15133
2007, 111, 15133-15136 Published on Web 10/03/2007
Improvement in the Durability of Pt Electrocatalysts by Coverage with Silica Layers Sakae Takenaka,*,† Hiroshi Matsumori,† Keizo Nakagawa,† Hideki Matsune,† Eishi Tanabe,‡ and Masahiro Kishida*,† Department of Chemical Engineering, Graduate School of Engineering, Kyushu UniVersity, Moto-oka 744, Nishi-ku, Fukuoka 819-0395, Japan, and Western Hiroshima Prefecture Industrial Institute, Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-0046, Japan ReceiVed: August 1, 2007; In Final Form: September 1, 2007
Carbon nanotube (CNT)-supported Pt metal nanoparticles were covered with silica layers by utilizing the successive hydrolysis of 3-aminopropyl-triethoxysilane and tetraethoxysilane on CNTs with Pt hydroxide. The CNT-supported Pt metal particles covered with silica layers (denoted as SiO2/Pt/CNT) were used as the electrocatalysts. SiO2/Pt/CNT electrocatalysts showed a high stability for the repeated potential cycling experiment, whereas Pt/CNT electrocatalysts were deactivated seriously for the experiment because of the growth of Pt metal particles in size. The silica layers in SiO2/Pt/CNT prevent the dissolution of Pt metal particles as well as the migration and agglomeration of Pt metal particles on the supports, which results in the improvement of the stability of Pt/CNT electrocatalysts.
Introduction Proton-exchange-membrane fuel cells (PEMFCs) are promising alternative power sources for transportation and portable applications because of advantages such as low emission, high efficiency, and rapid start-up. Highly dispersed Pt metal particles supported on a conductive support with a high surface area such as carbon black are usually utilized as electrocatalysts for both cathodes and anodes in PEMFCs. The durability of the Pt catalysts, especially the catalysts for the oxygen-reduction reaction (ORR) at cathodes, has been recognized recently as one of the most important issues that must be solved in order to make PEMFCs a reality.1-3 The electrochemically active surface area (ECSA) of Pt metal particles at the cathode reduces gradually during the repeated power-on and -off processes of the PEMFC because of the growth of the particles in size.4-6 Three different mechanisms for the loss of ECSA of Pt metal are well accepted. First, Pt metal particles migrate on carbon supports and subsequently agglomerate upon their collision.7 Second, Pt metal particles grow through Ostwald ripening; that is, single Pt atoms or molecules with Pt particles are transported onto other particles to form larger particles.8-10 Finally, unsupported Pt metal particles are formed due to the corrosion of carbon supports.11,12 These particles are agglomerated easily on the surface of carbons or ionomers. Various additives such as Ni and Co were added into Pt catalysts in order to suppress the growth of Pt metal particles at cathodes.13-17 However, most of the additives were also dissolved and diffused into the membrane of the PEMFC, which resulted in a decreased proton conductivity of the membrane.18 * Authors for correspondence. Sakae Takenaka, Department of Chemical Engineering, Graduate School of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan. Tel: +81-92-802-2752. Fax: +81-92-802-2752. E-mail:
[email protected]. † Kyushu University. ‡ Western Hiroshima Prefecture Industrial Institute.
10.1021/jp076120b CCC: $37.00
Here, we applied carbon nanotube (CNT)-supported Pt metal particles covered with silica layers (denoted as SiO2/Pt/CNT) to the electrocatalysts. Transportation of single Pt atoms or molecules with Pt particles onto other particles and/or the migration of Pt metal particles on CNT supports should not occur easily because Pt metal particles in SiO2/Pt/CNT are covered with silica layers. In addition, it is well accepted that CNT has a high tolerance to corrosion because of its high graphitization degree.19,20 In the present study, a superior stability is reported for SiO2/Pt/CNT electrocatalysts. Experimental Section Commercially available (supplied from Aldrich) multiwalled carbon nanotubes (denoted as CNTs) were used for the preparation of SiO2/Pt/CNT and Pt/CNT. The coverage of Pt/ CNT with silica layers was performed by utilizing the hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and tetraethoxysilane (TEOS) on the CNTs with Pt hydroxide.21 The preparation method of SiO2/Pt/CNT was described in detail in the Supporting Information. The TEM images of the samples were measured with a JEOL JEM-3000F. Measurement of Pt LIII-edge XANES spectra was performed at the Photon Factory of the Institute of Materials Structure Science for High-Energy Accelerator Research Organization, Tsukuba, Japan. The spectra were measured with a transmission mode with a Si(111) two-crystal monochromator on the beam line BL-9A at room temperature (Proposal No. 2007G532). Electrochemical measurements were carried out using a threecompartment electrochemical cell with a Pt mesh and a saturated Ag/AgCl electrode serving as the counter and reference electrode, respectively. The saturated Ag/AgCl electrode was separated from the working electrode compartment by a closed electrolyte bridge. All potentials are given relative to reversible hydrogen electrode (RHE). A glassy carbon disk electrode © 2007 American Chemical Society
15134 J. Phys. Chem. C, Vol. 111, No. 42, 2007
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Figure 2. Cyclic voltammograms for Pt/CNT (a) and SiO2/Pt/CNT (b) during the repeated potential cycling in 0.5 M H2SO4 at 303 K. Scan rate ) 50 mVs-1. The amount of Pt in the electrode: 0.0143 mg cm-2 for Pt/CNT; 0.0088 mg cm-2 for SiO2/Pt/CNT. Figure 1. TEM images of Pt/CNT (a and b), SiO2/Pt/CNT (c and d), and SiO2/Pt/CNT calcined at 1073 K in air (e and f).
(5 mm diameter) was used as the substrate for the catalysts and polished to a mirror finish. Catalyst ink was prepared by ultrasonically blending 0.020 g of the catalysts and 10.0 mL of methanol. A 20.0 µL aliquot of this ink was deposited on a glassy carbon disk and dried at 333 K. A 20 µL quantity of diluted Nafion with methanol was then dropped onto the catalysts to ensure the attachment of the catalysts with the disk. After the preparation of the catalysts, the electrode was applied with a potential between 0.05 and 1.20 V vs RHE in N2-purged 0.5 M H2SO4 at 303 K in order to perform the experiment for accelerated durability of the catalysts. Desorption of underpotentially deposited hydrogen was used to evaluate the ECSA of the Pt catalysts. Results and Discussion SiO2/Pt/CNT was prepared by the successive hydrolysis of APTES and TEOS on CNTs with Pt hydroxides. For comparison, CNT-supported Pt metal (denoted as Pt/CNT, 7 wt % as Pt) was prepared by a conventional impregnation method. The contents of Pt, CNT, and SiO2 in SiO2/Pt/CNT were evaluated by X-ray fluorescence spectroscopy and by thermogravimetric analysis under an air stream to be 3.9, 41.4, and 54.7 wt %, respectively. The structure of Pt species in SiO2/Pt/CNT and Pt/CNT was examined by Pt LIII-edge XANES spectra (Figure S1 in the Supporting Information).22 The XANES spectra show that the Pt species in these samples are present as Pt metal. Figure 1 shows TEM images of Pt/CNT and SiO2/Pt/CNT. In TEM images a and b for Pt/CNT, Pt metal particles with
diameters of 1-3 nm are observed on CNTs. In the TEM images for SiO2/Pt/CNT, CNTs, and Pt particles are also observed. However, the diameters of CNTs in SiO2/Pt/CNT seem larger than those in Pt/CNT. The result strongly suggests that CNTs in SiO2/Pt/CNT are covered with thin silica layers. The coverage of CNTs with silica layers is also clarified by the HRTEM image (d) for SiO2/Pt/CNT. It seems that Pt metal particles in SiO2/ Pt/CNT are always present in the silica layer; that is, Pt metal particles on CNT are covered with silica layers. Figure 1e and f shows TEM images for SiO2/Pt/CNT, which was calcined at 1073 K in air. In these TEM images, the formation of silica tubes is observed and Pt metal particles are always present in the channel of the tubes. When SiO2/Pt/CNT was calcined at 1073 K in air, CNTs were oxidized completely to form silica tubes, and Pt metal particles, which had been supported on CNTs, aggregated in the channel of the silica tubes. These results strongly suggest that Pt metal particles in SiO2/Pt/CNT are covered with silica layers. The hydrolysis of only TEOS on the CNTs with Pt hydroxide caused an irregular coverage of the CNT surfaces with thick silica layers (Figure S2 in the Supporting Information). Alternatively, the hydrolysis of only APTES on the CNTs with Pt hydroxide resulted in the uniform coverage of Pt/CNT with very thin silica layers (Figure S3 in the Supporting Information). It was reported that APTES became strongly adsorbed on the outer surface of CNT through the strong interaction between the amino groups in APTES and the graphene sheets of CNTs.21 It is likely that the primary particles of silica formed by the hydrolysis of TEOS are preferentially deposited on APTES adsorbed on the CNTs to uniformly cover the surfaces of CNTs with silica layers. Figure 2 shows cyclic voltammograms (CVs) of Pt/CNT and SiO2/Pt/CNT during the potential cycling experiments ranging
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Figure 3. Change of ECSAs for Pt/CNT and SiO2/Pt/CNT electrocatalysts during the repeated potential cycling.
Figure 4. TEM images of Pt/CNT (left) and SiO2/Pt/CNT (right) after the potential cycling experiments (1300 cycles).
from 0.05 to 1.20 V versus RHE at 50 mV s-1 at 303 K under a N2 atmosphere. In general, the durability of Pt electrocatalysts is examined by the potential cycling in electrolytes such as for aqueous H2SO4.16,17 In the CV for fresh Pt/CNT, two peaks observed correspond to the oxidation-reduction of Pt (0.61.2 V vs RHE) and the desorption-adsorption of hydrogen on Pt metal (0.05-0.4 V vs RHE). As the number of potential cycles increases, the intensities of these peaks reduce gradually. This result indicates that the ECSA of Pt/CNT decreases gradually with the repeated potential cycling. In the CV for SiO2/ Pt/CNT, two peaks similar to those for Pt/CNT are also observed. It should be noted that Pt metal particles in SiO2/Pt/ CNT are present on CNT and they are covered with silica layers that are insulators. Thus, the silica layers have a porous structure. Interestingly, the intensities of the peaks in the CV for SiO2/ Pt/CNT do not change over 1000 cycles, indicating that the ECSA of Pt metal for this sample does not decrease during the repeated potential cycling. ECSAs for Pt/CNT and SiO2/Pt/CNT at each cycle were evaluated based on the results shown in Figure 2. The results are shown in Figure 3. The ECSA for Pt/CNT was evaluated as 52 m2 g-1 at the initial stage of the potential cycling. However, the ECSA for Pt/CNT became lower monotonously with the potential cycling and finally decreased to ca. 9 m2 g-1 at 1300 cycles. Alternatively, the ECSA for SiO2/Pt/CNT at the initial stage of potential cycling was estimated to be 44 m2g-1, which was lower than that for Pt/ CNT. Some of the Pt metal particles in SiO2/Pt/CNT would not show activity for the electrochemical reaction because the silica layers prevents the contact of reactants (proton and water) to the Pt surface. As shown in Figure 3, the ECSA for SiO2/ Pt/CNT does not decrease significantly within 1300 cycles. Note that the ECSA for SiO2/Pt/CNT is larger than that for Pt/CNT after 200 cycles. Figure 4 shows TEM images of Pt/CNT and SiO2/Pt/CNT electrocatalysts after the potential cycling experiments (1300
J. Phys. Chem. C, Vol. 111, No. 42, 2007 15135 cycles) shown in Figure 2. In the TEM image of Pt/CNT after the potential cycling, Pt metal particles with diameters from 5 to 15 nm are observed, whereas the particle diameter in fresh Pt/CNT ranges from 1 to 3 nm. The TEM images indicate that Pt metal particles in Pt/CNT grow in size during the repeated potential cycling. In contrast, Pt metal particles larger than 5 nm cannot be found in the TEM image of SiO2/Pt/CNT after the potential cycling, although the size of the Pt metal particles in SiO2/Pt/CNT after the potential cycling is slightly larger than that in the fresh one. In addition, Pt LIII-edge EXAFS spectra for Pt/CNT and SiO2/Pt/CNT before and after the potential cycling experiments showed that the crystallite size of Pt metal in SiO2/Pt/CNT did not change appreciably during the potential cycling, whereas the Pt metal crystallites in Pt/CNT grew significantly in size (Figure S4 in the Supporting Information). Thus, it is concluded that the coverage of Pt metal particles with silica layers prevents the size growth of Pt metal particles on CNT. As described earlier, SiO2/Pt/CNT shows superior performance as an electrocatalyst. This is the first report on an improvement in the stability of Pt electrocatalysts by a coverage with silica layers, as far as we know. Silica layers that cover Pt metal particles have porous structures.23,24 The silica layers in SiO2/Pt/CNT would prevent the dissolution of Pt metal particles and the diffusion of dissolved cationic Pt species. In addition, the silica layers should inhibit the migration and agglomeration of Pt particles on the supports. Thus, SiO2/Pt/CNT electrocatalysts show a superior durability during potential cycling. Supporting Information Available: Preparation method of SiO2/Pt/CNT, XANES spectra of Pt/CNT and SiO2/Pt/CNT, TEM images of SiO2/Pt/CNT prepared by different methods, and EXAFS spectra of Pt/CNT and SiO2/Pt/CNT before and after the potential cycling experiments. This material is available free of charge via the Internet at http://pubs.acs. org. References and Notes (1) Bindra, P.; Clouser, S. J.; Yeager, E. J. Electrochem. Soc. 1979, 126, 1631. (2) Ferreira, P. J.; laO’, G. J.; Shao-Horn, Y.; Morgan, D.; Mokharia, R.; Kocha, S.; Gasteiger, H. A. J. Electrochem. Soc. 2005, 152, A2256. (3) Chen, Z.; Waje, M.; Li, W.; Yan, Y. Angew. Chem., Int. Ed. 2007, 46, 4060. (4) Kinoshita, K.; Lundquist, J. T.; Stonehart, P. J. Electroanal. Chem. 1973, 48, 157. (5) Yasuda, K.; Taniguchi, A.; Akita, T.; Ioroi, T.; Siroma, Z. Phys. Chem. Chem. Phys. 2006, 8, 746. (6) Guilminot, E.; Corcella, A.; Charlot, F.; Maillard, F.; Chatenet, M. J. Electrochem. Soc. 2007, 154, B96. (7) Gruver, G. A.; Pascoe, R. F.; Kunz, H. R. J. Electrochem. Soc. 1980, 127, 1219. (8) Tseung, A. C. C.; Dhara, S. C. Electrochim. Acta 1975, 20, 681. (9) Honji, A.; Mori, T.; Tamura, K.; Hishimura, Y. J. Electrochem. Soc. 1988, 135, 355. (10) Guilminot, E.; Corcella, A.; Charlot, F.; Maillard, F.; Chatenet, M. J. Electrochem. Soc. 2007, 154, B96. (11) Passalacqua, E.; Antonucci, P. L.; Vivaldi, M.; Patti, A.; Antonucci, V.; Giordano, N.; Kinoshita, K. Electrochim. Acta 1992, 37, 2725. (12) Kangasniemi, K. H.; Condit, D. A.; Jarvi, T. D. J. Electrochem. Soc. 2004, 151, E125. (13) Colo´n-Mercado, H. R.; Kim, H.; Popov, B. N. Electrochem. Commun. 2004, 6, 795. (14) Yu, P.; Pemberton, M.; Plasse, P. J. Power Sources 2005, 144, 11. (15) Takasu, Y.; Matsuyama, R.; Konishi, S.; Sugimoto, W.; Murakami, Y. Electrochem. Solid State Lett. 2005, 8, B34. (16) Colo´n-Mercado, H. R.; Popov, B. N. J. Power Sources 2006, 155, 253. (17) Koh, S.; Leisch, J.; Toney, M. F.; Strasser, P. J. Phys. Chem. C 2007, 111, 3744.
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