## 1 Introduction  
Solid oxide fuel cells (SOFCs) have shown great promise to contribute to the ultimate aim of environmentally friendly, efficient energy production and conversion [1; 2; 3; 4]. However, overcoming the slow kinetics of the oxygen reduction reaction (ORR) at the cathode is essential to fulfill the increasing demand for developing intermediate temperature SOFCs [5]. At high temperatures such as 1000 \({}^{\circ}\)C, La\({}_{1-\mathrm{x}}\)Sr\({}_{\mathrm{x}}\)MnO\({}_{3}\) (LSM\({}_{113}\)) is commonly used as a cathode for SOFCs owing to their good stability and high electronic conductivity, but simultaneously has low ionic conductivity and a narrow triple-phase boundary (TPB) region for the cathode reaction [1]. Therefore, new cathode materials with faster ORR kinetics are required to facilitate the development of intermediate temperature SOFCs. Cubic perovskite based mixed ionic and electronic conductors (MIECs) such as ABO\({}_{3}\) oxides [6; 7; 8; 9] and layered perovskite based MIECs such as Ruddlesden-Popper (RP) oxides [10; 11; 12; 13; 14] have been extensively studied to promote oxygen electrocatalysis at intermediate temperatures such as 600 \({}^{\circ}\)C. Compared to LSM\({}_{113}\), these MIECs possess considerably higher oxygen surface exchange kinetics, which enables to extend the electrochemically active region to the entire oxide surface from the electrode/electrolyte interface. In addition, they also have higher ionic conductivity than LSM\({}_{113}\), and thus are promising cathode materials for intermediate temperature SOFCs. The oxygen diffusion and surface exchange coefficients for ABO\({}_{3}\) and RP oxides are summarized in Table 1.  
In ABO\({}_{3}\) and RP oxides, the oxygen ion diffusion and oxygen surface exchange kinetics can be significantly influenced by cation substitution as substitutions on the A or B sites of MIECs are known to affect the oxygen stoichiometry. For example, substituting Sr for lanthanum on the A-site of La\({}_{1-x}\)Sr\({}_{x}\)CoO\({}_{3-\delta}\) (LSC\({}_{113}\)) results in an increase in oxygen vacancies, enhancing the oxygen transport and surface exchange kinetics [9; 15; 16; 17; 18; 19]. In addition, doping with different-size cations in ABO\({}_{3}\) perovskites can also facilitate oxygen migration owing to a reduction in the ion migration energy [20]. The effect of Sr substitution on the activation energy for oxygen ion diffusion and oxygen surface exchange kinetics in LSC\({}_{113}\), reviewed by [PERSON] et al. [21], is that the activation energies for oxygen diffusion and surface exchange kinetics decrease with increasing Sr content owing to the reduced vacancy formation energy. [PERSON] et al. [22] compared the oxygen diffusivity between Sr-doped cobaltites and Sr-doped manganites, concluding that B-site cations with a lower valence state increase in oxygen diffusivity. In the case of RP oxides, however, the effect of cation substitutions on the ORR kinetics is poorly understood due to difficulties in synthesizing RP oxides with a wide range of cation substitutions. Recently, a remarkable enhancement of the ORR kinetics has been achieved by using epitaxial oxide thin films, which have yielded many intriguing physical and chemical properties that cannot be achieved in bulk counterparts. Epitaxial thin films can also allow the understanding of the fundamental physicochemical properties of oxide materials, and thus have been used as model systems to develop design principles for enhancing the ORR kinetics [26; 27; 28]. In particular, the use of oxide thin films has the following advantages for manipulating and evaluating electrocatalytic properties [29]. First, epitaxial thin films can be grown with different orientations, which can allow evaluating anisotropic behaviors (appropriate selection of the single crystal substrates and optimization of the deposition conditions). Second, the termination plane of each surface structure can be controlled using epitaxial thin films, and thus the different surface exchange kinetics can be elucidated.