C (x = 0−0.5)

In this paper, we reported on the electrocatalytic activities for methanol oxidation of Pt0.495Ru0.505/C, Pt0.526-. Sn0.474/C, Pt0.467Ru0.495Sn0.038/C...
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J. Phys. Chem. C 2008, 112, 6337-6345

6337

Electrocatalytic Methanol Oxidation of Pt0.5Ru0.5-xSnx/C (x ) 0-0.5) Jing Zhu, Fangyi Cheng, Zhanliang Tao, and Jun Chen* Key Laboratory of Energy-Material Chemistry (Tianjin) and Engineering Research Center of Energy Storage & ConVersion (Ministry of Education), Chemistry College, Nankai UniVersity, Tianjin 300071, People’s Republic of China ReceiVed: January 3, 2008; In Final Form: February 22, 2008

In this paper, we reported on the electrocatalytic activities for methanol oxidation of Pt0.495Ru0.505/C, Pt0.526Sn0.474/C, Pt0.467Ru0.495Sn0.038/C, Pt0.472Ru0.413Sn0.115/C, Pt0.504Ru0.354Sn0.142/C, Pt0.537Ru0.243Sn0.22/C, and Pt0.487Ru0.152Sn0.361/C catalysts. The carbon-supported alloy catalysts were prepared by a modified electroless deposition method. The electrocatalytic activities for methanol oxidation were investigated by using cyclic voltammetry, chronoamperometry, slow linear sweep measurements, and Tafel steady-state plots. The results indicate that the Pt0.472Ru0.413Sn0.115/C catalyst exhibits superior electrochemical performance for methanol oxidation. Tafel plots demonstrate that the addition of Sn into the binary Pt-Ru system could accelerate the rate-determining reactions (RDS) of methanol oxidation rather than alter the RDS step. The apparent activation energies for methanol oxidation of the binary and ternary catalysts were determined by Tafel plots at different temperatures, showing that Pt0.472Ru0.413Sn0.115/C can significantly lower the activation energy in comparison with that of Pt-Ru/C and Pt-Sn/C. The present results confirm the synergy effect of Ru and Sn in the electrocatalysts and meanwhile suggest the applicability of Pt0.472Ru0.413Sn0.115/C for methanol oxidation with reduced catalyst price and enhanced catalytic activity.

Introduction In the past decade, fuel cells have emerged as an ideal device for energy storage and conversion owing to their high-energy conversion efficiency and low pollutant emission.1,2 Among various fuel cells, direct methanol fuel cells (DMFCs) appear to be one of the most promising systems because of their low operating temperatures and the use of a liquid fuel (methanol) that possesses high energy density, are facile to be stored and transported, and do not require any prior reforming.3,4 However, it is known that the widespread commercial application of these cells is hindered by the limited performance arising mainly from the sluggish methanol oxidation kinetics and limited durability of catalysts as well as their high cost due to the exclusive use of platinum and platinum alloy catalysts.1-5 Thus, it is of great scientific and practical importance to exploit relatively inexpensive and highly active electrocatalysts for methanol oxidation. The complete oxidation of methanol involves the sequence of methanol adsorption, dehydrogenation, and the successive oxidation of CO-like intermediates.6 Effective anode catalysts are required for not only accelerating dehydrogenation reaction but also improving the tolerance toward CO poisoning. Pt is recognized to be the most active metal for methanol dehydrogenation. On one hand, the formation of CO-like intermediates is prone to poison the Pt catalyst and thus causes efficiency loss. The addition of a secondary element such as Ru, Au, Mo, or Sn in the platinum system has been widely studied in the past decade and proved to be effective for oxidizing the adsorbed species on catalyst surface.6-10 Among them, ruthenium is the most common and successful promoter employed in the Pt-based catalyst due to their superior CO tolerance, which is generally * Corresponding author. Fax: (86) 22-2350-9118. E-mail: chenabc@ nankai.edu.cn.

attributed to a bifunctional effect.11 In addition, ternary systems such as PtRuW,12 PtRuSn,13,14 PtRuIr,15 and PtRuMo16 have also been investigated and found to further improve the performance of the Pt-based catalysts. On the other hand, the promotion mechanism of the ternary catalysts has remained somewhat elusive up to now. For example, in regard to ternary Pt-Ru-Sn catalyst, most studies have merely focused on evaluating the effect of the preparation method, the supports, and the composition or the redox state of Sn in the catalysts.14,17,18 Also, the reported results by different groups are even inconsistent with each other. For instance, Kobayashi and co-workers claimed that the catalytic activity of Pt3Ru2Sn/C for methanol oxidation was significantly higher than that of PtRu/C owing to the synergic effect of Ru as a water activator and Sn as an electronic modifier.13 Scott et al. compared the performance of titanium mesh supported PtRu and PtRuSn and found that PtRuSn exhibited better activity, which was associated with a significant increase of the surface area by the addition of Sn.17 Contrary results by Neto et al. indicated that Pt-Ru-Sn/C catalysts made by an alcohol reduction method were inferior to Pt-Sn/C.14 Pt-Ru-Sn (1: 1:1) catalysts prepared by an impregnation and colloid method were reported to be less active than Pt-Ru catalyst because of the blocking effect caused by the additive.16 Therefore, a systematic electrochemical study about the effect of Ru and Sn in the ternary catalysts is required to clearly elucidate the synergistic mechanism of Ru and Sn in the combination systems. In the present work, binary Pt-Ru and Pt-Sn catalysts and ternary Pt0.467Ru0.495Sn0.038, Pt0.472Ru0.413Sn0.115, Pt0.504Ru0.354Sn0.142, Pt0.537Ru0.243Sn0.22, and Pt0.487Ru0.152Sn0.361 catalysts supported on carbon were prepared by a modified electroless deposition method. Various electrochemical measurements including cyclic voltammetry (CV), chronoamperometry, slow linear sweep voltammetry (SLV), and Tafel plots were employed

10.1021/jp8000543 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/03/2008

6338 J. Phys. Chem. C, Vol. 112, No. 16, 2008 to systematically evaluate the catalytic activities of the asprepared catalysts for methanol oxidation. The results showed that the as-prepared Pt0.472Ru0.413Sn0.115/C exhibited better electrocatalytic performance than that of Pt-Ru/C or Pt-Sn/C catalysts. We also present in this paper the kinetic data of methanol oxidation over the obtained various catalysts using Tafel analysis. Furthermore, the rate-determining steps were analyzed from the Tafel slopes, and the apparent activation energies of methanol oxidation were determined from current densities at different over-potential range and different temperatures. Our results indicate the applicability of Pt0.472Ru0.413Sn0.115/C electrocatalyst for DMFCs with relatively enhanced activity and lowered dosage of noble metal, and reveal the synergistic effect of Ru and Sn in the ternary Pt-Ru-Sn systems from both the kinetic and the thermodynamic aspects. Experimental Section Catalysts Preparation. Vulcan XC-72 carbon black (Cabot, with a specific surface area of 250 m2 g-1) was used as the support. H2PtCl6‚6H2O, RuCl3, SnCl2‚2H2O, PdCl2, and other reagents were all of analytical grade and used without further purification. Nafion solution (5 wt %) was purchased from Aldrich and used as received. The carbon-supported Pt-Ru-Sn, Pt-Ru, and Pt-Sn catalysts were synthesized via a modified electroless deposition method described as follows.19 (1) Vulcan XC-72 carbon black was ultrasonicated in the activation-sensitization colloid solution containing SnCl2‚2H2O, PdCl2, Na2SnO3‚4H2O, and ethylene glycol for 10 min and then washed with distilled water. (2) The collected solid was re-dispersed in the solution containing a certain amount of H2PtCl6 (1.93 mM), SnCl2‚2H2O (4.43 mM), and RuCl3 (3.63 mM) and stirred vigorously for 1 h. (3) Na2CO3 aqueous solution was added slowly to adjust the pH value (pH ∼10). (4) Borohydride sodium solution (20 mg/L) was added and the resulting suspension was maintained under constant stirring at room temperature for 2 h to allow a complete reduction of metallic salt. Then the solid product was collected, washed thoroughly with water and ethanol to remove nitrate and chloride ions, and finally vacuum-dried at 60 °C for 4 h. The Pt/Ru/Sn atomic ratios of the as-prepared catalysts with the nominal compositions of Pt0.5Ru0.5-xSnx/C (x ) 0, 0.05, 0.1, 0.15, 0.25, 0.35, 0.5) were measured by inductively coupled plasma emission spectroscopy (ICP-9000, Thermo Jarrell-Ash Corp.) and the actual chemical compositions were Pt0.495Ru0.505/ C, Pt0.467Ru0.495Sn0.038/C, Pt0.472Ru0.413Sn0.115/C, Pt0.504Ru0.354Sn0.142/C, Pt0.537Ru0.243Sn0.22/C, Pt0.487Ru0.152Sn0.361/C, and Pt0.526Sn0.474/C. Characterization. The as-prepared samples were characterized by powder X-ray diffraction (XRD, Rigaku D/max 2500 X-ray generator, Cu KR radiation, λ ) 1.5405 Å) at a scanning rate of 4 deg/min from 3° to 80°. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were taken on a Philips Tecnai-F20 transmission electron microscope, using an accelerating voltage of 200 kV. The oxidation state was analyzed with X-ray photoelectron spectroscopy (XPS) in a Kratos Axis Ultra DLD electron spectrometer. Energy dispersive X-ray (EDX) was performed in the JEOL JSM-5600 scanning electron microscope with use of a Kevex Super 8000 detector. Electrochemical Measurements. Electrochemical measurements were carried out in the electrolyte consisting of 0.5 M H2SO4 and 1 M methanol by using conventional three-electrode cells with a Pt flake of 1 cm2 area as the counter electrode, an

Zhu et al. Ag/AgCl-KCl (saturated) electrode as the reference electrode, and a glassy carbon (GC, Φ ) 3 mm) loaded with active materials as the working electrode. All potential values were referenced to Ag/AgCl-KCl (saturated). Before each experiment, the GC electrode was polished with the slurry of alumina nanoparticles to a mirror. Five milligrams of the as-prepared catalysts was added into the solution containing 0.5 mL of Nafion and 0.5 mL of ethanol, which was ultrasonicated for 1 h to obtain a homogeneous slurry. Then, 5 µL of the slurry was dropped on the mirror-polished GC electrode with a diameter of 3 mm. The electrode was dried at 60 °C for 2 h before tests. The electrochemical performance of the electrodes was investigated by using a PARSTAT 2273 Advanced Electrochemical System at controlled temperatures. The cyclic voltammograms (CV) of methanol oxidation were measured at the scan rate of 50 mV/s in the potential range of -0.2 to 1.0 V at the temperature of 25 °C. The slow linear sweep measurements were carried out at the scan rate of 0.5 mV/s and at different temperatures of 26, 36, 46, and 56 °C for determining the Tafel slopes. The apparent activation energies of methanol oxidation on the as-synthesized catalysts were calculated from the Tafel plots at controlled temperatures. The chronoamperometry curves were obtained by polarizing the electrode at 0.5 V for 1500 s in the above-mentioned electrolyte. Before each test, the solution was purged with N2 for 15 min to eliminate the dissolved oxygen. Results and Discussion XRD and XPS Analysis. Figure 1A shows the typical XRD patterns of the as-prepared carbon-supported catalysts. All the XRD patterns clearly display the main characteristic peaks of the carbon support and the face-centered cubic (fcc) crystalline Pt, indicating that all catalysts mainly present in single phase with disordered structure (solid solution). The broad peak at low angle (approximately 2θ ) 25°) is attributed to the diffraction of the carbon support, and the peaks centered at about 2θ ) 40°, 46°, and 67° are readily indexed to the (111), (200), and (220) reflections of fcc Pt. The obvious broadening of peaks for all catalysts is due to the very fine grain size and defects on the metal particles. No peaks related to Ru, Sn, or their oxides can be detected in the diffraction patterns of all the catalysts. However, the presence of Ru oxides or Sn oxides with trace content or amorphous form cannot be ruled out since their diffraction peaks might be hidden by the broadened peaks of Pt.6 The shift of 2θ position for Pt peaks and the change in the lattice parameters reveals the conversion of Ru and Sn into the alloy state.14 As shown in Figure 1A, the main diffraction peaks of binary Pt-Ru/C catalyst shift toward higher 2θ values in comparison with those of the reported Pt/C (indicated by the vertical lines),20 whereas in the case of Pt-Sn/C the diffraction peaks move to lower 2θ values. For ternary Pt-Ru-Sn catalysts, intermediate 2θ values are observed between that of Pt-Ru and Pt-Sn alloy. Meanwhile, the 2θ angles of the main peaks decrease with increasing Sn content. The shifts of diffraction peaks were caused by the incorporation of Ru or Sn atoms in the Pt crystal lattice. The Pt(220) diffraction peak was selected to calculate the lattice constants and the average particle size. The lattice parameter of Pt0.495Ru0.505/C is 3.879 Å, which is smaller than that of Pt/C (3.916 Å)20 and reveals a combination of Ru and Pt lattice. For Pt0.526Sn0.474/C, the lattice parameter (3.982 Å) is much larger than that of Pt0.495Ru0.505/C and Pt/C, which can be ascribed to the incorporation of larger Sn atoms

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Figure 1. (A) XRD patterns of the as-prepared (a) Pt0.495Ru0.505/C, (b) Pt0.467Ru0.495Sn0.038/C, (c) Pt0.472Ru0.413Sn0.115/C, (d) Pt0.504Ru0.354Sn0.142/C, (e) Pt0.537Ru0.243Sn0.22/C, (f) Pt0.487Ru0.152Sn0.361/C, and (g) Pt0.526Sn0.474/C. (B) EDX spectrum of Pt0.472Ru0.413Sn0.115/C.

Figure 2. XPS patterns of the as-prepared Pt0.504Ru0.354Sn0.142/C catalyst: (a) Pt(4f), (b) Ru(3p), and (c) Sn(3d).

into the Pt lattice and the formation of PtSn alloy. Accordingly, the counteractive effect of Ru and Sn endows the Pt-Ru-Sn/C catalysts with intermediate lattice constant (e.g., 3.896 Å for Pt0.472Ru0.413Sn0.115/C). The average particle size was calculated from the XRD analysis according to the Scherrer formula.7 The average particle size for the carbon-supported Pt0.472Ru0.413Sn0.115, Pt-Ru, and Pt-Sn was about 1.6, 3.5, and 2.2 nm, respectively. Figure 1B shows the representative EDX spectrum of Pt0.472Ru0.413Sn0.115/C catalyst, confirming the presence of Pt, Ru, Sn, and C. The carbon signal in the EDX spectrum is attributed to the carbon support, while the Pt/Ru/Sn atomic ratios were 0.472: 0.412:0.116, being consistent with that (0.472:0.413:0.115) determined with ICP. XPS analysis was used to determine the surface oxidation states of the as-synthesized catalysts. Figure 2 shows the Pt(4f), Ru(3p), and Sn(3d) regions of the XPS spectra of Pt0.504Ru0.354Sn0.142/C as a representative example of the obtained ternary catalyst. The Pt(4f) signal (Figure 2a) can be deconvoluted into two pairs of doublets. The intense doublet at 71.4 and 74.8 eV is assigned to metallic Pt, while the second set of weak peaks at 72.7 and 76.1 eV are ascribed to Pt(II).20 The discrepancy from the standard Pt(0) value of 70.9 eV might be attributed to the interaction between metal and carbon support and/or the small cluster-size effect. The Ru(3p) instead of Ru(3d) spectrum was carried out to avoid the overlapping of Ru(3d) and C(1s) spectra, as shown in Figure 2b. The Ru(3p3/2) signal can be deconvoluted into two distinguishable peaks of different intensities at 461.2 and 462.8 eV, which correspond to Ru(0) and Ru(IV), respectively.21 XPS spectra of Sn(3d) shown in Figure 2c can be deconvoluted into two doublets: one

at 485.8 and 494.4 eV corresponds to Sn(0) and the other at 487.3 and 495.9 eV denotes tin oxide species although it is difficult to distinguish the peak of Sn(II) from that of Sn(IV). The results of XPS analysis suggest that the main part of Pt and Ru is in the metallic state, while Sn is mainly in the oxidized state on the surface of the ternary catalysts. TEM Characterization. Transmission electron microscopy provides further insight into the microstructure and dispersion of the as-synthesized catalyst particles. The typical images and histograms of particle size distribution for Pt0.472Ru0.413Sn0.115/ C, Pt0.495Ru0.505/C, and Pt0.526Sn0.474/C catalysts are shown in Figure 3. As is observed from the typical TEM images in Figure 3a-c, the metal particles of the as-prepared catalysts are nearly spherical in shape and well dispersed on the carbon surface, and no obvious agglomerations are observed. The inset of Figure 3a displays the typical HRTEM micrograph of a single nanoparticle, indicating the interlaced lattice fringes of the Pt0.472Ru0.413Sn0.115 alloy. Furthermore, the particle sizes in Figure 3a-c were estimated by measuring more than 300 nanoparticles selected randomly in the TEM images. The corresponding particle size distribution histogram (Figure 3d-f) of each sample shows a lognormal distribution accompanied by a relatively narrow size distribution. Most of the metal particles are in the range of 1.5-6 nm in diameters and the average particle sizes are approximately 2.6, 3.2, and 3.0 nm for Pt0.472Ru0.413Sn0.115/ C, Pt0.495Ru0.505/C, and Pt0.526Sn0.474/C, respectively, which are in good agreement with the calculated XRD results. Pt0.472Ru0.413Sn0.115/C exhibits a much narrower particle size distribution and smaller average particle size than that of the other two binary systems. In our previous work, we have shown that the activation-sensitization treatment procedure can effectively

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Figure 3. TEM images (a-c) and histograms( d-f) of particle size distribution of the as-obtained catalysts: (a, d) Pt0.472Ru0.413Sn0.115/C, (b, e) Pt0.495Ru0.505/C, and (c, f) Pt0.526Sn0.474/C. The inset in panel a shows the corresponding HRTEM micrograph of an individual nanocrystal.

Figure 4. (A) Cyclic voltammograms (CVs) and (B) the amplified part of the CVs at low potential range for the as-synthesized carbon-supported Pt-Ru, Pt-Sn, and Pt-Ru-Sn catalysts: (a) Pt0.472Ru0.413Sn0.115/C, (b) Pt0.504Ru0.354Sn0.142/C, (c) Pt0.467Ru0.495Sn0.038/C, (d) Pt0.495Ru0.505/C, (e) Pt0.537Ru0.243Sn0.22/C, (f) Pt0.487Ru0.152Sn0.361/C, and (g) Pt0.526Sn0.474/C. CVs were measured in the solution containing 1 M methanol and 0.5 M H2SO4 at room temperature and at the scan rate of 50 mV/s.

assist in the preparation of catalyst particles with good dispersion and small size.19 The decrease in particle size by the appropriate addition of Sn to the catalysts was also reported in other systems and the decreased particle size of PtRuSn/C may be advantageous to the enhanced catalytic activities for methanol oxidation.17,22 Cyclic Voltammograms. The electrochemical performances of the obtained carbon-supported Pt-Ru, Pt-Sn, and Pt-RuSn catalysts were initially investigated by using CV measurement. Figure 4 shows the resulting voltammograms. As shown in Figure 4A, all samples display two characteristic methanol oxidation peaks during the whole potential scan. There are one

oxidation peak centered at about 0.7 V on the forward scan curve and one peak at about 0.45 V on the reverse scan curve. The first peak is attributed to the primary oxidation of methanol on catalyst surfaces and the second anodic oxidation peak is attributed to the removal of the incompletely oxidized carbonaceous species formed during the forward scan.23 The performances of all catalysts examined in the study were evaluated in terms of the following three aspects: (1) forward anodic peak current density (at about 0.7 V) and (2) onset potential of methanol oxidation, indicative of the catalytic activity over methanol oxidation and (3) the ratio of the forward peak current density to reverse peak current density (If/Ir), demonstrating the

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TABLE 1: Comparison of the Catalytic Activity of the As-Prepared Catalysts for Methanol Oxidation. samples

Iap (mA/cm2)a

Eap (V)b

Eonset (V)

If/Ir ratioc

Pt0.495Ru0.505/C Pt0.467Ru0.495Sn0.038/C Pt0.472Ru0.413Sn0.115/C Pt0.504Ru0.354Sn0.142/C Pt0.537Ru0.243Sn0.22/C Pt0.487Ru0.152Sn0.361/C Pt0.526Sn0.474/C

22.71 23.65 31.76 26.61 15.91 11.39 10.25

0.697 0.681 0.682 0.692 0.699 0.702 0.686

0.190 0.166 0.137 0.136 0.220 0.246 0.263

0.93 1.34 1.77 1.49 1.90 1.23 1.32

a Anodic peak current density. b Anodic peak potential. c If and Ir, forward and reverse peak current density.

catalyst toleration toward the adsorbed carbonaceous intermediate species.7,10 The related test results are summarized in Table 1. First, the anodic peak current densities (at ∼0.7 V) of the binary and ternary catalysts toward methanol oxidation are in the order of Pt0.472Ru0.413Sn0.115/C > Pt0.504Ru0.354Sn0.142/C > Pt0.467Ru0.495Sn0.038/C > Pt0.495Ru0.505/C > Pt0.537Ru0.243Sn0.22/C > Pt0.487Ru0.152Sn0.361/C > Pt0.526Sn0.474/C. The anodic peak current density of methanol oxidation on Pt0.472Ru0.413Sn0.115/C is up to 1.39 and 3 times larger than that on Pt0.495Ru0.505/C and Pt0.526Sn0.474/C, respectively. Second, the sequence of the onset potentials (obtained from the amplified parts of CV in low potential range, Figure 4B) follows Pt0.504Ru0.354Sn0.142/C ≈ Pt0.472Ru0.413Sn0.115/C < Pt0.467Ru0.495Sn0.038/C < Pt0.495Ru0.505/C < Pt0.537Ru0.243Sn0.22/C < Pt0.487Ru0.152Sn0.361/C < Pt0.526Sn0.474/ C. The onset potentials of methanol oxidation range from 0.136 to 0.263 V for the catalysts with different Sn content. Third, the If/Ir ratios of all the ternary Pt-Ru-Sn/C catalysts are larger than that of the binary PtRu/C catalyst. For example, the If/Ir values of Pt0.472Ru0.413Sn0.115/C and Pt0.537Ru0.243Sn0.22/C are almost twice that of Pt0.495Ru0.505/C. Therefore, the results of CV measurement clearly reveal that the addition of an optimum amount of Sn is favorable for enhancing the catalytic activity of methanol electro-oxidation on the Pt-based catalyst. The asprepared ternary Pt0.472Ru0.413Sn0.115/C exhibited the most prominent electrochemical performance in terms of the largest peak current density and relatively lower onset potential and higher If/Ir ratio. Ru and Sn in Pt-based catalysts are believed to significantly lower the onset potential through a so-called bifunctional mechanism,11 in which the adsorption of oxygenous species occurs at much lower potential on additive Ru or Sn than that on Pt. The synergic effect of Ru and Sn inhibits the accumulation of CO on Pt sites and makes the Pt sites free for methanol oxidation, thus leading to a significant enhancement of anodic peak current density. In addition, both Ru and Sn in Pt-based alloy can assist in the oxidation of adsorbed CO-like carbonaceous species through different mechanisms according to Morimoto et al.24,25 Ruthenium is believed to be effective toward CO-like residues oxidation at higher potentials while tin mainly promotes the oxidation of COads that can be oxidized at more negative potentials on pure Pt sites. It is expected that ruthenium and tin would not compete but complement each other when they simultaneously exist on platinum sites, thus yielding even higher tolerance toward CO-like poisonous species. Also, the presence of Sn oxides (see XPS analysis in Figure 2) on the surface of the as-prepared catalysts can supply sufficient oxygencontaining species to remove the adsorbed carbonaceous species.26 Furthermore, an obvious decrease of particle size (shown in Figure 3) is attained by the addition of Sn to PtRu, which may be another advantageous factor for the enhanced catalytic activity of the ternary Pt-Ru-Sn/C catalysts. However, too

Figure 5. Chronoamperometry curves for carbon-supported Pt-Ru, Pt-Sn, and Pt-Ru-Sn catalysts with different Sn content at 0.5 V in a solution containing 1 M methanol and 0.5 M H2SO4.

much Sn or its oxides on the Pt sites may block the adsorption of methanol molecules and is hence detrimental to catalytic activity. Therefore, the Sn content in the ternary catalysts should be optimized to achieve superior performance. Chronoamperometry Measurement. The chronoamperometry measurement provides further information about the electroactivity and stability of the catalysts for methanol oxidation. Figure 5 presents the current-time curves recorded for the asprepared samples with different contents of Pt, Ru, and Sn. For all catalysts, the potential-static currents decrease rapidly in the initial stage, achieve a pseudosteady state after a period of ∼300 s, and then decay slightly with time. The decrement of Pt active sites caused by the accumulation of poisonous carbonaceous intermediates on the electrode surface is responsible for the slow long-term decay of current density after the initial stage.27 Similar to the CV results, the current densities of chronoamperometry measurement decrease in the following sequence of Pt0.472Ru0.413Sn0.115/C > Pt0.504Ru0.354Sn0.142/C > Pt0.467Ru0.495Sn0.038/C > Pt0.495Ru0.505/C > Pt0.537Ru0.243Sn0.22/C > Pt0.487Ru0.152Sn0.361/C > Pt0.526Sn0.474/C. Ternary Pt-Ru-Sn systems with Sn atomic content less than 15% show better performance than that of binary Pt-Ru and Pt-Sn. Again, Pt0.472Ru0.413Sn0.115/C maintains the highest current density and the lowest rate of current decay, further confirming their superior performance for methanol electro-oxidation. Pt0.472Ru0.413Sn0.115/C delivers a near-constant current density of 5.66 mA/cm2 after being polarized in acidic methanol solution for 500 s, which is almost twice the value for Pt0.495Ru0.505/C (2.95 mA/cm2). As a result, the ratio of Pt:Ru:Sn in the ternary catalysts should be optimized as 0.472:0.413:0.115 to attain both high activity and long-term stability. Slow Linear Sweep Voltammetry. Slow linear sweep voltammetry (SLV) measurements for the representative Pt0.495Ru0.505/C, Pt0.526Sn0.474/C, and Pt0.472Ru0.413Sn0.115/C catalysts were carried out in the solution containing 0.5 M H2SO4 and 1 M methanol at the scan rate of 0.5 mV/s. The electrolyte was stirred vigorously to smooth over the influence of mass transfer. Figure 6A shows the typical SLV curves of the as-prepared Pt0.472Ru0.413Sn0.115/C, Pt0.495Ru0.505/C, and Pt0.526Sn0.474/C at room temperature. Also, note that all SLV curves can be divided into several regions. In the low potential region, the current densities increase very slowly with increasing polarization potential and all three curves almost overlap, which is ascribed to the adsorbed hydrogen and water that partially inhibit the methanol adsorption.28 In the relatively high potential region

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Figure 6. Polarization plots (A) and the corresponding Tafel plots (B) of Pt0.472Ru0.413Sn0.115/C (black), Pt0.495Ru0.505/C (blue), and Pt0.526Sn0.474/C (red) in a solution containing 1 M methanol and 0.5 M H2SO4 at room temperature. The scan rate is set at 0.5 mV/s.

(above 0.27 V), the polarization current increases sharply with polarization potential and the current density of the ternary catalyst is significantly larger than those of binary catalysts. At 0.42 V the current densities of Pt0.472Ru0.413Sn0.115/C, Pt0.495Ru0.505/C, and Pt0.526Sn0.474/C are 1.15, 0.34, and 0.12 mA/cm2, respectively. A limiting current is achieved at about 0.6 V, which can be attributed to the blocking of active catalytic sites by the adsorbed CO-like species and/or the anions.29 Obviously, methanol oxidation reaction starts at more negative potential on the Pt0.472Ru0.413Sn0.115/C surface than that on the other two samples, which is consistent with the CV measurement data. Figure 6B displays the corresponding Tafel plots of the anode polarization of the three catalysts. Each plot can be fitted and divided into two linear regions according to the change of Tafel slopes. The first fitted Tafel slopes come within the scope of 112 to 203 mV/dec at low overpotentials (