Synthesis And Electrochemical Performance Of Mn, Co And Ni Based Functional Nanomaterials For Supercapacitor

As unique energy storage devices, Supercapacitors（SC）which exhibit high power density、long cycle life and fast charge-discharge capability have been widely used in hybrid electric vehicles, mobile electronic devices, etc. According to energy storage mechanism, supercapacitors can be divided into two catalogues:electrochemical double-layer capacitors（EDLCs）and pesudocapacitors. EDLCs store energy electrostatically through surface ion adsorption/desorption at the electrode/electrolyte interfaces, and pseudocapacitors utilize fast and reversible superficial Faradaic reactions between electrolyte ions and electroactive materials. Electrode materials as the core of supercapacitor have attracted considerable attention. Generally,carbon materials,metal oxides and conducting polymer are the three major electrode materials of supercapacitor. This thesis focuses on the research about the low-cost metal oxides including Mn/Co/Ni-based pseudocapacitance materials,and the structures,morphologies and element composition were intensively investigated by many characterization methods. In addition, the electrochemical performances of the as-prepared materials for SC applications have been detailly investigated by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectra. The main work is the following:1. Nanostructured graphene/amorphous α-MnO₂ composites have been synthesized by a facile coprecipitation method under the alkaline condition, in which graphene nanosheets as a supporting substrate to grow MnO₂. Characterizations of prepared samples’ morphology and microstructures indicate MnO₂ is successfully formed on the surface of graphene by chemical interaction. Moreover, the electrochemical properties of the synthesized electrode materials for supercapacitors are studied in a three-electrode experimental setup using a 1 M KOH aqueous solution as the electrolyte. As a result, the specific capacitance of graphene/MnO₂ composite（weight ratio of graphene to MnO₂ is 1:1） determined by a galvanostatic charge–discharge method at a current density of 1 A g^-1 reaches 367 F g^-1, which is 1.8 and 4.6 fold higher than that of pure graphene and MnO₂. The capacity retention of the graphene/MnO₂ composite is 73.9% of the original capacitance after 1000 cycles, indicating graphene/MnO₂ composite is a promising electrode material for supercapacitors.2. Hierarchical δ-MnO₂ nanosheets as electroactive materials have been directly deposited on nickel foam substrate by one-pot chelation-mediated aqueous method. The morphological evolution process has been investigated by scanning electron microscopy（SEM） at different time intervals in detail. The electrochemical test of the hierarchical δ-MnO₂ electrodes which are synthesized at 30oC, 40oC and 50oC are directly served as binder- and conductive-agent-free electrodes for supercapacitors have been explored. With the decrease of reaction temperature the specific capacitance of δ-MnO₂ electrode increases. The vertically aligned δ-MnO₂ nanosheets which have been synthesised at 30oC exhibit a highest capacitance of 325 F g^-1 at the current density of 1 A g^-1. The capacitance loss is less than 15% after 1000 cycles at the scan rate of 30 m V s-1. Furthermore, it is found that the equivalent series resistance and charge transfer resistance of the electrode are 0.36 Ω and 1.7 Ω, respectively. This not only provides a green, efficient synthetic method for the research of supercapacitor electrode materials, but also provides an innovative idea for preparing the supercapacitor electrode.3. The porous mixed-phase Co O/Co₃O₄ nanocomposites have been prepared by a facile ethanol-assisted solvothermal approach without a subsequent calcination process and the composites are formed by aggregations of many nanoparticles with an average diameter of 5±2 nm. The Co O/Co₃O₄ nanocomposites possess a high surface area(¹86.27 m2g^-1). Through electrochemical test, the Co O/Co₃O₄ electrode exhibits a high capacitance of 451 F g^-1 at a current density of 1 A g^-1, which is higher than that of the pure Co O electrode(203 F g^-1). Interestingly, the Co O/Co₃O₄ shows a low equivalent series resistance（0.16 Ω） and superior cycle stability（108% retention after 5000 cycles）. Moreover, an asymmetric supercapacitor based on this hybrid Co O/Co₃O₄ and activated carbon can deliver an energy density of 10.52 Wh kg^-1 at a power density of 140 W kg^-1.4. A template-free chemical co-precipitation method has been developed to fabricate the mesoporous Ni Co₂O₄ nanospheres with a high specific surface area of 215.98 m2 g^-1, which can be further self-assembled into 3D frameworks. The key to the formation of mesoporous Ni Co₂O₄ frameworks with a desired pore-size distribution centered at ⁴ nm is the unique preparation method assisted with sodium bicarbonate as pore-creating agent. When tested as electrode materials for SCs, the Ni Co₂O₄ electrodes deliver excellent electrochemical performances with high specific capacitance(842 F g^-1 at a current density of 2 A g^-1), superior cycling stability with no capacity decrease after 5000 cycles（103% initial capacity retention）, and great rate performance at a 10-time current density increase（79.9% specific capacitance retention）. Furthermore, as expected in Ni Co₂O₄-based asymmetric supercapacitor device, the energy density of as high as 29.76 Wh kg^-1 at a power density of 159.4 W kg^-1 can be achieved. This provides a general, eco-friendly, template-free synthetic strategy for the scale-up fabrication of the mesoporous Ni Co₂O₄ electrode material.Above all, for the synthetic methods, either co-precipitation or solvent process, the synthesis process, aggressively employed in this paper, are all simple, green, low energy consumption in order to achieve the electrode materials for supercapacitor with different morphology, excellent electrochemical properties. Moreover, materials involved in this paper（Mn, Co, Ni） are all abundant, low-cost, and environmental friendly. This works give us a theoretical and technical guidance for basic research and practical applications of the supercapacitor electrode materials.