脈衝式微波輔助合成鉑觸媒之研究

Preparation of Pt-based Catalysts by Pulse Microwave-assisted Deposition

羅安諾(AnupKumar Roy)

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(2012年)

https://doi.org/10.6838/YZU.2012.00164

摘要

本研究以鉑成份觸媒，結合觸媒化學氣相沉積法所製備之奈米碳管/碳紙基材，作為質子交換膜燃料電池陽極。實驗方法是以乙二醇作為還原劑並結合脈衝微波還原法直接將觸媒還原在奈米碳管/碳紙基材上。觸媒的特性則藉由電化學測試系統、熱重分析儀、X光繞射儀、掃描式電子顯微鏡、穿透式電子顯微鏡及膜電極組測試進行分析。分析結果顯示電化學觸媒均勻且一致的附著在酸化處理後的奈米碳管表面上，而觸媒的粒徑大小最低可達到2.1 nm且均勻分佈於碳材表面。本實驗設計了三種膜電極組，分別為自製陽極-商購陰極、商購陽極-自製陰極及商購陽極-商購陰極，在自製陽極-商購陰極之膜電極組測試中顯示觸媒製程CAT 3-1 (ton : toff = 3s : 1s)、CAT.2-1 (ton : toff = 2s : 1s)、CAT 3-2 (ton : toff = 3s : 2s)在燃料電池工作溫度40°C及80°C分別展現其最高功率密度為5.94、6.67、8.60 kW g Pt-1及6.39、8.00、11.41 kW g Pt-1，結果顯示觸媒製程CAT 3-2的表現效能最佳，再以此參數製作商購陽極-自製陰極膜電極組與商購陽極-商購陰極膜電極組做比較，在燃料電池工作溫度40°C及80°C其表現出最高功率密度為4.90、1.60 kW g Pt-1以及5.45、1.90 kW g Pt-1。由實驗結果可知，當脈衝比(ton / toff)較高時，觸媒顆粒大小隨之增加，展現出較差的分散性，並且影響其觸媒活性、穩定性及燃料電池功率密度。由膜電極組及循環伏安法測試發現觸媒表現出良好的活性與耐久性，此結果顯示奈米碳管的添加使得電子傳輸速度提升，並增加觸媒基材的耐久性。以奈米碳管為基材結合脈衝微波還原的快速製備方法，將可使鉑觸媒分散地更平均並且電子可進行一維傳輸，加速氧化還原的速度。膜電極組測試結果顯示，本研究所製備的電極比起商購電極表現出較高效能。這些觸媒效能差異，可歸因於奈米碳管的添加、觸媒粒徑及分散性等差異所造成。此結果證明了本研究所製備之氣體擴散電極(Gas Diffusion Electrode, GDE) 可大幅度提升燃料電池之效能，並提供一種快速及高性能的GDE製備技術。

關鍵字

脈衝微波還原法 ； 多元醇合成法 ； 鉑觸媒 ； 奈米碳管 ； 電化學分析 ； 燃料電池

並列摘要

This study represents a comprehensive pulse microwave assisted polyol synthesis method to deposit Pt catalyst onto the surface of multiwalled carbon nanotubes (CNTs). Three different catalysts were prepared using three different microwave pulse configuration namely CAT 3-1 (ton : toff = 3s : 1s); CAT.2-1 (ton : toff = 2s : 1s) and CAT 3-2 (ton : toff = 3s : 2s) The microstructure, morphology and Pt loading of the as-prepared catalysts were investigated by X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM) and thermogravimetric analysis (TGA). XRD and HR-TEM results revealed that the CAT 3-2 catalyst possessed the smallest mean particle size of 2.1 nm and most uniform distribution among all the synthesized catalysts. The electrochemical activity and stability of the Pt-CNT electrocatalysts were investigated in 1.0 M H2SO4 using cyclic voltammetry (CV). The results indicates that CAT 3-2 exhibits the highest SESA value and fairly good stability after potential cycling > 1000 cycles as compared to other two catalyst. The order of the catalysts in terms of SESA value was found to be CAT 3-2 > CAT 2-1 > CAT 3-1.
The performance of the as synthesized catalyst for hydrogen oxidation reaction (HOR) was analyzed using single cell proton exchange membrane fuel cell (PEMFC) measurements in a membrane electrode assembly (MEA) with 5 cm2 active area at two different operating temperatures (40°C and 80°C). The as synthesized catalyst constructs the anode electrode and a commercial GDE was the cathode. The maximum power density based on anode loading for all three catalysts were found to be 5.94, 6.67 and 8.60 kW g Pt-1 at 40°C and 6.39, 8.00 and 11.41 kW g Pt-1 at 80°C for CAT 3-1, CAT 2-1 and CAT 3-2, respectively. The overall performance of the catalysts was ordered as CAT 3-2 > CAT 2-1 > CAT 3-2.
Also from these investigations it was found that microwave pulse configuration have a profound effect on the performance of the Pt-CNT catalysts. As the MP factor (ton / toff) increases the bigger particle size and poor uniformity of Pt nanoparticles can be observed on the carbon support which in turn affects the catalytic activity, stability and the maximum power density of the electrocatalyst.

Based on the catalyst characterization results, CV results and single cell performance of three Pt-CNT catalyst fabricated, CAT 3-2 electrocatalyst was selected to test further its activity towards oxygen reduction reaction (ORR) in a single cell PEMFC. The MEA fabricated with 'Homemade Cathode-Commercial Anode' configuration using CAT 3-2 was investigated using single cell system. Also the 'Commercial Anode-Commercial Cathode' MEA was tested. The experimental results revealed that the maximum power density based on cathode loading for these MEAs are 4.90 and 1.60 kW g Pt-1 at 40°C, and 5.45 and 1.90 kW g Pt-1 at 80°C for CAT 3-2 and commercial MEA, respectively. i.e., The CAT 3-2 electrocatalyst performs better compared to the commercial GDE although it has a lower Pt loading then the commercial GDE. The long term durability test of CAT 3-2 MEA also show excellent durability comparable to the commercial MEA.
All the experimental results suggest that pulse microwave assisted polyol synthesis can be utilized as effective catalyst preparation method for synthesizing PEMFC electrocatalyst with improved catalytic activity and stability towards HOR and ORR.

並列關鍵字

Carbon nanotubes ； Electrochemical activity ； Fuel cells ； MP factor ； Pulse microwave ； Polyol synthesis ； Pt catalysts

參考文獻

[4] W.G. Colella, M.Z. Jacobson, D.M. Golden, J. Power Sources 150 (2005) 150.

[5] M. Granovskii, I. Dincer, M.A. Rosen, Int. J. Hydrogen Energy 31 (2006) 337.

[7] B. Rohland, J. Nitsch, H. Wendt, J. Power Sources 37 (1992) 271.

[8] L.J.J. Jansen, J. Appl. Electrochem. 37 (2007) 1383.

[9] S. Abanades, G. Flamant, Chem. Eng. Commun. 195 (2008) 1159.