Modified graphene for energy conversion and storage applications

• Main Author: Xiao, Ni
• Other Authors: School of Materials Science & Engineering
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Materials::Energy materials
• Online Access: http://hdl.handle.net/10356/55772
• Institution: Nanyang Technological University

In current society, global energy consumption has been rising at an astonishing speed because of the massive use of the electrical machines and equipments. Practically, electrical energy is the end product of many power processes. So study of electrical energy conversion and storage is a meaningful job towards practical applications. Graphene, one atomic sp2-bonded planar carbon sheet, has been theoretically and experimentally reported as a promising candidate in energy conversion and storage field due to its unique and outstanding properties. However, towards the advantage technologies of graphene-based materials, there still remain some challenges. Zero-band gap, low measured surface area caused by restacking, and poor reaction activity would all become bottlenecks for graphene in its great potential applications. These constitute the driving forces behind the exploration of graphene modification. In this thesis, we focused on the targeted synthesis of modified graphene using different physical or chemical methods. We have successfully modified graphene with local disorder, pores, and foreign nitrogen atoms respectively and achieved corresponding enhanced performances in electrical energy conversion (thermoelectric energy conversion) and storage (supercapacitor) applications. Firstly, local disorder was introduced into the CVD few-layer graphene films using plasma. For such disorder-modified few-layer graphene, the maximum thermopower could be strongly increased up to ~700 μV/K and the resulted power factor could reach as high as ~4.5×10-3 W K-2m-1, which was 15 times higher than the original one. Although the large-scale production of graphene using CVD method is still a challenge, this work experimentally opens up a new possibility for graphene in the application of thermoelectric energy conversion. Secondly, pores were generated in the structure of graphene frameworks through a facile thermal treatment of graphene oxides with the aid of intercalated nitric acid. The nitric acid not only favors the generation of pores into graphene but also facilitates the expansion of graphene. The specific surface area of such graphene frameworks was as high as 463 m2/g and the pore volume reached up to 2.23 cm3/g. When tested as supercapacitor electrodes, the graphene frameworks showed attractive energy storage performance, e.g. ~370 F/g at a current density of 1 A/g in 6 M NaOH electrolyte. This value was much higher than that of the sample without such treatment (~195 F/g at 1 A/g). Finally, nitrogen-modified few-layer graphene was prepared with the aid of melamine. The advantage of this approach lied in that the few-layer graphene was obtained directly from the graphite flakes and the structure avoided heavy damage like chemically derived graphene. We also studied the supercapacitor performances of such modified graphene and found the capacitances could reach ~220 F/g.