Hierarchical NiCo2O4@Ni(OH)2 core-shell nanoarrays as advanced electrodes for asymmetric supercapacitors with high energy density

Accepted Manuscript Hierarchical NiCo2O4@Ni(OH)2 core-shell nanoarrays as advanced electrodes for asymmetric supercapacitors with high energy density Ping Xia, Qiyuan Wang, Yintao Wang, Wei Quan, Deli Jiang, Min Chen PII:

S0925-8388(18)33046-9

DOI:

10.1016/j.jallcom.2018.08.163

Reference:

JALCOM 47251

To appear in:

Journal of Alloys and Compounds

Received Date: 5 June 2018 Revised Date:

14 August 2018

Accepted Date: 17 August 2018

Please cite this article as: P. Xia, Q. Wang, Y. Wang, W. Quan, D. Jiang, M. Chen, Hierarchical NiCo2O4@Ni(OH)2 core-shell nanoarrays as advanced electrodes for asymmetric supercapacitors with high energy density, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.08.163. 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.

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Hierarchical NiCo2O4@Ni(OH)2 core-shell nanoarrays as advanced electrodes for asymmetric supercapacitors with high energy density

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Ping Xia, Qiyuan Wang, Yintao Wang, Wei Quan, Deli Jiang, and Min Chen* School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang

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212013, China

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Corresponding author: Min Chen

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E-mail address: [email protected]

ACCEPTED MANUSCRIPT Abstract: Development of high-performance supercapacitors with high energy density is urgently required for energy storage device but remains challenging. In this work,

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novel NiCo2O4@Ni(OH)2 (denoted as NCO-NH) core-shell nanoarrays were synthesized. Benefiting from the structural and compositional advantages, the NCO-NH core-shell nanoarrays exhibited high specific capacitances (2218.0 and

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1796.0 F g-1 at current densities of 1 and 10 A g-1, respectively) and outstanding

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cycling stability (96.4% retention of the initial specific capacitance at 10 A g-1 after 5000 cycles). Using such NCO-NH nanoarrays as cathode and activated carbon as anode, an asymmetric supercapacitor (ASC) is further fabricated. This ASC device achieves a high energy density of 98.5 W h kg-1 at a power density of 800 W kg-1 (at

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current densities of 1 A g-1) and an energy density of 61.5 W h kg-1 at a high power density of 8000 W kg-1 (at current densities of 10 A g-1). This work highlights that construction of core-shell nanoarrays by coupling of bimetallic oxide with hydroxide

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could be a feasible way to obtain advanced electrode materials for high-performance

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supercapacitors.

Keywords: NiCo2O4; Ni(OH)2; Core-shell nanoarrays; Electrochemical performance; Asymmetric supercapacitor 1. Introduction

The development of high-performance energy storage systems has received great interests owing to the ever-increasing demands for renewable energy [1-3]. Supercapacitor has become a research focus due to its’ many advantages, such as high

ACCEPTED MANUSCRIPT power density, short charging time, long cycle life and environmental protection [4-7]. Nevertheless, the large scale application of supercapacitor is inhibited by its relatively lower energy density in contrast to other energy storage device such as lithium-ion

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batteries. To meet the increasing demand, development of high-performance electrode material is of great importance. Transition metal oxide has aroused tremendous concern due to their large theoretical specific capacitances derived from multiple

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oxidation states, which are desirable for pseudo capacitance generation [8]. Bimetallic

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oxides such as NCO [9-11], MnCo2O4 [12], ZnCo2O4 [13], NiMoO4 [14] and so on, exhibited enhanced electrochemical activity due to the synergistic effect of different metal elements which could endow the superior electronic conductivity and charge capacity compared with the corresponding single-component oxides. As a promising

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candidate, NCO has received much more attention due to its superior conductivity, capacity in offering richer Faradaic redox reactions, abundance on the earth, low cost and environment-friendly [15]. However, the rate capability and energy density of

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single NCO material are still not satisfactory, which will impair the performances of

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supercapacitor in particular at high-rate charge-discharge processes. To enhance the rate capability and achieve high energy density, construction of

hierarchical architecture consisting of multicomponent materials with more accessible electroactive sites has been considered as an effective strategy [16-18]. Ni(OH)2 has excellent theoretical specific capacitance over 2000 F g-1 and excellent chemical stability but with poor conductivity [19-24]. It has been considered that assembling this Ni(OH)2 active materials on the well-arranged NCO nanowire arrays to form

ACCEPTED MANUSCRIPT hierarchical core-shell nanostructures can improve the electrochemical performance of single-component system, since these hierarchical core-shell nanostructures not only offer a favorable interfacial electron transfer and ion diffusion path but also

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render more electroactive sites for the capacitive reaction. For example, Huang et al. constructed Ni(OH)2@NiCo2O4 hybrid composite on carbon fiber by the electrodeposition-annealing-electrodeposition method, and the Ni(OH)2@NiCo2O4

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electrodes demonstrated a high areal capacitance of 5.2 F/cm2 at a cycling current

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density of 2 mA/cm2 [23]. Similarly, Li et al. coated Ni(OH)2 on the NiCo2O4 nanowires grown on the carbon fiber by the electrodeposition method, and the composite possesses the highest areal capacitance of 6.04 F/cm2 at the current density of 5 mA/cm2 [24]. However, the compositional effect of the Ni(OH)2@NiCo2O4

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hybrids on their electrochemical activity was rarely studied. It is thus still desirable to improve the electrochemical performances of supercapacitor to meet the ever increasing renewable energy needs.

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Based on the above considerations, we constructed NH nanosheet wrapped NCO

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(NCO-NH) core-shell nanoarrays on Ni foam substrates as a high performance electrochemical electrode. This unique hierarchical have several features: (1) The NCO and NH individually enrich the redox reactions, resulting in the enhanced electrochemical capability. (2) The NH nanosheet wrapped on the nanowires could render more electroactive sites for the capacitive reaction which in turn overcome the problem of poor conductivity of NH. (3) The high conductivity of the NCO core of the NCO-NH heterostructure enabled the fast charge transport, which would

ACCEPTED MANUSCRIPT accelerate the reaction kinetics. (4) The core-shell nanowires arrays could enhance the stability of electrode in the backbone during charge/discharge cycles. Due to these advantages, the resulting NCO-NH nanoarrays exhibited high specific capacitances

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(2218.0 and 1796.0 F g-1 at current densities of 1 and 10 A g-1, respectively), which are much higher than those of reported values. Furthermore, the NCO-NH//AC asymmetric supercapacitor with a maximum operating voltage of 1.6 V was

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successfully assembled, achieving a high energy density of 98.5 W h kg-1 at a power

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density of 800 W kg-1 and an energy density of 61.5 W h kg-1 at a high power density of 8000 W kg-1 (at current densities of 10 A g-1). 2. Experimental section 2.1 Materials nitrate

(Ni(NO3)2·6H2O),

cobalt

nitrate(Co(NO3)2·6H2O),

urea

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Nickel

(CO(NH2)2), hydrochloric acid (HCl), potassiumhydroxide (KOH), absolute ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd., China and were used as

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received without any further purification. Ni foam (NF) was purchased from Jia Shide

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Form Metal Co., Ltd., China. Deionized water was used in the experiments. 2.2 Pretreatment of Ni foam (NF) A piece of NF (1×1×0.16 cm) was soaked in 0.5 M HCl for a few seconds before

synthesizing. In order to remove the surface oxidized layer, NF was ultrasonically washed using ethanol and acetone for three times, and then was dried. 2.3 Preparation of NCO nanoarrays NCO nanoarrays were synthesized by a facile hydrothermal method [25]. 4

ACCEPTED MANUSCRIPT mmol of Ni(NO3)2·6H2O, 8 mmol of Co(NO3)2·6H2O and 20 mmol of CO(NH2)2 were added in 35 mL distilled water. After 10 min vigorous stirring, the formed composite was transferred into a 50 mL Teflonlined stainless steel autoclave. A piece

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of cleaned NF was then immersed into the solution and then heated at 120 °C for 5 h. After cooling down to room temperature, the NF was taken out and washed with deionized water and ethanol three times, then vacuumed at 50 °C overnight. Finally,

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the NF was transferred to the tube-furnace and kept at 600 °C for 2 h in the air with a

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heating rate of 4 °C min-1. The mass loading of the NCO sample is approximately 2 mg cm-2.

2.4 Preparation of NCO-NH nanoarrays

NH was deposited on the surface of NCO nanoarrays by electrochemical

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deposition method [26]. Electrodeposition was carried out in a three electrode system using the NCO as the working electrode, Pt as the counter electrode, Hg/HgO as the reference electrode, and 0.1 M Ni(NO3)2·6H2O as the electrolyte solution. The

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electrodeposition was performed at the potential of -0.9 V. After electrodepositing,

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the NF with NCO-NH was washed with deionized water and ethanol three times, then vacuum dried at 50 °C overnight. By keeping the electrodeposition time at 100 s, 200 s, 400 s, 600 s, and 800 s, the corresponding electrode materials were prepared and donated as NCO-NH-100, NCO-NH-200, NCO-NH-400, NCO-NH-600 and NCO-NH-800, respectively. The mass loading of the NCO-NH-100, NCO-NH-200, NCO-NH-400, NCO-NH-600 and NCO-NH-800 is approximately 2.1, 2.2, 2.4, 2.7 and 2.9 mg cm-2.

ACCEPTED MANUSCRIPT 2.4 Materials characterization The crystalline phases of the as-prepared NCO-NH nanoarrays were characterized by powder X-ray diffraction (XRD) using Cu Ka radiation (λ = 0.15406

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nm) on a Bruker D8 Advance X-ray diffractometer in the 2θ range 10-80° at a scanning rate of 2° min-1. The morphology of the products were also examined by transmission electron microscopy (TEM) high-resolution TEM (HRTEM) images

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were recorded on a FEI JEM-2100 and FEI Tecnai G2 F20 operating at 200 kV.

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Surface analysis of nanoarrays were measured by X-ray photoelectron spectroscopy (XPS) using a ESCA PHI500 spectrometer. The nitrogen adsorption-desorption isotherms at 77 K were researched using a TriStar II 3020 surface area and porosity analyzer (Micromeritics Instrument Corporation, USA).

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2.5 Electrochemical measurements with a three-electrode cell

Electrochemical tests were carried out at room temperature through a three-electrode system. The NF which obtained sample was directly used as the

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working electrode, a saturated calomel electrode as the reference electrode, and a

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platinum plate as the counter electrode. 2 M KOH was used as electrolyte. The electrochemical performance was tested on an electrochemical workstation (CHI760E, CH Instrument In, Shanghai) by the techniques of electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge-discharge (GCD) [27]. The range of voltage window for cyclic voltammetry text is -0.4 to 1 V with a voltage scan rate ranging from 10 mV s-1 to 100 mV s-1. The maximum voltage of galvanostatic charge–discharge test is 0.5V and the current density is from 1 to 10

ACCEPTED MANUSCRIPT A g-1. Electrochemical impedance spectroscopy (EIS) measurements were carried out with a superimposed 5 mV sinusoidal voltage in the frequency range of 0.01 Hz to 100 kHz using a CHI 760E.

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The capacitance values of the NFs which obtained samples were calculated from the charge-discharge curve according to the following formula: Cs = (I ×∆t)/(m ×∆V)

(1)

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where Cs, I, ∆t, m, and ∆V are the specific capacitance (F g-1) of the electrodes,

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current (A), discharge time (s), mass of the electroactive electrode (mg) and potential window (V).

2.6 Electrochemical measurements of the asymmetric supercapacitors Asymmetric supercapacitors were assembled by electrolytes, activated carbon

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and prepared NCO-NH-400 composites. The electrochemical test was performed in a two electrode system with an electrolyte of 2M KOH and NCO-NH-400 was the

electrode.

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working electrode, and the activated carbon was the pair electrode and reference

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The capacitance values, energy density and power density of the NFs which obtained samples were calculated from the charge-discharge curve according to the following formulas:

Cs = (I ×∆t) / (m ×∆V)

(2)

E = 0.5 Cs (∆V) 2 / 3.6

(3)

P = 3600 E /∆t

(4)

where E and P are the energy density (Wh kg-1), and power density (W kg-1) of

ACCEPTED MANUSCRIPT the electrodes. 3. Results and discussion The synthetic schematic in Fig. 1 clearly shows the formation process of

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NCO-NH hierarchical core-shell heterostructures. Firstly, NCO precursor nanoarray was synthesized by a hydrothermal reaction of Ni(NO3)2·6H2O, Co(NO3)2·6H2O, and urea. Subsequently the pre-NCO nanoarray was calcined in the air to generate NCO

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nanoscale arrays. Finally, NCO-NH core-shell nanoarrays were prepared by a facile

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electrochemical deposition. The scanning electron microscopy (SEM) of in Fig. 2a clearly shows the NCO nanowire arrays grow uniformly and vertically on the surface of NF. These NiCo2O4 nanowire arrays are observed to be adequately separated with plenty of open spaces, which can thus serve as the ideal conductive scaffolds for the

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subsequent growth of the NH nanosheet [25]. The NH nanosheets were homogeneously deposited onto the surfaces of the NCO nanowire arrays, yielding uniform hierarchical core-shell hetero-structures (Fig. 2b). Other compound materials

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with hierarchical core-shell hetero-structures are shown in Fig. S1. With increasing

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the deposition time, the amount of NH nanosheets coated on the NCO nanowires increases gradually. Such well-defined three-dimensional core-shell structure was further demonstrated by TEM images (Fig. 2c and 2d). As can be seen, the tops of NCO-NH core-shell heterostructures become much rough compared to the bare NCO, indicating the outside coating of NH nanosheets. The HRTEM image (Fig. 2e) shows the two apparent lattice fringes with spacing of 0.25 nm and 0.48 nm, corresponding to the (311) and (111) planes of NCO nanowire arrays, while the lattice spacing of

ACCEPTED MANUSCRIPT 0.23 nm corresponds to the (101) plane of NH nanoscales. The EDS elemental mappingfurther shows that the cobalt, nickel, and oxygen elements are uniformly deposited in the NCO-NH-400 heterostructure (Fig. 2f-i).

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The X-ray diffraction (XRD) patterns of NCO, NCO-NH samples grown on the Ni foam are shown in Fig. 3a. The obvious diffraction peaks at 19.3°, 37°, 44.5°, 59°, and 65° can be ascribed to the diffraction of (111), (220), (311), (400), (511) and (440)

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planes of the cubic spinel NCO phase (JCPDS no. 73-1702). Six peaks at 19.3°, 33.2°,

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38.6°, 62.7°, 70.4° and 73.5° can be ascribed to the diffraction of (001), (100), (101), (111), (103) and (201) planes of NH (JCPDS no. 14-0117). The other diffraction peaks at 45°, 52° and 77° corresponding to the (111), (200), and (220) planes represent the presence of bare NFs (JCPDS card no. 1-1266).

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In order to study the element compositions and chemical forms, the electrode materials are characterized by X-ray photoelectron (XPS) measurements. It can be clearly seen that the Ni, Co, O elements exists in the NCO and NCO-NH-400

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electrode materials. By using a Gaussian fitting method, the Ni 2p (Fig. 3b) was fitted

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with two spin-orbit doublets, characteristic of Ni2+ and Ni3+, and two shakeup satellites (indicated as “Sat.”) [28]. The fitting peaks at binding energies of 854.5 eV and 872 eV are ascribed to the Ni2+, whereas the fitted peaks at binding energies of 856 eV and 873.5 eV correspond to the Ni3+ [16]. With the combination of NH, the intensities of Ni peaks of the composite electrode materials are enhanced. The emission spectrum curve of the Co 2p (Fig. 3c) was also fitted with two spin-orbit doublets, characteristic of Co2+ and Co3+, and two shakeup satellites. The fitted peaks

ACCEPTED MANUSCRIPT at 780.5eV and 795.3 eV are ascribed to Co3+, and the fitted peaks at 782.5 eV and 797.2 eV are assigned to Co2+ [29]. With the combination of NH, the intensities of Co peaks of the composite electrode materials are weakened. The component O 1s

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spectrum with three oxygen contributions is shown in Fig. 3d [30]. Explicitly, peaks at 529.6 and 529.8 eV are typical of metal-oxygen bonds [31]. The peak at the place of 531.2 eV is related to defects, pollutants and a series of surface substances, and the

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surface substances including hydroxyl groups and chemical oxygen adsorption. The

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strong peak at 532.4 eV is corresponding to the bound hydroxide groups (OH-). Peaks at 532.9 and 532.6 eV can be attributed to multiplicity of physi- and chemi-sorbed water at or near the surface [32]. Based on the above analyze, it can be concluded that the core-shell nanoarrays consisting of NCO nanowires and NH nanosheets are

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successfully constructed.

The cyclic voltammetry curve of NCO-NH-400 (Fig. 4a) with scanning rates from 10 to 100 mV, paired redox peaks located at about 0.22 and 0.52 V are observed

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for the NCO-NH-400 composite electrodes at a scan speed of 10 mV s-1. The

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anodic/cathodic peak current increases accordingly with the increasing of the sweep rate, indicating that the redox reaction proceeds rapidly on the surface of the electrode [32]. The charge and discharge curve of the NCO-NH-400 composite electrode (Fig. 4b) was tested under different current densities. The cyclic voltammetry and discharge curves of other samples are shown in Fig. S2 and S3. Apparently, the discharge curve can be separated into two parts in the voltage of 0.25V, of which the fast and slow potential reductions can be attributed to the internal resistance and redox reaction,

ACCEPTED MANUSCRIPT respectively. It can be observed that, all galvanostatic discharge curves demonstrated the typical pseudocapacitive behavior, and no obvious potential drop was observed. The

comparison

of

cyclic

voltammetry

(CV)

curves

(Fig.

4c)

of

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Ni(OH)2/NF-400S (denoted as NH-400), NCO and NCO-NH (100, 200, 400, 600 and 800) were texted at a sweep rate of 40 mV s-1. The sweep window and electrolyte of CV curves are -0.4 to 1.0V and 2 M KOH. The current output in the CV curves

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throughout the scan range for the NCO-NH composite is obviously higher than that of

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NCO and NH-400 composite electrode at the same scan rate, indicating the higher specific capacitance [33]. The current output of NCO-NH-400 is much higher than other composite electrodes, and the NCO-NH-400 composite electrode has better performance than other composites. The charge and discharge curves of NH-400,

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NCO electrodes and NCO-NH (100, 200, 400, 600 and 800) composite electrodes in the potential range of 0-0.5 V at a current density of 1 A g-1 are shown in Fig. 4d. It can be observed that the NCO-NH-400 shows the longest discharge time, which is

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consistent with the CV curves. The function of the specific capacitance and current

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density is shown in Fig. 4e. Strikingly, the NCO-NH-400 shows the highest specific capacitance. When the current density is 1, 2, 5 and 10A g-1, the specific capacitance is up to 2218.0, 2161.6, 2033.0 and 1796.0 F g-1. Most importantly, when the current density is 10 A g-1, the capacitance was still maintained 81%. The excellent cycling stability of the NCO-NH core-shell heterostructures in the three-electrode system was studied and the result was shown in Fig. 4f. It can be found that 96.4% of the initial capacitance can well preserved after 5000 cycles of charge–discharge at 10A g-1,

ACCEPTED MANUSCRIPT indicating a superior cycling stability. In order to characterize specific surface area of NH-400, NCO and NCO-NH-400 electrodes, the N2 adsorption–desorption isotherm was measured. The isotherm of

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different electrodes is shown in Fig. 5a. Compared with NH-400 (1.34m2 g-1) and NCO (14.65m2 g-1), the NCO-NH-400 (18.76m2 g-1) electrodes exhibited the increased specific surface area because of its rough surface. Electrochemical

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impedance spectroscopy (EIS) was carried out to further explore the electrochemical

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behaviors of as-prepared electrode materials. As shown in Fig. 5b, a semicircle in the high frequency region can be observed from the Nyquist plots. In the high frequency region, the intercepts of NH-400, NCO and NCO-NH-400 are similar, but NCO-NH-400 is much smaller, indicating a relatively low internal resistance (Rs) in

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this heterostructures. In the lower frequency area, the slope of the curve shows the Warburg impedance which represents the electrolyte diffusion in the porous electrode and proton diffusion in the host material [34]. Compared with NH-400 and NCO,

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NCO-NH-400 heterostructures has a more ideal line, which illustrates that the

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NCO-NH-400 nanoarray has a lower diffusion resistance of electrolyte to the electrode surface. Therefore, it can be induced that this improved conductivity of heterostructures NCO-NH-400 as well as the favorable ions diffusion dynamic could accounts for its excellent rate capability. To demonstrate the practical application of NCO-NH core-shell heterostructures, ASC device by using NCO-NH-400 as the positive electrode and active carbon (AC) as the negative electrode was fabricated (Fig. 6a). Before testing the ASC device, the

ACCEPTED MANUSCRIPT CV curves of cathode NCO-NH-400 and AC were carried out at a scan rate of 20 mV s-1 in a three-electrode configuration to determine the operating potential range. The stable potential range is between -0.8 and 0 V forAC electrode and between -0.4 and

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0.8 V for NCO-NH-400 electrode (Fig. S4a). Based on the discussion above, it is possible to fabrication an ACS with the potential window extending up to 1.6 V [35]. As shown in Fig. S4b, the cell voltage window of the asassembled ASC can be stably

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extended to 1.6 V. With the increase of working voltages, the curves exhibits a

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progressively larger closed area and a similar shape without obvious change even in a relatively high working voltage, indicating that the cell voltage of the fabricated asymmetric supercapacitor can be extended to 1.6 V. Fig. S4c displays the curves of GCD performance of the ASC device operated at various current densities. It can be

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clearly known that the curve is in triangular shape and corresponds to the discharge counterparts, showing the ideal coulombic efficiency. The specific capacitances of the NH-400//AC ASC were calculated to be 105.6, 88.6, 67.6, and 41.9 F g-1, and the

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specific capacitances of the NCO//AC ASC were 200.6, 193.8, 180.9, and 166.9 F g-1,

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and the specific capacitances of the NCO-NH-400//AC ASC were 277.1, 239.3, 203.2, and 173.1 F g-1at current densities of 1, 2, 5 and 10 A g-1, respectively (Fig. S4d). When the current density is 10 A g-1, the capacitance was still maintained 62.5%, further indicating good rate performance of NCO-NH-400//AC device. The cycle stability of NCO-NH-400//AC ASCs was further evaluated by long-term charging/discharging cycling at high current density of 10A g-1, as shown in Fig. 6b. Strikingly, 88.5% capacitance is retained for 5000 cycles, demonstrating

ACCEPTED MANUSCRIPT that the core-shell hetero-structures NCO-NH-400//AC electrode has excellent cycling stability [36]. To better highlight the high performance of the present NCO-NH-400//AC ASCs, the Ragone plot demonstrating the relationship between

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power density and the energy density were obtained according to the GCD curve. As shown in Fig. 6c, the NCO-NH-400//AC displays a maximum energy density of 98.5 W h kg-1 with a power density of 800 W kg-1 and still remains at 61.5 W h kg-1 even at

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a high power density of 8000 W kg−1. The obtained maximum energy density of

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present NCO-NH-400//AC is much higher is superior to previously reported ASC devices, such as NiCo2O4-CC [37], NiCo2O4-3D-RGO [38] and Co3O4@Ni(OH)2 [39]. The detailed comparison of electrochemical performances of these NCO- or NH-based electrode materials was listed in Table 1 [37-46]. In order to further

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demonstrate the practical application of the new ASC devices, two ASCs were assembled connected in series. As shown in Fig. 6d, after charging 3.2 V for a short time, the devices could power one red light emitting diode (LED) for 1 min and one

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storage.

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white LED for seconds indicating the practical application of the ASC in energy

Based on the above results, the high electrochemical performances of NCO-NH

can be ascribed to the following points. (1) The NCO and NH individually enrich the redox reactions, resulting in the enhanced electrochemical capability. (2) The NH nanosheet wrapped on the nanowires could render more electroactive sites for the capacitive reaction which in turn overcome the problem of poor conductivity of NH. (3) The high conductivity of the NCO core of the NCO-NH heterostructure enabled

ACCEPTED MANUSCRIPT the fast charge transport, which would accelerate the reaction kinetics. (4) The core-shell nanowires arrays could enhance the stability of electrode in the backbone during charge/discharge cycles. Due to these combined advantages, the resulting

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NCO-NH core-shell nanoarrays exhibited high specific capacitance and high stability. 4. Conclusions

In summary, novel NCO-NH core-shell heterostructure arrays have been

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successfully synthesized. This core-shell NCO-NH heterostructure arrays exhibited a

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high specific capacitance and excellent rate performance of 2218.0 and 1796.0 F g-1 at current densities of 1 and 10 A g-1 and outstanding cycling stability, with 96.4% retention of the initial specific capacitance at 10 A g-1 after 5000 cycles. Furthermore, the NCO-NH//AC ASC device with a maximum operating voltage of 1.6 V was

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successfully assembled, achieving a high energy density of 98.5 Wh kg-1 at a power density of 800 W kg-1 and an energy density of 61.5 Wh kg-1 at a high power density of 8000 W kg-1 (at current densities of 10 A g-1). This outstanding electrochemical

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performance of NCO-NH can be ascribed to the unique hierarchical which could

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enrich abundant electroactive sites for more redox reactions and could enhance the stability of electrode in the backbone during charge/discharge cycles. Therefore, this work provides a high-performance electrode material which can be used into promising energy storage system with high energy density. Acknowledgements This work was supported by the financial supports of National Nature Science Foundation of China (No. 21576121 and 21606111), Natural Science Foundation of

ACCEPTED MANUSCRIPT Jiangsu Province (BK20140530 and BK20150482) and Key Research Plan of Zhenjiang City (GY2015031 and GY2015044). Reference

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core-shell nanowire arrays for high performance supercapacitors, J. Mater. Chem.

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core/shell nanowires for electrochemical energy storage, J. Mater. Chem. A 3

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ACCEPTED MANUSCRIPT NiCo2O4@Ni(OH)2 supported on 3D N-doped graphene/carbon nanotubes sponge as an high performance electrode for Supercapacitor, Ceramics International 44 (2018) 3113–3121.

temperature

on

pseudocapacitance

fiber@NiCo2O4@Ni(OH)2

core–shell

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[22] J. Zhang, H. Gao, Q. Yang, X.T. Zhang, M.Y. Zhang, L.L. Xu, Effect of performance nanowire

array

carbon

composite

SC

electrodes,Applied Surface Science 356 (2015) 167–172.

of

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[23] L. Huang, D.C. Chen, Y. Ding, Z.L. Wang, Z.Z. Zeng, M.L. Liu, Hybrid composite Ni(OH)2@NiCo2O4 grown on carbon fiber paper for high-performance supercapacitors, ACS Appl. Mater. Interfaces 5 (2013) 11159-11162. [24] W. Li, L.P. Xin, X. Xu, Q.D. Liu, M. Zhang, S.J. Ding, M.S. Zhao, X.J. Lou,

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Facile synthesis of three-dimensional structured carbon fiber-NiCo2O4-Ni(OH)2 high-performance electrode for pseudocapacitors, Sci. Rep. 5 (2015) 9277. [25] D. Cheng, Y.F. Yang, J.L. Xie, C.J. Fang, G.Q. Zhang, J. Xiong, Hierarchical core-Shell

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NiCo2O4@NiMoO4

hybrid

nanowire/nanosheet

arrays

for

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high-performance pseudocapacitors, J. Mater. Chem. A 3 (2015) 14348-14357. [26] Y.F. Yang, D. Cheng, S.J. Chen, Y.L. Guan, J. Xiong, Construction of hierarchical NiCo2S4@Ni(OH)2 core-shell hybrid nanosheet arrays on Ni foam for high-performance aqueous hybrid supercapacitors, Electrochim. Acta 193 (2016) 116-127. [27] Y.X. Wen, S.L. Peng, Z.L. Wang, J.X. Hao, T.F. Qin, S.Q. Lu, J.C. Zhang, D.Y. He, X.Y. Fan, G.Z. Cao, Facile synthesis of ultrathin NiCo2S4 nano-petals inspired

ACCEPTED MANUSCRIPT by blooming buds for high-performance supercapacitors, J. Mater. Chem. A 5 (2017) 7144-7152. [28] C.Z. Yuan, J.Y. Li, L.R. Hou, X.G. Zhang, L.F. Shen, X.W. Lou, Ultrathin

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mesoporous NiCo2O4 nanosheets supported on Ni foam as advanced electrodes for supercapacitors, Adv. Funct. Mater. 22 (2012) 4592-4597.

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Fabrication of CoWO4@NiWO4 nanocomposites with good supercapacitve

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performances, Electrochim. Acta 174 (2015) 837-845.

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[31] J. Li, S. Xiong, Y. Liu, Z. Ju, Y. Qian, High electrochemical performance of monodisperse NiCo2O4 mesoporous microspheres as an anode material for Li-ion batteries, ACS Appl. Mater. Inter. 5 (2013) 981-988.

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[32] J.J. Shen, X.C. Li, L. Wan, K. Liang, B.K. Tay, L.B. Kong, X.B. Yan, An

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asymmetric supercapacitor with both ultra-high gravimetric and volumetric energy density based on 3D Ni(OH)2/MnO2@carbon nanotube and activated

polyaniline-derived carbon, ACS Appl. Mater. Inter. 9 (2017) 668-676.

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ACCEPTED MANUSCRIPT Bando, Ni(OH)2 nanosheet@Fe2O3 nanowire hybrid composite arrays for high-performance supercapacitor electrodes, Nano Energy 2 (2013) 754-763. [35] J.C. Chen, Y.M. Wang, J.Y. Cao, Y. Liu, J.H. Ouyang, D.C. Jia, Y. Zhou, Flexible solid-state

asymmetric

supercapacitor

based

on

ternary

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and

graphene/MnO2/carbon black hybrid film with high power performance, Electrochimi. Acta 182 (2015) 861-870.

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[36] Q.C. Zhang, J. Sun, Z.H. Pan, J. Zhang, J.X. Zhao, X.N. Wang, C.X. Zhang, Y.G.

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Yao, W.B. Lu, Q.W. Li, Y.G. Zhang, Z.X. Zhang, Stretchable fiber-shaped asymmetric supercapacitors with ultrahigh energy density, Nano Energy 39 (2017) 219-228.

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Electrodeposition of honeycomb-shaped NiCo2O4 on carbon cloth as binder-free electrode for asymmetric electrochemical capacitor with high energy density, RSC Adv. 6 (2016) 37562-37573.

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Fang, P.Y. Wang, Asymmetric supercapacitors based on NiCo2O4/three dimensional graphene composite and three dimensional graphene with high energy density, J. Mater. Chem. A 4 (2016) 18646-18653.

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performance, Chem. Eng. J. 315 (2017) 35-45.

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Chemelectrochem 4 (2017) 2414-2422. [41] X.Y. He, Q. Liu, J.Y. Liu, R. Li, H. Zhang, R. Chen, J. Wang, Hierarchical NiCo2O4@NiCoAl-layered double hydroxide core/shell nanoforest arrays as

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advanced electrodes for high-performance asymmetric supercapacitors, J. Alloy.

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Compd. 724 (2017) 130-138.

[42] X.W. Dong, Y.Y. Zhang, W.J. Wang, R. Zhao, Rational construction of 3D NiCo2O4@CoMoO4 core/shell nanoarrays as a positive electrode for asymmetric supercapacitor, J. Alloy. Compd. 729 (2017) 716-723.

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11834-11839.

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[44] F.F. Zhu, M. Yan, Y. Liu, H. Shen, Y. Lei, W.D. Shi, Hexagonal prism-like hierarchical Co9S8@Ni(OH)2 core–shell nanotubes on carbon fibers for high-performance asymmetric supercapacitors, J. Mater. Chem. A 5 (2017) 22782-22789.

[45] Y.M. Lv, A.F. Liu, H.W. Che, J.B. Mu, Z.C. Guo, X.L. Zhang, Y.M. Bai, Z.X. Zhang, G.S. Wang, Z.Z. Pei, Three-dimensional interconnected MnCo2O4 nanosheets@MnMoO4 nanosheets core-shell nanoarrays on Ni foam for

ACCEPTED MANUSCRIPT high-performance supercapacitors, Chem. Eng. J. 336 (2018) 64-73. [46] C.Q. Zhang, Q.D. Chen, H.B. Zhan, Supercapacitors based on reduced graphene oxide nanofibers supported Ni(OH)2 nanoplates with enhanced electrochemical

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performance, ACS Appl. Mater. Interfaces 8 (2016) 22977-22987.

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Fig. 1 Schematic illustration of the formation process of NCO-NH nanoarrays.

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Fig. 2 (a, b) SEM images of NCO and NCO-NH-400; (c, d) TEM images of NCO and

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NCO-NH-400; (e) HRTEM image of NCO-NH-400; (f-i) HAADF-STEM image and

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mapping of NCO-NH-400 nanoarray.

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(b)

NCO NH

2p2/3 3+

Intensity / a.u.

*

NCO-NH-400 NCO-NH-600

50

60

(201)

(511) (111) (440)

40

2+

NCO 2p2/3 3+

2+

2+

80

850

855

860

(d)

3+

3+

NCO

2+

2+ sat. 3+

3+

785

2p1/3

2+ 2+ sat. 3+

790 795 800 Binding Energy / eV

880

885

O 1s

NCO-NH-400

NCO-NH-400 780

875

NCO

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2p2/3

870

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2+

2+ sat.

2+

529.8

sat.

2+

Binding Energy / eV

Intensity / a.u.

Intensity / a.u.

865

2p1/3 3+

sat.

3+

NCO-NH-400

70

Co 2p 2+

3+

2+

529.5

2p2/3 3+

2p1/3

sat.

2Theta(degree)

(c)

2+

532.9

30

sat.

3+

532.6

20

NCO JCPDS 73-1702 NH JCPDS 14-0117 (400)

(111)

(220) (100) (311) (101)

NCO-NH-800

2p1/3 2+

531.2

Intensity

NCO-NH-200

sat.

2+

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* *

NCO-NH-100

10

Ni 2p

* NF

NCO

531.4

(a)

805

810

526

528

530

532

534

536

Binding Energy / eV

Fig. 3 (a) XRD patterns of NCO-NH nanoarrays on NFs; XPS spectra of NCO and

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NCO-NH-400, (b) Ni 2p, (c) Co 2p and (d) O 1s.

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(b)0.5

-1 10mV s 20mV s-1

100

40mV s-1 80mV s-1

50

Voltage (V)

Current Density (A g-1)

(a)150

100mV s-1

0 -50 -100

0.4

1.00 A g-1 2.00 A g-1

0.3

5.00 A g-1 10.00 A g-1

0.2

0.1

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-150 0.0

0.0

0.2 0.4 Voltage (V)

0.6

0.8

50

400

800

1200 1600 Time (s)

0 -50

2000

2400

2800

NH-400 NCO NCO-NH-100 NCO-NH-200 NCO-NH-400 NCO-NH-600 NCO-NH-800

0.4

Voltage (V)

100

0

(d)0.5

NH-400 NCO NCO-NH-100 NCO-NH-200 NCO-NH-400 NCO-NH-600 NCO-NH-800

0.3 0.2

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0.1

-100

0.0

-0.4

-0.2

0.0

0.2 0.4 Voltage (V)

0.6

(e)2400

0.8

1.0

0

400

800

1200 1600 Time (s)

2000

2400

2800

2500

(f)

NCO-NH-600 NCO-NH-200

NCO-NH-800

1200

NCO-NH-100 600

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NCO

2000

NH-400

0 0

2

4

6

8

Current density (A/g)

10

0.5

0.5

0.4

0.4 0.3

0.3

500

the last 10 cycles

the first 10 cycles

1500

1000

96.4%

Voltage (V)

NCO-NH-400

1800

Voltage (V)

Specific capacitance (F/g)

1.0

Specific capacitance (F/g)

Current Density (A g-1)

(c) 150

-0.2

SC

-0.4

0.2 0.1

0.2

0

500

1000

Time (s)

1500

2000

0.1

0

500

1000

Time (s)

1500

2000

0 0

1000

2000

3000

4000

5000

Cycle number

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Fig. 4 (a) CV curves of the NCO-NH-400 at various scan rates; (b) charge and discharge curves of the NCO-NH-400 at different current densities; (c) CV curves for

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Ni(OH)2/NF-400S (denoted as NH-400), NCO, and NCO-NH at a sweep rate of 40 mV s-1; (d) Comparison of GCD curves for NH-400, NCO, and NCO-NH at a current density of 1 A g-1; (e) Specific capacitance as a function of current density; (f) Cycling stability of NCO-NH-400 for 5000 cycles at a constant current density of 10 A g-1, the inset shows the corresponding charge-discharge curves of first and last 10 cycles.

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(b)

2.0

20

1.5

NH NCO NCO-NH-400

15

10

Z''/ohm

20

Z''/ohm

15

NH NCO NH-NCO-400

1.0 0.5 0.0

10

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Z'/ohm

CPE

5

5

0

0

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Volume adsorbed (cm3 STP g-1)

(a)

Rs

0.0

0.2 0.4 0.6 0.8 Relative pressure (p/p 0)

1.0

Rct

0

5

10

15

20

Zw

25

30

Z'/ohm

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Fig. 5 (a) Nitrogen sorption isotherm of the as-prepared NH-400, NCO, and

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NCO-NH-400; (b) Nyquist plots of EIS of the three electrodes; (c) Specific

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capacitance as a function of current density.

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Fig. 6 (a) Schematic illustration of the asymmetric supercapacitor configuration adopted by the NCO-NH cathode and AC anode; (b) Variation of capacitance value of the NCO-NH-400//AC ASCs device during the cycling test at a constant current

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density of 10 A g-1, the inset shows the corresponding charge-discharge curves of the first and last 10 cycles; (c) The Ragone plots related to energy and power densities of

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the NH-400//AC, NCO//AC and NCO-NH-400//AC ASCs; (d) Photograph of white

AC C

LED lit up by two NCO-NH-400//AC devices.

ACCEPTED MANUSCRIPT Table 1 Comparison studies for bimetallic oxide, hydroxide and their SC performances. Energy density (W h kg-1)

Ref.

3M

1.00

77.2% retention at 10 A g-1

750

32.00

[37]

NiCo2O4-3D-RGO

3M

5.00

36.1% retention at 10 A g-1

800

73.80

[38]

NiCo2O4-MnO2

3M

2.00

83.4% retention at 0.5 A g-1

400

27.80

[40]

NiCo2O4-NiCoAl-LDH

2M

4.00

78.5% retention at 10 A g-1

800

74.70

[41]

NiCo2O4@CoMoO4

2M

3.50

66.6% retention at 0.1 A cm-2

11427

29.52

[42]

NiCo2O4-NC

1M

1.00

37.7% retention at 10 A g-1

8500

28.00

[43]

Co3O4@Ni(OH)2

3M

4.00

36.4% retention at 4.0 A g-1

3455

40.00

[39]

NiCo2S4@Ni(OH)2

2M

2.80

60.6% retention at 0.1 A cm-2

290

53.30

[26]

Co9S8@Ni(OH)2

6M

2.00

50.2% retention at 10 A g-1

2500

12.50

[44]

MnCo2O4@MnMoO4

6M

1.01

49.5% retention at 10 A g-1

815

49.40

[45]

Ni(OH)2-RGO

6M

2-2.5

85.3 % retention at 40 mV s−1.

3185

37.60

[46]

NCO-NH-400

2M

800

98.53

This work

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NiCo2O4-CC

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Rate performance

Power density (W kg-1)

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Mass loading (mg cm-2)

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Material

Electrolyte (KOH)

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2.40

62.5% retention at 10 A g-1

ACCEPTED MANUSCRIPT Highlights 1. NiCo2O4@Ni(OH)2 core-shell nanoarrays have been successfully fabricated. 2. The core-shell nanoarrays electrodes showed high specific capacitance.

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3. The core-shell nanoarrays electrodes exhibited outstanding cycling stability.

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4. The ASC device based on core-shell nanoarrays can deliver high energy density.