Synthesis And Properties Of Carbon Fibre Cloth With Hierarchical Pore Structures As Electrode Of Supercapacitor

Supercapacitors are widely used in electronic devices because of their high power density and long cyclability.Carbon-based electrode materials for supercapacitors have shown many advantages,e.g.,high stability and high conductivity.Besides these virtues,carbon fiber cloth(CFC)can offer high flexibility and mechanic properties,thereof promising for flexible electronic devices.However,the specific capacitance of commercial CFCs is unsatisfactory due to the limited specific surface.Thus,such situation has severely restricted their further applications as the active material in high-performance supercapacitors.In order to address this issue,the commercial CFC is afforded with hierarchical pore structures to obtain high specific surface in this thesis,which is directly used as the electrode for supercapacitor.The hierarchical pore structure creates more electrochemical active surfaces which significantly improved supercapacitive properties of commercial CFCs.The CFCs with different hierarchical pore structures were prepared,and the relationship between pore structures and electrochemical performance was further investigated.Furthermore,the electrochemical effect of different electrolyte ions on the CFC electrodes was also studied,which promotes the application of commercial CFCs in capacitors.Moreover,further understanding of the structure-property relationship was obtained.Herein,the main contents of this thesis are concluded as follows:(1)Firstly,CFC with hierarchical pore structure was prepared by using a simple annealing treatment,which is also applicable for large-scale production.X-ray diffraction(XRD)and Raman spectroscopy are used to analyze the CFC structure.It shows that the activation process has forged a hierarchical and disordered pore structure indicated by an increasing D/G ratio while the phase structure of CFC remained.The contact angle and X-ray photoelectron spectroscopy(XPS)tests further proved that the surface hydrophilic groups increased and the surface changed from hydrophobic to super-hydrophilic after the annealing.These findings are beneficial to for improving the contact between CFC and electrolyte;Such activation method also effectively modified the surface morphology of CFC,and led to a porous structure on the surface,which was confirmed by scanning electron microscopy(SEM),transmission electron microscopy(TEM)and N2 adsorption experiments.As a result,a high specific surface area of 614 m~2/g can be reached.(2)Electrochemical performance of the activated CFC are characterized by using cyclic voltammograms,constant current charge-discharge and AC impedance tests.It was found that the activated CFC has achieved an ultrahigh area specific capacity of~1300 mF/cm~2 due to the modification of CFC surface through the annealing treatment.In addition,the electrochemical properties of activated CFC was studied in eight different kinds of electrolytes.It show that the capacity of activated CFC is almost unchanged in the electrolyte with the same anion,but the specific capacitance of activated CFC has a significantly correlation to the electrolytes with different anions.This result is correlated with ion exchange mechanisms and interfacial adsorption.A large amount of OH~-is beneficial to the increase of specific capacitance.Therefore,both asymmetric and symmetric supercapacitors were fabricated by matching Co(OH)2 and themselves,which harvested favorable power density and energy density.(3)The activated CFCs with different hierarchical pore structures are obtained after the annealing at different temperatures.The structure is characterized via XRD,integrated raman imaging and secondary electron microscopy(RISE).It was found that the annealing temperature has obvious effect on the ratio of peaks G to D.The pristinely disordered structure of the carbon fiber cloth becomes more disordered when it exceeds 400?.Infrared(IR)spectroscopy and XPS are used to analyze the surface functional groups.The hydrophilic groups on CFC surface increased with an increase of annealing temperature,which improved the wettability of CFC.SEM,N2adsorption/desorption tests and two dimensions non local density functional theory(2D-NLDFT)model are used to analyze the hierarchical pore structure of CFCs.As the annealing temperature increase,the surface became more coarse and the specific surface was enlarged.However,the hierarchical pore structures of activated CFC changed greatly after the annealing at different temperatures.The ratio of micropore to mesoporous volume gradually decreases when it exceeded 350?.(4)Activated CFC with different hierarchical pore structures are analyzed by using cyclic voltammetry and constant current charge-discharge tests.It was found that the increase of specific surface is unnecessarily correlated to the increase of specific capacity,but the hierarchical pore structure has a strong effect on the capacitance.The specific capacitance can be continuously improved by optimizing the specific surface and the ratio of micropore to mesoporous volumes.More detailed dynamic analysis show that in the case of small sweep speed,different hierarchical pore structures have different influences on capacitive control and diffusion control,which is beneficial to control the CFC performance.Such findings are crucial for designing and optimizing Li~+intercalation,Na~+storage and pseudo capacitive reaction.