Defect Chemistry and Proton Conductivity in Ba-based Perovskites

Author: Wu, Jian

Year: 2005

Degree: Dissertation (Ph.D.)

Advisor: Haile, Sossina M.

Committee Members: Johnson, William Lewis; Gavalas, George R.; Rossman, George Robert; Haile, Sossina M.; Goddard, William A., III

Option: Materials Science

DOI: 10.7907/VTCG-T978

Abstract

The site incorporation mechanism of M³⁺ dopants into A²⁺B⁴⁺O₃ perovskites controls the overall defect chemistry and thus their transport properties. For charge balance reasons, incorporation onto the A²⁺ site would require the creation of negatively charged point defects, such as cation vacancies, whereas incorporation onto the B⁴⁺ site is accompanied by the generation of positively charged defects, typically oxygen vacancies. Oxygen vacancy content, in turn, is relevant to proton conducting oxides in which protons are introduced via the dissolution of hydroxyl ions at vacant oxygen sites.

This work proposes that, on the basis of X-ray powder diffraction studies, electron microscopy, chemical analysis, thermal gravimetric analysis, AC impedance spectroscopy, extended X-ray fine structure (EXAFS) and atomistic simulation, that nominally B-site doped barium cerate can exhibit dopant partitioning partially as a consequence of barium evaporation at elevated temperatures. Such partitioning and the presence of significant dopant concentrations on the A-site negatively impact proton conductivity. As a consequence of the greater ability of larger cations to exist on the Ba site, the H₂O adsorption and proton conductivities of large-cation doped barium cerates are lower than those of small-cation doped analogs.

A series of dopants, La, Nd, Sm, Gd and Yb are adopted in doped BaCeO₃ with the composition BaCe0.85M0.15O3-δ. Yb doped BaCeO₃ yields the highest proton conductivity among all the doped samples. Compositional non-stoichiometry, which is closely tied to sample processing, is studied in a Bax0.85M0.15O3±δ series. It is indicated that low temperature synthesis is beneficial to reduce barium evaporation at elevated temperatures and in turn increase the proton conductivity. The chemical stability of BaCeO₃ is investigated and Zr is used to stabilize BaCeO₃ in CO₂-rich atmosphere effectively. This result helps to commercialize doped BaCeO₃ as the electrolyte material for SOFCs.

Files