Construction of CoreâShell NiMoO4@ Ni-Co-S Nanorods as

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Construction of Core-Shell NiMoO4@Ni-Co-S Nanorods as Advanced Electrodes for High-Performance Asymmetric Supercapacitors Chao Chen, Dan Yan, Xin Luo, Wenjia Gao, Guanjie Huang, Ziwu Han, Yan Zeng, and Zhihong Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16271 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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ACS Applied Materials & Interfaces

Construction of Core-Shell NiMoO4@Ni-Co-S Nanorods as Advanced Electrodes for High-Performance Asymmetric Supercapacitors Chao Chen,a Dan Yan,a Xin Luo,a Wenjia Gao,a Guanjie Huang,a Ziwu Han,a Yan Zengb and Zhihong Zhua* a

Institute of Nano-science and Nano-technology, College of Physical Science and Technology, Central

China Normal University, Wuhan, Hubei 430079, P. R. China. b

College of Chemistry, Central China Normal University, Wuhan, Hubei 430079, P. R China.

*Corresponding author: Zhihong Zhu: e-mail: [email protected]; Tel: +86—2767861185

ABSTRACT In this work, hierarchical core-shell NiMoO4@Ni-Co-S nanorods were first successfully grown on nickel foam by a facile two-step method to fabricate a bind-free electrode. The well-aligned electrode structure wrapped by Ni-Co-S nanosheets takes advantage of the excellent nanostructural properties and outstanding electrochemical performances owing to the synergistic effects of both nickel molybdenum oxides and nickel cobalt sulfides. The prepared core-shell nanorods in a threeelectrode cell yielded a high specific capacitance of 2.27 F cm-2 (1892 F g-1) at a current density of 5 mA cm-2 and retained a 91.7% specific capacitance even after 6000 cycles. Their electrochemical performances were further investigated as positive electrode for asymmetric supercapacitors. Notably, 1 ACS Paragon Plus Environment

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the energy density of the asymmetric supercapacitor device reached 2.45 mWh cm-3 at a power density of 0.131 W cm-3, and still retained a remarkable 80.3% specific capacitance after 3500 cycles. There is great potential for the electrode composed of the core-shell NiMoO4@Ni-Co-S nanorods for all-solidstate asymmetric supercapacitor device. KEYWORS: Core-shell nanostructure, NiMoO4 nanorods, Ni-Co-S nanosheets, Bind-free, Asymmetric supercapacitor device

1. INTRODUCTION To meet the urgent energy needs and search for the suitable substitution for traditional resources, numerous efforts have been paid to put research into pollution-free energy storage equipment, advanced energy conversion devices. For examples, supercapacitors (SCs),1-2 solar cells,3-4 Li-ion battery,5-6 and fuel cells.7-8 Among them, SCs are getting international concerns on account of their excellent characteristics, for instance, high capacity, rapid charging/discharging, urtra-long lifespan, and safety.9-15 However, despite these attractive characteristics, the performance improvement of SCs still remains challenging, and it becomes much more crucial for the research and development of advanced materials to satisfy the increasing needs.16 In recent years, asymmetric supercapacitors (ASCs), generally including pseudocapacitive and electric double-layer capacitive (EDLC) materials, are drawing increasing research interest due to their superior energy storage capacity.17-20 Additionally, to enhance the electrochemical performance of ASCs, building intriguing nanostructures for supercapacitor electrodes is regarded as an effective strategy.21 Hence, it is feasible to use advanced electrode materials owing optimized nanostructure, excellent electrochemical property to meet these demands, and the fabrication of advanced electrode materials is significant to develop highperformance ASCs.

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Transition metal oxides, which usually possess extremely high theoretical capacitance, practically provide higher specific capacitance and power densities than the typical EDLC, and have got great consideration. Thanks to their multiple oxidation states and excellent electronic conductivity, ternary metal oxides, especially those contain two different metal cations, are deemed to be promising candidates as advanced electrode materials for ASCs.22-25 More specifically, the primary interest in NiMoO4 lies on the fact that the high specific capacitance mainly from the Ni ion and the remarkable electrical conductivity mainly from the Mo ion can collectively contribute to the superior electrochemical behavior when used in energy storage devices.26 In the earlier work, Peng et al. reported the tunable fabrication of NiMoO4 nanorods on several kinds of metallic substrates as advanced electrodes, and the capacitance of the fabricated NiMoO4 nanorods based on nickel substrate can deliver a large value almost 1091.1 F g-1 at 1 A g-1 with good conductivity.27 So, the ternary NiMoO4 is confirmed to hold profound potential in terms of material development for ASCs. However, though it has such virtues, ternary metal oxides normally endure bad electrochemical stability. To overcome this disadvantage, nanostructure composite electrodes owning commendable properties, for instance, excellent conductivity, high specific or volumetric capacitance, and stable redox reaction characteristic need to be developed.28 Recent published works have also revealed the strong application potential of transition metal sulfides. For example, ternary nickel cobalt sulfides, as one of the most promising ternary metal sulfides, are universally utilized as the advanced nano materials for ASCs.29-34 Owing to the sulfion, the nickel cobalt sulfides represent an outstanding electronic conductivity, which is approximately twice higher that of the nickel cobalt oxides, and greatly superior than that of the binary metal sulfides.35-37 More significantly, similar to the function of Ni ion and Mo ion in NiMoO4, both the Ni ion and Co ion in this ternary sulfides can contribute to the overall electrochemical performance including specific capacitance and electronic conductivity, and can further offer abundant redox 3 ACS Paragon Plus Environment


chemistry sites.38-39 More recently, Chen et al. put forward a simple electrodeposition way to get NiCo-S cross-linked nanosheets grown around the surface of the processed carbon cloth as advanced materials for SCs. The capacitance of supercapacitor using this composite yields 1418 F g-1 even at 5 A g-1, owning an admirable rate capability.40 The ternary nickel cobalt sulfides serving as electrode materials are capable to represent robust material characteristics for energy storage applications. As for our work, we report the development of original core-shell NiMoO4@Ni-Co-S nanorods on 3D nickel substrate through two steps, their applications are also examined as positive electrodes for ASCs with excellent energy storage capacity, where Ni-Co-S nanosheets are wrapped on the wellorganized NiMoO4 nanorods to take the shape of hierarchical core-shell nanostructures by electrodeposition. This 3D core-shell structure supports abundant electrochemical active substance and promotes ion transmission speed between the materials and the nickel substrate with the synergistic effects of both NiMoO4 nanorods and Ni-Co-S nanosheets.

2. EXPERIMENTAL SECTION 2.1 Synthesis of pristine NiMoO4 nanorod arrays on nickel foam The chemicals in our work were used as received. In the synthesis process of the NiMoO4 nanorods, 2.5 mmol of nickel nitrate hexahydrate and 2.5 mmol of sodium molybdate dihydrate were mixed into 50 mL of DI water. The mixed solution was then put in a magnetic stir for a few minutes to form a uniform solution. The rectangular shaped nickel foam (2 × 4 × 0.03 cm3) was cleaned by diluted hydrochloric acid solution, ethanol, and DI water in turns a couple of times, and then the treated nickel foam was placed into a Teflon-lined stainless-steel autoclave that contained the prepared uniform solution. In the next step, the autoclave was heated under 160 °C for 360 minutes to get the Ni-Mo precursor. Afterwards, the Ni-Mo precursor was moved and cleaned with DI water thoroughly. Then it dried out in the drying oven under 70 °C. At last, the poorly crystallographic precursor was calcined 4 ACS Paragon Plus Environment

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under 400 °C for 120 minutes and the heating rate was set as 2 °C min-1 in a pure argon atmosphere to obtain crystallized NiMoO4 nanorods. 2.2 Synthesis of core-shell NiMoO4@Ni-Co-S nanorods The core-shell NiMoO4@Ni-Co-S nanorods were synthesized by the electrodeposition method. In detail, 5 mM cobalt nitrate hexahydrate, 5 mM nickel nitrate hexahydrate and 0.75 M thiourea were mixed into 100 mL of DI water. After stirring to form a uniform solution, the pH value of the uniform solution was then changed to approximately 6 with the addition of moderate ammonium hydroxide. Subsequently, an electrodeposition method was performed in a three-electrode cell, employing the pristine NiMoO4 electrode as the working electrode, Pt wire was chosen for the counter electrode, and Ag/AgCl was chosen for the reference electrode. A classic cyclic voltammetry method was used under a rate of 5 mV s-1 for 6, 8, and 10 cycles, and the working window was from -1.2 to 0.2 V vs. Ag/AgCl at ambient environment, respectively, resulting in the following electrodes, NiMoO4@Ni-Co-S-6 (NS6), NiMoO4@Ni-Co-S-8 (NS-8), and NiMoO4@Ni-Co-S-10 (NS-10), respectively. Finally, the supported nickel foam was carefully rinsed by DI water, air dried for 12 h, and then dried for another 12 h at 80 °C under vacuum. 2.3 Fabrication of the asymmetric supercapacitor device The all-solid-state asymmetric supercapacitor device was assembled by a NS-8 nanorods electrode working as the positive electrode, together with a carbon nanotube (CNT, obtained from local agency) electrode working as the negative electrode. The NS-8 electrode was tailored to a 1 × 2 cm2 rectangular shape, the CNT electrode was obtained through grinding 80 wt% CNT, 10 wt% PTFE and 10 wt% carbon black compounds with moderate absolute ethyl alcohol. The compounds was then pressed on the surface of a 1 × 2 cm2 nickel substrate (effective coating area is 1 × 1 cm2) through a tablet compressing machine. The PVA/KOH gel electrolyte was prepared through adding 2 g of PVA into 20 5 ACS Paragon Plus Environment


mL of 2 M KOH solution slowly, which was later warm-up in a thermostatic aqueous bath at 85 °C under constant stirring for 120 minutes, until the mixture became clear. Subsequently, the PVA/KOH electrolyte was removed under room temperature. When the electrolyte cooled down to a nearly semisolid state, each of positive and negative electrodes were immersed for several minutes. Next, the device was prepared through sandwiching the gel by using the CNT and NS-8 nanorods electrodes. The redundant gel was removed, and the device was dried. After the gel completely solidified, the asymmetric supercapacitor device was accomplished. The mean thickness of the assembled asymmetric supercapacitor device was calculated to approximately 0.65 mm (Figure S1a). 2.4 Characterization The morphology and crystal structure information were obtained by using X-ray diffraction (XRD; X’Pert PRO MRD, PANalytical), scanning electron microscopy (SEM; JEOL, JSM-6700F, Japan), and transmission electron microscopy (TEM; JEM-2100 (HR), Germanic). The composition of products was verified by Raman spectroscopy (LabRAM HR JY-Evolution), energy-dispersive X-ray spectroscopy (EDX; attached to the SEM), and Scanning X-ray photoelectron spectroscopy (XPS) Microprobe system (PHI5000 Versaprobe-II). 2.5 Electrochemical Measurement The electrochemical tests of single electrode were operated in a 2 M KOH solution for a threeelectrode cell on the CHI 440A workstation, where the NS-8 electrode was directly worked for the working electrode, Pt wire was chosen for the counter electrode, and a standard calomel electrode (SCE) was chosen for the reference electrode. The tests included cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), electrochemical impedance spectroscopy (ESI) from 0.1 Hz to 10 kHz, and a cycle ability test. Measurements of the as-fabricated device were evaluated for a twoelectrode cell on the same workstation where the NS-8 electrode was the positive materials, CNT were 6 ACS Paragon Plus Environment

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the negative materials, and the effective area of device was controlled to be 1 cm2. The relevant tests were carried out under room temperature. The special capacitance and the areal capacitance are calculated from the following formulas: ூௗ௧

‫ܥ‬௦௣ = ௠ ௗ௏ ூௗ௧

‫ܥ‬௔௥ = ௦ௗ௏

(1)

(2)

where Csp (F g-1) signifies the specific capacitance, Car (F cm-2) signifies the areal capacitance, m (g) at formula (1) signifies the mass loading of the active materials, and s (cm-2) signifies the effective area of the electrode materials. The volumetric capacitance is calculated from the following formula: ூௗ௧

‫ܥ‬௩ = ௩ௗ௏

(3)

where Cv (F cm-3) signifies the volumetric capacitance, v (cm-3) for this formula signifies the effective volume of the as-fabricated device. All the I (A), dt (s) and dV in formulas (1), (2), and (3) signify the same discharging current, discharging time, and potential window, respectively. The calculations of energy density (E), power density (P) of as-fabricated device are as the following formulas: ଵ

‫ ܸ∆ܥ = ܧ‬ଶ ଶ ܲ=

ா ∆௧

(4)

(5)

where C (F cm-3) signifies the volumetric capacitance, ∆V (V) signifies the highest potential during the charge-disfcharge stage and ∆t (s) signifies the discharge time.

3. RESULTS AND DISCUSSION



Scheme 1. Preparation process (a, b, c, d) of the core-shell NiMoO4@Ni-Co-S arrays and (e) the ion exchange diagram. The synthesis process of core-shell NiMoO4@Ni-Co-S arrays is shown in Scheme 1. In the first step, (Scheme a, b) the NiMoO4 nanorod arrays were obtained over the nickel substrate by the hydrothermal process. In second step, (Scheme b, c) the obtained arrays were annealed to obtain crystallized NiMoO4 nanorod arrays. In third step, (Scheme c, d) pristine NiMoO4 was electrodeposited to make core-shell NiMoO4@Ni-Co-S arrays. The deposited nanosheets placed on the NiMoO4 nanorod arrays (Scheme e) could offer abundant quick passages for mass transport and active sites for fast charge transfer behavior, which would greatly contribute to the overall specific capacitance.


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Figure 1. SEM images of (a, c) NiMoO4 nanorods and (b, d) core-shell NS-8 nanorods at different magnifications. The morphology of the NiMoO4 nanorods and core-shell NS-8 nanorods grown on bare nickel foam (Figure S1b) was first examined with scanning electron microscopy (SEM). Just as Figure 1(a, b) display, a large amount of uniform nanorod arrays fully covering the nickel foam can be observed. Their surface is extremely smooth and the maximum diameter of a single NiMoO4 nanorod is nearly 180 nm. From Figure 1(c, d), we can know the average diameter of the core-shell NS-8 nanorods has increased to approximately 200 nm, and the rough surface is due to the presence of deposited nanosheets that interconnect each other, leading to a uniform nanostructure. However, the adornment 9 ACS Paragon Plus Environment


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of the deposited nanosheets around the arrays does not destroy the fundamental structure, but makes the surface more wrinkled and provide more contact area, which contributes to the electrochemical performance tremendously.

Figure 2. (a) TEM image of the core-shell NiMoO4@Ni-Co-S structure; (b) HRTEM image and the inset SAED patterns of the NiMoO4 nanorods; (c) HRTEM and the SAED patterns of the Ni-Co-S nanosheets; (d) EDS spectrum for the NiMoO4@Ni-Co-S nanorods. Electron microscope (TEM) and High-resolution TEM (HRTEM) were used to obtain more precise nanostructure information of the NiMoO4@Ni-Co-S nanorods. Figure 2a presents a TEM image about the Ni-Co-S coatings around the NiMoO4 nanorod, and also indicates that the diameter of a single 10 ACS Paragon Plus Environment

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NiMoO4@Ni-Co-S nanorod is approximately 200 nm. The HRTEM image in Figure 2b shows lattice fringe owning an interplanar spacing about 0.409 nm, which is in accordance with the distance from the (111) plane of NiMoO4. The inset SAED pattern further shows the NiMoO4 nanorods is monocrystal. As displayed in Figure 2c, the interplanar crystal spacing are about 0.198 and 0.152 nm, corresponding with the nominal data of the Ni-Co-S crystal faces (103), (102) well.28 This is verified by the inset SAED pattern in Figure 2c as well, revealing conspicuous diffraction rings, and confirming its pleomorphic structure. The peaks of all elements in the EDS spectrum from Figure 2d are clearly observed, whose presence convincingly verify the S, O, Co, Mo, and Ni elements, corroborating the following XRD result.

Figure 3. (a) XRD pattern for the NiMoO4 nanorods scraped from the nickel substrate and (b) Raman spectrum for the NiMoO4 nanorods. The crystallographic nature of the NiMoO4 nanorods were revealed by X-ray diffraction (XRD). Every peak shown in Figure 3a was completely corresponded to the single α-phase NiMoO4 (JCPDS No. 86-0361) and metal nickel (JCPDS No. 87-0712) without impurities, such as peaks located at 2θ = 14.3°, 24.0°, 25.4°, 28.9°, 32.9°, 39.1°, 41.3°, 43.9°, 47.5° and 53.5°, respectively, corresponding to the (110), (021), (-112), (220), (022), (202), (040), (330), (-204) and (510) planes of NiMoO4 with the 11 ACS Paragon Plus Environment


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nominal lattice constant. As Figure 3b displays, the peaks located at 816, 337 cm-1 generating from the MoO4 vibrations,41 peaks at 857, 942 cm-1 generating from the effect of the Mo-O and Ni-O bonds are also clearly shown in the Raman results for the NiMoO4 nanorods,42 further proving the existence of NiMoO4.


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Figure 4. (a) XPS characterization of the NiMoO4@Ni-Co-S nanorods and XPS spectrum of (b) Ni 2p, (c) Co 2p, (d) Mo 3d, (e) O 1s and (f) S 2p. 13 ACS Paragon Plus Environment


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X-ray photoelectron spectroscopy (XPS) was also tested to obtain the valence state information from the superficial NiMoO4@Ni-Co-S. The XPS survey spectrum in Figure 2a shows convincing existence of Ni 2p, Co 2p, O 1s, S 2p, and Mo 3d. From Figure 4b, the Ni 2p spectrum can be well fitted into two main peaks, showing the characteristics of the Ni2+ and Ni3+ oxidation state, and each peak is along with its own satellite peak (identified as “Sat.”). The binding energy peaks located at 855.1 and 872.7 eV are consistent with the Ni 2p3/2 and Ni 2p1/2, respectively. The Co 2p spectrum in Figure 4c also exhibits two main peaks of both Co2+ and Co3+, and the Co 2p3/2 and Co 2p1/2 peaks correspond to 780.6 and 796.6 eV, respectively. The peak located at 529.7 eV in Figure 4d belongs to the O 1s, indicating a O2- oxidation state.23 The peaks located at 231.0 and 234.2 eV from the Mo 3d spectrum in Figure 4d belong to Mo 3d5/2 and Mo 3d3/2, indicating a Mo6+ oxidation state.26 Figure 4f presents the S 2p spectrum, its peaks located at 162.1 and 163.5 eV also match well with S 2p3/2 and S 2p1/2, showing a particular characteristic deriving from the S2- oxidation state.40 Therefore, the XPS data verify the existence of Ni2+, Ni3+, Co2+, Co3+, O2-, Mo6+ and S2- from the NiMoO4@Ni-Co-S.

Figure 5. SEM images of (a) NS-6, (b) NS-8, and (c) NS-10 nanorods. As indicated by SEM observations in Figure 5(a, b, c), the growth process of the core-shell structure was controllable with different electrodeposition cycles. It is clearly figured out that as the reaction cycle numbers increases from 6 to 10, more and more Ni-Co-S nanosheets appear on the pristine NiMoO4 nanorods, then the unique shell structure comes into being. Meanwhile, the core-shell 14 ACS Paragon Plus Environment

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NiMoO4@Ni-Co-S nanorods are observed with gradually-enhanced density from 1.0 to 1.5 mg cm-2 and have obviously-changed dimensionalities. Furthermore, to compare the NS-6, NS-8, and NS-10 electrodes concisely, we performed necessary electrochemical measurements to the three types of samples, and the optimal core-shell NS-8 electrode was then obtained. As displayed in Figure S5(a, b), the NS-8 electrode yields the highest capacitance at 5 mA cm-2, which is almost twice as much as the other two. It also exhibits the lowest equivalent series resistance (ESR) and the most subvertical line in low frequency regions,28 which means better electrical conductivity and electroactivity.



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Figure 6. (a) Comparative CV curves for the pristine NiMoO4 and NS-8 electrodes at 10 mV s-1; (b) GCD curves for NS-8 electrode at different current densities; (c) Comparative GCD curves for the pristine NiMoO4 and NS-8 electrodes at 5 mA cm-2; (d) The areal capacitance and specific capacitance for the pristine NiMoO4 and NS-8 electrodes; (e) EIS spectra for the pristine NiMoO4 and NS-8 16 ACS Paragon Plus Environment

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electrodes. Insets: equivalent circuit for the Nyquist plots, the enlarged high-frequency part; (f) Cycling performance of the pristine NiMoO4 and NS-8 electrodes measured at 20 mA cm-2. For the purpose of obtaining the performance evaluation of the NS-8 compared with the pristine NiMoO4 as a supercapacitor electrode, essential tests were investigated in a 2 M KOH solution. Figure 6a exhibits the comparative CV for the pristine NiMoO4 electrode and the NS-8 electrode under 10 mV s-1 with the potential range from 0 to 0.65 V. According to the curves, two redox peaks are visible from the pristine NiMoO4, mainly originating from the reversible redox reaction period of Ni2+/Ni3+, as the following redox equation: ܰ݅ ଶା ↔ ܰ݅ ଷା + ݁ ି

(6)

Another two redox peaks that originate from the reversible redox reaction period of Co2+/Co3+/Co4+ and Ni2+/Ni3+ are also visible from the NS-8 CV curve, the relevant redox equation as follows:29-30 ‫ ܵ݋ܥ‬+ ܱ‫ ܪܱܵ݋ܥ ↔ ି ܪ‬+ ݁ ି

(7)

‫ ܪܱܵ݋ܥ‬+ ܱ‫ ܱܵ݋ܥ ↔ ି ܪ‬+ ‫ܪ‬ଶ ܱ + ݁ ି

(8)

ܰ݅ܵ + ܱ‫ ܪܱܵ݅ܰ ↔ ି ܪ‬+ ݁ ି

(9)

which demonstrate the typical pseudocapacitive characteristics of the active materials. The area of the CV curve for the NS-8 electrode expanded obviously, demonstrating a much higher capacitance. The full CV curves of the pristine NiMoO4 and the NS-8 electrodes from 5 to 20 mV s-1 are also given in Figure S2(a, b). As we can see from the GCD curves for the NS-8 electrode in Figure 6b, its capacitance is 2.27 F cm-2 (1892 F g-1), 1.94 F cm-2 (1617 F g-1), 1.56 F cm-2 (1300 F g-1), and 1.01 F cm-2 (842 F g-1) at 5, 10, 20, and 40 mA cm-2, respectively, which shows a good rate capability. The capacitance comparison is further measured by GCD curves for the pristine NiMoO4 and the NS-8 17 ACS Paragon Plus Environment


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electrodes, displayed in Figure 6(c, d). The later one shows a much higher areal and specific capacitance, which is almost twice as much as the pristine NiMoO4 electrode (1.27, 0.93, 0.69, 0.44 F cm-2 at 5, 10, 20, and 40 mA cm-2, respectively, displayed in Figure S3), and the specific capacitance of both are also given in Figure S2(c, d). Figure 6e exhibits the impedance of the pristine NiMoO4 and the NS-8 electrodes. Both EIS curves consist of two same parts, the semicircle part in the highfrequency region and the linear part in the low-frequency region. In the high-frequency region, the intercept value on the x-coordinate corresponds to the Rs,43 the diameter of the semicircle corresponds to the Rct resulting from the Faradaic reactions and the double-layer capacitance (Cdl).44 In the lowfrequency region, the slope factor obtained from the curve equals the Warburg impedance (W).45 Obviously, the NS-8 electrode exhibits a lower Rs with the value of 1.35 ohm, comparing with the 1.41 ohm of the pristine NiMoO4 electrode, lower Rct and Cdl with a smaller semicircle diameter; its slope is also closer to 90 degrees perpendicular to the real axis. All of which makes for superior electrical conductivity and electroactivity. Besides, the cycling stability is another essential indicator for electrochemical property, Figure 6f exhibits the results of the pristine NiMoO4 and the NS-8 electrodes under 20 mA cm-2 over 3000 cycles and 6000 cycles, respectively. The capacitance retention of the NS-8 electrode remains ultra high after numerous rigorous reactions, due to the protection of the unique core-shell construction with satisfying stability, the active electrode materials are effectively prevent from destruction. After thousands of cycles, the NS-8 electrode exhibits good electrochemical stability with only a 8.3% loss to the initial value, and the stability has been significantly improved compared with the 33.4% loss of the pristine NiMoO4 electrode.


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Figure 7. (a) Configuration diagram for the all-solid-state device; (b) CV curves for the CNT and NS8 electrodes at 15 mV s-1; (c, d) CV curves for the NS-8//CNT device at different potential windows,



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and various scan rates; (e, f) GCD curves for the NS-8//CNT device at 4 mA cm-2 with different potentials, and various current densities with the same potential. To verify the practicality of the NS-8 electrode for energy storage, the all-solid-state device was assembled by combining the NS-8 sample as the positive electrode and the CNT as the negative electrode, and the PVA/KOH (2 M) gel as the separator and electrolyte. Figure 7a exhibits the configuration diagram for the device. From Figure 7b, the CNT and NS-8 electrodes scan from -1 V to 0 V, 0 V to 0.65 V at 15 mV s-1 accordingly, indicating the maximum potential of the prepared device is close to 1.6 V, and the CV curve displayed in Figure 7c begins to be polarized when the potential is more than 1.4 V, verifying that the potential window just reach the optimal 1.4 V. The GCD curves in Figure 4e provide further proof, showing the comparison under different potentials, from which we clearly observe the highest potential can be charged to 1.4 V with only a 0.03 V IR drop under 4 mA cm-2. The CV curves for the NS-8//CNT device under different scan rates with a potential window from 0 to 1.4 V are observable from Figure 7d. The CV shapes of the all-solid-state device show barely deformations responding to the incremental scan rates, even after increasing to 120 mV s-1, exhibiting the satisfactory rapid charge-discharge characteristic. From the GCD curves in Figure 7f, the volumetric capacitance of the prepared device was 540, 402, 336, 258, 192 and 150 mF cm-2 at 4, 8, 16, 32, 40 and 60 mA cm-2, respectively. Correspondingly, the energy density, also with the power densities of the prepared device was figured out, and the consequence are revealed in Figure 8a as a Ragone plot. The prepared device can give an energy density of 2.45 mWh cm-3 at the power density of 0.131 W cm-3. Even when the power density increases to 0.980 W cm-3, the device still possesses an energy density of 0.681 mWh cm-3. Clearly, the device performance compares favorably with other asymmetric supercapacitors reported previously in the literature.46-49 A commercial red LED is also powered (inset in Figure 8a) by a pair of devices brilliantly, meaning that the NS-8 electrode-based allsolid-state device owns highly practical-application potential for energy storage. The cycling 20 ACS Paragon Plus Environment

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performance of the NS-8//CNT device measured under 16 mA cm-2 is shown in Figure 8b. The capacitance remains 270 mF cm-2 after 3500 cycles, demonstrating the satisfactory stability of the device. The first and last 5 cycles were also displayed in an inset, and the coulombic efficiency of the first and last 5 cycles is about 96.3% and 85.4%, respectively, indicating a good coulombic efficiency during the whole cyclic process.

Figure 8. (a) Ragone plots of the NS-8//CNT device. Inset: an red LED powered by two devices in series. (b) Cycling performance of the NS-8//CNT device. Insert: the first and last 5 cycles.

4. CONCLUSIONS In general, through a reasonable nanostructure design and feasible synthesis method, a hierarchical core-shell NiMoO4@Ni-Co-S structure has been first synthesized on nickel foam as enhanced electrode materials with outstanding pseudocapacitive performance, through a simple hydrothermal method followed by another electrodeposition process. The ordered core-shell nanostructure and synergy generated by the pristine NiMoO4 nanorods and Ni-Co-S nanosheets presented in the composite enhance the electronic conductivity, capacitance, and electrochemistry stability that can not be realized through the pristine NiMoO4 nanorods. Additionally, composite nanorods of three different 21 ACS Paragon Plus Environment


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electrodeposition cycles were achieved, from which the optimal NS-8 nanorods electrode was obtained, which exhibited a specific capacitance of 2.27 F cm-2 (1892 F g-1) at 5 mA cm-2. Furthermore, the assembled device using this NS-8 nanorods electrode delivers an impressive energy density of 2.45 mWh cm-3 under a power density of 0.131 W cm-3, and possesses an outstanding cycle stability even after 3500 cycles, its energy storage ability for practical application has also been examined by successfully powering a commercial red LED powerfully. Our work demonstrates that core-shell NS-8 nanorods electrode is a promising, novel electrode for high-performance energy storage equipment.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Measure of an asymmetric supercapacitor device, SEM image of the bare nickel foam, CV curves, GCD curves and EIS spectra for different electrodes, matching process for positive and negative electrodes, and a table with corresponding specific capacitance of several other previously reported samples.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ACKNOWLEDGMENTS This work was financially supported by self-determined research funds of CCNU from the colleges’ basic research and operation of MOE (No. CCNU17TS007), the research funds of Key 22 ACS Paragon Plus Environment

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Laboratory of Analytical Chemistry for Biology and Medicine of MOE (ACBM2016003), the Key Scientific Project of Wuhan City (No. 2013011801010598), Henan science and Technology Cooperation Project (172106000064), the National Natural Science Foundation of China (No. 50802032 and 11275082) and the Basic Scientific Research Foundation of Central China Normal University (No. CCNU16A05004).



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