近百年來由於工業蓬勃發展及人口迅速成長，全球能源短缺問題日益加劇，氫經濟 (hydrogen economy) 一詞即在1970年為 J. O’M. Bockris 教授於美國通用汽車公司 (General Motors) 技術中心演講中所創，其主要以再生能源生產之氫燃料扮演世界能源供應主流為願景。氫燃料電池可將氫燃料中之化學能直接轉換為電能，是為近來倍受矚目的高效率綠色能源轉換技術；然而，成本抑減與材料穩定性是燃料電池目前所面臨的科學性及技術性之兩大挑戰，因此為了提供更多燃料電池於商業應用，廣泛精深的科學研究以發展新穎或改良之材料實屬高度必要。
本論文旨在開發低成本、高穩定性、環境友善之電極觸媒，係以電漿輔助原子層沉積技術 (Plasma-enhanced atomic layer deposition, PEALD) 製備氮化銅 (Cu3N) 奈米晶體為高性能異質結構之電極觸媒，提升燃料電池陰極氧還原反應 (Oxygen reduction reaction, ORR)。此外，並選擇具有高比表面積、優異導電性之商用碳黑 (Vulcan XC-72) 為觸媒載體，以降低電極成本。在電漿輔助原子層沉積製程中，六氟乙醯丙酮化銅 (copper (II) hexafluoroacetylacetonate, Cu(hfac)2) 與氨氣電漿 (NH3 plasma) 分別作為銅及氮元素之反應前驅物，以利可精確控制 Cu3N 尺寸，成長在 XC-72 表面。作為對照，Cu3N 奈米晶體亦直接成長於矽基板，再刮下來作為獨立的氧還原反應觸媒。X射線光電子能譜 (XPS) 分析證明電漿輔助原子層沉積技術能夠鍍製高純度一價銅 (Cu (I)) 氮化物之奈米晶體；X光繞射 (XRD) 圖譜證明沉積於矽基板之Cu3N奈米顆粒易沿著 [100] 優選晶向成長，而非 [111] 晶向；穿透式電子顯微鏡 (TEM) 觀測顯示電漿輔助原子層沉積製程初始階段可以忽略晶體成核孕育期；掃描式電子顯微鏡 (SEM) 影像顯示可精確控制 Cu3N 奈米顆粒尺寸並均勻鍍覆於 XC-72 之表面；原子層沉積之島狀成長機制造就碳黑表面上具有高度分散 Cu3N 奈米晶粒之形貌結構。
本研究以循環伏安法 (CV) 及線性掃描伏安法 (LSV) 於氧氣飽和之0.1 M氫氧化鉀 (KOH) 水溶液中量測 Cu3N 觸媒之電化學表現。原子層沉積系統循環數 (ALD cycle number) 能夠精確調控 Cu3N 奈米顆粒之尺度大小，進而影響其電極觸媒活性；氧還原反應極化曲線與循環伏安曲線皆顯示具有最佳大小 (約為 7.5 奈米) 的Cu3N奈米顆粒能夠提高此複合觸媒之氧還原反應活性。我們更進一步藉由 Koutecky-Levich 方程式計算電子轉移數 (electron transfer number)，以深入瞭解所有觸媒樣品的電子移轉過程，此結果顯示以二百次循環數製備之Cu3N200/C 擁有最佳的氧還原反應效率，並偏向四個電子的氧還原反應路徑。本研究在電漿輔助原子層沉積製程中，通入之氨氣電漿可將氮元素摻雜於原始的XC-72，因此氮摻雜 (N-doped) 的 XC-72 相較於原始 XC-72 在線性掃描伏安實驗中擁有較高的氧還原反應觸媒活性與較大的電子轉移數。此外，不同觸媒樣品的質量活性 (mass activity) 係藉由在 0.7 V (相對於可逆氫電極 (RHE) ) 的電流密度除以 Pt 或 Cu3N 實際重量後計算而得；雖然 Cu3N 重量百分比僅佔 Cu3N200/C 電極觸媒之 1.4 wt%，但其質量活性為 1707.0 mA mg-1，較商用 Pt/C電極觸媒 (白金重量百分比為20 wt%) 之質量活性 (264.7 mA mg-1) 高出 544 %。最後，Cu3N200/C 與 Pt/C 觸媒的加速衰退試驗 (accelerated degradation test) 係透過電位循環掃描 1,000 次，並分別記錄兩材料在試驗前後相對之氧還原反應極化曲線而得；Cu3N200/C 在電位 0.7 V (相對於可逆氫電極) 之質量活性於加速衰退試驗中降低 16.1 %，而 Pt/C 質量活性卻衰退 31.7 %，此結果顯示 Cu3N200/C 比 Pt/C 在鹼性環境有更優異之穩定性。
總結而言，本研究提出一種先進的乾式製程¬¬¬─電漿輔助原子層沉積技術─製備 Cu3N 奈米晶體，應用為非貴重金屬氧還原反應觸媒。其尺寸可調控且高度分散於普遍使用之觸媒載體─商用碳黑 XC-72。儘管單獨的 Cu3N 奈米晶粒或 XC-72 之觸媒活性不高，但兩者之複合結構展現出優異的氧還原反應活性，此活性亦受惠自碳黑於氨氣電漿中之氮摻雜。本研究係以兩百次原子層沉積循環數製備之 Cu3N200/C 電極觸媒在鹼性環境下的氧還原反應表現最為優異，此電極觸媒相較於典型商用 Pt/C 觸媒擁有相似的氧還原反應觸媒活性、顯著改善之質量活性、以及在電位上較佳之穩定性。Cu3N 電極觸媒優異的觸媒活性係歸功於 Cu3N 奈米顆粒與 XC-72 間之協同化學耦合效應，此複合觸媒在燃料電池領域具前瞻之應用潛能。

Global demand for energy has been rising inexorably in recent years by the reasons of industrial development and population growth. A term “hydrogen economy” was first coined in 1970 form a talk given by Prof. John O’M. Bockris at General Motors (GM) Technical Center. The hydrogen economy is a scenario where the world uses hydrogen fuel generated from renewable energy. As an efficient and environmentally friendly approach, hydrogen fuel cells have attracted increasing attention to convert available chemical energy from hydrogen fuel into electricity. However, the key scientific and technical challenges fuel cells encounter are cost reduction and material durability. Thus, in order to provide more commercially available fuel cells, extensive and intensive researches for developing novel or improved materials are highly required.
This thesis aims to develop electrocatalysts with low cost, high stability, and environmental benignity. A heterostructured electrocatalyst comprising Cu3N nanocrystals was fabricated by plasma-enhanced atomic layer deposition (PEALD) as a high-performance catalyst for enhanced oxygen reduction reaction (ORR). Herein, the commercial XC-72 carbon black, which possesses high specific surface area and excellent electrical conductivity, was chosen as the catalyst support to reduce the electrode cost. In the PEALD process, copper (II) hexafluoroacetylacetonate (Cu(hfac)2) and NH¬¬3 plasma were used as precursors of copper and nitrogen, respectively, to grow precisely size-controllable Cu3N on the surface of XC-72. For comparison, the Cu3N nanocrystals were also directly grown on the silicon substrate and then scrapped off as a free standing nanocatalyst for ORR. The XPS analysis indicates that highly pure nanocrystals of Cu (I) nitride could be deposited by PEALD. The XRD pattern evidences that as-deposited Cu3N nanocrystals on silicon substrate prefer to grown along the [100] rather than the [111] orientation. The TEM observation suggests that incubation period is negligible at the initial stage of PEALD process. The SEM micrographs show that Cu3N nanoparticles with precise control of size can be uniformly deposited on the XC-72. The island growth mechanism during ALD contributed to the configuration of well dispersed Cu3N nanoparticles on the surface of XC-72.
The electrochemical performance of Cu3N/C samples were measured by three-electrode cyclic voltammetry (CV) and linear sweep voltammetry (LSV) experiments in O2-saturated 0.1 M KOH. The electrocatalytic activity of Cu3N was affected by its dimension which can be precisely tailored by the cycle number of ALD. Both results of ORR polarization curves and CV curves suggest that an optimal size, approximately 7.5 nm, of Cu3N nanoparticles on XC-72 could improve the ORR activity of the Cu3N/C catalyst. The electron transfer numbers (n) for all samples are calculated by Koutecky-Levich equation to gain a further insight into the electron transfer process of the catalysts. The result reveals that Cu3N200/C prepared by 200 ALD cycles exhibits the highest ORR efficiency, and favors the 4e- reaction pathway. In this study, it can be considered that nitrogen was doped into the pristine XC-72 during the NH3 activated plasma pulse in the PEALD process. From the LSV experiment, it shows that the N-doped XC-72 exhibits higher ORR catalytic activity and larger n value than the pristine XC-72. Besides, the mass activities (MAs) for all samples are calculated by normalizing the current density (mA cm-2) at 0.7 V (vs RHE) to the actual weight of Pt or Cu3N. The Cu3N200/C electrocatalyst contains only 1.4 wt% Cu3N, but has a MA of 1707.0 mA mg-1 that is 544 % higher than that of commercial Pt/C electrocatalyst with 20 wt% Pt loading, 264.7 mA mg-1. The accelerated degradation test (ADT) was conducted by 1,000-cycle potential cycling, and the corresponding ORR polarization curves before and after ADT of Cu3N200/C and Pt/C catalyst were recorded, respectively. The MA at 0.7 V (vs RHE) of Cu3N200/C is reduced by 16.1 % after the ADT, whereas the MA degradation of Pt/C is approximately 31.7 %. This result suggests that Cu¬3N/C offers greater durability than Pt/C under alkaline condition.
In conclusion, here we report an advanced dry process, PEALD, to fabricate Cu3N nanocrystals as a non-noble metal ORR catalyst. Highly size-controllable and well-dispersed Cu¬3N nanoparticles were decorated on a widely used catalyst support, Vulcan XC-72 carbon black. Although Cu3N or XC-72 alone exhibited poor catalytic activity, their hybrid presented a high ORR activity which was further benefited by nitrogen doping into the carbon black. The Cu3N200/C electrocatlayst prepared with 200 ALD cycles exhibited the highest performance with similar catalytic activity, significantly improved mass activity, and potentially greater stability compared to a typical Pt/C catalyst in alkaline solution. The high electrocatalytic performance arising from the synergistic chemical coupling effect between Cu3N and XC-72 renders this nanocomposite promising application in fuel cells.

摘要
Abstract
誌謝
Chapter I Introduction 1
1.1. Hydrogen Energy 4
1.2. Motivation of the Research 5
1.3. Framework of the Research 7
Chapter II Literature Review 9
2.1. Basic Properties of Copper Nitride and Vulcan XC-72 9
2.1.1. Cu3N 9
2.1.2. Vulcan XC-72 11
2.2. Atomic Layer Deposition (ALD) 18
2.2.1. Principle of ALD 19
2.2.2. Plasma-Enhanced Atomic Layer Deposition (PEALD) 21
2.2.3. Fabrication of Cu3N by ALD 26
2.3. Fuel Cells 33
2.3.1. Basic Principle of Fuel Cell Operation 34
2.3.2. Mechanism of Oxygen Reduction Reaction (ORR) 38
2.4. Modification of Electrocatalyst for ORR 43
 
Chapter III Experimental Procedures 53
3.1. Preparation of Cu3N Nanocrystals by PEALD 53
3.2. Electrochemical Characterization 55
3.3. Analytical Instruments 56
Chapter IV Results and Discussion 59
4.1. Characterization of Hybrid Electrocatalysts 59
4.2. Electrochemical Performance 74
4.2.1. Oxygen Reduction Reaction 74
4.2.2. Electron Transfer Number 75
4.2.3. N-doped XC-72 82
4.2.4. Mass Activity (MA) 87
4.2.5. Accelerated Degradation Test (ADT) 88
Chapter V Conclusions 92
Chapter VI Suggested Future Work 93
References 95

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