p, Ptes eFs

GDE

ds ttlyid mred

des can be improved either byd/or improving the structure of

cathodsluggisures. D

particles are transported over the carbon support and coalesce [9,10].Another disadvantage comes from the fact that the catalyst is usuallyuniformly distributed throughout the gas diffusion layer. Not all thecatalyst particle

Electrochemistry Communications 12 (2010) 745748

Contents lists available at ScienceDirect

Electrochemistry C

e lsPd alloys [3,4] and nanostructered Pd-based catalysts [5], exhibitingan activity towards the oxygen reduction reaction (ORR) as good aspure Pt in acidic solution.

Regarding the structure of the catalyst layer, most of the researchhas been conducted envisaging a better three phase reactantelectrodeelectrolyte contact and electrocatalyst utilization [68],both strongly dependent on the electrocatalyst synthesis process.Typically, carbon black (XC-72CB) is impregnated by immersion into asolution containing the metal salt or complex, followed by chemicalreduction, giving rise to a powder type catalyst, which is then

deposition [15], to electrodeposition methods [1619], have beentested. The electrodeposition processes include pulse deposition[16,17] and voltammetric deposition [18]. These types of electrodesexhibit a lower catalyst layer thickness (210 m), resulting in abetter catalyst efciency and fuel cell performance when compared tocommercial ones [1820].

In this work an alternative method of preparation of the catalystlayer by direct deposition onto a porous carbon paper usingelectroless deposition has been studied. Alike electrodeposition, thismethod anchors the catalyst to the conductive substrate and candispersed in a Naon solution. This paste ielectrode support, a porous and conductivecloth or carbon paper. One of the main disadconcerns the catalyst sintering phenomen

Corresponding author.E-mail address: mcris@utad.pt (C. Oliveira).

1388-2481/$ see front matter 2010 Elsevier B.V. Aldoi:10.1016/j.elecom.2010.03.022espite Pt is generallythis reaction, successfult alloys [1,2], bimetallic

directly deposited on the membrane. A variety of different processesranging from vacuummethods, such as sputtering [12], physical vapordeposition [13], chemical vapor deposition [14] and electron-beamidentied as the best catalytic material forresults have been obtained with bimetallic P1. Introduction

The performance of fuel cell electrousing a more active electrocatalyst anthe catalyst layer.

Finding an effective catalyst for thebeen a major challenge due to themolecular oxygen at low temperate side of the fuel cell hash reduction kinetics of

electronic contacts, resulting in a low catalyst efciency [8,11]. Thiscan be avoided by the use of non-powder type processes in which thecatalyst can be preferentially located near the membrane, or evens then painted onto anmaterial such as carbonvantages of this methoda because the catalyst

prevent sinterincatalyst near thdeposition is coand it is inuencis particularly imother depositionis very simple afabricating electr

l rights reserved.s are then utilized due to the lack of ionic and/orA new route to prepare carbon paper-supreduction reaction

Rosa Rego a, Cristina Oliveira a,, Amado Velzquez b

a Departamento de Qumica, Centro de Qumica Vila Real, Universidade de Trs-os-Monb Laboratori de Cincia i Tecnologia Electroqumica de Materials, Departament de Qumica

a b s t r a c ta r t i c l e i n f o

Article history:Received 10 March 2010Accepted 17 March 2010Available online 25 March 2010

Keywords:ORRPd electrocatalystElectroless depositionCarbon paper

The catalytic activity towarof Pd nanoparticles direcinvestigated in sulphuric acactivity for the ORR, compa

j ourna l homepage: www.orted Pd catalyst for oxygen

ere-Llus Cabot b

Alto Douro, 5000-911 Vila Real, Portugalica, Universitat de Barcelona, 08028 Barcelona, Spain

he oxygen reduction reaction (ORR) of a novel material consisting of clustersdeposited on porous carbon paper by electroless deposition, has beenedium. It is shown that this new material exhibits a very high electrocatalyticto the commercial carbon paper-supported Pt.

2010 Elsevier B.V. All rights reserved.

ommunications

ev ie r.com/ locate /e lecomg. It should also lead to a preferential location of thee carbon paper surface because the electrolessntrolled by the diffusion of the electrolyte [21,22]ed by the hydrophilicity of the carbon surface, whichportant within the porous structure. In contrast tomethodologies, the electroless deposition techniquend easy to scale-up, thus being very attractive forodes economically on a large scale. Thismethodology

on Pdt1 and Pdt2 in 0.1 M H2SO4, as well as on commercial Pt/C forcomparison. A long straight segment in the polarization curve, starting

746 R. Rego et al. / Electrochemistry Communications 12 (2010) 745748will be applied to the preparation of a Pd-based cathode catalyst andits activity towards the oxygen reduction reaction (ORR) will beinvestigated.

2. Experimental

2.1. Preparation of the carbon paper-supported Pd

The Pd catalyst was deposited on a porous carbon paper (GDL 24 AC,Sigracet) by electroless deposition. In order to reduce the paperhydrophobicity, the carbon paper was rst immersed in a 0.1% (w/w)Triton X-100 (Plusone) solution for 24 h. The wet paper was thenimmersed in de-ionized water for approximately 2 h and then activatedby successive treatments in SnCl2 (1.0 g/l in 0.20 MHCl) and PdCl2 (0.1 g/l in0.20 MHCl). This sensibilization/activation stepwasnecessary inorderto seed the carbon surface with catalytic nucleus, as carbon surface is notcatalytic for the electroless deposition. After this procedure the paperwasimmersed in a Pd electroless solution (27 mM N2H4, 28 mM Pd2+, 0.1 MEDTA, 600 ml/lNH4OH) [3] at roomtemperature, keepingupwards on thetop of the plating solution the paper face that had not to be plated. Twodifferent specimens of carbon paper-supported Pd (Pd/Cpaper) gasdiffusion electrodes (GDEs) were prepared in this way, differing only onthe deposition time, Pdt1 and Pdt2, for 10 and 60 min, respectively.

2.2. Characterization of the catalyst

The Pd/Cpaper was used as the working electrode by sealing it in aTeon holder with an aperture of 5 mm diameter. The electrocatalyticactivity of the prepared catalyst for the oxygen reduction reaction wasinvestigated by linear sweep voltammetry (LSV) in an oxygen-saturated0.1 M H2SO4 solution using a Metrohm three-electrode electrochemicalcell and a series 100 Autolab potentiostat. A Pt foil and a double-junctionAg|AgCl,KCl (sat.) electrode were used as the counter and referenceelectrodes, respectively. However, all the potentials given in this workhave been referred to the Normal Hydrogen Electrode (NHE) scale. Priorto each electrochemical measurement several cyclic voltammograms in adeaerated solution were recorded in order to check the cleanness of thesurface.

For comparison, a commercially available carbon paper-supported Pt/C (0.5 mg cm2, Sigracet) and a Pd electroless lm (Pdeless) were alsoused. The latter was prepared on a Ni disk using the electroless solutionand plating conditions identical to those used for obtaining the Pd/Cpaper.However, in this case, the substrate was not submitted to thesensibilization/activation step because Ni is itself catalytic for theelectroless deposition [23].

Unless otherwise stated the current density is expressed against thegeometric surface area (0.196 cm2). The electrochemical active surfacearea (EASA) of Pdt1, Pdt2 and Pt/C was determined to be 1.35, 1.56 and42.2 cm2, respectively. The EASA of Pd-based electrodes was calculatedfrom the charge consumed in the formation of a PdO monolayer andassuming a charge of 405 C cm2 for the reduction of an adsorbed oxidemonolayer on a smooth Pd [24,25]. The EASA of Pt/C electrode wasdetermined from the charge consumed for CO stripping and also foratomic hydrogen adsorption/desorption of underpotentially depositedhydrogen, considering a charge of 220 C cm2 for a monolayer ofhydrogen adsorbed on Pt [26].

The morphology and composition of the prepared samples wereanalysed by a FEI Quanta 400FEG ESEM/EDAX Genesis X4M system. ThePd loading of the carbon paper was determined by electrothermalspectroscopy after metal dissolution in HCl/HNO3 (1:1). Pdt1 and Pdt2were found to contain 0.32 and 1.77 mg cm2, respectively.

Structural analysis of the Pd/Cpaper electrode was carried out in aPhilips X'Pert diffractometer by X-ray diffraction using Cu K radiation.Thepeakof Pd (111)wasused to calculate theaverage crystalline sizeby

employing the Scherrer equation.about 100 mVmore negative than Pt/C, is shown for Pdt1 and Pdt2 after30 minof the electrode immersion in theO2-saturated solution (Fig. 2a).However, if the electrode is immersed for a longer period, 23 h, a peakemerged at about 0.150.22 V (Fig. 2b). This behaviour reects a changeon the hydrophobicity of the carbon paper along the time of theelectrode immersion. Once the surface becomes more hydrophilic, theelectrolyte better penetrates into the interior of the porous carbon layer,giving rise to a peak characteristic of a diffusion-controlled process.Despite Pd/Cpaper exhibits a lower current density (normalized to thegeometric area) than Pt/C, its intrinsic high activity towards the oxygenreduction reaction is revealed by the exchange current density (j0),current density at 0.75 V (both normalized to the EASA) and Tafelslope (b), Table 1. Thesedata refer to thepolarization curveof Fig. 2b, butvery similar j0 and j (at 0.75 V) values were also obtained for thepolarization curve acquired after a shorter time immersion in the O2-saturated solution.

The exceptionally high j0 values of Pd/Cpaper reveal an intrinsic highability of the prepared electrodematerial towards theORR, comparable toPt. The similarity of j0 for both Pdt1 and Pdt2, allows concluding that thecatalytic activity of the preparedmaterial is independent of the depositiontime, i.e. it is not inuenced by the cluster size. Such j0 values aresignicantly higher than those reported in the literature for Pd depositedby magnetron sputtering (8.6108 and 2.2107 mA cm2) [27,28]or dispersed palladium nanoparticles (6.6106 mA cm2) [29]. Con-sidering Tafel slopes, even though an increase with immersion time wasobserved (from 49 to 81 mV dec1 for Pdt1, Fig. 2a and b), these arenoticeably smaller than that corresponding to Pt/C,which is indicative of abetter Pd/Cpaper electrode performance for the oxygen reduction,particularly important at high current densities. According to the

13. Results and discussion

3.1. Physical characterization of the catalyst

SEMand EDS analyses revealed that a short timedeposition (10 min,Pdt1) led towell dened spherical-type Pdparticles uniformly dispersedon the carbon paper surface, ranging from 100 to 200 nm in size(Fig. 1a), while a longer time deposition (60 min, Pdt2) results inparticles of larger size (Fig. 1b). A magnication of these images revealsthat such Pd spheroids are formed by the agglomeration of muchsmaller particles (Fig. 1c and d), approximately of the same size in bothsamples. This is conrmed by the XRD results, which, from the Scherrerequation, lead tomeancrystallite sizes of 16 and20 nmfor Pdt1 andPdt2,respectively, indicating thatduring electroless deposition,multilayers ofaggregated spherical-typenanoparticles are formed. Thismorphology istypical of an autocatalytic deposition process, in which small particles,1620 nm for the present experimental conditions, are depositedaround a catalytic core giving rise to a cluster, which itself catalysesfurther Pddeposition, resulting inmore nanoparticles all gathered in thesame cluster. The inuence of the experimental parameters such asconcentrations of palladium salt, reducing agent and surfactant, on thePd grain size, is out of the scope of the present paper and itwill be objectof investigation in the near future.

The cross-section analyses of Pdt1 and Pdt2 conrmed thepreferential allocation of the catalyst particles near the surface. TheEDS analyses of the surface of the Pd/Cpaper showed a small amount ofSn. The presence of this element has to be related with thesensibilization step, coming probably from the adsorption of Sn2+orSn4+ ion species on the carbon surface.

3.2. Catalyst activity towards ORR

Fig. 2 shows the linear sweep voltammograms for the O2 reductionliterature, the typical Tafel slope for Pt is 60 mV dec [30]. However,

Fig. 1. SEM images of as-deposited sam

Fig. 2. Polarization curves for Pdt1, Pdt2 and Pt/C in 0.1 M H2SO4 at 5 mV s1 after a)30 min; b) 120 min of the electrode immersion in the O2-saturated solution.

747R. Rego et al. / Electrochemistry Communications 12 (2010) 745748higher values have been found for porous electrodes,which are consistentwith the present data [31].

ples of Pdt1 (a, c) and Pdt2 (b, d).In order to evaluate whether the remarkable high intrinsiccatalytic activity of the prepared catalysts towards the ORR is relatedto the interaction between the Sn species (remaining after thesensibilization/activation step) with the deposited Pd, a Pd electrolesslm deposited on a Ni disk, i.e. on a substrate that has not beensubmitted to the sensibilization/activation step (Fig. 3), was preparedand its activity towards the ORR was investigated and compared tobulk Pt. The corresponding Tafel slope and j0 values (57 mV dec1 and1.08104 mA cm2, respectively) were found to be comparable tothose of Pd/Cpaper, pointing out that the high activity of Pd electrolessis not related to PdSn, but it is intrinsic to the Pd electrolessstructure/morphology. In fact, similarly to the Pd/Cpaper, the Pdelectroless lm is also formed by a layer of spherical-type particles,which are themselves aggregations of nanoparticles (inset of Fig. 3).Apparently, it is the typical morphology of the electroless depositwhich plays an important role on the anomalous high activity of Pdtowards the ORR.

In order to rationalize the remarkable activity enhancement ofelectroless Pd with respect to isolated nanoparticles of Pd, we brieyrecall literature data for the electrocatalytic behaviour of other clustertype deposits. It has been recently shown that agglomerates of Pt onGlassy Carbon (Pt/GC) containing a high concentration of grain

Table 1Exchange current densities (j0), Tafel slope (b) and current densities at 0.75 V for ORRin 0.1 M H2SO4 on Pdt1, Pd t2 (after 23 h immersion in the O2-saturated solution) andcommercial Pt/C.

Electrode material j0 (mA cm2) b (mV dec1) j (0.75 V) (mA cm2)

Pt/C 3.60104 108 0.21Pdt1 2.04104 81 0.02Pdt2 1.15104 70 0.12

boundaries have enhanced electrocatalytic activity compared toisolated Pt nanoparticles towards COads and methanol electrooxida-tion [32,33]. A likely reason for the enhanced activity of such

results reveal that this methodology affects the intrinsic activity of Pdtowards the ORR in sulphuric acid medium, with competitive resultscompared to the standard Pt/C catalyst.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.elecom.2010.03.022.

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Fig. 3. Polarization curves in 0.1 M H2SO4 solution saturated with O2 on Pdeless lm andPt bulk at 5 mV s1. SEM image of the Pdeless lm is shown in the inset.

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Further research concerning the preparation of the catalyst byelectroless deposition, on carbon paper impregnated with hydropho-bic material and Naon is under way, envisaging the application ofthis material to a PEMFC.

4. Conclusions

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A new route to prepare carbon paper-supported Pd catalyst for oxygen reduction reactionIntroductionExperimentalPreparation of the carbon paper-supported PdCharacterization of the catalyst

Results and discussionPhysical characterization of the catalystCatalyst activity towards ORR

ConclusionsSupplementary dataReferences