Synthesis and properties of morphology controllable copper sulphide nanosheets for supercapacitor application

Accepted Manuscript Title: Synthesis and properties of morphology controllable copper sulphide nanosheets for supercapacitor application Author: Wence Xu Yanqin Liang Yungao Su Shengli Zhu Zhenduo Cui Xianjin Yang Akihisa Inoue Qiang Wei Chunyong Liang PII: DOI: Reference:

S0013-4686(16)31435-9 http://dx.doi.org/doi:10.1016/j.electacta.2016.06.118 EA 27559

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

20-4-2016 30-5-2016 22-6-2016

Please cite this article as: Wence Xu, Yanqin Liang, Yungao Su, Shengli Zhu, Zhenduo Cui, Xianjin Yang, Akihisa Inoue, Qiang Wei, Chunyong Liang, Synthesis and properties of morphology controllable copper sulphide nanosheets for supercapacitor application, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.06.118 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and properties of morphology controllable copper sulphide nanosheets for supercapacitor application Wence Xua, Yanqin Lianga, Yungao Sua, Shengli Zhua, b, c, 1, Zhenduo Cuia,c, Xianjin Yanga,b,2, Akihisa Inouea, Qiang Weia, Chunyong Liangd a

School of Materials Science and Engineering, Tianjin University，Tianjin, 300072, China

b

c

Tianjin Key Laboratory of Composite and Functional Materials，Tianjin, 300072, China

Key Laboratory of Advanced Ceramics and Mchining Technology, Ministry of Education, Tianjin, 300072, China

d

Research Institute for Energy Equipment Materials, Hebei University of Technology, Tianjin 300130, China

1 2

Corresponding author. Tel: +86 22 27402494; Fax: +86 22 27404724; E-mail: [email protected] Corresponding author. Tel: +86 22 27402494; Fax: +86 22 27404724; E-mail: [email protected]

Abstract

CuS nanosheets and their various assembled forms prepared by dealloying a Ti-Cu amorphous alloy have been investigated as electrochemical pseudocapacitor materials for energy storage applications. The properties of the assembled forms of the CuS nanosheets are determined by using the concentration of H2SO4 employed in the dealloying process. Spherical clusters composed of CuS nanosheets were fabricated by this process. These clusters have the highest specific surface areas provided by the nanosheets and exhibit high electrochemical and charge storage-release performances. The CuS spherical clusters showed a high specific capacitance of ~276 F g-1 (at a scan rate of 5 mV s-1 in cyclic voltammetry tests) and ~713 F g-1 (at a charge-discharge current of 1.0 A g-1). The high pseudocapacitive performance can be attributed to the highest specific surface area in our case and absence of volume change of the nanosheets. These results suggest the importance of a rational nanostructure design for high-performance energy applications.

1. Introduction

Supercapacitors are rapidly emerging as complementary energy storage devices to conventional batteries[1, 2]. Compared to batteries, supercapacitors exhibit high power density, fast charge-discharge cycles, and long-term stability. These advantages make supercapacitors a promising power source and energy storage device for a variety of electrical applications. According to the storage mechanism, supercapacitors can be classified as electrochemical double layer capacitors (EDLCs), where energy storage is achieved predominantly via double-layer capacitance. This phenomenon of charge separation at an electrode-electrolyte interface (a Helmholtz double layer) can be widely observed in carbon materials such as nanotubes, graphite, and graphene[3, 4]. These large-surface-area carbon materials adsorb electrolytes on their surface to achieve energy storage and have been commercially employed[5]. However, their electrochemical performance cannot meet the demand of high power density and energy density in modern devices[6]. Hence, pseudocapacitive materials such as metal oxides are being considered as alternative materials for EDLCs. These materials have the advantage of fast and reversible electrochemical reactions occurring on their surfaces[7, 8].

Recently, extensive research efforts have been devoted to the development of transition-metal sulfides which have various possible stoichiometric compositions,

crystal structures, valence states, endowing them rich redox chemistry hence make it possible to fabricate higher specific capacity/capacitance than the other electrode materials [9, 10]. Compared to metal oxides, corresponding transition-metal sulfides exhibit better electrical conductivity, and thermal stability. In particular, ternary metal sulfides can provide richer redox reactions, resulting in higher specific capacitance [9, 11]. Covellite, a copper sulfide mineral with formula CuS, consists of a hexagonal structure with alternating CuS3-CuS2-CuS3 layers and S-S layers along the c-axis direction[12]. Recently, there have been some reports of CuS-based materials being used as electrode materials for lithium ion batteries[13] and photoelectrochemical cells[14, 15] due to their good electronic conductivity, high energy capacity, and better theoretical understanding of the anodic oxidation reaction[16]. In addition, CuS exhibits a “liquidus” property, characteristic of ion exchange processes. This allows one to fabricate various materials (such as CuSe and Ag2S) without the destruction of morphology by exchanging Cu or S for other elements via ion exchange[17], and therefore, is significant for CuS and CuS-derived materials for understanding the relationship between the morphology and supercapacitive property. The performance of electrode materials strongly depends on their morphologies. Recently, nanosheets have attracted much attention as supercapacitor electrode materials due to their large number of exposed surface-active sites[18].

In the present work, we aimed to fabricate three-dimensional spherical structures composed of CuS nanosheets. These structures would combine the advantages of nanosheets (large exposed surface active sites) with those of three-dimensional

spheres (the highest specific surface area). We report here an improved dealloying method to fabricate morphology-controllable CuS materials for use in the electrodes of supercapacitors. The morphologies of the as-prepared CuS can be tuned by adjusting the concentration of H2SO4 used in the dealloying process. Our study indicates that the three-dimensional spherical clusters exhibit better specific capacitance values than the nanosheets and nanoparticles due to their good conductivity, and mass transfer. In addition, the spherical CuS is stable for long-term charging and discharging cycles.

2. Experimental details

2.1 Synthesis of CuS nanosheets

All the CuS nanosheets were prepared by the chemical dealloying of the Ti70Cu30 amorphous alloy in H2SO4 solutions. The Ti-Cu amorphous alloy was fabricated by a previously reported melt-spinning method[19]. The amorphous alloy ribbons were cut into 20 mm × 1.5 mm × 25 μm and immersed in H2SO4 solutions with different concentrations. The dealloying process was carried out in a reaction kettle at 363 K for 3 d; subsequently, the samples were washed sequentially in deionized water and ethyl alcohol and dried in a vacuum oven for 10 h. The CuS samples prepared in 11 M, 13 M, and 15 M H2SO4 were named as sample A, sample B, and sample C, respectively.

2.2 Materials characterization

The morphologies of the CuS samples were analyzed by a Hitachi S-4800 scanning electron microscope (SEM) with an acceleration voltage of 5 kV. High-resolution transmission electron microscopy (HRTEM) analysis was performed using a JEOL 200 kV transmission electron microscope equipped with a CCD camera. X-ray diffraction (XRD) patterns were recorded by a Bruker D8 instrument with a Cu Kα radiation source operated as a rotating anode at 40 kV and 40 mA. The valence state of the CuS was analyzed using X-ray photoelectron spectroscopy (XPS, PHL1600ESCA). The specific surface area of the as-prepared samples were tested by nitrogen adsorption/desorption isotherms via a autosorb iQ instrument (Quantachrome U.S.).

2.3 Electrochemical measurements

All of the electrochemical experiments were performed in a standard three-electrode system using a Gamry Interface 1000 electrochemical workstation at room temperature. A platinum mesh and a saturated calomel electrode (SCE) served as the counter electrode and reference electrode, respectively. The working electrode was prepared from a mixture containing the as-prepared samples, acetylene black, and polyvinylidene difluoride in a mass ratio of 4:1:1 in the presence of N-methylpyrrolidinone. The mixed slurry was pasted onto nickel foam, which was then dried at 313 K under vacuum. The electrolytes used were 3 M KOH, NaOH, and LiOH solutions. Cyclic voltammetry (CV) and galvanostatic charge−discharge

methods were used to measure the charge storage capabilities and stabilities of the CuS materials.

3. Results and discussion

We had recently reported a method to fabricate nanoporous CuS with a uniform bicontinuous nanostructure using a Cu-rich Ti-Cu amorphous alloy. Here, we have expanded our approach to prepare CuS materials with different morphologies, which can be controlled simply by adjusting the concentration of H2SO4 used in the dealloying process. The amorphous Ti70Cu30 alloy was characterized by XRD, SEM and EDX. As shown in Figure S1, the precursor is amorphous state and there is no obvious nanostructure on the surface. According to the EDX pattern, the ratio between Ti and Cu is about 70:30. Figure 1(a) shows the XRD patterns of the CuS samples. All the samples were indexed as covellite CuS phase without metallic Cu, Ti, or other copper chalcogenides. The (101), (102), (103), (006), (110), (108), and (116) planes of covellite CuS hexagonal structure were marked in the XRD patterns. These results indicate that the concentration of H2SO4 used during the dealloying has little effect on the phase when the concentration of H2SO4 is over 11 M. However, low concentrations of H2SO4 result in the formation of a mixture of Cu and Cu2S (as shown in Figure S2a). To further investigate the materials composition, sample C was analyzed by XPS. Figure 1(b) shows the XPS pattern obtained for sample C. No Ti element was detected indicating that Ti would be totally dissolved in the dealloying process, which is in accordance with our XRD results. In addition, the chemometrics

of the Cu/S ratio of 1.01 suggests that the sample should be pure CuS. The valance of Cu element in CuS is uncertain until now, Liang et al. reported a (Cu1+)3(S22-)(S1-) or (Cu1+)3(S2-)(S2-) valence formalism for CuS [20] and Kumar suggested both Cu1+ and Cu2+ exist in covellite CuS [21]. The XPS pattern of Cu is shown in Figure 1(c). In our case, the primary peak is located at 932.5 eV and the sub-peak is located at 952.2 eV, which correspond to the Cu-S bond, respectively. These peaks indicate that Cu in the CuS material is Cu1+ [22]. Moreover, there are no satellite peaks which are regarded as the characteristic of Cu2+ , indicating only Cu1+ exists in our CuS material. Figure 1(d) is the fitted S pattern. The peaks located at 161.3 eV and 162.4 eV can be indexed as the sulfide and disulfide moieties in covellite, respectively. The peak at 163.7 eV can be attributed to the S-S bond. In addition, the ratio between the areas of disulfide and sulfide is 1.89: 1lose to 2:1, indicating the sample is pure covellite [23].

The morphologies of the CuS samples have been examined by SEM. As shown in Figure 2, the morphology of the sample changes significantly with the concentration of H2SO4 used in the process due to an Ostwald ripening mechanism, wherein, the small intermediate products get together to form large and smooth particles. In relatively high concentrations of H2SO4, the presence of hexagonal nanosheets with large diameter-to-thickness ratios is obvious. The length of these sheets lies in the range of 200–300 nm and the thickness is about 50 nm. The nanosheets stack together to form mesopores with sizes of ~20 nm. During the dealloying process, Ti was etched from the original amorphous Ti-Cu alloy allowing the acid efficient contact with Cu. With an increase in H2SO4 concentration (sample

C), three-dimensional spherical clusters composed of CuS nanosheets were generated. The diameters of these CuS clusters range from 500 nm to 700 nm. The thickness of nanosheets in the clusters is about 20 nm, which is smaller than that of sample B. Vacant spaces can be seen clearly between the sheets and clusters. This hybrid nanostructure promises efficient electrolyte immersion and shortens the distance for the diffusion of ions, which would contribute to faster charge/discharge cycles[24]. TEM was used to further examine the morphology of the sample C. As shown in Figure 2(d), the clusters were connected to each other by common nanosheets. The HRTEM image is shown in Figure 2(e). Lattice fringes of the sample were determined to be 0.30±0.01 nm, which correspond to the (102) plane of covellite CuS. According to the selected area electron diffraction, the CuS spherical structure is polycrystal. To understand the effect of H2SO4 concentrations used in the dealloying process on the specific surface area of these CuS materials, N2 adsorption–desorption isotherms of the samples were investigated. As shown in Figure 2(f), the specific surface areas of the samples A, B, and C are 15.68, 19.06, and 21.26 m2 g-1, respectively. The specific surface area is a key parameter for capacitor materials because a high specific surface area would contribute to faster material transfer, better contact with the carbon additive, and more efficient interactions between the electrode and electrolyte.

Time-dependent experiments were performed to investigate the evolution of the as-prepared CuS materials. Figure S3 shows the SEM images of the CuS materials prepared using different H2SO4 solutions at various reaction times. Figure 3 summarizes the growth mechanism of the CuS materials. There are two processes in

the formation of nanoporous CuS materials: (1) a dealloying process; and (2) nucleation and growth of CuS. At relatively low concentrations of H2SO4, the reaction exhibits the characteristics of the traditional dealloying process. Nanoporous structures composed of small nanoparticles were generated during the early stages (as shown in Figure S2(a)). Ti in the Ti-Cu amorphous alloy was etched in the presence of acid while Cu was transformed into copper sulfides. These stages can be described as follows [22]: 2Ti+6H+  2Ti3+ +3H2

(1)

5Cu+4H2SO4  3CuSO4 +Cu 2S+2H2O

(2)

Cu 2S+2H2SO4  CuSO4 +CuS+SO2 +2H2O

(3)

Additionally, CuS could dissolve in high concentrations of H2SO4:

CuS+2H2SO4  CuSO4 +SO2 +2H2O+S

(4)

As the reaction proceeds, the nanoparticles become larger due to the Ostwald ripening mechanism. At high concentrations of H2SO4, the nucleation and growth process dominates. This could be because high-concentrations of H2SO4 can accelerate the oxidation of Cu0 to Cu2+ by promoting the generation rate of sulfate radicals[25]. No obvious bicontinuous nanostructure is observed (as shown in Figure S2d). The distribution of the small particles is more compact with some formation of rudimentary of nanosheets. With an increase in reaction time, the nanosheets become larger. However, the nanosheets are restricted to grow in a two-dimensional mode due to insufficient growing space. The sheets coalesce with each other and finally they form a planar construction on a larger scale. On increasing the concentration of H2SO4

further, the surface of the original alloy is covered by the CuS nanosheets in 2 h, indicating that the generation of CuS would still be the dominating process. However, at extremely high concentrations of H2SO4 (13 M), the dissolution of CuS was promoted, and hence, the overall growth of CuS was suppressed. After an additional 2 h of reaction, bicontinuous nanostructures were formed. However, in contrast to the sample prepared in 11 M H2SO4, some nanosheets appear on the surface of the ligaments. Here, sufficient space can be obtained for the anisotropic growth of CuS and therefore, a three-dimensional spherical cluster composed of CuS nanosheets could be formed. Later on the nanosheets grew bigger and thicker and the clusters became hierarchical CuS nanospheres.

Ti is crucial to the improved dealloying process, because the dissolution of Ti can provide the opportunity for a direct contact between the H2SO4 and the Cu in the bulk of the amorphous alloy. To verify this hypothesis, we maximized the space hindrance effect. A pure Cu ribbon was used as a control to fabricate CuS. As shown in Figure S4, only large particles were generated after 3 days of reaction. The inner face (after scraping) was observed to be different from the surface, which indicates that the sulfurization reaction could not occur in the interior of the original Cu ribbons. This confirms that the space caused by the dissolution of Ti is very important for the growth of CuS in the aspect of morphology and sulfurization depth.

Figure 4 shows the CV curves for the different CuS electrodes at various scanning rates in 3 M KOH electrolyte. There are obvious pairs of reversible redox

waves in all the curves under different scanning rates. This indicates a typical pseudocapacitance behavior in the CuS material, which is similar to the other chalcogenide materials. Bodenez et al. suggested that the redox reactions in copper-based chalcogenide materials could be attributed to the replacement between cations in the electrolyte and CuS[26]. In our case however, the mechanism of the electrochemical reactions might be different. Figure S5 shows the initial two cycles of the CV curve in 3 M KOH. It is clear that the first cycle is different from the subsequent cycle due to a phase transformation from CuS to Cu(OH)2[27]. From previous studies[28], the phase transformation reactions can be deduced as follows:

The first cycle: 1 CuS+H 2O+ O2  Cu(OH)2 +S 2

(5)

The reversible redox reaction is as follows: Cu(OH)2 +KOH  CuOOK+H2O

(6)

When the scanning rate increases, the peak current density increases but the shape of the curves changes rarely, indicating that all the electrodes exhibit rapid redox reactions. As shown in Figure S6, a linear relationship can be observed for the wave current as a function of the square root of the scanning rate when the scanning rate is over 20 mV s-1. This suggests that the redox reactions might be controlled by diffusion processes. The specific capacitance under various scanning rates was calculated using the following equation: C

 Idt

mV

(7)

where I, dt, m, and ΔV are the oxidation or reduction currents, time differential, voltage range of one sweep segment, and mass of the active electrode material, respectively. As shown in Figure 4(d), the values of specific capacitance of all the CuS materials decrease with increasing scanning rates. At a high scanning rate, ions in electrolyte cannot enter the material efficiently, so the utilization rate of the electrode material is relative low, which results in a smaller specific capacitance. Sample C exhibits the highest specific capacitance (276 F g-1) at a scanning rate of 5 mV s-1.

The galvanostatic charge−discharge method was further used to evaluate the charge storage capacities of the as-prepared CuS materials. As shown in Figure 5, the charge-discharge curves display asymmetrical and non-linear behavior. The charge-discharge time decreases with increasing current densities. The non-linear characteristic of the charge-discharge curves indicates that the reaction should occur following a redox mechanism, which is in concordance with the CV results. The specific capacitance (Csp) of the CuS electrodes can also be calculated from the galvanostatic charge and discharge curves as follows: Csp 

I  t V  m

(8)

where I, △t, △V, and m are the discharge current, discharge time, potential window and mass of the material, respectively. The calculated Csp value of the CuS materials, as a function of discharge current, is shown in Figure 5(d). The specific capacitance decreases with increasing charge or discharge current density due to insufficient redox

reaction of the active materials under higher current densities[29]. The limited kinetics of the reactions can also be attributed to the insufficient number of ions, which diffuse and migrate into the active materials at high current density. Again, when the current density is high, a concentration polarization phenomenon could be observed, which would further affect the specific capacity[30]. The calculated capacitance values of the electrode materials were of the same order of those obtained from the CV curves. The specific capacitance of Sample C was still the highest. It was as high as ∼713 F g-1 at a current density of 1.0 A g-1 and ~500 F g-1 for a current density of 5 A g-1. As shown in Table S1, even though the specific capacitances of sample C is lower than the CuS composite materials, it is comparable to other pure CuS materials.

Electrochemical impedance spectroscopy (EIS) measurement was used to further confirm the electrochemical performance of the as-prepared samples. As shown in Figure S7, all the similar form with a semicircle at a higher frequency region and a spike at lower frequency. The semicircle diameter at higher frequency region represents the electron transfer resistance, which is attributed to the transfer kinetics of the redox couple at the electrode interface [31]. Sample C exhibits the lowest resistance demonstrating the fastest electron transfer between the active material and the charge collector, which is in accord with the CV and GCD results. This phenomenon can be attributed to the effective exposure of active sites provided by the higher specific surface area [32].

Several features make the CuS hexagonal materials unique building blocks for high capacity devices useful in energy storage and release. The interactions between Cu and S are covalent bonds, whereas the interactions between two sulfur species are the weaker van der Waals forces. These interactions can afford more opportunities for reactions between CuS and the cations in the solution, thereby yielding large capacities[33, 34]. There are two important parameters that should be considered to explain the differences in the electrochemical performance of these CuS materials: the specific surface area and the effective K+ concentration in the CuS materials. It has been proved that specific capacitance would be enhanced with increasing the specific surface area. In addition, the molecular weight would increase in the electrochemical process due to the presence of K+ in the CuOOK matrix[35]. The similar crystallinities of the samples indicate they have similar K+ concentrations. Hence, the specific surface area would now become the key parameter determining the specific capacitance of the materials. Higher the specific surface area of the sample, higher would be the specific capacitance it would exhibit. Some studies emphasize that excessive specific surface areas would lead to a decrease in the specific capacitance due to a suppression of K+ ions diffusion[36]. These results seem to be inconsistent with our case. However, morphology and crystallinity are also contributing factors to the electrochemical performances of these materials. The nanosheet morphology nature of the nanosheets are ideal to afford effective and rapid charging-discharging during cycling[37].

The ion diffusion coefficient and the diffusion distance are important indicators of efficient ion diffusion in the host electrode materials. The ion diffusion coefficient is an intrinsic property of the host materials and therefore cannot be controlled for a given material. Hence, thinning the nanoparticles to nanosheets can be an effective strategy to shorten the ion diffusion distance and enhance the efficiency of electrode materials. This could be the reason why the capacitance behaviors of the nanosheets plate and spherical clusters are better than those of the nanoparticles. On the other hand, effective contact between active materials and the carbon additive could be another important factor since the active materials generally have low electronic conductivity[38]. Sufficient connection between active materials and carbon additive would shorten the electron transport distance. The electron transport distance of sample C is shorter than that of the other two CuS materials because a large number of nanosheets can directly contact the carbon additive, indicating a smaller interfacial resistance. To sum up, sample C exhibits the highest electrochemical performance due to the advantages of the nanosheet structure, along with higher specific surface area, better carbon contact and mass transferability.

The capacitance of the materials can be strongly influenced by the electrolyte species because the adsorption/desorption of electrolyte ions into/from the electrode materials can be influenced by the size and diffusion speed of the solvated ions[39]. Hence, we performed CV tests for sample C in different electrolytes such as 3 M LiOH, 3 M NaOH, and 3 M KOH at various current densities (As shown in Fig. S8). Similar to the CV curves in 3 M KOH solution, a pair of redox peaks at 0.4 V and

0.25 V can be observed for all electrolytes, which indicate pseudocapacitance behavior. The galvanostatic charge−discharge curves of sample C in various electrolytes are shown in Figure 6. The electrochemical performance of sample C in the KOH solution is better than in other two electrolytes, indicating that the cations could influence both the power and energy performances of the supercapacitors[40]. As shown in Figure 6(e), generally, for ions with the same charge, a stronger polarization would happen in a smaller-radius solvent. Thereafter, more surrounding solvent molecules would be attracted on to the electrode, which would result in a larger hydrated ion. The electrolyte ion transport rate increases with decreasing hydrated ion radius, indicating that a smaller hydrated ion radius would result in higher capacity[41]. Hence, the smaller hydrated radius of K+ can promote better ionic mobility and interaction with the electrode material. As a result, the pseudocapacitive behavior of sample C was enhanced[39].

The stability of sample C, as required for rechargeable storage devices, was further measured by the cyclic charge–discharge method. Figure 7(b) shows the change in the specific capacitance of sample C for 2000 charge−discharge cycles at a current density of 2 A g-1. The sample retains 73% and 61% of the charge after 1000 and 2000 cycles, respectively. The inset of Figure 7 shows the last few charge−discharge cycles for sample C. As evident from Figure 7, these materials deliver good cyclic stability over 2000 cycles, indicating their potential as perdurable power storage and delivery materials. This good cyclic stability might be due to little

volume change of the nanosheets during the insertion and extraction of electrolyte ions in the stability tests.

Conclusions

In summary, we have designed a simple method for fabricating CuS materials with good morphology control and superior electrochemical performance towards use in supercapacitors. The morphologies and microstructure of the CuS nanosheets were controlled by the concentration of H2SO4 used in the dealloying process. In low concentration H2SO4 solutions, CuS nanoparticles and nanosheets were formed. With an increase in H2SO4 concentration, three-dimensional spherical clusters composed of the CuS nanosheets were generated. The CuS spherical clusters exhibit good electrochemical performance (specific capacitance of ~276 F g-1 at a scanning rate of 5 mV s-1 in cyclic voltammetry tests and ~713 F g-1 at a charge-discharge current of 1.0 A g-1). In addition, the CuS spherical clusters retains 73% and 61% of the charge after 1000 and 2000 cycles at a current density of 2 A g-1. These results may prove to be beneficial for the design and fabrication of improved energy storage devices.

Acknowledgements

We gratefully acknowledge the support by National Natural Science Foundation of China (51172159), Recruitment Program of Global Experts “1000 Talents Plan” of China (WQ20121200052) and Key project of Natural Science Foundation of Tianjin City (14JCZDJC38600).

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Figure captions Figure 1. (a) XRD patterns of the CuS materials prepared in different H2SO4 solution. (b) XPS spectra of the sample C and detailed XPS spectra of Cu 2p (c), and S 2p (d). Figure 2. SEM images of sample A (a), sample B (b), and sample C (c). TEM (d) and HRTEM (e) images of the as-prepared sample C. (f) N2 adsorption– desorption isotherms of the CuS samples prepared by various H 2SO4 solutions. The inset in (d) is selected area electron diffraction. Figure 3 Schematic illustration of the growth mechanism of the CuS materials.

Figure 4. CV curves of the sample A (a), sample B (b) and sample C (c) at various scan rates in 3 M KOH. (d) Variation of specific capacitance of the CuS materials with applied scan rates. Figure. 5 Galvanostatic charge−discharge characteristics of (a) sample A, (b) sample B and (c) sample C for various applied current densities. (d) Variation of specific capacitance of the CuS materials with applied current densities. Figure 6. CV curves of the sample C at various scan rates in 3 M LiOH (a), 3 M NaOH (b), and 3 M KOH (c). (d) Variation of specific capacitance of the CuS materials with applied current densities in different electrolytes. (e) Schematic representation showing the hydrated ionic radius in the CuS spherical clusters-based electrode. Figure 7. (b) Galvanostatic charge–discharge cycling test of the sample C at the current density of 2 A g-1. The inset is galvanostatic charge–discharge curves at

specific cycle numbers.

Fig. 1

Fig. 2

Fig. 3

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Fig. 5

Fig. 6

Fig. 7