Hydrothermal synthesis of an MoS₂/MnO₂ nanocomposite: a unique 3D-nanoflower/1D-nanorod structure for high-performance energy storage applications

Abstract

In this study, an MoS₂/MnO₂ nanocomposite electrode with a novel 3D nanoflower/1D nanorod architecture is effectively synthesized using a straightforward, cost-effective hydrothermal process. The addition of the 1D MnO₂ nanorod offers a structural backbone, while the 3D MoS₂ nanoflower generates additional reactive active sites. The hybrid structure makes fast electron and ion movement possible, which also increases the capacity for charge collection. This leads to 199.12 F g⁻¹ specific capacitance at 40 mA g⁻¹ in a designed 3D-1D MoS₂/MnO₂(6 wt%) electrode and exceptional rate capability with a tremendous cycling life (95% capacitance retention after 10 000 cycles). This discovery paves the way for the low-cost and straightforward construction of a hierarchical nanocomposite electrode with improved charge storage and electrical conductivity in energy storage applications like supercapacitors.

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1. Introduction

Energy from renewable sources is becoming a major topic due to the fast expansion of the world economy, rapid consumption of fossil fuels, and global climate change. Using renewable energy sources, such as wind, water, sun, and others, is a path to reduce current pollution and energy problems.^1–4 However, a continuous energy supply from renewable sources is not accessible due to the unavailability of sunlight throughout the entire time period and the unreliability of wind and tides. An alternative way to maintain a sustainable energy future by using energy from renewable sources is to use advanced energy storage devices, such as fuel cells, batteries, and supercapacitors (SCs).^1,5–9 Among the available energy storage alternatives, SCs are now in the limelight because of their low price, longer cycle life, high density of energy, etc. and are suitable for applications that require rapid and higher-power energy.^10–12 Because of their increased power density, safety, benefits of high charge–discharge rate, outstanding cycle stability, extended cycle life, low-temperature operation, low maintenance cost, etc., supercapacitors may replace batteries thus piquing the scientific and technological attention of many.^11,13–18 For such advantages, SCs find their applications in smart door locks, hybrid forklifts, cranes, smart grids, televisions, UPS (uninterruptible power supply) systems, electric vehicles, and other electronic devices.^19,20

A transition metal dichalcogenide called molybdenum disulfide (MoS₂) can be employed as an active component for designing energy storage devices, due to its electrical conductivity, surface area, and intrinsic ion conductivity which suggests good electrochemical properties.^21–25 The two-dimensional planar layered structure of MoS₂ is interconnected with sulfur–molybdenum–sulfur bonds. Mo atoms have different valence states (+2 to +6) and the interlayered space conducive to the electrolytic cation intercalation gives MoS₂ electrochemically pseudo-capacitance properties. But MoS₂ has moderate interfacial active sites to react with electrolyte ions, poor cycling stability, low energy density, and no stability in the interphase layer of electrolyte–solid, and it shows structural destruction especially aggregation during charge/discharge cycles, which are the drawbacks for supercapacitors.^26–28

Metal oxide nanoparticles such as Co₃O₄, NiO, V₂O₅, TiO₂, and MnO₂ have been added to the MoS₂ structure to enhance its capacitive properties.^29–35 Kanaujiya et al.²⁹ synthesized mesoporous MnO₂@MoS₂ nanocomposites using a versatile hydrothermal approach. Wang et al.³² have synthesized hierarchical MoS₂/Mn₃O₄ hybrid architectures where nanoparticles of Mn₃O₄ are uniformly incorporated into MoS₂ thin layers and achieved two times more capacitance than MoS₂ with 69.3% capacitance retention. Liao et al.³³ fabricated a MoS₂/MnO₂ (nanosheet/nanowire) hybrid using the lithography technique to analyze the interfacial impact on the storage of energy. But, MoS₂ nanosheet preparation methods take time and the yield is low. Those nanosheets have a restacking tendency at the time of charging–discharging that produces high surface energy and as a result, the electrical conductivity reduces.^36–40 To solve those drawbacks, a cost-effective procedure providing a higher yield, for example, a simple hydrothermal synthesis route, can play a vital role in producing a flower-like 3D-MoS₂ nanostructure. The MoS₂ nanoflower can give a high specific surface area, where a lot of interfacial active sites are present to react with electrolyte ions.²⁹ Due to the presence of a larger surface area and greater electrolytic ion migration active spots, the flower-shaped nanostructure has the ability to store more energy. Moreover, a longer cycle life can be maintained when it is used as the active material of electrodes.⁴¹ Additionally, the incorporation of nanoparticles into the flower-shaped structure may generate an increased number of active spots for the electrolytic ion diffusion, greater stability of the structure, and defect-rich surface.^42–45 However, as far as we know, the electrochemical behavior of nanocomposites made of a 3D MoS₂ nanoflower (NF) and 1D MnO₂ nanorods produced with various MoS₂ and MnO₂ weight ratios has not been thoroughly investigated.

In this study, an MoS₂ nanoflower/MnO₂ nanorod, a distinctive structurally combined nanocomposite, has been prepared to improve the capacitive performance of an MoS₂ nanoflower. Here, a hydrothermal method has been used to prepare MoS₂ nanoflowers and MnO₂ nanorods and to incorporate the rod-shaped MnO₂ nanoparticles into the MoS₂ nanoflowers in varying concentrations (2, 4, and 6 wt%). Manganese oxide (MnO₂) is a metal oxide semiconductor and has its own high theoretical capacitance, nontoxic nature, high cycling stability, electro-catalytic activity, lower cost, and abundance advantages.^46,47 MnO₂ has different polymorphs, and among them, our synthesized MnO₂ contains double chains of edge-sharing MnO₆ octahedral which exhibits high catalytic activities.⁴⁸ The addition of MnO₂ nanorods into MoS₂ nanoflowers may give more stability to the structure, rapid transportation of ions, and effective surface interactions with electrolyte ions; therefore, cycling stability and capacitance of MoS₂ nanoflowers may increase.⁴⁵

In this experiment, the physical and structural characteristics of synthesized samples were studied by FE-SEM (field emission scanning electron microscopy), HR-TEM (high-resolution transmission electron microscopy), and XRD (X-ray diffraction), respectively. FE-SEM results show that the as-synthesized MnO₂ has a 1D nanorod structure. The synthesized MoS₂/MnO₂ nanocomposites and pure MoS₂ have a 3D flower-shape morphology consisting of many nanolayered petals. HR-TEM results show that MnO₂ nanorods are present between the MoS₂ nanosheets in MoS₂/MnO₂ nanocomposites, and the interlayer spacing of MoS₂ increases with the MnO₂ concentration. XRD analysis reveals that the crystallite size decreases and the dislocation density increases for the MoS₂ NF when increased amounts of MnO₂ nanorod are added. Using a system of three electrodes, the electrochemical behavior of the as-synthesized samples was examined in 0.5 M Na₂SO₄ electrolyte at room temperature. Among all synthesized MoS₂/MnO₂ nanocomposites (NCs), MoS₂/MnO₂ (6 wt%) NC can store 199.12 F g⁻¹ at a 40 mA g⁻¹ current density. Here the density of energy was found to be 6.91 W h kg⁻¹ at a 10 W kg⁻¹ power density. At 170 mA g⁻¹ current density, the MoS₂/MnO₂ (6 wt%) NC undergoes 10 000 charge/discharge cycles and provides 95% capacitance retention which confirms its tremendous cycling stability. The MoS₂/MnO₂ nanocomposites show a higher capacitance than the bare MoS₂ electrode denoting that the 3D MoS₂ nanoflower incorporated with the MnO₂ nanorod gives a larger specific surface area, greater interlayer spacing, higher wettability, faster intrinsic electrical conductivity, improved electrolytic cation intercalation at the interface, and an overall defect-rich structure. The novelty of this work is the synthesis of 3D-nanoflower/1D-nanorod structural nanocomposites of MoS₂/MnO₂, and when compared (Table 5) to the capacitance of MoS₂ based supercapacitors that have been published in the different literature, our synthesized MoS₂/MnO₂ (6 wt%) NC shows good capacitance alongside capacitance retention. So, to prepare a high-performance energy storage device, this experiment can provide significant insights.

2. Section of experimentation

2.1. Chemical reagents

Sodium molybdate dihydrate (Na₂MoO₄·2H₂O), manganese sulfate monohydrate (MnSO₄·H₂O), potassium permanganate (KMnO₄), and dimethyl sulfoxide (C₂H₆OS) were obtained from Merck, Darmstadt, Germany. From Research Lab, India, thiourea (CH₄N₂S), sodium sulfate (Na₂SO₄) and polyvinyl alcohol (PVA) (C₂H₄O)_x were collected. Without any purification, analytical grade chemicals were used.

2.2. MoS₂ NF preparation

A clear precursor solution was prepared by dissolving 0.14 M sodium molybdate dihydrate and 0.65 M thiourea in 120 ml of deionized water. Then the solution was kept in a Teflon-lined autoclave for 24 hours at 200 °C. The resultant black precipitate was then collected from the autoclave and after drying at 70 °C for several hours the MoS₂ nanoflower was obtained.

2.3. MnO₂ nanorod preparation

5.6878 g of MnSO₄·H₂O and 2.3657 g of KMnO₄ were taken in 140 mL of deionized water. After stirring for 30 minutes, the solution was kept in a Teflon-lined autoclave for 12 h in an oven at 140°. After collecting the resultant brown precipitate, it was dried for a few hours at 80 °C yielding the MnO₂ nanorod.

2.4. Preparation of the MnO₂ incorporated MoS₂NF

Fig. 1(a) shows the schematic representation of nanocomposite preparation. At first, a desired amount of MnO₂ nanorod was added to 50 mL of deionized water and sonicated for 1 hour. After dissolving 0.14 M sodium molybdate dihydrate and 0.65 M thiourea in 70 mL of deionized water, the solution was transferred to the sonicated 50 mL MnO₂ solution. After stirring for 1 hour, the whole solution was kept in a Teflon-lined autoclave for 24 h at 200 °C. The desired black precipitate was collected and dried for a few hours at 80 °C. Various amounts of MnO₂ nanorods such as 2 wt%, 4 wt%, and 6 wt% were incorporated. The NCs are entitled as MoS₂/MnO₂ (2 wt%), MoS₂/MnO₂ (4 wt%) and MoS₂/MnO₂ (6 wt%) respectively.

Fig. 1 (a) Schematic representation of the synthesis of MoS₂/MnO₂ nanocomposites. (b) FE-SEM image of the MnO₂ nanorod at a low magnification.

2.5. Preparation of electrodes

To prepare working electrodes, a 100 μL slurry of the as-synthesized material was deposited on a cross-sectional surface (0.3 cm²) of a modified graphite electrode. To prepare the slurry, PVA (4% of the as-synthesized material) and dimethyl sulfoxide were added to the as-synthesized material. After sonicating for 1 h, the slurry was deposited and for drying, the working electrodes were kept at 70 °C. Here PVA acts as a binder and dimethyl sulfoxide works as a polar aprotic solvent.^49,50

2.6. Characterization

To get the microscopic information about the surface of the as-synthesized samples, FE-SEM (JSM 7600, Jeol) images were taken. Microstructural and morphological properties of the synthesized materials were also observed using HR-TEM (JEOL, JEM 2100 F) images. Using the radiation of CuK_α (λ = 1.5406 Å) of the X-ray diffractometer of 3040XPert PRO, Philips, the XRD data were obtained. The electrochemical capabilities of the as-synthesized samples were investigated by using a CS310 electrochemical workstation (Corrtest, China) in 0.5 M Na₂SO₄ electrolyte using a three-electrode system. Here the working electrode is a modified graphite electrode, the reference electrode is Ag/AgCl, and the counter electrode is a platinum plate (1 cm × 1 cm).

3. Results and discussion

3.1. Scanning electron microscopy

Fig. 1(b) presents the FESEM image of MnO₂ consisting of densely aligned disorderly arranged nanorods with several micrometers of length and a width of 47–98 nm. Some fragmented nanorods were observed and the formation of bundled nanorods could be seen. Under high magnification (Fig. S1, ESI†), the surface of the nanorods appears to be smooth.^45,51

From the FESEM image of MoS₂ (Fig. 2(a)), flower-like spherical 3D nanostructures were observed that confirm the successful synthesis of MoS₂ nanoflowers having a diameter between 5 and 6 μm. Those nanoflowers consist of plenty of aligned curved petals, which are assembled by several MoS₂ nanosheets, through a common inner center. The petal thickness is observed to be varied between 2 and 5 nm.⁵²Fig. 2(b–d) shows FESEM images of MoS₂/MnO₂ nanocomposites for different concentrations of MnO₂, where the curved petals of MoS₂ are more uniform and smoothly aligned. These nanoflowers with observed diameters in the range of 4–6 μm are in the cluster form or not dependent on each other.⁵³ When the MnO₂ nanorod was taken in the precursor solution of MoS₂, Mo⁴⁺ ions from the cationic precursor could be attracted on the surface of the MnO₂ nanorod because of van der Waals, cohesive or electrostatic, or other chemical forces.⁴⁵ S²⁻ ions from the anionic precursor combine with Mo⁴⁺ ions on the surface of the MnO₂ nanorod to form flower-shaped nanostructures.

Fig. 2 (a)–(d) FE-SEM images of MoS₂ NF, MoS₂/MnO₂ (2 wt%), MoS₂/MnO₂(4 wt%), and MoS₂/MnO₂ (6 wt%) nanocomposites, respectively.

From Fig. 2(b–d) we observe that the diameter of the MoS₂ nanoflower reduces with the addition of MnO₂ and the thickness of the petal for MoS₂/MnO₂(2 wt%) is in the range of 10–20 nm but for MoS₂/MnO₂(4 wt%) and MoS₂/MnO₂(6 wt%), the thickness got increased (20–40 nm) may be due to agglomeration of multiple layers of nanosheets.⁵⁴ Due to the increase of petal thickness, porosity and specific surface area would increase and as a result more electrical contact with the current collector would provide an excellent charge transfer rate and improvement in electrochemical capacity. As shown in Fig. 2(b and c), the existence of MnO₂ nanorods is not clearly observed which may be due to the low concentration of MnO₂. For higher concentrations of MnO₂, nanorods are clearly observed as shown in Fig. 2(d).

From EDX mapping shown in Fig. S2(a–d) (ESI†), the compositional information shown in Table 1 reveals that the surface of the MoS₂ NF is enriched with S atoms. In addition, in Fig. 2(b–d) the presence of dispersed fragmented particles on the surface of petals with Mo, S, Mn, and O elemental distribution from EDX mapping confirms that the petals consist of Mo and S and the fragmented particle has Mn and O. From the EDX results, we observe that the %atomic ratio of S and Mo was greater than 4, and this excess amount of sulfur atoms appeared as a result of many defects on the ultrathin MoS₂ nanosheets. The defective structures can provide more stability to the nanosheets by reducing the surface energy and generating more reactive sites.^55,56

Table 1 Elemental analysis from EDX mapping

MnO2 concentration (wt%) in the prepared samples	Mo (atom%)	S (atom%)	Mn (atom%)	O (atom%)	Mo : S
0	6.62	93.38	0	0	1 : 14
2	11.82	70.30	0.63	17.25	1 : 6
4	8.34	63.74	0.78	27.09	1 : 7
6	8.64	77.29	0.98	13.09	1 : 9

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3.2. Transmission electron microscopy

TEM images of MoS₂ NF and MoS₂/MnO₂ nanocomposites are shown in Fig. 3(a–d). From Fig. 3(a) it can be observed that the MoS₂ NF consists of many thin petals and those petals are composed of several MoS₂ nanosheets. As shown in Fig. 3(b–d), the curved and wrinkled nanosheets of MoS₂ are very thin and partially transparent. Light contrasts in various areas indicate the ultra-thin nature of the nanosheets. In the TEM image (Fig. 3(b–d)) of MoS₂/MnO₂(2, 4, 6 wt%) nanocomposites, the presence of MnO₂ nanorods between MoS₂ nanosheets is clearly visible. The transfer of electrons between MoS₂ and MnO₂ is facilitated due to the close integration between MnO₂ and MoS₂. Fig. 4(a–d) displays HR-TEM images along with the associated SAED (selected area electron diffraction) patterns. From the HRTEM image, in Fig. 4(a), we observed disorderly arranged lattice fringes and weak diffraction rings as shown in the inset SAED pattern confirming the poor crystallinity of the MoS₂ NF.⁵⁷

Fig. 3 (a)–(d) TEM images of MoS₂ NF, MoS₂/MnO₂ (2 wt%), MoS₂/MnO₂ (4 wt%) and MoS₂/MnO₂ (6 wt%) nanocomposites, respectively.

Fig. 4 (a)–(d) HR-TEM images of MoS₂ NF, MoS₂/MnO₂ (2 wt%), MoS₂/MnO₂ (4 wt%), and MoS₂/MnO₂ (6 wt%) nanocomposites, respectively. The insets of figures (a)–(d) show the corresponding SAED patterns.

The magnified HRTEM image (Fig. 4(a)) provides an interlayer spacing of 0.63 nm that agrees with the hexagonal MoS₂ (002) plane indicating the growth of hexagonal nanosheets along the vertical [002] crystal axis.⁵⁸ The interlayer spacings for MoS₂/MnO₂(2 wt%), MoS₂/MnO₂(4 wt%), and MoS₂/MnO₂(6 wt%) were found to be 0.64 nm, 0.65 nm, and 0.67 nm, respectively indicating a lattice expansion due to the addition of MnO₂.²⁸ Such an increase of interlayer spacing may be attributed to the diffusion of MnO₂ between MoS₂ layers.⁵⁹ The enlargement of interlayer spacing and unique nanoflower-nanorod morphology may provide better ions intercalation, more electroactive sites, and enhancement of electrolyte access that gives the desired electrochemical property with improved efficiency in attaining higher capacitance.^29,45,59

The SAED patterns for all the samples (inset of Fig. 4(a–d)) show bright concentric rings of diffraction that ensure the polycrystalline nature of the as-synthesized samples. Also, diffraction rings (inset of Fig. 4(a–d)) of the (100), (103), (105), and (110) lattice planes of MoS₂ are observed in all as-synthesized samples. Due to the low concentration of MnO₂, only lattice plane (521) is observed in (inset of Fig. 4(b and c)) MoS₂/MnO₂ (2 wt%) and MoS₂/MnO₂ (4 wt%). When the concentration of MnO₂ is increased, (400), (411), (541), and (600) lattice planes of MnO₂ nanorods are found in MoS₂/MnO₂ (6 wt%) nanocomposites.

The findings from the TEM analysis demonstrate that the MoS₂/MnO₂ nanocomposites were successfully prepared and confirm intimate contact between the MnO₂ nanorods and the MoS₂ NF. From the TEM images of the nanocomposite, discontinuity in the lattice fringes of the curled edges suggests that the crystals possess dislocations and defects.^53,56

3.3. X-ray diffraction

The XRD patterns were studied to confirm the phase, structure, and crystallinity of the MoS₂ nanoflower and MoS₂/MnO₂ nanocomposites, as shown in Fig. 5. As illustrated in Fig. 5(a), the hexagonal phase of MoS₂ (JCPDS No. 37-1492), which is a member of the space group P63/mmc, agrees well with the diffraction peaks of the pristine MoS₂. The peaks at 2θ = 14.04°, 33.49°, 39.46°, 48.57°, and 59.14° are attributed to the (002), (100), (103), (105) and (110) lattice planes, respectively.⁵³ Each reflection in the XRD pattern seen in Fig. S3 (ESI†) for the MnO₂ nanorod may easily be attributed to a pure tetragonal phase of the space group I4/m (no. 87), with a = 0.985 nm and c = 0.285 nm (for the lattice constants). The absence of peaks for any other manganese oxides suggests that the produced MnO₂ nanorods have a pure phase.⁶⁰ In Fig. 5(a) the sharp peak of pure MoS₂ nanoflower at 14.04°, which has a 0.63 nm d-spacing corresponding to the (002) plane, confirms that MoS₂ layers stacked toward the c axis during synthesis.⁶¹

Fig. 5 XRD patterns of pure MoS₂ NF and synthesized nanocomposites of MoS₂/MnO₂.

For MoS₂/MnO₂ nanocomposites, all the primary diffraction peaks of pure MoS₂ are present. Due to the low concentration of MnO₂ in MoS₂/MnO₂ (2 wt%), no diffraction peak of MnO₂ is observed. But the primary characteristic peaks of MnO₂ can be only clearly observed for MoS₂/MnO₂(4 wt%) and MoS₂/MnO₂(6 wt%) nanocomposites.^44,62,63 For MoS₂/MnO₂ NCs the broadened and weak diffraction of the (002) plane imply low crystallinity, indicating that the pristine MoS₂ NF has a larger average crystallite size than that of the MoS₂/MnO₂ nanocomposites.⁶⁴ A lower angle shift of the initial XRD peak occurs, which may be due to lattice distortion, indicating an increase in the MoS₂ interlayer distance in the nanocomposites.^64,65

The crystallite size (L), the density of dislocation (δ), and the microstrain (ε) of all samples were determined from the (002) peak using eqn (1)–(3), respectively.^66,67

δ = 1/L² (2)

where λ = X-ray wavelength, θ = diffraction angle, and β = diffraction peak's FWHM (full width at half-maximum).

At the (002) peak, the structural parameters were measured. Table 2 and Fig. 6 present the variation of structural parameters of the MoS₂ NF and MoS₂/MnO₂ NCs.^68–70 The crystallite size was found to be decreased from 7.51 nm to 5.59 nm when the concentration of MnO₂ increased from 2 wt% to 6 wt%.^68–70 Because the presence of MnO₂ nanorods helps to grow cross-linked MoS₂ nanosheets.^71,72 Microstrain and dislocation density increase with the concentration of MnO₂ suggesting increased defect states like dislocation generation, imperfect crystal structure, and vacancies in the MoS₂ NF.^73,74 Using the following equations, the values of lattice constants were calculated:

where a, b, and c denote lattice constants, d = atomic plane spacing, λ = wavelength (1.5406 Å), (h, k, l) denotes miller indices, and θ = incident angle. The calculated values (in Table 1) of lattice constants are less than standard values which represent less volume of lattice cell.⁷⁵ Increased dislocation density, smaller crystallite size, and difference in d spacing create lattice distortion. As a result, defects in lattice occur which gives profound active sites with an improved surface area, and at the time of charging/discharging, the charge transfer rate increases between active materials and solution.^76–79

Table 2 XRD study revealed the diffraction parameters for pristine MoS₂ NF and MoS₂/MnO₂ NCs

Synthesized particles	β (radian)	L (nm)	d (Å) (002)	d (Å) (100)	a = b (Å)	c (Å)	ε × 10^−3	δ × 10^−3 (nm^−2)
MoS2	1.12	7.51	6.30	2.672	3.08	12.60	39.41	17.71
MoS2/MnO2(2 wt%)	1.30	6.40	6.35	2.675	3.09	12.70	46.65	20.07
MoS2/MnO2(4 wt%)	1.34	6.23	6.34	2.671	3.08	12.68	47.85	20.61
MoS2/MnO2(6 wt%)	1.49	5.59	6.37	2.674	3.09	12.75	53.58	23.01

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Fig. 6 The variation of several X-ray diffraction parameters for different wt% of MnO₂.

3.4. Electrochemical performance analysis

3.4.1. Cyclic voltammetry. The cyclic voltammetry (CV) plots of the MoS₂ NF and MoS₂/MnO₂ NCs were analyzed with the applied potential between −0.3 and 0.2 V at the scan rates of 5, 10, 20, 30, 40, 50, and 70 mV s⁻¹ as shown in Fig. 7(a–d). For all the samples quasi-rectangular shapes of CV curves without any redox peaks were observed. The area for the CV plots of MoS₂/MnO₂ nanocomposites is larger than that of MoS₂ NF indicating an increase in capacitance due to the incorporation of MnO₂.

Fig. 7 (a)–(d) CV plots of pure MoS₂ NF and MoS₂/MnO₂ NCs at various scan rates.

Na⁺ cation has a great impact on the charge–discharge electrochemistry of the MoS₂ NF and MoS₂/MnO₂ nanocomposites. A mechanism could be suggested as follows:^57,80–83

MoS₂ + Na⁺ + e⁻ ↔ (MoS₂) − Na⁺ (surface)

MnO₂ + Na⁺ + e⁻ ↔ (MnO₂ − Na⁺) (surface)

(MoS₂/MnO₂)_surface + Na⁺ + e⁻ ↔ ((MoS₂/MnO₂) − Na⁺) (surface)

MoS₂ + Na⁺ + e⁻ ↔ Mo S − S Na (intercalation)

MnO₂ + Na⁺ + e⁻ ↔ MnO − ONa (intercalation)

MoS₂/MnO₂ + Na⁺ + e⁻ ↔ (MoS₂/MnO₂)Na (intercalation)

For a higher concentration of MnO₂ (6 wt%), the shape of the CV curve deviates from the rectangle shape. Such leaf shape of CV curves indicates the presence of electrical double-layer capacitance (EDLC).^84–86

3.4.2. Galvanostatic charge–discharge. The galvanostatic charge–discharge (GCD) measurements of the MoS₂ NF and MoS₂/MnO₂ NCs were taken between −0.30 V and 0.20 V at different current densities. A little deviation from the triangular and symmetrical form together with a small iR drop is observed in the GCD curves (Fig. 8(a–d)). This indicates the high reversibility of the materials and confirms the pseudocapacitive properties of the samples.⁸⁷ In the GCD curves, charging and discharging periods shorten as the density of current rises because surface adsorption and diffusion of electrolyte ions become slower into the electrode active material.^29,88,89 At low current densities, due to slow charge accumulation, access of electrolyte ions to active sites on the electrode increases through diffusion, thus providing a complete insertion/extraction reaction as well as better specific capacity.^88–90Fig. 8(e) shows GCD curves of all samples at a current density of 0.05 A g⁻¹ to compare their electrochemical performances. The specific capacitance (C_s) was estimated from the GCD curves using the formula:²⁹
where m = mass of active materials, Δt = discharge time, i = discharge current, and ΔV = potential window width. Fig. 9 shows the specific capacitance of the synthesized materials as a function of current densities. At 40 mA g⁻¹ current density, the specific capacitance (Fig. 9) obtained from the GCD curves was 199.12 F g⁻¹, 57.74 F g⁻¹, 48.11 F g⁻¹, for MoS₂/MnO₂ (6 wt%), MoS₂/MnO₂ (4 wt%), MoS₂/MnO₂ (2 wt%), respectively, while only 12.73 F g⁻¹ for the MoS₂ NF. Here, the MoS₂/MnO₂(6 wt%) NC shows the greatest specific capacitance of all the synthesized materials.

Fig. 8 (a)–(d) Galvanostatic charging–discharging curves of MoS₂ NF and MoS₂/MnO₂ NCs at various current densities. € GCD measurements at 0.05 A g⁻¹.

Fig. 9 Specific capacitance of MoS₂ NF and MoS₂/MnO₂ NCs at various current densities.

In the GCD curve, iR drop during discharging gives the intrinsic resistance characteristics of the samples.⁸⁷ At a high discharge current density of 140 mA g⁻¹(Fig. 10), for MoS₂/MnO₂ NCs short discharge occurs because of electrical double-layer capacitance. For longer discharge electric double-layer capacitance and faradaic capacitance are responsible.⁸⁷ The MoS₂/MnO₂ (6 wt%) nanocomposite showed the least iR drop of all the nanocomposites, suggesting less internal resistance of the active materials. During charging–discharging, energy dissipation was eliminated for lower internal resistance materials, and as a result, the energy storage performance improved.^87,91 So, to fabricate power-saving supercapacitors, the MoS₂/MnO₂ (6 wt%) nanocomposite is preferable.

Fig. 10 GCD curves of MoS₂ NF and MoS₂/MnO₂ NCs at 140 mA g⁻¹.

The energy density (E) and power density (P) were determined from the GCD curves of the synthesized samples by using eqn (4) and (5), respectively⁹²

where ΔV = voltage window width, C_s = specific capacitance, and Δt = discharge time.

Fig. 11 illustrates the Ragone curve for the MoS₂ NF and MoS₂/MnO₂ NCs and the values are presented in the supercapacitor region.⁹³

Fig. 11 Ragone curve (power density vs. energy density).

3.4.3. Electrochemical impedance spectroscopy. To investigate the capacitive and resistive properties, electrochemical impedance spectroscopy (EIS) was performed for the MoS₂ NF and MoS₂/MnO₂ NCs electrodes between 0.1 Hz and 100 kHz. Fig. 12 shows the Nyquist plots of the prepared materials. In that curve, the semicircular portion denotes the intrinsic resistance of electrodes, the middle frequency (Warburg zone) denotes the interaction between electrolyte ions and porosity of the material, and at low-frequency sudden increment in the values of impedance imaginary part can be denoted by the double-layer capacitive region.⁹⁴ The interception point in the high-frequency zone at the real impedance axis determines the effective series resistance value, and it can be represented as solution resistance (R_s).⁹⁵ EIS curves show that when the amount of MnO₂ increases in the MoS₂ NF, there is no interception point at the real impedance axis in the high-frequency zone. This suggests that the MoS₂ NF has more solution resistance than MoS₂/MnO₂ NCs.²⁹ So an increased concentration of MnO₂ in the MoS₂ NF has given better charge collection ability and faster ion transportation between electrode active materials and electrolytes.⁸⁴ The inset of Fig. 12(b) shows an equivalent circuit applied for EIS curve fitting (using ZView software). Here in the equivalent circuit W, R_ct, R_s, and CPE denote the Warburg impedance, charge transfer resistance, solution resistance, and constant phase element, respectively. The equivalent simulated curve is displayed in Fig. S4 (ESI†), from which all the different parameters of the equivalent circuit were determined and are listed in Table 3.

Fig. 12 (a) Nyquist curves of the synthesized samples. (b) High frequency zone Nyquist curves of the synthesized samples. The inset shows the equivalent circuit design of Randle's model.

Table 3 Fitting values of EIS for pure MoS₂ NF and MoS₂/MnO₂ NCs

Sample	R ct (Ω)	CPE-T (μF)	CPE-P	W (Ω)
MoS2	2.6	0.0835	0.99	10.16
MoS2/MnO2(2 wt%)	2.5	0.2520	0.95	6.55
MoS2/MnO2(4 wt%)	2.7	0.2600	0.98	6.39
MoS2/MnO2(6 wt%)	2.5	0.3309	1.00	5.93

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Table 3 shows that the R_ct (Ω) value has decreased and the CPE-T (μF) value has increased for high concentrations of MnO₂ in the MoS₂ NF indicating good electrical conductivity, high wettability, higher rate capability, increased reaction rates of the NC electrode, and better efficiency.^96,97 Quasi-semicircles and low R_ct are the results of double-layer capacitance as well as faradaic reactions (redox) on the electrode surface.⁹⁵ Due to enlarging interlayer distance the electrolyte gets easy access to the active material; as a result charge transfer resistance decreases when the concentration of MnO₂ increases in the MoS₂ NF.⁹⁸ Here CPE is utilized instead of a capacitor because of the compensation of the lack of uniformity in the process and represents the nonideal capacitive property, as, at the microscopic level the electrode surface is porous and rough.⁹⁸ In fact, the porosity and roughness of the surface may create a capacitance of double-layer type where the value of CPE-P in the range of 0.9 and 1 can show up as a constant phase element, and from Table 3 it can be seen that the CPE-P value of our NC samples satisfies the range. Also, when the MnO₂ concentration increases in the MoS₂ NF, the value of W (Ω) decreases. It denotes a shorter diffusion path and reduced diffusive resistance for MoS₂/MnO₂ NCs giving better electrochemical performance.⁹⁹

From the XRD data (Table 1) the MoS₂/MnO₂ (6 wt%) nanocomposite has the highest amount of dislocation density. Improvement in the double-layer capacitive property occurs due to higher dislocation density. Increased dislocation density reduces charge transfer resistance because of the smaller phase angle, so faster electron movement occurs between the active material and the electrolyte.^98,100

So, from the EIS data, we get that MoS₂/MnO₂ (6 wt%) NC shows better capacitive performance and this finding is compatible with CV and GCD tests.

3.4.4. Stability analysis. The stability analysis was performed for the MoS₂/MnO₂(6 wt%) sample as it provides better capacitive performance. Fig. 13(a) represents the cyclic stability plots taken up to 10 000 charge/discharge cycles at a current density of 170 mA g⁻¹. From Fig. 13(a) we observed that after 10 000 cycles retention was around 95%. The nanocomposite also gives an excellent coulombic efficiency of 106% as shown in Fig. 13(a). This superior coulombic efficiency as well as cycling stability can be attributed to the increased wettability, stability in structure, and faster ion diffusion.¹⁰¹ The Nyquist plots for the 1st cycle and 10 000th cycles are presented in Fig. 13(b) which demonstrates that after 10 000 cycles the radius of the semi-circular portion of the EIS spectrum becomes lower. This indicates reduced charge transfer resistance of the active materials after 10 000 cycles.^96,97 The insets of Fig. 13(b) show the fitted result and the values of the corresponding equivalent circuit components are presented in Table 4. R_ct and W values were found to be reduced after 10 000 cycles of operation which indicates the generation of more active sites and faster electron transport between the electrode and solution.^102,103

Fig. 13 (a) Coulombic efficiency and capacitive retention of the MoS₂/MnO₂ (6 wt%) nanocomposite over 10 000 cycles of charging–discharging, (b) Nyquist plots of the MoS₂/MnO₂ (6 wt%) nanocomposite before and after 10 000 cycles of charging–discharging. The insets show simulated and experimental Nyquist plots.

Table 4 Fitting EIS values for the MoS₂/MnO₂ (6 wt%) nanocomposite found in the first and ten thousandth cycle

Cycle	R ct (Ω)	CPE-T (μF)	CPE-P	W (Ω)
1	2.50	0.33	1.00	5.93
10 000	1.90	0.33	0.98	5.90

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3.4.5. Capacitive performance analysis of the MoS₂ NF after incorporation of the MnO₂ nanorod. For better comparison, the electrochemical performance of pure MnO₂ nanorod has been analyzed and is shown in Fig. S5 (ESI†). Fig. S6 (ESI†) exhibits the CV curve of all samples and MoS₂/MnO₂ (6 wt%) shows the highest CV area. The area of the CV curve increases with increasing the amount of MnO₂ nanorods and the largest CV area indicates better specific capacitance. The GCD curve displays a performance pattern that is similar to that. When the concentration of the MnO₂ nanorod increases in the MoS₂ NF the values of R_ct and W reduce. This denotes reduced resistance for charge transfer in the NCs and a shorter diffusion path of the ions in the NCs. So, the MnO₂ nanorod content affects the electrochemical performance significantly. The increased interplanar spacing, dislocation density, and surface area make the composite structure defect-rich and all these properties contribute to better electrochemical outcomes from the MoS₂/MnO₂ NCs. The interplanar spacing of the MoS₂ nanoflowers is increased by the incorporation of the MnO₂ nanorod, resulting in the defect-rich structure. This increased interplanar spacing might provide additional pathways for ion diffusion and electrons to the interface of the electrode while also improving the MoS₂ lattice electrical conductivity. The lower concentration of MnO₂ nanorod limits the transport of ions and electrons to the electrode surface from the electrolyte. The transport of electrons and ions from the electrolyte to the electrode surface is restricted by the lower concentration of the MnO₂ nanorod. For electrochemical reactions, MoS₂/MnO₂ (6%) offers more active sites on the surface of the electrode due to the lowest crystallite size and greater surface area among the synthesized NCs. However, as the MnO₂ amount is increased, the crystallite size decreases, and the surface area increases for chemical reactions. When the amount of MnO₂ is low, the size of the crystallite is high, limiting the number of reaction sites. Additionally, MnO₂ nanorods in the MoS₂ NF have improved the dislocation density of the produced NCs. The largest dislocation density is found in MoS₂/MnO₂ (6%) NC, which also results in an increase in capacitance value. The results and explanation above make it abundantly evident that the MoS₂/MnO₂ NCs perform better in terms of both capacitance and stability.

Table 5 presents the statistics of a MoS₂ based electrode.^{24,34,104–106} Among all the reported statistics, the MoS₂/MnO₂ (6%) provides better capacitive performance. This work yields better specific capacitance with superior stability. The collaborative effect of the MoS₂ NF and MnO₂ nanorod, reduced crystallite size, defect-mediated structure, and improved electrical conductivity are responsible for this amazing outcome. So, this composite has a great deal to offer as a contender for high-performance electrode material for capacitors.

Table 5 Comparing the capacitance of MoS₂ based supercapacitors that have been published in the literature

Materials of electrode	Electrolyte/electrode composition (mass ratio)	Current density (A g^−1)	Specific capacitance (F g^−1)	Cycle number	Retained specific capacitance %	Ref.
MoS2/Mn3O4	80 : 15 : 5	1	172	2000	69.3	35
MoS2 nanospheres//Active Carbon (AC)		1	65.33			104
MoS2NF	1 M KCl	1	168	3000	92.6	105
MoS2-G	1 M Na2SO4	5	130	1000	92.3	106
MoS2 nanowires/NiCo2O4//Active Carbon (AC)		1.5	51.7	8000	98.2	34
MoS2-CNT	1 M Na2SO4	2	74	1000	80.8	24
MoS2/MnO2	0.5Na2SO4	.04	199.12	10 000	95	This work

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4. Conclusion

The incorporation of MnO₂ nanorods into an MoS₂ NF provides an increased surface area that is electrochemically active for charge transfer and reduces the ion diffusion length at the time of charging–discharging. The synergistic effect of a 3D-1D structure reduces the inactive volume, opening the opportunity for ions of electrolyte to access the active material. The pseudocapacitive performance alongside the electrical double-layer capacitance of the MoS₂ NF can be considerably increased by the MnO₂ nanorod. The MoS₂/MnO₂(6 wt%) nanocomposite shows a higher specific capacitance (199.12 F g⁻¹ at 40 mA g⁻¹), improved capacitance retention, and cycle stability. High-performance energy storage devices can be made with this 3D-1D architectural electrode material that has exceptional electrochemical properties, enhanced stability, and coulombic efficiency.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ma00065j

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