Preparation And Propertites Of Manganese-based Oxide Nanomaterials For Supercapacitors

Electrochemical energy conservation and storage is one of the most important pathways to solve the energy crisis and environmental deterioration. In this research field, supercapacitors are getting more and more attention from people due to the high power density, long cycling life, fast speed for charging and discharging as well as safe reliability. Especially, electrode materials play a key role in supercapacitors, the performance of which was effected largely by them. Among these materials, base metals(such as Mn O2) always displaying excellent electrochemical performance owing to these special structure, low-cost, abundance and environmentally friendly nature, which not only offer the opportunity for us to pursue novel energy storage materials, but also provide a new platform to achieve function-oriented material design and property optimization.Mn O2 has stood out as the most promising electrode materials for supercapacitors, however, due to the intrinsic poor electrical conductivity and various crystal structures of Mn O2, which can deeply influence the electrochemical properties, thus hindering its wide application in an energy storage system. It is therefore the aim of this work to fully exploit the potential of Mn O2-based electrode materials. The dissertation proposes strategic designs and fabrication of high-performance nanomaterials to control the morphology and crystalline form of the products, and then explores the close relationship between synthesis technique, microstucture and the electrochemical properties. The optimization strategy by function-oriented design of Mn O2-based electrode materials and quantitative analysis for the energy storage mechanism of isomorphous core/shell nanostructure will shed new light on the design and fabrication of high-performance energy conversion and storage materials. The details of this dissertation are summarized briefly as follows:1. In fact, there is no study on combining the advantages of the magnetic field and the hydrothermal route to synthesize electrode materials with the aim of enhancing the electrochemical performance. Base on this phenomenon, we report an effective magnetic-field-assisted hydrothermal synthesis to induce the growth of Mn O2 nanostructures. As a result, three kinds of morphologies of Mn O2 are made, i.e, Mn O2 nanoflowers, Mn O2 nanoflower needles and Mn O2 nanoflower wires, the strengths of the magnetic fields were 0T, 0.3T and 0.6T, respectively. With the magnetic strength increasing, the crystal structure of Mn O2 changes from tetragonal phase of α-Mn O2 to orthorhombic phase of γ-Mn O2; importantly, as-fabricated electrode from the Mn O2 nanostructures exhibited an enhanced specific capacitance of 382 F/g at 1 m V/s, which considerably exceeds the value of Mn O2 material prepared without and with high magnetic field. In addition, the specific capacitance of α-Mn O2 electrode demonstrated an excellent cycle stability, i.e., nearly 95.3% retention after 5000 cycles. Therefore, the results imply that there was a close relationship between the magnetic strength with the morphology, crystal structure and magnetic properties of the Mn O2.2. This could be attributed to the following merits: nanoflower-needles with small diameters offer more accessible electroactive sites for charge transfer and the access of electrolyte ions, the relatively large 2 × 2 tunnels(4.6?) of α-Mn O2 formed by octahedral units significantly increase the electrolyte-material contact area and enhance ion diffusion, thus improving the utilization rate of the electrode materials.3. We have carried out an extensive study of hydrothermally-derived isomorphous Mn O2@Mn O2 core/shell nanostructures, which are consists of an isomorphous layer of β-Mn O2 nanosheets(shell) well grown on the surface of β-Mn O2 nanowires(core). As a model example, the smart electrode made of the isomorphous Mn O2@Mn O2 core/shell nanostructures delivers remarkable electrochemical performance, i.e., yielding greatly improved Csp with 4~5 times higher than that of Mn O2 nanowires. Moreover, we have proved that the increased Csp of the Mn O2@Mn O2 electrode is largely contributed by the capacitive processes including double-layer charging and Faradaic pseudocapacity from Conway and Dunn’s method. In addition, it also demonstrates a desirable cycle stability, i.e., nearly 92.2% retention after 20000 cycles at a current density of 5 A/g.4. This could be attributed to the following merits: Such intriguing capacitive behavior is attributed to the unique isomorphous core/shell hierarchical configuration and high mechanical stability as well as the better interfacial structures between the Mn O2 nanowire core and the ultrathin Mn O2 nanosheet shell. Furthermore, the defective and disordered regions throughout the whole core/shell architectures are the main cause for the unusual change in Csp along with cycling.