原子層化學氣相沉積法製備白金奈米粒以研究甲醇電催化氧化反應

Abstract

在直接甲醇燃料電池的應用上，好的陽極除了必須在最少量的白金觸媒負載下仍然對甲醇氧化反應具有好的電催化活性，還必須具備好的抗一氧化碳毒化能力。以二氧化鈦當作白金觸媒的載體能夠明顯地提升白金的電催化活性以及抗毒化的能力。在本研究裡，我們使用電漿輔助原子層沉積系統來製備負載在二氧化鈦基材上的白金奈米粒子，以便研究白金奈米粒子的尺寸對其電催化甲醇氧化反應之觸媒活性的影響。 沉積白金奈米顆粒於二氧化鈦基材上是利用電漿輔助原子層沉積系統於基板溫度2000C的條件下進行，並使用MeCpPtMe3為白金前驅物。沉積出的白金奈米粒子利用掃描式電子顯微鏡、穿透式電子顯微鏡及X光光電子能譜進行材料分析。隨著電漿輔助原子層沉積圈數的增加，白金奈米粒子的尺寸及密度也隨之增加，奈米粒子的尺寸介於2-5奈米之間。 電化學分析的結果顯示陽極在酸性電解液中對甲醇氧化反應的電催化活性會隨著電漿輔助原子層沉積系統的沉積圈數改變而改變。低原子層沉積圈數下的電極有較低電催化活性的表現，而這是因為分散的白金奈米粒子使電極的導電性不佳；但是，甲醇氧化反應中的If/Ib值較高，顯示低原子層沉積圈數下的電極具有較佳的抗毒化能力，其中If值表示正向掃描的甲醇氧化訊號，Ib值表示反向掃描的一氧化碳等中間產物氧化的訊號。當原子層沉積圈數增加到30圈以上，電極的抗毒化能力降低。利用原子層沉積系統的沉積圈數20-30圈之間所沉積出的白金奈米粒子具有最好的電催化活性及抗一氧化碳毒化能力。 白金沉積在二氧化鈦基材上電催化活性的提升歸功於兩者之間電荷轉移作用及雙官能基機制的協同效應。白金奈米粒子和二氧化鈦基材之間的電荷轉移能弱化一氧化碳在白金奈米粒子上的鍵結，使一氧化碳的氧化更容易進行。除此之外，二氧化鈦能夠在比白金更低的電位下產生吸附狀態的氫氧基，因此以二氧化鈦當作基材能夠藉著雙官能基機制來增加移除一氧化碳的能力，也因此提升白金奈米粒子對甲醇氧化反應的電催化活性。

For the application of direct methanol fuel cells (DMFC), it is desirable that the anode has a high electrocatalytic activity toward methanol oxidation reaction (MOR) with a minimized Pt catalyst loading and a high resistance against the CO poisoning effect. Titania supports for Pt catalyst loading are found to significantly enhance the electrocatalytic activity and the CO-tolerance of the anode. In the study, we used plasma enhanced atomic layer chemical vapor deposition (PEALD) to prepared Pt nanoparticles on TiO2 substrates and investigated the size effect of Pt nanoparticles on the improvement of the electrocatalytic activity toward MOR. The deposition of Pt nanoparticles on the TiO2 substrate by PEALD was performed at a substrate temperature of 2000C using (trimethyl)methylcyclopentadienylplatinum (MeCpPtMe3) as the Pt precursor. Material properties of the Pt nanoparticles were studied by secondary electron microscopy, x-ray photoelectron spectroscopy and transmission electron spectroscopy. The density and size of the PEALD Pt nanoparticles increases with the number of the PEALD reaction cycle, and the particle size was in the range of 2-5 nm. The electrochemical analysis showed that the electroactivity of the Pt/TiO2 anode toward MOR in acidic media depends on the PEALD cycle number. The anode prepared with a small PEALD cycle number exhibits a low electrochemical activity; this is because discrete Pt nanoparticles provide little electrical conductivity for the electrically insulating TiO2 support. However, the If/Ib ratio, which is defined by the ratio of the MOR peak in the forward scan to the CO-adspecies oxidation peak in the reverse scan in the cyclic voltammogram, reveals a good CO tolerance for the electrode. The CO tolerance of the electrode becomes lower when the PEALD cycle number is large (>30 cycles). The electrode with Pt nanoparticles deposited with a PECVD cycle number of 20-30 exhibits the best electrocatalytic performance, both in the electroactivity and in the CO tolerance. The excellent electrocatalytic performance of the Pt/TiO2 electrode is ascribed to the synergistic effect of the electronic effect and the bifunctional mechanism. Charge transfer (electronic effect) between Pt nanoparticles and the TiO2 support may weaken the bonding strength of CO on the Pt nanoparticles, facilitating the oxidation of CO adspecies. Moreover, the TiO2 support can improve the CO tolerance via the bifunctional mechanism effect because it dehydrogenates water forming adsorbed OH at a lower potential barrier than Pt, and thus enhance the electrocatalytic activity toward MOR.