Preparation of platinum-silver alloy nanoparticles and their catalytic performance in methanol electro-oxidation
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摘要: 采用一种无需使用任何有机表面活性剂或溶剂的方法，在熔融盐体系中制备了铂银纳米合金颗粒，考察了合金中元素银对碱性电解质中甲醇电氧化反应（MOR）的催化作用。透射电子显微镜表征结果显示，当前躯体铂银物质的量比为1时，可以得到组成为Pt52Ag48的合金纳米管。甲醇电氧化反应测试结果表明，具有干净表面的Pt52Ag48纳米管比常规的Pt黑具有更好的催化性能。Pt52Ag48合金纳米管的催化活性与其最大正扫电位密切相关，正扫电位从-1.0到0.5 V（vs.SCE），MOR峰值电流达到1.61 mA/μgPt，是从-1.0到0.1 V（vs.SCE）正扫电位的1.92倍。铂银合金表面层中的Ag元素主要通过在电化学循环中发生氧化还原反应来促进合金的MOR活性。研究结果可以为铂银合金在直接甲醇燃料电池（DMFC）中的应用提供理论支持。Abstract: Platinum-silver alloy nanoparticles (PtxAgy 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 PtxAgy NPs was investigated. The TEM images suggest that Pt52Ag48 nanotubes (NTs) can be obtained when the Pt/Ag ratio in the molten salt precursor reaches 1. The methanol electrooxidation reaction test results indicate that 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/μgPt 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 PtxAgy alloy may promote the MOR through a redox process during the electrochemical cycle. The insight shown in work should be beneficial to the application of PtxAgy 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
Figure 2 Particle size distribution histogram for Pt86Ag14 (a) and nanotube diameter distribution histogram for Pt52Ag48 (b), estimated from the SEM images
Figure 3 TEM images ((a), (b)) of the Pt52Ag48; HRTEM images ((c), (d)) of the local sites of Pt52Ag48 labelled in (b)
Figure 4 XRD patterns of the Pt, Pt86Ag14, Pt79Ag21, Pt52Ag48, Pt21Ag79, Pt11Ag89, and Ag
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
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
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)
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
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)
Table 1 PtxAgy 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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