合成核殼鉑釕奈米顆粒及官能基化碳載體

Abstract

本研究探討提升電化學甲醇氧化之方法，分別從製備雙元PtRu核殼奈米顆粒觸媒及觸媒之碳載體官能基化著手，旨在開發更廉價耐用之直接甲醇燃料電池陽極觸媒。首先，採用脈衝式定電流電鍍不同大小和組成之鉑釕奈米合金顆粒在XC-72R碳載體上。脈衝式電鍍的結果顯示鉑釕合金的比例隨著脈衝式電鍍周期 (Duty Cycle) 有規律趨勢變化。藉由XRD、TEM、ICP-MS證明PtRu(鉑釕)材料特性。利用循環伏安法(CV)和Pt金屬對氫離子吸脫附，鑑定PtRu NPs甲醇氧化的電化學行為。由XPS的結果分析得知Ru金屬的氧化態，可以推測出Pt及Ru有置換反應發生:脈衝式電鍍中，Ru金屬在電流通入的時段(Ton)有沉積在基材上，在電流停止的時間內(Toff)則被溶解;在Ton及Toff的時間內， Pt金屬則持續沉積。為了進一步了解置換反應的反應機制，利用X光吸收光譜(XAS)探討由碳材所支撐的Ru(釕)奈米粒子，浸泡在不同pH 環境下的 H2PtCl6 (氯鉑酸)水溶液中時，Pt(鉑)離子與Ru金屬奈米粒子的置換反應機制，並且形成雙元合金Pt為殼層、Ru為核心的奈米核殼結構。XAS結果顯示，Pt 離子在不同pH 環境下的擁有不同配位體種類及數量，這會影響Pt 離子活性，進一步地決定奈米粒子殼層上Ru和Pt 比例多寡與觸媒活性。電化學結果顯示在pH=1的H2PtCl6水溶液中所形成的雙合金核殼奈米粒子具有較低移除CO的電位以及穩定氧化H2 的催化效果，而在pH=8下，並沒有預期的表面雙合金產生，且奈米粒子有較差的CO移除以及氧化H2的特性。最後在碳載體上，經由在含氧硫酸中CV掃描處理可破壞Nafion ionomer 以快速製作含氧官能基在碳載體表面上。離子色譜法測量經由CV掃描後硫酸根離子的殘留含量。拉曼分析結果顯示碳材結構在CV掃描處理後只有微量的改變，證明碳材未經破壞。XPS survey分析結果也顯示出氟原子的成分比減少，是由於Nafion的降解所造成的原因，同時氧原子成份比也相對增加，官能基化電極相對於浸泡組電極，可增加170%的鉑離子吸附含量。藉由XRD、TEM、ICP-MS證明Pt材料特性，並藉由電化學甲醇氧化行為證明了Nafion含氧官能基也可以有效地協助Pt氧化甲醇的能力。

The objective for this research is to improve methanol electro-oxidation in direct methanol fuel cells via fabricating core-shell PtRu nanoparticles and functionalizing carbon supports simultaneously. First, galvanostatic deposition in rectangular pulses is employed to prepare PtRu nanoparticles on carbon cloths in various sizes and compositions. By adjusting duty cycle, we are able to control the surface composition of PtRu effectively. Material characterizations including XRD, TEM, XPS, and ICP-MS, as well as electrochemical analysis such as cyclic voltammetry and hydrogen desorption are carried out. We found that in a displacement reaction which Ru atoms are alternately deposited and dissolved during Ton and Toff, while Pt atoms are continuously deposited. To further investigate the extent of displacement reaction, we adopt XAS to explore the oxidation state and neighboring atoms for Pt and Ru in samples produced by immersing carbon-supported Ru nanoparticles in hexachloroplatinic acid solutions with pH of 1, 2.2 and 8, respectively. Spectra from XAS confirm that the pH value of hexachloroplatinic acidic solution determines the type of ligands complexing the Pt cations, and consequently affects the extent of displacement reaction and alloying degree of core-shell (Ru@Pt) nanoparticles. As a result, the samples from pH=1 bath reveal a desirable core-shell structure that displays a reduced onset potential in CO stripping and stable catalytic performance for the H2 oxidation reaction, while the samples from pH=8 bath indicate formation of Pt clusters on the Ru surface that leads to poor CO stripping and lower H2 oxidation performance. Lastly, we develop a facile electrochemical route to generate functional groups on the carbon surface via engaging the degradation of Nafion ionomer by multiple CV sweeps in oxygen-saturated H2SO4 electrolyte. Ion chromatography confirms the dissolution of sulfate anions upon CV scans. Raman analysis suggests a minor modification to carbon structure. XPS indicates a significant increase of oxygenated functional groups in conjunction with notable reduction in the fluorine content. The amount of the oxygenated functional groups is determined by curve-fitting of C1s spectra with known constituents. The functionalized electrode allows a 170% increment of Pt ion adsorption compared to that without functionalization. After electrochemical reductions, the functionalized electrode reveals significant improvements in electrocatalytic performance in methanol oxidation, which is attributed to the oxygenated functional groups that facilitate the oxidation of CO on Pt.