Design And Construction Of Carbon-based Flexible Materials And Their Energy Storage Properties

As the core technology in wearable portable electronic devices,smart clothing is expected to be the terminal of mobile medical,military and Internet.Flexible energy system,including energy storage and conversion devices,sensor and generator,the basis of functional combination and extension of clothing,has become a research hotspot in the past decade.Excellent flexibility and high-density energy are keys to promote the development of flexible energy storage electronic devices.Therefore,in this paper,functional multiwalled carbon nanotubes（MWCNT）was chosen as active materials and cotton fabric as flexible substrate to make a carbon-based flexible material with high bending stability,strong conductivity and adsorbility.Multi-dimensional structure based on the above carbon material for high-energy storage are also be designed.The proposed designing and construction technics will provide new possibilities for flexible electronic storage.The specific research content and result consist of the following four parts:（1）Design and construction of high flexible electrode exert significant influence on continuous flexible energy supply technology.In this part,a flexible electrode with 3D coordination network structure was constructed with cotton fabric as substrate and functional MWCNT with natural bending and winding properties in boiling point water medium.It is excellent flexibility and low resistance(20Ωsq^-1).Besides,the bending property and electrochemical performance are systematically studied,as well as the combination between the cotton and functional MWCNT.This technique endowed MWCNT with ultrafast thermal movement,so that they can embed among yarns,collided with fibers,and deposited in high density along with one-dimensional order of fibers by a large number of hydrogen bonds,van der Waals forces andπ-πbonds.Results show that the mass specific capacitance is 14.3 F g^-1(1 m V s^-1);after 3000 cycles,the capacitance retention rate can reach 95%;after 1000 times of bending test,the capacitance retention rate is 88.63%.This is a high efficiency and time-saving method,which improves the disadvantages of traditional repeated"dipping-drying"processing method and avoiding the problems of time-consuming and labor-consuming.In addition,there is no adhesive in the whole process of flexible electrode preparation,which makes the electrochemical performance of carbon nanotubes fully play,and solves the problem of the loss of activity of active materials due to the cover of adhesives,opening an new direction for the industrial production of flexible electrode without adhesive.（2）Excellent electrochemical energy storage and bending resistance are two essential factors for the development of flexible electrodes.In this chapter,a carbon-based flexible electrode was designed by the mean of carbonization,transforming the non-conductive cotton fiber into conductive carbon fiber skeleton with energy storage capacity and excellent conductivity.Specially,MWCNT plays a significant role in improving the crystal order and forming a resistance buffer layer outside the carbonized fiber to avoid fracture due to stress concentration during bending stress.Besides,the excellent adsorption performance of MWCNT endows the flexible carbon cloth with good adsorbability(BET specific capacitance is 103.69 m² g^-1),laying the foundation for the combination of high-performance energy storage materials.Electrochemical test results show that the specific capacitance of the material can reach 94.3 F g^-1 at the current density of 0.81 A g^-1,which is much higher than the same kind of flexible carbon materials.After 3000 cycles of testing,the capacitance retention rate is 96%;after bending at an angle of 60°,the capacitance retention rate is 93%.This technology creates a new theoretical guidance for the development and preparation of carbon-based flexible material for flexible supercapacitors.（3）Improving synergistic effect between electronic double layer capacitance（EDLC）and pseudocapacitance is one of the most effective methods for preparing high performance flexible electrode.Ni Co-LDH has special layer structure,rich electrochemical active sites and ultra-high electrochemical performance.However,its poor conductivity and inorganic powder status hindered its furthermore development.Therefore,in this part,porous honeycomb Ni Co-LDH flexible electrode with high power density was prepared with a mild reaction（55℃）by self-grown without any adhesions.It is a special 3D hierarchical structure,which provides high way for electron transportation and self-support stability for long cycle-life.The processing technique is to adsorb Ni²⁺and Co²⁺in the carbon-based flexible cloth in advance and react at 55℃,avoiding energy-consuming and have prospect for massive production.Electrochemical results show that the specific capacitance of the electrode is 811 F g^-1 at a scanning rate of 0.1 A g^-1.After 1500 times of cyclic stability test,the capacitance retention rate of the material is 89%,and after 1000 times of bending test,the capacitance retention rate is 99%.The specific capacitance is 47.25 F g^-1(0.1 A g^-1),the energy density is 9.45Wh kg^-1,and the power density is 61.96W kg^-1.In addition,the flexible super capacitor has good cycle stability.After 3000 cycles,the capacitor retention rate is 94%.When two flexible super capacitors are connected in series,the LED can be lighted for 60s.The prepared flexible electrode has the advantages of simple preparation,mild reaction conditions,strong operability and potential for industrial production.（4）Functionalization is one of the main directions for the development of flexible electrode in the future.Magnetic electrode can assemble automatically by magnetic attraction during the process of making flexible supercapacitor.Therefore,in this part,we made a magnetic flexible electrode with Fe₃O₄ and carbon-based flexible cloth in the magnetic field.In this composite,the magnetic Fe₃O₄ can not only enhance the excellent electrochemical energy storage properties of flexible electrode,but also endow the flexible electrode magnetic function.In addition,different from other flexible electrodes,the material has excellent magnetic properties,which is conducive to its self-adsorption in the assembly of flexible supercapacitor and improves the fit force of electrode assembly.Results show that the weight gain rate of Fe₃O₄ magnetic nanoparticles of the Fe₃O₄/carbon-based flexible cloth is about 32%,which is 30%more than non-magnetic field.Besides,at a scanning rate of 0.29 A g^-1,the specific capacitance can reach 396 F g^-1.After 3000cycles,the capacitance retention rate increased by 20%,after 10,000 cycles,the capacitance retention rate decreased to the initial value;this phenomenon can be ascribed to the activated electrochemical activity of Fe₃O₄ during the cycle test.After 1000 bending tests,the capacitance retention rate of the material was 98%.After assembling the flexible electronic device,the specific capacitance is 24.3 F g^-1(0.1 A g^-1),and the corresponding energy density and power density are4.86 Wh kg^-1 and 72.6 W kg^-1,respectively.This work provides an effective solution and strategy for the realization of high loading capacity of magnetic nanoparticles Fe₃O₄ on flexible materials without binding agent.