Reduction in PEM Fuel Cells - American Chemical Society

11238

J. Phys. Chem. B 2000, 104, 11238-11247

O2 Reduction in PEM Fuel Cells: Activity and Active Site Structural Information for Catalysts Obtained by the Pyrolysis at High Temperature of Fe Precursors M. Lefe` vre and J. P. Dodelet* INRS-EÄ nergie et Mate´ riaux, C. P. 1020, Varennes, Que., Canada, J3X 1S2

P. Bertrand PCPM, UniVersite´ Catholique de LouVain, 1348 LouVain-la-NeuVe, Belgium ReceiVed: July 7, 2000; In Final Form: September 7, 2000

Catalytic activity for O2 electroreduction in acidic medium has been studied. Catalysts have been produced by pyrolyzing perylene tetracarboxylic dianhydride (PTCDA) combined with Fe and N precursors. The Fe precursors used in this work are either FeII acetate or Cl-FeIII Tetramethoxyphenyl porphyrin (Cl-FeTMPP). The N precursors are NH3 and/or Cl-FeTMPP. To be able to vary the catalytic activity, two synthesis procedures have been used, varying either the heat-treatment temperature or the Fe content of the pyrolyzed materials. All the catalysts have been studied by Time-of-Flight Secondary Ion Mass Spectrometry (ToF SIMS) in order to find out if the relative intensity of one or several secondary ions was following the changes observed in the catalytic activity. Only one ion, FeN2C4+, may be considered as the signature of the catalytic site generated in this work. It has been detected in all prepared catalysts, whatever the Fe precursor or the synthesis procedure was used. It is proposed that this ion is produced under ToF SIMS analysis from part of a 1,10 phenanthrolinic-type structure, which is itself part of the catalytic site. This catalytic site is probably the same as the one obtained by the pyrolysis at high temperature (g 800 °C) of N4-Fe macrocycles adsorbed on carbon black. RDE and PEM fuel cell tests demonstrate that the most performing catalysts are those containing 2 wt % Fe as Cl-FeTMPP and 0.2 wt % Fe as Fe acetate. Increasing the Fe content beyond these limits mainly generates catalytically inactive Fe clusters and interferes with the characterization of the hightemperature active site.

Introduction In the preface of the Proceedings of the Symposium on Oxygen Electrochemistry held by the Electrochemical Society in 1995, the Editors (R. R. Adzic, F. C. Anson, and K. Kinoshita) wrote that: “Despite progress achieved in recent years, the electrochemical reactions of oxygen, in particular oxygen reduction, continue to be a challenge for electrochemists because of the complex kinetics and the need for better electrocatalysts. [...] The most notable need remains the improvement of the catalytic activity of the existing electrocatalysts and the development of new and better non noble metal electrocatalysts”.1 This statement is still true today and applies in particular to catalysts for the reduction of oxygen in polymer electrolyte membrane (PEM) fuel cells. H2/O2 PEM fuel cells are electrochemical power generators that function at the low pH of the Nafion protonic membrane (Nafion is a perfluoropolymer sulfonic acid with a gross equivalent concentration of about 0.1M H+).2 The use of an acidic reaction medium in PEM fuel cells has for consequence that, to this day, only noble metals such as Pt and its alloys have been found suitable as catalysts for oxygen reduction reaction in commercial prototypes. Even so, the cathode activation potential easily reaches values over 0.3 V (over 25% of the open circuit potential) for moderate current densities.3 Nonnoble metals are subject to rapid corrosion in the acidic medium of PEM fuel cells, but nonnoble metal chelates might provide an interesting way to obtain catalysts * To whom correspondence should be sent. [email protected]

for O2 reduction at low pH. It is therefore important to develop these catalysts because Pt is an expensive metal of low abundance (the Pt supply in 1998 was 5400 ounces troy or about 168 000 Kg),4 but also because in Direct Methanol Fuel Cells (another type of PEM fuel cells based on the direct electrooxidation of methanol instead of hydrogen), a fraction of the methanol fuel is able to diffuse across the membrane to end up in the cathode compartment.2 There, methanol is oxidized at a mass transport limited rate on the Pt-based catalyst interfering strongly with the oxygen electroreduction occurring at the same electrode. Since Jasinski’s discovery of the catalytic properties of Co phthalocyanine for oxygen reduction, more than 30 years ago,5 several authors have demonstrated that N4-Metal macrocycles (tetra-aza-annulenes, porphyrins, and phthalocyanines) with Metal ) Fe or Co, adsorbed on carbon and heat-treated at various temperatures are catalysts for the reduction of O2 in acidic medium.6-28 Furthermore, contrary to Pt, the same catalysts do not show activity for the oxidation of methanol.29-32 The best electrocatalytic activities have been shown to occur in the pyrolysis temperature range comprised between 500 and 700 °C. In that temperature range, the catalytic site is N4-Metal bound to the carbon support.15,33 It will be labeled the lowtemperature site. However, the use of these catalysts in a single membrane electrode assembly of a H2/O2 PEM fuel cell revealed that the low-temperature site is unstable and that pyrolysis temperatures g 800 °C are necessary to obtain more stable catalysts with, however, a lower catalytic activity.23,34 At these

10.1021/jp002444n CCC: $19.00 © 2000 American Chemical Society Published on Web 10/27/2000

O2 Reduction in PEM Fuel Cells SCHEME 1

pyrolysis temperatures, the catalytic site is not N4-Metal anymore.35 Its exact structure has not yet been elucidated. It will be labeled the high-temperature site. At these high pyrolysis temperatures, it is not even necessary to start from N4-Metal macrocycles to obtain catalysts for O2 reduction. Indeed, catalysts are obtained by pyrolyzing a metal precursor (a Fe or Co salt, a ferrocene derivative, etc..) adsorbed on carbon and a nitrogen precursor which is either coadsorbed on carbon or is a N-containing functionality at the surface of carbon, or a N-containing gas or vapor injected in the reactor during pyrolysis.36-46 Although the combined use of metal and nitrogen precursors always produces a catalytic material upon pyrolysis at high temperature, the performance of these catalysts is not identical but depends on the individual nature of each precursor. Most of the authors that prepared catalysts at high pyrolysis temperatures, either using Co or Fe macrocycles or individual metal and nitrogen precursors, report the presence of Co or Fe metallic clusters (eventually containing some metal carbides). Usually, a fraction of these clusters is covered with a thin carbon layer and therefore resists to corrosion in leaching solutions. We first believed that these clusters were the source of the catalytic activity at high pyrolysis temperatures, until we demonstrated that these particles were catalytically inactive.39,47 As a matter of fact, the presence of these clusters is a sign of an overloading of the carbon support in metallic precursor versus the amount of metallic precursor that can be used to generate the catalytic sites. The excess of metallic precursor is transformed into metal Co or Fe particles which are able at the high temperatures of pyrolysis to catalyze around them the graphitization of carbon containing materials.48,49 It was indeed demonstrated44 that (i) very small amounts of Fe sulfate (with Fe contents of the order of a few tens of ppm) adsorbed on carbon black were already able, at 900 °C and in the presence of a suitable nitrogen precursor, to generate measurable catalytic activity for the electrochemical reduction of oxygen, and (ii) that this catalytic activity increased by rising the Fe content up to 1000 ppm (0.1 wt %), then saturated for larger contents while metallic clusters began to appear. The saturation point of the catalytic activity was related to the total amount of available nitrogen at the surface of the carbon support. Given the importance of even traces of metallic impurities in the generation of some catalytic activity at high pyrolysis temperatures, we designed another procedure to obtain the catalysts in more controlled conditions than those prevailing when carbon black (with an uncontrollable impurity source) was used. In the new procedure, the carbon support is generated during the high temperature pyrolysis step. The advantage of that procedure which uses PTCDA (perylene tetracarboxylic dianhydride, see Scheme 1) as a precursor for the carbon support is that it is possible to purify PTCDA extensively from all its metallic impurities before proceeding to the adsorption of known amounts of the metal precursor: Fe acetate. The catalyst is obtained by pyrolyzing the resulting material at 900 °C in H2+NH3 (NH3 is the nitrogen precursor in this synthesis).

J. Phys. Chem. B, Vol. 104, No. 47, 2000 11239 Results obtained with this procedure50,51 were in agreement with the previous ones: as long as the particular type of nitrogen entering into the structure of the catalytic site was available at the surface of the carbon support, the catalytic activity increased with an increase of the Fe content. Furthermore, it was demonstrated that Fe was present as an ion in the catalytic site and that this ion was mainly coordinated to nitrogen of the pyridinic type, representing about 30% of the total N content at the surface of the carbon support. Considering this information, it became interesting to perform Time-of-Flight Secondary Ion Mass Spectrometry (ToF SIMS) experiments on these catalysts especially in the low Fe content range where catalytically inactive metallic clusters are absent. Therefore, this work presents ToF SIMS results obtained for catalysts prepared with two different precursors (FeII acetate and an iron porphyrin: ClFeIII Tetramethoxy phenylporphyrin or Cl-FeTMPP). By comparing the catalytic activity of materials prepared according to various procedures and the ToF SIMS spectra of the same materials, it will be shown that the relative intensity of one particular ion varies in accordance with the catalytic activity and, therefore, that this ion is the signature of the catalytic site for O2 electroreduction for the catalysts prepared in this work. Experimental Section Purification of PTCDA. The commercial dye, PTCDA (Aldrich), was washed overnight with a 1:2 solution of deionized water (d.H2O) and concentrated HCl under magnetic stirring to remove metallic impurities present in the commercial product. The main metallic impurity was Fe at 1540 ppm ( 5% (as determined by Neutron Activation Analysis; NAA); other transition metals such as Cr, Mn, Co, Ni, and Cu were all below 5 ppm.51 The suspension was filtered, rinsed with d.H2O, and dried at 75 °C. The above step was repeated twice. This purification procedure reduces the amount of iron present in the starting product to a level below the detection limit of NAA (∼40 ppm for this element). Pyrolysis of PTCDA. A fused silica boat containing PTCDA was introduced in a fused silica tube (5 cm diameter). Ar was circulated in the tube for 30 min. Then the tube was placed in a split-oven and a gas mixture containing NH3 + H2 was added to Ar in the 2:1:1 volumetric ratio. H2 is necessary to keep a reducing atmosphere during pyrolysis. The oven temperature was set at 400 °C for 1 h. Then the oven temperature was raised to 900 °C, where it was kept for another hour. Finally, the fused silica tube was removed from the oven and cooled under Ar. Pyrolyzed PTCDA was finely ground. Preparation of the Catalysts. Two procedures were used to prepare the catalysts. Procedure I (Pyrolysis of FeII acetate or Cl-FeTMPP adsorbed on PTCDA. For each sample, a quantity of FeII acetate was mixed with PTCDA in 100 mL of d.H2O to obtain the following nominal Fe contents: 50, 100, 200, 400, 800, 1600, 3200, 6400, 12 800, or 25 600 ppm. Each mixture was placed in an oven at 75 °C for one night to evaporate H2O. Each sample was then pyrolyzed at 900 °C in NH3 + H2 + Ar in the 2:1:1 volumetric ratio. After pyrolysis, the actual Fe contents of the catalysts, measured by NAA were 300, 790, 1030, 2030, 2530, 4660, 8100, 16 460, 25 160, and 55 640 ppm ( 5%, respectively. The decomposition of PTCDA with the loss of some of the organic mass during the pyrolysis step at 900 °C is at the origin of the Fe enrichment of the pyrolized material. The same procedure was used to prepare catalysts from the adsorption of Cl-FeTMPP (Aldrich) on PTCDA in an acetone solution. In this case, amounts of Cl-FeTMPP were used to obtain the fol-

11240 J. Phys. Chem. B, Vol. 104, No. 47, 2000 lowing nominal Fe contents: 50, 100, 200, 400, 800, 1600, 3200, and 6400 ppm. The actual Fe contents, measured by NAA, were 220, 460, 860, 1880, 3690, 6990, 11 880, and 20160 ppm ( 5%, respectively. Likewise, samples of Cl-FeTMPP adsorbed on PTCDA were prepared with a nominal Fe content of 400 ppm and pyrolyzed either at 500, 600, 700, 800, 900, or 1000 °C in NH3 + H2 + Ar in the volumetric ratio 2:1:1. Procedure II (FeII acetate and Cl-FeTMPP adsorbed on pyrolyzed PTCDA). An amount of FeII acetate required to obtain a nominal Fe content of 2000 ppm in the catalyst was mixed for 1 h with a suspension of pyrolyzed PTCDA in 100 mL of d.H2O. The mixture was placed in an oven at 75 °C for one night to evaporate the H2O. The resulting material was placed in a fused silica boat and introduced in the fused silica tube previously described. The pyrolysis temperature was set at either 400, 500, 600, 700, 800, 900, or 1000 °C. The pyrolysis atmosphere was Ar + H2 in a 1:1 volumetric ratio. The actual Fe content was measured by NAA for the catalyst prepared at 900 °C, and found to be 2011 ppm ( 5%. Procedure II was also used to prepare catalysts from Cl-FeTMPP. In that case, an amount of Cl-FeTMPP required to obtain a nominal Fe content of 2000 ppm in the catalyst was mixed for 1 h with a suspension of pyrolyzed PTCDA in 100 mL of acetone. The following operations were the same as for FeIIacetate, except that only one pyrolysis temperature, 600 °C, was used. Electrochemical Measurements. The catalysts were evaluated electrochemically in half and full cells. Measurements in half cells were obtained by rotating disk electrode technique (RDE). The experimental setup and procedure are described in detail elsewhere.23, 24 Briefly, 16 mg of finely ground catalyst, 0.400 mL of d.H2O, and 0.400 mL of a 5 wt % Nafion in alcohol-water solution (Aldrich) were ultrasonically blended for 10 min. Then 10 µL of this suspension were pipetted onto the 5 mm diameter vitreous carbon disk of the electrode. The suspension was dried in air at 75 °C. RDE measurements were performed at room temperature in a three electrode, one compartment cell containing H2SO4 (pH ) 1) as the electrolyte. It was saturated with O2 prior to the start of an experiment. Cyclic voltammograms were recorded at a scan rate of 10 mV/s between -0.3V and 0.7 V vs SCE. The measurements in full cells were obtained in a fuel cell test station. The catalyst suspension consisted of 12.9 mg of catalyst, 0.500 mL of d.H2O, and 0.300 mL of 5 wt % Nafion solution blended ultrasonically for 1 h. The anode consisted of a 1 cm2 ELAT electrode catalyzed with 0.37 mg/cm2 (20 wt %) Pt from ETEK. The cathode consisted of a 1 cm2 uncatalyzed ELAT electrode from ETEK. Two layers of the catalyst suspension were deposited on the active side of the cathode. Each layer was applied by pipetting 60 µL of the catalyst suspension onto the cathode, and then spreading it uniformly over the entire surface. A hot plate was used to accelerate the drying of the catalyst suspension between each application. The anode received one paint brush applied coating of 5 wt % Nafion solution. Both electrodes were then placed in a vacuum oven at 75 °C for 1 h. The amount of dried Nafion applied to the anode varied from 0.5 mg to 1.0 mg. The amount of dried catalyst applied to the cathode varied between 4 mg and 5 mg. Of this amount, 52% was calculated to be dried Nafion, the rest consisted of the catalyst itself. This calculation was done on the basis that 0.120 mL of Nafion suspension yields 5.5 mg of dried Nafion. A single cell assembly was prepared by pressing a Nafion 117 membrane between the anode and the cathode under 2500 pounds at 140 °C for 40 s. The same procedure

Lefe`vre et al. was used to prepare the reference membrane electrode assembly (MEA) using 2 wt % Pt from ETEK at the cathode. The cathode Pt loading for the reference MEA is therefore ∼0.04 to 0.05 mg Pt/cm2. All fuel cell measurements were performed at 80 °C. The O2 and H2 gas pressures were 60 and 30 psig, respectively. The O2 and H2 gas flow rates were 360 and 230 cm3/min, respectively. The two gases were humidified prior to admission into the fuel cell by passing them through stainless steel containers filled with d.H2O kept at 105 °C. Before performing any measurements, the fuel cell was left under open circuit conditions for 1 h. Next, the impedance of the fuel cell assembly was measured. Typical values obtained varied from 0.2 to 0.4 Ω. Then, a polarization curve was recorded by varying an applied potential from 0.9 to 0V vs RHE. This was followed by the application of a steady-state potential of 0.5V vs RHE in order to ascertain the stability of the catalyst. Surface Analysis. The surface of the catalyst was analyzed by ToF SIMS (Charles-Evans and Associates) using Ga+ 15 keV primary ions with a mass resolution of 10 000 for Si on a Si wafer. For the samples, a resolution of about 4000 at the Si mass was obtained. For the analysis, the catalysts were pressed into an In sheet and regions were analyzed in positive and negative ions. A post acceleration of 6 keV was used with a 0-10000 mass range analysis. The delivered dose was below 1012 ions/cm2, remaining therefore in the static SIMS range. Results and Discussion The goal of this work is to obtain information about the hightemperature site. To do so, we used two experimental procedures to prepare the catalysts. In the first procedure, an Fe precursor (Fe acetate or Cl-FeTMPP) is adsorbed onto PTCDA and the resulting material is pyrolyzed at various temperatures in H2 + NH3. In the second procedure, an Fe precursor (Fe acetate or Cl-TMPP) is adsorbed onto already pyrolized PTCDA at 900 °C in H2 + NH3. The resulting material is then submitted to a second pyrolysis in H2+Ar. The catalytic activity of all materials will first be analyzed. Then common trends will be searched between the catalytic activity and the occurrence of specific secondary ions that may be related to the catalytic site. 1. Catalytic Activity after Pyrolysis at 600 °C; Comparison between Procedures I and II. The difference between the two procedures is illustrated by the following experiments in which two materials are prepared from PTCDA and Cl-FeTMPP according to procedures I or II and a final pyrolysis temperature at 600 °C. The cyclic voltammograms obtained by RDE illustrates the catalytic activity of both materials. They are presented in Figure 1. The disk electrode is not rotated during the first scan giving only one peak for O2 reduction in O2 saturated H2SO4. The second scan is recorded at 1500 rpm. It shows an improvement of the reduction current due to an increase of O2 available at the electrode in rotation. It is possible to estimate the relative catalytic effect of both materials by measuring the voltage (Vpr) at which the maximum reduction current occurs at 0 rpm. The material prepared according to procedure I is practically inactive (Vpr ) -140 mV), whereas the one prepared according to procedure II is a catalyst (Vpr ) 415 mV) because values closest to the theoretical reversible potential of 0.985 V vs SCE indicate superior catalytic activity. The difference between these two materials is related to the properties of PTCDA. The latter molecule polymerizes into carbon fibers at temperatures above 520 °C by loosing its carboxylic moiety.52 Doing so, it decomposes into its elements adsorbed Cl-FeTMPP or any adsorbed fragment of Cl-

O2 Reduction in PEM Fuel Cells

Figure 1. RDE voltammograms, at 0 and 1500 rpm in O2 saturated H2SO4, of two materials prepared from PTCDA and Cl-FeTMPP according to procedures I or II and a final pyrolysis temperature at 600 °C.

J. Phys. Chem. B, Vol. 104, No. 47, 2000 11241

Figure 3. Polarization curves obtained at 80 °C in H2/O2 fuel cell using, at the cathode, catalysts prepared according to procedure I with Fe acetate or Cl-FeTMPP as Fe precursors.

Figure 4. Changes observed by RDE in the catalytic activity of materials prepared according to procedure II and at various heattreatment temperature, with Fe acetate as Fe precursor. Figure 2. Changes in RDE of the catalytic activity for O2 reduction, expressed as Vpr (Voltage at peak reduction for 0 rpm) with the Fe content for materials prepared according to procedure I with Fe acetate or Cl-FeTMPP as Fe precursors. Arrows indicate catalysts that will be used in fuel cell tests (Figure 3).

FeTMPP (including its Fe-N4 moiety). The material obtained after completion of the pyrolysis step at 600 °C is inactive. The catalytic properties of this material are as poor as those obtained by pyrolyzing at 600 °C and in H2 + NH3, Fe acetate adsorbed on PTCDA.50 (Catalytically active materials are only obtained with Fe acetate at higher pyrolysis temperatures (g 800 °C); similar results are therefore also expected with Cl-FeTMPP as Fe precursor in procedure I.) On the other hand, the material prepared by procedure II is catalytically active because PTCDA has been pyrolyzed first above its polymerization temperature to obtain a carbon support on which Cl-FeTMPP was then adsorbed. The catalytic properties of this material are similar to those obtained by pyrolyzing at 600 °C in Ar an FeII porphyrin adsorbed on carbon black.53 According to van Veen et al.,15,33

in these conditions, the catalytic site is Fe-N4 bound to the carbon support. 2. Catalytic Activity after Pyrolysis at 900 °C (Procedure I with Fe Acetate and Cl-FeTMPP). Figure 2 presents the catalytic activity measured by RDE of materials prepared with various Fe contents at 900 °C according to procedure I. The Fe precursor is either Fe acetate of Cl-FeTMPP. The two curves in Figure 2 are similar, and the following observations are made: (i) the catalytic activity of these materials improves drastically as soon as the Fe content on PTCDA is increased to a few tens of ppm; (ii) the catalytic activity saturates at about the same potential for both Fe precursors; (iii) the catalytic activity saturates at lower Fe contents for Fe acetate than for ClFeTMPP. This is due to a better dispersion of the Cl-FeTMPP on PTCDA. TEM results confirm this interpretation. Indeed, Fe clusters are detected by TEM for Fe loadings of Fe acetate as low as 5000 ppm (0.5 wt %), whereas clusters begin to appear for Fe loadings of 2 wt % when the Fe precursor is ClFeTMPP.

11242 J. Phys. Chem. B, Vol. 104, No. 47, 2000

Lefe`vre et al.



Figure 5. ToF SIMS positive ion spectra of (A) PTCDA pyrolyzed at 900 °C in H2 + NH3; (B) Vulcan XC-72R carbon black; (C) a catalyst obtained by procedure II using Fe acetate as Fe precursor and a second pyrolysis temperature of 800 °C.

Figure 3 presents the H2/O2 fuel cell polarization curves obtained in single membrane electrode assembly for some of the catalysts whose RDE measurements have been presented in Figure 2 (the chosen catalysts are marked with an arrow in Figure 2). A polarization curve with 2 wt % Pt on Vulcan (E-Tek) is given for comparison. When Figures 2 and 3 are compared, it is noticed that a smaller catalytic activity measured for Fe acetate in RDE corresponds to a less performing behavior in fuel cell. The same is also true for Cl-FeTMPP. Fuel cell performance of Cl-FeTMPP with 2 wt % Fe content is the best one recorded so far in our laboratory. It is approaching that of Pt 2 wt % on Vulcan measured in the same conditions. 3. Catalytic Activity and ToF SIMS Measurements after Pyrolysis at Various Temperatures (Procedure II with Fe Acetate). The Fe content of the catalysts in this section is 2000 ppm. This value is below the Fe content (∼5000 ppm) at which Fe clusters are detected at high pyrolysis temperature. When Fe content is constant, the only way to induce changes in the catalytic activity is by varying the pyrolysis temperature. RDE results will first be presented, followed by a ToF SIMS analysis of the same materials. Figure 4 presents changes observed by RDE in the catalytic activity of materials prepared according to procedure II with Fe acetate as Fe precursor. In that procedure, PTCDA is first pyrolyzed at 900 °C then the Fe precursor is loaded and the resulting material is divided into 8 parts, each part being heattreated a second time at a different temperatures from 20 to 1000 °C. The maximum activity occurs around 800 °C. The idea in recording ToF SIMS spectra of these samples was to

see if one or several secondary ions were following the relative intensity recorded in RDE experiments for the same catalysts (Figure 4). These ions would therefore be related to the catalytic site in action. Before entering into the detail of the ToF SIMS spectra of the catalysts, it is necessary to examine first the ToF SIMS spectrum of pyrolyzed (900 °C) PTCDA and justify our decision to use this carbon support rather than carbon black. Besides the fact already mentioned that PTCDA is a simple commercial molecule easy to purify from its metallic contaminants, purified PTCDA is also an ideal carbon support for ToF SIMS analysis because its positive ion spectrum displays a simpler background than the equivalent spectrum of Vulcan XC-72R carbon black. Both positive ion spectra are presented in Figures 5A and 5B, respectively. Each spectrum is composed of four consecutive ranges of 50 amu (atomic mass unit) each to encompass masses between 1 and 200 amu. The spectrum of pyrolyzed PTCDA contains less major ions than the spectrum of carbon black. The ‘y’ scale, which characterizes the total ion counts for ions in carbon black, is always larger than the equivalent ‘y’ scale for pyrolyzed PTCDA. Furthermore, the difference between the two ‘y’ scales increases as the amu of the ions increases. As an example, the maximum peak intensity in the 150-200 amu range for carbon black is more than 10 times that of pyrolyzed PTCDA. This is an important factor in the ToF SIMS analysis of the catalysts since small contributions of the catalytic site to an ion peak in the spectrum might be completely obliterated by the larger background of that ion in the carbon black spectrum. To be more specific, for instance, the peaks related to the aromatic


Lefe`vre et al.

Figure 6. Relative intensities of selected ions of catalysts obtained by procedure II using Fe acetate as Fe precursor and a second pyrolysis temperature ranging from 400 to 1000 °C; (A) positive and negative ions containing carbon; (B) positive and negative ions containing simultaneously C and N; (C) positive Fe ions.

ions appearing at 77, 91, 105, 115, 128, 141, 152, 165, 178, and 185 amu54 are more intense in the spectrum of carbon black than in the spectrum of pyrolyzed PTCDA. So are the peaks related to inorganic impurities: 39 (Ca), 40 and 42 (K). The large peaks at 113 and 115 amu are those of the In sheet on which both powders were pressed to be analyzed. Peaks at 28, 43, 73, 117, 147, and 191 amu are due to poly(dimethylsiloxane) (PDMS),54 a very common surface contaminant due to its very low surface free energy. There is no equivalent difference between the background of the negative ion spectrum of pyrolyzed PTCDA and that of carbon black. Figure 5C presents a typical positive ion spectrum of a catalyst obtained by procedure II, using Fe acetate as Fe

precursor and a second pyrolysis at a temperature of 800 °C. Changes in the intensities with the various heat-treatment temperature were monitored for several ions: (i) examples of positive and negative ions containing carbon are displayed in Figure 6A; (ii) examples of positive and negative ions containing simultaneously C and N are displayed in Figure 6B; (iii) the positive Fe+ ion is displayed in Figure 6C. All relative intensities reported in Figure 6 are obtained according to the relation

Relative Intensity′ of X+/- ) Intensity of X+/-/Intensity of Carbon+/- at 12 amu (1) where ( means either the positive or negative ion. It is obvious


Figure 7. Relative intensities of FeNC+ in catalysts obtained by procedure II using Fe acetate as Fe precursor and a second pyrolysis temperature ranging from 400 to 1000 °C; (A) Relative intensity expressed according to relation 1; (B) Relative intensities expressed according to relation 2; (C) Relative intensities expressed according to relation 2 for another ion, FeN2C+, detected in the same catalysts.


Figure 8. Comparison between changes, as a function of the pyrolysis temperature, of the catalytic activity and the relative intensity of FeN2C4+ measured in catalysts obtained by procedure II using Fe acetate as Fe precursor and a second pyrolysis temperature ranging from 400 to 1000 °C.

that none of the relative intensities of the ions in Figure 6 changes as the catalytic activity depicted in Figure 4. In our next attempt to find ions originating from the catalytic site, we recalled that the simultaneous presence in the reactor of carbon, nitrogen, and iron was a requirement to obtain catalytic activity.23 The same requirement was also verified for Co.46 Therefore, our next target was ions containing simultaneously C, N, and Fe. Figure 7A displays the relative intensity of FeNC+ as defined by eq 1. Again, the intensity in Figure 7A does not change as the catalytic activity in Figure 4. For reasons that will become obvious later on, it was decided to express the relative intensity of the ions containing C, N, and Fe according to the relation

Relative Intensity of FeNxCy+ ) Intensity of FeNxCy+/Σ Intensity of FeNxCy+ (2) In this case, the following ions were considered: (i) ions containing one Fe atom (there was no evidence of ions of the type FezNxCy); (ii) ions containing up to four nitrogen atoms; and (iii) ions containing up to 12 carbon atoms. Figure 7B presents the results of 7A expressed according to relation 2. Both figures are similar. The evolution of FeN2C2+ is presented in Figure 7C. Again, the relative intensity for this ion does not change as the catalytic activity in Figure 4. Among all the ions considered, only FeN2C4+ displays an evolution, which is similar to that of Figure 4. The catalytic activity for oxygen reduction and the relative intensity of FeN2C4+ in the same material are both illustrated in Figure 8. Except for FeNC+, FeN2C2+, and FeN2C4+, all other FeNxCy ions have very low occurrences (only a few counts) whereas FeN2C4+ reaches up to 45 counts. FeN2C4+ is therefore a genuine ion issued from the catalytic site. FeN2C4H+ is also observed. Its behavior is similar to that of FeN2C4+. 4. Catalytic Activity and ToF SIMS Measurements after Pyrolysis at Various Temperatures (Procedure I with ClFeTMPP). In this section, Cl-FeTMPP is loaded onto PTCDA to obtain a final Fe content of about 2000 ppm. In this case, the only way to vary the catalytic activity is to change the

Figure 9. Comparison between changes, as a function of the pyrolysis temperature, of the catalytic activity and relative intensity of FeN2C4+ measured in catalysts obtained by procedure I using Cl-FeTMPP as Fe precursor and a pyrolysis temperature ranging from 500 to 1000 °C.

pyrolysis temperature (in H2 + NH3) of the materials. All ions containing FeNxCy were monitored, and in this case, the situation is more complex than in the previous section because after a heat treatment at 500 °C, the following ions were detected with a relative intensity much higher than that of FeN2C4+. The detected ions were: FeN3C1+: FeN3C2+, FeN3C4+, FeN3C5+, FeN4C6H+, FeN4C8H+, FeN4C10H+. When the heat treatment was performed at a temperature g 600 °C, the intensity of all these ions decreased drastically. At the same time, the relative intensity of FeN2C4+ increased as seen in Figure 9. The disappearance of all FeNxCy+ ions except for FeN2C4+ is not difficult to understand if one remembers the experiment described in section 1. Indeed, the catalytically inactive material in section 1 is the same than the material heat-treated at 600 °C in this section. In section 1, it was concluded that the


Figure 10. Comparison between changes, as a function of the Fe content, of the catalytic activity and the relative intensity of FeN2C4+ measured in catalysts obtained by procedure I using Cl-FeTMPP as Fe precursor and a pyrolysis temperature of 900 °C. The dark triangle is the blank.

polymerization of PTCDA starting above 520 °C decomposed all molecules adsorbed on PTCDA into their elements. In these conditions, on one hand, the plethora of FeNxCy ions detected by ToF SIMS after a heat-treatment at 500 °C is understood because PTCDA is still intact at that temperature and all FeNxCy ions have their origin in what remains at that temperature of Cl-FeTMPP adsorbed on PTCDA. It is important to notice here that FeN2C4+ is not a major ion arising from the fragmentation during SIMS analysis of Cl-FeTMPP or its fragments. On the other hand, increasing the heat treatment temperature to g 600 °C induces the polymerization of PTCDA therefore decomposing all remaining fragments of Cl-FeTMPP into their elements. These elements will combine to produce a catalytic site, which is again characterized by the occurrence of FeN2C4+. Comparing the catalytic activity and relative intensity of FeN2C4+ in Figure 9, one may wonder why the materials prepared at 600 and 700 °C are quite inactive catalytically, whereas the relative intensity of FeN2C4+ at the same temperatures are already high. One possible explanation is related to the organization of the carbon support, which may not be fully conductive yet at these temperatures. An argument to consolidate this hypothesis is obtained from the RDE results reported in Figure 8. In that case, the catalytic site already functions at 600 °C. The difference between the two materials is that in Figure 8, the Fe precursor is loaded on already pyrolyzed PTCDA at 900 °C. 5. Catalytic Activity and ToF SIMS Measurements after Pyrolysis at 900 °C (Procedure I with Cl-FeTMPP). The analysis of FeNxCy ions has been performed on these catalysts for which Cl-FeTMPP is loaded onto PTCDA, and the resulting material is then pyrolyzed at 900 °C in H2+NH3. In this case, the only way to vary the catalytic activity is to change the Fe content. Again, FeN2C4+ was the only ion present and the changes in its relative intensity vs the Fe content are plotted in Figure 10. A similar trend is observed in Figure 10 between changes in the catalytic activity and FeN2C4+ relative intensity. Conclusions In this work, the following points have been demonstrated: 1. FeN2C4+ is the ToF SIMS signature of the catalytic site obtained at pyrolysis temperatures g 600 °C, by using procedure II with Fe acetate as Fe precursor. 2. FeN2C4+ is also the ToF SIMS signature of the catalytic site obtained at pyrolysis temperature g 800 °C, using procedure

Lefe`vre et al. I with Cl-FeTMPP as Fe precursor. This result is not surprising because it has been demonstrated that in procedure I, the polymerization of PTCDA induces the decomposition of ClFeTMPP (including the N4-Fe-Cl moiety) into its elements. In these conditions, the similarity between the two Fe precursors is therefore expected. It is proposed that the catalytic site characterized by the FeN2C4+ ToF SIMS signature is the high-temperature catalytic site as defined in the Introduction. Further experiments will be necessary to ascertain this hypothesis and show that the same signature is also found for catalysts prepared on carbon black. First, it was necessary to use PTCDA as precursor for the carbon support because it is characterized by a low background ToF SIMS signal. Now that the signature of the catalytic site has been identified, the interpretation of the ToF SIMS spectra of catalysts using a carbon black support will be facilitated. The origin of the FeN2C4+ ion is a structure for which an Fe ion is complexed by two nitrogens, which are themselves, bound to the carbon support. The occurrence of this structure is not surprising because it is found, for instance, in Fe complexes of the 1,10 phenanthroline derivatives which are used in the analytical determination of FeII and FeIII.54-57 Both nitrogens in the 1,10 phenanthroline structure are of the pyridinic-type (i.e., a N atom contributing to the π band with one electron). This is the main type of nitrogen, which is interacting with the Fe ion in the catalysts.50,51 It is therefore proposed that part of the catalytic high-temperature active site is made of two adjacent nitrogen of the phenanthrolinic type at the edge of the graphene layer of the carbon support. These two nitrogens are complexing the Fe ion. It is still unknown how the structure of the catalytic site is completed. It might be as simple as with CN ligands (CN- is one of the main negative ions detected by ToF SIMS in the negative ion spectrum of these materials), but more complex structures are not excluded. For instance, one of these might be a second phenanthrolinic group from another graphene plane, ending up with a N4-Fe non planar site. Acknowledgment. The authors want to thank C. Poleunis for his technical help with SIMS measurements. They also acknowledge the “ Que´bec/Wallonie-Bruxelles ” cooperation program for their support. References and Notes (1) Adzic, R. R.; Anson, F. C.; Kinoshita, K. In Proceedings of the Symposium on Oxygen Electrochemistry; The Electrochemical Society, Inc.: Pennington, NJ, 1966; 95-26, 3. (2) Gottesfeld, Sh.; Zawodzinski, T. A. AdV. Electrochem. Sci. and Eng. 1997, 5, 195. (3) Bernardi, D.; Verbrugge, M. W. J. Electrochem. Soc. 1992, 139, 2477. (4) Cowley, A., Ed. In Platinum 1999; Johnson Matthey: London, p 3. (5) Jasinski, R. Nature 1964, 201, 1212. (6) Wiesener, K. Electrochim. Acta 1986, 31, 1073. (7) Wiesener, K.; Ohms, D.; Neumann, V.; Franke, R. Mater. Chem. Phys. 1989, 22, 457. (8) Tarasevich, M. R.; Zhutaeva, G. V.; Radyushkina, K. A. Russ. J. Electrochem. (Trans. of Electrokhimiya) 1995, 31, 1064. (9) Tarasevich, M. R.; Radyushkina, K. A. Mater. Chem. Phys. 1989, 22, 477. (10) Scherson, D.; Tanaka, A. A.; Gupta, S. L.; Tryk, D.; Fierro, C.; Holze, R.; Yeager, E. B.; Latimer, R. P. Electrochim. Acta 1986, 31, 1247. (11) Tanaka, A.; Gupta, S. L.; Tryk, D.; Fierro, C.; Yeager E. B.; Scherson, D. A. In Structural Effects in Electrocatalysis and Oxygen Electrochemistry; Scherson, D., Tryk, D., Daroux, M., Xing, X., Ed.; The Electrochemical Society Inc.: Pennington, NJ, 1992; 92-11, 555. (12) Gojkovic, S. Lj.; Gupta, S.; Savinell, R. F. J. Electrochem. Soc. 1998, 145, 3493.

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Reduction in PEM Fuel Cells - American Chemical Society

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