## 3 Results and Discussion
### Oxygen Mobility in the NCNCO Materials
Such assumptions were made while analyzing electrochemical impedance spectroscopy data for the Pr\({}_{2}\)Ni\({}_{1-x}\)Cu\({}_{x}\)O\({}_{4+\delta}\)-based electrodes [82]. On the other hand, theoretical calculations predicted the positive effect of Cu cations on the oxygen transport characteristics due to the increase in the (Ni,Cu)-O bond lengths for Cu- and Ga-doped Pr\({}_{2}\)NiO\({}_{4+\delta}\)[83, 84]. In the present study, decreasing the oxygen transport characteristics byincreasing the copper content is assumed to be associated with a decrease in the highly-mobile interstitial oxygen content in agreement with the TOF NPD and in-situ XRD data (Sections 3.1 and 3.2) and the absolute oxygen over-stoichiometry values for the NCNCO samples determined in our recent study [49] from the sample mass change upon reduction in a 50 vol.% H\({}_{2}\)/50 vol.% Ar gas mixture at 1000 \({}^{\circ}\)C, showing their decrease from 0.07 (1) (y = 0.0) down to 0.00 (1) (y = 0.3). Independence of the effective activation energy of diffusion on Cu content (Table 6) and the literature data presented in works [35, 80] support this assumption as well. Electrochemical Performance of the Electrodes with Nd\({}_{1.6}\)Ca\({}_{0.4}\)Ni\({}_{1-y}\)Cu\({}_{y}\)O\({}_{4+\delta}\) Functional Layers. Factors Influencing the Electrode Performance  
Impedance data for the Nd\({}_{1.6}\)Ca\({}_{0.4}\)Ni\({}_{1-y}\)Cu\({}_{y}\)O\({}_{4+\delta}\)/SDC symmetrical cells with functional layers sintered at 1000 \({}^{\circ}\)C, collected in humidified air at 700 and 600 \({}^{\circ}\)C, are shown  
Figure 8: Arrhenius plots (**a**) and concentration dependencies (**b**) of the oxygen tracer diffusion coefficients for Nd\({}_{1.6}\)Ca\({}_{0.4}\)Ni\({}_{1-y}\)Cu\({}_{y}\)O\({}_{4+\delta}\) samples according to TPIE with C\({}^{18}\)O\({}_{2}\) data. in Figure 9a,b. The complex shape of the Nyquist plots indicates the presence of several different electrode processes contributing to the overall polarization resistance (\(R_{p}\)). The overall polarization resistance (\(R_{p}\)) of the electrochemical cell with symmetrically arranged electrodes was calculated as follows:  
\[R_{p}=\frac{(R_{dc}-R_{hf})\cdot S}{2} \tag{4}\]  
where \(S\) is the surface area of the electrode, \(R_{dc}\) is the \(dc\) resistance of the electrochemical cell, and \(R_{hf}\) is the ohmic resistance determined by extrapolation of the high-frequency part of an impedance spectrum to the intersection with the x-axis and related to the electrolyte resistance, contact electrolyte-electrode resistance, and a part of the lateral resistance of the electrode. It is seen that trends in changing the total electrode response are different at different temperatures; however, the electrode with the copper content equal to x = 0.2 shows the lowest polarization resistance equal to 1.28, 2.01, 3.59, and 5.70 \(\Omega\) cm\({}^{2}\) at 850, 800, 750, and 700 \({}^{\circ}\)C compared to the NCNCO pristine electrode equal to 2.48, 3.94, 7.45, and 12.28 \(\Omega\) cm\({}^{2}\) at 850, 800, 750, and 700 \({}^{\circ}\)C, respectively. To better illustrate this fact, the Arrhenius dependencies of the polarization conductivity, calculated as 1/\(R_{p}\), are presented in the supplementary materials in Figure S20. It is seen that the activation energy of the total polarization conductivity exhibits the minimum at the same concentration of copper (y = 0.2, \(E_{a}\) = 1.04 eV). It is worth noting that the Ea values for the doped samples, except for NCNCO02, were higher than those for the pristine NCNCO00 electrode (\(E_{a}\) = 1.03 eV). It is most probably related to lowering the oxygen surface exchange and diffusion kinetics byincreasing the copper content, which correlates well with the data presented in Section 3.4. The oxygen transport characteristics are directly related to the performance of MIEC electrodes and have a significant influence on the ORR rate, especially at low temperatures [84; 85].