| As a member of chemical power source family, supercapacitor combines the advantages of both conventional capacitors and rechargeable batteries. It has not only much more energy than conventional capacitors, but also much higher power density than rechargeable batteries. It is well known that the key for developing high performance supercapacitor depends upon the development of the high performance electrode materials, thus the research and development of electrode materials have become a hot topic in both academy and industry field during the recent years. In this study, the carbon microbeads (CMB) were prepared by glucose as raw materials following several steps:hydrothermal preparation, ambient drying and pyrolysis. The as-prepared CMB was treated in concentrated nitric acid in order to gain the highly activated carbon microbeads (ACMB). Besides, Mn3O4/ACMB composite, MnO2/ACMB composite and asymmetric supercapacitors were prepared, and their structure and electrochemical performances were investigated, respectively.The CMB were prepared by glucose as raw material. In order to improve the surface structure and electrochemical performance of CMB, the as-prepared CMB was treated in concentrated HNO3 to gain the highly ACMB. The morphology of CMB and ACMB were examined by fourier transform infrared spectrometer (IR) and scanning electron microscopy (SEM). The results showed that capacitive performance of CMB was greatly improved after being treated by concentrated HNO3 due to the increase of the oxygen-containing functional groups on their surface. The specific capacitance of ACMB is 291.9 F g-1 at a scanning rate of 1 mV s-1. Meanwhile, the specific capacitance of the button supercapacitor was as high as 75 F g-1 and the loss of specific capacitance was nearly neglectable after 5000 cycles.In order to combination of the high double layer capacitance of the ACMB and the high pseudo-capacitance of transition metal oxides, the MnO4/ACMB composites with various Mn3O4 ratios were prepared by a chemical co-precipitation method. The morphology of Mn3O4/ACMB composite was examined by SEM. The results showed that nano-sized Mn3O4 particles were randomly encapsulated on the surface of ACMB. The electrochemical measurements indicated that the Mn3O4/ACMB composite electrode had much better electrochemical performance than ACMB. The specific capacitance of Mn3O4/ACMB composite electrode reached 409.8 F g-1 when the content of Mn3O4 was 40%. Although the supercapacitor used Mn3O4/ACMB as active electrode material has an initial capacitance loss, the specific capacitance after 200 cycles stabilized nearly at a fixed value.The MnO2/ACMB composite electrode material was prepared by an in-situ coating technique. The SEM results show that y-MnO2 nanofibre was uniformly encapsulated on the surface of ACMB which demonstrated a dandelion-like appearance. As evidenced by cyclic voltammetry, galvanostatic charge/discharge test and cycle life measurements, MnO2/ACMB composite showed excellent capacitive performances. Moreover, the specific capacitance of MnO2/ACMB composite reached 375.9 F g-1. The capacitance retention of the supercapacitor using MnO2/ACMB composite as electrode active material was up to 93% after 1000 cycles.In order to get the higher energy density and power density, an asymmetric supercapacitor was constituted by using the MnO2 nanofiber with urchin-like appearance as positive electrode and the ACMB as negative electrode. The results showed that the asymmetric supercapacitor exhibited excellent electrochemical capacitive behavior. The specific capacitance of asymmetric supercapacitor is 85 F g-1 at 0.5 A g-1 in the voltage of 0-1.6 V, and 95% of initial capacity was retained after 100 cycles. The energy density is 43.2 Wh kg-1 at a power density of about 180 W kg-1 and still keeps 24.8 Wh kg-1 at a power density of 2880 W kg-1. |
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