Characterization of novel GdBa0.5Sr0.5CO2-xFexO5+δ perovskites for application in IT-SOFC cells

Novel, Sr-substituted A-site ordered perovskites with GdBa0.5Sr0.5Co2-xFexO5+delta (0 <= x <= 2) chemical composition were studied, and results of measurements of their phase composition, crystal structure, oxygen content 6, transport properties and chemical stability in relation to ceria electrolyte are presented in this work. It was found that despite 50% substitution of Ba by Sr, the tendency of ordering in A-sublattice is retained in Co-rich materials, but with the increase of iron content, a significant amount of unordered, but also perovskite phase appears. Compounds with high Co content possess highest electrical conductivity, which for GdBa0.5Sr0.5Co2O5+delta greatly exceeds 1000 S cm(-1) at temperatures above 400 degrees C. Seebeck coefficient remains positive for all studied compositions in 25-850 degrees C temperature range, indicating dominance of holes as main charge carriers. The double perovskite structure is responsible for a high deviation from the oxygen stoichiometry in studied materials, which increases considerably above 300 degrees C. GdBa0.5Sr0.5Co1.5Fe0.5O5+delta cathode material was selected for additional studies, which included evaluation of thermal expansion, determination of chemical diffusion coefficient D and surface exchange reaction coefficient K and electrochemical measurements in a custom made, electrolyte-supported Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs). At 800 degrees C value of D is close to 10(-5) cm(2) s(-1), which together with high concentration of oxygen vacancies points to a high ionic conductivity of the material. Together with total electrical conductivity exceeding 400 S cm(-1) in 600-800 degrees C range, and with high chemical stability in relation to ceria electrolyte, the observed properties are attractive from the point of view of application. However, due to a large chemical expansion, in order to obtain good adhesion between cathode and electrolyte layers, composite cathodes, consisting of GdBa0.5Sr0.5Co1.5Fe0.5O5+delta and ceria electrolyte powders had to be used. Recorded power density of IT-SOFC cell was about 0.27 W cm(-2) at 800 degrees C, while the performed impedance spectroscopy studies revealed that ohmic component was the dominant one above 700 degrees C in total polarization of the cell. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.