## 3 Thin Film Cathodes
### Ruddlesden-Popper (RP) Oxides  
The general formula of Ruddlesden-Popper (RP) phases can be written as A\({}_{n+1}\)B\({}_{n}\)O\({}_{3n+1}\)[123]. The RP phases are comprised of \(n\) consecutive perovskite layers (ABO\({}_{3}\)) alternating with rock-salt layers (AO) along the crystallographic \(\epsilon\)-axis direction. Their formula can be represented by (AO)(ABO\({}_{3}\))\({}_{n}\), where \(n\) represents the number of connected layers of vertex sharing BO\({}_{6}\) octahedra [124]. In the case of SOFCs, the \(n=1\) members of some RP series, notably those based on La\({}_{2}\)NiO\({}_{4+\delta}\) (LNO\({}_{214}\)) are of particular interest. These RP oxides were shown to accommodate oxygen interstitials in the AO layers [10], leading to fast ion conduction and hence potential application as cathodes at intermediate temperatures. Recently, lanthanum strontium cobaltites have attracted much attention as a cathode material for SOFCs, which exhibited interestingly fast oxygen exchange kinetics when interfaced with its perovskite counterpart LSC\({}_{113}\)[67; 125]. In addition, (La,Sr)\({}_{2}\)CuO\({}_{4}\) (LSCu\({}_{214}\)) has also demonstrated its availability as a promising cathode for SOFCs showing comparable oxygen surface exchange kinetics to that of LSCF\({}_{113}\) thin films at intermediate temperatures [126]. Figure 6 presents the ideal tetragonal unit-cells for \(n=1\), which correspond to the stoichiometric compounds all of same space group, _I4/mmm_. It is well-known that RP oxides exhibit strong anisotropic features as a result of the adjustment of two different structural units, i.e., ABO\({}_{3}\) and AO within the lattice. While the B-site cations are coordinated by six oxygen anions, the B-O bond lengths are different due to the Jahn-Teller effect caused by the valence state of the B-site cations [127]. This results in two types of oxygen species in the BO\({}_{6}\) octahedra, which are referred to as \"apical\" and \"equatorial\" oxygen. In the RP system, interstitial sites are located in the AO layer, where RP oxides can accommodate an excess oxygen as an interstitial oxygen defect. Oxygen vacancies can also be formed by appropriate doping in the system. Consequently, the oxygen transport properties of RP oxides can be strongly influenced by either oxygen interstitials or oxygen vacancies as discussed earlier. Nickelates are interesting alternative materials to LSC\({}_{113}\) for intermediate temperature SOFCs owing to its high oxygen surface exchange kinetics and oxygen transport properties [133; 10; 11; 13; 14; 61; 128; 129; 130]. However, due to difficulties in synthesizing epitaxial thin films of nickelates, few studies have examined the ORR kinetics of nickelates in a form of epitaxial films. [PERSON] et al. [134] first reported the oxygen electrocatalylysis of epitaxial RP oxide thin films using (011)-oriented epitaxial Nd\({}_{2}\)NiO\({}_{4+\delta}\) (NNO\({}_{214}\)) films on (001) YSZ substrates. According to this study, compression along the c -axis of NNO\({}_{214}\) films reduces the oxygen surface exchange kinetics. Later, [PERSON] et al. [135] compared the \(\delta\) values of tensile- and compressive-strained NNO\({}_{214}\) thin films, demonstrating that the larger \(\delta\) in tensile-strained NNO\({}_{214}\) can enhance the oxygen surface exchange kinetics relative to those of compressive-strained NNO\({}_{214}\) thin films (Figure 7a). These results are in good agreement with the previous observation by [PERSON] et al. [134]. Among various RP oxides, LNO\({}_{214}\) is the most widely studied member of the series. [PERSON] et al. [136] proposed that epitaxial LNO\({}_{214}\) thin films with a thickness of 300 nm may possess two different surface exchange rates as a result from the coexistence of two different film microstructures. Later, they demonstrated that the surface exchange reaction of polycrystalline LNO\({}_{214}\) samples limits the electrode reaction, showing comparable \(k^{q}\) values with those obtained previously for LNO\({}_{214}\) bulk [132].