TABLE OF CONTENTS A solid oxide fuel cell (SOFC) consists of three main components: a porous cathode (or air electrode), a porous anode (or fuel electrode), and an ion-conducting ceramic membrane, as shown in Figure 4.26. The operation temperature of SOFCs are high, typically between 700 C and 1000 C, to ensure sufficient ionic conductivity in the membrane and to promote electrochemical reactions. Similar to the PEM fuel cells, the advantages of the SOFCs include high-efficiency, high energy density, and low pollution. Furthermore, a major constraint of the PEM fuel cells, i.e., the requirement of hydrogen fuel, is relaxed in SOFCs. Although hydrogen is taken as a future alternative to the depleting fossil fuel, a number of issues related to hydrogen generation and storage limit the large-scale usage of hydrogen. About 96% of hydrogen is currently obtained by reforming hydrocarbons, and 20 to 30 percent of the chemical energy is wasted during the process [164]. Since the energy density of hydrogen gas is low compared to that of hydrocarbon liquid, hydrogen storage may be difficult and expensive.
Two types of geometries are commonly used for SOFCs, namely, tubular and planar configurations as shown in Figures 4.27(a) and 4.27(b), respectively. Some model power plants manufactured by Siemens-Westinghouse adopt the tubular design, which has demonstrated high efficiency and long term stability (3). However, two technical problems are associated with the tubular design, namely, high fabrication...
