奈米結構性材料之製作與合成及其於直接甲醇燃料電池之應用

Abstract

現今奈米尺度的科學與科技快速發展下，將化學能轉換成電能與低環境汙染優點的高效率直接甲醇燃料電池(DMFC) 提供了一個新的契機與方向。藉由奈米結構材料的大表面積，提供高分散性的鉑與鉑釕合金承載，因此提升直接甲醇燃料電池的功率密度。在這論文裡，我們研究奈米結構陽極觸媒的合成技術以及奈米結構觸媒的電催化特性。 製備觸媒載體採用兩個不同的方法，分別為奈米結構材料陽極氧化鋁(AAO)模板以及奈米孔洞的石墨化碳(g-C)。關於氧化鋁(AAO)模板方式，首先在矽晶片上成長矽奈米尖錐(SNCs)和非晶型碳(α-C)覆蓋的矽奈米尖錐陣列，用來做為觸媒的的載體，接著電鍍奈米鉑觸媒。有序排列的非晶型碳包覆矽奈米錐（ACNC）陣列產生，所造成的氧化鋁孔隙通道陣列安排模式，藉由電漿蝕刻在微波電漿化學氣相沉積(MPCVD)系統，轉移到有氧化鈦TiOx 光罩的矽載體。在MPCVD過程，一層約5奈米厚的α-C在矽奈米尖錐上沉澱。為了獲得高電流密度和重量活性，利用雙極性脈衝電鍍粒徑分佈均勻低於5奈米的奈米鉑沉積在有秩序的矽奈米尖錐和α-C包覆矽奈米尖錐上。藉著包覆一層奈米結晶石墨化碳，可以增加在矽奈米錐上的鉑奈米球分散度。根據電化學測量，奈米結構對甲醇氧化反應的催化活性優於全部覆蓋像平板狀的陽極。 此外，我們開發了一種新方法，形成奈米孔洞的石墨化碳，這是用來作為承載鉑和鉑釕合金催化劑，所使用的金剛烷火焰。電化學測試結果表明，鉑釕承載在奈米孔洞的石墨化碳對於甲醇電化學氧化有很好的催化活性和穩定性。良好的催化活性歸因於較高表面積的石墨化碳。 為了瞭解對於甲醇氧化反應在鉑觸媒的形態上的電催化活性的相關性，藉由在25○C下，直接雙極脈衝電化學沉積合成二維和三維鉑奈米結構材料。我們同時成功的在室溫下，合成了不同形狀的奈米鉑，如四面體和立方體在矽基板上。電化學研究中，二維和三維和形狀控制鉑的奈米結構顯示出對於直接甲醇燃料電池效率有潛在的應用。

Nanostructured materials can provide a large surface area for the loading of highly dispersed catalyst nanoparticles, such as Pt and Pt-Ru, thereby improving the power density of DMFCs. The thesis prepares various nanostructured anode electrocatalysts, and investigates electrocatalytic characteristics of the nanostructured catalysts. Two different approaches are adopted to prepare electrocatalyst supports, which are anodic aluminum oxide (AAO) templated nanostructured materials and nanoporous graphitic carbon (g-C). For the AAO templation approach, Si nanocones (SNCs) and amorphous carbon (α-C) coated Si nanocones arrays are first fabricated on the Si wafer for the use as the catalyst support, followed by electrodeposition of Pt nanoparticles. Well-ordered α-C coated Si nanocone (ACNC) arrays were produced as a result of the arrangement pattern transfer of AAO pore channel arrays to Si substrates with TiOx nanomasks by the plasma etch in the microwave plasma chemical vapor deposition (MPCVD) system. A layer of α-C about 5 nm thick was in-situ deposited on the SNCs during the MPCVD process. In order to obtain the high current density and mass activity, well dispersed Pt nanoparticles with a uniform size distribution below 5 nm were deposited on ordered SNC and ACNC by bipolar pulse electrodeposition. The dispersion of Pt nanoparticles on SNC was improved by coating a nanocrystalline g-C. The electrocatalytic activity of the nanostructured anodes toward methanol oxidation reaction (MOR) are superior to anodes with a blanket surface according to electrochemical measurements. In addition, we developed a new method to form nanoporous g-C, which was utilized as the support for Pt and Pt-Ru alloy catalysts, by the used of an adamantane flame. Electrochemical tests and results show the Pt-Ru supported on nanoporous g-C had excellent catalytic activity and stability toward methanol electrooxidation. The excellent catalytic activity may be attributed to the higher surface area of g-C. To investigate the dependence of electrocatalytic activity toward MOR on the morphology of Pt catalyst, the two dimensional (2D) and three dimensional (3D) Pt nanostrucutred materials were also synthesized by direct bipolar pulse electrochemical deposition at 25○C. We have also successfully synthesized Pt nanoparticles of different shapes such as tetrahedron and cube by fasten silicon at room temperature. An electrochemical study of 2D, 3D and the shape-controlled Pt nanostructures were showing its potential application for efficient direct methanol fuel cells.