質子交換膜燃料電池用之新型氣體擴散層與微孔層材料之開發及其性質研究

本研究致力於開發質子交換膜燃料電池之核心元件—氣體擴散層(GDL)。目標為改善質子交換膜燃料電池(PEMFC)於高溫或乾燥環境下，因缺水而造成膜電極組內「水管理」失衡之情形。因此著力開發新型氣體擴散層或氣體擴散層之微孔層材料以改善燃料電池於上述環境下因缺水而導致效能低下的情形。研究內容分成以下兩部分：第一部分: 石墨烯添加之單層氣體擴散層氣體擴散層為攸關質子交換膜燃料電池效能表現重要元件之一。然而，以傳統方式所製備的氣體擴散層，其過程複雜且所需成本高。因此，本實驗室在先前研究中，採用較簡單和低成本的製備方法研發出單層氣體擴散層(SL-GDL)，並應用於質子交換膜燃料電池中，其效率可達市售氣體擴散層的85 %。而本研究則透過石墨烯的添加及均勻分散在SL-GDL中，製備一系列石墨烯添加之單層氣體擴散層(SL-GDL-Gx(x = 1-3))，以改善原SL-GDL物理特性，並探討此單層氣體擴散層應用在質子交換膜燃料電池對效率的改善。結果顯示，石墨烯的添加造成SL-GDL-Gxs表面粗糙度的增加與不規則細長縫隙的形成，導致氣體滲透率的提高。同時，由其表面形貌可知其仍保持類微孔層的微結構，讓觸媒能有良好乘載及有效利用。孔洞結構的改變，在高溫與低濕環境中電池的水管理能有助益。此外，高導電度及機械性質的石墨烯添加，明顯降低SL-GDL-Gxs的電阻率，機械性質也得到改善。上述特性的改善，對電池效能有明顯的助益。由單電池效能測試結果顯示，在80 oC及相對濕度99.9 % 的測試條件下，以SL-GDL-G2為GDL所組成的FC-2展現最佳的效能，功率密度達527.4 mW/cm2，比FC-0 (無添加石墨烯的氣體擴散層SL-GDL-G0)及FC-3 (市售氣體擴散層SGL 35BC)分別高出46 %和15 %。此外，在50-80 oC及相對濕度15 %的測試條件下，FC-2效能表現皆高於以市售氣體擴散層SGL 35BC為GDL所組成的FC-3。結果顯示，本研究所製備有成本效益的SL-GDL-G2在質子交換膜燃料電池中是一具有實用潛力的氣體擴散層。第二部分: 新型核-殼二氧化矽@碳奈米纖維之製備及其作為氣體擴散層之微孔層材料本部分研究係採用同軸靜電紡絲技術，搭配最佳化的紡絲參數與後續之熱處理程序，製備一新型核-殼結構之二氧化矽@碳奈米纖維(SiO2@C)。此奈米纖維材料的鑑定包含掃描式及穿透式電子顯微鏡、X光繞射測量、電子導電度測試、拉伸測試、熱重分析、氮氣恆溫吸脫附分析和保水率(Water Uptake)。由結果顯示，核-殼纖維之吸濕性中孔洞二氧化矽存在於核之位置，而疏水性之電子導電碳存在殼之位置並具有多孔通道。與所製備之碳奈米纖維相較，核-殼結構之SiO2@C纖維的BET表面積、孔體積、電子導電度、機械強度和保水率皆具較高的數值。這些優異的性能使此SiO2@C纖維有利於燃料電池中之水管理能力，是質子交換膜燃料電池(PEMFC)中微孔層(MPL)的潛力材料。在相對濕度99.9 %條件下，以核-殼結構之二氧化矽@碳奈米纖維為微孔層材料之單電池FC-SiO2@C，在溫度範圍50-80 oC之功率密度均明顯高於以碳奈米纖維為微孔層材料之單電池FC-C，分別高出57 ~ 116 %，更比傳統疏水性碳黑粉末微孔層材料所組成的FC-V高出66 ~ 268 %。而另在相對濕度(RH) 15 %之環境下，FC-SiO2@C之功率密度在溫度範圍50-80 oC亦高出67 ~ 71 %，且比FC-V高出73 ~ 302 %。本研究結果顯示，此核-殼SiO2@C奈米纖維可應用在質子交換膜燃料電池之微孔層中，是一有實用前景的材料。

關鍵字

質子交換膜燃料電池；氣體擴散層；碳奈米纖維；微孔層

並列摘要

Part I. Graphene-contained Single-Layer Gas Diffusion Layer Gas diffusion layer (GDL) is an important part relating to the efficiency of proton exchange membrane fuel cells (PEMFCs). Nevertheless, the preparation cost of the conventional GDL is high. In our previous studies, a single-layer gas diffusion layer (SL-GDL) prepared with a simple and cost-effective process has been used in PEMFC, and reached 85 % efficiency of a commonly used commercial GDL. In this work, the physical properties improvement of a series of single-layer gas diffusion layers, SL-GDL-Gx (x = 1-3), by the good distribution of graphene in the SL-GDL, and the application on PEMFC are studied. The results indicate that the presence of good-distributed graphene in SL-GDL-Gxs causes an increase in surface roughness and the formation of irregular slender interstices, leading to the enhancement of gas permeability, while maintains the microporous layer (MPL)-like microstructure, retaining good loading and efficient utilization of the catalyst. The change of the microstructure of pores benefits the water management of PEMFC. Besides, the electrical resistivities are obviously decreased and the mechanical properties are improved. These improvements in physical properties are significantly beneficial to the PEMFC performance. The single cell performance tests show that the best performance measured at 80 oC under 99.9 % relative humidity (RH) is obtained from the PEMFC (FC-2) fabricated with SL-GDL-G2 and is 46 % higher than that from FC-0 with SL-GDL-G0 without graphene and 15 % higher than that from FC-3 with the commercial GDL. Besides, the performances of FC-2 measured in 50-80 oC under 15 % RH are all much higher than that of FC-3. The results indicate that SL-GDL-G2 prepared cost-effectively is a potential GDL for PEMFC. Part II. Fabrication of Novel Core-shell Silica@Carbon Nanofiber and usage in the Microporous Layer for Gas Diffusion Layer In this study, a novel core-shell silica@carbon nanofiber (SiO2@C) is successfully prepared via coaxial electrospinning technique with optimized parameters followed by heat treatment. The characterizations of the nanofiber are carried out by a combination of scanning electron microscope, transmission electron microscope, X-ray diffraction measurement, electrical conductivity test, tensile test, thermogravimetric analysis, analysis of nitrogen adsorption-desorption isotherm, mechanical strength test, and water uptake measurement. It is found that the hygroscopic mesoporous SiO2 is contained in a core and the hydrophobic electron-conductive carbon is in a shell that has porous channels. The BET surface area, pore volume, electrical conductivity, mechanical strength, and water uptake of SiO2@C are all superior to that of pure carbon nanofiber. These superior properties make SiO2@C a potential microporous layer (MPL) material, benefitting the water management ability of proton-exchange membrane fuel cells (PEMFCs). The result of the single-cell performance tests shows that regardless of relative humidity (RH) under 99.9 % or 15 %, the PEMFC fabricated with the SiO2@C-based MPL (FC- SiO2@C) demonstrates the highest performance. In the temperature range of 50–80 °C, under 99.9 % RH, the power densities of the PEMFC fabricated with the SiO2@C-based MPL (FC-SiO2@C) are all significantly higher than that of the pure carbon nanofiber-based MPL (FC-C) (57-116 %), and are 66~568 % higher than that of the traditional hydrophobic carbon black powder-based MPL (FC-V). As the relative humidity is adjusted to 15 % while the temperature range keeps the same, the power densities of the PEMFC fabricated with the SiO2@C-based MPL (FC- SiO2@C) are still all significantly higher than that of the pure carbon nanofiber-based MPL (FC-C) (67-71 %), and are 73-302 % higher than that of the traditional hydrophobic carbon black powder-based MPL (FC-V). This study indicates that the as-prepared novel core-shell SiO2@C nanofiber is a promising MPL material for PEMFC.