Fabrication And Electrochemical Properties Of Graphene-nio Sandwich Layered Nanocomposites

Supercapacitor has received considerable attention as a new type of energy storage media due to its excellent properties, such as higher power density compared with rechargeable batteries, higher energy density compared with conventional capacitors, large capacity, broad operating temperature range and long cycle life. Currently, the key point of supercapacitor technology is the development of advanced electrode materials and highly efficient electrolyte.Graphene, as a new type of two-dimensional material, has become a hot topic in science community due to its outstanding electrical, optical and mechanical properties. Many researchers focus on graphene nanosheets (GN) as basic units for various sp2 carbon materials and are trying to intentionally constitute nanoarchitectures and macrostructures with tuned properties using graphenes as the building blocks. One of these attempts is to prepare graphene nanosheets-metal oxide composites, which are potential candidates for highly efficient energy storage media.In this thesis, we propose two approaches for preparing GN-NiO composites: homogeneous symbiosis and co-precipitation methods. The co-precipitation method is based on the reduced graphene that behaves as the substrate for the precipitation reaction to obtain hydroxide and final oxides. The GN-NiO composite is obtained by a two-step process. That is, nickel hydroxide is formed on GN through the reaction of nickel salts and alkali, followed by a decomposition process to form nanosized NiO decorated on GN. SEM observations show that NiO particles are unevenly distributed on graphene nanosheets with lots of agglomeration. Electrochemical performance tests show that the maximum specific capacitance is 156 F/g with poor rate performance.The homogeneous symbiosis method starts from homogenous mixing of graphene oxide (GO) and nitrate aqueous solution, which guarantees the perfect contact of GN with nickel oxide nanoparticles in the final composite. Homogeneously mixed GO and nickel nitrate is subjected to a slow drying to form layered structure followed by a vacuum-promoted heat-treatment to simultaneously realize the transformation from single layered GO to GN and nickel nitrate to NiO, which constitute a novel GN/NiO/GN sandwich layered structure. The specific capacitance of GN-NiO composite is two times as high as that of the corresponding graphene, which maintains 88% of the initial value after 1000 cycles. The energy density and power density are 32.8Wh/kg and 983W/kg respectively at a constant current density of 1000 mA/g. We further prepare GN-Fe3O4 composites by this method, which hints us that this is a facile and general approach for producing graphene-based layered sandwich structure.