## 1 Introduction  
State-of-the-art high-temperature electrochemical devices have high potential for utilization due to their diverse applications, including power generation in fuel cells [1; 2; 3], electrolysis and gas utilization [4; 5; 6], purification and production of high-purity gases [7; 8; 9], gas analysis and detection [10; 11; 12], conversion of various gases, such as ammonia or hydrocarbons [13; 14; 15], etc. In most cases, the functional materials utilized in the primary components of such devices, including the electrolyte, cathode, and anode, are complex oxides that exhibit high stability at elevated temperatures and a variety of essential electrical, electrochemical, and functional properties [16; 17; 18; 19; 20; 21; 22; 23]. It is often difficult to achieve properties such as high electrolyte density and an optimal electrode/electrolyte interface, even at elevated temperatures. This is due to a number of factors, including evaporation, phase decomposition, chemical interaction, thermal expansion mismatch, segregation, and so forth [24; 25; 26]. In this context, an effective strategy for enhancing the functional properties of the electrolyte and electrodes is the incorporation of sintering aids. Sintering aids are typically binary compounds, such as fluorides [27; 28; 29], although they are more commonly oxides, which are introduced into the base material prior to the heat treatment. Sintering aids possess low melting points or form low-temperature intermediate phases or eutectic mixtures with the base material, thus improving material properties at low sintering temperatures [30; 31; 32]. In the case of solid electrolytes, the incorporation of sintering aids results in an enhancement of the density of the material or in the reduction in the temperature required for the formation of dense electrolytes. In [33], [PERSON] et al. note the positive effect of ZnO sintering aids on the properties of BaCe\({}_{0.2}\)Zr\({}_{0.7}\)Y\({}_{0.1}\)O\({}_{3-\delta}\) electrolytes. The use of the Bi\({}_{2}\)O\({}_{3}\)-CuO sintering aid allowed [PERSON] et al. to obtain dense (Ni\({}_{0.2}\)Cu\({}_{0.2}\)Zn\({}_{0.6}\)O)\({}_{1.03}\)(Fe\({}_{2}\)O\({}_{3}\))\({}_{0.97}\) ceramic at a low temperature [34]. Many studies have shown the positive effect of Fe\({}_{2}\)O\({}_{3}\) as a sintering aid on the properties of Gd\({}_{0.1}\)Ce\({}_{0.9}\)O\({}_{1.95}\)[35; 36], Gd\({}_{0.2}\)Ce\({}_{0.8}\)O\({}_{2-\delta}\)[37], GdSmZr\({}_{2}\)O\({}_{7}\)[38] and Sr\({}_{2}\)Fe\({}_{1.5}\)Mo\({}_{0.5}\)O\({}_{6-\delta}\)[39]. In [40], the effect of CuO, ZnO, Al\({}_{2}\)O\({}_{3}\), NiO, Fe\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\) and MoO\({}_{3}\) on the properties of the La\({}_{0.9}\)Sr\({}_{0.1}\)ScO\({}_{3-\delta}\) electrolyte was investigated. The findings revealed that the incorporation of Co\({}_{3}\)O\({}_{4}\) resulted in the most optimal outcome. The investigation of the solid proton-conducting electrolyte BaSn\({}_{0.8}\)Y\({}_{0.2}\)O\({}_{3-\delta}\) with the CuO sintering aid was performed by [PERSON] et al. in [41]. In the case of electrode materials, the utilization of sintering aids makes it possible to increase electrode conductivity and improve electrode adhesion to the electrolyte. In [42], it was demonstrated that the incorporation of the Bi\({}_{0.75}\)Y\({}_{0.25}\)O\({}_{2-\delta}\) + CuO aid markedly influences the conductivity and electrochemical activity of LaNi\({}_{0.6}\)Fe\({}_{0.4}\)O\({}_{3}-\delta\) electrodes. In [43], it was shown that the introduction of 1-3 wt.% CuO into the cathode material based on lanthanum nickelate increases its electrical conductivity by 2-3 times. The positive effect of copper oxide on the properties of electrode materials was also noted in [44]. Among the various high-temperature electrochemical devices, it is noteworthy to mention the so-called symmetrical electrochemical cells [45; 46; 47], which consist of a highly conductive supporting electrolyte and a cathode and anode of the same chemical composition.