白金觸媒之製備與鈦釩鉻合金氫化特性及其在燃料電池應用之研究

Synthesis of Platinum Catalyst and Hydrogenation Properties of Ti25V35Cr40 for Application in Fuel Cell

周哲平(Justin C.P. Chou)

指導教授 ： 彭宗平

國立清華大學/工學院/材料科學工程學系/碩士(2008年)

https://doi.org/10.6843/NTHU.2008.00453

摘要

能源危機不僅會造成國家間的紛爭，也隨時影響全球的經濟，其造成的通貨膨脹和空氣惡化皆影響人類的生活品質和環境，由於石油價格不斷的攀升，許多替代性能源的商業價格變得更具競爭力，例如豐田的油電混合車以及歐洲廣設的太陽能和風力發電設備。這些綠色能源是未來的希望，更是解決目前人類面臨的能源危機之方法。本論文分為兩部分，分別對綠色能源中的燃料電池之電極材料和儲氫合金之氫化特性進行研究。

氫氣燃料電池必須使用白金作為催化反應之觸媒，目前廣泛使用的白金觸媒利用碳黑作為分散的載體以增加反應面積。多壁奈米碳管有良好的機械和電性質，本研究嘗試將白金顆粒沉積於多壁奈米碳管的表面，希望能增加觸媒的穩定性和活性。當多壁奈米碳管經過酸洗處理後，其表面形成許多官能基，利用沉澱與氫氣還原法或硼氫化鈉的直接還原法皆能將白金均勻地分散在多壁奈米碳管和奈米鬚(carbon nanohorn)的表面。相較之下，沒有經過酸洗處理的奈米碳管則無法使白金均勻的分散在其表面。電化學量測燃料電池的結果得知白金在多壁奈米碳管上的發電效率每平方公分為0.25瓦，而使用碳黑的白金則有0.28瓦，兩個值相去不遠。其中又發現，如以多壁奈米碳管作為白金的載體，其氣體擴散電極中所需要的Nafion含量須降至10~20%才可達到最好發電效果，與表面積較高之碳黑所需的30%不同。

釩是一種可以吸附大量氫氣的儲氫合金，但是昂貴的價格讓純釩失去應用競爭力。如將釩鈦鉻熔成合金，其氫化特性與純釩相差不遠，但是便宜的鈦和鉻能增加合金的價格競爭力。本研究則在鈦釩鉻(Ti25V35Cr40)合金中置入少量的碳和硼元素，希望能增加其有效放氫量。利用電弧爐及氬氣電漿將金屬元素熔煉成合金，將試片在真空環境下以1200oC熱處理兩小時，使其均質化。從X光繞攝的結果和晶相顯微鏡的觀察下，發現所有的合金皆形成BCC相，但仍有一些析出物。研究發現，鈦釩鉻的有效放氫量為0.80H/M，但置入1 at%的硼和0.1 at%的碳元素之後，其有效放氫量則可分別提高至0.86H/M和0.87H/M。這九個百分比的差可以讓氫氣燃料電池汽車上的儲氫合金減少60到80公斤的重量，未來具有商品應用的潛力。

關鍵字

燃料電池 ； 白金觸媒 ； 鈦釩鉻合金

並列摘要

Energy crisis not only causes conflicts between nations, but also affects the world economy. Inflation and air-pollution both devaluate the living environment of human being. As crude oil price keeps increasing, many alternative energy systems become competitive for commercialization, such as the hybrid car from Toyota, solar, and wind power in European countries, etc. They are the hope to solve for the many problems people are encountering now. This study focuses on two building blocks of green energy: fuel cell and hydrogen storage.

Hydrogen fuel cell requires platinum (Pt) catalyst to accelerate the reaction, and most of the commercial Pt catalysts are deposited on amorphous carbon black for greater reaction surface area. With a hope to increase the activity and stability of Pt catalyst, another approach was made in this study by depositing Pt on multi-walled carbon nanotubes (MWNTs), which retain good mechanical and electrical properties. MWNTs were first surface modified with a mixture of nitric and sulfuric acid to produce surface functional groups. By either precipitation of Pt particles in a solution with an appropriate pH or direct reduction of Pt particles using sodium borohydride, Pt particles can be dispersed uniformly on the surface of modified MWNTs and carbon nanohorn (CNH). Moreover, the diameters of Pt can be controlled at 1 to 3 nm and 2 to 4 nm, respectively. In contrast, without surface modification, the Pt particles would tend to aggregate on the surface defects and tubular-ends of MWNTs. The cell performance with (17%) Pt/MWNT as the catalyst is 0.25 W/cm2, while that of commercial E-TEK (20%) Pt/XC-72 is 0.28 W/cm2. Unlike the optimal Nafion® loading in Pt/XC-72 gas diffusion electrode (GDE), the optimal Nafion® loading in Pt/MWNT GDE is determined to be between 10 and 20%.

As for hydrogen storage material, vanadium (V) can absorb a large amount of hydrogen; however, the high cost of V is its drawback. By alloying V with inexpensive titanium (Ti) and chromium (Cr), they turn into an inexpensive hydrogen storage alloy with respectable hydrogenation properties. In this study, 0.1 at% to 5 at% of boron (B) or carbon (C) was doped into the interstitial sites of the bcc Ti25V35C40 alloy, hoping to increase the effective desorption capacities for fuel cell application. The samples were prepared by arc melting and homogenized by annealing at 1200oC for 2 hours in vacuum. From X-ray diffrection and optical microscopic analyses, all specimens formed a bcc phase with the presence of some minor second phases. The effective desorption capacity of Ti25V35Cr40 is 0.80 H/M. The specimens that added with 1% B or 0.1% C exhibit greater effective desorption capacities, with values of 0.86 H/M and 0.87 H/M, respectively. The 9% increase of effective desorption capacity can reduce the weight of metal hydride in fuel cell vehicles by 60 to 80 kg, that demonstrates a greater potential for commercial applications.

並列關鍵字

PEMFC ； Pt catalyst ； TiVCr alloy

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