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Title: Electrospun one-dimensional carbon-based nanomaterials for energy application
Authors: Hu, Jing
Degree: Ph.D.
Issue Date: 2020
Abstract: The energy crisis, environmental problem, increasing hash demands of portable electronic devices and autonomous aircrafts have spurred tremendous research efforts on the exploration of advanced electrochemical energy storage systems. The performance of the energy storage systems essentially depends on the electrode materials. Therefore, selecting a proper combination of different materials according their intrinsic properties, employing suitable methods to fabricate optimized heterogeneous nanostructures, is considerable to overcome the challenges and improve the overall performance. Rechargeable lithium-sulfur batteries have been regarded as promising next generation energy storage systems due to the overwhelming advantage in energy density. However, their practical implementations are hindered by severe capacity fading and low sulfur utilization, which caused by polysulfide shuttling and the insulating nature of sulfur. Thus, we rationally design and fabricate MnO₂ nanosheets coated sulfur-embedded porous multichannel carbon nanofiber (CNF@S/MnO₂) as cathode for Li-S battery. The high conductivity of porous multichannel carbon nanofiber facilitates the kinetics of electron and ion transport in the electrodes, the porous structure encapsulates and sequesters sulfur in its interior void space to physically retard the dissolution of high-order polysulfides. Moreover, the MnO₂ shell exhibits acombination of physical and chemical adsorption for high-order polysulfides, which sequesters polysulfides leaked from the carbon matrix after long time charge/discharge cycles, resulting in enhanced cyclic stability. As a result, the electrode delivers a specific capacity of 1286 mA hg⁻¹ at 0.1 C and 728 mA h g⁻¹ at 3C. And the capacity remains 774 mAh g⁻¹ after cycling over 600 cycles at 1 C.
By rationally combining high conductivity multiple-channel porous CNF and polar MnO₂, the obtained electrode presents enhanced electrochemical performance with medium term cycle stability (600 cycle at 1 C current rate with fading rate of 0.028%). However, it does not solve the intrinsic problem with dissolution of polysufides due to the contained cyclo-S8 molecules. To radically eliminate "shuttling effects", electrochemical reactions along with severe polysulfides dissolution need to be inhibited. Herein, SPAN@CNT composite nanofibers are fabricated via electrospinning technique and calcination. Smaller sulfur molecular (Sx, x=2~4) entrapped in PAN derived carbon matrix via physical barriers and covalent bonding, which avoids formation of high-order polysulfides during lithiation process, resulting in solving the critical shuttle effects. Furthermore, the one-dimensional fibrous structure and high electrical conductivity CNTs encapsulated in the SPAN host effectively enhance reactive kinetics of the composite. As a result, the designed SPAN@CNT delivers superior rate performance (1005 mAh g-1 at 2C) and outstanding cycling stability (low capacity decay of 0.015% per cycle over 1500 times at 1 C). In-situ Raman technique combined with cyclic voltammetry (CV) is employed to explore the electrochemical reaction mechanism of SPAN electrode for Li-S battery. The formation of graphitized structure and S-N bond refresh our current knowledges of SPAN in Li-S batteries. Beyond Li-S batteries, we also keep eyes on supercapacitors (SCs) and lithium ion batteries (LIBs), which are two of the most dominated rechargeable energy storage devices in the current market. Partial graphitic hierarchical porous carbon nanofibers are prepared via electrospinning followed by pyrolysis, activation and acid treatment. The as-obtained carbon nanofiber presents synergy of ameliorating functional features, such as relatively high graphitic degree, high specific surface area, large pore volume with hierarchical porous structure, which are beneficial to excellent electrochemical properties. When applied in supercapacitors, the MNP-CNF delivers a high specific capacitance of 287 F g⁻¹ at 0.5 A g⁻¹, a high rate capacitance of 196 F g⁻¹ at 100 A g⁻¹ and a high capacity retention of 95.4% at 5 A g⁻¹ after 10000 cycles. When used as an anode for lithium ion batteries, MNP-CNF electrode displays an exceptionally high reversible capacity of 1495 mAh g⁻¹ at 0.1 A g⁻¹, superior cycle stability and an outstanding high-rate capacity of 391 mAh g⁻¹ at 10 A g⁻¹ for 1100 cycles. These values demonstrate the superiority of MNP-CNF as bi-functional electrodes in supercapacitors and Li-ion batteries with outstanding performance.
Subjects: Electric batteries -- Materials
Energy harvesting -- Materials
Hong Kong Polytechnic University -- Dissertations
Pages: xx, 157 pages : color illustrations
Appears in Collections:Thesis

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