Comparing TAP 900@Fe and TAP 900@ 57 Fe RDE ORR Ac�vity Thorough ex situ characteriza�on of TAP-derived materials was carried out in our previous work. Although some comparisons between TAP 900@ 57 Fe and TAP 900@Fe were missing. Considering O 2 reduc�on, reduced ac�vity has previously been reported for Fe enriched Fe-N-C samples compared to Fe-N-C prepared in the same manner but with natural abundance Fe precursor. The RDE O 2 reduc�on mass ac�vity for TAP 900@ 57 Fe and TAP 900@Fe can be found in Figure . The kine�c region and mass ac�vity at 0.8 V RHE, iR-free in O 2 -saturated RDE is lower in TAP 900@ Fe compared to previously reported TAP 900@Fe, with 3.77±0.54 and 5.01±0.79 A g FeNC -1 , respec�vely (Figure ). The lower ac�vity with Fe enrichment follows the previous report.