English

• 1.   Introduction

  Fuel cells have attracted increased research activity over the past few decades because of the ever-increasing environmental issues and the impending depletion of fossil fuels [1-4]. Lowtemperature fuel cell types include direct methanol fuel cells (DMFCs) and direct formic acid fuel cells (DFAFCs); these fuel cells present unique properties, including sufficiently high energy density and low operating temperatures, and are expected to become substituted power sources for portable electronic devices [5-8].

  Most low-temperature fuel cells use Pt metal as a catalyst, Pt-based alloys with other transition metals, especially nanostructured materials, exhibit higher performance than non-Pt-containing alloys because of the so-called bi-functional mechanism. However, the reaction intermediates easily poison Pt, such as CO. To address these problems, a strategy to improve catalytic activity by controlling the surface structure, alloy, and core-shell arrangement of the nanostructured catalysts was developed [9-11]. Pt-based catalysts with a stepped surface and/or concave facets have recently received renewed research interests at the theoretical and technological levels because of the presence of high-index facets on their surfaces, and their special optical and electrocatalytic properties [12-15]. It is reported that concave Pt-Pd-Cu NCs have demonstrated enhanced stability, resistance to catalyst poisoning, and markedly improved catalytic activity in methanol electrooxidation because of the presence of high density atomic steps, ledges, and kinks on their high-index-facets [10].

  In this work, Fe is used as structural modifier of Pt-Cu NCs via the simultaneous reduction of all precursors with the aid of PVP. We demonstrate the preparation, characterization, and electrooxidation reaction of concave Pt-Cu-Fe NCs obtained by a one-pot organic solvothermal method. The concave Pt-Cu-Fe NCs showed high electrocatalytic activity during methanol oxidation. Concave Pt-Cu-Fe NCs exhibited substantially improved electrochemical properties, such as improved durability and a more negative potential of the forward peak during methanol electro-oxidation compared with pure Pt-Cu NCs and commercial Pt/C catalysts.

  2.   Experimental

  2.1   Synthesis of concave Pt-Cu-Fe NCs

  To synthesize the Pt-Cu-Fe NCs, H₂PtCl₆·6H₂O (0.03 mmol), and PVP (50.0 mg, MW=40, 000, Aldrich) were added in absolute alcohol (5 mL). Meanwhile, Fe (acac)₃ (0.01 mmol, Acros, Geel, Belgium), and Cu (acac)₂ (0.06 mmol, Acros) were dispersed in oleylamine (10 mL, Acros). The dissolved Pt salt solution with yellow color was added into the oleylamine solution with continuous stirring. The solution was sonicated for about 30 min, and then poured into a Teflon-lined autoclave (25 mL). The autoclave was kept at 180 ℃ for 12 h, then cooled to 25 ℃. The obtained mixtures were precipitated by toluene and then centrifuged at 8000 r min^-1. The products were washed with alcohol three times and dried at 80 ℃ in a vacuum oven for 12 h. To compare electrochemical properties, commercial Pt/C catalyst was obtained from Alfa Aesar (Ward Hill, MA, USA).

  2.2   Structural and electrochemical analysis

  For the structural analysis of the catalysts, X-ray diffraction (XRD) analysis was carried out on Bruker D8-ADVANCE diffractometer with Cu Kα radiation of wavelength λ=1.5406 Å to study the crystallographic information of the samples. For all of the XRD measurements, the resolution in the scans was kept at 0.02°. The composition of the product was measured by the inductively coupled plasma-atomic emission spectrometer (ICP-AES; USA Thermo Jarrell-Ash Corp. ICP-9000(N+M)). Field-emission scanning electron microscopy (FESEM; JEOL, JSM-7500F, 10 kV) was used to analyze the morphology of the samples. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) equipped with energy-dispersive X-ray spectroscopy (EDX) were performed on a Philips Tecnai F20 microscope, working at 200 kV. The TEM samples were prepared by placing drops of catalyst suspension dispersed in ethanol on a carbon-coated nickel grid.

  All electrochemical measurements were carried out with a CHI 660D electrochemical workstation system and a conventional three-electrode cell at 25 ℃. The cell was composed of a glassy carbon electrode (0.07 cm²), Pt wire, and Ag/AgCl (in saturated KCl) as the working, counter, and reference electrodes, respectively. The catalyst ink was prepared by ultrasonically dispersing the catalyst powders in an appropriate amount of Millipore water Nafion (Sigma-Aldrich) solution (0.2%) and 2-propanol solution.

  3.   Results and discussion

  The chemical composition of the as-synthesized Pt-Cu-Fe NCs was characterized by EDX and XRD. EDX analysis demonstrated that Pt, Cu, and Fe were present at an atomic ratio of 0.32:0.58:0.11, no other elements were detected in all of the Pt-Cu-Fe NC samples (Fig. 1a). The XRD patterns of the Pt-Cu and Pt-Cu-Fe NCs are displayed in Fig. 1b. All of the samples exhibited similar disordered face-centered-cubic (fcc) structures. Compared with the standard (111) diffraction peaks, the alloyed Pt peak of Pt-Cu NCs indicated was not present in the XRD patterns. By comparison, Pt, Cu, and Fe single component peaks were not detected in the Pt-Cu-Fe NCs, thereby confirming the presence of only one alloy structure in the samples. The accurate molar ratios of Pt-Fe-Cu NCs in the concave NCs were verified by ICP-AES analysis; and the observed ratios were in good agreement with the ratio of metal sources used in the syntheses reactions.

  图 1

  

  图 1  (a) EDS spectrum and (b) XRD patterns of Pt-Cu-Fe NCs.

  Figure 1.  (a) EDS spectrum and (b) XRD patterns of Pt-Cu-Fe NCs.

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  Fig. 2a shows a representative FE-SEM image of the assynthesized materials (Pt-Cu-Fe NCs), which displayed uniform and well-dispersed nanoparticles. The morphology and structure of the NCs were further characterized by TEM (Fig. 2b, c). The selected-area electron-diffraction (SAED) pattern (inset of Fig. 2b) of the as-synthesized materials indicates the high crystallinity of the NCs. Careful measurements revealed that mainly one type of lattice fringe with an interplanar spacing of 1.90 Å (Fig. 2d), which can be ascribed to the (100) plane, is present in the sample. HAADFSTEM images (Fig. 2e) also confirmed the concave structure of the NCs. As shown in the single-particle STEM-EDS line-scan profiles (Fig. 2f), the distributions of Pt, Cu and Fe in the NCs were in good agreement with their cubic or concave shapes. According to our previous work [13], Pt-Cu-Fe NCs were synthesized in two major steps. First, Cu NCs were formed after reduction, and then Pt and Fe nucleated at the edges and faces of the Cu-rich NCs via galvanic replacement to form NCs with a concave surface morphology.

  图 2

  

  图 2  (a) FE-SEM, (b, c) TEM and magnified TEM, inset of (b) SAED pattern, (d) HAADF-TEM, (e) HAADF-STEM, and (f) STEM line-scan profile of Pt-Cu-Fe NCs.

  Figure 2.  (a) FE-SEM, (b, c) TEM and magnified TEM, inset of (b) SAED pattern, (d) HAADF-TEM, (e) HAADF-STEM, and (f) STEM line-scan profile of Pt-Cu-Fe NCs.

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  The XPS analysis was used to determine the electronic structure and valence state distribution of metals in the Pt-Cu-Fe NCs. As shown in Fig. 3a, a pair of asymmetric peaks constitutes the Pt 4f signal. The binding energies 71.1 eV and 74.5 eV were assigned to the Pt 4f_7/2 and Pt 4f_5/2. For Pt-Cu-Fe NCs, the Cu 2p peak (Fig. 3b) fitting results indicate that only trace amounts of Cu (Ⅱ) were found, suggesting that Cu (Ⅱ) was fully reduced. The remaining Cu in the catalysts mainly consisted of Cu (0) species, and Cu (Ⅰ) species may also exist in the catalyst. However, Cu (Ⅰ) is difficult to distinguish from Cu (0) because their binding energies are similar. Fig. 3c shows the Fe 2p spectrum of the Pt-Cu-Fe NCs, displaying the characteristics peak of Fe at 712.0 eV. By integrating the area of the Pt 4f, Cu 2p and Fe 2p core-level peaks, the atomic ratio of Pt:Cu:Fe on the surface of the Pt-Cu-Fe NCs is calculated to be 0.27:0.50:0.23, in contrast to the bulk value at 0.32:0.58:0.11 as calculated from the result of EDX. The large discrepancy may be attributed to the galvanic replacement of Cu at the surface NCs and the lower surface free energy of Pt compared to Fe, therefore more Pt atoms are preferentially deposited on the particle surface to acquire lower particle surface energy with higher stability.

  图 3

  

  图 3  (a) Pt 4f, (b) Cu2p, and (c) Fe 2p XPS spectra for Pt-Cu-Fe NCs.

  Figure 3.  (a) Pt 4f, (b) Cu2p, and (c) Fe 2p XPS spectra for Pt-Cu-Fe NCs.

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  Prior to electrocatalytic activity measurements, Pt_0.3Cu_0.6Fe_0.1 and Pt_0.3Cu_0.7 NCs were deposited on a carbon support by sonicating a mixture of NCs and carbon supports [13]. Fig. 4a shows the cyclic voltammograms (CVs) of Pt-Cu-Fe/C, Pt-Cu/C and Pt/C recorded at ambient temperature in N₂-purge 0.5 mol L^-1 H₂SO₄ solution at a sweep rate of 50.0 mV s^-1. The CV curves exhibited two distinct potential regions associated with hydrogen adsorption/desorption processes between (-0.05 V and +0.40 V), and the formation of adsorbed hydroxyl species layer beyond~0.5 V. The electrochemically active surface area (ECSA, Fig. 4b) of the samples was estimated using the electric charge accumulated during hydrogen adsorption, and found to decrease in following order: Pt/C > Pt-Cu/C > Pt-Cu-Fe/C (Table 1). The ECSAs of Pt-CuFe/C, Pt-Cu/C, and Pt/C were estimated to be 60.8, 68.3, and 82.1 m² g^-1 metal, respectively.

  图 4

  

  图 4  (a) Cyclic voltammograms (CVs) and (b) special ECSAs of Pt-Cu-Fe/C, Pt-Cu/C and Pt/C in 0.5 mol L^-1 H₂SO₄.

  Figure 4.  (a) Cyclic voltammograms (CVs) and (b) special ECSAs of Pt-Cu-Fe/C, Pt-Cu/C and Pt/C in 0.5 mol L^-1 H₂SO₄.

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  表 1

  

  表 1  Comparison of electrocatalytic properties of the prepared catalysts for methanol and formic acid oxidations.

  Table 1.  Comparison of electrocatalytic properties of the prepared catalysts for methanol and formic acid oxidations.

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  The electrocatalytic activities of Pt-Cu-Fe/C catalyst for methanol and formic acid oxidation were compared with those of Pt-Cu/C and commercial Pt/C catalysts (Fig. 5a and b). The currents were calculated by mass-transport correction and normalized to the loading amount of Pt because Pt has activity in the oxidation of the hydrocarbons used in this study. The electrochemical test results are summarized in Table 1. The PtCu-Fe/C catalysts exhibited enhanced activity compared with the other samples. The peak current densities of Pt-Cu-Fe/C for methanol and formic acid oxidation were 5.4 and 6.2 times higher than those on the Pt/C catalysts. The onset potentials of methanol and formic acid oxidation with the alloyed Pt-Cu-Fe/C occurred at much more negative values compared with those of the Pt-Cu/C and Pt/C catalysts, indicating a large enhancement in oxidation kinetics. The ratio of peak current densities of the two anodic peaks (Ⅰ_f/Ⅰ_b) of Pt-Cu-Fe/C were remarkably larger than those of Pt-Cu/C, and Pt/C, indicating the superior anti-poisoning property against carbonaceous species generated during the electrooxidation of methanol and formic acid [16]. Pt-Cu-Fe/C showed excellent poisoning resistance (Ⅰ_f/Ⅰ_b) during methanol (12.4) and formic acid (4.2) oxidation. Fig. 5b clearly shows the oxidation current of Pt-Cu-Fe/C, Pt-Cu/C, and Pt/C for formic acid. During the forward scan, the catalyst of Pt-Cu-Fe NCs showed a very small peak at about 0.11 V due to the hydrogen oxidation. The prepared Pt-Cu-Fe/C exhibited the most prominent electrochemical performance in terms of the largest forward peak current density, the lowest onset potential, and the highest Ⅰ_f/Ⅰ_b ratio.

  图 5

  

  图 5  Cyclic voltammograms (CVs) of Pt-Cu-Fe/C, Pt-Cu/C and Pt/C in (a) 0.5 mol L^-1 H₂SO₄/0.5 mol L^-1 CH₃OH and (b) 0.5 mol L^-1 H₂SO₄/0.5 mol L^-1 HCOOH.Chronoamperometric curves (CA) for the same samples in (c) 0.5 mol L^-1 H₂SO₄/0.5 mol L^-1 CH₃OH at 0.70 V and (d) 0.5 mol L^-1 H₂SO₄/0.5 mol L^-1 HCOOH at 0.40 V.

  Figure 5.  Cyclic voltammograms (CVs) of Pt-Cu-Fe/C, Pt-Cu/C and Pt/C in (a) 0.5 mol L^-1 H₂SO₄/0.5 mol L^-1 CH₃OH and (b) 0.5 mol L^-1 H₂SO₄/0.5 mol L^-1 HCOOH.Chronoamperometric curves (CA) for the same samples in (c) 0.5 mol L^-1 H₂SO₄/0.5 mol L^-1 CH₃OH at 0.70 V and (d) 0.5 mol L^-1 H₂SO₄/0.5 mol L^-1 HCOOH at 0.40 V.

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  To explore the observed enhancements in the electrocatalytic activity and CO-tolerance of the proposed catalyst further, we performed chronoamperometry (CA) experiments. The CA curves in Fig. 5c and d reveal that the current density of the concave Pt-Cu-Fe/C catalyst is higher than that of the commercial Pt/C catalyst. CA curves also demonstrated that the current densities in both oxidation cases decreased in the following order: Pt-Cu-Fe/C > PtCu/C > Pt/C. In particular, the Pt-Cu-Fe/C were able to maintain oxidation currents of 544 and 116 mA mg^-1 Pt after being polarized in the methanol and formic acid solutions for 3000 s; these currents are 6.9 and 29.0 times higher than those of the Pt/C catalysts.

  4.   Conclusion

  In summary, this work presents the synthesis, characterization, and electrochemical evaluation of concave Pt-Cu-Fe NCs via partial galvanic exchange. The electrocatalytic activity and stability of the prepared catalyst was comparatively investigated via methanol and formic acid electro-oxidation. The Pt-Cu-Fe/C catalyst exhibited relatively high durability and strong poisoning resistance; its Pt-mass activity was 5.4 and 6.2 times higher than those of Pt/C during methanol and formic acid oxidation, respectively. The Pt-Cu-Fe/C catalyst also exhibited excellent activity, including a more negative onset potential and higher CO tolerance, compared to those of Pt/C during formic acid electro-oxidation. The enhanced catalytic performance of Pt-Cu-Fe/C catalyst could be attributed to its concave structure and possible synergetic effects among its Pt, Cu, and Fe components. Further product analysis by in situ FTIR and HPLC is required to understand the enhancement effects of the concave Pt-Cu-Fe NCs better.

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• 

  Figure 1  (a) EDS spectrum and (b) XRD patterns of Pt-Cu-Fe NCs.

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  Figure 2  (a) FE-SEM, (b, c) TEM and magnified TEM, inset of (b) SAED pattern, (d) HAADF-TEM, (e) HAADF-STEM, and (f) STEM line-scan profile of Pt-Cu-Fe NCs.

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  Figure 3  (a) Pt 4f, (b) Cu2p, and (c) Fe 2p XPS spectra for Pt-Cu-Fe NCs.

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  Figure 4  (a) Cyclic voltammograms (CVs) and (b) special ECSAs of Pt-Cu-Fe/C, Pt-Cu/C and Pt/C in 0.5 mol L^-1 H₂SO₄.

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  Figure 5  Cyclic voltammograms (CVs) of Pt-Cu-Fe/C, Pt-Cu/C and Pt/C in (a) 0.5 mol L^-1 H₂SO₄/0.5 mol L^-1 CH₃OH and (b) 0.5 mol L^-1 H₂SO₄/0.5 mol L^-1 HCOOH.Chronoamperometric curves (CA) for the same samples in (c) 0.5 mol L^-1 H₂SO₄/0.5 mol L^-1 CH₃OH at 0.70 V and (d) 0.5 mol L^-1 H₂SO₄/0.5 mol L^-1 HCOOH at 0.40 V.

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  Table 1.  Comparison of electrocatalytic properties of the prepared catalysts for methanol and formic acid oxidations.

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