Perovskite Oxide for Cathode of SOFCs

The transport properties of perovskite-type oxides are dependent mainly on the B-site cations. Among them, Mn-based perovskites and Co/Fe-based perovskites are most frequently used for high-temperature and intermediate-temperature SOFCs, respectively. Recently, Ni-based K2NiF4-type oxides are also being investigated [5]. Their composition and microstructure are still to be optimized based on the defect chemistry, electrochemistry, and thermodynamics. 

The scientific bases for the electrode reaction kinetics are also to be established. In a porous cathode, oxygen adsorbed from the gas phase on the cathode particles is dissociated and transported via diffusion on the surface or through the bulk. The oxygen potential profile inside the cathode layer is established according to the rates of those processes under current flow. Thus, the properties of the electrode particles under operation are not correctly understood without the knowledge of electrode kinetics. 

In this chapter, general features of perovskite-type oxides are summarized from the point of view of the required properties as a cathode material. Then, 

Graduate School of Environmental Studies, Tohoku University, 1-1 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan e-mail kawada ee.mech.tohoku.ac.jp 

Ishihara (ed.), Perovskite Oxide for Solid Oxide Fuel Cells, 

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Table 6.2 Thennal expansion coefficients of perovskite oxides for SOFC cathode applications [33, 168, 184, 186]...

In this chapter the technological development in cathode materials, particularly the advances being made in the material s composition, fabrication, microstructure optimization, electrocatalytic activity, and stability of perovskite-based cathodes will be reviewed. The emphasis will be on the defect structure, conductivity, thermal expansion coefficient, and electrocatalytic activity of the extensively studied man-ganite-, cobaltite-, and ferrite-based perovskites. Alterative mixed ionic and electronic conducting perovskite-related oxides are discussed in relation to their potential application as cathodes for ITSOFCs. The interfacial reaction and compatibility of the perovskite-based cathode materials with electrolyte and metallic interconnect is also examined. Finally the degradation and performance stability of cathodes under SOFC operating conditions are described. 

The perovskite oxides used for SOFC cathodes can react with other fuel cell components especially with yttria-zirconia electrolyte and chromium-containing interconnect materials at high temperatures. However, the relative reactivity of the cathodes at a particular temperature and the formation of different phases in the fuel cell atmosphere... 

Certain oxides with a perovskite structure are generally applied to the cathode. For high temperature type SOFCs, doped-LaMn03 is used as the typical cathode. For low temperature SOFCs, LaSr(CoFe)03 or La(NiFe)03 are used as the cathode. Doped LaCo03 has a high electric conductivity and shows an excellent catalytic performance. However, the TEC of LaSrCo03 is larger than that of the electrolyte, and so Fe is substituted to reduce the TEC of the cathode. 

For FT SOFCs, perovskite oxides (La,Sr)(Co,Fe)O3 5 have attracted particular attention [50]. Due to chemical reactivity with yttria-stabilized zirconia (YSZ), the use of a protective interlayer between the cathode and electrolyte is required to increase the system s stability during long-term operation [51-53]. 

As in the case of the previous family of iron perovskites, this new series of compounds are of interest for their use as mixed ionic electronic conducting materials, mainly from the point of view of cathodes for Solid Oxide Fuel Cells (SOFC), although they could also be used as ceramic membranes for oxygen separation. In the ptresent case, the degree of lanthanide substitution was fixed to x=0.5 given that previous studies have shown that it is precisely at this degree of substitution when electronic and ionic conductivity are maximised (Hansen, 2010 Vidal et al., 2007). 

The oxygen reduction reaction occurs at the cathode. Nearly all candidate cathode materials are based on perovskite oxides such as lanthanum strontium manganite (LSM). However, the oxygen reduction reaction is particularly affected by lower operating temperatures with kinetics and transport processes that are thermally activated. The development of materials with higher electrochemical activities for the oxygen reduction reaction is critical for the development of IT-SOFCs with higher efficiencies. 

A large group of chemical compositions has been investigated as potential candidates for IT SOFC cathode materials. A recent article by Skinner has provided an overview of the progress of perovskite type oxides for the SOFC cathode, with an emphasis on the role of chemical compositions [41]. On the contrary, microstructure plays a major role in the cathode function as well. This is particularly true when the composite cathode, which shows a better performance compared to a single composition cathode, is used. Several authors have shown that electrode microstructure and transport properties have a profound effect on polarization. Tanner et al. [42] have shown that polarization resistance (Rp) depends upon the grain size, d, of the ionic conductor in the composite electrode and the volume fraction porosity, which was further derived as in (1) by considering the monolayer gas adsorption. A similar relation has been proposed as [43] ... 

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