The critical factor in increasing solid oxide fuel cells (SOFC) reliability and decreasing operating costs is lowering the operating temperature below 800 ^oC. Materials with the greatest prospects for application at these temperatures are transition metal perovskites because of a wide range of unique physical and thermal properties, stability against CO poisoning, no need for expensive catalysts, and possibility of use with variety of fuels. Currently used cathode materials such as La_1-xSr_xMnO₃ (LSM, x=0.1–0.4) with purely electronic conductivity are limiting the performance. Mixed ionic and electronic conducting cathodes are preferred for utilization because of an increased reaction zone of the three-phase boundary. Superior oxygen ion conductivity requires large amount of hoping sites such as oxygen vacancies. By using x-ray and neutron powder diffraction, thermogravimetric (TGA), conductivity, dilatometery, and area specific resistance (ASR) measurements we have recently studied several highly oxygen deficient perovskites, LSM (x>0.5) [1,2], SrMn_1-yFe_yO_z (SMF), and La_1-xSr_xFe_1-yCo_yO_z (LSFC, x>0.5)[3] to explore their potential as SOFC cathodes.

Polycrystalline samples were synthesized from stoichiometric mixtures of La₂O₃, SrCO₃, MnO₂, Fe₂O₃, and Co₃O₄. Single-phase perovskite SFM and LSFC were obtained in air, while LSM required special treatment in Ar followed by anneal in air. [4] Oxygen content under various atmospheres was studied with TGA apparatus (Cahn TG171). TGA was also used to obtain samples with desired oxygen content by “in-situ” anneal. Structural details, including oxygen content and ordering were investigated by NPD at the Intense Pulsed Neutron Source at ANL. High-resolution data were collected in situ by varying temperature, time, and oxygen partial pressure, and analyzed using the Rietveld method and the GSAS code. X-ray powder diffraction (Rigaku D/MAX) was used to determine the phases and structure types present after long-term annealing in air at 600, 800, and 1000 ^oC. ASR performance was studied using electrochemical impedance spectroscopy and dc polarization curves.

LSM samples with higher Sr content exhibit increasing range of oxygen deficiency, high ionic and electronic conductivity, and good chemical stability with YSZ. ASR measurements showed that compositions x=0.6–0.8 exhibit 2-3 times lower values than the standard LSM materials. While samples with x=0.9-1 decompose in air above 800 ^oC, all compositions show good thermal stability against decomposition at low oxygen pressures. [5] Three oxygen-vacancy ordered phases with z=2.71, 2.6, and 2.5, detrimental for both oxygen permeation and electrical conductivity, were found for x=0.6-1 up to 1100 ^oC. [6] The LSFC samples exhibit very high oxygen non-stoichiometry, 2.25≤z≤3.00 even at moderately reducing conditions, and excellent ionic and electronic conductivity. However, they show poor stability with YSZ and decomposition at low oxygen pressures. In addition, we observed several LSFC oxygen-vacancy ordered phases at z=2.875, 2.5, and 2.25+x/2, some of them stable to 800 ^oC. The SMF samples showed intermediate properties between LSM and LSFC; no oxygen vacancy ordered phases were observed to date. We have determined the compositions of LSM, SMF, and LSFC and their thermal stability ranges that exhibit only the favorable vacancy-disordered phases. Unfortunately, all these compositions exhibit very large thermal ((17-26)10^-6/K) and oxygen-chemical (0.024-0.030/mole) expansion coefficients not compatible with the known electrolytes.

Several new perovskite compositions satisfy quite a few requirements for low temperature SOFC cathode materials such as: fast oxygen-ion and high electronic conductivity, structural stability, chemical compatibility with YSZ electrolyte, and improved performance over the present materials. However, frequent presence of oxygen ordered phases at enhanced vacancy content and the a large thermal chemical expansion coefficients may limit their applicability. Requirements of an enhanced vacancy content for high ionic conductivity and the mixed-valency of transition metals for high electronic conductivity go up against the small expansion coefficients. Clever materials design of new compounds is required to overcome these impediments.

Acknowledgments

This work was supported by the NSF-DMR-0706610 and U.S. DOE-BES DE-AC02-06CH11357.

References

[1] B. Dabrowski, et al., JSSCh. 170, 154 (2003)

[2] O. Chmaissem, et al., Phys. Rev. B67, 094431, 2003

[3] K. Swierczek, et al., JSSCh. 182, 280 (2009)

[4] B. Dabrowski, et al., Acta Physica Polonica A 105, 45 (2004)

[5] L. Suescun, et al., JSSCh 180, 1698 (2007)

[6] L. Suescun, et al., Chem. Mater. 20, 1636 (2008)

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