| 摘要: | 直接甲醇燃料電池(DMFC)具有較高的能量轉換率和低污染的優點，但其高成本為實現大規模商業化應用的重大挑戰。在直接甲醇燃料電池中所需的陽極觸媒多以鉑金屬為基礎。因此開發有效的觸媒載體降低鉑金屬的使用以降低成本時，也必須兼顧增加觸媒的活性；此目標為發展燃料電池技術的首要課題。金屬氧化物材料作為觸媒載體近年來受到越來越受關注，主要是因其固有奈米尺寸的顆粒和電化學穩定性極有利於觸媒金屬和載體間的相互作用，可以穩定金屬顆粒協助燃料反應並提昇質子傳導性。然而金屬氧化物受限於低表面積和低電子導電性，抑制了觸媒活性。本論文研究方向是使用導電的碳材料修飾金屬氧化物材料改良以上缺點作為燃料電池奈米觸媒之載體。
研究第一部分以不同的金屬氧化物（二氧化鈦，二氧化矽和二氧化鋯）包覆一薄層的導電性聚合物（聚苯胺），接著在900 0C氮氣氣氛下石墨化形成含氮石墨化金屬氧化物（Nitrogen Graphitized Metal Oxide, NG-MO）。含氮的石墨層可以增加電子傳遞，提供穩定金屬顆粒。穿透式電子顯微鏡影像顯示鉑奈米粒子在NG-MO的分散比一般碳材(XC-72)或其他石墨碳材好，鉑金屬穩定顆粒大小大約3?4nm。電流密度循環伏安法顯示Pt/ NG-MO具有比其他載體像是XC-72或商用觸媒(E-TEK)更高的甲醇氧化活性。甲醇氧化活性會隨著金屬氧化物不同而有所改變，MOR最好的金屬氧化物載體為SiO2。但將Pt /NG-MO和商用觸媒(E-TEK)比較，穩定性都沒有一般商用觸媒(E-TEK)好。
第二部分研究中探討聚苯胺含量對催化行為的影響。在三種不同金屬氧化物系統中分別製作聚苯胺和金屬氧化物的比例為1:1、1:5、1:10三種載體並研究觸媒在甲醇氧化的活性。在TiO2系統中金屬氧化物含量多有比較好的甲醇氧化活性；然而在SiO2和ZrO2系統中金屬氧化物含量少反而有比較好的甲醇氧化活性。由此一結果顯示金屬氧化物有可能參與甲醇氧化反應，TiO2在甲醇中會有OH基吸附可以增加甲醇氧化的反應，TiO2較多可以幫助整個氧化反應的進行，而SiO2和ZrO2協助反應的趨勢較少，進行此反應的能力較低。
最後，第三部分的研究也探討雙合金系統在NG-MO載體的觸媒活性和穩定性。在文獻中MOR最好的合金系統是鉑銠合金，所以本研究使用鉑銠合金乘載在不同比例1:1含氮石墨化的金屬氧化物。結果顯示鉑銠合金的觸媒無論是觸媒活性還是穩定性都超越單一鉑金屬,但跟商用觸媒(E-TEK)相比較下穩定性在二氧化矽和二氧化鈦系統中比較好，而在二氧化鋯穩定性比商用觸媒差，這部分還可以做進一步的研究和改進。
Direct methanol fuel cell (DMFC) has the advantages of higher energy conversion rate less air pollution. However, its high cost becomes a major challenge for large-scale commercialization. The development of highly effective catalyst support to reduce the amount of platinum while increasing the activity of the catalyst has become the major goal in fuel cell technology. In recent years, the attempt of treating metal oxide material as catalyst support has drawn increasing attentions. This is primary due to high inherent dimensional and electrochemical stability enhance the interaction between the metal and the support, which help stabilize metal particles with improved fuel reaction and raised proton conductivity. However, limited by the low surface area and low electronic conductivity, metal oxide material inhibits catalytic activity. Purpose of this study is to modify the metal oxide with thin layer of conducting carbon in order to circumvent this deficiency.
The first part of this research examines the activity of such surface functionalized metal oxides (titanium dioxide, silicon dioxide and zirconium dioxide). The nitrogen-graphitized metal oxide (NG-MO) were prepared by first coating a thin layer of conducting polymer (e.g. polyaniline) on ceramic metal oxides (TiO2, SiO2, and ZrO2) followed by graphitization at 900℃under N2 atmosphere. The thin layer of N-containing graphite coated on the ceramics served as electron conductor. Furthermore, it served as stable anchorage for metal nano-catalysts. Surface topography mapping showed that Pt nanoparticle with stable size of 3~4 nm was homogeneously dispersed on NG-MO compared to that on XC-72 or on the other graphite-based carbons. The current density derived from cyclo voltametry suggested that Pt/NG-MO exhibits distinctively higher methanol catalytic performance compared to those at XC-72 supports. The topology of the Pt nanoparticle on NG-MO and its methanol oxidation activity depends heavily on the type of the ceramic metal oxide with SiO2 appeared to give the best results. However, all the Pt/NG-MO system displayed lower life-time durability compared to that of commercial catalyst (E-TEK).
The second part of the research studied the polyaniline content on catalytic behavior. Three supports with polyaniline to metal oxide ratio of 1:1、1:5、1:10 are prepared. In TiO2 system, higher content of metal oxide lead to higher methanol oxidation activity
however, in SiO2 and ZrO2 system, lower content of metal oxide lead to higher methanol oxidation activity .The results showed that metal oxide may be involved in the oxidation reaction of methanol. TiO2 performed better adsorption of OH group, enhancing the methanol oxidation reaction. Better methanol oxidation reaction activity is observed with higher TiO2 . In contrast, SiO2 and ZrO2 served as dormant substrate that the MOR only increases with increasing polyaniline coating.
Finally, the third part of the study explored the effect of NG-MO support on alloy system. We compared the activity and stability of alloy metal catalyst with that on carbon support. Since the platinum-rhodium alloy system displayed the best MOR in the research, we supported the platinum-rhodium alloy on graphite metal oxides containing nitrogen in the proportions of 1:1. The results showed that platinum-rhodium alloy catalyst displayed higher catalytic activity and stability than single-platinum metal. SiO2 and TiO2 system performed better stability than the commercial catalyst (E-TEK), but ZrO2 is much worse. Further work is required to verify such difference. |
