Research On Preparation Of Carbon-based Micro/Nano Hierarchical Structures For Microsupercapacitors

The carbon-based three dimensional micro/nano hierarchical structures have the merits of good conductivity, electrochemical stability, large effective surface area and structural stability. However, the fabrication of these structures is not easy to realize due to the cumbersome fabrication process, high-cost and poor repeatability. In this thesis, three cost-effective and feasible fabrication processeshave been developed for mass production of carbon-based micro/nano hierarchical structures. The morphologies of these structures aregreatly enriched and the shortages of the fabrication processes are overcome. As a highly potential application, high-performance microsupercapacitor eletrodes and prototypes have been fabricated based on the as-fabricated carbon-based micro/nano hierarchical structures. Their electrochemical performances are tested using cyclic voltametry, galvanostatic charging-discharging, and eletrochemical impedance spectroscopy. The fabricationprocessesof hierarchical micro/nano carbon-based structures and their electrochemical performanceare described below.Firstly, a unique fabrication process of carbon-based micro/nano hierarchical structures has been developed. Combined with carbon nanotubes (CNTs) modified photolighography process and pyrolysis process, C-MEMS/NEMS(Carbonmicro/nano electromechanical systems) structures arefabricatedwith the integration of CNTs. MnO2thin films are integrated onto CNTs/C-MEMS structures by electrochemical deposition method, and different morphologies of MnO2/CNTs/C-MEMS structures areobtained by controlling the deposition parameters. The MnO2/CNTs/C-MEMS microelectrode has the best electrochemical performance compared with individual C-MEMS, CNTs/C-MEMS and MnO2/C-MEMS microelectrodes;such performance is attributed to the high synergy of CNTs and MnO2thin films. In this research work, the developed MnO2/CNTs/C-MEMS microelectrodes achieve the specific areal capacitance of238mF cm-2at the current density of0.5mA cm-2, and retain81.8%of their initial capacitance after6000cycling tests.Secondly, the fabrication process of hemispherical micro/nano hierarchical structures have been developed based on the combination of nanoimprint process, oxygen plasma ethcing process and pyrolysis process. At first, the hemispherical structures areobtained by thermal melting process, which isused for the fabrication of polydimethylsiloxane hemispherical (PDMS) mold. During nanoimprinting process, PDMS mold and SU-8photoresist are used for the mass production of hemisphere array. SU-8micro/nano hierarchical structures with different morphologies are fabricated by oxygen plasma etching with controlled etching time. Combined with pyrolysis process and proper pyrolysis parameters, carbon micro/nano hierarchical structures areobtained. MnO2thin film isdepostied onto the as-fabricated carbon structures to explore the potential electromechical applications. The hierarchical deposited electrode shows excellent electrochemical performance compared to a smooth hemispherical deposited electrode. The MnO2/carbon micro/nano hierarchical microeletrode shows the gravimetrical capacitance of337Fg-1at the scan rate of5mV s-1.Finally, a surface activation process has been developed to fabricate carbon micro/nano structures, and based on these structures high-performance microsupercapacitor prototypes have been fabricated. Start with carbon cloth surface which is exfoliated as carbon nanotextures through wet chemical treatment, then the oxidized structures are reduced by annealing process in reductive atmosphere. The as-fabricated activated carbon micro/nano structures have large effective surface area, good conductivity and wettability. Such micro/nano structures,once preapred as microelectrodes then applied in a three-electrode system, they exhibit large specific areal capacitances, good rate performance and cycling performance. The all-solid-state flexible supercapacitor based on the activated carbon microelectrodes display excellent electrochemical performance, with a specific areal capacitance of222.05mF cm-2at the current density of0.31mA cm-2, and retaining104%of its initial capacitance value after30000cycling tests. In addition, the as-fabricated flexible supercapacitor keeps good performance even under bending and twisting contidions. Threesupercapacitors connected in series canpower-on light-emittingdiodes for several minutes. Furthermore, a simple nitrogen doping process has been developed to fabricate nitrogen-doped activated hierarchical carbon structures based on the surface activation process. The effect of the doping precursor concentration to the electrochemical performance of carbon microelectrode is studied. The nitrogen doped microeletrode demonstartessuperior electrochemical performance. At the current density of1mA cm-2, the specific areal capacitance of882.36mF cm-2is obtained in a three-electrode system. When assembled as symmetric supercapacitor, the specific areal capacitance of402.92mF cm-2is obtained at the current density of1mA cm-2. The maximum energy/power densities are0.946mW h cm-3and890mW cm-3respectively. To demonstrate the potential use of nitrogen-doped carbon micro/nano structures as current collectors, MnO2nanostructures are grown on themand the specific areal capacitance of the as-fabricated hybrid electrode of4.126F cm-2is obtained.