Highly active Pt nanoparticles on nickel phthalocyanine functionalized graphene nanosheets for methanol electrooxidation

Electrochimica Acta 113 (2013) 653–660

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Highly active Pt nanoparticles on nickel phthalocyanine functionalized graphene nanosheets for methanol electrooxidation Jing-Ping Zhong a , You-Jun Fan a,∗ , Hui Wang a , Rui-Xiang Wang a , Li-Li Fan a , Xing-Can Shen a , Zu-Jin Shi b a Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry of Education of China), College of Chemistry and Chemical Engineering, Guangxi Normal University, Guilin 541004, China b Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

a r t i c l e

i n f o

Article history: Received 27 June 2013 Received in revised form 12 September 2013 Accepted 12 September 2013 Available online 12 October 2013 Keywords: Platinum Graphene Functionalization Nickel phthalocyanine Methanol electrooxidation

a b s t r a c t A novel electrocatalyst using nickel (II) phthalocyanine-tetrasulfonic acid tetrasodium salt (TSNiPc) functionalized graphene (TSNiPc–graphene) composite as catalyst support for Pt nanoparticles is reported. The surface morphology, composition and structure of the prepared nanocomposites as well as their electrocatalytic properties toward methanol oxidation are characterized by UV–vis absorption spectroscopy, Raman spectroscopy, thermogravimetric analysis (TGA), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical tests. Pt nanoparticles are found uniformly dispersed on the surface of TSNiPc–graphene composite, with the small particle size of about 3.1 nm. Studies of cyclic voltammetry and chronoamperometry demonstrate that the Pt/TSNiPc–graphene exhibits much higher electrocatalytic activity and stability than the Pt/graphene catalyst for methanol oxidation. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Graphene, a two-dimensional carbon nanomaterial reported for the first time in 2004 [1], has attracted enormous interest in fuel cell applications due to its unique physicochemical properties, such as theoretically large surface area, excellent electrical conductivity, good chemical, thermal, optical and electrochemical stabilities, which make graphene a promising support materials [2,3]. In recent years, many research groups have reported that the graphene-supported Pt catalysts exhibit enhanced electrocatalytic performance for the methanol oxidation reaction [4–9]. For example, Sharma et al. [4] synthesized Pt/reduced graphene oxide (RGO) catalyst by a microwave-assisted polyol process, which exhibited better catalytic activity and CO poisoning tolerance for methanol oxidation compared to the commercial carbon-supported Pt electrocatalysts. Shi and Mu [8] prepared Pt/poly(pyrogallol)/graphene catalyst with higher electrocatalytic efficiency for methanol oxidation by the electrochemical deposition route. More recently, Xiong et al. [9] employed nitrogen-doped graphene as a conductive support for Pt nanoparticles and the electrochemical tests indicated that this catalyst showed higher methanol oxidation activity than a control catalyst of Pt loaded on undoped graphene. However,

∗ Corresponding author. Tel.: +86 773 5846279; fax: +86 773 2120958. E-mail address: [email protected] (Y.-J. Fan). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.09.092

similar to carbon nanotubes, pristine graphene is chemically inert and can not readily disperse in organic solvents or aqueous solutions [10,11], which would be disadvantageous for the assembly and dispersion of Pt-based nanoparticles. Therefore, it is necessary to further functionalize graphene sheets in order to improve their surface properties and dispersions in solvents. The functionalization of graphene can be performed by covalent or noncovalent methods [11–13]. Among them, noncovalent functionalization is preferable for catalyst support applications, because it enables attachment of molecules through ␲–␲ stacking or hydrophobic interactions, and thus preserves the intrinsic electronic and structural properties of graphene sheets, while improving their solubility quite remarkably. Metal phthalocyanines (MPcs), the two-dimensional 18 ␲-electron aromatic macrocycles with a metal atom located at the central cavity, are of great interest due to their excellent electronic properties and potential applications in some fields such as electrical devices, solar cells and biosensors [14–18]. Recently, several research groups have explored MPcs functionalized graphene composites for the achievement of well-dispersed graphene and the cathodic electrode materials of fuel cells [19–22]. Mensing et al. reported the electrochemical production of a stable aqueous dispersion of graphene–copper phthalocyanine hybrid material [19]. Zhang et al. prepared iron tetrasulfophthalocyanine functionalized graphene composites with enhanced activity for the oxygen reduction reaction (ORR) in a dual-chamber microbial fuel cell [21]. However,

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to the best of our knowledge, the studies on MPcs functionalized graphene composites used as the anodic catalyst support of fuel cells are very few. Only our group reported the synthesis of copper phthalocyanine-functionalized graphene with Pt catalysts, which exhibited superior electrocatalytic activity and stability for methanol oxidation [23]. It is known that the properties of MPcs molecules are closely related to the metal atom located at the central cavity [24]. Considering that the addition of nickel in Pt-based catalysts can obviously improve the electrocatalytic performance for the alcohol oxidation reaction [25,26], herein, we report the hydrothermal synthesis of a novel Pt electrocatalyst using nickel (II) phthalocyanine-tetrasulfonic acid tetrasodium salt (TSNiPc) functionalized graphene composite as catalyst support for Pt nanoparticles. The prepared nanocomposites were characterized by UV–vis absorption spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). The electrocatalytic performance of Pt/TSNiPc–graphene catalyst was evaluated by cyclic voltammetry (CV) and chronoamperometry methods. The results demonstrate that the Pt/TSNiPc–graphene catalyst exhibits much higher electrocatalytic activity and stability for methanol oxidation, in comparison with the Pt nanoparticles loaded on unfunctionalized graphene. 2. Experimental 2.1. Materials Graphene sheets (99.95%) used there was prepared by direct current arc-discharge method as reported in our previous work [27]. 5 wt% Nafion solution and nickel (II) phthalocyaninetetrasulfonic acid tetrasodium salt (TSNiPc) were purchased from Sigma–Aldrich. Hydrogen hexachloroplatinate (IV) hexahydrate (H2 PtCl6 ·6H2 O), methanol and sulfuric acid were obtained from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). All the chemicals are of analytical grade and used as received without further purification. All aqueous solutions were prepared using tridistilled water. 2.2. Preparation of Pt/TSNiPc–graphene catalyst 10 mg graphene and 20 mg TSNiPc were dispersed in 8 mL tridistilled water by ultrasonic treatment for 6 h, and then the asobtained suspension was allowed to stand overnight. Subsequently, 665 ␮L H2 PtCl6 solution (19 mM) was added into the suspension in 30 min with ultrasonication, and the pH value of system was adjusted to 10.5 by NaOH solution. An excess quantity of NaBH4 solution (10 mg mL−1 ) was then slowly added dropwise into the above solution under ultrasonication. After hydrothermal treatment for 20 h at 180 ◦ C, the resulting black solid was collected by centrifugation, washed repeatedly with tridistilled water and absolute ethanol several times to remove the ions possibly remaining in the final product, and then dried in vacuum at 60 ◦ C for 24 h. For comparison, Pt nanoparticles supported on graphene (Pt/graphene) was prepared with the similar procedure as described above.

confocal microscopic Raman spectrometer (Renishaw-InVia, UK). X-ray diffraction (XRD) was carried out on an X-ray diffractometer (Rigaku D/MAX 2500 v/pc, Japan) with a Cu K␣ radiation ˚ Thermogravimetric analysis (TGA) was persource ( = 1.5406 A). formed on a simultaneous thermogravimetric analyzer (Labsysevo TG-DSC/DTA, France). The samples were heated under a nitrogen atmosphere from room temperature to 1000 ◦ C at a rate of 10 ◦ C min−1 . X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Physical Electronics PHI Quantum 2000 system with an Al K␣ radiation source, and all reported values of electron binding energy were calibrated with respect to the principal peak of C 1s at 284.5 eV as an internal standard. To measure and compare the activity based on the Pt mass, an inductively coupled plasma-optical emission spectrophotometer (ICP-OES, Thermo Electron IRIS Intrepid II XSP, USA) was employed for composition measurements. In this study, The Pt contents of as-prepared Pt/TSNiPc–graphene and Pt/graphene catalysts were determined to be 12.6% and 23%, respectively. The higher Pt content of the Pt/graphene catalyst might be attributed to the poor solubility of pristine graphene in aqueous solutions. 2.4. Electrochemical measurements The electrochemical measurements were conducted at room temperature around 25 ◦ C using a standard three-electrode cell connected to a CHI 660D electrochemical workstation. A piece of Pt foil (1 cm2 ) and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. All potentials in the electrochemical tests are quoted versus the SCE scale. The working electrode is prepared as follows: the glass carbon (GC, Ф = 5 mm) electrode, polished with 5.0 ␮m, 1.0 ␮m, 0.3 ␮m Al2 O3 slurry and washed ultrasonically in water and ethanol for 5 min, was used as substrate for the catalysts. Then 1.5 mg of asprepared catalyst was dispersed ultrasonically in 400 ␮L Nafion solution (0.5 wt%), and 10 ␮L of the mixture was pipetted and airdried on the pretreated GC electrode at room temperature. For the Pt/TSNiPc–graphene and Pt/graphene catalysts, the Pt metal loading on the catalyst-modified GC electrodes was 23.63 and 43.13 ␮g cm−2 , respectively. The electrochemical active surface area (ECSA) of Pt nanoparticles was calculated from the hydrogen adsorption/desorption curve, which was recorded in 0.5 M H2 SO4 solution at a scan rate of 50 mV s−1 . The electrocatalytic efficiency of catalysts toward methanol oxidation was studied in 0.5 M CH3 OH + 0.5 M H2 SO4 solution. CO stripping voltammograms were obtained by oxidizing preadsorbed CO (COad ) in 0.5 M H2 SO4 solution at a scan rate of 50 mV s−1 . CO was bubbled into 0.5 M H2 SO4 for 10 min to allow saturated adsorption of CO onto the catalyst while maintaining the potential scan between −0.2 and 0 V, and then the dissolved CO in the electrolyte was removed by purging with nitrogen for 20 min. The current density in the electrochemical test was expressed by the normalized current per milligram of Pt loading. Before each electrochemical experiment, the electrolytic solution was purged with nitrogen for 15 min, and a flux of nitrogen was kept over the solution during measurements to prevent the interference of atmospheric oxygen.

2.3. Physical characterization 3. Results and discussion The size and morphology of the prepared catalysts were analyzed by high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100) with an accelerating voltage of 200 kV. Energy dispersive X-ray (EDX) spectroscopy characterization was carried out on the same apparatus (JEOL JEM-2100). UV–vis absorption spectra were recorded using a UV–vis spectrophotometer (Varian-Cary 100, USA). Raman spectra were obtained using a

The immobilization of TSNiPc on the surface of graphene was characterized by UV–vis absorption spectroscopy. Fig. 1A shows the UV–vis absorption spectra of TSNiPc, graphene, TSNiPc–graphene and Pt/TSNiPc–graphene in aqueous solution. It can be seen that TSNiPc exhibits strong Q-band absorption (curve a) in the region of 550–700 nm when compared to graphene without the

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Fig. 1. (A) UV–vis absorption spectra of TSNiPc (a), graphene (b), TSNiPc–graphene (c) and Pt/TSNiPc–graphene (d) in aqueous solution. (B) Raman spectra of graphene (a) and TSNiPc–graphene (b).

the G band appears at 1575 cm−1 , which is shifted toward higher frequency by 5 cm−1 compared to that of the graphene, indicating the charge transfer effect between the electron-acceptor TSNiPc and graphene [34]. The D band, which is a shoulder of the G band at about 1614 cm−1 , corresponds to second-order Raman scattering from the variation of the D band [35]. The ratio of the intensities of the two bands (ID /IG ) can be used to indicate the level of functionalization in a carbon material [36]. The ID /IG ratios for the pristine graphene and TSNiPc–graphene composite are 0.63 and 0.78, respectively. The higher ID /IG ratio and enhanced D band of the TSNiPc–graphene should be associated with a decrease in the average size of the sp2 domains in the composite, which is caused by the noncovalent attachment of TSNiPc to the graphene surface [37]. In order to evaluate the thermal stability of TSNiPc, graphene and TSNiPc–graphene composite, TGA was performed under a nitrogen atmosphere. As shown in Fig. 2A, the thermogram of TSNiPc demonstrates two major weight loss steps (curve a). The first step starts at 70 ◦ C to reach a plateau near 190 ◦ C with a weight loss of about 11 wt%. In this stage, TSNiPc has a stable molecular structure and, a small amount of weight loss due to volatilization of adsorbates. Another step starts at 500 ◦ C and ends at 550 ◦ C with 13.5 wt% weight loss, which may correspond to the thermal decomposition of TSNiPc. The fact is in agreement with the corresponding data reported by others [38]. Moreover, the thermogram (curve b) of the pristine graphene shows a weight loss of around

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characteristic spectrum (curve b), corresponding to the ␲–␲* transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the phthalocyanines ring [28–30]. The Q-band in the dilute aqueous solution is split in two, with contributions from both aggregate absorption (∼625 nm) and monomer absorption (∼657 nm) [30,31]. On the other hand, the addition of graphene to TSNiPc significantly decreases the intensity of the two peaks of Q-band, accompanied by a blue-shift of 2 nm and a red-shift of 7 nm, respectively (curve c). These results suggest an alteration of the electronic and optical properties in the composite due to the presence of ␲–␲ interactions between graphene and TSNiPc molecules [29]. The decrease in the peak intensities should be attributed to a small amount of TSNiPc molecules attached to graphene and a decrease in the density of trapped electrons in TSNiPc [16,32]. It is noted that, after the deposition of Pt nanoparticles on the TSNiPc–graphene surface (curve d), a peak of the Pt/TSNiPc–graphene composite is blue-shifted to some extent and another is red-shifted with respect to those of the TSNiPc–graphene, which might be ascribed to the interaction between the Pt nanoparticles and TSNiPc–graphene support. The surface-functionalization of graphene was also characterized by Raman spectroscopy. Fig. 1B displays the Raman spectra of graphene and TSNiPc–graphene. The Raman spectrum (curve a) of the graphene shows two prominent peaks at 1346 cm−1 (disordered vibration D band) and 1570 cm−1 (in-plane vibration G band) [5,33]. It is worth noting that, in the composite spectrum (curve b),

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Fig. 2. (A) TGA curves of TSNiPc (a), graphene (b) and TSNiPc–graphene (c) under the protection of N2 . (B) XRD patterns of Pt/TSNiPc–graphene (a) and Pt/graphene (b) catalysts.

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17.1% at 900 ◦ C, which excludes initial weight losses resulting from adsorbates. For the TSNiPc–graphene (curve c), the thermogram exhibits only 10.9% weight loss at this temperature. These results demonstrate that the functionalization of graphene with TSNiPc obviously improve the thermal stability of the composite, which is very important for the catalyst support material. Fig. 2B shows the XRD patterns of Pt/TSNiPc–graphene and Pt/graphene nanocomposites. The (0 0 2) peak located at the 2 value of around 26.1◦ in both catalysts evidently originates from the layered structure of graphene support [39,40]. The five other diffraction peaks at 39.7◦ , 46.3◦ , 67.5◦ , 81.5◦ and 85.8◦ can be ascribed to the diffractions of Pt(1 1 1), Pt(2 0 0), Pt(2 2 0), Pt(3 1 1) and Pt(2 2 2) planes, respectively, which represent the characteristics of face-centered cubic (fcc) structure of platinum [6,41]. Interestingly, the diffraction peaks of Pt for the Pt/TSNiPc–graphene catalyst shifted to a higher angle compared to that for the Pt/graphene. The calculated lattice constant of Pt ˚ is smaller than that for the for the Pt/TSNiPc–graphene (3.895 A) ˚ indicating at least partial formation of PtNi Pt/graphene (3.929 A), alloy [42,43]. Moreover, the Pt/TSNiPc–graphene displays significantly broader diffraction peak than the Pt/graphene catalyst, suggesting the smaller size of Pt nanoparticles supported on the TSNiPc–graphene. The average crystallite sizes of Pt nanoparticles were estimated by the Scherrer’s equation based on the peak assigned to the (2 2 0) plane [41]. The (2 2 0) plane was chosen for the calculation since this peak does not overlap with other peaks and therefore allows accurate analyses. The crystallite sizes are determined to be 3.5 and 7.4 nm for the Pt/TSNiPc–graphene and Pt/graphene catalysts, respectively. The formation of the Pt/TSNiPc–graphene nanocomposite was further characterized by energy dispersive X-ray (EDX)

Fig. 3. EDX spectrum of the Pt/TSNiPc–graphene catalyst.

spectroscopy, as shown in Fig. 3. The EDX spectrum shows the peaks corresponding to C, N, O, Ni, S and Pt elements, confirming the deposition of Pt nanoparticles on the surface of TSNiPc–graphene support. The morphology of as-prepared Pt catalysts was investigated with TEM, and their size distribution was evaluated statistically by measuring the diameter of 200 Pt nanoparticles in the magnified TEM images, as illustrated in Fig. 4. From Fig. 4A and B, Pt nanoparticles poorly distribute on the graphene surface and apparent Pt aggregation can be observed. The average particle size of Pt nanoparticles is 7.2 nm, which is close to the XRD result.

Fig. 4. TEM image (A) and the size distribution histogram (B) of Pt/graphene catalyst. TEM image (C) and the size distribution histogram (D) of Pt/TSNiPc–graphene catalyst.

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Fig. 5. (A) XPS survey spectra of Pt/TSNiPc–graphene and Pt/graphene catalysts. (B) N 1s spectrum of Pt/TSNiPc–graphene. (C) Pt 4f spectrum of Pt/TSNiPc–graphene. (D) Pt 4f spectrum of Pt/graphene.

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measured by the cyclic voltammograms of Pt/TSNiPc–graphene and Pt/graphene catalysts in nitrogen-saturated 0.5 M H2 SO4 solution. As shown in Fig. 6, the current peaks between −0.20 and 0.08 V are ascribed to the hydrogen adsorption/desorption behaviors at the electrodes, while the oxidation peak at 0.69 V is corresponding to the formation of Pt oxide and its related reduction peak is observed at about 0.51 V. The ECSA of Pt catalysts is calculated from measuring the hydrogen adsorption/desorption charges after double-layer correction and assuming a value of 210 ␮C cm−2 for the adsorption of a hydrogen monolayer [50]. Therefore, the ECSA of Pt/TSNiPc–graphene (curve a) can be calculated as 25.40 m2 g−1 ,

Current density / mA mg

From Fig. 4C and D, Pt nanoparticles are evenly dispersed on the TSNiPc–graphene support. The average particle size of Pt nanoparticles is 3.1 nm, which is also close to the XRD result. These results imply that the immobilization of TSNiPc on the graphene sheets produces the uniform distribution of surface functional groups, which are beneficial to the deposition of Pt nanoparticles. XPS was used to determine the surface composition and chemical oxidation states of the prepared composites. Fig. 5A shows the survey spectra of Pt/TSNiPc–graphene and Pt/graphene catalysts. Besides the signals of C 1s at 285.5 eV, O 1s at 534.1 eV, the signals of Pt 4f at 73.3 eV and Pt 4d at 314.4 eV appear in both samples [44,45]. Notice that the N 1s signal (400.3 eV) originating from TSNiPc is observed only in the Pt/TSNiPc–graphene but not in the Pt/graphene, indicating the attachment of TSNiPc on the surface of graphene. The high resolution spectrum of Pt/TSNiPc–graphene in Fig. 5B shows that the N 1s line is deconvoluted into three superimposed peaks: a main peak at 399.2 eV reflects the configuration of the NiN4 center as a mesomeric bridging of the Ni2+ ion with ideally four equivalent nitrogens [46], and the two peaks at 400.4 and 401.3 eV can be assigned to the pyrrolic-N and graphitic-N, respectively [47,48]. In addition, it is found that the Pt 4f signals of Pt/TSNiPc–graphene are similar to those of Pt/graphene (Fig. 5C and D). The two pairs of peaks indicate the existence of two different Pt oxidation states on the surface. The intense peaks of Pt 4f7/2 at 71.6 eV and Pt 4f5/2 at 75 eV are originated from the metallic platinum, Pt(0), while the peaks of Pt 4f7/2 at 72.8 eV and Pt 4f5/2 at 76.5 eV are attributed to the Pt(II) chemical state on PtO or Pt(OH)2 [9,49]. The relative amounts of the Pt(0) and Pt(II) species are nearly equal for both samples. The ECSA is a very important influence factor for the fuel cell reaction and the utilization ratio of Pt is closely interrelated with its dispersion situation and the ECSA. Typically, the ECSA can be

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Fig. 7. (A) Cyclic voltammograms (50 mV s−1 ) and (B) current–time curves, measured at 0.5 V, of methanol oxidation on Pt/TSNiPc–graphene (a) and Pt/graphene (b) catalyst in 0.5 M CH3 OH + 0.5 M H2 SO4 solution.

methanol oxidation on Pt/TSNiPc–graphene is 8.45 mA mg−1 (curve a), which is almost 6.8 times that of the Pt/graphene catalyst (curve b, 1.25 mA mg−1 ), illustrating that the Pt/TSNiPc–graphene catalyst has more excellent electrocatalytic activity and stability for methanol oxidation. The CO-tolerance abilities of the catalysts were evaluated by CO-stripping experiment. Fig. 8 shows the CO-stripping voltammograms and subsequent CV curves for the Pt/TSNiPc–graphene and Pt/graphene catalysts. It can be seen that for both catalysts the hydrogen adsorption/desorption in the low potential region is completely suppressed (the black line), which is due to the saturated coverage of COad species on the active sites of the Pt catalysts. After the removal of COad (the red line), the peaks associated with hydrogen adsorption/desorption appear. The peak potential of COad oxidation on the Pt/TSNiPc–graphene (Fig. 8A) is negatively shifted to 0.51 V, while the potential is 0.54 V on the Pt/graphene catalyst (Fig. 8B), indicating that the introduction of TSNiPc in the Pt/TSNiPc–graphene effectively improves the CO oxidation ability of the catalyst.

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much higher than that of the Pt/graphene catalyst (curve b, 6.35 m2 g−1 ). Higher ECSA of Pt/TSNiPc–graphene than Pt/graphene is clearly due to the higher dispersion of Pt nanoparticles and smaller Pt particle size on TSNiPc functionalized graphene sheets. It will be responsible for the enhanced electrocatalytic performance discussed below. Fig. 7A compares the cyclic voltammograms of Pt/TSNiPc–graphene and Pt/graphene catalysts in 0.5 M CH3 OH + 0.5 M H2 SO4 solution. As can be seen, the voltammograms are similar to those reported in the literature [9,50,51], with two irreversible current peaks during the methanol oxidation, attributing to methanol oxidation (forward scan peak at 0.62 V) and the oxidative removal of adsorbed intermediate species formed in the forward scan (backward scan peak at 0.47 V) [6,52]. The peak current densities of methanol oxidation on the Pt/TSNiPc–graphene catalyst (curve a) in the forward potential scan and in the backward potential scan are 1027.8 and 797.5 mA mg−1 , respectively, much higher than those on the Pt/graphene catalyst (curve b, 157.0 and 200.6 mA mg−1 ). Typically, the peak current density of Pt/TSNiPc–graphene in the forward scan is about 6.5 times that of the Pt/graphene catalyst. It should be noted that the mass activity of the Pt/TSNiPc–graphene is also higher than that of recent stateof-art Pt-based nanomaterials such as Pt@PDDA-GNSs [7], Pt NPs supported on nitrogen-doped graphene [9], PtRu/HPMo-CS-CNTs [49], Pt/PAMAM/ACS [50], and PtRu-NP/CNT-PCA nanohybrid [53]. Additionally, it is interesting to observe that the onset potential of methanol oxidation on the Pt/TSNiPc–graphene (curve a) is 0.15 V which is much lower than that on the Pt/graphene catalyst (curve b, 0.35 V). These results strongly indicate that the electrocatalytic activity of the Pt/TSNiPc–graphene for methanol oxidation is much higher than that of the Pt/graphene catalyst. To further investigate the role of TSNiPc, the specific activities of the catalysts based on the ECSA were also obtained. As depicted in Fig. S1, the specific activity of the Pt/TSNiPc–graphene is also higher than that of the Pt/graphene catalyst, which indicates that the activity enhancement for methanol oxidation on the Pt/TSNiPc–graphene can be attributed to the presence of TSNiPc. In order to compare the long-term performance of the Pt/TSNiPc–graphene and Pt/graphene catalysts toward methanol oxidation, the chronoamperometric measurements were carried out in 0.5 M CH3 OH + 0.5 M H2 SO4 solution at 0.5 V for 7200 s. As shown in Fig. 7B, in the initial period, the decay current density decrease rapidly for both catalysts, which can be ascribed to the formation of intermediate species, such as COads , CH3 OHads and CHOads during the methanol oxidation reaction [50]. After long-time operation (7200 s), the stable mass specific current of

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Potential/ V (SCE) Fig. 8. CO stripping voltammograms of Pt/TSNiPc–graphene (A) and Pt/graphene (B) catalyst in 0.5 M H2 SO4 at a scan rate of 50 mV s−1 .

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The reasons for the significant enhancement in the electrocatalytic performances of Pt/TSNiPc–graphene may be as follows: (1) Pt nanoparticles supported on the TSNiPc–graphene have small particle size, uniform distribution and high ECSA, which should be responsible for the enhanced electrocatalytic activity; (2) the formation of PtNi alloy in the Pt/TSNiPc–graphene catalyst enhances the electrocatalytic activity for methanol oxidation [42,43]; (3) the MPcs possess the unique molecular structure and outstanding physicochemical properties, such as exceptional thermal and chemical stability, and astounding electronic and photophysical features [19,24]. The synergistic cocatalytic effect between TSNiPc and Pt nanoparticles facilitates the methanol oxidation reaction [23]; (4) the sulfonic acid groups of TSNiPc can increase the hydrophilicity of the graphene by forming strong hydrogen bonds with water molecules, which promote their dissociation to produce –OHads during the methanol oxidation reaction [53,54], thus resulting in the more efficient oxidation of intermediate species generated in methanol oxidation and a better CO-poisoning tolerance of the catalyst. 4. Conclusions In summary, a novel nanostructured electrocatalyst (Pt/TSNiPc–graphene) with highly active Pt nanoparticles dispersed on the TSNiPc functionalized graphene sheets is reported. As-prepared nanocomposites are characterized by UV–vis, Raman, TGA, XRD, TEM, EDX, XPS and electrochemical tests. With the assistance of TSNiPc, Pt nanoparticles are uniformly deposited on the surface of graphene, and their dispersivity and ECSA are obviously enhanced. The cyclic voltammetry and chronoamperometry tests show that the prepared Pt/TSNiPc–graphene catalyst exhibits much higher electrocatalytic activity and stability than the Pt/graphene catalyst for methanol oxidation, demonstrating a new synthesis strategy for the development of the high-performance graphene-based electrocatalysts for DMFCs applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (21263002, 21161003), Guangxi Natural Science Foundation of China (2013GXNSFAA019024, 2010GXNSFF013001), the S&T Project of Guangxi Education Department of China (2013YB026) and the Foundation Project of Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Guangxi Normal University), Ministry of Education of China (CMEMR2012-A3). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta.2013. 09.092. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666. [2] D. Chen, L.H. Tang, J.H. Li, Graphene-based materials in electrochemistry, Chemical Society Reviews 39 (2010) 3157. [3] C.C. Huang, C. Li, G.Q. Shi, Graphene based catalysts, Energy & Environmental Science 5 (2012) 8848. [4] S. Sharma, A. Ganguly, P. Papakonstantinou, X. Miao, M. Li, J.L. Hutchison, M. Delichatsios, S. Ukleja, Rapid microwave synthesis of CO tolerant reduced graphene oxide-supported platinum electrocatalysts for oxidation of methanol, Journal of Physical Chemistry C 114 (2010) 19459. [5] S.M. Choi, M.H. Seo, H.J. Kim, W.B. Kim, Synthesis of surface-functionalized graphene nanosheets with high Pt-loadings and their applications to methanol electrooxidation, Carbon 49 (2011) 904.

659

[6] J.D. Qiu, G.C. Wang, R.P. Liang, X.H. Xia, H.W. Yu, Controllable deposition of platinum nanoparticles on graphene as an electrocatalyst for direct methanol fuel cells, Journal of Physical Chemistry C 115 (2011) 15639. [7] B.M. Luo, X.B. Yan, S. Xu, Q.J. Xue, Polyelectrolyte functionalization of graphene nanosheets as support for platinum nanoparticles and their applications to methanol oxidation, Electrochimica Acta 59 (2012) 429. [8] Q.F. Shi, S.L. Mu, Preparation of Pt/poly(pyrogallol)/graphene electrode and its electrocatalytic activity for methanol oxidation, Journal of Power Sources 203 (2012) 48. [9] B. Xiong, Y.K. Zhou, Y.Y. Zhao, J. Wang, X. Chen, R. O’Hayre, Z.P. Shao, The use of nitrogen-doped graphene supporting Pt nanoparticles as a catalyst for methanol electrocatalytic oxidation, Carbon 52 (2013) 181. [10] S.Y. Wang, X. Wang, S.P. Jiang, Self-assembly of mixed Pt and Au nanoparticles on PDDA-functionalized graphene as effective electrocatalysts for formic acid oxidation of fuel cells, Physical Chemistry Chemical Physics 13 (2011) 6883. [11] L. Yan, Y.B. Zheng, F. Zhao, S.J. Li, X.F. Gao, B.Q. Xu, P.S. Weiss, Y.L. Zhao, Chemistry and physics of a single atomic layer: strategies and challenges for functionalization of graphene and graphene-based materials, Chemical Society Reviews 41 (2012) 97. [12] K.S. Subrahmanyam, A. Ghosh, A. Gomathi, A. Govindaraj, C.N.R. Rao, Covalent and noncovalent functionalization and solubilization of graphene, Nanoscience and Nanotechnology Letters 1 (2009) 28. [13] S. Niyogi, E. Bekyarova, M.E. Itkis, J.L. McWilliams, M.A. Hamon, R.C. Haddon, Solution properties of graphite and graphene, Journal of the American Chemical Society 128 (2006) 7720. [14] A. Chunder, T. Pal, S.I. Khondaker, L. Zhai, Reduced graphene oxide/copper phthalocyanine composite and its optoelectrical properties, Journal of Physical Chemistry C 114 (2010) 15129. [15] S.S. Roy, D.J. Bindl, M.S. Arnold, Templating highly crystalline organic semiconductors using atomic membranes of graphene at the anode/organic interface, Journal of Physical Chemistry Letters 3 (2012) 873. [16] R.A. Hatton, N.P. Blanchard, V. Stolojan, A.J. Miller, S.R.P. Silva, Nanostructured copper phthalocyanine-sensitized multiwall carbon nanotube films, Langmuir 23 (2007) 6424. [17] I. Balan, I.G. David, V. David, A.I. Stoica, C. Mihailciuc, I. Stamatin, A.A. Ciucu, Electrocatalytic voltammetric determination of guanine at a cobalt phthalocyanine modified carbon nanotubes paste electrode, Journal of Electroanalytical Chemistry 654 (2011) 8. [18] H.S. Yin, Y.L. Zhou, J. Xu, S.Y. Ai, L. Cui, L.S. Zhu, Amperometric biosensor based on tyrosinase immobilized onto multiwalled carbon nanotubes-cobalt phthalocyanine-silk fibroin film and its application to determine bisphenol A, Analytica Chimica Acta 659 (2010) 144. [19] J.P. Mensing, T. Kerdcharoen, C. Sriprachuabwong, A. Wisitsoraat, D. Phokharatkul, T. Lomasa, A. Tuantranont, Facile preparation of graphene–metal phthalocyanine hybrid material by electrolytic exfoliation, Journal of Materials Chemistry 22 (2012) 17094. [20] J.H. Yang, Y. Gao, W. Zhang, P. Tang, J. Tan, A.H. Lu, D. Ma, Cobalt phthalocyanine–graphene oxide nanocomposite: complicated mutual electronic interaction, Journal of Physical Chemistry C 117 (2013) 3785. [21] Y.Z. Zhang, G.Q. Mo, X.W. Li, J.S. Ye, Iron tetrasulfophthalocyanine functionalized graphene as a platinum-free cathodic catalyst for efficient oxygen reduction in microbial fuel cells, Journal of Power Sources 197 (2012) 93. [22] Y.Y. Jiang, Y.Z. Lu, X.Y. Lv, D.X. Han, Q.X. Zhang, L. Niu, W. Chen, Enhanced catalytic performance of Pt-free iron phthalocyanine by graphene support for efficient oxygen reduction reaction, ACS Catalysis 3 (2013) 1263. [23] J.P. Zhong, Y.J. Fan, H. Wang, R.X. Wang, L.L. Fan, X.C. Shen, Z.J. Shi, Copper phthalocyanine functionalization of graphene nanosheets as support for platinum nanoparticles and their enhanced performance toward methanol oxidation, Journal of Power Sources 242 (2013) 208. [24] C.G. Claessens, U. Hahn, T. Torres, Phthalocyanines: from outstanding electronic properties to emerging applications, Chemical Record 8 (2008) 75. [25] Z.B. Wang, P.J. Zuo, G.J. Wang, C.Y. Du, G.P. Yin, Effect of Ni on PtRu/C catalyst performance for ethanol electrooxidation in acidic medium, Journal of Physical Chemistry C 112 (2008) 6582. [26] X.W. Zhou, R.H. Zhang, Z.Y. Zhou, S.G. Sun, Preparation of PtNi hollow nanospheres for the electrocatalytic oxidation of methanol, Journal of Power Sources 196 (2011) 5844. [27] Z.Y. Wang, N. Li, Z.J. Shi, Z.N. Gu, Low-cost and large-scale synthesis of graphene nanosheets by arc discharge in air, Nanotechnology 21 (2010) 175602/1. [28] M.S. A˘gırtas¸, B. Cabir, S. Özdemir, Novel metal (II) phthalocyanines with 3,4,5-trimethoxybenzyloxy-substituents: synthesis, characterization, aggregation behaviour and antioxidant activity, Dyes and Pigments 96 (2013) 152. [29] J. Thompson, A. Crossley, P.D. Nellist, V. Nicolosi, Single-step exfoliation and chemical functionalisation of graphene and hBN nanosheets with nickel phthalocyanine, Journal of Materials Chemistry 22 (2012) 23246. [30] P. Kluson, M. Drobek, A. Kalaji, M. Karaskova, J. Rakusan, Preparation, chemical modification and absorption properties of various phthalocyanines, Research on Chemical Intermediate 35 (2009) 103. [31] Y. Wang, H.Z. Chen, H.Y. Li, M. Wang, Fabrication of carbon nanotubes/copper phthalocyanine composites with improved compatibility, Materials Science and Engineering B 117 (2005) 296. [32] A. Baba, Y. Kanetsuna, S. Sriwichai, Y. Ohdaira, K. Shinbo, K. Kato, S. Phanichphant, F. Kaneko, Nanostructured carbon nanotubes/copper phthalocyanine hybrid multilayers prepared using layer-by-layer self-assembly approach, Thin Solid Films 518 (2010) 2200.

660

J.-P. Zhong et al. / Electrochimica Acta 113 (2013) 653–660

[33] S.J. Guo, S.J. Dong, E.K. Wang, Three-dimensional Pt-on-Pd bimetallic nanodendrites supported on graphene nanosheet: facile synthesis and used as an advanced nanoelectrocatalyst for methanol oxidation, ACS Nano 4 (2010) 547. [34] Q. Su, S. Pang, V. Alijani, C. Li, X. Feng, K. Müllen, Composites of graphene with large aromatic molecules, Advanced Materials 21 (2009) 3191. [35] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Raman spectroscopy in graphene, Physics Reports 473 (2009) 51. [36] G.B. Zhu, X. Zhang, P.B. Gai, X.H. Zhang, J.H. Chen, ␤-Cyclodextrin noncovalently functionalized single-walled carbon nanotubes bridged by 3,4,9,10perylene tetracarboxylic acid for ultrasensitive electrochemical sensing of 9-anthracenecarboxylic acid, Nanoscale 4 (2012) 5703. [37] G.H. Zeng, Y.B. Xing, J. Gao, Z.Q. Wang, X. Zhang, Unconventional layer-by-layer assembly of graphene multilayer films for enzyme-based glucose and maltose biosensing, Langmuir 26 (2010) 15022. [38] L. Ding, Q. Xin, X.J. Zhou, J.L. Qiao, H. Li, H.J. Wang, Electrochemical behavior of nanostructured nickel phthalocyanine (NiPc/C) for oxygen reduction reaction in alkaline media, Journal of Applied Electrochemistry 43 (2013) 43. [39] H.L. Guo, X.F. Wang, Q.Y. Qian, F.B. Wang, X.H. Xia, A green approach to the synthesis of graphene nanosheets, ACS Nano 3 (2009) 2653. [40] G. Wang, J. Yang, J. Park, X. Gou, B. Wang, H. Liu, J. Yao, Facile synthesis and characterization of graphene nanosheets, Journal of Physical Chemistry C 112 (2008) 8192. [41] Y.C. Si, E.T. Samulski, Exfoliated graphene separated by platinum nanoparticles, Chemistry of Materials 20 (2008) 6792. [42] Q. Jiang, L.H. Jiang, S.L. Wang, J. Qi, G.Q. Sun, A highly active PtNi/C electrocatalyst for methanol electro-oxidation in alkaline media, Catalysis Communications 12 (2010) 67. [43] H.C. He, P. Xiao, M. Zhou, F.L. Liu, S.J. Yu, L. Qiao, Y.H. Zhang, PtNi alloy nanoparticles supported on carbon-doped TiO2 nanotube arrays for photo-assisted methanol oxidation, Electrochimica Acta 88 (2013) 782. [44] Y.L. Hsin, K.C. Hwang, C.T. Yeh, Poly(vinylpyrrolidone)-modified graphite carbon nanofibers as promising supports for PtRu catalysts in direct methanol fuel cells, Journal of the American Chemical Society 129 (2007) 9999.

[45] D.P. He, C. Zeng, C. Xu, N.C. Cheng, H.G. Li, S.C. Mu, M. Pan, Polyanilinefunctionalized carbon nanotube supported platinum catalysts, Langmuir 27 (2011) 5582. [46] K. Müller, M. Richter, D. Friedrich, I. Paloumpa, U.I. Kramm, D. Schmeißer, Spectroscopic characterization of cobalt-phthalocyanine electrocatalysts for fuel cell applications, Solid State Ionics 216 (2012) 78. [47] K.C. Lee, L. Zhang, H.S. Liu, R. Hui, Z. Shi, J.J. Zhang, Oxygen reduction reaction (ORR) catalyzed by carbon-supported cobalt polypyrrole (Co-PPy/C) electrocatalysts, Electrochimica Acta 54 (2009) 4704. [48] L. Ding, J.L. Qiao, X.F. Dai, J. Zhang, J.J. Zhang, B.L. Tian, Highly active electrocatalysts for oxygen reduction from carbon-supported copper-phthalocyanine synthesized by high temperature treatment, International Journal of Hydrogen Energy 37 (2012) 14103. [49] Z.M. Cui, C.M. Li, S.P. Jiang, PtRu catalysts supported on heteropolyacid and chitosan functionalized carbon nanotubes for methanol oxidation reaction of fuel cells, Physical Chemistry Chemical Physics 13 (2011) 16349. [50] Y.Q. Huang, H.L. Huang, Y.J. Liu, Y. Xie, Z.R. Liang, C.H. Liu, Facile synthesis of poly(amidoamine)-modified carbon nanospheres supported Pt nanoparticles for direct methanol fuel cells, Journal of Power Sources 201 (2012) 81. [51] J.R.C. Salgado, F. Alcaide, G. Álvarez, L. Calvillo, M.J. Lázaro, E. Pastor, Pt–Ru electrocatalysts supported on ordered mesoporous carbon for direct methanol fuel cell, Journal of Power Sources 195 (2010) 4022. [52] K.L. Nagashree, N.H. Raviraj, M.F. Ahmed, Carbon paste electrodes modified by Pt and Pt–Ni microparticles dispersed in polyindole film for electrocatalytic oxidation of methanol, Electrochimica Acta 55 (2010) 2629. [53] Y.J. Kuang, Y. Cui, Y.S. Zhang, Y.M. Yu, X.H. Zhang, J.H. Chen, A strategy for the high dispersion of PtRu nanoparticles onto carbon nanotubes and their electrocatalytic oxidation of methanol, Chemistry – A European Journal 18 (2012) 1522. [54] L.Q. Hoa, M.C. Vestergaard, H. Yoshikawa, M. Satio, E. Tamiya, Functionalized multi-walled carbon nanotubes as supporting matrix for enhanced ethanol oxidation on Pt-based catalysts, Electrochemistry Communications 13 (2011) 746.