| 摘要: | 陽極觸媒的催化效能是影響直接乙醇燃料電池（Direct Ethanol Fuel cell, DEFC）輸出功率的最大因素。除了觸媒金屬外，支撐金屬的載體也影響觸媒的催化效能。目前常見之觸媒載體有碳黑與奈米碳管。然而奈米碳管表面石墨烯平滑的結構，不利於穩定奈米粒徑的金屬顆粒易於讓奈米粒子在反應中集結。許多研究已報導使用硝酸修飾奈米碳管表面，形成CNx（nitrogen-doped carbon (CNx) nanotubes）可以避免這些缺陷；另外也有許多研究開始使用導電高分子包覆奈米碳管表面來分散金屬顆粒，當鉑奈米粒子和其他過渡金屬固著在聚合物表面，觸媒可以展現出更優良的電子與質子的導電度、保持長時使用之熱穩定性、親水性和提高催化活性表面積。本研究顯示以導電高分子聚苯胺（Polyaniline）修飾之奈米碳管（PANICNT）作為載體，可以大幅提高乙醇燃料氧化活性並緩解Pt、Pt-Ru與Pt-Sn等奈米金屬粒子於長時間使用後的聚集和金屬流失的問題。承載於PANICNT表面之Pt-Sn奈米粒子可均勻分佈於載體上並保持顆粒尺寸範圍從2.0至4.0 nm之間；相較之下，PtSn顆粒於奈米碳管上則顯示嚴重聚集現象，在Vucan XC-72載體上之分散性也同樣較差。這是因為聚苯胺高分子上之氮官能基能與鉑金形成Pt-N鍵結，有助於均勻金屬顆粒大小展現出高分散性。從循環伏安法實驗中，PtSn/PANICNT觸媒氧化乙醇所產生的電流密度（748.7 A g-1 Pt），較其他未聚苯胺修飾之載體呈現更高的活性。其乙醇氧化之最高電流密度較PtSn/CNT高出458.9 A g-1 Pt。在經過耐受性測試（ADTs）後依然保持了77.8 %的電流密度顯示此技術可提升觸媒的穩定度。這項研究證實了的Pt-Sn二元金屬承載於PANICNT觸媒對於乙醇的氧化可以使鉑金的活性面積提高和催化活性上升並增加其長時使用之穩定性。進一步，本研究也探討在不同的合成條件下所製備PtSn/PANICNT觸媒的差異。這四種方法為EG-PtSn/PANICNT（以乙二醇為溶劑）、FA-PtSn/PANICNT（以甲酸為溶劑）、pH12-PtSn/PANICNT（以乙二醇為溶劑, pH=12）和R-PtSn/PANICNT（使用硼氫化鈉還原）。實驗結果發現，反應溶液之酸鹼度會影響觸媒金屬的承載以致於電化學測試結果較不佳，另外使用硼氫化鈉還原金屬會使觸媒金屬顆粒聚集嚴重，而單純使用乙二醇還原金屬可以使金屬粒子Pt-Sn分散最均勻。且利用循環伏安法實驗得到EG-PtSn/PANICNT顯現最好的電化學活性表面積以及乙醇氧化的催化活性。Catalytic activity of ethanol oxidation reaction is the most critical property dictating ethanol fuel cell performance. In addition to the metal nano-particle, the support material which fixes the nano-catalysts also contributes to the catalytic activity. The most common and widely used catalyst support is carbon black and carbon nanotube. However, the nano-catalysts tend to aggregate during reaction on the smooth graphene-like surface. Recently, there has been numerous report of heterogeneous catalysis that uses nitric acid to functionalize carbon nano-tubes surface which circumvent these deficiencies by forming CNx（nitrogen-doped carbon (CNx) nanotubes. Wrapping carbon nanotube with conducting polymers was recently explored to disperse metallic particles. When metallic platinum and compounds of transition metals are immobilized in the conducting polymer layer, the catalysts system delivered high electronic and protonic conductance, durable thermal stability, higher hydrophilicity, larger specific surface area, and considerable increase in active surface area. Current study demonstrated a novel support based on polyaniline-coated carbon nanotubes can substantially enhance ethanol oxidation activity and mitigate the problems of aggregation and leaching out related to Pt, Pt-Ru or Pt-Sn nano-catalysts. The Pt-Sn nanoparticle supported on PANICNT is sharply distributed with particle sizes ranging from 2.0 to 4.0 nm. For comparison, Pt-Sn particles loaded on bare CNT and XC-72 shows worse dispersion with larger particle size and lower surface area. This is attributed to the presence of strong Pt-N chelating bond between the nano-paticle with the nitrogen on polyaniline. The current densities derived from cyclovoltametry indicated PtSn/PANICNT yielded distinctively higher value （748.7 A g-1 Pt），which is 458.9 A g-1 Pt higher compared to PtSn/CNT without PANI functionalization. Through accelerated degradataion test（ADTs）, the novel catalysts system maintains 77.8 % or the current output after 5000 cycles, thus demonstrated its superior electrochemical stability compared to other supports. This study confirms Pt-Sn binary catalysts support on PANICNT yields superior catalytic activity for ethanol oxidation, higher Pt utilization efficiency, and displayed much better life-time durability when compared to that of PtSn/CNT or PtSn/XC-72. In second part of the work, we compared PtSn/PANICNT catalysts prepared by four different methods：EG- PtSn/PANICNT (ethylene glycol as solvent)、FA- PtSn/PANICNT(formic acid as solvent)、pH12- PtSn/PANICNT (ethylene glycol solvent at PH=12) and R- PtSn/PANICNT (NaBH4 as reducing agent). The result shows higher pH deteriorates the particle quality, while NaBH4 is too strong a reducing agent leading to particle aggregation. Ethylene glycol as a mild reducing agent, provided the best nanao-catalysts growth condition which lead to best Pt-Sn particle dispersion and most homogeneous particle size distribution. Cyclic voltammetry measurement shows EG- PtSn/PANICNT displayed the best electrochemical active surface area （ECSA）and highest catalytic activity for ethanol oxidation. |
