## 3 Thin Film Cathodes
### Oxide Heterostructure Interface  
To achieve enhanced surface exchange kinetics and stability, recent efforts have been focused on developing advanced cathode materials based on MIECs with surface modification [27; 33; 66; 68; 76; 82; 86; 125; 143; 144; 145; 146; 147; 148]. Oxide heterostructure interfaces, the combination of a Ruddlesden-Popper (RP, LSC\({}_{214}\)) layer on top of the perovskite LSC\({}_{113}\), have shown remarkably high oxygen surface exchange kinetics [27; 33; 68; 86; 125; 143; 148; 149]. [PERSON] et al. reported \(\sim\)1 order of magnitude enhancement in activity for the composite cathode screen-printed with the mixture of LSC64 and LSC\({}_{214}\)[143]. [PERSON] et al. [33] have also reported \(\sim\)3 orders of magnitude higher oxygen surface exchange coefficient (\(k^{\star}\)) at the interfacial region between polycrystalline LSC64 and LSC\({}_{214}\) compared to their bulk value [21]. Later, [PERSON] et al. [27; 148] utilized epitaxial LSC\({}_{113}\) and LSC\({}_{214}\) thin films, achieving higher oxygen surface exchange coefficients of LSC\({}_{113}\). Recently, [PERSON] et al. [150] also observed \(\sim\)10 times higher \(k^{q}\) values in heterostructure LSC\({}_{113}\)/LSC\({}_{214}\) thin films than those in LSC\({}_{113}\), which is attributed to increased number of active oxygen vacancies that manipulate the oxygen 2p-band center. A recent study using Coherent Bragg Rod Analysis (COBRA) revealed the atomic structure and concentrations of the (001)-oriented LSC\({}_{113}\) thin film on a STO substrate, which shows strontium (Sr) segregation toward the LSC\({}_{113}\) surface and Sr depletion near the interface between LSC\({}_{113}\) and STO [151]. It was also reported that heating the (001)-oriented LSC\({}_{113}\) surface leads to the formation of surface Sr-enriched particles upon annealing while the LSC\({}_{214}\)-decorated LSC\({}_{113}\) surface chemistry is stable upon heating [152]. More recently, [PERSON] et al. [86] compared the time-dependent surface exchange kinetics and stability of epitaxial LSC\({}_{113}\) and LSC\({}_{113}\) thin films with LSC\({}_{214}\) surface decoration. Using density functional theory (DFT) computation, the authors revealed greater surface Sr segregation for LSCF\({}_{113}\), which is predicted to have an SrO termination, than LSC\({}_{113}\), which is predicted to have a less Sr enriched (La\({}_{0.25}\)Sr\({}_{0.75}\))O termination. Furthermore, the authors pointed out a lower energy gain to move Sr from LSCF\({}_{113}\) into LSC\({}_{214}\) relative to the LSC\({}_{214}\)-LSC\({}_{113}\) surface (Figure 10). These observations have suggested that the surface decoration can modulate the surface Sr segregation and the surface phase stability, which can greatly influence the oxygen surface exchange kinetics and the surface stability in LSC\({}_{113}\) and LSCF\({}_{113}\). Therefore, understanding the surface decoration effect on the surface  
Figure 9: Arrhenius plots of surface exchange coefficients (\(k^{\star}\)) for different RP oxides: (**a**) \(k^{\star}\) along c-direction in La\({}_{2\text{-}}\)Sr\({}_{\text{x}}\)MO\({}_{4}\) (M = Cu, Co, Ni, [126; 133; 135; 144]); and (**b**) \(k^{\star}\) along a-b plane in A\({}_{2\text{-}}\)Sr\({}_{\text{x}}\)MO\({}_{4}\) (A = Sr, La, Nd and M = Co, Ni, [61; 65; 128; 133; 135; 144]) thin films. chemistry of perovskites is critical and other potential surface modification materials need to be investigated to design highly active and stable cathodes for SOFCs. Similar to the LSC\({}_{113}\) surface modification with LSC\({}_{214}\), several studies have also reported the enhanced surface electrocatalytic activity of porous LSCF cathodes with surface decoration [82; 147; 153; 154]. For example, depositing thin La\({}_{0.85}\)Sr\({}_{0.15}\)MnO\({}_{3-8}\) coatings on porous LSCF6428 electrodes using an infiltration process, [PERSON] et al. [82] showed the enhanced surface electrocatalytic activity of decorated LSCF\({}_{113}\) cathodes upon polarization.