Abstract
(Bi,Sr)FeO3−δ (BSF) cathode materials doped with either Co, Ni or Mn are synthesized by an ethylene diamine tetra-acetic acid (EDTA)-citrate complexing method, and the effects of the doping level on the mixed electronic-ionic conductivity at various temperatures are studied up to 800 °C. The phase purity and solid solution limit are investigated by X-ray diffraction (XRD). The ionic conductivity is measured by the four-probe direct current (DC) method, the valence state of Fe and Mn by X-ray photoelectron spectroscopy (XPS), and the oxygen non-stoichiometry by differential thermo-gravimetric analysis (TGA). The doped ferrites show interesting electronic conductivity dependent on the testing temperature, implying two conductive mechanisms, either controlled by double exchange at lower temperatures or small polaron (electron-oxygen vacancy) conduction at temperatures greater than 400 °C. The results of Co-doped BSF (S50C20) show the best mixed conductivity among the ferrites, and this is used to assemble cells. The cell with a S50C20 cathode in the region of 600–800 °C is improved by 15% in maximum power density greater than the cell with La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) due to the balanced contribution from oxygen ions, vacancies and electrons.
Highlights
Solid oxide fuel cells (SOFCs) are power generation devices that directly convert fuels into electricity via electrochemical reactions through clean and highly efficient operations
High performance cathodes based on La-perovskite (LaMO3 ) materials have been studied, and it was recently reported that cobalt-containing (La,Sr)CoO3 perovskites exhibit the best activity of electro-catalytic oxygen reduction reaction (ORR) at intermediate temperatures (600 ◦ C–800 ◦ C)
Since cobalt is a good catalyst for reduction oxygen molecules to ions and flexible redox behavior via valence transition [4], the cobalt containing perovskites, e.g., La0.6 Sr0.4 Fe0.8 Co0.2 O3−δ (LSFC6482), have a good electronic conductivity, they have very low ionic conductivity and the high coefficient of thermal expansion (CTE) of the LSFC cathode mismatches to that of zirconia-based electrolyte, which is unfavorable to thermal cycling of the cells [6,7,8,9,10]
Summary
Solid oxide fuel cells (SOFCs) are power generation devices that directly convert fuels into electricity via electrochemical reactions through clean and highly efficient operations. To improve the reliability and stability of the cells, lowering the operating temperature to an intermediate-temperature range (below 800 ◦ C) [1,2] is one of the main targets in the recent research efforts. These advantages of lower-temperature operation, including the ease of material selection, reduction in total cost and accelerating the start-up process, are generally recognized. A lower operating temperature results in sluggish oxygen reduction reaction (ORR) at the cathode and apparently increases the resistance of the electrolyte/cathode, potentially degrading the performance of SOFCs [4,5].
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