Controlled Synthesis And Formation Mechanism Study Of Graphene Based Nanomaterials For Supercapacitors

Graphene is an astonishing new carbon material possessing large surface area, high eletrical conductivity and excellent mechanical flexibility, which enable it to be a potential candidate material for highly efficient energy strorage. Our initiatives include designing straightforward strategies for cotrolled synthesis of graphene based nanocomposites, exploring their formation processes, and investigating their potential applications for supercapacitors. Supercapacitor is an intermediate device between a traditional capacitor and a battery thereby combining both high energy and power densities with various applications in daily lives. Essentially, an electrode material is the most important part in a supercapacitor which nevertheless can not rely on traditional electrochemically active materials due to their intrinsic weakness. While these materials, once combined with graphene sheets, can be remarkably improved in their electrochemical properties; thus it is probable to develop high performance supercapacitor devices based on such combinations. The main contributions of this dissertation are described as follows:(1) Graphene oxide (GO) is a widely used graphene derivatives with rich oxygen-containing groups like hydroxyl, carboxyl and epoxy, etc. Becuase of its good dispersibility in both water and many organic solvents, it is considered as an excellent substrate for constructing nanostructures. The functional groups can act as anchor sites and consequently make the in situ formed nanocrystals attach on the surface and edges of GO sheets. It was found that electrochemical properties of the nanocomposites were largely affected by the nanocrystal microstructres. As a typical example, a composite of graphene oxide supported needle-like manganese oxide has been successfully synthesized by virtue of the abundant functional groups of graphene oxide as anchor sites. It suggests that the electrochemical cycling performance of manganese oxide was greatly improved by adding graphene oxide to the system, leading to a hybrid with satisfied specific capacitance (216.0F·g-1) and good cycling capability, with84.1%of specific capacitance retened after1000cycles in comparison with that of69.0%for manganese oxide alone.(2) It is observed that the graphene oxide-manganese oxide composites have relatively large electrochemical resistances owing to their poor conductiviy of both individual components. To improve the condictivity, three chemical reagents, e.g. ethylene glycol, hydrazine and strong alkaline were used to deoxynate graphene oxide. Nevertheless, the resulting products, namely graphene-MnOOH, graphene-Mn3O4and graphene-MnO2 unexpectedly did not give improved electrochemical properties with all their specific capacitance shrinked.We then proposed a one-step route to synthesize a graphene-Co(OH)2composite. In the preparation process, Na2S is a vital multi-functional reagent as it can hydrolyze into alkaline to deposite Co(NO3)2into Co(OH)2, while simultaneously release HS-and H2S to deoxygnate graphene oxide into graphene. The composite presented an enhanced specific capacitance in strong alkaline electrolyte with the maximum Cs reached up to972.5F·g-1.(3) Furthermore, we have noted that some intrinsic properties of graphene-based composites derived from graphene oxide have been impaired owing to the presence of large amount of defects in carbon backbones. Therefore, a new soft chemical procedure was developed to synthesize composites from low defect density graphene sheets. In a typical preparation process, pristine graphite was exfoliated in solvent by sonication to give low defect density graphene dispersion. Then the hydrolysis of some metal nitrates like AgNO3, Co(NO3)2, Zn(NO3)2, Cu(NO3)2and Ni(NO3)2was used to release H+in aqueous solution. The H+and NO3-then can react with cabon atoms of graphene sheets to induce a small amount of oxygen-containing groups on the graphene surfaces which can act as anchor sites for metal ions. As H+was directly produced from metal nitrate precursors, its concentration can be intentionally tailored so seldom abundant fuctional groups are introduced, thereby the conjuction of graphene can be well preserved in the composites. We have obtained graphene-Ag2O, Co3O4, ZnO, Cu2O and Ni(OH)2and found that graphene-Ni(OH)2composite had the best electrochemical performances, with high specific capacitance (1087.9F g-1) and excellent stability (more than90%of specific capacitance was retened after2000cycles).Moreover, the low defect density graphene was employed as a structural template to produce MnO2, Co3O4and Cr2O3nanolamellas from the redox reactions between graphene and metal precursors. MnO2nanolamellas had a large surface area (50.3m2·g-1) and pores (average pore diameters were14.4nm). Such unique nanostructure facilitated the insertion/desertion of electrolyte ions into/out of the electrode materials, and can inhibited the aggregation of MnO2during electrochemical cycling, giving a large specific capacitance (206.2F·g-1) with good stability (more than90%of specific capacitance was retened after3000cycles).(4) A carbon nanotube can be viewed as a scrolled graphene sheet, which is excellent in properties and widely used. Therefore, we extend our low defect density graphene system to the low defect density carbon nanotube based materials. Various low defect density carbon nanotube composites, including Ag2O, CO3O4, Cu2O and Ni(OH)2have been dramatically prepared via hydrolysis of metal nitrates in low defect density carbon nanotube dispersions. The low-defect density carbon nanotube-Ni(OH)2composite shows a large specific capacitance of1244.2F·g-1As a one-dimensional nanostructure, the low defect density carbon naotube was used as a template to prepare metal oxide nanowires (MnO2, Co3O4and Cr2O3). The as-prepared MnO2nanowires had a high specific capacitance of201.0F·g-1.