Composite cathodes, microstructure

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. 

Although the electrode performance can be improved by the introduction of compositional and microstructural gradients into the anode or cathode, the processing required to produce such graded layers also increases in complexity as the number of discrete layers increases, particularly when separate deposition and firing steps are required for each increment in layer composition or structure. 

Song HS, Hyun SH, Moon J, and Song RH. Electrochemical and microstructural characterization of polymeric resin-derived multilayered composite cathode for SOFC. J. Power Sources 2005 145 272-277. 

Several recent breakthroughs in the design of solid oxide fuel cell (SOFC) anodes and cathodes are described in the Chapter of H. Uchida and M. Watanabe. The authors, who have pioneered several of these developments, provide a lucid presentation describing how careful fundamental investigations of interfacial electrocatalytic anode and cathode phenomena lead to novel electrode compositions and microstructures and to significant practical advances of SOFC anode and cathode stabihty and enhanced electrocatalysis. 

The project goals are to significantly improve both the kinetic performance of the electrocatalyst powder at low noble metal loading and its utilization in the cathode layers through layer structure development. Limitations in the catalyst performance will be addressed through combinatorial discovery of supported catalyst compositions and microstructures. The discovery of these new catalyst formulations will be carried out under conditions that have been scaled for commercial powder production. A large variation of binary, ternary and quaternary noble metal -transition metal alloys and mixed metal-metal oxide catalyst compositions will be screened. To improve the utilization/performance of the catalyst in MEAs,... 

A SEM micrograph of the cathode/electrolyte interface and preliminary results on the electrochemical activity of YSZ electrolyte-supported SOFCs containing Ni-YSZ anode and a LSCF-SDC composite cathode are shown in Fig. 14. As it can be seen in Fig. 14(a), the composite film not only has good adhesion to the electrolyte, but also possesses a porous microstructure which is required for the oxidant electrochemical reduction. It indicates that such a composite film can have a good performance as SOFC cathode. By the LSV technique, qualitative information about electrochemical activity of this SOFC was acquired. The power density curves (Fig. 14b) revealed that maximum power densities were 19, 26, 36 and 46 mW/cm2 at 800, 850, 900 and 950 °C. It is possible to compare these first results with literature data and safely state that the LSCF-SDC cathode composite is qualitatively better than other plain standard materials or cathode composites already reported. It should also be mentioned that the result obtained at 800 °C is similar to that reported by Mucdllo et al (Mucdllo et al., 2006) for a SOFC single cell with LSM-YSZ cathode, Ni-YSZ anode and 70 pm... 

A and B. Conductivity relaxations, related to the microstructure of the composite. Their activation energies are about 1 eV, similar to those of the grain or grain boundary conductivities and are independent of the oxygen partial pressure (Po ). One or both of these arcs may be absent in some composite cathodes, but are absent with point contact electrodes. 

The typical SOFC-MEA of IT-SOFC is with the structure of NiO+SDC SDC composite cathode (e g. SSC-SDC/SSC, etc.). The microstructure and the results of cell performance are shown in Figs. 8 9. The MPD is 607.68 mW/cm (at 650 °C) and the degradation rate is near 3.0%/Khr with 950 hours operation The redox problem of SDC electrolyte has been identified and the scheme to solve this issue will be proposed in the near future. LT-SOFC should be a technology of choice for these applications as long as we are in a... 

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] ... 

Button-size cells were manufaetured using both traditional and advanced (spark plasma) sintering techniques. Half cells eonsisting of eleetrolyte and anode were made by means of SPS and cathodes were applied afterwards by means of electrophoretie deposition. Polarization characteristics of cathode materials were obtained and analyzed with a due regard for their composition and microstructure. 

In addition to chemical composition, electrode microstructure has a profound effect on the cathode polarization. 

M. Wandel, J.R. Bowen, M. Mogensen, Performance, degradation and microstructure of LSCF/CGO composite cathodes, in European SOFC Symposium, Lucerne, 2008... 

Different microstructural regions in a material which has an almost uniform composition can also lead to the formation of corrosion cells (e.g., in the vicinity of welds). Basically, corrosion cells can be successfully overcome by cathodic protection. However, in practice, care has to be taken to avoid electrical shielding by large current-consuming cathode surfaces by keeping the area as small as possible. In general, with mixed installations of different metals, it must be remembered that the protection potentials and the protection range depend on the materials (Section 2.4). This can restrict the use of cathodic protection or make special potential control necessary. 

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