| Two-dimensional (2D) nanomaterials with atomic thickness and high anisotropy possess the characteristics of intrinsic quantum confined electrons and extremely high specific surface area, showing different physical properties compared with their bulk counterpart. During the past few years, the 2D nanomaterials bring in huge promise for the design of material with energy storage-related applications. The electrical behavior of 2D nanomaterials is closely tied with their surface and atomic structures. Through surface engineering and reconstructing of crystal lattice, the electronic structures of 2D nanomaterials could be effectively modulated their electrical behavior, providing the new way for designing and optimizing the materials with high electrochemical activity.In this dissertation, we developed several methods to modulate the electronic structure of 2D nanomaterials, realizing the engineering of their electrical behavior with high properties in energy storage application and establishing the model to understand the relationship of the electrical behavior and chemical properties in 2D nanomaterials. Varied chemical-modification such as subnanopore engineering strategy, surface engineering and hybridization with graphene for modulating the intrinsic electrical behavior have been applied in 2D nanomaterials, realizing the enhancement of their performance in energy application and providing platform to the understand the relationship between electronic behavior and properties in energy application. Detailed content includes the following several points:1:Developing new modification strategies for 2D nanomaterials is crucial for studying their intrinsic physical and chemical properties as well as expanding their application. Compared with traditional inorganic 2D materials,2D micro-porous polymers (2D-MPs) have unique periodic vacancies (also called as "subnanopores") in their lattices. These unique nanometer-sized pores in 2D-MPs could provide a new opportunity for modifying the electronic structure of 2D nanomaterials as well as retaining the integrity of their matrix. Based on g-C3N4 nanosheet, which have "nitrogen pots" in their crystal lattice, we developed a subnanopore engineering method with the molecular titanium-oxide incorporating into the "nitrogen pots". The XAFS results clearly verified that molecular titanium-oxide was successfully incorporated into the subnanopores of g-C3N4 coordinated by six nitrogen atoms and one oxygen atom. The successful incorporation of titanium-oxide into the subnanopores has effectively tuned the electronic band structure of the 2D carbon nitride nanosheets with band gap narrowing and the improvement of their photocatalytic activity under visible light. This work opens a new door to engineering the intrinsic properties of 2D subnanoporous nanomaterials.2:Conductive 2D nanomaterials have been discovered as good electrode materials for supercapacitors due to their high specific surface area and high electrical conductivity, providing an ideal platform to investigate the effect of two dimensional structures on the improved electrochemical performance in supercapacitors. As a proof of concept, the ultrathin TiN nanosheet with thickness of 2-3 nm was synthesized by a unique 2D spatially confined reaction. The synthesized 2D TiN nanosheets present relative high specific surface area, which were 3.7 times higher than that of bulk titanium nitride. While, the specific capacitance based on the TiN nanosheets electrode was ten times higher than that of bulk TiN. Obviously, the 3.7-fold increase in the specific surface area partly contributed to the tenfold increase in the electrochemical activity. Theoretical investigation demonstrated that when a dimensional confinement is imposed in the crystal, TiN could possess a higher electrical conductivity as well as maintain its metallic behavior, benefiting to the electrochemical supercapacitors. Furthermore, the EXAFS spectra disclose a different local atomic arrangement in the surface of TiN nanosheet with low coordination number compared with bulk TiN, which could provide more electroactive sites and thus benefit greatly to the performance of supercapacitor. Benefiting from the synergistic effect of enhanced specific effect of enhanced specific surface, improved electrical conductivity and the low coordination number of titanium, the TiN nanosheets realize intrinsic improved electrochemical activity toward supercapacitor comparing with bulk TiN. This finding would open a new door to design supercapacitor with high performance and inspire more scientific interest in metallic electrode applied in electrochemical reaction.3:Design 2D hybrid nanomaterials could bring to the synergy of high conductivity and electrochemical properties, which could promote the development of the high-performance flexible energy storage device. The concept of synergy can be seen in a layer-by-layer assembly structure of VOPO4/graphene 2D hybrid film. Graphene-like VOPO4 nanosheet was obtained with less than six atomic layers form bulk VOPO4·2H2O by 2-propanol-assisted exfoliation process. Because of the poor conductivity of VOPO4 nanosheet, a layer-by-layer strategy was adopted to assemble a VOPO4/graphene hybrid film with the VOPO4 nanosheets integrated on the graphene layers, which possessing the much improved electrical conductivity and making possible embodiment of outstanding electrochemical performance of VOPO4 nanosheets. The hybrid film possesses the synergic benefits of superior electrochemical performance of VOPO4 nanosheets and the high conductivity of graphene, showing high performance in flexible supercapacitor. The as-fabricated flexible supercapacitor based on hybrid structure exhibit high specific capacitance, long cycle life and excellent flexibility, leading to a high energy density and power density. Our findings represent a promising direction and a significant step towards exploring new quasi-2D materials for flexible energy device with higher energy density in the near future. |
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