Testing of carbon supported Pd–Pt electrocatalysts for methanol electrooxidation in direct methanol fuel cells

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 4 3 2 e4 4 3 9

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Testing of carbon supported PdePt electrocatalysts for methanol electrooxidation in direct methanol fuel cells Francisco Alcaide a,*, Garbin˜e A´lvarez a, Pere L. Cabot b, Hans-Ju¨rgen Grande a, Oscar Miguel a, Amaia Querejeta a Dpto. de Energı´a, CIDETEC-IK4, P
Miramo´n, 196, 20009 San Sebastia´n, Spain Laboratori d’Electroquı´mica de Materials i del Medi Ambient, Dept. Quı´mica Fı´sica, Universitat de Barcelona, Martı´ i Franque`s, 1-11, 08028 Barcelona, Spain a

b

article info

abstract

Article history:

In the present work, a detailed characterization of the electrochemical behavior of carbon

Received 2 November 2010

supported PdePt electrocatalysts toward CO and methanol electrooxidation in direct

Received in revised form

methanol fuel cells is reported. Technical electrodes containing an ionomer in their cata-

31 December 2010

lyst layer were prepared for this purpose. CO and methanol electrooxidation reactions were

Accepted 5 January 2011

used as test reactions to compare the electrocatalytic behavior of bimetallic supported

Available online 4 February 2011

nanoparticles in acidic liquid electrolyte and in solid polymer electrolyte (real fuel cell operating conditions). Experimental results in both environments are consistent and show

Keywords:

that the electrochemical behavior of carbon supported PdePt depends on their composi-

PdePt

tion, giving the best performance in direct methanol single fuel cell with a Pd:Pt atomic

Methanol electrooxidation

ratio of 25:75 in the catalyst.

CO tolerance

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Direct methanol fuel cell

1.

Introduction

The use of direct methanol fuel cells (DMFCs) as auxiliary power source for portable applications has been recently increased [1]. Methanol is easy handled, distributed, and stored. DMFCs are low-temperature devices with simple design, they do not need reformer or humidification, refueling is easy and fast, having high energy density compared to pressurized hydrogen (w6.1 vs. 1.0 kWh kg1, respectively). They find applications in leisure (mobile homes, cabins, boats), traffic regulation, security, and remote sensors, between others. However, some issues concerning the performance of the anode electrocatalyst, cost and system durability persist and must be solved to guarantee their commercialization on a large-scale [2].

reserved.

The activity of the anode electrocatalyst at temperatures below 60
C should be improved to gain in the fuel cell performance. It has been recognized that platinum shows the highest activity among single metals for methanol electrooxidation in acidic medium [3], but it is readily poisoned by the CO intermediates formed in this reaction [4]. Furthermore, the high cost and low availability of Pt limits its use in DMFCs [5]. The main strategies to enhance the catalyst activity are dispersing Pt on high surface area carbonaceous materials, the optimization of the catalyst layer structure in technical electrodes (the practical electrodes used in fuel cells), used in membrane electrode assemblies (MEAs), and the use of Ptbased alloys [6]. In this sense, PteRu alloys are nowadays the catalysts of choice for methanol oxidation [7], because of their high tolerance for CO poisoning, which is generally explained

* Corresponding author. Tel.: þ34 943 309 022; fax: þ34 943 309 136. E-mail address: [email protected] (F. Alcaide). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.01.015

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by a bifunctional mechanism, in which CO is oxidized by the oxygenated species formed on the Ru surface atoms [8]. However, there is much discussion about the effect of the Ru content in the literature, but in general the optimum Ru content in PteRu alloys for anodic methanol oxidation varies from 10% at room temperature to 50% at 60
C [9]. Furthermore, other systems than PteRu have been investigated for their suitability as methanol electrooxidation catalysts, including PteRh [10e12], PteSn [13e16], PteMo [17e19], PdeAg [20], PdeNi [21e23] and PtePd [24e28]. Good results have been reported for PtePd under some conditions, due to the synergistic effect between Pt and Pd, which has been explained by an electronic effect of Pd on Pt or the improvement of the reaction rate by decreasing the electrode poisoning. The use of palladium in fuel cell catalysts [29] is interesting, because it is more widespread in the Earth crust than platinum (abundance of 1.5  102 vs. 5  103 parts per million by mass, respectively [30]), and it is less expensive than Pt. Replacing Pt by Pd, if the electrocatalysis demands are satisfied, would then decrease the cost of the electrodes [31,32]. In addition, it is very stable in the acidic fuel cell environment [33] and it also exhibits interesting electrocatalytic properties for CO electrooxidation [34e37]. These properties have lead to consider Pd as a possible catalyst for those reactions in which CO appears as a poisoning intermediate, such as in the methanol or, more recently, in the ethanol [24,38e40], and formic acid [41e44] electrooxidation. The electrochemical properties of PtePd unsupported nanoparticles have been widely studied in terms of CO tolerance, methanol and formic acid oxidation [45e47]. However, a detailed study of the electrochemical behavior of carbon supported PtePd nanoparticles in a real DMFC environment has not been yet performed. The aim of this work is to study the electrochemical behavior of carbon supported PdePt electrocatalysts in the complete range of Pd content to approach the conditions for the best operation, which has not been previously done in such a wide range of compositions. A comparison between measurements in liquid and solid polymer electrolyte was made to experimentally confirm whether the conclusions achieved in previous studies using three-electrode cells could be extrapolated to real fuel cell operating conditions. In this sense, methanol oxidation has been selected as the probe reaction. Thus, technical electrodes, made of a diffusion layer and a catalysts layer, were prepared, and CO stripping and methanol electrooxidation experiments were performed in three-electrode cell and in single DMFC under practical conditions.

2.

Experimental

2.1.

Catalysts

The catalysts used in this work were commercial Pd, Pd75ePt25, Pd50ePt50, Pd25ePt75 (where subscripts indicate atomic percent, a/o), and Pt, all supported at 20 wt. % on carbon black Vulcan XC-72, from Premetek Co. They were characterized by X-Ray diffraction (XRD). The XRD diffractograms were obtained by a Bruker D8 Advance diffractometer

˚ ) and 2q scan from operating with Cu Ka radiation (l ¼ 1.5406 A 30 to 100
(at 0.02
min1). Diffraction peaks were assigned according to the International Center for Diffraction Data (ICDD) cards in PDF-2 database. The XRD data were used to determine the lattice parameter (from the interplanar distances), by refining the unit cell dimensions using WinPLOTR software, and the average crystallite size (from Scherrer equation) [48]. The instrumental and the microstrain broadening were accounted for in the calculation of the crystallite sizes by the given software, which allowed importing directly the experimental diffractograms without the need of the introduction of the parameters corresponding to the XRD instrumentation. The mass ratio of metal to carbon in the electrocatalysts and Pd:Pt atomic ratios were analyzed by the energy dispersive X-ray (EDX) technique. The EDX measurements were performed by means of an INCA-300 energy analyzer coupled to a JSM5910-LV JEOL scanning electron microscope (SEM). The compositions detailed in Table 1 show the average of five different measurements on the same sample, with relative error less than 1%.

2.2. MEAs

Preparation and testing of technical electrodes and

Porous diffusion electrodes for three-electrode cell experiments were prepared by the spraying method [49]. The diffusion layers were LT1400W ELAT GDL (E-TEK, Inc.). For each electrode, appropriate amounts of the supported catalyst and 5 wt. % Nafion ionomer dispersion (Aldrich) were ultrasonically dispersed in a mixture of isopropanol (Acros Organics, pur.) and ultrapure water (k  0.054 mS cm1, Millipore). Then, the ink was sprayed onto the diffusion layer by an air-brush gun fed with pure nitrogen. The metal loading in the electrode was 0.20 mg cm2 and the Nafion content was 20 wt. % (dry weight). Square shape MEAs (active geometric area of 16 cm2) were “catalyst coated membranes”, that is, the catalyst layers were sprayed directly onto both sides of a Nafion NR212 membrane. The composition of the anode catalyst layer was the same as the porous electrodes for the three-electrode cell explained above. The oxygen cathode catalyst layer contained 0.2 mgPt cm2 and a 30 wt. % (dry weight) of Nafion. Finally, gas diffusion layers ELAT GDL (E-TEK, Inc.) were placed beside the cathode and the anode electrodes, respectively. Electrochemical measurements in a conventional thermostatted three-electrode cell were performed with a

Table 1 e Composition from EDX and physical parameters from XRD analysis of PdLPt catalysts. Catalyst

Pd/C Pd75Pt25/C Pd50Pt50/C Pd25Pt75/C Pt/C

Metal loading (wt. %) 18.9 19.3 20.0 20.0 19.1

Pd:Pta (a/o) 100 73:27 44:56 30:70 100

Crystallite size (nm) 3.4  5.6  4.1  2.3  2.1 

0.31 0.43 0.61 0.57 0.24

Lattice parameter (nm) 0.3881  0.3902  0.3914  0.3902  0.3930 

a Pd:Pt atomic ratio determined by EDX measurements.

0.0015 0.0008 0.0003 0.0008 0.0019

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PGSTAT 30 (Eco Chemie) potentiostat driven by the GPES software. The working electrode was located inside a cylindrical holder, which had an inlet and an outlet that allowed the circulation of gases. The geometrical area exposed to the solution was 0.79 cm2. The counter electrode was a platinum wire, which was introduced in a compartment separated from the main solution by a glass frit. The reference electrode was a reversible hydrogen electrode, RHE (Gaskatel, GmbH) immersed in a glass tube connected with the main compartment of the cell by a LuggineHaber capillary. The electrolyte was 0.5 mol dm3 H2SO4 (Merck, Suprapur) aqueous solution deaerated by N2 bubbling for 20 min. Cyclic voltammograms were recorded under inert N2 atmosphere at 25.0
C and 20 mV s1 up to obtain reproducible voltammograms. CO voltammetric stripping experiments were performed at 25.0
C feeding the working electrode with a mixture of 0.1% CO in N2 at 0.250 cm3 min1 and atmospheric pressure for 50 min, while holding the electrode potential at 0.100 V vs. RHE. After adsorption and still holding the admission potential, CO was removed from the solution by N2 bubbling and the electrode gas feed was switched to N2 for 20 min. Then, two consecutive CO stripping voltammograms were recorded in the positive direction at 20 mV s1. Single cell experiments were performed placing a MEA in a commercial fuel cell hardware FCT 16SCH (Fuel Cell Technologies, Inc.), which was assembled using a uniform torque of 5 Nm. The MEA performance was evaluated by measuring the currentevoltage curves (potentiostatic polarization plots in steady-state conditions), using a home-made DMFC test station. Polarization curves were recorded after the MEA was activated following a procedure based on that reported in ref. [50]. Deionized water at 60
C was circulated for 12 h through the anodic and cathodic compartments of the single cell, each time that a fresh MEA was installed into the single cell hardware. Afterward, the cell temperature was set at 60
C. The anode feed was 2.0 mol dm3 aqueous solution of methanol at 1.5 mL min1 and zero backpressure, preheated at the cell temperature. The cathode feed was dry O2 (99.999%, Praxair) at 100 standard cm3 min1 and zero backpressure. Furthermore, the electrochemical characterization of the catalysts was performed in the MEA. In cyclic voltammetry measurements, the working electrode was the anode fed with humidified nitrogen at 50 standard cm3 min1, and the counter electrode and the reference electrode (DHE) was the cathode fed with humidified hydrogen at 50 standard cm3 min1 [51]. Cyclic voltammograms were recorded at a scan rate of 20 mV s1 and 60.0
C. CO stripping experiments were performed feeding the working electrode with a mixture of 0.1 vol. % CO in N2 at 250 standard cm3 min1 at atmospheric pressure for 50 min, while holding the electrode potential at 0.100 V vs. RHE. After adsorption and still holding the admission potential, the electrode gas feed was switched to N2 for 20 min at 250 standard cm3 min1. Then, two consecutive CO stripping voltammograms were recorded in the positive direction at 20 mV s1. The methanol oxidation activity was also measured in the MEA, feeding the working electrodes a with 2.0 mol dm3 aqueous methanol solution at 1.5 mL min1, and the reference/counter electrode with humidified hydrogen at 50 standard cm3 min1.

3.

Results and discussion

3.1. Structural and electrochemical characterization of PdePt catalysts Fig. 1 depicts the XRD diffractograms of the Pd/C, Pd75ePt25/C, Pd50ePt50/C, Pd25ePt75/C and Pt/C catalysts. Typical peaks characteristic of the face-centered cubic (FCC) lattice structure of Pd and Pt are also indicated in this figure. Although the atomic size of Pd and Pt is similar and their crystal structures are the same, explaining their similar diffraction peaks, the slight shift of the PdePt characteristic diffraction peaks toward lower 2q values when compared to those of Pd strongly indicates the incorporation of Pt and Pd atoms in the same lattice. Table 1 shows the lattice parameter of the PdePt catalysts as well as their mean crystallite sizes, determined as the mean values obtained for each reflection (the overlap f (100) and (200) reflections were deconvoluted with the help of the software). The values corresponding to Pd/C and Pt/C have been included for comparison. The lattice parameters of PdePt/C catalysts are higher than that for Pd/C, but lower than de value for Pt/C, which is indicative of alloy formation. Furthermore, the mean crystallite size of PdePt carbon supported catalysts increases with the Pd content. Both results are in agreement with those previously reported in the literature [29]. It is well known that the PtePd system shows complete solid solubility based on a FCC lattice. It has been recently demonstrated that this also applies to nanocrystalline materials and that Vegard’s rule applies [52]. A careful examination of the data presented in Table 1, including plotting the lattice parameter vs. at.% Pt, shows significant deviations from Vegard’s rule, particularly for the 75 at.% Pt sample, which could be attributed to differences in crystallite size in the nanosize range, which is known to affect the lattice parameter. The electrochemical characterization of the catalysts studied in this work was performed by cyclic voltammetry at 25
C in 0.5 M H2SO4 to simulate the proton exchange membrane electrolyte in the MEA. Reliable results were obtained after activation of the working electrodes by means of cyclic scans between 0.050 and 0.750 V at 0.100 V s1 until a stationary profile was obtained. Fig. 2 presents the steady

Fig. 1 e X-ray diffractograms for Pd/C, PdePt/C and Pt/C catalysts (in parenthesis planes corresponding to characteristic diffraction signals of FCC Pd).

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cyclic voltammograms at 20 mV s1, when the technical electrodes were prepared with Pd/C, PdePt/C, and Pt/C. The cyclic voltammogram corresponding to the Pd/C catalyst, curve a in Fig. 2, is similar to those reported in the literature in acidic electrolyte for Pd dispersed on graphite [53] and for unsupported Pd nanoparticles [54]. The two peaks observed before 0.250 V correspond to adsorption/desorption of hydrogen [55]. When going toward more positive potentials, the oxide region appears between 0.700 and 0.800 V. The shape of the cyclic voltammogram of the Pt/C catalyst, curve e in Fig. 2, displays the typical features found for smooth polycrystalline platinum in liquid electrolyte [56], slightly modified because of the contribution of the carbon support and the ionomer present in the catalyst layer. It can be seen that the adsorption/desorption hydrogen peaks were slightly less well resolved than those observed on polycrystalline Pt. There is also some contribution of the carbon support, as suggested by the slight curvature in the double layer region. As in the case

of Pd/C catalyst, a dominant hydrous oxide peak was observed. Finally, for the PdePt/C catalysts, curves b, c and d in Fig. 2, the hydrogen desorption only appears as a broad peak in the positive scan, in good agreement with the literature [37]. The oxide formation and reduction takes place at about the same potentials as for the Pt/C catalyst (ca. 0.700e0.800 V). Furthermore, CO stripping voltammograms were registered to gain more understanding about the CO electrooxidation characteristics on the catalysts. Fig. 3 shows the corresponding stripping voltammograms on Pd/C, PdePt/C, and Pt/C. All stripping voltammograms show a main CO oxidation peak, which moves toward less positive potentials as Pd is replaced by Pt in the catalysts. This indicates the easier oxidation of adsorbed CO on Pt/C when compared to Pd/ C. It is worth to note that each PdePt/C catalyst only originates one CO oxidation peak, showing the synergistic effect of both metals, as previously pointed out by Papageorgopoulos et al. [37]. The intermediate position of these peaks between those corresponding to Pd/C and Pt/C suggests an electronic modification affecting the CO adsorption characteristics on PdePt when compared to pure Pd and Pt metals [57]. This suggestion is supported by the fact that in the cyclic voltammograms of Fig. 2, the oxide formation occurs at about the same potential when all the catalysts are compared. Therefore, an easier CO oxidation on PdePt catalyst owing to the formation of oxides at lower potentials than those observed for pure Pt can be discarded. In addition, a close inspection of the CO oxidation peak potentials of PdePt/C catalysts reveals that the electrochemical CO adsorption behavior depends clearly on their bulk composition. This is shown in Fig. 4, in which one can see a linear dependence between the CO oxidation peak and the Pd content. This observation suggests that the surface and bulk composition of PdePt supported catalysts changes in a similar way. A similar result was previously reported for unsupported PdePt nanoparticles in ref. [47]. Note in addition the different cathodic reduction peaks of the oxides formed at the end of the anodic scan. The cathodic peaks are shifted to more negative values and wider when the Pd content decreases. In particular, the cathodic currents for Pt/C in the potential region of 0.4e0.6 V in Fig. 3 are quite high but not very different from that shown in Fig. 2e. This can be

Fig. 2 e Cyclic voltammograms for electrodes catalyzed with (a) Pd/C, (b) Pd50ePt50/C (c) Pd75ePt25/C, (d) Pd25ePt75/C, and (e) and Pt/C in 0.5 mol dmL3 H2SO4. T [ 25.0 ± 0.1
C. Scan rate of 20 mV sL1.

Fig. 3 e CO stripping cyclic voltammograms for Pd/C (B), Pd75ePt25/C (,), Pd50ePt50/C (>), Pd25ePt75/C (D), and Pt/C (3) catalysts in 0.5 mol dmL3 H2SO4. T [ 25.0 ± 0.1
C. Scan rate of 20 mV sL1.

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Fig. 4 e CO oxidation peak potential values vs. Pd content in catalysts (experiments performed using aqueous H2SO4 as electrolyte).

due to the higher anodic limit in Fig. 3, which allows a greater production of Pt oxides.

3.2. Performance of Pd/C, PdePt/C and Pt/C catalyzed anodes in MEAs To compare the electrochemical behavior of Pd/C, PdePt/C and Pt/C in three-electrode cell with that observed in real fuel cell operating conditions, we prepared MEAs containing these catalysts in their anodes and Pt/C in their cathodes. As the main object of this work is such a comparison, the metal loadings of the electrodes were lower than those usually used in DMFC, and therefore, not so high performances are expected. Fig. 5 shows the effect of the type of catalyst on CO electrooxidation in the MEAs. These experiments were performed at 60
C, because those given at room temperature have a limited validity regarding the catalyst behavior under practical conditions. It can be seen that CO stripping voltammograms follow the same trend observed in the three-electrode cell with liquid electrolyte

Fig. 5 e In situ CO stripping cyclic voltammograms for Pd/C (B), Pd75ePt25/C (,), Pd50ePt50/C (>), Pd25ePt75/C (D) and Pt/C (3) catalysts, after the adsorption of CO at 0.100 V vs. RHE. Working electrodes fed with N2; reference/counter electrode fed with H2. Both gases humidified by bubbling through water at the cell temperature of 60 ± 1
C. Only the positive scan at 20 mV sL1 is shown. MEA made of Nafion NR212.

(see Fig. 3): the CO peak is shifted toward more negative potentials when decreasing the amount of Pd in the catalyst. In addition, a linear variation of the CO oxidation peak potential with the Pd content is also found, as illustrated in Fig. 6, in good agreement with the results obtained in the three-electrode cell (Fig. 4). So in spite of the different conditions, the results of both types of experiments are consistent. Fig. 7 shows the linear sweep voltammograms of the anodes made of PdePt/C and Pt/C in 2.0 mol dm3 methanol at 60
C. The plotted current densities are referred to the real surface area for each working electrode, which was measured by determining the CO stripping charge and assuming that 420 mC cm2 corresponds to a monolayer of adsorbed CO [46,58,59]. As can be seen, the general shape of the curve is similar for all PdePt/C catalysts. The current density slowly increases with the applied potential. The best activity toward methanol electrooxidation is shown by the electrode catalyzed with Pd25ePt75/C. At 0.600 V, potential which gives an acceptable cell voltage in a DMFC, the currents decrease following the order: Pd25ePt75/C > Pd50ePt50/C z Pt/ C > Pd75ePt25/C, i.e. there is a maximum value of the catalyst activity for Pd contents around 25 at.%. On the other hand, it is shown that the electrocatalytic activity of pure Pd for methanol oxidation in acidic media in the low anodic potential region, when compared to Pt, is insignificant (see Fig. 5), in agreement with previous results in the literature [60]. The increase of the catalytic activity when Pd is added to Pt and its decrease when the Pd content is further increased can be explained considering two different reaction mechanisms for methanol oxidation on Pt and PdePt, in which different adsorbed intermediates are involved, as previously suggested [29]. According to this hypothesis, Platinum nanoparticles become covered by intermediates produced from methanol dehydrogenation, such as (:COH)ads, which require three neighboring platinum sites to be adsorbed. Thus, the Pt surface becomes slowly covered with this poison, according to reaction (1) of the following sequence:

Pt þ CH3OH / Pte(COH)ads þ 3Hþ þ 3e

(1)

Pte(COH)ads þ H2O / Pt þ CO2 þ 3Hþ þ 3e

(2)

Fig. 6 e In situ CO oxidation peak potential values vs. Pd content in catalysts (experiments carried out using Nafion NR212 proton exchange membrane as electrolyte).

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Fig. 7 e In situ j-E curves corresponding to the methanol electrooxidation on electrodes catalyzed with: Pd25ePt75/C (,), Pd50ePt50/C (>), Pd75ePt25/C (D), and Pt/C (B) at 60
C. Working electrodes fed with 2.0 mol dmL3 MeOH at 1.5 mL minL1. Reference/counter electrode fed with humidified H2 at the cell temperature. Scan rate of 1 mV sL1. MEA made of Nafion NR212.

The adsorbed intermediate is removed in the further oxidation, reaction (2), which appears to be slow to explain the accumulation of the poisoning adsorbed species. Nevertheless, when adding Pd to form PdePt nanoparticles, the presence of palladium has a diluting effect and limits the presence of three adjacent Pt sites that are necessary for the adsorption of the poisoning species. In this case, the presence of only two neighboring platinum sites would be more common, the methanol adsorption on two Pt sites probably favoring the adsorption of (:CO)ads species, as indicated by reaction (3) of the following sequence:

Pt þ CH3OH / Pte(CO)ads þ 4Hþ þ 4e

(3)

Pte(CO)ads þ H2O / Pt þ CO2 þ 2Hþ þ 2e

(4)

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To further remove the now different poisoning adsorbed species, reaction (4) should take place. If we assume that the rate of reaction (4) is higher than that of reaction (2), the poisoning effect of the (CO)ads adsorbed species should be smaller than that of (COH)ads ones. As this is the result of the Pd addition, the catalyst should have better electroactivity than pure Pt, as depicted in Fig. 7 for a Pd content of 25 at.%. However, when the Pd content increases, the amount of couples of neighboring Pt sites becomes smaller, with the concomitant decrease of the catalytic activity, and when it is too high, it further decreases due to the insignificant catalytic activity of pure Pd. Fig. 8 shows the voltage-current density, V-j, and the power density-current density, P-j, characteristics obtained in the single DMFC operating with a 2 mol dm3 aqueous methanol at 60
C and atmospheric pressure. The cathode was fed with pure O2 at atmospheric pressure, to maximize its activity and to minimize cathode effects on the relative activities of the cells. It is apparent that there are differences in performance for the different catalysts. In fact, the best performance was obtained when the anode was made of Pd25ePt75/C. Furthermore, the behavior of the catalysts in the single DMFC reflects the results of anode polarization experiments found in MEAs. Therefore, the results obtained in single cell experiments are consistent with those published carried out in electrochemical cells. This could be tentatively extended to previous results in the literature obtained in three-electrode cells to predict a good approach when applied to real fuel cells [27e29].

4.

Conclusion

In this work we have reported a detailed characterization of the electrochemical behavior of PdePt/C catalysts toward methanol electrooxidation in technical electrodes. The results obtained in three-electrode cell with liquid electrolyte and in single cell with solid electrolyte (real fuel cell operating conditions) are consistent, showing that, between the specimens studied, the electrocatalytic oxidation of methanol is more effective in the case of the bimetallic system Pd25ePt75/ C. The electrochemical measurements in three-electrode cell using technical electrodes can then be applied as a simple tool for characterizing the activity of the electrocatalysts. The positive electrocatalytic effect of small amounts of Pd can be explained by a decrease in the poisoning rate of Pt by CO species resulting from the electronic effect of Pd on Pt.

Acknowledgments The authors thank the Spanish Government MCINN and FEDER for financial support (MAT2008-06631-C03-03/MAT).

Fig. 8 e V-j and P-j curves for liquid-DMFCs with Pd25ePt75/ C (,), Pd50ePt50/C (>), Pd75ePt25/C (D), and Pt/C (B) catalyzed anodes fed with 2.0 mol dmL3 MeOH at 1.5 mL minL1. Cathode: 0.2 mgPt cmL2 of Pt/Vulcan catalyst fed with dry O2 at 50 mL minL1. Cell temperature of 60
C.

references

[1] Arico` AS, Baglio V, Antonucci V. Direct methanol fuel cells: history, status and perspectives. In: Liu H, Zhang J, editors. Electrocatalysis of direct methanol fuel cells. Weinheim: Wiley-WCH; 2009. p. 1e78.

4438

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 4 3 2 e4 4 3 9

[2] Bagotsky VS. Fuel cells: problems and solutions. Hoboken: John Wiley & Sons Inc; 2009. [3] Inada R, Shimazu K, Kita H. Analysis of time-dependent kinetics for the oxidation of methanol on a platinum electrode and reaction mechanism. J Electroanal Chem 1990; 277:315e26. [4] Iwasita T. Electrocatalysis of methanol oxidation. Electrochim Acta 2002;47:3663e74. [5] PGM prices. http://www.platinum.matthey.com/cgibin/ dynamic.pl?template¼historical [accessed July 2010]. ´ lvarez G, Alcaide F, Miguel O, Calvillo L, La´zaro MJ, [6] A Quintana JJ, et al. Technical electrodes catalyzed with PtRu on mesoporous ordered carbons for liquid direct methanol fuel cells. J Solid State Electrochem 2010;14:127e34. [7] Petrii OA. PteRu electrocatalysts for fuel cells: a representative review. J Solid State Electrochem 2008;12:609e42. [8] Ruth K, Vogt M, Zuber R. In: Vielstich W, Gasteiger HA, Lamm A, editors. Handbook of fuel cells e fundamentals, technology and applications, vol. 3. Chichester (UK): John Wiley & Sons, Ltd; 2003. [9] Lizcano-Valbuena WH, Caldas de Azevedo D, Gonzalez ER. Supported metal nanoparticles as electrocatalysts for lowtemperature fuel cells. Electrochim Acta 2004;49:1289.  ski A. Electro-oxidation [10] Siwek H, Tokarz W, Piela P, Czerwin of methanol on Pt-Rh alloys. J Power Sources 2008;181:24e30. [11] Kim YS, Nam SH, Shim H-S, Ahn H-J, Anand M, Kim WB. Electrospun bimetallic nanowires of PtRh and PtRu with compositional variation form methanol electrooxidation. Electrochem Commun 2008;10:1016e9. [12] Choi J-H, Park K-W, Park I-S, Nam W-H, Sung Y-E. Methanol electro-oxidation and direct methanol fuel cell using Pt/Rh and Pt/Ru/Rh alloy catalysts. Electrochim Acta 2004;50: 787e90. [13] Neto AO, Dias RR, Tusi MM, Linardi M, Spinace´ EV. Electrooxidation of methanol and ethanol using PtRu/C, PtSn/C and PtSnRu/C electrocatalysts prepared by na alcohol-reduction process. J Power Sources 2007;166:87e91. [14] Arico` AS, Antonucci V, Giordano N, Shukla AK, Ravikumar MK, Roy A, et al. Methanol oxidation on carbonsupported platinum-tin electrodes in sulfuric acid. J Power Sources 1994;50:295e309. [15] Hana DM, Guo ZP, Zeng R, Kim CJ, Meng YZ, Liub HK. Multiwalled carbon nanotube-supported Pt/Sn and Pt/Sn/ PMo12 electrocatalysts for methanol electro-oxidation. Int J Hydrogen Energy 2009;34:2426e34. [16] Colmati F, Antolini E, Gonzalez ER. PteSn/C electrocatalysts for methanol oxidation synthesized by reduction with formic acid. Electrochim Acta 2005;50:5496e503. [17] Martı´nez S, Martins ME, Zinola CF. Surface metal modifiers for methanol electrooxidation on platinum, molybdenum and tungsten. Int J Hydrogen Energy 2010;35:5343e55.  NR, Babic  BM, Radmilovic  VR, Gojkovic  SLJ, [18] Elezovic  NV, Vrac ar LJM. Pt/C doped by MoOx as the Krstajic electrocatalyst for oxygen reduction and methanol oxidation. J Power Sources 2008;175:250e5. [19] Ordo´n˜ez LC, Roquero P, Sebastian PJ, Ramı´rez J. Carbonsupported platinum-molybdenum electro-catalysts for methanol oxidation. Catal Today 2005;107e108:46e52. [20] Wang Y, Sheng ZM, Yang H, Jiang SP, Li CM. Electrocatalysis of carbon black e or activated carbon nanotubes-supported Pd-Ag towards methanol oxidation in alkaline media. Int J Hydrogen Energy 2010;35:10087e93. [21] Zhao Yang YX, Tian J, Wang F, Zhan L. Methanol electrooxidation on Ni@Pd core-shell nanoparticles supported on multi-walled carbon nanotubes in alkaline media. Int J Hydrogen Energy 2010;35:3249e57. [22] Singh RN, Singh A, Anindita. Electrocatalytic activity of binary and ternary composite films of Pd, MWCNT and Ni,

[23]

[24]

[25]

[26] [27]

[28]

[29] [30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

part II: methanol electrooxidation in 1 M KOH. Int J Hydrogen Energy 2009;34:2052e7. Wang M, Liu W, Huang C. Investigation of PdNiO/C catalyst for methanol electrooxidation. Int J Hydrogen Energy 2009; 34:2758e64. Kadirgan F, Beyhan S, Atilan T. Preparation and characterization of nano-sized PtePd/C catalysts and comparison of their electro-activity toward methanol and ethanol oxidation. Int J Hydrogen Energy 2009;34:4312e20. Wang H, Xu C, Cheng F, Zhang M, Wang S, Jiang SP. Pd/Pt coreeshell nanowire arrays as highly effective electrocatalysts for methanol electrooxidation in direct methanol fuel cells. Electrochem Commun 2008;10:1575e8. Xu Y, Lin X. Facile fabrication and electrocatalytic activity of Pt0.9Pd0.1 alloy film catalysts. J Power Sources 2007;170:13e9. Selvaraj V, Alagar M, Hamerton I. Electrocatalytic properties of monometallic and bimetallic nanoparticles-incorporated polypyrrole films for electro-oxidation of methanol. J Power Sources 2006;160:940e8. Kardigan F, Beden B, Le´ger JM, Lamy C. Synergistic effect in the electrocatalytic oxidation of methanol on platinum þ palladium electrodes. J Electroanal Chem 1981; 125:89e103. Antolini E. Palladium in fuel cell catalysis. Energy Environ Sci 2009;2:915e31. Section 14, geophysics, astronomy, and acoustics; abundance of elements in the earth’s crust and in the sea. In: Lide DR, editor. CRC Handbook of chemistry and physics. 85th ed. Boca Raton, Florida: CRC Press; 2005. Grigoriev SA, Lyutikova EK, Martemianov S, Fateev VN. On the possibility of replacement of Pt by Pd in a hydrogen electrode of PEM fuel cells. Int J Hydrogen Energy 2007;32: 4438e42. ´ lvarez G, Cabot PL, Miguel O, Querejeta A. Alcaide F, A Performance of carbon-supported PtPd as catalyst for hydrogen oxidation in the anodes of proton exchange membrane fuel cells. Int J Hydrogen Energy 2010;35: 11634e41.  M, qukaszewski M, Jerkiewicz G, Czerwin  ski A. Grden Electrochemical behaviour of palladium electrode: oxidation, electrodissolution and ionic adsorption. Electrochim Acta 2008;53:7583e98. Kadirgan F, Kannan AM, Atilan T, Beyhan S, Ozenler SS, Suzer S, et al. Carbon supported nano-sized Pt-Pd and Pt-Co electrocatalysts for proton exchange membrane fuel cells. Int J Hydrogen Energy 2009;34:9450e60. Garcia AC, Paganin VA, Ticianelli EA. CO tolerance of PdPt/C and PdPtRu/C anodes for PEMFC. Electrochim Acta 2008;53: 4309e15. Zhao M, Rice C, Masel RI, Waszczuk P, Wieckowski A. Kinetic study of electro-oxidation of formic acid on spontaneouslydeposited Pt/Pd nanoparticles. CO tolerant fuel cell chemistry. J Electrochem Soc 2004;151:A131e6. Papageorgopoulos DC, Keijzer M, Veldhuis JBJ, de Bruijn FA. CO tolerance of Pd-rich platinum palladium carbonsupported electrocatalysts. J Electrochem Soc 2002;149: A1400e4. Qin Y-H, Yang H-H, Zhang X-S, Li P, Ma C-A. Effect of carbon nanofibers microstructure on electrocatalytic activities of Pd electrocatalysts for ethanol oxidation in alkaline medium. Int J Hydrogen Energy 2010;35:7667e74. Yan Z, He G, Zhang G, Meng H, Shen PK. Pd nanoparticles supported on ultrahigh surface area honeycomb-like carbon for alcohol electrooxidation. Int J Hydrogen Energy 2010;35: 3263e9. Xu JB, Zhao TS, Shen SY, Li YS. Stabilization of the palladium electrocatalyst with alloyed gold for ethanol oxidation. Int J Hydrogen Energy 2010;35:6490e500.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 4 3 2 e4 4 3 9

[41] Wu YN, Liao SJ, Su YL, Zeng JF, Dang D. Enhancement of anodic oxidation of formic acid on palladium decorated Pt/C catalyst. J Power Sources 2010;195:6459e62. [42] Tang Y, Chen Y, Zhou P, Zhou Y, Lu L, Bao J, et al. Electrocatalytic performance of PdCo bimetallic hollow nanospheres for the oxidation of formic acid. J Solid State Electrochem 2010;14:2077e82. [43] Li RS, Hao H, Cai WB, Huang T, Yu AS. Preparation of carbon supported Pd-Pb hollow nanospheres and their electrocatalytic activities for formic acid oxidation. Electrochem Commun 2010;12:901e4. [44] Wang J, Yin G, Chen Y, Li R, Sun X. Pd nanoparticles deposited on vertically aligned carbon nanotubes grown on carbon paper for formic acid oxidation. Int J Hydrogen Energy 2009;34:8270e5. [45] Yu X, Pickup PG. Recent advances in direct formic acid fuel cells (DFAFC). J Power Sources 2008;182:124e32. [46] Rice C, Ha S, Masel RI, Wieckowski A. Catalysts for direct formic acid fuel cells. J Power Sources 2003;115:229e35. [47] Solla-Gullo´n J, Rodes A, Montiel V, Aldaz A, Clavilier J. Electrochemical characterization of platinum-palladium nanoparticles prepared in a water-in-oil microemulsion. J Electroanal Chem 2003;554e555:273e84. [48] Weibel A, Bouchet R, Boulc’h F, Knauth P. The big problem of small particles: a comparison of methods for determination of particle size in nanocrystalline anatase powders. Chem Mater 2005;17:2378e85. [49] Zelenay P, Xiaoming R, Sharon T, Gosttesfeld S. US Pat. 6,696,382, 2004. [50] Lim C, Allen RG, Scott K. Effect of dispersion methods of an unsupported Pt-Ru black anode catalyst on the power performance of a direct methanol fuel cell. J Power Sources 2006;161:11e8.

4439

[51] Kocha S. Principles of MEA preparation. In: Vielstich W, Lamm A, Gasteiger HA, editors. Handbook of fuel cellsfundamentals, technology and applications, vol. 3. Chichester (UK): John Wiley & Sons, Ltd; 2003. p. 538e65. [52] Cordero-Borboa AE, Sterling-Black E, Go´mez-Corte´s A, Va´zquez-Zavala A. X-ray diffraction evidence of the single solid solution character of bi-metallic Pt-Pd catalyst particles on an amorphous SiO2 substrate. Appl Surf Sci 2003;220:169e74. [53] Duarte MMM, Taberner PM, Mayer CE. Electrochemical behaviour of palladium/graphite and palladium/carbon systems. Electrochim Acta 1989;34:499e504. [54] Solla-Gullo´n J, Montiel V, Aldaz A, Clavilier J. Synthesis and electrochemical decontamination of platinum-palladium nanoparticles prepared by water-in-oil microemulsion. J Electrochem Soc 2003;150:E104e9. [55] Burke LD, Nagle LC. Anomalous behaviour of palladium in aqueous solution. J Electroanal Chem 1999;461:52e64. [56] Angerstein-Kozlowska H. In: Yeager E, Bockris JO’M, Conway BE, Saranpagani S, editors. Comprehensive treatise of electrohemistry, vol. 9. , New York: Plenum Press; 1984. p. 15e61. [57] Navarro RM, Pawelec B, Trejo JM, Mariscal R, Fierro JLG. Hydrogenation of aromatics on sulfur-resistant PtPd bimetallic catalysts. J Catal 2000;189:184e94. [58] Meng H, Sun S, Masse J-P, Dodelet J-P. Electrosynthesis of Pd single-crystal nanothorns and their application in the oxidation of formic acid. Chem Mater 2008;20:6998e7002. [59] Li X, Hsing I-M. Electrooxidation of formic acid on carbon supported PtxPd1x (x ¼ 0e1) nanocatalysts. Electrochim Acta 2006;51:3477e83. [60] Capon A, Parsons R. The oxidation of formic acid at noble metal electrodes part 4: platinum þ palladium alloys. J Electroanal Chem 1975;65:285e305.