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Electrochimica Acta 103 (2013) 66– 76

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom ep age: www.elsev ier .com/ locate /e lec tac ta

lectrochemical preparation and characterization of PdPt nanocagesith improved electrocatalytic activity toward oxygen reduction

eaction

eng Zhanga,b, Zhi-Gang Shaoa,∗, Wangting Lua,b, Feng Xiea, Xiaoping Qina, Baolian Yia

Fuel Cell System and Engineering Group, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, 116023 Dalian, PR ChinaGraduate School of Chinese Academy of Sciences, 19A Yuquan Road, 100049 Beijing, PR China

a r t i c l e i n f o

rticle history:eceived 11 January 2013eceived in revised form 3 April 2013ccepted 5 April 2013vailable online xxx

a b s t r a c t

Hollow PdPt nanocages were prepared by applying potential cycling treatment on core–shell Pd@Ptnanodendrites in 0.5 M sulfuric acid aqueous solutions, during which Pd core was dissolved and thePd@Pt structure was transformed to hollow nanocages. The nanocages were comprised of bimetallicPdPt nanoalloys, which was confirmed by high-resolution TEM (HRTEM), scanning TEM-energy disper-sive X-ray spectra (STEM-EDX), X-ray diffraction (XRD) and cyclic voltammetry (CV). The PdPt nanocages

eywords:ore–shelluel cellanocagexygen reduction

presented superior catalytic activity toward oxygen reduction reaction in comparison with Pd@Pt nan-odendrites and commercial Pt/C catalysts owing to the alloying effect and special morphology. Thenanocage morphology was still maintained after long-term durability testing, indicating good structurestability.

© 2013 Elsevier Ltd. All rights reserved.

dPt nanoalloy

. Introduction

Proton exchange membrane fuel cells (PEMFCs) are consid-red to be a promising candidate for the future generation ofower solutions [1]. However, the low activity of Pt/C towardxygen reduction reaction (ORR) on the cathode and the degra-ation of cathodic catalysts under dynamic potential cycling arewo of the most critical challenges that hinder the widespreadpplication of PEMFCs [2]. The development of nanostructured Pt-ased electrocatalysts is considered to be one of the most effectivepproaches to resolve these problems [3]. In the sight of compo-ition, Pt based alloys are believed to improve the ORR activity4–8], because the introduction of other metal compositions modi-es the electronic and crystallographic structures of Pt, resulting inhe decrease of the binding energy between Pt and oxygen [9]. Onhe other hand, the shape and morphology of nanocrystals are alsof great importance in the development of ORR catalysts. Therein,orous nanocatalysts [9–16] are attracting more and more atten-ion, because the porous structure gives birth to high surface areao volume ratios and creates a confined reaction environment that

mproves the reaction kinetics [14,15]. Moreover, the non-zeroimensional nanostructure makes the porous nanocrystals less vul-erable to dissolution, Ostwald ripening, and aggregation during

∗ Corresponding author. Tel.: +86 411 84379153; fax: +86 411 84379185.E-mail address: zhgshao@dicp.ac.cn (Z.-G. Shao).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.04.045

fuel cell operation than the commercial carbon black supportedzero dimensional Pt nanoparticles [13,17,18]. As a result, the com-bination of alloying and porous structure can be expected to be apromising candidate for the development of efficient ORR electro-catalysts, which has been proved by several very recent successfulexamples [9,10,15,16,18]. Hong et al. prepared Pd–Pt alloy hollownanostructures by sacrificial template method [9]. The octahedralor cubic Pd nanocrystals were used as the template, and Pt atomswere deposited by the galvanic replacement reaction between Pdtemplate and Pt precursors, resulting in the Pd–Pt alloy hollownanostructures at the expense of Pd nanocrystals. The obtainedoctahedral Pd–Pt alloy hollow nanostructures showed superiorORR activity than that of the Pt-on-Pd nanodendrites and commer-cial Pt/C catalyst. Besides, Snyder et al. synthesized porous PtNinanoalloys by dealloying method which is another frequently usedpathway in the preparation of porous alloys [15]. In the prepara-tion, Ni-rich solid PtNi nanoalloys were first synthesized, and thenmost of Ni atoms were dissolved in acid by potential cycling (adealloying treatment), and finally porous Pt-rich PtNi nanoalloyswere obtained. These porous PtNi nanoalloys presented significantenhancement on the ORR activity compared to Pt/C catalysts. Inaddition, Wang et al. adopted another common dealloying method,acid washing, to prepare nanoporous PtNi alloys which displayed

remarkable improvement in the activity and durability toward ORR[18]. On the basis of literature, nearly all porous Pt-based catalystswere synthesized by the above mentioned sacrificial template anddealloying pathway, other methods were seldom reported.

himica Acta 103 (2013) 66– 76 67

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In the present manuscript, we prepared hollow alloy PdPtanocages (NCs) by a new method, which is achieved by thelectrochemical transformation from core–shell Pd@Pt nanoden-rites (NDs) through potential cycling treatment. The structurend composition change was evidenced by physical and electro-hemical characterizations. The obtained PdPt nanocages showednhanced ORR activity in comparison with Pd@Pt nanodendritesnd commercial Pt/C catalysts. After accelerated durability testing,he structure of nanocage was maintained, indicating good stability.

. Experimental

.1. Synthesis of Pd@Pt NDs

Pd@Pt NDs with nominal Pd:Pt molar ratio of 2:1 are synthe-ized via a one-step method with modification [19]. In the presencef Pluronic® F127 (PEO106PPO70PEO106, Sigma Aldrich), Na2PdCl4nd K2PtCl4 were successively reduced by ascorbic acid (AA) atoom temperature in aqueous solutions for 12 h. The Pd@Pt NDsbtained were then supported onto hydrogen peroxide pre-treatedC-72R carbon blacks [20] with metal loading of 20% by addingarbon blacks into the as-prepared Pd@Pt NDs colloid and stirringhe mixture overnight. After repeated washing and centrifugationor several times, the supported electrocatalysts (denoted as Pd@PtDs/C) were obtained.

.2. Electrochemical preparation of PdPt NCs

Electrochemical preparation was carried out in a standard three-lectrode cell connected to a CHI-730D electrochemical station atoom temperature. Electrolyte was chosen to be 0.5 M H2SO4 aque-us solutions. A platinum foil counter electrode and a saturatedalomel electrode (SCE) served as the counter and reference elec-rode, respectively, but all electrode potentials were given versuseversible hydrogen electrode (RHE), and the conversion factor waseasured to be ERHE = ESCE − 0.263 V, followed by the method men-

ioned in the literature [21]. Rotating disk electrode (RDE) with alassy carbon disk (4 mm in diameter) was used as working elec-rode. Electrocatalysts slurry was prepared as follows: 5 mg Pd@PtDs/C was dispersed in a mixture of 5% Nafion solution (50 �L) and

so-propanol (4 mL). The mixture was sonicated for 20–30 min toorm an ink, and 10 �L of this ink was dropped on the glassy car-on disk and allowed to dry in air at room temperature. As a result,he metal loading on the RDE is 19.9 �g cm−2. Furthermore, the Ptoading on the RDE for Pd@Pt NDs/C is 9.5 �g cm−2 on the basis oft/Pd molar ratio measured by ICP (see below).

Before electrochemical preparation the catalysts were scannedetween 0.2 and 1.2 V at 100 mV s−1 in N2-purged electrolyteor several times in order to clean the surface of catalysts. Thenhe Pd@Pt NDs catalysts were cycled between 0.6 and 1.2 V for00 times and Pd@Pt NDs were transformed into PdPt nanocagesshown in Scheme 1).

.3. Physical characterization

X-ray diffraction (XRD) analysis was conducted on/MAX-2500/PC X-ray diffractometer using Cu K� radiation

� = 0.154056 nm). X-ray photoelectron spectra (XPS) werebtained from an ESCALAB 250Xi (Thermo Scientific) spectrometersing Al K� radiation. The composition of catalysts was determinedy inductively coupled plasma atomic emission spectroscopy (ICP-

ES, Perkin Elmer Optima 2000 DV). Transmission electronicroscopy (TEM) images were taken using JEOL JEM-2000EX

lectron microscope operating at 120 kV. The high resolutionEM (HRTEM) and high-angle annular dark-field scanning TEM

Scheme 1. Schematic illustration of the electrochemical preparation of PdPt NCs.

(HAADF-STEM) analysis were performed on FEI Tecnai G2 F30microscope.

2.4. Electrochemical study

The cyclic voltammograms (CVs) were recorded at 50 mV s−1 inN2-purged 0.5 M H2SO4. The Pt mass based specific electrochemicalsurface area (ECSA) is calculated from integrated hydrogen adsorp-tion and desorption cyclic voltammograms using 0.21 mC cm−2

Pt asthe conversion factor [22]. The ORR polarization curves were sweptpositively at 10 mV s−1 in O2-saturated 0.5 M H2SO4 with a rota-tion rate between 225 and 1600 rpm. The kinetic current of thecatalysts was calculated by using the well-known mass-transportcorrection for rotating disk electrodes: j−1 = j−1

k + j−1d [22], i.e.,

Koutecky–Levich equation, where j is the experimentally obtainedcurrent density, jd is the measured diffusion limiting current den-sity, and jk the kinetic current density. The accelerated degradationtest (ADT) was performed by cycling the potential between 0.6 and1.0 V at 100 mV s−1 in 0.5 M H2SO4 under continuous N2 flow. Cyclicvoltammetry curves and ORR polarization curves were measuredafter ADT via the method mentioned above. For comparison, thecommercial 20 wt.% Pt/C catalysts (Johnson Matthey) were studiedfollowing the same procedure described above. The Pt loading onthe RDE for Pt/C is 19.9 �g cm−2.

3. Results and discussion

3.1. Physical characterization of Pd@Pt NDs

In the presence of Pluronic® F127, Pd@Pt NDs can be obtained viathe spontaneously step-by-step reduction of Na2PdCl4 and K2PtCl4by ascorbic acid (AA) in an aqueous solution. Fig. 1a shows theTEM and HRTEM images of Pd@Pt NDs. The NDs have dendriticnanostructure and uniform shape. In the EDX spectrum shown inFig. 1b, there are peaks corresponding to five elements: C, O, Cu, Ptand Pd, where C, O and Cu are from carbon-coated copper grid,which suggests that the obtained NDs are composed of only Ptand Pd elements. The high-angle annular dark-field scanning TEM(HAADF-STEM) images in Fig. 1c and d further demonstrate the NDshave a three-dimensional dendritic morphology. Furthermore, theelemental line profiles (Fig. 1c) and maps (Fig. 1d) definitely con-firm that the NDs consist of a Pd core and Pt shell. The resultsabove indicate that Pd@Pt NDs with Pd core and dendritic Ptshell can be synthesized by a facile and one-step method withoutorganic solvents and harsh reaction conditions. Although the equi-

librium electrode potential of the PtCl42−/Pt couple [0.775 V vs. astandard hydrogen electrode (SHE)] is more positive than that of thePdCl42−/Pd (0.591 V vs. SHE) [23], Na2PdCl4 is reduced by AA first.If Na2PdCl4 or K2PtCl4 existed in the reaction system separately,

68 G. Zhang et al. / Electrochimica Acta 103 (2013) 66– 76

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ig. 1. (a) HRTEM images, (b) EDX spectrum, (c and d) HAADF-STEM image and eleepresents 5 nm and 50 nm, respectively.

he color of the system containing Na2PdCl4 turned to dark brownmmediately after AA was added, while that of the system contain-ng K2PtCl4 did not change obviously after 12 h. Similar phenomena

ere also observed by other researchers [24]. The TEM image of car-on supported Pd@Pt NDs is shown in Fig. 2a, and the NDs particleizes are distributed from 15 to 35 nm with an average particle sizet 22.7 nm (Fig. 2b).

Fig. 3 shows the XRD patterns of 20%Pt/C and Pd@Pt NDs/C. Theiffraction peak at approximately 25◦ comes from the carbon black,hile the diffraction peaks of the two catalysts at approximately

0◦, 47◦, 68◦, 81◦ and 86◦ are attributed to the (1 1 1), (2 0 0), (2 2 0),3 1 1) and (2 2 2) planes of the FCC structure of Pt and Pd, respec-ively. Because Pt and Pd possess the same crystal structure (FCC)nd very close lattice parameter, the sets of diffraction peaks oft and Pd are overlapped [25,26]. The diffraction peaks of Pd@PtDs/C are sharper than that of Pt/C, indicating larger particle sizend good crystallinity of Pd@Pt NDs.

.2. Physical characterization of PdPt NCs

As shown in TEM images (Fig. 2c), after potential cyclingetween 0.6 and 1.2 V for 300 cycles, solid core–shell Pd@Pt NDsere transformed to hollow nanocages, and the average particle

ize was decreased from 22.7 nm to 15.7 nm (Fig. 2d). The struc-ure change probably resulted from the dissolution of Pd duringhe potential cycling, because the standard electrochemical poten-ial for direct Pd dissolution is 0.92 V (Pd → Pd2+ + 2e−, U0 = 0.92 V)

al line profiles and maps of the Pd@Pt NDs. The scale bar in the inset of (a) and (d)

[27], so the thermodynamic conditions of Pd dissolution are metunder our experimental conditions.

In order to avoid the interference of support and observe thestructure more clearly, HRTEM images of NCs were taken with-out carbon support. As presented in Fig. 4a, the hollow structure isclearly displayed and the shell is made up of small nanoparticles.Surprisingly, the EDX spectrum (Fig. 4b) shows that Pd still exists,but the intensity of Pd peaks on NCs is weakened considerably incomparison with that of Pd@Pt NDs. In fact, the Pd/Pt molar ratioafter potential cycling is 50:50 by ICP measurements (Table 1). Fur-thermore, the elemental line profiles (Fig. 4c) and elemental maps(Fig. 4d) of one NC show that the contribution of Pt and Pd ele-ments is homogeneous and overlapped instead of Pt surroundingPd element, which suggests an alloy structure for the NCs [9,28,29].

For the sake of further observing the structure of PdPt NCs, thePd@Pt NDs/C catalyst slurries were casted onto a wet-proofed car-bon gas diffusion layer (GDL) commonly used in PEMFCs followedby the same potential cycling procedure to prepare PdPt NCs/C. TheGDL supported Pd@Pt NDs/C and PdPt NCs/C catalysts were takenXRD and XPS measurements, respectively.

The XRD patterns (part) of Pd@Pt NDs/C and PdPt NCs/C areshown in Fig. 5. Since Pd of Pd@Pt NDs was remarkably lost afterpotential cycling, the diffraction peaks of PdPt NCs should have

shifted negatively relative to Pd@Pt NDs owing to the larger latticeconstant of Pt. On the contrary, the (1 1 1) diffraction peaks for PdPtNCs shifted positively by 0.4◦ in comparison with Pd@Pt NDs. On thebasis of experimental results reported, the XRD diffraction peaks for

G. Zhang et al. / Electrochimica Acta 103 (2013) 66– 76 69

ons fo

tcl[

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Fig. 2. TEM images and corresponding particle size distributi

he bimetallic alloyed catalysts will shift positively relative to the

orresponding core–shell structures whose shell metal possessesarger lattice parameters than that of the core metal, e.g., Co–Pd30] and Co–Pt [31] systems. Similar results had been observed by

able 1d:Pt molar ratio, average particle size, ORR activities and Tafel slopes of Pd@Pt NDs/C, Pd

Catalysts Pd:Pt molarratio

Average particlesizea (nm)

Specific ECSA(m2 g−1

Pt )Areaactiv(�A

ICP XPS

Pd@Pt NDs/C 67:33 48:52 22.7 49.3 322.PdPt NCs/C 50:50 32:68 15.7 27.5 839.Pt/C n.a. 2.5 76.9 177.

a TEM data.b @0.85 V and calculated based on ECSA.c @0.85 V and calculated based on the initial Pt mass on RDE.d @0.85 V and calculated based on the initial Pt mass on RDE and Pd/Pt molar ratio meae Low overpotential region.f High overpotential region.

r (a, b) Pd@Pt NDs/C, (c, d) PdPt NCs/C and (e, f) 20%Pt/C (JM).

our groups on the Pt modified Pd/C catalysts [32]. The STEM-EDX

and XRD results confirmed that the PdPt NCs presented an alloystructure. During the repeated potential variation, Pd atoms in thecore were first dissolved, and then part of the Pd ions was deposited

Pt NCs/C and Pt/C.

-specificityb

cm−2ECSA)

Pt based massactivityc

(mA mg−1Pt )

PGM based massactivityd

(mA mg−1PGM)

Tafel slope ofLORe

(mV dec−1)

Tafel slope ofHORf

(mV dec−1)

2 158.8 75.8 72 1118 230.9 149.2 77 1170 136.1 136.1 72 102

sured by ICP.

70 G. Zhang et al. / Electrochimica Acta 103 (2013) 66– 76

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ap

Fr

Fig. 3. XRD spectra of Pd@Pt NDs/C and 20%Pt/C (JM).

t low potential. Since the lattice mismatch between Pd and Pt isnly 0.77% [33], it was easy for the deposited Pd to diffuse intohe lattice of Pt on the shell, i.e., the Pt nanoparticles become PdPtimetallic nanoalloys after potential cycling which composed thedPt NCs.

The XPS spectra of GDL supported Pd@Pt NDs/C and PdPt NCs/Cre displayed in Fig. 6. It can be seen in Fig. 6 that for PdPt NCs, the Pdeaks are decreased significantly in comparison with Pd@Pt NDs,

ig. 4. (a) HRTEM images, (b) EDX spectrum, (c and d) HAADF-STEM image and elemenepresents 2 nm and 50 nm, respectively.

Fig. 5. XRD spectra of GDL supported Pd@Pt NDs/C and PdPt NCs/C.

while the peak intensity of Pt reduces more slightly, suggestingremarkable Pd dissolution and little Pt loss after potential cycling[34]. Moreover, the surface composition of catalysts can be evalu-ated by XPS [35–37]. As shown in Table 1, the surface Pd/Pt molarratio of Pd@Pt NDs/C and PdPt NCs/C was 48:52 and 32:68, respec-tively, both of which were smaller than that of the bulk molar ratio

determined by ICP, indicating that both Pd@Pt NDs and PdPt NCshad Pt rich surfaces.

tal line profiles and maps of the PdPt NCs. The scale bar in the inset of (a) and (d)

G. Zhang et al. / Electrochimica Acta 103 (2013) 66– 76 71

Table 2Comparison of ORR activities of PdPt NCs with the literature data reported for Pt-based catalysts in 0.5 M H2SO4.

Area-specific activity @0.85 V vs. RHE (�A cm−2ECSA) Mass activity @0.85 V vs. RHE (mA mg−1) Reference

PdPtnanocages

839.8 230.9 (Pt-based) Thiswork149.2 (metal-based)

PtPd nanotubes ∼700 ∼115 (metal-based) [13]PtFe nanowires n.a. 77.1 (Pt-based) [39]Pt nanoassemblies ∼26 12.4 (Pt-based) [40]Pd@Pt nanostructures n.a. ∼14 (metal-based) [26]PtPd nanoalloy/C 39.7 n.a. [41]

3

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Fig. 6. XPS spectra of GDL supported Pd@Pt NDs/C and PdPt NCs/C.

.3. Electrocatalytic performances

The electrochemical behavior of Pd@Pt NDs during the forma-ion of PdPt NCs, i.e., the profile of CV curves during 300 cyclesetween 0.6 and 1.2 V, are shown in Fig. 7. The intense peak atround 0.05 V for Pd@Pt NDs/C was originated from the Pd surfacencovered by the porous Pt shell [26] and this peak was weakenedradually during the potential cycling, further confirming the dis-olution of Pd. After 300 cycles, PdPt NCs are obtained, and thatntense peak on the CV curve completely disappears.

In order to calculate the electrochemical surface area, theV curves of Pt/C, Pd@Pt NDs/C and PdPt NCs/C were recorded

rom 0.05 V to avoid the interference of the intense hydrogendsorption of Pd and hydrogen evolution reaction below 0.05 V.

s shown in Fig. 8a, there are hydrogen adsorption/desorptionegion (0.05–0.4 V) and surface metal oxidation/reduction region0.7–1.22 V) on the CV curves for all the three catalysts. The Pt massased ECSA for the three catalysts were listed in Table 1. The Pt mass

Fig. 7. CV curves of Pd@Pt NDs/C after different potential cycles between 0.6 and1.2 V at 50 mV s−1 in N2-purged 0.5 M H2SO4.

based ECSA of Pt/C is higher than that of Pd@Pt NDs and PdPt NCsowing to the smaller particle size of Pt nanoparticles of Pt/C. More-over, the decrease of Pt mass based ECSA for PdPt NCs relative toPd@Pt NDs is ascribed to the contraction and aggregation of Pt shellafter Pd core is dissolved (Fig. 2).

The polarization curves for ORR are demonstrated in Fig. 8b,and the Tafel plots based on area-specific activity and mass activityafter mass-transfer correction by the Koutecky–Levich equation areshown in Fig. 8c and d, respectively [38]. It can be found in Fig. 8cand d that Pd@Pt NDs/C has higher ORR activity than that of Pt/C,and the activity was further improved when the Pd@Pt NDs weretransformed to PdPt NCs. Specifically at 0.85 V, the area-specificactivity of PdPt NCs/C and Pd@Pt NDs/C is 4.7 and 1.8 times that ofPt/C (Table 1), respectively. In addition, the Pt mass activity of PdPtNCs/C, Pd@Pt NDs/C and Pt/C was 230.9, 158.8 and 136.1 mA mg−1

Pt ,respectively. More importantly, the PGM (Platinum Group Metal)based mass activity of PdPt NCs/C (149.2 mA mg−1

PGM) is still higherthan that of Pt/C. The catalytic results mentioned above indicatesuperior activity of PdPt NCs in comparison with Pd@Pt NDs andPt/C. In addition, the activity of PdPt NCs also possessed advantagethan that of many catalysts reported recently (Table 2).

The increased ORR activity for PdPt NCs may be attributed to thein situ alloying process, i.e., the diffusion of the Pd elements intothe Pt lattice, resulting in the lattice contraction of Pt. The latticecontraction will weaken the surface oxygen binding energy andreduce the coverage of intermediate oxygen species, giving riseto the enhanced ORR activity relative to the Pt nanoparticles ofPt/C and the Pt shell on the surface of Pd@Pt NDs [42,43]. Anotherreason can be ascribed to the relatively smooth surface feature ofthe NCs compared to that of the Pt/C and Pd@Pt NDs [9,44]. As

shown in the TEM and HRTEM images of PdPt NCs (Fig. 4a), thenanocages were built by the connection of nanoparticles and nano-bars which were larger than that of Pt nanoparticles of Pt/C andshell of Pd@Pt NDs, resulting in lower amount of low-coordinated

72 G. Zhang et al. / Electrochimica Acta 103 (2013) 66– 76

Fig. 8. (a) CV curves of Pd@Pt NDs/C, PdPt NCs/C and Pt/C at 50 mV s−1 in N2-purged 0.5 M H2SO4; (b) ORR polarization curves for these catalysts at 10 mV s−1 in O2-saturated0 activc

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.5 M H2SO4 with a rotation rate of 1600 rpm; (c) area-specific activity and (d) massatalysts at 1600 rpm, respectively.

t atoms on the surface of NCs (i.e., smoother), because the per-entage of low-coordinated Pt atoms decreases with the increase ofarticle size [45,46]. It is well known that these low-coordinated Pttoms have higher oxygen binding energies which is adverse to theRR activity [47]. As a result, the PdPt NCs showed the highest ORRctivity.

The Tafel plots of Pt/C, Pd@Pt NDs/C and PdPt NCs/C in Fig. 8chow two distinct slopes in the low overpotential region (LOR) andigh overpotential region (HOR), respectively (Table 1), and theurning point is approximately 0.88 V. As studied previously, thehange of the slope is attributed to the potential-dependent sur-ace coverage of oxygenated species that inhibit the adsorption of2 [48,49]. For these three catalysts, nearly identical Tafel slopesre found in the LOR. In the HOR, the slopes obtained on the Pd@PtDs/C and PdPt NCs/C are, however, slightly higher compared with

he slope obtained on Pt/C.The kinetics of the ORR for the three catalysts was studied

n RDE at different rotating speeds from 225 rpm to 1600 rpmnd the polarization curves are shown in Fig. 9a–c. All the ORRolarization curves have two well-defined regions, i.e., diffusionontrolled region (0.2–0.65 V) and mixed kinetic-diffusion con-rolled region (0.65–1.0 V). The relationship between the inverseurrent density (j−1) at four different potentials and the inverse ofhe square root of the rotating rate (ω−1/2) is plotted in Fig. 9d–fccording to the Koutecky–Levich equation [50]: j−1 = j−1

k + j−1d =

−1k + (0.62 nFcD2/3�−1/6)

−1ω−1/2, where n is the total number of

lectrons transferred, F is the Faraday constant (96,485 C mol−1), is the diffusion coefficient of oxygen in 0.5 M H2SO4 (cm2 s−1),

is the concentration of oxygen in 0.5 M H2SO4 (mol cm−3), � is

ity given as kinetic current densities normalized against ECSA and Pt mass of these

the kinematic viscosity of the electrolyte (cm2 s−1), and ω is therotation speed (rad s−1). As shown in Fig. 9, the linearity and paral-lelism of the Koutecky–Levich plots suggest first order kinetics withrespect to molecular oxygen [51,52]. The total number of electronstransferred (n) can be calculated according to Koutecky–Levichequation [53–55], which was 3.8, 3.8 and 3.9 for Pd@Pt NDs/C,PdPt NCs/C and Pt/C, respectively, with the value of D, c, and � tobe 1.9 × 10−5 cm2 s−1, 1.1 × 10−6 mol cm−3 and 1.0 × 10−2 cm2 s−1,respectively [54]. The result indicates that the ORR on the threecatalysts follows a preferential four-electron pathway, i.e., mostoxygen molecules are directly reduced to water and peroxide for-mation is little.

The long-term stability of PdPt NCs/C and Pt/C was evaluatedby applying linear potential cycling between 0.6 and 1.0 V in N2-purged 0.5 M H2SO4. As mentioned above, we used large potentialrange (between 0.6 and 1.2 V) to accelerate the dissolution of Pdand produce PdPt NCs in a short time. In contrast, small potentialrange (between 0.6 and 1.0 V) was adopted to study the durabilityof catalysts, because this potential window is close to the actualworking potential range of the fuel cells for the vehicle applicationand also suggested by U.S. Department of Energy to evaluate thedurability of cathodic electrocatalysts for PEMFCs [56]. The CV andORR polarization curves before and after degradation are shownin Fig. 10. After 10,000 cycles, the loss of ECSA was 33% for Pt/C(Fig. 10a), and the average particle size of Pt/C was increased from

2.5 nm (Fig. 2e and f) to 3.5 nm (Fig. 11a and b). In contrast, theECSA reduction for PdPt NCs/C was only 26% (Fig. 10c) and thePd/Pt molar ratio measured by ICP was 49:51, very close to theinitial value. The decrease of ECSA for PdPt NCs may come from the

G. Zhang et al. / Electrochimica Acta 103 (2013) 66– 76 73

Fig. 9. (a–c) ORR polarization curves at different rotating speed for (a) Pt/C, (b) Pd@Pt NDs/C and (c) PdPt NCs/C at 10 mV s−1 in O -saturated 0.5 M H SO ; (d–f) Koutecky–Levichp

cbnagt0s

lots for (d) Pt/C, (e) Pd@Pt NDs/C and (f) PdPt NCs/C.

oalescence of small PdPt nanoparticles composing the nanocage,ecause these nanoparticles contact with each other. However, theanocage structure did not collapse and had nearly the same aver-ge particle size with the initial value (Fig. 11c and d), indicating

ood structure stability. The half-wave potential of ORR polariza-ion curves for the Pt/C was negatively shifted by 0.013 V from.816 V to 0.803 V after degradation process (Fig. 10b), while thehift was 0.016 V for PdPt NCs/C from 0.805 V to 0.789 V (Fig. 10d).

2 2 4

The slightly smaller shift of Pt/C was probably ascribed to theso-called particle size effect [45,46,57]. There is a volcano rela-tionship between the mass activity of Pt nanoparticles for ORRand the average particle size. The highest mass activity was pre-

sented on the Pt nanoparticles at around 3.0 nm [57]. As a result,the degradation of mass activity of Pt/C was not severe owing to themoderate increase of the average particle size of Pt/C after potentialcycling.

74 G. Zhang et al. / Electrochimica Acta 103 (2013) 66– 76

Fig. 10. CV and ORR polarization curves of (a, b) Pt/C and (c, d) PdPt NCs after 10,000 potential cycles between 0.6 and 1.0 V at 50 mV s−1 in N2-purged 0.5 M H2SO4.

Fig. 11. TEM images and corresponding particle size distributions for (a, b) Pt/C and (c, d) PdPt NCs after 10,000 potential cycles between 0.6 and 1.0 V at 50 mV s−1 inN2-purged 0.5 M H2SO4.

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G. Zhang et al. / Electroc

. Conclusions

PdPt nanocages were transformed from core–shell Pd@Pt nan-dendrites by potential cycling due to the dissolution and diffusionf Pd element. This structure change was confirmed by HRTEM,TEM-EDX, XRD and CV measurements. The ORR activity of PdPtCs was enhanced in comparison with Pd@Pt NDs and commercialt/C catalysts; the nanocage structure was maintained during theegradation testing. This study is of significance for the electro-hemical synthesis of bimetallic hollow nanocages, as well as forhe preparation of electrocatalysts with high activity for oxygeneduction reaction.

cknowledgements

This work was financially supported by the National Highechnology Research and Development Program of China (Nos.011AA050701, 2011AA11A273) and the National Natural Scienceoundations of China (Nos. 20936008, 21076208).

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