## 1 Introduction
The minimal polarization resistance for the Nd\({}_{2}\)NiO\({}_{4+\delta}\) electrode was equal to 0.51 \(\Omega\) cm\({}^{2}\) at 700 \({}^{\circ}\)C, while for the Nd\({}_{1.7}\)Ca\({}_{0.3}\)NiO\({}_{4+\delta}\) electrode, it reached the minimal value at 0.71 \(\Omega\) cm\({}^{2}\) at 700 \({}^{\circ}\)C in air according to [36]. As an alternative, the utilization of a LaNi\({}_{0.6}\)Fe\({}_{0.4}\)O\({}_{3-\delta}\) (LNF) collector layer, developed in [36,37], provided a superior electrochemical performance of Ca-substituted Nd\({}_{2}\)NiO\({}_{4+\delta}\) on the Ce\({}_{0.8}\)Sm\({}_{0.2}\)O\({}_{1.9}\) electrolyte substrate. The optimum characteristic obtained for the Nd\({}_{1.6}\)Ca\({}_{0.4}\)NiO\({}_{4.08}\) electrode, with a functional layer sintered at 1200 \({}^{\circ}\)C, wasequal to 0.37 \(\Omega\) cm\({}^{2}\) at 700 \({}^{\circ}\)C, in comparison with the value of 0.71 \(\Omega\) cm\({}^{2}\) at 700 \({}^{\circ}\)C for the non-substituted Nd\({}_{2}\)NiO\({}_{4+\delta}\). [PERSON] et al. [32] reported the values of oxygen surface exchange constant (\(k_{\rm ex}\)) as being equal to \(\sim\)10\({}^{-4}\) cm s\({}^{-1}\) at 700 \({}^{\circ}\)C for Ln\({}_{2}\)NiO\({}_{4+\delta}\) and \(\sim\)10\({}^{-5}\) cm s\({}^{-1}\) for La\({}_{1.7}\)Ca\({}_{0.3}\)NiO\({}_{4.06}\). The values of the effective activation energy (\(E_{a,k}\)) were equal to 100-115 kJ mol\({}^{-1}\) for Ln\({}_{2}\)NiO\({}_{4+\delta}\) and 120-130 kJ mol\({}^{-1}\) for Ln\({}_{2-x}\)Ca\({}_{x}\)NiO\({}_{4+\delta}\). Data presented in [32] demonstrated a decrease in the surface oxygen reactivity with Ca-doping, a feature that was explained by the difficulties in the surface exchange process owing to the steric factors and lower oxygen coverage due to the carbonates forming on the active sites of the surface enriched with Ca. The trend for the oxygen diffusion mobility to decrease with Ca substitution was explained by the reduction in the highly mobile interstitial oxygen content, which hampered the cooperative mechanism of oxygen migration involving oxygen in both the interstitial and regular positions [31; 32]. The substitutions of Ni cations with Cu in Ln\({}_{2}\)NiO\({}_{4+\delta}\) (Ln = La [38; 39; 40]; Ln = Nd [41; 42]; Ln = Pr, Nd [43]) and the simultaneous substitution of Ln with Ca and Ni with Cu (Ln = La [44; 45; 46; 47; 48]; Ln = Nd [49]) had an effect on the level of oxygen over-stoichiometry [43; 49], the transport properties [39; 40; 41; 42; 44; 47; 49], and the electrochemical activity [39; 40; 41; 44; 46; 48] of the Cu-doped Ln\({}_{2}\)NiO\({}_{4+\delta}\) compounds. According to [PERSON] et al. [39], La\({}_{2}\)Ni\({}_{1-x}\)Cu\({}_{x}\)O\({}_{4+\delta}\)/ 8 YSZ samples at x = 0.0; 0.2; 0.6 gave the ASR values of 3.7 \(\Omega\) cm\({}^{2}\) at 850 \({}^{\circ}\)C. [PERSON] et al. [40] reported the polarization resistance (\(Rp\)) value for the La\({}_{2}\)Ni\({}_{0.8}\)Cu\({}_{2.0}\)O\({}_{4+\delta}\)/ BaCe\({}_{0.5}\)Zr\({}_{0.3}\)Dy\({}_{0.2}\)O\({}_{3-\delta}\) cell as equal to 0.17 \(\Omega\) cm\({}^{2}\) at 700 \({}^{\circ}\)C. The La\({}_{1.7}\)Ca\({}_{0.3}\)Ni\({}_{0.7}\)Cu\({}_{0.3}\)O\({}_{4+\delta}\) electrode in [44] exhibited a maximum conductivity value of 150-160 S cm\({}^{-1}\) and a minimal polarization resistance of 1.45 \(\Omega\) cm\({}^{2}\) at 650 \({}^{\circ}\)C, minimal in the series, whereas a Ni-YSZ/YSZ/\(\mathrm{La_{1.7}Ca_{0.3}Ni_{0.75}}\)Cu\({}_{0.25}\)O\({}_{4-\delta}\) cell, tested in [45], showed the power density of 0.71 W cm\({}^{-2}\) at 750 \({}^{\circ}\)C using H\({}_{2}\) and air as the fuel and oxidant, respectively. [PERSON] et al.