14617
2007, 111, 14617-14620 Published on Web 09/18/2007
Tungsten Monocarbide as Potential Replacement of Platinum for Methanol Electrooxidation Erich C. Weigert, Alan L. Stottlemyer, Michael B. Zellner, and Jingguang G. Chen* Center for Catalytic Science and Technology, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: July 13, 2007; In Final Form: August 31, 2007
Polycrystalline tungsten monocarbide (WC) surfaces are synthesized and evaluated as a potential replacement of platinum (Pt) electrocatalysts. A combined approach utilizing ultrahigh vacuum (UHV) and electrochemical techniques demonstrates that WC possesses the following prerequisites for alternative electrocatalysts: high activity toward methanol decomposition, improved resistance to poisoning by CO, and promising electrochemical stability.
Introduction We report herein a combined surface science and electrochemical study to demonstrate the feasibility of using tungsten monocarbide (WC) to replace platinum (Pt) as anode electrocatalysts for the electrooxidation of methanol. An effective anode electrocatalyst in direct methanol fuel cell (DMFC) should be active for the decomposition of methanol while remaining stable under the relatively harsh anode environment. It should also have high activity for the oxidation of carbon monoxide (CO), which is a reaction intermediate that poisons the surface by occupying the active sites. Although the Pt/Ru bimetallic alloy is currently the most effective anode electrocatalyst, both Pt and Ru are expensive due to limited supplies and both are susceptible to CO poisoning due to the strong bonding of CO on the active surface.1-3 Consequently, the discovery of less expensive and more CO tolerant alternative electrocatalysts is of both fundamental and practical importance for the understanding and development of DMFC. A large body of literature exists on the possibility of using transition metal carbides to mimic the catalytic properties of Pt-group metals4-7 since Levy and Boudart suggested that tungsten carbides displayed Pt-like behavior in catalytic reactions.7 Even though tungsten carbides have been identified as potential alternative electrocatalysts for DMFC, there is a lack of fundamental understanding, and correspondingly conflict between results from different groups,8-12 regarding the active phases and stability of this class of materials. In particular, depending on the synthesis conditions, tungsten carbides can exist in several phases and stoichiometries, including WC1-x, W2C, and WC. Previous XPS and electrochemical studies12 on well-characterized W2C and WC PVD films have demonstrated that the W2C phase was unstable in the electrochemical environment, with an onset oxidation at ∼0.4 V with respect to the normal hydrogen electrode (NHE). The WC phase, which is the subject of the current paper, is significantly more stable, with an onset of oxidation at ∼0.8 V. The objectives of the current Letter are to resolve the issues of phases and stability by performing electrochemical measurements on well-character* Corresponding author. Tel: 302-831-0642. Fax: 302-831-2085. Email:
[email protected].
10.1021/jp075504z CCC: $37.00
ized, phase pure WC films, and by performing XPS measurements of the surface oxidation before and after electrochemical measurements without exposing the WC films to air. Recently our group has performed surface science studies of the reaction pathways of methanol on carbide-modified tungsten single-crystal surfaces under ultrahigh vacuum (UHV) conditions.13,14 These results indicate that tungsten carbide surfaces are very active toward the decomposition of methanol. Equally important, the desorption temperature of the CO reaction intermediate from WC is at least 100 deg lower than that from Pt or Ru single-crystal surfaces, indicating that CO is more weakly bonded to tungsten carbides and can be more easily removed by desorption or electrooxidation. Both observations suggest the potential for utilizing tungsten carbides as alternative electrocatalysts to replace Pt or Pt/Ru. The focus of the current study is to bridge fundamental surface science studies on single crystals with the electrochemical evaluation on polycrystalline WC films. We have directly compared the activity of methanol dissociation and electrooxidation on polycrystalline WC films and Pt foils. The reaction pathways of methanol is investigated using temperature programmed desorption (TPD). In addition, a system combining in-situ electrochemical studies using cyclic voltammetry (CV), chronoamperometry (CA), and surface characterization using X-ray Photoelectron spectroscopy (XPS) is utilized to evaluate the electrochemical activity and stability. Experimental Methods The electrochemical measurements were performed using a three-electrode electrochemical half-cell, consisting of a counter (auxiliary) electrode, a reference electrode, and a working electrode. The half-cell assembly was attached directly to the UHV chamber via a gate valve. Implementation of a load-lock transfer system between the electrochemical half-cell and the UHV chamber eliminated exposure of the sample to air during and after CV or CA measurements. The TPD measurements were performed on similar Pt and WC polycrystalline films using a separate UHV system. More details about the experimental techniques, as well as the quantification methods for the values in Table 1, are provided in the Supporting Information. © 2007 American Chemical Society
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TABLE 1: Comparison of Activity of Methanol Reaction Pathways on Various Surfaces
surface
complete decomp activity per metal atom
CO activity per metal atom
CH4 activity per metal atom
total no. of CH3OH reacting per metal atom
WC 0.8 ML Pt/WC Pt
0.130 0.010 0.000
0.043 0.058 0.059
0.055 0.000 0.000
0.228 0.068 0.059
Results and Discussion The decomposition pathways of methanol on polycrystalline WC and Pt foils were evaluated using TPD; a WC surface that was modified by 0.8 ML (monolayer) of Pt was also studied to determine possible synergistic effects by supporting submonolayer coverages of Pt on WC. Figure 1 shows the TPD spectra following the desorption of CO, from the saturation coverage of molecularly adsorbed CO (Figure 1a) and from the decomposition of methanol (Figure 1b). Figure 1a clearly indicates that the desorption temperature of CO from the Pt foil is significantly higher than that of either WC or the 0.8 ML Pt/ WC surfaces, which is consistent with previous studies on the corresponding single-crystal surfaces.13,14 Figure 1a also shows that the peak areas of CO are within a factor of 2 for the three surfaces, indicating that the number of active sites on the three polycrystalline surfaces are comparable. This observation is important when comparing the oxidation currents of methanol on the three surfaces in the electrochemical measurements described later. The detection of the CO product in Figure 1b indicates that all three surfaces are active toward the decomposition of methanol under UHV conditions. These results are consistent with our previous studies on single-crystal surfaces of C/W(110)
and Pt/C/W(110),14 although the desorption peaks are less welldefined in the current study due to the polycrystalline nature of the surfaces. The desorption of CO from the WC surface occurs at two temperatures, with the 400 K peak corresponding to the desorption of molecular CO and the 865 K to the recombinative desorption from atomic carbon and oxygen. The decomposition of methanol on WC occurs through three reaction pathways: the complete decomposition to produce atomic C, O, and H2, the cleavage of the C-H bond to produce CO and H2, and the dissociation of the C-O bond to produce atomic O and CH4. The quantification of the different pathways on WC, 0.8 ML Pt/WC, and Pt surfaces is summarized in Table 1. More details about the quantification procedures of reaction pathways of methanol on the three surfaces will be described elsewhere. As discussed in earlier UHV surface science work on C/W(110) and Pt-modified C/W(110) surfaces, a synergistic effect was observed for the dissociation of methanol by supporting Pt on C/W(110).14 The combined TPD and vibrational study in that work demonstrated that WC was more active than Pt in the dissociation of methanol to produce methoxy (CH3O), whereas Pt was more active than WC for the subsequent cleavage of the C-H bonds in methoxy. Such a synergistic effect eliminated the undesirable reaction pathway
Figure 1. TPD spectra of carbon monoxide (28 amu) (a) from molecularly adsorbed CO at saturation coverage and (b) from CH3OH decomposition on Pt, 0.8 ML Pt/WC, and WC surfaces.
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Figure 2. CV curves of (a) Pt, (c) WC, and (d) 0.8 ML Pt/WC surfaces exposed to solutions of 0.05 M H2SO4 with 0.2 M CH3OH. For reference, CV curves of WC and 0.8 ML Pt/WC surfaces in 0.05 M H2SO4 without methanol are shown in (b).
that produced methane on C/W(110).14 As shown in Table 1, the presence of Pt also eliminates the formation of methane on the polycrystalline WC film, confirming the synergistic effect between Pt and tungsten carbide surfaces at low Pt coverages. The TPD results in Figure 1b also reveal that the desorption temperatures of molecular CO are significantly lower on WC and 0.8 ML Pt/WC than on Pt, suggesting that the WC and Pt/WC surfaces should have less chemisorbed CO as compared to Pt at operating temperatures of DMFC. The increased availability of catalytically active sites on WC and 0.8 ML Pt/ WC would then result in higher activity for methanol electrooxidation, as demonstrated later. The promising properties of WC from the UHV studies were tested in an electrochemical environment. The CV measurements (Figure 2) of methanol oxidation on the Pt foil are similar to those in the literature.15 The suppression of hydrogen and methanol-related features in the CV curve is most likely attributed to the strong adsorption of sulfate anions. This effect has been shown to occur on platinum surfaces, as discussed in previous studies.15-17 The scale has been adjusted in Figure 2a to permit a visual comparison between voltammetric responses. The CV curve of methanol oxidation on the Pt surface shows an onset beginning at ∼0.7 V, with a maximum oxidation current occurring at ∼0.9 V with respect to NHE. The decline
J. Phys. Chem. C, Vol. 111, No. 40, 2007 14619 of this oxidation feature is related to Pt oxidation at >0.9 V, leading to a reduction of catalytically active sites for methanol oxidation. On the negative sweep of the CV curve, an increase in the current is observed with a feature centered at ∼0.7 V, which then decreases due to surface poisoning by intermediates formed during the methanol oxidation.15 A comparison of the WC and Pt-modified WC surfaces is made in the H2SO4 electrolyte environment with (Figure 2c,d) and without (Figure 2b) the addition of 0.2 M CH3OH to demonstrate the difference between the current-potential responses produced as a result of adding the methanol fuel. The broad feature observed with the addition of the fuel molecule indicates that methanol is oxidized within this potential range. The CV curves over the WC surface reveal that the onset of methanol oxidation occurs at a lower voltage than that on Pt, beginning at ∼0.5 V and leading to a maximum current at ∼0.65 V. Unlike Pt, the negative sweep of the CV curve on WC does not display any additional features. This is tentatively attributed to the weaker interactions of surface intermediates, in particular chemisorbed CO, with the WC surface, consistent with the desorption temperatures in the TPD results in Figure 1. The 0.8 ML Pt/WC surface shows methanol oxidation CV curves similar to those of the WC surface, with an enhancement in the stability of the electrocatalyst because there is less deactivation of the voltammograms during successive scans. To evaluate the surface stability, XPS measurements were performed after 50 cycles of CV measurements between 0 and 1.0 V vs NHE, without exposure of the surfaces to air during and after CV measurements. As shown in Figure 3, the WC surface shows the presence of oxidized W features at 35.5 and 37.7 eV. In comparison, the oxidized W is not detected on the 0.8 ML Pt/WC surface, indicating that the electrochemical stability of the WC surface is enhanced by the presence of submonolayer coverages of Pt. This improvement of stability may be due to preferential bonding of Pt to defect sites present on the polycrystalline WC surface. A direct comparison of the activity could not be accurately made with the CV technique, due to the contributions of capacitive currents to the anodic peaks obtained with this method. CA measurements were therefore performed to compare the steady-state activity. Figure 4 shows the comparison of WC, 0.8 ML Pt/WC and Pt at oxidizing potentials of 0.65 and 0.8 V. Similar to previous CA measurements of Pt,18 an initial decay of the capacitive current contribution can be observed until an apparent steady-state current is reached, which is correlated to methanol oxidation activity. At the oxidation potential of 0.65 V (Figure 4a) the current from Pt quickly drops to a low steady-state value. This low current is most likely due to the presence of strongly adsorbed reaction intermediates such as CO. The activity shown by WC appears to coincide with the 0.8 ML Pt/WC surface and shows a higher current than that of the Pt surface. This enhancement in activity is attributed to an improved CO tolerance of the WC surface, which is consistent with the observation of lower CO desorption temperature in the TPD measurements. A comparison is also made at a potential of 0.8 V (Figure 4b), which is closer to the methanol oxidation maximum on polycrystalline Pt (Figure 2). The Pt surface shows a slightly increased steady-state current, but no substantial increase in the steady-state current is observed for the 0.8 ML Pt/WC surface as compared to the 0.65 V case. The WC surface shows a higher steady-state activity than either Pt or 0.8 ML Pt/WC. All three surfaces remain stable after the CA measurements at 0.8 V, as
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Figure 3. XPS spectra of the W 4f region before and after 50 cycles of CV measurements between 0 and 1.0 V for WC and 0.8 ML Pt/WC surfaces.
the stability without significantly decreasing the activity of WC. In our view the potential applications of tungsten carbides would require the combination of supporting submonolayer coverages of Pt on WC. When compared to pure Pt electrocatalysts, Pt/ WC offers two potential advantages: (1) Compared to pure Pt, where only the surface Pt atoms of the Pt particles are used for electrocatalysis, all Pt atoms in Pt/WC remain in the surface, which should lead to a significant reduction in the Pt loading. (2) Due to the strong interaction between Pt and the WC substrate, it would be less likely for Pt atoms on WC to agglomerate to produce larger Pt particles that would cause gradual decrease in electrochemical activity. Currently we are performing additional surface science and electrochemical studies to confirm and quantify these two potential advantages of the Pt/WC electrocatalysts. Acknowledgment. We acknowledge financial support from the National Science Foundation (Grant No. CTS-0518900). Supporting Information Available: Detailed explanations of the experimental procedures used to produce the results discussed in this letter are available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 4. CA measurements of Pt, WC, and 0.8 ML Pt/WC surfaces exposed to 0.05 M H2SO4 with 0.2 M CH3OH at potential step values of (a) 0.65 V and (b) 0.8 V.
confirmed by the absence of oxidized W in the corresponding XPS measurements (spectra not shown). Conclusions Our results demonstrate that WC is active to methanol oxidation and is a promising alternative electrocatalyst at voltages up to ∼0.8 V. At the same time, our results also show that WC is no longer stable for applications with oxidation voltages above 0.8 V. The stability of WC can be improved by the presence of submonolayer coverages of Pt, which enhances
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