## 3 Thin Film Cathodes
### Ruddlesden-Popper (RP) Oxides
However, to evaluate the effect of strain on the ORR kinetics in RP oxides, high quality epitaxial thin films are required. [PERSON] and co-workers [61] synthesized (001)-oriented epitaxial LNO\({}_{214}\) thin films on STO and NbGaO\({}_{3}\) (NGO) substrates, which introduce tensile and compressive strain states, respectively, into the LNO\({}_{214}\) films. They evaluated the oxygen diffusion and surface exchange kinetics for tensile- and compressive-strained LNO\({}_{214}\) films, concluding that both compressive and tensile strain led to a reduction in the oxygen diffusivity in the first 175-200 nm of the film, whereas neither strain affected the oxygen surface exchange. However, [PERSON] et al. [63] later demonstrated that tensile strain along the \(c\)-direction can lead to an increase in the driving force to form interstitial oxygen atoms in LNO\({}_{214}\), resulting in enhanced oxygen surface exchange kinetics of (100)-oriented epitaxial LNO\({}_{214}\) thin films (Figure 7b). In this study, the authors successfully controlled  
Figure 6: Schematic crystal structure of RP oxides, where A and B are a rare earth and transition metal atom. strain in LNO\({}_{214}\) films by modulating the film thickness. The discrepancy in the effects of strain on oxygen surface exchange kinetics in LNO\({}_{214}\) films may be attributed to the fact that the critical thickness required to maintain an acceptable strain state is fairly smaller than what [PERSON] et al. [61] used. However, the effect of strain on the ORR kinetics in RP oxides needs further investigation. Similar to the correlation between the oxygen 2p-band center and ORR kinetics in ABO\({}_{3}\) oxides, [PERSON] et al. [126] also proposed that the oxygen 2p-band center relative to the Fermi energy can be an effective descriptor to predict the oxygen surface exchange as well as the activation energy for oxygen transport in RP oxides. In contrast to ABO\({}_{3}\) oxides, the oxygen 2p-band center relative to the Fermi level was found to be inversely proportional to the oxygen surface exchange kinetics in RP oxides (Figure 8). According to the authors, the opposite slopes of the oxygen 2p-band center correlations between the RP oxides and the perovskite materials are due to the intrinsic mechanistic differences of their oxygen surface exchange kinetics and bulk anionic transport. The authors also found that the calculated interstitial formation energies of La\({}_{2}\)BO\({}_{4+\delta}\) (B = Co, Ni, and Cu) strongly depend on the B cation in the sequence LCuO\({}_{214}\) > LNO\({}_{214}\) > LCO\({}_{214}\), which corresponds to the trend of B-cation-dependent oxygen diffusivity. Recently, [PERSON] et al. [137] also calculated the formation energy of oxygen interstitials in the same RP oxide systems, demonstrating that, regardless of \(\delta\), the formation energy of oxygen interstitials decreases with decreasing the atomic number of the B cation, which is in good agreement with the data reported by [PERSON] et al. [126]. As discussed earlier, the anisotropy of oxygen migration is one of the intrinsic properties of RP oxides owing to its structural feature. Therefore, different oxygen diffusion and surface exchange kinetics along different crystallographic directions were reported as shown in Figure 9. Using NNO\({}_{214}\) and Pr\({}_{2}\)NiO\({}_{4+\delta}\) (PNO\({}_{214}\)) single crystalline oxides, [PERSON] et al. [128] observed a large anisotropy for the oxygen surface exchange kinetics for both oxides, where the \(k^{*}\) values in the \(a\)-\(b\) plane are 1.5 orders of magnitude larger than those along the \(c\)-axis. [PERSON] et al. [61] reported that the oxygen diffusion and oxygen surface exchange kinetics in the \(a\)-\(b\) plane are faster than those along the \(c\)-direction in epitaxial LNO\({}_{214}\) thin films.