## 3 Thin Film Cathodes
### ABO\({}_{3}\) Oxide Thin Films
#### 3.1.1 Cobaltites  
Epitaxial strain induced by lattice mismatch between a thin film and a substrate can effectively control the oxygen migration and oxygen defect formation in ABO3 perovskite oxides [87, 88, 89, 90, 91, 92, 93]. Using density functional theory (DFT) calculations, [PERSON] et al. [94] demonstrated that tensile strain increased the one-dimensional mobility of oxygen vacancy and adsorbed atomic oxygens migrations on the LCO113 surface, which can enhance the ORR kinetics. According to the authors, the maximum tensile strain for the enhanced ORR kinetics is 3%, which has a lower oxygen vacancy formation energy by 0.32 eV compared to the strain-free surface. Later, [PERSON] and [PERSON] [90] predicted the oxygen migration barriers in LaMO3 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Ga) perovskite oxides under tensile strain via DFT calculations. They showed that tensile strain can significantly reduce the oxygen migration barriers, resulting in enhanced oxygen diffusion (one order of magnitude) at 500 \({}^{\circ}\)C compared with the diffusion under unstrained conditions. [PERSON]' et al. [28] experimentally showed that tensile strained epitaxial La0.8 Sr0.2 CoO3-8 (LSC82) films exhibit significantly increased ORR activity compared to LSC82 bulk. According to the authors, the oxygen surface exchange coefficients (\(k^{q}\)) of epitaxial LSC82 films are about two orders of magnitude higher than those of LSC82 bulk, which is attributed to the increased oxygen vacancy concentration in LSC82 thin film relative to LSC bulk. Using secondary ion mass spectroscopy (SIMS) measurements, [PERSON] et al. [64] also investigated the effect of strain on the oxygen surface exchange kinetics of epitaxial LSC82 films. In this work, the epitaxial LSC82 films were synthesized on SrTiO3 (STO) and LaAlO3 (LAO) substrates to introduce tensile and compressive strain, respectively, into the LSC82 films, where the oxygen surface exchange kinetics was found to be faster in tensile strained films. The authors postulated that the lower formation energy of oxygen vacancy by tensile strain leads to the enhanced ORR kinetics. However, [PERSON] et al. [27] did not observe the strong dependency of the oxygen surface exchange kinetics of epitaxial La0.6 Sr0.4 CoO3-8 (LSC64) films on strain. The authors attempted to control the degree of tensile strain in LSC64 films with various film thicknesses but the \(k^{q}\) values of LSC64 films were thickness-independent and comparable with those of bulk LSC64 (\(k^{q}\) can be approximated as \(k^{\star}\)). Similarly, epitaxial LCO thin films also showed little dependence of their surface exchange kinetics as a function of film thickness although the films exhibited higher ORR kinetics than LCO bulk [95]. In contrast, a clear trend of the thickness-dependent ORR activity in epitaxial LSC64 thin films was recently demonstrated by [PERSON] et al. [96] (Figure 3a). According to this work, the oxygen surface exchange kinetics of LSC64 films can be enhanced as decreasing the film thickness until 50 nm, where strain relaxation occurs. The authors proposed that the increased ORR activity results from the increased oxygen vacancies in epitaxial LSC64 films due to the large tensile strain of high-quality films. Employing in situ impedance spectroscopy during thin film growth, [PERSON] et al. [97] also showed an enhancement in the oxygen surface exchange kinetics of tensile-strained epitaxial LSC64 films on (001)-oriented La0.95 Sr0.05 Ga0.95 Mg0.05O3-8 substrates compared to unstrained films on yttria-stabilized zirconia (YSZ) substrates. Similar to the results by [PERSON] et al. [96], the authors also found the same trend of enhanced oxygen surface exchange kinetics as film thickness decreases in epitaxial LSC64 films but they interpreted that this trend may be attributed to the capability of in-situ measurement as the films never exposed to any thermal cycling such as cooling from PLD preparation temperature or heating for impedance measurements.