C catalysts synthesized by a modified impregnation method for methanol electro-oxidation

Electrochimica Acta 54 (2009) 7274–7279

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High activity PtRu/C catalysts synthesized by a modified impregnation method for methanol electro-oxidation Liang Ma a,b , Changpeng Liu a , Jianhui Liao a , Tianhong Lu a , Wei Xing a,∗ , Jiujun Zhang c a b c

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, PR China Graduate University of the Chinese Academy of Sciences, Beijing 100049, PR China Institute for Fuel Cell Innovation, National Research Council of Canada, Vancouver, B.C., V6T 1W5 Canada

a r t i c l e

i n f o

Article history: Received 31 May 2009 Received in revised form 14 July 2009 Accepted 14 July 2009 Available online 23 July 2009 Keywords: PtRu/C Modified impregnation method Methanol electro-oxidation Specific activity Electrocatalyst X-ray photoelectron spectroscopy

a b s t r a c t A modified impregnation method was used to prepare highly dispersive carbon-supported PtRu catalyst (PtRu/C). Two modifications to the conventional impregnation method were performed: one was to precipitate the precursors ((NH4 )2 PtCl6 and Ru(OH)3 ) on the carbon support before metal reduction; the other was to add a buffer into the synthetic solution to stabilize the pH. The prepared catalyst showed a much higher activity for methanol electro-oxidation than a catalyst prepared by the conventional impregnation method, even higher than that of current commercially available, state-of-the-art catalysts. The morphology of the prepared catalyst was characterized using TEM and XRD measurements to determine particle sizes, alloying degree, and lattice parameters. Electrochemical methods were also used to ascertain the electrochemical active surface area and the specific activity of the catalyst. Based on XPS measurements, the high activity of this catalyst was found to originate from both metallic Ru (Ru0 ) and hydrous ruthenium oxides (RuOx Hy ) species on the catalyst surface. However, RuOx Hy was found to be more active than metallic Ru. In addition, the anhydrous ruthenium oxide (RuO2 ) species on the catalyst surface was found to be less active. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction The direct methanol fuel cell (DMFC) is considered a promising power conversion device for mobile and portable applications because it uses methanol, a renewable energy carrier, as fuel. However, several challenges, including cost and durability, still hinder its commercialization. One of the major factors in both challenges is the DMFC catalyst. Platinum (Pt)-based catalysts such as platinum–ruthenium (PtRu) alloys are the most practical for DMFC methanol electro-oxidation at our current level of technology [1–6]. However, these noble metal catalysts are not only expensive but also unstable, resulting in high cost and insufficient DMFC durability. In addition, the catalytic activity of these catalysts is still not high enough, resulting in a slow kinetics for the methanol electrooxidation reaction. Therefore, new catalyst development to further improve both catalytic activity and stability should be the first priority in DMFC development. To increase the catalytic activity of Pt–Ru catalysts, both experimental synthesis of catalysts and a fundamental understanding of catalytic mechanisms are necessary. With respect to experimental

∗ Corresponding author. Tel.: +86 431 85262223; fax: +86 431 85685653. E-mail address: [email protected] (W. Xing). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.07.045

work, the major strategies are optimizing existing catalysts, exploring new catalysts, and inventing innovative synthesis methods. Regarding fundamental understanding, current strategies should be focused on three aspects: exploring catalysis mechanisms, creating criteria for catalyst down-selection, and developing guides for new catalyst design. In the area of new catalyst synthesis, the most widely used methods for Pt–Ru catalyst preparation currently are the impregnation method [7–16], the colloidal method [17–26], and the microemulsion method [27–29]. The colloidal and microemulsion methods are relatively complex and costly, and therefore not appropriate for practical applications [3,30]. By comparison, the impregnation method is relatively simple and cost-effective. However, it has been difficult to prepare a homogeneous catalyst with a narrow size distribution using this method, especially when chlorine-containing precursors were used [7,10,11]. In order to improve the impregnation method, experimental optimization has been conducted by controlling synthetic conditions. For example, Yang et al. [14] and Wang et al. [31] demonstrated that some highdispersion PtRu/C catalysts with metal loadings as much as 60 wt% could be obtained by carefully controlling the synthetic conditions. In our previous work, by cautiously modifying synthetic conditions we used a simple impregnation method to obtain a uniformly distributed PtRu catalyst on carbon nanotubes [16]. Optimizing the experimental conditions thus seems fairly important in obtaining

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high activity PtRu/C catalysts when using the cost-effective and simple impregnation method. In respect of fundamental understanding, the two major focuses have been the catalytic mechanism and the catalyst structure–activity relationship. For example, the role of Ru in Pt–Ru catalysts has been emphasized in the literature. The promotion effect of Ru was identified as occurring through a bifunctional mechanism, in which Ru forms a labile oxygenated species at low potentials that converts the COads (an intermediate in methanol electro-oxidation) on neighboring Pt atoms to CO2 [3,32,33]. However, although significant understanding has been achieved, further investigation of the catalytic mechanism in methanol electrooxidation is still necessary. In other studies, it was shown that the electrocatalytic activity of PtRu catalysts was strongly dependent on catalyst bulk composition [7,17,33–36], dispersion [8,9,19,37,38], surface states [18,37,39–46], and morphology [47–49]. The effect of catalyst particle size on the activity of methanol electrooxidation has been studied extensively. Takasu et al. [8,9] showed that the specific activity decreased with diminishing PtRu particle size, but a particle size of ∼3 nm could yield maximum mass-specific activity toward methanol electro-oxidation. Another point of interest is the effect of Ru states on the catalytic activity of PtRu/C catalysts. Following the bifunctional mechanism, many researchers have emphasized the alloy degree of the PtRu catalyst. Dinh et al. [45] concluded from their in situ CO stripping experiments that the surface metal alloy domains with an atomic ratio close to 1:1 were key for high methanol electrooxidation activity. The more the metal alloy sites, the higher the catalyst activity. This finding was also supported by Arico et al. [18], who showed that catalysts with a higher alloy degree and metallic behavior on the surface appeared to be more active for methanol electro-oxidation. However, Rolison et al. [39], Long et al. [40], and Lu et al. [37,41] showed that the hydrous ruthenium oxides (RuOx Hy ) would be more beneficial for catalytic activity. As part of the continuing effort to improve PtRu/C catalyst activity by optimizing synthetic methods, in the present work the impregnation method was modified to produce a PtRu/C catalyst that displayed higher activity than (1) a catalyst prepared by the conventional impregnation method and (2) a commercially available catalyst. The potential reasons for this high activity were analyzed in terms of the structure and surface states of the catalyst.

2. Experimental 2.1. Catalyst preparation Carbon-supported PtRu catalyst (20 wt% Pt + 10 wt% Ru) was prepared from an aqueous solution of H2 PtCl6 and RuCl3 using a modified impregnation method [16]. First, Vulcan XC72 carbon (Cabot Corp.) was dispersed in deionized water with mechanical stirring and simultaneous ultrasonication. Then H2 PtCl6 and freshly prepared RuCl3 solution were added. The mixture was kept at room temperature and stirred for at least 2 h. 5% (v/v) NH3 ·H2 O was then added to raise the solution pH above 9, so that the metal precursors could precipitated to form (NH4 )2 PtCl6 and Ru(OH)3 and be loaded on carbon. After that, an excess of NaBH4 solution was added dropwise to reduce precipitation. The suspension was then filtered and washed with excess hot deionized water until no Cl− was detected. Finally, the resulting catalysts were dried at 80 ◦ C overnight in a vacuum oven. For comparison, carbon-supported PtRu catalyst was also prepared using a conventional impregnation method. In that case, instead of 5% (v/v) NH3 ·H2 O, sodium hydroxide was used to adjust the solution pH.

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2.2. Catalyst characterizations Transmission electron microscopy (TEM) was performed on a JEOL 2010 microscope operating at 200 kV. The X-ray diffraction (XRD) patterns for the catalysts were obtained using a RigakuD/MAX-PC 2500 X-ray diffractometer with the Cu K␣ ( = 1.5405 Å) radiation source operating at 40 kV and 200 mA. X-ray photoelectron spectroscopy (XPS) was carried out using an ESCALAB MKII photoelectron spectrometer (VG Scientific). The XPS experiments were performed in a spectroscopy chamber using a standard Al anode X-ray source. The Ru(3p) signals were collected and analyzed with deconvolution of the spectra using XPS Peak software. 2.3. Electrochemical measurements All electrochemical measurements were conducted using an EG&G model 273 potentiostat/galvanostat and a conventional three-electrode test cell at 25 ◦ C. To prepare the working electrode, 5 mg of PtRu/C was dispersed ultrasonically in 1 mL of diluted Nafion alcohol solution, which contained 950 ␮L ethanol and 50 ␮L Nafion solution (Aldrich, 5 w/o Nafion). Next, 10 ␮L of the suspension was pipetted onto a pre-cleaned glassy carbon substrate (Ø 3 mm). The coated electrode was then dried at room temperature for 30 min. Pt foil and Ag/AgCl were used as the counter and reference electrodes, respectively. All potentials in this report refer to the reversible hydrogen electrode (RHE). All electrolyte solutions were deaerated by high-purity nitrogen for at least 20 min prior to any measurement. Quasi-steady-state polarization and linear sweep voltammetry were carried out to evaluate the catalytic activity for methanol electro-oxidation. For quasi-steady-state polarization measurements, the working electrode was stepped from 0.05 V to potentials between 0.4 V and 0.55 V, in 0.05 V increments. The quasi-steadystate current, which was normalized to mass-specific current density (mA per mg of noble metal), was recorded after 15 min of polarization in 1 M CH3 OH + 0.5 M H2 SO4 solution at each potential. Each data set was recorded with a single electrode, i.e., after stepping from 0.05 V to 0.4 V, a further step to the following potential was performed. After the quasi-steady-state polarization measurements, linear sweep voltammetry was carried out between the potentials of 0.35 V and 0.75 V, at a scan rate of 1 mV s−1 . The surface area of the PtRu metal was measured by CO stripping voltammetry. CO was purged into 0.5 M H2 SO4 solution for 30 min to allow complete adsorption of CO onto the electrocatalyst, while maintaining a constant potential at 0 V. Excess CO was then purged with N2 for 30 min. The amount of COads was evaluated by integrating the COads stripping peak (electrode potential scan rate: 20 mV s−1 ), corrected for electric double-layer capacitance. The special surface area of the PtRu metal was estimated using two assumptions: (1) there was a monolayer of linearly adsorbed CO and (2) the Coulombic charge required for oxidation was 420 ␮C cm−2 [9,10]. 3. Results and discussion 3.1. Physicochemical characterization of PtRu/C electrocatalysts The dispersion of metal particles was analyzed by TEM measurements. Fig. 1 shows the TEM images and the corresponding particle size distributions of the PtRu/C catalysts. The size distributions were obtained by measuring the sizes of 100 randomly selected particles in the magnified TEM images. As shown in Fig. 1, the PtRu/C catalyst prepared by the modified impregnation method (this catalyst is expressed as PtRu/C-MI) shows relatively uniform and narrow size

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Fig. 1. TEM images of PtRu/C catalyst (20 wt% Pt + 10 wt% Ru). Upper: (a) prepared by modified impregnation method and (b) prepared by conventional impregnation method. Bottom: corresponding particle size distribution histograms of the catalysts: (c) prepared by modified impregnation method and (d) prepared by conventional impregnation method.

distribution, compared to the catalyst prepared by the conventional impregnation method (expressed as PtRu/C-CI). The average particle size of PtRu/C-MI was 2.3 nm, which was smaller than the size of particles formed by the conventional method (3.7 nm). This result shows that modifications to the synthetic method could improve catalyst dispersion as well as reduce particle size. Two modifications to the conventional impregnation method were likely responsible for these improvements. First was the precipitation of the precursors ((NH4 )2 PtCl6 and Ru(OH)3 ) before metal reduction. Their precipitation may have helped these precursors to load on the carbon support uniformly and prevent metal aggregation [16,50]. The second modification was altering the pH. Since the solution pH was changeable during metal reduction, the 5% (v/v) NH3 ·H2 O could have acted as a buffer agent, permitting adjustment of the pH range to optimize particle nucleation and growth rates [19,51,52]. The XRD patterns for the PtRu/C catalysts are shown in Fig. 2. Each of the catalysts displayed the characteristic patterns of Pt facecentered cubic (fcc) diffraction, except that the 2 values were all shifted to slightly higher values (shown in the inset at partial magnification). This shift was due to the incorporation of Ru atoms [20,36,44,53]. No distinct diffraction peaks related to the tetragonal RuO2 or hexagonal close-packed (hcp) Ru phases were observed. The Pt(2 2 0) peak was fitted to a Gaussian line shape on a linear background to calculate particle sizes and lattice parameters

Fig. 2. XRD patterns of PtRu/C catalysts: (a) prepared by modified impregnation method; (b) prepared by conventional impregnation method; and (c) commercially available PtRu/C catalyst.

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Table 1 Summary of structure parameters and mass-specific activities of different PtRu/C catalysts. Catalyst

PtRu/C-MI

PtRu/C-CI

PtRu/C-JM

TEM particle size (nm) XRD particle size (nm) Lattice parameter (nm) Ru atomic fraction in the alloy (%) Specific surface area (m2 g−1 metal) Mass-specific activity at 0.5 V (mA mg−1 metal)

2.3 2.4 0.38993 13.1

3.7 3.3 0.38998 12.7

– 2.5 0.38655 40.3

98.5

64

94.5

57.6

13.1

43.9

[20,31]. The particle sizes of the PtRu/C catalysts were calculated according to Scherrer formula, while the lattice parameters were calculated from XRD data according to Vegard’s law: √ 2K␣1 (1) a= sin max where K␣1 is the X-ray wavelength of Cu K␣1 radiation (=0.154056 nm) and  max is the peak position obtained from curve fitting. The Ru atomic fraction in the alloy, Ru , which is an effective parameter to determine the alloy degree of PtRu/C catalysts, was calculated using the formula proposed by Antolini and coworkers [53,54]: a = a0 − 0.124Ru

(2)

where a is the lattice parameter of the PtRu/C catalyst and a0 is the lattice parameter of pure Pt/C (=0.39155 nm). Table 1 summarizes the calculated particle sizes, lattice parameters, and Ru atomic fraction. It can be seen that the calculated sizes are in good agreement with those obtained by TEM. Moreover, the particle size of PtRu/C-MI is similar to that of commercially available 30 wt% PtRu/C catalyst (HiSPEC® 5000, Johnson Matthey); this latter catalyst is expressed as PtRu/C-JM. Both are smaller than the PtRu/C-CI particles. The Ru of PtRu/C-MI is 13.1%, which is very close to that of PtRu/C-CI (12.7%), indicating that only a small part of the Ru was alloyed while most of it existed in amorphous states. In contrast, the Ru of PtRu/C-JM is 40.3%, indicating a high degree of alloying. 3.2. Electrochemical performance Fig. 3 compares the mass-specific activity of different PtRu/C catalysts toward methanol electro-oxidation. The quasi-steady-state

Fig. 3. Comparison of the mass-specific activity of different PtRu/C catalysts toward methanol electro-oxidation. Mass-specific current densities were measured after 15 min of polarization in 1 M CH3 OH + 0.5 M H2 SO4 solution at 25 ◦ C for each potential.

Fig. 4. Comparison of the specific activities of different PtRu/C catalysts for methanol electro-oxidation: (a) prepared by modified impregnation method; (b) prepared by conventional impregnation method; and (c) baseline catalyst (Johnson Matthey). Detailed measurement conditions: 1 M CH3 OH + 0.5 M H2 SO4 , 25 ◦ C. Potential scan rate was 1 mV s−1 .

current density was recorded after 15 min in 1 M CH3 OH + 0.5 M H2 SO4 solution at 25 ◦ C. A potential region envisaged to be of interest for DMFC applications was selected for comparison [22,33,34]. It can be seen that PtRu/C-MI exhibited much better performance than both PtRu/C-CI and PtRu/C-JM for methanol electro-oxidation. For example, at 0.5 V, the mass-specific activity of PtRu/C-MI was 1.3 times higher than that of PtRu/C-JM and, remarkably, 5.2 times higher than that of PtRu/C-CI, as shown in Table 1. Moreover, all the PtRu/C catalysts showed similar Tafel slopes of about 140 mV dec−1 , probably suggesting a one-electron rate-determining step related to COads being removed from Pt sites by neighboring Ru–OH species [22,40]. To measure the catalysts’ intrinsic efficiency, CO stripping voltammetry was performed to determine the electrochemically active surface area. Table 1 also summarizes the specific electrochemical surface area of the catalysts measured by COads stripping analysis. PtRu/C-MI shows a larger specific electrochemical surface area (98.5 m2 g−1 ) compared to that of PtRu/C-CI (64 m2 g−1 ), which coincides with particle size order if assuming that the particles are spheres [36,54]. The specific activity was also measured by linear sweep voltammetry, with a scan rate of 1 mV s−1 . Fig. 4 compares the specific activity of different PtRu/C catalysts toward methanol electro-oxidation. At potential regions higher than 0.65 V, the specific activity shows an order of PtRu/C-JM > PtRu/C-MI > PtRu/CCI, while at potential regions lower than 0.65 V the order is PtRu/C-MI > PtRu/C-JM > PtRu/C-CI. It should be noted that the high potential regions (E > 0.55 V) are irrelevant for DMFC applications [22]. Therefore, the PtRu/C-MI in this work seems to be more active in the potential regions of interest for DMFC applications, compared to the baseline PtRu/C-JM catalyst. It would be interesting to investigate the origination of the PtRu/C-MI catalyst’s high activity. Some researchers have reported that the electrocatalytic activity of the PtRu/C catalyst was strongly dependent on the alloy degree; specifically, the higher the alloy degree, the more active the catalyst should be [17,18,45]. However, in the present study, PtRu/C-MI – with a lower alloy degree – showed a higher specific activity when compared to the baseline PtRu/C-JM catalyst, even though their average particle sizes were comparable. In addition, PtRu/C-MI showed a similar alloy degree to that of PtRu/C-CI, but its specific activity was much higher. This implies that the alloy degree may not be the main factor in determining catalyst activity. Apart from alloy degree and particle size, the surface states of Ru in the PtRu/C catalysts may significantly affect the electrocatalytic activity [18,37,39–46]. Furthermore, as discussed above, all the catalysts exhibited similar Tafel slopes, indicating a similar methanol electro-oxidation mechanism, which is

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Fig. 5. XPS spectra of different PtRu/C catalysts (Ru 3p3/2 line): (a) prepared by modified impregnation method; (b) prepared by conventional impregnation method; and (c) baseline catalyst (Johnson Matthey). Points: experimental data; lines: fitting curves.

related to the removal of COads from Pt sites by neighboring Ru–OH species. Therefore, in the following, we use XPS to analyze the surface states of Ru in PtRu/C catalysts.

3.3. XPS analysis To avoid interference from the C 1s signals, the Ru 3p signals (instead of the Ru 3d signals) of the different PtRu/C catalysts were analyzed and compared. Fig. 5 shows the curve-fitted Ru 3p3/2 spectra of the PtRu/C catalysts. The Ru 3p3/2 signals were deconvolved into three components, and the atomic ratio and binding energy of each component are listed in Table 2. The three components were attributed to metallic Ru (Ru0 ), anhydrous ruthenium oxide (RuO2 ), and hydrous ruthenium oxides (RuOx Hy ), respectively [14,18,55–57]. The component with the highest binding energy (466.2 eV) could not be assigned to the RuO3 species, since RuO3 is not thermodynamically stable [18,39]. Instead, we assigned it to the hydrous RuOx Hy species, which was reported to have a higher binding energy than anhydrous RuO2 [14,18,39,57].

As shown in Table 2, PtRu/C-MI contained the highest content of hydrous RuOx Hy species, which should explain its higher specific activity. The PtRu/C-CI catalyst contained much more content of the anhydrous RuO2 species, which is believed to be why it had the lowest specific activity. Rolison et al. [39,40] suggested that RuOx Hy , which is a mixed proton and electron conductor similar to Ru–OH species, should be the preferred Ru species, while other species should be less active. Cao et al. [58] directly showed the promotion effect of RuOx Hy for methanol oxidation. Based on our results and the literature, we believe that the high RuOx Hy species content on the catalyst surface should account for the high activity of PtRu/CMI. Unfortunately, although PtRu/C-CI showed a higher RuOx Hy species content than PtRu/C-JM, its activity was much lower. This may suggest that the presence of RuOx Hy species is not the only factor affecting catalyst activity. According to the bifunctional mechanism, metallic Ru could also act as an active species in the catalyst. On the other hand, the fact that PtRu/C-CI, with the highest anhydrous RuO2 species content, showed the lowest specific activity suggests the activity of RuO2 species should be much lower than that of metallic Ru and

Table 2 Binding energy and relative intensities of different species from curve-fitted XPS spectra and specific activity of different PtRu/C catalysts. Signal/sample

Binding energy (eV)

Species

Atomic ratio (%)

Specific activitya (␮A cm−2 )

Ru 3p3/2 /PtRu/C-MI

462.0 463.9 466.2

Ru RuO2 RuOx Hy

39.86 26.99 33.15

67.7

Ru 3p3/2 /PtRu/C-CI

462.0 463.9 466.2

Ru RuO2 RuOx Hy

28.44 40.11 31.44

30.1

Ru 3p3/2 /PtRu/C-JM

462.0 463.9 466.2

Ru RuO2 RuOx Hy

48.73 23.57 27.69

56.7

a

Specific activity measured at 0.5 V vs. RHE by linear sweep voltammetry; the scan rate was 1 mV s−1 , 25 ◦ C.

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hydrous RuOx Hy . In fact, many studies have failed to find any promotion effect from RuO2 [59–63]. Thus, because anhydrous RuO2 is a strongly oxidized species, it should be avoided on the catalyst surface. From the above it may be concluded that both metallic Ru and hydrous RuOx Hy are active surface species, while the anhydrous RuO2 species is inactive. Furthermore, as PtRu/C-MI showed a lower total content of metallic Ru and hydrous RuOx Hy species but a higher specific activity than PtRu/C-JM, it can be concluded that RuOx Hy species should be more active than metallic Ru species, since PtRu/C-MI has a relatively higher RuOx Hy species content. In addition, in this paper, it was found that the specific activities showed an order of PtRu/C-MI > PtRu/C-JM > PtRu/C-CI, whereas the particle sizes of these PtRu/C catalysts showed a reverse sequence. This is inconsistent with the “size effect” observed in the literature [8,9]. We believe that when a catalyst particle size is changed, its structural parameters will also alter. Such changes in structural parameters could have a larger effect on the catalytic activity than the effect of particle size, which makes the situation more complex. Nevertheless, the particle size effect should also be considered when investigating catalytic activity. 4. Conclusions In this study, a highly dispersed PtRu/C catalyst was synthesized using a modified impregnation method. The obtained catalyst showed a much higher activity for methanol electro-oxidation than a catalyst prepared by the conventional impregnation method, and an even higher activity than a current state-of-the-art commercially available catalyst. The origination of this catalyst’s high activity was also investigated using XPS measurements. Based on detailed structure and surface analysis, we have proposed that it was the surface species on the PtRu catalyst, rather than the alloy degree, that determined this high activity. Hydrous RuOx Hy and metallic Ru species are believed to be the active sites on the catalyst, while the anhydrous RuO2 species seemed to be less active. Acknowledgments This work is supported by the High Technology Research Program (863 programs 2007AA05Z159, 2007AA05Z143) of the Science and Technology Ministry of China, and the National Natural Science Foundation of China (Key Projects 20433060, 20703043). Professor Wei Xing would also like to thank the Institute for Fuel Cell Innovation, National Research Council of Canada (NRC-IFCI) for his visiting professorship. References [1] [2] [3] [4]

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