摻雜異價離子(Gd3+, Mg2+, Sr2+)對CeO2電解質之顯微結構及導電性的影響

Abstract

The aim of this study was to develop ceria-based solid electrolytes with high ionic conductivity for intermediate temperature (500〜800oC) SOFC (Solid Oxide Fuel Cell) instead of conventional YSZ electrolytes showing very low ionic conductivity during 500〜800oC. Fluorite structure with the higher oxygen ionic conductivity is the major electrolyte materials in SOFC. The CeO2 is stable cubic fluorite structure from the room temperature to melting point. Beside, as compared to the commercial YSZ (yttria stabilized zirconia) solid electrolyte, it has higher ionic conductivity and lower active energy, so it is applied to as the electrolyte material of intermediate temperature solid oxide fuel cell (SOFC). The Ce1-XGdXO2-δ (X = 0, 0.05, 0.1, 0.15, 0.2, 0.25) , Ce0.8Gd0.2-XMgXO2-δ (X = 0.05, 0.1), and Ce0.8Gd0.2-XSrXO2-δ (X = 0.05, 0.1) bulk electrolyte were sintered at 1350 oC〜1550 oC for 4 h by traditional solid-state reaction. Then, all sintered pellets were analyzed for crystal structure, appearance, atomic bonding, densification, and ionic conductivity by XRD, SEM, Raman, thermal expansion analyzer and DC-method. All pellets that sintered at 1550oC are cubic fluorite structure by XRD analysis. The Raman line broadens and becomes asymmetric with a low frequency tail, and a new broad feature appears at 〜570 cm-1 in the spectrum for aliovalent cations (Gd3+, Mg2+, Sr2+) doped ceria. These changes in the Raman spectrum are attributed to O vacancies, which were caused by Gd3+, Mg2+, and Sr2+ ions are substituted for Ce4+. The Ce0.8Gd0.2O2-δ (m-GDC20)(1550oC/4h) specimens possess the highest conductivity during 500∼800oC. Ionic conductivity increases from 2.18×10-5 S/cm (m-CeO2) to 0.077 S/cm (m-GDC20) at 800oC. The m-Ce0.8Gd0.2-XSrXO2-δ (X = 0.1)(1550oC/4h) specimen possesses the highest conductivity (0.082 S/cm) at 800 oC. The m-Ce0.8 Gd0.2-XMgXO2-δ (X = 0.05) specimen showed the lower ionic conductivity (0.065 S/cm) than the Ce0.8Gd0.2O2-δ (m-GDC20) with 0.077 S/cm at 800oC.