High-Performance Supercapacitor Electrode Obtained by Directly Bonding 2D Materials: Hierarchal MoS2 on Reduced Graphene Oxide

Highlights

• The hierarchical MoS₂/rGO nanohybrids have been synthesized for supercapacitors.

• Pure phase and flower shaped MoS₂ nanosheets have been synthesized using a mild hydrothermal method.

• The electrode showed a high specific capacitance of 850 F/g.

• The electrode exhibited 95.3% retention in capacitance after 10,000 cycles at 2 A/g.

• The electrochemical impedance revealed low resistance (0.325 Ω) as well as a small frequency response (2.778 s).

Introduction

Pseudo-capacitors being an important class of supercapacitors are widely employed for efficient energy storage. They have attracted great scientific and technical attention owing to the possible integration of the benefits of lithium-ion batteries (high energy density) and capacitors (high power density). Their capability to undergo rapid and reversible redox reactions made them a strong candidate to accumulate charges as compared to double-layer capacitors (Winter and Brodd, 2004; Tang et al., 2015; Binazadeh et al., 2016; Ghosh et al., 2019; Lv et al., 2019; Miao et al., 2019; He et al., 2020). However, the energy density of most commercially available electrochemical capacitors is much lower than that of lithium-ion batteries, creating a substantial barrier to the practical application of such supercapacitors on a large scale (Jabeen et al., 2017; Liu et al., 2018a; Zhao et al., 2018). The energy density depends on the capacitance and the potential window by relation E = ½CV². Therefore, considerable efforts have been made to maximize “E” by enhancing “V” and “C.” Among various types of electrochemical capacitors, 2D materials-based supercapacitors like graphene and MoS₂, are gaining significant interest because of their intrinsic features, such as their high conductivity, low cost, flexibility, wide potential, window and fast redox reaction (Firmiano et al., 2014; Hao et al., 2014; Sun et al., 2015; Lv et al., 2016; Mehran and Baig, 2019; Muthu and Gopalan, 2019; Khan et al., 2020; Rani et al., 2020; Rathinamala et al., 2020; Zhao et al., 2021).

Molybdenum disulfide has been widely investigated for various applications such as catalysis (electrocatalysis and photocatalysis) for H₂ production, sensors, lubricating agents, transistors, and electrode materials for rechargeable batteries (Chhowalla and Amaratunga, 2000; Chen et al., 2001; Sun et al., 2004; Murugan et al., 2006; Chakraborty et al., 2012; Firmiano et al., 2012; Wang et al., 2012). As a distinctive layered sulfide of a transition metal, MoS₂ is comprised of three atomic layers (S-Mo-S), which are stacked on top of each other and connected through “Van der Waals” forces (Geim and Grigorieva, 2013; Molina-Sánchez et al., 2015). This flexibility is because of its 2D nature, which is analogous to the graphene (Matte et al., 2010). Numerous approaches have been made to produce MoS_2, such as the hydrothermal method (Chang and Chen, 2011b), thermal evaporation-exfoliation (Balendhran et al., 2012), physical vapor deposition (Lee et al., 2012), chemical vapor deposition (Shaw et al., 2014), chemical exfoliation (Sadan et al., 2008), mechanical exfoliation (Zhou et al., 2011), and wet chemical approaches (Altavilla et al., 2011). However, during the synthesis process, these methods led to the formation of buckyball-like nanoparticles or nanotube of MoS₂ (Rosentsveig et al., 2001; Rapoport et al., 2005). An effective way to resolve this issue and to get the layered structured MoS₂ is to employ carbon materials such as graphene, as a template (Chang and Chen, 2011; Hwang et al., 2011; Li et al., 2011; Wang et al., 2018; An et al., 2019; Li et al., 2019b). The graphene impedes the formation of three dimensional MoS_2, resulting in the 2D growth of MoS₂ on graphene nanosheets during the synthesis process (Chang and Chen, 2011a; Hwang et al., 2011; Li et al., 2011). Moreover, besides the elemental support of the graphene during the synthesis process, the coupling effects among MoS₂ and graphene could create a hybrid material with unique characteristics. These coupling effects are controlled by nature of chemical bonds among these materials (Xu et al., 2020; Yang et al., 2020; Zhao et al., 2020). MoS₂ could hypothetically store charges in three ways: 1) “intersheet” and 2) “intrasheet” double layer storage on single atomic layers by diffusion process, or 3) the faradic redox process on a molybdenum center, as molybdenum possess numerous oxidation states (+2 to +6), making it an ideal pseudocapacitive material, like RuO₂ (Soon and Loh, 2007). However, MoS₂ has a stumpy specific capacitance which might be attributed to its low electric conductivity. Recently, Yun Lu et al. demonstrated that this issue could be resolved using FeS₂@carbon as a template for synthesis of MoS₂. Based on the same approach, graphene could be employed as a substrate to grow MoS₂. Graphene oxide could act as an ideal template for the growth of MoS₂ to enhance its electrochemical properties due to excellent electric conductivity, high surface area, and stability, which provides excessive ion diffusion pathways among MoS₂ and electrolyte interfaces. Another way to improve the performance of supercapacitors is using the element doping strategy (Ghosh et al., 2020; Wang et al., 2020a). For example, Wang et al. (2020b) employed the strategy of multivalent and isostructural anion substitution chemistry to enhance the electrochemical performance of electrode materials.

Herein, we have fabricated hierarchal MoS₂ flowers on GO sheets using a simplistic hydrothermal scheme, and prepared hybrid nanostructures are employed as potential electrode materials for supercapacitor applications. We observed that the MoS₂ layers grown on rGO retain highly efficient charge storage properties with excessive capacitance and energy density properties. The remarkable capacitive performance of the MoS₂/rGO hybrid electrode is likely due to the integration of faradic and non-radical processes of active MoS₂ layers in connection with highly conductive graphene oxide layers.

Experiment

Fabrication of Pure MoS₂ and MoS₂/rGO Nanohybrids

For the preparation of pure MoS₂ and its hybrid with reduced graphene oxide, a facile hydrothermal method is adopted with prior ultrasonication treatment. Analytical grade ammonium heptamolybdate (NH₄)₆Mo₇O₂₄, thiourea (CH₄N₂S), polyvinyl pyrrolidone (PVP, K30, Mol. wt. 40,000) (C₆H₉NO)_n, graphite flakes, sulfuric acid, sodium nitrate, potassium permanganate and acetic acid (CH₃COOH) were used as received. Graphene oxide was synthesized by a modified Hummers method (Xu et al., 2008; Zhu et al., 2011). Prior to the reaction in a Teflon-lined autoclave, 30 mg GO was suspended in 50 ml water using probe sonication for 2 h. 10 mmol CH₄N₂S was added in GO suspension with continuous stirring for 20 min. A further 5 mmol (NH₄)₆Mo₇O₂₄ was dissolved in 25 ml water at ambient conditions, followed by the addition of 150 mg PVP. Both the solutions (GO suspension and AHM solution) were mixed, keeping the temperature to a low value using an ice bath, and subjected to ultrasonication for 30 min. Just before the transfer of the mixture to autoclave, 1.5 ml CH₃COOH was added to the mixture. The autoclave was kept in an oven at 180°C for 16 h. Later, black precipitates were filtered, washed using ethanol and water repeatedly, and dried at 80°C for 8 h. For pure MoS₂, same procedure is adopted but in the absence of GO. After that, the Ni foam substrates (1 × 1 cm²) were treated with a 3 M HCl solution followed by washing and drying at 60°C overnight. The powered electrode materials were drop cast on pre-treated Ni foam by making an ink (10 mg of powder in a mixture of 500 µL DI water, 450 µL isopropanol, and 50 µL Nafion, followed by 1 h sonication) until the desired mass loading was achieved. After deposition, Ni foam was dried overnight at 80°C.

Electrodes Physicochemical Measurements

Phase analysis was carried out by powder X-ray diffraction (“STOE-Seifert X’Pert PRO”) using “CuKα” radiation at 2θ values from 10° to 80°. The morphology of electrodes was examined using scanning electron microscopy (“JEOL-instrument JSM-6490 A”) and transmission electron microscopy. The electrochemical testing was executed at an ambient temperature by “VMP3 multi-channel potentiostat/galvanostat.” For the three-electrode system, MoS₂, and MoS₂/rGO were employed as a working electrode, Ag/AgCl was used in 1 M KCl solution for the reference electrode, and a platinum mesh was used as a counter electrode. Cyclic voltammetry was executed at different scan rates (2–50 mV/s). Galvanostatic charge-discharge was performed at various current densities (1–10 Ag⁻¹). Electrochemical impedance spectroscopy was accomplished at 5 mV in the frequency range from 0.1 Hz to 100 kHz.

Results and Discussion

Physicochemical Characterization

To elucidate the growth mechanism of hierarchical MoS₂ flowers, a schematic diagram is presented in Figure 1. The growing mechanism is consistent with preceding literature (Burda et al., 2005; Zhang et al., 2015) of the “three-stage growth” mechanism, which includes rapid nucleation of amorphous primary particles, oriented aggregation of nanosheets, and self-assembly of hierarchical structures. Hydrolysis of thiourea to H₂S leads to the formation of the MoS₂ nuclei that consequently grows into nanosheets corresponding to their crystal growth tendency. When many nuclei were synthesized, it could result in the reduction of nutrients in the reaction medium for endowing formation of MoS₂ nuclei, and subsequent reduction of nanosheets in dimensions. Afterward, these nanosheets intertwined and rolled longitudinally to create flower-like nanospheres, constrained by the lowering of surface energy. The succeeding chemical reactions (Eqs 1-4) were proposed to have occurred throughout the entire transition progression (Nagaraju et al., 2007; Feng et al., 2009; Ma et al., 2013; Sun et al., 2014; Zhang et al., 2015; Sun et al., 2016)

${\text{CSN}}_2{\text{H}}_4+2{\text{H}}_2\text{O}\to 2{\text{NH}}_3+{\text{CO}}_2+{\text{H}}_2\text{S} (1)$

${\left({\text{NH}}_4\right)}_6{\text{Mo}}_7{\text{O}}_{24}\to 6{\text{NH}}_3+7{\text{MoO}}_3+3{\text{H}}_2\text{O} (2)$

${\text{MoO}}_3+3{\text{H}}_2\text{S}+2{\text{H}}_2\text{O}\to {\text{MoO}}_2+{\text{SO}}_4^{2-}+{2\text{H}}^{+}(3)$

${\text{MoO}}_2+2{\text{H}}_2\text{S}\to {\text{MoS}}_2+2{\text{H}}_2\text{O} (4)$

FIGURE 1

FIGURE 1. Schematic illustration of the morphological evolution process of the 3D flower-like MoS₂ on rGO.

The crystal structure and phase purity of synthesized nanomaterials were examined using X-ray diffraction. As displayed in Figure 2A, diffraction peaks located at 2θ = 14.2, 33.5, 40.2, 49.3, 58.9, and 69.5 were evident in both patterns of bare MoS₂ and MoS₂/rGO nanohybrids, which are well indexed to (d₀₀₂), (d₁₀₀), (d₁₀₃), (d₁₀₅), (d₁₁₀), and (d₂₀₁) crystal planes of hexagonal phase MoS₂ (JCPDS no. 37-1492). The well-broadened peaks could be ascribed to a lesser thickness of MoS₂ nanomaterial, confirming poor crystallinity of the MoS₂ nanomaterial and more defects with decreased nanometer-scale sizes. Figure 2B presents the nitrogen adsorption desorption isotherms for both pure MoS₂ and MoS₂/rGO hybrids. Adsorption isotherms depict type-II adsorption behavior, representing a multilayer adsorption taking place on the surface of both samples. A long plateau before saturation represents multilayer adsorption till it reaches maximum near the saturation pressure. The calculated multipoint BET analysis showed that the specific surface area of MoS₂/rGO nanohybrids was found to be 391 m²/g, which is a significantly higher value as compared to pure MoS₂ (124 m²/g). The resulting pore size-distribution exhibits manifestation of abundant mesopores, as shown in Figure 2C. The porous structure is advantageous for fast electrolyte diffusion and mass transportation. The average pore size of pure MoS₂ was 24 Å, while for those of MoS₂/rGO (average pore size 58 Å) an increment of 24 Å in pore size was seen, whereas the pore volume for pure MoS₂ was 0.08 cm³/g and pore volume for MoS₂/rGO was found to be 0.347 cm³/g. The increase in pore size is presumably because of gaps among MoS₂/rGO nanosheets as evident from the SEM images. SEM images (Figures 3A,B)) display the flower-like morphology of the resulting MoS₂ nanosheets containing hundreds of curved MoS₂ nanosheets, leading to hierarchical flowers. These hierarchical flowers possess a diameter of 115 nm and a thickness of 3.42 nm without agglomeration. These nanosheets are tangled together, causing many voids between them due to their curved morphology. Such small-sized nanoflowers provide abundant edge sites for MoS₂ nanosheets. Based on the growth mechanism, it can be inferred that thiourea is disintegrated to sulfur ions and fastened on the surface of rGO via nucleation, forming the MoS₂ nanoflower that entirely overlays all surfaces of rGO. Figure 3C shows the TEM image of MoS₂ while Figures 3D–I shows MoS₂ wrapped by rGO sheets, forming a 2D-2D network. TEM images revealed that MoS₂ nanosheets were completely in-situ anchored on rGO, which provides cross-linking with MoS₂ developing a well-interconnected network. Moreover, MoS₂ nanosheets were a few layers thick, and the void among layers was 0.63 and 00.36 nm for MoS₂/rGO, which corresponds with d-spacing of 0.63 nm (002), estimated by X-ray diffraction. Extended interlayer spaces are extremely advantageous in enhancing the kinetic capability of rapid and reversible intercalation/deintercalation of anions. The fringes twisted from the straight track could be elucidated as lattice defects in MoS₂ and rGO. These defects have the capacity for more electrolyte ions which might improve all electrochemical properties by delivering enough electroactive sites and extra electrical conductivity. This 2D-2D architecture is also advantageous to enhance the surface area of the nanocomposite. Moreover, the overlapping or merging of rGO creates an interrelated conducting framework and aids in fast electron transportation during a redox reaction. This 2D-2D nanostructure also improves the stability of MoS₂/rGO composites owing to the flexibility and toughness of rGO.

FIGURE 2

FIGURE 2. (A) XRD patterns of pure MoS₂ and MoS₂/rGO. (B) BET N₂ adsorption-desorption isotherms for pure MoS₂ and MoS₂/rGO nanocomposites. (C) Pore volume and pore size distribution for pure MoS₂ and MoS₂/rGO nanocomposites.

FIGURE 3

FIGURE 3. (A,B) SEM images of pure MoS₂(C) TEM images of pure MoS₂(D–I) TEM images of MoS₂/rGO nanocomposite.

Electrochemical Measurements

CV profiles of MoS₂ and MoS₂/rGO on Ni foam evaluated in scan rates from 2 mVs⁻¹ to 50 mVs⁻¹ are presented in Figures 4A,B. Cyclic voltammograms revealed a combination of rectangular (double-layer reaction) and redox peaks indicating the pseudocapacitive behavior with a fast-reversible redox reaction of MoS₂. A similar trend can be seen in most of the reported literature (Firmiano et al., 2014; Ji et al., 2015; Liu et al., 2018b; Tian et al., 2019). The CV curves retained their shapes after 50 mV/s, suggesting the good high rate electrochemical properties such as high stability, fast electrolyte ions diffusion into the active site, and good reversibility. Moreover, the current density of the MoS₂/rGO electrode has been improved greatly due to the addition of rGO instead of the pure MoS₂ electrode. The enhanced current density could be ascribed to high conductivity and effective utilization of active sites of MoS₂ after the growth on rGO (Tian et al., 2019). The peaks in the CV profiles of MoS₂-based electrodes could be ascribed to the redox reaction of layer structure MoS₂ i.e. Mo-IV ↔ Mo-V ↔ Mo-VI as per the following equations (Eqs 5and6) (Soon and Loh, 2007; Tian et al., 2019; Sun et al., 2020).

${\left({\text{MoS}}_2\right)}_{\text{Surface}}+{\text{K}}^{+}+{\text{e}}^{-}\leftrightarrow {\left({\text{MoS}}_2-{\text{K}}^{+}\right)}_{\text{Surface}}(5)$

${\text{MoS}}_2+x{\text{e}}^{-}+x{\text{K}}^{+}\leftrightarrow \left(\text{MoS}-\text{SK}\right)(6)$

FIGURE 4

FIGURE 4. Electrochemical performance of as-synthesized electrodes in a three-electrode configuration. (A) CV curves of MoS₂ on Ni foam. (B) CV curves of MoS₂/rGO nanocomposite on Ni foam. (C) Variation of peak current with scan rate and variation of peak current with square root scan rate (inset) for MoS₂/rGO nanocomposite on Ni Foam. (D) Histogram of different electrode materials.

The non-faradic process is ascribed to the adsorption and desorption of electrolyte ions (K⁺ ions) on the surface and intrasheet/intasheet of MoS₂ layers. The faradic reaction, on the other hand, is favored by Mo atoms. To further verify the charge storage mechanism and kinetics of oxidation and reduction scan, sweep voltammetry was performed by manipulating CV curves at different scan rates. Figure 4C displays a plot of anodic and cathodic peaks current as a function of scan rate with the inset as the square root of the scan rate for MoS₂/rGo. The peak current is determined using “Randles−Sevcik Equation (Eq. 7)” (Sarkar et al., 2018)

$i_P=0.4961 nFAC{\left(\frac{nFvD}{RT}\right)}^{\frac{1}{2}}(7)$

where “n” is the number of electrons transferred, “i_p”_P is peak current, “F” is Faraday constant, “A” electrode area, “C” is ions concentration, “D” is diffusion coefficient; and “v” represents scan rate. For the non-diffusion controlled process, the trend among scan rates and peak currents should be a straight line (Hu et al., 2009). Figure 4C shows a non-linear relation among scan rates and peak currents, indicating a diffusion-controlled process instead of a non-diffusion process (Hu et al., 2009). For comparative study, peak currents were independently sketched in Figure 4D which showed that both cathodic and anodic current was highest for MoS₂/rGO. However, the anodic current is higher as compared to the cathodic current, which could be due to the kinetic limitations of K+ ions over electrons (Kulkarni et al., 2014). rGO allows effective intercalation/de-intercalation of K+ ions by delivering additional conductive paths, ensuring a fast transference of charges through the interface which enhances all electrochemical properties (Gopalakrishnan et al., 2015). The specific capacitance “$C_s$” was computed by the equation below (Eq. 8) from CV curves (Kim et al., 2013)

$C_s=\frac{{{\displaystyle \int }}^{\text{}}idV}{2m{V}_s\Delta V}(8)$

Where “${{\displaystyle \int }}^{\text{}}idV$” is the integral area under the CV curve,” “m is the active mass of electrode material, “$V_s$” is the scan rate, and “$\Delta V$” is the potential window. The maximum capacitance of MoS₂ and MoS₂/rGO were found to be 281 F/g (62.5 mAh/g or 225 C/g) and 846 F/g (153 mAh/g or 550 C/g) at 2 mV/s, respectively.

“Trasatti method” was applied for exploring the electrochemical kinetics of active materials and processes through which charges are stored in electrodes (Ardizzone et al., 1990). This analysis relies on computing stored, charges precisely on external and internal surfaces of electrodes. The overall amount of charges stored is the sum of both surfaces’ charges stored as per the following equation (Eq. 9)

$q_T=q_i+q_o(9)$

Where q_T is total charges and q_o and q_i are charges on external and internal surfaces. The amount of charges on the external surface is an adsorption mechanism and does not depend on the scan rate. Whereas, at the internal surface, the quantity of charges is a “diffusion-controlled” mechanism. Therefore, the overall volumetric charges are dependent on scan rates as per the equation mentioned in our previous report (Baig et al., 2020). Figure 5 showed the plot between scan rates and the quantity of charges stored on the surface. From Figures 5A,D overall charges “$q_T$” could be estimated after the extrapolation of scan rates to zero, whereas “$q_o$” could be estimated after extrapolation of scan rates to infinity as shown in Figures 5B,E. Figured 5C,F showed capacitive (surface effects) and adsorption contributions of MoS₂/rGO and pure MoS₂ electrodes at various scan rates. The estimated capacitive contributions are significantly greater than a diffusion-controlled contribution at the same scan rates, demonstrating the highly efficient surface storage. The total charge contribution of the composite electrode was around 322 C/g, which is approximately 1.3 times greater than the pure MoS₂ electrode. Furthermore, the MoS₂/rGO electrode showed the capacitive contribution of 99% slightly increased from the MoS₂ counterpart, which accounts for 98% capacitive contribution. This means that most of the surfaces are fully accessible to electrolyte ions during the charge/discharge processes due to the increased interlayer spacing of MoS₂ layers resulting from the prevention of restacking of MoS₂ layers in the MoS₂/rGO electrode (Yan et al., 2017). The enhanced performance of the composite electrode could be attributed to high conductivity and efficient use of active sites of MoS₂ after the growth on rGO. Moreover, by increasing the scan rates, the diffusion-controlled process decreases due to a remarkable decline in ions’ diffusion time into the lattice, resulting in low overall capacity at high scan rates (Yan et al., 2017; Jiang et al., 2018).

FIGURE 5

FIGURE 5. Electrochemical performance of as-synthesized electrodes in a three-electrode configuration. (A) The inverse of the charge stored vs. the square root of the scan rate for MoS₂/rGO nanocomposite on Ni Foam. (B) The charge stored vs. the inverse of the square root of the scan rate for MoS₂/rGO nanocomposite on Ni Foam. (C) The capacity contribution at different scan rates of MoS₂/rGO nanocomposite on Ni Foam. (D) The inverse of the charge stored vs. the square root of the scan rate for pure MoS₂ on Ni Foam. (E) The charge stored vs. the inverse of the square root of the scan rate for pure MoS₂ on Ni Foam. (F) The capacity contribution at different scan rates of pure MoS₂ on Ni Foam.

GCD analysis was executed to authenticate the rate capabilities of electrode materials at current densities from 1 A/g to 10 A/g and is presented in Figure 6. Figures 6A,B signifies the GCD profile of MoS₂ and MoS₂/rGO (vs Ag/AgCl) at 1–10 A/g. The MoS₂/rGO/Ni has a remarkably higher discharge time (459 s) as compared to its counterpart, which has a discharge time of 266 s at 1 Ag⁻¹. The GCD analysis shows that, with the increase in current densities, discharge time decreases. This is because electrochemical kinetics could not cope with fast fluctuations in potential because of the sluggish transport of ions and ineffective utilization of active materials (Azad et al., 2020; Zhao et al., 2020). The charging profiles are nearly straight and identical to the discharge counterparts, which again reveals the outstanding reversibility of the MoS₂/rGO electrode. The Ohmic drop (“IR drop” or “ESR”) is insignificant even at greater current densities, demonstrating the exceptional electronic conductivity of electrodes. This low “Ohmic drop” could be because of the rapid intercalated/deintercalated phenomena of K⁺ and fast electrons transfer. In comparison, both capacitance (F/g) and capacity (mAh/g and C/g) were used. The maximum capacitance of MoS₂ and MoS₂/rGO was found to be 297 F/g (66 mAh/g or 238 C/g) and 850 F/g (153.5 mAh/g or 552.5 C/g) at 1 A/g respectively, and are presented in Figure 6C. The rate capability of MoS₂/rGO can be attributed to the increased interlayer spacing caused by the incorporation of rGO nanosheets in between MoS₂ layers while maintaining its good conductivity. These characteristics further improve the diffusion and transportation of electrolyte ions and increase the surface area accessible to the electrolyte ions. Table 1 presents the evaluation of capacitance from previously reported literature. Figure 6D showed the stability test of MoS₂/rGO was executed by periodic galvanostatic charge-discharge cycles for 10,000 cycles at 2 A/g. The MoS₂/rGO nanocomposite exhibited an excellent capacity retention of 95.3% after 10,000 cycles, whereas the MoS₂ exhibited a retention rate of only 79.4%. The excellent cyclic stability of the composite electrode is the direct result of the excellent mechanical and chemical properties of rGO. This might also be a result of the substrate which helps in sustaining the electrode structure and conductivity by having good compatibility with the electrolyte.

FIGURE 6

FIGURE 6. Electrochemical performance of as-synthesized electrodes in a three-electrode configuration. (A) Galvanostatic charge-discharge curves of pure MoS₂ on Ni Foam. (B) Galvanostatic charge-discharge curves of MoS₂/rGO nanocomposite on Ni Foam. (C) Gravimetric capacitance from GCD curves as a function of scan rate for MoS₂/rGO nanocomposite and MoS₂ on Ni Foam. (D) Cyclic stability of MoS₂/rGO nanocomposite measured at 2 A/g.

TABLE 1

TABLE 1. Specific capacitance of MoS₂-based electrodes reported by different groups.

Electrochemical impedance spectroscopy was employed to assess charging kinetics in relation to capacitive behavior. Figure 7A shows the Nyquist plot of synthesized electrodes. The inset in Figure 7A presents the equivalent circuit elements (Azad et al., 2020), where “R₁” is the ohmic resistance of the solution between the working and reference electrode, “R₂” is the polarization or the charge transfer resistance at the electrode and solution interface, “Q₂” is the constant phase element at this interface, “W₂” is Warburg impedance, and “C₃” is the faradic capacitance. Usually, a semi-circle (dia of this circle gives R_ct) could be observed in the “Nyquist plot” at higher frequencies, because of charge transfer resistances caused by faradic reactions. The R_ct values were 0.6542 and 0.3256 Ω for MoS₂ and MoS₂/rGO respectively (Table 2). The solution resistances for the MoS₂ and MoS₂/rGO were 1.089 and 0.785 Ω, respectively. The sudden ascend in impedance from the imaginary axis can be observed at the middle frequencies zone. This sudden rise gives “Warburg impedance” which is inclined at 45°, demonstrating a pseudo capacitance nature. The bode plot of MoS₂ and MoS₂/rGO is presented in Figure 7B. The impedance features of the supercapacitor fall among ideal resistors (phase angle 0°) and ideal capacitors (phase angle 90°) (Azad et al., 2020; Baig et al., 2020). The phase angle for MoS₂/rGO and MoS₂ was 79.8° and 65.6°, representing the outstanding pseudocapacitance performance of active materials. The charge storage mechanisms were additionally verified via normalize $C^{\prime }\left(\omega \right)$ and $C^{″}\left(\omega \right)$ responses as function of frequencies (Figures 7C,D) by means of a complex capacitance model (Jiang et al., 2018). The relaxation time for the MoS₂ electrode is 3.108 s, whereas for MoS₂/rGO the relaxation time is 2.778 s, demonstrating the fast frequency response and ion diffusion of the MoS₂/rGO. The electrochemical performance of synthesized electrodes is summarized in Table 3.

FIGURE 7

FIGURE 7. Electrochemical performance of as-synthesized electrodes in a three-electrode configuration. (A) Nyquist plot of MoS₂/rGO nanocomposite and MoS₂ on Ni foam. (B) Bode phase angle plot of MoS₂/rGO nanocomposite and MoS₂ on Ni foam. (C) Normalized real and imaginary capacitance vs. frequency of MoS₂ on Ni foam. (D) Normalized real and imaginary capacitance vs. frequency of MoS₂/rGO nanocomposite on Ni foam.

TABLE 2

TABLE 2. Equivalent circuit parameters of as-synthesized electrodes.

TABLE 3

TABLE 3. Summary of the electrochemical performance of synthesized electrodes.

Conclusion

The hierarchical MoS₂/rGO nanohybrids have been synthesized using a facile wet chemical approach as a potential electrode material for supercapacitor applications. The results revealed that the hierarchical nanohybrids have a significantly higher capacitance as compared to their single counterparts. The MoS₂/rGO electrode exhibit maximum capacitance of 850 F/g at 1 A/g, with capacitive retention of 95.3% after 10,000 cycles at a current density of 2 A/g. Furthermore, the electrochemical impedance revealed low resistance (0.325 Ω) as well as small frequency response (2.778 s) for the MoS₂/rGO electrode. rGO enhanced the electrochemical performance by offering conductive routes and excellent mechanical stability. The unusual synergistic effect in such a hybrid electrode is responsible for the efficient electron transfer process. The excellent performance of MoS₂/rGO is ascribed to the inherent electrochemical activity of individual rGO and MoS₂ by developing efficient boundaries among the two phases. The hierarchical 3D network supported on the NF substrate is also effective for exposure of numerous active sites, and thus, boosting the electrons/mass transference.

Data Availability Statement

All datasets presented in this study are included in the article.

Author Contributions

EP: Conceptualization, Supervision, Administration, Resources, Reviewing, and Editing. MB: Methodology, Investigation, Formal analysis, Writing- Original draft preparation, Validation, Data Curation. MY, and IG: Reviewing and Editing.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

Author EP would like to acknowledge HEC Pakistan for the financial grant 2017/HEC/NRPU/10482. MY would like to thank the National “Thousand Youth Talents” program of China and Ningbo 3315 program for the support. Author MB would like to acknowledge Indigenous Ph.D. fellowships for 5000 scholars, Higher Education Commission (HEC) Islamabad Pakistan (Phase-II Batch-V) for providing support for this work.

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