Design and Tailoring of the Nano-Structured Ruthenium Oxides for Next Generation Supercapacitors

Autor: Kuo-Hsin Chang, 張國興
Rok vydání: 2007
Druh dokumentu: 學位論文 ; thesis
Popis: 95
Supercapacitors are important energy storage/delivery devices in the modern society. Due to the unique pseudocapacitive properties of hydrous/anhydrous RuO2, RuO2-based supercapacitors have been commercialized in Taiwan. Based on the reversible redox transitions among Ru(VI), Ru(IV), and Ru(II) oxy-hydroxyl species, the specific capacitance of RuO2.xH2O synthesized via various methods was found to range form 100 to 1540 F/g and its theoretical value could even reach 2200 F/g. From the review on RuO2-based supercapacitors in chapter 1, the key factors dominating the performances of RuO2-based supercapacitors generally include (1) electrochemical reversibility of electroactive materials, (2) electronic resistance “between” and “within” electroactive particles, (3) contact resistance at the interface between current collector and active materials, (4) proton conductivity/diffusivity between and within particles, and (5) pore structure and surface area. Based on these key factors, we have the directions and rules to design Ru-based oxides for this application. Hence, how to design and grow Ru-based oxides with both high energy and high power characteristics is both academic and practical importance in the supercapacitor technologies, which will be carried out in this dissertation. In addition, the research motives and detailed experimental methods for the R&D of Ru-based oxides with both high energy and high power density were summarized in chapter 2. Amorphous RuOx.nH2O with highly thermal stability was successfully prepared by chemically oxidative method of RuCl3.xH2O with H2O2 in chapter 3. From the textural analyses, the main product of this chemical synthesis is RuO2.nH2O with a significant amount of RuO3. In addition, all RuOx.nH2O powders annealed in air between 150 and 400oC for 2 hrs show an amorphous structure because the presence of stable RuO3 uniformly trapped within the oxide matrix as an impurity to depress the crystallization. Due to the RuOx.nH2O maintain the amorphous structure after annealing, its specific capacitance almost keeps a constant of 500 F/g when the annealing temperature is between 200 and 400 oC. In chapter 4, RuO2.xH2O nanoparticulates with different crystal degree and various water contents were prepared via a hydrothermal synthesis route, successfully demonstrating the independent control of crystal size and water content of RuO2.xH2O. Prolonging the hydrothermal time is proposed as “hydrothermal annealing” to increase the crystalline degree but remain the water content of every RuO2.xH2O primary nanoparticulate due to the hydrothermal condition with mild temperature, high vapor pressure, and water-rich environment. The crystalline and hydrous nature of hydrothermally derived RuO2.xH2O particulates not only reduces the proton diffusion resistance but also enhances the electronic conductivity for the redox transitions of active species, which promotes the capacitive performances of RuO2.xH2O in supercapacitor applications. Coalescence of particulates accompanied with crystal growth upon annealing at the high temperature, found for RuO2.xH2O prepared by a sol-gel process, is effectively inhibited by using RuO2.xH2O nanocrystallites in chapter 5. This thermal stability, attributable to the barrier originated from the lattice energy of crystallites, maintains the high water content, nanocrystalline structure, and porous nature of RuO2.xH2O annealed at elevated temperatures from 200 to 400 oC. A hydrothermal derived RuO2-based supercapacitor with high specific capacitance (ca. 200 F/g measured at 100 mA/cm2) and a cycle-life time longer than 40,000 cycles, resulting from thermal stability, is demonstrated. In chapter 6, we use the membrane-templated synthesis route to prepare RuO2.xH2O nanotubular arrayed electrodes by means of the anodic deposition technique. The desired 3D mesoporous architecture of RuO2.xH2O nanotubular arrayed electrodes with annealing in air at 200 oC for 2 hrs simultaneously maintained the facility of electrolyte penetration, the ease of proton exchange/diffusion, and the metallic conductivity of crystalline RuO2, exhibiting unexpectedly ultrahigh power characteristics with its frequency “knee” reaching ca. 4.0~7.8 kHz, 20~40 times better than that of RuO2 single crystalline, arrayed nanorods. The specific power and specific energy of annealed RuO2.xH2O nanotubes measured at 0.8 V and 4 kHz is equal to 4320 kW/kg and 7.5 Wh/kg, respectively, demonstrating the characteristics of next generation supercapacitors. Finally, the significant contributions of this dissertation and further feasibility works were summarized in chapter 7.
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