Study On Porous Carbons And Their Rate Capability As Electrode Materials Of Supercapacitors

Supercapacitor is a new type of energy storage device that has attracted much attention owing to the high power density, long lifetime and so on. Porous carbons are the most widely used electrode materials for supercapacitor, and their microstructure, specific surface area, porous structure, electrical conductivity, surface chemistry property, etc. have great effects on their electrochemical performances. Nevertheless, porous carbons as electrode materials suffer from not only the low energy density but also the poor rate capability related to their unique porous structure, and thus show the limited power characteristics at high rate. In this study, we focus on the development of porous carbons as electrode materials of supercapacitors with high-rate performance by improving or optimizing their porous structure, electrical conductivity and microstructure.Phosphate-functionalized carbon nanotube/carbon composites with hierarchical porous structures have been prepared by a combined process including hydrothermal reaction and heat treatment, in which nonionic surfactant P123serves as soft template, and fructose and phosphoric acid are used as carbon source and phosphorus source, respectively. Carbon nanotube with suitable content helps increase the volumes of mesopores and macropores and the electrical conductivity of the composite. In contrast to the product free of carbon nanotube, the capacitance retention of the composite increases from6to78%at a current density of10A g-1. Moreover, phosphorus-containing functional groups help extend the operating voltage up to1.2V, and the composite also exhibits highly stable cycling performance at this voltage.Graphene/hydrothermal carbonaceous composites with high nitrogen content (10wt.%) have been synthesized by graphene oxide (GO) assisted hydrothermal reaction of glucosamine. Moreover, sheet-shaped carbonaceous composites are preferred to be generated since GO serves as a directing agent for deposition of intermediates from the hydrothermal reaction of glucosamine. Graphene with suitable content is beneficial for the KOH activation process of the carbonaceous composite, which makes the product possess higher specific surface area, micropore and mesopore volume, and helps increase the electrical conductivity by three orders-of-magnitude. Compared with the product without graphene, the activated composite with graphene has higher specific capacitance (300F g-1) and rate capacity, and the capacitance retention increases from36to76%at a current density of8A g-1Ultrathin polydopamine layers coated on nano-sized calcium carbonate nanoparticles have been prepared by using dopamine as precursor inspired by its self-polymerization property under basic and oxygen-containing reaction condition. Polydopamine layers are transformed to ultrathin carbon layers under heat treatment, and calcium carbonate nanoparticles release carbon dioxide simultaneously which in-situ activates the carbon layers and thus introduces micropores. Finally, graphene-like two dimensional ultrathin porous carbon nanosheets can be obtained when the templates are removed. The results of electrochemical measurements demonstrate that porous carbon nanosheets exhibit good rate capability, with a high capacitance retention of71%at a high current density of100A g-1.A general strategy for the synthesis of two dimensional carbon nanosheets have been developed with layered nanospace of montmorillonite as confined template, and gelatin and dopamine as the typical carbon source by using the intercalation reaction. Porous carbon nanosheets as electrode materials can be obtained by further KOH activation of carbon nanosheets. Gelatin derived porous carbon nanosheets possess a high specific capacitance of246F g-1at a high current density of100A g-1, and the capacitance retention increases from44to82%in comparison with the activated product directly from gelatin. Moreover, gelatin derived porous carbon nanosheets also exhibit excellent rate capability in the two electrode test with aqueous and organic electrolyte, showing a capacitance retention of83and81%at a current density of40A g-1, respectively.