A�er the ini�al 50 CVs at 50 mV s -1 , six CVs were conducted at 10 mV s -1 under Ar and then O 2 -satura�on. With increasing TAP 900@ 57 Fe loading under O 2 -satura�on, the limi�ng current density (below 0.65 V RHE ) only incrementally increases. This slight increase can be explained by the increasing thickness of the catalyst layer with loading, which penetrates deeper into the flowing O 2 -saturated electrolyte. Meanwhile, between 0.65-0.80 V RHE there is an increasing O 2 reduc�on peak in the cathodic direc�on (Figure ). This is caused by a build-up of O 2 concentra�on locally in the catalyst layer while scanning the poten�al region of 0.800-0.925 V RHE , where very litle ORR is observed.