Author: Fei, Linfeng
Title: Development of carbon-based nanocomposites for energy storage and environmental applications
Advisors: Wang, Yu (AP)
Chan, Helen L. W. (AP)
Degree: Ph.D.
Year: 2015
Subject: Storage batteries -- Materials.
Nanocomposites (Materials)
Hong Kong Polytechnic University -- Dissertations
Department: Department of Applied Physics
Pages: xii, 164 leaves : illustrations ; 30 cm
Language: English
Abstract: Over the past few decades, the rapid development of various electronic and electric devices including smartphones, wearable electronics, and electric vehicles have transformed the way we live and given rise to technologies associated with energy storage. Among all energy storage devices, lithium-ion batteries have been most popular and gained much commercial success. In the configuration of a lithium-ion battery, the cathode material has far more impact than the anode, the separator, or the electrolyte on overall performance, safety, and cost of the finished battery. In order to power large-scale systems, it would be essential to develop high performance cathode materials to ensure maximum performance. Anticipated to be one of the most promising solutions in next-generation revolutionary cathode materials for lithium-ion batteries, monoclinic Li₃V₂(PO₄)₃ has been frequently noted in the literature despite its low intrinsic electronic conductivity. To gain better electrochemical performance, a great deal of efforts have been made to improve its conductivity of monoclinic Li₃V₂(PO₄)₃ including size reduction, conductive surface coating, and lattice engineering. However, simple ways of synthesizing this electrode material with superior gravimetric/volumetric electrochemical characteristics and a robust structure has yet to be found. In this thesis, a rational Li₃V₂(PO₄)₃/C composite is first engineered by making porous carbon nanoflakes highly entangled on single-crystalline Li₃V₂(PO₄)₃ microrods. The resulting as-synthesized composite displays a number of structural features including a hierarchical mesoporous-macroporous microstructure with ultra-high tap density. The findings of the corresponding electrochemical evaluations reveal that our designed composite show greatly improved electrochemical performances, including good cycling stability and superior rate capability, which are directly resulted from its enhanced structural stability and reduced charge-transfer resistance.
To further improve the cycling stability of the Li₃V₂(PO₄)₃/C composite, we designed another novel architecture by incorporating carbon-coated Li₃V₂(PO₄)₃ nanospheres into a well-connected three-dimensional carbon network uniformly. Such a hierarchical porous framework combines multiple advantages including a rational mesoporous-macroporous structure to facilitate the electrolyte infiltration, a three-dimensional continuous electron pathway, and abundant porous space for accommodating volume expansion during the electrode charge/discharge. This material, as a cathode, is capable of delivering superior cycling stability with an excellent rate capability. In addition, in pursuit of solving various energy and environmental problems, high-performance photocatalytic materials are extensively studied, especially those worked upon visible-light. In this thesis, we also described a novel nanocomposite of porous Nb2O5 nanofibers decorated with graphene nanoclusters on their surfaces. By utilizing such a composite, the active region of Nb₂O₅ was significantly extended from UV region to UV-Vis region and hence, the photocatalytic performance was also greatly improved. Detailed structure analysis revealed that the performance enhancement was originated from the ample existence of chemical bondings (Nb-O-C) and the unique orientation of graphene layers (perpendicular to the Nb₂O₅/C interface). It should be noted that in the research of carbon-based composite, it has been a long-term challenge to fully understand the growth mechanism of carbon nanostructures on host particles at atomic level and in this thesis, we attempt to solve this problem by presenting one example, i.e., to document graphene growth from designed liquid-solid interface using in-situ transmission electron microscope technique coupled with a high-stable heating system. In the amorphous carbon-Pt₃Co modelling system, we discovered that the carbon first crystallized to nanoclusters at step-edges on liquid Pt₃Co surfaces before merging into graphene layers through kinetic restructuring via oriented-attachment, leading to the final formation of few-layered graphene nanostructures. The results from density-functional theory calculations further revealed that Co atoms rather than Pt atoms acted as initial nucleation centers, suggesting higher catalytic activity among transition metals than that of noble metals in this process.
Rights: All rights reserved
Access: open access

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