CHAPTER 3. Cathode Material Development

Abstract

Various cathode materials and cathode with different structures have been developed for SOFCs based on oxygen ion and proton conductors. Perovskite manganates such as strontium-doped lanthanum manganates are the conventional cathode materials widely used for oxygen-ion conducting SOFCs due to its stability at high temperature (800–1000 °C) and compatibility with the electrolyte. Their application is limited at intermediate temperature (<750 °C) because of the negligible oxygen ion conductivity and relatively low catalytic activity. Cobalt and iron are thus used to replace manganese in order to increase the catalytic activity as well as the oxygen ion conductivity so that the reaction zone is extended from the electrode-electrolyte physical interface to the electrode bulk. The reaction zone can be further extended by tailoring the cathode structures such as cooperating electrolytes to the cathode materials and forming nanostructures with ion impregnation. These materials and structures developed for the oxygen-ion conducting SOFCs are also applicable to the proton-conducting SOFCs, suggesting complicated cathode reaction steps, which might consist of proton and oxygen ion transportation. In addition, mixed electronic-proton conductor such as BaCe0.5Fe0.5O3 is developed. Proton conductors are also cooperated to the catalysts to enhance the cathode activity by extending the three-phase boundaries where proton, electron and oxygen molecular meet.