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 ion in low (D1), medium (D2) or high spin state (D3), 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− site−1 s−1 at 0.8V vs RHE. Moreover, all D1 sites and between 1/2 to 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.

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 porphyrinbased electrocatalysts. An iron porphyrin (FeTMPPCl)-based catalyst as well as a catalyst based on H 2 TMPP were prepared using the so-called foaming agent technique (FAT). The obtained iron catalyst was used as a generic material for the post-treatments. Structural changes were analyzed by 57 Fe Mo ¨ssbauer spectroscopy. The catalytic activity toward 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 Mo ¨ssbauer spectra of all catalysts can be fitted assuming two different ferrous Fe-N 4 centers, a CFeN 2 center (Fe 2+ , S ) 2) and an Fe 3 C center (Fe 0 ). After the intensities found in the Mo ¨ssbauer spectra were normalized 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-N 4 centers is found. Beside this, there is evidence for a correlation between the kinetic current density related to the hydrogen peroxide formation and CFeN 2 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 turnover frequencies for both the direct and the 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. (Appl. Catal., B 2005, 56, 9).

The high cost of proton-exchange-membrane fuel cells would be considerably reduced if platinumbased catalysts were replaced by iron-based substitutes, which have recently demonstrated comparable activity for oxygen reduction, but whose cause of activity decay in acidic medium has been elusive. Here, we reveal that the activity of Fe/N/C-catalysts prepared through a pyrolysis in NH3 is mostly imparted by acid-resistant FeN4-sites whose turnover frequency for the O2 reduction can be regulated by fine chemical changes of the catalyst surface. We show that surface N-groups protonate at pH 1 and subsequently bind anions. This results in decreased activity for the O2 reduction. The anions can be removed chemically or thermally, which restores the activity of acid-resistant FeN4-sites. These results are interpreted as an increased turnover frequency of FeN4-sites when specific surface N-groups protonate. These unprecedented findings provide new perspective for stabilizing the most active Fe/N/C-catalysts known to date.

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 FeN 4 -centers in which iron is mesomerically bonded to four nitrogen atoms are catalyzing the oxygen reduction reaction ͑ORR͒. X-ray induced photoelectron spectroscopy as well as Mössbauer 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 FeN 4 -center. Moreover, in accordance with these, higher kinetic current densities toward the oxygen reduction were observed. The above results suggest a relationship between the electron density of the FeN 4 -centers and the catalytic activity, where an increase in electron density on the iron centers enables an improvement in the turnover 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 centers during the ORR.

Pyrolysis of chloroiron-tetramethoxyphenyl-porphyrin (FeTMPP-Cl) in the presence of iron oxalate ({plus minus} 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 catalysts.