Hierarchical NiO@NiCo2O4 Core–shell Nanosheet Arrays on Ni

Apr 19, 2018 - *E-mail: [email protected] (W. Lei)., *E-mail: [email protected]; Fax: +86-25-84315190; Tel: +86-25-84315190 (Q. Hao)...
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Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 6246−6256

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Hierarchical NiO@NiCo2O4 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* Key Laboratory for Soft Chemistry and Functional Materials, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, China

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ABSTRACT: A facile solvothermal method followed by a postannealing 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. Recently, MnO2,10 Co3O4,11 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 NiCo2O4,13 ZnCo2O4,14,15 NiMoO4,16 and CoMoO47,17 have been favorable due to their multiple oxidation states for reversible Faradaic reactions as well as outstanding electrical conductivity.18 Among these bimetallic oxides, spinel NiCo2O4 is suggested as the most promising redox electrode material and is widely studied because of its excellent electronic conductivity, low cost, environmental friendliness and easily controllable morphologies.19 Various NiCo2O4 nanostructures with different morphologies have been investigated, such as 1D-nanowires,20 2D-nanosheets,21 and 3D-hierarchical structures composed of nanowires or nanosheets,13 which can efficiently improve the specific capacity due to the large surface area. The 3D hierarchical hybrid electrodes can exhibit higher capacitive performance due to the fast ion transportation and the synergetic effect between all components.13 For example, the Seaurchin-like core−shell nanoneedles of NiCo2O4@NiMoO4,22 3D nanosheet arrays of NiCo2O4@ MnO2,23 and 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 foam,18,24,25 carbon cloth,26 and titanium mesh,27 may

1. INTRODUCTION With the rapid development of portable electronic devices, such as smart phones, visual reality glasses, and wearable devices, sustainable and environmentally friendly energy storage and conversion devices including batteries, fuel cells, and electrochemical capacitors (ECs), have attracted great attention recently.1 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 stability.2,3 ECs can be divided into two types according to charge storage mechanisms, electrochemical double layer capacitors (EDLCs), and faradaic supercapacitors.3,4 EDLCs store energy by accumulating charge electrostatically at the interface of electrode/ electrolyte and possess high power density and excellent cycling performance, but theyhave limited specific capacitance. Being different from EDLCs, faradaic supercapacitors can store charges based on the fast reversible multielectron redox reactions occurring on the surface of an electrode, which usually generate high energy density and can be used as power devices.5 However, faradaic supercapacitors suffer from poor cyclic stability caused by volume change during the charge/discharge processes and the low conductivity of active materials.6 According to recent reports, the transition metal oxides, hydroxides, and conducting polymers are usually used as faradaic supercapacitors materials.7 Although RuO2 exhibits the most promising performance, it is limited by high cost, scarcity, and toxicity of RuO2 in commercial application.8,9 Therefore, exploring faradaic supercapacitor materials with high capacitance, nontoxicity, and low cost has become the main focus of the current study on supercapacitors. © 2018 American Chemical Society

Received: Revised: Accepted: Published: 6246

January 29, 2018 March 29, 2018 April 19, 2018 April 19, 2018 DOI: 10.1021/acs.iecr.8b00467 Ind. Eng. Chem. Res. 2018, 57, 6246−6256

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Industrial & Engineering Chemistry Research provide an integrated electrode, which avoids the “dead surface” caused by using binder to fix active materials.18 Therefore, to design 3D hierarchical nanostructures for electrode materials is of great popularity as a research orientation on fabricating highperformance supercapacitors.28 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 array supported on Ni foam without using binder. Due to the outstanding theoretical specific capacitance and thermal stability,29 NiO was constructed as an inner framework for supporting the 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 arrays.6 Moreover, the NiO@ NiCo2O4//Activated Carbon (AC) asymmetric supercapacitor showed 52.5 Wh kg−1 of specific energy at a specific power of 387.5 W kg−1 and retained 36.7 Wh kg−1 at a high specific power of 8000 W kg−1. In addition, the asymmetric supercapacitor possesses a good cyclic stability that the specific capacitance retention can retain 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 redox materials play an important role in greatly increasing the electrochemical performance of the supercapacitor electrode.

Then, the Teflon-lined stainless-steel autoclave was placed into an oven and heated at 130 °C 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 °C for 2 h, followed by an annealing process at 350 °C for 2 h the same as the previous postprocessing step. Meanwhile, Ni foam@NiO nanosheets, Ni foam@NiCo2O4 nanosheets, and Ni foam@NiO@NiCo2O4 nanosheet arrays with different reaction times 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 times 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 a field emission scanning electron microscope (FE-SEM; FEI Quanta 250FEG, America) operating at 20 kV and a transmission electron microscope (TEM; JEOL JEM-2100) included with an energy dispersive X-ray spectrometer (EDS). The surface elements 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. 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 6 M KOH aqueous solution. The tests were performed with a conventional three-electrode system by using an as-prepared electrode (1 cm × 1 cm) as the working electrode and 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 a Ni foam@ NiO@NiCo2O4 hierarchical core−shell array 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 nonwoven cloth separator and performed in a C2032 button coin cell. In the three-electrode system, specific capacitances were calculated according to the following equation:

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. All reagents used in experiments were of analytical purity and were used without further purification. 2.1.1. Synthesis of NiO Nanosheet 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 it was washed with distilled water and ethanol. One mmol of Ni(NO3)2·6H2O, 3 mmol of hexamethylenetetramine (HTMA), and 1 mmol of hexadecyl trimethylammonium bromide (CTAB) were dissolved in a mixture of 15 mL water and 15 mL of 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 the autoclave and maintained at 130 °C for 10 h. After cooling to 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 °C and then calcined at 350 °C for 2 h; thus, the NiO nanosheet 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 30 mL of 50% ethanol aqueous solution containing 0.5 mmol of Ni(NO3)2·6H2O, 1 mmol of Co(NO3)2·6H2O, 3 mmol of urea, and 2 mmol of CTAB was added to a stainless-steel autoclave containing a piece of Ni foam@NiO nanosheet array electrodes.

Csp = I × Δt /(m ·ΔV )

(1)

where I is the discharge current, Δt is the discharging time, ΔV 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 the twoelectrode cell can be calculated according to the following equations:

6247

E = C × V 2/2

(2)

P = E/t

(3) DOI: 10.1021/acs.iecr.8b00467 Ind. Eng. Chem. Res. 2018, 57, 6246−6256

Article

Industrial & Engineering Chemistry Research where E (W h kg−1) is the energy density, C (F g−1) is the specific capacitance of the 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.

increases to 160−220 nm, much thicker than NiO nanosheets of 30−40 nm. The surface area of the electrode material is also increased. It is suggested that this hierarchical core−shell network with high surface area may improve the electrochemical performance of the 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 vertical growth of the NiCo2O4 shell on the 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 the NiCo2O4 shell for 3 h (Figure S 1d), the thickness of the 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 (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 the 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

3. RESULTS AND DISCUSSION As illustrated in Figure 1, the hierarchical heterostructure of the Ni foam@NiO@NiCo2O4 core−shell nanosheet array was

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.

fabricated by a facile two-step solvothermal method, with each step followed by an annealing procedure. Initially, ultrathin Ni(OH)2 lamellar arrays were 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, and 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. The morphologies of the electrodes are shown in Figure 2. In Figure 2a and 2b are shown the NiO nanosheet arrays on

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.

Ni foam after solvothermal reaction and postannealing 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 a huge special surface area. Meanwhile, the interconnected nanosheets construct a tremendous porous open-framework structure, which may facilitate the efficient penetration of electrolyte ions and transport of electrons inside of active materials. 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

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. 6248

DOI: 10.1021/acs.iecr.8b00467 Ind. Eng. Chem. Res. 2018, 57, 6246−6256

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Industrial & Engineering Chemistry Research

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

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 7 nm, 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 arrays (Figure 3d), the lattice fringe existing in the dark area of the image may be assigned to the (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. EDS mapping analysis can be performed to determine the elemental compositions 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. The crystallographic structures of obtained products were further classified by XRD patterns. Powders of calcined product in each step were collected. As shown in Figure 5, different

Figure 5. XRD patterns of the NiO (a), NiCo2O4 (b), and NiO@ NiCo2O4 (c) scratched from Ni foam and hierarchical core−shell NiO@NiCo2O4 nanosheets grown on Ni foam (d), respectively.

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 the synthetic process. 6249

DOI: 10.1021/acs.iecr.8b00467 Ind. Eng. Chem. Res. 2018, 57, 6246−6256

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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.

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 4−7:

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. 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 Ni.30 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 780.7 eV; meanwhile, the other two fitted peaks at 794.9 and 780.1 eV are ascribed to Co3+.31,32 The high-resolution spectrum of the O 1s region presents three oxygen contributions. Specifically, the peak 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 surface.22 The peaks located at 532.3 eV (O3) can be ascribed to the physically or chemically absorbed H2O on or within the surface.14,29 Based on the analysis of the characterization above, the formation mechanism of NiO precursor is explained as follows:

Δ

C6H12N4 + 6H 2O → 6HCHO + 4NH3

(4)

NH3 + H 2O → NH4 + + OH‐

(5)

Ni+ + 2OH‐ → Ni(OH)2

(6)

Δ

Ni(OH)2 → NiO + H 2O

(7)

The formation mechanism of NiCo2O4 is shown as follows.13,33 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 were 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. Δ

CO(NH 2)2 + H 2O → 2NH3 + CO2

(8)

CO2 + H 2O → CO32 ‐ + 2H+

(9)

NH3 + H 2O → NH4 + + OH‐

(10)

2(Co2 +, Ni 2 +) + CO32 ‐ + 2OH‐ + nH 2O → (Co, Ni)2 CO3(OH)2 ·nH 2O 6250

(11)

DOI: 10.1021/acs.iecr.8b00467 Ind. Eng. Chem. Res. 2018, 57, 6246−6256

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Industrial & Engineering Chemistry Research

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. Δ

(Co, Ni)2 CO3(OH)2 ·nH 2O → NiCo2O4 + H 2O + CO2

suggesting a typical capacitive behavior in the fast redox process. The capacitive contribution can be calculated qualitatively by the equation i = k1ν + k2ν1/2, where k1 indicates capacitive effect and k2 represents diffusion process.36,37 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 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 is 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 toward positive and negative potential and the specific capacitance decreases respectively owing to enhancing internal diffusion resistance in the active material.38 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

(12)

In order to further explore the electrochemical properties of the Ni foam@NiO@NiCo2O4 electrode, a series of electrochemical tests were carried out in a three-electrode configuration system with a 6 M KOH aqueous solution as electrolyte. Figure 7a depicts the comparison on typical CV curves at different scan rates of 2−100 mV 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 + e‐

(13)

NiCo2O4 + H 2O + OH− ⇌ NiOOH + 2CoOOH + e− (14) −

CoOOH + OH ⇌ CoO2 + H 2O + e

−

(15)

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 (semiinfinite diffusion process) and 1 (capacitive process).35 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, 6251

DOI: 10.1021/acs.iecr.8b00467 Ind. Eng. Chem. Res. 2018, 57, 6246−6256

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the long time cycling. The superior cycle stability and Coulombic efficiency 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 solution.47 This process improves the degree of activation of the electrode materials during cycling.48 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 times (1−6 h). The presence of redox peaks in CV curves and long charge−recharge platforms in GCD curves demonstrates 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. By 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 nanothreads. While for the shorter time, the NiCo2O4 nanosheets on the NiO surface are too less, which leads to less capacitance. Therefore, the asprepared 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 the spectrum among all of the as-prepared electrodes, demonstrating that it has more efficient electrolyte ion 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 the 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 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 growth of NiCo 2 O 4 nanosheets. The 3D hierarchical electrodes have a larger specific surface, more accessible electrochemical active sites, and short pathways for ion diffusion and charge exchange at the

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 reports.39 Apparently, the CV curve of the Ni foam@NiO@NiCo2O4 has a larger integrated area than those of Ni foam@NiO and Ni foam@NiCo2O4 under the same conditions 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 a longer charge/discharge time than the individual components of NiO and NiCo2O4 under identical conditions. Besides, the Ni foam@NiO@NiCo2O4 electrode has a similar charge time and discharge time at different current densities ranging from 2 to 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 the 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 those of 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 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 remain for a 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. Its remarkable cycling stability and good rate performance are also superior to those of Ni(OH)245 and Ni [email protected] 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 determined that Ni foam@NiO@NiCo2O4 can exhibit an excellent Coulombic efficiency of nearly 100% during 6252

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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 10 mV 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 a red light LED by two ASC devices in series and cycling stability of the ASC device.

surface of core−shell nanosheet arrays.38 In addition, all components of Ni foam@NiO@NiCo2O4 electrode are transitionmetal 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 6253

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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 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 retain 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.

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 that 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 arrays.38 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 in 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 0.6 V and −1.0 to 0 V, respectively, in a threeelectrode system at 10 mV s−1 in Figure 8b. The loading mass of AC can be determined by the following equation: m− = m+ × (C+ × ΔV+)/(C− × ΔV−), 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, ΔV 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 twoelectrode supercapacitor device at the scan rates of 5−100 mV s−1 with the potential window of 0−1.6 V exhibit no obvious distortion in the Figure 8c. It indicates a good capacitive behavior. GCD curves of the asymmetric supercapacitor device at different current densities are further depicted in Figure 8d. The specific capacitances 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 the 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//NiCo2O4,50 AC//CQDs/ NiCo2O4,51 AC//RGO/NiCo2O4,52 AC//GNP/NiCo2O4,53 and AC//NiCo2O4 [email protected] 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 min 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.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b00467. Figure S1. Morphology comparision of Ni foam@NiO@ NiCo2O4 with different reaction times at the secondary solvothermal reaction. Figure S2. Brunauer−Emmett− Teller plot 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 times. Figure S8. EIS of the Ni foam@NiO, Ni foam@ NiCo2O4, and Ni foam@NiO@NiCo2O4 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Qingli Hao: 0000-0002-4491-4563 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 (XNY011), 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.

4. CONCLUSIONS In summary, a binder-free hierarchical NiO@NiCo2O4 core−shell nanosheet array 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 6254

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