CeO2 has received considerable interest as an electrolyte for intermediate temperature solid oxide fuel cells. Unfortunately, within this temperature range (~600-800°C), CeO2 displays poor ionic conductivity, while at high temperatures and low oxygen partial pressures Ce4+ can be reduced to Ce3+, due to facile oxygen vacancy formation, which results in electronic conductivity. In an attempt to improve the ionic conductivity of CeO2 aliovalent doping is commonly employed, with trivalent species (e.g. Gd3+, Sm3+) a particular focus due to their creation of charge compensating vacancies which forms an O2- diffusion pathway without the presence of Ce3+. Experimental evidence has corroborated the increased ionic conductivity from trivalent dopants, however a detailed understanding on the effect of the dopant on the electronic structure of CeO2 and a full exploration of the range of trivalent dopants has been lacking. This study considers two aspects of trivalent doping using density functional theory simulations: (i) the formation of charge compensating vacancies and their role in O2- conductivity, and (ii) their effect on the reducibility of CeO2. These aspects are explored for a range of p-, d- and f-block ions. The first part of the study focusses on the position of the dopant ions to the vacancies, as dopant-vacancy association can limit ionic diffusion. The results indicate a clear relationship between the dopant species ionic radius and its position relative to the vacancy. In addition, some rare earth dopants (e.g. Dy, Nd) are found to have low dopant-vacancy association energies, suggestive of better ionic conductivity. The second aspect of the study, which is largely ignored in the literature, shows that the reduction energy correlates with the defect structure, therefore being only indirectly influenced by the ionic radius.

Structure and reducibility of CeO2 doped with trivalent cations

253rd National Meeting (2017)

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