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 (YBa1+) and neutral as well as singly negatively charged Y on Zr site ( YZr0 and YZr1-). However, using recent results for the doubly positively charged oxygen vacancy (VO2+) and taking account charge compensation, Fermi level pinning occurs, so that under Ba-rich conditions YZr1- and VO2+ are really stable. A similar consideration yields YBa1+ and YZr1- as stable defects in the Zr-rich case. Concerning VO2+, 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 YZr0 and YZr1-. Under Ba-rich conditions Fermi level pinning results in a free formation energy for VO2+ of 0.481 eV which corresponds to a high VO2+ concentration and optimum conditions for proton conduction. In Zr-rich case the respective value is 0.863 eV which leads also to relatively high VO2+ occurrence but the situation is somewhat less favourable than for the Ba-rich environment.