## Results and Discussion
### Formation of the BCGCuO Electrolyte Films on LBNO\({}_{\text{ss}}\) Substrates  
The EPD of the BCGCuO electrolyte film on the LBNO\({}_{\text{ss}}\) substrate (designated as LBNO\({}_{\text{ss}}\)_1) was performed at a deposition voltage of 80 V for 1 min with the initial current being approximately 6 mA. Three cycles of deposition/sintering were carried out (intermediate sintering at temperatures of 1200 \({}^{\circ}\)C, 2 h and 1350 \({}^{\circ}\)C, 2 h, and final sintering--at 1450 \({}^{\circ}\)C, 2 h). Sintering at all the stages, including intermediate stages, was carried out in protective barium carbonate filler. The total coating thickness was 10 \(\upmu\)m. The film surface had well-developed grain structures with defects in the shape of closed pores and convex chambers, perhaps repeating the substrate surface structure (Figure 8a). The spatial distribution of the elements in the coating (Figure 8b,c) confirmed the presence of barium, the content of which is closer to the nominal level (\(\sim\)20.7 at.%) compared to that in the BCGCuO film obtained on the LBNO\({}_{\text{nc}}\)/LNFO\({}_{\text{P}}\) substrate (\(\sim\)17.1 at.%). However, the content of Gd (\(\sim\)0.8 at.%) still did not reach the nominal value and there was diffusion of La (\(\sim\)1.4 at.%) into the coating. Additional deposition/sintering cycles were performed to increase the film thickness. However, at the thickness of about 17 \(\upmu\)m, the BCGCuO coating partly peeled off from the substrate during sintering. This was probably due to internal stresses in the electrolyte layer caused by the difference in the BCGCuO and LBNO TEC values, as, according to [25], the TEC value of the BCGCuO electrolyte changes from 10.5 \(\times\) 10\({}^{-6}\) K\({}^{-1}\) in the temperature range 100-575 \({}^{\circ}\)C to 8.6 \(\times\) 10\({}^{-6}\) K\({}^{-1}\) in the temperature range 575-900 \({}^{\circ}\)C, and the TEC of LBNO is 15.2 \(\times\) 10\({}^{-6}\) K\({}^{-1}\) in the 100-900 \({}^{\circ}\)C range. The TEC mismatch influence, evening out in the case of thin films, becomes pronounced when increasing the BCGCuO film thickness deposited on the LBNO\({}_{\text{ss}}\) bulk substrate (\(\sim\)1 mm in this study). For further experiments, EPD of the BCGCuO electrolyte film on the LBNO\({}_{\text{ss}}\) substrate (designation LBNO\({}_{\text{ss}}\)_2) was repeated under the same conditions up to the electrolyte thickness of approximately 10 \(\upmu\)m, as it is seen on the cross-sectional image of the cell taken after the electrochemical study (Figure 8d). As in the case of the BCGCuO/LBNO\({}_{\text{ss}}\)_1 half-cell, Ba content in the electrolyte coating of the BCGCuO/LBNO\({}_{\text{ss}}\)_2 half-cell was close to nominal level. The cell parameters calculated for the BCGCuO film deposited on the LBNO\({}_{\text{ss}}\) substrate, equal to a = 8.7921(26) A, b = 6.2196(25) A, and c = 6.2225(21) A, are close to those for the as-prepared BCGCuO powder (Table 1), which reveals the EDX data, showing that in this film, Ba content was preserved during the sintering. However, there is still Gd-La interdiffusion between the substrate and the electrolyte layers which may influence the functional properties of the film. No diffusion of Ni into the film was detected. The results for all prepared half-cells are summarized in Table 2. It is seen that the presence of the bulk LBNO\({}_{\text{ss}}\) substrate and simultaneously using the BaCO\({}_{3}\) as a protective filler can be an effective means of preserving Ba content in the BCGCuO electrolyte close to the nominal level during sintering at high temperatures.