The online GDE ICP-MS concentra�on profile under 20 o C O 2 (Figure and) suggests Fe dissolu�on and subsequent detec�on by ICP-MS at low current density (-0.05 to -1.00 mA cm -2 geo ). Meanwhile, at higher current density (-15, -50 and -100 mA cm -2 geo ), a process of Fe dissolu�on and redeposi�on locally into Fe x O y in the catalyst layer is proposed. This is supported by the increased observa�on of Fe x O y a�er O 2 GDE protocol from HAADF-STEM and EDXS (Figure and Figure ). The reason for Fe x O y forma�on is hypothesized to arise based on the Fe Pourbaix diagram, where an increase in the local pH would form Fe 2 O 3 . This pH increase in the catalyst layer could occur due to the rapid consump�on of H + during increased O 2 reduc�on currents (4H + + O 2 + 4e -2H 2 O). It is then expected that some Fe x O y redissolves when returning anodically to low O 2 reduc�on current density (-1 mA cm - 2 geo ), due to a return to acidic pH. This redissolu�on is evidenced by the detected increase in Fe concentra�on at -1 mA cm -2 geo on the anodic step for 20 o C O 2 in GDE ICP-MS. The observa�on of Fe x O y corroborates previous findings from post-mortem O 2 AST protocols. Moreover, the increased Fe concentra�on detected when stepping the poten�al down in the cathodic direc�on a�er post AST (Figure ) for O 2 GDE at 20 o C and 75 o C supports the hypothesis that Fe x O y builds up in the catalyst layer at current densi�es of -50 mA cm geo during the AST and is only released at lower current density holds (-1 mA cm geo at 20 o C and 75 o C).