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Electrochimica Acta 56 (2011) 7064– 7070

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta

icrowave-assisted synthesis of graphene-supported Pd1Pt3 nanostructures andheir electrocatalytic activity for methanol oxidation

ui Zhang ∗, Xiaoqing Xu, Piao Gu, Chunyun Li, Ping Wu, Chenxin Cai ∗

iangsu Key Laboratory of New Power Batteries, Jiangsu Key Laboratory of Biofunctional Materials, Laboratory of Electrochemistry,ollege of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210097, PR China

r t i c l e i n f o

rticle history:eceived 21 March 2011eceived in revised form 28 May 2011ccepted 30 May 2011vailable online 12 June 2011

a b s t r a c t

This work reports the development of a facile, one-step microwave heating method for the synthesisof graphene-supported Pd1Pt3 (Pd core/Pt shell) electrocatalysts. The structure and composition of thesynthesized nanocomposites were characterized via transmission electron microscopy and atomic forcemicroscopy as well as energy-dispersive X-ray, X-ray powder diffraction, FTIR, and Raman spectroscopies.Using voltammetry, the electrocatalytic characteristics of the graphene-supported Pd1Pt3 nanostructures

eywords:icrowave methodraphened1Pt3 nanostructuresethanol oxidation

were evaluated for the oxidation of methanol as a model reaction. The results show that the introductionof graphene increases the electrochemically active surface area of the Pd1Pt3 nanostructures. As comparedto the unsupported Pd1Pt3 electrocatalyst, the graphene-supported Pd1Pt3 electrocatalyst exhibited 80%enhancement of the electrocatalytic specific mass current for the oxidation of methanol. This method mayserve as a general, facile approach for the synthesis of graphene-supported bimetallic PtM electrocatalysts

of th

lectrocatalytic activity with increased utilization

. Introduction

Currently, Pt or Pt-based bimetallic (PtM) catalysts are the mostidely used electrocatalysts in direct methanol fuel cells (DMFCs)

1–5]. However, pure Pt catalysts usually suffer from certain disad-antages, such as high cost and poisoning by CO-like intermediatepecies formed during the methanol oxidation reaction (MOR) [3,6].ombining Pt with another metal in a bimetallic catalyst is one ofhe most effective ways to resolve these problems. The introductionf another metal can significantly reduce the required Pt loading,odify the strength of surface adsorption by changing the d-band

tructure of Pt, and overcome the poisoning effects of adsorbed CO-ike species [3–11]. Among the reported bimetallic PtM (M = Ru, Pd,r, etc.) nanocatalysts, binary PtRu is the most widely accepted elec-rocatalytic material for the MOR at the DMFC anode because theoisoning species (CO species formed on the Pt sites) can be oxi-ized to CO2 by active oxygen atoms formed on the Ru [4,9–11].owever, PtRu catalysts exhibit poor stability in acid medium due

o the facile electrochemical dissolution of Ru at high potentialabove 0.5 V vs. SCE) [4].

A number of other catalytic materials have been studied in com-

ination with Pt. Of these materials, PtPd bimetallic catalysts haveaptured the interest of many researchers because they are con-iderably less expensive than pure Pt catalysts and exhibit unique

∗ Corresponding authors. Tel.: +86 25 85891780; fax: +86 25 5891767.E-mail addresses: zhangh@njnu.edu.cn (H. Zhang), cxcai@njnu.edu.cn (C. Cai).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.05.118

e Pt metal, which is expected to have promising applications in fuel cells.© 2011 Elsevier Ltd. All rights reserved.

characteristics that are not merely the sum of the properties of thetwo constituent metals [3,5–8,12]. The PtPd bimetallic system ishighly resistant to CO poisoning due to the large number of activeoxygen-containing species, such as PdO and PdOx, and the catalyticperformance of this system is significantly improved as comparedto the pure Pt catalyst [3,5–8]. Moreover, the PtPd catalyst is morestable than the PtRu catalyst at high potentials.

Although Pt-based bimetallic catalysts have exhibited greatimprovements in activity, the lack of routes for the controlled,large-scale synthesis of highly dispersed catalysts has limited theiruse in commercial devices [12]. Therefore, identification of a highyield route for the synthesis of PtM catalysts with good catalyticactivity is essential for both fundamental studies and practicalapplications. In a previous study [8], we reported a microwaveheating method for the synthesis of PdPt core–shell nanostructureswith controllable compositions. The results demonstrated that theobtained PdPt nanostructures have high catalytic activity for theMOR.

To further enhance the activity of the nanocatalysts and lowerthe usage of noble metals, loading the nanocatalysts on the surfaceof suitable supporting materials, i.e., those that are inexpensiveand have a large surface area and good electrical conductivity, ishighly desirable. Many types of carbon materials, such as carbonblack [3,13,14] and carbon nanotubes [5,15], have been explored

as potential supporting materials for immobilizing and stabiliz-ing Pt-based catalysts. The introduction of carbon nanomaterialscan effectively enhance the dispersion of the catalysts and reducethe usage of the metal catalysts. The recent discovery of graphene

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as opened up a new avenue of study into two-dimensional (2D)undamental science and potential technologies [16]. The superiorhermal and electric conductivities, high chemical stability, out-tanding mechanical strength, large surface-to-volume ratio, androad electrochemical window [16–20] make graphene an idealatalyst carrier among the next generation of carbon-based sup-ort materials [21–39]. Recently, considerable efforts have beenade to fabricate graphene-supported Pt [24,25,29] and Pt-based

atalysts [32,36,39] and to explore their performance in DMFCs.t/graphene, especially Pt-on-Pd/graphene, were found to exhibitigher activity for the MOR than Pt/Vulcan XC-72 [24,25,32,36],t/carbon nanotubes [29], or Pt/graphite [32] as the electrocatalystn DMFCs.

In this work, we report the synthesis and characterization ofraphene-supported Pd1Pt3 nanostructures, i.e., a Pt shell on a Pdore with a Pd/Pt molar ratio of 1:3, using ascorbic acid (AA) as

reducing agent to simultaneously reduce the graphene oxideGO) sheets, Pd and Pt precursors via a microwave heating method.ompared with the traditional chemical method, the microwaveethod provides more homogeneous reaction conditions and a

ast kinetic process. To the best of our knowledge, there is no cur-ent literature on the preparation of graphene-supported Pd1Pt3anostructures using this technique. The results demonstratedhat the introduction of graphene increases the electrochemicallyctive surface area (ECSA) of the Pd1Pt3 nanostructures; the elec-rocatalytic specific mass current for the MOR is enhanced by0% as compared with that of the unsupported Pd1Pt3 electro-atalyst. Our study demonstrates a simple and general strategyor the fabrication of graphene-based noble metal nanocompos-tes with increased Pt utilization that have potential utility in fuelells.

. Experimental

.1. Chemicals

Graphite powder (99.998%, 325 mesh, Alfa Aesar), potas-ium tetrachloroplatinate (K2PtCl4), palladium (II) chloride (PdCl2),scorbic acid (AA), and cetyltrimethylammonium bromide (CTAB)ere purchased from Shanghai Chemicals Regent Co. Ltd. (Shang-ai, China). All other chemicals were of analytical grade and usedithout further purification. All solutions were prepared withouble-distilled water.

.2. Synthesis of GO sheets

GO sheets were synthesized according to a published routenvolving graphite oxidation and exfoliation steps. Briefly, graphitexide was prepared from graphite powder by a modified Hummersethod [40,41]. Graphite was oxidized using concentrated H2SO4,

2S2O8, and P2O5 to produce preoxidized graphite, which was thenubjected to reoxidization using concentrated H2SO4 and KMnO4.xfoliation was carried out by sonicating a graphite oxide disper-ion (0.05 wt%) at ambient temperature for ca. 30 min. The resultingomogeneous yellow-brown dispersion was then subjected to cen-rifugation at 3000 rpm for 30 min to remove unexfoliated graphitexide (usually present in a very small amount), and the homoge-eous GO dispersion was obtained.

.3. Preparation of Pd1Pt3 and graphene-supported Pd1Pt3anostructures

The Pd1Pt3 nanostructures were prepared according to previ-usly reported procedures [8]. Briefly, 1.5 mL of K2PtCl4 solution24 mM) and 0.25 mL of PdCl2 solution (48 mM) were added into

cta 56 (2011) 7064– 7070 7065

50 mL of CTAB (20 mM) solution with stirring. The pH of the solu-tion was adjusted to 9 with NaOH solution (1 M). Next, 0.352 gof AA was added to the solution, and the mixture was heated ina homemade microwave-refluxing synthesis system at 200 W for3 min. The nanostructures were collected by centrifugation, wash-ing sequentially with double-distilled water and ethanol for severalcycles, and dried in air.

To prepare the graphene-supported Pd1Pt3 nanostructures,1 mL of GO solution (2 mg mL−1), 1.5 mL of K2PtCl4 solution(24 mM), and 0.25 mL of PdCl2 solution (48 mM) were added into50 mL of CTAB solution (25 mM) with stirring, and the pH of thesolution was adjusted to 9 with NaOH solution (1 M). Next, 0.402 gof AA was added, and the mixture was heated in a homemademicrowave-refluxing synthesis system at 200 W for 4 min, dur-ing which time the mixture turned black to indicate the formationof the graphene-supported Pd1Pt3 nanostructures. The nanostruc-tures were collected by centrifugation, washing sequentially withdouble-distilled water and ethanol for several cycles, and dried inair.

For comparison, Pt nanostructures and graphene were pre-pared by similar procedures. To prepare Pt nanostructures, 2 mLof K2PtCl4 solution (24 mM) was added to 50 mL of CTAB solution(20 mM) with stirring (the pH was adjusted to 9). Then, 0.352 g ofAA was added, the mixture was heated in a homemade microwave-refluxing synthesis system, and the Pt nanoparticles were collectedby centrifugation, washing, and drying in air as described for thePd1Pt3 nanostructures. To prepare graphene, 1 mL of GO solution(2 mg mL−1) was added into 50 mL of CTAB solution (10 mM) (pH 9).Then, 0.15 g of AA was added, the mixture was heated in a home-made microwave-refluxing synthesis system at 200 W for 4 min,and the graphene was collected by centrifugation, washing, anddrying in air as described previously.

2.4. Apparatus and procedures

A homemade microwave synthesis system with a maximumoutput power of 800 W was used for nanomaterial syntheses.The microwave synthesis system was connected to a refluxingsystem. Transmission electron microscopy (TEM) images wereobtained on a Hitachi H-7650 microscope with an acceleratingvoltage of 120 kV. Energy-dispersive X-ray spectroscopy (EDX)was obtained from an Oxford Link ISIS energy-dispersive spec-trometer fixed on the microscope. To obtain TEM images of GOand graphene-supported Pd1Pt3 nanostructures, the nanomateri-als were dispersed in ethanol, drop-cast on a carbon-coated Cugrid, dried for several hours at ambient temperature, and thenexamined with the TEM. Atomic force microscopy (AFM) imageswere recorded with a Nanoscope IIIa scanning probe microscope(Digital Instruments, USA) using a tapping mode. The sample usedfor measurements was prepared by casting the suspension of GO(0.1 mg mL−1) on the surface of a mica sheet. The solvent wasevaporated under vacuum before measurements were taken. X-raydiffraction (XRD) patterns were recorded on a Rigaku/Max-3A X-ray diffractometer with Cu K� radiation (� = 0.15418 nm). The FTIRspectrum was recorded on a Nexus 670 FTIR spectrophotometer(Nicolet Instruments) using a KBr disk at a resolution of 4 cm−1.Raman measurements were performed on a LabRam HR 800 UV(Jobin-Yvon) with a laser excitation wavelength of 514.5 nm. Thepowders of GO, graphene, or graphene-supported Pd1Pt3 nanos-tructures were placed on a clean glass substrate to collect Ramanmeasurements.

The electrochemical experiments were carried out with a CHI

760B electrochemical workstation (CH Instruments). A conven-tional three-electrode system was used with a modified glassycarbon (GC) electrode (3 mm in diameter) as the working elec-trode, a coiled platinum wire as the auxiliary electrode, and a

7066 H. Zhang et al. / Electrochimica Acta 56 (2011) 7064– 7070

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ig. 1. A typical TEM image (A1), tapping mode AFM image (samples supported onEM images of graphene-supported Pd1Pt3 nanostructures at different magnificatio

aturated calomel electrode (SCE) as the reference electrode. Theare GC electrodes were sequentially polished with 1.0-, 0.3-

and 0.05-�m alumina slurries, followed by thorough rinsingith double-distilled water, and then allowed to dry at room

emperature. To prepare the graphene-supported Pd1Pt3 modi-ed GC electrode, 8 mg of graphene-supported Pd1Pt3 catalystsas dispersed in 2 mL of DMF (1 mg mL−1) to yield a 4 mg mL−1

raphene-supported Pd1Pt3 suspension. Then, 5 �L of the disper-ion was drop-cast on the surface of the GC electrode with a

icrosyringe. The solvent was allowed to evaporate at ambient

emperature before use. Using similar procedures, the Pd1Pt3 andraphene modified electrodes were fabricated by casting 5 �L of thed1Pt3 suspension (4 mg mL−1 in water) and 5 �L of graphene sus-

and dried under vacuum) (A2), and the cross-sectional analysis (A3) of GO. Typical and B2), and an EDX spectrum of graphene-supported Pd1Pt3 nanostructures (B3).

pension (2 mg mL−1 in DMF) onto the surface of the GC electrodes.A solution of 0.5 M H2SO4 was used as the electrolyte. For MORexperiments, 0.5 M H2SO4 solution containing 0.5 M methanol wasdeaerated with high-purity nitrogen gas for at least 20 min prior toelectrochemical measurements, and a nitrogen environment wasmaintain over the solution to prevent oxygen from reaching thesolution during the cyclic voltammetric measurements. Before theMOR tests, the modified GC electrode was first activated in the 0.5 MH2SO4 solution by cyclic sweeping until a steady cyclic voltammet-

ric response was obtained. The CVs were recorded between −0.25and 1.2 V (vs. SCE) at a scan rate of 50 mV s−1. The chronoamper-ometry (current vs. time response) measurement was performed atthe stationary electrodes in 0.5 M H2SO4 solution containing 0.5 M

ica Acta 56 (2011) 7064– 7070 7067

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the G peak (ID/IG) can be used to evaluate the reduction of theGO [42,48–51]. As shown in Fig. 4, the G peaks of the grapheneand graphene-supported Pt1Pt3 nanostructures shift to lower fre-

H. Zhang et al. / Electrochim

ethanol under an applied potential of 0.60 V for a period of 1800s.ll electrochemical experiments were performed at room temper-ture (22 ± 1 ◦C).

. Results and discussion

.1. Characterization of graphene-supported Pd1Pt3anostructures

The morphology of the GO sheets was characterized by TEM andFM. As shown in panel A1 (Fig. 1), flake-like shapes with somerinkles are observed for the GO sheets in the TEM image, indicat-

ng that the graphite oxide was fully exfoliated into GO nanosheetsy ultrasonic treatment. The AFM image shows that the surface ofhe GO sheet is flat and smooth (panel A2). The cross-sectional anal-sis indicated that the thickness of the GO is ca. 1.1 nm (panel A3),hich is consistent with the reported apparent thickness of singleO sheets (∼1.2 nm) [42,43], suggesting that single-sheet GO coulde obtained with this method. Because the GO sheets contain manyxygen-containing functional groups, they can be readily dispersedn water as individual sheets. In addition, these groups could helpo anchor precursor metal ions or metal nanoparticles [25,44].

When AA was introduced into the mixture, the GO was reducedo graphene, and the Pt and Pd precursors were reduced in situ tod1Pt3 and deposited onto the surface of the graphene, produc-ng graphene-supported Pd1Pt3 nanostructures. As shown in panel1 (Fig. 1), the graphene sheets are uniformly decorated by thed1Pt3 nanostructures, indicative of a strong interaction betweenraphene and the Pd1Pt3 nanostructures. The Pd1Pt3 structures arepherical, and a mean diameter of 30 nm was estimated from theEM images (panel B2); the structural characteristics found herere consistent with those of the Pd1Pt3 nanospheres obtained in ourrevious work [8]. The graphene-supported Pd1Pt3 nanostructuresere also characterized by EDX (panel B3), and the results reveal

he presence of Pd, Pt, and C. The atomic ratio of Pd to Pt is about 1:3ased on the EDX analysis, which is in agreement with the Pd/Ptrecursor molar ratio in the reaction mixture. Graphene sheets pos-ess a large surface area and excellent electrical conductivity, andhe attached Pd1Pt3 nanostructures can inhibit aggregation of theraphene sheets; moreover, the rough surface of the Pt shell on thed1Pt3 is composed of tens of tiny Pt nanoparticles [8] and exhibitsood electrocatalytic performance. These characteristics can pro-uce very effective, advanced catalysts with high catalytic activity29].

The XRD patterns of graphite, GO, and graphene-supportedd1Pt3 nanostructures are shown in Fig. 2. As shown in Fig. 2a, theiffraction peak at ca. 26.5◦ can be assigned to the (0 0 2) reflec-ion of the hexagonal graphite structure [30,34], correspondingo an average interlayer spacing (d-spacing) of 0.336 nm. For GO,he characteristic diffraction peak (0 0 2) appears at 10.8◦, and the-spacing is about 0.82 nm (Fig. 2b), which is larger than that ofraphite due to the introduction of oxygenated functional groups45]. The diffraction peak at ca. 42.5◦ is associated with the (1 0 0)lane of the hexagonal structure of carbon [46]. After addition ofA, the characteristic peak of GO at 10.8◦ is completely absent,

ndicating that the GO was successfully reduced (Fig. 2c). The well-esolved peaks at 39.9, 46.4, and 67.9◦ in Fig. 2c can be indexed ashe (1 1 1), (2 0 0), and (2 2 0) crystalline planes of Pd and Pt, respec-ively (JCPDS card No. 04-0802 (Pt) and No. 46-1043 (Pd)). Theseesults confirm that the as-synthesized Pd1Pt3 nanostructures have

highly crystalline face-centered cubic phase with a Fm3m space

roup.

FTIR spectroscopy was also used to characterize the reduction ofO. In the IR spectrum of GO (Fig. 3a), the characteristic vibrationalodes of O–H groups (∼3420 cm−1) and C O groups (∼1726 cm−1),

Fig. 2. XRD patterns of graphite (a), GO (b), and graphene-supported Pd1Pt3 nanos-tructures (c).

the deformation peak of O–H groups (∼1410 cm−1), the stretch-ing peak of C–OH (∼1226 cm−1), and the stretching peak of C–O(∼1060 cm−1) are clearly observed, demonstrating that graphitewas successfully oxidized to graphite oxide under our experimen-tal conditions [19]. The peak at 1620 cm−1 can be assigned to theskeletal vibrations of the adsorbed water molecules and the bend-ing vibrations of unoxidized graphitic domains [19]. After additionof AA, both the graphene and graphene-supported Pd1Pt3 retainresidual oxygen functionalities on the graphene surface (Fig. 3band c); however, the IR intensities of these oxygen-containinggroups decrease significantly, and some of them disappear com-pletely, indicating that the GO was significantly deoxygenated andgraphene was successfully formed [47].

GO reduction was also characterized using Raman spectroscopy.As shown in Fig. 4, two major peaks, G and D, are observed inthe Raman spectra of GO, graphene, and the graphene-supportedPt1Pt3 nanostructures. The G peak can be ascribed to the E2g phononof the sp2 hybridized carbon atoms in the range of 1500–1600 cm−1

and the D peak can be ascribed to the breathing mode of the �-point phonons of A1g in the range of 1200–1500 cm−1 [42,48]. Theposition of the G peak and the intensity ratio of the D peak to

Fig. 3. FTIR spectra of GO (a), graphene (b), and graphene-supported Pd1Pt3 nanos-tructures (c).

7068 H. Zhang et al. / Electrochimica Acta 56 (2011) 7064– 7070

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uency as compared to that of GO, which is consistent with theaman spectrum of graphene obtained via chemical and thermaleduction of GO [48]. In addition, during the reduction of GO, thexygen functional groups in the GO sheets are partially removed,nd new smaller graphitic domains are established, which leadso an increase in the ID/IG ratio [34,49–51]. The values of ID/IG forraphene and graphene-supported Pd1Pt3 are 1.35 (curve b, Fig. 4)nd 1.27 (curve c), respectively. These values increase significantlys compared to that of GO (ID/IG = 0.90, curve a), indicating thatO was effectively reduced and graphene was largely established

35,50,51].

.2. Electrocatalytic activity of the graphene-supported Pd1Pt3anostructures toward the MOR

The electrocatalytic characteristics of the nanocatalysts werevaluated using the MOR as a model reaction. The performancef Pt-based electrocatalysts is highly dependent upon the ECSA ofhe catalyst, which is a measure of the number of electrochemi-ally active sites per gram of the catalyst [28]. Cyclic voltammetryCV) is an efficient and convenient tool that can be used to esti-

ate the ECSA of Pt-based catalysts. The CV curves of GC electrodesodified with Pt, unsupported Pd1Pt3, and graphene-supported

d1Pt3 in H2SO4 (0.5 M) are depicted in Fig. 5. The cathodic andnodic peaks appearing between −0.22 and 0.16 V (vs. SCE) orig-nate from the adsorption and desorption of atomic hydrogen on

ig. 5. CVs of GC electrodes modified with Pt (a), Pd1Pt3 (b), and graphene-supportedd1Pt3 (c) in 0.5 M H2SO4. Scan rate: 50 mV s−1.

Fig. 6. CVs of the oxidation of 0.5 M methanol at GC electrodes modified with Pt (a),Pd1Pt3 (b), and graphene-supported Pd1Pt3 (c) in 0.5 M H2SO4 solution containing0.5 M methanol. Scan rate: 50 mV s−1.

the electrocatalyst. The ECSA values can be evaluated by integrat-ing the area under the curve in the hydrogen adsorption range afterdouble-layer correction. A value of 210 �C cm−2 for the adsorp-tion of a monolayer of hydrogen atoms is used in this calculation[12,52]. The mass specific ECSA of the graphene-supported Pd1Pt3nanostructures is 49.8 m2 (gPd+Pt)−1, which is almost twice that ofunsupported Pd1Pt3 nanostructures obtained without graphene(25.1 m2 (gPd+Pt)−1). If only the Pt mass is considered, the ECSAof the graphene-supported Pd1Pt3 is 58.7 m2 (gPt)−1, which ishigher than those of Pt nanoparticles (11.0 m2 (gPt)−1, obtainedfrom Fig. 5a), Pt/multi-walled nanotubes (33.4 m2 (gPt)−1) [24],and Pt/graphene (44.6–36.3 m2 (gPt)−1) [30]). These results demon-strate that the graphene-supported Pd1Pt3 nanostructures canprovide a large surface area for electrocatalysis, which is attributedto the good distribution of Pd1Pt3 on the surface of the graphene.

Electrocatalytic oxidation of methanol in acidic solution (0.5 MH2SO4) was used as a model reaction to evaluate the catalyticactivity of the prepared graphene-supported Pd1Pt3 nanocatalysts.Cyclic voltammetric results indicate that the graphene and Pdnanostructures cannot catalyze the oxidation of methanol (datanot shown). However, the CVs for the Pt, Pd1Pt3, and graphene-supported Pd1Pt3 electrocatalysts display the typical methanolelectrooxidation features for Pt-based catalysts. As shown in Fig. 6,the methanol oxidation peak is observed at about 0.65 V in thepositive sweep. During the forward potential scan, Pt oxide inter-mediates are formed, which are inactive for the MOR. In thenegative potential scan, the surface Pt oxide is reduced, and theanodic current peak of the methanol oxidation is observed at about0.40 V. The values of the onset potential, peak potential, and cur-rent density in the forward scan are usually used for evaluating theactivity of a catalyst [24]. As the results show (Fig. 6), the onset of theMOR at the Pd1Pt3 catalyst occurred at a relatively lower potential(ca. 0.16 V) than it did at the Pt catalyst (ca. 0.27 V). The current den-sity of the positive sweep for Pd1Pt3 at 0.65 V is about 6-fold higherthan that for the Pt catalyst, which indicates that Pd1Pt3 exhibitedbetter performance for the MOR than pure Pt. These results can beattributed to the large number of active oxygen-containing species,such as PdO and PdOx, that are formed in the course of the MORand could address the problem of Pt catalyst poisoning by CO-likespecies [8]. Furthermore, the onset potentials of the MOR on thePd1Pt3 and graphene-supported Pd1Pt3 catalysts are the same, butthe current density for methanol oxidation at graphene-supported

Pd1Pt3 is higher than that at the Pd1Pt3 catalysts. The specificmass current was calculated to be 394 mA (mgPt)−1 for graphene-supported Pd1Pt3, which is about 80% higher than that calculatedfor the Pd1Pt3 catalysts (Fig. 6, inset), and this enhancement indi-

H. Zhang et al. / Electrochimica A

Fig. 7. Chronoamperometric curves of GC electrodes modified with Pt (a), Pd1Pt3

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b), and graphene-supported Pd1Pt3 (c) in 0.5 M H2SO4 solution containing 0.5 Methanol at a potential of 0.60 V (vs. SCE).

ates that the introduction of graphene effectively increases thet utilization. These results are in good agreement with the ECSAesults. Comparisons to other previously reported Pt-based electro-atalysts on modified electrodes with similar methanol transportonditions, i.e., preparation by casting the catalyst suspension onhe electrode surface and signal determination in quiescent solu-ions, show that the specific mass current at the electrode modifiedith the graphene-supported Pd1Pt3 electrocatalyst is higher than

hat for Pt/graphene nanocomposites (200 mA (mgPt)−1) [25] andommercial carbon-supported platinum catalysts (Pt/XC-72R, E-ek, 60% Pt, diameter of Pt = 6 nm, 51.0 mA (mgPt)−1) [53]. Thisesult suggests that the dispersion of the Pd1Pt3 bimetallic electro-atalyst on the graphene is a useful strategy for achieving relativelyigh electrocatalytic performance in fuel cells.

To measure the tolerance of the graphene-supported Pd1Pt3atalysts in the MOR, chronoamperometric measurements wereerformed in H2SO4 solution containing methanol for a durationf 1800s. Fig. 7 shows the current decay at a fixed potential of.60 V of GC electrodes modified with Pt, Pd1Pt3, and graphene-upported Pd1Pt3 in 0.5 M H2SO4 containing 0.5 M methanol. Theolarization currents at the Pt electrocatalyst decayed rapidly dur-

ng the initial period. However, at the GC electrodes modified withd1Pt3 and the graphene-supported Pd1Pt3, the current decayedlowly. The decrease in current density is due to surface poison-ng induced by the intermediate CO species [54]. The lower currentecay rate of the Pd1Pt3 and graphene-supported Pd1Pt3 may beue to effective cleaning of the electrode surface, which is consis-ent with the CV measurements described above. Furthermore, theraphene-supported Pd1Pt3 electrocatalyst was observed to exhibithe highest initial current, and this current remained much higherhan the other catalysts even after 1800s, which may be derivedrom the higher ECSA of the graphene-supported Pd1Pt3 catalyst asompared to that of the unsupported electrode Pd1Pt3 catalyst.

. Conclusions

In summary, a one-step, fast microwave heating methodas developed for the synthesis of graphene-supported Pd1Pt3anostructures. When compared with Pd1Pt3 catalysts, theseanostructures exhibit higher electrocatalytic current density forhe MOR with enhanced Pt utilization and good stability. The

eveloped approach is a useful method for preparing graphene-upported PtM electrocatalysts, which can be used in the field ofuel cells and other related fields.

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cta 56 (2011) 7064– 7070 7069

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (20833006, and 20905036), the Foundation of theJiangsu Education Committee (09KJA150001, 09KJB150006, and10KJB150009), a Project Funded by the Priority Academic Pro-gram Development of the Jiangsu Higher Education Institutions,the foundation of State Key Laboratory Breeding Base of GreenChemistry-Synthesis Technology (GCTKF2010001), and the Prim-ing Scientific Research Foundation for Advanced Talents in NanjingNormal University (2009103XGQ0064).

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