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One-nanometer-thick Pt3Ni Bimetallic Alloy Nanowires Advanced Oxygen Reduction Reaction: Integrating Multiple Advantages into One Catalyst Mingxing Gong, Zhiping Deng, Dongdong Xiao, Lili Han, Tonghui Zhao, Yun Lu, Tao Shen, Xupo Liu, Ruoqian Lin, Ting Huang, Guangwen Zhou, Huolin Xin, and Deli Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00603 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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ACS Catalysis

One-nanometer-thick Pt3Ni Bimetallic Alloy Nanowires Advanced Oxygen Reduction Reaction: Integrating Multiple Advantages into One Catalyst Mingxing Gonga, b, §, Zhiping Denga, §, Dongdong Xiaoc, Lili Hanb, Tonghui Zhaoa, Yun Lua, Tao Shena, Xupo Liua, Ruoqian Linb, Ting Huangb, Guangwen Zhouc, Huolin Xin*, b, d, Deli Wang*, a aKey

laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology. Wuhan, 430074, P.R. China. bCenter for Functional Nanomaterials, Brookhaven National Laboratory, Upton, 11973, NY, USA cMaterials Science and Engineering Program & Department of Mechanical Engineering, State University of New York, Binghamton, 13902, NY, USA. dDepartment of Physics and Astronomy, University of California, Irvine, 92697, CA, USA. KEYWORDS: Oxygen reduction reaction, Pt-Ni alloy, Ultrathin nanowires, Seed-mediated growth, Long-term durability

ABSTRACT: Developing highly active as well as durable oxygen reduction reaction (ORR) electrocatalysts are still imperative for clean and efficient energy conversion device, such as fuel cells and metal-air battery. For this purpose and maximize the utilization of noble Pt, we present here a facile, yet scalable strategy for the high-precise synthesis of 1 nm thick Pt3Ni bimetallic alloy nanowires (Pt3Ni BANWs). The seed-mediated growth mechanism of Pt3Ni BANWs was identified subsequently. As expected, the Pt3Ni BANWs delivered enhanced mass activity (0.546 A mg-1Pt, exceeding the 2020 target of DOE) in comparison to Pt nanowires assembly (Pt NWA, 0.098 A mg-1Pt) and Pt/C (Pt, 0.135 A mg-1Pt) due to the rational integration of multiple compositional and structural advantages. Moreover, the Pt3Ni BANWs displayed enhanced durability (37% MA retention) than Pt NWA and Pt after 50,000 potential cycles. All these results indicate that the ultrathin Pt3Ni BANWs are potential candidates for catalyzing ORR with acceptable activity and durability. The present work could not only provide a facile strategy but also a general guidance for the design of superb performance Pt-based nanowire catalysts for ORR.

conventional (111), (110) and (100) facets. Apart from size and composition regulation, the electrocatalytic activity of these nanocrystals can also be enhanced by tuning morphology and dimension. In this regard, Pt-TMs nanocrystals featured by various shapes, such as cube18, octahedron19, cage20, wire21, dendrite22, frameworks23 had been well prepared and showed enhanced activity compare with general Pt nanoparticles2. It is of importance to note that beyond activity, the durability of these desired Pt-based electrocatalysts is also a critical factor for practical operation. The ultrafine nanoparticles and/or alloy nanocrystals which aforementioned typically suffer from insufficient stability due to Ostwald ripening and/or TMs leaching during electrochemical cycling. Improving the durability of a given catalyst to meet the long term operation requirement is also a grand challenging. In the process of searching highly durable catalysts for ORR, many reported works manifest that elongated Pt nanowires or nanorods are less subject to Ostwald ripening, dissolution, and even aggregation than the Pt nanoparticles in acidic conditions24. Still further, 1D nanowires (NWs) have drawn increasing attention due to its intrinsic faster electron/mass transport, higher stability, more reaction active sites than

1. INTRODUCTION Improving the mass activity (MA, current normalized to mass) of platinum electrocatalysts to acceptable level (i.e. 0.44 A mg-1 @0.9 V, DOE target in 2020)1 for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) plays a decisive role in its widespread adoption2-6. As the MA is straightway determined by the specific activity (SA) as well as electrochemical active surface area (ECSA), it could be optimized via rationally tuning the exposed catalytic active site7, coordination environment8, and electronic effect9, etc. of Pt. For example, the size of Pt crystal has been reduced to several nanometer or even single atom to maximizing the atomic utilization10-12. Gradually, alloy strategy was widely adopted because it can not only reduce the Pt loading but also optimize the geometric and electronic structure of Pt13-15. Previous computational and experimental investigations have revealed that once alloyed with transition metals (TMs), the Pt-TMs alloy show dramatic activity enhancement2, 16. Moreover, combining selective etching approach with surface structure control technology, well-defined high-index facets were also been successfully designed and applied in many catalytic reactions17 due to their higher surface atom proportion as well as catalytic activity compared with the

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traditional 0D nanoparticles21. Furthermore, to maximize the utilization of Pt in these NWs, Zeng et al demonstrate the Rhdoped 1 nm Pt NWs, viz, only a few atomic layers, have ultrahigh fraction of surface atoms compressive surface strain and strong quantum confinement, all these factors are beneficial to accelerating the ORR. Subsequently, Rh-doped PtNi NWs with 6-7 atomic layers thick were also frabricated and deliver enhanced ORR performance25, 26. However, it is worth noting that the two steps synthesis strategy involved in the above works are slightly complicated. Environmentally friendly, one-step synthesis strategy without high temperature process is highly desired. To this end, the integration of alloy feature, ultrathin 1D nanostructure and high-index facets, etc. to one catalyst seems to be an ideal strategy for the creation of highly active Ptbased catalysts as well as the maximing utilization of noble Pt atoms. Herein, we demonstrate a new class of Pt3Ni bimetallic alloy nanowires (Pt3Ni BANWs) with only 1 nm thick from Pt nanowires assembly by rationally introducing a moderate amount of Ni(acac)2 in the precursor solution. Then, the asprepared Pt3Ni BANWs are well characterizated and the formation mechanism is identified. Finally, the electrochemical measurements are conducted to reveal the performance of these catalysts towards ORR. To investigate the outperformance of Pt3Ni BANWs than that on corresponding Pt nanowires assembly and Pt, further experiment and explanation are provided. 2.

EXPERIMENTAL SECTION

2.1. Reagents Chloroplatinic Acid (H2PtCl6), Nickel (II) Acetylacetonate (Ni(acac)2), Potassium Hydroxide (KOH) supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Solvents such as N, N-dimethylmethanamide (DMF), Ethylene Glycol (EG), Ethanol purchased from Aladdin. Other reagents were all of AR grade used without further purification. The water used in this work are ultrapure with the resistivity of 18.2 H1 2.2. Catalysts preparation Preparation of Pt nanowires assembly (Pt NWA). Typically, 0.125 mL of H2PtCl6 solution (0.2 M) and 400 mg of KOH were added to a 10 mL mixed solution (5 mL EG + 5 mL DMF). Mechanically stirring and ultrasonic for 3h to obtain the homogeneous and transparent solution, the resultant solution was then transferred into an autoclave. After solvothermal reaction for 8 h at 170 °C, the samples were collected by centrifugation and washed with ethanol and DI water (50:50, v/v) three times before drying at 60 °C for 10 h. Synthesis of the Pt3Ni bimetallic alloy nanowires (Pt3Ni BANWs). Similar procedure was adopted to produce Pt3Ni BANWs with 6.4 mg Ni(acac)2 in the precursor solution. 2.3. Physical characterization A 200 kV field-emission S/TEM (FEI-Talos) equipped with four energy-dispersive X-ray (EDX) spectroscopy detectors was adopted for TEM, HAADF-STEM, EDX and line scanning characterization. Powder X-ray diffraction (PXRD) patterns were obtained by using a X’Pert PRO diffractometer, and diffraction patterns were collected at a scanning rate of 4°/min with a step size of 0.02°. The composition of the catalysts was determined by X-ray fluorescence (XRF) using

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an EAGLE III spectrometer. Bruker diffractometer with Cu GM radiation (D8 Advance X-ray diffractometer, Cu GM N = 1.5406 Å, 40 kV, and 40 mA). The surface properties of these catalysts were analyzed by X-ray photoelectron spectroscopy (VG ESCALAB MKII instrument) which uses Mg GM X-ray source. The survey scans were obtained by setting a pass energy of 160 eV. High-resolution spectra of the individual elements were collected with the analyzer pass energy of 40 eV. The pressure of the analyzer chamber was keep at 10Q9 Pa. 2.4. Electrochemical measurements Electrochemical experiments were carried out in a traditional three-electrode system using Autolab electrochemical workstation (model: PGSTAT302N). A traditional threeelectrode system, in which a catalyst modified glassy carbon (GC) electrode, a graphite rod and a reversible hydrogen electrode (RHE) were served as working electrode, auxiliary electrode and reference electrode, respectively. All potentials in this work without specially mentioned were reported respect to RHE and all electrochemical tests were processed at 25 ± 1 °C. To prepare the catalyst ink, 5 mg of the carbonsupported catalysts was dispersed in a mixture solution which containing DI water (0.745 mL), isopropyl alcohol (0.250 mL), and Nafion solution (0.005 mL). The mixture was sonicated for 0.5 h to obtain a homogeneous catalyst ink. Finally, 3 microliters of ink was dropped onto the GC working electrode dry naturally. The Pt metal loading of the catalysts on the electrode was about 15 S cmQ2. The CVs were conducted in HClO4 (0.1 M) solution saturated by N2 with a sweep rate of 0.05 V sQ1. The ORR LSV were conducted in 0.1 M HClO4 electrolyte saturated by O2 with a sweep rate of 0.005 V sQ1 under the rotating speeds of 1600 rpm. The accelerated durability test (ADT) was conducted by applying cyclic sweeps at 0.6-1.0 V (vs. RHE) in 0.1 M HClO4 electrolyte with sweep rate of 0.1 V sQ1. 3.

RESULTS AND DISCUSSION

3.1. Morphological and Structural Characterizations. The Pt3Ni BANWs were prepared by a facile, scalable solvothermal method, meanwhile, the Pt nanowires assembly (Pt NWA) also obtained by the similar synthetic procedures except the presence of Ni(acac)2, (please see the Experimental section for details). Typically, the morphology and composition of the as-prepared Pt3Ni BANWs and Pt NWA were firstly characterized by TEM and HAADF-STEM equipped with four energy-dispersive X-ray (EDX) spectroscopy detectors. Low resolution TEM reveals that the highly pure and uniform 1D nanowires (Figure 1a) with average diameter of 1.0 nm (Figure 1b). The length of these NWs is up to one hundred nanometer or longer, displaying the aspect ratio of more than 100. HAADF-STEM image with atomically resolution confirming the single NWs make up by approximate 6 atomic layers, which would make almost 50% Pt utilization according to simulative model (Figure S1, Supporting Information, SI). While the Pt NWA with an average diameter of ca. 30 nm (Figure S2a-b) and the High resolution HAADF-STEM image of dispersive single Pt nanowire derived from Pt NWA reveals that the Pt nanowire also 1 nm thick but the surface is relatively smooth compared with Pt3Ni BANWs. We suppose that the abundant atomic defect in Pt3Ni BANWs (Figure 1c and Figure S2c) stem from the superficial Ni leaching during reaction process. The 2D elemental mapping (Figure 1d) images and 1D linescanning profile (Figure 1e) cleanly proved the homogeneous

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U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704 M. X. Gong thanks the scholarship supported by the China Scholarship Council (CSC) (201706160151).

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