Interim-Temperature SOFCs

The high working temperatures of solid-oxide fuel cells between 900 and 1000°C lead to numerous problems in the development, manufacture, and practical use 

A very limited number of materials exist that can be used to fabricate the various functional and strucmral components of SOFCs and that are sufficiently stable thermally and chemically to withstand their operating environment. As an example, take the interconnectors between cells for which mechanically weak lanthanum chromite ceramics or thermally stable high-alloy steels must be used. These materials are very expensive, so the interconnectors make a considerable contribution to the fuel cell s total cost. Numerous difficulties also arise in the selection of sealing materials. 

Since numerous ceramic components that are sensitive to thermal shocks are used in SOFCs, brute force must not be used when heating these cells from ambient temperature to the high working temperatures during startup. The same holds for cooling down during shutdown of the power plant. This implies that a rather long time is required for these processes. This is not very important 

The task of lowering the working temperature of SOFCs is made difficult by two facts (1) the conductivity of the conventional, YSZ-type electrolyte drops off sharply with decreasing temperature and (2) the rates of the electrochemical reactions occurring at the electrodes used in conventional SOFCs decrease with decreasing temperature, and the polarization of these electrodes (particularly that of the cathode) increases accordingly. 

To some extent, the first factor could be overcome by using thinner electrolytes. The ohmic resistance of a layer of YSZ electrolyte less than 5 p.m thick is not a limiting factor for designing fuel cells that have a large specific power. However, it is a rather difficult task to make defect-free electrolyte membranes so thin, yet sufficiently stable and reliable. For this reason, the search for SOFCs operable at lower temperatures has been primarily via the development of new types of material for the electrolyte and for electrodes that could work at these temperatures. 

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