Composition-Graded PdxNi1âx Nanospheres with Pt Monolayer

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Composition-Graded PdxNi1-x Nanospheres with Pt Monolayer Shells as High-Performance Electrocatalysts for Oxygen Reduction Reaction Liuxuan Luo, Fengjuan Zhu, Renxiu Tian, Lin Li, Shuiyun Shen, Xiaohui Yan, and Junliang Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01775 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017

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Composition-Graded PdxNi1-x Nanospheres with Pt Monolayer Shells as High-Performance Electrocatalysts for Oxygen Reduction Reaction Liuxuan Luo, Fengjuan Zhu, Renxiu Tian, Lin Li, Shuiyun Shen, Xiaohui Yan, and Junliang Zhang* Institute of Fuel Cells, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ABSTRACT: It is undoubtedly desirable, however, very challenging to appropriately balance the catalytic activity, electrochemical durability as well as noble-metal (NM) utilization when developing Pt-based catalysts for oxygen reduction reaction (ORR). Accordingly, in this work, a versatile and effective strategy that promises the nanostructure of both composition-graded core and mono- or multi-layer shell is proposed to synthesize highly uniform, sub-10 nm PdxNi1-x@Pt nanospheres (NSs) as high-performance ORR electrocatalysts. Highly uniform and composition-graded PdxNi1-x NSs are previously obtained via a facile one-pot Ni-substitution-based process, and then Pt mono- or multi-layer shells are coated onto them through Cu underpotential deposition coupled with Pt2+ galvanic displacement. Results show that carbon-supported PdxNi1-x@Pt electrocatalysts possess both high catalytic activity and high-efficient NM utilization toward ORR. The optimized Pd0.42Ni0.58@Pt/C exhibits 0.61 mA cm-2, 0.42 A mg-1Pd+Pt and 1.45 A mg-1Pt @ 0.9 V (vs. RHE) in the area-specific, NMmass-specific and Pt-mass-specific activity, respectively, reaching 2.8, 3.3 and 11.2 times relative to those of the commercial Pt/C. Moreover, Pd0.42Ni0.58@Pt/C also has a satisfactory electrochemical durability, preserving its high ORR catalytic activity even after 12000 potential cycles of the accelerated degradation test. The synthetic mechanism of PdxNi1-x NS core, Pt monolayer shell and their combined effects on the catalytic activity, electrochemical durability and NM utilization of PdxNi1-x@Pt/C toward ORR are comprehensively investigated.

KEYWORDS: composition-graded, Pt monolayer shell, core@shell, nanospheres, electrocatalysts, oxygen reduction reaction, proton exchange membrane fuel cell

INTRODUCTION A burgeoning global demand for clean, efficient and sustainable energy sources makes fuel cells, especially hydrogen-based proton exchange membrane fuel cells (PEMFCs), keep drawing increasing attention as alternatives to fossil-fuel-based power sources, owing to the high energy conversion efficiency, mild operating conditions and outstanding environmental friendliness.1-7 Although greatly developed, the commercialization of PEMFCs are still stagnated by two critical obstacles. One is the sluggish kinetics of cathodic oxygen reduction reaction (ORR) on the commercial Pt/C as well as the unsatisfactory electrochemical durability,4, 8-11 and the other is the high cost of PEMFCs caused by using the expensive (~$ 45 g-1) and scarce (37 p.p.b. in the Earth's crust) Pt materials,12, 13 which accounts for over 55% of the total cost.1, 14 In this regard, growing efforts are being devoted to improve the ORR catalytic activity and electrochemical durability of Pt-based catalysts, and three effective strategies were usually employed.15-21 (1) Alloying Pt with other transition or noble metals can greatly enhance the ORR catalytic activity, due to the “synergetic effects” on Pt imparted by the foreign metals.22-28 (2) In particular, it has been found that shape-controlled Pt-based catalysts can significantly improve the area-specific activity, owing to the desired exposure of certain more active Pt crystal planes toward

ORR.13, 16, 29-35 (3) Developing the core@Pt-shell nanostructures can lead to very high Pt utilization, since ORR only takes place on Pt shells that are deposited on non-Pt metal or alloy cores.21, 36-39 Very recently, shape-controlled Ptbased catalysts with core@Pt-shell nanostructures have been proven to perform remarkable Pt-mass-specific and area-specific activity via combining the strengths of the above-mentioned three strategies.20, 40-47 It is well known that under the operating conditions of PEMFCs, Pt/transition-metal (TM) alloy nanostructures tend to collapse due to the leaching of reactive transition metals in acidic environments, thus resulting in Pt atoms detachment, and fatally destroying the desired Pt crystal planes of shape-controlled Pt/TM alloy catalysts. Alloying Pt with other noble metals, such as Pd, has been demonstrated to be an effective method to avoid TM leaching in highly acidic and electrochemical environments.48, 49 In addition, it is very inspired that the incorporation of Au into Pt-based catalysts can retard the degradation through up-shifting the oxidation potential of Pt,50 and a series of Au-Pt alloy catalysts were therefore designed as highly durable ORR electrocatalysts for PEMFCs.15, 43, 51-54 However, so far, barely Pt/noble-metal (NM) alloy catalysts reported are simultaneously satisfactory in terms of not only the Pt-mass-specific/area-specific activity and electrochemical durability, but also the NM-mass-specific

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activity, which is thoroughly pivotal to reduce the cost of PEMFCs.

used in this work were prepared with ultrapure water (Millipore, > 18.2 MΩ cm).

It is noted that the Pt-monolayer-shell electrocatalysts which deposit monoatomic Pt shells on other NM substrates can demonstrate an ideally high Pt utilization with an extraordinary Pt-mass-specific activity,9, 12, 55-59 and some also exhibit remarkable electrochemical durability.17, 60, 61 However, the NM-mass-specific activity has been rarely investigated, and developing the ORR catalysts with a desired balance in the catalytic activity, electrochemical durability as well as NM utilization remains a great challenge.

Preparation of Carbon-Supported PdxNi1-x NSs. The series of highly uniform PdxNi1-x NSs were synthesized via a facile one-pot Ni-substitution-based process. In a typical synthesis, different amounts of Ni(acac)2 and PdBr2 with designed Ni/Pd atomic ratio were mixed with 10 mL of OAm and 75 uL of TOP in a three-neck flask. The mixture was firstly heated to 245 oC at a rated heating rate, and then kept for 1 h, before cooling down to room temperature. After being washed repeatedly with hexane and ethanol mixture, the as-synthesized PdxNi1-x NSs were well dispersed in chloroform for further deposition onto carbon supports. The deposition was performed by mixing the PdxNi1-x suspension with a given amount of carbon black powders dispersed in chloroform, and the product of carbon-supported PdxNi1-x NSs (PdxNi1-x/C) was washed repeatedly with ethanol and separated by high-speed centrifugation. It is noted that the residual surfactants on the surfaces of PdxNi1-x/C need to be removed with an acetic acid thermal treatment. The as-obtained PdxNi1-x/C was mixed with a given amount of acetic acid in an argon atmosphere, and then heated to 65 oC and kept for 2 h. For reference, carbon-supported Pd NSs (Pd1.00Ni0.00/C) were also prepared using only Pd(acac)2 as precursor. Formation of Pt Mono- or Multi-Layer Shells of PdxNi1-x@Pt NSs. The Pt monolayer shells were deposited onto the surfaces of PdxNi1-x NSs through the Cu underpotential deposition coupled with Pt2+ galvanic displacement.9, 17, 55 Specifically, a given amount of well-dispersed PdxNi1-x/C ink was pipetted onto a rotating disk electrode (RDE, glassy carbon, 5 mm diameter) as the working electrode. After being dried in air, the working electrode was transferred into a N2saturated 0.1 M HClO4 solution, and repeated potential cycling between 0.05 and 1.10 V was performed at a scan rate of 20 mV s-1 to completely eliminate Ni species in the outermost atomic layer of PdxNi1-x NSs as well as the residual organics. After being thoroughly rinsed with ultrapure water, the as-treated working electrode was transferred into an Ar-saturated solution containing 50 mM CuSO4 and 50 mM H2SO4, and stable potential cycling curves of Cu UPD between 0.34 and 0.81 V were obtained at a scan rate of 20 mV s-1. Subsequently, a linear anodic potential sweep to 0.9 V was performed to completely remove Cu atoms on the surfaces of PdxNi1-x NSs resulting from the potential cycling. Then, the Cu monolayer shells were deposited onto the surfaces of PdxNi1-x NSs through a cathodic potential sweeping from 0.81 to 0.34 V. Immediately, the modified electrode was immersed into an Arsaturated solution containing 1.0 mM K2PtCl4 and 50 mM H2SO4, and kept for 2 min to displace Cu with Pt totally. After being rinsed thoroughly with ultrapure water to completely remove PtCl42- from the solution film, the asprepared PdxNi1-x@Pt/C electrocatalysts were finally covered with a 4 uL drop of dilute Nafion solution (0.03%) and dried in air for further electrochemical measurement.

In this work, a versatile and effective strategy that promises the nanostructure of both composition-graded core and mono- or multi-layer shell is proposed to synthesize highly uniform, sub-10 nm PdxNi1-x@Pt nanospheres (NSs) as high-performance ORR electrocatalysts, promising not only high catalytic activity and satisfactory electrochemical durability but also high-efficient NM utilization, when benchmarked against the commercial Pt/C. Firstly, a facile one-pot Ni-substitution-based process was employed to synthesize highly uniform and compositiongraded PdxNi1-x NSs as the cores of PdxNi1-x@Pt NSs. Then, Pt mono- or multi-layer shells were coated onto PdxNi1-x NSs surfaces through the Cu underpotential deposition (UPD) coupled with Pt2+ galvanic displacement.9, 17, 55, 57 The synthetic mechanism for the composition-graded nanostructures of PdxNi1-x NSs was exploited in detail, and their effects combined with Pt monolayer shells on the fine balance among the catalytic activity, electrochemical durability and NM utilization of PdxNi1-x@Pt/C toward ORR were comprehensively investigated. The PdxNi1x@Pt/C electrocatalysts may be also applied in other ORRbased electrocatalytic applications, such as Li-air batteries, since developing highly active ORR catalysts is very crucial for enhancing their discharge performance.62-64 In addition, based on the facile one-pot Ni-substitutionbased process and the Pt2+ galvanic displacement method, through either or both replacing Ni/Pt with other transition/noble metals, this strategy can be readily expanded to synthesize various kinds of electrocatalysts with Pd-TM (TM = Co, Cu, Fe, etc.) NS cores and NM-monolayershells (NM = Au, Ag, Ru, etc.) for different electrocatalytic applications, showing great potential in the synthesis of advanced functional nanomaterials.

EXPERIMENTAL SECTION Chemicals and Materials. Nickel (II) acetylacetonate [Ni(acac)2, 95%], palladium (II) bromide (PdBr2, 99%), potassium tetrachloroplatinate (II) (K2PtCl4, ≥ 99.9%), copper (II) sulfate (CuSO4, ≥ 99%), oleylamine (OAm, 70%), trioctylphosphine (TOP, 90%), perchloric acid (HClO4, 70%), acetic acid (> 99%) and sulfuric acid (H2SO4, 95.0%~97.0%) were purchased from Sigma-Aldrich. Nafion stock solution (Dupont, 20%), commercial Pt/C catalyst (Tanaka Kikinzoku, Pt mass loading: 46.7%) and carbon black powders (AkzoNobel, EC-300J) were used as received. All the aqueous solutions


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For all the electrochemical processes in this work, a saturated calomel electrode (SCE) was used as the reference electrode, while a 1 cm2 platinum foil served as the counter electrode. A salt bridge containing 1.0 M KNO3 was used for the reference electrode, in order to avoid the influence of leaked Cl- on the electrochemical experiments. All the potentials in this paper are referenced to the reversible hydrogen electrode (RHE), and all the measured current densities are normalized to the geometric area of RDE (0.196 cm2).

Characterization. Transmission electron microscopy (TEM) images were acquired on both a JEOL 2100F field emission microscope and a JEOL JEM2010-HT analytical transmission electron microscope. Energy dispersive spectroscopy (EDS) elemental analyses were performed on the JEOL 2100F field emission microscope using an EDAX detector. High-angle annular dark field scanning TEM (HAADF-STEM) images were obtained on a JEOL JEM-ARM 200F spherical aberration correction transmission electron microscope, electron energy loss spectroscopy (EELS) tests and STEM-EDS elemental line-scanning tests were also conducted on this machine using the corresponding detectors. X-ray diffraction (XRD) patterns were recorded on a Bruker D8Advanced diffractometer with Cu Kα radiation (λ = 0.15404 nm) at a scan rate of 2 (°) min-1. Inductively coupled plasma (ICP) elemental analyses were performed using a Thermo iCAP6300 inductively coupled plasmaoptical emission spectrometer (ICP-OES). X-ray photoelectron spectroscopy (XPS) tests were carried out on a Shimadzu Kratos AXIS UltraDLD instrument. Electrochemical Measurements. The electrochemical performance of the series of PdxNi1x@Pt/C was evaluated with a PGSTAT302N Autolab electrochemical workstation (Metrohm) in a three-electrode cell at room temperature. Repeated potential sweeping between 0.05 and 1.10 V in a N2-saturated 0.1 M HClO4 solution was performed at a scan rate of 0.1 V s-1 to activate the PdxNi1-x@Pt/C electrocatalyst. Stable cyclic voltammogram (CV) curve was obtained at a scan rate of 20 mV s-1, and the corresponding ORR polarization curve was then scanned anodically in an O2-saturated 0.1 M HClO4 solution at a scan rate of 10 mV s-1. The electrochemical performance of commercial Pt/C catalyst was also measured for reference in the same way. The electrochemical active surface area (ECSA) was calculated according to the following equation. (1) / Where Q0 is the surface charge resulting from adsorption of hydrogen species, and the value of q0 was experimentally accepted as 210 uC cm-2.65 The ORR catalytic activity, i.e., the kinetic current density (jk), can be determined by the measured current density @ 0.9 V from the ORR polarization curve with a rotation rate of 1600 rpm, using the Koutechy-Levich (K-L) equation. 1 1 1 1 1 (2)

Where j (mA cm-2) is the measured current density, jd (mA cm-2) is the diffusion-limited current density and ω (rpm) is the RDE rotation rate. The parameter B is defined by the following equation.

0.201

(3)

Where n is the electron transfer number, F is the Faraday constant (96485 C mol-1), Co is the concentration of molecular oxygen in the electrolyte (1.26 × 10-3 mol L-1),30, 66, 67 Do is the diffusion coefficient of molecular oxygen in the electrolyte (1.93 × 10-5 cm2 s-1),30, 66, 68 and υ is the kinematic viscosity of the electrolyte (1.009 × 10-2 cm2 s-1).30, 66, 68

The electrochemical durability of PdxNi1-x@Pt/C in highly acidic environments was evaluated by the accelerated degradation test (ADT), through applying potential cycling between 0.7 and 1.0 V at a scan rate of 0.1 V s-1 in a N2-saturated 0.1 M HClO4 solution at room temperature. CV and the corresponding ORR polarization curves were recorded after every 6000 potential cycles.

RESULTS AND DISCUSSION Synthesis of Highly Uniform and CompositionGraded PdxNi1-x NSs with Pt Monolayer Shells. The series of highly uniform and composition-graded PdxNi1-x NSs were synthesized via a facile one-pot Nisubstitution-based process. In the synthetic protocol, OAm serves as the solvent, TOP works as the surfactant to stabilize the NS nanostructure,69 while the use of PdBr2 indispensably leads to the formation of fine spherical morphology.70 As illustrated in Figure 1, preferred nucleation of Ni atoms results from a home-made Ni-TOP complex due to its low decomposition temperature of ~190 o 69, 71 C. It was observed that the reactant color gradually changed from light-green to dark-brown when the reactants were heated from ~190 to 245 oC, indicating a continuous nucleation and growth of NSs. Subsequently, the replacement of part Ni atoms with Pd2+ from PdBr2 will be triggered by the huge difference in redox potentials (-0.25 VSHE for Ni/Ni2+ vs. +0.915 VSHE for Pd/Pd2+) combining with a higher temperature.72 In the meantime, Ni atoms continue to be deposited onto the surfaces of both Ni atoms and Pd atoms via successive decomposition of NiTOP complex. These two processes will not stop until either Ni(acac)2 or PdBr2 is depleted.

Figure 1. Schematic illustration for the synthetic mechanism of PdxNi1-x NS, the difference in the atomic radius between Pd and Ni is neglected.

The above speculation can be further confirmed by four other similar controlled synthetic procedures. When


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Ni(acac)2 was not added, the reactant color kept hardly changed even being heated to the temperature as high as 250 oC and maintained for 2 hours. The use of PdBr2 will not lead to the formation of Pd-TOP complex, which is supposed to decompose into Pd atoms at ~235 oC, as reported in the previous work.69 When further replacing PdBr2 with Pd(acac)2, the nucleation occurred at ~150 oC and led to Pd nanoparticles. By this way, the Pd1.00Ni0.00 NSs with similar size and morphology were successfully synthesized via a similar synthetic procedure as PdxNi1-x NSs (Figure S1, Supporting Information, SI). However, Ni NSs could still be obtained without PdBr2 or Pd(acac)2 in the precursors due to the low decomposition temperature of ~190 oC for the home-made Ni-TOP complex. Moreover, when the holding temperature was fixed at 195 oC in the presence of both PdBr2 and Ni(acac)2, only Ni NSs could be obtained after 30 min reaction (Figure S2, SI), proving the preferred nucleation of Ni atoms and the higher onset temperature of Pd2+ replacement. Therefore, such Ni-substitution-based process results in the unique composition-graded nanostructure, in which Ni content decreases while Pd increases along the radial direction from inner core to outer surface. Furthermore, according to the XPS results for the series of PdxNi1-x/C in Figure S3a (SI), the surface Ni atomic ratio increases with that in the bulk composition, thus, the structural models for the series of PdxNi1-x NSs can be speculated as Figure 2.

Figure 2. Structural models for the series of compositiongraded PdxNi1-x NSs.

Both EDS and ICP-OES elemental analyses were performed to determine the bulk composition of the series of PdxNi1-x NSs, while XPS elemental analyses to the surface Ni/Pd atomic ratio. It is noted that the ICP-OES and XPS analyses were performed on the PdxNi1-x/C after carbonsupporting and acetic acid thermal treatment, while the EDS analysis for the as-synthesized PdxNi1-x NSs. Table 1. Ni/Pd atomic ratios for the series of PdxNi1-x NSs determined by ICP-OES, EDS and XPS, respectively. Ni/Pd

Pd0.69Ni0.31

Pd0.54Ni0.46

Pd0.42Ni0.58

ICP-OES

0.31/0.69

0.46/0.54

0.58/0.42

EDS

0.31/0.69

0.45/0.55

0.54/0.46

XPS

0.19/0.81

0.25/0.75

0.35/0.65

As listed in Table 1, EDS and ICP-OES results prove a negligible deviation in the Ni/Pd atomic ratio, indicating a high uniformity in the bulk composition of all the

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PdxNi1-x samples and that their Ni/Pd atomic ratios hardly change after acetic acid thermal treatment. It is noted that all the surface Ni/Pd atomic ratios determined by XPS are smaller than those from ICP-OES, revealing that the series of PdxNi1-x NSs possess Pd-enriched surfaces, which agrees well with the theoretical speculation on the synthetic mechanism of PdxNi1-x NSs. Besides, as shown in Figure S3b (SI), the Ni/Pd atomic ratio for the series of PdxNi1-x NSs increases almost linearly with that in the initial precursors. Specifically, with the amount of PdBr2 fixed at 0.05 mmol, the addition of 0.0375, 0.05, 0.10 and 0.15 mmol of Ni(acac)2, respectively, led to the formation of Pd0.77Ni0.23, Pd0.69Ni0.31, Pd0.54Ni0.46 and Pd0.42Ni0.58 NSs. It is also noted that the Ni/Pd atomic ratio in the PdxNi1-x/C is smaller than that in the initial precursors, owing to the partial replacement of Ni atoms by Pd2+, further supporting the synthetic mechanism of PdxNi1-x NSs. As displayed in Figure S4 (SI), phosphorus can be clearly observed in the EDS patterns for all the PdxNi1-x samples, indicating that TOP is bonded to the PdxNi1-x NSs, which would reduce the active surface areas for the Cu UPD. Therefore, during the synthetic process, acetic acid thermal treating, repeated ethanol washing and electrochemical potential cycling were performed to remove the undesired TOP residual (Figure S5, SI). The nanostructures of the as-synthesized PdxNi1-x NSs were also analyzed via the STEM-EDS elemental linescanning tests. Figure 3a and b, respectively, show the Pd/Ni elemental distribution of single Pd0.69Ni0.31 and Pd0.42Ni0.58 NSs, along the scanning direction indicated by the yellow arrows in the insets. It is clearly seen that the Pd and Ni signals arise and descend almost simultaneously, which is obviously different from the well-defined core@shell nanostructures which possess clear signalarising and -descending position distances between the core- and shell-element.5, 52 This enables Ni atoms to influence the surface crystal lattice of PdxNi1-x NSs in a way that the well-defined core@shell nanostructures cannot, and such influence will be critical to the ORR catalytic activity of PdxNi1-x@Pt/C electrocatalysts, as proved in the following sections. Moreover, the fitting curve profile for Ni shows a “volcano” shape, indicating that Ni concentrates in the core region. By contrast, the fitting curve profile for Pd presents a “valley” shape in the center, revealing that Pd mostly distributes in the shell.38 The overall elemental signal intensity of Pd is stronger than that of Ni for the Pd0.69Ni0.31 NS, while it is opposite for the Pd0.42Ni0.58 NS, which are consistent with the ICP-OES results. Furthermore, in the surface region of these two NSs, the elemental signal intensity of Pd is always stronger than that of Ni, and the signal intensity difference between Pd and Ni decreases as Ni in the bulk composition increases, confirming the Pd-enriched surfaces of PdxNi1-x NSs and that the surface Ni atomic ratio increases with Ni in the bulk composition, which have been previously proved by the XPS results. The above elemental distribution are consistent with the composition-graded nanostructure of PdxNi1-x NS. For better understanding, schematic illustration for the composition-graded


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nanostructure of PdxNi1-x NS, in which Ni content decreases while Pd increases along the radial direction from

inner core to outer surface, is presented in Figure 3c.

Figure 3. STEM-EDS elemental line-scanning results of the as-synthesized single (a) Pd0.69Ni0.31 and (b) Pd0.42Ni0.58 NSs, the blue and green dash lines are the fitting curves for the scattered signal intensity dots of Pd and Ni, respectively. The insets are the HAADF-STEM images integrated with the corresponding fitting curves of signal intensity dots. (c) Schematic illustration for the composition-graded nanostructure of PdxNi1-x NS.

Figure 4. Representative TEM images and the corresponding histograms of particle diameter distribution (insets) for the assynthesized (a) Pd0.69Ni0.31, (b) Pd0.54Ni0.46, (c) Pd0.42Ni0.58 NSs and (d) Pd0.69Ni0.31/C.


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Figure 4a, b, and c show the typical TEM images of the Pd0.69Ni0.31, Pd0.54Ni0.46 and Pd0.42Ni0.58 NSs, respectively, and it is clearly observed that the as-synthesized PdxNi1-x NSs show well-defined spherical morphologies and are highly uniform. The histograms of particle diameter distribution reveal that the series of PdxNi1-x NSs have narrow particle diameter distribution. The average particle diameters of Pd0.69Ni0.31, Pd0.54Ni0.46 and Pd0.42Ni0.58 NSs are, respectively, 8.46, 8.14 and 7.30 nm, and the corresponding relative standard deviations are only 6.5%, 7.2% and 6.6%. Figure 4d demonstrates the uniform distribution of Pd0.69Ni0.31 NSs on carbon supports, which is in favor of the Cu UPD without the active surface area loss caused by the NSs aggregation. Furthermore, a series of experiments were also conducted to get the optimal synthetic conditions as briefly presented in Figure S6 and S7 (SI).

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distributed in a very thin shell, thus confirming the successful formation of Pt shell on the Pd0.69Ni0.31 NS core. To precisely determine the thickness and uniformity of the Pt shell, an effective analysis that combined elementsensitive EELS with HAADF intensity was employed.12 The background-subtracted EELS signal for the Pd peak around the L3 edge energy of 3173 eV (red, right) and the HAADF intensity (left) were all measured in twodimensional mappings, to determine the sizes and shapes of the Pd0.69Ni0.31 NS core and the entire nanosphere, respectively. Figure 5b shows a nearly 7.75 nm nanosphere with a 7.24 nm Pd0.69Ni0.31 NS core, which are obtained from the intensity cutoffs in the HAADF and EELS intensities, respectively. Combined with the STEM-EDS linescanning result, it can be confirmed that the difference of 0.51 nm in diameter corresponds to the existence of a monolayer thick Pt shell, since the Pt lattice spacing is ~0.24 nm.12

Figure 6. XRD patterns for the series of PdxNi1-x/C, the dash lines represent the diffraction peak positions, respectively.

Figure 5. (a) STEM-EDS elemental line-scanning result for the single Pd0.69Ni0.31@Pt NS, the setup is similar with Figure 3. (b) Its two-dimensional mapping results of the HAADF intensity (left) and the background-subtracted Pd EELS signal around the L3 edge energy of 3173 eV (red, right) with -1 0.44 nm pixel resolution, the scale bars in the insets are 2 nm.

The nanostructure of Pd0.69Ni0.31@Pt NS was also analyzed by the STEM-EDS elemental line-scanning test. As presented in Figure 5a, both the signal intensities and shapes for Pd and Ni show the similar elemental distribution with the as-synthesized Pd0.69Ni0.31 NS before the formation of Pt shell. More importantly, the fitting curve profile for Pt shows a flat “plateau” shape with a much lower intensity across the central region and a “bump” shape in the surface region, indicating that Pt is uniformly

According to the XRD patterns in Figure 6, the series of PdxNi1-x NSs all possess the low-crystalline structure. The broaden diffraction peaks observed in the XRD patterns may be ascribed to the bonding of TOP to PdxNi1-x NSs, which lowers the crystallinity during the growth of NSs.73 As presented in Figure S5b (SI), the HR-TEM image of the as-synthesized PdxNi1-x NSs further confirms their low crystallinity. It is also noted that, with the increase in Ni content of the PdxNi1-x NSs, the diffraction peak shifts slightly toward higher 2θ angle. This can be attributed to the increasing partial substitution of Pd atoms with Ni atoms as indicated in Figure S3a (SI) based on the Vegard’s law, since Ni (1.24 Å) has a smaller atomic radius than Pd (1.39 Å).31, 74 It is believed that such shifting can lead to the increased surface lattice contraction of PdxNi1-x NSs.21, 31, 32 Considering the instability of such nanostructure in highly acidic environments caused by the low crystallinity and the leaching of reactive Ni atoms, PdxNi1-x NSs with more Ni content than Pd0.42Ni0.58 have not been prepared.


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Greatly Enhanced Pd Utilization for the Cu UPD. The Cu monolayer shells can be underpotentially deposited onto the surfaces of only noble metals, including Pd, Au, and Pt.9, 17, 59, 75 The use of composition-graded PdxNi1-x NSs as the substrate will greatly reduce the amount of noble metals for Cu UPD, thus quite critical for improving the eventual NM utilization toward ORR. Figure 7 presents the CV curves for Cu UPD on the surfaces of carbon-supported Pd1.00Ni0.00, Pd0.69Ni0.31, Pd0.54Ni0.46 and Pd0.42Ni0.58 NSs, respectively. It shows that Cu monolayer shells can be deposited and removed from the PdxNi1-x NSs surfaces during the potential cycling, which is positive to the Cu bulk deposition potential (0.32 V) in this system. Differing from the previously reported work,9, 12, 55, 57, 58 no evident sharp peak is observed in the CV curves for Cu UPD. It is possibly ascribed to the low crystallinity of PdxNi1-x NSs, thus weakened the electrochemical characteristic peaks. The amount of deposited Cu atoms can be calculated from the Cu adsorption region in the CV curves as denoted in Figure 7. The inset in Figure 7 shows the normalized amounts of deposited Cu atoms for the series of PdxNi1-x/C with respect to unit mass of Pd (noted as Cu/Pd, which is normalized by that of the Pd1.00Ni0.00/C). It can be seen that with the increase in Ni content, the value of Cu/Pd for PdxNi1-x/C increases accordingly, indicating an enhancement in the Pd utilization for Cu UPD. This greatly enhanced Pd utilization is mainly attributed to the combined effects of the composition-graded nanostructures (Pd-enriched surface & Nienriched core) and the decrease of Pd content as the Ni content increases in the PdxNi1-x NSs.

Evaluation of Electrocatalytic Property toward ORR. The ORR catalytic activity of the series of PdxNi1-x@Pt/C were investigated in O2-saturated 0.1 M HClO4 solutions and compared with that for the commercial Pt/C. Figure 8a, b and c show the comparisons of Tafel plots in terms of the area-specific activity (mA cm-2), NM-mass-specific activity (A mg-1Pd+Pt) and Pt-mass-specific activity (A mg1 Pt), respectively. All the ORR catalytic activity were calculated without iR-correction or background calibration, which are visually presented in Figure 8d by bar charts. As listed in Table 2, the series of PdxNi1-x@Pt/C exhibit significantly enhanced ORR catalytic activity when benchmarked against the commercial Pt/C. Moreover, there exists a distinctive composition-dependence on the Ni In particular, the content in PdxNi1-x@Pt/C. Pd0.42Ni0.58@Pt/C shows the optimal ORR catalytic activity among all the electrocatalysts, and its area-specific activity reaches 0.61 mA cm-2, while 0.42 A mg-1Pd+Pt for the NMmass-specific activity and 1.45 A mg-1Pt for the Pt-massspecific activity, which are approximately 2.8, 3.3 and 11.2 times of those for the commercial Pt/C, respectively. Such distinctive composition-dependent enhancement in area-specific and Pt-mass-specific activity is mainly attributed to the increased surface lattice contraction of Pd substrate as the Ni content increases. As illustrated in Figure 9, the increased surface lattice contraction of Pd substrate can lead to an increased substrate-induced surface strain of the Pt monolayer shells,9, 17, 56, 61 thus resulting in the downshift of the strain-induced d-band center of Pt.36, 56 This downshift of Pt d-band center will further cause a weaker Pt-O bonding, thereby decreasing the intermediate OH-ad species adsorption on the Pt monolayer shells, thus, preserving more active sites for ORR and eventually resulting in the enhancement in area-specific activity.17, 21, 39 The Pt-mass-specific activity directly relates to the area-specific activity because of the Pt-monolayershell nanostructure, which ideally guarantees the involvement of every Pt atom in the ORR catalytic reaction. Besides, there also exists increasingly strong “synergetic effects” as the Ni content increases, which modifies the electronic structures of the Pt monolayer shells. This “synergetic effects” also plays an additional role in improving the area-specific activity, because the strong “synergetic effects” can also lower the energy of Pt d-band center.17, 41, 42, 45 Additionally, for the enhancement in NMmass-specific activity, it can also be ascribed to the greatFigure 7. CV curves of the Cu UPD for the series of PdxNi1ly enhanced Pd utilization for Cu UPD, which leads to x/C. The inset displays the comparison for their normalized even higher NM-mass-specific activity than that of the amounts of underpotentially deposited Cu atoms with rePd3Co@Pt/C in the previously reported work,12 even spect to unit mass of Pd, which are calculated from the Cu though the particle size of the Pd3Co@Pt/C (4.6 nm) in adsorption region. the previous work is much smaller. Table 2. ORR catalytic activity for the series of PdxNi1-x@Pt/C and the commercial Pt/C. Area-specific activity -2 (mA cm )

NM-mass-specific activity -1 (A mg Pd+Pt)

Pt-mass-specific activity -1 (A mg Pt)

Commercial Pt/C

0.22

0.13

0.13

Pd1.00Ni0.00@Pt/C

0.28

0.15

0.75


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Pd0.69Ni0.31@Pt/C

0.38

Pd0.54Ni0.46@Pt/C Pd0.42Ni0.58@Pt/C

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0.21

0.90

0.45

0.27

1.06

0.61

0.42

1.45

Figure 8. ORR catalytic activity comparisons (Tafel plots) among the series of PdxNi1-x@Pt/C and the commercial Pt/C in the (a) area-specific activity, (b) NM-mass-specific activity and (c) Pt-mass-specific activity. (d) Overall ORR catalytic activity comparisons in bar charts.

Figure 9. Schematic illustrations for the Pt monolayer shells ideally deposited on the surfaces of purified Pd1.00Ni0.00, Pd0.69Ni0.31, Pd0.54Ni0.46 and Pd0.42Ni0.58 NSs, respectively.

CV curves for the series of PdxNi1-x@Pt/C and the commercial Pt/C are shown in Figure S8a (SI). Compared with the commercial Pt/C, the onset oxidation potentials for the series of PdxNi1-x@Pt/C in the anodic scans all shift to more positive potentials, and so do the peak potentials of maximum oxides reduction in the cathodic scans, indicating that it is much harder to oxidize Pt atoms of the PdxNi1-x@Pt/C than those of the commercial Pt/C. Moreover, the onset oxidation potential and peak potential of the maximum oxides reduction all shift slightly toward more positive potential, from Pd1.00Ni0.00@Pt/C to

Pd0.42Ni0.58@Pt/C, revealing that the Pt monolayer shells are harder to be oxidized as Ni content increases, which is in well agreement with the above explanation on the increasing enhancement in area-specific activity. Furthermore, as shown in Figure S8b (SI), throughout the entire potential range, the series of PdxNi1-x@Pt/C clearly have lower intermediate OH-ad species coverage than that of the commercial Pt/C, and from Pd1.00Ni0.00@Pt/C to Pd0.42Ni0.58@Pt/C, a decrease in the intermediate OH-ad species coverage can be observed, confirming the above theory. In addition, the ORR catalytic pathway of Pd0.42Ni0.58@Pt/C is confirmed to be a desired fourelectron reaction (Figure S9, SI) and the Pt-monolayershell thickness effect has also been investigated (Figure S10, SI).


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ACS Catalysis

Figure 10. Area-normalized XPS spectra of Pt 4f7/2 for the commercial Pt/C and the series of PdxNi1-x@Pt/C, respectively, which are normalized by the corresponding total Pt 4f photoelectron intensity (area).

The XPS spectrum of Pt 4f7/2 can be utilized to probe the electronic structure of Pt, and evaluate its d-band center shifting.76 Figure 10 shows the Pt 4f7/2 XPS spectra for the commercial Pt/C and the series of PdxNi1-x@Pt/C, which are obtained from the Pt 4f XPS spectra (Figure S11, SI), respectively, and normalized by the corresponding total Pt 4f photoelectron intensity (area).76 A clear upshift of binding energy (BE) in the Pt 4f7/2 of PdxNi1-x@Pt/C can be observed compared with the commercial Pt/C, which is also as a function of Ni composition in the PdxNi1x@Pt/C. The Pt 4f7/2 BEs are 71.10, 71.22, 71.30, 71.45 and 71.57 eV, respectively, for the commercial Pt/C, Pd1.00Ni0.00@Pt/C, Pd0.69Ni0.31@Pt/C, Pd0.54Ni0.46@Pt/C and Pd0.42Ni0.58@Pt/C. The upshift of BE in Pt 4f7/2 implies a downshift in d-band center of Pt.15, 32, 51, 76 This BE upshift trend in Pt 4f7/2 for the series of PdxNi1-x@Pt/C directly confirms the aforementioned downshift trend of Pt dband center as the Ni composition in the PdxNi1-x@Pt/C increases.

Figure 11. Evolution of CV and the corresponding ORR polarization curves for the (a, b) Pd0.42Ni0.58@Pt/C and (c, d) commercial Pt/C after every 6000 potential cycles of the ADT. The electrochemical durability under highly acidic conditions of commercial Pt/C and Pd0.42Ni0.58@Pt/C were both tested utilizing the ADT protocol as described in the Experimental Section. According to Figure 11a and c, after, respectively, 6000 and 12000 potential cycles, the ECSAs for commercial Pt/C remains 91% and 83%; while still 97% and 91% for the Pd0.42Ni0.58@Pt/C. The loss in ECSA for Pd0.42Ni0.58@Pt/C can be mainly attributed to the prefer-

ential dissolution of Ni atoms in the Pd0.42Ni0.58 substrate caused by the harsh electrochemical and highly acidic environments, which can also lead to restructuring and further dissolution of Pt monolayer shell. As shown in Figure 11b and d, the ORR catalytic activity for commercial Pt/C directly decrease to 0.19 mA cm-2 and 0.11 mg-1Pd+Pt/Pt after 6000 potential cycles, and 0.17 mA cm2 and 0.09 mg-1Pd+Pt/Pt after 12000 potential cycles, remain-


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ing only 77% and 69% for the area-specific and Pt-massspecific activity, respectively. By contrast, after the initial 6000 potential cycles of the ADT, the area-specific, NMmass-specific and Pt-mass-specific activity for Pd0.42Ni0.58@Pt/C rise to 0.73 mA cm-2, 0.48 A mg-1Pd+Pt and 1.65 A mg-1Pt, respectively, while decrease to 0.69 mA cm-2, 0.43 A mg-1Pd+Pt and 1.46 A mg-1Pt after 12000 potential cycles. It is noted that, for better comparison, the calculations of mass-specific activity after the ADT are all based on the corresponding initial noble metal loading. The enhancement in ORR activity after the initial 6000 potential cycles may be attributed to the restructuring of Pt monolayer shell due to the slight dissolution of Ni atoms in the substrate. However, after 12000 potential cycles, as shown in Figure 11a, the shape of CV curve changes evidently, and the ECSA also has a relatively evident decline compared with that after the initial 6000 potential cycles, indicating the dissolution of Pt monolayer shells triggered by the further dissolution of Ni atoms in the substrate. The loss of Pt monolayer shell counteracts the effect of its restructuring, thus resulting in the decline in ORR catalytic activity. By contrast, there is no such “volcano” trend for the ORR catalytic activity of commercial Pt/C, due to its solid and single-metal nanostructure. Therefore, compared with the commercial Pt/C, the Pd0.42Ni0.58@Pt/C still preserves its high ORR catalytic activity even after 12000 potential cycles of the ADT, confirming its satisfactory electrochemical durability in highly acidic environments, which can be attributed to the enclosure of Pt monolayer shells which retards the dissolution of Ni atoms in the PdxNi1-x cores.

CONCLUSIONS In summary, a versatile and effective strategy that promises the nanostructure of both composition-graded core and mono- or multi-layer shell is developed to prepare the series of PdxNi1-x@Pt/C electrocatalysts. Firstly, a facile one-pot Ni-substitution-based process is employed to synthesize highly uniform and composition-graded PdxNi1-x NSs. Then, Pt mono- or multi-layer shells are coated onto them through Cu UPD coupled with Pt2+ galvanic displacement. The composition-graded nanostructures of PdxNi1-x NSs result from the preferential nucleation of Ni atoms and following replacement of partial Ni atoms with Pd2+ from PdBr2. Benefiting from the combined effects of composition-graded PdxNi1-x NS core and Pt monolayer shell, the series of PdxNi1-x@Pt/C exhibit both high catalytic activity and high-efficient NM utilization toward ORR. Especially, the most active Pd0.42Ni0.58@Pt/C shows 0.61 mA cm-2, 0.42 A mg-1Pd+Pt and 1.45 A mg-1Pt in the area-specific, NM-mass-specific and Pt-mass-specific activity, respectively, reaching an enhancement of 2.8, 3.3 and 11.2 times when benchmarked against the commercial Pt/C. Besides, Pd0.42Ni0.58@Pt/C also maintained its high ORR catalytic activity under highly acidic conditions even after 12000 potential cycles of the ADT, demonstrating its satisfactory electrochemical durability. Furthermore, the ORR catalytic pathway of Pd0.42Ni0.58@Pt/C is confirmed to be a desired four-

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electron reaction and the Pt-monolayer-shell thickness effect has also been investigated. This strategy has been demonstrated to be an effective approach for developing high-performance ORR electrocatalysts, which can be further expanded to the synthesis of various kinds of nanomaterials for different electrocatalytic applications.

ASSOCIATED CONTENT Supporting Information Figure S1 to S11. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21373135, 21533005 and 21503134), the National Key Research and Development Program of China (2016YFB0101201 and 2016YFB0101312), the Science Foundation of Ministry of Education of China (413064) and the Science and Technology Commission of Shanghai Municipality (14DZ1208103).

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ACS Catalysis

Graphical Table of Contents:


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ACS Catalysis

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Graphic Table of Contents 46x31mm (300 x 300 DPI)


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ACS Catalysis

Figure 1. Schematic illustration for the synthetic mechanism of PdxNi1-x NS, the difference in the atomic radius between Pd and Ni is neglected. 28x9mm (300 x 300 DPI)


ACS Catalysis

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Figure 2. Structural models for the series of composition-graded PdxNi1-x NSs. 34x14mm (300 x 300 DPI)


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ACS Catalysis

Figure 3. STEM-EDS elemental line-scanning results of the as-synthesized single (a) Pd0.69Ni0.31 and (b) Pd0.42Ni0.58 NSs, the blue and green dash lines are the fitting curves for the scattered signal intensity dots of Pd and Ni, respectively. The insets are the HAADF-STEM images integrated with the corresponding fitting curves of signal intensity dots. (c) Schematic illustration for the composition-graded nanostructure of PdxNi1-x NS. 93x58mm (300 x 300 DPI)


ACS Catalysis

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Figure 4. Representative TEM images and the corresponding histograms of particle diameter distribution (insets) for the as-synthesized (a) Pd0.69Ni0.31, (b) Pd0.54Ni0.46, (c) Pd0.42Ni0.58 NSs and (d) Pd0.69Ni0.31/C. 100x67mm (300 x 300 DPI)


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ACS Catalysis

Figure 5. (a) STEM-EDS elemental line-scanning result for the single Pd0.69Ni0.31@Pt NS, the setup is similar with Figure 3. (b) Its two-dimensional mapping results of the HAADF intensity (left) and the background-subtracted Pd EELS signal around the L3 edge energy of 3173 eV (red, right) with 0.44 nm pixel-1 resolution, the scale bars in the insets are 2 nm. 104x137mm (300 x 300 DPI)


ACS Catalysis

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Figure 6. XRD patterns for the series of PdxNi1-x/C, the dash lines represent the diffraction peak positions, respectively. 63x50mm (300 x 300 DPI)


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ACS Catalysis

Figure 7. CV curves of the Cu UPD for the series of PdxNi1-x/C. The inset displays the comparison for their normalized amounts of underpotentially deposited Cu atoms with re-spect to unit mass of Pd, which are calculated from the Cu adsorption region. 59x43mm (300 x 300 DPI)


ACS Catalysis

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Figure 8. ORR catalytic activity comparisons (Tafel plots) among the series of PdxNi1-x@Pt/C and the commercial Pt/C in the (a) area-specific activity, (b) NM-mass-specific activity and (c) Pt-mass-specific activity. (d) Overall ORR catalytic activity compari-sons in bar charts. 113x86mm (300 x 300 DPI)


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ACS Catalysis

Figure 9. Schematic illustrations for the Pt monolayer shells ideally deposited on the surfaces of purified Pd1.00Ni0.00, Pd0.69Ni0.31, Pd0.54Ni0.46 and Pd0.42Ni0.58 NSs, respectively. 27x9mm (300 x 300 DPI)


ACS Catalysis

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Figure 10. Area-normalized XPS spectra of Pt 4f7/2 for the commercial Pt/C and the series of PdxNi1x@Pt/C, respectively, which are normalized by the corresponding total Pt 4f photoelectron intensity (area). 64x52mm (300 x 300 DPI)


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ACS Catalysis

Figure 11. Evolution of CV and the corresponding ORR polarization curves for the (a, b) Pd0.42Ni0.58@Pt/C and (c, d) commercial Pt/C after every 6000 potential cycles of the ADT. 111x82mm (300 x 300 DPI)


Composition-Graded PdxNi1âx Nanospheres with Pt Monolayer

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