The Research On Electrochemical Capacitive Properties Of Graphene Sheets In Ionic Liquid Electrolytes

With a large amount of consumption of fossil fuel and global environment aggravating, there is a growing demand for green, sustainable and highly efficient energy storage devices. Supercapacitors, recognized as an important class of energy storage devices, have been recently broad applications due to their many advantages including high power density, fast charge/discharge processes, long cycle life, excellent safety and environmental benefits. Graphene sheets (GNSs), one-atom-thick planar sheets of sp2-bonded carbon atoms, have been identified as one of the most ideal electrode materials due to their superior electrical conductivity, high theoretical surface area and remarkable mechanical stiffness. Moreover, electrolyte is also an important component of supercapacitor. Ionic liquids (ILs), a class of room-temperature molten salts, have been recently recognized as a promising candidate to substitute the conventional electrolytes owing to their unique properties like wide operation voltage, low vapor pressure, non-flammability and excellent electrochemical/thermal stability.Firstly, we prepared high quality GNSs using modified Hummers’method with a little improvement. According to the result of FESEM, we find the GNSs are efficiently exfoliated to form separated thin sheets and wrinkled to form mesopore structure. Then, supercapacitors were assembled by GNSs electrodes and ILs electrolytes. We systematically study the electrochemical properties of GNSs in a series of ILs with the different anions. Combined with the formula derivation and building models, we conclude that the viscosity, ion size and molecular weight of ILs jointly affect the electrical conductivity of ILs which directly determines the electrochemical performances of GNSs.Secondly, we investigate a novel ionic liquid (IL) gel polymer electrolyte. The IL gel electrolyte shows excellent mechanical strength and high transmittance. Meanwhile, graphene electrode shows excellent electrochemical performance in this IL gel electrolyte. Thus, we prepared transparent conductive graphene film on poly(ethylene terephthalate)(PET) substrate by vacuum filtration method. Then, we fabricate a transparent, flexible and all-solid-state supercapacitor based on graphene film electrode using ionic liquid gel polymer electrolyte. The integrated device exhibit specific capacitance (30uF/cm) and a wide potential window (2.5V). More importantly, the capacitance of the device remains90%under mechanical deformations.Lastly, few layer graphene nanosheet were synthesized by arc discharge method. As-made graphene nanosheets (denoted as ad-GNSs) show few defects, high electrical conductivity and high thermal stability. We prepared a high conductivity IL gel polymer electrolyte modified by adding ad-GNSs as the ionic conducting promoter. This modified gel electrolyte shows excellent thermal stability up to400℃and a wide electrochemical window of3V. On this basic, an all-solid-state supercapacitor was fabricated using this modified IL gel polymer electrolyte, which shows obviously improved electrochemical behaviors compared with those of the all-solid-state supercapacitor using pure IL gel electrolyte. Specially, smaller internal resistance, higher specific capacitance, better rate performance and cycling stability are achieved.