Graphitic Carbon Nitride and Its Derivative Toward Energy Conversion
- Publication Type:
- Thesis
- Issue Date:
- 2020
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This Ph.D. project focus on graphitic carbon nitride (g-C₃N₄) and its derivatives toward energy conversion applications, including the photocatalytic hydrogen evolution and lithium sulfur (Li-S) batteries. Bulk g- C₃N₄ materials suffer from the insufficient supply of valid photocarriers due to the low surface area, poor visible light absorption, and fast photocarrier recombination. While Li-S batteries have been severely impeded by fast capacity fading and severe electrochemical polarization. Therefore, a rational morphology, defect, or hybrid modification on g- C₃N₄ and its derivatives is designed to boost the photocatalytic H₂ evolution or battery performance.
A facile structure and doping engineering strategy is proposed to obtain the atomic-thin mesoporous C/O-doped g- C₃N₄ nanosheets via an acid-assisted exfoliation route without any hard templates. The theoretical calculations reveal that C/O atoms would boost the charge transfer rate and charge separation efficiency due to the enhanced electronic polarization effect and shortened bond lengths. Additionally, the electronic conductivity is enhanced due to the formation of delocalized π-bonding. The synergic contribution of textural and electronic features renders an excellent photoelectrochemical (PEC) performance and superior H₂ evolution rates.
A broadband photocatalyst composed of defect engineered g- C₃N₄ (DCN) and upconversion NaYF₄: Yb³⁺, Tm³⁺ (NYF) nanocrystals is proposed to boost the utilization of solar energy. The simultaneous introduction of S dopants and C vacancies renders DCN with defect states to effectively extend its visible light absorption to 590 nm and provide a moderate electron-trapping ability, thus facilitating the re-absorption of upconverted photons and boosting photocarrier separation efficiency. Through the defect engineering, a promoted interfacial charge polarization between DCN and NYF is achieved, which favors the upconverted excited energy transfer from NYF onto DCN as verified both theoretically and experimentally. With the optimization of a 3D framework architecture, the NYF@DCN catalyst exhibits a superior solar H₂ evolution rate.
We report a strategy to trap polysulfides and boost Li-S redox kinetics by embedding, the g- C₃N₄ derivative, surface oxidized quantum-dot-size TiN (TiN-O) into the highly ordered mesoporous carbon matrix. While the carbon scaffold offers sufficient electrical contact to the insulate sulfur, benefiting the full usage of sulfur and physical confinement of polysulfides. The surface oxygen defects render TiN-O with a stronger charge polarization effect for polysulfides via the S-O-Ti bond as verified experimentally and theoretically. Remarkably, TiN-O based coin cells and prototype soft-package cells exhibit excellent cycling stability with great flexibility, demonstrating their potential for practical applications.
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