When Al-Doped Cobalt Sulfide Nanosheets Meet ... - ACS Publications

Feb 20, 2018 - College of Chemistry, Nanchang University, 999 Xuefu Avenue, ... of New Energy Chemistry/Institute of Polymers, Nanchang University, 99...
0 downloads 0 Views 9MB Size
Download PDF
www.acsnano.org

ACS Nano 2018.12:3030-3041. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/23/18. For personal use only.

When Al-Doped Cobalt Sulfide Nanosheets Meet Nickel Nanotube Arrays: A Highly Efficient and Stable Cathode for Asymmetric Supercapacitors Jun Huang,†,§ Junchao Wei,†,§,‡ Yingbo Xiao,† Yazhou Xu,† Yujuan Xiao,† Ying Wang,† Licheng Tan,† Kai Yuan,*,†,‡ and Yiwang Chen*,†,‡ †

College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China Jiangxi Provincial Key Laboratory of New Energy Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

‡

S Supporting Information *

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 combined with carbon cloth (denoted as CC/H-Ni@Al-Co-S) as an excellent self-standing cathode for asymmetric supercapacitors (ASCs). The combination of structural and compositional advantages endows the CC/H-Ni@Al-Co-S electrode with 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 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/CNT 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 10 000 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. KEYWORDS: carbon cloth, Ni nanotubes, Al-doped cobalt sulfide, self-standing, all-solid-state supercapacitors

T

dous efforts have been devoted to synthesizing high-capacitive cathode and anode materials. Compared with carbon-based materials, 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 made to explore cathode materials to achieve larger specific capacitance and good rate performance.12,13

he fast-growing use of electronic devices put forward a 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 the equation E = 1/2CV2, 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 Tremen© 2018 American Chemical Society

Received: February 3, 2018 Accepted: February 20, 2018 Published: February 20, 2018 3030

DOI: 10.1021/acsnano.8b00901 ACS Nano 2018, 12, 3030−3041

Article

Cite This: ACS Nano 2018, 12, 3030−3041

Article

ACS Nano

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

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 strategy for directly integrating active materials with current collectors is widely adopted to make an integrated electrode to avoid a “dead surface”. Specifically, well-aligned nanotubes array (NTA) 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 the electron/ion diffusion path to facilitate the reaction kinetics of electrodes for enhanced electrochemical performance.46,47 Based on the above consideration, herein, an 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 Aldoped cobalt sulfide nanosheets (CC/H-Ni@Al-Co-S) for a 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 a 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 10 000 cycles). Besides, the different temperature condition tests show that the ASC devices can work efficiently under 30 to 80 °C for further practical application.

Recently, electrochemically active transition metal sulfides (TMSs), especially cobalt sulfide (Co9S8, CoS2, Co3S4, etc.), have received tremendous research interest as promising cathodes for ASCs. The TMSs 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 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, carbon nanotubes (CNTs), and graphene to form TMS/carbon composites to achieve better performance.24−27 Although enhanced capacitance could be achieved through the assistance of 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 reactions.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-ion-doped transition metal oxides could improve the electric conductivity due to the oxygen vacancy generated.35,36 Instead, few works on the synthesis or electrochemical mechanisms of metal-ion-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 restrict 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 nanosheet based electrodes with both high electron and ion transport capabilities for high-performance

RESULTS AND DISCUSSION Morphology and Structure Characterization of CC/HNi@Al-Co-S. The 3D hierarchical structure of the CC/H-Ni@ Al-Co-S electrode was fabricated via a template-assisted method as schematically illustrated in Figure 1. A commercial woven carbon cloth (CC) with a smooth surface served as a conductive substrate (Figure S1). The aligned and ordered 3031

DOI: 10.1021/acsnano.8b00901 ACS Nano 2018, 12, 3030−3041

Article

ACS Nano

Figure 2. (a) SEM image 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 a Ni nanotube. (e) EDS element mapping images of CC/Ni nanotube arrays.

Figure 3. (a, b) Low- and (c) high-magnification SEM images of the CC/H-Ni@Al-Co-S nanosheet 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 the CC/H-Ni@ Al-Co-S nanosheet electrode; (h) XRD patterns of CC/ZnO, CC/Ni NTAs, and CC/H-Ni@Al-Co-S.

ZnO nanorod arrays (ZnO NRAs) were grown vertically on the surface of the 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). 3032

DOI: 10.1021/acsnano.8b00901 ACS Nano 2018, 12, 3030−3041

Article

ACS Nano

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) Specific capacitances of different electrodes calculated from CV and GCD curves.

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. The TEM images of a single Ni nanotube/Al-Co-S also confirm the hollow structure and that Al-Co-S nanosheets are uniformly coated on the surface of the 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 the bulk structure, as shown in Figure S4. The TEM image of Al-Co-S nanosheets exhibits 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 that 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 the CC, indicating the greater mass loading of the 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 an enhanced electrochemical performance. The prepared core−branch structure provides numerous interspaces for rapid electrolyte ion accessibility, shortens the ion diffusion paths, and facilitates more redox reactions, which simultaneously improves the energy storage performance. In contrast, inhomogeneous and low mass loading of nanosheets are found around the ZnO nanorods without the assistance of a Ni layer (Figure S6).

Then the ZnO NRAs were 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 the ZnO nanorods (Figure S3). The ZnO NRAs serve as sacrificial templates and could be dissolved in aqueous ammonia, resulting in hollow Ni nanotube arrays. The Ni NTAs have a shrinking surface (Figure 2c), and the thickness of the 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-reserving transformation, the final structure can also be referred to as an array. Finally, the shell of Al-doped cobalt sulfide nanosheet arrays was deposited on the exterior surface though an electrodeposition strategy to obtain the desired CC/H-Ni@ Al-Co-S electrode (for a detailed synthesis procedure, see the Experimental Section). As shown in Figure 3a−c, the typical SEM images of CC/HNi@Al-Co-S indicate that the well-preserved array structure and interconnected Al-doped cobalt sulfide nanosheets were anchored onto the entire Ni NTA surface. The Al-doped cobalt sulfide nanosheets with high uniformity in thickness were homogeneously distributed along the Ni NTAs, forming threedimensional hierarchical core−branch heterostructures. The low-magnification SEM images of CC/H-Ni@Al-Co-S show that our method can uniformly prepare in large scale the hybrid electrode with aligned H-Ni@Al-Co-S heterostructures of ca. 8 μm in length (Figure S5). Such a hierarchical and cross3033

DOI: 10.1021/acsnano.8b00901 ACS Nano 2018, 12, 3030−3041

Article

ACS Nano

Figure 5. (a) 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 the CC/H-Ni@Al-Co-S electrode at different scan rates. (d) Cycling stability of the CC/H-Ni@Al-Co-S electrode at consecutively various current densities. (e) Long-term cycling performance of the CC/H-Ni@Al-Co-S electrode at a current density of 10 A g−1 for 10 000 cycles. (f) Schematic illustration displaying the merits of the core−branch CC/H-Ni@Al-Co-S electrode for energy storage.

Besides, the CC/Al-Co-S electrode is also prepared via electrodeposition with the same conditions. As shown in Figure S7, the dense Al-doped cobalt sulfide nanosheet on the CC skeleton has a greater 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, the characteristic peaks of the 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@AlCo-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 characteristic of amorphous carbon.49 Furthermore, XPS analysis was carried out to understand the electronic structure of the as-prepared sample with Al doping. 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, matching well with the 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 metal sulfides and consistent with cobalt sulfides.52 The S 2p peak observed at 168.2 eV is attributed to surface sulfur with a high

oxide state of sulfates owing to partial oxidation of cobalt sulfide. The presence of the 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 performances of the CC/H-Ni@Al-Co-S and reference samples were first measured in a three-electrode cell with 2 M KOH as the 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, CC/ Co-S, CC/H-Ni@Co-S, and 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 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 the active material and substrate could decrease the contact resistance between them, leading 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 conditions, as confirmed by the SEM images (Figure S5), thus resulting in boosted capacitance performance. The Al doping 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 electric double-layer capacitive (EDLC) and redox reactions to substantially enhance 3034

DOI: 10.1021/acsnano.8b00901 ACS Nano 2018, 12, 3030−3041

Article

ACS Nano

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 nanosheet 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 the electrolyte into the dense center of the 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 nanosheet 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 10 000 cycles (Figure 5e), indicating the excellent cycling stability of the CC/H-Ni@Al-Co-S electrode. Impressively, after 10 000 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 of the core−branch CC/ H-Ni@Al-Co-S electrode. The enhanced specific capacity, rate capability, and cycling stability of the core−branch CC/H-Ni@Al-Co-S nanosheet electrode are mainly attributed to its synergic features of Ni nanotubes, Al doping, 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 the 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 the CC could reduce the interface resistance gap between the substrate and active materials and act as an electron “superhighway” to promote the ion/electron transfer rate. Third, the Al doping could improve the conductivity and electrochemical activity of cobalt sulfide, resulting in optimized utilization of cobalt sulfide nanosheets for an 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 for 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

the specific capacitance. The prepared core−branch CC/HNi@Al-Co-S electrode provides numerous interspaces for rapid electrolyte ion accessibility, shortens the ion diffusion paths, and facilitates 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 in the low-frequency region, suggesting the lower diffusion resistance (Rw) and charge transfer resistance (Rct). The low Rw and Rct benefit efficient ion diffusion of the 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 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 enables a faster charge transfer rate on the surface of the electrode. Therefore, the hybrid core−branch nanoarchitecture is essential for the electron collection and fast Faradaic reactions. Electrochemical impedance spectrometry (EIS) analytical results further verify that the 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 the discharging time for CC/H-Ni@AlCo-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 that of 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 doping. 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 a superior rate capability. The 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, with a retention of 72.3%, when the current density is increased to 100 A g−1. The structural and compositional advantages of the CC/ H-Ni@Al-Co-S electrode are responsible for the enhanced capacitance and redox reaction kinetics. To meet the requirement for real applications, the capacity based on the total mass of the electrode was 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 percentages were measured in a three-electrode cell. As shown in Figure 5a, at a constant current density, increasing the Al atomic percentage leads to an initial increase of the specific capacitance and then a rapid decrease. The electric conductivity of cobalt sulfide could be improved when doped with an appropriate amount of Al. The specific capacitances are all maximal at the Al atomic percentage of 2 at. % at different current densities. The reduction of the specific capacitance of 3035

DOI: 10.1021/acsnano.8b00901 ACS Nano 2018, 12, 3030−3041

Article

ACS Nano

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 at different temperatures. (f) Stability of ASCs at different temperature states.

the specific capacitance of the graphene/CNT 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/CNT film electrode shows outstanding electrochemical stability after 10 000 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 an anode for ASCs. The mass loading of the cathode and anode was balanced before assembling the full cell devices, and the CV curves under their corresponding voltage windows at a scan rate of 50 mV s−1 are 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 H2/O2 evolution in the CV plots, and 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 a 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 the three-electrode system, the CV curves of

exceptional electrochemical properties indicate that the CC/HNi@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 asobtained CC/H-Ni@Al-Co-S electrode for practical applications, asymmetric supercapacitors were further assembled by utilizing the CC/H-Ni@Al-Co-S electrode as the cathode and the flexible multilayer graphene/CNT film as the anode with a polyvinyl alcohol (PVA)−KOH gel electrolyte. Graphenebased materials with high electrical conductivity, good mechanical strength, large specific surface area, and superior electrochemical stability are expected to be promising candidates that can satisfy high-energy storage requirements for SCs. To prevent the restacking of graphene, onedimensional 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/CNT film exhibits a wrinkled characteristic. The cross-sectional SEM image shows a layer-by-layer structure with CNTs bridging the graphene film. Such a structure could allow electrolyte ions to fully access the whole film electrode, thus being ideal for supercapacitor performance. The electrochemical performance of the graphene/CNT film electrode was investigated in a three-electrode cell with 2 M KOH as the electrolyte. It is shown that the graphene/CNT 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, 3036

DOI: 10.1021/acsnano.8b00901 ACS Nano 2018, 12, 3030−3041

Article

ACS Nano

Figure 7. (a) Specific capacitance of ASCs calculated from CV and GCD curves. (b) 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.

kg−1 at a power density of 816 W kg−1, respectively (Figures 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 10 000 cycles, suggesting the prominent energy storage capability of the solid-state device. To demonstrate the potential applications of the asassembled 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 highperformance wearable energy storage devices.

the fabricated ASC exhibited an excellent capacitive behavior due to the inclusion of an EDLC material. With increasing the scan rate, as shown in the 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 an almost symmetrical property, indicating good capacitive behavior of the ASC device. Besides, CV curves under different temperatures at 10 mV s−1 (Figure 6e) show that polarization becomes more and more obvious with the increase of the test temperature, and the area of the CV curve increased by 40% when the test temperature increased 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 at 1 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 the active material on both electrodes, the ASC in this work delivered an energy density of 65.7 W h kg−1 at a 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 to or even higher than those of state-of-the-art cobalt-sulfide-based supercapacitors.54−60 We have also calculated the device performance based on the total mass of both electrodes and the entire mass of the device (including separator 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

CONCLUSION In summary, we have demonstrated the rational design and fabrication of a CC/H-Ni@Al-Co-S electrode with threedimensional 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 Aldoped cobalt sulfide nanosheets as well as the core−branch 3037

DOI: 10.1021/acsnano.8b00901 ACS Nano 2018, 12, 3030−3041

Article

ACS Nano

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/CNT 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 mixed with 45 mL of deionized water, and then the resulting 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 loadings of the cathode and anode were balanced according to the following equation:

hierarchical architecture, an improved electrochemical performance was achieved for CC/H-Ni@Al-Co-S. Therefore, the asfabricated CC/H-Ni@Al-Co-S electrode exhibits an ultrahigh specific capacitance (1830 F g−1 at 5 mV s−1), excellent rate capability (1047 F g−1 at 1000 mV s−1), and electrochemical stability, which can be attributed to the high 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 a multilayer graphene/CNT 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 a 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 a hierarchical core−branch heterostructure electrode with superior electrochemical properties provides a favorable strategy to synthesize high-performance electrode materials for next-generation flexible energy storage devices.

m+ C ΔV = s− − m− Cs +ΔV+ 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 (FEI, QuanTA-200F), TEM (JEOL, JEM-2100F), and 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 a single electrode and in a two-electrode configuration for asymmetric supercapacitor devices, respectively. In a 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. For a three-electrode configuration, for the CV curves, gravimetric specific capacitance C (F g−1) of the electrode materials was calculated by integrating the discharge portion using the following equation:

EXPERIMENTAL SECTION Synthesis of CC/ZnO Nanorod Arrays. ZnO NRAs were grown on a CC (3 × 5 cm2) by a facile hydrothermal method. Briefly, the carbon cloth was first wetted and cleaned with acetone and water, respectively. Then, the CC was dipped into a precursor solution (70 mL) containing 0.015 M Zn(NO3)2, 0.015 M hexamethylenetetramine (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 NRA 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 were first fabricated via the electrodeposition of a 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 were then immersed in 2.5 wt % aqueous ammonia for 3 h to remove ZnO completely to obtain the CC/Ni nanotube arrays. Synthesis of the 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), a platinum plate, and Ag/AgCl as working electrode, counter 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 three cycles within a voltage range of −1.2 to 0.2 V vs Ag/AgCl. The electrodeposited CC/Ni NTAs were 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 nanosheet electrode is about 1.5 mg cm−2. Preparation of Multilayer Graphene/CNT 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 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 CNT solution (1 mg mL−1) was dispersed in water followed by ultrasonication at 160 W for 1 h. Then, the GO/CNTs film was fabricated by pouring the GO and CNT suspension step by step into Teflon dishes and evaporation of the water under appropriate heating (60 °C) for film formation until drying. After peeling from the substrate, a free-standing, flexible, dark brown GO/CNT film was obtained. Finally, the GO/CNT film was annealed at high temperature (1000 °C) for 1 h under argon flow. The multilayer graphene/CNT film was used as an anode for the asymmetric supercapacitor.

C=

∫ I dV vmV

where I is current (A), v is the potential scan rate (mV s−1), m is the mass of the active material (mg), and V is the potential window (V). The specific capacitance (Cs) can also be calculated by integrating the area under the GCD curve by the following equation:

Cs =

I m dV /dt

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. For a two-electrode configuration (device measurements), the gravimetric capacitance (Cg) is calculated as

Cg =

∫ I dV vMV

where I (A) is the discharge current, v is the potential scan rate (mV s−1), M is the mass of the active material in both electrodes (mg), and V is the potential window (V). Gravimetric energy density (Eg) is calculated as

Eg =

∫ IV dt M

Gravimetric power density (Pg) is calculated as Pg =

Eg Δt

where Δt is the discharge time. 3038

DOI: 10.1021/acsnano.8b00901 ACS Nano 2018, 12, 3030−3041

Article

ACS Nano

Energy Storage Performance in Redox-Additive Electrolyte. J. Mater. Chem. A 2017, 5, 2224−2234. (10) Zhu, M. S.; Huang, Y.; Huang, Y.; Pei, Z. X.; Xue, Q.; Li, H. F.; Geng, H. Y.; Zhi, C. Y. Capacitance Enhancement in a Semiconductor Nanostructure-Based Supercapacitor by Solar Light and a SelfPowered Supercapacitor-Photodetector System. Adv. Funct. Mater. 2016, 26, 4481−4490. (11) Chen, H.; Hu, L. F.; Yan, Y.; Che, R. C.; Chen, M.; Wu, L. M. One-Step Fabrication of Ultrathin Porous Nickel Hydroxide Manganese Dioxide Hybrid Nanosheets for Supercapacitor Electrodes with Excellent Capacitive Performance. Adv. Energy Mater. 2013, 3, 1636. (12) Li, Y.; Xu, J.; Feng, T.; Yao, Q. F.; Xie, J. P.; Xia, H. Fe2O3 Nanoneedles on Ultrafine Nickel Nanotube Arrays as Efficient Anode for High-Performance Asymmetric Supercapacitors. Adv. Funct. Mater. 2017, 27, 1606728. (13) Huang, J.; Wei, J. C.; Xu, Y. Z.; Xiao, Y. B.; Chen, Y. W. A Pinecone-Inspired Hierarchical Vertically Aligned Nanosheet Array Electrode for High-Performance Asymmetric Supercapacitors. J. Mater. Chem. A 2017, 5, 23349−23360. (14) Zhou, W. J.; Cao, X. H.; Zeng, Z. Y.; Shi, W. H.; Zhu, Y. Y.; Yan, Q. Y.; Liu, H.; Wang, J. Y.; Zhang, H. One-Step Synthesis of Ni3S2 Nanorod@Ni(OH)2 Nanosheet Core-Shell Nanostructures on a Three-Dimensional Graphene Network for High-Performance Supercapacitors. Energy Environ. Sci. 2013, 6, 2216−2221. (15) Jiang, X. C.; Xie, Y.; Lu, J.; Zhu, L. Y.; He, W.; Qian, Y. T. Synthesis of Novel Nickel Sulfide Layer-Rolled Structures. Adv. Mater. 2001, 13, 1278−1281. (16) Kung, C. W.; Chen, H. W.; Lin, C. Y.; Huang, K. C.; Vittal, R.; Ho, K. C. CoS Acicular Nanorod Arrays for the Counter Electrode of an Efficient Dye-Sensitized Solar Cell. ACS Nano 2012, 6, 7016−7025. (17) Tang, S. C.; Zhu, B. G.; Shi, X. L.; Wu, J.; Meng, X. K. General Controlled Sulfidation toward Achieving Novel Nanosheet-Built Porous Square-FeCo2S4-Tube Arrays for High-Performance Asymmetric All-Solid-State Pseudocapacitors. Adv. Energy Mater. 2017, 7, 1601985. (18) Peng, S. J.; Li, L. L.; Li, C. C.; Tan, H. T.; Cai, R.; Yu, H.; Mhaisalkar, S.; Srinivasan, M.; Ramakrishna, S.; Yan, Q. Y. In situ Growth of NiCo2S4 Nanosheets on Graphene for High-Performance Supercapacitors. Chem. Commun. 2013, 49, 10178−10180. (19) Xiao, J. W.; Wan, L.; Yang, S. H.; Xiao, F.; Wang, S. Design Hierarchical Electrodes with Highly Conductive NiCo2S4 Nanotube Arrays Grown on Carbon Fiber Paper for High Performance Pseudocapacitors. Nano Lett. 2014, 14, 831−838. (20) Yu, L.; Zhang, L.; Wu, H. B.; Lou, X. W. Formation of NixCo3xS4 Hollow Nanoprisms with Enhanced Pseudocapacitive Properties. Angew. Chem. 2014, 126, 3785−3788. (21) Chen, H. C.; Jiang, J. J.; Zhang, L.; Xia, D. D.; Zhao, Y. D.; Guo, D. Q.; Qi, T.; Wan, H. Z. In situ growth of NiCo2S4 Nanotube Arrays on Ni Foam for Supercapacitors: Maximizing Utilization Efficiency at High Mass Loading to Achieve Ultrahigh Areal Pseudocapacitance. J. Power Sources 2014, 254, 249−257. (22) Lu, F.; Zhou, M.; Li, W. R.; Weng, Q. H.; Li, C. L.; Xue, Y. M.; Jiang, X. F.; Zeng, X. H.; Bando, Y.; Golberg, D. Engineering Sulfur Vacancies and Impurities in NiCo2S4 Nanostructures toward Optimal Supercapacitive Performance. Nano Energy 2016, 26, 313−323. (23) Zhang, G. Q.; Lou, X. W. General Solution Growth of Mesoporous NiCo2O4 Nanosheets on Various Conductive Substrates as High-Performance Electrodes for Supercapacitors. Adv. Mater. 2013, 25, 976−979. (24) Guo, Y.; Yu, L.; Wang, C.-Y.; Lin, Z.; Lou, X. W. Hierarchical Tubular Structures Composed of Mn-Based Mixed Metal Oxide Nanoflakes with Enhanced Electrochemical Properties. Adv. Funct. Mater. 2015, 25, 5184−5189. (25) Wu, J. B.; Gao, X.; Yu, H. M.; Ding, T. P.; Yan, Y. X.; Yao, B.; Xu, Y.; Chen, D. C.; Liu, M. L.; Huang, L. A Scalable Free-Standing V2O5/CNT Film Electrode for Supercapacitors with a Wide Operation Voltage (1.6 V) in an Aqueous Electrolyte. Adv. Funct. Mater. 2016, 26, 6114−6120.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b00901. 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@AlCo-S electrode, and corresponding ASC devices (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Kai Yuan: 0000-0002-4507-1510 Yiwang Chen: 0000-0003-4709-7623 Author Contributions §

J. Huang and J. Wei contributed equally to this work.

Notes

The authors declare no competing financial interest.

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). REFERENCES (1) Maher, F. E.; Veronica, S.; Sergey, D.; Richard, B. K. Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors. Science 2012, 335, 1326−1330. (2) Yu, Z. N.; Tetard, L.; Zhai, L.; Thomas, J. Supercapacitor Electrode Materials: Nanostructures from 0 to 3 Dimensions. Energy Environ. Sci. 2015, 8, 702−730. (3) Yuan, K.; Xu, Y. Z.; Uihlein, L.; Brunklaus, G.; Shi, L.; Heiderhoff, R.; Que, M. M.; Forster, M.; Chasse, T.; Pichler, T.; Riedl, T.; Chen, Y. W.; Scherf, U. Straightforward Generation of Pillared, Microporous Graphene Frameworks for Use in Supercapacitors. Adv. Mater. 2015, 27, 6714−6721. (4) Shen, L. F.; Yu, L.; Wu, H. B.; Yu, X.-Y.; Zhang, X. G.; Lou, X. W. Formation of Nickel Cobalt Sulfide Ball-In-Ball Hollow Spheres with Enhanced Electrochemical Pseudocapacitive Properties. Nat. Commun. 2015, 6, 6694−6702. (5) Chmiola, J.; Largeot, C.; Taberna, P.-L.; Simon, P.; Gogotsi, Y. Monolithic Carbide-Derived Carbon Films for Micro-Supercapacitors. Science 2010, 328, 480−483. (6) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845−854. (7) Liu, C.; Li, F.; Ma, L.-P.; Cheng, H.-M. Advanced Materials for Energy Storage. Adv. Mater. 2010, 22, E28−E62. (8) Nagaraju, G.; Sekhar, S. C.; Rama Raju, G. S.; Bharat, L. K.; Yu, J. S. Designed Construction of Yolk-Shell Structured Trimanganese Tetraoxide Nanospheres via Polar Solvent-Assisted Etching and Biomass-Derived Activated Porous Carbon Materials for High Performance Asymmetric Supercapacitors. J. Mater. Chem. A 2017, 5, 15808−15821. (9) Cha, S. M.; Nagaraju, G.; Chandra Sekhar, S.; Yu, J. S. A Facile Drop-Casting Approach to Nanostructured Copper Oxide-Painted Conductive Woven Textile as Binder-Free Electrode for Improved 3039

DOI: 10.1021/acsnano.8b00901 ACS Nano 2018, 12, 3030−3041

Article

ACS Nano (26) Ling, Z.; Wang, Z. Y.; Zhang, M. D.; Yu, C.; Wang, G.; Dong, Y. F.; Liu, S. H.; Wang, Y. W.; Qiu, J. S. Sustainable Synthesis and Assembly of Biomass-Derived B/N Co-Doped Carbon Nanosheets with Ultrahigh Aspect Ratio for High-Performance Supercapacitors. Adv. Funct. Mater. 2016, 26, 111−119. (27) Yang, J.; Yu, C.; Fan, X. M.; Zhao, C. T.; Qiu, J. H. Ultrafast Self-Assembly of Graphene Oxide-Induced Monolithic NiCo-Carbonate Hydroxide Nanowire Architectures with a Superior Volumetric Capacitance for Supercapacitors. Adv. Funct. Mater. 2015, 25, 2109− 2116. (28) Sun, S. N.; Nie, Y.; Sun, M. C.; Liang, T.; Sun, M. F.; Yang, H. X. Facile Synthesis of CoNi2S4 One-Dimensional Nanorods as Anode for High Performance Lithium Ion Batteries. Mater. Lett. 2016, 176, 87−90. (29) Guan, B. Y.; Yu, Y.; Wang, X.; Song, S. Y.; Lou, X. W. Formation of Onion-Like NiCo2S4 Particles via Sequential Ion-Exchange for Hybrid Supercapacitors. Adv. Mater. 2017, 29, 1605051. (30) Guo, M.; Balamurugan, J.; Thanh, T. D.; Kim, N. H.; Lee, J. H. Facile Fabrication of Co2CuS4 Nanoparticle Anchored N-Doped Graphene for High-Performance Asymmetric Supercapacitors. J. Mater. Chem. A 2016, 4, 17560−17571. (31) Li, H.; Su, Y.; Sun, W. W.; Wang, Y. Carbon Nanotubes Rooted in Porous Ternary Metal Sulfide@N/S-Doped Carbon Dodecahedron: Bimetal-Organic-Frameworks Derivation and Electrochemical Application for High-Capacity and Long-Life Lithium-Ion Batteries. Adv. Funct. Mater. 2016, 26, 8345−8353. (32) Chen, Y. M.; Li, Z.; Lou, X. W. General Formation of MxCo3‑xS4 (M = Ni, Mn, Zn) Hollow Tubular Structures for Hybrid Supercapacitors. Angew. Chem. 2015, 127, 10667−10670. (33) Chen, L. F.; Yu, Z. Y.; Wang, J. J.; Li, Q. X.; Tan, Z. Q.; Zhu, Y. W.; Yu, S. H. Metal-Like Fluorine-Doped β-FeOOH Nanorods Grown on Carbon Cloth for Scalable High-Performance Supercapacitors. Nano Energy 2015, 11, 119−128. (34) Kang, J. L.; Hirata, A.; Kang, L. J.; Zhang, X. M.; Hou, Y.; Chen, L. Y.; Li, C.; Fujita, T.; Akagi, K.; Chen, M. W. Enhanced Supercapacitor Performance of MnO2 by Atomic Doping. Angew. Chem. 2013, 125, 1708−1711. (35) Li, S.; Qiu, J. X.; Lai, C.; Ling, M.; Zhao, H. J.; Zhang, S. Q. Surface Capacitive Contributions: Towards High Rate Anode Materials for Sodium Ion Batteries. Nano Energy 2015, 12, 224−230. (36) Ruan, Y. J.; Zha, D. C.; Lv, L.; Zhang, B.; Liu, J.; Ji, X.; Wang, C. D.; Jiang, J. J. Al-doped b-NiS Mesoporous Nanoflowers for HybridType Electrodes toward Enhanced Electrochemical Performance. Electrochim. Acta 2017, 236, 307−318. (37) Zhao, M. T.; Wang, Y. X.; Ma, Q. L.; Huang, Y.; Zhang, X.; Ping, J. F.; Zhang, Z. C.; Lu, Q. P.; Yu, Y. F.; Xu, H.; Zhao, Y. L.; Zhang, H. Ultrathin 2D Metal-Organic Framework Nanosheets. Adv. Mater. 2015, 27, 7372−7378. (38) Chen, J. S.; Guan, C.; Gui, Y.; Blackwood, D. J. Rational Design of Self-Supported Ni3S2 Nanosheets Array for Advanced Asymmetric Supercapacitor with a Superior Energy Density. ACS Appl. Mater. Interfaces 2017, 9, 496−504. (39) Zhu, T.; Wu, H. B.; Wang, Y. B.; Xu, R.; Lou, X. W. Formation of 1D Hierarchical Structures Composed of Ni3S2 Nanosheets on CNTs Backbone for Supercapacitors and Photocatalytic H 2 Production. Adv. Energy Mater. 2012, 2, 1497−1502. (40) Xie, X. Q.; Ao, Z. M.; Su, D. W.; Zhang, J. Q.; Wang, G. X. MoS2/Graphene Composite Anodes with Enhanced Performance for Sodium-Ion Batteries: The Role of the Two-Dimensional Heterointerface. Adv. Funct. Mater. 2015, 25, 1393−1403. (41) Zhang, G. Q.; Lou, X. W. General Solution Growth of Mesoporous NiCo2O4 Nanosheets on Various Conductive Substrates as High-Performance Electrodes for Supercapacitors. Adv. Mater. 2013, 25, 976−979. (42) Wang, K.; Zhang, X.; Li, C.; Sun, X. Z.; Meng, Q. H.; Ma, Y. W.; Wei, Z. X. Chemically Crosslinked Hydrogel Film Leads to Integrated Flexible Supercapacitors with Superior Performance. Adv. Mater. 2015, 27, 7451−7457.

(43) Xiao, J. W.; Wan, L.; Yang, S. H.; Xiao, F.; Wang, S. Design Hierarchical Electrodes with Highly Conductive NiCo2S4 Nanotube Arrays Grown on Carbon Fiber Paper for High Performance Pseudocapacitors. Nano Lett. 2014, 14, 831−838. (44) Yang, P. H.; Ding, Y.; Lin, Z. Y.; Chen, Z. W.; Li, Y. Z.; Qiang, P. F.; Ebrahimi, M.; Mai, W. J.; Wong, C. P.; Wang, Z. L. Low-Cost High-Performance Solid-State Asymmetric Supercapacitors Based on MnO2 Nanowires and Fe2O3 Nanotubes. Nano Lett. 2014, 14, 731− 736. (45) Chen, G. F.; Li, X. X.; Zhang, L. Y.; Li, N.; Ma, T. Y.; Liu, Z. Q. A Porous Perchlorate-Doped Polypyrrole Nanocoating on Nickel Nanotube Arrays for Stable Wide-Potential-Window Supercapacitors. Adv. Mater. 2016, 28, 7680−7687. (46) Xia, X. H.; Zhang, Y. Q.; Fan, Z. X.; Chao, D. L.; Xiong, Q. Q.; Tu, J. P.; Zhang, H.; Fan, H. J. Tubular TiC Fiber Nanostructures as Supercapacitor Electrode Material with Stable Cycling Life and WideTemperature Performance. Energy Environ. Sci. 2015, 8, 1401709. (47) Lv, Q. Y.; Wang, S.; Sun, H. Y.; Luo, J.; Xiao, J.; Xiao, J. W.; Xiao, F.; Wang, S. Solid-State Thin-Film Supercapacitors with Ultrafast Charge/Discharge Based on N-Doped-Carbon-Tubes/Au-Nanoparticles Doped-MnO2 Nanocomposites. Nano Lett. 2016, 16, 40−47. (48) Wang, Q. H.; Jiao, L. F.; Han, Y.; Du, H. W.; Peng, W. X.; Huan, Q. N.; Song, D. W.; Si, Y. C.; Wang, Y. J.; Yuan, H. T. CoS2 Hollow Spheres: Fabrication and Their Application in Lithium-Ion Batteries. J. Phys. Chem. C 2011, 115, 8300−8304. (49) Zhao, J. G.; Yang, L. X.; Li, F. Y.; Yu, R. C.; Jin, C. Q. Structural Evolution in the Graphitization Process of Activated Carbon by HighPressure Sintering. Carbon 2009, 47, 744−751. (50) Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions. Adv. Funct. Mater. 2016, 26, 4661−4672. (51) Wen, Y. X.; Peng, S. L.; Wang, Z. L.; Hao, J. X.; Qin, T. F.; Lu, S. Q.; Zhang, J. C.; He, D. Y.; Fan, X. Y.; Cao, G. Z. Facile Synthesis of Ultrathin NiCo2S4 Nano-Petals Inspired by Blooming Buds for HighPerformance Supercapacitors. J. Mater. Chem. A 2017, 5, 7144−7152. (52) Ma, L. B.; Hu, Y.; Chen, R. P.; Zhu, G. Y.; Chen, T.; Lv, H. L.; Wang, Y. R.; Liang, J.; Liu, H. X.; Yan, C. Z.; Zhu, H. F.; Tie, Z. X.; Jin, Z.; Liu, J. Self-Assembled Ultrathin NiCo2S4 Nanoflakes Grown on Ni Foam as High-Performance Flexible 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 ZnCo-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 2018, 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. 3040

DOI: 10.1021/acsnano.8b00901 ACS Nano 2018, 12, 3030−3041

Article

ACS Nano (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.

3041

DOI: 10.1021/acsnano.8b00901 ACS Nano 2018, 12, 3030−3041