Preparation And Electrochemical Propetries Of Supercapacitor Electrode Materials Based On Manganese Oxide Composites

Electrochemical capacitors （ECs） frequently referred to as “supercapacitors” or“ultracapacitors” are a new class of energy storage device that can obtain greaterenergy density while maintaining high power density of conventional capacitors. Themain applications of ECs include as a backup power supply, used in electronicproducts field, such as the computer, timer, memory’s back-up power. As the drivingvoltage power supply, the ECs are used for high power occasions, combined withbatteries for electric vehicles. They can also be used in electric toys as the main powersupply. In recent years, with the rapid development of the global economy, theincreasing environment pollution and the depletion of fossil fuel, there is an urgentneed for a new technology associated with energy conversion and storage, includingresearch and development of ECs with high power density, energy density andlong-term cycling stability. To advance and realize their commercial potential, it isessential to enhance the performance/cost ratio of ECs devices which is leading to theinvestigation of new electrode materials and the development of new synthesismethodology.Manganese oxides, characterized by high specific capacitance, a variety of stableoxides, low-cost and environmentally friendly, are one of the most studied materialsas an alternative to RuO2. However, they are easy to dissolve, and the small specificsurface area, low electronic conductivity and ion conductivity are the obstacles thatneed to be overcame for further commercial applications. Many studies indicated thatthe development of new electrolyte salt is an available method to avoid the formationof acidic species in solution, therefore, to reduce the dissolution of the oxide ofmanganese. In addition, doping other metal oxide or carbon material into themanganese oxide, research on nanostructured manganese oxides, these are all thestrategies to increase the surface area, conductivity, and the efficient utilization of thepseudocapacitive materials. In this work, the room temperature liquid phase chemicalco-precipitation method and electrochemical deposition method were employed,respectively to introduce graphene and nickel hydroxide to the manganese oxide nanostructures to prepare manganese oxide composite electrode materials. Themicrostructure, morphology and chemical composition were characterized byscanning electron microscopy, X-ray diffraction, transmission electron microscopy,and X-ray photoelectron spectroscopy. Cyclic voltammetry and galvanostaticcharge/discharge measurements were applied to investigate the electrochemicalcapacitance of the electrode active materials.In the first part of this thesis, the room temperature liquid phase chemicalco-precipitation method was used to synthesize Mn3O4/graphene platelets compositeelectrode materials. It has been found that the composite was constituted by thegranular Mn3O4and film-like graphene. Both of them did not appear excessivestacking and aggregation but attached together uniform, thereby to obtain a largespecific surface area, which facilitated to provide a quick and easy path for the iondiffusion and electron transfer. The reversibility and capacitive properties of thematerial were relatively excellent. By recycled many times, the stability of thematerial was also proved to be very good.The second part of this thesis was relative to the use of the electrochemicaldeposition method to synthesize MnO_xand MnO_x/Ni（OH）₂composite electrodematerials. The results indicated that MnO_xacted as a template for growth of Ni（OH）₂with an inter-connected3D porous network nanostructure. The inter-connected3Dporous network nanostructure could provide a high specific surface area, so that moreof the active surface and the energy extracted from the composite. By electrochemicaltests, A maximum capacitance value of2334F/g at current density of5A/g in1MKOH electrolyte was achieved, much higher than that of pure Ni（OH）₂and MnO_x（992and179F/g, respectively）. Moreover, in the charge/discharge process at evenlarger current density of20A/g, the electrode could maintain82.8%of the initialspecific capacitance after500cycles, higher than that of pure Ni（OH）₂（only46.6%remains）.From electrochemical double layer capacitors （aqueous or organic electrolytes）to pseudocapacitors based on RuO2electrode, today a hybrid capacitor, combining acapacitive or pseudocapacitive electrode with a battery electrode, is being forwarded, which benefit from both the charter of capacitor and the battery. The MnO_x, as thepositive electrode, has a high porosity and3D nanostructures, combined with the lackof binder in the electrode are attributed to the improved performance ofpseudocapacitance, including electronic conductivity and cycling stability. Thissimple method will have potential applications in fabricating a wide range ofelectrode composite materials for ECs.