Medium-Temperature SOFCs

Design Concept of Catalyzed Reaction Layer for Medium-Temperature SOFC 

It is very important for a high-performance electrode to have both a highly active electrocatalytic reaction zone and a sufficient gas-supply network in its microstructure. The conventional anode material used so far is Ni-YSZ cermet prepared from pm-sized NiO and YSZ particles as shown in Fig. 3A. Because all reactants (fuel gas, electrons, and oxide ions) must meet together at the reaction sites, the so-called three phase boundary (TPB) zone is the effective 

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High-Performance Electrodes for Medium-Temperature SOFC... 

Uchida, H., Suzuki, S. and Watanabe, M. (2003) A high performance electrode for medium-temperature SOFC mixed conducting ceria based anode with highly dispersed Ni electrocatalysts. Proceedings of the eight International Symposium on Solid Oxide Fuel Cells (SOFC VIII), The Electrochemical Society, PV 2003-07, pp. 728-736. 

Our research results indeed contribute to the development of medium temperature operating SOFCs. For example, doped ceria has been employed in place of YSZ in the cermet-type anodes by many research groups. Effects of ojon on the activities of some electrodes have also been supported. ... 

The technical challenges posed by these systems are different from those facing low- to medium-temperature cells. For instance, there are no severe kinetic limitations at the electrodes or poisoning of electrocatalysts by impurities (other than sulfur) in the fuel gas. Instead, material science issues arise with (i) sintering of the electrodes and the electrolyte matrix, (ii) corrosion of cell components in molten salt electrolytes (MCFC), (iii) electrolyte migration in the external manifolds of MCFCs and (iv) differential expansion coefficients of the materials of construction in all-solid-state systems (SOFCs). 

Depending on operation temperature the material chosen for the bipolar plate is graphite (or graphite composites) or iron-based alloys (stainless steel) for low- and medium-temperature fuel cells (PEFC, PAFC, MCFC) and chromium or ceramic-based materials for the high-temperature systems (SOFC). In SOFC, for sealing reasons, tubular concepts have also been developed (see Section 8.1.4.6). 

SOFCs, like all fuel cell types, show an enhanced performance with increasing ceU pressure. Unlike low- and medium-temperature cells, the improvement is mainly due to the increase in the Nemst potential. We showed in Section 2.5.4 that the voltage change for an increase in pressure from Pi to P2 follows very closely the theoretical equation... 

We first consider these features common to all medium- and high-temperature fuel cells. Note that since these fuel cells use processed fuel and since use can be made of the exhaust heat, they have been mainly applied to stationary power generation systems. Although we shall see that the SOFC may find application in some mobile applications, the complexity of fuel processing usually rules out the application of high-temperature fuel cells for mobile use. 

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