Schematic Design of Conventional SOFCs

Conventional SOFCs exist in several design variants. The basic variants are tubular and planar cells. Monolithic cells joined the first two variants around 1990. The specific design and operating features of these and other variants are described in subsequent sections of this chapter. Those factors that are common for all variants of conventional SOFCs are described in the present section. 

The anodes of these cells consist of a cermet (ceramic-metal composite) of nickel and the zirconia electrolyte. This material is made from a mixture of nickel oxide (NiO) and the YSZ electrolyte. The nickel oxide is reduced in situ to metallic nickel, forming highly disperse particles that serve as the catalyst for anodic fuel gas oxidation reactions. These particles are distributed uniformly in the solid electrolyte, and are prevented from agglomerating during fuel cell operation by this electrolyte, thus retaining their catalytic activity. The YSZ material present in the anode also improves the contact between the nickel catalyst and the fuel cell s electrolyte layer. 

The cathodes consist of manganites or cobaltites of lanthanum doped with divalent metal ions [e.g., Lai-jcSrjcMnOs (LSM) or Lai- Sr CoOs (LSC), where 0.15 X 0.25]. Apart from their 0 ion conductivity, these cathode materials also have some electronic conductivity that secures a uniform current distribution over the entire electrode. LSC has a higher ionic conductivity than LSM but is more expensive and leads to problems, in particular because of a possible chemical interaction with the electrolyte. 

These three components of the fuel cell anode, cathode, and electrolyte, form a membrane-electrolyte assembly (MEA), since by analogy with PEMFCs, one may regard the thin layer of solid electrolyte as a membrane. Any one of the three MEA components can be selected as the entire fuel cell s support, and is then made relatively thick (up to 2 mm) to provide mechanical stability. The 

An important component of SOFCs that governs their operating reliability to a large extent (and sometimes their manufacturing cost) is that of the interconnectors needed to combine the individual cells in a battery. These must be purely electronically conducting, chemically sufficiently stable toward the oxidizing and reducing atmospheres within the fuel cells, and free of chemical interactions with the active materials on the electrodes. 

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