利用物理氣相沉積技術與超音波震盪技術添加可溼性金屬氧化物與金屬顆粒於陽極觸媒層中對於質子交換膜燃料電池效能的影響

鑒於國內外各單位對於燃料電池近幾年來的努力，目前已成功的解決許多技術及本質上的問題，然而距離商品化的目標仍面臨許多的挑戰，其中燃料電池陰陽兩極的水管理與CO毒化的問題為決定質子交換膜燃料電池發電效率及穩定性的重要因素。 為提升質子交換膜燃料電池於低溼度下的效能，大部分的文獻皆著重於改良Nafion&reg; 膜的可溼性。主要的改良方式為利用親水性過度金屬氧化物(Transition metal oxide)作為水分子吸附劑添加於Nafion&reg; 溶液中以製備高可溼性的Nafion&reg; 膜。而且，只有少數文獻提及改善觸媒層的可溼性及提升燃料電池於低濕度環境下效能的影響。本論文為水分子吸附劑添加之複合觸媒層在低溼度環境下提升膜電極的可溼性研究。首先將金屬親水性氧化鋅水分子吸附劑利用超音波震盪技術添加於陽極觸媒層中提高觸媒層的可溼性並提升質子交換膜燃料電池於低濕度下的效能。接著利用直流物理氣象沉積法將鈦與鈦氧化物濺鍍於陽極觸媒層上以減緩在相同增溼能力下鈦氧化物的沉積量，避免由於電阻增加而導至效能下降。同時也研究金屬水分子吸附劑的可行性。最後利用直流物理氣象沉積法將鈦釩鉻濺鍍於陽極觸媒層上探討偏壓對於可溼性的影響。根據實驗的結果，對於使用金屬氧化物水分子吸附劑，效能改善的程度主要為觸媒層可溼性與電阻的增加兩大關鍵因素競爭下的結果。而使用金屬水分子吸附劑，效能改善的程度主要為觸媒層可溼性與水氾濫兩大關鍵因素競爭下的結果。整體而言添加水分子吸附劑於陽極觸媒層中確實可以提升質子交換膜燃料電池於低溼度下的效能，並可以避免因為添加水分子吸附劑於Nafion&reg; 膜中對於膜的機械性質的影響。An adequate water management system to avoid the drying and flooding phenomena of the membrane electrode assembly (MEA) and an effective CO-tolerant catalyst are still the two main challenges needed to be overcome. Since the CO-poisoning phenomenon is induced by the low operation temperature (<100℃) of PEMFC limited by inappropriate water management, a well-established adequate water management system could solve these two challenges simultaneously. This study aims to investigate the feasibility of fabricating composite anode catalyst layer to increase the wettability of MEA at low humidity condition and then improve the performance of PEMFC. For fabricating composite anode catalyst layer, commercial and homemade ZnO hygroscopic particles were firstly added into the anode catalyst layer by ultrasonic technique. Secondly, island-like TiOx nano-particles were deposited on the surface of anode catalyst layer by direct sputtering for easing the negative effect caused by the inherent high electrical resistance of the hygroscopic metal oxide particles, by reducing the amount of hygroscopic metal oxide particles addition with same wettability improvement. Finally, Ti and Ti-V-Cr alloy were used as water adsorbent to be deposited on the surface of anode catalyst layer by direct sputtering for solving the dilemma caused by the inherent high electrical resistance of the hygroscopic metal oxide particles. To sum up, among all the specimens in which ZnO particles were added to the anode catalyst layer, the MEA with 10% ZnO particles addition exhibits the highest current density at different anode humidifier temperatures ranging from 25 to 65℃. Furthermore, the MEAs with anode sputtered by Ti all revealed better performance improvement than that sputtered with TiOx at low humidifying temperature (25, 45℃) even the TiOx-supttered anode showed better wettability than that of Ti-sputtered. At anode humidifier temperature 25℃ and 45℃, the highest improvement of Ti-V-Cr-sputtered MEAs with 100V bias were 35% and 26%, which are higher than the MEAs added with ZnO, sputtered with Ti and sputtered with TiOx. For the MEAs with transition metal oxide water adsorbent (ZnO and TiOx) at anode, the cell performance is determined by a competition mechanism between wettability and the variation of electrical resistance caused by transition metal oxide water adsorbent addition. Furthermore, for the MEAs with metal adsorbent, the cell performance was mainly determined by a competition mechanism between the positive effect arose from the enhancement of wettability of anodic catalyst layer and the negative effect of flooding induced by the excess hygroscopic metal (Ti and Ti-V-Cr).Abstract (Chinese) I Abstract (English) II Contents IV List of tables VI List of figures VII Chapter 1 Introduction 1 1-1 Development Background of Fuel Cells 1 1-2 Classification of Fuel Cells 3 1-3 Principle and construction of PEMFC 5 1-3-1 Structure of PEMFC 5 1-3-2 Bipolar plate 7 1-3-3 Catalyst layer 7 1-3-4 Gas diffusion layer 8 1-3-5 Proton exchange membrane 9 1-4 Polarization and over-potential 11 1-4-1 Activation polarization 11 1-4-2 Ohmic polarization 11 1-4-3 Concentration polarization 12 1-5 Water management 13 1-5-1 Migration of water molecular in PEMFC 14 1-5-2 Literature review of water management in PEMFC 15 1-6 Motivation and objective 18 Chapter 2 Experimental 20 2-1 Preparation of hygroscopic ZnO particles 20 2-2 Preparation of composite anode of PEMFC 21 2-2-1 ZnO-added composite anode 21 2-2-2 TiOx-deposited composite anode 21 2-2-3 TiO-deposited composite anode 22 2-2-4 Ti-V-Cr-deposited composite anode 23 2-3 Preparation of membrane electrode assembly 25 2-4 Analytical instruments 25 2-4-1 Field emission scanning electrode microscope 25 2-4-2 Transmission electrode microscope 26 2-4-3 Energy Dispersive X-ray Analysis 27 2-4-4 X-ray diffraction 28 2-4-5 Inductively coupled plasma mass spectrometry 28 2-4-6 Cyclic Voltammetry 29 2-4-7 Fuel cell polarization test 30 Chapter 3 Results and discussion 3-1 Improvement of the PEMFC performance at low-humidity conditions by adding hygroscopic ZnO particles into the catalyst layer 32 3-1-1 Characterization of ZnO particles and Pt/C-ZnO catalyst ink 32 3-1-2 Water contact angle of the catalyst layer 47 3-1-3 Single cell polarization test 52 3-2 Effect on PEMFC performance by coating Ti and TiOx on the anodic catalyst layer 61 3-2-1 Characterization of Ti and TiOx coated silicon wafer 61 3-2-2 Characterization of Ti and TiOx coated carbon cloth and anodic catalyst layer 67 3-2-3 Water contact angle of catalyst layer 73 3-2-4 Single cell polarization test 77 3-3 Sputtering Ti-V-Cr high-entropy alloy on the anodic catalyst for the improvement of PEMFC performance under low humidity condition 86 3-3-1 Characterization of Ti-V-Cr sputtered silicon wafer 86 3-3-2 Characterization of Ti-V-Cr sputtered carbon cloth and anodic catalyst layer 93 3-3-3 Water contact angle of Ti-V-Cr sputtered commercial carbon cloth and anodic catalyst layer 100 3-3-4 Single cell polarization test 109 Chapter 4 Conclusion 119 Reference 12

Performance Improvement in PEMFC by Adding Hydrophilic γ-alumina Particles to the Anode Catalyst Layer

近十年來，質子交換膜燃料電池(PEMFC)由於具有高能量密度、高能量轉換效率、操作簡易及零污染等優點，因此被視為最有可能取代現有的化石燃料，作為未來運輸設備、家用設備及可攜式設備的能源供應型態之一。儘管國內外各單位對於燃料電池的投入，使燃料電池得以快速地發展，但膜電極內部的水管理依然需要進一步地改善。 一般而言，水分子於膜電極(MEA)內遷移的驅動力主要為電遷移(electro-osmotic drag)與反擴散(back diffusion)，在理想操作狀態下，反擴散與電遷移兩者的合併效應使得水的淨輸送量接近零，然而在實際操作時，兩者往往呈現不平衡的狀態，尤其在高電流密度時電遷移現象更為強烈。此不平衡現象，會使陽極側的質子交換膜由於失水過多而脫水(de-watering)並累積水於流道、氣體擴散層與觸媒層造成陰極產生水氾濫(flooding)現象，進而降低PEMFC的發電效能。 本研究著重於改善陽極於低溼度時因膜脫水造成的效能下降。γ－氧化鋁由於其表面具有路易士酸基(Lewis acid sites)，可以吸附水分子的OH-基，因此可作為水分子吸附劑添加於陽極觸媒層中，以維持陽極的溼度。兩種不同比表面積的γ－氧化鋁皆經由溶膠-凝膠法合成，其比表面積分別為152、442 m2/g。利用超音波震盪及印刷技術，製備添加γ－氧化鋁的陽極。由接觸角量測的結果可以發現，隨著γ－氧化鋁添加量的增加，接觸角由136˚開始明顯地下降。從單電池測試的結果可知，添加10wt%的γ－氧化鋁於陽極觸媒層中，於不同的陽極增溼溫度(25℃、35℃、45℃、55℃)下，確實可以有效的提升低溼度條件下的發電效率；然而過量的γ－氧化鋁添加會造成內電阻的提升以及產生陽極水氾濫的現象，進而降低燃料電池的發電效率。In the past decade, proton exchange membrane fuel cells (PEMFC) have been regarded as a candidate for future power sources for transport, residential and portable applications, primarily due to the advantageous characteristics of high power density, high energy-conversion, simplicity of operation and near-zero pollutant emission. Although many problems of FEMFC have been solved, the management of water molecules inside the membrane electrode assembly (MEA) is still need to be farther improved. Typically, the main driving forces for transporting the water molecules inside the MEA are back diffusion and electro-osmotic drag. Theoretically, the effect of electro-osmotic drag and back diffusion should reach a balance, but the effect of electro-osmotic drag is stronger than the back diffusion in practice, especially at high current density. This unbalance tends to dry out the anode membrane and water molecules accumulate inside the flow channels, the gas-diffusion layer or the catalyst layer, resulting in flooding at the cathode of the membrane, and thus deteriorate the PEMFC performance. Therefore, management of the water content in MEA is recognized as a key requirement for PEMFC In this research, γ-alumina is used as a water-absorbent and was added into the anode catalyst layer of MEA, in order to raise the wettability and cell performance at low-humidity condition. Because of the Lewis acid sites on the surface, the hydroxyl groups of water molecules are attracted to the anode to maintain the humidity. The γ-aluminas were synthesized by a sol-gel procedure with BET surface areas of 152、442 m2/g. Anodes with γ-alumina addition were prepared by an ultrasonic and screen-printing technique. The water contact angle (WCA) of the anodes decreases with increasing γ-alumina addition. As can be seen in the results of single cell test, appropriate γ-alumina addition (10wt %) into anode catalyst layer can efficiently enhance the cell performance at low-humidity condition. In contrast, too much γ-alumina addition could increase the inner electrical resistance and cause flooding at anode thus degrading the cell performance.第一章 序論 1-1 前言-----------------------------------------------------------------------1 1-2 燃料電池簡介-----------------------------------------------------------1 1-2-1 燃料電池的發展史-------------------------------------------6 1-2-2 燃料電池的優點與發展-------------------------------------6 1-3 質子交換膜燃料電池(PEMFC)簡介-------------------------------8 1-3-1 質子交換模燃料電池(PEMFC)的構造-----------------10 1-3-2 雙極板--------------------------------------------------------11 1-3-3 膜電極組-----------------------------------------------------12 1-3-4 膜電極組的類型與製備方式-----------------------------17 1-4 研究動機與目的------------------------------------------------------ 20 第二章 文獻回顧 2-1 氧化鋁------------------------------------------------------------------ 21 2-1-1 γ－氧化鋁的發展-------------------------------------------21 2-1-2 溶膠－凝膠法-----------------------------------------------22 2-1-3 界面活性劑--------------------------------------------------24 2-1-4 熱處理--------------------------------------------------------25 2-2 碳黑的分散------------------------------------------------------------26 2-3 水管理------------------------------------------------------------------28 2-3-1 水分子的遷移方式-----------------------------------------28 2-3-2 脫水-----------------------------------------------------------30 2-4 電極反應動力學------------------------------------------------------31 2-4-1 極化-----------------------------------------------------------31 2-4-2 活性過電壓--------------------------------------------------33 2-4-3 歐姆過電壓--------------------------------------------------33 2-4-4 濃度過電壓--------------------------------------------------33 2-4-5 效率-----------------------------------------------------------34 第三章 實驗方法及步驟 3-1 藥品與材料------------------------------------------------------------35 3-2 儀器設備---------------------------------------------------------------36 3-3 實驗步驟---------------------------------------------------------------38 3-3-1 γ－氧化鋁的製備------------------------------------------38 3-3-2 Pt/C.B.觸媒的製備-----------------------------------------39 3-3-3 Nafion的前處理--------------------------------------------40 3-3-4 膜電極的製備-----------------------------------------------40 3-4 實驗分析儀器---------------------------------------------------------42 第四章 結果與討論 4-1 γ－氧化鋁的特性分析-----------------------------------------------44 4-1-1 X光光譜分析-----------------------------------------------44 4-1-2 熱重分析-----------------------------------------------------46 4-1-3 BET表面積及孔徑分佈分析-----------------------------47 4-1-4 掃描式電子顯微鏡及質譜儀分析-----------------------51 4-2 碳黑(C.B.)的分散-----------------------------------------------------56 4-3 觸媒漿料的特性分析------------------------------------------------63 4-4 接觸角測量分析------------------------------------------------------68 4-5 單電池測試------------------------------------------------------------72 第五章 結論----------------------------------------------------------------------77 參考文獻----------------------------------------------------------------------------78 附錄----------------------------------------------------------------------------------8

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