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Preparation and performance of nanosized tungsten carbides for electrocatalysis
Electrochimica Acta 55 (2010) 7969–7974
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Preparation and performance of nanosized tungsten carbides for electrocatalysis Pei Kang Shen ∗ , Shibin Yin, Zihui Li, Chan Chen State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China
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Article history: Received 29 November 2009 Received in revised form 3 March 2010 Accepted 5 March 2010 Available online 15 March 2010 Keywords: Tungsten carbides Intermittent microwave heating Continuous microwave heating Methanol oxidation Oxygen reduction
a b s t r a c t The principle of the intermittent microwave heating (IMH) method and the details on the working procedure for prepare nanosized materials were presented along with the comparison to the traditional continuous microwave heating (CMH) method. The nanosized tungsten carbides were synthesized as an example by this novel method. It produced WC with the average particle size of 21.4 nm at the procedure of 15 s-on and 15 s-off for 20 times, however, the particle size increased to 35.7 nm by CMH method for 5 min. The pure WC was obtained by post-treating the sample in NaOH solution, which gave the better performance as support. The nanosized WC was used as support for the Pt nanoparticles (Pt–WC/C(IMH)) for alcohol oxidation and oxygen reduction. It was proved that the Pt–WC/C(IMH) electrocatalysts gave the better performance than that prepared by CMH method (Pt–WC/C(CMH)) or Pt/C electrocatalysts in terms of the activity and CO-tolerance. The intermittent microwave heating method is easier to scale-up for mass production of the nanosized tungsten carbides and other nanosized materials as well. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Tungsten carbides (WC) have been intensively studied in chemical catalysis and electrochemical catalysis since the discovery of the Pt-like characteristics as reported by Levy and Boudart [1]. Recently, the enhanced activity was found by the addition of WC to Pt, Pd, Au and other precious or base metals toward oxygen reduction reaction (ORR) in both acidic and alkaline solutions [2–8]. Meanwhile, it has also been found that the introduction of WC into electrocatalysts has positive effect on hydrogen evolution [9,10], nitrophenol oxidation and reduction [11,12] and alcohol electrooxidation [13–16]. Moreover, WC as catalyst support in fuel cells is superior to the traditional carbon (e.g. Vulcan XC-72) in terms of the stability and conductivity [2,13,17]. The obstacle to use WC as catalyst supports is mainly due to the lack of synthesis methods to prepare high surface area WC. The commercial WC is very low in surface area (about 2 m2 g−1 ) with the particles in micrometer size. The surface areas of the WC particles synthesized by reported methods were mostly less than 10 m2 g−1 [18–28]. Therefore, it is necessary to develop novel methods to prepare high surface area WC by reducing the particle size used as catalyst supports since the particle size and the dispersion of the support are crucial for the catalytic reactions. In fact, nanosized or high surface area WC has been reported recently. Meng et al. [4,6,8,14] prepared Pt supported on W2 C/C
(Pt–W2 C/C) electrocatalysts by IMH method which showed a synergistic effect for ORR in acidic and alkaline solutions. The same effect was also found by the combination of W2 C/C and Ag nanoparticles for ORR in alkaline solutions [5] and alkaline alcohol fuel cells [8]. More evidence has been reported for hydrogen evolution reaction [29] and alcohol oxidation [7,9,30,31] as well. The WC with different morphologies and deposited on different matrixes were also prepared as catalysts supports [10,13,32,33]. Microwave irradiation method has exhibited remarkable advantages on nanoparticles preparation [34–37], and has been widely used in organic [38,39] and inorganic [40–43] nanoparticles synthesis. Unfortunately, the traditional, continuous microwave heating method is hardly to control the temperature during the heating process. Our group has developed a new way to rapid prepare nanosized WC by using intermittent microwave heating (IMH) method [4]. In this work, we report a modified IMH method to prepare nanosized WC. Compared to the traditional microwave heating methods, the temperatures in IMH method could be controlled during the synthesis processes by simply alternatively changing the heating time and the corresponding relaxation time for crystallization, phase transformation and structure re-arrangement of the preparing materials. 2. Experimental 2.1. Synthesis of tungsten carbides
∗ Corresponding author. Tel.: +86 20 84036736; fax: +86 20 84113369. E-mail address: [email protected] (P.K. Shen). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.03.025
Tungsten carbide was prepared by intermittent microwave heating (IMH) method. The ammonium metatungstate (AMT,
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(NH4 )6 H2 W12 O40 ·xH2 O, 3.7 g) was dissolved in 10 ml distilleddeionized water and 10 ml 2-propanol with magnetic stirring. Then, XC-72R carbon (2.5 g, Cabot corp., USA, SBET = 236.8 m2 g−1 ) as supporting material was added into the mixture and stirring until the ink uniformly dispersed. After heated at 80 ◦ C, the dried ink was further heated in a homemade program-controlled microwave oven (2000 W, 2.45 GHz) with heating procedures of 15 s-on/10 soff, 15 s-on/15 s-off, 15 s-on/20 s-off and 15 s-on/30 s-off for 20 times. The WC synthesis was carried out in air atmosphere. The well-dispersed powders were denoted as WC/C(IMH). For comparison, the continuous microwave heating method was also used to prepare WC and the corresponding product was denoted as WC/C(CMH). All the temperature data were obtained by recording the digital data from the dashboard during the synthesis processes. In order to get pure WC, the product was post-treated in 1.0 mol L−1 NaOH solution for more than 6 h with magnetic stirring. Afterwards, the resulting samples were filtered, washed and dried at 80 ◦ C for 12 h in a vacuum. The obtained samples were denoted as WC/C(IMH)-T and WC/C(CMH)-T, respectively.
and dried under infrared lamp to obtain an electrocatalysts thin film. The catalyst loading was kept constant as 0.25 mg cm−2 for the catalysts in this study. An aqueous solution containing 0.5 mol L−1 H2 SO4 with or without 1.0 mol L−1 CH3 OH was used as electrolyte, which was deaerated with high-pure nitrogen gas during the electrochemical characterization. The scanning potential was recorded at a scan rate of 50 mV s−1 . In the case of CO-stripping experiment, CO adsorption over Pt was conducted at −0.156 V in a CO saturated solution 0.5 mol L−1 H2 SO4 for 15 min. Subsequently, the electrolyte was purged with N2 for 30 min to remove dissolved CO from the solution. The potential scans between 0.156 and 1.20 V at a scan rate of 20 mV s−1 for two cycles. For oxygen reduction reaction (ORR) experiment, the rotating disk electrode (RDE) was carried out in an oxygen-saturated 0.5 mol L−1 H2 SO4 solution scanned between 1.1 to 0 V at a scan rate of 5 mV s−1 . All the potentials were referred to reversible hydrogen electrode (RHE) without specification.
2.2. Preparation of electrocatalysts
3. Results and discussion
The Pt–WC/C electrocatalysts were prepared by intermittent microwave heating method as described previously [44–46]. The main steps of this synthesis process were given as follows. The chloroplatinic acid precursor (2.71 ml, 18.5 mg Pt ml−1 ) was well mixed in a breaker with ethylene glycol (EG, 20 ml) in an ultrasonic bath. The WC/C(IMH) (200 mg) was then added into the mixture. After the pH value of the mixture was adjusted to ∼10 by a NaOH/EG solution. The well-dispersed slurry was obtained with stirring and ultrasonication for 15 min. Thereafter, the slurry was microwave-heated in the form of 5 s-on/5 s-off for five times. The WC synthesis was carried out in air atmosphere. After reacidification, the resulting black solid sample was filtered, washed and dried at 80 ◦ C for 12 h in a vacuum. The prepared electrocatalysts were denoted as Pt–WC/C(IMH) and the post-treated one as Pt–WC/C(IMH)-T. For the sake of comparison, the WC/C(CMH) with or without post-treatment and commonly used XC-72 as supports were also synthesized and used in the same way. The final electrocatalysts were denoted as Pt–WC/C(CMH)-T, Pt–WC/C(CMH) and Pt/C. The Pt loadings on the electrocatalysts were 20 wt%.
The temperature change during the continuous microwave heating or intermittent microwave heating process is shown in Fig. 1. As clearly displayed in the graph, the temperature continuously increases with time under the traditional heating mode. The final temperature depends on the nature of the materials being heated and the output power of microwave oven. The relationship between the heating time and the temperature for the continuous microwave heating method is shown in Fig. 1a. The principle of the IMH method can be explained by comparing the results as shown in Fig. 1. The temperature can be easily controlled as required by the IMH method. The required temperature can be alternatively reached to meet the requirements for the materials. In that case, we can adjust the upper temperature to control the degree of the crystallization of the material. More importantly, it is very convenience to control the growth of the crystallized or formed particles since the sustained time is very short at the highest temperature and this method also provides a relaxation time for crystallization, phase transformation and structure re-arrangement of the preparing materials. The longer the heat time the higher the temperature. The shorter the relaxation time the higher the low temperature limit. Fig. 2 shows the XRD patterns of the samples. The peaks at the 2 of 31.51◦ , 35.64◦ , 48.30◦ , 64.02◦ , 73.10◦ and 77.10◦ with the d values of 2.8431, 2.5170, 1.8813, 1.4531, 1.2934 and 1.2360 are corresponding to the (0 0 1), (1 0 0), (1 0 1), (1 1 0), (1 1 1) and (1 0 2) facets of WC [48–50], the 2 of 38.05◦ and 39.56◦ with the d values of 2.3630 and 2.2760 are corresponding to the (2 0 0) and (1 0 2) facets of W2 C, the 2 of 23.08◦ , 23.71◦ , 24.10◦ , 26.59◦ , 28.78◦ , 33.33◦ and 34.02◦ with the d values of 3.8500, 3.7500, 3.6900, 3.3500, 3.1000, 2.6860 and 2.6330 are corresponding to (0 0 1), (0 2 0), (0 2 0), (1 2 0), (1 1 1), (0 2 1) and (2 2 0) facets of WO3 [4], and the 2 of 40.26◦ with the d values of 2.2380 is corresponding to (1 1 0) facet of W. As clearly shown in Fig. 2A, the components of the products depend on the heating mode. The WO3 appeared in all the samples. It is due to the reaction of tungsten with trace oxygen in the container during the heating process. The WO3 could be significantly reduced in the N2 or Ar atmosphere. On the other hand, the WO3 would be gradually transferred into tungsten carbides at higher temperatures. The percentage of WC increased by reducing the relaxation time like the samples heated at 15 s-on/10 s-off and 15 s-on/15 s-off for 20 times. It was concluded that the WC can form at the temperature over 1000 ◦ C according to the temperature measurement during the heating processes.
2.3. Characterization of electrocatalysts The XRD measurements were carried out on a D/Max-III (Rigaku Co., Japan) using Cu K␣ radiation ( = 0.15406 nm), and operating at 40 kV and 20 mA. The 2 angular regions between 20◦ and 80◦ were explored at a scan rate of 5◦ min−1 in order to obtain the particle size of the prepared WC according to the Scherrer formula [47]. TEM investigations were carried out in a JEOL TEM-2010 (HR) at 200 kV to get the information of the mean particle size distribution of WC prepared by IMH method and CMH method. Electrochemical measurements were conducted on a PARETAT 2273 instrument (Princeton Applied Research, USA) in a thermostat-controlled standard three-electrode cell at room temperature using a saturated calomel electrode (SCE) and a platinum foil (1.0 × 1.0 cm2 ) as the reference and counter electrodes, respectively. A glass carbon (GC) disk electrode was used as the substrate for depositing the electrocatalysts thin film in the electrochemical measurements. The thin film electrocatalysts working electrode was prepared as follows. A mixture containing 10.0 mg electrocatalysts, 1.9 ml ethanol and 0.1 ml Nafion suspension (5 wt.%) was dispersed under ultrasonic for 30 min to obtain a well-dispersed electrocatalysts ink. The geometric area of the electrode was 0.1963 cm2 . The electrocatalysts ink was then quantitatively transferred onto the surface of the GC electrode by using a micropipette,
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Fig. 1. The relationship between the temperature and the heating time during the WC synthesis processes. (a) CMH, (b) IMH, 15 s-on/10 s-off, (c) IMH, 15 s-on/15 s-off and (d) IMH, 15 s-on/30 s-off.
Fig. 2. (A) The XRD patterns of the samples prepared (a) by continuous microwave heating (CMH) for 5 min and by intermittent microwave heating (IMH) at different on/off procedures of (b) 15 s-on/30 s-off, (c) 15 s-on/20 s-off, (d) 15 s-on/15 s-off and (e) 15 s-on/10 s-off for 20 times. (B) The XRD patterns of the WC synthesized by (a) CMH for 5 min and (b) IMH at 15 s-on/15 s-off for 20 times after the post-treatment in 1.0 mol L−1 NaOH solution. (C) The XRD patterns of the Pt nanoparticles supported on WC/C(IMH) prepared at 15 s-on/15 s-off for 20 times (a) after and (b) before the post-treatment in 1.0 mol L−1 NaOH solution. (D) The XRD patterns of the Pt nanoparticles supported on the WC/C(CMH) (a) after and (b) before the post-treatment in 1.0 mol L−1 NaOH solution. All data obtained at a scan rate of 5◦ min−1 , () WC, (♦) WO3 and (䊉) W2 C.
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Fig. 3. The TEM images of (a) WC/C(IMH) and (b) Pt–WC/C(IMH)-T. The corresponding EDX patterns of (c) WC/C(IMH) and (d) Pt–WC/C(CMH)-T.
In the case that the samples were heated by continuous microwave heating (CMH) method for 5 min, the average particle size was 35.7 nm calculated by the Scherrer formula [47], which is larger than that of the samples prepared by IMH at 15 s-on/10 soff of 24.8 nm and 15 s-on/15 s-off of 21.4 nm. The results indicated that the relaxation during the IMH processes could significantly protect the particles from the growth. The new particles produced in the every irradiation on period instead of the continuous growth of the formed particles. However, when the sample was heated by CMH, the temperature increases with time and consequently to form large particles. It can be concluded that the on and off time during the IMH processes were selected according to the temperature needed for the formation of WC. In this study, the temperature was controlled at around 1000 ◦ C as shown in Fig. 1. The shorter heating time (or longer relaxation time) gives the lower temperature to form WO3 dominantly. The longer heating time (or shorter relaxation time) results in higher temperature to form larger particles. The repeat time depends on the chemical reaction and the amount of the material. In the present case, the formation of WC proceeds via W → W2 C → WC. The WO3 and W2 C will dominant the product if not enough repeat time is applied. The WC particle size will increase at too much repeat time. For example, the dominant product is WO3 at 15 s-on and 15 s-off for 10 times, but the dominant product is changed to WC at 15 s-on and 15 s-off for 20 times. On the other hand, the particle size of the WC could be less than 20 nm at 15 s-on and 15 s-off for 20 times, while more than 30 nm at 15 s-on and 15 s-off for 30 times.
On the other hand, it has been reported that W2 C and WO3 are effective to enhance activity of the electrocatalysts as supports in fuel cells [6,51,52], however, they are not stable both in alkaline and acidic electrolytes [53]. Therefore, we treated the samples in 1.0 mol L−1 NaOH solution to remove the W2 C and WO3 containing in the as-prepared samples. The corresponding XRD results of WC/C(CMH)-T and the WC/C(IMH)-T prepared at 15 s-on/15 s-off are shown in Fig. 2B. It can be clearly seen that the W2 C and WO3 have be completely removed after the post-treatment and only WC left. The XRD patterns of WC/C(IMH) prepared at 15 s-on/15 s-off for 20 times and WC/C(CMH) before and after supported by Pt nanoparticles are shown in Fig. 2C and D. Obviously, all the electrocatalysts exhibited the typical characteristics of a crystalline Pt face centered cubic (fcc) structure with the average particle size of Pt on WC/C(IMH)-T, Pt on WC/C(IMH), Pt on WC/C(CMH)-T and Pt on WC/C(CMH) were 3.1 nm, 3.3 nm, 3.3 nm and 3.6 nm, respectively. The typical TEM images of WC/C prepared by IMH method at 15 s-on/15 s-off and Pt–WC/C(IMH)-T and the corresponding EDX patterns are shown in Fig. 3. It gives the similar results in the particle size both for WC and Pt nanoparticles that the average WC size was 25.0 nm and the corresponding Pt nanoparticles on WC/C was 3.0 nm. The methanol oxidation on Pt nanoparticles supported on WC/C(CMH) and WC/C(IMH) before and after post-treated were evaluated in 0.5 mol L−1 H2 SO4 with or without 1.0 mol L−1 CH3 OH and the results are shown in Fig. 4. It is clear that the Pt–WC/C(IMH)-T electrocatalysts showed the better per-
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Fig. 4. (a) The cyclic voltammograms (CV) of (a) Pt–WC/C(IMH)-T, (b) Pt–WC/C(IMH), (c) Pt–WC/C(CMH)-T, (d) Pt–WC/C(CMH) and (e) Pt/C electrocatalysts tested in 0.5 mol L−1 H2 SO4 + 1.0 mol L−1 CH3 OH at a scan rate of 50 mV s−1 at 25 ◦ C. The insert is the CV curves of the same electrocatalysts in 0.5 mol L−1 H2 SO4 solution. (b) CO-stripping voltammograms of Pt/C and Pt–WC/C(IMH)-T electrocatalysts in 0.5 mol L−1 H2 SO4 at a scan rate of 20 mV s−1 . Pre-adsorption of CO over Pt at −0.156 V (vs. RHE) for 15 min.
formance for methanol oxidation than that of Pt–WC/C(IMH), Pt–WC/C(CMH)-T, Pt–WC/C(CMH) and Pt/C electrocatalysts. The reason is that the Pt–WC/C(IMH)-T electrocatalysts has the larger electrochemical active surface area due to the smaller WC particle size and smaller Pt nanoparticle size as shown in the inset in Fig. 4. It is a challenge to increase the CO poisoning-resistance for a novel electrocatalysts since normally the by-products generated during the alcohol oxidation will occupy the active sites of Pt and consequently decreases the performance of Pt for further alcohol oxidation [54]. The numerous publications concerned the formation mechanism of CO during the alcohol oxidation, the kinetics of CO on the electrode and the effect of the catalyst on the adsorption and oxidation of CO [55–60]. We performed the CO-stripping experiment to compare the performance of the Pt–WC/C(IMH)-T and traditional Pt/C electrocatalyst as shown in Fig. 4b. The active surface area of Pt can be calculated by assuming a full coverage of CO monolayer on Pt associated with 420 C cm−2 charge [61]. However, in the present case, we fell difficult to calculate the active surface area of the electrocatalysts. The more active Pt–WC/C(IMH)-T gave smaller CO oxidation peak. Another reason could be the presence of WC reduces the adsorption of CO on the electrocatalyst. Meanwhile, it can also be seen from Fig. 4b that
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Fig. 5. (a) Polarization curves for the oxygen reduction reaction on Pt–WC/C(IMH)T electrocatalyst in oxygen-saturated 0.5 mol L−1 H2 SO4 at different rotating rates, ´ plot for oxyat 25 ◦ C, with a scan rate of 5 mV s−1 and (b) the Koutechy–Levich gen reduction reaction at 0.82 V (vs. RHE) on Pt–WC/C(IMH)-T electrocatalyst in O2 saturated 0.5 mol L−1 H2 SO4 solution.
CO could be oxidized more easily on Pt–WC/C(IMH)-T to give more negative onset potential which has 70 mV difference compared to that on Pt/C, indicating an improvement in the CO anti-poisoning resistance. Fig. 5a shows the oxygen reduction reaction (ORR) curves on Pt–WC/C(IMH)-T electrocatalyst in oxygen-saturated 0.5 mol L−1 H2 SO4 solution at different rotating rates with a scan rate of 5 mV s−1 . The kinetics for oxygen reduction on the electrode was ´ analyzed by Koutechy–Levich relationship. The reciprocal of the observed currents at 0.82 V on the RDE coated with Pt–WC/C(IMH)T from Fig. 5a were plotted against the reciprocal of the square root of rotating rates, as shown in Fig. 5b in O2 saturated 0.5 mol L−1 H2 SO4 aqueous solution. From the following equation [62], the slope (1/B) is directly related to the total number of electrons (n) involved in oxygen reduction: 1 1 1 = + I IK B(ω)1/2
(1)
where I is the experimentally observed current, Ik the kinetic current, B equals to 0.62nFACO2 (DO2 )2/3 v1/6 , v the rotation rate in rad s−1 , F the Faraday constant, A the electrode area, CO2 the bulk concentration of O2 , DO2 the diffusion coefficient for O2 and v the kinetics viscosity of the electrolyte. It can be found that the linear curve obtained experimentally is strictly parallel to the theoretical
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line of the four-electron reduction mechanism, indicating a fourelectron pathway for the ORR on Pt–WC/C(IMH)-T electrocatalyst. 4. Conclusions The principle of the intermittent microwave heating (IMH) method was described along with the comparison to the traditional continuous microwave heating method. The advantages of this novel method were evidenced during the synthesis of nanosized tungsten carbides. It resulted in average particle size of 21.4 nm at the procedure of 15 s-on and 15 s-off for repeat 20 times, however, the particle size increased to 35.7 nm by CMH method for 5 min. The pure WC on carbon could be reached by post-treating the sample in NaOH solution, which gave the better performance as support. The comparison of the Pt nanoparticles supported on WC-based supports indicated that the Pt nanoparticles on post-treated WC/C electrocatalyst prepared by IMH is the best for the alcohol oxidation. The intermittent microwave heating method is easier to scale-up for mass production of the nanosized tungsten carbides and other nanosized materials as well. Acknowledgements The work was supported by the China National 863 Program (2009AA05Z110), the Guangdong Sci. & Technol. Key Projects (2007A010700001, 2007B090400032) and Guangzhou Sci. & Technol. Key Projects (2007Z1-D0051, SKT[2007]17-11). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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Preparation and performance of nanosized tungsten carbides for electrocatalysis Pei Kang Shen ∗ , Shibin Yin, Zihui Li, Chan Chen State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China
a r t i c l e
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Article history: Received 29 November 2009 Received in revised form 3 March 2010 Accepted 5 March 2010 Available online 15 March 2010 Keywords: Tungsten carbides Intermittent microwave heating Continuous microwave heating Methanol oxidation Oxygen reduction
a b s t r a c t The principle of the intermittent microwave heating (IMH) method and the details on the working procedure for prepare nanosized materials were presented along with the comparison to the traditional continuous microwave heating (CMH) method. The nanosized tungsten carbides were synthesized as an example by this novel method. It produced WC with the average particle size of 21.4 nm at the procedure of 15 s-on and 15 s-off for 20 times, however, the particle size increased to 35.7 nm by CMH method for 5 min. The pure WC was obtained by post-treating the sample in NaOH solution, which gave the better performance as support. The nanosized WC was used as support for the Pt nanoparticles (Pt–WC/C(IMH)) for alcohol oxidation and oxygen reduction. It was proved that the Pt–WC/C(IMH) electrocatalysts gave the better performance than that prepared by CMH method (Pt–WC/C(CMH)) or Pt/C electrocatalysts in terms of the activity and CO-tolerance. The intermittent microwave heating method is easier to scale-up for mass production of the nanosized tungsten carbides and other nanosized materials as well. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Tungsten carbides (WC) have been intensively studied in chemical catalysis and electrochemical catalysis since the discovery of the Pt-like characteristics as reported by Levy and Boudart [1]. Recently, the enhanced activity was found by the addition of WC to Pt, Pd, Au and other precious or base metals toward oxygen reduction reaction (ORR) in both acidic and alkaline solutions [2–8]. Meanwhile, it has also been found that the introduction of WC into electrocatalysts has positive effect on hydrogen evolution [9,10], nitrophenol oxidation and reduction [11,12] and alcohol electrooxidation [13–16]. Moreover, WC as catalyst support in fuel cells is superior to the traditional carbon (e.g. Vulcan XC-72) in terms of the stability and conductivity [2,13,17]. The obstacle to use WC as catalyst supports is mainly due to the lack of synthesis methods to prepare high surface area WC. The commercial WC is very low in surface area (about 2 m2 g−1 ) with the particles in micrometer size. The surface areas of the WC particles synthesized by reported methods were mostly less than 10 m2 g−1 [18–28]. Therefore, it is necessary to develop novel methods to prepare high surface area WC by reducing the particle size used as catalyst supports since the particle size and the dispersion of the support are crucial for the catalytic reactions. In fact, nanosized or high surface area WC has been reported recently. Meng et al. [4,6,8,14] prepared Pt supported on W2 C/C
(Pt–W2 C/C) electrocatalysts by IMH method which showed a synergistic effect for ORR in acidic and alkaline solutions. The same effect was also found by the combination of W2 C/C and Ag nanoparticles for ORR in alkaline solutions [5] and alkaline alcohol fuel cells [8]. More evidence has been reported for hydrogen evolution reaction [29] and alcohol oxidation [7,9,30,31] as well. The WC with different morphologies and deposited on different matrixes were also prepared as catalysts supports [10,13,32,33]. Microwave irradiation method has exhibited remarkable advantages on nanoparticles preparation [34–37], and has been widely used in organic [38,39] and inorganic [40–43] nanoparticles synthesis. Unfortunately, the traditional, continuous microwave heating method is hardly to control the temperature during the heating process. Our group has developed a new way to rapid prepare nanosized WC by using intermittent microwave heating (IMH) method [4]. In this work, we report a modified IMH method to prepare nanosized WC. Compared to the traditional microwave heating methods, the temperatures in IMH method could be controlled during the synthesis processes by simply alternatively changing the heating time and the corresponding relaxation time for crystallization, phase transformation and structure re-arrangement of the preparing materials. 2. Experimental 2.1. Synthesis of tungsten carbides
∗ Corresponding author. Tel.: +86 20 84036736; fax: +86 20 84113369. E-mail address: [email protected] (P.K. Shen). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.03.025
Tungsten carbide was prepared by intermittent microwave heating (IMH) method. The ammonium metatungstate (AMT,
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(NH4 )6 H2 W12 O40 ·xH2 O, 3.7 g) was dissolved in 10 ml distilleddeionized water and 10 ml 2-propanol with magnetic stirring. Then, XC-72R carbon (2.5 g, Cabot corp., USA, SBET = 236.8 m2 g−1 ) as supporting material was added into the mixture and stirring until the ink uniformly dispersed. After heated at 80 ◦ C, the dried ink was further heated in a homemade program-controlled microwave oven (2000 W, 2.45 GHz) with heating procedures of 15 s-on/10 soff, 15 s-on/15 s-off, 15 s-on/20 s-off and 15 s-on/30 s-off for 20 times. The WC synthesis was carried out in air atmosphere. The well-dispersed powders were denoted as WC/C(IMH). For comparison, the continuous microwave heating method was also used to prepare WC and the corresponding product was denoted as WC/C(CMH). All the temperature data were obtained by recording the digital data from the dashboard during the synthesis processes. In order to get pure WC, the product was post-treated in 1.0 mol L−1 NaOH solution for more than 6 h with magnetic stirring. Afterwards, the resulting samples were filtered, washed and dried at 80 ◦ C for 12 h in a vacuum. The obtained samples were denoted as WC/C(IMH)-T and WC/C(CMH)-T, respectively.
and dried under infrared lamp to obtain an electrocatalysts thin film. The catalyst loading was kept constant as 0.25 mg cm−2 for the catalysts in this study. An aqueous solution containing 0.5 mol L−1 H2 SO4 with or without 1.0 mol L−1 CH3 OH was used as electrolyte, which was deaerated with high-pure nitrogen gas during the electrochemical characterization. The scanning potential was recorded at a scan rate of 50 mV s−1 . In the case of CO-stripping experiment, CO adsorption over Pt was conducted at −0.156 V in a CO saturated solution 0.5 mol L−1 H2 SO4 for 15 min. Subsequently, the electrolyte was purged with N2 for 30 min to remove dissolved CO from the solution. The potential scans between 0.156 and 1.20 V at a scan rate of 20 mV s−1 for two cycles. For oxygen reduction reaction (ORR) experiment, the rotating disk electrode (RDE) was carried out in an oxygen-saturated 0.5 mol L−1 H2 SO4 solution scanned between 1.1 to 0 V at a scan rate of 5 mV s−1 . All the potentials were referred to reversible hydrogen electrode (RHE) without specification.
2.2. Preparation of electrocatalysts
3. Results and discussion
The Pt–WC/C electrocatalysts were prepared by intermittent microwave heating method as described previously [44–46]. The main steps of this synthesis process were given as follows. The chloroplatinic acid precursor (2.71 ml, 18.5 mg Pt ml−1 ) was well mixed in a breaker with ethylene glycol (EG, 20 ml) in an ultrasonic bath. The WC/C(IMH) (200 mg) was then added into the mixture. After the pH value of the mixture was adjusted to ∼10 by a NaOH/EG solution. The well-dispersed slurry was obtained with stirring and ultrasonication for 15 min. Thereafter, the slurry was microwave-heated in the form of 5 s-on/5 s-off for five times. The WC synthesis was carried out in air atmosphere. After reacidification, the resulting black solid sample was filtered, washed and dried at 80 ◦ C for 12 h in a vacuum. The prepared electrocatalysts were denoted as Pt–WC/C(IMH) and the post-treated one as Pt–WC/C(IMH)-T. For the sake of comparison, the WC/C(CMH) with or without post-treatment and commonly used XC-72 as supports were also synthesized and used in the same way. The final electrocatalysts were denoted as Pt–WC/C(CMH)-T, Pt–WC/C(CMH) and Pt/C. The Pt loadings on the electrocatalysts were 20 wt%.
The temperature change during the continuous microwave heating or intermittent microwave heating process is shown in Fig. 1. As clearly displayed in the graph, the temperature continuously increases with time under the traditional heating mode. The final temperature depends on the nature of the materials being heated and the output power of microwave oven. The relationship between the heating time and the temperature for the continuous microwave heating method is shown in Fig. 1a. The principle of the IMH method can be explained by comparing the results as shown in Fig. 1. The temperature can be easily controlled as required by the IMH method. The required temperature can be alternatively reached to meet the requirements for the materials. In that case, we can adjust the upper temperature to control the degree of the crystallization of the material. More importantly, it is very convenience to control the growth of the crystallized or formed particles since the sustained time is very short at the highest temperature and this method also provides a relaxation time for crystallization, phase transformation and structure re-arrangement of the preparing materials. The longer the heat time the higher the temperature. The shorter the relaxation time the higher the low temperature limit. Fig. 2 shows the XRD patterns of the samples. The peaks at the 2 of 31.51◦ , 35.64◦ , 48.30◦ , 64.02◦ , 73.10◦ and 77.10◦ with the d values of 2.8431, 2.5170, 1.8813, 1.4531, 1.2934 and 1.2360 are corresponding to the (0 0 1), (1 0 0), (1 0 1), (1 1 0), (1 1 1) and (1 0 2) facets of WC [48–50], the 2 of 38.05◦ and 39.56◦ with the d values of 2.3630 and 2.2760 are corresponding to the (2 0 0) and (1 0 2) facets of W2 C, the 2 of 23.08◦ , 23.71◦ , 24.10◦ , 26.59◦ , 28.78◦ , 33.33◦ and 34.02◦ with the d values of 3.8500, 3.7500, 3.6900, 3.3500, 3.1000, 2.6860 and 2.6330 are corresponding to (0 0 1), (0 2 0), (0 2 0), (1 2 0), (1 1 1), (0 2 1) and (2 2 0) facets of WO3 [4], and the 2 of 40.26◦ with the d values of 2.2380 is corresponding to (1 1 0) facet of W. As clearly shown in Fig. 2A, the components of the products depend on the heating mode. The WO3 appeared in all the samples. It is due to the reaction of tungsten with trace oxygen in the container during the heating process. The WO3 could be significantly reduced in the N2 or Ar atmosphere. On the other hand, the WO3 would be gradually transferred into tungsten carbides at higher temperatures. The percentage of WC increased by reducing the relaxation time like the samples heated at 15 s-on/10 s-off and 15 s-on/15 s-off for 20 times. It was concluded that the WC can form at the temperature over 1000 ◦ C according to the temperature measurement during the heating processes.
2.3. Characterization of electrocatalysts The XRD measurements were carried out on a D/Max-III (Rigaku Co., Japan) using Cu K␣ radiation ( = 0.15406 nm), and operating at 40 kV and 20 mA. The 2 angular regions between 20◦ and 80◦ were explored at a scan rate of 5◦ min−1 in order to obtain the particle size of the prepared WC according to the Scherrer formula [47]. TEM investigations were carried out in a JEOL TEM-2010 (HR) at 200 kV to get the information of the mean particle size distribution of WC prepared by IMH method and CMH method. Electrochemical measurements were conducted on a PARETAT 2273 instrument (Princeton Applied Research, USA) in a thermostat-controlled standard three-electrode cell at room temperature using a saturated calomel electrode (SCE) and a platinum foil (1.0 × 1.0 cm2 ) as the reference and counter electrodes, respectively. A glass carbon (GC) disk electrode was used as the substrate for depositing the electrocatalysts thin film in the electrochemical measurements. The thin film electrocatalysts working electrode was prepared as follows. A mixture containing 10.0 mg electrocatalysts, 1.9 ml ethanol and 0.1 ml Nafion suspension (5 wt.%) was dispersed under ultrasonic for 30 min to obtain a well-dispersed electrocatalysts ink. The geometric area of the electrode was 0.1963 cm2 . The electrocatalysts ink was then quantitatively transferred onto the surface of the GC electrode by using a micropipette,
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Fig. 1. The relationship between the temperature and the heating time during the WC synthesis processes. (a) CMH, (b) IMH, 15 s-on/10 s-off, (c) IMH, 15 s-on/15 s-off and (d) IMH, 15 s-on/30 s-off.
Fig. 2. (A) The XRD patterns of the samples prepared (a) by continuous microwave heating (CMH) for 5 min and by intermittent microwave heating (IMH) at different on/off procedures of (b) 15 s-on/30 s-off, (c) 15 s-on/20 s-off, (d) 15 s-on/15 s-off and (e) 15 s-on/10 s-off for 20 times. (B) The XRD patterns of the WC synthesized by (a) CMH for 5 min and (b) IMH at 15 s-on/15 s-off for 20 times after the post-treatment in 1.0 mol L−1 NaOH solution. (C) The XRD patterns of the Pt nanoparticles supported on WC/C(IMH) prepared at 15 s-on/15 s-off for 20 times (a) after and (b) before the post-treatment in 1.0 mol L−1 NaOH solution. (D) The XRD patterns of the Pt nanoparticles supported on the WC/C(CMH) (a) after and (b) before the post-treatment in 1.0 mol L−1 NaOH solution. All data obtained at a scan rate of 5◦ min−1 , () WC, (♦) WO3 and (䊉) W2 C.
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Fig. 3. The TEM images of (a) WC/C(IMH) and (b) Pt–WC/C(IMH)-T. The corresponding EDX patterns of (c) WC/C(IMH) and (d) Pt–WC/C(CMH)-T.
In the case that the samples were heated by continuous microwave heating (CMH) method for 5 min, the average particle size was 35.7 nm calculated by the Scherrer formula [47], which is larger than that of the samples prepared by IMH at 15 s-on/10 soff of 24.8 nm and 15 s-on/15 s-off of 21.4 nm. The results indicated that the relaxation during the IMH processes could significantly protect the particles from the growth. The new particles produced in the every irradiation on period instead of the continuous growth of the formed particles. However, when the sample was heated by CMH, the temperature increases with time and consequently to form large particles. It can be concluded that the on and off time during the IMH processes were selected according to the temperature needed for the formation of WC. In this study, the temperature was controlled at around 1000 ◦ C as shown in Fig. 1. The shorter heating time (or longer relaxation time) gives the lower temperature to form WO3 dominantly. The longer heating time (or shorter relaxation time) results in higher temperature to form larger particles. The repeat time depends on the chemical reaction and the amount of the material. In the present case, the formation of WC proceeds via W → W2 C → WC. The WO3 and W2 C will dominant the product if not enough repeat time is applied. The WC particle size will increase at too much repeat time. For example, the dominant product is WO3 at 15 s-on and 15 s-off for 10 times, but the dominant product is changed to WC at 15 s-on and 15 s-off for 20 times. On the other hand, the particle size of the WC could be less than 20 nm at 15 s-on and 15 s-off for 20 times, while more than 30 nm at 15 s-on and 15 s-off for 30 times.
On the other hand, it has been reported that W2 C and WO3 are effective to enhance activity of the electrocatalysts as supports in fuel cells [6,51,52], however, they are not stable both in alkaline and acidic electrolytes [53]. Therefore, we treated the samples in 1.0 mol L−1 NaOH solution to remove the W2 C and WO3 containing in the as-prepared samples. The corresponding XRD results of WC/C(CMH)-T and the WC/C(IMH)-T prepared at 15 s-on/15 s-off are shown in Fig. 2B. It can be clearly seen that the W2 C and WO3 have be completely removed after the post-treatment and only WC left. The XRD patterns of WC/C(IMH) prepared at 15 s-on/15 s-off for 20 times and WC/C(CMH) before and after supported by Pt nanoparticles are shown in Fig. 2C and D. Obviously, all the electrocatalysts exhibited the typical characteristics of a crystalline Pt face centered cubic (fcc) structure with the average particle size of Pt on WC/C(IMH)-T, Pt on WC/C(IMH), Pt on WC/C(CMH)-T and Pt on WC/C(CMH) were 3.1 nm, 3.3 nm, 3.3 nm and 3.6 nm, respectively. The typical TEM images of WC/C prepared by IMH method at 15 s-on/15 s-off and Pt–WC/C(IMH)-T and the corresponding EDX patterns are shown in Fig. 3. It gives the similar results in the particle size both for WC and Pt nanoparticles that the average WC size was 25.0 nm and the corresponding Pt nanoparticles on WC/C was 3.0 nm. The methanol oxidation on Pt nanoparticles supported on WC/C(CMH) and WC/C(IMH) before and after post-treated were evaluated in 0.5 mol L−1 H2 SO4 with or without 1.0 mol L−1 CH3 OH and the results are shown in Fig. 4. It is clear that the Pt–WC/C(IMH)-T electrocatalysts showed the better per-
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Fig. 4. (a) The cyclic voltammograms (CV) of (a) Pt–WC/C(IMH)-T, (b) Pt–WC/C(IMH), (c) Pt–WC/C(CMH)-T, (d) Pt–WC/C(CMH) and (e) Pt/C electrocatalysts tested in 0.5 mol L−1 H2 SO4 + 1.0 mol L−1 CH3 OH at a scan rate of 50 mV s−1 at 25 ◦ C. The insert is the CV curves of the same electrocatalysts in 0.5 mol L−1 H2 SO4 solution. (b) CO-stripping voltammograms of Pt/C and Pt–WC/C(IMH)-T electrocatalysts in 0.5 mol L−1 H2 SO4 at a scan rate of 20 mV s−1 . Pre-adsorption of CO over Pt at −0.156 V (vs. RHE) for 15 min.
formance for methanol oxidation than that of Pt–WC/C(IMH), Pt–WC/C(CMH)-T, Pt–WC/C(CMH) and Pt/C electrocatalysts. The reason is that the Pt–WC/C(IMH)-T electrocatalysts has the larger electrochemical active surface area due to the smaller WC particle size and smaller Pt nanoparticle size as shown in the inset in Fig. 4. It is a challenge to increase the CO poisoning-resistance for a novel electrocatalysts since normally the by-products generated during the alcohol oxidation will occupy the active sites of Pt and consequently decreases the performance of Pt for further alcohol oxidation [54]. The numerous publications concerned the formation mechanism of CO during the alcohol oxidation, the kinetics of CO on the electrode and the effect of the catalyst on the adsorption and oxidation of CO [55–60]. We performed the CO-stripping experiment to compare the performance of the Pt–WC/C(IMH)-T and traditional Pt/C electrocatalyst as shown in Fig. 4b. The active surface area of Pt can be calculated by assuming a full coverage of CO monolayer on Pt associated with 420 C cm−2 charge [61]. However, in the present case, we fell difficult to calculate the active surface area of the electrocatalysts. The more active Pt–WC/C(IMH)-T gave smaller CO oxidation peak. Another reason could be the presence of WC reduces the adsorption of CO on the electrocatalyst. Meanwhile, it can also be seen from Fig. 4b that
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Fig. 5. (a) Polarization curves for the oxygen reduction reaction on Pt–WC/C(IMH)T electrocatalyst in oxygen-saturated 0.5 mol L−1 H2 SO4 at different rotating rates, ´ plot for oxyat 25 ◦ C, with a scan rate of 5 mV s−1 and (b) the Koutechy–Levich gen reduction reaction at 0.82 V (vs. RHE) on Pt–WC/C(IMH)-T electrocatalyst in O2 saturated 0.5 mol L−1 H2 SO4 solution.
CO could be oxidized more easily on Pt–WC/C(IMH)-T to give more negative onset potential which has 70 mV difference compared to that on Pt/C, indicating an improvement in the CO anti-poisoning resistance. Fig. 5a shows the oxygen reduction reaction (ORR) curves on Pt–WC/C(IMH)-T electrocatalyst in oxygen-saturated 0.5 mol L−1 H2 SO4 solution at different rotating rates with a scan rate of 5 mV s−1 . The kinetics for oxygen reduction on the electrode was ´ analyzed by Koutechy–Levich relationship. The reciprocal of the observed currents at 0.82 V on the RDE coated with Pt–WC/C(IMH)T from Fig. 5a were plotted against the reciprocal of the square root of rotating rates, as shown in Fig. 5b in O2 saturated 0.5 mol L−1 H2 SO4 aqueous solution. From the following equation [62], the slope (1/B) is directly related to the total number of electrons (n) involved in oxygen reduction: 1 1 1 = + I IK B(ω)1/2
(1)
where I is the experimentally observed current, Ik the kinetic current, B equals to 0.62nFACO2 (DO2 )2/3 v1/6 , v the rotation rate in rad s−1 , F the Faraday constant, A the electrode area, CO2 the bulk concentration of O2 , DO2 the diffusion coefficient for O2 and v the kinetics viscosity of the electrolyte. It can be found that the linear curve obtained experimentally is strictly parallel to the theoretical
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line of the four-electron reduction mechanism, indicating a fourelectron pathway for the ORR on Pt–WC/C(IMH)-T electrocatalyst. 4. Conclusions The principle of the intermittent microwave heating (IMH) method was described along with the comparison to the traditional continuous microwave heating method. The advantages of this novel method were evidenced during the synthesis of nanosized tungsten carbides. It resulted in average particle size of 21.4 nm at the procedure of 15 s-on and 15 s-off for repeat 20 times, however, the particle size increased to 35.7 nm by CMH method for 5 min. The pure WC on carbon could be reached by post-treating the sample in NaOH solution, which gave the better performance as support. The comparison of the Pt nanoparticles supported on WC-based supports indicated that the Pt nanoparticles on post-treated WC/C electrocatalyst prepared by IMH is the best for the alcohol oxidation. The intermittent microwave heating method is easier to scale-up for mass production of the nanosized tungsten carbides and other nanosized materials as well. Acknowledgements The work was supported by the China National 863 Program (2009AA05Z110), the Guangdong Sci. & Technol. Key Projects (2007A010700001, 2007B090400032) and Guangzhou Sci. & Technol. Key Projects (2007Z1-D0051, SKT[2007]17-11). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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