| Author: | Wang, Man |
| Title: | Synthesis and characterization of novel electrode materials for high-performance supercapacitors |
| Advisors: | Chen, Guohua (ME) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Supercapacitors Electrodes, Carbon Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xx, 134 pages : color illustrations |
| Language: | English |
| Abstract: | Supercapacitors (SCs), as emerging advanced high-efficiency energy-storage devices, have been extensively investigated in recent years because of their high rate capability and excellent cycling stability. However, the unsatisfactory energy density (E<10 Wh kg–1) of SCs severely hinders their further applications. The optimization of composition and structure of electrode materials to match with various electrolytes has been regarded as an effective strategy to enhance the overall electrochemical performance of SCs. Among them, carbon-based electrode materials stand out by virtue ·of their excellent chemical and physical characteristics including large specific surface area, high electrical conductivity and excellent structural stability, and are potentially suitable for different kinds of aqueous/organic electrolytes. In the present study, three kinds of carbon-based electrode materials are designed and fabricated for SCs with increased energy density by using various electrolytes. First of all, two-dimensional nitrogen-doped porous carbon nanosheets (denoted as NC-x with x being the carbonization temperature in Degree Celsius) were systematically synthesized by using general dual-crystals templating assisted strategy with pore structure and surface composition optimized. Employed as both positive and negative electrodes, the as-prepared NC-900 delivered an energy density of 22.4 Wh kg–1 with a working voltage up to 1.6 V and a capacitance retention of 95% after 30,000 cycles using redox electrolyte (2 M Na2SO4 and 0.05 M KI). The experimental and theoretical analyses reveal the contributions of nitrogen configurations on the carbon scaffolds to accelerate the redox chemistry of I–/I3–. The enhanced redox performance of the porous carbon nanosheets is linearly proportional to the graphitic N content, which is consistent with the enlarged electron-donating area and the strong adsorption capacity towards iodine species (-0.69 eV for I* and -0.73 eV for I2) at the graphitic N sites. Then, nickel cobalt layer double hydroxide (NiCoLDH) was deposited onto carbon fiber through the electrodeposition process, followed by the introduction of poly(3,4-ethylenedioxythiophene) (PEDOT) skin onto the surface of NiCoLDH via oxidative chemical vapor deposition technique. By analyzing multiple experimental results and conducting COMSOL multiphysics simulation, it was found that such ionically permeable and electronically conductive PEDOT skin with optimized thickness can facilitate the electrons transportation along the interface, inducing uniformly distributed potential and optimizing pathway of ions to active sites. Further DFT theoretical analysis indicated the presence of PEDOT layer can build an embedded electric field and induce lower desorption energy of hydrogen for electrochemical redox reaction. Thus, the resulting electrode material exhibited an improved rate capability from 55% to 79% and an enhanced cycling stability from 70% to 92% after 6000 cycles. As a proof-of-concept application, a supercapacitor was made of 10nm PEDOT coated LDH as positive electrode and activated carbon as negative electrode, LDH/PEDOT-10//AC. It can deliver a high energy density up to 58 Wh kg-1 with a working voltage of 1.6 V in 6 M KOH electrolyte. Besides, 80% capacitance retention can be achieved after 6,000 cycles. Finally, boron-doped porous carbon nanosheet (BPCN) was prepared through a simple one-step calcination method. Experimental results and density functional theory simulations revealed that not only the sufficient porous structure (89% mesopore volume) can provide the efficient pathway for sodium-ions, p-type boron doping into the carbon matrix can also generate strong adsorption energy (-2.3 eV) toward sodium-ions, leading to double capacitive contribution in slope region compared to the undoped carbon material. As a result, the BPCN electrode delivered a high capacity of 357 mAh g-1 and 164 mAh g-1 at 0.1 A g-1 and 10 A g-1, respectively. Additionally, the as-assembled sodium ion capacitors with a working voltage as high as 4 V exhibited a maximum energy density 64.8 Wh kg−1 and maintained 80% capacity after 1,500 cycles at 1.0 A g−1. |
| Rights: | All rights reserved |
| Access: | open access |
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