- Title
- Synthesis and characterization of N-rich mesoporous carbon nitrides and their hybrids towards applications in electrochemical energy storage and conversion
- Creator
- Kim, Sungho
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Carbon-based materials play a significant role in the development of alternative clean and sustainable energy technologies to overcome an increasing number of issues such as global warming and the energy crisis. Carbon nitrides (CNs) are interesting materials with semi-metallic properties and diverse functionalities, including high basicity, hardness and chemical robustness. Research on CNs has gained momentum owing to their exciting properties and the possibilities of the development of an endless number of materials with different weight ratios of C and N. The unique semiconducting properties of CNs with a band gap of ~2.7 eV has allowed for their application in wider areas of research including fuel cells, water splitting, CO2 capture and conversion. However, the application of CNs in the field of energy storage and conversion is limited due to their monotonous stoichiometry, small surface area and low electrochemical properties. The drawbacks could be overcome by creating the nanoporosity in the CN and modifying the framework composition, including variations in the carbon/nitrogen content and atomic structure of the CN. The electronic conductivity of the CN nanostructures, which is a key parameter for energy storage and conversion applications, could also be altered by their hybridization with conducting nanostructures. Chapter 1 of the thesis outlines the recent progress and challenges in CN based research, which includes developing new synthetic strategies for the design of novel N-rich mesoporous nanomaterials with different chemical compositions, core molecular structures and band structures. One of the prominent research areas is to hybridize CNs with two-dimensional (2D) nanosheets for enhancing their efficacy in energy storage and conversion applications. This thesis is mainly focused on the: (1) synthesis and characterization of N-rich carbon nitrides (C3N5, C3N6 and C3N7) with mesoporous and tunable band structures using various single molecular precursors including triazole and tetrazole through a simple nanotemplating approach involving mesoporous silica templates; (2) hybridization of the N-rich carbon nitrides with organic/inorganic nanostructures to tailor their physicochemical properties, especially the electronic conductivity and the electrochemical stability of the final hybrids; (3) improvement of functionalities of CNs in energy storage and conversion applications such as electrocatalysis and secondary batteries; (4) understanding the relationship between the properties and functionalities of CNs via computational calculations and synchrotron X-ray absorption spectroscopic analysis. Chapter 2 describes thermodynamically stable triazole-based mesoporous C3N7 and C3N6 with ordered structure and their enhanced oxygen reduction reaction (ORR) activities. The triazole-based C3N7 and C3N6 were synthesized for the first time by pyrolysis of 5-amino-1H-tetrazole (5-ATTZ) using mesoporous silica as a template at a temperature of 250 and 300 oC, respectively. Polymerization of the 5-ATTZ into the mesochannels of the template and its conversion to the ordered mesoporous C3N7 and C3N6 is investigated by X-ray diffraction, high-resolution transmission electron microscopy (HR-TEM), field emission-scanning electron microscope (FE-SEM) and N2 adsorption-desorption analyses. A combined characterization technique including soft X-ray absorption spectroscopy (XAS), Fourier transform infrared spectroscopy (FR-IR) and density functional theory (DFT) calculations demonstrates that the N-N bonds are stabilized in the form of tetrazine and triazole moieties in the C3N7 and C3N6. The mesoporous C3N7 exhibits excellent oxygen reduction reaction (ORR) performance and large electron transfer number as compared to those of graphitic carbon nitride (g-C3N4) and mesoporous C3N6. The high performance of the most optimised mesoporous C3N7 is attributed to the nitrogen in the tetrazine moiety that acts as an active site for ORR. Chapter 3 deals with N-rich carbon nitride/MoS2 hybrids as electrode materials and investigates their electrochemical functionalities for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). The N-rich C3N5 exhibits higher nitrogen content and the expanded gallery height than g-C3N4, which benefits the attraction and intercalation of lithium and sodium ions. Furthermore, its highly ordered mesoporous structure offers surface-derived storage of lithium and sodium ions. The density functional theory (DFT) calculations demonstrate that the adsorption energy of lithium and sodium ions on the C3N5/MoS2 hybrid is in the desired range for their reversible intercalation. Among present mesoporous C3N5/MoS2 hybrids with tailored MoS2 content, the most optimized C3N5/MoS2 hybrid shows 3.86 and 10.80 times of reversible capacities for LIBs and SIBs as compared to mesoporous g-C3N4. Chapter 4 demonstrates the crucial effect of the N-rich environment in electrode performance of N-rich carbon nitrides for LIBs and SIBs. The triazole-based mesoporous C3N7 shows a reversible capacity of 250.0 mAhg-1 at 0.1 Ag-1 for LIBs, which is the best anode performance among the pure carbon nitrides reported. It would be attributed to the synergistic effect of plenty of active sites and suitable lithium-ion adsorption energy, maximizing the lithium-ion storage. Moreover, the hybridization of C3N7 with MoS2 nanosheets much improves the capacity at 0.1 Ag-1 over 100 cycles which is up to 298.4 mAhg-1 and 111.6 mAhg-1 for LIBs and SIBs, respectively. The outstanding specific capacities of these materials is attributed to the benefits of their smaller pore channels and pyrrolic nitrogen moiety. Chapter 5 presents MoS2 coupled with ordered mesoporous carbon nitride (CN/MoS2) as an anode electrode material for SIBs with high performance. The material was prepared via the nanotemplating approach that includes single-step pyrolysis of phosphomolybdic acid hydrate (PMA), dithiooxamide (DTO) and 5-ATTZ. The crystallinity and chemical composition of the CN/MoS2 hybrid were tailored by controlling the calcination temperature from 500 to 800 oC. Chemical features of the hybrid were examined by CHNS and near-edge X-ray absorption fine structure (NEXAFS) analyses, X-ray photoemission spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). The MoS2 layers with a uniform lateral size of 3.5 nm and 5-layer stacking are homogeneously distributed in a void space of highly ordered porous carbon nitride. The most optimized CN/MoS2 hybrid that was prepared at 600 oC (CN/MoS2-600 hybrid) presents not only a high discharge specific capacity of 605 mAhg-1 at 1 Ag-1 and 100th cycle but also stable rate capability and long-life cyclability for 200 cycles. Chapter 6 summarises each chapter and future perspectives on N-rich carbon nitrides for energy storage and conversion.
- Subject
- N-rich carbon nitrides; porous materials; hybrid; triazole; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1506971
- Identifier
- uon:55941
- Rights
- Copyright 2021 Sungho Kim
- Language
- eng
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