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This chapter provides an overview of the key features and characteristics of SOFC, along with descriptions of the main types of SOFC and their performance. Those readers interested in greater detail, as well as an excellent history of SOFC development, are referred to Singhal and Kendall (1), and other references listed at the end of this chapter. 7.1 Cell Components The major components of an individual SOFC cell include the electrolyte, the cathode, and the anode. Fuel cell stacks contain an electrical interconnect, which links individual cells together in series or parallel. The electrolyte is made from a ceramic such as yttria-stabilized zirconia (YSZ) and functions as a conductor of oxide ions. Oxygen atoms are reduced into oxide ions on the porous cathode surface by electrons, and then flow through the ceramic electrolyte to the fuel- rich porous anode where the oxide ions react with fuel (hydrogen), giving up electrons. The interconnect serves to conduct the electrons through an external circuit. 7.1.1 ElectrolyteMaterials As indicated by their name, SOFCs use solid oxide ceramics, typically perovskites, as the electrolyte. Nernst (2) realized in the 1890s that certain perovskites, stabilized zirconias, conducted ions in a certain temperature range. Baur and Preis (3) demonstrated in 1943 that such electrolytes could be used as (oxygen) ion conductors in fuel cells. Currently, yttrium stablilized zirconia (3, 8, or 10 percent yttria, abbreviated to YSZ) is the most commonly used electrolyte for SOFC. YSZ provides high conductivity at temperatures above 700 °C (Figure 7-1, (4, 5, 6)), while exhibiting negligible electronic conductivity at these temperatures (above 1500 °C it becomes an electronic conductor). In a fuel cell operating with a current density of 250 mA/cm2 at 1000 °C and an electrolyte of 200 μm thickness, the resistance loss in the electrolyte would be 50 mV. However, for mechanical reasons it is desirable to operate the SOFC at lower temperatures. To operate at 800 °C, the electrolyte thickness would have to be reduced by about an order of magnitude to maintain a similar ohmic loss in the electrolyte. Colloidal fabrication and co-sintering processes have emerged, whereby YSZ membranes are produced as thin films (~10 μm) on porous electrode structures. These thin-film membranes improve performance and reduce operating temperatures of SOFCs. To enable these colloidal processes to be successful, finer YSZ powders are needed. These applications require nano-scale powders with BET surface areas of 100 to 120 m2/g and the use of suspensions ranging from 10 to 40 percent solid content (7, 8). Alternative electrolytes have been considered and are being developed. As shown in Figure 7-1, scandium-doped zirconia (SDZ) is more conductive than YSZ, permitting a further reduction of the operating temperature by 50 to 100 °C. Gadolinium-doped ceria is even more conductive, but is partially reduced in hydrogen at temperatures above 600 °C; formation of Ce+3 ions generates electron holes that make ceria electronically conductive, thus short-circuiting the cell. A substantially more conductive material that is stable in air and hydrogen was discovered by Goodenough (9). Lanthanum gallate with strontium doping on the A-site of the perovskite and magnesium on the B-site could be used at temperatures as low as 600 °C even on a thick electrolyte. Laboratory fuel cells with this electrolyte have been tested, but the typical 7-2PDF Image | Fuel Cell Handbook (Seventh Edition)
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