多孔性硫酸化二氧化鋯結構與表面酸性對質子導度影響之研究

Abstract

在本文研究，我們製備了多孔性的硫酸化二氧化鋯並且應用其特性在質子交換膜燃料電池中質子傳導膜的基材。除了檢測其材料的多孔結構性質及表面酸度外，並加以討論此兩者的相互關係及兩者對質子導度的單方及相互的影響。製備多孔結構的硫酸化二氧化鋯，是利用不同長碳鏈的介面活性劑當作模板以共沉澱法或水熱法合成。中孔洞硫酸化二氧化鋯隨著界面活性劑的增加，其比表面積從78增加至128 m2/g，相對的質子導度也從1.2×10-2提升至2.0 ×10-2 S/cm。微孔洞硫酸化二氧化鋯有較高的質子導度約2.6×10-2 S/cm。推測其微孔的小孔徑及其高的表面酸度使得質子易在表面傳遞。而利用中孔洞硫酸化二氧化鋯(C16TAB/Zr= 0.5, average pore size= 2.8, surface area= 128 m2/g)的樣品用0.9M的硫酸溶液做再披覆後發現，質子導度提升為原本2.0 ×10-2 S/cm至9.5 ×10-2 S/cm，且此數值比現今商業質子交換膜(Nafion, 5.2×10-2 S/cm)之效益高約兩倍。孔徑大小與表面酸性皆影響著水含量並且控制著質子傳導的能力。即便微孔洞的硫酸化二氧化鋯有著最高的水分吸附能力，但其質子導度卻並沒有如經過再披覆硫酸的樣品來的高，推測小於0.6 nm的微孔洞材料，水分會因為太緊密的吸附在表面而導致質子不易傳導，因此適當的孔徑大小和表面酸性結合可使材料具有高的質子導度特性。

In this study, porous sulfated ZrO2 (S-ZrO2) powders were prepared as a promising alternative proton-conducting material for fuel cells. The porous structure, surface acidity and proton conductivity were examined and their relationships were investigated. The S-ZO2 samples were prepared through templating precipitation and hydrothermal method. The mesoporous S-ZrO2 samples exhibited the proton conductivities of 1.2-2.0×10-2 S/cm, and the conductivities were highly dependent on their specific surface areas (78-128 m2/g). The microporous S-ZrO2 sample templated with octyltrimethylammonium bromide (C8TAB) had a higher proton conductivity of 2.6 ×10-2 S/cm. Small pore sizes assist protons hopping between bulk water and surface acidic sites to promote conductive efficiency. Post impregnation of the mesoporous S-ZrO2 sample (C16TAB/Zr= 0.5, average pore size= 2.8, surface area= 128 m2/g) with a 0.9 M H2SO4 solution remarkably improved its proton conductivity from 2.0 ×10-2 to 9.5 ×10-2 S/cm. This value is twice higher than that of the commercial Nafion (5.2×10-2 S/cm). Both the pore size and surface acidity determine the water content and control the proton conductivity. Even though the microporous S-ZrO2 samples showed the highest capability for keeping water molecules, their proton conductivity were not higher than the post sulfation powders. Microporous channels with the pore size smaller than 0.6 nm block water tightly and retard proton diffusion. Therefore, the optimal pore size (0.6-2.8nm) and surface acidity can contribute to high proton conductivity.