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The resistivities of typical cell components at 1,000 °C under fuel cell gaseous environments (38) are 10 ohm-cm (ionic) for the electrolyte (8-10 mol percent Y2O3 doped ZrO2), 1 ohm-cm (electronic) for the cell interconnect (doped LaCrO3), 0.01 ohm-cm (electronic) for the cathode (doped LaMnO3), and 3 x 10-6 ohm-cm (electronic) for the anode (Ni/ZrO2 cermet). It is apparent that the solid oxide electrolyte is the least conductive of the cell components, followed by the cell interconnect. Furthermore, an operating temperature of about 1,000 °C is necessary if the ionic conductivity of the solid electrolyte (i.e., 0.02/ohm-cm at 800 °C and 0.1/ohm-cm at 1,000 °C) is to be within an order of magnitude of that of aqueous electrolytes. The solid electrolyte in SOFCs must be only about 25 to 50 μm thick if its ohmic loss at 1,000 °C is to be comparable to the electrolyte in PAFCs (39). Fortunately, thin electrolyte structures of about 40 μm thickness can be fabricated by EVD, as well as by tape casting and other ceramic processing techniques. Operation of SOFCs requires individual cell components that are thermally compatible so that stable interfaces are established at 1,000 °C, i.e., CTEs for cell components must be closely matched to reduce thermal stress arising from differential expansion between components. Fortunately, the electrolyte, interconnect, and cathode listed in Table 7-1 have reasonably close CTEs (i.e., ~10-5 cm/cm °C from room temperature to 1,000 °C). An anode made of 100 percent nickel would have excellent electrical conductivity. However, the CTE of 100 percent nickel would be 50 percent greater than the ceramic electrolyte and the cathode tube, which causes a thermal mismatch. This thermal mismatch has been resolved by mixing ceramic powders with Ni or NiO. The trade-off in the amounts of Ni (to achieve high conductivity) and ceramic (to better match the CTE) is approximately 30/70 Ni/YSZ by volume (40). Schematic representations of the gas manifold design and cross section of a typical tube bundle (41) are presented in Figure 7-12. In this design, the tubular cathode is formed by extrusion. The electrolyte and cell interconnect are deposited by electrochemical vapor deposition (EVD) and plasma spraying, respectively, on the cathode. The anode is subsequently formed on the electrolyte by slurry deposition. A major advantage of this design is that relatively large single tubular cells can be constructed in which the successive active layers can be deposited without chemical or material interference with previously-deposited layers. The support tube is closed at one end, which eliminates gas seals between cells. 7-18PDF Image | Fuel Cell Handbook (Seventh Edition)
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