Flexible Supercapacitors Based On Carbon Nanotube Fibers

With the rapid growth of wearable and portable electronics,it is extremely important to develop matchable energy storage devices which should be simultaneously lightweight,flexible,portable and integrable.As an important branch of flexible energy storage devices,fiber supercapacitors not only have the advantages of high power density,fast charge and discharge process,long cyclic life,and environmental friendliness of ordinary supercapacitors,but also have the advantages of light-weight,tiny volume,weavability and mechanical flexibility.It can be woven into textiles to form an integrated multifunctional system to meet the wearable requirements of flexibility,miniaturization and integration.As a typical fiber electrode material,carbon nanotube fiber(CNTF)demonstrates many advantages such as good conductivity,good flexibility,and strong mechanical properties.However,the specific capacitance of CNTF is low because its energy storage performance is mainly from electrical double layer capacitance.Being directly combined with the pseudo-capacitor materials,the cyclic stability and mechanical stability are poor.To this end,in this thesis,highperformance fiber supercapacitor is prepared by surface modification of the CNTF electrode material,electrode structure design and the compositing with pseudocapacitor materials.The main works are as follows:(1)The surface modification of CNTF was carried out by simple electrochemical activation.The modified fiber surface exhibits wrinkled structure which can increase the specific surface area of the fiber.Meanwhile,oxygen-containing groups such as hydroxyl group and carboxyl group are introduced into CNTF,and thus increases the pseudo capacitance of the fiber.The results show that the specific capacitance of the fibers is improved,which is 8.2 times of that of the initial CNTF.The all-solid symmetric supercapacitor assembled with activated CNTF electrode has a high specific capacitance of 78.6 F/g at 1 A/g.After 100,000 charge-discharge cycles,only about 4%of the specific capacitance is lost,showing excellent cycle stability.(2)This "branch/trunk" structured N-CNT@CNTF electrode was prepared with nitrogen-doped carbon nanotubes(N-CNT)grown on the surface of CNTF by chemical vapor deposition(CVD).The CNTF acts as the trunk.N-CNT grown on CNT fiber,which presents a helical structure on the CNTF surface,acts as the branch forming a three-dimensional network space.Moreover,N-CNT has good electrical conductivity,which not only increases the specific surface area of the fiber surface,but also facilitates the charge transmission.The cyclic voltammetry curves of N-CNT@CNTF electrode at5 000 m V/s still maintain the rectangular shape,indicating that the sample can be charged and discharged rapidly.The fiber electrode exhibited excellent energy storage properties with a high specific capacitance of 29.9 F/g at 1 A/g.The fiber electrode also exhibits outstanding rate capability with 20.2 F/g maintained at ultra-high current density of 50 A/g.Moreover,89.5% of the capacitance can be remained after 100,000charge-discharge cycles at 20 A/g.In addition,it can be repeatedly bent without obvious specific capacity deduction.Although the specific capacity of the branch@trunk structure electrode needs to be improved,its good charge conductivity and flexibility make it can be used as a fiber substrate.Combining it with pseudo-capacitance materials can develop fiber electrodes that have both good energy storage performance and flexibility.(3)Tree-like structured PANI/N-CNT@CNTF electrode was prepared by electrochemical deposition of polyaniline(PANI)on the surface of N-CNT@CNTF.The tree-like structured PANI/N-CNT@CNT fiber exhibits high specific capacitance,excellent rate capability and cyclic stability.The specific capacitance reaches 323.8 F/g at current density of 1 A/g and 221.3 F/g can be kept at high current density of 50 A/g.The specific capacitance of the PANI/N-CNT@CNT fiber electrode material in symmetric supercapacitor reached 264.8 F/g at 1 A/g and 192.5 F/g can be kept at high current density of 50 A/g,with excellent rate capability.This outstanding rate capability should be ascribed to the advantages of the special tree-like structure with both large effective surface area,uniform PANI loading and modified charge transfer route.The assembled supercapacitor possesses excellent cyclic stability with 92.1% of the original capacitance maintained after 10,000 charge-discharge cycles at 20 A/g.Beyond excellent energy storage properties,the fiber also has good flexibility,with almost no change in capacitance at different bending angles.The capacitance can be maintained95.5% after repeated bending processes for up to 10,000 cycles.In conclusion,through surface modification,structural design,doping and compositing with PANI,fiber electrodes with good energy storage performance and flexibility are developed based on oriented CNTF.This method can be well applied to the research of high-performance fiber electrodes for wearable devices.