Under ini�al Ar in GDE ICP-MS (Figure ), the most significant increase in Fe concentra�on occurs when the poten�al drops from 0.83 to 0.74 V RHE . This can be explained by the Fe 3+ /Fe 2+ redox transi�on at 0.76 V RHE (Figure ). It is worth no�ng that with a Fe-N-C, Fe atoms possess different formal redox and dissolu�on poten�als depending on their coordina�ng ligands and extended local environment (number and size of graphene sheets, oxygen func�onal groups ). This broad Fe 3+ /Fe 2+ redox range is also ini�ally observed in Figure . Moreover, a�er the increases of Fe concentra�on during cathodic poten�al shi�s, gradual declines in the Fe concentra�on are frequently observed. This is related to the fact that the loca�on of the Fe within the Fe-N-C structure (outer catalyst layer surface or deeper within) affects the transfer func�on and hence residence �me.