F2-Fluorination of Fe/N/C catalysts poisons FeN4, but not CNx sites. Main causes of instability in PEMFCs are either FeN4 demetalation for Fe/N/C or H2O2 when FeN4 sites are poisoned by fluorination or absent as in MOF_CNx_Ar + NH3.

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Nitrogen and phosphorus-codoped graphene dots supported on nitrogen-doped three-dimensional graphene (N, P-GDs/N-3DG) have been synthesized by a facile freeze-annealing process. On the surface of the 3D interconnected porous structure, the N, P-GDs are uniformly dispersed. The as-prepared N, P-GDs/ N-3DG material served as a metal-free catalyst for oxygen reduction reaction (ORR) in an alkaline medium and evaluated by a rotating ring-disk electrode. The N, P-GDs/N-3DG catalyst exhibits excellent ORR activity, which is comparable to that of the commercial Pt/C catalyst. Furthermore, it exhibits a higher tolerance to methanol and better stability than the Pt/C. This enhanced electrochemical catalytic performance can be ascribed to the presence of abundant functional groups and edge defects. This study indicates that P−N bonded structures play a vital role as the active sites in ORR.

Two types of FeN 4 -based catalytic sites have been proposed in the literature to perform the oxygen reduction reaction (ORR) in PEM fuel cells running with a Fe-based catalyst. They are FeN 4 /C, derived from the molecular structure of iron porphyrin, and FeN (2+2) /C, derived from the structure of the iron complex with two 1,10-phenanthroline molecules. The energetics and thermodynamic stability (equilibrium constant, K c , for iron acid leaching at pH ∼ 0) of these two types of sites were determined and were already reported in this journal [J. Electrochem. Soc., 164(9) F948-F957 ( 2017)]. This work deals with the same FeN 4 sites but this time after their fluorination on the central Fe atom and on their carbon support. All evaluated fluorinated FeN 4 -based sites are stable at both 298 and 353 K, especially F-FeN 4 /C(F) and F 2 -FeN (2+2) /C(F). Interatomic d Fe−N , d Fe−F , and bond energies E Fe−F derived in the frame of the thermodynamic calculations were compared with the values obtained by density functional theory (DFT) calculations, and an agreement was found. DFT was also used to assess the ORR capability of both F-FeN 4 /C and F-FeN (2+2) /C. Both fluorinated sites were found to be ORR active when O 2 was bonded to the free side of the Fe atom, opposite to the F atom.

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