Haile - Proton Conducting Perovites

Proton Conducting Perovskites

A number of Me³⁺ (Me = Y, Gd, Nd, La) doped perovskites (BaCeO₃, BaZrO₃, SrZrO₃) exhibit high proton conductivities after exposure to water vapor, and thus have potential as electrolytes in mid-temperature fuel cells. My early work in this system focused on the relationship between microstructure, defect chemistry and proton transport. My group has shown, for example, that the conductivity of Gd-doped BaCeO₃ is highly sensitive to slight changes in stoichiometry, which may occur as a result of extreme processing conditions. However, glassy grain boundary phases, which were predicted to result from either excess barium content or dopant incorporation onto the Ba rather than Ce site, are not relevant to the transport mechanism. Our transmission electron microscopy studies revealed the grain boundaries in polycrystalline BaCe_0.9Gd_0.1O₃ and BaCe_0.85Gd_0.15O₃ to be free of any secondary phases that could influence proton transport. In a comprehensive study of microstructural influences on proton transport, we measured the conductivities of samples with grain sizes ranging from 1 to 11 mm, and established that the neither the bulk nor the specific grain boundary conductivity depend on grain size . Moreover, we demonstrated quite clearly that the grain boundary impedance is in fact higher than it is the bulk. Thus, for fuel cell and other applications, materials with large grain sizes are preferable in order to minimize the total grain boundary volume and maximize overall conductivity. As a part of this study, we also developed a complete methodology for determining the specific grain boundary conductivity from impedance measurements, without the need for direct microstructural investigations. Normally, when one performs an impedance measurement, one obtains a value for the “effective” grain boundary resistance, but is unable to convert this to a specific grain boundary resistivity because the grain size and thus the number of grain boundaries that contribute to this effective resistance is not known. If one makes the assumption that the bulk and grain boundary dielectric constants are equal (and they certainly do not differ by more than an order of magnitude), we have demonstrated that excellent estimates of the specific grain boundary resistivity can be obtained directly from the impedance data, in the absence of any microstructural knowledge. This has significant impact for enabling the grain boundary properties of different samples prepared by different routes to be directly compared.

Current efforts aim on improving the chemical stability of barium cerate in CO₂ containing atmospheres, as well as expanding our investigations to related chemical systems, with a particular emphasis on developing mixed electron and proton conductors.

Acknowledgments ($$$)

Former: Department of Energy; General Motors; EPRI; HRL (Hughes Research Labs); Honeywell-Allied Signal; Defense Advanced Research Projects Agency (DARPA)

Selected Publications

C. Y. Jones, J. Wu, L.P. Li and S. M. Haile, "Hydrogen Content in Doped and Undoped BaPrO3 and BaCeO3 by Cold Neutron Prompt-Gamma Activation Analysis," J. Appl. Phy. 97, 114908-1-4. (2005).

J. Wu, R. A. Davies, M. S. Islam, and S. M. Haile, "Atomistic Study of Doped BaCeO3: Dopant Site-Selectivity and Cation Non-stoichiometry," Chem. Mat. 17, 846-851 (2005).

T. A. Mary, C. Y. Jones, G. J. Snyder and S. M. Haile, "Nonstoichiometry, Structure and Electrical Properties of 'SrPrO3'," Chem. Mat. 17, 5146-5154 (2005).

P. Babilo and S. M. Haile, "Enhanced Sintering of Yttrium doped Barium Zirconate by Addition of ZnO," J. Amer. Cer. Soc. 88, 2362-2368 (2005).

J. Wu, S. M. Webb, S. Brennan and S. M. Haile, "Dopant Site Selectivity in BaCe0.85M0.15O3-d by Extended X-ray Absorption Fine Structure," J. Appl. Phys. 97, 054101 (2005).

J. Wu, L. P. Li, W. T. P. Espinosa, and S. M. Haile, "Defect Chemistry and Transport Properties of BaxCe0.85M0.15O3-d," J. Mat. Res. 19, 2366-2376 (2004).

S.M. Haile, G. Staneff and K.H. Ryu "Non-Stoichiometry, Grain Boundary Transport and Chemical Stability of Proton Conducting Perovskites," J. Mat. Sci. 36 (2001) 1149-1160.

K.H. Ryu and S.M. Haile "Chemical Stability and Proton Conductivity of Doped Perovskite Oxides in the BaCeO₃-BaZrO₃ System," Solid State Ionics 125 (1999) 355-367.

S. M. Haile, D. L. West and J. Campbell "The Role of Microstructure and Processing on the Proton Conducting Properties of Gadolinium-Doped Barium Cerate," J. Mat. Res. 13 (1998) 1576-1595.

D. Shima and S. M. Haile "The Influence of Cation Non-Stoichiometry on the Properties of Undoped and Gadolinia-Doped Barium Cerate," Solid State Ionics 97 (1997) 443-447.

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