C Nanocomposite Electrocatalysts for Proton-Exchange

16138

J. Phys. Chem. C 2007, 111, 16138-16146

Pt/SWNT-Pt/C Nanocomposite Electrocatalysts for Proton-Exchange Membrane Fuel Cells A. Leela Mohana Reddy and S. Ramaprabhu* AlternatiVe Energy Technology Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India ReceiVed: October 25, 2006; In Final Form: January 11, 2007

Single-walled carbon nanotubes (SWNT) have been synthesized by pyrolysis of methane over Mm (Mischmetal)-based AB3 alloy hydride catalyst. Purification of as-grown SWNT was carried out by air oxidation followed by acid treatment. The as-grown and purified SWNT have been characterized by XRD, SEM, TEM, HRTEM, TGA, IR, and Raman spectroscopy studies. Well-dispersed Pt catalysts on SWNT catalyst support for polymer electrolyte membrane fuel cells (PEMFC) have been prepared by a simple chemical reduction method using prefunctionalized SWNT. The anode and cathode electrodes for PEMFC have been fabricated using Pt-supported SWNT and commercial Pt/C electrocatalysts with different compositions of Pt/SWNT and Pt/C. The dependence of the fuel cell performance on the dispersion and accessibility of SWNT support and Pt electrocatalysts in the electrocatalyst mixture for the oxygen reduction reaction in PEMFC has been discussed. These results open up a way to use Pt/SWNT + Pt/C nanocomposites as electrocatalysts in PEMFC.

1. Introduction Polymer electrolyte membrane fuel cells (PEMFC) have attracted much attention for in-house power generation due to their advantageous features such as a low operating temperature, sustained operation at high current density, low weight, compactness, long stack life, fast start-up, and suitability during discontinuous operation.1-5 However, wide application is hindered by their high cost. It is generally believed that the large amount of depleting platinum required as a catalyst in PEMFC is one of the main reasons why fuel cells are excluded from commercialization. In the past 2 decades, continuous efforts have been devoted to increase the utilization of Pt and reduce the amount of Pt used in PEMFC. Electrocatalysts with small size and high dispersion result in high electrocatalytic activity.6 This suggests that it is highly desirable to have good Pt supporting materials with high surface area, which will enhance the Pt dispersion and hence reduce the catalyst loading, thereby improving the fuel cell performance.7-10 CNTs are attractive materials for catalyst support in PEMFC due to their morphology and interesting properties such as nanometer size, high accessible surface area, corrosion resistance, good electrical conductivity, and high stability.11 Systematic study of the performance of carbon nanotubes as catalyst support materials in DMFC electrodes has shown that single-walled carbon nanotubes (SWNT) are much superior when compared to other carbonbased supporting materials.12 In principle, SWNT are seamless cylinders, but they often have defect sites, where the attachment of platinum or platinum-ruthenium catalyst particles is most likely to occur. The density of defect sites and differences between different sources of SWNT can play an important role in determining the performance of a PEMFC. Achieving a higher degree of dispersion of Pt on the SWNT support is an important goal for maximum utilization of the high surface area, conductivity, and porosity of SWNT. Common strategies to disperse SWNT are to functionalize the side wall of the nanotubes13 or * Corresponding author. Phone: +91-44-22574862. Fax: +91-4422570509. E-mail: [email protected].

to use surfactants.14-20 Since the aromatic ring system of the carbon nanotubes can be disrupted by the application of extremely aggressive reagents, such as HNO3 or H2SO4, or a mixture of two, the nanotubes can be functionalized with anchor groups such as hydroxyl (-OH), carboxyl (-COOH), and carbonyl (>CdO), which can act as anchoring sites for the metal complexes.21-24 The Pt/SWNT has an exceptionally high electrochemically active surface area (ECSA), exhibits excellent electrocatalytic activities toward oxygen reduction, and has potential application in designing a membrane assembly for PEMFC. Recently, we showed that by using alloy hydride catalysts, we have the ability to synthesize SWNT, MWNT, and metal-encapsulated MWNT in bulk quantities.25 We now report a successful method of preparing Pt/SWNT electrocatalysts (Scheme 1). The key idea is to combine Pt nanoparticles and SWNT to modify the electrodes in order to improve their electroactivity for better fuel cell performance. Further, in PEMFC, most of the recent work is being focused on the development of cathode electrocatalyst materials. Hence, further we focus here on the development of both anode and cathode electrodes using Pt/SWNT and commercial Pt/C nanocomposite mixtures and study the performance of fuel cells by systematic variation in the composition of Pt/SWNT and Pt/C. We report the synthesis of SWNT by the pyrolysis of methane over AB3 alloy hydride catalyst, prepared by a hydrogen decrepitation technique.26 These alloy hydride particles are catalytically very active toward the growth of SWNT, due to the presence of transition metals such as Fe, Co, or Ni, and are free from being oxidized. SWNT-supported Pt (Pt/SWNT) is synthesized by a chemical reduction method using carboxyl-functionalized SWNT. We describe the preparation of nanocomposites of Pt-supported SWNT and Pt-loaded carbon black (Pt/C) (Pt content of 20 wt %) as the anode and cathode electrodes materials in PEMFC. The performance of the PEMFC with different compositions of Pt/SWNT and Pt/C as the electrocatalyst is systematically studied, and the role of SWNT and the optimum composition of Pt/SWNT for better performance of PEMFC are discussed.

10.1021/jp066985+ CCC: $37.00 © 2007 American Chemical Society Published on Web 10/17/2007

Pt/SWNT-Pt/C Nanocomposite Electrocatalysts

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16139

SCHEME 1: Schematic Representation of Pt/SWNT Preparation

2. Experimental Section 2.1. Synthesis of SWNT. SWNT have been synthesized by catalytic chemical vapor deposition of methane over Mischmetal (Mm)-based AB3 alloy hydride catalyst using a single-furnace technique. The Mischmetal (Bharat Rare Earths Metals, India) used has the following composition : Ce 50%, La 35%, Pr 8%, Nd 5%, Fe 0.5%, and other rare earth elements 1.5%. Alloy hydride catalyst has been obtained by a hydrogen decrepitation technique using a high-pressure hydrogen absorption/desorption facility. About 250 mg of the hydride powder was placed in a quartz boat and then introduced to the flow reactor (quartz tube with an i.d. of 30 mm and a length of 100 mm) for the synthesis of SWNT. The alloy hydride powders were heated at 500 °C in a hydrogen flow hydrogen (50 mL/min) for 1 h in order to reduce any surface-oxidized catalyst particles. The hydrogen gas flow was stopped, and the temperature of the furnace was raised to 1050 °C for the production of SWNT. Methane was then allowed to flow (100 mL/min) for 30 min. The deposition was carried out at atmospheric pressure and in an argon flow. After completion of the deposition, the reactor was allowed to cool to room temperature in the presence of argon flow. The quartz boat was carefully removed from the reactor, and carbon deposits from the quartz boat were taken out. The purification of the as-grown sample was carried out by air oxidation at 500 °C for 2 h to remove the amorphous carbon and to open the ends of the carbon nanotubes. The above air-oxidized CNTs were then refluxed with concentrated HNO3 for 24 h, followed by washing with deionized water several times, and then the sample was dried in air for 30 min at 100 °C. 2.2. Functionalization of SWNT. Purified SWNT were ultrasonicated in concentrated nitric acid for 3 h. The ultrasonic waves produce microscopic bubbles in the liquid; collapsing of these microscopic bubbles results in shock waves, which are highly effective in increasing the nanotube wetting. Xing and co-workers27,28 have reported in detail the effect of sonication on CNTs, using an ultrasonic bath with a power of 130 W and a frequency of 40 kHz (Fisher). They reported that 2 h of sonication time is required to produce enough surface groups for Pt decoration. In the present experiments we have used an ultrasonic bath with a power of 100 W and a frequency of 40 kHz. The lower power was used mainly to reduce the damage to CNTs that will result during the ultrasonication. Accordingly,

we have increased the time of sonication from 2 to 3 h to achieve good surface modification. After the sonication procedure, refluxing in nitric acid under constant agitation in 30 mL of 70% HNO3 at 110 °C for 12 h has been done, followed by washing with deionized water several times and drying the sample in air for 30 min at 100 °C. 2.3. Preparation and Characterization of Pt/SWNT. The above-prepared carboxyl group functionalized SWNT were sonicated in acetone (organic solvent) in order to remove any agglomeration of the CNTs that might result during the washing and filtering. The solid phase was removed by centrifugation and washed with distilled water; the recovered SWNT were dried at 80 °C for 12 h. The dried sample was ultrasonicated in 10 mL of acetone for 1 h, and then 0.075 M H2PtCl6 solution was added slowly during stirring. After 12 h, the mixture was reduced by adding reducing solution containing 0.1 M NaBH4 and 1 M NaOH. After completion of reaction, the solution was washed with deionized water, filtered, and dried by vacuum filtration using a filter. The recovered Pt-loaded SWNT were dried at 80 °C for 3 h. The crystallinity of the samples was obtained by X-ray powder diffraction (XRD) analysis, performed with monochromatic Cu KR radiation. Morphological characteristics of CNTs were obtained using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM measurements were carried out by ultrasonicating the sample in acetone for 1 h for good dispersion and then depositing it onto copper grids. Functional group identification and characterization were carried out using a Bruker IFS66v FTIR spectrometer in the range of 1000-4000 cm-1. Sample preparation for FTIR studies involved dispersing the SWNT in 2-propanol using low-power sonication. The solution then was deposited dropwise on ZnSe substrates maintained in air at ∼60 °C. The solvent from each drop was allowed to evaporate before the next drop was added. The final film transmittance was ∼60% at 1000 cm-1. The spectrum obtained from the thus-prepared sample was analyzed after removing the reflection/absorption loses of the ZnSe substrate. 2.4. Preparation of Membrane Electrode Assembly (MEA). The required amount of catalyst was suspended in deionized water and ultrasonicated by adding 5 wt % Nafion solution. The suspension was spread uniformly over the gas diffusion layered carbon paper (Toray) by a spin-coating technique. The

16140 J. Phys. Chem. C, Vol. 111, No. 44, 2007

Reddy and Ramaprabhu different electrodes were recorded by scanning the potential from -0.25 to +1.0 V at 0.02 V/s in oxygen-saturated 0.1 M HClO4. The electrolyte and reference electrodes were refreshed every 4-6 h to avoid contamination, such as from chloride, during the test. Desorption of underpotentially deposited hydrogen in a stationary voltammogram was used to evaluate the electrochemically active surface area (ECSA) of the Pt catalyst on the electrode. The ECSA of an electrocatalyst is a measure of the number of electrochemically active sites per gram of the catalyst. ECSA is an important parameter to compare different electrocatalytic supports of electrode materials. ECSA is determined by integrating the area under the potential window for H+ adsorption/desorption peaks after subtracting the charge from the double-layer region. 3. Results and Discussion

Figure 1. Powder X-ray diffractograms of (a) as-grown SWNT, (b) purified SWNT prepared by pyrolysis of acetylene over alloy hydride catalyst, and (c) Pt-loaded SWNT.

electrode surface area used was 11.56 cm2 with Pt loadings of 0.25 and 0.50 mg cm-2 on the anode and cathode, respectively. A Nafion membrane (Nafion R) cut in the dimension of 5 cm × 5 cm was pretreated by boiling in a solution of 5% H2O2 and 1 M H2SO4 at 80 °C for 30 min and washing several times with deionized water. The cathodes and anodes were then pressed on either side of the pretreated Nafion 1135 membrane at a pressure of 50 kg/cm2 at 130 °C for 2 min. By varying the composition of the electrocatalyst (mixture of Pt/SWNT and Pt/C) on both the anode and cathode, five different MEAs were prepared for the systematic analysis of performance of PEMFC by keeping the other parameters constant. A single PEMFC was assembled using the MEA, two graphite plates with gas channels machined to a serpentine geometry, two Teflon gaskets, and two aluminum end plates. The performance of the PEMFC was studied by an indigenously fabricated fuel cell test station, using a dc electronic load box. Since hydration of the electrolyte membrane is important for attaining good performance of the PEMFC, reactant gases were humidified. 2.5. Electrochemical Characterization. Half-cell reactions were carried out in a three-arm electrochemical cell using PSGSTAT-30 (AUTOLAB). In the preparation of the working electrode, about 5 mg of Pt/SWCNT and 5 µL of diluted Nafion (0.05 wt % Nafion) in ethanol were mixed using an ultrasonic bath. A measured volume of this mixture was then pipetted onto the glassy carbon electrode (0.03 cm2) substrate and dried at room temperature for 24 h. The thickness of the film is estimated to be less than 0.1 µm with a platinum loading of 15 µg of Pt/cm2. A thin film is necessary in order to minimize diffusion resistance in the Nafion film. For comparison, Pt/C, 50 wt % Pt/SWNT + 50 wt % Pt/C electrodes were also prepared under the same preparation conditions. Pt foil and saturated calomel electrodes (SCE) were used as the counter and the reference electrodes, respectively. Initially, the electrode was operated by applying a potential between -0.2 and +1.2 V at 0.05 V/s for 20 min in 0.1 M HClO4 solution to eliminate any contamination in the Nafion membrane. Hydrodynamic voltammograms of

Mm-based AB3 alloys, after several hydrogenation and dehydrogenation cycles, were found to be finely powdered to about 5-10 µm. These novel hydride catalysts prepared using a hydrogen decrepitation technique provide fresh surfaces with large surface area, free from oxidation, which further increases the catalytic sites for the formation of CNTs. High hydrogen absorption, large decrepitation, and low cost make these hydrides better catalysts for large-scale production of CNTs. The yield of carbon deposit was calculated using the equation

carbon yield (% ) )

(mtot - mcat) mcat

(1)

where mcat is the initial amount of the catalyst (before reaction) and mtot is the total weight of the sample after reaction. Carbon deposited using Mm-based AB3 alloy hydride catalysts showed a weight gain of ∼95%. Figure 1a shows the XRD pattern of as-prepared SWNT using alloy hydrides as catalysts. The peaks are indexed to the reflections of hexagonal graphite. A few peaks corresponding to the catalytic impurities are also seen. The removal of metallic impurities by acid treatment is clearly shown for purified SWNT (Figure 1b). The broader diffraction peaks of Pt along with the reflections of hexagonal graphite for Pt-loaded SWNT are shown in Figure 1c. Figure 2 shows the SEM, TEM, and HRTEM images of purified SWNT, respectively. From Figure 2 it is clear that good quality SWNT have been obtained by a CCVD technique using the Mm-based AB3 alloy hydride catalyst. Further, the HRTEM image (Figure 2d) reveals the bundle nature of single-walled carbon nanotubes with each graphene layer being clearly distinguishable. Raman spectroscopy has been used to investigate the vibrational properties of the carbon samples. Figure 3a shows the Raman spectra of purified SWNT. The peak at 268 cm-1, corresponds to the radial breathing mode (RBM) of SWNT. Tangential modes corresponding to the Raman-allowed optical mode E2g of two-dimensional graphite, centered around 1590 cm-1 (G-band), is observed. In addition, a peak centered at around 1350 cm-1 (D-band) is mainly due to defects and carbonaceous particles present in the sample. The intensity of the D-band gives the degree of disorder present along the tube. These can be pentagons, heptagonal defects, the pentagonheptagon pairs, or line defects.29 The peak still remains with low intensity, implying that some degree of disorder is present along the tube even after purification. Acid-treated SWNT shows a clear increase in the intensity of the D-band (Figure 3b), which can be attributed due to the defects created along the nanotube surface during the vigorous acid treatment.



Figure 2. (a) SEM, (b and c) TEM, and (d) HRTEM images of purified SWNT.

Figure 4. FTIR spectra of acid-treated purified SWNT.

Figure 3. FT-Raman spectra of (a) purified SWNT and (b) functionalized SWNT.

The FTIR spectra of acid-treated SWNT in the range of 1000-4000 cm-1 is shown in Figure 4. A broad absorption band at 3452 cm-1 is attributed to the hydroxyl group (νOH).30,31 This band might have resulted due to water νOH and δH2O22 and also the -OH functional groups resulting due to the chemical treatment during the purification and functionalization processes, respectively.32 Bands at 2927 and 2848 cm-1 are due

to asymmetric and symmetric CH stretching. A small peak at 1734 cm-1 is associated with the CdO stretching of the carboxylic acid (-COOH) group.33 The peak at 1643 cm-1 is due to CdC stretching of the CNTs.34 The peak at 1460 cm-1 is due to O-H bending deformation in -COOH. A small peak at 1087 cm-1 is assigned to C-O bond stretching.33 Thus, the generation of -OH and -COOH groups on CNTs due to functionalization is observed. Damages in the graphene layers of SWNT bundles were observed after chemical treatment during functionalization.35 HRTEM of SWNT (Figure 5a) shows the loss of morphology resulting due to the vigorous acid treatment procedure. Figure 5, parts b and c, shows the TEM and HRTEM images of Pt-loaded SWNT. The TEM images of Pt/SWNT show a more or less uniform distribution of noble metal particles of a


Reddy and Ramaprabhu

Figure 5. (a) HRTEM image of SWNT bundles after functionalization; (b) TEM, (c) HRTEM, and (d) EDAX patterns of Pt/SWNT.

size of about 3-5 nm on the CNTs. The HRTEM image of Pt/SWNT clearly shows lattice planes of Pt particles indicating the crystalline nature of catalytic Pt. The energy-dispersive analysis (EDAX) (Figure 5d) shows that the amount of Pt loaded on the carbon nanotube support with reference to carbon can be evaluated qualitatively as 20%. The basic electrochemical reactions occurring at the anode and cathode of the proton-exchange fuel cell are, at the anode,

/2H2 S H+ + e-

(2)

/2O2 + 2H+ + e- S H2O

(3)

1

and at the cathode, 1

The overall performance of PEMFC is controlled by different processes like dissolution of gases at the gas-electrolyte interface, diffusion of the dissolved hydrogen/oxygen to the electrolyte-catalyst interface, electrochemical reaction at the catalyst-electrolyte interface, transport of charges by the current collector, and proton transport across the membrane. Among the different issues involved in commercializing PEMFC, development of electrode materials is being focused on in recent days for improving catalyst surface area by dispersing them on suitable supporting materials.36-38 In search of different catalyst support materials, SWNT seem to be a better option due to their high conductivity and nanostructure morphology.21,38-40 In order to find the enhanced performance of PEMFC using Pt/SWNT as the anode and cathode electrode materials and to compare

with that of commercial Pt/C, five different MEAs were prepared by varying the composition of the electrocatalyst (mixture of Pt/SWNT and Pt/C) on both the anode and cathode. The polarization curves obtained from the single-cell PEMFC using the above MEAs at different temperatures under an operating pressure of 1 bar are shown in Figure 6. Prior to polarization studies, the electrodes were activated between open-circuit potential and high current densities. The activation cycle is necessary to activate the catalyst for the oxygen reduction reaction. In the low current density region, rapid voltage drop in the potential-current curve, generally known as activation polarization, reflects the sluggish kinetics intrinsic to the oxygen reduction reaction at the cathode surface. As the current density increases, a mild drop in voltage is observed because of the cell resistance. Mass transport limitations account for the rapid drop seen at higher current densities. Initially, Pt/C has been kept as the anode catalyst and the cathode has been varied as Pt/C (Figure 6a), 100 wt % Pt/SWNT (Figure 6b), 50 wt % Pt/C + 50 wt % Pt/SWNT (Figure 6c), and membrane electrode assemblies were prepared for PEMFC. The performance of the PEMFC is observed to be good for cathode catalyst having a mixture of 50 wt % Pt/SWNT + 50 wt % Pt/C. Therefore, 50 wt % Pt/SWNT + 50% Pt/C has been kept as the cathode catalyst, and variation in the anode materials (100 wt % Pt/ SWNT, 50 wt % Pt/SWNT + 50% Pt/C) was done for comparison of performance of PEMFC (Figure 6, parts d and e). From the above studies, performance of PEMFC with 50 wt % Pt/SWNT + 50% Pt/C as both the anode and cathode



Figure 6. Polarization curves of PEMFC at various temperatures with the anode and cathode catalyst containing a mixture of Pt/C and Pt/SWNT electrocatalysts: (a) Pt/C on both the anode and the cathode, (b) Pt/C on the anode and Pt/SWNT on the cathode, (c) Pt/C on the anode and 50 wt % Pt/SWNT + 50 wt % Pt/C on the cathode, (d) Pt/SWNT on the anode and 50 wt % Pt/SWNT + 50 wt % Pt/C on the cathode, (e) 50 wt % Pt/SWNT + 50 wt % Pt/C on both the anode and the cathode.

catalyst is found to be the best combination for the enhanced performance of PEMFC. From Figure 7, the comparative polarization and power curves for all MEAs at a constant temperature of 60 °C shows a maximum power density of 294 mW cm-2 for MEA with 50 wt % Pt/SWNT + 50% Pt/C composites as electrode materials on both electrodes. A comparison of the fuel cell performance with the cathode and anode catalyst containing a mixture of different compositions of Pt/SWNT, prepared using SWNT synthesized over alloy hydride catalyst, and Pt/C is given in Table 1. Commercial 20% Pt on C as the cathode and anode catalyst in PEMFC shows a

lesser performance with a potential of about 540 mV at a current density of around 258 mA cm-2. Under the same operating conditions, PEMFC with both cathode and anode catalysts containing 50 wt % Pt/SWNT + 50 wt % Pt/C shows the maximum performance, with a potential of 540 mV at a current density of around 485 mA cm-2 and power density of 262 mW cm-2 and thus seems to be the optimal composition for better performance of PEMFC, under the present study. Thus, the composition of Pt/SWNT in the catalyst mixture plays the key role in the overall performance of the PEMFC. Since the Pt loading is around 20% in both Pt/SWNT and Pt/C, the loading


Reddy and Ramaprabhu

Figure 7. Comparative polarization and power curves of different MEAs at a constant cell temperature of 60 °C.

TABLE 1: Kinetic Parameters from Regression Analysis of Polarization and Current Density (at 540 mV) of PEMFC with Cathode and Anode Catalyst Containing a Mixture of Pt/SWNT and Pt/C Electrocatalysts Taken at 60 °C anode catalyst Pt/C (E-Tek) Pt/C (E-Tek) Pt/C (E-Tek) Pt/SWNT 50 wt % Pt/C + 50 wt % Pt/SWNT

E0 (V)

current density (mA cm-2) at 540 mV

power density (mW cm-2) at 540 mV

b (V dec-1)

R (Ω cm2)

Pt/C (E-Tek) Pt/SWNT

0.992

258

139

0.072

1.06

0.975

418

225

0.067

0.62

50 wt % Pt/C + 50 wt % Pt/SWNT 50 wt % Pt/C + 50 wt % Pt/SWNT 50 wt % Pt/C + 50 wt % Pt/SWNT

0.98

460

248

0.071

0.52

0.986

472

255

0.077

0.49

0.994

485

262

0.076

0.46

cathode catalyst

of Pt in the mixture at different compositions remains the same. Better performance of PEMFC was observed for cathode catalysts with Pt/SWNT compared to those containing commercial Pt/C, which could be attributed to the higher catalytic reactivity of smaller Pt particles with uniform sizes decorated on the SWNT. The higher performance of the Pt/SWNT electrodes compared to that of the Pt/C electrodes could be attributed to the networks and interiors of CNTs consisting of spaces for gas diffusion and the high electrical conductivity of SWNT. TEM analysis (Figure 5a) shows a homogeneous distribution of nanosize Pt particles on the SWNT surface, which results in an enhanced interaction between the Pt and SWNT, leading to higher performance. Furthermore, the pretreatment of SWNT with concentrated HNO3 results in functionalization of the SWNT surface with carboxylic acid groups, which would act as additional anchoring sites for better adherence of Pt nanoparticles onto the SWNT surface, thereby giving a better performance at higher current densities. The PEMFC performance also depends on the electrical conductivity of the SWNT support and its ability to transport electrons to the current collector of MEA. In the present study, the performance of the PEMFC is maximum for the catalyst with 50 wt % Pt/SWNT + 50 wt % Pt/C on both the cathode and anode sides. This can be attributed to the good accessibility and dispersion of the SWNT support and the Pt electrocatalysts in the mixture for the oxygen reduction reaction. The TEM image of 50 wt % Pt/SWNT + 50 wt % Pt/C composite (Figure 8) clearly shows the uniform distribution of Pt/C over Pt/SWNT. This results in more efficient

Pt usage in the nanocomposite with Pt/SWNT and Pt/C in equal weight percentage, thereby increasing the gas-electrolytecatalyst boundaries. Therefore, the present study indicates that the accessibility and dispersion of SWNT and hence the Pt electrocatalysts in the mixture for the oxygen reduction reaction may be the determining factor in deciding the performance of the cell. The experimental polarization data were analyzed using the semiempirical equation proposed by Srinivasan et al.41

V ) V0 - b log(I) - RI

(4)

where V and I are the experimentally measured cell voltage and current, b and R are the Tafel slope and total dc resistance. The dc resistance constitutes the contributions from membrane resistance and other electrode components responsible for the linear variation of potential with current. The membrane resistance was found to be 0.1072 Ω cm2, for a membrane of thickness of 89 µm. The experimental data were fitted to the above equation by a nonlinear least-squares method in order to evaluate the kinetic parameters of different electrocatalysts from regression analysis, and the values are given in Table 1. From Table 1 it is clear that the high performance is obtained with MEA having 50 wt % Pt/SWNT on both electrodes, which naturally shows a low dc resistance. The Tafel slope (dV/d(log I)) for the oxygen reduction reaction was around 60-80 mV/ decade over the entire current density range for the all combinations of electrocatalysts.


J. Phys. Chem. C, Vol. 111, No. 44, 2007 16145 to be 56 m2/gPt, 102 m2/gPt, and 118 m2/gPt, respectively. No degradation in the ECSA was observed for the Pt/SWNT after storing it in a suspension for over 1 week. 4. Conclusion

Figure 8. TEM image of the 50 wt % Pt/SWNT + 50 wt % Pt/C composite.

Functionalization of SWNT with carboxyl groups results in improved adhesion of Pt nanoparticles of around 3-5 nm diameter onto the SWNT surface. PEMFC with both the anode and cathode containing 50 wt % Pt/SWNT + 50 wt % Pt/C nanocomposite gives a higher performance due to the good accessibility and dispersibility of Pt electrocatalysts on the SWNT support resulting in more efficient Pt usage in the mixture for the oxygen reduction reaction. Better performance of the Pt/SWNT cathode catalyst over that of commercial Pt/C is due to the improved electron transfer and Pt utilization, showing the suitability of Pt/SWNT + Pt/C nanocomposites as electrodes for PEMFC. The higher catalytic activity has been attributed to the larger surface area provided by the carbon nanotube architecture and good electrical properties of SWNT and a high density of functional groups such as carboxyl, hydroxyl, and carbonyl groups on SWNT. The results presented in this study highlight the use of SWNT as a potential candidate as support material in PEMFC. Acknowledgment. Financial support from DRDO, RCI, TNSCST, MHRD, and IIT Madras for the present work is gratefully acknowledged. References and Notes

Figure 9. Cyclic voltammograms for Pt/C, Pt/SWNT, and 50 wt % Pt/C + 50 wt % Pt/SWNT recorded in 0.1 M HClO4 at a scan rate of 0.02 V/s. The Pt loading was 15 µg cm-2.

Figure 9 shows cyclic voltammograms for H adsorption on Pt/C, Pt/SWNT, and 50 wt % Pt/C + 50 wt % Pt/SWNT composite. Characteristic peaks in the negative region (0-0.1 V) are attributed to atomic hydrogen adsorption on the Pt surface and reflect the ECSA of Pt. The integrated area of the cyclic voltammogram represents QH (charge arising from hydrogen evolution reactions) and can be used to determine ECSA by employing the expression38,42

ECSA [cm2/g of Pt] ) QH [µC cm-2] {210 [µC cm-2] × electrode loading [g of Pt cm-2]}

(5)

Pt/SWNT electrodes show increased characteristic peaks, compared to those of Pt/C, whereas 50 wt % Pt/C + 50 wt % Pt/ SWNT composite electrodes show a remarkably larger peak, reflecting their high surface area. The ECSA of Pt/C, Pt/SWNT, and 50 wt % Pt/C + 50 wt % Pt/SWNT composites has found

(1) Costamagna, P.; Srinivasan, S. J. Power Sources 2001, 102, 242. (2) Haile, S. M. Acta Mater. 2003, 51, 5981. (3) Ghenciu, A. F. Curr. Opin. Solid State Mater. 2002, 6, 389. (4) Mehta, V.; Cooper, J. S. J. Power Sources 2003, 114, 32. (5) Costamagna, P.; Srinivasan, S. J. Power Sources 2001, 102, 253. (6) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (7) Serp, P.; Corrias, M.; Kalck, P. Appl. Catal., A 2003, 253, 337. (8) Che, G. L.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998, 393, 346. (9) Rajesh, B.; Thampi, K. R.; Bonard, J. M.; Xanthopoulos, N.; Mathieu, H. J.; Viswanathan, B. J. Phys. Chem. B 2003, 107, 2701. (10) Mastragostino, M.; Missiroli, A.; Soavi, F. J. Electrochem. Soc. 2004, 151, A1919. (11) Shao, Y.; Yin, G.; Gao, Y.; Shi, P. J. Electrochem. Soc. 2006, 153, A1093. (12) Girishkumar, G.; Hall, T. D.; Vinodgopal, K.; Kamat, P. V. J. Phys. Chem. B 2006, 110, 107. (13) Yu, R. Q.; Chen, L. W.; Liu, Q. P.; Lin, J. Y.; Tan, K. L.; Ng, S. C.; Chan, H. S. O.; Xu, G. Q.; Hor, T. S. A. Chem. Mater. 1998, 10, 718. (14) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J. P.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593. (15) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E.; Schmidt, J.; Talmon, Y. Nano Lett. 2003, 3, 1379. (16) O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y. H.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265. (17) Li, W. Z.; Liang, C. H.; Zhou, W. J.; Qiu, J. S.; Li, H. Q.; Sun, G. Q.; Xin, Q. Carbon 2004, 42, 436. (18) Che, G.; Lakshmi, B. B.; Martin, C. R.; Fisher, E. R. Langmuir 1999, 15, 750. (19) Liu, Z.; Lin, X.; Lee, J. Y.; Zhang, W.; Han, M.; Gan, L. M. Langmuir 2002, 18, 4054. (20) Girishkumar, G.; Vinodgopal, K.; Meisel, D.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 19960. (21) Michelson, E. T.; Chiang, I. W.; Zimmermann, J. L.; Boul, P. J.; Lozano, J.; Liu, J.; Smalley, R. E.; Hauge, R. H.; Margrave, J. L. J. Phys. Chem. B 1999, 103, 4318. (22) Holzinger, M.; Vostrovsky, O.; Hirsch, A.; Hennrich, F.; Kappes, M.; Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002. (23) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760.

16146 J. Phys. Chem. C, Vol. 111, No. 44, 2007 (24) Kongkanand, A.; Kuwabata, S.; Girishkumar, G.; Kamat, P. V. Langmuir 2006, 22, 2392. (25) Leela Mohana Reddy, A.; Shaijumon, M. M.; Ramaprabhu, S. Nanotechnology 2006, 17, 5299. (26) Shaijumon, M. M.; Ramaprabhu, S. Chem. Phys. Lett. 2003, 374, 513. (27) Xing, Y. J. Phys. Chem. B 2004, 108, 19255. (28) Xing, Y.; Li, L.; Chusuei., C. C.; Hull, R. V. Langmuir 2005, 21, 4185. (29) Qian, W.; Liu, T.; Wei, F.; Wang, Z.; Luo, G.; Yu, H.; Li, Z. Carbon 2003, 41, 2613. (30) Kovtyukhova, N. I.; Malouk, T. E.; Pan, L.; Dickey, E. C. J. Am. Chem. Soc. 2003, 125, 9761. (31) Colthup, N. B.; Daly, L. H.; Wiberlay, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: Boston, 1990. (32) Kim, U. J.; Furtado, C. A.; Liu, X.; Chen, G.; Eklund, P. C. J. Am. Chem. Soc. 2005, 127, 15437. (33) Kuznetsova, A.; Douglas, D.; Namneko, V.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. Chem. Phys. Lett. 2000, 321, 292.

Reddy and Ramaprabhu (34) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1999; pp 183 -4. (35) Lu, K. L.; Lago, R. M.; Chen, Y. K.; Green, M. L. H.; Harris, P. J. F.; Tsang, S. C. Carbon 1996, 34, 814. (36) He, Z. B.; Chen, J. H.; Liu, D. Y.; Zhou, H. H.; Kuang, Y. F. Diamond Relat. Mater. 2004, 13, 1764. (37) Savadogo, O.; Essalik, A. J. Electrochem. Soc. 1996, 143, 1814. (38) Girishkumar, G.; Rettker, M.; Underhile, R.; Binz, D.; Vinodgopal, K.; McGinn, P.; Kamat, P. V. Langmuir 2005, 21, 8487. (39) McElrath, K. O.; Smith, K. A.; Bahr, J. L.; Wainerdi, T. J.; Karohl, D. A.; Colbert, D. T.; Miller, M. A.; Oviatt, H. W.; Cline, E. D. U.S. Patent Application, 2004; p A1. (40) Choi, W. B.; Chu, J. U.; Pak, C. H.; Chang, H. (Samsung SDI Co., Ltd., South Korea). U.S. Patent Application, 2004, p A1. (41) Srinivasan, S.; Ticianelli, E. A.; Derouin, C. R.; Redondo, A. J. Power Sources 1988, 22, 359. (42) Sogaard, M.; Odgaard, M.; Skou, E. M. Solid State Ionics 2001, 145, 31.

C Nanocomposite Electrocatalysts for Proton-Exchange

Recommend Documents