| 摘要: | 鉑基合金做為質子交換膜燃料電池(polymer electrolyte membrane fuel cells, PEMFC)的陰極觸媒一直以來被廣泛研究，然而，鉑的昂貴且稀少性成為發展陰極觸媒之重要挑戰，因此設計低鉑或不含鉑的陰極觸媒為PEMFC實際應用的重要課題。因此，本研究利用不同的還原方法製備出不同結構之鈷基低鉑觸媒。隨後透過觸媒的設計，在鈷基低鉑觸媒中添加金，其利用總體效應(ensemble effects)改質結構，並可防止在長時間測試時觸媒的自主溶解。對於觸媒藉由金的添加以及合金化的加成效應對於氧氣還原反應(oxygen reduction reaction, ORR)之活性增益將在本研究中闡述。所製備觸媒之ORR活性、形貌、結構、表面組成、化學組成以及未填滿d軌域(number of unoccupied d-states, hTs)分別使用旋轉盤電極(rotating disk electrode),高解析度穿透式電子顯微鏡(high resolution transmission electron microscopy), X光繞射儀(X-ray diffraction),光電子能譜儀(X-ray photoelectron spectroscopy), 感應耦合電漿原子發射光譜分析儀(inductively coupled plasma-atomic emission spectrometer)以及X光吸收光譜(X-ray absorption spectroscopy)等儀器鑑定。
研究結果分為兩部分，第一部分為用不同還原條件製備金屬負載量為40 wt%與不同結構之鈷基低鉑觸媒且依其還原條件命名為CoPt-CO570, CoPt-H470, CoPt-H570和CoPt-NaBH4。而鈷基低鉑觸媒之所以具有明顯的ORR活性增益在於鈷鉑合金相的存在以及表面富含鉑。經由加速穩定度測試(accelerated durability test, ADT)，CoPt-H470有最好的ORR穩定度是由於氧化鈷的存在，其排斥吸附於鉑上的含氧物種，使其從鉑上較易脫附，可增加鉑的活性位置。
第二部分分別用3, 10, 12,和25 at %的金作為改質劑添加於金屬負載量為40 wt % 之鈷基低鉑觸媒中，並分別命名為CoPtAu-1/C, CoPtAu-2/C, CoPtAu-3/C和CoPtAu-4/C。在所有以金做為改質劑的鈷基低鉑觸媒中，其活性約為商用觸媒Pt/C 2.6-9.2倍，而CoPtAu-1/C 有著最好的ORR活性表現，當1000圈ADT之後，CoPtAu-1/C 的ORR活性只衰退了原本的15 %，其對ORR穩定性的增益是源於以金為核，底層的金會避免表面的鉑收縮進底層進而破壞觸媒的結構。
;Pt-based alloys as cathode catalysts of polymer electrolyte membrane fuel cells (PEMFC) have been widely researched; however, the main challenges of Pt-based cathode catalysts are the high cost and Pt scarcity. Thus, the design of low or non Pt cathodic materials is an important task for the practical application of PEMFCs. Therefore, in this study, carbon supported CoPt catalysts with different structures can be prepared by different reducing conditions. Then through catalysts design, we have prepared carbon-support CoPtAu nanoparticles with low Pt loading and Au modification, which takes the advantages of ensemble effects and prevents catalysts from vulnerable dissolution during long term test. Their synergistic effect of alloying and Au modification on the oxygen reduction reaction (ORR) performance is also elucidated. The ORR activities, morphologies, structures, surface compositions, chemical compositions, and un-filled d-states (hTs) of the carbon-supported CoPt and CoPtAu catalysts are analyzed by rotating disc electrode, high resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, inductively coupled plasma-atomic emission spectrometer and X-ray absorption spectroscopy, respectively.
This study is divided into two parts. In the first part, carbon-supported CoPt catalysts with metal loading of 40 wt% and different structures have been prepared (named as CoPt-CO570, CoPt-H470, CoPt-H570 and CoPt-NaBH4). A significant enhancement of ORR activity for CoPt catalysts can be owing to the existence of CoPt alloy phase and an enrichment of surface Pt. With respect to accelerated durability test (ADT), the CoPt-H470 presents the best ORR durability among all catalysts, suggesting that the existence of the Co oxide, leading to OH repulsion between Pt–OH and Co oxides, as decreasing the OH coverage on surface Pt, and increasing the number of free Pt active sites.
In the second part, carbon-supported CoPt catalysts with metal loading of 40 wt% and Au addition of 3, 10, 12, and 25 at % (named as CoPtAu-1/C, CoPtAu-2/C, CoPtAu-3/C and CoPtAu-4/C, respectively) have been prepared. For the Au-modified CoPt/C catalysts with low Pt contents, their mass activity is about 2.6-9.2 times higher than that of commercial Pt/C. Besides, a significant enhancement in ORR performance based on the ADT test is noted for CoPtAu-1/C with Au core with a decay of 15 % after 1000 potential cycles. The promotion of ORR durability for CoPtAu-1/C is attributed to the Au core, in which the subsurface Au prevents the surface Pt from shrinking in the subsurface to damage the structure. |
