Core-shell Cu2-xS @ CoS2 heterogeneous nanowire array with superior electrochemical performance for supercapacitor application

Electrochimica Acta 323 (2019) 134839

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Core-shell Cu2-xS @ CoS2 heterogeneous nanowire array with superior electrochemical performance for supercapacitor application Mengjiao Gao a, Kai Le b, Guanwen Wang a, Zhou Wang a, Fenglong Wang a, Wei Liu b, Jiurong Liu a, * a Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education and College of Materials Science and Engineering, Shandong University, No. 17923 Jingshi Road, Lixia District, Jinan, Shandong, 250061, China b Institute of Crystal Materials, Shandong University, No. 27 Shanda South Road, Jinan, Shandong, 250100, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 July 2019 Received in revised form 3 September 2019 Accepted 5 September 2019 Available online 9 September 2019

Tuning the composition and microstructure of heterogeneous nanocomposites is an effective way to get high-performance electrode materials. In this study, we demonstrate an optimized structuralengineering strategy to fabricate the core-shell Cu2-xS @ CoS2 (CCS) nanowire array to maximize the electrochemical properties of composite materials. Structure and composition of the resultant are determined by various characterization methods, like transmission electron microscopy and X-ray photoelectron spectrometer. When applied as electrode material, the well-designed CCS sample exhibits large reversible capacity (1007 F g
1 at 2 A g
1) and superior cycling stability (113.6% over 5000 cycles). Assembled asymmetric supercapacitor employing CCS and reduced graphene oxide electrode also shows high energy density (35.4 W h kg
1) and power density (8250 W kg
1), as well as excellent cycling performance (104.7% after 8000 cycles). Microscopic characterizations and electrochemical measurements demonstrate that the excellent supercapacitive performance is attributed to the abundant reactive sites originated from the synergistic effects between the heterogeneous composition and superior structural stability from ingenious core-shell nanowires. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Cu2S CoS2 Core-shell structure Supercapacitor In-situ growth

1. Introduction The development of electrical energy storage devices is one of the most critical issues in the world [1]. Electricity storage generated from intermittent sources and the large-scale application of electrical vehicles is heavily dependent on the widespread utilization of devices with high energy and power [2,3]. Supercapacitors are regarded as promising devices with merits of fast chargedischarge time, long cycling life and high power density, but the low energy density restricts its application. Design and synthesis of high-capacity electrode material is a key solution to overcome this problem [4]. In recent years, metal sulfides, like NiS [5,6], CoS2 [7e9], Co9S8 [10,11], NiCo2S4 [12,13] have been widely investigated due to high electrochemical activity, decent electrical conductivity and diversified crystal structure [14]. In contrast, copper sulfides have received less attention although it has low band gap [15e17], abundant resources and environmental friendliness [18,19].

* Corresponding author. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.electacta.2019.134839 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

However, the capacity and energy density of the pristine Cu2S is still too limited to meet the requirement of practical application. To overcome this drawback and improve electrochemical performance, CoS2 with high capacity is chosen to synthesize hybrid material with Cu2S for high-performance electrode of supercapacitor. Nanostructure plays an important role in the properties of hybrid materials. Compared to ordinary mixture, core-shell structure can fully achieve synergistic effect of materials, and promote electrochemical reaction [20]. First, core-shell structure is beneficial for facilitating electrolyte infiltration and shortening electron/ ion transport pathway of electrode to promote electrochemical reaction and expedite reaction rate, thus boosting the capacitive properties [21e23]. Second, the shell can accommodate strain caused by the redox reaction occurring within electrode and the core could offer mechanical support to avoid the morphological collapse during electrochemical reaction, thus improving the cycling stability of materials [24]. Third, direct growth on current collector can avoid the stack of materials and increase specific surface area [25]. And deadweight of inactive polymer binder can

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also be removed, so the overall energy density of electrode materials is improved [26,27]. Here, we synthesized Cu2-xS @ CoS2 nanowire array directly grown on Cu foam (CCS) by a combined approach of anodized oxidation, chemical-bath deposition and hydrothermal sulfidation process. Benefited from hybrid composition and core-shell structure, the material exhibits outstanding electrochemical performance. The specific capacitance reaches up to 1007 F g
1 at current density of 2 A g
1, and cycling stability is as good as 113.6% capacity retention after 5000 cycles, simultaneously. Asymmetric supercapacitor assembled using CCS as positive and rGO as negative electrodes presents maximum energy density of 35.4 W h kg
1 and power density of 8250 W kg
1. Cycling test indicates that 104.7% capacity remains after 8000 cycles, with Coulombic efficiency keeping over 95%. The results suggest the well-designed core-shell composite can be a promising candidate material for energy storage applications. 2. Experimental 2.1. Materials All chemicals used are of analytical grade without further purification, including Cu foam (thickness of 0.3 mm), potassium hydroxide (KOH), cobalt sulfate heptahydrate (CoSO4$7H2O), urea (CH4N2O), and sodium sulfide nonahydrate (Na2S$9H2O). 2.2. Synthesis of CuO/Cu2O nanowire array The schematic diagram of synthesis procedure is shown in Scheme 1. First, the Cu foam was rinsed by HCl solution and acetone to remove the oxide layer and organic impurity. Then the cleaned Cu foam was electrochemically anodized in 2 M KOH solution on a three-electrode system with current density of 2.5 mA cm
2 for 15 min. Subsequently, the Cu foam with Cu(OH)2 nanowires were flushed with water and ethanol, and then dried at 60  C overnight. Finally, the Cu(OH)2 nanowires were calcined in a muffle furnace at 300  C for 3 h to obtain CuO/Cu2O nanowire array (Scheme 1, I). 2.3. Synthesis of Cu2-xS @ CoS2 core-shell nanowire array 5 mmol CoSO4$7H2O and 70 mmol urea were dissolved in 60 mL deionized water under stirring, and Cu foam with CuO/Cu2O nanowire array were immersed in the mixed solution. After sealed by aluminium foil, the mixture was heated to 85  C and maintained for 10 h to obtain CuO/Cu2O @ CoO2 (CCO) nanowires (Scheme 1, II). Then, the CCO sample was hydrothermally treated in 60 mL Na2S solution (10 mM) at 120  C for 10 h to preparing Cu2-xS @ CoS2 (CCS) nanowires (Scheme 1, III). For comparison, Cu2-xS nanowires (CS) were synthesized by direct sulfidation of CuO/Cu2O nanowires

under the same condition.

3. Results and discussion 3.1. Materials characterization As is shown in Figure S1, the Cu foam is uniformly coated with CuO/Cu2O nanowires, and in the enlarged images, the nanowires exhibit smooth surface with diameter of 100e300 nm (Fig. 1a, e). After the growth of CoO2 shell, the columnar CuO/Cu2O nanowire changed into conical CCO nanowire, arranging densely on Cu foam substrate (Fig. 1b, f). After sulfidation process, the core-shell nanowires structure of CCS sample is well remained, as is shown in Fig. 1c and g. This structure has large specific surface area and great amount of void spaces, which is beneficial for electrolyte penetration and ions exchange, thus facilitating chemical reactions. Moreover, some small particles appear at the edge of CCS nanowire (Fig. 1g), which is owing to fast recrystallization and growth of grain arising from active reaction of dangling bonds on the outer edge and Na2S solution. Similarly, in the image of CS sample (Fig. 1d, h), the surface of nanowires is covered by delicate nanosheet after sulfidation, which is also due to recrystallization and oriented growth of Cu2S crystal. The elemental mapping images of CCS coreshell nanowire are shown in Fig. 1i-l, and Co, Cu, S elements are uniformly distributed on the selected area. Figure S2 shows XRD pattern of CuO/Cu2O nanowires and confirms its composition. XRD pattern of CS and CCS nanowires is shown in Fig. 2a. The three strong peaks at 43.3, 50.4 and 74.1 derived from Cu foam substrate (PDF# 04e0836), and peaks at 27.9, 32.3, 36.2, 39.8 and 57.6 can be assigned to (111), (200), (210), (211) and (222) planes of CoS2 (PDF# 41e1471). Besides, the other peaks at 39.1, 45.4, 45.5, 53.8 and 71.0 marked by diamond symbol can be indexed to Cu1.96S (PDF# 29e0578). And the extra peaks can be assigned to monoclinic Cu2S (PDF# 33e0490). The results indicate that CCS nanowires mainly consist of Cu2S, Cu1.96S and CoS2, so the compound is indicated as Cu2-xS @ CoS2. Similarly, the pattern of CS sample indicates the composite includes Cu substrate, Cu2S and Cu1.96S substance. The Raman measurement was carried out and the results are shown in Fig. 2b. There are four typical peaks located at 72, 120, 266 and 472 cm
1, corresponding to characteristic band of CueS, which reveals the existence of copper sulfide [28,29]. And the peak at 394 cm
1 indicates the in-phase stretching vibration of sulfur atoms in the dumbbells of CoS2 materials [30].The results suggest the composition of copper sulfide and cobalt sulfide. TEM images are shown in Fig. 3 to observe the nanostructure of CCS sample. Fig. 3a presents the core-shell CCS nanowire with diameter of about 300 nm, and the inner Cu2S nanowire with diameter of about 150 nm is uniformly coated by CoS2 shell. Besides, the nanowire is composed by a number of tiny grains with

Scheme 1. The synthesis of (I) CuO/Cu2O nanowires, (II) CuO/Cu2O @ CoO2 (CCO), and (III) Cu2-xS @ CoS2 (CCS) core-shell nanowire array.

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Fig. 1. SEM images of (a,e) CuO/Cu2O nanowires, (b,f) CCO, (c,g) CCS, and (d,h) CS samples at different magnification. (i)e(l). The EDX mapping of CCS nanowire.

Fig. 2. (a) XRD pattern of CCS and CS sample with scanning range from 15 to 85 . (b) Raman spectra of CCS sample.

porous structure, which is beneficial for electrolyte penetration, ions diffusion and electron transportation. The high magnification image of the nanowire is shown in Fig. 3b and c. In Fig. 3b, lattices with spacing of 0.30 and 0.47 nm correspond to (312) and (211) crystal planes of Cu2S, and in Fig. 3c, fringes of 0.27 and 0.32 nm can be assigned to (200) and (111) planes of CoS2, which is consistent with many reported works [7,31,32]. In the pattern of selected area electron diffraction (SAED), six clear concentric diffraction rings in Fig. 3d indicate polycrystalline characteristic of the nanowire, corresponding to (211), (312), (106) crystal planes of Cu2S and (200), (211), (222) planes of CoS2, respectively, which is in coincidence with XRD results. XPS measurement is used to further confirm the valence states and electronic properties of the composite. In Figure S3, peaks of Cu 2p, Co 2p, O 1s, S 2p can be observed at the corresponding binding energy in survey scan. Fig. 4a shows the spectrum of Cu 2p. Peaks at 952.3 and 932.5 eV with splitting of 19.8 eV can be indexed to Cu 2p1/2 and Cu 2p3/2, and peaks at 962.5 and 942.3 eV are the satellite

peaks of them, indicating the existence of Cuþ in Cu2S phase [18]. Besides, peaks at 954.3 and 934.4 eV can be ascribed to Cu 2p1/2 and Cu 2p3/2 of Cu2þ [33], which is because of the coexistence of Cuþ and Cu2þ in Cu1.96S, and oxidized copper sulfides on the sample surface. In the spectrum of Co 2p of Fig. 4b, peaks at 796.9 and 781.0 eV correspond to Co 2p1/2 and Co 2p 3/2, with two satellite peaks at 803.1 and 786.6 eV, respectively, which demonstrates the Co2þ ions of CoS2 [31,34]. And the fluctuating spectrum line suggests the low content of CoS2, which is also confirmed by the short peak of Co 2p in Figure S3. O 1s spectrum is shown in Fig. 4c. The strong peak at 531.4 eV indicates OH
from water molecule absorbed on surface, and the residual minor peak at 529.9 eV is due to metal-oxygen bond, indicating a partial oxidation of the sulfides on surface [33]. Spectrum of S 2p is shown in Fig. 4d, the S 2p1/2 and S 2p3/2 peaks are located in 163.1 and 162.0 eV, respectively. Along with the satellite peak at 168.3 eV, the existence of S2
can be verified [35,36]. According to the above analysis as well as XRD and TEM results, the composition of Cu2-xS and CoS2 is confirmed.

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Fig. 3. TEM images of CCS core-shell nanowire at (a) low and (b, c) high magnification, as well as (d) SAED pattern with the inset as the selected area.

Fig. 4. XPS spectra of (a) Cu 2p, (b) Co 2p, (c) O 1s and (d) S 2p.

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Fig. 5. (a) CV curves of CCS and CS samples at scanning rate of 20 mV s
1. (b) GCD curves of CCS and CS samples at current density of 2 A g
1. (c) CV curves of CCS sample at various scanning rates. (d) GCD curves of CCS sample with current densities of 2e20 A g
1. (e) CV curves of CS sample at various scanning rates. (f) GCD curves of CS sample with different current densities. (g) Cycling performance of CCS sample at current density of 3 A g
1. (h) EIS measurement of CCS and CS samples, and the inset is the enlarged plots of lowfrequency region.

3.2. Electrochemical properties Fig. 5a shows the CV curves of CCS and CS samples with the same scanning rate of 20 mV s
1 at potential range from
0.1e0.6 V. It can be clearly seen that the curve of CCS covers larger area than CS, indicating higher specific capacitance from better charge storage ability, due to synergistic effect of CoS2 and Cu2-xS as well as core-shell structure [37]. Besides, there are a pair of peaks at 0.45/0.35 V on the CV curve of CS sample, indicating pseudocapacitive behavior of Cu2S, which can be expressed as follows [18].

Cu2 S þ OH
4Cu2 SOH þ e


(1)

While in the curve of CCS sample, expect for redox peaks of Cu2S at 0.43/0.33 V (the unnoticeable reduction peaks may be due to the fact that compared to activated redox reaction of CoS2, the contribution of Cu2S to the total capacity is much less), there are other two pairs of peaks at
0.07/-0.04 V and 0.20/0.25 V, which exhibit pseudocapacitive behavior, owing to reversible Faradic reaction of CoS2 and CoS2O [38,39].

CoS2 þ OH
4CoS2 OH þ H2 O þ e


(2)

CoS2 OH þ OH
4CoS2 O þ H2 O þ e


(3)
1

GCD curves of the two samples at current density of 2 A g are shown in Fig. 5b, with the distinct plateaus suggesting typical battery-type behavior due to Faradic reaction [40]. Besides, the longer discharge time of CCS sample also indicates higher capacity. Fig. 5c and e exhibit the CV curves of CCS and CS with obvious scanning rates of 5, 10, 20, 50 mV s
1. With the scanning rate increases, these curves exhibit higher current density and larger area, but maintain similar shape, indicating highly efficient electrons and ions transportation behavior of host material [24]. Additionally, under higher scanning rate, the oxidation and reduction peak shifts to higher and lower potential, respectively, which is attributed to the effect of charge transfer kinetics on redox reaction [41]. And the slight shift of redox peaks also suggests good reversibility in high scanning rate [42]. The GCD curves of CCS with current densities from 2 to 20 A g
1 are shown in Fig. 5d. These almost symmetric curves demonstrate reversible redox reaction, and the low iR drop reflects good electric conductivity of the material. According to

equation (4), the specific capacitance (Cs) can be calculated as 1007.3, 879.1, 694.6, 616.4, 527.3 F g
1 at current density of 2, 5, 10, 15, 20 A g
1. The result is much higher than that of CS sample (Fig. 5f, 264.0 and 246.4, 240.9, 240.0, 205.5 F g
1 at 2, 5, 10, 15, 20 A g
1) and many other cobalt sulfides [31,43e48], verifying the great effect of synergistic composition and core-shell structure on the improvement of electrochemical performance.

Cs ¼

I  Dt m  DV

(4)

where I means charge/discharge current, Dt and DV represents discharge time and potential window, respectively, and m is the unit mass of active material on electrode, which is 1.5 mg cm
2. Moreover, the bare Cu foam was hydrothermally sulfurized in Na2S solution under the same condition and its electrochemical performance is depicted in Figure S4a, b. Compared with CCS sample, the narrow CV curve and short charge-discharge time indicates lower capacitance. The result suggests that the high capacity of CCS sample is attributed to the synergistic effect of CoS2 and Cu2-xS as well as core-shell structure, rather than the sulfurized Cu foam. The cycling performance is measured by GCD test at current density of 3 A g
1 with voltage range of
0.1-0.45 V. As is shown in Fig. 5g, during the first 300 cycles, the Cs of CCS sample increases gradually from 772 to 880 F g
1, which is attributed to the activation of electrode material arising from slow infiltration of electrolyte into hybrid structure [49,50]. In the subsequent cycles, the capacitance presents slow attenuation, but the final capacity maintains 877 F g
1, 113.6% retention of the first cycle, showing superior cycling stability. And the Coulombic efficiency reaches from 81% to 97%, also indicating outstanding reversibility. The cycling performance of CS sample is shown in Figure S4c, after 5000 cycles, the Cs decreases from 239.4 to 237.7 F g
1, and the capacitance retention is 99.3%. The result suggests that the CoS2 shell can not only increase the capacitance, but improve cycling stability, which may because the core-shell structure provides sufficient space to accommodate volume change due to the redox reaction occurring in the electrode, thus preventing the destruction of nanostructure. The SEM images of CCS electrode after 5000 cycles are shown in Figure S5. It can be seen that the core-shell nanowire structure is well-maintained, confirming good cycling stability of the sample in three-electrode system. The reaction kinetic behavior is investigated by EIS measurement in Fig. 5h. The EIS curve consists

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of the semi-circle part in high-frequency region and the straight line in low-frequency region. The radius of the semi-circle represents charge-transfer resistance (Rct) between electrode surface and electrolyte, which of CCS sample (Rct ¼ 0.77 U) is much lower than CS (Rct ¼ 3.46 U). The result can be attributed to the fact that the porous shell is beneficial for the electrolyte penetration, thus facilitating the ion/charge transportation at the electrodeelectrolyte interface. The intercept on Z’ axis indicates internal resistance (Rs), including the resistance of electrolyte and active materials, as well as contact resistance between active material and current collector [51]. The CS sample exhibits low Rs value (Rs ¼ 1.74 U), which is due to the high electrical conductivity derived from Cu foam substrate with high conductivity and in-situ growth of Cu2-xS nanowire [18,52]. Besides, the performance of CCS sample is further enhanced with highly-conductive CoS2 shell and core-shell structure, so lower Rs (Rs ¼ 1.30 U) is exhibited. In lowfrequency region, the straight line reflects Warburg resistance, representing the diffusion process of ions from electrolyte into inner electrode. And the steeper curve of CCS sample clearly demonstrates better diffusion property obtained from porous core-shell nanowires structure. According to the above discussion, the high performance of CCS sample is attributed to the mechanism illustrated in Fig. 6. First, the CoS2 shows high conductivity, and the multiple valence states of Co element can trigger abundant redox reactions, thus resulting in high capacity. Besides, the porous conical shell possesses large specific surface area and sufficient active sites, which can expedite the diffusion process of OH
, thus promoting redox action and increasing capacity. Second, benefited from the growth of Cu2-xS nanowires by anodized-oxidation method on Cu foam, the CCS sample can in-situ grow on the substrate directly. The structure can bring in the decreased charge-transfer resistance between CCS sample and current collector, and prevent exfoliation of the active material. Moreover, the Cu2-xS nanowire core can not only provide effective electron transportation pathway to improve the conductivity of electrode, but also offer mechanical support for the coreshell nanowires to avoid structure collapse due to volume changes during redox reaction process, thus enhancing cycling stability. Third, the Cu substrate provides high conductivity and interconnected tunnel, which can further improve conductivity and promote redox reaction. Fourth, the growth of Cu2-xS nanowires on Cu foam and the CoS2 shell on the outside of Cu2-xS nanowires can form hierarchical architecture and heterogeneous interface, the former is beneficial for the migration of electrolyte ions into

Fig. 6. Structure and mechanism illustration of CCS sample.

electrode, and the latter could create more reaction active sites [53,54]. Benefit from the above reasons, electrochemical properties of CCS electrode can be improved. An asymmetric supercapacitor device (ASC) is assembled using CCS sample and reduced graphene oxide (rGO) as positive and negative electrode, respectively. Figure S6a shows the CV curves of rGO with potential window from
1.0 to 0.0 V. Different from the distorted curve with obvious redox peaks of CCS sample, the quasirectangular shape illustrates electric-double-layer capacitor (EDLC) characteristic of rGO material. The symmetric GCD curves also confirm EDLC characteristic, and the Cs is calculated to be 164.2, 154.4, 145.8, 140.8, 137.0 F g
1 at current density from 1 to 5 A g
1 (Figure S6b). Therefore, the charge between CCS and rGO electrode is balanced by adjusting the mass (m) of active material according to the following equation:

mþ = m
¼ Cs
V
=Csþ Vþ

(5)

Fig. 7a shows CV curves of ASC device at different potential window with scanning rate of 10 mV s
1. It can be clearly seen that the curve of 1.7 V exhibits conspicuous redox peaks without obvious polarization, so it is applied as the appropriate working potential window. Under this voltage range, CV curves with scanning rates from 5 to 50 mV s
1 are depicted in Fig. 7b. The overall profile of these curves looks like quasi-rectangular shape, indicating the existence of electric-double-layer capacitance from rGO electrode. And the obvious redox peaks demonstrate the pesudocapacitance contribution of CCS electrode. GCD curves of the device are shown in Fig. 7c. The calculated Cs is 93.5 F g
1 at current density of 1 A g
1, and retains 24.6 F g
1 at 10 A g
1, showing outstanding rate capability (Fig. 7d). Cycling performance is tested at current density of 3 A g
1, and after 8000 cycles, the Cs increases from 51.5 to 53.9 F g
1, 104.7% of the first cycle, with Coulombic efficiency maintaining more than 95%, indicating excellent cycling stability (Fig. 7e). The SEM and TEM images of CCS electrode after 8000 cycles in ASC device are shown in Figure S7. In SEM images (Figure S7a, b), it can be clearly seen that the tip of CCS nanowires is slightly damaged due to higher strain during redox reaction, but the core-shell nanowire profile is well-maintained. The detailed TEM images (Figure S7c, d) also show similar results, indicating that core-shell structure is beneficial for alleviating the structural damage caused by the volume change in cycles and improving stability of the sample. The XPS measurements of CCS sample after 8000 cycles in device were conducted and shown in Figure S8. From the spectra, it can be seen that the spectral lines and the peaks position of Cu, Co, O, S elements showed no significant change, indicating that the composition of CCS sample maintained stable after cycles. As is shown in Figure S8a, compared to Cu 2p spectrum in Fig. 4a, the relative intensity of Cuþ/Cu2þ increases, which is may be attributed to the transformation of Cu2þ and Cuþ during redox reaction. In Figure S8b, the Co 2p spectrum confirms the existence of Co2þ, indicating the composition stability of CoS2 shell during long cycles. And in Figure S8c, the intensity of O2
arising from surface absorption is much higher than the O2
in metal-oxygen bond. The result is due to the absorption of much OH
arising from redox reaction between active materials and electrolyte ions. Spectrum of S 2p is shown in Figure S8d, the fluctuating spectrum line indicates low content of S element on surface, caused by the slight oxidation of sulfide during reaction. According to the above results, metal oxide/hydroxide was generated in redox reaction, and the structure and composition of CCS sample maintained well during long cycles [55,56]. Ragone plot between energy density (E) and power density (P) is depicted to evaluate the overall performance of device, the

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Fig. 7. (a) CV curves of ASC device at different potential window with scanning rate of 10 mV s
1. (b) CV curves of ASC device at various scanning rates. (c) GCD curves of ASC device at current density from 1 to 10 A g
1. (d) Specific capacitance of ASC device at different current density. (e) Cycling performance of ASC device at current density of 3 A g
1. (f) Ragone plots of ASC device with many reported works.

calculation of which can be formulated as follows:

1 E ¼ Cs V 2 2

(6)

P ¼ E=t

(7)

The maximum energy density is 35.4 W h kg
1 with power density of 825 W kg
1, and remains 9.3 W h kg
1 at maximum power density of 8250 W kg
1 (Fig. 7f). The result is higher than or comparable to many related materials, as listed in Table 1 [31,43e49,57,58]. 4. Conclusions In summary, core-shell Cu2-xS @ CoS2 nanowire array on Cu

foam was prepared by chemical-bath growth followed by a subsequent sulfidation process. Benefited from the synergistic effect of Cu2-xS with structural support and CoS2 with high capacitance, as well as porous core-shell structure with large specific surface area and fast ion diffusion pathway, the product presents high specific capacitance (1007 F g
1 at 2 A g
1) and stable cycling performance (113.6% capacity retention after 5000 cycles). Asymmetric supercapacitor assembled by CCS and rGO materials also exhibits high energy density of 35.4 W h kg
1 and maximum power density of 8250 W kg
1, as well as excellent cycling stability of 104.7% retention after 8000 cycles. These results indicate that through rational design of composition and structure of the nanocomposites, improved electrochemical performance can be expected and this study will provide more guidance for the future fabrication of highperformance electrode materials.

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Table 1 Comparison of the electrochemical properties of CCS with reported works. Sample

MoS2/CoS2 nanotube [57] CoS2 hollow nanotube [31] CoS2 hollow nanoframe [45] CoS2 octahedron [46] rGO/CoS2 nanocomposite [43] Co9S8 nanosheet [48] Co9S8 @ Ni(OH)2 nanotube [47] Cu2O/CuO/Co3O4 nanowire [58] Cobalt sulfide nanotubes [49] Hollow cobalt sulfide [44] Cu2S @ CoS2 nanowires (this work)

Specific Capacitance

142.5 mF cm
2 (1 mA cm
2) 883.2 F g
1 (2 A g
1) 421 F g
1 (2 A g
1) 225.3 F g
1 (2 A g
1) 621.3 F g
1 (2 A g
1) 847.7 F g
1 (2 A g
1) 900 F g
1 (2 A g
1) 318 F g
1 (0.5 A g
1) 285 F g
1 (0.5 A g
1) 420 F g
1 (1 A g
1) 1007 F g
1 (2 A g
1)

Cycling Performance

Energy density/Power density

Three-electrode System

ASC Device

92.7%-1000cycles (1 mA cm
2) 83%-5000cycles (5 A g
1) 88.33%-5000cycles (0.5 A g
1) 92.6%-2000cycles (2 A g
1) 95.4%-2000cycles (8 A g
1) 87.4%-1000cycles (10 A g
1) e 80%-3000 cycles (5 A g
1) 86.5%-1000cycles (0.5 A g
1) 100%-5000cycles 113.6%-4000 cycles (3 A g
1)

e e 86.7%-5000 cycles (0.5 A g
1) e e 84.4%-1000 cycles (5 A g
1) 97.3%-5000 cycles (3 A g
1) e e e 104.7%-8000 cycles (3 A g
1)

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51572157 and No. 51502159); the Fundamental Research Funds of Shandong University (2015JC016, 2015JC036, and 2018JC046); the Natural Science Foundation of Shandong Province (ZR2016BM16); and the Qilu Young Scholar Scheme of Shandong University (No. 31370088963043). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.134839. References [1] P. Geng, S. Zheng, H. Tang, R. Zhu, L. Zhang, S. Cao, H. Xue, H. Pang, Transition metal sulfides based on graphene for electrochemical energy storage, Adv. Energy Mater. 8 (2018) 1703259. [2] F. Beguin, V. Presser, A. Balducci, E. Frackowiak, Carbons and electrolytes for advanced supercapacitors, Adv. Mater. 26 (2014) 2219e2251. [3] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845e854. [4] S. Chandra Sekhar, G. Nagaraju, J.S. Yu, High-performance pouch-type hybrid supercapacitor based on hierarchical NiO-Co3O4-NiO composite nanoarchitectures as an advanced electrode material, Nano Energy 48 (2018) 81e92. [5] T. Liu, C. Jiang, B. Cheng, W. You, J. Yu, Hierarchical NiS/N-doped carbon composite hollow spheres with excellent supercapacitor performance, J. Mater. Chem. 5 (2017) 21257e21265. [6] Q. Qin, L. Chen, T. Wei, X. Liu, MoS2/NiS yolk-shell microsphere-based electrodes for overall water splitting and asymmetric supercapacitor, Small 15 (2019), 1803639. [7] S. Wang, Y. Song, Y. Ma, Z. Zhu, C. Zhao, C. Zhao, Attaining a high energy density of 106 Wh kg-1 for aqueous supercapacitor based on VS4/rGO/CoS2 @ Co electrode, Chem. Eng. J. 365 (2019) 88e98. [8] X.D. Lou, Electronic modulation of CoO/CoS2/Cu1.81S hierarchical tubular heterostructures for high energy density hybrid supercapacitors, Angew. Chem. Int. Ed. (2019), https://doi.org/10.1002/anie.201907516. Accepted Author Manuscript. [9] Y. Su, C. Wu, Y. Song, Y. Li, Y. Guo, S. Xu, Sulfides/3D reduced graphene oxide composite with a large specific surface area for high-performance all-solidstate pseudocapacitors, Appl. Surf. Sci. 488 (2019) 134e141. [10] S. Sun, J. Luo, Y. Qian, Y. Jin, Y. Liu, Y. Qiu, X. Li, C. Fang, J. Han, Y. Huang, Metalorganic framework derived honeycomb Co9S8 @ C composites for highperformance supercapacitors, Adv. Energy Mater. 8 (2018) 1801080. [11] J. Xu, Q. Wang, X. Wang, Q. Xiang, B. Hang, D. Chen, G. Shen, Flexible asymmetric supercapacitors based upon Co9S8 nanorod//Co3O4 @ RuO2 nanosheet arrays on carbon cloth, ACS Nano 7 (2013) 5453e5462. [12] 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) 24525e24535. [13] 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. [14] S. Tang, B. Zhu, X. Shi, J. Wu, X. Meng, General controlled sulfidation toward achieving novel nanosheet-built porous square-FeCo2S4-tube arrays for highperformance asymmetric all-solid-state pseudocapacitors, Adv. Energy Mater.

11.11 W h kg
1/480 W kg
1 34.68 W h kg
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1 33.8 W h kg
1/400 W kg
1 11.8 W h kg
1/300 W kg
1 13.8 W h kg
1/824.6 W kg
1 20 W h kg
1/828.5 W kg
1 12.5 W h kg
1/2500 W kg
1 6 W h kg
1/1100 W kg
1 14.25 W h kg
1/150 W kg
1 19.4 W h kg
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1/825 W kg
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