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Electrocatalytic Activity and CO Tolerance Properties of Mesostructured Pt/WO3 Composite as an Anode Catalyst for PEMFCs Xiangzhi Cui, Limin Guo, Fangming Cui, Qianjun He, and Jianlin Shi* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China ReceiVed: September 6, 2008; ReVised Manuscript ReceiVed: December 8, 2008

A mesostructured Pt/WO3 electrochemical catalyst has been prepared by a loading small amount of Pt (7.5 wt %) on mesoporous tungsten oxide, which was synthesized by a simple one-step casting method using mesoporous silica (KIT-6) as a hard template. The resultant mesostructured Pt/WO3 catalyst showed high electrocatalytic activity for hydrogen electrooxidation, and the mass activity (mA · mg-Pt-1, per mass of Pt) of it for hydrogen electrooxidation was more than three times of that of the commercial 20 wt % Pt/C catalyst (E-TEK). In addition, it exhibited much improved resistance to CO poisoning relative to the 20 wt % Pt/C catalyst. Since the catalyst is also stable in an electrochemical environment, it could serve as an alternative electrocatalyst for proton exchange membrane fuel cells with high electrocatalytic activity and CO tolerance. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) of low operation temperature and high energy density are one of the most promising clean energy sources suitable for applications in automobiles, residential heat and power supply, etc.1 One of the key components that determines the performance and cost of PEMFCs is the electrode catalyst. It is well-known that platinum is the dominant electrocatalytic material in both anode and cathode of PEMFCs because Pt-based catalysts show high and stable activity for both electrooxidation of hydrogen and reduction of oxygen.2 However, Pt is susceptible to CO poisoning, which is a byproduct generated during the processing of fossil fuels such as methane and gasoline, and will lose its catalytic activity with respect to time, and these electrochemical properties of Pt catalyst have been studied extensively.3 The volume ratio of CO in these fossil fuels after redisposing and selected oxidation can be reduced to 1 × 10-4, but at practical operation temperatures of 333∼373 K for PEMFCs, CO with a volume ratio as low as 1 × 10-6 in these fossil fuels would make the Pt catalyst become poisoned and lose its catalytic activity.4 So, much research has been devoted to develop new electrocatalysts to eliminate or reduce the CO poisoning in order to prompt the large-scale application of PEMFCs.5 In addition, Pt is expensive and its supply is limited. Thus, for a widespread application of PEMFCs, searching for low-cost electrocatalysts that possesses high CO resistance as well as high catalytic activity toward hydrogen oxidation remains a great challenge.6 Presently, the most promising anode materials for PEMFCs are Pt-Ru bimetallic catalysts dispersed on carbon materials. In order to make up for the slow oxidation kinetics, however, high loadings (as high as 50 wt %) of these expensive noble metals are required, and this leads to the very high cost of PEMFCs.7,8 Accordingly, there has been considerable research in recent years to replace noble metals for PEMFC electrodes.9 Tungsten oxide-based electrocatalyst materials have received considerable attention most recently because of its assistant electrocatalytic effect with Pt on methanol electrooxidation.10 * Corresponding author. Tel.: 86-21-52412714. Fax: 86-21-52413122. E-mail: [email protected].

As an electrocatalyst, Pt-WO3 supported on carbon matrix has been reported, in which WO3 can form a nonstoichiometric and electroconductive hydrogen tungsten bronze (HxWO3) compound in acidic solution, and this compound can facilitate the dehydrogenation during the reaction of methanol oxidation and lighten the CO poisoning of Pt catalyst.11-13 As we know, mesoporous materials with ordered pore structures and high surface areas are technologically important for a variety of applications such as in heterogeneous catalysis, adsorption, chemical sensing, electrodes, and transportation/ storage of fluids and gases, and also in some biological areas.14 We recently prepared a mesoporous tungsten oxide of hexagonal WO3 phase with a high surface area (86 m2 · g-1) by a hard template replicating method.15 This mesoporous WO3 with appropriate amount of carbon black addition shows clear electrocatalytic activity toward hydrogen oxidation. We also investigated the electrochemical activity of Pt supported on mesoporous WO3 toward methanol oxidation, and the resultant Pt/mesoporous WO3 composite showed high activity for methanol oxidation.16 In the present study, a slightly platinized mesoporous WO3 was studied in terms of the electrooxidation properties of hydrogen in acidic solution with a particular attention to its resistance to CO poisoning. Its performance as a potential anode electrocatalyst for PEMFCs is compared with a commercial 20 wt % Pt/C catalyst (E-TEK). Experimental Section Synthesis of Electrocatalyst. The mesoporous WO3 supports were prepared by a replicating method using mesoporous silica with cubic Ia3d symmetry (designated as KIT-6) as a hard template. The parent mesoporous silica (KIT-6) template was synthesized according to the published procedure using triblock copolymer Pluronic P123 (EO20PO70EO20) as a soft template by adding n-butanol in an acidic aqueous solution.17 The synthesis procedure of mesoporous WO3 was as follows: 1.2 g of 12phosphotungstic acid was dissolved in 5 mL of ethanol, and this solution was incorporated into 0.4 g of as-prepared KIT-6 by an incipient wetness impregnation technique. After the ethanol evaporation at 313 K, the composite was calcined at 773 K for 3 h to give a decomposition product of tungsten

10.1021/jp8079205 CCC: $40.75  2009 American Chemical Society Published on Web 02/12/2009

Mesostructured Pt/WO3 Catalyst for PEMFCs trioxide inside the silica template. The silica template was removed with 2 M HF solution under stirring for 6 h. This template-free WO3 was collected by centrifugation, washed with enough distilled water, dried at room temperature in vacuum, and named as m-WO3. The platinum particles were loaded on mesoporous tungsten oxide by the conventional borohydride reduction method with the Pt loading amount of 7.5 wt % and named as m-7.5Pt/WO3.18 A commercial 20 wt % Pt/C was purchased from E-TEK and used as received. Characterization. X-ray diffraction (XRD) patterns of prepared samples were recorded on a Rigaku D/Max-2550V X-ray diffractometer with a Cu Ka radiation target (40 kV, 40 mA). The N2 sorption measurement was performed using Micromeritics Tristar 3000 at 77 K, and the specific surface area and the pore size distribution were calculated using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. X-ray photoelectron spectroscopy (XPS) signals were collected on a VG Micro MK∏ instrument using monochromatic Al KR X-rays at 1253.6 eV operated at 150 W. Field emission scanning electron microscopy (FE-SEM) analysis was performed on a JEOL JSM-6700F field emission scanning electron microscope. Transmittance electron microscopy (TEM) images were obtained on a JEOL 200CX electron microscope operating at 160 kV. Energy-dispersive X-ray spectra (EDS) were collected from an attached Oxford Link ISIS energy-dispersive spectrometer fixed on a JEM-2010 electron microscope operated at 200 kV. Electrochemical Characterization. Electrochemical measurements were carried out on a CHI660A electrochemical workstation. The working electrodes for the electrochemical measurements were fabricated by dispersing 5 mg of catalyst in a mixed solution of 1 mL ethanol/water (1:1, volume scale) and 25 µL Nafion (5 wt %) solutions. The suspension was ultrasonicated evenly for 20 min, 20 µL of the suspension was dropped onto the glassy carbon electrode (φ ) 6 mm), and the solvent was slowly evaporated at 333 K. Platinum sheet and Ag/AgCl/3 M KCl were used as the counter and the reference electrodes, respectively. Highly pure N2 was introduced prior to the measurements to deaerate the electrolyte. A solution of 0.5 M H2SO4 was used as electrolyte for all electrochemical experiments. For CO tolerance experiments, a solution of 0.5 M H2SO4 was first purged with a 1% CO and 99% H2 (volume scale) gas mixture for 1 h, and then the measurement vessel was air-proofed to prevent the solution from contacting the atmosphere. Cyclic voltammetry (CV) and chronoamperometry (CA) were done to investigate the electrochemical properties of the catalysts. Results and Discussion The small-angle XRD patterns of m-WO3 and m-7.5Pt/WO3 (see Supporting Information) show that the m-WO3 replica has inherited the ordered structure of the template, and this ordered structure has been retained after a small amount of Pt loading. In addition, a wide-angle XRD peak of the m-7.5Pt/WO3 composite at 2θ ) 39.8° (Supporting Information) represents the Pt (111) lattice plane (JCPDS card no. 04-0802). The surface compositional information of the prepared m-7.5Pt/WO3 was collected by XPS (Supporting Information). From the region where the XPS signals of W4f are expected to appear, it can be seen that the mesoporous WO3 support exhibits two principal peaks at 35.9 and 38.0 eV, assigned to the 6+ oxidation state of tungsten judged from the W 4f7/2 and W 4f5/2 spectral lines, respectively, which are in accordance with the reported data of W 4f7/2 and W 4f5/2 in WO3.19 The XPS signals of metal Pt

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Figure 1. Nitrogen sorption isotherm of the prepared samples and the corresponding pore size distribution curves in the inset.

4f7/2 and Pt 4f5/2 maximized at 71.0 and 74.2 eV, respectively, are in accordance with the reported data of Pt 4f7/2 with the binding energy 70.8 eV.20 The nitrogen sorption isotherms of the prepared m-WO3 and m-7.5Pt/WO3 are shown in Figure 1. The similar hysteresis profiles for the two samples indicate the typical mesoporous character for both materials. This demonstrates that the prepared m-WO3 retains the mesoporous structure of the template, and the mesostructure is not affected significantly after Pt nanoparticle loading. The pore size distribution was calculated from the desorption branch and is shown in the inset. The bimodal pores in the ranges 2∼4 and 9∼12 nm for the prepared samples can be seen in the pore size distribution curve. The average pore size and pore volume are 8.5 and 8.3 nm, and 0.12 cm3 · g-1 and 0.11 cm3 · g-1, for the m-/WO3 and m-7.5Pt/WO3, respectively, and the corresponding BET surface area of the latter (80 m2 · g-1) is a little lower than that of the former (86 m2 · g-1) because of metal Pt nanoparticles loading. The FESEM image of m-WO3 in Figure 2a shows a curled and flakelike morphology with a flake thickness of about 50 nm. TEM images in Figure 2b and 2c show that the prepared m-WO3 possesses a well-ordered mesoporous framework in the view of (111) and (100) directions of cubic Ia3d symmetry, respectively. The Si signal can be hardly detected in the EDS pattern of Figure 2d, meaning the complete removal of silica template. The selected area electron diffraction (SAED) pattern in the inset in Figure 2c indicates that the mesoporous WO3 is highly crystallized. High-resolution TEM (HRTEM) of the prepared m-7.5Pt/WO3 shows two dominant crystal lattice stripes (Figure 2e). The smaller one of d ) 0.23 nm in mesopores can be attributed to Pt metal (d ) 0.226 nm, JCPDS card no. 04-0802), and the other one of d ) 0.27 nm in the framework to WO3 according to JCPDS card no. 20-1324, implying the existence of Pt nanoparticles in the pores of the mesoporous WO3 support. The average Pt particle size on WO3 is around 5 nm according to the HRTEM image, which is in agreement with the value (5.2 nm) calculated from the broadening of the characteristic XRD peak of Pt metal using the Debye-Scherrer equation. Figure 3 gives the cyclic voltammogram (CV) curves of the samples. The electrochemical areas (ECAs), specific current (mA · cm-2 electrode), and mass activity (mA · mg-Pt-1) of the catalysts were calculated and are listed in Table 1. From the data, the commercial 20 wt % Pt/C catalyst showed an ECA value of 134 m2 · g-Pt-1; however, the mesostructured m-Pt/WO3 catalyst showed a much higher ECA of 355 m2 · g-Pt-1. This electrochemical surface area of m-Pt/WO3 corresponds to ca. 0.5 nm diameter of spherical Pt particles. However, from TEM

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Figure 2. Typical FE-SEM image of m-WO3 (a) and the TEM images (b, c) of it in the [111] and [100] directions, respectively, with the SAED pattern of it in the inset in (c), and the EDS pattern of WO3 (d); (e) is the HRTEM image of m-7.5Pt/WO3.

and XRD measurements, the diameter of Pt particles in m-7.5Pt/ WO3 was found to be 5-6 nm, 1 order higher than that calculated from the ECA value. In the present case, the much higher ECA value for the m-Pt/WO3 could only be accounted for by the active participation of mesoporous WO3 in electrochemical hydrogen oxidation together with Pt. This difference in ECA values between Pt/WO3 and Pt/C catalysts is also reflected in the maximum mass activity data. The maximum mass activity of m-7.5Pt/WO3 (256.6 mA · mg-Pt-1) is more than three times of that of the commercial 20 wt % Pt/C E-TEK catalyst (72 mA · mg-Pt-1). The maximum specific activity of m-7.5Pt/WO3 (6.8 mA · cm-2) was also higher than that of Pt/C catalyst (5.1 mA · cm-2).

It could also be observed from Figure 3a that the voltage ranges of different catalysts where hydrogen oxidation is taking place are different from each other. The Pt/C catalyst showed a multipeak CV profile for hydrogen oxidation covering a wide range of voltage from -0.2 to 0.1 V, which can be related to, for example, the different exposed lattice planes of Pt having different reactivities for hydrogen oxidation.21 Comparatively, for the m-7.5Pt/WO3, the CV curve is single peaked at about 0.05 V covering even a wider voltage range from -0.2 to 0.2 V. Mesoporous WO3 with a small amount of Pt loaded shows similar electrochemical behavior to pure WO3 with significantly enhanced electrocatalytic activity, but not to that of Pt in Pt/C catalyst (E-TEK); i.e., the mesostructured Pt/WO3 catalyst is

Mesostructured Pt/WO3 Catalyst for PEMFCs

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Figure 3. Cyclic voltammograms of samples under a scan rate of 0.05 V · s-1 at 298 K: (a) CV curves of various samples without CO introduction; (b) and (d) CV curves for m-7.5Pt/WO3 and commercial 20 wt % Pt/C (E-TEK), respectively, in 0.5 M H2SO4 solution (1) and 0.5 M H2SO4 solution purged with a gas mixture of 1% CO and 99% H2 (2) for an hour; (c) CV curves of m-7.5Pt/WO3 cycled for 20 times under condition 2.

TABLE 1: Electrocatalytic Activities and CO Tolerance Performance of the Mesostructured m-7.5Pt/WO3 and a Commercial 20 wt % Pt/C Catalyst for Hydrogen Electrooxidation catalysts m-7.5Pt/WO3 20 wt % Pt/C (E-TEK)

condition of electrolyte

maximum specific activitya (mA.cm-2)

maximum mass activityb (mA · mg-Pt-1)

ECA c (m2 · g-Pt-1)

without CO with CO without CO with CO

6.8 (0.05 V) 6.7 (0.05 V) 5.1 (-0.05 V) 4.5 (-0.05 V)

256.6 (0.05 V) 252.8 (0.05 V) 72 (-0.05 V) 63.7 (-0.05 V)

355 348 134 107

a Intrinsic activity of the catalysts normalized to the electrochemical active surface areas. b Intrinsic activity of the catalysts normalized to the mass of Pt. c Electrochemical area (ECA) of the catalysts, ECA (cm2 Pt/g Pt)22 ) charge(H) (µC/cm2)/[210 (µC/cm2 Pt) × electrode loading (g Pt/cm2)].

more mesoporous WO3-like rather than Pt-like. Our previous study has shown that the formation of a hydrogen tungsten bronze HxWO3 compound (WO3 + xH ) HxWO3, 0 < x < 1) should be responsible for the electrocatalytic activity,15 and in the present study, based on the similar CV profile of mesoporous WO3 and Pt/WO3, we believe that there is a cooperative effect between Pt and WO3 in hydrogen electrooxidation, in which Pt assists in the formation of HxWO3 (hydrogen adsorption) by easily adsorbing H2 and then the adsorbed hydrogen species (hydrogen atoms, for example) spill over onto the WO3 surface,22 promoting the formation of HxWO3 compound. To test the CO poisoning effect on the prepared catalysts, CO was introduced into the 0.5 M H2SO4 electrolyte solution by purging a mixed gas of 1% CO and 99% H2 into the electrolyte. Figure 3b-d and Table 1 compare the CO tolerance performances of m-7.5Pt/WO3 and 20 wt % Pt/C. It is shown that the current density and ECA values of 20 wt % Pt/C catalyst decreased by ca. 16% when CO was introduced in the electrolyte. However, in the case of m-7.5Pt/WO3 electrocatalyst, such a decrease is not higher than 7%. This indicates that mesostructured Pt/WO3 catalyst has significantly higher CO

tolerance than Pt/C catalyst. It has also been found that the m-7.5Pt/WO3 electrocatalyst shows no apparent current decrease when cycled 20 times under the presence of CO in the electrochemical test, indicating the high electrocatalytic stability of the prepared catalyst against the CO poisoning. The CO tolerance was also tested by chronoamperometry (CA) at a fixed voltage of -0.02 V where both the prepared m-7.5Pt/WO3 and commercial Pt/C catalysts share the same current values in the CV curves in Figure 3a. As shown in Figure 4, 20 wt % Pt/C catalyst shows an obviouse decrease of current in 100 s when CO is introduced, and the end current of the test is about 0.5 A · g-Pt-1, two-thirds of the value under the absence of CO (0.75 A · g-Pt-1). In contrast, the m-7.5Pt/WO3 electrocatalyst showed much better CO tolerance than the Pt/C catalyst, and the mass current output of the former is very stable at about 1.8 A · g-Pt-1, 86% of that under the absence of CO. This result is in accordance with our previous conclusion that Pt/mesoporous WO3 showed comparable or even higher electrocatalytic activity toward the methanol electrooxidation than a commercial PtRu/C catalyst.16 One of the factors contributing to the high activity was suggested to be the facile removal of CO

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Cui et al. m-7.5Pt/WO3 composite. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 4. Chronoamperometry curves of (A) m-7.5Pt/WO3 and (B) 20 wt % Pt/C (E-TEK) in 0.5 M H2SO4 solution (1) and 0.5 M H2SO4 solution purged with a gas mixture of 1% CO and 99% H2 for an hour (2) under a scan rate of 0.05 V · s-1 at 298 K.

intermediate formed on Pt surface by high activity provided by metallic platinum and the Bro¨nsted acid function of WOx (HxWO3). In addition, the high surface area and unique mesostructure of WO3 support, together with the high dispersion of Pt nanoparticles in mesoporous WO3, would greatly facilitate the electrochemical catalytic process. In summary, a mesotructured Pt/WO3 composite catalyst with 7.5 wt % Pt supported on mesoporous tungsten oxide (WO3phase) has been synthesized and serves as an effective CO tolerant electroanode catalyst for hydrogen oxidation. This Pt/ mesoporous WO3 catalyst with a low amount of Pt loading shows not only more than three times the Pt mass activity of the commercial 20 wt % Pt/C catalyst but also much improved resistance to CO poisoning for hydrogen electrooxidation. The high electrocatalytic activity and the high CO tolerance should be attributed to the formation of a tungsten bronze compound, and the high surface area and mesopore structure of the prepared WO3 should have additional but indispensable contribution to the high electrocatalytic performance of the Pt/mesoporous WO3 catalyst. Acknowledgment. The authors gratefully acknowledge the financial support from National Natural Science Foundation of China with Contract 20633090, Qiming Star Project of Shanghai with Contract 05QMX1458, and Foundation of Shanghai Nanotechnology (0552nm030). Supporting Information Available: Low and high XRD patterns of different samples and XPS spectra of the prepared

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