English

• Recently, the direct methanol fuel cells (DMFCs) have drawn extensive attention in the field of fuel cells because of their high conversion efficiency and low cost^{[1, 2]}. As well known, platinum (Pt) is widely used as the catalyst in DMFCs for its high activity. However, the high price^{[3, 4]} and low durability^[5-7]of Pt have caused critical problems for their commercialization. Therefore, it is an urgent task to improve the catalytic performance and reduce the consumption of Pt in the electro-catalysts for the application of fuel cells^[8-10].

  Fortunately, Pt-based alloys show great promise. For instance, many Pt-based bimetallic materials have been prepared and applied in fuel cells, such as PtPd^[11], PtCu^[12-15], PtFe^[16], PtCo^[17-19], PtNi^[20], PtMn^[21], PtPb^[22], PtAg^[23-28], etc. These bimetallic alloys exhibited higher catalytic performance than single platinum in the fuel cell reactions, but with much lower cost^[29-31]. Besides, Pt-based alloys are also more stable and corroded much more slowly in alkaline medium than in strong acidic medium^{[32, 33]}, which can enhance the stability of the catalyst and electrode. In addition, the alkaline environment is friendlier to Pt-based alloys, since the methanol electrooxidation reaction (MOR) and oxygen reduction reaction (ORR) are much easier in the alkaline medium^{[34, 35]}.

  It was reported that a proper amount of Ag in PtAg alloy could enhance the resistance of Pt catalyst against CO poisoning and thus greatly improve the catalytic activity^{[36, 37]}. Later, Feng et al^{[38, 39]} also verified the positive role of Ag oxide in promoting the methanol electrooxidation in alkaline electrolyte. Moreover, the catalyst surface cleanness also plays a crucial role in the catalytic performance^[40]. Conventional methods often used the surfactants and capping agents, generating residues on the catalyst surface; these residues were difficult to remove completely and had a great influence on the catalytic performance^[41-43]. Recently, some alternative methods have been employed to prepare the Pt-based alloy nanoparticles (NPs) of controllable shapes and clean surfaces^[44-46]. For example, Pt nanosheets^[47], Pt nanocave^[48], PtCu nanocave^[15], and PtAg nanotube^[23] were obtained with much cleaner surfaces and significantly enhanced MOR and formic acid electrooxidation catalytic performance.

  In this work, a group of Pt_xAg_y alloy NPs with clean surface was prepared in the molten salts system and their performance as anodic catalysts in alkaline media for MOR was then investigated, to explore the important role of Ag in enhancing the catalytic performance of Pt_xAg_y alloy NPs for MOR in alkaline electrolyte.

  1.   Experimental

  1.1   Preparation of the Pt_xAg_y alloy NPs

  The Pt_xAg_y alloy NPs were prepared following the reported procedures^[23]. 6.6 g KNO₃ and 3.4 g LiNO₃ were thoroughly grounded and uniformly mixed in a glass flask and placed in a 170 ℃ salt bath, until the mixture was melted. After that, a designed amount of silver acetate (CH₃COOAg) and tetra-ammine platinum oxalate {Pt(NH₃)₄C₂O₄} (15.6 mg Pt(NH₃)₄C₂O₄ and 0.9 mg CH₃COOAg for Pt₈₆Ag₁₄; 14.1 mg Pt(NH₃)₄C₂O₄ and 1.7 mg CH₃COOAg for Pt₇₉Ag₂₁; 8.8 mg Pt(NH₃)₄C₂O₄ and 4.2 mg CH₃COOAg for Pt₅₂Ag₄₈; 3.5 mg Pt(NH₃)₄C₂O₄ and 6.7 mg CH₃COOAg for Pt₂₁Ag₇₉; 2.0 mg Pt(NH₃)₄C₂O₄ and 7.4 mg CH₃COOAg for Pt₁₁Ag₈₉) and 22.4 mg KOH were added into the mixed molten salt with vigorously stirring for 2 h. After the reaction was complete, the molten salt was cooled to room temperature and washed three times with distilled water to remove the soluble salt.

  1.2   Characterization of the Pt_xAg_y alloy NPs

  Scanning electron microscope (SEM) images were taken on a JEOL 7000F field emission electron microscope; the energy dispersive X-ray (EDX) was used to analyse the average atomic distribution of the Pt_xAg_y NPs. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained by using a JEOL 3010 with 300 kV acceleration voltages. The Powder X-ray diffraction (XRD) was conducted on a Bruker D8 advance X-ray diffractometer.

  1.3   Electro-catalytic activity of the Pt_xAg_y alloy NPs

  The electro-catalytic performance of the Pt_xAg_y NPs in alkaline electrolytes (0.5 moL/L KOH) media for MOR was evaluated by using cyclic voltammetry (CV) technique with a three-electrode system that consisted of a saturated calomel electrode (SCE) reference electrode, a 1 cm×1 cm platinum plate (99.9%) counter electrode and a glassy carbon (GC) working electrode. The catalyst ink was prepared by dispersing 5 mg Pt_xAg_y catalyst in 5 mL mixed solvent of distilled water, 2-propanol (AR, 99.8%) and 5% Nafion solution (with a volume ratio of 4 :1 :0.025). The working electrode was obtained by dropping 10 μL of catalyst ink onto the GC and dried in air. All potentials in this work were relative to SCE. The stable CV curves were recorded between -1.0 to 0.5 V (vs. SCE) after 50 cycles with the scanning rate of 50 mV/s. CV tests for MOR were carried out in 0.5 moL/L KOH + 2 moL/L CH₃OH (HPLC, 99.9%) solution between -1.0 to 0.5 V (vs. SCE) with the scanning rate of 20 mV/s.

  2.   Results and discussion

  2.1   SEM, TEM, EDX, and XRD analysis of Pt_xAg_y NPs

  Figure 1 shows the SEM and TEM images of the as-prepared Pt_xAg_y alloy NPs with different Pt(NH₃)₄C₂O₄/CH₃COOAg molar ratios (8, 4, 1, 1/4 and 1/8) in the molten salt precursor; on the basis of the EDX results (Table 1), the obtained products are denoted as Pt₈₆Ag₁₄, Pt₇₉Ag₂₁, Pt₅₂Ag₄₈, Pt₂₁Ag₇₉ and Pt₁₁Ag₈₉, respectively.

  Figure 1

  

  Figure 1.  SEM and TEM images of ((a), (f)) Pt₈₆Ag₁₄, ((b), (g)) Pt₇₉Ag₂₁, ((c), (h)) Pt₅₂Ag₄₈, ((d), (i)) Pt₂₁Ag₇₉, and ((e), (j)) Pt₁₁Ag₈₉

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  Table 1

  

  Table 1.  Pt_xAg_y atomic molar ratios in the alloy NPs prepared with different Pt(NH₃)₄C₂O₄/CH₃COOAg ratios in the molten salt precursor, analyzed by EDX

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  PtxAgy sample	Pt(NH3)4C2O4/CH3COOAg	Pt/Ag, by EDX
  Pt86Ag14	8:1	6.09
  Pt79Ag21	4:1	3.89:1
  Pt52Ag48	1:1	1.10:1
  Pt21Ag79	1:4	1:3.88
  Pt11Ag89	1:8	1:8.51

  The as-prepared Pt_xAg_y alloy NPs have a very clean surface, since the preparation process was operated in the system of molten salts without using any organic surfactant or solvent^[23]. As shown in Figure 1(a)-(e), (f)-(j), with a decrease in the content of Pt, the morphology of products changes gradually from NPs to nanotubes (NTs) and to mixture of NPs and NTs. A proper content of silver in the molten salt can promote the change in the product shape from NPs to NTs; in particular, NT becomes the main products with a Pt/Ag ratio of 1 in the molten salt precursor. The average size displays a great decrease from (82.5±12.1) nm for Pt₈₆Ag₁₄ to (32.1±5.9) nm for Pt₅₂Ag₄₈, as shown in Figure 2.

  Figure 2

  

  Figure 2.  Particle size distribution histogram for Pt₈₆Ag₁₄ (a) and nanotube diameter distribution histogram for Pt₅₂Ag₄₈ (b), estimated from the SEM images

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  2.2   TEM and HRTEM analysis of Pt₅₂Ag₄₈ NTs

  Figure 3 displays the TEM and HRTEM images of Pt₅₂Ag₄₈ NTs. The Pt₅₂Ag₄₈ NTs shows a worm-like morphology with hollow or solid interior (Figure 3(a), (b)); the tube diameter and wall thickness are about 25.6 and 6.5 nm, respectively. It seems that the Pt₅₂Ag₄₈ NTs are aggregated with several NPs with or without hollow interiors. Meanwhile, the HRTEM images shown in Figure 3(c), (d) indicate that the lattice spacing of Pt₅₂Ag₄₈ NTs was about 0.230 and 0.198 nm, which could be indexed to the (111) and (200) crystal planes of Pt_xAg_y alloy, respectively^[40].

  Figure 3

  

  Figure 3.  TEM images ((a), (b)) of the Pt₅₂Ag₄₈; HRTEM images ((c), (d)) of the local sites of Pt₅₂Ag₄₈ labelled in (b)

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  Furthermore, the peaks in the XRD patterns of Pt_xAg_y NPs can be labelled as (111), (200), (220), (311), and (222) planes of a face-centred-cubic (fcc) structure, as shown in Figure 4. With a high content of Pt (or Ag), the diffraction peak is very close to pure metallic Pt (or Ag), indicating a Pt-rich (or Ag-rich) alloy phase^[49]. The redundant diffraction peaks appear in the XRD patterns of Pt₇₉Ag₂₁ and Pt₅₂Ag₄₈, probably due to the formation of trace amounts of Ag-rich alloy or pure Ag.

  Figure 4

  

  Figure 4.  XRD patterns of the Pt, Pt₈₆Ag₁₄, Pt₇₉Ag₂₁, Pt₅₂Ag₄₈, Pt₂₁Ag₇₉, Pt₁₁Ag₈₉, and Ag

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  2.3   Cyclic voltammograms analysis of Pt_xAg_y NPs

  Figure 5 shows the typical cyclic voltammograms (CVs) of Pt₁₁Ag₈₉, Pt₂₁Ag₇₉, Pt₅₂Ag₄₈, Pt₇₉Ag₂₁ and Pt₈₆Ag₁₄ electrodes with the same loading in the electrolyte of N₂-saturated 0.5 moL/L KOH.

  Figure 5

  

  Figure 5.  Cyclic voltammograms of Pt₁₁Ag₈₉, Pt₂₁Ag₇₉, Pt₅₂Ag₄₈, Pt₇₉Ag₂₁ and Pt₈₆Ag₁₄ NPs performed in the electrolyte of N₂-saturated 0.5 moL/L KOH. the inset is the enlarge image of CV between -1.0 and -0.7 V

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  Two anode peaks appear in the forward sweep, one shoulder at low potential (about 0.20 V) and another strong peak at higher potential (about 0.25 V). These two anodization peaks were generally attributed to the oxidation of Ag to AgO^- and Ag₂O, respectively^[50]. In the backward sweep, an obvious cathodic peak appears at about 0.0 V, which could be assigned to the reduction of Ag₂O to metallic Ag^[51]. With a decrease in the Ag content in Pt_xAg_y, the redox peak current of Ag decreases gradually, indicating that the number of Ag atoms exposed on the surface is declining. In the low-potential region of the CVs curve, the peaks appeared at -1.0 to -0.7 V are attributed to the hydrogen adsorption-desorption characteristics of Pt surface, as shown in the inset of Figure 5^[52]. In addition, in the backward sweep, the broad cathode peak at -0.2 to -0.7 V can be assigned to the reduction of Pt oxide during the anodization process at high potential^[53].

  As shown in Figure 6, when the cyclic voltammetry is applied to Pt₅₂Ag₄₈ NTs, the Ag atoms in the alloy are de-alloyed, which will expose more Pt surface (about -0.5 V) and gave a larger electrochemical surface area (ECSA) for the hydrogen adsorption-desorption (-1.0 to -0.7 V). Furthermore, the intensity of Ag redox peaks increases with an increase in the number of CV cycles and tends to stabilize after 50 cycles, suggesting that Ag element exists in the surface layer of the Pt_xAg_y alloys.

  Figure 6

  

  Figure 6.  Cyclic voltammograms of Pt₅₂Ag₄₈ in N₂-saturated 0.5 moL/L KOH electrolyte

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  2.4   Effect of positive potential limit to the cyclic voltammograms of Pt₅₂Ag₄₈ NTs

  Figure 7 shows the CV curves of Pt black (JM) and Pt₅₂Ag₄₈ catalysts performed with different positive potential limits (-1.0 to 0.5 V and -1.0 to 0.1 V). The commercial Pt black displays a classical H adsorption-desorption region (-1.0 to -0.7 V) and a Pt oxides formation and reduction region (-0.2 to -0.5 V). The potential limit has almost no effect on the Pt oxide reduction peaks position; however, a decrease in the positive potential limit leads to a slight decline of the peak current. In contrast, the CV feature of the Pt₅₂Ag₄₈ NTs is greatly dependent on the potential limit. With the positive potential limit of -1.0 to 0.5 V, the Ag redox peak and the H adsorption-desorption characteristics are observed. However, if the positive potential is limited in -1.0 to 0.1 V, the features of Ag redox and hydrogen adsorption-desorption disappear.

  Figure 7

  

  Figure 7.  Cyclic voltammograms of Pt black and Pt₅₂Ag₄₈ performed with different positive potential limits (-1.0 to 0.5 V and -1.0 to 0.1 V) in the electrolyte of N₂-saturated 0.5 mol/L KOH

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  It is well known that the adsorption and desorption of hydrogen on silver atoms are inactive^[54]. When the potential is applied in the range of -1.0 to 0.1 V, Ag atoms cannot be de-alloyed from the catalyst surface and it is then difficult to reflect the hydrogen adsorption-desorption characteristics. In addition, the electric double layer for Pt₅₂Ag₄₈ NTs subjected to 0.5 V is much broader than that at 0.1 V, suggesting that the surface of Pt₅₂Ag₄₈ NTs can be roughened by the Ag redox process^{[55, 56]}.

  2.5   Evaluation of the activity of Pt_xAg_y NPs in MOR

  Figure 8 a shows the CV and linear polarization curves of MOR over the Pt black, Pt₁₁Ag₈₉, Pt₂₁Ag₇₉, Pt₅₂Ag₄₈, Pt₇₉Ag₂₁, and Pt₈₆Ag₁₄ catalysts in the electrolyte of 0.5 moL/L KOH + 2 moL/L CH₃OH. During the forward scanning process, substantially no current can be detected when the potential is lower than -0.6 V. If the scan is moved to more positive potential, a very distinct symmetrical anodic peak appears at around -0.2 V. The peak potential and peak current density are important indicators to evaluate the catalyst performance. As given in the Tbale 2, in the forward scanning, the peak current densities (I_f) for the MOR are 1.43 and 1.61 mA/ μg_Pt for the Pt black and Pt₅₂Ag₄₈ catalysts, respectively. Meanwhile, the mass current densities of the Pt₅₂Ag₄₈ NTs measured at -0.25 V is about 1.19 times higher than that of Pt black (insert in Figure 8(a)). Furthermore, all Pt_xAg_y catalysts presents a more negative peak potential (E_p), in comparison with the Pt black, suggesting a higher activity of Pt_xAg_y in CO_ads oxidation, as given in Table 2. For example, E_p of Pt₅₂Ag₄₈ NTs is -0.12 V, which shifts negatively by 50 mV compared with that of the Pt black (-0.07 V).

  Figure 8

  

  Figure 8.  Cyclic voltammograms (a) and linear polarization curves (b) for MOR on the Pt black (JM), Pt₁₁Ag₈₉, Pt₂₁Ag₇₉, Pt₅₂Ag₄₈, Pt₇₉Ag₂₁ and Pt₈₆Ag₁₄ catalysts in the electrolyte of 0.5 moL/L KOH + 2 moL/L CH₃OH, the insets in (a) and (b) show the corresponding activities at -0.25 V and the dependence between the onset potential of MOR and the catalysts composition, respectively (the scan rate is 20 mV/s)

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  Table 2

  

  Table 2.  MOR performance of Pt black, Pt₁₁Ag₈₉, Pt₂₁Ag₇₉, Pt₅₂Ag₄₈, Pt₇₉Ag₂₁ and Pt₈₆Ag₁₄ catalysts in 0.5 moL/L KOH + 2 moL/L CH₃OH

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  Sample	E0 /V	Ep /V	If /(mA·μgPt-1)	Ib /(mA·μgPt-1)	If/Ib	I@-0.25V /(mA·μgPt-1)
  Pt black	-0.77	-0.07	1.43	0.56	2.55	0.86
  Pt11Ag89	-0.51	-0.28	0.02	-	-	0.02
  Pt21Ag79	-0.71	-0.25	0.34	0.03	11.33	0.33
  Pt52Ag48	-0.91	-0.20	1.61	0.19	8.47	1.02
  Pt79Ag21	-0.82	-0.21	0.43	0.06	7.17	0.39
  Pt86Ag14	-0.66	-0.24	0.25	0.02	12.50	0.26

  In the backward scanning process, another anodic peak is observed at about -0.4 V for the catalysts of Pt black and Pt_xAg_y NPs. The tolerance ability of the catalyst to the accumulation of carbonaceous species can be evaluated by the ratio of the value of forward peak current (I_f) to the backward peak current (I_b); a higher I_f/I_b ratio means a higher capacity in the oxidation of methanol and the carbonaceous residues accumulated on the catalyst surface to CO₂ during the forward scan^[22]. The I_f/I_b values for the Pt black, Pt₂₁Ag₇₉, Pt₅₂Ag₄₈, Pt₇₉Ag₂₁, and Pt₈₆Ag₁₄ catalyst are 2.55, 11.33, 8.47, 7.17 and 12.50, respectively, as given in Table 2, indicating that the Pt_xAg_y alloy NPs are more active to directly oxidize methanol to CO₂ than Pt black.

  To investigate the electrocatalytic performance of the catalysts at low potential, the onset potential (E₀) of MOR was measured based on the CV curve. As shown in Figure 8(b), among all the Pt_xAg_y and Pt black catalysts, the Pt₅₂Ag₄₈ NPs shows the most negative E₀ and highest I_f values (Table 2), which indicates that the Pt₅₂Ag₄₈ NTs catalyst has a much better MOR activity than that the Pt black. That is, the presence of Ag can remarkably improve the catalytic performance of Pt-Ag alloy in MOR. In view of the extremely limited catalytic activity of Ag compared to Pt in alkaline media^[57], the enhancement in the catalytic performance of the Pt_xAg_y alloy in MOR is mainly ascribed to the promoting effect of Ag.

  2.6   Promoting effect of Ag on the catalytic activity of Pt₅₂Ag₄₈ NTs in MOR

  In addition to the catalyst composition, the positive potential limit of CV also has a great effect on the catalytic activity of Pt_xAg_y alloys in MOR. For comparison, the catalytic activity of Pt black in MOR with the different potential limits was also tested. Figures 9(a) and 9(b) show the CVs of Pt black and Pt₅₂Ag₄₈, in the electrolyte of 0.5 moL/L KOH + 2 moL/L CH₃OH, with the potential limit of -1.0 to 0.5 V and -1.0 to 0.1 V, respectively. For the Pt black, the MOR performance is approximately the same when the positive potential limits are changed from 0.1 to 0.5 V (Figure 9(a)). In contrast, for the Pt₅₂Ag₄₈ catalyst, the catalytic characteristics have a large discrepancy among different positive potential limits (Figure 9(b)).

  Figure 9

  

  Figure 9.  Cyclic voltammograms of (a) Pt black and (b) Pt₅₂Ag₄₈ in the electrolyte of 0.5 moL/L KOH + 2 moL/L CH₃OH with different potential limits at a scan rate of 20 mV/s

  black solid line: -1.0 to 0.5 V; red dotted line: -1.0 to 0.1 V

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  With a relatively lower potential limit of 0.1 V, the Pt₅₂Ag₄₈ catalyst shows a mass peak current density of 0.84 mA/μg_Pt. As the potential limit increases to 0.5 V, the peak current density of the Pt₅₂Ag₄₈ catalyst increases to 1.61 mA/μg_Pt, which is 1.92 times higher than that subjected to the 0.1 V potential limit. The catalytic activity of Pt-Ag alloy is improved significantly, as the redox reaction of Ag can be initiated with the positive potential limit of -1.0 to 0.5 V. Moreover, the Ag oxide formed in the redox reaction can greatly enhance the catalytic activity of the Pt_xAg_y alloy in MOR^[38]. The promoting effect of Ag oxide for MOR can be ascribed to the trace Ag oxide wrapped around the active Pt atoms in the Pt_xAg_y alloy NPs, which may play an important role in the reaction to remove the CO-like intermediate species adsorbed on Pt atoms, to release the the active Pt sites; in this way, the bifunctional effect between Pt and Ag oxide leads to a significant improvement of MOR activity^[38].

  To further verify the promoting effect of Ag on the methanol oxidation performance of Pt_xAg_y alloy, the sequential cyclic voltammogram technique was used to identify the MOR activity of Pt₅₂Ag₄₈ catalyst in different potential limits. For the potential limit of -1.0 to 0.5 V, the Pt₅₂Ag₄₈ catalyst was subjected to CV in 0.5 moL/L KOH solution for 50 cycles until the electrode was stable, and the sequential CV was then tested in 0.5 moL/L KOH + 2 moL/L CH₃OH. A parallel operation was conducted with the same process in the solution of 0.5 moL/L KOH for 50 cycles with the potential limit of -1.0 to 0.1 V.

  As shown in Figure 10(a), the values of peak current for the Pt₅₂Ag₄₈ NTs shows a rapid increase from the first to the fifth cycle, when the potential is applied within the range -1.0 to 0.5 V. After the tenth cycle, the peak current tends to be stable and increases no longer. On the contrary, the peak current decreases continuously when the positive potential is limited in the range of -1.0 to 0.1 V (Figure 10(b)). The value of peak current at the 50th cycle (0.84 mA/μg_Pt) is only 67.2% of that at the first cycle (1.25 mA/ μg_Pt). With the potential limit of -1.0 to 0.1 V, it is difficult to perform the redox reaction of Ag during the oxidation of methanol and the CO-like intermediate species adsorbed on the surfaces of Pt atoms cannot be removed immediately. As a result, the catalytic performance of Pt-Ag alloy in MOR degrades gradually, suggesting that the promoting effect of Ag on the MOR activityof Pt_xAg_y alloy is in effect mainly in the redox reaction process.

  Figure 10

  

  Figure 10.  Sequential cyclic voltammograms of Pt₅₂Ag₄₈ in the electrolyte of 0.5 moL/L KOH + 2 moL/L CH₃OH with the potential ranges of (a) -1.0 to 0.5 V and (b) -1.0 to 0.1 V (scan rate 20 mV/s)

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  3.   Conclusions

  A series of platinum-silver alloy nanoparticles (Pt_xAg_y NPs) were synthesized in a molten salt system without using any organic surfactants or solvents; the catalytic role of Ag in the methanol electrooxidation reaction (MOR) in alkaline electrolyte over Pt_xAg_y NPs was investigated.

  The results indicate that Pt₅₂Ag₄₈ nanotubes (NTs) can be obtained when the Pt/Ag ratio in the molten salt precursor reaches 1. The catalytic performance of Pt_xAg_y alloy in MOR in alkaline electrolyte is greatly enhanced by the Ag element. The Pt₅₂Ag₄₈ NTs with a clean surface exhibits a much better catalytic performance than the conventional Pt black in MOR. Meanwhile, the catalytic activity of the Pt₅₂Ag₄₈ NTs is greatly related to the positive potential limit; the peak current of MOR reaches 1.61 mA/ μg_Pt with a positive potential limit from -1.0 to 0.5 V (vs. SCE), which is 1.92 times higher than that with a positive potential limit from -1.0 to 0.1 V (vs. SCE). The Ag element in the surface layer of Pt_xAg_y alloy may promote the MOR through a redox process during the electrochemical cycle. The findings in this work are valuable in explaining the mechanism of Ag in promoting catalytic performance of Pt-Ag alloy and designing bimetallic catalysts in MOR, which should be beneficial to the application of Pt_xAg_y alloy in the direct methanol fuel cells (DMFCs).

  
  
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  Figure 1  SEM and TEM images of ((a), (f)) Pt₈₆Ag₁₄, ((b), (g)) Pt₇₉Ag₂₁, ((c), (h)) Pt₅₂Ag₄₈, ((d), (i)) Pt₂₁Ag₇₉, and ((e), (j)) Pt₁₁Ag₈₉

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  Figure 2  Particle size distribution histogram for Pt₈₆Ag₁₄ (a) and nanotube diameter distribution histogram for Pt₅₂Ag₄₈ (b), estimated from the SEM images

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  Figure 3  TEM images ((a), (b)) of the Pt₅₂Ag₄₈; HRTEM images ((c), (d)) of the local sites of Pt₅₂Ag₄₈ labelled in (b)

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  Figure 4  XRD patterns of the Pt, Pt₈₆Ag₁₄, Pt₇₉Ag₂₁, Pt₅₂Ag₄₈, Pt₂₁Ag₇₉, Pt₁₁Ag₈₉, and Ag

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  Figure 5  Cyclic voltammograms of Pt₁₁Ag₈₉, Pt₂₁Ag₇₉, Pt₅₂Ag₄₈, Pt₇₉Ag₂₁ and Pt₈₆Ag₁₄ NPs performed in the electrolyte of N₂-saturated 0.5 moL/L KOH. the inset is the enlarge image of CV between -1.0 and -0.7 V

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  Figure 6  Cyclic voltammograms of Pt₅₂Ag₄₈ in N₂-saturated 0.5 moL/L KOH electrolyte

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  Figure 7  Cyclic voltammograms of Pt black and Pt₅₂Ag₄₈ performed with different positive potential limits (-1.0 to 0.5 V and -1.0 to 0.1 V) in the electrolyte of N₂-saturated 0.5 mol/L KOH

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  Figure 8  Cyclic voltammograms (a) and linear polarization curves (b) for MOR on the Pt black (JM), Pt₁₁Ag₈₉, Pt₂₁Ag₇₉, Pt₅₂Ag₄₈, Pt₇₉Ag₂₁ and Pt₈₆Ag₁₄ catalysts in the electrolyte of 0.5 moL/L KOH + 2 moL/L CH₃OH, the insets in (a) and (b) show the corresponding activities at -0.25 V and the dependence between the onset potential of MOR and the catalysts composition, respectively (the scan rate is 20 mV/s)

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  Figure 9  Cyclic voltammograms of (a) Pt black and (b) Pt₅₂Ag₄₈ in the electrolyte of 0.5 moL/L KOH + 2 moL/L CH₃OH with different potential limits at a scan rate of 20 mV/s

  black solid line: -1.0 to 0.5 V; red dotted line: -1.0 to 0.1 V

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  Figure 10  Sequential cyclic voltammograms of Pt₅₂Ag₄₈ in the electrolyte of 0.5 moL/L KOH + 2 moL/L CH₃OH with the potential ranges of (a) -1.0 to 0.5 V and (b) -1.0 to 0.1 V (scan rate 20 mV/s)

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  Table 1.  Pt_xAg_y atomic molar ratios in the alloy NPs prepared with different Pt(NH₃)₄C₂O₄/CH₃COOAg ratios in the molten salt precursor, analyzed by EDX

  PtxAgy sample	Pt(NH3)4C2O4/CH3COOAg	Pt/Ag, by EDX
  Pt86Ag14	8:1	6.09
  Pt79Ag21	4:1	3.89:1
  Pt52Ag48	1:1	1.10:1
  Pt21Ag79	1:4	1:3.88
  Pt11Ag89	1:8	1:8.51

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  Table 2.  MOR performance of Pt black, Pt₁₁Ag₈₉, Pt₂₁Ag₇₉, Pt₅₂Ag₄₈, Pt₇₉Ag₂₁ and Pt₈₆Ag₁₄ catalysts in 0.5 moL/L KOH + 2 moL/L CH₃OH

  Sample	E0 /V	Ep /V	If /(mA·μgPt-1)	Ib /(mA·μgPt-1)	If/Ib	I@-0.25V /(mA·μgPt-1)
  Pt black	-0.77	-0.07	1.43	0.56	2.55	0.86
  Pt11Ag89	-0.51	-0.28	0.02	-	-	0.02
  Pt21Ag79	-0.71	-0.25	0.34	0.03	11.33	0.33
  Pt52Ag48	-0.91	-0.20	1.61	0.19	8.47	1.02
  Pt79Ag21	-0.82	-0.21	0.43	0.06	7.17	0.39
  Pt86Ag14	-0.66	-0.24	0.25	0.02	12.50	0.26

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