Moving to operando flow cell ICP-MS measurements in 0.1 M HClO 4 , TAP 900@ 57 Fe was used to avoid polyatomic interference from ArO + and maximize spectrometric signal. The setup and experimental protocol are depicted in Figure and Figure , respec�vely. First, ICP-MS calibra�on, electrochemical impedance spectroscopy and open circuit poten�al (OCP) measurements were conducted to ensure correct installa�on and opera�on. Next, 50 fast (50 mV s -1 ) cyclic voltammograms (CV) between 0.925-0.200 V RHE were measured in Ar-saturated electrolyte to allow the catalyst to reach a stable electrochemical and dissolu�on measurement (Figure , 0.2 mg FeNC cm -2 geo ). Mg was also monitored during the ini�al 50 cycles due to its use as a templa�ng agent during synthesis, with 0.06 wt.% detected from ex situ ICP-MS in our previous work. Mg dissolu�on did not vary with poten�al (Figure ) and so is not considered further. Meanwhile, the rate of Fe dissolu�on followed an exponen�al decay.