Preparation And Electrochemical Properties Of Manganese Dioxides And Their Nanocomposite For Supercapacitors

In recent years, supercapacitor has emerged to be a new energy storage device due to its merits such as high power density, longer cycle life, wide operating temperature range and fast charging-discharging, etc. There have been three major types of electrode materials for ECs, i.e., carbon materials, conducting polymers and transition metal oxides. Among these electrode materials, transition metal oxides, especially in the form of nanomaterial, have been extensively studied as electrode materials of supercapacitors. This can be interpreted as their species richness, low cost, especially abundant Faradic redox reactions which contribute to high specific capacitances and energy density. Generally, transition metal oxides including MnO2, NiO, and Co3O4, etc., of which MnO2received a keen interest for its nature abundance, wide working potential window and environmental friendly.In this dissertation, a facile electrodeposition method was used to synthesize several MnO2nanomaterials. Firstly, the impact from three kinds of current collector, i.e., nickel foam, graphite paper and carbon fiber cloth on synthesis electrode was taken into account. Carbon fiber cloth had a strong disturbance on accuracy of estimating electrochemical performance of electrode due to its large electric double-layer capacitance. Meanwhile, acid should be used to clean nickel oxide which covered on nickel foam, thus the waste acid does harm to environment, and the Ni substrate could be oxidation during the long tests. While the graphite paper has low background capacitance and is very stable, so it was selected as the best current collector. After that, graphite paper was selected as current collector to study the impact from precursor solution on synthesis MnO2nanomaterials. Electrodepositions were conducted in three kinds of solution, i.e., Mn(CH3COO)2. Mn(NO3)2and MnCl2. The morphology of two MnO2nanomaterials acquired in Mn(NO3)2and MnCl2solution were both nanoflake balls, while the morphology of MnO2electrodeposited in Mn(CH3COO)2solution was nanosheets. Electrochemical measurements indicated that the MnO2electrode which was acquired in MnCl2electrolyte exhibited the highest specific capacitance (318F g-1at the scan rate of2mV s-1) and good cycling stability (maintaining98%capacitance of initialization after1000cycles). Compared with Mn(CH3COO)2and Mn(NO3)2solutions, MnCl2acted as precursor solution is more suitable for synthesis high performance MnO2nanomaterials.Based on above research, we further synthesized MnO2/NiCo204composite in a fluffy sponge-like structure with ultrathin porous nanoflakes via electrodeposition. The fluffy sponge-like structure is advantageous to the permeation of the electrolyte to inner nanostructures to take part in reactions as well as the emission of joule heat to the electrolyte, thus increasing the cycling stability of the electrode. Meantime, ultrathin porous nanoflakes would enable a fast, reversible Faradaic reaction; hence a good rate performance can be available. The MnO2/NiCo2O4electrode exhibites an enhanced specific capacitance of935Fg-1at1Ag-1with high rate capability (74.9%capacity retention at50A g-1) and excellent cycling stability (maintaining103.1%capacitance of initialization after25000cycles). High specific capacitance, high rate capability and ultra-long cycling stability make it to be a very promising electrode material for high performance supercapacitors.