A Facile Preparation of Hollow Palladium Nanosphere Catalysts for

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A Facile Preparation of Hollow Palladium Nanosphere Catalysts for Direct Formic Acid Fuel Cell Zhengyu Bai,† Lin Yang,*,† Lei Li,‡ Jing Lv,† Kui Wang,† and Jie Zhang† College of Chemistry and EnVironmental Science, Henan Normal UniVersity, Xinxiang 453007, People’s Republic of China, and School of Chemistry and Chemical Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, People’s Republic of China ReceiVed: January 6, 2009; ReVised Manuscript ReceiVed: April 27, 2009

A facile and low-cost preparation of a polypyrrole-modified Vulcan XC-72 carbon-supported Pd/PPy-XC72 hollow nanosphere catalyst was introduced in this paper. To increase the stability and utilization ratio of the catalyst, a two-step strategy was employed, which included the synthesis of a polypyrrole-carbon composite and the preparation of a hollow nanosphere catalyst. Co nanoparticles as sacrificial templates were deposited on the prepared polypyrrole-carbon composites by chemical reduction, and then Pd hollow nanospheres formed through the surface replacement reaction between the surface of Co nanoparticles and PdCl2 at room temperature. The novel Pd/PPy-XC-72 hollow nanosphere catalyst was 30-40 nm in diameter and 3-4 nm in shell thickness. FT-IR and TGA characterizations confirmed the existence and content of polypyrrole in the catalysts. The catalysts with 40 and 20% Pd showed a very high electrochemically active surface area and significant increase in electrocatalytic activity toward formic acid oxidation, which make them the preferable catalysts for direct formic acid fuel cells. 1. Introduction Fuel cells are an attractive option for power generation due to their high efficiency and little or no pollution.1-3 Among various types of fuel cells, the direct formic acid fuel cell (DFAFC) is considered as a promising system for automotive and portable electronic applications owing to its high energy density and low operating temperature.4-7 Formic acid is nontoxic, and DFAFCs are able to achieve a higher power density than that of direct methanol fuel cells, although the energy density of methanol is higher than that of formic acid.8,9 However, the high cost of catalysts caused by the exclusive use of platinum and platinum-based catalysts in the fuel-cell electrodes is one of the major barriers to limit the commercial application of DFAFCs. Many investigations in this field were focused on the exploration of less expensive, more abundant nonplatinum catalysts that can offer acceptable performance.10-12 Recently, Pd catalysts were found to possess superior performance in formic acid oxidation compared with that of Pt-based catalysts and were considered as a substitute of Pt for the catalyst in DFAFCs.13-17 In the latest decade, core-shell or hollow metal nanostructures have attracted extensive interest due to their distinguished chemical and physical properties resulting from their special shape and composition. An alternative strategy can be selected to prepare catalysts with core-shell or hollow metal nanostructures, of which the ultranoble metals are deposited onto the surface of the non-noble metal core through the surface replacement reaction involving sacrificial templates.18-23 It not only achieves high catalytic performance and utilization efficiency because of their relatively lower densities and higher surface areas than those of their solid counterparts but also * Corresponding author. Phone: +86-373-3328117. Fax: +86-3733328507. E-mail: [email protected]. † Henan Normal University. ‡ Shanghai Jiao Tong University.

reduces the cost of the catalyst. Zhong and his co-workers described the novel findings of the investigation of core/shell nanoparticles (Au@Pt, Pt@Au, Fe3O4@Au@Pt) for their nanostructural correlation of the electrocatalytic properties for the methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR).21 Dong et al. reported a spongelike Au/Pt core/ shell electrocatalyst with hollow cavity obtained via a wet chemical process.22 Xing et al. investigated the Pd hollow nanosphere catalyst for DFAFCs and controlled the particle size by adjusting the pH value.23 Au, Ag, Pt, and Au/Pt alloy hollow nanostructures have also been prepared via employing Co nanoparticles as sacrificial templates produced in situ.24,25 However, the effective control of wall thicknesses and dispersivity of the resulting hollow particles, as well as the stability of catalysts, is still under improvement to increase the electrocatalytic activity and utilization ratio of catalysts. In this paper, our objectives are to control the structure and particle size of catalysts and increase the dispersivity. Herein, a non-noble metal, cobalt, employed as a sacrificial template was entrapped in the structure of conducting polymers, and then Pd hollow nanospheres were prepared through the surface replacement reaction. Compared with the prevenient preparation methods of hollow metal nanostructures, the difference is that the surface of carbon particles is modified by polypyrrole (PPy). It is well-known that conducting polymers, such as PPy and polyaniline (PANI), have good electronic and proton conductivity, dispervisity, and stability. Furthermore, the addition of PPy could control the dispersivity of the product and generate a matrix for entrapping cobalt and MeNx (Me Co, Fe) active ORR sites, which could increase the electrocatalytic activity and utilization ratio of the catalyst.26 Because the hollow nanospheres were entrapped in the structure of PPy, the as-prepared catalyst showed a very high electrochemically active surface (EAS) area, and the electrocatalytic activity increased significantly.

10.1021/jp902713k CCC: $40.75  2009 American Chemical Society Published on Web 05/26/2009

Hollow Palladium Nanosphere Catalysts

Figure 1. Formation mechanism of Pd hollow nanosphere catalyst.

2. Experimental Section 2.1. Synthesis of PPy-Carbon Composite. Briefly, Vulcan XC-72 (Cabot Corp., USA) was pretreated in boiled 6 M HNO3

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10569 for 8 h, then washed with double-distilled water (DD water) and dried at 60 °C in vacuum condition for 12 h. PPy-carbon composites were synthesized by in situ chemical oxidative polymerization of pyrrole monomer on carbon powders. The above pretreated Vulcan XC-72 (0.6 g) was dispersed in 150 mL of isopropanol aqueous solution (volume ratio ) 1:1) by ultrasonic treatment for 30 min. Three millimoles of pyrrole monomer (dissolved in 10 mL of ethanol) was added to the above suspension and stirred for 30 min. Then 200 mL of (NH4)2S2O8 (0.82 g) aqueous solution was added slowly to the suspension with constant stirring for 4 h at room temperature. After reaction, the resulting PPy-XC-72 powder was filtered and rinsed with DD water and absolute ethanol until the filtrate became colorless. The obtained black powder was dried under vacuum condition at room temperature for 24 h. 2.2. Preparation of Hollow Nanosphere Pd/PPy-XC-72 Catalyst. Pretreated PPy-XC-72 (100 mg) and 217 mg of Co(NO3)3 · 6H2O were added in 100 mL of ethanol aqueous solution (volume ratio ) 1:1) and ultrasonically treated for 30 min, then the mixture was stirred and purged with N2 for 1 h to keep the saturated N2 atmosphere. A freshly prepared solution

Figure 2. TEM images of the as-prepared Co nanocrystal cores (a) and Pd nanoparticle catalyst with PPy (b, c) and without PPy (d).

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Figure 3. X-ray diffraction pattern (a) and EDS pattern (b) of hollow nanosphere Pd/PPy-XC-72 (40% Pd) catalyst.

of NaBH4 (0.5 mg in 10 mL of 0.1 M NaOH solution) was added dropwise into the above solution under stirring for 2 h to form Co nanocrystal cores, and then the reaction was continued for another 10 h to ensure the complete decomposition of the redundant NaBH4. Afterward, a precursor aqueous solution of the right amount PdCl2 was added dropwise with vigorous stirring for 5 h. The whole process was at room temperature. Finally, the product was collected by filtration, washed several times with absolute ethanol and DD water, and then dried at 50 °C under vacuum condition for 12 h to obtain the product to afford measurement. 2.3. Measurements. The morphology of the catalyst was measured by high-resolution transmission electron microscopy (HRTEM) (JEOL-100CX) with an energy-dispersive spectrometer (EDS) operated at 200 kV. Nanoparticles were first immersed into ethanol and, subsequently, dispersed ultrasonically for 5 min. A drop of the suspension was then deposited on a lacey carbon grid and dried in air for TEM observations. X-ray diffraction (XRD) analysis was performed using a D/max2200/PC X-ray diffractometer with Cu KR radiation source. The Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a Bio-Rad FTS-40 Fourier transform infrared spectrometer in the wavenumber range of 4000-400 cm-1, and thermogravimetric analysis (TGA) was performed on an NETZSCH STA 449C instrument. Cyclic voltammetry measurements were carried out in a threeelectrode cell by using a model 616 rotating disk electrode from

Autolab PGSTAT302 electrochemical test system. A glassy carbon disk (3 mm o.d.) coating catalyst was used as working electrode, a platinum foil (1 cm-2) as counter-electrode, and an Ag/AgCl electrode as reference. A 0.5 M H2SO4 aqueous solution served as electrolyte for hydrogen oxidation measurements and 0.5 M HCOOH in 0.5 M H2SO4 for formic acid oxidation measurements. High-purity N2 was bubbled into the electrolytes during the experiments. 3. Results and Discussion The Pd/PPy-XC-72 hollow nanospheres were prepared through the surface replacement reaction, which was supported by in situ chemical polymerization of pyrrole on Vulcan XC72 carbon powders. Here, Co(NO3)3 · 6H2O was used as the source of the Co core, PdCl2 as the source of the Pd shell, PPy as the electric polymer and stabilizer, and XC-72 carbon as the support. First, (NH4)2S2O8 was used as the oxidant for polymerization of pyrrole monomers. PPy modification can increase dispersivity and the EAS area, so it is able to improve the electrocatalysis utilization ratio of the catalyst. Second, the cobalt source was entrapped in the structure of PPy and then was reduced to Co nanocrystals, which were employed as seeds for the further growth. At the same time, active Co-N sites were formed without destroying the initial polymer structure. Third, the Co@Pd/PPy-XC-72 core-shell nanospheres were formed through a surface replacement reaction between Co nanoparticles

Hollow Palladium Nanosphere Catalysts

Figure 4. FT-IR spectra of the samples: (a) PPy, (b) Pd/PPy, and (c) Pd/PPy-XC-72 (40% Pd) catalyst.

and PdCl2 around the cobalt nanoparticles. Finally, the residual cobalt was removed carefully by dilute hydrochloric acid, then the Pd/PPy-XC-72 hollow nanospheres were formed. The formation mechanism of hollow nanosphere Pd/PPy-XC-72 catalyst is shown in Figure 1. Here, PPy makes a strong impact on the catalyst because the cobalt atoms are entrapped in the structure of PPy and link to pyrrole units, thus allowing for the formation of active Co-N sites without destroying the initial polymer structure. The hypothesis involving Me-N active sites has been supported by the studies correlating fuel cell performance with nitrogen concentration in the porphyrin catalyst.26,27 Figure 2 shows transmission electron microscopy (TEM) images of Co nanocrystal cores and the resulting Pd samples. As observed in Figure 2a, Co nanoparticles formed in the second step are welldispersed on the surface of the XC-72 carbon with a relatively narrow particle size distribution, and the mean particle diameter is ca. 10 nm. From Figure 2b, the final product, hollow Pd nanospheres, is well-dispersed on the surface of the Vulcan XC-72 carbon support with a relatively narrow particle size distribution. The particle diameters vary from 30 to 40 nm, and the average thickness of the shell is about 3 nm. The selected area electron diffraction (SAED) pattern (inset in Figure 2b) of a Pd nanoparticle shows a lattice spacing of d ) 2.3022, 1.9827, 1.3529, and 1.1735 Å that, respectively, corresponds to the (111), (200), (220), and (311) facets of the Pd nanoparticle. All of the spacings are very close to the standard d values. The corresponding HRTEM image of the hollow Pd nanospheres in Figure 2c shows that the Pd crystalline plane distance is 1.176 Å, which is very close to the standard d value (1.172 Å) of the pure Pd (311) plane distance. To investigate the influence of PPy on the formation of the hollow Pd nanospheres, a control experiment was carried out in the absence of PPy (see Figure 2d), in which the hollow nanospheres disappeared, while carbon-supported Pd nanoparticles with an average size of about 3 nm were formed with serious agglomeration. The results demonstrated that the presence of PPy was a key factor in controlling and regulating the shape and size of the hollow Pd nanospheres. It can be believed that cobalt particles were entrapped in the structure of PPy, allowing for the formation of active MeNx sites, which contributed to the synthesis of hollow nanospheric Pd without agglomeration. Figure 3 shows the XRD pattern and EDS pattern of the asprepared hollow nanosphere Pd/PPy-XC-72 catalysts. It could be observed from the XRD pattern that the four peaks at 39.8°, 46.3°, 67.6°, and 81.9° are characteristic of face-centered cubic

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Figure 5. TGA curves of (a) Pd/XC-72 catalyst, (b) hollow nanosphere Pd/PPy-XC-72 catalyst, and (c) pure PPy. All of them were under the protection of N2.

(fcc) crystalline Pd, which corresponds to the (111), (200), (220), and (311) facets, respectively, except for the characteristic peak of amorphous Vulcan XC-72 carbon support at 24.5°. Furthermore, no peaks related to Co were observed, showing that most of the cobalt has been redissolved into solution by the replacement reaction, and this result was also proved by the succedent EDS measurement (Figure 3b). In Figure 3b, the Pd and Co content in the sample is about 39 and 4 wt %, respectively. It may be related to the fact that there were some cobalt leftovers in the Pd/PPy-XC-72 catalyst, in accordance with the theoretical palladium content of 40 wt %. To confirm the existence of PPy, the Pd/PPy-XC-72 composite was characterized by FT-IR and TGA. Figure 4 shows the FT-IR spectra of PPy, Pd/PPy, and the as-prepared Pd/ PPy-XC-72 catalyst. In Figure 4, spectrum a, all characteristic peaks of PPy can be observed. The peak located at about 1649 cm-1 is assigned to the coupling between CdC and the unsymmetrical stretching vibration of the pyrrole rings, 1438 cm-1 to stretching vibration of pyrrole rings, 3420 cm-1 to N-H, and 1192 cm-1 to C-N stretching of the PPy. PPy is characterized by major peaks at 1540 and 1450 cm-1 that are attributed to the unsymmetrical stretching vibration and stretching vibration of pyrrole rings, respectively. These data are consistent with other reports.30,31 As shown in Figure 4, spectrum b, the FT-IR spectrum of Pd/PPy nanocomposite is different from that of PPy. Compared with pure PPy, the coupling peak at 1649 cm-1 split and shifted to low wavenumber, and the peak of PPy in the Pd/PPy nanocomposite at 1438 cm-1 shifted to low wavenumber. These may due to the presence of the Pd atoms entrapped in the structure of PPy. Similar results have been reported in the system of polymers combining with Au.32,33 In Figure 4, spectrum c, the decrease of band intensity is ascribed to C adsorbing IR spectra; however, absorption peaks of the unsymmetrical stretching and stretching vibration of pyrrole rings at 1577 and 1407 cm-1 are also observed. Overall, these FT-IR spectra provided supportive evidence that Pd/PPy-XC72 nanocomposites with Pd atoms entrapped in the structure of PPy have been successfully prepared. The TGA curves shown in Figure 5 compare the thermal decomposition curves of pure PPy and as-prepared Pd/XC-72 and Pd/PPy-XC-72 (40% Pd) catalysts under the protection of N2. The TGA curve of pure PPy shows a two-step weight loss. The weight loss in the first step (100-200 °C) is attributed to the loss of some small polymers via degradation. The second step, which starts at

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Figure 6. Cyclic voltammograms of as-prepared hollow nanosphere Pd/PPy-XC-72 catalysts on a glassy carbon electrode: electrolyte, 0.5 M H2SO4 aqueous solution; sweep rate, 50 mV s-1; at 25 °C.

around 300 °C, corresponds to polymer degradation.34,35 The Pd/PPy-XC-72 catalyst shows an earlier initial mass loss and the fastest mass loss as compared to that of the palladium-free samples. The TGA data suggest that decomposition of the composite appears to be activated in the presence of palladium.36 Figure 6 shows the electrochemical reactivity and EAS areas of different catalysts determined by cyclic voltammetry measurement performed in 0.5 M H2SO4 electrolyte at a scan rate of 50 mV · s-1. For a reference, catalysts Pd/PPy-XC-72 with 20% Pd and Pd/XC-72 were synthesized under the same conditions to compare the electrochemical activity. As shown in Figure 6, the hydrogen adsorption peaks of hollow nanosphere Pd/PPy-XC-72 catalyst with 40% Pd and 20% Pd are both larger than those of JM commercial 40% Pt/C. According to the Coulombic amount (Q) associated with the peak area, the EAS area can be calculated using the following equation.28,29

EAS )

Q m·C

(1)

where C denotes the quantity of electricity when hydrogen molecules are adsorbed on palladium with a homogeneous and single layer (here, it is 210 µC · cm2) and m is the mass of palladium on the catalyst surface. The EAS values of the catalysts are shown in Table 1. The EAS value of as-prepared hollow nanosphere Pd/PPy-XC-72 catalysts is significantly larger than that of JM commercial 40% Pt/C catalyst and Pd/XC-72 catalyst, indicating the enhanced electrochemical activity compared to that of commercial catalysts. More importantly, the as-prepared hollow nanosphere Pd/ PPy-XC-72 catalyst samples show excellent stability during cycling. There is almost no drop in the peak current density after the 50 cycle cyclic voltammetry measurement process. The cyclic voltammetry curve of methanol oxidation on as-prepared Pd/PPy-XC-72 catalysts was also tested; the result was the same as the curve tested in H2SO4 solution.

Figure 7. Cyclic voltammograms of as-prepared hollow nanosphere Pd/PPy-XC-72 catalysts on a glassy carbon electrode: electrolyte, 0.5 M H2SO4 + 0.5 M HCOOH aqueous solution; sweep rate, 50 mV s-1; at 25 °C.

Figure 7 is a comparison of formic acid oxidation on asprepared catalysts with different Pd content, which was tested in 0.5 M H2SO4 + 0.5 M HCOOH aqueous solution under halfcell conditions. From the curves, two main peaks for the formic acid oxidation at about 0.35 V in both the positive and negative scan directions are observed in the two catalyst electrodes, and the corresponding peak current densities are both over 150 mA/ cm2, which are much higher than those for recently reported catalysts.15,16 The higher activity of the as-prepared hollow nanosphere Pd/PPy-XC-72 catalysts may be also attributed to its hollow nanosphere structure and high EAS area. In addition, the cobalt leftovers (4 wt %) in the product may also be helpful to the catalytic oxidation of formic acid. It can be seen clearly that the peak current densities of the Pd/PPy-XC-72 (40% Pd) catalyst and 20% Pd catalyst are both much higher than that of the Pd/XC-72 (40% Pd) catalyst. It further proves that the catalytic activity of as-prepared Pd/PPy-XC-72 catalyst is much superior to that of Pd/XC-72 catalyst without PPy. This suggests that the existence of PPy in catalysts is an important factor for their electrocatalytic activities. 4. Conclusions In conclusion, the Pd/PPy-XC-72 hollow nanospheric catalyst was prepared with a very simple chemical reduction method. Herein, a non-noble metal cobalt employed as a sacrificial template was entrapped in the structure of conducting polymers, and then Pd hollow nanospheres were formed through the surface replacement reaction. PPy modification can increase dispersivity and the electrochemical surface area significantly, which could improve the electrocatalytic activity and utilization ratio of the catalyst. The electrocatalytic activity of the as-prepared catalysts for the oxidation of formic acid is much higher than that of recently reported catalysts. The hollow nanosphere structure enlarged the EAS of catalysts through the direct pathway, so that

TABLE 1: Electrochemically Active Surface of Different Catalysts catalyst

hollow nanospheres Pd/PPy-XC-72 with 40% Pd

hollow nanospheres Pd/PPy-XC-72 with 20% Pd

Pd/PPy-XC-72

JM commercial 40%Pt/C

EAS (m2 g-1 Pt/Pd)

118

68

54

62

Hollow Palladium Nanosphere Catalysts enhanced the electrocatalytic activity. The significantly lower cost and higher electrocatalytic activity of Pd/PPy-XC-72 hollow nanospheres make them a priori catalysts for future DFAFCs. Acknowledgment. This work was financially supported by the National Basic Research Program of China (Grant No. 2005CB724306), National Key Basic Research and Development Program of China (Grant No. 2009CB626610), the National Science Foundation of China (Grant No. 20771036), and the National Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20070476001). Henan Key Proposed Program for Basic and Frontier Research (2009). Supporting Information Available: The stability of the Pd/ PPy-XC-72 catalysts in the acidic solution and SEM image of the as-prepared Pd/PPy-XC-72 catalyst. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Rajesh, B.; Ravindranathan Thampi, K.; Bonard, J. M.; Mathieu, H. J.; Xanthopoulos, N.; Viswanathan, B. Electrochem. Solid-State Lett. 2004, 7, 404. (2) Rajesh, B.; Piotr, Z. Nature 2006, 443, 63. (3) Jarvi, T. D.; Sriramulu, S.; Stuve, E. M. Colloids Surf., A 1998, 134, 145. (4) Rice, C.; Ha, S.; Masel, R. I.; Waszczuk, P.; Wieckowski, A.; Barnard, T. J. Power Sources 2002, 111, 83. (5) Rice, C.; Ha, S.; Masel, R. I.; Wieckowski, A. J. Power Sources 2003, 115, 229. (6) Zhu, Y. M.; Ha, S.; Masel, R. I. J. Power Sources 2004, 130, 8. (7) Kang, S.; Lee, J.; Lee, J. K.; Chung, S. Y.; Tak, Y. J. Phys. Chem. B 2006, 110, 7270. (8) Zhu, Y. M.; Khana, Z.; Masela, R. I. J. Power Sources 2005, 139, 15. (9) Jayashree, R. S.; Spendelow, J. S.; Yeom, J.; Rastogi, C.; Shannon, M. A.; Kenis, P. J. A. Electrochim. Acta 2005, 50, 4674. (10) Zhou, W. J.; Lee, J. Y. Electrochem. Commun. 2007, 9, 1725. (11) Li, X. G.; Hsing, I. M. Electrochim. Acta 2006, 51, 3477.

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