Microwave-assisted Synthesis And Supercapacitance Applications Of Manganese Dioxide/rGO Nanocomposites

Compared with traditional energy storage devices, supercapacitors with higher power-density and longer cycle-life, are seen as a new type of energy storage device, and have been widely studied and used. Manganese dioxide(Mn O2) is a supercapacitor electrode material because of low cost, environmental safety, and high theoretical capacities. However, the poor conductivity of manganese dioxide limits its practical application in supercapacitors. As another supercapacitor electrode material, carbon-based nanomaterials have higher electronic conductivity and specific surface areas, but their theoretical capacity is relatively low. In this thesis, we combined the advantages of manganese dioxide and carbon-based nanomaterials and designed a series of experiments to improve the actual specific capacity and electrical conductivity of Mn O2-based electrode materials. The major research contents and results were shown as followings.(1) Synthesis and electrochemical properties of Mn O2 nanostructures via a microwave-assisted hydrothermal method. Mn Ox nanostructures with different morphologies and phases were synthesized via a microwave-assisted solvothermal method at 60-160 oC for 30 min, using KMn O4 as the Mn source and ethanol as the reducing agent. The electrochemical performance suggests that the Mn O2 sample synthesized at 60 oC with poor crystallinity has a better specific capacitance as high as 338 F/g.(2) Synthesis and electrochemical properties of Mn O2 nanostructures and Mn O2/r GO nanocomposites via a microwave irradiation in air. A series of Mn O2 samples with poor crystallinity have been synthesized at 60 oC under a microwave irradiation in air by adjusting the reaction durations and the concentrations of KMn O4. We found that the sample synthesized at 60 oC, with a reaction duration of 30 min and a [KMn O4] concentration of 0.0424 mol/L, shows a higher specific capacitance. Then we prepared a Mn O2/r GO composite material by adding a grapheme oxide(GO) solution into the above precursor. When the amount of the r GO added is 5 wt%, the as-obtained Mn O2/5% r GO sample shows a high specific area of 417.3 m2/g, and has a specific capacitance of 394.7 F g-1, with a 22.2% increase over the specific capacity comparing with the pure Mn O2 sample without r GO.(3) Synthesis and electrochemical properties of Mn O2/CNT nanocomposites via a microwave irradiation in air. By using the reducibility of CNTs, we prepared the Mn O2/CNT nanocomposites by controlling the amounts of potassium permanganate and CNT under a microwave irradiation in air. The as-prepared Mn O2/2% CNT sample shows a hierarchical microstructure composed of nanosheets, and has a specific capacitance of 270.6 F/g at a scan rate of 10 m V/s, and a specific capacitance of 208.3 F/g at a constant current charge and discharge current of 1 A/g.(4) Synthesis and electrochemical properties of Mn O2/CNT/r GO nanocomposites. The Mn O2/2%CNT/2%r GO nanocomposites with a three-dimensional(3D) network composed of nanosheets are synthesized by using the Mn O2/2% CNT nanocomposites and GO as the raw materials, and the amount of r GO is kept 2 wt% of the Mn O2 sample. The as- obtained Mn O2/2% CNT/2% r GO nanocomposite electrode has a specific capacitance of 361.3 F/g at a scan rate of 10 m V/s, a 33.5% increase over the sample of Mn O2/2% CNT composite; its specific capacitance is up to 301.9 F/g at a constant current charge/discharge current of 1 A/g, 44.9% higher than the Mn O2/2% CNT sample without r GO.