控制三元合金形狀成長及其對燃料電池催化之應用

The Shape Control of Ternary Alloy Nanocrystals and Their Application of the catalytic reaction in the Fuel Cell

錢佳華(Chien Chia Hua)

指導教授 ： 陳家俊

國立臺灣師範大學/理學院/化學系/碩士(2010年)

摘要

本論文是利用多元醇還原合成油相系統(polyol reduction)，用以控制三合金(ternary alloy)奈米粒子的形狀，歸納相關變因對大小和形狀上影響，並用於燃料電池(fuel cell)，希望粒子形狀影響催化效果。
　　實驗中所用的金屬鹽於高溫下與介面活性劑的交互作用，改變不同反應條件，得到FePtCo及FePtNi的polyhedron、cube、wire不同形狀奈米結構。研究發現以Oleic acid(OA)和Oleylamine(OLA)為介面活性劑時，粒子的大小及形狀對於介面活性劑比例非常敏感，歸納出OA/OLA=4 mL/4 mL，能合成出大小較統一的polyhedron粒子，甚至將OLA比例提高到12ml，沒有OA的參與，達臨界微胞濃度(critical micelle concentration，CMC)時形成束狀微胞(micelle)，合成出wire粒子；當以磷酸根和OLA為介面活性劑時，提高反應時間，由於(111)和(100)晶面動力學和熱力學成長的差異，造成兩階段的成長機制，合成出cube粒子。接著我們將所有不同形狀的觸媒，測試材料電化學活性。發現雖然對於甲醇氧化反應(methanol oxidation reaction, MOR)，效果沒有達到預期。但於氧氣還原反應(oxygen reduction reaction, ORR)，FePtCo中不同形狀的奈米粒子ORR mass activity皆比Pt-JM好兩倍左右，其效果大小程度依序為cube > polyhedron > wire > Pt-JM，而FePtNi中不同形狀的奈米粒子表現皆不好。且在理論計算上證明FePtCo材料，對於氧氣吸附能力較好，也證明FePtCo中Fe和Co對於Pt的charge transfer貢獻較大，使得進行ORR催化反應時會擁有更高的催化活性。

關鍵字

三元合金 ； 形狀控制 ； 燃料電池

並列摘要

We developed the morphological control of ternary alloy nanocystal (FePtCo and FePtNi) through the polyol reduction in high-boiling solvent. The metallic precursors were reacted with different surfactant combination, including, oleicacid/oleylamine (OLA/OA) and oleylamine/alkyl-phosphonic acid (OLA/TDPA or OLA/ODPA). The uniform polyhedron was synthesized in the OA/OLA = 4 mL/4 mL. The formation of uniform polyhedron was reasonably explained by the classical nucleation theory. Moreover, the nanowire was synthesized in the excess amount of OLA (12 mL) and the absence of OA. The growth mechanism was suggested that the soft template of OLA was formatted due to the achievement of critical micelle concentration (CMC). On the other hand, the nanocubes were synthesized in the presence of OLA/TDPA and/or OLA/ODPA. The two-step growth mechanism was observed because of the difference between the kinetic and thermodynamic growth trend on {111} and {100} facets of cuboctahedral seed. Also, several experimental parameters, including reaction time and concentrations of reducing agent were also considered as the key factor in the morphological.
　　Finally, we used FePtCo and FePtNi with different shapes (polyhedron, nanocube and nanowire) as catalysts. The results in methanol oxidation reaction (MOR) showed the onset potential of all samples were more negative than that of PtRu-JM. However, the measurements in oxygen reduction reaction (ORR) showed that mass activity of FePtCo were 2-fold higher than that of Pt-JM. Further, the ORR efficacy was in the sequence: nanocube > polyhedron > nanowire. The results showed that the difference in structural facets differentiated the ORR activity. We also demonstrated that FePtCo exhibited lower O2 adsorption energy and Co and Fe atoms significantly transferred electrons into Pt atoms through the computation of density functional theory (DFT). According to the results, the catalytic activity of FePtCo nanocrystal outperformed that of FePtNi in ORR.

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