Density functional theory + U analysis of the electronic structure and defect chemistry of LSCF (La_0.5Sr_0.5Co_0.25Fe_0.75O_3-:δ)

Reducing operating temperatures is a key step in making solid oxide fuel cell (SOFC) technology viable. A promising strategy for accomplishing this goal is employing mixed ion-electron conducting (MIEC) cathodes. La_1-xSr_xCo_1-yFe_yO_3-δ (LSCF) is the most widely employed MIEC cathode material; however, rational optimization of the composition of LSCF requires fundamental insight linking its electronic structure to its defect chemistry. To provide the necessary insight, density functional theory plus U (DFT+U) calculations are used to investigate the electronic structure of LSCF (x_Sr = 0.50, y_Co = 0.25). The DFT+U calculations show that LSCF has a significantly different electronic structure than La_1-xSr_xFeO₃ because of the addition of cobalt, but that minimal electronic structure differences exist between La_0.5Sr_0.5Co_0.25Fe_0.75O₃ and La_0.5Sr_0.5Co_0.5Fe_0.5O₃. The oxygen vacancy formation energy (ΔE_f,vac) is calculated for residing in different local environments within La_0.5Sr_0.5Co_0.25Fe_0.75O₃. These results show that configurations have the highest ΔE_f,vac, while have the lowest ΔE_f,vac and may act as traps for. We conclude that compositions with more Fe than Co are preferred because the additional sites would lead to higher overall ΔE_f,vac (and lower concentrations), while the trapping strength of the sites is relatively weak (∼0.3 eV).