Acceptor-Doped BaZrO3 Perovskite: Synthesis, Structure and Proton Conductivity.

Doctoral thesis, 2008

Acceptor-doped perovskite oxides exhibit significant proton conductivity in hydrogen containing atmospheres. Therefore, they have potential for use as separator materials for various electrochemical devices including gas sensors, electrolysers and fuel cells. For example, yttrium doped BaZrO3 perovskites have shown high bulk proton conductivity and high chemical stability. However, significant grain boundary resistance has subdued the technological applications. The work constituting this thesis has mainly been focused on synthesis, structural characterisation and electrochemical investigations of acceptor-doped BaZr1-xMxO3-d (M = Ga3+, Sc3+, In3+, Yb3+ and Y3+: x = 0-0.75) perovskite oxides in bulk form. The polycrystalline samples were prepared via a traditional solid-state sintering route and by wet chemical routes. A combination of techniques such as x-ray and neutron powder diffraction, thermogravimetry, scanning electron microscopy (SEM) and impedance spectroscopy (IS) have been used to characterize the samples. Rietveld analysis of high resolution low temperature neutron powder diffraction data revealed that the deuteron site was localised at, or close to, the 12h crystallographic position of BaZr0.5In0.5O2.5(OD)0.5. It was also confirmed that the hydration/deuteration reaction caused an expansion of the cell parameter, while keeping the lattice symmetry, and thus the basic structure intact. Moreover, from Rietveld analysis of the neutron data the oxygen vacancy concentration was determined and they were found to be statistically distributed in the structure. Heavily doped samples showed higher proton conductivity compared to lightly doped samples, indeed this is an option to improve the conductivity in the material for certain dopants, e.g., In, Sc and Yb. Interestingly, at the same level of doping the proton conductivity differs significantly for different acceptors dopants. The effects of co-doping at the B-site, e.g. BaZr0.9In0.05M0.05O3-d (M = Ga and Yb) were investigated. Surprisingly, lower proton conductivity was obtained for these co-doped samples compared to the sample containing a single dopant ion e.g. BaZr0.9M0.1O3-d (M = Ga, In and Yb). It was also confirmed that samples with smaller grain size show lower total proton conductivity due to high grain-boundary resistance. Through the present work an increased understanding of the factors influencing the proton conductivity in acceptor doped BaZrO3 has been obtained. In particular, the highly doped materials were found to be of considerable interest in the effort of producing materials with high proton conductivity.