Low temperature proton exchange membrane fuel cells (PEMFCs) powered by green hydrogen provide a means to sustainable energy conversion for sta�onary and transport applica�ons. Their widespread commercializa�on is par�ally limited by the cost of the pla�num (Pt)-based nanopar�cles supported on high surface area carbon (Pt/C) at the cathode, where oxygen reduc�on reac�on (ORR) occurs. Single iron (Fe), cobalt (Co), manganese (Mn) or �n (Sn) atoms (and their combina�ons) coordinated to nitrogen-doped carbon (M-N-C, where M is the metal) exhibit the most promising non-precious metal ac�vity for ORR. Of these, Fe-N-C has exhibited the greatest PEMFC performance. S�ll, ~60-100 µm Fe-N-C thick Fe-N-C cathodes are commonly used to compete with the PEMFC performance of ~5 µm Pt/C thick Pt-based cathodes, due mainly to Fe-N-Cs lower specific and volumetric ac�ve site density. With further improvements, Fe-N-C offer a poten�ally less expensive and less environmentally impac�ul alterna�ve to Pt/C, although highly ac�ve Fe-N-C typically suffer from lower durability. Researchers have improved the stability of Fe-N-C by improved synthesis pathways, producing atomically dispersed ac�ve sites, rather than encapsulated nanopar�cles, which induce instability. Most recently adding atomically thin protec�ve coa�ngs or reduc�ve pyrolysis condi�ons has led to Fe-N-C durability beyond 300 h in PEMFC under H 2 /Air. However, Fe-N-C durability is s�ll below commercial realiza�on for transport applica�ons (>5,000 h) owing to several degrada�on routes, which can be separated into two categories. Firstly, support modifica�on, such as oxida�on of the N-C matrix, and N-protona�on (especially for materials synthesized through pyrolysis under ammonia). Second is direct ac�ve metal atom modifica�on by agglomera�on/ aggrega�on, and demetalla�on/dissolu�on. The demetalla�on of the ac�ve site can also take place indirectly through chemical or electrochemical corrosion of the N-C matrix. Steps can be taken to deconvolute these degrada�on pathways and also minimize them, or even temporarily reverse them by reac�va�on. However, studies point towards the demetalla�on of FeN x ac�ve sites being the primary irreversible performance degrada�on mechanism in PEMFCs and the first step in the aggrega�on scenario. Induc�vely coupled plasma mass spectrometry (ICP-MS) is a highly sensi�ve technique which can provide �me-and poten�al-resolved Fe dissolu�on profiles from Fe-N-C catalysts. Monitoring Fe dissolu�on from ex situ ICP-MS, in tandem with other characteriza�on techniques, in rota�ng disc electrode (RDE) and PEMFC has revealed significant dissolu�on of Fe, although probing the mechanism requires operando measurements. In the first online flow cell ICP-MS study, Choi et al. suggested forma�on of insoluble ferric (Fe 3+ ) species, which dissolve under PEMFC opera�ng condi�ons (E cathode < 0.7 V RHE ) due to operando reduc�on to soluble ferrous (Fe 2+ ) ca�ons. This is in line with former ex situ ICP-MS findings of Zelenay and coworkers who suggested higher solubility of Fe species in acid solu�ons compared to Fe 3+ species. Previous online flow cell ICP-MS studies also provided cri�cal informa�on on the effects of pyrolysis atmosphere, bulk electrolyte pH, and catalyst modifica�on on the extent of Fe dissolu�on. Nonetheless, flow cell ICP-MS studies are limited to low current densi�es, and cannot reproduce all the prac�cal condi�ons occurring in an opera�ng PEMFC device (O 2 par�al pressure and current density, lower rela�ve humidity). In this respect, online gas diffusion electrode (GDE) ICP-MS is an adequate tool to simulate the environment of a PEMFC cathode more realis�cally, and gain PEMFCrelevant durability trends. For instance, Ehelebe et al. first demonstrated significantly lower dissolu�on of Pt/C catalysts in GDE configura�on compared to flow cell systems due to varying mass transport condi�ons of Pt species, 34 as previously proposed. Very recently, Choi and coworkers monitored in situ changes in ac�ve site density and operando Fe dissolu�on of a Fe-N-C under Ar and O 2 at different temperatures using GDE ICP-MS cell in acidic condi�ons. From site density monitoring, the reduced turnover frequency confirmed a reac�ve oxygen species catalyzed carbon corrosion scenario. However, despite using a GDE, Choi and coworkers current densi�es at 0.6 V chronoamperometric holds (<10 mA cm -2 geo ) were comparable to values achievable in flow cell (~1-2 mA cm -2 geo ), and not prac�cal PEMFCs. They observed from post-mortem transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDXS) elemental mapping that Fe deposited as Fe x O y nanopar�cles a�er O 2 reduc�on in their Fe-N-C derived from microporous zeoli�c imidazolate framework-8 (ZIF-8), confirming earlier findings from Kumar et al. Evidence of Fe x O y nanopar�cle forma�on in PEMFC-relevant condi�ons has previously been ascribed to highly ac�ve but unstable high-spin FeN 4 C 12 moie�es, via Mössbauer spectroscopy. Temperature is also a cri�cal parameter for durability of Fe-N-C catalysts. Goellner et al. first evidenced that the rate of corrosion of a N-C matrix (150 square wave cycling between 0.9-1.4 V RHE , 3 s holds in RDE) increases 14-fold when temperatures increase from 20 to 80 o C. This resulted in 18-fold larger O 2 reduc�on ac�vity decay (at 0.8 V RHE ), which was assigned to N-C corrosion. Carbon corrosion can be avoided at 25 o C in RDE by keeping poten�al < 0.9 V RHE , although some carbon corrosion (<7 mA cm - 2 geo ) is reported in PEMFC at 80 o C. Kumar et al. reported Fe cluster forma�on under load cycling (Arsaturated 0.1 M H 2 SO 4 , 0.6-1.0 V RHE ) at 80 o C, but did not observe Fe clusters at 60 o C, providing strong evidence of the effect of temperature on the fate of Fe species. Finally, we note that Osmieri et al. reported greater performance loss under air-fed vs. N 2 -fed PEMFC cathode (3 s holds at 0.95 and 0.6 V vs anode , 80 o C), although with no nanopar�cle forma�on. There are thus conflic�ng results in literature, which is could be due to opera�on condi�ons, (temperature, gas atmosphere, Nafion content, current densi�es, poten�al etc.), storage condi�ons, electrode prepara�on and synthesized Fe-N-C proper�es. Moreover, most of Fe-N-C catalysts studied by operando ICP-MS have consisted of low ac�ve site u�liza�on Fe-N-C derived from ZIF-8. Our laboratory, and others, have highlighted that such catalysts display a predominantly or purely microporous structure. This limits the mass transport and electrochemical ac�ve site u�liza�on (number of electrochemically accessible FeN x sites to the total number of FeN x sites) to typically <10%. This prompted us to revisit Fe dissolu�on and the fate of Fe in FeN x ac�ve sites from our recently developed high FeN x u�lisa�on (>50%) Fe-N-C with high micro-and meso-porosity. This pore structure can facilitate mass transport of reactants for improved ac�vity, while also enabling transport of dissolved Fe ions for operando ICP-MS detec�on.