One-Step Synthesis of 3D Network-like NixCo1–xMoO4 Porous

Sep 14, 2017 - 3D network-like Ni0.75Co0.25MoO4 porous nanosheets were rationally ... possess tremendously promising applications in energy storage de...
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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10139-10147

One-Step Synthesis of 3D Network-like NixCo1−xMoO4 Porous Nanosheets for High Performance Battery-type Hybrid Supercapacitors Pengxiao Sun,† Chenggang Wang,† Weidong He, Peiyu Hou, and Xijin Xu* School of Physics and Technology, University of Jinan, 336 West Road of Nan Xinzhuang, Jinan 250022, Shandong Province, Peoples Republic of China S Supporting Information *

ABSTRACT: 3D network-like, ultrathin, and controllable NixCo1−xMoO4 (0 ≤ x ≤ 1) nanosheets on carbon cloth have been successfully designed and synthesized by a one-step hydrothermal process. All these NixCo1−xMoO4 (0 ≤ x ≤ 1) nanosheets possess similar morphologies and structures with thin nanosheets. The electrochemical performance can be optimized when x = 0.75, owing to the synergistic effect of nickel and cobalt ions. The Ni0.75Co0.25MoO4 nanosheets exhibit high specific capacity of 726.6 C g−1 at 1 A g−1, excellent rate capability maintaining about 73% at 20 A g−1, and favorable cycling stability with about 97.2% retention of the initial after 3000 cycles. The corresponding hybrid supercapacitor exhibits high specific capacity of 99 F g−1 at 1 A g−1 and high energy density of 35.2 Wh kg−1 at the power density of 800 W kg−1. The high specific capacity, excellent rate capability, and favorable cycling performance elucidate that these NixCo1−xMoO4 (0 ≤ x ≤ 1) nanosheets possess tremendously promising applications in energy storage devices. KEYWORDS: One-step hydrothermal, NixCo1−xMoO4 nanosheets, Hybrid supercapacitors

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INTRODUCTION In recent years, the high demand for energy storage for portable electronic devices, power tools, and electric vehicles has induced a great amount of efforts devoted to high-performance, low-cost, and lower environmental polution energy storage systems.1−5 Supercapacitors have been regarded as one of the most promising energy storage devices because of their large power density, rapid charge/discharge ability, and longer life span.6−9 However, the lower energy densities than those of the lithium ion batteries which currently dominate the market greatly restrict the commercialization of supercapacitors. The electrode materials are one of the key components to improve supercapacitors performance.10−13 Nickel/cobalt-based nanomaterials have been widely investigated as the electrodes materials of supercapacitors on account of their multivalence for rich redox reactions, high capacity, low toxicity and pollution, and rich resources reserves.14−16 However, Ni/Co-based electrode materials commonly suffer low power density and poor electrochemical stability because these materials are diffusion-controlled and the energy storage reactions are not limited to the surface. Therefore, Ni/Co-based electrode materials could provide several times higher charge storage and energy densities than carbonaceous materials but lower power densities and structure stabilities.17−19 These issues can be improved by enhancing the © 2017 American Chemical Society

conductivity, enlarging the specific area, and decreasing diffusion distances. Nickel/cobalt-based ultrathin nanosheets such as Ni(OH)2, NiO and Co(OH)2 nanosheets, etc. display high specific area; however, these solitary nickel/cobalt-based materials usually exhibit lower conductivity.20−22 Nickel−cobalt binary metal compounds possess higher conductivity and structure stabilities than solitary nickel/cobalt-based compounds such as NiCo2O4 and cobalt−nickel-layered double hydroxides etc.23−25 which is mainly due to the synergistic effect of nickel and cobalt. Additionally, molybdate nanomaterials exhibit excellent intrinsic properties and good electrochemical performance. However, poor cycling performance and low specific capacity have restricted the development of molybdate nanomaterials. Xia26 et al. have synthesized CoMoO4/graphene composites that show low specific capacity of 394.5 F g−1. Wang et al.27 obtained network-like porous NiMoO4 nanoarchitectures assembled with ultrathin mesoporous nanosheets on three-dimensional graphene foam, and only 89% of the initial reversible capacity are remained after 120 cycles. Received: June 29, 2017 Revised: September 2, 2017 Published: September 14, 2017 10139

DOI: 10.1021/acssuschemeng.7b02143 ACS Sustainable Chem. Eng. 2017, 5, 10139−10147

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration of the one-step strategy for the synthesis of the 3D network-like NixCo1−xMoO4 (0 ≤ x ≤ 1) nanosheets coating on carbon cloth.

Cm−ΔV − m+ = m− Cm+ΔV +

In order to further trigger the performance of molybdate nanomaterials, the carbon cloth with excellent flexibility and conductivity was chosen as conductive substrates, on which 3D network-like NixCo1−xMoO4 (0 ≤ x ≤ 1) porous nanosheets were synthesized with a facile hydrothermal method and directly used as binder-free electrodes. The electrochemical properties of as-prepared NixCo1−xMoO4 nanosheets were carefully investigated for the samples with the changing mole ratio of nickel and cobalt. Meanwhile, the corresponding hybrid supercapacitor device with the Ni0.75Co0.25MoO4 as the positive electrode and activated carbon (AC) as the negative electrode was also assembled.

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where q is the charge of the electrode, Cm refers to the specific capacitance of the electrode, ΔV represents the potential window of the electrode, and m is the mass of the electrode material. Characterizations. The compositions and morphologies of the asprepared NixCo1−xMoO4 nanosheets were characterized by a field emission scanning electron microscope (FESEM, FEI QUANTA FEG250). The transmission electron microscopy (TEM) was performed on a FEI Tecnai G2 F20 at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250XI electron spectrometer equipped with Al Kα X-ray radiation (hν = 1486.6 eV) as the source for excitation. Electrochemical measurements. The electrochemical measurements were carried out by using an electrochemical workstation (CHI 660E) with a three-electrode configuration in a 3 M KOH aqueous solution. The as-prepared materials, Pt foil and a saturated calomel electrode (SCE), were used as working electrode, counter electrode, and reference electrode, respectively. The electrochemical properties of the supercapacitor electrodes were evaluated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS). The specific capacities of the NixCo1−xMoO4 nanosheets under the three-electrode configuration were calculated from the GCD curves by the following equation:30

EXPERIMENTAL SECTION

Synthesis of 3D network-like NixCo1−xMoO4 porous nanosheets on carbon cloth. All the reagents were of analytical grade, purchased from Sinopharm, and were directly used without further purification. The carbon cloth was purchased from (WOS 1002, 360 μm thick, 125 g m−2) CeTech co., Ltd. First, 1 mmol of acetates of Ni(CH3COO)2·4H2O and Co(CH3COO)2·4H2O (with different mole ratios of 0:1, 1:1, 1:3, 3:1, 1:0) was dissolved into 30 mL of deionized water under vigorous magnetic stirring to form a homogeneous mixture solution. Then 0.2 g of ammonium molybdate ((NH4)6Mo7O24·4H2O) and 0.24 g of urea were added to this solution. After stirring for 30 min, the solution was transferred into a Teflon-lined stainless steel autoclave and kept at 160 °C for 10 h. After cooling down to room temperature naturally, the carbon cloth coated by the samples was taken out and washed by DI water and ethanol to remove the residual impurities and then dried in an oven at 60 °C for 12 h. Finally, the precursor was annealed at 400 °C for 2 h to obtain NixCo1−xMoO4 nanosheets. Fabrication of hybrid supercapacitor. The two-electrode hybrid supercapacitor device was fabricated by using NixCo1−xMoO4 nanosheets, active carbon (AC), cellulose separators (TF4035, NKK), and 3 M KOH aqueous as the positive and negative electrodes, separators, and electrolyte, respectively. The NixCo1−xMoO4 nanosheets coated on carbon cloth were directly treated as positive electrode. AC, acetylene black, and PVDF with a mass ratio of 80:10:10 were mixed into a homogeneous slurry by N-methylpyrrolidinone (NMP) and pasted on carbon cloth as the negative electrode. In order to optimize the electrochemical properties, we balanced the charge storage in positive and negative electrodes, and the equations were as follows:28,29

q = CmΔVm

(1)

q + = q−

(2)

(3)

qc =

I Δt m

(4)

where qc is the specific capacity, I is the charge/discharge current, Δt is the discharge time, and m is the mass of active materials. The specific capacitances of the as-fabricated device were obtained from the following formula:31

Cdevice =

I Δt mtotal ΔV

(5) −1

where the Cdevice (F g ) values are the specific capacitances of the hybrid supercapacitor device; I (A) is the discharge current of the discharge process; Δt (s) is the discharge time; mtotal (g) is the total mass of the positive and negative active materials; and ΔV (V) is the potential operating voltage. The energy density and power density of the as-assembled device were calculated according to the following equations:32 E=

P= 10140

∫ Iv(t ) dt mtotal

E Δt

(6)

(7) DOI: 10.1021/acssuschemeng.7b02143 ACS Sustainable Chem. Eng. 2017, 5, 10139−10147

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. SEM images: (a) carbon cloth; (b) NixCo1−xMoO4 nanosheets with x = 0.25; (c) NixCo1−xMoO4 nanosheets with x = 0.5; (d) NixCo1−xMoO4 nanosheets with x = 0.75; (e) NiMoO4 nanosheets; (f) CoMoO4 nanosheets.

Figure 3. (a−b) TEM images of Ni0.75Co0.25MoO4 nanosheets; (c) EDS mapping of Ni0.75Co0.25MoO4 nanosheets. where E refers to the energy density (Wh kg−1), I is the discharge current of the discharge process, v(t) is the cell voltage, dt refers to the time differential, mtotal is the total mass of the positive and negative electrode materials, P (W kg−1) is the power density, and Δt is the discharging time.

NixCo1−xMoO4 nanosheets keep changing with the concentration of Ni2+ and Co2+ owing to the synergistic effect of Ni and Co ions. Higher-magnified SEM images (insets of Figure 2b−f) reveal that the densely packed, highly ordered NixCo1−xMoO4 (0 ≤ x ≤ 1) nanosheets are interconnected with each other forming a 3D network-like structure. The 3D network-like nanosheets offer shortcuts for charge transport with plenty of spaces remaining which promote the penetration of electrolyte and ensure full contact between electrode materials and electrolyte.33 Moreover, it can be observed that the surfaces of NixCo1−xMoO4 nanosheets are rougher than CoMoO4 nanosheets as revealed in Figure S1. These features will greatly enlarge the surface area and provide a large amount of electroactive sites for redox reactions. What’s more, these 3D network-like nanosheets tightly adhere to the surface of carbon cloth which guarantees sufficient mechanical strength and stability for flexible device.

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RESULTS AND DISCUSSION 3D network-like NixCo1−xMoO4 (0 ≤ x ≤ 1) porous nanosheets via a one-step hydrothermal process were synthesized as shown in Figure 1. The NixCo1−xMoO 4 nanosheets were directly grown on the entire surface of the carbon cloth, and all these NixCo1−xMoO4 nanosheets delivered similar morphologies regardless of the concentration of Ni2+ and Co2+ as depict in Figure 2. Obviously, all the individual fibers are totally coated by NixCo1−xMoO4 nanosheets compared with Figure 2a. Additionally, all these NixCo1−xMoO4 (0 ≤ x ≤ 1) nanosheets are uniformly vertically adhered to the surfaces of carbon cloth. Besides, the sizes of these 10141

DOI: 10.1021/acssuschemeng.7b02143 ACS Sustainable Chem. Eng. 2017, 5, 10139−10147

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Figure 4. XPS spectra of Ni0.75Co0.25MoO4: (a) Ni 2p spectrum; (b) Co 2p spectrum; (c) Mo 3d spectrum; (d) O 1s spectrum.

energy peaks located at 781.6 and 796.9 eV. These peaks are corresponding to Co 2p3/2 and Co 2p1/2, respectively, indicating the existence of Co2+ associated with the tetrahedral sites of spinel CoMoO4 nanosheets.34 In addition, two peaks with binding energy of 232.8 and 235.8 eV, corresponding to Mo 3d5/2 and Mo 3d3/2, respectively, are observed in the Mo 3d spectrum (Figure 4c), which indicates the existence of Mo6+ in molybdate.35 Additionally, the spectrum of O 1s shows a peak at 531.6 eV, which is typical of a metal−oxygen bond (Figure 4d).36 The above results suggest the successful synthesis of Ni0.75Co0.25MoO4 nanosheets. The electrochemical performances of the NixCo1−xMoO4 nanosheets in three electrodes were investigated by directly using them as the working electrode. As observed from the CV curves of carbon cloth and Ni0.75Co0.25MoO4 nanosheets (as shown in Figure 5a), it is obvious that the carbon cloth makes negligible contributions to the capacity of the as-synthesized NixCo1−xMoO4 nanosheets electrode. The CV curves of asprepared NixCo1−xMoO4 nanosheets at 5 mV s−1 in Figure 5b exhibit clearly peaks between 0 and 0.6 V (vs Hg/HgO), implying the presence of faradaic redox reactions in all assynthesized NixCo1−xMoO4 nanosheets. The possible electrochemical mechanisms in 3 M KOH aqueous solution of NixCo1−xMoO4 (0 ≤ x ≤ 1) are described by the following equations:37−39

The morphologies and structures of the NixCo1−xMoO4 nanosheets were further investigated by analyzing individual nanosheets using TEM. Figure 3a indicates that the surface of Ni0.75Co0.25MoO4 nanosheets is rough and rugged, matching well with the SEM images. As depicted in the high magnification TEM images of Figure 3b, Ni0.75Co0.25MoO4 nanosheets are actually porous, which maintains a large number of active sites for redox reactions. The HRTEM images of Ni0.75Co0.25MoO4 nanosheets (insets in Figure 3b) exhibit that the interplanar spacings are 0.204 and 0.30 nm, corresponding well to the (300) planes of NiMoO4 and the (310) planes of CoMoO4. These results demonstrate that the Ni0.75Co0.25MoO4 nanosheets are formed by the coexistence of NiMoO4 and CoMoO4. To further determine the elements, the Ni0.75Co0.25MoO4 nanosheets were detected by EDS (Figure S2) and EDS mapping (Figure 3c), where the type of elements and distributions of Ni0.75Co0.25MoO4 nanosheets is displayed . The EDS presents the element signals of Ni, Co, Mo, and O in consistence with these elements in Ni0.75Co0.25MoO4 nanosheets. The compositional distributions in Figure 3c indicate the homogeneous distributions of Ni, Co, Mo, and O throughout the whole Ni0.75Co0.25MoO4 nanosheets. XPS spectra shown in Figure 4 are employed to determine the detailed compositions and oxidation state of the asprepared Ni0.75Co0.25MoO4 nanosheets. From the survey spectra of XPS (Figure S3), the signals of Ni, Co, Mo, and O elements are clearly detected, which match well with the results of EDS. The Ni 2p spectrum (Figure 4a) exhibits two main peaks at 856.2 and 873.9 eV corresponding to Ni 2p3/2 and Ni 2p1/2, respectively, which are accompanied by two shakeup satellite peaks (denoted as “sat.”). These two main peaks are separated by 17.7 eV, proving the existence of the Ni2+ oxidation state which is consistent with NiMoO4. The Co 2p spectrum (Figure 4b) delivers two main peaks with binding

3[NixCo1 − x (OH)3 ]− ↔ 3NixCo1 − xOOH + 3H 2O + 3e− (8)

CoOOH + OH− ↔ CoO2 + H 2O + e−

(9)

Remarkably, the electrochemical capacitance of as-obtained NixCo1−xMoO4 nanosheets originating from the quasi reversible electron transfer process that mainly involves the Co2+/ Co3+ and Ni2+/Ni3+ redox couple.38,39 The electrochemical performance of as-obtained NixCo1−xMoO4 nanosheets can be 10142

DOI: 10.1021/acssuschemeng.7b02143 ACS Sustainable Chem. Eng. 2017, 5, 10139−10147

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ACS Sustainable Chemistry & Engineering

Figure 5. (a) CV curves of carbon cloth and Ni0.75Co0.25MoO4 nanosheets at 2 mV s−1; (b) CV curves of NixCo1−xMoO4 nanosheets at 5 mV s−1; (c) CV curves of Ni0.75Co0.25MoO4 nanosheets with different scan rates; (d) GCD curves of NixCo1−xMoO4 nanosheets at 5A g−1; (e) GCD curves of Ni0.75Co0.25MoO4 at different current densities; and (f) specific capacitances at different current densities of NixCo1−xMoO4 nanosheets.

optimized when the ratio of nickel and cobalt ions is 3:1. The CV curves of Ni0.75Co0.25MoO4 nanosheets show the maximum area, demonstrating the highest specific capacity among these NixCo1−xMoO4 nanosheets electrodes. The CV curves of Ni0.75Co0.25MoO4 nanosheets at different scan rates are recorded in Figure 5c. The shapes of the CV curves exhibit negligible changes except for the progressive shift of the redox peak positions with increasing scan rates from 2 mV s−1 to 30 mV s−1. The symmetrical shapes of all these CV curves manifest that these NixCo1−xMoO4 nanosheets possess highly reversible redox reactions, fast charge transport, and superior rate capability. Other NixCo1−xMoO4 nanosheets also exhibited similar properties as shown in Figure S4. The GCD curves in Figure 5d and Figure S5 present voltage plateaus, further indicating the existence of faradaic processes.

Additionally, the longest discharge time of Ni0.75Co0.25MoO4 nanosheets prove their larger capacity comparing with other Ni x Co 1 − x MoO 4 nanosheets. The GCD curves of Ni0.75Co0.25MoO4 nanosheets in Figure 5e display almost symmetric shapes with current density ranging from 1 to 20 A g−1, demonstrating high Coulombic efficiency due to the highly reversible redox reactions of NixCo1−xMoO4 nanosheets. The specific capacities of NixCo1−xMoO4 nanosheets (Figure 5f) are calculated based on the GCD curves (Figure 5e and Figure S5) according to the eq 4. Clearly, Ni0.75Co0.25MoO4 nanosheets possess highest capacity of 726.6 C g−1 at 1 A g−1 and 529.1 C g−1 at 20 A g−1. About 73% of the initial specific capacity is still retained even at a high current density of 20 A g−1 elucidating these Ni0.75Co0.25MoO4 nanosheets have high rate capability and power performance. Additionally, CoMoO4 10143

DOI: 10.1021/acssuschemeng.7b02143 ACS Sustainable Chem. Eng. 2017, 5, 10139−10147

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Figure 6. (a) Cycling performance of NixCo1−xMoO4 nanosheets; (b) EIS of the as-prepared materials.

ranging from 1 to 20 A g−1. The reasons why Ni0.75Co0.25MoO4 nanosheets hold the larger specific capacity and excellent rate capability can be attributed to the synergistic effect of the multiple metal ions in NixCo1−xMoO4, which provide not only rich redox reactions and better conductivity but also outstanding structural stability. The cycling performance is one of the critical properties of supercapacitors, which determines the practical applications. Figure 6a shows the cycling stability of as-synthesized electrode materials at 5 A g−1 under consecutive charge/discharge processes. After 3000 cycles, the specific capacity of CoMoO4 nanosheets increases from 169 C g−1 to 184 C g−1 (about 109.1% of initial capacity), which demonstrates that the CoMoO4 nanosheets possess superior structural stability during the cycling. The slow increase may be ascribed to the full contact of CoMoO4 electrode with the electrolytes with the increase of cycles. Ni0.25Co0.75MoO4 nanosheets display higher capacity retentions of 213 C g−1 (about 105.1% of the initial capacity) after 3000 cycles, and the capacity of Ni0.5Co0.5MoO4 nanosheets reaches 355.5 C g−1 (97.2% of the initial capacity) after 3000 cycles. Additionally, the capacity of Ni0.75Co0.25MoO4 nanosheets still maintains 664.8 C g−1 (97.2% of the initial capacity) after 3000 cycles. However, the capacity of NiMoO4 nanosheets decreases from 244.6 C g−1 to 208.4 F g−1, which only remains approximately 85.1% of the initial capacity after 3000 cycles. The quick capacity degradation of NiMoO4 nanosheets can be ascribed to the structure destruction during the faradic reactions, revealing that these pure NiMoO4 nanosheets have lower structural stability. It is clear that the cycling performances of NixCo1−xMoO4 nanosheets are greatly affected by the contents of nickel elements. The morphologies and structures of

Table 1. EIS Simulation Electrodes

Rs/Ω

Rct/Ω

CoMoO4 Ni0.25Co0.75MoO4 Ni0.5Co0.5MoO4 Ni0.75Co0.25MoO4 NiMoO4

0.81 0.66 0.73 0.78 0.88

0.24 0.08 0.03 0.2 0.3

Figure 7. Schematic illustration of the as-assembled hybrid supercapacitor (HSCs).

nanosheets display 156.5 C g−1 at 1 A g−1 and 137.5 C g−1 at 20 A g−1 (about 88% of initial capacity). Ni0.25Co0.75MoO4 nanosheets exhibit 191.3 C g−1 at 1 A g−1 and 155.8 C g−1 at 20 A g−1 (about 83% of initial capacity). And Ni0.5Co0.5MoO4 nanosheets show the capacity of 389.5 C g−1 at 1 A g−1 and 296.0 C g−1 at 20 A g−1 (about 76% of initial capacity). However, the capacities of NiMoO4 nanosheets only remained approximately 69.3% of the initial capacity, which have decreased to 158 C g−1 from 228 C g−1 with current density 10144

DOI: 10.1021/acssuschemeng.7b02143 ACS Sustainable Chem. Eng. 2017, 5, 10139−10147

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Figure 8. Electrochemical characterizations of Ni0.75Co0.25MoO4//AC hybrid supercapacitors: (a) CV curves of the AC and Ni0.75Co0.25MoO4 at 5 mV s−1 based on three electrode configurations; (b) CV curves of the as-assembled device at 5 mV s−1 ranging from 0.9 V to 1.6 V; (c) CV curves of as-assembled hybrid supercapacitors at various scan rates ranging from 5 to 30 mV s−1; (d) specific capacitance of as-assembled hybrid supercapacitor at different current densities, with the inset the corresponding GCD curves; (e) the cycling performance of the as-assembled hybrid supercapacitors; (f) ragone plot of the as-assembled hybrid supercapacitors.

charge transfer resistance (Rct). The Rs decreases with the decreasing contents of nickel elements for Ni0.75Co0.25MoO4, Ni0.5Co0.5MoO4, and Ni0.25Co0.75MoO4. These results illustrate that the electrical conductivity and ion diffusion of NixCo1−xMoO4 electrode are greatly affected by the contents of nickel elements. As Ni0.75Co0.25MoO4 possesses high electrochemical performances; therefore, we choose Ni0.75Co0.25MoO4 nanosheets as the positive electrode which exhibit optimized performance among these NixCo1−xMoO4 nanosheets. The corresponding hybrid supercapacitor is obtained and processed as shown in Figure 7. In order to balance the charge storage in positive and negative electrodes, we observe the CV curves of AC with different scan rates and GCD curves of activated carbon at different current densities (Figure S6), and the AC is pasted

Ni0.75Co0.25MoO4 nanosheets remain almost unchanged after 3000 cycles, which also proves the stability of the structure as shown in Figure 6b. The high specific capacity, excellent rate capability, and favorable cycling performance of Ni 0 . 7 5 Co 0 . 2 5 MoO 4 nanosheets indicate that these Ni0.75Co0.25MoO4 nanosheets are promising candidates for the potential application. To further understand the electrical conductivity and ion diffusion of the NixCo1−xMoO4 electrode, the electrochemical impedance spectroscopy (EIS) was measured in the frequency range 100 kHz ∼ 0.01 Hz. The Nyquist plots and equivalent circuit diagram are recorded in Figure 6c, and the corresponding simulated results are listed in Table 1. Compared with other samples, the NiMoO4 and CoMoO4 electrode possess the highest internal resistance (Rs) and higher 10145

DOI: 10.1021/acssuschemeng.7b02143 ACS Sustainable Chem. Eng. 2017, 5, 10139−10147

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ACS Sustainable Chemistry & Engineering onto carbon cloth used as the negative. The CV curves of the AC and Ni0.75Co0.25MoO4 nanosheets at 5 mV s−1 implemented in the three electrodes configuration are shown in Figure 8a, where the Ni0.75Co0.25MoO4 nanosheets and AC work at 0−0.6 V and −0.9−0 V, respectively, manifesting the feasibility of these two electrodes to assemble a hybrid supercapacitor. Figure 8b delivers CV curves of the as-assembled hybrid supercapacitor ranging from 0 to 0.9 V to 0−1.6 V at a scan rate of 10 mV s−1. It is worth noting that polarization occurs when the voltage is extended to be more than 1.6 V, and no polarization exists before 1.55 V. Thus, 1.55 V is used as the cell voltage to investigate the electrochemical performance by considering the security and stability of the device. As shown in Figure 8c, the CV curves of the as-fabricated supercapacitor at different scan rates ranging from 5 mV s−1 to 30 mV s−1 do not display obvious polarization, further demonstrating the cell voltage is reasonable. The GCD curves in Figure 8d show that the curves at all current densities are almost symmetric, indicating that the asprepared device possesses high Coulombic efficiency. The specific capacitances calculated from typical GCD curves at different current densities show a highest specific capacitance of about 99 F g−1 at a current density of 1 A g−1 (Figure 8d), which is still maintained at 46 F g−1 at 10 A g−1. Additionally, the as-assembled device retains about 89.2% of the initial specific capacitance at a high current density of 5 A g−1 after 3000 cycles as shown in Figure 8e, indicating favorable cycling performance. The Nyquist plots are showed in Figure S7, and the corresponding simulated results are listed in Table S1. Remarkably, the as-assembled Ni0.75Co0.25MoO4//AC hybrid supercapacitor possesses lower internal resistance (Rs) and lower charge transfer resistance (Rct). The power density and energy density are calculated and delivered as a Ragone plot in Figure 8f. A maximum energy density of 35.2 Wh kg −1 at the power density of 800 W kg−1 is obtained, which is found superior than those of Ni0.67Co0.33MoO4//RGO (25.6 Wh kg−1 at 775 W kg−1)3, 3D graphene/CoMoO4//AC (26.8 Wh kg−1 at a power density of 532 W kg−1),40 and CoMoO4//AC asymmetric supercapacitor (14.5 Wh kg−1 at a power density of 200 W kg−1).41 The high energy density and power density and favorable cycling performance of the as-assembled Ni0.75Co0.25MoO4//AC hybrid supercapacitor further indicate that these Ni0.75Co0.25MoO4 nanosheets are promising candidates for the potential application.

nanosheets coated on carbon cloth can be used as a promising electrode for the next-generation energy storage applications.

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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/acssuschemeng.7b02143. Additional experimental information, SEM, EDS curve, XPS spectrum, CV curves, GCD curves, and EIS results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mails: [email protected]. ORCID

Peiyu Hou: 0000-0003-0476-5812 Xijin Xu: 0000-0002-3877-6483 Author Contributions †

P.S. and C.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51672109) and the Natural Science Foundation of Shandong Province for Excellent Young Scholars (ZR2016JL015).

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CONCLUSION In summary, 3D network-like NixCo1−xMoO4 (0 ≤ x ≤ 1) porous nanosheets on carbon cloth have been successfully designed and synthesized. The electrochemical performance can be optimized when x = 0.75, and the Ni0.75Co0.25MoO4 nanosheets display the highest specific capacity of 726.6 C g−1 at 1 A g−1 and outstanding rate capability (still maintained 529.1 C g−1 at 20 A g−1), which is far greater than those of other NixCo1−xMoO4 (0 ≤ x ≤ 1) nanosheets. Furthermore, Ni0.75Co0.25MoO4 nanosheets exhibit excellent cycling performance with the performance maintained 97.2% of the initial capacity after 3000 cycles by virtue of the synergistic effect of n i c k e l a n d c o b a lt . M o r e o v e r , t h e a s - a s s e m b l e d Ni0.75Co0.25MoO4//AC hybrid supercapacitor device manifests excellent specific capacitance as high as 99 F g−1 at 1 A g−1 and a high energy density of 35.2 W h kg−1 at a power density of 800 W kg−1. Therefore, the 3D network-like Ni0.75Co0.25MoO4 10146

DOI: 10.1021/acssuschemeng.7b02143 ACS Sustainable Chem. Eng. 2017, 5, 10139−10147

Research Article

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DOI: 10.1021/acssuschemeng.7b02143 ACS Sustainable Chem. Eng. 2017, 5, 10139−10147