Improvement of Oxygen Storage Capacity by Charge Transfer via Oxygen Ions in Sr₃Fe_2–xNi_xO_7.0−δ

Abstract

In this study, based on the first-principles theoretical analysis of the atomic and electronic structures in Sr 3 Fe 2−x Ni x O 7.0−δ , we propose a mechanism where charge transfer via oxygen ions plays an important role in the high oxygen storage capacity. In the pristine Sr 3 Fe 2 O 6.0 , removing an oxygen ion from Sr 3 Fe 2 O 6.0 locally increases the total number of 3d electrons in the Fe ion near the oxygen vacancy. This increase of 3d electrons in the Fe ion results in difficulty in reducing Sr 3 Fe 2 O 6.0 , limiting the oxygen storage capacity. When an Fe ion in Sr 3 Fe 2 O 6.0 is substituted with a Ni ion, electrons in 2p orbitals of oxygen ions are transferred to 3d orbitals in Ni ions due to the more negative charge transfer energy and the system becomes metallic. When an oxygen ion is extracted from Sr 3 Fe 1.5 Ni 0.5 O 6.0 , the excess electrons are accommodated into oxygen 2p orbitals and the number of 3d electrons in the Fe or Ni ions does not increase, making it possible for the oxygen ions to be released and stored much more easily. Furthermore, the effect of Ni substitution is not localized at the nearest-neighbor oxygen ions but also extended to distant oxygen ions because the excess electrons by removing an oxygen ion can be delocalized due to the metallic nature of the system, activating the oxygen ions distant from the substituted Ni to store and release oxygen ions. We also found that Ni substitution suppresses the repulsive energy between the oxygen and Ni ions during oxygen migration owing to the larger migration space caused by the shorter Ni−O ionic bond length. The suppression of repulsive energy by Ni substitution thereby increases the speed of oxygen migration and further increases the oxygen storage capacity.