In Fig. and, the polarization curves of CCMs with the core-shell and the Umicore catalyst are displayed together with the high frequency resistance (HFR) and HFR-corrected polarization curves. Fig. shows the baseline experiment with a relatively high iridium loading of ~ 1.3 mgIr cm -2 . As expected from the in-plane conductivity measurements (Fig. ), the core-shell CLs exhibit a higher ohmic resistance (as can be seen from the steeper slope of the polarization curve) and a higher HFR compared to the Umicore CL. For the HFR-free polarization curves, the performance of the two CCMs is similar. This leads us to the conclusion that the performance difference mainly stems from resistances included in the HFR. Since all components, except the anode catalyst layer, are kept the same, contributions to the resistance stem from the anode CL bulk and interfacial contacts between the anode CL and PTL. An additional bulk electrical resistance of the core-shell CL (as opposed to the Umicore CL) could stem from the lower iridium content and decreased conductivity of amorphous IrOx. Additionally, two structural factors have an impact. First, the electrical pathway is longer as the CL thickness for the core-shell catalyst is significantly increased compared to the Umicore catalyst for the same absolute iridium loading (core-shell ~ 14 µm vs. Umicore ~ 5 µm at 1.3 mg cm -2 ; from thickness factor in Fig. ). Second, the cross-sectional area for electrical conduction is reduced since it only takes place via the thin shell for the core-shell CL. This combination leads to the inferior overall performance of the core-shell CL compared to the CCM with Umicore catalyst at high loadings (filled symbols in Fig. ). However, the HFR-corrected performance at low current densities of the core-shell catalyst is superior to the reference (see inset Fig. ). This can be assigned to a higher intrinsic activity of the substoichiometric and amorphous IrOx of the core-shell catalyst compared to the rutile IrO2 of the Umicore catalyst, as evident from RDE measurements (Fig. ). In contrast, starting from 3 A cm -2 the HFR-free polarization curves are crossing. This could be explicable by two scenarios. First, mass transport issues could lead to an upward bending of the polarization curve due to additional voltage losses. When regarding the FIB-SEM cross-sections in Fig. , a difference in pore size is apparent. Besides the pore size, the wettability and surface structure inside the pores of the material was shown to have an influence on the bubble formation and the resulting two phase flow of water and oxygen. Further investigation is required to determine the influence of these parameters and their interaction. A second explanation could be an increased ionic resistance of the catalyst layer (which is not included in the HFR ), resulting in a larger slope of the HFR-free polarization curve for the core-shell CL. When keeping the absolute iridium loading constant, the CL thickness for the core-shell catalyst is significantly increased compared to the Umicore catalyst (~ 14 µm vs. ~ 5 µm at 1.3 mg cm -2 ). Even though the tortuosity of the ionic pathway within the core-shell CL seems to be decreased (compare Fig. to Fig. ), the threefold increased thickness could lead to a longer ionic percolation path resulting in a higher resistance (as sketched in blue in Fig. ). When decreasing the loading to 0.2 -0.3 mgIr cm -2 as shown in Fig. , the core-shell catalyst noticeably outperforms the Umicore reference catalyst throughout the full current-density range. The CCMs with the core-shell catalyst show a decreased cell potential by 50 mV @ 2 A cm -2 compared to the reference CCMs (1.77 V vs. 1.82 V @ 2 A cm -2 ). Regarding the HFR-values, opposite trends can be observed. While the CCMs with the Umicore catalyst on the anode CL exhibit an increased HFR for low loadings (as already observed in literature ), the HFR of the CCMs with the core-shell catalyst decreases with decreasing loading. For the low loading case, the CLs are thinner (core-shell ~ 2.4 µm vs. Umicore ~ 1.1 µm), which influences the performance of the two CLs in different ways. In the case of the Umicore catalyst, we assign the observed decrease in performance (Fig. ) to a cracking of the layer upon membrane swelling during operation. If the layer thickness decreases below the threshold of 2 µm, cracks can lead to a disconnection of the CL and to a decreased in-plane conductivity resulting in a higher electron transport resistance within the CL. In combination with a rather coarse PTL with large pores that do not compensate for a disconnection, this can explain an increased HFR with decreased loading. When using uncoated titanium PTLs, the native TiO2 layer acts as semiconductor and might induce a Schottky barrier between CL and PTL and thereby additional resistance. However, such a band bending is probably not applicable to explain an increased HFR in our case, because platinum-coated PTLs are used in all measurements. With Pt coating (metal-metal contact between PTL and CL), no Schottky barrier should be present at the PTL|CL interface, resulting in the same effect for both catalysts independent of their oxidation state and work function. This was also observed for a system with rutile IrO2 or amorphous IrOx, where the implementation of a platinum coating on the PTL resulted in a very similar HFR of 50 mΩ cm 2 and 47 mΩ cm 2 for the rutile and amorphous iridium oxide catalyst, respectively. It could be argued that the advantage of decreasing the iridium loading is diminished when the PTL needs to be coated with Pt, as additional platinum group metal (PGM)-content is added to the system. However, in our work the mean amount of added Pt to each cell yields 0.035 ± 0.025 mgPt cm -2 while the tenfold decrease in iridium loading compared to a state-of-the-art loading of 2 mg cm -2 saves 1.8 mgIr cm -2 . Additionally, platinum is more abundant than iridium so that it is currently not the bottleneck for a scale-up of PEMWE. Comparing the high-and low-loading performance of the core-shell CL, the kinetic penalty due to a decreased loading is balanced out by a decrease in HFR, leading to a superior performance compared to the reference CL. For current densities higher than 2 A cm -2 , the low-loading CL even outperforms the high-loading CL with the core-shell catalyst (1.99 V vs. 2.02 V at 4 A cm -2 ). The decrease in HFR for the low-loading core-shell CL is assigned to a combination of two phenomena: First, a layer cracking is prevented due to a higher thickness factor. Secondly, the electrical resistance of the CL is reduced from high-to low-loaded CLs due to the thickness reduction and as a result a shorter electrical pathway. In this regard, an optimal layer thickness needs to be determined. A comparison of the HFR-free polarization curves at low iridium-loadings shows the higher activity of the core-shell catalyst, outperforming the CCM with Umicore catalyst. The HFR-free polarization curves maintain a constant difference. Thus, neither the ionic resistance contribution nor a mass transport limitation appears in the low-loading case with the core-shell catalyst where the layer thickness is reduced compared to the high loading case (~ 14 µm vs. ~ 2.4 µm). The HFR-free performance of the Umicore catalyst with low loading is decreased in comparison to the high-loading case. This could stem from a cracking of the CL leading to parts of the CL not being active and thus increasing the kinetic overpotential. Note that the deviations in the case of low loading with IrO2/TiO2 Umicore CL are larger (Fig. , grey curves) hinting towards an issue with the homogeneity of the electrodes in general, which is not observed for higher loadings or for any loading with our TiO2@IrOx core-shell catalyst. Moreover, Fig. and displays an increase in HFR with current density for the core-shell CL for both, high and low loadings. In general, this can be caused by either in-plane or through-plane effects. In-plane inhomogeneities can cause an apparent higher bulk electrolyte resistance either by local dry out of the membrane (which can be caused by large land-structures of the PTL ) or by inhomogeneous usage of the bulk electrolyte phase for the ion transport. The latter is often observed for alkaline water electrolysis systems where bubble accumulation can block direct ionic pathways between the electrodes. However, also for membrane-based electrolysis systems such an effect can be envisioned to occur in case the evolved gases accumulate in larger domains of the CL (i.e., on the same scale as the bulk electrolyte thickness). Such domains could then not contribute to the overall reaction rate efficiently, leading to inhomogeneous catalyst utilization and therewith, a higher HFR. Minding the larger pore size for the core-shell CL, however, we deem this effect not to be the cause for the observed trend, especially in comparison to the nano-porous reference CL. Through-plane effects of the membrane on the other hand could be the reason for the behavior of the core-shell CL HFR. The high osmotic drag at high current densities could lead to a reduced humidification close to the interface of the anode CL with the membrane. While a change in water profile across the membrane with current density was shown for thick membranes, the same effect can also be explained for thin membranes (as in our case ~ 51 µm) paired with thick electrodes . At high current densities the void volume of the anode CL will be filled with the oxygen product gas, limiting the water transport towards the active site to the ionomer film in the CL. Due to the larger thickness of the core-shell CL, this increase in HFR with higher current densities is apparent in Fig. and (green curves). The stability of the newly developed core-shell catalyst and the commercial references (IrOx Alfa Aesar Premion and IrO2/TiO2 Umicore Elyst Ir75) was examined in a scanning flow cell (SFC) coupled to an inductively coupled plasma mass spectrometer (ICP-MS) as well as in singlecell testing. In the SFC-ICP-MS measurement (Fig. ), the commercial catalyst from Umicore exhibits the highest dissolution stability, which is attributed to the complete oxidation to rutile IrO2. Promisingly, the stability of the TiO2@IrOx core-shell catalyst is similar to the Alfa Aesar reference catalyst. The same trend between the core-shell catalyst and the reference from Umicore is observed from the single-cell stability testing over 200 h @ 2 A cm -2 (Fig. ). We observe a higher degradation rate for the core-shell catalyst compared to the reference from Umicore. However, compared to various literature values for low iridium loading PEMWE stability testing with newly developed catalysts, the degradation rate of our catalyst is in a similar range (cf. ESI 'Stability testing'). Additionally, the core-shell structure was examined via HAADF-STEM after 200 h of testing. As shown in Fig. , the particles appear unchanged. It can be concluded that the catalyst already possesses good stability, which needs to be further improved. To directly compare the technological suitability of the two catalysts, the iridium-specific power density can be used. It is calculated based on the current density reached at a cell voltage of 1.79 V (equals 70% efficiency with regard to the lower heating value). With the values from Fig. (filled symbols, not HFR-corrected), the core-shell catalyst possesses an iridium-specific power density of 17.9 kW gIr -1 which is 1.7-times higher than the Umicore catalyst with a power density of 10.4 kW gIr -1 . However, the value is still ~ 3 -6-times lower than the postulated mass activity of 50 -100 kW gIr -1 needed for a successful large-scale implementation of PEMWE. Nevertheless, there is potential to further decrease the loading of the core-shell catalyst without disconnecting the catalyst layer by decreasing the iridium content in the catalyst powder below 40 wt%, which is currently under investigation. Decreasing the iridium loading in the anode catalyst layer for single-cell measurements was the subject of several previous studies where various approaches were tested. Since it is difficult to directly compare results across labs due to differences in cell hardware, components, and operation conditions, the examples here should serve as a context in the effort to employ low iridium loadings. The examples are further used in the comparison for our stability test results (cf. ESI 'Stability testing'). In an earlier publication from our group, TiO2 microparticles were coated with IrO2 and used to prepare porous transport electrodes. At a loading of 0.4 mgIr cm -2 the developed catalyst outperformed the reference from Umicore IrO2/TiO2 with 0.5 mgIr cm -2 . Hegge et al. implemented an IrOx-nanofiber interlayer between an IrOxnanoparticle-based CL and the PTL to improve their interfacial electrical contact and combine high porosity (interlayer) and high surface area (nanoparticles) within the CL. With an overall low loading of 0.2 mgIr cm -2 , it was possible to achieve a similar performance compared to a cell with 1.2 mgIr cm -2 of only IrOx nanoparticles. Taie et al. were able to decrease the loading of a bulk IrO2 catalyst down to 0.026 mgIr cm -2 while observing a better performance than a 2.5 mgIr cm -2 loaded commercial reference MEA (Greenerity E400). They argue that a control of CL morphology makes low loadings possible, which they achieve by preparing CCMs via direct CL deposition onto the membrane with spray coating. Möckl et al. tested a hydrous iridium oxide supported on TiO2 with a loading of 0.25 mgIr cm -1 . The new catalyst outperformed the Umicore IrO2/TiO2 benchmark catalyst at a state-of-the-art loading of 2 mgIr cm -2 clearly. These few examples show that there are several different approaches in research to decrease the anode catalyst loading for a scale-up of the PEMWE technology. Our performance is not in all cases superior to the presented literature examples. However, we emphasize that our synthesis route is facile and scalable, and the coating method used to prepare electrodes is industrially applicable. Besides, our approach is open to be combined with other approaches for example the implementation of an interlayer (cf. Hegge et al. ) to improve interfacial contact and cope for the decreased (in-plane) conductivity of our core-shell catalyst.