Perovskite cathodes

Air flow When air blower malfunctions, systems will be in trouble. Oxygen starvation takes place in cathodes. Perovskite oxide cathodes will be reduced to provide oxygen if oxygen gas is not sufficiently available. As a result, such cathodes are heavily damaged. 

Table 3.3 CTE and conductivity of various cathode perovskite materials...

Complex Base-Metal Oxides Complex oxide systems include the mixed oxides of some metals which have perovskite or spinel structure. Both the perovskites and the spinels exhibit catalytic activity toward cathodic oxygen reduction, but important differences exist in the behavior of these systems. 

An example for a compound of the perovskite type is LaNiOj. In other com-ponnds of the perovskite type, nickel may be replaced by cobalt or iron, and lan-thannm in part by alkaline-earth metals, an example being Lag 8Sro2Co03. The activity of perovskites toward cathodic oxygen reduction is low at room temperature but rises drastically with increasing temperature (particularly so above 150°C). In certain cases the activity rises so much that the equilibrium potential of the oxygen electrode is established. 

Fig. 43. Full-cell performance with hot-pressed membrane, perovskite electrodes. Cathode removal and anode generation as a function of applied current. Lines calculated from stoichiometry, 1 mol/2 F.

The Brouwer diagram approach can be illustrated with reference to the perovskite structure oxide system BaYbvPr VC>3, which has been explored as a potential cathode material for use in solid oxide fuel cells. The parent phase... 

Mai A, HaanappelVAC, UhlenbruckS, TietzF, and Stover D. Ferrite-based perovskites as cathode materials for anode-supported solid oxide fuel cells, Part I. Variation of composition. Solid State Ionics 2006 176 1341-1350. 

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. 

LaMn03 is an intrinsic p-type conductor. Electronic conductivity is enhanced by substitution of the La3+ site with divalent ions such as strontium or calcium. Of the alkaline-earth dopants, Sr substitution is preferred for SOLC applications because the resultant perovskite forms stable compounds with high conductivity in the oxidizing atmosphere found at the cathode [41], Extensive data show that La, xSi. MnO where x = 0.1 - 0.2, provides high conductivity while maintaining mechanical and chemical stability with YSZ [41, 42],... 

FIGURE 3.6 Cathodic overpotential of some Mn-based perovskites at a current density of 0.1 A cut2 as a function of operating temperature. (From Ishihara, T. et al., J. Electrochem. Soc., 142 1519-1524, 1995. With permission.)... 

An alternative to the Co-rich perovskites is the Sr-doped LaFe03 which has a lower thermal expansion coefficient and a superior chemical compatibility with doped Ce02 electrolyte. LaFe03 is expected to be more stable than Ni- and Co-based perovskites because the Fe3+ ion has the stable electronic configuration 3d5. It is, therefore, expected that compositions in the system (La,Sr)(Co,Fe)03 will have desirable properties for intermediate temperature SOFC cathode applications. 

The Sr-doped praseodymium manganites and cobaltites have been studied by several groups as potential cathodes for ITSOFC. Kostogloudis et al. [116-118] systematically investigated the crystal structure, electrical conductivity, and thermal expansion properties of (Pr, Sr)Mn03, (Pr, Sr)Co03, and (Pr, Sr)(Co, Mn)03 systems. All compounds have the orthorhombic perovskite GdFe03-type structure (Pbnm... 

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... 

Besides the glass seal interfaces, interactions have also been reported at the interfaces of the metallic interconnect with electrical contact layers, which are inserted between the cathode and the interconnect to minimize interfacial electrical resistance and facilitate stack assembly. For example, perovskites that are typically used for cathodes and considered as potential contact materials have been reported to react with interconnect alloys. Reaction between manganites- and chromia-forming alloys lead to formation of a manganese-containing spinel interlayer that appears to help minimize the contact ASR [219,220], Sr in the perovskite conductive oxides can react with the chromia scale on alloys to form SrCr04 [219,221],... 

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... 

Single-phase perovskite MIECs such as Sr-doped lanthanum cobaltite (LSC), lanthanum ferrite (LSF), lanthanum cobalt ferrite (LSCF) and samarium cobaltite (SSC), and Ca-doped lanthanum ferrite (LCF) [13] are sometimes used alone in SOFC cathodes, as depicted in the lower right-hand comer of Figure 6.1, but combining an... 

MIEC with an additional ionically conductive phase, such as GDC or SDC, typically extends the electrochemically active region still further due to the higher ionic conductivity of GDC and SDC compared to that of the perovskites. The optimal composition of a two-phase composite depends in part on the operation temperature, due to the larger dependence of ionic conductivity on temperature compared to electronic conductivity. A two-phase composite of LSCF-GDC therefore has an increasingly large optimal GDC content as the operating temperature is reduced [14], A minimum cathode Rp for temperatures above approximately 650°C has been found for 70-30 wt% LSCF-GDC composite cathodes, while at lower temperatures, a 50-50 wt% LSCF-SDC composite cathode was found to have a lower Rp [15]. 

To prevent inter-reactions between YSZ electrolytes and perovskite cathode materials [32, 33],... 

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