Figure shows the polarization curves recorded after 24 h, 283 h and 479 h, indicating stable performance of the STz25 membrane and a polarization behavior similar to that of the Zirfon reference material. Additionally, Figure shows that the ASR calculated from EIS for STz25 was 0.419 Ω cm 2 compared with 0.570 Ω cm 2 for Zirfon. Assuming that the main contribution to the resistance comes from the membrane, this corresponds to an ionic conductivity of 36 mS cm -1 for STz25 and 88 mS cm -1 for Zirfon. It is worth noting that the ionic conductivity value for STz25 in the single cell test was about half of what was observed in the ex-situ measurements. This discrepancy could be influenced by resistance contributions at the membrane-electrode interface or contributions from K + conductivity in the ex-situ measurements. The hydrogen crossover was also evaluated during the cell test. The data for H2 in O2 levels corrected for nitrogen gas that was used to flush the system is presented in Figure for both STz25 and Zirfon. In the case of Zirfon, the decrease in H2/O2 level with increasing current density is expected, as it is diluted by the generated O2, and the rate of O2 generation also increases. However, for STz25, the decrease in H2 level was less pronounced. At 50 mA cm -2 , the permeability calculated from the H2 flux density shown in Figure , were quite similar for both Zirfon and STz25, with values of 7.71 × 10 -9 and 8.13 × 10 -9 mol s -1 cm -1 bar -1 , respectively.