Y doping of BaZrO3 may lead to optimum conditions for proton conduction at operating temperature of solid oxide fuel cells: a first principles study

Abstract

First-principle calculations are performed to investigate Y substitutional defects at ground state and at 1000 K, for Ba- and Zr-rich chemical environments. In dependence on the Fermi level, at ground state singly positively charged Y may be potentially stable on Ba site ( ${{\rm{Y}}}_{Ba}^{1+}$ ) and neutral as well as singly negatively charged Y on Zr site ( ${\,{\rm{Y}}}_{Zr}^{0}$ and ${{\rm{Y}}}_{Zr}^{1-}$ ). However, using recent results for the doubly positively charged oxygen vacancy ( ${{\rm{V}}}_{{\rm{O}}}^{2+}$ ) and taking account charge compensation, Fermi level pinning occurs, so that under Ba-rich conditions ${{\rm{Y}}}_{{\rm{Zr}}}^{1-}\,$ and ${{\rm{V}}}_{{\rm{O}}}^{2+}$ are really stable. A similar consideration yields ${{\rm{Y}}}_{{\rm{Ba}}}^{1+}$ and ${{\rm{Y}}}_{{\rm{Zr}}}^{1-}$ as stable defects in the Zr-rich case. Concerning ${{\rm{V}}}_{{\rm{O}}}^{2+},$ which occurrence is a prerequisite to obtain a good proton conductor, by Y doping, at ground state only in the Ba-rich case a moderate concentration can be formed. At 1000 K the situation is improved importantly. The consideration of vibrational contributions to the free formation energy of Y on Zr site shows an increase of the stability of ${\,{\rm{Y}}}_{{\rm{Zr}}}^{0}$ and ${{\rm{Y}}}_{{\rm{Zr}}}^{1-}.$ Under Ba-rich conditions Fermi level pinning results in a free formation energy for the doubly positively charged O vacancy of 0.481 eV which corresponds to a high ${{\rm{V}}}_{{\rm{O}}}^{2+}$ concentration and optimum conditions for proton conduction. In Zr-rich case the respective value is 0.863 eV which leads also to relatively high ${{\rm{V}}}_{{\rm{O}}}^{2+}\,$ occurrence but the situation is less favourable than for the Ba-rich environment.

Keywords

• density functional theory
• ab initio thermodynamics
• phonon calculations
• solid oxide fuel cells