## 3 Results
### Oxygen Surface Exchange Kinetics  
Figure 2 shows an ECR normalized curve corresponding to the 0.5 to 0.1 atm of the reduction step measured with the LSCO film decorated with \(\sim\)3 nm thick LNO at 450 \({}^{\circ}\)C. ECR measures the electrical conductivity of the thin film as a function of time during a change in the oxygen partial pressure, \(p\)(O2) [37,38]. The electrical conductivity is very sensitive to the changes in oxygen chemical potential depending on the defect chemistry of the thin film, which can enable the evaluation of oxygen transport properties [39,40]. The raw data of the relaxations were fitted by a solution to [PERSON]'s second law [41,42]. A simple exponential function  
\[g(t)=1-\exp\ (-\frac{t}{\tau})\]  
with  
\[\tau=\frac{l}{k_{\text{chem}}}\]  
where \(\tau\), \(l\), and \(k_{\text{chem}}\) are the time constant, film thickness, and oxygen surface exchange coefficient, respectively, was used to fit the experimental results. As shown in Figure 2, the fitted data with the simple model was found to well-match the experimental data, and thus this simple model was used in this work. Figure 3 shows the \(k_{\rm chem}\) values of the undecorated LSCO and LNO-decorated LSCO films collected from ECR measurements. Considering the film thicknesses were much smaller than the critical thickness (estimated to be 3 \(\upmu\)m for bulk La\({}_{2}\)CuO\({}_{4}\)[43]), the \(p\)(O\({}_{2}\))-dependent \(k_{\rm chem}\) values (Figure 3b) suggest that the oxygen surface exchange kinetics govern the ORR activity on the film surface. As shown in Figure 3, the formation of heterointerfaces has a strong influence on the oxygen surface exchange kinetics of the LSCO film. Interestingly, the \(k_{\rm chem}\) values of the LSCO films were enhanced with LNO coverage less than or equal to \(\sim\)3 nm in thickness, whereas those were reduced with \(\sim\)15 nm LNO coverage (Figure 3a). Figure 3b shows the \(p\)(O\({}_{2}\))-dependent \(k_{\rm chem}\) values of the undecorated LSCO and LNO-decorated LSCO films at 350 \({}^{\circ}\)C. At a \(p\)(O\({}_{2}\)) of 10\({}^{-2}\) atm, the LSCO films with partial LNO coverages showed increased \(k_{\rm chem}\) values by up to approximately one order of magnitude compared with the undecorated LSCO film. The \(p\)(O\({}_{2}\)) dependency of  
Figure 1: (**a**) XRD 0–20 patterns of the epitaxial reference LSCO and LNO-decorated LSCO films; (**b**) XRD \(\phi\) scans of the 202 reflection of a YSZ substrate, a GDC buffer layer (\(\sim\)20 nm), and a LNO film (\(\sim\)15 nm), and of the 103 reflection of a LSCO film (\(\sim\)60 nm). GDC and YSZ substrate peaks are indicated with # and \(\star\), respectively. RSMs around the 2 2-4 Bragg peak from YSZ of (**c**) reference LSCO, (**d**) \(\sim\)3 nm LNO-decorated LSCO, and (**e**) \(\sim\)15 nm LNO-decorated LSCO film. Figure 2: Experimental and fitted ECR data obtained from the \(\sim\)3 nm LNO-decorated LSCO film under a gas switch from 0.5 to 0.1 atm at 450 \({}^{\circ}\)C.  
\(k_{\text{chem}}\) (\(k\propto\text{ }P_{\text{O}_{2}}^{\text{m}}\)) can be indicative of the rate-limiting step of oxygen surface exchange [44, 45]. The undecorated LSCO film exhibited an \(m\) value close to 0.5, which indicates that the dissociation of molecular oxygen to atomic oxygen is likely to be the rate-limiting step (O\({}_{\text{2, ads}}\leftrightarrow 2\)O\({}_{\text{ads}}\)). With partial LNO coverages (-1.5 and -3 nm), however, the dependence of \(k_{\text{chem}}\) on \(p\)(O\({}_{\text{2}}\)) exhibited noticeable changes. The \(m\) values for the LSCO films with partial LNO coverages were found to be close to 0.25, which implies that the charge transfer process (O\({}_{\text{2,ads}}+4\text{e}^{-}+2\text{V}_{\text{O}^{-}}\leftrightarrow 2 \text{O}_{\text{O}^{\circ}}\)) can be the rate-limiting step.