Enhanced Oxygen Reduction Reaction Activity and Characterization

Article pubs.acs.org/JPCC

Enhanced Oxygen Reduction Reaction Activity and Characterization of Pt−Pd/C Bimetallic Fuel Cell Catalysts with Pt-Enriched Surfaces in Acid Media Licheng Liu,† Gabor Samjeske,† Shin-ichi Nagamatsu,† Oki Sekizawa,† Kensaku Nagasawa,† Shinobu Takao,† Yoshiaki Imaizumi,† Takashi Yamamoto,§ Tomoya Uruga,†,∥ and Yasuhiro Iwasawa*,†,‡ †

Innovation Research Center for Fuel Cells, The University of Electro-Communications, Choufugaoka, Chofu, Tokyo 182-8585, Japan ‡ Department of Engineering Science, Graduate School of Information Engineering Science, The University of Electro-Communications, Choufugaoka, Chofu, Tokyo 182-8585, Japan § Department of Mathmatical and Material Sciences, Faculty of Integrated Arts and Sciences, The University of Tokushima, Minamijosanjima-cho, Tokushima 770-8502, Japan ∥ Japan Synchrotron Radiation Research Institute, Spring-8, Koto, Sayo, Hyogo 679-5198, Japan S Supporting Information *

ABSTRACT: Three types of bimetallic Pt−Pd nanoparticles with different core−shell structures besides Pt and Pd nanoparticles were synthesized by coreduction and sequential reduction methods in ethylene glycol. The synthesized nanoparticles were supported on carbon to prepare five different electrocatalysts Pt/C, Pd/C, PdPt alloy/C, Pd(core)−Pt(shell)/C, and Pt(core)−Pd(shell)/C for oxygen reduction reaction (ORR) in fuel cells. The nanoparticles and supported catalysts were characterized by means of transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), extended X-ray absorption fine structure (EXAFS), and cyclic voltammetry (CV). It was proposed by these characterizations that the PdPt alloy/C, Pd(core)− Pt(shell)/C, and Pt(core)−Pd(shell)/C catalysts constituted Pd4Pt1(core)− Pt(two-layers shell), Pd (core)−Pd2Pt1(three-layers)−Pt(three-layers shell), and Pt(core)−Pt2Pd1(two-layers)−Pd (microcrystal shell), respectively. The Pt surface-enriched catalysts were more stable than the Pd surface-enriched catalysts in long-term CV scanning in acid electrolyte. The Pt/C, PdPt alloy/C, and Pd(core)−Pt(shell)/C catalysts with Pt-enriched surfaces showed much higher ORR specific activity than the Pd/C and Pt(core)−Pd(shell)/C catalysts with Pd-enriched surfaces. The Pt surface-enriched bimetal catalysts with core−shell structures showed the higher Pt-based mass activity than the Pt monometal catalyst. The PdPt catalysts with Pd/ Pt = 2 and 4 in an atomic ratio were also prepared by the coreduction method. The Pt-enriched surfaces formed also with these samples, but the ORR specific activity and (Pd + Pt)-based mass activity decreased with increasing Pd/Pt ratios (1, 2, and 4). The present study provided core−shell catalysts with better ORR activity, which may be useful for understanding key issues to develop next-generation fuel-cell cathode catalysts.

1. INTRODUCTION

The potential-loss in the initial part of fuel-cell polarization curves is attributed to hindered O2 reduction caused by adsorbed oxygenates like OH species on Pt in the potential region of about 0.75−1.0 V.15−18 A promising approach to improve the ORR activity and durability of the Pt/C cathode catalysts is to alloy Pt with nonnoble transition metals such as Co, Ni, Cu, Fe, Cr, Ti, etc., where their Pt alloys have been reported to be 2−10 times more active than polycrystalline Pt for the ORR.19−40 The

While carbon-supported Pt catalysts are still among the most active and durable cathode catalysts for polymer electrolyte fuel cells (PEFCs), the high cost and limited supply of platinum restrict wide applications of the fuel cell technology particularly to automobiles.1−9 To reduce the loading of Pt used as cathode catalysts, the insufficient oxygen reduction reaction (ORR) activity (slow kinetics) and durability of Pt/C cathode catalysts under PEFC operating conditions must be improved.10−14 The detailed mechanisms for the ORR with H2 (O2 + 4H+ +4e− → 2H2O) and deactivation of the cathode catalysts in an atomic scale still remain unclear, which must be understood more thoroughly for development of next generation PEFC catalysts. © 2012 American Chemical Society

Received: August 12, 2012 Revised: October 6, 2012 Published: October 18, 2012 23453

dx.doi.org/10.1021/jp308021a | J. Phys. Chem. C 2012, 116, 23453−23464

The Journal of Physical Chemistry C

Article

(EG) was used as reductant, and polyvinylpyrrolidone (PVP) (MW = 55 000 g/mol) was used as protecting reagent. 2.1.1. Preparation of Pd and Pt Nanoparticles. Either 55.5 mg of NaPdCl4 and 62.8 mg of PVP or 52.2 mg of H2PtCl6·6H2O and 34 mg of PVP were dissolved into 15 mL of EG under stirring. The resultant pale-yellow solution was heated to 413 K in an oil bath under stirring, at which temperature it was kept for 2.5 h. After cooling down, the produced nanoparticles were separated by centrifugation and washed by ethanol and n-hexane. The colloidal nanoparticles were suspended and stored in 10 mL of ethanol. 2.1.2. Preparation of Pd(core)−Pt(shell) and Pt(core)− Pd(shell) Nanoparticles by Sequential Reduction. Either 19.4 mg of NaPdCl4 or 33.9 mg of H2PtCl6·6H2O was first used to synthesize Pd or Pt nanoparticles as seeded cores for the next step by the same procedure as above. The isolated Pd or Pt nanoparticles were suspended in 15 mL of EG. Then, 33.9 mg of H2PtCl6·6H2O or 19.4 mg of NaPdCl4 and PVP were dissolved in the core-seeds suspension. The reaction was carried out at the same temperature and duration time as those for the core-metal nanoparticle synthesis. The produced nanoparticles were centrifuged and washed as described above and stored in ethanol. The Pd/Pt ratios for these two samples were 1:1 in an atomic ratio. 2.1.3. Preparation of Bimetallic PdPt Alloy Nanoparticles with Pt-Enriched Surface by Coreduction. NaPdCl4 (19.4 mg) and 33.9 mg of H2PtCl6·6H2O (Pd/Pt = 1); 28.8 mg of NaPdCl4 and 25.1 mg of H2PtCl6·6H2O (Pd/Pt = 2); or 37.9 mg of NaPdCl4 and 16.5 mg of H2PtCl6·6H2O (Pd/Pt = 4), together with PVP, were dissolved into 15 mL of EG under stirring. Subsequent procedures were identical to the description above. The samples with different Pd/Pt molar ratios of 1, 2, and 4 are denoted as PdPt, 2PdPt, and 4PdPt, respectively. 2.1.4. Preparation of Carbon-Supported Nanoparticle Catalysts. Thirty milligrams of carbon (Vulcan XC-72) was added to the ethanol suspension in which the as-synthesized nanoparticles were dispersed. The resultant suspension was placed into an ultrasonic bath for 1 h and then stirred overnight. The obtained carbon-supported catalysts were collected by centrifugation and dried in air. For removing PVP residuals on the catalyst, the catalysts were calcined at 473 K for 2 h in 20% O2/N2 gas flow. 2.2. Characterization. TEM images were taken with a JEOL JEM-2010 at an accelerating voltage of 200 kV. After diluting the ethanol suspension containing the metal nanoparticles with additional ethanol and ultrasonic treatment, the solution was applied onto copper grids covered with carbon film and dried in vacuum before inserting into the main chamber of JEM-2010 TEM. FT-IR spectra for CO adsorbed on nanoparticle surfaces were measured using a JASCO FT-IR 4200 spectrometer. To facilitate CO-adsorption, the ethanol dispersion containing assynthesized nanoparticles was purged with CO gas for 15 min. Then, 3 μL of the suspension was transferred on a KBr plate with a micropipet. After drying, the KBr plate was mounted into the sample stage for measuring FT-IR spectra in a transmitting mode. XRD patterns were recorded on a Rigaku Ultima III diffractometer with Cu Kα radiation (λ = 0.154178) at 40 kV and 40 mA. The scanning range (2θ) was from 20° to 90° at a scanning rate of 5 °/min. The XRD data were analyzed with MDI Jade 6.0 software.

higher ORR activities of Pt alloys have been demonstrated to depend on nanoparticle size and shape (plane), surface roughness, compressive strain effect (bond distance), downward Pt d-band center, ligand (electronic) effect, etc.41−79 However, most of Pt−M alloy catalysts are not stable due to leaching and segregation of M over time.24,28,49 Another approach is to synthesize Pt mono- or bimetallic nanocomposites with various shapes and morphologies exposing active crystal planes toward the ORR.80,81 For example, Pt nanocubes,82 nanowires,83 and Pd−Pt nanodentrites84 have been reported to exhibit enhanced electrocatalytic activities in fuel cells. However, these nanomaterials have practical problems in large-scale production and long-term stability. The other promising approach to develop high-performance cathode catalysts is fabrication of Pt surface-enriched nanoparticles with so-called skin, skeleton, or core−shell structures, involving Pt monolayer deposition on second metal nanoparticles, where Pt can be sufficiently used and hence catalyst cost can be reduced.41−48,85−88 The Pt mass-specific ORR activity of Pd(core)−Pt(shell)/C has been reported to be 5−8 times higher than that of Pt/C, while the noble-metal (Pt + Pd) mass activity was two times higher.45 Another advantage of the core−shell catalysts is a wide range of possibility of tailoring the activity by substituting either the core metals or a part of the Pt monolayer-shell with one of the metals above-mentioned.85,86 A comparative study among the core metals Ru, Ir, Rh, Au, and Pd for Pt shell revealed that the Pd(core)−Pt(shell) catalyst showed the highest ORR activity.48 The various methods for synthesizing Pt surface-enriched core−shell or core−shell-like nanoparticles have been outlined as follows.43 After an initial colloidal chemical reduction to fabricate the core, sequential reduction allows to grow Pt shells on the premade core-metal nanoparticle seeds.89,90 Surface fabrication is also possible by electrochemical methods, including underpotential deposition (UPD) replacement reactions.45−48,85 Alternatively, electrochemical dealloying (selective leaching),91 chemical-reaction-driven reconstruction,92 and post-treatment ways43 can also be used to form Pt-shell structures enclosing the metal-core particles. Although surface design of core−shell catalysts and their characterization at the atomic level have extensively been studied for both model systems such as single crystals, polycrystals, or carbonsupported nanoparticles and practical PEFCs, these issues have not been thoroughly established yet. Herein, we have investigated the fabrication of PtPd nanoparticles by a colloidal polyol reduction method. The fabricated nanoparticles have been characterized by X-ray absorption fine structure (XAFS), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), cyclic voltammetry (CV), linear sweep voltammetry, etc. The results showed that Pt surface-enriched PtPd nanoparticles can be prepared by both sequential reduction and coreduction of Pt/Pd precursors using ethylene glycol as reductant. The Pt surface-enriched nanoparticle catalysts exhibited significant enhancement of the Ptbased mass activity for the ORR.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. Carbon-supported metal catalysts were prepared by supporting premade metal nanoparticles on carbon support. All metal nanoparticles were synthesized by a polyol process in a batch. Ethylene glycol 23454



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from being oxidized during the nanoparticle formation.89,94 However, in this study, we carried out the preparation of nanoparticles in solution exposed to air. 3.1.1. TEM, FT-IR, and XRD. Figure 1 shows TEM images of the as-synthesized PdPt alloy, Pd(core)−Pt(shell), Pt(core)−

XAFS spectra at Pt LIII-edge and Pd K-edge for carbonsupported monometallic and bimetallic catalysts were measured at BL01B1 station in Spring-8 using a Si(111) monochromator in a transmission mode at room temperature. The background in XAFS spectra were removed using REX2000 software (version 2.5, RIGAKU). The Victoreen function was employed for the background subtraction, and the spline smoothing method with Cook and Sayers criteria was used as the μ0 method. The extracted EXAFS oscillation was k3-weighted and Fourier transformed to r-space over k = 30−160 nm−1 or 30− 150 nm−1, respectively, for Pd K-edge data and Pt LIII-edge data. The fittings of k3-weighted EXAFS data in r-space were performed with Artemis. The phase shift and amplitude functions for Pt−O, Pt−Pt, Pt−Pd, Pd−O, Pd−Pt, and Pd− Pd were obtained from FEFF (version 8.4). The EXAFS curvefittings at both Pt LIII-edge and Pd K-edge were conducted simultaneously, where Pt−Pd bond distance at Pt LIII-edge and Pd−Pt bond distance at Pd K-edge were fixed to be the same value. 2.3. Electrochemical Measurements. Electrochemical measurements of the synthesized catalysts were performed using a CHI 760D potentiostat/galvanostat in a standard threeelectrode configuration with a reversible hydrogen electrode (RHE) as reference and a platinum foil as counter electrode at room temperature. A glassy carbon (GC) disk electrode (PINE instruments) with a diameter of 5 mm and a geometric area of 0.1963 cm2 was polished with 0.05 μm alumina before each experiment and used as a working electrode and a substrate for electrocatalyst film. The thin film catalyst layer on the GC electrode was prepared as follows. A mixture containing electrocatalyst, Millipore water, and Nafion solution was ultrasonicated for 20 min to obtain a well-dispersed ink. The catalyst ink was then quantitatively transferred onto the GC electrode surface by using a micropipet and dried in vacuum to obtain a catalyst thin film. The amount of catalyst ink transferred was individually chosen so that the metal loading was regulated to be 25.5 μg/cm2 for all catalysts. All electrochemical experiments were conducted using a 0.1 M HClO4 (Merck) solution and Milipore-Q water. The CVs were recorded between potential limits of 0.05 and 1.2 V (vs RHE) at a sweep rate of 50 mV s−1 and under continuous high-purity nitrogen purging. Accelerated catalyst aging experiments were performed by increasing the cycles up to 1000 under the same CV conditions. The ORR experiments were carried out using LSV in a potential range between 0.2 and 1.0 V and at a scan rate of 10 mV s−1 in the electrolyte saturated with high-purity oxygen and at rotation speeds of 400, 900, 1600, and 2500 rpm using a MSR rotator from PINE instruments. Anodic CO electro-oxidation was carried out under potentiodynamic conditions at a scan rate of 50 mV s−1 in the same electrolyte, after CO-adsorption at 0.1 V for 30 min in CO-saturated electrolyte and subsequent removal of CO dissolved in the 0.1 M HClO4 by N2-purging.

Figure 1. TEM images of the as-synthesized PdPt alloy (A), Pd(core)−Pt(shell) (B), Pt(core)−Pd(shell) (C), Pd (D), and Pt (E) nanoparticles.

Pd(shell), and Pd and Pt nanoparticles (unsupported samples). As shown in Figure 1, spherical nanoparticles with relatively narrow size distribution were synthesized for all samples. The averaged particle sizes for the as-synthesized PdPt alloy, Pd(core)−Pt(shell), Pt(core)−Pd(shell), and Pd and Pt samples were estimated to be 11.2, 8.1, 7.6, 6.5, and 7.2 nm, respectively. Bimetallic nanoparticles, irrespective alloys or core−shell assemblies, had larger sizes than monometallic Pd and Pt nanoparticles. Among the as-synthesized samples, the PdPt alloy nanoparticles obtained by the coreduction method possessed the biggest particle size (av.). Since CO adsorption on metals in the gas phase is surface-sensitive, FT-IR spectra of CO adsorbed on the as-synthesized nanoparticles were measured in Figure 2. Distinctive absorption peaks at 2051, 2047, and 2051 cm−1 were observed with PdPt alloy, Pd(core)−Pt(shell), and Pt samples. The absorption peaks around 2050 cm−1 are ascribed to CO molecules adsorbed at the Pt surface in an atop configuration. The outermost Pt layers in the PdPt, Pd(core)−Pt(shell), and Pt samples may not significantly be influenced electronically from the underlying layers because of their similar CO stretching vibration frequencies (negligible ligand effect). However, absorption peaks around 1900 cm−1 appeared for the Pt(core)−Pd(shell) and Pd samples, which are assigned to CO adsorbed at Pd surface in a bridge configuration. Although the linear CO peak

3. RESULTS AND DISCUSSION 3.1. Characterizations of As-Synthesized and Supported Pd, Pt, and Pt−Pd Nanoparticles. Synthesis of the monometallic and bimetallic nanoparticles using EG and PVP as a reducing reagent and a protective stabilizer, respectively, is well-established as outlined in various publications.89,90,93,94 Usually, nitrogen or argon gas purging is necessary for the synthesis reactions at elevated temperatures since it might be useful for reducing the particle size and preventing the surface 23455



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Figure 2. FT-IR spectra for CO adsorbed on different nanoparticles: (1) PdPt alloy (2051 cm−1), (2) Pd(core)−Pt(shell) (2047 cm−1), (3) Pt(core)−Pd(shell) (1904 cm−1), (4) Pd (1895 cm−1), and (5) Pt (2051 cm−1).

Figure 3. XRD patterns of different catalysts and enlargement of (111) diffraction peaks (inset, top right): (1) PdPt alloy/C, (2) Pd(core)− Pt(shell)/C, (3) Pt(core)−Pd(shell)/C, (4) Pd/C, and (5) Pt/C.

Table 1. XRD and CV Data for the Five Catalysts

at Pd surface should also be located around 2050 cm−1, bridgetype CO is preferable at Pd surfaces, and linear-type CO is usually minor in contrast to the preferable atop configuration at Pt surfaces (Figure 2). This is in good agreement with previous reports.89,95 As for the PdPt alloy, Pd(core)−Pt(shell), and Pt samples, besides the peak around 2050 cm−1, weak and broad peaks around 1830 cm−1 were observed (Figure 2). They may be due to CO adsorbed at Pt surfaces in a bridge configuration,96 where any peaks at around 1900 cm−1 assigned to bridge-CO at Pd surfaces were not observed with PdPt alloy and Pd(core)−Pt(shell) samples. These results provide definite information on the surfaces of PdPt alloy, Pd(core)−Pt(shell), and Pt(core)−Pd(shell) samples. Regardless of the bulk structures, the surfaces of the PdPt nanoparticles prepared by the coreduction method and Pd(core)−Pt(shell) nanoparticles prepared by the sequential reduction method are suggested to have the outermost Pt layers. Reversely, the surface of the Pt(core)−Pd(shell) nanoparticles prepared by the sequential reduction method is suggested to possess Pd layers. It is to be noted that the PdPt alloy nanoparticles synthesized by the coreduction method showed a feature of a Pt shell structure, instead of a uniform PdPt alloy or a Pd enriched shell. This issue will be further discussed hereinafter. The as-synthesized nanoparticles were loaded onto carbon support to prepare electrocatalysts. The obtained five carbonsupported catalysts were characterized by XRD as shown in Figure 3. A typical fcc crystal phase can be found for these Pd/ Pt based catalysts.87,97,98 The Pd/C and Pt/C catalysts showed the same diffraction patterns as the standard material cards [PDF-46-1043 and PDF-04-0802]. For the bimetallic PdPt alloy/C, Pd(core)−Pt(shell)/C, and Pt(core)−Pd(shell)/C catalysts, XRD fcc peaks were observed at the 2θ angles between those for Pd and Pt metals. The XRD patterns of the PdPt alloy/C and Pd(core)−Pt(shell)/C in the inset of Figure 3 were similar to each other, indicating similar bulk compositions. Independent diffraction peaks assigned to Pd or Pt layers suggested by FT-IR were hard to observe probably due to very thin layers. The XRD pattern (inset of Figure 3) for the Pt(core)−Pd(shell)/C showed a definite peak of Pt(111). The particle sizes of catalysts were calculated by the Scherrer equation,87 and the estimated values are listed in Table 1. The estimated particle sizes ranged between 6.5 and 10.1 nm. The PdPt alloy/C catalyst prepared by the coreduction method had the biggest particle size (10.1 nm) among the examined

catalyst 2θ(111) (deg) d (nm)a Eoxy (V)b ETS (V)c

PdPt alloy/C

Pd(core)− Pt(shell)/C

Pt(core)− Pd(shell)/C

Pd/C

Pt/C

39.98

39.94

39.96

40.12

39.76

10.1 0.801 0.905

8.0 0.795 0.913

8.6 0.734 0.879

8.5 0.737 0.885

6.5 0.791 0.917

a

Particle size estimated from the Scherrer equation. bReduction peak potential of oxygenates in CV. cTransition potential of the Tafel slopes from −120 to −60 mV/dec.

catalysts. The PdPt alloy/C (10.1 nm), Pd(core)−Pt(shell)/C (8.0 nm), Pt(core)−Pd(shell)/C (8.6 nm), and Pt/C (6.5 nm) catalysts showed similar nanoparticle sizes to those of the assynthesized unsupported nanoparticles (11.2, 8.1, 7.6, and 7.2 nm, respectively), but the Pd nanoparticles on the Pd/C catalysts (8.5 nm) became bigger than the as-synthesized Pd nanoparticles (6.5 nm) upon supporting. The particle size (8.5 nm) determined by XRD (Table 1) was similar to the averaged particle size (8.7 nm) estimated by TEM (Supporting Information, Figure S1). It has been reported that Pd is easy to grow into larger nanoparticles no matter oxidized or reduced conditions compared to Pt.99−101 The treatments at 473 K to remove the residual PVP under existing O2 may bring about the particle growth in the Pd/C catalyst and a little also in the Pt(core)−Pd(shell)/C with Pd-enriched surface layers. It is to be noted that the Pt-enriched surfaces are more stable and resistive to heating in air, which may be favorable for improving the catalyst stability in the ORR.102 3.1.2. XAFS. The carbon-supported bimetal catalysts PdPt alloy/C, Pd(core)-Pt(shell)/C and Pt(core)-Pd(shell)/C were further characterized by XAFS to obtain structural parameters of the catalysts at both Pt and Pd sites. XAFS is a powerful tool for investigating the local coordination structures and oxidation states of supported metal nanoparticle catalysts in any forms.103−118 Figure 4 shows the Fourier transforms of the EXAFS data at Pt LIII-edge and their curve-fittings for the bimetallic electrocatalysts with Pd/Pt = 1 in an atomic ratio; PdPt alloy/C, Pd(core)−Pt(shell)/C, and Pt(core)−Pd(shell)/C. The Pt LIIIedge EXAFS curve-fitting results for the four electrocatalysts Pt/C, PdPt alloy/C, Pd(core)−Pt(shell)/C, and Pt(core)− Pd(shell)/C are listed in Table 2. The interatomic distance (R) 23456



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respectively (totaling 12 for fcc crystal structure) though the total CN (CNPt−Pt + CNPt−Pd) determined by EXAFS is a little smaller than 12 for the crystal bulk due to the coordinatively unsaturated outermost layer. However, their CNs were determined to be 7.1 and 3.9, respectively, by the EXAFS curve-fitting analysis (Figure 4 and Table 2), which demonstrates segregation of Pt at the catalyst surface. When Pd core is covered by Pt monatomic layer or double layers, the CNs of Pt−Pt and Pt−Pd would be approximately 6 and 3 or 8 and 1, respectively. The real aspect of the PdPt alloy/C may not be so simple because the cores of PdPt nanoparticles make PdPtx alloys as suggested by XRD of Figure 3. As for the local coordination structure around Pd sites, the catalysts were analyzed by Pd K-edge EXAFS. The Pd K-edge EXAFS Fourier transforms and their curve-fittings for the Pd/C, PdPt alloy/C, Pd(core)−Pt(shell)/C, and Pt(core)−Pd(shell)/C are shown in Figure 5, and the curve-fitting results are listed in Table 3. The Pd K-edge EXAFS analysis for the PdPt alloy/C catalyst revealed CNPd−Pt and CNPd−Pd to be 1.9 and 8.7, respectively. This result also indicates the formation of Pt-enriched surface and Pd-enriched core. In the case of the structure of Pd core covered with Pt surface layers, the expected CNPd−Pd for the 10.1 nm nanoparticles is approximately 11. The observed value of 8.7 is much smaller than 11, but the total value is 10.6 (1.9 + 8.7), which demonstrates that the core is not pure Pd but PdPtx alloy. We assume the composition Pd4Pt1 from the relative value of CNPd−Pt and CNPd−Pd for the PdPt alloy/C. As consequence, it is suggested that the PdPt alloy is composed of Pd-enriched core (Pd4Pt1) and Pt shell (probably double layers, judging from the particle size, the Pd4Pt composition, and the Pd/Pt = 1 ratio) as proposed in Figure 6. This core−shell structure agrees with the conclusion obtained from CO adsorption experiments in Figure 2. The structural parameters for the Pd(core)−Pt(shell)/C (Pd/Pt = 1) catalyst determined by Pt LIII-edge and Pd K-edge EXAFS are listed in Tables 2 and 3, respectively. The CNPt−Pt and CNPt−Pd around Pt sites were determined as 7.8 at 0.276 nm and 3.1 at 0.275 nm, respectively. If it is conceived that the top four layers at the surface are Pt layers and that the core is Pd metal as the Pd/Pt ratio is unity, the expected CNPt−Pt and CNPt−Pd would be 10 and 0.6, respectively, but they are far from the observed values. However, CNPd−Pt and CNPd−Pd around Pd sites in the Pd(core)−Pt(shell)/C catalyst were also determined to be 2.2 and 8.4, respectively by Pd K-edge EXAFS analysis (Figure 5 and Table 3). We have calculated the expected CNPt−Pt and CNPt−Pd at Pt sites and the expected CNPd−Pt and CNPd−Pd at Pd sites, replacing a part of the Pt layers by PtPdx bimetal layers with different compositions (x).

Figure 4. Pt LIII-edge EXAFS Fourier transforms and their curvefitting results for the PdPt alloy/C (a), Pd(core)−Pt(shell)/C (b), and Pt(core)−Pd(shell)/C (c) catalysts. Dotted lines, obs; solid lines, calcd.

and coordination number (CN) for Pt−Pt bonds in the Pt/C were determined to be 0.276 nm and 10.4, respectively, which are close to the values expected from fcc Pt metal nanoparticles with 6.5 nm in diameter. The Pt LIII-edge EXAFS analysis for the PdPt alloy nanoparticles (10.1 nm size) in the PdPt alloy/C catalyst prepared by the coreduction method revealed the similar bond length of 0.275 nm (±0.002 nm) for Pt−Pt and Pt−Pd, indicating Pd−Pt alloying as suggested by XRD in Figure 3. If the PdPt alloy nanoparticles constitute a single alloy phase, the CNs of Pt−Pt and Pt−Pd should be around 6.0,

Table 2. Structural Parameters Determined by the Curve-Fitting Analysis of Pt LIII-Edge EXAFS for Pt/C, Pd/C, PdPt alloy/C (Pd/Pt = 1), Pd(core)−Pt(shell)/C (Pd/Pt = 1), and Pt(core)−Pd(shell)/C (Pd/Pt = 1)a sample (residual factor) Pt foil (Rf = 0.74%) Pt/C (Rf = 0.91%) PdPt/C (Rf = 0.62%) Pd(core)−Pt(shell)/C (Rf = 0.39%) Pt(core)−Pd(shell)/Cb (Rf = 0.74%)

a

shell Pt−Pt Pt−Pt Pt−Pt Pt−Pd Pt−Pt Pt−Pd Pt−Pt Pt−Pd

R (nm)

CN 12.4 10.4 7.1 3.9 7.8 3.1 10.5 1.9

± ± ± ± ± ± ± ±

1.2 0.9 0.7 0.8 0.7 0.9 0.7 0.8

0.277 0.276 0.276 0.275 0.276 0.275 0.276 0.276

± ± ± ± ± ± ± ±

0.0002 0.0003 0.005 0.008 0.004 0.011 0.003 0.011

ΔE0 (eV) 4.3 5.0 −0.3 −0.1 −1.3 −0.3 −1.0

± ± ± ± ± ± ±

0.8 1.0 1.1 1.5 1.1 1.9 0.6

σ2 (10−5 nm2) 5.4 6.4 6.0 6.3 6.5 8.2 6.2 9.5

± ± ± ± ± ± ± ±

0.3 0.4 0.4 1.1 0.3 1.6 0.3 3.3

k = 30−150 nm−1; R = 0.16−0.32 nm. bΔE0 of both scattering paths was fixed at the same value. 23457



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Figure 5. Pd K-edge EXAFS Fourier transforms and their curve-fitting results for Pd foil (a), Pd/C (b), PdPt alloy/C (c), Pd(core)−Pt(shell)/C (d), and Pt(core)−Pd(shell)/C (e). Dotted lines, obs; solid lines, calcd.

Table 3. Structural Parameters Determined by the Curve-Fitting Analysis of Pd K-Edge EXAFS for Pt/C, Pd/C, PdPt alloy/C (Pd/Pt = 1), Pd(core)−Pt(shell)/C (Pd/Pt = 1), and Pt(core)−Pd(shell)/C (Pd/Pt = 1)a sample (residual factor) Pd foil (Rf = 0.32%) Pd/Cb,d (Rf = 0.85%) PdPt/Cc (Rf = 2.30%) Pd(core)−Pt(shell)/Cc,d (Rf = 1.40%) Pt(core)−Pd(shell)/Cc,d,e (Rf = 3.08%)

shell Pd−Pd Pd−Pd Pd−Pt Pd−Pd Pd−Pt Pd−Pd Pd−Pt Pd−Pd

R (nm)

CN 12.1 11.0 1.9 8.7 2.2 8.4 0.4 6.2

± 0.8 ± ± ± ± ± ±

2.5 1.7 1.5 0.6 0.6 0.9

0.275 0.275 0.275 0.276 0.275 0.275 0.276 0.276

± ± ± ± ± ± ± ±

0.0002 0.0003 0.008 0.016 0.011 0.006 0.011 0.017

ΔE0 (eV) −3.3 −1.7 −7.9 0.1 1.6

± ± ± ± ±

σ2 (10−5 nm2)

0.5 0.9 8.2 3.5 1.3

6.6 7.8 3.8 7.8 8.5

−0.2 ± 3.0

3.8 7.8

± 0.3 ± 0.1 ± 3.1 ± 3.1

a k = 30−160 nm−1; R = 0.16−0.32 nm. bCN was fixed at 11.0, the value estimated from the particle size. cσ2 of Pd−Pd scattering path was fixed at the value for Pd/C. dΔE0 of both scattering paths was fixed at the same value. eσ2 of Pd−Pt scattering path was fixed at the value for PdPt/C.

EXAFS are also listed in Tables 2 and 3, respectively. The CNPt−Pt and CNPt−Pd around Pt sites were determined as 10.6 at 0.276 nm and 1.9 at 0.276 nm, respectively. We also simulated the CNPt−Pt and CNPt−Pd at Pt sites and the CNPd−Pt and CNPd−Pd at Pd sites on the basis of a layer-by-layer deposition mode of Pd on the Pt core during the Pd/Pt nanoparticle synthesis, changing the number, and composition of both shell and core, to find a structure that satisfies all the CNs. If the top four layers at the surface are Pd layers and the core is Pt metal, corresponding to Pd/Pt = 1 approximately, the

Thus, among the plausible models, it was found that a Pd core−Pt shell (three layers) structure with three interfacial layers of Pt1Pd2 composition between the Pd core and the Pt shell can reproduce the observed CNs at Pt and Pd sites simultaneously as a whole; expected CNPt−Pt and CNPt−Pd are 7.8 and 1.8, respectively, and expected CNPd−Pt and CNPd−Pd are 1.6 and 9.0, respectively. The proposed structure is shown in Figure 6. The structural parameters for the Pt(core)−Pd(shell)/C (Pd/Pt = 1) catalyst determined by Pt LIII-edge and Pd K-edge 23458



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the structure−performance relationship and further development of next-generation FC catalysts. 3.2. Comparison of the Catalytic Properties of Pd, Pt, and Pt−Pd Nanoparticles. 3.2.1. Electrochemical Experiments. The cyclic voltammograms (CVs) for Pd/C, Pt/C, and the three bimetallic Pt−Pd/C catalysts were recorded at a scan speed of 50 mV/s in deaerated 0.1 M HClO4. The CVs show three clearly distinguishable potential regions for H 2 adsorption/desorption (0.05−0.34 V), double-layer charging (0.34−0.7 V), and OH(ad)/O(ad) formation/reduction (0.7− 1.2 V), respectively, as shown in Figure 7. The CV curve shapes

Figure 7. Cyclic voltammograms of the catalysts in N2-saturated 0.1 M HClO4 solution at a scan rate of 50 mV s−1: (1) PdPt alloy/C (black), (2) Pd(core)−Pt(shell)/C (red), (3) Pt(core)−Pd(shell)/C (blue), (4) Pd/C (olive), and (5) Pt/C (cyan). Figure 6. Proposed structures of the bimetallic nanoparticles in the (a) PdPt alloy/C, (b) Pd(core)−Pt(shell)/C, and (c) Pt(core)−Pd(shell)/C catalysts.

in the H2 adsorption/desorption region are characteristic for each of the catalysts (Figure 7). The Pd/C and Pt(core)− Pd(shell)/C catalysts showed much larger currents and peak areas than the other catalysts for both, H2 adsorption/ desorption and OH(ad)/O(ad) formation/reduction. The Pd/C and Pt(core)−Pd(shell)/C catalysts exhibited very sharp current peaks at 0.086 and 0.07 V, respectively, which originate from H2 gas generation and oxidation, indicating that the process take place more easily on the Pd surfaces than on the Pt surfaces in PdPt alloy/C, Pd(core)−Pt(shell)/C, and Pt/ C catalysts. The electric double layers were identical for all the catalysts investigated in this study. Cathodic reduction of the oxygenated species on the surface was different from each other for the catalysts with Pd-rich surfaces and Pt-rich surfaces. The very large O-species reduction peaks in the Pd/C and Pt(core)−Pd(shell)/C catalysts indicate that abundant oxygenates such as Pd(OH)2 and PdO can form in the Pd surface layers. The similarity of the reduction potentials (Table 1) was observed with the oxygenated species for the catalysts having Pd-rich surfaces on one hand and having Pt-rich surfaces on the other hand, which corroborated the findings from FT-IR, XRD, and XAFS experiments. These results also indicate the surface segregation on the bimetallic nanoparticles, which remains stable during the supporting processes on the carbon. CV was repeated up to 1000 cycles in the potential window between 0.05 and 1.2 V to investigate the stability of the nanoparticle surfaces. The electrochemical surface areas (ECSAs) of the catalyst nanoparticles were calculated by integrating the charge of the H2 adsorption peak area from 0.068−0.114 V (depending on different catalyst) to 0.4 V.119,120 Using different start potential for integration is to eliminate possible H2 spillover in Pd-containing materials, which is believed to happen with sharp H2 deposition peak at low

expected CNPt−Pt and CNPt−Pd are estimated to be 11.4 and 0.6, respectively, and the expected CNPd−Pt and CNPd−Pd are 0.6 and 10.5, respectively. The values of CNPt−Pd and CNPd−Pd are far from the observed CNPt−Pd and CNPd−Pd . Then, the fourth layer of the top four layers was replaced by a Pt2Pd1 layer, and the fifth Pt layer was also replaced by a Pt2Pd1 layer to have nearly Pd/Pt = 1 totally. In this structure model, the CNPt−Pt and CNPt−Pd around Pt sites were calculated to be 10.7 and 1.3, which reproduce the observed values 10.6 (±0.7) and 1.9 (±0.8), respectively. However, the CNPd−Pt and CNPd−Pd around Pd sites expected from the model are 1.0 and 9.5, respectively, where the expected CNPd−Pd is much larger than the observed value of 6.2 (±0.7), while the expected CNPd−Pt (1.0) at Pd sites in the interface layer is close to the observed value of 0.4 (±0.6). Only the CNPd−Pd among the four CNs around Pt and Pd sites was different from the observed value. These results allow us to assume that the top three layers are microcrystallized, while keeping the rest of the structure without change. When the top three layers are composed of microcrystals with 0.8 nm dimension (corresponding to three layers), the expected CNPd−Pd is 5.6. Thus, this structural model reproduces not only the observed CNPt−Pt and CNPt−Pd around Pt sites but also the observed CNPd−Pd and CNPd−Pt around Pd sites, respectively. The structure of Pt(core)−Pt2Pd1(two-layers shell)−Pd(three-layers equivalent microcrystals shell) is tentatively illustrated as a most plausible structure in Figure 6. The proposed core−shell structures are based on the careful EXAFS analysis with aid of XRD and TEM, but further study on the core−shell structures is necessary. Nevertheless, these pieces of information may be useful in obtaining insight into 23459



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potential.119−121 The calculated ECSAs were plotted against the cycle number in Figure 8. The ECSA values usually decrease

Figure 9. CO-stripping patterns on the different catalysts: (1) PdPt alloy/C, (2) Pd(core)−Pt(shell)/C, (3) Pt(core)−Pd(shell)/C, (4) Pd/C, and (5) Pt/C. Figure 8. ECSA variation with the CV cycles for the catalysts: (1) PdPt alloy/C, (2) Pd(core)−Pt(shell)/C, (3) Pt(core)−Pd(shell)/C, (4) Pd/C, and (5) Pt/C.

order of CO oxidation peak potentials for the catalysts decreased as follows, Pd/C > Pt(core)−Pd(shell)/C > PdPt alloy/C > Pd(core)−Pt(shell)/C > Pt/C. This means that Pt shows the highest activity for the electrooxidation of adsorbed CO and the three bimetallic catalysts show moderate activities between Pt and Pd. It is noteworthy that some samples showed overlapped multipeaks, especially on Pt(core)−Pd(shell), Pd(core)−Pt(shell), and Pt. This may be attributed to the oxidation of CO adsorbed on different surface sites and nonuniform surface sites (crystal plane, edge, and vertex).127,128 For example, the CO electro-oxidation potential on Pd/C was very close to that on Pd(111) single crystal in HClO4 solution, while the CO electro-oxidation on Pt(core)−Pd(shell)/C exhibited two overlapped peaks, which are assigned to the CO electro-oxidation on Pd(110) and Pd(111), respectively.129 The CO electro-oxidation on Pt/C, PdPt alloy/C, and Pd(core)−Pt(shell)/C resembled the peak and potential on Pt(111),130 Pt(110),131 and polycrystalline Pt.132 By integrating the CO stripping peak area, the ECSA of the electrocatalyst can be calculated accurately, particularly for Pd-based catalysts because, in the case of H2 adsorption/desorption on Pd, there is possible spillover of H2 when deducting CV in aqueous electrolyte. The surface specific activity that would be shown in the followings section is based on the surface area obtained by CO stripping. 3.2.2. Electrocatalytic Activity toward ORR. The specific activity (SA) normalized to the surface area and the mass activity (MA) normalized to the metal amount for the ORR are presented in Figures 9 (inset) and 10 (inset), respectively, where the SA and MA are both expressed as normalized kinetic current (ik) at both 0.85 and 0.9 V. The SA and MA were calculated from the LSV experiments as depicted in Figure 10 by using the following Koutecky−Levich equation (neglecting the Nafion film-diffusion-limited current), 1 1 1 = + i id ik

with an increase in the CV cycles due to increasing metal particle size. Among the five catalysts, PdPt alloy/C and Pd(core)−Pt(shell)/C catalysts with Pt-enriched surfaces showed high stability, and they were even better than the pure Pt/C catalyst, where the ECSAs of the Pt surface-enriched PdPt alloy and Pd(core)−Pt(shell) nanoparticles lost less than 20% after 1000 CV cycles. However, the ECSAs of the Pd/C and the Pt(core)−Pd(shell)/C with Pd-enriched surface decreased to 53.8% and 41.6% of their initial values, respectively, after 1000 CV cycles in 0.1 M HClO4. The Pd/ C and Pt(core)−Pd(shell)/C catalysts with Pd surfaces may suffer irreversible growth of the metal nanoparticles and Pd dissolution into the Nafion electrolyte during the potential scans due to the lower oxidation potential (0.915 V) compared to Pt (1.18 V).101 As for the PdPt alloy nanoparticles synthesized by the coreduction method, Pt was segregated to the surface to make two Pt layers on Pd4Pt1 core instead of uniform PdPt alloys. However, Pd-enriched surfaces are preferable in a thermodynamic equilibrium due to the lower surface energy of Pd.121 This has been observed and proved theoretically and experimentally.122,123 Pd nanoparticles synthesized using EG as solvent and reducing reagent are redissolved by oxidative etching, resulting in the formation of uniform Pd nanoparticles.124 A similar result can be seen in Ag nanoparticle synthesis.125 In the present article, Pd atoms at the surface of Pt−Pd alloy nanoparticles may selectively dissolve by the similar oxidative etching to leave Pt surface layers. Recent DFT calculations demonstrated that a kind of patchy multishell segregation in Pt−Pd alloy nanoparticles was favored, which is rationalized in terms of coordination-dependent bond-energy variations in the metal−metal interactions.126 The aforementioned Pd dissolution may occur if this patchy multishell structure forms. Actually, the Pd4Pt1(core)−Pt(shell) formation may not be explained monocausally, and further investigation on the synthesis mechanism is necessary. To further examine the electrochemical properties of the synthesized catalysts, CO electro-oxidation experiments were also carried out in Figure 9, which shows the CO oxidation current peaks at different potentials for the five catalysts, indicating that the different surface structures of the catalysts lead to the different oxidation potentials for adsorbed CO. The

where i and id are experimentally measured current and diffusion limited current, respectively. The SA based on the ECSA from the CO stripping peak varies with the different catalysts (Figure 9 (inset)). The highest SA was observed with the Pt/C catalyst. The PdPt alloy/C and Pd(core)−Pt(shell)/ C, which both possess Pt surface layers, had the similar SA to each other but a little smaller than the Pt/C. The SAs of the Pt(core)−Pd(shell)/C and Pd/C, which both possess the Pd surface layers, were very low, compared to the other three 23460



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observed in Figure 11.119 The break in the Tafel slope on the polycrystalline Pt electrode has been attributed to a change from Temkin type to Langmuir type for the adsorption of reaction intermediates, or a change in the surface coverage of OHad/Oad species and specifically adsorbing anions, which control the availability of free adsorption sites for the adsorption of molecular O2.133 The explanation can be applied to the present catalysts. The transition potentials of the Tafel slopes are listed in Table 1. The Pt-surface catalysts had higher transition potentials than the Pd-surface catalysts, which means that the Pt-surface catalysts have the higher current densities at the lower overpotentials. The production of core−shell (Pt shell) nanoparticles or Pt surface-enriched nanoparticles are a useful way to improve the activity of Pd−Pt bimetallic catalysts. The conventional sequential reduction method to synthesize Pd(core)−Pt(shell) nanoparticles is complex and difficult to control the core−shell structures due to a possible nucleation of separate nanoparticles in the reduction process of the second metal. However, as shown in the present study, the Pt surface-enriched nanoparticles were successfully synthesized by the coreduction method in EG in air as characterized by FT-IR and XAFS. The PdPt alloy/C catalyst fabricated by the coreduction of the Pt and Pd precursors in EG showed the similar property and activity to the Pd(core)−Pt(shell)/C catalyst fabricated by the sequential reduction method. 3.3. Pt Surface-Enriched PdPt Alloy/C Catalysts with Different Pd/Pt Ratios. PdPt alloy nanoparticles with the Pd/ Pt atomic ratios of 2 and 4 were also prepared by the coreduction method for comparison. These catalysts are denoted as 2PdPt and 4PdPt, respectively, hereinafter. FT-IR spectra of CO adsorbed on PdPt, 2PdPt, and 4PdPt are shown in Figure S2 (Supporting Information). Similar to the PdPt (2051 cm−1) in Figure 2, the 2PdPt and 4PdPt samples also showed strong absorption peaks at 2037 and 2035 cm−1, respectively, and broad absorption peaks at around 1850 cm−1 were also observed. These absorption peaks are attributed to CO molecules adsorbed on atop sites and bridge sites at Pt surfaces, respectively,96 which suggests that the 2PdPt and 4PdPt nanoparticles are also covered by Pt surface layers in spite of the Pt quantity. The XRD patterns for carbonsupported 2PdPt and 4PdPt catalysts showed fcc (111) and (200) diffraction peaks with slight shifts to the higher 2θ angles by increasing Pd/Pt ratios due to alloying in the bulk as shown in Figure S3 (Supporting Information). The Pd/Pt atomic ratios of the catalysts measured by XRF were 1.06, 2.01, and 4.02 for PdPt/C, 2PdPt/C, and 4PdPt/C, respectively, which are consistent with the feeding ratios. Figure 12 shows the linear scan voltammograms for the PdPt alloy/C, 2PdPt alloy/C, and 4PdPt alloy/C catalysts recorded in O2-saturated 0.1 M HClO4 and the calculated specific activities at 0.9 V in the inset. The SAs decreased with an increase in the Pd/Pt ratio, where the ECSAs were estimated from the CV data in Figure S4 (Supporting Information). The SA of PdPt (1.7) was about 1.4 times higher than that of 2PdPt (1.25) and 2.3 times higher than that of 4PdPt (0.74). Their SA values for the ORR were all higher than those of 4.0 nm Pd(core)−Pt(1−3 ML shell)/C catalysts.53 The mass activities of the PdPt alloy/C, 2PdPt alloy/C, and 4PdPt alloy/C catalysts were also calculated based on the Pt quantity and the Pd + Pt quantity (PGM) as shown in Figure 13. Among the three samples, the PdPt alloy/C and 2PdPt alloy/C catalysts showed the similar highest Pt-based MA, and the 4PdPt alloy/

Figure 10. Linear scan voltammograms with anodic sweep 10 mV/s, 1600 rpm, and Coutecky− Levich plots (inset) of the catalysts: (1) PdPt alloy/C (black), (2) Pd(core)−Pt(shell)/C (red), (3) Pt(core)− Pd(shell)/C (blue), (4) Pd/C (olive), and (5) Pt/C (cyan).

catalysts with the Pt surface layers. It is obvious that the Pt surface-enriched catalysts show higher SAs than the Pd surfaceenriched catalysts at both 0.85 and 0.9 V. The SAs of the PdPt alloy/C and Pd(core)−Pt(shell)/C catalysts are in good agreement with the previous results on a Pd(core)−Pt(monolayer shell)/C catalyst, which are around 0.5 mA/cm2 at 0.9 V.53 The order of the CO oxidation potentials is inversely proportional to the SAs, which suggests that a similar site requirement is necessary for both oxygen reduction and CO oxidation on the Pd- and Pt-based catalysts. The MAs for the bimetallic catalysts, which were calculated based on only the Pt loading, are shown in Figure 10 (inset). At 0.85 and 0.9 V, the PdPt alloy/C and Pd(core)−Pt(shell)/C catalsyts exhibited similar and highest MAs among the five catalysts, even better than the pure Pt catalyst. On the contrary, the two Pd-surface catalysts, Pd/C and Pt(core)−Pd(shell)/C, showed the lowest MAs among the five catalysts. Figure 11 shows the Tafel curves, where the overpotentials are plotted against log(ik) (logarithm of current per ECSA).

Figure 11. Tafel plots for the ORR on the different catalysts: (1) PdPt alloy/C, (2) Pd(core)−Pt(shell)/C, (3) Pt(core)−Pd(shell)/C, (4) Pd/C, and (5) Pt/C.

The normalized kinetic activities are compared in the order, Pt/ C > PdPt alloy/C ≈ Pd(core)−Pt(shell)/C ≫ Pt(core)− Pd(shell)/C > Pd/C. For noble metals, two different Tafel slopes are usually observed in the different overpotential regions. In fact, two Tafel slopes of −60 mV/dec in the low overpotential region (low current density) and −120 mV/dec in the high overpotential region (high current density) were 23461



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specific activity than Pd surface-enriched catalysts Pt(core)− Pd(shell)/C and Pd/C for oxygen reduction reaction (ORR). The PdPt alloy/C and Pd(core)−Pt(shell)/C catalysts with core−shell structures showed higher Pt-based mass activity than Pt catalysts. Neither electronic (ligand) effect nor compressive strain effect were significantly observed in the bimetal systems. When the Pt quantity in the PdPt alloy/C decreased from Pd/Pt = 1/1 to 2/1 and 4/1, the Pt-based mass activity did not decrease so much, indicating the similar Pt environment and surface layers. The present study provides core−shell catalysts with better ORR activity, but establishment of reliable design and characterization of less Pt robust electrocatalysts are still needed for development of nextgeneration fuel-cell cathode catalysts.

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Figure 12. Linear scan voltammograms in O2-saturated 0.1 M HClO4 electrolyte at a scan rate of 10 mV s−1 and specific activities at 0.9 V for (1) PdPt alloy/C, (2) 2PdPt alloy/C, and (3) 4PdPt alloy/C catalysts.

ASSOCIATED CONTENT

S Supporting Information *

TEM images, FT-IR spectra, XRD patterns, and cyclic voltammograms. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: +81-42-443-5483. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This study was supported by New Energy and Industrial Technology Development Organization (NEDO).

Figure 13. ORR mass activities at 0.9 V for (1) PdPt alloy/C, (2) 2PdPt alloy/C, and (3) 4PdPt alloy/C catalysts.

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4. CONCLUSIONS PdPt alloy, Pd(core)−Pt(shell), and Pt(core)−Pd(shell) nanoparticles with different core−shell structures were synthesized by coreduction and sequential reduction methods in ethylene glycol. The carbon-supported Pd−Pt bimetallic catalysts were prepared from the obtained bimetallic nanoparticles, while keeping their structures. The core−shell structures in the PdPt alloy/C, Pd(core)−Pt(shell)/C, and Pt(core)−Pd(shell)/C catalysts were suggested by EXAFS, XRD, FT-IR, and CV to possess Pd 4 Pt 1 (core)−Pt(two-layers shell), Pd(core)− Pd2Pt1(three-layers)−Pt(three-layers shell), and Pt(core)− Pt2Pd1(two-layers)−Pd(three-layers equivalent microcrystals shell), respectively. Further study to elucidate the core−shell structures is necessary. The Pt surface-enriched catalysts PdPt alloy/C, Pd(core)−Pt(shell)/C, and Pt/C showed higher 23462



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