To test the novel core-shell structure in a PEMWE single cell, CLs were fabricated containing the TiO2@IrOx particles. For a better understanding of the single-cell results, the CLs were analyzed regarding structure and conductivity. The nano-CT measurement of a CL incorporating the TiO2@IrOx core-shell particles at a low loading of 0.2 mgIr cm -2 is displayed in Fig. . The nano-CT tilt-series (Fig. and Video V.1) and reconstructed slices (Fig. and Video V.2) show a homogeneous layer structure with interconnection of the particles throughout the layer. Thus, electrical percolation through the CL should be enabled even at this low loading. For the reconstructed volume in Fig. , a catalyst volume fraction in the CL of 31.4 vol% was determined (see Fig. for segmentation of the volume). From the ink formulation, the volume fraction of the wet ionomer is calculated to be 35.1 vol%. Based on this, the void volume fraction amounts to 33.5 vol%. In literature, an optimal value for void and ionomer fraction of ~ 35 vol% each was found for a good balance between mass transport, ionic and electrical conductivity. The highly porous structure of the TiO2@IrOx CL can also be seen from the FIB-SEM crosssectional image in Fig. . In the following, the Umicore IrO2/TiO2 catalyst is chosen as a reference since it is, with regards to structure, closer to the core-shell catalyst as both are supported catalysts. The FIB-SEM cross-sectional image of a CL with the Umicore IrO2/TiO2 catalyst is shown in Fig. . When comparing Fig. and, it becomes clear that the layer morphology differs strongly due to the difference in support particle size and iridium content between the two catalysts. A difference in CL thickness was determined by SEM imaging of embedded CCM samples for differently loaded anodes with the core-shell and Umicore catalyst shown in Fig. . As the slope of the graph in Fig. , we derived a thickness factor defined as the CL thickness normalized by the iridium loading. The thickness factor of the core-shell CL of 10.9 ± 1.1 µm (mgIr cm -2 ) -1 is almost three times higher than for the Umicore CL with only 3.9 ± 0.8 µm (mgIr cm -2 ) -1 . The latter value is in good agreement with a literature value of 4.3 ± 0.3 µm (mgIr cm -2 ) -1 for the same catalyst material and CL fabrication method. As further reference, a CL fabricated via decal transfer with the IrOx catalyst from Alfa Aesar yields a thickness factor of 4 -5 µm (mgIr cm -2 ) -1 . The difference in thickness factor mainly originates from the difference in iridium content in the catalyst powder (core-shell: 40 wt% vs. Umicore: 75 wt% and Alfa Aesar: 86 wt% Ir). A higher porosity due to the particle size can further contribute to this effect. As discussed above, a higher thickness factor is necessary to reach the goal of decreasing the amount of iridium used. With a higher thickness factor, thicker layers at reduced loadings can be realized compared to a catalyst with a lower thickness factor (e.g., the Umicore catalyst). When swelling of the ionomer and membrane upon water contact causes layer cracking during PEMWE operation, a thicker CL is assumed to avoid a disconnection of the layer and therefore preventing a deactivation of parts of the catalyst and increased electric in-plane resistance. This effect is schematically depicted in Fig. and for a high-and low-thickness-factor catalyst, respectively. A cracking of CLs was visualized in literature after contact with water at 80 °C and was shown to be more pronounced for thinner layers. For the extreme case of reaching a full decarbonization of the transportation sector with hydrogen produced by PEMWE, calculations suggest that the iridium loading needs to be decreased to 0.05 mgIr cm -2 while a thickness of > 2 µm is maintained (optimal thickness ≈ 4 -8 µm for Umicore Elyst Ir75). It becomes clear that this goal cannot be reached with unsupported or high Ir-content catalysts due to the low thickness factor. With our core-shell catalyst, the thickness factor is almost tripled, bringing us closer to this goal. To fall below the threshold of 2 µm for the CL with our catalyst, the loading needs to be decreased below 0.18 mgIr cm -2 . Admittedly, for the core-shell structures at loadings decreased below 0.37 mgIr cm -2 , the layer is thinner than the aforementioned optimum of 4 µm. However, the optimal layer thickness needs to be reevaluated for our material, since, in comparison to the reference material, the layer morphology differs due to a larger particle size. Moreover, due to the core-shell structure and decreased iridium content, the electrical layer connection is changed compared to the Umicore CL. Thus, in our future work we aim for further decreasing the iridium content in the catalyst powder to achieve even lower loadings at maintained layer thickness. Another positive effect of thicker layers is the higher robustness when handling catalyst layers. This is particularly important for industrial applications, where large electrodes are used. Furthermore, we faced the issue that with the Umicore catalyst, although using the Mayer rod with lowest wet-film thickness, we were not able to reach loadings below 0.3 mgIr cm -2 . In literature, loadings down to 0.026 mgIr cm -2 with good performance (1.87 V @ 2 A cm -2 ) are reported. Nevertheless, CCMs with such low loadings are usually fabricated via spray coating, which is less suitable for a scale-up to industrial level. In contrast, the CCM fabrication used herein (Mayer rod coating and decal transfer) is closer to slot-die coating as truly scalable method. The in-plane conductivities of pristine and dry CLs were measured for the core-shell and the Umicore catalyst. The results are shown as a function of varying iridium loadings in Fig. . As aforementioned, the in-plane conductivity does not play a large role for RDE measurements due to thin catalyst layers coated onto a highly conductive gold electrode so that an effect cannot be seen from these experiments. In singlecell measurements, however, a high in-plane conductivity is important to ensure a good catalyst utilization. To assess if the connection of the layer is maintained when decreasing the loading, the in-plane conductivity is plotted dependent on the iridium loading in Fig. . If the layer integrity were not maintained, a drop in in-plane conductivity would be expected. This is not the case for any of the catalysts. We emphasize that the measurements are performed in dry conditions. A cracking of the layer is expected to occur upon membrane and ionomer swelling when they are in contact with water. Insights into the performance of CCMs in wet state is provided by the single-cell testing in the following section.