Low-Dimensional Functional Nanomaterials:Design And Applications In Energy Storage And Conversion

Ever-increasing energy demands and growing global environmental concerns associated with excessive fossil fuels usage are stimulating a broad, intensive search for clean and sustainable alternative energy sources. In that field, lithium ion battery, supercapacitor and electrocatalytic water splitting have recently emerged as three kinds of attractive approaches to realize efficient energy conversion and storage. As the important energy storage devices, the lithium ion battery with high energy density and the supercapacitor with high power density are widely used in commercial life. While the electrocatalytic water splitting for H2production/O2evolution could converse the energy of electricity to the clean H2energy. Thus, it is necessary to design the electrode material with beneficial structure to improve the development of the energy conversion and storage.The goal of this dissertation is to design and fabrication novel functional electrode materials with beneficial structure based on the analyses of the restrictive factors of the electrochemical process on the lithium ion battery, supercapacitor and electrocatalytic water splitting, thus improving their performance of the energy conversion and storage. The details of this dissertation are summarized briefly as follows:1. The ionic and electronic conductivity are the restrictive factors for the capacity and rate performance of the aqueous lithium ion batteries. Based on the fact, we report a novel electrode material with a3D tunneled crystal structure, Lio.3V2O5, which shows both high ionic and electronic conductivity. Through the AC-impendence, the calculation data of diffusion coefficient of lithium ion of the material is10-8-10-12cm2/s, thus demonstrating the high ionic conductivity of Lio.3V2O5. For the first time, the novel electrode material was fabricated into an aqueous lithium ion battery coupled with LiCoO2, which exhibits the superior capacity and rate performance. The first discharge capacity achieves182mAh/g at60mA/g, which is much higher than the most reported70-120mAh/g. In order to investigate the charge/discharge mechanism of Lio.3V2O5, XRD and XPS technique was taken for the characterization of the electrode at different charge/discharge position. For the first time, we propose a possible charge/discharge mechanism and three-insertion sites for lithium ion in Li0.3V2O5. The novel and beneficial structure of the material will provide a new pathway for design and optimization of the functional electrode in the future.2. Base on the understanding of the double-layer capacitance with low capacity and the pseudocapacitance with low electricity conductivity, we proposed the nanocomposite electrode combined with pseudocapacitive ZnCo2O4and highly conductive graphene with ultrathin thickness for the application of all-solid-state flexible thin-film supercapactors (ASSTFSs). The novel strucuture of the nanocomposite maximally integrates both the merites of each component with the high conductivity and ultrathin thickness, which enhances the electron transfer, shorts the ion diffusion paths and increases the electrode/electrolyte contact in ASSTFSs, leading to the high electrochemical performance. The as-fabricated ASSTFS achieves a high areal capacitance of5100μF/cm2at0.2A/m2, and long-term cycling stability for1000cycles, demonstrating the superior electrochemical performance and rendering it a promising candidate for portable electronics.3. For the first time, a series of spinel-structured nanosheets with oxygen deficiency and ultrathin thickness were designed to increase the reactivity and number of the active sites of catalysts, which were then taken as an excellent platform for promoting the oxygen evolution reaction (OER). Theoretical investigation shows that the oxygen vacancies confined in ultrathin nanosheet could lower the adsorption energy of H2O, leading to increased water oxidation electrocatalystic efficiency. As expected, the NiCo2O4ultrathin nanosheets with rich oxygen vacancies exhibit large current density of285mA/cm2at0.8V and a small overpotential of0.32V, both of which are superior to the corresponding bulk/oxygen deficient-poor samples and even higher than that of most reported non-precious metal catalysts. This work would provide a new pathway for the designing of advanced OER catalysts.4. Based on the understanding of the benefits and drawbacks of the layered double-hydroxide used for OER, such as MnCo hydroxide, with low overpotential and superior durability, but low electronic conductivity which restricts its improvement of the OER performance, we achieve the MnCo hydroxide/graphene composite. Benefiting for the highly catalytic activity of MnCo hydroxide and the superior electronic conductivity of graphene, the nanocomposite shows the high OER performance. The current density could reach461mA/cm2and it also shows a low overpotential of0.33V for10mA/cm2. The OER performance is not only better than most reported non-noble metal OER catalyst, but also compatable to the most active noble metal-RuO2catalyst. This will provide the opportunity for enhancing the electrocatalytic activity of catalysts, and offer new insight into the design and optimization of novel electrocatalysts.