Local atomic structure and ORR kinetics of Fe-based perovskite oxides for IT-SOFCs
LIM T. 1, JO K. 1, LEE S. 2, YONGNAM K. 3, HEESOO L. 1, BEOM-KYEONG P. 1
1 Pusan National University, Busan, Korea (Republic of); 2 Korea Institute of Ceramic Engineering and Technology, Jinju, Korea (Republic of); 3 Korea Testing Laboratory, Seoul, Korea (Republic of)
Intermediate temperature solid oxide fuel cells (IT-SOFCs) have used Co-based perovskite materials (e.g., LaxSr1-xCoyFe1-yO3-δ and Ba0.5Sr0.5Co0.8Fe0.2O3-δ) as cathode materials. However, Co has the disadvantage of a high thermal expansion coefficient and low structural stability. To enhance the durability of IT-SOFCs, Fe-based perovskite materials with high thermal stability have been studied, and Ba0.5Sr0.5FeO3-δ have advantages such as a disordered oxygen vacancy and a three-dimensional oxygen diffusion path, which are beneficial for oxygen ion conduction. The relatively low oxygen reduction reaction (ORR) activity of Ba0.5Sr0.5FeO3-δ, compared to Co-based perovskite materials, can be improved by substituting Fe with other transition metals to promote charge transfer process and oxygen ion transport.
We studied the oxygen vacancy formation and electronic structure changes of Cu-doped Ba0.5Sr0.5FeO3-δ (Ba0.5Sr0.5Fe1-xCuxO3-δ, BSFCux, x=0-0.5), and its ORR kinetics. The BSFCux perovskite powders were synthesized by a solid-state method, and its crystal structure and lattice constant were analyzed through Rietveld refinement. The increase in oxygen vacancy concentration within the lattice due to Cu doping was confirmed through iodometric titration and thermogravimetric analysis.
Using local atomic structure analysis with Fe K-edge EXAFS, the changes in coordination number of Fe resulting from oxygen vacancy formation were analyzed, and oxidation state of Fe was analyzed using Fe K-edge XANES. The average oxidation state of Fe was decreased due to the oxygen vacancy formation within the lattice. The changes in electronic structure induced by Cu doping in BSFCux were analyzed through X-ray photoelectron spectroscopy (XPS), and it was confirmed that the valence band maximum (VBM) of BSFCux increased with Cu doping. Therefore, the increased VBM is expected to enhance the charge transfer process of the ORR, and the changes in the charge transfer process by Cu doping will be discussed with the electrical conductivity.