利用成長於碳布的奈米碳管為載體之直接甲醇燃料電池鉑與鉑釕觸媒之電化學及結構特性分析

Electrochemical and Structural Characterizations of Platinum and Platinum-Ruthenium Particles Electrodeposited on Carbon Nanotubes Directly Grown on Carbon Cloths for Direct Methanol Fuel Cell Applications

蔡銘麒(Ming-Chi Tsai)

指導教授 ： 蔡春鴻 ； 葉宗洸

國立清華大學/原子科學院/工程與系統科學系/博士(2006年)

摘要

由於直接甲醇燃料電池(Direct Methanol Fuel Cell, DMFC)的高能量密度、低操作溫度、對環境的友善度等優點，目前已成為下世代可攜式電子產品電源的重要選項之一。DMFC係以固態質子交換膜作為固態電解質，二邊各為陰、陽極的觸媒電極層。其中，陽極的觸媒為碳支撐鉑-釕(Pt-Ru)合金，陰極則為碳支撐的Pt金屬。現今DMFC的作法是將觸媒漿料直接塗佈於碳布或碳紙表面後，再與固態電解質熱壓後而形成。一般來說，DMFC的效率大多取決於觸媒金屬的分散性及粒徑大小。為了能得到高分散性及較小粒徑的觸媒顆粒，使用高比表面積的奈米碳材料做為觸媒支撐層為目前廣為採納的方法。其中，奈米碳管(Carbon Nanotubes, CNTs)由於其絕佳的化學、物理性質及高比表面積的特性，目前已被視為最有潛力的碳支撐材料。除了使用高比表面積的CNTs做為觸媒支撐層之外，發展新穎的製備方法製備分散性佳的奈米Pt或Pt-Ru觸媒也是相當重要的課題。另外，抑制一氧化碳對陽極的Pt觸媒毒化而提高觸媒效能的問題，減少貴重金屬的使用量同時不影響電池效率的成本問題等，都是發展商業化DMFC的挑戰。而本研究的主要目的是製備高效能的陽極觸媒，同時減少一氧化碳的毒化現象，以獲得較高的電池功率密度。
研究分為二個方向：一為利用熱化學沈積法，將CNTs直接長成於碳布上以作為Pt及Pt-Ru觸媒的支撐層以提高觸媒層與擴散層的導電率。二為利用改良式電鍍技術將奈米尺寸的Pt及Pt-Ru觸媒沈積於上述之CNTs表面。實驗結果發現，添加乙二醇於電鍍液中可以得到分散性佳的奈米觸媒顆粒。利用X光粉末繞射儀(X-ray Powder Diffraction)、穿透式電子顯微鏡(Transmission Electron Microscopy)、感應耦合電漿質譜(Inductively Coupled Plasma-Mass Spectrometer)，分析觸媒型態與組成，並使用循環伏安法(Cyclic Voltammetry)分析觸媒在半電池時的催化活性和抑制CO的毒化能力；之後再將Pt-Ru/CNT觸媒當作陽極觸媒，利用Johnson Matthey Pt/C商業觸媒當作陰極觸媒，組裝成單電池(single-DMFC)，進行電壓電流特性分析，以獲得其功率密度。將電化學的結果和Johnson Matthey的PtRu/C商業觸媒進行比較，可發現自製的觸媒在半電池實驗的甲醇氧化電流密度、抑制CO毒化能力已優於商業觸媒。使用自製的觸媒所組成的全電池和利用商業觸媒組成的全電池其功率密度已提升約65%，顯示自製之Pt-Ru/CNT觸媒電極的活性已優於商業化之觸媒電極。

關鍵字

奈米碳管 ； 電鍍 ； 白金 ； 鉑釕合金 ； 活性 ； 甲醇氧化 ； 直接甲醇燃料電池

並列摘要

Direct methanol fuel cells (DMFCs) are attracting much more attention as a power source in portable electronic devices due to their advantages such as environment-friendly, high power density, easy handling of fuel, and low operating temperature. The core of a DMFC is the membrane electrode assembly that in general comprises a thin flat proton exchange membrane electrolyte with catalyst layers (in general, carbon-based material supported platinum (Pt) or platinum-ruthenium (Pt-Ru) nanoparticles with a mixture of proton conducting ionomer) bonded to both sides. The catalyst layers are conventionally pasted on the electron-conducting carbon materials such as carbon papers or carbon cloths. In order for a DMFC to operate more efficiently, the nature of catalysts with a smaller particle size and better dispersed characteristics is readily considered. One way to enhancing the dispersion of the catalysts is to construct catalyst supports with nanostructure of relatively high surface areas to volume (or mass ratio). Carbon nanotubes (CNTs) that can bear a high and accessible surface area through a deliberately engineered process have been shown a promising candidate supporting material as the catalyst layer of a low temperature fuel cell. In addition to the preparation of a more effective carbon supporting material, a novl method of depositing well-dispersed nano-sized Pt or Pt-Ru catalyst particles is also essential. Carbon monoxide poisoning on Pt or Pt-Ru catalysts surface during methanol oxidation is an issue of equal important to be addressed in pursuing the high performance catalyst layer for DMFC applications. Besides, another challenge for the commercialization of DMFC is to reduce the precious metal loading of the catalyst layers for cost-reduction without jeopardizing the fuel cell efficiency.
In this study, for the simultaneous accessibility of catalysts to the fuels, the electron-conducting diffusion layer and the proton-conducting electrolyte, we employed a novel method to directly grow the CNTs on carbon cloths as the electron-conducting and supporting materials of the catalysts. Then, an improved electrodeposition technique to deposit Pt and Pt-Ru nanoparticles on the surfaces of these CNTs was adopted. Through an extensive effort on the parametric study of the electrodeposition process, including temperature, deposition period, deposition sequence of Pt and Ru, and molar ratio between the electrolyte and the metal precursor salts, a better dispersed nano-sized Pt and Pt-Ru catalysts electrodeposited on the surfaces of the CNTs was achieved when the ethylene glycol (EG) contained deposition electrolyte was adopted. The methanol oxidation efficiency of Pt and Pt-Ru/CNT electrodes analyzed by cyclic voltammetry (CV) revealed that the methanol oxidation efficiency of the Pt-Ru/CNT electrodeposited in ethylene glycol containing H2SO4 aqueous solutions was better than that of commercialized Pt-Ru/C electrode. The structural and composition characteristics of Pt/CNT and Pt-Ru/CNT electrodes were also analyzed by scanning electron microscopy, transmission electron microscopy, X-ray diffractometer, X-ray adsorption near-edge spectroscopy, and inductively coupled plasma mass spectroscopy. Furthermore, the power density of a DMFC using Pt-Ru/CNT as the anode was ~65% greater than that of another DMFC with a commercial Pt-Ru/C anode, clearly indicating a significantly improved catalytic activity of the new Pt-Ru/CNT electrode.

並列關鍵字

Carbon nanotubes ； Electrodeposition ； Pt ； Pt-Ru ； Activity ； Methanol oxidation ； Direct methanol fuel cell

參考文獻

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