Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeN_x Moieties in Fe-N-C Catalysts

Abstract

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Identification of Durable and Non-Durable FeNx Sites in Fe-N-C Materials for Proton Exchange Membrane Fuel Cells

“…28 Exacerbated demetallation was recently reported in acid for highly-active NH3-pyrolyzed Fe-N-C catalysts, explaining their poor durability in PEMFC. 22,29 In contrast, high stability in acidic medium was reported by us for two catalysts exclusively comprising FeNx moieties, with only 25 % activity decrease after 30,000 loadcycles in Ar-saturated 0.1 M H2SO4 at 80°C. 21 However, after 10,000 load-cycles in the same conditions but in O2-saturated electrolyte, the decrease in activity and number of FeNx moieties was 65 and 83 %, respectively, forming iron oxide particles during cycling.…”

Identification of durable and non-durable FeNx sites in Fe–N–C materials for proton exchange membrane fuel cells

“…18,22,36 NH3 pyrolysis introduces highly-basic nitrogen groups, increasing the turnover frequency (TOF) of FeNx sites but leading to decreased durability in PEMFC. 15,17 This was recently explained by higher demetallation rates for NH3-vs. Ar-treated Fe-N-C catalysts in acid medium, 17,29 while demetallation rates were equally low in alkaline medium. 29 Due to its high stability in acid, 21 Fe0.5 was selected for in situ 57 Fe Mössbauer spectroscopy.…”

Stable, Active, and Methanol-Tolerant PGM-Free Surfaces in an Acidic Medium: Electron Tunneling at Play in Pt/FeNC Hybrid Catalysts for Direct Methanol Fuel Cell Cathodes

“…2 Recent reports indicate that high initial ORR activity for FeNC materials corroborates with high microporous surface area and also with high Fe leaching rates in oxygenated acidic medium. [39][40][41] In particular, the simultaneous combination of low electrochemical potential and presence of oxygen seems to trigger Fe demetallation from FeN x sites, little or not observed in the absence of O 2 . [41][42][43] Such demetallation and degradation is likely related to the in situ production of H 2 O 2 and reactive oxygen species via Fenton reactions, catalyzed by Fe cations and other 3d transition metal cations.…”