Design and fabrication of highly open nickel cobalt sulfide nanosheets on Ni foam for asymmetric supercapacitors with high energy density and long cycle-life

Journal of Power Sources 378 (2018) 31–39

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Design and fabrication of highly open nickel cobalt sulfide nanosheets on Ni foam for asymmetric supercapacitors with high energy density and long cycle-life

T

Daosong Zhaa, Yongsheng Fua,∗, Lili Zhangb, Junwu Zhua,∗∗, Xin Wanga,∗∗∗ a b

Key Laboratory for Soft Chemistry and Functional Materials of Ministry Education, Nanjing University of Science and Technology, Nanjing 210094, China Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, Huaiyin Normal University, Huai'an 223300, China

H I G H L I G H T S

G RA P H I C A L AB S T R A C T

open nickel cobalt sulfide na• Highly nosheets on Ni foam were rationally designed.

optimized electrode showed ex• The cellent specific capacitance and rate capability.

assembled asymmetric super• The capacitor delivered a high energy of

•

58.1 Wh kg−1. The device exhibited remarkable longterm cycling durability after 70,000 cycles.

A R T I C L E I N F O

A B S T R A C T

Keywords: Nickel cobalt sulfides Binder-free Asymmetric supercapacitor High energy density Long-term stability

Nickel cobalt sulfides (NiCo-S) are promising electrode materials for high-performance supercapacitors but normally show poor rate capability and unsatisfactory long-term endurance. To overcome these disadvantages, a properly constructed electrode architecture with abundant electron transport channels, excellent electronic conductivity and robust structural stability is required. Herein, considering that in situ transformation can mostly retain the specific structural advantages of the precursors, a two-step strategy is purposefully developed to construct a binder-free electrode composed of interconnected NiCo-S nanosheets on Ni foam (NiCo-S/NF), in which NiCo-S/NF is synthesized via the in situ sulfuration of networked acetate anion-intercalated nickel cobalt layered double hydroxide nanosheets loaded on Ni foam (A-NiCo-LDH/NF). Noticeably, the optimized Ni1Co1-S/ NF exhibits an ultrahigh specific capacitance of 2553.9 F g−1 at 0.5 A g−1, excellent rate capability (1898.1 F g−1 at 50 A g−1) and superior cycling stability (nearly 90% capacitance retention after 10,000 cycles). Furthermore, the assembled asymmetric supercapacitor based on Ni1Co1-S/NF demonstrates a high energy density of 58.1 Wh kg−1 at a power density of 796 W kg−1 and impressive long-term durability even after a repeated charge/discharge process as long as 70,000 cycles (∼92% capacitance retention). The attractive properties endow the Ni1Co1-S/NF electrode with significant potential for high-performance energy storage devices.

∗

Corresponding author. Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Fu), [email protected] (J. Zhu), [email protected] (X. Wang).

∗∗

∗∗∗

https://doi.org/10.1016/j.jpowsour.2017.12.020 Received 28 June 2017; Received in revised form 20 November 2017; Accepted 8 December 2017 0378-7753/ © 2017 Elsevier B.V. All rights reserved.

Journal of Power Sources 378 (2018) 31–39

D. Zha et al.

1. Introduction

appropriate precursors with special properties are also of great importance. Recently, we constructed binder-free electrodes composed of highly interconnected acetate anion-intercalated nickel cobalt layered double hydroxide nanosheets on Ni foam (A-NiCo-LDH/NF) through a feasible alkali-free “solvothermal and in situ hydrolysis” strategy [36]. These electrodes showed distinctly enhanced electrochemical properties and cycling stability, which were attributed to the intercalate-induced expanded interlayer spacing and stable architecture with unique morphology. In consideration of retaining the abovementioned structural merits, we attempted to transform the A-NiCo-LDH/NFs into their associated binder-free nickel cobalt sulfide electrodes (NiCo-S/NFs) via an in situ procedure. Herein, we performed another solvothermal treatment to sulfurize the as-obtained A-NiCo-LDH/NFs into the corresponding NiCo-S/NFs electrodes. As expected, sulfuration was achieved in situ, and the original structural and morphological features of the A-NiCo-LDH/NFs were basically maintained. Benefiting from the special nanostructure and significant synergetic effects among the components, the optimized Ni1Co1-S/NF electrode presented extraordinary electrochemical performance, including ultrahigh specific capacitance, excellent rate capability and brilliant long-term endurance. Furthermore, an asymmetric supercapacitor (ASC) device based on Ni1Co1-S/NF and nitrogen-doped graphene (NG) was assembled. Notably, the assembled ASC device delivered an impressively high energy density (58.1 Wh kg−1) and power density (25,081.7 W kg−1), as well as a spectacular cycling durability even after a long charge/discharge process repeated for 70,000 cycles.

Due to the urgent situations of ever-increasing energy demand and environmental pollution, intense enthusiasm to exploit clean and renewable energy sources as well as the corresponding advanced energy storage devices has been sparked worldwide. Of which, supercapacitors have drawn extensive attention in virtue of their high power output, quick charge/discharge rate, long service life with low maintenance cost and prominent reliability [1–3]. Considering the predominant effect of the electrode material on the overall performance of a supercapacitor, it is imperative to explore advanced electrode materials with prominent advantages, such as high specific capacitance and rate capability, remarkable energy density and power density, inexpensive cost and ultralong lifespan [3–6]. Considering the energy storage mechanism, a rough classification of electrode materials into two types can be achieved. Where conventional carbon-based electrical double layer capacitive (EDLC) materials normally suffer from limited specific capacitance and inferior energy density [7,8], pseudocapacitive materials, which store charge by undergoing reversible redox reactions at (or near) the electrode-electrolyte interface, can deliver distinctly higher specific capacitance and energy density than carbonaceous materials [9,10]. Among various pseudocapacitive materials, transition metal sulfides have been widely explored as high-performance electrode materials for advanced electrochemical energy storage devices [11–13]. In which, bimetallic nickel cobalt sulfides have being widely investigated as promising candidates on account of the obviously higher electrochemical activity and electrical conductivity than their oxides/hydroxides counterparts as well as the better mechanical and thermal stability [13–19]. Furthermore, benefiting from the effective integration of both Ni and Co elements, which have multiple valence states, bimetallic nickel cobalt sulfides normally possess richer redox-active sites than single component sulfides, thus giving rise to improved electrochemical activity and specific capacitance [17–20]. Nevertheless, the intrinsic electrical conductivity of nickel cobalt sulfides alone is still insufficient for high-rate charge transfer, which inevitably causes deteriorated capacitance and unsatisfactory rate capability [21–23]. Moreover, the poor electrochemical stability during long-term charge-discharge processes of nickel cobalt sulfide electrode materials is another problem that needs to be solved [10,16,21]. To this end, ideal high-performance nickel cobalt sulfide electrode materials should have a specific structure with more accessible electroactive sites, faster charge transfer ability, shortened ion diffusion paths and enhanced electrochemical stability [24]. To date, many studies have been carried out to thoroughly overcome these barriers. Among which, the proper hybridization of nickel cobalt sulfides with various carbon materials has been regarded as a feasible solution [19,25–28]. Whereas, when these powdery hybrids are treated with a conventional slurry-coating procedure to prepare working electrodes, the tedious process, including grinding and high-pressure tabletting, unavoidably damages the as-constructed structures and restricts charge transport [16]. Besides, the addition of low-capacitance conductive additives and insulative polymer binders also obstructs charge transfer and depresses the electrochemical properties [29–31]. As another route to effectively realizing the supposed performance without sacrificing the intrinsic merits of the electrodes, the rational construction of a binder-free system via directly growing nickel cobalt sulfides onto conductive substrates has also been proven valid and discussed in detail [16,17,22,24,32,33]. Up to now, among the various methods to fabricate high-performance nickel cobalt sulfides, in situ transformation from the corresponding oxide/hydroxide precursors has been proven highly efficient [24,32–35]. During this transformation, the structure and morphology of precursors can be basically reserved, which inspired us to prepare nickel cobalt sulfide-based materials with specific nanostructures by controlling the precursors. Therefore, the selection and investigation of

2. Experimental section 2.1. Chemicals and materials Prior to the experiments, Ni foam substrates were treated with the procedure given in the supplementary information to ensure a clean surface. All other analytical reagents were directly used as received. Nickel acetate tetrahydrate (Ni(Ac)2·4H2O, 99%), cobalt acetate tetrahydrate (Co(Ac)2·4H2O, 99%) and sodium sulfide nonahydrate (Na2S·9H2O, 98%) were sourced from Aladdin Industrial Cooperation. Anhydrous methanol was purchased from Sinopharm Chemical Reagent Co., Ltd. All involved aqueous solutions were prepared using deionized water (DW). 2.2. Synthesis of NiCo-S/NF electrodes All NiCo-S/NF electrodes were prepared through a two-step strategy, as depicted in Fig. 1a. First, A-NiCo-LDH/NFs were obtained via our reported approach [36]. Afterwards, an in situ solvothermal sulfuration was applied using A-NiCo-LDH/NFs as precursors to acquire the desired NiCo-S/NFs. Typically, 15 mL of homogeneous methanol solution containing 0.5 mmol Ni(Ac)2·4H2O and 0.5 mmol Co (Ac)2·4H2O was added into a 25 mL Teflon-lined autoclave together with three pieces of treated Ni foam (1 × 1 cm). To ensure the complete surface wetting of the Ni foams, the autoclave was sealed for 1 h before heating. Subsequently, the autoclave was heated at 180 °C for 12 h and then naturally cooled. The substrates were collected and repeatedly washed with DW under ultrasonication to obtain A-NiCo-LDH/NFs. Then, these A-NiCo-LDH/NFs were placed into a 25 mL autoclave containing 15 mL of 0.1 M Na2S aqueous solution and kept at 120 °C for 6 h. After the reaction was complete, the black products were sufficiently washed with DW and ethanol 5 times each and then dried at 60 °C for 24 h. The ultimate product was denoted as Ni1Co1-S/NF with an average active mass loading of ∼1.4 mg on the projected area of per square centimeter. To clarify the influence of substrate, the individual Ni foams also underwent the same two-step solvothermal procedure, except for the absence of Ni(Ac)2·4H2O and Co(Ac)2·4H2O in the firststep solvothermal process. Notably, the influence of the substrate can 32

Journal of Power Sources 378 (2018) 31–39

D. Zha et al.

Fig. 1. (a) Schematic illustrating the fabrication process of the NiCo-S/NF electrode; FESEM images of Ni foam (b), A-NiCo-LDH/NF (c) and Ni1Co1-S/NF (d) with the corresponding digital photographs inset; FESEM images of Ni1Co1-S/NF at higher magnifications (e and f); (g) FESEM image and corresponding elemental mapping images of Ni, Co, S, O and C for Ni1Co1-S/NF; TEM images of the Ni1Co1-S nanosheets scraped from Ni1Co1-S/NF (h and i); HRTEM image of Ni1Co1-S nanosheets (j).

source (λ = 1.5406 Å). Raman spectra were collected on a LABRAM Aramis Raman Microprobe with an excitation wavelength of 532 nm. Xray photoelectron spectra (XPS) were recorded on an RBD upgraded PHI-5000C ESCA system (Perkin Elmer) using Al Kα as the excitation source (1486.6 eV).

be neglected because of its nearly unchanged weight before and after the treatment (Table S1), and the mostly overlapped CV curves (Fig. S1). For comparison, a series of NiCo-S/NF electrodes with various Ni/ Co feed ratios (1:3, 1:2, 2:1 and 3:1) were also produced under a constant quantity of metal salts (1 mmol) and methanol (15 mL). Meanwhile, relevant powdery NiCo-S samples were prepared under the same conditions without Ni foam.

2.4. Electrochemical measurements All parameter setting and corresponding calculation in this work were based on the active mass loading on the electrodes, excluding the Ni foam current collectors. The electrochemical properties of all electrodes were first evaluated using a three-electrode configuration with a Hg/HgO electrode and platinum foil as the reference and counter electrode, respectively. For the working electrodes, all NiCo-S/NFs electrodes were directly employed, while the powdery materials underwent a reported slurry-coating technique, the details for which are clarified in the supplementary data [37]. In addition, an ASC device was constructed by using Ni1Co1-S/NF

2.3. Materials characterization The morphologies and microstructure were probed using a HITACHI S-4800 II field emission scanning electron microscope (FESEM) and JEOL JEM-2100 transmission electron microscope (TEM). Fourier transform infrared (FTIR) spectra were acquired on a Bruker Tensor 27 FTIR spectrophotometer over a wavenumber range of 400 to 4000 cm−1. X-ray diffraction (XRD) measurements were conducted on a Bruker D8 Advance diffractometer equipped with a Cu Kα radiation 33

Journal of Power Sources 378 (2018) 31–39

D. Zha et al.

without destroying the intrinsic structure. As depicted in Figs. S2c and S2d, the nearly unchanged flower-like appearance of the powdery ANiCo-LDH and Ni1Co1-S samples further confirm the in situ conversion. As observed at high resolution (Fig. 1f), Ni1Co1-S nanosheets display a relatively rougher surface with numerous ultrafine nanoparticles. Based on the EDX results of Ni1Co1-S powders scraped from Ni1Co1-S/NF (Fig. S3), the measured Ni/Co atom ratio (∼1.03/1) was quite approximate to the feeding ratio (1/1). Fig. 1h and Fig. S4 display the elemental mapping images of Ni1Co1-S/NF and powdery Ni1Co1-S, respectively. Wherein, the evenly distributed Ni, Co and S elements reflect the uniform formation of Ni1Co1-S, while the homogenous C and O elements can be ascribed to the intercalated acetate anions inherited from the ANiCo-LDH precursors. The existence of acetate anions can also be verified by the FTIR results (detailed discussions in Fig. S5), which are conducive to maintaining the expanded interlayer spacing for rapid ion diffusion and promoting the electrochemical performance [36]. Furthermore, the FTIR spectrum also suggests the presence of abundant hydrophilic hydroxyl groups, which can effectively improve the wettability of the electrode material and accelerate electrolyte ion penetration within the electrode, thus enhancing the rate performance [38]. The morphological and structural characteristics were further elucidated by TEM. Consistent with the FESEM results, the scraped Ni1Co1S exhibits an oversize sheet-like structure with good integrality and only slight wrinkles (Fig. 1h), further confirming the in-situ sulfuration without destroying the basic structure. Notably, unlike the nearly transparent A-NiCo-LDH sheets, which have a silk-like appearance (Fig. S6), the Ni1Co1-S sheets possess an apparently rougher surface and well-defined shape. In the magnified TEM image (Fig. 1i), numerous nanosized nanocrystals and tiny pores can be seen throughout the Ni1Co1-S nanosheets due to the etching-like effect of S2− during sulfuration [31]. The same results can also be gained from the comparative TEM images of powdery A-NiCo-LDH and Ni1Co1-S (Fig. S7), further proving the sulfuration proceeded in situ and homogenously. The highresolution transmission electron microscopy (HRTEM) image (Fig. 1j) displays three sets of well-defined lattice fringes with d-spacings of 0.21, 0.24 and 0.28 nm, which can be well assigned to the (202), (003) and (110) crystal planes of nickel cobalt sulfide, respectively [24]. The crystal structure and phase purity of the samples were examined by XRD, and their corresponding patterns are plotted in Fig. 2a. The results of A-NiCo-LDH/NF indicate the successful formation of hydrotalcite-like NiCo-LDH [36]. After sulfuration, the characteristic peaks of A-NiCo-LDH almost disappeared, while several diffraction peaks at 2θ values of 21.7°, 30.9°, 37.8°, 49.7° and 55.1° can be clearly detected in the pattern of Ni1Co1-S/NF, which correspond to the (101), (110), (003), (113) and (122) crystal planes of hexagonal nickel cobalt sulfide (JCPDS NO. 44–1418), respectively [24]. Meanwhile, no impurity signals were detected, verifying the high phase purity of the products obtained via the specified procedure. To gain deeper insight into the structural features and confirm the successful fabrication, Raman analysis of Ni1Co1-S/NF was conducted (Fig. 2b), and the results are greatly in line with the reported conclusions of nickel cobalt sulfides [2,27,39,40]. The Raman peaks at 649 and 528 cm−1 can be ascribed to the Raman active A1g and T2g modes, respectively [27,39]. Besides, the peaks at 379 and 246 cm−1 are well assigned to the A1g and Eg modes caused by the stretching of S atoms towards the tetrahedral site Ni atom and the bending of the S-Nitetra-S bonds, respectively [2,27,39]. In addition, resulting from the asymmetric bending of the S-Nitetra-S bonds, three T2g modes can also be detected at 150, 305, and 351 cm−1 [2,27,40]. XPS was adopted to explore the surface elemental composition and chemical valence states of Ni1Co1-S/NF. The survey spectrum illustrates the co-presence of Ni, Co, S, C and O elements (Fig. 2c), which agrees with the elemental mapping results (Fig. 1g). The measured Ni/Co atom ratio was ∼1.06/1, which is close to the EDX result (Fig. S3). Through a Gaussian fitting method, both the Ni 2p and Co 2p spectra can be

and nitrogen-doped graphene (NG) as the positive and negative electrode material, respectively, which is labelled as Ni1Co1-S/NF//NG. The detailed synthetic route for NG and the preparation of the negative electrode are provided in the supplementary information. To achieve ideal ASC behaviour, the charge quantity (Q) on the two electrodes should be balanced, and the optimal mass ratio between the positive and negative electrode was determined according to formula (1) [14,17,23]:

m+ C × ΔV− = − m− C+ × ΔV+

(1)

where C (F g−1), ΔV (V) and m (g) represent the specific capacitance, discharge potential range and active mass of each single electrode, respectively. All electrochemical tests in both the three-electrode and two-electrode system were performed at 25 °C in 2 M KOH aqueous electrolyte. All parameter settings and associated calculations were based on the active mass loading. Cyclic voltammetry (CV) and galvanostatic chargedischarge (GCD) tests were conducted on a CHI 760D electrochemical workstation. Electrochemical impedance spectroscopy (EIS) measurements were performed on a PGSTAT 302N Autolab over a frequency range of 105 to 0.1 Hz with an amplitude of 5 mV. The long-term cycling stability was tested on a Land CT2001A battery testing system. On basis of GCD results, the specific capacitances in the threeelectrode system (Cs, F g−1) and the ASC device (CA, F g−1) can be calculated according to formulae (2) and (3), respectively [17,18]:

Cs =

I × Δt ms × ΔV

(2)

CA =

I × Δt mt × ΔV

(3)

where ΔV (V) represents the potential change excluding the potential drop; ms (g) is the active mass on a single electrode; and mt refers to the total active weight on both the positive electrode and negative electrode (m+ + m−). I (A) and Δt (s) represent the current value setting and duration time during the discharge process, respectively. The energy densities (E, Wh kg−1) and power densities (P, W kg−1) of the ASC device were acquired using formulae (4) and (5), respectively [14,23]:

E = CA × P=

(ΔV )2 2 × 3.6

3600 × E Δt

(4) (5)

−1

where CA (F g ) is the calculated specific capacitance value of the ASC, ΔV (V) and Δt (s) are the voltage variation and lasting time during the discharge process. 3. Results and discussion 3.1. Fabrication and structural characterization of NiCo-S/NF The two-step fabrication procedure of NiCo-S/NF is schematically demonstrated in Fig. 1a. Take Ni1Co1-S/NF as an example, A-NiCoLDH/NF was achieved via a one-pot solvothermal process combined with in situ hydrolysis [36]. During which, the originally bare substrate was uniformly covered with numerous interconnected A-NiCo-LDH nanosheets, which transformed the Ni foam from metalescent to matt (Fig. 1b and c). Thereafter, another solvothermal process was adopted to sulfurize A-NiCo-LDH/NF to Ni1Co1-S/NF. After sulfuration, the structural integrity of the electrode was greatly maintained, except the macroscopic appearance became homogeneously black (Fig. 1d). Compared with A-NiCo-LDH/NF (Figs. S2a and S2b), the highly open morphology composed of crosslinked nanosheets was almost unchanged (Fig. 1e), indicating the sulfuration process occurred in situ 34

Journal of Power Sources 378 (2018) 31–39

D. Zha et al.

Fig. 2. XRD patterns (a) of A-NiCo-LDH/NF and Ni1Co1-S/NF; Raman spectrum (b) of the scraped Ni1Co1-S sample; XPS spectra of the scraped Ni1Co1-S sample: survey (c), Ni 2p (d), Co 2p (e) and S 2p (f) spectra.

manifesting the excellent reversibility and low resistance of the electrode towards fast redox reactions [16,17]. As shown in Fig. S9, both the cathodic and anodic peak densities (Ip) deliver a strong linear dependence on the square root of the scan rate (v1/2), demonstrating a diffusion-controlled electrochemical process [14,19]. The high slope values of both Ip-v1/2 plots also manifest the fluent electrolyte ion diffusion and rapid charge transfer rate within the Ni1Co1-S/NF electrode [36]. As demonstrated in Fig. 3d and e, all GCD curves of Ni1Co1-S/NF at different current densities show apparent voltage platforms arising from the pseudocapacitive nature, which agrees with the CV results. Notably, all GCD plots maintained a similar symmetric appearance with unnoticeable IR drops, manifesting the superior reversibility and columbic efficiency, outstanding rate performance and excellent electronic conductivity [36,37]. Fig. S10a exhibits the discharge plots of Ni1Co1-S/NF and powdery Ni1Co1-S at 40 A g−1, the marked difference in IR drop directly reveals the dramatically improved conductivity of the Ni1Co1-S/NF electrode [42]. According to formula (2), the dependency of the specific capacitance on the current density of all NiCoS/NF electrodes is depicted in Fig. 3f, of which the specific capacitances of Ni1Co1-S/NF at all current densities are distinctly higher than those of its competitors. In detail, Ni1Co1-S/NF delivers a maximum specific capacitance of 2553.9 F g−1 at 0.5 A g−1, followed by a slight decline due to the essence of diffusion-controlled redox reactions [25]. It is striking that Ni1Co1-S/NF still possesses specific capacitances as high as 2144.2 and 1898.1 F g−1 even at the high current densities of 20 and 50 A g−1, respectively. In contrast, powdery Ni1Co1-S has an obviously lower specific capacitance of 1865.8 F g−1 at 0.5 A g−1, which sharply decreases to 411.3 F g−1 at 40 A g−1 (Fig. S10b). The distinct gap between self-assembled Ni1Co1-S and Ni1Co1-S/NF in both capacitance and rate capability can be ascribed to the distinction between the powdery sample and binder-free electrode system. For powdery Ni1Co1S, the working electrode was fabricated by the slurry-coating method. The use of insulative polymer binders and conductive additives would inevitably depress the electron conductivity and generate some “dead volume” that is not accessible for electrolyte diffusion, thus resulting in sluggish electron transfer and insufficient utilization of electroactive materials. Besides, the high-pressure tableting of working electrode before use would also toughly damage the as-constructed structure and impede electrolyte diffusion, thus leading to the depressed electrochemical properties of powdery Ni1Co1-S. While for Ni1Co1-S/NF, the

properly deconvoluted into two spin-orbit doublets accompanied by two shake-up satellites. In the Ni 2p spectrum (Fig. 2d), the doublet fitting peaks located at binding energies of 853.1 and 870.4 eV are associated with Ni2+, while the peaks at 855.5 and 873.1 eV are assigned to Ni3+. Moreover, the intense satellite peaks reveal that Ni2+ is the major component of Ni elements in the sample [17,41]. Likewise, in the Co 2p spectrum (Fig. 2e), one pair of fitting peaks at 778.6 and 793.5 eV are indexed to Co3+, while the other pair of peaks at 780.2 and 796.3 eV are characteristic of Co2+. In addition, the weak intensity of the satellite peaks implies that the dominant proportion of Co elements is occupied by Co3+ [17,22]. The coexisting Ni3+/Ni2+ and Co3+/Co2+ redox couples endow Ni1Co1-S/NF with great potential to undergo sufficient multiple redox reactions to boost the electrochemical performance [18,24]. In the S 2p spectrum (Fig. 2f), the fitting peaks at 162.6 eV (S 2p1/2) and 161.5 eV (S 2p3/2) are ascribed to the sulfurmetal bonds in NiCo-S and S2− in low coordination on the surface, respectively [18,20,34]. Meanwhile, the peak centred at 168.5 eV results from surface sulfur species in higher oxidized states [16,34]. 3.2. Electrochemical properties of NiCo-S/NF To systematically investigate the electrochemical properties of the involved samples, repeated electrochemical tests were first performed using a three-electrode system. Fig. 3a illustrates the CV curves of all NiCo-S/NF electrodes with various Ni/Co feeding ratios at 5 mV s−1. The multiple redox peaks clearly indicate pseudocapacitive features arising from the reversible Faradaic reactions of the Ni2+/Ni3+ and Co2+/Co3+ redox couples [10,17,21]. Among all the NiCo-S/NF electrodes with different Ni/Co feeding ratios, the CV curve of Ni1Co1-S/NF exhibits the largest integral area. Therefore, Ni1Co1-S/NF delivers the highest specific capacitance, which can be further confirmed by the longest discharge duration at 0.5 A g−1 (Fig. S8). As the CV plots of Ni foam, Ni1Co1-S and Ni1Co1-S/NF electrodes at 5 mV s−1 exhibited in Fig. 3b, the capacitance contribution from the substrate is negligible. Therefore, the distinct augment in specific capacitance of the Ni1Co1-S/ NF electrode can be attributed to the special nanostructure and significant synergistic effects of the components within the binder-free electrode construction [36]. Fig. 3c presents the CV curves of Ni1Co1-S/ NF at various scan rates ranging from 5 to 50 mV s−1. It is noted that the peak current densities increased accordingly with increased scan rates, while the shape of the CV curves was basically maintained, 35

Journal of Power Sources 378 (2018) 31–39

D. Zha et al.

Fig. 3. (a) CV curves at 5 mV s−1 for the NiCo-S/NF electrodes with various Ni/Co ratios; (b) CV curves of the Ni foam, Ni1Co1-S and Ni1Co1-S/NF electrodes at 5 mV s−1; (c) CV curves of Ni1Co1-S/NF at various sweep rates; (d and e) GCD curves of Ni1Co1-S/NF at different current densities; (f) Specific capacitance as a function of current density for the NiCo-S/NF electrodes with various Ni/Co ratios.

implying the intriguing long-term stability. It is also noted that the charge-discharge coulombic efficiency was maintained at nearly 100% throughout the whole period after the rapid increase during the initial cycles, further manifesting the excellent electrochemical reversibility [37]. Fig. 4b illustrates the Nyquist plots of Ni1Co1-S/NF before and after the cycling test. As reported, the charge-transfer resistance (Rct) and equivalent series resistance (ESR) are reflected by the diameter of the semicircle at high frequency region and the intersection of the semicircle on the real axis, respectively [16]. It is noted that both plots display similar appearance with no obvious semicircles at high-frequency region, demonstrating the significantly low Rct arising from the high-rate charge transfer process and fast electrode kinetics [42,43]. After the cycles ended, the intersection on the real axis at the highfrequency region only shifted from 0.599 to 0.655 Ω, indicating a very slight change in ESR after 10,000 cycles. Additionally, the slope of the

robust combination of the highly open Ni1Co1-S network with conductive Ni foam substrate can effectively avoid all disadvantages mentioned above, further manifesting the superiority of the constructed binder-free Ni1Co1-S/NF system. Moreover, through a rough comparison with dozens of recently reported nickel cobalt sulfide-based electrode materials (Table S2, Refs. S14-S36), Ni1Co1-S/NF showed impressive competitiveness and superiority for application in highperformance supercapacitors. As another crucial standard for evaluating supercapacitors, the cycling stabilities of the associated samples were examined by repeating the GCD process at 20 A g−1 for 10,000 cycles. For powdery Ni1Co1-S, the capacitance sharply decreased to only 39.7% of original value after 10,000 cycles (Fig. S11), while for Ni1Co1-S/NF (Fig. 4a), the capacitance decay is quite slight. Notably, ∼92.2% and ∼89.3% of the initial capacitance was retained after 5000 and 10,000 cycles, respectively,

Fig. 4. (a) Cycling performance and coulombic efficiencies of Ni1Co1-S/NF at a constant current density of 20 A g−1 for 10,000 cycles; (b) Nyquist plots of Ni1Co1-S/NF before and after the cycling measurements; (c) FESEM images of Ni1Co1-S/NF after 10,000 charge/discharge cycles.

36

Journal of Power Sources 378 (2018) 31–39

D. Zha et al.

drop. According to formula (3), the specific capacitances at various current densities of the ASC device were calculated (Fig. 5d), showing a high specific capacitance of 164.9 F g−1 at a current density of 1 A g−1. When the current density was increased to 40 A g−1, a specific capacitance of 98.9 F g−1 was still retained, reflecting the good rate capability. Meanwhile, Fig. 5d also illustrates the dependence of IR drops on the current densities of the device. The rather low IR drop values with a strong linear fit reveal the rapid I-V response and considerably low internal resistance, which favour high power delivery to satisfy practical applications [19,42]. As a reference, The Nyquist plot of the as-assembled ASC (Fig. S13) exhibits a rather low ESR of ∼0.16 Ω, resulting from the shortened ion pathway and improved electron transport [46]. In addition, the negligible semicircle at high frequency also indicates high speed charge transfer within the device for high power output. To examine the long-term durability of the ASC device, consecutive GCD tests at a constant current density of 10 A g−1 were carried out for 70,000 cycles (Fig. 5e). It is worth noting that the slight capacitance decay during the initial cycles can be ascribed to the insufficient wettability and reduced utilization factor of the electroactive material [23,36,47]. As cycling continued, the surface wetting and utilization of the active materials were improved, leading to the gradual increase in the capacitance [35,46]. Thereafter, the specific capacitance decreased quite slowly and ended up with a considerable retention of ∼92% even after 70,000 cycles, implying the long lifespan and brilliant long-term durability. Additionally, the coulombic efficiency was maintained at nearly 100% throughout the whole cycling examination, further illustrating the remarkable electrochemical reversibility, which can also be established by the almost unvaried GCD curves of the initial and last cycles (inset of Fig. 5e). As another two critical criteria for evaluating the practicability of asymmetric supercapacitors, the energy density and power density of the Ni1Co1-S/NF//NG ASC were calculated and illustrated as a Ragone plot in Fig. 6. Inspiringly, the ASC device delivers a maximum energy density of 58.1 Wh kg−1 at a power density of 796 W kg−1. When the power density was increased to as high as 25,081.7 W kg−1, an energy density of 21.6 Wh kg−1 was still retained, indicating the excellent high-power performance. It is noted that the obtained energy densities and specific power output are also superior to those of recently reported aqueous nickel cobalt sulfide-based asymmetric supercapacitors, such as VNCS//AC/G (44.9 Wh kg−1 at 870 W kg−1) [10], H-NiCo2S4//AC (35.6 Wh kg−1 at 819.5 W kg−1) [17], NiCo2S4/MWCNTs-5//rGO (51.8 Wh kg−1 at 865 W kg−1) [19], Ni@CNTs@Ni-Co-S//CC@CNTs (46.5 Wh kg−1 at 800 W kg−1) [22], NiCo2S4/NCF//OMC/NCF (45.5 Wh kg−1 at 512 W kg−1) [30], NiCo2S4@Ni3V2O8//AC (42.7 Wh kg−1 at 200 W kg−1) [33], NiCo2S4@Co(OH)2//AC (35.89 Wh kg−1 at 400 W kg−1) [35], and NiCo2S4@Ni(OH)2//AC (53.3 Wh kg−1 at 290 W kg−1) [48]. The high energy density can be comparable to the ASC devices using redox-active electrolyte, such as vanadium oxide/ carbon onion composite-based ASC (45 Wh kg−1 at 0.05 A g−1) [49]. Given that volumetric energy and power densities are crucial for practical applications, the corresponding volumetric energy density (Ev, mWh cm−3) and volumetric power density (Pv, mW cm−3) were also calculated based on the volumes of the two electrodes with a size of 1 × 1 × 0.15 cm3 (Fig. S14). The ASC device delivers a maximum volumetric energy density of 3.1 mWh cm−3 at a power density of 42.4 mW cm−3 and an energy density of 1.2 mWh cm−3 was still retained even at a high power density of 1336.9 mW cm−3. Encouragingly, the volumetric energy and power densities of as-constructed ASC device are also comparable to several recently reported asymmetric supercapacitors (Fig. S14, Refs. S37-S42). These abovementioned results clearly indicate that such an ASC device possesses attractive prospects in applications in advanced energy storage devices.

line section at low frequency region is related to the diffusion rate of electrolyte ions within electrode [16,44]. Therefore, the nearly vertical straight line section with only slightly varied slope after 10,000 cycles indicates the almost unaffected rapid ion diffusion ability and supercapacitive behaviour [16,44]. The nearly unvaried plots further verify the remarkable stability and highly reversible redox reactions of the Ni1Co1-S/NF electrode [36,45]. Fig. 4c presents the FESEM images of Ni1Co1-S/NF after the cycling examination. Inspiringly, apart from slight wrinkles on the surface of the nanosheets, the structural integrity was well maintained without obvious deformation or destruction. This steady structure intuitively suggests the excellent long-term stability of the Ni1Co1-S/NF electrode. Arguably, the impressive electrochemical performance of the Ni1Co1-S/NF electrode can be attributed to the significant synergistic effects of the components and the special nanostructure, which has the following merits. (1) The abundant ultrafine nanocrystals and small pores throughout the nanosheets, along with the expanded interlayer spacing caused by intercalated acetate anions, can effectively facilitate electrolyte diffusion into the matrix through the shortened diffusion paths. Therefore, more active sites can be efficiently exploited even at large current densities, leading to the significantly improved capacitance and rate capability. (2) The high crosslinking of oversized Ni1Co1S nanosheets with good integrality on the 3D Ni foam can provide massive channels for fast charge transport and facile electrolyte diffusion, thus resulting in enhanced capacitance and high-rate performance. (3) The strong connection between Ni1Co1-S nanosheets can stabilize the NiCo-S/NF structure with plentiful voids, which can effectively accommodate the volume variation during the repeated charge/discharge process and improve the long-term endurance [16,29]. (4) The robust combination with a highly conductive substrate not only enhances the electrical conductivity and mechanical stability of the electrode but also avoids disadvantages due to the use of conductive additives or polymer binders. 3.3. Electrochemical evaluation of a Ni1Co1-S/NF//NG asymmetric supercapacitor To further demonstrate the potential of as-obtained Ni1Co1-S/NF in practical application, an ASC device was assembled using Ni1Co1-S/NF and NG as the positive and negative electrode, respectively (Fig. 5a). The CV and GCD results of NG clearly illustrate the typical EDLC features within a stable potential window of −1∼0 V (Figs. S12a and S12b). Fig. S12c exhibits the specific capacitances of NG at different current densities. Prior to the construction of the ASC device, the active mass ratio of Ni1Co1-S/NF to NG was determined to be ∼1/4.7 (as described in supplementary data) according to charge balance formula (1). The active mass loading on positive and negative electrode was set as 1.4 and 6.6 mg, respectively. In the CV curves of Ni1Co1-S/NF and NG electrodes at 20 mV s−1 (Fig. S13a), the working voltage window of the ASC device was estimated to be 1.6 V. To further confirm the stable voltage, a series of CV tests within various operation voltages were conducted at 10 mV s−1 (Fig. S13b). When the upper potential limit was extended to 1.7 V, an obvious distortion induced by irreversible oxygen evolution reactions can be observed [13]. To this end, we selected the working voltage of 1.6 V to further investigate the electrochemical performance of the assembled ASC device. Fig. 5b displays the CV curves of the Ni1Co1-S/NF//NG ASC within the voltage of 0–1.6 V at various sweep rates. The quasi-rectangular CV shapes with obvious redox peaks imply the effective integration of electrical double layer capacitance and pseudocapacitance in the ASC device [17,32]. Meanwhile, the CV shape was nearly undistorted even at a high scan rate of 200 mV s−1, revealing the desirable reversibility and excellent rate capability [17,19,23]. Under the same potential window of 0–1.6 V, GCD measurements were performed at various current densities to evaluate the assembled ASC device. Fig. 5c shows the corresponding galvanostatic discharge curves without an obvious IR 37

Journal of Power Sources 378 (2018) 31–39

D. Zha et al.

Fig. 5. (a) Schematic illustration of the assembled Ni1Co1-S/NF//NG ASC device; (b) CV curves of the ASC device at various scan rates; (c) Galvanostatic discharge plots of the ASC device at various current densities; (d) Specific capacitance and internal resistance (or IR drop) variation with current density for the ASC device; (e) Cycling performance and coulombic efficiencies of the Ni1Co1-S/NF//NG ASC device at a constant current density of 10 A g−1 for 70,000 cycles. The inset of (e) displays the GCD curves of the initial 10 cycles (left) and last 10 cycles (middle) as well as the comparison of the first and last cycles (right).

to the specific architecture with noticeable synergistic effects among the components. The strongly integrated nanostructure, with excellent structural integrity, abundant electron transport channels and shortened diffusion pathways, significantly improves the electrical conductivity and ensures the sufficient utilization of electroactive species even during high-rate redox processes, thus distinctly enhancing the energy storage capacity, power output and long-term durability. These prominent supercapacitive behaviours not only suggest the great potential of our Ni1Co1-S/NF system in high-performance supercapacitors but also demonstrate the high feasibility of the precursor-controlling strategy to synthesize ideal electrode materials. Acknowledgements Fig. 6. Ragone plot of the Ni1Co1-S/NF//NG ASC device compared with several reported ASCs based on nickel cobalt sulfides.

This work was supported by the NNSF of China (No. 51572125, 51472101), the Natural Science Foundation of Jiangsu Province (No. BK20171423), the Fundamental Research Funds for the Central Universities (No. 30917015102, No. 30916014103), the Opening Project of the Jiangsu Key Laboratory forChemistry of Low-Dimensional Materials (No. 51472101) and PAPD of Jiangsu.

4. Conclusions In summary, a highly open binder-free electrode composed of interconnected oversized NiCo-S nanosheets robustly grown on Ni foam was purposefully designed and fabricated through a two-step method. The optimal Ni1Co1-S/NF electrode exhibited extraordinary supercapacitive properties, including ultrahigh specific capacitance (2553.9 F g−1 at 0.5 A g−1), superior rate capability (1898.1 F g−1 at 50 A g−1) and remarkable long-term cycling stability (nearly 90% capacitance retention after 10,000 cycles). Additionally, the assembled asymmetric supercapacitor device based on Ni1Co1-S/NF presented a prominent energy density of 58.1 Wh kg−1 at 796 W kg−1 and outstanding power density of 25,081.7 W kg−1 at 21.6 Wh kg−1. Moreover, the ASC device also delivered an incredibly long cycle life and striking cycling stability (∼92% capacitance retention after 70,000 cycles). The impressive electrochemical performance can be attributed

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.jpowsour.2017.12.020. References [1] F. Bonaccorso, L. Colombo, G. Yu, M. Stoller, V. Tozzini, A.C. Ferrari, R.S. Ruoff, V. Pellegrini, Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage, Science 347 (2015) 1246501. [2] J. Shen, J. Wu, L. Pei, M.-T.F. Rodrigues, Z. Zhang, F. Zhang, X. Zhang, P.M. Ajayan, M. Ye, CoNi2S4-graphene-2D-MoSe2 as an advanced electrode material for supercapacitors, Adv. Energy Mater. 6 (2016) 1600341.

38

Journal of Power Sources 378 (2018) 31–39

D. Zha et al.

[3] S. Ortaboy, J.P. Alper, F. Rossi, G. Bertoni, G. Salviati, C. Carraro, R. Maboudian, MnOx-decorated carbonized porous silicon nanowire electrodes for high performance supercapacitors, Energy Environ. Sci. 10 (2017) 1505–1516. [4] P. Xiong, J. Zhu, X. Wang, Recent advances on multi-component hybrid nanostructures for electrochemical capacitors, J. Power Sources 294 (2015) 31–50. [5] M.-S. Balogun, Y. Huang, W. Qiu, H. Yang, H. Ji, Y. Tong, Updates on the development of nanostructured transition metal nitrides for electrochemical energy storage and water splitting, Mater. Today, https://doi.org/10.1016/j.mattod.2017. 03.019. [6] M.-S. Balogun, W. Qiu, W. Wang, P. Fang, X. Lu, Y. Tong, Recent advances in metal nitrides as high performance electrode materials for energy storage devices, J. Mater. Chem. A 3 (2015) 1364–1387. [7] M. Boota, C. Chen, M. Bécuwe, L. Miao, Y. Gogotsi, Pseudocapacitance and excellent cyclability of 2,5-dimethoxy-1,4-benzoquinone on graphene, Energy Environ. Sci. 9 (2016) 2586–2594. [8] D. Zha, Y. Fu, X. Gao, L. Zhang, X. Wang, Intimately coupled hybrid of carbon black/nickel cobaltite for supercapacitors with enhanced energy-storage properties and ultra-long cycle life, Electrochim. Acta 257 (2017) 494–503. [9] Z. Zhu, W. Du, W. Guo, W. Zhu, Research progress of preparation and application of transition metal ternary compounds in supercapacitors, Chin. J. Appl. Chem. 33 (2016) 267–276. [10] S. Abureden, F.M. Hassan, G. Lui, S. Sy, R. Batmaz, W. Ahn, A. Yu, Z. Chen, Modified chalcogens with a tuned nanoarchitecture for high energy density and long life hybrid super capacitors, J. Mater. Chem. A 5 (2017) 7523–7532. [11] Y. Luo, M.S. Balogun, W. Qiu, R. Zhao, P. Liu, Y. Tong, Sulfurization of FeOOH nanorods on a carbon cloth and their conversion into Fe2O3/Fe3O4–S core-shell nanorods for lithium storage, Chem. Commun. 51 (2015) 13016–13019. [12] M.S. Balogun, W. Qiu, J. Jian, Y. Huang, Y. Luo, H. Yang, C. Liang, X. Lu, Y. Tong, Vanadium nitride nanowire supported SnS2 nanosheets with high reversible capacity as anode material for lithium ion batteries, ACS Appl. Mater. Interfaces 7 (2015) 23205–23215. [13] X.-Y. Yu, L. Yu, X.W.D. Lou, Metal sulfide hollow nanostructures for electrochemical energy storage, Adv. Energy Mater. 6 (2016) 1501333. [14] B.Y. Guan, L. Yu, X. Wang, S. Song, X.W. Lou, Formation of onion-like NiCo2S4 particles via sequential ion-exchange for hybrid supercapacitors, Adv. Mater. 29 (2017) 1605051. [15] J. Zhu, S. Tang, J. Wu, X. Shi, B. Zhu, X. Meng, Wearable high-performance supercapacitors based on silver-sputtered textiles with FeCo2S4-NiCo2S4 composite nanotube-built multitripod architectures as advanced flexible electrodes, Adv. Energy Mater. 7 (2016) 1601234. [16] J. Zhao, Z. Li, M. Zhang, A. Meng, Q. Li, Vertically cross-linked and porous CoNi2S4 nanosheets-decorated SiC nanowires with exceptional capacitive performance as a free-standing electrode for asymmetric supercapacitors, J. Power Sources 332 (2016) 355–365. [17] Y. Wen, S. Peng, Z. Wang, J. Hao, T. Qin, S. Lu, J. Zhang, D. He, X. Fan, G. Cao, Facile synthesis of ultrathin NiCo2S4 nano-petals inspired by blooming buds for high-performance supercapacitors, J. Mater. Chem. A 5 (2017) 7144–7152. [18] J. Yang, C. Yu, X. Fan, S. Liang, S. Li, H. Huang, Z. Ling, C. Hao, J. Qiu, Electroactive edge site-enriched nickel-cobalt sulfide into graphene frameworks for high-performance asymmetric supercapacitors, Energy Environ. Sci. 9 (2016) 1299–1307. [19] P. Wen, M. Fan, D. Yang, Y. Wang, H. Cheng, J. Wang, An asymmetric supercapacitor with ultrahigh energy density based on nickle cobalt sulfide nanocluster anchoring multi-wall carbon nanotubes hybrid, J. Power Sources 320 (2016) 28–36. [20] J.-G. Wang, D. Jin, R. Zhou, C. Shen, K. Xie, B. Wei, One-step synthesis of NiCo2S4 ultrathin nanosheets on conductive substrates as advanced electrodes for high-efficient energy storage, J. Power Sources 306 (2016) 100–106. [21] X. Liu, Z. Wu, Y. Yin, Hierarchical NiCo2S4@PANI core/shell nanowires grown on carbon fiber with enhanced electrochemical performance for hybrid supercapacitors, Chem. Eng. J. 323 (2017) 330–339. [22] T. Peng, H. Yi, P. Sun, Y. Jing, R. Wang, H. Wang, X. Wang, In situ growth of binderfree CNTs@Ni-Co-S nanosheets core/shell hybrids on Ni mesh for high energy density asymmetric supercapacitors, J. Mater. Chem. A 4 (2016) 8888–8897. [23] K.P. Annamalai, L. Liu, Y. Tao, Highly exposed nickel cobalt sulfide-rGO nanoporous structures: an advanced energy-storage electrode material, J. Mater. Chem. A 5 (2017) 9991–9997. [24] Z. Li, X. Li, L. Xiang, X. Xie, X. Li, D.-R. Xiao, J. Shen, W. Lu, L. Lu, S. Liu, Threedimensional hierarchical nickel-cobalt-sulfide nanostructures for high performance electrochemical energy storage electrodes, J. Mater. Chem. A 4 (2016) 18335–18341. [25] X. Cai, X. Shen, Z. Ji, X. Sheng, L. Kong, A. Yuan, Synthesis and remarkable capacitive performance of reduced grapheneoxide/silver/nickel-cobalt sulfide ternary nanocomposites, Chem. Eng. J. 308 (2017) 184–192. [26] T. Wang, Q. Le, G. Zhang, S. Zhu, B. Guan, J. Zhang, S. Xing, Y. Zhang, Facile preparation and sulfidation analysis for activated multiporous carbon@NiCo2S4 nanostructure with enhanced supercapacitive properties, Electrochim. Acta 211 (2016) 627–635. [27] Y. Tingting, L. Ruiyi, L. Zaijun, G. Zhiguo, W. Guangli, L. Junkang, Hybrid of

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40] [41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

39

NiCo2S4 and nitrogen and sulphur-functionalized multiple graphene aerogel for application in supercapacitors and oxygen reduction with significant electrochemical synergy, Electrochim. Acta 211 (2016) 59–70. Y. Zhu, X. Ji, Z. Wu, Y. Liu, NiCo2S4 hollow microsphere decorated by acetylene black for high-performance asymmetric supercapacitor, Electrochim. Acta 186 (2015) 562–571. X. He, Q. Liu, J. Liu, R. Li, H. Zhang, R. Chen, J. Wang, High-performance all-solidstate asymmetrical supercapacitors based on petal-like NiCo2S4/Polyaniline nanosheets, Chem. Eng. J. 325 (2017) 134–143. L. Shen, J. Wang, G. Xu, H. Li, H. Dou, X. Zhang, NiCo2S4 Nanosheets grown on nitrogen-doped carbon foams as an advanced electrode for supercapacitors, Adv. Energy Mater. 5 (2015) 1400977. R. Zou, M.F. Yuen, L. Yu, J. Hu, C.S. Lee, W. Zhang, Electrochemical energy storage application and degradation analysis of carbon-coated hierarchical NiCo2S4 coreshell nanowire arrays grown directly on graphene/nickel foam, Sci. Rep. 6 (2016) 20264. M. Yan, Y. Yao, J. Wen, L. Long, M. Kong, G. Zhang, X. Liao, G. Yin, Z. Huang, Construction of a hierarchical NiCo2S4@PPy core-shell heterostructure nanotube array on Ni Foam for a high-performance asymmetric supercapacitor, ACS Appl. Mater. Interfaces 8 (2016) 24525–24535. L. Niu, Y. Wang, F. Ruan, C. Shen, S. Shan, M. Xu, Z. Sun, C. Li, X. Liu, Y. Gong, In situ growth of NiCo2S4@Ni3V2O8 on Ni foam as a binder-free electrode for asymmetric supercapacitors, J. Mater. Chem. A 4 (2016) 5669–5677. L. Ma, Y. Hu, R. Chen, G. Zhu, T. Chen, H. Lv, Y. Wang, J. Liang, H. Liu, C. Yan, H. Zhu, Z. Tie, Z. Jin, J. Liu, Self-assembled ultrathin NiCo2S4 nanoflakes grown on Ni foam as high-performance flexible electrodes for hydrogen evolution reaction in alkaline solution, Nano Energy 24 (2016) 139–147. R. Li, S. Wang, Z. Huang, F. Lu, T. He, NiCo2S4@Co(OH)2 core-shell nanotube arrays in situ grown on Ni foam for high performances asymmetric supercapacitors, J. Power Sources 312 (2016) 156–164. D. Zha, H. Sun, Y. Fu, X. Ouyang, X. Wang, Acetate anion-intercalated nickel-cobalt layered double hydroxide nanosheets supported on Ni foam for high-performance supercapacitors with excellent long-term cycling stability, Electrochim. Acta 236 (2017) 18–27. D. Zha, P. Xiong, X. Wang, Strongly coupled manganese ferrite/carbon black/ polyaniline hybrid for low-cost supercapacitors with high rate capability, Electrochim. Acta 185 (2015) 218–228. H. Chen, L. Hu, M. Chen, Y. Yan, L. Wu, Nickel–cobalt layered double hydroxide nanosheets for high-performance supercapacitor electrode materials, Adv. Funct. Mater. 24 (2014) 934–942. Y. Song, Z. Chen, Y. Li, Q. Wang, F. Fang, Y.-N. Zhou, L. Hu, D. Sun, Pseudocapacitance-tuned high-rate and long-term cyclability of NiCo2S4 hexagonal nanosheets prepared by vapor transformation for lithium storage, J. Mater. Chem. A 5 (2017) 9022–9031. C. Xia, P. Li, A.N. Gandi, U. Schwingenschlögl, H.N. Alshareef, Is NiCo2S4 really a semiconductor? Chem. Mater. 27 (2015) 6482–6485. X. Xiong, G. Waller, D. Ding, D. Chen, B. Rainwater, B. Zhao, Z. Wang, M. Liu, Controlled synthesis of NiCo2S4 nanostructured arrays on carbon fiber paper for high-performance pseudocapacitors, Nano Energy 16 (2015) 71–80. F. Lai, Y.E. Miao, L. Zuo, H. Lu, Y. Huang, T. Liu, Biomass-derived nitrogen-doped carbon nanofiber network: a facile template for decoration of ultrathin nickel-cobalt layered double hydroxide nanosheets as high-performance asymmetric supercapacitor electrode, Small 12 (2016) 3235–3244. K. Chi, Z. Zhang, Q. Lv, C. Xie, J. Xiao, F. Xiao, S. Wang, Well-ordered oxygendeficient CoMoO4 and Fe2O3 nanoplate arrays on 3D graphene foam: toward flexible asymmetric supercapacitors with enhanced capacitive properties, ACS Appl. Mater. Interfaces 9 (2017) 6044–6053. Y. Zhao, H. Ma, S. Huang, X. Zhang, M. Xia, Y. Tang, Z.F. Ma, Monolayer nickel cobalt hydroxyl carbonate for high performance all-solid-state asymmetric supercapacitors, ACS Appl. Mater. Interfaces 8 (2016) 22997–23005. X. Zang, Z. Dai, J. Guo, Q. Dong, J. Yang, W. Huang, X. Dong, Controllable synthesis of triangular Ni(HCO3)2 nanosheets for supercapacitor, Nano Res. 9 (2016) 1358–1365. T. Li, W. Zhang, L. Zhi, H. Yu, L. Dang, F. Shi, H. Xu, F. Hu, Z. Liu, Z. Lei, J. Qiu, High-energy asymmetric electrochemical capacitors based on oxides functionalized hollow carbon fibers electrodes, Nano Energy 30 (2016) 9–17. S. Zeng, H. Chen, F. Cai, Y. Kang, M. Chen, Q. Li, Electrochemical fabrication of carbon nanotube/polyaniline hydrogelfilm for all-solid-state flexible supercapacitor with high areal capacitance, J. Mater. Chem. A 3 (2015) 23864–23870. Y. Yang, D. Cheng, S. Chen, Y. Guan, J. Xiong, Construction of hierarchical NiCo2S4@Ni(OH)2 core-shell hybrid nanosheet arrays on Ni foam for high-performance aqueous hybrid supercapacitors, Electrochim. Acta 193 (2016) 116–127. S. Fleischmann, M. Zeiger, N. Jäckel, B. Krüner, V. Lemkova, M. Widmaier, V. Presser, Tuning pseudocapacitive and battery-like lithium intercalation in vanadium dioxide/carbon onion hybrids for asymmetric supercapacitor anodes, J. Mater. Chem. A 5 (2017) 13039–13051.