## 3 Results
### Electrochemical Performance of the Electrodes
A distinct trend can be observed in the case of utilizing copper oxide as a sintering aid. The nature of the rate-determining steps of the oxygen reduction reaction changes with increasing sintering temperature, and their specific frequencies are shifted to the low-frequency region. In the case of the 1050-Fe sample, the DRT function is comparable to that of the 1000-Cu, albeit exhibiting a slight shift towards the high-frequency region (Figure 5c). This behavior can be attributed to the higher melting temperature of iron oxide in comparison to copper oxide. The number of peaks and their relaxation frequencies, as derived from the DRT functions, allow us to hypothesize about the nature of the stages that limit the rate of the oxygen reduction reaction. In the case of the 1000-Cu sample, these processes are observed at the electrode/electrolyte boundaries and between the electrode particles. They appear to be related to the slow transport of electrons and/or oxygen ions across the boundaries, which can be attributed to the low sintering temperature of the electrode and the poor quality of the interfacial boundaries. As the sintering temperature increases, the electrode adhesion increases and the rate of electron and ion transport across the boundaries increases. For electrodes sintered at higher temperatures of 1050-Cu and 1100-Cu, the rate-determining stage of oxygen reduction is the interfacial exchange of electrode oxygen with the gas phase [39]. The DRT function for the 1150-Cu sample exhibits a low-frequency peak, which is characteristic of gas diffusion processes [65]. This peak is a consequence of a notable reduction in electrode porosity at the sintering temperature of 1150 \({}^{\circ}\)C. Consequently, the porosity estimation derived from micrographs using the ImageJ software (version 1.5 li8) package indicated a value of approximately 62% for the 1050-Cu sample and approximately 35% for the 1150-Cu sample (Figure 5d). As previously stated in the Introduction, symmetrical electrochemical cell electrodes must operate efficiently in both oxidizing and reducing atmospheres. Accordingly, we proceeded to examine the electrochemical behavior of LSF electrodes in a wet hydrogen atmosphere. Figure 6a illustrates the temperature dependence of the polarization resistance of LSF electrodes in a wet hydrogen atmosphere. A comparison of these data with the results presented in Figure 5a reveals that the sintering temperature and the type of sintering aid exert disparate effects on electrode performance in different atmospheres. In particular, the polarization resistance of the samples with copper oxide was observed to increase with an elevated sintering temperature. The lowest polarization resistance was observed for the 1000-Cu sample, reaching approximately 1.7 \(\Omega\) cm\({}^{2}\) at 800 \({}^{\circ}\)C. Conversely, the highest polarization resistance was recorded for the 1150-Cu sample, approximately 6.5 \(\Omega\) cm\({}^{2}\) at the same temperature. It is noteworthy that the polarization resistance of the 1050-Fe electrode exhibited the highest performance among all studied samples, reaching approximately 1.3 \(\Omega\) cm\({}^{2}\) at 800 \({}^{\circ}\)C. The results demonstrated that the activation energy of the polarization resistance in a wet hydrogen atmosphere is nearly identical to that observed in air (Figures 5a and 6a). This finding suggests that the rate of the hydrogen oxidation reaction may be constrained by the same processes that regulate the oxygen reduction reaction. Figure 6b presents a comparison of the polarization resistance for all investigated electrodes in various atmospheres at 800 \({}^{\circ}\)C. From the perspective of utilizing the electrodes in symmetrical cells, electrode 1050-Fe demonstrates the most effective performance. It exhibits the most optimal values of polarization resistance in both oxidizing and reducing atmospheres.