Distribution of Relaxation Times and Equivalent Circuits Analysis of Ba0.5Sr0.5Co0.8Fe0.2O3-delta

The phenomena taking place in Ba0.5Sr0.5Co0.8Fe0.2O3-? (BSCF) as cathodic material in solid oxide fuel cells are investigated by electrochemical impedance spectroscopy. BSCF powders are prepared by solution combustion synthesis. Measurements are collected at different temperatures, under various bias potentials and also recorded after long-term operation. Impedance spectra are thoroughly analyzed by the distribution of relaxation times (DRT) approach and compared to the standard equivalent circuits method. At 700 °C, losses are dominated by ionic conduction and charge transfer at the electrode/electrolyte interface, while oxygen adsorption and bulk diffusion provide a minor contribution to polarization. The performances of pristine materials are remarkable as a very low polarization resistance is measured at 700 °C. After prolonged testing at operative temperature, the BSCF cathodes show increasing total polarization resistance, especially due to progressive limitations in the migration of oxygen ions, caused by secondary phase formation. DRT analysis supports the physical interpretation of phenomena taking place in the material and shows the formation of a new contribution at low frequency which can be ascribed to partial decomposition of BSCF.