Cathodes for Intermediate Temperature SOFCs

In addition to compatibility with the electrolyte, compatibility of the cathode with the interconnect is also important. Both oxide ceramic and metallic materials are used as interconnects in SOFCs. As expected, these two types of interconnects present quite different issues in their compatibility with the cathode. 

Since the A-site-deficient lanthanum manganite shows better compatibility with YSZ, its compatibility with oxide interconnect is of particular interest. Nishiyama et al. [62] found the following interesting behaviour of the A-site-deficient manganite at the interface with (La,Ca)Cr03 (LCC) that had excess CaO to enhance its air sinterabillty  

LaCrOs is formed as a dense layer next to the original LCC while CaMnOs is formed as a porous layer next to the LaCrOs layer. This reaction is triggered by the presence of calcium oxychromates in the original LCC which can be squeezed out of grain boundaries under oxygen potential gradient. 

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Fu C, Sun K, Zhang N, Chen X, and Zhou D. Electrochemical characteristics of LSCF-SDC composite cathode for intermediate temperature SOFC. Electrochim. Acta 2007 52 4589-4594. 

Liu Y, Compson C, and Liu M. Nanostructured and functionally graded cathodes for intermediate-temperature SOFCs. Fuel Cells Bull. 2004 10 12-15. 

L.-W. Tai, M.M. Nasrallah and H.U. Anderson, Lai-xSrxCoi-yFeyOs-a, A potential cathode for intermediate temperature SOFC applications, in S.C. Singhal and H. Iwahara (Eds.), Proceedings of the 3rd International Symposium on Solid Oxide Fuel Cells. The Electrochemical Society, Pennington, NJ, 1994, pp. 241-251. 

Liang, F.L., Chen, X, Jiang, S.P., Chi, B., Pu, J. Jian, L. Development of nanostructured and palladium promoted (La,Sr)Mn03-based cathodes for intermediate-temperature SOFCs. Electrochem. Solid-State Lett. 11 (2008), pp. B213-B216. 

It is generally considered that the cathodes for intermediate-temperature SOFCs should be electrochemically more active than those cathodes in the first generation, namely, LSM. When LSF, LSC, or LSCF is used as cathode, an interlayer made of doped ceria becomes inevitable to avoid chemical reactions between cathode and YSZ. In addition, such cathodes should be also protected against Cr vapors. This approach gives rise to a complicated layer structure across the electrolyte to current collector and makes it difficult to fabricate such complicated layers. For example, the following points are important ... 

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. 

Li Q, Zhao H, Huo L, Sun L, Cheng X, and Grenier JC. Electrode properties of Sr doped La2Cu04 as new cathode material for intermediate-temperature SOFCs. Electrochem. Commun. 2007 9 1508-1512. 

Chiba, R., Yoshimura, F., Sakurai, Y., Tabata, Y., and Arakawa, M. A Study of Cathode Materials for Intermediate Temperature SOFCs Prepared by the Sol-gel Method, Solid State Ionics, 175,23 (2004). 

Mixed-conducting (A,Ln)M03 (M = Fe, Co), promising as cathodes of intermediate-temperature SOFCs, oxygen-separation membranes, and electrocatalysts for high- and low-temperature processes involving oxygen. 

E. Maguire, B. Gharbage, F. Marquesb, and J. Labrinchab. Cathode materials for intermediate temperature SOFCs. Solid State Ionics 127, (2000) 329-335. 

LSCF is another promising cathode material for intermediate-temperature SOFCs [81, 82]. Owing to its much higher oxygen ionic conductivity than LSM,... 

LSCs are attractive cathode materials for intermediate temperature SOFCs in terms of its high catalytic activity and surface exchange coefficient. However, the material also has a large coefficient of thermal expansion, which is about 23 x 10 almost twice that of typical electrolytes, such as YSZ and doped ceria electrolytes (11-12 x 10 K ). The TEC mismatch between the electrolyte and cathode will result in a thermal stress, which is responsible for delamination at the... 

For intermediate temperature SOFCs, a composite cathode consisting of strontium-doped lanthanum manganite (LSM) and YSZ has shown good performance [10]. For use with ceria-based electrolytes [11], a (La,Sr)(Co,Fe)03 (LSCF)-based cathode has been developed [12],... 

A serious challenge in the use of LSM as a cathode material in intermediate temperature SOFC stems from the use of metallic interconnects. Many of these metals contain chromium, which forms a stable protective oxide (chromia) layer with reasonable conductivity (see Section 7.1.4 on interconnects for more details). However, chromia vapors can lead to serious poisoning of the cathode (21, 22). Although one might attribute this problem more to the interconnect material than to the cathode, the poisoning effect was found to depend strongly on the electrolyte/cathode material combination. 

Similarly, for certain degradation mechanisms, temperature becomes a dominant factor. Particularly, cathode materials have the tendency of being more reactive to CO2, Cr03(g), etc. In view of this, in the intermediate-temperature SOFCs, it becomes important to know degradation mechanisms associated with materials utilized in such SOFCs. 

Ceria doped with gadolinia (GDC) or samaria (SDC) is used as the interlayer in some intermediate-temperature SOFCs. Even with the interlayer, however, issues of long-term stability may still remain [39]. If LaGaOs-based oxide is used as the electrolyte, the interface stability problem is less significant. LSC-related cathodes are widely used on LaGa03. For long-term stability, however, the interdiffusion of Co and Ga should be avoided. 

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