Under Ar-saturated condi�ons the current density increases propor�onal to the catalyst loading; we note there is an increasing peak on the cathodic scan (Figure ). We atribute this cathodic peak to the reduc�on of trace O 2 , arising from air ingress at the junc�on of the Kalrez O-ring and cell (or cavita�on from the peristal�c pump). S�ll, the amount of O 2 appears negligible. Normalizing the Fe detected to charge passed and catalyst loading shows the amount of Fe detected is constant under O 2 but increases with reduced catalyst loading under Ar (Figure ). Meanwhile, the amount of Fe detected is equivalent under either gas satura�on, with 1.3-2.0% of total Fe detected, and linear dependence with Fe-N-C loading (Figure ). Focusing on the dissolu�on at 0.2 mg FeNC cm geo , similar profiles are observed under Ar and O 2 -satura�on (Figure ).