Heteroatom Doped Porous Carbon Materials Derived From Pyrolysis Of Polymer And Their Electrochemical Performance

Supercapacitors, also known as electrochemical capacitors, are novel power storage device based on electrode materials of high specific surface area carbon material, conducting polymer and transition metal oxide. Supercapacitors have significant potential, including high power density, long cycle life, fast charge/discharge rates, and low manufacturing costs. Supercapacitors have been extensively used in civilian and military areas because of their distinctive electrical properties. According to the mechanism of charge storage, supercapacitors can be divided into two types, namely, electric double-layer capacitors(EDLCs) and pseudocapacitors. Various types of carbon materials with high surface area and high electrical conductivity have been used as electrode materials in this technology. Carbon-based supercapacitors derive from a wealth of sources, show excellent electrochemical performance and popular price, which is the first commercialized supercapacitor electrode materials. Compared with most pseudocapacitor electrode materials such as metal oxides and conductive polymers, carbon-based electrode materials offer lower specific capacitance and energy density in supercapacitor applications. This thesis focuses on the design of carbon materials for EDLCs with different morphologies from polymer precursors. The polymer precursors with special morphologies were carbonized under inert atmosphere and transformed to heteroatom doped porous carbon materials. Our work aims at providing theoretical and scientific basis for the high capacitance carbon-based supercapacitors. The details are as follows:(1) Porous nitrogen-doped hollow carbon spheres(PNHCS) have been prepared by pyrolysis of hollow polyaniline spheres(HPS) synthesized by sulfonated polystyrene spheres(SPS) as a hard template. At a current density of 0.5 A·g-1, the specific capacitance of the PNHCS is ca. 213 F·g-1. The capacity retention after 5000 charge/discharge cycles at a current density of 1 A·g-1 is more than 91%. The enhanced electrochemical performance can be attributed to the unique carbon nanostructure and nitrogen-doping of the PNHCS. The hollow macro-structure plays the role of an “ion-buffering” reservoir. The micropores of the PNHCS enlarge the specific surface area, while the mesopores offer larger channels for liquid electrolyte penetration. Nitrogen groups in the PNHCS not only improve the wettability of the carbon surface, but also enhance the capacitance by addition of a pseudocapacitive redox process.(2) Phytic acid was used as protonic acid dopant and soft template to synthesize 3D network polyaniline fibers(P-PANI). The P-PANI was transformed to nitrogen and phosphorus co-doped carbon nanofiber(NPCNF) after pyrolysis. The KOH activation process was used to tune the pore texture and yield to the porous nitrogen and phosphorus co-doped carbon nanofibers(PNPCNF). The PNPCNF have a high specific surface area of 2586 m2·g-1 and large pore volume of 1.43 cm3·g-1. High specific surface area and abundant porous structure are benifical to enhancing of electrochemical performance of carbon materials. Nitrogen doping in the carbon material enhances the capacitance by addition of a pseudocapacitive redox process, and phosphorus doping widens the potential window of carbon electrode. The highest specific capacitance of PNPCNF was 280 F·g-1 measured in three-electrode system(1 A·g-1) and highest energy density was 16.3Wh·kg-1 measured in two-electrode system.(3) Flexible carbon nanotube(CNT) paper was used as substrate to fabricate polyaniline nanowire array/CNT paper composites(CNT/PANI). The CNT/PANI was transformed to nitrogen-doped carbon/CNT paper composites(CNT/NC) after pyrolysis. High graphitized CNT paper improves the electrical conductivity of composites. NC array enlarge the specific surface area and build a three-dimensional structure. Nitrogen groups doped in the CNT/NC enhance the capacitance by addition of a pseudocapacitive redox process. NC array offer larger channels for liquid electrolyte access and ion transport. The binder-free CNT/NC indicates excellent rate capability. At a current density of 2 A·g-1, the highest specific capacitances of the CNT/NC are ca. 211 F·g-1. The capacity retention is more than 60% at the charge-discharge current density of 50 A·g-1.(4) Microporous polymeric organic frameworks(POF) were used as precursors to synthesize the porous nitrogen-doped carbon microspheres(PNCM). The KOH activation process was used to tune porous texture of the PNCM and yield to the activated-PNCM(A-PNCM). The specific surface areas of the PNCM and A-PNCM are 921 and 1303 m2·g-1, respectively. The nitrogen doped PNCM and A-PNCM with abundant micropores and mesopores exhibit excellent electrochemical performance. At a current density of 0.5 A·g-1, the specific capacitances of the PNCM and A-PNCM are ca. 248.4 and 281.8 F·g-1, respectively, which are superior to the reported carbonaceous electrode materials. Even at a current density up to 20 A·g-1, the specific capacitances of the PNCM and A-PNCM still remain 94.9 and 154.3 F·g-1. These results clearly suggest that the PNCM and A-PNCM are promising candidates for EDLCs.