Synthesis Of Coal Based Carbon Nanofibers Composite Materials And Its Application In Supercapacitor

As novel energy storage devices, supercapacitor with high power density, fast charge/dischargerate and long cycle life, have the enormous potential applications in the field of energy storage. With the development of customer electronics（mobile phones, digital cameras and laptop） toward being portable, flexible, miniaturized, supercapacitor with high power density have to be designed and fabricated to meet the requirements of market. At present, how to further improve the electrochemistry properties has become a research hotspot in the field of flexible supercapacitor. The electrode materials are crucial for the performance of supercapaciors. The preparation of high-performance electrode materials is an attractive areas of research in the study of supercapacitor. In this paper, heishan coal as raw material was oxidized by using a mixture of HNO3（63%） and H2SO4（98%）（1:3, volume ratio）. The acid treatment process broke down the weak bond of coal macromolecules. The solubility of acid treated coal will improve in DMF. We used the solution of polyacrylonitrile and acid treated coal as the precursor of electrospinning, finally obtained the flexible coal based carbon nanofiber（CBCNFs）. We use CBCNFs as support to fabricate the CBCNFs/PANI and CBCNFs/MnO₂ composites. The electrochemical properties of the samples were tested. This thesis mainly includes the following four parts:（1） We used the acid treated coal/polyacrylonitrile as the precursor of electrospinning. The flexible CBCNFs was obtained after the preoxidation and carbonization of the precursor fiber cloth, and the electrochemical properties of the CBCNFs were explored. The results show that the CBCNFs exhibit a high specific capacitance of 233 F/g at a current density of 0.5 A/g in 6 M KOH electrolyte. The capacitance retention ratio of the CBCNFs remained 90% after 2000 cycles at a current density of 2 A/g.（2） The CBCNFs were well modified by using a mixture of HNO3（63%） and H2SO4（98%）（1:3, volume ratio）, a large number of oxygen containing functional groups were introduced. The coal based carbon nanofibers/polyaniline（CBCNFs/PANI） were prepared by a simple rapid-mixture polymerization of aniline（ANI）. Ammonium peroxydisulfate（APS） was used as the oxidant, H2SO4 was selected as the doping agents. The morphology of CBCNFs/PANI nanocomposites can be controlled by changing the concentration of the aniline monomer. In order to acquire the optimal supercapacitive performance, we change the concentration of H2SO4 and molar ratio of APS/ANI to fabricate CBCNFs/PANI nanocomposites. The results of electrochemical tests indicated that at the aniline concentrations of 0.015 M, the H2SO4 concentration of 2 M, and the molar ratio of APS/ANI of 1:1,the obtained CBCNFs/PANI-2 hybrid electrode not only exhibits a high specific capacitance of 458 F/g at a current density of 0.5 A/g in 1 M H2SO4 electrolyte, but also gives excellent retention ration of 87% after 1000 cycles at the current density of 2 A/g. Such excellent performance of CBCNFs/PANI is attributed to its nanostructure and the synergistic effects between the CBCNFs and ordered needle-like polyaniline nanowire arrays.（3） Structure and morphology of polyaniline influenced by different acid doped. In this section, HCl was selected as the doping agents. CBCNFs/PANI nanocomposites were prepared by changing the concentration of HCl. The results of electrochemical tests indicated that when the HCl concentrations was 1 M, the obtained CBCNFs/PANI-1M hybrid electrode exhibit a high specific capacitance of 480 F/g at a current density of 0.5 A/g in 1 M H2SO4 electrolyte.（4） The CBCNFs were well modified by using the low temperature plasma surface modification technology. The CBCNFs/MnO₂ composites were prepared by in situ reduction reaction using plasma-modified CBCNFs as reductant to react with KMnO4. The results exhibit that the nanoflakes of MnO₂ were well coated on the CBCNFs. Furthermore, the electrochemical performance of the electrode material as flexible supercapacitor was investigated. The prepared material（CBCNFs/MnO₂-2） with 2/1（mass ratio） KMnO4/CBCNFs shows good electrochemical performance. It was found that the CBCNFs/MnO₂-2 electrodes showed a specific capacitance of 118 F/g at a current density of 0.1 A/g in 0.5 M Na2SO4 electrolyte, which is 4.5 times higher than that of the CBCNFs（26 F/g）. The capacitance retention ratio of the CBCNFs/MnO₂-2 remained 97% after 1000 cycles at a current density of 1 A/g.