合成單層白金於鈀釕合金奈米顆粒觸媒應用於酸性燃料電池之氧氣還原反應

Abstract

本研究使用濕式迴流化學法製備附著於奈米碳黑載體的PdRu、Pd3Ru及Pd9Ru鈀釕合金奈米顆粒觸媒。經由X光繞射儀分析得知所有比例的鈀釕合金觸媒都以合金的形式存在，並沒有單獨的鈀及釕奈米顆粒。在掃描穿透式電子顯微鏡分析結果顯現出其三種組成的奈米顆粒都為不規則形狀及平均粒徑皆小於3奈米。從鈀及釕的K-edge的延伸X光吸收細微結構得知，釕原子傾向聚集在PdxRu/C的表面。在氧氣飽和的0.1 M過氯酸水溶液中進行氧氣還原反應的測試，相對於另外兩種組成的觸媒，Pd9Ru/C具有較好的電化學催化活性。接著將Pd9Ru/C經過銅的欠電位沉積，其次為進行伽凡尼置換反應，將白金單一原子層沉積於Pd9Ru的表面上，形成核殼狀的Pd9Ru@Pt結構。相較於商用Pt觸媒，Pd9Ru@Pt觸媒擁有良好的氧化還原反應的催化活性，其質量活性比為0.38 mA μg-1Pt，優於商用Pt觸媒的0.11 mA μg-1Pt。除此之外，由Koutecky-Levich方程式所得的斜率，推論出氧氣是經由直接四電子的還原路徑被還原成水。在觸媒穩定度測試中，將Pd9Ru@Pt觸媒置於0.1M的過氯酸水溶液進行10,000圈的電化學循環掃描，其質量活性下降了50%，代表其結構仍有改進的空間。此核殼結構的Pd9Ru@Pt電催化劑顯示出可大幅降低白金的使用量以及原物料的價格。

Nanoparticles of PdRu, Pd3Ru, and Pd9Ru are synthesized and impregnated on carbon black via a wet chemical reflux process. X-ray diffraction patterns of the as-synthesized samples, PdxRu/C (x=1/3/9), suggest succesful formation of alloy without presence of individual Pd and Ru nanoparticles. Images from transmission electron microscope confirm irregularly-shaped nanoparticles with average size below 3 nm. Analysis from extended X-ray absorption fine structure on both Pd and Ru K-edge absorption spectra indicate that Ru atoms are enriched on the surface of PdxRu/C. Among these samples, the Pd9Ru/C exhibits the highest electrocatalytic activity for oxygen reduction reaction (ORR) in an oxygen-saturated 0.1 M aqueous HClO4 solution. Subsequently, the Pd9Ru/C undegoes Cu under potential deposition, followed by a galvanic displacement reaction to deposit a Pt monolayer on the Pd9Ru surface (Pd9Ru@Pt). The Pd9Ru@Pt reveals better ORR performance than that of Pt, reaching a mass activity of 0.38 mA μg-1Pt, as compared to that of commercially available Pt nanoparticles (0.11 mA μg-1Pt). In addition, a four-electron mechanism is responsible for the ORR actions occurring on the Pd9Ru@Pt/C. In the stability test, the Pd9Ru@Pt/C demonstrates a loss of 50 % in the mass activity after employing CV scans for 10,000 cycles in 0.1 M aqueous solution, indicating a large room for improvement. This core-shell Pd9Ru@Pt electrocatalyst represents a substantial reduction in the amount of Pt consumption and hence the raw material cost.