## 3 Thin Film Cathodes
### ABO\({}_{3}\) Oxide Thin Films
#### 3.1.1 Cobaltites
However, regardless of strain, epitaxial LSC113 thin films were found to have higher \(k^{q}\) values compared to their bulk counterpart (Figure 3b). It is known that lattice strain can tune the electronic structure and charge transfer in ABO\({}_{3}\) oxides [98, 99]. Using scanning tunneling spectroscopy and density functional theory calculations, [PERSON] et al. [55] showed that the surface electronic state depends on the strain state of the epitaxial LSC82 thin films. In this work, STO and LAO substrates were utilized to induce tensile and compressive strains, respectively, into the LSC82 films, where tensile-strained LSC films exhibited a more increased electronic density of states (DOS) near the Fermi level compared to compressive strained LSC films. It was hypothesized that the increased DOS and the transition to the metal-like state are attributed to the easier formation of oxygen vacancies on tensile strained LSC films, which was supported by DFT calculations. A recent study for a wide range of bulk perovskites demonstrated that the position of the oxygen 2p-band center with respect to the Fermi level is linearly correlated with the activation barrier of oxygen surface exchange, acting as a descriptor for the oxygen surface exchange kinetics [100]. According to this work, the formation energy of bulk oxygen vacancies decreases with increasing the oxygen 2p band center relative to the Fermi level, indicating the enhanced oxygen reduction reaction (Figure 4a). Later, [PERSON] et al. [96] demonstrated that the oxygen 2p-band center relative to the Fermi level can be linearly correlated with strain state, where tensile strain results in the upshift of the oxygen 2p-band center (Figure 4b). According to the authors, As the oxygen 2p-band center also has a linear relationship with the vacancy formation energy in perovskite oxides [100], and thus the tensile strain is expected to lower the energy of vacancy formation due to an increase in the oxygen 2p-band center in the LSC\({}_{113}\) films. [PERSON] et al. [95] showed that in epitaxial LCO\({}_{113}\) films, strain can be used to reduce the spin transition temperature and promote the occupation of higher spin states that weaken the Co-O bond, which results in an increase in the oxygen surface exchange kinetics. The authors also pointed out that the decreased Co-O bond strength due to the increased fractional occupancy of high-spin states can lead to the oxygen 2p-band center upshift relative to the Fermi energy. Recently, [PERSON] et al. [101] measured the ORR activities of polycrystalline LSC\({}_{113}\) thin films with a wide range of Sr concentration (0 \(\leq\) Sr \(\leq\)0.8). The authors found that with increasing the Sr content, the O 2p-band center can be close to the Fermi level, which favors the oxygen vacancy formation. However, they claimed that at Sr \(>\) 0.4, Co\({}^{2+}\) becomes optimal for the surface instead of Co\({}^{3+}\)/Co\({}^{4+}\) and may act as electron traps, which may decrease the electronic conductivity, leading to an overall poor ORR activity of LSC  
Figure 3: Arrhenius plots of surface exchange coefficients (\(k^{\star}\)) for epitaxial La\({}_{1-x}\)Sr\({}_{x}\)CoO\({}_{3-\delta}\) thin films. (**a**) Effect of thickness difference on \(k^{\star}\) for La\({}_{0.6}\)Sr\({}_{0.4}\)CoO\({}_{3-\delta}\)[96] and of substrate difference (STO and LAO) on \(k^{\star}\) for La\({}_{0.8}\)Sr\({}_{0.2}\)CoO\({}_{3-\delta}\)[64] thin films and La\({}_{0.8}\)Sr\({}_{0.2}\)CoO\({}_{3-\delta}\)[9] polycrystalline samples. (**b**) Effect of Sr substitution on \(k^{\star}\) for La\({}_{1-x}\)Sr\({}_{x}\)CoO\({}_{3-\delta}\) (x = 0, 0.2 and 0.4, [64, 95, 96]) thin films and La\({}_{0.8}\)Sr\({}_{0.2}\)CoO\({}_{3-\delta}\)[9] polycrystalline oxides. films. These results showed a different trend with the previously reported data [95; 96] which showed enhanced ORR kinetics due to an upshift of the oxygen 2p-band center to the Fermi level.