# GDC-Based Infiltrated Electrodes for Solid Oxide Electrolyzer Cells (SOECs)
## 3 Results and Discussion
Subfigure (**A**) (lowest magnification) gives evidence of the homogeneity of cathode’s porous structure, while the red circles give evidence of the well-dispersed clusters (**B**) of doped-ceria nanoparticles (**C**) alongside the whole electrode’s surface. Figure 3: SEM micrograph of the LSCM/GDCW cathode cross section. At low magnification, the LSCM/GDCE surface (Figure 4A) seems porous, with the presence of micrometric clusters (Figure 4B) covered by GDC nanometric grains of about 100 nm (Figure 4C). The measured size value is very similar to the GDC crystallite size calculated using the Scherrer equation. Otherwise, the LSCM/GDCW surface shows dense and compact zones separated by micrometric cracks (Figure 5A). At higher magnification, the LSCM/GDCW compact areas look quite dense (Figure 5B), different to the LSCM/GDCE (Figure 4B). Thus, agglomeration phenomena tend to fill up the surface porosity and these features reasonably occur after each calcination step. The tendency to agglomerate is well known for oxides deriving from aqueous solution precipitation [29; 30; 31]. To overcome this limitation, the use of some final washing cycles with ethanol can remove the water molecules from the surface of the particles and hinder the formation of an oxo-bridging bond [35]. Thus, when an aqueous solution was used for the infiltration, the GDC did not properly penetrate inside the porous LSCM backbone and only a small amount of the GDC reached the interface with the electrolyte. Figure 5C,D confirms the presence of dispersed and not interconnected GDC nanoparticles in the interior electrode. Figure 5: Top-view micrographs (fully completed infiltration) of the LSCM/GDCW cathode at different magnifications: external layer (**A–D**) and internal electrode (**E**,**F**). Subfigures from (**A–D**) have been taken on the outer surface, while subfigures (**E**,**F**) have been taken inside the crevices clearly visible in (**A**,**D**). Doped-ceria nanoparticles (in red circles) are poorly accessible, as they can only be observed considerably deep inside the electrode. Thus, the microstructure observed can be related to the infiltration procedure reported in Figure 1. The constant GDC loading and the reduced number of infiltrations of the GDCE are in agreement with the observed open-surface porosity that induces a continuous GDC pathway in the backbone; differently, the non-linear loading and the larger amount of infiltration of the GDCW can be related to a less porous electrode surface that prevents a proper and homogeneous infiltration. So, in the LSCM/GDCW cathode, the GDC particles stack together on the external surface of the electrode, while in the LSCM/GDCE a much more uniform distribution of Gd-doped ceria inside the electrode occurs. Therefore, the noticeable morphological differences emerging from the SEM micrographs between infiltrated cathodes may affect the electrochemical behavior. Figure 6 shows the Nyquist plots at OCV of the LSCM/GDCE and LSCM/GDCE cells operating in a CO/CO\({}_{2}\) = 30/70 mixture in the 700-800 \({}^{\circ}\)C temperature range. As commonly reported, the impedance spectra show depressed semicircles due to the overlapping of different electrochemical phenomena. Table 1 reports the electrochemical data of the LSCM/GDCW and LSCM/GDCE cells: the ohmic resistance (R\({}_{\text{ohm}}\)) being the high frequency intercept with the real axis, and the polarization resistance (R\({}_{\text{pol}}\)) corresponding to the difference between the high and low frequency intercepts. At all temperatures investigated, the R\({}_{\text{ohm}}\) and R\({}_{\text{pol}}\) values of the LSCM/GDCE are lower than those of the LSCM/GDCW. The ohmic resistance (R\({}_{\text{ohm}}\)) is mainly ascribable to the electrolyte but it can be also due to the ohmic contribution of electrodes, thus it is likely that the larger R\({}_{\text{ohm}}\) values of the LSCM/GDCW cell can be ascribable to the agglomeration of GDC nanoparticles on the cathode surface that enhance the ohmic resistance.