Facile Synthesis of Highly Active Three-Dimensional Urchin-Like Pd

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Facile Synthesis of Highly Active Three-Dimensional Urchin-Like Pd@PtNi Nanostructures for Improved Methanol and Ethanol Electrochemical Oxidation Guohong Ren, Yajun Liu, Weigang Wang, Mingqian Wang, Zhicheng Zhang, Ying Liang, Shishan Wu, and Jian Shen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00438 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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ACS Applied Nano Materials

Facile Synthesis of Highly Active Three-Dimensional Urchin-Like Pd@PtNi Nanostructures for Improved Methanol and Ethanol Electrochemical Oxidation Guohong Ren,† Yajun Liu,† Weigang Wang,† Mingqian Wang,† Zhicheng Zhang,† Ying Liang,† Shishan Wu,*,† and Jian Shen*,†,‡

†

Key Laboratory of High Performance Polymer Material and Technology of Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, 163 Xianlin Avenue, Qixia District, Nanjing, 210023, China ‡ Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Key Laboratory of Biomedical Materials, College of Chemistry and Materials Science, Nanjing Normal University, 1 Wenyuan Road, Qixia District, Nanjing, 210046, China

ABSTRACT Exploitation of highly active catalysts for alcohol electrooxidation is urgent for direct alcohol fuel cells (DAFCs). In this research, a facile and mild synthetic approach is utilized to control and tailor the morphology of the three-dimensional (3D) urchin-like Pd@PtNi nanostructures (NSs), and the formation mechanism of the as-prepared nanostructures is expounded in detail. The Pd@PtNi NSs exhibit outstanding electrochemical properties and remarkable durability toward both methanol and ethanol oxidation reaction (MOR and EOR) in alkaline solution. The electrochemically active surface area (ECSA) of the Pd@PtNi NSs is 59.5 m2 g-1, and their mass activity for MOR and EOR are 1614.3 mA mg-1 and 1502.3 mA mg-1, respectively, which are much higher than those of their ternary or binary alloy counterparts as well as commercial Pt black catalysts. Moreover, it still retains high current densities after catalyzing 10000 s, while the current densities of other nanocatalysts reduce to nearly zero. The outstanding electrochemical activities and durability are owing to the specific 3D urchin-like nanostructures providing enormous active sites for catalytic reaction and the synergy effects between Pt, Pd and Ni atoms. The 3D urchin-like Pd@PtNi NSs will enrich the electrocatalysts for DAFCs.

KEYWORDS: 3D urchin-like nanostructures, reaction active sites, electrochemically active surface area, electrochemical oxidation reaction, electrochemical activity and durability 1

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1. INTRODUCTION Direct alcohol fuel cells have received intensive attentions because of their high energy conversion efficiency, easy operation system, low reaction temperature and environment friendliness.1-3 In recent decades, direct methanol fuel cells (DMFCs) and direct ethanol fuel cells (DEFCs) are considered as prospective energy conversion devices because the sources methanol and ethanol are abundant and renewable, easily to transport and store, as well as cost-efficient.4,5 However, their practical applications are confronted to many obstacles, such as the high cost and low electrochemical performance of catalysts for MOR and EOR.6 It has been reported that platinum (Pt) catalysts could improve the electrocatalytic activities of DAFCs owing to their distinct reactivity. However, the commercialization applications of Pt and Pt-based nanocatalysts are seriously hindered for their high cost, poor durability and easy poison.7-9 Therefore, many efforts have been made to solve these problems, such as controlling the structure and tailoring the morphology of the nanocatalysts, as well as constructing the bimetal or multi-metal nanocatalysts.10,11 In order to increase the ECSA and active sites of the Pt-based nanocatalysts and improve their catalytic ability and stability simultaneously, a great many attentions have been paid to exploring the 3D nanostructured morphologies. The 3D nanostructures are not incline to aggregate as compared with the zero-dimensional (0D) or one-dimensional (1D) nanomaterials, and they can also provide tremendous corners, thorns, accessible active sites as well as abundant electron transfer pathways which endow the catalysts superior electrochemical performance and stability.12 Wang and his co-workers fabricate the well-dispersed 3D dendritic platinum nanostructures (DPNs) that contain sufficient reaction sites and high surface area for reactant molecules. The prepared 3D DPNs are used to catalyze the reduction of dioxygen and oxidation for methanol, and show superior catalytic performances.13 It is known that the morphologies of nanocatalysts and the distribution of the Pt atoms, which play an important role for their catalytic properties, could be influenced by the foreign metals. Moreover, it’s also an effective device that coupling the Pt with other earth-abundant metals (such as Pd, Au, Cu, Co and Ni) to reduce the mass-loading and cost of the Pt-based nanocatalysts.4,14 The prepared bimetal or multi-metal Pt-based nanocatalysts significantly enhance the electrocatalytic performance through the synergistic and electronic effects between different metals. For instance, Chou et al. prepare the 3D Pt-Ni multipods by a simple synthetic protocol and elaborate its formation mechanism. These 3D Pt-Ni multipods have excellent solution process 2


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ability and improve the heterojunction polymer solar cells efficiency.15 Generally, the catalytic properties of Pt-based nanocatalysts are tightly dependent on their sizes, morphologies, structures and compositions.16 So the Pt-based nanocatalysts with different structures and morphologies, such as nanowires,17 nanocubes,18 nanospheres,19 nanoflowers20 and nanoclusters21 and so on, are synthesized and investigated. The precise control of the morphologies and sizes of nanocatalysts is important for improving the electrochemical performance and reducing the loading amount of Pt-based nanocatalysts. In recent years, the core-shell structure nanocatalysts with thin Pt shells have been extensively explored. The special core-shell nanostructures could effectively reduce the mass of Pt and increase the number of the metal (sub)surfacial active sites which are beneficial to the electrocatalytic performance.5,10 From this point, a lot of works have been devoted to preparing the core-shell Pt-based nanocatalysts with various shapes. Among them, urchin-like Pt-based nanocatalysts possessing a lot of branches and rough surface strongly promote the enhancement of their specific surface areas. Shen and co-workers synthesize high-yield and multi-branched Pt3Ni alloy nanourchins by modulating the heating rates of the Pt and Ni acetylacetonates during the reduction process.22 However, there are some disadvantages for this synthesis method, including the tedious operation progress, high experimental temperature, long reaction time, as well as troublesomely to control the morphologies and growth kinetics of Pt-based nanocrystals. Therefore, a facile and efficient strategy is still urgent to obtain the Pt-based nanocatalysts with perfect structures. In this research, we synthesize 3D urchin-like Pd@PtNi NSs through a mild and effective one-pot strategy. The Pd@PtNi NSs are featured with abundant branches and rough surfaces, which offer more contact surfaces and reaction active sites that observably enhance the catalytic activities. The as-prepared NSs are used to catalyze MOR and EOR in alkaline medium, where the electrocatalytic property and durability for enduring operation significantly improve. As compared with commercial Pt black catalysts, it displays higher current density and superior stability after repeating 1000 circles. This excellent performance is ascribed to the special urchin-like nanostructures, enormous active sites, large electrochemically active surface area and the synergy effects between Pt, Pd and Ni. The 3D urchin-like Pd@PtNi NSs will be a promising candidate for the electrocatalysts toward MOR and EOR.

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2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Chloroplatinic acid (H2PtCl6), palladium chloride (PdCl2), nickel chloride hexahydrate (NiCl2·6H2O), ascorbic acid (AA), commercial Pt black catalysts are purchased from Shanghai Aladdin Chemical Reagent Co. Ltd., China. H2SO4 (98%), HCl (37%), potassium hydroxide (KOH), methanol and ethanol are purchased from Shanghai Sinopharm Chemicals Reagent Co., Ltd., China. The triblock copolymer poly(ethylene

glycol)-block-poly(propylene

glycol)-block-poly(ethylene

glycol)

(PEG-PPG-PEG

Pluronic® P-123, Mw=5800) is purchased from Sigma-Aldrich (Shanghai, China). All the chemicals are used as received without further purification. The H2PdCl4 solution is prepared according to the reported literature.23 All the aqueous solutions are prepared with twice-distilled water in the whole experiments.

2.2. Preparation of the 3D Urchin-Like Pd@PtNi NSs. For typical synthesis of 3D urchin-like Pd@PtNi NSs, 0.03 g of triblock copolymer P-123 is dispersed in 8.5 mL twice-distilled water by ultrasonication for 10 min. Then, 648 µL of H2PtCl6 (38.62 mM), 200 µL of H2PdCl4 (100 mM) and 200 µL of NiCl2 (100 mM) are successively added to the above solution under stirring. After stirring for 10 min, 400 µL of the fresh AA solution (0.5 M) is injected into the homogeneous solution dropwise and react for 30 min. The black precipitates are obtained by centrifugation and thoroughly wash several times using two-distilled water. The final black products are dried in vacuum at 60 °C for further use. For comparison, the bimetal PtPd nanoparticles (NPs), PtNi NPs and PdNi NPs are also prepared in the same method without adding the precursors NiCl2, H2PdCl4 and H2PtCl6, respectively. The PdPtNi nanoparticles are fabricated use a similar way without adding the triblock copolymer in control experiments. Moreover, the carbon-support NPs are synthesized using the similar procedure of the Pd@PtNi NSs by adding the graphene oxide (GO) and carbon nanotubes (CNTs) as support, and the products are marked as PdPtNi NPs/GO and PdPtNi NPs/CNTs, respectively.

2.3. Characterization. The detailed morphologies of the products are investigated by the transmission electron microscopy 4


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(TEM, JEM-1011) and high-resolution transmission electron microscopy which equipped with the selective area electron diffraction (HR-TEM-SAED, Philips-FEI Tecnai F20). Meanwhile, the elemental compositions are examined by the energy-dispersive X-ray spectroscopy (EDS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) on FEI Tecnai F20 operated at an acceleration voltage of 200 kV. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Optima 5300DV, Perkin Elmer Inc.) analysis is performed to determine the composition and content of metallic element. The crystal structures are analyzed by X-ray diffraction (XRD) spectra on a Shimadzu XRD-6000 diffractometer. X-ray photoelectron spectroscopy (XPS) analysis are carried out on a PHI 5000 Versa Probe.

2.4. Electrochemical Investigations. All the electrochemical measurements are performed with a three-electrode cell configuration on a CHI 660D electrochemical workstation (CH Instruments, Chenhua Company, Shanghai, China). In the standard three-electrode cell, the glassy carbon electrode (GCE, 3 mm diameter), a saturated calomel electrode (SCE) and a platinum wire are used as working electrode, reference electrode and counter electrode, respectively. The electrocatalytic performance and stability of the catalysts modified electrodes are investigated in 1.0 M KOH containing 1.0 M methanol or 1.0 M ethanol at a scan rate of 50 mV s-1. All of the potentials here are against with reversible hydrogen electrode (RHE), unless otherwise stated. For the construction of the 3D urchin-like Pd@PtNi NSs modified electrode, 2 mg of the catalysts are dispersed into 2 mL twice-distilled water and obtain a homogeneous suspension by ultrasonication for 30 min. Then, 7.5 µL of the suspensions are uniformly casted onto the clean GCE and dried at the room temperature. Subsequently, 5 µL of Nafion solution (0.05 wt%) is used to facilitate tight adhesion of the catalyst on the electrode surface. The PdPtNi NPs, PdPt NPs, PtNi NPs, PdNi NPs and commercial Pt black catalysts modified electrodes are prepared in a similar way for comparison. The electrochemically active surface area of the catalyst is determined by CO-stripping analysis. Firstly, CO is adsorbed on the surface of the catalyst through bubbling CO in 0.5 M H2SO4 at 0.1 V for 30 min. The excess CO in the solution is purged by bubbling N2 through the solution for another 30 min. The CO-stripping voltammograms are carried out in 0.5 M H2SO4 with a scan rate of 50 mV s-1. The following equation is used to calculate the ECSA value of the catalyst:24 5



ECSA = Q / (m × 420)

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(1)

where Q (µC) is the charge of CO desorption electrooxidation, m is the mass of the active metal on the electrode surface, and 420 is the charge required to oxidize the adsorbed monolayer CO on the surface of the catalyst (µC cm-2).

3. RESULTS AND DISCUSSION 3.1. Characterization. The morphology and size of the as-prepared Pd@PtNi NSs are analyzed by transmission electron microscopy (Figure 1A-D). As observed in the low-resolution TEM image (Figure 1A and 1B), the 3D Pd@PtNi NSs are consist of numerous grains and dendritic structures which provide enormous reaction active sites and corner atoms. The size distribution of the Pd@PtNi NSs from 35 to 55 nm illustrates the NSs are about 47 nm (inset Figure 1A). The core-shell NSs with rough surfaces are proved to be polycrystalline by the selective area electron diffraction pattern (inset in Figure 1B). More detailed information of the Pd@PtNi NSs is observed in the HR-TEM images (Figure 1C and D). The typical lattice fringe spacings are estimated to be 0.228, 0.221 and 0.205 nm in Figure 1D, where the 0.228 nm is consistent with the (111) planes of the Pd core,25 and the 0.221 and 0.205 nm are indexed to the (111) and (200) planes of the PtNi shell.26,27 The values of the PtNi lattice spacing are among the (111) plane of Pt (0.226 nm, JCPDS-04-0802) and Ni (0.204 nm, JCPDS-04-0850) because the substitution of Pt with Ni atoms leads to the contraction of the Pt lattice.28,29 According to the EDS analysis (Figure 1E), the Pt, Pd and Ni are co-existence and their atomic ratio is approximated to 3:6:1, which matches well with the inductively coupled plasma-atomic emission spectroscopy (ICP-AES) calculation result (Pt:Pd:Ni = 3:6.13:0.96). The elemental ratio is not consistent with the initial precursor solution, which may due to the incomplete reduction of the precursor ions. The XRD patterns are used to investigate the crystal structure and phase composition of the Pd@PtNi NSs as shown in Figure 1F. In the XRD spectrum (Figure 1F), the characteristic diffraction peaks at 40.0o, 46.5o, 68.0o and 82.0o are consistent of the (111), (200), (220), (311) of the PtNi alloy (PDF No. 65-9445).30-32 Moreover, there are no obvious peaks around 41.9o, 48.8o and 73.9o owing to the existence of Ni in PtNi alloy and/or the minor amount of Ni NPs, which indicates the formation of the PtNi shells in the Pd@PtNi NSs.33 Based on the EDX and ICP-AES results, the reduction ratio of Ni2+ ions to the Ni NPs 6


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is calculated to be about 13.7%. The diffraction peaks at 86.4o could be ascribed to the (222) lattice plane of the Pd cores in the Pd@PtNi NSs, which is consistent with the previous report.28 The XRD results further indicate that the NSs are composed of Pd cores and outer PtNi shells. Notably, the diffraction peak intensity of the (111) planes is larger than that of the others, suggesting the abundant (111) planes in the Pd@PtNi NSs.

Figure 1. TEM (A,B) and HR-TEM (C,D) images of the as-prepared 3D urchin-like Pd@PtNi NSs; EDS spectrum (E) and XRD spectra (F) of the 3D urchin-like Pd@PtNi NSs. Insets show the corresponding histogram of particle size distribution (in Figure 1A) and SAED pattern (in Figure 1B).

The high-angle annular dark-field scanning transmission electron microscopy-energy dispersive X-ray spectroscopy experiments are conducted to further determine the elemental distribution in the Pd@PtNi 7



NSs. As illustrated from Figure 2A-D, the selected-area elemental analysis mapping images of Pt, Pd, Ni and the overlap show the uniform dispersion of the metal elements. And the mapping images demonstrate that the Pd distributes in the center of nanostructures forming a core (Figure 2B), while the Pt and Ni are detected in the outer shell (Figure 2A and B), powerfully confirming the constitution of core-shell structures. The EDS line scanning profiles, taken along the yellow line marked in Figure 2E, also reveal that the Pd element is mainly located in the center region as well as the Pt and Ni elements are in the outside layer (Figure 2F), which further demonstrates their core-shell structures.34 These results are consistent with the TEM and XRD analysis in Figure 1.

Figure 2. The HAADF-STEM-EDS mappings (A-D) marked with the red box in Figure 2E and EDS line scanning profiles (F) of the 3D urchin-like Pd@PtNi NSs (the yellow line marked in Figure 2E). Purple, green, and red lines in the EDS line scan profiles are the signals from Pd, Pt and Ni elements, respectively. 8


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In order to make a thorough inquiry about the surface chemical compositions and oxidation states of the Pd@PtNi NSs, X-ray photoelectron spectroscopy (XPS) measurements are employed. The XPS spectrum (Figure S1A) shows that the Pd@PtNi NSs mainly contain Pt, Pd, Ni, C and O with the XPS peaks at 71, 334, 860, 284 and 531 eV, respectively. The C signal in XPS spectrum may be from the background C element for calibration, the interference carbonaceous species in the surroundings or very small amount of residual reagents on the surface of the Pd@PtNi NSs. The XPS spectra demonstrate the variation in the electronic structures of the metallic elements in Pd@PtNi NSs.35 Compared with pristine Pt, the Pt 4f peaks of the Pd@PtNi NSs around at 71.1 and 74.4 eV negatively shift about 0.2 eV (Figure S1B). The high-resolution XPS spectra of the Pt 4f are divided into two doublets, one is at the binding energy of 71.02 and 74.05 eV ascribed to the Pt (0) and the other at 72.48 and 76.17 eV assigned to the Pt (+2) (Figure S1B), revealing the existence of metallic Pt and PtO or Pt(OH)2 in the NSs. It is same to the Pd 3d peaks as displayed in Figure S1C, where the binding energies of Pd 3d shift to the positive positions about 0.3 eV. The high-resolution Pd 3d spectra are resolved into two pairs of peaks, which are the stronger peaks around 335.24 and 340.59 eV correspond to the metallic Pd (0) and the weaker ones at 338.50 and 343.56 eV ascribed to Pd (+2) species of PdO or Pd(OH)2. The corresponding Ni 2p XPS spectra are also researched, the unique intense shakeup satellite at the higher binding energy and the primary peaks at lower binding energy are observed in the Figure S1D. The main peaks at the position of 853.96, 855.78 and 858.52 eV are attributed to the Ni (0), Ni(OH)2 and Ni(OOH), respectively.36 The metallic Pt (0) and Pd (0), which are effective to the catalytic reaction, are the major species in the Pd@PtNi NSs by measuring the relative peak intensities. It has been reported that the shift of the binding energy could modify the electronic structure and change the d-band center that relative to the Fermi level.20,37,38 The shifts of the Pt 4f peaks (in Figure S1B) indicate the electron transfer from Pd and Ni to Pt because of the higher electronegativity of Pt, which leads to the change of the electronic structure and the downshift of the d-band center of the Pt in the Pd@PtNi NSs.39 As a result, the downshift of the d-band center of Pt can reduce the binding strength of the COads species that produced on the surface of the Pd@PtNi NSs in alkaline solution and facilitate to react with the OHads which is the active oxygen species in MOR and EOR. Therefore, the electrochemical performance of the Pd@PtNi NSs for methanol and ethanol can be enhanced.40

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3.2. Formation Mechanism. The possible formation mechanism of the 3D urchin-like core-shell Pd@PtNi NSs could be described to the nucleation and growth progress of the metal NPs in the present of direction agent (Figure 3A).41,42 In the reaction system, the [PdCl4]2- ions are preferentially reduced to Pd NPs because of its higher reduction rate than that of [PtCl6]2- under the same conditions. The reductive Pd NPs could form the inner core that served as “seeds” for the subsequent construction of the Pt and Ni NPs.43 At the growth stage of the reaction, the [PtCl6]2- and Ni2+ are co-reduced and form the outer shell by the influence of the structural director and stabilizer P-123. However, the reduction ability of the [PtCl6]2- ([PtCl6]2-/Pt, 0.68 V vs. SHE) and Ni2+ (Ni2+/Ni, -0.25 V vs. SHE) ions and diffusion rates of Pt and Ni NPs are different, which affect the distribution and amount of the Pt and Ni atoms in the Pd@PtNi NSs. The [PtCl6]2- ions are easier to be reduced than the Ni2+ ions by using the weak reducing agent. Additionally, the separation of the core Pd and the shell PtNi is caused by the different reduction kinetics of [PdCl4]2-, [PtCl6]2- and Ni2+, which are necessary for the formation of core-shell structures.44 In order to verify the formation mechanism, we investigate the morphology evolution of the Pd@PtNi NSs. The temporal HRTEM and EDS elemental mapping images with different reaction times are shown in Figure 3B-I and Figure S2A-D. In the initial stage (5 min), the [PdCl4]2- ions are reduced to the irregular nanoparticles which are similar to the sphere with the size about 20 nm (Figure 3B). The elemental analysis also display that there is only Pd element in the NPs (Figure 3F). The irregular Pd NPs could serve as the “seeds” for the assembling of Pt and Ni atoms and evolve into the Pd core. Although the shapes of NPs are unchanged, their sizes become larger after reacting 10 min (Figure S2A). As shown in Figure 3C, the surfaces of the nanoparticles are rough when the time up to 15 min. And the Pd, Pt and Ni elements are detected in the EDS elemental mapping images, which indicate that the Pt and Ni ions are reduced simultaneously and grow on the Pd core (Figure 3G). However, the content of the Pt NPs is more than that of the Ni NPs, revealing the better reduction ability of the Pt ions. With the increase of the reduction time, the assemblies of the Pt and Ni atoms make the shell thicken and form the obvious branch structures (Figure S2B, Figure 3D and 3H). The magnified TEM image in the Figure S2C (25 min) shows the lattice plane spacing of the PtNi branches, which further proves the formation of the core-shell structure as marked in the red circle.45 Up to 30 min, the perfect urchin-like Pd@PtNi NSs with distinct dendritic structures are obtained (Figure 3E and 3I). And the amplifying dendritic structure TEM image indicates the 10


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overgrowth of the PtNi shell (Figure S2D). The PtNi branches provide a large number of active sites for the nanostructures toward the electrochemical reaction. It is worth noting that the contents of the Pt and Ni elements increase with reaction time (Figure 3G-I).

Figure 3. Schematic illustration of the formation mechanism of the 3D urchin-like Pd@PtNi NSs (A). The HRTEM, HAADF-STEM and corresponding EDS elemental mapping images with different reaction time: 5 min (B and F), 15 min (C and G), 25 min (D and H) and 30 min (E and I). The scale bar is 10 nm in the G, H and I.

To investigate the influence of the shape director and stabilizer (P-123) for the formation of 3D urchin-like NSs, the experiments with different amount P-123 are executed. Without structural director, the aggregated NPs with various sizes and shapes are observed in Figure S3A. As shown in Figure S3B, the NPs with flower-like structures are synthesized when only add 0.01 g P-123. However, the morphology of the flower-like structures is difficult to control. It is illustrated in Figure S3C that the excessive P-123 (0.05 g) could not effectively regulate the morphology of the Pd@PtNi NSs and the urchin-like core-shell 11



structures become ambiguous. Therefore, the 0.03 g P-123 are added in this experiment. Moreover, the binary metal nanoparticles are prepared in the contrast experiments. The TEM images of PtPd NPs, PtNi NPs and PdNi NPs are displayed in Figure S3D-F, respectively. The PtPd NPs are composed of nanospheres with different diameters and tended to aggregate (Figure S3D), while the PtNi NPs and PdNi NPs possess of a large number of grains without regular shapes (Figure S3E and F). In addition, the carbon-support NPs are synthesized by using the GO and CNTs as the support. As shown in Figure S4, the PdPtNi NPs disperse irregularly on the surface of GO and CNTs (Figure S4A and C). In the enlarged view (Figure S4B and D), the morphology of the PdPtNi NPs anchoring on the surface of GO and CNTs are not as good as the Pd@PtNi NSs (Figure 1A and B) and their sizes are out of control.

3.3. Electrochemical Measurements. The ECSA is an important parameter for the as-prepared nanocatalysts. The CO-stripping voltammetry experiments are conducted to estimate the ECSA values. All the cyclic voltammetry curves exhibit the distinctive peaks of the CO oxidation in the first cycle scanning but disappear in the second cycle (Figure S5). The ECSA of the Pd@PtNi NSs is calculated to be 59.5 m2g-1, that is much larger in comparison with that of PdPtNi NPs (52.7 m2g-1), PdPt NPs (46.8 m2g-1), PtNi NPs (43.2 m2g-1), PdNi NPs (40.9 m2g-1) and commercial Pt black catalysts (29.7 m2g-1). This suggests that the Pd@PtNi NSs possess more active sites and better ability to remove the COads on the surface of the NSs, which could improve their catalytic property. In addition, the onset potential of the Pd@PtNi NSs (0.64 V) shifts negatively about 20 mV, 160 mV, 175 mV, 243 mV and 187 mV than that of PdPtNi NPs, PdPt NPs, PtNi NPs, PdNi NPs and commercial Pt black catalysts (the green line marked in Figure S5), implying the Pd@PtNi NSs are beneficial to remove the poisoning intermediate species from their surface. The larger ECSA and more negative onset potential benefit from the 3D nanostructures of the Pd@PtNi NSs and the addition of the Pd and Ni atoms changing the electronic structure and d-bond center of the Pt atoms. Therefore, the Pd@PtNi NSs can facilitate the enhancement of the electrochemical performance for MOR and EOR. The cyclic voltammetry is used to evaluated the electrochemical properties of the obtained Pd@PtNi NSs, the PdPtNi NPs, binary metal NPs and commercial Pt black catalysts. The methanol and ethanol electrooxidation reactions are performed in 1 M KOH solution containing 1 M methanol or ethanol at a scanning rate of 50 mVs-1, respectively. The electrochemical cyclic voltammograms (CVs) for MOR and 12


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EOR are normalized by the loading amount of the active metallic catalysts on the working electrodes. In the CVs, the characteristic oxidation peaks of MOR and EOR for all the catalysts are observed (Figure 4A and B). The peaks of the forward scanning are ascribed to the oxidation of the chemisorbed methanol and ethanol, and the reverse scanning peaks are mainly related with the successive oxidation of the intermediate carbonaceous species produced in the forward sweep.46 As described in Figure 4A, the onset potential of the Pd@PtNi NSs for methanol electrochemical oxidation is 362 mV which negatively shifts 116, 98, 109, 125 and 116 mV than that of the PdPtNi NPs, PdPt NPs, PtNi NPs, PdNi NPs and commercial Pt black catalysts. In the Figure 4B, the onset potential of the ethanol electrochemical oxidation (353 mV) also down shifts 105, 55, 33, 25 and 115 mV than that of the PdPtNi NPs, PdPt NPs, PtNi NPs, PdNi NPs and commercial Pt black catalysts. Therefore, the Pd@PtNi NSs can observably reduce the overpotential in the methanol and ethanol electrochemical oxidation.9 The positive scan oxidation peak current intensity of the Pd@PtNi NSs for MOR are 1614.3 mA mg-1, which are much larger than that of the PdPtNi NPs (1167.3 mA mg-1), PdPt NPs (970.5 mA mg-1), PtNi NPs (879.1 mA mg-1), PdNi NPs (765.8 mA mg-1) and commercial Pt black catalysts (467.6 mA mg-1). And the peak potential of the Pd@PtNi NSs toward MOR is 961 mV, which is more negative than that of the PdPtNi NPs (997 mV), PdPt NPs (964 mV), PtNi NPs (977 mV), PdNi NPs (974 mV) and commercial Pt black catalysts (1028 mV). Similarly, the peak current densities of the EOR are 1502.3, 1174.8, 1099.1, 835.2, 784.3 and 431.9 mA mg-1 for the Pd@PtNi NSs, PdPtNi NPs, PdPt NPs, PtNi NPs, PdNi NPs and commercial Pt black catalysts, respectively. The oxidation peak potential of the Pd@PtNi NSs (859 mV) negatively shifts about 60, 3, 8, 185 and 63 mV as compared to that of the PdPtNi NPs (919 mV), PdPt NPs (862 mV), PtNi NPs (867 mV), PdNi NPs (1044 mV) and commercial Pt black catalysts (922 mV). These results indicate that the Pd@PtNi NSs show higher electrochemical performance and more negative peak potential than that of the other NPs and the commercial Pt black catalysts. The current density of the forward sweep (jf) to that of the reverse sweep (jr) is marked as jf/jr, and according to the literature report it’s related to the tolerance of the catalyst to the intermediate carbonaceous species.39 The jf/jr ratio of the Pd@PtNi NSs for MOR is calculated to be 5.07, which is higher than that of PdPtNi NPs (4.52), PdPt NPs (4.66), PtNi NPs (4.03), PdNi NPs (2.16) and commercial Pt black catalysts (2.14). As well, the jf/jr ratios toward EOR are 1.97, 1.95, 1.83, 1.58, 1.67 and 1.88 for the Pd@PtNi NSs, PdPtNi NPs, PdPt NPs, PtNi NPs, PdNi NPs and commercial Pt black catalysts, respectively. The Pd@PtNi NSs perform better resistance to the carbonaceous species and are easier to remove the COads poisoning as compared with other catalysts. The 13



negative onset and oxidation peak potentials, as well as high peak current intensities of the Pd@PtNi NSs toward MOR and EOR are owing to their branches and rough surfaces providing a large number of accessible active sites. As a comparison, the electrochemical activities of all the samples for MOR and EOR in the acidic medium are also measured. As shown in Figure S6, the oxidation peaks of MOR and EOR for all the catalysts are observed in the CVs. And the current densities of the Pd@PtNi NSs for methanol and ethanol oxidation are 351.2 mA mg-1 and 383.8 mA mg-1, respcetively. Although the current densities of the Pd@PtNi NSs are the highest compared to the other catalysts, their current intensities are poorer than that in the alkaline electrolyte. This may due to the alkaline media accelerate the transfer rates from reactants to the anode, improve the reaction kinetics and reduce the corrosion of the catalysts.47 Therefore, the Pd@PtNi NSs could be considered as the promising catalyst for DAFCs under alkaline conditions. Furthermore, the corresponding mass activity (MA) and specific activity (SA), normalized in reference to the loading amounts of active metals and ECSA for MOR and EOR, are illustrated in Figure 4C and D. The jMA and jSA of the Pd@PtNi NSs for methanol are 1614.3 mA mg-1 and 2.68 mA cm-2, which are higher than that of the PdPtNi NPs (1161.9 mA mg-1, 2.18 mA cm-2), PdPt NPs (948.4 mA mg-1, 2.04 mA cm-2), PtNi NPs (872.7 mA mg-1, 2.02 mA cm-2), PdNi NPs (765.8 mA mg-1, 1.87 mA cm-2) and commercial Pt black catalysts (467.6 mA mg-1, 1.41 mA cm-2). As the same to ethanol (Figure 4D), the jMA and jSA of the Pd@PtNi NSs (1502.3 mA mg-1, 2.52 mA cm-2) are higher when compared to the PdPtNi NPs (1174.8 mA mg-1, 2.12 mA cm-2), PdPt NPs (1099.1 mA mg-1, 2.21 mA cm-2), PtNi NPs (835.2 mA mg-1, 1.81 mA cm-2), PdNi NPs (784.3 mA mg-1, 1.83 mA cm-2) and commercial Pt black catalysts (431.9 mA mg-1, 1.40 mA cm-2). Additionally, the electrochemical performances of the as-synthesized Pd@PtNi NSs are comparable or higher than that of the published literatures (Table 1).35,38,41,48-50 The superior electrocatalytic activities of the Pd@PtNi NSs may due to the large ECSA of the 3D NSs with numerous reaction active sites and the synergistic integration of the promotional effect between Pt, Pd and Ni.51 The addition of the Pd and Ni can modify the Pt d-band center and the electronic properties of the Pt atoms, which is related with the electron transfer.39 In the electrooxidation process, the synergetic effect of Pt, Pd and Ni can generate numerous reaction active sites and accelerate electron transfer that are beneficial to the formation of the reactive oxygen species (OHads) on the surface of the Pd@PtNi NSs. Moreover, the synergetic effect is able to weaken the electronic interaction between the surface of Pt atoms and the intermediate species (such as COads), and facilitate to remove the Pt poisoning substances.52,53 Therefore, 14


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the synergetic effect enhances the electrochemical performance of the Pd@PtNi NSs toward MOR and EOR.

Figure 4. The CVs of the different nanocatalysts modified electrodes for methanol (A) and ethanol (B) oxidation in 1.0 M KOH solution with a scan rate of 50 mV s-1. The mass activity and specific activity of all the nanocatalysts for methanol (C) and ethanol (D) oxidation. Table 1 The comparison of the electrocatalytic performance of different nanocatalysts for MOR a and EOR b.

Materials

Pd@Pt DNC/rGO a b

ECSA

jMA

jSA

(m2 g-1)

(mA mg-1)

(mA cm-2)

114.2

1210.0 a

-

1128.5 b

-

Reference

[48]

PtNiCo nanodendrites a

30.5

1500.0 a

4.90

[38]

Pt85Cu15 nanoalloys b

14.1

810.0 b

5.76

[41]

AgAu@Pt nanoframes a

24.6

483.1 a

1.96

[49]

Pd@Pt3Ni/C b

42.5

5780.0 b

13.61

[35]

Pt NPs/NiFe-LDH/RGO a

24.6

949.3 a

-

[50]

15



Pd@PtNi NSs a b

PdPtNi NPs a b

PdPt NPs a b

PtNi NPs a b

PdNi NPs a b

Pt black a b

59.5

52.7

46.8

43.2

40.9

29.7

1614.3 a

2.68 a

1502.3 b

2.52 b

1167.3 a

2.18 a

1174.8 b

2.12 b

970.5 a

2.04 a

1099.1 b

2.21 b

879.1 a

2.02 a

835.2 b

1.81 b

765.8 a

1.87 a

784.3 b

1.83 b

467.6 a

1.41 a

431.9 b

1.40 b

Page 16 of 25

This work

This work

This work

This work

This work

This work

It is seriously important for the catalysts to have an outstanding stability. The chronoamperometry experiments are applied to evaluate the stabilities of different catalysts for methanol and ethanol electrooxidation reactions in 1 M KOH solution with the fixed potential at 0.9 V (vs. RHE) for 10000 s. Though the current intensities of all the catalysts decrease in the primary stage as displayed in the i-t curves (Figure 5A and B), the Pd@PtNi NSs reduce more slowly when compared with the PdPtNi NPs, PdPt NPs, PtNi NPs, PdNi NPs and commercial Pt black catalysts. The formation of the intermediates, such as CO-like species adsorbed on the surface, may poison the Pt-based nanocatalysts, leading to the decline of the current density. The Pd@PtNi NSs still maintain the highest mass activity after testing10000 s, while the current density of the PdPtNi NPs, PdPt NPs, PtNi NPs, PdNi NPs and commercial Pt black catalysts are nearly to zero (Figure 5A and B). Meanwhile, the current densities of different cycle circles for all nanocatalysts are also performed from 1st to 1000th cycle. The histograms demonstrate that the Pd@PtNi NSs, PdPtNi NPs, PdPt NPs, PtNi NPs, PdNi NPs and commercial Pt black catalysts lose respectively 39%, 53%, 63%, 59%, 75% and 86% in current intensity for MOR at the 1000th cycle (Figure 5C). And Figure 5D shows the results of the EOR after 1000 cycles, and 38%, 62%, 57%, 61%, 69% and 81% mass activity are lost for the Pd@PtNi NSs, PdPtNi NPs, PdPt NPs, PtNi NPs, PdNi NPs and commercial Pt black catalysts, respectively. The TEM images of the Pd@PtNi NSs in Figure S7 display that there is no distinct morphology change after the MOR and EOR stability tests. These results imply the 16


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outstanding electrochemical performances and durability of the Pd@PtNi NSs for MOR and EOR.

Figure 5. The chronoamperometric curves of the catalysts modified electrodes toward MOR (A) and EOR (B) recorded at 0.9 V with the scan rate of 50 mV s-1. The current densities for MOR (C) and EOR (D) at different cycles.

The enlarged ECSA, negatively shifted onset and oxidation peak potential, outstanding electrochemical properties and toleration could be ascribed to the advantages of the 3D urchin-like Pd@PtNi NSs: (i) the 3D nanostructures increase the specific surface area and the contract surface between the catalysts and reactants, as well as the rough surfaces with many branches provide numerous reaction active sites which are favorable to the electrocatalysis. (ii) the special core-shell structures enhance the mutual promotion effect between the Pd cores and the PtNi shells, leading to the improved electrocatalytic performance. (iii) the addition of the Pd and Ni atoms could not only reduce the loading and cost of the Pt-based nanocatalysts, but also modify the electronic structures of the Pt atoms and increase the electrochemical activities and durability of the Pd@PtNi NSs toward MOR and EOR.

4. CONCLUSIONS In this research, a simple and facile method is reported to fabricate the 3D urchin-like Pd@PtNi NSs 17



for MOR and EOR. The probable formation mechanism of the 3D urchin-like Pd@PtNi NSs is also elucidated in detail. The 3D nanostructures possess lots of merits such as the larger ECSA, numerous reaction active sites and the synergistic promotion function between the Pd core and the PtNi shell. The Pd@PtNi NSs modified electrodes are used to catalyze the electrooxidation of the methanol and ethanol, and display outstanding electrochemical properties and remarkable stability as compared with the ternary PdPtNi NPs, corresponding binary metal NPs and commercial Pt black catalysts. The onset potential of the Pd@PtNi NSs negatively shifts and the mass activity as well as specific activity are much higher than that of the other nanocatalysts. After test 10000 s, the 3D urchin-like Pd@PtNi NSs still remain high mass activity for MOR and EOR, while the current densities of other nanocatalysts reduce to nearly zero. The excellent electrochemical performance and outstanding stability are attributed to the particular 3D urchin-like core-shell structures which enhance the electrocatalytic property and the introduction of Pd and Ni that reduces the loading of Pt-based nanocatalysts and the poisoning of carbonaceous species. Thus, the unique 3D urchin-like core-shell nanostructures could open a new avenue for the enhancement of electrocatalytic performance toward DAFCs.

ASSOCIATED CONTENT Supporting Information XPS spectrum and high resolution XPS spectra of the 3D urchin-like Pd@PtNi NSs; The TEM images of the 3D urchin-like Pd@PtNi NSs with different reaction time; The TEM images of the PdPtNi NPs with the effect of P-123 and the corresponding binary metal nanoparticles; The ECSA of the nanocatalysts modified electrodes; The TEM images of the Pd@PtNi NSs after stability tests.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Nos. 51272100).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (Shishan Wu) *E-mail: [email protected]. (Jian Shen)

Notes There are no conflicts to declare.

18


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The Table of Contents Graphic

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Facile Synthesis of Highly Active Three-Dimensional Urchin-Like Pd

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