Structural Properties of Unsupported Pt−Ru Nanoparticles as Anodic Catalyst for Proton Exchange Membrane Fuel Cells

Structural Properties of Unsupported Pt-Ru Nanoparticles as Anodic Catalyst for ProtonExchange Membrane Fuel Cells

Amado Velazquez-Palenzuela, Francesc Centellas, Jose Antonio Garrido, Conchita Arias,Rosa Marıa Rodrıguez, Enric Brillas, and Pere-Lluıs Cabot*Laboratori d’Electroquımica dels Materials i del Medi Ambient, Departament de Quımica Fısica,UniVersitat de Barcelona, Martı i Franques 1-11, 08028 Barcelona, Spain

ReceiVed: December 18, 2009; ReVised Manuscript ReceiVed: January 29, 2010

The structural properties of the nanoparticles of a high-performance commercial unsupported Pt-Ruelectrocatalyst with a nominal equiatomic relationship have been studied. They were exhaustively determinedby transmission electron microscopy (TEM), high-resolution TEM, fast Fourier transform, electron diffraction,X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) techniques. TEM and its coupledtechniques showed that polyoriented Pt-Ru nanoparticles present an average size of 3.0 ( 0.5 nm withnondetection of metal oxides. XPS spectra, however, indicate the existence of oxidized Pt and Ru species. Inparticular, the proton- and electron-conducting hydrous Ru oxide involved in the electrocatalytic oxidation ofCO and methanol is detected. XRD confirmed that a Pt-Ru solid solution with a 41 at.% Ru is the maincrystallographic phase in the electrocatalyst, whereas Pt and Ru oxides appear to be amorphous or very thin.The strain corrected Williamson-Hall models confirmed that nanoparticles have crystalline defects with amean size of 2.8-2.9 nm. Cyclic voltammetry in 0.5 M H2SO4 was employed to determine the effect of thevariation of the anodic limit scan and the potential range for the formation of hydroxylated species and metaloxides. The nanoparticles surface was active regardless of the metal oxides detected by structural analysis,which were stable under cycling and not protective.

1. Introduction

Unsupported and carbon-supported Pt-Ru nanoparticles arebeing developed as catalytic material in anodes for protonexchange membrane fuel cells (PEMFCs), generally to oxidizeCO-contaminated hydrogen or methanol (in direct methanol fuelcells, DMFCs).1 The better performance of Pt-Ru alloy whencompared to pure Pt can be explained by the so-calledbifunctional mechanism, in which the CO poisoning the activesites of the Pt catalyst is converted into CO2 with participationof oxygenated species adsorbed on Ru:2

A second explanation is based on the electronic effectproduced by alloyed Ru on CO-poisoned Pt atoms yielding aweakening of the Pt-CO bond that enhances its oxidation toCO2.3,4

Carbon-supported Pt-Ru electrocatalysts are more widelyemployed because they allow us to obtain a good dispersion ofmetallic nanoparticles. Several innovative syntheses, includingmicroemulsion, colloidal, impregnation, and polyol methods,5-11

have been proposed for the preparation of electrocatalysts forPEMFCs with better structural properties (particle size, disper-sion, composition, and so forth). In addition, carbonaceousmaterials in the form of Vulcan carbon black, carbon nanotubes,carbon nanofibers, mesoporous carbon, and graphitic carbonhave also been recently reported as good supports for Pt-Runanoparticles.12-18 As an example, the authors have recently

reported the good electrocatalytic behavior of high-performance(HP) equiatomic Pt-Ru 20 wt.% on Vulcan XC-72.18 However,several works have focused on the use of unsupported Pt-Runanoparticles and the proton exchange membrane material,generally Nafion copolymer, for the membrane electrode as-sembly (MEA) preparation of DMFCs, with good results,too.19-21

High-resolution transmission electron microscopy (HRTEM)and X-ray diffraction (XRD) are the most extended techniquesto determine the structural parameters of the nanoparticles suchas their size distribution, morphology, and composition. Note,however, that the mean size obtained from XRD measurementscan be strongly affected by the lattice strain derived from thepresence of dislocations or vacancies.22,23 Composition informa-tion is also available when using the XRD results, specially thealloy degree of the Pt-Ru nanoparticle that has been restrictedto phases with crystallographic order because amorphousstructures cannot be detected. In this scenario, the X-rayphotoelectron spectroscopy (XPS) analysis is of special interestbecause of its ability for the quantitative detection of metaloxides in the electrocatalyst. These compounds can play animportant role in electrocatalysis, as is the case of hydrousruthenium oxide RuOxHy.24-29 It has a suitable proton andelectron conductivity, which allows a significant reduction ofthe ionomer content in the Pt-Ru catalyst layer for achievingan optimum performance.21 Furthermore, oxygen chemisorptionon RuOxHy can be useful,26,28 unlike on anhydrous RuO2 whichadsorbs a smaller amount of this molecule. Adsorbed oxygenwould act similarly to oxygenated species adsorbed on Ru inPt-Ru nanoparticles, providing a better tolerance to COpoisoning and increasing the performance for methanol oxida-tion. The different properties of the anhydrous and hydrousruthenium oxide are an example of the importance of the

* To whom correspondence should be addressed. Tel: +34 93 4039236;Fax: +34 93 4021231; E-mail: [email protected].

Pt-CO + Ru-OH f Pt + Ru + CO2 + H+ + e-

(1)

J. Phys. Chem. C 2010, 114, 4399–4407 4399

10.1021/jp9119815 2010 American Chemical SocietyPublished on Web 02/18/2010

identification of the chemical species present in the analyzedelectrocatalyst.

In this work, we present a detailed study on the structuralcharacterization of a HP commercial unsupported Pt-Ruelectrocatalyst with a nominal equiatomic stoichiometry, whichshowed good electrocatalytic performance in carbon-supportedform.18 The in-depth knowledge of these Pt-Ru catalysts isimportant because they can be considered as reference materialsin the development of catalysts for low-temperature fuel cells.The morphological properties have been analyzed by means ofTEM, HRTEM, fast Fourier transform (FFT), electron diffrac-tion, XPS, and XRD. In this way, size distribution, shape, latticedefects, oxidation states of the elements, and bulk compositionof unsupported Pt-Ru nanoparticles have been determined.Cyclic voltammetry in a 0.5 M H2SO4 electrolyte solution wasalso carried out as additional technique to check the surfacecomposition of the nanoparticles; we took particular interest inthe influence of the anodic scan limit in the voltammogrampattern. Some comparative studies have been made with carbon-supported Pt-Ru electrocatalyst to clarify the different oxidationstates of the metallic components.

2. Experimental Section

2.1. Chemicals and Materials. Unsupported HP 1:1 Pt-Rualloy (Pt-Ru electrocatalyst) and supported HP 20% 1:1 Pt-Rualloy on Vulcan XC-72 carbon black (Pt-Ru/C electrocatalyst)were purchased from E-Tek. The ionomer was a 5% solutionof Nafion perfluorinated ion-exchange resin in a mixture ofaliphatic low molecular-weight alcohols (isopropanol:n-propanolin weight ratio 55:45) and water (15-25 wt.%), supplied byAldrich. Glassy-carbon (GC) disk electrodes of 3 mm diameterwere provided by Metrohm. Analytical grade 96% H2SO4 fromMerck was used to prepare 0.5 M H2SO4 as the electrolyte forthe electrochemical experiments. All solutions were preparedwith high-purity water obtained with a Millipore Milli-Q system(resistivity > 18 MΩ cm). H2 and Ar gases were Linde 5.0(purity g 99.999%).

2.2. Physical Characterization of the Pt-Ru Electrocata-lyst. The size distribution, crystallographic phases, and mor-phologic quantification of the Pt-Ru nanoparticles from un-supported electrocatalyst were analyzed by TEM and HRTEMby using a JEOL JEM 2100 TEM 200 KV, which allowed usto obtain the corresponding images and the electron diffractionpatterns. The samples were prepared by placing a drop of asuspension obtained by ultrasonic dispersion of 0.5 mg of thePt-Ru electrocatalyst in 3 mL of n-hexane for 10 min over aholley-carbon copper grid and evaporating the solvent until totaldrying by using a 40 W lamp for 15 min. TEM and HRTEMimages were recorded with a Gatan MultiScan 794 CCD(charge-coupled device) camera. Gatan Digital Micrograph 3.7.0software was used for the digital treatment of images andanalysis of selected areas of interest by FFT. Crystallographicdata obtained from electron diffraction pattern and FFT werecontrasted with CaRIne Crystallography 3.1 and PCPDFWIN2.3 softwares, respectively.

Comparative XPS experiments for Pt-Ru and Pt-Ru/Celectrocatalysts were performed in a Physical Electronics PHI5500 Multitechnique System spectrometer with a monochro-matic X-ray source (Al KR line of 1486.6 eV energy and 350W), placed perpendicularly to the analyzer axis and calibratedby using the 4f7/2 line of Pt region, which was located at 71.5eV. The analyzed area was a circle of 0.8 mm diameter. Asurvey spectrum (187.5 eV of Pass Energy and 0.8 eV/step)was collected before high-resolution spectra (23.5 eV of Pass

Energy and 0.1 eV/step) were recorded. All measurements weremade in an ultra-high-vacuum (UHV) chamber pressure between5.0 × 10-9 and 2.0 × 10-8 torr. The deconvolution of the XPSspectra was carried out by using Ulvac-phi MultiPak V8.2Bsoftware. The global peak contribution of corresponding de-convoluted signal areas was used for the relative quantificationof the species concentration. Because of the size of thenanoparticles and the X-ray penetration, the results of theanalyses were considered to be related to the whole material,not only from their surface.

The XRD pattern of the unsupported electrocatalyst wasobtained by using a PANAlytical X’Pert PRO Alpha-1 diffrac-tometer equipped with a Cu KR radiation (λ ) 0.15406 nm).The samples were placed on a Si-Xtal support, and the 2θ anglewas varied between 10 and 140°. Experimental diffractionpatterns were modeled by using a FullProf Suite WinPLOTR2008 software to fit the corresponding diffraction signals toindividual peaks for calculating the exact value of 2θ peak angleand the width at half height (fwhm), as well as the mean sizeof the nanoparticles.

2.3. Electrochemical Experiments. The electrochemicalmeasurements were performed at 25.0 ( 0.1 °C by using aconventional thermostatted double-wall three-electrode glass cellfrom Metrohm of 200 mL and an Ecochemie Autolab PG-STAT100 potentiostat-galvanostat with computerized controlby an Autolab GPES software. A 3.78 cm2 Pt rod served as theauxiliary electrode, whereas a double junction Ag|AgCl|KCl(saturated) was used as the reference electrode. All potentialsgiven in this work are referred to the reversible hydrogenelectrode (RHE). The working electrode was prepared byfollowing the thin-layer technique.30 It consisted of catalyst inksdeposited in one step on the GC electrode, as previouslydescribed.18,31 Before the ink deposition, the GC tip wasconditioned through polishing with aluminum oxide pastes of0.3 and 0.05 µm (Buehler Micropolish II deagglomeratedR-alumina and γ-alumina, respectively) on a Buehler PSA-backed White Felt polishing cloth until achieving a mirror finish,being rinsed with Millipore Milli-Q water in ultrasonic bathbetween the polishing steps. Aqueous inks of electrocatalystwith a concentration of 5.0 mg mL-1 were prepared bysonicating for 45 min different amounts of Pt-Ru electrocata-lyst, Millipore Milli-Q water, and the Nafion solution. Theionomer served to attach the nanoparticles on the GC electrode,and its composition in the dry inks was varied in the range30-50 wt.%. Stirred volumes of each ink in the range 1.5-2.5 µL were deposited by means of a digital micropipet,Labopette Variabel from Hirschmann or Witopet from Witeg,on the surface of the GC disk electrode, by carefully weightingsuch volumes with an AG 245 Mettler-Toledo analytical balance(accuracy of ( 0.01 mg). The recently prepared electrodes weredried for 24 h in a clean desiccator at room temperature.Afterward, the working electrode was ready to be used in theEcochemie Autolab RDE with final Pt loads on the GC surfacein the range 25-30 µg cm-2. After the electrolyte deaerationby Ar bubbling, consecutive cyclic voltammograms at 100, 50,and 20 mV s-1 within the potential range of either 0.02-0.65V or 0.02-0.98 V were performed under Ar atmosphere. Afast quasi-stationary shape of the cyclic voltammogram wasalways obtained, suggesting stability and cleanness of the GCanode.

3. Results and Discussion

3.1. TEM and HRTEM Analyses. TEM and HRTEMimages for the Pt-Ru electrocatalyst are shown in Figure 1a,b,

4400 J. Phys. Chem. C, Vol. 114, No. 10, 2010 Velazquez-Palenzuela et al.

respectively. The structures exhibited confirm that the electro-catalyst is composed of agglomerated metal nanoparticles withcrystalline order. From these images, 100 nanoparticles werecounted in order to determine their size. The size distributionhistogram depicted in Figure 2 reveals that the size of Pt-Runanoparticles in the electrocatalyst surface varies between 2.0and 5.0 nm, following a Gaussian distribution centered in anaverage value of 3.0 ( 0.5 nm.

The quantitative determination of predominant three-dimen-sional shape of nanoparticles obtained by using image-contrastsoftware was not carried out because of their high agglomera-tion, which prevented an accurate measurement of their two-dimensional projection areas with a digital contrast treatment.Despite this problem, the recorded HRTEM images showedPt-Ru nanoparticles with different two-dimensional morphol-ogies, mainly hexagonal, square, and elliptical geometries,

indicating that the corresponding three-dimensional shape isvariable. The existence of different morphologies then leads toa global polyoriented structure with a regular distribution ofsurface planes for the electrocatalysis of hydrogen and carbonmonoxide oxidation on Pt-Ru nanoparticles.

Techniques coupled to TEM were employed to characterizethe composition of the unsupported nanoparticles observed inTEM images. Figure 3a presents the electron diffraction patternfrom a global image of the Pt-Ru electrocatalyst. The observedsignals are attributed to the face-centered cubic (FCC) structureof platinum. Interplanar distances (d) of 0.222, 0.137, 0.116,and 0.088 nm were determined, which correspond to the groupof (111), (220), (311), and (420) planes of Pt (FCC), respec-tively. On the other hand, the FFT analysis performed by usingthe HRTEM images corroborated the existence of platinumstructure in the unsupported electrocatalyst. Figure 3b revealsthe FFT pattern for the nanoparticle shown in the inset. In thiscase, the interplanar distances obtained from the pair of points1-2 and 3-4 are 0.226 and 0.224 nm, respectively, in goodagreement with the d value associated with Pt (111) planes.Signals of crystalline structure for Pt or Ru oxides, or segregatedRu, were not detected by electron diffraction or FFT analyses,confirming the Pt (FCC) structure as the main crystallographicphase.

3.2. XPS Measurements. The XPS spectra were recordedto obtain data about composition and oxidation states of theelements in the unsupported Pt-Ru electrocatalyst. A similaranalysis was also carried out for the carbon-supported Pt-Ruelectrocatalyst for comparison purposes. Experimental XPSsignals were compared with data in the litterature.24,26,32-35

Figure 1. (a) TEM and (b) HRTEM images of the HP 1:1 Pt-Rualloy showing agglomerated nanoparticles.

Figure 2. Size distribution of 100 HP 1:1 Pt-Ru electrocatalystnanoparticles, obtained from HRTEM measurements.

Figure 3. (a) Electron diffraction pattern of the HP 1:1 Pt-Ru alloyshowing the diffraction signals associated to the Pt FCC structure. (b)FFT analysis of the nanoparticle of the inset, with the points 1-4 beingcompatible with the Pt unit cell.

Structural Properties of Unsupported Pt-Ru Nanoparticles J. Phys. Chem. C, Vol. 114, No. 10, 2010 4401

Figure 4 exhibits the general XPS spectrum in the range ofbinding energies up to 1100 eV for the unsupported Pt-Ruelectrocatalyst. As can be observed, all the appearing signalsare identified and attributed to platinum, ruthenium, and oxygenelements. The corresponding XPS spectrum for the carbon-supported electrocatalyst (not shown) offered identical morphol-ogy but with the presence of a very narrow additional peak at285 eV assigned to the C 1s from carbon support that overlapswith the Ru 3d doublet. Note that a very weak C 1s signal wasalso found in the XPS spectrum of unsupported electrocatalystas a result of the presence of environmental carbonaceousimpurities adsorbed on the surface nanoparticle. The inset inFigure 4 reveals the existence of a single broad peak located atabout 529 eV for both electrocatalysts due to O 1s band,indicating the existence of oxygen species in them.

Figure 5 depicts the Pt 4f binding energy region of theelectrocatalyst. For both materials, a doublet is displayed becauseof the spin-orbital splitting that originates lower-energy(Pt 4f7/2) and higher-energy (Pt 4f5/2) bands with relativeintensities (3:4). These bands appear around 71.5 and 74.9 eV

as considerably broad peaks, as expected by the existence ofvarious Pt species. To quantify the different oxidation statesof Pt, the general doublet was deconvoluted, and three pairs ofpeaks with different areas were obtained. These results indicatethat Pt is found in the samples in three different oxidation states:metallic Pt, Pt(II), and Pt(IV). The existence of Pt(II) can beassigned to PtO or Pt(OH)2 species, whereas Pt(IV) is generallyattributed to PtO2. Pt(II) and Pt(IV) can also form a mixture ofoxides with formula Pt3O4 ((Pt(II)2)(Pt(IV))O4), like we previ-ously confirmed for the carbon-supported Pt-Ru electrocatalystby electron diffraction and FFT techniques.18 The correspondingbinding energies and relative concentrations of each elementfor both electrocatalysts are shown in Table 1. In both cases,metallic Pt appears as the predominant species (68 and 53%for unsupported and carbon-supported electrocatalyst, respec-tively). Note the higher fraction of oxidized Pt determined forthe carbon-supported electrocatalyst (47%), which explains theability of detecting Pt oxides in it through electron diffractionand FFT local analyses,18 in contrast with the negative resultsobtained for the unsupported Pt-Ru nanoparticles.

Figure 6 presents the Ru 3p region XPS spectra of the twoPt-Ru electrocatalysts. This region was selected because theC 1s peak partially overlaps with the slightly more intense Ru3d doublet (3d3/2 and 3d5/2 bands);33 therefore, it cannot be usedfor quantitative estimation of oxidation states. The Ru3p3/2 bandat 461.5 eV was deconvoluted into three contributions ofdifferent intensities for both samples. The corresponding bindingenergies and normalized areas are presented in Table 1. As canbeen seen, the lowest energy band at 461.6 and 461.9 eV forthe corresponding unsupported and carbon-supported electro-catalyst is attributed to metallic Ru. The signal at mediumenergy, about 464 eV, is assigned to oxidized Ru species withoxidation state (IV), particularly anhydrous RuO2. The thirdcomponent located at higher binding energy, about 466 eV, canbe associated with hydrous RuO2 or RuOxHy, because its bindingenergy is comparatively greater than that of the anhydrousstructure.26,32 RuO3 is another oxide that could generate a higherenergy band, but its existence is discarded because this structureis only stable in the gas phase from 1200 to 1500 °C,32 notbeing thermodynamically stable under our working conditions.For both electrocatalysts, metallic Ru is found as the predomi-

Figure 4. General X-ray photoelectron spectra for the HP 1:1 Pt-Ruelectrocatalyst showing signal of Pt, Ru, and O. The inset shows thecorresponding spectra in the O 1s region.

Figure 5. X-ray photoelectron spectra in the Pt 4f region for (a)unsupported and (b) carbon-supported Pt-Ru alloys showing thedeconvolution of the doublet signals of metallic Pt, Pt(II), and Pt(IV).

TABLE 1: Binding Energies and Relative ConcentrationValues for Pt and Ru Species Obtained from Pt 4f and Ru3p XPS Spectra, Respectively, For Unsupported andCarbon-Supported Pt-Ru Electrocatalysts

species BEpeak (eV)a relative concentration (%)b

Electrocatalyst: Pt-Ruc

Pt 71.5-74.9 68Pt(II) 72.2-75.2 23Pt(IV) 74.2-77.2 9Ru 461.6 71RuO2 464.2 11RuOxHy 466.1 18

Electrocatalyst: Pt-Ru/Cd

Pt 71.5-74.8 53Pt(II) 72.3-75.6 33Pt(IV) 74.2-77.2 14Ru 461.9 62RuO2 463.9 12RuOxHy 466.2 26

a Peak position of the deconvoluted band. b Referred to theatomic Pt or Ru in all the Pt or Ru species, respectively.c Unsupported HP 1:1 Pt-Ru alloy. d Supported HP 20% 1:1 Pt-Rualloy on Vulcan XC-72 carbon black.


nant species (71 and 62%, respectively), again with a greaterpresence of its oxidized form in the carbon-supported electro-catalyst, whereas the RuOxHy structure is dominant. Theexistence of this oxide plays an important role in the electro-catalyst oxidation of CO and alcohols. Thus, RuOxHy is a mixedelectron and proton conductor that can act as water dissociator(the mechanism for the proton conduction) to form hydroxylatedspecies,24 necessary for CO elimination such as described byreaction 1. Therefore, it is expected that the presence of thisoxide at the electrocatalyst surface gives a higher tolerance toCO poisoning.

3.3. XRD Analysis. Experimental XRD pattern of the Pt-Ruelectrocatalyst and its respective modeling are shown in Figure7. Typical peaks of the Pt FCC structure can be observed,whereas crystalline hexagonal (HCP) Ru and metal oxides arenot detected. Although this finding is in agreement with electron

diffraction and FFT data, it should be remarked that the twolatter analyses involve a reduced amount of nanoparticles,whereas XRD is considered a more representative techniquebecause it analyzes all nanoparticles present in the sample tested.The absence of the Pt and Ru oxides detected by XPS suggeststhat these phases are amorphous. On the other hand, a shift ofall diffraction peaks to higher 2θ values compared to pure Ptwas always found, owing to the incorporation of Ru into the Ptstructure, which results in the contraction of the unit cell. Forexample, the (111) peak for pure Pt appears at 39.89°, whereasfor Pt-Ru electrocatalyst, it is located at 40.49°. The latticeparameter for the Pt-Ru unit cell was aPt-Ru ) 0.3862 nm, asdetermined by eq 2 from the corresponding interplanar distances(dhkl) obtained for the different diffraction peaks of Miller index(hkl):

The values of dhkl were previously calculated by using thewell-known Bragg’s law:

In eq 3, n is the diffraction order (generally 1), λ is thewavelength of the Cu KR radiation, and θ is the diffraction angle.Vegard’s law was then applied to estimate the nanoparticlecomposition. It relates the lattice parameter aPt-Ru of the Pt-RuFCC structure with the alloy degree of Ru (XRu, between 0 and1) by means of eq 4:36

where aPt is the lattice parameter of pure Pt (0.3923 nm). Avalue of XRu ) 0.41 was obtained, slightly lower than thenominal 0.50 atomic composition. Note in addition that theatomic fraction of Ru incorporated in the FCC lattice of Pt inthe HP 20% 1:1 Pt-Ru/C electrocatalyst was 0.26,18 which issmaller than the value for the unsupported one. Ru or metaloxides were not detected neither by electronic diffraction norXRD, suggesting that a fraction of Ru could be present inamorphous form. On the other hand, the metal oxides, possiblymixed oxides of Pt and Ru, could be also amorphous or evenpresent as very thin films.

On the other hand, the average nanoparticle size (d) wasdetermined from the Debye-Scherrer eq 5:37

where k ) 0.9, λ is the wavelength of the Cu KR radiationmentioned above, θ is the angle at the maximum of the peak,and B2θ is the width of the corresponding fwhm. A d value of2.4 nm was obtained for the Pt-Ru nanoparticles, slightly lowerthan 3.0 ( 0.5 nm found from TEM analysis, as stated above.However, the d value determined by XRD can be affected bythe lattice strain because of dislocations or atom vacancies. TheWilliamson-Hall analysis was then used to correct the contri-bution of nanocrystal stress to the size measured.22,23 Threedifferent approaches of the Williamson-Hall model werechecked, as accurately described for nanocrystalline silver.22 The

Figure 6. X-ray photoelectron spectra in the Ru 3p region for (a)unsupported and (b) carbon-supported Pt-Ru electrocatalysts showingthe deconvolution in the signals of metallic Ru, RuO2, and RuOxHy.

Figure 7. XRD pattern for HP 1:1 Pt-Ru alloy. The experimentaldata (Yexp) and fitting (Yfit) are plotted together. The difference betweenboth, Yexp - Yfit, is close to zero, and the magnification in the insetshows the good quality of the fitting. The arrows 1-6 indicate the 2θvalues where peaks for crystalline RuO2 (1,2), Ru (3,4), PtO2 (5), andPt3O4 (6) should appear, but they are not detected.

dhkl )aPt-Ru

√h2 + k2 + l2(2)

nλ ) 2dhkl sin θ (3)

aPt-Ru ) aPt - 0.0149XRu (4)

d ) kλB2θ cos θ

(5)


first approach assumes the existence of a three-dimensionaluniform deformation or isotropic microstrain (ε) that allows thecalculation of the corresponding nanoparticle size D fromeq 6:

The other two Williamson-Hall approaches are more realisticbecause they include the effect of the anisotropy of the Young’smodulus (E). Thus, the so-called uniform deformation stressmodel replaces ε in eq 6 by the anisotropic microstrainεhkl () σ/Ehkl) to give eq 7:

where D′ represents the nanoparticle size, σ is the uniformdeformation stress, and Ehkl is the Young’s modulus in thenormal direction to the corresponding plane of Miller index(hkl). For a cubic crystal, Ehkl is calculated as a function of theelastic compliances s11, s12, and s44 by means of eq 8:

To apply eq 8, we take the values related to the FCC platinum,that is, s11 ) 7.35 TPa-1, s44 ) 13.1 TPa-1, and s12 ) -3.08TPa-1,38 as acceptable elastic compliances.

The third Williamson-Hall approach is the so-called uniformdeformation energy density model and considers Hooke’s lawto define a uniform deformation energy density or resilience(u) as eq 9 indicates. In the elastic range, εhkl) σ/Ehkl; therefore,eq 9 can be reformulated as eq 10.

From these considerations, the Williamson-Hall equation canbe written as eq 11:

where D′′ is the corresponding nanoparticle size in this thirdmodel.

The average sizes of the Pt-Ru nanoparticles associated witheqs 6, 7, and 11 for the above Williamson-Hall approacheswere calculated from the plots of B2θ cos θ with 4sin θ for theuniform deformation model (Figure 8a), 4sin θ Ehkl

-1 for theuniform deformation stress model (Figure 8b), and 25/2 sin θEhkl

-1/2 for the uniform deformation energy density model(Figure 8c). The three Williamson-Hall models lead to goodfits of the experimental results to the expected trends. Fromthese linear representations, quite similar values of D ) 2.9nm, D’ ) 2.8 nm, and D′′ ) 2.9 nm were obtained, which are

closer to the average size of 3.0 nm measured from TEMimages. Consequently, the existence of defects in the form ofdislocations or atom vacancies in the Pt-Ru nanoparticles isconfirmed.

Finally, the specific area normalized by mass of Pt-Ru alloy(SPt-Ru) or by Pt load (SPt) was determined by considering theaverage nanoparticle size (measured by TEM or calculated bythe three Williamson-Hall approaches tested from XRD data)with the assumption of a spherical structure according toeq 12:

B2θ cos θ ) kλD

+ 4ε sin θ (6)

B2θ cos θ ) kλD′ +

4σ sin θEhkl

(7)

Ehkl ) s11 - (2s11 - 2s12 - s44)k2l2 + l2h2 + h2k2

(h2 + k2 + l2)2

(8)

u )εhkl

2 Ehkl

2(9)

σEhkl

) ( 2uEhkl

)1/2(10)

B2θ cos θ ) kλD′′ + 4( 2u

Ehkl)1/2

sin θ (11)

Figure 8. Williamson-Hall plots for the HP 1:1 Pt-Ru electrocatalystobtained by assuming (a) uniform deformation, (b) uniform deformationstress, and (c) uniform deformation energy density models.

Si )3

FPt-Rur(12)


where FPt-Ru is the density of Pt-Ru nanoparticles and r is halfof the average nanoparticle size. The corresponding density ofPt-Ru nanoparticles was calculated from eq 13:

By taking the fraction of Ru in the alloy (XRu ) 0.41) andthe density of the pure platinum and ruthenium as FPt ) 21.47g cm-3 and FRu ) 12.37 g cm-3,39 one obtains Falloy ) 17.73g cm-3.

The specific areas obtained from this density in eq 12 arecollected in Table 2. An average value of 117 m2 g-1 for SPt-Ru

and 177 m2 g-1 for SPt was determined by the different methods.3.4. Cyclic Voltammetry Analysis. The cyclic voltammetry

technique was applied to detect the surface nanoparticlecomposition because this area behaves as a true active phasefor the electrocatalysis.40 Similar electrochemical behavior wasfound for both the unsupported and the carbon-supportedsamples (Figures 9 and 10). Apart from the expected differentheight of the peaks due to the different Pt loads, the anodic andcathodic peaks appear to be less marked for the carbon-supported one, probably because of the already indicated slight

differences in the structural properties of the nanoparticles andthe effect of the carbon support. As an example, Figure 9 depictsthe first five cycles recorded between 0.02 and 0.98 V at 100mV s-1 in 0.5 M H2SO4 for a Nafion ink with HP Pt-Ru (Figure9a) and HP 20 wt.% 1:1 Pt-Ru/C Vulcan XC-72 (Figure 9b),both deposited on a GC disk electrode. After the second scan,the voltammograms become quasi-stationary, indicating theexistence of a stable and clean active surface of the electro-catalyst without electroactive impurities.

Cyclic voltammograms recorded for inks made of unsup-ported and carbon-supported Pt-Ru with Nafion on GC indeaerated 0.5 M H2SO4 solution are shown in Figure 10a,b,respectively, each one for two different anodic limits of 0.65and 0.98 V. The upper anodic limit potential was limited toless than 1.0 V to avoid the loss of Ru atoms by dissolution,which could lead to a decrease of the performance of theelectrocatalyst both for CO tolerance and for methanoloxidation.41,42 It was found that the shape of the cyclicvoltammogram did not change with the Nafion content in thecatalyst ink; therefore, the I-E curves shown in Figure 10 canbe considered representative of the behavior of this electrocata-lyst in acid media. The presence of Pt-Ru alloy on the surface,not only in the nanoparticle bulk, is evidenced from severalfacts. Thus, the reversible and relatively symmetrical hydrogenmonatomic adsorption/desorption peaks appearing in the lowpotential range of 0.01-0.20 V of Figure 10a,b are verysuppressed, corresponding to a charge much lower than thatobtained for pure Pt electrocatalyst.31 Because a difference of2 orders of magnitude exists between the exchange currentdensity for hydrogen oxidation on pure Pt and pure Ru (0.3and 0.003 A cm-2,43 respectively), one can consider that thepresence of Ru causes the strong decay of the hydrogenadsorption/desorption processes when compared to pure Pt.

TABLE 2: Morphological Properties of UnsupportedPt-Ru Nanoparticles Determined by Different Methods

method particle size (nm) SPt-Ru (m2 g-1)a SPt (m2 g-1)b

TEM 3.0 113 171XRD, uniform φ 2.9 117 177XRD, uniform σ 2.8 121 184XRD, uniform u 2.9 117 177Average value 2.9 117 177

a Specific area normalized by mass of Pt-Ru alloy. b Specificarea normalized by Pt mass.

Figure 9. First five cyclic voltammograms recorded for (a) an ink ofHP 1:1 Pt-Ru alloy (load of 20 µg Pt cm-2) with 50 wt.% Nafion and(b) an ink of HP 20 wt.% 1:1 Pt-Ru/C Vulcan XC-72 (load of 30 µgPt cm-2) with 30 wt.% Nafion deposited on a GC disk electrode, in0.5 M H2SO4 at a scan rate of 100 mV s-1 and 25.0 °C.

FPt-Ru ) FRuXRu + FPt(1 - XRu) (13)

Figure 10. Cyclic voltammograms recorded for (a) an ink of HP 1:1Pt-Ru alloy (load of 30 µg Pt cm-2) with 50 wt.% Nafion and (b) anink of HP 20 wt.% 1:1 Pt-Ru/C Vulcan XC-72 electrocatalyst (loadof 25 µg Pt cm-2) with 33 wt.% Nafion deposited on a GC diskelectrode, in 0.5 M H2SO4 at a scan rate of 20 mV s-1 and 25.0 °C.Initial potential, 0.02 V; reversal potential, 0.65 V (dashed line) and0.98 V (solid line).


However, the loss of activity for HOR due to Ru incorporationshould be compensated by a higher CO tolerance.2

Note that the cyclic voltammograms of Figure 10a,b exhibita slight dependence of the current with potential in the range0.20-0.65 V. This trend can be related to a pseudocapacitiveactivity derived from the discharge of water molecules on Rusurface atoms to form hydroxylated species with structureRu-OH from reaction 14:44

In this way, Gojkovic et al.45 reported that the pseudocapaci-tance in cyclic voltammograms increased with increasing therelative proportion of Ru on the electrocatalyst because thenuclei of formation of oxygenated species are the alloyed Ruatoms in the Pt-Ru nanoparticles. This means that there areexposed Ru atoms on which hydroxyl groups are adsorbed,regardless of the metallic oxides existing in the electrocatalystand found through structural analysis. The lack of significantchanges in the first consecutive cycles shows that the metaloxides are also stable, suggesting that they essentially participateas electron and proton conductors.

In contrast, the cyclic voltammograms of Figure 10 show anincrease in the anodic current at potentials higher than 0.85 Vwhen the reversal potential reaches a value near 1.0 V. Ananodic peak is not fully developed because of the potential limitrestriction imposed in our working potential range to avoid Rudissolution. The cathodic current peak for the reduction ofsurface oxides of Pt is not appreciated, because it is typicallylocated at 0.80 V, as found for pure Pt electrocatalyst.31

Nevertheless, a broad cathodic peak can be observed at about0.45 V that can be related to the reduction of metal oxidesgenerated in the anodic scan. The potential shift of about 0.35V obtained for the metal oxide reduction peak is anotherconsequence of the presence of Ru on the nanoparticle surface.In fact, Frelink et al.46 evaluated the mass change of a Ptelectrode during Ru deposition by using an electrochemicalquartz crystal microbalance and proposed a linear correlationbetween the potential change of the metallic oxide reductionpeak on the Pt-Ru electrocatalyst and the surface Ru content.Therefore, an easy way to elucidate the compositional stabilityof the Pt-Ru nanoparticles during the electrocatalytic operationcould consist in the evaluation of the position of the metallicoxide reduction peak in relation to that of Pt. In the presentwork, no modification of the potential peak was detected afterrepetitive potential cycles, as expected if the Pt-Ru nanopar-ticles tested possess a high stability and Ru is not lost afteroxidation.

Although this was not explored in this paper, Jeon et al.47

and Lu et al.48 have concluded that anodic treatments atpotentials higher than 1.0 V improve the activity of the Pt-Ruelectrocatalysts for methanol oxidation. This behavior is at-tributed to the lower formation of the irreversible hydrous Ruoxide and, in the case of high Ru content in the electrocatalyst,to the production of reversible RuOxHy species, which are moreactive for methanol oxidation than pure Ru. According to thestructural characterization and to the initial cyclic voltammetrystudies reported in this paper, the HP Pt:Ru 1:1 electrocatalystappears to have suitable properties for a good CO tolerance andability for methanol oxidation. The kinetics of H2, CO, andmethanol electro-oxidation on this catalyst together with thestudy of the possible influence of the metal oxides present inthe Pt-Ru electrocatalyst or generated by potential scan at high

potentials, which are out of the scope of this paper, will be theobject of future work.

4. Conclusions

TEM analysis of unsupported HP 1:1 Pt-Ru alloy electro-catalyst revealed that it is composed of nanoparticles with anaverage size of 3.0 ( 0.5 nm and variable morphology. Metaloxides were not found through local analyses by electrondiffraction and FFT techniques. However, the detailed study ofthe XPS spectra showed the presence of Pt and Ru in differentoxidation states, with larger proportion of the metallic forms.Similar characteristics were found for the carbon-supportedPt-Ru alloy but with a major proportion of oxides. HydrousRu oxide, a proton and electron conductor which favors COand methanol electro-oxidation, was detected as one of theconstituent phases in the electrocatalyst. XRD measurementsindicated an alloy degree of 41 at.% Ru in the FCC Pt latticeas the principal crystallographic phase. The absence of XRDpeaks for Pt and Ru oxide species indicated that they wereamorphous or very thin. Williamson-Hall models involvingisotropic (uniform deformation) and anisotropic (uniform de-formation stress and uniform deformation energy density)approaches were taken into account for the analyses of straincontribution to the XRD pattern. The three different models ledto nanoparticle sizes in the range of 2.8-2.9 nm, a value higherthan 2.4 nm calculated by using the Debye-Scherrer equationbut comparable to the TEM data, thereby confirming thepresence of crystallographic defects (dislocations and vacancies)in the Pt-Ru nanoparticles. Cyclic voltammetry in deaerated0.5 M H2SO4 solution obtained by using a thin-film Pt-Ru/Nafion catalyst ink as working electrode allowed us to provethe presence of Ru on the nanoparticle surface. The slow currentincrease appearing in the range 0.2-0.8 V was assigned to theformation of hydroxylated Ru species, whereas Ru oxides weregenerated in the electrochemically active surface at potentialshigher than 0.8 V and reduced at about 0.45 V. The Pt and Ruoxides detected in the previous structural analysis appeared tobe grown close to the nanoparticles, without protective character,and stable during cycling.

Acknowledgment. The authors thank the financial supportgiven by the Spanish MEC (Ministerio de Educacion y Ciencia)through Project NAN2004-09333-C05-03. The grant from theFundacio Pedro Pons (Universitat de Barcelona) and the FPUfellowship from Spanish MEC received by A.V.P. to do thiswork are also acknowledged. The authors also thank the SCT-UB (Serveis Cientıfico-Tecnics de la Universitat de Barcelona)for the TEM, XPS, and XRD facilities. This work is dedicatedto the memory of Amado Velazquez Mendez.

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