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 e xamined experimentally and by computational simulations. These materials ha ve the potential to combine high proton conductivity and thermodynamic stab ility. While any structural and chemical perturbation originating from the B-site occupation (poor chemical matching of the acceptor-dopant or Zr/Ti-m ixing) 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 lat tice oxygen, which is influenced by both A- and B-site occupations. The hig hest 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 show s that Y does not change the oxide's basicity (i.e. it chemically matches o n the Zr-site of BaZrO3). The highest proton conductivities have been obser ved for high Y-dopant concentrations (15-20 mol%). For temperatures below a bout 700 degreesC, the observed proton conductivities clearly exceed the ox ide ion conductivities of the best oxide ion conductors. The high conductiv ity and thermodynamic stability make these materials interesting alternativ es for oxide ion conductors such as Y-stabilized zirconia, which are curren tly used as separator material for high drain electrochemical applications, such as solid oxide fuel cells. (C) 2001 Elsevier Science B.V. All rights reserved.