Density functional theory + U analysis of the electronic structure and defect chemistry of LSCF (LaSrCoFeO)

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− x Sr x Co 1− y Fe y O 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− x Sr x FeO 3 because of the addition of cobalt, but that minimal electronic structure differences exist between La 0.5 Sr 0.5 Co 0.25 Fe 0.75 O 3 and La 0.5 Sr 0.5 Co 0.5 Fe 0.5 O 3 . The oxygen vacancy formation energy (Δ E f,vac ) is calculated for residing in different local environments within La 0.5 Sr 0.5 Co 0.25 Fe 0.75 O 3 . These results show that configurations have the highest Δ E f,vac , while have the lowest Δ E 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 Δ E f,vac (and lower concentrations), while the trapping strength of the sites is relatively weak (∼0.3 eV). Density functional theory + U calculations of oxygen vacancy formation in La 0.5 Sr 0.5 Co 0.25 Fe 0.75 O 3 reveal a preference for configurations.