To understand the solvent effect on the morphology of SS-NiS@3DNF electrode nanostructures, we investigated this effect by changing the solvent during synthesis at fixed ethylene glycol amount, reaction temperature, and reaction time. When the synthesis proceeds in the presence of methanol, a few-layered sheet-like morphology of the NiS grown on a conductive 3DNF (Supplementary Fig. S1 online) substrate (SS-NiS@3DNF-M, Fig. 1a) is observed. At high magnification, the SEM image displayed in Fig. 1b shows that the SS-NiS@3DNF-M (area marked by the yellow dotted line) is composed of few-layered nanosheets with a diameter of 2–3 µm and has macro- and microsized pores inside the structure. In contrast, when the solvent is changed from methanol to ethanol during the solvothermal experiment, the SEM image of the SS-NiS@3DNF-E electrode shows an irregular spherical morphology (Fig. 1c) with a size of 10 µm to 15 µm (Fig. 1d). However, at higher magnification, the SEM image (Fig. 1d) of SS-NiS@3DNF-E shows that each sphere is interconnected and interlinked to each other, which is beneficial for contact of the electrolyte and active electrode material surface during the electrochemical process. In addition, the SS-NiS@3DNF-P electrode displays an aggregated sphere-shaped morphology (Fig. 1e and f).
Figure 1SEM images of the (a–c) SS-NiS@3DNF-M electrode, (d–f) SS-NiS@3DNF-E electrode, and (g–i) SS-NiS@3DNF-P electrode.
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SEM images of the other electrode, i.e., SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, SS-NiS@3DNF-E-3, and SS-NiS@3DNF-E-4 prepared using different amounts of thiourea as the sulfur precursor, were also examined by FESEM, and the results are displayed in Fig. 2. The SEM analysis results show that thiourea played an important role in controlling the morphology and size of the nickel sulfide grown at the surface of 3DNF; thiourea is a very inexpensive sulfur precursor and is easily available compared to other sulfur-based compounds27.
Figure 2High- and low-magnification SEM images of (a) SS-NiS@3DNF-E-1, (b) SS-NiS@3DNF-E-2, (c) SS-NiS@3DNF-E-3, and (d) SS-NiS@3DNF-E-4.
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Figure 2a shows an SEM image of the SS-NiS@3DNF-E-1 electrode, which shows an aggregate interconnected sheet-like structure deposited on 3DNF, whereas an irregular sphere-shaped morphology was observed in the case of the SS-NiS@3DNF-E-2 electrode (Fig. 2b). With a further increment of the thiourea concentration from 0.15 to 1.5 mM, the morphology of the SS-NiS@3DNF-E-3 electrode remains the same as that of the SS-NiS@3DNF-E-2 electrode, but the size of the spherical particles entirely changes into a spherical morphology with a uniform structure between 2 and 4 µm in size (Fig. 2c and Supplementary Fig. S2 online). The figure shows that all sphere shapes were interconnected and interlinked to each other, which helped with contact of the electrolyte and active electrode material surface during the electrochemical supercapacitive process. Figure 2d shows an SEM image of the SS-NiS@3DNF-E-4 electrode, which displays irregular morphology and shape, and the shape is not very clear compared to the other electrodes. These results show that the appropriate concentration of thiourea played an important role in the formation of well-defined SS-NiS@3DNF, which is discussed in detail in the following synthesis mechanism section.
The above morphology was further examined by the TEM and HRTEM analysis which shows that the dark colored spheres interlinked with the other spheres through the thin sheets (Supplementary Fig. S3 online). The HRTEM images shows that the grown nickel sulfide at the surface of the three dimensional nickel foam has well-ordered crystalline structure which well matched with previously reported work 22,25.
Morphological studies support the role of the solvent and the effect of the sulfur precursor concentration as follows. The nickel salt and thiourea were dissolved in an ethylene glycol and ethanol solvent, which subsequently formed strong complexation between nickel ions (Ni2+) and thiourea, leading to the formation of a nickel-thiourea complex compound, which subsequently decomposed during the thermal process. Additionally, thermal treatment prevents the production of a large number of S2− ions in the solution, which provides favorable conditions for the formation of nickel sulfide products. The above statements can be expressed in terms of the following suggested reaction27:
$$Ni^{ + 2} + H_{2} NCSNH_{2} \frac{Ethylene\, glycol}{{Ethanol}} \to \left[ {Ni\left( {SCN_{2} H_{4} } \right)_{2} } \right]^{ + 2} Reaction 1$$
$$\left[ {Ni\left( {SCN_{2} H_{4} } \right)_{2} } \right]^{ + 2} \frac{Decomposition}{{in\, thermal\, condition}} \to NiS Reaction 2$$
The morphology of the SS-NiS@3DNF electrodes was significantly affected by various reaction parameters, such as the thiourea concentration, reaction time, and solvent effect, which are systematically shown in Fig. 3. The shape and size of the nanomaterial during synthesis have a significant effect on the reaction rate. In the present case, ethylene glycol and ethanol play an important role in the fabrication of SS-NiS@3DNF electrodes. Ethylene glycol and ethanol act as both reaction media and dispersion media and can efficiently absorb and stabilize the surface of the particles, producing monodisperse metal sulfide crystals with good dispersity28. However, ethylene glycol has a high permanent dipole moment and is an excellent susceptor of reactor heat during hydrothermal treatment; it can take energy, which helps decompose the thiourea and nickel complex compound ([Ni(SCN2H4)2]2+) and initiate the formation of the product on the provided substrate29,30. During the synthesis procedure, at the beginning of the reaction inside the Teflon reactor, frequent formation of nuclei started, and after time, the nuclei tended to aggregate (3 h Supplementary Fig. S4a online) with nonbearing nuclei, leading to the formation of a spherical shell (6 h, Supplementary Fig. S4b online) on 3DNF. After 12 h (Supplementary Fig. S4c online), the sphere-shaped nickel sulfide started to interconnect and interlink with the neighboring spheres.
Figure 3Schematic presentation of the SS-NiS@3DNF electrode fabrication.
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After increasing the synthesis time (24 h, Supplementary Fig. S4d online), the spheres became larger in size, and the interconnection between spheres decreased compared to the SS-NiS@3DNF after 12 h. The growth procedure of the SS-NiS@3DNF electrode spheres was performed under controlled time and solution environment, resulting in interlinked-interconnected spheres of nickel sulfide grown on the nickel foam. When the concentration of thiourea was low, the reaction rate was low due to the low ability of sulfur ions to react with the crystal faces. Under low sulfur conditions, the nuclei tended to undergo isotropic growth and start forming a thermodynamically favorable spherical morphology. In contrast, when the concentration of thiourea was high, the nucleation rate was also high, and the consistent growth of nucleated particles was reduced; under this condition, the synthesized nickel sulfide was large in size.
Supplementary Fig. S5 online shows the powder XRD pattern of the optimized SS-NiS@3DNF-E-3, which showed smooth lines with sharp peaks at 30.09°, 34.58°, 45.80,° 53.5°, 60.7°, 62.64°, 65.3°, 70.61° and 73.2° 2θ values corresponding to the (100), (101), (102), (110), (103), (200), (201), (004), and (202) planes, respectively. All the peaks of the optimized and fabricated SS-NiS@3DNF-E-3 electrode were indexed and well matched JCPDF No: 10–075-06136,22. XPS was also performed to determine the chemical composition and surface characteristics, such as the surface percentage and nature of nickel and sulfur bonds, of the optimized SS-NiS@3DNF-E-3 material (Supplementary Fig. S6)22,25. Figure 4a shows the XPS high-resolution Ni 2p core level spectrum, which is divided into two broad peaks at binding energies of 871.80 and 853.3 eV assigned to Ni2+, while those observed at 855.85 and 874.20 eV correspond to Ni3+ and two shakeup satellite bands, which further demonstrate the presence of Ni 2p in SS-NiS@3DNF-E-325. Figure 4b shows the high-resolution XPS S 2p spectrum, which contains two main peaks at binding energies of 160.0 to 164.0 eV, which shows that sulfur is present in the sulfide phase over the SS-NiS@3DNF-E-3 electrode25. The band observed at 168.2 eV was attributed to SO3. The presence of other sulfur species was still obviously expected because the surface of sulfide easily oxidized in ambient air and formed other forms of sulfur. However, the presence of high oxidation state sulfur does not appear to affect the electrochemical performance.
Figure 4High-resolution XPS spectra of (a) Ni 2p and (b) S 2p of the SS-NiS@3DNF-E-3 electrode.
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The comparative electrochemical performances of all the fabricated electrodes, i.e., SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, SS-NiS@3DNF-E-3, and SS-NiS@3DNF-E-4, as possible binder-free electrodes for ES application were evaluated. The selection of the electrolyte is also an important parameter in electrochemical supercapacitive applications because its characteristics should include a high ionic concentration in a small amount of electrolyte and a low resistance. Therefore, KOH electrolyte is better than the other electrolytes owing to its low resistance and high ionic concentration. The initial electrochemical supercapacitive behavior of all the electrodes (SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, SS-NiS@3DNF-E-3, and SS-NiS@3DNF-E-4) was analyzed by CV in a 2 M KOH electrolyte, and the results are displayed in Fig. 5a and Supplementary Fig. S7 online. Figure 5a shows the comparative CV graph of the SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, SS-NiS@3DNF-E-3, and SS-NiS@3DNF-E-4 electrodes recorded at a fixed scan rate of 10 mV/s within the potential range of −0.2 to 0.4 V. Figure S 7 shows the CV graph of the SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, SS-NiS@3DNF-E-3, and SS-NiS@3DNF-E-4 electrodes recorded at different scan rates within the potential range of -0.2–0.4 V. The comparative CV curves of all the electrodes (Fig. 5a) revealed that all the electrodes show the presence of the redox peak corresponding to the reversible faradic reaction over the electrode due to the possible reversible reaction from Ni-S to Ni-S-OH in the charge storage mechanism, which can be summarized as follows22,25:
$${\text{NiS }} + {\text{ OH}}^{ - } { \leftrightarrows }{\text{NiSOH }} + {\text{ e}}^{ - }$$
Figure 5(a) Comparative CV graph of the SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, SS-NiS@3DNF-E-3, and SS-NiS@3DNF-E-4 electrodes., (b) comparative GCD graph of the SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, SS-NiS@3DNF-E-3, and SS-NiS@3DNF-E-4 electrodes., (c) comparative specific capacitance bar graph of the SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, SS-NiS@3DNF-E-3, and SS-NiS@3DNF-E-4 electrodes., and (d) specific capacitance at different current densities of the SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, SS-NiS@3DNF-E-3, and SS-NiS@3DNF-E-4 electrodes.
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The observed peak at approximately 0.34 V was assigned to oxidation of Ni-S to Ni-S-OH, and the corresponding reduction peak at 0.2 V was attributed to the reversible reaction process. In addition, among all the electrodes, SS-NiS@3DNF-E-3 showed a high current response with a large integrated area compared to SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, and SS-NiS@3DNF-E-4 in the CV results, which indicates that the electrochemical capacitive performance of the SS-NiS@3DNF-E-3 electrode may be higher than that of the rest of the electrodes. From the CV curves, we can clearly see that when we increased the thiourea amount, the CV integrated area became large for the SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, and SS-NiS@3DNF-E-3 electrodes, whereas in the case of SS-NiS@3DNF-E-3, the CV integrated area decreased, which may be due to a decrease in the interconnectivity between neighboring spheres. The large CV integrated area might also be due to each sphere being interconnected and interlinked to each other, which helps the flow of electrons during the electrochemical process over the electrode during the electrochemical measurements. The anodic and cathodic peaks of all the electrodes were shifted to the right and left with increasing scan rate (Supplementary Fig. S7 online). The small shift indicates that a more reversible and faster charge transfer phenomenon occurs during electrochemical analysis. For better recognition of the charge storage mechanism and potential specific capacitance of the fabricated binder-free SS-NiS@3DNF electrodes, GCD analysis was performed in the potential range of −0.1 to 0.4 V, and the results are displayed in Fig. 5 and Supplementary Fig. S8 online. Figure 5b shows the comparative GCD curves of the SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, SS-NiS@3DNF-E-3, and SS-NiS@3DNF-E-4 electrodes recorded at a fixed current density of 1 A/g, whereas Supplementary Fig. S8 online shows the GCD graph of the individual electrodes recorded at different current densities. The comparative GCD graph and individual electrode GCD curves revealed that all the electrodes exhibited a pseudocapacitive nature, which is also in accordance with the above CV results. The GCD graph also shows the presence of a voltage plateau from 0.31 to 0.40 V, again suggesting that the redox reaction plays an important role in the overall charge–discharge process occurring over the electrode surface during the electrochemical process.
The comparative GCD graph and specific capacitance graph (Fig. 5c and d) clearly reveal that the SS-NiS@3DNF-E-3 electrode shows a much better specific capacitance than the SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, and SS-NiS@3DNF-E-4 electrodes, which may be due to all the spheres being interconnected and interlinked to each other, which provides a larger expose area and more electroactive sites for ions and electrons during the redox reaction. Additionally, direct growth of NiS on the conductive 3DNF substrate facilitates ionic and electronic transport, which enhances the overall performance of the SS-NiS@3DNF-E-3 electrode. Moreover, the specific capacitance of all the fabricated electrodes was calculated from the discharge curves using the equation mentioned in the electronic supplementary information, in which the highest specific capacitances of the SS-NiS@3DNF-E-1, SS-NiS@3DNF-E-2, SS-NiS@3DNF-E-3, and SS-NiS@3DNF-E-4 electrodes were 188.0, 470.0, 694.0 and 230.0 F/g at a current density of 1 A/g. Additionally, the GCD profiles of each electrode were also examined at different current densities. The specific capacitance of the SS-NiS@3DNF-E-1 electrode was 188.2, 180.4, 156.6, 140.0, 128.0, and 49.5 Fg−1 (Supplementary Fig. S8a online), whereas for the SS-NiS@3DNF-E-2 electrode, it was 470.0, 464.0, 432.0, 350.0, and 224.0 F/g (Supplementary Fig. S8b online). Similarly, for the SS-NiS@3DNF-E-3 electrode, it was 694.0, 780.0, 688.0, 660.0, 504.0, and 288.0 F/g (Supplementary Fig. S8c online), and for the SS-NiS@3DNF-E-4 electrode, the calculated specific capacitance was 230.0, 228.0, 210.0, 154.0, 112.0, and 48.0 F/g (Supplementary Fig. S8d online) at current densities of 1, 2, 3, 3.5, 4, and 6 A/g, respectively.
The GCD curves of all the individual electrodes and the corresponding specific capacitance performance at different current densities clearly show that with increasing current density, the restriction of the electron and electrolyte transport gradually decreases, which is responsible for the decrease in the capacitance of the electrode. The relationship between the current density and specific capacitance is presented in Fig. 5d. The specific capacitance of all the fabricated electrodes gradually decreased with increasing current density. This phenomenon occurs due to the internal voltage drop and insufficient active material involved in the redox reaction at higher currents. Furthermore, the specific capacitance of the optimized SS-NiS@3DNF-E-3 electrode is also compared with those of previously reported nickel sulfide-based electrode materials in detail in Table 1. The optimized SS-NiS@3DNF-E-3 electrode shows good specific capacitance even at high current density, which confirms the good rate capability of the optimized electrode.
Table 1 Comparison of the specific capacitance, energy density and power density of the SS-NiS@3DNF-E-3 electrode with those of other reported nickel sulfide-based electrode materials.Full size table
Long-term continuous charge–discharge cycling or the cycling stability of the electrode is a critical issue and an important parameter for practical SC applications because MO-based electrodes usually suffer from a lack of long-term stability due to electrode material degradation. The cycling stability of the optimized SS-NiS@3DNF-E-3 electrode was evaluated by continuous GCD measurements for up to 6700 cycles at a fixed current density of 3.0 A/g. According to the cycling stability curve (Fig. 6a), at the start of the cycling stability test, the specific capacitance of the electrode increased due to the self-activation effect, and after that, it started slightly decreasing and stabilized at more than 88% after 6700 cycles. The cycling stability results were also compared with previous results, and the cycling stability in the present case was significantly higher than that of the other reported nickel sulfide-based electrode materials in Table 1.
Figure 6(a) Cyclic retention up to 6700 cycles, and (b) energy and power density curve of the SS-NiS@3DNF-E-3 electrode.
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The energy density and power density are other major concerns for practical application of all electrodes in SC applications. The energy and power densities of the optimized electrode were calculated from the GCD curve using the equation mentioned in the electronic supplementary information at different current densities and plotted on the Ragone diagram shown in Fig. 6b. The Ragone plot shows that the highest energy density of the SS-NiS@3DNF-E-3 electrode was approximately 24.9 Wh/kg at a power density of 250.93 W/kg, and the electrode maintained an energy density of 7.5 Wh/kg at a power density of 1500 W/kg at the current load. The excellent electrochemical performance of the fabricated optimized binder-free SS-NiS@3DNF-E-3 electrode is due first to its interconnected interlinked structure between spheres, which helps provide a large exposed area and more electroactive sites during the redox reaction. Second, direct growth on Ni foam facilitates ionic and electronic transport, which enhances the performance of the electrode. Third, direct growth on the 3DNF substrate prevents binder and conductive additive adhesion, which decreases the resistance. These conditions create an effective and stable pathway for charge transfer during the electrochemical supercapacitive process.