The activity of the TiO2@IrOx core-shell catalyst was analyzed in rotating disk electrode (RDE) measurements and compared to two commercial reference catalysts, IrOx (Alfa Aesar Premion) and IrO2/TiO2 (Umicore Elyst Ir75 0480). Additionally, for a detailed investigation of the core-shell structure, we tested the novel catalyst without the thin shell (cf. Fig. ), labelled as IrOx/TiO2. The results are shown in Fig. . The activity of the core-shell catalyst is in between the two reference catalysts, with the Alfa Aesar catalyst showing the highest and the Umicore catalyst showing the lowest activity. The difference in activity of the Alfa Aesar and the Umicore catalyst is a well-known behavior reported in literature and can be explained by a substoichiometric IrOx phase for the Alfa Aesar catalyst, which results in an increased activity. The fully oxidized rutile IrO2 phase of the Umicore catalyst, on the contrary, is highly crystalline and yields lower activities. Interestingly, the activity of the novel catalyst with and without shell is almost identical. Although this result may suggest that the thin shell has no advantage, we emphasize that the conductivity of the catalyst only plays a minor role in RDE measurements due to the thin catalyst layers and the highly conductive electrodes. To further investigate the surface-specific activity of the core-shell catalyst, RDE results were normalized to the BET surface area (BET surface area values are shown in Fig. ). For this analysis, we assumed that all the TiO2 support is covered with iridium oxide so that the BET analysis equates to the active catalyst surface area. Thus, this analysis is not possible for the IrOx/TiO2 catalyst (without closed shell). As can be seen in Fig. , the surface-specific activity of the core-shell catalyst still lies in between the two commercial references. However, it is even closer to the activity of the Alfa Aesar catalyst, indicating that the activity per catalytic site is relatively high. This finding is further supported by similar Tafel slopes for the core-shell catalyst and the Alfa Aesar catalyst, with 47 and 48 mV dec -1 , whereas the Umicore catalyst exhibits a higher Tafel slope of 56 mV dec -1 (Fig. ). We explain this high surface-specific activity of the core-shell catalyst by the incomplete oxidation and the substoichiometric IrOx surface phase as evidenced by physical characterization. In the end, the iridium-mass activity of the catalyst depicts the most important descriptor for the activity regarding the scarcity of iridium. The mass activity of the different catalysts is evaluated by determining the mass-specific current densities at 1.6 V vs. RHE which are given in Fig. . The core-shell catalyst exhibits a moderately high mass activity of 233 ± 18 A gIr , which lies in between the two commercial references with mass activities of 530 ± 50 A gIr -1 and 65 ± 1 A gIr -1 for the Alfa Aesar and the Umicore catalyst, respectively. Noticeably, the synthesis method for the TiO2@IrOx core-shell catalyst shows a high reproducibility with the same OER activities for three different synthesis batches as shown in the linear sweep voltammograms (LSVs) in Fig. . It has been shown that the conductivity of the anodic catalyst layer is highly important in PEMWE, particularly the in-plane conductivity, which is needed to contact catalyst particles that are not in direct contact with the PTL. For this reason, the powder conductivities of the catalysts were measured and the results are presented in Fig. . The Alfa Aesar and Umicore commercial catalysts exhibit conductivities of 20.8 ± 3.5 S cm -1 and 50.0 ± 9.4 S cm -1 , respectively. The TiO2@IrOx core-shell catalyst exhibits a lower powder conductivity of 3.5 ± 0.5 S cm -1 , which is attributed to the lower iridium content and the incomplete oxidation of the core-shell catalyst. Again, the high crystallinity comes into play for the Umicore catalyst, resulting in a higher conductivity than that of the Alfa Aesar catalyst. The powder conductivity of ~ 50 S cm -1 for the Umicore catalyst is in good agreement with literature values. However, a higher conductivity (416 S cm -1 ) was also reported by the same group which is attributed to a higher powder compression force. Thus, we emphasize the importance of measuring reference samples during powder conductivity testing of novel catalysts. Although the conductivity of the novel core-shell catalyst is comparably low, the conductivity increases by a factor of three when the catalyst underwent the photodeposition step. This significant increase in conductivity can thus be directly attributed to the formation of the IrOx shell. We emphasize that the synthesis of a thin and homogeneous IrOx shell paves the way towards producing OER catalysts with low iridium contents while maintaining a sufficient conductivity.