Study of Heteroatom-doped Carbon Materials derived from Polyimide Precursors for Supercapacitor Applications

Abstract

Carbon materials are most widely used as supercapacitor electrode materials because of high power density, high electrical conductivity, and stable long cycle life. However, it needs to overcome their relatively low energy density properties. The various design strategies to enhance specific capacitance of carbon materials have been developed to figure out the problems. <br/> This thesis is mainly focused on the synthesis, characterization and supercapacitor applications of various types of heteroatom doped carbon materials derived from PI precursors by in situ doping thechnique. In the following chapters, we will introduce different kinds of heteroatoms, which are doped into carbon matrix and investigate their doping effects in electrochemical performances. <br/> Firstly, nitrogen-doped carbon derived from polyimide/MWCNT composites for flexible all-solid-state symmetric supercapacitors was developed. As a nitrogen-doped carbon precursor, aminophenyl multiwall carbon nanotube grafted PI precursor was synthesized by in situ polymerization method. The synthesized PI precursor solution was dropped to the CC surface which was served as the flexible substrate and current collector and coating followed by direct carbonization at high temperature. These suggested drop, paste and pyrolysis process for the fabrication of electrodes effectively solved the technical problems of the control of the loading mass of active material on the CC surface, and the difficulty of large scale production. The obtained electrode showed a high specific capacitance of 333.4 F g−1 at 1 A g−1 (based on active material mass) in a three electrode system. Fabricated an all-solid-state flexible supercapacitor device exhibited a high volumetric capacitance of 3.88 F cm−3 at a current density of 0.02 mA cm−3. This flexible supercapacitor device can deliver the maximum volumetric energy density of 0.50 mWh cm−3 and presents a good cycling stability with capacitance retention of 85.3% after 10,000 cycles. Furthermore, this device displays superior flexibility with stable electrochemical performance and good capacitance retention. <br/> Secondly, multiple-heteroatom-doped carbons (from single- to triple-doped) by the pyrolysis of polyimide precursors using a simple and facile in situ approach were developed. This approach can be tuned heteroatom compositions by controlling the desired polyimide monomer functional groups as well as introducing external doping sources into the polyimide precursor solutions. Various types of multiple-heteroatom-doped carbons such as N, N,S-, N,F-, N,S,B-, and N,F,B-doped carbon were synthesized. In comparison with single N-doped carbons, the specific capacitance of N,F,B-triple-doped carbon was remarkably enhanced, to 350.3 F g−1 at 1 A g−1 in a three-electrode system. Moreover, a flexible all-solid-state supercapacitor device was fabricated based on the N,F,B-triple-doped carbon, which exhibited a high volumetric capacitance of 4.45 F cm−3 at a current density of 0.01 mA cm−3. The maximum volumetric energy density of the flexible supercapacitor device was achieved as 0.58 mWh cm−3. <br/> Finally, N,B-co-doped carbon by pyrolysis using polyimide precursors incorporating ammonia borane (NH3BH3) was synthesized with a simple and effective process. Polyimide is an attractive N-doping carbon source for supercapacitor applications. In addition, NH3BH3 is an efficient heteroatom-doping source for introducing boron as well as nitrogen atoms during thermal processes. N,B-co-doped carbon was prepared by high-temperature pyrolysis of a precursor solution pasted on carbon cloth. The effects of dual doping of the carbon surface were investigated by X-ray photoelectron spectroscopy, Raman spectroscopy, and sheet resistance measurements. In comparison with N-doped carbon, the specific capacitance of N,B-co-doped carbon in a three-electrode system was enhanced (277.8 F g−1 at 1 A g−1) owing to the synergetic effects of dual heteroatom doping. An as-fabricated flexible all-solid-state supercapacitor device exhibited a volumetric capacitance of 2.97 F cm−3 at a current density of 0.01 mA cm−3 and a maximum energy density of 0.38 mWh cm−3.<br/>