Honeycomb-like mesoporous nitrogen-doped carbon supported Pt catalyst for methanol electrooxidation

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Honeycomb-like mesoporous nitrogen-doped carbon supported Pt catalyst for methanol electrooxidation Li-Mei Zhang a, Zhen-Bo Wang Da-Ming Gu b a b

a,* ,

Jing-Jia Zhang a, Xu-Lei Sui

a,b

, Lei Zhao a,

School of Chemical Engineering and Technology, Harbin Institute of Technology, No. 92 West-Da Zhi Street, Harbin 150001, China School of Science, Harbin Institute of Technology, No. 92 West-Da Zhi Street, Harbin 150001, China

A R T I C L E I N F O

A B S T R A C T

Article history:

A novel nitrogen-doped carbon material with honeycomb-like mesoporous structure was

Received 27 January 2015

synthesized via carbonization of polyaniline under template and acted as support for the

Accepted 13 June 2015

Pt catalyst. Pyrolysis temperature plays a significant role in enhancing electrochemical

Available online 18 June 2015

performance of Pt/mesoporous nitrogen-doped carbon (Pt/MNC) catalyst. Considering comprehensively electrocatalytic activity and stability for methanol electrooxidation, MNC-900 (MNC prepared at pyrolysis temperature of 900 °C) is an appropriate support for Pt catalyst. Moreover, the electrochemical performance of Pt/MNC-900 catalyst is significantly superior to that of Pt/mesoporous carbon (Pt/MC) catalyst. The enhancement is attributed to N species introduced to the support and optimal proportion of pyridinic N, pyrrolic N and graphitic N. When the contents of pyridinic N, pyrrolic N and graphitic N achieve optimal proportion, MNC support reveals not only intense anchoring effect of Pt nanoparticles but also enhanced electric conductivity, further improving electrocatalytic activity and stability of the catalyst. Ó 2015 Elsevier Ltd. All rights reserved.

1.

Introduction

Direct methanol fuel cell (DMFC) has recently come to be regarded as one of the most promising power sources for portable electronic devices and electric vehicles on account of their high energy conversion efficiency and environmental benign nature [1–5]. With the current state of technology, carbon supported Pt based alloys are the most popular and effective catalysts for DMFC. However, the high cost of Pt and susceptible to corrosion of carbon support remain the two biggest hurdles to the commercialization of DMFC [6,7]. Therefore, great efforts have been focused on developing * Corresponding author: Fax: +86 451 86418616. E-mail address: [email protected] (Z.-B. Wang). http://dx.doi.org/10.1016/j.carbon.2015.06.022 0008-6223/Ó 2015 Elsevier Ltd. All rights reserved.

advanced carbon materials, such as carbon nanotubes [8,9], graphene [10–12] and ordered mesoporous carbon [13–15], with large surface area, high chemical stability, and excellent electrical conductivity. In particular, ordered mesoporous carbon supported Pt based catalyst exhibits improved electrocatalytic properties due to the high surface area from the mesopores for high dispersion of Pt nanoparticles and efficient transport of molecules and ions from the macropores for increasing accessibility to the active sites [16]. Nitrogen-doped carbon material has been proposed as support for the metal-based catalyst [17–19]. As reported, N species on the carbon support can lead to high dispersion of

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fine Pt nanoparticles with the synergistic interaction of Pt and support, resulting in the improved catalytic activity and stability toward methanol oxidation reaction (MOR) [20]. Nitrogen doping methods can be divided into two main ways: in situ doping and postsynthesis doping methods [21–23]. Pyrolysis of nitrogen-containing precursors (e.g. polyacrylonitrile, polypyrrole, polyaniline) [24–27] and chemical vapor deposition of nitrogen-containing compounds (e.g. phthalocyanine, ethylenediamine, pyridine) [28–31] are the most straightforward methods for in situ doping. Postsynthesis doping method is achieved by heat treatment of preformed carbon material in a nitrogen-containing atmosphere (e.g. NH3) [32,33]. In situ doping method has received much more attention, because of nitrogen species uniform distribution in the support with high nitrogen content, being beneficial for immobilization of Pt nanoparticles with an improved catalytic activity and stability [34–37]. In this work, honeycomb-like mesoporous nitrogen-doped carbon (MNC) was successfully synthesized by using polyaniline as carbon and nitrogen precursor, and silica nanoparticles as template for achieving mesoporous structure. The N contents and distribution proportion of different types N of MNC were closely related to the pyrolysis temperature. Subsequently, Pt nanoparticles were dispersed on MNC support through a microwave-assisted polyol process to obtain Pt/MNC catalyst. Effects of different pyrolysis temperatures on the physical and electrochemical performance of catalyst were also systematically investigated.

2.

Experiments

2.1.

Materials

Aniline and ammonium persulfate (APS) was purchased from Aladdinâ reagent Inc. Silica colloid (Ludox HS-40, 12 nm SiO2 nanoparticles dispersed in water) was purchased from Shanghai Seebio Biotech Inc. Hexachloroplatinic acid (H2PtCl6Æ6H2O) was purchased from Shanghai Jiuyue Chemical Co. 5 wt.% Nafion solution was purchased from Dupont. Except where specified, all chemicals were of analytical grade and used as received.

2.2.

Synthesis of MNC support and Pt/MNC catalyst

MNC support was synthesized as follows: 500 mg aniline monomer was dissolved in 10 mL 1.0 mol L 1 HCl successively and then 7.5 g silica colloid (40 wt.%) was added. After stirring for 1 h, 2.5 mL 1.0 mol L 1 HCl solution containing 1.23 g APS was added dropwise with vigorous stirring. The polymerization was conducted in an ice bath for 8 h. After evaporation of water at 70 °C, the obtained polyaniline coated silica nanoparticle (PANI@SiO2) composite was then pyrolyzed under flowing argon at different temperatures (700, 800, 900 and 1000 °C) for 2 h. The silica template was removed by 10 wt.% HF etching. Pt/MNC catalyst was prepared by a microwave-assisted polyol process. Briefly, 40 mg MNC support was dispersed into 60 mL mixture solvent of ethylene glycol and isopropyl alcohol under ultrasonic treatment for 1 h. Then 1.34 mL

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0.0383 mol L 1 H2PtCl6-EG solution was added into the uniform carbon ink with urgent agitation for 3 h. The pH value of the mixture was then adjusted to 12.0 by adding dropwise 1 mol L 1 NaOH-EG solution. After being saturated with argon for 20 min, the mixture was heated using a microwave oven for 55 s. When the mixture cooled down to room temperature, dilute HNO3 was added dropwise to adjust the pH value of the mixture to 2.0–3.0. The mixture was kept stirring for 12 h and then the product was washed repeatedly with ultrapure water (18.2 M XÆcm). The obtained Pt/MNC catalyst was dried for 5 h at 80 °C and then stored in a vacuum vessel. For comparison, homemade mesoporous carbon [38,39] supported Pt catalyst (Pt/MC catalyst) was also prepared by a microwave-assisted polyol process.

2.3.

Physical characterization

Brunauer–Emmett–Teller (BET) surface area of the sample was examined via nitrogen adsorption experiments at 77 K by using a QUADRASORB SI analyzer. X-ray diffraction (XRD) analysis was carried out with the D/max-RB diffractometer (Rigaku, Japan) to determine crystal structure of the sample. Raman spectrum of the sample was measured by using Renishaw inVia (Renishaw Instruments, England). X-ray photoelectron spectroscopy (XPS) analysis was carried out with a Physical Electronics PHI model 5700 instrument to determine the surface property of the sample. Hitachi-S-4700 analyzer was coupled to a scanning electron microscope (SEM, Hitachi Ltd. S-4700) for a rapid energy dispersive analysis of X-ray (EDAX) of elemental composition of the sample. Transmission electron microscopy (TEM) was taken by a TECNAI G2 F30 field emission transmission electron microscope. Before taking the electron micrograph, the sample was finely ground and ultrasonically dispersed in alcohol, and a drop of the resultant dispersion was deposited and dried on a standard copper grid coated with carbon film. The applied voltage was 300 kV.

2.4.

Electrochemical measurements

The electrochemical measurements were carried out in a conventional three-electrode cell by using CHI650E electrochemical workstation controlled at room temperature. The cell consists of a working electrode, an Hg/Hg2SO4 (0.68 V relative to reversible hydrogen electrode, RHE) reference electrode and a platinum foil counter electrode. All of the potentials are relative to the RHE electrode, unless otherwise noted. The working electrode was prepared as follows: 4 mg catalyst was dispersed in 2 mL mixed solution of ethanol and ultrapure water (v/v = 1:1) and ultrasonicated for 20 min to form a uniform catalyst ink. A total of 5 lL of well-dispersed catalyst ink was pipetted onto the glassy carbon electrode surface and onto which 5 lL of a dilute aqueous Nafion solution (5 wt.% solution in a mixture of lower aliphatic alcohols and DuPont water) was added. The prepared electrode was dried at room temperature before electrochemical tests. The cyclic voltammograms (CV) were recorded within a potential range from 0.05 V to 1.2 V (vs. RHE). The electrochemically active specific surface area (ESA) of catalyst was calculated from the formula ESAPt = QH/(0.21 MPt). QH is the

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charge due to the hydrogen adsorption/desorption in the hydrogen region of the CV, 0.21 is the electrical charge associated with monolayer adsorption of hydrogen on Pt, and MPt is the loading of Pt metal on the working electrode [40]. In order to activate and clean the catalyst surface, the working electrode was treated by continuous cycle at a scan rate of 50 mV s 1 in argon-purged 0.5 mol L 1 H2SO4 solution until a stable response was obtained. Electrochemical impedance spectroscopy (EIS) was obtained at frequencies between 100 kHz and 0.01 Hz with 12 points per decade.

3.

Results and discussion

The schematic diagram describing the preparation of Pt/MNC catalyst is shown in Fig. 1. It can be obtained that three principal steps of catalyst synthesis are as follows: (A) oxidative polymerization of aniline by addition of APS, (B) heat treatment in argon atmosphere and subsequently acid etching, (C) the deposition of Pt nanoparticles onto MNC support. The morphology of PANI@SiO2 composite before and after heat treatment is investigated by SEM. From Figs. S1(a–e) and 2(a and b), it can be clearly observed that PANI@SiO2 composite displays the amorphous chunk structure. After heat treatment and acid etching, spherical particles with a honeycomblike structure appear in MNC support, demonstrating that carbonization occurs during the heat treatment process and a porous structure is successfully introduced through acid etching SiO2 nanoparticles (12 nm). The result can be further confirmed by N2 adsorption–desorption isotherms (Figs. S2 and 2(c)). The remarkable hysteresis loops indicate the mesoporous nature of MNC support. For the sake of convenience and concision, MNC obtained at different pyrolysis temperatures of 700, 800, 900 and 1000 °C are denoted as MNC-700, MNC-800, MNC-900 and MNC-1000. The BET surface areas of MNC-700, MNC-800, MNC-900 and MNC-1000 are 639, 787, 787 and 675 m2 g 1, respectively, showing that MNC support

Fig. 1 – The schematic diagram of the preparation of Pt/MNC catalyst. (A color version of this figure can be viewed online.)

has a high BET surface area, can provide plenty of active sites for the deposition of Pt nanoparticles. Moreover, according to the Barrett–Joyer–Halenda (BJH) model (insets in Figs. S2 and 2(c)), the mesopore size distributions of MNC-700, MNC-800, MNC-900 and MNC-1000 are centered at 12.4, 12.4, 12.3 and 12.4 nm, respectively. Raman spectra further verify the decomposition of PANI@SiO2 composite and the formation of the carbon structure. As shown in Figs. S1(f) and 2(d), the Raman spectrum of PANI@SiO2 composite exhibits the characteristic bands of PANI, such as the C–H bending of the quinoid ring at 1165.8 cm 1, C–N+ stretching of the bipolaron structure at 1337.5 cm 1, C@N stretching vibrations at 1493.0 cm 1 and C–C stretching of the benzenoid ring at 1588.1 cm 1 [41]. However, MNC support obtained after heat treatment displays only two distinctive broad D and G bands at 1341.1 and 1582.8 cm 1, respectively. The D band is a common feature of all disordered graphitic carbon, while the G band is closely related to a graphitic carbon phase with a sp2 electronic configuration, such as graphene layer [42]. The morphology and nanostructure of Pt/MNC catalysts are examined by TEM (Figs. 3(a) and S3). The mesoporous with the pore size about 12 nm in the MNC support is clearly visible, which is consistent with the result of BET measurement. In addition, it can be clearly seen that Pt nanoparticles are homogeneously deposited onto MNC-700, MNC-800 and MNC-900 supports. The possible reason is that MNC supports can provide enough N sites for the deposition of Pt nanoparticles. However, Pt nanoparticles tend to aggregate in the Pt/MNC-1000 catalyst, which is attributed to insufficient N sites. HRTEM image of Pt/MNC-900 catalyst is also measured and the typical result is shown in Fig. 3(b). It can be seen clearly the regular lattice fringes with a spacing of 0.23 nm, which is highly consistent with the (1 1 1) plane of Pt. The crystalline structures of Pt/MNC catalysts are characterized by XRD. Representative diffraction peaks of Pt [(1 1 1), (2 0 0), (2 2 0), and (3 1 1)] are distinctly observed in the XRD patterns (Fig. 4(a)), which means that Pt forms the face-centered cubic (fcc) crystal structure. In addition, it can be seen that Pt diffraction peak width for the Pt/MNC-1000 catalyst is obviously narrower than that of other catalysts. The possible reason is Pt nanoparticle size in the Pt/MNC-1000 catalyst is bigger than that of other catalysts, which agrees with the result of TEM. Moreover, the EDAX results of Pt/MNC catalysts (Fig. S4) show that the mass fractions of Pt in the Pt/MNC-700, Pt/MNC-800, Pt/MNC-900 and Pt/MNC-1000 catalysts are 20.58, 21.20, 21.92 and 19.45 wt.%, respectively, which is consistent with the theoretical value of 20 wt.%. XPS measurements are performed to probe the surface composition and chemical states of Pt/MNC catalysts. The N/C surface atomic ratios of Pt/MNC catalysts in the XPS survey spectra (Fig. 4(b)) gradually decrease with the increasing of pyrolysis temperature, with N/C ratio of 11.29, 10.75, 7.77 and 5.03 at.%, respectively. The N 1s spectrum (Fig. 4(c)) of Pt/MNC-900 catalyst can be deconvoluted into four peaks at 398.3, 400.2, 401.1 and 402.3 eV, which can be assigned to N6 (pyridinic N) , N-5 (pyrrolic N or pyridinic-N in association with phenolic or carbonyl group), N-Q (graphitic N) and N-O (oxidized N) [43,44]. It has been reported that N species at the edges of graphene layers (N-5 and N-6) may provide the

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Fig. 2 – SEM images (a and b), N2 adsorption–desorption isotherms and the pore size distribution from the BJH method (inset) (c), and Raman spectrum (d) of MNC-900. (A color version of this figure can be viewed online.)

Fig. 3 – TEM image (a), Pt size distribution (statistic number 100) (inset in (a)), and HRTEM image (b) of Pt/MNC-900 catalyst. (A color version of this figure can be viewed online.)

main initial nucleation sites for the deposition of Pt nanoparticles [45] and also active sites for Pt immobilization avoiding surface Pt diffusion and coalescence even at the high Pt loading [46,47]. On the other hand, the N atom substitutes (N-Q) within graphene layer could enhance the electric conductivity of carbon material [48]. In addition, the high resolution XPS spectra of N 1s and distribution proportion of different types N in the Pt/MNC catalysts are displayed in Fig. S5 and Table S1. From Table S1, it can be observed that the contents of N-5 and N-6 gradually decrease, while the content of N-Q increases continually with the increasing of pyrolysis temperature. The decrease of N-5 and N-6 can bring disadvantageous effect to disperse and anchor Pt nanoparticles, while the increase of N-Q can enhance electric conductivity of support.

When the contents of N-5, N-6 and N-Q achieve optimal proportion, MNC support reveals not only intense anchoring effect of Pt nanoparticles but also enhanced electric conductivity, further improving catalytic activity and stability of the catalyst. The Pt 4f spectrum (Figs. 4(d) and S6) of Pt/MNC catalyst can be deconvoluted into three pairs of doublets, which can be attributed to metallic Pt, Pt2+ and Pt4+, respectively. Moreover, from Table S2, it can be obtained that the binding energies of metallic Pt for Pt/MNC-700, Pt/MNC-800 and Pt/MNC-900 catalysts appear a positive shift of 0.4 eV in comparison with that of Pt/C [49]. It is known that when finely dispersed metal particles are deposited on a support, there is always a slight shift in the binding energy due to size effects

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Fig. 4 – XRD patterns (a), XPS survey spectra (b) of Pt/MNC catalysts; the high resolution XPS spectra of N 1s (c) and Pt 4f (d) of Pt/MNC-900 catalyst. (A color version of this figure can be viewed online.)

in metal nanoparticle/cluster [50,51]. As Pt nanoparticle/cluster size decreases, there is a reduction in the lineshape asymmetry, a broadening in the linewidth, and a shift of the corelevel toward the higher binding energies [52]. Therefore, the slight shift in the Pt peak toward higher binding energies is likely due to the presence of N sites and the effect of small particles in the MNC supports. However, the positive shift of metallic Pt for Pt/MNC-1000 catalyst has not been observed, which can be attributed to the aggregation of Pt nanoparticles caused by insufficient N sites. The cyclic voltammograms of Pt/MNC catalysts in acidic medium (0.5 mol L 1 H2SO4) are illustrated in Fig. 5(a). The electrochemically active specific surface areas (ESAPt) are obtained by the measurements of the hydrogen adsorption– desorption (HAD) integrals. The ESAPt of Pt/MNC-700, Pt/MNC-800, Pt/MNC-900 and Pt/MNC-1000 catalysts are 50.8, 79.1, 97.9 and 44.7 m2 g 1, respectively. The result demonstrates that Pt/MNC-900 catalyst exhibits the best electrochemical active area. Moreover, the electrocatalytic activities of the catalysts toward methanol oxidation reaction (MOR) are shown in Fig. 5(b). The peak current densities of Pt/MNC catalysts increase initially and then decrease with the increasing of pyrolysis temperature. The methanol oxidation activity are maximized for Pt/MNC-900 catalyst with the peak current density of 10.2 mA cm 2 and decrease successively for Pt/MNC-800, Pt/MNC-1000 and Pt/MNC-700 catalysts, with the peak current density of 9.4, 8.4 and

7.4 mA cm 2, respectively. Pt/MNC-900 catalyst exhibits the best electrocatalytic activity toward MOR, which is attributed to optimal proportion of pyridinic N, pyrrolic N and graphitic N. With the increasing of pyrolysis temperature, the contents of pyridinic N and pyrrolic N gradually decrease, while the content of graphitic N increases continually. The decrease of pyridinic N and pyrrolic N can bring disadvantageous effect to disperse and anchor Pt nanoparticles, while the increase of graphitic N can enhance electric conductivity of support. When the contents of pyridinic N, pyrrolic N and graphitic N achieve optimal proportion, MNC support reveals not only intense anchoring effect of Pt nanoparticles but also enhanced electric conductivity, further improving electrocatalytic activity of the catalyst. Electrochemical impedance spectroscopy (EIS) can be used as an effective method for measuring the charge transfer resistance (Rct), which reflects the electrocatalytic activity of the catalyst toward MOR. The Nyquist plots of Pt/MNC catalysts in methanol acidic medium are displayed in Fig. 5(c). It can be clearly seen that the spectra exhibit strong contributions of inductive components at high frequencies. This can be ascribed to the external circuit inductance and usually does not involve an electrochemical process [53]. The large arc that appears in the medium frequency range relates to the electro-oxidation of methanol, and at the low frequency, this arc extends into the fourth quadrant and forms an induction loop that represents the electro-oxidation of (CO)ad

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Fig. 5 – Cyclic voltammograms of Pt/MNC catalysts in acidic medium (a), cyclic voltammograms of Pt/MNC catalysts in methanol acidic medium (b), Nyquist plots of Pt/MNC catalysts in methanol acidic medium (c) and relationship of normalized peak current density and cycle number of Pt/MNC catalysts in methanol acidic medium (d). (A color version of this figure can be viewed online.)

[54,55]. In order to quantitatively analyze the impedance behavior, the resistances can be obtained from the analysis of EIS by using the software of ZsimpWin based on an equivalent electric circuit [56] (Table S3). The Rct of Pt/MNC-700, Pt/MNC-800, Pt/MNC-900 and Pt/MNC-1000 catalysts are 106.6, 71.6, 54.9 and 127.8 X cm2, respectively, which further reflects the higher electrocatalytic activity for Pt/MNC-900 catalyst. The long-term stability of the catalyst is another important factor influencing their practical application in DMFCs. For comparison, the cycling aging of Pt/MNC catalysts are measured in methanol acidic medium (Figs. 5(d) and S7(a–d)). The retention rates of Pt/MNC-700, Pt/MNC-800, Pt/MNC-900 and Pt/MNC-1000 catalysts after 1000 cycles are 63.2%, 66.4%, 70.4% and 75.5%, respectively, indicating that the electrocatalytic stabilities of Pt/MNC catalysts for MOR are gradually strengthened with the increasing of pyrolysis temperature. One possible reason is that the stabilities of supports are gradually enhanced with the decrease of defects. Considering comprehensively electrocatalytic activity and stability toward MOR, MNC-900 is an appropriate support for Pt catalyst. To verify the potential application of the catalyst, a further comparison is conducted between Pt/MNC-900 and Pt/MC catalysts. Fig. 6(a) shows the cyclic voltammograms in acidic medium. According to the ESAPt formula, the ESAPt of

Pt/MNC-900 catalyst with 97.9 m2 g 1 is significantly higher than 73.6 m2 g 1 of Pt/MC catalyst, demonstrating that the electrochemical active area of Pt/MNC-900 catalyst is evidently superior to that of Pt/MC catalyst. In addition, the electrochemical performance of Pt/MNC-900 and Pt/MC catalysts toward MOR are shown in Fig. 6(b–d). From Fig. 6(b), it can be clearly seen that the peak current density of Pt/MNC-900 catalyst is 10.2 mA cm 2, which is 1.4 times higher than 7.4 mA cm 2 of Pt/MC catalyst. Meanwhile, the Rct of Pt/MNC-900 catalyst is 54.9 X cm2, which is much smaller than 90.2 X cm2 of Pt/MC catalyst (Fig. 6(c) and Table S3). The results of the cyclic voltammograms and Nyquist plots indicate that methanol electrooxidation activity of Pt/MNC900 catalyst is obviously better than that of Pt/MC catalyst. The cycling aging (Figs. 6(d) and S7(c and e)) can be used for reflecting the electrocatalytic stability toward MOR. As shown in Fig. 6(d), the retention rate of Pt/MNC-900 catalyst after 1000 cycles is 70.4%, which is higher than 62.4% of Pt/MC catalyst. The results of electrochemical measurements demonstrate that Pt/MNC-900 catalyst has the better electrocatalytic activity and stability, comparing with Pt/MC catalyst. The improvement of electrochemical performance for Pt/MNC-900 catalyst can be ascribed to N species introduced to the support. N species on the support play a significant role in anchoring Pt nanoparticles and enhancing the electric conductivity of carbon material.

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Fig. 6 – Electrochemical performance of Pt/MNC-900 and Pt/MC catalysts: cyclic voltammograms in acidic medium (a) and in methanol acidic medium (b), Nyquist plots (c) and relationship of normalized peak current density and cycle number (d) in methanol acidic medium. (A color version of this figure can be viewed online.)

4.

Conclusions

Honeycomb-like mesoporous nitrogen-doped carbon (MNC) was successfully prepared by carbonization of polyaniline with silica nanoparticles as template and acted as Pt catalyst support. The pyrolysis temperature is closely related to physical and electrochemical performances of Pt/MNC catalysts. The Pt/MNC-900 catalyst exhibits the best electrochemical performances for methanol electrooxidation ascribed to optimal proportion of pyridinic N, pyrrolic N and graphitic N. When the contents of pyridinic N, pyrrolic N and graphitic N achieve optimal proportion, MNC support reveals not only intense anchoring effect of Pt nanoparticles but also enhanced electric conductivity, further improving electrocatalytic activity and stability of the catalyst. Moreover, the peak current density of Pt/MNC-900 catalyst is 10.2 mA cm 2, which is 1.4 times higher than 7.4 mA cm 2 of Pt/MC catalyst. The retention rate of Pt/MNC-900 catalyst after 1000 cycles is 70.4%, which is better than 62.4% of Pt/MC catalyst. The excellent electrocatalytic activity and stability originates from N species introduced to the support. N species on the support play a significant role in anchoring Pt nanoparticles and enhancing the electric conductivity of carbon material. As a result, the Pt/MNC-900 catalyst will be a promising candidate as anode catalyst for DMFC.

Acknowledgments We acknowledge the National Natural Science Foundation of China (Grant No. 21273058), China Postdoctoral Science Foundation (Grant Nos. 2012M520731 and 2014T70350), Heilongjiang Postdoctoral Financial Assistance (LBH-Z12089) for their financial support.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2015.06.022.

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