Lanthanum manganite anodes

Fig. 13.22. The monolithic SOFC concept of Argonne National Laboratory. Anode nickel-yttria-stabilized zirconia. Cathode strontium-doped lanthanum manganite. Interconnect doped lanthanum chromite, a, Interconnection b, electron-ion path c, anode d, electrolyte e, cathode. (Reprinted from K. Kordesch,...

The anode material in SOFCs is a cermet (metal/ceramic composite material) of 30 to 40 percent nickel in zirconia, and the cathode is lanthanum manganite doped with calcium oxide or strontium oxide. Both of these materials are porous and mixed ionic/electronic conductors. The bipolar separator typically is doped lanthanum chromite, but a metal can be used in cells operating below 1073 K (1472°F). The bipolar plate materials are dense and electronically conductive. 

Electrodes The anodes of SOFC consist of Ni cermet, a composite of metallic Ni and YSZ, Ni provides the high electrical conductivity and catalytic activity, zirconia provides the mechanical, thermal, and chemical stability. In addition, it confers to the anode the same expansion coefficient of the electrolyte and renders compatible anode and electrolyte. The electrical conductivity of such anodes is predominantly electronic. Figure 14 shows the three-phase boundary at the interface porous anode YSZ and the reactions which take place. The cathode of the SOFC consists of mixed conductive oxides with perovskite crystalline structure. Sr doped lanthanum manganite is mostly used, it is a good /7-type conductor and can contain noble metals. 

Fig. 2 Schematic of SOFC operation presenting the four-step anodic. H2O formation and electron transfer were shown in the insert. LSM strontium-doped lanthanum manganite (cathode for SOFC), Vq" vacancy of oxygen...

Cells to be operated in the temperature region of 900-1,000°C. Usually oxide interconnects are used together with electrolyte made of YSZ. Since materials compatibility is severe at higher temperatures, stable lanthanum manganite-base cathode is adopted, whereas nickel anodes are used in a similar manner to other types. Electrolyte-self-support or cathode-support types are adopted. 

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].)...

The best known type of SOFC anode-supported cells consists of a Ni/Zirconia cermet support layer, a thin functional anode, a thin dense yttria doped zirconia electrolyte layer and a thin Strontium doped lanthanum manganite (LSM) cathode layer (material alternatives are given in Fig. 8). Such cells have proved to be stable and durable for thousands of operation hours when operated at relatively mild conditions [9]. The degradation rate increases with increasing cell polarisation and increasing current density. The polarisation-dependent degradation has been identified to mainly originate from the cathode/electrolyte interface. However,... 

To meet the requirements for electronic conductivity in both the SOFC anode and cathode, a metallic electronic conductor, usually nickel, is typically used in the anode, and a conductive perovskite, such as lanthanum strontium manganite (LSM), is typically used in the cathode. Because the electrochemical reactions in fuel cell electrodes can only occur at surfaces where electronic and ionically conductive phases and the gas phase are in contact with each other (Figure 6.1), it is common... 

Currently, the standard SOFCs use yttria-stabilized zirconia (YSZ) (see Section 2.4.3) containing typically 8 mol% of Y02 as the electrolyte, a ceramic metal (a cermet) composed of Ni plus YSZ as the anode, and lanthanum strontium manganite (LSM) perovskite (La Sr, MnO,, ) as the cathode material [5,7,9,135],... 

In a 1999 letter to Nature (Perry Murray etal., 1999), from North Western University, Illinois, the authors record the first laboratory achievement of useful oxidation rates for direct methane electrochemical oxidation, using an IT/SOFC. The cathode structures were porous lanthanum strontium manganite (LSM) on porous ( 203)0.15 (Ce02)o,85 or YDC. The anodes were cermets, porous YSZ with nickel in the pores. The laboratory operating temperatures were in the range 500-700 °C. The account of the North Western work, reporting on new anode types, continues on pp. 921-924 of Williams (2002). 

The electronic conductors commonly used for SOFCs are lanthanum strontium manganite (LSM) for cathodes and nickel metal for anodes. The anodes are prepared in situ by reduction of a nickel oxide/ electrolyte composite. 

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