Solid oxide-molten carbonate nanocomposite fuel cells: particle size effect
Shalima, Shawuti and Öncel, Çınar and Gülgün, Mehmet Ali (2012) Solid oxide-molten carbonate nanocomposite fuel cells: particle size effect. (Submitted)
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Nanocomposite electrolytes made up of samarium doped ceria nanoparticles dispersed in a sodium carbonate matrix demonstrated an ionic conductivity of 0.1 Scm-1 around 500 oC. This temperature is about 300 oC lower than the temperatures where some of the best solid oxide electrolytes provided the same conductivity. The exact mechanism that yielded these high ionic conductivities is yet unknown. The interfaces between the oxide particles and the matrix are suspected to play a crucial role in ionic conductivity. This study is focused on shedding some light on the relationship between improved ionic conductivity and the composite microstructure. Model composites fabricated with samarium doped ceria particles with nanometer and/or micrometer sizes were investigated. Using oxide particles with different particle sizes the amount of interfacial areas in the composite was controlled. Micrographic and diffraction studies showed that the composites were two-phase materials, where the sodium carbonate matrix phase was amorphous. The maximum amount of the carbonate phase used in this study was 10 weight percent (wt%) 24 volume percent (vol%). Even at this high level of carbonate phase volume, both phases (oxide particles and carbonate phases) were interconnected in the microstructure. Impedance analysis results demonstrated that nanocomposites with nano sized ceria particles had higher ionic conductivities from 200 to 500 oC compared to the microcomposites with the micrometer sized ceria particles. However, at 600 oC, both composites had very similar impedance responses. X-ray diffraction pattern of the two SDC particles revealed the same fluorite structure. High resolution electron microscopy studies of sintered samples failed to illustrate an obvious reaction interphase between the components of the composite. The EDS analysis of the composite microstructre is complicated by strongly overlapping signals between Na-K, Sm-L, and Ce-L lines.
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