TY - GEN A1 - Kramm, Ulrike Ingrid A1 - Lefèvre, Michel A1 - Larouche, Nicholas A1 - Schmeißer, Dieter A1 - Dodelet, Jean-Pol T1 - Correlations between Mass Activity and Physicochemical Properties of Fe/N/C Catalysts for the ORR in PEM Fuel Cell via 57 Fe Mössbauer Spectroscopy and Other Techniques T2 - Journal of the American Chemica Society N2 - The aim of this work is to clarify the origin of the enhanced PEM-FC performance of catalysts prepared by the procedures described in Science 2009, 324, 71 and Nat. Commun. 2011, 2, 416. Catalysts were characterized after a first heat treatment in argon at 1050 °C (Ar) and a second heat treatment in ammonia at 950 °C (Ar + NH3). For the NC catalysts a variation of the nitrogen precursor was also implemented. 57Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, neutron activation analysis, and N2 sorption measurements were used to characterize all catalysts. The results were correlated to the mass activity of these catalysts measured at 0.8 V in H2/O2 PEM-FC. It was found that all catalysts contain the same FeN4-like species already found in INRS Standard (Phys. Chem. Chem. Phys. 2012, 14, 11673). Among all FeN4-like species, only D1 sites, assigned to FeN4/C, and D3, assigned to N-FeN2+2 /C sites, were active for the oxygen reduction reaction (ORR). The difference between INRS Standard and the new catalysts is simply that there are many more D1 and D3 sites available in the new catalysts. All (Ar + NH3)-type catalysts have a much larger porosity than Ar-type catalysts, while the maximum number of their active sites is only slightly larger after a second heat treatment in NH3. The large difference in activity between the Ar-type catalysts and the Ar + NH3 ones stems from the availability of the sites to perform ORR, as many sites of the Ar-type catalysts are secluded in the material, while they are available at the surface of the Ar + NH3-type catalysts. KW - Fe-N-C catalyst KW - 57 Fe Mössbauer Spectroscopy KW - X-ray photo-electron spectroscopy KW - neutron activation analysis KW - nitrogen sorption measurements KW - oxygen reduction reaction (ORR) KW - PEM Fuel Cell Y1 - 2014 U6 - https://doi.org/10.1021/ja410076f SN - 0002-7863 SN - 1520-5126 VL - 136 IS - 3 SP - 978 EP - 985 ER - TY - GEN A1 - Kramm, Ulrike Ingrid A1 - Lefèvre, Michel A1 - Bogdanoff, Peter A1 - Schmeißer, Dieter A1 - Dodelet, Jean-Pol T1 - Analyzing Structural Changes of Fe-N-C Cathode Catalysts in PEM Fuel Cell by Mößbauer Spectroscopy of Complete Membrane-Electrode-Assemblies T2 - The Journal of Physical Chemistry Letters N2 - The applicability of analyzing by Mößbauer spectroscopy the structural changes of Fe–N–C catalysts that have been tested at the cathode of membrane electrode assemblies in proton exchange membrane (PEM) fuel cells is demonstrated. The Mößbauer characterization of powders of the same catalysts was recently described in our previous publication. A possible change of the iron species upon testing in fuel cell was investigated here by Mößbauer spectroscopy, energy-dispersive X-ray cross-sectional imaging, and neutron activation analysis. Our results show that the absorption probability of γ rays by the iron nuclei in Fe–N–C is strongly affected by the presence of Nafion and water content. A detailed investigation of the effect of an oxidizing treatment (1.2 V) of the non-noble cathode in PEM fuel cell indicates that the observed activity decay is mainly attributable to carbon oxidation causing a leaching of active iron sites hosted in the carbon matrix. KW - iron-based catalyst KW - iron−nitrogen−carbon catalyst KW - non-noble metal catalyst KW - nonprecious metal catalyst KW - oxygen reduction reaction KW - ORR KW - polymer electrolyte membrane fuel cell KW - PEM-FC KW - degradation Y1 - 2014 U6 - https://doi.org/10.1021/jz501955g SN - 1948-7185 VL - 2014 IS - 5 SP - 3750 EP - 3756 ER - TY - GEN A1 - Szakacs, Csaba E. A1 - Lefèvre, Michel A1 - Kramm, Ulrike Ingrid A1 - Dodelet, Jean-Pol A1 - Vidal, François T1 - A density functional theory study of catalytic sites for oxygen reduction in Fe/N/C catalysts used in H2/O2 fuel cells T2 - Physical Chemistry Chemical Physics N2 - The oxygen reduction catalytic activity of carbon-supported FeN4 moieties bridging micropores between two graphene sheets was investigated by density functional theory (DFT). Based on the FeN2+2/C structure proposed earlier by our group, two types of FeN2+2/C structures were considered: one mostly planar and one in which the Fe ion is significantly displaced out of the graphitic plane. A structure in which the FeN4 moiety is embedded in an extended graphene sheet (FeNpyri4/C) was also considered. In addition, we have investigated the influence of an axial pyridine group approaching the Fe centre. The formation energy is lowest for the planar FeN2+2/C structure. The overall downhill behaviour of the relative free energy vs. the reaction step suggests that most structures have catalytic activity near zero potential. This conclusion is further supported by calculations of the binding energies of adsorbed O2 and H2O and of the O–O bond lengths of adsorbed O2 and OOH. The side-on interaction of adsorbed O2 is preferred over the end-on interaction for the three basic structures without the axial pyridine. The pyridine coordination produces a stronger binding of O2 for the planar FeN2+2/C and the FeNpyri4/C structures as well as a dominant end-on interaction of O2. The energy levels of the planar FeN2+2/C structure with and without the pyridine ligand are nearly equal for iron spin states S = 1 and S = 2, suggesting that both configurations are formed with similar concentration during the preparation process, as also previously found for two of the iron sites by Mössbauer spectroscopy experiments. KW - density functional theory (DFT) KW - iron-based catalyst KW - iron−nitrogen−carbon catalyst KW - non-noble metal catalyst KW - oxygen reduction reaction (ORR) KW - polymer electrolyte membrane fuel cell (PEM-FC) Y1 - 2014 U6 - https://doi.org/10.1039/C3CP55331K SN - 1463-9076 SN - 1463-9084 VL - 2014 IS - 16 SP - 13654 EP - 13661 ER -