以原子層沉積法製備奈米鉑觸媒於新穎多孔性載體應用於高效能質子交換膜燃料電池及其加速老化之研究

Fabrication of Platinum Catalyst on Novel Porous Supports by Atomic Layer Deposition for High Specific Power Density Proton Exchange Membrane Fuel Cell and Its Accelerated Degradation Test

薛仰志(Yang-Chih Hsueh)

指導教授 ： 彭宗平

國立清華大學/工學院/材料科學工程學系/博士(2013年)

https://doi.org/10.6843/NTHU.2013.00505

摘要

由於燃料電池有低汙染、無噪音、高效率等之優異性能，被公認為是最具有潛力的綠色能源之一。即使具有多項的優異特性，燃料電池在商品化的過程中仍然有不少的挑戰，其中最大的課題就是如何降低電池的成本以及增加耐久度。觸媒的成本在整個燃料電池中佔了將近一半，所以如何降低觸媒的使用量，並同時維持甚至提升燃料電池的效率，是我們所面臨的一大課題。
本論文的研究目的係以此作為出發點，經由原子層沉積技術(atomic layer deposition, ALD)製備鉑奈米晶粒，沈積均勻分散且顆粒大小一致之白金奈米顆粒，作為質子交換膜燃料電池之觸媒，為國內首創；並以創新的奈米結構材取代傳統碳載體，俾降低觸媒使用量，並提高燃料電池的效率。本論文分為兩部分，分別對製備不同的奈米結構載體和燃料電池耐久度測試進行研究。
第一部分中，奈米碳管(carbon nanotube, CNT)因具有高導電性、高比表面積與高化學穩定性等特點而作為觸媒載體，但由於表面反應性不佳，須經過前處理才得以在碳管表面提供反應缺陷或官能基以供沉積觸媒。本論文分別利用氧氣電漿以及酸洗處理在CNT表面形成反應缺陷或官能基，並以ALD於其上成長鉑奈米觸媒，在分散性及均勻性上皆表現優異。在電池性能方面，酸洗處理後製備而成的膜電極組(membrane electrode assembly, MEA)結果顯示較氧氣電漿處理的為佳，接近使用E-Tek商用電極的對照組，但ALD製備鉑觸媒擔載量(0.019 mg/cm2)僅為商用電極使用量(0.5 mg/cm2)的二十五分之一，其比功率密度高於E-Tek商用電極十倍之多，在降低觸媒使用量的目標上實為一大進展。
除了CNT之外，本論文中亦製備奈米蜂巢狀(nanohoneycomb)及反蛋白石光子晶體(inverse opal)結構作為觸媒載體。奈米蜂巢狀結構具有三維的多孔性結構，在本文中此種結構是由金屬鎳所構成，除了高比表面積、高導電性的特點之外，亦有研究指出，利用鎳作為基材成長白金觸媒，在電化學的表現上即有合金的效果產生；反蛋白石光子晶體亦為三維的多孔性結構，由於此種結構的多層規則性排列，作為燃料電池的觸媒載體的比表面積得以提升，而為了應用在燃料電池上，材料選定為氮化鈦(TiN)，除了符合燃料電池載體的材料特性外，更具有優於碳黑的導電性，使得在提升比表面積的同時又能夠在導電性上得到兼顧。
論文的第二部分為燃料電池耐久度測試，係採用CNT作為觸媒載體製備而成的膜電極組進行實驗。耐久度測試動輒上千小時，為節省實驗時間，使用動態負載的方式加速膜電極組老化機制(accelerated degradation test, ADT)，以在短時間達到老化目標，在100小時的老化測試中，可以達到60000次循環電流週期，並利用電化學及表面分析儀器分析觸媒及加速老化後的燃料電池性能。

關鍵字

原子層沉積 ； 質子交換膜燃料電池 ； 白金

並列摘要

Since fuel cells have the high energy density and low pollution in the reaction process, they are recognized as one of the most promising green energy devices. Even with many excellent properties, commercialization of fuel cells still faces many challenges. The two most significant issues are how to reduce the cost and increase the durability of the fuel cell. The cost of the catalyst accounts for about fifty percent of the fuel cell, so how to reduce the amount of catalyst used and maintain the efficiency of fuel cell at the same time is a major issue that we are facing. It is also the main purpose of this research.
Atomic layer deposition (ALD) technique was adopted to prepare platinum catalyst nanoparticles with uniform particle size and well dispersion. Furthermore, innovative nanostructures were used to replace the traditional carbon support. It was attempted to reduce the amount of catalyst and to improve the efficiency of proton exchange membrane fuel cells (PEMFCs). The dissertation is divided into two parts, the first part focuses on the preparations of various innovative nanostructured catalyst and supports, and the second part is to study the fuel cell durability.
In the first section, carbon nanotube (CNT) was chosen to be as the support because of its high electrical conductivity, high specific surface area, and high chemical stability. Because the surface reactivity of CNT is poor, pre-treatment is needed to create defects and functional groups on the surface of CNT for depositing the catalyst. In this study, two different pre-treatment processes are chosen to modify the surface of CNT. The first one is oxygen plasma treatment and the other one is acid treatment. After the pre-treatment, Pt nanoparticles with good dispersion and uniformity are deposited by ALD. The membrane electrode assembly (MEA) performances of PEMFCs made with acid treated CNT are better than that made with oxygen plasma treated and close to that of commercial E-Tek electrodes. The most remarkable finding is that the ultra-low Pt loading of electrode, 0.019 mg/cm2. This is much lower than commercial one (0.5 mg/cm2), has the specific power density 11 times higher than that made with commercial E-Tek electrodes.
In addition to CNT, Ni nanohoneycomb and TiN inverse opal structures as the catalyst supports are also fabricated. The Ni nanohoneycomb structure is a three-dimensional porous structure. Apart from high surface area and high conductivity, the electrical property of Pt deposited on Ni substrate is similar to that of Pt-Ni alloys. Inverse opal structure is also a three-dimensional porous structure, and the multilayer structure with a regular arrangement would enhance the specific surface area of the support. In order to apply to fuel cells, TiN is chosen as the support material. In addition to the characteristics that are suitable for the fuel cell, the conductivity of TiN is better than that of carbon black. Therefore the TiN inverse opal structure could enhance the specific surface area and the support conductivity at the same time.
The second part of the dissertation is to study the fuel cell durability. MEA made by acid treated CNTs as the catalyst support is used in the experiment. In general, durability test often takes thousands of hours. In order to reduce the test time, a dynamic load method is used to accelerate the aging process (accelerated degradation test, ADT), which could achieve the degradation target in a shorter time. It could achieve 60,000 circulating current cycles in 100 hours of ADT test. The electrochemical and surface analysis methods are adopted to analyze the catalyst degradation after ADT of the fuel cells.

並列關鍵字

Atomic layer deposition ； Proton exchange membrane fuel cell ； Platinum

參考文獻

2 Boudghene, S. A. and Traversa, E., “Solid oxide fuel cells (SOFCs): A review of an environmentally clean and efficient source of energy.” Renew Sustain. Energ. Rev., 6, 433–455 (2002).

3 Behling, N. H., Fuel Cells: Current Technology Challenges and Future Research Needs, Elsevier, Amsterdam, (2013).

5 Damberger, T. A., “Fuel cells for hospitals.” J. Power Sources, 71, 45–50 (1998).

6 Fuel Cell Today. The fuel cell today industry review 2012. Corporate report

7 Kirubakaran, A., Jain, S., and Nema, R. K., “A review on fuel cell technologies and power electronic interface.” Renew. Sust. Energ. Rev., 13, 2430–2440 (2009).