Clay as a Dispersant in Catalyst Layer for Fuel Cells

摘要 本論文分為四個討論主軸(4-1~4-4)包含利用天然黏土分散碳黑、置備碳黑/黏土觸媒層分別用於燃料電池與鋅空氣燃料電池，與利用黏土置備奈米銀粒子。本論文提出幾何分散機制，用於分散不同幾何形狀之奈米粒子(球狀與片狀)。不需添加任何有機分散劑的情況下，片狀結構的黏土可以粉碎聚集的碳黑粒子並分散於水相(約50-70 nm)。均勻分散的碳黑/黏土溶液可進一步被應用於燃料電池觸媒層與鋅空氣電池觸媒層。此外，藉由幾何分散機制，可進一步分散奈米銀粒子於的片狀黏土表面。堆疊的黏土載體利用負表面電荷可捕獲與固定20-30nm奈米銀粒子。 (1) 本論文(4-1)中利用不同幾何形狀的奈米片狀黏土，將球狀碳黑的奈米顆粒分散在水中。以天然水性黏土做新型分散劑，可分散親油性碳黑於水相。利用黏土的片狀結構,分散球狀顆粒的碳黑，此法有別於有機界面活性劑分散碳黑。幾何分散系統利用兩種不同幾何形狀的奈米粒子,會互相抵銷原本的物性與幾何性質,進而減低各自聚集的能量。碳黑會被層狀的黏土所阻隔並因带電黏土的親水性質，將黏土與碳黑分散於水相。由於不需要添加有機分散劑，此分散系統的熱穩定性可達烈裂解點600 oC。 (2) 本論文中(4-2)利用不同幾何形狀的奈米片狀黏土，將球狀碳黑的奈米顆粒分散在水中。不需添加分散劑，黏土可有效分散碳黑於5wt% Nafion 共溶液。利用SEM觀察發現碳黑粒子約40-60 nm。利用含浸法製備鉑金觸媒粒子於上述碳黑黏土溶液中，利用TEM觀察發現碳黑粒子約4-5 nm。 利用電化學分析儀得知添加適量黏土可有效提升燃料電池效率。 (3) 本論文(4-3)設計一四層薄膜電極組應用於鋅空氣燃料電池。此四層薄膜電極組由空氣電極(陰極)、質子交換膜、鋅片電極(陽極)與鋼網所構成，電極組厚度僅達0.5cm。空氣電極為碳黑、黏土、PTFE高分子與觸媒所構成。研究結果指出此新型薄膜模組化鋅空氣電池可有效避免電解質漏液損害電極的現象並達到6 mW/cm2 (固定電流= 90 mA)電流密度。此外，藉由加入黏土分散劑於空氣電極，開路電壓與電池效能皆可被有效提升。結果更指出優異的碳黑分散程度與薄膜電極組設計有助於電池的發電與提升效能，因天然黏土可有效分散碳黑與觸媒在PTFE高分子中。於polarization測試中發現 開路電壓可達90% 的發電效率，此薄膜型鋅空氣電池可連續操作48小時不間斷。 (4) 本論文(4-4)將黏土(Na+-MMT)層間Na+電荷以Ag+代換(Ag+-MMT)，利用NaBH4或甲醇還原劑還原銀離子。藉由黏土片狀構造與黏土表面負電荷造成立體障礙，避免Ag粒子聚集的現象，達到均勻分散的效果。研究發現，FE-SEM可觀察到奈米銀粒子均勻被吸附於黏土表面，奈米銀粒子的粒徑約26 nm。最後，提出相同組成下，可控制不同粒徑的方法。在還原銀離子的過程中，利用氙燈光束照射反應溶液，所形成的奈米銀粒子之粒約10 nm。ABSTRACT In this dissertation, four parts (4-1~4-4) of studies involve the dispersion of carbon black (CB) particles by clay, and the preparation of CB/clay/Pt catalyst layer for fuel cell and the preparation of CB/clay/MnO2 electrode for zinc-air battery. This dissertation proposes a geometric heterogeneity factor that substantially enhances the dispersion of nanoparticles of different geometric shapes.(i.e., particle vs. platelet). Without using any organic dispersant, the platelet-like clays enable to finely disperse Carbon Black (CB) particles (50-70 nm diameter) in pulverized powder and also in the water suspension. Finely dispersion of the CB/clay paste was further used in catalyst layer for proton-exchange membrane fuel cells and zinc-air battery. Furthermore, through geometric heterogeneity factor, silver nanoparticles were finely dispersed on the negative charge surface of platelet-like clays. Stacked platelet clay provided a surface support in capturing and immobilizing silver nanoparticles (20-30 nm diameter) during the silver nitrate reduction. (1) In chapter 4-1, it proposes a method that substantially enhances the dispersion of nanoparticles of different geometric shapes. When pulverizing the mixture of spherical carbon black (CB) and platelet-like clay particles, an energy barrier is present in mitigating the individual particle aggregation. Without using any organic dispersant, the platelet-like clays enable to finely disperse CB particles in pulverized powder and also in the water suspension. The field emission scanning electronic microscopy (FE-SEM) surface analyses showed the fine dispersion of CB particles in average diameter of 50-70 nm on clay surface. Such a dispersion system has instant advantages of circumventing the use of thermally unstable organic dispersants and easily generating a highly stable CB dispersion up to 600 oC TGA decomposition temperature. (2) In chapter 4-2, this study reports a hybrid support that substantially improves the dispersion of nanoparticles of different geometric shapes. Without using any organic dispersant, the plate-like clays effectively disperse spherical carbon black (CB) particles in pulverized powder and 5 wt% Nafion co-solvent suspension. Fine dispersion of CB particles with an average diameter of 40-60 nm was achieved based on the SEM morphology analyses. A 20 wt% Pt catalyst was then prepared by the impregnation method on various weight ratios of CB/clay hybrids and TEM showed the fine dispersion of Pt nanoparticles with an average diameter of 4-5 nm. Polarization measurements verified that by applying an appropriate quantity of clay modules in preparing anode catalyst layer can indeed enhance the fuel cell performance. (3) In chapter 4-3, it investigates a 4-layer membrane electrode assembly (MEA) consisting of an air-electrode, proton exchange membrane, zinc platelet and steel supporter. The experimental results indicate that the 4-layer MEA for zinc-air fuel cells avoids electrolyte leakage from battery and achieves a power density of 6 mW/cm2 (at 90 mA). A series of air-electrode films are constructed of CB/PTFE hybrid or CB/clay/PTFE hybrid. The open current voltage (OCV) and current performance can be improved by adding clay to these membranes as clay can pulverize CB aggregation. In a polarization test, the OCV (E) performance reached approximately 90% based on standard potential (E0) and no electrolyte leaked over 48 hours. These experimental results indicate that good CB dispersion by clay significantly improves cell performance. (4) In chapter 4-4, Silver nanoparticles (Ag NPs) of high uniformity and widespread over a large dimension are generated by using a novel method. Through the difference in geometric shapes and hence the noncovalent bonding interaction, Ag NPs of spherical shape were finely dispersed on the negative charge surface of platelet-like clays. Stacked platelet clay provided a surface support in capturing and immobilizing Ag NPs during the silver nitrate reduction. High density of cationic exchange sites of the clay enabled to tightly immobilize and hence homogeneously disperse Ag NPs (Dn=26 nm) in narrow size distribution. During the reduction process, a photo-irradiation method of supplying full-wave light source was found to be effective for controlling the Ag-NPs growths. Unusually fine dispersion of Ag NPs in an average 10 nm (Dn) diameter has been achieved.ACKNOWEDGEMENT..........................................I 摘要....................................................II ABSTRACT................................................IV CHAPTER 1 Introduction...................................1 1-1 Dispersion of Carbon Black...........................1 1-2 Structure and Property of Type of Layered Silicate...3 1-3 Proton Exchange Membrane Fuel Cell (PEMFC) ..........3 1-4 Zn-air Fuel Cell.....................................5 1-5 Silver Nanoparticles Immobilized on Clay Surface.....6 CHAPTER 2 Scientific and Technical Literature Review.....8 2-1 Application, Structure and Property of Clay..........8 2-2 Carbon Black........................................12 2-3 Principle and Construction of PEMFC.................13 2-4 Principle and Construction of Zn–Air Battery.......19 CHAPTER 3 Experimental Section..........................21 3-1 Materials...........................................21 3-2 Experimental Procedures. ...........................21 CHAPTER 4 Results and Discussion........................28 4-1 Preparation of Well- Dispersing Carbon Black Using Inorganic Clay.........................................28 4-2 Clay as a Dispersion Agent in Anode Catalyst Layer for PEMFC..................................................37 4-3 Clay as a Dispersant in Catalyst Layer for Zinc–Air Fuel Cell..............................................43 4-4 Synthesis and Immobilization of Silver Nanoparticles on Clay Surface...........................................54 CHAPTER 5 Conclusions..................................62 REFERENES ..............................................66 VITA...................................................6