Interlaced Ni-Co LDH nanosheets wrapped Co9S8 nanotube with hierarchical structure toward high performance supercapacitors

Accepted Manuscript Interlaced Ni-Co LDH nanosheets wrapped Co9S8 nanotube with hierarchical structure toward high performance supercapacitors Henan Jia, Zhaoyue Wang, Xiaohang Zheng, Jinghuang Lin, Haoyan Liang, Yifei Cai, Junlei Qi, Jian Cao, Jicai Feng, Weidong Fei PII: DOI: Reference:

S1385-8947(18)31151-3 https://doi.org/10.1016/j.cej.2018.06.113 CEJ 19324

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

4 May 2018 17 June 2018 18 June 2018

Please cite this article as: H. Jia, Z. Wang, X. Zheng, J. Lin, H. Liang, Y. Cai, J. Qi, J. Cao, J. Feng, W. Fei, Interlaced Ni-Co LDH nanosheets wrapped Co9S8 nanotube with hierarchical structure toward high performance supercapacitors, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.06.113

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Interlaced Ni-Co LDH nanosheets wrapped Co9S8 nanotube with hierarchical structure toward high performance supercapacitors Henan Jia†, Zhaoyue Wang†, Xiaohang Zheng*, Jinghuang Lin, Haoyan Liang, Yifei Cai, Junlei Qi*, Jian Cao, Jicai Feng, Weidong Fei* State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China

*Corresponding authors: Tel. /fax: 86-451-86418146; †These authors contributed equally. E-mail: [email protected] (J. L. Qi) 1

Abstract High electrochemical performance of supercapacitor electrodes largely rely on the smart design of nanoarchitectures with scrupulous combination of different active materials. We present a facile method for the multicomponent design of hierarchical Co9S8 hollow nanotubes (CS NTs) wrapped with Ni-Co layered double hydroxides nanosheets (Ni-Co LDH NSs) core-shell high-performance supercapacitors. The CS NTs serve as an ideal backbone with pentagonal cross-section to enhance the conductivity for acting as a “superhighway” for electron transport, whereas the Ni-Co LDH NSs with high electrochemical activity electrodeposited on the surface of CS NTs provide the electroactive sites and enhance the structural stability for faradaic reaction. The CS NTs@Ni-Co LDH NSs hierarchical electrode presents a high specific capacitance of 1020 C g-1 and high cycling stability of 90.4% after 10000 cycles. Moreover, an asymmetric supercapacitor (ASC) was assembled with CS NTs@Ni-Co LDH NSs as the anode and active carbon as cathode. The as-fabricated ASC shows high energy density of 50 Wh kg-1 and exhibit superior cyclic stability of 86.4% retention over 5000 cycles, suggesting this electrode is promising for efficient electrochemical capacitors. Keywords: hollow nanotube arrays, Co 9S8, Ni-Co LDH, multicomponent, supercapacitor

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1. Introduction To meet the urgent needs in electric energy storage for electric vehicles and mobile electronics, more efforts have been focused on developing the environmentally friendly and sustainable alternative energy sources.[1-3] Electrochemical capacitors, or supercapacitors, have attracted tremendous research interest because of their high power performance, fast recharge ability and long cycle life.[4-6] Among the supercapacitor technologies, there is a promising choice in research towards hybrid supercapacitors (HSCs).[7, 8] Based on Simon et al. and Lou et al. reports, HSC is a special type of capacitor, which usually combines two parts, one battery-like electrodes as energy source and one capacitive electrode as power source.[9-12] Among the multitudinous battery-like electrode materials for HSC, transition metal sulfides are of wide interest as the effective HSC electrode, mainly due to their superior electrochemical properties and higher conductivity caused by their smaller band gap when compared with corresponding metal oxides.[13-16] Among the transition metal sulfide, Late et al. reported that Co9S8 exhibits many intriguing characteristics, such as excellent electrochemical properties, high conductivity, and greatly improved redox reactions, suggesting it has been investigated as the promising electrode material.[17-19] Although great attention has been concentrated, so far the available Co9S8-based electrodes still suffer from unsatisfactory specific capacitance and poor cycling life, which restricts its applications in high performance capacitors.[20,21] Some effective and feasible strategies have been employed to enhance the capacitive performance and improve the cycling stability of transition metal sulfide electrodes, as Zhu et al. reported, including the construction with other well-known battery-like materials to form core-shell structures to release the strain and 3

accommodate the volume expansion as well as enhancing the cycling stability.[22-26] For instance, a specific capacitance of 1620 F g-1 at 0.5 A g-1 observed form Co9S8 nanorod@Ni(OH)2 nanosheets core-shell structures was much higher than that of pristine Ni(OH)2 nanosheets (~900 F g-1).[27] Pan et al. synthesized multi-shelled CoO/Co9S8 microspheres exhibited obviously enhanced specific capacitance of 1100 F g-1 than pure multi-shelled CoO microspheres.[28] These demonstrated that nanosized Co9S8 with high specific surface and electrical conductivity can effectively shorten the diffusion path of ions. In addition, transition metal layered double hydroxides (LDH) have been considered as the most promising battery-like materials because of their high theoretical specific capacitance and satisfactory stability. Recently, KCu7S4@NiMn LDHs core-shell nanostructures have been developed and yield enhanced specific capacitance and ultra-high cycling stability (96% retain after 10000 cycles) compared with pristine KCu7S4 nanowires.[29] Li et al. reported that core-shell FeCo2O4@NiCo LDH exhibited much enhanced specific capacitance, as well as good structural stability with a good retention of 91.6% than pure FeCo 2O4 (72.4%).[30] These results imply that LDH as shell can not only increase the specific surface and active sites for capacitance but also enhance the structural stability for whole electrode.[31, 32] Therefore, constructing core-shell electrodes with Co 9S8 and transition metal LDH is expecting to obtain higher electrochemical performance from synergistic effects. Inspired by the above discussions, we develop a cost-effective strategy to design novel Co9S8 nanotubes@Ni-Co LDH nanosheets core-shell electrode (CS NTs@NC LDH NSs) on Ni foam for high performance supercapacitors, in which one-dimensional Co9S8 nanotubes (CS NTs) as core materials and NC LDH nanosheets (NC LDH NSs) as the shell can obtain the benefits from both two 4

individual components. As cores, the CS NTs with high conductivity and large surface area can shorten the ion and electron diffusion pathway and improve the connection area with electrolyte. As shells, the NC LDH NSs wrapped CS NTs can not only enlarge the specific surface area for capacitance but also enhance the structural stability for whole electrode. Because of the synergistic effects, the core-shell CS NTs@NC LDH NSs showed superior specific capacitance of 1020 C g-1 and long-term cycling stability (90.4% retention after 10000 cycles). Moreover, the CS NTs@NC LDH NSs//active carbon asymmetric supercapacitor exhibits a specific capacitance (236 C g-1), high energy density (50 Wh kg-1) and good cycling stability (86.4% after 5000 cycles). 2. Results and discussions Figure 1 schematically described the synthesis and fabrication process of CS NTs@NC LDH NSs electrodes on Ni foam through a three-step method (detailed experiment in supporting information). Firstly, the uniform assembly of Co precursor was directly grown on Ni foam by a hydrothermal process. Secondly, the anion-exchange reactions with S2- ions results in the conversion of Co precursor nanoneedles to the CS NTs. The obtained Co9S8 nanotubes exhibit rough surface and polygonal cross-section. And they were directed used as scaffold for the fabrication of NC LDH NSs through the controlled electrodeposition to obtain NC LDH NSs wrapped CS NTs. Field emission scanning electron microscope (FESEM) image in Figure 2a illustrates that the surface of Ni foam is homogeneously covered by the vertical-aligned Co precursor arrays. As shown in Figure 2b, the high magnification image reveals that the Co precursor arrays with diameters of ~200 nm are solid hexagonal prisms with the length of ~3 μm, and the precursor arrays show no 5

self-aggregation phenomenon. Furthermore, as shown in Figure 2c, transmission electron microscopy (TEM) image of Co precursor arrays reveals that the precursor arrays have smooth surface and solid structure, which is associated with SEM image in Figure 2b. After sulfurization process, the order and vertical-aligned nanostructures of the Co precursor arrays were well retained. As shown in Figure 2d, the CS NTs are well separated apart from each other and show homogeneous distribution on the surface of Ni foam. Furthermore, as shown in Figure 2e, the surfaces of sulfureted precursor arrays become rough. And as shown in Figure S1, the broken Co 9S8 nanotube suggests that the cross-section still exhibits polygonal morphology and solid precursor arrays have become hollow nanotubes. Figure 2f shows the TEM image of CS NTs, after vulcanization, the solid structure of Co precursor array become hollow nanotubes with a highly porous structure, which consists of numerous particles with size of tens of nanometers. This porous structure offers the additional room to enhance the expansion tendency.[33] The diameter of hollow nanotube is about 200 nm and the wall thickness is 10~30 nm. The formation of CS NTs with rough surface can be explained by outward diffusion effect during vulcanization that offers the potential to design various hollow structures without any template.[34,35] In the hydrothermal process with Na2S, the unequal diffusion of cationic ions and S2-, which means the outward diffusion of Co ions was higher than the inner flow of S 2-, resulting in the void space in the inner of nanotube and resulting in hollow CS NTs.[36,37] This unique architecture will obviously increase the surface area of active materials and facilitate the electron transportation and thus benefit for the improved electrochemical properties.[38] In summary, the CS NTs are realized through the outward diffusion effect and still maintain the order structure. Furthermore, the ordered and vertical-aligned structure of hollow CS NTs is not 6

influenced by the electrodeposition of NC LDH NSs. As shown in Figure 3a, the CS NTs@NC LDH NSs are still homogeneously distributed on the Ni foam and still separated from each other with no obvious stack. Figure 3b shows enlarged SEM image of Figure 3a, the nanosheets were homogeneously wrapped on the surface of CS NTs in a twisting fashion. The nanosheets show ultrathin thickness and interlace with each other. Moreover, the bottom of nanosheets are connected with the surface of CS NTs, and the top of nanosheets were well separated from interlaced neighboring nanosheets, this unique structure does not completely disguise the inner CS NTs and thus increasing the specific area and active sites. Meanwhile, the interconnected and interlaced nanosheets create the void space and this neighboring interlaced architecture is benefit for increasing the charge storage. The loading structure of NC LDH NSs can also be controlled through adjusting the deposition time. As shown in Figure S2, the NC LDH NSs with different deposition time show different morphology. As shown in Figure S2a, the NC LDH NSs deposited at 100 s show small lateral size and only partial cover the surface of CS NTs with a thin shell. When the deposition time prolongs to 300 s, the NC LDH NSs exhibit the ultrathin sheet with suitable length and interconnected with each other and maintain the distance between each nanosheet (Figure S2b). Further increasing deposition time to 600 s would result in a dense layer of NC LDH NSs on CS NTs with reduced distance between the neighboring nanosheet, and this dense layer will completely disguise the inner CS NTs (Figure S2c). The thickness of NC LDH NSs shell increases with prolonging the deposition time, and the NC LDH NSs was also deposited into the cavities between the nanotubes, which results in the agglomeration of CS NTs@NC LDH NSs. Therefore, the ultrathin nanosheets which were deposited at 300 s show good separation. And directly constructing them as a shell on the surface of core CS 7

NTs would increase the specific surface area and active sites and protect the core during electrochemical process, while not reduce the distance between the nanotubes and cause the clusters of CS NTs@NC LDH NSs. Figure 3c clearly shows the TEM image of the hybrid CS NTs@NC LDH NSs core-shell structure. It can be seen that numerous NC LDH NSs as shell tightly wrap the core CS NTs, and the NC LDH NSs are linked with each other in a wrinkled fashion. Moreover, the contrast difference between the core and shell exhibits the shell thickness of 90 nm. The HRTEM image of CS NTs@NC LDH NSs marked by red cycle presented in Figure 3d reveals that the fringe spacings of 0.452 and 0.232 nm correspond to (006) and (110) planes of NC LDH.[39-41] The HRTEM image marked by green cycle presented in Figure 3e reveals that the fringe spacing of 0.29 nm corresponds well to the (311) plane of Co 9S8.[42] The compositional distributions of a CS NTs@NC LDH NSs are confirmed by HAADF-STEM-EDS. And mapping analysis (Figure 3f) show that the element S is located in the core part of the hybrid nanotube, while both Co, Ni and O are uniformly distributed in the shell region, confirming the core-shell CS NTs@NC LDH NSs hierarchical structure. Based on above discussion, the unique CS NTs@NC LDH NSs core-shell electrodes have been realized. X-ray diffraction (XRD) pattern is measured to evaluate the phase composition of the electrodes. Figure 4a shows the XRD patterns of CS NTs and CS NTs@NC LDH NSs. Besides three strong diffraction peaks located at 44.4o, 52.0o and 76.6o which can be assigned to Ni foam, the CS NTs exhibit the diffraction peaks at 2θ, 29.8°, 31.2°, 36.2° and 54.6° attributed to the (311), (222), (400) and (531) planes of the standard cubic phase Co9S8 (JCPDS card No. 65-1765), respectively.[42] In addition to the peaks associated with Co 9S8 and Ni, the core-shell CS NTs@NC LDH NSs 8

exhibit the diffraction peaks at 23.1o, 34.9o, 37.6o, 39.4o, 60.8o and 62.4o can be indexed as (006), (012), (104), (015), (110) and (113) planes of hexagonal NC LDH (JCPDS card No. 40-0216).[39] The XRD spectra of core-shell CS NTs@NC LDH NSs indicates the existence of Co9S8, NC LDH, and Ni foam substrate, and the absence of extra peaks implies the purity of core-shell CS NTs@NC LDH NSs electrodes without any residues and contaminants. The relative weak and unconspicuous peaks of Co 9S8 and Ni-Co LDH peaks might be attributed to the poor crystallinity after the low-temperature hydrothermal treatment and relatively small size of crystallites.[43] The chemical composition and the valence states are evaluated by XPS. Figure 4b shows the XPS survey spectrum of CS NTs@NC LDH NSs. It indicates the existence of Co, S, Ni and O elements in CS NTs@NC LDH NSs. Moreover, the C 1s in CS NTs@NC LDH NSs may generate from the carbon tape which was used to fix the samples. As shown in Figure 4c and d, the XPS spectra of Co 2p and Ni 2p can be fitted through Gaussian fitting with two main peaks and two shakeup satellites (marked as “Sat.”). The Co 2p3/2 and Co 2p1/2 spectra can be fitted to two spin-orbit doublets of Co 2+ and Co3+. The Co3+ peaks are centered at binding energies of 780.3 and 795.8 eV, and the other peaks at 781.9 and 797.1 eV are attributed from Co 2+. The satellite peaks at around 785.5 and 802.5 eV are two shakeup type peaks of Co. These results confirm that the Co species have Co2+/Co3+ in the hybrid electrodes. Usually, the Co element with different valence states are co-existing in Co9S8 and Ni-Co LDH.[42,44] and the high presence of Co 3+ peak in the Co 2p spectra is attributed to high atomic percentage of Co 3+ which is caused by the sulfidation reaction leading to the conversion of Co2+ in Co-precursor to Co3+.[45] Moreover, due to the formation of H2S in the sulfidation process, there is still the existence of Co2+.[46] Meanwhile, the Ni 2p spectra displayed Ni 2p3/2 and Ni 2p1/2 9

with two shakeup satellites. In addition, the peaks of Ni 2p 3/2 and Ni 2p1/2 can be broadened by several peaks, whose binding energies centered at 854.8, 855.9, 872.4 and 873.7 eV, indicating the coexistence of Ni2+ and Ni3+ in hybrid electrodes.[47,48] Moreover, the peaks for Ni3+ are higher than those for Ni2+, which indicates that the Ni3+ is the main valence state. The higher Ni3+ might be due to the fast heterogeneous nucleation of NiOOH with oxygen vacancy defects on the surface of Co 9S8 nanotubes during deposition process.[49] These results illustrate that the chemical composition of the CS NTs@NC LDH NSs electrodes own a mixed composition containing Co2+/Co3+, Ni2+/Ni3+, which is consistent with previous reports.[48] And different cations will play a role in enhanced electrochemical performance.[50] On the basis of above discussion of XRD, XPS and TEM, it can be confirmed that this core-shell electrode is composed of CS NTs and NC LDH NSs. To further indicate the nanostructure change of CS NTs before and after deposition of interlaced Ni-Co LDH nanosheets (NC LDH NSs), Brunauer-Emmett-Teller (BET) N2 adsorption/desorption isotherms of the typical CS NTs and the corresponding CS NTS@NC LDH NSs materials were provided, as shown in Figure S3. The BET surface area of CS NTS@NC LDH NSs is 37.5 m2 g−1, higher than that for the CS NTs (17.7 m2 g−1). The BET surface area increase is due to the interlaced NC LDH NSs on the surface of hollow CS NTs. In the respective insets, Barrett-Joyner-Halenda (BJH) pore-size distributions are presented. And the pores of CS NTs are mostly 3-7 nm, and CS NTS@NC LDH NSs show an increase of 8-12 nm in pore size. The increase in pore size is due to the presence of spaces between the NC LDH NSs. To evaluate the electrochemical properties of CS NTs and CS NTs@NC LDH NSs materials as the electrodes, the electrochemical tests were carried out in a three electrode system in 6 M KOH aqueous electrolyte, and Hg/HgO and Pt were used as 10

the reference and counter electrodes. Fig. 5a shows the cyclic voltammetry (CV) curves of the typical CS NTs@NC LDH NSs electrode with the deposition time 300 s at the scan rates from 5 to 30 mV s-1. Typically, a couple of symmetric faradaic redox peaks can be observed within the potential window from -0.1 to 0.5 V, suggesting the presence of CS NTs@NC LDH NSs referring to M–O and M–O–OH during the electrochemical process, in which M indicates the Ni and Co ions. The reactions are described as follows:[51] Co9S8 + 9H2O + 9/2O2 → 9Co(OH)2 + 8S Ni(OH)2 + OH- ⇋ NiOOH + H2O + eCo(OH)2 + OH- ⇋ CoOOH + H2O + eAs the scan rates increase, the similar CV curve shapes and the symmetric redox peaks are still observed, suggesting the fast electronic and ionic transport and ideal rate capabilities of the electrodes. Furthermore, with increase of the scan rate, the cathodic peaks move to a negative potential while anodic peaks shift towards a positive potential, which mainly arise from the insufficient intercalation of electrolyte ions into the dense center of nanostructure.[33] Meanwhile, the increasing of anodic peak current density and decreasing of cathodic peak current density indicate the low resistance and relatively fast redox reactions.[52] The GCD curves of the CS NTs@NC LDH NSs electrode with the deposition time 300 s at current densities from 1.25 to 25 A g-1 are shown in Figure 5b. All curves are almost symmetric, indicating the high coulombic efficiency which is caused by the highly reversibility of redox reactions during charge/discharge process. Moreover, the nonlinear charge/discharge profile confirms the battery-like behavior of CS NTs@NC LDH NSs, which is consistent with the redox peaks observed in CV curves. Figure 5c shows the CV curves of the Co-precursors, CS NTs and CS NTs@NC LDH NSs at a constant scan 11

rate of 30 mV s-1. The as-prepared CS NTs@NC LDH NSs electrode exhibits the largest curve area and current and peak intensity compared with other electrodes, indicating the obviously enhanced specific capacitance and fast reaction kinetics. And the significantly lower current intensity of Co-precursors than that of others implies that the capacitance can be ruled out. Figure S4 shows the GCD curves of CS NTs and CS NTs@NC LDH NSs at 10 mA. And the discharging time of CS NTs@NC LDH NSs (384 s) is much higher than that of CS NTs (140 s) and Co-precursors (30 s). As shown in Figure 5d, the values of the specific capacitance for CS NTs@NC LDH NSs are as high as 1020, 929.9, 790, 686.8 and 559 C g-1 with the current densities ranging from 1.25 to 25 A g-1, whereas the CS NTs electrode shows the specific capacitance of 455.7, 417, 378, 330 and 287.7 C g-1. We also directly grow NC LDH NSs on Ni form through electrodeposition, and the deposition time is 300 s. The mass loading of NC LDH NSs on Ni foam can be calculated to be about 1.7 mg cm-2. The electrochemical performance of NC LDH NSs was evaluated in a three-electrode system. Figure S5a shows CV curves of the NC LDH NSs at various scan rates ranging from 5 to 30 mV s−1. As expected, a couple of prominent faradaic redox peaks are apparent within the potential window from -0.1 to 0.5 V. The galvanostatic charge/discharge (GCD) curves in Figure S5b show that the potential-time curves at all current densities from 1 to 20 A g−1 are almost symmetric. Figure S5c shows the specific capacitance values of NC LDH NSs, the values are 496, 442, 377, 312, 277 C g-1 with the current densities ranging from 1 to 20 A g-1, which is small than CS NTs@NC LDH NSs. Obviously, in-situ forming Ni-Co LDH nanosheets on Co9S8 can significant improve the electrochemical performance. Even at a high current density of 25 A g-1, CS NTs@NC LDH NSs retains 54.7 % of the pristine capacitance, which is larger than that of CS NTs electrode. These improved specific capacitance and rate capacity could be 12

attributed to the hierarchical core-shell structures, which could improve the conductivity and shorten the diffusion path for electrons and ions. The CV curves of CS NTs@NC LDH NSs with different deposition time is shown in Figure S6, and the area of CS NTs@NC LDH NSs with 300 s is obviously larger than those of other CS NTs@NC LDH NSs electrodes. And we also can see that the CS NTs@NC LDH NSs with 300 s presents the much longer discharge time than other electrodes. As shown in Figure S7a, the optimized deposition time is 300 s, and the highest Cs is 1020 C g-1, which is higher than CS NTs@NC LDH NSs-100 (669 C g-1) and CS NTs@NC LDH NSs-600 (576 C g-1). The electrochemical impedance spectroscopy (EIS) was carried out in the frequency between 105 Hz to 0.1 Hz which provides the electrochemical response of the electrodes, and the detailed Nyquist plots are shown in Figure 5e. Generally, the small semicircle at high frequency region followed by a straight line at the low frequency represents the ideal behavior of electrodes. The impedance spectra of both electrodes exhibit similar semicircles and straight lines. However, the slope of the straight line for CS NTs@NC LDH NSs (~80o) is large than that of CS NTs at low frequency region, which demonstrates the lower diffusion resistance indicating fast electric responses during redox reaction.[53-55] In addition, the core-shell CS NTs@NC LDH NSs electrode exhibits the smaller serial resistance (intercept of real axis, Rs=0.45 Ω) and charge transfer resistance (diameter of the semicircle, Rct=0.15 Ω) than that of CS NTs (Rs=0.55 Ω, Rct=0.25 Ω), which further confirms the enhanced conductivity and rapid electron transfer kinetics. The smaller resistance could be attributed to the unique porous morphology, the interlaced NC LDH NSs wrap the CS NTs and are well separated apart from each other, which increase the active sites for electrochemical reaction and maintain the space for the electrolyte to reach the Co9S8 core. The EIS analytical results indicate that the CS NTs@NC LDH 13

NSs exhibits a much enhanced conductivity which caused by the tailored configuration of NC LDH and the high conductivity of Co 9S8. In order to further evaluate the long-term cycling stability of the electrodes, the cyclic performance was performed at a current of 25 mA, as shown in Figure 5f. It can be clearly seen that the core-shell CS NTs@NC LDH NSs electrode displays excellent cycling stability with about 90.4% capacitance after 10000 cycles, which is higher than CS NTs electrodes (64.6%). Moreover, the capacitance retentions are 81.8% and 74.2% for CS NTs@NC LDH NSs with 100 s and 300 s, respectively, as shown in Figure S7b. The excellent cycling stability of CS NTs@NC LDH NSs could be attributed to the enhanced conductivity of Co9S8 and the wrapping of NC LDH NSs. The CS NTs provide the electron superhighways for charge storage, and the wrapping of NC LDH NSs on the surface of Co9S8 core could enhance the stability and prevent the structural breakdown during long-term electrochemical reactions. This suggests that this core-shell structure is benefit for the cycling stability. This phenomenon can also be evidenced by the SEM image of CS NTs and CS NTs@NC LDH NSs after cycling stability, as shown in Figure S8. The morphology of core-shell CS NTs@NC LDH NSs still preserved well after cycling test, and the CS NTs@NC LDH NSs still adhered to the Ni foam. It can be clearly observed that the morphology of CS NTs has changed significantly when compared with it before cycling. The vertical-aligned CS NTs have been fractured and destroyed, which may due to the potential dissolution and stress effect during charge/discharge process and the harsh redox reaction in strong KOH solution. Meanwhile, the NC LDH NSs show little destruction sacrifice themselves during charge/discharge process due to the harsh redox reactions in concentrated KOH electrolyte (6 M).[46] The superior electrochemical performance of CS NTs@NC LDH NSs with core-shell nanostructures could be expected as the 14

potential electrode for high performance supercapacitors. To further evaluate the possibility of the CS NTs@NC LDH NSs electrodes for practical applications, an asymmetric supercapacitor (ASC) was assembled by utilizing the CS NTs@NC LDH NSs electrode as the positive electrode and the activated carbon (AC) as the negative electrode in 6 M KOH electrolyte. Figure S9 shows the ASC device at different voltage windows at the scan rate of 30 mV s -1. There is a dramatic hump occurred at a potential window of beyond 1.5 V, indicating that some irreversible oxidations happen.[56] This suggests that the stable working potential of the ASC device can achieve 1.5 V. Figure 6a shows the CV curves of AC and CS NTs@NC LDH NSs electrode measured at the scan rate of 30 mV s-1. The working potential window of CS NTs@NC LDH NSs was -0.1 to 0.5 V, and the AC electrode was measured at the potential window of -1 to 0 V. The CV curve of AC shows a nearly rectangular shape, suggesting the excellent EDLCs property. Figure 6b shows the CV curves of CS NTs@NC LDH NSs//AC ASC in the potential window of 0-1.5 V at the scan rate of 5, 10, 20, 30 and 50 mV s-1. When the scan rate increases, all curves of CS NTs@NC LDH NSs//AC ASC exhibit the similar shape, indicating a good rate capability. And small redox peaks can also be observed due to the presence of battery-like CS NTs@NC LDH NSs electrode. As shown in Figure 6c, the electrochemical performance of ASC was further evaluated by GCD curves at various scan rate. The specific capacitance of as-prepared ASC is calculated to be 236, 225.7, 210.8, 200.6 and 182 C g-1 at current densities of 1, 2, 4, 6 and 10 A g-1, respectively, which is still retaining 77.3% of initial capacitance at 10 A g-1, further illustrating the high capacitance and rate capacity of CS NTs@NC LDH NSs//AC ASC. The cycling stability of as-prepared ASC was evaluated at the current density of 6 A g-1, and the results are shown in Figure 6e. It can be clearly seen that the CS NTs@NC LDH 15

NSs//AC ASC exhibits a long cycling life with about 86.4% retention after 5000 cycles. The Ragone plot of ASC is shown in Figure 6f. According to the discharge curves, the ASC delivers the high energy density of 50 Wh kg -1 at a power density of 839 W kg-1. This value is superior over that of many other ASCs using similar anode Co9S8 or Ni-Co LDH with various morphologies, such as GO//Ni-Co LDH (33.7 Wh kg-1 at 551 W kg-1)[57], Ni-Co LDH//AC (17.5 Wh kg -1 at 10500 W kg-1)[58], Ni-Co LDH//CBCN (36.3 Wh kg-1 at 800.2 W kg-1)[59], ZnO//Ni-Co LDH (36 Wh kg-1 at 374.7 W kg-1)[60], Co9S8-3DG//RGO (31.6 Wh kg-1 at 910 W kg-1)[61], NixCo1-x(OH)2//GS (33 Wh kg-1 at 970 W kg-1)[62], CoMoO4-Co9S8//AC (42 Wh kg-1 at 181.1 W kg-1)[63], where CBCN is nitrogen doped graphene

and GS is graphene

sheet. Even at a high power density of 9118 W kg-1, our CS NTs@NC LDH NSs//AC ASC still exhibits the energy density of 38 Wh kg-1. In order to further evaluate the practical applicability of the CS NTs@NC LDH NSs//AC ASC, two samples of ASC devices are assembled in series and can power 15 red light emitting diodes (LED) in parallel (one LED working voltage ≈2 V) for more than 30 min when the ASCs were charging only 30 s at a current density of 10 A g-1, as shown in Figure S10. The superior electrochemical performance of CS NTs@NC LDH NSs electrode mainly caused by the following plausible reasons. Firstly, the sulfide-based Co9S8 core materials provide enhanced conductivity and more quickly Faradic redox reactions compared to metal Co based oxides; the hollow structure with polygonal cross-section and porous wall of Co 9S8 nanotubes maintain the electrolyte to diffuse to the inner part to the nanotube, which enhances the utilization of the active materials. Secondly, the Ni-Co LDH as porous shell improves the utilization of Co 9S8 by increasing the active sites in electrochemical process, and the Ni-Co LDH nanosheets interlaced with each other to form the void between the nanosheet, which presents a 16

trap for ions transport and remits the volume change during charge/discharge process;[20] and Ni-Co LDH nanosheets as shell sacrifice themselves to protect the inner Co9S8 and enhance the cycling stability (supported by Figure 5f). Finally, the direct growth CS NTs@NC LDH NSs on Ni foam without the use of binding agents reduce the internal resistance and enhance the mechanical stability. Therefore, the high performance is due to the solitary of Co 9S8 core and Ni-Co LDH shell and well-designed CS NTs@NC LDH NSs hierarchical structure. This unique structure opens the opportunity for energy storage devices. 3. Conclusion In conclusion, a unique hierarchical hollow CS NTs@NC LDH NSs has been developed through a stepwise simple synthesis approach. The anion-exchange reaction through S2- was optimized to obtain hollow Co 9S8 and electrodeposition allows the formation of ultrathin Ni-Co LDH nanosheets wrapped the nanotubes. Benefit from the unique hierarchical structure, the hybrid CS NTs@NC LDH NSs electrodes achieve the high specific capacitance of 1020 C g-1 at the current density of 1.25 A g-1 with high rate capability and excellent cycling stability (90.4% capacitance after 10000 cycles), which are due to the interlaced NC LDH nanosheets, the high conductivity of CS NTs and their synergy lead to the structural stability. Furthermore, an ASC based on CS NTs@NC LDH NSs as anode and AC as cathode has been fabricated. The as-fabricated ASC exhibits superior specific capacitance of 236 C g-1 at a current density of 1 A g-1 with a stable voltage window of 1.5 V. Furthermore, the ASC can deliver the high energy density of 50 Wh kg -1 at a power density of 839 W kg-1 and an energy density of 38 Wh kg -1 at a high power density of 9118 W kg -1. The ASC also exhibits good cycling stability with 86.4% retention after 5000 cycles. Therefore, such electrodes with remarkable electrochemical performance are 17

promising candidates for sustainable supercapacitors

Supporting Information Electronic supplementary information (ESI) available. See DOI:

Acknowledgements The support from the National Natural Science Foundation of China (Grant Nos. 51575135, 51622503, U1537206 and 51621091), China, is highly appreciated.

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of CoMoO4/Co9S8

Nanorod Arrays

on Nickel

Foam

for

High-Performance Asymmetric Supercapacitors with High Energy Density, Electrochim. Acta 252 (2017) 470-481. 63. W. Zhou, X. Cao, Z. Zeng, W. Shi, Y. Zhu, Q. Yan, H. Liu, J. Wang, H. Zhang, One-step

synthesis

of

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nanorod@Ni(OH)2

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26

Figure captions:

Figure 1. Schematic illustration for the fabrication of CS NTs@NC LDH NSs electrodes on Ni foam by stepwise synthesis approach.

Figure 2. SEM images of a) large area uniform distribution of Co precursors, b) Co precursors at high magnification; TEM image of c) Co precursors with solid structure; SEM image of d) large area uniform distribution of CS NTs, e) CS NTs at high magnification; TEM image of e) CS NTs with hollow structure.

Figure 3. SEM images of CS NTs@NC LDH NSs at a) low magnification and b) high magnification; c) TEM image of CS NTs@NC LDH NSs; d,e) HRTEM images of CS NTs@NC LDH NSs obtained from the red and green region in c); f,g) HAADF STEM and EDS elemental mappings of different elements of S, Co, Ni and O recorded from a single nanotube separated from CS NTs@NC LDH NSs.

Figure 4. a) The representative XRD patterns for CS NTs and CS NTs@NC LDH NSs. XPS spectra (b) survey, (c) Co 2p, (d) Ni 2p for CS NTs@NC LDH NSs.

Figure 5. Electrochemical performance of the samples for supercapacitors. a) CV curves of the CS NTs@NC LDH NSs. b) Galvanostatic charge/discharge curves of CS NTs@NC LDH NSs at different current densities. c) CV curves of the Co precursors, CS NTs and CS NTs@NC LDH NSs at a scan rate of 30 mV s−1. d) specific capacitances of CS NTs and CS NTs@NC LDH NSs at different current densities. e) Nyquist plots in a frequency range from 0.1 Hz to 100 kHz for CS NTs and CS 27

NTs@NC LDH NSs. f) cycling stability tests over 10000 cycles for CS NTs and CS NTs@NC LDH NSs.

Figure 6. a) CV curves of the CS NTs@NC LDH NSs and AC at 30 mV s -1. b) CV curves of the CS NTs@NC LDH NSs//AC device at different scan rates; (c) GCD curves of the ASC device measured at different current densities; d) specific capacitances of CS NTs@NC LDH NSs//AC at different current densities. e) Cycling performance at a constant current density of 6 A g−1. f) The Ragone plot related to energy and power densities of the CS NTs@NC LDH NSs//AC asymmetric supercapacitors.

28

Step I

Step II

Hydrothermal

Step III

Anion exchange

Electrodeposition

Porous Ni-Co LDH

Hollow

Ni foam

Co9S8 nanotubes (CS NTs)

Co-precursors

Figure 1

29

Co9S8 nanotubes@Ni-Co LDH nanosheets (CS NTs@NC LDH NSs)

祄

(b)

(c)

3μm

500nm (e)

(d)

500nm (f)

3μm

500nm

500nm 2 0 0

Figure 2

30

n m

0 . .5 5祄

(a)

(a)

(b)

(c) e d

3μm (d)

500nm (f)

(e)

200nm (g) 1 0 0

d~0.232 nm

S

Ni

O

n m

d~0.29 nm

d~0.452 nm

5nm

Co

50nm

5nm

Figure 3

31

(c)

20

30

40

50

60

70

80

2 Theta(degree)

(d)

Co 2p3/2 Co 2p1/2 Co 2+ Co 2+ 3+ “sat.” “sat.” Co

1000

Co 3+

Binding Energy(eV)

S 2p

C 1s

CS NTs 800

600

400

200

Binding Energy(eV) Ni 2p3/2 Ni 2p1/2 Ni 2+ 2+ Ni “sat.” “sat.” Ni 3+

880

805 800 795 790 785 780 775

O 1s

Ni 2p

CS NTs@NC LDH NSs

Co 2p Co (LMM) Ni (LMM)

Intensity(a.u.)

CS NTs

10

Intensity(a.u.)

(b)

Co9S8 CS NTs@NC LDH NSs JCPDS No. 65-1765 Ni-Co LDH JCPDS No. 40-0216

Intensity(a.u.)

Intensity(a.u.)

(a)

870

860

Ni 3+

850

Binding Energy(eV)

Figure 4

32

0

(b)0.5

100 50 0 -50

(c)150 25 A/g 12.5 A/g 6.25 A/g 2.5 A/g 1.25 A/g

0.4 0.3

Current (A/g)

30 mV/s 25 mV/s 20 mV/s 15 mV/s 10 mV/s 5 mV/s

Potential(V)

0.2 0.1

0.0

0.1

0.2

0.3

0.4

0.5

Potential(V)

1200

(e)

CS NTs CS NTs@NC LDH NSs

1000

0.0

800 600 400

-Z'' (Ohms)

-0.1

(d) Capacitance(C/g)

50 0 -50

30 mV/s

-100

-100 0

400

800

5

10

15

20

25

Current density (A/g)

1600

2000

CS NTs CS NTs@NC LDH NSs 0.5 0.4 0.3 0.2

2

0.1

0

0

2

0.2

0.4

4

0.6

Z' (Ohms)

Figure 5

33

0.8

6

1.0

8

0.1

0.2

0.3

0.4

0.5

Potential(V) CS NTs@NC LDH NSs 90.4%

100

6 4

-0.1 0.0

(f)

8

0.0 0.0

0

1200

Time(s)

200 0

Co-precursor CS NTs CS NTs@NC LDH NSs

100

Normalized capacitance(%)

Current (A/g)

(a)150

80

CS NTs 64.6% 60

0

0

2000

4000

6000

Cycle number

8000

10000

0 -50

-100

10 5

-5 -10

200 160 120 80 40 2

4

6

Potential(V)

(e)

240

8

Current Density (A

10 g-1)

0.8 0.6 0.4

80

CS NTs@NC LDH NSs//AC 86.4% 60

0

1000

2000

3000

Cycle number

Figure 6

34

0

(f) 100

100

0

1.0

0.0

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Normalized capacitance(%)

Capacitance(C/g)

(d)

1.2

0.2

-15

Potential(V)

10 A/g 6 A/g 4 A/g 2 A/g 1 A/g

1.4

0

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

0

1.6

Potential(V)

50

0

(c) 50 mV/s 30 mV/s 20 mV/s 10 mV/s 5 mV/s

15

4000

5000

Energy density(Wh/kg)

100

(b) 20 Active Carbon CS NTs@NC LDH NSs

Current (A/g)

Current (A/g)

(a)150

10

100

200

300

400

500

Time(s)

This work Ref.57 GOMC//Ni-Co LDH Ref.58 Ni-Co LDH//AC Ref.59 Ni-Co LDH//CBCN Ref.60 ZnONFs//Ni-Co LDH Ref.61 Co9S8-3DG//RGO Ref.62 NixCo1-x(OH)2//GS Ref.63 CoMoO4-Co9S8//AC

1 100

1000

Power density(W/kg)

10000

Highlights 1. Ni-Co LDH nanosheets wrapped Co9S8 nanotubes on Ni foam was prepared by chemical ropute 2. The obtained Co9S8@Ni-Co LDH exhibited a high specific capacitance of 1020 C g-1 and high cycling stability of 90.4% 3. The positive synergistic effect of Co9S8 and Ni-Co LDH is observed.

35

Graphical Abstract

Step I

Step II

Hydrothermal

Step III

Anion exchange

Electrodeposition

Porous Ni-Co LDH

Hollow

Ni foam

Co9S8 nanotubes (CS NTs)

Co-precursors

36

Co9S8 nanotubes@Ni-Co LDH nanosheets (CS NTs@NC LDH NSs)