When Al-Doped Cobalt Sulfide Nanosheets Meet Nickel Nanotube

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When Al-Doped Cobalt-Sulfide Nanosheets Meet Nickel Nanotube Arrays: A Highly Efficient and Stable Cathode for Asymmetric Supercapacitors Jun Huang, Junchao Wei, Yinbo Xiao, Yazhou Xu, Yujuan Xiao, Ying Wang, Licheng Tan, Kai Yuan, and Yiwang Chen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00901 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

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When Al-Doped Cobalt-Sulfide Nanosheets Meet Nickel Nanotube Arrays: A Highly Efficient and Stable Cathode for Asymmetric Supercapacitors Jun Huanga, Junchao Weia,b Yingbo Xiaoa, Yazhou Xua, Yujuan Xiaoa, Ying Wanga, Licheng Tana, Kai Yuan*a,b, and Yiwang Chen*a,b

a

College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031,

China b

Jiangxi Provincial Key Laboratory of New Energy Chemistry/Institute of Polymers,

Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China E-mail: [email protected] (K.Y.); E-mail: [email protected] (Y.C.).

Keywords: carbon cloth, Ni nanotubes, Al-doped cobalt sulfide, self-standing, all-solid-state supercapacitors

Abstract Although cobalt sulfide is a promising electrode material for supercapacitors, its wide application is limited by relative poor electrochemical performance, low electrical conductivity, and inefficient nanostructure. Here, we demonstrated that the electrochemical activity of cobalt sulfide could be significantly improved by Al doping. We designed and fabricated hierarchical core-branch Al-doped cobalt sulfide nanosheets anchored on Ni nanotube arrays combining with carbon cloth (denoted as CC/H-Ni@Al-Co-S)

as

excellent

self-standing

cathode

for

asymmetric

supercapacitors (ASCs). The combination of structural and compositional advantages endows CC/H-Ni@Al-Co-S electrode superior electrochemical performance with high specific capacitance (1830 F g-1/2434 F g-1 at 5 mV s-1/1 A g-1) and excellent rate

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capability (57.2%/72.3% retention at 1000 mV s-1/100 A g-1). The corresponding all-solid-state ASCs with CC/H-Ni@Al-Co-S and multilayer graphene/CNTs film as cathode and anode, respectively, achieve a high energy density up to 65.7 W h kg-1 as well as superb cycling stability (90.6 % retention after 10000 cycles). Moreover, the ASCs also exhibit good flexibility and stability under different bending conditions. This work provides a general, effective route to prepare high performance electrode materials for flexible all-solid-state energy storage devices.

The fast-growing use of electronics devices put forward new challenge to high performance energy storage devices and technologies, in which supercapacitors (SCs) with fast charge-discharge ability, high power density, long cycling life, no memory effect, and eco-friendly nature have been demonstrated as efficient power sources in energy storage devices.1-5 However, the development and practical application of SCs have been largely restricted by their low energy density.6,7 According to equation E = 1/2 CV2, the effective strategy for increasing the energy density (E) is obviously to extend the specific capacitance (C) and device voltage (V) by assembly of different cathode and anode materials for asymmetric supercapacitors (ASCs).8,9 Tremendous efforts have been devoted to synthesis high capacitive cathode and anode materials. Compare to the carbon-based materials, the pseudocapacitive/battery-type materials possess better energy storage properties because of their high-capacity values that result from the fast electrochemical redox reactions with electrolyte ions.10,11 Active efforts have been attracted to explore cathode materials to achieve larger specific capacitance and good rate performance.12,13

Recently, electrochemically active transition metal sulfides (TMS), especially cobalt sulfide (Co9S8, CoS2, and Co3S4 etc.), have received tremendous research interest as promising cathode for ASCs. The TMS have better electrical conductivity and higher electrochemical activity than those of the oxide counterparts due to the lower electronegativity of sulfur compared to that of oxygen, resulting in better ﬂexibility and stability for electrode materials.14-16 However, so far, the available


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cobalt sulfide based electrode materials still suffer from low rate capability and/or poor electrochemical stability because of their high dependence on surface Faradaic redox reactions and sluggish reaction kinetics under high charge/discharge rates.17-19 Many works have focused on engineering nanosized/nanostructured cobalt sulfide-based materials to shorten the ion diffusion path,20-23 hybridizing with high conductive agents such as carbon fiber, CNT and graphene to form TMS/carbon composite to achieve better performance.24-27 Although enhanced capacitance could be achieved through the assistance with carbon materials, the synthesis procedures are usually complicated. Compared with single-metal sulfides, ternary transition metal sulfides such as M–Co–S (M = Ni, Cu, Zn and Mn) possess much higher electrochemical activity and capacity, due to their multiple oxidation states and smaller band-gaps for richer oxidation reaction.28-32 Nevertheless, the resultant improvement in electronic conductivity and capacitance is still obscure. Recent works have demonstrated that metal doping is an effective, simple method to adjust the band gap and tune the conductivity of electrode materials for boosting the capacity performance.33,34 A large number of papers have reported that metal ions doped transition metal oxides could improve the electric conductivity due to the oxygen vacancy generated.35,36 Instead, few works about the synthesis or electrochemical mechanisms of metal ions doped cobalt sulfides for SCs were reported.

2D nanosheets have demonstrated their merits due to the anisotropic structure and high surface-to-bulk ratio, which grant a shorter diffusion path for electrons and ions. 37,38

Nevertheless, the self-aggregation and disordered arrangement of nanosheets on

conductive substrates restricts the exposure of the active sites, which may result in the low specific capacitance and decreased redox reaction kinetics.39,40 It is still a great challenge to develop 2D nanosheets based electrodes with both high electron and ion transport capabilities for high performance ASCs. In addition, the conventional slurry-derived electrode requires a polymer binder to immobilize the active materials, which inevitably results in buried active sites and insufficient mass and electron transport ability, thus impairing the energy storage performance.41,42 Alternatively, the


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strategy for directly integrating active materials with current collectors is widely adopted to make an integrated electrode to avoid the “dead surface”. Specifically, well-aligned nanotubes arrays (NTAs) structures grown on current collectors are extremely interesting owing to their intrinsic advantages, including high exposed surface area, enhanced electron transport, and improved mechanical stability.43-45 The configuration with a built-in electron conductor can not only maximize the utilization rate of active materials, but also shorten electron/ion diffusion path to facilitate the reaction kinetics of electrodes for enhanced electrochemical performance.46,47 Based on the above consideration, herein, ultrafine hierarchical core-branch cobalt sulfide-based electrode was rationally designed and successfully synthesized by using carbon cloth as the conductive substrate and Ni NTAs as the secondary substrate for further

coating

interconnected

Al-doped

cobalt

sulfide

nanosheets

(CC/H-Ni@Al-Co-S) for high performance cathode of ASCs. The electrochemical performance of CC/H-Ni@Al-Co-S was significantly extended by synergistic effects of the ultrafine interconnected Al-doped cobalt sulfide nanosheets and the hierarchical core-branch architecture. The hybrid electrode exhibits a high specific capacitance of 1830 F g-1 at a scan rate of 5 mV s-1 and 2434 F g-1 at a current density of 1 A g-1 as well as excellent rate capability. The corresponding ASC achieves a high energy density of 65.7 W h kg-1 at the power density of 765.3 W kg-1, high specific capacitance (156 F g-1 at 10 mV s-1) and electrochemical cycling stability (90.6 % retention after 10000 cycles). Besides, the different temperatures condition tests present the ASC devices can efficient work under 30 to 80 °C for further practical application.

Result and Discussion Morphology and Structure Characterization of CC/H-Ni@Al-Co-S. The 3D hierarchical

structure

CC/H-Ni@Al-Co-S

electrode

was

fabricated

via

a

template-assisted method as schematically illustrated in Figure 1. The commercial woven carbon cloth (CC) with a smooth surface serve as conductive substrate (Figure S1). The aligned and ordered ZnO nanorod arrays (ZnO NRAs) were grown vertically


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on the surface of CC via a hydrothermal method. After growth, the CC was uniformly covered by vertically aligned ZnO NRAs of ca. 200 nm in diameter and 5-8 µm in length (Figure 2a,b). Then the ZnO NRAs used as the backbone and template for the following electrodeposition of a homogeneous and continuous Ni layer. The surface roughness of ZnO NRAs is increased after the Ni coating (Figure S2). The energy dispersive X-ray spectroscopy (EDS) element mapping images show a homogeneous distribution of the elements (Zn, O, Ni, and C), indicating that Ni is uniformly distributed on the surface of ZnO nanorods (Figure S3). The ZnO NRAs serve as sacrificial template and could be dissolved in aqueous ammonia, resulting in the hollow Ni nanotube arrays (NTAs). The Ni NTAs with a shrinking surface (Figure 2c) and the thickness of Ni nanotube wall is about 71 nm (Figure 2d). The EDS mapping images of CC/Ni NTAs show that the ZnO core is completely removed and the Ni element is uniformly distributed along the nanotubes (Figure 2e). Because of a direct contact with the CC substrate and a shape-reservation transformation, the final structure can be also referred as an array. Finally, a shell of Al-doped cobalt sulfide nanosheet arrays were deposited on the exterior surface though an electrodeposition strategy to obtain the desired CC/H-Ni@Al-Co-S electrode (for detailed synthesis procedure, see Experimental Section).

As shown in Figure 3a-c, the typical SEM images of CC/H-Ni@Al-Co-S, indicate the well-preserved array structure and interconnected Al-doped cobalt sulfide nanosheets were anchored onto the entire Ni NTAs surface. The Al-doped cobalt sulfide nanosheets with high uniformity in thickness and homogeneously distributed along

the

Ni

NTAs,

forming

three-dimensional

hierarchical

core-branch

heterostructures. The low magnification SEM images of CC/H-Ni@Al-Co-S show that our method can uniformly, large scale prepare the hybrid electrode with aligned H-Ni@Al-Co-S heterostructures of ca. 8 µm in length (Figure S5). Such hierarchical and cross-connected nanonetwork of Al-Co-S nanosheets on Ni NTAs enables a large accessible surface area for electrolyte penetration, initiates the rapid electrochemical reactions with electrolyte ions, and enhances the specific capacity and rate capability.


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The TEM images of a single Ni nanotube/Al-Co-S also confirm the hollow structure and Al-Co-S nanosheets are uniformly coated on the surface of Ni nanotube (Figure 3d,e). The hollow structure could provide guiding channels in assisting the insertion/extrusion of electrolyte ions in charge–discharge cycles against with the bulk structure, as shown in Figure S4. The TEM image of Al-Co-S nanosheets exhibit the interconnected morphology and the high-resolution transmission electron microscopy (HRTEM) displays the lattice fringe spacing of 0.57 nm, which corresponds well to the (111) plane of Co9S8 (Figure 3f). The corresponding selected area electron diffraction (SAED) pattern further confirms the crystalline structure of Co9S8. The EDS mapping exhibits Co, S, Al, Ni and C elements are uniformly distributed through the entire CC/H-Ni@Al-Co-S (Figure 3g).

Due to the open spaces of the array structure, Al-doped cobalt sulfide nanosheets are also partly deposited on the skeleton of CC, indicating the more mass loading of active material, resulting in the further enhancement of the energy storage performance (Figure S5). The Ni NTAs are favorable for accelerating the ion and electron transportation through the active material, leading to the enhanced electrochemical performance. The prepared core-branch structure provides numerous interspaces for rapid electrolyte ion accessibility, shorten the ion diffusion paths, and facilitate more redox reactions, which simultaneously improves the energy storage performance. In contrast, inhomogeneous and low mass loading of nanosheets are formed around the ZnO nanorods without the assistance of Ni layer (Figure S6). Besides, the CC/Al-Co-S electrode is also prepared via electrodeposition with the same condition. As shown in Figure S7, the dense Al-doped cobalt sulfide nanosheets on the CC skeleton has a larger thickness with many cracks, which may result in high contact resistance, limited accessible surface area, and weak stability, implying the deteriorated electrochemical performance for energy storage.

The phase and composition of products are further confirmed by X-ray diffraction (XRD) analysis and X-ray photoelectron spectroscopy (XPS). As shown in Figure 3h,


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the characteristic peaks of ZnO phase (JCPDS 36-1451) are clearly seen in the XRD pattern for the sample after the hydrothermal growth of ZnO nanorods. The two Ni peaks indicate the presence of Ni nanotubes after etching the ZnO core. The diffraction peaks of the as-synthesized CC/H-Ni@Al-Co-S can be well assigned to the cobalt sulfide phase solely with Co9S8 (JCPDS 19-0364) and Co3S4 (JCPDS 47-1738) rather than a pure phase, which might be attributed to the complex stoichiometry of cobalt chalcogenides.48 A broad peak around 26.5°is the characteristic of amorphous carbon.49 Furthermore, XPS analysis was carried out to understand the electronic structure of the as-prepared sample with Al doped. The survey XPS spectrum shows that the sample consists of Ni, Co, C, S, and Al without other impurities. High-resolution XPS spectra of Ni 2p, Co 2p, C 1s, S 2p, and Al 2p core levels have been recorded (Figure S8). Two peaks located at 855.8 and 873.2 eV correspond to Ni 2p1/3 and Ni 2p2/3, respectively, match well with electronic state of Ni2+ species.50 The Co 2p3/2 peaks observed at 781.2 and 796.6 eV are characteristic of Co3+ and Co2+ species, respectively.51 For the S 2p, the peak observed at 163.4 eV is typical of metal-sulfur bonds in the metal sulfides and consistent with cobalt sulfides.52 The S 2p peak observed at 168.2 eV is attributed to surface sulfur with high oxide state of sulfates owing to partial oxidation of cobalt sulfide. The presence of Al 2p peak further confirmed Al was successfully doped, which is in good agreement with the EDS mapping results.

Electrochemical

Properties

of

CC/H-Ni@Al-Co-S.

The

electrochemical

performance of the CC/H-Ni@Al-Co-S and reference samples were first measured in a three-electrode cell with 2M KOH as electrolyte in a potential window from -0.2 to 0.6 V (vs Ag/AgCl) at room temperature. To demonstrate the superior capacitive performance of the core-branch CC/H-Ni@Al-Co-S electrode, three counterparts including CC/Co-S, CC/H-Ni@Co-S, CC/Al-Co-S were also tested for comparison. From the cyclic voltammetry (CV) curves (Figure 4a), it is evident that the pair of redox peaks can be observed in all the samples, which mainly originated from the Faradaic reactions of cobalt sulfide in KOH solution. The CC/H-Ni@Co-S exhibits a


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larger CV curve area and redox peak intensity compared to CC/Co-S and CC/Al-Co-S, implying a significantly improved specific capacitance and faster redox reaction kinetics processes. The hollow Ni NTAs acting as the bridge of active material and substrate could decrease the contact resistance between them, lead to enhanced ion and electron transportation through the active material. Besides, the conducting Ni layer could increase the mass loading of active material under the same electrodeposition condition, as confirmed by the SEM images (Figure S5), thus resulting in boosted capacitance performance. The Al doped is also favorable for accelerating the conductivity of cobalt sulfide, leading to the enhancement of energy storage performance. As expected, the expanded CV curve of the CC/H-Ni@Al-Co-S electrode suggests that the hollow core-branch heterostructure can provide abundant active sites for both EDLC and redox reactions to substantially enhance the specific capacitance. The prepared core-branch CC/H-Ni@Al-Co-S electrode provide numerous interspaces for rapid electrolyte ion accessibility, shorten the ion diffusion paths and facilitate more redox reactions, which simultaneously improves the energy storage performance.

The impedance spectra show that the slope of the straight line for CC/H-Ni@Co-S and CC/Al-Co-S is larger than that of CC/Co-S at low frequency region, suggesting the lower diffusion resistance (Rw) and charge transfer resistance (Rct). The low Rw and Rct are benefit for efficient ions diffusion of electrolyte during redox reaction and fast charge transfer during the charge/discharge processes (Figure 4b). Moreover, among the four electrodes, the CC/H-Ni@Al-Co-S electrode presents the smallest equivalent

series

resistance

(Rs)

of

0.85

Ω,

due

to

the

favorable

structural/compositional advantages. The Ni nanotubes act as efficient electron transportation bridges between active materials and current collectors. The ultrafine hierarchical and cross-connected network of Al-doped cobalt sulfide nanosheets also endow faster charge transfer rate on the surface of electrode. Therefore, the hybrid core-branch nanoarchitecture is essential for the electron collection and fast Faradaic reactions. Electrochemical impedance spectrometry (EIS) analytical results further


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verify that CC/H-Ni@Al-Co-S electrode exhibits favorable reaction kinetics and lower internal resistance compared with its counterparts. The galvanostatic charge/discharge (GCD) curves (Figure 4c) of the four samples show that discharging time for CC/H-Ni@Al-Co-S is much longer than those for other samples, indicating the highest capacitance. Furthermore, the internal resistance drop (IR drop) of CC/H-Ni@Al-Co-S

(0.014

V)

is

smaller

than

CC/Al-Co-S

(0.023

V),

CC/H-Ni@Co-S (0.019 V) and CC/Co-S (0.034 V), benefiting from the synergistic effect of Ni nanotubes and Al doped. The specific capacitances of the electrodes are calculated from CV and GCD curves (Figure 4d). It can be seen that the CC/H-Ni@Al-Co-S electrode delivers the highest specific capacitance of 1830 F g-1 at 5 mV s-1, whereas only 1239, 855 and 496 F g-1 are obtained for CC/Al-Co-S, CC/H-Ni@Co-S and CC/Co-S, respectively. Surprisingly, 57.2% of capacity was retained even at a high scan rate of 1000 mV s-1, signifying that the hollow core-branch electrode exhibits the superior rate capability. CC/H-Ni@Al-Co-S electrode could deliver a high specific capacitance of 2434 F g-1 at a current density of 1 A g-1, and with a retention of 72.3% when the current density increased to 100 A g-1. The structural and compositional advantages of CC/H-Ni@Al-Co-S electrode are responsible for the enhanced capacitance and redox reaction kinetics. To meet the requirement for real application, the capacity based on the total mass of the electrode were also investigated by CV and GCD. As shown in Figure S9, at a scan rate of 5 mV s-1, the hybrid electrode delivers a capacitance of 189 F g-1 and maintains a capacitance of 108 F g-1 when the scan rate is increased to 1000 mV s-1.

To

investigate

the

effect

of

Al

on

the

electrochemical

performance,

CC/H-Ni@Al-Co-S samples doped with different Al atomic percentage were measured in a three-electrode cell. As shown in Figure 5a, at a constant current density, increasing Al atomic percentage leads to that the specific capacitance increases initially and then decreases rapidly. The electric conductivity of cobalt sulfide could be improved when doped with appropriate amount of Al. The specific capacitances are all maximal at the Al atomic percentage of 2 at% at different current


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densities. The reduction of the specific capacitance of CC/H-Ni@Al-Co-S with more Al doping may be due to the excessive Al prohibiting the electrochemical redox reactions between cobalt sulfide and the electrolyte. Figure 5b shows the CV curves of the CC/H-Ni@Al-Co-S nanosheets electrode at various scan rates from 5 to 1000 mV s-1. Even at the high scan rate of 1000 mV s-1, the CV curve can still retain a definite pair of redox peaks without obvious distortion, suggesting the ideal pseudocapacitance behavior and rate capabilities of the electrode. Furthermore, the anodic peaks shift to a higher potential while the cathodic peaks move to a lower potential ascribed to the insufficient intercalation of ions from electrolyte into the dense center of nanostructure. Meanwhile, the anodic peak current density increases and the cathodic peak current density decreases, implying a relatively low resistance and fast redox reactions at the interface. The GCD curves at all current densities from 1 to 40 A g-1 are almost symmetric (Figure 5c), indicating a high coulombic efficiency due to the highly reversible redox reactions of the electrode on the charge/discharge process.

To further investigate the cycling stability of the electrode, the CC/H-Ni@Al-Co-S nanosheets electrode was tested at 1, 5, 10, 20 and 40 A g-1 for 50 cycles, once the charge/discharge rate was set back to 1 A g-1, corresponding to 98.2 % recovery of the initial capacity (Figure 5d). In addition, the long-term cycling test at a charge/discharge current density of 10 A g-1 was carried out, and 94.8% of the initial capacitance remained after 10000 cycles (Figure 5e), indicating the excellent cycling stability of the CC/H-Ni@Al-Co-S electrode. Impressively, after 10000 cycles, the morphology of H-Ni@Al-Co-S nanosheets was conserved well without obvious desquamation, demonstrating the strong structural and electrochemical stability of the hybrid electrode. The long-term cycling stability is always a bottleneck for the solitary cobalt sulfide based materials as revealed by numerous previous reports,14,18,20,50,53

which

is

optimized

by

the

fabrication

CC/H-Ni@Al-Co-S electrode.


of

core-branch

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The enhanced specific capacity, rate capability, and cycling stability of the core-branch CC/H-Ni@Al-Co-S nanosheets electrode are mainly attributed to its synergic features of Ni nanotubes, Al doped and hierarchical nanoarchitecture configuration of the material, as schematically illustrated in Figure 5f. First, the commercial woven carbon cloth serves as the high conductive and ﬂexible current collector without tedious process of polymer binder/conductive additive mixing, which could significantly reduce the “dead volume” and improve the electrochemical activity of the electrode. Second, the direct growth of Ni NTAs on the surface of CC could reduce the interface resistance gap between the substrate and active materials, and act as the electron “superhighways” to promote the ion/electron transfer rate. Third, the Al doped could improve the conductivity and electrochemical activity of cobalt sulfide, resulting in optimized utilization of cobalt sulfide nanosheets for the improved specific capacity. Fourth, the interconnected vertically aligned Al-doped cobalt sulfide nanosheets adhered on Ni NTAs with dense electrochemical active sites and high specific surface provide numerous pathways for electrolyte ion penetration to charge storage and delivery. Moreover, the electrons can be directly and rapidly transferred within the hierarchical nanoarchitecture of CC/H-Ni@Al-Co-S, which is also favorable for elevating capacity. As a result, most of the active material would be available for rapid redox reactions with electrolyte ions, leading to the enhancement of energy storage properties. Such exceptional electrochemical properties indicate that CC/H-Ni@Al-Co-S

electrode

could

be

an

outstanding

cathode

for

high-energy-density flexible supercapacitors.

All-Solid-State Asymmetric Supercapacitors Based on CC/H-Ni@Al-Co-S. To evaluate the possibility of the as-obtained CC/H-Ni@Al-Co-S electrode for practical application, asymmetric supercapacitors (ASCs) were further assembled by utilizing the CC/H-Ni@Al-Co-S electrode as the cathode and the flexible multilayer graphene/carbon nanotube (CNT) film as the anode with PVA-KOH gel electrolyte. Graphene-based materials with high electrical conductivity, good mechanical strength, large specific surface area, and superior electrochemical stability are expected as


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promising candidates that can satisfy high-energy storage requirements for SCs. To prevent the restacking of graphene, one-dimensional CNTs were used as a smart spacer. CNTs not only prevent graphene from restacking to increase the accessible surface area but also act as efficient electrical conducting paths to improve the power density. As shown in Figure S10, the top-view SEM image of the graphene/CNTs film exhibits a wrinkled characteristic. The cross-sectional SEM image shows a layer-by-layer structure with CNTs bridge the graphene film. Such structure could provide electrolyte ions fully access to the whole film electrode, thus ideal to supercapacitor performance. The electrochemical performance of the graphene/CNTs film electrode was investigated in a three-electrode cell with 2M KOH as the electrolyte. It is shown that the graphene/CNTs film electrode has excellent electrochemical performance in terms of rectangular cyclic voltammetry curves (Figure S11a), symmetric charge-discharge behaviors (Figure S11b), and long-term cycling stability (Figure S11c). In a voltage window from -1 to 0 V, the specific capacitance of the graphene/CNTs film electrode can reach 263 F g-1 at a current density of 1 A g-1, and it still remains 143 F g-1 at a high current density of 100 A g-1 (Figure S11d). Moreover, the graphene/CNTs film electrode shows outstanding electrochemical stability after 10000 cycles of charge and discharge at a high current density of 20 A g-1. The excellent electrochemical properties of the graphene/CNT film electrode indicate that it is ideal to act as a anode for ASCs.

The mass loading of cathode and anode was balanced before assemble the full cell devices, and the CV curves under their corresponding voltage windows at a scan rate of 50 mV s-1 were shown in Figure S12. According to the CV curve, the overall capacitance of the ASC device derives from the combined contribution of pseudocapacitance and electrical double layer capacitance. From CV curves of the ASC device (Figure 6a) recorded at different voltage windows, the stable operating voltage can be extended up to 1.8 V at a scan rate of 10 mV s-1. This can also be demonstrated by GCD measurements of the ASC device with different voltage windows at a current density of 20 A g-1 (Figure 6b). There is no obvious of H2/O2


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evolution in the CV plots as well as no overcharging region in the GCD plots was observed, signifying the good electrochemical operation stability and maximum potential window of the device (1.8 V). The CV curves of the ASC were tested at various scan rates of 10-1000 mV s-1 with a stable potential window of 0-1.8 V (Figure 6c). Markedly, the CV curves can still retain a definite rectangular shape without obvious distortion even at the high scan rate of 1000 mV s-1, suggesting the ideal capacitance behavior and good rate capabilities of the ASC device. Distinctive from the solid redox peaks observed in three-electrode system, the CV curves of the fabricated ASC exhibited the excellent capacitive behavior due to the inclusion of EDLC material. With increasing the scan rate, as shown in CV curves, the current response also distinctively increased, signifying the good I-V response of the device. Figure 6d displays the GCD curves of the ASC at different charge/discharge currents of 1-40 A g-1. It is noticeable that all the charge-discharge curves exhibited almost symmetrical property, indicating good capacitive behavior of the ASC device. Besides, CV curves under different temperatures at 10 mV s-1 (Figure 6e) present that polarization becomes more and more obvious with the increase of test temperature, and the area of CV curve increased by 40% when the test temperature increases from 30 to 80 °C (Figure 6f).

The low bulk solution resistance (Rs) value of 0.48 Ω for ASC (Figure S15) proves the high electrical conductivity of both electrodes. The specific capacitance was calculated to be 159 F g-1 at 10 mV s-1 (Figure 7a), which was retained 71 F g-1 with a capacitance retention rate of 44.6 % even at 1000 mV s-1, further evidencing the high capacitance and good rate performance of the ASC devices. Based on the mass of active material on both electrode, the ASC in this work delivered an energy density of 65.7 W h kg-1 at the power density of 765.3 W kg-1, which maintained 40.9 W h kg-1 with a high power density of 9816 W kg-1 (Figure 7b). The obtained energy density of our device is comparable or even higher than those of state-of-the-art cobalt sulfide-based supercapacitors. We have also calculated the device performance based on the total mass of both electrode and the entire mass of device (including separator


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and electrolyte) and find that the device delivers an energy density of 6.53 W h kg -1 at a power density of 825 W kg-1, and an energy density of 3.58 W h kg-1 at a power density of 816 W kg-1, respectively (Figure S13 and S14). The cycling performance of the ASC was evaluated at a current density of 5 A g-1, as shown in Figure 7c. It can be clearly seen that the ASC displays excellent cycling stability with about 90.6% of the initial specific capacitance even after 10000 cycles, suggesting the prominent energy storage capability of the solid-state device.

To demonstrate the potential application of the as assembled ASC in portable and wearable electronics, a series of mechanical flexibility tests were also performed. As illustrated in Figure 7d, negligible performance degradation and almost completely overlapped CV curves were achieved at a scan rate of 10 mV s-1 with different bending conditions, highlighting the exceptional flexibility and stability of our ASCs. Even at the twisted condition, the device also exhibited a good electrochemical durability and flexibility. As the device had a maximum working potential of 1.8 V and high energy density, the charged solitary ASC can effectively operate a red light-emitting diode (LED) (Figure 7e) and an electrical motor fan (Figure 7f). Considering the superior electrochemical properties, the produced hierarchical core-branch CC/H-Ni@Al-Co-S electrode could serve as a flexible and efficient cathode for high-performance wearable energy storage devices.

Conclusion In summary, we have demonstrated the rational design and fabrication of CC/H-Ni@Al-Co-S electrode with three-dimensionally well-aligned core-branch Ni NTAs@Al-doped cobalt sulfide nanosheets on a commercial carbon cloth. Benefiting from the high electrical conductivity of Ni nanotubes and the synergistic effects of the ultrafine interconnected Al doped cobalt sulfide nanosheets as well as the core-branch hierarchical architecture, improved electrochemical performance was achieved for CC/H-Ni@Al-Co-S. Therefore, the as-fabricated CC/H-Ni@Al-Co-S electrode exhibits an ultrahigh specific capacitance (1830 F g-1 at 5 mV s-1), excellent rate


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capability (1047 F g-1 at 1000 mV s-1) and electrochemical stability, which can be attributed to the highly utilization rate of active material and fast charge transport in the electrode. Furthermore, the flexible asymmetric supercapacitors fabricated with CC/H-Ni@Al-Co-S as cathode and multilayer graphene/CNTs film as anode demonstrate outstanding electrochemical performance. The device (maximum operating voltage of 1.8 V) delivers a high energy density of 65.7 W h kg-1 at the power density of 765.3 W kg-1, high specific capacitance (159 F g-1 at 10 mV s-1) and excellent electrochemical stability. Our simple approach to hierarchical core-branch heterostructure electrode with superior electrochemical properties provides a favorable

strategy

to

synthesis

high-performance

electrode

materials

for

next-generation flexible energy storage devices.

Experimental Section Synthesis of CC/ZnO Nanorod Arrays: ZnO NRAs were grown on CC (3 × 5 cm2) by a facile hydrothermal method. Briefly, the carbon cloth was first wetted and cleaned with acetone and water, respectively. Then, CC was dipped into a precursor solution (70 mL) containing 0.015 M Zn(NO3)2, 0.015 M HMTA, and ammonia (4 mL). The sealed bottle was placed into an oven at 90 °C for 24 h. After that, the white-colored CC/ZnO NRAs substrate was obtained by washing with water and drying at 80 °C for 6 h.

Synthesis of CC/Ni Nanotube Arrays: CC/ZnO@Ni NRAs was first fabricated via the electrodeposition of Ni layer on the surface of ZnO NRAs at a scan rate of 5 mV S-1 in a mixed aqueous solution of 0.04 M NiSO4 and 0.04 M NH4Cl for 10 min. The synthesized CC/ZnO@Ni NRAs was then immersed in 2.5 wt% aqueous ammonia for 3 h to remove ZnO completely to obtain the CC/Ni nanotube arrays (CC/Ni NTAs).

Synthesis of CC/H-Ni@Al-Co-S electrode: The CC/H-Ni@Al-Co-S electrode was prepared via the electrodeposition method using a three-electrode configuration with CC/Ni NTAs (3 × 5 cm2), platinum plate, and Ag/AgCl as working electrode, counter


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electrode, and reference electrode, respectively. The electrodeposition bath was an ethanol-water solution with a volume ratio of 3:7, containing 0.05 M Co(NO3)2·6H2O with desired concentrations of Al(NO3)3·9H2O and 0.5 M thiourea (CS(NH2)2). The pH value of the solution was adjusted to 6. The electrodynamic deposition was carried out in a three-electrode cell using CC/Ni NTAs as the working electrode, Pt as counter electrode, and Ag/AgCl as reference electrode by cyclic voltammetry at a scan rate of 5 mV s-1 for 3 cycles within a voltage range of -1.2 to 0.2 V vs Ag/AgCl. The electrodeposited CC/Ni NTAs was cleaned by rinsing with a large amount of water, followed by drying in air for 12 h and vacuum drying at 80 °C for 12 h. The typical mass loading of the positive CC/H-Ni@Al-Co-S nanosheets electrode is about 1.5 mg cm-2.

Preparation of multilayer graphene/CNTs film: Specifically, the graphene oxide (GO) prepared by a modified Hummer's method was dispersed in water with a concentration of 1 mg mL-1 and followed by ultrasonication at 160 W for 1 h. The pH value of the obtained GO dispersion was adjusted to ∼10 by NH3∙H2O. The CNTs solution (1 mg mL-1) was dispersed in water and followed by ultrasonication at 160 W for 1 h. Then, GO/CNTs film was fabricated by pouring GO and CNTs suspension step by step into Teflon dishes and kept evaporation of water under appropriate heating (60 °C) for film formation until drying. After peeled from the substrate, a free-standing, flexible, dark brown GO/CNTs film was obtained. Finally, the GO/CNTs film was annealed at high temperature (1000 °C) for 1 hour under argon flow. The multilayer graphene/CNTs film was used as anode for the asymmetric supercapacitor.

Assembly of the solid-state ASC: The fabrication of the asymmetric supercapacitor was conducted by taking the CC/H-Ni@Al-Co-S nanosheets and multilayer graphene/CNTs film as cathode and anode, respectively. A PVA/KOH gel electrolyte was used as the electrolyte, and a porous polymer membrane as the separator (Celgard 3501). In a typical gel electrolyte preparation, 4.5 g of KOH and 4 g of PVA were


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mixed with 45 mL of deionized water, and then the resulted mixture was heated at 90 °C for 2 h to get a clear sticky solution. Prior to the fabrication of the asymmetric supercapacitor, the mass loading of the cathode and anode were balanced according to the following equation:

= where m is the mass, Cs is the specific capacitance, and ΔV is the voltage range for positive and negative electrodes, respectively.

Materials Characterizations: The crystallographic information and phase purity of the samples were characterized by XRD (PERSEE, XD-3 with Cu Kα radiation), EDS (Tecnai G2 F30 S-TWIN), and XPS (Thermo-VG; ESCALAB 250). The morphology and microstructure were investigated by field emission scanning electron microscope (FE-SEM, FEI, QuanTA-200F) and transmission electron microscopy (TEM, JEOL, JEM-2100F), and high-resolution transmission electron microscopy (HRTEM).

Electrochemical Measurements: The electrochemical performances of as-prepared electrodes were investigated with CV, GCD, and EIS measurements using a CHI 660E electrochemical workstation (Chenhua, Shanghai) in a three-electrode configuration for single electrode and in a two-electrode configuration for asymmetric supercapacitor devices, respectively. In half-cell test system, 2 M KOH was used as the electrolyte, while in the quasi-solid-state supercapacitor system, PVA/KOH gel electrolyte was used.

Calculations: Three-electrode configuration For the CV curves, gravimetric-specific capacitance C (F g-1) of electrode materials was calculated by integrating the discharge portion using the following equation:


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C= Where I is current (A), v is the potential scan rate (mV s-1), m is mass of active material (mg), and V is the potential window (V).

The specific capacitance (Cs) can be also calculated by integrating the area under the GCD curve by the following equation:

= Where I (A) is the discharge current, m (g) represents the mass of the active material, and the value of dV (V)/dt (s) indicates the slope of the discharge curve in the GCD measurement.

Two-electrode configuration (device measurements) Gravimetric capacitance (Cg)

= Where I (A) is the discharge current, v is the potential scan rate (mV s-1), M is mass of active material in both electrodes (mg), and V is the potential window (V).

Gravimetric energy density (Eg),

= Gravimetric power density (Pg),

= where Δt is the discharge time.


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ASSOCIATED CONTENT The authors declare no competing financial interest. Supporting Information Structure and morphology characterizations of ZnO, ZnO@Ni, ZnO/Ni@Al-Co-S, CC/ZnO@Ni nanorod arrays, CC/Al-Co-S, CC/ZnO@Al-Co-S, and graphene/CNTs film electrodes. Electrochemical performance of the multilayer graphene/CNTs electrode, CC/H-Ni@Al-Co-S electrode, and corresponding ASC devices. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions J.H. and J.W. contributed equally to this work.

ACKNOWLEDGMENTS This work is financially supported by the National Science Fund for Distinguished Young Scholars (51425304), the NSFC-DFG Joint Research Project (51761135114), the National Natural Science Foundation of China (21704038, 51763018), the Natural Science Foundation of Jiangxi Province (20171ACB21009), and the National Postdoctoral Program for Innovative Talents (BX201700112).


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Electrodes for Hydrogen Evolution Reaction in Alkaline Solution. Nano Energy 2016, 24, 139-147. (53) Surendran, S.; Selvan, R. K. Growth and Characterization of 3D Flower-Like β-NiS on Carbon Cloth: A Dexterous and Flexible Multifunctional Electrode for Supercapattery and Water-Splitting Applications. Adv. Mater. Interfaces 2017, 4, 1701056. (54) Zhang, C. Y.; Cai, X. Y.; Qian, Y.; Jiang, H. F.; Zhou, L. J.; Li, B. S.; Lai, L. F.; Shen, Z. X.; Huang, W. Electrochemically Synthesis of Nickel Cobalt Sulfide for High-Performance Flexible Asymmetric Supercapacitors. Adv. Sci. 2017, 4, 1700375. (55) Li, C.; Balamurugan, J.; Kim, N. H.; Lee, J. H. Hierarchical Zn-Co-S Nanowires as Advanced Electrodes for All Solid State Asymmetric Supercapacitors. Adv. Energy Mater. 2017, 7, 1702014. (56) Qi, J. Q.; Chang, Y.; Sui, Y. W.; He, Y. Z.; Meng, Q. K.; Wei, F. X.; Ren, Y. J.; Jin, Y. X. Facile Synthesis of Ag-Decorated Ni3S2 Nanosheets with 3D Bush Structure Grown on rGO and Its Application as Positive Electrode Material in Asymmetric Supercapacitor. Adv. Mater. Interfaces 2017, 4, 1700985. (57) Chen, W.; Xia, C.; Alshareef, H. N. One-Step Electrodeposited Nickel Cobalt Sulfide Nanosheet Arrays for High-Performance Asymmetric Supercapacitors. ACS Nano 2014, 8, 9531-9541. (58) Shen, L. F.; Wang, J.; Xu, G. Y.; Li, H. S.; Dou, H.; Zhang, X. G. NiCo2S4 Nanosheets Grown on Nitrogen-Doped Carbon Foams as an Advanced Electrode for Supercapacitors. Adv. Energy Mater. 2015, 5, 1400977. (59) Moosavifard, S. E.; Fani, S. Rahmanian, M. Hierarchical CuCo2S4 Hollow Nanoneedle Arrays as Novel Binder-Free Electrodes for High-Performance Asymmetric Supercapacitors. Chem. Commun. 2016, 52, 4517-4520. (60) Hu, W.; Chen, R. Q.; Xie, W.; Zou, L. L.; Qin, N.; Bao, D. H. CoNi2S4 Nanosheet Arrays Supported on Nickel Foams with Ultrahigh Capacitance for Aqueous Asymmetric Supercapacitor Applications. ACS Appl. Mater. Interfaces 2014, 6, 19318-19326.


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ACS Nano

Ni Coating

ZnO NRAs Growth

CC

CC/ZnO@Ni NRAs

CC/ZnO NRAs

ZnO Etching

Al-Co-S NSAs Growth

CC/H-Ni@Al-Co-S NSAs

CC/Ni NTAs

Figure 1. Schematic illustration of the fabrication of hierarchical core-branch CC/H-Ni@Al-Co-S nanosheets electrode.


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Figure 2. (a) SEM images of CC/ZnO nanorod arrays. (b) TEM image of a single ZnO nanorod. (c) SEM image of CC/Ni nanotube arrays. (d) TEM image of Ni nanotube. (e) EDS element mapping images of CC/Ni nanotube arrays.


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ACS Nano

Figure 3. (a, b) Low and (c) high magnification SEM images of CC/H-Ni@Al-Co-S nanosheets electrode, (d, e) TEM images of Ni nanotube@Al-doping cobalt sulfide nanosheets, (f) TEM image of Al-doped cobalt sulfide nanosheets, (g) EDS mapping of CC/H-Ni@Al-Co-S nanosheets electrode, (h) XRD patterns of CC/ZnO, CC/Ni NTAs and CC/H-Ni@Al-Co-S.


ACS Nano

(a) 0.08

10 mV s

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4

3

-1

-Z'' (ohm)

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 0

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Figure 4. (a) CV curves of CC/Co-S, CC/H-Ni@Co-S, CC/Al-Co-S and CC/H-Ni@Al-Co-S electrodes at a scan rate of 10 mV s-1. (b) EIS spectra of CC/Co-S, CC/H-Ni@Co-S, CC/Al-Co-S and CC/H-Ni@Al-Co-S. (c) GCD curves of the CC/Co-S, CC/H-Ni@Co-S, CC/Al-Co-S and CC/H-Ni@Al-Co-S electrodes at a current density of 1 A g-1. (d) The specific capacitances of different electrodes calculated from CV and GCD curves.


(a)2600 -1


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-1

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Figure 5. (a) The specific capacitances of CC/H-Ni@Al-Co-S electrodes at different current densities versus the Al atomic percentages. (b) CV and (c) GCD curves of CC/H-Ni@Al-Co-S electrode at different scan rates. (d) Cycling stability of CC/H-Ni@Al-Co-S electrode at consecutively various current densities. (e) The long-term cycling performance of CC/H-Ni@Al-Co-S electrode at a current density of 10 A g-1 for 10000 cycles. (f) Schematic illustration displaying the merits of the core-branch CC/H-Ni@Al-Co-S electrode for energy storage.


ACS Nano

-1

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0.0

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Figure 6. (a) CV curves of the as-assembled ASCs measured at different operating voltages at a constant scan rate of 10 mV s-1. (b) GCD curves of the ASCs collected over different voltages (from 1.0 to 1.8 V) at a current density of 20 A g-1. (c) CV curves of ASCs at different scan rates from 10 to 1000 mV s-1. (d) GCD curves of ASCs at different current densities from 1 to 40 A g-1. (e) CV curves of ASCs under different temperature. (f) The stability of ASCs at different temperature states.


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ACS Nano

Figure 7. (a) Specific capacitance of ASCs calculated from CV and GCD curves. (b) The Ragone plot related to energy and power densities of the ASCs compared with literature results. (c) Cycling performance of ASCs at a constant current density of 5 A g-1. (d) CV curves of the flexible ASC under different bending conditions. The insets of (d) show the photographic images of the device under different bending states. The fully charged solitary ASC operates (e) a red LED and (f) an electric motor fan, demonstrating its potential suitability for wearable electronic applications.


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When Al-Doped Cobalt Sulfide Nanosheets Meet Nickel Nanotube

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