| With the rapid development of the industry, traditional fossil fuels are increasingly consumed by human beings, such as coal, petroleum and nature gas. Therefore, developing the renewable and clean energy sources related to the energy storage and conversion technologies, such as supercapacitors, lithium batteries and solar cells has become one of the hot issues studied nowadays. Compared to traditional batteries, supercapacitors are a new-type potential electrochemical device with many advantages like high power density, excellent cyclic performance, long service life and less environmental pollution. It is well known that the electrode materials are the key factor that can influence the performance of supercapacitors, and thus developing new style carbon materials with high capacity have very important significance. Currently, metal-organic frameworks (MOFs) are often considered as potential precursor to prepare porous carbons as electrode materials owing to their high specific areas, high porosity, tunable pore size and multidimensional networks. ZIFs, as a series of MOFs, have received more attention because of their high thermal stability and containing rich nitrogen. Therefore, in this thesis, we adopted ZIFs as main precursor together with second carbon sources to prepare nitrogen-doped porous carbons, and further investigated the electrochemical and adsorption properties of nitrogen-doped porous carbons.The main content of the thesis is as follows.(l)Due to the high specific surface area, high thermal stability as well as rich nitrogen sources of ZIF-8, we choose ZIF-8 as main precursor together with four different and pollution-free secondary carbon sources (including melamine, urea, xylitol and sucrose) to prepare ZIF-derived nitrogen-doped porous carbons. We explore the effects of additional carbon sources on the structure, morphology, pore size distribution, specific area, graphitization degree and nitrogen content of ZIF-derived porous carbons.(2)We investigated the electrochemical properties of ZIF-derived nitrogen-doped porous carbons, including the cyclic voltammetry, galvanostatic charge/discharge (GC), electrochemical impedance spectroscopy (EIS) and cycling stability measurements by adopting a three-electrode system in alkaline electrolyte solution. Results indicate that macromolecular carbon sources, say, sucrose, can effectively protect the nitrogen loss from ZIF backbone owing to the pre-melting and polymerization of the sucrose adsorbed on the ZIF surface in the carbonization process, which makes the corresponding ZIF-derived nitrogen-doped porous Carbon-ZS have high nitrogen content, especially the highest N-Q content, leading to excellent capacitive performance. Moreover, Carbon-ZS has good electrical conductivity and suitable hierarchical micro-/mesoporous structures which is advantage for quick mass transfer and smooth diffusion of electrolytes on the electrode. The specific capacitance of Carbon-ZS in 6 M KOH solution reaches 285.8 F g-1 at a current density of 0.1 A g"1. Furthermore, besides high capacitance, Carbon-ZS also shows excellent cycling stability as electrode material for supercapacitors over 1000 discharge-charge cycles.(3) We adopted ZIF-8 and ZIF-8/xylitol composite as precursors by programmable temperature carbonization method to prepare nitrogen-doped porous carbons, marked as Carbon-Z and Carbon-ZX, respectively, and further investigated the Xe adsorption and separation in the nitrogen-doped carbons at room temperature. Further adsorption measurements indicate that ZIF-derived nitrogen-doped Carbon-ZX exhibits extremely high Xe capacity of 4.42 mmol g-1 at 298 K and 1 bar, which is even more than three times of the matrix ZIF-8 at similar conditions. Moreover, Carbon-ZX also shows the highest Xe/N2 selectivity about-120, and this value is larger than all other reported MOFs. These remarkable features illustrate that ZIF-derived nitrogen-doped porous carbon is an excellent adsorbent for Xe adsorption and separation at room temperature. |
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