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Electrochimica Acta 56 (2011) 6860– 6865

Contents lists available at ScienceDirect

Electrochimica Acta

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

ltrasonic-assisted synthesis of Pd–Ni alloy catalysts supported on multi-walledarbon nanotubes for formic acid electrooxidation

uoshi Li, Zhen Wei, Tao Huang ∗, Aishui Yu ∗∗

epartment of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200438, China

r t i c l e i n f o

rticle history:eceived 22 April 2011eceived in revised form 18 May 2011ccepted 23 May 2011

a b s t r a c t

Pd–Ni alloys with different compositions (i.e. Pd2Ni, PdNi, PdNi2) dispersed on multi-walled carbon nan-otubes (MWCNTs) are prepared by ultrasonic-assisted chemical reduction. The X-ray diffraction (XRD)patterns indicate that all Pd and Pd–Ni nanoparticles exist as Pd face-centered cubic structure, while Nialloys with Pd. The transmission electron microscopy (TEM) images show the addition of nickel decreases

vailable online 1 June 2011

eywords:d–Ni alloyWCNTsltrasonic-assisted reduction

the particle size and improves the dispersion. The X-ray photoelectron spectroscopy (XPS) spectra demon-strate the electronic modification of Pd by nickel doping. The electrochemical measurements reveal thatthe PdNi catalysts have better catalytic activity and stability for formic acid electrooxidation, among themPdNi/MWCNTs is the best. The performance enhancement is ascribed to the increase of electroactivesurface area (EASA) and nickel doping effect which might modify the electronic structure.

ormic acid electrooxdiation

. Introduction

With the increasing environmental concerns and acceleratedonsumption of fossil fuels, the polymer electrolyte membraneuel cells (PEMFCs) have aroused wide research interests in recentears [1,2]. Compared to other fuels, formic acid has the advan-age of nontoxicity, uninflammability at room temperature, higherheoretical open circuit potential and lower crossover through the

embrane, thus direct formic acid fuel cells (DFAFCs) are thoughto be promising power sources for portable electronic devices [3,4].owever, for practical applications one of the key issues is theesign of effective and low cost anode catalysts for DFAFCs.

On one hand, Pd-based catalysts have been widely investigatedecently because of its lower cost and superior performance com-ared to Pt-based catalysts [5–8]. However, the stability of pured is not satisfactory and its activity needs further improvementor commercialization. It is believed that the activity of catalyststrongly depends on the electronic, structural, and geometric prop-rties of metallic materials. Therefore, much effort has been madey controllable synthesis of designed nanostructures or alloyingith a second element to achieve the above goal. Several Pd-based

lloy catalysts have been investigated towards formic acid elec-

rooxidation, such as PdAu, PdCo, PdSn, PdPt and so on [9–12].ecently Du et al. reported formic acid oxidation on PdNi2 mod-

fied glassy carbon electrode (GCE) prepared by electrodeposition,

∗ Corresponding author. Tel.: +86 21 51630321; fax: +86 21 51630320.∗∗ Corresponding author. Tel.: +86 21 51630320; fax: +86 21 51630320.

E-mail addresses: huangt@fudan.edu.cn (T. Huang), asyu@fudan.edu.cn (A. Yu).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.05.097

© 2011 Elsevier Ltd. All rights reserved.

the PdNi2 alloy showed much enhanced activity [13]. However, thecarbon supported nanoparticle is the only practical form for fuel cellcatalysts for its high utilization rate of noble metal. And how thecomposition of PdNi alloy affects the activity has not been reportedyet.

On the other hand, it is well known that the type of carbonsupport affects the performance and stability of the catalysts. Theunique properties of carbon nanotubes (CNTs), such as high spe-cific surface area, electrical conductivity, and good thermal andchemical stability, make it a good catalyst support for fuel cells.A few multi-walled carbon nanotubes (MWCNTs) supported Pd-based catalysts have been reported and showed enhanced activity[14–16].

In this work, Pd and Pd–Ni catalysts with different com-positions dispersed on MWCNTs were prepared by a simpleultrasonic-assisted chemical reduction procedure, aiming to finda less expensive and more efficient electrocatalyst for DFAFCs. Thephysicochemical properties and electrochemical activities of thenanoparticles for formic acid oxidation were investigated. And thereasons for the performance enhancement of PdNi catalyst are dis-cussed.

2. Experimental

2.1. Preparation of catalysts

Hydroxyl multi-wall carbon nanotubes with a diameter of8 nm and 30 �m length were used as the supports. PdCl2,Ni(CH3COO)2·4H2O, Vulcan XC-72 carbon and other chemicalswere all of analytical grade and used without further purification.

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For MWCNTs supported catalyst synthesis, firstly PdCl2 andi(CH3COO)2 salts with different atomic ratios were dissolved

nto an HCl solution, the pH value was adjusted to 9 using KOHolution. Then a certain amount of MWCNTs slurry was addednder ultrasonication. After ultrasonicated for 20 min, a solutionontaining 120 mg KBH4 was added dropwise and the resultinguspension was maintained under ultrasonication for 2 h to allow

complete reduction of metallic salt. Then the slurry was cen-rifuged, washed and dried under vacuum at 60 ◦C overnight. Theesultant Pd–Ni/MWCNTs catalysts with 20 wt.% Pd and the atomicatios of Pd:Ni = 2:1, 1:1 and 1:2 were denoted as Pd2Ni/MWCNTs,dNi/MWCNTs and PdNi2/MWCNTs, respectively. For comparison,0 wt.% Pd/MWCNTs and 20 wt.% Pd/XC-72 was prepared with theame method.

.2. Characterization

The as-prepared catalysts were characterized by XRD, whichas performed on a Bruker D8 Advance X-ray diffractometer usingu K� radiation with a k of 1.5406 A at a scan rate of 4◦ min−1. Theorphology was observed by scanning electron microscope (SEM,itachi, FE-SEM S-4800) and transmission electron microscopy

TEM, JEOL JEM-2100F UHR). The composition was analyzed byn energy dispersive X-ray spectrometer (EDX) operated at 20 keVBruker, QUANTAX 400, attached to SEM), and the element analysis

apping was carried out on the same equipment. X-ray photoelec-ron spectroscopy (XPS) experiments were carried out on a RBDpgraded PHI-5000C ESCA system (Perkin Elmer) with Mg K� radi-tion (h� = 1253.6 eV).

.3. Electrochemical measurement

The working electrode for electrochemical experiment was pre-ared by thin film electrode method [17]. A polished glassy carbonGC, 5 mm) sealed by poly(tetrafluoroethylene) was used as theubstrate. And then 5 �L suspension of well-dispersed catalysts insopropanol solution was carefully transferred onto a GC substrate.fter solution evaporation, the deposited catalysts were coveredy 3 �L Nafion solution (0.5 wt.%, Dupont) and typical Pd loading isbout 13 �g cm−2.

The working electrodes were first characterized in 0.5 M H2SO4olution at a scan rate of 50 mV s−1 by cyclic voltammetry (CV).he CV measurements about catalytic activity were carried out in aolution of 0.5 M HCOOH and 0.5 M H2SO4 at a scan rate of 50 mV s−1

nd chronoamperometry (CA) tests were performed by polarizingt −0.34 V for 1000s in the same solution.

All electrochemical measurements were carried out on an elec-rochemical workstation (CH Instrument 660A, CHI Company)sing a conventional three-electrode glassy cell with a platinumheet and an Hg/Hg2SO4, K2SO4 electrode (0.64 V vs. NHE) as theounter and reference electrodes at room temperature. Potentialsn the text are referred to the Hg/Hg2SO4, K2SO4 electrode.

. Results and discussion

.1. Physicochemical characterization

Fig. 1 shows the XRD patterns of the as-prepared catalysts. Theeak located at 26◦ corresponds to the graphite (0 0 2) plane ofWCNTs support. The other principal peaks exhibit the character-

stics of a single face-centered-cubic (fcc) crystallographic structure

f Pd (JCPDS, Card No. 5-681), corresponding to the planes (1 1 1),2 0 0), (2 2 0) and (3 1 1) located at 40◦, 47◦, 68◦ and 82◦, respec-ively. It should be noticed the PdNi/MWCNTs and PdNi2/MWCNTsnly show the peak of plane (1 1 1), the other peaks are too wide and

Fig. 1. XRD patterns of the as-prepared catalysts: (a) Pd/MWCNTs, (b)Pd2Ni/MWCNTs, (c) PdNi/MWCNTs and (d) PdNi2/MWCNTs.

small to be seen. It is ascribed to so-called peak broadening phe-nomenon from the remarkable decrease of particle size. It couldbe also seen the diffraction peak of MWCNTs located in the sameangle of all the catalysts, by contrast, the characteristic peak of Pdin all Pd–Ni/MWCNTs catalysts are shifted to higher 2� values withrespect to the corresponding peaks in the Pd/MWCNTs, indicatingthe formation of alloy between Pd and Ni, and an contraction inthe Pd–Pd interatomic distance. It is in accordance to the literature[18]. The average size of the particles is calculated according to theDebye–Scherrer formula [19]:

d (Å ) = 0.89�

cos �

where is the width of half peak, � is the incident wavelength, dis the particle diameter, and � is the diffraction angle. The averageparticle size estimated from the Pd (1 1 1) diffraction peaks are 5.2,3.8, 1.7 and 1.6 nm for Pd, Pd2Ni, PdNi and PdNi2, respectively. Itindicates that the addition of Ni decreases the particle size, whichwould result in the increase of electroactive surface areas (EASA).

The SEM images in Fig. 2 display the overall morphology ofthe as-prepared catalysts. It could be seen that the nanoparti-cles were entangled in the outer surface of MWCNTs. However,the particle size and the difference between the Pd/MWCNTs andPdNi/MWCNTs are not clearly shown in this image.

The morphology of the as-prepared catalysts are characterizedby TEM and shown in Fig. 3. It reveals the particle size of Pd andPd2Ni are similar, about 4–5 nm, that of PdNi and PdNi2 are smaller,about 2–3 nm, which is in accordance to the XRD calculation results.Comparison of the TEM images of four catalysts indicates that someof the Pd particles are agglomerated, while Pd–Ni alloy nanoparti-cles are well dispersed on the surface of MWCNTs. From the highresolution transmission electron microscopy (HRTEM) image inFig. 3(E), it could be further seen the small particle size and gooddispersion of PdNi nanoparticles clearly. Fig. 3(F) shows a typicalTEM image of Pd/XC-72, the particle size is larger and some particlesare agglomerated.

In order to confirm the elemental composition of the as-prepared catalysts, the EDX investigation was carried out and theEDX pattern of PdNi/MWCNTs is displayed in Fig. 4(D). It manifeststhe existence of Pd and Ni. The atomic ratios of Pd:Ni for Pd2Ni, PdNi,PdNi2 from EDX are listed in Table 1, similar to the preset values,which means all the metal ions can be completely reduced by ourprocedure. The distribution of Pd and Ni in the whole catalysts was

studied with element analysis mapping at low magnification. Fromthe Pd, Ni element maps shown in Fig. 4(B) and (C), the two met-als are uniformly distributed in the catalysts. It coincides with the

6862 R. Li et al. / Electrochimica Acta 56 (2011) 6860– 6865

Fig. 2. SEM images of the as-prepared catalysts: (A) Pd/MWCNTs and (B) PdNi/MWCNTs.

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ig. 3. TEM images of the as-prepared catalysts: (A) Pd/MWCNTs, (B) Pd2Ni/MWCNTsdNi/MWCNTs.

able 1hysicochemical and electrochemical characteristics of the catalysts.

Catalysts Atomic ratio of Pd:Ni (EDX) EASA (m

Pd/MWCNTs 100:0 77.2

Pd2Ni/MWCNTs 70:30 93.2

PdNi/MWCNTs 52:48 104.8

PdNi2/MWCNTs 35:65 116.0

Pd/XC-72 100:0 63.2

, (C) PdNi/MWCNTs, (D) PdNi2/MWCNTs, (F) Pd/XC-72 and (E) HRTEM image of

2 g−1) jp(MSC) (A mg−1 Pd) jp(SSC) (mA cm−2)

1.07 1.391.62 1.732.25 2.151.97 1.700.82 1.28

R. Li et al. / Electrochimica Acta 56 (2011) 6860– 6865 6863

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Fig. 4. (A) SEM image of PdNi/MWCNTs at low magnification, the correspo

bove conclusion of the formation of alloy (solid solution) betweend and Ni from XRD results.

According to Hammer and Nørskov’s d-band center model24,25], the Pd d-band center shifts down when Pd is alloyed withi, which suggests weaker Pd-adsorbate bonds. Moreover, it wasemonstrated by Kibler et al. that a suitable downshift of the d-and center of Pd increased the electrocatalysis of formic acid on ad surface [26]. Combining the theoretical and fundamental exper-mental studies above, the electronic structure modification of Pdy Ni is expectable. To confirm that, XPS was employed to char-cterize the electronic property of catalyst surface. Fig. 5 displayshe representative spectra for Pd (3d) regions of Pd/MWCNTs and

dNi/MWCNTs. Apparently, both of the spectra include a doublethat consist of a high-energy peak (Pd 3d3/2) and a low-energy peakPd 3d5/2). To inspect any shift more accurately, binding energies

ig. 5. Representative spectra for Pd (3d) regions of the as-prepared catalysts: (a)d/MWCNTs and (b) PdNi/MWCNTs.

element mapping of Ni (B) and Pd (C). (D) EDX pattern of PdNi/MWCNTs.

were calibrated using the containment carbon (C1s = 284.6 eV). Forthe Pd/MWCNTs, the Pd 3d5/2 peak is located at ∼335.6 eV, while itshifts negatively by ∼0.2 eV for PdNi/MWCNTs. This shift of bindingenergy is due to the slight electron transfer from Ni to Pd, consis-tent with the electronegativity of Pd and Ni (Pd 2.2, Ni 1.8) [23].Therefore, the addition of Ni modifies the electronic structure ofPd and produces an electronic effect. Furthermore, Zhang et al.have reported the lowering of Pd (3d) binding energy influencedthe chemisorptions energy of adsorbate and benefited the formicacid eletrooxidation [27]. It is probably the same case for Pd–Nialloy catalysts.

3.2. Electrochemical characterization

In order to evaluate the electronic and geometric structure ofthe Pd surfaces, CV experiments were conducted in 0.5 M H2SO4(Fig. 6). The shape of the profile is similar to the literature [6]. The

Fig. 6. Cyclic voltammograms of the catalysts in 0.5 M H2SO4 solution at a scan rateof 50 mV s−1.

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ultiple peaks between −0.7 V and −0.4 V are attributed to the so-alled hydrogen absorption/desorption (Hab) process (peak I) andydrogen adsorption/desorption (Had) process (peak II) [20]. Themaller pair of peaks at the more positive potentials is thought toe an indication of real surface area of catalysts and is missing orerged into the broad Hab peaks sometimes. The significant aug-entation of them indicates that the real surface area increasesith the addition of Ni, which may result from the decrease ofarticle size and better dispersion. This result corroborates the con-lusion from XRD and TEM described above. The double layer regionetween −0.4 V and 0.2 V is mainly contributed by the carbon sup-ort. The observed difference in current of Pd oxidation after 0.1 Van be ascribed to the difference in surface area. In addition, theurface Pd oxide reduction peaks of Pd–Ni catalyst shift negatively,hich may be due to an increase in the OH adsorption strength [7].

he accurate EASA of each catalyst was calculated from the densityf charge associated with the reduction of a full monolayer of Pdxides [10,21] and the values are presented in Table 1.

.3. Electrocatalytic properties for formic acid oxidation

Fig. 7 shows cyclic voltammograms of formic acid electrooxi-ation on the as-prepared catalysts in a solution of 0.5 M H2SO4

nd 0.5 M HCOOH. The catalytic activity was evaluated as masspecific current (MSC, normalized to the mass of Pd, shown inig. 7A) and surface specific current (SSC, normalized to EASA,hown in Fig. 7B). A large current peak at potential range from −0.4

ig. 7. Cyclic voltammograms of formic acid electrooxidation on different catalystsn 0.5 M H2SO4 + 0.5 M HCOOH at a scan rate of 50 mV s−1. (A) Mass specific currentnd (B) surface specific current.

a 56 (2011) 6860– 6865

to −0.1 V and an indistinct shoulder at potential range from 0 to0.2 V were observed, which corresponded to the reaction of formicacid oxidation (FAO) via the direct pathway and the CO pathway,respectively [21]. Apparently, the FAO on the Pd and PdNi alloycatalysts is mainly through the direct pathway, in agreement withthe previous report [22]. The direct oxidation peak in the forwardscan is usually used to evaluate the electrocatalytic activity ofthe catalysts. As observed, the rank of MSC of the five catalystsfor formic acid electroxidation is in the order of PdNi/MWCNTs >PdNi2/MWCNTs > Pd2Ni/MWCNTs > Pd/MWCNTs > Pd/XC-72,which indicates the activity of catalysts is increased by the addi-tion of Ni. The peak currents (MSC) are list in Table 1. It could beseen that the current density of the PdNi/MWCNTs is about 2.1times of the Pd/MWCNTs and 2.7 times of Pd/XC-72. In order toevaluate the intrinsic activity of the catalysts, the current is nor-malized to the real surface areas for five catalysts. The rank of SSCfor formic acid electroxidation is in the order of PdNi/MWCNTs >Pd2Ni/MWCNTs > PdNi2/MWCNTs > Pd/MWCNTs ≈ Pd/XC-72. Itindicates that the superiority of PdNi alloy catalysts is not onlydue to the much larger EASA, it also benefits from the elec-tronic modification effect of Ni verified by the XPS experimentabove. It also should be mentioned that by removing the EASAinfluences the peak current (SSC) of Pd2Ni/MWCNTs is higherthan PdNi2/MWCNTs in contrast to MSC (Table 1), which meanstoo much Ni will cause the drop of the intrinsic activity of thecatalysts. In addition, the SSC of Pd/MWCNTs and Pd/XC-72 aresimilar, which indicates the activity enhancement of Pd/MWCNTSis mainly due to the increase of EASA from smaller particle andbetter dispersion.

The stability of the electrocatalysts is extremely importantfor their real applications in DFAFCs. The long-term activity anddurability of the Pd-based catalysts were further assessed bychronoamperometry test, conducted on catalyst-coated GC withthe potential fixed at −0.34 V, a typical working potential for formicacid oxidation in DFAFCs (Fig. 8). As observed, PdNi alloy catalystsshow superior activity and durability as compared to their Pd coun-terparts. Among them, PdNi/MWCNTs possess the highest initialand limiting current densities, which indicate the best catalyticactivity and stability for formic acid electrooxidation.

By combining cyclic voltammetry and chronoamperometricresults, it is concluded that the PdNi/MWCNTs catalyst has the high-est catalytic performance for formic acid electrooxidation, whichdemonstrates that the addition of an appropriate amount of Ni

can greatly improve the catalytic performance of Pd/MWCNTs cat-alysts. The enhancement can be ascribed to the largely increasedEASA from smaller particle size and better dispersion combiningwith the electronic modification effect of nickel.

Fig. 8. Chronoamperometric curves on the catalysts in 0.5 M H2SO4 + 0.5 M HCOOHat −0.34 V.

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. Conclusions

In summary, a series of Pd–Ni alloy nanoparticles with dif-erent compositions dispersed on MWCNTs were synthesized vian ultrasonic-assisted chemical reduction method. The electrocat-lytic properties of the as-prepared catalyst were evaluated byypical electrochemical methods. It is found that PdNi/MWCNTsatalyst shows excellent catalytic activity. In addition, it is moreconomic because with the doping of nickel, the using of nobleetal Pd is less. The PdNi/MWCNTs catalyst shows great potential

s less expensive electrocatalyst for formic acid electrooxidation inFAFCs. And further work will aim to understand the formic acidxidation mechanism on the PdNi alloy nanoparticles.

cknowledgments

This work is financially supported by a grant from Nationalatural Science Foundation of China (No. 21003025) and Sci-nce & Technology Commission of Shanghai Municipality (No.8DZ2270500), China.

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