Chemical Stability of Doped BaCe0.9Y0.1O3-x as a Proton Conducting Electrolyte for IT-SOFC

Abstract

BaCe0.9Y0.1O3-δ (BCY) has been one of the most studied materials known for its highest proton conductivity at temperatures between 500 and 700 ºC, which allows its application as a proton conducting electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFC). The proton conductivity is an exclusive property of mixed oxides with perovskite structure and large unit cell volume, such as BaCeO3 or SrCeO3. Doping with aliovalent cations (Y3+) that replace Ce4+ induces formation of point defects (oxygen vacancies), which in wet or hydrogen containing atmosphere allow proton mobility. The main disadvantage of this material is its instability in CO2-rich atmosphere due to the basic character of the crystal lattice, thus limiting its application in SOFCs in respect to fuel selection. However, the stability of BCY can be enhanced by doping with cations that may raise the acidic character of the material, such as Nb5+, Ta5+ or In3+. Introduction of pentivalent cations will lead to reduced amount of point defects and consequently lower proton conductivity and it is therefore recommended that their molar concentration should not exceed 5 %. On the other hand, trivalent In3+ is more suitable as it can completely replace Y3+ since it can both serve as a point defect source and increase acidity of the crystal lattice. Because of these properties it can be introduced in much larger amounts than Nb5+ or Ta5+. In this study BaCe0.9-xNbxY0.1O3-δ (where x = 0.01, 0.03 and 0.05) and BaCe1-xInxO3-δ (where x = 0.15, 0.20 and 0.25) powders were synthesized by the method of autocombustion, while BaCe0.9-xTaxY0.1O3-δ (where x = 0.01, 0.03 and 0.05) powders were prepared by the classical solid state route. Much higher specific surface areas were observed for the samples synthesized by the autocombustion method. In the case of Nb and Ta doped samples, the dense electrolytes were formed after sintering at 1550 ºC for 5 h in air. Temperature of 1300 ºC was enough to complete sintering of the samples doped with In after 5 h in air, which was another advantage of In as a dopant. The conductivities determined by impedance measurements in temperature range of 550-700 ºC in wet hydrogen showed a decreasing trend with increase of Nb and Ta content, while it was the opposite in the case of In. Interestingly, the total conductivity of the samples BaCe0.85Nb0.05Y0.1O3-δ, BaCe0.85Ta0.05Y0.1O3-δ and BaCe0.75In0.25O3-δ reached around 5×10–3 S/cm in wet hydrogen atmosphere at 700 ºC. After exposure in 100 % CO2 atmosphere at 700 ºC for 5 h, the samples were investigated by X-ray analysis. It was found that even 15 % In could completely supress degradation of electrolyte, while the highest concentrations of Nb and Ta (5%) were necessary to secure sufficient stability in CO2.