Non-PGM Electrocatalysts for PEM Fuel Cells: Thermodynamic Stability and DFT Evaluation of Fluorinated FeN4-Based ORR Catalysts

Abstract

Two types of FeN4-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 FeN4/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, Kc, 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 FeN4 sites but this time after their fluorination on the central Fe atom and on their carbon support. All evaluated fluorinated FeN4-based sites are stable at both 298 and 353 K, especially F-FeN4/C(F) and F2-FeN(2+2)/C(F). Interatomic dFe − N, dFe − F, and bond energies EFe − 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-FeN4/C and F-FeN(2+2)/C. Both fluorinated sites were found to be ORR active when O2 was bonded to the free side of the Fe atom, opposite to the F atom.