Design And Construction Of Carbon Nanotube Fabric Electrodes And Study On Their Performance In Flexible Supercapacitors

With the development of wearable electronics,energy storage devices are required to be light and flexible.Supercapacitors have attracted much attention in many energy storage devices due to their merits of high power density,long cycle life and fast charging and discharging speed.However,the traditional supercapacitor is rigid and heavy,which cannot meet the needs of wearable electronics.The fabric is light and flexible,and its pore structure can effectively load electrode materials.If the electrodes of supercapacitors are made into fabrics,they could be used for wearable devices.However,the performance of the available fabric based flexible supercapacitors are still difficult to effectively meet the requirements.This is mainly because the available fabric electrodes are difficult to combine high conductivity,high flexibility and high capacitance.As a new type of fiber material,carbon nanotube（CNT）fiber retains the excellent electrical and mechanical properties of microscopic carbon nanotubes to a certain extent,which is an ideal material for constructing new multifunctional fabric.In this thesis,carbon nanotube fabric-based electrodes with high conductivity,high flexibility and high capacitance have been constructed from the CNT cylinders（hollow cylindrical CNT）prepared by floating catalytic chemical vapor deposition,and combined with twisting and knitting technologies.The application of carbon nanotube fabric-based electrodes in flexible supercapacitors was also studied.Finally,an asymmetric supercapacitor was constructed to improve the energy density of the device.The main contents and results are as follows:（1）Oriented CNT fibers were prepared by floating catalytic chemical vapor deposition,and CNT fiber fabric electrodes with high conductivity were further designed and constructed.The CNT cylinders prepared by floating catalytic chemical vapor deposition method was contracted by water bath to obtain CNT fibers,and then multiple fibers were twisted into CNT yarns,and finally multiple yarns were knitted into CNT fiber fabrics（CNTFF）.The CNTFF was activated by thermal oxidation and acid treatment,and the resulting fabric was directly used as current collector and electrode material.The results showed that the CNT fibers had good orientation and certain mechanical and electrical properties.The surface structure of the twisted CNT yarns was dense,which could be used as a new type of textile material for fabricating fabrics.The prepared CNTFF retained the excellent electrical conductivity of CNT to a certain extent.The activated CNTFF electrode still had excellent conductivity,and its structural integrity was also retained.After activation,the oxygen-containing functional groups were introduced and the specific surface area was increased,which significantly improved the electrochemical performance of the fabric electrode.The activated CNTFF electrode showed an areal specific capacitance of 1988.0 m F cm^-2at the current density is 2 m A cm^-2.When the current density increased to 100 m A cm^-2,the capacitance retention of the electrode was 44.7%,indicating good rate performance.After 10000cycles of charging and discharging,the capacitance retention of the electrode was 96.6%,showing good cycling stability.The quasi-solid-state supercapacitor based on activated CNTFF possessed an areal specific capacitance of 1033.4 m F cm^-2 at a current density of 2 m A cm^-2,a maximum energy density of 143.0μW h cm^-2,and a maximum power density of 30600.0μW h cm^-2.In addition,the device had good rate performance and cycling stability.（2）Carbon nanotubes were compounded with polymer yarns to further design and construct carbon nanotube composite yarn fabric electrodes with high flexibility and conductivity.The CNT cylinders were continuously prepared from a floating catalyst chemical vapor deposition process,which were then wrapped on the surface of polymer yarns by using the shrinkage force provided by ethanol molecules.The carbon nanotube composite yarn with uniform and stable conductive layer was obtained by multilayer wrapping.Then,carbon nanotube composite yarn fabric（CPYF）was constructed by knitting process.The fabric electrode（CPYF-ZIF-67-PPy）was prepared by loading zeolite imidazolate framework material（ZIF-67）and polypyrrole（PPy）on the surface of fabric by in-situ growth and chemical polymerization.Benefiting from the electrical conductivity provided by outer carbon nanotubes and the flexibility provided by inner polymer yarn,the CPYF constructed from the composite yarn had both excellent electrical conductivity and flexibility.PPy not only acted as a"bridge"connecting ZIF-67 to form a conductive path,effectively promoting ion diffusion,but also provided a pseudocapacitance.CPYF-ZIF-67-PPy fabric electrode had good electrochemical performance,and its areal specific capacitance was related to pyrrole concentration.CPYF-ZIF-67-PPy（0.2M）electrode prepared at 0.2 mol L^-1 had the highest areal specific capacitance(2308.8 m F cm^-2)at a current density of 0.5 m A cm^-2.In addition,the capacitance retention of CPYF-ZIF-67-PPy electrode was 87.6%after 10000 cycles of charging and discharging,showing good cycling stability.The flexible supercapacitor based on CPYF-ZIF-67-PPy electrode had a maximum energy density of 112.0μW h cm^-2(at a power density of 201.5μW cm^-2),and the capacitance was 98.0%after 5000 times of bending at 180°,showing good cycling stability and flexibility.（3）By weaving carbon nanotube yarns and cotton yarns,carbon nanotube blended fabric electrodes with conductivity,flexibility and high capacitance were designed and constructed,which improved the comprehensive performance of flexible electrodes.The CNT yarns and cotton yarns were knitted together to obtain CNT/cotton blended fabric.The fabric electrodes were prepared by loading PPy on the blended fabrics by chemical polymerization.Finally,a flexible supercapacitor was assembled with optimized fabric electrode.The results showed that,due to the synergistic action of CNT yarns and cotton yarns,the prepared blended fabric had excellent electrical conductivity and flexibility,and could be directly used as a current collector.Meanwhile,the prepared blended fabric had multistage pore structure,which offered the blended fabric higher load capacity of active materials.The mass loading of PPy could be adjusted by the concentration of pyrrole monomer.When the concentration of pyrrole was 0.2 mol L^-1,the mass loading reached to 13.2 mg cm^-2,which met the commercial requirement of 10 mg cm^-2.The areal specific capacitance of PPy-coated blended fabric electrode was related to the mass loading.The areal specific capacitance of the fabric electrode with the mass loading of 13.2 mg cm^-2 could reach to4192.6 m F cm^-2(at a current density of 5 m A cm^-2).When the current density was increased to 100m A cm^-2,the areal specific capacitance of the fabric electrode was 2077.7 m F cm^-2,and the capacitance retention was 50.0%,indicating good rate capability.The areal specific capacitance of the assembled supercapacitor was 1947.9 m F cm^-2(at a current density of 5 m A cm^-2).After 5000cycles of charging and discharging,the capacitance retention of this device was 93.4%,showing good cycle stability.In addition,the device could deliver a maximum energy density of 170.3μW h cm^-2,and had good electrochemical property under bending conditions.（4）A flexible asymmetric supercapacitor based on carbon nanotube fabric was constructed to improve the energy density of the device.Nitrogen-doped carbon nanotube fiber fabric（NCNTFF）was prepared by in-situ polymerization and carbonization process,and then the NCNTFF-Ni Co₂O₄ electrode was prepared by hydrothermal method and annealing treatment.The asymmetric supercapacitor was assembled by using NCNTFF-Ni Co₂O₄ as positive electrode and activated CNTFF as negative electrode.The results showed that the introduced nitrogen-doped carbon layer not only improved the hydrophilicity of the fabric surface,but also increased the specific surface area of the fabric,which was conducive to the uniform growth and distribution of Ni Co₂O₄ nanowires.The prepared NCNTFF-Ni Co₂O₄ retained the excellent flexibility of CNTFF,and had good electrical conductivity.Benefit from the conductivity of the fabric substrate and the good interface bonding between the nitrogen-doped carbon layer and Ni Co₂O₄,the prepared NCNTFF-Ni Co₂O₄ electrode had a high areal specific capacitance(2430.5 m F cm^-2 at 2 m A cm^-2).Even under the high current density of 100 m A cm^-2,the capacitance value of NCNTFF-Ni Co₂O₄remained 62.1%,showing excellent rate performance.The voltage window of the constructed asymmetric supercapacitor could be extended to 1.4 V with an areal specific capacitance of 883.6m F cm^-2(at a current density of 2 m A cm^-2).Meanwhile,the device had a high energy density of240.5μW h cm^-2(at the power density of 1403.7μW cm^-2),indicating high energy storage performance.In addition,the assembled devices possessed good cycling stability and flexibility.