Preparation And Properties Of Manganese-Based Metallic Oxides And Their Composites For Supercapacitors

Supercapacitors have become the most promising candidates for next-generation power devices in recent years because of their excellent properties such as high power density, fast charge-discharge rate, long cycle life and safe operation for time-dependent power needs of modern electronics and power systems. According to the charge-storage mechanism, they are generally divided into two categories, i.e., electrical double-layer capacitors using carbon-active materials, and pseudocapacitors using redox-active materials. Up to now, for pseudocapacitors, there has been extensive interest in developing commercial attractive transition-metal oxide (TMO) electrodes, as these TMOs are natural abundance, low cost, and environmental friendliness, and particularly can provide various oxidation states for efficient redox charge transfer and enable a higher energy density and a high theoretical specific capacitance. As with different microstructures and compositions, they show substantial differences in supercapacitor performance due to dissimilarities in the electrode/electrolyte interface properties and ion transfer rates during the charge storage processes. Therefore, in this doctoral dissertation, beginning with manganese dioxide, and then developing some Mn-based metal oxide composites to meet some challenges in the transition metal oxide electrode materials. A variety of one-dimensional, two-dimensional and three-dimensional Mn-based TMOs or multi-component composite TMO materials have been synthesized by hydrothermal method and electrochemical deposition, and then were characterized by means of XRD, SEM and TEM, and finally fabricated into electrodes to examine the electrochemical performances, in particular, improving their practical electrode applications. The main points of this dissertation are summarized as follows:1. Single-crystal α-MnO2ultralong nanowires (-40μm in length and～15nm in diameter) were synthesized by a simple process of hydrothermal treatment. By varying the surfactants, the other two kinds of a-MnO2nanostructures (nanorods and nanoflowers) were also prepared. The electrical conductivity of as-prepared MnO2nanowires and nanorods was then studied by a new STM-TEM holder, and their obtained results showed that the MnO2ultralong nanowires possess better electrical conductivity compared with MnO2nanorods, and both of the voltages were ranged from-5to5V, and the current value of MnO2ultralong nanowires varies from-252.5to206.7nA, while the MnO2NR varies from-122.3to92.3nA. Compared with as-synthesized a-MnO2nanorods and nanoflowers, individual α-MnO2ultralong nanowires showed a better electrochemical performance, and their based electrodes exhibited an enhanced specific capacitance of345F g-1at1A/g with high rate capability (54.7%at10A g-1).2. Ultrafine a-MnO2nanobelts (8-10nm in diameter) were synthesized by a facile, template-free and effective electrochemical method on Ni foam, and an extensive study of the electrode properties with respect to changes of the temperature has been carried out. Through both CV and galvanostatic CD investigation, the specific capacitance has been seen to increase with increasing temperature, with values as high as509F g-1observed at50℃, higher than that of in0and25℃. Also, this electrode demonstrated a good rate capability with39.1%retention even the scan rate increasing to100times (1to100mV s-1). More importantly, the specific capacitance of the MnO2nanobelt electrode has91.3%retention after5000cycles with repeated heating and cooling during temperature of0to50℃, showing good high temperature-resistive long-term cycle stability.3.3D hierarchical heterostructures of MnO2nanosheets or nanorods grown on Au-coated Co3O4porous nanowall arrays were designed and synthesized, looking like a sandwiched configuration of Co3O4@Au@MnO2, by a facial and controllable electrochemical deposition process. Due to their unique self-assembling architecture characteristics involving porous Co3O4nanowalls, ultrathin MnO2nanosheets, and a high conductivity Au layer sandwiched between them, each component provides much needed critical function for efficient use of metal oxide for energy storage. The synthesized3D hierarchical heterostructures exhibited favorable electrochemical performances such as a high specific capacitance as851.4F g-1at10mV s-1and1532.4F g-1at1A g-1, good rate performance and an excellent long-term cycle stability (nearly no degradation after5000cycles), thus could be considered as perspective materials for high-performance electrochemical capacitors.4. An urchin-like cubic phase MnCo2O5hierarchical structure with4～6μm in diameter was synthesized by a facile hydrothermal method followed by calculation in400℃. SEM and TEM characterizations confirmed that the unique hierarchical structure was composed of1D nanochains consisting of nanoparticles, making the MnCo204.5becomes a highly porous texture. As an electrode material, it exhibited a specific capacitance of151.2F g-1at scan rate of5mV s-1. More importantly, this unique electrode demonstrated an outstanding rate capability with83.6%retention even at the current density increasing to50times (0.1to5A g-1), and showed excellent long-term cycle stability that it nearly had no decrease after2100cycles at progressively varied current densities.