@misc{MuellerRichterFriedrichetal., author = {M{\"u}ller, Klaus and Richter, Matthias and Friedrich, Daniel and Paloumpa, Ioanna and Kramm, Ulrike Ingrid and Schmeißer, Dieter}, title = {Spectroscopic characterization of Cobalt-Phthalocyanine electrocatalysts for fuel cell applications}, series = {Solid State Ionics}, journal = {Solid State Ionics}, number = {216}, issn = {1872-7689}, doi = {10.1016/j.ssi.2011.12.013}, pages = {78 -- 82}, abstract = {For polymer-electrolyte-membrane fuel cells (PEM-FC), platinumcatalysts on carbon based substrates are state of the art, due to high electrochemical activity and chemical stability in acid electrolytes. High costs of platinum force investigations of alternative catalysts. With respect to the oxygen reduction reaction, possible candidates are transition metal (TM) complexes like TM-porphyrines or TM-phthalocyanines. Pyrolysis on carbon based substrates improves the catalytic activity. Highest activities were found for Fe or Co as transition metal centers. We present results of the pyrolysis of Co-phthalocyanine (CoPc) on carbon black. Besides an investigation of morphology by scanning electron microscopy (SEM) and chemical composition by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) during pyrolysis, we present a near edge X-ray absorption fine structure (NEXAFS) study of the chemical composition for the related annealing steps. This investigation is accompanied by an electrochemical characterization of the catalytic activity and selectivity. The XPS and NEXAFS data indicate a decomposition of the CoN4(chelate-) complex during pyrolysis. The N1s data for 800 °C show graphite-like, formally pyrrolic and pyridinic like features, without any significant contri-bution of Co. The Co2p data for both, XPS and NEXAFS are dominated by CoO/Co2O3. It can be concluded that during the pyrolysis cobalt particles are formed by the disintegration of CoN4-centers, after pyrolysis these par-ticles become oxidized upon contact with air.}, language = {en} } @misc{KrammHerrmannGeppertFiechteretal., author = {Kramm, Ulrike Ingrid and Herrmann-Geppert, Iris and Fiechter, Sebastian and Zehl, Gerald and Zizak, Ivo and Dorbandt, Iris and Schmeißer, Dieter and Bogdanoff, Peter}, title = {Effect of iron carbide formation on the number of active sites in Fe-N-C catalysts for the oxygen reduction reaction in acidic media}, series = {Journals of materials chemistry A}, journal = {Journals of materials chemistry A}, number = {8}, issn = {2050-7488}, doi = {10.1039/c3ta13821f}, pages = {2663 -- 2670}, language = {en} } @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{KrammZanaVoschetal., author = {Kramm, Ulrike Ingrid and Zana, Alessandro and Vosch, Tom and Fiechter, Sebastian and Arenz, Matthias and Schmeißer, Dieter}, title = {On the structural composition and stability of Fe-N-C catalysts prepared by an intermediate acid leaching}, series = {Journal of Solid State Electrochemistry}, volume = {20}, journal = {Journal of Solid State Electrochemistry}, number = {4}, issn = {1432-8488}, doi = {10.1007/s10008-015-3060-z}, pages = {969 -- 981}, abstract = {The development of highly active and stable non-noble metal catalysts (NNMC) for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEM-FC) becomes of importance in order to enable cost reduction. In this work, we discuss the structural composition as derived from Fe-57M{\"o}ßbauer spectroscopy and X-ray diffraction, catalytic performance determined by a rotating (ring)disk electrode (RRDE) technique and stability evaluation of our Fe-N-C catalysts prepared by an intermediate acid leaching (IAL). The advantage of this IAL is given by a high density of active sites within the catalyst, as even without sulphur addition, an iron carbide formation and related disintegration of active sites are inhibited. In addition, our accelerated stress tests illustrate better stability of the sulphur-free IAL catalyst in comparison to the sulphur-added one.}, language = {en} }