Preparation of zinc oxide/carbon composites and their application for Energy Storage Devices

Abstract

There has been an increasing demand for environmental friendly, high performance energy storage systems in the past decade. Supercapacitors, also known as ultracapacitors, are the energy devices that store and release energy through ion adsorption/desorption process at the surface between an electrode and electrolyte. Supercapacitors exhibit extremely high power density, reasonable energy density, longer cycle life, and minimum charge separation compared to those of conventional capacitors. However, research still exists from the perspective of enhancing the specific capacitance of electrode materials, so that they find their way into the market by replacing batteries. High surface area and optimum pore size of the electrode materials for an electrolyte solution play an important role in ensuring the excellent performance of a supercapacitor in terms of power density and energy storage capability. This study was about the development of new synthetic electrode material using the diverse carbon and zinc oxide. Zinc oxide has many advantages economically, environmentally, and high energy density, but it is not proper to use the raw material directly as the electrode due to low conductivity. Therefore, the composites such as activated carbon, and MWNT was performed to improve the electrical performances. Also, effect of heteroatom doping for enhancing the specific capacitance was investigated. In chapter 2, we present a systematic study on the morphological variation of zinc oxide nanostruc-ture by varying the reaction time and precursor via precipitation method. Zinc nitrate hexahydrate and zinc acetate dehydrate were used as a precursor, which was refluxed at 90 oC with reductant for different reaction time. A simple and facile precipitation route has been studied for the shape-control of crystalline zinc oxide nanostructures like rods, flowers, and porous flower structures. The ratio of precursor and reaction time are affected in the morphological control of zinc oxide nanostructures. On the basis of our shape-control of the zinc oxide structures by the precipitation technique, growth mechanisms were proposed. Also, the electro-chemical properties of the zinc oxide electrodes obtained from different precursors and reaction time were investigated by cyclic voltammetry, and galvanostatic charge/discharge measurements. In chapter 3, the capacitances of partially oxidized multi-walled carbon nanotubes (MWNTs) elec-trodes treated with nitric acid were investigated for use as the electrode materials of electric double-layer ca-pacitors (EDLCs). The oxidized MWNTs had a much higher specific surface area, pore volume and average pore width than those of pristine MWNTs. Moreover, functional groups such as hydroxyl, carbonyl and car-boxyl groups were observed on the surfaces of oxidized MWNTs. The specific capacitance of the oxidized MWNTs after 3 h a nitric acid treatment was 68.8 F/g in the highest oxidized state, more than 7 times greater than that of the pristine MWNTs. Further, the capacitance trends apparently follow the change of the zeta potentials, not the surface areas, indicating that this trend depends on the adsorption of electrolytes as compared to the migration path structure. Further, the oxidized MWNT electrode materials showed good long-term stability during repeated cycles and no decay for more than 1,000 cycles. To improve electrochemical performance of electrode materials in different ways, zinc oxide decorated onto the multiwalled carbon nanotubes (MWNTs) were prepared by simple and inexpensive precipitation method. The zinc oxide/MWNTs composites were fabricated with different zinc precursor and reaction time. The surface morphological analysis showed that the zinc oxide with the zinc precursor and reaction time uniformly decorated onto the MWNT surface. Furthermore, the electrochemical properties carried out for composites that these were the efficient electrodes for electrochemical charge-storage applications. In chapter 4, we investigated a sustainable approach to producing activated carbon doped with nitro-gen utilizing the inherent nitrogen and carbon in microalgae, and its potential application of microalgae as the electrode materials of a supercapacitor. Simple carbonization of microalgae in the presence of potassium hydroxide efficiently promoted the incorporation of stable hetero nitrogen atoms on carbon skeletons. Moreover, pseudo-capacitive behavior resulting from reversible faradic electron-transfer chemical reactions is expected to be associated with the EDLC and exhibit synergistic enhancement of specific capacitance. Besides, A low mass ratio of zinc oxide was advantageous to their dispersion in the composites. The composite with the optimal concentration at 0.001 M zinc oxide has homogeneous incorporation of activated carbon and achieves a specific capacitance as high as 106 F g-1 at 0.5 A g-1 in the potential range -1.0~0.0 V. In chapter 5, N, S-codoped activated carbon derived asphaltene was prepared by potassium hydroxide and their applications for electrode materials of electric double-layer capacitors. The samples inherently contained the heteroatom such as nitrogen, and sulfur, which enhanced positive effects on capacitance through improved electron transfer, was obtained without treatment. The obtained activated carbon exhibited both content of nitrogen (1.17 wt.%) and sulfur (0.32 wt.%) after activation process. Also, it had very high surface area of $2558 m^2/g $ and 0.98 cm3/g of mesopore volume, while raw asphaltene, and asphaltene carbonized at $700 ^circ C$ were high content of heteroatom and low BET surface area. The activated carbon derived asphaltene was the high specific capacitance 128 F/g at 0.5 A/g in 1 M $Na_2SO_4$ compared with other samples. This results show the content and species of heteroatoms were not almost affected without an effect of microstructure of electrode materials such as high BET specific area, and mesopore volume. To improve electrochemical performance of electrode materials, zinc oxide/activated carbon derived asphaltene were fabricated and then investigated electrochemical properties. The zinc oxide/activated carbon derived asphaltene complexes were the high specific capacitance 155 F/g at 0.5 A/g in 1 M $Na_2SO_4$ .