利用電漿輔助原子層沉積系統成長白金奈米顆粒於二氧化鈦基材─直接甲醇燃料電池陽極觸媒之應用

Abstract

直接甲醇燃料電池最常使用的觸媒為白金，其原因為在甲醇氧化反應 (methanol oxidation reaction, MOR) 當中有優異的電催化特性。在直接甲醇燃料電池研究中，減少觸媒尺寸與優化觸媒分散性為廣泛研究的課題，目的為減少觸媒負載量並增加觸媒對甲醇氧化反應之電化學活性面積。除此之外，若使用金屬氧化物作為白金觸媒的基材，也可增加觸媒之電催化效率。在本研究中，我們使用電漿輔助原子層沉積系統 (plasma enhance atomic layer deposition, PEALD) 沉積白金奈米粒子於二氧化鈦基材上，並且研究隨著白金奈米粒子尺寸的改變對甲醇氧化反應之影響。 利用電漿輔助原子層沉積系統沉積白金奈米粒子於二氧化鈦薄膜上，沉積白金的前驅物為MeCpPtMe3與氧氣，沉積溫度為200oC。二氧化鈦基材的製備方式為使用電子束蒸鍍系統沉積於矽晶圓上。控制原子沉積系統的反應圈數可以沉積出不同大小的白金奈米粒子。由掃描式電子顯微鏡和穿透式電子顯微鏡可以觀察到隨著原子層沉積系統圈數的改變，白金奈米粒子的尺寸在3nm至13nm之間。 利用循環伏安法和一氧化碳氧化移除法來研究白金/二氧化鈦電極在酸性溶液中對甲醇氧化反應之電催化活性。電極的MOR電催化活性和一氧化碳容忍度會隨著原子層沉積系統之反應圈數而改變。我們發現在PEALD反應圈數為第35圈時，有最好之電化學表現。當PEALD反應圈數大於50圈時，白金奈米粒子連接成膜，並且對甲醇氧化反應之電化學表現類似於白金薄膜。白金奈米粒子有良好的電催化活性可歸功於白金奈米粒子之尺寸與和二氧化鈦基材之間電荷轉移作用、雙官能基機制發揮的協同效應。白金奈米粒子和二氧化鈦基材之間的電荷轉移作用改變白金表面一氧化碳吸附之性質，有助於一氧化碳被氫氧基氧化之反應，而增進對甲醇氧化之電催化活性。

Platinum has a superior electrocatalytic activity toward methanol oxidation reaction (MOR) and, therefore, Pt is the most preferred catalyst used in direct methanol fuel cells (DMFCs). In the present time, the most widely studied subject in DMFC research is to reduce the size and to optimize the distribution of Pt nanoparticles for minimizing the use of the precious Pt catalyst and concurrent increasing the electroactivity surface area (ESA) for methanol oxidation. In addition, metal oxides can be used as the Pt support to enhance the electrocatalytic efficiency of the Pt catalyst. In this works, we used plasma-enhanced atomic layer chemical vapor deposition (PEALD) to deposit Pt nanoparticles on the TiO2 substrate, and study the electrocatalytic activity of the Pt nanoparticles toward MOR as a function of the particle size. A TiO2 thin film was prepared on the Si substrate by electron beam evaporation, and Pt nanoparticles were deposited on the TiO2 thin film by PEALD at 200oC using MeCpPtMe3 as the Pt precursor and O2 as the oxidant of the precursor. The size of Pt nanoparticles can be well controlled by varying the number of the ALD reaction cycle. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that well-dispersed Pt nanoparticles were deposited on the TiO2 surface and the particle size of Pt nanoparticles was in the range between 3 nm and 13 nm depending on the ALD cycle number. Cyclic voltammetry (CV) and CO stripping analysis were performed to study the electrocatalytic activity of the Pt/TiO2 electrode toward MOR under acidic media. The electrocatalytic activity and the CO tolerance of the electrode are a function of the Pt ALD cycle number. We found that the electrode with a Pt ALD cycle number of 35 had the best electrochemical performance. When the ALD cycle number was larger than 50, Pt nanoparticles coalesced, and the electrode exhibited a electrochemical performance for MOR similar to a Pt thin film. The excellent electrocatalytic activity is ascribed to the synergistic effect of the nanometer sized of Pt nanoparticles and the electronic interaction between the TiO2 support and the Pt nanoparticles. Charge transfer between Pt nanoparticles and the TiO2 support may modify CO chemisorption properties on the Pt nanoparticles, thereby facilitating CO oxidation via the bi-functional mechanism and thus improving the electrocatalytic activity of the Pt catalyst toward methanol oxidation.