The Effects of Dopants and A:B Site Nonstoichiometry on Properties of Perovskite-Type Proton Conductors

Investigations of perovskite-type BaCeO3 and SrCeO2 with various dopants (Y, Gd, Nd, and Ni) indicate that their microstructures and electrical properties are strongly influenced by the type and amount of dopants. Grain growth and densification of sintered samples are influenced by dopant level and A:B site nonstoichiometry. The conductivity of BaCe1_Y039 increases with the yttrium content in hydrogen and wet Ar; and exhibits a maximum in oxygen at an yttrium content of 10 to 20%. BaCe08Y0203_1 has the highest conductivity in a hydrogen atmosphere: —4.54 X 10-2 11' cm' at 600°C, and —-4.16 >< 10 fl1 cm1 at 800°C. The effect of BaO excess depends on the concentration of dopant. Compared with BaCe091Y01503_0, doped BaCeO3 with BaO excess (Ba00.90Ce020.025Y203) has a higher total conductivity in all atmospheres studied (02, H2, and wet Ar), whereas the conductivity of BaCeO3 with excess BaO (Ba00.85Ce020.05Y203) is lower than that of BaCe09Y61O26. BaCeO3 based materials have higher conductivities than those of SrCeO2 based materials, whereas SrCeO3 based materials show higher proton transference numbers. Introduction Mixed ionic electronic conductors (MIECs) are used in many solid-state electrochemical systems such as solid oxide fuel cells (SOFCs), solid-state gas sensors, and membranes for gas separation.12 Two well-known MIECs are the partially substituted perovskite-type oxides BaCeO2 and SrCeO3, in which substitution for Ce by trivalent cations causes the formation of oxygen vacancies and other charged defects and gives rise to mixed conduction in atmospheres containing 02, H2, and H20 vapor. Because much of the charge transport in BaCeO2 and SrCeO2 is by protons, these materials are being investigated as possible hydrogen separation membranes. To be suitable for hydrogen separation, a material must have a high selectivity for hydrogen, so its proton transference number must be much higher than its transference number for oxygen ion conduction. To be useful in a nongalvanic mode, the transference number for electronic conduction should be comparable to that for protonic conduction, and the protonic and electronic conductivities should be sufficiently high (> X 10 fl' cm'). In addition, the materials must exhibit high catalytic activity for the dissociation and recombination of hydrogen at the gas/solid interfaces. Transport properties of perovskites are strongly influenced by the ionic radii of dopants. Kilner and Brook3 used lattice simulation techniques to model ionic conduction in perovskites and concluded that the overall activation energy for conduction should be minimal when the host and dopant cations have similar ionic radii. Based on this criterion, Bonanos et al. suggested that Gd would maximize the conductivity of BaCeO2. In fact, BaCe61Gd22023 has been widely studied for SOFCs because of its high conductivity in 02-containing atmospheres and under fuel cell conditions.4-6 Iwahara et al.7 studied the mixed conduction of Yb-, Y-, Dy-, Gd-, Sm-, and Nd-doped BaCeO2, and reported that the proton transference number decreased while the oxygen ion transference number increased with increasing dopant ionic radius. They reasoned that dopants with large ionic radii made oxygen ion conduction more favorable by enlarging the spacing along the a axis. In atmospheres containing water vapor, protons can be formed through the reaction H20+V+0#20H [1] Liu and Nowick found that Nd-doped BaCeO3 was a very good proton conductor when exposed to a water vaporcontaining atmosphere8 but that Eu-, Yb-, and Gd-doped BaCeO3 were not.9 Proton conduction in moist atmospheres was also studied by Slade and Singh, who showed that Gd-doped BaCeO2 had the highest conductivity Electrochemical Society Student Member. Electrochemical Society Active Member. among compounds doped with Y, Gd, Nd, and La. 10 Stevenson et al. also found that 15% Gd-doped BaCeO3 had the highest total conductivity among Yb-, Nd-, and Gd-doped samples in wet Ar. Formation of protons in alkaline earth cerates occurs readily in hydrogen-containing atmospheres according to