製備金屬觸媒/奈米碳管複合電極以應用於直接甲醇燃料電池

Fabrication of Metallic Catalyst/Carbon Nanotube Composite Electrodes for Direct Methanol Fuel Cells

魏金龍(Jin-Long Wei)

指導教授 ： 謝建德

元智大學/工程學院/化學工程與材料科學學系/碩士(2009年)

https://doi.org/10.6838/YZU.2009.00318

摘要

本實驗分成兩部份進行探討，第一部份是使用三種不同還原法製備相同比例的Pt–Co/CNT觸媒；第二部份則是使用微波還原法製備不同比例的Pt–Co/MCMB觸媒。
第一部分研究中，分別以三種不同方法製備鉑-鈷/奈米碳管(Pt–Co/CNT) 金屬觸媒，並選用循環伏安法(CV) 和甲醇氧化法去探討其電化學性質。由X光繞射分析圖(XRD) 顯示出，這三組觸媒擁有不同的結晶大小，且有不同程度的原子分散度。使用硼氫化鈉當作還原劑時，鉑表面上會覆蓋一層鈷，會產生雙金屬Pt–Co 粒子，直接使用熱還原法會產生Pt–Co的奈米合金，且合金化程度會變高。由實驗結果指出活性表面覆蓋率會隨著Pt–Co觸媒合金化程度的增加而提升，此結果顯示原子分散的重要性。由循環伏安法(CV) 中可判別出合金化程度有利於電化學活性的提升。在甲醇氧化測試中其電化學活性較佳的樣品，原因歸咎於觸媒的雙功能機制(bifunctional mechanism)，Pt–Co/CNT觸媒提供很多Pt–Co，在甲醇氧化時，有OH基產生即與Co產生Co–OH鍵結，形成中間產物，因此Pt就會得到更多的活性位置。有鑑於此，合成條件是影響表面原子分佈的重要因素之ㄧ，故推斷其觸媒活性與表面原子分佈有關。
第二部分研究中，以微波還原法製備不同比例鉑–鈷/石墨(Pt–Co/MCMB) 金屬觸媒，並以循環伏安法和甲醇氧化法探討其電化學性質。由XRD透露出這三組觸媒不但擁有不同的結晶大小，而且有不同程度的原子分散度。從CV圖中，我們可看出電化學比表面積會隨著掃瞄圈數的增加而減少，又以PtCo/MCMB (Pt/Co=50/50) 這組觸媒擁有較佳的電化學特性，顯示出Pt/Co不同的重要性。在甲醇氧化測試中，經過循環圈數100圈後其IF/IB (Pt/Co=25/75，50/50，75/25) 的值1.08、1.12與1.13，我們把原因歸咎於使用微波還原法所製備的觸媒粒子大小是影響其電化學活性的重要因素之ㄧ，且使用微波還原法可大幅減少還原時間。

關鍵字

奈米碳管、石墨 ； 電化學活性 ； 合金化程度 ； 直接甲醇燃料電池

並列摘要

This research was divided into two parts to explore. In the part 1, we used three different reduction methods to prepare the same ratios of Pt–Co/CNT catalysts. In the part 2, we used the method of microwave reduction to prepare three kinds of proportion of Pt–Co/MCMB catalysts.
Part 1: The electrochemical activities of three types of Pt–Co/CNT catalysts, prepared from different Co depositions, in methanol oxidation have been investigated. X-ray diffraction reveals that these Pt–Co/CNT catalysts possess not only different crystalline sizes but also different levels of atomic distribution. The use of strong reducing agent (NaBH4) enables the formation of a cobalt layer over the Pt surface, inducing bimetallic Pt–Co particles, whereas direct thermal reduction enables the formation of Pt–Co nanoalloy with a high degree of alloying. It has been shown that the normalized active surface coverage increases the alloying degree of Pt–Co catalysts, indicating the importance of atomic distribution. Cyclic voltammetric measurement also reveals that the Pt–Co/CNT catalyst with a good alloying degree exhibits a better electrochemical activity, high CO tolerance, and long-term durability (> 100 cycles). This activity improvement in methanol oxidation can be attributed to the bifunctional mechanism of binary catalysts: since the Pt–Co/CNT catalyst offers a large amount of Pt–Co pairs, the Co site serves as a promoting center for the generation of Co–OH species, and thus more Pt sites are available for methanol oxidation. Accordingly, the synthesis condition is one of the key factors affecting the distribution of surface atoms, significantly related to their catalytic ability in methanol oxidation.
Part 2: The method of microwave reduction was used to prepare Pt–Co/MCMB catalysts with different Pt/Co atomic ratios. It revealed that three kinds of catalysts, Pt25Co75/MCMB, Pt50Co50/MCMB and Pt75Co25/MCMB, exhibit different crystal sizes showing different electrochemical activities in methanol oxidation. From CV curve, the electrochemical surface area is the decrease function of scan number. The Pt50Co50/MCMB catalyst displays the best electrochemical properties among the Pt-Co/MCMB catalysts, revealing importance of different proportion. In the methanol oxidization, IF/IB value of Pt25Co75/MCMB, Pt50Co50/MCMB and Pt75Co25/MCMB are 1.08, 1.12 and 1.13 after 100 cycles, respectively. We inference the particle size of the method of microwave reduction to affect the electrochemical activity is one of importance factors, and the method of microwave reduction can substantial decrease reduction time.

並列關鍵字

Pt–Co catalysts ； Carbon nanotubes ； Electrochemical activity ； Degree of alloying ； Direct methanol fuel cells

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