Oxide Modified Carbon Fiber And Its Use In High Energy Density Supercapacitors

Supercapacitor,also known as electrochemical capacitor,is characterized by fast charge-discharge rate,excellent cycling stability and high power density.However,the relatively low energy density largely restricts its wide applications.Building high energy density asymmetric supercapacitor（ASC）with outstanding cycling stability has received considerable attentions.Rational design of the high capacity electrodes and the good matching of positive and negative electrodes are crucial to fabricate an asymmetric supercapacitor.Fe₂O₃ has been considered as one of the attractive negative electrode materials due to its low cost,environment friendly,high theoretical capacitance and more negative operating potential in KOH electrolyte.On the other hand,NiCo-DH and Co₃O₄ electrodes,having high theoretical capacitance（>3000 F g-1）and easily controllable nanostructure and morphology,are promising positive electrodes in configuring ASC.However,the main challenge for these pseudocapacitive materials is improving their low rate capability due to their low conductivity and poor cycling stability resulted from the weak interaction between active materials and current collectors,which greatly limits their applications.To address those problems,in chapter III,we design the nanostructure to increase the effective contact area between the electrode and electrolyte,enhance the electrode capacitance and shorten the ion migration path.On the other hand,the tubular aCF with ultralong one-dimensional（ID）structure can provide a fast electron and ion transport,bringing a high-rate performance.The high-capacity α-Fe₂O₃ and nickel-cobalt double hydroxides（NiCo-DH）electrodes were grown on the hollow carbon fibers（aCF）by a chemical bath deposition technique.The strong interaction between pseudocapacitive materials and aCF reduces the interface resistance and effectively improves the cycling stability of pseudocapacitive materials.At the same time,using aCF-Fe₂O₃ of high capacity to replace traditional carbon electrode can improve the energy density of the supercapacitor greatly.The fabricated asymmetric capacitor（ASC）shows an extended working voltage window up to 1.6 V in 2.0 M KOH electrolyte and exhibits a specific capacitance of 245 F g-1 at a current density of 0.5 A g-1.More importantly,the ASC delivers an energy density as high as 83.7 Wh kg-1 at the power density of 392.3 W kg-1 and exhibits remarkable cycling stability with 94.6%capacitance retention even after 10000 continuous charging-discharging cycles.After being charged at 3 A g-1 for 76 s,two ASCs connected in series can light up a red LED indicator for about 50 min,drive a toy car with a total weight of 72 g to run 1.85 m or enable an electronic timer to work for 17 h.In order to meet the requirement of fast development of flexible wearable electronic products,the ASC should not only have high energy density,but also possess excellent mechanical flexibility.In chapter Ⅳ,Fe₂O₃ arnd Co₃O₄ nanosheets have been vertically grown on flexible carbon cloth（CC）through galvanostatic and potentiostatic technique in combination with low temperature thermal annealing method.By adjusting the deposition time,we can obtain a series of CC-Fe₂O₃ and CC-Co₃O₄ composite electrodes with controllable content,tunable sheet thickness and good electrical conductivity.Results based on three-electrode cell show that the maximum areal capacitance of CC-Fe₂O₃ electrodes reaches 1.56 F cm-2 at the current density of 10 mA cm-2 and capacitance retention is 102%after 5000 continuous cycles.CC-Co₃O₄ electrodes exhibit a maximum areal capacitance of 2.02 F cm-2 at 5 mA cm-2.An all-solid-state supercapacitor with CC-Fe₂O₃-4h as negative electrode,CC-Co₃O₄-20m as positive electrode and PVA-KOH as gel electrolyte can deliver an areal capacitance up to 842 mF cm-2,volumetric capacitance of 17.5 F cm-3 and remarkable cycling stability with 93%capacitance retention after 4000 cycles of galvanostatic charge-discharge.Moreover,maximum volumetric energy density of 6.75 mWh cm-3 at power density of 104 mW cm-3 is also achieved.In addition,the device can well maintain its capacitive performances at various bending or twisting states.