J. Phys. Chem. C 2009, 113, 17407–17410
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EXAFS Analysis of Electrocatalytic WC Materials B. Ingham,*,† C. D. A. Brady,‡ G. T. Burstein,‡ N. Gaston,† and M. P. Ryan§ Industrial Research Limited, 69 Gracefield Road, Lower Hutt 5011, New Zealand, Department of Materials Science and Metallurgy, UniVersity of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom, and Department of Materials and London Centre for Nanotechnology, Imperial College London, Exhibition Road, South Kensington, London SW7 2AZ, United Kingdom ReceiVed: June 4, 2009; ReVised Manuscript ReceiVed: August 19, 2009
Tungsten carbide is an interesting non-noble metal anode electrocatalyst that can be passivated against corrosion in acidic electrolytes for use in low-temperature fuel cells. The structure of nanocrystalline WC passivated in 1.5 M H2SO4 at 65 °C was studied using synchrotron extended X-ray absorption fine structure spectroscopy and compared with the as-prepared material. Changes in the average local structure are noted that correspond to the development of an oxide phase in the material. This is most likely to be present as a WIV-oxide phase on the surfaces of the particles. Introduction Tungsten carbide shows particular promise as the basis of a heterogeneous catalyst1 as well as a non-noble metal anode electrocatalyst in low-temperature (T < 80 °C) fuel cells, because of its electrocatalytic activity toward the anodic oxidation of hydrogen,2 its resistance to carbon monoxide poisoning,3 and crucially, its passivity toward corrosion in strongly acidic solutions.4-7 However, the origin of the passivity in these electrolytes is still a question. It has been suggested that the carbon component plays a critical role in the passivation mechanism, but the presence of a passivating oxide layer has also been mooted.3-7 Pourbaix’ calculation of the equilibrium thermodynamics of the metallic W/H2O system8 shows considerable stability of the oxides WO2 and WO3 in aqueous acidic solutions. Supporting evidence for a significant role for carbon in this material is that the stoichiometric WC carbide is significantly more electrocatalytically active than the lower carbide, W2C, and is also more passive against corrosion.4 Moreover, passivation in acidic solution has been observed in itself to increase the electrocatalytic activity of the surface,4 suggesting that a carbon-enriched surface may be not only passive but also more active toward oxidation of hydrogen than the as-prepared surface. Such a mechanism has been proposed to account for the electrocatalytic activity of other carbides prepared by nonequilibrium methods.9,10 The hypothesis is consistent with theoretical calculations of the energetics of adsorption of hydrogen on such a surface; however the role of any oxide phase has not yet been addressed in this context.11 The aim of the current work is to understand the chemical and structural changes induced by the electrochemical passivation treatment in aqueous sulfuric acid. Experimental Section Nanocrystalline tungsten carbide (particle size ∼100 nm) was synthesized by solid-state thermal reaction of WO3, and high surface-area carbon (X272R carbon, Cabot Corp., BET surface * To whom correspondence should be addressed. E-mail: b.ingham@ irl.cri.nz. † Industrial Research Limited. ‡ University of Cambridge. § Imperial College London.
area: 237 m2 g-1). A detailed account of this reaction has been described earlier.4 The electrocatalyst was made into Nafionbound particulate disk electrodes using Nafion 117 solution (containing 5 wt % Nafion polymer to 0.9 g WC) by sonication of the suspension followed by evaporation to give an ink that could be applied to a 25 mm Toray carbon paper (TGHP-090) disk. The disk electrode thus formed was dried at 80 °C for 60 min and then pressed for 5 min at a pressure of 100 bar. This electrode was then treated potentiostatically in 1.5 M H2SO4 at 65 °C using an applied electrode potential range of -0.61 to -0.26 V against a saturated Hg/Hg2SO4 electrode (MMS), equivalent to +0.017 to +0.367 V(SHE) (standard hydrogen electrode) over the course of 4 h. The electrode showed passivity against corrosion and enhanced electrocatalytic activity for the anodic hydrogen oxidation reaction. After the electrochemical treatment the WC powder was recovered by soaking the material in isopropanol to remove the Nafion binder. This material is referred to below as the passivated WC sample. W L3 EXAFS spectra were collected in transmission geometry using beamline 10-2 at the Stanford Synchrotron Radiation Lightsource. The standards and samples in powder form were mixed with BN (Sigma-Aldrich, 98%) as a diluent and packed into transmission holders to a thickness of 1 mm with a concentration yielding one absorption length of material in the beam at the W L3 edge (10207 eV). The standards comprised a W foil, and commercially obtained WO2 (Sigma-Aldrich, 99.999%) and WC (Sigma-Aldrich, 99%). Data were collected up to k ) 16.25 (11225 eV). WO2 was used as an oxide standard rather than WO3 because WO3 readily converts to substoichiometric anion-deficient material (WO3-x). The EXAFS data were prepared and analyzed using Athena and Artemis12 with FEFF6.13 The averaging, background subtraction and normalization procedures were performed using standard routines. The data were fitted in R-space with k1, k2, and k3 weightings simultaneously. Results and Discussion The W, WO2, and WC standards were fitted with models constructed from their appropriate structures as found in the literature. W is body-centered cubic (Im3m, a ) 3.1648 Å; W, 0,0,0).14 WO2 has a distorted rutile structure (P21/c, a ) 5.563,
10.1021/jp905283f CCC: $40.75 2009 American Chemical Society Published on Web 09/16/2009
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Ingham et al.
TABLE 1: Fitting Details for the Three Standards W -1
3 - 15 1 - 5.3 32.44 5 0.544 ( 0.034 6.09 ( 0.47 -0.00227 ( 0.00059 0.00222 ( 0.00020 0.00305 ( 0.00024
k-range (Å ) R-range (Å) independent points number of parameters S02 ∆E0 scalea σ2 (first shell) σ2 (SS paths) σ2 (MS paths) R-factor a
b
0.0102
WO2 3 - 12 1-5 22.55 5 0.573 ( 0.057 12.11 ( 0.85 0.0035 ( 0.0026 b
0.00056 ( 0.00092 0.0063 ( 0.0058 0.0221
WC 4 - 14 1-5 25.07 6 0.674 ( 0.038 9.08 ( 0.54 0.00024 ( 0.00092 0.00295 ( 0.00100 0.00197 ( 0.00026 0.00271 ( 0.00169 0.0142
∆r ) reff*scale. Set to σ (SS paths). b
2
Figure 1. Data and fits for WC standard in (a) k- and (b) R-space; and as-prepared WC nanopowder sample in (c) k- and (d) R-space.
b ) 4.896, c ) 5.663, β ) 120.47; W, 0.2278, -0.0102, 0.0111; O1, 0.1119, 0.2186, 0.2334; O2, 0.3900, 0.7024, 0.2986)15 and WC is hexagonal (P-63m, a ) 2.9065, c ) 2.8366; W, 0, 0, 0; C, 0.6667, 0.3333, 0.5).16 All paths up to R ) 6 Å were used for W and WC, but in the case of WO2 (which, owing to its distorted structure has a much larger number of paths) only the first 256 paths were used (up to R ) 5.47 Å). The fitting constraints and parameters obtained are shown in Table 1. Since the singlescattering (SS) and multiple-scattering (MS) paths can often have different disorder parameters σ2, and the first shell usually has a lower σ2 than subsequent SS paths, these were assigned as different parameters in an effort to improve the fit. The S02 (amplitude) values are consistent for the three standards, as are the ∆E0 values. The “scale” parameters are close to zero, indicating that the input models are appropriate. The σ2 for the WC first shell paths is higher than for subsequent SS paths, which is unexpected. This is most likely due to a slight difference between the reported and actual atomic structure of WC. The higher σ2 may indicate that the actual structure has
a lower symmetry of the first W-C shell than obtained from using the model. Therefore the first W-C shell (six equidistant paths) ought to be expressed as 6 paths of slightly different length. However we believe that changing the model to reflect this would only complicate the structure at longer distances. In contrast, the σ2 for the WO2 SS paths is very small, which suggests that the actual structure is less disordered than the model (recalling that WO2 has a distorted rutile structure and that not all paths could be included in the fitting). In all cases, the ratio of the number of parameters to the number of independent points is good, and the R-factors are close to 0.02 or less. The maximum correlation was 0.87 (between S02 and σ2 for W). Figure 1 shows the data and fits for the WC bulk powder standard and the WC as-prepared nanopowder sample. The parameters for the WC as-prepared sample fit, using the WC model as for the standard, are given in Table 2. By comparing the S02 value with that obtained for the WC standard, we obtain a first shell co-ordination number for the WC as-prepared sample of 5.7 ( 0.4 (expected 6). There are no significant particle size attenuation
EXAFS Analysis of Electrocatalytic WC Materials TABLE 2: Fitting Details for WC As-Prepared Sample
J. Phys. Chem. C, Vol. 113, No. 40, 2009 17409 TABLE 3: Fitting Details for Passivated WC Sample
WC as-prepared -1
k-range (Å ) R-range (Å) independent points number of parameters S02 ∆E0 scalea σ2 (all paths) R-factor a
passivated WC -1
4-14 1-5 25.07 4 0.643 ( 0.058 7.45 ( 0.82 -0.00081 ( 0.00093 0.00209 ( 0.00031 0.0112
k-range (Å ) R-range (Å) independent points number of parameters S02 (WC) S02 (W-O) ∆E0 scale (WC)a scale (W-O)a σ2 (WC, first shell) σ2 (WC, SS paths)b σ2 (W-O) R-factor
∆r ) reff*scale.
a
Figure 2. Data and fits for passivated WC nanopowder sample in (a) k- and (b) R-space.
effects. From the high degree of similarity between the two fits and their corresponding parameters, we conclude that the asprepared WC sample has the same average local structure as bulk WC. All of the W standards and samples studied have an intense “white line” in the XANES region. This posed some issues for the automatic background subtraction routine, which were overcome by increasing the lower limit of the k-range being fitted from 2 (a typical value) to 3 or 4. The intense white line is indicative of strong ionic bonding.17 Figure 2 shows the data and fits for the passivated WC nanopowder sample. The data are significantly different from the as-prepared WC sample, and different from all three of the standards. The model that gave the best fit to the data used all SS paths and all MS paths up to R ) 5.65 Å from the WC structure as before, plus the six paths from the first W-O shell of the WO2 structure. Including other WO2 paths did not improve the fit. Eight parameters were used to fit the data for the passivated WC sample. Their values are given in Table 3. Restrictions were
4-14 1-5 25.07 8 0.359 ( 0.038 0.331 ( 0.142 4.98 ( 1.11 -0.0024 ( 0.0011 -0.0915 ( 0.0061 0.00380 ( 0.00175 0.00209 ( 0.00036 0.00994 ( 0.00566 0.0169
∆r ) reff*scale. b σ2 (WC, MS paths) ) 2*σ2 (WC, SS paths).
placed on the σ2 parameters, and the ∆E0 parameters were defined as being the same for the WC and W-O components. First we consider the immediate local structure around W. By comparing the S02 values with the values obtained from fitting the respective standards, we obtain co-ordination numbers for the first shell of 3.20 ( 0.25 W-C and 3.4 ( 0.9 W-O. The respective bond distances are 2.1918 ( 0.0024 Å for W-C (c.f. 2.1971 Å for bulk WC) and 1.835 ( 0.012 Å for W-O (c.f. 2.020 Å average in bulk WO2). The ∆E0 obtained is reasonable. The two σ2 disorder parameters used in the WC part of the model are similar to the WC standard, and σ2 for the first shell is again larger than for the subsequent SS paths, indicating possible oversimplification of the WC structure. σ2 for the W-O paths is larger than the W-C paths, indicating a high degree of disorder (particularly compared with the WO2 standard, which had an abnormally low σ2 value for the SS paths). This could explain why adding additional paths from the WO2 structure did not improve the fit. These results are for the average local structure throughout the entire sample. Several geometries could in principle satisfy the model as presented here, for example, (a) every W atom is coordinated to 3 C atoms and 3 O atoms; (b) there are some WC particles and some WO2 particles; (c) the surfaces of the WC particles have been converted to a W-oxide. Of these, description (c) is the most likely. A 50% mixture of W-C and W-O in a 100 nm particle would require a surface W-oxide layer about 10 nm thick. The overall structure has significantly more disorder than the standards, as evidenced by (i) the higher σ2 values, (ii) the fact that adding MS paths from the WC model does not improve the fit, and (iii) adding additional paths from WO2 past the first shell does not improve the fit. This has implications on the fitting of the higher order W-C shells. Increasing the number of fitted parameters to allow for a separate S02 for the first W-C shell did not give a statistically different value than that for the longer paths. The S02 value obtained implies that the bulk co-ordination numbers are roughly half that expected for WC. This could be due to (i) the disorder in WC mentioned above, which will give rise to destructive interference of the EXAFS (which affects longer paths more than shorter paths), and/or (ii) the presence of a significantly thick boundary layer between the “surface oxide layer” and the “core”, where the presence of both W-O and W-C bonds give rise to a disordered local structure. W forms a number of oxide phases of which WO3 and WO2 have the simplest stoichiometry. A slight substoichiometry of WO3 is thermodynamically favored (as shown both by densityfunctional theory (DFT) calculations and experimentally).18 The
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oxidation state of tungsten (WIV) in WC indicated that WO2 would be a better oxide standard to use in this analysis. (Since EXAFS is not sensitive to direction within the structure, either compound could have been used as the W-O standard.) The oxide phase that forms on the surfaces of the particles is most likely to be a WIV-oxide. The EXAFS fitting results indicate that this has a high degree of disorder compared with the WO2 standard. In addition, the electrically conducting nature of bulk WO219 makes it a better starting model than (insulating) WO3 for an electrocatalytically active surface oxide phase. In reality, the surface oxide film probably has no bulk counterpart. We mention here that Zellner et al.20,21 have used X-ray photoelectron spectroscopy (XPS) of the W 4f electron to demonstrate the presence of an oxide on sputtered thin films of WC after polarization in H2SO4 solution, but they referred to this oxide as WxOy, and did not specify x and y. Their XPS work20,21 was performed after potential sweeps (rather than after constant potential treatment as used here), and the sweeps extended to the high electrode potential of 1.0 V(SHE). Because of the high potential, it is likely that the WxOy species detected by XPS was indeed WO3. Our own earlier work shows that tungsten carbide does in fact corrode at a significant rate at higher potentials in H2SO4 solution and this might be expected to precipitate the oxide WO3 from solution by virtue of its insolubility.3-7 We have deliberately confined the present work to low potentials at which a hydrogen anode in a fuel cell might be expected to operate, and thereby confining the reactions to those which impart passivity. Our own experiments have shown that potentiostatic current transients of polycrystalline WC electrodes in sulfuric acid show behavior consistent with some oxidation of the carbide surface.3-7 Since the thermodynamics of tungsten oxides show them to be very insoluble at low pH,8 we take this to mean that there is likely to be some oxide on the surface.3-7 The low solubility of the oxides in acidic solution8 does not of course, preclude entirely some anodic dissolution of tiny amounts of surface tungsten, but the data described above are consistent with the presence of the oxide. The fact that the data described above for passivated WC differ from those of all three of the standards is fully consistent with the fact the passivated surface does indeed carry some oxide. In addition, we have performed DFT calculations to estimate the energy required of the carbide to oxide transformation, that is, the energy of substitution of oxygen within the ideal WC (0001) surface.22-26 This is to represent the interface between the WC core and the W-oxide surface film (expected to be ∼10 nm from the EXAFS results). A thermodynamic barrier of 0.14 eV to the substitution of surface carbon for a monolayer of oxygen at low coverage (25%) was obtained. At 50% coverage, the substitution is thermodynamically favorable by a small amount (-0.02 eV) and at 100% coverage this increases to -0.34 eV. However subsurface substitution of oxygen remains thermodynamically unfavorable at all coverages. In order to consider the more complex model of a thin WO2 surface layer on WC, additional knowledge of the mechanism of formation of the surface oxide phase would be required. However the thermodynamic stability of the first oxygen monolayer (from DFT) indicates that oxide formation on the surface is possible. The results of fitting the EXAFS spectrum with the WC bulk structure with an S02 parameter of roughly half that of the bulk standard (and hence indicating a lower co-ordination) are compatible with the DFT observations. C is able to occupy effective interstitial sites between W atoms in WC, whereas the replacement of C with O leads to a complicated distortion of the structure.
Ingham et al. Conclusions We have compared the EXAFS spectra of both as-prepared and electrochemically passivated WC samples with WO2, WC, and W metal standards. The effect of electrochemical treatment on the WC structure is evident in the analysis. The passivated material contains a significant proportion (∼50%) of oxide in addition to the original carbide material. A model of WC with a thin, disordered WIV-oxide surface shell has been proposed as a possible structure. Simple DFT calculations support the proposed formation of a surface oxide. This interpretation of the EXAFS fitting shows that the original hypothesis of a carbon-enriched surface being formed is incomplete. While there may be carbon enrichment occurring, the formation of a WIVoxide phase at the surface is also evident. This result is unexpected and the implications for the enhanced electrocatalytic activity are being explored. Insight into the structural changes in WC upon passivation will help to understand the origin of its electrocatalytic activity and allow these properties to be exploited in fuel cell applications. Acknowledgment. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. Funding was provided in part by the New Zealand Foundation for Research, Science and Technology under contract CO8X0409. We also acknowledge partial funding by the EPSRC. References and Notes (1) Levy, R. B.; Boudart, M. Science 1973, 181, 547. (2) Binder, H.; Ko¨hling, A.; Kuhn, W.; Lindner, W.; Sandstede, G. Nature 1969, 224, 1299. (3) McIntyre, D. R.; Burstein, G. T.; Vossen, A. J. Power Sources 2002, 107, 67. (4) Brady, C. D. A.; Rees, E. J.; Burstein, G. T. J. Power Sources 2008, 179, 17. (5) Burstein, G. T.; Barnett, C. J.; Kucernak, A. R. J.; Williams, K. R. J. Electrochem. Soc. 1996, 143, L139. (6) Barnett, C. J.; Burstein, G. T.; Kucernak, A. R. J.; Williams, K. R. Electrochim. Acta 1997, 42, 2381. (7) Burstein, G. T.; Barnett, C. J.; Kucernak, A. R. J.; Williams, K. R. Catal. Today 1997, 38, 425. (8) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; Pergamon/CEBELCOR: Oxford, 1966; p 280-285. (9) Burstein, G. T.; McIntyre, D. R.; Vossen, A. Electrochem. Solid State Lett. 2002, 5, A80. (10) Brady, C. D. A.; Rees, E. J.; Burstein, G. T. J. Power Sources 2008, 179, 17. (11) Gaston, N.; Hendy, S. C. Catal. Today [Online early access.] DOI: 10.1016/j.cattod.2008.10.050. (12) Ravel, B.; Newville, M. J. Synchrotron Rad. 2005, 12, 537. (13) Newville, M. J. Synchrotron Rad. 2001, 8, 322. (14) Dubrovinsky, L. S.; Saxena, S. K. Phys. Chem. Miner. 1997, 24, 547. (15) Palmer, D. J.; Dickens, P. G. Acta Cryst. B 1979, 35, 2199. (16) Leciejewicz, J. Acta Crystallogr. 1961, 14, 200. (17) Yang, N.; Mickelson, G. E.; Greenlay, N.; Kelly, S. D.; Bare, S. R. AIP Conf. Proc. 2007, 882, 663. (18) Ingham, B.; Hendy, S. C.; Chong, S. V.; Tallon, J. L. Phys. ReV. B 2005, 72, 075109. (19) Dissanayake, M. A. K. L.; Chase, L. L. Phys. ReV. B 1978, 18, 6872. (20) Zellner, M. B.; Chen, J. G. Catal. Today 2005, 99, 299. (21) Weigert, E. C.; Stottlemyer, A. L.; Zellner, M. B.; Chen, J. G. J. Phys. Chem. C. 2007, 111, 14617. (22) Calculations have been performed with the code VASP23 within the framework of DFT,24,25 using the PW91 functional.26 A seven-layer slab model of the hexagonal (0001) surface was employed. Further details of the model are given in ref 11. (23) (a) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558. (b) Kresse, G.; Hafner, J. Phys. ReV. B 1994, 49, 14251. (24) Hohenberg, P.; Kohn, W. Phys. ReV. 1964, 136, B864. (25) Kohn, W.; Sham, L. J. Phys. ReV. 1965, 140, A1133. (26) Perdew, J. P.; Wang, Y. Phys ReV. B 1992, 45, 13244.
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