Reparation Of Carbon Cloth-based Nano-composites And Their Electrochemical Performance

As a kind of novel energy storage devices, supercapacitors exhibit promisingprospects for applications with their high power density, fast charge/discharge rates andlong life cycles. Flexible supercapacitors have potential applications in wearable,miniaturization and portable electronic devices. However, the electrode material is the keyto the high performance of supercapacitors. The design and fabrication of new compositesby combining carbon substrates with high pseudocapacitance materials is an effective wayto have complementary advantages and improve the properties of the composites.Carbon-based electrodes can take a number of manufactured forms such as graphene,carbon nanotubes, activated carbon, carbon fibers and so on, which have already attracteda lot of attentions. Manganese dioxide is one of the most attractive inorganic materialsbecause of its high specific capacitance, large amount in resources, simple preparation andenvironmentally friendly properties. As one of widely used conducting polymers,polyaniline(PANI) not only offers all the advantages above-mentioned but also has highconductivity. Therefore, they have become research hotspot recently. In this work, carboncloth wove by carbon fibers (CC) was selected as substrate due to its good electricalconductivity, chemical stability, light weight and flexibility.MnO2/CC, PANI/CC,MnO2/PANI/CC and PANI/MnO2/CC composites were prepared by chemical andelectrochemical methods and used as freestanding electrodes for supercapacitors. Themorphologies and structures of composites were analyzed and the electrochemicalperformances were also measured and investigated. The carbon cloth (CC) substrate wasfirstly treated by the nitrogen plasma etching process. MnO2/CC composites werefabricated by hydrothermal method with5mM KMnO4as Mn source and, the carbon fibersserving as reagent and flexible substrate. Various samples with different thickness ofMnO2layers on CC were obtained by controlling the reaction time. The morphologies andstructures of MnO2deposited on CC were investigated by Field-emission scanning electron microscopy (FESEM), Raman, X-ray diffraction (XRD) and X-ray photoelectronspectroscopy (XPS). Electrochemical properties were studied by cyclic voltammetry,galvonostatic charging/discharging and impedance measurement techniques. The resultsshow that MnO2/CC composites fabricated by hydrothermal method exhibit a not gooddistribution and are sensitive to the reaction environment. The capacitances of thecomposites increase with the increasing reaction time. The MnO2/CC composites withreaction time of0.5h demonstrated a high specific capacitance of76.9mF/cm2at a current densityof0.2mA/cm2, and the specific capacitance still remained about89%of its initial value aftercontinuous1700charge/discharge cycles.To further improve the uniformity of distribution and the controllability ofmorphology, a simple electrochemical deposition method was introduced to synthesize aseries of MnO2/CC composites with different thickness of MnO2film, which werecontrolled by tuning the deposition time. The results show that the well ordered nanosheetsMnO2were obtained though electrochemical deposition method. With the shortestdeposition time of100s, MnO2were already uniformly coated on the surface of carbonfibers. As the deposition time increase, the size of MnO2nanosheets became larger and thespecific capacitances increased with the larger mass loading of MnO2. When theelectrochemical deposition time reached to2000s, the composites showed a good electrochemicalperformance with an areal specific capacitance of291mF/cm2at0.2mA/cm2but poor utilizationof inner MnO2. When normalized to MnO2, the specific capacitance of MnO2/CC-400s couldachieve to338.7F/g at1A/g, and the capacitance remains essentially unchanged for2000cycles,showing a good cycling stability. Both the capacitance and stability of the obtained MnO2/CCcomposites have improved comparing to the composites obtained though hydrothermal method.Because of the poor electrical conductivity of MnO2, conducting polymers werepromising materials to improve the conductivity of composites, which could furtherenhance electrochemical performance. The PANI/CC composites were synthesized via insitu polymerization of aniline though controlling the ratio of oxidant and aniline (ANI)monomer, temperature and reaction time. In this work, the molar ratio of ANI to APS was3:1, which has the tendency to form nanowires. When the concentration of aniline is up to0.015mmol/mL, collapsion of polyaniline nanowires occurred on the CC surface. Theelectrochemical performance of resulting PANI/CC composites electrode showed thatfirstly increased and then decreased with the increase of adding ANI concentration. Whenthe concentration of aniline is0.01mmol/mL, the resulting PANI/CC composites electrodeexhibit a high capacitance of225mF/cm2at the current density of0.2mA/cm2, butsuffered from poor rate capability of70%capacitance retention after1600cycles. In order to further improve the conductivity of MnO2/CC composites and the cyclestability of PANI/CC composites, The MnO2and PANI were loaded on surface of CCaccording to the different order, the synergy effects of the MnO2and PANI were utilized tofurther improve the electrical performance of composites. Two different core-shellstructures of MnO2/PANI/CC and PANI/MnO2/CC ternary composites were fabricatedthough the optimum synthesis condition mentioned above. The morphologies ofPANI/MnO2/CC composites showed that the nanosheets-structured manganese dioxide layerprovide template for the growth of polyaniline, and at the same time the nanowires PANI wereintercalated into the interlayer of the MnO2, leading to the increasing of the cohesive forceand providing high conductive channels. While the morphologies of MnO2/PANI/CCexhibited that the existence of MnO2nanosheets and relatively independent, with thedeposition time of400s, PANI nanowires were uniformly coated by the outermost MnO2layer completely. Using three electrode test system, the obtained PANI/MnO2/CC-400sternary composites exhibited a high capacitance of421.6mF/cm2at the current density of0.2mA/cm2, while MnO2/PANI/CC-400s only exhibited284.7mF/cm2. When the currentdensity increased by5times the specific capacitance of PANI/MnO2/CC composites still kept63%of the original value which was much larger than48.6%retention that MnO2/PANI/CCcomposites retained.