Three-Dimensional MoS2 Nanodot-Impregnated Nickel Foam Electrodes for High-Performance Supercapacitor Applications.

An economical and binder-free electrode was fabricated by impregnation of sub-5 nm MoS2 nanodots (MoS2 NDs) onto a three-dimensional (3D) nickel substrate using the facile dip-coating method. The MoS2 NDs were successfully synthesized by controlled bath sonication of highly crystalline MoS2 nanosheets. The as-fabricated high-surface area electrode demonstrated promising electrochemical properties. It was observed that the as-synthesized NDs outperformed the layered MoS2 peers as the electrode for supercapacitors. MoS2 NDs exhibited an excellent specific capacitance (C sp) of 395 F/g at a current load of 1.5 A/g in a three-electrode configuration. In addition, the fabricated symmetric supercapacitor demonstrated a C sp value of 122 F/g at 1 A/g and a cyclic performance of 86% over 1000 cycles with a gravimetric power and energy density of 10,000 W/kg and 22 W h/kg, respectively. Owing to its simple and efficient fabrication and high surface area, such 3D electrodes show high promise for various energy storage devices.

Introduction

Recently, with ever-growing energy concerns, the quest for high-performance multifunctional materials that display high performance toward energy storage and conversion with effective usage is on high demand. On this front, supercapacitors, owing to its ability to outburst power instantaneously, high specific energy, fast charge/discharge rates, and source of green energy have attracted a great deal of interests.[1,2] Supercapacitors, otherwise electric double layer capacitor (EDLC), possess excellent energy and power density that opens portal for myriad applications in industries such as consumer electronics, hybrid automobiles, military devices, industrial power, and memory backup systems.[3,4] Therefore, development of high-performance supercapacitors always remains a thrust area of research in the scientific community. In general, supercapacitors can be categorized into pseudo and double-layer capacitor, respectively, based on their charge storage mechanism. In a pseudocapacitor, energy storage occurs primarily via redox mechanism on the periphery of electrode materials such as transition metal oxides and hydroxides, conducting polymers such as PANI, poly-oxometalates, EDOT, and so forth are typical electrode materials in pseudo-capacitors.[5−7] However, in EDLC charge separation occurs at the interface of electrode and electrolyte; usually carbon materials like graphene, CNTs, carbon nanospheres, and activated carbon are utilized as the electrode material.[8−10] In addition to metal oxides and conducting polymer-based pseudocapacitors, metal organic frameworks, conductive metal sulfides such as WS2, MoS2, CoS, NiS, SnS, and ZnS and their hybrids have also emerged as distinctive[1] materials for supercapacitor applications.[11−18] Amongst, molybdenum disulfide (MoS2) possess a unique S–Mo–S trilayer atomic structure that is restrained by weak van der Waals forces, analogous to that of graphene.[19,20] Also, due to its high electrical conductivity, and unique layered morphology, MoS2-based materials offer better prospects as an electrode material for high-performance supercapacitor compared to metal oxides and graphitic materials. Similarly, nanostructured MoS2 can be synthesized via simple and economical processes such as hydrothermal, sono-chemical, and chemical vapor deposition techniques.[21−23] Nowadays, MoS2 has also attracted plenty of recognition in electrochemical energy storage devices attributed to its innate ionic conductivity and higher theoretical capacity than graphite, owing to its graphene analogous structure.[24,25] The aforementioned characteristics of MoS2 make them excellent electrode material compared to metal oxides and graphite. However, adorned with such fascinating properties, the studies that explore supercapacitive nature of MoS2 electrodes are still limited and mostly focus nanostructured MoS2 and its composites. Soon and Loh reported MoS2 micro-supercapacitor with an areal capacitive value of 8 mF/cm2.[26] Similarly, Sun et al. fabricated a tubular C/MoS2 electrode material with a Csp value of 210 F/g at 1 A/g.[27] Huang et al. reported high-performance supercapacitor with polyaniline/graphene equivalent MoS2 as electrode material.[28] Kim et al. reported MoS2 nanostructures obtained via hydrothermal reaction and utilized as negative electrode material in 1 M Na2SO4 electrolyte and maximum capacitive performance of 92 F/g was observed at a current load of 0.5 mA/cm2.[11] A flower-like MoS2 structure was communicated by Ma et al. with a Csp value of 122 F/g at 1 A/g.[29] Wang et al. also reported flower like morphology with an improved Csp value of 168 F/g at 1 A/g.[30] Karade et al., illustrated MoS2 nanoflakes prepared via chemical bath deposition with a high Csp value of ∼210 F/g at 1.5 A/g.[2] Apart from this, MoS2/carbon composites such as MoS2/graphene, MoS2/graphene oxide, and MoS2/MWCNT electrode materials are also reported.[31,32] In further advancement, the recent reports show the synthesis of zero-dimensional nanodots (NDs) of MoS2 which are expected to possess different electronics and physico-chemical properties than their 2D nano and bulk counterparts, owing to modulated size confinements and edge effects.[33] Currently, the MoS2 NDs are being investigated for applications such as electrocatalysts.[34] However, no literary reports investigating the MoS2 NDs supercapacitive performance has been noted so far.

Hence, in the present report we have attempted to investigate the electrochemical and supercapacitor performance of MoS2 NDs impregnated on the 3D nickel foam. The uniformly sized MoS2 NDs were synthesized from controlled bath sonication of crystalline MoS2 nanosheet in isopropanol at ambient temperature. The resulting NDs were successfully surface impregnated on Ni foam using a simple and facile dip-coating method. Subsequently, the electrochemical performance of MoS2 NDs@ Ni foam was evaluated using galvanostatic charge/discharge, chrono-potentiometry (CD), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) in a basic electrolyte (6 M KOH). Moreover, the supercapacitor performance of electrode material was also evaluated in a symmetric two-electrode configuration using MoS2 ND-doped electrode material as cathode and anode, respectively. The resulting electrochemical properties suggest that the 3D MoS2 NDs/Ni foam exhibit superior supercapacitive performance and cyclic efficiency compared to the bulk and nanostructured MoS2 and its hybrid materials.

Results and Discussion

Experimental Section

Chemical and Materials

Ammonium tetrathiomolybdate (ATTM) (99.97% pure), potassium hydroxide (ACS Reagent, >85%), and isopropanol was procured from Sigma-Aldrich and was utilized as-received. High grade nickel foam (99.8% purity) was purchased from XIAMEN Tob New Energy Technology, China.

Fabrication of 3D MoS2 NDs/Ni Foam Electrodes (3D MoS2 NDs/NF)

First, uniformly distributed MoS2 NDs were synthesized by top-down approach and by ultrasonication of a nanocrystalline MoS2 in isopropanol. The MoS2 was freshly prepared by thermal annealing of ATTM at 600 °C for 2 h inside the tube furnace and under a continuous flow of argon. After sonication, a color change of the dispersion to yellowish-green was observed. Further, the resulting dispersion was subjected to centrifugation at 4000 rpm for 1 h, which enabled the separation of highly dispersed MoS2 NDs and large-sized MoS2 particles. Thus the resulting MoS2 NDs dispersion was recovered and used for further structural and electrochemical characterizations. In the second step, 3D Ni foam was coated with MoS2 NDs by simple repetitive dip-coating into the as obtained MoS2 ND dispersion with known concentration. Pale yellow NDs impregnated Ni Foam was dried at 80 °C under vacuum and used for further studies. The active material loading (0.02 mg) was calculated using the weight difference of nickel foam before and after coating by high precision weighing balance which is accurate up to 0.0001 mg. A detailed synthesis mechanism is shown schematic illustration presented in Figure . The areal volumetric loading of MoS2 QD on 3D nickel foam of dimension 0.8 cm × 0.5 cm × 0.1 cm was 0.5 mg/cm3.

Figure 1

TEM image of MoS2 nanolayers (a), and MoS2 NDs (inset SAED pattern) (b), SEM image of NDs impregnated 3D nickel foam, inset showing higher magnification of ND doped nickel foam strut (c), HRTEM image of MoS2 NDs (d).

Material Characterization

The scanning electron microscopy (SEM) images were taken with a Quanta- FEG-250 scanning electron microscope. X-ray diffraction (XRD) patterns were obtained on X’Pert PRO Powder Diffractometer with Cu Kα radiation. A Raman spectrum of MoS2 NDs was collected using a Jobin Yvon HORIBA LabRAM spectrometer with back-scattered confocal configuration using a HeNe laser (633 nm). Transmission electron microscopy (TEM) images were obtained using FEI Tecnai G20 with 0.11 nm point resolution operated at 200 kV using Gatan digital camera.

Electrochemical Measurements

Electrochemical behavior of 3D MoS2 NDs/NF was studied via CV, charge–discharge (CD), and impedance (EIS) using a potentiostat–galvanostat (VMP-300, BioLogic) instrument. In a three-electrode configuration, a 3D MoS2 NDs/Ni electrode was utilized as a working electrode, and platinum wire (pt) and saturated calomel electrode (SCE) were employed as counter and reference electrode, respectively. The electrochemical analysis was carried out in 6 M KOH electrolyte for scan rates 10–100 mV/s, current densities 1–20 A/g, and frequency ranging from 100 kHz to 1 Hz, respectively. The specific capacitance of 3D MoS2 NDs/NF was evaluated from discharge curve (CD) by using eq where Csp is the specific capacitance in F/g, I is the current in mA, Δt is the discharge time in seconds, m is the amount of active material in mg, and ΔV (V) is the operational window. Also, the supercapacitor performance of the electrode material was evaluated by fabricating a symmetric supercapacitor device using 3D MoS2 NDs/NF as a positive and negative electrode. Whatman filter paper, cat no.: 1441 110, soaked in 6 M KOH was used as a separator. Specific capacitance, power density (W/kg) and energy density (W h/kg) of fabricated supercapacitor was estimated using the following equations eqs –4.[35] EIS was performed from 1 Hz to 100 kHz for a sinusoidal voltage of 10 mV.

The synthesis protocol was schematically demonstrated in Scheme . The highly crystalline as-obtained MoS2 nanosheets were subjected to the controlled bath sonication in iso-propanol suspension for 1 h. After the stipulated time, a stable yellowish-green dispersion of the sub-5 nm monodisperse MoS2 NDs were obtained. The forces exerted by fluid while bath sonication could lead the breakdown of MoS2 nanosheets, resulting in the formation of such well-dispersed NDs. Subsequently, the impregnation of these NDs on to 3D nickel foam substrate was achieved by facile dip coating method. The morphological evolution of MoS2 phases was ascertained by high-resolution TEM (HRTEM) images provided in Figures , and S1 respectively. Figure a shows the TEM image of highly crystalline MoS2 nanosheets obtained after thermal annealing of ATTM precursor at 600 °C for 2 h, which clearly shows a characteristic layered structure of MoS2. Upon sonication treatment, the layered MoS2 was disintegrated into well-dispersed sub—5 nm NDs was observed (Figure b) Previously, Gopalakrishnan et al. reported the growth of MoS2 nanoparticles distributed within the in MoS2 nanosheets via bath and probe sonication of bulk MoS2 for hydrogen evolution kinetics study.[36] However, the average sizes of MoS2 nanoparticles were not informed. Herein, we report much smaller dimensions and uniform distribution of MoS2 NDs. This could be attributed to the controlled bath sonication subjected to highly crystalline MoS2 nanosheets. Henceforth, the controlled bath sonication facilitated very precise scission of MoS2 nanosheets than latter. A detailed study on earlier reported MoS2 NDs is shown in Table S1.

Scheme 1

Schematic Representation of Synthesis Procedure to Obtain MoS2 NDs and Their Impregnation on to 3D Nickel Foam

Furthermore, the resulting 3D MoS2 NDs/NF was subjected to various structural characterizations. The SEM image of dip-coated nickel foam in MoS2 ND dispersion is shown in Figure c (inset provided, displays the cell strut of dip coated nickel foam). The high-resolution TEM image obtained for the latter is shown in Figure d.

The formation of sub-5 nm MoS2 NDs was clearly evident. Moreover, the absence of any clear and distinct lattice fringes indicated the amorphous nature of the resulting MoS2 NDs. Similarly, the SAED pattern (inset of Figure c) did not show any visible diffraction spots or rings which further support the amorphous nature of these NDs. Also, these results confirm the simple dip-coating facilitated good adherence between NDs and nickel foam. This can be due to weak van der Waals force between NDs and the nickel substrate. Additionally, the nickel substrate will provide good conductive pathway and bulk level interaction for electron/ion transportation during oxidation/reduction reaction due to its high conductivity and foam structure, respectively. The Raman study reveals the formation of nanoscopic MoS2. The characteristic Raman peaks (Figure a) near 378 and 407 cm–1 associated with E2g1 and A1g active modes, respectively, ascertains the presence of MoS2 nanostructure.[37−39] The additional Raman peak near 450 cm–1 is associated with oxidation of MoS2 to MoO3 in the presence of laser light. Generation of hetero-dimensional MoS2 NDs can be attributed to the sonication assisted scission of MoS2 nanosheets. Here, sonication parameters were controlled to obtain MoS2 NDs. The standing waves induced during ultrasonic vibration vibrate the planar MoS2 nanosheets and prolonged sonication resulted in the formation of NDs, as observed in TEM images (Figure b,d). Hence, a simple bath sonication over a prolonged time led to the formation of MoS2 NDs. The XRD pattern of 3D MoS2 NDs/NF foam was shown in Figure b,c. It can be seen that the no characteristic peaks associated with MoS2 diffraction was observed and only peaks associated with Ni foam are present. This indicates that the existence of noncrystalline nanodots of MoS2 successfully impregnated on the Ni foam substrate. Moreover, the absence of any MoS2 specific peaks in short-range XRD (Figure c) indicates the existence of amorphous MoS2 NDs. For comparison, the XRD data of crystalline MoS2 and pristine MoS2 NDs are presented in Figure S2, indicating highly crystalline MoS2 nanosheets and amorphous MoS2 NDs. Similarly, the X-ray photoelectron spectroscopy and SEM–energy-dispersive X-ray spectrometry characterization indicates that the precursor MoS2 crystalline starting material is free from contamination (Figures S3 and S4). Furthermore, an amorphous material possesses low lattice energy and therefore enhances the utility ratio of active material. This allows easy de-intercalation process, which is beneficial in electrochemical performance of the electrode material.

Figure 2

Raman spectra recorded using 633 nm laser (a) and XRD pattern for pristine and 3D MoS2 NDs/NF (b,c).

Electrochemical Studies

Under three electrode configuration, the CV curves were obtained for scan rates varying from 10 to 100 mV/s. As observed in Figure a, presence of redox shoulder in CV plots indicates the redox reaction occurring between strong basic electrolyte, (KOH), and MoS2 NDs. During the faradaic charge transfer process, the ions such as H+ and K+ distribute into the stacked layer of MoS2 structure resulting in a reaction mechanism as shown in eq .[2]

Figure 3

Electrochemical characterization of MoS2 NDs in three electrode configuration. (a) CV profile of MoS2 NDs for varying scan rate. (b) Peak current density as a function of scan rate. (c) CD profile for varying current load. (d) Specific capacitance as a function of current load.

However nonfaradaic nature is exhibited along the electrode/electrolyte interface, as shown in eq .[2]

In addition, the CV curve area increases proportionally for scan rates 10–100 mV/s, implying the capacitive nature of the electrode. Besides, the shape of the CV plot more or less remains the same even at higher scan rates indicating good rate and capacitive performance of MoS2 NDs. Figure S5 provides a qualitative comparison between CV plots obtained for pristine nickel and MoS2 ND impregnated nickel foam. It can be concluded that the charge storage mechanism primarily happens in MoS2 ND-doped nickel foam and electrochemical inertness of utilized nickel foam. Figure d shows the change in specific capacitance for different current densities. The decrease in capacitance was observed with increase in current densities, which can be associated with an internal resistance of electrode material. MoS2 NDs demonstrated excellent Csp value of 395 F/g at 1.5 A/g. Several researchers have reported capacitive behavior of MoS2 nanostructures. Recent studies that include MoS2 nanoflakes reported by Karade et al.[2] reported a specific capacitance of 576 F/g at 5 mV/s. Ilanchezhiyan et al.[12] reported a capacitance value of 122 F/g at 5 mV/s in the 1 M Na2SO4 electrolyte. Ramadoss et al.[13] reported spherical aggregates of MoS2 nanostructures with a specific capacitance of 403 F/g at 1 mV/s. Linear relation obtained for cathodic and anodic peak current for the corresponding square root of scan rate (Figure b) indicates the diffusion-controlled behavior on the MoS2 ND electrode surface.[40] The latter behavior is observed very often in the presence of strong basic electrolytes (6 M KOH). The CD curves were obtained at varying current load (1.5, 2, 2.5, 3, 5, and 10 A/g) and showed triangular and symmetrical behavior indicating good capacitive behavior of electrode material. The specific capacitance of the MoS2 ND electrode was obtained using eq , from CD curve shown in Figure c. Specific capacitance (Cs) of MoS2 NDs obtained are 395, 147, 106, 86, 62, and 47 F/g for current loads 1.5, 2, 2.5, 3, 5, and 10 A/g, respectively (Figure d). Observed specific capacitance of MoS2 NDs (1.5 A/g) is significantly large than that of pure MoS2 (1 A/g) reported earlier 40, 98, 122, and 168 F/g, respectively.[27−30] This indicates that the MoS2 NDs exhibited superior performance compared to the aforementioned MoS2 peers. The observed high capacitance for MoS2 NDs was primarily attributed due to pseudo behavior in the presence of a strong basic electrolyte (6 M KOH), facilitated by MoS2 NDs uniformly impregnated on the surface of the nickel substrate via simple dip coating. In addition, nickel substrate provides a better conductive pathway for intercalation/de-intercalation of ions during redox reaction. Previously reported capacitive behavior of MoS2 electrode materials along with present work are tabulated in Table S1.

The EIS studies were performed to understand the charge transfer process occurring between the MoS2 ND electrode and electrolyte interface and resistance developed within the system (Figure a). The semicircle in the higher frequency region accommodates the electrode/electrolyte charge transfer resistance and electrolyte resistance.

Figure 4

(a) Nyquist plot of electrode material inset showing the equivalent circuit (b) assembly of symmetric supercapacitor and charge storage mechanism, schematics.

The linear line accounts for the diffusive resistance, known as Warburg impedance. The obtained slope is greater than 45°, suggesting good capacitive nature of the electrode material. The Nyquist plot obtained was fitted with an equivalent circuit (Figure a, inset) containing R and C components. An equivalent series resistance (Rs) ∼1.19 Ω was obtained from the intercept on real axis. Semi arc in the high-frequency region can be associated with double-layer capacitance Cdl and charge-transfer resistance, Rct ≈ 3.75 Ω at the electrode/electrolyte interface.[41] The presence of the Cf component implies the faradaic charge-transfer process occurring in the alkali medium. Similarly, the performance of the resulting 3D MoS2 NDs/NF was compared with the previously reported MoS2-based electrode materials for supercapacitor application and tabulated in the Table .

Table 1

Comparison of 3D MoS2 ND Performance with Preciously Reported MoS2 Electrode Materials

sample description	synthesis method	specific capacitance	electrolyte	references
MoS2 NDs	bath sonication/probe sonication	395 F/g @ 1 A/g	6 M KOH	present work
MoS2/graphene	hydrothermal, in cysteine solvent	243 F/g @ 1 A/g	1 M Na2SO4	(42)
MoS2/chemically modified graphene	hydrothermal reduction reaction	268 F/g @ 0.5 A/g	1 M Na2SO4	(43)
edge oriented MoS2 nanoporous film	oxidation of Mo foil by sulfur vapors	15 mF/cm2 @ 1 mA/cm2	1 M LiOH	(44)
flower like MoS2 nanostructure	hydrothermal	168 F/g @ 1 A/g	1 M KCl	(45)
MoS2 thin film electrode	physical deposition	330 F/cm3 @ 17 mA/cm2	0.5 M H2SO4	(46)

Symmetric Supercapacitor

To further investigate electrochemical properties of MoS2 ND as an electrode material, a symmetrical supercapacitor was fabricated as shown in Figure b. The charge storage mechanism is also illustrated in the schematics provided in Figure b. A 6 M KOH soaked filter paper was used as electrolyte and separator, respectively. The operational voltage of symmetrical supercapacitor was determined by investigating the CV plots of negative and positive MoS2 ND electrode material in a three electrode configuration (Figure a). As observed in Figure a, MoS2 ND electrodes exhibited nearly rectangular shape except toward the extreme potential, where instability of electrode material was pronounced. This can be attributed to hydrogen and oxygen evolution reaction, thereby narrowing the operating voltage.[47−49] Hence, the determined operating potential of 0.6 V was used for carrying out electrochemical analysis in a symmetric supercapacitor. The CV plot obtained for symmetric supercapacitor at 30 mV/s with the determined operational voltage is shown in Figure b. As observed, nearly a rectangular curve was obtained in a two-electrode configuration, confirming the capacitive nature of MoS2 ND supercapacitor.

Figure 5

(a) Operational voltage of cathode and anode at 30 mV/s. (b) Working potential of symmetric supercapacitor (30 mV/s).

Figure a shows the CV curves of MoS2 NDs symmetric supercapacitor for scan rates 20–100 mV/s, with an operational window of 0.6 V. As seen in Figure a, all of the curves are identical and exhibited an approaching rectangular shape indicating good rate capability of the two-electrode supercapacitor. Figure b shows the capacitance (Csp) obtained for MoS2 ND supercapacitor for different scan rates. Specific capacitance from CV plots was calculated using eq .where I(V)dV is the area enclosed in CV curves, v is the scan rate in V/s, and other units were as stated elsewhere in the text.

Figure 6

(a) CV plot for a symmetric supercapacitor (20–100 mV/s). (b) Specific capacitance obtained for varying scan rates. (c) Constant current charge–discharge performance of a symmetric supercapacitor. (d) Specific capacitance obtained for current load applied.

Specific capacitances obtained are 105, 100, 104, 102, and 100 F/g for scan rates 20–100 mV/s. The electrode material demonstrated excellent rate capability as there was a capacitance retention of 95% for varying scan rates (Figure b). Galvanostatic charge–discharge plots were obtained for MoS2 ND symmetric supercapacitor as shown in Figure c. A voltage drop of ∼0.25 Ω can be observed. The specific capacitances obtained using eq is 122, 98, 82, 80, and 48 F/g for current loads 1, 1.5, 2, 2.5, and 10 A/g, respectively (Figure d). MoS2 ND symmetric supercapacitor exhibited 65% of capacitance retention for current loads varying from 1 to 2.5 A/g.

Figure , illustrates the cyclic stability and Coulombic efficiency of MoS2 ND supercapacitor for an input load of 2.5 A/g. As seen in Figure a, as-assembled symmetric supercapacitor demonstrated a capacitance retention of ∼100% in the first 300 cycles and then begins to flatten. The electrode material demonstrated a capacitance retention of ∼86% over 1000 cycles. CD curve obtained in first and last few cycles are shown in Figure a, inset. Similarly, the MoS2 NDs displayed an excellent Coulombic efficiency of 100% at the end of 1000 CD cycles, which further highlights the advantage of the MoS2 ND electrode. Hence, we can conclude that the MoS2 ND symmetric supercapacitor exhibits good reversibility of energy storage. The Ragone plot obtained for the MoS2 ND supercapacitor is shown in Figure b. As observed, the fabricated supercapacitor exhibited high energy density without comprising much power density. A high energy density and power density of 22 W h/kg and 1300 W/kg were obtained for a current load of 1 A/g. The fabricated supercapacitor exhibited gravimetric energy and power density of 2.4 W h/kg and ∼10,000 W/kg, respectively, though the input current load was 10 A/g. The energy and power characteristics obtained in the present work were compared to previously reported work as shown in Figure b.[2,11,50−53] As illustrated in Figure b, present work demonstrated better energy density compared to Krishamoorthy et al.,[11] Javed et al.,[50] and Patil et al.[52] EIS was also performed on supercapacitor for a frequency range of 100 kHz to 1 Hz. Nyquist plot obtained before and after 1000 cycles is shown in Figure c. As observed, the Nyquist plot obtained exhibited a similar trend even after 1000 cycles. The slope of the linear curve in lower frequency that accounts for diffusive resistance (Warburg impedance) is greater than 45°, indicating the capacitive behavior.[2,11,30,41,54,55] An equivalent circuit for the same is provided in Figure c (inset). An equivalent series resistance (Rs) of ∼4.01 Ω was observed for supercapacitor before and after 1000 cycles, demonstrating the stability of fabricated supercapacitor.

Figure 7

Electrochemical properties of symmetric MoS2 ND supercapacitor (a) capacitance retention of supercapacitor for current load of 2.5 A/g and inset illustrating first few and last few cycles (b) Ragone plot for MoS2 ND supercapacitor. (c) Nyquist plot (d) Bode plot.

Bode plot shown in Figure d indicates, at lower frequencies phase angle (Φ) is closer to 90°, exhibiting the behavior of an ideal capacitor (Cdl). However, a deviation from the ideal capacitive behavior is associated with faradaic charge transfer process occurring as shown in eq .[2,54] The presence of Cf component in the equivalent circuit shown in Figure c; inset ascertains the redox nature of the symmetric supercapacitor.

Conclusions

In this preliminary study, we have demonstrated the fabrication of 3D MoS2 ND-impregnated Ni foam as binder-free electrode for supercapacitor applications. The sub-5 nm MoS2 NDs were successfully synthesized by controlled bath sonication of 2D nanocrystalline MoS2 sheets that were obtained via thermal annealing of the ATTM precursor. The fabrication of the electrode was achieved by simple and economical dip-coating procedure. The morphological and structural characterizations confirmed the formation NDs as well as successful deposition of MoS2 NDs on the 3D nickel substrate. The resulting electrode material exhibited a superior capacitance value (Csp) 395 F/g for an input load of 1.5 A/g compared to bulk and nano MoS2 based electrodes in a three-electrode configuration. In addition, a symmetric supercapacitor was also fabricated and successfully evaluated the supercapacitive performance of the electrode material. The latter demonstrated promising values (Csp = 122 F/g at 1 A/g) and a cyclic performance of 86% over 1000 runs. Furthermore, a high energy and power density of 22 W h/kg and 10,000 W/kg were achieved, respectively. Overall, the presented approach can be extended to the large-scale production of economical binder-free electrode based on MoS2 NDs for various energy storage applications.