One-step Preparation Of Porous Carbon For Supercapacitors With Inorganic Salt Electrolytes

As a new type of energy storage devices, the main limitation of the supercapacitors is their lower energy density. Novel carbon materials with high surface area, a range of shapes, sizes and pore size distributions, as well as high electrical conductivity are being constantly developed and tested as potential supercapacitor electrodes. In addition, aqueous medium as promising electrolytes for supercapacitors because they are high conductivity, low viscosity, cheap, non-corrosive and electrochemically stable. The current research is focused on how to improve the energy density of the surpercapacitors by choosing suitable precursors and method to prepare porous carbon or nitrogen-doped porous carbon materials with excellent properties, and studying on supercapacitive performance of aqueous medium. In this thesis, therefore, we concentrate on the research of the preparation and characterization of high performance porous carbon materials and their hetero-atom doping. The electrochemical performance of the porous carbon materials derived from organic and aqueous medium, as well as the relationship between the conductivity and the electrochemical performance of the nitrate solutions was investigated in detail. Main conclusions and findings were drawn as follows:(1) Synthesis and characterization of the porous carbon materials derived from organic salts:This paper presents the direct synthesis of porous carbon materials derived from organic salts via the carbonization of tartrate salt, humate salt and potassium acid phthalate in inert atmosphere without any activation. The carbonization temperature and time is a very important parameter that ensures formation of the desired carbon structures and enhances their electrochemical performance in supercapacitors. The porous carbon materials derived from organic salts exhibited a high specific surface area, developed pore structure, high conductivity, and supercapacitive performance, which are ideal for electrodes of supercapacitors. The porous carbon materials derived from potassium humate has a high specific surface area(890 m2 g-1), as well as a large specific capacitance of 232 F g-1(26.1 μF cm–2) at a current density of 0.5 A g-1. The sponge-like nanoporous carbons derived from potassium tartrate has a high specific surface area(1290 m2 g–1), as well as a large specific capacitance of 296 F g–1(29.1μF cm–2) at a current density of 0.7 A g-1. The interconnected microporous carbon derived from potassium acid phthalate has a high specific surface area(1118 m2 g-1), as well as a large specific capacitance of 243 F g-1(21.7 μF cm-2) at a current density of 0.1 A g-1.(2) Synthesis and characterization of the Nitrogen-doped porous carbon materials derived from organic:Nitrogen-doped porous carbons are very simply prepared by direct carbonization of a nitrogen-containing organic salt, with low-biotechnology fulvic acid potassium salts as a precursor. The prepared Nitrogen-doped porous carbons had a high surface area(2142 m2 g-1), large pore volumes(1.35 cm3 g-1), and nitrogen content(4.66%), making them suitable electrode materials for supercapacitors. It can also deliver excellent electrochemical performance(large specific capacitance of 235 F g-1(14.50 μF cm-2) at a current density of 0.5 A g-1) and cycle durability.(3) Thermal decomposition of potassium acid phthalate:The reaction mechanism of the preparation of porous carbon materials by direct carbonization of organic salts was discussed. The metallic potassium serves a crucial function in increasing material porosity as their vapors efficiently intercalate into the carbon lattices of the carbon matrix, which results in disruption and swelling of the carbon lattices. Thus, it is beneficial to the formation of the pore structure. In addition, the formation of CO2 gas in the carbonization process is helpful to the formation of micropore structure.(4) The electrochemical performance of nitrate solution:The relationship between the conductivity and the electrochemical performance of nitrate solution is studied by using the different concentration of the nitrate solution as the electrolyte solution. Results indicate that the conductivity of electrolyte solution is a key factor in obtaining optimal electrochemical behavior, it is not necessarily the most important factor. Interesting,Electrochemical investigations showed that the nitrate solution can be used in an efficient capacitor working in a wide voltage range, while maintaining a high capacitance retention(after 5,000 cycles) even more than water theoretical decomposition voltage. Due to this reason they can be considered as one of the most promising electrolyte solutions.