Improved properties of doped BaCe0.9Y0.1O3-δ as a proton conducting electrolyte for IT-SOFC

Abstract

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 produce proton defects highly mobile at elevated temperatures. BaCe0.9Y0.1O3-δ (BCY) is one of the best proton conducting electrolyte at temperatures between 500 and 700 ºC, which allows its application in intermediate-temperature solid oxide fuel cells (ITSOFC). Yet, one of the main drawbacks of this material is its instability in CO2-rich atmospheres. Since BCY is basic in character, it normally reacts with CO2 to form BaCO3 and yttria doped ceria. Both products exhibit no proton conductivity, thus limiting application of BCY in IT-SOFCs where CO2 appears as a product of electrochemical performance. However, the stability of BCY can be improved by doping with cations that may raise the acidic character of the material, such as Nb5+, Ta5+ or In3+. Introduction of Nb5+ and Ta5+ will reduce the amount of point defects and consequently decrease the proton conductivity. This relation has been known as trade-off effect. Nevertheless, if their molar concentration exceed no more than 5% it is possible to obtain functional electrolytes with satisfying stability and conductivity. On the other hand, trivalent In3+ can completely replace Y3+ since it can both serve as a point defect source and increase acidity of the crystal lattice. Accordingly, 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 the samples doped with Nb and Ta, the dense electrolytes were formed after sintering at 1550 ºC for 5 h in air. Temperature of 1300 °C 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 °C 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 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 °C. After exposure in 100 % CO2 atmosphere at 700 °C for 5 h, the samples were investigated by X-ray analysis. It was found that even 15 % In could completely supress degradation of the electrolyte, while the highest concentrations of Nb and Ta (5%) were necessary to secure sufficient stability in CO2.