高效能直接甲醇燃料電池

High Performance of Direct Methanol Fuel Cell

王丞浩(Chen-Hao Wang)

指導教授 ： 施漢章 ； 陳貴賢 ； 林麗瓊

國立清華大學/工學院/材料科學工程學系/博士(2007年)

https://doi.org/10.6843/NTHU.2007.00157

摘要

直接甲醇燃料電池（Direct methanol fuel cell, DMFC）是一種將甲醇燃料與氧氣利用電化學方式產生電能的能源產生裝置，被認為能應用在個人式行動電子產品、各種的感測器甚至是交通工具上。然而，由於甲醇的催化效率緩慢，以及甲醇會由陽極滲透過質子交換膜至陰極造成陰極毒化，都將導致直接甲醇燃料電池效率下降。此外，高含量的貴金屬觸媒成本亦不利直接甲醇燃料電池的商品化。本論文提出兩個方向以解決這些問題：一、製作高效率陽極以提升甲醇催化效率並且降低貴金屬觸媒使用量；二、在質子交換膜覆蓋一層甲醇阻擋層以降低甲醇滲透率。
為了製作一個高效率並且使用低含量貴金屬觸媒的直接甲醇燃料電池，吾人將奈米碳管直接成長在碳布上，並將白金和釕觸媒濺鍍在直接成長的奈米碳管上作為陽極。奈米碳管的合成是使用微波輔助電漿化學沈積法以CH4/H2/N2的混合氣體做為合成原料。利用電化學的循環伏安法與阻抗分析在1 mM Fe(CN)63-/4-的環境下測量，吾人發現直接成長奈米碳管電極擁有較快的電子轉移與較低的阻抗。由於在製備奈米碳管時有摻入氮，吾人使用掃瞄式電子顯微鏡和穿透式電子顯微鏡觀察到這些摻入氮的地方可以形成活性點使得白金和釕等觸媒可以均勻地被分佈在奈米碳管上並可進一步製作膜電極組。膜電極組是一種陽極/膜/陰極的三明治結構。使用低含量白金和釕觸媒（0.4 mg cm-2）的奈米碳管電極作為陽極，和白金觸媒（3.0 mg cm-2）作為陰極，並且將Nafion 117夾在中間的膜電極組，其表現優於一般傳統使用高含量貴金屬觸媒陽極的膜電極組。從微結構分析，使用奈米碳管電極的膜電極組擁有薄和均勻的觸媒層，並且觸媒層與質子交換膜和氣體擴散層擁有良好的界面連續性。
為了減緩甲醇由陽極滲透至陰極速率，吾人選用化學穩定且電傳導良好的質子化polyaniline（PANI）覆蓋在Nafion 117（N117）形成一個擁有甲醇阻擋層的複合膜（PANI/N117）。覆蓋在N117的PANI厚度約為100 nm且其電導約為13.24 S cm-1。與N117作比較，在室溫下PANI/N117減少了41%的甲醇滲透率，證明後者具有降低甲醇滲透的能力。使用N117和PANI/N117作為的膜電極組操作在1，2，4，6和8 M的甲醇環境與60 oC的操作溫度，吾人發現兩者有截然不同的現象。使用N117的膜電極組，其輸出功率隨著甲醇濃度增加而降低，這是由於甲醇滲透至陰極導致陰極觸媒毒化所致。使用PANI/N117的膜電極組，其輸出功率隨著甲醇濃度增加而增加；最大的輸出功率在6 M可以達到70 mW cm-2。此外，吾人亦發現當電池操作在1 M到 6 M的甲醇濃度時，使用PANI/N117的膜電極組可以比N117的膜電極組有效地降低甲醇滲透率，使得前者在甲醇濃度增加時表現良好。由於PANI/N117在高濃度的甲醇操作中表現良好，吾人認為其將適合在直接甲醇燃料電池在長時間的操作。

關鍵字

燃料電池 ； 奈米探管 ； 甲醇 ； 質子交換膜

並列摘要

The direct methanol fuel cell (DMFC) is an attractive and promising power generator, which generates electricity by an electrochemical redox reaction of methanol and oxygen, enabling a wide range of applications from small sensors, portable electronic devices, to automobiles. However, the slow methanol electro-oxidation and severe methanol crossover undermine the DMFC performance. On the other hand, a high loading of noble metal electrocatalysts make it too expensive for commercialization. In this thesis, two solutions are proposed to tackle these issues: an efficient anode with a low loading of noble metal electrocatalysts to enhance the methanol electro-oxidation; a proton exchange membrane coated with a methanol blocking layer to reduce the methanol crossover.
Firstly, the study demonstrated the feasibility of a high-performance membrane-electrode-assembly (MEA), with low electrocatalyst loading on carbon nanotubes (CNTs), which were grown directly on carbon cloth as an anode. The direct growth of CNTs was realized by microwave plasma-enhanced chemical vapor deposition using CH4/H2/N2 as precursors. The cyclic voltammetry and electrochemical impedance measurements with 1 mM Fe(CN)63-/4- redox reaction reveal a fast electron transport and a low resistance on the direct grown CNT. The electrocatalysts, platinum and ruthenium, were coated on CNTs by sputtering technique to form the Pt-Ru/CNTs-CC anode (Pt-Ru/CNTs-CC). The MEA, the sandwiched structure which comprises 0.4 mg cm-2 Pt-Ru/CNTs-CC as the anode, 3.0 mg cm-2 Pt black as the cathode and Nafion 117 membrane at the center, performs very well in a direct methanol fuel cell (DMFC) test. The micro-structural MEA analysis shows that the thin electrocatalyst layer is uniform, with good interfacial continuity between membrane and the gas diffusion layer.
Secondly, protonated polyaniline (PANI), a stable and electrically conducting polymer, was directly polymerized on a Nafion 117 membrane (N117), forming a composite membrane, to act as a methanol blocking layer (PANI/N117), whose was evaluated to reduce the methanol crossover in the DMFC. A PANI layer coated on the N117 has a thickness of 100 nm, with an electrical conductivity of about 13.24 S cm-1. The methanol permeability of the PANI/N117 is 41% less than that of the N117 at room temperature, suggesting that the PANI/N117 can effectively reduce the methanol crossover in the DMFC. The MEAs using the conventional N117 (N117-based MEA) and the new developed PANI/N117 (PANI/N117-based MEA) were compared to the feeding of 1, 2, 4, 6 and 8 M methanol at 60 oC. The output power of the N117-based MEA is reduced at higher methanol concentration, which is due to the methanol crossover of the N117. However, the PANI/N117-based MEA exhibits higher output power at higher methanol concentration. The maximum power density of the PANI/N117-based MEA is 70 mW cm-2 at 6 M methanol solution. This value is double that of the N117-based MEA under identical conditions. This work also suggests that the methanol-crossover rate of the PANI/N117-based MEA is about 60% lower than that of the N117-based MEA from 1 M to 6 M methanol solutions. The PANI/N117-based MEA performs well at elevated methanol concentration, suggesting the potential for long-term operation of small-scale DMFCs.

並列關鍵字

Fuel cell ； Carbon nanotube ； methanol ； proton exchange membrane