Partial Exfoliation Of Carbon Electrodes And Their Incorporation With Pseudocapacitive Materials

In order to further improve capacitive properties of supercapacitor,it is important to develop advanced electrode materials with superior performance.In the last decade,development on carbon-based composite materials.especially graphene/pseodocapacitve materials,opens up a new direction to develop advanced supercapacitors with improved performance.However,for the composite materials fabricated by the conventional methods,the highly conductive pathway between the graphene sheets is partially blocked by the semi-conductive pseudocapacitive compounds,leading to limited energy storage performances.Therefore,developing integrated 3D conductive network for carbon-based composite materials to further improve the supercapacitive performance is still a scientific challenge.In this thesis,unique 3D porous graphene frameworks are tried to be constructed through partial exfoliation of carbon electrode,using facile electrochemical methods.Scanning electron microscope(SEM),transmission electron microscopy(TEM),atomic force microscope(AFM),X-ray photoelectron spectroscopy(XPS),Raman spectra are conducted to characterize the materials.Contact angle is collected to test the wetting ability.Cyclic voltammetry,constant current charging/discharging technique,and electrochemical impedance spectra(EIS)are used to study the capacitive performance of the fabricated electrodes.Integration of the partially exfoliated carbon electrodes with pseudocapcitive materials,such as MnO2,polypyrrole,vanadium oxide,and Ni-Co double hydroxide,is conducted by in-situ electrodeposition strategies.The physical/chemical properties of these composites are characterized by SEM,TEM,XPS,Raman,and X-ray powder diffraction(XRD).The electrochemical performance of these materials is studied by cyclic voltammetry,constant current charging/discharging technique,and EIS.These materials exhibit good electrical conductivity and enhanced cycling stability.Our strategies provide a new opportunity for fabricating high performance carbon-based pseudocapacitive composite materials for energy storage applications.The main work and findings of this thesis are listed below:(1)Partial exfoliation of carbon electrodes(e.i.graphite foil and carbon cloth)is conducted by facile electrochemical methods.First,functionalized partially exfoliated graphite(FEG)is fabricated by a two-step electrochemical method.Second,electrochemically partial exfoliation is conducted using commercial conbon cloth as the substrate(ECC).Third,a unique tri-layered graphite structure(RTG),which contains the top graphene layer,middle ion intercalated graphite layer and bottom bulk graphite layer,is constructed through electrochemical ion intercalation and subsequent electrochemical reduction process.The small carbon units,such as graphene and graphite sheets,are partially exfoliated from the graphite base rather than fully exfoliated into the electrolyte.The partially exfoliated carbon is seamlessly connected with the substrate,constructing a 3D highly conductive network.Owing to their excellent electrochemical performance and unique structures,these partially exfoliated carbon electrodes have great potential to be used as advanced current collector to support pseudocapacitive materials.The functional groups on the carbon electrodes can act as growth sites to enhance the interaction between the carbon and the pseudocapacitive materials,facilitating their synergistic effect.In addition,the flexible exfoliated carbon can also act as structural buffer layer to release the mechanical stress of the pseudocapacitive materials generated during the charging and discharging process,further enhancing their cycling stability.Furthermore,RTG electrode exhibits excellent areal capacitance of 820 mF cm-1 at a current density of 2 nA cm-2,corresponding to a high gravimetric capacitance of 96.5 F g-1 based on the whole mass of the electrode(?8.5 mg cm-2),which is nearly 400 times higher than that of pristine graphite foil.RTG also delivers a good capacitive retention of 75%when the current density increased from 2 to 100 mAcm-2.This remarkable rate capability can be attributed to its excellent electrical conductivity.In addition,RTG also shows 94%capacitive retention after 10000 charging/discharging cycles,indicating its good cycling stability.These results are among the best values for carbon and carbon based composite materials with comparable mass loadings.(2)Manganese dioxide(MnO2)nanosheet arrays are electrochemically deposited on functional partially exfoliated graphite(FEG)to afford FEG/MnO2.The deposition of MnO2 nanosheet array on the partially exfoliated graphene in FEG constructs a unique hierarchical porous structure,ensuring excellent electrical conductivity and fast diffusion rate.FEG/MnO2 electrode delivers a high specific capacitance of 1061 F g-1 at current density of 1 A g-1,approaching its theoretical value.In addition,this electrode also exhibit good rate capability and cycling stability.An assembled supercapacitive device using RTG as anode and RTG/MnO2 as cathode can yields a high energy density of 9 Wh kg-1 at power density of 97W kg-1(calculated based on the total mass of both electrodes,?19 mg cm-2).Even at a high power density of 2466 W kg-1,the device can still retain a high energy density of 6.4 Wh kg-1.Moreover,RTG//RTG/MnO2 also exhibits good cycling stability with only 6%capacitive decay after 10000 cycles.(3)A two-prong strategy is designed to stabilize PPy film by growing it on FEG substrate and doping it with ?-naphthalene sulfonate anions(NS-).The PPy electrode achieves a remarkable capacitance retention rate of 97.5%after cycling between-0.8 and 0 V vs.SCE for 10000 cycles.The exceptional stability of PPy electrode can be attributed to two factors:1)the flexible nature of FEG substrate accommodates large volumetric deformation;2)the presence of immobile NS-dopants suppresses the counterion drain effect during charge-discharge cycling,thus ensures good electrical conductivity.An asymmetric pseudocapacitor using the stabilized PPy film as anode also retains 97%of the capacitance after 10000 cycles,which is the best value reported for PPy based supercapacitors.In addition,the assembled supercapacitor device also delivers a remarkable energy density of 75 Wh kg-1 at power density of 1000 W kg-1.(4)Ultrathin Ni-Co double hydroxide(Ni-Co DH)nanosheets are electro-deposited on the partially exfoliated graphene on FEG.The interconnected hydroxide nanosheets deposited on the graphene construct hierarchical conductive network,increasing the surface area and decreasing the ion diffusion distances.The seamless connection between the partially exfoliated graphene and the graphite base ensures fast charge transport for the deposited Ni-Co double hydroxide materials.Therefore,the battery-type Ni-Co DH deposited on FEG exhibits pseudocapacitive properties and excellent rate capability.FEG/Ni-Co DH delivers a remarkable capacitance of 2442 F g-1 at current density of 1 A g-1.A capacitive retention of 83.5%is also retained when the current density increased 50 times from 1 to 50 A g-1.In addition,a supercapacitor device is assembled using FEG/PPy as anode and FEG/Ni-Co DH as cathode.It is shown that the fast decay of the device's capacitance during cycling test can be ascribed to the over-oxidation of the PPy anode.Therefore,we propose an "unbalanced charge matching" strategy to optimize the charge storage potential of both electrodes during long term charging/discharging process to suppress the over-xoidation of PPy anode.Cycling stability of the supercapacitor device has been greatly improved,91%capacitance retention can be achieved after 5000 charge-discharge cycles.The device can also deliver a high energy density of 60 Wh kg-1 at power density of 650 W kg-1.(5)Amorphous mixed-valence vanadium oxide is electrochemically deposited on exfoliated carbon cloth.The valence state of V in the oxide is adjusted by an additional electrochemical reduction process.Our results suggest that tuning the V(?)V(?)ratio of vanadium oxide can efficiently suppress the dissolution of the active materials.The oxygen-functionalized carbon shell on exfoliated carbon cloth can bind strongly with VO,via the formation of C-O-V bonding,which retains the electrode integrity and suppresses the structural degradation of the oxide during charging/discharging.The amorphous mixed-valence vanadium oxide without any protective coating exhibits record-high cycling stability in the aqueous electrolyte with no capacitive decay in 100,000 cycles.In addition,SEM,XPS,and EIS spectra were collected to further study the "active process" of vanadium oxide during cycling.