Edoardo Magnone

Structural Properties and Electrochemical Characteristics of Ba0.5Sr0.5Co1 − x Fe x O3 − δ Phases in Different Atmospheres

Ba0.5Sr0.5Co1�xFexO3� 0.2 x 0.8 powders were synthesized using a wet chemical procedure starting from nitrates as precursors and citric acid and ethylenediaminetetraacetic acid as complexing agents. The structural properties and the electrochemical behavior of the materials were investigated as a function of temperature T, atmosphere air, oxygen, and hydrogen, and Fe content x using simultaneous thermogravimetric and differential thermal analysis, X-ray diffraction analysis, and electrochemical impedance spectroscopy measurements. Significant differences were observed in the crystalline structure of the Ba0.5Sr0.5Co1�xFexO3� samples when they were fired in different atmospheres inert and air; oxidizing and reducing. The formation of a cubic-type single phase in the 0.2 and 0.8 Fe content x range was confirmed under oxidizing conditions. The unit cell volume increased with decreasing x and in more oxidizing atmospheres. Further structural transformation with the collapse of the cubic structure was observed with increasing Fe content x under reducing conditions. The cubic-type Ba0.5Sr0.5Co1�xFexO3� solid solutions were tested as electrodes on Ce0.8Sm0.2O2�x pellets in symmetrical cells. The lowest obtained values of area specific resistance were 0.56 for Ba0.5Sr0.5Co0.2Fe0.8O3� in air and 0.25 cm 2 for Ba0.5Sr0.5Co0.8Fe0.2O3� in oxygen atmosphere at 1073 K.

Electrochemical Impedance Spectroscopy Analysis of Pr0.8Sr0.2Co0.5Fe0.5O3 − δ as Cathode Material for Intermediate Temperature Solid Oxide Fuel Cells

The electrochemical properties of Pr 0.8 Sr 0.2 Co 0.5 Fe 0.5 O 3-δ used as cathode material for solid oxide fuel cells were investigated. Powder samples were prepared using the combustion synthesis technique. Pr 0.8 Sr 0.2 Co 0.5 Fe 0.5 O 3-δ crystallized in the orthorhombic space group Pnma (no. 62) with a = 5.447(4) A, b = 5.425(1) A, and c = 7.680(3) A. The final reliable factors of the Rietveld refinement were R wp = 3.75%, R p = 2.95%, and R exp = 3.15%. Electrochemical impedance spectroscopy (EIS) measurements were used to characterize the overpotential of the Pr 0.8 Sr 0.2 Co 0.5 Fe 0.5 O 3-δ electrodes on the Ce 0.8 Sm 0.2 O 2-δ electrolyte as a function of temperature and applied dc voltages. The EIS data showed the presence of two relaxation processes at different frequencies. The resistance associated with the high frequency process was almost independent from the applied bias, whereas the resistance associated with the low frequency semicircle sharply decreased with increasing temperature and applied dc voltage. These findings suggested that the temperature-voltage-activated process could be attributed to the oxygen reduction reaction at the electrode surface, whereas the high frequency process was associated with the electrochemical charge-transfer reactions across the electrode-electrolyte interface. This demonstrated that the cathode performance can be improved by optimizing its microstructure, especially by extending the electrochemically active three-phase boundary length.