合成不同形貌之銀奈米催化劑於氧氣還原反應之應用

Abstract

發展非鉑系觸媒催化氧氣還原反應之研究對於燃料電池普 及化與便利化具有極重大的影響，其中銀系催化劑在成本上可以 有效地降低，且在催化活性上也有優異的表現，另外，銀系催化 劑的活性與其形貌有明顯的關係，因此，有效控制不同形貌的奈 米銀可以有效提升其氧氣還原的反應。本研究分為三個主軸進行， 首先，接續實驗室之前的研究，在立方體結構上進行形貌的改變， 合成出具有較優異的氧氣還原反應的反應面；另一方面，合成不 同深寬比的棒狀奈米銀結構探討形狀上對於催化氧氣還原反應 之影響；最後一部分合成不同碳厚度的截角立方體奈米銀(Ag@C) 結構，分別探討碳包覆銀對於催化氧氣還原反應之影響。 首先截角立方體奈米銀觸媒部分，利用水熱法合成。由TEM 結果確認藉由控制界面活性劑與還原劑的濃度可以有效改變不 同成長面的成長速度。由XRD 分析探討截角立方體奈米銀的繞 射峰值有明顯的改變。經由電化學分析，截角立方體之奈米銀結 構對於催化氧氣還原反應具有優於立方體之效果，且進一步進行 截角立方銀奈米顆粒尺寸對於氧氣還原反應的探討，。 在不同深寬比的奈米銀觸媒部分則利用添加鐵離子來控制 晶種，藉由改變鐵離子濃度合成不同深寬比的奈米銀結構。經由 TEM 圖譜確認合成不同深寬比的奈米銀。由XRD 分析探討不同 深寬比的奈米銀皆為fcc 結構。最後藉由電化學分析得知，不同 深寬比的奈米銀有不同的電化學活性，以最大的深寬比其電化學 活性最佳。 最後，Ag@C 的部分，利用第二階段的升溫的方法合成。由 TEM 結果確認不同碳化時間可以控制碳膜的厚度，且在碳化時間 過長時會導致結構的團聚現象。由XRD 分析證實，在高溫處理 過後所合成的銀依舊為fcc 結構。最後進行電化學的分析，可以 得知在不同碳膜下電化學有不同的活性，當碳膜太厚時，導致電 子傳導上不易降低了電化學活性，找到最佳的碳膜厚度可以有效 穩定進行電化學反應下的銀奈米結構。

Non-platinum based electrochemical active materials were developed to catalyze the oxygen reduction reaction (ORR) in an alkaline electrolyte. Many research indicated that the silver has demonstrate excellent ability for ORR. In addition, the morphology of silver affected its ability greatly. For this reason, it is our objective to control the morphology of silver particles to promote ORR ability. First, we followed the self-established synthesis condition to fabricate silver cubes and truncated cubes to increase the specific facets with better catalysis for ORR. Second, silver nanorods in various aspect ratios were synthesized and the ORR analysis was carried out. Finally, to increase the stability of silver for ORR, Ag@C was synthesized to stabilize the structure of silver. In the synthesis for truncated cubes, we employed a hydrothermal method to form the desirable morphology. From the IV TEM images, we confirmed that an effective control for the concentration of surfactant and reducing agent could change the growth rate of at different faces. From the XRD, it revealed that the ratio of (111) to (200) has changed notably. Finally, we determine that the morphology with better performance is truncated cube instead of regular cube for ORR. As expected, the silver nanoparticles with different sizes have different electro-catalytic activities for ORR. In the second part, silver nanorods were synthesized by adding different concentrations of iron ions to control silver seed. From TEM images, they confirmed that different aspect ratios of silver nanorods were synthesized successfully. We determined that the structure of silver nanorod was fcc from XRD. The potentiodynamic test demonstrated that the ORR was improved by increasing the aspect ratio of silver nanorods. Carbon-decorated silver truncated cubes with a core-shell structure were formed by a two-stage hydrothermal synthesis. From TEM images, they confirmed that the thickness of carbon shell was effectively controlled depending on processing time. From XRD, the structure was still fcc. Finally, the Ag@C structure was prepared in optimized parameter, revealing outstanding performances for ORR.