Design.Fabrication And Assembly Of The3D Graphene Based Flexible Supercapactiors

As a new type of energy storage device, supercapacitors have been widely studied, because they have higher energy density compared to traditional capacitors, and higher power density and longer cycling life compared to Li-ion batteries. With the development of the flexible electronics, flexible supercapacitors show more and more potentials for applications, such as flexible display and energy storage system. Thus, a lightweight, flexible, and highly efficient energy management strategy is needed for flexible energy-storage devices to meet a rapidly growing demand. Graphene-based flexible supercapacitors are one of the most promising candidates because of their intriguing features.In this thesis, we describe the use of freestanding, lightweight (0.75mg/cm2), ultrathin (<200μm), highly conductive (55S/cm), large specific surface area (392m2/g) and flexible three-dimensional (3D) graphene networks, loaded with MnO2by electrodeposition, as the electrodes of a flexible supercapacitor. It was found that the3D graphene networks showed an ideal supporter for active materials and permitted a large MnO2mass loading of9.8mg/cm2(～92.9%of the mass of the entire electrode), leading to a high area capacitance of1.42F/cm2at a scan rate of2mV/s. With a view to practical applications, we have further optimized the MnO2content with respect to the entire electrode and achieved a maximum specific capacitance of130F/g. In addition, we have also explored the excellent electrochemical performance of a symmetric supercapacitor (of weight less than10mg and thickness～0.8mm) consisting of a sandwich structure of two pieces of3D graphene/MnO2composite network separated by a membrane and encapsulated in polyethylene terephthalate (PET) membranes. This research might provide a method for flexible, lightweight, high-performance, low-cost, and environmentally friendly materials used in energy conversion and storage systems for the effective use of renewable energy.In order to develop the flexible supercapacitors with high power density, free-standing and flexible three-dimensional graphene/carbon nanotubes/MnO2(3DG/CNTs/MnO2) composite electrodes with interconnected ternary3D structures were fabricated, and the fast electron and ion transport channels were effectively constructed in the rationally designed electrodes. Consequently, the obtained3DG/CNTs/MnO2composite electrodes exhibit superior specific capacitance and rate capability to3DG/MnO2electrodes. Furthermore, the3DG/CNTs/MnO2based asymmetric supercapacitor demonstrates the maximum energy and power densities of33.71Wh/kg and up to22727.3W/kg, respectively. Moreover, the asymmetric supercapacitor exhibits excellent cycling stability with95.3%of the specific capacitance maintained after1000cycle tests. Our proposed synthesis strategy to construct the novel ternary3D structured electrodes can be efficiently applied to other high performance energy storage/conversion systems.To obtain freestanding and binder-free negative electrodes, the ZnO nanowires were vertically grown on3DG skeletons by hydrothermal method, and the obtained ZnO nanowires were subsequently used as sacrificial template to fabricate the Fe2O3nanotubes (NTs) via the reaction between ZnO NTs and Fe3+in Fe(NO3)3aqueous solution. The constructed3DG/Fe2O3NTs composite electrode exhibits a high areal capacitance of435mF/cm2, which can be attributed to the synergetic effect that comes from the unique structure of3DG and Fe2O3NTs. The optimizing for the structures and electrochemical properties of the3DG/Fe2O3NTs electrode as well as its asymmetric device is still under way.