Controlled Synthesis Of Graphene/Graphene-based Materials For Their Supercapacitor Application

1. Large scale synthesis of graphene with controlled size and layer numbers has been realized by chemical exfoliation method. The impact of graphene size and layer numbers on the electrical conductivity of transparent conductive film electrodes and their specific surface area (SSA) has also been well studied. It is found with increasing amount of oxidants, the size of graphene oxide (GO) is dramatically decreased from20000to550nm2. And if we decrease the amount of oxidants, we can get few layered graphene oxide (FGO) which exhibits both larger size (～1μm) and number of layers (2-4) than single layered GO. Transparent conductive film electrodes based on reduced GO and FGO have been fabricated by spin coating process and chemical or thermal reduction. It is demonstrated that transparent conductive film electrodes based on reduced FGO exhibit much higher electrical conductivity of～480S/m than those based on reduced GO (-38S/m), which indicates that the size and number of layers of graphene have played important roles on the electrical conductivities of transparent conductive film electrodes. Furthermore, based on the results from theoretical calculation and experimental characterizations, we also find out that the size and number of layers in graphene materials have great influence on their SSA. That is, the SSA of graphene materials will highly decrease with increasing size and number of layers, as an example shown in FGO (60m2/g) and GO (4m2/g).2. Until now, few sp2carbon materials simultaneously exhibit superior performance for SSA and electrical conductivity at bulk state. Thus, by combining the good conductivity of graphene and high SSA of activated carbon, we prepare a series of graphene-based sp2carbon materials at the bulk scale, which exhibit ultrahigh SSA and excellent bulk conductivity. It is found that both the SSA and conductivity of the products can be controlled by varying the proportion between the graphene and activated carbon in the products. The conductivity of the products is increased with increasing amount of graphene. But the SSA of the products is firstly increased but then decreased with increasing amount of graphene. Thorough characterizations have suggested that the structure and morphology of carbon precursors played important roles on the SSA of the products. This is further identified by using porous pollens and firm solid phenolic resin as carbon precursors of activated carbons. And activated carbons based on porous pollens have exhibited much higher SSA than phenolic resin based activated carbons. Furthermore, comprehensive study and structural characterization have been performed for exploring the structure of graphene-based sp2carbon materials and explaining the origin of its ultrahigh specific surface (even higher than the theoretical SSA of graphene,2680m2/g). We conclude that these graphene-based sp2carbon materials consist of mainly defected/wrinkled single layer graphene sheets in the dimensional size of a few nanometers. And the ultrahigh SSA cannot be due to the existence of pentagon and/or heptagon in the graphene-based sp2carbon materials but should mainly come from the edge (or defects) of the graphene sheets with dimensional size of a few nanometers. We believe this should be also applicable to other sp2carbon materials with high SSA, such as activated carbon, thus has important implication for the structural and property study of activated carbon.3. A series of sp2carbon materials with different SSA and controlled pore size distribution (PSD) were prepared at large scale through a facile and low-cost method. The SSA and PSD of these carbon materials were controlled by adjusting preparation methods and using different carbon sources. Experimental capacitance performance has been tested for all the sp carbon materials and the highest specific capacitance in ionic liquid is220F/g, with an ultrahigh energy density up to90Wh/kg. The impacts of SSA and PSD on their capacitance performance were thoroughly investigated, which demonstrated that both SSA and PSD played the most important role on their effective SSA (E-SSA) and capacitance performance. And a linear relationship has been observed between the E-SSA and experimental specific capacitance, with a coefficient of determination of0.96. Furthermore, a general theoretical model using the slit/cylindrical NL-DFT approach is proposed for the estimation of the specific capacitance of sp2carbon materials, which is in good agreement with the experimental specific capacitance. These results offer a simple but reliable method to predict the capacitance performance of these materials, thus speeding up the designing and screening the materials for high performance supercapacitor and other surface area related devices.4. A versatile new strategy for producing graphene/cobalt magnetic nanocomposites by combining the sol-gel method and autocombustion is presented. GO, cobalt nitrate and citric acid are used as starting materials and a dry gel of the mixture is prepared through a routine sol-gel approach. The autocombustion reaction was activated at300℃in a tube furnace under an argon atmosphere, which produces lots of reducing agents such as H2and CH4and then in situ reduce GO and cobalt oxide to get graphene/cobalt magnetic nanocomposites. It is demonstrated that the cobalt nanoparticles (but not cobalt oxide) with size of～10nm are homogeneously loaded on graphene sheets. Further more, other metal nanoparticles such as Ni, Cu, Ag and Bi can also be loaded on graphene using the same method, which have great potential for the application on the electromagnetic shield and microwave absorption.