Density functional theory plus U analysis of the electronic structure and defect chemistry of LSCF (La0.5Sr0.5Co0.25Fe0.75O3-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. La1-xSrxCo1-yFeyO3-delta (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 La1-xSrxFeO3 because of the addition of cobalt, but that minimal electronic structure differences exist between La0.5Sr0.5Co0.25Fe0.75O3 and La0.5Sr0.5Co0.5Fe0.5O3. The oxygen vacancy (V-O(center dot center dot))) formation energy (Delta E-f,E-vac) is calculated for V-O(center dot center dot) residing in different local environments within La0.5Sr0.5Co0.25Fe0.75O3. These results show that Co - V-O(center dot center dot) - Co configurations have the highest DEf, vac, while Co - V-O(center dot center dot) - Fe have the lowest Delta E-f,E-vac and may act as traps for V-O(center dot center dot). We conclude that compositions with more Fe than Co are preferred because the additional Co - V-O(center dot center dot) - Co sites would lead to higher overall Delta E-f,E-vac (and lower V-O(center dot center dot) concentrations), while the trapping strength of the Co - V-O(center dot center dot) - Fe sites is relatively weak (similar to 0.3 eV).