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J. Phys. Chem. B 2002, 106, 760-766
Pt-Ru/Carbon Fiber Nanocomposites: Synthesis, Characterization, and Performance as Anode Catalysts of Direct Methanol Fuel Cells. A Search for Exceptional Performance Eve S. Steigerwalt,† Gregg A. Deluga,‡ and C. M. Lukehart*,† Contribution from the Department of Chemistry, Vanderbilt UniVersity, NashVille, Tennessee 37235, and the Corrosion Research Center, Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, Minneapolis, Minnesota 55455 ReceiVed: July 13, 2001; In Final Form: October 18, 2001
Six Pt-Ru/carbon fiber nanocomposites have been prepared by using a bimetallic precursor as a source of metal. Carbon fiber supports include singled-walled nanotubes, multiwalled nanotubes, or graphitic carbon nanofibers having either platelet, wide herringbone, or narrow tubular herringbone atomic structures. Preparative procedures have been optimized to enhance the performance of these nanocomposites as anode electrocatalysts in direct methanol fuel cells. Pt-Ru nanoparticles are the major metal-containing component of these nanocomposites along with variable amounts of Ru metal. A range of direct methanol fuel cell performance is observed with a Pt-Ru/narrow tubular herringbone graphitic carbon nanofiber nanocomposite showing the highest performance. This performance is equivalent to that recorded for an unsupported Pt-Ru colloid at an anode catalyst loading of 2.7 mg total metal/cm2, but 64% greater than that of the unsupported Pt-Ru colloid at a lower catalyst loading of 1.5 mg/cm2.
Introduction In direct methanol fuel cells (DMFCs), aqueous methanol is electrooxidized to produce CO2 and electrical current.1 Electrocatalysts having higher activity for methanol oxidation are critically needed to achieve enhanced DMFC performance. Such catalysts are usually prepared as unsupported metal colloids or as composites in which metal nanoparticles are deposited on an electrically conducting carbon of high surface area. The search for active methanol oxidation catalysts has involved the variation of catalyst preparation and composition, the use of supported or unsupported catalysts, and, more recently, recognition of the importance of the atomic structure of the carbon support on catalytic activity. Mixed-metal catalysts containing Pt are currently favored for methanol oxidation.1 Pt activates the C-H bonds of methanol producing a Pt-CO surface species, whereas an oxophilic metal activates water and accelerates oxidation of surface-adsorbed CO to CO2. The absence of an oxophilic metal leads to CO poisoning of the anode catalyst in working DMFCs. Although combinatorial studies indicate that ternary and quaternary alloy compositions, such as Pt1Ru1W1 or Pt44Ru41Os10Ir5, possess superior activity as DMFC anode catalysts,2 much interest remains in improving the activity of more established binary catalysts containing Pt and Ru. Unsupported Pt-Ru colloids and supported Pt-Ru/carbon nanocomposites have been prepared by a variety of chemical methods including solution-phase reduction of metal ions and thermal decomposition of either single-source or dual-source molecular precursors.1,3 Carbon support materials have included carbon powder, carbon blacks, desulfurized carbon blacks, and fullerene soot.3d DMFC testing data indicate high performance when either unsupported or supported Pt-Ru catalysts are used depending * To whom correspondence should be addressed. E-mail: charles.
[email protected]. † Vanderbilt University. ‡ The University of Minnesota.
on anode catalyst composition and particle size.3g,4 We have reported that a Pt-Ru/Vulcan carbon nanocomposite, prepared with a bimetallic precursor as the source of metal and tested as an anode catalyst in a DMFC, gives a cell performance equivalent to that of an unsupported Pt-Ru catalyst.5 However, electrochemical measurements reported recently in this journal by Bessel and co-workers challenge conventional wisdom regarding both the preferred catalyst composition and carbon support used for methanol oxidation.6 Pt/graphitic carbon nanofiber (GCNF) nanocomposites having the known “platelet”, “ribbon”, or “herringbone” carbon support structures were tested electrochemically as catalysts for methanol oxidation. The Pt/ GCNF nanocomposites having the “platelet” or “ribbon” GCNF structures give appreciably higher activity than Pt/Vulcan carbon nanocomposites. In addition, these Pt/GCNF nanocomposites seem to be less susceptible to CO poisoning than Pt/Vulcan carbon electrocatalysts. Although both the activity and CO tolerance of Pt/GCNF nanocomposites needs to be determined in working DMFCs, it has been suggested that the unique atomic structure of GCNF supports can influence Pt nanocrystal morphology and impart enhanced catalytic activity to these nanocomposites.6 As part of an ongoing investigation of new synthetic strategies for preparing metal alloy/carbon nanocomposites exhibiting high performance as DMFC anode catalysts,7 we now report the preparation and characterization of six Pt-Ru/carbon fiber nanocomposites along with DMFC testing results. Each nanocomposite is prepared by using a bimetallic precursor as the source of Pt and Ru metal. Synthetic procedures have been optimized to produce Pt-Ru/carbon fiber nanocomposites showing measurable performance as DMFC anode catalysts. DMFC testing is conducted in a uniform fashion with the intention of identifying any Pt-Ru/carbon fiber nanocomposites that might show exceptionally high DMFC performance. Each nanocomposite is prepared at high total metal loading while maintaining acceptably small alloy nanocrystals to provide a
10.1021/jp012707t CCC: $22.00 © 2002 American Chemical Society Published on Web 01/04/2002
Pt-Ru/Carbon Fiber Nanocomposite Performance good test of DMFC relative performance.5 A range of DMFC performance is observed. Metal nanoparticles supported on narrow tubular GCNFs give the highest relative DMFC performance. This performance is equivalent to that of unsupported Pt-Ru anode catalyst at high catalyst loading (2.7 mg total metal/cm2), but 64% higher than that of unsupported Pt-Ru catalyst at lower loading (1.5 mg total metal/cm2).7f Nanocomposites using other carbon fiber supports give considerably lower performance. Metal/carbon fiber nanocomposites containing some degree of Ru metal phase separation give the highest performance as DMFC anode catalysts. Experimental Section General Methods. Solvents were distilled before use. All gaseous reagents were procured from Aire Liquide Gas. The complex, (η-C2H4)(Cl)Pt(µ-Cl)2Ru(Cl)(η3:η3-2,7-dimethyloctadienediyl), 1, was prepared by using a published procedure5 and was used as a single-source molecular precursor of Pt and Ru metal. GCNF supports having platelet, wide herringbone (width ) 75 nm ( 50 nm), or narrow tubular herringbone (width ) 25 nm ( 10 nm) atomic structures were prepared according to reported procedures.8 Edge-enhanced GCNFs having the platelet atomic structure were obtained in very limited supply from MER Corporation as proprietary material. Single-walled carbon nanotubes (SWNTs) were obtained as a toluene suspension of ultrapure, demineralized nanotubes from Tubes@Rice. The demineralization procedure consisted of subjecting SWNTs to 12 h of reflux in 2.6 M nitric acid, followed by rinsing in pH 8.0 aqueous solution, isolation by centrifugation, and finally extraction into toluene. The Co and Ni observed by an energydispersive spectrometer (EDS) in the nanocomposite prepared on this SWNT support results from residual growth catalyst that survived this demineralization procedure. Residual Co/Ni growth catalyst is present as particles of 3- to 15-nm diameter overcoated with onionlike layers of graphitic carbon. Dry samples of SWNTs were isolated by evaporation of the toluene phase at reduced pressure. Multiwalled carbon nanotubes (MWNTs) were purchased from Aldrich Chemical Co. All thermal treatments were performed in Lindberg tube furnaces equipped with quartz tubes having either 1.5-in. (nanocomposite preparations) or 3-in. (GCNF preparations) diameter. Flow rates of gaseous reactants were measured with gas flow meters. Elemental analysis was performed by Galbraith Laboratories, Knoxville, TN. Transmission electron microscopy (TEM) was performed on a Philips CM-20T operating at 200 keV equipped with an EDS. Samples were prepared by sonicating a small amount of the nanocomposite to be analyzed in acetone. A drop of this slurry was deposited onto a 3-mm holey carbon-copper grid (SPI Supplies), and the acetone was allowed to evaporate. Graphene plane orientations of GCNF supports prepared in this study were confirmed by high-resolution TEM. Powder X-ray diffraction (XRD) scans were obtained by using a Scintag X1 θ/θ automated powder X-ray diffractometer equipped with a Cu target, a Peltier-cooled solid-state detector, and a Buhler high-temperature/controlled atmosphere attachment. Samples were supported on zero-background Si (510) plates. Surface area measurements were performed on selected carbon fiber supports by using a NOVA-1000 Gas Sorption Analyzer (Quantachrome). Surface areas were calculated by the Brunauer-Emmett-Teller method with NOVA/DRP software. The following surface areas were determined: narrow tubular herringbone GCNFs (111 m2/g), wide herringbone GCNFs (95 m2/g), SWNTs (109 m2/g), MWNTs (8 m2/g).
J. Phys. Chem. B, Vol. 106, No. 4, 2002 761 Thermogravimetric analysis (TGA) was performed on a TA Instruments high-resolution TGA model 2950. Metal oxides on the surface of freshly reduced nanocomposite were removed in situ by heating TGA samples in a atmosphere of getter gas (90: 10 N2/H2) at a rate of 5 °C/min from room temperature to 100 °C and holding at that temperature for 120 min. To prevent any explosive mixture of hydrogen and oxygen, as well as to improve the quality of the data, the TGA instrument was placed inside a large glovebag and surrounded with an inert atmosphere at positive pressure during the course of the experiment. Reductive Decomposition of Complex 1. A flask containing 45 mg of complex 1 was connected in sequence with glass fittings to a cold trap (-78 °C), an acid trap containing 50 mL of standardized aqueous NaOH solution (0.0136 M), and finally to an oil bubbler. The system was flushed for 15 min with getter gas (90:10 N2/H2) at ambient temperature with no sign of reaction. The flask containing complex 1 was then heated to 150 °C by using an oil bath for 30 min under a continuous flow of getter gas. During this period, complex 1 reactively decomposed to a black solid. The content of the cold trap was extracted by using organic solvents and was analyzed by 1H NMR and gas chromatography-mass spectrometry (GC-MS). GC-MS analysis indicated a major organic product (92%) having a mass of 142 amu and three partially hydrogenated minor products (8%) each having a mass of 140 amu. The mass spectrum and 1H NMR spectrum of the major organic product matched those of a commercial sample of 2,7-dimethyloctane. Titration of the NaOH trap solution with 0.01395 M aqueous HCl to a phenolphthalein end point indicated that 2.5 equiv of NaOH had been neutralized during the reaction. Powder XRD analysis of the black residue revealed a face-centered cubic (fcc) pattern having a cell constant and peak widths consistent with that expected for Pt-Ru nanoparticles having an average diameter of 4 nm. General Procedure for the Preparation of Pt-Ru/carbon Nanocomposites, 2a-2f. Nanocomposites 2a-2f were prepared by a multideposition procedure consisting of three deposition cycles and a final thermal treatment optimized to give Pt-Ru/ carbon fiber nanocomposites having measurable DMFC performance. For each cycle, approximately one-third of the required portion of precursor 1 was dissolved in acetone. To this solution was added the appropriate mass of carbon support. The resulting slurry was stirred for 1 h under N2 to permit adsorption of precursor 1, and the precursor/carbon composite was isolated as a dry black solid upon removal of the solvent at reduced pressure. The precursor/carbon composite was placed in a glazed alumina boat, inserted in a tube furnace, and heated under getter gas (90:10 N2/H2) from room temperature to 250350 °C at 15 °C/min. After holding at the maximum temperature for 2 min, the sample was cooled to ambient temperature under N2. This procedure was repeated two additional times to give the desired metal loading. As a final thermal treatment, nanocomposite samples were placed in a tube furnace and were heated at 15 °C/min under air from ambient temperature to 350 °C. After a N2 purge for 15 min, getter gas was introduced, and the sample was heated at 15 °C/min to 650 °C. The gas flow was changed to N2. The sample was then annealed at 650 °C for 5-60 min and was cooled slowly in the tube furnace to ambient temperature or was displaced from the furnace to cool more rapidly to room temperature under a flow of N2. DMFC Testing. Pt-Ru/carbon nanocomposites prepared in this study along with unsupported Pt1Ru1 colloid (U.S. Patent No. 5773162, 1998) were tested as anode catalysts in working
762 J. Phys. Chem. B, Vol. 106, No. 4, 2002
Figure 1. TGA analysis of the thermal decomposition of complex 1 in Ar (trace A) or in getter gas (trace B).
DMFCs at the Corrosion Research Center, University of Minnesota, under a common set of conditions. Detailed procedures5 for the preparation of membrane electrode assemblies (MEAs) and testing procedures have been published (see also U.S. Patent 5,599,638). Anode catalyst loading is typically 2.7 mg total metal/cm2 unless otherwise indicated. Results and Discussion Reductive decomposition of complex 1 leads to loss of the organic and chloro ancillary ligands and to the formation of Pt-Ru nanocrystals. As shown in Figure 1, TGA analysis of the thermal decomposition of complex 1 in Ar (trace A) from ambient temperature to 800 °C is consistent with gradual sequential loss of chloro and ethylene ligands (25-220 °C), chloro and bis-allyl ligands (220-435 °C), and incomplete loss of the remaining chlorine up to 800 °C, giving a residual mass 2.59 wt % greater than the calculated total metal mass content of 49.17 wt %. However, thermal decomposition of complex 1 under getter gas (trace B) leads to rapid loss of the bis-allyl, ethylene, and two chloro ligands by 90 °C and the remaining chloro ligands by 120 °C giving a residual mass content of 50.39 wt % consistent with only slightly impure Pt-Ru metal. Reductive decomposition of complex 1 by getter gas has been studied in greater detail through product analysis. Organic products isolated from this reaction include 2,7-dimethyloctane as the major product (92%) identified by 1H NMR and GC-MS along with minor amounts of monounsaturated octanes. A significant amount of acid is also formed and detected by acidbase titration, consistent with loss of chlorine as HCl. XRD and TEM analysis of the reaction residue reveals Pt-Ru alloy nanoparticles having an average diameter of 4 nm. Reductive decomposition of complex 1 on six different carbon fiber supports leads to loss of the organic and chloro ancillary ligands and to the formation of Pt-Ru/carbon fiber nanocomposites 2a-2f as distinguished by the type of carbon fiber support (2a, platelet GCNFs; 2b, wide herringbone GCNFs; 2c, narrow tubular herringbone GCNFs; 2d, edge-enhanced platelet GCNFs; 2e, SWNTs; and 2f, MWNTs). Selected preparative and characterization data for these metal/carbon fiber nanocomposites are provided in Table 1. Each nanocomposite is prepared by using a three-step deposition procedure with a final oxidative/reductive thermal treatment to ensure both high total metal loading (43-62 wt %) and metal alloy particles giving measurable performance as anode catalysts in subsequent DMFC testing.9 During the final thermal treatment, samples are heated in air up to 350 °C and are then reduced in getter gas from 350 to 650 °C. Samples are annealed at that temperature for 5-60 min under N2. Although most of the carbon supports suffer only nominal weight loss upon heating to 350 °C in air, independent TGA analysis confirms that the SWNT support used in this study loses 54 wt
Steigerwalt et al. % during this oxidative thermal treatment, thus accounting for the higher total metal loading of 61.5 wt % obtained for nanocomposite 2e. Bulk elemental analyses of these nanocomposites indicate Pt/Ru atomic ratios of 0.98-1.14 consistent within experimental error5 with the 1:1 metal stoichiometry of precursor 1. Representative bright-field TEM images of nanocomposites 2a-2f, as displayed in Figure 2, reveal metal particles of high contrast having average diameters of 4.8-8.6 nm dispersed on the appropriate carbon support. Images at higher magnification confirm the structure expected of the carbon fiber support. TEM analysis of metal alloy particle size at each step of nanocomposite synthesis reveals a doubling of average nanocluster diameter during the final thermal treatment (see below). Highresolution TEM images of metal particles in nanocomposite 2c (Figure 3) reveal lattice fringes of 2.24 Å d-spacing across the entire visible diameter of the nanoparticles consistent with the (111) lattice spacing of 2.24 ( 0.01 Å known for fcc Pt-Ru alloys.10 Broad-area EDS spectra of nanocomposites 2a-2f (see Figure 3) show emission from Pt and Ru with relative intensities corresponding to Pt/Ru atomic ratios of 0.9 (1)-1.3 (4) on the micron scale. Standard deviations for Pt/Ru atomic ratios determined by EDS are determined from multiple-spot EDS analysis of Pt and Ru emission intensities. The Pt/Ru atomic ratio of 1.3 (4) for nanocomposite 2e has an abnormally large standard deviation consistent with significant compositional heterogeneity within this sample. XRD analysis confirms significant metal phase separation within this sample, as discussed below. EDS emission from Co and Ni is evident in nanocomposite 2e because of the presence of residual amounts of SWNT growth catalyst. Low-intensity Si emission is sometimes observed as a detector artifact or, for nanocomposite 2c, because of the presence of residual trace amounts of the fumed silica GCNF, a growth substrate. XRD scans of nanocomposites 2a-2f are displayed in Figure 4. Diffraction from the (002) planes of the graphitic supports is evident at ∼27° in 2θ with peak widths reflecting the degree of long-range order within these crystalline graphite supports. Each scan reveals a predominant fcc pattern of peaks consistent with that expected for Pt-Ru alloys having cell constants of 3.88 ( 0.02 Å.10 Peaks associated with this alloy phase are identified by the appropriate Miller indices. Scherrer’s analysis of experimentally measured XRD peak widths gives average crystalline domain sizes of 5.8-17.5 nm for the alloy nanoparticles.11 Nanoparticle sizes determined from XRD peak widths are volume-weighted, whereas those determined by direct measurement from TEM images are number-averaged. Nanocomposites having volume-weighted average particle sizes much greater than number-average particle sizes, such as with 2a, probably contain a significant fraction of atypically large nanoparticles. The longer annealing time used in the synthesis of nanocomposite 2a increases nanoparticle size and polydispersity within this sample. Each XRD scan shows peaks of varying relative intensity near 38 and 44° in 2θ corresponding to diffraction from the (100) and (101) planes of Ru metal. The presence of Ru metal in these nanocomposites indicates some degree of metal phase separation. Diffraction from any equivalent amount of Pt metal would be obscured by peaks of the Pt-Ru alloy phase. The presence of Ru metal is not observed in nanocomposites prepared under solely reducing conditions. In situ XRD analysis using a controlled-atmosphere, hightemperature attachment demonstrates a link between the ap-
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J. Phys. Chem. B, Vol. 106, No. 4, 2002 763
TABLE 1: Preparation and Characterization Data for the Pt-Ru/Carbon Nanocomposites 2a-2f 2a
2b
2c
2d
2e
2f
carbon support
sample no.
platelet GCNF
edge-enhanced platelet GCNF 250 30 -a -a 0.93 (8) 5.1 9.4 17.1
MWNT
350 60 47.0 1.12 1.1 (2) 4.8 17.5 10.6
herringbone GCNF (narrow) 250 30 42.6 1.06 1.0 (1) 7.0 5.8 5.5
SWNT
reaction T (°C) anneal t (min) metal wt % Pt:Ru atomic ratio (bulk) Pt:Ru atomic ratio (EDS)b aver. diameter (TEM, nm) aver. diameter (XRD, nm) MEA resistivity (mΩ)c
herringbone GCNF (wide) 250 5 46.3 1.14 0.94 (7) 8.6 6.4 10.9
250 30 61.5 1.01 1.3 (4) 6.9 6.5 6.9
250 30 47.1 0.98 0.9 (1) 8.1 12.6 10.7
a Insufficient mass for bulk elemental analysis. b Standard deviations in parentheses; determined by multiple-spot EDS analysis of each sample on the micron scale. c MEA resistivity for the unsupported Pt-Ru colloid catalyst is 8.4 mΩ.
Figure 2. Representative bright-field TEM micrographs of the Pt-Ru/carbon fiber nanocomposites 2a-2f.
pearance of metal phase separation and prior oxidative treatment of the alloy nanocomposite. As shown in Figure 5, an XRD scan of nanocomposite 2b prepared under solely reducing conditions (scan A) shows diffraction only from fcc Pt-Ru alloy nanoclusters of 2.3-nm average diameter. Heating the sample under air to 350 °C leads to formation of RuO2 as indicated by the appearance of the (101) peak known for this substance near 35° in 2θ (scan B).12a Subsequent reduction under getter gas from 350 to 500 °C gives scan C. At this point the oxidized Ru species has been reduced to Ru metal, as indicated by the appearance of peaks near 38° and 44° in 2θ, and the Pt-Ru alloy particles have increased in size to 2.8 nm. Heating the sample to 650 °C under getter gas, followed by a 30-min anneal at that temperature, gives scan D in which only particle growth has occurred giving Pt-Ru nanoclusters with an average diameter of 7.3 nm. These results demonstrate that Ru metal
separates from Pt-Ru alloy upon air oxidation to form oxidized Ru species. During subsequent reduction and annealing, some amount of Ru metal remains phase separated from the Pt-Ru alloy phase. The XRD scan of nanocomposite 2c, as prepared and stored under ambient conditions, reveals a peak of very weak intensity near 36° in 2θ. Although this species might represent a small amount of unreduced Ru oxide, it could also be hydrous ruthenium oxide formed by the oxidation of nanoscale Ru metal upon exposure to ambient atmosphere. The principal diffraction peak of hydrous ruthenium oxide appears near 36.3° in 2θ, slightly higher in value than the (101) peak of RuO2 observed at 35.1° in 2θ.12a However, electrochemical capacitance measurements recorded for nanocomposite 2c reveal no detectable amount of hydrous ruthenium oxide,12b so nanocomposite 2c probably contains a small amount of unreduced Ru oxide.
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Figure 6. Powder XRD scans of nanocomposite 2b prepared under solely reducing conditions (scan A) followed by in situ sequential air oxidation up to 350 °C (scan B), reduction by getter gas from 350 to 500 °C (scan C), and annealing at 650 °C under getter gas for 30 min (scan D). Figure 3. Bright-field TEM micrograph of the Pt-Ru/GCNF nanocomposite 2c at high magnification showing (111) lattice fringes of the alloy nanoparticles. The double-headed arrow indicates the direction of the long axis of the GCNF.
Figure 4. Broad area EDS spectra of the Pt-Ru/carbon fiber nanocomposites 2a-2f. X-ray emission from the Cu grid of the sample holder is also evident.
Figure 5. Powder XRD scans of the Pt-Ru/carbon fiber nanocomposites 2a-2f.
Current-voltage curves of DMFCs fabricated by using nanocomposites 2a-2f or an unsupported Pt-Ru colloid as anode catalyst are shown in Figure 6. For each test, the anode catalyst loading is 2.7 mg total metal/cm2 except for nanocomposite 2e where the loading is 1.5 mg/cm2. Nanocomposite 2d physically breaks down during DMFC testing giving a performance curve atypical of that expected for an active Pt-Ru anode catalyst. The edge-enhanced GCNF support appears to be
unstable under the DMFC operating conditions, although insufficient quantities of this proprietary support prevented duplication of this observation. Although considerable care must be exercised when interpreting relative DMFC performance among catalysts differing in more than one feature, several interesting observations are evident from DMFC performance data acquired for nanocomposites 2a-2f: First, DMFC performance apparently is enhanced when some degree of Ru metal phase separation is present within the asprepared nanocomposite. Nanocomposite 2b′ tests very poorly as an anode catalyst with DMFC cell potential declining rapidly with increasing current density. This nanocomposite was prepared under solely reducing conditions and contains Pt-Ru alloy as the only metal-containing species detectable by XRD. However, a nanocomposite, 2b, prepared by using the identical procedure but including an intermediate oxidative thermal treatment in the final step of the synthesis, gives significantly higher DMFC performance. An oxidative thermal treatment was therefore incorporated into the general nanocomposite preparative procedure to enhance DMFC performance. As discussed above, introduction of an oxidative thermal treatment in the nanocomposite preparation induces some degree of Ru metal phase separation. Nanocomposite 2e, which contains the greatest relative amount of Ru metal phase separation among the asprepared nanocomposites, gives the second best DMFC performance of all of the nanocomposites tested even at a slightly lower loading. We speculate that phase separation of Ru metal enhances DMFC performance by forming more active Pt-rich alloy nanoparticles and/or by providing a reactive source of Ru metal that oxidizes to hydrous ruthenium oxide under the operating conditions of a DMFC. Second, DMFC performance as a function of carbon support decreases in the trend narrow tubular herringbone GCNF (2c) > SWNTs (2e) > MWNTs (2f) > wide herringbone GCNF (2b) > platelet GCNF (2a). Although the performance curve shown in Figure 6 for nanocomposite 2e is at a lower catalyst loading of 1.5 mg total metal/cm2, the DMFC performance of nanocomposite 2c recorded at that same loading is significantly greater than that of nanocomposite 2e at all current densities. Relative DMFC performance does not parallel any trend in the surface areas of the carbon supports used, so the intrinsic surface area of the carbon fiber support does not seem to be a dominant factor in determining relative DMFC performance. We speculate, however, that relative DMFC performance depends strongly on the electrical conductance of the carbon
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J. Phys. Chem. B, Vol. 106, No. 4, 2002 765
Figure 7. DMFC current density-voltage (V) curves comparing the performance of nanocomposites 2a-2f with that of an unsupported Pt-Ru colloid as anode catalysts. Catalyst loading is 2.7 mg total metal/cm2 for the unsupported catalyst and nanocomposites 2a, 2b, 2b′, 2c, 2d, and 2f, and 1.5 mg/cm2 for nanocomposite 2e.
Figure 8. DMFC current density-voltage (V) curves comparing the performance of nanocomposite 2c (connected data points) with that of an unsupported Pt-Ru colloid (unconnected data points) at anode catalyst loading of 2.7 mg total metal/cm2 (circles), 1.5 mg total metal/cm2 (squares), or 0.5 mg total metal/cm2 (triangles).
fiber support and the ability of the catalyst support to transport electrons along the long axis of the fiber and eventually to the current collector of the MEA. The observed trend in increasing MEA resistivity, 2c (5.5 mΩ) < 2e (6.9 mΩ) < 2a (10.6 mΩ) < 2f (10.7 mΩ) < 2b (10.9 mΩ) < 2d (17.1 mΩ), generally parallels the trend in decreasing DMFC performance discussed above. The highest performing nanocomposite, 2c, has the lowest MEA resistivity of all the nanocomposites tested. This nanocomposite contains narrow tubular herringbone GCNFs as the carbon support. Electrons produced on the surfaces of PtRu catalyst particles could flow across the graphene sheets of the herringbone layers to the highly conducting tubular graphitic core without encountering significant ohmic barriers. The high performance of nanocomposites having SWNTs (2e) or MWNTs (2f) as supports could be attributed to the high intrinsic electrical conductance of these bulk materials. The electrical conductance of SWNTs varies from insulator to metal-like depending on nanotube width and degree of helicity of the carbon nanotube.13 Since a mixture of nanotube structures is expected for the supports used in this study, a high degree of electrical conductivity is expected for these supports. Also, GCNFs having the platelet (2a) or wide herringbone (2b) atomic structures give relatively low DMFC performance. These GCNF structures should have high electrical conductance within individual graphene sheets but poor electrical conductance along the long fiber axis because of the pi-stacking of individual graphene
sheets in this direction.14 More detailed study of ohmic loses encountered within carbon fiber electrodes is clearly needed to verify such speculation. Third, nanocomposite 2c gives the highest DMFC performance of all samples tested and maintains a cell potential comparable to that of an unsupported Pt-Ru colloid at all current densities with a loading of 2.7 mg total metal/cm2. A comparison of DMFC performance of nanocomposite 2c and the unsupported Pt-Ru colloid as anode catalysts at three different loadings is shown in Figure 7. As expected, overall performance levels decrease with catalyst loading; however, the relative performance of nanocomposite 2c increasingly exceeds that of the unsupported Pt-Ru colloid as catalyst loading decreases. This trend probably reflects a greater ability of supported catalyst particles to maintain electrical continuity with the MEA current collector as more dilute catalyst suspensions are used in MEA fabrication. With a catalyst loading of 1.5 mg total metal/cm2, both the absolute and relative performance produced by nanocomposite 2c are of practical interest. At 0.4 V, the current density recorded for nanocomposite 2c of 0.23 A/cm2 is 64% greater than that measured for the unsupported catalyst (0.14 A/cm2) (Figure 8). Similarly, at a current density of 0.4 A/cm2, nanocomposite 2c produces a cell potential 56 mV higher than that of the unsupported catalyst. Nanocomposite 2c seems to have the optimal combination of catalyst composition and carbon fiber
766 J. Phys. Chem. B, Vol. 106, No. 4, 2002 support of the Pt-Ru/carbon fiber nanocomposites prepared and tested in this study. Conclusions Six Pt-Ru/carbon fiber nanocomposites have been synthesized by using a molecular precursor as a source of metal. As prepared and stored under ambient conditions, these nanocomposites contain Pt-Ru nanocrystals widely dispersed on carbon fiber supports along with detectable amounts of phase-separated Ru metal nanoparticles and, in one case, an amount of oxidized Ru. The performance of these nanocomposites as anode catalysts has been measured in a DMFC. Pt-Ru/carbon fiber nanocomposites having some degree of Ru metal phase separation give the highest DMFC performance. In addition, of the carbon fiber supports examined, GCNFs having a narrow tubular herringbone structure show the highest DMFC performance. The greatest performance achieved is 64% greater than that measured for an unsupported Pt-Ru colloid catalyst. More specific definition of the dependence of DMFC performance on metal catalyst composition and the structure of carbon fiber supports is needed. Acknowledgment. Research support provided by the U.S. Army Research Office under Grants DAAH04-95-1-0146, DAAH04-96-1-0179, DAAH04-96-1-0302, and DAAG55-981-0362 is gratefully acknowledged by C.M.L. Support from MURI Contract DA/DAAH04-95-1-0094 is gratefully acknowledged by G.A.D. We also thank Dr. D. A. Shores for helpful discussions and Dr. S. McElvaney of the Office of Naval Research for a gift of SWNTs. References and Notes (1) (a) Hamnett, A. Catal. Today 1997, 38, 445. (b) Hogarth, M. P.; Hards, G. A. Platinum Met. ReV. 1996, 40, 150. (c) Chandler, G. K.; Genders, J. D.; Pletcher, D. Platinum Met. ReV. 1997, 41, 54. (d) Ralph, T. R. Platinum Met. ReV. 1997, 41, 102. (e) Hamnett A.; Troughton, G. L. Chem. Ind. 1992, 480. (f) Ren, X.; Wilson, M. S.; Gottesfeld, S. J. Electrochem. Soc. 1996, 143, L12. (g) Hogarth, M. P.; Christensenand, P. A.; Hammet, A. Proc. First Int. Symp. New Mater. Fuel Cells 1995, 310. (h) Lin, W. F.; Wang, J. T.; Savinell, R. F. J. Electrochem. Soc. 1997, 144, 1917. (i) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J. Electrochem. Soc. 1994, 141, 1795. (j) Surampudi, S.; Narayanan, S. R.; Vamos, E.; Frank, H.; Halpert, G.; LaConti, A.; Kosek, J.; Surya Prakash, G. K.; Olah, G. A. J. Power Sources 1994, 47, 377. (k) Wang, K.; Gasteiger, H. A.; Markovic, N. M.; Ross, P. N., Jr. Electrochim. Acta 1996, 41, 2587. (l) Pathanjali, G. A.; Krishnamurthy, B.; Chireau, R. F.; Mital, C. K. Bull. Electrochem. 1996, 12, 193.
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