|Abstract: ||本研究利用甲酸還原法(formic acid method, FAM)製備不同長寬比之鉑奈米棒(nanorod)及鉑金，鉑銀，鉑鈀奈米棒以應用於氧氣還原反應(oxygen reduction reaction, ORR)。對於鉑奈米材料之一維(one-dimensional)結構及合金效應對ORR之增益將在本研究中探討。所製備觸媒之結構、表面物種、表面組成、化學組成、形貌、電催化活性以及未填滿d軌域(number of unoccupied d-states, hTs)分析分別使用X光繞射儀(X-ray diffraction, XRD), 程式溫控還原儀(temperature programmed reduction, TPR),光電子能譜儀(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), 高解析度穿透式電子顯微鏡(high resolution transmission electron microscopy, HRTEM), 旋轉盤電極(rotating disk electrode, RDE)以及X光吸收光譜(X-ray absorption spectroscopy, XAS)等儀器鑑定。|
研究結果分為兩部分，第一部分以FAM製備金屬負載量為45 wt%之不同長寬比的碳擔載鉑及鉑金奈米棒，其長寬比分別為1.84, 2.34, 3.75和4.00，且命名為Pt-1, Pt-2, Pt-3和PtAu。在ORR活性的表現上，Pt-3有最好之質量活性(mass activity, MA)，而PtAu則由於金的表面覆蓋，其活性略遜於Pt-3；然而在經過1000圈加速穩定度測試(accelerated durability test, ADT)後，PtAu奈米棒有最佳的ORR活性及穩定度，其活性僅下降24 %。另外，從XAS計算可鑑定出PtAu奈米棒有較低之hTs，其意指有較多之電子轉移至鉑之d軌域，降低與氧鍵結，進而提升ORR活性。
第二部分同樣使用FAM製備45 wt%之碳擔載鉑金、鉑銀、鉑鈀奈米棒，命名為PtAu, PtAg及PtPd，其具有鉑在內核，且第二金屬在外殼之核殼結構，其奈米棒長寬比約為4.0且沿著(111)方向生長。PtAu相較於其他合金觸媒有較好之ORR活性，其次為PtAg奈米棒，其活性為商用材Pt/C之1.2倍。然而在1000圈ADT後，PtAu因為鉑之表面偏析使其結構不穩定，進而劣化ORR活性與穩定性。相反的，由於PtAg奈米棒有較穩定之結構，經由ADT之後PtAg奈米棒之活性及穩定度為最佳，僅下降了9 %，歸因於銀之合金化效應。;In this study, Pt nanorods (NRs) with different aspect ratios and Pt3M (M= Au, Ag or Pd) NRs are prepared by the formic acid method (FAM) for the oxygen reduction reaction (ORR). For Pt nanomaterials, the effect of one-dimensional (1-D) morphology and alloying on the promotion of their ORR performance has been investigated. The structures, surface species, surface compositions, chemical compositions, morphologies, electrochemical properties and the number of unoccupied d-state (hTs) of prepared catalysts are characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-atomic emission spectrometer (ICP-AES), high resolution transmission electron microscopy (HRTEM), rotating disk electrode (RDE) technique and X-ray absorption spectroscopy (XAS), respectively.
This study is divided into two parts. In the first part, the 45 wt% carbon-supported Pt and PtAu NRs with aspect ratios of 1.84, 2.34, 3.75 and 4.00 (named as Pt-1, Pt-2, Pt-3 and PtAu, respectively) are prepared via FAM. The mass activity (MA) of Pt-3 is the highest, and PtAu has lower MA than Pt-3, due to the coverage of some Pt surface atoms by Au. Nevertheless, after 1000 cycles of accelerated durability test (ADT), PtAu NRs show the best ORR activity and stability with only a decay of 24 %. The hTs value measured by XAS spectra shows that PtAu NRs have lower hTs, implying that more electrons transfer to Pt d-state, the Pt-O- binding is weaker and the ORR performance is promoted.
In the second part, the 45 wt% of carbon-supported PtAu, PtAg and PtPd NRs with a Pt core/M shell structure, aspect ratio of 4.0, and growth along (111) direction are prepared by FAM. The ORR activity of PtAu is the best among the Pt-based catalysts, and that of PtAg NRs is the second, which is 1.2 times higher than that of Pt/C. However, after 1000 cycles of ADT, due to the surface Pt segregation during ADT, the structure of PtAu becomes unstable, deteriorating the ORR performance. On the contrary, PtAg NRs show the best stability among the Pt3M catalysts with only a slight decay of 9 % attributed to the stable structure through Ag alloying during ADT.