Tuning the electrochemical properties of NiS2 2D-nanoflakes by one-zone sulfurization for supercapacitor applications

Nickel-based sulfides (particularly NiS2) are regarded as promising materials for highly efficient electrochemical generation and storage devices. The conventional fabrication methods of nanostructured NiSX electrodes involve several complex steps using multiple precursors and techniques. In this paper, the NiSX electrodes are prepared by a plain one-step process of one-zone sulfurization of Ni foam. The evolution of highly electroactive 2D-nanoflakes reliant on sulfurization temperature is studied. Scanning electron microscopy, x-ray diffractometry, and energy-dispersive x-ray spectroscopy confirmed the presence of NiSX (x = 1 and 2) in the prepared structures. A strong dependence of sample morphology and 2D-nanoflakes density on sulfurization temperature was demonstrated. The electrochemical properties of samples were characterized by cyclic voltammetry and electrochemical impedance spectroscopy measurements. Owing to the 2D-nanoflake structure, the NiS2 showed attractive electrochemical performance, including a high specific capacitance of 648 mF cm−2 and a capacitance retention rate of 90,7% after 3000 cycles. Our study shows that the composition and crystal growth of NiSX can be tuned by reaction temperature during the sulfurization and high perspective of sulfurization in the synthesis of highly electroactive large-scale electrodes for supercapacitors.


Introduction
The current transition towards decarbonized economy increases the importance of clean energy conversion and storage systems including electrochemical water splitting technologies, batteries, and supercapacitors. To increase the performance of such systems, strong activities are focusing on the research and synthesis of new materials with the ability to provide higher storage capabilities or catalytic activities. Thanks to their chemical and structural properties, transition metal sulfides (TMS) have been quite recently under intensive investigation for applications in different modern energy and environmental technologies. Among TMS, nickel sulfides, namely NiS and NiS 2 , have been reported as promising candidates for Li-ion batteries, supercapacitors, and water splitting electrodes [1][2][3][4]. The excellent electrical conductivity, high theoretical capacitance, large active sites, non-tocity and abundant availability in nature, as well as achieved fast charge and discharge rates and impressive cycle life of NiS and NiS 2 based structures makes these materials appealing for preparation of new energy storage and conversion devices with unique properties and high performance [5][6][7]. Thanks to the Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. pseudocapacitive character of charge storage mechanisms for such materials, a high energy density is expected for supercapacitor devices prepared from Ni-based TMS [8].
Owning to the unique properties of Ni-based TMS materials a lot of studies were published about their fabrication and utilization for energy storage and conversion applications.
In the work of Lu et al [9], sulphur-doped Co 3 O 4 (S-Co 3 O 4 ) electrode material was developed by doping sulphur into Co 3 O 4 . The S-Co 3 O 4 nanowires were synthesized on carbon cloth by a hydrothermal method. Asprepared electrodes achieved an areal capacitance of 0.55 F cm −2 at 10 mV s −1 in a 5 M LiCl solution, with a long-term cycling stability with 92% capacitance retention after 10 000 cycles.
Tong et al demonstrated the cost-effective MoO X /NiS 2 hybrid structures as the electrode material for supercapacitors via hydrothermal method. The hybrid electrodes delivered a specific capacitance of 1050 F g −1 at 1 A g −1 with high stability [2].
In another work, Iqbal et al have synthesized nickel sulfide by employing hydrothermal and sonochemical proceses. Final active material was deposited on nickel foam. The as synthesized electrode obtained specific capacity of 434.16 C g −1 at 1.6 A g −1 [10].
In their work Wu et al [11] assembled NiSNF/CF@NiSNP-3 by 3-step preparation using carbonization of polyacrylonitrill and NiS which were suseqently impregnated through electrospinning technology with Ni precursor and sulfurized. The NiSNF/CF@NiSNP-3 electrode delivers a reversible specific capacitance of 1691.1 F g −1 at 1 A g −1 and a coulombic efficiency of 98.5%.
Ahmed et al developed NiS/MoS 2 anchored multiwall carbon nanotubes (MWCNT/NiS/MoS 2 ) as the hybrid composite electrode with specific capacitance of 371 F g −1 at 2 A g −1 , along with the capacitance retention of 82% after 2000 cycles. This material was also used as a catalyst for hydrogen evolution reaction (HER). The composite offers the lowest overpotential of 193 mV to achieve a current density of 10 mA cm −2 for HER [12].
Another promising application for Ni-based TMS is hydrogen generation. The use of noble metals as electrocatalysts prevents large-scale application in the generation of green hydrogen because of cost issues. The use of Earth-abundant elements based on TMS is considered a promising noble metal-free material for watersplitting applications [13,14].
For instance, Li et al synthesized the S, N co-doped graphite carbon-coated Ni/NiS/NiS 2 nanocomposites by the arc discharge combined with the sulfuration process. The structures possessed efficient electrocatalytic activities and intense stability in HER, UOR, and urea electrolysis. Prepared structures only needed the overpotential of 60 mV to achieve the current density of −10 mA cm −2 towards HER. [15].
Zhou et al prepared MoS 2 /NiS/Ni heterostructure on 3D nanoporous Ni via one-pot de-alloying combined hydro-thermal synthesis. The as-prepared samples were presented with low overpotential of 108 mV for HER [16].
The performance of all mentioned electrochemical devices depends mostly on the material properties of their structures. Structures for energy conversion and storage require a large surface area and appropriate stoichiometry to achieve a high performance. The preparation and material properties engineering of structures on the nanoscale level are crucial for achieving a better performance of energy-related devices [17][18][19].
The conventional fabrication methods of electrodes generally involve several complex steps using multiple expensive, or environmentally hazardous precursors (such as thiourea) and techniques [20][21][22][23][24][25]. Fabrication processes often involve mixing active materials with different additions as conductive materials and binders, which often causes a formation of electrochemically less active interfaces. These low-active interfaces are responsible for reduction of a charge transfer efficiency [10].
In this paper, we demonstrate a one-step straightforward, low-cost, nontoxic procedure with the ability to synthesize NiS 2 2D-nanocrystals with controlled crystallography, that can yield nanoflakes and other more complicated 2D-shapes through changes in the reaction temperature.
Here, we report the fabrication of nickel sulfides using a one-zone sulfurization of nickel foam for possible utilization in supercapacitor devices. Sulfurization is an effective method to produce transition metal sulfides [26][27][28] and large-area can be obtained with this method [29]. By tuning the annealing temperature of the sulfurization, we were able to control the morphology and stoichiometry of nickel sulfides and thus tune the performance of prepared electrodes. The influence of different sulfurization temperatures on structure and charge storage mechanism was investigated by electrochemical characterization.

Electrode fabrication
The nickel sulfides (NiS 2 , NiS) were synthesized by a one-zone sulfurization process from the porous nickel foam substrate [30]. The nickel foam (1 cm × 2 cm) with porosity 95%, 80-110 ppi was immersed into 3% HCl for 30 min and then cleaned with deionized water. Since nickel oxidize easily, this procedure was realized to remove the nickel oxide from the top of the Ni foam. The clean nickel foam was sulfurized in a custom-designed chemical vapor deposition (CVD) chamber [30]. Horizontal tube furnace (Clasic 4011T) with quartz tube (1 m long) was used for the annealing. The nickel foam was placed in the center of the alumina crucible (10 cm long) on the sample holder made of quartz. The sulfur powder (1.5 g) was put on both ends of the crucible. The schematic representation of the sulfurization is shown in the Sojkova et al [31]. The crucible with a sulfur powder and the nickel foam was placed in the center of the furnace. The sulfurization temperature was in the range of 200°C-400°C. The sulfurization process was carried out in a nitrogen atmosphere, at ambient pressure. The furnace was heated up with a heating rate of 25°C per minute. The sample was hold on the annealing temperature for 5 min and then naturally cooled down.

Structural and material characterization
Samples were examined by x-ray diffractometry (XRD) to confirm the presence of sulfides. XRD analysis was provided by a Bruker D8 Advance diffractometer equipped with a Co anode operating at 12 kW for highintensity radiation (λ = 1.5406 Å). The Bragg-Brentano geometry was used for all measurements. The DIFFRAC.EVA and TOPAS software were used for the analysis of diffraction patterns.

Electrochemical characterization
All electrochemical measurements were conducted in a typical three-electrode system (prepared electrodes acted as a working electrode, platinum electrode as the counter electrode, and saturated Ag/AgCl in 3 M KCl as a reference electrode) in 1 M KOH aqueous electrolyte measured by a Gamry potentiostat Interface 1010E. The specific capacitance C S (F cm −2 ) was calculated from cyclic voltammetry (CV) scans by the following equation: is the potential window, v (V s −1 ) stands for the scan rate, and S (cm 2 ) is the surface area of the active electrodes. CV scans were done in the voltage window of 0.0 and 0.5 V versus Ag/AgCl at different scan rates. Electrochemical impedance spectroscopy (EIS) data were measured in a frequency range of 0.1-100 000 Hz (10 points per decade) with 30 μA amplitude. Analysis of measured data was carried out using Gamry Echem Analyst software and Kramers-Kronig tool was used for fitting purposes.

Structural and material characterization
The as-prepared samples of nickel sulfides were characterized by SEM, EDX, and XRD measurements.
The SEM was conducted to examine the effect of sulfurization parameters on the morphology of nickel sulfides. Figure 1 shows the SEM images of pure Ni foam and the NiS X samples synthesized at temperatures of 200°C, 250°C, 300°C, 375°C, and 400°C. The structured surface with a high density of 2D nanoflakes is clearly observed on the samples prepared at the temperatures of 250°C, 300°C and 375°C, with the highest density at 300°C. The presence of these nanoflakes is not observed for 200°C and 400°C sulfurization temperature. In different published works [23,[32][33][34][35], the NiS 2 -based 2D structures are reported to have high energy storage capabilities. Hence, these nanoflakes are strongly desirable to be present at supercapacitor structures. The improved capacitive properties of electrodes with nano-sized objects (like nanoneedles or nanoflakes) have been published in comparison to other NiS films, where the improved properties are due to the structure allowing easier de/intercalation of electrolyte ions into electrode material by reducing the diffusion resistance of electroactive material [36,37].
EDX study was performed to further identify the chemical composition of the samples. The EDX results are available as supplementary information in table 1. Sulfur and nickel were present in all prepared samples at different concentration. At lower sulfurization temperature, almost no sulfur was present in the samples indicating that a higher sulfurization temperature is necessary to obtain nickel sulphide phase. With increasing temperature for samples prepared at 5 min sulfurization time, the atomic ratio of Ni:S gradually decreased from 200°C to 400°C, with the most optimal stoichiometric ratio for NiS 2 at 300°C. The results obtained from EDX together with SEM revealed that the growth of high-density 2D-NiS 2 nanoflakes is strongly supported for sulfurization temperatures between 300°C and 375°C.
XRD analysis was performed to identify the nickel sulphide phases present in the samples. Figure 2(a) shows XRD patterns of the NiS X samples prepared at sulfurization temperatures of 200°C and 250°C. At 200°C, only metallic phase of nickel can be seen at 44.6°. With increased sulfurization temperature to 250°C, small peak of NiS 2 is observed at 31.1°. This increase corresponds to a small increase of 2D-NiS 2 nanoflakes observed at SEM images. Figure 2(b) shows XRD patterns of the NiS X samples prepared at higher sulfurization temperatures between 300°C-400°C. A significantly higher NiS X peaks are observed than at the temperatures below 250°C. Peaks at 32.1°and 38.6°, 44.9°are observed for electrodes prepared between 300°C-400°C, corresponding for S and Ni 3 S 2 phase, respectively. The characteristic peaks at 31.1°and 30.0°, 34.5°, 35.2°, 45.7°, 48.8°correspond to the NiS 2 phase and NiS phase, respectively. Although NiS, Ni 3 S 2 and NiS 2 phases are present on samples prepared at 300°C and 375°C, the highest intensity of NiS 2 is observed for sample prepared at 300°C for 5-minute sulfurization process. In the same way, the increased concentration of 2D-nanostructured NiS 2 observed through SEM correlates with XRD results, where intensity peaks for NiS 2 are higher on sample prepared at 300°C compared to 375°C. Through these observations it can be stated that these 2D-nanostructures correspond to-NiS 2 type. This is further confirmed through sample prepared at 400°C, where neither NiS 2 peak on XRD or 2D nanostructure at SEM is observed.  Figure 3(a) shows a comparison of the CV curves of NiS X samples prepared at different sulfurization temperatures and measured at a scan rate of 5 mV s −1 . Cathodic and anodic peaks indicate the pseudocapacitive character of the charge storage mechanism and can be associated with the following redox reactions [34,38,39]:

Electrochemical characterization
In next step, the electrochemical impedance spectroscopy (EIS) was performed on all NiS X samples in a frequency range from 0.1 Hz to 100 kHz and analysed in Nyquist representation. EIS was used to determine the equivalent series resistance (ESR) of NiS X samples. Figure 3(b) shows the Nyquist plots of as-prepared samples at different sulfurization properties. Inset of the figure shows the equivalent electrical circuit used for analysis of the measured data. In the equivalent circuit, R S stands for the series resistance of samples, CPE represents doublelayer capacitance, R CT is the charge transfer resistance, Z W is the Warburg impedance and C L is the Faradaic pseudocapacitive contribution. It can be observed that EIS curves follow a depressed semicircle at high frequencies. The depressed semicircle can be correlated with a non-ideal capacitive behaviour [40]. A constant phase element (CPE) instead of a capacitance was selected for this reason in the equivalent electrical circuit. The charge transfer resistance, R CT is associated with the diameter of this semicircle. The greater the semicircle diameter is, the greater the energy that is lost during the charge transfer (charging/discharging). Hence, the smaller semicircles for samples prepared at 250, 300, and 375°C sulfurization temperature exhibited in the Nyquist plot suggest lower charge transfer resistance, manifesting the superior properties of 2D-NiS 2 nanoflakes against NiS structure. The specific capacitances and serial resistances of all NiS X -based samples evaluated from the CV curves measured at 5 mV s −1 scan rate and EIS curves, respectively, are shown in figure 3. Apparent trend can be seen amongst the samples. The samples prepared at 200°C and 400°C show lower specific capacitances of 328 mF cm −2 and 105 mF cm −2 , respectively. These lower capacitances consent with SEM, EDX, and XRD results, where no NiS 2 2D-nanostructures were observed. Moreover, on the samples with higher specific capacitances of 510 mF cm −2 (250°C) and 543 mF cm −2 (375°C), respectively, 2D-nanoflakes are observed.  The maximum specific capacitance of 648 mF cm −2 was obtained in the sample prepared at 300°C. Based on the SEM (figure 1), XRD (figure 2), and EDX analyses, the highest value of specific capacity is provided by the presence of the highest density of 2D-NiS 2 nanoflakes on the sample surface. It has been proven that the capacitive properties of 2D-structures based on NiS 2 are significantly higher compared to NiS [23,[32][33][34][35]. Therefore, it can be assumed that the dominant contribution to the capacitive behaviour of the NiS X samples originates from the presence of 2D NiS 2 nanoflakes.
To support our conclusion, it is necessary to corelate the results with serial resistance of samples. The figure 4 revealed a low series resistance of all samples in the range of 1.7-3.8 ohm. The values of the resistance, R S do not follow the trend of the capacitance obtained at varied sulfurization temperatures. The measurements at low scan rates and thus the capacity calculated from these measurements are not significantly influenced by series resistance. Therefore, we can associate the evaluated specific capacitance with the structural and morphological properties of the structures.
The sample prepared at 300°C exhibiting the highest specific capacitance was further analysed by means of CV characterization at different scan rate ( figure 3(a)). Results revealed the negative shift of reduction peak positions upon the increase of the scan rate. This process can be associated with a slower diffusion rate of OHions at higher scan rates [41,42], which indicates limited ion intercalation into the lattice of as-prepared NiS X samples. As a result of this, an increase of specific capacitance with the decreasing scan rate is observed in the figure 5(b). A low discharging rates are required to fully exploit a high capacitance of the prepared NiS X based supercapacitor electrodes.
The cyclic stability as a critical parameter to determine the energy storage performance of prepared electrodes was tested through a CV. Figure 6(a) shows CV curves at 100 mV s −1 of electrode prepared at 300°C. The capacity retention of prepared electrodes is shown in figure 6(b). The electrode prepared at 300°C obtained a specific capacitance retention ratio of 90,7% after 3000 CV cycles, illustrating the excellent electrochemical stability of the NiS 2 2D-nanostructures. The electrode prepared at 300°C is capable of obtaining 103% and 100% of the initial specific capacitance after 500 and 1000 cycles, respectively. The capacitance increase in the first 500 cycles can be interpreted by the fact that the electrochemical activity of the active materials is activated gradually at the beginning. Therefore, these results demonstrate the perspective electrochemical performances and excellent cyclic stability of the prepared NiS X electrodes.

Conclusion
In this paper we presented the successful straightforward one-step preparation and characterization of nanostructured Ni-based sulfides using a one-zone sulfurization process of nickel foam for binder free supercapacitor applications. It was shown that the composition of nickel sulfides and crystal growth can be well controlled by tuning the temperature of the sulfurization process. Nanosized 2D-NiS 2 structures were prepared with optimal sulfurization temperature at 300°C. Presence of 2D-NiS 2 nanoflakes at sulfurization temperatures of 250, 300, and 375°C was confirmed with SEM, EDX, and XRD. Pseudocapacitive charge storage mechanism of as-prepared electrodes was confirmed. The presence of NiS 2 structures led to increased electrochemical properties of fabricated samples. The highest specific capacitance of 648 mF cm −2 at 5 mV s −1 and cyclic retention of 90,7% after 3000 cycles was achieved on a sample prepared by sulfurization temperature of 300°C for 5 min The high capacitance of this sample correlates with a high density of 2D NiS 2 nanoflakes. These achieved results manifest that the sulfurization temperature can exert a great influence on the electrochemical properties of nickel sulfides.