NANO LETTERS
Fullerene-Based Carbon Nanostructures for Methanol Oxidation
2004 Vol. 4, No. 3 415-418
K. Vinodgopal,†,‡ Mehul Haria,‡ Dan Meisel,†,§ and Prashant Kamat*,†,| The Radiation Laboratory and The Center for Molecularly Engineered Materials, UniVersity of Notre Dame, Notre Dame, Indiana 46556, and Department of Chemistry, Indiana UniVersity Northwest, Gary, Indiana 46408 Received November 14, 2003; Revised Manuscript Received December 18, 2003
ABSTRACT Films of C60 clusters were electrophoretically deposited on optically transparent electrode surfaces. These C60 films constitute a new class of carbon electrodes with properties that differ from graphite and diamond electrodes. The electrophoretically deposited C60 cluster film is highly porous and is quite stable to oxidative potentials. Hence C60 film provides an electrochemical window to carry out oxidation processes. Upon electrodeposition of platinum particles, these nanostructured carbon films show remarkable activity toward methanol oxidation. The dependence of methanol oxidation on the amount of platinum and C60 in a half-cell reaction demonstrates the role of fullerene nanoclusters as new form of carbon support.
Introduction. Fuel cells in general and direct methanol fuel cells (DMFC) in particular are attracting much attention because of their potential use in powering portable electronic devices.1-5 Most DMFC anodes where the oxidation of methanol is carried out are based on electrocatalysts in which metal nanoparticles such as a 1:1 mixture of platinum and ruthenium are deposited on electrically conducting highsurface-area carbon films. Developing miniaturized fuel cells requires design of submicron-sized carbon support electrodes in a controlled way. Carbon nanotubes and carbon nanoclusters are attractive candidates for designing such miniaturized electrodes. Recent demonstration of platinum clusters supported on single-walled and multiwalled carbon nanotubes as highsurface-area carbon supports has opened up new approaches for developing electrode materials for DMFC.3,6-10 Lukehart and co-workers11 have prepared Pt-Ru/graphite nanofiber composites that show appreciably higher activity for methanol oxidation. However, the synthesis, metal loading, and deposition of these nanotubes to produce a working anode remains a nontrivial task. Recent efforts to assemble threedimensional arrays of fullerene (C60 and analogues) clusters have provided new avenues to design high-surface-area electrode materials.12,13 Nanostructured fullerene films cast on a conducting surface constitute a new class of carbon electrodes with properties that differ from graphite and * Corresponding author. E-mail:
[email protected]. † Radiation Laboratory, University of Notre Dame. ‡ Department of Chemistry, Indiana University Northwest (kvinod@ iun.edu). § Department of Chemistry, University of Notre Dame, (
[email protected]). | Department of Chemical and Biomolecular Engineering, University of Notre Dame (
[email protected]). 10.1021/nl035028y CCC: $27.50 Published on Web 02/03/2004
© 2004 American Chemical Society
diamond electrodes. These films are quite stable to oxidative potentials (Eox ) 1.9 V vs SCE)14 and hence provide a wide electrochemical window to carryout oxidation processes. The fullerene films are known to exhibit electrocatalytic15 and electrochemical sensing16 properties. The high surface area of these fullerene films makes them particularly useful for fuel cell applications. We now demonstrate the feasibility of using electrophoretically deposited C60 nanoclusters as high-surface-area carbon supports for methanol oxidation. The nanostructured carbon fullerene film serves as a conductive dispersing support for depositing the Pt nanoparticles and promotes catalytic activity toward methanol oxidation. The half-cell reactions involving methanol oxidation at platinized fullerene clusters demonstrate for the first time the use of molecular clusters in designing fuel cell electrodes. Experimental Section. High purity C60 was obtained from SES Research. Clusters of C60 were prepared by injecting 40 µL of a 1 mM solution of C60 in toluene into 1.4 mL of acetonitrile to obtain a brown cluster suspension. The C60 cluster suspension in acetonitrile was transferred to a small cell in which two optically transparent electrodes (OTE) were kept at a distance of ∼6 mm by a Teflon spacer. The OTEs were cut from an electrically conducting glass sheet (TEC Glass) obtained from Pilkington. A DC voltage of 100 V was applied between the two electrodes using a Fluka 415 DC power supply. Deposition of a light yellowish brown film on the electrode surface connected to the positive terminal confirms the deposition of the film on the OTE. The film was then dried in air. These electrophoretically
deposited films were quite robust and did not deteriorate during electrochemical measurements. Platinum nanoparticles were loaded on the OTE/C60 films by electrochemical reduction from a solution of Pt(IV). A 1 cm2 area of the fullerene film on the OTE was exposed to an aqueous solution of hexachloroplatinic acid, and reduction was carried out at a potential of -350 mV versus SCE. Electrolysis was carried out in the presence of 1 M LiClO4 as a supporting electrolyte using a BAS 100 electrochemical analyzer. The amount of platinum deposited as determined from coulometry in a typical experiment is ∼52 µg/cm2. Cyclic voltammetry measurements were carried out using the BAS100 analyzer and a three-electrode cell consisting of the C60 electrode as the working electrode, platinum counter electrode, and SCE reference electrode. Absorption spectra were recorded using a Shimadzu 3101 PC spectrophotometer. AFM images were obtained using a Digital Instruments (DI) Nanoscope III in the tapping mode. An etched silicon tip was used as the AFM probe for imaging the C60 films on the OTE in air. Results and Discussion. Fullerenes and their derivatives form clusters in mixed solvents arising from the strong hydrophobic interactions between the fullerene units.13 These optically transparent clusters typically have diameters in the range of 50 and 150 nm. High-surface-area fullerene films with well controlled morphology are prepared by electrophoretic methods from a solution of C60 clusters suspended in a solvent mixture of 3:1(v/v) acetonitrile and toluene. Although C60 molecules are uncharged, they attain a net negative charge when dispersed in a polar solvent and subjected to an electric field. The details of the electrophoretic deposition of C60 clusters have been presented earlier.15 The clusters deposited on an OTE exhibit a yellowish-brown color. The thickness of the film is controlled by changing the concentration of the cluster solution, varying the applied voltage, or altering the deposition time. Electrophoretically deposited C60 clusters are electroactive, capable of promoting redox processes at controlled potentials.15,17-19 Platinum metal particles were deposited on OTE/C60 electrodes using an electrolytic reduction process. The deposition was carried out by immersing the electrode in an aqueous solution of 10 mM H2PtCl6 and applying a constant potential of -350 mV versus SCE. At this negative potential PtCl62- ions are reduced and a deposit of Pt clusters can be seen. Figure 1 compares the AFM image of an OTE/C60 electrode before (Figure 1A) and after deposition of platinum (Figure 1B). The AFM image of the OTE/C60 electrode (Figure 1A) shows an assembly of 50-100 nm diameter clusters, thus yielding a high surface area for the nanostructured carbon electrode. Comparison of the two AFM images indicates that the larger size platinum crystallites (100-150 nm) are being deposited on the C60 cluster film. Thus, we can qualitatively infer that the platinum deposited on the C60 clusters is well dispersed and provides nanostructured morphology. Note that only the topology is characterized in these experiments; we were unable to differentiate the underlying C60 clusters from the surface deposited platinum 416
Figure 1. (A) Tapping mode AFM image of C60 clusters (54 µg/ cm2) deposited by electrophoresis on OTE. (B) Tapping mode AFM image of platinum nanocrystallites electrochemically deposited on OTE/C60 films. The electrode used was the same as in (A) but the image is of a different spot on the same electrode. The amount of Pt deposited was determined by coulometry to be 54 µg/cm2.
Figure 2. Cyclic voltammogram of 2M methanol oxidation in 1 M H2SO4 at (a) OTE/C60, (b) OTE/Pt, and (c) OTE/C60/Pt. Scan rate was 20 mV/s. The amount of C60 deposited was determined to be 6 µg/cm2 while the amount of Pt deposited in both (b) and (c) was determined by coulometry to be 54 µg/cm2.
particles. Nonetheless, it is evident from Figure 1B that the Pt is dispersed as small particles rather than in the form of a smooth film. The electrochemical activity of the platinum-coated OTE/ C60 electrode was tested for methanol oxidation in a halfcell reaction. The cyclic voltammetry experiments were performed in 1 M H2SO4 solution containing 2 M methanol. For comparison, a bare OTE (without C60) coated with platinum using the same electrodeposition conditions was also employed. In Figure 2 the cyclic voltammograms for methanol oxidation at the C60 film on OTE (curve a) and platinized OTE (curve b) are compared with the OTE/C60/ Pt (curve c). C60 cluster film alone deposited on OTE is unable to oxidize methanol. However, deposition of Pt islands on C60 film enables the methanol oxidation. While the platinum deposited on OTE is able to oxidize methanol, it is evident that the oxidation current observed with the C60/ Pt combination is nearly double the current on bare platinum. The typical methanol oxidation in a cyclic voltammogram is observed with a prominent irreversible current peak around 650 mV vs SCE. The primary products formed at the electrode yield a secondary oxidation wave in the return scan. Nano Lett., Vol. 4, No. 3, 2004
Figure 3. Absorption spectra of C60 films deposited by electrophoresis from solution on OTE (prior to Pt deposition). The amount of C60 deposited on the film is controlled by varying the deposition time.
Figure 4. Cyclic voltammogram of 2 M methanol oxidation in 1 M H2SO4 at OTE/C60/Pt shown as a function of increasing amounts of C60 on the OTE. Scan rate was 20 mV/s. The amount of Pt deposited was determined by coulometry in each case to be 54 µg/ cm2.
Such cyclic voltammogram behavior has been used as a probe to investigate the effectiveness of carbon support electrodes in a half-cell reaction. At potentials greater than 650 mV, methanol is oxidized to CO2 via a dual-path mechanism consisting of adsorbed CO and non-CO reactive intermediates (reaction 1 and 2).20,21 CH3OH f CO + 4H+ + 4e
(1)
CH3OH + H2O f CO2 + 6H+ + 6e
(2)
Because of the catalyst poisoning effect, a non-CO reaction pathway is preferred to drive the electrochemical oxidative process. Osawa and co-workers22,23 have recently pointed out that formate is a major reactive intermediate in the non-CO pathway of the reaction mechanism for MeOH electrooxidation. Efforts were made to optimize both the C60 and the platinum coverages so that the methanol oxidation is maximized. For varying the coverage of C60, different concentration of C60 clusters in acetonitrile/toluene solution were employed prior to electrophoretic deposition. The time of electrophoretic deposition was extended until all the C60 clusters were deposited on the OTE surface. Thicker films exhibit dark brown coloration as compared to yellow coloration of thin films. The absorbance of the C60 in toluene at 330 nm ( ) 50 000 M-1 cm-1) was used to determine the net amount of C60 deposited. The corresponding absorption spectra of the fullerene films are shown in Figure 3. The absorbance of the C60 film increases linearly with increasing thickness of C60 cluster film. These electrodes were then immersed in an aqueous solution of H2PtCl6 and held at potential of -350 V vs SCE. By controlling the net charge that was used to reduce Pt(IV) (0.1 coulomb) the same amount of Pt was deposited on all these OTE/C60 electrodes. The cyclic voltammetry of the C60 electrodes with different coverages but with the same amount of platinum for the methanol oxidation is shown in Nano Lett., Vol. 4, No. 3, 2004
Figure 5. Methanol oxidation currents monitored at 660 mV plotted as a function of increasing amounts of platinum deposit on OTE/C60. The amount of C60 deposited on OTE was maintained constant at 75 nmol/cm2. Scan rate was 20mV/s.
Figure 4. The peak current for methanol oxidation increases with increasing coverage of C60 on the electrode surface. At the highest coverage of 75 nmol/cm2, a maximum anodic current of 1.8 mA/cm2 was obtained. Multilayer deposition of C60 clusters is expected at higher coverages. These results further demonstrate the role of C60 clusters as an effective support in promoting the methanol oxidation at Pt crystallites. The loading of Pt on the C60 electrode was optimized by increasing the deposition time at a controlled potential. The net charge consumed during the reduction of (PtCl6)2- was taken as a measure of the amount of Pt deposited. Figure 5 shows the current for oxidation of methanol at a OTE/C60/ Pt electrode as a function of platinum coverage. With increased deposition of platinum, an enhanced current for methanol oxidation is clearly observed. For about 100 µg of Pt/cm2, we obtain a maximum current of 3.6 mA/cm2 and a C/Pt ratio of 1:2. Further optimization is necessary to 417
minimize the use of Pt while maintaining the high methanol oxidation efficiency. The half cell reaction studied in this investigation demonstrates the ability to employ molecular clusters of fullerenes as a new form of carbon support for inducing electrochemical oxidation of methanol. The enhancement in the oxidation current observed at the platinized fullerene film is indicative of the important role that fullerene clusters play toward methanol oxidation. The origin of this beneficial effect of fullerene film is attributed to the fact that these fullerene films provide a high surface area for the electrode morphology. Furthermore, earlier studies have shown their ability to catalyze the oxidation of redox couples such as ferrocene. We are currently investigating whether such a catalytic role contributes to the observed enhancement in the methanol oxidation as well. In any event, the ease of preparing nanostructured C60 films is an important consideration for developing miniaturized fuel cell electrodes. Conclusions. The electrophoretic deposition of C60 clusters provides a simple and convenient method to design nanostructured carbon supports. These films are electrochemically active and can readily be modified with deposits of Pt to promote methanol oxidation. The results discussed in the present report demonstrate the ability of C60 clusters to serve as a new type of carbon supports for fuel cell applications. Further experiments are underway to improve the performance of OTE/C60/Pt electrodes by modifying the deposition methods and to determine the underlying mechanism for the improved performance. The process is also potentially useful for developing nanometer-submicron scale carbon electrodes on which controlled metallic catalysts can be deposited. Acknowledgment. This work was supported by the U.S. Army CECOM RDEC through Agreement DAAB07-03-3K414. Such support does not constitute endorsement by the U.S. Army of the views expressed in this publication. This
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NL035028Y
Nano Lett., Vol. 4, No. 3, 2004
