| 摘要: | 在質子交換膜燃料電池的陰極端，其主要挑戰為鉑的稀少且昂貴及氧氣還原反應(oxygen reduction reaction, ORR)之效率不佳，因此，發展具有成本優勢且高性能的陰極觸媒，成為近年來最熱門的研究之一。在本研究中，藉由銀添加修飾鈷基低鉑觸媒之活性與穩定度。我們首先研究碳支撐之鈷鉑觸媒之最佳銀添加量，接著更進一步的探討細微結構與電催化性質的關係。所製備觸媒之結構相、表面組成、形貌、電催化性質以及未填滿d軌域(number of unoccupied d-states, hTs)分析分別使用X光繞射儀、光電子能譜儀、高解析度穿透式電子顯微鏡(high resolution transmission electron microscopy, HRTEM)、旋轉環-盤電極以及X光吸收光譜(X-ray absorption spectroscopy, XAS)等儀器鑑定。
本研究結果分為兩部分，第一部分為以一氧化碳製備不同銀添加之碳支撐鈷鉑銀奈米顆粒。銀的添加會增益電化學表面積與ORR活性，其中鈷鉑銀原子比為90:5:5的Co90樣品，展現了最佳的ORR性能與穩定度，這可以歸因於高電化學表面積以及添加適當量的銀，其在0.85伏特的質量活性為商用純鉑觸媒的8.4倍高，因此我們論證了，添加適當的銀可以有效地增益ORR性能。另一方面，添加45 %銀原子的Co50樣品在穩定度測試後發現了大量的銀溶解，導致結構的瓦解，ORR活性衰退了75 %。
在第二部分中，藉由不同的還原劑將碳支撐鈷鉑銀合成出不同的結構，且固定其鈷鉑銀原子比為90:5:5。比起商用純鉑觸媒，這些低鉑含量的觸媒展現了相當優異的ORR活性及穩定性。根據HRTEM、XAS以及電化學測試，我們系統性地闡述出原子排列與ORR性能的關係。鉑-鈷及鈷-鈷的配位數分別與ORR之活性及穩定性有關聯性，Co90的結構為鉑與銀的原子隨機散布在鈷主體中，其具有最高的鈷-鈷配位數，這對ORR的性能並沒有正向效應；另一方面，Co90-1的結構為鉑與銀各自團簇在鈷主體中，其具有最高的鉑-鈷配位數，也代表著具有最大的電子修飾效應而擁有最高的ORR活性，然而，在穩定度測試1000圈之後，其損失了32 %的ORR活性，這歸咎於較高的鈷-鈷配位數。Co90-2的結構為銀聚集在鉑團簇的周圍，其具有中等的鉑-鈷配位數與最低的鈷-鈷配位數和hTs值，代表具有最穩定的ORR性能，在穩定度測試1000圈之後，其ORR性能僅衰退21 %。
;The main challenges of polymer electrolyte membrane fuel cells are Pt scarcity, cost, and the sluggish kinetics of oxygen reduction reaction (ORR). Therefore, the development of cost-effective and high performance cathode catalysts have become one of the most popular research topics in recent years. In this study, carbon-supported CoPtAg nanoparticles (NPs) with different atomic ratios and different structures are prepared for the ORR. We have tried to find the appropriate amount of Ag addition into CoPt catalysts and then the correlation between the fine structures and the electrochemical properties of CoPtAg catalysts is elucidated. The phases and structures, surface compositions, morphologies, electrochemical properties, and the number of unoccupied d-state of prepared catalysts are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy (HRTEM), rotating ring-disk electrode technique and X-ray absorption spectroscopy (XAS), respectively.
This study is divided into two parts. In the first part, carbon-supported CoPtAg NPs with different atomic ratios are reduced in CO atmosphere. The electrochemical surface area (ECSA) and ORR activity of CoPt is enhanced by the Ag addition. Among these CoPt samples, Co90 with Co/Pt/Ag atomic ratio of 90/5/5 displays the best ORR performance and stability attributed to the highest ECSA and appropriate amount of Ag addition. The mass activity at 0.85 V of Co90 is 8.4 times higher than that of commercial pure Pt. Thus, we have demonstrated that adding appropriate amount of Ag into CoPt/C catalyst can effectively promote ORR performance. However, Co50 with 45 at. % of Ag addition has a decay of 75 % after ADT of 1000 cycles, caused by significant Ag dissolution, leading to structure collapse.
In the second part, carbon-supported Co90Pt5Ag5 NPs with different atomic arrangements have been prepared by different reduction agents. These samples with low Pt loading has superior ORR activity and stability to Pt/C. Based on the analysis of the HRTEM, XAS, and electrochemical measurements, the relationship between atomic arrangement and ORR performance of Co90Pt5Ag5 is systematically elucidated. The coordination numbers of Pt-Co and Co-Co (CNPt-Co and CNCo-Co) of catalysts are related to their ORR activity and stability, respectively. Co90 with Pt and Ag randomly distributing within the Co matrix has the highest CNCo-Co, which does not have positive effect on the ORR performance. On the other hand, Co90-1 sample with Pt and Ag cores within the Co matrix has the highest CNPt-Co and ORR activity ascribed to the electronic modification effect. However, it losses 32 % ORR activity after accelerated durability test (ADT) of 1000 cycles due to high CNCo-Co. Co90-2 with Pt in the inner core and Ag on the outer shell structure has moderate CNPt-Co, very low CNCo-Co, and un-filled d-states, which benefit the ORR stability and the decay rate is 21 % after ADT of 1000 cycles. |
