A Comparison of Electrochemical Performance of Double Perovskite REBaCo 2 O 5+x Cathodes in Symmetrical Solid Oxide Fuel
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1126-S08-07
A Comparison of Electrochemical Performance of Double Perovskite REBaCo2O5+x Cathodes in Symmetrical Solid Oxide Fuel Cells W. Gong, M. Yadav and Allan J. Jacobson Texas Center for Superconductivity and Department of Chemistry, University of Houston, Houston, Texas 77204-5003 ABSTRACT The segregated vacancies in the A site-ordered oxygen-deficient double perovskites REBaCo2O5+x (RE = La, Pr, Nd, Sm, Eu) (RBCO) are thought to greatly enhance the diffusivity of oxide ions in the bulk of these materials and possibly supply surface defect sites with enhanced reactivity towards molecular oxygen. Some materials in this family of REBaCo2O5+x compounds, such as PrBaCo2O5+x, (PBCO), have already demonstrated high electronic conductivity, rapid oxygen ion diffusion and surface exchange kinetics. Therefore, the family of REBaCo2O5+x compounds were synthesized and evaluated as cathode materials for intermediate temperature solid oxide fuel cells (SOFCs) based on gadolinium doped ceria (CGO) electrolytes. The electrochemical performance of symmetrical cells (REBaCo2O5+x + CGO composite cathodes on the CGO electrolytes) was evaluated by using AC impedance spectroscopy. The area specific resistance (ASR) performance was measured as a function of temperature as well as oxygen partial pressure. INTRODUCTION In the development of solid oxide fuel cells, a critical goal for wide-spread commercialization is to lower cost and enhance stability by reducing the operating temperature to 400 -700 °C. In recent years, many mixed ionic-electronic conducting oxide materials containing Mn, Fe, Co and Ni have been studied for use as cathode materials at intermediate operating temperature with both zirconia and ceria electrolytes [1-4]. Cobalt containing perovskite oxides have been widely studied since they exhibit high electrocatalytic activity for the oxygen reduction reaction. They are usually too reactive for use with zirconia electrolytes but can be used with ceria. As part of an investigation of new cathode materials for intermediate temperature SOFCs, we have investigated a series of double perovskite oxides REBaCo2O5+x (RE = La, Pr, Nd, Sm, Eu) (RBCO) with ordered A cations which, in turn, localize the oxygen vacancies into layers. This class of compounds has the general formula AA’B2O5+x. The segregated vacancies in these materials are thought to greatly enhance the diffusivity of oxide ions in the bulk of the material and possibly supply surface defect sites with enhanced reactivity towards molecular oxygen. The low temperature structural properties of these compounds have been studied in detail [5-9]. High temperature measurements of the oxygen-deficient double perovskite PBCO demonstrated remarkably high electronic conductivity and rapid oxygen ion diffusion and surface exchange kinetics at intermediate temperature [10]. The structure of a representative double perovskite oxide PrBaCo2O5+x is shown in Figure 1.
Perovskite
Fluorite
Ba
Pr
Figure 1. The idealized structure of PrBaCo2O5+x; purple, gray and red spheres r
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