Non-noble bimetallic alloy encased in nitrogen-doped nanotubes as a highly active and durable electrocatalyst for oxygen reduction reaction

Accepted Manuscript Non-noble bimetallic alloy encased in nitrogen-doped nanotubes as a highly active and durable electrocatalyst for oxygen reduction reaction Liming Zeng, Xiangzhi Cui, Lisong Chen, Ting Ye, Weimin Huang, Ruguang Ma, Xiaohua Zhang, Jianlin Shi PII:

S0008-6223(16)31095-8

DOI:

10.1016/j.carbon.2016.12.017

Reference:

CARBON 11540

To appear in:

Carbon

Received Date: 1 October 2016 Revised Date:

21 November 2016

Accepted Date: 6 December 2016

Please cite this article as: L. Zeng, X. Cui, L. Chen, T. Ye, W. Huang, R. Ma, X. Zhang, J. Shi, Non-noble bimetallic alloy encased in nitrogen-doped nanotubes as a highly active and durable electrocatalyst for oxygen reduction reaction, Carbon (2017), doi: 10.1016/j.carbon.2016.12.017. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Graphical Abstract:

A new type of noble metal-free ORR electrocatalyst of Ni-Co nanocrystal alloy encapsulated in N-doped carbon nanotubes (NiCo@NCNT-700) is fabricated through a facile pyrolysis route. The NiCo@NCNT-700 demonstrates outstanding ORR activity highly comparable to the commercial Pt/C

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and excellent operational durability, markedly better than commercial Pt/C under alkaline conditions.

Non-noble bimetallic alloy encased in nitrogen-doped nanotubes as a highly active and durable electrocatalyst for oxygen reduction reaction.

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Liming Zeng, Xiangzhi Cui*, Lisong Chen, Ting Ye, Weimin Huang, Ruguang Ma, Xiaohua Zhang,

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Jianlin Shi*

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ToC figure:

ACCEPTED MANUSCRIPT Non-noble bimetallic alloy encased in nitrogen-doped nanotubes as a highly active and durable electrocatalyst for oxygen reduction reaction Liming Zeng1,2, Xiangzhi Cui1*, Lisong Chen1, Ting Ye1,2, Weimin Huang1, Ruguang

1

State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of

Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China.

University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China. E-mail: [email protected](X. Z. Cui); [email protected](J. L. Shi);

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*corresponding author.

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2

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Ma1, Xiaohua Zhang1,2, Jianlin Shi1*

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Fax: + 86-21-52413122; Tel: + 86-21-52412712

ACCEPTED MANUSCRIPT ABSTRACT Exploring highly active, cost-effective and durable oxygen reduction reaction (ORR) electrocatalysts as substitutes for the rare platinum-based catalysts is of great

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significance in energy conversion and storage devices, such as alkaline fuel cells, metal-air batteries, etc. Herein, we fabricated a new type ORR electrocatalyst of Ni-Co nanocrystal alloy encapsulated in N-doped carbon nanotubes (NCNTs) through

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a facile, scalable route utilizing nickel acetate and cobalt chloride as metallic Ni and

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Co precursors, thiourea as a nitrogen and carbon source, respectively, under a Ni/Co atomic ratio of 3:7 at 700°C. The obtained nanocomposite catalyst NiCo@NCNT-700 exhibited an outstanding ORR activity close to that of the state-of-the-art Pt/C catalyst and superior operational durability under alkaline conditions, which could be

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attributed to the co-contributions among the uniformly distributed Ni-Co alloy nanoparticles, graphitic NCNTs and the formation of Co-N species. This work provides a new insight for the rational design and development of efficient non-noble

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metal electrocatalysts by integrating electrochemically active units into the

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nanocomposite for challenging electrochemical energy-related technologies. 1. Introduction

Oxygen reduction reaction (ORR) is an important electrochemical process for

energy-related devices associated with efficient energy conversion and storage, such as fuel cells, metal-air batteries and certain electrolysers (for example, chlor-alkali ones)[1, 2].The sluggish ORR kinetics and performance deterioration at the cathode has been being one of the greatest challenges against these electrochemical energy

ACCEPTED MANUSCRIPT storage and conversion systems[3]. Therefore, it is urgent to develop highly active and durable electrocatalysts to ensure the high performance during the ORR processes. The commercial Pt/C-based composites are still the commonly used ORR catalysts

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exhibiting relatively high activity, while its large-scale application is greatly hampered by the quick activity reduction, rarity and the consequent high cost of the noble metal[4]. Hence, searching for high-active, cost-effective and stable precious

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metal-free ORR electrocatalysts has attracted tremendous attentions[5-8].

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Recently, the metal-based catalysts, especially from the 3d transition metals, have been extensively demonstrated to be highly active to ORR and other catalytic reactions[8-11].Among them, nickel and cobalt, which are abundant and environment-friendly, have been widely explored to show great potentials in the

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applications in energy conversion and storage devices[12, 13]. Furthermore, bimetallic-based catalysts including bimetallic alloy, such as FeCo, NiCo, FeNi, are increasingly becoming promising substitutes for the electrochemical noble metal

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catalysts[10, 14-16]. Very surprisingly, however, no reports of bimetallic alloy

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catalysts for ORR can be found to date. Apart from the metal-based catalysts, the N-incorporated carbon nanotubes (CNTs) have been found to facilitate the formation of defects in regularly arranged carbon atoms, leading to the enhancement of ORR activity in alkaline medium[17, 18]. According to the previous report, the encapsulated metal-related particles were favorable in activating the outer graphitic layers, contributing to the enhancement of active site density toward ORR[19]. Considering the current status of ORR catalyst exploration, a nanocomposite between Ni-Co

ACCEPTED MANUSCRIPT bimetallic alloy nanoparticles and N-doped carbon nanotubes was proposed to desirably provide more catalytically active sites to accelerate the ORR kinetics and improve the long-term operational stability.

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Herein, we report the fabrication of N-doped carbon nanotube-supported Ni-Co bimetallic alloy nanoparticles via a facile pyrolysis method using metal salts (nickel acetate and cobalt chloride) as metal precursors, and thiourea as carbon and nitrogen

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sources. The catalyst, labelled as NiCo@NCNT-700, exhibits highly catalytic activity

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and excellent operational durability for ORR in basic medium. A control experiment of metal-etching was examined, which indicate that the metal-related species have played a key role in the contribution to the ORR activity. The co-contributions from the Ni-Co alloyed particles, N-doped CNTs and the formation of Co-N are proposed

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to account for the high ORR activity comparable to commercial Pt/C. 2. Experimental section

2.1. Preparation of NiCo@NCNT Electrocatalysts

modifications[11].

Briefly,

The

nickel

acetate

tetrahydrate

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several

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The NiCo@NCNTs samples were prepared following a reported procedure with

(Ni(CH3COO)2•4H2O) and cobalt(II) chloride hexahydrate (CoCl2•4H2O) were dissolved in 10 ml of ethanol with varying metal molar ratios (0:10, 3:7, 5:5, 8:2, 10:0), keeping a total molar content of 1 mmol. Then, 2 g of thiourea was introduced into the solution, followed by sonication treatment for 5 min. Next, the mixture was subjected to water bath under stirring at 80°C until the ethanol was completely evaporated. The resulting dried mixture was thoroughly ground into a homogeneous

ACCEPTED MANUSCRIPT fine powder and subsequently placed at the center of corundum tube furnace. Then the temperature was ramped from 25°C to 450°C at a rate of 1oC/min under Ar atmosphere and maintained at 450oC for 2 h. After that, the samples was heated to

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700oC within 1 h and kept at 700oC for 2 h. Finally, NiCo@NCNTs were collected after cooled to ambient temperature under Ar flow. As a control, a part of the collected NiCo@NCNT-700 was dispersed in 3 M HCl aqueous solution and stirred for 12 h at

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80°C. Then the etched sample was collected through centrifugation, rinsed with

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deionized water and lyophilized. Thus as-prepared Ni-Co alloyed particles encapsulated by graphitic carbon nanotubes (denoted as NiCo@NCNT-X, X refers to pyrolysis temperature) were obtained. 2.2 Materials Characterizations

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The powder X-ray diffraction (PXRD) was performed on a Rigaku D/Max-2550 V X-ray diffractometer with a Cu Kα radiation target (40 KV, 40 mA). Scanning electron microscope (SEM) imaging was carried out using a Hitachi-S4800 scanning electron

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microscope (10 kV). Transmission electron microscopy (TEM), energy-dispersive

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X-ray (EDX) mapping was conducted using a JEOL-2100F high resolution transmission electron microscope (200 kV). X-ray photoelectron spectroscopy (XPS) signals were measured on a VG Micro MK II instrument using monochromatic Mg Kα X-rays (150 W, 1253.6 eV), and the C 1s electron peak (BE = 285 eV) was used as internal reference to perform spectrum calibration. Raman spectra were recorded on a DXR Raman Microscope (Thermal Scientific, USA) with a 532 nm excitation length. The

nitrogen

adsorption-desorption

measurements

were

carried

out

using

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were

obtained

by

the

Brunauer-Emmett-Teller

(BET)

and

Barrett-Joyner-Halenda (BJH) methods, respectively. A CHI 760E electrochemical

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workstation (CH Instruments) was used to measure the electrocatalytic properties of the samples. 2.3. Electrochemical measurements

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The electrochemical measurements were carried out in a standard three-electrode

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glass cell filled with 0.1 M KOH solution on a CH Instruments 760E electrochemical workstation. A glassy carbon electrode (GCE) coated with catalysts, a Pt foil and an Ag/AgCl electrode were employed as the working electrode, counter electrode and reference electrode, respectively. To prepare catalyst ink for ORR test, 6 mg of

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catalyst and 50 µL of 5% Nafion solution (DuPont) were dispersed in 950 µL of deionized water and absolute ethanol solution (v: v=1:1). Then the mixture was sonicated for at least 30 min to form a homogeneous ink. An aliquot of 2.5 µL of the

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catalyst ink was pipetted onto a glassy carbon electrode, giving a catalyst loading of

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0.212 mg cm−2. A commercial Pt/Vulcan XC-72 sample (20 wt. % Pt on Vulcan carbon black) was measured for comparison with the same loading amount as the prepared catalysts. All potentials were calibrated relative to the reversible hydrogen electrode (RHE) scale according to the Nernst equation (ERHE = EAg/AgCl + 0.059 × pH + 0.198 V), where EAg/AgCl is the external potential measured against the Ag/AgCl reference electrode. For all the measurements, high-purity N2/O2 gas was bubbled into the solution for 30 min before the electrochemical measurements and throughout the

ACCEPTED MANUSCRIPT whole testing process. Prior to linear sweep voltammetry (LSV) measurements, the electrodes were scanned at 50 mV s−1 until reproducible cyclic voltammograms (CVs) were achieved. The LSV measurements were performed in an O2-saturated KOH

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solution with a scan rate of 5 mV s−1 at a rotating speed of 1600 rpm. Likewise, the LSV curves were also measured in an O2-saturated HClO4 solution with the same rotating speed and scan rate. The electrochemical impedance spectroscopy

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measurements were conducted in a frequency range of 0.01 Hz-100 kHz with an

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amplitude of 5 mV at a fixed voltage of 0.82V vs. RHE. The polarization plots collected at various rotating speeds were employed to analyze the kinetics for the catalysts. The kinetics parameters including electron transfer number (n) and kinetic current density (jk) could be calculated by the Koutecky-Levich equations shown as



















= + =




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follows:[8] + 

/



(1) (2)

 = 

(3)

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B = 0.2   
/

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where j is the measured current density, jk and jl are the kinetic and diffusion limiting current densities, ω is the rotating rate of electrode (rpm). n is the overall number of electrons transferred in oxygen reduction, F is Faraday constant (96 485 C mol-1), C0 is the bulk concentration of O2 (1.2 × 10-3 mol L-1), and υ is the kinetic viscosity of electrolyte (0.01 cm2 s-1). DO2 is the diffusion coefficient of O2 in 0.1 M KOH (1.9 × 10-5 cm s-1), k is the electron transfer rate constant. B is determined from the slope of the Koutecky-Levich plots (j-1 vs ω-1/2). The constant 0.2 is adopted when the rotating

ACCEPTED MANUSCRIPT speed is expressed in rpm. 3. Results and Discussion 3.1. Characterization of the as-prepared catalysts

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Among the prepared samples of various molar ratios between Ni and Co, the sample NiCo@NCNT-700 (Ni:Co=3:7) shows a pure alloy phase (Fig.S1a), and its XRD pattern displays three distinctive peaks (44.38°, 51.68°, 76.12°) located in

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between those of Co (PDF-# 15-0806) and Ni (PDF-# 04-0850), which could be indexed to the (111), (200), (220) lattice planes for a metallic Co or Ni[20, 21]

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fcc-structure. (Fig. 1 and inset). This suggests that the Ni/Co ratio of 3:7 is the optimal molar ratio for formation of the single phase alloyed Ni-Co nanoparticles among the compositions investigated. The weak and broad peak centered at ca. 25.92° was

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assigned to the (002) facets of graphite carbon, demonstrating the existence of several graphitic layers in the NiCo@MCNT-700. At the same Ni/Co ratio of 3:7, sample NiCo@MCNT-700, NiCo@MCNT-800 and NiCo@MCNT-900 fabricated by heating

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at 700, 800 and 900oC, respectively, display similar XRD patterns but

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NiCo@MCNT-700 demonstrates relatively weaker and more broadened peaks (Fig.S1b), indicating smaller Ni-Co alloy nanoparticle size of NiCo@MCNT-700.

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Fig. 1. XRD pattern of sample NiCo@NCNT-700 and the magnification area with 2θ ranging from 43o

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to 78o in the inset.

The Brunauer-Emmett-Teller (BET) specific surface area and pore size distribution were obtained via N2 adsorption-desorption analysis. From Fig. S2a, all the NiCo@NCNT-X samples display type IV isotherm curves with a hysteresis loop,

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which was characteristic of mesoporous structure. In addition, the nitrogen adsorption volume increases with the increasing pyrolysis temperature, and BJH pore size distribution becomes more intense according to Fig. S2a and 2b. The mesopore sizes

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are centered at 3.82, 3.84, 3.89 nm for NiCo@NCNT-700, NiCo@MCNT-800,

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NiCo@NCNT-900 (Table S1), respectively. Typically, the mesopore channels has been demonstrated to promote the transport of reactants and/or products, such as O2, H+, OH- and H2O, to access or detach from catalytically active sites during the ORR process[22, 23]. The sample NiCo@NCNT-700 displayed a slightly low BET specific surface area (116.1 m2 g-1) and pore volume (0.266 cm3 g-1) compared with those of other samples (see data in Table S1), whereas the highest ORR catalytic activity was obtained. It can be therefore known that the surface area of the as-obtained samples

ACCEPTED MANUSCRIPT plays an insignificant role in determining the ORR activity. Raman spectra of the NiCo@NCNT-X samples were also investigated, showing two clear bands at 1362 cm-1 and 1585 cm-1 for D band and G band, respectively (Fig.

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S2c), which was attributed to the disordered carbon structure and the vibration of the sp2-hybridization of graphitic carbon atoms in a hexagonal lattice[24]. The ID/IG ratios are widely used to evaluate the degree of disorder in graphitic carbon materials, and

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the values were calculated to be 1.14, 1.03 and 1.01 for NiCo@NCNT-700,

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NiCo@NCNT-800, NiCo@NCNT-900, respectively, indicating a slightly lower graphitization degree of NiCo@NCNT-700 because of its higher amount of structural defects.

From the SEM image in Fig. 2a, a fluffy and crumpled morphology is observed for

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the sample NiCo@MCNT-700, in which very dense and thin CNTs can be discerned. It can also be seen that the metallic particles of 15-39 nm in size are mainly distributed at the epitaxial ends of the nanotubes according to the magnified SEM

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image in Fig. 2a (inset) and Fig. 2b. In contrast, samples NiCo@MCNT-800 and

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NiCo@MCNT-900 exhibit lower amount of CNTs, larger nanotube diameter and larger particle size than those of NiCo@MCNT-700 (see SEM images in Fig. S3). Therefore, it can be understood that the smaller Ni-Co alloy nanoparticle size of sample NiCo@MCNT-700 could be favorable for oxygen reduction reactions. In addition, as can be seen from the SEM images in Fig. S4, very fewer or even no CNTs are present when only Ni or Co precursor has been used in the synthesis, indicating that only the alloyed metal nanoparticles are capable of catalyzing the extensive

ACCEPTED MANUSCRIPT formation of the nanotubes. The morphological features were further observed using transmission electron microscopy (TEM), and it can be found in Fig. 2c that the alloy nanoparticles are encased in the top end of CNTs and coated by several graphitic

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layers (Fig. 2d). The high-resolution TEM (HR-TEM) image of the nanoparticles presents a d-spacing of 0.215 nm (Fig. 2e), which can be assigned to the (111) crystal plane of the Ni-Co alloy, in accordance with the XRD results. From the high-angle

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angular dark-field TEM (HAADF-TEM) image (Fig. 2f and 2j) and the corresponding

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EDS mappings (Fig. 2g-i), it can be found that the encapsulated nanoparticles are composed of homogeneously dispersed Ni and Co elements, which supports the formation of metallic Ni-Co alloyed nanoparticles, in line with the results from XRD. As shown in the electron energy loss spectroscopy (EELS) ( Fig. 2k), two groups of

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characteristic peaks corresponding to Ni (L2, L3) and Co (L2, L3) can observed, further

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verifying the Ni-Co alloying nature.

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Fig. 2. (a, b) SEM images of NiCo@NCNT-700 and the corresponding particle size distribution histogram in the inset; (c) TEM and (d, e) HRTEM images of the NiCo@NCNT-700 with the inset

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showing the FTT diffraction image of the dotted rectangle area in (d); (f) HAADF-TEM image and (g, h) the corresponding EDS mappings of Ni and Co in (f) of NiCo@NCNT-700; (i) EDX spectrum of the red rectangle area in (f); (j) HAADF-TEM image and the corresponding EELS spectrum (k) of the red solid line area in (j) of NiCo@NCNT-700.

X-ray photoelectron spectroscopy (XPS) measurement was carried out to

investigate the surface composition and chemical valence state of the as-prepared NiCo@NCNT-X samples. The survey spectra of NiCo@NCNT-X show clearly the peaks centered at around 285, 532, 780.5, 855.0 eV, corresponding to C1s, O 1s, Co

ACCEPTED MANUSCRIPT 2p, Ni 2p, respectively (Fig. 3a and Fig. S5). Meanwhile, a peak belonging to N 1s (ca. 399 eV) also presents, indicating that nitrogen has been well incorporated into the carbon frameworks. The successful doping of N into carbon materials could facilitate

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the enhancement of chemical/thermal stability, i.e., oxidation resistance of carbon, and enables the co-contributions between the catalytically active species and carbon supports[25, 26]. Additionally, a quite weak peak located at ca.165 eV is also detected,

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which belongs to S 2p.

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The XPS data of NiCo@NCNT-X are summarized in Table. S2. The ratios of N/C increased from 0.12 to 0.17 for the samples pyrolyzed from 700 to 800°C, but subsequently decreased to 0.10 upon elevating to 900°C. The results reveal that the doping of N species in the carbon structure largely depends on the pyrolysis

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temperature, which agrees with the previous reports[27-29]. However, the concentrations of Ni and Co exhibit gradual decreases at elevated pyrolysis temperatures.

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The ORR performance has not only been markedly influenced by the doping level

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of heteroatom N, but also by the bonding types of N with the graphitic carbon scaffold and other species[30, 31]. As shown in Fig. 3b and Fig. S6, the XPS N 1s spectra can be deconvoluted into five independent bands at 398.4, 399.1, 399.9, 401.1 and 403.3eV, attributing to pyridinic N, Co-coordinated N, pyrrolic N, graphitic N and oxides of nitrogen, respectively[32, 33]. Reportedly, the pyridinic N, pyrrolic N, and quaternary N can enhance current density and promote oxygen reduction via inducing the chemical/electronic environment changes of the neighbouring carbon atoms, but

ACCEPTED MANUSCRIPT the contribution of the oxidized N species is unknown[34-37]. Importantly, relatively high levels of Co-coordinated N have been obtained in the NiCo@NCNT-X samples (Fig. S6a, 6d, and 6g), which have been known to be catalytically active for ORR[4].

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From Fig. S7, the S 2p can be divided into four peaks at 162.2, 163.5, 164.7, 168.7 eV, which could be assigned to thiol group (162.2 eV), thiophene group (163.5 eV and 164.7 eV) and SOx moieties (168.7 eV), respectively. Reportedly, the thiophene S

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might be active for ORR[38, 39]. From the as-prepared samples, as temperature

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increased from 700 to 900oC, the amount of thiophene S also increased accordingly (Fig. S7), however the sample NiCo@NCNT-700 prepared at 700oC shows the highest ORR activity, implying that the insignificant role of thiophene S. The XPS Ni 2p high-resolution spectrum of NiCo@NCNT-700 shows two distinct

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peaks located at 853.70 and 872.01 eV, confirming the metallic state of Ni[40, 41]. Additionally, three peaks (855.48, 861.24, and 878.67 eV) may be ascribed to satellite Ni2+ or the readily oxidized Ni ion located at the outer surface of Ni-Co alloy

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nanoparticles owing to the exposure to air (Fig. 3c)[16, 41]. The metallic state of Co

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can also be affirmed by the peak centered at 779.24 eV as shown in the fitted spectrum of Co (Fig. 3d). Likewise, four bands of binding energy centered at 780.81, 784.26, 796.62, 803.09 eV, respectively, may be attributed to the satellite peaks or the oxidized state of Co due to its susceptibility to atmosphere as well[40, 42]. The XPS spectra of Ni 2p and Co 2p from NiCo@NCNT-800 and NiCo@NCNT-900 are also shown in Fig. S6.

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Fig. 3. (a) Survey XPS spectrum of the NiCo@NCNT-700; (b) the N 1s, (c) Ni 2p, (d) Co 2p high-resolution spectra of the NiCo@NCNT-700, along with their corresponding fitting curves.

3.2 Electrochemical activity evaluation for ORR

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To assess the electrocatalytic activities of the prepared samples toward ORR, cyclic voltammetric (CV) measurements was initially conducted in N2 and O2-saturated 0.1 M KOH solutions at a scan rate of 100 mV s-1. It can be found in Fig. 4a that all the

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catalysts, including NiCo@NCNT-700, NiCo@NCNT-800 and NiCo@NCNT-900

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obtained at the Ni/Co ratio of 3:7, exhibit oxygen reduction activity in O2-saturated electrolyte, but quasi-rectangular voltammograms without redox peaks in the N2-saturated electrolyte. Clearly, NiCo@NCNT-700 shows a much higher electrochemical active surface area (ECSA) and the peak current density than the samples NiCo@NCNT-800 and NiCo@NCNT-900 (Fig. 4b), indicating the much higher electrocatalytic activity of the former. To further investigate the electrochemical performance, the linear scanning voltammogram (LSV) curves were

ACCEPTED MANUSCRIPT recorded on a rotating disk electrode (RDE) at a rotation speed of 1600 rpm. Among prepared samples of various molar ratios between Ni and Co, the sample NiCo@MCNT-700 of the molar ratio of Ni/Co=3:7 demonstrates a much more

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positive onset and half-wave potentials than the others (Fig. 4c), verifying the importance of the Ni-Co alloy structure at the mole ratio. At the same Ni/Co ratio of 3:7, sample NiCo@NCNT-700 shows a relatively high onset and half-wave potentials

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of 0.93 V and 0.82 V versus reversible hydrogen electrode (RHE), respectively, which

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were comparable to that of the commercial Pt/C (Fig. 5a). On the contrary, samples NiCo@NCNT-800 and NiCo@NCNT-900 show relatively lower onset and half-wave potentials than NiCo@NCNT-700. Furthermore, the limited current density of NiCo@NCNT-700 is about 0.4 mA/cm2 smaller than that of Pt/C, but 1.1 and 1.3

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mA/cm2 larger than those of NiCo@NCNT-800 and NiCo@NCNT-900, respectively. The results stress the importance of small size and uniform alloying nanostructure of Ni-Co nanoparticles in the top end of N-doped carbon nanotubes for the ORR

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catalytic activity[18]. Moreover, the ORR activity of the NiCo@NCNT-700 is

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comparable to, or significantly higher than reported non-precious catalysts listed in Table. S3.

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Fig. 4. (a) Cycle voltammetry (CV) curves of NiCo@NCNT-X (X=700, 800, 900) in O2-saturated (red solid curves) and N2-saturated (black dashed curves) 0.1 M KOH at a scan rate of 100 mV s-1; (b) the

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CV curves of NiCo@NCNT-X (X=700, 800, 900); (c) LSV curves of the as-prepared samples with various Ni/Co atomic ratios at a scan rate of 5 mV s-1; (d) Nyquist plots of samples obtained from EIS measurements in O2-saturated 0.1 M KOH solution at constant potential of 0.82 V (vs. RHE).

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ORR kinetics were investigated by using the Koutecky-Levich (K-L) equation with

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the LSV curves obtained at different rotation rates (Fig. 5b). The K-L plots of NiCo@NCNT-700 at various electrode potentials shows good linear relationships, from which the electron transfer number per O2 molecules was determined to be 3.7-3.9, indicating an approximate four-electron pathway similar to that of Pt/C[43]. In comparison, NiCo@NCNT-800 and NiCo@NCNT-900 show much lower electron transfer numbers of ~3.4 and ~3.2, respectively, according to the histograms in Fig. 5c, suggesting a more effective ORR catalyst of NiCo@NCNT-700. The kinetic current

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excellent ORR activity of the NiCo@NCNT-700 catalyst was further proved by its slightly higher Tafel slope of 71 mV decade-1 than that (67 mV decade-1) of Pt/C catalyst at low overpotentials, which is much lower than the others (> 80 mV

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decade-1)(Fig. 5d). From the LSV scans before and after accelerated degradation tests

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as shown in Fig. 5e, it can be found that the NiCo@NCNT-700 has experienced a remarkably smaller half-wave potential negative shift than that of the Pt/C catalyst (28 mV for NiCo@NCNT-700 versus 47 mV for Pt/C) in 2000 consecutive cycles under the same conditions,

even

after 5000

consecutive cycles

(46

mV for

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NiCo@NCNT-700 versus 66 mV for Pt/C). The operational durability was further examined by chronoamperometric (CA) measurements within 20 000 s at 0.82 V (Fig. 5f), and the NiCo@NCNT-700 catalyst still retained ~80% of the original current, in

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stability.

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contrast to 35% for Pt/C, further confirming the excellent catalytic operational

In electrochemical impedance spectroscopy (EIS), the semicircle diameter from

Nyquist plot is usually employed to elucidate the charge transfer resistance at the electrode surface. In Fig. 4d, the NiCo@NCNT-700 shows the lowest charge transfer resistance among the samples as revealed by the smallest semicircle diameter, hence indicating that the NiCo@NCNT-700 performs more effectively than the others in shuttling charges from electrode to solution, which consequently results in the

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significant enhancement of the catalytic activity for O2 reduction[38].

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Fig. 5. (a) Linear scanning voltammograms (LSVs) for NiCo@NCNT-X (X=700, 800, 900) and commercial Pt/C on rotating disk electrode at a rotation speed of 1,600 rpm and a scan rate of 5 mV s-1 in O2-saturated 0.1 M KOH solution; (b) LSVs of NiCo@NCNT-700 in O2-saturated 0.1 M KOH at various rotation speeds with a scan rate of 5 mV s-1 and the corresponding K-L plots (inset) for NiCo@NCNT-700 at various potentials and Pt/C (0.60 V vs. RHE); (c) kinetic current density (JK) of various samples in comparison with that of a commercial Pt/C and the corresponding electron-transfer numbers at 0.60 V (vs. RHE); (d) Tafel plots of the prepared samples and Pt/C; (e) LSV curves before and after accelerated degradation test conducted in 0.1 M KOH at a scan rate of 100 mV s-1 for the NiCo@NCNT-700 and Pt/C; (f) current-time (I-t) chronoamperometric response of NiCo@NCNT-700

ACCEPTED MANUSCRIPT and Pt/C at 0.82 V(vs. RHE) in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm.

The samples NiCo@NCNT-X (X=700, 800, 900) also demonstrate certain ORR activity in acidic medium as shown in Fig. S8. The onset and half-wave potentials of

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NiCo@NCNT-700 are 0.79 V and 0.54 V respectively, which are more positive than those of NiCo@NCNT-800 and NiCo@NCNT-900, but negative than that of Pt/C.

The high ORR catalytic activity of NiCo@NCNT-700 in alkaline medium is

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ascribed to the cooperative effects among the Ni-Co alloyed nanoparticles, the formed

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Co-N species and N-doped CNTs (Fig. 6). As reported previously, the cobalt species could be extracted by nitrogen atoms to form Co-N sites in the carbon support, which were also believed to be catalytically active for ORR[4]. It has been revealed that the encased Co-coordinated nitrogen complex is capable of reducing the oxygen

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adsorption energy at carbon surfaces favoring the oxygen molecule reduction according to the theoretical calculations[45]. Moreover, the geometric confinement of

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cobalt-nickel nanoparticles within the nitrogen-doped carbon structures could generate highly active Co-N sites[46, 47]. Here, the largely differentiated chemical/

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electronic environment(s) of bimetallic Ni-Co alloy from that of only Ni or Co, is proposed to promote the formation of Co-N active species via weakening the Co-Co bond by Ni. As depicted in the blue area in Fig. 6, the Ni species in the Ni-Co alloy tends to weaken the Co-Co bond because of the relatively stronger electropositive feature of Co than Ni, favorably leading to the formation of Co-N. From Fig. 3b, a rather intense deconvoluted peak at 399.1 eV can be found and assigned to Co-N[33]. Thus for the NiCo@NCNT-700 sample, the Co-N species is supposed to be the key

ACCEPTED MANUSCRIPT origin of its high ORR activity in alkaline medium, together with the Ni-Co alloy nanoparticles being the other important factor which promotes the Co-N formation. To clarify the important role of Ni-Co nanoparticles for ORR, the control experiments

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were conducted. After 12 h etching, the ORR activity of NiCo@NCNT-700 was remarkably deteriorated as demonstrated by the large negative shift of onset and half-wave potentials as well as the ECSA compared with the sample before etched, as

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shown in Fig. S9 and the inset. Obviously, the drastic drop of the ORR activity largely

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results from the leaching of the N-doped CNTs coated Ni-Co nanoparticles. As shown in Fig. S10, the XRD pattern of NiCo@NCNT-700 after being etched displays much lowered peak intensities as compared to that before etching, indicating the considerable reduction of the alloy nanoparticle size. It is also demonstrated by the

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SEM image in Fig. S10 that the alloy nanoparticles become much smaller or even disappeared after acid etching, resulting in the substantial reduction of the amount of Co-N active sites, which further confirms the indispensable role of Ni-Co alloy

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nanoparticles for the outstanding ORR catalytic performance. In addition, the ORR

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activity can still be retained partly in acidic and alkaline media though the amount reduction of Ni-Co alloy nanoparticles, along with the collapse of nanotubes to a certain extent (Fig. S8, S9 and S10), which implies that the N-doped C species also play a certain role in ORR. Thus, the excellent ORR performance of NiCo@NCNT-700 would be attributed to the co-contribution by Ni-Co alloy nanoparticles, Co-N species and N-doped CNTs, in which both the Co-N and N-doped C species are active for ORR and the bimetallic Ni-Co alloy boost the formation of

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Co-N active sites.

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Fig. 6. Schematic illustration of the possible mechanism for the favorably promoted formation of Co-Nx species by Ni-alloying with Co (red dotted line area). Both Co-N species and N-doped CNTs are

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active toward ORR in NiCo@NCNT-700, while the bimetallic Ni-Co alloy promote the formation of Co-N active species.

4. Conclusions

In conclusion, Ni-Co alloy nanoparticles fully encapsulated in N-doped graphitic

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carbon nanotubes were fabricated by a facile, low-cost process. To the best of our knowledge, this is the first report of Ni-Co bimetallic nanoalloys encased in N-doped

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CNTs nanocomposite for the ORR catalysis. Importantly, the composite catalyst demonstrates excellent electrocatalytic ORR activity with an approximate

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four-electron pathway and robust operational stability in alkaline medium, which could be attributed to the unique confined architecture and host-guest electronic interaction. The encapsulated Ni-Co alloyed nanoparticles, N-doped CNTs and the formation of Co-N species enable cooperatively enhanced electrocatalytic kinetics for ORR, which may provide an effective way for the rational design and preparation of high-performance nonprecious metal electrocatalysts for energy conversion and storage applications.

ACCEPTED MANUSCRIPT Acknowledgements We gratefully acknowledge the financial support from National Key Basic Research Program of China (2013CB933200), Natural Science Foundation of Shanghai

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National Natural Science Foundation of China (U1510107).

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(16ZR1440600), state key laboratory of heavy oil processing (SKLOP201402003),

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