Preparation Of Nitrogen-doped Porous Carbons And Their Electrochemical Properties

Porous carbon electrode materials attracted increasing attention in the field of electrochemical capacitors owing to their excellent physical and chemical properties, such as rich resource, low cost, high specific surface area and controllable pore structure. This dissertation is focusing on the fine-tuning of the structure-texture and surface properties of heteroatom-doped porous carbons. Porous carbons with different structural characteristics and surface properties were prepared by gas-phase heteroatom doping, carbonization/activation bulk heteroatom-doped materials or the solid-phase post-introduced heteroatom doping, combining with the conventional physical or chemical activation. The effects of specific surface area, pore structure, surface properties and particle size of the synthesized carbon materials on the electrochemical performances were studied in detail.The main results in this thesis were summarized as follows:Nitrogen-doped single-walled carbon nanohorns (N-SWCNHs) with high nitrogen content (more than6at.%) were synthesized by a flowing nitrogen assisted arc discharge method at atmospheric pressure in a tubular reactor. Most of the nitrogen chemical states were in pyridinic, pyrrolic, pyridone, and triple-bonded CN bonding configuration presented at the defect sites or the edges of graphene layers. And nitrogen-doping was helpful for the formation of positive curvature structure of N-SWCNHs. Air oxidation can open cone-shaped caps and increase the specific surface area and micropore volume of N-SWCNHs (N-SWCNH-ox). The specific surface area and micropore volume were increased from423m2·g-1and0.03cm3·g-1to741m2·g-1and0.18cm3·g-1, respectively. The pore opening of N-SWCNHs occurs mainly at the defect sites in pyridinic configuration. N-SWCNHs showed a better frequency response and a shorter time constant than those of N-SWCNH-ox in6M KOH and1M H2SO4electrolytes (0.39vs.1.01s and0.47vs.1.02s), indicating that electrical energy storage can be improved by exploiting the positive surface curvature structure. However, N-SWCNH-ox as the electrode materials exhibited higher specific capacitance and capacitance retention ratio than that of N-SWCNHs. The specific capacitances of N-SWCNH-ox were107and105F·g-1at the scan rate of5mV·s-1, and with the capacitance retention ratios of78and81%at the scan rate200mV·s-1in6M KOH and1M H2SO4electrolytes, respectively. Amorphous nitrogen-doped activated carbons (NACs) were prepared by either chemical or physical activation of nitrogen-enriched precursor, which was prepared by carbonization of aminated precursor synthesized through nucleophilic substitution reaction between polyvinyl chloride (PVC) and ethylenediamine. The activation of nitrogen-enriched precursor was much easier than that of the pure carbonized PVC precursor at the same KOH/carbon ratio. KOH activated NACs exhibited higher specific surface areas and larger porosities than those of physical activated NACs. The specific surface areas and pore volumes of the KOH activated NACs were up to2000m2·g-1and1.21cm3·g-1, respectively, while those of physical activated NACs were only less than700m2·g-1and0.43cm3·g-1. Moreover, KOH activation showed a strong selective etching effect on the nitrogen atoms in the nitrogen-enriched precursor with nitrogen contents less than1.3at.%, and the nitrogen chemical states in the final products are predominantly in pyrrolic or pyridone configuration. However, for physical activation, the activation agent showed much weaker selective etching effect on the nitrogen atoms than KOH activation. The nitrogen contents of physical activated NACs were in the range of6at.%. The correlation between activating agent/activation time and the nitrogen chemical states of NACs was not found, and N atoms are mainly pyridinic and quaternary nitrogen. KOH activated NACs showed higher electrochemical performance than the physical activated samples. The Cg/SBET, specific capacitances per surface area, of KOH activated NACs were less than0.2F·m-2, indicating the specific capacitance is mainly the contribution of the electrochemical double layer capacitance. However, the physical activated NACs with high nitrogen content showed a large pseudo-capacitance contribution with a high Cg/SBET more than0.2F·m2, and the contribution of the electrochemical double layer capacitance increased with the increase of the specific surface area.Nanostructured activated carbons or heteroatom-doped porous carbons (N, B, P, BN and NP-doping) were prepared from nanoscale fractal structured depleted fullerene soot (DFS) by the activation method or the chemical reagent impregnated oxidation-carbonization method. The specific surface area and pore volume of the KOH activated DFS were up to2153m2·g-1and1.37cm3·g-1, respectively. While the oxidation-carbonization method can simultaneously tune the specific surface area, pore structure and surface properties of the DFS. The fractal nanostructure and short microporous of both CO2activated DFS and KOH activated samples exhibited high capacitances and high rate performances. The KOH activated DFS exhibited a high specific capacitance of244F·g-1at the scan rate of5mV·s-1in6M KOH electrolyte, and a high capacitance retention ratio up to80%at the scan rate of200mV·s-1. A further analysis of the electrochemical behavior of heteroatom-doped DFS in6M KOH,1M H2SO4and0.5M K2SO4electrolytes were investigated. Compared with the N or P doping, the B-doped DFS showed prominent effect on the improvement of the capacitance performance in acidic electrolyte. The specific capacitances of BDFS, NDFS and PDFS were274,182and206F·g-1at the current density of0.05A·g-1, respectively. The heteroatom doping is an effective method to achieve a high capacitance and improve its rate capability. The maximum specific capacitances of the heteroatom-doped DFS were264,274and98F·g-1at the current density of0.05A·g-1in6M KOH,1M H2SO4and0.5M K2SO4electrolytes, respectively. Furthermore, the co-doped NPDFS and BNDFS presented an enhanced electrochemical performance than the individual doped samples due to the synergistic effects.