鉑錫合金奈米棒觸媒之氧化程度對直接甲醇燃料電池的電化學催化效果研究

Abstract

直接甲醇燃料電池(DMFCs)是透過將甲醇燃料以化學能形式直接轉換成電能的一種電池，其可攜帶性使之成為極具發展潛力的供電裝置。在此研究中分別藉由實驗與理論計算兩個面向來檢定DMFCs中的甲醇氧化反應(MOR)，並通過此研究揭示將Pt與具高度親氧性Sn進行氧化後，其對陽極觸媒PtSn所造成的重要影響。關於實驗部分，原先的Pt3Sn nanorods(NRs)是透過甲酸還原法所合成，隨後透過改變不同溫度(150, 200, 250與300oC)與加溫時間(1, 1.5, 3與5 hr)的氧化後處理過程進行各式樣品的製備。其中經由不同的氧化條件所得到的PtSn NRs氧化程度皆不盡相同，所以藉HRTEM, XRD, EDX, XPS對觸媒的表徵進行鑑定，並由電化學測試瞭解其MOR的催化能力。透過實驗的結果可以發現，當Pt3Sn NRs在經過200oC加熱氧化三小時的條件下擁有約54 %的表面氧化度，也具備最為優異的MOR活性與觸媒穩定性。計算的部分則分別探討甲醇在乾淨與經過氧化(表面具有氧原子吸附)的Pt表面、NR模型的脫氫反應及氧化反應。由結果顯示出，無論是乾淨的Pt表面亦或是NR，(100)面皆擁有較低的脫氫反應能與反應能障。之於經過氧化的表面，(100)面的氧化反應可以得到更進一步的提升。NR則因為同時具備(100)與(111)表面，且在side位點擁有最穩定的氧吸附。因此亦如實驗的結果，其將展現最優異的MOR催化活性與觸媒穩定性。Direct methanol fuel cells (DMFCs), which directly convert the chemical energy of methanol fuel into electricity, are a promising candidate for portable electronic devices. In this study, we experimentally and computationally examined methanol oxidation reaction (MOR), the key anodic reaction in DMFCs, on PtSn materials to mechanistically reveal the important oxide effect from Pt and the high-oxophilic Sn.Experimentally, Pt3Sn nanorods (NRs) was initially prepared by formic acid reduction method; post-oxidation processes with varied oxidation temperatures (150, 200, 250, and 300oC) and time (1, 1.5, 3 and 5 hours) have been further employed. Those PtSn NRs with different degrees of oxidations have been characterized by HRTEM, XRD, EDX and XPS and their MOR performance have been examined by electrochemical tests. The experimental results showed that Pt3Sn NRs oxidized at 200oC for 3 hours have a surface oxidation degree of 54 % and showed the best MOR activity and stability.Computationally, we examined methanol decomposition and oxidation, key steps in MOR, on clean and oxidized (with surface oxygen) Pt surfaces and NR. Our results found that (100) facet on clean Pt surface and NR have lower reaction energies and activation barriers for decomposition reactions. On oxide surface, (100) facets can further assisted for the oxidation reactions. NR, which contains both (100) and (111) facets and has the most stable oxide on side sites, is expected to show the best MOR activity and stability, as observed from the experiment.