Flexible Asymmetric Supercapacitors Based On Carbon Nanotube Fibers

Recently,wearable electronic devices such as flexible displays and sensors have been gradually applied in various fields such as medical,military and outdoor sports.With further development of these electronics,conventional bulk or planar flexible energy-supplying devices have been difficult to meet the requirements for miniaturization,portability,light weight and integration of these electronic devices.Thus,it is extremely desired to develop lightweight,flexible,stretchable,safe,stable and high-performance energy storage devices.In addition to inheriting the advantages of traditional capacitor such as high power density,long cycle life,good stability,non-toxicity and environmental adaptability,fiber-shaped supercapacitor?FSC?also possesses the advantages of tiny volume,higher flexibility and easy weaving.Therefore,it has very prominent advantages in the field of wearable electronics.However,the energy density of FSC is still too low toward practical application.Therefore,how to increase its energy density without sacrificing its power density is a key challenge.Constructing an asymmetrical supercapacitor?ASC?to widen its operating voltage to accordingly increase the energy density is an effective method to solve the above problem.Here,we select carbon nanotube fiber?CNTF?with excellent electrical conductivity,good flexibility and strong mechanical properties as current collectors and conductive substrates to design and synthesize highly aligned three-dimensional nanostructured composites.Then,these hybrid fibers were applied to construct ASC with high energy density.The main research contents are as follows:?1?We develop a facile and effective method to directly grow dandelion-like molybdenum-nickel-cobalt ternary oxide?MNCO?nanowire array?NWA?on CNTF with a high specific capacitance of 490.7 F/cm³?1840 mF/cm²?at a current density of1 mA/cm².Benefiting from the three-dimensional nanostructure,high conductivity and excellent pseudocapacitance properties,we successfully fabricate a fiber-shaped asymmetric supercapacitor?FASC?with a maximum operational voltage of 1.6 V,which is assembled by twisting MNCO/CNTF positive electrode and thin carbon-coated VN NWA on CNTF negative electrode together with KOH/poly?vinyl alcohol??PVA?as gel electrolyte.The optimized FASC delivers a remarkable specific capacitance of 62.3 F/cm³?233.7 mF/cm²?and an exceeding energy density of 22.2mWh/cm³?83.1?Wh/cm²?.Additionally,it exhibits outstanding flexibility with capacitance retention maintaining 90.2%after bending 3,500 times.Thus,the high performance MNCO/CNTF electrode opens a new avenue to fabricate high-performance FASC for next-generation wearable energy storage devices.?2?We successfully fabricated a hierarchically structured three-dimensional VO₂@polypyrrole?VO₂@PPy?core-shell NWA on CNTF via a facile and cost-effective route.Such a composite structure not only enables a highly pseudo-capacitive VO₂ core to provide a large surface area for reversible Faradic redox reactions,but also uses a highly conductive PPy shell to suppress the dissolution of VO₂,thus making the hybrid fiber a new state of fibrous electrode with exceptional specific capacitance and rate behavior.To benefit from these superior features,we assembled a high-performance FASC with an operating voltage of 1.8 V and achieved a remarkable specific capacitance of 60.6 F/cm³ and an extraordinary energy density up to 27.3 mWh/cm³ at a current density of 0.25 A/cm³.In addition,the FASC device exhibited excellent flexibility;its capacitance retention remained at88.9%after bending 4000 times.Thus,these findings should help to develop advanced VO₂-based cathodes for next-generation wearable energy storage devices.