SOFC cathodes three-phase-boundaries

Figure 2.5 Schematic illustration of electrolyte-cathode three phase boundary, typical of high temperature YSZ-based SOFCs...

In the case of SOFCs, a large volume of work shows that for many SOFC electrodes, overall performance scales with the ID geometric length of this three-phase boundary. As such, the TBP concept and electrode performance models based on it have proven to be some of the most useful phenomenological concepts for guiding design and fabrication of SOFC cathodes, particularly the microstructure. 

Although the SOFC community has generally maintained an empirical approach to the three-phase boundary longer than the aqueous and polymer literature, the last 20 years have seen a similar transformation of our understanding of SOFC cathode kinetics. Few examples remain today of solid-state electrochemical reactions that are not known to be at least partially limited by solid-state or surface diffusion processes or chemical catalytic processes remote from the electrochemical—kinetic interface. 

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. 

There are, however, a number of important systems where this situation does not hold, for example ceramic-metal composites, and ceramic composites of electronic and ionic conductors, used as electrodes in sohd oxide fuel cells. Composite electrodes are important in a solid oxide fuel cell (SOFC), as they provide the contact area necessary for the electrode processes to occur. This is usually visualized as the three-phase boundary (TPB), the boundary line where electronic conductor, ionic conductor, and pores meet. A composite cathode is shown schematically in Figure 4.1.14, after Costamagna et al. [1998]. The processes occurring in a composite electrode are briefly as follows ... 

Figure 38.3 (a) CH4 fuel processing for the SOFC and PEMFC. (b) Two types of SOFCs planar SOFC and tubular SOFC and typical three-phase boundary at the anode and the cathode. 

Figure 7.24 Illustration of the three-phase boundary regions of different SOFC anode materials. Similar extension of the boundary is obtained in mixed conducting cathode materials.

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