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研究生： 吳宜萱
Wu, Yi-Shiuan
論文名稱： 高效能奈米結構化三相區電極製作與電化學穩定性測試應用於微型直接甲醇燃料電池
Fabrication of a Highly Efficient Nano-Structured Three-Phase-Zone Electrode and the Electrochemical Stability Tests for DMFC Applications
指導教授： 曾繁根
Tseng, Fan-Gang
口試委員: 薛富盛
薛康琳
葉宗洸
蘇育全
曾繁根
學位類別： 博士
Doctor
系所名稱： 原子科學院 - 工程與系統科學系
Department of Engineering and System Science
論文出版年： 2012
畢業學年度： 100
語文別： 英文
論文頁數： 109
中文關鍵詞： 白金奈米觸媒 、奈米碳管 、直接甲醇燃料電池 、開放迴路還原系統 、電化學穩定性 、電位循環掃描技術 、三相區 、旋轉塗佈
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    • In the present study, highly homogeneous platinum nanocatalysts with enhanced electrocatalytic activity were uniformly deposited on carbon nanotubes directly grown on a silicon plate (Pt/CNTs/Si) as the electrodes for direct methanol fuel cells (DMFCs) by a novel homemade open-loop reduction system (OLRS). Compared with a traditional reflux system that maintains the ratio of water to ethylene glycol (EG) at ~160 °C for ~4 h, the gradual concentration increase of EG in the precursor solution can be accomplished by distilling off water in the OLRS while increasing the temperature to 130 °C. This process with simultaneous increases in precursor concentration and in reaction temperature rendered high-quality Pt nanoparticles to precipitate with high-density dispersion on the pretreated CNTs. The OLRS is not only able to shorten the reduction time (<1.5 h) but also able to enhance the electrocatalytic activity of the electrodes by creating a preferential orientation of Pt (111) facets for the methanol oxidation reaction (MOR). Cyclic voltammetry and electrochemical impedance spectroscopy were conducted to evaluate the mass activity (MA) and charge transfer resistance (Rct) of the fabricated electrodes for the MOR. Compared with the electrodes prepared by traditional Pt reductions (MA: 100-360 A gPt-1 and Rct: 40-80 Ω-cm2), the Pt/CNTs/Si-based electrodes prepared at 130 °C in the OLRS exhibited superior electrocatalytic properties, including an MA of 435 A gPt-1 and an Rct of ~30 Ω-cm2. By the potential cycling technique of startup and shutdown cycles under strongly oxidizing conditions, the electrochemical stability of the above-prepared CNTs and Pt/CNTs electrodes was evaluated to mimic the real electrode operating environment in DMFCs. The cyclic voltammetry (CV) curves for MOR revealed that the performance degradation of the electrochemically-treated electrodes at 60 °C was 1.7 times higher than those at 25 °C after the electrochemical oxidation tests for 5 h, resulting from the loss of electrochemical surface area (ESA) of Pt catalysts during the electrode operation. This is mainly due to carbon corrosion or rearrangement of Pt catalysts on CNTs, resulting in the Pt agglomeration (or growth of Pt particles) on and Pt detachment from the surface of the CNTs during the electrochemical oxidation process. With regard to the membrane electrode assembly (MEA), a Pt/CNTs electrode with a thin and uniform ionomer layer as the proton-conducting electrolyte on a nano patterned three-phase zone (TPZ) was fabricated in this study, aiming at high electrocatalytic activity and high charge transfer rate for MOR. Unlike conventional paste or spray methods that produced thick and non-uniform ionomer layers to form TPZs within the catalyst layers (50-100 um) of electrodes, thin and uniform ionomer layers (5-10 nm) on the hydrophilic-treated Pt/CNTs electrodes were harvested by spin-coating. The thickness of the ionomer layer was controlled by altering the spin-coating speed, and the effect of the ionomer thickness on the surface of the catalyst layer and on the electrochemical properties of the electrodes for MOR was studied. Compared to the electrode fabricated by spraying (MA: 355 A gPt-1, Rct: 48 Ω-cm2), the ionomer-coated electrode spin-coated at 4000 rpm exhibited superior properties for MOR (MA: 381 A gPt-1, Rct: 15 Ω-cm2). The outcome renders this new electrode to embrace potential applications in micro DMFCs with the design of a thin and uniform TPZ.

    ABSTRACT i ACKNOWLEDGEMENTS iv CONTENTS v FIGURE CAPTIONS viii TABLE CAPTIONS xvii NOMENCLATURE xviii Chapter 1. Introduction 1 1.1. Principle of fuel cells 2 1.2. Classification of fuel cells 3 1.2.1. Alkaline fuel cell (AFC) 4 1.2.2. Phosphoric acid fuel cell (PAFC) 4 1.2.3. Solid oxide fuel cell (SOFC) 5 1.2.4. Molten carbonate fuel cell (MCFC) 6 1.2.5. Proton exchange membrane fuel cell (PEMFC) 6 1.2.6. Direct methanol fuel cell (DMFC) 7 1.3. Motivation and objectives 8 Chapter 2. Literature review 11 2.1. Catalyst supports 11 2.2. Nanocatalysts 16 2.3. Electrochemical oxidation 21 2.4. Three-phase zone electrodes 30 Chapter 3. Experimental 41 3.1. Growth of CNTs 42 3.2. Functionalization of CNTs 43 3.3. Preparation of Pt catalysts 44 3.4. Preparation of ionomer-coated electrodes 47 3.5. Electrochemical measurements 48 3.6. Electrochemical oxidation 51 Chapter 4. Results and discussion 53 4.1. Morphology of CNTs 53 4.2. Hydrophilicity of CNTs 55 4.3. Catalyst preparation 57 4.4. Morphology of Pt/CNTs/Si-based electrodes 58 4.5. Morphology of ionomer-coated electrodes 61 4.6. Electrocatalytic properties of Pt catalysts 69 4.7. Electrocatalytic properties of ionomer-coated electrodes 72 4.8. EIS analysis of Pt/CNTs/Si-based electrodes 74 4.9. EIS analysis of ionomer-coated electrodes 80 4.10. Electrochemical oxidation 84 4.10.1. Stability of plain CNTs 84 4.10.2. Stability of Pt/CNTs 88 Chapter 5. Conclusions 95 Chapter 6. Future work 97 Appendix: Publication 98 Journal Papers 98 International Conference Papers 98 Domestic Conference Papers 101 Patents 102 References 103

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