Ni-doping effects on formation and migration of oxygen vacancies in SrFe_1-NiO_3- oxygen carriers

Ni is a promising B-site doping element capable of improving the oxygen carrier performance of SrFeO₃ perovskite. In this work, the effect of Ni doping on the formation and migration of oxygen vacancies in SrFe_1-NiO_3- (x = 0, 0.0625, 0.125, 0.1875, and 0.25) is investigated using density functional theory calculations. Our results show that the oxygen vacancies formed from Ni-O-Fe chains exhibit lower formation energy (E_f) compared to those from Fe-O-Fe chains in each doping system. Additionally, E_f generally decreases with an increase of Ni content. This Ni-promoted formation of V_O is attributed to three factors: weakened Ni-O bonding, the closure of O-2p states to the Fermi level by Ni-O hybridization, and Ni³⁺ decreasing the positive charges to be compensated by V_O formation. Due to these multiple advantages, a modest Ni doping of x = 0.25 can induce a higher P_O and a lower T comparted to the relatively larger Co doping of x = 0.5, thermodynamically. Kinetically, Ni-doping appears to be a disadvantage as it hinders oxygen migration, due to a higher oxygen migration barrier through SrSrNi compared to the SrSrFe pathway. However, the overall oxygen ion conduction would not be significantly influenced by hopping through a nearby pathway of SrSrFe with a low migration barrier in a system doped with a small amount of Ni. In a word, a small amount of Ni doping has an advantage over Co doping in terms of enhancing the oxygen carrier performance of the parent SrFeO₃ system.