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Pt–W bimetallic alloys as CO-tolerant PEMFC anode catalysts
Electrochimica Acta 89 (2013) 744–748
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Pt–W bimetallic alloys as CO-tolerant PEMFC anode catalysts Yu Dai a,b , Yuwen Liu a , Shengli Chen a,∗ a Hubei Electrochemical Power Sources Key Laboratory, Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan 430072, China b Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China
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Article history: Received 20 September 2012 Received in revised form 19 October 2012 Accepted 3 November 2012 Available online 10 November 2012 Keywords: PEM fuel cells Hydrogen oxidation reaction CO tolerant electrocatalyst Pt–W alloys
a b s t r a c t Pt–W nanoalloys with compositions ranging from Pt3 W to PtW2 were explored as electrocatalysts for hydrogen oxidation reaction (HOR). It is shown that alloying Pt with W can lead to significantly enhanced electrocatalytic activity for HOR, with nearly 4 times increase in the exchange current density as compared with pure Pt. What’s more, these Pt–W alloys possess superior CO tolerance to Pt and PtRu, mainly due to the weakened bonding of CO on their Pt-enriched surfaces. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Proton-exchange membrane fuel cells (PEMFCs) hold great promise for low/zero-emission electric vehicles and distributed power stations [1]. Differing from the oxygen reduction reaction (ORR) on the cathode, which has slow kinetics, the hydrogen oxidation reaction (HOR) on the anode of PEMFCs is rather facile on Pt-based catalysts, with overpotential less than 20 mV under typical fuel cell operation conditions. However, Pt as anode electrocatalyst for PEMFCs easily suffers from CO poisoning even when the fuel contains trace amounts of CO, which can lead to significant activity decline [2]. Bleeding air/O2 [3] or using pure instead of reformed H2 [4] in the anode could solve the CO poison problem. However, the resulted system complication may limit the viability of fuel cells. Air bleeding would also cause membrane degradation due to the H2 O2 formation through O2 reduction at anode [3]. A relatively more viable alternative will be using CO tolerant electrocatalysts [5–18]. Pt–Ru alloys are currently recognized as the best anode electrocatalysts for PEMFCs using reformed H2 as fuel [8,9]. The CO tolerance of Pt–Ru catalysts is known to operate through the socalled bifunctional mechanism. On the surface of Pt–Ru alloys, CO adsorbs mainly on the Pt atom sites, while Ru atom sites are prone to absorb oxygenated species from water dissociation at relatively negative potentials. These oxygenated species could oxidize the adsorbed CO at the neighboring Pt atom sites. Currently, the
∗ Corresponding author. E-mail address: [email protected] (S. Chen). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.11.011
CO-tolerance of Pt–Ru alloys remain limited for efficient hydrogen oxidation, so that the anode has to be highly catalyst loaded. This will preclude the practical use of PEMFCs because that Pt and Ru are both precious metals. In addition, the stability of Ru in acidic media is unsatisfactory for PEMFC application [8]. Therefore, durable and precious metal lean electrocatalysts for CO tolerant hydrogen oxidation are highly expected. There have been extensive recent studies on increasing the CO tolerance of Pt or Pt–Ru catalysts by addition of Mo and/or W. Mukerjee et al. reported 2- to 3-folds of activity enhancement over PtRu/C for HOR in H2 containing 100 ppm CO could be achieved by using a catalyst of Pt4 Mo/C [14]. A similar bifunctional mechanism was proposed for the CO-tolerance of this catalyst, in which the oxygenated species formed on Mo oxidatively eliminate the adsorbed CO on the neighboring Pt. It was also reported that CO-tolerance better than the state-of-the-art PtRu catalyst could be achieved by depositing a Pt shell on MoOx . The CO tolerance was attributed to the weakened Pt–CO interaction by the MoOx core [6]. Similarly, Pt nanoparticles supported on tungsten oxide (Pt/WOx ) also showed better CO tolerance than the carbon supported Pt and PtRu catalysts [10,11]. In addition, introduction of Mo or W or their oxides into the Pt–Ru catalysts was found to give much higher current densities than Pt–Ru in fuel cells operated with H2 /CO mixture [12,13]. Recently, Wang et al. reported that Pt/Ti0.7 W0.3 O2 exhibited much lower onset potential for H2 oxidation in the presence of 2% CO relative to Pt/C and PtRu/C [5]. CO-tolerant HOR were also reported on Pt–W alloys [15,16]. For examples, Passalacqua et al. reported that carbon supported Pt–W alloy with a Pt/W atomic ratio of 50:50 exhibited better CO tolerant HOR activity even than the Pt–Ru with the same atomic ratio [16].
Y. Dai et al. / Electrochimica Acta 89 (2013) 744–748
Watanabe et al. have shown that Pt–Fe, Pt–Ni, Pt–Co and PtMo alloys with core–shell structures are also good CO tolerant HOR catalysts. Since that these bimetallic alloy catalysts possessed a Ptenriched surface, the authors argued that the CO tolerant hydrogen oxidation on these catalysts was through a so-called detoxification mechanism rather than the bifunctional mechanism mentioned above. They argued that the underlying transition metals can modify the electronic properties of the surface Pt through the so-called strain and/or ligand effects, which could result in weakened adsorption of CO on Pt [17,18]. The coverage of CO would therefore be much lower than pure Pt in the same conditions. Recently, we found that alloying Pt with W can form a stable Pt-enriched surface even at composition of high W content such as PtW2 , due to the strong surface segregation tendency of Pt in Pt–W alloys [19]. In addition, W can modify the electronic structure of the surface Pt so that it binds oxygenated species more weakly than pure Pt [19]. In the present study, we show that these Pt–W alloys are also superior CO tolerant anode catalysts for PEMFCs due to their much enhanced HOR exchange current densities over pure Pt and even better CO tolerance than PtRu. In addition, it is shown that the CO-tolerance of these Pt surface-enriched alloys is also due to their weak binding to CO. In most of previous reports, the CO-tolerant hydrogen oxidation of Pt–W and/or Pt–WO3 systems has been explained in terms of the bifunctional mechanism [11], strong metal-support interaction [10] or the formation of hydrogen tungsten bronzes [15]. 2. Experimental
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prepared Pt–W/C catalysts (Pt–W/C) of various compositions have relatively uniform size distribution with alloy particle sizes of ∼5 nm (Fig. 1, also see Ref. [19]). The XRD responses indicated that the Pt–W particles possess a Pt fcc structure [19]. For catalysts with compositions from Pt3 W to PtW, no obvious diffraction characteristics for tungsten or its oxides were seen on the obtained XRD patterns. For PtW2 , some diffraction peaks associated with tungsten oxides appeared. The EDS and the ICP-AES data showed that the alloy particles have Pt/W ratios close to the nominal values expected from the amounts of the starting materials [19]. Therefore, we will denote the prepared catalysts with their nominal compositions in the following. 2.2. Electrochemical measurements Electrochemical measurements were performed with a threeelectrode configuration. The working electrodes were the commonly used thin-film rotating-disk-electrode (RDE) made by coating the studying catalysts as a thin film onto a glass carbon (GC) RDE substrate (diameter: 5 mm) with Nafion as the binding agent. In addition to Pt–W/C catalysts, measurements with two reference catalysts, namely, 20 wt% Pt/C and 40 wt% PtRu/C from JohnsonMatthey (JM) were also conducted for comparison. The counter electrode was a Pt foil, and the reference electrode was a saturated calomel electrode (SCE), which was separated from the working electrode by a Luggin capillary. However, all the potentials were expressed on the scale of the reversible hydrogen electrode (RHE) in this paper. The working electrolyte solution was 0.5 M H2 SO4 aqueous solution.
2.1. Materials preparation and characterizations 3. Results and discussion The details for the synthesis of the carbon supported Pt–W alloy catalysts (Pt–W/C) have been given elsewhere [19]. In brief, the required amounts of chloroplatinic acid, tungsten hexacarbonyl and Vulcan XC-72R carbon black were mixed thoroughly in tetrahydrofuran (THF) by ultrasonic stirring and heating. The resulted paste was first treated in a tube furnace at ∼120 ◦ C to reduce the impregnated Pt precursors and then at ∼700 ◦ C to decompose the tungsten hexacarbonyl, after which the sample was cooled down naturally to room temperature to allow Pt segregating to the surface of the catalyst. Such segregation should be thermodynamically favored due to that Pt has much lower surface energy than W. The Pt/W ratio was controlled by adjusting the amounts of the chloroplatinic acid and tungsten hexacarbonyl precursors added in THF. The total metal loading (Pt + W) on carbon was maintained at 20 wt% for various Pt–W/C catalysts. Transmission electron microscopy (TEM) obtained on a JEOL JEM-2010 transmission electron microscope showed that the
3.1. Hydrogen oxidation in the absence of CO Fig. 2 gives the cyclic voltammograms (CVs) for various Pt–W/C and Pt/C (20 wt%, Johnson-Matthey) catalysts in electrolyte solution saturated with inert gas, which show typical Pt features for the underpotential deposition (UPD) adsorption/desorption of hydrogen, indicating the surface enrichment of Pt in these Pt–W/C catalysts even in Pt-lean PtW2 /C catalyst. In fact, the surface enrichment of Pt in these catalysts was also implied by the results of the angular-resolved XPS measurements. By deconvoluting the XPS spectra in binding energy region between 68 eV and 80 eV into components of Pt4f and W5s through Gauss-Lorentzian fitting and correcting the sensitivity of each element, the Pt/W atom ratios in the near surface region were estimated to be 4.2:1, 3.6:1, 4.2:1 and 2.5:1 for the Pt3 W/C, Pt2 W/C, PtW/C and PtW2 /C respectively [19], which are much higher than the corresponding nominal
Fig. 1. (a) Representative TEM images and (b) the corresponding histograms of particle size distribution of the prepared Pt2 W/C.
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(Fig. 3b). Besides, the currents are obviously lower in magnitude than those under the high catalyst loading, especially when high electrode rotation rates are used. For instance, the current obtained at 6400 rpm under the low catalyst loading (Fig. 3b) is only ∼3/4 of that obtained under the high catalyst loading (Fig. 3a). According to the Koutecky–Levich equation, i.e., I −1 = Ik−1 +
Fig. 2. CVs in Ar-saturated 0.5 M H2 SO4 for the prepared Pt–W/C and the JM Pt/C catalysts. Potential scanning rates: 50 mV/s. The total metal loadings: 51 g cm−2 .
compositions. The estimation performed with the W4f spectra in the binding energy region between 31 eV and 40 eV gave very similar results. It is known that HOR has a very facile kinetics which is difficult to measure with the normal RDE method. Recently, we have shown that the HOR kinetics may be studied by using a catalyst poor thinfilm RDE method [20]. The main strategy is to reduce the amount of the catalyst on the catalytically inert GC substrate so that the kinetic current (Ik ) on the RDE is significantly decreased. The measured polarization curves thus would be governed mainly by the reaction kinetics rather than the diffusion of H2 in solution. It was found that reliable extraction of the exchange current density (j0 ) of the HOR may be achieved when the Pt loading on GC RDE substrate is below 2 g Pt cm−2 for Pt/C catalysts [20]. In this study, we will use 1 g cm−2 total metal loading to determine the j0 values for HOR on Pt–W/C catalysts. To verify that the polarization curves given by the thin-film RDE with such a low catalyst loading can be used to extract the kinetics of the HOR, we measured the steady-state polarization curves of the 20 wt% Pt/C catalyst for HOR under a high metal loading (20 g Pt cm−2 ) and a low metal loading (1 g Pt cm−2 ) respectively. The results are given in Fig. 3. It can be seen that the polarization curves under the high catalyst loading exhibit well-defined limiting currents (Fig. 3a). On the polarization curves obtained under the catalyst loading of 1 g Pt cm−2 , however, no well-defined limiting currents are seen. In the potential region where the limiting currents occur under high catalyst loading, the currents show slight increase with the positive going of potential
−1 1/IdL , the RDE current would reach the limiting diffusion current (IdL ) as Ik IdL . Since Ik = AmSjk , in which A, m and S and jk refer to the geometric area of the GC substrate, loading of catalyst, the specific surface area per mass of catalyst and the kinetic current density produced by per surface area of catalyst, the magnitude of Ik will be determined by m and jk for certain catalyst. Considering that the HOR on Pt-based catalysts proceed with either the Tafel reaction (1/2H2 + * ↔ H*) or the Heyrovsky reaction (H2 + * ↔ H* + H+ ) being the rate determining step, we should have jk ∝(1 − ) at relatively positive potentials, where is the surface coverage of the hydrogen ad-atoms. Since that decreases with the positive going of potential, the kinetic current for HOR would increase with the positive going of potential. The measured current (I) thus would increase according to the Koutecky–Levich equation unless Ik is much larger than IdL , which may be satisfied under high catalyst loading (m) and large jk (low ). Thus, the limiting currents on the polarization curves obtained under high catalyst loading should be the limiting diffusion current. As seen in inset of Fig. 3a, these limiting currents give a linear Levich plot going through the origin of coordinates, which is the typical feature of the diffusion limiting current. We found that further increase in the catalyst loading above 20 g Pt cm−2 changed little in the limiting current for certain ω, which further suggests that the limiting currents under high catalyst loadings are purely diffusion controlled. Under the low catalyst loadings, the condition of Ik IdL may not be satisfied. The continuous increase of current with the positive going of potential is thus understandable. This means that the polarization curves under the low catalyst loading are at least partially governed by kinetic currents even in the potential region where the limiting diffusion currents occur under high catalyst loading. In the potential region near the equilibrium potential, the value of Ik should be much lower than those in the limiting current region. In this case, the measured currents could be predominantly governed by Ik , which would allow reliable determination of Ik from the polarization curves. As shown by the inset of Fig. 3b, the currents in the limiting current region under the low catalyst loading give linear Koutecky–Levich plots, in which the reciprocal of the current (I−1 ) at certain potential is plotted against the reciprocal of the square
Fig. 3. Steady-state polarization curves in H2 -saturated 0.5 M H2 SO4 for the JM Pt/C catalysts at different electrode rotation rates under (a) high metal loading (20 g cm−2 ) and (b) low metal loading (1 g cm−2 ) respectively. Potential scanning rate: 5 mV. Inset of (a): The dependence of the limiting currents and the square roots of the electrode rotation rates. Inset of (b): The relationship between reciprocal of currents and the reciprocal of the square roots of the electrode rotation rates.
Y. Dai et al. / Electrochimica Acta 89 (2013) 744–748
Fig. 4. Steady-state polarization curves in H2 -saturated 0.5 M H2 SO4 for the prepared Pt–W/C and the JM Pt/C catalysts. The total metal loadings: 1.0 g cm−2 . Potential scanning rates: 5 mV/s. Inset of: Polarization curves near the equilibrium potential.
root of the electrode rotation rate (ω−1/2 ). Furthermore, the slopes of these Koutecky–Levich plots hardly change with the potential. This means that the polarization curves obtained under such a low catalyst loading remain follow the Koutecky–Levich equation and that the limiting diffusion currents (therefore the diffusion mode) of the RDE are not changed by lowering the catalyst loading. Fig. 4 shows the steady-state polarization curves of the prepared Pt–W/C and JM Pt/C catalysts for HOR under a catalyst loading of 1 g cm−2 (total metal). It can be seen that the electrodes loaded with Pt–W/C catalysts give higher limiting current than the Pt/C under the same metal loading, indicating that the kinetics of HOR are more facile on Pt–W/C catalysts. According to Chen and Kucernak [21], the current–potential relation near the equilibrium potential can be expressed by Eq. (1), in which I0 is the exchange current for HOR on the catalyst-loaded RDE, and F, R and T have their usual meanings. Thus, the exchange current density (j0 ) for HOR at an interested catalyst can be estimated according to slope of the relatively linear current–potential dependence (I ∼ ) near the equilibrium potential (inset of Fig. 4) and the electrochemical active areas of Pt (ESA) given by the catalyst, according to j0 = I0 /ESA. The ESA can be estimated with the UPD H charges in CVs for various catalysts (Fig. 2). The HOR j0 values thus determined are similarly around 22 mA (cm2 Pt)−1 for various Pt–W/C catalysts, which is about 3.5 times higher than that determined for the JM Pt/C catalyst (∼6.0 mA (cm2 Pt)−1 ). When normalized by the Pt mass in the catalysts, the corresponding mass activities for HOR are 4.7, 8.6, 9.3 and 18.2 A (mg Pt)−1 respectively for Pt/C, Pt3 W/C, Pt2 W/C and PtW2 /C. Thus, a nearly 4 times increase in Pt mass activity for HOR would be achieved as PtW2 is used as catalyst to replace Pt in the anode of PEMFCs. 1 2F 1 = · − RT I IdL I0
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Fig. 5. CO stripping curves in Ar-saturated solution for the prepared Pt–W/C, the JM Pt/C and JM PtRu/C catalysts. Scanning rates: 20 mV/s. Total metal loadings: 51 g cm−2 .
holding the potential at 0.05 V for 120 min in H2 /CO-saturated solution. For the sake of simplicity, we will denote the HOR under H2 /CO as HOR-CO in the following. As compared with the Pt/C and PtRu/C, the onset potentials for CO-stripping on Pt–W/C catalysts show a negative shift of ∼50 mV and a positive shift of ∼100 mV respectively (Fig. 5), which indicates that the stripping of the CO adsorbed on the surface of the Pt–W nanoalloys is more facile than that on pure Pt, but more difficult than that on PtRu. In order to compare the CO coverage on the catalyst surface, the charges associated with UPD H desorption and the charges involved in the stripping of the adsorbed CO were evaluated by integrating the corresponding voltammetric peaks. For Pt/C, the charge of CO stripping was found to be about twice of that for UPD H desorption, whereas the charge ratios of CO stripping and UPD H desorption were less than 1.8 for the Pt–W/C catalysts. This seems to suggest that CO does not form a full monolayer on the surface of the Pt–W/C catalysts due to the more weakly boding. When the HOR-CO polarization curves are compared, the Pt–W/C catalysts exhibit HOR onset potentials about 150 mV and 200 mV more negative than that on the Pt–Ru/C and Pt/C respectively, suggesting that the Pt–W alloy catalysts are more CO tolerant for HOR than Pt/C and PtRu/C. The enhanced CO tolerance of Pt–W/C catalysts over Pt/C is in consistence with their more negative CO stripping potentials than Pt/C. The better CO tolerance of Pt–W/C catalysts than PtRu/C, however, contradicts with their more positive CO stripping potentials than PtRu/C. This suggests that the mechanism of CO-tolerant oxidation of H2 on Pt–W alloys differs from that on Pt–Ru alloys.
(1)
3.2. Hydrogen oxidation in the presence of CO To explore the CO-tolerance of the Pt–W/C catalysts for hydrogen oxidation, they are compared with the JM Pt/C and PtRu/C reference catalysts according to the voltammetric curves for COstripping and the HOR polarization curves obtained with H2 gas containing 1000 ppm of CO (H2 /CO). To record the voltammetric curves for CO stripping, the catalysts-loaded electrode was first held at 0.125 V for 30 min in CO-saturated solution, after which the solution was purged with argon for 30 min. The current responses were then recorded by applying a potential sweep of 0.125 V → 0.05 V → 1.275 V → 0.05 V → 1.275 V (Fig. 5). The HOR polarization curves (Fig. 6) are recorded on the anodic sweep after
Fig. 6. Steady-state HOR polarization curves in H2 /CO-saturated solution for the prepared Pt–W/C, the JM Pt/C and JM PtRu/C catalysts. Scanning rates: 5 mV/s. Total metal loadings: 20 g cm−2 .
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As stated in Section 1, the CO removal from Pt–Ru alloy surface is realized mainly through the bifunctional mechanism [8,9,22], in which oxygenated species adsorbed on the highly oxophilic Ru atoms sites oxidize CO adsorbed on the neighboring Pt sites. In H2 /CO-saturated solution, visible hydrogen oxidation should take place as the removal of CO adsorbed at surface Pt atom sites is initialized. It can be seen in Figs. 5 and 6 that the CO stripping and the HOR-CO on the PtRu/C catalyst both start at potential around 0.45 V, suggesting that the CO removal in the two processes undergo the similar bifunctional mechanism. The surfaces of Pt and Pt–W alloys mainly consist of Pt atoms, at which the adsorption of oxygenated species would be more difficult than that at Ru atoms on PtRu surfaces. This should be the reason why the CO stripping on Pt/C and Pt–W/C occurs at more positive potentials than that on PtRu/C. The more negative CO stripping potential on Pt–W alloys than that on Pt/C should be due to the more weakly bonding of CO on the segregated Pt surfaces of the alloys, as a result of the electronic structure change, for example, downshift of the d-band centers [19]. As shown in Figs. 5 and 6, the HOR-CO on Pt/C and Pt–W/C catalysts starts at more negative potentials than that for the CO stripping, which suggests that the CO removal on these catalysts in the H2 /CO-saturated solution do not necessarily require the formation of the oxygenated species. Instead, CO removal may be initialized through the competitive adsorption of H through H2 dissociation as proposed by Watanabe et al. [17,18]. If a surface binds CO more weakly, it would be easier for H species to compete with CO for occupying the catalyst’s surface, so the dissociation of H2 on CO-covered surface would occur at more negative potential. The superior CO-tolerant hydrogen oxidation on Pt–W/C catalysts should proceed through this detoxification mechanism. 4. Conclusion The activities of Pt–W alloy catalysts for HOR are investigated in the presence and absence of a trace level of CO respectively. Nearly 4 times increase in the exchange current density for the HOR are found for Pt–W catalysts as compared with the pure Pt catalyst. Pt–W alloys have superior CO-tolerance for HOR to the PtRu catalyst due to the weakened bonding of CO on their segregated Pt surfaces. Especially, the CO-tolerant hydrogen oxidation on the PtW2 alloy may offer an opportunity for realizing low precious metal anode electrocatalysis in PEMFCs operating with reformed hydrogen. Acknowledgments This work is supported by the Ministry of Science and Technology (Grant Nos. 2012CB932800 and 2012AA110601) and the National Natural Science Foundation of China (Grant No. 20973131).
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Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Pt–W bimetallic alloys as CO-tolerant PEMFC anode catalysts Yu Dai a,b , Yuwen Liu a , Shengli Chen a,∗ a Hubei Electrochemical Power Sources Key Laboratory, Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan 430072, China b Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China
a r t i c l e
i n f o
Article history: Received 20 September 2012 Received in revised form 19 October 2012 Accepted 3 November 2012 Available online 10 November 2012 Keywords: PEM fuel cells Hydrogen oxidation reaction CO tolerant electrocatalyst Pt–W alloys
a b s t r a c t Pt–W nanoalloys with compositions ranging from Pt3 W to PtW2 were explored as electrocatalysts for hydrogen oxidation reaction (HOR). It is shown that alloying Pt with W can lead to significantly enhanced electrocatalytic activity for HOR, with nearly 4 times increase in the exchange current density as compared with pure Pt. What’s more, these Pt–W alloys possess superior CO tolerance to Pt and PtRu, mainly due to the weakened bonding of CO on their Pt-enriched surfaces. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Proton-exchange membrane fuel cells (PEMFCs) hold great promise for low/zero-emission electric vehicles and distributed power stations [1]. Differing from the oxygen reduction reaction (ORR) on the cathode, which has slow kinetics, the hydrogen oxidation reaction (HOR) on the anode of PEMFCs is rather facile on Pt-based catalysts, with overpotential less than 20 mV under typical fuel cell operation conditions. However, Pt as anode electrocatalyst for PEMFCs easily suffers from CO poisoning even when the fuel contains trace amounts of CO, which can lead to significant activity decline [2]. Bleeding air/O2 [3] or using pure instead of reformed H2 [4] in the anode could solve the CO poison problem. However, the resulted system complication may limit the viability of fuel cells. Air bleeding would also cause membrane degradation due to the H2 O2 formation through O2 reduction at anode [3]. A relatively more viable alternative will be using CO tolerant electrocatalysts [5–18]. Pt–Ru alloys are currently recognized as the best anode electrocatalysts for PEMFCs using reformed H2 as fuel [8,9]. The CO tolerance of Pt–Ru catalysts is known to operate through the socalled bifunctional mechanism. On the surface of Pt–Ru alloys, CO adsorbs mainly on the Pt atom sites, while Ru atom sites are prone to absorb oxygenated species from water dissociation at relatively negative potentials. These oxygenated species could oxidize the adsorbed CO at the neighboring Pt atom sites. Currently, the
∗ Corresponding author. E-mail address: [email protected] (S. Chen). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.11.011
CO-tolerance of Pt–Ru alloys remain limited for efficient hydrogen oxidation, so that the anode has to be highly catalyst loaded. This will preclude the practical use of PEMFCs because that Pt and Ru are both precious metals. In addition, the stability of Ru in acidic media is unsatisfactory for PEMFC application [8]. Therefore, durable and precious metal lean electrocatalysts for CO tolerant hydrogen oxidation are highly expected. There have been extensive recent studies on increasing the CO tolerance of Pt or Pt–Ru catalysts by addition of Mo and/or W. Mukerjee et al. reported 2- to 3-folds of activity enhancement over PtRu/C for HOR in H2 containing 100 ppm CO could be achieved by using a catalyst of Pt4 Mo/C [14]. A similar bifunctional mechanism was proposed for the CO-tolerance of this catalyst, in which the oxygenated species formed on Mo oxidatively eliminate the adsorbed CO on the neighboring Pt. It was also reported that CO-tolerance better than the state-of-the-art PtRu catalyst could be achieved by depositing a Pt shell on MoOx . The CO tolerance was attributed to the weakened Pt–CO interaction by the MoOx core [6]. Similarly, Pt nanoparticles supported on tungsten oxide (Pt/WOx ) also showed better CO tolerance than the carbon supported Pt and PtRu catalysts [10,11]. In addition, introduction of Mo or W or their oxides into the Pt–Ru catalysts was found to give much higher current densities than Pt–Ru in fuel cells operated with H2 /CO mixture [12,13]. Recently, Wang et al. reported that Pt/Ti0.7 W0.3 O2 exhibited much lower onset potential for H2 oxidation in the presence of 2% CO relative to Pt/C and PtRu/C [5]. CO-tolerant HOR were also reported on Pt–W alloys [15,16]. For examples, Passalacqua et al. reported that carbon supported Pt–W alloy with a Pt/W atomic ratio of 50:50 exhibited better CO tolerant HOR activity even than the Pt–Ru with the same atomic ratio [16].
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Watanabe et al. have shown that Pt–Fe, Pt–Ni, Pt–Co and PtMo alloys with core–shell structures are also good CO tolerant HOR catalysts. Since that these bimetallic alloy catalysts possessed a Ptenriched surface, the authors argued that the CO tolerant hydrogen oxidation on these catalysts was through a so-called detoxification mechanism rather than the bifunctional mechanism mentioned above. They argued that the underlying transition metals can modify the electronic properties of the surface Pt through the so-called strain and/or ligand effects, which could result in weakened adsorption of CO on Pt [17,18]. The coverage of CO would therefore be much lower than pure Pt in the same conditions. Recently, we found that alloying Pt with W can form a stable Pt-enriched surface even at composition of high W content such as PtW2 , due to the strong surface segregation tendency of Pt in Pt–W alloys [19]. In addition, W can modify the electronic structure of the surface Pt so that it binds oxygenated species more weakly than pure Pt [19]. In the present study, we show that these Pt–W alloys are also superior CO tolerant anode catalysts for PEMFCs due to their much enhanced HOR exchange current densities over pure Pt and even better CO tolerance than PtRu. In addition, it is shown that the CO-tolerance of these Pt surface-enriched alloys is also due to their weak binding to CO. In most of previous reports, the CO-tolerant hydrogen oxidation of Pt–W and/or Pt–WO3 systems has been explained in terms of the bifunctional mechanism [11], strong metal-support interaction [10] or the formation of hydrogen tungsten bronzes [15]. 2. Experimental
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prepared Pt–W/C catalysts (Pt–W/C) of various compositions have relatively uniform size distribution with alloy particle sizes of ∼5 nm (Fig. 1, also see Ref. [19]). The XRD responses indicated that the Pt–W particles possess a Pt fcc structure [19]. For catalysts with compositions from Pt3 W to PtW, no obvious diffraction characteristics for tungsten or its oxides were seen on the obtained XRD patterns. For PtW2 , some diffraction peaks associated with tungsten oxides appeared. The EDS and the ICP-AES data showed that the alloy particles have Pt/W ratios close to the nominal values expected from the amounts of the starting materials [19]. Therefore, we will denote the prepared catalysts with their nominal compositions in the following. 2.2. Electrochemical measurements Electrochemical measurements were performed with a threeelectrode configuration. The working electrodes were the commonly used thin-film rotating-disk-electrode (RDE) made by coating the studying catalysts as a thin film onto a glass carbon (GC) RDE substrate (diameter: 5 mm) with Nafion as the binding agent. In addition to Pt–W/C catalysts, measurements with two reference catalysts, namely, 20 wt% Pt/C and 40 wt% PtRu/C from JohnsonMatthey (JM) were also conducted for comparison. The counter electrode was a Pt foil, and the reference electrode was a saturated calomel electrode (SCE), which was separated from the working electrode by a Luggin capillary. However, all the potentials were expressed on the scale of the reversible hydrogen electrode (RHE) in this paper. The working electrolyte solution was 0.5 M H2 SO4 aqueous solution.
2.1. Materials preparation and characterizations 3. Results and discussion The details for the synthesis of the carbon supported Pt–W alloy catalysts (Pt–W/C) have been given elsewhere [19]. In brief, the required amounts of chloroplatinic acid, tungsten hexacarbonyl and Vulcan XC-72R carbon black were mixed thoroughly in tetrahydrofuran (THF) by ultrasonic stirring and heating. The resulted paste was first treated in a tube furnace at ∼120 ◦ C to reduce the impregnated Pt precursors and then at ∼700 ◦ C to decompose the tungsten hexacarbonyl, after which the sample was cooled down naturally to room temperature to allow Pt segregating to the surface of the catalyst. Such segregation should be thermodynamically favored due to that Pt has much lower surface energy than W. The Pt/W ratio was controlled by adjusting the amounts of the chloroplatinic acid and tungsten hexacarbonyl precursors added in THF. The total metal loading (Pt + W) on carbon was maintained at 20 wt% for various Pt–W/C catalysts. Transmission electron microscopy (TEM) obtained on a JEOL JEM-2010 transmission electron microscope showed that the
3.1. Hydrogen oxidation in the absence of CO Fig. 2 gives the cyclic voltammograms (CVs) for various Pt–W/C and Pt/C (20 wt%, Johnson-Matthey) catalysts in electrolyte solution saturated with inert gas, which show typical Pt features for the underpotential deposition (UPD) adsorption/desorption of hydrogen, indicating the surface enrichment of Pt in these Pt–W/C catalysts even in Pt-lean PtW2 /C catalyst. In fact, the surface enrichment of Pt in these catalysts was also implied by the results of the angular-resolved XPS measurements. By deconvoluting the XPS spectra in binding energy region between 68 eV and 80 eV into components of Pt4f and W5s through Gauss-Lorentzian fitting and correcting the sensitivity of each element, the Pt/W atom ratios in the near surface region were estimated to be 4.2:1, 3.6:1, 4.2:1 and 2.5:1 for the Pt3 W/C, Pt2 W/C, PtW/C and PtW2 /C respectively [19], which are much higher than the corresponding nominal
Fig. 1. (a) Representative TEM images and (b) the corresponding histograms of particle size distribution of the prepared Pt2 W/C.
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(Fig. 3b). Besides, the currents are obviously lower in magnitude than those under the high catalyst loading, especially when high electrode rotation rates are used. For instance, the current obtained at 6400 rpm under the low catalyst loading (Fig. 3b) is only ∼3/4 of that obtained under the high catalyst loading (Fig. 3a). According to the Koutecky–Levich equation, i.e., I −1 = Ik−1 +
Fig. 2. CVs in Ar-saturated 0.5 M H2 SO4 for the prepared Pt–W/C and the JM Pt/C catalysts. Potential scanning rates: 50 mV/s. The total metal loadings: 51 g cm−2 .
compositions. The estimation performed with the W4f spectra in the binding energy region between 31 eV and 40 eV gave very similar results. It is known that HOR has a very facile kinetics which is difficult to measure with the normal RDE method. Recently, we have shown that the HOR kinetics may be studied by using a catalyst poor thinfilm RDE method [20]. The main strategy is to reduce the amount of the catalyst on the catalytically inert GC substrate so that the kinetic current (Ik ) on the RDE is significantly decreased. The measured polarization curves thus would be governed mainly by the reaction kinetics rather than the diffusion of H2 in solution. It was found that reliable extraction of the exchange current density (j0 ) of the HOR may be achieved when the Pt loading on GC RDE substrate is below 2 g Pt cm−2 for Pt/C catalysts [20]. In this study, we will use 1 g cm−2 total metal loading to determine the j0 values for HOR on Pt–W/C catalysts. To verify that the polarization curves given by the thin-film RDE with such a low catalyst loading can be used to extract the kinetics of the HOR, we measured the steady-state polarization curves of the 20 wt% Pt/C catalyst for HOR under a high metal loading (20 g Pt cm−2 ) and a low metal loading (1 g Pt cm−2 ) respectively. The results are given in Fig. 3. It can be seen that the polarization curves under the high catalyst loading exhibit well-defined limiting currents (Fig. 3a). On the polarization curves obtained under the catalyst loading of 1 g Pt cm−2 , however, no well-defined limiting currents are seen. In the potential region where the limiting currents occur under high catalyst loading, the currents show slight increase with the positive going of potential
−1 1/IdL , the RDE current would reach the limiting diffusion current (IdL ) as Ik IdL . Since Ik = AmSjk , in which A, m and S and jk refer to the geometric area of the GC substrate, loading of catalyst, the specific surface area per mass of catalyst and the kinetic current density produced by per surface area of catalyst, the magnitude of Ik will be determined by m and jk for certain catalyst. Considering that the HOR on Pt-based catalysts proceed with either the Tafel reaction (1/2H2 + * ↔ H*) or the Heyrovsky reaction (H2 + * ↔ H* + H+ ) being the rate determining step, we should have jk ∝(1 − ) at relatively positive potentials, where is the surface coverage of the hydrogen ad-atoms. Since that decreases with the positive going of potential, the kinetic current for HOR would increase with the positive going of potential. The measured current (I) thus would increase according to the Koutecky–Levich equation unless Ik is much larger than IdL , which may be satisfied under high catalyst loading (m) and large jk (low ). Thus, the limiting currents on the polarization curves obtained under high catalyst loading should be the limiting diffusion current. As seen in inset of Fig. 3a, these limiting currents give a linear Levich plot going through the origin of coordinates, which is the typical feature of the diffusion limiting current. We found that further increase in the catalyst loading above 20 g Pt cm−2 changed little in the limiting current for certain ω, which further suggests that the limiting currents under high catalyst loadings are purely diffusion controlled. Under the low catalyst loadings, the condition of Ik IdL may not be satisfied. The continuous increase of current with the positive going of potential is thus understandable. This means that the polarization curves under the low catalyst loading are at least partially governed by kinetic currents even in the potential region where the limiting diffusion currents occur under high catalyst loading. In the potential region near the equilibrium potential, the value of Ik should be much lower than those in the limiting current region. In this case, the measured currents could be predominantly governed by Ik , which would allow reliable determination of Ik from the polarization curves. As shown by the inset of Fig. 3b, the currents in the limiting current region under the low catalyst loading give linear Koutecky–Levich plots, in which the reciprocal of the current (I−1 ) at certain potential is plotted against the reciprocal of the square
Fig. 3. Steady-state polarization curves in H2 -saturated 0.5 M H2 SO4 for the JM Pt/C catalysts at different electrode rotation rates under (a) high metal loading (20 g cm−2 ) and (b) low metal loading (1 g cm−2 ) respectively. Potential scanning rate: 5 mV. Inset of (a): The dependence of the limiting currents and the square roots of the electrode rotation rates. Inset of (b): The relationship between reciprocal of currents and the reciprocal of the square roots of the electrode rotation rates.
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Fig. 4. Steady-state polarization curves in H2 -saturated 0.5 M H2 SO4 for the prepared Pt–W/C and the JM Pt/C catalysts. The total metal loadings: 1.0 g cm−2 . Potential scanning rates: 5 mV/s. Inset of: Polarization curves near the equilibrium potential.
root of the electrode rotation rate (ω−1/2 ). Furthermore, the slopes of these Koutecky–Levich plots hardly change with the potential. This means that the polarization curves obtained under such a low catalyst loading remain follow the Koutecky–Levich equation and that the limiting diffusion currents (therefore the diffusion mode) of the RDE are not changed by lowering the catalyst loading. Fig. 4 shows the steady-state polarization curves of the prepared Pt–W/C and JM Pt/C catalysts for HOR under a catalyst loading of 1 g cm−2 (total metal). It can be seen that the electrodes loaded with Pt–W/C catalysts give higher limiting current than the Pt/C under the same metal loading, indicating that the kinetics of HOR are more facile on Pt–W/C catalysts. According to Chen and Kucernak [21], the current–potential relation near the equilibrium potential can be expressed by Eq. (1), in which I0 is the exchange current for HOR on the catalyst-loaded RDE, and F, R and T have their usual meanings. Thus, the exchange current density (j0 ) for HOR at an interested catalyst can be estimated according to slope of the relatively linear current–potential dependence (I ∼ ) near the equilibrium potential (inset of Fig. 4) and the electrochemical active areas of Pt (ESA) given by the catalyst, according to j0 = I0 /ESA. The ESA can be estimated with the UPD H charges in CVs for various catalysts (Fig. 2). The HOR j0 values thus determined are similarly around 22 mA (cm2 Pt)−1 for various Pt–W/C catalysts, which is about 3.5 times higher than that determined for the JM Pt/C catalyst (∼6.0 mA (cm2 Pt)−1 ). When normalized by the Pt mass in the catalysts, the corresponding mass activities for HOR are 4.7, 8.6, 9.3 and 18.2 A (mg Pt)−1 respectively for Pt/C, Pt3 W/C, Pt2 W/C and PtW2 /C. Thus, a nearly 4 times increase in Pt mass activity for HOR would be achieved as PtW2 is used as catalyst to replace Pt in the anode of PEMFCs. 1 2F 1 = · − RT I IdL I0
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Fig. 5. CO stripping curves in Ar-saturated solution for the prepared Pt–W/C, the JM Pt/C and JM PtRu/C catalysts. Scanning rates: 20 mV/s. Total metal loadings: 51 g cm−2 .
holding the potential at 0.05 V for 120 min in H2 /CO-saturated solution. For the sake of simplicity, we will denote the HOR under H2 /CO as HOR-CO in the following. As compared with the Pt/C and PtRu/C, the onset potentials for CO-stripping on Pt–W/C catalysts show a negative shift of ∼50 mV and a positive shift of ∼100 mV respectively (Fig. 5), which indicates that the stripping of the CO adsorbed on the surface of the Pt–W nanoalloys is more facile than that on pure Pt, but more difficult than that on PtRu. In order to compare the CO coverage on the catalyst surface, the charges associated with UPD H desorption and the charges involved in the stripping of the adsorbed CO were evaluated by integrating the corresponding voltammetric peaks. For Pt/C, the charge of CO stripping was found to be about twice of that for UPD H desorption, whereas the charge ratios of CO stripping and UPD H desorption were less than 1.8 for the Pt–W/C catalysts. This seems to suggest that CO does not form a full monolayer on the surface of the Pt–W/C catalysts due to the more weakly boding. When the HOR-CO polarization curves are compared, the Pt–W/C catalysts exhibit HOR onset potentials about 150 mV and 200 mV more negative than that on the Pt–Ru/C and Pt/C respectively, suggesting that the Pt–W alloy catalysts are more CO tolerant for HOR than Pt/C and PtRu/C. The enhanced CO tolerance of Pt–W/C catalysts over Pt/C is in consistence with their more negative CO stripping potentials than Pt/C. The better CO tolerance of Pt–W/C catalysts than PtRu/C, however, contradicts with their more positive CO stripping potentials than PtRu/C. This suggests that the mechanism of CO-tolerant oxidation of H2 on Pt–W alloys differs from that on Pt–Ru alloys.
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3.2. Hydrogen oxidation in the presence of CO To explore the CO-tolerance of the Pt–W/C catalysts for hydrogen oxidation, they are compared with the JM Pt/C and PtRu/C reference catalysts according to the voltammetric curves for COstripping and the HOR polarization curves obtained with H2 gas containing 1000 ppm of CO (H2 /CO). To record the voltammetric curves for CO stripping, the catalysts-loaded electrode was first held at 0.125 V for 30 min in CO-saturated solution, after which the solution was purged with argon for 30 min. The current responses were then recorded by applying a potential sweep of 0.125 V → 0.05 V → 1.275 V → 0.05 V → 1.275 V (Fig. 5). The HOR polarization curves (Fig. 6) are recorded on the anodic sweep after
Fig. 6. Steady-state HOR polarization curves in H2 /CO-saturated solution for the prepared Pt–W/C, the JM Pt/C and JM PtRu/C catalysts. Scanning rates: 5 mV/s. Total metal loadings: 20 g cm−2 .
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As stated in Section 1, the CO removal from Pt–Ru alloy surface is realized mainly through the bifunctional mechanism [8,9,22], in which oxygenated species adsorbed on the highly oxophilic Ru atoms sites oxidize CO adsorbed on the neighboring Pt sites. In H2 /CO-saturated solution, visible hydrogen oxidation should take place as the removal of CO adsorbed at surface Pt atom sites is initialized. It can be seen in Figs. 5 and 6 that the CO stripping and the HOR-CO on the PtRu/C catalyst both start at potential around 0.45 V, suggesting that the CO removal in the two processes undergo the similar bifunctional mechanism. The surfaces of Pt and Pt–W alloys mainly consist of Pt atoms, at which the adsorption of oxygenated species would be more difficult than that at Ru atoms on PtRu surfaces. This should be the reason why the CO stripping on Pt/C and Pt–W/C occurs at more positive potentials than that on PtRu/C. The more negative CO stripping potential on Pt–W alloys than that on Pt/C should be due to the more weakly bonding of CO on the segregated Pt surfaces of the alloys, as a result of the electronic structure change, for example, downshift of the d-band centers [19]. As shown in Figs. 5 and 6, the HOR-CO on Pt/C and Pt–W/C catalysts starts at more negative potentials than that for the CO stripping, which suggests that the CO removal on these catalysts in the H2 /CO-saturated solution do not necessarily require the formation of the oxygenated species. Instead, CO removal may be initialized through the competitive adsorption of H through H2 dissociation as proposed by Watanabe et al. [17,18]. If a surface binds CO more weakly, it would be easier for H species to compete with CO for occupying the catalyst’s surface, so the dissociation of H2 on CO-covered surface would occur at more negative potential. The superior CO-tolerant hydrogen oxidation on Pt–W/C catalysts should proceed through this detoxification mechanism. 4. Conclusion The activities of Pt–W alloy catalysts for HOR are investigated in the presence and absence of a trace level of CO respectively. Nearly 4 times increase in the exchange current density for the HOR are found for Pt–W catalysts as compared with the pure Pt catalyst. Pt–W alloys have superior CO-tolerance for HOR to the PtRu catalyst due to the weakened bonding of CO on their segregated Pt surfaces. Especially, the CO-tolerant hydrogen oxidation on the PtW2 alloy may offer an opportunity for realizing low precious metal anode electrocatalysis in PEMFCs operating with reformed hydrogen. Acknowledgments This work is supported by the Ministry of Science and Technology (Grant Nos. 2012CB932800 and 2012AA110601) and the National Natural Science Foundation of China (Grant No. 20973131).
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