coreâshell nanosheet arrays on Ni foam for high-perform

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Materials and Interfaces 2

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Hierarchical NiO@NiCoO core–shell nanosheet arrays on Ni foam for high-performance electrochemical supercapacitors Di Yao, Yu Ouyang, Xinyan Jiao, Haitao Ye, Wu Lei, Xifeng Xia, Lei Lu, and Qingli Hao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00467 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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Industrial & Engineering Chemistry Research

Hierarchical NiO@NiCo2O4 core–shell nanosheet arrays on Ni foam for high-performance electrochemical supercapacitors Di Yaoa, Yu Ouyanga, Xinyan Jiaoa, Haitao Yea, Wu Leia.*, Xifeng Xiaa, Lei Lua, Qingli Haoa.* a

Key Laboratory for Soft Chemistry and Functional Materials, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu,China

*Corresponding author. E-mail: [email protected] (Q. Hao); [email protected] (W. Lei); Fax: +86-25-84315190(Q. Hao); Tel: +86-25-84315190(Q. Hao)

1. Abstract A facile solvothermal method followed by a post-annealing process is used to prepare NiO@NiCo2O4 core–shell nanosheet arrays supported on Ni foam substrate for a high-performance supercapacitor. The hybrid electrode possesses a three-dimensional structure with the ‘shell’ of NiCo2O4 nanoflakes anchored on the ‘core’ of ordered NiO nanosheets. It shows high specific capacitance of 1623.6 F g-1 (or specific capacity of 225.5 mAh g-1) at 2 A g-1 and excellent rate performance with a 96% capacitance retention rate at 20 A g-1. The high cycling stability is proved by nearly 90% capacitance retention at 10 A g-1 after 10000 cycles. Its asymmetric supercapacitor, assembled with NiO@NiCo2O4/Ni foam and the activated carbon/Ni foam as the positive and negative electrode, respectively, displays the specific energy of 52.5 W h kg-1 at 387.5 W kg-1. The excellent electrochemical performance of NiO@NiCo2O4 electrode indicates its great potential in applications of energy storage devices. Keywords: Heterostructure; Asymmetric supercapacitors; NiCo2O4; NiO

2. Introduction 1

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As the rapid development of portable electronic devices, such as smart phones, visual reality glasses and wearable devices, sustainable and environmental friendly energy storage and conversion devices including batteries, fuel cells, and electrochemical capacitors (ECs), have attracted great attention recently1. In this respect, ECs stand out as a new strong competitor in the energy storage race due to their high power density, superior rate performance and favorable cyclic stability2,3. ECs can be divided into two types according to charge storage mechanisms, electrochemical double layer capacitors (EDLCs) and faradaic supercapacitors3,4. EDLCs storage energy by accumulating charge electrostatically at the interface of electrode/electrolyte and possess high power density and excellent cycling performance, but have limited specific capacitance. Being different from EDLCs, faradaic supercapacitors can store charges based on the fast reversible multi-electron redox reactions occurring on the surface of electrode, which usually generate high energy density and can be used as power devices5. However, faradaic supercapacitors suffer from poor cyclic stability caused by volume change during the charge/discharge processes and the low conductivity of active materials6. According to recent reports, the transition metal oxides, hydroxides and conducting polymers are usually used as faradaic supercapacitors materials7. Although RuO2 exhibits the most promising performance, it is limited by high cost, scarcity and toxicity of RuO2 in commercial application8,9. Therefore, exploring faradaic supercapacitors materials with high capacitance, non-toxic and low cost have become the main focus of the current study on supercapacitors. 2


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Recently, MnO210, Co3O411 and NiO12 as the candidates for high-performance supercapacitors have been widely reported due to the high theoretical capacitance and low cost. Moreover, some bimetallic redox oxides, such as NiCo2O413, ZnCo2O414,15, NiMoO416, and CoMoO47, 17 have been favorable due to their multiple oxidation states for reversible Faradaic reactions as well as outstanding electrical conductivity18. Among these bimetallic oxides, spinel NiCo2O4 is suggested as the most promising redox electrode material and widely studied because of its excellent electronic conductivity,

low

cost,

environmental

friendliness

and easily

controllable

morphologies19. Various NiCo2O4 nanostructures with different morphologies have been investigated, such as 1D-nanowires20, 2D-nanosheets21 and 3D-hierarchical structures composed of nanowires or nanosheets13, which can efficiently improve the specific capacity due to the large surface area. The 3D hierarchical hybrid electrodes can exhibit the higher capacitive performance due to the fast ion transportation and the synergetic effect between all components13. For example, the Seaurchin-like core-shell

nanoneedles

of

NiCo2O4@NiMoO422,

3D

nanosheet

arrays

of

NiCo2O4@MnO223, core–sheath nanowires of ZnCo2O4@NiCo2O414 demonstrated the better electrochemical performance than individual metal oxides. Besides, a hierarchical core-shell structure of hybrid materials, supported on conductive substrates like Ni foam18, 24,25, carbon cloth26 and titanium mesh27, may provide an integrated electrode, which avoid ‘dead surface’ caused by using binder to fix active materials18. Therefore, to design 3D hierarchical nanostructures for electrode materials is of great popularity as a research orientation on fabricating 3



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high-performance supercapacitors28. But there is still a wide space for us to explore the hybrid electrode materials based on NiCo2O4 with 3D hierarchical nanostructures and high performance. In this article, we report a hierarchical NiO@NiCo2O4 core–shell nanosheet arrays supported on Ni foam without using binder. Due to the outstanding theoretical specific capacitance and thermal stability29, NiO was constructed as inner framework for supporting outer shell of NiCo2O4 nanosheets. Furthermore，the Ni@NiO@NiCo2O4 contained the same Ni element in the interface and easily formed a whole intergrated architecture. Such a structure exhibited strong interface affinity and effectively decreased the contact resistance, which could improve the efficiency of electron transport. The optimal integrated electrode showed a high specific capacitance of 1623.6 F g-1(or specific capacity: 225.5 mAh g-1) at a discharge current density of 2 A g-1 and 1560 F g-1(or 216.7 mAh g-1) at 20 A g-1 that retained about 96% of initial capacitance, proving its excellent rate performance. Additionally, it exhibited a high cycling stability that retained about 90% specific capacitance of its initial value after 10000 cycles, which is superior to the single-layer NiCo2O4 nanosheet arrays6. Moreover, the NiO@NiCo2O4//Activated Carbon (AC) asymmetric supercapacitor showed 52.5 Wh kg-1 of specific energy at a specific power of 387.5W kg-1 and retained 36.7 Wh kg-1 at a high specific power of 8000W kg-1. In addition, the asymmetric supercapacitor possesses a good cyclic stability that the specific capacitance retention can remain nearly 90% of its initial value after 3000 cycles at 2 A g-1. The ordered core–shell nanosheet arrays consisting of two kinds of faradaic 4


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redox materials play an important role in greatly increasing the electrochemical performance of the supercapacitor electrode.

2. Experimental 2.1 Materials synthesis All reagents used in experiments were of analytical purity and without further purification. 2.1.1 Synthesis of NiO nanosheets arrays on Ni foam Typically, a piece of Ni foam (1×1 cm2) was completely immersed into 3 M HCl solution for half an hour to remove oxides, and then washed with distilled water and ethanol. 1 mmol Ni(NO3)2·6H2O, 3 mmol hexamethylenetetramine (HTMA) and 1 mmol Hexadecyl trimethyl ammonium Bromide (CTAB) were dissolved in a mixture of 15 mL water and 15 mL ethanol by magnetic stirring at room temperature for 0.5 h to form a uniform solution. Then, the dissolved solution was poured into a 50 mL Teflon-lined stainless-steel autoclave. The Ni foam was put into autoclave and maintained at 130 ℃ for 10 h. After cooling to the room temperature in air, the as-prepared precursor was taken out and washed with distilled water and ethanol several times in ultrasonic cleaner successively. Finally, it was dried in an oven at 60 ℃ and then calcined at 350 ℃ for 2 h, thus the NiO nanosheets arrays (about 0.37 mg) grown on Ni foam were obtained, and it was named Ni foam@NiO. 2.1.2 Synthesis of hierarchical NiO@NiCo2O4 core–shell nanosheet arrays on Ni foam In a typical process the 30mL 50% ethanol aqueous solution containing 0.5 mmol Ni(NO3)2·6H2O, 1 mmol Co(NO3)2·6H2O, 3 mmol urea and 2 mmol CTAB were added to a stainless-steel autoclave containing a piece of Ni foam@NiO 5



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nanosheet array electrodes. Then, the Teflon-lined stainless-steel autoclave was placed into oven and heated at 130 ℃ for different time periods. Subsequently, the autoclave was cooled to room temperature. Finally, the sample was taken out and washed by distilled water and ethanol, and then dried at 60 ℃ for 2 h, followed by an annealing process at 350 ℃ for 2 h as same as the previous post-processing step. Meanwhile, Ni foam@NiO nanosheets, Ni foam@NiCo2O4 nanosheets and Ni foam@NiO@NiCo2O4 nanosheet arrays with different reaction time were synthesized for comparison. The NiCo2O4 nanosheets were directly grown on a nickel foam by the same procedure without NiO substrate. The products obtained with various reaction time in the secondary solvothermal reaction were also compared with each other and characterized electrochemically under the same conditions. 2.2 Materials characterization methods The composition of the products scratched from Ni foam was examined by X-ray diffraction (XRD) recorded on an X-ray diffractometer (Bruker D8-Advance diffractometer, Germany) at a scan rate of 10 degrees per min in the 2θ range from 10° to 80° using Cu Kα radiation (λ=1.5406 Å). The size and microstructure of products were investigated using field emission scanning electron microscope (FE-SEM; FEI Quanta 250FEG, America) operating at 20 kV and transmission electron microscope (TEM; JEOL JEM-2100) included with an energy dispersive X-ray spectrometer (EDS). The surface element of electrodes were investigated by X-ray photoelectron spectrophotometer (XPS, Thermo-VG Scientific ESCALAB 250X, America), which was performed under the condition of using Mg K radiation. 2.3 Electrochemical measurements 6


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The regular electrochemical tests such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were carried out on an electrochemical workstation (CHI600E electrochemical workstation) in 6M KOH aqueous solution. The tests were performed with a conventional three-electrode system by using as-prepared electrode (1 cm × 1 cm) as the working electrode, Pt foil and Hg / HgO as the counter and reference electrodes, respectively. The EIS was measured at perturbation amplitude of 5 mV with a frequency range of 0.01 Hz to 100 kHz. In order to evaluate the potential application of Ni foam@NiO@NiCo2O4 hierarchical core-shell arrays electrode, an asymmetric supercapacitor (ASC) was assembled by using Ni foam@NiO@NiCo2O4 nanosheet arrays as a cathode and activated carbon (AC, provided friendly by Jiangsu JF Advanced Technologies, Inc.) as an anode in 6 M KOH electrolyte, which were separated by non-woven cloth separator and performed in a C2032 button coin cell. In the three-electrode system, specific capacitances were calculated according to the following equation: Csp= I×∆t/(m·∆V)

(1)

where I is the discharge current, ∆t is the discharging time, ∆V is the potential window during the discharge process and m is the mass of active materials. The energy density and power density of the ASC in two-electrode cell can be calculated according to the following equations: E = C×V2/2

(2) 7



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P = E/t

(3)

where E (W h kg-1) is the energy density, C (F g-1) is the specific capacitance of ASCs device, V (V) is the potential window of discharge, P (W kg-1) is the power density and t (s) is the discharge time.

3. Results and discussion As

illustrated

in

Figure

1,

the

hierarchical

heterostructure

of

Ni

foam@NiO@NiCo2O4 core–shell nanosheet array was fabricated by a facile two-step solvothermal method, each step followed by an annealing procedure. Initially, ultrathin Ni(OH)2 lamellar arrays vertically formed by a mild solvothermal reaction. After annealing in air, NiO nanosheet arrays were obtained. Subsequently, the second facile solvothermal process followed by the same annealing treatment was carried out, all NiCo2O4 nanosheets vertically grew on the surface of NiO sheets to form a hierarchical NiO@NiCo2O4 core–shell nanosheet array, which was regarded as a flexible electrochemical energy electrode material.

Figure 1. Schematic diagram of the overall preparation process for the hierarchical heterostructure of Ni foam@NiO@NiCo2O4 nanosheet arrays. (a) Ni foam substrate; (b) NiO nanosheet arrays; (c) Ni foam@NiO@NiCo2O4 hierarchical core–shell nanosheet arrays. 8


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The morphologies of the electrodes are shown on Figure 2. As shown in Figure 2a and 2b, the NiO nanosheet arrays on Ni foam after solvothermal reaction and post-annealing procedure. The surface of Ni foam is completely covered with NiO nanosheet arrays which have a length of 1-3 µm. NiO nanosheets are cross-linked with each other and vertically grow on the Ni foam surface to form a 3D network structure with huge special surface area. Meanwhile, the interconnected nanosheets construct tremendous porous open-framework structure, which may facilitate the efficient penetration of electrolyte ions and transport of electrons inside of active materials.

Figure 2. SEM images of NiO nanosheet arrays (a, b) and NiO@NiCo2O4 hierarchical core–shell nanosheet arrays (c, d) on Ni foam substrates. The solvothermal reaction at 9



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the second step was 2.5 h. Figure 2c and 2d illustrate that, after the second solvothermal reaction and annealing procedure, the smaller NiCo2O4 nanosheets vertically deposit on the surface of NiO nanosheets, forming the hierarchical NiO@NiCo2O4 core–shell arrays on Ni foam. On account of the growth of NiCo2O4 nanosheets, the thickness of the core– shell nanosheet arrays approximately increase to 160-220 nm, much thicker than NiO nanosheets of 30-40 nm. The surface area of electrode material is also increased. It is suggested that this hierarchical core–shell network with high surface area may improve the electrochemical performance of electrode. The effect of reaction time at the secondary solvothermal reaction on the morphology of Ni foam@NiO@NiCo2O4 was also investigated. The result was demonstrated in Figure S1, and the formation process of the hybrid electrodes was explored. For the NiO@NiCo2O4 obtained after 1.5 h (Figure S1a), the NiO nanosheets are clearly observed, but no visible NiCo2O4 nanosheets grown on the NiO nanosheet arrays can be found. With the increase of reaction time, the NiCo2O4 nanosheets become evident and the increasing thickness of NiO@NiCo2O4 walls can well prove the vertically growth of NiCo2O4 shell on surface of NiO nanosheet arrays, as presented in Figure S1b-d (2, 2.5, and 3h). As the reaction time is 2.5 h, the ‘shell’ of NiCo2O4 already turns into the thin nanosheets that cover the whole NiO nanosheet arrays (Figure S1c). With the growth of NiCo2O4 shell for 3h (Figure S1d), the thickness of NiO@NiCo2O4 core–shell walls becomes around 500 nm, which leads to the decrease in the size of large pores. When the reaction time is higher than 4 h 10


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(Figure S1e-g), NiCo2O4 nanosheets grow bigger and bigger, leading to the blocked porous structure. Moreover, the longer reaction time produces the more nanoneedle-like NiCo2O4 aggregation, which completely blocks the porous structure of NiO interconnected nanosheets and reaches the highest mass loading (Figure S1g). The blocked porous structure would reduce the electrochemical utilization of the hybrid electrode of NiO@NiCo2O4, due to the limit of electrolyte ions accessing to surface of both active materials. Therefore, the optimized reaction time is 2.5 h in the current work. The obtained NiO@NiCo2O4 on Ni foam shows the perfect 3D hierarchical nanostructure which can be confirmed by N2 adsorption/desorption isotherms (Figure S2). In order to further investigate the evident morphology of the hybrid materials by TEM, the NiO and NiO@NiCo2O4 core–shell nanosheets were obtained by ultrasonication of their integrated Ni foam electrodes. Figure 3a exhibits the corresponding TEM image of NiO nanosheets. The HRTEM image of NiO nanosheets is shown in Figure 3b, and it can clearly present the lattice fringes with an interplanar spacing of 0.15 and 0.24 nm, which perfectly match with the (220) and (111) planes of the halite NiO phase. Moreover, there are many mesopores in the sheets with the diameter of about 7nm, which result from the post-treatment of calcination. Figure 3c shows the TEM of the hierarchical NiO@NiCo2O4 core–shell arrays. It is apparently demonstrated that many small NiCo2O4 nanosheets vertically grow on the large surface of NiO nanosheets, because the edges of NiCo2O4 sheets are presented as dark lines in Figure 3c. In the HRTEM image of NiO@NiCo2O4 core–shell nanosheet 11



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arrays (Figure 3d), the lattice fringe existing in the dark area of the image may be assigned to (311) plane of spinel NiCo2O4, while the lattice fringes in the light area of the image are corresponding to the (220) and (111) planes of the halite NiO phase with the lattice spacing of 0.24 and 0.15 nm, respectively.

Figure 3 TEM images of NiO nanosheet (a, b) and NiO@NiCo2O4 core–shell nanosheet (c, d) arrays obtained from the Ni foams. The solvothermal reaction at the second step was 2.5 h. EDS mapping analysis can be performed to determine the elemental compositions 12


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of nanosheet arrays. As shown in Figure 4a-e, the interface element distribution images depict that the hierarchical NiO@NiCo2O4 hybrid electrode mainly consists of Ni, Co and O elements. The appearance of Co element in the EDS map suggests that the NiCo2O4 nanosheets were successfully produced on the surface of NiO nanosheets to form the hierarchical NiO@NiCo2O4 hybrid nanostructure, which is expected to enhance the electrochemical performance of the hybrid electrode.

Figure 4. EDS analysis (a-e) of the selected area from NiO@NiCo2O4 nanosheet 13



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arrays. The crystallographic structures of obtained products were further classified by XRD patterns. Powder of calcined product in each step were collected. As shown in Figure 5, different XRD patterns are assigned to NiO (a), NiCo2O4 (b), hierarchical core–shell NiO@NiCo2O4 nanosheet arrays scratched form Ni foam (c) and NiO@NiCo2O4 hierarchical nanosheets aggregated on Ni foam (d), respectively. The diffraction peaks at 2θ = 37.1°, 43.1°, 62.6°, 75° and 79° are indexed to (111), (200), (220), (331) and (222) planes of halite NiO (JCPDS no. 65-2901) (Figure 5a). Meanwhile, as shown in Figure 5b, the diffraction peaks at 18.9°, 31.1°, 36.7°, 44.6°, 59.1° and 65° are assigned to (111), (220), (311), (400) , (511) and (440) planes of spinel phase NiCo2O4, according to JCPDS card (no. 20-0781). The results demonstrate the formation of NiO and NiCo2O4 during the two steps of synthetic process.

14


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Figure 5 XRD patterns of the NiO (a), NiCo2O4 (b), NiO@NiCo2O4 (c) scratched form Ni foam and hierarchical core–shell NiO@NiCo2O4 nanosheets grown on Ni foam (d), respectively. As demonstrated in Figure 5c, the diffraction peaks of NiO@NiCo2O4 scratched from Ni foam indicate a complex of both NiO phase and NiCo2O4 phase. The pattern of Ni foam@NiO@NiCo2O4 (Figure 5d) exhibits three obvious strong peaks, which are attributed to Ni foam (JCPDS no.03-1051).Moreover, five recognizable peaks at 2θ = 18.9°, 37.1°, 43.1°, 59.1° and 79° can be indexed to (111), (200), (222) planes of NiO (JCPDS no. 65-2901) and (111), (511) planes of cubic spinel NiCo2O4 (JCPDS no. 20-0781), which indicate that NiCo2O4 nanoflakes were successfully grown on the NiO nanosheet arrays.

15



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Figure 6 XPS survey spectrum (a), Ni 2p (b), Co 2p (c) and O 1s (d) for hierarchical NiO@NiCo2O4 nanosheet arrays. XPS was used to characterize the surface composition and chemical states of electrode materials. Figure 6 indicates the characteristic peaks of Ni, Co and O from the survey spectrum and without any impurity peaks detected. The two peaks at 853.8 and 871.9 eV are indexed to Ni2+, while those at 855.4 and 873.7 eV are assigned to Ni3+ 27. The satellite peaks located at 861.1 and 879.4 eV are corresponding to high binding energy sides of Ni 2p3/2 and Ni 2p1/2, which are two shakeup-type peaks of Ni30. Besides, the Co 2p spectrum shows a characteristic of two spin–orbit doublets, which can be ascribed to two kinds of ions. The fitted peaks of Co2+ 2p1/2 and 2p3/2 are recorded at binding energies of 796.3 and 780.7eV, meanwhile the rest two fitted peaks at the 794.9 and 780.1eV are ascribed to Co3+

31,32

. The high-resolution

spectrum of the O1s region presents three oxygen contributions. Specifically, the peak 16


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at 529.5 eV (O1) belongs to the typical metal–oxygen bonds and the peak sitting at 531.1 eV (O2) is associated with oxygen ions with low coordination state at the surface22. The peaks located at 532.3 eV (O3) can be ascribed to the physically or chemically absorbed H2O on or within the surface14, 29. Based on the analysis of the characterization above, the formation mechanism of NiO precursor is explained as follows: the decomposition and hydrolysis of HTMA during the process of solvothermal lead to release OH-. The generated ions interact with metal ions, which result in the Ni(OH)2 nanosheets anchored on the surface of Ni foam. With the annealing treatment, the Ni(OH)2 was decomposed into NiO by releasing gaseous CO2 and H2O. This process can be demonstrated by XRD patterns shown in Figure S3 and the equations involved are expressed in Eqs (1)-(4): ∆

C6H12N4 + 6H2O → 6HCHO + 4NH3

(1)

NH3 + H2O → NH4+ + OH-

(2)

Ni+ + 2OH- → Ni(OH)2

(3)

∆

Ni(OH)2 → NiO + H2O The formation mechanism of NiCo2O4 is shown as follow13,

(4) 33

. With the

decomposition and hydrolysis of Urea, the OH- and CO32- ions were released and then interacted with metal ions, which lead to generated metal carbonate hydroxide salts. After annealing treatment, metal oxides prepared accompanied by the release of gaseous CO2 and H2O. The equations for preparing NiCo2O4 in the second solvothermal and annealing process can be classified as follows. ∆

CO(NH2)2 + H2O → 2NH3 + CO2

(5) 17



Page 18 of 38

CO2 + H2O → CO32- + 2H+

(6)

NH3 + H2O → NH4+ + OH-

(7)

2(Co2+, Ni2+) + CO32- + 2OH- + nH2O → (Co, Ni)2CO3(OH)2nH2O

(8)

∆

(Co, Ni)2CO3(OH)2nH2O → NiCo2O4 + H2O + CO2 In

order

to

further

explore

the

electrochemical

properties

(9) of

Ni

foam@NiO@NiCo2O4 electrode, a series of electrochemical tests were carried out in a three-electrode configuration system with a 6M KOH aqueous solution as electrolyte. Figure 7a depicts the comparison on typical CV curves at different scan rates of 2 ~ 100mV s-1 with a potential window ranging from 0 to 0.6 V. The shape of CV curves indicates the characteristics of faradaic supercapacitors. The contribution of redox peaks is expressed as follows: NiO + OH- ⇌ NiOOH + eNiCo2O4 + H2O + OH- ⇌ NiOOH + 2CoOOH + eCoOOH + OH- ⇌ CoO2+ H2O + e-

(10) (11) (12)

The degree of capacitive effect can be qualitatively determined by some related analysis. According to a power-law relationship : i = avb which contains two measured parameters of current (i) and scan rate (v) obtained from CV curves (both a and b are constants), the b-value can be given from the slope of log(i) versus log(v)34. The b-value is between 0.5 (semi-infinite diffusion process) and 1 (capacitive process)35.

18


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Figure 7 (a) CV curves at 2 ~100 mV s-1 and (b) galvanostatic charge/discharge curves at 2~ 20 A g-1 of Ni foam@NiO@NiCo2O4. Comparison in (c) CV curves at 25 mV s-1, (d) galvanostatic charge/discharge curves at 2 A g-1, (e) specific capacitances at different current densities, and (f) cycling performance at 10 A g-1 for 10000 cycles for Ni foam@NiO, Ni foam@NiCo2O4 and Ni foam@NiO@NiCo2O4 electrodes. As shown in Figure S4a, the b-value of NiO@NiCo2O4 is approximately equal to 1 in the sweep rate range of 2-10 mV s-1, suggesting a typical capacitive behavior in the fast redox process. The capacitive contribution can be calculated qualitatively by 19



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the equation: i = k1ν + k2ν1/2, where the k1 indicates capacitive effect and the k2 represents diffusion process36,37. As shown in Figure S4b, k1 and k2 can be determined from the slope and the y-axis intercept of a straight line by plotting ν1/2 versus i/ν1/2. What we noticed in the Figure S4c is the shade region (k1ν) inside the CV curve at 5 mV s−1 occupies about 82.3% of the total area, which can determine the capacitive contribution to the whole capacitance. With the increase of the scan rate range from 2 to 10 mV s−1, the capacitive contribution is improved while the diffusion contribution depressed as expected (Figure S4d). The peak current increases with the increasing of scan rate, suggesting the rapid enhancement of electronic and ionic transport rates. In addition, the redox peaks shift towards positive and negative potential and the specific capacitance decreases respectively owing to enhancing internal diffusion resistance in the active material38. As shown in Figure 7b, the pseudocapacitive properties of the Ni foam@NiO@NiCo2O4 electrode can be obtained by galvanostatic charge-discharge tests at different current densities ranging from 2 to 20 A g-1 with a potential window of 0 to 0.5 V (vs. Hg/ HgO).The charge-discharge curves display that the as-prepared electrode has almost the same charging and discharging time, demonstrating its excellent Coulombic efficiency. Also, the distinct voltage plateau regions emerge during the charging/ discharging process, which can be ascribed to redox reactions on the surface of the pseudocapacitive or battery materials. This result is consistent with the expression in previous literature reports39. Apparently, the CV curve of the Ni foam@NiO@NiCo2O4 has a larger integrated 20


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area than those of Ni foam@NiO and Ni foam@NiCo2O4 under the same condition in Figure 7c, suggesting that the Ni foam@NiO@NiCo2O4 electrode possesses the largest specific capacitance of all electrodes. Furthermore, compared with the redox peaks of Ni foam@NiO electrode, Ni foam@NiO@NiCo2O4 exhibits an obvious enhancement in specific capacitance, due to the introduction of NiCo2O4 nanosheets. The comparison of galvanostatic charge-discharge curves for three electrodes at 2 A g-1 is exhibited in Figure 7d. It is displayed that the GCD curve of Ni foam@NiO@NiCo2O4 reveals the longer charge/discharge time than the individual component of NiO and NiCo2O4 under identical conditions. Besides, the Ni foam@NiO@NiCo2O4 electrode has the similar charge time and discharge time at different current densities ranging from 2-20 A g-1 as shown in Figure 7b, demonstrating that such pseudocapacitive electrode not only possesses an enhanced capacitance but also has excellent Coulombic efficiency. As shown in Figure 7e, the specific capacitance of Ni foam@NiO@NiCo2O4 electrode is up to 1623.6 F g-1 (225.5 mAh g-1) at the current density of 2 A g-1 which is much higher than those of Ni foam@NiO and Ni foam@NiCo2O4 nanosheet arrays electrodes. Moreover, its capacitance performance is superior to various hierarchical electrodes reported such as NiCo2O4@MnMoO4 core–shell flowers/Ni foam (1118 F g-1 at 1 A g-1)40, NiCo2O4@MnO2 core–shell nanosheet arrays/Ni foam (1595.1 F g-1 at 3 mA cm-2)23, ZnCo2O4@NiCo2O4 core–shell nanowires/PAN nanofibers (1476 F g-1 at 1 A g-1)14, 3D hierarchical porous rose-like NiCo2O4/MnCo2O4 (911.3 F g-1 at 5 A g-1)41 , NiCo2O4–GO nanosheets/Ni foam(1078 F g-1 at 1 mA cm-2)42, hierarchical 21



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NiCo2O4@NiO core–shell nanowires/carbon cloth(1501 F g-1 at 5 mA cm-2)26 and so on. Even if the current density increases to 20 A g-1, the specific capacitance of Ni foam@NiO@NiCo2O4 is still up to 1560 F g-1 (or 216.7 mAh g-1) and retains about 96% of its initial capacitance, indicating its superior rate capability. For the evaluation in areal capacitance, the areal capacitance of NiO@NiCo2O4 is 1.3 F cm-2 at 1.6 mA cm-2 and can still retain 1.25 F cm-2 at 16 mA cm-2, which is superior to Co3O4@MnO2(0.56 F cm-2 at 11.25 mA cm-2)43 and MnO2@NiO(0.35 F cm-2 at 9.5 mA cm-2)44. The excellent cycle performance is another advantage for practical applications of supercapacitors. As depicted in Figure 7f, with an activated period, the specific capacitance of Ni foam@NiO@NiCo2O4 can remain 100% after 5000 cycles of charge/discharge at 10 A g-1 and keep for further several thousand cycles (Figure S5). After 10000 cycles, it still maintains nearly 90% of its initial specific capacitance, much higher than those of Ni foam@NiO and Ni foam@NiCo2O4 electrodes. It remarkable cycling stability and good rate performance are also superior to those of Ni(OH)245 and Ni foam@NiCo2S446. The Figure S6a shows the first few cycles of charge/discharge curves. It demonstrates that such a nanosheet arrays network of NiO@NiCo2O4 has high electrochemical stability. Moreover, the Coulomb efficiency of Ni foam@NiO@NiCo2O4 electrode during 10000 cycles of charge/discharge at 10 A g-1 is shown in Figure S6b. It is obviously determinated that Ni foam@NiO@NiCo2O4 can exhibit an excellent Coulombic efficiency of nearly 100% during the long time cycling. The superior cycle stability and Coulombic efficiency 22


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are mainly ascribed to the 3D hierarchical nanosheets network structure with a high mechanical strength, avoiding collapse of the structure caused by volume change during the long term charge-discharge cycles. A continuous increase of the specific capacitance displayed in the figure at the beginning cycles is attributed to the enhancement of wettability between electrode materials and the electrolyte solution47. This process improves the degree of activation of the electrode materials during cycling48. The electrochemical performance of electrode materials can be improved by controlling reaction time. Figure S7(a-b) shows the CV curves at 25 mV s-1 and the GCD curves at 2 A g-1 among all of the as-prepared electrodes with different reaction time (1-6 h). The presence of redox peaks in CV curves and long charge-recharge platform in GCD curves demonstrate the faradaic pseudocapacitive behaviors of these electrodes. The corresponding specific capacitances of all electrodes at different current densities were calculated and shown in Figure S7c. With comparison, the product obtained after 2.5 h reached the highest capacitance, which means the hybrid NiO@NiCo2O4 produced after 2.5 h has the optimized morphology and combination. Considering the morphology shown in Figure S1, we can suggest that the thickness, size and morphology or mass loading of NiCo2O4 nanosheets have a great influence on electrochemical properties. For the longer time, the penetration and diffusion of electrolytes on the surface of active materials would be blocked because the nanosheets of NiCo2O4 are too thick and even turn into long nano threads. While for the shorter time, the NiCo2O4 nanosheets on NiO surface are too less, which leads to 23



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the less capacitance. Therefore, the as-prepared NiO@NiCo2O4 core–shell nanosheet arrays with appropriate thickness, size and morphology display the most remarkable electrochemical performance. The EIS measurement was carried out in the frequency range of 0.01 Hz to 100 kHz to further explore the electrochemical performance of the Ni foam@NiO, Ni foam@NiCo2O4 and Ni foam@NiO@NiCo2O4 electrodes. As shown in Figure S8, the charge transfer resistance (Rct) can be estimated by a semicircle diameter in the high frequency region. Ambiguous arcs of all electrodes in the high frequency region indicate its remarkable electronic and ionic conductivity between electrode and electrolyte, especially for the Ni foam@NiO@NiCo2O4 electrode with a less arc due to the synergetic effect (inset of Figure S8). Obviously, the Ni foam@NiO@NiCo2O4 electrode exhibits a more ideal straight line with larger slope in the low frequency area of spectrum among all of as-prepared electrodes, demonstrating that it has more efficient electrolyte ions diffusion and large double-layered capacitance. This can be ascribed to the hierarchical mesoporous nanosheet arrays. Due to the open-framework structure with relatively large surface area and abundant space, the active materials of electrode can be utilized as much as possible to contact with OH- 49. Compared with Ni foam@NiO and Ni foam@NiCo2O4 electrodes, the Ni foam@NiO@NiCo2O4 exhibits the outstanding performance. The enhanced electrochemical performance is mainly attributed to the 3D hierarchical nanosheet arrays anchored on Ni foam. The interconnected NiO nanosheets construct the framework and open pores of hierarchical nanosheet arrays that can support the 24


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growth of NiCo2O4 nanosheets. The 3D hierarchical electrode have a larger specific surface, more accessible electrochemical active sites, and short pathways for on diffusion and charge exchange at the surface of core-shell nanosheet arrays38. In addition, all components of Ni foam@NiO@NiCo2O4 electrode are transition-metal oxides with high theoretical capacitance. The NiO nanosheet arrays are anchored on the surface of Ni foam as the supporting structure. Vertically growing NiCo2O4 nanoflakes on NiO arrays can further increase the specific surface area to facilitate diffusion of the electrolyte ions and accelerate redox reactions of active materials. The thin nanosheets of NiO and NiCo2O4 may be more fully used than the bulk ones. The possible synergistic effect resulted from the well-combination of these two components with good adhesion can further contribute to the enhancement of the electrochemical performance of Ni foam@NiO@NiCo2O4. Besides, the integrated NiO@NiCo2O4 nanosheets growing on Ni foam directly can exhibit low inner resistance due to no conductive additive and binder used. All these advantages of such a 3D hierarchical structure of Ni foam@NiO@NiCo2O4 can efficiently facilitate ion diffusion and charge exchange at the surface of core-shell nanosheet arrays38. In order to evaluate the potential application of Ni foam@NiO@NiCo2O4 core-shell nanosheet arrays electrode, an asymmetric supercapacitor (ASC) was assembled by using Ni foam@NiO@NiCo2O4 nanosheet arrays as a cathode and activated carbon pressed on a Ni foam as an anode in 6 M KOH electrolyte. As shown Figure 8a, the electrodes and electrolyte were packed in a pair of cell shells. The CV curves of the cathode and anode were characterized with the potential window of 0 to 25



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0.6 V and -1.0 to 0 V respectively in a three-electrode system at 10 mV s−1 in Figure 8b.

26


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Figure 8 (a) Schematic illustration of the as-fabricated Ni foam@NiO@NiCo2O4//AC ASC device. (b) CV curves of Ni foam@NiO@NiCo2O4 and AC electrodes at 10mV s-1 in a three-electrode system. (c) CV curves of Ni foam@NiO@NiCo2O4//AC ASC at various scan rates. (d) GCD curves and (e) specific capacitance values of the Ni foam@NiO@NiCo2O4//AC ASC at various current densities; (f) Ragone plots of different

ASC

devices

related

to

NiCo2O4.

(g)

Photograph

of

the

Ni

foam@NiO@NiCo2O4//AC ASC used to drive a red LED of 5 mm diameter. (h) Display of light a red LED by two ASC devices in series and cycling stability of the ASC device. The loading mass of AC can be determined by the following equation: m− = m+ ×(C+ × ∆V+)/(C− × ∆V−) which corresponds to the charge balance relationship of q+ = q− , where m is the active mass of the electrode, C is the specific areal capacitance, ∆V is the working potential window. The average mass of NiO@NiCo2O4 is 0.8 mg cm−2. Therefore, the calculated result indicates that the specific mass of AC required for the anode is 3.2 mg cm−2. The specific capacitance of activated carbon used in this study is about 190.8 F g-1 at a discharge current density of 2 A g-1, according to its galvanostatic charge-discharge data (not shown here). The different potential windows of cathode and anode constitute the potential range for the ASC device. The rectangular CV curve of the AC electrode indicates its EDLC behavior. The CV curves of the two-electrode supercapacitor device at the scan rates of 5–100 mV s-1 with the potential window of 0–1.6 V exhibits no obvious distortion in the Figure 8c. It indicates a good capacitive behavior. 27



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GCD curves of the asymmetric supercapacitor device at different current densities are further depicted in Figure 8d. The specific capacitance at different current densities for the ASC calculated are shown in Figure 8e. From the Ragone figure

(Figure

8f),

it can

be

shown

that the

specific

energy

of

Ni

foam@NiO@NiCo2O4//AC device can reach to 52.5 Wh kg-1 at a specific power of 387.5 W kg-1. It is much higher than reported values such as AC//NiCo2O450, AC//CQDs/NiCo2O451, AC//RGO/NiCo2O452, AC//GNP/NiCo2O453 and AC//NiCo2O4 NSs@HMRAs30. Also, it still retains 36.7 Wh kg-1 at a high specific power of 8000 W kg-1, as much as 70% of its original specific energy. Two ASC devices in series can successfully light a red LED of 5 mm diameter for 8 minutes approximately (Figure 8g). Figure 8h reveals 90% capacitance retention over 3000 cycles at 2 A g-1, demonstrating the good cycle stability of ASC electrode. All these attractive results indicate that the core–shell Ni foam@NiO@NiCo2O4 nanosheet arrays electrode has potential practical value for energy storage.

Conclusions In summary, a binder-free hierarchical NiO@NiCo2O4 core–shell nanosheet arrays supported on Ni foam was successfully prepared by a facile two-step solvothermal method. The optimized NiO@NiCo2O4 electrode shows a high specific capacitance of 1624 F g-1(or 225.5 mAh g-1 in specific capacity) at a discharge current density of 2 A g-1 and also possesses capacitance of 1560 F g-1 (or 216.7 mAh g-1) at 20 A g-1 (96% of initial capacitance), indicating a good rate performance. Additionally, it exhibits a high capacitance retention of 90% initial specific capacitance after 10000 28


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cycles of charge-discharge. Furthermore, the ASC of Ni foam@NiO@NiCo2O4//AC exhibits the specific energy of 52.5 Wh kg-1 at a specific power of 387.5 W kg-1, and retains 36.7 Wh kg-1 at a high specific power of 8000 W kg-1. After 3000 cycles at 2 A g-1, the specific capacitance retention can remain nearly 90% of its initial value, suggesting its good cyclic stability. The ordered core–shell nanosheet arrays structure and the synergistic effects of the two pseudocapacitive materials can greatly increase the electrochemical performance of Ni foam@NiO@NiCo2O4 electrode so that it becomes a promising electrode material which has a potential application for energy storage devices.

Associated content Supporting information Figure S1. Morphology comparation of Ni foam@NiO@NiCo2O4 with different reaction

time

at

the

secondary

solvothermal

reaction.

Figure

S2.

Brunauer−Emmett−Teller of Ni foam@NiO@NiCo2O4. Figure S3. XRD patterns of the solvothermal product prepared in the first step before annealing and after annealing. Figure S4. Analysis of the degree of capacitive effect. Figure S5. Morphology of NiO@NiCo2O4 after 5000 cycles. Figure S6. The first few cycles of galvanostatic charge–discharge curves and Coulomb efficiency Sensitivity of NiO@NiCo2O4. Figure S7. Comparison of the electrochemical properties of NiO@NiCo2O4 with different reaction time. Figure S8. EIS of the Ni foam@NiO, Ni foam@NiCo2O4 and Ni foam@NiO@NiCo2O4.

Acknowledgements 29



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The work was supported by the National Natural Science Foundation of China (No. 21576138, 51572127), China-Israel Cooperative Program (2016YFE0129900), Program for NCET-12-0629, PhD Program Foundation of Ministry of Education of China (No.20133219110018), Natural Science Foundation of Jiangsu Province (BK20160828), Postdoctoral Foundation (1501016B), Six Major Talent Summit (XNY-011), and PAPD of Jiangsu Province, and the program for Science and Technology Innovative Research Team in Universities of Jiangsu Province, China. We also thank Dr. Huaping Bai and Dr. Wanying Tang for the XRD and Raman data collection, at Analysis and Test Center Nanjing University of Science and Technology.

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Co3O4@Pt@MnO2 Nanowire Arrays for High-Performance Supercapacitors. Sci. Rep. 2013, 3, 2978. (49) Cai, D.; Wang, D.; Liu, B.; Wang, L.; Liu, Y.; Li, H.; Wang, Y.; Li, Q.; Wang, T. Three-Dimensional Co3O4@NiMoO4 Core/Shell Nanowire Arrays on Ni Foam for Electrochemical Energy Storage. ACS Appl. Mater. Interfaces 2014, 6 (7), 5050-5055. (50) Ding, R.; Qi, L.; Jia, M.; Wang, H. Facile and Large-Scale Chemical Synthesis of Highly Porous Secondary Submicron/Micron-Sized NiCo2O4 Materials for High-Performance Aqueous Hybrid AC-NiCo2O4 Electrochemical Capacitors. Electrochim. Acta 2013, 107, 494-502. (51) Zhu, Y.; Wu, Z.; Jing, M.; Hou, H.; Yang, Y.; Zhang, Y.; Yang, X.; Song, W.; Jia, X.; Ji, X. Porous NiCo2O4 Spheres Tuned through Carbon Quantum Dots Utilised as Advanced Materials for an Asymmetric Supercapacitor. J. Mater. Chem. A 2015, 3 (2), 866-877. (52) Wang, X.; Liu, W. S.; Lu, X.; Lee, P. S. Dodecyl Sulfate-Induced Fast Faradic Process in Nickel Cobalt Oxide-Reduced Graphite Oxide Composite Material and its Application for Asymmetric Supercapacitor Device. J. Mater. Chem. 2012, 22 (43), 23114-23119. (53) Wang, H.; Holt, C. M. B.; Li, Z.; Tan, X.; Amirkhiz, B. S.; Xu, Z.; Olsen, B. C.; Stephenson, T.; Mitlin, D. Graphene-Nickel Cobaltite Nanocomposite Asymmetrical Supercapacitor with Commercial Level Mass Loading. Nano Res. 2012, 5 (9), 605-617.

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Figure Legends Figure 1. Schematic diagram of the overall preparation process for the hierarchical heterostructure of Ni foam@NiO@NiCo2O4 nanosheet arrays. (a) Ni foam substrate; (b) NiO nanosheet arrays; (c) Ni foam@NiO@NiCo2O4 hierarchical core–shell nanosheet arrays. Figure 2. SEM images of NiO nanosheet arrays (a, b) and NiO@NiCo2O4 hierarchical core–shell nanosheet arrays (c, d) on Ni foam substrates. The solvothermal reaction at the second step was 2.5 h. Figure 3. TEM images of NiO nanosheet (a, b) and NiO@NiCo2O4 core–shell nanosheet (c, d) arrays obtained from the Ni foams. The solvothermal reaction at the second step was 2.5 h. Figure 4. EDS analysis (a-e) of the selected area from NiO@NiCo2O4 nanosheet arrays. Figure 5. XRD patterns of the NiO (a), NiCo2O4 (b), NiO@NiCo2O4(c) scratched 35



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form Ni foam and hierarchical core–shell NiO@NiCo2O4 nanosheets grown on Ni foam (d), respectively. Figure 6. XPS survey spectrum (a), Ni 2p (b), Co 2p (c) and O 1s (d) for hierarchical NiO@NiCo2O4 nanosheet arrays. Figure 7. (a) CV curves at 2 ~100 mV s-1 and (b) galvanostatic charge/discharge curves at 2~ 20 A g-1 of Ni foam@NiO@NiCo2O4. Comparison in (c) CV curves at 25 mV s-1, (d) galvanostatic charge/discharge curves at 2 A g-1, (e) specific capacitances at different current densities, and (f) cycling performance at 10 A g-1 for 10000 cycles for Ni foam@NiO, Ni foam@NiCo2O4 and Ni foam@NiO@NiCo2O4 electrodes. Figure 8. (a) Schematic illustration of the as-fabricated Ni foam@NiO@NiCo2O4//AC ASC device. (b) CV curves of Ni foam@NiO@NiCo2O4 and AC electrodes at 10mV s-1 in a three-electrode system. (c) CV curves of Ni foam@NiO@NiCo2O4//AC ASC at various scan rates. (d) GCD curves and (e) specific capacitance values of the Ni foam@NiO@NiCo2O4//AC ASC at various current densities; (f) Ragone plots of different

ASC

devices

related

to

NiCo2O4.

(g)

Photograph

of

the

Ni

foam@NiO@NiCo2O4//AC ASC used to drive a red LED of 5 mm diameter. (h) Display of light a red LED by two ASC devices in series and cycling stability of the ASC device.

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