This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys., 2013, 15, 3803--3813 3803

Cite this: Phys. Chem.Chem.Phys.,2013,15, 3803

Towards a highly-efficient fuel-cell catalyst:optimization of Pt particle size, supportsand surface-oxygen group concentration

Navaneethan Muthuswamy,a Jose Luis Gomez de la Fuente,b Piotr Ochal,b

Rajiv Giri,b Steinar Raaen,c Svein Sunde,b Magnus Rønninga and De Chen*a

In the present work, methanol oxidation reaction was investigated on Pt particles of various diameters

on carbon-nanofibers and carbon-black supports with different surface-oxygen concentrations, aiming

for a better understanding of the relationship between the catalyst properties and the electrochemical

performance. The pre-synthesized Pt nanoparticles in ethylene glycol, prepared by the polyol method

without using any capping agents, were deposited on different carbon supports. Removal of oxygen-

groups from the carbon supports had profound positive effects on not only the Pt dispersion but also

the specific activity. The edge structures on the stacked graphene sheets in the platelet carbon-

nanofibers provided a strong interaction with the Pt particles, significantly reconstructing them in the

process. Such reconstruction resulted in the formation of more plated Pt particles on the CNF than on

the carbon-black and exposure of more Pt atoms with relatively high co-ordination numbers, and

thereby higher specific activity. Owing to the combined advantages of optimum Pt particle diameter, an

oxygen-free surface and the unique properties of CNFs, Pt supported on heat-treated CNFs exhibited a

higher mass activity twice of that of its commercial counterpart.

1. Introduction

The rapid depletion of fossil fuels and the challenges related toglobal warming necessitate the introduction of novel fuels andfuel chains, such as those based on hydrogen, its storage inchemicals such as methanol, and back-conversion to energy infuel cells. Generating electricity from methanol directly in themethanol oxidation reaction (MOR) in direct methanol fuelcells (DMFCs) is an attractive alternative to reforming themethanol first and subsequent utilization of the hydrogen asthe fuel.1–3 Pt supported on commercial carbons such as VulcanXC-72, Black Pearls BP 2000, Ketjen Black, etc. are normallyused as catalysts for MOR in DMFCs.4,5 However, poisoning ofPt by carbon monoxide (CO) and intermediates producedduring methanol oxidation at the anode is a major issue inimproving the performance.6–10 Generally, CO poisoning can bereduced by tailoring the properties of Pt catalysts either by

addition of a secondary metal such as ruthenium (Ru), tin (Sn),etc. or through favorable metal–support interactions.9–22

Recently, Pt-based catalysts supported on graphitic supportssuch as carbon-nanotubes (CNTs), nitrogen-doped CNTs (N-CNTs),carbon-nanofibers (CNFs) and carbon-nanocages (CNCs) werereported to have improved CO tolerance.15–21,23–27 It has beensuggested that the improved CO tolerance is due to electronicperturbations exerted by graphitic supports on Pt. Moreover,surface-science studies on Pt/graphite model catalysts by Oh et al.28

illustrate the significant decrease in the Pt–CO bond strength.However, unlike CNTs, where more basal planes are exposed, onlyedges are dominating in CNFs.29 This feature could make CNFs abetter catalyst support than CNTs as the presence of defects maylead to a stronger anchoring of the metal particles on the CNF.30

Tsuji et al.31 reported that a PtRu catalyst supported on a plateletCNF support exhibited higher activity towards methanol oxidationthan those supported on tubular CNF catalysts due to the presenceof a higher ratio of the edge sites compared to basal sites on CNFs.Moreover, Bessel et al.32 found that the catalysts containing 5% Ptsupported on platelet CNFs exhibited activities comparable to thatdisplayed by about 25 wt% Pt on Vulcan carbon.

Usually, DMFC anodes are fabricated with high metal loading(>20 wt%) on carbon-blacks to avoid drop in performance

a Department of Chemical Engineering, Norwegian University of Science and

Technology (NTNU), N-7491 Trondheim, Norway. E-mail: chen@nt.ntnu.nob Department of Materials Science and Engineering, Norwegian University of Science

and Technology (NTNU), N-7491 Trondheim, Norwayc Department of Physics, Norwegian University of Science and Technology (NTNU),

N-7491 Trondheim, Norway

Received 16th October 2012,Accepted 11th January 2013

DOI: 10.1039/c3cp43659d

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because of kinetic and mass transport limitations.33 Therefore,during catalyst synthesis with higher metal loadings, it is vitalto control the size of particles without any significant agglo-meration. Recent studies have shown that by the polyol method,it is possible to control the particle size at high metal loading ondifferent carbon supports.34,35 Also, a strong size-dependence inthe specific (per Pt-area) electro-catalytic activity for the MOR iscommonly observed.36

It has been argued that during the preparation of catalysts bythe polyol method, oxygenated groups on the support-surfaceintroduced through acid treatment are both beneficial anddetrimental to the catalyst dispersion.37–41 Some studies suggestthat these oxygen groups anchor the metal precursors and act asnucleation sites for the subsequent reduction of metal atoms,leaving them bound to the support surface.37,41,42 In this case,the dispersion of catalysts on carbon supports can be improvedby the presence of surface-oxygen groups. Other studies suggestthat the introduction of oxygen groups reduces the potentialdifference between the CNF surfaces and the metal colloids andthereby, reduces the electrostatic metal–support interaction,43,44

where metal NPs are formed via reduction of metal precursorsand anchored on the support surfaces.35,43,44

In addition, some studies were dedicated to the introductionof surface-oxygen groups on carbon supports to improve theCO poisoning tolerance behavior.19,45–48 However, the relationbetween the effect of these surface-oxygen groups on COpoisoning tolerance and the particle size of catalysts is notcompletely correlated.49–51

Therefore, a fundamental understanding of the relationbetween the surface chemistry of carbons, metal particle prop-erties, and electro-chemical performance is highly desirable,and is necessary for a rational design of the fuel cell catalysts.In this work, the catalytic implications of the carbon surfacestructure in the MOR will be addressed by a comparative studybetween the carbon-black and carbon-nanofiber supports. Thesurface concentration of oxygen groups on these supports wastuned by nitric acid treatment and heat treatment, followed byPt deposition. The catalyst properties towards the MOR wereinvestigated and the activity of the catalysts was compared atidentical particle diameters. The specific activity and massactivity of the catalysts were determined with respect to thePt particle diameter, carbon support type and oxygen groups onthe support surface, displaying a strong interdependence ofelectrocatalytic activity on both support type and Pt particlediameter.

2. Experimental method2.1. Materials and chemicals

Fe2(NO3)3�9H2O (99%) and ethylene glycol (99.5%) were pro-vided by Fluka. Al2O3 (PURALOX SCCa-5/200) came fromCondea, H2PtCl6 (99.995%) from Sigma-Aldrich and NaOH(99%) from Merck.

2.1.1. CNF synthesis. Carbon-nanofibers (CNFs) were syn-thesized by chemical vapor deposition using an Al2O3 supportediron catalyst in a tubular quartz reactor. About 1.0 g of growth

catalyst (20% Fe/Al2O3) was initially calcined at 600 1C instationary air for 3 hours and reduced at 700 1C in a hydrogen–argon – H2–Ar (40/160 ml min�1) flow for 2 hours. CNFs werethen grown at 550 1C in a H2–CO–Ar (27/80/160 ml min�1) gasflow for 18 hours.

2.1.2. Acid treatment of CNFs and carbon-black. The rawCNFs were treated thrice with concentrated nitric acid for 3 hoursto remove the growth catalyst and then repeatedly washed withdeionized water. The washed CNFs were then dried at 110 1C. Thisbatch will be referred to as CNF_N below. For comparison,carbon-black (Vulcan XC72) provided by Cabot Corporation wasalso treated in the same way, and is labeled XC72_N below.

2.1.3. Heat treatment of oxidized CNFs. About 1.0 g ofCNF_N support was subjected to heat treatment in a N2 atmo-sphere for one hour at 700 1C, and is labeled CNF_N_700 below.Another gram of CNF_N support was subjected to heat treat-ment in an Ar atmosphere for 2 hours at 1000 1C, and is labeledCNF_N_1K below.

2.1.4. Synthesis of Pt nanoparticles. The Pt colloidalsolution was prepared by a polyol method and without theuse of any capping agents.52 About 1.0 g of hexachloroplatinicacid was dissolved in 250 ml of ethylene glycol and the pH wasadjusted to 12 using 0.5 M NaOH. The reaction mixture washeld at 145 1C for 3 hours and then cooled down to roomtemperature. In the polyol process, the (PtCl6)2� ions arereduced to form Pt colloids by the concomitant oxidation ofethylene glycol to glycolic acid, which is present as glycolateanions in an alkaline medium.35 The reduction of Pt wasconfirmed by the color change from yellowish to black. Duringthe reduction process, the glycolate anions adsorb at the sur-face of Pt particles act as a stabilizer by generating an electro-static repulsive force between the particles.34,53 The synthesizedPt colloids were stored in air-tight sample containers. Noprecipitates were observed even after several days of storage.

2.1.5. Deposition of Pt nanoparticles on carbon supports.The appropriate volume of the colloidal solution was mixedwith each carbon support listed in Table 1 under sonicationand the pH-adjustment to acidic conditions was done using1.0 M HCl. The suspension was stirred for 18 hours at 55 1C,and subsequently centrifuged at 6000 rpm for 5 min. Thesupernatant solution remained clear and transparent, givingdirect evidence for the absence of any suspended Pt colloidalparticles. It follows that almost 100% of the Pt particles insolution were deposited on the carbon supports.

The supported catalysts were washed repeatedly with water,which leads to a clean Pt particle surface on the carbonsupports. The name of the catalyst, the Pt loading, the type ofcarbon support, and the average particle diameter (vide infra)are listed in Table 2. The present procedure will be referred tohere as adsorption polyol method because pre-synthesized Ptcolloids are deposited on different carbon supports.

2.2. Physical characterization

2.2.1. N2-adsorption measurements. The physical propertiesof carbon supports were characterized using a MicrometricsTristar 3000. Approximately 0.1 g of the carbon support was

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degassed at 200 1C overnight prior to the analysis. The surfacearea was calculated by the Brunauer–Emmett–Teller (BET)method. The total pore volume and pore-size distribution wereobtained from nitrogen adsorption using the Barrett–Joyner–Halenda (BJH) method.

2.2.2. Temperature programmed desorption (TPD). Tem-perature programmed desorption (TPD) was carried out on thecarbon supports using a thermogravimetric analyzer (NetzschSTA-429 instrument). The carbon supports (10–15 mg) wereheated in Ar flow (60 ml min�1) at 100 1C for 30 min. Later, thetemperature was raised to 1000 1C at a rate of 5 1C min�1. Gasesevolved during the heating process were monitored using aquadrupole mass spectrometer (Netzsch QMS403C instru-ment). The weight loss in the range of 100 1C to 1000 1C wasconsidered reflecting the percentage of oxygen groups presenton the carbon supports.

2.2.3. Temperature programmed oxidation (TPO). Tem-perature programmed oxidation (TPO) was carried out on thesupported metal catalysts using a thermogravimetric analyzer(TGA Q500, TA Instruments). 10–15 mg of the catalyst washeated to 850 1C (rate 10 1C min�1) in a mixture of air and Ar inthe ratio of 90 : 10 ml min�1. The metal residue left aftercombustion of the carbon material was collected for energydispersive X-ray studies.

2.2.4. Energy dispersive X-ray spectroscopy (EDX). Energydispersive X-ray spectroscopy (EDX) was carried out on themetal residues obtained from TPO in scanning electron micro-scopy (SEM Hitachi S-3400N) with an accelerating voltage of20 kV. From the Pt : O ratio detected from EDX signal on metalresidues the Pt loading present on each catalyst was estimatedand listed in Table 2.

2.2.5. Transmission electron microscopy (TEM). TEM char-acterization for the supported Pt catalysts was performed in a

JEOL JEM-2010 electron microscope equipped with a tungstenfilament. The samples were prepared by dispersing the powdersamples in ethanol and a drop of this suspension was placed ona holey carbon film on a copper grid.

2.2.6. X-ray photoelectron spectroscopy (XPS). X-ray photo-electron spectroscopy (XPS) was performed to investigate thePt-4f7/2 binding energy of Pt nanoparticles supported oncarbon-black and carbon-nanofibers. The samples were pre-pared by covering a carbon tape with an even layer of catalystpowder. The measurements were conducted using a hemi-spherical SCIENTA SES 2002 electron energy analyzer. A mono-chromatised Al-Ka X-ray source (Gammadata Scienta) was usedto obtain the excited radiation.

2.3. Electrochemical characterization

2.3.1. Electrochemical setup. The electrochemical studieswere conducted with a conventional three-electrode system,with a reversible hydrogen electrode (RHE) as the referenceelectrode and platinum foil as the counter electrode. Theworking electrode was prepared by coating the catalyst ink ona disk-type glassy carbon as described by Schmidt et al.54 Thecatalyst ink was prepared by sonicating 1.0 mg of the preparedcatalyst in 1.0 ml of water. About 20.0 ml of the catalyst wereplaced on the disk-type glassy carbon and dried in N2 flow,followed by addition of 20.0 ml of diluted Nafiont and driedagain. Prior to the electrochemical measurements the workingelectrode was electrochemically conditioned between 0.02 and1.2 V at a scan rate of 100 mV s�1 and 10 mV s�1 to obtain stablecyclic-voltammograms (CVs).

2.3.2. CO stripping. Carbon monoxide (CO) adsorption wascarried out from a CO saturated 0.5 M HClO4 solution at anadsorption potential of 50 mV for 120 s. The excess of COdissolved in the solution was then removed by bubbling Ar for

Table 2 Catalyst names, Pt loadings and estimates of ESA, and Pt particle size

Catalyst name Carbon code Pt loading(%) ESA (m2 g�1 Pt) The average Pt particles sizea (nm)

Pt–XC72 XC72 18.6 109 2.5Pt–XC72_N XC72_N 18.5 86 3.2Pt–CNF_N CNF_N 16.7 65 4.3Pt–CNF_N_700 CNF_N_700 17.6 72 3.8Pt–CNF_N_1K CNF_N_1K 17.8 95 2.9ETEK — 20.0 67 4.110.6% Pt–CNF_N CNF_N 10.6 92 3.04.6% Pt–CNF_N CNF_N 4.6 105 2.7

a The average Pt particle size is calculated from CO stripping based on the relation d = 6 � 103/ESA � r, where, r is the density of Pt (21.45 g cm�3).

Table 1 Carbon supports, pre-treatments and estimates of surface area, pore size, pore volume, and percentage of surface functional groups

Carbon code Carbon supports Pre-treatmentsBET surfacearea (m2 g�1)

Pore size(nm)

Micropore volume(cm3 g�1)

Mesopore volume(cm3 g�1)

% of oxygengroups

XC72 Vulcan carbon black — 235 9.3 0.028 0.44 1.6XC72_N ’’ Conc. HNO3 169 9.8 0.020 0.34 5.5CNF_N Platelet CNF Conc. HNO3 127 11.0 0.006 0.31 6CNF_N_700 ’’ Conc. HNO3 and heated at

700 1C in a N2 atm. for 1 h— — — — 2.4

CNF_N_1K ’’ Conc. HNO3 and heated at1000 1C in an Ar atm for 2 h

166 12 0.006 0.46 1.8

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30 min by holding the electrode at the adsorption potential.After the removal of excess dissolved CO, the potential wasscanned from 0.02 V to 1.2 V at a 10 mV scan rate.

The CO surface area in cm2 (SCO) for each catalyst isestimated using the following relation,

SCO ¼QCO

0:420mC cm�2 (1)

In the relation above, QCO is the CO stripping charge obtainedby taking the second positive sweep (a CO removed curve) as abaseline and subtracting this baseline from the area under theCO oxidation peak. This area is then divided by the sweep rateto obtain QCO in coulombs (C). A monolayer of adsorbed COcorresponds to a stripping charge of 0.420 mC cm�2.

The average particle diameter of Pt on each carbon supportwas determined by the relation,

d ¼ 6� 103

ESA� rnm (2)

In the equation above, r is the density of Pt (21.45 g cm�3), andESA is the electrochemical surface area in m2 g�1.55

2.3.3. Electro-oxidation of methanol. Methanol electro-oxidation experiments were carried out in a 0.5 M H2SO4 and2.0 M methanol solution by cyclic-voltammetry and chrono-amperometry at room temperature. The cyclic voltammogramswere recorded at 10 mV s�1 from 0.02 to 1.2 V. The chrono-amperometry experiments were also carried out in the samesolution by holding the potential at 0.5 V for 6000 s.

3. Results3.1. Properties of the supports

The relation between the pre-treatment method applied tovarious carbon supports and the resulting textural propertiesis summarized in Table 1. The average pore size of CNFs isslightly larger than that of carbon-black. The micropore volumeof CNFs is much smaller than the Vulcan carbon. In CNFs, themesopores are generated by the entanglement of the 1D CNF.56

The pore volume and the surface area are reduced from 0.44 to0.34 cm3 g�1 and from 235 to 169 m2 g�1, respectively, by theintroduction of oxygen groups on the surface of carbon-black bynitric acid treatment. As the oxygen groups are removed fromthe CNF surface by heat treatment in the inert atmosphere, thepore volume and surface area are increased from 0.31 to0.46 cm3 g�1 and from 127 to 166 m2 g�1, respectively, for thistype of carbon.

Fig. 1 shows the temperature programmed desorption (TPD)profiles for the carbon supports in the absence of Pt catalyst tomeasure the oxygen groups. The oxygen groups on carbonsurfaces decompose at high temperatures,57–59 which seemsto be the case here also. About 6 wt% of oxygen groups is foundon CNFs oxidized with nitric acid (CNF_N). This is close to8 wt% reported by Ros et al.57 on fishbone carbon-nanofibersoxidized with nitric acid. The amount of the oxygen groupsis very low on the non-oxidized carbon-black, while it increasedto about 5.5 wt% when oxidized in nitric acid. The weight

losses in Fig. 1 were accompanied by the release of H2O, COand CO2 as detected by mass spectroscopy. The observedH2O signal might stem from physisorbed water and/or bydehydration of neighboring carboxylic groups forming carboxylicanhydride groups.58 CO may originate from phenol, carbonyl,quinone, ether and anhydride groups while CO2 may be formedby the decomposition of carboxylic groups, carboxylic anhydridesand lactones.59 The concentration of oxygen groups is lower onheat-treated supports CNF_700 (2.4 wt%) and CNF_1K (1.8 wt%)as expected.

3.2. Properties of Pt particles

The Pt NPs are well dispersed in the colloid as seen in the TEMimage in Fig. 2. The average diameter of the Pt NPs is 2.4 �1 nm. In addition, a few particles with a diameter below 1.0 nmare also observed.

The Pt loading for the various carbon-supported catalysts inTable 1 as obtained from TGA is listed in Table 2. Obviously, thepresent procedure results in good control of the Pt loading,which is a prerequisite for evaluating the effect of supports onPt dispersion.

TEM images of Pt–XC72 and Pt–XC72_N catalysts are shownin Fig. 3a and b. The corresponding Pt dispersions are shown as

Fig. 1 Temperature programmed desorption profile of carbon supportsin the absence of Pt catalyst. Conditions: 5 1C min�1, an Ar (60 ml min�1)atmosphere.

Fig. 2 High magnification TEM image of Pt NPs in colloidal solution. The inset isa Pt size distribution histogram.

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histograms in the insets. Pt is well dispersed on the non-oxidized carbon-black support (XC72) and the average diameteris 2.9 � 1.5 nm (Table 2). However, the Pt dispersion on theoxidized carbon-black support (XC72_N) is relatively poor andits average diameter is slightly larger (3.5 nm) than the Ptaverage diameter on the non-oxidized carbon-black support.Some particles in the range of 3.5–6 nm are also observed onthis support (Fig. 3b). Closer inspection of this catalyst atdifferent regions reveals that two or three Pt particles appeartogether in the form of agglomerates. This can be associatedwith a relatively weak Pt–support interaction.

The Pt dispersion is poor on oxidized CNFs (CNF_N) asshown in Fig. 4a and b, and a clear bimodal particle sizedistribution in the range of 1.5–3.2 nm and 3.5–6 nm (theFig. 4a inset) is apparent.

In addition, a few particles with the diameter of 10 nm arealso observed. However, the good dispersion of Pt on pre-heated CNFs at 1000 1C (Pt–CNF_N_1K) (Fig. 5a and b) isinteresting. The particle diameters are predominantly in therange of 1.5–3.2 nm and only very few particles in the range of3.5–6 nm are observed (the Fig. 5a inset). This shows thatduring the deposition process Pt particles form agglomerateson the carbon supports whose surfaces are oxygen-groupenriched (Fig. 4). The pre-heating treatment, applied on oxygen-group enriched CNFs prior to Pt deposition, improved the Ptdispersion (Fig. 5).

The metal dispersion is further illustrated by CO strippingresults. Fig. 6 shows the CO stripping-voltammograms andthe subsequent CV obtained for the catalysts Pt–XC72 (Fig. 6A),

Pt–XC72_N (Fig. 6B), Pt–CNF_N (Fig. 6C), and Pt–CNF_N_1K(Fig. 6D). The solid line on all four panels represents the COstripping-voltammetry and the dashed line represents the CVafter CO oxidation. The absence of H2 adsorption peaks inthe hydrogen-upd region (a lower potential range) in the COstripping-voltammograms for all catalysts confirms the com-plete coverage of CO molecules on the Pt surface.

A single peak at a potential of around 0.77 V is observed forthe Pt–XC72 catalyst (Fig. 6A), whereas, for the Pt–XC72_Ncatalyst, a shoulder with an onset potential of 0.62 V isalso observed along with the main peak (Fig. 6B). This shouldercan be attributed to Pt agglomerates as discussed in thestudies on carbon-black and glassy carbon (GC) supported Ptcatalysts.51,60–62

For the Pt–CNF_N catalyst, a minor peak at 0.68 V isobserved along with the main peak, indicating significantagglomeration (Fig. 6C). However, for the catalyst Pt–CNF_N_1K,this is not so. Instead, only a shoulder is observed along with themain peak, indicating less agglomeration (Fig. 6D). This is in goodagreement with the TEM observations above.

To confirm that the shoulder and minor peak observed inCO stripping-voltammetry on Pt–CNF_N are from Pt agglo-merates, catalysts with a low Pt loading (4.6 wt% and 10.6 wt%)on the same support (CNF_N) were also investigated withCO-stripping voltammetry, and the results are shown in Fig. 7.

Fig. 3 High magnification TEM images of (a) Pt–XC72 (18.6 wt% Pt) and(b) Pt–XC72_N (18.5 wt% Pt) catalysts. The inset is Pt size distribution histograms.

Fig. 4 (a) TEM image and (b) high magnification TEM image of Pt–CNF_N(16.7 wt% Pt) catalyst. The inset is Pt size distribution histograms.

Fig. 5 (a) TEM image and (b) high magnification TEM image of Pt–CNF_N_1K(17.8 wt% Pt) catalyst. The inset is Pt size distribution histograms.

Fig. 6 CO stripping voltammograms for Pt–XC72 (A), Pt–XC72_N (B), Pt–CNF_N(C) and Pt–CNF_N_1K (D). Conditions: 10 mV s�1, 0.5 M HClO4, and CO isadsorbed at 50 mV.

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For a catalyst with the lowest loading of Pt (4.6 wt%) on theoxidized CNF support (CNF_N), only the main peak at 0.77 V isobserved and no shoulder or minor peak is observed (Fig. 7A).For the catalyst with a loading of 10.6 wt%, a shoulder isapparent with an onset potential of 0.62 V (Fig. 7B). For aloading of 16.7 wt% a clear peak is obtained (Fig. 7C). Thisshows that once the Pt loading is increased the Pt particlesagglomerate on the oxidized CNF support (CNF_N).

The average Pt particle diameter was calculated from the COstripping experiment as described in Section 4 and listed inTable 2. The average Pt particle diameter calculated from theCO stripping follows the order: Pt–CNF_N > Pt–CNF_N_700 >Pt–XC72_N > Pt–CNF_N_1K > Pt–XC72 (Table 2). The TEMand electrochemical results thus appear to indicate thefollowing; the surface-oxygen groups introduced on CNFsfacilitate the Pt agglomeration. The heat-treatment of CNFsbefore Pt deposition at 700 1C and 1000 1C in an inert atmo-sphere helps in improving the Pt dispersion. The electrostaticinteraction between the Pt colloids and the carbon supports hasprofound effects on the catalyst preparation. It has beenobserved that the presence of surface-oxygen groups on CNFsdecreases the electrostatic attraction between the CNF surfaceand the Pt NPs.43 This causes Pt agglomeration by affecting thekinetics of Pt adsorption on an oxygen group enriched CNFsurface. Pre heat-treatment at 700 1C and 1000 1C in an inertatmosphere helps in removing surface-oxygen groups to acertain extent (Table 1). In addition, the heat treatmentincreases the surface area and generates a positively-chargedsurface as indicated by the higher point of zero charge (PZC) ofCNFs.43,44 As a result, a strong static interaction betweenpositively-charged CNFs and negatively-charged Pt NPs will beinduced at relatively low pHs (pH of 3 in the present work) andleads to well-dispersed Pt NPs on these high-temperaturetreated CNFs. This further supports our previous findings onthe importance of oxygen-free sites for anchoring metallicNPs.44 The present work demonstrates much better dispersionof Pt NPs on the CNF treated at 1000 1C than on the one treatedat 700 1C, possibly due to less oxygen on the former CNF asshown in Fig. 1.

In order to study the morphology of Pt NPs on differentsupports, HRTEM images were taken on Pt NPs in colloids andthe same supported on XC72 and CNF supports. The catalystsPt–XC72 and Pt–CNF_N_1K were selected for this study as bothhave identical Pt particle diameters. The inset in Fig. 8a showsthat the Pt NPs have a cuboctahedral shape in the colloidsolution, which is a generally accepted structure for Pt NPs.63,64

These NPs can be considered to be representative of poly-oriented particles. The inset in Fig. 8b shows that Pt NPs havea semi-cuboctahedral shape on carbon-black, which can beconsidered again representative of poly-oriented particles.However, the Pt NPs have an elongated shape on CNFs withthe principal direction parallel to the length of the fiber(Fig. 8c). This is most clearly seen in the particles when viewedat the edge of the fibers. The studies on lattice reflections showthat the (111) plane is more dominant. The interaction betweenPt particles with 100 atoms (B2 nm) and the edges of plateletcarbon-nanofibers has been investigated by a moleculardynamic (MD) simulation using the reactive force field in ourprevious studies. The results reveal a strong interactionbetween them, and that the mismatch between the atomicstructure of the platelet edge and the adsorbed Pt particlesleads to a significant reconstruction of Pt NPs, resulting in asubsequent flattening of the Pt particle.65 This is in goodagreement with the TEM observation (Fig. 8b and c).These results suggested that the unique edge structure of theplatelets provided a stronger interaction with Pt NPs thancarbon-blacks.

3.3. Electrochemical oxidation of methanol

For assessment of the electrocatalytic activity for the MOR, theelectrode potential was scanned between 0.02 V and 1.2 V at ascan rate of 100 mV s�1 and 10 mV s�1 in an aqueous solutionof 0.5 M H2SO4 and 2 M CH3OH. Examples of voltammetriccurves recorded at 10 mV s�1 are shown in Fig. 9. The current

Fig. 7 CO stripping voltammograms for 4.6% Pt–CNF_N (A), 10.6% Pt–CNF_N(B) and Pt–CNF_N (C). Conditions: 10 mV s�1, 0.5 M HClO4 and CO is adsorbed at50 mV.

Fig. 8 HRTEM images indicating the shape of Pt particles in colloidal solution(a), and on carbon black (b) and CNF (c).

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has been normalized by the corresponding CO surface area(Sco) obtained from the CO stripping experiment. The onsetpotential for the methanol oxidation during positive scan(curve I) starts at around 0.4 V and the maximum current isobserved at around 0.9 V.

The CVs of Fig. 9 indicate significant variations in electro-catalytic activity for the MOR, since they display significantvariations in the magnitude of the current at various potentials,including the peak potential. Since we wished to relate thisvariation in the peak potential for the different catalysts todifferences in the Pt particulate diameter and type of support,we will consider the value corresponding to the maximumcurrent in the forward scan (curve I) a measure of theelectrocatalytic activity for the MOR. The specific activity (themaximum current normalized with Sco) thus follows the order:Pt–CNF_N_700 > Pt–CNF_N > Pt–CNF_N_1K E E-TEK EPt–XC72_N > Pt–XC72. Except for Pt–CNF_N_1K, the specificactivity of CNF-supported catalysts (Pt–CNF_N_700 and Pt–CNF_N)is generally higher than that of the carbon-black-supportedcatalysts. The mass activity follows the order: Pt–CNF_N_700 >Pt–CNF_N_1K > Pt–XC72 E Pt–CNF_N E Pt–XC72_N > E-TEK.For these, the mass activity of CNF-supported catalysts(Pt–CNF_N_700 and Pt–CNF_N_1K) is generally higher thanthat of the carbon-black-supported counterparts; with theexception of Pt–CNF_N.

An important trend in the specific and mass activities isdisplayed in Fig. 10 in which these quantities are plotted as

functions of ESA and related to the average particle diametercalculated from CO stripping. The Pt particle diameter on theidentical support was varied by varying the Pt loading (4.6, 10.6and about 20 wt%) as well as the surface-oxygen content. Theresults in Fig. 10 can be classified into three categories, wherethe lines (I), (II) and (III) represent the CNF with high Ocontent, Vulcan carbon, and CNF treated at high temperatures,respectively. The trends in specific and mass activities are moreor less the same for all the supports, but the slopes are not thesame and are shifted with respect to one another. In general,the activity at a given Pt particle diameter follows the order:CNF with less O > CNF with rich O > Vulcan carbon. Evenregarded as purely empirical results, Fig. 10a and b displayclear effects of the particle diameter, the type of support andthe surface-oxygen content, which allows to an optimization ofthe catalyst with respect to MOR.

The de-activation of the catalysts was also evaluated bychronoamperometry. These experiments were performed at0.5 V in the same solution (0.5 M H2SO4 and 2 M CH3OH)used for the voltammetric measurement. Current vs. time for anumber of supports is shown in Fig. 11, where the currentagain has been normalized by the corresponding Sco value.Initially, the current density on all the catalysts drops rapidly.Thereafter, Pt–CNF_N and Pt–CNF_N_1K show remarkably lessdeactivation. We observed that the activity of these two catalystsdrops gradually and reaches steady state after approximately45 min. Finally, it is apparent that Pt–CNF_N_1K showslesser degree of deactivation when compared to all othercatalysts. The stationary specific activity follows the order:Pt–CNF_N_1K > Pt–CNF_N E Pt–CNF_N_700 > Pt–XC72_N >E-TEK > Pt–XC72. The stabilized mass activity follows the order:Pt–CNF_N_1K > Pt–CNF_N_700 > Pt–CNF_N E Pt–XC72_N EPt–XC72 > E-TEK.

Fig. 12a and b show the stabilized (stationary) specific andmass activity as a function of ESA related to average particlediameter. Again, the specific and mass activities of the CNF-supported catalysts outperform those of Vulcan carbon sup-ported catalysts with identical initial Pt particle size. Amongstall catalysts, the Pt–CNF_N_1K shows the highest stabilizedmass activity as well as stabilized specific activity. Thus, a CNFsupport pre-heated at 1000 1C appears to be the most promisingsupport for MOR among those investigated here.

Fig. 9 Voltammetric curves for methanol electro-oxidation. Conditions: 10 mV s�1,0.5 M H2SO4 � 2 M CH3OH solution, room temperature.

Fig. 10 (a) Specific and (b) mass activities as functions of ESA. The forwardpeak current in voltammetric curve is used to calculate the currentdensities. Conditions: 10 mV s�1, 0.5 M H2SO4 + 2 M CH3OH solution, roomtemperature.

Fig. 11 Chronoamperometric curves for methanol electro-oxidation. Condi-tions: 0.5 V, 0.5 M H2SO4 + 2 M CH3OH solution, room temperature.

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4. Discussion4.1. Effect of the carbon structures

The most important result presented here is that the electro-catalytic activity of carbon-supported catalysts depends criti-cally on the structure of that support. This may in principle bedue to the different surface structures of the Pt NPs on thedifferent substrates or due to substrate influence on electrondensity of Pt NPs i.e. variations in the density of states (DOS)near the Fermi level (the ligand effect).

The results above indicate strongly that the surface structureplays an important role in changing the catalytic properties ofthe catalysts with the different supports. In general, the surfacestructure/morphology of metal NPs has an influence on theadsorption of intermediates and a significant catalytic con-sequence of activity.66–72 Recently, Solla-Gullon et al.73 investi-gated the methanol oxidation activity of Pt NPs with differentshapes. They confirmed that the Pt NPs with different shapeshave different crystallographic domains exposed on the surface.The MOR studied by chronoamperometry revealed that amongthe Pt NPs with different orientation, the Pt NPs containing apreferential (111) orientation have the highest activity. Theshape-dependent activity for methanol oxidation was alsoreported by Susut et al.74 Therefore, if the carbon support signifi-cantly influences the structure of our Pt catalyst in this respect, theobserved differences in catalytic activity would result.

The XPS results indicate that d-band shifts and changes inelectron density are less consequential for the variations inelectrocatalytic activity. We therefore dismiss this as a majorcause for the difference in electrocatalytic activity of carbon-supported catalysts.

In summary, therefore, we hold the most likely cause for thedifferences in electrocatalytic activity demonstrated with thedifferent supports in Fig. 10 and 12 to be due to differences inthe surface structure of the catalyst induced by the supports.

4.2. Effect of oxygen-content on CNF surfaces

Another most important result presented here is that thesurface-oxygen content on carbon supports and the specificactivity have a strong correlation. This may in principle be dueto the fact that the surface-oxygen content plays a major role inPt dispersion or in the electron transfer process during MOR.

The results above support the inference that the Pt deposi-tion on an oxygen-enriched support results in bigger Pt parti-cles. In general, the Pt particle diameter has a greater influenceon MOR activity. In MOR, the CO is considered to be the mostprominent intermediate species that hinder the direct oxida-tion of methanol to CO2.14,75,76 The poison, CO, is oxidized byOHads according to the following equation:7,13,77

COads + OHads - CO2 + H+ + e� (3)

Here, the OHads species is supplied by the dissociation of wateron the Pt surface.7 The majority of the kinetic studies revealedthat the oxidation of CO (eqn (3)) is the rate-determining step(rds)7–10 for MOR. Therefore, the specific activity for MORdepends on the Pt–CO and Pt–OH bond strength. Mukerjeeand McBreen78 reported that a decrease in Pt particle size willmonotonically increase the Pt–CO and Pt–OH bond strengthand thus increase the intrinsic activation energies for the MOR.The decrease in particle size increases the fraction of lowco-ordination sites (edge, corner and step sites). A detaileddescription of these geometric characteristics is given byKinoshita for cubo-octahedral particles.79 The in situ X-rayabsorption spectroscopy (XAS) studies revealed that the adsorp-tion of CO and OH on these low co-ordinated sites are toostrong.78 The Brønsted–Evans–Polanyi relation suggests thatthe barriers for new bond formation involving strongly-adsorbed species increases proportionately with adsorptionenergies. It can be simply understood that more energy isrequired to break a stronger bond on Pt to make new bonds.As a result, the increase in the fraction of co-ordinativelyunsaturated sites that results from decreasing the particle sizewill monotonically increase the CO and OH binding energy andthus increase the intrinsic activation energies for the MOR.Therefore, the lower specific activity on smaller Pt NPs reflectsthe prevalence of low co-ordination corner, edge and step sites,which bind all adsorbed species more strongly. The relation-ship between the adsorption strength and activity is alsoconsistent with our previous kinetic study of heterogeneouscatalytic oxidation of hydrogen on different Pt catalysts, wherethe O binding energy with the Pt surface was found to be themost dominating factor determining the activity, and a larger Obinding energy resulted in a lower activity.80

In addition, Maillard et al.62 reported that larger particlesform OHads at a relatively low potential as compared to smallerparticles. The CO stripping studies (Fig. 7) appear to supportthese notions. The pre-adsorbed COads gets oxidized at arelatively low potential in the range of 0.64 V on larger Ptparticles and at a relatively high potential in the range of 0.77 Von smaller Pt particles (Fig. 7), revealing a low COads adsorptionstrength and also perhaps the formation of OHads at a relativelylow potential on larger Pt particles. This is reflected in methanoloxidation by the easy removal of COads at lower potentials onlarger particles as the scan progresses from lower potential tohigher potential, whereas, the COads remains on smaller Ptparticles and blocks the methanol molecules for further adsorp-tion. Therefore, the specific activity decreases with decreasing

Fig. 12 (a) Specific and (b) mass activities as a functions of ESA. The steady-statecurrent measured for 90 min by the chronoamperometry method is used tocalculate the current densities. Conditions: 0.5 V, 0.5 M H2SO4 + 2 M CH3OHsolution, room temperature.

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Pt particle diameter, which is in good agreement with theprevious results.36,49,50,81

Since the Pt particle diameter has great influence on MOR,the catalysts should be compared with the identical Pt particlediameter as shown in Fig. 10. Lines I and III in Fig. 10 indicatethat the Pt NPs supported on oxygen-enriched CNFs have lessconsequences on the activity. It has been recognized that theelectron-withdrawing properties of the oxygen groups on theCNF surfaces could reduce the p-electron density in grapheneand enhance the charge transfer from Pt NPs particles to thegraphene. As a consequence, it increases CO and OH adsorp-tion and thus increases the energy barrier of the reaction. Thisis in good agreement with our previously-reported results oflower activity and higher ignition temperature for hydrogenoxidation in the presence of CO for Pt NPs supported onoxygen-enriched CNFs compared to oxygen-deficient CNFs.80

This result is contradictory to earlier results, where it wasreported that the introduction of oxygen groups on graphiticplanes enhances the performance of electro-catalysts forthe methanol oxidation reaction.19,40,45,82,83 They argue thatthe electron transfer from Pt clusters to oxygen atoms on thesupport surface is beneficial to the enhancement of the cata-lytic properties and to improve the electrocatalyst’s stability.Li et al.19 proposed that some active oxygen groups introducedon the CNF surface might contribute directly to oxidation of COadsorbed on the Pt surface. Further, it is stated that higherpercentage of mesopores enhances the mass diffusion in theelectro-chemical reaction. On the other hand, it is also statedthat the introduction of oxygen groups on the carbon surfaceresults in a decrease in electrical conductivity, which is a veryimportant property for fuel cell catalysts.38 In addition, Calvilloet al.48 proposed that the surface oxygen groups facilitate the Ptagglomeration during the catalyst preparation step. As a result,the Pt agglomeration enhances the methanol oxidation reac-tion. This is in good agreement with the present work. Althoughthe exact nature of the effect of oxygen on the activity andstability of Pt NPs could not be fully resolved in this work, theresults here are very encouraging. The removal of the oxygengroups from the CNF has profound positive effects on MOR.

In summary, therefore, we hold that the most-likely causefor the higher specific activity of Pt NPs supported on oxygen-enriched CNFs (Fig. 9) is due to the formation of biggerparticles during the deposition stage, and the oxygen groupson the surface of the CNF surface have negative influence onthe specific activity.

5. Conclusions

The present synthesis procedure was found to be more suitableto study the effect of different novel support materials for MOR.

Based on the results and discussions, the Pt particle diameter,the carbon structure and the concentration of oxygen-groups onthe carbon surfaces have been identified as the three importantparameters for the Pt activity in MOR.

The carbon structure plays an important role in the stabili-zation of Pt particles. The edge sites of the stacked graphene

nanosheets in platelet CNFs provide strong interaction with thePt particles, leading to more faceted Pt surfaces exposedthrough a significant reconstruction of the Pt particles. As aconsequence, both specific activity and mass activity of theCNF-supported catalysts are found to be higher than for thecarbon-black-supported catalysts in the Pt particle diameterrange of 2.5–4.3 nm.

The Pt deposited on CNFs with an oxygen-group-enrichedsurface exhibits poor dispersion, assigned to reduced electro-static interaction between Pt NPs and CNFs. The removal ofoxygen groups on this support prior to Pt deposition throughheat treatment improved Pt dispersion, and also the activity forMOR. This could be due to the increase in p-electron density onheat-treated CNFs which may perhaps reduce the charge transferfrom Pt to CNF, thereby lowering the adsorption strength of COand OH species. Therefore, the high temperature treatment is asimple but effective way to manipulate the properties of bothCNF and Pt particles.

The active Pt catalyst for MOR can be obtained by optimizingthese three parameters. The preference of supports for anodecatalysts in DMFC follows the order: CNF with less O > CNFwith rich O > Vulcan carbon. The Pt–CNF_N_1K catalyst com-bines the advantages of the unique properties of CNF andoxygen-free surface sites. As a consequence, the Pt–CNF_N_1Kcatalyst has a mass activity that is twice as much that of thecommercial E-TEK catalyst. Finally, it should be pointed outthat the principles of rational design of the Pt catalysts isexpected to be applicable for bi- or multi-metallic catalysts,not just for methanol but also for hydrogen fuel cells.

Acknowledgements

The Research Council of Norway (NFR) is acknowledged for thefinancial support through the NANOMAT program, GrantNo. 10329500. The support from Tie-Jun Zhao for helping inpreparation of CNF is greatly appreciated. Dung T. Tran isacknowledged for TEM assistance.

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