Our experiments in flow cell ICP-MS first confirm that the dissolu�on of Fe atoms is indeed the predominant degrada�on mechanism in this type of catalyst. In CV, two well-defined Fe dissolu�on peaks can be observed, with onset of 0.73 and 0.33 V RHE on the cathodic scan (Figure ). The two Fe concentra�on peaks could represent two different Fe species dissolving at different poten�als, or different dissolu�on process with different formal poten�als. Only one Fe concentra�on peak was resolved by Santori et al., with an onset of Fe concentra�on at ca. 0.75 V RHE for their Ar-pyrolysed Fe-N-C in O 2 -saturated 0.1 M H 2 SO 4 at 2 mV s -1 (data reproduced in Figure ). Meanwhile Choi et al. observed the onset of increased Fe concentra�on at 0.77 V RHE , with two dis�nguishable Fe dissolu�on peaks, as observed here. The poten�al at which peak Fe concentra�on occurs is not discussed as this depends on mass transport, which changes with the electrochemical cell design and opera�ng condi�ons.