奈米Pt/CeO2/MWCNTs混成電觸媒之合成與特性研究

Synthesis and Characterization of Nanocrystalline Pt/CeO2 /MWCNTs Hybrid Electrocatalysts

陳正瀚(Cheng-Han Chen)

指導教授 ： 林鴻明

大同大學/工程學院/材料工程學系所/博士(2009年)

摘要

本研究動機在於合成奈米混成觸媒，以開發燃料電池之陽極材料，並對此材料之結構特性與電化學反應之間做相關性的實驗與討論。期望經由導入多壁奈米碳管作為載體，製作鉑與氧化鈰混成觸媒系統，研究此三相結構對於電子傳輸行為改變所造成的電催化影響。
第一部分，使用改良式溶膠凝膠法，可成功合成氧化鈰/奈米碳管混成材料。在不同熱處理溫度下，300oC時生成非晶質氧化鈰披覆於碳管表面，500oC時為10-15奈米的氧化鈰顆粒沈積於碳管表面，700oC時則產生20-30奈米的氧化鈰顆粒披覆於碳管表面。經實驗檢測，700oC熱處理後的氧化鈰/奈米碳管混成材料，不僅有較完整的結晶構造，電化學反應行為較為優異，可做為設計燃料電池觸媒的合成條件依據。
第二部份，以多元醇法合成奈米碳管、碳黑(Vulcan xc-72)或氧化鈰為載體的鉑系混成觸媒，藉由一氧化碳轉化率與電催化實驗來探討載體效益。實驗結果顯示，鉑/氧化鈰觸媒可在較低溫下將一氧化碳轉換成二氧化碳；電催化方面，鉑/氧化鈰觸媒雖然可以在低電位時轉換甲醇而產生電流，但整體電流強度與電位轉換的比較下，仍以鉑/碳黑觸媒具有較優異的表現。
第三部份，結合改良式溶膠凝膠法與多元醇法，可成功合成鉑/氧化鈰/奈米碳管混成材料。經多元醇法還原出奈米白金微粒，3-7奈米的白金微粒可均勻分散於氧化鈰-700oC/奈米碳管表面。以靜電噴塗法製備電極進行循環伏安量測，比較鉑/氧化鈰-700oC /奈米碳管混成材料、商業粉鉑-釕雙金屬/碳黑(E-tek)、鉑/奈米碳管等三種樣品的電催化反應。實驗結果顯示，鉑/氧化鈰-700oC /奈米碳管混成觸媒具有最低氧化還原電位與最高電流密度。由此可知，氧化鈰可以提供氧，將吸附於白金表面的一氧化碳予以氧化，解決燃料電池陽極觸媒金屬被一氧化碳毒化之問題。同時，在一氧化碳氧化轉換率實驗中，此鉑/氧化鈰-700oC /奈米碳管混成觸媒在室溫下仍可保持60%轉化率。所以，本研究驗證此新型奈米混成觸媒可在燃料電池工作溫度內，不會因毒化問題而失去活性造成電池效能降低。

關鍵字

甲醇燃料電池 ； 電催化 ； 奈米碳管 ； 二氧化鈰 ； 奈米混成觸媒

並列摘要

The motivation of this research work is focused on synthesis and electro-catalytic characterization of nano-hybrid catalysts for anode materials in fuel cell application. Carbon nanotubes are used as support to study the supporting effect for electro-catalysts. Noble metal, Pt, and metal oxide semiconductor, CeO2, have been combined with MWCNTs to study the structure and conductivity influence of electro-catalytic behaviors.
Serious samples of CeO2/MWCNTs are synthesized by modified sol-gel method with 300, 500, or 700oC annealing. CeO2-300oC/MWCNTs have amorphous ceria coverage on MWCNTs surface. For 500oC heat treatment, the 10-15nm of CeO2 is decorated on MWCNTs with partial crystallization. After 700oC annealing, a distinct hybrid CeO2/MWCNTs nanomaterials are prepared with perfect crystalline structure and better particle dispersion. In electrochemistry analysis, the electrochemical active surface area and methanol electro-oxidation of CeO2-700oC are shown better catalytic activity than that of other heat-treated samples.
Pt/Vulcan-xc72, Pt/CeO2, and Pt/MWCNTs are synthesized via polyol method successfully for study the supporting effects. CO oxidation results indicate Pt/CeO2 has lower transferred temperature than that of Pt/Vulcan-xc72 and Pt/MWCNTs. From the result of C-V scanning curves, Pt/Vulcan-xc72 has a larger electro-oxidizing current and steady onset potential than that of Pt/CeO2, and Pt/MWCNTs.
Heterogeneous catalysts, Pt/CeO2/MWCNTs, are synthesized via combined polyol and modified sol-gel methods. Because CeO2-700oC has been proved its excellent catalytic activity in previous study, PtRu/Vuclan-xc72 (E-tek), Pt/MWCNTs, and Pt/CeO2-700oC/MWCNTs are chosen to compare and examine the methanol electro-oxidation kinetics. The structures of catalysts are characterized by FESEM, FETEM, and XRD. TGA is used to determine the content of components in hybrid system. The electrochemical active surface area obtained from electro-oxidation curve indicates Pt active sites of Pt/CeO2-700oC/MWCNTs have the largest specific surface area than that of PtRu/Vuclan-xc72 and Pt/MWCNTs. This indicates there are better metal dispersion and good support effect for Pt/CeO2-700oC/MWCNTs. The CO poisoning behavior of catalysts during methanol oxidation reactions have been investigated in C-V curves. The electro-oxidizing currents and onset potentials of Pt/CeO2-700oC/MWCNTs have the steady in long-term electro-oxidation reaction and it indicates better electro-catalytic activity. In CO oxidation conversion analysis, Pt/CeO2-700oC/MWCNTs have lower 100% conversion temperature of CO to CO2 than that of others and still maintain 60% conversion rate after 250 minutes at room temperature. The present study indicates cerium oxide in Pt/CeO2-700oC/MWCNTs catalyst will enhance significantly oxygen ions transportation between interface of Pt and MWCNTs to eliminate CO poison problem that may use for fuel cell applications.

並列關鍵字

DMFC ； Nano Hybrid Catalysts ； MWCNTs ； CeO2 ； Electro-catalysis

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