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合成核殼鉑釕奈米顆粒及官能基化碳載體

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國立交通大學

材料科學與工程學系

博士論文

合成核殼鉑釕奈米顆粒及官能基化碳載體

提升甲醇電化學氧化性能

Enhancement of Methanol Electro-oxidation via Core-shell PtRu

Nanoparticles and Functionalized Carbon Supports

研究生:謝育淇

指導教授:吳樸偉 教授

李志甫 博士

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National Chiao Tung University

Department of Materials Science and

Engineering

Ph.D. Dissertation

合成核殼鉑釕奈米顆粒及官能基化碳載體

提升甲醇電化學氧化性能

Enhancement of Methanol Electro-oxidation via Core-shell PtRu

Nanoparticles and Functionalized Carbon Supports

Student: Yu-Chi Hsieh

Advisors: Prof. Pu-Wei Wu

Dr. Jyh-Fu Lee

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2

合成核殼鉑釕奈米顆粒及官能基化碳載體

提升甲醇電化學氧化性能

研 究 生:謝育淇 Student: Yu-Chi Hsieh

指導教授:吳樸偉 教授 Advisor: Prof. Pu-Wei Wu

李志甫 博士 Dr. Jyh-Fu Lee

國立交通大學

材料科學與工程博士學位

博士論文

A Dissertation

Submitted to Department of Materials Science and Engineering College of Engineering

National Chiao Tung University in partial Fulfillment of the Requirements

for the Degree of Doctor Philosophy in

Materials Science and Engineering

July 2012

Hsinchu, Taiwan, Republic of China

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合成核殼鉑釕奈米顆粒及官能基化碳載體提升甲醇電化學氧化性能

研究生:謝育淇

指導教授:吳樸偉 教授

李志甫 博士

國立交通大學

材料科學與工程學系

摘要

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

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ii 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。

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Enhancement of Methanol Electro-oxidation via Core-shell PtRu

Nanoparticles and Functionalized Carbon Supports

Student: Yu-Chi Hsieh

Advisors: Prof. Pu-Wei Wu

Dr. Jyh Fu Lee

Department of Materials Science and Engineering

National Chiao Tung University

Abstract

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

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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.

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致謝

人生的際遇總是難以莫測,原以為完成碩士學位後,應不會再在學術殿堂裡追逐。在 服兵役期間,站哨時、與同僚聊天時,滿腦子都是科學,發現自己還是熱衷于科學。因此, 攻讀博士學位的緣由是單純熱愛科學的心與夢想。但若沒有上帝巧妙的安排,與主的帶領, 我想我也不會順利進入這學術殿堂。感謝耶和華與主耶穌讓我經歷這豐富的人生過程。 首先感謝我的博士班指導教授們——吳樸偉教授與李志甫博士。在學術專業、人生路 途、人際關係、生活處事上,您們有如學生的父親與兄長一般教導我,在這五年讓學生成長 與成熟許多。首先我非常榮幸在吳樸偉老師的團隊下作研究,在研究路途上您給予學生很大 的自由度,讓我盡情地享受我的科學夢,並且在關鍵時刻給予學生正確的方向與想法。您對 學術的嚴謹態度、開放式的思考、邏輯性的推測還有學術道德的嚴格要求,都是學生在博士 期間收穫最多的智慧。您的美式教學風格也常鼓勵學生們多接觸國際會議。在您的鼓勵下, 學生有機會多次參加國際會議。2008 年在夏威夷電化學會議,學生有機會認識並接觸布魯克 海文國家實驗室的 Dr. Adzic ,因此埋下在美國進修一年的種子。2010 年在拉斯維加斯電化 學會議,學生更進一步地與 Dr. Jia X. Wang 討論在美國進修一年的計畫。2011 年學生在您的 推薦下前往美國布魯克海文國家實驗室進修一年,在專業知識上學生收穫良多,都是老師您 的功勞。求學期間與吳老師英文信件來往,老師您都是字句用心地教導學生的英文成為字字 珠璣、優美的英文文章。在生活上,老師您也常常教導學生,不論是學術職業倫理上的關係 應答,人際關係的進退還是豐富的生活經驗教導,都讓學生在人生和人格上更加成熟茁壯。 同時也非常感謝我另一位指導教授,李志甫博士。您的帶領和教導,讓學生學習到專業的 X 光吸收光譜技術,並從中獲得許多科學啟發,因此在專業上使學生連續獲得兩屆同步輻中心 的博士生培育計畫獎學金。李老師您對實驗的嚴謹態度與對分析數據的精準要求,讓學生在 實際實驗上受益良多,因此在成果上使學生在同步輻射年會中連續兩屆獲得學生組的佳作。 最後感謝李老師的寬宏大量,因學生的粗心常在實驗室與實驗站上搞得一團糟。古人說[一日 為師,終生為父],吳教授與李博士您們的教導,令學生受益一生,學生畢生難忘。 一起奮鬥的實驗夥伴們,感謝你們。最年長的張玉塵學長與周兆玲學姊,感謝你們提 供了不少中科院的資源。尤其是疏水性碳布,只有中科院生產,實驗上的需求都靠你們了。 謝謝你們常常帶我參觀中科院的環境生活,讓我認識了中華民國軍事研究最高單位的生活。 回想起玉塵學長買的股票漲時,那一天就有免費的中餐,真是懷念啊。林勝結大哥與謝逸凡 大哥,實驗室有你們這兩大支柱撐著,才可讓小弟我在實驗室胡搞瞎搞。感謝勝結大哥解決 我生活上的許多疑難雜症,在新竹生活有任何問題找你就對啦!感謝逸凡大哥樂活的心態,讓 我學習如何排解和舒緩壓力。逸凡大哥你總是帶著微笑面對事情,從你臉上沒有看到過憂愁, 這正是我學習的地方。實驗室超級戰將黃苡叡學長與張雲閔學長,能認識你們是我的榮幸。 你們對科學與工程的熱情,總是鼓勵著我,並且在不同的角度審查我們彼此的研究。感謝苡

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vi 叡學長你總是用工業界的角度與環境觀看並提醒我,學術必須要學以致用。至於張雲閔學長, 我跟你的恩怨最長久了,在博士期間除了老師以外,我跟你生活共事最久,所以整個恩恩怨 怨可以再寫個博士論文了,我想這就是同袍情誼。感謝你在 TEM 上熱情的幫忙;感謝你在學 術研究上與我彼此激烈的爭辯討論,讓我們倆可以認真思考並維持自己的觀點;感謝你讓我 知道學術並不是自己爽就好,總是會有不一樣的聲音;感謝你在生活上與研究上總是對我有 個照應。實驗室老大陳境妤,感謝你細心負責實驗室的大小事務,以致於我這常常不負責公 共事務的學長可以開心的作研究。你身上一步步扎實的學習與做事態度,讓我學習到按部就 班,認真學習,就沒有學不會的地方。感謝你在拉曼、IR 光譜、XRD 儀器上的協助。感性的 廖晨宏,從你專題生,一路看著你到現在都已博二了。看著你成長真是欣慰,感謝你全力以 赴認真完成我們的實驗。感謝姚奕全在 HR-TEM 上熱情的協助。感謝王仁君,時常在新竹陪 我聊天吃飯的對象,聽我訴說心中的苦悶。感謝詹丁山博士在 BL01C1 XAS 實驗上的熱情協 助與教學。感謝包志文博士在 BL17C1 XAS 實驗上的熱情協助與教學。感謝謝承安與陳姿亘 在 XAS 實作與分析軟體上的教學。感謝陳重守在 XPS 儀器上的幫忙。感謝陳偉達在美國一 起共患難的支援和鼓勵。感謝劉怡玲在離子分析儀上的協助。許多細節未提到的學長同學們, 蔣國章、車牧龍、彭俊彥、章詠湟、張修誠等等在此一起感謝。 碩士班學弟妹們,有了你們我的生活才精彩。首先感謝張立忠,在我博士最艱苦的時 間出現,是個強而有力的夥伴。感謝你超強的英文能力,幫助我完成了許多任務。感謝你, 我們一起完成了我博士論文中重要的一篇研究。你讓我知道宅男,也可以是很有品味的。接 下來感謝郭哲瑋,在我在美國的期間,幫我完成在台灣的任務。謝謝你在美工上的幫助。感 謝你任勞任怨地完成計畫的工作與研究。感謝你,讓我知道出國旅遊,可以很簡單的。感謝 李依叡與陳詠民在研究計畫上,認真付出。感謝呂永錚在釕(Ru)前驅物上給我專業的建議。 感謝邱于凡、蔡佳芬在專題研究上認真的學習與貢獻。感謝一起奮鬥畢業過程的學弟們,許 議文、李孟翰、邱尊偉。感謝三位出國進修博士的優秀學弟蔡合成、賴俊翰、陸意德。蔡合 成感謝你與我一起奮鬥博士資格考試的階段。感謝你願意與我分享你的生活與情感部份。賴 俊翰感謝你,實驗室有你的地方就充滿了歡樂。陸意德感謝你,讓我看到一位熱愛書本到會 忘了睡覺的人,也感謝你在 EQCM 上的協助。感謝葉耕余和李佳勳,在研究與友誼上熱情的 相挺。感謝陳國豐,提供許多台積電專業的知識。實驗室的眾多學弟妹們,黃柏翰、林韋霖、 林建程、梁雁汝、陳欣儀、傅宥閔、黃冠傑、陳琪、陳婉瑩、周亮余、黃筱琳、邱于凡、王 儷曄、陳儷尹(大小粒魚丸)、陳致源、張詠策、梁茹夢、張瀠方、陳柏均、張庭瑜、賴欣君、 蔡致芳、林映眉、羅世儒等人,我在此感謝你們,博士班的生活有你們真好。

在美國精彩豐富的生活。首先感謝 Dr. Jia X. Wang 擔任我在美國的指導教授。Jia 在 您的帶領下,讓我學習到專業的電化學知識,及要有遠見的目標與培養深厚專業知識的能力。 更重要的是,你在繁忙工作的同時,又能兼顧好家庭生活,也讓我學習到對時間的有效管理。 真的是非常感謝你,讓我有機會前往美國進修一年,見識到廣闊的世界。我也非常感謝 Dr. Radoslav Adzic,讓我加入您的團隊,從而接觸到頂尖優秀的人才。您的智慧是個百寶庫,遇 到問題總是有答案與很多想法。雖然您已年邁七十,您對科學的求知慾與新鮮感猶如年輕人

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一般。您溫和和藹的態度,讓我學習到作爲一位領導者的風範。我也非常感謝廖世軍教授, 您待我如您兒子一般的照顧。您樂觀的態度、認真學習的精神,都是值得我學習的地方。我 也非常喜歡和你聊有關中國的事務。感謝孟祥波博士,您也待我如您弟弟一般的照顧。謝謝 您常鼓勵我要努力向上,鼓勵我快成家。謝謝您,我會努力盡快達成你的願望。感謝 Wei-Fu Chen 和 Chloe Wang 夫婦,讓我感受到,我人在美國也可以享受到台灣家庭的生活,謝謝你 們的盛情款待與生活上的幫忙。感謝 Yun Cai, 在 Cu UPD 技術上的教學,還有生活上的協助, 也很懷念你的美食。感謝楊莉君,一起在美國實習研究一年,並且互相鼓勵,一起遊玩的日 子。感謝 YongMan Choi, 經常的鼓勵我,並相信主。感謝 Kotaro Sasaki,在 In-site XAS 上專 業的教學。感謝 Miomir 和 Stoyan, 在實驗室安全細節上的教導。還有一群未提起的朋友們, 謝謝你們讓我在美國的生活很精彩。 家庭是最後的堡壘。最感激的就是我的家人,阿嬤、爸爸、媽媽、翠菱、育婷,女友 張宇。阿嬤您最疼我了,常常塞零用錢給我,謝謝您貼心的照顧,當孫子總是幸福的。爸, 從念博士的第一天,您就告誡我說[不要整天待實驗室,要多出去追女孩子]。我知道我會努 力完成您的願望,趕快娶妻生兒,謝謝您讓我知道家庭是最重要的。媽,謝謝您不論我發生 什麼事,您總是最挺我的。謝謝您辛勞地為家裡的大小事務付出。翠菱,謝謝你平時細心地 陪伴家人,使我可以從你身上獲得家人的需求。育婷,感謝你平時提供的一些潮流物品,讓 我可以放鬆心情。你未來也好好努力加油!最後感謝我的女友張宇,謝謝你在我最後關鍵煎熬 的日子裡,一直陪在我身旁,還經常鼓勵我,並相信我的能力。 感謝主!我相信這一切都是主的安排。

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Table of Contents

Abstract (Chinese) ... i

Abstract (English) ... iii

Acknowledgement (Chinese) ... v

Table of Contents ... viii

List of Tables ... x

List of Figures ... xii

Chapter 1 Introduction ... 1

1.1 Background ... 1

1.2 Motivation ... 6

Chapter 2 Literature Review ... 7

2.1 Anode electrocatalysts for direct methanol fuel cell ... 7

2.2 Platinum monolayer core-shell electrocatalysts ... 12

2.3 Carbon based materials ... 18

2.3.1 Applications in fuel cells ... 18

2.3.2 Surface functionalization ... 19

2.4 Nafion ionomer... 20

Chapter 3 Enhancement of Methanol Electro-oxidaiton Performances via Core-Shell PtRu Nanoparticles Prepared by Pulse Current Deposition ... 21

3.1 Introduction ... 21

3.2 Experimental ... 23

3.3 Results and discussion ... 25

3.3.1 Various Toff for pulse electroplating ... 25

3.3.2 Various Ton for pulse electroplating ... 31

3.3.1 Duty Cycle for pulse electroplating ... 35

3.3.4 X-ray absorption spectroscopy analysis ... 43

3.4 Conclusions... 51

Chapter 4 Investigation of Formation Mechanisms for PtRu Core-Shell Nanostructures during Galvanic Displacement by XAS and EQCM ... 52

4.1 Introduction ... 52

4.2 Experimental ... 55

4.3 Results and discussion ... 59

4.3.1 Materials characterizations on Ru@Pt/XC72/CC ... 59

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4.3.3 Electrochemical analysis of Ru@Pt/XC72/CC ... 82

4.3.4 EQCM analysis ... 86

4.4 Conclusions... 90

Chapter 5 Enhancement of Methanol Electro-oxidation via Functionalization of Carbon Supports by the Electrochemical Degradation of Nafion Ionomer ... 91

5.1 Introduction ... 91

5.2 Experimental ... 92

5.3 Results and discussion ... 94

5.3.1 Electrochemical degradation of Nafion ionomer ... 94

5.3.2 Carbon functionalization ... 99

5.3.3 Methanol electro-oxidation ... 109

5.4 Conclusions... 116

Chapter 6 Conclusion and Future Work ... 117

6.1 Conclusions... 117

6.2 Future work ... 119

6.2.1 Development of electrocatalysts with different nanostructures by the advanced pulse electroplating ... 119

6.2.2 Development of electrocatalysts with different compositions for other catalytic reactions ... 119

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List of Tables

Table 3.1 Results from materials characterizations on the PtRu nanoparticles with fixed values of Ton (50 ms),

Ja (50 mA/cm2), and total coulombic charge (8.0 C/cm2)………..………. 26

Table 3.2 Electrochemical parameters from the CV scans in mass activity of the PtRu-Catalyzed carbon cloths with fixed values of Ton (50 ms), Ja (50 mA/cm2), and total coulombic charge (8.0 C/cm2)………….. 29

Table 3.3 Results from materials characterizations on the PtRu nanoparticles with fixed values of Toff (400

ms), Ja (50 mA/cm2), and total coulombic charge (8.0 C/cm2)………... 32

Table 3.4 Electrochemical parameters from the CV scans in mass activity of the PtRu-catalyzed carbon cloths with fixed values of Toff (400 ms), Ja (50 mA/cm2), and total coulombic charge (8.0 C/cm2)………… 32

Table 3.5 Results from materials characterizations on PtRu nanoparticles with fixed values of Ton (50 ms), Toff

(400 ms), and total coulombic charge (8.0 C/cm2)……….. 36 Table 3.6 Electrochemical parameters from the CV scans in mass activity of PtRu-catalyzed carbon cloths

with fixed values of Ton (50 ms), Toff (400 ms), and total coulombic charge (8.0 C/cm2)…………... 36

Table 3.7 Lattice parameter and alloyed Ru for the PtRu nanoparticles with fixed values of Ton (50 ms), Ja (50

mA/cm2), and coulombic charge (8.0 C/cm2)………... 40 Table 3.8 Results from XPS and curve fitting of PtRu nanoparticles with fixed values of Ton (50 ms), Ja (50

mA/cm2), and total coulombic charge (8.0 C/cm2)………. 42 Table 3.9 EXAFS fitting parameters at Pt L

III-edge and at the Ru K-edge for PtRu nanoparticles with different

values Toff (100, 400 and 600 ms) and fixed values of Ton (50 ms), Ja (50 mA/cm2) and coulombic

charge (8.0 C/cm2)………... 50 Table 4.1 EDX results on Ru@Pt/XC72 from group A of pH 1, pH 2.2, and pH 8……… 64 Table 4.2 ICP-MS results on Ru@Pt/XC72/CC from group A of pH 1, pH 2.2, and pH 8, as well as their

corresponding H2PtCl6 solution………... 67

Table 4.3 EXAFS fitting parameters at the Ru K-edge for Ru/XC72/CC and Ru@Pt/XC72/CC under various conditions... ... 73 Table 4.4 EXAFS fitting parameters at the Pt LIII-edge for Ru/XC72/CC and Ru@Pt/XC72/CC under various

conditions... 78 Table 5.1 The atomic ratios for carbon, oxygen, and fluorine from XPS profiles for as-prepared electrode, as

well as electrodes after CV scans with and without the supply of ambient oxygen……… 101 Table 5.2 The atomic ratios for the C−C, −OH, −C=O, −COOH, and C−F from XPS curve fitting for

as-prepared electrode, as well as electrodes after CV scans with ambient oxygen and without

ambient oxygen……… 103

Table 5.3 The atomic ratios for C−C, −OH, −C=O, −COOH, and C−F from C1s XPS curve fitting for as-prepared electrode, as well as electrodes made of XC-72R/carbon cloth with and without immersion in HCl solution containing concentrated residues from Nafion ionomer decomposition…. 109

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Table 5.4 Electrochemical parameters obtained from CV profiles on functionalized and reference electrodes for methanol electro-oxidation……… 113

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List of Figures

Figure 1.1 Possible operation modes for electrodeposition……….. 2 Figure 1.2 A schematic demonstration for the functional group formation and physical adsorption of

selective cations………... 4 Figure 2.1 Schematic illustration of methanol oxidation on Pt surface in different potential regions, studied

by FTIR and attenuated total reflection (ATR)………... 9 Figure 2.2 Schematic illustration of methanol oxidation on PtRu surface in different potential regions,

studied by FTIR and ATR………... 10 Figure 2.3 Illustration structures of PtRu alloy, Ru@Pt core-shell, monometallic nanoparticles……… 11 Figure 2.4 Cyclic voltammogram for the Au (111) single crystal electrode in 1 mM CuSO4 with 0.05 M

H2SO4. The sweep rate was 1 mV/s……… 13

Figure 2.5 Interfacial structure of the Cu UPD on Au (111) single crystal surface after the first UPD peak: (a) top view (b) side view………. 13 Figure 2.6 The top view of the Cu-(1×1) monolayer on Au (111) surface after the second UPD peak……... 14 Figure 2.7 Polarization curves for the ORR on the Pt monolayer supported on different single crystal

surfaces……… 15

Figure 2.8 STEM HAADF image (left) and the EDS line scan profile (right) indicating the core-shell structure of the Pt monolayer catalyst on Pd nanoparticles………...………. 15 Figure 2.9 The Pt mass activities for the ORR as a function of the potential cycle number n during fuel cell

testing for the Pt monolayer catalyst on Pd nanoparticles (red), compared to those for commercial Pt/C catalysts (green and blue). The potential cycles were square waves with a 30s dwell time at 0.7 and 0.9 V each (vs. RHE) at 80 °C……… 16 Figure 2.10 The structure for the Pt submonolayer catalyst on Ru core (the inset) PtRu20 and its HOR behavior

for the CO tolerance test, compared to that of a commercial Pt2Ru3 catalyst………. 17

Figure 2.11 (a) Methanol-oxidation currents normalized by the total noble-metal mass, and (b) the chrono-potentiometric measurements at 0.69 V for the MOR, for the Pt monolayer catalyst on Ru nanoparticles, compared to that on the commercial PtRu………... 17 Figure 2.12 (a) TEM image of PtRu nanoparticles decorated on CNTs (sample B02). (b) Cyclic

voltammograms for the MOR of PtRu nanoparticles on CNTs (B02) and commercial PtRu catalyst produced by Johnson Matthey (J-M), in N2-saturated 0.5 M H2SO4 + 1 M CH3OH

electrolytes, 20 mV/s………. 18 Figure 2.13 (a) The equilibrium potential diagram showing the potential difference between carbon (CNTs)

and metal ions (PtCl6

2-). (b) Polarization curves for the MOR on Pt nanocatalysts supported on CNTs with (line 1) and without (line 2) surface functionalization………. 19 Figure 2.14 Example structure of a sulphonated fluoroethylene from Dupont……….. 20 Figure 3.1 Representative TEM images for the PtRu nanoparticles with fixed values of Ton (50 ms), Ja (50

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Figure 3.2 CV profiles in mass activity for the PtRu-catalyzed carbon cloths with fixed values of Ton (50

ms), Ja (50 mA/cm2), and coulombic charge (8.0 C/cm2), as well as Toff of (a) 100, (b) 300, (c)

500, (d) 200, (e) 400, and (f) 600 ms……….. 30 Figure 3.3 Representative TEM images for the PtRu nanoparticles with fixed values of Toff (400 ms), Ja (50

mA/cm2), and coulombic charge (8.0 C/cm2), as well as Ton of (A) 25 and (B) 400 ms……… 33

Figure 3.4 CV profiles in mass activity for the PtRu-catalyzed carbon cloths with fixed values of Toff (400

ms), Ja (50 mAcm2), and coulombic charge (8.0 Ccm2), as well as Ton of (a) 25, (b) 50, (c) 100,

and (d) 400 ms………. 34

Figure 3.5 The effect of duty cycle on the Pt atomic ratio for the PtRu nanoparticles. Data are from Tables 3.1(

), III (

), and V (×)………... 35 Figure 3.6 The XRD patterns for the PtRu nanoparticles with fixed values of Ton (50 ms), Ja (50 mA/cm2),

and coulombic charge (8.0 C/cm2), as well as Toff of (a) 100, (b) 400, and (c) 600 ms……….. 39

Figure 3.7 XRD patterns from Fig. 3.6 with an enlarged range between 38 and 42° for lattice parameter

determination………... 37

Figure 3.8 (A) XPS signals of Ru (3p3/2) from PtRu nanoparticles with fixed values of Ton (50 ms), Ja (50

mA/cm2), and coulombic charge (8.0 C/cm2), as well as Toff in (a) 100, (b) 400, and (c) 600 ms.

(B) The results of curve fitting using Ru0, RuO2, and RuO2·nH2O……… 41

Figure 3.9 (a) The Pt LIII-edge XANES and EXAFS spectra and (b) the Pt LIII-edge k-space spectra for PtRu

nanoparticles with different values of Toff (100, 400, and 600 ms) and fixed values of Ton (50 ms),

Ja (50 mA/cm2), and coulombic charge (8.0 C/cm2), along with Pt foil serving as the reference in

(b)……… 44

Figure 3.10 (a) The Ru K-edge XANES spectra and (b) the Pt K-edge k-space spectra for PtRu nanoparticles with different values of Toff (100, 400 and 600 ms) and fixed values of Ton (50 ms), Ja (50

mA/cm2), and coulombic charge (8.0 C/cm2), along with Ru metal serving as the reference in (b).. 46 Figure 3.11 (a) The Pt L

III-edge and (b) the Ru K-edge Fourier-transformed EXAFS spectra for PtRu

nanoparticles with different values Toff (100, 400 and 600 ms) and fixed values of Ton (50 ms), Ja

(50 mA/cm2) and coulombic charge (8.0 C/cm2)……… 49 Figure 3.12 Schematic diagrams for cross sections of PtRu nanoparticles with different values Toff (100, 400

and 600 ms) and fixed values of Ton (50 ms), Ja (50 mA/cm 2

) and coulombic charge (8.0 C/cm2)... 51 Figure 4.1 A flow chart for the processing steps involved to prepare samples of group A, group B and

reference group……… 57

Figure 4.2 The XRD patterns for the XC72/CC, Ru/XC72, Ru/XC72/CC, and Ru@Pt/XC72/CC from group A of pH 1, pH 2.2, and pH 8 in scan range of (a) 30◦–90◦ and (b) 36◦–42◦………. 61 Figure 4.3 The XRD patterns for the Ru@Pt/XC72/CC from group B of pH 1, pH 2.2, and pH 8 in scan

range of (a) 30◦–90◦ and (b) 36◦–42◦……… 63 Figure 4.4 The TEM images for (a) Ru/XC72 and Ru@Pt/XC72/CC from group A of (b) pH 1, (c) pH 2.2,

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Figure 4.5 The Ru K-edge XANES spectra of Ru, RuO2, and Ru/XC72/CC from reference group of pH 1,

pH 7, and pH 8……… 68

Figure 4.6 The Ru K-edge XANES spectra of Ru@Pt/XC72/CC from group A and group B……… 69 Figure 4.7 The Ru K-edge Fourier-transformed EXAFS spectra from Figs. 4.4 and 4.5……… 71 Figure 4.8 The Pt LIII-edge XANES spectra of Pt foil and H2PtCl6 solution of pH 1, pH 2.2, and pH 8……… 72

Figure 4.9 The Pt LIII-edge XANES spectra of Ru@Pt/XC72/CC from group A and group B…... 75

Figure 4.10 The Pt LIII-edge Fourier-transformed EXAFS spectra from Figs. 4.8 and 4.9………... 76

Figure 4.11 The Ru K-edge XANES spectra of Ru ions after displacement reaction, along with Ru metal, Ru/C, RuCl3(aq) and RuO2(s) serving as references... 79

Figure 412 Schematic diagrams for PtRu displacement reaction occurring at (a) low pH and (b) high pH

conditions……… 81

Figure 4.13 The cyclic voltammetric curves for CO oxidation on E-TEK/CC and Ru@Pt/XC72/CC from group B of pH 1, pH 2.2, and pH 8………. 83 Figure 4.14 The H2 oxidation curves in (a) apparent current density and (b) mass activity on E-TEK/CC and

Ru@Pt/XC72/CC from group B of pH 1, pH 2.2, and pH 8……….. 85 Figure 4.15 EQCM measurements using Pt plating bath; (a), (b), and (c), as well as PtRu plating bath; (d), (e),

and (f). The (a) and (d) are the current profile during pulse deposition. The (b) and (e) are their respective mass variation in each pulse. The (c) and (f) are the voltage reading during plating time and open circuit voltage during resting time... 87 Figure 4.16 EQCM profiles in a single pulse; (a), (b), and (c) for Pt plating bath, and (d), (e), and (f) for PtRu

plating bath. The (a) and (d) are the current profile during pulse deposition. The (b) and (e) are their respective mass variation in each pulse. The (c) and (f) are the voltage reading during plating time and open circuit voltage during resting time... 88 Figure 4.17 Ratio of (a) the surface oxide mass change (for the sample in Pt bath) or (b) the displacement

mass change (for the sample in PtRu bath) to the electrodeposition mass change. These data were obtained for the first ten pulses from EQCM measurements in Fig. 4.15... 89 Figure 5.1 A schematic of the electrochemical cell for CV scans in 0.5 M H2SO4 aqueous solution. The

carbon cloth is partially exposed to ambient oxygen……….. 93 Figure 5.2 Profiles from multiple CV scans with ambient oxygen for electrodes containing carbon cloth,

XC-72R, and Nafion ionomer………. 95 Figure 5.3 Profiles from multiple CV scans (a) with ambient oxygen and (b) without ambient oxygen for

electrodes containing carbon cloth and Nafion ionomer. Also shown in (a) is the electrode with

carbon cloth only………. 97

Figure 5.4 Comparison in the current value obtained at 0.5 V from the 20th CV cycle for electrodes containing carbon cloth (CC), Nafion ionomer, CC/Nafion ionomer,and CC/XC-72R/Nafion ionomer. These CV experiments are performed with ambient oxygen and without ambient oxygen, respectively……… 98

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Figure 5.5 Raman spectra for electrodes after CV scans with ambient oxygen and H2SO4 immersion only.

These electrodes contain carbon cloth, XC-72R, and Nafion ionomer………... 100 Figure 5.6 XPS surveys for (a) as-prepared electrode, as well as electrodes after CV scans (b) without

ambient oxygen and (c) with ambient oxygen. These electrodes contain carbon cloth, XC-72R, and Nafion ionomer………. 100 Figure 5.7 (a) C1s XPS profiles for as-prepared electrode, as well as electrodes after CV scans without

ambient oxygen and with ambient oxygen. (b) Curve fitting for the C 1s XPS profile from electrode after CV scans with ambient oxygen. These electrodes contain carbon cloth, XC-72R, and Nafion ionomer………. 102 Figure 5.8 Ion chromatogram for Nafion ionomer degradation in 0.1 M HCl aqueous solution………. 104 Figure 5.9 Variation of sulfate concentration as a function of CV scans with ambient oxygen. The data at 0th

cycle is obtained from the electrode immersed in 0.1 M HCl aqueous solution……… 105 Figure 5.10 C1s XPS profiles for (a) as-prepared electrode (carbon cloth/XC-72R/Nafion ionomer), as well as

electrodes (carbon cloth/XC-72R) (b) before and (c) after immersion in HCl solution ontaining concentrated residues from Nafion ionomer decomposition……….. 107 Figure 5.11 S2p XPS profiles for (a) as-prepared electrode (carbon cloth/XC-72R/Nafion ionomer), as well as

electrodes (carbon cloth/XC-72R) (b) before and (c) after immersion in HCl solution containing concentrated residues from Nafion ionomer decomposition……….. 108 Figure 5.12 TEM images for deposited Pt nanoparticles on (a) functionalized and (b) baseline electrodes……. 110 Figure 5.13 ECSA profiles for functionalized and baseline lectrodes………... 111 Figure 5.14 CV profiles for functionalized and baseline electrodes on methanol electrooxidation in (a)

apparent current density, (b) mass activity, and (c) unit Pt electrochemical surface area………….. 114 Figure 5.15 Chronoamperograms for functionalized and baseline electrodes on methanol electro-oxidation at

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Chapter 1 Introduction

1.1 Background

Fuel cells are of considerable interests as alternative energy-generating systems for sustainable future with reduced emission. To render the fuel cells commercially viable, it is necessary to reduce system cost and operation life time simultaneously[1, 2]. Unfortunately, conventional electrocatalysts are primarily based on precious metals and they tend to aggregate or break off during cell operations. Therefore, one particular aspect to overcome these obstacles is to fabricate electrocatalysts in desirable core-shell structures with reduced particle size, and establish uniform dispersion and solid anchoring onto suitable carbonaceous supports. In a core-shell arrangement, the inexpensive element can constitute the core while the expensive one can reside on the surface instead. In such way, the electrocatalytic activity for the shell element remains intact but the catalyst cost is expected to be reduced substantially[3-5]. Alternatively, it is suggested that the carbon surface can be deliberately functionalized so the anchoring sites for depositing ions can be increased, leading to larger catalyst loading and stronger bonding between the carbon support and active metal[6]. It is surmised that interaction like this could relieve catalyst loss or aggregation.

Among many materials investigated for fuel cell applications, the development of bimetallic PtRu nanoparticles has attracted substantial attention recently because the PtRu is not only an effective electrocatalyst for methanol oxidation reaction in direct methanol fuel cells (DMFCs) but also demonstrates impressive CO oxidation ability for reformate hydrogen fuel cells[7, 8]. In DMFCs, methanol electro-oxidation entails consecutive removal of hydrogen that leaves a CO strongly bonded to the Pt, resulting in a gradual loss of catalytic activity known as CO poisoning. For the reformate hydrogen fuel cells, there is often minute presence of residual CO in the hydrogen feeds so it becomes a concern once the Pt is employed for hydrogen oxidation at the anode. To alleviate the CO poisoning effect, the Ru is purposely alloyed with Pt because the Ru can either

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provide the oxygenated species for CO oxidation to CO2 (known as bifunctional model) or alter the electronic structure of Pt so the CO-Pt bond is weakened significantly (known as ligand effect)[3, 9, 10].

To prepare PtRu nanoparticle, it is established that the electrochemical pulse electroplating method allows interrupted time for mass transport so better control over composition and morphology is possible over conventional galvanostatic or potentiostatic counterparts. For example, Tsai et al. (蔡春鴻教授) hasemployed the pulse deposition, with the addition of chemical additives, to prepare fine PtRu on carbon nanotube surface for enhanced catalytic actions[11]. In general, many electrochemical variables can be adjusted to attain desirable deposit properties (See Fig. 1.1 as follows). However, one of the drawbacks is that there are nucleation and growth occurring in each pulse so the deposits are known to reveal a wide size distribution of particle sizes. In addition, for PtRu the replenishment of individual cations depends on their respective concentrations and diffusion coefficients. As a result, this is likely to produce unnecessary variation in deposit composition in each pulse.

Figure 1.1 Possible operation modes for electrodeposition.

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elucidate the detailed arrangement for Pt and Ru in nanoparticulate forms[12, 13]. It is because with light source from NSRRC (National Synchrotron Radiation Research Center), any minute variation in the absorption coefficient can be diagnosed. For example, spectra of XANES (X-ray Absorption Near Edge Structure Spectroscopy) and EXAFS (Extended X-ray Absorption Fine Structure) are routinely used to determine the oxidation state, fractional d-electron density, atomic environment of the absorbing atom, as well as its short-range ordering and geometric arrangement. In this regard, with XAS, we can follow the formation mechanism of PtRu nanoparticles and analyze Pt and Ru for both deposit and solution states. Moreover, recent studies have adopted the in-situ XAS to characterize the surface rearrangement in PtRu nanoparticles during fuel cell operation (under polarizations) so better understanding over life time performance can be established[14, 15].

Another important factor affecting the performance of electrocatalysts is the catalyst supports. Amount many conductive materials, carbonaceous materials have been widely employed as the substrates for catalyst impregnations in room tempeaturare fuel cells[11, 16, 17]. It has been found that nanoparticulate PtRu are able to distribute uniformly, leading to reduced loading and better catalyst utilization[6]. Untreated carbon is usually hydrophobic that allows poor adsorption of catalyst precursors and active metals. After proper surface functionalizations to render a hydrophilic surface, the carbon is expected to adsorb more catalyst precursors for a larger amount of catalyst deposition. In literature, carbon functionalization involves anodization treatments in corrosive acids at moderate temperature. For example, Kangasniemi et al. imposed potentiostatic treatments on Vulcan XC72 (XC72) in 1 M H2SO4 solution, and observed a signficant oxidation for the anodizing voltage of 1.2 V for 16 h. The degree of surface functionalization also depends on the type of carbon because its surface area and microstructure differ considerably. After functionalization, surface oxidized groups such as phenols, carbonyls, carboxylic acids, ethers, quinones, and lactones have been identified17. The exact mechanism responsible for the formation of selective functional groups is contingent on the processing steps employed and the type of carbon.

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Another route to functionalize carbons is via the chemical changes of polymeric binders. In electrode fabrications, Nafion ionomer is often added in mixture with carbon, serving not only as a binder but also conductive channels for proton transports. Therefore, it is expected that the Nafion ionomer would suffer from structural alteration and loss of sulfonic acid side chains if deliberate electrochemical treatments are imposed. Previously, extensive efforts have been devoted to understand the responsible mechanism for Nafion membrane degradation in different environments and factors including humidity, temperature, and oxygen concentration are found to be critical[18, 19]. According to Bruijn et al., hydroxyl (‧OH) and peroxy (‧OOH) radicals formed during fuel cell operations are able to attack polymer end groups that still contain residual terminal H-groups[20]. Further studies also indicate that the sulfonic acid side groups are more susceptible to radical attacks than poly(tetrafluoroethylene) backbone[21]. The broken species of Nafion ionomer contain free radicals that attach to the carbon which catalyze further carbon oxidation[22]. Presence of functionalized groups has been established to catalyze additional oxidized groups. Therefore, we realize that the intentional degradation of Nafion ionomer provides a facile route for carbon support functionalization. Fig. 1.2 depicts a schematic showing the formation of functionalized groups and adsorption of selective cations.

Figure 1.2 A schematic demonstration for the functional group formation and physical adsorption of selective cations.

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An alternative route to manipulate the surface composition of PtRu is to take advantage of the displacement reaction. The displacement reaction is also known as redox-transmetallation reaction or spontaneous deposition, and it often occurs in multi-component systems with constituents revealing distinct values of redox potentials[23, 24]. In principle, when a binary deposit is in contact with their respective cations in electrolyte, the constituent of lower redox potential is dissolved from the deposit while the one with a higher redox potential is reduced from the electrolyte. Consequently, the deposit on the surface can be tailored for a desirable makeup which is different from that of bulk if the displacement reaction is carefully controlled. For PtRu, once the Ru is immersed in the electrolyte containing Pt cations, the Ru would undergo an oxidation reaction in conjunction with the reduction of Pt cations. Previously, Adzic et al. and Huang et al. (黃炳照教授) have adopted the displacement reaction to tailor core-shell nanoparticles with impressive results[12, 25].

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1.2 Motivation

In this study, I attempt to prepare desirable PtRu core-shell nanoparticles for improved catalytic behaviors. In addition, I explore the functionalization of catalyst support in order to obtain a better life time performance. To realize the core-shell structured PtRu nanoparticles, I adopt galvanostatic pulse deposition. The surface composition of PtRu nanoparticles is controlled via the displacement reaction. To prolong life time activity, I carry out functionalization treatment to anchor electrocatalysts on the carbon support. In Chapter 2, more literature reviews are provided so that readers can be acquainted with our work.

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Chapter 2 Literature Review

2.1 Anode electrocatalysts for direct methanol fuel cell

Fuel cells using hydrogen or methanol as fuels and oxygen or air as oxidants convert chemical energy in the fuels directly into useable electrical energy, and thus they are suitable as power sources for automobiles and portable electronics. In particular, using fuel cells in electric vehicles has been considered as a promising route to reduce energy consumption since they can generate electricity in a cleaner and more efficient way as compared to conventional gasoline-based engines. To date, there are many fuel cell systems competing for commercial viability. Among them, the direct methanol fuel cell (DMFC) has been studied extensively because of its simple fuel usage and feed strategies[26, 27] Comparing to hydrogen gas for other fuel cell systems, the DMFC uses methanol as the feedstock. Methanol is liquid and easy to be handled, stored, and transported. In addition, methanol is renewable and is of high energy density. Therefore using methanol as the fuel can simplify the entire fuel cell system.

So far, the most popular electrocatalyst for methanol electro-oxidation is Pt and its alloys. Methanol oxidation reaction (MOR) on the Pt metal involves several reaction steps including (1) methanol adsorption; (2) C-H bond activation (methanol dissociation); (3) water adsorption; (4) water activation; and (5) CO oxidation[28]. Previously, the detailed MOR mechanisms on the Pt have been investigated by in-situ Fourier transform infrared spectroscopy (FTIR) and electrochemical quartz crystal microbalance (EQCM)[29, 30], and they are discussed below at different potential regimes.

Electrochemical mechanism for the methanol oxidation reaction (MOR) on platinum (Pt) metal is composed of series of reaction steps including (1) methanol adsorption; (2) C-H bond activation (methanol dissociation); (3) water adsorption; (4) water activation; and (5) CO oxidation [28]. The detailed MOR mechanisms on Pt have been investigated by in situ Fourier transform infrared spectroscopy (FTIR) and electrochemical quartz crystal microbalance (EQCM)[29, 30] in different

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8 potential regions, explained as follows.

1) At potential region around 0.05V:

Methanol molecules dissociate and adsorb on the Pt surface by dehydrogenation and the subsequent formation of formyl (-CHO) and CO, as shown in Equation (2.1-2.2) and Figure 2.1.

4Pt + CH3OH  Pt-CHO + 3Pt-H (2.1)

Pt-CHO + Pt  Pt-CO + Pt-H (2.2) 2) At potential region around 0.05V-0.2V:

The intermediate CO at Pt sites and the adsorbed H2O molecules form formic acid (HCOOH)2, as shown in Equation (2.3) and Figure 2.1.

2Pt-CO + 2Pt-OH2  Pt2-(HCOOH)2 + 2Pt (2.3) 3) At potential region around 0.2V-0.6V:

(HCOOH)2 is oxidized via two pathways as shown in Equation (2.4-2.5) and Figure 2.1 Pt2-(HCOOH)2  Pt-(CO2) + Pt-COOH + 3H+ + 3e- (2.4)

Pt2-(HCOOH)2  2Pt-(CO2) + 4H+ + 4e- (2.5) 4) At high potential region (over 0.6V):

The adsorbed H2O molecules on the Pt surface form the surface oxide Pt-OH, and consequently, the CO can be oxidized to CO2 via the reaction between Pt-OH and Pt-CO, as shown in Equation (2.6-2.7) and Figure 2.1

Pt-OH2  Pt-OH + H+ + e- (2.6)

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Figure 2.1 Schematic illustration of methanol oxidation on Pt surface in different potential regions, studied by FTIR and attenuated total reflection (ATR)[30].

From the above discussion of MOR mechanism on the Pt surface, we note that the Pt surface can form sufficient bonding with the adsorbed CO. However, the CO is difficult to desorb once it occupies the Pt surface. Therefore one rational solution is to add second or even third elements into the system for the purpose of promoting the oxidation of Pt-CO to Pt-CO2. To suit this function, the PtRu catalyst is well known as a promising DMFC catalyst because it can easily remove CO from the Pt sites. The function of Ru can be explained by two mechanisms[31-43].

1) Bifunctional effect

In an aqueous solution, the H2O molecules can adsorb onto the Ru surface and form Ru-OH at low potentials. And the CO adsorbed on neighboring Pt sites can be oxidatively removed by the reaction between Ru-OH and Pt-CO. Alloying Ru with Pt is able to increase the CO tolerance. The detailed reaction mechanisms are shown in Figure 2.2.

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Figure 2.2 Schematic illustration of methanol oxidation on PtRu surface in different potential regions, studied by FTIR and ATR[44].

2) Ligand effect (electronic effect)

The second element added to the Pt-based catalysts will directly or indirectly modify the electronic structure of Pt, decrease the adsorption ability of CO on Pt, and consequently enhance the performance toward the MOR[45].

Usually the variables affecting the activities of PtRu electrocatalysts include particle size, the dispersion of nanoparticles on carbon supports, and etc. By further study of the atomic structure in bimetallic PtRu nanoparticles, it is found that different nanostructures such as core-shell structures or perfect alloying ones impose significant effects on the electrochemical behaviors of PtRu catalysts for the MOR[9, 37, 38, 42, 43, 46]. For example, Selim et al.[3] proposed that the in H2 atmosphere with 1000 ppm CO, the effective CO oxidation to CO2 can be catalyzed at 30 °C by

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Ru@Pt core -shell nanostructured catalyst, which is superior as compared to 85 °C by commercial PtRu alloy nanocatalyst, at 93 °C by monometallic mixtures of Pt and Ru nanoparticles, and at 170 °C by pure Pt nanoparticles. Figure 2.3 illustrates the structures of these nanoparticles.

Figure 2.3 illustration structures of PtRu alloy, Ru@Pt core-shell, monometallic nanoparticles[3].

Huang et al. (黃炳照教授)[9, 25, 47]. have also studied the structural impact of PtRu catalysts on the anodic MOR currents by X-ray absorption spectroscopy (XAS). They found out that the MOR performances were enhanced with the increased PRu values.

PRu = NRu–Pt/(NRu–Pt + NRu–Ru)

The PRu value represents the possibility of forming a heterostructure for Ru atoms with Pt atoms. The increased PRu suggests the increased NRu–Pt and the decreased NRu–Ru, and the more profound bifunctional effect of PtRu. And therefore the PtRu heterostructure can promote the performance toward MOR.

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2.2 Platinum monolayer core-shell electrocatalysts

Further decreasing the Pt amount and enhancing the Pt utilization remain to be one major goal for the development of noble-metal-based catalysts in recent years. The Pt monolayer electrocatalysts developed by R.R. Adzic’s group provide the ultra-low Pt content, the highest Pt utilization, and impressive activity[12, 48]. The Pt monolayer is prepared via a technique involving copper underpotential deposition (Cu UPD).

Cu UPD deposits one monolayer of copper on different electrically conductive substrates with higher reduction potentials than that of Cu. It entails the chemisorptions of copper atoms on the substrate at the underpotential with respect to the potential for Cu bulk deposition which occurs at a more positive potential than the thermodynamic potential for Cu deposition in the electrolytes. UPD is a surface-limited monolayer-deposition process and it can be performed with or without the coadsorption of anions in the electrolytes. Take the formation of Cu monolayer on Au (111) single crystal surface as an example[49, 50], there are two Cu UPD peaks on the Au (111) in CuSO4 electrolyte (Fig. 2.4). After the first UPD peak around 0.2 V (vs. SCE) (Fig. 2.4), the commensurate (√3×√3)R30° Cu adlayer occupying the 3-fold hollow sites on Au (111) is formed with the coadsorption of Cu2+ cations and SO42- anions, as shown in Fig. 2.5. In Fig. 2.5, the lowest layer is the Au (111) substrate, the middle layer is the Cu adlayer, and the top layer is the coadsorbed SO42- anion overlayer. With additional UPD after the second UPD twin-peak around 0.05 V (vs. SCE) (Fig. 2.4), the Cu monolayer, i.e. the Cu-(1×1) structure is formed on Au (111), as shown in Fig. 2.6. The coadsobed SO42- anions adopt an unchanged (√3×√3)R30° arrangement forming an overlayer on top of the two Cu adlayers with different arrangements. By controlling the potential, a complete Cu monolayer can be formed on the substrate.

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Figure 2.4 Cyclic voltammogram for the Au (111) single crystal electrode in 1 mM CuSO4 with 0.05 M H2SO4. The sweep rate was 1 mV/s [49].

Figure 2.5 Interfacial structure of the Cu UPD on Au (111) single crystal surface after the first UPD peak[49]: (a) top view (b) side view.

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Figure 2.6 The top view of the Cu-(1×1) monolayer on Au (111) surface after the second UPD peak[50].

In the study of Adzic’s group, the Cu monolayer adsorbed on different substrates is subsequently galvanically displaced with PtCl42- ions in order to place a Pt monolayer on the substrates. They have placed the Pt monolayer on different single crystal electrodes (Au, Pd, Ir, Ru and etc.) and studied the catalytic activities toward the oxygen reduction reaction (ORR), and their results are provided in Fig. 2.7. They have also developed a scale-up synthesis method based on Cu UPD to produce gram-quantity Pt monolayer catalysts[51]. The core-shell structure for the Pt monolayer on Pd nanoparticles, prepared by this scale-up Cu UPD synthesis method, is studied by the scanning transmission electron microscopy (STEM), high-angle annular dark field (HAADF) image, and the elemental line scan profile by the energy dispersive spectroscopy (EDS), as shown in Fig. 2.8. The Pt monolayer nanocatalysts supported on Pd cores exhibite enhanced and stable activities for the ORR at the cathode in fuel cells[52], as shown in Fig. 2.9.

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Figure 2.7 Polarization curves for the ORR on the Pt monolayer supported on different single crystal surfaces[53, 54].

Figure 2.8 STEM HAADF image (left) and the EDS line scan profile (right) indicating the core-shell structure of the Pt monolayer catalyst on Pd nanoparticles[52].

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Figure 2.9 The Pt mass activities for the ORR as a function of the potential cycle number n during fuel cell testing for the Pt monolayer catalyst on Pd nanoparticles (red), compared to those for commercial Pt/C catalysts (green and blue). The potential cycles were square waves with a 30s dwell time at 0.7 and 0.9 V each (vs. RHE) at 80 °C[52].

The Pt monolayer catalysts prepared by the Cu UPD have also been applied as the anode catalyst in fuel cells. Pt reveal reasonable activities for hydrogen oxidation reaction (HOR), but suffers from undesirable CO tolerance. The CO exists either as an inevitable impurity in the H2 gas feed produced by re-forming ethanol or hydrocarbons used as the fuel for the proton-exchange membrane H2/O2 fuel cells (PEMFC), or as the byproduct produced in the process of methanol oxidation reaction (MOR) for the direct methanol fuel cells (DMFC). The Pt monolayer catalysts on Ru nanoparticles exhibit higher CO tolerance for the HOR (Fig. 2.10) and more stable activity for MOR (Fig. 2.11). All these results in literatures suggest that the CO-tolerant ability for PtRu catalysts depends strongly on their geometrical structures and atomic distributions.

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Figure 2.10 The structure for the Pt submonolayer catalyst on Ru core (the inset) PtRu20 and its HOR behavior for the CO tolerance test, compared to that of a commercial Pt2Ru3 catalyst[4, 12, 55].

Figure 2.11 (a) Methanol-oxidation currents normalized by the total noble-metal mass, and (b) the chrono-potentiometric measurements at 0.69 V for the MOR, for the Pt monolayer catalyst on Ru nanoparticles, compared to that on the commercial PtRu catalyst[12].

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2.3 Carbon based materials

2.3.1 Applications in fuel cells

Carbon-based materials display sufficient electrical conductivity, chemically inert structure, large surface area, porosity, and most importantly, inexpensive material cost. As a result, carbon demonstrates potential applications in many electrochemical fields such as capacitors and fuel cells. Nanostructured carbon materials exhibit enhanced electrical conductivity and chemical stability, and thus become rather promising for applications in electrochemical fields. For example, carbon nanotubes (CNTs) decorated with metal nanoparticles as catalysts have improved the performance of fuel cells. One primary reason is the extraordinary electrical conductivity of CNTs that can reduce unnecessary energy dissipation. The other secondary reason is that the special two-dimensional hexagonal structure of CNTs improves the dispersion uniformity and thus the utilization and catalytic activities of metal nanocatalysts. As shown in Fig. 2.12, PtRu nanoparticles decorated on CNTs exhibit a higher activity for the MOR than the commercial PtRu catalyst.

Figure 2.12 (a) TEM image of PtRu nanoparticles decorated on CNTs (sample B02). (b) Cyclic voltammograms for the MOR of PtRu nanoparticles on CNTs (B02) and commercial PtRu catalyst produced by Johnson Matthey (J-M), in N2-saturated 0.5 M H2SO4 + 1 M CH3OH electrolytes, 20 mV/s[56].

(a)

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19 2.3.2 Surface functionalization

The catalytic activities for metal nanocatalysts supported on carbon-based materials are affected by the chemical properties of the carbon supports. First, most of the chemically-stable carbon structures are graphitic arrangements, in which carbon atoms are bound together by sp2 hybrid bonds. This ring-like structure reveals a significant hydrophobicity. However, oxygen-containing functional groups can be generated on the carbon surface by the interaction between the defective sites on the carbon surface with unsaturated bonding and water molecules in aqueous solutions. Such hydrophilic oxygen-containing functional groups on the carbon surface have strong adsorption tendency for metal ions in precursor solutions and thus enables more loading of metal catalysts on carbon supports.[57] In addition, spontaneous deposition of metal precursors on the carbon supports can occur because of the driving force from the reduction potential difference between carbon and metal ions[58, 59], as shown in Fig. 2.13 (a). Furthermore, the oxygen-containing functional groups (-OH, C=O and COOH) on the carbon supports can also promote MOR behaviors for Pt nanocatalysts[58], as shown in Fig. 2.12 (b).

Figure 2.13 (a) The equilibrium potential diagram showing the potential difference between carbon (CNTs) and metal ions (PtCl62-). (b) Polarization curves for the MOR on Pt nanocatalysts supported on CNTs with (line 1) and without (line 2) surface functionalization.[58]

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2.4 Nafion ionomer

In the fuel cell, the solid polymer membrane acts as the proton conductor between the two electrodes allowing the rapid proton transport. The most commonly used solid electrolyte is Nafion ionomer (Dupont).

As shown in Fig. 2.14, Nafion consists of a strongly hydrophobic polytetrafluoroethylene (PTFE) backbone, and a strongly hydrophilic side chain ending with the sulphonic acid (SO

3H). Indeed, the –SO

3H group is ionically bonded, which makes the end of the side chain to be actually – SO

3

-. Due to the high hydrophilicity of the side chain, the regions around it will become hydrated via the absorption of large quantities of water. Protons are first gathered within these hydrated regions, and then they are mobile along the side chain because they are only weakly attracted by – SO

3

-, and they are eventually transported by the Nafion ionomer. This is how the Nafion ionomer works as the proton conductor[18, 19, 60, 61].

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Chapter 3 Enhancement of Methanol Electro-oxidaiton Performances via

Core-Shell PtRu Nanoparticles Prepared by Pulse Current Deposition

3.1 Introduction

Development of clean and affordable energy has attracted considerable attention due to rising concerns over oil price and harmful CO2 emission. Among the possible systems under study, the direct methanol fuel cell (DMFC) is recognized as a promising power source for applications in portable electronics and transportations.[1, 2] Because electro-oxidation of methanol is intrinsically slow, many materials have been investigated as electrocatalysts at the anode. They include alloys in binary, tertiary, and quaternary compositions such as PtRu, PtCo, PtRuCo, and PtRuNiZr.[27, 62-65] So far, the PtRu has appeared as the leading candidate with superb1 ectrocatalytic performance. It is because by alloying with Ru, the undesirable Pt poisoning by CO could be largely reduced. Mechanisms including bifunctional effect and ligand model are proposed to explain the contributory role of Ru while alloying with Pt.[66, 67] Moreover, the catalytic behaviors of PtRu depend greatly on its surface composition. For example, Richarz et al. prepared the PtxRu1−x in various compositions and determined the Pt0.5Ru0.5 to possess the highest activity for methanol electro-oxidation.[32]

In practice, the PtRu is impregnated on appropriate carbon supports for an extended reaction interface. Conventional synthetic approaches for the PtRu-catalyzed electrodes entail techniques in chemical reduction and hydrogen annealing.[42, 68] These methods add substantial difficulties in controlling the locations and compositions of the resulting PtRu nanoparticles. In contrast, approaches involving electrochemical reductions are rather straightforward. Because the growths of PtRu nanoparticles are occurring selectively at the interface between electrode and electrolyte, the electrodeposition routes are recognized to produce electrodes with exceptional efficiencies in the catalyst utilization, albeit with moderate size distributions.[56, 69] Because the nuclei formation and growth are extremely sensitive to the overpotentials imposed, potentiostatic and galvanostatic

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depositions are known to produce distinct morphologies and compositions in the resulting PtRu nanoparticles. Between them, the galvanostatic deposition is suitable to prepare catalysts in a larger geometric area and better distributions, in addition to simpler operation setups.[38, 40, 70, 71]

In the galvanostatic depositions, the driving forces are imposed in manners of direct current or pulse current (pc). With a single variable in current density (Ja), the dc deposition is known to produce dendritic morphologies, because growths of the deposit are capped by the mass transport at a diffusion-limiting current.[72] In contrast, deposition in the pc mode allows independent adjustments of Ja, current on-time (Ton), and current off-time (Toff), offering more opportunities to obtain deposits with desirable attributes. As a result, many groups have employed pc depositions to fabricate electrocatalysts in Pt and PtRu.[38, 40, 70, 73-75] For example, Choi et al. reported notable advantages of pc deposition in particle sizes, adhesions, and uniform distributions.[73]

During the Toff in a pc deposition for binary alloys, differences in the redox potentials for the deposited metals often render a spontaneous galvanostatic displacement reaction in which the constituent of less positive redox potential dissolves from the deposit while the one with a higher redox potential is reduced from the electrolyte. A well-studied system is the CuNi alloy where detailed theoretic modeling and experimental results were discussed.[76-78] In this system, the Ni was alternately deposited and dissolved during Ton and Toff, while the Cu was deposited continuously. Hence, the ratio for the Ton/Toff played an important role in determining the resulting CuNi composition. So far, many groups have employed the displacement reaction to prepare substrates with unique surface layers.[23, 79-82] For instance, noble films of Au, Pd, and Pt were deposited on Ge substrates with reasonable adhesions.[79] In addition, Brankovic et al. have explored the spontaneous depositions of Pt on both singlecrystalline and nanoparticulate Ru surfaces.[34, 83, 84] In Ru single crystals, they believe the surface oxidations are responsible for the reduction of PtCl62− from the electrolyte. However, in the case of nanoparticles, partial dissolutions of Ru are likely to contribute to the PtCl62− reductions.

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To date, many groups have employed pc deposition to fabricate PtRu nanoparticle and characterize their electrochemical performances.[38, 70, 72, 74, 75] However, none of them discussed the possible influences of displacement reaction in determining the resulting PtRu compositions. In this work, we investigate relevant variables to identify the effect of displacement reaction by carrying out careful analysis on the compositions and associated methanol electro-oxidation behaviors.

3.2 Experimental

Commercial carbon (XC-72R) on carbon cloth

A carbon cloth (E-TEK) is used as the starting substrate for the growth of PtRu. Prior to the PC deposition process, the carbon cloth is coated with an ink dispersion in which 5.0 mg Nafion solution (5.0 wt %) and 8.0 mg carbon powders (Vulcan XC-72R) are mixed in 5.0 mL 99.5 wt % ethanol for 30 min. The ink dispersion is deposited repeatedly on a 2×2 cm2 carbon cloth which is kept at 80 ˚C atop a hotplate to evaporate the residual solvent. The weight of the coated electrode is 26.3 mg/cm2. Subsequently, an electrochemical conditioning step is conducted by imposing five voltammetric scans on the coated carbon cloth at potentials between -0.2 and +1.1 V (vs. Ag/AgCl) at a scan rate of 50 mV/s in an electrolyte of 0.5 M H2SO4. The purpose for this treatment is to homogenize the coated carbon cloth and expose a larger effective surface area.

PtRu pulse current electroplating

The plating bath for the electrodeposition is formulated by mixing 99.9 wt % RuCl3 (Sigma-Aldrich) and 97.0 wt % NaNO2 (Showa) in an aqueous solution at 100 ˚C for 1.0 h, followed by dissolution of 99.9 wt % H2PtCl6. Afterward, the solution is cooled to room temperature with the addition of 97.0 wt % H2SO4 (Showa) to increase the conductivity of the electrolyte. The resulting concentrations for the H2PtCl6, RuCl3, NaNO2, and H2SO4 are 0.005,

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0.005, 0.050, and 0.250 M, respectively. The solution is aged for several days to reach a steady state for the complex ions. In the PC depositions, rectangular pulses with independent parameters in Ton,

Toff, and Ja are explored. Three sets of experiments are designed to elucidate the effect of displacement reaction. First, Ton of 50 ms and Ja of 50 mA/cm2 are selected with the Toff varied between 100 and 600 ms. Second, Toff of 400 ms and Ja of 50 mA/cm2 are chosen with the Ton varied between 25 and 400 ms. Lastly, we maintain the Ton and Toff at 50 and 400 ms, but adjust the

Ja between 75 and 200 mA/cm2. Throughout our experiments, the total coulombic charge is kept at 8.0 C/cm2. Once the deposition is completed, the carbon cloth was removed and washed for subsequent methanol electro-oxidation characterizations.

Electrochemical analysis

The electrochemical measurements were conducted at 26°C in a three-electrode arrangement using an EG&G 263A. First, to evaluate the electrochemical surface area (ECSA), the PtRu-catalyzed carbon cloths were subjected to cyclic voltammetric (CV) scans in the voltage range of -0.2 and 0.9 V in H2SO4 at a scan rate of 50 mV/s. The ECSA was estimated by the integrated charge in the hydrogen desorption region.[85, 86] Next, for catalytic abilities on the methanol electro-oxidation, multiple CV sweeps were performed in a potential range of -0.2 and 0.9 V at a scan rate of 20 mV/s in 500 mL of 0.5M H2SO4 and 1.0 M CH3OH, The area for the working electrode was 1.0 cm2. The Ag/AgCl and Pt foil (10 cm2) were used as the reference and counter electrodes, respectively. The CV scan at the second cycle was used for comparison purposes.

Materials characterizations

For phase confirmation of the deposited PtRu nanoparticles, X-ray diffraction (XRD, Siemens D5000) with a Kα of 1.54 Å was employed. A transmission electron microscope (TEM, Philips

數據

Figure 1.2 A schematic demonstration for the functional group formation and physical adsorption of  selective cations
Figure 2.1 Schematic illustration of methanol oxidation on Pt surface in different potential regions,  studied by FTIR and attenuated total reflection (ATR)[30]
Figure  2.2  Schematic  illustration  of  methanol  oxidation  on  PtRu  surface  in  different  potential  regions, studied by FTIR and ATR[44]
Figure 2.5 Interfacial structure of the Cu UPD on Au (111) single crystal surface after the first UPD  peak[49]: (a) top view (b) side view
+7

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