功能性奈米碳管於燃料電池之研究

Functional Carbon Nanotubes for Fuel Cell Applications

杜鶴芸(He-Yun Du)

指導教授 ： 顏溪成

共同指導教授 ： 陳貴賢 ； 林麗瓊

國立臺灣大學/工學院/化學工程學研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.10254

摘要

質子交換膜燃料電池（PEMFC）和直接甲醇燃料電池（DMFC）使用固體高分子電解質，即 Nafion膜，夾在兩個電極中間作為質子通道。 Nafion膜可迅速傳導質子並維持高電流密度，而且提供的電子絕緣體和氣體阻隔的功能。通過加濕燃料和氧氣可保持兩個電極高濃度的質子傳導。燃料電池目前研究都將貴金屬觸媒分散在碳載體上，由於催化劑的活性取決於鉑粒子尺寸與分散程度，研究發現理想的碳載體應當具有以下的結構和性質：（一）高比表面積和良好的電子傳導能力（二）順暢的反應氣體途徑（三）高穩定性。因此，許多研究努力尋找新的觸媒載體，其中，奈米碳管（CNTs）就其具有獨特的一維奈米結構和性能就成為最佳選擇。
本文第一部分介紹使用乙二醇法還原奈米鉑觸媒顆粒(Pt NPs)在直接成長的摻氮奈米碳管上(CNT-CC)。由於奈米碳管直接成長於碳布上，故提供了一個快速的電子轉移路徑，導致減少能量損失並提高了奈米鉑觸媒的活性。藉由摻氮碳管表面的氮原子製造出類似缺陷的表面以利奈米鉑觸媒均勻成核分散。乙二醇法藉由調整反應溶液之pH值可得到不同尺寸之鉑觸媒顆粒。由電子顯微鏡影像可知在15 mM 氫氧化鈉溶液下乙二醇法可製備出2.8 nm 鉑觸媒顆粒均勻分散於直接成長之碳管上(Pt/CNT-CC)。由電化學測量Pt/CNT-CC擁有極佳之甲醇氧化能力，故是應用在直接甲醇燃料電池的理想觸媒。
另一方面我們成功的製備出低鉑觸媒顆粒(0.05毫克每平方公分)負載於不含鐵氟龍(PTFE)之直接成長碳管電極(Pt/MPL-CNT)。碳管能降低電極阻值並本身疏水，所以不須另外加入疏水劑。在70度氫氣與氧氣條件下，鉑奈米觸媒為0.05毫克每平方公分之Pt/MPL-CNT之單電池效能為902毫瓦每平方公分的功率輸出，而商用碳粉系統需要0.25毫克每平方公分的觸媒量才能達到824毫瓦每平方公分。Pt/MPL-CNT電極較商用電極有五倍以上的觸媒重量效率提升。這裡顯示出直接生長於碳布的碳管具有優良之的電子傳導能力與其內在的疏水性，故有助於得到較佳效能單電池性能。
總而言之，將奈米碳管直接成長在碳布上作為觸媒的載體，可以提升碳材表面積以及降低介面電阻，以致有效的提升甲醇電池之甲醇氧化能力及氫氧燃料電池之輸出功率，並且降低鉑觸媒使用量至五分之一以下，對將來燃料電池的價格及推廣都有極大幫助。

關鍵字

燃料電池

並列摘要

Proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC) use solid polymer electrolyte, namely Nafion sandwiched between the two electrodes. Nafion not only plays the role as an electronic insulator and gas barrier but also allows rapid proton transport and supports high current densities. In order to maintain the high proton conductivity of Nafion, humidified fuel and O2 are passed through the two electrodes. The fuel cell utilizes precious metal catalysts that dispersed on carbon substrate. It is well known that the activity of a catalyst depends significantly on the size of the Pt particles and their dispersion pattern over the support structures. The ideal support should have the following structure and properties: (i) high surface area and good electrical properties, (ii) reactant gas access to the electrocatalysts, and (iii) high electrochemical stability under fuel cell operating conditions. Among them, carbon nanotubes (CNTs) become the top choice due to their unique one-dimensional structures and properties.
The first part of this work presents the synthesis of platinum nanoparticles (Pt NPs) and their subsequent deposition on the nitrogen-doped carbon nanotubes by ethylene (EG) method, which have been directly grown on a carbon cloth (CNT-CC electrode). The CNT-CC electrode provides a fast electron-transfer path to the carbon cloth, resulting in energy-loss reduction and enhancing catalytic activity of Pt NPs. The N-dopant in CNT serves as the defect sites to enhance nucleation of Pt particles. The EG method enable to control the Pt NP size by adjusting the pH of the reaction solution. The Pt NPs dispersed on the CNT-CC have an average size of 2.8 nm (Pt/CNT-CC) prepared using 15mM NaOH, with high uniformity, as confirmed by electron microscopy. Cyclic voltammetry measurements of the electrocatalytic activity of the Pt/CNT-CC for methanol oxidation indicate excellent electrocatalytic activity and are ideal for direct methanol fuel cell applications of the Pt/CNT-CC electrode.
In the second part of this study, we successfully demonstrate a PEMFC with catalyst layer comprising of low loading of platinum nanoparticles (0.05 mg cm-2) supported by a directly grown micro-porous carbon nanotube (CNT) layer without incorporation of PTFE, (Pt/MPL-CNT). The directly grown micro-porous CNT layer has low electronic resistance and is intrinsically hydrophobic, which are the properties replaced the PTFE incorporation. In the single cell tests, PEMFCs with 0.05 mg cm-2 Pt/MPL-CNT and 0.25 mg cm-2 Pt/ PTFE-MPL-CP were used at the cathodes. These cells yielded maximum power densities of 902 mW cm-2 and 824 mW cm-2, respectively, at 70 C when operated with H2/O2. Notably, the cell performance of Pt/MPL-CNT is four times larger than commercial electrode under the same Pt NPs amount. It is shown that the directly grown micro-porous CNT layer has low electronic resistance and is intrinsically hydrophobic, which account for the high performance obtained here.
To summarize, the CNTs directly grows on CC for catalyst support in FCs effectively enhances the performance of PEMFC and DMFC by increasing the surface area and minimizing the internal resistance of carbon support. Moreover, substantial reduction in the catalyst loading (one-fifth) is also demonstrated. The finding can be important for future applications of FCs.

並列關鍵字

fuel cell

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