Study On The Preparation And Properties Of Fiber-Shaped Flexible Supercapacitors Based On Carbon Fiber

The fiber-shaped flexible supercapacitor is a new type energy storage device with the advantages of high power density,long cycle life,fast charging/discharging,and excellent flexibility.It has broad application prospects in the field of flexible electronic devices,but the relatively low energy density restricts its practical application.How to prepare electrode materials with high specific capacitance and widen the working voltage is the key to effectively improve the energy density of fiber-shaped flexible supercapacitor.In this paper,carbon fibers(CF)were used as the flexible substrate.Firstly,carbon nanotubes(CNT)were deposited onto the CF surface by electrophoretic deposition to form a conductive network,and then manganese dioxide(MnO2)and polypyrrole(PPy)were deposited separately to obtain CF/CNT/MnO2 and CF/CNT/PPy composite electrodes with different working voltage windows.Finally,the composite electrodes were assembled into an asymmetric fiber-shaped supercapacitor using gel electrolyte.The structure of electrode materials was studied by SEM,XRD,XPS and FT-IR.Electrochemical properties of electrodes and devices were studied by Cyclic Voltammetry(CV),Galvanostatic Charge-Discharge(GCD)and Electrochemical Impedance Spectroscopy(EIS).Firstly,CF/MnO2 electrodes were prepared by electrodeposition with constant current,and the influences of current density and deposition time on electrochemical performance of electrodes were studied.The results showed that when the current density was 4 mA/cm,the specific capacitance of the electrode was the maximum,but the excessive current density will lead to the uneven structure of the deposited MnO2,resulting in the electrochemical performance degradation.When the current density was constant,the deposition amount of MnO2 increased with the deposition time,and the specific capacitance of electrodes decreased significantly.CNT and MnO2 were successively deposited on the CF surface to obtain CF/CNT/MnO2 composite electrode.When the deposition amount of MnO2 was 2.1 mg,the specific capacitance of CF/CNT/MnO2 composite electrode was 267.5 F/g,11.7%higher than that of CF/MnO2 electrode(239.5 F/g),indicating that CNT can effectively improve the specific capacitance of MnO2 electrode.The reason is that the synergistic effect of carbon materials and pseudo-capacitive materials makes the composite electrode has excellent electrochemical performance.CF/PPy electrodes were prepared by electropolymerization with constant current.The effects of polymerization electrolyte,current density and polymerization time on electrochemical performance were studied.The results showed that the specific capacitance of CF/PPy electrode doped with Cl-prepared in a mixture of pyrrole and LiCl was significantly higher than that of CF/PPy electrode prepared in pyrrole aqueous solution.In a small range,the current density has little effect on the performance of electrodes.When the polymerization time was prolonged,the mass of PPy increased and the specific capacitance of the electrodes decreased.CNT and PPy were successively deposited on the surface of CF to obtain CF/CNT/PPy composite electrode.When the deposition amount of PPy was 3.0 mg,the specific capacitance of CF/CNT/PPy composite electrode was 277.1 F/g,14.1%higher than that of CF/PPy electrode(237.9 F/g),indicating that the addition of CNT could effectively improve the specific capacitance of PPy electrode.The polyvinyl alcohol gel electrolyte(PVA-LiCl)with LiCl concentration of 2 mol/L was prepared and assembled with the electrodes to obtain twisted fiber-shaped flexible supercapacitors.By comparing the asymmetric devices CF/CNT/MnO2//CF/CNT/PPy and CF/MnO2//CF/PPy,when the current density was 4 A/g,the energy density and power density of the former were 411%and 61%higher than that of the latter respectively,indicating that the device assembled by composite electrodes had better electrochemical performance under large current charging and discharging.Comparing the asymmetric device CF/MnO2//CF/PPy with the symmetric devices CF/MnO2//CF/MnO2 and CF/PPy//CF/PPy,the asymmetric device has higher working voltage(1.7 V).The energy density of CF/MnO2//CF/PPy is 3.6 times and 6.5 times higher than that of CF/MnO2//CF/MnO2 and CF/PPy//CF/PPy respectively at the current density of 0.5 A/g,and the power density is also significantly improved,indicating that the method of assembling electrodes with different voltage windows into asymmetric structure to broaden the working voltage of devices can effectively improve the electrochemical performance of devices.The CV curves of the device under different bending states are basically consistent,and the capacitance retention rate is 91.6%after 500 times of repeated bending,indicating that the device has good flexibility.