Single-Wall Carbon Nanohorns Supporting Pt Catalyst in Direct

Apr 23, 2009 - Mayumi Kosaka*, Sadanori Kuroshima, Kenji Kobayashi, Shoji Sekino, Toshinari Ichihashi, Shin Nakamura, Tsutomu Yoshitake and Yoshimi Ku...
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J. Phys. Chem. C 2009, 113, 8660–8667

Single-Wall Carbon Nanohorns Supporting Pt Catalyst in Direct Methanol Fuel Cells Mayumi Kosaka,* Sadanori Kuroshima, Kenji Kobayashi, Shoji Sekino, Toshinari Ichihashi, Shin Nakamura, Tsutomu Yoshitake, and Yoshimi Kubo*,‡ Nano Electronics Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan ReceiVed: NoVember 30, 2008; ReVised Manuscript ReceiVed: March 23, 2009

We made high-power direct methanol fuel cells (DMFCs) by using single-wall carbon nanohorns (SWNHs) to support the Pt catalyst. The unique structure of the SWNH aggregate is advantageous to support the fine Pt catalysts. To prevent the growth of Pt particles even under high-Pt-content conditions, defects were intentionally created on the surface of SWNHs by oxidizing them with H2O2. This reduced the mean diameter of the Pt particles supported on the SWNHs to 2.9 nm, which is roughly 2/3 that of the Pt particles on asgrown SWNHs, for 60 wt % Pt content. We also improved the process used to make a membrane electrode assembly (MEA) by immersing the catalyst electrodes in 50 vol % MeOH before the hot-pressing process. This increased the active surface area of the Pt catalysts to nearly 100% of the Pt surface and strengthened the joint between the electrodes and the membrane. These improvements enabled us to obtain a power density of 76 mW/cm2 at 0.4 V for a passive-type DMFC operated at 40 °C. 1. Introduction 1-7

A direct methanol fuel cell (DMFC), using methanol and air to produce electricity, is expected to be a useful portable power source because its energy density per unit weight is potentially greater than 1000 W h/kg, more than several times that of currently available lithium ion batteries. One serious problem with DMFCs, however, has been that at room temperature the power density of their membrane electrode assemblies (MEAs) is too small, at most ∼10 mW/cm2. This means that the area of the MEA in a DMFC powering a small 1-W cell phone would have to be more than 100 cm2. Because convenient portable devices have to be compact, DMFCs cannot be put to practical use unless the power density of their MEAs is increased by at least an order of magnitude. One way to do this is by using a highly efficient catalyst supported on optimized carbon. A variety of carbon materialsscarbon nanospheres,8 carbon nanofibers9-12 carbon nanocoils,13,14 nanoporous carbon,15 carbon black,16 carbon nanotubes,17-26 and carbon nanotubes grown on carbon paper27-29shave recently been investigated as electrocatalyst supports. We have previously reported the use of singlewall carbon nanohorn (SWNH) aggregates as catalyst supports for polymer-electrolyte fuel cells (PEFCs) using H2 and O2.30 Platinum particles about 2 nm in diameter were homogeneously dispersed on the SWNHs to make a supported catalyst containing 20 wt % Pt (see Movie S1 in the Supporting Information). SWNH aggregates have a unique structure in which the wall of each SWNH consists of a single graphene sheet, each SWNH is horn-shaped, and thousands of SWNHs form a “dahliaflower”-like aggregate about 100 nm in diameter that has a spherical shape like that of a sea urchin or a chestnut bur.31 The structure would be advantageous for supporting fine particles of Pt catalyst for fuel cell electrodes because each SWNH aggregate has thousands of nanospaces between the * To whom correspondence should be addressed. E-mail: m-kosaka@ az.jp.nec.com and [email protected]. Phone: +81-29-850-1138. Fax: +81-29-856-6137. ‡ Present address: NEC Tokin Corporation, 1120 Simokuzawa, Sagamihara 229-1198, Japan.

horns, the horns would prevent catalyst migration, and the graphitic carbon structure would be more stable than the amorphous-like carbon black. The remarkable features of the structure of SWNHs would also improve the durability32-38 of catalysts. In this study, we improved the Pt/SWNHs supported catalyst for the DMFC cathode used for portable power applications. It is reported that in PEFCs using H2 and air, the Pt loading for the anode can be reduced to 0.05 mg/cm2 and that for the cathode can be reduced to 0.2 mg/cm2.39 The electrodes in DMFCs, in contrast, need much higher Pt loadings (>1 mg/ cm2) because the methanol oxidation is much slower than hydrogen oxidation and DMFCs for portable applications usually operate at lower temperatures than H2-air fuel cells do. It should be noted, however, that the thickness of a catalyst electrode increases with increasing Pt loading, considerably increasing the electrical resistance as well as the material transport resistance in the cell. In a Pt/C catalyst containing 20 wt % Pt, for example, the electrode thickness is only ∼20 µm for a Pt loading of 0.2 mg/cm2 but becomes ∼500 µm for a Pt loading of 5 mg/cm2. Such a thick electrode, an order of magnitude thicker than the polymer electrolyte membrane (several tens of micrometers thick), would never work well in practical use. To make the material transport easy and to reduce the electrical resistance, it is necessary to increase the Pt content in the supported catalyst because the electrode thickness is mainly determined by the volume of the carbon support. (Note that the density of Pt, 21.4 g/cm3, is much higher than that of C, 2.25 g/cm3). The roughly 500-µm thickness of the electrode with a 5-mg/cm2 loading can be reduced to ∼125, ∼80, ∼50, and ∼30 µm if the Pt content is increased to 50, 60, 70, and 80 wt %, respectively. It is therefore necessary to increase the Pt content in the supported catalyst for DMFCs that requires high Pt loadings. On the other hand, the size of the Pt particles on a carbon support tends to increase with increasing the Pt content, and this reduces the mass activity of Pt catalyst through the reduction of surface area. Thus it is also very important to suppress the growth of Pt particles during the preparation of supported

10.1021/jp8105293 CCC: $40.75  2009 American Chemical Society Published on Web 04/23/2009

Production of High-Power Direct Methanol Fuel Cells catalysts, especially those with higher Pt contents. In this study, we intentionally created atomic defects on the surface of SWNHs by oxidizing them in a H2O2 solution to increase the number of adsorption sites for Pt particles. As a result, sufficiently small Pt particles could be formed even under highPt-content conditions because the H2O2 oxidation resulted in up to three times as many Pt particles adsorbing to the individual SWNHs. We have also improved the process for preparing the membrane electrode assembly (MEA) to increase the output power density of a passive-type DMFC operating at room temperature. Immersing the catalyst electrodes in a methanol solution before the MEA hot-pressing process was found to be very effective in increasing the active Pt surface area and strengthening the joint at the electrode-membrane interface. We obtained power densities as high as 76 mW/cm2 at 0.4 V for passive-type air-breathing DMFCs operating at 40 °C. 2. Experimental Section Dahlia-flower-type SWNH aggregates were produced by CO2 laser ablation of a graphite rod (100 mm in diameter and 500 mm long) in an Ar atmosphere at ambient pressure by using a large-scale three-chamber apparatus under conditions in which the laser power density was 30 kW/cm2 and the target rotated at 2 rpm.31,40 More than 60 g of SWNHs were obtained within an hour. The SWNHs were approximately 95% pure, and the impurities consisted of amorphous carbon and graphite-like particles.41 Since the as-produced SWNHs were fluffy (apparent density ∼0.02 g/cm3) and highly hydrophobic, they were agglomerated by immersing them in ethanol, sonicating them for 3 min, and then drying them at room temperature under flowing N2. This made them much denser (∼0.2 g/cm3) and easier to disperse in aqueous solutions. We used such agglomerated and less hydrophobic SWNHs (hereafter referred to as “asSWNHs”) as a starting material in the experiments reported here. To create defects on the surface of the SWNHs’ rolled graphene structure, we oxidized as-SWNHs in an aqueous solution containing 30% H2O2.42 Samples containing about 5 g of as-SWNHs dispersed in 2.5 L of 30% H2O2 were stirred for 3 h while a water/oil bath kept them at a temperature between room temperature and 90 °C. After the SWNHs were extracted from the dispersion by filtration and washed five times with water, they were dried in vacuo at 100 °C for 48 h. The Brunauer-Emmett-Teller (BET) specific surface areas of the H2O2-treated SWNHs (treated-SWNHs) were calculated from the isothermal nitrogen adsorption measured by a surface area analyzer (Shimadzu MIC-2000). The surface area of the asSWNHs was also measured for reference. The as-SWNHs and treated-SWNHs were also observed by using a transmission electron microscope (Topcon EM-002B) operated at a 120-kV accelerating voltage. The Pt catalyst was supported on the SWNHs by using a colloidal method.43,44 The as-SWNHs and the treated-SWNHs at 70 °C were used as the catalyst supports. After the NaHSO3 and H2PtCl6 solution was stirred and pH adjusted to 5-6 by using a Na2CO3 solution, the SWNHs powder was put into the mixed solution and dispersed with a homogenizer. H2O2 was added to the mixture to form Pt oxide colloids that were immediately adsorbed on the surface of the SWNHs. The excess of H2O2 was then removed by boiling the solution. The platinum content (wt % Pt) was adjusted by changing the nominal Pt: SWNHs ratio. After Cl-, Na+, and other ions were eliminated by filtrating the solution, the obtained Pt oxide particles supported on the SWNHs were reduced in hydrazine solution.

J. Phys. Chem. C, Vol. 113, No. 20, 2009 8661 The reduced samples were dried and further reduced in H2 gas at room temperature for 2 days. The obtained Pt catalyst supported on the SWNHs (Pt/SWNHs) was characterized by thermogravimetric analysis (TGA, Bruker, TG-DTA2000SA), transmission electron microscopy (TEM), and cyclic voltammetry (CV) with an electrochemical analyzer (BAS, ALS/ CHI600A). The membrane electrode assembly (MEA) was prepared as follows. The Pt/SWNHs catalyst paste for a cathode was prepared by blending 6.25 g of Pt/SWNHs catalyst powder (60 wt % Pt), 18 mL of isopropyl alcohol, and 30 mL of 5 wt % Nafion ionomer solution (Du Pont) in a Teflon container under N2. The mixture was stirred with a planetary stirrer (∼1500 rpm revolution with 600 rpm rotation) for about 1 h intermittently until the catalyst and the Nafion solution were completely mixed. For the preparation of an anode, a mixture of 5 g of Pt-Ru catalyst powder (supported on carbon black, Tanaka TEC61E54) containing 50 wt % Pt-Ru, 12 mL of water, and 50 mL of the 5 wt % Nafion solution was also stirred for about 1 h. Note that the solvent for Pt/SWNHs catalyst contained no water because the hydrophobicity of SWNHs makes them rather cohesive. The catalyst pastes for anodes and cathodes were sprayed onto 0.2-mm-thick (40 × 40)-mm2 porous electrodes (so-called “diffusion electrodes”) made of Fe-Cr-Si fiber (NHK Spring) at about 0.2 mg/cm2 per minute while being dried under N2 on a hot plate at 140 °C. We used these “metal paper” electrodes as the diffusion electrodes instead of the widely used carbon paper because of the higher electrical conductivity and toughness of the metal material. A cathode contained approximately 5 mg/ cm2 of Pt, and an anode contained approximately 4 mg/cm2 of Pt-Ru. A hydrocarbon membrane (Tokuyama, C255) 25-µm thick was used as the electrolyte membrane. A hydrocarbon electrolyte ionomer (Tokuyama, C-solution) sprayed on both sides of the membrane and dried at 140 °C acted as adhesion layers. For full swelling, the membranes were then stored in distilled water until use. The anode and cathode were immersed in a 50 vol % MeOH solution for 18 h before the hot-pressing process. This MeOH immersion process was found to be indispensable to obtain a high cell performance and thus will be discussed in more detail later in this paper. The wet electrodes with methanol solution were placed on both sides of the wet membrane and hot pressed at 115 °C under the pressure of 50 kg/cm2 for 10 min to form a MEA. The fuel cell performance of the (40 × 40)-mm2 MEA was evaluated by using a passive-type DMFC test cell at room temperature. The anode side of the MEA was immersed in the methanol fuel, while the cathode side was exposed to air (air breathing). The current-voltage (I-V) characteristics of the cell were measured, in an oven kept at 40 °C and 95% humidity, by using a Keithley 2440 SourceMeter. To keep the heat generated during the measurement from increasing the measurement temperature, the methanol fuel was circulated slowly (∼10 cm3/min) between the test cell (fuel volume ∼10 cm3) and a larger reservoir (∼500 cm3) placed in the same oven. The cell temperature was monitored by thermocouples attached to both sides of the cell. The anode temperature was rather stable for the lower current region (0.45 V) and increased by only a few degrees with increasing the current up to 4 A. The cathode showed slightly higher temperatures than the anode, but the difference was at most 2 °C. Note that such a slow circulation of the methanol fuel did not in any way increase the power density of the cell. The series resistance of the cell was estimated from the impedance spectra measured at open-

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Figure 1. BET specific surface areas of the treated-SWNHs versus H2O2 treatment temperature.

circuit potential in the frequency range between 0.1 Hz and 100 kHz (ac voltage of 10 mV) by using an impedance analyzer (Solartron 1255B/1287). 3. Results and Discussion SWNHs and Catalyst Support. The BET surface areas are plotted in Figure 1 against the H2O2 treatment temperature. The surface area of the as-SWNHs (not shown) was 400-420 m2/ g. This value seems rather small if we consider that each SWNH has a closed graphene structure and that the theoretical surface area of a graphene sheet is about 1300 m2/g (for one side). Thus more than 2/3 of the SWNH’s surface area seems to be inaccessible because of overlapping of horns in the aggregated structure of SWNHs. The observed surface area increased with increasing H2O2 treatment temperature and at 90 °C reached 1100 m2/g. These results suggest that an internal surface area as well as a part of the external surface area of SWNHs was measured as a result of gas diffusion through defects larger than N2 molecules. The increasing surface area is an indication of the increased number and/or size of the defects. Previous studies have also found the surface area of SWNHs to be increased by other oxidation methods: treatment with 70% HNO3 at 130 °C for 15 min yielded a surface area of 1200 m2/g,45 and combustion at 550 °C in 100% oxygen for 10 min yielded one of 1400 m2/g.46 Compared with those methods, the present H2O2 treatment seems easier to perform and to provide better control of the amount and size of defects on the surface of SWNHs. If the defects were larger than the H2PtCl6 ion (diameter ∼0.8 nm), the H2PtCl6 ion would penetrate into the inner space of nanohorns and form Pt clusters inside SWNHs during the supporting Pt catalyst process on the SWNHs. Those Pt clusters inside SWNHs, however, would never be active as catalyst for fuel cells. We therefore checked the size of the defects in the treated-SWNHs by using C60 molecules, (diameter ∼0.7 nm) whose size is similar to that of the H2PtCl6 ion.45,47 We put the treated-SWNHs into a C60-saturated toluene solution and ultrasonicated it for 1 min to disperse the SWNHs homogeneously. The toluene was then evaporated at room temperature under a nitrogen gas flow. The number of C60 molecules inside the SWNHs was counted by TEM observation. No C60 molecules were evident inside SWNHs treated at 55 °C (Figure 2a), a few were evident inside SWNHs treated at 75 °C (Figure 2b), and many were evident inside SWNHs treated at 90 °C (Figure 2c). Note that no disintegration of the horn-shaped tips was observed by TEM even when the sample was treated at 90 °C, although incorporated C60 molecules indicated the presence of large defects (>0.7 nm) on the surface of SWNHs. The original structure of dahlia-flower-like aggregate seems quite robust

Figure 2. TEM images of treated-SWNHs with C60 incorporated. H2O2treatment temperatures: (a) 55, (b) 75, and (c) 90 °C.

against such oxidation treatment. From these results, the H2O2 treatment temperature of 65-75 °C was judged to be most appropriate for making the largest number of small defects on the surface of SWNHs. In this study, we therefore treated SWNHs with H2O2 at 70 °C. Platinum was supported on the as-SWNHs and the treatedSWNHs (at 70 °C) at various levels of Pt content csdefined as the Pt/(Pt + SWNHs) weight ratiosfrom 10 to 80 wt % by changing the nominal compositions. The actual Pt wt % for each sample was determined from the TGA measurement in a pure O2 gas flow at a heating rate of 10 deg/min. The actual and nominal contents were within 7% for each sample, so values specified and referred to in the rest of this paper will be nominal values. The particle size of Pt catalyst was estimated from TEM images by measuring sizes of about 500 particles for each sample and calculating the mean. The histograms of the Pt particle size (diameter) distribution for samples supported on both as- and treated-SWNHs are shown in Figure S2, Supporting Information. The mean Pt particle sizes are plotted against Pt content in Figure 3a. The mean Pt particle sizes of both as- and treated-SWNHs increase with the Pt content, and the former is always larger than the latter for the same Pt content. These results imply that the number of Pt particles on the as-SWNHs is smaller than that on the treated-SWNHs. To estimate the number of Pt particles, we plotted the Pt particle size against the cubic root of the Pt/SWNHs weight ratio R ≡ c/(1 - c) in

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Figure 3. (a) Mean Pt particle size versus Pt wt %. (b) Mean Pt particle size plotted against the cubic root of the Pt/SWNHs weight ratio R.

Figure 3b. In a simple sphere model, the number of Pt particles on 1 g of SWNHs is expressed as (6/πFPt)(R/D3), where FPt is the density of Pt (21.4 g/cm3), and D is the diameter of the Pt particles. As shown in Figure 3b, the Pt particle diameter D is proportional to R1/3 for both as- and treated-SWNHs, which demonstrates that the number of Pt particles is kept constant even when the Pt content is largely changed. This strongly suggests that for each SWNH there are a fixed number of adsorption sites where Pt particles are preferentially adsorbed. From the slopes (D/R1/3) fitted to both SWNHs data, the treatedSWNHs are considered to have 2.5 times more adsorption sites (5.0 × 1018 per g-SWNHs) than the as-SWNHs (2.0 × 1018 per g-SWNHs) do. Such a large increase in the number of adsorption sites on the treated-SWNHs should be attributed to the creation of defects by the H2O2 treatment. Considering the SWNHs’ specific surface area, about 400 m2/g, one can calculate that the average area allowed for each Pt particle is about 200 nm2 for as-SWNHs and 80 nm2 for treated-SWNHs. Now we consider the SWNHs’ aggregate structure in more detail. The spherical SWNHs aggregate with a diameter of ∼100 nm is considered to consist of about 2000 nanohorns 2-4 nm in diameter and 40-50 nm long.31,48 This model roughly gives a surface area of ∼400 nm2 for an individual nanohorn and an apparent density of ∼1.2 g/cm3 for a single aggregate. Note that because the observed surface area (∼400 m2/g) is about 1/3 of the theoretical one (∼1300 m2/g), only about 1/3 of the nanohorn’s surface area near the tip is available for supporting catalyst. Thus the available area for an individual SWNH is ∼130 nm2, which allows a few Pt catalyst particles to be supported on it. The total number of Pt particles supported on a single aggregate would be about 1300 for as-SWNHs and 3300 for treated-SWNHs. These considerations are roughly consistent with the TEM observations shown in Figure 4. Characterization of Catalyst Electrode. The Pt catalysts supported on the SWNHs were characterized by steady-state cyclic voltammetry (CV). The CV measurements were carried out in a conventional three-electrode cell containing 0.5 M H2SO4 electrolyte purged with N2 at room temperature. The working electrode was prepared by spreading about 4 mg of a mixture of the Pt/SWNHs catalyst, isopropyl alcohol, and 5 wt % Nafion solution on carbon-fiber paper (Toray, TGP-H-120) and drying it at 140 °C on a hot plate. The electrode geometric

Figure 4. TEM images of the supported catalyst: (a) 20 wt % Pt on as-SWNHs, (b) 20 wt % Pt on treated-SWNHs, (c) 60 wt % Pt on as-SWNHs, and (d) 60 wt % Pt on treated-SWNHs.

Figure 5. Cyclic voltammograms for 50 wt % Pt on as-SWNHs catalyst electrodes: (a) data for electrodes immersed for 18 h in water containing various concentrations of MeOH and (b) data for electrodes immersed in 50 vol % MeOH solution for various times.

area exposed to the electrolyte was 0.09 cm2. A platinum-wire electrode was placed as the counter electrode, and an Ag/AgCl electrode was used as the reference electrode. The potentials reported here are potentials relative to the normal hydrogen electrode (NHE). The surface of the Pt catalyst was refreshed by cycling the potential between 0 and 1.2 V at 50 mV/s. Then the CV was measured at 5 mV/s. It should be noted that the as-prepared (dried) catalyst electrode showed very strong hydrophobicity and repelled the H2SO4 solution. Thus the electrode surface was mostly covered with air bubbles, resulting in a quite small CV profile as shown in Figure 5a. Thinking that this hydrophobicity was probably due to the nature of the dried Nafion ionomer, we tried to fully swell the electrode (Nafion) prior to the measurement. The CV profile, however, was not improved much even after the electrode had been immersed in water for 18 h (Figure 5a, red curve). We therefore tried immersing it in methanol, which produced much better swelling, and found the CV profile to be improved markedly. The working electrodes were thus immersed in various concentrations of MeOH for 18 h at room temperature and then rinsed with H2SO4 to remove the methanol before the

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CV measurements. The results are summarized in Figure 5a. The amplitude of the CV profile increased with increases in MeOH concentration and saturated at 30-50 vol %, where typical CV profiles for Pt catalyst particles were observed. The observed hydrogen adsorption and desorption peaks (0-0.3 V) are associated with Pt(110), Pt(111), and Pt(100) sites,49 and the peak in the positive region at 0.7 V is associated with remaining methanol. The samples immersed in MeOH at concentrations of 60 vol % or more separated from the carbon paper within 18 h. As shown in Figure 5b for 50 vol % MeOH, the amplitude of the CV profile also depended on the immersion time. The amplitude increased markedly with increases in immersion time up to 30 min. The active Pt surface area of the electrodes was estimated from the hydrogen adsorption and desorption waves in the CV profiles50 measured after the electrodes were immersed in 50 vol % MeOH for 18 h. The relatively large double-layer current due to the large surface area of the carbon support was subtracted in the calculation of this estimate. The hydrogen adsorption and desorption charges for the electrode consisting of 50 wt % Pt supported on as-SWNHs were respectively 181 and 156 mC/mg of Pt (mean ) 169 mC/mg of Pt), from which the active Pt surface area was calculated, assuming a specific charge of 210 µC/cm2, to be 80 m2/g. This surface area corresponds to a Pt particle size of 3.5 nm, which is consistent with the 3.4 nm particle size estimated from the TEM images. In the case of 60 wt % Pt, the mean of the hydrogen adsorption and desorption charges was 139 mC/mg of Pt for as-SWNHs and 195 mC/mg of Pt for treated-SWNHs. These values correspond to the active Pt surface areas and Pt particle sizes of 66 m2/g and 4.2 nm for the Pt on as-SWNHs and 93 m2/g and 3.0 nm for the Pt on treated-SWNHs. These particle sizes are also consistent with the mean Pt particle sizes estimated from the TEM images, 4.2 and 2.9 nm. These results clearly demonstrate that almost all the surfaces of the Pt catalyst particles dispersed in the electrode can be activated only by immersing the electrode in solution with a high concentration of MeOH, such as 50 vol %, for more than 30 min at room temperature. Although the detailed mechanism of this activation is unknown, we suggest some points as follows. First, the high-concentration MeOH solution can penetrate the strongly hydrophobic electrode and swell the Nafion component enough, thus making it a good ionic conductor (electrolyte). Note that the ionic conductivity of dried Nafion is usually very low. Second, the swelled (thus expanded) Nafion is so soft that it will wrap around the Pt particles tightly. Finally, some chemical effects might be responsible. After the 140 °C drying process the Pt surface may be contaminated with impurities or covered with an oxide monolayer that would prevent catalytic reaction. The high concentration of MeOH can possibly remove those impurities or oxides chemically through its reduction ability. As already mentioned in the Experimental Section, we used this MeOH-immersion method when preparing the membrane electrode assemblies evaluated in the work reported here. MEA Preparation and Fuel Cell Performance. Figure 6 shows the morphology, observed by a scanning electron microscope (Hitachi S-4800), of the cathode catalyst electrode (60 wt % Pt on treated-SWNHs) for a (40 × 40)-mm2 MEA after the spray and drying process. At first sight, the catalyst electrode looks very porous, consisting of particles about 100 µm in diameter and even larger pores (Figure 6a). Such a porous structure seems to be formed through the deposition of droplets (∼100 µm) produced by the spray. At higher magnifications (Figures 6b and 6c), these large particles are seen to have finer

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Figure 6. SEM images of the surface of cathodes with 60 wt % Pt on treated-SWNHs: (a) three-dimensional network structure of SWNH electrode, (b) enlarged structure of SWNH electrode, (c) close-up view of SWNH aggregates, and (d) SWNH aggregates supporting Pt particles (bright dots).

structures made of porous particles several micrometers in diameter that at the highest magnification (Figure 6d) turn out to be agglomerates of a number of spherical SWNHs ∼100 nm in diameter. Note that the supported Pt particles are visible in Figure 6d as bright dots on the spherical SWNHs. To estimate the porosity of the catalyst electrode, we measured the thickness of the electrode with various Pt contents prepared for a fixed Pt loading of 5 mg/cm2. When the catalyst pastes for those electrodes was being prepared, the amount of isopropyl alcohol and Nafion solution was varied in proportion to the amount of SWNHs to ensure that the pastes had similar viscosities. (Note that the volume of a Pt/SWNHs catalyst is nearly equal to that of the SWNHs in it because of the high density of Pt. And the viscosity of the paste is mainly determined by the volume ratio of the components.) Thus the weight ratio of Nafion/SWNHs prepared according to the recipe described in the Experimental Section was about 3/5. For a constant Pt loading of 5 mg/cm2, the amounts of (SWNHs + Nafion) are calculated to be about (20 + 12), (5 + 3), (3.3 + 2.0), and (2.1 + 1.3) mg/cm2 for the Pt contents of 20, 50, 60, and 70 wt %. Assuming the densities of SWNHs (∼1 g/cm3) and Nafion (∼2 g/cm3), the theoretical thickness would be about 0.26, 0.065, 0.043, and 0.028 mm. The measured electrode thicknesses were about six times greater: 1.6, 0.42, 0.27, and 0.17 mm for Pt contents of 20, 50, 60, and 70 wt %. Therefore, the average porosity of the cathode catalyst electrode (before hot pressing) is estimated to be about 84%. Such a porous structure of the cathode electrode seems to be quite suitable to breathe in the air and drain (vaporize) the water produced during the fuel cell reaction. In particular, the multiplenetwork structure of pores consisting of large (∼100 µm), medium (several micrometers), and small (∼0.1 µm) pores would be very suitable for delivering the air to every Pt catalyst particle efficiently and vaporizing the water quickly. On the other hand, the optimized thickness of the anode (50 wt % Pt-Ru supported on carbon black) was 0.16 mm for a Pt-Ru loading of 4 mg/cm2. Thus the porosity of the anode is estimated to be about 74%, assuming the carbon-black density of ∼1.8. Note that the optimized structure for the anode is denser than that for the cathode. This is probably due to the fact that the methanol fuel can easily penetrate into the Nafion ionomer to

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Figure 7. (a) I-V characteristics of MEA whose catalyst electrodes were immersed in a 50 vol % MeOH solution for 0 to 18 h prior to the hot pressing. (b) Current densities at 0.6 V in the I-V curves (red symbols) and series resistances of MEA (black symbols) plotted against the MeOH immersion time. The 60 wt % Pt catalyst on the treatedSWNHs was used for the cathode.

Figure 8. (a) I-V characteristics of MEA measured with various concentrations of the MeOH fuel. (b) I-V characteristics of MEAs using as-SWNHs and treated-SWNHs as catalyst supports for the cathode (60 wt % Pt). The measurements were carried out at 40 °C with a passive-type DMFC test cell with use of 4 vol % MeOH (anode) as fuel and air breathing (cathode).

reach the Pt catalysts. Moreover, a denser (and thus thinner) electrode generally has less electrical and ionic resistance. After hot pressing, the cathode (60 wt % Pt, 0.27-mm thick) was compressed to 0.12 mm thickness and the anode to 0.10 mm thickness. Thus the porosities after hot pressing are estimated to be 64% for the cathode and 58% for the anode. SEM observation revealed that the multiple-pore structure of the cathode was basically unchanged by hot pressing. The effect of the MeOH immersion of the electrodes on the DMFC performance was examined by using different immersions. The current-voltage (I-V) characteristics at 40 °C of passive-type DMFC test cells (using 4 vol % MeOH as fuel and air as oxidant) with MEAs whose electrodes had been immersed in 50 vol % MeOH for 0 to 18 h are shown in Figure 7a. The treated-SWNHs with 60 wt % Pt were used for the cathode. The current densities at 0.6 V (red circles) and the series resistances (impedances at 100 kHz) of the cell (black circles) are also plotted in Figure 7b against the MeOH immersion time. The notation “dry” in the figure designates data points for a sample that was prepared by using nonswollen membranes and electrodes (ones that had not been immersed in MeOH). The high series resistance for the dry sample indicates low ionic conductivity of membrane as well as poor adhesion between membrane and electrodes. The series resistance is considerably reduced by the use of swollen membrane and even more reduced by the MeOH immersion of electrodes, saturating at ∼0.3 Ω cm2 for immersions lasting 30 min or more. Note that this series resistance is an order of magnitude larger than the membrane resistance at room temperature, ∼0.04 Ω cm2. (This membrane resistance corresponds to an ionic conductivity of ∼0.06 S/cm, which is similar to or even higher than that for Nafion.) Therefore, most of the series resistance is ascribed to the contact resistance at the membrane/electrode interface and/or the intrinsic resistance of the catalyst electrode. It should be noted that the thicknesses of both the cathode (120 µm) and anode (100 µm) are considerably greater than that of the electrolyte membrane (25 µm). The two electrodesswith a total thickness of 220 µm and consisting of the Nafion ionomer,

carbon-supported catalyst, and poressare probably responsible for most of the 0.3-Ω cm2 series resistance. In any case, the series resistance is reduced rather quickly (within 30 min) by the MeOH immersion. The current density at 0.6 V, on the other hand, increases more slowly and saturates after more than 6 h of immersion (Figure 7b). Since the electrochemical activation process is dominant at 0.6 V, the increase of the current density indicates the increase of the active surface area of the Pt catalysts that was brought about by the MeOH immersion of the electrodes. As mentioned before, the active surface area of catalyst particles is thought to be increased by the swollen Nafion making better contact with catalyst particles and by the high concentration of MeOH removing contaminants from the catalyst surface by reduction reactions. Since the series resistance is reduced and saturated within 30 min (Figure 7b), the swelling of Nafion seems to be completed rather quickly. The activation (refreshing) of the catalyst surface by reduction reactions, on the other hand, would take more time than the swelling of Nafion. (In the case of CV measurements, note that the catalyst surface is refreshed by cycling the potential up to 1.2 V prior to the measurement. Therefore the CV profile saturates when the Nafion is swollen (at ∼30 min) as demonstrated in Figure 5b.) As shown in Figure 7a, the I-V curves shift upward with increasing MeOH immersion time, while the slopes above 0.05 A/cm2 are almost unchanged. Those slopes correspond to the internal resistances of 1.0-1.1 Ω cm2, which is more than three times the series resistances obtained in the impedance measurements. Thus it is suggested that the material diffusion processes within the “thick” catalyst electrodes are responsible to those resistances. The I-V curves start to deviate downward above 0.2 A/cm2, probably because of insufficient transport of fuel and/or air. The methanol crossover through the membrane from the anode side to the cathode side also influences the I-V characteristics of the MEA. Figure 8a shows the I-V curves for MEAs with 60 wt % Pt/treated-SWNHs catalyst measured

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with various concentrations of MeOH. As the MeOH concentration decreases from 10 to 3 vol %, the I-V curve as well as the open circuit voltage shifts upward because the rate of MeOH crossover decreases. The voltage for 3-vol % MeOH, however, shows a remarkable decline at current densities above 0.15 A/cm2 because of the lack of fuel supply. In those I-V curves, the highest current density at 0.4 V was obtained for 4 vol % MeOH. Using the MEA preparation method described above, we prepared two MEAs with different cathodes: one with 60 wt % Pt supported on treated-SWNHs and the other with 60 wt % Pt supported on as-SWNHs. The I-V characteristics measured in an oven at 40 °C for a passive-type DMFC test cell that used 4 vol % MeOH and air (air breathing) are shown in Figure 8b. The voltages for the catalyst on the treated-SWNHs are higher than those for the catalyst on the as-SWNHs. The current density at 0.6 V is 0.026 A/cm2 for the treated-SWNHs and 0.010 A/cm2 for the as-SWNHs, which reflects the difference in the Pt surface area between the two catalysts. (The ratio of Pt surface area is about 3:2 in this case.) The Pt/treated-SWNHs catalyst yielded an output power density as high as 76 mW/cm2 at 0.4 V, at which the passive-type DMFC usually operates around room temperature. It is well-known that the output power density of DMFCs is considerably enhanced by increasing the operating temperature or by using O2 instead of air. Most of the previous measurements of DMFCs have been done at high temperatures, 50-90 °C, and supplying pure O2 to the cathode.16-21,24-26,29 The highest power densities obtained with pure O2 were 145 mW/cm2 at 90 °C,24 75 mW/cm2 at 80 °C,19 45 mW/cm2 at 70 °C,18 90 mW/cm2 at 60 °C,24 117 mW/cm2 at 50 °C,16 and 30 mW/cm2 at 30 °C.24 On the other hand, there have been only a few reports of measurements made with air flow or air breathing for the cathode. The highest power densities obtained supplying air to the cathode at 0.2 L/min were 100 mW/cm2 at 80 °C, 75 mW/ cm2 at 60 °C, and 50 mW/cm2 at 40 °C.51 In the case of air breathing, a power density of 13 mW/cm2 was obtained at 22 °C.28 The 76 mW/cm2 at 0.4 V obtained in the present study is thus extremely high for an air-breathing DMFC at 40 °C. We tested the cell performance under these severe conditions because the small DMFCs for consumer electronics applications would be mostly used with air breathing at temperatures near room temperature. We also prepared other MEAs using the Pt/treated-SWNHs catalysts with the Pt contents of 70 and 80 wt % under the same Pt loading condition (5 mg/cm2). Although the thickness of the catalyst electrode became thinner for those high-Pt-content catalysts, the active surface area of catalyst was reduced due to the growth of Pt particles. As a result, the power densities for those MEAs did not exceed the highest value mentioned above. Note that for a given Pt loading there is a trade-off relationship between electrode thickness and catalyst activity. 4. Conclusion SWNH aggregates were used to support Pt catalyst in the DMFC cathode. Their unique structure is advantageous to support fine particles of catalyst for fuel cell electrodes because the SWNH aggregate has thousands of nanospaces between the horns. In addition, we have created defects on the surface of the SWNHs by oxidizing them with H2O2 solutions and thereby increased the number of effective adsorption sites for Pt particles by a factor of 3. Thus we could obtain well-dispersed fine Pt particles even in high-Pt weight percent conditions. The Pt particle size for 60 wt % Pt on treated-SWNHs was 2.9 nm, which was about 2/3 that for 60 wt % Pt on as-SWNHs.

Kosaka et al. We also improved the MEA preparation process. The cathode catalyst electrode was formed to be very porous, with multiplepore structure found to be very effective for breathing the air and removing the water. Immersing the catalyst electrodes in high concentrations of MeOH before hot pressing makes the active surface area of the catalyst increase to nearly 100% of the Pt surface. The MeOH immersion also strengthened the adhesion between the membrane and electrodes, thus helping increase the power density of the MEA. The highest power density of 76 mW/cm2 at 0.4 V was obtained for a passivetype DMFC test cell operated at 40 °C with a cathode made of treated-SWNHs containing 60 wt % Pt. One of the remaining problems is that the catalyst electrode is still so much thicker than the electrolyte membrane. This is caused by the fact that high Pt loadings are necessary for DMFCs because of the slow catalyst reactions. Too thick an electrode, however, not only has a high series resistance but also obstructs the diffusion of fuel and air, thereby increasing the internal resistance of the cell. To solve this problem, it is necessary to further increase the Pt content of the supported catalyst while suppressing the growth of the Pt particles. Therefore, the method used for treatment of SWNHs as well as the catalyst supporting method should be further improved. Acknowledgment. This work was in part performed under the management of the Nano Carbon Technology Project supported by NEDO. The authors are thankful to Dr. Takeshi Azami and Dr. Daisuke Kasuya for preparing the SWNHs. We would like to thank Dr. Masako Yudasaka for suggesting the H2O2 treatment of SWNHs. We would also like to thank Dr. Ryota Yuge, Dr. Sumio Iijima, Dr. Hideto Imai, and Dr. Takashi Manako for useful discussions. Supporting Information Available: A movie showing the TEM images of the Pt particles supported on SWNHs and a figure showing the Pt particle size distribution for samples of as-SWNHs and treated-SWNHs with various Pt contents. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Hampson, N. A.; Wilars, M. J. J. Power Sources 1979, 4, 191. (2) Surampudi, S.; Narayanan, S. R.; Vamos, E.; Frank, H.; Halpert, G.; Laconti, A.; Kosek, J.; Prakash, G. K. S.; Olah, G. A. J. Power Souces 1994, 47, 377. (3) Ren, X.; Wilson, M. S.; Gottesfeld, S. J. Electrochem. Soc. 1996, 143, L12. (4) Wasmus, S.; Kuver, A. J. Electroanal. Chem. 1999, 461, 14. (5) Shukla, A. K.; Christensen, P. A.; Hamnett, A.; Hogarth, M. P. J. Power Sources 1995, 55, 87. (6) Arico, A. S.; Creti, P.; Kim, H.; Mantegna, R.; Glordano, N.; Antonucci, V. J. Electrochem. Soc. 1996, 143, 3950. (7) Baldauf, M.; Preidel, W. J. Power Sources 1999, 84, 161. (8) Yang, R.; Qiu, X.; Zhang, H.; Li, J.; Zhu, W.; Wang, Huang, X.; Chen, L. Carbon 2005, 43, 11. (9) Steigerwalt, E. S.; Deluga, G. A.; Lukehart, C. M. J. Phys. Chem. B 2002, 106, 760. (10) Bessel, C. A.; Laubernds, K.; Rodriguez, N. M.; Baker, R. T. K. J. Phys. Chem. B 2001, 105, 1115. (11) Guo, J.; Sun, G.; Wang, Q.; Wang, G.; Zhou, Z.; Tang, S.; Jiang, L.; Zhou, B.; Xin, Q. Carbon 2006, 44, 152. (12) Steigerwalt, E. S.; Deluga, G. A.; Cliffel, D. E.; Lukehart, C. M. J. Phys. Chem. B 2001, 105, 8079. (13) Hyeon, T.; Han, S.; Sung, Y.; Park, K.; Kim, Y. Angew. Chem., Int. Ed. 2003, 42, 4352. (14) Park, K.; Sung, Y.; Han, S.; Yun, Y.; Hyeon, T. J. Phys. Chem. B 2004, 108, 939. (15) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature (London) 2001, 412, 169. (16) Kuroki, H.; Yamaguchi, T. J. Electrochem. Soc. 2006, 153, A1417.

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