While flow cell measurements proved insigh�ul, the degrada�on rate in real PEMFCs may be different due to higher ORR rates and therefore ORR charge passed. To reach higher current densi�es and condi�ons comparable to PEMFCs, TAP 900@Fe was tested in a GDE half-cell coupled to online ICP-MS in 0. geo (Figure ). This is assigned to improved we�ng of TAP 900@Fe during the 20 o C Ar AST. Meanwhile, 20 o C O 2 AST led to no�ceable performance degrada�on a�er only 200 cycles, with poten�al shi� at -50 mA cm geo of -50±30 mV (from 0.61±0.03 to 0.56±0.00 V RHE, iR-free ) compared to pris�ne 20 o C TAP 900@Fe (Figure ). At -50 mA cm geo , 75 o C GDE pris�ne TAP 900@Fe shows an ), with slight reduc�ons in C=O and C-O peaks for AST samples (Figure ). Meanwhile, a�er 75 o C O 2 protocol, a clear overall O1s increase is found, equivalent to 12.2 at.% O1s (Figure ). There is less discernible change in the C1s spectra, aside from reduc�on in C-N and C-C and increase in CF2 in all AST samples compared to the pris�ne TAP 900@Fe GDE (Figure ). Raman spectra (Figure Within the pris�ne TAP 900@Fe GDE no visible nanopar�cles are detected using HAADF-STEM and STEM-EDXS spectrum imaging (Figure , Figure ). A�er 20 o C Ar protocol, one large Fe x O y nanopar�cle is detected in the spectrum image, while, at higher magnifica�on, small clusters are observed (Figure , Figure ). Numerous Fe nanopar�cles are observed following 20 o C O 2 protocol in GDE, which are assigned to Fe x O y based on overlaying the Fe and O EDXS mapping (Figure , Figure ). HAADF-STEM combined with EDXS and EELS reveals clusters containing Ca and Fe in fresh and post Ar and O 2 AST GDE (Figure ). The presence of Ca remains unexplained, as we consistently used MQ water for all our electrochemical experiments and rinsing steps. No trace of Ca was also detected in the na�ve catalyst. We therefore atribute it to contamina�on by tap water. The peak at 695 eV is from Fe-K. STEM-EELS analysis in regions without Fe par�cles cannot resolve any Fe peak (Figure ), likely owing to the concentra�on of FeN x sites being below the limit of detec�on.