Pd Tetragonal Nanosheets Enhance

Jan 10, 2019 - While surface strain engineering in shaped and bimetallic nanostructures offers additional variables for manoeuvring the catalysis, ...
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Fully tensile strained Pd3Pb/Pd tetragonal nanosheets enhance oxygen reduction catalysis Chongyang Tang, Nan Zhang, Yujin Ji, Qi Shao, Youyong Li, Xiangheng Xiao, and Xiaoqing Huang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04921 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Fully tensile strained Pd3Pb/Pd tetragonal nanosheets enhance oxygen reduction catalysis Chongyang Tang1,2#, Nan Zhang2#, Yujin Ji3#, Qi Shao1, Youyong Li3*, Xiangheng Xiao2, Xiaoqing Huang1* C. Tang, N. Zhang, Q. Shao, Prof. X. Huang College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, China. E-mail: [email protected] C. Tang, N. Zhang, Prof. X. Xiao Department of Physics and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Hubei Nuclear Solid Physics Key Laboratory, Wuhan University, Wuhan 430072, China. Y. Ji, Prof. Y. Li Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, China. E-mail: [email protected]

ABSTRACT: While surface strain engineering in shaped and bimetallic nanostructures offers additional variables for manoeuvring the catalysis, manipulating isotropic strain distributions in nanostructures remains great challenge to reach higher tiers of catalyst’s design. Herein, we report an efficient approach to construct a unique class of core/shell palladium-lead (PdPb)/Pd nanosheets (NSs) and nanocubes (NCs) with homogeneous tensile strain along [001] on both the top-Pd and edge-Pd surfaces for boosting oxygen reduction reaction (ORR). These core/shell Pd-Pb/Pd NSs and Pd-Pb/Pd NCs exhibit over 160% and 140% increases in mass activity and over 114% and 98% increases in specific activity when compared with these unshelled counterparts, respectively. Especially, the Pd3Pb/Pd NSs show the ORR mass and specific activities of 0.57 A/mgPd and 1.31 mA/cm2 at 0.90 V versus reversible hydrogen electrode, which are 8.8 (6.5) and 9.4 (9.8) times higher than those of the commercial Pd/C 1 ACS Paragon Plus Environment

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(Pt/C), respectively. The valence band photoemission spectra and first-principles calculations collectively show that the tensile strained Pd shell results in an upshift of the d-band-center of Pd, weakening the chemisorption of oxygenated species due to the contribution of the antibonding orbital. In addition, the Pd3Pb/Pd NSs and NCs with intermetallic core and homogeneous few layers of Pd shell can sustain at least 20000 potential cycles with negligible activity decay and composition changes. The present work provides a new direction for the design of highly active and stable catalysts for fuel cells and beyond. KEYWORDS: Tensile Strain, Palladium, Lead, Nanosheet, Oxygen Reduction Reaction

The sluggish kinetics of oxygen reduction reaction (ORR) at the cathode and the high cost of the indispensable noble metal nanocatalysts are the key challenges for the commercial viability of electrochemical energy-conversion devices and proton exchange membrane fuel cells.1-5 To overcome these limitations, numerous efforts have been devoted to exploring more efficient noble metal catalysts.6-9 Among them, considerable research has been carried out by alloying noble metal with transition metal, which induces electronic charge transfer between different atoms (ligand effect) for enhanced ORR catalysis,10-12 while the limited activity improvement and unsatisfied stability of the conventional alloys necessitate the development of more advanced catalysts with superior ORR performance.13,14 Strain-engineering is an important approach to modulate the surface electronic structure for upgrading the ORR catalysis.15-19 Previous studies have demonstrated that the 5d-band center of platinum (Pt) can be shifted by ~0.1 eV with only 1% lattice strain,20 which can appreciably optimize the adsorption of reaction intermediates. Several other approaches have been also devoted to engineering the surface strain of catalysts, for example, inducing surface strain by forming structural disorder alloy,21 physically stretching the supporting substrate,18,22 as well as selectively removing the alloy atoms.6 However, inhomogeneous strain and ligand 2 ACS Paragon Plus Environment

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effects existing in the conventional catalysts make the determination of the effects on catalysis very complicated. In addition, the strain effect on the metal-oxygen adsorbate binding is highly facet dependent.23 For example, the effect of tensile strain on the Pt (111) facet is not desirable for ORR but favorable on (110) facet because such surface strain on (110) facet can reduce the overlap of the wave functions and downshift the σ* antibonding states near the Fermi level, decreasing binding of the oxygen species to the surfaces during the catalysis.19,24 To this end, advanced methods that can flexibly and effectively control the strain in catalysts without introducing additional effects are highly needed. Constructing core-shell architecture with intermetallic phases and 3d transition metals (Fe, Ni, Co and Cu) can be harnessed to isolate ligand effect appeared in bimetallic catalysts as well as provide the optimized control over the compressive strain to enhance oxygen reduction catalysis.25-28 Although the strain effect has been investigated in the Pt-based electrocatalysts,18,19,24 the control over the strain in Pd-based nanostructures for ORR enhancement have not been explored yet, not to mention the introduction of homogenous strain on boosting Pd-based electrocatalysts. In this work, we report on a new class of Pd3Pb/Pd core/shell nanosheets (NSs) and nanocubes (NCs) with homogenous tensile strain for largely boosting ORR. Rather than using compressive strain to optimize the oxygen adsorption energy, we show that at very high tensile strain, the Pd (100) plane located outside the intermetallic Pd-Pb nanoparticles (NPs) can exhibit superior activity for ORR.29 By integrating the strong tensile strain of Pd3Pb to Pd (100) facet along [001] direction, the NCs and NSs exhibits over 160% and 140% increases in mass activity and over 114% and 98% increases in specific activity when compared with the unshelled counterparts, respectively. In particular, the Pd3Pb/Pd NSs show the ORR mass and specific activities of 0.57 A/mgPd and 1.31 mA/cm2 at 0.90 V versus reversible hydrogen electrode (RHE), which are 8.8 (6.5) and 9.4 (9.8) times higher than those of the commercial Pd/C (Pt/C), respectively. In addition, the Pd3Pb/Pd NCs and NSs show negligible activity 3 ACS Paragon Plus Environment

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decay and no obvious structure/composition changes after 20000 potential cycles, much superior to their unshelled counterparts, as well as the commercial Pt/C and Pd/C. The valence band photoemission spectra and first-principles calculations show that the tensile strained Pd shell results in upshift of the d-band-center of Pd, weakening the chemisorption of oxygenated species due to more contribution of the anti-bonding orbital. The Pd3Pb/Pd NSs were synthesized using palladium (II) acetylacetonate (Pd(acac)2) and lead (II) formate (Pb(HCOO)2) as the metal precursors, oleylamine (OAm)/1-octadecene (ODE) mixture as the solvents and surfactants, glucose as the reducing agent, and ammonium bromide (NH4Br) as the structure-directing agent (see Materials and methods for details). Through tuning the amounts of Pd(acac)2 and Pb(HCOO)2, the intermetallic Pd3Pb NSs and Pd3Pb/Pd NSs can be selectively generated (Figure 1a and Figure S1). The transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADFSTEM) images clearly show the typical two-dimensional (2D) architecture of the Pd3Pb/Pd NSs (Figure 1b and 1c), which is similar with the Pd3Pb NSs (Figure S1). The powder X-ray diffraction (PXRD) pattern of Pd3Pb/Pd NSs displays intermetallic primitive cubic phase (JDPDS No. 65-3266) with no typical peaks indexed to Pd, indicating that the concomitant lattice expansion due to the relatively larger Pb atoms incorporated into the Pd structure (Figure S1d). Meanwhile, the peak positions of Pd3Pb/Pd NSs are shifted to higher angles relative to Pd3Pb NSs, which can be explained that the lattice of Pd3Pb was contracted as the ratio of Pd to Pb increases. X-ray photoelectron spectroscopy (XPS) reveals that Pd0 3d5/2 and Pd0 3d3/2 peaks of Pd3Pb/Pd NSs were centered at 341.13 and 335.82 eV, shifting to higher binding energy with respect to those of Pd/C at 340.93 and 335.67 eV, respectively,30,31 suggesting the modulated surface electronic structure of Pd3Pb/Pd NSs. The surface Pd/Pb ratio of Pd3Pb/Pd NSs was examined to be 76.7/23.3, higher than that of the Pd3Pb NSs (73.7/26.3), indicating their Pd rich surface structures (Figure 1e). The composition of Pd to 4 ACS Paragon Plus Environment

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Pb of Pd3Pd/Pd NSs was 78.6/21.4, which is higher than Pd3Pb NSs (74.4/25.6), as revealed by scanning electron microscopy energy dispersive spectrometer (SEM-EDS), consistent with the inductively coupled plasma atomic emission spectroscopy (ICP-AES) result (Figure 1f and Figure S1c). The line-profile analysis confirms that Pd and Pb concentrate in the core region with Pd-rich surface, being consistent with the TEM-EDS mappings (Figure 1g,h). The high-resolution TEM (HRTEM) was further carried out to characterize the structures of the Pd3Pb/Pd NSs. Figure 2a is an individual NS imaged from the top-down view. The selected area electron diffraction (SAED) along the [001] zone axis of the Pd3Pb/Pd NSs (Figure 2b) shows the single crystalline feature with primitive cubic phase (Pm-3m) (Figure 2c). The absence of superlattice reflections (100) is revealed (Figure 2c and Figure S1), which can be interpreted by the fact that the formation of (100) Pd layers weakens superlattice reflections of interior Pd3Pb.32 Figure 2d is a HRTEM image from the [001] Pd3Pb zone axis (top view). Figure 2e and 2f are HRTEM images taken at higher magnifications from the areas indicated by the yellow rectangles. The Pd and Pd3Pb phases can be identified from their different stacking sequences in the HRTEM images. Image simulation with a multisliced method as well as the projection of atoms was overlapped on Figure 2e and 2f. We can see that there are Pd layers on the surfaces of NSs forming the PdPb (simple cube (sc))/Pd(face centered cube (fcc)) core/shell structure. We speculate that the Pd layer epitaxially grow on Pd3Pb NSs with {100}PdPb//{100}Pd. More importantly, the lattice parameter of the Pd shell is smaller than that of intermetallic Pd3Pb and it is therefore strained tensilely. The selectedarea fast Fourier transform (FFT) analysis, simulative SAED and strain-model (Figures S2-4) further collectively revealed that the Pd layers were fully confined to the Pd3Pb core, resulting in a 5.4% tensile strain along [100] and [010] Pd (Figure S2). The large homogeneous tensile strain in Pd {100} layers is supposed to make a significant impact on the ORR activity of the NSs. 5 ACS Paragon Plus Environment

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Impressively, the prepared Pd3Pb/Pd NCs also exhibit a homogenous tensile strain along [001] on both the top-Pd and edge-Pd surfaces (Figure 3a,b). The composition of Pd to Pb for Pd3Pb/Pd NCs is 78.1/21.9 (Pd/Pb), which is higher than that of Pd3Pb NCs (74.5/25.5), as revealed by SEM-EDS, consistent with the XPS result (Figures S5-7). The XRD pattern of Pd3Pb/Pd NCs shows the pronounced (100) and (110) peaks, characteristic of an intermetallic primitive cubic phase (JDPDS No. 65-3266) (Figure 3c). The line-profile analysis confirms that the Pd and Pb concentrate in the core region with Pd-rich surface (Figure 3d). The HRTEM image was performed to investigate the Pd3Pb/Pd NCs at the atomic scale. As shown in Figure 3e, the edge is assigned to the (100) plane of Pd, while the interior is in consistent with (100) plane of ordered Pd3Pb. The continuous lattice fringes across the interface between the Pd3Pb core and the Pd surface indicate the epitaxial growth of Pd on the Pd3Pb NCs. The FFT patterns taken from regions of f and g, marked as yellow dashed squares (Figure 3e) clearly demonstrate that the f region has a characteristic diffraction pattern of a [001] zone with sc phase, while the g region presents fcc diffraction pattern along the [001] zone due to absence of (100) superlatttice spots, further confirming the core/shell structure. The intensities profiles for Pd3Pb and Pd lattices are shown in Figure 3h, where the lattice spacing changes from 2.05 Å in the core (the green solid rectangle in Figure 1e) to 2.00 Å in edge region (red solid rectangle), resulting in tensile strain along [100] Pd. The elemental distribution of Pd and Pb was characterized using STEM-EDS mappings, where the Pd (green), Pb (red), and the combined images confirmed the presence of Pd-rich surface around the Pd3Pb core (Figure 3i). Combining the XPS, HRTEM, FFT, line-scan and STEM-EDS mapping results together, we can conclude that Pd is formed by a conformal and uniform shell on the surface of Pd3Pb NCs with large tensile strain along both the [001] top-Pd and edge-Pd surfaces. The ORR properties of the Pd3Pb/Pd NSs, Pd3Pb/Pd NCs, Pd3Pb NSs as well as Pd3Pb NCs were investigated and compared with the commercial Pt/C (Johnson Matthey (JM), 20 6 ACS Paragon Plus Environment

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wt% Pt on VulcanXC72R carbon, Pt particle size 2 to 5 nm) and the commercial Pd/C (Sigma-Aldrich, 10 wt% Pd). Before the electrochemical evaluation, all the Pd-Pb NPs were deposited on a commercial carbon (C, Vulcan) via the sonication and washed with acetic acid at room temperature (Figures S8-10, named as Pd3Pb/Pd NSs/C, Pd3Pb/Pd NCs/C, Pd3Pb NSs/C and Pd3Pb NCs/C). Based on the CO stripping (Figure S11), the electrochemical active surface areas (ECSAs) of the Pd3Pb/Pd NSs/C, Pd3Pb/Pd NCs/C, Pd3Pb NSs/C and Pd3Pb NCs/C were calculated to be 43.7, 31.9, 35.8 and 26.2 m2 g-1, respectively. To evaluate the electrocatalytic activities toward ORR, the ORR polarization curves of different Pd-Pb NPs/C, the commercial Pd/C and the commercial Pt/C were measured in O2-saturated 0.1 M KOH solution (Figure 4a). As shown in Figure 4b, the mass activities of Pd3Pb/Pd NSs/C and Pd3Pb/Pd NCs/C are 0.574 and 0.469 A/mgPd at 0.9 V versus RHE, which are 2.7 and 2.4 times than their corresponding unshelled counterparts (0.217 and 0.195 A/mgPd), 6.5 and 5.3 times than the commercial Pt/C (0.088 A/mgPt), and 8.8 and 7.2 times than the commercial Pd/C (0.065 A/mgPd), respectively. The specific activity of Pd3Pb/Pd NSs/C and Pd3Pb/Pd NCs/C could reach 1.31 and 1.47 mA/cm2 at 0.9 V versus RHE, 2.2 and 2.0 times greater than those of their unshelled counterparts (0.61 and 0.744 mA/cm2), 9.8 and 11.1 times than the commercial Pt/C (0.133 mA/cm2), and 9.4 and 10.5 times than the commercial Pd/C (0.140 mA/cm2), respectively, which are among the highest alkaline ORR activities of all the Pdbased catalysts reported to date (Table S1), even better than many Pt-based catalysts (Table S2). In addition to the greatly enhanced specific and mass activities, the Pd3Pb/Pd NSs/C and Pd3Pb/Pd NCs/C also exhibited excellent durability as compared with their unshelled counterparts. The electrochemical durability tests were measured in N2-saturated 0.1 M KOH solution at the potential between 0.6 V and 1.1 V versus RHE by using accelerated durability tests (ADTs) at a sweep rate of 200 mV s-1. Figure 4c-d show the ORR polarization curves of 7 ACS Paragon Plus Environment

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the Pd3Pb/Pd NSs/C and Pd3Pb/Pd NCs/C before and after 20000 sweeping cycles. After 20000 sweeping cycles, the mass activities of Pd3Pb/Pd NSs/C and Pd3Pb/Pd NCs/C dropped by only ~8.1% and 9.2%, respectively, in contrast to those of their unshelled counterparts with loss up to 28.7% and 30.0%, respectively (Figure 4e). The commercial Pt/C and Pd/C showed much larger negative shifts in ORR polarization curves (Figure S12) with 46.6% and 50.8 % mass activity loss, respectively (Figure 4e). In addition, the Pd3Pb/Pd NSs and Pd3Pb/Pd NCs could largely retain their mophologies and composition after long-term ADT test (Figures S8-9). Under the same condition, the commercial Pt/C and Pd/C exhibit large size changes and substantial aggregation (Figures S13-14), clearly showing that the welldefined Pd shell of the Pd3Pb/Pd NSs and Pd3Pb/Pd NCs can deter the loss of interior metal, resulting in the enhanced ORR durability.28,29 To understand the exceptional ORR performance of the Pd-Pb/Pd NPs, we performed the first-principles calculations to investigate the underlying structure-activity relationship of the strained Pd systems. By comparing the ORR processes on the pristine Pd (100), Pd3Pb (100) and Pd3Pb/Pd (100), the concrete adsorption configurations of ORR intermediates (O2, *OOH, *O and *OH) on three surface models are shown in Figure 5a-b. It is found that the height of Pd atoms on Pd3Pb surface were lower than that of Pb atoms, resulting to the ORR intermediates adsorbed on the hollow site of triangle Pb-Pb-Pd motifs while the ORR intermediates bounded on the hollow sites and bridge sites of planar Pd (100) surfaces. The Gibbs free energy change in Figure 5c revealed that the ORR performance on Pd3Pb (100) surface could be superior than that on pristine Pd (100) surface due to a lower overpotential . However, for Pd3Pb/Pd (100) its overpotential was further decreased to 0.71 V, showing that the strain effect plays a vital role on enhancing the performance of Pd-Pb/Pd NPs.

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Nøskov et al. found the classical volcanic relationship of ORR catalysts in which the activity of Pd catalyst is lower than that of Pt catalyst due to a larger oxygen binding energy E(*O).33 As shown in Figure 5d, the tensile lattice strain in the Pd3Pb/Pd systems could weaken the E(*O) and enhance the adsorption of *OH, thus decreasing its ORR overpotential. In contrast, the ORR of Pd3Pb under the compressed strain shows better activity. This ORR enhancement trigged by strain effect originated from the classical d-band theory and the external d-orbital shift on catalytic surface is a determining factor to modulate the adsorption of ORR intermediates. A higher d-band-center (closer to Fermi level) could occupy more antibonding orbital when coupling with the p orbital of oxygen, thus weakening E(*O). The surface valance band spectra analysis of Pd-Pb NPs and commercial Pd/C collected by XPS revealed that the d-band-center of unshelled Pd-Pb has an upward shift compared with Pd/C but less than that of tensile strained Pd-Pb (Figure 5e). Based on the d-band model, introducing the tensile strain effect on the ordered Pd3Pb surface weakens the oxygen binding energy to an optimal value, leading to the enhanced ORR activities. In summary, for the first time we have created a class of Pd3Pb/Pd core/shell NSs and NCs with homogeneous tensile strain along the Pd (100) for boosting ORR. The valence band photoemission spectra and first-principles calculations collectively show that the tensile strained Pd shell results in an upshift of the electronic band structure of Pd and weakens the chemisorption of oxygenated species to an optimal value due to the contribution of the antibonding orbital, and thus lowers the activation barriers for proton- and electron-transfer processes. As a result, these strained Pd3Pb/Pd NSs and Pd3Pb/Pd NCs exhibit over 160% and 140% increase in mass activity and over 114% and 98% increase in specific activity compared with these unshelled counterparts, respectively. Especially, Pd3Pb/Pd NSs show the mass and specific activities of 0.574 A/mgPd and 1.31 mA/cm2 at 0.90 V versus RHE, which are 8.8 (6.5) and 9.4 (9.8) times higher than those of the commercial Pd/C (Pt/C), respectively. 9 ACS Paragon Plus Environment

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The well-defined Pd shell largely deters the loss of interior metal, ensuring the enhanced durability of the Pd3Pb/Pd NSs with negligible activity decay and composition change after at least 20000 potential cycles. The present work opens new door for the design of highly active and stable catalysts for fuel cells and beyond.

ASSOCIATED CONTENT Supporting Information Supporting Information is available from the ACS Publications website or from the author. AUTHOR INFORMATION Corresponding Author [email protected] [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was financially supported by Ministry of Science and Technology (2016YFA0204100, 2017YFA0208200), the National Natural Science Foundation of China (21571135), Young Thousand Talented Program, Jiangsu Province Natural Science Fund for Distinguished Young Scholars (BK20170003), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the start-up supports from Soochow University. C.T., N.Z. and Y. J. contributed equally. REFERENCES (1) Hunt, S. T.; Milina, M.; Alba-Rubio, A. C.; Hendon, C. H.; Dumesic, J. A.; RománLeshkov, Y. Science 2016, 352, 974-978. (2) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; van Schalkwijk, W. Nat. Mater. 2005, 4, 366-377. (3) Long, R.; Zhou ,S.; Wiley, B. J.; Xiong, Y. J. Chem. Soc. Rev. 2014, 43, 6288-6310. (4) Wang, X.; Choi, S.; Roling, L. T.; Luo, M.; Ma C.; Zhang, L.; Chi, M. F.; Liu J. Y.; Xie Z. X; Herron J. A.; Mavrikakis M.; Xia Y. N. Nat. Commun. 2015, 6, 7594. 10 ACS Paragon Plus Environment

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(20) Wang, H. T.; Xu, S. C.; Tsai, C.; Li, Y. Z.; Liu, C.; Zhao, J.; Liu, Y. Y.; Yuan, H. Y.; Abild-Pedersen, F.; Prinz, F. B.; Nørskov, J. K.; Cui, Y. Science 2016, 354, 1031-1036. 21. Gilbert, B.; Huang, F.; Zhang, H.; Waychunas, G. A.; Banfield, J. F. Science 2004, 305, 651-654. (22) Castellanos-Gomez, A.; Roldán, R.; Cappelluti, E.; Buscema, M.; Guinea, F.; Zant, H. S. J.; Steele, G. A. Nano Lett. 2013, 13, 5361-5366. (23) Francis, M. F.; Curtin, W. A. Nat. Commun. 2015, 6, 6261. (24) Luo, M. C.; Guo, S. J. Nat. Rev. Mater. 2017, 2, 201759. (25) Xia, X. H.; Wang, Y.; Ruditskiy, A., Xia, Y. N. Adv. Mater. 2014, 25, 6313-6333. (26) Luo, M. C.; Sun, Y. J.; Wang, L.; Guo, S. J. Adv. Energy Mater. 2017, 7, 1602073. (27) Wang, J. X.; Inada, H.; Wu, L.; Zhu, Y.; Choi, Y.; Liu, P.; Zhou, W. P.; Adzic, R. R. J. Am. Chem. Soc. 2009, 131, 17298-17302. (28) Bu, L. Z.; Guo, S. J.; Zhang, X.; Shen, X.; Su, D.; Lu, G.; Zhu, X.; Yao, J. L.; Guo, J.; Huang, X. Q. Nat. Commun. 2016, 7, 11850. (29) Chen, Z. Z.; Zhang, X.; Lu, G. J. Phys. Chem. C 2017, 121, 1964-1973. (30) Li, M. Q.; Zhang, N.; Long, R.; Ye, W.; Wang, C. M.; Xiong, Y. J. Small 2017, 13, 1604173. (31) Zhang, Z. C.; Liu, Y.; Chen, B.; Gong, Y.; Gu, L.; Fan, Z. X.; Yang, N. L.; Lai, Z. C.; Chen, Y.; Wang, J.; Huang, Y.; Sindoro, M.; Niu, W. X.; Li, B.; Zong, Y.; Yang, Y. H.; Huang, X.; Huo, F. W.; Huang, W.; Zhang, H. Adv. Mater. 2016, 28, 10282-10286. (32) Douglas, A.; Neethling, J. H.; Santamarta, R.; Schryvers, D.; Cornish, L. A. J. Alloys Compd. 2007, 432, 96-102. (33) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L. J. Phys. Chem. B 2004, 108, 17886-17892.

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FIGURES

Figure 1. Morphological and structural characterizations of Pd3Pb/Pd NSs. (a) Schematic illustration of the Pd3Pb NSs and Pd3Pb/Pd NSs. (b) TEM image and (c) HAADF-STEM image of Pd3Pb/Pd NSs. (d) PXRD pattern of Pd3Pb NSs and Pd3Pb/Pd NSs, (e) Pd 3d XPS spectra of Pd3Pb/Pd NSs, Pd3Pb NSs and Pd/C. The inset in (d) shows the PXRD pattern from the selected area in (d). (f) SEM-EDS spectrum, (g) line scan analysis, and (h) HAADFSTEM image and corresponding elemental mappings of Pd3Pb/Pd NSs.

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Figure 2. Structure analysis of Pd3Pb/Pd NSs. (a) A model, (b) TEM image from out-of-plate view, (c) SAED pattern and (d) HRTEM image of one single NS. (e) and (f) are HRTEM images from the selected area in (d). Simulated HRTEM images as well as the atomic models in (e) and (f) are superimposed on the experimental images. (g) The schematic atom models of the NS showing the top interface [(100)Pd//(100)Pd3Pb] and the side interface [(001)Pd//(001)Pd3Pb].

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Figure 3. Morphology and structure characterization of Pd3Pb/Pd NCs. (a) Schematic illustration of the Pd3Pb NCs and Pd3Pb/Pd NCs. (b) STEM image, (c) PXRD pattern, (d) Line scan analysis and (e) HRTEM image of Pd3Pb/Pd NCs. (f) and (g) are FFT images taken from the two yellow dashed squares (areas f and g) in (e). (h) Intensity profiles taken along the atomic layers marked by green and red rectangles in (e). (i) HAADF-STEM image and corresponding elemental mappings of Pd3Pb/Pd NCs.

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Figure 4. Electrocatalytic performances of different Pd-Pb NPs/C, the commercial Pt/C and the commercial Pd/C for ORR. (a) ORR polarization curves and (b) mass and specific activities of different catalysts. The ORR polarization curves were recorded at room temperature in an O2-saturated 0.1 M KOH aqueous solution. ORR polarization curves of (c) the Pd3Pb/Pd NSs/C and (d) the Pd3Pb/Pd NCs/C before and after different potential cycles between 0.6 and 1.1 V versus RHE. (e) The changes on mass activities of the different Pd-Pb NPs/C, the commercial Pt/C and the commercial Pd/C before and after different potential cycles.

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Figure 5. Reaction pathway and valence electron characterization. Top view and side view of the adsorption configurations for ORR intermediates (O2, *OOH, *O and *OH) on (a) Pd3Pb/Pb (100) and (b) Pd3Pb (100) surfaces. (c) The Gibbs free energy evolution along 4ereaction processes without considering the effect of pH. (d) The relationship of overpotential and lattice strain on Pd3Pb/Pb (100) and Pd3Pb (100) surfaces. (e) Surface valence band photoemission spectra of different Pd-Pb NPs/C and the commercial Pd/C.

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