@misc{KrammLefevreLaroucheetal., author = {Kramm, Ulrike Ingrid and Lef{\`e}vre, Michel and Larouche, Nicholas and Schmeißer, Dieter and Dodelet, Jean-Pol}, title = {Correlations between Mass Activity and Physicochemical Properties of Fe/N/C Catalysts for the ORR in PEM Fuel Cell via 57 Fe M{\"o}ssbauer Spectroscopy and Other Techniques}, series = {Journal of the American Chemica Society}, volume = {136}, journal = {Journal of the American Chemica Society}, number = {3}, issn = {0002-7863}, doi = {10.1021/ja410076f}, pages = {978 -- 985}, abstract = {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 M{\"o}ssbauer 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.}, language = {en} } @misc{KrammLefevreBogdanoffetal., author = {Kramm, Ulrike Ingrid and Lef{\`e}vre, Michel and Bogdanoff, Peter and Schmeißer, Dieter and Dodelet, Jean-Pol}, title = {Analyzing Structural Changes of Fe-N-C Cathode Catalysts in PEM Fuel Cell by M{\"o}ßbauer Spectroscopy of Complete Membrane-Electrode-Assemblies}, series = {The Journal of Physical Chemistry Letters}, volume = {2014}, journal = {The Journal of Physical Chemistry Letters}, number = {5}, issn = {1948-7185}, doi = {10.1021/jz501955g}, pages = {3750 -- 3756}, abstract = {The applicability of analyzing by M{\"o}ßbauer 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 M{\"o}ßbauer 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 M{\"o}ßbauer 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.}, language = {en} } @misc{SzakacsLefevreKrammetal., author = {Szakacs, Csaba E. and Lef{\`e}vre, Michel and Kramm, Ulrike Ingrid and Dodelet, Jean-Pol and Vidal, Fran{\c{c}}ois}, title = {A density functional theory study of catalytic sites for oxygen reduction in Fe/N/C catalysts used in H2/O2 fuel cells}, series = {Physical Chemistry Chemical Physics}, volume = {2014}, journal = {Physical Chemistry Chemical Physics}, number = {16}, issn = {1463-9076}, doi = {10.1039/C3CP55331K}, pages = {13654 -- 13661}, abstract = {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 M{\"o}ssbauer spectroscopy experiments.}, language = {en} }