Ultrathin nickel sulfide nano-flames as an electrode for high performance supercapacitor; comparison of symmetric FSS-SCs and electrochemical SCs device

Abstract

Metal sulfides have received well deserved attention due to their excellent electrical conductivity and thermal stability, as compared to metal oxides, allowing them to achieve a high capacitance and energy density for portable energy storage devices. In this study, the preparation of highly porous nano-flames composed of nickel sulfide (NiS) thin film on a cost effective, flexible stainless steel substrate through a trouble free, inexpensive and simple chemical bath deposition (CBD) method is reported. The prepared nano-flames composed of a NiS thin film demonstrates the excellent electrochemical features with a maximum specific capacitance (C_s) of 750.6 F g⁻¹ at a scan rate of 5 mV s⁻¹ in a three electrode system. Furthermore, the portable symmetric flexible solid state supercapacitor (FSS-SC) and electrochemical supercapacitor (SC) are fabricated and tested. In comparison with the symmetric electrochemical SC, the symmetric FSS-SC shows an excellent electrochemical performance with a high C_s of 104 F g⁻¹ at 5 mV s⁻¹ with a good electrochemical stability of 85.3% over 3000 CV cycles. This study constitutes the first comparison of symmetric FSS-SCs and electrochemical SCs formed with NiS nano-flames. Such an impressive symmetric FSS-SC is predicted to be an exceptionally promising candidate for energy storage systems.

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1 Introduction

Electrical energy storage materials have been extensively investigated to counter the increasing demand of high power, high energy and inexpensive storage systems. The various electrical energy storing devices such as batteries and supercapacitors have been developed with different charge storage mechanisms.¹ Batteries have more contributions from an electroactive material showing a high specific energy density (SED) but suffer from a lower specific power density (SPD) and electrochemical cycling stability.² In contrast to batteries, supercapacitors store electric charges along with surface adsorption, i.e., non-faradaic (electrochemical double layer capacitors (EDLCs)), and a redox reaction on the electrode surface (pseudocapacitor), ultimately elevating the SPD as well as the electrochemical cycling stability, fulfilling the energy storage ability of the electrode material.³ EDLC shows a lower specific capacitance (C_s) as well as a lower SPD and SED compared to a pseudocapacitor. The pseudocapacitor offers a higher C_s than EDLCs because of their fast and reversible redox reactions. The hydride electrode, which is a combination of EDLCs and a pseudocapacitor, improves the SPD, cycling stability as well as SED of the electrode.⁴ Even after improving the SED, SPD and stability of the hybrid device, it has not reached up to the requirements.

Recently, the preparation of nano-structured morphologies with a controlled shape, size, internal structure and composition has received an enormous amount of attention to accomplish novel morphology dependent chemical properties for electrode materials.^5,6 The open-porous architecture composed of ultrathin nano-sheets, allowing for an electronically conductive framework, is highly favorable for increasing the electron transport along with maintaining a good ionic conductivity.⁷ Moreover, the porous morphology is beneficial for supercapacitor applications due to the maximum utilization of the electroactive material in addition to a fast charge transfer reaction between the electrode material and electrolyte ions.⁸ Furthermore, there is a need to synthesize porous morphological materials having pseudocapacitive behavior with better cycling stability.

Presently, metal sulfides (NiS, CoS, and CuS) are in the influence of research in the supercapacitor field due to their excellent intrinsic and electrochemical properties.^9–11 Nickel sulfides inaugurate an important type of metal sulfides having different phases such as NiS, NiS₂, Ni₃S₂, Ni₃S₄, Ni₇S₆, and Ni₉S₈. Moreover, the nickel sulfides are extensively applicable in dye-sensitized solar cells, supercapacitors and lithium ion batteries.^12–14 NiS is one of the most imperative phases of nickel sulfide as it offers a number of benefits such as a high theoretical capacity, good rate performance and better electrical conductivity. To prefer supercapacitor materials, nickel sulfide is a most promising material and has been investigated by many researchers. Yu et al.¹⁵ synthesized a NiS material by an ultrasound-assisted soaking method and the electrode exhibited a C_s of 107.9 F g⁻¹. A C_s of 800 F g⁻¹ in a 6 M KOH electrolyte is reported by Wang et al.¹⁶ for a NiS/GO electrode prepared using a hydrothermal method. The α-NiS nanoparticles grown on reduced graphene oxide (RGO) and single-wall carbon nanotubes (CNT) exhibited high-performance (857.7 F g⁻¹), which was synthesized by Yang et al.¹⁷ using a one-step solvothermal method. In addition to the above C_s values reported by investigators, further improvement of the long term electrochemical stability, the SPD and the SED of the supercapacitor is required. Moreover, there is still a need to investigate the performance of NiS electrodes fabricated using different inexpensive techniques.

Herein, based on the above deliberation, NiS thin films are prepared by a simple, low cost chemical bath deposition (CBD) technique. The films are characterized using different techniques. The electrochemical performance is tested in a three electrode system using a battery cycler. Furthermore, a symmetric FSS-SC device is fabricated using NiS electrodes and a polyvinyl alcohol–potassium hydroxide (PVA–KOH) polymer gel electrolyte as a separator as well as the electrolyte. Subsequently, the electrochemical SC device is fabricated using two electrodes of NiS in an aqueous 2 M KOH electrolyte. Finally, the performance is compared by measuring the C_s, SED, SPD and electrochemical stability. All these investigations denote that the NiS electrode is quite promising for supercapacitor applications.

2 Experimental section

2.1 Synthesis of NiS thin films

All the analytical grade (AR) grade chemicals were used without further purification. In a typical synthesis, 0.1 M NiSO₄ was dissolved in 30 mL of double distilled water (DDW) by continuous magnetic stirring. Furthermore, 1.5 mL of trimethylamine (TEA) was mixed with 0.1 M NiSO₄. The solution resulted into a green colored transparent homogeneous solution. Then, 0.15 M of Na₂S₂O₃·5H₂O was added into the above mentioned solution and stirred well for 10 min. The pH of the solution was maintained at 2.5 (±0.1) by adding 1 M HCl. The color of the solution turned greenish white. The well emery polished and cleaned stainless steel (SS) substrate (304 grade) was immersed in the above mentioned precursor solution. Thereafter, the reaction bath was maintained at a constant temperature of 80 °C. After 5 h, a black colored NiS thin film was developed on the SS substrate. The reaction mechanism of the film formation is as follows:¹⁸

NiSO₄ + TEA → [Ni[TEA]]⁺ + SO₄⁻ (1)

Na₂S₂O₃ + H₂O → H₂S + Na₂SO₄ (2)

[Ni[TEA]]⁺ + H₂S + HCl → NiS + TEA + HCl + 2H⁺ (3)

2.2 Characterization of the NiS thin film

The X-ray diffraction (XRD) technique (Bruker AXS D8 Advance Model (K_α of λ = 1.54 Å)) was used for structural property determination. Fourier transform infrared (FTIR) spectroscopy was used to collect high spectral resolution data over a wide spectral range of wavelengths. The X-ray photoelectron spectra (XPS) (VG Multilab 2000, Thermo VG Scientific, UK) were used for state confirmation. A field emission scanning electron microscope (FE-SEM) (Model JEOL JSM 6390) was used for surface morphology determination. The atomic percentage of nickel and sulfur present in the thin film material was calculated from EDAX using a JEOL JSM 6390 unit. A Rame Hart instrument with the drop image advanced software unit was used for the wettability test. A high-resolution transmission electron microscopy (TEM) study was carried out using high resolution JEOL-3010 microscope unit. The specific surface area and pore size distribution of the NiS powder was studied by Brunauer, Emmett and Teller (BET) as well as Barrett–Joyner–Halenda (BJH) analysis using a Quantachrome Instruments v11.02 model.

2.3 Electrochemical measurements

Electrochemical performance of the NiS electrode was performed using a conventional three electrode system in which the NiS thin film (1.0 cm² area) on a SS substrate, a platinum wire and a SCE were used as working, counter and reference electrodes, respectively. Furthermore, the aqueous solution of 2 M KOH was used as an electrolyte. The symmetric FSS-SC and electrochemical SC devices are fabricated by a PVA-KOH gel and aqueous 2 M KOH electrolytes, respectively. The cyclic voltammetry (CV), galvanostatic charge discharge (GCD) measurements as well as the electrochemical stability of the devices were measured using an automatic battery cycler unit (WBCS3000). Subsequently, the electrochemical impedance spectroscopy (EIS) analysis was carried out through an electrochemical workstation (ZIVE SP5).

3 Results and discussion

To deposit a NiS thin film on a SS substrate, NiSO₄ is used as the nickel source, Na₂S₂O₃ as the sulfur source and TEA as the complexing agent. After addition of TEA in a NiSO₄ solution, the [Ni(TEA)]⁺ complex is formed (eqn (1)). The Na₂S₂O₃ in DDW gives H₂S (eqn (2)), which reacts with [Ni(TEA)]⁺ complex species and finally NiS developed on the SS substrate by replacing TEA with a sulfur molecule (eqn (3)). The schematic of the formation of NiS ultrathin nano-flames with nucleation, aggregation, coalescence and growth is shown in Scheme 1. The formation of the NiS material and phase confirmation was studied using X-ray diffraction analysis. Fig. 1(A) shows the XRD pattern of the NiS thin film deposited on the SS substrate. The diffraction peaks located at 30.16°, 31.58°, 35.16°, 40.41°, 48.65°, 52.55°, 57.16°, 59.59°, 67.30°, 72.64° and 81.50° are assigned as (101), (300), (021), (211), (131), (401), (330), (012), (600), (312) and (161), respectively, crystalline planes of the rhombohedral crystal structure with lattice constants of a = b = 9.5890 Å and c = 3.1650 Å, which are well in accordance with the JCPDS card no. 00-002-0693. The asterisk (*) represents the SS substrate peaks. The lower intensity peak, represented by Δ, corresponds to the formation of a small amount of the Ni₃S₂ impurity phase of nickel sulfide.

Scheme 1 The schematic of the NiS thin film by a simple chemical bath deposition method using NiSO₄ and Na₂S₂O₃ as the nickel and sulfur sources, respectively. The ultrathin nano-flame like morphology obtained in the order of nucleation, aggregation, coalescence and growth of NiS nanoparticles on a SS substrate. The surface morphology changed after 2500 CV cycles with a degradation of the NiS electroactive materials.

Fig. 1 (A) The XRD pattern of the NiS thin film on a SS substrate, and (B) the FT-IR spectrum of the NiS sample in the frequency range of 360 to 4000 cm⁻¹, (C) XPS survey spectrum, (D) Ni 2p spectrum, (E) S 2p spectrum of the NiS material, and (F) nitrogen adsorption–desorption isotherms of the NiS material (inset shows corresponding BJH pore size distribution plot).

Fig. 1(B) depicts the FT-IR spectrum of the synthesized NiS thin film within the scanning range of 360–4000 cm⁻¹. The peaks observed at 2921 and 2852 cm⁻¹ correspond to the C–H stretching modes. The broad peak at 3450 cm⁻¹ corresponds to the O–H stretching vibrations, while the peak at 1627 cm⁻¹ is related to O–H rocking vibrations. The peak at 1385 cm⁻¹ denotes reactivity of the C–N and N–H bonds in organic ligands. The bending vibrations of the sulfide group and absorption of H₂O link to peaks at 1108 and 1627 cm⁻¹, respectively.¹⁹ The two bands observed at 610 and 3450 cm⁻¹ are due to the δ_OH as well as O–H vibrations of the hydrogen-bonded hydroxyl group and intercalated water molecules, respectively.

X-ray photoelectron spectroscopy (XPS) is preferred to determine the chemical composition of nickel sulfide and oxidation state of Ni in the deposited NiS thin film. Fig. 1(C) shows the survey spectrum, which displays no other elemental peaks different from the peaks of Ni, S, C, and O, confirming that the sample is pure. The Ni 2p spectrum is fitted to two spin–orbit doublet peaks as Ni 2p_3/2 and Ni 2p_1/2 at binding energy of 853.26 and 857.40 eV, respectively²⁰ (see Fig. 1(D)). The corresponding satellite peaks at 861.5 and 880.0 eV indicate the presence of nickel-oxygen species on the surface of the NiS thin film.²¹ The peak at 853.26 eV, near Ni 2p_3/2, corresponds to the characteristic peak of NiS materials.²² Moreover, the XPS spectrum of sulfur in the state of S 2p is displayed in Fig. 1(E). The peak observed at a binding energy of 169.5 eV corresponds to the presence of sulfur in thin film material.²³ The fitting peaks at 168.8 and 170.77 eV correspond to S 2p_3/2. This results are well matches to the previous report.²⁴ The observed peaks at particular binding energies confirm the presence of Ni and S in thin film material with a stable phase.

The BET surface area and pore size distribution of NiS thin film surface are directly correlated with specific capacitance.²⁵ The BET and pore size dispersal analysis accompanied by N₂ adsorption–desorption isotherms are reported in Fig. 1(F). It displays a hysteresis loop with designated adsorption-desorption characteristics of porous NiS nano-flames. A considerably acceptable BET surface area of 54.3 m² g⁻¹ is observed. More importantly, the higher surface area contributed with formation of porous nano-flames on the surface. The BJH pore size distribution of the NiS powder material is deliberated using the desorption curve (see inset of Fig. 1(F)). The NiS sample comprises a higher macro/mesoporosity, as displays by the comparatively high intensity peaks at around 5.4 to 21.4 nm, which can offer low-resistant pathways for the ions through the porous nano-flames structure and a small diffusion route due to the well-ordered macro/mesoporous channels. More prominently, pores at the positions 44.5 and 67 nm reduce ion diffusion resistance, which contributes to fast charge transfer and storage capacity.

Fig. 2(A and B) shows FE-SEM images of the deposited NiS thin film surface at magnifications of 10 000× and 25 000×. The surface morphology exhibits an interconnected, porous 3D framework structure composed of ultrathin nano-flames, which are electronically conductive and highly beneficial for extending the electron transport, while also maintaining a better ionic conductivity. The more number of electroactive sides offered by the NiS nano-flames could increase the effective contact area between the active material and OH⁻ ions in the KOH electrolyte.²⁶ The ultrathin nano-flames provide fast electrolyte penetration into the NiS material and react with the electroactive surface. The nano-flames of NiS give easy paths for the transfer of electrolyte ions and ionic diffusion resistance reduction, with fast charge transfer by a decrease in the charge transfer resistance (R_ct) in the NiS material. An EDAX spectrum of the NiS sample is shown in Fig. 2(C). The peaks of nickel and sulfur elements are clearly consistent with the elemental composition of NiS. The atomic percentage is found to be 56.30% for nickel and 43.70% for sulfur, denoting good stoichiometry. The results from EDAX analysis are in well agreement with elemental mapping results. Fig. 2(D and E) shows the TEM images of the NiS thin film at two different ranges. The different levels of brightness in the TEM images indicate the porous surface of NiS nano-flames. This confirms that the porous and interconnected 3D framework structure, composed of ultrathin nano-flames, is grown randomly over the surface. The nano-flame-like structure is observed in the TEM, which is analogous to a surface morphology analysis. The contact angle on the surface of the NiS thin film is found to be 55.6° (see Fig. 2(F)). The wettability test is carried out to investigate the interaction between aqueous 2 M KOH and the NiS thin film. This hydrophilic property of NiS is useful for making proper contact between the electroactive material and the aqueous 2 M KOH electrolyte. This essential condition of the material provides a more specific response towards electrochemical capacitor applications. The percent wise elemental mapping of the NiS thin film is shown in Fig. 2(G). The distributions of nickel and sulfur present in thin film are illustrated in Fig. 2(H and I), respectively. The uniform distribution of Ni and S in the NiS nano-flames is clearly visualized in the elemental maps.

Fig. 2 (A, B) The FE-SEM images of the NiS thin film surface at magnifications of 10 000× and 25 000×, respectively, (C) the EDAX spectrum showing elemental atomic and weight percentage of nickel and sulfur (D, E) TEM images of the NiS thin film at two different magnifications (inset shows higher magnification TEM image), (F) the contact angle of the NiS thin film surface, (G) elemental analysis of the NiS thin film, and (H, I) the elemental mapping of nickel and sulfur.

4 Electrochemical supercapacitive properties

The electrochemical performance of the NiS electrode was tested according to the 3 electrode system in a 2 M KOH electrolyte. Fig. 3(A) shows the CV curves of NiS electrode observed at scan rates of 5, 20, 50 and 100 mV s⁻¹ within the potential of +0.1 to +0.45 V. A pair of redox peaks appeared at +0.27 and +0.37 V per SCE and are attributed to the faradaic reaction at surface of the NiS thin film.²⁷ The shape of the CV curves indicate typical faradaic charge transfer behavior dissimilar from the ideal rectangular CV shapes of conventional EDLCs. The electron transfer process in electrochemically reversible reactions mainly include a variation of Ni²⁺/Ni³⁺ redox couple interposed by the OH⁻ ions from the 2 M KOH electrolyte.²⁸ More significantly, the current densities of the oxidation peak increase with an increase in the scan rate from 5–100 mV s⁻¹, confirming that the NiS electrode has an abundantly electroactive surface area with a high reversibility along with charge–discharge cycles. At a lower scan rate (5 mV s⁻¹), OH⁻ ions can easily diffuse into the NiS electrode. Nevertheless, at a higher scan rate, OH⁻ ions will only interact with the outer surface of the NiS electrode and the effective interaction between the electrode–electrolyte is reduced giving little contribution from the interior electrode material. The following intercalation/deintercalation reaction at the electrode-electrolyte interface occurs,

NiS + OH⁻ ↔ NiSOH + e⁻ (4)

Fig. 3 (A) The CV curves of the NiS electrode at scan rates of 5, 20, 50 and 100 mV s⁻¹ in an aqueous 2 M KOH electrolyte, (B) the graph of the specific capacitance (C_s) versus scan rate of the NiS electrode, (C) the GCD curves at different current densities of 1, 3, 5 and 10 mA cm⁻², and (D) the graph of C_s versus current density of the NiS electrode in the 2 M KOH electrolyte, (E) Nyquist plot of the NiS electrode before CV cycling and after CV cycling (inset shows the magnified Nyquist plot), and (F) the Bode plot of phase angle (Φ), log|Z| vs. log frequency (f) of the NiS electrode (inset figure illustrates difference of capacitance (C′_r and C′′_im) with log frequency (f) of the NiS electrode.

The OH⁻ ions from the KOH electrolyte participate in the reaction.

Fig. 3(B) shows the variation of C_s with scan rate. At a lower scan rate (5 mV s⁻¹), more time is available for the intercalation/deintercalation reaction. Therefore, the C_s value is higher at a lower scan rate than at a higher scan rate. The maximum utilization of an electroactive material in an electrochemical reaction gives more charge storage. Fig. 3(C) depicts the charge–discharge curves at different current densities from 0.5 to 3 mA cm⁻² in the 2 M KOH electrolyte. Initially, the charging curve shows a vertical line, which is commonly observed in the GCD measurement of NiS.²⁹ There are voltage plateaus in the discharge curves of the NiS electrode, which is strong indication of a typical faradaic reaction at the electrode–electrolyte interface. Fig. 3(D) illustrates the C_s values at various discharge current densities calculated by the following equation.²⁷

where I is discharging current (A) at time dt (s), m is the mass (g) of the NiS electrode material for a 1 cm² area and (V₂ − V₁) (V per SCE) is the operating potential window. The C_s of the NiS electrode is calculated as 576, 524, 495 and 371 F g⁻¹ with a high columbic efficiency at current densities of 0.5, 1, 2 and 3 mA cm⁻², respectively. The C_s value slowly decreases with an increase in current density because of the increase in the IR drop and insufficient active material at higher current densities compared to lower current densities. Moreover, the concentration of OH⁻ ions at the electrode interface is reduced at a higher current density due to the slower diffusion of OH⁻ ions, whereas there is a requirement for a large number of OH⁻ ions. The obtained C_s value of the NiS electrode is much better than that of previous reports (118, 717 and 710 F g⁻¹).^15,30,31 The specific energy density (SED) and specific power density (SPD) are evaluated at various current densities using the following equations,andwhere C_s is the specific capacitance (F g⁻¹), dt is the discharging time (s) and V₂ − V₁ is the potential window of the NiS electrode.

Fig. 3(E) shows the Nyquist plot of the NiS electrode before CV cycling and after cycling in the 2 M KOH electrolyte. The figure inset shows the Nyquist plot magnified. The first non-zero intercept of the Nyquist plot with the real axis gives the solution resistance (R_s), whereas the diameter of the semicircle in the lower frequency region gives R_ct. The semicircle in the higher frequency region indicates the capacitive behavior of the NiS electrode. After the semicircle, a straight line at an angle of 45° originated due to Warburg's constant. From these analyses, an R_s of 0.57 and 1.26 Ω cm⁻² are observed for before and after CV cycling of the NiS electrode. R_ct values of 0.67 and 1.41 Ω cm⁻² are calculated for before and after CV cycling, respectively. A low value for R_ct tends to improve the rate capability as R_ct acts as a restrictive factor for the rapid charge and discharge of a supercapacitor.³² The resistance values for after 2500 CV cycling are more as compared to before CV cycling owing to degradation of the electroactive material after cycling. The ultrathin nano-flames of the NiS electrode display a low Warburg impedance before CV cycling, which assists the diffusion of OH⁻ ions into the NiS electrode. The Bode plot of the NiS electrode is shown in Fig. 3(F). The phase angle at a higher frequency range is lower because of the ionic resistance of the electrolyte, designating a lower capacitance.³³ The phase angle increases toward −90° at the lower frequency range marinating the capacitive behavior of the NiS electrode. The phase angle for the NiS electrode is found to be −80.69°. The figure inset displays the link between the real (C′_r) and imaginary (C′′_Im) parts of the capacitance, which changes with respect to frequency. This result supports the Nyquist plot analysis and CV study. At a higher frequency range, only R_s represents the total impedance of the system. Subsequently, the NiS electrode shows a plateau line in the higher frequency region.

Fig. 4(A) shows the Ragone plot of the NiS electrode. The NiS electrode in 2 M KOH electrolyte shows a maximum SED and SPD of 28 W h kg⁻¹ and 4.98 kW kg⁻¹, respectively. The previously reported SPD and SED values are also included in Ragone plot for comparison. Long term electrochemical stability is a critical requirement for the commercial use of supercapacitors. The electrochemical stability of the NiS electrode up to 2500 CV cycles is reported in Fig. 4(B). The figure inset shows the CV curves at a constant scan rate of 100 mV s⁻¹ for the 2nd and 2500^th CV cycles. A capacity retention after 2500 CV cycles of 92% is observed. The degradation of the electroactive material during the cycling process reduces the performance of the electrode. Fig. 4(C and D) shows FE-SEM images of the NiS film surface after 2500 CV cycles. The degradation of the electroactive material is cleanly observed. The ultrathin nano-flames on the surface vanish with a changing morphology. Obviously, the porosity and surface area are both reduced after CV cycling. The contribution of electroactive material decreases with CV cycling, resulting in a reduction in the performance of the electrode. The instability of the NiS electrode may be due to some irreversible oxidation reaction, phase change, and/or the detachment of nano-flames from the SS substrate. The comparative results from previous reports are included in Table 1, including material morphology, electrolyte and electrochemical stability. In addition, the XRD pattern and SEM image of the stainless steel substrate is included in Fig. 4(E and F). The three major peaks in the XRD pattern are observed at 2θ positions of 44.3°, 50.14° and 74.2°. The SEM image shows no particular morphology, but it has a plane structure. The 304 grade SS substrates that was used has a higher corrosion resistance compared with regular steel, while it has the ability to change into various shapes.

Fig. 4 (A) The Ragone plot of the NiS electrode, (B) the graph depicts capacity retention after 2500 CV cycles for the NiS electrode (inset shows CV curves at a constant 100 mV s⁻¹ scan rate for the 2nd and 2500^th cycles), (C, D) the FE-SEM images of the NiS thin film after 2500 CV cycles at two different magnification of 10, 000× and 25 000×, respectively, (E and F) the XRD pattern and SEM image of the SS substrate.

Table 1 The comparative chart of electrochemical supercapacitor performance for the prepared NiS electrodes with other reports

Material	Preparation method	Surface morphology	Electrolyte	Specific capacitance (F g^−1)	Capacity retention (%)	Reference
NiS	Template assisted	Hollow structure	3 M KOH	668	72 (for 1000 cycles)	42
NiS/GO	Hydrothermal	Spherical	1 M H2SO4	109.37	Not reported	43
β-NiS	Solvothermal	Hierarchical flower-like	2 M KOH	512.96	Not reported	17
NiS	Chemical bath deposition	Ultrathin nano-flames	2 M KOH	750.6	92 (for 2500 cycles)	This work
β-NiS	Anionic exchange	Nano needles	2 M KOH	415	90 (for 2000 cycles)	8
NiS	Hydrothermal	Nano-rods	2 M KOH	583.2	72 (for 1000 cycles)	44
α-NiS	One-step template free	Hollow spheres	3 M KOH	717.2	85.2 (for 1000 cycles)	45
NiS	Ultrasound-assisted	Nano-sheet	2 M KOH	107.9	89 (for 1000 cycles)	15
NiS	Hydrothermal	Not reported	6 M KOH	143	Reported up to 200 cycles	46

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5 Fabrication of symmetric FSS-SC and electrochemical SC devices

Our strategy is to achieve higher SED and cycling stability from a supercapacitor cell. For that purpose, the symmetric FSS-SC device is fabricated using a PVA-KOH gel polymer electrolyte. The PVA-KOH gel electrolyte can prevent the chemical dissolution of NiS by decreasing the water content, which avoids structure crushing of the NiS through maintaining the shortest interaction between the electroactive NiS and the SS substrate during cycling. Moreover, this gel electrolyte with a limited amount of water can successfully suppress the electrochemical oxidation reaction of NiS nano-flames with a rising mechanical stability. Based on such effective properties of the PVA-KOH gel electrolyte, the inexpensive, lightweight, flexible and portable symmetric FSS-SC device is fabricated using the NiS thin film on a flexible SS substrate. To compare the advantages of the symmetric FSS-SC, an electrochemical SC device is also fabricated with an aqueous 2 M KOH electrolyte. In the symmetric FSS-SC device, PVA-KOH acts as a separator as well as an electrolyte. Fig. 5(a–f) shows the various steps involved in the fabrication of the symmetric FSS-SC device. These images show (a) the NiS thin film on a flexible SS substrate, (b) the flexibility of the SS substrate, (c) one side of the NiS electrode, (d) painting of the PVA-KOH gel electrolyte on the electrode, (e) the whole FSS-SCs device, and (f) the flexibility of the device at different bending angles. Fig. 5(g–l) depicts the steps involved in the fabrication of the electrochemical SC device. The images show (g) a small plastic bottle, (h) seal cutting, (i) sandwiched and wounded NiS electrodes, (j) whole device assembly in the plastic bottle tube, (k) filling the bottle with the aqueous 2 M KOH electrolyte, and (l) fabrication of the electrochemical SC device.

Fig. 5 The image displays (a) the NiS thin film on a flexible SS substrate, (b) flexibility of the NiS electrode, (c) one electrode with perfect cutting, (d) the painting of the PVA-KOH gel polymer electrolyte on the NiS electrode, (e) fabricated symmetric FSS-SC device, and (f) flexibility of the FSS-SC device for different bending angles, (g) plastic bottle tube, (h) cutting of the seal of the bottle tube to get the electrode contacts out, (i) assembly of the device electrode, (j) whole electrode assembly in the bottle tube, (k) filling of the 2 M KOH electrolyte, and (l) the fabricated electrochemical SC device.

A comparative electrochemical performance study of the symmetric FSS-SC and the electrochemical SC is carried out. Fig. 6(a) depicts the CV curves of the FSS-SC device at scan rates of 5-100 mV s⁻¹ within the potential window of 0 to +1.0 V. Above a +1.0 V potential, a dramatically drop in current during cycling was observed. This may be due to the incremental IR drop and insufficient active material involvement in the reaction. The CV curve shows increase in current as the scan rate goes from 5 to 100 mV s⁻¹, which designates capacitive and fast charge–discharge behavior of the symmetric FSS-SCs device. Fig. 6(b) shows CV curves of the electrochemical SC device at various scan rates from 5 to 100 mV s⁻¹ within potential of 0 to +0.8 V. Fig. 6(c) shows the comparison of the CV curves of the symmetric FSS-SC and electrochemical SCs devices. It is clearly observed that the area under the CV curve for the symmetric FSS-SC is high as compared to the electrochemical SC. More importantly, the potential window for the symmetric FSS-SC is greater than that of the electrochemical SC. The C_s versus scan rate graph of these two different devices shows a larger difference in the performance (see Fig. 6(d)). The figure inset shows the histogram of the device performance. An excellent specific capacitance is clearly observed. A maximum C_s of 104.2 F g⁻¹ is achieved at a scan rate of 5 mV s⁻¹ for the symmetric FSS-SC device. The major role of a higher C_s is more time availability during the charge–discharge process recommended at a lower scan rate (5 mV s⁻¹) than a higher scan rate (100 mV s⁻¹). A lower C_s of 15.7 F g⁻¹ is observed for the electrochemical SC device. This may be due to large anions and the low dissociation energy of the PVA-KOH gel polymer electrolyte, providing more free ions than the aqueous 2 M KOH electrolyte. The achieved C_s values confirmed the hypothesis that the PVA-KOH gel electrolyte can dominantly suppress the irreversible oxidation reaction as well as structural pulverization of the NiS nano-flames, without losing the electrochemical performance. The major factor for improvement of the supercapacitor performance of the symmetric FSS-SCs device is the ionic conductivity of the polymer gel electrolyte. An ionic conductivity for the PVA-KOH polymer gel electrolyte on the order of 10⁻² S cm⁻¹ is reported by Yang et al.^34,35

Fig. 6 (a and b) The CV curves of the symmetric FSS-SC device and electrochemical SC device within a scan rates of 5–100 mV s⁻¹, respectively, (c) the comparative CV curves of the symmetric FSS-SC and electrochemical SC at a constant scan rate of 100 mV s⁻¹, and (d) the C_s versus scan rate plot of the symmetric FSS-SC and electrochemical SC devices at a fix scan rate of 5 mV s⁻¹ (inset shows the histogram of the comparison of the C_s values), (e and f) the GCD curves of the symmetric FSS-SC and electrochemical SC devices at current densities of 1.2, 2, 3, and 5 mA, respectively, (g) the comparative GCD curves of symmetric FSS-SCs and electrochemical SCs at constant current of 1.2 mA, (h) the C_s versus current density graph of the symmetric FSS-SC and electrochemical SC devices, and (i) the Ragone plot showing a comparison between the SED and SPD values of the symmetric FSS-SC and electrochemical SC devices.

Fig. 6(e and f) shows the GCD curves of the symmetric FSS-SC device at various current densities varying from 1.2 to 5 mA with a potential window ranging from 0 to +1.0 V for the symmetric FSS-SC, and 0 to +0.8 V for the electrochemical SC device. From GCD analysis, it is observed that at the start of the charging process, almost vertical curves are generally observed in the GCD curve of nickel sulfide.¹³ Furthermore, the GCD curve shows a plateau line in the charge process revealing the redox process of the nickel sulfide. The triangle shape of charge–discharge curves confirm the excellent capacitive behavior of the FSS-SC device. Fig. 6(g) shows GCD curves of the above mentioned two devices at a constant current of 1.2 mA. The higher potential window and higher discharge time of the symmetric FSS-SC device provides a higher C_s as compared to the electrochemical SC. Fig. 6(h) depicts the graph of C_s versus current density of the symmetric FSS-SC device and the electrochemical SC device. The performance difference between these two devices shows that the FSS-SC has the ability to store more electrical energy than the electrochemical SC device. The symmetric FSS-SC device achieved a C_s of 98 F g⁻¹ at a current density of 1.2 mA. This is due to a lower ion diffusion matrix, whereas the PVA-KOH gel electrolyte can prevent the chemical dissolution of NiS by reducing the water content of electrolyte. Moreover, the PVA-KOH electrolyte acts as an elastic coating which is useful for avoiding structure crushing, favorable for the mechanical strength of nickel sulfide. Fig. 6(i) illustrates the Ragone plots of these two devices, which displays a comparison between the performances of the high and low SPD of the devices. The positions of the SED and SPD values of these two devices confirm the supercapacitive nature of the NiS electrode. The Nyquist plot for the symmetric FSS-SC and electrochemical SC device in the frequency range of 100 mHz to 1 MHz at an AC amplitude of 10 mV is shown in Fig. 7(A and C), while figure insets show equivalent circuit and magnified Nyquist plots. At the higher frequency region, the small semicircle indicates that the charge is transferred at the NiS electrode and the PVA-KOH polymer gel electrolyte interface due to redox reactions.^36–39 Equivalent series resistance (ESR) (R₁) for the symmetric FSS-SC and electrochemical SC of 0.23 and 0.76 Ω, respectively, are observed due to the combination of the ionic resistance of the PVA-KOH electrolyte and the aqueous 2 M KOH electrolyte. The semicircle illustrates the combination of R_ct and pseudocapacitance (C₁), which appears due to the surface roughness and conductivity difference between the electrode and electrolytes. Total R_ct values of 0.22 and 0.15 Ω are observed. More importantly, a line at nearly 45° in the mid frequency range relative to the diffusion ions in the NiS electrode is denoted as the Warburg constant (W). Fig. 7(B and D) illustrates the Bode plots of the symmetric FSS-SC and electrochemical SC devices in which the phase angle rises up to −78.15 and −37.56, respectively, demonstrating the capacitive behavior, supporting the Nyquist plot results. The higher difference in phase angle indicates the capability of the symmetric FSS-SC device over the electrochemical SC device. Fig. 8(A and B) depicts the imaginary (C′′_im) and real (C′_r) parts of capacitance. This higher frequency region manifests a plateau line indicative of the capacitive behavior of the symmetric FSS-SC and electrochemical SC devices. To prove that a gel electrolyte can increase the mechanical stability as well as reduce the chemical dissolution of NiS, further cycling performance tests for the symmetric FSS-SC and aqueous electrochemical SC devices were performed for 3000 CV cycles. Fig. 8(C and D) shows long term electrochemical stability of the symmetric FSS-SC and electrochemical SC devices for 3000 CV cycles. The area under the CV curve is slightly reduced after 3000 CV cycles, denoting degradation of the electroactive material during the charging-discharging process. The capacity retention after 3000 CV cycles for the symmetric FSS-SC and electrochemical SC devices is about 85.3% and 61.5%, respectively. The comparative electrochemical stability plots of the symmetric FSS-SC and electrochemical SC are represented in Fig. 8(E). The result shows that the supercapacitive performance of the symmetric FSS-SC device is better than the electrochemical SC device. Furthermore, the impact of bending on the flexible SC device is studied at different bending angles. Fig. 8(F) shows CV curves for the symmetric FSS-SC device for different bending angles. The result displays no larger change in the area under the CV curves, indicating that there is less impact of bending of the device on the supercapacitive performance. This proves the flexibility of the symmetric FSS-SC device without reducing the performance. Fig. 9 shows the C_s values at different bending angles of the device. The inset image shows actual images of the device at different bending angles. There is a slight change in the supercapacitive performance of the device at different bending angles. The actual demonstration of the fabricated symmetric FSS-SC device is shown in Fig. 10(a and b). The device is charged for 20 s, and during discharging the red light emitting diode (LED) glows up to 30 s. This confirms the scope of the NiS electrode in future supercapacitor electrode materials. From all the above mentioned results, the final conclusion is that the symmetric FSS-SC device is better than the electrochemical SC device. Furthermore, the drawbacks, such as leakage of the aqueous electrolyte, inflexibility, difficulty in maintaining cost, inconvenience in fabrication and maximum space requirements, of the electrochemical SC motivate toward the fabrication of the FSS-SC device.

Fig. 7 (A and C) Nyquist plot of the symmetric FSS-SC device and the electrochemical SC device (inset figures shows the magnified Nyquist plot and equivalent circuit from which the Nyquist plot is plotted), and (B and D) Bode plot of the symmetric FSS-SC device and the electrochemical SC device.

Fig. 8 (A and B) Disparity of capacitance C′_r and C′′_im with log frequency (f) of the symmetric FSS-SC device and the electrochemical SC device, (C and D) electrochemical stability of the symmetric FSS-SC and electrochemical SC devices up to 3000 CV cycles, (E) capacity retention with CV cycles, and (F) the CV curves at different bending angles at a constant scan rate of 100 mV s⁻¹ for the symmetric FSS-SC device.

Fig. 9 The plot of C_s values at different bending angles with a constant scan rate of 100 mV s⁻¹ for the symmetric FSS-SC device (inset shows images of the FSS-SC device for different bending angles).

Fig. 10 Digital images of the actual demonstration of the symmetric FSS-SC device with the glow of a red LED at (a) 0 s and (b) 30 s.

6 Conclusions

In summary, the nano-flame morphological NiS thin film is successfully deposited on a SS substrate by the CBD method, which exhibits a higher specific capacitance of 750.6 F g⁻¹ at scan rate of 5 mV s⁻¹ with a 92% electrochemical stability up to 2500 CV cycles. Furthermore, the symmetric FSS-SC and electrochemical SC devices are fabricated using PVA-KOH gel and aqueous 2 M KOH electrolytes, respectively. This study gives an effective approach to stabilizing the NiS nano-flame morphology without sacrificing its electrochemical performance in a PVA-KOH gel electrolyte. In addition, the PVA-KOH electrolyte suppresses oxidation reactions and restricts structural pulverization of the NiS electrode. The symmetric FSS-SC exhibits a maximum specific capacitance of 104.2 F g⁻¹ with an acceptable energy density of 14.6 W h kg⁻¹, which is much higher than that obtained for the electrochemical SC. The present study shows the first demonstration of using a NiS high-energy storage electrode, which could hypothetically expand the performance of energy storage devices.

Acknowledgements

Present study was sustained by the Human Resources Development program (No. 20124010203180) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea government Ministry. The basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (NRF-2015R1A2A2A01006856).

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