The optimal co-doping of SrFeCoO oxygen carriers in redox applications.

Although the oxygen carrier SrCoO has higher redox activity than SrFeO, cobalt is both more expensive and scarcer than iron, which would hinder the wide implementation of SrCoO. For these reasons, doping SrFeO with Co is a potential compromise, benefitting the redox properties of SrFeO, 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 SrFeCoO (x = 0, 0.125, 0.25, 0.375, 0.5). Our findings show that the oxygen vacancy formation energies (E) 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 decreases nearly 0.5 eV between the x = 0 and x = 0.25 samples while E 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 SrFeCoO.