Density functional theory + U analysis of the electronic structure and defect chemistry of LSCF (La 0.5 Sr 0.5 Co 0.25 Fe 0.75 O 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-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 (xSr = 0.50, yCo = 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 (V$-\atop{o}$) formation energy (ΔEf,vac) is calculated for V$-\atop{o}$ residing in different local environments within La 0.5 Sr 0.5 Co 0.25 Fe 0.75 O 3 . These results show that Co-V$-\atop{o}$-Co configurations have the highest ΔEf,vac, while Co-V$-\atop{o}$-Fe have the lowest ΔEf,vac and may act as traps for V$-\atop{o}$. We conclude that compositions with more Fe than Co are preferred because the additional Co-V$-\atop{o}$-Co sites would lead to higher overall ΔEf,vac (and lower V$-\atop{o}$ concentrations), while the trapping strength of the Image Co-V$-\atop{o}$-Fe sites is relatively weak (~0.3 eV).