4 nanowires are affected by the amount of NiCo2O4 loaded onto GO. The electrochemical properties of the NiCo2O4/GO composites were studied using cyclic voltammetry and galvanostatic charge/discharge measurement by a three-electrode system. The NiCo2O4/GO sample, which was prepared with 10 mL GO solution (0.5mg/mL),4 mmol CoCl2·6H2O?2 mmol NiCl2·6H2O and 2 mmol CO(NH2)2, exhibited the best electrochemical performance. A specific capacitance as high as 645.5 F/g was observed at 1 A/g within the operated voltage window (0 to 0.55 V) in 2 mol/L KOH aqueous solution. The value of specific capacitance is about 60% higher than that of pure NiCo2O4 (387 F/g). The excellent electrochemical performance of NiCo2O4/GO was interpreted to be due to the synergistic effect of NiCo2O4 nanowires and GO.By the same hydrothermal methods, the ZnCo2O4/GO and MnCo2O4/GO composite materials were also synthesized. The effect of GO solution concentration on the electrochemical behavior of the composite materials was investigated. It was also found that that with the use of GO solution (0.5 mg/mL), the specific capacity of ZnCo2O4/GO and MnCo2O4/GO composite materials were increased by 207% and 121%, respectively, at a current density of 1 A/g. The enhanced electrochemical performance can be attributed to the synergistic effect of ZnCo2O4(or MnCo2O4) and GO. |
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