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Single-Wall Carbon Nanotubes Supported Platinum Nanoparticles with Improved Electrocatalytic Activity for Oxygen Reduction Reaction Anusorn Kongkanand,†,‡ Susumu Kuwabata,*,‡ G. Girishkumar,† and Prashant Kamat*,† Radiation Laboratory, Departments of Chemistry & Biochemistry and Chemical & Biomolecular Engineering, UniVersity of Notre Dame, Notre Dame, Indiana 46556-0579, and Department of Applied Chemistry, Graduate School of Engineering, Osaka UniVersity, Suita, Osaka 565-0871, Japan ReceiVed October 12, 2005. In Final Form: December 7, 2005 Significant enhancement in the electrocatalytic activity of Pt particles toward oxygen reduction reaction (ORR) has been achieved by depositing them on a single wall carbon nanotubes (SWCNT) support. Compared to a commercial Pt/carbon black catalyst, Pt/SWCNT films cast on a rotating disk electrode exhibit a lower onset potential and a higher electron-transfer rate constant for oxygen reduction. Improved stability of the SWCNT support is also confirmed from the minimal change in the oxygen reduction current during repeated cycling over a period of 36 h. These studies open up ways to utilize SWCNT/Pt electrocatalyst as a cathode in the proton-exchange-membrane-based hydrogen and methanol fuel cells.
Introduction The performance of a cathode is a major limiting factor in optimizing the power output of hydrogen and direct methanol based fuel cells.1,2 The electrocatalyst dispersed on a carbon support is tailored to induce four-electron reduction of oxygen to water by utilizing the protons that permeate from the anode compartment. Although the anode compartment provides optimum performance, the improvement in the cathode compartment is being sought continuously. A low reduction oxygen rate at the cathode is one of the major challenges to overcome.3 Recent research focus has been diverted toward the development of electrocatalysts that make use of bimetallic alloys.4 Of particular interest is the Pt-Co alloys that have shown an enhanced rate constant for O2 reduction.5,6 Recent efforts in our laboratory7-9 and elsewhere10-18 have focused on the utilization of carbon nanostructures as support * Corresponding authors. E-mail:
[email protected] (S.K.). † University of Notre Dame. ‡ Osaka University.
[email protected] (P.K.);
(1) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal. B-EnViron. 2005, 56, 9. (2) Costamagna, P.; Srinivasan, S. J. Power Sources 2001, 102, 242. (3) Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. J. Phys. Chem. B 2004, 108, 17886. (4) Shukla, A. K.; Raman, R. K. Annu. ReV. Mater. Res. 2003, 33, 155. (5) Paulus, U. A.; Wokaun, A.; Scherer, G. G.; Schmidt, T. J.; Stamenkovic, V.; Radmilovic, V.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. B 2002, 106, 4181. (6) Yu, P.; Pemberton, M.; Plasse, P. J. Power Sources 2005, 144, 11. (7) Girishkumar, G.; Vinodgopal, K.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 19960. (8) Girishkumar, G.; Rettker, M.; Underhile, R.; Binz, D.; Vinodgopal, K.; McGinn, P.; Kamat, P. Langmuir 2005, 21, 8487. (9) Drew, K.; Girishkumar, G.; Vinodgopal, K.; Kamat, P. V. J. Phys. Chem. B 2005, 109, 11851. (10) Che, G.; Lakshmi, B. B.; Martin, C. R.; Fisher, E. R. Langmuir 1999, 15, 750. (11) Steigerwalt, E. S.; Deluga, G. A.; Lukehart, C. M. J. Phys. Chem. B 2002, 106, 760. (12) Maynard, H. L.; Meyers, J. P. J. Vac. Sci. Technol. B 2002, 20, 1287. (13) Rajesh, B.; Thampi, K. R.; Bonard, J. M.; Viswanathan, B. Bull. Mater. Sci. 2000, 23, 341. (14) Choi, W.-B. C., J.-U.; Pak, C.-H.; Chang, H. Method of fabrication of carbon nanotubes for fuel cells. U. S. Pat. Appl. Publ. S. Korea 2004, 1, A1. (15) Kim, C.; Kim, Y. J.; Kim, Y. A.; Yanagisawa, T.; Park, K. C.; Endo, M. J. Appl. Phys. 2004, 96, 5903. (16) Sun, X.; Li, R.; Villers, D.; Dodelet, J. P.; Desilets, S. Chem. Phys. Lett. 2003, 379, 99.
Scheme 1. Reduction of O2 at the Pt/SWCNT Electrocatalyst
material to disperse the electrocatalysts. The preparation of single wall carbon nanotube (SWCNT)-based PEM assembly and its performance in the hydrogen fuel cell has been investigated. Compared to commercial carbon black/Pt catalyst, SWCNT/ Pt-catalyst-based hydrogen fuel cells show improved performance at higher anodic pressures.8 Although few preliminary studies demonstrate the use of carbon nanotubes as the support material for developing an O2 reduction electrocatalyst,19 detailed characterization is lacking. Furthermore, the performance comparison with a commercial electrocatalyst can also provide the basis for elucidating the role of carbon support in the reduction of oxygen. Electrocatalytic experiments that characterize oxygen reduction reaction (ORR) at the SWCNT/Pt electrode (Scheme 1) and highlight the beneficial aspect of SWCNT support are presented here. Experimental Section Preparation of SWCNT-Supported Pt. SWCNTs were obtained from SES Research and used as is for the present study. A total of 5 mg of a suspension of Pt/SWCNT was achieved by the following procedure: 3 mg of SWCNT was mixed with 0.12 mL of 5 wt % (17) McElrath, K. O. S., K. A.; Bahr, J. L.; Wainerdi, T. J.; Karohl, D. A.; Colbert, D. T.; Miller, M. A.; Oviatt, H. W.; Cline, E. D. Fuel cell electrode comprising carbon nanotubes. In United States Patent Application, 2004; pp A1. (18) Wang, C.; Waje, M.; Wang, X.; Tang, J. M.; Haddon, R. C.; Yan, Y. S. Nano Lett. 2004, 4, 345. (19) Cui, H.-F.; Ye, J.-S.; Zhang, W.-D.; Wang, J.; Sheu, F.-S. J. Electroanal. Chem. 2005, 577, 295.
10.1021/la052753a CCC: $33.50 © 2006 American Chemical Society Published on Web 01/31/2006
SWCNT Supported Platinum Nanoparticles Nafion (Aldrich) in 3 mL of 2-propanol. The mixture was sonicated for 1 h in cold water. Then the suspension was vigorously stirred, while 0.88 mL of 2 mg of Pt black (HiSPEC 1000, from JohnsonMatthey) in water was added to the suspension dropwise at a rate of 0.25 mL/min. An additional 1 h sonication was employed to obtain a suspension of 40 wt % Pt supported on SWCNT. A Pt particle size of 2-5 nm was claimed by the manufacturer. For comparison, a commercially available catalyst of 40 wt % Pt supported on Vulcan XC-72 from E-TEK was used. A total of 5 mg of the Pt/C catalyst was mixed with 0.12 mL of 5 wt % Nafion in 3.88 mL of 2-propanol to obtain Pt/C suspension. Electrode Preparation. Electrode preparation was similar to that described by Paulus et al.20 A glassy carbon rotating disk electrode (0.283 cm2, from Pine Instruments) was used as a substrate for the catalyst and was polished with diamond paste and alumina down to 0.05 µm to obtain a mirror finish. A total of 8 µL of Pt/SWCNT or Pt/C suspension was pipetted onto the substrate and was dried at room temperature for 15 min. This leads to a Pt loading of 14 µgPt/cm2. Then, 5 µL of diluted Nafion (0.05 wt. % Nafion) in ethanol was dropped onto the catalyst to ensure attachment of the nanotube film to the substrate. Thickness of the film is estimated to be less than 0.1 µm. A thin film is necessary in order to minimize diffusion resistance in the Nafion film. 2-Propanol was used as a solvent for preparing catalyst suspension since it provided better dispersion. For recording the scanning electron microscopy (SEM) image, a suspension of Pt/SWCNT was drop-casted onto a piece of carbon fiber paper (Toray) and dried in air. The Pt loading corresponded to 40 µg /cm2. The images were recorded using a Hitachi S-4500 scanning electron microscope. Apparatus. Electrochemical measurements were carried out using a three-compartment electrochemical cell with a Pt foil and a saturated calomel electrode (SCE) serving as the counter and the reference electrodes, respectively. The reference electrode was connected to the electrochemical cell with a double-junction electrolyte to prevent chloride contamination and was cooled to room temperature (24 °C). All potentials cited in the report are those with respect to this reference electrode. Electrochemical measurements were recorded using BAS 100B electrochemical analyzer. Temperature dependence experiments of oxygen reduction were carried out using an electrochemical cell with a liquid jacket for temperature control. Electrochemical Measurements. After preparation of the catalyst on the substrate the electrode was applied with potential between -0.275 and +1.2 V at 0.05 V/s for 20 min in deaerated 0.1 M HClO4 solution to eliminate any contamination which may exist in the Nafion membrane. Desorption of underpotentially deposited hydrogen in a stationary voltammogram was used to evaluate electrochemically active surface area (ECSA) of the Pt catalyst on the electrode. The oxidation of adsorbed carbon monoxide on Pt was also taken into account in evaluating the ECSA. Electrode potential of -0.2 V was applied for 5 min in CO saturated 0.1 M HClO4 prior to the potential sweep from -0.2 to +1.2 V in deaerated 0.1 M HClO4. Values of 210 and 420 µC cm-2 were used for determining the ECSA from adsorbed hydrogen and CO, respectively. An average value of ECSA estimated from both methods was used to evaluate the electrocatalyst. Hydrodynamic voltammograms at rotating disk electrode were recorded by scanning its potential from -0.25 to +1.0 V at 0.02 V/s in oxygen saturated 0.1 M HClO4. Measurements were employed after the electrolyte temperature and oxygen concentration reach equilibrium (ca. 20 min). Accelerated durability tests (ADT) were employed by cycling the electrode potential between -0.2 and +1.0 V at 0.02 V/s in air. The electrolyte and reference electrode were refreshed every 4-6 h to avoid contamination such as chloride during the test.
Results and Discussion Characterizations of Pt-Deposited SWCNT. A good dispersion of the Pt catalyst on the carbon support is a precondition (20) Paulus, U. A.; Schmidt, T. J.; Gasteiger, H. A.; Behm, R. J. J. Electroanal. Chem. 2001, 495, 134.
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Figure 1. Scanning electron micrograph of a SWCNT film cast on carbon fiber (Toray) paper after deposition of Pt particles under different magnifications. Pt particles anchored on SWCNT can be seen in the magnified image (C).
for attaining good electrocatalytic activity in a fuel cell reaction. High surface area carbon supports serve as the backbone for the dispersion of electrocatalyst particles. Carbon black with a BET surface area of ca. 80 m2/g is commonly employed as a support. Carbon nanotubes exhibit a surface area of 50-500 m2/g. In the present study, we employed SWCNT sample (SES Research) with a BET surface area of 81 m2/g, as it was comparable to the commercial catalyst. Although SWCNTs can be isolated in Nafion suspension by sonication, they form bundles of diameter 5-10 nm upon deposition as a film on Toray paper. Any improvement achieved in their distribution will aid in the effective dispersion of catalyst particles. Figure 1 shows a scanning electron micrograph of Pt/SWCNT deposited on carbon fiber electrode (Toray paper). The distribution of SWCNT is random, and they are bundled because of van der Waals interactions. The SEM shows a highly porous morphology for the SWCNT films that serves as a support for anchoring Pt nanoparticles. The extremely small size of Pt nanoparticles (diameter 2-5 nm) makes it difficult to distinguish at higher catalyst loadings. However, the morphology of Pt particles anchored on SWCNT can be visualized at low loadings (Figure 1C). Further characterizations of SWCNT/Pt electrodes have been characterized in our earlier studies. The Pt/SWCNT film deposited on a glassy carbon electrode is electrochemically active. Hydrogen adsorption characteristics are presented in the cyclic voltammogram in Figure 2a. The cyclic voltammetric behavior is reproducible and represents adsorption and desorption of hydrogen on Pt/SWCNT electrocatalyst. These measurements confirm the availability of a clean Pt surface for carrying out electrochemical reactions. We further estimated the electrochemically active surface area (ECSA) by integrating the voltammogram corresponding to hydrogen desorption from the electrode surface.21 The ECSA estimated from the hydrogen adsorption-desorption curve was 17.2 m2/gPt. (21) Sogaard, M.; Odgaard, M.; Skou, E. M. Solid State Ionics 2001, 145, 31.
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Figure 2. (a) Cyclic voltammogram recorded using Pt/SWCNT (14 µg/cm2) in deaerated 0.1 M perchloric acid at a scan rate of 0.02 V/s. (b) Linear sweep voltammogram recorded after adsorption of CO at the Pt/SWCNT electrode in the same solution. The peak area in panel b corresponds to stripping of adsorbed CO. The inset is the stripping voltammograms of CO at Pt/SWCNT and Pt/C.
Figure 3. Rotating disk voltammograms for the oxygen reduction on Pt/SWCNT taken in O2 saturated 0.1 M HClO4 solution at 24-60 °C. Rotating rate ) 1600 rpm. Potential scan rate ) 0.02 V/s. The inset is the experimental values of limiting currents and the theoretical values of limiting currents based on physical and chemical properties of pure water.
In addition, we also estimated ECSA using CO adsorption measurements. The Pt/SWCNT electrode was kept in a COsaturated perchloric acid (0.1 M) solution for 10 min, and then, a deaerated perchloric acid solution was transferred to the electrochemical cell while applying the electrode potential at -0.2 V. The linear sweep voltammogram (trace b in Figure 2) shows the stripping of the adsorbed CO on the Pt/SWCNT film. The ECSA measured from the CO stripping peak was 18.4 m2/ gPt and is similar to the value obtained from hydrogen desorption measurements. (The difference in the two values is within the error of 5% in the experimental measurements.). The lower ECSA values indicate aggregation of Pt clusters on the SWCNT surface. Efforts are underway to increase the distribution of the catalyst particles on the SWCNT surface. Desorption of the CO from the Pt catalyst surface is important from the point of minimizing the poisoning effect in DMFC. Pt-Ru catalysts are routinely used to overcome the CO poisoning effect.22 To see whether the SWCNT support has influence on the CO binding to the Pt surface, we compared the CO stripping behavior of Pt/SWCNT with that of the Pt dispersed on a commercial (E-TEK) carbon support (referred as Pt/C catalyst). The stripping of CO with Pt/SWCNT occurs at potential ∼120 mV lower than the Pt/C catalyst (see inset, Figure 2). The fact that we observe lower CO stripping with no change in the characteristics of reduction and formation of the surface Pt oxide suggests lower adsorption energy for CO on Pt. Since oxidation of CO is often a limiting factor in the methanol oxidation, we would expect a beneficial effect of SWCNT/Pt electrocatalyst in DMFC. Oxygen Reduction on Pt-Deposited SWCNT. Although attempts have been made to employ carbon nanotubes as support for promoting oxygen reduction, little is known about the role it plays in promoting catalytic activity.7,8,19,23 A number of factors such as kinetics, electrochemical active surface area, conductivity, and porosity of the support are involved in determining the overall catalytic activity. To explore these factors in detail, we carried out rotating disk electrode experiments at different temperatures. Hydrodynamic voltammograms of Pt/SWCNT for oxygen reduction are shown in Figure 3. In the range of 24-40 °C, an increase in the limiting current is seen. At temperatures greater
than 40 °C, we observe a decrease in the limiting current. As the temperature increases, the viscosity of the medium decreases thus facilitating diffusion of oxygen in the medium. However, the oxygen concentration and the density of electrolyte decrease at elevated temperatures. This in turn induces a mixed behavior in the limiting current (inset of Figure 3). Paulus et al.20 derived an expression using the physical-chemical properties of pure water to fit the temperature-dependent limiting currents. The solid line presented in the inset of Figure 3 shows the fit using this analysis. Since the oxygen concentration decreases with increasing temperature, one needs to account for this variation while employing the limiting current values for the determination of the oxygen reduction rate. Wakabayashi et al.24,25 noted that the kinetic current, ik, needs to be defined in terms of apparent rate constant using eq 1 when considering the kinetics of electrocatalyst at elevated temperatures
(22) Watanabe, M.; Zhu, Y.; Igarashi, H.; Uchida, H. Electrochemistry 2000, 68, 244. (23) He, Z.; Chen, J.; Liu, D.; Zhou, H.; Kuang, Y. Diamond Relat. Mater. 2004, 13, 1764.
(24) Wakabayashi, N.; Takeichi, M.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 5836. (25) Wakabayashi, N.; Takeichi, M.; Itagaki, M.; Uchida, H.; Watanabe, M. J. Electroanal. Chem. 2005, 574, 339.
kapp ) -ik/nFA[O2][H+]
(1)
where A is the specific surface area and [O2] and [H+] are the concentrations of oxygen and proton, respectively. The kinetic currents, ik, were estimated from the rotating disk experiments (ik ) idi/(id - i)), where id is diffusion-limited current and i is the apparent current obtained from the voltammograms. The apparent rate constants for oxygen reduction, kapp, were determined at two different potentials (625 mV and 575 mV) using Pt/SWCNT and Pt/C electrodes. The potentials chosen for the determination of kapp correspond to two different parts of the rising current in the voltammogram (see for example Figure 3), where the currents are electron transfer-limiting and are not affected by the diffusion of O2 in the film. Hence the difference in porosity of the film between carbon black and SWCNT can be excluded. The experiments were conducted at different temperatures to determine the activation barrier for O2 reduction. Arrhenius plots of log(kapp) versus 1/T are shown in Figure 4. Various electrochemical parameters that compare the electrocatalytic activity of Pt/SWCNT and PT/C electrodes are compared in Table 1. The rate constants for oxygen reduction at both of these potentials are 2-fold higher for the Pt/SWCNT electrode
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Figure 4. Arrhenius plots for the apparent rate constant kapp on Pt/SWCNT and Pt/C at 625 and 575 mV vs SCE.
than those for Pt/C. These results further provide credence for our argument that the SWCNT support plays an important role in enhancing the catalytic activity toward oxygen reduction. The activation enthalpy determined from the slopes for both Pt/ SWCNT and Pt/C at 625 and 575 mV are ca. 25 kJ/mol. These values are in agreement with the activation enthalpy (21-28 kJ/mol) observed for Pt catalysts anchored on other carbon supports.20,26-29 This implies that the activation barriers for oxygen reduction at Pt/SWCNT and Pt/C are similar and the basic mechanism of oxygen reduction remains unchanged. The only difference is the increased rate constant for oxygen reduction at Pt electrocatalysts on SWCNT support. Comparison of Oxygen Reduction at Pt/SWCNT and Pt/C Electrodes. The comparison of oxygen reduction at commercially obtained Pt/C (Pt 40 wt %/Vulcan XC-72 from E-TEK) was made at similar Pt loading (14 µg/cm2). The rotating disk voltammograms of Pt/SWCNT and Pt/C are shown in Figure 5A. The ECSAs determined for these two electrodes were 17.8 m2/gPt compared to 33.5 m2/gPt for Pt/C. The discrepancy in the ECSA shows that the commercial catalyst has a better dispersion of Pt nanoparticles than the one we prepared with SWCNT. Despite the lower ECSA, we observe a 10 mV shift in the onset potential for oxygen reduction at Pt/SWCNT electrode. Lowering of the onset potential using the SWCNT support is further evidence for its catalytic role in the oxygen reduction. The Tafel plots derived from the voltammograms (Figure 5A) are shown in Figure 5B. Tafel slopes of -59 mV/decade are found for both Pt/SWCNT and Pt/C in the low current region. This region corresponds to oxygen reduction when the adsorbed hydroxyl species at the Pt surface (OHads) determines the electrode activity.28,30,31 In the high current region, Pt/SWCNT showed a slightly higher Tafel slope -131 mV/decade than Pt/C (-111 mV/decade). This is probably due to strong van der Waals interactions between the nanotubes, which causes the nanotubes to bundle and results in poor dispersion of Pt/SWCNT on the substrate. This fact becomes obvious in the transition region between charge-transfer-controlled and mass-transfer-controlled regions. Despite the difference in ESCA and Tafel slopes, the limiting current observed for O2 reduction at both Pt/C and Pt/SWCNT were similar. The high porosity of the nanotubes is likely to allow the reactants to diffuse through and promote the oxygen
Figure 5. (A) Rotating disk voltammograms comparing the oxygen reduction on Pt/SWCNT (a) and Pt/C (b) in O2 saturated 0.1 M HClO4 solution at 24 °C. Rotating rate ) 1600 rpm. Potential scan rate ) 0.02 V/s. (B) The corresponding Tafel plots.
reduction at high current density. In addition, an improved chargetransfer rate constant at Pt/SWCNT also makes the reduction efficient. Accelerated Durability Tests for Pt/C and Pt/SWCNT. Though Pt is known to be stable over a wide potential range, it exhibits some instability at potentials where oxygen reduction is seen. During the long-term use of Pt electrocatalyst, the active area of the Pt electrocatalyst decreases mainly because of agglomeration and surface passivation of Pt nanoparticles. Evaluating the durability of electrocatalyst requires long time runs. However, corrosion of the electrocatalyst can be accelerated by cycling the electrode potential in acidic solutions.6,32,33 Kinoshita et al. found an ECSA loss of 70% when they performed potential cycling of Pt/C between -0.2 and +1.0 V for 3500 cycles.32 The loss in ECSA was mainly attributed to agglomeration of Pt particles. We performed the accelerated durability tests (ADT), by cycling the electrode potential between -0.2 and +1.0 V at a scan rate of 0.5 cycles per min in air, to examine the stability Pt particles anchored on the carbon supports. As expected, the active area of Pt particles supported on carbon black (Figure 6A) decreased as we cycled the electrode continuously over a long period. Interestingly, the decrease in the active area of Pt/SWCNT is smaller than that of Pt/C (Figure 6B). A shift of about 15 mV to a higher potential for the reduction of surface oxides was observed for Pt/C after 36 h of potential cycling, whereas no such observable shift could be seen for Pt/SWCNT. These results
Table 1. Electrocatalytic Properties of Pt/SWCNT and Pt/C Electrodes
Pt/SWCNT Pt/C
ESCA (m2/gPt)
onset potential for O2 reduction (mV)
17.8 33.5
639 632
Tafel slope (mV/dec) high i low i -59 -59
-131 -111
kapp at 625mV, 24°C (cm4/mol s)
∆E (kJ/mol)
6.42 3.28
24.8 ( 0.3 24.8 ( 0.3
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Figure 7. Electrochemically active surface areas of Pt particles on SWCNT (a) and on C (b) during the accelerated durability tests (ADT).
its ECSA, whereas Pt/SWCNT lost only 16% of its ECSA. These accelerated stability tests suggests that SWCNT is a superior support to anchor Pt particles. In addition to the improved catalytic activity, the SWCNT support minimizes the Pt aggregation effect during long-term usage. Efforts are underway to improve the ECSA of Pt/SWCNT by dispersing the SWCNT support on the electrode surface without the bundling effects.
Conclusions Figure 6. Cyclic voltammograms recorded using Pt/C (A) and Pt/ SWCNT (B) in deaerated 0.1 M perchloric acid at a scan rate of 0.05 V/s at the starting of the ADT, after 7 h, and after 36 h of ADT. Pt loading is 14 µg/cm2 for both electrodes.
indicate a lower degree of recrystallization of Pt particles on SWCNT and a greater stability of SWCNT to anchor Pt particles. The decreases in ECSA values with time of potential cycling are summarized in Figure 7. Although, Pt/C has a higher ECSA than Pt/SWCNT before the durability test, the ECSA of Pt/C decreased continuously with potential cycling and finally decreased below ECSA of Pt/SWCNT after 36 h. After 36 h of potential cycling, which is comparable to 2160 cycles when performed at a scan rate of 1 cycle per min, Pt/C lost 50% of (26) Sepa, D. B.; Vojnovic, M. V.; Vracar, L. M.; Damjanovic, A. Electrochim. Acta 1986, 31, 91. (27) Sepa, D. B.; Vojnovic, M. V.; Vracar, L. M.; Damjanovic, A. Electrochim. Acta 1986, 31, 97. (28) Damjanovic, A.; Sepa, D. B. Electrochim. Acta 1990, 35, 1157. (29) Damjanovic, A.; Walsh, A. T.; Sepa, D. B. J. Phys. Chem. 1990, 94, 1967. (30) Markovic, N. M.; Gasteiger, H. A.; Grgur, B. N.; Ross, P. N. J. Electroanal. Chem. 1999, 467, 157. (31) Markovic, N. M.; Adzic, R. R.; Cahan, B. D.; Yeager, E. B. J. Electroanal. Chem. 1994, 377, 249. (32) Kinoshita, K.; Lundquist, J. T.; Stonehart, P. Electroanal. Chem. Interfacial Electrochem. 1973, 48, 157. (33) Colon-Mercado, H. R.; Kim, H.; Popov, B. N. Electrochem. Commun. 2004, 6, 795.
Oxygen reduction at Pt/SWCNT films has been evaluated using a thin film rotating disk electrode at elevated temperature. Pt deposited on SWCNT exhibited a 2-fold higher rate constant, kapp, than Pt/C. Activation enthalpies for O2 reduction at Pt/ SWCNT and Pt/C exhibited similar values, thus confirming the rate-determining step involved in the oxygen reduction were same. The high porosity of SWCNT facilitates diffusion of the reactant and facilitates interaction with the Pt surface. It is evident from the accelerated durability tests that SWCNT enhance the stability of the electrocatalyst. Furthermore, the lower energy of CO adsorption observed with the Pt/SWCNT electrode also demonstrates the CO-tolerance electrocatalyst and thus gives credence for the use of SWCNT in direct methanol fuel cells. Acknowledgment. The research described herein was supported by the Indiana 21st Century Research and Technology Fund and the U.S. Army CECOM RDEC through Agreement DAAB07-03-3-K414. Such support does not constitute endorsement by the U.S. Army of the views expressed in this publication. A.K. expresses his thanks to the Center of Excellence (21COE) Program “Creation of Integrated Eco Chemistry” of Osaka University. We would like to thank Timothy Hall for his assistance in recording SEM images and Prof. Vinodgopal for helpful discussions. This is Contribution No. NDRL 4631 from Notre Dame Radiation Laboratory. LA052753A
