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Fast Preparation of PtRu Catalysts Supported on Carbon Nanofibers by the Microwave-Polyol Method and Their Application to Fuel Cells Masaharu Tsuji,*,§,# Masatoshi Kubokawa,# Ryuto Yano,# Nobuhiro Miyamae,# Takeshi Tsuji,§,# Mun-Suk Jun,#,† Seonghwa Hong,† Seongyop Lim,† Seong-Ho Yoon,§,# and Isao Mochida† Institute for Materials Chemistry and Engineering, Kyushu UniVersity, Kasuga 816-8580, Japan, Graduate School of Engineering Sciences, Kyushu UniVersity, Kasuga 816-8580, Japan, and Art, Science and Technology Center for CooperatiVe Research, Kyushu UniVersity, Kasuga 816-8580, Japan ReceiVed July 28, 2006. In Final Form: NoVember 18, 2006 PtRu alloy nanoparticles (24 ( 1 wt %, Ru/Pt atomic ratios ) 0.91-0.97) supported on carbon nanofibers (CNFs) were prepared within a few minutes by using a microwave-polyol method. Three types of CNFs with very different surface structures, such as platelet, herringbone, and tubular ones, were used as new carbon supports. The dependence of particles sizes and electrochemical properties on the structures of CNFs was examined. It was found that the methanol fuel cell activities of PtRu/CNF catalysts were in the order of platelet > tubular > herringbone. The methanol fuel cell activities of PtRu/CNFs measured at 60 °C were 1.7-3.0 times higher than that of a standard PtRu (29 wt %, Ru/Pt atomic ratio ) 0.92) catalyst loaded on carbon black (Vulcan XC72R) support. The best electrocatalytic activity was obtained for the platelet CNF, which is characterized by its edge surface and high graphitization degree.
Introduction Carbon-supported PtRu alloy nanoparticles have been most widely applied in fuel cell due to their high electrocatalytic activity accompanied by high protection ability for carbon monoxide poisoning. Their electrocatalytic activity strongly depends on the particle size and distribution, carbon supports, and dispersion of catalysts on the supports. It has been reported that PtRu particles about 3.0 nm in diameter displayed the highest catalytic activity for methanol electrooxidation.1 It is still a challenge to prepare the PtRu particles with a suitable and uniform size on carbon supports for preparing high-performance catalysts for fuel cells. If high electrochemical activity can be obtained at low PtRu concentrations, we can save expensive rare metals such as Pt and Ru. When Pt and PtRu particles loaded on carbon supports are prepared by using such conventional methods as impregnation and pre-precipitation methods, it usually takes 1 day at least to carefully reduce and precipitate these particles under controlled environments.2,3 In recent years, a microwave (MW)-assisted heating method has received great attention as a promising method for the preparation of monodispersed Pt and PtRu catalysts on carbon supports. The MW-assisted method is superior in terms of its simplicity and rapidity, compared to the above conventional methods. Until now, Pt and PtRu catalysts supported on carbon black (e.g., Vulcan XC72) and carbon nanotubes have been * To whom correspondence should be addressed. E-mail: tsuji@ cm.kyushu-u.ac.jp. § Institute for Materials Chemistry and Engineering. # Graduate School of Engineering Sciences. † Art, Science and Technology Center for Cooperative Research. (1) Takasu, Y.; Itaya, H.; Iwazaki, T.; Miyoshi, R.; Ohnuma, T.; Susimoto, W.; Murakami, Y. Chem. Commun. 2001, 341. (2) Bock, C.; Paquet, C.; Couillard, M.; Botton, G. A.; MacDougall, B. R. J. Am. Chem. Soc. 2004, 126, 8028. (3) Liu, C.; Xue, X.; Lu, T.; Xing, W. J. Power Sources 2006, 161, 68. (4) Chen, W. X.; Lee, J. Y.; Liu, J. L. Chem. Commun. 2002, 2588. (5) Liu, Z.; Ling, X. Y.; Lee, J. Y.; Su, X.; Gan, L. M. J. Mater. Chem. 2003, 13, 3049. (6) Liu Z.; Ling, X. Y.; Su, X.; Lee, J. Y. J. Phys. Chem. B 2004, 108, 8234.
prepared by using an MW-assisted heating method, and their electrochemical properties have been measured.4-7 Physicochemical properties of catalyst supports such as surface area, electrical conductivity, substrate wettability, and so forth are also no less important than dispersion methods of metal particles. Recently, the edge surface of carbons was quantitatively proven to be much more active to the electrochemical capacitance than the basal plane,8 indicating that the electrocatalytic activity of catalyst particles can be significantly affected by the surface of the carbon on which they are placed. In our preliminary report, we demonstrated that the MWpolyol method is useful for the preparation of monodispersed Pt catalysts on herringbone carbon nanofibers (CNFs), although their electrocatalytic activity has not been measured.9 Here, we applied the MW-assisted polyol process to prepare CNF-supported PtRu nanoparticles, denoted as PtRu/CNF. Not only herringbone CNFs, but also platelet and tubular CNFs with different stacking structures of graphene sheets were used as new carbon supports. Single-cell tests for methanol oxidation were carried out to examine the electrochemical properties of PtRu catalysts. The dependence of particles sizes and electrochemical properties on the structures of CNFs was examined. The electrocatalytic activity of PtRu/CNF catalysts was compared with that of standard Johnson Matthey PtRu catalysts loaded on conventional Vulcan XC72R carbon black. We demonstrate here that PtRu/CNF catalysts give significantly higher electrocatalytic activity than that of the standard Johnson Matthey PtRu catalyst loaded on conventional carbon black. Experimental Section Platelet, herringbone, and tubular CNFs were synthesized from a CO/H2 mixture or a C2H4/H2 mixture on the catalysts of Fe, Ni(7) Liu, Z. L.; Lee, J. Y.; Chen, W. X.; Han, M.; Gan, L. M. Langmuir 2004, 20, 181. (8) Kim, T.; Lim, S.; Kwon, K.; Hong, S.-H.; Qiao, W.; Rhee, C. K.; Yoon, S.-H.; Mochida, I. Langmuir 2006, 22, 9086. (9) Tsuji, M.; Hashimoto, M.; Nishizawa, Y.; Kubokawa, M.; Tsuji, T. Chem.s Eur. J. 2005, 11, 440.
10.1021/la062223u CCC: $37.00 © 2007 American Chemical Society Published on Web 12/13/2006
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Figure 1. HRTEM images of (a) platelet, (b) herringbone, and (c) tubular CNFs. Table 1. Composition and Particle Size of PtRu Catalysts Prepared by the MW-Polyol Method metal contenta Ru/Ptb (wt %) (atomic ratio) PtRu/platelet CNF PtRu/herringbone CNF PtRu/tubular CNF PtRu/Vulcan carbond
23.1 23.2 24.7 29.0
0.93 ( 0.08 0.97 ( 0.12 0.91 ( 0.19 0.92 ( 0.18
TEM (nm)
XRDc (nm)
3.4 ( 0.3 3.5 ( 0.3 3.7 ( 0.5
2.9 3.2 3.1 2.4
a Metal content was measured by TG. b Atomic ratio was obtained from TEM-EDS. c The size of catalyst particle was calculated from (220) peaks (2θ ) 65-69°) of XRD spectra using Scherrer’s equation. d Johnson Matthey commercial PtRu catalyst loaded on Vulcan XC72 carbon black.
Figure 3. XRD patterns of (a) PtRu/platelet CNF, (b) PtRu/ herringbone CNF, (c) PtRu/tubular CNF, (d) Johnson Matthey PtRu/ Vulcan carbon catalysts.
Figure 2. TEM images PtRu nanoparticles (24 ( 1 wt %, Ru/Pt atomic ratios ) 0.91-0.97) loaded on (a) platelet, (b) herringbone, and (c) tubular CNFs. These PtRu catalysts were prepared under MW heating for 2.5 min. Cu (8:2), and Fe-Ni (2:8), respectively, as described elsewhere.10 Figure 1a-c shows high-resolution transmission electron microscope (HRTEM) images of three kinds of CNFs. Platelet CNF, which consists of a graphene alignment perpendicular to the fiber axis, had the shape of a ribbon where the longer width was around (10) Yoon, S.-H.; Lim, S.; Hong, S.-H.; Qiao, W.-M.; Whitehurst, D. D.; Mochida, I.; An, B.; Yokogawa, K. Carbon 2005, 43, 1828.
80-350 nm. Herringbone CNF had a graphene alignment angled by 50-70° to the fiber axis. The cross-section of the fiber appeared polygonal, such as tetragonal, pentagonal, and hexagonal. The diameter of the herringbone CNF ranged widely from 50 to 450 nm. Tubular CNF with the graphene alignment parallel to the fiber axis showed quite homogeneous diameters of 20-40 nm. These data indicate that the diameters of CNFs are herringbone = platelet > tubular, and the relative BET surface areas are in the order of herringbone (150-240 m2/g) > tubular (90-120 m2/g) = platelet (70-90 m2/g). From X-ray diffraction (XRD) results, platelet and tubular types showed a quite high graphitization degree comparable to the natural graphite, while the herringbone type had relatively low crystallinity, summarized in the order of platelet > tubular . herringbone.11 The surfaces of platelet and herringbone CNFs were characterized by the graphitic edges as the graphitic layers in the
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Figure 4. Electrocatalytic activity of PtRu catalysts loaded on (a) platelet, (b) herringbone, (c) tubular CNFs, and (d) carbon black (Johnson Matthey fuel cell catalyst HiSPEC 5000). Open and solid symbols represent voltage and power density, respectively. fibers were aligned in a way perpendicular and declined to the fiber axis, respectively, while tubular CNFs consisted of the surface of a rounded basal plane, as shown in Figure 1. The MW-polyol apparatus used in this study was similar to that reported previously for the preparation of metallic nanoparticles except for the presence of CNFs.9 An MW oven was modified by installing a condenser and thermocouple through holes of the ceiling and a magnetic stirrer coated with Teflon at the bottom. A threenecked flask (1000 mL) was placed in the MW oven and connected to the condenser. The solution containing 713 mg of H2PtCl6‚6H2O, 345 mg of RuCl3‚2H2O, 20.7 mL of KOH (0.4 M in ethylene glycol (EG)), and 1.38 g of CNFs in 890 mL of EG was irradiated by MW (Shikoku Keisoku) in a continuous wave mode at 650 W for only 2.5 min. These values correspond to 30 wt % PtRu/CNF catalysts with Pt/Ru weight ratios of 2:1 (Pt/Ru atomic ratios of about 1:1). The solution was rapidly heated to the boiling point of EG (198 °C) after about 1.5 min under MW irradiation and held in this temperature for 1 min. Products were filtered and dried before TEM observation and measurements of electrochemical properties. After MW irradiation, PtRu/CNFs were redispersed in deionized water for TEM observation (JEOL JEM-2100 and JEM 3000F). Specimens containing PtRu/CNFs were prepared by dropping the solutions on Cu grids covered with carbon. Single-cell tests were carried out under the following conditions at 30, 60, and 90 °C. For a membrane electrode assembly (MEA) of a direct methanol fuel cell (DMFC), an anode (supported catalysts) prepared from PtRu and a cathode prepared from Pt black (5 mg/ cm2; Johnson Matthey Co.) was formed on Teflon-coated carbon (11) Chu, D.; Gilman, S. J. Electrochem. Soc. 1996, 143, 1685.
paper substrates using catalyst inks containing the appropriate weight percentage of Nafion ionomer solution (Aldrich). The MEA for unit cell tests was fabricated by pressing as-prepared cathode and anode layers onto both sides of a pretreated Nafion 117 electrolyte membrane at 130 °C under a pressure of 100 kg/cm2 for 3 min. Cell performance was evaluated in a DMFC unit cell with a 2.5 × 2.5 cm2 crosssectional area and measured with an FC impedance meter (Kikusui, Tokyo, Japan; KFM2030). Both fuel and oxidant flow paths were machined into graphite block end-plates, which also served as current collectors. A 2 M methanol solution with a flow rate of 4 mL/min was supplied, and a dry O2 flow was regulated at 200 cm-3 min-1 using a flowmeter.
Results and Discussion Figure 2a-c shows TEM images of PtRu nanoparticles loaded on platelet, herringbone, and tubular CNFs. It is clear that welldispersed PtRu nanoparticles are loaded on each CNF. The average diameters of PtRu nanoparticles were determined to be 3.4 ( 0.3, 3.5 ( 0.3, and 3.7 ( 0.5 nm for platelet, herringbone, and tubular CNFs, respectively. PtRu nanoparticles were also characterized from thermogravimetry (TG), TEM energydispersive X-ray spectroscopy (EDS), and XRD measurements. Metal contents on CNFs and Vulcan XC72R carbon black and Ru/Pt atomic ratios are given in Table 1. PtRu contents in PtRu/ CNF catalysts were 24 ( 1 wt %, and that in PtRu/Vulcan XC72R carbon black was 29 wt %. This indicated that about 6 ( 1 wt % of PtRu was not loaded on CNFs dominantly due to preparation
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of PtRu particles in solvent, and it was removed through the filtering process. The Ru/Pt atomic ratios were determined to be 0.91-0.97. Figure 3a-d shows XRD patterns of three PtRu/CNF catalysts prepared by the MW-polyol method and commercial PtRu/Vulcan carbon catalyst. The XRD patterns of PtRu/CNF catalysts are similar to those of the commercial PtRu (29 wt %)/Vulcan carbon catalyst and reported PtRu/CNTs.6,11 On the basis of these TEMEDS and XRD data, it was concluded that nanoparticles deposited on CNFs were PtRu alloy particles. Average particle sizes of PtRu catalysts were estimated from the width of XRD peaks (220: 2θ ) 65-69°) using Scherrer’s equation. They are compared with those obtained from TEM images in Table 1. The average sizes of PtRu/CNFs obtained from XRD peaks (3.1 ( 0.2 nm) are comparable with those measured by TEM images (∼3.5 nm). TEM data suggest that PtRu nanoparticles with narrow size distributions can be prepared on the platelet and herringbone CNFs having edges of graphene sheets. The formation of PtRu nanoparticles initiates from the following reactions in EG solution under MW heating:9
CH2OH - CH2OH f CH3CHO + H2O 4CH3CHO + Pt4+ f Pt + 4H+ + 2CH3COCOCH3
(1)
(2a)
6CH3CHO + 2Ru3+ f 2Ru + 6H+ + 3CH3COCOCH3 (2b) CNFs, which consist of graphene sheets, are good electric conductive materials. They have free conductive electrons, which can be accelerated by absorbing MWs. This process is called a conduction loss of MWs.12 Accelerated electrons enhance the vibration of the carbon lattice in graphene sheets, so that higher temperature is established on the surface of CNFs relative to that in EG solution. Since the reduction of metallic ions occurs on hotter spots preferentially, PtRu particles are deposited preferentially on the CNF surfaces. PtRu particles adhere strongly to CNFs at high temperature. Therefore, leaching of catalysts from CNF surfaces is greatly suppressed under MW heating. This is an advantage of the MW-assisted heating method for the preparation of metallic catalysts on CNFs. Figure 4a-d shows the dependence of voltage and power density on the current density for three PtRu/CNF catalysts along with the corresponding data obtained by using a standard PtRu/ Vulcan carbon catalyst (Johnson Matthey HiSPEC 5000) at 30, 60, and 90 °C. When the temperature of measurements is increased from 30 to 90 °C, both the voltage and power density increase in all cases. In the cases of three CNFs, the peaks of power density were observed at 120-150, 240-320, and 420-450 (12) Kingston, H. M.; Haswell, S. J. MicrowaVe-Enhanced Chemistry: Fundamentals, Sample Preparation, and Applications; American Chemical Society: Washington, DC, 1997.
mA/cm2 at 30, 60, and 90 °C, respectively. On the other hand, they were observed at lower current densities of 110, 120, and 250 mA/cm2 in the case of Vulcan XC72R carbon black at 30, 60, and 90 °C, respectively. At a standard temperature of 60 °C, the peak power densities were about 90, 50, and 60 mW/cm2 for platelet, herringbone, and tubular CNFs, respectively, indicating that the electrocatalytic activities are in the order of platelet > tubular > herringbone CNFs. These values are 1.7-3.0 times higher than that of PtRu/carbon black, for which the maximum value was about 30 mW/cm2. On the basis of these facts, it was concluded that electrocatalytic activities of PtRu/CNFs are higher than that of PtRu/carbon black by 70-200%, and the highest activity can be obtained by using platelet CNF. The measured relative Brunauer-Emmett-Teller (BET) surface areas were in the order of herringbone > tubular = platelet, while the electrocatalytic activity was in the order of platelet > tubular = herringbone CNFs. Thus, we can find no clear relationship between the electrocatalytic activity of PtRu/CNF catalysts and the relative BET surface areas. There are little differences in the size and dispersion of PtRu alloy particles over all CNFs. However, the single-cell tests showed a large difference in the catalytic activity of PtRu/CNFs. On the basis of the present findings, the present MW-polyol method appears to be superior to generate homogeneous nanosized PtRu alloy particles irrespective of catalyst supports, at least in the case of CNFs with quite different surface structure. The structure and physicochemical properties of CNFs may be more important in the final single-cell performance, considering that catalyst particles formed into similar size by the same method should have similar catalytic activity. The platelet CNF, which showed the highest single-cell performance in this study, has a lot of flat edges of well-developed graphitic layers along the fiber axis. This flat edge can trap PtRu nanoparticles strongly. Therefore, a high electrocatalytic activity may be obtained. In order to clarify why the platelet CNF has the best electrocatalytic activity, further detailed experimental studies will be required.
Conclusion The MW-polyol method was applied to the fast synthesis of CNF-supported PtRu (23 ( 1 wt %, Ru/Pt atomic ratios ) 0.910.97) catalysts. PtRu/CNF catalysts can be synthesized under MW heating for only 2.5 min in a one pot. Platelet, herringbone, and tubular CNFs were used as new carbon supports. The electrocatalytic activities for methanol oxidation using these CNFs were 70-200% higher than that using standard PtRu (29 wt %, Ru/Pt atomic ratio ) 0.92)/Vulcan carbon obtained from Johnson Matthey. Thus, CNFs, especially platelet CNFs, are useful as new carbon supports of catalysts for the application to DMFCs. Acknowledgment. This work was partially supported by JSTCREST and the Joint Project of Chemical Synthesis Core Research Institutions. LA062223U
