Preparation And Electrochemical Properties Of Carbon Nanotube-based Electrode Materials For Supercapacitor

Because of its high power density, high energy density, fast charging&discharging, longcyclye life, Supercapacitors-a new type energy storage device-have attracted more and moreattention. Among all kinds of electrode materials for supercapacitors, carbon materials havebeen widely used. Since first discovered in1991, carbon nanotubes have triggered a new waveof research on carbon materials, for its unique nano-scale hollow structure, high crystallinity,large specific surface area, high conductivity. And much researches that carbon nanotubesused as electrode materials for supercapacitors have been done. However mere carbonnanotubes uesd as electrode materials have relatively low specific capcatiance. So carbonnanotubes were combined with metal oxides or conductive polymer as electrode materials forthe supercapacitors, that is the current trend.In this paper, we utilized carbon nanotubes as substrate, prepared double-walled carbonnanotubes（DWCNTs）/manganese dioxide composites and multi-walled carbon nanotube/polyaniline （MWNT/PANI） composites, which was further carbonized to prepare Carbonizedcarbon composites. The morphologies and microstructures of the samples were examined byscanning electron microscopy, transition electron microscopy and X-ray diffraction.Electrochemical properties were characterized by cyclic voltammetry, galvanostaticcharge/discharge and electrochemical impedance spectroscopy.DWCNTs/MnO₂composites were synthesized by a microwave irradiation method. Then,the mechanism of manganese dioxide loaded in the surface of carbon nanotubes was studied.Meanwhile, we investigated the effect of different concent of MnO₂on electrochemicalproperties of DWCNTs/MnO₂composites. The birnessite-type MnO₂mainly deposits insidethe tubes at low MnO4-content, while on the surfaces of the tubes at high MnO4-. Because ofthe synergistic contribution from MnO₂coated on DWCNTs, the composites not only hasgood conductivity but also good electrochemical reversibility and strong capacitive reactance.Specially, DWCNTs-85%MnO₂composite exhibits high specific capacitance (240.2F·g^-1)and good cycling stability （6.8%capacity loss after2000cycles）.Multi-walled carbon nanotubes/polyaniline （MWNT/PANI） composites were synthesizedby in situ polymerization. Meanwhile, we investigated the effects of MWNT contents onMWNT/PANI composites’ electrochemical performance. The results show that the capacity of the composites were related with multi-walled tube content, when MWNT content incomposites is low （such as MWNT/PANI-1:50, MWNT/PANI-1:80）, composite has lowerspecific capacitance than that of pure PANI, While MWNT content is little higher（MWNT/PANI-1:20）, the composite has higher specific capacitance than that of pure PANI.Thus, MWNT/PANI composites were further carbonized to properties of carboncomposite materials, whose electrochemical properities were thorough studied. Aftercarbonation, compared to their pre-polymer, the capacity of carbonized carbon compositeshave increased more or less. For example, at the scan rate of2mV·s^-1, the specificcapacitance of C-MWNT/PANI-1:50composite is261.7F·g^-1, which is far higher than120F·g^-1of MWNT/PANI-1:50. Pure polyaniline carbonazed carbon （C-PANI） andC-MWNT/PANI-1:50composite were treated by the cycle life test. From the results of cyclelife test, we have found that the specific capacitance of the two samples at the scan rate of100mV s^-1after2000cycles times, compared to their initial specific capacitance, reduced only13.5%and12.6%respectively, indicating that they have high retention rate of the cycle life.Finally, we also studied the microwave reduction of graphite oxide （GO）, and preparedMWGO sample, whose specific capacitance is182.6F·g^-1at2mV·s^-1. We Added KOH to theinitial GO solution to prepare MWKGO samples. The addition of KOH can be further modifythe microwave sample’s surface, making about2³nm diameter pores on its surface, which isthe optimum diameter, and can effectively utilize the active electrochemical materials. So atthe same scan rate, MWKGO sample has higher specific capacitance, about100F g^-1higherthan that of MWGO sample averagely. MWKGO sample was treated by the cycle life test. Wehave found that the specific capacitance of the two samples at the scan rate of100mV s^-1,after5000cycles times, compared to their initial specific capacitance, reduced only11.2%,indicating its high electrochemical performance and stability.