Study Of Solid-state Supercapacitor Based On 3D Graphene/transition Metal Oxide Composites

Recently,with the rapid development of wearable electronics,the demand of energy storage devices for wearable electronics is growing.Therefore,study of energy storage devices for flexible wearable electronics have been one of the emphases and hotspots at the field of energy storage.As one of the main types of flexible energy storage devices,the research and development of flexible solid-state supercapacitor(FSSC)has attracted extensive attention.There are many kinds of electrode materials for supercapacitors,among which transition metal oxides are widely used in supercapacitors because of their high theoretical specific capacitance.Moreover,the low conductivity of transition metal oxides limits their practical application.Therefore,it is urgent to study and develop new methods for improving the conductivity of transition metal oxides to enhance their specific capacitance.In addition,conventional electrode materials generally require a conductive substrate without capacitive contribution as the collector of the electrode material,which increases the overall weight of the whole electrode and decreases the specific energy density based on whole electrode.Therefore,it is of great significance to construct 3D flexible self-supporting electrodes with high specific surface area and high specific capacitance for constructing flexible self-supporting solid supercapacitors.In view of the above problems,this thesis firstly studied the strategy for improving the specific capacitance of transition metal oxide by iron doping.Then 3D G-PPy flexible self-supporting composite materials of graphene(G)and polypyrrole(PPy)were prepared by chemical vapor deposition and electrodeposition methods,which was further used as electrode collector and active material to vertically grow Fe-doped MnCo₂O₄(Fe-MnCo₂O₄)nanowires on its surface.Thus,the 3D flexible self-supporting G-PPy@Fe-MnCo₂O₄ electrode was constructed and its application in flexible solid-state supercapacitors was further investigated.The main research contents are as follows:(1)3D Fe-MnCo₂O₄ nanowire arrays on nickel foam were prepared by in situ one-step hydrothermal method.The influence of Fe-doping ratios on the electrochemical performance of the electrode materials were investigated,and the doping mechanism was simulated by DFT simulation.The results show that the 3D network of nickel foams can load more mass of active materials,and the porous structure can provide more channels for full contact between electrolyte and active materials,and electrolyte ions can transfer quickly during charge and discharge which is important for the cycle stability of electrode materials.Fe-MnCo₂O₄ NWs obtain metallic characteristics by doping Fe into MnCo₂O₄ lattice which induces the free propagation of electrons in the electrode.And the density of the electron cloud presents de-localization.As a result,the conductivity of the electrode material is enhanced greatly which increases the bulk phase active sites of the electrode material,improving the utilization efficiency of the electrode material,electron transport rate and specific capacitance.The experimental results show that the enclosed CV area of Fe-MnCo₂O₄ nanowire electrode is obviously larger than that of MnCo₂O₄ nanowire electrode.When Fe:Co is 1:2,the area specific capacitance of Fe-MnCo₂O₄ nanowire electrode achieves the highest value which is up to 6500 m F cm^-2.Therefore,the specific capacity of MnCo₂O₄ electrode materials can be effectively improved by Fe-doping method.(2)3D graphene was prepared on nickel foam by chemical vapor deposition(CVD)method,and PPy was deposited on 3D graphene by electrodeposition method to construct a self-supporting 3D G-PPy substrate,and then Fe-MnCo₂O₄ nanowire arrays were grown on the 3D G-PPy substrate by one-step hydrothermal method.As a flexible self-supporting substrate,the 3D network skeleton of G-PPy greatly improves the specific surface area of electrode materials.Compared with the conventional current collector,the 3D G-PPy self-supporting substrate can both be the active material and current collector which is beneficial to the capacity of the whole electrode.After that,Fe-MnCo₂O₄ composite nanowire arrays were vertically grown on 3D G-PPy by hydrothermal method,and the influence of different Fe-doping ratios on the performance of electrode materials were studied to obtain the best Fe-doping ratio of the 3D G-ppy@Fe-MnCo₂O₄ electrode.The electrochemical performance of the electrode material shows that the area specific capacitance of the electrode material reaches to 5136 m F cm^-2 at the current density of 2m A cm^-2,and the capacitance retention can be maintained up to 94.7%after charging and discharging 7000 cycles,presenting excellent stability.Finally,3D G-ppy@Fe-MnCo₂O₄ was used as electrode to construct symmetrical flexible solid-state supercapacitor(FSSC).The CV curves have no obvious change at different bending angles,and the self-supporting FSSC around finger can power LED Light successfully.It demonstrates that the fiexible self-supporting FSSC has excellent electrochemical stability and good wearable characteristics.Therefore,a novel flexible self-supporting 3D electrode is developed in this thesis,which provides a new concept and approach to explore next energy storage devices for flexible wearable electronics.