Enhanced Electrochemical Performance of Hydrothermally Synthesized NiS/ZnS Composites as an Electrode for Super-Capacitors

In this study, nickel sulfide (NiS), zinc sulfide (ZnS), and their composites have been synthesized by using surfactant driven hydrothermal method. Synthesized materials are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy, UV–Vis and Photoluminescence spectroscopy. XRD results have shown the presence of corresponding structural planes. Crystallite size was much smaller (15 nm) in the case of ZnS nanomaterials, whereas, composite materials have shown size comparable to NiS nanomaterials. SEM images presented morphology of star-like, spherical, and mixture of two for NiS, ZnS, and NiS/ZnS nanocomposites respectively. EDX spectrum of composite materials showed Nickel, Zinc, and Sulfur, indicating the purity of the synthesized composite. Electrochemical measurements i.e. cyclic voltammetry and galvanostatic charge–discharge were determined for all three materials. Maximum specific capacitance is obtained as 1594.68 F g⁻¹ at a scan rate of 5 mV S⁻¹ for NiS/ZnS composite materials whereas a charging/discharging time of 461.97 s is observed. The composite materials have shown 95.4% retention for applied for 3000 charging–discharging cycles. The favorable behavior of NiS/ZnS composites indicated their potential as an electrode material for pseudo-capacitors.

With the decrease of accessibility to fossil fuels and environmental changes, society is driving towards supportable and renewable sources of energy with higher proficiency and little to zero carbon emission. Solar and wind energy are the natural and major renewable resources of energy but cannot supply energy continuously. When we consider the electrical energy storage devices, innovative devices are using electrochemical capacitors, batteries, and super-capacitors [1,2,3]. Super-capacitors and energy storage devices have been assumed very interesting due to their higher and reliable power density, fast charge–discharge rates, long lifetime, and more environment compatibility, when compared with rechargeable batteries. Super-capacitors are promising energy storage devices that show a wide range of applications i.e. electronics, hybrid electrical vehicles, etc. [4, 5]. There are two types of super-capacitors based on their charging-discharging mechanisms: (1) Electrical double-layer capacitors use electro-sorption in an electric double layer on porous electrodes, show higher electrical conductivity and specific surface area. Electrical double-layer capacitors are built using carbon porous nanomaterials such as graphene, carbon aerogels, and nanotubes [6, 7], and (2) Pseudo-capacitors store energy by charge transferring between electrode and electrolyte and exhibit high capacitance and energy density as compare to Electrical double-layer capacitors. Pseudo-capacitors also show reversible redox reactions for storage of charging between the electroactive candidates and ions of electrodes. Pseudo-capacitors mostly use hydroxides, transition metal oxides, and conductive polymers as electrode materials [8, 9].

Transition metal sulfides possess enhanced technological properties such as higher electrical conductivity, outstanding specific capacitance (Cs), low cost, excellent electrochemical behavior, and fast and reversible redox reaction [10,11,12,13]. Due to these characteristics, the transition metal sulfides such as SnS, NiS, CoS, ZnS, and NiS [14,15,16,17] are extensively reported as electrode materials for super-capacitors [18]. Amongst these metal sulfides, NiS presents various beneficial properties, such as easy synthesis, economical, less toxic and efficient material for super-capacitor application [19,20,21]. Nickel species can change valency which improve the capacity of NiS, however, Ni species have poor conductivity in single metal sulfide, consequently, inhibits the charge transfer [20]. Conversely, zinc species provide higher electrical conductivity due to lower valence states and support the charge transport in composite/binary sulfides upon charging-discharging process [18].

Several methods to synthesize the metal sulfide are being reported in the literature such as the chemical vapor deposition method [22], the hydrothermal method [23], the chemical bath deposition method [24], and the chemical co-precipitation method [25]. The solvothermal method has been used to fabricate hierarchical like the structure of β-NiS which approaches the specific discharging capacitance of 857.67–513 F g⁻¹ at current density 2–5 A g⁻¹ [24]. Nanoflake arrays based NiS was synthesized through the chemical bath deposition process which arrived Cs of 718 F g⁻¹ at current density 2 A g⁻¹ [26]. A simple hydrothermal process has been used to fabricate the zinc cobalt sulfide nanoparticles which achieved the energy density of 45.4 Wh kg⁻¹ at 805 W kg⁻¹ power density [27]. ZnS nanotubes synthesized by the hydrothermal process exhibited high specific discharge capacity (950 mAh g⁻¹) [23]. NiS thin films synthesized via the chemical bath deposition procedure used to identify the charge–discharge behavior and exhibited maximum energy and power density of 11.7 Wh kg⁻¹ and 4.3 kW kg⁻¹ respectively at current density 1 mA cm⁻² [28]. Composite of NiS/ZnS has fabricated on nickel foam through the CVD process that arrived Cs 1533 F g⁻¹ at 7.5 A g⁻¹ [22]. In this study, star-like NiS and spherical ZnS nanocomposites have been synthesized using the hydrothermal method. These nanocomposites have been characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy (FTIR), and photoluminescence spectroscopy. Finally, electrochemical behavior as an electrode for super-capacitor is evaluated.

Materials

All the chemicals, NiCl₂·6H₂O (BDH), ZnCl₂ (Daejung, 98%), and thiourea (H₂NCSNH₂) have been used as the received. CTAB was used as a surfactant. All the solutions have been prepared in distilled water.

Fabrication of Materials

The hydrothermal method has been used to fabricate the NiS and ZnS NPs. In a typical experiment, 11.89 g NiCl₂·6H₂O and 3.81 g thiourea dissolved in 100 ml deionized water separately by using ultrasonication. Then thiourea is mixed dropwise in NiCl₂·6H₂O during continuous stirring for 2 h on the magnetic hot plate to make the homogenous solution. 0.5 g of CTAB is also added as a surfactant. After that, the homogenous solution is transferred into the Teflon lined autoclave for the hydrothermal treatment at 200 °C for 15 h. The obtained precipitates are collected by centrifugation and washed several times using deionized water. Finally, the extracted precipitates are dried in an oven for 16 h at 60 °C and grinded to obtain the fine powder. A similar procedure was repeated to obtain the ZnS NPs. For the fabrication of NiS/ZnS nanocomposite, 8.32 g of NiCl₂·6H₂O, 4.77 g of ZnCl₂, and 5.32 g thiourea are used during the hydrothermal process as discussed above.

Characterization Tools

The structural investigation of the fabricated materials is performed by XRD using a Philips (model Xpert) diffractometer with a Cu Kα radiation line (λ = 0.15406 nm) within the 2θ angle range of 20°–80°. The morphology along with grain size distribution are investigated using SEM (JEOL, model JSM-6480 from Japan). The chemical composition is examined by EDX incorporated with the SEM. The FTIR analysis is performed using a Perklin Elmer apparatus of version 10.4.3.

Electrochemical Measurements

The aqueous solution of 2 M KOH has been used to perform the electrochemical measurements. The electrochemical properties are studied by three-electrode system with the synthesized NiS, ZnS, and their composite being utilized as a working electrode, Pt wire as a counter electrode and Ag/AgCl as a reference electrode. The working electrode is prepared by an active mass ratio (80:15:5) of synthesized material (NiS, ZnS, and its nanocomposite), graphite, and PVA. The material is mixed, coated on the nickel foam (2 cm*2 cm) and dry at room temperature. The active mass on the working electrode was 5 mg. Cyclic voltammetry (CV) along with galvanostatic charge–discharge (GCD) are used to identify the electrochemical properties of synthesized electrodes. CV is conducted over the potential range within 0.0 to 0.5 V and scan rates 5, 10. 15, 20, 25, 30, 40 and 50 mV s⁻¹. GCD experiments have been performed in the identical electrolyte for various current densities. The Cs, power density, and energy density of the electrodes have been analyzed through the GCD curves applying the subsequent equations:

where the I and dV communicate the current and potential values obtained from the redox peaks. The variables: m, S, and \(\Delta\)V communicate the active mass, utilized scan rate, and the potential window established for the CV experiment. Where, the dV/dt indicates the slope in the GCD curl, energy (E_d), and power densities (P_d).

Characterization of Nanomaterials

Synthesized materials have been characterized by XRD initially for understanding the changes in the structural transformations in composites. Figure 1 displays the XRD patterns of NiS, ZnS, and its nanocomposite prepared by the hydrothermal method. The XRD patterns of sulfides present different diffraction peaks refer to the formation of metal sulfide as desired. The majority of NiS diffraction peaks match with JCPDS card 00-012-0041 which presents NiS phase, whereas peaks present at diffraction angles 26.5°, 31.3°, and 57.5° belong to JCPDS card 00-047-1739 with chemical formula Ni₃S₄. This indicates the multiphase nature of NiS nanomaterials. For ZnS, the XRD pattern matches with JCPDS card 01-077-2100 and presents a single-phase cubic structure. Composite material shows diffraction peaks of both the sulfide materials i.e. NiS and ZnS, however, there is slight shift in principle peaks of NiS towards lower angles and intensity variations relative to pristine NiS. These changes can be linked with the formation of composite materials. XRD peaks are principally linked with the crystallite size of the sample powder. Small crystalline domains lead toward fewer diffracting material and result in lower diffraction intensity. This is associated with diffracting volume, specific crystal orientation, and constructive interference that can affect the diffraction peaks. To study the effect of composite formation on the structural properties of the materials, the crystallite size is calculated. The NiS, ZnS, and composite materials have crystallite size as 41.63, 14.91, and 33.84 nm respectively. The texture coefficients of the synthesized material are calculated by Eq. (4);

where I and I₀ are relative intensities of measured and reference spectrums and N is number of diffraction peaks. The preferred growth orientation for NiS, ZnS ad NiS/ZnS is [410], [220] and [300] respectively (Table 1).

XRD patterns of a NiS, b ZnS and c NiS/ZnS composites

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The crystal distortion and imperfections by strain-induced broadening are generally related by formula, \(\varepsilon =\frac{{\beta }_{s}}{\mathrm{tan}\theta }\), where β_s is full width at half maximum of specific diffraction peak. While calculating the crystallite size using Scherrer’s formula, the strain broadening factor is usually ignored, hence provides ambiguous information about crystallite size. Therefore, Williamson–Hall plots were used to calculate the crystallite size. This method is based upon identifying the lattice strain and crystalline size with additive components of the total breath integral of Bragg peaks. The Williamson–Hall equation can be written as:

We can write Eq. (6) as

Using Eq. (7), we can plot a graph between β_hkl cosθ and sinθ to calculate the crystallite size and lattice strain (Figure S1). A comparison of crystallite size and lattice strain determined by Scherer and Williamson–Hall method is given in Table 2. The observed discrepancy is due to the presence of internal stress. It is observed that the ZnS nanoparticles have high percentage of defects and consequently higher strain broadening, on the other hand, the nanocomposites showed a small variation between two crystallite sizes, hence indicative of lesser crystal defects [25].

Figure 2 presents the SEM images of NiS, ZnS, and its nanocomposite at different magnifications. Figure 2a shows the star-like morphology of NiS nanomaterials; low magnification image presents abundant yield of the materials and homogenous morphology. It is believed that presence of a surfactant during the hydrothermal reaction can tune the morphology of the NiS nanostructure and promotes star-like morphology. The formation of star-like NiS materials follows the diffusion-limited growth mechanism [29]. In this process, upon the formation of the nucleus with a fewer number of atoms, the transition from nucleation to growth process is very fast due to the availability of many monomers. This large quantity of monomers promotes the diffusion process of atoms at the nearest point as more supplies of monomers are available and results in dendrites type growth [30]. However, in our case, the presence of CTAB as surfactant limits rough formation of dendrites and sharp tips are formed because of diffusion. This process provides star-like morphology of NiS nanomaterials rather than irregular dendrite like growth due to the presence of surfactant [30]. Figure 2b shows SEM images of ZnS and presents spherical morphology. Low magnification image shows homogeneity of the morphology and narrow size distribution. Most of the particles are distributed homogenously and make some bunch of NPs by agglomeration of particles. SEM images of composite materials have shown a combination of spherical and star-like morphologies (Fig. 2c). SEM images present unique morphologies of nanomaterials that can be applied for super-capacitor applications. The average particle size of NiS, ZnS and its composites estimated from SEM image is ~ 68.4 ± 4.5 nm, 79.4 ± 10.9 nm, and 83.4 ± 7.8 nm respectively. Chemical composition determined by EDX spectroscopy of composite materials has shown peaks for nickel, zinc, and sulfur (Fig. 2d). EDX result confirms the chemical purity of the synthesized nanocomposites.

SEM images of a NiS with star-like morphology, b ZnS with spheres, c NiS/ZnS nanocomposites, d EDX spectrum of NiS/ZnS nanocomposites

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FTIR is a useful technique that tells us about the functional groups, vibrational, and bond stretching functionalized on the surface of the synthesized NiS, ZnS, and their nanocomposites. FTIR spectra of the NiS, ZnS, and their nanocomposites are illustrated in Fig. 3a. FTIR spectra for different samples are showing their unique bands with organic-based bonding and metal bonds. Inorganic materials typically offered broader and rare counts of bands as compared to organic materials. FTIR spectra reveal a broad band of the metal sulfide (symmetric sulfide) at 415, 436, 469, 480, and 557 cm⁻¹. The NiS stretching is present at 610 cm⁻¹. The transmittance band located near 2100 cm⁻¹ belongs to a carbon–carbon triple stretching bond, whereas, 2360 cm⁻¹ represents the CO₂ bond. The asymmetric and symmetric vibrational stretching of CH₂ present at 2851 and 2916 cm⁻¹ [31]. The band at 1468 cm⁻¹ is assigned to C–O stretching vibration. The stretching and bending vibration of OH groups are present at 3630 and 3650 cm⁻¹ [32]. In ZnS, the metal sulfide bands attain at 419, 441, 491, and stretching vibration at 518 cm⁻¹. The OH stretching and bending peaks obtain at 3588 and 3649 cm⁻¹ in the case of ZnS [33]. Composite materials have shown a mix of bands belong to NiS and ZnS in the NiS/ZnS spectrum. Figure 5b shows the UV–visible spectra of NiS, ZnS, and NiS/ZnS nanocomposites recorded between 200 and 900 nm. The spectra shows a sharp peak at 250 nm and the optical absorption peak edge of NiS, ZnS, and its composite are approximately located at 223 nm, 221 nm, and 220 nm, respectively in the ultraviolet region. The optical spectrum provides an approximation about the bandgap of the semiconductor materials, useful for light related applications. The optical band gap can be calculated by using tauc’s plot using the Eq. (8) given below,

where exponent n base on the transition. An estimation of the bandgap is obtained by drawing a tangent line on the curve towards the horizontal line (Figure S2). The obtained band gap values of NiS, ZnS, and its nanocomposite is about 4.0 eV, 3.8 eV, and 4.2 eV, respectively. The comparable values for NiS and ZnS have been reported in the literature [34, 35]. Optical properties are further studied by obtaining room temperature photoluminescence spectra of the nanomaterials with excitation at a wavelength of 325 nm (Fig. 3c). NiS nanomaterials have PL band edge around 375 nm and 525 nm, whereas ZnS has an absorption peak at 516 nm. The composite materials have shown a blue shift in photoluminescence absorption around 475 nm. This also shows that optical properties are changed in composite materials.

a FTIR spectra of NiS, ZnS, and their composites, indicating the presence of metal sulfide bonds and other organic functionalities, b UV–vis and c Photoluminescence spectrums of NiS, ZnS, and NiS/ZnS nanocomposites

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Electrochemical Measurements

CV and GCD are used to characterize the electrochemical properties of NiS, ZnS, and NiS/ZnS nanocomposite using the three-electrode system. The curves of cyclic voltammetry are recorded at various scan rates from 5 to 50 mV/S as shown in Fig. 4a–c, with potential window 0–0.5 V (vs. Ag/AgCl). CV curves reveal the redox peaks which typically explain the pseudo-capacitor behavior as suggested by [36] and indicate the dominant Faradic reaction performance during the energy charge and discharge procedure for the NiS, ZnS, and NiS/ZnS nanocomposites [37]. The faradic process of these materials can be explained by the reactions [38] given in the Eqs. (9–11),

CV curves determine at different scan rates i.e. 5 mV/s, 10 mV/s, 15 mV/s, 20 mV/s, 25 mV/s, 30 mV/s, 40 mV/s, 50 mV/s for a NiS, b ZnS, c NiS/ZnS nanocomposite, d the specific capacitance variation with scan rate for sulfide materials calculated from CV curves

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With increasing the scan rate, shifting in cathodic and anodic peak position is observed which indicates the rapid ionic transportation [39].

Figure 4d presents Cs variations of NiS, ZnS, and composite materials at dissimilar scan rates. Cs provides an estimation of charge storage and discharging capacity. These measurements are used to optimize appropriate capacitance values at a selected scan rate. The Cs values of NiS/ZnS composite materials are considerably improved as compared to pure NiS and ZnS. All materials have shown capacitive behavior, NiS and ZnS curves show that Cs increases till 15 mV/S and then reduce with the further increase of scan rate. Using the Eq. (1), the Cs of NiS is observed as 603.85, 576.50, 555.07, 512.5, 499.73, 492.87, 450.71 and 447.03 F g⁻¹, for ZnS; 295.78, 294.44, 288.78, 285.44, 276.44, 271.64, 243.13 and 225.02 F g⁻¹ at scan rate 5, 10, 15, 20, 25, 30, 40 and 50 mV s⁻¹. The Css of NiS/ZnS nanocomposite are considerably higher i.e. 1594.68, 1292, 967.80, 837.80, 870.58, 834.99, 713.05, and 689.31 F g⁻¹ at scan rate range 5–50 mV s⁻¹. The Cs drops with an increase in the scan rate in all-metal sulfide materials due to a rise in resistance. The higher improvement occurs in the Cs of the NiS/ZnS nanocomposite as compare to individual NiS and ZnS due to higher electrical range as well as the larger specific area of the surface [40]. The value of the Cs of NiS/ZnS nanocomposite is higher than the individual NiS and ZnS as shown in Fig. 4. This is the highest reported value of Cs for NiS/ZnS composites as compare to Ikkurthi et al. [13] with a value of 1523 F g⁻¹.

GCD graphs for individual NiS, ZnS, and NiS/ZnS nanocomposite using the three electrodes method are display in Fig. 5a–c. Equations 2 and 3 are used to compute energy and power density, respectively. For individual NiS and ZnS, the charging and discharging time is 396.36, 134.23, 29.71 and 7.89 s, and 186.26, 76.76, 10, and 2.5 s at 0.5, 1, 5, and 10 mA/cm² respectively. However, NiS/ZnS nanocomposites have longer time i.e. 461.97, 233.87, 48.57, and 23.48 s at 0.5, 1, 5, and 10 mA/cm² respectively. Additionally, the stage region of the CD curves is also correspond with the CV profile and verified the Pseudocapacitor nature of the materials (Fig. 5d). The nanocomposite of NiS/ZnS has shown a large GCD time as compare to individual NiS and ZnS at a similar current density that could be due to comparatively superior electrical conductivity. Consequently, ions and electrons cannot achieve sufficient time for insertion/desertion on the electrode surface under the control at the greater current density, and such a component expands the resistive performance by displaying extra polarization clear by GCD profile [41]. Table 3 is showing the energy density as well as power density of metal sulfide and their composite materials. The maximum power density obtains 1313.25, 1057.33 and 1363.2 W kg⁻¹ correspond to energy density 1.0, 0.5, and 3.4 Wh kg⁻¹ at current density 10 mA cm⁻² respectively for NiS, ZnS, and NiS/ZnS nanocomposites. The GCD curves of NiS, ZnS, and NiS/ZnS nanocomposite electrodes also exhibit the typical performance of the faradic reaction, which are reliable with the results attained through the CV curves. The cyclic stability of the materials is probed by conducting cyclic charging-discharging measurements for 3000 cycles (Fig. 6). The Cs of NiS/ZnS composite declined slowly from 1594.68 to 1521.81 F g⁻¹ after 3000 cycles. In case of pristine NiS and ZnS, value reduced from 603.85 to 509.84 F g⁻¹ and 295.78 F g⁻¹ to 210.79 F g⁻¹ respectively at current density of 10 mA cm⁻². This shows that NiS–ZnS composite materials have excellent stability with 95.4% retention as compare to 84.4% and 71.2% for NiS and ZnS materials respectively. This shows that distinguished electrochemical stability of composite materials can be useful for practical energy devices. Reduced retention of pristine NiS and ZnS can be ascribed to the structural annihilation of the material instigated by massive ion exchange.

GCD curves of a NiS b ZnS and c NiS/ZnS nanocomposite, d Schematic mechanism with NiS/ZnS composite materials as an electrode

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Cyclic performance of synthesized materials in 2 M KOH solution with current density 10 mA cm⁻² for 3000 cycles

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The exceptional electrochemical behavior of the NiS@ZnS composite is ascribed to the reasons: (1) the synergistic effect due to the NiS and ZnS interface eases electron transport in the course of charge–discharge cycles due to enhanced electrical conductivity; and (2) the star like morphology of NiS in composite materials offers larger specific surface area as compare to the pristine NiS and ZnS and cause higher Cs [42]. Consequently, the composite materials display improved electrochemical properties. Additionally, the Cs revealed in present work is reasonably higher than different previously reported pristine and composite materials as summarized in Table 4.

Electrochemical Impedance Spectroscopy (EIS)

The conductive properties, electron transport and surface induced mechanisms of the prepared electrodes have been analyzed using electrochemical impedance spectroscopy (EIS) measurements. Figure 7 shows the comparison among bare and materials modified electrode. The EIS spectrum of NiS/ZnS composite materials is comprised of a semicircle at higher frequency and a straight line at low frequency. The outcomes establish the idea that composite modified electrode is more efficient for electron transfer and limited diffusion processes at high and low frequency respectively. The representative Nyquist circle diameter of composite is smaller than bare and pristine NiS and ZnS, indicating higher electrochemical activeness of material, additionally, transfers electrons more rapidly. Therefore, this can be concluded from the EIS studies that composite modified electrode have lower electron impedance and boost the electron transfer process.

EIS spectrums of bare, pristine NiS, ZnS and NiS/ZnS composite materials modified GCE

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In this study, NiS, ZnS, and their composites have been prepared by using the hydrothermal method using CTAB as a surfactant. NiS nanomaterials have shown multiphase nature, whereas, composite materials have presented diffraction peaks of both NiS and ZnS with a slight shift in peaks. SEM images showed star-like morphology of NiS, spheres for ZnS, and combine morphologies for composite materials. FITR spectrums of materials presented bands of metals sulfides and other functionalities. Finally, the potential of synthesized materials and their composite have been tested for super-capacitor application by measuring electrochemical properties. Higher Cs was observed for NiS/ZnS composite materials. In addition, power density value of 1313.25, 1057.33 and 1363.2 W kg⁻¹ correspond to energy density 1.0, 0.5 and 3.4 Wh kg⁻¹ at current density 10 mA cm⁻² respectively was observed for NiS/ZnS nanocomposites. This suggests that NiS/ZnS composites are a better option for utilizing as an electrode in super-capacitor.

Affiliations

1. Department of Physics, University of Agriculture, Faisalabad, Pakistan

   Ali Asghar & Yasir Javed

2. Department of Applied Sciences, National Textile University, Faisalabad, Pakistan

   Muhammad Imran Yousaf

3. National Institute of Biotechnology and Genetic Engineering, Jhang Road, Faisalabad, Pakistan

   Naveed Akhtar Shad & Muhammad Sarwar

4. Henan Key Laboratory of Photovoltic Materials, School of Physics, Henan Normal University, Xinxiang, 453007, China

   M. Munir Sajid

5. Department of Physics, Riphah International University, 13-km Raiwind Road, Lahore Campus, Pakistan

   Amir Muhammad Afzal

6. Department of Physics, COMSATS University Islamabad, Lahore Campus, Lahore, Pakistan

   Aamir Razzaq

7. Department of Physics, Faculty of Science, Jazan University, Jazan, 45142, Saudi Arabia

   Mohammad Shariq

8. Department of Physics, College of Science, University of Bahrain, Zallaq, Kingdom of Bahrain

   Qurrat-ul-ain Gulfam

9. Department of Physics, Federal University of Maranhao, Sao Luis, Brazil

   Surender K. Sharma

10. Department of Physics, Central University of Punjab, Bathinda, India

    Surender K. Sharma

Corresponding authors

Correspondence to Naveed Akhtar Shad or Yasir Javed.

Conflict of interest

Authors have no conflict of interest to declare.

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