Electrochemistry Communications 12 (2010) 901–904

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Electrochemistry Communications

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Preparation of carbon supported Pd–Pb hollow nanospheres and theirelectrocatalytic activities for formic acid oxidation

Ruoshi Li, Hao Hao, Wen-Bin Cai, Tao Huang ⁎, Aishui Yu ⁎Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai, 200438, China

⁎ Corresponding authors. Yu is to be contacted at Tel./Tel.: +86 21 51630321; fax: +86 21 51630320.

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

1388-2481/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.elecom.2010.04.016

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 March 2010Received in revised form 19 April 2010Accepted 21 April 2010Available online 29 April 2010

Keywords:Formic acid oxidationHollow nanospheresGalvanic replacement reactionPd–Pb alloyElectrocatalysis

Pd–Pb hollow nanospheres dispersed on carbon black were developed by a galvanic replacement reactionbetween sacrificial cobalt nanoparticles and Pd2+, Pb2+ ions. The as-prepared catalysts were characterizedby transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and X-raydiffraction (XRD). The electrochemical measurements show that the as-prepared catalysts have excellentcatalytic activity for formic acid electrooxidation, which is attributed to the large surface area caused by thehollow structure and the lead doping effect which might modify the electronic structure of the catalysts.

fax: +86 21 51630320. Huang,

asyu@fudan.edu.cn (A. Yu).

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Direct formic acid fuel cells (DFAFCs) have aroused considerableattention in the last decade due to its nontoxicity, uninflammability atroom temperature, higher theoretical open circuit potential and lowercrossover through the membrane. Thus they are thought to bepromising power sources for portable electronic devices [1,2]. How-ever, for practical applications one of the key issues is the design ofeffective and low cost anode catalysts for DFAFCs.

Pd-based catalysts have been investigated widely because of itslower cost and superior performance compared to Pt-based catalysts[3–5]. Since electrocatalytic activity of a catalyst strongly depends onthe electronic and structural properties, controllable synthetic routesof Pd-based catalysts with designed microstructure are highlydesirable [6–10]. Noble metal nanomaterials with hollow interiorshave been synthesized and have shown different catalytic activitiesfrom their solid counterparts, which is mainly due to their low densityand high surface area [7–9,11,12]. They also have the advantage ofsaving of materials and reduction of cost. The galvanic replacementreactions involving desired metal ions and nanoparticles provide asimple route to synthesize metal hollow structure [7–9,11,12]. Therehave been reports about the synthesis of palladium hollow nano-

spheres and their increased catalytic activities towards formic acidelectrooxidation [8,13]. Alloying with a second element is also aregular way to improve the activity of catalysts. [14–18].

Combining the advantages of both hollow structure and alloying,here we developed a facile way to synthesize carbon supported Pd–Pb hollow nanospheres. The Pd2+, Pb2+ ions were co-reduced by thesacrificial cobalt nanoparticles. To our knowledge, this is the firstreport about the synthesis of hollow alloy nanospheres of Pd andnon-noble metal and its electrocatalytic properties towards formicacid.

2. Experimental

2.1. Preparation of catalysts

The Co nanoparticles were fabricated according to the literature[11]. Briefly 1.3 mL 0.1 M CoCl2 was added to a deaerated aqueoussolution containing 2 mmol KBH4 and 0.25 mmol citric acid rapidly.Then the solution turns brown, indicating the formation of Conanoparticles. H2 was evolved during the reaction and continued forseveral minutes. When the bubbles ceased, a solution containing0.1 mmol H2PdCl4 and 0.025 mmol Pb(CH3COO)2 was added understirring. The solution turned blackish and stirred for 2 h. Finally thesolution became transparent with some black grains in it. Theproducts were centrifuged and washed with water. Then the Pd–Pbhollow nanospheres were obtained.

The Pd–Pb hollow nanospheres were added to a certain amount ofVulcan XC-72 carbon slurry to let the metal loading 20 wt.% and

Fig. 1. XRD patterns of the as-prepared catalysts. The vertical lines correspond to the Pdfcc phase (PDF-#65-6174).

902 R. Li et al. / Electrochemistry Communications 12 (2010) 901–904

ultrasonicated for 1 h. Then the slurry was centrifuged, washed anddried under vacuum at 60 °C overnight. The obtained catalysts werenoted as HN Pd–Pb/C.

For comparison, HN Pd/C catalysts was prepared through aprocedure similar to that mentioned above only without the additionof Pb(CH3COO)2. Pd/C catalysts were prepared by the conventionalliquid phase reduction method with KBH4 as the reducing agent.

2.2. Materials characterization

The as-prepared catalysts were characterized by XRD, which wasperformed on a Bruker D8 Advance X-ray diffractometer using Cu Kαradiation with a k of 1.5406 Å at a scan rate of 4 deg min−1. Themorphology of the obtained Pd–Pb hollow nanospheres was observedby transmission electronmicroscopy (TEM, JEOL JEM-2010F UHR) andthe composition was analyzed by an energy dispersive X-rayspectrometer (EDX) operated at 200 kV (Oxford, IET 200, attachedto TEM).

2.3. Electrochemical measurement

The working electrode for electrochemical experiment wasprepared by thin film electrode method [19]. A polished glassy carbon(GC,Φ 3 mm) sealed by PTFE was used as the substrate. And then 5 μLsuspension of well-dispersed catalysts in water solution was carefullytransferred onto a GC substrate. After water evaporation, thedeposited catalysts were covered by 2 μL Nafion solution (0.5 wt.%,Dupont) and typical metal loading is about 36 μg cm−2.

The working electrodes were first characterized in 0.5 M H2SO4

solution at a scan rate of 50 mV s−1 by cyclic voltammetry (CV).Electrochemical CO stripping voltammograms were measured by theoxidation of pre-adsorbed CO (COad) in 0.5 M H2SO4 at a scan rate of50 mV s−1. CO was introduced into the solution for 15 min to allowfor complete adsorption of CO onto the catalysts. During this process,the working electrode was maintained at −0.25 V (vs. Hg/Hg2SO4,K2SO4). Excess CO in the electrolyte was then purged with high purityAr for 15 min. The CV measurements about catalytic activity werecarried out in a solution of 0.5 M HCOOH and 0.5 M H2SO4 at a scanrate of 50 mV s−1 and chronoamperometry (CA) tests were per-formed by polarizing at −0.2 V for 1000 s in the same solution. Thechronopotentiometry measurements were carried out with an anodiccurrent of 1 mA for 60 s.

All electrochemical measurements were carried out on anelectrochemical workstation (CH Instrument 660A, CHI Company)using a conventional three-electrode glassy cell with a platinum sheetand an Hg/Hg2SO4, K2SO4 electrode (0.64 V vs. NHE) as the counterand reference electrodes.

3. Results and discussion

3.1. Physicochemical characterization

Fig. 1 shows XRD patterns of the as-prepared catalysts. The peaksaround 40.1, 46.6, 68.1 and 82.1° can be attributed to the diffractionpeaks of Pd crystal faces (111), (200), (220) and (311) respectivelywhich are characteristics of Pd face-centered cubic phase. They areadapted fromPDF-#65-6174 and indicated by the vertical lines in Fig. 1.Both Pd/C and HN Pd/C show the typical diffraction peaksmatchedwellwith the standard lines. By contrast, the peaks for HN Pd–Pb/C shift tolower 2θ values, indicating an enlargement in the Pd–Pd interatomicdistance. It is in accordance to the literature [18]. It can be seen there areno Pb peaks, indicating Pb was alloyed with Pd.

Fig. 2a–d shows the morphologies of the as-prepared catalysts.Fig. 2a shows TEM image of the Pd/C nanoparticles, its morphologyand distribution is similar to the literature [20]. Fig. 2b and c showsclear character of hollow structure with a dark edge and brighter

center. Those hollow nanospheres formed on the basis of thereplacement reaction between Co nanoparticles and metallic ions[11]. The shell is porous because as the replacement reactioncontinued, Co2+, Pd2+, Pb2+ diffused continuously in the reversedirection at different rates. The formation mechanism can beattributed to a so-called Kirkendall effect, in which pores formbecause of the difference in diffusion rates between the diffusioncouple [21]. It also can be seen the HN Pd–Pb is more porous than theHN Pd, this may be because that the diffusion components wereincreasing with the Pb2+, the diffusion rates were all different andwould be favorable to the formation of porous structure. It also can beseen the nanoparticles in the HN Pd–Pb were smaller than that in theHN Pd, which may be due to the lead doping effect. Fig. 2d shows thehigh resolution TEM of HN Pd. In order to confirm the elementalcomposition of HN Pd–Pb, the EDX investigation was carried out andthe results are displayed in Fig. 2e. It indicates the existence of Pd andPb, and the atomic ratio is a little lower than the targeted values, thismay be due to the inherent uncertainty in EDX data.

3.2. Electrochemical characterization

In order to evaluate the electronic and geometric structure of thePd surfaces, CV experiments were conducted in 0.5 M H2SO4 (Fig. 3a).The shape of the profile is similar to the literature [20]. The multiplepeaks between−0.7 V to−0.4 V are attributed to the adsorption anddesorption of hydrogen. It can be seen the hollow nanospherescatalysts show bigger hydrogen peaks, which means the increase ofelectrochemically active surface area (EAS) due to the specialstructure. The HN Pd–Pb peaks are even bigger, which may be dueto the more porous structure and smaller particle size. And the peaksare wider, which may be assigned to the Pb-doping effect. Enlarge-ment in the Pd–Pd lattice causes a more open Pd structure in favor ofhydrogen diffusion. According to the literature, a dilated Pd latticeshowed higher activity towards formic acid electrooxidation [22].

CO stripping voltammetry was used to reasonably evaluate theEAS of each catalyst [23]. Fig. 3b shows the typical CO strippingvoltammograms for different catalysts. The stripping peak area onthe HN Pd–Pb/C electrode is much larger than that of other two,indicating that there are more active surface sites. The calculatedEAS are 1.08, 1.49 and 2.22 cm2 for Pd/C, HN Pd/C and HN Pd–Pb/C,respectively [23], which is in accordance with the results of the abovecharacterization.

Fig. 3c and d shows cyclic voltammograms of formic acidelectrooxidation on the as-prepared catalysts in a solution of 0.5 MH2SO4 and 0.5 M HCOOH. The catalytic activity was evaluated as massspecific current (MSC, normalized to the mass of Pd, shown in Fig. 3c)and surface specific current (SSC, normalized to EAS, shown in

Fig. 2. (a) TEM images of Pd/C (b). TEM images of HN Pd. (c) TEM images of HN Pd–Pb. (d) HRTEM of HN Pd–Pb. (e) EDX pattern of the HN Pd–Pb.

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Fig. 3d). In the forward scan, formic acid oxidation produced an anodicpeak and in the reverse scan, there was also an oxidation peak, which isattributed to formic acid oxidation after the reduction of the oxidized Pdoxide and the removal of the incompletely oxidized carbonaceousspecies formed in the forward scan. The oxidation peak in the forwardscan is usually used to evaluate the electrocatalytic activity of thecatalysts. As observed, MSC of the three catalysts are 0.61, 1.08 and2.63 A mg−1, respectively, in theorder ofHNPd–Pb/C NHNPd/C NPd/C.The current density of the HN Pd/C is about 1.8 times of the Pd/C, this ismainly due to thespecial hollowstructure. And the currentdensity of theHNPd–Pb/C is nearly 2.4 times of HNPd/C and 4.3 times of the Pd/C. Thisis attributed to the combinationof the advantage of hollowstructure andthe doping effect of Pb. In order to evaluate the intrinsic activity of thecatalysts, the current is normalized to the real surface areas for threecatalysts. The rank of SSC for formic acid electrooxidation is still in theorder ofHNPd–Pb/C NHNPd/C NPd/C. It indicates that thesuperiority ofHN Pd–Pb/C is not only due to the much larger EAS, it also benefits fromtheelectronicmodification effect of the lead. It also should bementionedthat the shape of the CV profiles of the HN Pd–Pb/C is different from theHN Pd/C and Pd/C, the oxidation peak in the reverse scan is higher thanthe forward scan. It indicates the carbonaceous species are formed in theforward scan. The main reason for this may be that the doping of Pbchanges the electronic structure of Pd surface.

The results of chronoamperometry tests on different catalystswere compared in Fig. 3e, reflecting the activity and stability ofdifferent catalysts. The formic acid electrooxidation on HN Pd–Pb/Cpossesses the highest initial and limiting current densities, whichindicates the best catalytic activity and stability for formic acidelectrooxidation, whereas Pd/C has the lowest.

Fig. 3f gives the chronopotentiometry measurements of formicacid oxidation on different catalysts. The potential rank is in the orderof HN Pd–Pb/C bHN Pd/C bPd/C, which demonstrates that HN Pd/Cand HN Pd–Pb/C have better catalytic activity.

4. Conclusions

In summary, we synthesized a noble catalyst, i.e. carbon supportedhollow Pd–Pb nanospheres for formic acid electrooxidation via a facilemethod. The electrocatalytic properties of the as-prepared catalystwere evaluated by typical electrochemical methods. Combining theadvantage of hollow structure and Pb-doping, the catalyst showsexcellent catalytic activity. In addition, it is more economic becausewith the doping of Pb, the using of noble metal Pd is less. And furtherwork will aim to seek a better doping non-noble metal withoutforming carbonaceous species on the Pd surface.

Fig. 3. (a) Cyclic voltammograms of the catalysts in 0.5 MH2SO4 at a scan rate of 50 mV s−1; (b) CO stripping voltammograms on the catalysts in 0.5 MH2SO4 at a scan rate of 50 mV s−1

(only the oxidative CO removal region is shown); (c, d) cyclic voltammograms of formic acid electrooxidation on the catalysts in 0.5 MH2SO4+0.5 MHCOOH at a scan rate of 50 mV s−1.(e) Chronoamperometric curves on the catalysts in 0.5 MH2SO4+0.5 MHCOOHat−0.2 V. (f) Chronopotentiometrymeasurements on the catalysts in 0.5 MH2SO4+0.5 MHCOOHwithan anodic current of 1 mA.

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Acknowledgments

This work is financially supported by a grant from the Key Programof Basic Research of the Shanghai Committee of Science andTechnology (No. 08JC1402000) and Science and Technology Com-mission of Shanghai Municipality (08DZ2270500), China.

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