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Optimization of ionic conductivity in doped ceria
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
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2006 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 103, no 10, 3518-3521 p.Article in journal (Refereed) Published
Abstract [en]

Oxides with the cubic fluorite structure, e.g., ceria (CeO2), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations. The high ionic conductivity of doped ceria makes it an attractive electrolyte for solid oxide fuel cells, whose prospects as an environmentally friendly power source are very promising. In these electrolytes, the current is carried by oxygen ions that are transported by oxygen vacancies, present to compensate for the lower charge of the dopant cations. Ionic conductivity in ceria is closely related to oxygen-vacancy formation and migration properties. A clear physical picture of the connection between the choice of a dopant and the improvement of ionic conductivity in ceria is still lacking. Here we present a quantum-mechanical first-principles study of the influence of different trivalent impurities on these properties. Our results reveal a remarkable correspondence between vacancy properties at the atomic level and the macroscopic ionic conductivity. The key parameters comprise migration barriers for bulk diffusion and vacancy-dopant interactions, represented by association (binding) energies of vacancy-dopant clusters. The interactions can be divided into repulsive elastic and attractive electronic parts. In the optimal electrolyte, these parts should balance. This finding offers a simple and clear way to narrow the search for superior dopants and combinations of dopants. The ideal dopant should have an effective atomic number between 61 (Pm) and 62 (Sm), and we elaborate that combinations of Nd/Sm and Pr/Gd show enhanced ionic conductivity, as compared with that for each element separately.

Place, publisher, year, edition, pages
2006. Vol. 103, no 10, 3518-3521 p.
Keyword [en]
density functional theory; diffusion; point defects; solid oxide fuel cells; CeO2
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
URN: urn:nbn:se:kth:diva-6887DOI: 10.1073/pnas.0509537103ISI: 000236225300006Scopus ID: 2-s2.0-33644866625OAI: oai:DiVA.org:kth-6887DiVA: diva2:11727
Note
QC 20100622Available from: 2007-03-14 Created: 2007-03-14 Last updated: 2012-03-22Bibliographically approved
In thesis
1. From the Electronic Structure of Point Defects to Functional Properties of Metals and Ceramics
Open this publication in new window or tab >>From the Electronic Structure of Point Defects to Functional Properties of Metals and Ceramics
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Point defects are an inherent part of crystalline materials and they influence important physical and chemical properties, such as diffusion, hardness, catalytic activity and phase stability. Increased understanding of point defects enables us to tailor the defect-related properties to the application at hand. Modeling and simulation have a prominent role in acquiring this knowledge. In this thesis thermodynamic and kinetic properties of point defects in metals and ceramics are studied using first-principles calculations based on density functional theory. Phenomenological models are used to translate the atomic level properties, obtained from the first-principles calculations, into functional materials properties. The next paragraph presents the particular problems under study.

The formation and migration of vacancies and simple vacancy clusters in copper are investigated by calculating the energies associated with these processes. The structure, stability and electronic properties of the low-oxygen oxides of titanium, TiOx with 1/3 < x < 3/2, are studied and the importance of structural vacancies is demonstrated. We develop an integrated first-principles and Calphad approach to calculate phase diagrams in the titanium-carbon-nitrogen system, with particular focus on vacancy-induced ordering of the substoichiometric

carbonitride phase, TiCxNy (x+y < 1). The possibility of forming higher oxides of plutonium than plutonium dioxide is explored by calculating the enthalpies for nonstoichiometric defect-containing compounds and the analysis shows that such oxidation is only produced by strong oxidants. For ceria (CeO2) doped with trivalent ions from the lanthanide series we probe the connection between the choice of a dopant and the improvement of ionic conductivity by studying the oxygen-vacancy formation and migration properties. The significance of minimizing the dopant-vacancy interactions is highlighted. We investigate the redox thermodynamics of CeO2-MO2 solid solutions with M being Ti, Zr, Hf, Th, Si, Ge, Sn or Pb and show that reduction is facilitated by small solutes.

The results in this thesis are relevant for the performance of solid electrolytes, which are an integral part of solid oxide fuel cells, oxygen storage materials in automotive three-way catalysts, nuclear waste materials and cutting tool materials.

Place, publisher, year, edition, pages
Stockholm: Materialvetenskap, 2007
Keyword
first principles, ab initio, density functional theory, Calphad, point defects, diffusion, solid electrolytes, oxygen storage materials
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-4309 (URN)978-91-7178-590-9 (ISBN)
Public defence
2007-03-30, F2, Lindstedtsvägen 26, Stockholm, 10:00
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Note
QC 20100622Available from: 2007-03-14 Created: 2007-03-14 Last updated: 2012-03-22Bibliographically approved

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