| 摘要: | 直接甲醇燃料電池具有高能量密度、進料容易、工作溫度低且是乾淨的能源，目前常應用於燃料電池觸媒，不論陽極的甲醇氧化或是陰極的氧還原，還是以白金及其合金觸媒催化效果較好，不過白金是價格昂貴的貴金屬，使得燃料電池成本居高不下，難以商業化，因此目前以減低白金用量(提升單位白金催化活性)的研究方向為主。所以有將白金奈米化的趨勢，而結合能固定奈米粒子的載體材料以利穩定奈米化的催化金屬是常見的策略。
目前常應用於觸媒載體的碳材雖然有高導電度和高表面積的優勢，然而在正電位下，碳容易氧化成二氧化碳使耐久度受到了限制，因此選用具有電化學、熱穩定性和抗腐蝕性的金屬陶瓷材料為一趨勢。根據文獻記載，鈣鈦礦結構LaTiO2N(LTON)的能隙是2.1eV比常應用於燃料電池觸媒載體的TiO2 3.2eV還要低而預測可得到更好的導電性，且此材料有鑭系金屬的參與，這種二元金屬陶瓷材料常有特殊的性質，因此本研究我們選用了鈣鈦礦結構LTON當作載體於應用在燃料電池觸媒的可能性。
實驗結果指出，Ce0.1La0.9TiO2N(Ce0.1-LTON)相較於其它微量Ce摻雜所承載白金具有較好催化甲醇氧化的活性，並由HR-TEM影像中得知Ce0.1-LTON可合成出較小的白金奈米粒徑(約3.7nm)，並比其它的Ce莫耳比，發現Ce含量不同可以有效控制合成時白金奈米金屬的粒徑大小。由於Ce-O-Pt鍵結產生改變還原奈米金屬時的成核環境，然而單純白金金屬易受甲醇氧化時產生的中間產物一氧化碳佔住活性位置降低催化效率，稱為CO毒化(CO-poisoning)，因此通常會添加第二金屬釕形成鉑釕合金當觸媒催化中心降低此效應。承載PtRu後發現需在Ce0.5-LTON才有較佳的催化活性(187A/g per Pt)，並於HR-TEM影像得到較小奈米粒徑(3.5nm)。Ru的加入使系統較複雜，因為還要考慮在還原的環境下，Ru和Ce會形成Ru-Ce complex，因此推測當Ce0.5-LTON承載PtRu時，Ce-O-Pt鍵結和Ru-Ce complex的競爭反應而得到最佳化結果。
不過導電度並沒有我們預期的要好，仍需要添加XC-72碳材或導電高分子PEDOT的修飾彌補這個缺陷，期待有更好非使用碳的改質方法。
;Direct methanol fuel cells (DMFCs) have high energy conversion efficiency, low operating temperature, easy fuel-feeding and it is environmentally friendly. Platinum-based catalyst has been commonly used in DMFCs, but it suffered from high cost. Thus, development of effective catalyst support which argues the catalyst activity and reduces amounts of Pt loading becomes the primary goal.
According to previous literature, traditional supports which composed of carbon material, such as XC-72, were oxidized easily to CO2 in positive potential and limited catalysts life time. In view of this, a recent research trend is to substituted carbon by ceramic materials which possess several advantages on electrochemical, thermal stability and corrosion-resistance. We examined perovskite structure, LaTiO2N(LTON), as the support. Earlier study shows the band gap of LTON is about 2.1 eV, lower than 3.2eV of TiO2 which usually applies in fuel cell catalyst support and promises better electron conductivity. In addition, utilization of lanthanum element is synthesized in LTON to make perovskite structure. The kind of binary metal ceramics has particular properties so we expect to know whether LTON applies in fuel cell catalyst support
In our experiment, we have discovered Ce doped LTON can reduce Pt particle size. HR-TEM shows that the size of Pt NPs prepared with the composition of Ce0.1La0.9TiO2N(Ce0.1-LTON) exhibited smaller particles
(3.7 nm) than other molar ratio of Ce, and exhibited more efficient methanol oxidation reaction (MOR) because production of Ce-O-Pt bond affects nucleation in reducing metal step. However, platinum NPs in MOR suffers from CO poisoning. To solve the problem, we have introduced PtRu alloy NPs to support on Cex-LTON and discovered changing composition of Ce0.5-LTON exhibited more efficient MOR with higher mass current (184A/g per Pt).
HR-TEM also shows that the size of PtRu NPs was smaller particles(3.4nm) than other ratio of Ce. In the situation, it is complicated to get explanation when Ru is mixed in the system due to considering production of Ru-Ce complex in reduction environment. We just guess optimization of two competitive factor which are production of Ce-O-Pt and Ce-Ru complex is at the ratio amount.
The electron conductivity issue is not entirely resolved from the lower band gap, and other carbon materials such as XC-72 and electron conductive polymer PEDOT are still required to amend the conductivity deficiency. Better improvement are being explored to amend this situation. |
