Non-Noble Electrocatalysts for O2 Reduction: How Does Heat

J. Phys. Chem. C 2007, 111, 5971-5976

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Non-Noble Electrocatalysts for O2 Reduction: How Does Heat Treatment Affect Their Activity and Structure? Part II. Structural Changes Observed by Electron Microscopy, Raman, and Mass Spectroscopy Fre´ de´ ric Jaouen,*,† Alessandra Maria Serventi,† Michel Lefe` vre,† Jean-Pol Dodelet,*,† and Patrick Bertrand‡ INRS EÄ nergie, Mate´ riaux et Te´ le´ communications, 1650 BouleVard Lionel Boulet, Varennes (Que´ bec) Canada, J3X 1S2 and PCPM, UniVersite´ Catholique de LouVain, 1348 LouVain-la-NeuVe, Belgium ReceiVed: December 1, 2006; In Final Form: February 28, 2007

In part I of this paper, a model was proposed to (i) describe the gasification of a spherical carbon black particle by ammonia at high temperature and (ii) explain the activity for the O2 electroreduction of catalysts based on Fe and carbon heat-treated in NH3. In this second part, such catalysts, heat-treated for various times in ammonia, are characterized by electron microscopy, Raman spectroscopy, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Electron microscopy shows an average particle diameter of 38 nm for the pristine furnace black, decreasing to 28 nm after the carbon powder loses 90% of its mass. This agrees with model prediction. Raman spectroscopy shows that the experimental width at half-maximum of both the graphitic peak (∼1595-1600 cm-1) and the disorder and graphitic-edge peak (∼1345-1355 cm-1) constantly decreases with increasing weight loss. It is found that the width at half-maximum of these peaks correlates with the fraction of disordered carbon phase calculated by the model. Last, changes of the ToF-SIMS total and Fe+ signals upon gasification of the carbon black also support the model.

1. Introduction The origin for this work and the interest in developing nonnoble catalysts for polymer electrolyte fuel cells (PEFC) were already introduced in part I of this paper. Here is a summary. Non-noble catalysts for O2 electroreduction in PEFC are obtained by adsorbing small quantities of Fe or Co (0.2-1 wt %) onto a carbon black followed by a heat treatment in ammonia at 900-1000 °C. The core of the active sites is an FeN4 or a CoN4 moiety where N atoms are either of pyridinic1-3 or pyrrolic type.4-7 However, the moiety is covalently bonded to the carbon support in a yet unknown fashion. Recently, we studied the evolution of the activity for the O2 reduction reaction (ORR) of catalysts obtained after various times of heat treatment in NH3 at 950 °C.8 The activity was found to initially increase by two decades to reach a maximum and, upon longer reaction times, to decrease by a factor 4-5 relative to the maximum. Here it is stressed that the carbon black is gasified by NH3. Therefore, the microstructure and surface topology of the catalysts change tremendously with the time of heat treatment. The evolution of the ORR catalytic activity with time of reaction was mimicked by that of the micropore specific area. From these results, it was concluded that (i) active sites of such catalysts may only be formed in micropores of the carbon black and (ii) the density of micropores in a carbon black presently limits the density of active sites (i.e., it limits the overall activity of such catalysts). Thus, our attention then focused on gaining a better understanding of the evolution of micropores in a carbon black upon reaction with ammonia at high temperature. With this aim, * Corresponding authors. (F.J.) E-mail: [email protected]. Fax: +1 450 929 8102. (J.-P.D.) E-mail: [email protected]. Fax: +1 450 929 8198. † INRS E Ä nergie, Mate´riaux et Te´le´communications. ‡ Universite ´ Catholique de Louvain.

a model was developed and presented in part I of this paper. The model predicts (i) the overall rate of reaction of a carbon black particle with NH3 and (ii) the induced structural changes. The model is based on the premise of two carbon phases; ordered (graphite crystallites) and disordered. The latter reacts faster with NH3, enabling the creation of an internal pore network. The fraction of disordered phase initially present in the particle as well as kinetic parameters of the reaction of each carbon phase with ammonia were assessed by fitting the model to experimental curves of specific area and weight loss percentage versus time of heat treatment in ammonia. The objectives of the present paper are (i) to experimentally investigate, as a function of the time of reaction with NH3, the particle size, Raman signal, and fraction of disordered phase in the Fe- and carbon black-based catalysts for the ORR and (ii) to quantitatively compare the experimental values (particle size, time-of-flight secondary ion mass spectrometry (ToF-SIMS signal)) to the values predicted by the model (part I of this paper). In the remainder of this paper, the modeling results refer exclusively to the calculation based on the model presented in part I with the parameter values found in Table 1 of part I except for the initial value of fraction of disordered phase, which is assumed to vary according to the profile shown in Figure 5D of part I (the fraction of disordered phase is 0.35 in the center of the particle and only 0.20 on the periphery). This profile was shown to best reproduce the evolution of the experimentally measured microporous specific area (part I). 2. Experimental Section 2.1. Catalyst Synthesis. The catalysts investigated here are the same samples that were investigated in ref 8 (heat treatment temperature 950 °C) and the same that the model parameters were calibrated to in part I of this paper. The catalysts are all

10.1021/jp068274h CCC: $37.00 © 2007 American Chemical Society Published on Web 04/04/2007

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Figure 2. Particle size distributions obtained from SEM micrograph analysis. Same samples A, B, and C as shown in Figure 1.

Figure 3. Average particle diameter versus percentage of weight loss during heat treatment in NH3. Model prediction (solid line), experimental measurement from SEM micrographs (9), and diameter calculated from the simple hypothesis of a shrinking core (b). The experimental volume-averaged diameter, dav, was calculated from daV N ) (Σi)1 di3)1/3/N1/3 in which di is the experimental diameter of particle i and N is the total number of particles in each distribution. The diameter d (in nanometer), corresponding to a simple shrinking-core hypothesis, was calculated from d ) 6000/(FS) with F ) 2 g cm-3 and S in m2 g-1 measured experimentally by multipoint BET analysis of N2 adsorption isotherms.

Figure 1. SEM micrographs of (A) the pristine furnace black, (B) a catalyst having lost 47% of its carbon mass through reaction with ammonia, and (C) a catalyst having lost 87% of its carbon mass.

based on a single furnace black in loose form having an initial Brunnauer-Emmett-Teller (BET) area of 71 m2 g-1. This furnace black is a developmental product from the Sid Richardson Carbon Company. The catalysts were prepared by adsorbing in water solution 0.2 wt % Fe as iron acetate onto the pristine furnace black, stirring the solution and drying it overnight before heat-treating the resulting powder in NH3 at 950 °C for various times. This paper investigates the effect of the time of heat treatment on the structure of the furnace black

particles that are found in the catalysts. More details about the catalyst synthesis procedure are found in ref 8. 2.2. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and High-Resolution TEM (HRTEM). SEM images were obtained with a field emission Hitachi S-4700 microscope at a low voltage of 2 kV to avoid charge effects. The micrographs were analyzed to retrieve particle size distributions. One micrograph was chosen for each time of reaction in ammonia, and as many spherical particles as possible were identified, in the range of 130-190 particles per micrograph. TEM observations were performed with a Hitachi H-9000NAR microscope at an accelerating voltage of 300 kV. HRTEM images were obtained with a JEOL JEM 2100F operated at 200 kV.

Non-Noble Electrocatalysts for O2 Reduction, Part II

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Figure 4. (A) fwhm of the D peak seen in experimental Raman spectra against the percentage of weight loss (9). (B) Model-predicted fraction of disordered carbon phase in the total carbon solid phase against percentage of weight loss.

2.3. Raman Spectroscopy. The equipment and analysis procedures are identical to those used in ref 9. Raman spectroscopy was performed in backscattering mode on catalytic powders with a Renishaw Raman microscope and a 514.5 nm laser. The light power was 5 mW and the beam was focused with a 50× objective. The spectra were analyzed by fitting with four symmetric Lorentzian peaks, although only two peaks are visible (a typical spectrum is found in ref 9). Recently, Sadezky and co-workers studied soot by Raman spectroscopy.10 Nine fitting procedures were tested differing by either the number of peaks (between 3 and 5) or the peak shape (Lorentzian or Gaussian). The fitting procedure employed here corresponds to the second best fitting procedure investigated by these authors, differing from the best one only in the shape of the peak found at ∼1530 cm-1. 2.4. ToF-SIMS. The surface of the catalysts was analyzed by ToF-SIMS (Charles-Evans and Associates) using Ga+ 15 keV primary ions. For the catalyst samples, the resolution on the ion mass is m/∆m ∼ 4000. The catalysts were deposited on an Ag foil precoated with conductor glue. Four regions were analyzed for each sample in positive and negative ions and a mean value was obtained. A 7 keV postacceleration was used with a 0-10 000 mass range analysis. The delivered dose was below 1012 ions/cm2, remaining therefore in the static SIMS range. The absolute intensity used in this paper is the count of all ions positively charged, I+, except for H+ ions and polydimethylsiloxane ions. 2.5. Neutron Activation Analysis. Analysis of the bulk Fe content of the catalysts was made by neutron activation analysis at the EÄ cole Polytechnique de Montre´al. 3. Results 3.1. Change of the Particle Structure with Time of Reaction in Ammonia. The model calculations (part I of this paper) predicted that the average particle diameter does not decrease much up to a weight loss percentage of 80%. A mere 20% reduction relative to the initial value was predicted. This startling prediction required a thorough experimental investigation. The distribution of particle sizes of the powder after various times of heat treatment in NH3 was obtained from the analysis of SEM micrographs. Figure 1 shows SEM micrographs of the pristine carbon black (Figure 1A) of a catalyst having lost 47% of its carbon mass (Figure 1B) and of a second catalyst having lost 87% of its mass (Figure 1C). At first sight, there is no salient

Figure 5. HRTEM micrographs for (A) the pristine furnace black and for catalysts having lost a mass percentage of carbon during the heat treatment in NH3 of (B) 47% and (C) 87%.

difference between these three micrographs despite the chasm between their specific areas: 70, 690, and 860 m2 g-1 for the samples of Figure 1A, 1B, and 1C, respectively. The distribution of particle sizes obtained from the same micrographs is presented in Figure 2. Clearly, large particles (d > 40 nm) are more frequently observed at 0 wt % loss than at 87 wt % loss. Also, the diameter for which a maximum number of particles is observed has been shifted from about 42 nm (0 wt % loss) down to 28 nm (87 wt % loss). The information obtained from the particle size distributions is presented in a more condensed form in Figure 3. The average particle diameter (9) is plotted against the weight loss percentage (the latter being of course an increasing function of the time of heat treatment). The solid line is the model prediction for a single particle of initial diameter 41.8 nm. The experimental and predicted diameters

5974 J. Phys. Chem. C, Vol. 111, No. 16, 2007 agree although there is an offset of about 5 nm between the two curves. This offset mainly arises because the initial diameter d0 (subscript 0 for 0% weight loss) used in the model was obtained not from a SEM micrograph but from the equation d0 ) 6000/(FS0); where S0 is the specific area of the pristine carbon black (0% weight loss, 71 m2 g-1) and F the particle density (assumed 2 g cm-3). This equation is valid only for spherical particles without internal pores;11 thus it applies only at time zero of reaction with NH3 in this study. The offset between the two curves of Figure 3 might also come from the difficulty in distinguishing individual particles from aggregates of particles in SEM micrographs. This applies especially to large particles because of their low curvature. Overall, Figure 3 supports the model calculations: Particle shrinking is minor up to 40-50% weight loss according to both the model and to the SEM analysis. This sustains the idea that the carbon mass that is gasified between 0 and 50% weight loss mainly originates from the inside of the particle and not from the outer surface of the particle. Moreover, the experimental diameters observed with SEM clearly rule out the possibility of a simple shrinking-core model, where no internal pores would be created and the carbon mass would be peeled off successively from the outer spherical layer. Such a hypothesis leads to very small theoretical diameters (Figure 3, b). For example, at 50% weight loss, the powder exhibits a BET area, noted S, of 700 m2 g-1. Thus, according to a pure shrinking-core model, the average particle diameter should be d ) 6000/(FS) ) 4.3 nm, with F ) 2 g cm-3. Such a value is out of the range of the particle diameters observed with SEM (Figure 3, 9). Raman spectroscopy is a powerful technique to follow changes in the structure of carbon blacks. Two peaks are visible on such spectra (Figure 1 in ref 9): the graphitic peak, which is a vibration mode of the benzene rings allowed at any location in the crystals (E2g mode, with change in bond angle, 15901600 cm-1), and a second peak, which is due to a vibration mode allowed only at edges of the graphitic crystallites (breathing mode, 1340-1360 cm-1) or in smaller aromatic hydrocarbon molecules. The latter peak has been somewhat misleadingly labeled as the D-peak in the literature with D standing for disorder. For carbon blacks, the intensity of this peak is probably the sum of two intensities: one coming from crystallites edges, which is not considered disorder per se, and another one coming from the disordered carbon phase that was defined in the model described in part I of this paper. Looking at the experimental values of the full width at half-maximum (fwhm) of the D peak, wD, one observes a constant decrease with increasing weight loss (Figure 4A). The width of the D peak, wD, decreases from 215 to 150 cm-1 between 0 and 90 wt % loss and the width of the G peak, wG, (not shown) decreases from 75 to 57 cm-1. On the other hand, the peak positions are nearly constant, as well as the ratio of the height of the G peak to that of D peak. No theory exists for interpreting changes in peak widths of such materials. The Tuinstra and Koenig equation12 relates the ratio of areas (product of peak width and height) below the G and D peaks to the crystallite size, but peak widths are not separately considered. Thus, the variation of the widths of the G and D peaks is unexpected from a theoretical viewpoint. Intuitively, the thinner a peak is, the more ordered the material is. Thus, one could think that the decrease of wD and wG to be related to the decrease of the fraction of disordered carbon phase. Indeed, if one plots the model-predicted volume fraction of disordered carbon phase in the total carbon volume against the predicted percentage of weight loss, the shape of that modeled curve (Figure 4B) is

Jaouen et al.

Figure 6. (A) Absolute signal of all positive ions (except H+ and polydimethylsiloxane ions) measured by ToF-SIMS (9) against percentage of weight loss during heat treatment. (B) Absolute signal of aromatic ions (C6H5+, C7H7+, C8H9+, C9H7+, C10H8+, C11H9+, C12H8+, C13H9+) measured by ToF-SIMS (9) against percentage of weight loss. In both (A) and (B), the solid line corresponds to the model prediction. The calculation is explained in section 3.1.

very similar to the shape of the experimental curve wD versus percentage of weight loss (Figure 4A). In conclusion, we propose that for carbon blacks the broader the D peak (or G peak) is, the larger the fraction of disordered carbon phase in the material is. This result enables us to rationalize a previous finding: the catalytic activity obtained from such catalysts based on various carbon blacks (initial BET ranging from 70 to 1500 m2 g-1) increased with increasing width of the D peak of these carbon blacks before reaction with NH3.9 This result is now explained as follows: the broader the D peak in the pristine carbon black, the more disordered phase there is in the carbon prior to heat treatment. When heat-treated at high temperature, a carbon black with high-disordered phase content will give rise to high micropore specific area due to the removal of disordered carbon located between the ordered crystallites. Because active sites for O2 electroreduction of such catalysts are formed only in micropores,8 such a carbon will give a good catalytic activity after heat treatment. The samples were also analyzed using HRTEM to observe where and how the micropores are formed inside the carbon black particles. Surprisingly, the HRTEM micrographs of the different samples (0 to 87% weight loss; corresponding BET areas of 70 to 860 m2 g-1) are very similar (Figure 5). This can only be explained by the fact that HRTEM images are the result of a superposition of layers of graphitic crystallites occupying a volume (i.e. the images are not a cross-sectional view). This would prevent pores from being seen because all pores are delimited by walls made of crystallites. In conclusion, HRTEM is not helpful in studying the evolution of the catalysts microstructure with the time of reaction in NH3. The last technique used to study these catalysts was ToFSIMS. It is a surface analysis technique with a maximum probing depth of about 10 Å.13 Experimentally, it is observed that the absolute signal from the samples decreases by a factor of 3 with increasing percentage of weight loss (increasing time of reaction in ammonia) (Figure 6A, 9). At first sight, this might seem surprising. Intuitively, one would expect the signal to increase instead because the BET area of the samples increases with increasing weight loss. However, the ToF-SIMS technique

Non-Noble Electrocatalysts for O2 Reduction, Part II probes only the surface that is directly exposed to the bombardment by Ga+ ions. This condition also applies in the other direction (i.e., the ions expelled from the surface by the Ga+ bombardment must be able to leave the sample without colliding with pore walls). Therefore, the surface area in the bulk of carbon black particles is not sensed by this technique. According to the model presented in part I, the surface area created by NH3 etching is entirely situated inside the particles, and thus it cannot contribute to the ToF-SIMS signal. On the other hand, the external surface of a single particle decreases with weight loss, first due to the rapid etching of disordered carbon by NH3 and, second, due to slow shrinking of the graphitic crystallites on the external surface. With the model (part I), this effect can be followed quantitatively. For a single particle, the surface area dwelling in the three shells nearest to the outer particle surface (three spherical shells of thickness 3 Å as defined in the model) was calculated and assumed to be the surface probed by ToFSIMS. This area is noted as Sext. Finally, before comparing to the experimental ToF-SIMS signal intensity, another factor must be accounted for, namely, the number of carbon black particles that are analyzed by this technique. With samples being subjected to the Ga+ bombardment on a fixed geometric area, it is obvious that the smaller the particles, the larger their number in that area. Thus, the experimental ToF-SIMS signal should be proportional to Sext/d2, not to Sext, in which d is the particle diameter predicted by the model (Figure 3, model line) and d2 is thus proportional to the two-dimensional projected area of a spherical particle. Indeed, the curve Sext/d2 versus weight loss predicted by the model looks similar to the experimental ToF-SIMS signal intensity (solid line in Figure 6A; the whole predicted curve Sext/d2 has been multiplied by an arbitrarily number so that its maximum is comparable to the maximum count experimentally measured for all positive ions, i.e., about 300 000). Between 0 and 10% weight loss, the predicted signal rises by a factor of 2.5, then decreases linearly until 40% weight loss, and levels off between 40 and 90% weight loss. The predicted signal agrees with the experimental signal except in the region of 0-10% weight loss. The large signal experimentally observed at 0% weight loss might be due to species adsorbed on the surface of the pristine furnace black. In Figure 6B, only the aromatic compounds found in the total I+ signal are looked at (i.e., C6H5+, C7H7+, C8H9+, C9H7+, C10H8+, C11H9+, C12H8+, C13H9+; they are assumed to be specific to the carbon back structure). There, the experimental signal at low weight loss (0-5%) is smaller than that at 15-30% weight loss, validating the initial rise of the signal that is predicted by model calculations. Physically, the model predicts that at time zero, the surface giving a ToFSIMS signal in a single particle is only the outer surface of the sphere. Then, disordered carbon is removed from the first layers and some edge surface is created. That surface contributes to the ToF-SIMS signal as well because it is situated close to the outer surface. Then, as more disordered carbon is removed from the particle, the micropores become deeper and the newly created surface is undetectable using ToF-SIMS because it is situated too deeply inside the particle. At the same time, the graphite crystals that contribute to the ToF-SIMS signal start to shrink because NH3 attacks their edges. Consequently, the ToF-SIMS signal decreases (region 5-35% weight loss). If one could analyze a single particle, the signal should continue to decrease even for W > 50% weight loss. However, at W > 50% the particles start to shrink slightly (Figure 3). Consequently, more particles can occupy the fixed geometric area analyzed by the ToF-SIMS instrument. Thus, the decreasing

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Figure 7. (A) Fe bulk concentration as measured by neutron activation analysis (9) against the percentage of weight loss during heat treatment in NH3. The dashed line corresponds to the hypothesis of carbon gasification while all initial Fe atoms remain on the solid; thus Fe bulk ) (0.18 wt %) × 100/(100 - W) where W is the experimental weight loss in %. (B) Absolute signal of Fe+ ions measured by ToF-SIMS (9) against the percentage of weight loss during heat treatment in NH3. The line corresponds to the model prediction and the calculation is explained in section 3.2.

trend per particle is offset by the increased number of analyzed particles. This results in a ratio Sext/d2 that is about constant between 40 and 90% weight loss (Figure 6A). In conclusion, the model-predicted shrinking of graphitic crystallites on the outer surface of the particles is supported by the decrease of the experimental ToF-SIMS signal intensity in the region 5-30% weight loss. In the region 40-90% weight loss, the decrease of the particle size (thus, increase in number of particles per area) predicted by the model offsets the above trend yielding a constant ToF-SIMS signal intensity, which is also experimentally observed. Previously, the experimental decrease of the ToF-SIMS signal with increasing BET area of carbon blacks had been observed14 but could not be explained. 3.2. The Fate of Iron in the Course of Heat Treatment. An iron ion is the center of the site(s) catalyzing the O2 electroreduction in such materials. It is therefore of interest to investigate the evolution of iron in the course of heat treatment in ammonia. The iron bulk content of these catalysts (measured by neutron activation) increases with increasing weight loss (Figure 7A, 9). All catalysts are prepared by adding initially 0.2 wt % Fe (measured to be 0.18 wt % by neutron activation analysis) on the pristine carbon black. The experimental Fe bulk concentration increases according to the eq 0.18% × 100/(100 - W) in which W is the experimental percentage of weight loss (dashed line in Figure 7A). Thus, Figure 7A tells us that although in the course of the heat treatment carbon is gasified, all Fe atoms initially present remain on the solid carbon that is left. What is not known is whether the iron atoms form a cluster, are embedded in a carbon shell, or simply remain on the surface in an active or inactive form for the O2 electroreduction. Consider a single particle. If all the initial Fe atoms remain close to the outer surface of the particle and if they are not embedded in a carbon shell, then the ToF-SIMS signal for iron should be independent of the weight loss. For measurements taken on a fixed geometric area of a powder (many particles), one must divide a constant signal per particle by the square of the particle diameter for the reason explained in the previous

5976 J. Phys. Chem. C, Vol. 111, No. 16, 2007 section. Figure 7B compares the experimental Fe+ signal (9) to the model-predicted curve C/d2 (solid line); where C is a suitable constant scalar so that the predicted line matches the number of counts experimentally measured at 0-10% weight loss, and d is the model-predicted particle diameter (Figure 3). It is seen that the experimental and model lines agree with each other, each yielding a constant value up to 50% weight loss (owing to a constant particle diameter, Figure 3) followed by an increase by a factor of 2-3 between 50 and 90 wt % due to particle shrinking (increased number of particles probed by ToFSIMS). The conclusion is that Fe atoms initially present remain detectable using ToF-SIMS up to 90% weight loss. Thus, the Fe atoms (or ions) must sit on the carbon black surface; they are not embedded in a carbon shell, and they do not aggregate with other Fe atoms (no Fe2+ ions detected) to form large crystallites. On the other hand, this does not necessarily mean that all Fe atoms form catalytic sites for the ORR. An Fe ion must sit in a micropore having a proper size and be coordinated to nitrogen atoms to be active for the ORR. In part I of this paper, the activity per carbon black particle (number of Fe atoms independent of weight loss %) was shown to increase greatly between 0 and 35% weight loss, then to decrease by a decade upon larger weight loss (part I, Figure 6B). This decrease was explained by a decrease of the micropore area per particle (part I, Figure 6A). Thus, when micropores become too large, active sites are destroyed and the Fe ions cannot retain their active form. 4. Conclusions 1. SEM, Raman spectroscopy, and ToF-SIMS techniques support the mathematical model for carbon black gasification by NH3 presented in part I of this paper. HRTEM could not bring to the forefront any difference between samples (e.g., carbon blacks with BET areas of 70 and 860 m2 g-1) and was therefore not useful in this investigation. 2. The average diameter measured from SEM decreases from about 37 nm (pristine furnace black) to 27 nm (same furnace black but after 90% of its mass having been gasified by NH3). This agrees with the model prediction. 3. The experimental width at half-maximum of the D peak

Jaouen et al. (or G peak) of the Raman spectrum correlates with the fraction of disordered carbon phase predicted by the model. Both widths decrease with increasing weight loss of the carbon by reaction with NH3. 4. The signal intensity measured by ToF-SIMS on non-noble catalysts decreases with increasing time of reaction in ammonia. This is likely explained by the fact that only the external shell (9 Å thick) of the carbon black particles is detectable using ToFSIMS. The model predicts a decrease of the surface area dwelling in the external shell due to shrinking of the graphitic crystallites that are present therein. Acknowledgment. This work is supported by NSERC and General Motors of Canada. Support from the Que´bec/WallonieBruxelles cooperation is also acknowledged. References and Notes (1) Faubert, G.; Coˆte´, R.; Dodelet, J. P.; Lefe`vre, M.; Bertrand, P. Electrochim. Acta 1999, 44, 2589. (2) Lefe`vre, M.; Dodelet, J. P.; Bertrand, P. J. Phys. Chem. B 2002, 106, 8705. (3) Schulenburg, H. S.; Stankov, S.; Schu¨nemann, V.; Radnik, J.; Dorbandt, I.; Fiechter, S.; Bogdanoff, P.; Tributsch, H. J. Phys. Chem. B 2003, 107, 9034. (4) van Veen, J. A. R.; Colijn, H. A.; van Baar, H. F. Electrochim. Acta 1988, 33, 801. (5) Bouwkamp-Wijnoltz, A. L.; Visscher, W.; Van Veen, J. A. R.; Boellaard, E.; Van der Kraan, A. M.; Tang, S. C. J. Phys. Chem. B 2002, 106, 12993. (6) Bron, M.; Radnik, J.; Fieber-Erdmann, M.; Bogdanoff, P.; Fiechter, S. J. Electroanal. Chem. 2002, 535, 113. (7) Yuasa, M.; Yamaguchi, A.; Itsuki, H.; Tanaka, K.; Yamamoto, M.; Oyaizu, K. Chem. Mater. 2005, 17, 4278. (8) Jaouen, F.; Lefe`vre, M.; Dodelet, J. P.; Cai, M. J. Phys. Chem. B 2006, 110, 5553. (9) Jaouen, F.; Charreteur, F.; Dodelet, J. P. J. Electrochem. Soc. 2006, 153, A689. (10) Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Po¨schl, U. Carbon 2005, 43, 1731. (11) Hess, W. M.; Herd, C. R. Microstructure, Morphology, and General Physical Properties. In Carbon Black; Donnet, J-B., Bansal, R. C., Wang, M-J., eds.; Dekker: New York, 1993; Chapter 3, p 89-173. (12) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53, 1126. (13) Feldman, L. C.; Mayer, J. W. Fundamentals of Surface and Thin Film Analysis; North-Holland: Holland, 1986; p 353. (14) Poleunis, C.; Van den Eynde, X.; Grivei, E.; Smet, H.; Probst, N.; Bertrand, P. Surf. Interface Anal. 2000, 30, 420.