Controllable Synthesis Graphene Based Materials And Their Electrochemical Applications

Graphene is a monolayer of carbon atoms tightly packed intoa two-dimensional(2D) honeycomb lattice. Owing to its unique properties, includinghigh electrical andthermal conductivities, huge theoretical specific surface area (SSA), andgreatmechanicalstrength,graphene has attracted a great deal of attention in the flied ofenergy storage and conversion,composite materials,catalysis and so on.However,chemical modified graphene has relatively low conducitivty compared with that ofperfect graphene.The reduction of graphene oxide (GO) also increases thehydrophobic and π–π stacking interactionsbetween graphene sheets, which enhancestheir agglomerationand heavily decreases their specific surface areas. Meanwhile,although graphene materials have excellent properties, it can not meet all therequirements of practical applications. In this dissertation, we focus on the synthesisof graphene-based materials with controllable sizes, structures and compositions, andinvestigate their electrochemicalcatalytic activity for oxygen reduction reaction (ORR)and the electrochemical performance for supericapacitors.Graphene quantum dots weresynthesizedby directly oxidizingand etching graphite powders. The yield of GQDs is as high as63±7%, suggestingthis technique is suitable for producing GQDs on a large scale. The GQDsare nanocrystals with lateral dimensions in the range of2–4nm and an averagethickness of around1.3nm. The emission peaks of as-prepared GQDs can be tuned inthe range of440to510nm by varying the reaction conditions. Their fluorescencequantum yields were tested to be around1%, which could be further increased toabout3%by hydrothermal treatment.Graphitic carbon nitridewas immobilizedon the surfaces of chemically convertedgraphene sheets to form a layered composite, which exhibited greatly improvedconductivityand electrocatalytic performance on oxygen reduction reaction.Considering the low costs of chemically modified graphene (CCG) and graphiticcarbon nitride (GCN), and being freefrom CO poisoning, CCG/GCN composite is a cheap metal-freecatalyst with high catalysis performance for ORR.Nanoporous nitrogen doped carbon was used to modify the surfaces of graphenesheets by carbonizing a mixture of graphene oxide andphenol-melamine-formaldehyde pre-polymer in the presence of a soft template(F127). The resulting graphene based composite sheets (G-PMFs) have a sandwichstructure with one graphene layer and two nanoporous nitrogen-doped carbon layers.G-PMFs have large specific surface areas of190to630m2/g and exhibited highelectrocatalytic activity, good durability and high selectivity for the oxygen reductionreaction.Mesoporous graphene/onion carbon composite films (GOCs) have beensuccessfully prepared by graphitizing the composite films of GO and nanodiamond.The insertion of OC nanoparticles between graphene layers increased the SSAs ofGOC composites. They have high SSAs and conductivities of about420m2/g and7400S/m, respectively. Thus, they can be used as an electrode material forsupercapacitor and its specific capacitance was tested to be143F/g at0.2A/g.Furthermore, these thick mesoporous graphitic films have excellent electrochemicalstability and their rate performance is also much higher than that of a CCG film witha thickness of only10μm.Self-assembld graphene organogel (SGO) with a3D macrostructure has beensuccessfully prepared by solvothermal reduction of GO dispersed in PC. The resultingSGO has a good affinity with this solvent. Furthermore, it has a highly conductiveporous3D network. Therefore, the ion diffusion property of SGO basedsupercapacitors in TEABF/PC electrolyte was greatly improved. The supercapacitorbased on SGO exhibited a high specific capacity of140F/g and a high maximumenergy density of43.5Wh/kg. They also showed excellent rate capabilities.