Incorporation of Carbon Quantum Dots for Improvement of Supercapacitor Performance of Nickel Sulfide.

A facile hydrothermal method is adopted for the synthesis of hierarchical flowerlike nickel sulfide nanostructure materials and their composite with carbon quantum dot (NiS/C-dot) composite. The composite material exhibited good performance for electrochemical energy-storage devices as supercapacitor with a specific capacity of 880 F g-1 at a current density of 2 A g-1. The material remained stable up to the tested 2000 charge-discharge cycles. Carbon quantum dots of size 1.3 nm were synthesized from natural sources and the favorable electronic and surface property of C-dots were utilized for improvement of the supercapacitor performance of NiS. The results from Tafel analysis, double-layer capacitance, and the impedance measurement reveal that the incorporation of C-dots inside the NiS matrix has improved the charge-transfer process, which is mainly responsible for the enhancement of the supercapacitive property of the composite materials.

Introduction

Electrochemical supercapacitors have attracted many researchers in the recent years because of their high power density than batteries, long cyclic life, and higher energy density than conventional dielectric capacitors as energy-storage devices. In addition to that supercapacitors score high in terms of environmental friendliness among the energy-storage devices. In recent years, various metal sulfides, oxides, and their nanocomposites such as NiCo2S4, Ni3S2, CuS, SnS2, CoS2, and CoS have been reported for their highly reversible electrochemical redox process, long cyclic stability, high charge-storage capacity through pseudocapacitance, and unique physical and chemical properties. Among different metal sulfides, nickel sulfides and their composites are particularly important due to their high electrical conductivity, cost-effectiveness, and good mechanical stability. There have been various reports utilizing different procedures for the synthesis of nickel sulfide and its different composite materials and their application as supercapacitor.[1−32] Nickel sulfide present in different stoichiometric forms such as NiS, Ni3S2, Ni6S5, Ni7S6, and Ni9S8 and all of these stoichiometric forms have importance in energy-storage applications, fuel cells, water splitting, and Li-ion batteries. Conductivity of nickel sulfide is generally good for its application as supercapacitor; however, there are contradiction in its reported conductivity and the materials in prolonged use suffer loss in conductivity owing to the nonuniformity in grain size and their distribution.[9,10] NiS also reported to have undergone decay in the supercapacitive property on repetitive charge–discharge cycles, and the incorporation of the conducting support materials improves the cycle stability of the materials.[33] Therefore, the requirement of the addition of other agents with NiS to improve the supercapacitive property is twofold: it increases the conductivity of the NiS and also stabilizes the materials on long-term charge–discharge cycles. Incorporation of conducting carbon-based materials is one of the preferred choices to improve the conductivity of the metal oxide- and sulfide-based supercapacitors. Carbon has been used as activated carbon, carbon nanotube,[34,35] and graphene.[33] Recently, carbon quantum dots (C-dots)[36] and graphene quantum dots[37] have shown promising results in the improvement of the supercapacitors. C-dots are quasi zero-dimensional nanomaterials with very high chemical stability and these are highly soluble in water with very good possibility of functionalization. The excellent electrochemical and photochemical properties of the C-dots make them a promising candidate in catalysis,[38−41] sensing,[42] and supercapacitor applications.[43]

Under the present investigation, the C-dots have been synthesized from lemon juice by adapting the hydrothermal route to prepare NiS with C-dots nanocomposite. A single-step novel approach has been adapted to the synthesis of hierarchical flowerlike NiS/C-dots nanostructure by hydrothermal method without using any precipitating reagent or any template. To the best of our knowledge, no report on the supercapacitor property of NiS/C-dots composite exists in the literature. Here, flowerlike NiS nanostructures were obtained. The supercapacitive properties of the NiS/C-dots nanocomposites have been investigated and compared to the pristine NiS. The specific capacity was significantly higher in the case of hybrid composite material than the pristine NiS, and the synergistic effect of C-dots incorporation inside the NiS nanostructures toward the improvements of supercapacitance has been discussed in the manuscript. In addition to providing better conductivity of charge to the composite materials, C-dots have the additional advantage that they can increase the capacitance through the incorporation of additional positive charges into the matrix.[44] In contrast, there is a possibility of decrease in the surface area of porous materials in the presence of C-dots, which is essentially due to the filling of the pores of NiS materials by the C-dots. It would therefore be interesting to investigate and discuss the electrochemical properties of the NiS and C-dots composite materials and to compare the behavior with pristine NiS materials, which would fetch out some of the interesting aspects of the electrochemical behavior of NiS in relation to the supercapacitance properties.

Results and Discussion

Spectroscopic and Microscopic Characterization of the Material

Fourier transform infrared (FTIR) and transmission electron microscopy (TEM) measurements of the materials are shown in Figure . FTIR measurements indicated the presence of C=O, C–H, O–H, C–O–C, and C=C groups on the surface of the carbon dots (Figure A). The NiS/C-dots composite material showed three prominent FTIR signals at 3214, 1639, and 1074 cm–1, corresponding to the O–H, C=C, and C–O bonds, respectively (Figure B). Therefore, during the formation of NiS/C-dots composite materials by hydrothermal treatment, most of the functional groups of the C-dots were reduced and the corresponding FTIR signals were not observed. This in turn would form partially reduced C-dots in the NiS/C-dots composite materials. The TEM images of the C-dots with two different resolutions are shown in Figure C,D. C-dots were nearly uniform in size and the high-resolution image has revealed the mean diameter of the C-dots as 2.5 nm. The size of the C-dots was also measured using dynamic light scattering method, and the results are shown in Figure S1 of the Supporting Information. The mean diameter of the particles obtained was 1.3 nm.

Figure 1

FTIR plot of (A) C-dots (B) NiS/C-dots composite, and (C, D) TEM image of C-dots at two different resolutions.

The composite materials as synthesized using hydrothermal method were characterized using X-ray powder diffraction (XRD) to predict the crystal structure, and the corresponding diffraction patterns are shown in Figure A,B. All of the diffraction peaks were indexed to pure NiS in a rhombohedral structure with crystal lattice parameter a = 9.620, c = 3.149 (JCPDF #12-0041). The diffraction pattern of NiS/C-dots nanocomposite material matches that of the pristine NiS, signifying no significant change in the crystal structure of the NiS due to the formation of composite with C-dots.

Figure 2

XRD patterns of (A) NiS/C-dots composite and (B) pristine NiS.

The morphology of the materials was obtained using field emission scanning electron microscopy (FESEM) measurements. Figure A,B shows the FESEM images of the NiS/C-dots composite material, and Figure C,D shows the FESEM images of pristine NiS. Results indicate the synthesis of well-defined hierarchical flowerlike NiS nanostructure. The length of each flower petal is ∼0.8 μm in NiS/C-dots composite. In the case of pristine NiS, the SEM patterns are different from those of the composite material, which showed rodlike patterns. NiS has two major morphological forms: flowerlike NiS (f-NiS) and rodlike NiS (r-NiS). Both the morphological forms are reported to be having the same crystal structure,[45] which has also been observed in the present case. The f-NiS is reported to be formed through the diffusion-limited aggregation.[46] Since the f-NiS has been formed only with the presence of C-dots, the C-dots are responsible for the formation of f-NiS morphology. The rodlike NiS is formed through the flake-cracking mechanism, and it has been reported that NiS initially formed a sheetlike morphology, later the flakes are cracked in the form of nanorod and materials are transformed into rodlike morphology. The flake-cracking mechanism has also been reported in the case of biomolecule-assisted synthesis of single-crystalline selenium nanowires and nanoribbons.[47] It would therefore be interesting to observe the difference in the electrochemical property from the two materials by adding the contribution from the improvement in conductivity for introduction of C-dots in the NiS and also due to the difference in the morphology of the two materials as revealed from the microscopic measurements.

Figure 3

SEM images of NiS/C-dots at different resolutions (A, B); pristine NiS at different resolutions (C, D).

Atomic force microscopy (AFM) measurements of the NiS/C-dots composite and the pristine NiS were recorded, and the results are shown in Figure S2. The nanopetals as observed in the SEM and TEM measurements could not be resolved in the AFM image. However, the difference in the morphology of the NiS/C-dots composite and the pristine NiS could be well observed. The composite has shown structural arrangements of elongated grains, which can be considered as the poorly resolved nanopetal as observed from the SEM and TEM measurements. The clusters of the grains were distributed all along the composite materials for pristine NiS, the AFM morphology was characterized as needle-type shape of the grains, and the morphology resembles with the SEM image as shown in Figure D.

TEM images of the NiS/C-dots composite and pristine NiS are shown in Figure A,B. The composite material is characterized with the nanoflower-type morphology and the flower petals were very well spread all over the substrate. The pristine material has shown nanowire-type pattern; however, the nanowires were not well organized all over the substrate of the materials. The selected area electron diffraction (SAED) patterns of the composite and the pristine materials were also obtained and are shown in Figure C,D, The d-spacing of 0.25 nm in the case of NiS/C-dots composite materials indicates the presence of (002) plane, which has also been supported from the XRD measurements with the peak at 35.9°. In the case of pristine NiS material, the d spacing was obtained as 0.47 nm, which corresponds to the (110) plane of the NiS material. This has also been supported by the X-ray diffraction peak, where the diffraction pattern from the (110) plane of NiS was observed at 18.5°.[32] In composite materials, the C-dots are not separately seen; however, their presence strongly dictates the morphology of the NiS.

Figure 4

TEM image and SAED pattern of NiS/C-dots composite (A, C) and pristine NiS (B, D).

The chemical state and molecular environment of the NiS/C-dots composite material were investigated through X-ray photoelectron spectroscopy (XPS). The peaks 166, 284, 529, and 854.4 eV in the wide-scan XPS image of Figure A referring to S 2p, C 1s, O 1s, and Ni 2p, respectively, confirm the presence of S, C, O, and Ni elements in NiS/C-dots composite material.[48] The high-resolution XPS images of Ni 2p in the NiS/C-dots composite material are shown in the Figure B; the peaks at 854.2 and 872.4 eV are assigned to the binding energies of Ni 2p3/2 and Ni 2p1/2, respectively.[49] Two other peaks at binding energies 859.1 and 878.1 eV are the satellite peaks of Ni 2p3/2 and Ni 2p1/2, respectively, which indicated the presence of coulomb interaction between core holes and valence electrons.[50,51] The peaks at 161.4 and 162.5 eV in the high-resolution S 2p spectrum of NiS/C-dots composite materials (Figure C), corresponding to S 2p3/2 and S 2p1/2, suggest the presence of divalent sulfide ions (S2–) in the NiS/C-dots composite materials.[52] The peak at 166.5 eV could be recognized as S–O bond due to the surface oxidation.[52] The deconvoluted C 1s spectra of NiS/C-dots composite materials in Figure D consist of a main peak at 284.5 eV, which corresponds to the C–C bond with sp2 orbital. The other peaks at the binding energies 285.6, 286.7, 287.2, and 288.6 eV correspond to the sp3 C–C-bonded carbon, C–O–C, C=O, and C–OH, suggesting the surface functional groups on the NiS/C-dots composite material. These XPS results indicated that NiS/C-dots composite materials were successfully formed, which are quite consistent with the XRD and FTIR analyses.

Figure 5

XPS images of the NiS/C-dots composite material: (A) wide-scan XPS images, (B) Ni 2p, (C) S 2p, and (D) C 1s.

Electrochemical Investigations

After spectroscopic and microscopic characterizations of the materials, the electrochemical properties of the materials have been investigated by cyclic voltammetry (CV), galvanostatic charge–discharge measurements, and electrochemical impedance measurements.[53,54] CV measurements were carried out in 2 M KOH as the electrolyte in multiple cycles at a scan rate of 10 mV s–1, and the results are shown in Figure A,B. CV plot has indicated that the composite material exhibited pseudocapacitive nature and one oxidation peak and corresponding reduction peak appeared in the CV plot; both the materials have shown stable cyclic voltammetry profile. The reversibility of the material was due to the symmetric nature of the redox process as obtained from the CV measurements. On multiple cycles, the NiS/C-dots composite materials showed sharp increase in the capacitive nature of the electrochemical process and the enhancement in current was saturated after around 50 cycles. The CV plot of the pristine NiS has also been characterized with one oxidation peak and the corresponding one reduction peak. The capacitive and redox current did not increase significantly with the multiple cycles in the cyclic voltammetry measurements. The measured current density in the case of the NiS/C-dots composite materials is significantly high compared to the pristine NiS. The separation between the oxidation and the reduction peaks in the case of NiS/C-dots composite is around 0.21 V, whereas the same in pristine NiS material is around 0.29 V. The larger peak separation in pristine NiS compared to the NiS/C-dots composite materials is related to the rate of the charge-transfer process. The composite materials thus have shown better charge-transport property compared to the pristine materials. CV experiments were further carried out at different scan rates to delineate the relative charge-transfer process in two materials, and the results are shown in Figure C,D. The current density increased with increase in the scan rate of the measurements. The anodic peak current and the cathodic peak current were plotted with respect to the scan rates, and the plots are shown in Figure S3A,B in the Supporting Information. No linear correlation between the anodic and cathodic peak currents with the scan rates of the measurements is observed. The peak currents were also plotted with respect to the square root of the scan rate of the measurements and are shown in the same figure (Figure S3C,D). The linear correlation between the peak currents and the square root of the scan rate of the measurements indicated the diffusion-controlled nature of the electrochemical oxidation and the reduction process. The linear fitting resulted higher slope (almost double) for the anodic peak than the cathodic peak for both the materials. In the anodic scans, the anions ingress inside the materials, and the higher slope is indicative of the higher rate of ingress of the ions from the composite materials compared to the rate of regress of the ions out of the material matrix. Larger intercept for the cathodic peak is due to the fact that cathodic scan was recorded immediately after the anodic scan and there might not be complete discharge of the electrochemical interface during the anodic scan; therefore, some charge will always remain attached prior to the another charging process.

Figure 6

Multiple cyclic voltammetric plot of (A) NiS/C-dots composite, (B) pristine NiS and CV plot with scan rate variation, (C) NiS/C-dots composite, and (D) pristine NiS.

When the slope and the intercept of two materials were compared, the values are observed to be much higher in the case of the NiS/C-dots composite compared to the pristine NiS. Higher slope for the composite materials is indicative of the faster charge-transfer process, and the larger intercept is due to the enhanced amount of electrostatic charge associated in the composite materials compared to the pristine NiS.

After having the idea about the electron-transfer process, the pure capacitive nature of the substrates was obtained from the double-layer capacitive measurements by recording the CV at different scan rates at the double-layer region, and the corresponding plots are shown in Figure A,B. The current density at the capacitive region is higher in the case of pristine NiS compared to the composite materials. The double-layer capacitance was calculated and plotted with respect to the scan rate of the measurements, and is shown in Figure S4A,B of the Supporting Information. The measured double-layer capacitance was marginally higher in the case of pristine materials than the composite materials at lower scan rates. However, the double-layer capacitance decreased drastically in pristine materials with increase in the scan rate and its values are significantly lower compared to the composite material at moderate scan rate of the measurements. Therefore, the number of electrochemically active sites available for the electrostatic charging as reflected by low scan rate is nearly similar in the two materials. In the case of pristine materials, some of these active sites might be deep inside the matrix, and at higher scan rates, those sites could not be accessed, which resulted in the fast drop in the measured capacitance values. On the other hand, in NiS/C-dots composite materials, the NiS is infused with highly conducting C-dots; therefore, the transport of charge deep inside the matrix of the materials is quite fast.

Figure 7

Scan rate variation at low double-layer region: (A) NiS/C-dots composite. (B) Pristine NiS Tafel plot of (C) NiS/C-dots composite and (D) pristine NiS.

The specific capacity of the composite materials can be calculated from the measured current density of CV curve on dividing the specific current by the scan rate of the measurements. The maximum specific capacity of 650 F g–1 was obtained from the CV measurements at 10 mV s–1 scan rate in the case of NiS/C-dots composite materials; similarly, in the case of pristine NiS materials, the specific capacity was obtained as 480 F g–1 at the scan rate of 10 mV s–1. It is therefore realized that the specific capacity in NiS/C-dots composite materials is considerably higher compared to the pristine NiS materials under similar experimental conditions. Incorporation of carbon dots into the pristine NiS matrix thus improved the specific capacity of the materials.

The NiS/C-dots composite materials have shown physical surface area of around 40% higher than the pristine NiS substrate as obtained from the Brunauer–Emmett–Teller measurements (for NiS/C-dots composite, it is 40 m2g–1, and for the pristine NiS, it is 25 m2 g–1). The electrochemically active surface area of the materials was measured from the cyclic voltammetry measurements at different scan rates in the double-layer region, and the corresponding plots are shown in Figure A,B. Double-layer capacitance of NiS/C-dots composite is about twice that of the pristine NiS, and the results are shown in Figure S4 of the Supporting Information. With increase in the scan rates, the double-layer capacitance increases in the composite materials, whereas in pristine materials, it was decreased with increase in the scan rates of the measurements at all scan rates. The higher double-layer capacitance and maintaining its value up to the scan rates of 60 mV s–1 indicate the enhancement in the electrochemically active centers on inclusion of C-dots with NiS matrix, and the rates of the arrangements of the charges over the double layers became higher. Higher double-layer capacitance at large scan rates indicates faster charge-transport process.

To delineate the electrochemical processes involved in two materials, the Tafel plots were constructed from the CV plot, and are shown in Figure C,D, The Tafel slope values as obtained from the CV measurements are placed as inset of the plots. The Tafel slope for the composite material was lower compared to the pristine materials. Lower Tafel slope is indicative of the better charge-transfer process in NiS/C-dots composite materials compared to the pristine NiS materials. It was further observed that with multiple cycles, the value of the Tafel slope decreased in both the materials. The structural/morphological changes, which could be responsible for the enhancement of the charge-transfer process with multiple cycles were ascertained. No structural changes are observed after the samples undergo electrochemical testing, which has been confirmed from XRD measurements, as shown in Figure S5. Morphological analysis of the samples, after the electrochemical test was carried out using SEM and AFM measurements, showed no significant change in the morphology of the materials after the electrochemical test of 2000 charge–discharge cycles, and the figures are shown in Figure S6 of the Supporting Information. Other possibility for the betterment in the charge-transfer property is due to the modification in the charge distribution of the materials after initial cycles. A similar observation has been reported previously in NiS, which has been explained due to the activation of the electrode materials after initial charge–discharge cycles.[55]

The galvanostatic charge–discharge experiments were carried out in 2 M KOH supporting electrolyte medium at a potential range of 0–0.5 V, and the plots are shown in Figure S7 in the Supporting Information. Multiple charge–discharge experiments were carried out at a current density of 2 A g–1. The specific capacities of the composite materials and the pristine NiS were obtained as 880 and 710 F g–1, respectively. The charge–discharge curve indicates that the material is stable up to 2000 charge–discharge cycles. Both the materials have shown stable charge–discharge characteristics, and the charging and discharging of the materials are asymmetric in nature. Toward the end of the charging process, both the materials have shown battery behavior. Charge–discharge experiments were also carried out at different applied current densities, and the plots are shown in Figure A,B. The slope of the initial portion of the discharge plot up to 0.3 V is much steeper in the case of pristine NiS material compared to the NiS/C-dots composite material. Later portion of the discharge plot from 0.3 to 0 V is almost similar in shape. For the same current density, the steep slope of the discharge plot is indicative of the lower capacitive property of the material. The shape of the discharge plot is similar to that of the NiS materials reported earlier.[8,56] The observed plateau during the discharge process is indicative of the pseudocapacitive behavior of the material. The discharge characteristics of the nickel oxide materials, however, are different from those of the NiS, and the plateau region as observed in the NiS is not observed in nickel oxide. Although nickel oxide shows good supercapacitive performance, it suffers low electrical conductivity, and its performance is improved using nickel oxide in its composite form with carbon fiber.[57]

Figure 8

Galvanostatic charge–discharge plot at different current densities (A) NiS/C-dots composite, (B) pristine NiS, and specific capacity vs current density, (C) NiS/C-dots composite, (D) pristine NiS, (E) Nyquist plot of NiS/C-dots composite at different applied potentials, and (F) Nyquist plot of pristine NiS at different applied potentials.

The discharge capacity of the materials was measured at different current densities, and the measured specific capacity was plotted against the current density of the measurements (cf. Figure C,D). The specific capacity of the composite material is higher compared to the pristine material; with increase in the current density, the specific capacity increased initially and then decreased very fast in the case of pristine material and slowly in the case of composite material. The specific capacity was measured at multiple charge–discharge cycles and plotted up to 2000 cycles (from the charge–discharge cycles as in Figure S7), and the results are shown in Figure S8 of the Supporting information. Specific capacity is increased with multiple cycling up to the charge–discharge cycle of around 50, after which the measured specific capacity is stabilized. It remained stable up to the charge–discharge cycles of 2000 recorded in the present measurements.

Electrochemical Impedance Measurement

Electrochemical impedance measurement provides information about the electrochemical interface by separating the transport of charge by overcoming the solution resistance, resistance in the double-layer region, resistance due to the charge transfer, and diffusion resistance. For a porous surface, additional pore resistance and pore capacitance also require to be included for explaining the complete electrochemical behavior. Supercapacitors have all such electrochemical processes requiring separate analysis to zero down the complete characterization of the electrochemical process involved during charging and discharging process.[58] In the present case, impedance measurements were carried out at different applied potentials of 0.16, 0.46, 0.56, and 0.70 V, and the Nyquist plot are shown in Figure E for NiS/C-dots composite materials. In Figure F, the impedance response of pristine NiS is shown. Results showed low charge-transfer resistance (R2 in the equivalent circuit of Figure S9) in the case of NiS/C-dots composite materials (456 Ω, at 0.46 V) compared to its pristine form NiS (690 Ω, at 0.46 V); lowering of the charge-transfer resistance is due to the increase in the charge transport through the incorporation of highly conducting C-dots. The series resistance is indicative of the electronic conductivity of the charge since the solution resistances in both the materials are the same. The series resistance (R1 in the equivalent circuit of Figure S9) decreased from 150 Ω in pristine materials to 75 Ω in composite material, which indicates the improvement of the electronic conductivity of the materials due to the incorporation of C-dots and the improvement in conductivity will improve the transfer of charge to deep inside the composite materials. About 1.5 times increase of the charge-transfer resistance indicates the easy charging and discharging process by improving the interaction of electrolyte ions over the composite material matrix and improving the redox process of the pseudocapacitor. The improvement of the conductivity of the material and the charge-transfer process has been reflected in the cyclic voltammetry measurements, where the peak potential of NiS has been shifted to lower potential due to the incorporation of C-dots in the composite material compared to the pristine material.

The nature of the Nyquist plot varied with the variation in the applied potential at which the impedance was recorded. At the applied potential of 0.16 V, the Nyquist plot showed the restricted finite space Warburg impedance, and with the shift in the applied potential from 0.16 to 0.46 V, the diffusion characteristics has been modified to the semi-infinite Warburg diffusion resistance. On further enhancing the applied potential toward more positive potentials, the Warburg component ceased. As seen from the SEM measurements, the materials were having a porous nature due to the flower petal and rod types of morphology and the electrolyte ions would get in and out during the electrochemical processes. At a lower applied potential (0.16 V), since the driving force is not so large, the Warburg component is high and the restricted finite diffusion type of nature is revealed. Diffusion process is easier when 0.46 V was applied and ions could diffuse out from dipper part of the composite materials. At even higher applied potential, no significant diffusion resistance is observed.

Energy density of the materials was calculated using the equationThe capacitor is considered as the symmetric type, and the mass change on the counter electrode is same as that over the working electrode. Energy densities of 30 and 24 Wh kg–1 were obtained for NiS/C-dots composite and pristine NiS materials, respectively. The power densities when calculated from the energy densities were obtained as 108 and 86 kW kg–1 for NiS/C-dots composite and pristine NiS materials, respectively.

Considering the materials to be purely capacitive in nature, the power density (Pmax) was calculated from the electrode voltage and the electrochemical series resistance of the electrode using the following relation.[59,60]where Vmax is the maximum voltage of the cell, m is mass of the material, and Resr is the electrochemical series resistance. The electrochemical series resistance was obtained from the initial drop in potential of the discharge plot. The maximum power density was thus obtained as 33 and 16 kW kg–1 for NiS/C-dots and pristine NiS, respectively. The values obtained from eq are significantly lower than the power density calculated from the energy density, which indicates the presence of a significant amount of pseudocapacitive property of the materials. Calculation of energy and power density using the measurements from the three-electrode geometry has its issue of overestimation of the values. There are chances of overestimation of the values as the mass change at the counter electrode is not exactly known in three-electrode geometry. The present calculation is made based on the equal mass change in both working and counter electrodes, which has been used for the calculation of capacitive properties of similar materials using three-electrode geometry.[61−66]

Previous reports have reported a specific capacity of 500–800 F g–1 with flowerlike β NiS materials.[8] 3D NiS-rGO aerogel nanocomposite showed a reported capacity of 852 F g–1,[23] whereas the NiS/GO nanocomposite showed a specific capacity 800 F g–1.[9] A comparison of the performance of the present material with the similar materials reported in the literature is presented in Table .[67−73] The presently developed material thus shows superior electrochemical performance to the similar materials reported in the literature. Further, the electrochemical performance of NiS/C-dots is improved on incorporation of carbon dots inside the NiS matrix. This improvement is attributed to the improved charge-transfer process through the favorable entanglement of C-dots along the NiS matrix. Zeta potential was measured, and the values were obtained for the NiS/C-dots composite and the pristine NiS materials as −11.5 and −5.6 mV, respectively. High negative value of the zeta potential indicates that the NiS/C-dots composite materials could accommodate more negative charge compared to the pristine materials. The faradic process involved at the interface of the NiS/C-dots composite modified electrode is[74,75]Therefore, more negative zeta potential would facilitate more accumulation of charge through the formation of NiS(OH) species during the charging process. While analyzing the results from Tafel slope, the larger slope of the anodic potential scan compared to the cathodic scan indicated the better kinetics for the ingress of the OH– ions inside the matrix of the composite materials, which has also been supported from the zeta potential measurements.

Table 1

Comparison of the Energy Density and Power Density of Some Literature Reported Materials with the Present Material

sl. no.	materials used	energy density (Wh kg–1)	power density (W kg–1)	references
1	graphene/Ni3S2	10.8	8000	ref [67]
2	Ni/Co sulfide	22.9	10 208	ref [68]
3	porous Ni/Co sulfide	17.7	2325	ref [69]
4	mesoporous NiCo2S4	10.8	8000	ref [70]
5	NiS hollow cubes	15.84	6200	ref [71]
6	NiS hollow structures with double shells	21.8	8000	ref [72]
7	porous square rodlike nickel persulfide	11.19	13 520	ref [73]
8	NiS and C-dots composite	30	33 000	present work

NiS has reasonably good electrical conductivity; however, its conductivity depends strongly on the phase of the NiS, thickness of the films, and the grain size. Charge conduction through a film depends significantly on the orientation of the films. Incorporation of carbon dots inside the NiS matrix has improved the supercapacitive property of the composite materials due to (1) the incorporation of highly conducting C-dots, which minimizes the orientation restriction and the size restriction of the NiS grains and improves the conductivity of the composite materials and (2) the presence of C-dots all along the composite materials matrix, which would improve the electrical conductivity of the materials and reduce the diffusion length of the electrolyte during the charge–discharge process. Because of all of these factors, the supercapacitance of NiS improved when in composite with C-dots.

Conclusions

Carbon quantum dots were synthesized using the hydrothermal method from lemon juice, and the size of the carbon dots was obtained to be around 1–2 nm. Nanocomposite materials of NiS with carbon dots were synthesized using the hydrothermal method. Microscopic investigation revealed the nanoflower structure of the NiS nanophase with the carbon quantum dots entangled all along the NiS phase. The NiS and its composite with carbon quantum dots showed significant improvement in the capacitance properties compared to the pristine NiS materials. Charge transport of the composite material has improved significantly compared to the pristine NiS, which has been indicated from the cyclic voltammetry, charge–discharge, and the Tafel analysis of the electrochemical investigations. The specific capacity has been significantly improved with the specific capacity of around 880 and 710 F g–1 obtained in the case of Nis/C-dots and pristine NiS materials, respectively. Such improvement of the charge-storage property is related to the enhancement of the charge transport and the surface area of the composite materials compared to the pristine materials. The present study therefore indicates the significant improvement of capacitive property of the transition-metal sulfide material upon having composite with carbon quantum dots. There is an enormous scope of the incorporation of carbon quantum dots in making composite with transition-metal oxides and sulfides for improvement of supercapacitive properties, and research in this direction would evolve many interesting materials for supercapacitor.

Experimental Section

Preparation of Carbon Dots

Carbon dots were synthesized using hydrothermal method from lemon juice. Pulp-free lemon juice (40 mL) and absolute ethanol (20 mL) were mixed well and then taken in a 100 mL Teflon vessel of the autoclave. Hydrogen peroxide (2 mL) was added dropwise to it, mixed well, and the autoclave was closed. The Teflon-lined autoclave was kept in a muffle furnace at 200 °C for 24 h.[76,77] After cooling, the autoclave was opened to obtain a brownish yellow C-dots solution; the suspension was centrifuged at 5000 rpm to separate black residues. The supernatant was again centrifuged at 6000 rpm to separate larger particle and finally the supernatant solution was stored at room temperature for further use.

Preparation of NiS/C-Dots Composite Material

NiS/C-dots composite was prepared through the hydrothermal method of synthesis. During the synthesis process, 0.50 mM NiSO4 and 100 mM thiourea were added and made up to the volume of 50 mL. This solution was sonicated for 10 min. The solution was then transferred to a Teflon-lined stainless steel autoclave vessel of 100 mL capacity, and 10 mL of C-dots solution with C-dots concentration of 0.1 mg/mL was added to it. Then, the autoclave was kept in a muffle furnace at 200 °C for 24 h, after which it was allowed to cool at room temperature. The final product was washed with deionized water four to five times and then with ethanol two to three times and finally kept in a vacuum oven for drying at 80 °C for 12 h. This material was then drop-cast over a glassy carbon substrate with a surface area of 1.5 cm2 with sample loading of 20 mg for making the electrodes.

Instrumentation

Electrochemical investigations were carried out using Hg/HgO (alkaline) as the reference electrode, a glassy carbon rod as the counter electrode, and the NiS or NiS/C-dots-modified substrate as the working electrode. All of the experiments were performed at room temperature (298 K). Electrochemical measurements were performed using an EcoChemie Potentiostat/Galvanostat, Autolab 302N; the data acquisition and analysis were carried out using GPES 4.9 and FRA software. The electrochemical properties of the supercapacitor materials were investigated in 2 M KOH solution. All of the electrochemical studies were carried out in static condition. Field Emission Gun-Scanning Electron Microscopes (FEG-SEM) system model JSM-7600F was used for SEM measurements. Mini-Flex XRD system from Rigaku was used for X-ray diffraction (XRD) measurements. Transmission electron microscopy (TEM) was carried out using the Phillips-CM 200 electron microscope operated at 200 kV. FTIR measurements were carried out using FTIR spectrometer model Tensor II from Bruker. Particle size and zeta potential were measured using particle size analyzer model Litesizer 500 from Anton Paar. XPS measurements were conducted on MULTILAB (Thermo VG Scientific) using Al Kα radiation as monochromator.