Ex situ/Operando X-Ray Absorption Spectroscopy on Fe_0.07Zr_0.93O_2-δ/C vs. Fe−N−C as Pt-Group-Metal-Free Oxygen Reduction Reaction Catalysts in Proton Exchange Membrane Fuel Cells

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_0.07Zr_0.93O_2-δ/C catalyst confirms the incorporation of Fe³⁺ in the ZrO₂ 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 (Fe³⁺ and Fe²⁺), consistent with Fe demetalation from the ZrO₂ 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 Fe³⁺) 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_0.07Zr_0.93O_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.