Construction of nickel cobalt sulfide nanosheet arrays on carbon cloth for performance-enhanced supercapacitor

Journal Pre-proof Construction of nickel cobalt sulfide nanosheet arrays on carbon cloth for performance-enhanced supercapacitor Tao Liu, Jiahao Liu, Liuyang Zhang, Bei Cheng, Jiaguo Yu

PII:

S1005-0302(20)30170-5

DOI:

https://doi.org/10.1016/j.jmst.2019.12.027

Reference:

JMST 2017

To appear in:

Journal of Materials Science & Technology

Received Date:

25 November 2019

Revised Date:

24 December 2019

Accepted Date:

31 December 2019

Please cite this article as: Liu T, Liu J, Zhang L, Cheng B, Yu J, Construction of nickel cobalt sulfide nanosheet arrays on carbon cloth for performance-enhanced supercapacitor, Journal of Materials Science and amp; Technology (2020), doi: https://doi.org/10.1016/j.jmst.2019.12.027

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Research Article

Construction of nickel cobalt sulfide nanosheet arrays on carbon cloth for performance-enhanced supercapacitor

Tao Liu1, Jiahao Liu1, Liuyang Zhang1*, Bei Cheng1, Jiaguo Yu1,2*

State Key Laboratory of Advanced Technology for Materials Synthesis and

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Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, China

School of Materials Science and Engineering, Zhengzhou University, Zhengzhou

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* Corresponding authors.

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450001, China

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Zhang)

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E-mail addresses: [email protected] (J. Yu), [email protected] (L.

Abstract: Materials featured with self-supported three-dimensional network,

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hierarchical pores and rich electrochemical active sites are considered as promising electrodes for pseudocapacitors. Herein, a novel strategy for the growth of nickel-cobalt bisulfide

(NiCoS) nanosheets arrays on carbon cloth (CC) as

supercapacitor electrodes is reported, involving deposition of two-dimensional metal-organic framework (MOF) precursors on the CC skeletons, conversion of MOF

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into nickel-cobalt layered double-hydroxide by ion exchange process and formation of NiCoS by a sulfidation treatment. The NiCoS nanosheets with rough surface and porous structures are uniformly anchored on the CC skeletons. The unique architecture endows the composite (NiCoS/CC) with abundant accessible active sites. Besides, robust electrical/mechanical joint between the nanosheets and the substrates is attained, leading to the improved electrochemical performance. Moreover, an

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asymmetric supercapacitor device is constructed by using NiCoS/CC and activated

carbon as a positive electrode and a negative electrode, respectively. The optimized device exhibits a high specific capacitance, large energy density and long cycle life.

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The NiCoS/CC electrode with intriguing electrochemical properties and mechanical

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flexibility holds great prospect for next-generation wearable devices.

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Keywords: Metal-organic framework, Nickel sulfide, cobalt sulfide, Carbon cloth,

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Supercapacitor

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1. Introduction

With increasing consumption of fossil fuels (e.g., coal, petroleum, and natural

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gas), the sustainable and renewable energy resources (e.g., solar, wind, and tidal energy) have aroused extensive attention [1-3]. It is greatly crucial to store the energy by electrochemical energy storage devices. Supercapacitors have been considered as one of the most promising energy storage devices for the next-generation electronics and

electric

vehicles,

owing

to

their

2

high

power

density

and

short

charging/discharging time together with long life span [4-9]. Based on the storage mechanism, supercapacitors can be divided into electrical double-layer capacitors (EDLCs) and pseudocapacitors, which store the electrochemical energy via charge accumulation and redox reactions, respectively [10-15]. Carbonaceous materials are one typical case for EDLCs; while transition metal oxides/sulfides are classic pseudocapacitive materials. However, both of them suffer from relatively low energy

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density, which becomes a huge barrier to the application of supercapacitors. According to the equation (E=0.5×CV2), the energy density can be greatly enhanced

by either improving specific capacitances (C) of electrode materials or by designing

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asymmetric supercapacitors to expand their operation voltage. In this work, targeted

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at high energy density, the aforementioned two means are collectively adopted. Transition metal sulfides are chosen as the electrode materials because of their

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unique physiochemical properties and high theoretical capacitances. Their high

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capacitances originate from the multiple oxidation states, which can consume or produce more electrons during redox reactions. For instance, nickel sulfides [16,17],

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cobalt sulfides [18,19] and iron sulfides [20] as well as their ternary sulfides [21-24] exhibited excellent electrochemical performances. Moreover, when transition metal

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sulfides incorporated with conductive carbon materials (graphene [25-28], carbon nanotube [29-31], hollow carbon sphere [32-34] and carbon fiber [35,36]), their composites possessed high electrical conductivity and enhanced electrochemical active surface area. Although these efforts have made considerable progress in improving electrochemical performances, the “dead surface” of metal sulfide

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electrodes caused by using the polymer binders still exists, limiting the permeation of the electrolyte to the inner electrode and the transfer of electrons in the electrode. Therefore, diverse three-dimension (3D) conductive networks (nickel foam [37-39], carbon cloths (CC) [40-42], graphene foam [43-46], et al.) have been introduced into the electrode materials. The active materials can directly grow on the 3D conductive substrates, averting the use of binders or conductive additives. Besides, under this

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circumstance, a short ion diffusion path is realized, enabling fast reversible charge transfer and boosting the adhesion between the active materials and substrates.

Based on the previous analysis, nickel-cobalt sulfide (NiCoS) nanosheet arrays

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have been grown on the CC skeletons in this work. A novel method was proposed,

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including in-situ growth of two-dimensional cobalt metal-organic framework (Co-MOF) on the CC surface, and formation of nickel cobalt layered double

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hydroxide (NiCo-LDH) by ion exchange, followed by the hydrothermal sulfidation. The NiCoS/CC composites were directly evaluated as supercapacitor electrodes and

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exhibited a high specific capacitance of 1653 F g–1 at the current density of 1 A g–1.

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Additionally, a flexible asymmetric supercapacitor was assembled by using NiCoS/CC as a positive electrode and AC as a negative electrode. The flexible device

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delivered a high energy density of 40 Wh kg–1 at a power density of 379 W kg–1.

2. Experimental section 2.1 Preparation of Co-MOF/CC Carbon cloth (CC) was cleaned by sequential sonication in the mixture of

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acetone and ethanol for 30 mins each, followed by UV-ozone treatment of 15 mins. Then CC was cut into small pieces (1.5×6 cm2, back side protected with tape). 45 mL of an aqueous solution containing 2-methylimidazole (MIM, 0.4 M) was quickly mixed with 45 mL of Co(NO3)2·6H2O aqueous solution (50 mM). Then a piece of CC substrate was vertically immersed into the mixture solution for 1 h. The Co-MOF nanosheets anchored on the CC skeletons were obtained after washing with purified

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water and drying for 12 h. The mass loading of Co-MOF on CC was ~1.5 mg cm–2. 2.2 Synthesis of NiCo-LDH/CC

NiCo-LDH/CC was prepared by immersing the prepared Co-MOF/CC in 35 mL

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of ethanol solution containing 45 mg of Ni(NO3)2·6H2O. Next, solvothermal

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treatment was carried out at the temperature of 120 ℃ for 2 h. Finally, NiCo-LDH/CC was obtained after washing with ethanol and drying in a vacuum oven.

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The mass loading of NiCo-LDH nanosheets on the CC was ~0.75 mg cm–2.

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2.3 Synthesis of NiCo-bisulfide/CC

NiCo-LDH/CC was immersed in 40 mL of an aqueous solution containing

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Na2S·6H2O (600 mg), and sealed in a 50 mL Teflon steel autoclave. NiCoS/CC was harvested after incubating at 90 °C for 10 h. The mass loading of NiCoS nanosheets

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on the CC substrate was determined to be about 0.6 mg cm–2. The control sample, i.e. powdered NiCoS was obtained without the immersion of CC. 2.4 Material characterization Powder X-ray diffraction (XRD) patterns were collected on a Shimadzu XRD 6100 diffractometer (Cu Kα, λ = 0.15418 nm). Field emission scanning electron

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microscopy (FESEM, JSM-7500) and transmission electron microscopy (TEM, Titan G2) were employed to observe the microstructure morphology. X-ray photoelectron spectroscopy (XPS) results were obtained via a Thermo ESCALA 250. Contact angle measurement was conducted on a Theta instrument (Biolin Scientific). 2.5 Electrochemical characterization The NiCoS/CC was directly served as the working electrode. To prepare the

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powdered NiCoS or activated carbon electrodes, active materials, acetylene black and polyvinylidene fluoride with the mass ratio of 8:1:1 were mixed in N-Methyl

pyrrolidone (NMP) solvent, and then the obtained slurry was brushed over carbon

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cloth. After air-drying at 100°C overnight, the electrodes were obtained. These

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electrodes were tested in 2 M KOH aqueous electrolyte. For the three-electrode configuration, platinum foil and a Hg/HgO electrode were served as the counter

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electrode and the reference electrode, respectively. For the asymmetric

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supercapacitors, NiCoS/CC and activated carbon electrodes were used as a positive electrode and a negative electrode, respectively. Cyclic voltammetry (CV),

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galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) was recorded through an electrochemical workstation (CHI 760E). For the

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three-electrode system, GCD curves were measured by using the electrodes (1 cm × 2 cm) with the current densities of 1, 2, 3, 5 and 10 A g–1. CV curves were obtained at the scan rate from 5 to 50 mV s–1 within the potential window of 0~0.6 V. EIS measurements were conducted at open circuit potential with the frequency ranging

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from 100 kHz to 10 mHz at an amplitude of 5 mV. And the electrochemical analysis software Zview and electrical equivalent circuit were used to simulate the EIS data.

3. Result and discussion

The synthesis procedure of NiCoS/CC electrodes is illustrated in Fig. 1,

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embracing in-situ growth, ion exchange and sulfidation. Firstly, CC substrate was

treated with ultraviolet (UV) and O3 to improve its surface wettability. As shown in Fig. S1, the contact angle of CC substrate before and after UV and O3 treatment were

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129.5° and 46.2°, respectively, indicating the improvement of hydrophilicity. In

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the step I, two-dimensional Co-MOF flakes were uniformly deposited on the CC skeletons via a facile reaction between cobalt ions and MIM in aqueous solution.

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Subsequently, Co-MOF/CC was converted into NiCo-LDH/CC by ion exchange and

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etching process [47] (step II). Due to the release of hydroxyl and nickel ions from the hydrolysis of nickel nitrate hexahydrate in ethanol, Co-MOF templates were gradually

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etched by the protons and released cobalt ions. Afterwards, NiCo-LDH nanosheets were formed by coprecipitation of cobalt and nickel ions. Finally, the NiCo-LDH/CC

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was sulfurized to generate NiCoS/CC via hydrothermal treatment at the presence of Na2S (step III).

Obviously, 1 hour synthesis yielded Co-MOF flakes, which were uniformly and vertically anchored on the CC substrate (Fig. 2(a) and 2(b)). The corresponding

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energy-dispersive X-ray spectroscopy (EDX) spectra (Fig. S2) verify that N, C and Co elements coexisted in the Co-MOF/CC. From the magnified images in Fig. 2(c), the surface of Co-MOF flakes with triangular cross-section were smooth. After the transformation of Co-MOF/CC into NiCo-LDH/CC in the presence of Ni(NO3)2, NiCo-LDH nanosheets were still closely deposited on the CC skeletons (Fig 2(d) and 2(e)), and their enlarged images showed that abundant tiny nanoflakes appeared on

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their surface (Fig. 2(f)). The formation of an additional layer of thin slices on the

NiCo-LDH surface was ascribed to the coprecipitation of cobalt and nickel ions during the hydrolysis of nickel nitride hexahydrate. The EDX spectra of

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NiCo-LDH/CC confirmed that Ni element was successfully incorporated into the

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nanosheets (Fig. S3). For NiCoS/CC, it integrally inherited the nanoarray structure of NiCo-LDH/CC (Fig. 2(g) and 2(h)). And the surface of NiCoS nanosheet was covered

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with chiffon-like nanoflakes (Fig. 2(i)). The EDX images of NiCoS/CC illustrated that

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Ni, Co and S elements were uniformly dispersed on the surface of carbon fiber (Fig. 2(j)).

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The microstructure features and chemical composition of NiCoS nanosheet were further measured by TEM. The NiCoS nanosheet was were composed of numerous

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small nanoplates (Fig. 3(a)). As exhibited in the HRTEM of NiCoS nanosheets (Fig. 2(b)), the spacing of the lattice fringes was calculated to be 2.873 and 2.277 Å, matching well with the (110) d-spacing of Ni3S2 and the (331) d-spacing of Co9S8, respectively. The high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image with the corresponding elemental mapping

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images (Fig. 2(c)) manifested the uniform distribution of Ni, Co and S across the structure, indicating the successful generation of NiCoS nanosheets.

The crystal structures of the products in the synthesis process were evaluated by XRD (Fig. 4(a) and Fig. S4). The XRD patterns of Co-MOF/CC agree with those reported Co-MOF[41], indicating the successful deposition of Co-MOF on the CC

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substrate. After the etching treatment by Ni(NO3)2, four peaks located at 11.1°, 22.2°, 33.2° and 60.5° emerged, which can be respectively indexed to (003), (006), (009)

and (110) planes, revealing NiCo-LDH was yielded. XRD peaks of NiCoS can be

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assigned to Ni3S2 (JCPDS 44-1418) [48-50] and Co9S8 (JCPDS 65-6801) [51]. The

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evolution of these composites instead of binary sulfides can be attributed to the different solubility product constant between nickel sulfide and cobalt sulfide. Briefly,

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the XRD patterns confirmed the transformation from Co-MOF/CC to NiCoS/CC.

The XPS survey spectrum of NiCoS/CC (Fig. S5) presents the existence of Co,

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Ni, S, C and O elements. And Table S1 exhibits the atomic percentages of each element in NiCoS. The atomic ratio between Co and Ni was calculated to be about

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1.5:1. The high resolution XPS spectrum of Ni 2p (Fig. 4(b)) is deconvoluted into two spin-orbit doublets. Two doublets centered at 855.5 and 852.6 eV and at 872.8 and 869.9 eV were assigned to Ni2+ and Ni3+, respectively, agreement with the characteristic peaks of Ni3S2 [52]. For Co 2p XPS spectrum (Fig. 4(c)), a significant spin-orbit doublet located at 781.8 and 797.3 eV was ascribed to the Co 2p3/2 and 2p1/2

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signals of Co2+, respectively, while another doublet located at 778.2 and 793.7 eV was consistent with the XPS results of Co9S8 [53]. Meanwhile, the high resolution XPS spectrum of S 2p displayed two peaks at 162.0 and 163.18 eV, which were indexed to the S 2p3/2 and 2p1/2 signals of S2–, respectively. Besides, an additional peak at 168.2 eV on the S 2p spectrum corresponds to oxidized sulfur groups (sulfate or sulfonate).Overall, XPS results were pointed to Ni3S2 and Co9S8, in agreement with

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XRD analysis.

To evaluate the supercapacitive performances of NiCoS/CC and pristine NiCoS,

we utilized a three-electrode system in 2 M KOH aqueous electrolyte for

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electrochemical measurement. As illustrated in Fig. 5(a), the NiCoS/CC electrode

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delivered exceeding electrochemical behaviors and larger current density are compared with the pristine NiCoS electrode. Additionally, oxidation peaks at

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0.35-0.55 V and reduction peaks at 0.1-0.3 V were obviously observed on the CV

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curves, indicating that the reversible redox reaction could occur within the potential window of 0-0.6 V. The integrated CV area of NiCoS/CC at the sweeping rate of 10

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mV s–1 was much larger than that of pristine NiCoS, suggesting that NiCoS/CC could store more electrochemical energy than powdered NiCoS. With increasing scan rates

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from 5 to 30 mV s–1 (Fig. S6 (b)), the anodic and cathodic peaks shifted toward higher and lower potentials, respectively, due to the polarization of the electrode and the decline of the ion diffusion rate under high scan rates. Moreover, the GCD curves of NiCoS/CC and pristine NiCoS electrodes (Fig. S6 (c) and (d)) were fairly symmetric, indicating good electrochemical reversibility. As shown in Fig. 5(b), a clear voltage

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plateau at ~0.35 V was greatly consistent with the CV curves. However, the internal resistance (iR) drop of pristine NiCoS electrode was serious while that of NiCoS/CC electrode was almost negligible; illustrating that NiCoS/CC electrode had small contact resistance and a fast I–V response. In comparison with pristine NiCoS, longer discharging time of NiCoS/CC represented higher specific capacitance. The rate performances of NiCoS/CC and pristine NiCoS have been estimated by

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the GCD test. As shown in Fig. 5(c), NiCoS/CC electrode exhibited high specific

capacitances of 1653, 1595, 1491, 1375 and 1344 F g–1 at the current densities of 1, 2,

4, 8 and 10 A g–1, respectively. Even at 20 A g–1, it still maintained a high specific

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capacitance of 1275 F g–1, indicating the electrode possessed outstanding rate

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capability and superior pseudocapacitance. The decrease of the specific capacitance with increasing current density was due to the increment of voltage drop and the

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insufficient participation of active materials in the electrochemical reactions at higher

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current density. By contrast, the specific capacitances of pristine NiCoS electrode were only one third of those of NiCoS/CC at the same current densities. Cycling

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stability of the NiCoS/CC material was evaluated by measuring the values of specific capacitances as a function of the cycling number at the current density of 10 A g–1

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(Fig. 5(d)). It should be mentioned that the specific capacitance of NiCoS/CC in the whole cycling process surpassed that of pure NiCoS. After 3000 successive cycles, the retention of the initial specific capacitance for NiCoS/CC was still about 84%, which was much higher than that for pristine NiCoS (73%). It therefore implied that NiCoS/CC show superiority over NiCoS in terms of stability. To delve into the

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reasons for its preponderance, we examined its structure variation after testing 3000 cycles. From the FESEM images of NiCoS/CC electrode (Fig. S7), the morphology integrity of NiCoS/CC electrode was successfully preserved. The NiCoS nanosheets were still closely attached on the CC skeletons without any delamination from the substrate. To summarize, the high specific capacitance, excellent rate performance and long cycling life of NiCoS/CC suggest that it is a promising alternative as electrode

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material for high energy density supercapacitor. Besides, the direct growth method showed great advantages over the traditional slurry-coating method.

Electrochemical impedance spectroscopy was utilized to appraise the transport

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kinetics of electrolyte ions and electrons [54,55]. Fig. 6(a) shows the Nyquist plots of

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NiCoS/CC and pristine NiCoS electrodes. The real axis intercept at high-frequency region (Fig. 6(a) inset) represented the equivalent series resistance (Rs), which

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consists of the internal resistance of active material together with the contact resistant

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between active material and current collector. Based on an equivalent circuit (inset in Fig. 6(a)), the NiCoS/CC had a lower Rs (0.864 Ω) than pristine NiCoS (1.25 Ω),

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which was attributed to the direct growth of NiCoS nanosheets on the surface of CC. In the high-frequency region, the NiCoS/CC electrode had a Faradaic charge-transfer

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resistance (Rct) of 0.11 Ω based on the radius of the semicircle, which was smaller than pristine NiCoS (0.26 Ω), suggesting the accelerated charge transfer. In the medium-frequency region, the Warburg-type line of NiCoS/CC was much shorter than pristine NiCoS, indicating the fast diffusion of electrolyte ions in the NiCoS/CC electrode. This was mainly due to the sufficient diffusion paths induced by the vertical

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deposition of NiCoS nanosheets on the CC surface. In the Bode plots (Fig. 6(b)), the characteristic frequency (f0) at the phase angle of ‒45o for NiCoS/CC and pristine NiCoS were 8.33 and 5.55 Hz which can be converted to the time constants (τ0) of 0.12 and 0.18 s, respectively. That validates faster frequency response of NiCoS/CC. To get a clearer visualization, structural features and charge storage mechanism of the NiCoS/CC electrode are combined in Fig. 6(c). The direct growth of NiCoS

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nanosheets on the CC skeletons can offer enough interspace for electrolyte

penetration, which avail most of the active materials. The surface roughness and porosity of NiCoS nanosheets enable sufficient contact area with electrolyte, leading

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to fast diffusion of electrolyte ions from the surface to the interior. Moreover, the

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NiCoS/CC electrode free of conductive additives and polymer binders can significantly get rid of the “dead surface”, enhancing the utilization of the active

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materials. Importantly, the CC framework is in tight junction with the NiCoS

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nanosheets, serving as a conductive path to accelerate the electron transfer during the charging/discharging process. Therefore, the unique architecture of NiCoS/CC

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endows it with accelerated ion diffusion and electron transfer. The electrochemical performances of supercapacitor device were evaluated by

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assembling NiCoS/CC as a positive electrode and activated carbon as a negative electrode in 2 M KOH electrolyte. Based on the charge balance theory between an anode and a cathode, the optimal mass ratio between NiCoS/CC and AC was 1:3.3 (calculated based on the electrochemical performance of AC in Fig. S8). Therefore, the mass loading of AC on the carbon cloth was about 2 mg cm-2. Fig. S9 exhibits

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stable CV curves in the potential windows from –1.0 to 0.0 V for AC with ideal EDLC behavior and from 0.0 to 0.6 V for NiCoS/CC with typical Faradaic behavior, respectively. In Fig. 7(a), negligible distortion has been witnessed in the shapes of CV curves even at a high sweep rate of 50 mV s–1, suggesting outstanding supercapacitive behavior with fast and reversible electrochemical energy storage capability. As shown in Fig. 7(b), the GCD curves of the NiCoS/CC device were obtained within the

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voltage window of 0–1.5 V. The charging time was almost equal to the discharging

time, declaring impressive reversibility of the NiCoS/CC electrode. The specific capacitances of the NiCoS/CC devices were calculated according to the mass of active

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materials in both electrodes. The device delivered a high capacitance of 128 F g–1 at

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the current density of 0.5 A g–1, and a capacitance of 98 F g–1 was still maintained when the current density was increased ten times, revealing distinguished

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electrochemical kinetic behavior.

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The Ragone plot (Fig. 7(c)) shows the relationship between the power density and the energy density for the NiCoS/CC device and other reported counterparts. The

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NiCoS/CC device delivered a high energy density of 40 Wh kg–1 at a power density of W

kg–1,

which

was

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CC@NiCo2O4//CC@N-carbon

higher

(31.9

W

than h

kg–1

its

counterparts, at

2900

W

such kg–1)

as [41],

NiCo2O4//NiCo2O4 (38.3 W h kg–1 at 396 W kg–1) [56], hollow double-shelled NiO//AC (21.4 W h kg–1 at 375 W kg–1) [57] and CNT/Ni(OH)2//rGO (35 W h kg–1 at 1800 W kg–1) [58]. The energy density of NiCoS/CC device comparing with the previously-reported NiCo-based asymmetric supercapacitor is shown in Table 1. Even

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at a relatively high power density of 8860 W kg–1, an energy density of 23 W h kg–1 was still retained. The high energy density of the NiCoS/CC device sprang from the high capacitance of each electrode and the enlarged voltage window. Moreover, the cycling performance of the device was also measured (Fig. 7(d)). After 7000 cycles, the capacitance of the NiCoS/CC device still retained 84% of its initial capacitance, manifesting a competitive cycling stability. The long lifespan was attributed to the

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desirable architecture of NiCoS/CC electrode. 4. Conclusion

In summary, we proposed a binder-free supercapacitor electrode in which NiCoS

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nanosheets were deposited on the surface of CC matrix. This design has several

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benefits: namely, (1) strong mechanical adhesion between NiCoS and CC backbone stemming from the direct growth guarantees rapid charge transfer kinetics; (2)

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abundant active sites provided from the ultrathin NiCoS flakes is conductive to Benefiting

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electrochemical storage; (3) conductive CC expedites electron transfer.

from the structural assets and optimized chemical composition, the NiCoS/CC

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electrode exhibited an excellent pseudocapacitive performance including high specific capacitance, good rate capability and long cycling life. Meanwhile, the NiCoS/CC

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asymmetric supercapacitor delivered a high energy density without sacrificing its high power density. We believed that the strategy in the present work would become prevalent in preparing performance-enhanced supercapacitor electrodes with outstanding electrochemical performance.

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Conflicts of interest The authors declare no competing financial interests.

Acknowledgements This work was supported by NSFC (21801200, U1905215, U1705251 and 51872220), Innovative Research Funds of SKLWUT (2017-ZD-4) and Fundamental

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Research Funds for the Central Universities (WUT:2019IVB050).

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22

Fig. 1. Synthetic scheme for the growth of NiCoS/CC via a facile chemical bath

ur

na

lP

re

-p

ro of

deposition method followed with ion exchange and sulfidation treatment.

Jo

Fig. 2 FESEM images of (a-c) Co-MOF/CC, (d-f) NiCo-LDH/CC and (g-i) NiCoS/CC. (j) elemental mapping images of NiCoS/CC.

23

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Fig. 3 (a) TEM image, (b) HRTEM and (c) HAADF-STEM image and elemental

Jo

ur

na

lP

re

-p

mapping images for NiCoS nanosheet.

Fig.4 (a) XRD patterns of CC, Co-MOF/CC, NiCo-LHD/CC and NiCoS/CC. (b) Ni 2p, (c) Co 2p and (d) S 2p high-resolution XPS spectra of NiCoS/CC. 24

ro of -p

Fig. 5 (a) CV curves, (b) GCD curves, (c) specific capacitance at various current

re

densities and (d) cycling performance of pure NiCoS and NiCoS/CC measured at 20

Jo

ur

na

lP

A g–1.

25

ro of -p re

lP

Fig. 6 (a) Nyquist plot and (b) Bode phase angle plot of NiCoS/CC and pure NiCoS electrodes. (c) Schematic mechanism of the NiCoS/CC electrode for electrochemical

Jo

ur

na

energy storage.

26

ro of -p

Fig. 7 The device constructed by NiCoS/CC as the positive electrode and AC as the

re

negative electrode: (a) CV curves, (b) GCD curves, (c) Ragone plot and (d) cycle

Jo

ur

na

lP

performance.

27

Table 1. Comparison of our work with related literature on energy density and power

NiCo2S4/graphene aerogel

3 M KOH

1.6

1600

34.9

[59]

6 M KOH

1.0

100

10.2

[60]

3 M KOH

1.6

390

36.2

[61]

2 M KOH

1.6

800

35.8 

[62]

800

20.9

[63]

689

41.4

[64]

701

37.9

[65]

725

35.2

[50]

700

26.6

[66]

PVA-KOH 1.6 gel 3 M KOH 1.6 6 M KOH

1.4

1 M KOH

1.5

3 M KOH

1.4

2 M KOH

0.55

242

17.0

[67]

1 M KOH

1.4

1823

35.6

[68]

1 M KOH

1.6

793

47.3

[69]

3 M KOH

1.6

800

29.1

[70]

6 M KOH

1.6

400

42.5

[71]

0.9

200

16.7

[72]

1.6

1600

36.0

1.5

379

40.0

[73] This work

ur

na

lP

NiCo2S4 nanocages NiCo hydroxide/carbon nanotube NiCo2S4@Ni3S2 hybrid nanoarray on Ni foam NiCo2S4 nanoneedles on mesocarbon microbeads NiCo2S4@CoS2 on carbon cloth Nickel–cobalt phosphide hollow microspheres Nickel nanowire @ NiCo2S4 arrays Nickel cobalt selenide complex hollow spheres NiCo double hydroxide microspheres

Energy density Ref. (W h kg– 1 )

NiCoS/CC

2 M KOH

Jo NiCo2S4-rGO

2 M NaOH 1 M KOH

NiCo2S4/g-C3N4

ro of

NiCo hydroxides/graphene aerogel Hollow NiCoP nanosphere NiCoMn ternary sulfide nanoneedles on Ni foam CoSx/Ni-Co LDH nanocages

Power density (W kg–1)

re

Materials

Voltage Electrolyte range (V)

-p

density.

28