Stability of Bi₂O₃-based ionic conductor and its application on composite cathode

In recent years, solid oxide fuel cells (SOFCs) have been considered as desired devices for generating electricity by electrochemical combination of a fuel with an oxidant. Oxygen ion conductors are usually used as the electrolytes for SOFCs. Among several types of oxygen ion conductors such as CeO₂, stabilized Bi₂O₃ and ZrO₂, Bi₂O₃ with stabilizers exhibit the highest oxygen ion conductivity. Although Bi₂O₃ tends to be reduced to Bi metal, Bi₂O₃ incorporated with proper dopants will show enhanced stability against hydrogen. The highest OCV for SOFC using Bi₂O₃-based electrolyte is around 0.5V which is still too low to be practical. On the other hand, Bi₂O₃-based system may make a good ionic component for a composite cathode. In this study, we choose yttria-stabilized bismuth oxides (YSB) as ionic conductor and mixed with strontium-doped lanthanum manganite (LSM) for composite cathode. Size effect of LSM on the polarization resistance was investigated by using conventional solid-state reacted and nano-sized LSM. The electrochemical performances of composite cathodes with different LSM:YSB ratios have been investigated at temperature ranging from 500°C to 650°C using AC impedance spectroscopy. The polarization resistance measured from a symmetric cell consisting of nano-sized LSM-YSB electrodes on a YSB electrolyte is 50% lower than that of submicron-sized LSM-YSB cathodes by SSR (0.2 ohmcm²) at 650°C. The polarization of cathode is reduced because of the mixing of high surface area electric conducting materials and ionic conducting materials. The nanostructure of composite cathode is able to extend triple phase boundary (electronic conductor, ionic conductor, and oxygen gas) and provide numerous reaction sites for electrochemical reaction to occur.