Graphene Based Supercapacitors:Preparation And Electrochemical Energy Storage Mechanism Of Electrode Materials

As a fascinating green energy storage and conversion system, supercapacitor withexcellent electrochemical performance urgently desires electrode materials with highperformance. Graphene, as a building block of sp2carbon materials, is characterizedby unique structure and fascinating properties, and shows great potential in the fieldof supercapacitors. Many methods have been developed to control the structure andproperty of graphene. However, there is a great gap between the obtained specificcapacitances and the theoretical one, and the fabricated supercapacitor usually suffersfrom an inferior energy density. Therefore, this dissertation mainly investigates theenergy storage mechanism of graphene-based electrode materials, controllablyprepares the electrode materials with unique structure to optimize theirelectrochemical performances, and fabricate highly efficient electrochemical devices.In order to confirm the main factors influencing the electrochemical performanceof graphene-based electrode materials, two typical types of graphene-based carbonmaterials, graphene nanosheets (GNs) with nonporous structure and zeolite templatedcarbons (ZTCs) with highly microporous structure were first exploited as foundationalmodel materials to investigate their electrochemical behavior and capacitive propertyin an aqueous electrolyte. We found that both pore structure and surface chemistryhave effects on the electrochemical performance, but their roles in determining thecapacitive properties of carbons with different structure should be distinguished. Thesurface chemistry plays a more vital role in affecting the capacitance properties of lessporous carbons. While for the electrochemical performances of porous carbon, bothfactors exert a synergetic effect on the specific capacitance of carbon materials, butpore structure is a dominant factor. We choose three-dimensional porous graphenemacroassembly to investigate the relationship between microstructure, specificsurface area and oxygen-containing functional groups of carbon materials and theirelectrochemical performances. Optimizing the amounts and species ofoxygen-containing functional groups can improve the affinity of electrodes andreduce the ion transport resistance, which are beneficial for enhancing the specificcapacitances and rate performances of carbon electrodes.Based on the fundamental research, we develop some novel methods to fabricateelectrodes materials for supercapacitors with outstanding electrochemical performance in order to widen their applications. An evaporation-induced dryingmethod was employed to prepare high density graphene macroassembly withabundant pores. The macroassembly as an electrode for a supercapacitor exhibits anultrahigh volumetric capacitance. The graphene macroassembly as a carbon backbonealso could be utilized to homogenously disperse metal oxide nanoparticles onto thegraphene surface. We propose a facile solution method to prepare RuO2/graphenecomposite macroassembly with higher density based on the same evaporation-induceddrying mechanism. The synergetic energy storage mechanism between RuO2andgraphene ensures the composite electrode shows ultrahigh volumetric energy densityand power density. A facile and novel reactive template method at mild condition wasdeveloped to synthesis three-dimensional porous MnO2nanomaterials. The excellentrate performance of MnO2electrodes widens the application of the correspondingsupercapacitors. Furthermore, MnO2/graphene composite macroassembly withthree-dimensional structure can be obtained by adjusting the molar ratio of thereactants.