以鐠摻雜之氧化鈰為主體之材料的合成與鑑定及其在燃料電池上的應用

Preparation, Characterization and Application of Praseodymium-doped Ceria-based Materials in Fuel Cells

陳美如(Mei-Ju Chen)

指導教授 ： 鄭淑芬

國立臺灣大學/理學院/化學研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.02092

摘要

摘要
此論文內容囊括四大部分。首先，複合陰極材料的研究結果發現，
以重量比50% La0.6Sr0.4Co0.2Fe0.8O3 (6428LSCoF) to Ce0.7Pr0.3O2-δ(30CPO) or Ce0.7Pr0.28Mg0.02O2-δ(28,2-CPMgO)的複合陰極，可有效降低陰極極化電阻，進而提升單電池效能。相較於20CGO-6428LSCoF, 30CPO-6428LSCoF複合陰極，以50% 28,2-CPMgO-6428LSCoF複合陰極組裝之單電池，可獲得開路電壓0.74V、最大電流密度1110 mA/cm2與最高功率密度297 mW/cm2之最佳單電池效能。
第二部份研究結果顯示，氧化鈰材料於還原氣氛下之電子導電特性，可藉由鐠的摻雜達到抑制的效果，抑制程度隨鐠的摻雜量增加而增加。其中，以25CPO材料於空氣、還原氣氛下，具有相當之氧離子與電子導電率，但其電子導電率卻隨時間逐漸降低。此外，引入少量鎂的共摻雜，不僅可提升CPO材料於空氣下之氧離子導電率，還可維持其於還原氣氛下電子導電率，不隨時間改變。進而發現，25,2-CPMgO材料，於空氣、還原氣氛下之氧離子、電子導電率幾近相當且不隨時間衰退。第三，藉由引入CPO材料作為陽極與電解質層間之緩衝層，
不僅可抑制因20CGO電解質電子導電特性導致開路電壓降低的現象，進而提升單電池之開路電壓，也因CPO材料之高氧離子導電率特性，降低了歐姆極化電阻，更加改善單電池效能。相較於沒有CPO緩衝層的引入，引入緩衝層的單電池具有較佳電池效能。以25,2-CPMgO作為緩衝層的單電池，具有開路電壓0.8V、最大電流密度1090 mA/cm2與最高功率密度為285 mW/cm2之單電池效能。
除此之外，開路電壓可維持至少140小時，只有3.75%的衰減。
最後，將CPO材料，作為富氫環境下CO選擇性氧化反應觸媒之載體的應用。利用原位X光吸收光譜(In-situ XAS)的鑑定技術，探討CuO/CeO2觸媒於富氫環境下CO選擇性反應條件下，反應位置與觸媒失活現象的研究與探討。CO轉換率隨反應溫度升高而增加，於140-160度時可達100% CO轉換率。反之，當反應溫度高於120度，CO2選擇率隨溫度升高而降低。此外，原位X光吸收光譜的結果顯示，當反應溫度高於120度時，氧化銅開始還原成金屬態銅。因此，金屬態銅為氫氧化成水之活性位置，而Cu2+可能會是CO氧化成CO2的活性位置。相較於氧化鈰，雖然鐠摻雜氧化鈰材料具有較高氧離子導電率，但以鐠摻雜氧化鈰作為載體支撐氧化銅之觸媒，對於富氫環境下CO選擇性氧化反應的催化活性卻是負效應。其中，以dd4-20CuCeO2-δ 觸媒可達最佳催化活性。於130度，可達95% CO轉換率與98% CO2選擇率。

關鍵字：固態氧化物燃料電池，鐠摻雜氧化鈰，複合陰極，CO選擇性氧化反應，原位X光吸收光譜與銅鈰氧化物觸媒

關鍵字

固態氧化物燃料電池 ； 鐠摻雜氧化鈰 ； 複合陰極 ； CO選擇性氧化反應 ； 原位X光吸收光譜與銅鈰氧化物觸媒

並列摘要

Abstract
The thesis includes four parts. In the first part, optimal weight ratio of 50% of La0.6Sr0.4Co0.2Fe0.8O3 (6428LSCoF)
to Ce0.7Pr0.3O2-δ(30CPO) or Ce0.7Pr0.28Mg0.02O2-δ(28,2-CPMgO) composite cathodes can effectively reduce polarization resistance and result in high cell performance of the single cells assembled with 60NiO-20CGO anode, 20CGO electrolyte and the composite cathodes. The optimal cell performance of 0.74 V, 1110 mA/cm2 and 297 mW/cm2 was obtained at 700˚C on the cell with the 50% 28,2-CPMgO-6428LSCoF composite cathode. In the second part, it was found that the degree of suppression of electronic conductivity of doped ceria material increases with the amount of Pr dopant. Of Pr doped ceria materials, 25CPO has nearly the same conductivities in air and in reducing atmosphere, but the conductivity in reducing atmosphere declines with time. It was also found that co-doping small amount of Mg in CPO not only enhances the oxide ion conductivity in air, but also retains the conductivity in reducing atmosphere with time. The nearly equal conductivities under air and reducing atmosphere
could be obtained by the 2% Mg co-doped 25CPO (25,2-CPMgO) material. In the third part, the Pr doped ceria was introduced in between the anode and electrolyte as a buffer layer to impede the reduction of electrolyte during cell operation. The single cells with the buffer layers had better cell performances than that without the buffer layer. The optimal cell OCV, current density and power density of 0.8 V, 1090 mA/cm2 and 285 mW/cm2 were obtained on the cell with 25,2-CPMgO buffer layer and 50% 28,2-CPMgO-6428LSCoF composite cathode at 700˚C. Moreover, the OCV could be obtained for 140 h with only 3.75% degradation.
In the last part, Pr-doped CeO2 was used as the catalyst support of preferential oxidation (PROX) of CO.
The active sites and deactivation behaviors of CuO/CeO2 catalysts for the preferential oxidation (PROX) of CO
in H2-rich environment were examined by in-situ X-ray absorption spectroscopy. The CO conversion increased with
increasing reaction temperature, while the CO2 selectivity decreased, especially at temperatures higher than 120˚C.
100% CO conversions can be achieved over CuO/dx-CeO2 at 140-160˚C. In-situ Cu K-edge XAS revealed that CuO was reduced
to metallic Cu when the reaction temperature was higher than 120˚C, accompanying with the decline in CO2 selectivity. Thus, the active site for H2 oxidation to water would be metallic Cu, and the Cu2+ might be the active species for CO oxidation of to CO2. For Pr-doped ceria supported CuO catalysts, although the Pr-doped ceria materials have higher oxide ion conductivity than CeO2, it has a negative effect on the catalytic activity of CO preferential oxidation under H2-rich atmosphere. The best catalytic activity was obtained over the dd4-20CuCeO2-δ catalyst in CO preferential oxidation under H2-rich atmosphere. The T95 was 130˚C with S95 of 98%.

Keyword: SOFC, Pr-doped ceria, composite cathode, CO PROX, in-situ XAS and CuO/CeO2 catalyst.

並列關鍵字

SOFC ； Pr-doped ceria ； composite cathode ； CO PROX ； in-situ XAS and CuO/CeO2 catalyst.

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