Nanocrystalline β-NiS: a redox-mediated electrode in aqueous electrolyte for pseudocapacitor/supercapacitor applications

Abstract

Currently, enhancing the performance of electrochemical supercapacitors is the subject of intense research to fulfill the ever-increasing demand for grid-scale energy storage and delivery solution, thereby utilizing the full potential of renewable energy resources and decreasing our dependence on fossil fuels. Metal sulfides, such as cobalt sulfide (CoS), nickel sulfide (NiS), molybdenum sulfide (MoS), copper sulfide (CuS), and others, have recently emerged as a promising class of active electrode materials, alongside other supercapacitor electrode materials, due to their relatively high specific capacitance values and exceptional reversible redox reaction activities. The synthesis, characterizations, and electrochemical performances of single-phase nanocrystalline β-NiS are presented here and the electrode based on this material shows a specific capacitance of 1578 F g⁻¹ at 1 A g⁻¹ from the galvanostatic discharge profile, whereas a capacitance of 1611 F g⁻¹ at 1 mV s⁻¹ was obtained through the CV curve in 2 M KOH aqueous electrolyte. Additionally, the electrode also performs well in neutral 0.5 M Na₂SO₄ electrolytes resulting in specific capacitance equivalent to 403 F g⁻¹ at 1 mV s⁻¹ scan rate. The high charge storage capacity of the material is due to the superior intercalative (inner) charge storage coupled with the surface (outer) charges stored by the β-NiS electrode and was found to be 72% and 28%, respectively, in aqueous 2 M KOH electrolyte. This intercalative charge storage mechanism is also responsible for its excellent cycling stability. Additionally, we assembled aqueous asymmetric supercapacitors (ASCs) with activated carbon (AC) as the negative electrode and the β-NiS electrode as the positive electrode. The combination of the β-NiS electrode and AC with excellent cycling stability resulted in the highest specific energy equivalent to ∼163 W h kg⁻¹ and a specific power of ∼507 W kg⁻¹ at 1 A g⁻¹ current rate.

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Introduction

The tremendous growth of the worldwide economy resulting in climate changes, a rise in pollution, a steady depletion of fossil resources, and the energy crisis due to the demand vs. supply gap has boosted and pushed the demand for clean, renewable energy resources that are both sustainable and efficient. The most significant technological challenge the world is facing today is the efficient utilization of renewable energy resources and, for that, the grid-scale energy storage/delivery solution is required due to the fluctuating nature of power generation through renewables. In recent years, because of their outstanding characteristics of long cycling life, high power density, rapid recharge capability, higher capacitance, and flexibility, electrochemical supercapacitors (ECs) have received much attention for application in electric vehicles, portable devices, and other electronic devices that require tremendous electrical energy at elevated power levels in tiny pulses.^1–5

ECs are the subject of intense research in order to enhance their performance and they are capable of bridging the performance gap between dielectric capacitors and rechargeable batteries by providing higher power and energy density values. ECs are divided into two categories based on the charge storage mechanisms and active materials: electric double-layer capacitors (EDLCs) and pseudocapacitors. The capacity of EDLCs is derived from electrostatic charges accumulating at the electrode/electrolyte interface. In addition to using an electrostatic electric double layer, pseudocapacitors store electrical energy in a faradaic component, which is the insertion of the electrolyte's mobile cation into the host electrode, reducing the host-electrode cation.^6,7

Various materials such as carbonaceous materials, transition metal oxides or hydroxides, and conducting polymers have all been investigated for use as supercapacitor electrodes but are constrained by their low rate capability. Carbon materials in actual supercapacitors typically have specific capacity (C_s) values of 75–175 F g⁻¹ for aqueous electrolytes and 40–100 F g⁻¹ for organic electrolytes,⁸ which are lower than that of transition metal oxides or hydroxides. As pseudocapacitors have a greater theoretical specific capacitance than ordinary EDLCs, they find more applications as cathodes for supercapacitors. In this regard, considerable research efforts have been made towards designing and synthesizing various high-performance pseudocapacitive anode materials, such as Fe₂O₃,⁹ Fe₃O₄,¹⁰ Co₃O₄,¹¹ V₂O₅,¹² MoO₃,¹³ and transition-metal sulfides,^14,15 in recent times. Among them, transition metal sulfides such as WS₂,¹⁶ FeS₂,¹⁷ MoS₂,¹⁸ CuS,¹⁹ CoS,²⁰ and NiS^21–31 have recently gained more interest and have been widely used as the anode materials for supercapacitors due to their low electronegativity, excellent electrical conductivity, high specific capacitance, and rich electrochemical redox sites in comparison with their oxide counterparts. Hou et al.²¹ reported the first use of NiS as a supercapacitor electrode. Further, they proposed an electrochemical reaction mechanism. Zhu et al.²² designed and fabricated hierarchical hollow structures to examine the increased electrochemical performance of nickel sulfide.

NiS has drawn much attention because of its exceptional electrical conductivity, low toxicity, affordability, simplicity of manufacture, and variety of valence states.^23–28 For instance, it is frequently used as a catalyst in hydrogenation and carbon liquefaction processes and to store solar energy.²⁹ Additionally, its impacts on toughened glass and its carcinogenic effects have drawn much attention.

NiS, NiS₂, Ni₃S₂, Ni₃S₄, and Ni₉S₈, as well as amorphous and nanocrystalline phases, are among the known solid phases of nickel sulfides. The most prevalent nickel sulfide minerals among these solid phases are millerite (NiS), heazlewoodite (Ni₃S₂), polydymite (Ni₃S₄), and vaesite (NiS₂). There are two types of nickel(II) monosulfide (NiS): NiAs-type NiS (also known as α-NiS; space group: P6₃/mmc)³⁰ and millerite (also known as β-NiS; space group: R3m). At low temperatures (below 652 K), millerite is the stable form of NiS, but at higher temperatures, NiAs-type NiS is more stable. Millerite is the most prevalent nickel sulfide mineral and nickel ore mineral.³¹

In this article, we present the synthesis, characterizations, and electrochemical performances of β-NiS nanoparticles, and they show the phonon confinement effect confirmed by Raman and UV-visible spectra. The β-NiS electrode shows a specific capacitance of 1578 F g⁻¹ at 1 A g⁻¹ from the galvanostatic discharge profile, whereas a capacitance of 1611 F g⁻¹ at 1 mV s⁻¹ was obtained through CV curves in 2 M KOH aqueous electrolyte. Additionally, we assembled aqueous asymmetric supercapacitors (ASCs) with activated carbon (AC) as the negative electrode and the β-NiS electrode as the positive electrode. The ASCs type full cell containing the β-NiS active electrode and AC (counter electrode) show excellent cycling stability resulting in the highest specific energy equivalent to ∼163 W h kg⁻¹ and a specific power of ∼507 W kg⁻¹.

Experimental

Synthesis

For the synthesis of β-NiS, 5.94 g (1 M) of nickel(II) chloride hexahydrate (NiCl₂·6H₂O) was dissolved in 250 mL of deionized water with continuous stirring for 4–5 hours to obtain a homogeneous solution, followed by passing H₂S gas (synthesized by the Kipp generator method) into the solution. A black precipitate of the product β-NiS was obtained after a while, which was then filtered and washed several times with deionized water. The product was dried in a hot air oven at 120 °C for 12 h under a N₂ atmosphere. The final product was further characterized and tested for its electrochemical performance.

NiCl₂·6H₂O + H₂S → NiS + 2HCl + 6H₂O. (1)

Characterization

The phase purity and structure of the material were examined through a Rigaku MiniFlex desktop X-ray diffractometer (XRD) with Cu-Kα radiation (λ = 0.154 nm) in the 2θ range of 15°–65° with a step size of 0.02°. The Rietveld refinement of the powder XRD patterns was carried out using FullProf Suite. X’Pert HighScore (PANalytical) software was used to identify the critical phase. SEM (EVO-Scanning Electron Microscope MA15/18, CARL ZEISS Microscopy Ltd) was used to determine the surface morphology and structure of the sample. Microstructural investigations were performed on a high-resolution transmission electron microscope (FEI Tecnai G² 20 TWIN, USA) at a 200 kV acceleration voltage. For TEM imaging, the samples were dispersed in 20 mL of anhydrous ethanol and sonicated for 30 minutes; then, a drop was placed onto a holey-carbon-coated copper grid (Pelco International, USA). HRTEM micrographs were recorded and analyzed with Gatan Microscopy Suite (DigitalMicrograph). The optical properties were studied in the 200–800 nm range using an Agilent Cary 60 UV-visible spectrophotometer. The thermal stability of the as-prepared sample was examined using a TGA thermal analyzer (STA 6000, PerkinElmer Pvt. Ltd); the analysis was carried out at a heating rate of 10 °C min⁻¹ in the temperature range of 30 to 700 °C. The Raman spectra of the samples were determined using a Renishaw inVia Raman microscope under laser irradiation of 633 nm. The infrared spectra of the samples were recorded using a Nicolet iS5 FT-IR spectrometer in the range of 400 to 4000 cm⁻¹. The pore size distribution and specific surface area of the sample were measured by BET (BELCAT-II, MicrotracBEL Corp.). All electrochemical performances of the sample, including cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS), were conducted using a conventional three-electrode arrangement and measured using a Metrohm Autolab (PGSTAT204) equipped with the FRA32M module; electrochemical measurements were analyzed using NOVA 1.1 software.

Preparation of electrodes

The working electrode was prepared by taking the required amounts of the active material, activated carbon, and polyvinylidene difluoride (PVDF) binder in a ratio of 7 : 2 : 1 in N-methyl-2-pyrrolidone (NMP) solvent. The homogeneous viscous slurry was prepared in a mortar and pestle to prepare the working electrode, accompanied by casting over a 1 cm² area of Toray carbon paper. Further, the coated electrode was dried at 80 °C for 12 h. Activated carbon was used to increase the conductivity of the electrodes in electrode preparation. The electrode loading was calculated by taking the weight of the electrode on an electronic balance (error limit: 0.01 mg). For that, first, the weight of Toray paper was taken and then the weight of Toray paper coated with electrode ink (after drying the ink on the Toray paper on a 1 × 1 cm² area). The total electrode load was found to be ∼1 mg (0.7 mg active material : 0.2 mg activated carbon : 0.1 mg PVDF binder). Similarly, activated carbon (AC) electrodes were prepared by taking the required amounts of activated carbon and PVDF binder in a ratio of 9 : 1 in N-methyl-2-pyrrolidone (NMP) solvent.

Results and discussion

Structural properties

The XRD peak pattern of the as-prepared powder sample confirms the purity and formation of the β-phase of NiS nanoparticles. Fig. 1a shows the Rietveld refined XRD profile of β-NiS nanoparticles; refinement was done using the program FullProf Suite. Parameters were refined in the space group R3m and converged to a final R_Bragg = 4.22%, R_f = 3.25%, R_p = 21.4%, R_wp = 14.8 and a goodness of fit (χ²) of 1.53. The sharp prominent diffraction peaks at 2θ values of 18.46, 30.52, 32.25, 35.9, and 40.66 represent the [110], [101], [300], [021], and [211] planes of β-NiS nanoparticles in the rhombohedral cell (space group: R3m) with lattice parameters a = 9.61(3) Å, b = 9.61(3) Å, c = 3.13(7) Å, α = β = γ = 90° and match very well with the diffraction peaks obtained for β-NiS nanoparticles of ICDD file no. 00-003-0760. Table 1 shows the crystallographic diffraction data of β-NiS nanoparticles, and the crystallite size was calculated using the Debye–Scherrer equation, and the average crystallite size was found to be ∼12.6 (±1) nm. The VESTA image in Fig. 1b shows that β-NiS nanoparticles adopt a rhombohedral millerite phase.

Fig. 1 (a) Rietveld refinement of the XRD profile of β-NiS nanoparticles and (b) VESTA image of β-NiS nanoparticles.

Table 1 X-ray crystallographic representative diffraction data for β-NiS nanoparticles

Position (2θ)	d-spacing (Å)	hkl (plane)	Crystallite size (nm)
18.457	4.803092	1 1 0	33.18836
30.521	2.926489	1 0 1	18.67821
32.255	2.773067	3 0 0	17.44846
35.904	2.499140	0 2 1	15.22944
37.416	2.401546	2 2 0	14.43066
40.662	2.216989	2 1 1	12.90767
49.028	1.856471	1 3 1	9.88285
50.213	1.815398	4 1 0	9.534162
52.825	1.731624	4 0 1	9.062734
56.440	1.628991	3 2 1	7.942084
57.516	1.601031	3 3 0	7.702197
60.185	1.536270	0 1 2	7.145272
63.260	1.468781	0 5 1	6.563816
63.528	1.463245	2 0 2	6.515981
Average crystallite size (nm)			12.58799

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Thermogravimetric analysis (TGA) under a N₂ atmosphere (Fig. 2a) was performed to understand the thermal stability of β-NiS nanoparticles. The as-precipitated black powder was utilized for the TGA study. The first two decomposition stages are located between 30 °C and 220 °C (weight loss of about 6.64%, equivalent to ∼0.3H₂O molecules), which could be due to the removal of residual moisture and the hydroxide group. The third stage of weight loss, about 9.12% (equivalent to ∼0.4 sulfur loss), occurs in the temperature range of 220–550 °C and is attributed to the structural collapse and gradual decomposition of the sample and release of sulfur.³² The DTA (differential thermogravimetry analysis) curve in the inset clearly shows two endothermic peaks at 83 °C and 200 °C which are attributed to the desorption of the absorbed water and the removal of the structural water molecule, while the endothermic peak at 530 °C may be attributed to structural collapse and the gradual loss of sulfur in which a weight loss of 9.12% was observed. Hence the samples were dried at 120 °C in a N₂ atmosphere to remove the water present in the sample to make it moisture free. The weight losses that were observed in the TGA study are represented by eqn (2) and (3):

NiS·xH₂O → NiS + xH₂O; x ∼ 0.3 (T ∼ 220 °C) (2)

NiS → NiS_y + (1 − y)S; y ∼ 0.6 (T ∼ 550 °C) (3)

Fig. 2b presents the FT-IR spectrum of the as-prepared β-NiS nanoparticles. The broad band at 3297 cm⁻¹ and 1615 cm⁻¹ can be attributed to the stretching and bending vibrations of the OH groups of water adsorbed to the surface, respectively.³³ The bands at 442 cm⁻¹, 420 cm⁻¹, 666 cm⁻¹, 766 cm⁻¹, 1096 cm⁻¹, and 984 cm⁻¹ are due to the symmetric and asymmetric stretching of the Ni–S band in NiS particles, which is in good agreement with the results published for β-NiS nanoparticles.³⁴

Fig. 2 (a) TGA of β-NiS nanoparticles in an N₂ atmosphere (the inset shows the DTA plot), (b) FT-IR spectra of β-NiS nanoparticles, (c) Raman spectra of β-NiS nanoparticles, (d) UV-Visible spectra of β-NiS nanoparticles (the inset shows the Tauc plot for β-NiS) and (e) BET nitrogen adsorption/desorption isotherm of β-NiS nanoparticles.

Raman lines (Fig. 2c) for β-NiS nanoparticles (<10 nm) are asymmetrically widened and shifted toward lower wavenumbers due to the quantum confinement of optical phonons related to the particle size of the materials, i.e., red-shifted, indicating the phonon confinement effect. β-NiS nanoparticles show a stretching vibration band around 471 cm⁻¹ due to the presence of NiS nanocrystals and a high intensity stretching band at 1087 cm⁻¹ confirms the phonon confinement effect³¹ due to the nanosize of β-NiS particles.^35,36Fig. 2d presents the UV-visible spectrum showing an absorption peak around 212 nm, confirming the formation of β-NiS nanoparticles,²⁴ while their uniformity and narrow particle size distribution are confirmed based on the narrow width of this absorption peak which is in good agreement with the results of XRD and Raman spectroscopy.

The band gap of β-NiS nanoparticles was obtained using the Tauc plot (inset of Fig. 2d) and was found to be 2.08 eV. Fig. 2e presents the BET surface area measurements of the β-NiS nanoparticles. The nitrogen adsorption and desorption isotherms exhibit type IV isotherm characteristics, which correlate with a complex mixture of micro- and mesoporous structures for β-NiS nanoparticles. The calculated BET specific surface area is 64 m² g⁻¹, and both micropores and mesopores with an average diameter of 23 nm. The high surface area and mesoporous structure of β-NiS nanoparticles will offer better contact for the electrolyte and contribute to excellent electrochemical kinetics due to increasing charge carriers.³⁷ The calculated mesopore diameter of β-NiS is much bigger than that of the OH⁻ ions in an aqueous KOH electrolyte.³⁸

The electronic structure of the β-NiS sample was studied by X-ray photoelectron spectroscopy (XPS). The high-resolution Ni (2p) spectrum shown in Fig. 3a could be assigned to 2p_3/2 of Ni²⁺ (855.82 eV) and 2p_1/2 of Ni²⁺ (873.32 eV) ions, as well as the corresponding satellite peaks at 861.09 and 873.31 eV. The high-resolution S (2p) spectrum shown in Fig. 3b could be assigned to 2p_3/2 of S²⁻ at 162.16 eV.³⁹

Fig. 3 XPS plot of β-NiS nanoparticles: (a) Ni (2p) spectra and (b) S (2p) spectra.

The SEM image in Fig. 4a displays the particle distribution and sponge-like arrangement of the β-NiS nanoparticles. β-NiS nanoparticles are highly agglomerated, and sub-micron size grains are visible in the SEM image. Fig. 4b shows the EDX (energy dispersive X-ray) analysis image confirming the composition of the material. The HRTEM image in Fig. 4c of the as-synthesized β-NiS nanoparticles under optimum conditions shows atomic arrangements in localized regions within the sample. Fig. 4d shows the selected area electron diffraction (SAED) pattern showing clear well-aligned diffraction spots, with lattice spacing of 0.29, 0.27, 0.22, 0.18, 0.17 and 0.16 nm corresponding to [101], [300], [211], [131], [401], and [321] planes, respectively. Fig. 4e shows the lattice fringes of β-NiS nanoparticles confirming the crystalline nature of the nanoparticles. Insets (i, ii, and iii) show the FFT (fast Fourier transform) image, inverse FFT, and d-spacing of β-NiS nanoparticles measured with Gatan Microscopy Suite (Digital Micrograph). The calculated d-spacing value was 0.276 nm, which matches the [300] plane of the β-NiS nanoparticles.

Fig. 4 (a) FE-SEM images, (b) EDX analysis image of the prepared β-NiS nanoparticles, (c) HRTEM images of β-NiS nanoparticles, (d) SAED pattern showing the lattice and (e) lattice fringes of β-NiS nanoparticles. Insets (i, ii, and iii) of (e) show the FFT image, inverse FFT, and d-spacing of β-NiS nanoparticles.

Electrochemical studies

The charge-storage capacity of the β-NiS nanoparticle electrodes was first evaluated by cyclic voltammetry (CV) in a three-electrode system where β-NiS nanoparticles were employed as the working electrode, a platinum electrode as the counter electrode, and saturated Hg/HgO (1 M KOH) as the reference electrode in KOH electrolyte. The CV curves were measured in KOH of different concentrations, namely 0.5, 1, 2, 4, and 6 M. From the KOH concentration vs. mass-specific capacitance plot depicted in Fig. 5a, the mass-specific capacitance was calculated using eqn (4); the mass-specific capacitances are 723 F g⁻¹, 806 F g⁻¹, 1611 F g⁻¹, 1094 F g⁻¹, and 864 F g⁻¹ at 1 mV s⁻¹ in 0.5, 1, 2, 4 and 6 M KOH. The specific capacitance initially rises as the OH⁻ concentration increases and reaches its maximum in a 2 M KOH solution and then falls as the OH⁻ concentration increases further.⁴⁰where C is the capacitance in F g⁻¹, (E₂ − E₁) is the width of the potential window (V), and the integral is the total voltammetric charge obtained by the integration of both positive and negative sweeps in a CV curve. Hence, a factor of 1/2 is used to calculate the charge in either the forward or backward scan, i(E) is the instantaneous current (A), m is the mass (g) of the active material and υ is the scan rate (V s⁻¹).

Fig. 5 (a) The plot of KOH concentration vs. specific capacitance at different scan rates. Typical CV curves for the β-NiS electrode in (b) 1 M, (c) 2 M and (d) 4 M KOH solutions at different scan rates of 1, 5, 10, 60 and 100 mV s⁻¹. (e) The comparative CV study in electrolytes of different concentrations at 10 mV s⁻¹. (f) The plot of log (peak current) vs. log (square root of the scan rate) for diffusion kinetics of the β-NiS electrode.

The CV curve shown in Fig. 5c (2 M KOH) depicts the presence of one pair of well-separated redox peaks at +0.37/+0.20 V. The nature of the curve explains the pseudocapacitive behavior coupled with surface redox (electrosorption).⁴¹ Redox peaks originated due to the reversible transformation of Ni²⁺ to Ni³⁺ through electrosorption (redox) of OH⁻ ions, as represented in eqn (5).⁴²

Ni²⁺S²⁻ + KOH ↔ K⁺[Ni³⁺S²⁻–OH⁻]e⁻ (5)

The CV curves at different scan rates ranging from 1 to 100 mV s⁻¹ are shown in Fig. 5(b)–(d). The specific capacitances from the CV curves were calculated using eqn (4), and the capacitance was found to be close to 1611 F g⁻¹, 1211 F g⁻¹, 996 F g⁻¹, 861 F g⁻¹, 751 F g⁻¹, 631 F g⁻¹, 556 F g⁻¹, 492 F g⁻¹, and 435 F g⁻¹ at scan rates of 1, 2, 5, 10, 20, 40, 60, 80, and 100 mV s⁻¹, respectively, for 2 M KOH. Because the active areas of the electrode are more accessible for the electrochemical reaction at lower scan rates, the specific capacitance drops as the scan rate increases. With higher scan rates, it was shown that the anodic peak potential shifted to a more favorable position, while the cathodic peak potential shifted to a more hostile position.

Fig. 5e shows the comparative CV study in electrolytes of different concentrations ranging from 1 M to 4 M. According to this study, the redox potential is shifted to a lower value with increasing concentration of the electrolyte KOH, which is not desirable for high-voltage supercapacitor applications. Further, the current density values also confirmed an increase in capacitance value from 0.5 M to 2 M KOH concentration and then a decrease in the capacitance value above 2 M KOH concentration.⁴³

Fig. 5f shows the linear relationship between cathodic and anodic peak currents concerning the square root of scan rate (υ^1/2), showing that the CV curve exhibits a semi-infinite diffusion-controlled process. Furthermore, the diffusion kinetics of electrodes can be better understood by determining the diffusion coefficient. The diffusion coefficient for the electrode was determined using the Randles–Sevcik equation as represented in eqn (6):^44,45

i_p = 2.686 × 10⁵ × n^3/2AD^1/2C₀υ^1/2 (6)

where i_p is the peak current (A), n is the number of electrons transferred in the redox reaction (usually 1), A is the electrode area in cm², D is the diffusion coefficient in cm² s⁻¹, C₀ is the concentration of OH⁻ ions in mol cm⁻³, and υ is the scan rate in V s⁻¹. The diffusion coefficient (D) of hydroxide (OH⁻) ions was found to be 3.13 × 10⁻⁷ cm² s⁻¹ for oxidation and 0.92 × 10⁻⁷ cm² s⁻¹ for reduction for the β-NiS electrode.

The power law provided in eqn (7) was employed to further evaluate the qualitative contribution of the various charge storage kinetics/mechanisms of the β-NiS electrode:⁴⁶

i = aυ^b (7)

where i is the current (A), the values of a and b are adjustable, and υ is the scan rate (V s⁻¹). The b value lies between 0.5 and 1, where b = 0.5 stands for the semi-infinite diffusion control reaction for a battery (intercalative behavior) type material, while b = 1 stands for the surface control capacitive reaction or electrosorption.⁴⁷

From Fig. 6a, the slopes of the corresponding log(υ) versus log(i) plots for the b-values of oxidative and reductive currents were found to be 0.58 and 0.58, respectively, indicating the dominance of semi-infinite diffusion-controlled intercalative processes during the electrochemical reaction resulting in battery-type supercapacitor behavior. The high specific capacitance of β-NiS nanoparticles is attributed to both semi-infinite diffusion-controlled and surface-control redox (non-diffusion) mediated pseudocapacitance processes. Trasatti⁴⁸ and Dunn's⁴⁹ methods are adopted to evaluate the contributions of diffusion-controlled and non-diffusion-controlled capacitance concerning the experimental data.

Fig. 6 Electrodynamic characteristics of the β-NiS electrode: (a) plot of the linear relationship between log (peak current) and log (scan rate) in two different scan rate regions, (b) plot of the power law of the charged state at a potential and the discharged state at a potential, (c) diffusive and capacitive contributions at different scan rates, (d) analysis of the kinetic contribution at 10 mV s⁻¹, and (e and f) Trasatti plot at different scan rates.

Fig. 6b shows the quantitative voltammetry sweep rate dependence which can distinguish the capacitive contribution to the current response. The current response at a fixed potential is the contribution of two separate mechanisms: surface capacitive effects and diffusion-controlled insertion or intercalation.

i(υ) = k₁υ + k₂υ^1/2 (8)

For better understanding, eqn (8) was modified asIn eqn (8), k₁υ and k₂υ^1/2 represent the current contributions from the surface capacitive and diffusion-controlled intercalation processes, respectively. Thus, after the determination of k₁ and k₂, from the obtained slope and intercept of the y-axis from linear fit, we can quantify their contributions to the current density at specific potentials.⁵⁰

The representative curve {(V)/υ^1/2vs. υ^1/2} shown in Fig. 6c represents the contribution of surface capacitance and diffusion-controlled intercalation at different scan rates. Fig. 6d represents a specific contribution at a 10 mV s⁻¹ scan rate, and the contribution of surface capacitance or electrosorption was found to be 28%, and that of diffusion-controlled intercalation was found to be close to 72%. The Trasatti plot was further used to determine the amount of charge stored in the outer and inner surfaces. According to Trassati, the inner and outer surface capacitance of the electrode adds to the total specific capacitance. It can be expressed as

C_Total = C_in + C_out (F g⁻¹) (10)

The charge storage method depends on the scan rate from CV analysis. Fig. 6e shows the linear fit of C⁻¹vs. υ^1/2 at different scan rates, and the y-intercept represents the amount of total charge storage or capacitance of the electrode. Fig. 6f shows the linear fit of C vs. υ^−1/2, and the y-intercept represents the outer surface charge storage or capacitance of the electrode. The total capacitance value (C_total), which was determined by computing the y-intercept value used on the Trassati plot, was found to be 1751 F g⁻¹, C_in was found to be 1341 F g⁻¹ (76.6% of the total capacitance value), and C_out was found to be 410 F g⁻¹ (23.4% of the total capacitance value). This confirms that 77% of the charge stored in β-NiS electrodes was due to pseudocapacitive intercalation-type behavior. Overall, it can be concluded that β-NiS nanoparticles store charge predominately through the intercalative pseudocapacitive charge storage process.

Galvanostatic studies were conducted for a more precise capacity assessment of β-NiS electrodes. The specific capacitance of the electrode can be calculated from the charge–discharge curve using eqn (11):²⁶

where I is the discharge current (A), Δt is the discharge time (s), m is the mass of the active material in the electrode (g), and ΔV is the potential change during discharge (V). Fig. 7a shows specific capacitances of 1578 F g⁻¹, 1516 F g⁻¹, 1410 F g⁻¹, 1177 F g⁻¹, and 944 F g⁻¹ at current densities of 1, 2, 3, 5, and 10 A g⁻¹, respectively. According to the above GCD observations, the specific capacitance decreased gradually as the current density increased. The specific capacitance fell to 40% of its initial value in the required current density range.

Fig. 7 (a) Charge/discharge curve of the β-NiS electrode, (b) capacitance performance of the β-NiS electrode at different constant current density values, (c) capacitance retention and coulombic efficiency of the β-NiS electrode, (d) the comparative study of GCD at 1 M, 2 M and 4 M KOH concentrations and (e) Nyquist and (f) Bode plots of the β-NiS electrode at 10 mV between 0.1 Hz and 100 kHz (the inset displays an enlarged view of the Nyquist plot in the high-frequency region).

Fig. 7b shows the capacitance value of the cycle number with a different current density of the β-NiS electrode. Fig. 7c shows the excellent long-term cycling stability of the β-NiS electrode at 10 A g⁻¹ for 10 000 cycles. The 86% capacity retention reflects that the specific capacitance of the electrode did not change that much from the initial capacitance after 10 000 cycles. The coulombic efficiency of the electrode was 90.5% after 10 000 cycles of continuous charge/discharge, which reveals the high reversibility of the β-NiS electrode. Fig. 7d shows the comparative study of GCD at 1 M, 2 M, and 4 M KOH concentrations. It is clear from the discharge time that the electrode delivers a longer discharge time in 2 M KOH electrolyte compared to 1 M and 4 M KOH electrolyte concentrations at a 1 A g⁻¹ current rate. The specific capacitances of the electrode calculated from the charge–discharge curve are 801 F g⁻¹, 1578 F g⁻¹, and 1042 F g⁻¹ for 1 M, 2 M, and 4 M KOH electrolyte concentrations, respectively. According to comparative GCD observations, we confirmed an increase in the specific capacitance value from 1 M to 2 M KOH concentration and then a decrease in capacitance at 4 M KOH concentration which is in good agreement with the results of the CV study. In addition to electrochemical stability, we performed AC electrochemical impedance spectroscopy (EIS) in the frequency range of 100 kHz to 0.1 Hz at 10 mV applied potential, as shown in the Nyquist plot in Fig. 7e. The Bode plot shown in Fig. 7f illustrates the phase angle of the β-NiS electrode with respect to the applied frequency. The phase angle of the β-NiS electrode was 57.2° and 53.2° for the first cycle and after 2500 cycles, respectively. The lower phase angle (around 50°) affirms the pseudocapacitive nature of the β-NiS electrode, which is in good agreement with the Nyquist plot.^51,52 The specific impedance contribution was attributed to the impedance distributions over charge transfer resistance (R_ct), electric series resistance (R_s), and Warburg impedance (R_w). The R_s and R_ct values for the β-NiS electrode were 1.34 Ω and 0.33 Ω, respectively (see the enlarged view in the inset of Fig. 7e). Lower frequency data show the Warburg diffusion resistance for the β-NiS electrode. The straight line in the low-frequency region indicates a fast OH⁻ ion diffusion-controlled process. EIS spectra after 2500 repeated charge–discharge cycles compared with the first cycle demonstrate the good electrochemical stability of the β-NiS electrode.

Further, studies were conducted to understand the charge storage behavior of β-NiS electrodes in a neutral electrolyte of 0.5 M Na₂SO₄. The CV curve at various scan rates, shown in Fig. 8a, indicates well-separated redox peaks (+0.68/+0.48 V at 1 mV s⁻¹ scan rate). The specific capacitance, which is a function of the scan rate, values were estimated in 0.5 M Na₂SO₄ electrolyte, which were 403 F g⁻¹, 323 F g⁻¹, 238 F g⁻¹, 117 F g⁻¹, and 79 F g⁻¹ at 1 mV s⁻¹, 5 mV s⁻¹, 10 mV s⁻¹, 60 mV s⁻¹, and 100 mV s⁻¹, respectively, for the β-NiS electrode. Fig. 8b displays the findings of the quantitative galvanostatic charge/discharge storage measurement, and the values were found to be 386 F g⁻¹, 308 F g⁻¹, 195 F g⁻¹, 115 F g⁻¹, and 71 F g⁻¹ at 1 A g⁻¹, 2 A g⁻¹, 3 A g⁻¹, 5 A g⁻¹, and 10 A g⁻¹, respectively.

Fig. 8 (a) CV curve and (b) GCD plot of the β-NiS electrode in 0.5 M Na₂SO₄ electrolyte. (c) Comparative CV diagram of the β-NiS electrode in 2 M KOH and 0.5 M Na₂SO₄ electrolyte at 10 mV s⁻¹, and (d) comparative charge/discharge curve of the β-NiS electrode in KOH and the Na₂SO₄ medium at 1 A g⁻¹.

Subsequently, a comparative study was carried out to study the effects of anions present in the electrolyte. Fig. 8c shows the comparative CV curve of the β-NiS electrode in 0.5 M Na₂SO₄ and 2 M KOH electrolytes. There were changes in the redox peaks from KOH to Na₂SO₄ electrolyte. Further, in comparison to 0.5 M Na₂SO₄, the redox peaks were found to be more dominant in 2 M KOH; this may be due to the difference in the hydration radii of hydroxyl ions (3 Å) as compared to sulfate ions (3.79 Å). A larger SO₄²⁻ hydrate sphere further reduces the number of ions that enter the pores, which results in slower diffusion and reduced production of electric double layers.⁵³ Additionally, the greater molar conductance of the OH⁻ ion (198 cm² Ω mol⁻¹) in the KOH electrolyte compared to SO₄²⁻ (79.8 cm² Ω mol⁻¹) in Na₂SO₄ exhibits a more robust current response in the CV curve.⁵³

GCD experiments were carried out in 2 M KOH and 0.5 M Na₂SO₄ electrolytes to determine the quantitative capacitance of the electrode. It is clear from Fig. 8d (see the discharge time) that the electrode delivers a longer discharge time in 2 M KOH electrolyte as compared to 0.5 M Na₂SO₄ electrolyte at a 1 A g⁻¹ current rate. These studies corroborate the applicability of an aqueous 2 M KOH electrolyte for high electrode performance because the smaller OH⁻ anions match the pore width of the electrode well, resulting in improved performance.

Two electrode test

Two electrode measurements were conducted in 2 M KOH to understand the useful charge storage behavior of the β-NiS electrode using activated carbon (AC) as the counter electrode. To evaluate the maximum specific capacitance during the full cell test, the storage capacity of positive and negative electrodes must be balanced as shown in eqn (12).Eqn (13) was used to calculate the mass ratio (m⁺/m⁻) of the positive and negative electrode materials to balance the charge storage capacity of the cell.where m⁺, m⁻, and C₊, C₋, ΔE₊, ΔE₋ are the mass, specific capacitance, and potential window of positive and negative electrodes estimated by three-electrode measurements.⁵⁴

Fig. 9a shows CV curves at a 10 mV s⁻¹ scan rate, where the β-NiS electrode was used as the positive electrode and AC (activated carbon) was used as the negative electrode. The electrochemical properties of the fabricated AC (activated carbon) electrode are shown in the ESI† (Fig. S1 and S2) using cyclic voltammetry and galvanostatic charge discharge. The specific capacitance of the AC electrode was 381 F g⁻¹ from the GCD at 1 A g⁻¹ and 384 F g⁻¹ in CV at 1 mV s⁻¹. As activated carbon is used to increase the conductivity of the active electrode, while calculating the electrochemical charge storage capacity of the NiS electrode, the contribution to capacitance due to activated carbon is extracted and it shows the capacitance value of the NiS electrode only. The calculated mass ratio (m⁺/m⁻) was 1 : 1.67 for the asymmetric cell, and the weight of the active material was measured to be 4.45 mg (excluding the weight of acetylene black and PVDF). Fig. 9b shows the CV plot of the β-NiS electrode//AC, a two-electrode ASC [asymmetric supercapacitor cell at scan rates ranging from 1 to 100 mV s⁻¹ in this potential window (0–1.5 V)]. Subsequently, Fig. 9c shows the GCD curve, and the specific capacitance values were calculated using eqn (11). The capacitance values were 780, 716, 622, 494 and 326 F g⁻¹ at current densities of 1, 2, 3, 5, and 10 A g⁻¹, respectively.

Fig. 9 (a) Separate CV plots for activated carbon and the β-NiS electrode in 2 M KOH electrolyte at 10 mV s⁻¹, (b) CV curve at different scan rates, (c) charge–discharge curve at different current rates, (d) Nyquist and (e) Bode plots at 10 mV for the initial cycle and after 2500 cycles of full cell formation, (f) capacitance retention and coulombic efficiency and (g) Ragone plot of the β-NiS//activated carbon fuel cell in the ASC mode in comparison with reported supercapacitor devices.

Fig. 9d and e show the EIS (Nyquist and Bode) plots in the frequency range of 0.1 Hz to 100 kHz at 10 mV s⁻¹ applied potential, confirming the retention of the electronic structure and resistance of the fuel cell (β-NiS electrode//AC) as the impedance of the material decreases after completion of 2500 cycles compared to the first cycle. From Fig. 9e, it is evident that the phase angle at the tail is 54.6° and 51.6° for the first cycle and after 2500 cycles, respectively, which further confirms the pseudocapacitive nature of the β-NiS//AC cell. The EIS results corroborate the cycling stability by demonstrating a little difference in the internal and charge transfer resistance of the cell before and after the test. The β-NiS electrode//AC (ASC) full cell shows brilliant long-term cycling stability with 90% capacity retention after 2500 cycles, as shown in Fig. 9c. The two-electrode cell has lost only 4% of its initial coulombic efficiency after 2500 cycles.

The specific power and specific energy of asymmetric cell capacitors were calculated using eqn (14) and (15).

where C_sp is the specific capacitance, V is the operating voltage, and t_dis is the discharge time. Fig. 9f displays the Ragone plot (specific energy vs. specific power) of the β-NiS electrode//AC (ASC) full-cell device in comparison with reported supercapacitor devices. The resultant values of asymmetric cells confirm the highest specific energy equivalent to ∼163 W h kg⁻¹ at 1 A g⁻¹ current density and specific power equivalent to ∼507 W kg⁻¹. A maximum specific power of ∼5021 W kg⁻¹ was obtained when the specific energy was decreased to ∼ 68 W h kg⁻¹ at 10 A g⁻¹ of current density. The obtained energy and power density values of the β-NiS electrode//AC (ASC) cell are superior/comparable to reported supercapacitor devices using Ni–Fe cells, Mn incorporated MoS₂, Ni_0.55Co_0.45C₂O₄//AC, SnS₂, and others as electrode materials as shown in Table 2.⁵⁵

Table 2 The Ragone plot of the β-NiS electrode//AC (ASC) device in comparison with reported supercapacitor devices

Material	Energy density (W h kg^−1)	Power density (kW kg^−1)	Ref.
Ni–Fe cells	38.78	11.54	62
Mn incorporated MoS2	48.8	5	56
(Ni0.33Co0.67)Se2 complex	29.1	0.799	57
MnS thin films	66.41	0.3	58
CF-SnS2	3.1	0.123	60
NiCo2S4 nanoplate	35	0.12	59
NiSe@MoSe2//N-PMCN	32	0.425	63
Ni(OH)2/graphene//porous graphene	77.8	0.174	64
β-NiS nanoparticles	68	5	Present work
	103	2.51
	129.6	1.5
	149.2	1
	162.5	0.5

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Conclusions

In summary, β-NiS nanoparticles were synthesized using a simple precipitation route where H₂S generated through the Kipp generator method was used as the precipitating agent for sulfide formation in an aqueous medium. High diffusion of the gas into the liquid phase resulted in the formation of single-phase β-NiS nanoparticles. The XRD patterns show that the formation of more stable β-NiS nanoparticles is in the rhombohedral millerite phase, and the grain size estimated using the Debye–Scherrer equation was approximately 10 nm. Nanocrystalline β-NiS shows the quantum confinement effect due to the smaller/nanocrystalline size of the particles by Raman spectra and supported by XRD and UV-Visible spectroscopy data. The nanocrystalline β-NiS electrode showed high pseudocapacitive performance with a specific capacitance of 1611 F g⁻¹ at 1 mV s⁻¹ in the potential window of 0–0.6 V in CV and 1578 F g⁻¹ at a current density of 1 A g⁻¹ in the potential window of 0–0.45 V in the GCD measurement. Further, the excellent cycling stability and predominant intercalative mechanism seem to be the reasons behind the high charge storage capacity of the materials as the intercalative (inner) and surface (outer) charges stored by β-NiS electrodes were found to be 72% and 28%, respectively. In 2 M KOH electrolyte, the β-NiS//AC fuel cell resulted in a maximum specific energy of ∼163 W h kg⁻¹ with a specific power equivalent to ∼507 W kg⁻¹ in the voltage window of 1.5 V in 2 M KOH electrolyte at 1 A g⁻¹ current density. The capacitances of different transition-metal-sulfide-based pseudocapacitors are summarized in Table 3, and the results show that the charge storage behavior and the capacitance value of the β-NiS electrode are comparable or superior to most of the transition-metal-sulfide-based pseudocapacitors reported to date. These findings demonstrate the potential of nanocrystalline β-NiS to serve as a pseudocapacitive electrode for large-scale energy storage applications.

Table 3 Values of the specific capacitance of some transition-metal sulfides

Material	Capacitance (F g^−1)	Current density or scan rate	Electrolyte	Ref.
Pyrite FeS2 nanobelts	308	5 mV s^−1	1 M Na2SO4	17
Mn incorporated MoS2	430	5 mV s^−1	0.5 M Na2SO4	56
(Ni0.33Co0.67)Se2 complex	828	1 A g^−1	3 M KOH	57
MnS thin films	747	1 mA cm^−2	1 M KOH	58
NiS nanorods/nanoplates	788	1 mA cm^−2	1 M KOH	26
NiCo2S4 nanoplate	437	1 A g^−1	3 M KOH	59
Carnation flower-like SnS2	524	0.08 A g^−1	1 M KCl	60
CoS-AC composite	798	10 A g^−1	K3Fe(CN)6 and KOH	61
β-NiS nanoparticles	1611	1 mV s^−1	2 M KOH	Present work
	1578	1 A g^−1

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Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank the Department of Chemistry, IIT (BHU), for its facility and support. The authors also gratefully acknowledge the use of characterization facilities and services at the Central Instrument Facility Centre (CIFC), IIT-BHU, Varanasi. Dr Preetam Singh thanks the Science and Engineering Research Board (SERB) India for the financial support (Project no. EMR/2016/006840). Dr Asha Gupta thanks the Science and Engineering Research Board (SERB) India for the financial support (Project no. R&D/SERB/Chy/19-20/03).

References

1. B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamental and Technological Applications, Kluwer academic/Plenum Press, New York, 1999.
2. M. Winter and R. Brodd, What Are Batteries, Fuel Cells, and Supercapacitors?, Chem. Rev., 2004, 104, 4245–4269.
3. S. Meher and G. Rao, Ultralayered Co₃O₄ for High-Performance Supercapacitor Applications, J. Phys. Chem. C, 2011, 115, 15646–15654.
4. X. Xia, C. Zhu, J. Luo, Z. Zeng, C. Guan, C. F. Ng, H. Zhang and H. Fan, Synthesis of free-standing metal sulfide nanoarrays via anion exchange reaction and their electrochemical energy storage application, Small, 2014, 10, 766–773.
5. Q. Wang, L. Jiao, H. Du, J. Yang, Q. Huan, W. Peng, Y. Si, Y. Wang and H. Yuan, Facile synthesis and superior supercapacitor performances of three-dimensional cobalt sulfide hierarchitectures, CrystEngComm, 2011, 13, 6960–6963.
6. P. Simon and Y. Gogotsi, Materials for electrochemicalcapacitors, Nat. Mater., 2008, 7, 845–854.
7. B. E. Conway and W. G. Pell, Double-layer and pseudocapacitance types of electrochemical capacitors and their applications to the development of hybrid devices, J. Solid State Electrochem., 2003, 7, 637–644.
8. G. Wang, L. Zhang and J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev., 2012, 41, 797–828.
9. S. Cheng and Y. Zhang, et al., Energizing Fe₂O₃-based supercapacitors with tunable surface pseudocapacitance via physical spatial-confining strategy, Chem. Eng. J., 2021, 406, 126875.
10. C. C. Chen and C.-Y. Chen, et al., Influence of Fe₃O₄ nanoparticles on pseudocapacitive behavior of the charge storage process, J. Alloys Compd., 2015, 645, 250–258.
11. M. Mirzaeian and N. Akhanova, et al., Improvement of the Pseudocapacitive Performance of Cobalt Oxide-Based Electrodes for Electrochemical Capacitors, Energies, 2020, 13, 5228.
12. S. J. Patil and N. R. Chodankar, et al., Carbon alternative pseudocapacitive V₂O₅ nanobricks and δ-MnO₂ nanoflakes @ α-MnO₂ nanowires hetero-phase for high-energy pseudocapacitor, J. Power Sources, 2020, 453, 227766.
13. H. S. Kim and J. B. Cook, Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO_3−x, Nat. Mater., 2017, 16, 454–460.
14. X. Rui, H. Tan and Q. Yan, Nanostructured metal sulfides for energy storage, Nanoscale, 2014, 6, 9889–9924.
15. J. Theerthagiria and R. A. Senthil, et al., Recent progress and emerging challenges of transition metal sulfides based composite electrodes for electrochemical supercapacitive energy storage, Ceram. Int., 2020, 46, 14317–14345.
16. O. Ola and K. Thummavichai, Layered tungsten-based composites and their pseudocapacitive and electrocatalytic performance, Mater. Chem. Front., 2022, 6, 737–747.
17. J. Chen, X. Zhou, C. Mei, J. Xu, S. Zhou and C. P. Wong, Pyrite FeS₂ nanobelts as high-performance anode material for aqueous pseudocapacitor, Electrochim. Acta, 2016, 222, 172–176.
18. S. S. Singha, S. Rudra, S. Mondal, M. Pradhan, A. K. Nayak, B. Satpati, P. Pal, K. Das and A. Sinha, Mn incorporated MoS₂ nanoflowers: a high performance electrode material for symmetric supercapacitor, Electrochim. Acta, 2020, 338, 135815.
19. Y. Hu and L. Zhang, Boosting High-Rate Sodium Storage of CuS via a Hollow Spherical Nanostructure and Surface Pseudocapacitive Behavior, ACS Appl. Energy Mater., 2021, 4, 8901–8909.
20. X. Chen and Q. Liu, Nickel and cobalt sulfide-based nanostructured materials for electrochemical energy storage devices, Chem. Eng. J., 2021, 409, 127237.
21. L. Hou, C. Yuan, D. Li, L. Yang, L. Shen, F. Zhang and X. Zhang, Electrochemically induced transformation of NiS nanoparticles into Ni(OH)₂ in KOH aqueous solution toward electrochemical capacitors, Electrochim. Acta, 2011, 56, 7454–7459.
22. T. Zhu, Z. Wang, S. Ding, J. Chen and X. W. Lou, Hierarchical nickel sulfide hollow spheres for high performance supercapacitors, RSC Adv., 2011, 1, 397–400.
23. J. Yang, X. Duan, W. Guo, D. Li, H. Zhang and W. Zheng, Electrochemical performances investigation of NiS/rGO composite as electrode material for supercapacitors, Nano Energy, 2014, 5, 74–81.
24. Y. Fazli and S. M. Pourmortazavi, et al., Synthesis, characterization and photocatalytic property of nickel sulfide nanoparticles, J. Mater. Sci.: Mater. Electron., 2016, 27, 7192–7199.
25. X. Lei, M. Li, M. Lu and X. Guan, Electrochemical Performances Investigation of New Carbon-Coated Nickel Sulfides as Electrode Material for Supercapacitors, Materials, 2019, 12, 3509.
26. P. Gaikar and S. P. Pawar, et al., Synthesis of nickel sulfide as a promising electrode material for pseudocapacitor application, RSC Adv., 2016, 6, 112589–112593.
27. H. Wang, J. Wang and M. Liang, et al., Novel Dealloying-Fabricated NiS/NiO Nanoparticles with Superior Cycling Stability for Supercapacitors, ACS Omega, 2021, 6, 17999–18007.
28. L. Peng and X. Ji, et al., Nickel Sulfide Nanoparticles Synthesized by Microwave-assisted Method as Promising Supercapacitor Electrodes: An Experimental and Computational Study, Electrochim. Acta, 2015, 182, 361–367.
29. Y. Li and H. X. Zhang, et al., New design of oriented NiS nanoflower arrays as platinum-free counter electrode for high-efficient dye-sensitized solar cells, Superlattices Microstruct., 2019, 125, 66–71.
30. H. Sowa, H. Ahsbahs and W. Schmitz, X-ray diffraction studies of millerite NiS under non-ambient conditions, Phys. Chem. Miner., 2004, 31, 321–327.
31. F. Guillaume and L. Zhang, et al., Optical phonons in millerite (NiS) from single-crystal polarized Raman spectroscopy, J. Raman Spectrosc., 2008, 39, 1419–1422.
32. H. Guo and Y. Ke, et al., Efficient adsorption and photocatalytic degradation of congo red onto hydrothermally synthesized NiS nanoparticles, J. Nanopart. Res., 2013, 15, 1475.
33. A. B. Vakylabad and M. Schaffie, et al., A procedure for processing of pregnant leach solution (PLS) produced from a chalcopyrite-ore bio-heap: CuO Nano-powder fabrication, Hydrometallurgy, 2016, 163, 24–32.
34. M. Kristl, B. Dojer, S. Gyergyek and J. Kristl, Synthesis of nickel and cobalt sulphide nanoparticles using a low cost sonochemical method, Heliyon, 2017, 3, e00273.
35. M. Shajudheen and S. Kumar, et al., Structural and Optical Properties of NiS nanoparticles Synthesized by Chemical Precipitation Method, IJIRSET, 2016, 5(8), 2319–8753.
36. M. Indhumathy and A. Prakasam, Controllable Synthesis of NiS/rGO Hybrid Composite: An Excellent Counter Electrode for Dye Sensitized Solar Cell, J. Cluster Sci., 2020, 31, 91–98.
37. F. Behnoudnia and H. Dehghani, Anion effect on the control of morphology for NiC₂O₄·2H₂O nanostructures as precursors for synthesis of Ni(OH)₂ and NiO nanostructures and their application for removing heavy metal ions of cadmium(II) and lead(II), Dalton Trans., 2014, 43(9), 3471–3478.
38. L. An, Q. Ren, W. Li, K. Xu, Y. Cao, T. Ji, R. Zou, Z. Chen and J. Hu, Highly ordered mesoporous NiCo₂O₄ with superior pseudocapacitance performance for supercapacitors, J. Mater. Chem. A, 2015, 3(21), 11503–11510.
39. G. B. Shombe and M. D. Khan, et al., Direct solvent free synthesis of bare α-NiS, β-NiS and α-β-NiS composite as excellent electrocatalysts: effect of selfcapping on supercapacitance and overall water splitting activity, Sci. Rep., 2020, 10, 3260.
40. A. Gupta and V. Kushwaha, et al., SrFeO_3−δ: a novel Fe⁴⁺–Fe²⁺ redox mediated pseudocapacitive electrode in aqueous electrolyte, Phys. Chem. Chem. Phys., 2022, 24, 11066–11078.
41. B. Evanko, S. W. Boettcher, S. J. Yoo and G. D. Stucky, Redox-Enhanced Electrochemical Capacitors: Status, Opportunity, and Best Practices for Performance Evaluation, ACS Energy Lett., 2017, 2, 2581–2590.
42. S. Nachimuthu and K. Kannan, et al., Electrochemical and magnetic properties of 3D porous NiS/CuS nanocomposites, Appl. Surf. Sci. Adv., 2022, 7, 100209.
43. N. K. Mishra and A. K. Singh, NiC₂O₄·2H₂O Nanoflakes: A Novel Redox-mediated Intercalative Pseudocapacitive Electrode for Supercapacitor Applications in Aqueous KOH and Neutral Na₂SO₄ electrolytes, ChemistrySelect, 2022, 7(21), e202201134.
44. B. Ramkumar, S. Yuvaraj, S. Surendran, K. Pandi, H. V. Ramasamy, Y. S. Lee and R. K. Selvan, Synthesis and characterization of carbon coated LiCo_1/3Ni_1/3Mn_1/3O₂ and bio-mass derived graphene like porous carbon electrodes for aqueous Li-ion hybrid supercapacitor, J. Phys. Chem. Solids, 2018, 112, 270–279.
45. N. Mayedwa, N. Mongwaketsi, S. Khamlich, K. Kaviyarasu, N. Matinise and M. Maaza, Green synthesis of zinc tin oxide (ZnSnO₃) nanoparticles using Aspalathus Linearis natural extracts: structural, morphological, optical and electrochemistry study, Appl. Surf. Sci., 2018, 446, 250–257.
46. D. Su, A. McDonagh, S. Z. Qiao and G. Wang, High-Capacity Aqueous Potassium-Ion Batteries for Large-Scale Energy Storage, Adv. Mater., 2017, 29(1), 1–8.
47. S. Fleischmann, J. B. Mitchell, R. Wang, C. Zhan, D. Jiang, V. Presser and V. Augustyn, Pseudocapacitance: From Fundamental Understanding to High Power Energy Storage Materials, Chem. Rev., 2020, 120, 6738–6782.
48. S. Ardizzone, G. Fregonara and S. Trasatti, ‘Inner’’ And ‘‘Outer’’ Active Surface of RuO2 Electrodes, Electrochim. Acta, 1990, 35, 263–267.
49. J. Wang, J. Polleux, J. Lim and B. Dunn, Pseudocapacitive Contributions to Electrochemical Energy Storage in TiO₂ (Anatase) Nanoparticles, J. Phys. Chem. C, 2007, 111, 14925–14931.
50. S. H. Kim, B. J. Cook, H. Lin and B. Dunn, et al., Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO_3−x, Nat. Mater., 2017, 16, 454–460.
51. D. Mohanadas and M. A. A. M. Abdah, Facile synthesis of PEDOT-rGO/HKUST-1 for high performancesymmetrical supercapacitor device, Sci. Rep., 2021, 11, 11747.
52. K. Krishnamoorthy, G. K. Veerasubramani, S. Radhakrishnan and S. J. Kim, One pot hydrothermal growth of hierarchical nanostructured Ni₃S₂ on Ni foam for supercapacitor application, Chem. Eng. J., 2014, 251, 116–122.
53. B. Pal, S. Yang, S. Ramesh, V. Thangadurai and R. Jose, Electrolyte selection for supercapacitive devices: a critical review, Nanoscale Adv., 2019, 1(10), 3807–3835.
54. K. Subramani, N. Sudhan, R. Divya and M. Sathish, All-solidstate asymmetric supercapacitors based on cobalt hexacyanoferrate derived CoS and activated carbon, RSC Adv., 2017, 7, 6648–6659.
55. S. Manoharan and P. Pazhamalai, et al., Proton conducting solid electrolyte-piezoelectric PVDF hybrids: novel bifunctional separator for self-charging supercapacitor power cell, Nano Energy, 2021, 83, 105753.
56. S. S. Singha, S. Rudra, S. Mondal, M. Pradhan, A. K. Nayak, B. Satpati, P. Pal, K. Das and A. Sinha, Mn incorporated MoS₂ nanoflowers: a high performance electrode material for symmetric supercapacitor, Electrochim. Acta, 2020, 338, 135815.
57. L. Quan, T. Liu, M. Yi, Q. Chen, D. Cai and H. Zhan, Construction of hierarchical nickel cobalt selenide complex hollow spheres for pseudocapacitors with enhanced performance, Electrochim. Acta, 2018, 281, 109–116.
58. R. B. Pujari and A. C. Lokhande, Synthesis of MnS microfibers for high performance flexible supercapacitors, Mater. Des., 2016, 108, 510–517.
59. J. Pu and F. Cui, et al., Preparation and Electrochemical Characterization of Hollow Hexagonal NiCo₂S₄ Nanoplates as Pseudocapacitor Materials, ACS Sustainable Chem. Eng., 2014, 2, 809–815.
60. R. K. Mishra and G. W. Baek, One-step solvothermal synthesis of carnation flower-like SnS₂ as superior electrodes for supercapacitor applications, Appl. Surf. Sci., 2017, 425, 923–931.
61. T. Xu and Z. Wang, et al., One-pot synthesis of a CoS–AC electrode in a redox electrolyte for high-performance supercapacitors, J. Appl. Electrochem., 2019, 49, 1069–1077.
62. J. Yao and Y. Liu, et al., Configuring Optimal FeS₂@Carbon Nanoreactor Anod: TowardmInsights into Pyrite Phase Change/Failure Mechanism in Rechargeablees Ni–Fe Cells, ACS Appl. Mater. Interfaces, 2019, 11, 42032–42041.
63. H. Peng and J. Zhou, et al., High-Performance Asymmetric Supercapacitor Designed with a Novel NiSe@MoSe₂ Nanosheet Array and Nitrogen-Doped Carbon Nanosheet, ACS Sustainable Chem. Eng., 2017, 5, 5951–5963.
64. Y. F. Liu, G. H. Yuan, Z. H. Jiang, Z. P. Yao and M. Yue, Preparation of Ni(OH)₂-Graphene Sheet-Carbon Nanotube Composite as Electrode Material for Supercapacitors, J. Alloys Compd., 2015, 618, 37–43.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2cp03847a

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