High temperature, SOFC

For high temperature SOFCs, a conventional commercial alloy cannot be used as the interconnector because they are easily oxidized under a high temperature oxidizing atmosphere. Therefore presently, special rare metals or doped-LaCr03 are used as the interconnector. For low temperature SOFCs, a commercial alloy can be used as the interconnector and thus, various alloys are considered as the interconnector. Austenite stainless steels have a high TEC in comparison to YSZ, and only ferritic stainless steels are considered as a candidate for the interconnector. 

In the case of the high temperature SOFC discussed below the principles outlined above equally apply. The technical differences are that the cell runs typically on hydrocarbon fuels (e.g. natural or coal-gas) and that the electrolyte is an oxygen ion conductor rather than a proton conductor. The complex fuel molecules, in the presence of the water molecule and at the high operating... 

At 600 °C in the MCFC, the dynamic equilibrium conditions are ideal for anode reform. The voracious oxidation reaction swallows both reform and shift reaction products as they are formed. The latter reactions are left striving to equilibrate. In the high-temperature SOFC the reform reaction is very vigorous, and uneven temperature distribution can occur. To avoid that irreversibility, Siemens Westinghouse still employs separate reformers. More irreversibility, but SOFC temperatures are on their way down The intermediate-temperature SOFC is emerging. 

Figure 3.20 shows a cylindrical layout often used for high-temperature SOFCs. Alternatives are a stack of planar cells or a disk concept with feed tubes in the centre. A consideration of efficient heat exchange is the cormnon design strategy for the high-temperature fuel cell geometry. 

Solid oxide fuel cells (SOFCs) are one of the most efficient energy conversion devices [1]. The main demand in the current SOFC development is lowering operation temperature to the range of 600-800 C - intermediate temperature SOFC (IT-SOFC). In order to lower operational temperature and increase or at least sustain performance comparable to that at high temperature SOFCs, it is necessary to decrease the resistance of the electrolyte and lower the overpotential of the electrodes. One of the ways to achieve this goal is to decrease the thickness of the electrolyte and optimize the structure of the electrodes. 

A SER based on low-pressure interconnected bubbling fluidised beds applied to power plants with CO2 capture is proposed by Ref. [54], In this case, the H2-rich gas produced is converted in a high-temperature SOFC and regeneration is carried out by recovering the waste heat from the fuel cell through an internal heat transfer loop. 

T. Klemenso, C. Chung, P. H. Larsen and M. Mogensen, The mechanism behind redox instability of anodes in high-temperature SOFCs, J. Electrochem. Soc., 152 (II), p. A2186 (2005)... 

Anodes perform electrooxidation of fuel by catalyzing the reaction and facilitating fuel access and product removal [1]. These require sufficient reaction sites for the fuel oxidation and electronic conductivity to transfer electrons from the oxidant to the cell components, i.e., electrol34e and current collectors. In high-temperature SOFCs, porous cermets, made from a percolating metal phase and... 

For many practical catalysts, b is known from experiments. The characteristic values of b for the low- (PEFC and DMFC) and high-temperature (SOFC) fuel cells are listed in Table 1.1. Note that in the electrochemical hterature the dependence of Q on 77 is often represented as a power of 10, rather than the exponent the respective value bw 2.36. 

One of the problems with anode-supported cells is that any difference in thermal expansion between anode and electrolyte becomes more significant than in conventional high-temperature SOFCs. For this reason many developers use porous nickel cermet anodes with interfacial regions made of NiA SZ doped with ceria. Operating at temperatures below about 700°C means that metallic bipolar plates can be used, and the lower the temperature, the less exotic the steel needs to be. Ferritic stainless steels can be used below about 600°C, and these have the advantage that they have a low thermal expansion coefficient. Conventional doped LSM-YSZ cathodes can be used but there is much development in progress to improve cathode materials as the cathode overpotentials become more significant as the cell temperatures are lowered. A recent review of cathode materials has been published by Ralph (2001). 

The type of the catalyst employed in the fuel cell depends on the type of fuel, the solid electrolyte used, and the operating temperature. Here we will consider the recent trends in catalysis for the two major types of the fuel cells, including low-temperature proton exchange membrane fuel cells and high-temperature SOFCs. Depending on the fuel used, low-temperature PEMFCs fall into two major categories hydrogen and direct methanol fuel cells (DMFCs). 

Typical Parameters for Low-Temperature PEMFC and High-Temperature SOFC... 

YSZ is so far the most widely used solid electrolyte for application in high temperature SOFC. For many years, the zirconium oxide is already known as a conductor of oxygen ions. 

Manganese-based perovskites are widely recognized as the materials best suited for the cathode of a high-temperature SOFC that uses a zirconia-based electrolyte and operates at temperatures higher than 800°C. In this section, (La, Sr)Mn03 (LSM) is chosen for further discussion. 

Perovskite-type oxides based on Mn, Co, Fe, or K2NiF4-type oxide with Ni are studied as cathode materials for SOFCs. For high-temperature SOFCs, LaMnOs-based materials are mainly used because of the high compatibility... 

Figure 4.19 Typical polarization curves for high-temperature SOFC in different gas environments at 1000° C. The high operating temperature enables the use of low-cost catalyst materials such as nickel (anode) and strontium-doped lanthanum manganite (cathode), with very low kinetic polarization losses. (Reproduced with permission from [5].)...

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