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Well-Dispersed High-Loading Pt Nanoparticles Supported by Shell-Core Nanostructured Carbon for Methanol Electrooxidation Gang Wu,* Deyu Li, Changsong Dai, Dianlong Wang, and Ning Li Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China ReceiVed September 20, 2007. In Final Form: December 17, 2007 Shell-core nanostructured carbon materials with a nitrogen-doped graphitic layer as a shell and pristine carbon black particle as a core were synthesized by carbonizing the hybrid materials containing in situ polymerized aniline onto carbon black. In an N-doped carbon layer, the nitrogen atoms substitute carbon atoms at the edge and interior of the graphene structure to form pyridinic N and quaternary N structures, respectively. As a result, the carbon structure becomes more compact, showing curvatures and disorder in the graphene stacking. In comparison with nondoped carbon, the N-doped one was proved to be a suitable supporting material to synthesize high-loading Pt catalysts (up to 60 wt %) with a more uniform size distribution and stronger metal-support interactions due to its high electrochemically accessible surface area, richness of disorder and defects, and high electron density. Moreover, the more rapid chargetransfer rates over the N-doped carbon material are evidenced by the high crystallinity of the graphitic shell layer with nitrogen doping as well as the low charge-transfer resistance at the electrolyte/electrode interface. Beneficial roles of nitrogen doping can be found to enhance the CO tolerance of Pt catalysts. Accordingly, an improved performance in methanol oxidation was achieved on a high-loading Pt catalyst supported by N-doped carbon. The enhanced catalytic properties were extensively discussed based on mass activity (Pt utilization) and intrinsic activity (charge-transfer rate). Therefore, N-doped carbon layers present many advantages over nondoped ones and would emerge as an interesting supporting carbon material for fuel cell electrocatalysts.

Introduction Comparing with proton exchange membrane fuel cells (PEMFCs) fed by hydrogen, direct methanol fuel cells (DMFCs) possess many advantages such as compatibility with the existing petroleum distribution network due to the consumption of liquid fuel, and feasibility to handle water and heat management. However, performance has been a major problem for the DMFC, and it typically produces only one-third of the PEMFC’s power density. Besides the limitation caused by the sluggish electrochemical activity of oxygen reduction in the cathode, the DMFC anode is also limited by poor electrochemical activity for methanol oxidation. As a result, a large amount of Pt-based electrocatalysts must be used in a DMFC to achieve high power density. However, in the optimization of a membrane electrode assembly (MEA), the electrocatalyst layers should be reasonably thin, which can reduce the inner electrical resistance of the catalytic layer and enhance the rate of proton diffusion and reactant permeability in the depth of the electrocatalyst layer.1-3 Thus, the effective solution is using the high-loading Pt/C catalyst to maintain the larger amount of Pt in the limited volume space. High-loading Pt-based catalysts can not only optimize the catalyst layer structure but also improve the complete oxidation of methanol. Behm and co-workers4 once indicated that the electrochemical efficiencies, the product distribution, and the turnover frequencies (TOF) of partial reactions (methanol oxidation to formaldehyde, formic acid, and CO2) show a pronounced dependence on Pt loading. The increasing loading would lead to significant enhancement in the efficiency of * To whom correspondence should be addressed. Telephone: 505-6673060. Fax: 505-665-4292. E-mail: [email protected]. Present address: Los Alamos National Laboratory, MPA-11, Los Alamos, New Mexico 87545. (1) Song, S. Q.; Wang, Y.; Shen, P. K. J. Power Sources 2007, 170, 46-49. (2) Yan, S. Y.; Sun, G. Q.; Tian, J.; Jiang, L. H.; Qi, J.; Xin, Q. Electrochim. Acta 2006, 52, 1692-1696. (3) Ralph, T. R.; Hogarth, M. P. Platinum Met. ReV. 2002, 46, 117-135. (4) Jusys, Z.; Kaiser, J.; Behm, R. J. Langmuir 2003, 19, 6759-6769.

complete oxidation to CO2, while the percentage of oxidation to formaldehyde and formic acid also can decline to almost zero. The pronounced variations in product distribution/TOF values with catalyst loading are attributed to increasing readsorption and subsequent complete oxidation of desorbed reaction intermediates (formaldehyde and formic acid) at higher Pt loadings, while at low loadings these intermediates are more likely to survive. These loading effects will become even more pronounced at elevated temperatures, where desorption is expected to become more significant. However, since particle size and agglomeration of Pt usually increase with its loading onto supports, synthesis procedures of a high-loading Pt catalyst with a narrow size distribution remain an enormous challenge.5,6 The morphology and structure of supported Pt are strongly affected by the synthesis method,7,8 metal precursor,9,10 and support materials.11-15 Conventional methods for the synthesis of high-loading Pt and Pt-based electrocatalysts are mainly impregnation16 and colloid methods17 (5) Maillard, F.; Savinova, E. R.; Stimming, U. J. Electroanal. Chem. 2007, 599, 221-232. (6) Friedrich, K. A.; Henglein, F.; Stimming, U.; Unkauf, W. Electrochim. Acta 2000, 45, 3283-3293. (7) Njoki, P. N.; Luo, J.; Wang, L. Y.; Maye, M. M.; Quaizar, H.; Zhong, C. J. Langmuir 2005, 21, 1623-1628. (8) Gu, Y. J.; Wong, W. T. Langmuir 2006, 22, 11447-11452. (9) Sen, F.; Gokagac, G. J. Phys. Chem. C 2007, 111, 1467-1473. (10) Basnayake, R.; Li, Z. R.; Katar, S.; Zhou, W.; Rivera, H.; Smotkin, E. S.; Casadonte, D. J.; Korzeniewski, C. Langmuir 2006, 22, 10446-10450. (11) Guha, A.; Lu, W. J.; Zawodzinski, T. A.; Schiraldi, D. A. Carbon 2007, 45, 1506-1517. (12) Wu, G.; Li, L.; Li, J. H.; Xu, B. Q. J. Power Sources 2006, 155, 118-127. (13) Wu, G.; Chen, Y. S.; Xu, B. Q. Electrochem. Commun. 2005, 7, 12371243. (14) Ding, J.; Chan, K. Y.; Ren, J. W.; Xiao, F. S. Electrochim. Acta 2005, 50, 3131-3141. (15) Kongkanand, A.; Kuwabata, S.; Girishkumar, G.; Kamat, P. Langmuir 2006, 22, 2392-2396. (16) Miller, J. T.; Schreier, M.; Kropf, A. J.; Regalbuto, J. R. J. Catal. 2004, 225, 203-212. (17) Xiong, Y. J.; Washio, I.; Chen, J. Y.; Cai, H. G.; Li, Z. Y.; Xia, Y. N. Langmuir 2006, 22, 8563-8570.

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Shell-Core Nanostructured N-Doped Carbon Supports

by using a complex route and stabilizer. However, the impregnation method is limited because the average particle size is usually large and the size distribution is broad. Although the surfactantassisted routes can produce well-homogenized ultrafine Pt electrocatalysts, the removal of surfactant makes these syntheses more complex, thus hindering their application. Generally, the preparation procedures of Pt/C catalysts involve the adsorption of active compounds onto supporting carbon materials. Thus, the morphology and nanostructure of carbon materials are considered to be main factors in obtaining a high dispersion of nanoparticle catalysts, which motivated numerous studies to improve metal dispersions on carbons, mainly through optimization in the metal-supporting interaction, functionalization of the carbon surface, electronic conductivity, proper surface area, and pore structure.18-20 It was confirmed that a high surface area and mesoporous structure of the carbon support are highly important in acquiring a high dispersion for high-loading Pt/C, because the micropores may cause loss of the catalyst dispersion. Moreover, the reduction and deposition of metal particles would be influenced by the surface nanostructure of carbon materials for the synthesis of high-loading Pt catalysts, explained by the interaction between metal particles and support materials.21 Recently, Sung and coworkers22 demonstrated that carbon nanofiber (CNF) has an advantage over carbon black to prepare high-loading PtRu/C catalysts (up to 60 wt %,) due to the highly ordered surface structure of CNF. Moreover, Ryoo and co-workers23 also described a general strategy for the synthesis of highly ordered, rigid arrays of nanoporous carbon having uniform but tunable diameters. The resulting material supports a high dispersion of platinum nanoparticles, exceeding that of other common microporous carbon materials. On the other hand, there is increasing evidence showing that the electrochemical and physical properties of carbon materials are extremely sensitive to doping by heteroatoms.24,25 In particular, nitrogen can dope into carbon, modifying the surface structures and states of a carbon material but maintaining the robust framework of the carbon and most of its intrinsic properties. Moreover, the doping of nitrogen into carbon showed a profound effect on surface chemical activity, electron-transfer rates, and adsorption in redox reactions,25-28 so it is highly important to distinguish the impact on the preparation of supported Pt-based catalysts. In this work, a simple and effective strategy to dope nitrogen into carbon by carbonization of in situ synthesized polyaniline was demonstrated. It was found that the N-doped carbon is capable of supporting highloading Pt particles with a well-dispersed morphology and unique properties, thereby enhancing catalytic activity for methanol electrooxidation in fuel cell technology. Experimental Section Chemicals and Preparations. Polyaniline coated on carbon black (commercially available Ketjen Black EC 300J; BET surface area: about 950 m2/g) was prepared by in situ polymerization using (18) Wu, G.; Li, L.; Li, J. H.; Xu, B. Q. Carbon 2005, 43, 2579-2587. (19) Vijayaraghavan, G.; Stevenson, K. J. Langmuir 2007, 23, 5279-5282. (20) Metz, K. M.; Goel, D.; Hamers, R. J. J. Phys. Chem. C 2007, 111, 72607265. (21) Adamczyk, Z. AdV. Colloid Interface Sci. 2003, 100, 267-347. (22) Park, I. S.; Park, K. W.; Choi, J. H.; Park, C. R.; Sung, Y. E. Carbon 2007, 45, 28-33. (23) Sang, H. J.; Choi, S. J.; Ilwhan, O.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 12-33. (24) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641-1653. (25) Maldonado, S.; Stevenson, K. J. J. Phys. Chem. B 2004, 108, 1137511383. (26) Souza, F. G.; Sirelli, L.; Michel, R. C.; Soares, B. G.; Herbst, M. H. J. Appl. Polym. Sci. 2006, 102, 535-541. (27) Strelko, V. V.; Kuts, V. S.; Thrower, P. A. Carbon 2000, 38, 1499-1503. (28) Huang, M. C.; Teng, H. S. Carbon 2003, 41, 951-957.

Langmuir, Vol. 24, No. 7, 2008 3567 potassium peroxydisulfate as the oxidizing agent in acidic (0.5 M H2SO4) aqueous methanol (70 vol %) medium. The hybrid carbon materials were treated at 900 °C in an argon gas flow for 1 h to dope nitrogen into carbon. Finally, the resulting N-doped carbon samples were thoroughly washed with diluted H2SO4 and deionized water. The carbon supported Pt catalysts were prepared by the so-called formic acid method (FAM) with various Pt loading from 5 to 60 wt %. The detailed procedure is described in the Supporting Information. For comparison, similar procedures were carried out using normal carbon black as the support to prepare the Pt catalyst with 60 wt % loading. The N-doped and nondoped carbon supported Pt catalysts in this work are denoted as Pt/N-C and Pt/C, respectively. Characterizations. X-ray photoelectron spectroscopy (XPS) analysis was used to analyze the presence of functional groups and the chemical composition of the nitrogen-doped carbon supports as well as their loaded Pt particles. The crystallinity of the N-doped carbon and loaded Pt catalysts with 60 wt % loading were determined by X-ray diffraction (XRD) performed on an automated Rigaku diffractometer. High-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL JEM-2010F microscope with a resolution of 0.102 nm operating at 200 kV to study the morphology of nitrogen-doped carbon and loaded Pt particles. Meanwhile, high-angle annular dark-field scanning transmission electron microscopy (STEM) was employed to compare the Pt/N-C (60 wt %) and Pt/C (60 wt %) catalysts. The EG&G model 273 potentiostat/galvanostat and 1025 frequency response detector were used for all electrochemical measurements in a conventional three electrode cell, using a Hg/HgSO4 electrode (0.64 V vs NHE) as the reference electrode and a large-area Pt sheet as the counter electrode at room temperature (23 ( 2 °C). Paste electrodes prepared by mixing each of the carbon materials (Ndoped and nondoped) with 5% Nafion isopropanol solution were applied to a glassy carbon rotating disc electrode (GC-RDE), which was characterized as the working electrode in 0.5 M H2SO4 solution by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) to evaluate their electrochemical properties. CO Stripping and Methanol Electrooxidation. CO stripping was carried out in 0.5 M H2SO4 solution over Pt/N-C and Pt/C (60 wt %) catalysts. The electrolyte solution was first purged with high purity nitrogen gas. Adsorption of CO on the electrode catalyst was conducted by bubbling CO gas (UHP grade) through the electrolyte solution for 15 min, followed by purging with nitrogen for 20 min to remove residual CO from the solution. The CO stripping CV curve and blank CV curve can be obtained from two consecutive scan cycles in the potential between 0.05 and 1.2 V at a sweep rate of 10 mV/s. Methanol electrooxidation over the Pt/N-C (60 wt %) and Pt/C (60 wt %) catalysts was performed by CV and EIS measurements in 0.5 M CH3OH + 0.5 M H2SO4 solutions at room temperature. The CV curves were recorded from 0 to 1.2 V at a sweep of 5 mV/s. EIS measurements were conducted at a constant potential of 0.65 V in the frequency range between 100 kHz and 0.01 Hz, using an excitation signal of 5 mV on a Solarton 1025 frequency response analyzer. The impedance parameters were fitted and analyzed using the Zsimpwin program.

Results 1. Physical Properties of the N-Doped Carbon Support. Scheme 1 demonstrates the primary approach for the synthesis of shell-core nanostructured nitrogen-doped carbon materials. First, polyaniline was in situ polymerized and deposited on carbon black particles, forming a thin film layer. During pyrolysis, the hybrid polyaniline coated carbon black was carbonized to generate a shell-core nanostructure with nitrogen-doped graphitic carbon as a shell and pristine carbon particle as a core. The advantage of using polyaniline is that it contains both the carbon source and nitrogen precursor, which nitrogen can dope into a graphitic carbon structure during its carbonization. Moreover, the employment of a polymer for the synthesis of nitrogen-doped carbon allows the polymer to uniformly cover the carbon materials,

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Figure 1. Representative high-resolution TEM images for shell-core nanostructured N-doped carbon materials at various magnifications. Scheme 1. Basic Strategy to Synthesize Shell-Core Nanostructured N-Doped Carbon Materials

which easily incorporates the desired heteroatoms and controls their compositions.29 The shell-core structure in N-doped carbon materials was further evidenced by the representative HRTEM images in Figure 1 exhibiting carbon particles surrounded by graphitic carbon shells at various magnifications. The thickness of the graphitic carbon layer is in the 5-8 nm range. During the thermal treatment, the polyaniline covering the carbon black particle was carbonized to form the graphitic shell. The carbon black mainly remains as amorphous carbon. This can be clearly seen in Figure 1b and c where both types of carbon structures are displayed. The electron conductivity in electrocatalysis would benefit from the layers of graphitic carbon nanostructures covering the regular carbon black particles due to their high crystallinity30 as illustrated by the HRTEM images. In a comparison of these HRTEM images for nondoped (shown in the Supporting Information) and N-doped carbon (Figure 1d), subtle structural differences are clearly seen. The N-doped carbon surface contains more curvature and (29) Lahaye, J.; Nanse, G.; Bagreev, A.; Strelko, V. Carbon 1999, 37, 585590. (30) Wu, G.; Xu, B. Q. J. Power Sources 2007, 174, 148-158.

dislocations in the graphene stacking (turbostratic disorder), which is probably due to the propensity of substituted nitrogen for the formation of pentagonal defects in the graphene sheets.31 Additionally, the N-doped carbon structure appears more compact than the nondoped analogues. The shorter compartment distances caused by nitrogen doping were also detected in the CNx films32 and N-doped multiwalled carbon nanotubes (MWNTs) grown by chemical vapor deposition (CVD)33 as explained by the fact that N inclusion encourages nanocrystallite formation and suppresses surface diffusion of the carbon during graphitization. Figure 2 shows a comparison of the XPS and XRD patterns for the N-doped and nondoped carbon materials. The representative N 1s spectra for N-doped and nondoped carbon are shown in Figure 2a. The XPS survey scan indicated that the approximate N atom content is 5.2 atom % on the carbon surface for the N-doped sample. No obvious peak referring to surface nitrogen (31) Pollak, E.; Salitra, G.; Soffer, A.; Aurbach, D. Carbon 2006, 44, 33023307. (32) Fuge, G. M.; Rennick, C. J.; Pearce, S. R. J.; May, P. W.; Ashfold, M. N. R. Diamond Relat. Mater. 2003, 12, 1049-1054. (33) Jang, J. W.; Lee, C. E.; Lyu, S. C.; Lee, T. J.; Lee, C. J. Appl. Phys. Lett. 2004, 84, 2877-2879.

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Figure 2. X-ray photoelectron spectroscopy (XPS) analysis (a) and X-ray diffraction (XRD) patterns (b) for N-doped and nondoped carbon samples.

functional groups can be detected for the nondoped carbon. Moreover, the N-doped carbon shows two comparable peaks at 398.6 and 401.3 eV, respectively. Generally, during the pyrolysis process (above 700 °C), the nitrogen atoms can incorporate into the graphene layers to replace carbon atoms at different sites, and in doing so they show various binding energies in the XPS spectra: pyridinic N (398.6 ( 0.3 eV), pyrrolic N (400.5 ( 0.3 eV), and quaternary N (401.3 ( 0.3 eV).29 The pyridinic forms of nitrogen, doped at the edges of the graphitic carbon layers, will be referred to as N-6. The quaternary nitrogen, defined as doping inside the graphitic carbon plane, will be referred to as N-Q. Pyrrole nitrogen (pyrrolic-N) can be represented as N-5.24,34 Therefore, the two dominant peaks at 398.6 and 401.3 eV can be assigned to pyridinic and quaternary nitrogen, respectively, suggesting that the N atoms were successfully incorporated into the carbon structures to replace the carbon atoms which were located at the edges and inside of the graphitic carbon layers. It has been reported24 that, during pyrolysis, N-6 can be converted to N-Q gradually until equilibrium is achieved with a constant ratio of N-6/N-Q. It can therefore be presumed that the distributions of doping sites of N at the edge and interior sites of the plane are tending to the steady state. Since nitrogen doping via the replacement of C atoms with N atoms has been confirmed by means of XPS, the effects of nitrogen doping on the carbon graphitic structures and crystallinity can be assessed using X-ray diffraction (XRD). In the XRD (34) Maldonado, S.; Morin, S.; Stevenson, K. J. Carbon 2006, 44, 14291437.

Figure 3. Cyclic voltammograms (a) and Nyquist plots of EIS (b) for nondoped and N-doped carbon electrodes recorded in 0.5 M H2SO4. The inset of (b) is the equivalent circuit used for fitting the experimental data; the solid lines show the fitted curves.

patterns of these samples shown in Figure 2b, the strongest peaks at 2θ ) 24.5-25.5 °C correspond to the (002) basal plane diffraction in the graphitic structure. It was found that the relative intensity of carbon (002) peaks for N-doped samples is higher than that of carbon samples without nitrogen doping, which means a better graphitic crystalline structure. The broad peaks can be attributed to two separate forms of carbon cited previously as turbostratic carbon (carbon black) and graphene carbon (graphitic carbon layers with nitrogen doping). In the case of CNx films,32 both electron spin resonance (ESR) and Raman analyses indicated that the carbon materials become more graphitic after N doping. In addition, it should be noted that the nitrogen-doped carbon samples display a positive shift in the C(002) peak when compared with the carbon black sample without N doping. This is probably partially attributed to a decrease in the d-space of the (002) crystal plane in the graphitic structure of the N-doped carbon, which is in good agreement with other cases of N-doped carbon materials.32,33 2. Electrochemical Characterizations of N-Doped Carbon. Figure 3 compares the electrochemical properties of nondoped and N-doped carbon samples in 0.5 M H2SO4 solution using CV and EIS techniques. In the CV curves shown in Figure 3a, the potential window between 0.4 and 0.8 V is an indicator of capacitive current which depends on the electrochemically accessible area Sa (m2/g), an area where the electrolyte can reach

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in the porous structure of carbon materials. According to the equations shown in the Supporting Information, the calculated Sa of N-doped carbon (205.7 m2/g) is significantly greater than that of nondoped carbon (55.5 m2/g). Since it was reported that nitrogen-plasma treated inorganic (metal, oxide)35,36 and organic materials (polymer and wool fabrics)37,38 exhibited a dramatic improvement in hydrophilicity and wettability due to the generation of CsN, CdN, and amide bonds after plasma irradiation, the facilitating access of solvated and charged ions on N-doped carbon can associate with the doped functional N atoms. Besides the electrochemically accessible area, charge (electron and proton)-transfer rates in the catalysts are of great importance for a high oxidation rate of methanol.39 Here, EIS was considered to study the characteristics of the carbon material including conductivity, structures, and charge transport at the carbon/ electrolyte interface. The Nyquist plots for nondoped and N-doped carbon at open circuit potentials are shown in Figure 3b. It can be seen that the impedance spectra of both carbon samples display similar characteristics. The depressed semicircle in the highfrequency region is ascribed to the charge (electron and proton) exchange at the carbon/electrolyte interface. A straight line at low frequencies can be expressed by using a Warburg diffusion element due to the semi-infinite diffusion of ions at the carbon/ electrolyte interface.40 Thus, an equivalent circuit to represent the impedance behaviors of the porous electrode at open circuit potential40,41 was employed in this study and it is shown as an inset image in Figure 3b. According to the simulated parameters shown in the Supporting Information, the charge-transfer resistance (Rct) in N-doped carbon (1065 Ω cm2) is almost 3 times lower than that of nondoped carbon (2990 Ω cm2), which reveals the easier charge (electron and proton) transfer at the interface of N-doped carbon/electrolyte. Meanwhile, larger constant phase element (CPE) and smaller n values were found for N-doped carbon, corresponding to highly available surface area for electrochemical reactions. In addition, the lower W values of N-doped carbon also reveal that the diffusion of ions in the N-doped carbon is facile, which is in good agreement with CV analysis. Given its higher electrochemically accessible surface area and lower charge-transfer resistance at the carbon support/electrolyte interface, N-doped carbon is more functional to act as the supporting material for electrocatalysts than traditional carbon black. 3. Physical Properties of Pt Particles Supported by N-Doped Carbon. When traditional carbon black was used for the supporting material, the particle size of deposited Pt would be significantly increasing with Pt loading.42 To study the effect of Pt loading over N-doped carbon on the particle size, the HRTEM images for Pt/N-C catalysts with varying Pt loadings ranging from 5 to 60 wt % were compared in Figure 4. It can be seen from the images that aggregation of the Pt nanoparticles is minimal with increasing Pt loading. Based on the measurements of Pt (35) Weibel, D. E.; Vilani, C.; Habert, A. C.; Achete, C. A. Surf. Coat. Technol. 2006, 201, 4190-4194. (36) Canal, C.; Molina, R.; Erra, P.; Ricard, A. Eur. Phys. J.: Appl. Phys. 2006, 36, 35-41. (37) Iriyama, Y.; Ohbayashi, K.; Ihara, T. J. Photopolym. Sci. Technol. 2006, 19, 215-219. (38) Wang, Y. J.; Lu, L.; Zheng, Y. D.; Chen, X. F. J. Biomed. Mater. Res., Part A 2006, 76A, 589-595. (39) Tarola, A.; Dini, D.; Salatelli, E.; Andreani, F.; Decker, F. Electrochim. Acta 1999, 44, 4189-4193. (40) Beden, B.; Lamy, C. Electrochim. Acta 1990, 35, 691-704. (41) Kim, M.; Park, J. N.; Kim, H.; Song, S.; Lee, W. H. J. Power Sources 2006, 163, 93-97. (42) Yasuda, K.; Nishimura, Y. Mater. Chem. Phys. 2003, 82, 921-928.

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particles in random regions, the average sizes (diameter) were estimated to be 2.0, 2.2, 2.7, and 3.2 nm for the 5, 15, 30, and 60 wt % loadings, respectively. These observations indicated that particle size variation is insignificant with respect to an increase in Pt loading, revealing that increased loading results in an increase in the number of Pt particles and not a significant increase in the size of the particles when N-doped carbon was used as the supporting material. For further comparison, STEM images of N-doped and nondoped carbon supported Pt with 60 wt % loading prepared by an identical procedure were compared in Figure 5. The metal particle size distributions (PSDs) based on 300 particles at random determined from the images are also shown for each Pt catalyst. The observed mean particle size for Pt supported by nondoped carbon is about 5.2 ( 3.0 nm, with a relatively wide size distribution and obvious agglomeration. The substantially smaller Pt particle size (3.2 ( 1.2 nm) over N-doped carbon with the feature of a lognormal distribution revealed that the N-doped carbon is more capable of accommodating Pt seeds to form a uniform and well-dispersed metal particle morphology. Bergamaski et al.43 once proposed that an optimum particle size range for efficiently electrooxidizing methanol to CO2 was found between 3 and 10 nm, and loss in efficiency is mostly related to the partial oxidation of methanol to formaldehyde on either too small or too large particles. Since the particle sizes for both Pt/C and Pt/N-C are in this range, the size effect on oxidation efficiency will be ignored in this work. Besides the comparable electrochemically accessible surface areas in N-doped carbon contributed to the high dispersion of Pt, the well-dispersed Pt particles onto N-doped carbon probably also benefit from the stabilizing function of CsN groups, CdN groups, or similar structures. Detailed explanations will be presented in subsequent discussion parts. Thus, HRTEM images of carbon supported high-loading Pt particles directly validate the important role of nitrogen doping in carbon and the applicability of the synthesis for high-loading Pt catalysts we described herein. XPS was used to study Pt/N-C (60 wt %) and Pt/C (60 wt %) catalysts shown in the Supporting Information. As compared with the non-carbon supported Pt catalyst, the most intense doublet peaks (near 70.8 and 74.2 eV) assigned to Pt(0) for the N-doped carbon were found to positively shift toward higher binding energies (71.2 and 74.6 eV), probably due to stronger Pt-support interactions or smaller particle size.44 4. CO Stripping Study of Pt Catalysts Supported on N-Doped Carbon. In situ electrochemical Fourier transform infrared (FTIR) spectroscopy and electrochemical mass spectrometry45 were employed to study the reaction mechanism of methanol oxidation onto the Pt catalyst, which proposed that the adsorption intermediate COad significantly limits the reaction rate. So, CO stripping voltammograms for the Pt catalysts supported by nondoped and N-doped carbon were compared in Figure 6 to determine the effects of Pt loading and nitrogen doping on the CO tolerance of Pt catalysts. As for the regular carbon black, Guerin and co-workers46 systematically varied Pt loading in Pt/Vulcan catalysts from 10% to 78% in CO stripping experiments. When the Pt content reached 40%, a shoulder at 0.69 V appeared, which developed into a peak with the further increase of Pt content and became overwhelming for 78% Pt/ (43) Bergamaski, K.; Pinheiro, A. L. N.; Teixeira-Neto, E.; Nart, F. C. J. Phys. Chem. B 2006, 110, 19271-19279. (44) Arico, A. S.; Shukla, A. K.; Kim, H.; Park, S.; Min, M.; Antonucci, V. Appl. Surf. Sci. 2001, 172, 3340. (45) Munk, J.; Christensen, P. A.; Hamnett, A.; Skou, E. J. Electroanal. Chem. 1996, 401, 215-222. (46) Guerin, S.; Hayden, B. E.; Lee, C. E.; Mormiche, C.; Owen, J. R.; Russell, A. E.; Theobald, B.; Thompsett, D. J. Comb. Chem. 2004, 6, 149-158.

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Figure 4. HRTEM images for N-doped carbon supported Pt with varying Pt loadings: (a,b) 5 wt %, (c,d) 15 wt %, (e,f) 30 wt %, and (g,h) 60 wt %.

Vulcan. Likewise, a low potential shoulder was detected for the Pt/C (60 wt %) catalyst in our work. Using FTIR spectroscopy

and CO stripping, Stimming and co-workers47 demonstrated that multiple stripping peaks are a consequence of interparticle

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Figure 5. STEM images and particle size distributions (PSDs): (a,c) Pt/C (60 wt %) and (b,d) Pt/N-C (60 wt %) catalysts.

Figure 6. CO stripping over Pt/N-C (5, 30, and 60 wt %) and Pt/C (60 wt %) catalysts.

heterogeneity such as particle agglomeration and wide particle distribution. However, in the cases of Pt/N-C catalysts with varying loadings from 5% to 60%, all CO stripping showed symmetric shapes with single peak potentials at 0.88-0.77 V, suggesting that CO adsorption and removal occur on Pt particles of similar size for each Pt/N-C catalyst. It is thus very likely to confirm again that a more uniform dispersion of Pt can be supported by N-doped carbon when compared with nondoped carbon. Upon increasing the Pt loading from 5 to 60 wt %, the CO oxidation peak potentials gradually shift to the negative direction from 0.88 to 0.77 V, an indication of more facile CO removal and potentially improved CO tolerance in practice. The negative shift in the CO oxidation peak potential with increasing (47) Maillard, F.; Savinova, E.; Simonov, P. A.; Zaikovskii, V. I.; Stimming, U. J. Phys. Chem. B 2004, 108, 17893-17904.

Pt loading partially could be attributed to the larger particles having “rougher” surfaces than the small particles, which have some fairly smooth Pt(111) facets containing irregular steps and occasionally twinned particles.48 Meanwhile, the oxidative removal of CO is mainly controlled by the number of defects serving as an active center for OH adsorption. Similar results also were once reported by Arenz et al.48 and Stimming et al.49 An increase of the particle size from 1.8 to 3.3 nm for isolated Pt nanoparticles without agglomeration results in a negative shift of the CO oxidation peak. On the other hand, the enhancement in the CO tolerance of Pt catalysts at high loading also can be considered as physical agglomerating effects that the single Pt site is always surround by other adjoining Pt sites with high density, increasing the availability to supply OH groups from adjacent Pt sites and their transfer rate, while at low loadings these OH groups just can be provided at more positive potentials. Therefore, according to the size and agglomeration effects, when compared with the Pt/N-C (60 wt %) catalyst (ca. 3.2 nm), the Pt/C (60 wt %) catalyst with a larger particle size (ca. 5.0 nm) and obvious agglomeration should exhibit a more negative peak potential in CO oxidation. However, the results are opposite: the peak potential of CO oxidation for the Pt/N-C catalyst (60 wt %) is slightly shifted to the negative potential. The opposite results reveal that some beneficial influences from the N-doped carbon support (stronger metal-support interaction44 and easier formation of active groups50 from N-doped sites) overwhelm the size effect in CO oxidation. (48) Arenz, M.; Mayrhofer, K. J. J.; Stamenkovic, V.; Blizanac, B. B.; Tomoyuki, T.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2005, 127, 6819-6829. (49) Maillard, F.; Schreier, S.; Hanzlik, M.; Savinova, E. R.; Weinkauf, S.; Stimming, U. Phys. Chem. Chem. Phys. 2005, 7, 385-393. (50) Ebbesen, S. D.; Mojet, B. L.; Lefferts, L. Langmuir 2006, 22, 10791085.

Shell-Core Nanostructured N-Doped Carbon Supports

The Pt electrochemical active surface area (EAS) of these Pt/N-C and Pt/C catalysts can be calculated according to the CO stripping peak area from Figure 6. Among them, 5 wt % Pt/N-C and 30 wt % Pt/C possess similar EASs, which are 87 and 83 m2 g-1, respectively, while the Pt/N-C (60 wt %) catalyst gives the lower EAS of 68 m2 g-1 with a slightly larger particle size. Due to significant agglomerations of Pt particles over the normal Pt/C (60 wt %) catalyst, its EAS is only 45 m2 g-1. Based on the EAS and the specific activity (mA/cm2 Pt), the values for the turnover frequency (TOF) of CO oxidation on surface Pt atoms at 0.75 V were calculated for these catalysts. The Pt/N-C (60 wt %) catalyst appeared to show the highest TOF (0.081 s-1) followed by the Pt/C (60 wt %) (0.070 s-1), Pt/N-C (30 wt %) (0.048 s-1), and Pt/N-C (5 wt %) (0.019 s-1) catalysts. Thus, the unique surface graphitic nanostructure (richness of disordered structures and defects at the edge or interior of the carbon plane) and high density of surface functional groups (associated with nitrogen) on the N-doped carbon would be responsible for the higher CO tolerance of the Pt/N-C catalyst, since they could strengthen the metal-support interactions51 and enhance charge transfer at the electrolyte/electrode interface.30 The concept that stronger metalsupport interactions would benefit the electron transfer between the metal and support during the electrochemical reactions once was proven by an electron spin resonance (ESR) study.52 5. Methanol Electrooxidation on Pt/N-C Catalysts with High Loading. Typical CV curves for methanol electrooxidation over Pt/N-C (60 wt %) and Pt/C (60 wt %) catalysts are shown in Figure 7 a. The enhanced activity of the Pt catalyst supported on the nitrogen-doped carbon is evidenced by a slightly lower onset potential and significantly higher current density. The onset potential for Pt/N-C was 0.48 V, while that for Pt/C was 0.55 V. In principle, with respect to the mechanism of alcohol electrooxidation on Pt sites, the onset potential is related to the breaking of C-H bonds and subsequent removal of intermediates such as COad by oxidation with OHad supplied by Pt-OH sites or other sources.53 In our CO stripping measurements, TOF (turnover frequency) values in CO oxidation for each Pt catalyst at the same loading (60 wt %) showed that the activity based on each active site (or unit of surface area) of Pt/N-C (0.081 s-1 at 0.75 V) is higher than that of Pt/C (0.070 s-1 at 0.75 V). In a similar way, the oxidative removal of C1 intermediates would easily occur on Pt/N-C catalysts. We normalized the current of methanol oxidation by using the Pt surface area and plotted the CV curves, which are shown in the inset of Figure 7a, for comparison with the curves normalized using the mass of Pt. Thus, it can be seen that an improvement of Pt catalytic activity results from the N-doped support, probably attributed to the stronger Pt-support interactions in the promotion of C-H breaking and COad tolerance. Moreover, the significantly higher methanol oxidation currents in both forward and subsequently reversed scans for the Pt/N-C catalyst clearly demonstrate a higher mass activity of Pt metal in the Pt/N-C catalyst, mainly due to the higher utilization of well-dispersed Pt nanoparticles supported by N-doped carbon. EIS measurements were carried out at 0.65 V to evaluate the charge-transfer property of methanol oxidation over both Pt/ N-C (60 wt %) and Pt/C (60 wt %) catalysts. The Nyquist plots are shown as Figure 7b. In our previous analysis of EIS for methanol oxidation over the PtRu/C catalyst,54 the impedance behaviors relating to the rate-determining step are dependent on (51) Nagai, Y.; Hirabayashi, T.; Dohmae, K.; Takagi, N.; Minami, T.; Shinjoh, H.; Matsumoto, S. J. Catal. 2006, 242, 103-109. (52) Baschuk, J. J.; Li, X. Int. J. Energy Res. 2001, 25, 695-713. (53) Wu, G.; Swaidan, R.; Cui, G. J. Power Sources 2007, 172, 180-188. (54) Wu, G.; Li, L.; Xu, B. Q. Electrochim. Acta 2004, 50, 1-10.

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Figure 7. Cyclic voltammograms (a) and Nyquist plots of EIS (b) for methanol oxidation over Pt/N-C (60 wt %) and Pt/C (60 wt %) catalysts in 0.5 M CH3OH + 0.5 M H2SO4 solutions. The inset in (a) shows the CV curves normalized using the Pt surface area (mA/ m2 Pt). The CV was recorded at a sweep rate of 5 mV/s, and EIS was performed at 0.65 V. Solid red line, Pt/N-C (60 wt %); blue dashed line, Pt/C (60 wt %).

the reaction potentials. At low potentials, methanol dehydrogenation is the rate-determining step, whereas at high potentials, the oxidative removal of COad became the rate-determining step. Here, an intermediate potential (0.65 V), involved at the transition of the rate-determining step from methanol dehydrogenation to COad oxidation, was chosen to compare the reactivity of Pt/N-C (60 wt %) and Pt/C (60 wt %) catalysts. As expected, the Nyquist plots featured an inductive loop in low-frequency range for both Pt catalysts. The inductive loop in methanol electrooxidation indicates that the COad coverage on Pt metal decreases with increasing reaction potential (b < 0), and the decrease in COad leads to an increase of the Faradaic current (m < 0).54 A reasonable explanation for this observation is that enough OHad groups on Pt sites can be formed with increasing potential. The as-formed OHad groups are then used to oxidize COad to decrease CO poisoning, generating more refreshed Pt sites for methanol adsorption to enhance the Faradaic current. Thus, an equivalent circuit representing the impedance behavior54 was employed and it is shown as an inset in Figure 7b. The charge-transfer resistances obtained from simulation of the impedance spectra were 62 and 85 Ω cm2 for Pt/N-C (60 wt %) and Pt/C (60 wt %) catalysts, respectively, thereby proving much faster charge-transfer rates during methanol electrooxidation over the Pt/N-C catalyst.

Discussion In this work, N-doped carbon materials with a shell-core structure were synthesized using a convenient strategy, where

3574 Langmuir, Vol. 24, No. 7, 2008

in situ polymerized aniline was carbonized to form a layer of an N-doped graphitic shell on a core of pristine carbon black particles. The graphitic carbon shells onto carbon materials can effectively modify the surface states and structures of carbon black materials showing positive physical and electrochemical properties as the supporting material. In general, the remarkable enhancement in catalytic activity for methanol electrooxidation using N-doped carbon supported Pt catalysts can be explained by two principles: (i) loaded nanosized Pt particles with well-dispersed morphology at high loading contribute to an improvement in Pt utilization showing higher mass activity (mA/mg Pt) and (ii) better conductivity and stronger metal-support interaction also facilitate charge transfer, increasing the turnover frequency in the methanol oxidation reaction (intrinsic activity). First, concerning Pt utilization at high loading, the HRTEM images clearly presented a uniform distribution of Pt particles on N-doped carbon, suggesting higher Pt utilization. Moreover, when N-doped carbon was used as the supporting material, increasing Pt loadings only resulted in an increase in the number of Pt particles and not an obvious growth in the size of the particles. In principle, it should be recognized that the formation of platinum clusters has two distinct stages. The first stage is the formation of nuclei of the new phase. The nuclei then grow into bigger particles with time. Usually, the increase of nanoparticle size with loading is attributed to the fact that the number densities of surface functional sites onto carbon are insufficient to provide the nuclei for Pt seeds and particles would grow bigger. Compared with nondoped carbon black, the graphitic layer with nitrogen doping exhibited a higher electrochemically accessible surface area and faster charge-transfer rate the electrode/electrolyte interface, which would be suitable for subsequent accommodation of catalytic metal (like Pt) particles and facilitating mass transfer (proton and methanol) during methanol oxidation. The increasing electrochemical surface area can be partially attributed to the richness of the functional groups associated with the doped N atoms probably enhancing the hydrophilic ability and facilitating the access of solvated and charged ions. Since the Pt morphology on nondoped and N-doped carbon materials is strongly dependent on the surface area of supporting materials, a question that the higher electrochemical surface area for N-doped carbon is derived from heat treatment or nitrogen doping is raised. Thus, it is necessary to investigate the effect of heat treatment on BrunauerEmmett-Teller (BET) and electrochemically accessible surface areas of carbon black. The results shown in the Supporting Information indicated that heat treatment at 900 °C for 1 h at inert atmosphere leads to the BET surface area of carbon black (nondoped carbon) decreasing from 945 to 820 m2/g and to the electrochemically accessible surface area also declining from 55.5 to 41.3 m2/g. The high pyrolysis temperature may induce shrinkage of the carbon structure, resulting in a lower porosity, as described by Ahmadpour and Do.55 Therefore, the difference of the electrochemically accessible surface area between nondoped and N-doped carbon cannot be attributed to heat treatment and just associated with changes in surface states due to nitrogen doping. Second, the beneficial morphology of nanosized Pt particles with high utilization onto N-doped graphitic shells also can be attributed to the subtle change of the surface nanostructure caused by nitrogen doping. On N-doped carbon, a large number of C-N functionalities have been assigned to the pyridinic and quarternary N, replacing the carbon atoms at the edge and interior of the graphene, respectively. The N-doped graphitic carbon nanostructure shows more disorder (curvature and dislocations) in (55) Ahmadpour, A.; Do, D. D. Carbon 1996, 34, 471-479.

Wu et al.

the graphene stacking due to the propensity of incorporated nitrogen to form pentagonal defects in the graphene sheets.34,56 Using Raman analysis, Stevenson and co-workers34 once found that the disorder in N-doped carbon nanotubes (CNTs) linearly increased with nitrogen content, providing direct evidence that the surface disorder is related to nitrogen doping. Therefore, the disordered nanostructure and defects in the edge and interior sites in the N-doped graphitic layer can serve as active sites for anchoring Pt seeds easily onto the supporting materials. This is probably the second explanation for uniformly dispersed Pt particles on N-doped carbon at a high loading compared with traditional carbon black supports, enhancing Pt utilization in methanol oxidation. Third, nitrogen doping not only modifies the carbon surface by forming a graphitic nanostructure layer but also influences the surface pKa values of carbon supports for polar surfaces. For traditional carbon materials, pHpzc values are typically near or below 7, dictated by acidic oxygen functionalities. It was reported34 that the N-doped CNTs with 4.0 atom % nitrogen possess a much more alkaline pHpzc value of 9.3 due to the prevalence of negatively charged nitrogen functionalities. Thus, the reductive character of graphitic carbon may also result from nitrogen doping. Strelko and co-workers27 have also proposed that nitrogenated graphitic carbons exhibit increased electron donation determined by the reductive gas phase adsorption of molecular oxygen. Moreover, Nevidomskyy and co-workers57 predicted that nitrogen doping in carbon nanotubes will result in chemically active localized areas with higher electron density. Using iodimetric analysis, a correlation between nitrogen content and the number of reducing sites is apparent, with triple the number of active sites observed for N-CNT containing 7.5 atom % nitrogen over nondoped CNTs. Szymanski and co-workers58 also indicated that the increasing reductive sites or high local electron density onto N-doped carbon materials can be tracked with the increase in the amounts of pyridinic and quaternary type nitrogen. From the chemical point of view, the pyridinic N linked only to two sp2 carbon atoms contributes to one Pπ electron in the graphitic π system, while the quaternary N atom in the center position, where it substitutes a carbon shared by three adjacent ring positions, contributes to two Pπ electrons in the graphitic π system.29 Lahaye and co-workers29 also confirmed that the N-doped carbon materials present a higher surface polarity than carbons without nitrogen. Thus, it is reasonable to expect that the doped nitrogen atoms in the carbon structure behave as reactive sites and show increased reactivity for Pt reduction due to high electron density generated from the incorporation of nitrogen. Therefore, we believe that the increased electron density of nitrogen doping sites on graphitic carbon layers would enhance the reduction of Pt salt and lead to stronger interactions between metal particles and supports in Pt/N-C catalysts. Especially, the stronger metal-support interaction can be confirmed by positive shifting of Pt(0) peaks in XPS analysis. Additionally, to further check the interaction strength between Pt particles and carbon supports, we compared both nondoped and N-doped carbon supported Pt catalysts in terms of the degradation of the electrochemical active surface area (EAS) of Pt with CV cycling number in nitrogen saturated H2SO4 solution (shown in the Supporting Information). The results indicated that the decay of the EAS for Pt/N-C with CV cycling is significantly less than that for traditional Pt/C, which determines the strong binding (56) Sjostrom, H.; Stafstrom, S.; Boman, M.; Sundgren, J. E. Phys. ReV. Lett. 1995, 75, 1336-1339. (57) Nevidomskyy, A. H.; Csanyi, G.; Payne, M. C. Phys. ReV. Lett. 2003, 91, 1055021-1055025. (58) Biniak, S.; Szymanski, G.; Siedlewski, J.; Swiatkowski, A. Carbon 1997, 35, 1799-1810.

Shell-Core Nanostructured N-Doped Carbon Supports

between Pt nanoparticles and N-doped carbon from the point of nanoparticle agglomeration. All of above factors including high electrochemically accessible surface area, richness of disordered nanostructures and defects, and high electron density on N-doped carbon supports contribute to the synthesis of an electrocatalyst with high-loading welldispersed Pt particles with stronger metal-support interactions, thereby enhancing the Pt utilization and catalytic activity of methanol oxidation. On the other hand, it was reported that nitrogen atoms, known as electron donors, increase the electrical conductivity in semiconductors or oxide ceramics.59 Moreover, examinations of these prepared N-doped carbon materials by HRTEM reveal that the shell layer of the graphitic nanostructure onto carbon black bulk has a high crystallinity and displays very well-defined (002) lattice fringes. A higher graphitization degree would be an important quality of supporting carbon materials to electrocatalysts since it induces higher electron conductivity and stronger electronic interaction with the loaded catalytic metal particles,60 which suggests that the N-doped carbon could a better supporting carbon material than traditional carbon black for electron transfer during electrochemical reactions. Since graphitization is a timeconsuming and expensive process, the surface graphitization with nitrogen doping in this work would be an attractive method to improve conductivity for traditional carbon materials. Thus, the higher performance of Pt/N-C can be partially attributed to the surface properties of the N-doping in the graphitic framework, which can facilitate electron transfer through the surface graphitic shell layer.61

Conclusion In summary, shell-core nanostructured carbon materials were synthesized using a hybrid carbon black coated by polyaniline which was carbonized to generate a graphitic carbon layer as a shell and pristine carbon particle as a core. In graphitic layers, nitrogen atoms substitute carbon atoms at the edge and interior sites of graphene to form pyridinic N and quaternary N, respectively. The formed graphitic layer with nitrogen doping shows high crystallinity and more disorder (curvature and (59) Liu, C. J.; Sheu, C. S.; Huang, M. S. Phys. ReV. B 2000, 61, 1432314326. (60) Park, K. W.; Sung, Y. E.; Han, S.; Yun, Y.; Hyeon, T. J. Phys. Chem. B 2004, 108, 939-944. (61) Su, F.; Zhao, X. S.; Wang, Y.; Zeng, J.; Zhou, Z.; Lee J. Y. J. Phys. Chem. B 2005, 109, 20200-20206.

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dislocations) in the graphene stacking and also exhibits a higher accessible surface area in electrochemical reactions and lower charge-transfer resistance at the carbon/electrolyte interface. In comparison with nondoped carbon, the morphology of welldispersed Pt particles with high loading (60 wt %) on the N-doped carbon supports was validated directly using HRTEM and STEM techniques. The comparisons of CO stripping over high-loading Pt electrocatalysts using nondoped carbon and N-doped carbon supports indicated that higher EAS (m2/g Pt) and beneficial support effects on the CO tolerance of Pt can originate from nitrogen doping. In methanol oxidation, under identical conditions, the Pt particles supported by the N-doped carbon have a much higher electrocatalytic activity (higher current, more negative onset potentials, lower charge-transfer resistance at constant potential) than those supported by nondoped carbon. The improved activity for methanol oxidation is attributed to higher Pt utilization (mass activity) and more rapid charge (electron and proton) transfer (intrinsic activity). The higher Pt utilization is defined by a more uniform dispersion of Pt particles due to a high electrochemically accessible surface area, richness of disordered nanostructures and defects, and high electron density on nitrogen-doped graphitic carbon, thereby enhancing the catalytic activity of methanol oxidation. The quicker chargetransfer rates are evidenced by the high crystallinity in the graphitic shell layer with nitrogen doping as well as the weakening of Pt-CO bonds to easily oxidize the adsorbed intermediate COad. Thus, it is clear that the shell-core nanostructured N-doped carbon material is a promising support for preparing high-loading Pt-based electrocatalysts to enhance the performance of methanol oxidation in advanced DMFCs. Acknowledgment. The authors acknowledge financial support for this work from the National Natural Science Foundation of China (NSFC) (20435010, 20503012). We also appreciate Dr. Huang Jiugui at Baoshan Iron & Steel Co. for his help with HRTEM and other analyses. Supporting Information Available: Nonessential experimental details and characterizations for N-doped carbon and loaded Pt catalysts. This material is available free of charge via the Internet at http:// pubs.acs.org. LA7029278