Facile In Situ Synthesis of Multiwall Carbon Nanotube Supported

May 27, 2010 - Wei Ye , Shulian He , Liping Ding , Yunjin Yao , Lei Wan , Shiding Miao , and Jinzhang Xu. The Journal of Physical Chemistry C 2013 117...
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J. Phys. Chem. C 2010, 114, 10843–10849

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Facile In Situ Synthesis of Multiwall Carbon Nanotube Supported Flowerlike Pt Nanostructures: An Efficient Electrocatalyst for Fuel Cell Application Sourov Ghosh and C. Retna Raj* Department of Chemistry, Indian Institute of Technology, Kharagpur, West Bengal, India ReceiVed: January 20, 2010; ReVised Manuscript ReceiVed: April 21, 2010

Multiwall carbon nanotube (MWCNT)-supported flowerlike Pt nanostructure with pronounced electrocatalytic activity in the reduction of oxygen and oxidation of methanol was synthesized by wet chemical hydrogen reduction route. The Pt nanostructures on the MWCNT were characterized by transmission electron microscopic, field emission scanning electron microscopic, X-ray diffraction (XRD), X-ray photoelectron spectroscopic, and electrochemical measurements. The Pt nanostructures on MWCNT have flowerlike morphology with an average size of 80 nm. XRD and selective area electron diffraction measurements show that the Pt nanoflowers are crystalline and have face centered cubic structure. The flowerlike Pt nanostructure shows excellent electrocatalytic activity toward oxygen reduction and methanol oxidation reactions. The electrocatalytic performance of the nanoelectrocatalyst was evaluated in terms of catalytic current density, stability, and reduction/oxidation potential. The particle loading strongly controls the electrocatalytic activity. High-catalytic current density was obtained at lower loading of the nanoelectrocatalyst. The kinetics of oxygen reduction reaction was analyzed using rotating ring-disk electrode system. The nanoelectrocatalyst favors the 4-electron pathway for the reduction of oxygen at favorable potential. The electrochemical impedance spectroscopic (EIS) measurement was used to evaluate the performance of the catalyst toward methanol oxidation. The EIS response of the electrode toward oxidation of methanol strongly depends on the electrode potential. Capacitive, inductive, and pseudoinductive behaviors, depending on the electrode potential, were observed. The charge transfer resistance decreases gradually while increasing the potential from 0.5 to 0.8 V. Negative impedance was obtained at the potential of 1.0 V. The electrocatalytic performance of flowerlike nanostructure is significantly higher than the conventional spherical nanoparticles. The shape and surface morphology of the nanoparticles have profound effect in their electrocatalytic activity. Introduction The increasing concern about the environmental consequence of the use of fossil fuel and significant raise in the oil price due to depletion of fossil fuel demands for economically viable and environment friendly alternative energy source for the daily needs. Fuel cells have emerged as one of the promising green energy source for portable electronic devices and automobiles.1 They offer the attributes necessary to meet the environmental demands of sustainable energy. The direct fuel cells based on the electrochemical oxidation of small organic molecules (SOM) such as formic acid, alcohols, and so forth are attracting enormous interest in recent years owing to their advantages over the other fuel cells. The market for the SOMs-based fuel cell is raising rapidly as it can be operated at room temperature and it does not require pretreatment of fuel. One of the major advantages with SOMs-based fuel cells is the high energy conversion efficiency.2,3 Moreover, unlike the conventional H2/ O2 fuel cells, the storage and handling of SOMs-based fuel cells are very easy. Direct methanol fuel cells (DMFCs) are very promising, as they are capable of delivering high-power density at low and intermediate temperatures. The basic function of DMFCs involves the cathodic reduction of oxygen and anodic oxidation of methanol.1-3 The precious metal Pt-based electrocatalysts are widely used for the anodic and cathodic reactions.4-11 However, * To whom correspondence should be addressed. E-mail: crraj@ chem.iitkgp.ernet.in. Fax: 91-3222-282252. Tel: 91-3222-283348.

the major concerns with Pt-based catalyst are (i) slow electron transfer kinetics for the oxygen reduction reaction (ORR) at cathode, and (ii) catalyst poisoning during the anode reaction due to the adsorption of CO-like intermediates.4,7,12 The high overpotential for the electrode reaction and poisoning of the catalyst significantly reduces the efficiency of fuel cell. Although the standard reversible potential for the ORR is 1.23 V (NHE), significant overpotential loss due to the sluggish electron transfer kinetics and adsorption of supporting electrolytes anions have been observed with the fuel cell cathodes.1 The high cost of Pt-based catalyst is a barrier in the large scale commercialization of DMFCs. For the successful commercialization of such devices at low cost, the amount of Pt catalyst should be reduced significantly without sacrificing the efficiency of the device. The performance of DMFCs can be improved by (i) using nanosized Pt-based electrocatalysts and (ii) facilitating the easy accesses of the catalyst to the reactants. The Pt-based nanoparticles of different shape and size have been chemically and electrochemically synthesized by several groups in the past.5-11,13-20 The electrocatalytic performance of Pt particles toward ORR and oxidation of methanol depends on several factors like size, shape, and surface structure of the particles, interparticle distance, particle loading, nature of supporting electrolyte anions, and so forth.21-28 It is generally accepted that the electrocatalysts with stepped surface of low coordination number have high catalytic activity;5,29 the Pt nanostructures with sharp edges and corners are known to have high catalytic activity.5 The studies on the Pt single crystal electrode suggests that the (110) electrode

10.1021/jp100551e  2010 American Chemical Society Published on Web 05/27/2010

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has high activity toward ORR.4 The catalyst can be made easily accessible for the reactant by uniformly distributing them on a suitable support. High surface area carbon like carbon black is traditionally used as a catalyst support. However, the catalysts loaded are trapped inside the pores of carbon and are almost not accessible for the substrate.30 Nanoscale carbon materials are of great interest owing to their unique properties.31,32 The high mechanical strength, electronic conductivity, and large surface area of carbon nanotubes (CNTs) make them a suitable candidate as catalyst support for fuel cell applications.32-34 The CNT-supported metal particles exhibit high catalytic activity. It has been demonstrated that the CNT-supported catalyst shows significant enhancement in the performance of fuel cells and charge storage devices.33-37 It is well understood that the catalytic properties of the metal particle depends on their shape and surface structure and the catalyst support.21-26,34 Various approaches have been employed to synthesis CNTsupported metal nanoparticle of different shape and size.33,34,38-41 The metal nanoparticles are loaded on the surface of CNT by physical adsorption, chemical reduction of impregnated metal precursors, chemical vapor deposition, electrochemical deposition, and so forth.35-42 For instance, Quinn et al. electrodeposited noble metal nanoparticles on CNTs under potential control.38 Girishkumar et al. electrochemically deposited Pt nanoparticles on single wall carbon nanotubes for fuel cell applications.33 Dong’s group extensively synthesized CNT-supported metal nanoparticles for various applications.39,40 Hrapovic et al. deposited nanoscale Pt particles on single wall carbon nanotube for biosensing applications.41 Karousis et al. chemically decorated CNTs with Pd nanoparticle to investigate its catalytic activity in organic reactions.43 Synthesis of metal nanoparticles of high electrocatalytic activity on the walls of CNTs for various electrochemical reactions is still a challenge for the synthetic nanochemist. In the present investigation, we describe the shape controlled in situ chemical synthesis of electrocatalytically active Pt nanostructures on the walls of MWCNTs for the electrocatalytic reduction of oxygen and oxidation of methanol. The electrocatalytic performance of the Pt nanostructures is compared with the commercial Pt (10%)-loaded activated carbon. Rotating ring-disk electrode (RRDE) and electrochemical impedance measurements were performed to evaluate the electrocatalytic activity. Experimental Section Material. H2PtCl6, Nafion, and MWCNTs (g95% purity) were obtained from Sigma-Aldrich. MWCNTs were used after purification according to the standard procedures. All other chemicals used in this investigation were of analytical grade. All the solutions were prepared with Milli-Q (18 MΩ cm) water. The activated carbon supported Pt (10 wt %) was obtained from Sigma-Aldrich. Carbon-coated copper grids for transmission electron microscopic measurement were obtained from Pelco International. Instrumentation. Transmission electron microscopy (TEM) images were obtained from JEOL JEM 2010 transmission electron microscope operating at 200 kV. JEOL JEM 6700F field-emission scanning electron microscope (FESEM) was used to acquire surface morphology and particle size. The X-ray diffraction (XRD) analysis was accomplished with the Phillips X’part PRO X-ray diffraction unit using Ni-filtered Cu-KR (λ ) 1.54 Å) radiation. X-ray photoelectron spectroscopy (XPS) measurements were performed with Specs (German) using the energy source Mg (KR, hν ) 1253.6 eV). Electrochemical measurements were performed in a two compartment three-

Ghosh and Raj electrode cell with a glassy carbon (0.07 cm2) working, a Pt wire auxiliary, and Ag/AgCl (3 M KCl) reference electrodes. Cyclic voltammograms were recorded using a computercontrolled CHI842B (Austin, TX) electrochemical analyzer attached to a picoamp booster-Faraday cage. Chronoamperometry and impedance analysis were performed with Autolab potentiostat-galvanostat (302N) using computer-controlled NOVA 1.5 software. The kinetics of ORR was analyzed using rotating GC disk Pt ring electrode system (disk area, 0.247, and ring area, 0.169 cm2; collection efficiency, 37%) from PINE instruments (U.S.A.). The potentials in the impedance plots are referred against Ag/AgCl. All other potentials are referred against SHE. Synthesis of MWCNT-Supported Pt Nanoflowers. All the glassware used in the synthetic procedure was cleaned with freshly prepared aqua regia and rinsed thoroughly with Milli-Q water. In a typical synthesis, 5 mg of purified MWCNTs were mixed with 5 mL of aqueous solution of H2PtCl6.6H2O (6 mM) and stirred in a two neck round-bottom flask for the adsorption of metal precursor on the walls of MWCNTs. Then 85 µL of 2% aqueous nafion solution was added to the mixture. The stirring was continued and the reaction vessel was purged with pure argon gas for 15 min to remove the dissolved oxygen. Then pure hydrogen gas was bubbled through the mixture for 10 min for the reduction of surface bound metal precursor. Resulting solution was then filtered off and dried under argon atmosphere. The colorless filtrate was subjected to UV-visible spectral measurement and it did not show any characteristic spectral features for H2PtCl6, confirming the complete adsorption and reduction of the precursor. Modification of Electrode with Nanoparticle-Decorated MWCNTs. The GC disk electrode and GC-Pt RRDEs were polished well with fine emery paper and alumina (0.05 µm) slurry and sonicated in Millipore water for 10-15 min to remove the physically adsorbed impurities. These electrodes were washed repeatedly with copious amount of Millipore water and dried under argon atmosphere. For the electrode modification, the Pt nanoparticle-decorated MWCNT (0.5 mg/mL) was mixed with 1% alcoholic nafion solution and stirred vigorously for 15 min. The cleaned electrodes were modified with 10 µL of the nanocomposite suspension and dried in a decicator for 30 min at room temperature. These nanoparticle-modified electrodes were subjected to electrochemical experiments. Electrodes with different particle loading were prepared by controlling the amount of nanoparticle decorated MWCNT on the electrode surface (0.5, 1 and 2 mg/mL). All the measurements have been performed at least three times. Results and Discussion Surface Characterization. Figure 1 displays the FESEM and TEM images obtained for the MWCNT-supported Pt nanoparticles. The FESEM image shows that the nanoparticles have an average size distribution of 80 nm. The particles have flowerand budlike morphology and are well dispersed over MWCNT. Further insight on the morphology of the Pt nanoflowers (nPtFs) was obtained by TEM measurements. TEM measurement confirms the existence of nPtFs on the walls of MWCNT; the size of nPtFs is almost the same as obtained with the FESEM measurement. The HRTEM image of selected nPtFs shows the fringe spacing of 0.23 nm, which corresponds to the interplanar distance of (111) plane. The selected area electron diffraction (SAED) pattern obtained for nPtF are indexed to (111), (200), and (220) diffractions, indicating that the nanoparticles have face-centered cubic structure. The XRD pattern shows three

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Figure 3. Cyclic voltammogram illustrating the electrocatalytic activity of nPtFs toward ORR in 0.5 M H2SO4. Scan rate: 25 mV/s. Inset shows the voltammogram obtained for ORR on the commercial catalystmodified electrode.

Figure 1. FESEM (a) and TEM (b,c) images and (d) SAED pattern obtained for the Pt nanoparticle-decorated MWCNTs. The scale bar in (a) and (b) is 100 nm.

positive than the bulk Pt electrode; (ii) the position of cathodic peak associated with the oxide reduction is less positive than that of bulk Pt electrode. This can be ascribed to the higher binding energy of the electrons in nPtFs. The electrochemically accessible surface area (ECSA) of nPtF was obtained by integrating the charge associated with the hydrogen adsorption/ desorption using the following relation29

ECSA )

Figure 2. X-ray photoelectron spectrum obtained for nPtFs.

peaks at 2θ ) 39.8, 46.5, and 67.8° (Figure 1S, Supporting Information), corresponding to the (111), (200), (220) reflections, respectively. The XRD profile is consistent with the SAED pattern, and it confirms the crystalline nature of nPtFs. Energy dispersive spectral analysis further confirms the existence of zerovalent Pt on the walls of MWCNTs (Figure 2S, Supporting Information). The chemical composition of nPtFs was further examined with XPS measurements (Figure 2); two peaks corresponding to 4f7/2 and 4f5/2 were obtained at 71.1 and 74.4 eV for 4f7/2 and 4f5/2 electrons, respectively. The peak position confirms the existence of metallic Pt. The binding energies are slightly higher (∼0.3 eV) than those of the bulk Pt presumably due to the particle size effect. Electrochemical Characterization. The cyclic voltammogram obtained for nPtFs modified electrode in 0.5 M H2SO4 under argon atmosphere exhibits characteristic voltammetric profile associated with the oxide formation and reduction and hydrogen adsorption/desorption peaks (Figure 3S, Supporting Information). Careful analysis of the voltammetric profile reveals that (i) onset potential for the peak associated with the oxide formation on the nanoparticles modified electrode is more

QH Qref

(1)

where QH is the double layer-corrected charge obtained for hydrogen adsorption/desorption (mC/cm2) and Qref ) 0.21 mC/ cm2. The ECSA depends on the amount of CNT-supported nanoparticles loaded on the electrode surface; it increases with the increase of loading. However, we found that the specific ECSA (area per unit weight of Pt) is significantly large at lower loading of the nanoparticle (Figure 4S, Supporting Information). Electrocatalytic Reduction of Oxygen. The electrocatalytic activity of nPtF decorated MWCNT in the ORR was evaluated by cyclic voltammetric and RRDE measurements. Figure 3 is the cyclic voltammogram obtained for ORR in 0.5 M H2SO4 with the nanoparticle-based and 10% Pt on activated carbon (inset)-based electrodes. Well-defined voltammetric peak corresponding to the four-electron reduction of oxygen to water was obtained at the potential of 0.72 V. The half-wave potential for ORR on the nPtF-based electrode is 0.76 V, which is 230 mV more positive than that of the commercial catalyst (10% Pt-loaded activated carbon). The commercial catalyst-modified electrode shows sluggish voltammetric response for ORR with a half-wave potential of 0.53 V. The positive shift in the peak potential on the nanoparticle-based electrode indicates that the flowerlike nanoparticles efficiently catalyze the reduction of oxygen. The catalyst with more positive half-wave potential is considered to have enhanced electrocatalytic activity. More importantly, the onset potential for ORR on the nanoparticlebased electrode is significantly more positive (>150 mV) than the commercial catalyst, strongly supporting the high catalytic activity of the nanoparticles. Particle Loading Dependent Electrocatalytic Performance. The electrocatalytic response of the electrode depends on the loading of nanoparticles on the electrode surface. Although the voltammetric peak current gradually increases with increasing the loading, the surface area normalized current is significantly

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TABLE 1: Electrocatalytic Performance of nPtFs in the Oxygen Reduction and Methanol Oxidation Reactions oxygen reductionb

methanol oxidationb

loading of Pt (µg/cm2)

surface area (cm2)

particle coveragea

J (µA/cm )

Ep (mV)

Ip (µA)

J (µA/cm )

Ep (mV)

If/Ir

56 28 14

1.982 1.527 1.281

28.3 21.8 18.2

128.6 132.7 140.5

727 721 726

255 202 180

378.7 439.7 551

881 895 878

1.587 1.387 1.363

2

2

a Particle coverage is the ratio of electrochemically accessible surface area to the geometrical surface area. b The mass normalized catalytic current is significantly high at lower loading of the catalyst; 10 µA/µg for ORR and 40 µA/µg for methanol oxidation at the loading of 14 µg/ cm2.

high at lower loading (Table 1 and Figure 5S, Supporting Information). It should be highlighted here that 14 µg/cm2 loading of nPtF yields the catalytic current density of 140.5 µA/cm2, which is higher than that obtained at 56 µg/cm2 (Table 1). It is worth noting that the mass-normalized catalytic current for ORR at lower loading (14 µg/cm2) is 4.3 times higher than that at higher loading (56 µg/cm2). The catalytic current density is remarkably high (∼10 times) with respect to the commercial Pt catalyst. Very recently, Wang et al. reported the influence of particle loading on the electrocatalytic activity of CNT-supported monodispersed spherical Pt nanoparticles in the reduction of oxygen and oxidation of methanol.44 It has been shown that the catalytic current density for these electrochemical reactions increases with increase in the particle loading and attained the saturation at higher loading. However, in our case high current density for the ORR was observed at lower loading of nPtF. This could be due to the shape and surface structure of the nanoparticle and nanoparticle coverage on the electrode surface. It is generally accepted that the anisotropic nanostructures with stepped surface of low coordination number exhibits high catalytic activity with respect to the spherical nanostructures.5,29,45 Formo et al. recently reported that the Pt nanowires have significantly high electrocatalytic activity in comparison with conventional Pt nanoparticles.45 Our ongoing preliminary investigation on the CNT-supported spherical Pt nanoparticle shows that the catalytic current density increases while increasing the particle loading (Figure 6S, Supporting Information) and is in agreement with the reported literature.44 The profound electrocatalytic activity of our flowerlike nanoparticles at lower loading can be ascribed to their surface morphology and shape. The comparison of the electrocatalytic performance of nPtFs with our earlier report17 on the spherical nanoparticles shows that nPtFs have excellent electrocatalytic activity. Rotating Ring-Disk Voltammetric Studies. RRDE experiments were performed to further evaluate the catalytic effect and for the better realization of ORR kinetics. Because we observed high catalytic current density at lower loading, all the RRDE experiments were performed at a fixed loading of nPtFs 14 µg/cm2. The polarization curves obtained for ORR at different rotation rates are displayed in Figure 4. The disk potential was scanned from 1.1 to 0.35 V while holding the ring potential at 0.85 V. The H2O2 produced, if any, during the ORR can oxidize at the ring. The onset and half-wave potential for the ORR was measured to be 0.93 and 0.76 V, respectively. The onset potential for ORR on commercial catalyst is 230 mV less positive than that of the nPtF-based electrode (Figure 7S, Supporting Information), revealing that the nanoparticle efficiently catalyze the reduction of oxygen. The number of electrons (n) involved in ORR was calculated from the ratio of ring and disk currents using the relation46

n ) 4 - 2(iR /iDN)

(2)

Figure 4. Rotation dependent polarization curves obtained for ORR with nPtF-modified RRDE in oxygen saturated 0.5 M H2SO4. Scan rate: 2 mV/s. The potential of ring was held at 0.85 V.

where iR and iD are the ring and disk currents, respectively, and N is the collection efficiency (0.37). The number of electrons involved was calculated to be 3.7, which is very close to the expected value of 4 for the reduction of oxygen to water. The limiting current for the reduction of oxygen gradually increases while increasing the speed of rotation. The kinetics of ORR was analyzed using Kouckety-Levich equation (eqs 3 and 4)

1 1 1 + ) J Jd Jk

(3)

1 1 1 + ) J nFkC0 0.62nFC0D2/3ν-1/6ω1/2

(4)

where Jd and Jk are the diffusion limited and kinetic current density, respectively; ω is the angular velocity, n is the number of electrons transferred, F is Faraday’s constant, C is the concentration of dissolved oxygen, D is the diffusion coefficient of oxygen, ν is the kinematic viscosity of solution, and k is the apparent electron transfer rate constant. The plot of 1/J versus 1/ω1/2 is a straight line with intercept of 1/Jk and slope of 1/Jd. The kinetic current density at the potential of 0.6 V was 0.653 mA/cm2. The Tafel plot was made by plotting the kinetic current density obtained at different potential (Figure 8S, Supporting Information). Two distinct slopes of 163 and 123 mV/dec were obtained at the low and high current density region, respectively. The value at low current density region is higher than the expected value for ORR on Pt-based electrode. Such high slopes have been reported by Crooks et al. for ORR.47 They have obtained the Tafel slopes of 157 and 106 mV/decade at the high and low current density regions, respectively. Wang et al. explored the deviation of Tafel slope from the intrinsic values and is accounted for the site blocking and electronic effects.48 The actual reason for the high slope in our case is not known

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Figure 5. Cyclic voltammogram illustrating the electrocatalytic performance of nPtFs toward oxidation of methanol (0.1 M) in 0.5 M H2SO4. Scan rate: 25 mV/s. Inset shows the voltammetric response of electrode modified with commercial catalyst toward methanol oxidation.

Figure 6. Chronoamperometric curves obtained for the oxidation of methanol (0.1 M) on (a) nPtFs and (b) commercial catalyst-modified electrodes. The potential of the electrode was held at 0.85 V and the current was measured for 2 h in 0.5 M H2SO4.

at the present stage. However, presently we are exploring the possible reason for the high value of Tafel slopes. Electrocatalytic Oxidation of Methanol. The electrocatalytic performance of nPtFs toward methanol oxidation was evaluated by cyclic voltammetric, chronoampreometric, and electrochemical impedance measurements. Figure 5 illustrates the electrocatalytic activity of nPtFs in the oxidation of methanol in acidic solution. The anodic peak for the oxidation of methanol is observed at ∼0.9 V. The onset potential for the oxidation of methanol (∼0.42 V) on the nPtF-based electrode is significantly less positive (250 mV) than that of the commercial catalyst, suggesting the high catalytic activity of the nanoparticle. The anodic peak in the forward scan is ascribed to the oxidation of methanol and the peak observed during reverse scan is due to the adsorbed intermediate produced during the forward sweep.49 The catalytic current density obtained with the nPtF-based electrode is ∼72 times higher than that of the commercial catalyst. The ratio of the anodic peak current obtained in the forward and reverse scan can be considered as the measure of the tolerance of electrocatalyst toward carbonaceous species. The ratio obtained in the present case (Table 1) is high with respect to the bulk Pt electrode and Pt nanoparticle on Vulcan XC-72 carbon.50 The mass-normalized catalytic current for the oxidation of methanol is 42 µA/µg. The high catalytic activity can be ascribed to the shape and surface structures of nPtFs. It has been demonstrated recently that Pt nanowires have high electrocatalytic activity for methanol oxidation with respect to the conventional Pt catalyst.10,45 It is known that surface structure has a strong influence on the methanol oxidation reaction and (110) surface has high electrocatalytic activity.51 Moreover, the catalytic current density obtained for the oxidation of methanol with nPtF-based electrode is significantly higher than our earlier studies on the spherical Pt nanoparticle immobilized on sol-gel nework,17 indicating that the shape and surface structure of the nanoparticle have control over the electrocatalytic activity. As in the case of oxygen reduction, the particle loading significantly influences the electrocatalytic performance. The catalytic current density obtained at 14 µg/cm2 loading of the nanoparticles is higher (1.25 to 1.45 times) than those of the other loading, highlighting that low amount of nPtF is sufficient enough to obtain high current density (Table 1). The electrocatalytic activity of the nanoparticles was further examined by recording the steady-state current at a fixed potential. The potential of the electrode was held at 0.95 V and the current flowing through the electrode was monitored for a

period of 7200 s (Figure 6). The high current observed in the initial stage originates from the double layer charging. The initial current decayed sharply and attained the steady state. Very stable steady-state response was obtained with the nPtF electrodes. The slower decay of steady-state current indicates that the nPtFs have high tolerance toward poisoning by CO-like intermediates. To compare the electrocatalytic activity of the nanoparticles, steady-state experiment was performed with the commercial catalyst. As can be seen from Figure 6b, a sudden decrease in the current on the Pt-loaded activated carbon was observed. The current density at the end of 7200 s on the nPtF electrode is ∼29 times higher than that obtained with the commercial catalyst. The steady state current on the nPtF electrode is highly stable, suggesting that the electrode surface does not undergo deactivation during the oxidation process. Electrochemical Impedance Study. Electrochemical impedance measurements have been performed to further evaluate the electron transfer kinetics of methanol oxidation on the nanostructured electrocatalyst. The impedance behavior of the electrode is strongly dependent on the electrode potential. Figure 7 is the Nyquist complex impedance plot obtained for the oxidation of methanol at different potentials. At the potentials of 0.3 and 0.4 V, the Nyquist plot does not show arc, supporting the capacitive behavior. The double layer charging and adsorption of methanol on the electrode surface influence the impedance of the electrode at these potentials. A large arc is observed at the potential of 0.5 V, indicating the presence of resistive component. The electron transfer kinetics for the oxidation of methanol is rather slow due to the strong adsorption of poisoning intermediate CO. Pseudoinductive behavior was observed at the potentials of 0.6, 0.7, and 0.8 V. The impedance arc extends to the fourth quadrant at these potentials. The diameter of the arc in the first quadrant significantly decreased while switching the potential from 0.6 to 0.8 V, suggesting a fast electron transfer. The small arc in the fourth quadrant at low frequency is welldefined at 0.8 V. The pseudoinductive behavior is ascribed to the oxidative removal of adsorbed CO on the nanoparticle. Interestingly, at the potential of 1.0 V a sudden change in the impedance pattern was observed (Figure 7g). The impedance arc appears in the second quadrant at this potential. The negative Faradaic impedance suggests the existence of an inductive component.52 Such negative impedance has been observed recently by Lee et al. and Chen et al. for the oxidation of methanol and formic acid with Pt-based electrocatalyst.10,53 The negative impedance can be ascribed to the formation of chemisorbed hydroxyl species in the potential range; the

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Figure 7. Potential dependent 3D Nyquist complex impedance plot (A and B) obtained for the oxidation of methanol (0.1 M) on nPtF modified electrode in 0.5 M H2SO4. Electrode potential (a) 0.3, (b) 0.4, (c) 0.5, (d) 0.6, (e) 0.7, (f) 0.8, (g) 1.0, (h) 1.1 V (SHE). The equivalent circuit X was used to fit the curve c, g, and h and Y was used to fit the curve d, e, and f, shown in Figure 7A,B. Circuit components: Rct, charge transfer resistance; Rs, solution resistance; CPE, constant phase element; L, inductance; and Ro, resistance due to adsorbed species.

TABLE 2: Electrochemical Impedance Fitting Parameters Obtained for the Impedance Data Shown in Figure 7a

a

E (V)

Rs (Ω)

Rct (kΩ)

Q/Y0 (µMho)

0.5b 0.6c 0.7c 0.8c 1.0b 1.1b

55.5 (4.902) 52.3 (2.132) 50.5 (0.737) 50.8 (0.965) 50.7 (1.189) 51.1 (0.822)

22.8 (4.05) 3.7 (3.16) 2.4 (2.42) 2.3 (3.40) -7.3 (2.11) 3.2 (1.53)

226 (1.42) 220 (1.31) 210 (2.35) 212 (3.35) 259 (1.73) 272 (1.83)

R0 (Ω) 2.46 (2.58) 2.85 (4.19) 3.45 (6.26)

n 0.8 (1.32) 0.81 (0.67) 0.8 (0.71) 0.8 (1.00) 0.78 (0.69) 0.8 (0.64)

L (kH) 6 (7.53) 10 (6.80) 7 (8.20)

Values given in parenthesis are the percentage of error in the fitting. b Equivalent circuit “X”. c Equivalent circuit “Y”.

hydroxyl radical facilitates the oxidation of adsorbed CO. At more positive potential (1.1 V), the impedance arc appears again in the first quadrant (Figure 7h). The diameter of the arc is slightly higher than those at the potentials 0.7 to 1.0 V. The lower charge transfer resistance at the potentials 0.7 to 1.0 V indicates faster electron transfer kinetics. Thus the impedance analysis for the oxidation of methanol reveals that the property of the nPtF electrode toward oxidation of methanol strongly dependent on the potential and the impedance changes from capacitive to resistive and to pseudoinductive and then to inductive and resistive behavior. The equivalent circuits54 shown in Figure 7 (X and Y) were used to fit the impedance data. The charge transfer resistance varies with increase in the electrode potential (Table 2). It decreases up to the oxidation peak potential of methanol and further increase in the electrode potential leads to slight increase in the charge transfer resistance. It is worth pointing out here that the impedance behavior for the oxidation of methanol on CNT-supported spherical nanoparticle-based electrode is largely different from that of the nPtFbased electrode. The spherical nanoparticle-based electrode does not show any inductive or pseudoinductive behavior (Figure 10S, Supporting Information) for the oxidation of methanol; this electrode shows capacitive and resistive behavior toward

oxidation of methanol. It strongly suggests that the shape and surface structure play key role in controlling the electrocatalytic activity of the nanoparticles. The impedance behavior was further analyzed with Bode plot (Figure 11S, Supporting Information). The change of impedance behavior from capacitive to resistive is clearly seen in this plot. The phase angle remains almost constant at the potential of 0.3 and 0.4 V, indicating that the capacitance dominates the electrode reaction. Negative phase angle appears at the potentials of 0.6, 0.7, and 0.8 V. This further indicates the transition of electron transfer kinetics behavior from resistive to pseudoinductive behavior. The Bode plot shows zero phase angles at these potentials. The decrease in the phase angle with potential supports the fast electron transfer kinetics. All these results are in excellent agreement with those of the Nyquist plot and cyclic voltammograms. Conclusion We have demonstrated a facile synthesis of carbon nanotubesupported electrocatalytically highly active flowerlike Pt nanoparticle and the electrocatalytic performance toward fuel cell reaction. The electrocatalytic properties of Pt nanostructures

Synthesis of MWCNT Supported Flowerlike Pt Nanostructures were investigated by voltammetry, RRDE, amperometry, and impedance measurements. The electrocatalytic performance is strongly dependent on the particle loading and shape of the nanoparticles. The flowerlike nanostructures have high electrocatalytic performance with respect to the spherical nanoparticles. Interestingly, the catalytic current density for the reduction of oxygen and oxidation of methanol is significantly high at lower loading of nanoparticles. The nanocatalyst favors the four electron pathway for the reduction of oxygen to water. Potential dependent impedance measurements indicate the formation of different reaction intermediates during the oxidation process. The transition of capacitive to resistive at the lower potentials and resistive to pseudoinductive and then to inductive behaviors at high potential have been observed. The electron transfer kinetics for the oxidation of methanol is fast at the potential of 0.7 to 1.0 V. Carbon nanotube-supported flowerlike nanostructure shows high catalytic activity with respect to the commercial catalyst. Shape and surface structure controls the electrocatalytic performance. Acknowledgment. This work was financially supported by defense research and development organization, New Delhi. We thank Dr. R. K. Sahu (National Metallurgical Laboratory, Jamshedpur, India) for XPS measurements. S.G. is a recipient of CSIR fellowship. Supporting Information Available: XRD and EDS spectral pattern, cyclic voltammograms obtained in H2SO4 for surface area measurement, plot for the loading-dependent specific area, comparison of polarization curve for ORR with commercial catalyst, plot of Tafel slope obtained, loading-dependent catalytic activity for methanol oxidation, and oxygen-reduction reactions, Bode plot obtained for methanol oxidation and Nyquist plot for the oxidation of methanol on spherical nanoparticle-based electrode. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kordesch, K. V.; Simader, G. R. Chem. ReV. 1995, 95, 191. (2) Carrette, L.; Friedrch, K. A.; Stimming, U. Fuel Cell 2001, 1, 5. (3) Wang, C. Y. Chem. ReV. 2004, 104, 4727. (4) Markovic, N. M.; Gasteiger, H. A.; Ross, P. N., Jr. J. Phys. Chem. 1995, 99, 3411. (5) Lim, B.; Jiang, M.; Camargo, P. H. C.; Cho, E. C.; Tao, J.; Lu, X.; Zhu, Y.; Xia, Y. Science 2009, 324, 1302. (6) Shao, M. H.; Huang, T.; Liu, P.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Adzic, R. R. Langmuir 2006, 22, 10409. (7) Kinoshita, K. Electrochemical oxygen technology; John Wiley & Sons, Inc.; New York, 1992; p 307. (8) Chang, G.; Oyama, M.; Hirao, K. J. Phys. Chem. B 2006, 110, 1860. (9) Lou, Y.; Maye, M. M.; Han, L.; Luo, J.; Zhong, C. J. Chem. Commun 2001, 473. (10) Lee, E. P.; Peng, Z.; Chen, W.; Chen, S.; Yang, H.; Xia, Y. ACS Nano 2008, 2, 2167. (11) Zhang, J.; Mo, Y.; Vukmirovic, M. B.; Klie, R.; Sasaki, K.; Adzic, R. R. J. Phys. Chem. B 2004, 108, 10955. (12) Jarvi, T. D.; Stuve, E. M. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH New York, 1998; p 75. (13) Wang, C.; Daimon, H.; Sun, S. Nano Lett. 2009, 9, 1493.

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