All cell tests with CCMs were performed on a commercial test bench (600 Electrolyzer Test System, Scribner LLC) equipped with a potentiostat with a current booster (BioLogic VSP-300). An in-house designed cell fixture adapted from previous reports with 5 cm 2 active area was used. Two sets of CCMs were manufactured for both catalysts. One set with high loading anodes of 1.33 ± 0.20 mgIr cm -2 with the core-shell TiO2@IrOx and the reference IrO2/TiO2 catalyst (Umicore Elyst Ir75), respectively. For the low loading set, anodes with 0.22 ± 0.01 mgIr cm -2 and 0.30 ± 0.03 mgIr cm -2 for core-shell and Umicore catalyst were fabricated, respectively. For all CCMs the cathode loading was kept to 0.19 ± 0.08 mgPt cm -2 . The respective CCM was assembled with a platinum-coated porous transport layer (PTL) consisting of titanium (2GDL10-0,25 from NV Bekaert SA, Belgium) on the anode and a gas diffusion layer (GDL) with microporous layer (MPL) consisting of carbon (H24C5 Freudenberg & Co. KG) on the cathode. The PTL was platinum coated on both sides via sputter deposition (Cressington sputter coater 108) with an average loading of 0.035 ± 0.025 mgPt cm -2 . Gasket thicknesses were chosen so that a compression of the carbon GDL of 23.7 ± 2.2% was achieved. The anode was flushed with 100 mL min -1 DI water which was additionally purged with nitrogen for 10 min prior to testing to remove any dissolved CO2 from refilling of the tank. The anode feed was heated to 80 °C and the cathode was kept dry and only the venting line behind the cell was purged with nitrogen for safety reasons. The cell was heated to 80 °C with cartridge heaters placed into both endplates and regulated by a thermocouple placed into the cathodic flow field plate. Temperatures were kept constant throughout the whole testing time. When cell temperature and anode feed reached 80 °C, the temperature was held for 30 min before starting the electrochemical testing. The test starts with a test for electrical short at 1 V for 1 min and a conditioning step of 30 min at 1 A cm -2 . In the following, three subsequent polarization curves were recorded. The polarization curves are measured between 0.01 and 4 A cm -2 , holding each current density step for 5 min to ensure steady-state conditions, and averaging the last 15 s for analysis. Galvanostatic electrochemical impedance spectroscopy (EIS) was performed from 100 kHz to 500 mHz at each point of the polarization curve after the respective current density hold. The perturbation of each AC current was chosen ≤ 10% of the AC current but not smaller than 20 mA so that a good signal-to-noise ratio was achieved, and a linear system response was ensured. Since the first two polarization curves were considered part of conditioning, overlap of the last two polarization curves was checked, and only the last polarization curve was taken for analysis. All tests were repeated three times with pristine CCMs for reproducibility.