A Facile Method to Synthesize Pt-Ni Octahedral Nanoparticles with


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A Facile Method to Synthesize Pt-Ni Octahedral Nanoparticles with Porous and Open-Structure Features for Enhanced Oxygen Reduction Catalysis Zheng Jiang, Yang Liu, Lei Huang, Wen Hao Gong, and Pei Kang Shen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05444 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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A Facile Method to Synthesize Pt-Ni Octahedral Nanoparticles with Porous and Open-Structure Features for Enhanced Oxygen Reduction Catalysis. Zheng Jiang, Yang Liu, Lei Huang, Wen Hao Gong and Pei Kang Shen*

Collaborative Innovation Center of Renewable Energy Materials, Guangxi Key Laboratory of Electrochemical Energy Materials, College of Chemistry and Chemical Engineering, State Key Laboratory of Processing for Non-Ferrous Metal and Featured Materials, Guangxi University, Nanning 530004, P. R. China

Keywords: platinum; alloy-phase; anisotropic growth; octahedral; porous and openstructure Abstract: In recent years, the nanocatalysts with opening structure are fascinating for electrocatalytic reactions due to their high specific area, large void space and potentially highly active sites. Meanwhile, porous structure is more beneficial to create a confined environment which would enhance catalytic activity. Herein, we show the porous PtNi octahedral nanoparticles which exhibit abundant Pt-Ni alloy structures with transparent features and tiny Pt-Ni octahedral nanoparticles. By controlling the anisotropy of Pt with surfactant in solvothermal synthesis, we have synthsised composition-segregated octahedral Pt-Ni nanoparticles and tiny Pt-Ni octahedral nanoparticles at different precursor ratios. After subsequent chemical etching, the structure of composition-segregated octahedral Pt-Ni nanoparticles evolved a porous and open structure. The porous octahedral Pt-Ni nanoparticles show ~ 7.1 times higher 1

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specific activity and ~ 5.7 times higher mass activity for the oxygen reduction reaction than that of TKK-Pt/C catalyst, and 1.6 times mass activity of tiny Pt-Ni octahedral nanoparticles. The activity enhance is attributed to some of Pt atoms deposited on the faces of octahedron and form Pt-Ni alloy with Ni atoms. After chemical etching, lots of active sites and alloy phases expose into electrochemical environment which lead to enhanced electrochemical activity.

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Introduction

As the development of society, accelerated depletion of fossil fuels and environment problems got attention.1-2 Polymer electrolyte membrane fuel cells (PEMFCs) have been aroused more attention because of renewable energy sources and friendly to the environment since many years ago. Fuel cell is regarded as the hope for solving energy issues. Furthermore, electrode catalyst plays an important role in fuel cell, affecting the application of fuel cells. Platinum (Pt) is an effective catalyst in many significant applications, especially in fuel cells.3-6 Although, Pt catalyst has well catalytic property for electrochemical reaction, unfortunately, the limited resource of Pt, expensive price and poor durability confining the application of Pt catalyst in fuel cells. However, lots of previous work suggest that Pt-M alloys (M=Ni, Co, Cu, etc.) have more positive catalytic property than pure Pt catalyst,

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and the dosage of Pt is fewer. Thus, creating new Pt-based

catalyst has become a primary mission for fuel cell application during the past few decades. Pt-Ni nanocrystals have showed favorable performance in oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) among various bimetallic Ptbased alloys.10-11 Various of morphologies of Pt-Ni nanocrystals have been created, such as octahedra,12-13 rhombic dodecahedron,14 dendrites11 and frame. 6, 14-15 As we know that the surface and structure of nanocrystals greatly affect catalytic activity, such as nanoscale phase segregation, resulting from rearrangement of atom positions in alloy nanoparticles may improve electrocatalytic performance; 3

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high-index facets, which

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exhibits higher activity than low-index faces, because of higher surface energy.

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6, 16-17

In recent reports, Pt-rich edges could be formed in Pt-Ni rhombic dodecahedra and octahedral, after chemical etching, those nanoparticles could expose more alloy-phase facets which show more positive electrode performance than single-phase facets 16-17 because of synergistic effect. They also provide higher atomic utilization and expose more active sites for catalytic reaction, these features can enhance catalytic activity and lower the dosage of Pt. Therefore, such fascinating structures have been actively pursued in order to prepare the nanocatalysts with great catalytic property. In this work, we have prepared porous and open-structure octahedral Pt-Ni nanoparticles (PON), which display abundant pores and defects on the facets of Pt-Ni nanoparticle. Compared to the solid particles, the catalysts with nanoporous structures can offer more reaction sites and enhance activity via the so-called nanoconfinement effect.18 This intriguing alloy structure exhibits wonderful catalytic property for ORR. Two-step methods are used to prepare PON. At the first step, we concentrate on preparing composition-segregated octahedral Pt-Ni nanoparticle (CON) by one-pot solvothermal synthesis. Unlike others reports, without CO serve as reductant and surfactant, the Pt atoms can not easily diffuse to the surface of octahedron15, a small amount of Pt atoms would stay at octahedron’s facets and combine with Ni atoms to formed alloy phase. At the second step, CON would be etched by acetic acid, which would create countless micropores even more penetrate nanoparticles in the mid temperate condition, lots of Ni components are removed from CON lead to alloy-phase

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facets exposed. Such treatment can be effectively expose more active sites and improve catalytic property.

Results and discussion

In our recipe, H2PtCl6·6H2O is added to the reaction solvent directly to avoid the formation of Nickle oxide19. In addition, acetylacetonate (derived from Ni(acac)2) in the DMF solution is essential for octahedral nanoparticles formed, which selectively absorb on {111} of Pt crystal, induce subsequent selective deposition and promote the formation of hexapod structures9, 12-13. Furthermore, as the standard reduction potential of Pt is much higher than the standard reduction potential of Ni, which likely leads to Pt reduce earlier than Ni. Therefore, a fascinated structure can be image directly: Ptrich hexapod holder located at interior of octahedral particle, and eight Ni-rich {111} faces constitute external octahedral structure. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of a CON are shown in Figure 1. The average size of CON is 63±2 nm, and all nanoparticles are highly symmetrical. The STEM image in Figure 1b obviously exhibits more bight white lines along the edges of octahedron. We could find Pt-rich regions by energy-dispersive X-ray spectroscopy (EDS) elemental mapping analysis. Eight {111} Pt-Ni facets constitute octahedral nanoparticle and Pt element enrich at edges, corners and center, which results from priority reduction of Pt and subsequent Pt-Ni co-reduction. The HRTEM image reveals 5

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a mismatch boundary at 4 or 5 layers from the surface of the {111} facet (Figure 1d). The lattice spacing of (111) in this region is about 0.214 nm, which is about 5.1% more than the lattice spacing of pure Ni phase (0.2035 nm), but less than the lattice spacing of pure Pt phase (0.2239 nm) about 4.4%. It demonstrates that Pt-Ni alloy has formed in this region. The XRD patterns (Figure S1) of CON exhibits similar face-centeredcubic (FCC) structure, peaks of CON of (111) at 43.1°which fall in the position between those of monometallic Pt and monometallic Ni, also confirms the formation of a Pt-Ni alloy structure, which is consistent with Pt, Ni elements distribution showed in mapping analysis. Line scanning for the nanoparticles (Figure S2) reveals that there is a Pt-core in the center of CON. No matter what scanning direction passes through the center of particle, Ni element content always higher than Pt element content, but at center region, Pt content curve peak appears while Ni content decreases, at the same position. Although anisotropic growth process has been used for many the synthesis process of nanoparticles, we could not describe the interaction between CTA+ and different crystal facets completely. The TEM images of intermediates growing at different times has been captured to reveal the grow process of CON. Figure 2 presents 4 kinds of intermediates at different times. After an initial growth period of 10 minutes, Pt-Ni nanoparticles exhibited near-spherical cuboctahedra around 2 to 4 nm (Figure 2a), which result from fast reduction of PtCl62-. Cuboctahedron, a typical Wulff polyhedron, represents a thermodynamically stable structure at the nucleation stage because of the lowest surface energy.9 The EDS shows an average composition is Pt0.83Ni0.17, 6

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suggesting Pt ions would be easier reduced than Ni ions. When reaction lasts 30 minutes, the particles grow up to 13-19 nm, which exhibit two different morphologies (Figure 2b). One is a stretched cuboctahedron (13-15 nm), the other one is hexapod structure (17-19 nm). In this stage, new reduced atoms begin to deposit at crystal nucleus to promote the nanocrystals grow up, anisotropic growth begins to control the morphologies of nanoparticles, six asymmetrical arms grow along directions. Then, the hexapod structure formed. Branched and spherical shapes are coexisted results from the competition between the anisotropic growth and an opposing surface diffusion. Spherical shapes have lower surface energy, but once a critical particle size is exceeded, surface diffusion become slowed, the anisotropic growth be dominated.9 In addition, in the dimethylfomamide (DMF) solvent, acetylacetonate ligand has stronger adsorption on the {111} facets of nanoparticles,9,

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which is the key for

octahedral structures formed. It selective absorbs on the {111} facets of cuboctahedra, meanwhile, and leads to subsequent selective growth so that the arms faster growth along directions and hexapod structures formed. When extend the reaction time to 1 hour, most nanocrystals have evolved into concave-octahedral particles, but some hexapod particles still exit (Figure 2c), the average size is 18-26 nm, and the average composition is Pt0.76Ni0.24, Ni element content begin increased. In this stage, most of Pt precursors has been reduced, but the reduction of Ni precursors is increased, a mass of Ni ions and some Pt ions are co-deposition at concave surface, while the growth of Ptrich phase along the hexapod arms are slow down because the concentration of Pt precursors decreased. CTA+ is vital for the deposition of Ni ions because long alkyl 7

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chain is comparable in size to the atomic spacing on the high-index facet,21-22 it would selective absorb on high-index facet of arms then, induce the selective deposition of Ni ions and Pt ions on concave face. Furthermore, it could control the reduction rate of Ni ions lead to Pt-Ni co-deposition because of weak reduction ability.23 If no CTAC is added during synthesis process, the nanoparticles will become smaller and undefined compare with original recipe (Figure S3c). What’s more, PVP also is an essential for the formation of well-defined octahedral nanoparticles, which serves ass reductant and stabilizer.10-11, 24 It is usually used for shape-controlled of nanocatalyst, especially in a relatively high temperature situation.25-26 If without PVP in our recipe, uniform polyhedron alloy nanoparticles with ~ 30 nm diameter are obtained (Figure S3b). The next stage extends the reaction time to 6 hours, octahedral structures have initial shaped (Figure 2d), the average size is 43-48 nm and the average composition is Pt0.42Ni0.58. Those octahedral particles without sharp vertex may indicate that the process of Pt, Ni co-deposition and Pt diffusion perhaps would not finish. After 12 hours, the octahedral particles with sharp vertex formed (Figure 2e), further etched with acetic acid, sonication for 3h at 80 °C, porous octahedral Pt-Ni nanoparticle obtained (Figure 2f). To explain why Pt atoms of vertex position favor to move to edge, we performed density functional theory (DFT) calculations and theoretical analysis. Figure 3a shows the relative energy of a whole octahedral particle, when the Ni atom is replaced by Pt atom at vertex (0 eV), edge (-0.045 eV) and surface (1.123 eV) position respectively. It is obvious that the octahedral particles have the lowest relative energy, when Pt atom at edge position of octahedron. Therefore, we infer Pt atoms would move and rearrange 8

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toward the edge for lowest relative energy, so that, in the STEM image of CON, while lines edges of octahedron can be observed. The partial density of state (PDOS) of Pt atoms at different positions is calculated as shown in Figure 3b. Due to high degree of localization of the d orbital, the Pt atoms of vertex have the lowest electrocatalytic activity compare to other Pt atoms at different positions. The PDOS of Pt d electrons at surface and edge are more gently than vertex’s Pt atoms, but main electronic state of Pt atoms at surface located in lower energy range compare to the Pt atoms of edge. It suggests that the Pt atoms at surface have widest d-band and lowest d-band center compare to Pt atoms of vertex and edge, which lead to the highest electrocatalytic activity.27-28 Therefore, PON has a fascinating ORR activity. It is well known that the atom radius of Pt is larger than that of Ni, when there are enough Ni atoms surrounding a Pt atom, the atomic separation would be compressed, d-band of Pt atom would be wider and d-band center has a negative shift keep away from Fermi energy level.27 Figure 3c shows the environment of Pt atoms at vertex, edge and surface, respectively. Pt atoms on Vertex touch the fewest Ni atoms, more Ni atoms near the edge, while, Pt atoms of surface are surrounded by a mass of Ni atoms, therefore, surface Pt atoms have richest coordinated atoms that result to the strongest synergistic effect and highest activity. Figure 4 shows STEM images for PON that demonstrated its porous structure countless pores attached to {111} of Pt-Ni nanoparticles, those pores are deep enough to penetrate whole nanoparticle so that we can observe the Pt-axes in the octahedral center. This kind of nanostructures would be more beneficial for enhancing the activity 9

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and stability of nanoparticles. For catalytic applications, relatively transparent structures expose inner atoms to participate a reaction and provide channels for interchange of materials. Furthermore, the confined environment of the porous structures would be lead to the activity enhancement.18 The interior Pt-rich hexapod structure and the external Pt-rich edges and corner are co-constituted octahedral frame which is supporting whole structure, let PON exhibits superior stability and durability. The EDS shows Ni element content of PON (Pt0.65Ni0.35) is lower than CON (Pt0.29Ni0.71) (Figure S4), demonstrating that lots of Ni element dissolve out from CON during the chemically etched process. The position of micropores has further proofed Ni atoms distributed at surface and inner of CON, it mutual corroborate with mapping analysis of CON. Weak acid (such as acetic acid) can dissolve Ni species slowly and mildly in a warm situation and retain Pt atoms and keeping hold octahedral structure. But if we use strong acid (such as hydrochloric acid) to deal with chemical etching process, the octahedral structure may be destroyed (Figure S5). After strong acid etching, the Ptaxes and Pt-core in the center of nanoparticle are clearly visible, which demonstrate excellent stability of Pt-frame, but octahedral nanostructure has been warped. In the STEM image, we still can visit white lines along the edges and axes, the shiny cores are especially noticeable, but those dilapidated structures are not stable in the electrochemical environment. Figure 4(c-1) and Figure 4(c-2) exhibit HRTEM image of a PON of surface and edge, at (111) surface region, six diffracting points appear in the fast Fourier transform (FFT) images demonstrate that after chemical etching, the crystalline structure of octahedral nanoparticles have been changed, different tendency 10

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of {111} appear. Dominant X-ray diffraction (XRD) peaks of PON has moved negatively relative to CON because of decreased of Ni content, but it still suggests PtNi alloy exist. Pt-Ni alloys are superior to pure Pt for enhancing ORR activity, because they have a downshift in the surface d-band center position compared to pure Pt, which lead to weaker binding with surface adsorbates.14 Therefore, this kind of nanoparticles with relatively transparent structure and Pt-Ni alloy phase is more suitable to ORR. If we change the amount of Ni precursor from 35 mg to 7 mg in our recipe, the tiny Pt-Ni octahedral nanoparticle (TON) can be obtained. Figure S6 shows the characterizations of TON. The average particle size is 7.2 nm (Figure S7). Only used 8 h at 160 ℃, this tiny structure formed. Maybe due to H2PtCl6 being reduced easier than Pt(acac)2,29 the reaction time can be shortened a lot than other’s work. As the decrease of Ni precursors, the average particle size reduced. Following previous reports, solvothermal techniques at mild temperatures (120℃),the DMF solvent acts a complex agent solvent, and reducing agent, but the reaction time is long to 42 h.13 However, in this study, relatively high temperatures (160℃) and shaper Pt precursor (H2PtCl6) are utilized (see experimental details), the reaction time is greatly reduced. The EDS shows Pt-Ni ratio approaches 1:1 (Pt0.53Ni0.47) suggests that the amount of Ni precursor could affect average size of nanoparticles directly, when the amount of Ni precursor and Pt precursor is close, nanocatalyst are favorable to form tiny-scale particles, maybe due to lower surface energy. By taking different ratios of precursors, we have obtained TON and CON. PON are obtained via subsequent chemical etched of CON. TON has smaller particle size than 11

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PON, which is better for enhancing atomic utilization ratio, but PON with open and porous structure also provide more active site and high atomic utilization ratio. So, we investigate the electrocatalytic performances of the TON and the PON for ORR by disk electrode measurements (Figure 5). To assess the ORR activity of Pt-Ni nanoparticles, the carbon supported catalysts are prepared by mixing with commercial carbon support (Cabot, Vulcant-72) in the n-butylamine and subsequently annealing in air at 200 °C (See Electrochemical study). The commercial Pt/C (TKK, Japan) as a benchmark catalyst is used for comparison. The electrochemically active surface areas (ECSAs) of the catalysts are obtained by calculating based on the charge collected in the hydrogen region from the cyclic voltammetry (Figure 5a). Their ORR activity is evaluated by linear sweep voltammetry (LSV) in O2-saturated 0.1 M HClO4 solution, at a scan rate of 10 mV s-1 and a rotating speed of 1600 rpm. Figure 5b shows the ORR polarization curves of different catalysts. Both TON and PON exhibited enhanced catalytic activity than TKK-Pt/C. What is more, the PON exhibited the best mass activity (914 mA mgPt-1) and specific activity (2.45 mA cm-2) at 0.9 V, which is 5.7 times and 7.1 times greater than that of TKK-Pt/C. In addition, the half-wave potential of PON is positively shifted 50 mV compared to TKK-Pt/C. TON also shows better ORR activity than TKK-Pt/C, with the mass activity of 573mA/mgPt and specific activity of 1.82 mA/cm2 which are higher than that of the TKK-Pt/C of 3.5 times and 5.3 times, respectively. The ORR activity enhanced could be attributed to Pt-Ni alloy phase can downshift d-band position, lead to the synergistic effect bewteen Pt and Ni components, weak adsorption of intermediate species on the 12

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catalysts.14, 30-31 On the other hand, the structure of those catalyst provide high atomic utilization ratio also is a main reason for ORR activity enhanced. The durability of nanocatalyst is evaluated by an accelerated durability test (ADT) between 0.6 and 1.0 V for 6,000 potential cycles. As shown in Figure S11, PON exhibits good durability, the half-wave potential of PON is slightly negative shifted about 14 mV, while the TKK-Pt/C is negative shifted about 27 mV after ADT. The mass activity of PON reduce to 683 mA mg Pt-1, about 25.3% lost , which is 6.1 times higher than TKK-Pt/C (112 mA mg Pt-1) after ADT. The CV measurements show a loss of 18.1% in ECSA for the PON nanocrystals, 27.8 % for the commercial Pt/C catalyst, also reveals that the durability of PON is better than that of TKK-Pt/C. The ORR activity reduce maybe lead to the corrosion of Ni atoms. After ADT test, the Ni component of PON has changed to 24%, and the corrosion of excess metal is a common issue in the Pt-based catalysts.12, 32

Conclusions

In summary, we have prepared the porous octahedral Pt-Ni nanoparticle with facile solvothermal synthesis and chemical etched method. Duo to high energy barrier, the inner Pt atoms can not diffuse to surface smoothly and be stayed in the facets of octahedron. After chemical etching, PONs exhibit a lot of Pt-Ni alloy-phase with porous and open-structure features on faces of octahedral nanoparticles which be confirmed to be most active for ORR comparing to other sites of octahedron. Therefore, 13

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PONs can exhibit wonderful catalytic property for ORR. This work provides a facile stategy for preparation and optimization of the nanocatalysts. Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

Corresponding Author *E-mail: [email protected] ORCID Pei Kang Shen: 0000-0001-6244-5978 Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the Guangxi Science and Technology Project (AA17204083, AB16380030), National Basic Research Program of China (2015CB932304), the link project of the National Natural Science Foundation of China and Fujian Province (U1705252), the Natural Science Foundation of Guangdong Province (2015A030312007) , the Mutil-function Computer Center of Guangxi Univerity, LvLiang Cloud Computing Center of China (TianHe-2) and the Danish project of Initiative toward Nonprecious Metal Polymer Fuel Cells (4106-000012B). Prof. Tsiakaras has been financially supported by the Ministry of Education and Science of the Russian Federation (Mega-Grant, contract no. 14.Z50.31.0001) and co-financed by the European Union and Greek national funds through the Operational Program

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Competitiveness, Entrepreneurship and Innovation, under the call RESEARCH – CREATE – INNOVATE (project code:T1EDK-02442). References: (1). Liu, H. L.; Nosheen, F.; Wang, X., Noble metal alloy complex nanostructures: controllable synthesis and their electrochemical property. Chem. Soc. Rev. 2015, 44, 3056-78. (2). Yu, X.; Wang, D.; Peng, Q.; Li, Y., High performance electrocatalyst: Pt-Cu hollow nanocrystals. Chem. Commun. (Camb.) 2011, 47, 8094-6. (3). Lim, B.; Jiang, M.; Camargo, P. H.; Cho, E. C.; Tao, J.; Lu, X.; Zhu, Y.; Xia, Y., Pd-Pt bimetallic nanodendrites with high activity for oxygen reduction. Science 2009, 324, 1302-5. (4). Luo, S.; Shen, P. K., Concave Platinum-Copper Octopod Nanoframes Bounded with Multiple High-Index Facets for Efficient Electrooxidation Catalysis. ACS Nano 2017, 11, 11946-11953. (5). Qi, Y.; Bian, T.; Choi, S. I.; Jiang, Y.; Jin, C.; Fu, M.; Zhang, H.; Yang, D., Kinetically controlled synthesis of Pt-Cu alloy concave nanocubes with high-index facets for methanol electro-oxidation. Chem. Commun. (Camb.) 2014, 50, 560-2. (6). Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J. A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.; Stamenkovic, V. R., Highly crystalline multimetallic nanoframes with three-dimensional electrocatalytic surfaces. Science 2014, 343, 1339-43. (7). Du, X. W.; Luo, S. P.; Du, H. Y.; Tang, M.; Huang, X. D.; Shen, P. K., Monodisperse and self-assembled Pt-Cu nanoparticles as an efficient electrocatalyst for the methanol oxidation reaction. Journal of Materials Chemistry A 2016, 4, 1579-1585. (8). Jung, N.; Sohn, Y.; Park, J. H.; Nahm, K. S.; Kim, P.; Yoo, S. J., High-performance [email protected] core-shell nanoparticles decorated with nanoporous Pt surfaces for oxygen reduction reaction. Applied Catalysis B-Environmental 2016, 196, 199-206.

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(18). Shao, M.; Chang, Q.; Dodelet, J. P.; Chenitz, R., Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chemical reviews 2016, 116, 3594657. (19). Choi, S. I.; Shao, M.; Lu, N.; Ruditskiy, A.; Peng, H. C.; Park, J.; Guerrero, S.; Wang, J.; Kim, M. J.; Xia, Y., Synthesis and characterization of [email protected] core-shell octahedra with high activity toward oxygen reduction. ACS Nano 2014, 8, 10363-71. (20). Xia, T.; Liu, J.; Wang, S.; Wang, C.; Sun, Y.; Gu, L.; Wang, R., Enhanced catalytic activities of NiPt truncated octahedral nanoparticles toward ethylene glycol oxidation and oxygen reduction in alkaline electrolyte. ACS Appl. Mater. Interfaces 2016, 8. (21). Yu, Y.; Zhang, Q.; Lu, X.; Lee, J. Y., Seed-Mediated Synthesis of Monodisperse Concave Trisoctahedral Gold Nanocrystals with Controllable Sizes. J. Phys. Chem. C 2010, 114, 11119-11126. (22). Zhang, H.; Jin, M.; Xia, Y., Noble-metal nanocrystals with concave surfaces: synthesis and applications. Cheminform 2012, 51, 7604-7604. (23). Lee, Y. W.; Kim, M.; Kim, Z. H.; Sang, W. H., One-Step Synthesis of [email protected] Core−Shell Nanooctahedron. 2009. (24). Zhang, P.; Dai, X.; Zhang, X.; Chen, Z.; Yang, Y.; Sun, H.; Wang, X.; Wang, H.; Wang, M.; Su, H., One-Pot Synthesis of Ternary Pt–Ni–Cu Nanocrystals with High Catalytic Performance. Chem. Mater. 2015, 27, 6402-6410. (25). Huang, X.; Li, Y.; Chen, Y.; Zhou, H.; Duan, X.; Huang, Y., Plasmonic and Catalytic AuPd Nanowheels for the Efficient Conversion of Light into Chemical Energy. Angewandte Chemie International Edition 2013, 125, 6179-6183. (26). Xiong, Y.; Washio, I.; Chen, J.; Cai, H.; Li, Z. Y.; Xia, Y., Poly(vinyl pyrrolidone): a dual functional reductant and stabilizer for the facile synthesis of noble metal nanoplates in aqueous solutions. Langmuir 2006, 22, 8563-8570. (27). Hammer, B.; Norskov, J. K., Theoretical Surface Science and Catalysis: Calculations and Concepts. Adv. Catal. 2000, 45, 71-129. (28). Liu, Y.; Yin, S.; Shen, P. K., Asymmetric 3d Electronic Structure for Enhanced Oxygen Evolution Catalysis. ACS Appl. Mater. Interfaces 2018, 10, 23131-23139. 17

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(29). Xia, Y.; Xia, X.; Peng, H. C., Shape-Controlled Synthesis of Colloidal Metal Nanocrystals: Thermodynamic versus Kinetic Products. J. Am. Chem. Soc. 2015, 137, 7947-66. (30). Hodnik, N.; Baldizzone, C.; Cherevko, S.; Zeradjanin, A.; Mayrhofer, K. J. J., The Effect of the Voltage Scan Rate on the Determination of the Oxygen Reduction Activity of Pt/C Fuel Cell Catalyst. Electrocatalysis 2015, 6, 237-241. (31). Li, Y.; Quan, F.; Chen, K.; Chen, L.; Chen, C., Synthesis of highly monodispersed PtCuNi nanocrystals with high electro-catalytic activities towards oxygen reduction reaction. Catal. Today 2016, 278, 247-254. (32). Baldizzone, C.; Gan, L.; Hodnik, N.; Keeley, G. P.; Kostka, A.; Heggen, M.; Strasser, P.; Mayrhofer, K. J. J., Stability of Dealloyed Porous Pt/Ni Nanoparticles. Acs Catalysis 2015, 5, 5000-5007.

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Figure 1: Structural analysis of CON. (a) TEM image. (b) HAADF-STEM image and EDS elemental mapping analysis of CON, which shows the distribution of Pt (green) and Ni (red). (c) TEM image of a single nanoparticle of CON. (d) HRTEM image of a CON along the zone axis. (e) and (f) FFT images corresponding two different regions in (d).

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Figure 2: TEM images of structure of a CON at different reaction times: (a) after 10 minutes, (b) after 30 minutes, (c) after 1 hour, (d) after 6 hours, (e) after 12 hours (CON). (f) TME image of PON. (g) Schematic illustration for the structural evolution into a CON.

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Figure 3:a) The relative energy of octahedral particles, Pt red, Ni cyan. b) the PDOS of the Pt 5d electrons and c) environment of Pt atoms when Pt atoms located at different positions (Pt: yellow, Ni: blue).

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Figure:4:Structure analysis of PON. (a) STEM and HAADF-STEM images of PON. (b) EDS elemental mapping analysis of PON. (c) TEM image of a single PON. (c-1), (c-2) HRTEM image of edge and surface of a PON. (i), (ii) FFT images were corresponding (c-2) surface region and (c-1) edge region, respectively.

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Figure 5: (a) CV curves of TON, PON and TKK-Pt/C catalysts in N2 -saturated 0.1 M HClO4 aqueous solution at scan rate of 50 mV s -1. (b) The ORR polarization curves of TON, PON and TKK-Pt/C catalysts in O2 -saturated 0.1 M HClO4 aqueous solution at scan rate of 10 mV s-1 and rotation rate of 1600 rpm. The geometric area of rotating disk electrode is 0.196 cm2. (c), (d) The comparison of ECSA and mass activity of TON, PON and TKK-Pt/C catalysts.

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A facile method to synthesize highly porous and open octahedral Pt-Ni nanoparticle displays superior catalytic activity and stability.

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TOC

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