Nanocrystaline mixed ionic-electronic conducting thin-film cathodes
Considerable interest has emerged in low-to-medium temperature (500 - 800 °C) solid oxide fuel cells (SOFCs) implicating less-severe problems with thermally activated degradation compared to high-temperature SOFCs which operate in the 800-1000°C temperature range. To compensate for the lower diffusivity of oxygen ions and the reduced catalytic activity at these temperatures, the evaluation of new materials is motivated. One concept for the improvement of the cathode performance is the use of nanostructured mixed ionic-electronic conductors (MIEC) like La1-xSrxCoO3-δ (LSCO) with a large surface area and high density of grain boundaries.
The goal within this project was the characterization of the chemical composition / phase stability and the microstructure with special focus on the grain boundaries, which influence the transport properties of nanocrystalline material. A detailed characterization of the structural assembly and chemical composition was performed by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED), energy-dispersive X-ray spectroscopy (EDXS), energy-filtered TEM (EFTEM) and electron-energy-loss spectroscopy (EELS).
Nanocrystalline cathode layers with nominal compositions of La0.5Sr0.5CoO3-δ and La0.6Sr0.4CoO3-δ were fabricated by metal organic deposition (cooperation with the Fraunhofer Institut für Silikatforschung (Würzburg)). As substrates, common electrolytes like polycrystalline Gd-doped Ceria (CGO) and Yttria-stabilized Zirconia (YSZ) were used. The decomposition of the material is visualized in the figure below which shows the microstructure (left colum, TEM bright-field images) after annealing at 1000 °C/8 h, 700 °C/8 h and 700 °C/100 h (top to bottom) as well as the element distribution of La, Sr, Co, Zr and Ce (left to right) obtained by EFTEM.
L. Dieterle, D. Bach, R. Schneider, H. Störmer, D. Gerthsen, C. Peters, A. Weber, E. Ivers-Tiffée, U. Guntov, H. Yokokawa, Structural properties and chemical stability of nanocrystalline La0.5Sr0.5CoO3-d layers on Yttria-stabilized Zirconia, J. Mat. Sci. 43, 3135 (2008)
L. Dieterle, P. Bockstaller, D. Gerthsen, J. Hayd, E. Ivers-Tiffée, U. Guntow, Microstructure of Nanoscaled La0.5Sr0.5CoO3-d Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells, Adv. Energy Mat. 1, 249 (2011) DOI: 10.1002/aenm.201000036
J. Hayd, L. Dieterle, U. Guntow, D. Gerthsen, E. Ivers-Tiffée, Nanoscaled La0.6Sr0.4CoO3-δ as intermediate temperature SOFC cathode: Microstructure and electrochemical performance, J. Power Souces 196, 7263 (2011)
L. Dieterle, Electron microscopy study of pure and Sr-substituted LaCoO3, Dissertation KIT, (2012) http://digbib.ubka.uni-karlsruhe.de/volltexte/1000029250