利用奈米碳管與鉑釕化學沉積法製備直接甲醇燃料電池陽極觸媒

Preparation multi-walled carbon nanotubes supported PtRu catalysts for Direct Methanol fuel cells anode by chemical reduction

賴羿如

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/碩士(2006年)

https://doi.org/10.6843/NTHU.2006.00543

摘要

直接甲醇燃料電池被視為解決未來能源危機的重要選擇，主要是由於其擁有高能量轉換效能，低污染。在低溫環境下操作，可應用於可攜式電池。甲醇氧化的過程當中，其中一項中間產物為一氧化碳，而ㄧ氧化碳對於甲醇燃料電池的白金觸媒有毒化的現象，使得燃料電池降低其效能，解決的方法是希望透過其他合金防止ㄧ氧化碳毒化現象，目前以PtRu合金為最佳的雙合金組合。
為了增加金屬觸媒的分散度，有效利用且穩定奈米尺寸的金屬觸媒，必須使用高孔隙度載體。目前所使用的載體為碳載體，因為碳載體無論在酸鹼當中都有好的穩定性，具有好的導電度，高比表面積。過去廣為使用的碳載體為碳黑，主要是因為碳黑具有好的導電性，與極大的比表面積，抗腐蝕性等等取勝。近來研究顯示，因為奈米碳管具有獨特的結構，電子、電機、機械、物理與化學性質逐，可以提供足夠的反應面積，希望可以取代傳統碳黑，提供直接甲醇燃料電池有更好的效能。
對於金屬觸媒，大致上希望可以達到下列幾項原則：（1）奈米尺度的顆粒大小;（2）鉑釕形成合金;（3）能夠在載體上面有良好的分散情形;（4）低成本。觸媒製備方面，化學還原方法為目前廣泛被使用的，簡單且直接的化學技術，將鉑釕前驅物跟碳載體形成均勻的混合。本篇所使用的還原劑為乙二醇，不但具有還原金屬觸媒的效用，同時可穩定金屬觸媒。
觸媒製備完成後，必須做定性與定量分析。透過掃描式電子顯微鏡（Scanning Electron Microscopy，SEM），穿透式電子顯微鏡（Transmission Electron Microscpoy，TEM），可以觀察奈米碳管、觸媒在奈米碳管上面的形貌與分佈情形，EDS(energy dispersion spectroscope)、XRD（X-ray Diffraction）、ICP-MS可作為定性分析，循環伏安法( Cyclic Voltammetry)得知金屬觸媒的活性等等。
研究結果發現，奈米碳管經過親水處理之後，膠狀觸媒才有辦法進一步還原於奈米碳管上。氫氣濕式還原可以加強觸媒還原於載體上面的效果，但是對於觸媒的活性沒有明顯的提升。乙二醇的使用可以改善觸媒的分散度，但無法有效形成PtRu合金。控制溶液NaOH濃度的初始值可達到控制觸媒顆粒尺寸。空氣處理過後的觸媒活性改善，但是一氧化碳再氧化峰變的明顯。

關鍵字

直接甲醇燃料電池

並列摘要

The direct methanol fuel cell (DMFC) is a low temperature fuel cell. It can be used for mobile applications. The advantages of direct methanol fuel cells (DMFC) over hydrogen fuel cells include easy storage of the high energy density liquid fuel. CO can adsorb very strongly on the Pt surface in the fuel cell anode , blocking the active sites and causing a large decrease in the electrode performance.PtRu alloys are currently the most active anode catalyst for the oxidation of methanol.
To achieve economical Pt loadings in the MEA the electrocatalysts are supported
on high surface area carbon support with a high mesoporous area. The support material must provide a high electrical conductivity, give good reactant gas access to the electrocatalyst, and also show good corrosion resistance. Carbon nanotubes are prominent materials that exhibit special properties which make them suitable for application in several technological areas.Carbon nanotubes have been studied as an electrocatalyst support for direct methanol fuel cells (DMFCs).
The common criteria for a high catalyst are: (1) a narrow nanoscale size distribution; (2) a fully alloyed degree; (3) high dispersion on carbon support; (4) low cost. All of these chemical reduction methods include a chemical step for forming nanoparticles, and a deposit step for dispersing the catalyst onto the carbon particles. We use Ethylene glycol as a reductive agent here.
The catalysts were characterized by transmission electron microscopy (TEM), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and inductively coupled plasma-mass spectrometer (ICP-MS). Their electrochemical behaviors in a half cell are analyzed by cyclic voltammetry (CV).
Because the pristine surface of CNTs is inert, it is difficult to attach metal nanoparticles to the substrate surface. Through surface pre-treatments, the metal nanoparticles could easily attach onto the CNTs surface. In order to achieve high dispersion and maximum utilization, we use ethylene glycol as a reductive agent. The results show that the synthesis solution pH is a key factor that influences the catalyst particle size. Heat treatments at these “low” temperatures free up valuable catalyst sites without resulting in changes of the PtRu catalyst properties.After air treated, CO poisioning is obvious.

並列關鍵字

DMFC ； Anode catalyst ； Methanol Oxidation ； Electrochemistry ； PtRu

參考文獻

[6] Xuan Cheng, Ling Chen, Cheng Peng, Zhiwu Chen, Ying Zhang, and

[2] T. R. Ralph and M. P. Hogarth,Platinum Metals Rev., 2002, 46, (3)

[5] T.Frelink,W.Visscher,J.A.R.Van Veen, Journal of the Electroanalytical Chemistry 382(1995)65-72

Qinbai Fanc, Journal of The Electrochemical Society, 151 (1) A48-A52 ,(2004)

[8] T. Iwasita, Electrochimica Acta 47 (2002) 3663_/3674