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

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 YBa 1+ and neutral as well as singly negatively charged Y on Zr site YZr 0 and YZr 1-. However, using recent results for the doubly positively charged oxygen vacancy VO 2+ and taking account charge compensation, Fermi level pinning occurs, so that under Ba-rich conditions YZr 1- and VO 2+ are really stable. A similar consideration yields YBa 1+ and YZr 1- as stable defects in the Zr-rich case. Concerning VO 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 YZr 0 and YZr 1-. Under Ba-rich conditions Fermi level pinning results in a free formation energy of 0.481 eV which corresponds to a high VO 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 VO 2+ occurrence but the situation is somewhat less favourable than for the Ba-rich environment.