Michele Piana

In this study, ex situ and operando X‐ray absorption spectroscopy (XAS) is employed to shed light on structure and degradation mechanism of Fe‐based catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Ex situ XAS on pristine Fe 0.07 Zr 0.93 O 2‐δ /C catalyst confirms the incorporation of Fe 3+ in the ZrO 2 structure and clearly exclude any significant presence of Fe−N−C‐type structures. The edge shift in data on in‐house aged samples demonstrates a mixed oxidation state of Fe (Fe 3+ and Fe 2+ ), consistent with Fe demetalation from the ZrO 2 structure. Furthermore, a more symmetric coordination in the pre‐edge shape points towards the formation of oxidic Fe clusters upon aging. Fe demetalation is inferred also from the edge shift to higher energy (presence of Fe 3+ ) in operando XAS data at 0.3 V, due to Fe phases not electrically polarizable/reducible at the applied voltage. Electrochemical data exclude any correlation between extent of aging and type of test, also for a commercial Fe−N−C catalyst by Pajarito Powder. The observed faster aging for Fe 0.07 Zr 0.93 O 2‐δ compared to Fe−N−C is attributed to an improved mass transport to/from active sites, manifest also in very similar initial current densities at 0.3 V, despite much higher catalyst activity for Fe−N−C.