Polyallylamine Functionalized Palladium Icosahedra: One-Pot Water

Mar 12, 2013 - Aleksey Ruditskiy , Ming Zhao , Kyle D. Gilroy , Madeline Vara , and ... Ayral , Salvatore Abate , Tony Chave , Sergey I. Nikitenko , V...
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Polyallylamine Functionalized Palladium Icosahedra: One-Pot WaterBased Synthesis and Their Superior Electrocatalytic Activity and Ethanol Tolerant Ability in Alkaline Media Gengtao Fu, Xian Jiang, Lin Tao, Yu Chen,* Jun Lin, Yiming Zhou, Yawen Tang, and Tianhong Lu Jiangsu Key Laboratory of Power Batteries, Laboratory of Electrochemistry, School of Chemistry and Materials Science, Nanjing Normal University, 1# Wenyuan Road, Nanjing 210023, P. R. China S Supporting Information *

ABSTRACT: Polyallylamine (PAH) functionalized Pd icosahedra are synthesized through a simple, one-pot, seedless and hydrothermal growth method. Herein, PAH is used efficiently as a complex-forming agent, capping agent, and facet-selective agent. The strong interaction between PAH and Pd atom sharply changes the electronic structure of Pd atom in the Pd icosahedra. The protective function of PAH layers and enhanced antietching capability of Pd atom are responsible for the formation of the Pd icosahedra. Very importantly, the as-prepared PAH functionalized Pd icosahedra exhibit superior electrocatalytic activity and ethanol tolerant ability toward the oxygen reduction reaction (ORR) compared to the commercially available Pt black in alkaline media. At 0.95 V (vs RHE), the ORR specific kinetic current density at the Pd icosahedra is 4.48 times higher than that at commercial Pt black. The fact demonstrates the appropriate surface modification of the Pd nanoparticles by nonmetallic molecules can be regarded as an effective way to enhance the electrocatalytic activity toward the ORR.

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INTRODUCTION The synthesis of palladium (Pd) nanoparticles with specific morphologies, such as spheres, octahedra, plates, wires, tetrahedra, cubes, flowers, springs, tubes, shells, rods, thorns, bipyramids, and bars, has attracted enormous interest due to the fascinating size- and shape-dependent properties and invaluable applications in organic synthesis, fuel cells, and hydrogen storage/sensing, etc.1−8 Compared with singlecrystalline nanoparticles, multitwinned nanoparticles (MTPs), such as dendrites, icosahedra, decahedra, multipods, and bipyramids, exhibit significantly enhanced activity for a number of catalysis owning to the existence of high density of twinned defects.9−19 Pd icosahedron is one of MTPs, which is made of 20 tetrahedral subunits with 30 twin boundaries, resulting in a surface enclosed by 20 {111} facets.13−15 Since edge and corner atoms in MTPs exhibit open coordination sites that may result in significantly different bond enthalpies, desorption energies, and adsorption geometries compared to adsorption on terrace sites,19 the Pd icosahedron with 12 corners and 30 edges is anticipated to display high performance in catalysis. For example, among Pd nanostructures with various morphologies such as icosahedra, tetrahedra, decahedra, and triangular plates, Pd icosahedra with the most corners exhibit the highest electrocatalytic activity toward the formic acid oxidation reaction.18 At present, less expensive and widely available Pd has been widely exploited as the Pt-alternative catalysts toward the oxygen reduction reaction (ORR) because Pd possesses © 2013 American Chemical Society

properties similar to those of Pt (same group of the periodic table, same crystal structure, and similar atomic size).20−22 After adjusting appropriately the geometric and electronic structures of Pd, the catalytic activity of Pd becomes comparable to that of Pt toward the ORR. Traditionally, these modulations are achieved by alloying Pd with other transition metals such as iron, cobalt, and nickel.20−22 Very recently, the chemical functionalization of metal surface by nonmetallic molecules becomes a new trend in catalyst design. In brief, nonmetallic molecules adsorbed on metal surface can influence the electronic property of the metal or block the adsorption of spectrator anions.23,24 For example, the cyanide functionalized Pt{111}−CNad showed a 25-fold increase in the H2SO4 solution toward the ORR compared to the naked Pt{111} facets.24 As an advantage over organic systems, we here report a convenient and environmentally friendly water-based route to synthesize the Pd icosahedra with uniform size by using PdII− polyallylamine (PdII−PAH) complex as precursor. The strong N−Pt bond that originates from the interaction between PAH and Pd atom sharply changes the electronic structure of Pd atom in the Pd icosahedra. The as-prepared PAH functionalized Pd icosahedra exhibit the remarkably improved electrocatalytic activity and ethanol tolerant ability toward the ORR over the commercially available Pt black in alkaline media. Received: December 11, 2012 Revised: February 1, 2013 Published: March 12, 2013 4413

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polarization curves, the kinetic current density (ik) was calculated using the Koutecky−Levich equation (eq 1), which was described as follows:33,34

EXPERIMENTAL SECTION

Reagents and Chemicals. Poly(allylamine hydrochloride) (PAH, weight-average molecular weight 150 000) was supplied from Nitto Boseki Co., Ltd. (Tokyo, Japan) Palladium chloride (PdCl2) and formaldehyde solution (HCHO, 40%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Commercial Pt black was purchased from Johnson Matthey Corporation. Other reagents were of analytical reagent grade and used without further purification. Synthesis of the Pd Icosahedra. Similar to the case of [PdII(NH3)4]Cl2 in the PdCl2−NH3·H2O system, PAH can interact with PdCl2 to generate the colorless PAH−PdII complex by coordination to the amine groups.25 In a typical synthesis, 1.0 mL of 0.1 M PdCl2 and 1.7 mL of 0.52 M PAH were added into 16.7 mL of water (molar ratio of PAH monomer to PdII was 9:126). After adjusting solution pH to 3.0, 0.5 mL of formaldehyde solution (40%) was added into the resulting PAH−PdII complex solution. Then, the resultant colorless mixture solution was transferred to a 50 mL Teflonlined stainless-steel autoclave and was then heated at 120 °C for 6 h under hydrothermal conditions. After being cooled to room temperature, the Pd icosahedra were obtained by centrifugation at 15 000 rpm for 10 min, washed several times with water, and then dried at 60 °C for 5 h in a vacuum dryer. Electrochemical Instrument. All electrochemical experiments were performed by using a CHI 660 C electrochemical analyzer (CH Instruments, Shanghai, Chenghua Co.). A standard three-electrode system was used for all electrochemical experiments, which consisted of a platinum wire as the auxiliary electrode, a saturated calomel reference electrode protected by Luggin capillary with KCl solution as the reference electrode, and a catalyst modified glassy carbon electrode as the working electrode. Rotating disk electrode test was performed on Gamry’s Rotating Disk Electrode (RDE710) with a glassy carbon disk. Potentials in this study were reported with respect to the reversible hydrogen electrode (RHE). All electrochemical measurements were carried out at 30 ± 1 °C. Preparation of Working Electrode. Prior to electrochemical test, the Pd icosahedra were treated with UV/ozone27 (wavelength at 185 and 254 nm in air for 4 h) to remove the most capping agent (i.e., PAH). For preparation of working electrode, a previously reported procedure was used.28,29 An evenly distributed suspension of catalyst was prepared by ultrasonic the mixture of 5 mg of catalyst and 4 mL of H2O for 30 min, and 11 μL of the resulting suspension was laid on the surface of the precleared glassy carbon electrode (5 mm diameter, 0.196 cm2). After drying at room temperature, 3 μL of Nafion solution (5 wt %) was covered on the modified electrode surface and allowed drying again. Thus, the working electrode was obtained, and the specific loading of Pd metal on the electrode surface was about 70.15 μg cm−2. All working electrodes were pretreated by cycling the potential between 0.167 and 1.367 V (vs RHE) for 50 cycles in order to further remove surface contamination prior to electrochemical test. Measurement of the Electrochemically Active Surface Area. Cyclic voltammetry (CV) measurements were carried out in N2saturated 0.1 M NaOH or 0.1 M HClO4 solutions at a sweep rate of 50 mV s−1. The electrochemically active surface area (ECSA) of Pd catalysts were calculated from CVs in 0.1 M HClO4 solution by integrating the reduction charge of surface Pd(OH)2 and assuming a value of 420 μC cm−2 for the reduction charge of a Pd(OH)2 monolayer on the Pd surface30,31 because ECSA of Pd catalysts cannot be precisely assessed by coulometry in the “hydrogen region” due to the interference of hydrogen absorption in bulk Pd. The ECSA of Pt black was also calculated from CVs in 0.1 M HClO4 solution by measuring the charge collected in the hydrogen adsorption/desorption region after double-layer correction and assuming a value of 210 μC cm−2 for the adsorption of a hydrogen monolayer.32,33 Electrochemical Parameters of the Oxygen Reduction Reaction. The oxygen reduction reaction (ORR) measurements were conducted at room temperature in 0.1 M NaOH solutions under a flow of O2 using the rotating disk electrode (RDE) at a rotation rate of 1600 rpm and a sweep rate of 5 mV s−1. Based on the ORR

1 1 1 = + i ik id

(1)

where id and i were the limited diffusion current density and the measured current density, respectively. Ethanol Tolerant Measurements. The electrocatalytic oxidation of ethanol at catalysts was done in an N2-saturated 0.1 M NaOH solution with 1.0 M ethanol at a scan rate of 50 mV s−1. The study of ethanol tolerant ability for ORR was done in an O2-saturated 0.1 M NaOH solutions with 1.0 M ethanol using RDE at a rotation rate of 1600 rpm and a sweep rate of 5 mV s−1. Instruments. Ultraviolet and visible spectroscopy (UV−vis) was recorded at room temperature on a Shimadzu UV3600 spectrophotometer equipped with 1.0 cm quartz cells. Fourier transform infrared (FT-IR) was carried out using a Nicolet 520 SXFTIR spectrometer. Xray diffraction (XRD) patterns of the Pd catalysts were obtained with Model D/max-rC X-ray diffractometer using Cu Kα radiation source (λ = 1.5406 Å) and operating at 40 kV and 100 mA. Transmission electron microscopy (TEM) measurements were made on a JEOL JEM-2100F transmission electron microscopy operated at an accelerating voltage of 200 kV. High-resolution X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo VG Scientific ESCALAB 250 spectrometer with an Al Kα radiator, and the vacuum in the analysis chamber was maintained at about 10−9 mbar. The binding energy was calibrated by means of the C 1s peak energy of 284.6 eV. The rotating disk electrode test was performed on Gamry’s Rotating Disk Electrode (RDE710) with a glassy carbon disk.

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RESULTS AND DISCUSSION Figure 1A shows TEM image of the as-synthesized products. As observed, nearly all of the projections of the Pd nanoparticles illustrate the hexagonal projections and the multiply twinned structure with an edge length of 14 ± 3 nm, which are outstanding features of icosahedra (insert).35,36 Each icosahedron has three types of rotational axes: 2-fold, 3-fold, and 5-

Figure 1. (A) Representative TEM image of the Pd icosahedra. Inset: a schematic structure model of icosahedron. (B) HRTEM image of an individual Pd icosahedraon. Inset: SAED pattern of an individual Pd icosahedron. 4414

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fold.36 Generally, 5-fold axial direction is hard to obtain in experiment since an icosahedron prefers to lie on a flat surface against one of its faces rather than one of its corners.37,38 Figure 1B depicts the high-resolution TEM (HRTEM) image of an individual Pd icosahedron. Multiply twinned structures can be observed, and different growth directions can be identified for the two planes adjoining the boundary. The interval between the two lattice fringes is found to be 0.225 nm, close to the {111} lattice spacing of face-centered cubic (fcc) Pd. Moreover, butterfly-like contrast characteristic indicates the as-prepared Pd icosahedra possess 3-fold twinned structure.16 Unlike typical crystalline solids, the icosahedra cannot exist as a single crystal due to the existence of high density of twinned defects.39 Selected area electron diffraction (SAED) pattern clearly shows diffraction rings corresponding to various facets of fcc Pd (inset in Figure 1B), confirming that the Pd icosahedra are polycrystalline. Figure 2A shows XRD pattern of the Pd icosahedra. The Pd icosahedra show all characteristic diffraction peaks of fcc Pd

to monitor the reaction process momentarily (Figure 3). At the beginning, the characteristic peak of PAH−PdII complex at

Figure 3. UV−vis absorption spectra of the reaction system at different stages.

∼210 nm decreased slowly in intensity with time, accompanying the appearance of localized surface plasmon resonance of Pd nanoparticles (i.e., the very wide UV absorption in the wavelength range of 500−200 nm). After 4 h, the absorbance value reaction solution remained almost invariable, indicating a near-complete conversion of PdII−PAH complex to Pd nanoparticles. To further elucidate the growth mode of the Pd icosahedra, Pd nanocrystals from different growth stages were investigated by TEM. As observed, the ultrafine Pd nanoparticles (average size ∼4 nm) were produced at 1 h (Figure 4A). The top inset of Figure 4A shows that most of the

Figure 2. (A) XRD pattern of the Pd icosahedra. (B) XPS spectrum of the Pd icosahedra in the Pd 3d region.

(JCPDS standard 5-681 Pd). The ratio between the intensities of the {111} and {200} peaks is higher than the value reported for a conventional powder sample (2.85 versus 2.38), indicating that the Pd icosahedra are mainly covered with {111} facets.14,35,36 According to the Scherrer’s equation, the particle size of the Pd icosahedra is calculated to be 13.6 nm, in good agreement with the statistical TEM result. Figure 2B shows the Pd 3d spectrum of the Pd icosahedra. The Pd 3d signal of the Pd icosahedra is deconvoluted into two components: Pd 3d3/2 (340.5 eV), Pd 3d5/2 (335.2 eV) and Pd 3d3/2 (341.5 eV), Pd 3d5/2 (336.2 eV), which are assigned to Pd0 and PdII species, respectively. By measuring the relative peak areas, the percentage of Pd0 species in the Pd icosahedra is calculated to be 91.3%, much higher than the reported value of Pd nanoparticles,40,41 demonstrating the interaction between Pd atom and O2 in air occurs difficultly. Considering that the PdII−PAH complex has characteristic absorption behavior, we first used UV−vis absorption spectra

Figure 4. TEM images of the as-synthesized Pd icosahedra at different growth stages: (A) 1, (B) 2, (C) 4, and (D) 6 h. The top inset in each TEM image: HRTEM image of an individual Pd icosahedron. The middle inset in each TEM image: the FFT pattern of the HRTEM image. The bottom inset in each TEM image: the corresponding size distribution histogram of the Pd icosahedra.

ultrafine Pd nanoparticles have a projected hexagonal shape, indicating the Pd icosahedra have already formed at 1 h. The corresponding fast Fourier transform (FFT) pattern indicates that the Pd icosahedra are polycrystalline (the middle insert in Figure 4A). At t = 2 h, the products contained mainly the small icosahedra with an average size of 9 nm (Figure 4B). At t = 4 h, the products were almost transformed into well-defined Pd icosahedra with ∼14 nm size (Figure 4C). When the reaction 4415

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obtained (Figure S5). At 100 °C, however, PdII−PAH complex cannot react with HCHO within 6 h reaction time. Considering the economy and cost issue, a 120 °C reaction temperature is selected in our synthesis. The advanced ORR electrocatalytic activity of the Pd icosahedra was evaluated in alkaline media. Prior to electrochemical test, the Pd icosahedra were consecutively treated with UV/ozone and electrochemical cleaning to remove the most capping agent (i.e., PAH). After treatment, N 1s peak is still observed distinctly (Figure S6), indicating a few PAH molecules (or its oxidation production) still adsorb on the Pd icosahedra surface. Moreover, the Pd 3d binding energy of the Pd icosahedra negatively shifts ca. 0.5 eV compared to the PdNPs-control (Figure S7), indicating the residual PAH on the Pd icosahedra surface still strongly influence the d-band center of Pd. Figure 5A shows cyclic voltammetry (CV) curves of the Pd

time was further extended to 6 h, no significant change was observed for the Pd icosahedra in terms of both size and shape (Figure 4D). The trend was in accordance with the above UV− vis measurements. These results demonstrate that the Pd icosahedra are formed in a stage as early as nucleation and concomitant with the growth process,42 rather than through the evolution of other morphology in the growth process (Scheme 1). Scheme 1. Proposed Evolution Mode of the Pd Icosahedra

It is well-known that the well-defined Pd icosahedra with high yields usually are not the major products of typical solution-phase synthesis in the presence of air owing to highly oxidative etching.43 So, one of the keys to the formation of the Pd icosahedra is possibly to eliminate oxidative etching. For example, Xia and co-workers successfully synthesize the Pd icosahedra by using citrate ions as an oxygen elimination agent to block oxidative etching.36 In our synthesis, PAH adsorb preferentially on Pd{111} facets owing to strong amine−Pd interaction,8,25 which facilitates the generation of the Pd icosahedra nuclei due to thermodynamic stability of Pd icosahedral structure under small particle size conditions.44 The adsorption of PAH on the Pd icosahedra surface was verified by FT-IR (Figure S1). Obviously, the adsorbed PAH layer on the Pd icosahedra can act as an obstacle to protect the defects of twinned seeds against oxidative etching.15 Furthermore, it is observed that the binding energy of Pd 3d in the Pd icosahedra negatively shift ca. 0.7 eV compared to that of Pd nanoparticles obtained by the controlled experiment without PAH (termed as Pd-NPs-control) (Figure S2). The d-band center theory indicates that lowering of the d-band center results in the decrease in interaction strength of the various adsorbates to the substrates.45,46 Therefore, the negative shift of binding energy of Pd induces a weak oxophilicity of the Pd icosahedra and consequently enhances antioxidation etching capability. Therefore, the protection function of adsorbed PAH layer and antioxidation etching capability of Pd are responsible for the formation of the Pd icosahedra due to the suppression of oxidative etching. In a controlled experiment without PAH, the Pd products (i.e., Pd-NPs-control) consist mainly of irregular shapes and show obvious aggregation (Figure S3), indicating the bulky molecule size, excellent hydrophilic property, and positively charged polyelectrolytic nature of PAH molecules47 can effectively prevent the aggregation of the Pd icosahedra. In order to investigate the influence of more reaction parameters on the morphologies, a series of controlled experiments were carried out. It is observed that the solution pH influences morphology of Pd nanocrystals. When the initial solution pH is adjusted to 1.0 and 6.0, Pd icosahedra cannot be obtained (Figure S4). However, the detailed mechanism is not clear for us, and the relevant investigations are currently in progress. Meanwhile, the morphology of Pd nanocrystals was temperature independent at a pH 3.0 solution. At 140 °C, the Pd icosahedra with uniform size distribution can also be

Figure 5. (A) CV curves for the Pd icosahedra and Pd-NPs-control in N2-saturated 0.1 M NaOH solution at a scan rate of 50 mV s−1. (B) ORR polarization curves for the Pd icosahedra, Pd-NPs-control, and commercial Pt black in O2-saturated 0.1 M NaOH solution at a scan rate of 5 mV s−1 and rotation rate of 1600 rpm. (C) Specific kinetic current densities (ik) for the Pd icosahedra, Pd-NPs-control, and commercial Pt black at different potentials. (D) Specific kinetic current densities for the Pd icosahedra, Pd-NPs-control, and commercial Pt black at 0.95 V.

icosahedra and Pd-NPs-control in N2-saturated 0.1 M NaOH solution. Compared with the Pd-NPs-control, palladium oxide reduction peak of the Pd icosahedra positively shifts ca. 120 mV, indicating the strong interaction between PAH and Pd icosahedra decreases the oxophilicity on Pd (i.e., the Pd icosahedra have the lower hydroxyl surface coverage than the Pd-NPs-control).22,48 Figure 5B shows the typical ORR polarization curves of the Pd icosahedra, Pd-NPs-control, and commercial Pt black in O2-saturated 0.1 M NaOH solution at a rotation rate of 1600 rpm. All polarization curves display the diffusion-limiting current region below 0.8 V, and the mixed kinetic−diffusion control region between ∼0.8 and ∼1.0 V. All ORR polarization curves for Pd catalysts show double steps, which may be ascribed to the generation of the trace amounts of H2O2.49 The onset potential of ORR (EORR), which was defined as the potential corresponding to −10 μA cm−2, was used to indicate ORR activity.50,51 EORR of the Pd icosahedra, Pd-NPs-control, and commercial Pt black locate at 1.05, 0.98, and 1.03 V, respectively. The Pd icosahedra show a 70 and 20 mV shift, to more positive potentials, relative to the Pd-NPs4416

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control and commercial Pt black, indicating dramatically improved ORR kinetics at lower overpotential for the Pd icosahedra. In general, the mass activity is taken as an index to assess the applicability of the catalyst toward the ORR.52,53 At 1.0 V, the mass activity of the Pd icosahedra is 1.95 A g−1, which is ∼9.20 times higher than that of Pd-NPs-control (0.21 A g−1) and ∼1.56 times higher than that of the Pt black (1.25 A g−1). Meanwhile, it is observed that the mass activity of the Pd icosahedra is also ∼1.60 times higher than that of commercial 20 wt % Pt/C at 1.0 V (Figure S8), further confirming excellent catalytic activity of the Pd icosahedra toward the ORR. Since the specific kinetic activity (normalized to ECSA) represented the intrinsic electrocatalytic activity, the specific kinetic activities of catalysts toward the ORR were further investigated. The ECSA values of catalysts were measured in a N2-saturated 0.1 M HClO4 solution (Figure S9; see Experimental Section for calculation details). As seen, the Pd icosahedra show greatly improved specific kinetic activity than the Pd-NPs-control and Pt black in the whole potential range (0.85−0.95 V) (Figure 5C). For instance, the specific kinetic current density at 0.95 V is about 1.30 A m−2 for the Pd icosahedra, which is enhanced by a factor of 7.65 and 4.48 as compared with these of Pd-NPscontrol (0.17 A m−2) and Pt black (0.29 A m−2) (Figure 5D). ORR in aqueous alkaline media is a complicated electrocatalytic reaction. It is now widely accepted that proton transfer to O2, producing adsorbed HO2• or HO2− intermediates, occurs prior to cleavage of the O−O bond during the ORR on Pd surface.54 The d-band model developed by Nørskov has been successful in relating the adsorption properties of ratelimiting intermediates in catalytic processes to the electronic structure of the catalyst.45,46 Although the detailed reaction mechanism in the ORR is still not clear, a lot of investigations have indicated that an appropriate downshift of the d-band center can weaken the adsorption energy of reactive intermediates and result in an increase in the catalytic reactivity.55,56 Since the residual PAH affects the binding energy of the Pd icosahedra due to the strong N−Pd interaction, the change of d-band center of Pd is responsible for the enhanced electrocatalytic activity toward the ORR. Meanwhile, the ORR kinetics is also controlled by the amount of available active sites on the catalyst’s surface.57 Thus, the higher ORR activity at the Pd icosahedra can also be ascribed to their lower hydroxyl coverage and consequently more available reaction sites. Similar to the case of Au icosahedra-catalyzed ORR,17 multiple-twinned structure of Pd icosahedral likely improves the ORR activity because the actively low-coordinated defective atoms benefit the ORR kinetics.58,59 The alcohol tolerant ability of cathode electrocatalysts is very important for the practical applications of direct alcohol fuel cells. The Pd is generally known as one of the good catalysts for ethanol electrooxidation under alkaline conditions even compared with Pt.60 However, it is surprisingly observed that specific peak current density of the Pd icosahedra toward the ethanol electrooxidation is 4.3 times lower than that of the PdNPs-control (Figure 6A), indicating the Pd icosahedra have low electrocatalytic acitivity toward the ethanol oxidation. Likely, the residual PAH on the Pd icosahedra can act as barrier networks to restrain accessibility of ethanol with larger molecule size on Pd surface (Scheme 2). In order to confirm the mechanism, the electrooxidation of methanol and 1propanol on catalysts were also investigated by CV. It is observed that specific activity of the Pd icosahedra decreases with increasing molecule size of alcohol, relative to the Pd-NPs-

Figure 6. (A) CV curves for the Pd icosahedra and Pd-NPs-control in N2-saturated 0.1 M NaOH solutions with 1 M CH3CH2OH at a scan rate of 50 mV s−1. (B) The ratios of specific peak current densities of methanol, ethanol, and 1-propanol at the Pd icosahedra (Iico) to these at the Pd-NPs-control (INPs). For convenience, the specific peak current densities of methanol, ethanol, and 1-propanol at the Pd-NPscontrol were normalized to 1. As observed, the value of the Iico/INPs decreases with increasing molecule size of alcohol. A smaller value of the Iico/INPs implies better alcohol tolerant ability.

Scheme 2. Schematic Representation of the Influence of the Residual PAH Barrier Networks at the Pd Icosahedra Surface on the Accessibility of Ethanol Molecules with Big Molecule Size

control (Figure 6B), confirming the block function of PAH barrier networks for the alcohol accessibility. The ethanol tolerance of the Pd icosahedra for ORR was further tested in O2-saturated 0.1 M NaOH with 1 M ethanol at a scan rate of 5 mV s−1. For the Pd-NPs-control, the current transitions from negative to positive at about +0.70 V, and there is a large current peak at +0.83 V in the presence of ethanol (Figure 7A). The observed phenomenon is attributed to the

Figure 7. ORR polarization curves for the Pd-NPs-control and Pd icosahedra in O2-saturated 0.1 M NaOH solutions with 1 M CH3CH2OH at a scan rate of 50 mV s−1.

competition between ORR and ethanol oxidation reaction, in which the ORR current is overwhelmed by the ethanol oxidation current. On the contrary, the ORR activity of the Pd icosahedra is much less affected in the presence of ethanol (Figure 7B), confirming excellent ethanol tolerant ability of the Pd icosahedra. The accelerated durability test of the ORR was carried out by applying linear potential sweeps between 0.4 and 1.1 V (vs RHE) with a scan rate of 5 mV s−1 in O2-saturated 0.1 M 4417

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Notes

NaOH solution. After 1000 cycles, there is only about a 5 mV negative shift in the half-wave potential for the Pd icosahedra (Figure 8A). The CVs of the Pd icosahedra before and after the

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21005039, 21073094, and 21273116), the United Fund of NSFC and Yunnan Province (U1137602), Industry-Academia Cooperation Innovation Fund Project of Jiangsu Province (BY2012001), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Figure 8. (A) ORR polarization curves for the as-prepared Pd icosahedra in O2-saturated 0.1 M NaOH solution before and after 1000 potential cycles at a scan rate of 5 mV s−1 and rotation rate of 1600 rpm. (B) CV curves for the Pd icosahedra in N2-saturated 0.1 M NaOH solution before and after 1000 cycles at a scan rate of 50 mV s−1.

durability tests were also measured. The Pd icosahedra exhibit a negligible loss of 6.4% in reduction charge of surface Pd(OH)2 after durability test (Figure 8B), suggesting that the Pd icosahedra have excellent durability toward the ORR. These results confirm that the Pd icosahedra are highly promising for use in low-temperature alkaline fuel cells, such as direct ethanol fuel cells. Finally, the ORR activity of Pd icosahedra in acidic media was also investigated. It is observed that the half-wave potential of the ORR at Pd icosahedra is close to that at commercial Pt black in O2-saturated 0.1 M HClO4 solution (Figure S10), indicating the Pd icosahedra may likely be used as the Pt alternative catalysts toward the ORR in acidic media.

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CONCLUSIONS In summary, the Pd icosahedra with uniform size have been obtained through a facile, one-pot, seedless, hydrothermal growth method. Herein, PAH is used efficiently as a complexforming agent, capping agent, and facet-selective agent. The growth mechanism indicates that the Pd icosahedra are formed from multiple-twinned seeds via “nucleation−growth” process. The strong N−Pd interaction sharply changes the electronic structure of the Pd icosahedra. The PAH functionalized Pd icosahedra exhibit superior electrocatalytic activity toward the ORR in alkaline media relative to the commercial Pt black, which can be ascribed to the downshift of the d-band center, lower hydroxyl coverage, and high density of twinned defects of the Pd icosahedra. Moreover, the residual PAH on the Pd icosahedra surface can act as barrier networks to restrain accessibility of ethanol, which leads to particular ethanol tolerant ability in alkaline media. Therefore, the PAH functionalized Pd icosahedra may be a novel, cheap replacement for Pt as a cathode electrocatalyst in alkaline media.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental section and additional characterization data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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