鐠摻雜之氧化鈰電解質於中溫固態氧化物燃料電池之研究

Abstract

本文成功地利用Pechini method製備出，以二氧化鈰為主體摻雜鐠之中溫固態氧化物燃料電池(intermediate temperature solid oxide fuel cell, 縮寫為IT-SOFC)的電解質材料。研究材料包括鐠單一摻雜之Ce1-xPrxO2-δ，x = 0.05-0.35和鐠與鹼土金屬共摻雜之Ce0.7Pr0.3-yMyO2-δ，M = Mg或Ca，樣品經1500℃高溫燒結後，以X光粉末繞射儀鑑定出皆為單一物相之立方瑩石結構。以Pr LⅢ-edge之X光近邊緣吸收光譜(X-ray absorption near-edge spectroscopy, 縮寫為XANES)的測量來鑑定樣品中Ce與Pr之氧化態。藉由與Pr2O3、Pr6O11和CeO2為標準品之XANES圖譜的比對，發現所有經1500℃高溫燒結過後鐠摻雜樣品之XANES圖譜相似於Pr6O11之XANES，因此樣品中Pr應是以混價(3+、4+)之氧化態存在。進一步研究發現樣品於700℃煅燒的反應過程中就有部分Pr3+會氧化成Pr4+。因此，樣品中氧空缺濃度應該低於實際鐠摻雜量所應產生之氧空缺濃度。 所製備樣品的離子導電度以交流阻抗儀於400-700℃空氣下作量測。CPO體系中，樣品30CPO於700℃具有最佳氧離子導電度0.0668 Scm-1，此數值高於目前被報導於700℃下具有最佳氧離子導電度之Ce1-xGdxO2-δ (0.0316 scm-1)與Ce1-xSmxO2-δ(0.041 Scm-1)電解質材料。 為了達到降低材料成本，本文也嘗試引入鹼土金屬，製備雙摻雜的CPO材料，並且探討鹼土金屬共摻雜CPO對於導電度與氧化態的影響。結果顯示，共摻雜鹼土金屬並不影響CPO之物相結構，以及Ce與Pr於樣品中之氧化態形式。導電度的影響，鎂離子共摻雜具有正面的提升效果，反之鈣離子的共摻雜反而降低了原先CPO之導電度。其中，以成分組成為Ce0.7Pr0.28Mg0.02O2-δ是所有樣品中具有最佳導電度的材料。

Pr-doped ceria-based electrolyte for intermediate temperature solid oxide fuel cell (IT-SOFC) was successfully synthesized by Pechini process (citric acid process). All the Pr-doped ceria (CPO) of the formula Ce1-xPrxO2-δ where x = 0.05-0.35 and those co-doped with alkaline earth of the formula Ce0.7Pr0.3-yMyO2-δ, 0.01 ≦ y ≦0.05 were ceria based solid solutions of fluorite type structures, characterized by X-ray powder diffraction. A portion of Pr3+ ions doped in ceria was oxidized to Pr4+ during the calcination process at 700℃ in air. Compared with the Pr LⅢ-edge X-ray absorption near-edge spectroscopy (XANES) profiles of Pr2O3 and Pr6O11, all the XANES profiles of the Pr-doped ceria materials after calcination at 1500℃ in air were similar to that of Pr6O11. As a result, lower number of oxygen vacancies was created in the samples than that expected from the original doping ratios. The ionic conductivities of the Pr-doped ceria-based materials were measured at 400-700℃ in air by using AC impedance spectroscopy. In the CPO system, Ce0.7Pr0.3O2-δ (30CPO) gave the highest conductivity of 0.0668 Scm-1, which was higher than 0.0316 Scm-1 for Ce0.8Gd0.2O2-δ (CGO) and 0.041 Scm-1 for Ce1-xSmxO2-δ (CSO), and the latter two were reported to give the highest conductivities among ceria-based materials applied in IT-SOFC. The effect of co-doping of alkaline earth in CPMO system on ionic conductivity was also investigated. Compared with CPO, it was found that Mg2+-dopant had positive improvement in conductivity, but the Ca2+-dopant did not. The material doped with small amount of Mg2+, Ce0.7Pr0.28Mg0.02O2-δ was found to retain the unit cell dimension as that of 30CPO and gave the highest conductivity.