The cell with the LSCF air electrode showed better performance than the LSM cell in both fuel cell and electrolysis operation modes and for all the gas compositions investigated (see Fig. 2 and Table 1). In particular, the ASR of cell A at 800 °C 50/50 H2/H2O was about 1.6 times lower than that of cell B in both SOFC/SOEC modes, and still lower than the ASR of cell B test at 850 °C. The SOFC and SOEC polarizations of the LSM cell were almost symmetric with both investigated fuel mixtures, while the LSCF cell showed higher losses in the electrolysis mode at high current density. This is in agreement with the literature that reports asymmetric polarizations for LSCF electrodes . This behavior can be attributed to the depletion of oxygen vacancies at the electrode/electrolyte interface that occurs under SOEC mode. The air electrode shows a limiting current behavior under anodic polarization because the oxygen transport is slowed down by the depletion of vacancies; this YPYDVPDYA effect is evident in highly oxygen deficient materials, as LSCF above 600 °C, while is less discernible for LSM that has an ionic conductivity of several orders of magnitude lower than LSCF  and . At a current density of 0.5 A cm−2 (RU ∼70%) the 50/50 H2/H2O test on cell A showed a difference of 34 mV between SOEC and SOFC total overpotential against only 2 mV of cell B in the same operating conditions. It must be emphasized that the effect of the back-flow diffusion can favor SOEC operation by generating new reactant at the cell border; therefore with a sealed-housing testing rig it could be expected an even worse behavior of the cells in SOEC at high RU. In Fig. 4 is highlighted the asymmetrical behavior of the LSCF cell, which had lower total cell resistance in SOFC mode at all the tested RUs. The increase of the total cell resistance observed in Fig. 4 in both operational modes with increasing RU is due to enhanced conversion resistance (low frequency arcs are connected to fuel conversion phenomena ).