Structure of the catalytic sites in Fe/N/C-catalysts for O-2-reduction in PEM fuel cells

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by 57Fe Mössbauer spectroscopy of Fe/N/C catalysts containing 0.03 to 1.55 wt% Fe, which were prepared by impregnation of iron acetate on carbon black followed by heat-treatment in NH3 at 950 °C. Four different Fe-species were detected at all iron concentrations: three doublets assigned to molecular FeN4-like sites with their ferrous ions in a low (D1), intermediate (D2) or high (D3) spin state, and two other doublets assigned to a single Fe-species (D4 and D5) consisting of surface oxidized nitride nanoparticles (FexN, with x ≤ 2.1). A fifth Fe-species appears only in those catalysts with Fe-contents ≥0.27 wt%. It is characterized by a very broad singlet, which has been assigned to incomplete FeN4-like sites that quickly dissolve in contact with an acid. Among the five Fe-species identified in these catalysts, only D1 and D3 display catalytic activity for the oxygen reduction reaction (ORR) in the acid medium, with D3 featuring a composite structure with a protonated neighbour basic nitrogen and being by far the most active species, with an estimated turn over frequency for the ORR of 11.4 e− per site per s at 0.8 V vs. RHE. Moreover, all D1 sites and between 1/2 and 2/3 of the D3 sites are acid-resistant. A scheme for the mechanism of site formation upon heat-treatment is also proposed. This identification of the ORR-active sites in these catalysts is of crucial importance to design strategies to improve the catalytic activity and stability of these materials

Nature of the catalytic centers of porphyrin based electrocatalysts for the ORR A correlation of kinetic current density with the site density of Fe N4 centers

In this work it has been shown that structural changes of an as prepared catalyst enable the assignment of the catalytic centers responsible for the direct and indirect oxygen reduction reaction, respectively, of porphyrin based electrocatalysts. An iron porphyrin FeTMPPCl based catalyst as well as a catalyst based on H2TMPP were prepared using the so called Foaming Agent Technique FAT . The obtained iron catalyst was used as generic material for the post treatments. Structural changes were analyzed by 57Fe Mö bauer spectroscopy. The catalytic activity towards the oxygen reduction reaction ORR was determined using Rotating Ring Disc Electrode R R DE experiments. The catalysts exhibit a variation in the iron content between 2.9 and 4.5 wt caused by the post treatments. It has been found that the Mö bauer spectra of all catalysts can be fitted assuming two different ferrous Fe N4 centers, a CFeN2 center Fe2 , S 2 , and an Fe3C center Fe0 . After normalizing the intensities found in the Mö bauer spectra relative to the iron content, a linear correlation between the kinetic current density related to the direct oxygen reduction and the amount of in plane Fe N4 centers is found. Beside this, there are evidences for a correlation between the kinetic current density related to the hydrogen peroxide formation and CFeN2 centers. Heat treated carbon supported iron porphyrin, prepared as reference material, exhibits the same behavior as our FAT catalysts. The correlation enables us to obtain the turn over frequencies for both, the direct and indirect oxygen reduction reaction and to determine the site densities, in which we reach a third of the target value published by Gasteiger et al.

New insight into the nature of catalytic activity of pyrolysed iron porphyrin based electro catalysts for the oxygen reduction reaction ORR in acidic media

In this contribution, catalysts, prepared by an impregnation technique have been characterized structurally and chemically via 57Fe Mö bauer spectroscopy, X ray photoelectron spectroscopy XPS and by elemental analysis, respectively. On the basis of the structural characterization it was concluded that those FeN4 centres in which iron is mesomericly bonded to four nitrogen atoms, are catalyzing the ORR. Furthermore, XPS as well as Mö bauer spectroscopy revealed that with increasing pyrolysis temperature the electron density at iron atoms was increased while that of the coordinating nitrogen atoms was reduced. This behaviour was connected with a decreased electric field gradient EFG as estimated by quadrupole splitting. The smaller EFG is interpreted in terms of a larger iron nitrogen bond distance and or a slight movement of iron out of plane. We propose that this characteristic of the centres is one parameter that enables the higher turn over frequency during ORR of pyrolysed porphyrin

Influence of the electron density of FeN4 centers towards the catalytic activity of pyrolyzed FeTMPPCl based ORR electrocatalysts

FeTMPPCl impregnated on a carbon black was heat treated to different temperatures. The obtained catalysts were characterized before and after acid leaching by structural and chemical analyses. On the basis of the structural characterization it was concluded that those FeN4 centres in which iron is mesomerically bonded to four nitrogen atoms, are catalyzing the oxygen reduction reaction ORR . XPS as well as Mö bauer spectroscopy revealed that higher pyrolysis temperatures cause a partial shift of electron density from the coordinating nitrogen atoms to the iron atom of the active FeN4 centre. Moreover, in accordance to these higher kinetic current densities towards the oxygen reduction were observed. The above results suggest a relationship between the electron density of the FeN4 centres and the catalytic activity, where an increase in electron density on the iron centres enable an improvement in the turn over frequency during ORR, thus, compensating the lower concentration of active sites. This finding makes it most likely that on heat treated porphyrin based Fe N C catalysts the oxygen molecules coordinate to these iron centres during the OR

On the Influence of Sulphur on the Pyrolysis Process of FeTMPP Cl based Electro Catalysts with Respect to Oxygen Reduction Reaction ORR in Acidic Media

Pyrolysis of chloroiron tetramethoxyphenyl porphyrin FeTMPP Cl in the presence of iron oxalate sulphur leads to the formation of higly porous and active catalysts for the oxygen reduction reaction ORR . In order to clarify the influence of sulphur the pyrolysis process is analyzed by thermogravimetry TG and by high temperature X ray diffraction HT XRD . In the absence of sulphur iron carbide FexC is formed which catalyses the proceeding graphitisation of the pyrolysis products. As a result catalytic active centres are decomposed by this reaction. This can be avoided by the addition of sulphur because iron monosulphide FeS; troilite is formed instead of FexC. Furthermore, FeS can easily be removed in a successive etching step so that nearly all inactive by products can be removed. The results are in accordance with the higher electrochemical performance of the sulphur containing catalyst