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Comparison of Pd–Co–Au electrocatalysts prepared by conventional borohydride and microemulsion methods for oxygen reduction in fuel cells
Electrochemistry Communications 8 (2006) 807–814 www.elsevier.com/locate/elecom
Comparison of Pd–Co–Au electrocatalysts prepared by conventional borohydride and microemulsion methods for oxygen reduction in fuel cells V. Raghuveer, P.J. Ferreira, A. Manthiram
*
Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX 78712, United States Received 27 February 2006; received in revised form 10 March 2006; accepted 13 March 2006 Available online 17 April 2006
Abstract Pd–Co–Au/C catalysts synthesized by the conventional borohydride reduction method and reverse microemulsion method followed by heat treatment at 500–900 °C in a reducing atmosphere have been characterized by X-ray diffraction, transmission electron microscopy, and polarization measurements for oxygen reduction reaction (ORR) in proton exchange membrane fuels cells. While a single phase Pd–Co–Au alloy is formed at temperatures as low as 500 °C with the microemulsion method, heat treatment at 900 °C is needed to obtain single phase with the conventional borohydride method. The samples prepared by the microemulsion method show better catalytic activity for ORR compared to those prepared by the conventional borohydride method due to a higher degree of alloying at lower temperatures while keeping the particle size small and the surface area high. The activity of the samples prepared by the microemulsion method is comparable or slightly better than that of commercial Pt/C catalyst at 60 °C. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Fuel cells; Catalysts; Chemical synthesis; Catalytic activity; Microstructure
1. Introduction Platinum supported on carbon black is widely used as an electrocatalyst for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). However, the high cost and limited abundance of Pt pose serious problems for a widespread commercialization of the fuel cell technology. To overcome this difficulty, alloying of Pt with other less expensive metals as well as a reduction in the catalyst loading have been pursued [1–9]. In addition, platinum-free catalysts such as non-platinum based metal combinations [10–13], metal oxides [14,15], carbides [16,17], chalcogenides [18,19], enzymes [20,21], inorganic and organometallic complexes [22,23] have been investigated over the years for ORR. Recently, we reported three palladium-based electrocatalysts, Pd–Co–Au (Pd:Co:Au = *
Corresponding author. Tel.: +1 512 471 1791; fax: +1 512 471 7681. E-mail address: [email protected] (A. Manthiram).
1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.03.022
70:20:10 atom%), Pd–Ti (Pd:Ti = 50:50 atom%), and Pd– Co–Mo (Pd:Co:Mo = 70:20:10 atom%) [24,25] that show catalytic activity comparable to that of Pt for ORR in PEMFC at 60 °C. However, for structure and surface sensitive reactions such as electrocatalysis for ORR, the particle size, dispersion, and compositional homogeneity of the alloy clusters on the carbon support are important factors to obtain good catalytic activity [26], which become particularly challenging with multiple metals involved. Usually, the Pt-based [7,27] and Pd-based [24] alloy catalysts are prepared and/or post-treated at high temperatures in inert or reducing atmospheres in order to promote alloy formation. However, the thermal treatment at high temperatures leads to an undesired particle growth, which results in a decrease in the surface area and catalytic activity. Therefore, catalyst preparation methods that can offer high degree of alloy homogeneity with small particle size and high surface area at moderate temperatures are needed. In this context,
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colloidal assemblies such as reverse micelles (water in oil microemulsion) offer an attractive approach to prepare multi-metallic alloy compositions with a high degree of homogeneity and a good control of particle size. Surfactant stabilized reverse micelles not only serve as microreactors for chemical reactions but also as steric stabilizers to inhibit particle growth. The main advantage of the microemulsion method is the ability to easily control the particle size by varying the synthesis conditions. Various nanoparticles including metals and alloys [28–30] metal oxides [31–33], and organic polymers [34] have been prepared by the reverse microemulsion (RME) method. For example, Pt– Ru/C and Pt–M/C (M = Fe and Co) catalysts prepared by the RME method show high catalytic activity for methanol oxidation, hydrogen oxidation, and oxygen reduction in fuel cells [35–38]. We present here the synthesis of the newly identified Pd– Co–Au/C catalysts by the reverse microemulsion method and a comparison of their structure and properties with those prepared by the conventional borohydride reduction method. The Pd–Co–Au/C catalysts prepared by the two methods are subjected to heat treatment at various temperatures in a reducing atmosphere and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) equipped with an energy dispersive spectroscopic (EDS) analysis to investigate the structure and phase identification, alloy composition, homogeneity, and particle size and distribution of the catalysts. The Pd– Co–Au/C catalysts prepared by the two methods are also compared as cathode catalysts in single cell PEMFC for ORR. 2. Experimental 2.1. Synthesis by conventional borohydride reduction method Carbon supported Pd–Co–Au catalysts with a metal(s) loading of 20 wt% were prepared by adding required amounts of metal salts (ammonium tetrachloropalladate, cobalt nitrate, and dihydrogen tetrachloroaurate) into a constantly stirring carbon slurry that was prepared by suspending the Vulcan XC-72R carbon black in deionized water. The mixture was agitated in an ultrasonic bath at room temperature for 30 min. A few drops of 1 M NaOH solution was then added to the mixture to raise the pH to 10 before adding 5 wt% of sodium borohydride. The resulting reaction mixture was stirred for 15 min, kept aside for overnight, filtered, washed with deionized water, and dried in air. The powder thus obtained was finally heattreated at various temperatures between 500 and 900 °C in a flowing mixture of 10% H2–90% Ar for 1 h and cooled to room temperature with a cooling rate of 5 °C/min. 2.2. Synthesis by reverse microemulsion method Carbon supported Pd–Co–Au catalysts with a metal(s) loading of 20 wt% were prepared by the microemulsion
method with the use of sodium dioctylsulfosuccinate (AOT) as the surfactant and heptane as the oil phase. Microemulsion 1 was prepared by mixing required amounts of ammonium hexachloropalladate, cobalt nitrate, dihydrogen tetrachloroaurate, AOT, deionized water, and heptane under constant stirring followed by ultrasonication for 20 min. Microemulsion 2 was prepared by mixing sodium borohydride, AOT, deionized water, and heptane under constant stirring followed by ultrasonication for 20 min. In both the microemulsions, the molar ratio of water to AOT was kept at 10:1. Microemulsions 1 and 2 were then mixed together and ultrasonicated for 2 h. Subsequently, an appropriate amount of carbon (Vulcan XC 72R) was added to the mixture to give a metal(s):C weight ratio of 20:80. The resultant slurry was kept under constant stirring for 2 h, filtered, washed copiously with acetone and deionized water, and dried in an air oven at 100 °C for 2 h. The powder samples thus obtained were heated at various temperatures as described for the conventional borohydride reduction method. 2.3. Materials characterization The catalyst samples were characterized by XRD patterns recorded at a slow scan rate between 10° and 70° with a counting time of 10 s per 0.02°. A guide to the percentage degree of alloying DA was obtained as DA ¼ faPd aalloyðTÞ g=faPd aalloyð900Þ g 100 where aPd, aalloy(T), and aalloy(900) refer to the cell parameter values of, respectively, Pd, Pd–Co–Au alloy heat treated at various temperatures (T = 500, 650, 750 °C), and Pd–Co– Au alloy heat treated at 900 °C. The morphology of the catalyst samples was characterized with a JEOL 2010F high-resolution transmission electron microscope (TEM) operated at 200 keV. The compositional analysis of the catalyst samples was carried out with the EDS system available in the TEM. The average size of the alloy particles in the catalyst samples was calculated from the TEM images based on the random selection of 75 particles. The surface area values were calculated using the equation S = 6000/rd, where r is the crystallite size in nm obtained from the TEM data and d is the density of the Pd metal or alloy [12,37–40]; the density values used were 21.5 and 12.2 g/cm3, respectively, for Pt and Pd–Co–Au (70:20:10 atom%) alloy. 2.4. Membrane-electrode assembly fabrication and fuel cell evaluation The gas diffusion electrodes and membrane electrode assemblies (MEA) were fabricated as described elsewhere [7,9,24,25]. The performances of the MEAs in PEMFC, using the Pd alloys synthesized in this study as cathodes and commercial Pt/C as anodes were evaluated with a commercial fuel cell test system (Compucell GT, Electrochem) and a single cell test rig with 5 cm2 active geometrical area.
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Fig. 1 compares the XRD patterns of the carbon-supported Pd–Co–Au (70:20:10 atom%) catalyst samples prepared by the conventional borohydride method before and after heat treating at various temperatures in the reducing atmosphere. The as-prepared sample shows broad reflections corresponding to two face centered cubic (fcc) phases: one close to that of Pd (dotted line refers to the 2h value expected for pure Pd) and the other close to that of Au (solid line refers to the 2h value expected for pure Au). The two fcc phases are present even after heat treating
at 500 or 650 °C although the intensity of the Au-rich phase relative to the Pd-rich phase appears to decrease with heating. The shifting of the reflections to higher angles compared to those of pure Pd (dotted line) or Au (solid line) as the heat treatment temperature increases to 650 °C suggests an increasing incorporation of the smaller Co into the Pd lattice or the smaller Co and Pd into the Au lattice. The changes in the relative intensities of the Au-rich and Pd-rich phases on heating at 500 and 650 °C could be due to the changes in the alloy composition as well as the degree of crystallinity. However, reflections corresponding to only a single fcc phase are found on heat treating at 900 °C, indicating the formation of a ternary Pd–Co–Au alloy phase at higher temperatures. Fig. 2 compares the XRD patterns of the carbon-supported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalyst samples prepared by the reverse microemulsion method after heat treatment at various temperatures in the reducing atmosphere. Samples heat-treated at moderate to high temperatures (500–900 °C) all show reflections that are characteristic of a single phase fcc lattice unlike the samples obtained by the conventional borohydride method. The reflections are shifted to higher 2h values compared to that
Fig. 1. XRD patterns of the carbon-supported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalysts prepared by the conventional borohydride reduction method before and after heat treatment in 10% H2–90% Ar atmosphere at various temperatures. The dotted and solid lines indicate, respectively, the standard 2h values corresponding to the (1 1 1) reflections of Pd and Au metals.
Fig. 2. XRD patterns of the carbon-supported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalysts prepared by the reverse microemulsion method followed by heat treatment in 10% H2–90% Ar atmosphere at various temperatures. The dotted and solid lines indicate, respectively, the standard 2h values corresponding to the (1 1 1) reflections of Pd and Au metals.
Galvanostatic polarization studies were conducted at 60 °C with humidified hydrogen and oxygen gas as reactants; the hydrogen and oxygen pressures were 18 and 20 psi, respectively. For a comparison, a commercial sample of platinized carbon (20 wt% platinum on carbon, Alfa Aesar) was also examined as cathode. 3. Results and discussion 3.1. Structural comparison of the Pd–Co–Au/C catalysts obtained by conventional borohydride and reverse microemulsion methods
0a 0.7 4 3 33 27 18 14 15 18 28 35 6.1 10.2 11.2 13.8 0.3883 0.3878 0.3872 0.3863 a
Specific activity is zero at 0.7 V since the activation onset potential is <0.7 V.
– – – – 79:14:07 73:17:09 71:21:08 72:20:08 Single Single Single Single 500 650 750 900
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– – 0a – – 11 – – 46 – – 25.4 – – 0.3861 68:30:02 68:19:13 –
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Specific area (m2/g metal(s)) Average particle size from TEM (nm) Standard deviation (nm) Lattice parameter from XRD (nm) Composition from EDS analysis Phase analysis Annealing temperature in 10% H2–90% Ar (°C) Synthesis method
of pure Pd (indicated by the dotted line), indicating a contraction of the lattice due to alloy formation. The extent of shifting increases slightly and the lattice parameter decreases with increasing heat treatment temperature (Table 1), suggesting an increase in the degree of alloying. At lower heat treatment temperatures (<900 °C), it is possible that a fraction of Co or Au may be present as free metals or as oxides. It is interesting to note that the extent of alloying after heat treatment at 900 °C is almost the same for the samples obtained by the conventional borohydride reduction and the reverse microemulsion methods as indicated by the XRD data and lattice parameter values (Figs. 1 and 2 and Table 1). Fig. 3 compares the TEM images of the carbon-supported Pd–Co–Au catalyst samples that were prepared by the conventional borohydride method (Figs. 3a and b) and the reverse microemulsion method (Figs. 3c and d) before and after heat treatment at various temperatures in the reducing atmosphere. With the conventional borohydride method, although a relatively uniform dispersion of metal particles is observed for the 500 °C sample, particles with spherical and irregular shapes with some agglomeration are noted (Fig. 3a). On the other hand, particles with a predominantly spherical shape with a uniform dispersion are found for the 900 °C sample (Fig. 3b). EDS elemental analysis of the as-prepared and 500 °C samples (Table 1) indicates the existence of Pd-rich and Au-rich compositions in different regions, which is consistent with the observation of mixed phases in the XRD patterns (Fig. 1). This reveals a poor dispersion of metals obtained by the conventional borohydride method. However, EDS analysis of the 900 °C sample indicates a single phase Pd–Co–Au composition (Table 1) with a Pd:Co:Au ratio close to the nominal composition of 70:20:10 atom %, which is consistent with the XRD data (Fig. 1). The TEM and XRD data indicate that heat treatment at high temperatures (900 °C) and chemical diffusion of the three metals over large atomic distances are essential to get the single phase Pd–Co–Au alloy due to the poor chemical homogeneity of the samples derived by the conventional borohydride reduction method. The poor chemical homogeneity in the as-prepared sample originates from the differences in the reducibility (reduction potentials) of the Pd2+, Au2+, and Co2+ ions. With the reverse microemulsion method, particles with more or less spherical morphologies with a uniform dispersion on the carbon support are observed for all the temperatures (Figs. 3c and d and Table 1). EDS elemental analysis indicates all the samples to be single phase (Table 1) with compositions close to that of the nominal composition (Pd:Co:Au = 70:20:10 atom%), which is consistent with the XRD data in Fig. 2. Unlike the conventional borohydride reduction method, the reverse microemulsion method gives single phase samples at temperatures as low as 500 °C (Figs. 1 and 2). In the reverse microemulsion method, a slow and controlled reduction of the three metal ions present within the nanometer sized aqueous domains
Specific activity at 0.7 V (A/m2 area of metal(s))
V. Raghuveer et al. / Electrochemistry Communications 8 (2006) 807–814 Table 1 XRD and TEM analysis data and catalytic activity of the carbon supported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalysts prepared by the conventional borohydride and reverse microemulsion methods
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Fig. 3. TEM images of the carbon-supported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalysts prepared by the conventional borohydride reduction (CBR) method and reverse microemulsion (RME) method: (a) CBR method followed by heating at 500 °C; (b) CBR method followed by heating at 900 °C; (c) RME method followed by heating at 500 °C; and (d) RME method followed by heating at 900 °C.
or droplets (miroreactors), resulting from a collision of the two microemulsions (one consisting of the three metal ions and the other consisting of the borohydrides), gives smaller particles with an intimate mixing or alloying of the three metals. A small diffusion distance in the smaller particles facilitates the formation of single-phase alloys with uniform composition and good degree of alloying at moderate temperatures as low as 500 °C. Fig. 4 compares the histograms of the carbon-supported Pd–Co–Au (single phase) catalyst samples prepared by the reverse microemulsion method after heat treatment at various temperatures (500–900 °C). In order to quantify the particle size distribution, a Gaussian distribution was fitted to the experimental data of the samples heat-treated at various temperatures (Fig. 4). The R2 values indicate a good fit. The average mean particle size and standard deviation (SD) values obtained from the raw TEM data increase with increasing heat treatment temperature as seen in Table 1.
The lower standard deviation value of the 500 °C sample indicates a narrow particle size distribution while the slightly higher standard deviation values obtained for the 650, 750 and 900 °C samples indicate a relatively broader distribution. The particle size and particle size distribution of the 900 °C samples obtained by both the reverse microemulsion and the conventional borohydride methods are also compared in Figs. 4d and e. The 900 °C samples are chosen for this comparison due to the formation of single phase alloy irrespective of the preparation method. The alloy catalyst synthesized by the reverse microemulsion method has a narrow particle size distribution with smaller particle size (Fig. 4d) compared to that prepared by the conventional method (Fig. 4e), as evident from the lower mean particle size and standard deviation values (Table 1). Both the XRD and TEM results thus clearly demonstrate that the reverse microemulsion method leads to alloy particles with
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(b) 25
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R = 0.8889 Mean = 15 SD = 6.1
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Particle size (nm) Fig. 4. Histograms of the carbon-supported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalyst particle size distribution measured over a random selection of 75 particles. The samples were prepared by the reverse microemulsion method followed by heat treatment at (a) 500 °C, (b) 650 °C, (c) 750 °C, and (d) 900 °C. For a comparison, the sample prepared by the conventional borohydride method followed by heating at 900 °C is shown in (e).
good degree of alloying and homogeneity at lower heat treatment temperatures while keeping the particle size small compared to conventional borohydride method, which could be beneficial to achieve high catalytic activity. 3.2. Electrochemical comparison of the Pd–Co–Au/C catalysts obtained by conventional borohydride and reverse microemulsion methods Fig. 5 compares the polarization curves recorded in single cell PEMFC with the Pd–Co–Au (70:20:10 atom%) cathode catalysts obtained by the conventional borohydride method (Fig. 5a) and reverse microemulsion method (Fig. 5b) before and after heat treatment at various temperatures. It also gives the data obtained with a commercial Pt/C (20 wt% Pt on carbon, Johnson-Matthey) cathode catalyst. With the conventional borohydride method (Fig. 5a), the as-prepared Pd–Co–Au sample shows poor electrocatalytic activity for ORR with a large polarization loss due to a higher activation overpotential, while the
500 °C sample shows performance close to that of the commercial Pt/C catalyst with a low polarization loss due to an increasing degree of alloying as evident from the XRD data (Fig. 1). However, the 900 °C sample shows electrocatalytic activity lower than that of the 500 °C sample despite an increase in the degree of alloying due to an increase in the average particle size (Fig. 3b and Table 1) and a consequent decrease in the surface area. With the reverse microemulsion method (Fig. 5b), the as-prepared sample was not considered for the comparison as it contains adsorbed organic surfactant impurities. The catalytic activity of the Pd–Co–Au samples increase with increasing heat treatment temperature from 500 to 750 °C and then decreases on going to the 900 °C sample. In fact, the 650 and 750 °C samples exhibit electrochemical performances better than that of the commercial Pt/C catalyst with open-circuit voltages of 0.89 V, which is close to that found with the commercial Pt/C catalyst. While the increase in catalytic activity on going from 500 to 750 °C is due to an increase in the degree of alloying, the decrease
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1.0 (a) Commercial Pt Pd (70) Co (20) Au (10) , As-prepared
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Fig. 5. Comparison of the steady-state polarization curves of the carbonsupported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalysts prepared by the (a) conventional borohydride method and (b) reverse microemulsion method followed by heat treatment at various temperatures for ORR in single cell PEMFC at 60 °C with those of a commercial (JohnsonMatthey) Pt catalyst with a metal(s) loading of 0.2 mg/cm2. The current density values are with respect to electrode geometrical area.
in catalytic activity on going to the 900 °C sample is due to a significant increase in particle size (Table 1), despite a further increase in the degree of alloying. Comparing the electrochemical performances in Fig. 5b, we see that the 750 °C Pd–Co–Au/C sample exhibits higher catalytic activity than the 500, 650 and 900 °C Pd–Co–Au/ C samples. Both the electronic factors arising from the alloy content and geometric factors arising from particle size are known to influence the chemisorption behavior of oxygenated species and consequently the catalytic activity for ORR [5–9]. Fig. 6 shows the effect of heat treatment temperature on the degree of alloying and surface area (Fig. 6a) derived, respectively, from the XRD and TEM data and on the catalytic activity for ORR of the Pd– Co–Au/C catalysts (Fig. 6b) prepared by the reverse microemulsion method. As seen in Fig. 6a, the degree of alloying (DA) increases while the surface area decreases with increasing heat treatment temperature. The decrease in the surface area is due to an increase in the particle size (Table 1). A comparison of the catalytic activity of the catalysts heat treated at different temperatures is made in
Fig. 6. Effect of heat treatment temperature on the (a) degree of alloying and surface area and (b) catalytic activity for ORR of the carbonsupported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalysts prepared by the reverse microemulsion method. The current density values are with respect to electrode geometrical area in (b).
Fig. 6b by measuring the current density (per unit geometrical area) values at 0.7 V. In order to have a better comparison of the catalytic activity for ORR, we show in Table 1 the specific activities (mA/m2 of metal or alloy), which are defined as the current per unit surface area of the catalyst at 0.7 V in the polarization curves given in Fig. 5b. It is interesting to note that the specific activity of the 750 and 900 °C samples are comparable or better than that of the commercial Pt. With the Pd–Co–Au alloy, the catalytic activity (Fig. 6b) and specific activity (Table 1) increase with increasing heat treatment temperature, reaches a maximum at 750 °C, and then decreases on going to 900 °C. As pointed out earlier, while the initial increase is due to an increase in the degree of alloying, the decrease at higher temperature is due to an increase in particle size and a decrease in surface area (Fig. 6a and Table 1). This conclusion is further supported by a comparison of the activities of the 900 °C (singlephase alloy) samples obtained by the reverse microemulsion and the conventional borohydride methods. Both the 900 °C samples possess almost the same degree of alloying as indicated by the lattice parameter values, but have different particle sizes (Table 1). The higher activity of
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the 900 °C sample obtained by the reverse microemulsion method (Fig. 5b) compared to that obtained by the conventional borohydride method (Fig. 5a) could be due to a smaller particle size and higher surface area, indicating that the particle size plays an important role in addition to the degree of alloying. Additionally, differences in surface characteristics (e.g., crystallographic plane) and particle size distribution depending upon the synthesis method and heating temperature may influence the electrochemical activity. Thus, synthetic approaches that can give a high degree of alloying and homogeneity at lower temperature while keeping the particle size small with optimal surface characteristics have the possibility of improving the catalytic activity further. 4. Conclusions Pd–Co–Au/C catalysts prepared by the reverse microemulsion method exhibit a higher degree of alloying at lower temperatures compared to those prepared by the conventional borohydride reduction method due to a slow and controlled reduction of the metal ions within the nanometer sized aqueous domains or droplets. The high degree of alloying while keeping the particle size small and surface area high leads to a better catalytic activity for the oxygen reduction reaction in PEMFC. The activity of the Pd–Co–Au alloy samples obtained by the reverse microemulsion method is in fact comparable to or slightly better than that of commercial Pt/C catalyst at 60 °C. The study demonstrates that controlled synthesis approaches such as the reverse microemulsion method adopted here can help to improve the chemical homogeneity and degree of alloying with multi-metallic compositions. Furthermore, the comparable activity of the Pd–Co–Au alloy catalysts to that of Pt with good tolerance to methanol poisoning [24,25] may provide important advantages with respect to lowering the fuel cell cost and enhancing the overall performance, particularly in direct methanol fuel cells. Acknowledgment This work was supported by the Welch Foundation Grant F-1254. References [1] S.H. Joo, S.J. Choi, M. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. [2] M.C.R. Martinez, D.C. Amoros, A.L. Salano, C.S. Martinez, H. Yamashita, M. Anpo, Carbon 33 (1995) 3.
[3] V. Raghuveer, A. Manthiram, Electrochem. Solid-State Lett. 7 (2003) A336. [4] M. Watanabe, T. Tsurumu, T. Mizukami, T. Nakamura, P. Stonehart, J. Electrochem. Soc. 141 (1994) 2659. [5] S. Mukerjee, S. Srinivasan, M. Soriaga, J. Electrochem. Soc. 142 (1995) 1409. [6] T. Toda, H. Igarashi, H. Uchida, M. Watanabe, J. Electrochem. Soc. 146 (1999) 3750. [7] L. Xiong, A. Manthiram, J. Mater. Chem. 14 (2004) 1454. [8] T. Toda, H. Igarashi, M. Watanabe, J. Eletroanal. Chem. 460 (1999) 258. [9] L. Xiong, A. Manthiram, J. Electrochem. Soc. 152 (2005) A697. [10] R. Rivera-Noriega, N. Castillo-Hernandez, A.B. Soto-Guzman, O. Solorza-Feria, Int. J. Hydrogen Energy 27 (2002) 457. [11] S. Ye, A.K. Vijh, Electrochem. Commun. 27 (2003) 272. [12] R. Pattabhiraman, Appl. Catal. A 153 (1997) 9. [13] O. Savadago, K. Lee, K. Oishi, S. Mitsushima, N. Kamiya, K.-I. Ota, Electrochem. Commun. 105 (2004) 6. [14] J.-M. Zen, R. Manoharan, J.B. Goodenough, J. Appl. Electrochem. 22 (1992) 140. [15] J.-M. Zen, C.-B. Wang, J. Electrochem. Soc. 141 (1994) L51. [16] J.G. Chen, Chem. Rev. 96 (1996) 1477. [17] R. Cote, G. Lalande, G. Faubert, D. Guay, J.P. Dodelet, G.J. Denes, J. New Mater. Electrochem. Syst. 1 (1998) 7. [18] N.A. Vante, H. Tributsch, Nature 323 (1986) 431. [19] V. Trapp, P. Christensen, A. Hamnett, J. Chem. Soc., Faraday Trans. 144 (1996) 218. [20] N. Mano, J.L. Fernandez, Y. Kim, W. Shin, A.J. Bard, A. Heller, J. Am. Chem. Soc. 125 (2003) 15290. [21] N. Mano, H.-H. Kim, Y. Zhang, A. Heller, J. Am. Chem. Soc. 124 (2002) 6480. [22] K. Sawai, N. Suzuki, J. Electrochem. Soc 151 (2004) A682. [23] J.P. Collman, P. Deniesevich, Y. Konai, M. Marrocco, C. Koval, F.C. Anson, J. Am. Chem. Soc. 102 (1980) 6027. [24] J.L. Fernandez, V. Raghuveer, A. Manthiram, A.J. Bard, J. Am. Chem. Soc. 127 (2005) 13100. [25] V. Raghuveer, A. Manthiram, A.J. Bard, J. Phys. Chem. 109 (2005) 22909. [26] G.A. Somarjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994. [27] T.R. Ralph, M. Hogarth, Platinum Metals. Rev. 46 (2002) 1. [28] J. Solla-Gullon, V. Montiel, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 491 (2000) 69. [29] M. Wu, D. Chen, T. Huang, Langmuir 17 (2001) 3877. [30] J. Solla-Gullon, A. Rodes, V. Montiel, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 554–555 (2003) 273. [31] C.R. Vestal, Z.J. Zhang, Chem. Mater. 14 (2002) 3817. [32] S. Santra, R. Tapec, N. Theodoropoulou, J. Dobson, A. Hebard, W. Tan, Langmuir 17 (2001) 2900. [33] C. Lu, P. Yeh, J. Mater. Chem. 10 (2000) 599. [34] M. Antonietti, R. Basten, S. Lohmann, Macromol. Chem. Phys. 196 (1995) 441. [35] Y. Liu, X. Qui, Z. Zhen, W. Zhu, Electrochem. Commun. 4 (2002) 550. [36] M.J. Escudero, E. Hontanon, S. Schwartz, M. Boutonnet, L. Daza, J. Power Sources 106 (2002) 206. [37] L. Xiong, A. Manthiram, Solid State Ionics 176 (2005) 385. [38] L. Xiong, A. Manthiram, Electrochim. Acta 50 (2005) 2323. [39] C.R. Heal, L.K. Mkayulla, Carbon 26 (1988) 815. [40] X.Y. Wang, S.H. Moon, M.A. Vannice, J. Catal. 71 (1981) 167.
Comparison of Pd–Co–Au electrocatalysts prepared by conventional borohydride and microemulsion methods for oxygen reduction in fuel cells V. Raghuveer, P.J. Ferreira, A. Manthiram
*
Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX 78712, United States Received 27 February 2006; received in revised form 10 March 2006; accepted 13 March 2006 Available online 17 April 2006
Abstract Pd–Co–Au/C catalysts synthesized by the conventional borohydride reduction method and reverse microemulsion method followed by heat treatment at 500–900 °C in a reducing atmosphere have been characterized by X-ray diffraction, transmission electron microscopy, and polarization measurements for oxygen reduction reaction (ORR) in proton exchange membrane fuels cells. While a single phase Pd–Co–Au alloy is formed at temperatures as low as 500 °C with the microemulsion method, heat treatment at 900 °C is needed to obtain single phase with the conventional borohydride method. The samples prepared by the microemulsion method show better catalytic activity for ORR compared to those prepared by the conventional borohydride method due to a higher degree of alloying at lower temperatures while keeping the particle size small and the surface area high. The activity of the samples prepared by the microemulsion method is comparable or slightly better than that of commercial Pt/C catalyst at 60 °C. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Fuel cells; Catalysts; Chemical synthesis; Catalytic activity; Microstructure
1. Introduction Platinum supported on carbon black is widely used as an electrocatalyst for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). However, the high cost and limited abundance of Pt pose serious problems for a widespread commercialization of the fuel cell technology. To overcome this difficulty, alloying of Pt with other less expensive metals as well as a reduction in the catalyst loading have been pursued [1–9]. In addition, platinum-free catalysts such as non-platinum based metal combinations [10–13], metal oxides [14,15], carbides [16,17], chalcogenides [18,19], enzymes [20,21], inorganic and organometallic complexes [22,23] have been investigated over the years for ORR. Recently, we reported three palladium-based electrocatalysts, Pd–Co–Au (Pd:Co:Au = *
Corresponding author. Tel.: +1 512 471 1791; fax: +1 512 471 7681. E-mail address: [email protected] (A. Manthiram).
1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.03.022
70:20:10 atom%), Pd–Ti (Pd:Ti = 50:50 atom%), and Pd– Co–Mo (Pd:Co:Mo = 70:20:10 atom%) [24,25] that show catalytic activity comparable to that of Pt for ORR in PEMFC at 60 °C. However, for structure and surface sensitive reactions such as electrocatalysis for ORR, the particle size, dispersion, and compositional homogeneity of the alloy clusters on the carbon support are important factors to obtain good catalytic activity [26], which become particularly challenging with multiple metals involved. Usually, the Pt-based [7,27] and Pd-based [24] alloy catalysts are prepared and/or post-treated at high temperatures in inert or reducing atmospheres in order to promote alloy formation. However, the thermal treatment at high temperatures leads to an undesired particle growth, which results in a decrease in the surface area and catalytic activity. Therefore, catalyst preparation methods that can offer high degree of alloy homogeneity with small particle size and high surface area at moderate temperatures are needed. In this context,
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colloidal assemblies such as reverse micelles (water in oil microemulsion) offer an attractive approach to prepare multi-metallic alloy compositions with a high degree of homogeneity and a good control of particle size. Surfactant stabilized reverse micelles not only serve as microreactors for chemical reactions but also as steric stabilizers to inhibit particle growth. The main advantage of the microemulsion method is the ability to easily control the particle size by varying the synthesis conditions. Various nanoparticles including metals and alloys [28–30] metal oxides [31–33], and organic polymers [34] have been prepared by the reverse microemulsion (RME) method. For example, Pt– Ru/C and Pt–M/C (M = Fe and Co) catalysts prepared by the RME method show high catalytic activity for methanol oxidation, hydrogen oxidation, and oxygen reduction in fuel cells [35–38]. We present here the synthesis of the newly identified Pd– Co–Au/C catalysts by the reverse microemulsion method and a comparison of their structure and properties with those prepared by the conventional borohydride reduction method. The Pd–Co–Au/C catalysts prepared by the two methods are subjected to heat treatment at various temperatures in a reducing atmosphere and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) equipped with an energy dispersive spectroscopic (EDS) analysis to investigate the structure and phase identification, alloy composition, homogeneity, and particle size and distribution of the catalysts. The Pd– Co–Au/C catalysts prepared by the two methods are also compared as cathode catalysts in single cell PEMFC for ORR. 2. Experimental 2.1. Synthesis by conventional borohydride reduction method Carbon supported Pd–Co–Au catalysts with a metal(s) loading of 20 wt% were prepared by adding required amounts of metal salts (ammonium tetrachloropalladate, cobalt nitrate, and dihydrogen tetrachloroaurate) into a constantly stirring carbon slurry that was prepared by suspending the Vulcan XC-72R carbon black in deionized water. The mixture was agitated in an ultrasonic bath at room temperature for 30 min. A few drops of 1 M NaOH solution was then added to the mixture to raise the pH to 10 before adding 5 wt% of sodium borohydride. The resulting reaction mixture was stirred for 15 min, kept aside for overnight, filtered, washed with deionized water, and dried in air. The powder thus obtained was finally heattreated at various temperatures between 500 and 900 °C in a flowing mixture of 10% H2–90% Ar for 1 h and cooled to room temperature with a cooling rate of 5 °C/min. 2.2. Synthesis by reverse microemulsion method Carbon supported Pd–Co–Au catalysts with a metal(s) loading of 20 wt% were prepared by the microemulsion
method with the use of sodium dioctylsulfosuccinate (AOT) as the surfactant and heptane as the oil phase. Microemulsion 1 was prepared by mixing required amounts of ammonium hexachloropalladate, cobalt nitrate, dihydrogen tetrachloroaurate, AOT, deionized water, and heptane under constant stirring followed by ultrasonication for 20 min. Microemulsion 2 was prepared by mixing sodium borohydride, AOT, deionized water, and heptane under constant stirring followed by ultrasonication for 20 min. In both the microemulsions, the molar ratio of water to AOT was kept at 10:1. Microemulsions 1 and 2 were then mixed together and ultrasonicated for 2 h. Subsequently, an appropriate amount of carbon (Vulcan XC 72R) was added to the mixture to give a metal(s):C weight ratio of 20:80. The resultant slurry was kept under constant stirring for 2 h, filtered, washed copiously with acetone and deionized water, and dried in an air oven at 100 °C for 2 h. The powder samples thus obtained were heated at various temperatures as described for the conventional borohydride reduction method. 2.3. Materials characterization The catalyst samples were characterized by XRD patterns recorded at a slow scan rate between 10° and 70° with a counting time of 10 s per 0.02°. A guide to the percentage degree of alloying DA was obtained as DA ¼ faPd aalloyðTÞ g=faPd aalloyð900Þ g 100 where aPd, aalloy(T), and aalloy(900) refer to the cell parameter values of, respectively, Pd, Pd–Co–Au alloy heat treated at various temperatures (T = 500, 650, 750 °C), and Pd–Co– Au alloy heat treated at 900 °C. The morphology of the catalyst samples was characterized with a JEOL 2010F high-resolution transmission electron microscope (TEM) operated at 200 keV. The compositional analysis of the catalyst samples was carried out with the EDS system available in the TEM. The average size of the alloy particles in the catalyst samples was calculated from the TEM images based on the random selection of 75 particles. The surface area values were calculated using the equation S = 6000/rd, where r is the crystallite size in nm obtained from the TEM data and d is the density of the Pd metal or alloy [12,37–40]; the density values used were 21.5 and 12.2 g/cm3, respectively, for Pt and Pd–Co–Au (70:20:10 atom%) alloy. 2.4. Membrane-electrode assembly fabrication and fuel cell evaluation The gas diffusion electrodes and membrane electrode assemblies (MEA) were fabricated as described elsewhere [7,9,24,25]. The performances of the MEAs in PEMFC, using the Pd alloys synthesized in this study as cathodes and commercial Pt/C as anodes were evaluated with a commercial fuel cell test system (Compucell GT, Electrochem) and a single cell test rig with 5 cm2 active geometrical area.
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Fig. 1 compares the XRD patterns of the carbon-supported Pd–Co–Au (70:20:10 atom%) catalyst samples prepared by the conventional borohydride method before and after heat treating at various temperatures in the reducing atmosphere. The as-prepared sample shows broad reflections corresponding to two face centered cubic (fcc) phases: one close to that of Pd (dotted line refers to the 2h value expected for pure Pd) and the other close to that of Au (solid line refers to the 2h value expected for pure Au). The two fcc phases are present even after heat treating
at 500 or 650 °C although the intensity of the Au-rich phase relative to the Pd-rich phase appears to decrease with heating. The shifting of the reflections to higher angles compared to those of pure Pd (dotted line) or Au (solid line) as the heat treatment temperature increases to 650 °C suggests an increasing incorporation of the smaller Co into the Pd lattice or the smaller Co and Pd into the Au lattice. The changes in the relative intensities of the Au-rich and Pd-rich phases on heating at 500 and 650 °C could be due to the changes in the alloy composition as well as the degree of crystallinity. However, reflections corresponding to only a single fcc phase are found on heat treating at 900 °C, indicating the formation of a ternary Pd–Co–Au alloy phase at higher temperatures. Fig. 2 compares the XRD patterns of the carbon-supported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalyst samples prepared by the reverse microemulsion method after heat treatment at various temperatures in the reducing atmosphere. Samples heat-treated at moderate to high temperatures (500–900 °C) all show reflections that are characteristic of a single phase fcc lattice unlike the samples obtained by the conventional borohydride method. The reflections are shifted to higher 2h values compared to that
Fig. 1. XRD patterns of the carbon-supported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalysts prepared by the conventional borohydride reduction method before and after heat treatment in 10% H2–90% Ar atmosphere at various temperatures. The dotted and solid lines indicate, respectively, the standard 2h values corresponding to the (1 1 1) reflections of Pd and Au metals.
Fig. 2. XRD patterns of the carbon-supported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalysts prepared by the reverse microemulsion method followed by heat treatment in 10% H2–90% Ar atmosphere at various temperatures. The dotted and solid lines indicate, respectively, the standard 2h values corresponding to the (1 1 1) reflections of Pd and Au metals.
Galvanostatic polarization studies were conducted at 60 °C with humidified hydrogen and oxygen gas as reactants; the hydrogen and oxygen pressures were 18 and 20 psi, respectively. For a comparison, a commercial sample of platinized carbon (20 wt% platinum on carbon, Alfa Aesar) was also examined as cathode. 3. Results and discussion 3.1. Structural comparison of the Pd–Co–Au/C catalysts obtained by conventional borohydride and reverse microemulsion methods
0a 0.7 4 3 33 27 18 14 15 18 28 35 6.1 10.2 11.2 13.8 0.3883 0.3878 0.3872 0.3863 a
Specific activity is zero at 0.7 V since the activation onset potential is <0.7 V.
– – – – 79:14:07 73:17:09 71:21:08 72:20:08 Single Single Single Single 500 650 750 900
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– – 0a – – 11 – – 46 – – 25.4 – – 0.3861 68:30:02 68:19:13 –
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Specific area (m2/g metal(s)) Average particle size from TEM (nm) Standard deviation (nm) Lattice parameter from XRD (nm) Composition from EDS analysis Phase analysis Annealing temperature in 10% H2–90% Ar (°C) Synthesis method
of pure Pd (indicated by the dotted line), indicating a contraction of the lattice due to alloy formation. The extent of shifting increases slightly and the lattice parameter decreases with increasing heat treatment temperature (Table 1), suggesting an increase in the degree of alloying. At lower heat treatment temperatures (<900 °C), it is possible that a fraction of Co or Au may be present as free metals or as oxides. It is interesting to note that the extent of alloying after heat treatment at 900 °C is almost the same for the samples obtained by the conventional borohydride reduction and the reverse microemulsion methods as indicated by the XRD data and lattice parameter values (Figs. 1 and 2 and Table 1). Fig. 3 compares the TEM images of the carbon-supported Pd–Co–Au catalyst samples that were prepared by the conventional borohydride method (Figs. 3a and b) and the reverse microemulsion method (Figs. 3c and d) before and after heat treatment at various temperatures in the reducing atmosphere. With the conventional borohydride method, although a relatively uniform dispersion of metal particles is observed for the 500 °C sample, particles with spherical and irregular shapes with some agglomeration are noted (Fig. 3a). On the other hand, particles with a predominantly spherical shape with a uniform dispersion are found for the 900 °C sample (Fig. 3b). EDS elemental analysis of the as-prepared and 500 °C samples (Table 1) indicates the existence of Pd-rich and Au-rich compositions in different regions, which is consistent with the observation of mixed phases in the XRD patterns (Fig. 1). This reveals a poor dispersion of metals obtained by the conventional borohydride method. However, EDS analysis of the 900 °C sample indicates a single phase Pd–Co–Au composition (Table 1) with a Pd:Co:Au ratio close to the nominal composition of 70:20:10 atom %, which is consistent with the XRD data (Fig. 1). The TEM and XRD data indicate that heat treatment at high temperatures (900 °C) and chemical diffusion of the three metals over large atomic distances are essential to get the single phase Pd–Co–Au alloy due to the poor chemical homogeneity of the samples derived by the conventional borohydride reduction method. The poor chemical homogeneity in the as-prepared sample originates from the differences in the reducibility (reduction potentials) of the Pd2+, Au2+, and Co2+ ions. With the reverse microemulsion method, particles with more or less spherical morphologies with a uniform dispersion on the carbon support are observed for all the temperatures (Figs. 3c and d and Table 1). EDS elemental analysis indicates all the samples to be single phase (Table 1) with compositions close to that of the nominal composition (Pd:Co:Au = 70:20:10 atom%), which is consistent with the XRD data in Fig. 2. Unlike the conventional borohydride reduction method, the reverse microemulsion method gives single phase samples at temperatures as low as 500 °C (Figs. 1 and 2). In the reverse microemulsion method, a slow and controlled reduction of the three metal ions present within the nanometer sized aqueous domains
Specific activity at 0.7 V (A/m2 area of metal(s))
V. Raghuveer et al. / Electrochemistry Communications 8 (2006) 807–814 Table 1 XRD and TEM analysis data and catalytic activity of the carbon supported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalysts prepared by the conventional borohydride and reverse microemulsion methods
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Fig. 3. TEM images of the carbon-supported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalysts prepared by the conventional borohydride reduction (CBR) method and reverse microemulsion (RME) method: (a) CBR method followed by heating at 500 °C; (b) CBR method followed by heating at 900 °C; (c) RME method followed by heating at 500 °C; and (d) RME method followed by heating at 900 °C.
or droplets (miroreactors), resulting from a collision of the two microemulsions (one consisting of the three metal ions and the other consisting of the borohydrides), gives smaller particles with an intimate mixing or alloying of the three metals. A small diffusion distance in the smaller particles facilitates the formation of single-phase alloys with uniform composition and good degree of alloying at moderate temperatures as low as 500 °C. Fig. 4 compares the histograms of the carbon-supported Pd–Co–Au (single phase) catalyst samples prepared by the reverse microemulsion method after heat treatment at various temperatures (500–900 °C). In order to quantify the particle size distribution, a Gaussian distribution was fitted to the experimental data of the samples heat-treated at various temperatures (Fig. 4). The R2 values indicate a good fit. The average mean particle size and standard deviation (SD) values obtained from the raw TEM data increase with increasing heat treatment temperature as seen in Table 1.
The lower standard deviation value of the 500 °C sample indicates a narrow particle size distribution while the slightly higher standard deviation values obtained for the 650, 750 and 900 °C samples indicate a relatively broader distribution. The particle size and particle size distribution of the 900 °C samples obtained by both the reverse microemulsion and the conventional borohydride methods are also compared in Figs. 4d and e. The 900 °C samples are chosen for this comparison due to the formation of single phase alloy irrespective of the preparation method. The alloy catalyst synthesized by the reverse microemulsion method has a narrow particle size distribution with smaller particle size (Fig. 4d) compared to that prepared by the conventional method (Fig. 4e), as evident from the lower mean particle size and standard deviation values (Table 1). Both the XRD and TEM results thus clearly demonstrate that the reverse microemulsion method leads to alloy particles with
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(b) 25
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R = 0.8889 Mean = 15 SD = 6.1
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Particle size (nm) Fig. 4. Histograms of the carbon-supported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalyst particle size distribution measured over a random selection of 75 particles. The samples were prepared by the reverse microemulsion method followed by heat treatment at (a) 500 °C, (b) 650 °C, (c) 750 °C, and (d) 900 °C. For a comparison, the sample prepared by the conventional borohydride method followed by heating at 900 °C is shown in (e).
good degree of alloying and homogeneity at lower heat treatment temperatures while keeping the particle size small compared to conventional borohydride method, which could be beneficial to achieve high catalytic activity. 3.2. Electrochemical comparison of the Pd–Co–Au/C catalysts obtained by conventional borohydride and reverse microemulsion methods Fig. 5 compares the polarization curves recorded in single cell PEMFC with the Pd–Co–Au (70:20:10 atom%) cathode catalysts obtained by the conventional borohydride method (Fig. 5a) and reverse microemulsion method (Fig. 5b) before and after heat treatment at various temperatures. It also gives the data obtained with a commercial Pt/C (20 wt% Pt on carbon, Johnson-Matthey) cathode catalyst. With the conventional borohydride method (Fig. 5a), the as-prepared Pd–Co–Au sample shows poor electrocatalytic activity for ORR with a large polarization loss due to a higher activation overpotential, while the
500 °C sample shows performance close to that of the commercial Pt/C catalyst with a low polarization loss due to an increasing degree of alloying as evident from the XRD data (Fig. 1). However, the 900 °C sample shows electrocatalytic activity lower than that of the 500 °C sample despite an increase in the degree of alloying due to an increase in the average particle size (Fig. 3b and Table 1) and a consequent decrease in the surface area. With the reverse microemulsion method (Fig. 5b), the as-prepared sample was not considered for the comparison as it contains adsorbed organic surfactant impurities. The catalytic activity of the Pd–Co–Au samples increase with increasing heat treatment temperature from 500 to 750 °C and then decreases on going to the 900 °C sample. In fact, the 650 and 750 °C samples exhibit electrochemical performances better than that of the commercial Pt/C catalyst with open-circuit voltages of 0.89 V, which is close to that found with the commercial Pt/C catalyst. While the increase in catalytic activity on going from 500 to 750 °C is due to an increase in the degree of alloying, the decrease
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1.0 (a) Commercial Pt Pd (70) Co (20) Au (10) , As-prepared
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Fig. 5. Comparison of the steady-state polarization curves of the carbonsupported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalysts prepared by the (a) conventional borohydride method and (b) reverse microemulsion method followed by heat treatment at various temperatures for ORR in single cell PEMFC at 60 °C with those of a commercial (JohnsonMatthey) Pt catalyst with a metal(s) loading of 0.2 mg/cm2. The current density values are with respect to electrode geometrical area.
in catalytic activity on going to the 900 °C sample is due to a significant increase in particle size (Table 1), despite a further increase in the degree of alloying. Comparing the electrochemical performances in Fig. 5b, we see that the 750 °C Pd–Co–Au/C sample exhibits higher catalytic activity than the 500, 650 and 900 °C Pd–Co–Au/ C samples. Both the electronic factors arising from the alloy content and geometric factors arising from particle size are known to influence the chemisorption behavior of oxygenated species and consequently the catalytic activity for ORR [5–9]. Fig. 6 shows the effect of heat treatment temperature on the degree of alloying and surface area (Fig. 6a) derived, respectively, from the XRD and TEM data and on the catalytic activity for ORR of the Pd– Co–Au/C catalysts (Fig. 6b) prepared by the reverse microemulsion method. As seen in Fig. 6a, the degree of alloying (DA) increases while the surface area decreases with increasing heat treatment temperature. The decrease in the surface area is due to an increase in the particle size (Table 1). A comparison of the catalytic activity of the catalysts heat treated at different temperatures is made in
Fig. 6. Effect of heat treatment temperature on the (a) degree of alloying and surface area and (b) catalytic activity for ORR of the carbonsupported Pd–Co–Au (Pd:Co:Au = 70:20:10 atom%) catalysts prepared by the reverse microemulsion method. The current density values are with respect to electrode geometrical area in (b).
Fig. 6b by measuring the current density (per unit geometrical area) values at 0.7 V. In order to have a better comparison of the catalytic activity for ORR, we show in Table 1 the specific activities (mA/m2 of metal or alloy), which are defined as the current per unit surface area of the catalyst at 0.7 V in the polarization curves given in Fig. 5b. It is interesting to note that the specific activity of the 750 and 900 °C samples are comparable or better than that of the commercial Pt. With the Pd–Co–Au alloy, the catalytic activity (Fig. 6b) and specific activity (Table 1) increase with increasing heat treatment temperature, reaches a maximum at 750 °C, and then decreases on going to 900 °C. As pointed out earlier, while the initial increase is due to an increase in the degree of alloying, the decrease at higher temperature is due to an increase in particle size and a decrease in surface area (Fig. 6a and Table 1). This conclusion is further supported by a comparison of the activities of the 900 °C (singlephase alloy) samples obtained by the reverse microemulsion and the conventional borohydride methods. Both the 900 °C samples possess almost the same degree of alloying as indicated by the lattice parameter values, but have different particle sizes (Table 1). The higher activity of
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the 900 °C sample obtained by the reverse microemulsion method (Fig. 5b) compared to that obtained by the conventional borohydride method (Fig. 5a) could be due to a smaller particle size and higher surface area, indicating that the particle size plays an important role in addition to the degree of alloying. Additionally, differences in surface characteristics (e.g., crystallographic plane) and particle size distribution depending upon the synthesis method and heating temperature may influence the electrochemical activity. Thus, synthetic approaches that can give a high degree of alloying and homogeneity at lower temperature while keeping the particle size small with optimal surface characteristics have the possibility of improving the catalytic activity further. 4. Conclusions Pd–Co–Au/C catalysts prepared by the reverse microemulsion method exhibit a higher degree of alloying at lower temperatures compared to those prepared by the conventional borohydride reduction method due to a slow and controlled reduction of the metal ions within the nanometer sized aqueous domains or droplets. The high degree of alloying while keeping the particle size small and surface area high leads to a better catalytic activity for the oxygen reduction reaction in PEMFC. The activity of the Pd–Co–Au alloy samples obtained by the reverse microemulsion method is in fact comparable to or slightly better than that of commercial Pt/C catalyst at 60 °C. The study demonstrates that controlled synthesis approaches such as the reverse microemulsion method adopted here can help to improve the chemical homogeneity and degree of alloying with multi-metallic compositions. Furthermore, the comparable activity of the Pd–Co–Au alloy catalysts to that of Pt with good tolerance to methanol poisoning [24,25] may provide important advantages with respect to lowering the fuel cell cost and enhancing the overall performance, particularly in direct methanol fuel cells. Acknowledgment This work was supported by the Welch Foundation Grant F-1254. References [1] S.H. Joo, S.J. Choi, M. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. [2] M.C.R. Martinez, D.C. Amoros, A.L. Salano, C.S. Martinez, H. Yamashita, M. Anpo, Carbon 33 (1995) 3.
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