Proton conducting alkaline earth zirconates and titanates for high drain electrochemical applications

The mobility and stability of protonic defects in acceptor-doped perovskite-type oxides (ABO(3)) in the system SrTiO3-SrZrO3-BaZrO3-BaTiO3 have been examined experimentally and by computational simulations. These materials have the potential to combine high proton conductivity and thermodynamic stability. While any structural and chemical perturbation originating from the B-site occupation (poor chemical matching of the acceptor-dopant or Zr/Ti-mixing) leads to a significant reduction of the mobility of protonic defects, Sr/Ba-mixing on the A-site appears to be less critical. The stability of protonic defects is found to essentially scale with the basicity of the lattice oxygen, which is influenced by both A- and B-site occupations. The highest proton conductivities are observed for acceptor-doped BaZrO3. Despite its significantly higher ionic radius compared to Zr4+, Y3+ is found to be optimal as an acceptor dopant for BaZrO3. Mulliken population analysis shows that Y does not change the oxide's basicity (i.e. it chemically matches on the Zr-site of BaZrO3). The highest proton conductivities have been observed for high Y-dopant concentrations (15-20 mol%). For temperatures below about 700 degreesC, the observed proton conductivities clearly exceed the oxide ion conductivities of the best oxide ion conductors. The high conductivity and thermodynamic stability make these materials interesting alternatives for oxide ion conductors such as Y-stabilized zirconia, which are currently used as separator material for high drain electrochemical applications, such as solid oxide fuel cells. (C) 2001 Elsevier Science B.V. All rights reserved.