PtNi Alloy Nanoparticles Supported on Polyelectrolyte Functionalized ...

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Jan 19, 2013 - aLaboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of ... bState Key Laboratory of Solid Lubrication, Lanzhou Institute of ...
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Journal of The Electrochemical Society, 160 (3) F262-F268 (2013) 0013-4651/2013/160(3)/F262/7/$31.00 © The Electrochemical Society

PtNi Alloy Nanoparticles Supported on Polyelectrolyte Functionalized Graphene as Effective Electrocatalysts for Methanol Oxidation Baomin Luo,a,b,c Shan Xu,d Xingbin Yan,a,b,z and Qunji Xueb a Laboratory

of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China c Graduate School of Chinese Academy of Sciences, Beijing 100039, China d State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b State

This paper reports the use of a polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA), for fabricating bimetallic, highly active PtNi nanocatalysts supported on PDDA-functionalized graphene (P-G). The PDDA-Coating creates a distribution of positive charges on the surfaces of G, which is in favor of the electrostatic self-assembly of negatively charged PtCl6 2− ions and positively charged Ni2+ ions on the P-G, respectively. The subsequent reduction process by ethylene glycol gives PtNi nanoparticles with uniform distribution on the P-G. The as-made PtNi nanocatalysts supported on the P-G show better activity and stability for the electro-oxidation of methanol compared with pure Pt nanocatalyst supported on the P-G and PtNi nanocatalysts supported on G. Most importantly, the as-made Pt2 Ni1 /P-G and Pt1 Ni1 /P-G catalysts display higher activity and stability than the commercial Johnson Matthey PtRu/C (20%) catalyst (JM PtRu/C). © 2013 The Electrochemical Society. [DOI: 10.1149/2.056303jes] All rights reserved. Manuscript submitted November 20, 2012; revised manuscript received December 31, 2012. Published January 19, 2013.

Direct methanol fuel cells (DMFCs) have drawn great attention recently due to their high energy-conversion efficiency and energy density, low operating temperature, low-to-zero pollutant emissions, as well as the simple handling and processing of the fuel.1 Pt is the most effective catalyst for DMFCs: it facilitates oxygen reduction at a cathode and methanol oxidation at an anode. However, the high cost of the Pt electro-catalyst is still an obstacle inhibiting broad applications of DMFCs. Moreover, pure Pt catalysts are readily poisoned by carbon monoxide, which is a by-product of methano electrooxidation. One approach that has been demonstrated to improve the catalytic activity and to lower the cost of the catalysts is the alloying of Pt with other transition metals, such as Ru, Fe, Sn, Ni, etc.2–18 Incorporation nonprecious metals into the catalysts nanostructure not only reduces the dosage of Pt but also remits the poisoning of the Pt catalysts by carbon monoxide. Among them, PtNi alloy is a promising anode catalyst for DMFCs.5–18 Theoretical calculations have shown that the segregation process which generally leads to Pt surface enrichment is unlikely to occur in the Pt-Ni system.19 Furthermore, in the potential range useful for methanol oxidation, Ni from the PtNi alloy would not dissolve in the electrolyte. The resistance to electrolyte dissolution has been attributed to a nickel hydroxide passivated surface and by the enhanced stability of Ni in the Pt lattice.5 Another approach to reduce the Pt dosage is to use a carbon support with high surface area, such as carbon black, carbon nanotubes, mesoporous carbon and carbon nanofibers, to enhance the dispersion of metal nanoparticles (NPs), which can increase the utilization and efficiency of Pt and Pt-based alloy electro-catalysts.20–23 Graphene (G) has attracted an enormous amount of interests from both theoretical and experimental scientists due to their potential applications, such as lithium ion batteries,24–28 electrochemical capacitors,29–32 solar cells,33–36 field emission,37–41 and biosensors.42–46 G also has potential application as heterogeneous catalyst supports in direct methanol fuel cells due to their specific electronic conductivity and extremely high specific surface area.47–54 Here, an attempt to synthesize uniform PtNi alloy NPs dispersed on G for high-performance methanol oxidation was reported. It is difficult to deposit metal nanoparticles on G with uniform distribution if no surface functionalization. It has been reported that functionalization G or graphene oxide with poly(diallyldimethylammoniumchloride) (PDDA) facilities the uniform distribution of Pt, PtAu and PtPd NPs z

E-mail: [email protected]

on G.55–61 In order to deposit PtNi NPs with uniform distribution, the G are functionalizated by PDDA before loading PtNi NPs. To the best of our knowledge, this is the first report on functionalization of G by PDDA and using it as the support for PtNi NPs. The PtNi NPs are uniformly deposited on G after the functionalization. Most importantly, the as-prepared PtNi NPs supported on the PDDA-functionalized G (P-G) exhibit improved electro-catalytic activity and stability toward methanol oxidation. Experimental Materials.— PDDA (20 wt% in water, MW = 400,000–500,000) and 5 wt% nafion solution were purchased from Sigma. Graphite powder (325 mesh), Carbon black (Vulcan, XC-72R), H2 PtCl6 · 6H2 O, NiCl2 · 6H2 O, NaOH, H2 SO4 , and ethanol were purchased from Qingdao Huatai Tech. Co. Ltd., Cabot Corp., Shenyang Keda reagent factory, Shanghai Hunter Special Chemicals Co. Ltd., Tianjin Dehn Chemicals Co. Ltd., Baiyin Liangyou Chemicals Co. Ltd., and China National Pharmaceutical Corp., respectively. Ethylene glycol (EG), methanol and acetone were purchased from Tianjin Fuyu Special Chemicals Co. Ltd. All reagents were used as received without further purification. Deionized water (DI water) with resistance of approximately 18 M cm was used throughout the experiment. Synthesis of PtNi NPs on the P-G.— The procedures for preparations of G and P-G were present in our previous work.58 To synthesize PtNi NPs with different Pt:Ni atomic ratio, 30 mg of P-G, 3 mL of H2 PtCl6 solution (7.9 mg mL−1 ) and 30 mL of EG were mixed under ultrasonication, and then 7, 13.92, and 27.8 mg NiCl2 · 6H2 O were added into the solution, respectively. The pH value of the reaction system was then adjusted to 12 by adding dropwise 2.5 M NaOH solution, followed by refluxing the solution at 130◦ C for 3 h to ensure the complete reduction of metal salts. After the reduction, the pH value was adjusted to 3–4 and the reaction system was kept on stirring for 1 hour. The PDDA-Coating creates a distribution of positive charges on the surfaces of G. This leads to the self-assembly the negatively charged PtCl6 2− , followed by the subsequent self-assembly of positively charged Ni2+ on P-G. The following reduction of metal precursors using EG realizes the formation of the PtNi NPs on P-G. Fig. 1 illustrates schematically the principle of the synthesis of PtNi electrocatalysts on P-G. After the deposition of NPs, the solid products were obtained by filtration using a polyvinylidene fluoride membrane (pore

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Figure 2. TEM (a) and SEM (b) images of G.

Results and Discussion

Figure 1. Schematic diagram of the synthesis of PtNi/P-G catalysts.

size: 0.45 μm) and washing with DI water and acetone, followed by drying in air at 60◦ C for 24 h. According to the nominal atomic ratio between Pt and Ni (2:1, 1:1, and 1:2, respectively), the as-prepared composites were denoted as Pt2 Ni1 /P-G, Pt1 Ni1 /P-G, and Pt1 Ni2 /P-G, respectively. For the sake of comparison, pure Pt NPs deposited on P-G (denoted as Pt/P-G), PtNi NPs with atomic ratio of 1:1 supported on G and PDDA-functionalized carbon black (denoted as Pt1 Ni1 /G and Pt1 Ni1 /P-C) were also prepared. The procedure for preparation of Pt/P-G was similar with that for preparation of Pt1 Ni1 /P-G except without NiCl2 · 6H2 O in reaction system. The procedure for functionalization of carbon black with PDDA was presented in our previous work.58 The procedures for preparation of Pt1 Ni1 /G and Pt1 Ni1 /P-C were similar with that for preparation of Pt1 Ni1 /P-G and the P-G was replaced by G and P-C, respectively.

Characterization of G.— To characterize the morphology of G, TEM and SEM were carried out and the corresponding results are presented in Fig. 2a and 2b. It can be seen the G exhibit the typical tulle-like structure. XPS was used to analyze the element chemical states of GO and G. The C1s XPS spectra of GO and G are shown in Fig. 3a and 3b. The C1s XPS spectrum of GO clearly indicates a considerable degree of oxidation with four components: (a) nonoxygenated C–C=C at 284.6 eV, (b) C–O at 286.7 eV, (c) C=O at 287.8 eV and (d) O–C=O at 288.6 eV. In contrast to the C1s XPS

Characterization.— Structural and morphological investigations of the samples were performed by X-ray diffraction using Cu Kα radiation (XRD, Panalytical X’ Pert Pro), transmission electron microscope (TEM, Tecnai-G2-F30) and field-emission scanning electron microscope (FESEM, JSM-6701F). The thickness of G was investigated using Nanoscope III, a multimode atomic force microscope (AFM, Veeco) in tapping mode. An IRIS Advantage ER/S inductively coupled plasma spectrometer (ICP, TJA) was used for all metaldeterminations. The surface chemical species of graphene oxide (GO, precursor of G), G and Pt1 Ni1 /P-G were examined on a Perkin-Elmer PHI-5702 multifunctional X-ray photoelectron spectroscope (XPS, Physical Electronics) using Al Ka radiation of 1486.6 eV as the excitation source.

Electrochemical measurement.— Cyclic voltammetry (CV) and chronoamperometry (CA) measurements were performed with a CHI 660D electrochemical workstation at room temperature. As a typical process, 5 mg catalyst was ultrasonically mixed with 2 mL of ethanol to form a homogeneous ink, 10 μL of this ink was dropped onto the surface of a glassy carbon electrode (GCE, 5 mm in diameter), and then 3 μL of diluted nafion solution (5 wt% nafion solution was diluted by 10 times ethanol by volume) was dropped to fix the catalyst sample on the electrode. Before each experiment, the GCE was polished using 0.05 μm alumina powders followed by washing in ethanol and distilled water. Pt wire and a saturated calomel electrode were used as counter electrode and reference electrode, respectively. All the potentials in the paper are presented with respect to normal hydrogen electrode (NHE).

Figure 3. The C 1s XPS spectra of GO and G.

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Table I. The loading of Pt and Ni and atomic ratios of Pt to Ni.

Electrocatalyst

Figure 4. (a) AFM image of G, (b) cross-sectional analysis along the lines shown in AFM image.

spectrum of GO, the spectrum of G shows the presence of the same functionalities but with much weaker intensities, indicating that most of the oxygen functional groups were reduced. AFM image and its corresponding height profile (Fig. 4a and 4b) show that an individual G on a silicon wafer surface has a thickness of 0.8∼1.0 nm. In a manner that parallels previous results, the observed G thicknesses are significantly greater than the theoretical values of G layer (0.34 nm) because a small number of oxygen-containing groups still remain on the surface of G after reduction. XRD and XPS analysis of the catalysts.— Fig. 5 shows the XRD patterns of the Pt/P-G, Pt1 Ni1 /P-C, Pt1 Ni1 /G, Pt2 Ni1 /P-G, Pt1 Ni1 /P-G and Pt1 Ni2 /P-G catalysts. The broad diffraction peak observed around 2θ = 24◦ is due to the hexagonal structure of G and carbon black.58 For the Pt/P-G, the face-centered cubic (fcc) structure of Pt is confirmed by the presence of diffraction peaks at 39.8◦ , 46.2◦ , and 67.5◦ , which are assigned to Pt (111), Pt (200), and Pt (220) planes, respectively. Compared with the pattern of pure Pt, the diffraction peaks for all PtNi alloy catalysts all shift slightly to higher 2θ values. The shift reveals that Ni atoms have entered into the Pt lattice and PtNi alloys have formed. Moreover, the atomic ratios of Pt to Ni are calculated by the shift of the peaks using Vegard’s law and calculated using ICP results, which are shown in Table I. The atomic ratios of Pt to Ni calculated from ICP results are different from those calculated from XRD results. The reason for the different results obtained from XRD and ICP is because part of Pt and Ni atoms do not form alloy. The atomic ratios calculated from XRD peak shift reveal the real atomic ratios of Pt to Ni atoms in PtNi alloy, whereas those calculated from ICP results reveal the total atomic ratios of Pt to Ni atoms including the non-alloy Pt and Ni atoms. Take Pt1 Ni1 /P-G catalyst as an example, the XPS

Figure 5. XRD patterns of (a) Pt/P-G, (b) Pt1 Ni1 /P-C, (c) Pt1 Ni1 /G, (d) Pt2 Ni1 /P-G, (e) Pt1 Ni1 /P-G, and (f) Pt1 Ni2 /P-G.

Pt/P-G Pt1 Ni1 /P-C Pt1 Ni1 /G Pt2 Ni1 /P-G Pt1 Ni1 /P-G Pt1 Ni2 /P-G

Pt loading (wt%)

Ni loading (wt%)

Pt:Ni from XRD

Pt:Ni from ICP

13.04 17.60 14.93 13.69 14.44 14.37

0 3.22 4.93 1.87 3.52 8.93

– 7:1 4:1 4:1 4:1 4:1

– 8:5 1:1 2:1 1:1 1:2

results show that Pt and Ni atoms do not entirely form alloy. Figure 6a shows the Pt 4f region of the spectrum, which could be deconvoluted into two pairs of doublets. The most intense doublet (at 70.8 and 74.1 eV) is the signature of metallic Pt. The second and weaker doublet (at 72.3 and 75.6 eV), with binding energy at 1.5 eV higher than Pt(0), could be assigned to the Pt(II) oxidation state, such as PtO and Pt(OH)2 .62 The Ni 2p3/2 spectrum shows a complex structure with intense satellite signals of high binding energy adjacent to the main peaks, which may be ascribed to a multi-electron excitation (shake-up peaks) (Fig. 6b). Curve fitting of the Ni 2p3/2 signals gives different nickel species. After considering the shake-up peaks, the Ni 2p3/2 XPS peaks at the binding energies of 852.7 eV, 853.8 eV, 855.6 eV, and 857.3 eV are ascribed to Ni(0), NiO, Ni(OH)2 , and

Figure 6. XPS spectra of (a) Pt 4f and (b) Ni 2p of Pt1 Ni1 /P-G.

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Table II. Comparison of the electrochemical properties of the Pt1 Ni1 /P-C, PtRu/C, Pt1 Ni1 /G, and Pt1 Ni1 /P-G catalysts. ECSA If-mass If-specific I3600s Electrocatalyst (m2 g−1 Pt ) (mA mg−1 Pt ) (mA cm−2 ) (mA mg−1 Pt ) Pt1 Ni1 /P-C PtRu/C Pt1 Ni1 /G Pt1 Ni1 /P-G

54.5 36.8 38.2 40.3

498.5 926.4 1044.8 1220.2

0.9 2.5 2.7 3.0

4.4 6.9 32.7 64.3

interplanar distance of fcc Pt and Ni, respectively. All the Pt and PtNi NPs are enclosed mainly by (111) facets.

Figure 7. TEM images of (a) Pt/P-G, (b) Pt1 Ni1 /P-C, (c) Pt1 Ni1 /G, (d) Pt2 Ni1 /P-G, (e) Pt1 Ni1 /P-G, and (f) Pt1 Ni2 /P-G. The insets show the corresponding HRTEM images of Pt and PtNi NPs. The scale bar of the insets is 5 nm.

NiOOH, respectively.5 Furthermore, compared with pure Pt, the Pt 4f XPS spectrum has a peak shift of −0.4 eV for Pt 4f7/2 (71.20 eV for pure Pt) and Pt 4f5/2 (74.53 eV), respectively, indicating an electronic structural change of Pt when it is alloyed with Ni. The result is in agreement with previous report.7 It can be seen that only part of Ni atoms form alloy with Pt atoms. Most likely, Ni(0) occupies part of platinum sites, and the metallic grains are intermixed with amorphous Ni oxides, such as NiO, Ni(OH)2 and NiOOH revealed in the XPS spectra.7 Morphology analysis.— The morphology and dispersion of Pt and PtNi nanocatalysts were examined by TEM. As shown in Fig. 7, both Pt and PtNi NPs are uniformly supported on the surfaces of the P-G and P-C with narrow size distributions. In the case of Pt1 Ni1 /G, poor dispersion of PtNi alloy NPs on G with a large number of aggregates is found (Fig. 7c). The result indicates that the P-G can avoid the aggregation of PtNi NPs supported on them compared with G. The dispersion of the PtNi NPs is also better the former report on synthesis of PtNi NPs on G.6 The crystalline state of the as-prepared PtNi NPs is confirmed by the high-resolution TEM (HRTEM) analysis. The analysis presents lattice fringes with an interplanar distance of 0.226 nm for Pt/P-G, 0.224 nm for Pt1 Ni1 /P-C, and 0.222 nm for Pt2 Ni1 /P-G, Pt1 Ni1 /P-G and Pt1 Ni2 /P-G. The spacing of 0.224 nm and 0.222 nm is between 0.226 and 0.203 nm, corresponding to the (111)

Electrochemical measurement.— The electrochemically active surface area (ECSA) of the catalysts can be calculated from the areas of hydrogen desorption after deduction of the double-layer region. Fig. 8a shows the cyclic voltammograms of the PtRu/C, Pt1 Ni1 /P-C, Pt1 Ni1 /G, and Pt1 Ni1 /P-G catalysts in N2 -purged 0.5 M H2 SO4 at a sweep rate of 50 mV s−1 . The ECSA (m2 g−1 Pt ) of the catalysts is estimated according to the equation ECSA = QH /(210×WPt ), where WPt represents the Pt loading (μg) on the electrode, QH is the total charge (μC) for hydrogen desorption, and 210 represents the charge (μC cm−2 Pt ) required to oxidize a monolayer of hydrogen on a bright Pt surface.16 The ECSA of the catalysts are summarized in Table II. The Pt1 Ni1 /P-G catalyst shows higer ECSA value than the Pt1 Ni1 /G catalyst, apparently due to the better dispersion of the PtNi NPs on P-G. The catalytic activity of the PtRu/C, Pt1 Ni1 /P-C, Pt1 Ni1 /G, and Pt1 Ni1 /P-G catalysts was characterized by CV in 0.5 M H2 SO4 + 1.0 M CH3 OH aqueous solution at a potential scan rate of 50 mV s−1 , and the corresponding results are shown in Fig. 8b and 8c. The scans in both the positive and negative directions show characteristic CV peaks for methanol oxidation. The forward scan peak (at ca. 0.89 V vs NHE) current density (If ) can be used to judge the catalyst activity. Here we use the mass activity (If-mass ), which is defined as the ratio of current obtained from the forward CV scans to the mass of Pt on the GCE, and the specific activity (If-specific ), which is defined as the ratio of peak current obtained from the forward scans of CV curve to the ECSA, to evaluate the activity of catalysts. The mass activities and specific activities of the Pt1 Ni1 /P-G catalyst are higher than these of the Pt1 Ni1 /P-C catalyst, indicating P-G is superior to P-C as support. Moreover, the mass and specific activities of the Pt1 Ni1 /P-G catalyst are higher than these of the Pt1 Ni1 /G, apparently due to the better dispersion of the PtNi NPs on P-G, and indicating P-G is a more effective support than P-C. Most importantly, the Pt1 Ni1 /P-G catalyst shows higher activities than the commercial JM PtRu catalyst. The detailed data have been shown in Table II. Because durability is one of the major concerns in current fuelcell technology, the stability of the catalysts was further tested by CA measurements in 0.5 M H2 SO4 + 1.0 M CH3 OH aqueous solution at 0.894 V (vs. NHE) for 3600 s, and the corresponding results are shown in Fig. 8d and Table II. All samples display an initial fast current decay, which is attributed to the poisoning of the catalysts by intermediate species such as COads , CH3 OHads , and CHOads formed during the methanol oxidation reaction. The residual current density after testing for 3600 s (I3600s ) is shown in Table II. The I3600s of the Pt1 Ni1 /P-G catalyst is the largest, indicating that the Pt1 Ni1 /P-G catalyst has excellent stability toward the electro-oxidation methanol, indicating it possesses good tolerance toward reaction intermediates and favors long-term application as the anode materials in DMFCs. To study the impact of Ni content on the electrochemical properties of the PtNi catalysts, the ECSAs, activities and stabilities of the Pt/ P-G, Pt2 Ni1 /P-G, Pt1 Ni1 /P-G, and Pt1 Ni2 /P-G catalysts were tested and the corresponding results are shown in Fig. 9 and Table III. It can be seen from Table III that the proper addition of Ni can increase the ECSA of the catalysts. The increased ECSA may be due to Ni atoms occupy the sites of interior Pt atoms and thus more Pt atoms

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Figure 8. (a) Cyclic voltammograms of the PtRu/C, Pt1 Ni1 /P-C, Pt1 Ni1 /G and Pt1 Ni1 /P-G catalysts in 0.5 M H2 SO4 aqueous solution; cyclic voltammograms of the PtRu/C, Pt1 Ni1 /P-C, Pt1 Ni1 /G and Pt1 Ni1 /P-G catalysts in 0.5 M H2 SO4 + 1.0 M methanol aqueous solution, (b) normalized by the mass of Pt, and (c) normalized by the ECSA; (d) Current-time plots of the PtRu/C, Pt1 Ni1 /P-C, Pt1 Ni1 /G and Pt1 Ni1 /P-G catalysts in 0.5 M H2 SO4 + 1.0 M methanol aqueous solution at 0.894 V.

can be arranged at particle surface. When the Ni atoms are excess, however, part of Pt atoms at surface will be covered by NiO, Ni(OH)2 and NiOOH molecules. The mass activity of the catalysts follow the trend of Pt2 Ni1 /P-G > Pt1 Ni1 /P-G > Pt1 Ni2 /P-G > Pt/P-G (Fig. 9b). The result indicates that the proper addition of Ni can markedly increase the mass activity of the catalysts. But the excess addition of Ni is harmful for the mass activity of the catalysts. The enhanced CH3 OH oxidation activity on the PtNi alloy catalysts can be explained as follows: (1) The electronic structural of Pt is changed when it is alloyed with Ni, which lowers the bonding energy between Pt and COads . It is well known that COads is an intermediate species of methanol oxidation, which poisons the Pt catalyst and reduce the Pt activity. In other words, COads is more

easily removed from Pt atom.11,18,64 (2) As we all known that COads can be removed by following reactions: Pt + H2 O = Pt-OH + H+

[1]

M-OH + Pt-CO = M + Pt + CO2 + H+

[2]

where M is Pt, Ru, Ni etc. NiO, Ni(OH)2 and NiOOH could provide Ni-OH to remove COads from Pt active sites, thus providing more active Pt sites for methanol oxidation.18,63,64 (3) The addition of Ni to Pt lattice makes more Pt atoms grow on the grain surface, thus the catalytic active sites of catalysts are increased, which is reflected from the ECSA values of the catalysts. The Ni contents in reaction

Table III. Comparison of the electrochemical properties of the Pt/P-G, Pt2 Ni1 /P-G, Pt1 Ni1 /P-G, and Pt1 Ni2 /P-G catalysts.

Electrocatalyst Pt/P-G Pt2 Ni1 /P-G Pt1 Ni1 /P-G Pt1 Ni2 /P-G

ECSA (m2 g−1 Pt )

If-mass (mA mg−1 Pt )

If-specific (mA cm−2 )

I7200s (mA mg−1 Pt )

If-310 (mA mg−1 Pt )

If-310 :If-mass (%)

27.8 53.8 40.3 15.7

696.6 1452.3 1220.2 734.7

2.5 2.7 3.0 4.7

4.1 31.7 11.7 5.8

505.4 1170.3 1045.4 1154.6

72.6 80.6 85.7 81.7

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Figure 9. (a) Cyclic voltammograms of the Pt/P-G, Pt2 Ni1 /P-G, Pt1 Ni1 /P-G and Pt1 Ni2 /P-G catalysts in 0.5 M H2 SO4 aqueous solution; cyclic voltammograms of the Pt/P-G, Pt2 Ni1 /P-G, Pt1 Ni1 /P-G and Pt1 Ni2 /P-G catalysts in 0.5 M H2 SO4 + 1.0 M methanol aqueous solution, (b) normalized by the mass of Pt, and (c) normalized by the ECSA; (d) Current-time plots of the Pt/P-G, Pt2 Ni1 /P-G, Pt1 Ni1 /P-G and Pt1 Ni2 /P-G catalysts in 0.5 M H2 SO4 + 1.0 M methanol aqueous solution at 0.894 V.

system strongly impact on the activity of the as-prepared PtNi/P-G. It presents a facile method to tune the catalytic activity of catalysts by changing Ni content in reaction system. However, the specific activity of the catalysts increases with the increase of Ni content, it may be because more sufficient NiO, NiOOH, and Ni(OH)2 around Pt atoms are available with the increase of the Ni content for the removing of COads species (Fig. 9c). The stability of the samples was tested by CA. To get a steadystate current density, the test duration was set to 7200 s. The residual current density after testing for 7200 s (I7200s ) is shown in Table III. The PtNi alloy catalyst shows higher stability than the pure Pt catalyst, and the I7200s of the Pt2 Ni1 /P-G catalyst is the highest indicating its stability is the best in the CA test. The stability of the four samples was also tested by CV. As shown in the inset of Fig. 9d, the activity of the samples increases at beginning and then decreases gradually. This may result from three reasons: change of the surface structure of the catalysts; accumulation of poisonous species (such as COads ) on the surface of the catalysts and methanol consumption during the successive scans.65 For the third reason, a control experiment was carried out: after 300 cycles, the catalysts were investigated again by CV in fresh electrolyte for another 10 cycles. The ratios of the peak current density in the last cycle (If-310 ) to the highest peak current density (If-mass ) for the PtNi/P-G catalysts are larger than that for the

Pt/P-G catalyst, indicating the cycle stability of the PtNi/P-G catalysts is better than that of the Pt/P-G catalyst. Moreover, the If-310 of the Pt2 Ni1 /P-G catalyst is the highest. Conclusions In conclusion, we have reported a way to synthesize PtNi alloy catalysts on G by the functionalization of G with PDDA, the electrostatic self-assembly of PtCl6 2− and Ni2+ ions on the P-G, and subsequent the reduction using ethylene glycol. The PtNi NPs are uniformly deposited on the PDDA-functionalized G. The PtNi/P-G catalysts show higher electrocatalytic activity and better stability than the Pt/P-G catalyst. Most importantly, the Pt2 Ni1 /P-G and Pt1 Ni1 /P-G catalysts show much better activity and stability than the commercial PtRu/C catalyst and the activity and stability of the Pt2 Ni1 /P-G is the best. In this sense, the Pt2 Ni1 /P-G catalysts can be a promising catalyst for methanol oxidation in fuel cells. Such synthetic procedure can be extended to prepare G-supported various metal catalysts. Acknowledgments This work was supported by the Top Hundred Talents Program of Chinese Academy of Sciences, the National Nature Science Foundations of China (51005225 and 21103205).

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Journal of The Electrochemical Society, 160 (3) F262-F268 (2013) References

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