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Electrochimica Acta 62 (2012) 242– 249

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

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

reparation and electrochemistry of graphene nanosheets–multiwalled carbonanotubes hybrid nanomaterials as Pd electrocatalyst support for formic acidxidation

udong Yanga, Chengmin Shenb, Xiangjun Lua, Hao Tonga, Jiajia Zhua, Xiaogang Zhanga,∗,ong-jun Gaob,∗∗

College of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR ChinaBeijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China

r t i c l e i n f o

rticle history:eceived 1 November 2011eceived in revised form 7 December 2011ccepted 8 December 2011vailable online 16 December 2011

a b s t r a c t

Graphene nanosheets–MWCNTs (GNS–CNTs) composites were synthesized by in situ reduction method,and then palladium nanoparticles (NPs) were supported on the GNS–CNTs by a microwave-assisted polyolprocess. Microstructure measurements showed that the graphene nanosheets and the CNTs formed auniform nanocomposite with CNTs absorbed on the graphene nanosheets surface and/or filled betweenthe graphene nanosheets. Compared to Pd/Vulcan XC-72R carbon, Pd/GNS, or Pd/CNTs catalysts, the

eywords:uel cellraphene nanosheetsalladiumatalysts

Pd/GNS–CNTs catalysts exhibit excellent electrocatalytic activity and stability for formic acid electro-oxidation when the mass ratio of GO to CNTs is 5:1. The superior performance of Pd/GNS–CNTs catalystsmay arise from large surface area utilization for NPs and enhanced electronic conductivity of the sup-ports. Therefore, the GNS–CNTs composite should be a promising carbon material for application aselectrocatalyst support in fuel cells.

ormic acid

. Introduction

The electrochemical oxidation of formic acid has attracted a lotf attention in the past few years due to the great potential ofirect formic acid fuel cells (DFAFCs) as efficient energy suppli-rs for mobile and portable applications [1–3]. Recent progress ind-based catalysts has revealed that Pd nanoparticles (NPs) are aromising catalyst employed in DFAFCs due to their high catalyticctivity [4–6]. The most commonly accepted mechanism of formiccid oxidation is the so-called ‘parallel or dual pathway mecha-ism’ and is described below [7]. The first mechanism, called “directathway,” involves direct oxidation of the acid to carbon dioxide:

HCOOH + Me → CO2 + 2H+

+ Me + 2e− (“Me′′ = Pt, Pd and other) (1)

A second mechanism occurs when carbon monoxide adsorbsnto a “Me” surface, and two electrochemical steps follow:

COOH + Me → Me–CO + H2O (2)

∗ Corresponding author. Tel.: +86 25 52112902; fax: +86 25 52112626.∗∗ Corresponding author. Tel.: +86 10 82648035; fax: +86 10 62556598.

E-mail addresses: azhangxg@163.com (X. Zhang), hjgao@aphy.iphy.ac.cnH.-j. Gao).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.12.022

© 2011 Elsevier Ltd. All rights reserved.

Me + H2O → Me–OH + H+ + e− (3)

Me–CO + Me–OH → 2Me + CO2 + H+ + e− (4)

It is well established that Pd promotes the “direct pathway”mechanism of formic acid oxidation (Eq. (1)). A more recent inves-tigation has shown that the slow adsorption of the “CO-like”intermediate might be the principal reason for the deactivation ofPd catalysts during formic acid oxidation [8]; such a conclusion issupported by a prior investigation using in situ infrared absorptionspectroscopy [9]. On the other hand, a Pt electrode promotes theindirect mechanism (Eqs. (2)–(4)).

In order to improve the catalytic character and lower theoverall cost of fuel cells, Pd NPs are used to load on the con-ductive carbon materials [10–12], which not only maximize theavailability of nanosized electrocatalyst surface area for electrontransfer but also provide better mass transport of reactants to theelectrocatalyst. Therefore, much effort has been devoted to devel-oping novel catalyst supports. The recent emergence of graphenenanosheet has opened a new avenue for utilizing two-dimensionalnew carbon material as a support because of its unique proper-ties [13–15]. Recently, Pd–graphene catalysts have become a hot

topic of interest in fuel cells [16–19]. In particular, Chen et al. [16]showed Pd/graphene exhibited better catalytic performance andstability compared to the commercial Pd/C catalyst. Fu et al. [17]found that Pd/graphene shows better electrochemical activity for

S. Yang et al. / Electrochimica A

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Fig. 1. Schematic representation of the synthesis of Pd/GNS–CNTs catalysts.

lectro-oxidation of formic acid than the Pd/C catalyst. Zhang et al.18] reported about the deposition of Pd on graphene sheets andheir electrochemical activity for ethanol oxidation was studied.raphene sheets prepared through the exfoliation of graphite oxide

eave behind some defects and vacancies, which can act as goodnchoring sites for the deposition of metal NPs that can be used foruel cell applications.

However, as-reduced graphene sheets tend to form irreversiblegglomerates through van der Waals interactions and even restacko form graphite, which sacrificed both large specific surface areand outstanding single-layer electric property of graphene [20]. Inddition, graphene sheets prepared through the chemical reduc-ng with hydrazine leave behind some defects and vacancies that

ill reduce the conductivity of the composite [21]. These problemseverely restrict the further applications for the catalyst supports.he restacking can be prevented by using CNTs as spacers [22]hich will result in the increase in the surface area and electronic

onductivity and thereby the enhancement in performance. Thus,ombination of graphene nanosheets and CNTs as novel catalystupport materials may increase catalytic activity of NPs. As a result,he CNTs increase the electronic conductivity of the nanosheets.nd meanwhile, the CNTs ensure the high electrochemical utiliza-

ion of graphene layers as well as the open nano-channels providedy three-dimensional GNS–CNTs hybrid material.

In this paper, GNS–CNTs hybrid materials as catalyst supportith different mass ratios were prepared by in situ chemical reduc-

ion method. Then the palladium was directly reduced onto theybrid support (Fig. 1). Catalysts with different CNTs compositionsere investigated. The results demonstrate that Pd/GNS–CNTs

xhibits excellent catalytic activity and stability than Pd/GNS andd/CNTs catalyst for formic acid oxidation.

. Experimental

.1. Materials

The majority of the chemicals including graphite power (SPrade), palladium chloride (PdCl2), hydrazine hydrate, and ethylenelycol (EG) were purchased from Sinopharm Chemical Reagent Co.,td., China, with their purity in analytical grade. Multi-walled car-on nanotubes (CNTs, 20–40 nm in diameter) were purchased fromanotech Port Co., Ltd. (Shenzhen, China) and purified by reflux-

ng them in nitric acid (HNO3) for 6 h before use. Other chemicaleagents were analytical grade and used as received without furtherurification.

.2. Synthesis of the GNS–CNTs materials

The graphite oxide (GO) was prepared according to modifiedummer’s method [23]. Exfoliation of GO and CNTs was achievedy ultrasonication of the dispersion using an ultrasonic bath. In a

cta 62 (2012) 242– 249 243

typical synthetic procedure for GNS–CNTs, 1 mL hydrazine hydrateis added into the resulting dispersion (100 mL) and the reactionmixture was kept at 100 ◦C for 24 h under constant stirring. Finally,the solid was filtered, washed several times with distilled waterand alcohol, and dried at 60 ◦C for 12 h in a vacuum oven. The massratio of GO to CNTs for each of the composite supports was 1:1, 3:1,5:1, and 7:1 denoted in this report as GNS–CNTs (1:1), GNS–CNTs(3:1), GNS–CNTs (5:1), and GNS–CNTs (7:1), respectively. The puregraphene nanosheets were prepared through the above-mentionedchemical process without the presence of CNTs.

2.3. Synthesis of the Pd/GNS–CNTs materials

To obtain the Pd/GNS–CNTs (5:1) composites, in a typical pro-cedure, 20 mg GNS–CNTs (5:1) powder was dispersed in 30 mL EGsolvent by ultrasonic treatment for approximately 1 h and then0.71 mL of 7 mg mL−1 PdCl2 solution was added under magneticstirring. The PH value of this mixture was adjusted to 10.0 by adding1 M NaOH aqueous solution. Subsequently, the solution was putinto a microwave oven (1000 W, 2.45 GHz) and then was alter-natively heated for 2 min and paused for 30 s for eight times ata maximum temperature of 140 ◦C. The resulting slurry was cen-trifuged, washed with deionized water and then dried in a vacuumoven. For comparative purposes, a sample of Pd-loaded GNS–CNTswas also prepared under identical conditions. The same procedurewas followed for the synthesis of Pd/GNS, Pd/CNTs, and Pd/VulcanXC-72R carbon catalyst (denoted as Pd/C).

2.4. Preparation of electrode

Glassy carbon (GC) electrode, 5 mm in diameter (electrode area0.2 cm2), polished with 0.05 �m alumina to a mirror-finish beforeeach experiment, was used as substrates for supported catalysts.For the electrode preparation, typically, 3 mg catalyst was addedinto 0.5 mL of 0.05 wt.% Nafion solution, and then the mixturewas treated for 1 h with ultrasonication for uniform dispersion.A measured volume (30 �L) of this mixture was dropped by amicrosyringe onto the top surface of the GC electrode. The as-obtained catalyst modified GC electrode was employed as theworking electrode in our experiments.

2.5. Instrument and measurement

X-ray diffraction (XRD) analysis was carried out on BrukerD8-ADVANCE diffractometer with Cu K� radiation of wavelength� = 0.15418 nm. The Raman spectroscopy was used to study theintegrity and electronic structure of the samples on the Ramansystem YJ-HR800 with confocal microscopy. Transmission electronmicroscopy (TEM, JEOL JEM-2100) was applied to characterize themorphology. The analysis of the composition of the catalyst wasobtained with a Thermo IRIS Intrepid II inductively coupled plasmaatom emission spectrometry (ICP-AES) system. The electronic con-ductivities of the samples were measured by a four-point probemethod (SDY-5 Four-Point probe meter).

All electrochemical measurements were carried out with aCHI 660C electrochemical workstation system and a conventionalthree-electrode system. A Pt wire, the saturated calomel electrode(SCE) and the GC electrode were used as the counter electrode,

the reference electrode and the working electrode, respectively.All electrolytes were deaerated by bubbling N2 for 20 min and pro-tected with a nitrogen atmosphere during the entire experimentalprocedure. All experiments were performed at 25 ± 1 ◦C.

244 S. Yang et al. / Electrochimica A

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ig. 2. XRD patterns of (a) Pd/CNTs, (b) Pd/GNS, (c) Pd/GNS–CNTs (1:1), (d)d/GNS–CNTs (3:1), (e) Pd/GNS–CNTs (5:1), and (f) Pd/GNS–CNTs (7:1).

. Results and discussion

The XRD patterns of Pd/CNTs, Pd/GNS, and Pd/GNS–CNTs arehown in Fig. 2. For Pd/GNS, the position of the (0 0 2) diffrac-ion peak corresponds to the interlayer spacing 3.61 A in graphene,hich is a little larger than the d-spacing of well ordered graphite

3.4 A). The small amount of functional groups and hydrogenemaining might be the main reason for this difference. Afterhe reduction by hydrazine, the graphene tends to form veryense agglomerates with layered structure in dry state, whichesults from the van der Waals interactions between the lay-rs of graphene [24]. Compared with Pd/CNTs, with the lowermount of CNTs in GNS–CNTs composites, the characteristic diffrac-ion peak of the C (0 0 2) with a broad peak is slightly shiftedo a lower angle in the case of Pd/GNS–CNTs, which also can bettributed to the effect from the CNTs. The all as-prepared cata-ysts exhibit the strong diffraction peaks at 2� = 39.9◦, 46.3◦, 67.8◦

nd 81.6◦ that can be assigned to the characteristic (1 1 1), (2 0 0),2 2 0), and (3 1 1) crystalline planes of Pd, respectively, which

eans palladium exists as face-centered-cubic (fcc) structure.mong them, the average particle sizes of the Pd/CNTs, Pd/GNS,nd Pd/GNS–CNTs (5:1) catalysts, calculated from the (2 2 0) peaksing the Debye–Scherrer equation, were 4.3, 4.1, and 4.0 nm,espectively.

As shown in Fig. 3, the Raman spectrum of the graphene cane decomposed into two features: the well-documented D and G

eaks, and the D band at 1351 cm−1 corresponding to defects ordge areas and G band at 1586 cm−1 related to the vibration ofp2-hybridized carbon. Similarly, the Raman spectra of the

Fig. 3. Raman spectra of the GNS and GNS–CNTs (5:1).

cta 62 (2012) 242– 249

GNS–CNTs (5:1) both display two prominent peaks at 1351 and1586 cm−1, corresponding to the D and G peaks, respectively. It isworth noting that, due to the incorporation of the CNTs, the inten-sity ratio of D/G in the GNS–CNTs (5:1) composite decreases incomparison with that in the graphene.

The morphologies of Pd/CNTs, Pd/GNS, GNS–CNTs (5:1), andPd/GNS–CNTs (5:1) were characterized by TEM. From the TEMimages in Fig. 4a and b, the Pd NPs are slightly more uniformlydispersed on the graphene nanosheets than the CNTs due to theremained oxygen-containing functionalities on the surface of thegraphene sheets. As shown in Fig. 4c, the graphene nanosheetsand the CNTs formed a uniform nanocomposite with the CNTsabsorbed on the graphene nanosheet surface and/or filled betweenthe graphene nanosheets (the arrows indicate the presence of CNTsin the graphene sheets). It is clearly seen that the GNS–CNTs sup-ports are decorated by the nanosized Pd particles with very fewaggregations. Further high-resolution TEM (HRTEM) image (theinsert in Fig. 4d) analysis indicated lattice fringes with an interfringedistance of approximately 2.2 A, which is close to the interplane dis-tance of the [1 1 1] planes in the fcc structured Pd. From TEM images,the mean size of the Pd NPs decorated on the GNS–CNTs (5:1) wasabout 4.0 nm. Clearly, in this study, the microwave-assisted polyolprocess plays a key role in keeping a similar Pd particle size on thethree supports, while the different Pd particle dispersion mainlydepends on the carbon support. The slightly lower Pd dispersionon CNTs is mainly due to the inert graphite layers and the highersurface tension on the CNTs support. As can be seen in Fig. 4d, thegraphene sheets in the GNS–CNTs exist distributed CNTs betweenor on the graphene sheets. Based on these, we speculate thatCNTs in the hybrid materials of GNS–CNTs set up a fairly conduc-tive network, which may facile charge-transfer and mass-transferprocesses.

The practical composition of Pd in different samples was evalu-ated by ICP-AES analysis. The obtained ICP-AES composition of thePd/C, Pd/GNS, Pd/CNTs, Pd/GNS–CNTs (1:1), Pd/GNS–CNTs (3:1),Pd/GNS–CNTs (5:1), and Pd/GNS–CNTs (7:1) catalysts for metalloading was 18.6, 20.1, 18.5, 18.8, 19.0, 19.2, and 19.6 wt.%, respec-tively. The electrochemically active surface area (ECSA) providesimportant information regarding the number of available activesites [25]. Fig. 5a shows the cyclic voltammograms of differentelectrocatalyst composites by scanning the potential from −0.3to 0.8 V vs SCE at a scan rate of 20 mV s−1 in 0.25 M H2SO4.From Fig. 5a, it can be seen that the area of hydrogen adsorp-tion and desorption peak for Pd/GNS–CNTs (5:1) electrocatalystsis bigger than the other electrocatalysts. The difference of currentdensity can be also ascribed mainly to the difference in surfaceareas and only to an insignificant extent to the presence. TheECSA was estimated by integrating the voltammogram correspond-ing to hydrogen desorption (QH) by adapting the assumption of212 �C cm−2 from the electrode surface. The ECSA for Pd/C, Pd/GNS,Pd/CNTs, Pd/GNS–CNTs (1:1), Pd/GNS–CNTs (3:1), Pd/GNS–CNTs(5:1), and Pd/GNS–CNTs (7:1) was estimated to be 69.3, 75.2, 72.1,80.3, 83.6, 87.8, and 78.1 m2 g−1, respectively. The results show thatthe Pd/GNS–CNTs (5:1) electrode has the highest ECSA and Pd uti-lization, while the Pd/C has the lowest. The lowest ECSA and Pdutilization for the Pd/CNTs are most likely due to the inert graphitelayers and the higher surface tension of the CNTs support. Com-pared with Pd/GNS–CNTs (5:1), the higher ECSA of Pd/GNS–CNTs(5:1) is likely due to the fact that the electrode possesses a threedimensional structure and better conductive paths. In addition, thedouble layer capacitance was also obtained from the cyclic voltam-metry data. The electrical capacitance is a measure of the surface

area, both of Pd and support composites that can be accessedapproached by electrons as well as protons [10]. The increaseddouble-layer thickness of the Pd/GNS–CNTs (5:1) based electrodesreflects the higher specific surface area of the support composite.

S. Yang et al. / Electrochimica Acta 62 (2012) 242– 249 245

and (d

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Fig. 4. TEM images of (a) Pd/CNTs, (b) Pd/GNS, (c) GNS–CNTs (5:1),

To optimize the electrode activity, different ratios of GO to CNTsere examined by cyclic voltammetry and chronoamperometry.

ig. 5b shows the cyclic voltammogram in 0.25 M H2SO4 containing.25 M formic acid. As shown in Fig. 5b, the cyclic voltammogrameatures are in good agreement with the literature [10]. The twoormic acid oxidation peaks during positive potential scanning,robably correspond to the two reaction steps (Eq. (5)) and (Eq.6)) of reaction:

COOH → reactive intermediates (5)

eactive intermediates → CO2 + 2H+ + 2e− (6)

ig. 5. (a) Cyclic voltammograms of the as-prepared catalysts in 0.25 M H2SO4 solution ats-prepared catalysts in 0.25 M H2SO4 solution containing 0.25 M HCOOH at a scan rate o

) Pd/GNS–CNTs (5:1). The insert is HRTEM image of a single Pd NP.

The positive scan oxidation peak current density and peakpotential of the catalysts are summarized in Table 1. It is clear thatthe electrocatalyst with the mass ratio of GO to CNTs equal to 5:1gave the best performance. This value is substantially higher thanthose for pristine graphene nanosheets, Vulcan XC-72R carbon, orCNTs. The highest peak currents were observed on Pd/GNS–CNTs(5:1), indicating the highest catalytic activity for HCOOH oxidation,nearly twice than that on Pd/GNS electrode catalysts. And their

corresponding peak potentials are located at 0.09 V, 30 mV morenegative than that for the Pd/GNS catalysts. As shown in Table 1,the content of CNTs in the Pd/GNS–CNTs catalysts affects the cat-alytic activity for formic acid oxidation. With the CNTs content

a scan rate of 20 mV s−1; (b) cyclic voltammograms of formic acid oxidation on thef 50 mV s−1.

246 S. Yang et al. / Electrochimica Acta 62 (2012) 242– 249

Table 1Comparison of electrochemical characterization of Pd-based catalysts.

Sample Ep (V) ip (mA cm−2) i600 (mA cm−2)

Pd/C 0.12 15.79 0.82Pd/GNS 0.12 17.15 0.54Pd/CNTs 0.12 24.34 1.18Pd/GNS–CNTs (1:1) 0.10 24.03 1.47Pd/GNS–CNTs (3:1) 0.11 26.79 2.89Pd/GNS–CNTs (5:1) 0.09 33.61 3.50

Es

iwtwcitGssgrolt

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Fig. 6. Relationship between the specific activity and CNTs contents, the specificactivity was obtained from the current density of forward peaks for formic acidelectrooxidation on the Pd/GNS–CNTs catalysts.

F0

Pd/GNS–CNTs (7:1) 0.10 20.17 1.20

p, the peak potential of the positive scan; ip, the peak current density of the positivecan; i600, the current density after 600 s.

ncreasing, the current density increases at first and then decreasesith the excess amount of CNTs in the catalysts. It can be seen that

he best performance of the Pd/GNS–CNTs catalysts was obtainedhen the mass ratio of GO to CNTs is 5:1; too high and too low CNTs

ontents both cause a decrease in the activity of catalyst. The signif-cant enhancement for the catalytic activity of HCOOH oxidation onhe Pd/GNS–CNTs electrode was related to the three-dimensionalNS–CNTs electrode structure. Furthermore, these nanosheets pos-ess large surface areas, and particles can be deposited on bothides of these sheets [26]. Additionally, the CNTs attached onto theraphene surface can prevent the reduced GO from aggregation andestacking, and possess large surface areas because both the facesf graphene are accessible in their applications. Given single or fewayered graphene with less agglomeration may help to facilitate theransmission of the electrolyte through the surface of the catalyst.

Furthermore, the relationship between the specific activity andNTs contents in the Pd/GNS–CNTs catalysts is displayed in Fig. 6.

t was found that the content of CNTs in the Pd/GNS–CNTs catalystsffected the catalytic activity for formic acid oxidation. With theNTs content increasing, the current density increases at first andhen decreases with the excess amount of CNTs in the catalysts.hese results proved that CNTs as one of the composite supportingaterials loaded Pd NPs played a key role on the oxidation of formic

cid.The stability of Pd-based electrocatalyts is extremely important

or their real applications in DFAFCs. Chronoamperometric exper-ments were widely applied to explore the catalytic stability andeaction mechanism [27]. The long-term activity and durabilityf the Pd-based catalysts were further assessed by chronoamper-metry test (i–t curve) with the potential fixed at 0.1 V for 600 ss shown in Fig. 7. A gradual decrease can be seen in the simi-ar model for all catalysts in the oxidation current density with

ime, indicating thereby the poisoning of electrocatalysts. How-ver, the initial current densities and limiting current densities forormic acid oxidation on Pd/GNS–CNTs (5:1) catalyst are found toe higher than the other catalysts in the whole process. The current

ig. 8. (a) Cyclic voltammograms of formic acid oxidation on the as-prepared catalysts a.1 V in 0.25 M H2SO4 solution containing 0.25 M HCOOH.

Fig. 7. Chronoamperometry curves for glass carbon electrodes modified with dif-ferent catalysts in 0.25 M H2SO4 solution containing 0.25 M HCOOH at 0.1 V.

densities at the end of each test are listed in Table 1. Notably,Pd/GNS–CNTs showed superior activity and durability as comparedto their Pd/GNS counterparts. The electro-oxidation current onPd/GNS–CNTs (5:1) electrode at 600 s is 1.2–6.5 times as high as thaton the others. The much better long-term electrocatalytic activityof Pd/GNS–CNTs (5:1) catalysts is due to the electrode struc-ture, which is advantageous for efficient diffusion and transportof intermediates or by-product. The results further demonstrate

that the Pd/GNS–CNTs (5:1) catalyst exhibits the best perfor-mance. This result is in agreement with its behavior in the cyclicvoltammogram.

t a scan rate of 50 mV s−1; (b) chronoamperometry curves for different catalysts at

S. Yang et al. / Electrochimica Acta 62 (2012) 242– 249 247

F Pd/GNc specs

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ig. 9. (a and b) The linear sweep voltammetry curves for formic acid oxidation on

urrent density with the square root of scan rates; (d) electrochemical impedanceolution containing 0.25 M HCOOH.

For comparison, Pd/GNS–CNTs (5:1)-2 was synthesized by ultra-onication of graphene and CNTs suspensions with the same CNTsontent. Graphene was prepared by the chemical process withouthe presence of CNTs. As shown in Fig. 8, Pd/GNS–CNTs (5:1) cat-lyst showed superior activity and durability as compared withd/GNS–CNTs (5:1)-2 catalyst for formic acid electro-oxidation.he enhanced performance could attribute to the well-dispersedd NPs on the uniform morphology of GNS–CNTs (5:1) composite.t suggests that CNTs is a better “separator” material to separatend stabilize the graphene in the GNS–CNTs (5:1) material. TheNS–CNTs (5:1) material has a larger surface contact or cover-ge area as compared to GNS–CNTs (5:1)-2. Hence, the chance ofraphene agglomerated to form graphite platelets is less in theNS–CNTs (5:1) composite. In addition, the composite maintains

he larger surface to facilitate the transport of the electrolyte andormic acid through the surface of the catalyst. Thus more active

aterial will participate in the electrochemical reaction to give aigher activity.

The linear sweep voltammetry (LSV) curves of the Pd/GNS–CNTs5:1) and Pd/GNS electrode at various rates in 0.25 M H2SO4 solu-ion containing 0.25 M HCOOH are shown in Fig. 9a and b. It can beeen that oxidation potential and peak current density for formiccid oxidation become more prominent with the scan rates increas-ng. It is indicated that the oxidation of formic acid is an irreversiblelectrode process. The peak current density of Pd/GNS–CNTs (5:1)s greater than that of Pd/GNS, anticipating higher electrochemi-al activity. Fig. 9c shows the relation of peak current (ip) to thequare root of scan rates (v1/2) for Pd/GNS–CNTs (5:1) compositelectrodes as a comparison with Pd/G. It can be seen that ip depends

n v1/2 linearly, confirming that a diffusion-controlled process takeslace. According to the Eq. (7) [28], provided that both electrodesave same number of n, A, and C0*, where n, A, C0* stand for elec-ron transfer numbers, electrode area, and initial concentration,

S and Pd/GNS–CNTs (5:1) electrode at different scan rates; (c) relationship of peaktroscopy from 100 kHz to 10 mHz for different catalyst materials in 0.25 M H2SO4

respectively, which is almost the case, diffusion coefficients(DGNS–CNTs and DGNS) for Pd/GNS and Pd/GNS–CNTs (5:1) compos-ite electrodes are compared. From Eq. (8), it is evident that CNTsimprove the diffusion coefficient of the electrode. Additionally, theimproved electron-transfer kinetics at the Pd/GNS–CNTs catalystcan limit the amount of intermediates. Furthermore, the three-dimensional electrode structure of Pd/GNS–CNTs is expected to beadvantageous for efficient diffusion and transport of by-product[29,30]. Therefore, the GNS–CNTs as a support of Pd-based electro-catalyst for formic acid oxidation have a better performance.

ip = (2.69 × 105)n3/2AD1/20 C0

∗v1/2 (7)

DGNS–CNTs

DGNS=

((ip/v1/2)GNS–CNTs

(ip/v1/2)GNS

)2

= 2.6 (8)

The electronic conductivity of Pd/GNS–CNTs (5:1) hybrid mate-rials (ca. 5.3 S m−1) is about four times that of the Pd/GNS (ca.1.2 S m−1), demonstrating the improved electron transport due tothe existence of the CNTs. The fact that CNTs increase the conduc-tivity of the electrode can also be derived from the electrochemicalimpedance spectroscopy (EIS). EIS spectra of formic acid oxida-tion on electrodes Pd/GNS and Pd/GNS–CNTs (5:1) are shown inFig. 9d. Both electrodes exhibit a semicircle at higher frequencyregion and a straight line at lower frequency region (Fig. 9d). Athigh frequencies, the diameter of the semicircle has been con-sidered as the charge transfer resistance representing the rate ofcharge exchange between ions in aqueous and composite at elec-trochemical interface [31]. It can be seen that the diameter of thesemicircle of Pd/GNS–CNTs (5:1) is smaller than that of Pd/GNS.

This suggests that formic acid oxidation proceeds much easier onelectrode Pd/GNS–CNTs (5:1) than on electrode Pd/GNS. Thestraight line at low frequency in the EIS suggests the presence ofWarburg diffusion resistance [32]. The results confirm that CNTs

248 S. Yang et al. / Electrochimica A

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ig. 10. Schematic diagram explaining the conversion of adsorbed COads species toO2 on Pd/GNS–CNTs hybrids.

lay a key role in increasing the conductivity of the compositelectrode.

A more recent investigation demonstrated that the slowdsorption of the “CO-like” intermediate might be the principaleason for the deactivation of Pd catalysts during formic acidxidation [8]. As we know, the graphene nanosheets prepared byhe chemical reduction with hydrazine leave behind some residualxygen groups on the graphene. The remarkably strong antipoi-oning activity of the Pd/GNS electrocatalysts has been found to bessociated with the type and surface density of covalently boundxygen containing groups remained on the GNS support. Theresence of residual oxygen groups on the graphene support canromote the oxidation of CO adsorbed, COads, on the active Pd sitesia the mechanism [33]. The proposed mechanism is described inhe following equations and in the schematic of Fig. 10:

NS + H2O → GNS–(OH)ads + H+ + e− (forward scan) (9)

d–COads + GNS–(OH)ads → CO2 + Pd + GNS + H+ + e

− (reverse scan) (10)

Dissociative adsorption of water molecules on the GNS supportreates GNS–(OH)ads surface groups adjacent to Pd NPs (Eq. (9)),hich readily oxidize COads groups on the peripheral Pd atoms (Eq.

10)). The hydrophilic nature of GNS promotes water activation ands the major driver in this mechanism.

Based on the above results, these favorable properties maye attributed to three aspects. First, the CNTs are a better “sep-rator” material as regards the separation and stabilizationf graphene in the GNS–CNTs (5:1) material. The GNS–CNTs5:1) material has a larger surface contact or coverage areahich can provide a support for anchoring well-dispersed PdPs. Additionally, the CNTs work as a highly conductive matrix

or quickly providing electrons which might be favorable forlectrochemical reaction. In addition, the special frameworksnd properties of Pd/GNS–CNTs hybrids were helpful to facil-tate the transport of the electrolyte onto the surface of theatalysts and thus reduced the liquid sealing effect, which wouldnhance the active surface area for electrochemical reactions.inally, a possible functional effect of support may also occururing electrode reactions. The presence of residual oxygenroups on GNS–CNTs (5:1) plays a role on the removal of carbona-

eous species from the adjacent Pd sites, which can promote thexidation of formic acid [33].

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4. Conclusions

In summary, graphene nanosheets–MWCNTs hybrid materialswith different mass ratios were prepared by the in situ reduc-tion methods. The GNS–CNTs were used as the support of thePd NPs (Pd/GNS–CNTs) for formic acid electrooxidation and theirelectrochemical properties have been investigated. The improvedperformance of Pd/GNS–CNTs compared to Pd/GNS and Pd/CNTshas been attributed to inhibit the agglomeration of graphene sheetsand increased electrical conductivity brought about by CNTs. It isbelieved that the stability and electrical conductivity of GNS–CNTscomposite are increased by CNTs, on the other hand, the aggre-gation or restacking of graphene to form graphite platelets iseffectively prevented by CNTs. Thus, GNS–CNTs can be used as amore suitable and promising electrode material for formic acid fuelcells.

Acknowledgements

The work was supported by National Natural Science Founda-tion of China (No. 20873064), Natural Science Foundation of JiangsuProvince (No. BK2011030). S.D. Yang also gratefully acknowl-edged the support by Graduate Student Innovation Foundation ofJiangsu Province (CX09B 075Z) and NUAA Research Funding (No.NS2010165).

References

[1] B. Lim, M.J. Jiang, P.H.C. Camargo, E.C. Cho, J. Tao, X.M. Lu, Y.M. Zhu, Y.N. Xia,Science 324 (2009) 1302.

[2] Y.M. Zhu, S.Y. Ha, R.I. Masel, J. Power Sources 130 (2004) 8.[3] S.J. Kang, J. Lee, J.K. Lee, S.Y. Chung, Y. Tak, J. Phys. Chem. B 110 (2006)

7270.[4] W.P. Zhou, A. Lewera, R. Larsen, R.I. Masel, P.S. Bagus, A. Wieckowski, J. Phys.

Chem. B 110 (2006) 13393.[5] Y. Zhu, Y.Y. Kang, Z.Q. Zou, Q. Zhou, J.W. Zheng, B.J. Xia, H. Yang, Electrochem.

Commun. 10 (2008) 802.[6] J.T. Zhang, C.C. Qiu, H.Y. Ma, X.Y. Liu, J. Phys. Chem. C 112 (2008) 13970.[7] J.M. Feliu, E. Herrero, in: W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), Handbook

of Fuel Cells, vol. 2, Wiley, New York, 2003, p. 679.[8] X.W. Yu, P.G. Pickup, Electrochem. Commun. 11 (2009) 2012.[9] H. Miyake, T. Okada, G. Samjeské, M. Osawa, Phys. Chem. Chem. Phys. 10 (2008)

3662.10] S.D. Yang, C.M. Shen, Y.Y. Liang, H. Tong, W. He, X.Z. Shi, X.G. Zhang, H.J. Gao,

Nanoscale 3 (2011) 3277.11] J.S. Zheng, X.S. Zhang, P. Li, J. Zhu, X.G. Zhou, W.K. Yuan, Electrochem. Commun.

9 (2007) 895.12] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001)

169.13] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach,

R.D. Piner, S.T. Nguyen, R.S. Ruoff, Nature 442 (2006) 282.14] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183.15] N. Behabtu, J.R. Lomeda, M.J. Green, A.L. Higginbotham, A. Sinitskii, D.V.

Kosynkin, D. Tsentalovich, A.N.G. Parra-Vasquez, J. Schmidt, E. Kesselman,Y. Cohen, Y. Talmon, J.M. Tour, M. Pasquali, Nat. Nanotechnol. 5 (2010)406.

16] X.M. Chen, G.H. Wu, J.M. Chen, X. Chen, Z.X. Xie, X.R. Wang, J. Am. Chem. Soc.133 (2011) 3693.

17] J. Yang, C.G. Tian, L. Wang, H.G. Fu, J. Mater. Chem. 21 (2011) 3384.18] Z.L. Wen, S.D. Yang, Q.J. Song, L. Hao, X.G. Zhang, Acta Phys.-Chim. Sin. 26 (2010)

1570.19] M.H. Seo, S.M. Choi, H.J. Kim, W.B. Kim, Electrochem. Commun. 13 (2011)

182.20] S. Stankovich, D. Dikin, R. Piner, K. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.

Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558.21] C. Gomez-Navarro, J.C. Meyers, R.S. Sundaram, A. Chuvilin, S. Kurash, M.

Burghard, K. Kern, U. Kaizer, Nano Lett. 10 (2010) 1144.22] R.I. Jafri, T. Arockiados, N. Rajalakshmi, S. Ramaprabhu, J. Electrochem. Soc. 157

(2010) B874.23] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339.24] J. Yan, T. Wei, B. Shao, F.Q. Ma, Z.J. Fan, M.L. Zhang, C. Zheng, Y.C. Shang, W.Z.

Qian, F. Wei, Carbon 48 (2010) 1731.

Adv. Mater. 20 (2008) 3050.27] M.C. Zhao, C. Rice, R.I. Masel, P. Waszczuk, A. Wieckowskib, J. Electrochem. Soc.

151 (2004) A131.

mica A

[[[[

S. Yang et al. / Electrochi

28] L.H. Su, X.G. Zhang, Y. Liu, J. Solid State Electrochem. 12 (2008) 1129.29] M. Zhou, J.D. Guo, L.P. Guo, J. Bai, Anal. Chem. 80 (2008) 4642.30] H. Chang, S.H. Joo, C. Pak, J. Mater. Chem. 17 (2007) 3078.31] G. Wu, L. Li, J.H. Li, B.Q. Xu, J. Power Sources 155 (2006) 118.

[

[

cta 62 (2012) 242– 249 249

32] A. Tarola, D. Dini, E. Salatelli, F. Andreani, F. Decker, Electrochim. Acta 1999(1999) 4189.

33] S. Sharma, A. Ganguly, P. Papakonstantinou, X.P. Miao, M.X. Li, J.L. Hutchison,M. Delichatsios, S. Ukleja, J. Phys. Chem. C 114 (2010) 19459.