Non-PGM Electrocatalysts for PEM Fuel Cells: A DFT Study on the Effects of Fluorination of FeNx-Doped and N-Doped Carbon Catalysts

Abstract

Fluorination is considered as a means of reducing the degradation of Fe/N/C, a highly active FeNx-doped disorganized carbon catalyst for the oxygen reduction reaction (ORR) in PEM fuel cells. Our recent experiments have, however, revealed that fluorination poisons the FeNx moiety of the Fe/N/C catalytic site, considerably reducing the activity of the resulting catalyst to that of carbon only doped with nitrogen. Using the density functional theory (DFT), we clarify in this work the mechanisms by which fluorine interacts with the catalyst. We studied 10 possible FeNx site configurations as well as 2 metal-free sites in the absence or presence of fluorine molecules and atoms. When the FeNx moiety is located on a single graphene layer accessible on both sides, we found that fluorine binds strongly to Fe but that two F atoms, one on each side of the FeNx plane, are necessary to completely inhibit the catalytic activity of the FeNx sites. When considering the more realistic model of a stack of graphene layers, only one F atom is needed to poison the FeNx moiety on the top layer since ORR hardly takes place between carbon layers. We also found that metal-free catalytic N-sites are immune to poisoning by fluorination, in accordance with our experiments. Finally, we explain how most of the catalytic activity can be recovered by heating to 900 °C after fluorination. This research helps to clarify the role of metallic sites compared to non-metallic ones upon the fluorination of FeNx-doped disorganized carbon catalysts.

Energetics and thermodynamic stability of potential Fe(II)‐hexa‐aza‐active sites for O2 reduction in PEM fuel cells

“…The FeN (2+2) /C site has been the subject of many DFT studies. So were also other sites of the same nature, but with fewer pyridinic nitrogen atoms in close vicinity with the Fe ion that renders these sites less stable, yet able to produce ORR 75–77 . A particularity of the FeN (2+2) /C site is that it is easily embedded in a graphene layer (see Figure 6D).…”

“…So were also other sites of the same nature, but with fewer pyridinic nitrogen atoms in close vicinity with the Fe ion that renders these sites less stable, yet able to produce ORR. [75][76][77] A particularity of the FeN (2+2) /C site is that it is easily embedded in a graphene layer (see Figure 6D). To form such a structure, one only has to complete the graphene structure which is missing in the micropore.…”

Fluorination and its Effects on Electrocatalysts for Low‐Temperature Fuel Cells

“…The INRS team just pioneered both DFT and thermodynamic calculations on the Fe-based active sites and their fluorinated ones, providing interesting results to understand the instability of the catalyst. [177,178] As mentioned in early sections, some researchers have adopted fluorination strategies to reduce the degradation of the catalysts for ORR in PEM fuel cells. However, the experiments by Zhang and co-workers have revealed that fluorination can poison the FeN x active sites in the catalyst.…”

“…Reproduced according to the terms of the CC BY license. [ 177 ] Copyright 2021, The authors, published by MDPI.…”

“…The tables show the adsorption of F atoms on these structures. Reproduced according to the terms of the CC BY license [177]. Copyright 2021, The authors, published by MDPI.…”