Chapter 5 - Cathode

Abstract

This chapter discusses the cathode used in a solid oxide fuel cell (SOFC). The cathode possesses sufficient electronic conductivity to support electron flow in the oxidizing environment at the operating temperature. In general, maximum possible cathode conductivity is desirable to minimize ohmic losses. The processing conditions of each fabrication method are tailored to produce the required cathode structure. The cathode should have sufficient catalytic activity, thus low polarization, for the electrochemical reduction of the oxidant. In addition to these requirements, other desirable properties for the SOFC cathode are (1) high strength and toughness, (2) fabricability, and (3) low cost. The main function of the cathode is to provide reaction sites for the electrochemical reduction of the oxidant. Thus, the cathode material should be stable in the oxidant oxidizing environment and have sufficient electronic conductivity and catalytic activity for the oxidant gas reaction at the operating conditions. Because the SOFC operates at high temperatures (600º to 1000ºC), the cathode must be chemically and thermally compatible with the other cell components, from room temperature to those operating temperatures and to the even higher temperatures at which the fuel cell is fabricated. The key requirements for the anode in the SOFC are dependent on the following factors: stability, conductivity, compatibility, thermal expansion, porosity, and catalytic activity. Noble metals such as platinum and palladium are unsuitable for practical applications because of prohibitive cost. Many doped oxides are available, but only a few meet the requirements of thermal expansion match and compatibility with the electrolyte. Currently, doped lanthanum manganite (LaMnO3) is most commonly used.