High-Performance MnO2 Nanowire/MoS2 Nanosheet Composite for a Symmetrical Solid-State Supercapacitor.

To improve the production rate of MoS2 nanosheets as an excellent supercapacitor (SC) material and enhance the performance of the MoS2-based solid-state SC, a liquid phase exfoliation method is used to prepare MoS2 nanosheets on a large scale. Then, the MnO2 nanowire sample is synthesized by a one-step hydrothermal method to make a composite with the as-synthesized MoS2 nanosheets to achieve a better performance of the solid-state SC. The interaction between the MoS2 nanosheets and MnO2 nanowires produces a synergistic effect, resulting in a decent energy storage performance. For practical applications, all-solid-state SC devices are fabricated with different molar ratios of MoS2 nanosheets and MnO2 nanowires. From the experimental results, it can be seen that the synthesized nanocomposite with a 1:4 M ratio of MoS2 nanosheets and MnO2 nanowires exhibits a high Brunauer-Emmett-Teller surface area (∼118 m2/g), optimum pore size distribution, a specific capacitance value of 212 F/g at 0.8 A/g, an energy density of 29.5 W h/kg, and a power density of 1316 W/kg. Besides, cyclic charging-discharging and retention tests manifest significant cycling stability with 84.1% capacitive retention after completing 5000 rapid charge-discharge cycles. It is believed that this unique, symmetric, lightweight, solid-state SC device may help accomplish a scalable approach toward powering forthcoming portable energy storage applications.

Introduction

The energy crisis persists worldwide and is evolving into a more intense concern due to the deficiency of fossil fuel storage, ozone layer depletion, harm to biodiversity, and geopolitical and military disputes. Renewable energy sources’ production and storage techniques are required to reduce greenhouse gas emissions and prevent the rapid waste of fossil fuels. Most renewable energy resources are mainly dependent on regional weather and climatic conditions. Therefore, improving the performance of energy storage devices is essential for developing robust, stable, safe, and affordable energy supplies of these resources. Supercapacitors (SCs) have been studied more due to their high power density, fast charging–discharging rate, and stability over long cycle life. Remarkable progress in energy storage device performance has been accomplished through current improvements to maximize the specific capacitance by using nanostructured, conducting, and porous materials.[1−4] According to the charge storage phenomenon, electric double-layer capacitors (EDLCs) and pseudocapacitors are conventional energy storage capacitor types in SCs. In addition, a pseudocapacitor is operated on the principle of the active materials’ fast and reversible surface or near-surface redox Faradaic reactions. Advancement of high performance and stable energy sources of lightweight, higher energy density, and enormous power density is needed to fulfill the rapidly growing demand for transportable and wearable electronic gadgets.[5,6] Recent studies have focused explicitly on liquid-based SCs with aqueous solvents, organic solvents, or ionic fluids as electrolytes. These SCs have two notable drawbacks which restrict their use for transportable electronic devices: (i) the device fabrication needs high-cost packaging materials and approaches to avoid the feasible leakage of electrolytes and (ii) most electrolytes are highly toxic and corrosive. Unlike conventional SCs, solid-state SCs do not have liquid electrolytes and do not need robust, rigid packaging for containment. As a result, they can be thinner, lighter, and more flexible.[7−10]

It has been widely researched that a pseudocapacitor is an essential technological approach for superior electrochemical energy storage applications due to its excellent charge transfer reactions.[11−15,23] Usually, transition metal oxides, such as MnO2, are promising for active substances in pseudocapacitors because of the striking cation exchange. The pseudocapacitors, such as the ruthenium oxide-based devices (RuO2), have a large specific capacitance of 1580 F/g (at 1 mV/s). However, their toxic properties and high cost make RuO2 less appealing for practical uses.[16−19,37] Among other low-cost alternatives, MnO2 has been a promising material because of its outstanding environmental friendliness, low cost, and high theoretical capacitance. However, this value is challenging to achieve due to the low electrical conductivity of MnO2 (10 –5 to 10 –6 S cm –1).

Furthermore, electrodes prepared using different phases of MnO2 such as α, β, γ, and δ provided excellent electrochemical features for SC applications. However, it has been studied that increasing the mass loading of MnO2 on the current collector plate reduces the available surface area and lowers the electrical conductivity. This decreases the ionic diffusion rate and ultimately leads to poor specific capacitance. The MnO2 nanostructure performs well in neutral aqueous electrolytes and stores charge through non-Faradaic reactions. Still, the highly productive utilization of MnO2 in the electrochemical application is limited by mobility and lower conductivity. Thus, instead of pure MnO2, MnO2-based nanocomposites should be used as illustrative materials to enhance the ion conductivity of the SC.[20,21]

As a promising candidate for energy storage application, metallic molybdenum disulfide (1T-MoS2) has gained enormous research interest due to its typical layered structure and outstanding electrochemical performance. The layered structure provides more surface area for charge storage and different oxidation states from +2 to +6 of the central Mo atoms to increase the pseudocapacitance. Generally, the 1T phase exhibits an improved hydrophilic behavior and an excellent electrical conductivity higher than that of 2H–MoS2, which directs to a higher specific capacitance in a liquid electrolyte.[22−26] Accordingly, 1T MoS2 could be a suitable electrode material for manufacturing high-performance solid-state SCs. On the other hand, layered 1T-MoS2 nanosheets lead to restacking due to the out-of-plane van der Waals (vdW) force of attraction between adjacent layers, reducing the active surface area and preventing effective electrolyte ion diffusion, ensuing in a faster decay of electrochemical performance.[27−36] To control the reaggregation and possible phase change of 1T-MoS2, which leads to poor performance, a solution-processable method to get MoS2 nanosheets with stable dispersion is required to explore the spectrum of practical uses.

Regardless, two-dimensional (2D) transition metal disulfides, such as layered MoS2, have been broadly studied for energy storage applications due to their unique structures and higher conductivity. However, the practical use of the MoS2 material is limited by the low energy density. Some studies have shown that this problem can be fixed by forming heterostructures of MoS2 nanostructures with materials having higher energy density. As investigated earlier, the heterojunction structure between pseudocapacitive metal oxide materials, such as MnO2 nanomaterials, and 2D materials, such as MoS2, could improve the performance of SC devices largely. Almost all metal oxide materials have poor electrical conductivity, limiting their applications in SCs due to inadequate electron transportation. Therefore, many studies have shown a practical advancement in the electric conductivity by composing a heterojunction structure between metal oxide materials and 2D materials, which enhances the SC’s performance due to the fast electron transportation. Although there are several studies on the improvement of the electrochemical performance of SCs by utilizing the heterojunction structures, a more facile and scalable fabrication method is still required to study the actual applications of SCs. Thus, considerable efforts have been made to produce SCs with suitable characteristics by composing metal oxides.[37]

In this study, the binary nanocomposites of MoS2 nanosheets and MnO2 nanowires were used as electrodes to fabricate symmetrical solid-state SC devices for the first time. In this work, we have synthesized few-layer 2H–MoS2 nanosheets by liquid-phase exfoliation in a nontoxic solvent isopropanol without any surfactant stabilizer; this was mixed with β-MnO2 nanowires prepared from the hydrothermal synthesis in various molar ratios to produce MoS2:MnO2 electrodes (see the Experimental Methods). The as-prepared MoS2/MnO2 composites exhibit superior electrochemical performances as an electrode material for SCs.

The specific capacitance of 212 F/g at 0.8 A/g of the MoS2:MnO2 (1:4) solid-state SC is attributed to the synergistic effect between layered MoS2 nanosheets and MnO2 nanowires. This nanocomposite enhances the surface area with a specific aspect ratio, boosting the specific capacitance and SC performance. In addition, the MoS2:MnO2 nanocomposite enhances the pseudocapacitance of the composite by forming active sites and a Faradic reaction. All-solid-state symmetric SCs have been fabricated using the poly(vinyl alcohol) (PVA)/H3O4 gel as the electrolyte and MoS2/MnO2 as electrodes, respectively, to ensure the practical applications of the as-synthesized electrode materials. The assembled MoS2/MnO2 (1:4) SC delivers an energy density of 29.5 W h/kg and a power density of 1316 W/kg (at 0.8 A/g), exceeding the performance of other devices. In addition, the fabricated device reveals an excellent capacitance retention of 84.1% and remarkable cycling stability after 5000 cycles. This study could facilitate the design and manufacturing of high-energy storage devices with ultrahigh power and energy density. This strategy may be another avenue for manufacturing other high-performance SC devices.

Experimental Methods

Synthesis of MoS2 Nanosheets

The MoS2 powder was procured from Sigma Aldrich (product code: 69860), having particle size ∼6 μm (maximum 40 μm); isopropyl alcohol (IPA) was utilized as the solvent. All the chemicals used in this work were of scientific grade and were employed without further refinement. In detail, MoS2 nanosheets were synthesized from the bulk MoS2 powder by using the collective phenomena of sonication and centrifugation. 30 mg of the bulk MoS2 powder was ground by hand using a mortar and pestle for 30 min and dissolved in 30 mL of IPA. The suspension was then transferred to an ultrasonicating bath for sonication up to 6 h to reduce the weak vdW forces between adjacent layers of bulk MoS2. Finally, the solution was centrifuged for 1 h at 1400 rcf using a NEYA 10 REMI centrifuge to separate the supernatant containing MoS2 nanosheets from bulk MoS2.

Synthesis of MnO2 Nanowires

The synthesis of MnO2 nanowires was carried out via a hydrothermal route. Potassium permanganate (KMnO4) and manganese sulfate monohydrate (MnSO4·H2O) were obtained from Thermo Fisher and Sigma Aldrich, respectively. 10 mM KMnO4 and 15 mM MnSO4·H2O were dissolved in 80 ml of deionized water and magnetically stirred for 30 min for homogeneous mixing. The whole solution mixture was transferred into a 100 ml capacity autoclave and kept inside a muffle furnace at 90 °C for 24 h. The as-synthesized materials were then filtered after the solution mixture was naturally cooled to room temperature and then frequently washed with ultrapure water and diluted ethanol. After that, the samples were dried at 70 °C for 12 h inside a vacuum oven.

Three-Electrode System

The working electrode was prepared separately for the solution-state measurement of MoS2 nanosheet and MnO2 nanowire samples by drop-casting over a glassy carbon electrode of 2 mm diameter (mass loading 0.05 mg/cm2). A Pt wire was used as the counter electrode and saturated Ag/AgCl as the reference electrode. H3PO4 (0.5 M) solution was used as an electrolyte.

Device Fabrication

The gel polymer electrolyte was produced as follows. PVA (2 g) was added to deionized water (10 ml) at 90 °C and stirred for 3 h to form a homogeneous solution. Subsequently, H3PO4 (1 ml) was gradually mixed with the PVA solution to obtain the PVA/H3PO4 gel solution. The gel solution was discharged into a glass Petri dish, and the extra water was evaporated at room temperature to form a gel polymer electrolyte. The polymeric gel electrolyte was used as both an electrolyte and a separator.

MoS2 nanosheets and MnO2 nanowires were homogeneously mixed in a molar ratio of 1:1, 1:2, and 1:4 and sonicated for 20 min. The devices were fabricated by mixing the MoS2 nanosheets and MnO2 nanowires in different weight ratios of MoS2:MnO2 (1:1), MoS2:MnO2 (1:2), and MoS2:MnO2 (1:4). The mixture was then painted using a spin-coater on a 40 μm thick gold sheet supported on a mica sheet, which acts as a collector plate. After drying overnight at 50 °C inside a vacuum oven, a copper wire contact was made from both electrodes for electrochemical testing. The mass of the active material was calculated by weighing the electrodes after drying, and it weighed ∼0.87 mg (mass loading: 0.65 mg/cm2). Two electrodes were sandwiched using a PVA/H3PO4 gel electrolyte to fabricate solid-state SC devices. It was dried under a vacuum at 50 °C, and a carbon rag was used as the separator. The PVA/H3PO4 gel was selected as an electrolyte due to its good water solubility, excellent compatibility with the electrode, and good chemical and thermal stability. All fabricated devices were further sealed using a GE varnish as a packaging material to protect the gel electrolyte from overdrying. The performance of the fabricated devices was evaluated by the two-electrode measurement system.

Characterization

The surface morphology of the samples was examined using a scanning electron microscope (ULTRA 55, Karl Zeiss). The powder samples were spread on a clean sticky carbon tape and put onto an SEM stub. The scanning electron microscopy (SEM) images were collected by applying an accelerating voltage of 5 kV. Structural analysis and mapping were carried out using a transmission electron microscope [Model JEOL 2000 FX-II with an ultrathin window (Oxford Instruments) and an energy-dispersive spectrometer system and a charge-coupled device (CCD) image recording system] conducted at an accelerating voltage of 200 kV. Transmission electron microscopy (TEM) analysis was performed by drop-casting the diluted MoS2 solution over the carbon-coated copper grid, followed by drying. SHIMAZU’s spectrophotometer (UV–vis 3700) was used to check the absorption spectra of MoS2 nanosheets. The proper baseline correction was made by placing an identical cuvette filled with IPA at both source and reference sites to eliminate the absorption/reflection from the cuvette and the noise effect due to air and pure solvent. Diffraction peaks were analyzed using a Rigaku Miniflex diffractometer with a typical X-ray tube (Cu Kα radiation, 40 kV, 30 mA) and a Hypix-400 MF 2D hybrid pixel array detector. The Raman spectra of the as-synthesized samples were collected using a Horiba LabRAM HR Evolution Raman spectrometer using a 532 nm (0.3 mW) laser; 10 s scans were acquired with the laser using the 50 × objectives of a Zeiss stereomicroscope. X-ray photoelectron spectroscopy (XPS) was used to distinguish the chemical composition using a Thermo Scientific K-Alpha ESCA equipped with a monochromatic Al Kα X-ray source at 1486.6 eV. The Brunauer–Emmett–Teller specific surface area was determined using nitrogen adsorption–desorption isotherms at 77 K (NOVA 2200e). Before measuring, the samples were degassed in a 10–6 Torr vacuum at 150 °C for 12 h to remove the adsorbed moisture. The electrochemical performance was studied via cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) measurements, and electrochemical impedance spectroscopy (EIS) using an AUTO LAB:/PGSTAT128N instrument.

Results and Discussion

Figure a shows the SEM image of the synthesized MoS2 nanosheets, and the typical morphology at the micron level provides more surface area for excellent electrochemical activity. TEM results for the MoS2 nanosheets, as shown in Figure b, show the sheet-like structure with a thin layered structure morphology.[38] A typical SEM image of MnO2 nanowires is exhibited in Figure c. The result shows that the vertical nanowire has a stem-like structure with a diameter of 50 ± 5 nm and a length in the range of 300–800 nm. The nanowires are homogeneously distributed over the electrode surface, as observed from the SEM morphology analysis. High-angle annular dark-field-scanning TEM (HAADF-STEM) was conducted to examine the morphology and distribution of MoS2 nanosheets and MnO2 nanowires in the nanocomposite. Figure d shows the HAADF-STEM image of the as-grown MnO2 nanowires. It can also be seen distinctly from Figure e that the composite consists of many large MoS2 nanosheets (1–3 μm) forming a highly warped surface. It is recognized as a more irregular surface with a widespread surface area of the composite sample. Figure f shows the d-spacing of the MoS2 nanosheets and MnO2 nanowires, measured to be 0.204 and 0.11 nm, respectively, which matches the d-spacing values calculated from X-ray diffraction (XRD) results (Figure S5a).[39,40] The distinct ring pattern of selected-area electron diffraction (SAED) shows that it is well crystallized. The SAED pattern of the composite (Figure g) shows a circular ring of bright dots due to the crystalline nature of the MoS2 nanosheets, and the circular ring of the diffused bright dots shows the morphology of MnO2 nanowires.

Figure 1

(a) SEM and (b) TEM images of MoS2 nanosheets. (c) SEM and (d) corresponding HAADF-STEM images of MnO2 nanowires. (e) Low-magnification TEM image of the MoS2/MnO2 composite. (f) Lattice fringes of MoS2 nanosheets and MnO2 nanowires. (g) SAED pattern of the corresponding MoS2:MnO2 composite. (h) Elemental distribution of the MoS2/MnO2 composite. The elemental distributions of (i) Mo, (j) S, (k) Mn, and (l) O in the sample.

The SAED patterns exhibit the hexagonal and tetragonal phases of MoS2 nanosheets and MnO2 nanowires, consistent with the XRD results (Figure S4a). Energy-dispersive X-ray elemental mapping was also conducted for the composites, as shown in Figure h. The corresponding elemental compositions were examined separately for each element. Sequentially, the elemental concentrations of Mo, S, Mn, and O are shown in Figure i–l. The elemental mapping shows that all the elements (Mo, S, Mn, and O) are uniformly distributed in the composite sample.

As shown in Figure a, two significant peaks were found at 228.6 and 232.07 eV, resulting from Mo 3d5/2 and Mo 3d3/2 in Mo (+4), implying the existence of the phase of MoS2 in the sample. A short peak at 234.7 eV arises from the Mo (+6) partial oxidation state of MoS2, while the peaks at 162.12 and 163.5 eV could be assigned to the S 2p3/2 and S 2p1/2 states, respectively, of the S 2p orbital (shown in Figure b). The high-resolution XPS (HRXPS) Mn 2p exhibits a doublet (shown in Figure c) at 642.4 and 653.6 eV, corresponding to the binding energies of Mn 2p3/2 and Mn 2p1/2, respectively, of the as-synthesized α-MnO2 nanowires. However, the pair of Mn 2p3/2 and Mn 2p1/2 was noticed due to the mixed valence states of Mn (Mn4+/Mn3+), showing that oxygen vacancies are present on the surface of β-MnO2 nanowires. In contrast, O 1s (shown in Figure d) consists of three peaks at 529.5, 529.78, and 531 eV, which are associated with the formation of Mn–O–Mn, Mn–O–H, and H–O–H bonds, respectively. All the designated XPS results are compatible with the earlier reported results and confirm the MnO2 and MoS2 phases in the composite.[41]

Figure 2

HRXPS spectra of different elements. (a) Mo 3d, (b) S 2p, (c) Mn 2p, and (d) O 1s.

CV, GCD, and cyclic stability were analyzed using an electrochemical working station (Autolab/PGSTAT128N). The symmetric solid-state SCs were assembled using MoS2/MnO2 composites as the anode and cathode without further treatment. CV measurements were carried out individually at various scan rates (varying from 20 to 100 mV s–1).

Furthermore, the near-mirror image current acknowledgment on voltage reversal characteristic of the CV curves reveals an excellent capacitance feature and excellent reversibility. The pattern of the CV curves is quasilinear with the increment of scanning speed, indicating higher pseudocapacitive properties and a high-rate performance. However, the absence of distinct peaks in the CV curves of MoS2:MnO2 (1:4), MoS2:MnO2 (1:2), and MoS2:MnO2 (1:1) SCs imply the dominance of pseudocapacitance over the EDLC, as shown in Figure a–c. The cathodic and anodic currents increased gradually with an increase in scan rates.[42−45,50,51] The areal specific capacitance of the electrode can be determined according to eq (46−48)

Figure 3

CV response of (a) MoS2/MnO2 (1:4), (b) MoS2/MnO2 (1:2), and (c) MoS2/MnO2 (1:1) at different scan rates ranging from 20 to 100 mV/s and (d) average peak current vs square root of scan rate.

The gravimetric specific capacitance from CV is calculated using eq .where Csp is the areal specific capacitance (F/cm2), I(V) is the instantaneous current (in ampere), ∫ I(V)·dV denotes the integral area of the cyclic voltammogram loop, A = area of the active material, m = the total mass of the electrodes, dV/dt is the scan rate, and V2–V1 is the width of the potential window. Figure d shows the peak current variation with the scan rate. This is because the total amount of redox-active mass is constant on the device’s electrode surface, which implies that the total amount of charge to be transferred at the explored potential window is also a constant. Therefore, at a low scan rate, the ions have enough time to diffuse in the material’s tiny pores, encouraging more charge storage. At a higher scan rate, the rate-limited diffusion of ions decreases the chance of storage and hence capacity drops. The improved specific capacitance is ascribed to the MoS2’s nanosheet-type morphology combined with MnO2 nanowires. This aggregate of sheet-and-wire-type morphology facilitates a much more significant accumulation of ions (within MoS2 nanosheets) and ion diffusion by MnO2 nanowires. Faradaic electron transfer occurs with the change in oxidation states of the Mo and S atoms at edge sites due to the adsorption of electrolyte ions onto the MoS2 surface and intercalation of ions between neighboring layers.

The GCD technique is performed at different current densities ranging from 0.8 to 3 A/g to understand its charge storage phenomenon and performance. The GCD profiles of all SCs acquired at different currents are shown in Figure a–c. In addition, the time required for charging and discharging the solid-state SC is high for lower currents and low for higher currents. This happens for the following reasons: at higher current density, the electrolyte ions cannot use the electroactive area completely, but at a lower current density, the electrolyte ions have enough time to reach the electroactive site of the electrode materials. An instant voltage drop (IR drop) happens when the SC swaps from charging to discharging due to the collective ohmic resistance of the electrodes, gel electrolyte, and electrical contact resistances of the devices. A specific capacitance of 212 F/g of the MoS2:MnO2 (1:4) SC was achieved at a current density of 0.8 A/g. After that, as the current density was increased, the specific capacitance decreased to 59 F/g at 3 A/g, as exhibited in Figure d.

Figure 4

GCD plots of (a) MoS2/MnO2 (1:4), (b) MoS2/MnO2 (1:2), and (c) MoS2/MnO2 (1:1)and (d) corresponding plots of specific capacitance vs current density.

The specific capacitance drops at a high current density corresponded with the results in Figure d due to the higher resistance of the gel electrolyte. The specific capacitances were calculated from the GCD plots according to eq .where I = discharge current, Δt = time for a complete discharge, m = mass of the electrode, and ΔV = discharging voltage.[8,9]

The specific capacitance value determined from the CV curves versus the scan rate is summarized in Figure a. The specific capacitance for the MoS2:MnO2 (1:4) SC is 118 mF/cm2 at a scan rate of 20 mV/s, which is higher than that for other SCs. As the scan rate increased to 100 mV/s, the areal-specific capacitance reduced to 32 mF/cm2. Figure b shows the variation in specific capacitance with scan rates. Note that the MoS2/MnO2 (1:4) SC showed the highest specific capacitance of 180 F/g at a scan rate of 20 mV/s compared to other SCs. Moreover, a lower capacitance with an increased scan rate is marked due to the charge-resistive behavior of the electrode material at a higher scan rate.

Figure 5

(a) Areal specific capacitance of all three devices with the variation of scan rates ranging from 20 to 100 mV/s, (b) gravimetric specific capacitance as determined from the GCD curve, (c) cycling stability performance of the devices recorded at 0.8 A/g, and (d) Ragone plots of the SC devices.

The capacitance retention as a function of the number of cycles is plotted in Figure c. Outstanding retention (∼84.1%) of the MoS2/MnO2 (1:4) device after 5000 cycles has been observed, demonstrating the electrode’s excellent stability. For the practical applications of the SC device, long-term cycling stability is an essential requirement. It is fascinating to observe from Figure c that the Csp value moderately rises during the initial CCD cycles, which may be associated with the precise activation of the electrode materials with the electrolyte, overcoming the internal resistances of the device.[50,51]Figure d shows the Ragone plot of all MoS2/MnO2 SCs. The energy and power density were derived from galvanostatic charging–discharging at various current densities. The energy density and power density are evaluated using eqs and 5, respectively.where E is the energy density, P is the power density, Csp is the gravimetric specific capacitance calculated through GCDs, V is the voltage window, and Δt is the discharge time.[20,21,23−26]

As observed from Ragone plots, the power density of the MoS2/MnO2 (1:4) SC increases from 1316 to 2683 W/kg as the current density increases from 0.8 to 3 A/g, which is higher than those of the other two devices. Meanwhile, the MoS2/MnO2 (1:4) SC exhibits an ultrahigh power density of 1316 W/kg and an energy density of 29.5 W h/kg at the current density of 0.8 A/g. By contrast, other planar SCs hardly achieve such a high power density at a relatively high energy density.

EIS analysis is an efficient technique to investigate the charge transfer at the electrode–electrolyte interface. As shown in Figure a, the Nyquist plot of the MoS2/MnO2 (1:4) device reveals a much shorter radius than that of MoS2/MnO2 (1:2) and MoS2/MnO2 (1:1). It signifies that few-layer MoS2 nanosheets dramatically affect the transport of charge carriers in the devices. The radius of these curves is identified from the magnified image in Figure b. Based on the proposed matching circuit exhibited in Figure d, the high-frequency intercept on the real axis denotes the ohmic series resistance (RS), corresponding to the bulk resistance of the electrolyte and electrodes. The first half of the semicircle at high frequency can be described by resistance (RCT1), suggesting the ease of electrolyte ions to the electroactive materials, the electrode’s better conductivity, and chemical capacitance (CPE1) of the solid-state interface layer built due to the passivation reaction. The second half of the semicircle at medium frequency corresponds to the charge transfer resistance (RCT2) and double-layer capacitance (CPE2). Moreover, Warburg impedance (W) arising from the diffusion of ions on the electrodes’ surface can also be seen in the low-frequency range. The impedance of the constant phase element (CPE) has the expression Z = 1/Y0 (jω)−α, where Y0 = 1/C for α = 1 and Y0 = R for α = 0; C and R denote the capacitance and resistance, respectively. α is specified as the exponent of the CPE.[52,53] The α component is merely resistive for α = 0, and for α = 1, the component is merely capacitive. Compared to MoS2/MnO2 (1:1) and MoS2/MnO2 (1:2) devices, the much small internal impedance (i.e., RS, RCT1, and RCT2) of MoS2/MnO2 (1:4) signifies decreasing solid-state interface layer resistance and charge transfer resistance. The goodness of fit (χ2) for MoS2/MnO2 (1:4), MoS2/MnO2 (1:2), and MoS2/MnO2 (1:1) devices is determined to be 0.002, 0.002, and 0.001, respectively, signifying an excellent fitting. The circuit parameters obtained from the standard equivalent circuits are shown in Table S1. The series resistance (RS) for the MoS2/MnO2 (1:4) device is ∼312 Ω, much lower than those of the MoS2/MnO2 (1:1) and MoS2/MnO2 (1:2) devices. Accurate RCT1 and RCT2 were obtained by fitting the equivalent circuit. For the MoS2/MnO2 (1:4) device, the RCT1 and RCT2 values (0.9 and 0.7 kΩ) are much lower than those of MoS2/MnO2 (1:1) and MoS2/MnO2 (1:2) SCs. Thus, the better performance of the MoS2/MnO2 (1:4) device can be related to the proportion of MoS2 nanosheets and MnO2 nanowires where MoS2 nanosheets are increasing the specific capacitance by contributing a large specific surface area and improving charge transfer kinetic. Energy density and power density are essential parameters in measuring energy storage device practicability. Also, the Bode plot (shown in Figure c) shows the plot of the phase angle versus applied frequency. It is known that the phase angle of an ideal capacitor is nearer to −90°, whereas for pseudocapacitors, it is roughly −45°. The Bode plot demonstrated that the phase angle varies from −60° to −75° at 0.05 Hz, indicating the combination of the double layer and pseudocapacitor nature of the MoS2/MnO2 devices.[51] The equivalent circuit used for fitting the EIS plots in this analysis is depicted in Figure d.

Figure 6

(a) Nyquist plots for experimental data and fitted data and a (b) magnified portion of Nyquist plots; (c) corresponding Bode plots of the devices; and (d) equivalent electrical circuit model for the EIS data analysis.

More details on the performance comparison of MoS2:MnO2 SCs with the state-of-the-art SC devices are given in Table . The excellent performance of the MoS2:MnO2 (1:4) device is due to the high conductivity and the available surface area of the electrode. The data presented in this study imply that the device can store the charge with enormous capacity and excellent cyclic stability. Here, the existence of MnO2 nanowires over MoS2 nanosheets has been shown by TEM (Figure e) images. As the proportion of MnO2 in the electrode increases, the evolution of different individual morphologies, that is, MnO2 nanowires and MoS2 nanosheets, becomes productive (see Figure h). Accordingly, it can be figured out that the MoS2/MnO2 (1:4) composite possesses sufficient amounts of MnO2 and MoS2 nanostructures. Also, the superior electrochemical activity of the MoS2/MnO2 (1:4) SC is the result of synergistic effects of MoS2 and MnO2, which occur between two or more materials to give a combined result. Furthermore, we understand that the execution of such a heterogeneous material can be improved in the future by tuning the layer thickness and electrical conductivity (by introducing pentavalent or trivalent impurities) and by incorporating any pseudocapacitive nanostructure material, such as metal oxide nanowires like a silver nanowire, Co3O4 nanowire, or tungsten nanowire.[45−50]

Table 1

Comparison of the SC Performance in the Present Work with That in Earlier Reports of Different MoS2-Based SCs

device material	electrolyte	specific capacitance (Cs)	energy density (W h/kg)	power density (W/kg)	stability (%)	refs
MoS2/CNT	1 M Na2SO4	74.05 F/g at 2 A/g (three electrode)			80.8% after 1000 cycles	(52)
carbon nanosheets/flower-like MoS2	1 M KOH	381 F/g at 1 A/g (three electrode)			92% after 3000 cycles	(53)
MoS2/carbon cloth	1 M Na2SO4	151.1 F/g at 10 mA/cm2 (symmetric coin cell SC)	11.13	250	86.1% after 2000 cycles	(54)
carbon-MoS2 yolk–shell microspheres	1 M Na2SO4	122.6 F/g at 1A/g (three electrode)	17.03	500.1	95% after 3000 cycles	(46)
MoO2/carbon nanosheets	1 M Na2SO4	190.9 F/g at 1 A/g (symmetric coin cell SC)	10.3	378	79% after 2000 cycles	(47)
MnO2/carbon cloth	1 M Na2SO4	67.8 F/g at 1 A/g (asymmetric coin cell SC)	18.46	699.54	97.3% after 2000 cycles	(48)
Ti/TiO2/MoS2 coaxial fiber	PVA/H3PO4	230.2 F/g (symmetric solid-state SC)	2.70	530.9	89% after 2000 cycles	(55)
sheared MnO2/MoS2	PVA/Na2SO4	27.5 mF/cm2 at 0.1 mA/cm2 (symmetric solid-state SC)	0.0011 mWh/cm2	0.5 mW/cm2	90% after 3000 cycles	(37)
MoS2 nanosheets/MnO2 nanowires	PVA/H3PO4	212 F/g at 0.8 A/g (symmetric solid-state SC)	29.5	1316	84.1% after 5000 cycles	This work

Conclusions

In conclusion, it has been studied that MoS2 nanosheet-embedded MnO2 nanowire electrodes with different weight ratios have attractive electrochemical properties that make them an efficient electrode material for designing a solid-state- SC. The MoS2 nanosheets were added to prevent fast oxidation of the MnO2 nanowires and provide additional pseudocapacitance. The two materials have unique advantages and produce an excellent synergetic interaction in the interface, significantly increasing the electrochemical performance. Electrochemical experiments reveal that the solid-state SC [MoS2/MnO2 (1:4)] has an outstanding specific capacitance of 118 F/cm2 at the scan rate of 20 mV/s and 212 F/g at the current density of 0.8 A/g. After 5000 long cycles, the capacitance of solid-state devices can be retained at around 84.1% with outstanding cyclic stability. The fabricated solid-state SC [MoS2/MnO2 (1:4)] produces a tremendous energy density of 29.5 W h/kg at a power density of 1316 W/kg. Hence, the MoS2/MnO2 composite is an advantageous electrode material for next-generation storage devices. As might be intended, all devices exhibit high pseudocapacitance, excellent charging–discharging, and good cycle stability. These remarkable characteristics demonstrate that the composite of MoS2 nanosheets and MnO2 nanowire electrodes can be used to produce high-performing solid-state SCs. This facile preparation method may lead to the development of other metal oxide–transition metal dichalcogenide composites for a stable and high-performance SC, which is essential and appealing for various energy storage applications.