NF heterojunction as both

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In-situ synthesis of Petal-like MoO2@MoN/NF heterojunction as both Advanced Binder-free Anode and Electrocatalyst for Lithium Ion Battery and Water Splitting Yan Sun, Yinlong Zhou, Yaping Zhu, Yuhua Shen, and Anjian Xie ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06321 • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 28, 2019

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ACS Sustainable Chemistry & Engineering

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In-situ synthesis of Petal-like MoO2@MoN/NF heterojunction as both Advanced Binder-free Anode and Electrocatalyst for Lithium Ion Battery and Water Splitting

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Yan Sun, Yinlong Zhou, Yaping Zhu, Yuhua Shen*, Anjian Xie*

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*E-mail: [email protected]; [email protected]

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College of Chemistry and Chemical Engineering, Lab for Clean Energy & Green Catalysis, Anhui University, Hefei 230601, China

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Abstract Designing and synthesizing heterostructure as well as binder-free electrodes or

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electrocatalysts

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performance of lithium ion battery and water splitting are still significant challenges

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to scientists. Here, it is the first time to in situ synthesize the porous petal-like MoN

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nanolayer-coated MoO heterojunction (MoO @MoN) on commercial nickel foam

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(NF) by a localized nitrided transformation method as both binder-free electrode for

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LIBs and electrocatalyst for water splitting. The in-situ formation of MoN nanolayer

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could create MoO @MoN heterostructure, therefore enhancing the electronic

with

porous

nanostructure

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for

enhancing

electrochemical

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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conductivity and the electron/ion transfer. The XRD and XPS measurements

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confirmed the wonderful reversibility of the MoN layer during lithiation/delithiation

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cycling, which effectively promoted the long-life cycling performance (1190.1 mA

4

h g after 500 cycles at the current density of 0.5 A g ). Meanwhile, the

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MoO @MoN/NF/LiFePO full cell displayed stable capacity after 100 cycles.

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Moreover, the product also showed the improved electrocatalytic activity for

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hydrogen evolution and oxygen evolution. The excellent results suggest that our

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work opens a simple in situ heterojunction formation way for the synthesis of other

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multi-functional materials applied in energy conversion and storage, etc.

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Key

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Binder-free; Lithium ion battery; Water splitting

words:

Molybdenum dioxide;

Molybdenum nitride;

Heterojunction;

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Introduction

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Recently, increasing energy consumption and worsening environmental problems

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have accelerated the development of sustainable and clean energy storage systems,

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such as LIBs and hydrogen and oxygen productions through water electrolysis .

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Transition metal oxides (TMOs) are promising materials for lithium ion battery

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(LIBs), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)

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because of their low cost and environmental friendliness

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considered as one of the most promising candidates for both LIBs and water

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splitting due to its high theoretical capacity (800 mA h g ), large density (6.5 g cm )

1-4

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5-10

. MoO has been 2

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to store more energy at the same size of the LIBs compared to that of graphite (2.3 g

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cm ), environmental friendliness and excellent catalytic activity . However, the

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practical application of MoO is hindered for some reasons, such as large and

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uneven volume changes during charge/discharge processes leading to electrode

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pulverization, low electronic conductivity and harsh particle aggregation. In order to

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overcome these barriers, a popular method is to composite MoO

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graphene . For example, Xia et al. prepared MoO with coated carbon presenting

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excellent cycling stability because the carbon matrix enhanced the electrical

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conductivity and avoided the volume change . Tang et al. mixed MoO

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11-13

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with carbon or

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14-19

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2

with

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phosphorus-doped nanoporous carbon and graphene substrates as high-performance

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catalytic was due to that graphene increased the electrochemical conductivity and

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surface area . 18

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Applying transition metal nitrides (TMNs) to modify corresponding TMOs is

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another way to improve the electrochemical performance . Some TMNs (Mo N,

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TiN, VN) attract researcher's attention because they present high electrocatalytic

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activities, surface stability and functional physical properties such as hardness and

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electrical conductivity . Wan et al. synthesized TiN modified Li Ti O that exhibited

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an excellent rate capability and an importantly enhanced cycling performance . Li

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et al. reported that the Fe N coated Fe O prepared by grinding method and then

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calcining under the nitrogen atmosphere showed a high reversible capacity of 620

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mA h g after 60 cycles at the current density of 200 mA g . In addition, it is

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interesting that MoN also has superior properties such as high Pt-like

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electrocatalytic activities, chemical stability, and great conductivity for LIBs and

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water splitting. Abbas et al. synthesized MoN in nitrogen doped carbon nanotube

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(CNT), which presented the reversible capacity of 1323 mA h g after 200 cycles at

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a current density of 100 mA g . Zhu et al. demonstrated that MoN@nitrogen-doped

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carbon exhibited the enhanced electrocatalytic activity for HER . However, MoN

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coated MoO nanocomposits for both LIBs and water splitting have not been

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reported.

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Heterostructures can be synthesized by regulating different compositions or

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doping to enable passivation of interfaces, which is proposed to improve the surface

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reaction kinetics and reversible electron/ion transport at the interface, and own

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synergistic effects for enhancing electrochemical performance . For example,

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two-dimensional (2D) carbon/MoS heterostructures presented excellent cycling and

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high-rate performance in lithium storage . SnO /MXene heterostructure showed

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favorable capacity as anode materials in LIBs . Herein, a simple nitride

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transformation was applied to in situ preparation of MoO @MoN heterojunction

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nanopetals, which uniformly grew on NF. The petal-like MoO @MoN/NF as the

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anode material exhibited excellent cyclic performance and high rate capacity.

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Moreover, beyond application in LIBs, the product also can be used as a non-noble

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high performance electrocatalyst for water splitting.

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Experimental Section 4


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Materials: Ammonium molybdate tetrahydrate ((NH ) Mo O ×4H O) and sodium

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dodecyl benzene sulfonate (SDBS) were purchased from Aladdin industrial

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Corporation (P. R. China). Hydrochloric acid (HCl) was obtained from Shanghai

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Hushi Laborator Equipment Co., Ltd (P. R. China). All reagents were utilized

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without further purification. Milli-Q water (Millipore Corp., with resistivity of

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18.2MW•cm) was used for the experiments. NFs were purchased from Jinghong

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New Energy Corporation (P. R. China) in Taobao.

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Synthesis of MoO @MoN/NF: NFs with diameter of 14mm were cleaned by HCl

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(3M) solution under ultrasound bath for 30min, then washed with deionized water

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and absolute ethanol for removing the nickel oxide. 1g of (NH ) Mo O ×4H O and

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2.8g of SDBS were dissolved in 60mL of deionized water, and then transferred into

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a 100mL Teflon-lined stainless autoclave. Subsequently, a NF was immersed into

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the above mixed solution. The autoclave was heated to 100°C in an electric oven for

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18h. After the reaction, the NF coated with brown product was washed by ethanol

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and deionized water for three times before dried at 60°C for 3h in vacuum drying

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oven, the obtained sample was named as precursor. Then precursors were annealed

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at 600°C in Ar/H (5%) for 2h to get the MoO on NF (MoO /NF), followed by

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calcining at 450°C in NH for 1h, MoO @MoN on NF (MoO @MoN/NF) were

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obtained.

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Characterization: The phase analysis of as-prepared products was carried out by

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power X-ray diffraction (XRD, DX-2700) with Cu-Kα radiation (30 kV, 25 mA, λ =

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1.54056 Å) at a scan rate of 0.05° s . The morphologies of the samples were

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characterized by field emission scanning electron microscope (FESEM, Hitachi

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S-4800, Japan) at a voltage of 80 kV, Transmission electron microscopy (TEM,

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Japan Electron Co., Ltd.) was operated at an accelerating voltage of 200 kV. The

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surface elemental analysis of the samples was surveyed with X-ray photoelectron

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spectroscopy (XPS, Thermo ESCALAB250, US). X-ray photoelectron spectroscopy

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(XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed on a

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Thermo ESCALAB250. He I (hν = 21.22 eV) was employed as the exciting source

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during the UPS measurement.

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Electrochemical Performance Measurement

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LIBs test: The coin cell was assembled in an argon-filled glove box. The NF

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supported MoO @MoN (1.58cm area, the mass loading: 2.0-2.3mg, the thickness:

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1mm) was directly used as the working electrode. For calculating the mass loading

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of MoO @MoN on NF, the prepared MoO @MoN/NF was immersed into a 40%

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HCl solution overnight for the remove of NF, and the obtained precipitate was

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centrifuged, washed, dried and weighed. The metallic lithium and polyethylene film

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(Celgard, 2400) were employed as the counter electrode and separator, respectively.

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1 M LiPF in an ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl

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carbonate (DEC) mixture (1:1:1, in wt %) was used as the nonaqueous electrolyte.

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For the full Li-ion cell tests, a commercial LiFeO took place of lithium foil in the

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cathode. The cathode was prepared from commercial LiFeO powder, acetylene

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black, and polyvinylidene fluoride (PVDF) in a weight ratio of 8:1:1. The mixed

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slurry was spread on Al foil (14 µm) and dried under vacuum at 120 °C for 6 h. The

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mass loadings of the LiFeO as the electrode were 1.38 mg cm , 3.11 mg cm and

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4.46 mg cm , corresponding to the mass cathode/anode(C/A) ratio of 1.1, 2 and 3.

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The thickness of cathode electrode was in the range of 60-90 µm. Before measuring

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full cell, the MoO @MoN/NF was pre-treated by lithiation process. The

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charge-discharge measurements were performed on a CT-3008W-5V 10mA

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(Neware Technology Co., Ltd., P. R. China) within the voltage range of 0.01-3.0V.

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Cyclic voltammetry (CV) curves at a scanning rate of 0.1 mV s were measured on

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an electrochemical workstation (CHI660D, Shanghai CH instruments Co., Ltd. P. R.

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China). Electrochemical impedance spectroscopy (EIS) was measured in the range

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of 100 MHz-0.01 Hz.

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HER and OER tests: Electrochemical performance measurements were enforced in

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a typical three-electrode cell on CHI660D electrochemical workstation. The

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MoO @MoN/NF was directly used as working electrode, an Ag/AgCl electrode as

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the reference electrode, and a graphite rod as the counter electrode. The 1M KOH

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was applied as electrolyte. All potential values were calibrated to a reversible

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hydrogen electrode (RHE) for appraising the electrocatalytic performances of

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various catalysts.

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Results and Discussion

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Scheme 1. Schematic illustration for the synthesis process of MoO @MoN

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nanopetals on NF

2

6 7

NF is an ideal 3D microporous skeleton as basement for synthesizing different

8

composites, which can effectively promote electrolyte diffusion through the

9

electrode, serve as a solid support to keep the integrity of electrode, and eliminate

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the use of insulating polymer binders and conducting additives. Scheme 1 illustrates

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the schematic of the synthesis approach for MoO @MoN on NF in the present study.

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Firstly, the gray NF (Scheme 1a) as a substrate was immersed in (NH ) Mo O ×4H O

13

aqueous solution, the brown precursor was acquired by using the hydrothermal

14

method (Scheme 1b). Then the black MoO (Scheme 1c) was obtained by calcining

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the brown precursor in an Ar/H (5%) gas atmosphere at 600°C for 2h. Lastly, the

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part of MoO was nitrided in suit in the NH gas atmosphere at 450°C for 1h to

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2

2

2

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3

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obtain the final product of black MoO @MoN heterojunction (Scheme 1d). The

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color change of samples on NF exhibited some reactions maybe occurred, XRD and

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XPS results can further confirm them later.

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Figure 1. SEM images of (a, b) the precursor, (d, e) MoO /NF and (g, h)

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MoO @MoN/NF; XRD patterns of (c) precursor, (f) MoO /NF and (i)

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MoO @MoN/NF. (Ni, MoO and MoN are marked as “¨”, “§” and “©”,

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respectively)

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2

2

2

2

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Figures 1a, b present SEM images of precursor obtained by hydrothermal way

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without annealing, consisted of many nanosheets. From the XRD results in Figure

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1c, except for the three strong peaks derived from the NF substrate (marked as “¨”)

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(JPCDS card no. 04-0850), no other peaks could be seen, indicating that 9



Page 10 of 32

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nanosheet-like precursor was amorphous material. As shown in Figures. 2d, e, there

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was no obvious morphology change for the annealed precursor in Ar/H (5%)

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compared with Figures 1a, b, but the color of samples varied from brown to black

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(Scheme 1b, c). In Figure 1f, the XRD pattern maintained original NF peaks and

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three new diffraction peaks at 2q of 26.03°, 37.03° and 53.52°(marked as “§”) were

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assigned to the (-111), (-211) and (211) planes of MoO (JPCDS card no. 32-0671),

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demonstrating the formation of MoO /NF. After further annealing in NH

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atmosphere, the resulting product exhibited different shape (Figure 1g) from

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MoO /NF. The higher-magnification SEM image (Figure 1h) presented that the

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product possesses petal-like structure composed of many sheets with the thickness

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of 2-3 nm, and sheets interconnected with each other. The ultrathin nanopetals can

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provide an enlarged electrolyte/electrode contact area for lithium storage. The XRD

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result in Figure 1i showed besides the presence of MoO /NF, the diffraction peaks at

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2q values of 31.91°, 36.21°, 49.01° and 65.13° (marked as“©”) can be well indexed

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to the (002), (200), (211) and (220) planes of MoN (JPCDS card no. 25-1367). The

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results demonstrate the successful synthesis of MoO @MoN/NF nanocomposites

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via an in-situ nitrided conversion reaction of MoO /NF calcined in the NH

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atmosphere.

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1 2 3

Figure 2. (a, b) TEM images, (c) HRTEM image, (d) SAED image and (e) STEM

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image of MoO @MoN/NF and the corresponding EDS mappings of (f)

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molybdenum, (g) oxygen and (h) nitrogen.

2

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The heterostructure and morphology of samples were further investigated by

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TEM images. Figures 2a, b exhibited the low magnification TEM images of

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MoO @MoN/NF, which presented petal-like structure with widths of ~60nm. An

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interplanar spacing of 0.186 nm, characteristic of the (211) plans of MoN, could be

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found on the boundary (Figure 2c), while the internal lattice spacing of 0.241nm

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corresponded to the (-211) lattice planes of MoO , indicating the MoN grew on the

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border and on the surface of MoO via in situ nitrided transformation process. And

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the formation of heterojunction between MoO and MoN was indicated by purple

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line. The selected area electron diffraction (SAED) pattern (Figure 2d) presented the

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polycrystalline nature of MoO @MoN heterojunctions, consisting with MoO and

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MoN. The results confirm that the MoO and MoN co-existed, which is agreed with

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the XRD and XPS results. STEM image and EDS elemental mappings of the

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product were shown in Figures 2e-h, indicating three elements of Mo, O and N were

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homogeneously distribution. The in situ localized nitrided transformation and

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formation of MoN on the border and surface can improve the electrical conductivity,

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shorten the electron/ion-transfer pathway and generate MoO @MoN heterostructure,

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which can lead to the phase interface with lattice defects to improve contact

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between material and electrolyte , and tailor the physicochemical characteristics of

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the two components and endow MoO @MoN with remarkable performance because

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of synergistic effects .

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Figure 3. (a) Nitrogen adsorption-desorption isotherm; (b) XPS survey, (c) Mo 3d,

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(d) N 1s and (e) O 1s core-level spectra and (f) UPS spectra of porous

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MoO @MoN/NF. The inset in Figure. 3a presents the pore size distribution. 2

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Nitrogen adsorption-desorption measurement and Barrett-Joyner-Halenda (BJH)

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pore-size analysis (Figure 3a) can be applied to researching specific area and porous

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characteristics of MoO @MoN/NF nanocomposites. The Brunauer–Emmett–Teller

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(BET) specific surface area was calculated to be around 40.52 m g and the average

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pore size was 18 nm. It is significant that MoO @MoN/NF owns porous structure,

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which can provide extra active sites and improve the contact of active material with

13

electrolyte. In addition, the porous structure can decrease the Li diffusion distance

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and buffer the volume change during charge-discharge process. To identify the

15

surface composition and chemical states on the sample, the XPS spectra of

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MoO @MoN/NF are obtained as shown in Figures 3b-e. The survey XPS spectrum

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in Figure 3b confirmed the principle core levels of Ni 2p, N1s, O1s, and Mo3d. The

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high-resolution XPS spectrum of the Mo 3d peak is shown in Figure 3c. The

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binding energy at around 229.6 eV and 232.7 eV were assigned to the Mo (IV)

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oxidation state of MoO and the other peak at 229.2 eV revealed the Mo-N bonds in

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MoN, indicating the formation of MoO and MoN. The peak located at 235.6 was

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attributed to the Mo (VI) oxidation state.

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slight oxidation during the sample preparation for XPS measurement. In the

8

corresponding N 1s spectrum (Figure 3d), the two peaks at 397.9 eV and 395.7 eV

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belonged to the formation of Mo-N bond.

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32-34

The presence of Mo (VI) was due to the 26

26, 35-36

The O 1s peak at 530.8 eV (Figure. 4e)

10

was derived from MoO . The XPS spectra provide further evidence for the

11

synthesis of MoO @MoN/NF. The UPS valence band spectra of MoO /NF and

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MoO @MoN/NF are shown in Figure 3f The valence band maximum position of

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MoO /NF and MoO @MoN/NF were found to be at 0.48 eV and 1.81 eV,

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respectively. Because the valence electrons close to the Fermi level of MoO mainly

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supported the d states and the valence band of MoO /NF had a negative shift after

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nitriding, the d states of MoO @MoN/NF changed. The antibonding states of

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MoO @MoN/NF were filled with more electrons compared to that of MoO /NF ,

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resulting in the subdued adsorption of O and the rapid dissociation of H O absorbed

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on the catalyst surface, which could enhance the kinetics of OER and HER .

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Figure 4. (a) The first four CV curves of MoO @MoN/NF, the voltage range was

3

from 0.01 to 3.0 V at a scan rate of 0.1 mV s ; (b) discharge-charge curves of

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MoO @MoN/NF during the first two and twentieth cycles, the cell was tested

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between 0.01 and 3.0 V at a current density of 0.2 A g ; (c) rate-capability tests for

6

MoO /NF and MoO @MoN/NF at various current densities; (d) cycle-life

7

performances of different samples at a current density of 0.5 A g over 500 cycles

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and (e) comparison of the electrochemical performances of similar LIB anode

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materials.

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Figure 5. Electrochemical impedance spectra for MoO /NF and MoO @MoN/NF. 2

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Page 16 of 32

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The as-synthesized MoO @MoN/NF was used as a binder-free anode to evaluate

2

the lithium-ion storage performance. Figure 4a presents the first four CV curves

3

between 0.01 and 3.0 V at a scan rate of 0.1 mV s . The reversible reaction of

4

MoO @MoN/NF with lithium is described by the electrochemical reactions shown

5

in Eqs. (1-3)

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-1

2

25, 41

6

MoO + xLi + xe « Li MoO (1)

7

Li MoO + (4-x) Li + (4-x) e « 2Li O + Mo (2)

8

MoN + xLi + xe « Mo + Li N (3)

+

-

2

x

2

+

x

-

2

2

+

-

x

9

In the first cycle, redox peaks at around 1.23V/1.5 V could be observed, which was

10

due to the partial insertion of Li into MoO to form Li MoO (Eq. 1). An irreversible

11

reduction peak at 0.59 V and then disappeared in the following cycles, which

12

corresponded to the irreversible reduction of electrolyte and the formation of solid

13

electrolyte interface (SEI) film. In the next three cycles, the reduction peak located

14

at 0.3V, indicating that the conversion reaction of Li MoO with Li (Eq 2) .The

15

second, third and fourth CV curves mostly overlapped, indicating that the

16

electrochemical reaction was reversible. However, the wide reduction and oxidation

17

peaks were different from the reports, which might be ascribed to the role of the

18

MoN coating. Meanwhile, MoN presented the conversion reaction with Li (Eq. 3),

19

the cathodic peak at 1.5V corresponded to the conversion reaction of MoN with Li

20

overlaps , while the anodic peak at 1.23V was attributed to the Li released from

21

Li N . The first two and twentieth charge-discharge curves of MoO @MoN/NF at

22

current density of 0.2 A g over a potential range of 0.01-3.0V are shown in Figure

+

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x

2

x

+

2

41

42

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+

+

+

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x

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4b. In the first cycle, the initial discharge and charge capacities of MoO @MoN/NF

2

were 1727.3 mA h g and 1328.8 mA h g , respectively. An irreversible capacity

3

loss of 76.9% was mainly caused by forming the SEI film and trapping of Li in the

4

MoO @MoN/NF lattice. In the second cycle, the discharge and charge capacities

5

could also achieve to 1296.4 mA h g and 1294.9 mA h g , and there was no further

6

obvious decrease until the 20 cycle. The MoO @MoN/NF also presents the more

7

excellent rate performances than MoO /NF at various current densities, as shown in

8

Figure 4c. In the initial 10 cycles, the MoO @MoN/NF exhibited reversible

9

capacities from 1752.7 mA h g to 1328.1 mA h g at 0.1 A g . The reversible

10

capacity still retained around 810.3 mA h g even at a high current density of 1.0 A

11

g during 41 -50 cycles. When the current density was dropped from 1.0 to 0.1 A g ,

12

the reversible capacity was still high (up to 1390.2 mA h g ). To further confirmed

13

the excellent rate performance of MoO @MoN/NF, the cycles were continuing.

14

After 90 cycles, the reversible capacity could also reach to 1480 mA h g at 0.1 A g .

15

On the contrary, the specific capacity of MoO /NF only maintained around 418.5

16

mA h g at 1A g and restored around 1005.3 mA h g when the current density

17

returned to 0.1 A g . The results show that the Li storage and release performances

18

of MoO @MoN/NF are much more superior to MoO /NF. Besides the excellent rate

19

capacity, the cycling stability and coulombic efficiencies (CEs) of MoO @MoN/NF

20

compared with MoO /NF are exhibited in Figure 4d at a current density of 0.5 A g .

21

It was seen that the initial discharge capacity of MoO @MoN/NF was 1727.3 mA h

22

g , and then slight dropped in the 2 to 24 cycle. From 25 to 100 cycle, the

2

-1

-1

+

2

-1

-1

th

2

2

2

-1

-1

-1

-1

-1

th

th

-1

-1

2

-1

-1

2

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-1

-1

+

2

2

2

-1

2

2

-1

nd

th

17


th

th


Page 18 of 32

1

capacity of MoO @MoN/NF increased to 1187.6 mA h g and then slightly declined

2

from 100 to 150 cycle. Afterwards, the reversible capacity became stable and

3

retained as high as 1056.1 mA h g even 400 cycles. Interestingly, the reversible

4

capacity of MoO @MoN/NF improved to 1190.1 mA h g after 500 cycles.

5

However, the capacity of MoO /NF faded directly after 50 cycle, which might be

6

due to the pulverization issue. To the best of our knowledge, this sample exhibits

7

excellent electrochemical performance at the high density compared with other

8

previously reported graphene/MoO , MoO /carbon

9

nanotubes/MoN , MoO @Mo N , MoO /Mo C/C , MoO /Mo C/graphene

-1

2

th

th

-1

-1

2

th

2

12

2

25

2

2

22

2

2

, graphene/MoN , carbon

14, 43

2

44-45

36

2

2

and

46

10

MoO /Mo S/graphene (Figure 4e). The enhanced electrochemical performances of

11

MoO @MoN/NF are further confirmed by EIS. Figure 5 shows Nyquist plots of the

12

MoO /NF and MoO @MoN/NF before cycling. The depressed high-frequency

13

semicircle is related to the charge transfer resistance at electrode/electrolyte

14

interface, and low-frequency straight line corresponds to the lithium ion diffusion in

15

the electrode . The MoO @MoN/NF presented lower resistance than MoO /NF,

16

indicating that the MoN coating improved the electrical conductivity, which

17

resulted in the excellent electrochemical performances of MoO @MoN/NF.

47

2

2

2

2

2

48

2

2

2

18


Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Figure 6. XRD patterns of (a) MoO /NF and (b) MoO @MoN/NF (Ni, MoO and

3

MoN are marked as “ª”, “#” and “*”, respectively), (c) Mo 3d and (d) N 1s XPS

4

spectra and (e, f) SEM images of MoO @MoN/NF electrodes after 500 cycles at

5

current density of 0.5 A g .The inset in Figure 6f exhibits the photograph of

6

MoO @MoN/NF electrode after 500 cycles.

2

2

2

2

-1

2

7 8

In the XRD pattern of MoO /NF measured after 500 cycles fully charge at current

9

density of 0.5 A g (Figure 6a), there were no obvious peaks of MoO , which was

2

-1

2

19



Page 20 of 32

1

possibly ascribed to the decomposition of MoO during cycling. Interestingly, the

2

MoO peaks could be seen in the MoO @MoN/NF electrode (Figure 6b) after 500

3

lithiation/delithiation process, although the peaks became broad and weak, and the

4

MoN peaks also maintained well. And Mo 3d and N 1s core level XPS spectra

5

(Figures 6c, d) present the Mo (IV) oxidation state and Mo-N bonds, indicating high

6

reversibility and stability for MoO @MoN/NF electrode. SEM images can further

7

confirm the structure stability of MoO @MoN/NF after 500 cycles. It was seen from

8

Figures 6e, f that the nanopetal structure on the NF remained hardly changed and

9

active material had no aggregation on NF. The inset in Figure 6f presented the

10

photograph of MoO @MoN/NF electrode after 500 cycles, there was no obviously

11

break and disintegration on the anode because the coating structure with MoN

12

nanolayer could contribute to structural stability and prevent agglomeration of

13

active materials for enhancing the cycling performance of the electrode.

2

th

2

2

2

2

2

14

20


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1 2

Figure 7. Charge–discharge curves of MoO @MoN/NF/LiFePO full cell at the

3

C/A mass ratios of (a)1.1, (b)2 and (c)3; (d) cycle performances of the

4

MoO @MoN/NF/LiFePO full cell with different C/A ratios over 100 cycles at a

5

current density of 100 mA·g .

2

2

4

4

-1

6 7

In order to further identify its potential in practical applications, the petal-like

8

MoO @MoN/NF

9

commercial LiFePO cathode in full cell. The charge-discharge curves and cycling

10

performances of the full cells were measured at different cathode (C) /anode (A)

11

mass ratios. The properties of full cell were normalized to the mass of LiFePO

12

cathode material. Figures 7a-c show the first and 10 charge-discharge curves of

13

MoO @MoN/NF/LiFePO full cell between 1.0 and 3.6V at a current density of 0.1

2

heterojunction

nanocomposite

was

assembled

with

the

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th

2

4

21



Page 22 of 32

1

A g with three different C/A mass ratios. The initial cycle discharge capacities of

2

the C/A mass ratios of 1.1, 2 and 3 are 122.8 mA h g , 123.4 mA h g and 126.9 mA

3

h g , and the charge capacities are 139.60 mA h g , 140.98 mA h g and 141.5 mA h

4

g , respectively, while the coulombic efficiencies (CEs) of 1st cycle are 87.9%,

5

87.5% and 89.7%, respectively. And the discharge capacity in the 10 cycle of the

6

C/A mass ratio of 3 is higher than that of 1.1 and 2, indicating the excellent

7

electrochemical performance of C/A mass ratio of 3. The cycling stability of the

8

MoO @MoN/NF/LiFePO full cells with three types C/A mass ratios at current

9

density of 0.1 A g are shown in Figure 7d. We observe that the full cell with C/A

10

mass ratio of 1.1, 2 and 3 exhibited the discharge capacity of 69.2 mA h g , 89.4 mA

11

h g and 117.9 mA h g at 100 cycle, suggesting the retention of capacity was

12

56.3%, 72.4% and 83%. The above results show that full cell with C/A mass ratio of

13

3 possesses the most stable cycling performance, ascribing to deeper lithium ion

14

intercalation for the anode electrode49. And the specific capacities of

15

MoO2@MoN/NF and LiFePO4 are ~1100 mA h g and 170 mA h g , the mass C/A

16

ratio of 3 is enough high to balance lithium trapping inside the MoO2@MoN/NF

17

anode50. The assembled MoO @MoN/NF /LiFePO full cell could power a green,

18

yellow and red LED lighting (Figure S1). Therefore, the MoO @MoN/NF with

19

petal-like heterostructure possesses long and stability cycle life performance,

20

suggesting it is a promising anode material for LIB applications.

-1

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1 2

Figure 8. IR corrected LSV curves of NF, MoO /NF and MoO @MoN/NF 2

2

3

electrodes in 1.0M KOH at scanning rate of 5 mV s for (a) HER and (d) OER; LSV

4

curves of MoO @MoN/NF before and after 2000 CV cycles for (b) HER and (e)

5

OER; Tafel plots of NF, MoO /NF and MoO @MoN/NF for (c) HER and (f) OER

-1

2

2

2

6 7

In addition to remarkable electrochemical properties as an anode for LIB,

8

MoO @MoN/NF also exhibited excellent electrocatalytic HER and OER

9

performances, which were evaluated in 1.0M KOH electrolyte using a

10

three-electrode system. Figure 8a presents the HER linear sweep voltammetry (LSV)

11

curves over NF (without loading), MoO /NF (loading: 1.4 mg cm ) and

12

MoO @MoN/NF (loading: 1.4 mg cm ), respectively. The NF and MoO /NF shown

13

poor performance for the HER (corrected using internal resistance (iR)

14

compensation), which owned large initial overpotentials of 210 mV and 150 mV

15

and large overpotentials of 256 mV and 187 mV at the current density of 10 mA

16

cm , respectively. In complete comparison, the as synthesized petal-like

2

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2

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Page 24 of 32

1

MoO @MoN/NF heterojunction presented small onset potential value (near zero)

2

and the corresponding overpotential reached 152 mV at current density of 10 mA

3

cm . Stability was a parameter for evaluating electrocatalytic activity, the durable

4

test had been conducted by 2000 cycles shown in Figure 8b. No obvious decrease

5

had been observed, indicating good electrocatalytic stability of MoO @MoN/NF

6

toward HER process. Furthermore, Tafel slop is another critical parameter to

7

estimate the HER reaction kinetics of electrocatalysts. As shown in Figure 8c, the

8

Tafel plots was calculated from polarization measurements for proving the HER

9

activity. Tafel slope for MoO @MoN/NF (98 mV dec ) was smaller than that for NF

10

(140 mV dec ) and MoO /NF (102 mV dec ), meaning superior HER kinetics for

11

MoO @MoN/NF. The results clearly demonstrate that the MoO @MoN/NF

12

possesses more excellent HER catalytic performance than NF and MoO /NF in

13

alkaline media. The OER catalytic performances of the NF, MoO /NF and

14

MoO @MoN/NF were also evaluated in 1.0M KOH solution. Figure 8d shows the

15

OER polarization curves over NF, MoO /NF and MoO @MoN/NF electrodes, which

16

are corrected by iR compensation. MoO @MoN/NF showed a good catalytic OER

17

activity and took only 290 mV overpotential at the current density of 10 mA cm ,

18

which was 60 mV lower than MoO /NF (350 mV). The high stability of porous

19

MoO @MoN/NF after the continuous measurements for 2000 cycles is shown in

20

Figure 8e. The Tafel slopes (Figure 8f) for NF, MoO /NF and MoO @MoN/NF

21

were 124 mV dec , 108 mV dec and 98 mV dec , respectively, which indicated the

22

presence of porous petal-like MoO @MoN/NF possesses a favorable OER catalytic

2

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2

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2

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2

2

2

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2

2

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2

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1

dynamics. Abundant hydrogen and oxygen were generated at the cathode and anode

2

by using MoO @MoN/NF as the catalyst and MoO @MoN/NF /LiFePO full cell as

3

the electricity source (Figure S2). These results further confirm that the improved

4

activity of MoO @MoN/NF over MoO /NF and NF implies the important role of

5

heterojunction for OER and HER due to them synergistic effect and strong

6

electronic interaction. Meanwhile, the MoN coating can enhance the electrical

7

conductivity and 3D skeleton structure of NF with good conductive and porous

8

structure can favor the electron transfer during reaction. Furthermore, the

9

MoO @MoN is directly synthesized on NF without binder which can decrease valid

10

activity of catalysts, block active sites and mass transfer, and reduce gas

11

permeability.

2

2

2

4

2

51

2

52

12 13

Conclusion

14

In summary, the MoO @MoN heterojunction on NF was successfully synthesized

15

by an in-situ growth and nitrided transformation way. The excellent electrochemical

16

performances of MoO @MoN/NF are ascribed to the following reasons: (a) the

17

porous structure can provide extra space to remit the volumetric variation during

18

charge-discharge process, shorten the transmission path for Li and provide full

19

contact of electrolyte with electrode for Li-ion storage; (b) the in situ nitrided

20

conversion for the formation of

21

improve the electronic conductivity; (c) the coating structure could prevent active

22

materials from aggregation during lithiation-delithiation process, making a

2

2

+

MoN nanolayer at the border and on surface can

25



Page 26 of 32

1

stabilized structure of MoO @MoN; (d) the binder-free electrode without using

2

insulating polymer binder and conductive material can further improve Li and

3

electron kinetics of redox reactions. The MoO @MoN/NF presents excellent

4

specific capacity of 1190.1 mA h g at 0.5 A g after 500 cycles, which is much

5

higher than MoO /NF (320.5 mA h g ). A MoO @MoN/NF/LiFePO full cell also

6

delivers a long lifetime and stable cyclability. Furthermore, the MoO @MoN/NF

7

also exhibits enhanced electrocatalytic activity for OER and HER. It is expected

8

that Mo-based nanocomposites on different substrates can be as candidates in

9

energy conversion for the next generation energy transfer and storage devices

2

+

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-1

-1

2

2

4

2

10 11 12 13

ASSOCIATED CONTENT

14

Supporting Information

15

LED bulbs and splitting devices by using MoO2@MoN/NF as electrode and catalyst

16

and MoO2@MoN/NF /LiFePO4 full cell as electricity source.

17 18

AUTHOR INFORMATION

19

Corresponding Author

20

*E-mail: [email protected]; [email protected]

21

ORCID

22

Yan Sun：0000-0003-2639-6106 26


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1

Yuhua Shen：0000-0002-0527-4735

2

Anjian Xie：0000-0002-2864-4823

3

Author Contributions

4

The manuscript was written through the contributions of all authors. All authors

5

have given approval to the final version of the manuscript.

6

Notes

7

The authors declare no competing financial interest.

8

Acknowledgments

9

This work is supported by the National Natural Science Foundation of China (Nos.

10

21671001 and 21571002).

11 12 13 14 15 16 17 18 19 20 21 22

References 27



Page 28 of 32

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Free-Standing Membrane Anode for Lithium/Sodium Ion Batteries. ACS Appl. Mater. Interfaces

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The porous MoO @MoN/NF/NF heterojunction was synthesized by a localized

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nitrided transformation method, which exhibits outstanding electrochemical

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performances in both LIBs and water splitting.

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NF heterojunction as both

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