Nanostructured Coral-like Carbon as Pt Support for Fuel Cells

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Nanostructured Coral-like Carbon as Pt Support for Fuel Cells Wei-Fu Chen, Jing-Ping Wang, Chun-Han Hsu, Jing-Yi Jhan, Hsisheng Teng, and Ping-Lin Kuo* Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan City, 70101 Taiwan ReceiVed: January 15, 2010; ReVised Manuscript ReceiVed: March 13, 2010

A coral-like carbon material (Coral-C) is synthesized by growing curled carbon nanotubes (CNTs) onto carbon black (Lamp Black) to incorporate the unique structures and properties of the two nanostructured carbons, CNTs and carbon spheres. This Coral-C, having a good electronic conductivity, is used as a supporting material for Pt nanocatalyst for application in fuel cell electrodes. The Pt nanoparticles, being synthesized by a ligand exchange method, are stabilized on Coral-C though an enhanced deposition process with poly(oxyproplyene)diamines. The Coral-C supported Pt catalyst shows excellent electrochemical active area (102.5 m2 g-1 Pt), good catalytic activity toward methanol oxidation (1.5 times higher than E-TEK Pt/C), and good power output in single DMFC (1.3 times better than E-TEK Pt/C), which could be attributed to the unique nanostructure of the catalyst: high conductivity of the surface accessible support and highly distributed Pt nanoparticles. The successful advancement in this coral-like nanostructure design for fuel cell catalyst presents a significant achievement in both the scientific and engineering fields. 1. Introduction Currently, carbon-based materials are attracting widespread scientific and technological attention because of their exceptional chemical, mechanical and thermal stability, and have proven to be a very interesting material for a variety of applications.1-4 remarkable progress has been recently made in the fabrication of carbon materials, for example, carbon nanotubes (CNTs), nanocages, nanofibers, carbon black, and mesoporous carbon, as electrochemical devices5 because of their unique properties such as high electrochemically accessible surface area and high electronic conductivity.6 A major commercial application of CNTs is their use as supporting materials for catalysts, especially in the electrode of fuel cells. Electrodes for fuel cells are generally gas diffusible to ensure the supply of the fuel to the reaction sites where the noble metal catalyst is in contact with the ionic and electronic conductor. The key characteristic for a high-efficiency electrode is a triphase boundary between the gas supply on the one hand, and the catalyst particle and the ionic conductor on the other hand. The particles must be in direct contact with an electronic conductor to ensure the electrons are supplied to or taken away from the reaction site. Electronic conductivity is usually provided by a carbon support onto which the catalyst particles are deposited. The triphase boundary is usually produced by impregnating the catalyst/support powder with some ionomer binder, that is, Nafion dispersion, before pressing the electrode onto the membrane. This ensures good contact of most catalyst particles with the ionomer material, that is, Nafion. Moreover, when using humidified gas or a methanol/water mixture, the catalyst layer must be sufficiently hydrophobic to prevent the pores from flooding.7,8 To address these issues, several strategies for the fabrication of high-efficiency electrodes have been developed. Efforts have been made in optimizing the ionomer content in the cathode and anode so as to maximize the triphase boundary.9,10 Sun et al. demonstrated an interesting nanostructure comprising Pt * To whom correspondence should be addressed. Telephone: +886-62757575-62658. Fax: +886-6-2762331. E-mail: [email protected].

nanowires on carbon nanospheres,11 which enhanced and increased mass activity toward an oxygen reduction reaction by 50%. Additionally, Yan’s group developed supportless Ptalloy nanotubes that improved mass transport and catalyst utilization,12 while the 3M Fuel Cell team engineered a vertically aligned nanostructured thin film showing improved kinetics.13 To solve the flooding problem in the pores of the catalyst layer, researchers have endeavored to increase the hydrophobicity of the catalyst layer. This hydrophobicity can be provided by introducing PTFE as a binder in combination with Nafion, which is hydrophilic.14,15 However, the presence of PTFE in the electrode may increase the contact resistance and hinder the transportation of gas. Studies concerning this issue have been conducted, for example, Li et al developed an antiflooding catalyst layer with oxygen permeable silicon oil which improved water balance and gas transport.16 The requirements for an appropriate catalyst support are high accessible surface area, sufficient electron conductivity, sound chemical stability, and optimized hydrophobicity. The present study has incorporated the unique structures and properties of the two nanostructured carbons, CNTs and carbon spheres, and developed a new material, coral-like carbon (CoralC), by growing curled CNT on carbon black (Lamp Black). This new material offers the advantageous features of high electronic conductivity, chemical stability, and hydrophobicity. Similar nanostructured carbon materials have been extensively studied previously in interconnected structures,17 electron conduction18 and capacitor behavior.19 Subsequently, this Coral-C was used as a supporting material in Pt nanocatalysts for application in fuel cell electrodes in which the Pt nanoparticles, being synthesized by a ligand exchange method, were stabilized on Coral-C through an enhanced deposition process with poly(oxyproplyene)diamines. The successful advancement in this coral-like nanostructure design for fuel cell catalysts presents a significant achievement in both the scientific and engineering fields. On the one hand, the unique three-dimensional architecture provides a highly accessible surface favoring the formation of the triphase boundary as well as better utilization of the costly

10.1021/jp100397h  2010 American Chemical Society Published on Web 03/31/2010

Nanostructured Coral-like Carbon as Pt Support for Fuel Cells

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SCHEME 1: Illustration of the Synthetic Process Comprising the Amination of the Lamp Black (LB), Deposition of Fe Nanoparticles on LB, and Growth of CNTs on LB to Form Nanostructured Coral-C

noble metals. On the other hand, its hydrophobicity could improve the water balance and gas transport in the catalyst layer. For the Coral-C-supported Pt catalyst, both the electrochemical activity (toward methanol) and the performance in single direct methanol fuel cells (DMFCs) were explored. For an electrode assembled with this catalyst, the coral-like nanostructure is shown to significantly enhance the conduction of electrons and the fine dispersion of Pt nanoparticles. 2. Experimental Section Preparation of Coral-like Carbon. The synthesis of corallike carbon materials (Coral-C) is shown in Scheme 1. The pristine carbon, Lamp Black 101 (LB, Degussa), was pretreated with HNO3 for 6 h at 80 °C. After cooling to room temperature, the reaction mixture was washed with deionized water until a pH of 7 was obtained, subsequent to which the product was dried under vacuum at 60 °C for 24 h, yielding carboxylfunctionalized carbon. The prepared carboxyl-functionalized carbons were refluxed with excess neat thionyl chloride, SOCl2 (Aldrich), at 65 °C for 24 h. The residual SOCl2 was removed by distillation, giving acyl chloride-functionalized carbons. The acyl chloride-functionalized carbons were immediately reacted with excess triethylenetetramine (TETA, Aldrich) at 120 °C for 48 h. Unreacted TETA was removed by washing with ethanol and deionized water until the filtrate became clear, yielding the ethylenimine-functionalized LB carbon. Fe(NO3)3 · 9H2O (Aldrich) was dissolved in water and mixed thoroughly with aqueous suspensions of the as-prepared ethylenimine-functionalized LB carbon. The solvent was then evaporated and the resultant cake heated to 80 °C for 3 h, removed from the furnace and ground in an agate mortar. The CVD growth of MWCNT used in this work was carried out in a quartz tube flow reactor centered in a horizontal tube furnace. Carefully weighed catalyst samples were placed in a quartz boat at the center of the reactor tube in the furnace. A catalyst reduction step was performed in situ in the CVD reactor by

first passing 20 sccm flow of H2 at 500 °C for 2 h. The reducing atmosphere was then replaced by argon (99.99%) and the temperature was raised at 10 °C min-1 to the desired growth temperature. MWCNTs were then grown by passing a mixture of C2H2 (20 sccm) diluted in argon (100 sccm) over the catalyst at a temperature of 750 °C for 30 min to obtain the Coral-C powder. The powder that was collected was soaked in 37% HCl aqueous solution to remove iron particles. Syntheses of Coral-like Carbon-Supported Pt Nanocatalysts. Coral-C-supported Pt nanoparticles were prepared according to the following procedures. Typically, 50 mL of 10-4 M H2PtCl6 · 6H2O (Alfa Aesor) aqueous solution was mixed with 2 × 10-4 mol trisodium citrate at 100 °C with stirring under reflux for 30 min. A transparent brown solution of Pt colloids was obtained, which indicates the formation of Pt nanoparticles. After cooling down, poly(oxypropylene)diamines (JEFFAMINE D-400, Huntsman Corp.), being dissolved in a 50 mL ethanol solution, was added to the Pt colloidal solution, keeping the molar ratio of H2PtCl6 to poly(oxypropylene)diamines of 0.1. The Coral-C-supported Pt catalyst was prepared by combining the Pt colloidal solution with a suspension of Coral-C in ethanol/ water (vol. ratio ) 50/50). The solution was stirred vigorously for 1 h, kept still overnight, and then centrifugation process was performed to separate the solid residue and solvent at a speed of 10 000 rpm for 30 min. The resulting solids were washed with a copious amount of distilled water to remove excess Cland finally dried in oven at 90 °C, giving the Coral-C-supported Pt catalyst (Pt/Coral-C). MWCNT- (Desunnano Co. Ltd.) and LB- supported Pt catalysts were prepared by the same procedure as well. A nominal metal loading of 20 wt % was taken on all the catalysts. These carbon-supported nanoparticles were then subjected to a thermal treatment at 400 °C for 5 h under Ar atmosphere. The resulting powder was used to prepare working electrodes. Methods of Characterization. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a VG

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Figure 1. The XPS photoelectron spectra in the (a) C 1s and (b) N 1s regions of TETA-functionalized LB carbon, (c) Fe 2p region of the LBsupported Fe2O3 nanoparticles. (d) The Pt 4f line for Coral-C supported Pt after thermal treatment in H2 at 400 °C for 5hrs.

Scientific ESCALAB 210 electron spectrometer using Mg-KR radiation under a vacuum of 2 × 10-8 Pa. The morphological characterization was performed by high-resolution field emission scanning electron microscope (HR FESEM) using a JEOL JEM6700 FESEM operating at 10 kV. Transmission electron microscopy (TEM) was conducted using a Hitachi H-7500 microscope operating at 120 kV. TG analysis was performed on a thermogravimetric analyzer (Perkin-Elmer TGA 7) over a temperature range of 50-900 °C at a heating rate of 20 °C min-1. X-ray powder diffraction (XRD) was performed on a Rigaku RINT2100 X-ray diffractometer with Cu-KR radiation operated at 30 kV and 30 mA. A CHI-608A potentiostat/ galvanostat and a conventional three-electrode test cell were used for electrochemical measurements. An Ag/AgCl/KCl (3.0 M) leak-free electrode was used as a reference. Single fuel cell test was evaluated using a unit cell with an active area of 5 cm2 fed with a 2.0 M methanol aqueous solution at the anode with a rate of 2 mL min-1 by a peristaltic micropump and oxygen at the cathode with a rate of 100 mL min-1. The membrane electrode assemblies for the single cell test were fabricated as follows. Catalyst ink was prepared by mixing Pt/C catalyst powder with water (2 mL for 1 g of electrocatalyst), and then adding isopropanol (20 mL for 1 g of electrocatalyst) to avoid any ignition. Five percent Nafion dispersion (Dupont) was added (0.8 g solid Nafion for 1 g of catalyst) to the catalyst slurry. Catalyst coating on gas diffusion layer (50 wt % wet-proofing carbon paper, Toray) with 5 cm2 active area was fabricated by brushing Pt/C catalyst ink. The catalyst loadings on the anode and cathode layers were both 2 mg Pt cm-2. The catalyst-coated GDLs were hot-pressed with Nafion-117 membrane (Du Pont) at 140 °C under 30 kg cm-2 of pressure.

3. Results and Discussion Synthesis and Characterization of Coral-like Carbon Materials. The synthesis of nanostructured coral-like carbon material (Coral-C) involves five main steps, which are illustrated in Scheme 1. First, carboxyl-rich carbon black was prepared by oxidizing Lamp Black 101 carbon (LB) with 60% HNO3 to produce a significant number of carboxyl groups on the LB’s surface, which were then reacted with SOCl2 to produce acyl chloride-functionalized LB. Tetraethylenetriamine (TETA) was then grafted to the LB’s surface by the reaction between the acyl chloride and the amine groups. These grafted ethylenimine chains, in turn, serve as the chelate sites for the immobilization of Fe nanoparticles. Fe-covered LB can be obtained by reducing the complexed Fe(III) ions with H2. The ethylenimine chains stabilize the Fe nanoparticles through steric effects and thereby serve to immobilize Fe on the LB’s surface to form a dense particulate layer. The subsequent chemical vapor deposition (CVD) of acetylene at 750 °C for 30 min produced carbon nanotubes (CNTs) on carbon particles. The chemical composition of the functionalized LB, Fe2O3, and Pt nanoparticles were characterized by X-ray photoemission spectroscopy (XPS), as shown in Figure 1. The XPS peak observed at 284.6 eV as shown in Figure 1a could be assigned to the binding energy of C 1s. The band position matched graphitic carbon, indicating the existence of a large amount of sp2-hybridized carbon.20 In Figure 1a, the peaks at 286.5 and 288.4 eV21 are ascribed to the C-N and CdO bonding, respectively. Derived from the N 1s line shown in Figure 1b is an intense peak, which appeared at 400.9 eV. This peak demonstrates the presence of protonated ammonium ions.22,23 The inset of Figure 1a shows the C 1s line for the LB carbon-



Figure 2. Typical SEM images of (a) pristine LB carbon particles and (b) the Coral-C. (c) TEM image of the Coral-C, and (d) the corresponding histogram of the tube diameter distribution.

supported Fe2O3 after thermal treatment at 500 °C in H2 for 1 h. The peaks at 286.5 and 288.4 eV disappear, indicating that the surface amine groups on TETA-functionalized LB carbon are removed following the reduction of the Fe ions. The presence of the Fe species on LB after thermal treatment at 500 °C in H2 for 1 h was confirmed by the Fe 2p XPS spectrum. XPS analysis of the aforementioned Fe nanoparticles, as shown in Figure 1c, exhibited two main peaks at 712.6 and 725.8 eV, which are characteristic peaks for Fe(III) 2p3/2 and Fe(III) 2p1/2 lines,24,25 respectively. No peak associated with zerovalent iron (Fe(0) 2p3/2) was observed at around 706 eV. These two peaks reveal that the Fe particles on LB carbon exist as iron(II or III) oxides. Using TETA-functionalized LB carbon-supported Fe catalysts and acetylene as a carbon source, we prepared a Coral-C with CNTs on LB carbons. The SEM image of the pristine LB (Figure 2a) showed that the primary particle size of LB was about 200 nm. The resulting morphology of the Coral-C was investigated by SEM and TEM in terms of the quality of the distribution and the diameter of the nanotubes. For Coral-C, CNTs were clearly produced (Figure 2b) on the surface of LB. The morphology and structure of the as-synthesized Coral-C is shown in the TEM image (Figure 2c). The spherical LB particles are connected and enwrapped by the highly curled CNTs with lengths of several hundred nanometers. It is known that the curled structure of CNTs is the result of defects of the geometric configurations of hexagonal, pentagonal, and heptagonal carbon rings.26 The histogram of the TEM image presented in Figure 2c shows that the tube diameter distribution is wide (Figure 2d). The average diameter of the nanotubes was calculated as being 27.2 nm. The electron conductivity of the Coral-C was further characterized, as shown in Table 1. The electron conductivities were measured by adding 5% PTFE in the samples to be 8.8, 277.4, 64.6, and 11.0 S cm-1 for LB, commercial MWCNT (Figure S1), Coral-C, and XC-72, respectively. Evidently, the growth of the CNTs on LB brought about good electron conductivity, which is much higher than XC-72 carbon. This reveals that the

TABLE 1: The Electron Conductivity of Various Carbon Supportsa

a

carbon type

electron conductivity (S cm-1)

Coral-C LB MWCNT XC-72

64.6 8.8 277.4 11.0

All samples for conductivity measures were added 5% PTFE.

curled CNTs on LB particles enable the conduction of electrons among carbon particles more effectively. Deposition of Pt Nanoparticles on Coral-like Carbon Supports. The Coral-C was used as platinum support for application in fuel cells. In this study, Pt nanoparticles were prepared by a ligand-exchange process27 involving the reduction of Pt precursors by trisodium citrate, and the stabilization and deposition of Pt particles on the carbon supports by the aminocontaining polymer, poly(oxypropylene)diamine, which was used to prepare noble metal nanoparticles in our previous work.28,29 Stabilization of the Pt particles by poly(oxypropylene)diamine can be discussed mainly from the viewpoint of complexing ability and steric effect. On the one hand, amino groups, which have been reported to interact with metal ions as well as with the corresponding reduced metal,30-32 can be considered as a highly effective chelating agent for metal atoms, but on the other hand, the poly(oxypropylene) chain surrounding the complexed metal core prevents the particles from undergoing any kind of agglomeration. The resulting morphologies of the Pt catalyst supported by carbon materials after thermal treatment at 400 °C for 5 h were investigated by TEM in terms of the quality of the dispersion and the quantity of the nanoparticles. The TEM image of the Pt/Coral-C sample (Figure 3a) shows that the deposition of welldispersed Pt as small metal clusters on the surface of the Coral-C is achieved. Attempts at depositing remarkably uniform ∼2.3 nm diameter Pt clusters were successful, indicating that poly(oxypropylene)diamine provides effective steric stabilization to

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Figure 3. TEM micrographs and corresponding size histograms of (a) Pt/Coral-C, (b) Pt/MWCNT, and (c) Pt/LB catalysts having thermal treated at 400 °C for 5 h in N2.

prevent Pt nanoparticles from aggregating. The “free” amine groups on poly(oxypropylene)diamine-stabilized Pt(0) can further attach onto the carbon surface through electrostatic interaction and/or hydrogen bonding force with the surface carboxyl groups, as reported previously.28 The hydrophobic poly(oxypropylene) segments can also promote the macromolecules loaded with Pt nanoparticles to adsorb onto the carbon surface through hydrophobic interaction while simultaneously separating the particles from each other. Moreover, during the thermal activation process, these macromolecules can also serve as blocks to effectively prevent metal particles from aggregation or fusion. We also compared the MWCNT and LB-supported Pt with Coral-C-supported Pt. Figure 3b,c shows aggregated particles, composed of nanoparticles 3.9 and 5.6 nm in size for Pt/ MWCNT and Pt/LB, respectively. The result indicated that the particle size and the quantity of dispersion of Pt nanoparticles were also influenced by the type of supporting carbon. In Figure 3b, the Pt/MWCNT shows some aggregation which resulted in a particle size of about 2-5 nm and a slightly inferior distribution, as compared with Pt/Coral-C. Commercial MWCNTs possess fewer defect structures on tube walls, and therefore have fewer polar sites for Pt deposition. Figure 3c shows a serious aggregation of Pt nanoparticles which results in a wide size distribution of large nanoparticles in the range of 4-8 nm. This result can be ascribed to a less defective structure with a very low surface area of LB carbons. Even with the presence of poly(oxypropylene)diamine, aggregation of Pt particles occurred on LB carbon due to the insufficient surface area for Pt deposition. Structural Analysis and Loading Percentage of Pt on Supporting Carbon. The obtained Pt catalysts were analyzed by powder XRD patterns (Figure S2). The patterns show characteristic diffraction peaks at 39, 46, 67, and 81°, which are designated to Pt{111}, {200}, {220}, and {311} facets of the face-centered cubic (fcc) crystal structure, respectively.33 The grain size obtained from the pattern for Pt/Coral-C is 3.1 nm, which is close to the diameter obtained by TEM images (2.9 nm).

Although the thermal activation of the catalysts allows the Pt nanocrystals to expose their active sites, it is the state of the surface Pt atoms that determines the activity of Pt-based catalysts. In Figure 1d, the Pt 4f line shows two pairs of peaks from the spin-orbital splitting of the 4f7/2 and 4f5/2. The most intense doublets observed, namely those at 70.9 and 74.3 eV, are attributed to zerovalent platinum (Pt(0)). The second set of doublets obtained at 72.0 and 75.3 eV can be ascribed to Pt (II) chemical states such as the PtO and Pt(OH)2 species,34 indicating the presence of Pt oxides. The percentage of each component is obtained from the relative area of these peaks. The percentages of Pt(0) are given as 68.2% for the Pt/C catalyst; this figure is slightly lower than that of the E-TEK Pt/C catalyst, namely 73.5%, due to the size effect. Finely dispersed metallic Pt(0)-dominated catalysts were successfully prepared by this ligand-exchange method. The loading percentages of these carbon-supported Pt catalysts were analyzed by measuring the residual percentages in TGA by using air to decompose all the carbonaceous components (Figure S3). All carbon components should be eliminated by air at 850 °C. As shown in Table 2, the Pt-loading percentages calculated from the residue values are 17.0% for Pt/Coral-C, which is slightly lower than the nominal value of 20%. This result can be explained by the complexing ability and the reversible equilibrium inherent in the depositing process of the D400-coordinated Pt. Some of the D400-coordinated Pt may reversibly dissolve in solvent during the deposition. The other Pt-loading percentages of Pt/LB and Pt/MWCNT are 20.1 and 16.2%, respectively. Electrocatalytic Activity and DMFC Performance of Pt/ Coral-C Catalyst. The electrocatalytic properties of these carbon-supported Pt catalysts in direct methanol oxidation were examined using a three-electrode chemical station. Figure 4 shows the typical voltammetric curves recorded from the various Pt/C electrodes in N2 saturated 0.5 M H2SO4 solution with (Figure 4b) and without (Figure 4a) 1.0 M methanol. The Pt/ Coral-C shows prominent peaks in the hydrogen adsorption/ desorption area.35 The calculated electrochemical active surface areas (Ae) are listed in Table 2 and, as can be seen, Pt/Coral-C



Figure 4. Cyclic voltammograms of the Pt catalysts with different carbon supports in N2 saturated (a) 0.50 M H2SO4 solution at 20 mV s-1 and (b) 0.50 M H2SO4/1.0 M CH3OH solution at 20 mV s-1.

Figure 5. Comparison of (a) polarization curves and (b) the power density curves of in-house MEA with different Pt/C catalysts as anode. (Cell temperature, 60 °C; electrolyte membrane, Nafion-117; anode, 2 mL min-1 2.0 M methanol; cathode, O2 flow rate, 100 mL min-1, atmospheric pressure.

TABLE 2: Pt-Loading Percent, Particle Size, and CV Results of Various Pt Catalysts Supported with Different Carbon Materials forward peak catalyst

Pt loading [wt%]

Pt Size [nm]

Ae [m2 g-1 Pt]

onset potential [mV)

Epa [mV vs SCE]

If [mA cm-2]

Pt/Coral-C Pt/LB Pt/MWCNT E-TEK Pt/C

17.0 20.1 16.2 20.0

2.3 5.6 3.9 3.2

102.5 28.0 63.9 65.2

391 440 386 402

596 591 594 597

9.6 1.4 7.0 6.1

with a value of 102.5 m2 g-1 Pt is higher than E-TEK Pt/C, Pt/LB, and Pt/MWCNT. Figure 4b shows representative cyclic voltammograms of direct methanol oxidation of the Pt/Coral-C catalyst. Experiments with an LB carbon-supported Pt catalyst, a MWCNT-supported Pt catalyst and E-TEK 20% Pt/C were carried out for comparison. The voltammetric features are in good agreement with those from literature.36-38 Pt/LB shows a poor activity due to the large Pt particle size and the poor electron conduction of LB carbon. As shown in Table 2, Pt/ Coral-C displays a 1.5 times higher current density of the forward anodic peak than the E-TEK Pt/C catalyst and 1.37 times higher than the MWCNT-supported Pt catalyst. Considering that the Pt/Coral-C and E-TEK Pt/C have almost identical Pt loadings, Pt/Coral-C appears to be significantly more active for methanol oxidation. This significant improvement in the activity toward methanol oxidation can be attributed to the high level of dispersion of Pt nanoparticles on Coral-C supports. Furthermore, the onset potential of Pt/Coral-C is at 391 mV vs SCE, which is more negative than that of E-TEK Pt/C. It is well-known that the activation potential depends on the effective

surface area and the type of supporting material. It should be noted that the Pt/MWCNT, having a similar electrochemically active area as E-TEK Pt/C, shows a much lower onset potential (386 mV vs SCE) as well. Thus the enhanced onset potential appears to result not only from the large effective area, but also from the increased electron conductivity of the Coral-C. The performance of the membrane electrode assemblies (MEAs) in DMFC with different catalysts as anode and E-TEK Pt/C, 20% Pt on carbon as cathode, was evaluated in a single cell DMFC. The anode was the aforementioned Pt/Coral-C catalyst, and Nafion-117 was sandwiched between the cathode and the anode by hot pressing. The performance was compared to the MEAs with Pt/MWCNT, Pt/LB, and E-TEK 20% Pt/C as anode, respectively. The Pt/Coral-C catalyst exhibited a better performance than that of the E-TEK Pt/C (Figure 5a) with about 70 mV higher in open circuit potential. Pt/Coral-C shows a limiting current density of 230 mA cm-2 (Table 3), which is 30 mA cm-2 higher than that of E-TEK Pt/C. Figure 5b displays the corresponding power density curves of these MEAs. Pt/ Coral-C shows a good power density of 21.0 mW cm-2, which

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TABLE 3: The Performance for Single DMFC at 60 °C by Using Different Catalysts As Anode

catalyst Pt/Coral-C Pt/LB Pt/MWCNT E-TEK Pt/C

OCP [V]

power density [mW cm-2]

limiting current density [mA cm-2]

0.64 0.66a 0.61 0.63 0.57

21.0 27.9a 14.0 18.0 16.2

230 297a 175 223 200

a MEA composed of Pt/Coral-C as cathode and E-TEK 20% Pt/ XC-72 as anode.

is about 1.3 times higher than that of E-TEK Pt/C. The DMFC performance of these catalysts succinctly answers the above results in methanol oxidation. Furthermore, we applied the Pt/ Coral-C catalyst in the cathode of MEA and E-TEK 20% Pt/C in the cathode side. Quite surprisingly, as shown in the curve Pt/Coral-C*, Figure 5a, a more pronounced polarization curve with a open circuit potential of 0.66 V, and a much higher limiting current density of 297 mA cm-2 were obtained. This leads to a significant increase in power density as shown in Figure 5b. The Pt/Coral-C in the cathode side produced a power density of 27.9 mW cm-2, which is 1.7 times higher than E-TEK Pt/C (see Table 3). From the origin for the enhanced kinetics of the in-house made cathode (Pt/Coral-C), it can be clearly seen that the improvement extends to small current densities where O2 mass transport is not limited. Interestingly, the Coral-C supported Pt catalyst seems to have a promising potential for acting as cathode materials in fuel cells. Further studies in the electrochemical activity and performance of these cathodes for fuel cells are in progress. 4. Conclusions The aim of this study was to investigate unique Coral-C by growing CNTs on LB carbon particles for fabricating a carbonsupported Pt catalyst. The Pt nanoparticles, which were prepared by a citrate reduction method, were coordinated, deposited, and stabilized on the Coral-C by a so-called “ligand exchange method” with difunctional polyoxypropylenediamines. Such coral-like nanostructures were realized through the in situ growth of CNTs after having deposited iron nanoparticles on TETA functionalized LB carbon particles. The Coral-C possessed good electronic conductivity, and the supported Pt catalyst showed excellent electrochemically active areas and good catalytic activity toward methanol oxidation, which could be attributed to the unique nanostructure of the catalyst: high conductivity of the defect-rich support and highly distributed Pt nanoparticles. It is envisaged that the excellent properties obtained from the in-house Pt/Coral-C may be of great importance for catalysts in proton exchange membrane fuel cells. Acknowledgment. The supports under the National Science Council (95-2221-E-006-227-MY3) and the National Nanoscience and Nanotechnology Program (96-2120-M-006-006) are gratefully acknowledged. Supporting Information Available: Figures S1-S3 (TEM images, XRD patterns, and TGA thermograms). This material is available free of charge via the Internet at http://pubs.acs.org.

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JP100397H

Nanostructured Coral-like Carbon as Pt Support for Fuel Cells

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