Electrochemical Energy Storage Of Cobalt Oxide,Vanadium Oxide/Graphene Nanocomposites

With the rapid development of science and technology,more and more countries pay attention to new green energy,such as solar energy,wind energy,tidal energy.However,the intermittent of green energy limits its large-scale applications.So energy storage technology has been widely concerned.As a common energy storage device,lithium ion battery does not meet the requirements of fast charging and high power output.Therefore,the battery cannot meet the requirements in many practical applications.Supercapacitors with high power density and long cycle life can make up for lithium ion battery,which will have a broad application prospect in the future.In this thesis,the key purpose was to design a reasonable material structure for supercapacitor electrode and improve its electrochemical properties.First of all,the CVD method was used to prepare three-dimensional graphene as current collector.Then,active materials of metal oxide were grown on the surface of graphene by hydrothermal method,which was directly used to supercapacitor electrode to characterize its electrochemical performances.Finally,the electrodes were assembled into supercapacitors to evaluate the overall electrochemical energy storage.The main contents of this thesis were as follows:?1?Three-dimensional graphene?3DG?,as current collector,was prepared by CVD method at 1000°C.Then,Ag quantum dots?AgQDs?embedded Co₃O₄ nanosheets were grown on 3DG surface by combining hydrothermal process and annealing.The advantage of the binder-free and self-supported Ag/Co₃O₄/3DG electrode was that the superlight electrode quality and the small interface resistance.The areal specific capacitance?Ca?of Ag/Co₃O₄/3DG electrode was 304 mF cm-2 at a charge/discharge current density of 1 mA cm-2,which was about twice higher than the pristine Co₃O₄/3DG electrode.The cycling stability of the hybrid electrode was examined over 10000 charge/discharge cycles at the current density of 1 mA cm-2,which showed that over 120% of the areal specific capacitance was maintained,and the largest Ca of 421 mF cm-2 could be reached at the 4000 th cycle.Such superior performance was attributed to the reasonable electrode structure and the embedded AgQDs in Co₃O₄ nanosheets.The AgQDs integration improved the conductivity of Ag/Co₃O₄/3DG electrode,increased the specific capacitance,and enhanced the rate performance.Finally,the Ag/Co₃O₄/3DG electrode and AC electrode were successfully assembled into a flexible asymmetric supercapacitor,which exhibited the areal specific capacitance of 53.3 mF cm-2 at 1 mA cm-2,the high energy density of 26.7 Wh kg-1 and the large power density of 15000 W kg-1.The flexible asymmetric supercapacitor charged for 30 s at 2 mA cm-2 can successfully light red LED for half an hour.?2?Based on the structure advantage of the Ag/Co₃O₄/3DG electrodes,vanadium oxide with wide voltage window was prepared by hydrothermal method.The nanobelts?NBs?and nanotubes?NTs?were successfully grown on the 3DG current collect by the reduction of lauryl amine for V2O5 raw material.The VO_x/3DG electrode showed the Ca of 225 mF cm-2 at the current density of 1 mA cm-2.The CV curves showed a negligible change as the scan rates increase from 5 to 100 mV s-1.This revealed the good electrochemical reversibility and the high power capabilities.With the increase of electrochemical cycles,the areal-specific capacitance of the VO_x NTs/3DG remained 76.7% compared to the initial Ca after 2000 cycles.?3?To further optimize the electrochemical properties of VO_x/3DG electrodes,the lauryl amine in hydrothermal solution was modulated.VO₂ nanoflowers was achieved on the 3DG surface?VO₂ NFs/3DG?.The VO₂ nanoflowers can increase the specific surface area of the electrodes,which was more beneficial to electrolyte ion transport for the redox reaction.In the three-electrode measurement configuration of VO₂ NFs/3DG electrode,the Ca was 466 mF cm-2 at the current density of 3 mA cm-2.The cycling stability of the hybrid electrode was also examined over 3000 charge/discharge cycles,which showed that over 63.5% of the capacitance was maintained.Compared with other reports,the capacitance and cycle stability of the VO₂ NFs/3DG electrode were wonderful.In addition,the electrochemical performances in two-electrode configuration were also studied to check the practical application of the VO₂ NFs/3DG electrodes.