English

• Direct methanol fuel cells (DMFCs) are highly attractive energy conversion devices for powering portable electronics with the advantages of high energy conversion efficiency, safe storage and less pollution^[1-3]. Platinum is generally accepted as the most effective electro-catalyst towards the methanol oxidation reaction (MOR). However, Pt-based catalysts suffer from several important drawbacks, including the high cost, insufficient activity and low tolerance to carbon monoxide, which greatly hinder the commercialization of DMFCs. In addition, the performance and stability of the Pt-based catalyst are also affected by the corrosion behavior and inert essence of the carbon black support (e.g., Vulcan XC-72). Therefore, achieving a low-cost, high-efficient and stable catalyst remains a major challenge.

  The deposition of Pt nanoparticles on tungsten carbide (Pt/WC) are considered as an effective strategy to address the above-mentioned issues since WC displays its platinum-like activity, high stability in acid solutions and CO poisoning tolerance^[4-6]. But surface science results show that tungsten carbide alone indicates low activity towards MOR because its surface is less active for the C-H bond scission compared with high activity towards the O-H bond cleavage to produce a methoxy intermediate^[7]. Further research shows that adding low coverages of Pt on WC films enhances the scission of the C-H bonds of methoxy, suggesting a synergistic effect between Pt and WC capable of promoting dehydrogenation of methanol to CO₂^{[8, 9]}. Unfortunately, current WC synthesis using a solid state method^[10-12] requires high carburization temperatures in order to overcome the energy barriers for carbon incorporation into the metal lattice, which leads to uncontrollable particle sintering, exceedingly low surface area and surface impurity such as graphitic coke and nonstoichiometricoxide (WO_x)^[13]. Thus, tremendous efforts have been made to effectively control the size and morphology of WC with a large specific surface area.

  Along with the size and morphology, the surface compositions have an important effect on the catalytic activity of WC. The high synthesis temperature results in embedding WC within high excesses of surface carbon under an atmosphere of hydrocarbon and hydrogen. Although the surface carbon of WC could protect WC from oxidation, the catalytic activity of WC is negatively affected by it in the MOR. In order to partially remove surface carbon and activate the catalyst, various surface treatment methods have been investigated, including electrochemical activation^[14], oxygen plasma^[15] and hydrogen reduction^[16]. In particular, Pt nanoparticles tend to bind strongly to W-terminated WC surfaces in comparison with C-terminated WC surfaces^[17]. Unfortunately, W-terminated WC surfaces cannot readily form stable carbides and is prone to oxidation even in ambient air, leading to the formation of an oxide surface layer^[18]. To the best our knowledge, no report is available on the effect of the surface oxide on the activity and stability of Pt/WC catalysts.

  Tungsten trioxide (WO₃) has attracted considerable attention in recent years, mainly due to its insolubility in acid solution and hydrogen spill-over effect and the bifunctional mechanism, especially for DMFC applications^{[19, 20]}. Previous studies by our groups have demonstrated that WO₃ cannot only form tungsten bronze compounds to promote the dehydrogenation of methanol, but also remove the adsorbed intermediates during the MOR^[21]. Herein an in-situ reduction carbonization technology to synthesis WC microspheres modified with tungsten oxide (WO₂) or surface carbon is proposed, which is used as an electro-catalyst support for MOR.

  1.   Experimental

  1.1   Preparation of tungsten-based samples

  Hollow ammonium met tungstate microspheres (AMT) precursor was prepared by spray drying method, which was previously reported by our group^[22]. The as-prepared precursor was sent into a tube furnace that had been free from oxygen, followed by undergoing the gas-solid reaction under an atmosphere of CO/CO₂. The flow rates of CO and CO₂ were 200 and 20 mL/min, respectively. Initially, the temperature of the furnace was raised to 673 K and maintained for 1 h, and then raised to 1123 K for different reaction time (6, 10 and 15 h) while a mixture of CO and CO₂ at a ratio of 10:1 was always passed through the furnace. After cooled to room temperature under an atmosphere of nitrogen, all the samples were passivated for 6 h in a 1% oxygen/ 99% nitrogen environment at room temperature before exposure to air in order to prevent rapid oxidation. The as-prepared products were respectively designated as WCO-6 h, WCO-10 h and WC-15 h.

  1.2   Preparation of electrocatalysts

  Subsequently, Pt/WCO-6 h catalyst was fabricated by the improved liquid phase reduction method^[23]. In a typical procedure, 50 mg of the as-prepared WCO-6 h support and 11.4 mL of 5 mmol/L H₂PtCl₆ solution were dispersed in 50 mL of water by sonication. After the pH value was adjusted to nearly 9 using 1 mol/L NaOH solution, a freshly prepared 10 mL of 0.1 mol/L NaBH₄ solution was added to the solution under vigorous stirring for 6 h. Then, the mixture was centrifuged and washed with deionized water, and finally dried at room temperature, yielding 20% Pt loading on the supports. For comparison, Pt/WC-15 h catalysts with the same loading amounts were respectively prepared by the same procedure above. A commercial 20% Pt/C (JM Pt/C) was purchased from Johnson Matthy Co., Ltd. and used as contrast samples.

  1.3   Electrochemical measurements

  The electrochemical measurements of the catalysts were carried out with an Ivium electrochemical workstation. The workstation was connected to a standard three-electrode cell with a glassy carbon (GC, with a diameter of 3 mm) as the working electrode, a Pt foil and saturated calomel electrode (SCE) as counter and reference electrodes, respectively. For the working electrode preparation, 5 mg of electrocatalysts were ultrasonically mixed in 200 μL of ethanol-water solution to form a homogeneous ink. After the GC electrode was polished using aqueous alumina, 2 μL of the electrocatalyst ink was pipetted using a micropipettor onto the GC surface to make a Pt loading of about 0.14 mg/cm², followed by 5 μL of Nafion solution of 1.0% (DuPont, USA) in ethanol dropped to fix the electrocatalyst on the GCE surface. The cyclic voltammetry(CV) measurements were carried out in the aqueous solution of 0.5 mol/L H₂SO₄ containing 1.0 mol/L CH₃OH at a scan rate of 50 mV/s. Chronoamperometry measurements were performed under a potential of 0.3 V or 0.6 V for 6000 s in 0.5 mol/L H₂SO₄ aqueous solution containing 1.0 mol/L CH₃OH. All solutions were fully purged with Ar gas before the measurements.

  1.4   Characterization

  The phases present in the synthesized materials were identified using XRD (Panalytical X′Pert Pro, Cu Kα₁ radiation source (λ = 0.1541 nm), voltage of 40 kV, current of 40 mA). The morphology and structure of the products were characterized using FE-SEM (Hitachi S-4700 Ⅱ) and TEM (Tecnai G2 F30) with Energy-dispersive X-ray spectroscopic (EDS). XPS was carried out on Kratos AXIS Ultra DLD. All XPS spectra were corrected using the C 1s line at 284.6 eV. Curve fitting and back ground subtraction were accomplished.

  2.   Results and discussion

  2.1   Structure and morphology

  Figure 1 shows that XRD patterns of tungsten carbides microsphere prepared under CO/CO₂ mixture atmosphere using the as-prepared AMT precursor as a function of reaction time.

  Figure 1

  

  Figure 1.  XRD patterns of (a) WCO-6 h, (b) WCO-10 h and (c) WC-15 h

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  It can be found that the main diffraction peaks matched well with those of the hexagonal phase WC (JCPDS No. 72-0097) except for peaks appearing at 25.7°, 37.0° and 52.9° which can be indexed to the monoclinic phase WO₂ (JCPDS No. 32-1393). Meanwhile, the characteristic (011) peak of WO₂ at 25.7° becomes weaker with the increase of reaction time and thoroughly disappears at 15 h, indicating the formation of pure hexagonal phase WC. Combined with the results of in situ XRD^[24], it could be concluded that the phase transition of the sample follows the pattern AMT, WO₃, WO₂, WC with increasing reaction time during the reduction and carbonization.

  Figure 2 shows SEM images of tungsten carbide microspheres prepared at different reaction time under CO/CO₂ mixture atmosphere using the as-prepared AMT precursor, which has well-defined spherical structures with an average diameter of ca. 3 mm previously reported by our group. This result indicates that WO₃ is gradually in situ carburized due to homogeneous substitution of oxygen atoms in the lattice with carbon atoms without serious collapse and aggregation during the carburization process. In particular, the surface of WC microspheres becomes rough and porous structure with increasing reaction time due to the reverse diffusion of internal ammonia gas generated from AMT decomposition and external CO/CO₂ mixture atmosphere.

  Figure 2

  

  Figure 2.  SEM images of ((a), (b)) WCO-6 h and ((c), (d)) WC-15 rowspan=""

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  The composition and chemical state on the surface of WC microspheres was further investigated by XPS measurement. A comparison of the XPS C 1s and W 4f peaks in the light of reaction time is given in Figure 3. Two types of carbon (Figure 3(a)) are mainly observed in peaks at 282.7 and 284.7 eV, which are assigned as carbide and graphite, respectively. Other C 1s peaks at higher binding energy can be assigned to various oxidized carbon species, indicating that surface impurities such as the carbon deposition increase with reaction time. Inspection of the XPS W 4f spectra (Figure 3(b)) displays a significant increase in the carbide-modified W at 34.0 and 32.0 eV, and a decrease in the oxygen-modified W at 37.8 and 35.4 eV^[25], indicating oxygen is gradually replaced by carbon with increasing the reaction time, consistent with the result of XRD. A relatively small amount of oxidic oxygen (WO_x) is always present on the freshly prepared WC surface regardless of the method of preparation.

  Figure 3

  

  Figure 3.  XPS spectrum of Pt/WCO-6 h and Pt/WC-15 h, showing the peaks for the (a) C 1s and (b) W 4f elements

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  The crystalline phases, particle size and distributionof Pt nanoparticles deposited on the surface of as-prepared WC microspheres were characterized by XRD (Figure 4) and TEM (Figure 5). The intense diffraction peaks at 31.3°, 35.5° and 48.2° are indexed as the (001), (100) and (101) planes of the hexagonal phase WC^[26]. The slight diffraction peaks at 39.8°, 46.2° and 67.5° are corresponding to the Pt (111), (200) and (220) planes, respectively. No characteristic diffraction peaks of WO₂ is observed because of its lower loading content and weak crystallization. According to Debye-Scherrer′s formula, the crystal sizes of Pt nanoparticles are calculated to be 4.8 and 4.2 nm for Pt/WCO-6 h and Pt/WC-15 h catalysts, respectively.

  Figure 4

  

  Figure 4.  XRD patterns of (a) Pt/WCO-6 h and (b) Pt/WC-15 h

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  Figure 5

  

  Figure 5.  (a), (b) Typical TEM images of the as-prepared Pt/WCO-6 h with different magnifications; (c) EDS of Pt/WCO-6 h catalyst

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  It can be seen from the TEM images (Figure 5(a)) that Pt nanoparticles with the average size of 4.6 nm are uniformly dispersed on the WCO-6 h supports, which are consistent with the results of XRD. The high resolution TEM (HRTEM) image (Figure 5(b)) indicates a lattice spacing of 0.23 nm with the lattice orientation of (111) of Pt as marked by the white double lines, which are assigned to the face-centered cubic (fcc) (111) planes of Pt. Figure 5(c) shows the EDS spectra of Pt/WCO-6 h confirming the Pt loading concentration of 18.63%, which is close to actual loading amounts.

  2.2   Activity toward methanol electrooxidation

  The electrocatalytic properties of Pt/WCO-6 h catalyst were compared with those of Pt/WC-15 h and JM Pt/C catalysts.The hydrogen underpotential deposition (H-UPD) measurement (Figure 6(a)) is also used to calculate the electrochemically active surface area (ECSA).The result indicates that the Pt/WCO-6 h catalyst possesses a much larger electrochemically active surface area (ECSA) value (112 m²/g) than those of Pt/WC-15h (35.7 m²/g), JM Pt/C (48 m²/g) and platinum-based catalysts previously reported such as Pd-Pt bimetallic nanodendrites (57.1 m²/g)^[27], Pt/polyaniline/C (67.5 m²/g)^[28], Pt/ionic liquid/CNT (67.6 m²/g)^[29], Pt-Ni₂P/C (69.3 m²/g)^[30], etc. As shown in Figure 6(b), the onset potential of Pt/WCO-6 h catalyst is negatively shifted in comparison with that of the commercial Pt/WC-15 h and JM Pt/C catalysts, indicating that MOR can take place at a lower potential on Pt/WCO-6 h. In particular, Pt/WCO-6 h catalyst achieves the highest mass activity of 655.4 mA/mg, which is 3.6 and 2.2 times as high as those of Pt/WC-15 h (183.7 mA/mg) and JM Pt/C (300 mA/mg), respectively. This significant enhancement in the electrocatalytic activity of Pt/WCO-6 h might be related to the surface tungsten oxide, which accelerates the dehydrogenation of methanol on Pt and WC surface. On the contrary, the negative effect of impurities such as residue carbon leads to poor activity of Pt/WC-15 h towards MOR.

  Figure 6

  

  Figure 6.  Cyclic voltammetric curves of Pt/WCO-6 h, Pt/WC-15 h and JM Pt/C catalysts in (a) 0.5 mol/L H₂SO₄ and (b) 0.5 mol/L H₂SO₄ + 1.0 mol/L CH₃OH solution at the scan rate of 50 mV/s

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  It was also well known that the peak current ratio of the forward to backward scans (I_f/I_b) can be used to evaluate the anti-poisoning ability of catalysts in DMFCs. In this study, the I_f/I_b ratio of Pt/WCO-6 h catalyst (1.4) exhibites higher than those of Pt/WC-15 h (1.3) and JM Pt/C (0.7), displaying that Pt/WCO-6 h catalyst has better anti-poisoning ability during the MOR. Chronoamperometric (CA) measurements are also performed at 0.4 V or 0.6 V for 6000 s in order to evaluate the stability of catalysts (Figure 7).

  Figure 7

  

  Figure 7.  Chronoamperometric curves of Pt/WCO-6 h, Pt/WC-15 h and JM Pt/C catalysts at (a) 0.4 V and (b) 0.6 V for 6000 s in 0.5 mol/L H₂SO₄ + 1.0 mol/L CH₃OH solution

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  It is obvious that Pt/WCO-6 h catalyst exhibits a slower current decay over time in comparison with Pt/WC-15 h and JM Pt/C catalysts, indicating a higher tolerance to the carbonaceous species generated whether high voltage or low voltage during MOR. According to the experimental results combined with the literature reports, the reaction pathway of dehydrogenation on the Pt/WCO-6 h catalyst might involve the following steps^[31-33]:

  ${\rm{Pt}} + {\rm{C}}{{\rm{H}}_3}{\rm{OH}} \to {\rm{Pt}} - {\rm{C}}{{\rm{H}}_3}{\rm{O}}{{\rm{H}}_{{\rm{ads}}}}$

  (1)

  ${\rm{Pt - C}}{{\rm{H}}_3}{\rm{O}}{{\rm{H}}_{{\rm{ads}}}} \to {\rm{Pt - C}}{{\rm{O}}_{{\rm{ads}}}} + 4{{\rm{H}}^ + } + 4{\rm{e}}$

  (2)

  ${\rm{WC + C}}{{\rm{H}}_{\rm{3}}}{\rm{OH}} \to {\rm{WC}} - {\rm{OC}}{{\rm{H}}_{\rm{3}}}_{{\rm{ads}}} + {{\rm{H}}^ + } + {\rm{e}}$

  (3)

  $\text{WC}-\text{OC}{{\text{H}}_{\text{3}}}_{\text{ads}}\xrightarrow{\text{Pt}}\text{WC}-\text{C}{{\text{O}}_{\text{ads}}}+3{{\text{H}}^{+}}+3\text{e}$

  (4)

  The hydrogen adsorbed on the Pt spills over onto the surface of the WO₃ and formed H_xWO₃, which speeded up the dehydrogenation of methanoland promoted the adsorption and transformation of CO_ads intermediates. Subsequently, H_xWO₃ could be readily oxidized to release hydrogen ions, electronsand WO₃. This cyclic process could be explained as follows^{[34, 35]}:

  ${\rm{W}}{{\rm{O}}_{\rm{3}}} + x{\rm{Pt}} - {\rm{H}} \to {{\rm{H}}_x}{\rm{W}}{{\rm{O}}_{\rm{3}}} + x{\rm{Pt}}$

  (5)

  ${{\rm{H}}_x}{\rm{W}}{{\rm{O}}_{\rm{3}}} - {\rm{O}}{{\rm{H}}_{{\rm{ads}}}} + x{\rm{Pt}} - {\rm{C}}{{\rm{O}}_{{\rm{ads}}}} \to {{\rm{H}}_x}{\rm{W}}{{\rm{O}}_{\rm{3}}} + {\rm{Pt}} + {\rm{C}}{{\rm{O}}_2} + {{\rm{H}}^ + } + {\rm{e}}$

  (6)

  ${{\rm{H}}_x}{\rm{W}}{{\rm{O}}_{\rm{3}}} - {\rm{O}}{{\rm{H}}_{{\rm{ads}}}} + x{\rm{WC}} - {\rm{C}}{{\rm{O}}_{{\rm{ads}}}} \to {{\rm{H}}_x}{\rm{W}}{{\rm{O}}_{\rm{3}}} + {\rm{Pt}} + {\rm{C}}{{\rm{O}}_2} + {{\rm{H}}^ + } + {\rm{e}}$

  (7)

  ${{\rm{H}}_x}{\rm{W}}{{\rm{O}}_{\rm{3}}} \to {\rm{W}}{{\rm{O}}_{\rm{3}}} + x{{\rm{H}}^ + } + x{\rm{e}}$

  (8)

  Moreover, the WC surface is active toward the dissociation of H₂O to producesurface hydroxyl groups^[9], which also favors the removal of CO_ads intermediates during MOR.

  3.   Conclusions

  In summary, a novel method for the synthesis of WC microspheres with various surface compositions as a function of reaction time is developed, which are used as a highly efficient support for methanol electro-oxidation. The resultant Pt/WCO-6 h catalyst exhibits excellent catalytic performances towards MOR. It could be concluded that the surface tungsten oxide species have a positive effect on the electrocatalytic performances of Pt/WC catalyst. This work not only provides a facile method for the synthesis of WC microspheres with well-defined shapes, but also opens up new opportunities to design Pt/WC catalyst with enhanced performance for MOR.

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• 

  Figure 1  XRD patterns of (a) WCO-6 h, (b) WCO-10 h and (c) WC-15 h

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  Figure 2  SEM images of ((a), (b)) WCO-6 h and ((c), (d)) WC-15 rowspan=""

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  Figure 3  XPS spectrum of Pt/WCO-6 h and Pt/WC-15 h, showing the peaks for the (a) C 1s and (b) W 4f elements

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  Figure 4  XRD patterns of (a) Pt/WCO-6 h and (b) Pt/WC-15 h

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  Figure 5  (a), (b) Typical TEM images of the as-prepared Pt/WCO-6 h with different magnifications; (c) EDS of Pt/WCO-6 h catalyst

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  Figure 6  Cyclic voltammetric curves of Pt/WCO-6 h, Pt/WC-15 h and JM Pt/C catalysts in (a) 0.5 mol/L H₂SO₄ and (b) 0.5 mol/L H₂SO₄ + 1.0 mol/L CH₃OH solution at the scan rate of 50 mV/s

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  Figure 7  Chronoamperometric curves of Pt/WCO-6 h, Pt/WC-15 h and JM Pt/C catalysts at (a) 0.4 V and (b) 0.6 V for 6000 s in 0.5 mol/L H₂SO₄ + 1.0 mol/L CH₃OH solution

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