| 摘要: | 陰離子交換膜燃料電池（anion exchange membrane fuel cells, AEMFCs）的陰極端，氧氣還原反應（oxygen reduction reaction, ORR）是一個緩慢且多步驟之反應，過電位和明顯的功率損耗也降低AEMFCs的整體能源效率。因此，發展能提升ORR效率的方法便成為能源材料研究的重點之一。在本研究中，製備具有不同結構的鉑鈀和鉑鈀釕觸媒並探討精細結構和觸媒的電化學性能之間的關係。所製備觸媒之晶格結構、表面組成、化學組成、微結構、形貌和電化學特性分別使用X光繞射儀（X-ray diffraction, XRD）、光電子能譜（X-ray photoelectron spectroscopy, XPS）、感應耦合電漿原子發射光譜分析儀（inductively coupled plasma-atomic emission spectrometer, ICP-AES）、X光吸收光譜(X-ray absorption spectroscopy, XAS)、高解析度穿透式電子顯微鏡（high resolution transmission electron microscopy, HRTEM）和旋轉盤電極（rotating disk electrode, RDE）等儀器做分析。
本研究分成兩部分。在第一部分中，利用鈀和鉑的不同沉積條件製備具有不同核/殼結構之碳負載鉑鈀觸媒。相比於單金屬鈀觸媒，加入鉑的雙金屬觸媒都顯示出較大的ORR性能和穩定性。在這些鉑鈀樣品中，最高的鉑-鉑配位數和最高程度的核/殼結構(PtPd-1)促進ORR性能，因其有適當的電子修飾效應。PtPd-1在0.85伏特的質量活性是PtPd-3之近2倍。此外，PtPd-3具有最低鉑-鉑配位數和最高的鉑覆蓋在表面，這有利於其在ORR中的穩定度，其衰減率在5000圈的加速穩定度測試 (accelerated durability test, ADT)中只有15 ％。
在第二部分中，我們製備出金屬附載量20 %之低鉑鉑鈀釕三元觸媒，將其與鈀釕雙元觸媒比較，皆具有優異的電化學性能。根據HRTEM、XAS、電化學測試的分析，所製備的樣品皆顯示出釕核、鈀間層、Pt殼的結構，且我們系統性地闡述出鉑鈀釕觸媒其精細結構與催化性質之間的關係。觸媒的鉑-鈀配位數都與他們的ORR性能有關。Pt-5具有最高的鉑-鈀配位數，這對其ORR活性有正面的影響。然而，5000圈的加速穩定度測試後，其損失了37.3％的ORR活性。另一方面，Pt-20具有最低鉑-鈀配位數和最高的鉑覆蓋在表面，這有利於觸媒在ORR中的穩定性，其衰減率在5000圈的加速穩定度測試中只有最低的29 ％。本研究闡明在鹼性電解質中核殼結構會促進觸媒活性，但表面鉑則會提升觸媒之穩定度。
;At the cathode of anion exchange membrane fuel cells (AEMFCs), the oxygen reduction reaction (ORR) is a sluggish reaction and consists of multiple reaction steps. The overpotential phenomena and significant power losses also reduce the overall energy efficiency of AEMFCs. Therefore, one of the important tasks for the energy materials is to d promote ORR efficiency. In this study, the PtPd and PtPdRu catalysts with different structures have been prepared. The relationship between their fine structures and electrochemical properties is elucidated. The lattice structures, surface compositions, chemical compositions, local structural parameters, morphologies and electrochemical performances are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-atomic emission spectrometer (ICP-AES), XAS, high resolution transmission electron microscopy (HRTEM), and rotating disc electrode (RDE), respectively.
This study is divided into two parts. In the first part, carbon-supported PtPd/C catalysts with different degrees of core/shell structure can be prepared at different deposition conditions The addition of Pt into Pd catalysts have demonstrated great promotion of ORR performance and stability compared to Pd/C reference. Among these PtPd samples, PtPd-1 with the highest coordination numbers of Pt-Pt (CNPt-Pt) and the highest degree of core/shell structure displays the best ORR performance attributed to the appropriate electronic effects. The mass activity at 0.85 V of PtPd-1 is nearly 2 times higher than that of PtPd-3. Besides, PtPd-3 sample with the lowest CNPt-Pt and the highest Pt coverage on the surface has the best ORR stability and the decay rate is only 15 % after accelerated durability test (ADT) of 5000 cycles.
In the second part, carbon-supported PtPdRu catalysts with metal loading of 20 wt % and different amounts of Pt adlayers have been prepared. These samples with low Pt loading have superior electrochemical properties to reference PdRu/C. Based on the analysis of the HRTEM, XAS, and electrochemical measurement, the as-prepared samples exhibit Ru core, Pd inter-layer, and Pt shell structure, and the correlation between fine structure and catalytic properties of PtPdRu catalysts is methodically elucidated. The CNPt-Pd of catalysts is related to their ORR performance. Pt-5 has the highest CNPt-Pd, which has positive effect on the ORR activity, and losses 37.3 % ORR activity after ADT of 5000 cycles. On the other hand, Pt-20 has the lowest CNPt-Pd and the highest Pt coverage on the surface, benefiting the ORR stability with the lowest decay rate of 28.9 % after ADT of 5000 cycles. We have demonstrated that the core/shell structure can promote the ORR activity while the surface Pt enhances the stability. |
