Study On The Control Of Pore Structures And The Enhancement Of Electrochemical Properties For Carbon Materials

In this dissertation, template carbonization method were used to prepare a series of nanoporous carbon with high specific surface areas and pore volumes by regulating carbonization temperature and the mass ratio between carbon precursors and templates. In addition, the capacitor performances are measured in two-electrode system and three-electrode system, and further improved by introducing redox active additive into electrolyte. The major research contents of the dissertation are summarized as follows:1. Nitrogen-containing nanoporous carbon with spherical network structure have been prepared using a template carbonization method, in which diphenylguanidine serves as carbon precursor, commercially available CaCO₃ as template, and azodicarbonamide as nitridation agent. All the obtained nanoporous carbon materials exhibit the amorphous features with low graphitization degree and show nanoscale spherical structures. Besides, the nitrogen species doped mainly include pyridine nitrogen, pyrrolic nitrogen, and graphitic nitrogen. The experimental results indicate that the mass ratio of starting materials can effectively control the specific surface areas, pore structures and nitrogen contents of the obtained samples. The Carbon-DC sample exhibits a specific surface area of 652.3m² g^-1 and a nitrogen content of 6.9 at.%. However, the specific surface area of Carbon-DCA which is obtained after nitriding treatment can reach up to 1022.6 m² g^-1, and the nitrogen content is 12.45 at.%.Electrochemical performances are first measured in a three-electrode system using KOH as electrolyte. At a current density of 1 A g^-1, the Carbon-DC and Carbon-DCA samples present the specific capacitances of 176.2 and 220.0 F g^-1. Next, azodicarbonamide is incorporated into the carbon precursor as nitridation agent during the carbonization process to enhance the nitrogen content. Carbon-DCA is measured in a three-electrode system using H₂SO₄ as electrolyte, at a current density of 1 A g^-1, the specific capacitance is improved from 220.0 F g^-1 to 263.0 F g^-1, and shows good electrochemical stability.2. A straightforward multi-template carbonization method for producing nanoporous carbons has been implemented by using magnesium citrate as carbon source and magnesium powder as template. The mass ratio of magnesium citrate and magnesium powder as well as carbonization temperature have significant impact on the specific surface areas, pore structures and electrochemical performances of the obtained samples. C-4:1-900 obtained at 900℃ shows a large specific surface area of 1972.1 m² g^-1 and a high pore volume of 4.78 cm³ g^-1. Besides, higher temperature can lead to larger crystallinity, C-4:1-1000 shows the best crystallinity.In a three-electrode system, the specific capacitance of C-4:1-900 can reach up to 236.5 F g^-1 at 1 A g^-1. Even at a current density of 20 A g^-1, the capacitance retention is still 66.3%. In a two-electrode system, the C-4:1-900 sample was measured at different operation temperatures of 25/50/80℃, and the specific capacitances are 132.6,158.4 and 178.2 F g^-1, respectively. Notably, the energy density of the cell configuration measured at 80℃ is 15 Wh kg^-1 even at a high power density of 10 W kg^-1.3. Highly nanoporous carbon materials have been produced by a synchronous carbonization/graphitization process, using magnesium citrate serves as the carbon source and nickel nitrate as graphitization catalyst. The lower temperature favors for the formation of larger porosity, whilst higher temperature for better crystallinity. The specific surface area of the C-800 sample can reach up to 2587.13 m²g^-1, which is larger than most of the reported nanoporous carbon materials. Moreover, the corresponding pore volume also displays a high value of 4.64 cm³ g^-1, but the crystallinity of C-800 is lowest. The specific surface area of the C-1000 sample is 967.96 m² g^-1 but the lattice spacing has slightly increased and the diffraction ring becomes a little bright, indicate its best crystallinity.At a current density of 1 A g^-1, C-800 shows the best specific capacitance of 305.3 F g^-1, but C-1000 with largest graphitization shows slightly decreased specific capacitance while retaining better rate capabilitie, namely a steadier rate in capacitance. Then, a novel redox active additive of p-nitroaniline （PNA） was introduced into the 6 mol L-1 KOH electrolyte to largely improve the capacitance. All of GCD curves deliver the geometry with evident charging/discharging platforms, and the CV curves present a pair of broad and symmetric oxidation and reduction peaks. The C-800-2 sample with the PNA concentration of 2 mmol delivers largely improved capacitance of 502.1 F g^-1 at 1 A g^-1, which is almost 1.65 fold increase. The retention of specific capacitances of the C-8001/2 samples can still reach up to 88.9% and 86.8% after 5000 consecutive charging-discharging cycles, implying good cycling stabilities of the C-8001/2 samples. Apparently, the present PNA is commercially available and highly effective for elevating the specific capacitance and might be implemented for the wide supercapacitor application.