## 3 Thin Film Cathodes
### Ruddlesden-Popper (RP) Oxides
However, later the authors also evaluated the anisotropy for the oxygen surface exchange kinetics of LNO\({}_{214}\) single crystalline, reporting the lack of anisotropy in the oxygen surface exchange values, which is attributed to the surface segregation process [133]. The anisotropy of the oxygen surface exchange kinetics was also observed in Sr-doped LNO\({}_{214}\) films. [PERSON] et al. [65] demonstrated that substituting Sr for lanthanum in LNO\({}_{214}\) thin films can result in the structural  
Figure 7: Arrhenius plots of surface exchange coefficients (\(k^{*}\)) for A\({}_{2}\)NiO\({}_{4+\delta}\) (A = La and Nd): (**a**) Tensile (Tens.) and compressive (Comp.) strained (001)-oriented La\({}_{2}\)NiO\({}_{4+\delta}\) films [61] and tensile, zero, and compressive strained (100)-oriented Nd\({}_{2}\)NiO\({}_{4+\delta}\) thin films [135]; (**b**) (001)- and (100)-oriented La\({}_{2}\)NiO\({}_{4+\delta}\)[61; 63] thin films with various thickness and substrates. reorientation of the films because of the reduction in the surface energy of the (001) surface, reporting the anisotropic oxygen surface exchange kinetics. La2-SrxCuO4\(\pm\delta\) (LSCu214) oxides are famous for their high T\({}_{\rm c}\) superconductivity [138; 139; 140]. Interestingly, the superconducting properties of LSCu214 depend on the Sr concentration and oxygen stoichiometry, which also strongly influence the kinetics of oxygen exchange and oxygen diffusion. [PERSON] et al. reported the anisotropic oxygen transport properties and proposed the defect model for Sr-doped curpates [38; 141]. Recently, using epitaxial LSCu214 thin films, the oxygen surface exchange kinetics were evaluated at intermediate temperatures [126]. In this study, the \(k^{\rm d}\) values of undoped La2 CuO4 (LCuO214) thin films were found to be higher than those of Sr-doped La2 CuO4 thin films (Figure 9a), which is attributed to that adding Sr content resulted in changes of the oxygen 2p-band center relative to the Fermi energy. More recently, [PERSON] and co-workers [142] revealed that the oxygen nonstoichiometry commonly reported for strained LSCu214 is mediated by the strain-modified surface exchange kinetics, rather than reduced thermodynamic oxygen formation energies. The authors also demonstrated that tensile-strained LSCu214 shows nearly an order of magnitude faster oxygen exchange rate in tensile-strained LSCu214 films compared to a compressively strained film. La2-SrxCoO4\(\pm\delta\) (LSC214) oxides are widely used as a perovskite counterpart to promote the oxygen electrocatalysis of ABO3 perovskite such as LSC113 [33; 68; 125; 143]. However, LSC214 showed very attractive transport and surface exchange properties at intermediate temperatures. For instance, La2 CoO4\(\pm\delta\) (LCO214) showed higher oxygen diffusivity and lower activation energy in the temperature range of 450-700 \({}^{\circ}\)C as compared to nickelates and cuprates [11; 13; 25]. Later, [PERSON] et al. [126] calculated the interstitial formation energies of LCO214, LNO214, and LCuO214 and found the minimum energy for interstitial formation in LCO214. In addition, the authors also reported that the \(k^{\rm d}\) values of LSC214 thin films are comparable to LSC113 thin films, indicating a promising alternative cathode material to LSC113. Recently, [PERSON] et al. [144] successfully fabricated LSC214 (Sr = 0.25) thin films grown in two different crystallographic orientations, i.e., (100) and (001), using (100) LaSrAlO4 (LSAO) and (001) SrTiO3 (STO) substrates. They demonstrated that the oxygen diffusivity along the \(a\)-\(b\) plane in the (100)-oriented LSC214 film was three orders of magnitude lower than that in the (001)-oriented LSC214 film (Figure 9b).