## 3 Thin Film Cathodes
### ABO\({}_{3}\) Oxide Thin Films
#### 3.1.2 Ferrites  
Iron-based perovskites (LaFeO\({}_{3}\), LFeO\({}_{113}\)) are typically oxygen deficient materials [102; 103]. Therefore, LFO\({}_{113}\) has high oxygen mobility within the material [104] and is suitable for catalytic applications such as oxygen reduction and ion transport in SOFCs [105; 106]. Substituting Sr for La on the A-site of LFO\({}_{113}\) is widely used to promote oxygen defects, enhancing oxygen diffusion and surface exchange processes due to the charge neutrality [107]. However, the ORR activity of La\({}_{1-x}\)Sr\({}_{x}\)FeO\({}_{3}\) (LSF\({}_{113}\)) thin films is still insufficient for the use of oxygen electrocatalysis. For example, using electrochemical impedance spectroscopy (EIS) measurements, [PERSON] et al. [108] found that the oxygen surface exchange coefficients (\(k^{q}\)) of La\({}_{0.8}\)Sr\({}_{0.2}\)FeO\({}_{3}\) (LSF82) thin films are comparable to those of La\({}_{0.8}\)Sr\({}_{0.2}\)MnO\({}_{3}\) (LSM82) thin films. Compared to LSF\({}_{113}\), LSCF\({}_{113}\) exhibits better ionic conductivity, reasonable high-temperature stability, and catalytic activity [109; 110; 111; 112; 113]. However, there was no systematic study on the strain dependent ORR kinetics (Figure 5a). [PERSON] et al. [47] and [PERSON] et al. [85] reported the ORR kinetics of La\({}_{0.6}\)Sr\({}_{0.4}\)Co\({}_{0.8}\)Fe\({}_{0.2}\)O\({}_{3-8}\) (LSCF6482) thin films (Figure 5b). The authors concluded that LSCF6428 films can enhance the ORR kinetics compared to bulk LSCF6428 due to the increased oxygen vacancies. So far, substantial efforts have been focused on tailoring the surface microstructure of LSCF\({}_{113}\) thin films to enhance the ORR kinetics, based on the hypothesis that many grain boundaries on the surface of LSCF\({}_{113}\) films may modify the oxygen bulk diffusion and catalytic properties to enhance the ORR kinetics. For instance, [PERSON] et al. [114] investigated the ORR kinetics of La\({}_{0.52}\)Sr\({}_{0.48}\)Co\({}_{0.18}\)Fe\({}_{0.82}\)O\({}_{3-8}\) thin films with various thicknesses (Figure 5b). In this study, the polarization resistance of LSCF films was reduced by decreasing the film thickness, indicating that the number of grain boundaries on the surface increases. Consequently, the oxygen surface exchange and diffusion coefficients were significantly enhanced in LSCF thin films compared to bulk LSCF [84]. [PERSON] et al. [115] investigated the ORR kinetics of LSCF6428 thin films with a Ce\({}_{0.8}\)Gd\({}_{0.2}\)O\({}_{1.9}\) (CGO) layer. Adding CGO to LSCF6428 films reduced the polarization resistance for LSCF-CGO films, which leads to a higher density of grain boundaries. [PERSON] et al. [116] discussed the oxygen surface exchange and diffusion kinetics of LSCF6428 thin films by comparing a non-organized microstructure  
Figure 4: **(a)** Schematic electronic structure plots to illustrate the correlation of the ORR energetics vs. the O p-band center based on the rigid band model. Reprinted from [100] with permission of The Royal Society of Chemistry. **(b)** DFT calculations for the oxygen 2p-band center with respect to the Fermi level as a function of strain in LSCO films. The calculated oxygen 2p-band centers are averaged over LSCO films with five different Sr orderings. Reprinted from [96] with permission of The American Chemical Society. film. The authors found that introducing grain boundaries on LSCF6428 films can enhance the oxygen surface exchange reactivity, which is in good agreement with previously reported data [115]. It was also shown that the ORR kinetics can be enhanced by using the LSCF\({}_{113}\) thin film as an interfacial layer at the electrode-electrolyte interface. [PERSON] et al. [120] investigated the ORR kinetics of adding LSCF6482 thin films between porous LSCF113 cathode and Gd-doped ceria (GDC) substrate. In this study, the addition of LSCF6482 thin films was found to result in lower polarization resistance, which may be attributed to the increased active area due to the enhanced adhesion between cathode/substrate. [PERSON] et al.