Nature of the Catalytic Centers of Porphyrin-Based Electrocatalysts for

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J. Phys. Chem. C 2008, 112, 15356–15366

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 Ulrike I. Koslowski,*,‡ Irmgard Abs-Wurmbach,† Sebastian Fiechter,*,‡ and Peter Bogdanoff‡ Helmholtz-Zentrum Berlin fu¨r Materialen and Energie (former Hahn-Meitner-Institut), Lise-Meitner-Campus, Glienicker Strasse 100, D-14109 Berlin, Germany, and Technical UniVersity Berlin, Faculty VI, Ackerstrasse 76, D-13355 Berlin, Germany ReceiVed: March 20, 2008; ReVised Manuscript ReceiVed: July 21, 2008

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 H2TMPP 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 57Fe 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-N4 centers, a CFeN2 center (Fe2+, S ) 2) and an Fe3C center (Fe0). 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-N4 centers is found. Beside this, there is evidence 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 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). 1. Introduction To date, platinum and its alloys are commercially utilized to catalyze the oxygen reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cells (FC). Because platinum is rare and expensive, alternative catalyst materials with a high catalytic activity toward the ORR and sufficient chemical stability are requested, enabling the production of a low-cost catalyst. For the purpose of an application in FCs, Gasteiger et al.1 calculated target values concerning the turnover frequency and site density, which are necessary for an implementation of low-cost, non-noble catalysts in fuel cells. As an alternative catalyst, special attention has been given to N4-metallomacrocycles, which were heat-treated at temperatures ranging from 600 to 1000 °C in inert gas atmospheres to increase the catalytic activity and also the chemical stability. Such a procedure was first published by Jahnke2 (1976) and Bagotzky3 (1977), and later adapted by several other authors.4-9 In contrast to platinum, the pyrolyzed macrocycles show a high methanol tolerance, making them suitable in DMFC applications.8,10 Investigations of N4-metallomacrocycles were undertaken to determine the structure of the catalytic centers in heat-treated materials. In most publications, the catalytic activity was attributed to Me-N4 or Me-N2 centers,4,5,7,8,11-22 whereby * Corresponding author. Phone: +49-30-8062-2927. Fax: +49-30-80622434 (S.F.). E-mail: [email protected] (U.I.K.); fiechter@ helmholtz-berlin.de (S.F.). † Technical University Berlin. ‡ Helmholtz-Center Berlin fu ¨ r Materialen and Energie.

Wiesener et al.23 favored special structures in the formed carbon network as catalytic sites. Some others attributed the catalytic activity to metallic particles encapsulated by carbon.24,25 In later publications, Gojkovic,7 Lalande,9 and Faubert16 demonstrated that these metallic particles can be excluded from contributing in the catalytic process. It had also been shown that highly active catalysts can be obtained from heat-treated carbon supported iron salts at high temperatures in a N-rich atmosphere (e.g., acetonitril or NH3),9,26-28 hereafter labeled as Fe/CB + AN or Fe/CB + NH3, respectively. As a result of TOF-SIMS measurements of these materials, it was assumed by Lefe`vre et al.19 that both materials, heat-treated N4 metallo-macrocycles and Fe/CB + NH3, occur in the same metal structures, whereby the distribution of FeNxCy sites changes. Using 57Fe Mo¨ssbauer spectroscopy, only some structural investigations on heat-treated carbon-supported iron porphyrins were previously performed.4,11,13,15,29 The conditions to obtain the catalysts vary between publications: The authors used different porphyrin loadings, heating ramps, end temperatures, or remaining times. The temperature dependency of the structural features detected for the catalysts was therefore not uniform. In most cases, an etching step13 is also missing or was performed after contact with air. Therefore, Mo¨ssbauer spectra are dominated by inorganic byproducts like iron oxides, hindering the detection of the active centers. Nevertheless, investigations by van Veen,4 Blomquist,13 and BouwkampWijnoltz15 show the presence of Fe2+/3+N4 species also after pyrolysis at 700 or 800 °C, respectively. Although most of these

10.1021/jp802456e CCC: $40.75  2008 American Chemical Society Published on Web 09/06/2008

Correlation of Kinetic Current with In-Plane FeN4 Species authors expect FeN4 as the catalytic center, a convincing correlation between this structure and the catalytic activity toward the oxygen reduction has not been proven so far. The use of other techniques enabled the correlation of some other structural features to the catalytic activity. Herrmann30 presented a correlation between the catalytic activity and the extension of the in situ-formed graphene layers obtained by Raman spectroscopy. Bouwkamp-Wijnoltz15 proposed a correlation between the catalytic activity and the amount of electrochemically accessible Fe2+ centers. Results of Dodelet’s group revealed correlations between the catalytic activity and the amount of FeN2C4+ (ref 31), the nitrogen content,21,27,32 and the micropore ratio.27,33 Most of these results were obtained by the characterization of catalysts prepared by pyrolysis of iron salts impregnated on a carbon support and pyrolyzed in a nitrogen-rich atmosphere. From a theoretical point of view, calculations performed by Anderson and Sidik34 showed that by comparing Fe3+N4 and Fe2+N4 centers only the latter are able to reduce oxygen by a 4-electron transfer process. The Fe3+N4 center was excluded due to its strong bonding to water. In the case of Fe2+N4, the formation of hydrogen peroxide is inhibited by coadsorption of hydrogen to one of the surrounding nitrogen atoms (see reaction step 15 in their calculated reaction scheme). They also performed calculations on nitrogen-doped carbon35 and found that only one of the carbons directly bonded to nitrogen (C-N catalytic site) could act as a catalytic site but only enabling a 2-electron reduction to hydrogen peroxide. Furthermore, the calculations exhibit higher activities if nitrogen is bonded inside the graphene layer, as compared to that at the edges. This result may also explain the increasing current density with increasing graphene-layer extension of CoTMPP-based catalysts,30 assuming a comparable role of nitrogen bonded to CoN4 centers, which are embedded in the graphene layers. This Article describes several post-treatments of one generic iron catalyst to induce structural changes as well as changes in the catalytic activity. The generic catalyst was prepared by the pyrolysis of iron porphyrin in the presence of iron oxalate and sulfur (so-called foaming agent technique (FAT)). The advantage of this preparation procedure is an in situ-formed carbon matrix with a homogeneous distribution of catalytic centers over the complete material (not only on the surface). The processes involved by the foaming agent technique are published by Bogdanoff et al.36,37 and Hermann et al.38 The prepared materials show promising high catalytic activity toward ORR in rotating disk electrode (RDE) measurements. Former structural investigations of FAT catalysts allow one to reduce the number of possible iron coordination for the assignment in Mo¨ssbauer investigations. X-ray-induced photoelectron spectroscopy (XPS) performed by Herrmann30 showed that after the heat-treatment amounts of 2-5 wt % sulfur remain bonded in the carbon matrix, whereas no Me-S bondings were detected. This result is underlined by extended X-ray absorption fine structure (EXAFS) analysis by Schmithals.22 His investigation showed that no coordination of iron by sulfur or oxygen can be found. It is proposed that iron is coordinated by nitrogen or carbon with a coordination number varying from 3 to 5. In this Article, we will concentrate on the characterization of the generic and the post-treated catalysts, using 57Fe Mo¨ssbauer spectroscopy and electrochemical measurements (RRDE). Correlations between the characteristics of the Mo¨ssbauer spectra and the obtained current densities are discussed. A linear dependency between the catalytic activity and the amount of in-plane Fe2+N4 centers, characterized by 57Fe Mo¨ssbauer

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15357 spectroscopy, could be inferred. A comparison between these catalysts and a reference material (a carbon-supported FeTMPPCl) shows that all catalysts exhibit identical iron structures, visible by identical features in the Mo¨ssbauer spectra. 2. Materials and Methods 2.1. Catalyst Preparation. Preparation of the Generic Catalyst Using the Foaming Agent Technique (FAT). Employing the molar ratios given in Tributsch et al.,39 a generic catalyst material (named A) has been prepared using chloro-iron(III)5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphyrin (FeTMPPCl, 94%, TriPorTech), iron(II) oxalate dihydrate (g99%, Riedel de Hae¨n), and sulfur. Additionally, a catalyst (named B) has been obtained using 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphyrin (H2TMPP, 94%, Porphyrin Systems), iron(II) oxalate dihydrate (g99%, Riedel de Hae¨n), and sulfur. Both obtained precursors were subjected to a heating procedure under a N2 gas flow. Samples were heated to 800 °C at a rate of 7.5 K/min. During the heat-treatment, the samples were allowed to dwell at 450 and 800 °C for 15 and 30 min, respectively. The leaching process was performed in 1 M hydrochloric acid and stirred for 1 h in an ultrasonic bath before being held at boiling temperature for 15 min. Finally, samples were washed with distilled water and dried. This procedure resulted in reproducible properties and can be given in terms of yields (50-55 wt % related to the porphyrin mass), iron amounts (2.7-3.3 wt %), and obtained current densities (RDE) (2.0-2.8 A/g). Preparation of the Post-Treated Catalysts. To modify the structure and composition of the catalyst (which impact the catalytic behavior), various post-treatments of the generic catalyst A, each using 120 mg of catalyst powder, were performed. Catalyst C was obtained by a treatment in 10 mL of concentrated nitric acid and catalyst D by a treatment in 10 mL of concentrated hydrogen peroxide. Both processes were performed for 10 min at room temperature and were followed by a washing of the remaining catalyst materials with distilled water. Catalysts E and F were subsequently heated in nitrogen, and 10 mg of sulfur was added to catalyst F prior to this second heat-treatment. A subsequent heat-treatment in CO2 was carried out to obtain catalyst G. All of these secondary heat-treatments (catalysts E-G) were performed at 800 °C for 30 min, before quenching to room temperature, without any additional etching step (see also Figure 1). Preparation of a Reference Catalyst by the Impregnation Technique (Catalyst Labeled H). To compare the structurecatalytic property relationship of FAT catalysts to a reference material, a carbon-supported FeTMPPCl catalyst was prepared similar to the preparation described in Herrmann et al.,40 employing 310 mg of FeTMPPCl (enriched with 54% of 57Fe) and 0.9 g of Ketjen Black EC600JD (AKZO NOBEL). About 90 mg of this precursor was heated (5 K/min) under constant N2 flow to 800 °C, where it was kept for 30 min. After cooling, the remaining product was etched in 1 M HCl and washed with H2Odest. Related to the initial precursor mass, a yield of 93 wt % was obtained. A scheme of all preparations can be followed in Figure 1. 2.2. Electrochemical Measurements. The electrochemical experiments were carried out at room temperature in a threeelectrode system with a platinum wire as counter electrode and a Hg/Hg2SO4/0.5 M H2SO4 reference electrode (0.68 V vs SHE). Catalyst ink was prepared by suspending 1 mg of the catalyst in 200 µL of a 1:1 water-ethanol mixture containing 0.2% Nafion. Five microliters of this suspension was dropped onto

15358 J. Phys. Chem. C, Vol. 112, No. 39, 2008

Koslowski et al.

Figure 1. Scheme of all catalyst preparations discussed in this Article.

the 0.1963 cm2 glassy carbon disk of the rotating ring disc electrode (RRDE), obtaining a catalyst load of 0.13 mg/cm2. First, the Pt-ring was activated by cyclic voltammetry (CV) in N2-saturated 0.5 M H2SO4. The working electrode containing the catalyst was then cycled until a steady state was reached. The open circuit potential was measured after flushing the electrolyte with oxygen for a period of 20 min. Rotating ring disk electrode (RRDE) measurements were conducted with a sweep rate of 0.3 V/min in the same O2-saturated electrolyte at 100, 200, 400, 576, 729, and 900 rpm (in this order). During the RRDE measurement, oxygen was only passed over the surface of the electrolyte. The potential of the Pt-ring was kept at 1.4 V (SHE), and hydrogen peroxide production was determined with a collection efficiency of 0.22. The collection efficiency of the Pt-ring of this RRDE was determined using the Fe2+/Fe3+ redox couple, by performing a RRDE measurement in an oxygen-free potassium sulfate/ hexacyanoferrat solution. To determine the kinetic current density, the Koutecky-Levich equation was applied.41 All given potentials are related to the standard hydrogen electrode (SHE). All given current densities are mass related. 2.3. 57Fe Mo¨ssbauer Spectroscopy. Mo¨ssbauer spectra were obtained using a 57Co/Rh-source. Each spectrum was recorded at room temperature with a 1024 multichannel analyzer equipped with a constant electronical drive system with triangular reference waveform (Halder Electronics, Germany). Velocity scale and isomer shift δiso were calibrated against natural iron (R-Fe-foil, 25 µm thick, 99.99% purity). The isomer shift from the calibration was used as a gravity center for the fitting procedure. Spectra were recorded in a velocity range of (4 mm/s. To check the samples on magnetic contribution, control measurements (not shown) were performed in a range of (10 mm/s for some catalysts. The spectroscopic data were obtained by leastsquares fitting using the modified program “MBF”,42 assuming a Lorentzian line shape.43,44 If possible, fitting parameters were kept free; in some cases, it was necessary to fix parameters. These values are marked by (f) in the fit parameters (see Table 2). FAT samples were not enriched by 57Fe (2.1% Fe-57). The amount of Mo¨ssbauer active 57Fe used was 40-63 µg for all FAT samples. In the impregnation catalyst, the lower Fe amount (1 wt % Fe only placed on the surface of the catalyst) made a

57Fe

enrichment necessary, resulting in an amount of 230 µg of during Mo¨ssbauer performance. For all catalysts, the metal contents were determined using neutron activation analysis (NAA). 57Fe

3. Results and Discussion 3.1. Preparation of FAT Catalysts. To obtain a catalyst for the post-treatments, a generic FAT catalyst was prepared, as described in section 2.1. During the heat-treatment of a FAT precursor mixture, most of the porphyrin is transformed into a carbon matrix with a homogeneous distribution of catalytic centers over the complete material (not only on the surface). The process of catalyst formation was described earlier for the CoTMPP-iron oxalate system.36-39 Performing the heat-treatment of the H2TMPP/iron-oxalate/S precursor mixture, which was used to obtain catalyst B, it is assumed that some of the iron of the decomposed iron oxalate reacts with H2N4 sites. This forms iron centers identical to those found in catalyst A. This suggestion is underlined by the results of extended X-ray absorption fine structure (EXAFS) analysis performed by Schmithals.22 He demonstrated that the final catalysts exhibited both CoNx and FeNx centers, when starting from a precursor mixture of iron oxalate, sulfur, and CoTMPP (instead of FeTMPPCl or H2TMPP). These findings have been confirmed by 57Fe Mo¨ssbauer spectroscopy, exhibiting the same spectra for all catalysts prepared with iron oxalate and sulfur in combination with CoTMPP, FeTMPPCl, or H2TMPP. TABLE 1: Mass-Related Kinetic Current Densities J and Hydrogen Peroxide Production (Both from RRDE Measurements for a Potential of 0.75 V (SHE)) and Iron Contents, Determined by Neutron Activation Analysis (NAA), of All Discussed Catalystsa catalyst Fe/wt % A B C D E F G H a

3.0 (0.1) 2.9 (0.1) 2.9 (0.1) 3.1 (0.1) 3.5 (0.1) 3.3 (0.1) 4.5 (0.1) 1.0 (0.1)

J/A/g 2.81 (0.14) 1.39 (0.07) 2.77 (0.14) 2.85 (0.14) 4.38 (0.22) 4.48 (0.22) 6.74 (0.34) 2.37 (0.12)

H2O2 (900 rpm)/% JH2O2/A/g 4.9 (0.5) 6.7 (0.7) 10.1 (1.0) 9.0 (0.9) 1.7 (0.2) 3.6 (0.4) 6.6 (0.7) 5.2 (0.5)

0.14 (0.04) 0.09 (0.03) 0.28 (0.06) 0.26 (0.06) 0.07 (0.03) 0.16 (0.04) 0.45 (0.08) 0.12 (0.04)

JH2O/A/g 2.67 (0.12) 1.29 (0.14) 2.49 (0.10) 2.59 (0.11) 4.31 (0.14) 4.32 (0.12) 6.29 (0.09) 2.25 (0.13)

Errors are given in brackets; for the classification of each catalyst, see Figure 1.

13.2 (0.1) 11 (5) 19.3 (0.1) 18.3 (0.1) 21.4 (0.2) 9.3 (0.1) 6.2 (1.4) 4.9 (1.8)

Errors are given in brackets; (f) represents assigned fixed values. a

A/% ∆w/mm/s

0.51 (0.11) 0.5 (f) 0.55 (0.08) 0.48 (0.05) 0.47 (0.12) 0.37 (0.07) 0.4 (f) 0.5 (f) 1.75 (0.05) 1.45 (0.10) 1.5 (f) 1.5 (f) 1.53 (0.06) 1.57 (0.03) 1.5 (f) 1.70 (0.10)

EQ/mm/s δiso/mm/s

0.38 (0.01) 0.35 (f) 0.32 (0.01) 0.34 (0.01) 0.36 (0.01) 0.35 (0.01) 0.33 (0.03) 0.40 (0.01) 28.6 (0.1) 48.8 (0.1) 31.3 (0.1) 33.6 (0.1) 22.8 (0.1) 43.0 (0.1) 25.8 (1.1) 60.7 (0.1)

A/% ∆w/mm/s

1.02 (0.08) 1.06 (0.05) 1.19 (0.08) 1.1 (0.1) 0.91 (0.12) 1.32 (0.07) 0.8 (f) 1.39 (0.07) 2.93 (0.08) 2.62 (0.05) 2.74 (0.09) 2.72 (0.06) 2.64 (0.12) 2.5 (0.1) 2.42 (0.04) 2.61 (0.08)

EQ/mm/s δiso/mm/s

0.31 (0.02) 0.4 (f) 0.37 (0.02) 0.37 (0.01) 0.42 (0.03) 0.41 (0.02 0.38 (0.02) 0.3 (f) 54.7 (0.1) 35.4 (0.1) 46.4 (0.1) 44.6 (0.1) 49.7 (0.1) 44.4 (0.1) 65.1 (0.02) 32.0 (0.1)

A/% ∆w/mm/s

0.62 (0.02) 0.61 (0.06) 0.55 (0.02) 0.52 (0.01) 0.57 (0.04) 0.57 (0.02) 0.53 (0.01) 0.66 (0.04) 0.92 (0.02) 0.98 (0.07) 0.87 (0.01) 0.84 (0.01) 0.88 (0.05) 0.92 (0.02) 0.78 (0.01) 0.99 (0.03)

EQ/mm/s δiso/mm/s

0.33 (0.01) 0.34 (0.01) 0.34 (0.01) 0.34 (0.01) 0.35 (0.01) 0.33 (0.01) 0.33 (0.01) 0.3 (f) -0.1 (f) -0.1 (f) -0.1 (f) -0.1 (f) -0.1 (f) -0.1 (f) -0.1 (f) -0.12 (f)

A/% ∆w/mm/s

0.45 (f) 0.45 (0.12) 0.4 (f) 0.45 (f) 0.4 (f) 0.4 (f) 0.4 (f) 0.4 (f)

δiso/mm/s no.

A B C D E F G H

3.6 (0.5) 4.8 (0.1) 3.0 (0.5) 3.5 (0.4) 6.1 (0.8) 3.3 (0.4) 3.0 (0.7) 2.4 (0.5)

doublet 3 doublet 2 doublet 1 singlet

TABLE 2: Isomer Shift δiso (Relative to r-Fe at Room Temperature), Quadrupole Splitting EQ, and Full Width at Half-Maximum ∆w Inferred from Mo¨ssbauer Spectra by Means of the Code MBFa

Correlation of Kinetic Current with In-Plane FeN4 Species

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15359 3.2. Performing the Post-Treatments. The aim of performing the post-treatments was to induce structural changes in the generic catalyst material, enabling a correlation between structure and the catalytic behavior. Catalyst A was used as a generic material for different post-treatments as described in section 2.1. Treatment with HNO3 leads to a small decrease of the iron amount (catalyst assigned as C), while the current density is similar. Ten minutes of conditioning with hydrogen peroxide left the catalyst nearly unaffected with respect to the iron amount as well as current density (catalyst D). Schulenburg et al.11 performed a hydrogen peroxide treatment (for 100 h) of an Fe-N-type catalyst and determined nearly complete inactivity toward ORR. It is assumed that such a long-term treatment might have the same effect on our catalyst. Both treatments (catalysts C and D) caused a doubling of the hydrogen peroxide production. A second heat-treatment of catalyst A caused a burnoff of catalyst material by 16% (N2 atmosphere, catalyst E), 13% (addition of sulfur, N2 atmosphere, catalyst F), and 37% (CO2 atmosphere, catalyst G), respectively. In all cases, the received iron amounts are higher than that of the generic (A) but a little lower as would be suggested by the burnoff. Catalysts E and F reveal similar catalytic activities and iron amounts. Because sulfur leads to significant improvement during the preparation of the FAT catalysts (like A and B), a further improvement during a second heat-treatment is not found. A second heat-treatment in CO2 (G) is most effective in improving the generic catalyst (A). Caused by the burnoff of carbon, the iron amount rose to 1.3-fold of the generic catalyst, and the catalytic activity increased to 2.4-fold. An overview of the different treatments is depicted in Figure 1, and the results of the electrochemical investigations, as well as the iron contents, are given in Table 1. 3.3. Electrochemical Activity. To investigate the influence of structure and composition with respect to catalytic activity, all catalysts were characterized by R(R)DE. This enables the determination of the current density and the hydrogen peroxide formation (%) in the kinetically controlled region of the Tafel plot. The results are summarized in Table 1 for a potential of 0.75 V (SHE). Depending on the kind of post-treatment, we achieved kinetic current densities ranging from 1.4 to 6.7 A/g. 3.4. Interpretation of 57Fe Mo¨ssbauer Results. In Figure 2, Mo¨ssbauer spectra of the studied samples are shown. Results of the analysis are given in Table 2. All catalysts can be fitted42 applying three doublets (D1-D3) and one singlet structure (sing); see Figure 2. By comparing catalysts obtained by the FAT (catalysts A-G) and by the impregnation technique (catalyst H), Fe coordinated by S can be excluded because in the impregnation process no sulfur was used, yet similar Mo¨ssbauer spectra were obtained. As mentioned in the Introduction, EXAFS measurements (Schmithals22) reveal a 3-5fold coordination of iron by nitrogen or carbon. Therefore, the most probable bondings of iron are formed to nitrogen or carbon. In the following, coordination that can be assigned to the attained parameters (δiso and EQ) of each doublet and the singlet (δiso) will be discussed. Doublet 1. The values of δiso and EQ of our doublet D1 (average values: δiso ) 0.33 mm/s, EQ ) 0.90 mm/s), shown in Figure 2, are well established typical for Fe2+N4 centers.45 However, the assignment of these parameters in heat-treated porphyrin-based catalysts is inconsistent. Bouwkamp-Wijnoltz et al.15 investigated an FeTPPCl/CB catalyst (pyrolyzed at 700 °C) by 57Fe Mo¨ssbauer spectroscopy. A doublet was assigned to the obtained spectrum using the


Figure 2. 57Fe Mo¨ssbauer spectra, including the fits obtained from the MBF program, of all discussed catalysts. Calibrations of the velocity scale and the isomer shift are relative to R-Fe at room temperature. The differently prepared and treated catalysts are assigned by the letters A-H, given in Figure 1. The assignment of Mo¨ssbauer patterns to three doublets and one singlet is visualized in the spectrum of catalyst B. The same color is used for the same subspectra.

parameters δiso ) 0.37 mm/s, EQ ) 0.96 mm/s. This result is comparable to our first doublet D1. They explained this doublet as an Fe3+N4 center, whereas also an Fe2+N4 (S ) 0) structure was taken into account. From Mo¨ssbauer spectra measured in biological structures, it is known that, without combination with other techniques, the differentiation between Fe2+ and Fe3+ in FeN4 centers is difficult. This is explained by the large covalency of the bondings.46 Discrimination might be possible combining Mo¨ssbauer spectroscopy with an external magnetic field (e.g., 20 mT) or by electron paramagnetic resonance (EPR) spectroscopy: In contrast to a low-spin (LS) Fe(II), the Fe3+-N4 center should be visible in X-band EPR. However, according to EPR investigations, not discussed in this Article, the preparation of highly active FAT catalysts is possible without any Fe3+ signal in EPR, whereas Mo¨ssbauer spectra behave qualitatively unaffected. Because of the fact that EPR is much more sensitive than 57Fe Mo¨ssbauer spectroscopy, signals in EPR might be attributed to even very low concentrations of Fe3+ or mixed states, which are not visible in our Mo¨ssbauer spectra. Therefore, we can exclude Fe3+ spin states to explain iron centers of highly active catalysts in our Mo¨ssbauer patterns. Bouwkamp-Wijnoltz15 compared ex situ with in situ 57Fe Mo¨ssbauer measurements taken at open circuit potential (OCP). No changes in Mo¨ssbauer signals were detectable. However, DFT calculations of Anderson et al.34 simulating the ORR process on Fe3+/2+N4 centers predict that contact of Fe3+N4 with water should result in a strong bonding of Fe3+ to H2O molecules. This bonding blocks the center for further oxygen reduction. It can also be assumed that such a bonding will cause changes in the coordination number of the Fe3+ being detectable by in situ Mo¨ssbauer spectroscopy. It is assumed that the assignment of an oxidation state of 2+ of Bouwkamp-Wijnoltz’s center 1 (FeN4) is therefore probable. Blomquist et al.13 also investigated heat-treated FeTPPCl and associated their site 1 (δiso ) 0.36 mm/s, EQ ) 1.11 mm/s) with an Fe2+-N4 center in the low-spin state. Some other authors assigned it as R-Fe2O3 with particle sizes less than 10 nm (ref 11) or FeOOH (ref 47). Following the argumentation given above, both of these Fe3+ compounds can be excluded due to their oxidation state. X-ray diffractograms

Koslowski et al. as well as TEM images did not show any metallic or inorganic iron phases. By applying a final etching step directly after pyrolysis, it is ensured that metallic iron is accessible by the acid and will thus be removed. No iron oxide can therefore be formed afterward, when handling the catalyst in air. This is a big advantage of the preparation by our foaming agent technique (FAT), as it leads to catalysts with nearly no inorganic byproducts. In conclusion, we assign doublet 1 (δiso ) 0.33 mm/s and EQ ) 0.90 mm/s) as characteristic for the presence of Fe2+N4 centers (S ) 0) in our catalysts. Doublet 2. The parameters of our second doublet D2 (δIso, EQ, ∆w) are ambiguous. A second doublet was also found in investigations of Bouwkamp-Wijnoltz et al.15 (δiso ) 0.54 mm/ s, EQ ) 2.4 mm/s). This doublet was attributed to an Fe2+N4 site with iron in the low-spin state, which is distinguished from our D1 by an iron atom out of the N4 plane.15 From our isomer shift δiso and quadrupole splitting EQ, having average values of δiso ) 0.37 mm/s and EQ ) 2.65 mm/s, we can conclude that we have a higher s-electron density at the nucleus. This is caused by a lower isomer shift, as compared to the value obtained by Bouwkamp-Wijnoltz. It is suggested that an out-of-plane iron position could be forced by adsorption of, for example, O2 in the axial position of the FeN4 structure. It might also be possible that a distorted 4-fold coordination of iron by nitrogen itself causes the outof-plane position of the iron. We suggest that such a structure can be formed when two adjacent (nitrogen doped) graphene layers form an angle among each other and are connected by iron bonded by two nitrogen atoms at the edges of each of these layers. Such FeN2+2/CB structures were proposed by Charreteur et al. as active sites.48 Another possible configuration could be a second in-plane ferrous FeN4 center, where iron occurs in the midspin state similar to the iron phthalocyanine published by Taube et al.49 At room temperature, they found values of δiso ) 0.37 mm/s and EQ ) 2.58 mm/s. In contrast to the other Mo¨ssbauer patterns in our spectra, this doublet exhibits an anomalous large full width at halfmaximum (∆w, Table 2), which is, in most cases, larger than 1 mm/s. The reason for the large ∆w can be explained assuming slight changes in the configuration of the iron coordination. This causes an overlap of several doublets having nearly the same parameters of isomer shift and quadrupole splitting. Following the assignment by Bouwkamp-Wijnoltz,15 the reason could be a slight variation in the out-of-plane position of iron by distortion of the graphene network. Next to Charreteur’s FeN2+2/CB site,48 the distances and positions of two adjacent graphene layers may vary. Following the correlation according to Taube,49 the effect could be explained by changes in the coordination in the vicinity of the center (nearest-neighbor effects), for example, by implementation of sulfur in the surrounding aromatic system leading to a distortion of the graphene like environment. Because it is known that the substituents in the periphery of a macrocyclic ring affect the physical and chemical properties of the metal center,50 these changes evoke deviations in Mo¨ssbauer patterns. This is in agreement with the above hypothesis. The FAT preparation technique used makes it obvious that an irregular environment of iron centers slightly changes Mo¨ssbauer absorption behavior. As a result, we conclude that our second doublet (δiso ) 0.37 mm/s, EQ ) 2.65 mm/s) can be attributed to a distorted ferrous FeN4 center (out-of-plane position of iron or the bonding to nitrogen belonging to two adjacent graphene layers or an


J. Phys. Chem. C, Vol. 112, No. 39, 2008 15361

TABLE 3: Assignment of Mo¨ssbauer Data (δiso, EQ, ∆w) to Different Iron Centers and Their Oxidation and Spin Statea δiso/mm/s D1 D2 D3 sing a

0.33 0.37 0.35 -0.10

EQ/mm/s

∆w/mm/s

oxid./spin state

structure

lit.

0.58 1.10 0.47 0.42

Fe /S ) 0 Fe2+/S ) 0 or 1 Fe2+/S ) 2 Fe0

Fe-N4 (in-plane) Fe-N4 (distorted) C-Fe-N2 metallic Fe/Fe3C

13, 15, 45 15, 48, 49 52 11, 12

0.90 2.65 1.56

2+

For the assignment of the abbreviations, see Table 2.

inhomogeneous chemical environment in the periphery of the FeN4 center). Doublet 3. The interpretation of the third doublet D3 (δiso ) 0.35 mm/s, EQ ) 1.56 mm/s) is more complex. The quadrupole splitting is typical for Fe3+ LS porphyrin complexes,51 but this oxidation state can be excluded by our EPR measurements; the reasons for that are given above. Another possible explanation might be a Fe2+, high-spin (HS), 3-fold coordinated by two nitrogen and one carbon atom. Andres et al.52 found similar values for the isomer shift δiso and the quadrupole splitting EQ studying polycrystalline N2FeCH3 at 200 K (δiso ) 0.37 mm/s, EQ ) 1.59 mm/s). In contrast to this pure phase, it is expected that our third doublet D3 has a similar composition, but is embedded in the carbon matrix. In the case of N2FeCH3, iron appears as Fe2+ in the high-spin state, associated by an EPR signal at g ) 11.4 when measuring in the parallel mode. Unfortunately, the configuration of our EPR system does not allow measurements in the parallel mode for our high conducting catalysts. Explaining doublet 3, we tentatively suppose a 3-fold coordination of iron by two nitrogen atoms and one carbon atom (CFeN2, Fe2+, high-spin) motivated by the findings of Andres et al.52 However, further investigations are necessary to establish this assumption. Singlet. The singlet (δiso ) -0.10 mm/s) that is found in all samples can be interpreted as super paramagnetic iron11,12 (Fe or Fe3C) protected by the carbon matrix, because all nonencapsulated iron should be removed during the acid treatment. Electron microscopic pictures of our catalyst39 and also XRD measurements did not show any metallic particles in the nanometer scale. This observation is a precondition for the explanation of a super paramagnetic behavior. All results that deviate from the Mo¨ssbauer investigations are summarized in Table 3. 3.5. Correlation of the Quantity of the Different Iron Centers to the Catalytic Activity in the Oxygen Reduction Reaction (ORR). To estimate the absolute Fe amount, which contributes to the related Mo¨ssbauer center FeCenterX,CatY, the relative area A [%] of each Mo¨ssbauer structure (determined by MBF-Fit) was multiplied by the Fe-content of each catalyst, using eq 1.

FeCenterX,CatY[wt %] )

A[%]CenterX,CatY · FeCatY[wt %] (1) 100

∑ FeCenterX,CatY[wt %] ) FeCatY[wt %]

(2)

X

The sum of all normalized MBF areas, expressed as FeCenterX,CatY for each sample, is consequently equal to the iron content of the related sample (compare to eq 2). The calculated concentration of each type of center is summarized in Table 4. The absorption in Mo¨ssbauer spectroscopy of the different iron centers in our catalysts is dependent on the bonding properties of each iron species, which are represented by the Debye-Waller factor.53,54 This factor denotes the recoil-free fraction of nuclear transition. In a first approximation, it depends

TABLE 4: Calculated Amounts of Fe of Each Mo¨ssbauer Center FeCenterX,CatY (sing, D1-D3)a sing Fe doublet 1 doublet 2 doublet 3 no. (errFe)/wt % Fe (errFe)/wt % Fe (errFe)/wt % Fe (errFe)/wt % A B C D E F G H

0.11 (0.05) 0.14 (0.02) 0.09 (0.05) 0.11 (0.04) 0.21 (0.06) 0.11 (0.04) 0.14 (0.09) 0.02 (0.04)

1.64 (0.04) 1.03 (0.04) 1.35 (0.04) 1.38 (0.04) 1.74 (0.04) 1.47 (0.04) 2.93 (0.06) 0.32 (0.06)

0.86 (0.03) 1.42 (0.04) 0.91 (0.03) 1.04 (0.03) 0.80 (0.03) 1.42 (0.04) 1.16 (0.05) 0.61 (0.08)

0.40 (0.02) 0.32 (0.26) 0.56 (0.03) 0.57 (0.02) 0.74 (0.03) 0.31 (0.02) 0.28 (0.12) 0.05 (0.08)

a Determination was done under the assumption of comparable Debye-Waller factors for all iron species. Errors are given in brackets.

on the temperature of the sample, the energy of the gamma quantum Eγ, and the bonding properties of each iron species. It increases with decreasing temperature, decreasing Eγ, and increasing bond strength.55 In the case of our catalyst material, the Debye-Waller factors are unknown, but the temperature and also the energy of the gamma quantum are equal for all samples. Therefore, the absorption only depends on the bonding properties of iron, including next and peripheral neighbor effects. In this approach, we presume Debye-Waller factors of the same order of magnitude for all species. As a result, the obtained relative absorption of our Mo¨ssbauer patterns can directly be correlated to the amount of Fe bonded in the particular structure, as it is expressed in eq 1. Concerning the different absorption intensities of the doublets D1-D3, we assume a predominantly covalent bonding for all three centers. An exception might be the singlet exhibiting a different Debye-Waller factor. By the fact that the relative amount of the singlet is small (as compared to the other structures) and similar in all catalysts (Arel ) 3.0-5.7%), a comparison of the samples among each other is justified. The results of the calculated iron amount related to each center, as a function of the total current densities, determined by RDE are given in Figure 3a-d. From Figure 3a, a principal correlation of the amount of iron bonded in doublet D1 to the current density is evident. Please note that this correlation even remains visible using current densities measured at other potentials inside the kinetic-controlled ORR regime (U > 0.65 V). For all other iron centers, no such correlation has been found. With respect to our RRDE measurements, we are able to calculate the current densities related to a direct oxygen reduction (4 electrons) and the hydrogen peroxide formation (2 electrons) from the total current densities and the relative hydrogen peroxide production of each sample according to the following equations:

2-electron pathway: JH2O2 ) J(0.75 V) · H2O2%(0.75 V) (3) 4-electron pathway: JH2O ) J(0.75 V) - JH2O2

(4)

Plotting the iron amount of structure D1 as a function of current density related to direct ORR, a linear correlation


Koslowski et al.

Figure 3. Correlation of the iron nominated contribution of each Mo¨ssbauer pattern FeCenterX to the overall mass related current density J (at 0.75V (SHE)) of the ORR (in A/g). The related iron centers are given in the boxes for each species. Errors are marked by bars.

Figure 4. (a) Correlation between iron amount of the in-plane FeN4 structure (related to doublet D1) and the kinetic current density (at 0.75 V vs SHE) calculated for the direct oxygen reduction to water, J(H2O), and (b) correlation of iron in doublet 3 (assumed as CFeN2 center) with the current at 0.75 V (SHE) related to the hydrogen peroxide formation, J(H2O2). Errors are marked by bars.

between iron concentration and current density appears, shown in Figure 4a (relative error of the slope: 22%). Therefore, it can be concluded that this center is mainly responsible for the 4-electron transfer in the process of oxygen reduction. The fit is best, however, when the overall current density is used. Therefore, it is assumed that beside the direct oxygen reduction, some oxygen is only partially reduced under formation of hydrogen peroxide. No correlation, however, can be found plotting the amount of iron bonded in D1 as a function of the current related to a 2-electron oxygen reduction process. On the other hand, Figure 4b gives evidence for a linear correlation between hydrogen peroxide formation and the site density of our third doublet D3 (CFeN2), if neglecting the values of the catalysts E and G. This linear trend is visible in the potential range from 0.8 to 0.3 V (SHE). Parameters used for the linear fits are listed in Table 5. The reasons why samples E and G do not fit into this curve can only be speculated: One important aspect is that both D1 centers (only partially) and in general nitrogen in the graphene network (previously named C-N catalytic sites)35 have the ability to produce hydrogen peroxide. It has been postulated

TABLE 5: Linear Fits of FAT Catalysts Separated into Values for the Direct and Indirect Oxygen Reduction (FeFeN4 ) k0 + k1*JH2O or FeCFeN2 ) k0 + k1*JH2O2), See Figure 3a and ba

4-electron pathway 2-electron pathway

k0

k1 (slope)

Fe structure

Mo¨ssbauer site

0.52 (0.28)

0.33 (0.07)

FeN4 (in-plane)

D1

0.17 (0.07)

1.44 (0.37)

CFeN2

D3

a Samples E and G were neglected in the fit of the 2-electron pathway. Errors are given in brackets.

that a second treatment in the presence of CO2 (catalyst G) may favor the formation of such surface sites, improving hydrogen peroxide formation. Because CO2 treatment also has a destructive influence on Fe centers, it might lead to an increase in the concentration of nitrogen available for subsequent formation of C-N or similar sites. In comparison to the secondary heat-treatment in CO2, following a heat-treatment in the presence of N2 (catalyst E) might evoke other catalytic sites in the graphene network. From


J. Phys. Chem. C, Vol. 112, No. 39, 2008 15363

2-electron pathway: FeCFeN2,surf ) FeCFeN2(D3) - 0.17 wt % (5) 4-electron pathway: FeFeN4,surf ) FeFeN4(D1) - 0.52 wt % (6)

Figure 5. Correlation between the relative amounts of iron on the surface calculated for D1 and D3.

a former publication, it is known that these sites might cause a reduction of hydrogen peroxide to water.56 Assuming a large number of those sites in catalyst E, and taking the high porosity of our material into account, it is proposed that after the formation of hydrogen peroxide on CFeN2 centers this is further reduced on those sites, causing a lowering of percental hydrogen peroxide as detected at the Pt-ring in RRDE. The accuracy of the obtained fit (Figure 4b) can clearly be retraced, as described next. The intersections of the curves with the ordinates in Figures 4a and b are different from zero. This phenomenon could be explained by the amount of Fe centers that are embedded in the volume of the catalyst material (0.52 wt % for FeN4 (D1) and 0.17 wt % for CFeN2 (D3)). We assume a homogeneous distribution of all iron centers in our carbon matrix, whereas only the iron centers on (or near) the surface are active in ORR. The homogeneous distribution of elements is evident from the similarity of XPS and combustion analysis, giving the surface and the integral element composition, respectively, performed on selected catalysts. The amounts of CFeN2,surf and FeN4,surf that contribute to the hydrogen peroxide formation or the oxygen reduction to water, respectively, are calculated by subtracting the amount of centers not participating in catalysis from the overall iron amount bonded in structures D3 (FeCFeN2) and D1 (FeFeN4), respectively (see eqs 5 and 6):

By post-treatments, we vary the surface area of our catalysts, and as a consequence the number of iron centers participating in the ORR is changed. The relative amount of D1 and D3 participating in the ORR process can be calculated. If a homogeneous distribution is given, it should lead to similar values of Fesurf/Fe for D1 and D3 for each catalyst. As it can be seen in Figure 5, there is a good agreement in this respect. This is a further indication to confirm that most of the hydrogen peroxide formation is connected with the CFeN2 center of doublet D3. Only catalyst G (neglected in the obtained fit for the 2-electron pathway, see Figure 4b) does not fit into this graph. It can be concluded that at minimum 1.66 wt % iron is not catalytically active or not accessible during the ORR process. In contrast, 0.66-2.52 wt % iron, depending on the preparation and post-treatment procedure, contributes to the oxygen reduction reaction. (It should be kept in mind that beside the embedded iron centers D1 (0.52 wt %) and D3 (0.17 the site density of iron centers on the surface of our foaming agent catalysts, we are able to compare their catalytic activity directly with our catalyst generated via the impregnation technique, where the iron centers are naturally located on the surface of the carbon support. In Figure 6a and b, the correlation between site density of iron centers and the kinetic current density related to water and hydrogen peroxide formation, respectively, together with a measurement obtained from an impregnation catalyst (catalyst H, not filled symbols) are shown. Catalyst H fits well into the graphs, underlining that here all D1 and D3 centers are positioned on or nearby the surface. Taking all catalysts into account (except samples E and G in Figure 6b), linear fits were performed (with an intercept of zero), calculating slopes of 0.32 wt %/(A/g) (err: 9%) in the case of center D1 and 1.36 wt %/(A/ g) (err: 11%) for center D3, similar to those of Figure 4a and b. 3.6. Determination of the Reaction Rate (Turnover Frequency fTO) and Site Densities SD of the Catalysts. From the slopes of the curves in Figure 6a and b, the reaction rates

Figure 6. (a) Correlation between the total amount of all active FeN4 centers (FeD1_surf) of all catalysts (foaming agent and impregnation technique) and the current density J(H2O), related to the formation of water. (b) Correlation between the total amount of active CFeN2 sites (FeD3_surf) and the current density J(H2O2), related to the formation of hydrogen peroxide. Measured points related to foaming agent catalysts are marked by filled symbols (2 and b), while those belonging to the impregnation catalyst are labeled in (a) by 4 and in (b) by O. Currents are given for a potential of 0.75 V (SHE). Errors are marked by bars. Linear fits were calculated assuming an intersection of the straight line with the origin of the coordinates, whereas in (b) catalysts E and G were neglected during the fitting procedure. Slopes of the linear fits are given as insets.


Koslowski et al.

TABLE 6: Turnover Frequencies fTO and Site Densities SD for Different Fe Centersa fTO of 47% Pt/C target non-Pt Fe/C (0.3 wt %) Fe/C (0-0.25 wt %) FePhen 10% Pt/C averages

catalytic site

ORR

T/°C

Pt ? FeNxCy FeNxCy FeN4 Pt FeN4 CFeN2

d d d, i d, i d, i d d i

80 80 80 20 80b 80b 25

fTO (0.75 V)/ e-/site · s

0.95 0.18 0.04

fTO(0.8 V)/ e-/site · s 25 2.5 0.40 0.14 1.0 10 0.04 0.009

SD*1019/sites/cm3

IK (0.8 V)/ A/cm3

32 32 1.29

1300 130 0.82

0.9 3.4 4.9 1.0

0.03 55 0.31 0.01

lit. 1 1 27 27 1 1 this work this work

a For reasons of comparison, values found in the literature and related temperature data of the electrochemically measurements are added. Column “ORR”: d ) direct water formation (4-electron transfer), and i ) indirect oxygen reduction (hydrogen peroxide formation, 2-electron transfer). b Calculated assuming an activation energy comparable to that of Pt/C.

of oxygen turnover in the process of water and hydrogen peroxide formation can be calculated according to eq 7:

∂[O2]Red ∂Q ≈ ) J ≈ NFeNx ∂t ∂t

(7)

where [O2]Red denotes the number of moles of oxygen reduced in the time interval δt. This value is proportional to the charge per time, which is equal to the current determined in RDE that is again proportional to the number of catalytic centers NFeNx. The inverse values of the slopes in Figure 6a and b show that the formation of hydrogen peroxide at center D3 is by a factor of 4.3 smaller than the formation of water on Fe2+N4 centers (D1). This is in agreement with results of Dodelet’s group19,21 reporting on a higher frequency for their centers catalyzing the direct reduction to water. However, while they propose that the FeN4 cores are responsible for the 2-electron transfer, we claim this coordination type to be responsible for the 4-electron process. Dodelet et al.19 obtained their results from time-of-flight (TOF) measurements correlating different fragments of FeNxCy+ ions to different iron coordinations. In our investigation, we attribute the active structures detected in the Mo¨ssbauer spectra by comparison with Mo¨ssbauer data given in the literature. Dodelet’s group21,48 assigned the direct reduction of oxygen to water to the presence of FeN2+2/CB centers. However, we discussed this structure as a possibility to explain doublet D2, which did not show any correlation to the ORR. It should be noted that the assignment of D2 to this specific center has to be regarded as an assumption and that the Mo¨ssbauer patterns belonging to certain FeN2+2 centers still remain speculative. The NH3 treatment used by Dodelet’s group could result in the formation of a further catalytic center not present in any of our catalysts. If this is the case, a different Mo¨ssbauer signal should be expected. On the other hand, it can also be possible that only parts of doublet D2 fit to their FeN2+2/C structure overlaid with a further doublet, which eliminates the visualization of a dependency for the FeN2+2/CB center. Further investigation studying the influence of NH3 during a heat-treatment might help to explain this discrepancy. Nevertheless, the presented direct reduction of oxygen to water catalyzed by Fe2+N4 centers (doublet D1) is in agreement with results from DFT calculations described by Anderson and Sidik,34 and also with 57Fe Mo¨ssbauer measurements published by Bouwkamp-Wijnoltz et al.,15 as mentioned above. Going back to Figure 6, the turnover frequencies fTO [electrons/site · s] for both reaction pathways and the number of active sites per volume named as site density SD [sites/cm3] can be determined. Gasteiger et al.1 defined target values for

fTO and SD, which are prerequisitions to make non-noble catalysts applicable in fuel cells. In a first step, the number of Fe-centers that contribute to the current density are calculated using the iron amount FeFeNx,surf (in wt %) (calculated by eqs 5 and 6), the molar weight of iron (MFe ) 55.845 g/mol), and Avogadro’s number (NA ) 6.022 × 1023 atoms/mol); see eq 8.

NFeNX,surf /(atoms/g) )

FeFeNX,surf /wt % 100 · MFe/(g/mol)

· NA/(atoms/mol) (8)

As in each D1 and D3 center, only one Fe atom is bonded in the catalytic center, and the number of iron atoms NFeinspeciesX,surf is equivalent to the number of catalytic sites Nsites,speciesX,surf. By multiplying the latter unit with the density of the catalyst material Fcatalyst, the number of active sites per volume SD can be determined, as given in eq 9:

SD/(sites/cm3) ) NFeNX,surf/(sites/g) · Fcatalyst/(g/cm3) (9) The density of our material is not known, but we expect a value close to F ) 0.4 g/cm3, as reported by Gasteiger et al.1 By using the slopes given in Figure 6 (s4e- and s2e-), the elementary charge (e), and the number of iron sites, the turnover frequencies fTO can be calculated using the following equation:

fTO/(Nelectrons/(site · s)) ) 100 · MFe/(g/mol) (10) sxe-/(A/gFeNx) · NA/(atoms/mol) · e/A · s In Table 6, the turnover frequency for our reference potential of 0.75 V fTO(0.75V), the determined value for fTO(0.8V), and the site density SD are summarized. For comparisons sake, values obtained by other authors1,27 are also given. The turnover frequency fTO at 0.8 V can be calculated using the mean Tafel slope of our catalysts (75.8 V/dec) and the related potential offset (off - 50 mV), according to eq 11:

ln(10)) ( -50 75.8

fTO(0.8 V) ≈ fTO(0.75 V) · exp

(11)

The obtained turnover frequencies for the 4-electron pathway are of the same order of magnitude, but by a factor of 5 lower than the overall turnover frequencies of catalysts reported by Dodelet’s group.27 While we argue that the oxygen reduction to water is catalyzed by in-plane Fe2+N4 centers (D1), Dodelet et al. proposed an FeN2+2/C center mainly responsible for the ORR.48 The presence of a further than our center D1, emerging from ammonia treatment of Fe/CB, is thought to be the most

Correlation of Kinetic Current with In-Plane FeN4 Species probable explanation for the different behavior as compared to center D1. It seems that the bonding of iron to two different, but adjacent graphitic crystallites, each contributing by two nitrogen atoms at the edge of the crystallite, is responsible for the pronounced oxygen reduction ability. Taking into account the saturation effect of catalysts based on heat-treated Fe/CB in NH3, as published by Jaouen and Dodelet,27 the authors propose that the high-active FeN2+2/C centers are only formed at lower iron contents (up to 0.1 wt % Fe). It is suggested by them that a higher iron content led to the formation of FeN4 centers. Recalculation of the slope, obtained in Figure 3 by Jaouen et al.,27 indicated a considerably lower slope in the saturated region (0.1-0.8 wt % Fe) of the curve. It seems to be a slope of ∼0.2 (Table 1, ref 27), instead of 1.0 for iron contents up to 0.2 wt %. The factor of 5, between both slopes, fits very well to the turnover frequencies determined in this Article. The target values published by Gasteiger et al.1 of low-cost catalysts are defined for a potential of 0.8 V (SHE), an O2 partial pressure pO2 of 100 kPaabs, and a temperature of 80 °C in a fuel cell. Assuming activation energy for FAT catalysts in the same order of magnitude as Pt/C, the turnover frequency at 80 °C should be increased by a factor of about 50 as compared to our conditions. A turnover frequency of fTO,0.8V (80 °C) ≈ 2 electrons/site · s could be calculated, reaching 80% of Gasteiger’s target value (2.5 electrons/site · s). Nevertheless, activation energies of Pt/C vary in the range from 22 to 76 kJ/mol (ref 1), causing a high uncertainty. From these results, it has to be concluded that temperature-dependent measurements of kinetic current densities are necessary to estimate the activation energy of the Fe2+N4 centers. As compared to Jaouen et al.,27 significantly higher site densities SD (1.0 × 1020sites/cm3 maximum, catalyst G) can be realized using our foaming agent technique (FAT). However, this value is still 70% off from the target value, defined by Gasteiger et al.1 4. Conclusion and Outlook Several post-treatments of a generic FAT catalyst enable correlations of the amount of Fe centers with kinetic current densities measured by R(R)DE. The following conclusions can be made: (1) The total number of in-plane FeN4 (D1) embedded in a graphene-type matrix and accessible by the electrolyte is linearly correlated to the kinetic current density in the oxygen reduction reaction to water. (2) A linear correlation between doublet 3 (addressed as CFeN2 centers) and the hydrogen peroxide formation of the catalyst is proposed. (3) The best obtained catalyst (G) exhibits 30% of the site density of the target value determined by Gasteiger et al. (4) Turnover frequencies for the 4-electron pathway (fTO,4e (0.75 V) ) 0.18 Ne/site · s) and for the reduction to hydrogen peroxide (fTO,2e (0.75V) ) 0.01 Ne/site · s) were determined. Extrapolating the turnover frequencies to fuel cell conditions (T ) 80 °C, pO2 ) 100 kPaabs), these values are of the same order of magnitude as that demanded by Gasteiger et al.1 as a target value. (5) Results reported by Jaouen et al.27 give evidence that catalysts of high iron content (>0.1 wt %), prepared by NH3 treatment at temperatures g900 °C, have the tendency to form Fe2+N4 centers. With respect to the high amount of iron that is not participating in the ORR process of our FAT catalysts, further efforts should be made to increase the number of Fe centers catalyzing

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15365 ORR by increasing the surface area. As a consequence, it should be possible to double the catalytic activity by increasing the site density. Acknowledgment. We would like to thank our colleagues Iris Dorbandt, Iris Herrmann, Ingrid Rother, Gerald Zehl, and Andreas Ziegler for supporting this work through fruitful discussions and technical assistance. Support by Klaus Lips and his group in EPR measurements and the performance of neutron activation measurements of Mrs. Alber and her group is gratefully acknowledged. We thank Maria Nowotny, Sophie Gledhill, and the referees for their valuable comments, in form and content, to improve this paper. References and Notes (1) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal., B 2005, 56, 9. (2) Jahnke, H.; Scho¨nborn, M.; Zimmermann, G. Top. Curr. Chem. 1976, 61, 133. (3) Bagotzky, V. S.; Tarasevich, M. R.; Radyushkina, K. A.; A., L. O.; Andrusyova, S. I. J. Power Sources 1977-1978, 2, 233. (4) Veen, P. D. J. A. R. v.; Baar, J. F. v.; Kroese, K. J. J. Chem. Soc., Farraday Trans. 1981, 77, 2827. (5) Blomquist, J.; Helgeson, U.; Moberg, L. C.; Johansson, L. Y.; Larsson, R. Electrochim. Acta 1982, 27, 1445. (6) Contamin, O.; Debiemme-Chouvy, C.; Savy, M.; Scarbeck, G. Electrochim. Acta 1999, 45, 721. (7) Gojkovıć, S. L.; Gupta, S.; Savinell, R. F. J. Electroanal. Chem. 1999, 462, 63. (8) Gupta, S. L.; Tryk, D.; Zecevic, S. K.; Aldred, W.; Guo, D.; Savinell, R. F. J. Appl. Electrochem. 1998, 28, 673. (9) Lalande, G.; Coˆte´, R.; Guay, D.; Dodelet, J.-P.; Weng, L. T.; Bertrand, P. Electrochim. Acta 1997, 42, 1379. (10) Sun, G.-Q.; Wang, J.-T.; Savinell, R. F. J. Appl. Electrochem. 1998, 28, 1087. (11) Schulenburg, H.; Stankov, S.; Schu¨nemann, V.; Radnik, J.; Dorbandt, I.; Fiechter, S.; Bogdanoff, P.; Tributsch, H. J. Phys. Chem. B 2003, 107, 9034. (12) Veen, J. A. R. v.; Colijn, H. A.; Baar, J. F. v. Electrochim. Acta 1988, 33, 801. (13) Blomquist, J.; Lang, H.; Larsson, R.; Widelo¨v, A. J. Chem. Soc., Faraday Trans. 1992, 88, 2007. (14) Bron, M.; Fiechter, S.; Hilgendorf, M.; Bogdanoff, P. J. Appl. Electrochem. 2002, 32, 211. (15) Bouwkamp-Wijnoltz, A. L.; Visscher, W.; Veen, J. A. R. v.; Boellaard, E.; Kraan, A. M. v. d.; Tang, S. C. J. Phys. Chem. B 2002, 106, 12993. (16) Faubert, G.; Coˆte´, R.; Dodelet, J.-P.; Lefe`vre, M.; Bertrand, P. Electrochim. Acta 1999, 44, 2589. (17) Gupta, S. L.; Tryk, D.; Bae, I.; Aldred, W.; Yeager, E. B. J. Appl. Electrochem. 1989, 19, 19. (18) Jaouen, F.; Marcotte, S.; Dodelet, J.-P.; Lindbergh, G. J. Phys. Chem. B 2003, 107, 1376. (19) Lefe`vre, M.; Dodelet, J.-P.; Bertrand, P. J. Phys. Chem. B 2002, 106, 8705. (20) Scherson, D. A.; Tanaka, A. A.; Gupta, G. P.; Tryk, D. A.; Fierro, C.; Holze, R.; Yeager, E. B.; Lattimer, R. P. Electrochim. Acta 1986, 31, 1247. (21) Me´dard, C.; Lefe`vre, M.; Dodelet, J.-P.; Jaouen, F.; Lindbergh, G. Electrochim. Acta 2006, 51, 3202. (22) Schmithals, G. Structural and electrochemical characterization of the catalytic sites in noble metal free catalysts for oxygen reduction. Ph.D. Thesis, Freie Universita¨t, Berlin, 2005. (23) Wiesener, K. Electrochim. Acta 1986, 31, 1073. (24) Faubert, G.; Lalande, G.; Coˆte´, R.; Guay, D.; Dodelet, J.-P.; Weng, L. T.; Bertrand, P.; Deńe`s, G. Electrochim. Acta 1996, 41, 1689. (25) Lalande, G.; Faubert, G.; Coˆte´, R.; Guay, D.; Dodelet, J.-P.; Weng, L. T.; Bertrand, P. J. Power Sources 1996, 61, 227. (26) He, P.; Lefe`vre, M.; Faubert, G.; Dodelet, J.-P. J. New Mater. Electrochem. Syst. 1999, 2, 243. (27) Jaouen, F.; Dodelet, J.-P. Electrochim. Acta 2007, 52, 5975. (28) Faubert, G.; Coˆte´, R.; Guay, D.; Dodelet, J.-P.; Deńe`s, G.; Poleunis, C.; Bertrand, P. Electrochim. Acta 1998, 43, 1969. (29) Tarasevich, M. R.; Radyushkina, K. A. Mater. Chem. Phys. 1989, 22, 477. (30) Herrmann, I. Development and optimization of innovative preparation strategies for transition metal based electrocatalysts for the oxygen reduction. Ph.D. Thesis, Freie Universita¨t, Berlin, 2005.

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