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Exfoliated Graphene Separated by Platinum Nanoparticles Yongchao Si and Edward T. Samulski* Department of Chemistry, UniVersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599 ReceiVed May 20, 2008. ReVised Manuscript ReceiVed July 31, 2008
Aggregation of isolated graphene sheets during drying graphene dispersions leads to a loss of its ultrahigh surface area advantage as a two-dimensional nanomaterial. We report a metal nanoparticle-graphene composite with a partially exfoliated graphene morphology derived from drying aqueous dispersions of platinum nanoparticles adhered to graphene. Platinum nanoparticles with diameters spanning several nanometers are adhered to graphene by a chemical route involving the reduction of metal precursors in a graphene dispersion. Face-to-face aggregation of graphene sheets is arrested by 3-4 nm fcc Pt crystallites on the graphene surfaces, and in the resulting jammed Pt-graphene composite, the Pt acts as spacers resulting in mechanically exfoliated, high-surface-area material of potential interest for supercapacitors and fuel cells.
Introduction Graphene, a single-atom-thick sheet of hexagonally arrayed sp2-bonded carbon atoms, is a two-dimensional (2D) macromolecule exhibiting extremely high specific surface area (2600 m2/g).1-3 The properties of graphene rapidly devolve with the number of aggregated sheets, approaching its 3D limiting form, graphite.1,2 In fact, aggregates in excess of ∼10 graphene sheets are considered to be thin platelets of graphite. Although graphene can be isolated by manually peeling off the top surface of small mesas of pyrolytic graphite, large-scale applications require a more general source of material. Stable dispersions of graphene could become a reliable and economically feasible source of an ultrahigh surface area substrate. Submirometer graphene suspensions were recently reported by direct chemical exfoliation of graphite in dimethylformatmide,4 but exfoliation of graphite oxide followed by chemical reduction is the primary route to “graphene,” isolated 2D carbon sheets with varying degrees of residual chemical modification.5,6 In solvated dispersions of graphene prepared by chemical reduction, graphene sheets are separated by solvent apparently stabilized by electrostatic forces associated with ionizable groups introduced during exfoliation.5,6 However, like other dispersions of nanomaterials with high aspect ratios, on drying the graphene dispersion isolated sheets aggregate * Corresponding author. E-mail:
[email protected]. Telephone: (919) 962-1561. Fax: (919) 962-2388.
(1) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451–10453. (2) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183–191. (3) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. A.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282–286. (4) Blake, P.; Brimicombe, P. D.; Nair, R. R.; Booth, T. J.; Jiang, D.; Schedin, F.; Ponomarenko, L. A.; Morozov, S. V.; Gleeson, H. F.; Hill, E. W.; Geim, A. K.; Novoselov, K. S. Nano Lett 2008, 8, 1704– 1708. (5) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101–105. (6) Si, Y.; Samulski, E. T. Nano Lett. 2008, 8, 1679–1682.
and form an irreversibly precipitated agglomerate. As a result, the aggregated graphene behaves no differently than particulate graphite platelets having relatively low surface area. That is, the ultrahigh surface area advantages of 2D graphene is lost, and this loss deleteriously affects the potential applications of graphene, for example, its use in supercapacitors, batteries, fuel cells, composite materials, emissive displays, micromechanical resonators, transistors, and ultrasensitive chemical detectors.1,3,7-11 In sum, the intrinsic ultrahigh surface area is paramount to advancing applications of graphene. However, to exploit this unique feature, the aggregation of graphene sheets has to be minimized or. ideally, prevented when graphene dispersions are dried. As demonstrated with other allotropes of carbon (carbon nanotube and graphite), the metal nanoparticles-carbon composites exhibit promising potential in applications such as chemical sensor, energy storage, catalysis, hydrogen storage, and electromagnetic interference shielding.12-18 Hence a new class of functional composites could be created (7) Novoselov, K. S.; Jiang, Z.; Zhang, Y.; Morozov, S. V.; Stormer, H. L.; Zeitler, U.; Maan, J. C.; Boebinger, G. S.; Kim, P.; Geim, A. K. Science 2007, 315, 1379–1379. (8) Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Nano Lett. 2007, 7, 3499–3503. (9) Geim, A. K.; MacDonald, A. H. Phys. Today 2007, 60, 35–41. (10) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. (11) Castro, E. V.; Novoselov, K. S.; Morozov, S. V.; Peres, N. M. R.; Santos, J. M. B. L. D.; Nilsson, J.; Guinea, F.; Geim, A. K.; Neto, A. H. C. Phys. ReV. Lett. 2007, 99, 216801–216804. (12) Che, G.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998, 393, 346–349. (13) Kauffman, D. R.; Star, A. Nano Lett. 2007, 7, 1863–1868. (14) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Klang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377–379. (15) Huang, Y.; Xu, Z.; Shen, J.; Tang, T.; Huang, R. Appl. Phys. Lett. 2007, 90, 133117–133113. (16) Kong, J.; Chapline, M. G.; Dai, H. AdV. Mater. 2001, 13, 1384–1386. (17) Lee, K. T.; Jung, Y. S.; Oh, S. M. J. Am. Chem. Soc. 2003, 125, 5652–5653. (18) Tang, J.; Jensen, K.; Waje, M.; Li, W.; Itikis, M. E.; Larsen, P.; Pauley, K.; Yan, Y.; Haddon, R. C. J. Phys. Chem. C 2006, 111, 17901– 17904.
10.1021/cm801356a CCC: $40.75 2008 American Chemical Society Published on Web 10/15/2008
Exfoliated Graphene Separated by Pt Nanoparticles
Figure 1. Schematic of graphene sheets and nanoparticle-modified graphene sheets in its dispersion and dry state. (a) Graphene sheets, isolated graphene sheets exist in its dispersion; in the dry state, graphene sheets aggregate and stack into a layered structure like graphite. (b) Nanoparticle modified graphene sheets; isolated sheets exist in its dispersion; in the dry state the nanoparticles with diameters spanning several nanometers act as nanoscale spacers to increase the interlayer spacing between graphene sheets, thus making both faces of graphene accessible.
from 2D graphene with metal nanoparticles. Adhering metal nanoparticles to the 2D graphene sheets could inhibit the aggregation of graphene sheets and result in a mechanically jammed, exfoliated graphene agglomerate with very high surface area. To demonstrate this, metal nanoparticles are synthesized on graphene by a chemical route involving the reduction of metal precursors in a graphene dispersion. As shown in the schematics of Figure 1a, when an aqueous dispersion of isolated graphene sheets is dried, aggregates having graphitic stacks with a very small interlayer spacing are formed. However, the introduction of nanoparticles into the dispersion of graphene sheets impedes the formation of a stacked graphitic structure (Figure 1b). By functioning as a “spacer”, the metal nanoparticles increase the distance between the graphene sheets to several nanometers, thereby making both the faces of graphene accessible. As a result, the nanoparticle spacers ensure that the high specific surface area as well as other unique properties exhibited by 2D graphene is retained in its dry state. In this paper we show that metal nanoparticles with diameters spanning several nanometers inhibit face-to-face aggregation of graphene sheets yielding a high specific surface area metal-graphene composite with potential applications in supercapacitors and catalysis. We illustrate this by preparing a composite of platinum nanoparticles on sparingly sulfonated graphene (Pt-graphene). We also demonstrate a potential application for this new graphene morphology with preliminary data using the Pt-graphene as the catalyst for oxygen reduction in a fuel cell configuration. Experimental Section Synthesis of Pt-Graphene Composite. Platinum nanoparticles are deposited on graphene sheets by the chemical reduction of
Chem. Mater., Vol. 20, No. 21, 2008 6793 chloroplatinic acid (H2PtCl6) with methanol in the presence of the surfactant 3-(N,N-dimethyldodecylammonio) propanesulfonate (SB12). The details of synthesis of water soluble graphene can be found in ref 6. In a typical procedure, 60 mg of chloroplatinic acid hexahydrate (Sigma-Aldrich) in 4 g of water (pH ) 7, after neutralized with sodium carbonate) was added into 44 g of aqueous dispersion of graphene that contains 20 mg of graphene. After 39 mg of SB12 (Aldrich) in 12.5 g of methanol was added into the mixture, the pH of the reaction mixture was adjusted to ∼7 with sodium carbonate. The reaction mixture was kept at 80 °C for 90 min under constant stirring. A few drops of dilute sulfuric acid (1 N) solution were then added into the mixture to precipitate the Pt-graphene. The product was isolated by filtration, and the filtrate is colorless if all chloroplatinic acid has been reduced. After rinsing with water and methanol thoroughly, the Pt-graphene thus prepared was dried at 70 °C for 15 h. For comparison, aggregated graphene sheets were also prepared by drying an aqueous dispersion at 70 °C for 15 h. Material Characterization. Atomic force microscopy (AFM) images of graphene deposited from a dilute aqueous dispersion on a freshly cleaved mica surface were taken with a Nanoscope III in tapping mode using a NSC14/no Al probe (MikroMash, Wilsonville, Oregen). After sonication for 5 min with a Fisher Scientific FSH20H ultrasonic bath cleaner, a droplet of graphene dispersion (∼0.01 mg/mL) was cast onto a freshly cleaved mica surface, followed by drying at room temperature overnight. Scanning electron microscopy (SEM) characterization of Pt-graphene (or graphene after being dried at 70 °C for 15 h) was performed with a FEI Helios 600 Nanolab Dual Beam System. X-ray diffraction (XRD) of dried Pt-graphene (or graphene powder) was performed with a Rigaku Multiflex powder diffractometer with Cu radiation between 5° and 90° with a scan rate of 0.5°/min and an incident wavelength of 0.154 056 nm (Cu KR). Transmission electron microscopy characterization of Pt-graphene was performed using a Philips CM12 TEM with an accelerating voltage of 100 kV. After being sonicated for 5 min with a Fisher Scientific FSH20H ultrasonic bath cleaner, a droplet of aqueous Pt-graphene dispersion (∼0.02 mg/mL) was cast onto a TEM copper grid followed by drying overnight at room temperature. A thin slice of dried Pt-graphene film was prepared from the solid film of dried Pt-graphene sheets imbedded in epoxy and subsequently microtomed. The Pt-graphene film (ca. a few micrometers thick) was prepared by drop-casting the concentrated aqueous dispersion (3-4 mg/mL) onto a glass slide at room temperature. The sample was kept at room temperature overnight followed by drying in the oven for 15 h. Then the black film was peeled off the glass for microtoming. Surface area measurement of dried graphene (or Pt-graphene) was performed with a Quantachrome Autosorb-1 surface area and pore size analyzer using the BET method with nitrogen gas adsorption. After being dried at 70 °C for 15 h in the oven, the samples were outgassed at 3 mTorr and 150 °C for 8 h before the measurement. Electrochemical Characterization. Cyclic voltammetry of dried Pt-graphene (or graphene) on a glassy carbon electrode was performed with a BAS 100B Potentiostat. Data were acquired at a scan rate of 50 mV/s between 0.02-1.2 V in 0.5 N H2SO4 with a three-electrode configuration; platinum wire is used as counter electrode and an Ag/AgCl electrode as the reference electrode. Electrodes for the fuel cell test were prepared by painting an ink of Nafion and the catalyst in methanol/water (1:1 by weight) onto a single-sided Elat gas diffusion substrate. Pt-graphene is used on the cathode, and 40 wt % Pt/C (Alfa) is on the anode. The Nafion content in the catalyst ink is 30 wt % for both Pt-graphene and Pt/C, and the Pt loading is ∼0.4 mg/cm.2 A membrane electrode assembly consisting of one anode, one cathode, and a Nafion112
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Figure 2. (a) Image of graphene from its dilute aqueous dispersion on freshly cleaved mica visualized by AFM. (b) Zooming AFM image of a graphene sheet in (a); the height of graphene sheet measured by AFM between two arrows is 1 nm. (c) TEM image of platinum separated graphene sheet from its aqueous dispersion; platinum nanoparticles appear as dark dots. (d) Powder X-ray diffractograms of dried graphene sheets and platinum separated graphene sheets (Pt-graphene).
membrane was tested with a Scribner 890B fuel cell test stand at 65 °C and a constant total pressure of 101 kPa. H2 and O2 were humidified at 65 °C before entering the fuel cell, and their flow rate was 50 cm3/min.
Results and Discussion Recently we developed a facile and scalable preparation of aqueous solutions of sparingly sulfonated graphene starting from oxidized graphite.6 The graphene is readily dispersed in the form of single 2D sheets in water and forms a stable dispersion due to the presence of covalently attached sulfonic acid groups. Evaporated contiguous films from the graphene aqueous dispersion exhibit a high electrical conductivity (1250 S/m), suggesting the presence of an extensive conjugated hexagonal network of sp2 carbons in graphene. Figure 2a,b shows the atomic force microscopy (AFM) images of isolated graphene sheets evaporated from its dilute aqueous dispersion on freshly cleaved mica. The graphene has lateral dimensions ranging from several hundred nanometers to several micrometers and a thickness of 1 nm, which is characteristic of a fully exfoliated carbon sheet3 confirming that the dispersion is comprised of isolated graphene sheets. (Isolated small fragments of graphene and folded wrinklelike features are observed on the surface of some graphene sheets.) The graphene sheets used to prepare metal-graphene composites have an aspect ratio ranging from several hundred to several thousand. Many types of metals can be deposited on graphene sheets in the form of nanoparticles to impart new functionality to
this increasingly popular 2D nanomaterial. For example, the combination of ferromagnetic elements like Ni, Co, and Fe with graphene could create composites with the capability of shielding electromagnetic interference, and Pd (or Au)graphene composites could be used to make ultrasensitive chemical sensors to detect hydrogen (or NO gas). Here we use platinum in a proof of concept demonstration. However, the same methodology applies to metals like Ag, Au, Cu, Pd, Ni, Al, Co, Cr, and Fe; all of these metals can be deposited on isolated graphene sheets and serve as a spacer when the graphene dispersion is dried. To prepare the platinum-graphene composite, platinum nanoparticles are deposited on graphene sheets by the chemical reduction of chloroplatinic acid (H2PtCl6) with methanol in the presence of the surfactant 3-(N,N-dimethyldodecylammonio) propanesulfonate (SB12). The lightly sulfonated graphene forms stable dispersion in water at reasonable concentrations (2 mg/mL) in the pH range of 3-10. Direct addition of H2PtCl6, a strong acid, causes the significant change in the pH of the dispersion, and coagulation of aggregated graphene sheets occurs. However, this can be avoided by neutralization of H2PtCl6 with sodium carbonate before its addition. Isolated graphene sheets can exist in a mixture of water/methanol (3:1 by volume), thus making it possible to reduce the platinum precursor with methanol in the presence of graphene. The function of the surfactant SB12 is to control the size of the platinum nanoparticles and also prevent aggregation.19,20 To be effective as spacers, the nanoparticles have to adhere on the
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Figure 3. Electrochemical properties of dried graphene sheets and platinum separated graphene sheets (Pt-graphene). (a) Cyclic volammogram of graphene on a glass carbon electrode. (b) Cyclic volammogram of Pt-graphene on a glass carbon electrode; data were acquired at a scan rate of 50 mV/s between 0.02-1.2 V in 0.5 N H2SO4 with a three-electrode configuration; platinum wire is used as counter electrode and Ag/AgCl electrode is the reference electrode. (c) Performance curve of a PEM fuel cell using Pt-graphene on the cathode; fuel cell is operated at 65 °C using H2 on the anode and O2 on the cathode, reactants are humidified at 65 °C with a constant total pressure of 101 kPa.
surface of graphene as uniformly as possible. The nanopaticles’ effectiveness as a spacer to maintain separate graphene sheets would be compromised by the aggregation of nanoparticles, so the surfactant SB12 is essential to prevent the aggregation of platinum nanoparticles during reduction. We found that a stoichiometric ratio of 1 surfactant to 1 platinum precursor inhibits aggregation of platinum nanoparticles during reduction. Figure 2c shows a TEM image of platinum nanoparticles supported on graphene sheets. In this image, platinum nanoparticles appear as dark dots with a diameter of 3 to 4 nm on a lighter shaded substrate corresponding to the planar graphene sheet. The nanoparticles cover the graphene sheets with an interparticle distance ranging from several nanometers to several tens of nanometers, occupying only a very small portion of the surface of the graphene sheet. In Figure 2d, powder X-ray diffraction of the Pt-graphene composite exhibits the characteristic face-centered cubic (fcc) platinum lattice: diffraction peaks at 39.9° for Pt(111), 46.3° for Pt(200), 67.7° for Pt(220), and 81.4° for Pt(311)21 confirm that the platinum precursor H2PtCl6 has been reduced to platinum by methanol. The diffraction peak for Pt(220) is used to estimate the platinum crystallite size since there is no interference from other diffraction peaks. The Scherrer equation yields an average crystallite size of Pt (normal to Pt(220)) on graphene of 4.2 nm, which is consistent with the TEM results. By assuming (19) Li, X.; Hsing, I.-M. Electrochim. Acta 2006, 51, 5250–5258. (20) Bonnemann, H.; Braun, G.; BrijouxW.; Brinkmann, R. S. T. A.; Seevogel, K.; Siepen, K. J. Organomet. Chem. 1996, 520, 143–162. (21) Carmo, M.; Paganin, V. A.; Rosolen, J. M.; Gonzalez, E. R. J. Power Sources 2005, 142, 169–176.
that platinum nanoparticles are spherical, we calculate that separated particles of this size correspond to a Pt surface area of 66 m2/g. The presence of platinum crystallites in the composite is further confirmed by cyclic voltammetry measurement. Figure 3a,b shows the voltammograms of dried graphene and Pt-graphene, respectively, on glassy-carbon electrodes in 0.5 N sulfuric acid between 0.02 and 1.2 V. Pt-graphene shows the characteristic hydrogen absorption/desorption on platinum between 0.05 and 0.3 V,22 confirming the presence of platinum on graphene. In contrast, there is no noticeable hydrogen desorption between 0.05 and 0.3 V in the voltammogram of graphene as shown in Figure 3a. Between 0.02 V and 1 V, graphene behaves like a capacitor, and the oxidation/reduction peaks (0.5-0.6 V) associated with the surface oxides (e.g., phenolic and ester groups) are hardly noticeable; they are typically present on the surface of carbon black.22,23 If the metal nanoparticles act as nanoscale spacers and increase the interlayer spacing between graphene sheets, the accessible surface of graphene in the dry Pt-graphene agglomerate should be comparable to exfoliated graphite. The surface area of dried graphene and dried Pt-graphene were measured by nitrogen gas absorption. Residual water must be removed before the absorption measurement, and this accomplished in two stages: (1) drying at 70 °C for 15 h; (2) drying at 150 °C for 8 h. Isolated graphene sheets have (22) Reddy, A. L. M.; Ramaprabhu, S. J. Phys. Chem. C 2007, 111, 16138– 16146. (23) Kinoshita, K.; Bett, J. A. S. Carbon 1973, 11, 403–411.
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Figure 4. Images of dried graphene sheets and platinum separated graphene sheets (Pt-graphene). (a) SEM image of dried graphene sheets, layered structure is observed. (b) SEM image of dried Pt-graphene sheets. (c) TEM image of thin slice of dried Pt-graphene sheets, black dots are platinum nanoparticles and lighter background are graphene sheets.
a theoretical surface area of 2600 m2/g, so decreases in measured surface area are an indicator of the extent to which the graphene sheets aggregate face-to-face. Dried graphene exhibits a BET value of 44 m2/g, which is significantly lower than theoretical predictions for isolated graphene sheets. This value indicates that on average, about 60 graphene sheets aggregate into graphitic stacks that, in turn, inhibit intercalation of nitrogen. In contrast, the dried Pt-graphene composite exhibits a BET value of 862 m2/g (after correcting for the contribution from the surface area of the Pt nanoparticles, ∼66 m2/g calculated from X-ray diffraction). The roughly 20 times larger accessible surface area in condensed Pt-graphene suggests that face-to-face stacking is effectively limited to ∼3 aggregated sheets per stack. We attribute this enhanced surface area in Pt-graphene to the arrested aggregationsmechanical exfoliation in the jammed network of 3D Pt nanoparticles and 2D graphene sheets. This idea of mechanically exfoliated graphene is corroborated by X-ray diffraction: Dried graphene shows a broad diffraction peak for C(002) at 2θ ) 25° similar to that of carbon black (Figure 2d)24 and is interpreted in terms of short-range order in stacked graphene sheets. Similar short-
range order is observed in dried graphene dispersions. But in Pt-graphene the diffraction peak for C(002) is negligible indicating that significant face-to-face stacking is absent. Figure 4 a shows the SEM image of aggregated graphene in its dry state. The very large anisometric shapes of the two-dimensional graphene causes coplanar alignment of sheets into layered aggregates with an interlayer spacing close to that of graphite. The surface of the aggregated sheets is fairly smooth, and a layered structure is observed at the edge of the agglomerates. By contrast, deposition of several nanometer diameter particles on the 2D graphene sheets impedes stacking, and the SEM of the Pt-modified, mechanically exfoliated layered structure (Figure 4 b) exhibits nanoscale textures indicative of a much rougher surface. The Pt nanoparticles on the graphene surface act as nanoscale spacers and increase the spacing between adjacent carbon sheets preventing face-to-face van der Waals contact between neighboring graphene sheets and a corresponding consolidated graphitic structure. As a result, both surfaces of graphene sheets can be accessed, and the available surface of Pt-modified graphene in the dry state is significantly increased.
(24) Su, F.; Zeng, J.; Bao, X.; Yu, Y.; Lee, J.; Zhao, X. S. Chem. Mater. 2005, 17, 3960–3967.
The nanoparticles are not observed on the graphene surface with SEM, but their presence is confirmed using TEM
Exfoliated Graphene Separated by Pt Nanoparticles
(Figure 4 c). To get TEM images, a thin slice of dried Pt-graphene film was prepared from dried Pt-graphene sheets imbedded in epoxy and subsequently microtomed. The image of Pt-graphene agglomerate is dramatically different from the isolated Pt-graphene sheets (Figure 2c) in terms of density of particles. In Figure 4c Pt nanoparticles from multiple layers of graphene sheets in the agglomerate are observed, and this apparent increase in Pt nanoparticle density is indicative of partially exfoliated graphene sheets separated by Pt nanoparticles in the Pt-graphene agglomerate, that is, a layered structure in the schematic of Figure 1b. Cyclic voltammetry is one technique used to estimate the surface area of carbon materials.25 The surface area of graphene can be derived from the capacitance of graphene electrode in contact with a liquid acid, which provides another datum to support the findings of the BET measurement. By dividing the double layer charge/discharge current by the potential scan rate, the capacitance of graphene assessable to liquid electrolyte can be determined.22 The dried graphene gives a capacitance of 14 F/g. In contrast, Pt-graphene has a significantly larger capacitance of 269 F/g. In aggregated graphene, only the outer surface of the agglomerate can form an electrochemical double layer with the electrolyte and thereby contribute to the generation of a double layer charge/discharge current. Since the capacitance is proportional to the surface area accessible to the liquid electrolyte, it is not surprising that aggregated graphene sheets have a small capacitance. Assuming a value for the double layer capacitance of carbon (25 µF/cm2 at 0.35 V),25 the surface area of Pt-graphene sheets is estimated to be 1078 m2/g, which is 18 times higher than that of aggregated graphene. The magnitude of the increased surface area in Pt-graphene measured by cyclic voltammetry is consistent with what is observed using the BET surface area measurement. The platinum nanoparticles function as nanospacers and result in a mechanically arrested aggregate of exfoliated graphene that is accessible to the electrolyte. Moreover, this high-surface-area composite with good electrical conductivity could be a promising electrode material for supercapacitors. The capacitance (surface area) of graphene measured in the three-electrode cyclic voltammetry is merely used for the purpose of corroborating the BET surface area measurement
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of the Pt-graphene composite. However, to critically evaluate potential practical applications (e.g., supercapacitors), the capacitance of a graphene electrode measured in a twoelectrode cell would be more accurate than the three-electrode setup since the cell configuration affects the measurement capacitance of electrode material.26 One potential application for Pt-graphene composite is in fuel cell electrodes. In current fuel cell technology, platinum or platinum alloys are dispersed in the form of nanoparticles onto carbon black to electrocatalyze hydrogen oxidation or oxygen reduction. 2D graphene sheets promise a superior support material for a high-surface-area platinum catalyst, a major challenge in advancing fuel cell technology. Electrodes using Pt-graphene were prepared and tested for oxygen reduction on the cathode in a fuel cell. As shown in Figure 3c, the fuel cell exhibits good open-circuit voltage (∼0.99 V with H2 on the anode and O2 on the cathode). When the fuel cell is tested at 65 °C, the cell voltage is 0.65 V at a current density of 300 mA/cm.2 The initial test result indicates that Pt-graphene composites are electrochemically active and catalyze oxygen reduction in a fuel cell environment. With further optimization of the electrode structure and the Pt-graphene catalyst composition, fuel cell operation is anticipated to improve. Conclusions We propose a Pt-graphene composite with a novel graphene morphology derived from drying aqueous dispersions of platinum nanoparticles adhered to graphene. Faceto-face aggregation of graphene sheets is arrested by 3-4 nm fcc Pt crystallites on the graphene surfaces, and in the resulting jammed Pt-graphene composite, the Pt acts as spacers resulting in mechanically exfoliated, high-surfacearea material. Metal nanoparticle-graphene composites using other metals can be prepared by the strategy similar to that used to make Pt-graphene. The resulting highly expanded lamellar structure retains attributes of the 2D graphene hexagonal carbon network with a high specific surface area, a promising composite for supercapacitors and fuel cells. Acknowledgment. We acknowledge Dr. Wallace Ambrose for help in TEM sample preparation and Dr. Nick Zafiropoulos for help with surface area measurement. CM801356A
(25) Kocha, S. S. Handbook of fuel cells; fundamentals, technology and applications; Vielstich, V., Gasteiger, H. A., Lamm, A., Ed.; John Wiley & Sons: Hoboken, NJ, 2003; Vol. 3, pp 538-565.
(26) Khomenko, V.; Frackowiak, E.; Beguin, F. Electrochim. Acta 2005, 50, 2499–2506.
