Prior to electrochemical tes�ng, GDEs were immersed in ultrapure water for 1 hour. The electrolyte, reference and counter electrodes were 0.1 M HClO 4 (Suprapur, Sigma Aldrich), Ag/AgCl (inner and outer compartments filled with 3M KCl and 0.1 M HClO 4 , respec�vely, Metrohm) and Ti/Ir mixed oxide grid (METAKEM), respec�vely. Ag/AgCl/3M KCl was calibrated every day at the temperature of interest (E Ag/AgCl/Cl-= 0.316 ± 0.011 V RHE at 20 o C and E Ag/AgCl = 0.297 ± 0.013 V RHE at 75 °C). A gas humidifica�on system built with two gas washing botles (Duran) and a hea�ng plate (IKA TM RCT Basic Hot Plate S�rrer) was used to heat the purged gases to 75 °C. The GDE half-cell was heated to 74 ± 1 °C using an electrolyte recircula�on system via a hea�ng bath (AQUAline, LAUDA). In GDE, following the previously reported protocol, 64 100% post iR correc�on was applied for O 2 measurements, while for Ar measurements, 50% was applied in situ and 50% post Ar experiment. Details of GDE ICP-MS opera�on and protocol are detailed in the Supplementary Informa�on, Table and. The online Fe dissolu�on was measured with our previously reported GDE ICP-MS setup, shown in Figure .