Cite this: DOI: 10.1039/c3ra23112g

3-Dimensionally self-assembled single crystallineplatinum nanostructures on few-layer graphene as anefficient oxygen reduction electrocatalyst3

Received 30th November 2012,Accepted 17th February 2013

DOI: 10.1039/c3ra23112g

www.rsc.org/advances

Sreekuttan M. Unni,a Vijayamohanan K. Pillaib and Sreekumar Kurungot*a

Here, we report for the first time the synthesis of a 3-D self-assembled single crystalline platinum

nanostructure directly on the graphene surface (PtNAGE) without using any harmful structural directing

agents. A slow reduction method is used to prepare the desired platinum morphology. Initial formation of

platinum nanoparticles and their homogenous dispersion on the surface of graphene have been observed

10 h after the commencement of the reduction using formic acid as the reducing agent. From these

initially deposited seed particles, the growth starts on the {111} facets along the ,111. direction and the

nanostructure formation is completed within 72 h of the commencement of the reaction. The individual

assembly has a diameter of y80 nm. PtNAGE shows superior electrocatalytic activity towards oxygen

reduction compared to graphene supported platinum (PtGE) and commercial carbon supported platinum

(PtC) catalysts. PtNAGE is less vulnerable to strong hydroxyl adsorption compared to PtC and PtGE. Specific

activity and mass activity of the catalyst are high compared to PtC by a factor of 6.50 and 1.80, respectively,

and 4.00 and 3.05, respectively, compared to PtGE. The limiting current density of PtNAGE is 1.28 and 1.20

times higher than PtGE and PtC, respectively. Kinetic analysis of PtNAGE shows that the oxygen reduction

reaction follows first order kinetics involving a four electron transfer mechanism with the direct formation

of water. In addition to this, it has been observed that PtNAGE also prevents surface area degradation

better than the commercial platinised carbon under potential induced conditions.

1 Introduction

The size and shape dependent changes induced in thephysico-chemical properties of metal nanoparticles can beattributed to the electron movement in a restricted environ-ment which is commonly known as quantum confinement.Even diminutive variations in morphologies show unprece-dented alterations in the property characteristics of nanopar-ticles. Such changes significantly broaden the scope of thematerials for distinctive applications in nanotechnology.1–6

Fascinated by such dramatic changes in the propertiesinduced by the variations in the size and shape, developmentof appropriate synthetic strategies for metal nanoparticles withthe desired morphologies has become a challenging endea-vour in nanotechnology.3,4,7,8 Platinum (Pt) morphologies inthis context are widely studied because of their excellenttuneable properties that span into various applications.8–11 Inenergy sector, Pt catalysts are vital to Polymer Electrolyte

Membrane Fuel Cells (PEMFCs), which are generally poweredby hydrogen, methanol, ethanol, formic acid, etc.12–14 TheOxygen Reduction Reaction (ORR) is one of the mostchallenging quests concerning a PEMFC electrode, for whichPt has become an inevitable component to minimize theoverpotentials.15 However, due to the high cost and availabilityissues with Pt, current PEMFC research is mainly focused onthe use of a minimal amount of Pt pertaining to a higherefficiency.16 Catalytic efficiency of Pt based nanomaterials canbe enhanced by fine tuning their morphology, and hencerevealing that the shape of the nanoparticles is significant forassessing the catalytic activity. Many reports are available onnanowires, nanotubes, nanorods, nanodendrites, multipods,yolk shells, nanoflowers, nanocapsules and urchin likestructures of Pt and most of these morphologies showenhanced ORR activity over simple Pt nanoparticles with asize range of 2–4 nm.17–35

Among the different morphologies of Pt, three dimension-ally (3-D) assembled nanostructures show enhanced ORRactivity compared to zero dimensional nanoparticles. Ptnanodentrite, nanourchin, nanowire, etc. assemblies areshown to possess far fewer defective sites as well as theminimum number of low coordination sites due to theirdowny crystalline planes.36,37 These factors inevitably reduce

aPhysical and Materials Chemistry Division, National Chemical Laboratory, Pune,

Maharashtra-411008, India. E-mail: k.sreekumar@ncl.res.in;

Fax: (+91) 20 2590 2636; Tel: (+91) 20 2590 2566bCentral Electrochemical Research Institute, Karaikudi, Tamil Nadu-630006, India

3 Electronic supplementary information (ESI) available: TEM images of PtNAGEat different time intervals of the reaction. See DOI: 10.1039/c3ra23112g

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PAPER

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the surface energy and interfacial resistance towards oxygenadsorption. In this regard, the 3-D assembled nanostructuresynthesis is a daunting challenge for the desired applications.Most of the reports currently dealing with the synthesis ofthese nanostructures depict the significance of structuredirecting moieties. However, for fuel cell catalysis, supportedcatalysts are preferred. Since most of the reported literature onPt nanostructures deals with unsupported particles, develop-ment of feasible strategies for the direct growth of the 3-Dassembly of Pt nanostructures without any surfactant on thesupport materials is very important.

Slow reduction of the Pt ion is very effective for thesynthesis of nanowires or dentrites directly on the supportmaterials. Few reports are available on the direct growth of Ptnanowires, dentrites and stars on the surface of conductingcarbon and carbon nanotubes (CNTs) using HCOOH as a slowreducing agent.23,38,39 Recently, Adzic et al. reported thesynthesis of carbon supported Pt–Pd core–shell nanowireswith enhanced ORR activity.40 However, these core–shellnanowires were prepared through a multistep processes whichincludes removal of the structure directing groups as amandatory requirement in order to activate the surface ofthe nanowires. Such post synthesis treatments not onlyadversely affect the morphologies, but it also appeared to bevery difficult to get rid of the contamination of the noble metalsurface by the surfactants.

The work we presented here is important in this contextbecause it involves a unique preparation strategy of a 3-Dassembled platinum nanostructure on graphene nanosheetsby using HCOOH as the reducing agent, devoid of anycatalytically harmful structure-directing moieties. It is wellknown that graphene has a high conductivity and surface areacompared to the other allotropes of carbon.41–43 Grapheneitself improves the catalytic activity of Pt by localizing theexcess electrons closer to the Pt and thereby making it morefavourable for electron withdrawing reactions. The 3-D self-assembled Pt nanostructure on graphene developed by theapproach adopted here shows enhanced ORR activity which issuperior to Pt nanoparticles supported on graphene or onconducting carbon. Moreover, these PtNAGEs are less vulner-

able to dissolution, aggregation and ripening during theoperating electrochemical environments. Thus, it may beexpected that the combined effects of the nanostructure andthe features of the graphene nanosheets, such as their highconductivity, enhance the total ORR activity.

2 Experimental section

Materials

Hydrogen hexachloroplatinate hexahydrate (H2PtCl6?6H2O)was purchased from Aldrich Chemicals. Formic acid(HCOOH) was procured from Rankem Chemicals. Few layergraphene was purchased from J K Impex, India. All thechemicals were used as received without any further purifica-tion. A copper grid with a carbon support (Icon Analytical) wasused for the HR-TEM observations.

Synthesis of the 3-D self-assembled Pt nanostructure

PtNAGEs were synthesized by the chemical reduction of a Ptprecursor with formic acid. All the experiments were con-ducted in aqueous solution at room temperature and under anambient atmosphere. In a typical synthesis of PtNAGE (40wt%) on a graphene sheet, 106 mg hexachloroplatinic acid and6 mL formic acid were dissolved in 20 mL H2O and sonicatedwith acid treated graphene sheets for 30 min. All the aqueoussolutions for the reaction were prepared in deionized water.After this initial dispersion, the solution was stored at roomtemperature for 72 h. The product was collected by centrifuga-tion and washed several times with water, and then dried in anoven at 60 uC for further use in characterization andelectrochemical measurements. To study the growth mechan-ism of the catalysts, the samples were collected at differenttime intervals. For an effective comparison, a 40 wt% Ptsupported graphene catalyst was prepared using a conven-tional polyol method as reported elsewhere12 and Pt (40 wt%)on carbon was purchased from Johnson Matthey Pvt. Ltd.

Sample characterization

High-Resolution Transmission Electron Microscopy (HR-TEM)images were obtained with a Tecnai-T 30 model at anaccelerated voltage of 300 kV. The samples for TEM wereprepared by placing a drop of the catalyst sample inisopropanol onto a carbon coated Cu grid. The Pt loadingwas confirmed by Thermo Gravimetric Analysis (TGA). X-rayDiffraction (XRD) was recorded on an Xpert Highscore Plusinstrument using Cu-Ka radiation at a step of 0.02u (2h).Scherrer and Bragg formulae were employed to calculate themean crystalline size and the lattice parameters of thecatalysts. X-ray Photoelectron Spectroscopy (XPS) measure-ments were carried out on a VG Micro Tech ESCA 300uinstrument at a pressure of . 1 6 1029 Torr (pass energy of 50eV, electron take off angle of 60u and the overall resolution ofy0.1 eV).

Electrochemical studies

The electrochemical properties of the catalysts were measuredby cyclic voltammetry (CV) using an Autolab PGSTAT30 (Eco-

Scheme 1 Illustration of the formation of the 3-D self-assembled singlecrystalline platinum nanostructure on few-layered graphene (PtNAGE).

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Chemie) instrument and rotating disk electrode (RDE) usingPine Research Instrumentation in a conventional three-electrode test cell with a reference hydrogen electrode (RHE)and a platinum foil as the reference and counter electrodes,respectively.

To prepare the working electrode, after polishing the glassycarbon (GC) electrode using 0.3 and 0.05 mm alumina slurries,a 10 ml aliquot of the catalyst slurry, made by sonicating 10 mgof the catalyst in 2 ml 3 : 2 ethanol–water mixture, was drop-coated on the electrode surface. Subsequently, 1.5 ml of 0.01wt% Nafion diluted with ethanol was applied on the wholesurface of the electrode to yield a uniform thin film. Thiselectrode was then dried in air and was used as the workingelectrode for all the electrochemical studies.

The CV measurements were conducted with a sweep rate of50 mV s21 in a 0.1 M HClO4 solution. The electrochemicalactive surface areas (EASs), were calculated by measuring thecharge associated with the Hads (QH) between 0.0 and 0.4 V andassuming Qref = 0.21 mC cm22. The EAS of Pt was calculatedbased on the relation EAS = QH/(Qref 6 m), where QH is thecharge for H adsorption (mC cm22), m is the Pt loading on theelectrode (mg cm22), and Qref is the charge required for themonolayer adsorption of hydrogen on a Pt surface (0.21 mCcm22). The adsorption of the hydroxyl species was calculatedbased on the OHad peak in the CV curves at the potential largerthan 0.6 V. Dividing the hydroxyl adsorption charge by theoverall active surface area resulted in the surface coverage ofthe OHad species. Kinetics of the oxygen reduction reactions ofall the catalysts were studied using a rotating disk electrode(RDE) with a platinum loading in the working electrode of0.102 mg cm22, in 0.1 M HClO4, using a three-electrode cellassembly at a scan rate of 10 mV s21 and electrode rotationspeeds of 900, 1200, 1600, 2000 and 2500 rpm. The stabilitytests were conducted at ambient temperature in 0.1 M HClO4

solution.

3 Results and discussion

Morphological characterization was carried out using HighResolution Transmission Electron Microscopy (HR-TEM). Thehigh magnification images of the formed particles clearlyshow the 3-D self-assembled nanostructure (Fig. 1 and 2). Aclose inspection of the structure of the PtNAGE reveals that thediameter of the assembly varies from 80 to 100 nm (Fig. 1d).Interestingly, the HR-TEM images reveal the single crystallinenature of the formed PtNAGE. Selected Area ElectronDiffraction (SAED) of the PtNAGE shows a dotted pattern,which reasserts the single crystalline nature. This dotted SAEDpattern indicates that the nanostructure is not an assembly ofnanowires, but possibly originates from a seed growthmechanism. Sun et al. have reported a similar mechanismfor a seed assisted growth of single crystalline Pt nanostarsinvolving multiarms originating from the predeposited seedparticles.44 It has been assumed that the growth mechanism issimilar to the Pt nanowire reported on the other carbonallotropes. Nanoparticle assembly grows in the ,111.

direction as marked in the HR-TEM image shown in Fig. 3.

More interestingly, the crystallographic alignment of each ofthe projections is single crystalline with a lattice spacing of2.2638 Å for the (111) plane. Typically, HCOOH reduces a Ption to Pt nanoparticles which get attached to the surface ofgraphene. These small nanoparticles act as the nucleationsites for further adsorbing ions and nanoparticles. Since E{111}

, E{100} , E{110} (where ‘E’ stands for the surface energy), mostof the Pt particles will be adsorbed onto the minimum surfaceenergy plane and hence the growth of the petals will beenhanced in the ,111. direction.

The nanoparticles which are reduced initially on thesurface of graphene may exhibit mixed faces of (111) and(100) planes so as to minimize the surface energy. Thesenanoparticles with truncated octahedral geometry lead to the

Fig. 1 (a–c) HR-TEM images of the PtNAGE after 72 h commencement of thereaction taken at different magnifications, inset SAED of the PtNAGE sampledafter 72 h, (d) size distribution histogram of PtNAGE.

Fig. 2 (a and b) The HR-TEM images of PtNAGE on graphene, (c) represents theSAED pattern of image (a).

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growth of the arms in the ,111. direction. PtNAGE is thusformed from the truncated octahedron which has largenumber of the {111} facets. During the slow reduction processusing HCOOH, at room temperature, preferential anisotropicgrowth on the (111) plane along the ,111. direction may bejustified by the lowest energy principle.38 However, the slowrate of the reduction and presence of the graphene supportalso play significant roles in facilitating the growth pattern.Further to these observations, X-ray Diffraction (XRD) analysisconfirms that the PtNAGE crystallized in a face centered cubic(fcc) lattice similar to that of bulk Pt (Fig. 4).

To understand the proposed growth mechanism of thePtNAGE, we collected samples at different time intervals (10,35 and 72 h) and performed the HR-TEM analysis.Interestingly, the sample collected 10 h after the commence-ment of the reaction shows the presence of Pt nanoparticleswith a truncated octahedron well dispersed on the graphenenanosheets, together with free Pt nanoparticles in the reactionmedium (Fig. S1 in the ESI3). This observation suggests thatthe reduction of Pt ions mainly takes place in the solution

phase and these reduced nanoparticles subsequently getslowly attached to the low energy surface of the truncatedoctahedron deposited on the graphene surface. This leads tothe one dimensional growth of Pt along the ,111. direction.The anisotropic factor, R, is a useful parameter that sheds lighton the morphology of nanoparticles as the X-ray can scanaround 1011 nanoparticles at a time, which is significantlymore than an electron beam could. The ratio D(100)/D(111),defined as the anisotropic factor, R, which is the ratio of thegrowth rates along the ,100. and ,111. direction(fwhm111cos(h200)/fwhm200cos(h111)), is .1.75 for the lowestenergy close packed structure of the octahedron and 0.58 forthe cubic structure.14,45 XRD analysis of the nanoparticles after10 h shows an R value of y0.8036, indicating that the formedstructure is a truncated octahedron (Fig. 4). Platinised carbon(PtC, 40 wt%) and Pt on graphene (PtGE, 40 wt%) prepared bythe polyol method also have matching R values (y0.8035).

After 35 h, it is observed from the TEM image that the (111)plane of the truncated octahedron starts growing slowly,resulting in PtNAGE possessing an R value of 0.8274 (Fig. S2 inthe ESI3). After 72 h, the R value of these PtNAGE structuresincreases to 0.8673, which unambiguously confirms thegrowth of the petals along the ,111. direction. Even thoughwith the combination of the HR-TEM and XRD analysis wecould postulate the mechanism of the PtNAGE formationduring the slow reduction process, a detailed study is requiredfor a comprehensive understanding of the coexisting factorsdictating the growth process.

With the aforementioned TEM and XRD data, we canconclude that the Pt nanoparticles with truncated octahedralplanes are forming first by the reduction of the Pt ions. Thesenanoparticles with a large number of {111} faces disperse verywell on the surface of the graphene sheets. The dispersed Ptnanoparticle thereafter acts as a seed for further growth of thenanostructure through the adsorption of Pt ions. Thisfacilitates the three dimensional growth of the clusterpossessing the PtNAGE morphology. The planar surfacemorphology of graphene also appeared to play a crucial rolein mobilizing the three dimensional self assembling processas we could not observe similar growth patterns on othercarbon nano morphologies such as CNTs.

The X-ray photoelectron spectroscopy (XPS) spectra of thePt 4f core level in PtNAGE and PtGE are given in Fig. 5.Different peaks are observed for Pt 4f due to its multipleoxidation states. The Pt 4f(7/2,5/2) peak could be deconvolutedinto three sets of spin–orbit doublets. For PtNAGE, the Pt 4f(7/2,5/2) peaks observed at 71.35 and 75.08, 73.79 and 78.81,76.65 and 80.76 eV can be ascribed to the Pt0, Pt2+ and Pt4+

moieties respectively.46,47 Similarly, PtGE also shows three setsof peaks at 71.35 and 75.00, 73.92 and 78.81, 76.69 and 81.17eV respectively due to the Pt0, Pt2+ and Pt4+ moieties. Anadditional two sets of doublets of Pt2+ and Pt4+ are seen mainlydue to the interactions between Pt and the oxygen present ongraphene and oxygen on Pt (GE–O–Pt and GE–Pt–O).48 XPSspectra of Pt 4f in both PtNAGE and PtGE show similarbinding energy which indicates that the interaction between

Fig. 4 X-ray diffraction pattern of the PtNAGE supported on graphene sampledat different reaction times.

Fig. 3 HR-TEM image of the PtNAGE with an image width of 2 nm.

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the Pt and graphene supports are similar for the systemevolved by the slow reduction as well as by the polyol methods.

Many factors influence the ORR activity of Pt. A fewprevailing factors are: (1) Pt–Pt distance, (2) coordinationnumber of Pt, and (3) electron density state of the 5d orbital ofPt.49–51 PtNAGE meets all these criteria and hence shows anenhanced ORR activity. The XRD analysis shows a decreasedd-spacing of the Pt (111) plane compared to the correspondingplanes of Pt on carbon (PtC) and graphene (PtGE) (0.225 nm,0.226 nm and 0.226 nm respectively for PtNAGE, PtC andPtGE). Analysis of the samples collected at different timeintervals clearly depicts the transition in the d-spacing as theone dimensional growth proceeds. Samples corresponding to10 and 35 h after the commencement of the reaction displaythe same d-spacing, viz. 0.226 nm, whereas the one sampledafter 72 h displays a slightly lower d-spacing of 0.225 nm.Along with the reduced d-spacing, most of these PtNAGE havesmooth crystalline planes which minimize the low-coordina-tion defect sites, which fulfils another criterion to meet theenhanced ORR and durability characteristics. Randomlylocalized holes and the electrons of graphene will migratecloser to the platinum metal nanoparticle after metal decora-tion.50 Once this happens, these localized electrons concur-rently modify the electron density of the 5d orbitals, andsubsequently facilitate the reduction process of oxygen. Morestudies to understand the role of the support materials on

modifying the electronic properties of the dispersed metalnanoparticles are ongoing in our laboratory.

PtNAGE, PtC and PtGE exhibit well defined characteristicpeaks for hydrogen desorption and adsorption, oxide forma-tion and reduction (Fig. 6a) in cyclic voltammetry (CV)analysis. The electrochemically active surface areas (EASs)calculated from the charge corresponding to the hydrogendesorption peaks from the CV analysis are 15.55, 56.80 and20.21 m2 g21 respectively for PtNAGE, PtC and PtGE. Thisdifference in EAS is mainly related to the particle size. Ptnanoparticle size is around 4.5 and 8 nm respectively for PtCand PtGE, compared to PtNAGE. Due to the small size of Ptparticle in PtC, it shows higher EAS compared to rest of thematerials. Hydroxyl (OH) adsorption on crystalline planesstrongly depends on the ORR activity. Strong adsorption of theOH groups always retards the oxygen reduction process.15,52

The OH adsorption at E . 0.6 V (Fig. 6b) (the area ofadsorption of OH at a potential higher than 0.6 V isnormalized by EAS53) of PtNAGE is significantly low comparedto PtGE and PtC. This indicates that the 1-D projections ofPtNAGE, which have more exposed (110) planes due to thegrowth along the {111} facets of the truncated octahedronseeds, do not bind with the OH species too strongly.Consequently this feature provides a favourable adsorptionstate required for the oxygen reduction. However, for PtGE and

Fig. 5 X-ray photoelectron spectra of the Pt 4f core level of (a) PtNAGE and (b)PtGE.

Fig. 6 (a) Cyclic voltammograms of the three catalysts (40 wt% Pt) in 0.1 MHClO4 at room temperature with a scan rate of 50 mV s21; (b) hydroxyl surfacecoverage (hOH) for the three catalysts in 0.1 M HClO4 at a potential higher than0.6 V in room temperature with a scan rate of 50 mV s21.

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PtC, the more exposed planes are (111) and (100), which have arelatively high affinity towards the OH adsorption.54,55

The peak potential and the onset are more positive forPtNAGE on the backward sweep, which indicates fastdesorption of the adsorbed OH species from the Pt surface.

The enhanced ORR activity of PtNAGE was studied using aRotating Disc Electrode (RDE) in O2 saturated 0.1 M HClO4 at arotation speed of 1600 rpm and a scan rate of 10 mV s21

(Fig. 7a). Half-wave potential (E1/2) of PtNAGE is 0.81 V, whichis significantly higher than the E1/2 of 0.78 V displayed by PtCand 0.73 by PtGE. Specific activity56,57 (i.e. the normalizedvalue with respect to the EAS) of PtNAGE at a potential 0.85 Vis found to be 1.04 A m22 Pt which is higher than that for PtGE(0.26 A m22 Pt) and PtC (0.16 A m22 Pt). Similarly, massactivity (i.e. the normalized value with respect to the weight ofthe catalyst) of PtNAGE at a potential 0.85 V is found to be0.159 A mg21, which is superior to PtGE (0.052 A mg21) andPtC (0.088 A mg21). These results unambiguously confirm

that, for PtNAGE, despite its low active surface area, thecatalyst quality determined by the specific activity and massactivity is high by a factor of 6.50 and 1.80, respectively,compared to PtC and 4 and 3.05, respectively, compared toPtGE (Table 1). As evident from Fig. 7a, the limiting currentdensity (jl) of PtNAGE (25.5 mA cm22) is also higher than thatof PtGE and PtC (24.2 and 24.5 mA cm22, respectively). Thegreater the magnitude of jl, the more enhanced the ORRactivity is.

The Koutecky–Levich (K–L) equation was applied tocalculate kinetic current density based on the ORR polariza-tion curves, which can be described as follows:

1

j~

1

jlz

1

jkz

1

jf

where j is the measured current density, jk is the kinetic currentdensity, jl is the diffusion (mass-transfer) limited current densityand jf is the film diffusion current. Here, jf can be neglected as theamount of Nafion is significantly low and hence will not affect thelimiting current density. In the laminar flow region, the diffusioncurrent density is a function of the rotational velocity and hencethe above equation may be approximated as follows:

1

j~

1

nFkCO2

z1

0:62nFACO2D

2=3O2

u{1=6v1=2

Here, n is the number of electrons, F is the Faradayconstant (96 485.5 C), A is the area of the electrode (0.196 cm2)and CO2

is the concentration of the dissolved oxygen in theelectrolye solution (1.22 6 1026 mol cm23). This equation isvery useful to find out the rate constant of the ORR.

jl~0:62nFACO2D

2=3O2

u{1=6v1=2

where n is the number of electrons, F is the Faraday constant(96 485.5 C), CO2

is the bulk O2 concentration (1.38 6 1026 molcm23), DO2

is the diffusion coefficient (1.69 6 1025 cm2 s21) of O2

in the electrolyte, A is the area of the electrode (0.196 cm2), n is thekinematic viscosity of the electrolyte (0.01009 cm2 s21), and v isthe rotation rate of the electrode in radians per second (2prpm/60).The value obtained for the kinetic current was independent ofdiffusion and could be used to evaluate the intrinsic activity of thecatalysts. The plot of the inverse of current density (1/j) as afunction of the inverse of the square root of the rate (v21/2), at aparticular potential obtained from the hydrodynamic voltammo-gram, assists in the evaluation of the useful kinetic parameterssuch as kinetic current density (jk), number of electron transfer (n)and rate constant for ORR (k). The slope of the K–L plots (Fig. 7b)

Fig. 7 (a) Linear sweep voltammogram of the catalysts in O2 saturated 0.1 MHClO4 solution at room temperature (1600 rpm and 10 mV s21 scan); (b)Koutecky–Levich plot of the catalysts at different rotation speeds at a potentialof 0.7 V in O2 saturated 0.1 M HClO4 at room temperature with a scan rate of 10mV s21.

Table 1 Summary of the electrochemical active area, mass and specific activities for the ORR of the different catalysts prepared in this study

CatalystElectrochemical activesurface area (m2 g21)

Mass activity at0.85 V vs. RHE (A mg21)

Specific activity at0.85 V vs. RHE (A m22 Pt)

Limiting currentdensity (mA cm22)

PtNAGE 15.55 0.16 1.04 25.51PtGE 20.21 0.052 0.26 24.27PtC 56.80 0.088 0.16 24.58

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gives crucial insights on the number of electron transfer (n) and inthe present case ‘n’ is estimated to be 3.9, 3.7 and 3.8 for PtNAGE,PtC and PtGE, respectively. The nearly parallel nature of the linearplots corresponding to PtC and PtNAGE shows first orderreduction with the possible involvement of the same reactionmechanism. However, PtGE is found to exhibit a slightly differentmode of plot under the similar conditions. The K–L plots of thecatalysts clearly highlight the higher jk value of PtNAGE comparedto the other two catalysts. For example, the jk value measured at0.7 V for PtNAGE is found to be 1.3 and 5.1 fold higher than thatfor PtC and PtGE, respectively. A plot of jk vs. the applied potential,as shown in Fig. 8a, clearly highlights the level of superioritymaintained by PtNAGE along the potential window. Mostsignificantly, at higher potentials, the activation loss for PtNAGEis smaller compared to that of PtC and PtGE. Also, the Tafel plotsderived from the hydrodynamic voltammograms at an electroderotation speed of 1600 rpm, corrected for diffusion effect, suggestfine correlation in the mechanism of ORR for both PtNAGE andPtC (Fig. 8b). Clearly, the Tafel plot of PtGE exhibits a slightdeviation indicating a variation from the mechanism followed bythe other two systems. At the low overpotential region, the jk valueof PtNAGE is high compared to the other two catalysts.

This is significant because the current at the low over-potential region determines the efficiency of the electrocata-lyst.58 At 0.95 V, PtNAGE shows a 1.86 and 4.01 fold higher jk

value compared to PtC and PtGE, respectively. This high jk

value at the lower overpotential region thus gives conclusiveevidence on the enhanced ORR activity of PtNAGE due to itsunique morphological features and modulated adsorptioncharacteristics.

Durability analysis of these catalysts was carried out bypotential cycling between 0.0 and 1.2 V in 0.1 M HClO4 at roomtemperature with a scan rate of 50 mV s21 for 250 cycles(Fig. 9). This leads to 16% reduction in the EAS of PtNAGEcompared to 56 and 59% reduction observed, respectively, forPtGE and PtC. The reduction in the EAS of nanoparticles ismainly due to the ripening where the support also plays asignificant role. Potential cycling triggers carbon corrosion,and Pt significantly catalyzes this process. The enhanceddurability of the graphene-supported catalysts compared toPtC reveals that graphene is more resistant to corrosion underthe electrochemical environments.

4 Conclusions

We have presented a facile method for preparing a threedimensionally self-assembled platinum nanostructure on fewlayer graphene without employing any surfactants and byfollowing the simple in situ reduction of platinic acid byHCOOH. The formed PtNAGEs are single crystalline in nature,which show enhanced oxygen reduction activity. In a broadperspective, this PtNAGE may also find other potentialapplications especially in catalysis and studies to explore thepotential benefits of the system are underway in ourlaboratory.

Fig. 8 (a) Polarization plots of the catalysts, where kinetic current density (jk) iscalculated from the K–L plots at different potentials in O2 saturated 0.1 M HClO4

at room temperature and at a scan rate of 10 mV s21; (b) Tafel plots correctedto diffusion of the catalysts at the lower current density region derived from thelinear sweep voltammogram in O2 saturated 0.1 M HClO4 solution at a scan rateof 10 mV s21.

Fig. 9 Loss of electrochemical surface area as a function of potential cyclingnumbers in 0.1 M HClO4 at room temperature.

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Acknowledgements

This work was supported by CSIR through the HYDEN project(Project Code: CSC0122). Special thanks go to Dr S. Pal,Director, NCL, Pune. SMU thanks CSIR-UGC for JRF.

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