Sulfur Nanoparticle-Decorated Nickel Cobalt Sulfide Hetero-Nanostructures with Enhanced Energy Storage for High-Performance Supercapacitors

Transition-metal sulfides exaggerate higher theoretical capacities and were considered a type of prospective nanomaterials for energy storage; their inherent weaker conductivities and lower electrochemical active sites limited the commercial applications of the electrodes. The sheet-like nickel cobalt sulfide nanoparticles with richer sulfur vacancies were fabricated by a two-step hydrothermal technique. The sheet-like nanoparticles self-combination by ultrathin nanoparticles brought active electrodes entirely contacted with the electrolytes, benefiting ion diffusion and charges/discharges. Nevertheless, defect engineers of sulfur vacancy at the atomic level raise the intrinsic conductivities and improve the active sites for energy storage functions. As a result, the gained sulfur-deficient NiCo2S4 nanosheets consist of good specific capacitances of 971 F g−1 at 2 A g−1 and an excellent cycle span, retaining 88.7% of the initial capacitance over 3500 cyclings. Moreover, the values of capacitance results exhibited that the fulfilling characteristic of the sample was a combination of the hydrothermal procedure and the surface capacitances behavior. This novel investigation proposes a new perspective to importantly improve the electrochemical performances of the electrode by the absolute engineering of defects and morphologies in the supercapacitor field.


Introduction
Worldwide, research is rising investigating safe, portable, renewable electrochemical electrical applications, and lightweight electrical gadgets, such as supercapacitors (SCs), which consider eco-friendly stabilities as well as higher energy/power densities [1][2][3]. In energy storage applications, supercapacitors (SCs) attained widespread consciousness on account of their extraordinary benefits in terms of rapid charging/discharging procedures, high reliability, excellent power densities, cheaper price, and environmental natures [4][5][6][7]. Meanwhile, the weaker energy densities of SCs have constricted them from laboratory scales to practical-span productivity [8][9][10]. Thus, higher performance sample-electrodes are worth finding for enhancing the energy densities of SCs.
Among them, transition-metal sulfides were advised as an essential source for the SCs application, owing to their higher capacity results, cheaper prices, innate abundances, and eco-friendliness [11]. Comparable to binary metal sulfides, bimetallic sulfide types (e.g., CuCo 2 S 4 [12], NiCo 2 S 4 [13], FeCo 2 S 4 [14], MnCo 2 S 4 [15]) normally manifest good added electrochemical behaviors for the given two causes: (i) the synergetic reaction of bimetallic sulfide types would consider a richer redox reaction; and (ii) bimetallic sulfide types normally exhibit higher conductivities [16][17][18]. Nevertheless, the insufficient energy storage active sites and weaker ion diffusion kinetics were reasoned by the limit of ion diffusion connections, which led to poorer rate capabilities and specific capacities [19]. Recent research has been maintained to enhance the electrochemical performances of the samples' electrodes by morphology optimization and defect formations [20][21][22]. For instance, Wan et al. constructed NiCo 2 S 4 porous nanotubes using sacrifices precursors to reason redox reactions at intrinsic activate sites, allow for faster ion transportations in the aqueous electrolyte, and represent a higher specific capacity of 933 F g −1 at 1 A g −1 [23]; Chen et al. fabricated caterpillar-type NiCoS crystals, exhibiting the higher specific capacitance of 1777 F g −1 at 1 A g −1 and retaining a 90.9% capacity of the starting cycles over 3000 cycles at 10 A g −1 [24]; Zhou et al. synthesized oxygen-deficient (V o -ZnO/CoO) nanowires [25], in which the oxygen defect improves the redox processes, allowing for faster electron transportation rates; and Huang et al. fabricated NiS@C with a sulfur vacancy (H-NiS 1−X /C−50), where the sulfur vacancy formations provide lattice distortion reactions, exhibiting improved conductivities and good electrochemical activities (1728 F g −1 , 1 A g −1 ) [26]. Evidentially, the combination of sulfur-deficient NiCo 2 S 4 with a conductive matrix-type nickel foam can enhance the cycling performances and rate capabilities of materials. Meanwhile, it has been noted that bimetallic nickel cobalt sulfide consists of numerous redox sites and abundant valence states [27]; therefore, it might be a favoring electrode applicant for SCs, in which its less efficient ion/electron transportations hinder the electrochemical activity. To the best of our understanding, the performances of a sulfur-deficient NiCo 2 S 4 were not adequate, and it was the main endeavor to further boost the electrochemical activities of SCs. Compared to the recently reported composited bimetallic sulfides, it could be anticipated that the sulfur-deficient NiCo 2 S 4 composite might provide stronger cooperative effects and assist volume alterations, promising to enhance the electrochemical performances for SCs applications.
Based on the above findings, we integrate deficient engineering with rational nanomorphology construction to grow sheet-like nickel cobalt sulfide nanoparticles, self-assembling by ultrathin nanoparticles with richer sulfur vacancies through hydrothermal treatment. The fabricated electrode consists of higher conductivities and numerous abundant activation sites, providing good specific capacitances of 971 F g −1 at 2 A g −1 and the 88.7% retained rate of the prior capacitances over 3500 long cycles. Moreover, the sulfur-deficient NiCo 2 S 4 nanosheets electrode exhibits lower resistances, revealing the extraordinary current conductivity of the sample material. In the investigation, the interconnected nanoparticle-type sulfur-deficient NiCo 2 S 4 nanosheets composite was illustrated to be perfect for SCs applications.

Preparation of Sheet-like NiCo 2 S 4 Nanoparticles with Richer Sulfur Vacancies on Ni Foam
In the preparation of sulfur-deficient sheet-like NiCo 2 S 4 nanoparticles, 0.5 mmol Ni(NO 3 ) 2 ·6H 2 O, 0.9 mmol Co(NO 3 ) 2 ·6H 2 O, and 1.17 mmol Na 2 S·9H 2 O were dissolved in 40 mL of DI water under continuous stirring to gain a pink product. Next, the Ni foam with product precursors was kept in a Teflon-lined stainless autoclave and optimized at 110 • C for 13 h. After reaching room temperature, the samples were rinsed with DI water and subjected to the drying process to gain the needed products. After being dried, the black precipitate product was annealed at 400 • C for 1 h in a furnace in a continual Ar atmosphere. The representative mass load of the sulfur-deficient NiCo 2 S 4 composite was ∼2.6 mg cm −2 .

Characterizations and Instruments
The surface morphology and structures of the samples were observed using a scanning electron microscope (SEM, JSM-7800F equipping) and transmission electron microscopy (TEM, JEM-2100F, Busan, Korea). The crystal structures of the samples were determined using characteristic diffraction peaks measured by X-ray diffraction (XRD) analysis, which was performed using Bruker D8 Advance. X-ray photoelectron spectroscopy (ESCCALAB 250Xi, Busan, Korea) was organized to investigate the chemical bonding details of the electrodes.

Electrochemical Performance Test
The evaluation of all the electrochemical calculations for the electrodes was organized in 3: an electrode setup employing the current precursor products as the working electrodes, Platinum (Pt) sheet as the counter, and Ag/AgCl as the references electrode. Moreover, the current working electrode was assessed utilizing a 3-electrode cell with 2 M KOH electrolytes. Galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopies (EIS) are evaluated by an electrochemical workstation. The specific capacitances (C sp , F g −1 ) of the samples were calculated by Equation (1) based on CD plots [28][29][30]: where ∆t (s), I (A), m (g), and ∆V (V) designate the discharge times, currents, the mass of the electrode, and the applied potential, respectively.

Structure Characterization
The fabrication of the sulfur-deficient sheet-like NiCo 2 S 4 nanoparticles on nickel foam is reported in Figure 1. The Ni foam consists of a porous nature composed of interconnected nickel skeletons, which possess the benefits of excellent electrical conductivities and richer porosity, consequently supplying numerous active sites for the redox process. Thus, the growth of sheet-like NiCo 2 S 4 particles on the nickel skeleton was a reasonable possibility. First, the NiCo 2 S 4 particles were acquired by a hydrothermal process accompanied by annealing at 400 • C for 1 h in the Ar atmosphere. The gained thin nanosheets were collected from nanosheets with a porous nature.  Figure 2 displays the microscopic morphology of the as-synthesized pure NiCo2S4 nanoparticles electrode and sulfur-deficient NiCo2S4 nanosheet composite samples. As illustrated in Figure 2a,b, the pure NiCo2S4 nanoparticles sample holds slightly incomplete developed and shapeless particle-type morphologies. Dissimilar to the pure Ni-Co2S4 nanoparticles, the sulfur-deficient NiCo2S4 nanosheets composite was made up of ultrathin-type nanosheets, self-assembling sheet-like particles with a median diameter of  Figure 2 displays the microscopic morphology of the as-synthesized pure NiCo 2 S 4 nanoparticles electrode and sulfur-deficient NiCo 2 S 4 nanosheet composite samples. As illustrated in Figure 2a,b, the pure NiCo 2 S 4 nanoparticles sample holds slightly incomplete developed and shapeless particle-type morphologies. Dissimilar to the pure NiCo 2 S 4 nanoparticles, the sulfur-deficient NiCo 2 S 4 nanosheets composite was made up of ultrathintype nanosheets, self-assembling sheet-like particles with a median diameter of around 7 µm (Figure 2c,d). This hierarchical nano-micro unique construction would supply abundant redox sites with higher active sites, condensing the diffusion networks of electrons/ions throughout the electrochemical reactions. In addition, the hydrothermal treatment affected the microstructure and crystallinity of the sulfur-deficient NiCo 2 S 4 nanosheets composite, with the composite leading to the agglomeration of metallic nanoparticles and the electrochemical reactions of the electrode.   Figure 3a further clarified that the metallic particles were flat and immobilized on the nickel foam skeleton porous structure. Figure 3b shows that, in the high-resolution TEM images, sulfur-deficient NiCo2S4 nanosheet composites possess lattice fringes, such that spaces of 0.24 and 0.28 nm were acquired; which are in excellent accordance with the (400) and (511) planes of NiCo2S4, respectively [31,32]. On the other hand, the EDS elemental maps in Figure 3c-e depicted the uniform distributions of Ni, C, and S, revealing that the sulfur-deficient NiCo2S4 nanosheet composites are victoriously synthesized by a two-pot hydrothermal route.   Figure 3a further clarified that the metallic particles were flat and immobilized on the nickel foam skeleton porous structure. Figure 3b shows that, in the high-resolution TEM images, sulfur-deficient NiCo 2 S 4 nanosheet composites possess lattice fringes, such that spaces of 0.24 and 0.28 nm were acquired; which are in excellent accordance with the (400) and (511) planes of NiCo 2 S 4 , respectively [31,32]. On the other hand, the EDS elemental maps in Figure 3c-e depicted the uniform distributions of Ni, C, and S, revealing that the sulfur-deficient NiCo 2 S 4 nanosheet composites are victoriously synthesized by a two-pot hydrothermal route.
The diffraction phase and purity of the as-reported pure NiCo 2 S 4 nanoparticles electrode and sulfur-deficient NiCo 2 S 4 nanosheet composites are investigated using X-ray diffraction analysis (Figure 4). The characteristic peaks at 26.7 • , 31.5 • , 38.4 • , 47.5 • , and 50.6 • are attributed with the (220), (311), (400), (422), and (511) planes of the cubic NiCo 2 S 4 phases, respectively (PDF#20-0782) [33,34]. It is obvious that all the good diffraction peaks of the NiCo 2 S 4 would be ascribed to NiCo 2 S 4 without clear diffractions correlated to the im-purities, which evidences the successful preparation of sulfur-deficient NiCo 2 S 4 nanosheet composites. Moreover, no additional peaks from the remaining crystallized planes could be gained from the electrode, noticing the excellent form of a pure NiCo 2 S 4 sample. The diffraction phase and purity of the as-reported pure NiCo2S4 nanoparticles electrode and sulfur-deficient NiCo2S4 nanosheet composites are investigated using X-ray diffraction analysis (Figure 4). The characteristic peaks at 26.7°, 31.5°, 38.4°, 47.5°, and 50.6° are attributed with the (220), (311), (400), (422), and (511) planes of the cubic NiCo2S4 phases, respectively (PDF#20-0782) [33,34]. It is obvious that all the good diffraction peaks of the NiCo2S4 would be ascribed to NiCo2S4 without clear diffractions correlated to the impurities, which evidences the successful preparation of sulfur-deficient NiCo2S4 nanosheet composites. Moreover, no additional peaks from the remaining crystallized planes could be gained from the electrode, noticing the excellent form of a pure NiCo2S4 sample. The chemical composition and elemental bonding states of the as-reported electrodes are fixed by using XPS spectra (Figure 4b-e). Figure 4b indicates a survey spectra of the sulfur-deficient NiCo 2 S 4 nanosheets composites., which reports a dominance of Ni, C, and S [35]. The typical spectrum of Ni 2p (Figure 5b) and Co 2p (Figure 5c) corroborate the binary spin-orbits, respectively. In Figure 5b, in which there are the Ni 2p spectra, the 875.7 and 856.5 eV peaks are located from the Ni 2p 1/2 and Ni 2p 3/2 , respectively [36,37]. Accordingly, the Co 2p spectra in Figure 5c illustrate that the 802.4 and 783.3 peaks are located therein and associated with Co 2p [38][39][40]. Figure 5d displays the XPS spectrum of S 2p, revealing the existence of sulfide ions in the binary Ni-Co-S. The foremost peaks of S 2p were allocated at 168.9 eV. These noticed data were related to the earlier reporting [41]. The XPS findings recognize that the atomic ratios of Ni, Co, and S elements in the NiCo 2 S 4 nanosheets were 1:2.93:4.94, which correlates with the findings output of the NiCo 2 S 4 samples. The XRD and XPS results were fitting, considering the chemical elemental states and the formation compositions of the pure NiCo 2 S 4 phases. Furthermore, the XPS spectra displayed the mixed chemical elemental states necessity of Ni 2p/Ni 3p and Co 2p/Co 3p, which play an important role in developing the entire electrochemical activities of the NiCo 2 S 4 sample. The chemical composition and elemental bonding states of the as-reported electrodes are fixed by using XPS spectra (Figure 4b-e). Figure 4b indicates a survey spectra of the sulfur-deficient NiCo2S4 nanosheets composites., which reports a dominance of Ni, C, and S [35]. The typical spectrum of Ni 2p (Figure 5b) and Co 2p (Figure 5c) corroborate the binary spin-orbits, respectively. In Figure 5b, in which there are the Ni 2p spectra, the 875.7 and 856.5 eV peaks are located from the Ni 2p1/2 and Ni 2p3/2, respectively [36,37]. Accordingly, the Co 2p spectra in Figure 5c illustrate that the 802.4 and 783.3 peaks are located therein and associated with Co 2p [38][39][40]. Figure 5d displays the XPS spectrum of S 2p, revealing the existence of sulfide ions in the binary Ni-Co-S. The foremost peaks of S 2p were allocated at 168.9 eV. These noticed data were related to the earlier reporting [41]. The XPS findings recognize that the atomic ratios of Ni, Co, and S elements in the NiCo2S4 nanosheets were 1:2.93:4.94, which correlates with the findings output of the NiCo2S4 samples. The XRD and XPS results were fitting, considering the chemical ele-

Electrochemical Performance
The CV and GCD plots were first organized to find out the electrochemical activities of the pure NiCo2S4 nanoparticles electrode and the sulfur-deficient NiCo2S4 nanosheet composites in the three-electrode setup. As illustrated in Figure 5a, a symmetric Faraday redox pairs plot was noticed from the CV plot, which is because of their highly reversible

Electrochemical Performance
The CV and GCD plots were first organized to find out the electrochemical activities of the pure NiCo 2 S 4 nanoparticles electrode and the sulfur-deficient NiCo 2 S 4 nanosheet composites in the three-electrode setup. As illustrated in Figure 5a, a symmetric Faraday redox pairs plot was noticed from the CV plot, which is because of their highly reversible redox process amid Ni 3+ /Ni 2+ and Co 3+ / Co 2+ beneath alkaline solutions, resulting in the typical nature of PCs. The Faradaic possible reaction of the sulfur-deficient NiCo 2 S 4 nanosheets composite was suggested as given [42,43]: The CV plot region of the sulfur-deficient NiCo 2 S 4 nanosheet composites was much larger than that of the pure NiCo 2 S 4 nanoparticles electrode, revealing its high charging storing capability. Figure 5b illustrates the GCD plots of both electrodes, and clear charge/discharge platforms were denoted in all the particulars, further proposing its PCs properties. As expected, the sulfur-deficient NiCo 2 S 4 nanosheet composites showed longer discharging times, which disclosed their higher capacitances. Figure 6a discloses the CV plots of sulfur-deficient NiCo 2 S 4 nanosheet composites at various sweep rates. Acquiescing with the polarizing effects of the sample electrodes, improving the sweep rates (5 to 100 mV s −1 ) led to the cathode's peak shifting to fewer potentials, whereas the anode, peaking according to the reverse trends, moved to a higher potential. Figure 6b displays the GCD plot of the sulfur-deficient NiCo 2 S 4 nanosheets composite electrode at different current values (2 to 20 A g −1 ). The plot shapes are nearly similar and symmetrical, indicating its good electrochemical reversibility. Figure S1 demonstrates the CV and GCD plots of the pure NiCo 2 S 4 nanoparticles electrode at various scan rates (5 to 100 mV s −1 ) and current densities (2 to 20 A g −1 ) in the 2M KOH aqueous electrolyte; this further exhibited its superior rate capabilities. According to the GCD results, the measured specific capacitance of both electrodes was reported in Figure 6c. At 1 A/g, the specific capacitances of the pure NiCo 2 S 4 nanoparticles electrode and sulfur-deficient NiCo 2 S 4 nanosheet composites were 564 and 971 F g −1 , respectively. These data values confirm that the hydrothermal route significantly overblows the specific capacitances of the composite. On the contrary, the higher sulfur-deficient NiCo 2 S 4 nanosheets composite leads to the agglomeration of the metallic particle; thus, hindering the electrical migrations and holding back their electrochemical process. When the sulfur-deficient NiCo 2 S 4 nanosheets composite is at 2 A g −1 , a specific surface region is large enough to reveal the required electrochemical active speeds to enhance the networks of the electrode sample and electrolytes.
In Figure 6d, the pure NiCo 2 S 4 nanoparticles electrode and sulfur-deficient NiCo 2 S 4 nanosheet composites reported smaller semi-circled diameters in the higher-frequencies area, and have roughly vertical lines in the lower frequencies region; this hinted that the sulfur-deficient NiCo 2 S 4 nanosheet composites have optimal conductivities as well as quick charging transportation capabilities. The EIS curve is composed of the equivalent series resistance (Rs), the charge transfer resistance (Rct), and Warburg diffusion resistance (R w ), which is related to the resistance of ionic diffusion in the electrolyte. The fitting results show that the Rct values of the sulfur-deficient NiCo 2 S 4 nanosheets composite and pure NiCo 2 S 4 are 0.145 and 0.194 Ω, respectively. Therefore, it can be determined that the charge transfer resistance of the sulfur-deficient NiCo 2 S 4 nanosheets composite is the smallest. Furthermore, the equivalent series resistance values of the sulfur-deficient NiCo 2 S 4 nanosheets composite and pure NiCo 2 S 4 are 1.732 and 0.831 Ω, respectively. The slope of the sulfur-deficient NiCo 2 S 4 nanosheets composite is the largest, indicating that it has a high diffusivity of the electrolyte. In both cases, the Rs and Rct values of the sulfur-deficient NiCo 2 S 4 nanosheets composite has a strong dynamic response and electrical conductivity. Figure 7 illustrates the long cycles of the sulfur-deficient NiCo 2 S 4 nanosheet composites at 3 A g −1 . After 3500 long cycles of charges/discharges, the capacity retention of the sulfur-deficient NiCo 2 S 4 nanosheet composites is still retained at 88.7%, further illustrating the outstanding life span of cycling stability analysis. From Table 1, the specified capacitance results data of the electrodes are demonstrated for comparison. Figure S2 illustrates the EIS curves of the sulfur-deficient NiCo 2 S 4 before and after 3500 cycles of the stability test at 3 Ag −1 .
These data values confirm that the hydrothermal route significantly overblows the specific capacitances of the composite. On the contrary, the higher sulfur-deficient NiCo2S4 nanosheets composite leads to the agglomeration of the metallic particle; thus, hindering the electrical migrations and holding back their electrochemical process. When the sulfur-deficient NiCo2S4 nanosheets composite is at 2 A g −1 , a specific surface region is large enough to reveal the required electrochemical active speeds to enhance the networks of the electrode sample and electrolytes. In Figure 6d, the pure NiCo2S4 nanoparticles electrode and sulfur-deficient NiCo2S4 nanosheet composites reported smaller semi-circled diameters in the higher-frequencies area, and have roughly vertical lines in the lower frequencies region; this hinted that the sulfur-deficient NiCo2S4 nanosheet composites have optimal conductivities as well as quick charging transportation capabilities. The EIS curve is composed of the equivalent series resistance (Rs), the charge transfer resistance (Rct), and Warburg diffusion resistance (Rw), which is related to the resistance of ionic diffusion in the electrolyte. The fitting results show that the Rct values of the sulfur-deficient NiCo2S4 nanosheets composite and pure NiCo2S4 are 0.145 and 0.194 Ω, respectively. Therefore, it can be determined that the charge transfer resistance of the sulfur-deficient NiCo2S4 nanosheets composite is the smallest. Furthermore, the equivalent series resistance values of the sulfur-deficient NiCo2S4 nanosheets composite and pure NiCo2S4 are 1.732 and 0.831 Ω, respectively. The slope of the sulfur-deficient NiCo2S4 nanosheets composite is the larg- est, indicating that it has a high diffusivity of the electrolyte. In both cases, the Rs and Rct values of the sulfur-deficient NiCo2S4 nanosheets composite has a strong dynamic response and electrical conductivity. Figure 7 illustrates the long cycles of the sulfur-deficient NiCo2S4 nanosheet composites at 3 A g −1 . After 3500 long cycles of charges/discharges, the capacity retention of the sulfur-deficient NiCo2S4 nanosheet composites is still retained at 88.7%, further illustrating the outstanding life span of cycling stability analysis. From Table 1, the specified capacitance results data of the electrodes are demonstrated for comparison. Figure S2 illustrates the EIS curves of the sulfur-deficient NiCo2S4 before and after 3500 cycles of the stability test at 3 Ag −1 .

Conclusions
We demonstrated a higher-efficiency approach to enhance the higher-performance electrode materials for supercapacitors by constructing rational nanomorphologies and

Conclusions
We demonstrated a higher-efficiency approach to enhance the higher-performance electrode materials for supercapacitors by constructing rational nanomorphologies and increasing sulfur vacancies in NiCo 2 S 4 . The physical morphology data results exhibited that the hydrothermal route corresponds to the composite material crystallinity. By the data values, the as-developed sulfur-deficient NiCo 2 S 4 nanosheet composites display a superior specific capacitance of 971 F g −1 at 2 A g −1 and capacitance retentions of 88.7% at 3 A g −1 over 3500 long cycles. The excellent electrochemical capabilities would have corresponded to the involvement of sulfur vacancies and well-constructive superior specific area particlelike morphology that improves the electrode's conductivities, enhances activated sites for the electrochemical reactions, and encourages the electrolyte penetrations into the sample material. This study would open unique and favoring paths for the succession of future generations of high-performance materials for SCs. The developed electrode exhibited attractive potential in SCs applications.

Supplementary Materials:
The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/molecules27217458/s1, Figure S1: CV and GCD plots of pure NiCo 2 S 4 nanoparticles electrode at various scan rates and current densities in aqueous electrolyte; Figure S2: EIS curves of the sulfur deficient NiCo 2 S 4 before and after 3500 cycles of stability test at 3 Ag −1 .

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.