A Study on the Defect Chemistry of Barium-Strontium-Cobalt-Iron-Oxide System

Abstract

BSCF, or Ba0.5Sr0.5Co0.8Fe0.2O3-δ, is a well known mixed electronic and ionic conductor (MEIC) that has been considered as potential cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). Initial 4-point conductivity measurements of Ba0.5Sr0.5CoxFe1-xO3-δ (x = 0, 0.2, 0.4, 0.6, and 0.8) as a function of temperature in air were conducted with Pt-paste electrodes. Below 400oC, conductivity was thermally activated (Ea = 0.21 – 0.4eV). Above 400oC, conductivity decreased with increasing temperature, which was attributed to a decrease in p-type carriers resulting from the formation of oxygen vacancies associated with the reduction of Fe4+. The conductivity of BSCF (x = 0.8) as a function of pO2 was also measured. Above 400oC for pO2 > 0.01atm, the conductivity increased with increasing pO2, confirming p-type conduction. For pO2 < 0.01atm, a slight increase in conductivity with decreasing pO2 was observed. To further investigate the possibility of n-type conduction in BSCF at low pO2, the conductivity and thermopower simultaneously as a function of temperature and pO2 for five compositions of Ba0.5Sr0.5CoxFe1-xO3-δ (0 < x < 0.8) were measured. These measurements enable better understanding of the conduction mechanism of BSCF according to the changes of temperatures and oxygen partial pressures. With XPS analysis, as cobalt concentration increases, the trend of peak shifts in O1s and cations (Co, Fe) has been studied. This study provides the fundamental background of bonding state on the surface of BSCF ceramics.