The optimal co-doping of SrFe1−xCoxO3−δ oxygen carriers in redox applications

Abstract

Although the oxygen carrier SrCoO₃ has higher redox activity than SrFeO₃, cobalt is both more expensive and scarcer than iron, which would hinder the wide implementation of SrCoO₃. For these reasons, doping SrFeO₃ with Co is a potential compromise, benefitting the redox properties of SrFeO₃, while still limiting the overall amount of cobalt being used. To find the optimal level of Co-doping, density functional theory calculations were performed to investigate the Co-doping effect on the oxygen vacancy formation and oxygen migration in SrFe_1−xCo_xO_3−δ (x = 0, 0.125, 0.25, 0.375, 0.5). Our findings show that the oxygen vacancy formation energies (E_f) decrease with the increase of Co content resulting from the increased composition of the O-2p band at the Fermi level upon Co doping. In particular, the E_f decreases nearly 0.5 eV between the x = 0 and x = 0.25 samples while E_f only decreases 0.1 eV further as Co content is increased to x = 0.5. We obtain that x = 0.25 is an optimal cost/benefit ratio for Co doping, which is preserved at both low oxygen vacancy concentrations (δ = 0.0625 values listed above) and at high concentrations of δ = 0.1875 and 0.375. Kinetically, the oxygen migration barrier has slight change upon Co doping due to the similar size of Co and Fe. Therefore, considering both redox activity and economics in reversible oxygen storage applications, x = 0.25 is suggested as the optimal Co-doping value in SrFe_1−xCo_xO_3−δ.