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High-Performance Pd3Pb Intermetallic Catalyst for Electrochemical Oxygen Reduction Zhiming Cui, Hao Chen, Mengtian Zhao, and Francis J. DiSalvo Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b00121 • Publication Date (Web): 05 Feb 2016 Downloaded from http://pubs.acs.org on February 8, 2016

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High-Performance Pd3Pb Intermetallic Catalyst for Electrochemical Oxygen Reduction Zhiming Cui, Hao Chen, Mengtian Zhao and Francis J DiSalvo*. Cornell University, Department of Chemistry and Chemical Biology, Baker Laboratory, Ithaca, NY 14853-1301, USA

KEYWORDS: Oxygen Reduction Reaction; Intermetallic; Palladium; Lead; Metal-air battery

ABSTRACT: Extensive efforts to develop highly active and strongly durable electrocatalyst for oxygen reduction are motivated by a need for metal-air batteries and fuel cells. Here, we report a very promising catalyst prototype of structurally ordered Pdbased alloys, Pd3Pb intermetallic compound. Such structurally ordered Pd3Pb/C exhibits significant increase in mass activity. More importantly, ordered Pd3Pb/C is highly durable and achieves higher cell efficiency and several times longer cycle life than conventional Pt/C catalysts in Zn-air batteries. Interestingly, ordered Pd3Pb/C possesses very high methanol tolerance during electrochemical oxygen reduction, which make it an excellent methanol-tolerant cathode catalyst for alkaline polymer electrolyte membrane 1 ACS Paragon Plus Environment

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fuel cells. This study provides a promising route to optimize the synthesis of ordered Pdbased intermetallic catalysts for fuel cells and metal-air batteries.

Increasing energy demands have stimulated intense research on alternative energy conversion and storage systems, which can provide high efficiency, low cost and environmental benignity.1,2 The oxygen reduction reaction (ORR) plays a critical role in many energy conversion and storage technologies including metal–air batteries, fuel cells and electrolysers.3-5 The sluggish kinetics of the ORR at the cathode has been the major limiting factor for cost, activity and stability of such devices.4,6 Pt and Pt-based alloys remain the most efficient ORR catalysts in both acidic electrolytes and alkaline electrolytes. However, high cost and declining activity during operation have made such alloys increasingly unattractive.7 To substitute the relatively active but ultimately expensive Pt and Pt-based alloy catalysts, low cost alternatives such as N-doped carbon materials, 8-11 nonprecious metals (Fe, Co, etc), metal oxides (MnO2, Fe3O4, Co3O4, IrO2, etc),4,12-16 as well as various perovskites and spinels6 have been explored; each has initially looked promising but none have advanced far enough to replace Pt alloys in systems under development. Pd and its alloys have emerged as strong candidates for replacing Pt owing to their high catalytic activity and durability, especially in alkaline media.17,18 Many effective approaches are reported to increase catalytic activity and stability of such Pd-based catalysts, including controlling and tuning particle size and morphology,19 the use of various support materials,18,20 and the combination of Pd with other metals to form bimetallic catalysts.17,21,22 In the latter case, alloying Pd with transition metals is the most frequently used strategy. However, in these alloys, the

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atomic positions are occupied randomly by Pd and the secondary metal, so that they have varying surface composition and randomly distributed active sites. Apparently, such the alloys do not provide predictable control over structure, geometric, and electronic effects. Unlike disordered alloys, in an ordered intermetallic compound each crystallographic site is occupied by a specific type of atom, resulting in compositional and positional order, thus uniform active sites on the same surface plane.23,24 In such compounds, strong bonding between Pd and the second transition metal provides long-term stability and avoids deactivation.25 Ordered intermetallic compounds such as Pd2Ga and PdGa have been proven to show superior catalytic selectivity and stability over the commercial Pdbased catalysts for the selective semihydrogenation of acetylene.26 However, so far the reports on ordered Pd-based intermetallic catalysts are still very limited. The lack of studies on the ordered phases is largely due to the concerns about particle sintering during the high temperature annealing that is generally necessary to obtain ordered phase.

27,28

The synthesis of ordered phase with controlled small particle sizes remain a great challenge. Generally, room temperature preparation leads to alloyed or even amorphous products, whereas annealing at elevated temperatures can lead to ordered structures. This raises two general but important questions: what annealing temperature is suitable for ordering and what factors control the ordering kinetics? To address the above issue, it is necessary to explore the factors that control the ordering and particle size, and then develop a suitable synthetic route. Herein we report a successful synthesis of ordered Pd3Pb intermetallic phase with small particle size. The as-synthesized ordered Pd3Pb/C catalyst exhibits significant enhancement in catalytic activity and durability for the ORR when compared to disordered Pd3Pb/C, Pd/C and Pt/C. Ordered Pd3Pb/C achieve higher 3 ACS Paragon Plus Environment

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cell efficiency and several times longer cycle life than those of conventional Pt/C catalysts in Zn-air batteries. More interestingly, ordered Pd3Pb/C also shows excellent catalytic performance for the ORR in the present of methanol, suggesting that it can act as a promising methanol-tolerance cathode catalyst. Structurally ordered Pd3Pb intermetallic nanoparticles can be obtained by a modified impregnation-reduction approach including the mixture of Pd and Pb precursors with high-surface-area carbon black in THF, coreduction of Pd and Pd precursors and heat treatment at high temperatures. Potassium triethylborohydride (KEt3BH) and lithium triethylborohydride (LiEt3BH) are used as reducing agents because they both have fast reduction kinetics and can address the co-reduction issue of Pd and Pb precursors. The formation of Pd3Pb nanoparticles in a THF solution is shown in the following equations: 3 + + 8 = + 8 + 4 + 8

(Equation 1)

3 + + 8 = + 8 + 4 + 8

(Equation 2)

For the two reactions above, either KCl or LiCl is produced as a byproduct. Since KCl is not soluble in THF,29 reduction using the KEt3BH results in an insoluble KCl matrix that can trap as-produced Pd3Pb particles, preventing their agglomeration during synthesis as well as during annealing. The experimental details are presented in Supporting Information (SI). A schematic diagram for the preparation of ordered Pd3Pb/C is presented in Figure S1 (SI). LiCl, unlike KCl, is soluble in THF and is completely removed by the washing process; therefore, for the products extensive sintering of the Pd3Pb nanoparticles occurs upon annealing. Figure 1a and 1b show the images of Pd3Pb/C samples synthesized with KEt3BH and LiEt3BH, respectively. These two samples are annealed at 400 oC for 12h. It is easily observed that the particle sizes of 4 ACS Paragon Plus Environment

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Pd3Pb prepared with KEt3BH (5.2 ±0.4 nm) are much smaller than those of Pd3Pb prepared with LiEt3BH (7.4±0.6 nm), suggesting KEt3BH is more suitable than LiEt3BH for the synthesis of ordered Pd3Pb with small particle size. We further investigate the effect of annealing conditions (annealing temperature and annealing time) on the ordering and particle sizes of Pd3Pb phases. One of the issues of using hydride as reducing agent is the potential formation of the stable hydride phase during the initial reduction reaction, PdH0.5, which may impede the formation of homogeneous alloy or ordered intermetallic phases. 12. + 4 = 4 + 3

(Equation 3)

Assuming PdH0.5 does form during reduction, the enthalpy of PdH0.5 and Pd3Pb can then be used to estimate the thermodynamic transition temperature between the hydride and the alloy (The detailed calculation is given in page 5 of SI). As a result, ∆G for the above reaction is always negative regardless of temperature. Therefore, the reaction to form Pd3Pb intermetallic phases can take place spontaneously without annealing. The rate of formation of the intermetallic phase from an alloy is dominated by solid state diffusion which increases exponentially with annealing temperature. Figure 2a shows XRD patterns of the samples annealed for 12 h at 200 oC, 400 oC, 600 oC and 800 oC, respectively. When the annealing temperature exceeds 400

o

C, there are visible

superlattice peaks (so-called “ordering peaks”), indicating that the intermetallic phases are formed. With increasing temperature or annealing time, the ordering peaks become visible and sharper. For the samples annealed at 600 oC and 800 oC, six additional peaks are assigned to the 110, 110, 210, 211, 300, and 310 ordering reflections of Pd3Pb in the Cu3Au structure-type. The domain sizes for these four Pd3Pb samples are 3.8 nm, 4.7 nm,


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6.9 nm 12.2 nm, respectively, as calculated from the diffraction peaks widths by the Debye-Scherrer equation. Obviously, a high annealing temperature favors the formation of the ordered phase, but at the same time annealing promotes the sintering of Pd3Pb nanoparticles. The agglomeration of Pd3Pb nanoparticles is accelerated at 800 oC due to the melting of the KCl matrix. Therefore, we choose a lower annealing temperature of 600 °C to study the effect of annealing time on ordering. As shown in Figure 2b, the ordering peaks begin to appear after 3h, and as expected, the intensity of the ordering peaks increase with time. The degree of ordering can be roughly gauged by comparing the intensity ratio of (110) peak to (111) peak. The ratios of the (110) peak to the (111) peak are 0.018, 0.045, 0.057 and 0.061 for the samples annealed for 3h, 6h, 12h and 24h, respectively. The ratio for the samples annealed for 24h is very close to the ratio of 0.063 from the reference bulk X-ray patterns (PDF# 03-065-3259), suggesting that these nanoparticles are fully ordered (within error). In addition, as shown in Figure S2 (SI), at a fixed annealing temperature of 600 oC, the domain size of Pd3Pb increases with increasing annealing time, but the growth rates gradually decrease. In other words, the domain size of Pd3Pb particles doesn’t increase significantly with increasing annealing time. Therefore, it is a good strategy to synthesize the ordered phase while maintaining small particle size by increasing annealing time at relatively lower temperature. There are some other methods such as carbothermal reduction which could generate Pt-based or Pdbased ordered phases.30,31 Most of the previous reported methods only apply to some late 3d-transition metals due to relatively slow reducing kinetics of carbon or hydrogen. The Pd3Pb/C sample annealed at 600 oC for 24 h is denoted as ordered Pd3Pb/C, and the sample without annealing is denoted as disordered Pd3Pb/C. The XRD patterns for 6 ACS Paragon Plus Environment

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ordered Pd3Pb/C and disordered Pd3Pb/C are shown in Figure 3a, along with the diffraction pattern of the Pd/C catalyst as a comparison. For Pd/C, the Pd particles crystallize in space group Fm-3m with refined lattice parameter a = 3.891Å. In the case of disordered Pd3Pb/C, its XRD patterns clearly show the five main characteristic peaks of the face-centered cubic (FCC) crystalline Pd, demonstrating that this is the disordered alloy structure (solid solution). The XRD peaks of disordered Pd3Pb/C alloys shift to lower angles relative to Pd/C, which is due to the increase of the lattice constant upon alloying Pd with the larger Pb. Ordered Pd3Pb/C crystallizes in a cubic structure and is consistent with space group Pm-3m with a refined lattice parameter, a = 4.035 Å. Figure 3b shows an overview TEM image of ordered Pd3Pb/C catalyst and its corresponding histogram of metal nanoparticle diameters (insert). Pd3Pb particles are highly dispersed on the surface of the carbon support. The histogram of metal nanoparticle diameters was obtained from a sampling size of 100 particles in random regions. It shows a relatively narrow particle size distribution, with an average size of 7.2±0.5 nm. The TEM image of disordered Pd3Pb/C, Pd/C and Pt/C, as well as their corresponding histograms of particle sizes are presented in Figure S3 (SI). The average particle sizes for disordered Pd3Pb/C, Pd/C and Pt/C are 4.6±0.5, 6.3±0.4 nm and 5.1±0.3 nm, respectively. To visualize better the ordered intermetallic structure, we have also investigated the atomic level arrangement of Pd and Pb by high-resolution TEM (HRTEM). Figure 3c shows clear lattice fringes (0.232 nm), corresponding to the (111) plane of cubic phase Pd3Pb. Ordered arrangements can be directly obtained from the diffraction patterns by an electron diffraction technique, diffraction STEM (D-STEM), which allows 1-2 nm spatial resolution electron diffraction.32 Figure 3d shows the diffraction patterns


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on the [001] zone axis collected from the inset particle, which align with the simulated intermetallic phase (Figure S4, SI). The presence of superlattice reflections such as (110), (210) confirms the formation of ordered Pd3Pb intermetallic structure. STEM-EDS line spectra of Pd3Pb/C in Figure S5 (SI) suggest that Pd and Pb are uniformly distributed in the nanoparticles. As shown in Figure S6 (SI), the atomic ratio of Pd/Pb is close to 3, which is consistent with ratio of reactant precursors. The actual metal loadings on the carbon support was determined by TGA are 37.4wt.% for ordered Pd3Pb/C, 33.2 wt.% for disordered Pd3Pb/C and 20.6wt.% for Pd/C, respectively. Figure 4a shows polarization curves for ordered Pd3Pb/C, disordered Pd3Pb/C, Pd/C and Pt/C catalysts in 0.1 M KOH saturated with oxygen, using a rotating disk electrode (RDE). The onset potential of ORR on ordered Pd3Pb/C catalysts is ~1.03 V, which is more positive than those on disordered Pd3Pb/C, Pd/C and Pt/C. This indicates that oxygen is more easily reduced on ordered Pd3Pb/C catalysts. In the mixed kinetic/diffusion regime (0.7 V to 1 V), the half-wave potentials of the ORR polarization is often used to evaluate the electrocatalytic activity of a catalyst. The half–wave potential follows the order: Pd/C = Pt/C (0.88V) < disordered Pd3Pb/C (0.90 V) < ordered Pd3Pb/C (0.92 V). The ordered Pd3Pb/C catalyst shows a significant 20-40 mV shift to more positive potentials relative to the other three catalysts. The kinetic current (jk) is calculated using Koutecky-Levich equation (details can be found in experimental section). As seen in Figure 4b, ordered Pd3Pb/C exhibits much higher mass activity than disordered Pd3Pb/C, Pd/C and Pt/C. For instance, ordered Pd3Pb/C produces currents as high as 168.9 mA·mg-1Pd at 0.9 V, which is ~3.8 time higher than that of as-prepared Pt/C sample. The enhanced activity from the ordered intermetallic Pd3Pb may arise from the 8 ACS Paragon Plus Environment

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ordering of active sites and changes in the Pd−Pd bond distance as well as modification of the electron configuration. Figure 4c and Figure 4d show the rotation-rate-dependent current–potential curves of ordered Pd3Pb/C and disordered Pd3Pb/C and their corresponding Koutecky–Levich plots at different potentials. Similar curves for disordered Pd/C and Pt/C are given in Figure S7 (SI). The electron transfer numbers (n) are calculated to be ~4 for both ordered Pd3Pb/C and disordered Pd3Pb/C at 0.6V-0.8 V from the slops of Koutecky-Levich plots, suggesting that Pd3Pb alloy catalysts favors a 4e- oxygen reduction process. To verify the ORR catalytic pathways of ordered Pd3Pb/C catalysts, rotating ring-disk electrode (RRDE) measurements were performed to monitor the formation of peroxide species. The measured HO2- yields are below 3% over the potential range of 0.4-0.9V (vs. RHE), corresponding to a high electron transfer number of >3.94 (Figure S8, SI). This is consistent with the result obtained from the KouteckyLevich plots based on RDE measurements. The stability of the catalysts was assessed by applying potential between 0.6 and 1.0 V in O2-saturated 0.1 M KOH at 100 mV s-1. Figure S9 (SI) shows ORR polarization curves in O2-saturated 0.1 M KOH after stability tests of 5000 cycles. Ordered Pd3Pb/C showed a degradation of only 10 mV in its half-wave potential, which is much lower than disordered Pd3Pb/C (15 mV), Pd/C (30 mV) and Pt/C (40 mV), indicating the enhanced durability of ordered Pd3Pb/C intermetallic catalysts. The mass activities of the catalysts before and after the stability test are depicted in Figure 5a. Ordered Pd3Pb/C is more stable under ORR conditions than the other three catalysts. After 5000 cycles, the currents on ordered Pd3Pb/C are 140.4 mA·mg-1Pd at 0.9 V, which is reduced by 16.8% from the start of the cycling. In contrast, a larger activity loss is observed in the other


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three catalysts (28.9 % for disordered Pd3Pb/C, 27.8% for Pd/C and 25.7% for Pt/C). Excellent stability of ordered Pd3Pb/C is also proved by chronoamperometry method as shown in Figure 5b. Continuous oxygen reduction (12 hours) at 0.8V (vs. RHE) on ordered Pd3Pb/C, disordered Pd3Pb/C, Pd/C and Pt/C electrode cause 14.1%, 20.5%, 22.6% and 24.7% decrease in current density, respectively. Although the ordered Pd3Pb/C is more durable than other catalysts, it undergoes obvious performance degradation, which is mainly due to the corrosion of carbon support. Carbon materials are thermodynamically unstable at relatively high potentials due to its low equilibrium potential of 0.207 V (vs. RHE).33,34 The performance of ordered Pd3Pb/C catalyst is also evaluated in Zn-air batteries with NiCo2O4 grown onto a Ni foam (NCONF@Ni) serving as the catalyst for the oxygen evolution reaction (OER). The characterization of NCONF@Ni is presented in Figure S10 (SI) and the battery configuration is shown in Figure S11 (SI). For comparison, a conventional Pt/C catalyst is also tested under the same conditions. Figure 6a and 6b show the total 135 cycles voltage profiles as well as the enlarged 1st and 135th cycle voltage profiles. Since the discharge and charge voltage profiles of Zn-air batteries are almost flat, we represent the activity of the catalysts by the cell voltage at the end of discharge and charge of each cycle. The voltaic efficiency is calculated based on the discharge end voltage divided by charge end voltage. The initial round-trip overpotential is 0.72V, contributing to a voltaic efficiency of 64.7 %. After 135 cycles, the round-trip overpotential increased to 0.86 V, leading to a residue voltaic efficiency of 58.8%. This is among the best cycle performances reported for Zn-air batteries with alkaline catholyte so far (see a comparison of state-of-the-art Zn-air batteries in Table S1). As a comparison,


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the cycling performance of Pt/C enabled Zn-air batteries is shown in Figure 6c and 6d. The round-trip overpotential increases from 0.77 V at the 1st cycle to 1.34 V at the 37th cycle, while the voltaic efficiency decreased dramatically from 62.5% at the 1st cycle to 35.7 % at the 37th cycle. Apparently, structurally ordered Pd3Pb nanoparticles possess much higher durability than Pt nanoparticles. When normalized to the mass of consumed Zn, the battery using ordered Pd3Pb catalyst exhibits a specific capacity of 574 mAh gZn-1 (corresponding to an energy density of ~710 Wh KgZn-1) at a current density of 10 mA cm-2. Since Pd is also used in methanol fuel cells, we also examined the ORR behavior of Pd based catalysts when some methanol is present in the electrolyte. Methanol crossover from anode to cathode often results in reducing the open-circuit potential and poisoning the electrocatalysts at the cathode.35,36 Recent studies have indicated Pd-based cathode catalyst such as Pd3Co possess higher methanol tolerance than Pt and Pt-based alloys.17,37-41 But these methanol-tolerant cathode catalyst are mainly applied in acid media with very few reports for the application in alkaline media. Here, ordered Pd3Pb catalysts are evaluated as methanol-tolerant cathode catalyst for alkaline fuel cells. Figure 7a shows ORR polarization curves on ordered Pd3Pb/C and Pt/C in O2-saturated 0.1 M KOH+0.5 M CH3OH. As compared to the ORR in 0.1 M KOH, both ordered Pd3Pb/C and Pt/C show significant increase in overpotential in the presence of methanol due to the competitive reactions between oxygen reduction and methanol oxidation. Apparently, the anodic peaks at 1.01V for ordered Pd3Pb/C and the 0.99 V peak for Pt/C are mainly due to the electrooxidation of methanol. The peak current density on ordered Pd3Pb/C is one tenth that on Pt/C, suggesting that ordered Pd3Pb/C has a very high methanol tolerance.


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We also evaluate and compare the methanol tolerance of ordered Pd3Pb/C and Pt/C by a chronoamperometric method at 0.8V in O2-saturated 0.1 M KOH+0.5M CH3OH at a rotation rate of 1600 rpm. As shown in Figure 7b, after 2h, the current density on ordered Pd3Pb/C is reduced by ~31%, which is significantly less than that on Pt/C (~90%). Both ORR polarization curves and chronoamperometric curves indicate that ordered Pd3Pb/C is a promising methanol-tolerant cathode catalyst for alkaline polymer electrolyte membrane fuel cells. In conclusion, we demonstrated an innovative and effective method for the synthesis of structurally ordered Pd3Pb with controlled particle sizes. Elevated temperatures and increasing annealing time can accelerate the formation of ordered phases. HRTEM and XRD show the prepared Pd3Pb catalyst are crystalline and fully ordered. Ordered Pd3Pb/C catalysts exhibit exhibits ~2-4 time increase in mass activity relative to disordered Pd3Pb/C, Pd/C and Pt/C. Moreover, ordered Pd3Pb/C catalysts are extremely stable, enabling a Zn-air battery to exhibit excellent long-term cycling performance (over 560 h with only a 0.14 V increase in round-trip overpotential). More interestingly, ordered Pd3Pb/C is found to be a promising methanol-tolerant cathode catalyst for alkaline polymer electrolyte membrane fuel cells. The extraordinarily high performance of ordered Pd3Pb/C shows that Pd-based intermetallic compounds are worthy of further study, and the synthetic strategy reported here offers new opportunities for preparing durable and active Pd-based electrocatalysts for metal-air batteries and fuel cells.

ASSOCIATED CONTENT


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Supporting Information. The synthesis of Pd3Pb/C catalyst; TEM images, EDX, STEM-EDX line scan, and electrochemical test results; Zn-air batteries test. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Tel.: +1 607 255 7328. Fax: +1 607 255 4137. E-mail: [email protected] (Francis J. DiSalvo). Notes The authors declare no competing financial interest ACKNOWLEDGMENT This work was supported by the Energy Materials Center at Cornell, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DE-SC0001086. We thank Dr. Longjun Li for assistance with Zn-air batteries test and analysis.

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18. Lu, Y.; Jiang, Y.; Gao, X.; Wang, X.; Chen, W. J. Am. Chem. Soc. 2014, 136, 1168711697. 19. Poon, K. C.; Tan, D. C. L.; Vo, T. D. T.; Khezri, B.; Su, H.; Webster, R. D.; Sato, H. J. Am. Chem. Soc. 2014, 136, 5217-5220. 20. Sharma, C. S.; Awasthi, R.; Singh, R. N.; Sinha, A. S. K. Phys. Chem. Chem. Phys. 2013, 15, 20333-20344. 21. Slanac, D. A.; Hardin, W. G.; Johnston, K. P.; Stevenson, K. J. J. Am. Chem. Soc. 2012, 134, 9812-9819. 22. Altamirano-Gutierrez, A.; Fernandez, A. M.; Varela, F. J. R. J. Hydrogen Energy 2013, 38, 12657-12666. 23. Casado-Rivera, E.; Volpe, D. J.; Alden, L.; Lind, C.; Downie, C.; Vázquez-Alvarez, T.; Angelo, A. C. D.; DiSalvo, F. J.; Abruña, H. D. J. Am. Chem. Soc. 2004, 126, 40434049. 24. Alden, L. R.; Han, D. K.; Matsumoto, F.; Abruna, H. D.; DiSalvo, F. J. Chem. Mater. 2006, 18, 5591-5596. 25. Wencka, M.; Hahne, M.; Kocjan, A.; Vrtnik, S.; Koželj, P.; Korže, D.; Jagličić, Z.; Sorić, M.; Popčević, P.; Ivkov, J.; Smontara, A.; Gille, P.; Jurga, S.; Tomeš, P.; Paschen, S.; Ormeci, A.; Armbrüster, M.; Grin, Y.; Dolinšek, J. Intermetallics 2014, 55, 56-65. 26. Armbrüster, M.; Kovnir, K.; Behrens, M.; Teschner, D.; Grin, Y.; Schlögl, R. J. Am. Chem. Soc. 2010, 132, 14745-14747. 27. Chen, H.; Yu, Y. C.; Xin, H. L. L.; Newton, K. A.; Holtz, M. E.; Wang, D. L.; Muller, D. A.; Abruna, H. D.; DiSalvo, F. J. . Chem. Mater. 2013, 25, 1436-1442.


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28. Chen, H.; Wang, D.; Yu, Y.; Newton, K. A.; Muller, D. A.; Abruña, H.; DiSalvo, F. J. J. Am. Chem. Soc. 2012, 134, 18453-18459. 29. Strong, J.; Tuttle, T. R. J. Phys. Chem. 1973, 77, 533-539. 30. Stassi, A.; Gatto, I.; Monforte, G.; Baglio, V.; Passalacqua, E.; Antonucci, V.; Arico, A. S. J. Power Sources 2012, 208, 35-45. 31. Kuttiyiel, K. A.; Sasaki, K.; Su, D.; Wu, L. J.; Zhu, Y. M.; Adzic, R. R. Nat. Commun. 2014, 10.1038/ncomms6185. 32. Ganesh, K.; Kawasaki, M.; Zhou, J.; Ferreira, P. Microsc. Microanal. 2009, 15, 752753. 33. Wang, Y. J.; Wilkinson, D. P.; Zhang, J. J. Chem. Rev. 2011, 111, 7625-7651. 34. Parrondo, J.; Han, T.; Niangar, E.; Wang, C.; Dale, N.; Adjemian, K.; Ramani, V. P. Natl. Acad. Sci., 2014, 111, 45-50. 35. Cui, Z. M.; Li, N. W.; Zhou, X. C.; Liu, C. P.; Liao, J. H.; Zhang, S. B.; Xing, W. J. Power Sources 2007, 173, 162-165. 36. Cui, Z. M.; Xing, W.; Liu, C. P.; Liao, J. H.; Zhang, H. J. Power Sources 2009, 188, 24-29. 37. Liu, M. M.; Lu, Y. Z.; Chen, W. Adv. Funct. Mater. 2013, 23, 1289-1296. 38. Arico, A. S.; Stassi, A.; D'Urso, C.; Sebastian, D.; Baglio, V. Chem-Eur. J., 2014, 20, 10679-10684. 39. Maheswari, S.; Sridhar, P.; Pitchumani, S. Electrochem. Commun. 2013, 26, 97-100. 40. Zhang, H.; Hao, Q.; Geng, H. R.; Xu, C. X. J. Hydrogen Energy 2013, 38, 1002910038. 41. Kim, J.; Momma, T.; Osaka, T. J. Power Sources 2009, 189, 999-1002.


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Figures

Figure 1. TEM images of Pd3Pb/C samples synthesized with different reducing agents: (a) KEt3BH; (b) LiEt3BH.


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Figure 2 (a) XRD patterns of Pd3Pb/C samples annealed at different temperatures for 12h; (b) XRD patterns of Pd3Pb/C samples annealed at 600 oC for different annealing time.


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Figure 3 (a) XRD patterns of Pd/C (1), disordered Pd3Pb/C (2) and ordered Pd3Pb/C (3). Drop lines correspond to Pd3Pb (PDF Card 65-3266). (b) Overview TEM image of ordered Pd3Pb/C. The inset shows the corresponding histogram of metal nanoparticles; (c) HR-TEM images of ordered Pd3Pb/C; (d) D-STEM patterns on the [001] zone axis collected from one particle in the insert.


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Figure 4 (a) ORR polarization curves in O2-saturated 0.1 M KOH. Rotation rate: 1600 rmp; Sweep rate: 10 mV s-1.; (b) Kinetic current (mass activities) at different potentials; (c) and (d) Rotating–disk voltammograms of ordered Pd3Pb/C and disordered Pd3Pb/C in O2-saturated 0.1M KOH with a sweep rate of 10 mV s-1 at the different rotation rates. The insets show corresponding Koutecky-Levich plots (j-1 versus ω-1/2 at 0.6 V-0.8V).


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Figure 5 (a) Bar plot of the kinetic current density at 0.9 V in 0.1 M KOH before and after stability tests of 5000 cycles; (b) Chronoamperometric responses of four samples at 0.8 V in O2-saturated 0.1 M KOH (percentage of current retained versus operation time).


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Figure 6 The discharge and charge voltage profiles of Zn-air batteries with (a, b) ordered Pd3Pb/C + NCONF@Ni decoupled catalysts, and (c, d) conventional Pt/C + NCONF@Ni decoupled catalysts at a current density of 10 mA cm-2 at room temperature.


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Figure 7 (a) ORR polarization curves in O2-saturated 0.1 M KOH+0.5M CH3OH. Rotation rate: 1600 rmp; Sweep rate: 10 mV s-1; (b) Chronoamperometric responses of ordered Pd3Pb/C and Pt/C at 0.7 V in O2-saturated 0.1 M KOH+0.5M CH3OH.


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