| 摘要: | 燃料電池為藉由燃料的化學能透過電化學反應的方式轉化為電能的裝置，因其具有高功率密度、相對較低的工作溫度和零排放，而成為具潛力之運輸和攜帶式應用中的替代電源。鉑 (platinum, Pt) 因其獨特的吸附能和高活性，被認為是燃料電池應用中最佳之觸媒材料。然而Pt有較強的親氧性會降低陰極氧還原反應 (oxygen reduction reaction, ORR) 的活性並使活性位點氧化，導致穩定性不佳，並不利於燃料電池技術的商業性。因此開發高效且穩定的Pt基觸媒對於質子交換膜燃料電池 (proton exchange membrane fuel cells, PEMFCs) 具有相當重要意義。本研究開發了幾種用於PEMFCs中的Pt基觸媒，探討其精細結構的控制和表面改質對於ORR性能增益的重要性。
本論文的研究主題分為三部分，第一部分探討Pt奈米棒 (nanorods, NRs)上金屬氧化物的修飾對ORR的性能影響，並證明了各種金屬氧化物 (metal oxide, MO) (M = Ni, Rh, Pd, Ag, Ir, Sn) 對Pt NRs觸媒的活性和穩定性有不同程度的增益。透過質量活性 (MAextra) 的計算，我們進一步的揭示了各種PtMOx觸媒的MAextra，其趨勢為PtSnOx > PtAgOx > PtNiOx > PtPdOx > PtRhOx > PtIrOx，可發現PtSnOx觸媒表現出最佳的活性 (MAtotal = 638 mA/mgPt)，在加速穩定度測試 (accelerated durability test, ADT) 後仍保有高的質量活性 (MAADT = 638 mA/mgPt)，表示SnOx修飾可以改變Pt的電子結構，且在SnOx上的含氧物種 (oxygen-containing species, OCS) 透過排斥效應抑制了Pt的氧化。另一方面，亦發現PtSnOx觸媒的MAextra和MAtotal數值非常相近，進一步的證明了ADT期間ORR活性的貢獻主要來自於Pt-SnOx的界面。
第二部分透過精細結構的調控，探討PtAuSn殼/核三元NRs觸媒的ORR活性和穩定性。值得注意的是相比於合金奈米觸媒，殼/核分離的奈米結構觸媒通常表現出更高的活性和穩定性，因其具有較高的貴金屬利用率和電子修飾效應。在ORR的效能中， 以AuSnOx修飾的Pt觸媒在0.85 V下具有很高的活性 (MA = 1069 mA/mgPt)。此外，表面具有SnOx保護層的Au@SnOx修飾Pt觸媒，在10000圈ADT測試後仍表現出高度的穩定性 (MA = 251 mA/mgPt)。這部分的研究強調了精細結構的控制可以有效地提升觸媒的活性及穩定性。
第三部分探討有序和無序結構的Pt3Co觸媒和其修飾效應對ORR性能的影響。通過氫氣熱處理製備有序結構的Pt3Co觸媒，其表現出高的活性 (MA = 386 mA/mgPt)，表明有序結構可以有效地提升ORR活性。接著透過PtSn表面修飾的有序Pt3Co觸媒，在有序結構和修飾效應的協同相互作用下，進一步增強了ORR活性 (MA = 504 mA/mgPt)，在5000圈ADT測試後仍表現出高度的穩定度 (MA = 428 mA/mgPt)。這部分的研究提供了關於Pt3Co基觸媒結構設計的見解，以實現增強ORR性能的目的。
本篇研究主要探討，各種製備和改質方法，包括金屬氧化物修飾、殼核結構控制、有序結構的表面修飾以及SnOx的排斥作用，以合成各種具有高效能ORR活性和穩定性的Pt基觸媒。採用了上述的策略，所製備的PtSnOx、PtAuSn和Pt3CoSn觸媒皆優於Pt商用材，其活性提升了超過3倍以上，在未來將這些高效能Pt基觸媒運用於PEMFCs當中，預期將是極具潛力之陰極觸媒。
;Fuel cells, converting chemical energy from fuels into electricity through an electrochemical reaction, are promising alternative power sources for transportation and portable applications due to their high power density, relatively low operation temperature, and zero emissions. Platinum (Pt) has been regarded as the state-of-the art catalysts materials in the fuel cell application due to its unique adsorption energy and high activity. However, the high oxophilicity of Pt demotes the activity of oxygen reduction reaction (ORR) at the cathode and oxidizes the active sites, resulting in a vulnerable durability and disadvantaging the commercial viability of the fuel cell technology. Therefore, the development of highly effective and stable Pt-based electrocatalysts is of importance for the proton exchange membrane fuel cells (PEMFCs). In this study, we have developed several Pt-based catalysts for PEMFCs to discuss the importance of fine structures control and surface modification to promote ORR performance.
This study is divided into three parts. In the first part, we have investigated the ORR performance of oxide decorated Pt nanorodss (NRs) catalysts and demonstrated that various metal oxides (MO, M = Ni, Rh, Pd, Ag, Ir, and Sn) influence the activity and stability of Pt NRs. Through mass activity (MAextra) calculation, our study has further revealed that the MAextra of various PtMOx have the trend: PtSnOx > PtAgOx > PtNiOx > PtPdOx > PtRhOx > PtIrOx. As a result, the optimized Pt-SnOx catalyst indeed demonstrates the best activity with enhanced mass activity (MAtotal = 860 mA/mgPt) and much improved stability after accelerated durability test (ADT)(MAADT = 638 mA/mgPt), suggesting that SnOx addition can modify Pt electronic properties and the oxygen-containing species (OCS) on SnOx can inhibit Pt oxidation by the repulsion effect. In addition, it is also found that MAextra of PtSnOx is very close to MAADT, further proving that the primary contribution of ORR activity during the ADT is from the Pt-SnOx interfaces.
In the second part, we have studied that the ORR activity and stability of PtAuSn core/shell ternary NRs catalysts, which are fine structure-dependent. Notably, core/shell segregated nanocatalysts exhibit improved activity and stability compared with alloyed nanocatalysts because of the high noble metal utilization and electronic modification effect. AuSnOx decorated Pt catalysts can reach high ORR mass activity of 1069 mA/mgPt at 0.85 V (IR-free). Moreover, the Au@SnOx decorated Pt catalysts with surface SnOx protective layer show high ORR stability after ADT of 10,000 cycles, which yet maintain high mass activity (251 mA/mgPt). This part highlights that controlling the fine structure of catalysts can effectively promote the activity and durability of the electrocatalysts for fuel cells.
In the third part, we have investigated the ORR performance of the ordered/disordered Pt3Co structure and the decoration effect. Herein, an ordered O-Pt3Co catalysts have been prepared via H2 heat treatment, showing high activity (MA = 386 mA/mgPt), suggesting that the ordered structure can effectively improve ORR activity. Subsequently, through PtSn decoration on ordered Pt3Co catalyst, the synergistic interaction between ordered structure and decoration effect further enhance the ORR activity (MA = 504 mA/mgPt) and maintain the activity after ADT (MA = 428 mA/mgPt). This part provides insights about structural design of Pt3Co-based catalyst to achieve enhanced ORR performance.
In this study, the highly effective Pt-based catalysts are prepared and promoted for ORR. We have used several methods of preparation and modification, including metal oxide decoration, core/shell structure control, surface decoration on the ordered structure, and repulsion effect of SnOx to synthesize various Pt-based catalysts with better ORR activity and stability. Taking the above strategies, all of theprepared PtSnOx, PtAuSn, and Pt3CoSn catalysts have ORR activity 3 times higher than commercial Pt/C catalyst. It is believed that these highly effective Pt-based catalysts are promising cathode catalysts in PEMFCs. |
