# Sustainable and Scalable Approach for Enhancing the Electrochemical Performance of Molybdenum Disulfide (MoS2)

Authors: Sunil Kumar; Vinay Kumar; Raman Devi; Avnish Kumar Sisodia; Anushree Jatrana; Raj Bahadur Singh; Rita Dahiya; Ajay Kumar Mishra
Journal: Advances in Materials Science and Engineering
 (2022)

---

## Abstract

Molybdenum disulfide (MoS2), the second most thoroughly investigated two-dimensional material after graphene, has attracted considerable interest in energy storage applications owing to its exceptional qualities, including its unique crystal structure, low electronegativity, and high specific capacity. In this study, we showed that a simple ball-milling procedure causes significant improvement in the capacitive properties of the bulk MoS2 (BL-MoS2). We characterized the material before and after the milling process using X-ray diffraction (XRD) and a BET surface area analyzer to find the material’s structural, crystalline features, and surface area, respectively. We prepared electrodes of BL-MoS2 and ball-milled MoS2 (BM-MoS2) for electrochemical investigation. The charge storage characteristics were examined using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). The BM-MoS2 and BL-MoS2 have a specific capacitance of 114 F/g and 96 F/g, respectively.


---

## Body

## 1. Introduction

The world has recently turned its focus to the energy crisis, which has emerged as one of the biggest issues. This problem arises because of our overdependence on automatic and advanced technological devices. They ease our routine life, but they also consume electric power. The demand for energy increases very fast, but on the other hand, the energy harvesting and storage mechanism is not growing. To meet the global market for “energy,” the scientific community has to find new solutions and search for new methods and materials that possess prodigious performances in the energy harvesting, conversion, and storage sector [1].Supercapacitors can fulfill the gap of hardship that we are facing and help us get rid of these challenges. The ability of supercapacitors to carry significantly greater energy-power efficiency, specific power density, faster charge-discharge rate, and a longer lifespan compared to rechargeable batteries make them a favorite for next-generation energy storage devices [2, 3]. Supercapacitors are having 1000 times more power density than that of batteries [4]. Despite extensive research and significant achievements in recent years, energy storage devices continuously face numerous challenges, including high energy density, fast charging, slow discharging, and long life, as well as their sustainable production using readily available and abundant materials.To solve these restrictions, there is a significant demand for developing new concepts and materials with outstanding performance while being scalable, low-cost, and ecologically benign. Two-dimensional (2D) materials, especially layer-structuredtransition-metal dichalcogenides (TMDs), have gained much attention in the supercapacitor sector due to their unique physiochemical features such as large surface area, micro or mesoporous structure, and strong conductivity. Among them, molybdenum disulfide (MoS2) has been identified as one of the most appealing materials and has recently been investigated in electrochemical energy storage applications. The structure of MoS2 is similar to that of graphite, which has a layer-like structure made up of three atom layers with a Mo layer sandwiched between two S layers, all of which are held together by weak van der Waals forces [5]. The recent development in supercapacitor research has also focused on MoS2 mainly because of its higher intrinsic ionic conductivity compared to oxides and high theoretical capacity than graphite [6]. For example, the study on MoS2, used as an electrode material for supercapacitors due to its large surface area and sheet-like morphology, results in double-layer charge storage (EDLC) in the supercapacitor by Soon and Loh [7].Along with the EDLC, the faradic capacitance also contributes to capacitance increment due to the diffusion of the ions into the MoS2 layers and causes a significant role in enhancing charge storage capabilities. Therefore, a lot of work focused on synthesizing nanosized MoS2 structures, and various methods are used for this purpose. All these methods make the material much more useful by altering its properties like surface area and conductivity and increasing the edge sites, which causes enhancement in the active sites. Every method has advantages and disadvantages, but scalability, environmental hazard, and accessible material production are the challenges. In order to address all these problems, the ball-milling process provides an excellent way to synthesize the desired material with large surface area and also resolves the issue of scalability [8]. In this study, we present a straightforward and scalable approach based on dry ball milling that improves the electrochemical characteristics of BL-MoS2 without using any chemical agents or reactions. We observed that the BM-MoS2 had high specific capacitance; hence, enhanced electrochemical features can be utilized for electrochemical supercapacitor applications. This work aims to study the effect of ball milling on the charge storage mechanism of BL-MoS2.


## 2. Material and Methods

### 2.1. Chemicals

Molybdenum disulfide and potassium chloride were purchased from Sigma-Aldrich, and these chemicals were not further refined before being utilized.


### 2.2. Procedure of Ball-Milling of Bulk MoS2

Using a “Fritsch Pulveristee 5 planetary” ball milling machine, the BL-MoS2 was subjected to a 12-hour ball milling procedure. We employed 30 g of BL-MoS2 powder and 300 g of stainless steel balls into a ball milling jar. The ball milling process was carried out at 300 rpm for 12 hours.


### 2.3. Characterization Techniques

The crystal structure of BL-MoS2 and BM-MoS2 was recorded by X-ray diffraction (XRD) on “Rigaku Mini Flex II X-ray Diffractometer” and surface area analysis was done using “Quantachrome Instruments v5.21” of Anton Paar. A Perkin-Elmer FTIR spectrometer was used to record the FTIR spectra in the range of 4000 cm−1 to 400 cm−1 to examine the different functional groups in the sample. The morphology of the samples was evaluated with the help of a field emission scanning electron microscope (FESEM) of JEOL iT-800, and an electrochemical workstation (Metrohm Autolab PGSTAT-204M) was used to accomplish the electrochemical investigation.


### 2.4. Preparation of an Electrode for Electrochemical Investigation

By mixing BM-MoS2 and carbon black using a mortar and pestle, the working electrode was prepared to explore the electrochemical behavior of the processed material. This mixture was used to form a slurry. To make a slurry, we dissolve 3 mg of polyvinylidene fluoride (PVDF) in 2 ml in an organic solvent “N-methyl-2- pyrrolidine” using a magnetic stirrer. The stirring process continued until a transparent solution was formed, and then, the mixture of BM-MoS2 (24 mg) and carbon black (3 mg) was added to this solution and stirred for 12 hours. Through this process, we obtained a thick slurry paste spread on the area of one centimeter square of the current collector (graphite sheet) with a mass loading of 1.39 mg of the active material and dried it in a vacuum oven for 12 hours at 80°. The ingredient’s mass ratio was 8 : 1:1 in the slurry. The same method was adopted to prepare the BL-MoS2 working electrode. KCl (1 M) was used as an electrolyte to test the fabricated electrode at room temperature using a conventional three electrodes setup. The BL-MoS2 and BM-MoS2, Ag/AgCl electrodes, and spiral platinum wire were used as working, reference, and counter electrodes, respectively. At different scan rates of 10 mV/s to 200 mV/s, the CV recorded in the voltage window of −0.7 V to 0.7 V. GCD profiles were carried out at different current densities 1, 1.5, 2, 2.5, and 3 A/g. Further, the EIS analysis was carried out in the frequency range of 10 MHz to 100 kHz. Using equation (1), the specific capacitance was calculated from the GCD curve. The detailed discussion about how the specific capacitance was calculated is given in Table 1 of supplementary file.(1)Cp=I×tm×∆V,where Cp, I, t, m, and ΔV represent their usual meanings, i.e., the specific capacitance (F/g), discharge current (A), time duration of discharging (s), mass deposited on the electrode (g), and potential window (V), respectively.


## 2.1. Chemicals

Molybdenum disulfide and potassium chloride were purchased from Sigma-Aldrich, and these chemicals were not further refined before being utilized.


## 2.2. Procedure of Ball-Milling of Bulk MoS2

Using a “Fritsch Pulveristee 5 planetary” ball milling machine, the BL-MoS2 was subjected to a 12-hour ball milling procedure. We employed 30 g of BL-MoS2 powder and 300 g of stainless steel balls into a ball milling jar. The ball milling process was carried out at 300 rpm for 12 hours.


## 2.3. Characterization Techniques

The crystal structure of BL-MoS2 and BM-MoS2 was recorded by X-ray diffraction (XRD) on “Rigaku Mini Flex II X-ray Diffractometer” and surface area analysis was done using “Quantachrome Instruments v5.21” of Anton Paar. A Perkin-Elmer FTIR spectrometer was used to record the FTIR spectra in the range of 4000 cm−1 to 400 cm−1 to examine the different functional groups in the sample. The morphology of the samples was evaluated with the help of a field emission scanning electron microscope (FESEM) of JEOL iT-800, and an electrochemical workstation (Metrohm Autolab PGSTAT-204M) was used to accomplish the electrochemical investigation.


## 2.4. Preparation of an Electrode for Electrochemical Investigation

By mixing BM-MoS2 and carbon black using a mortar and pestle, the working electrode was prepared to explore the electrochemical behavior of the processed material. This mixture was used to form a slurry. To make a slurry, we dissolve 3 mg of polyvinylidene fluoride (PVDF) in 2 ml in an organic solvent “N-methyl-2- pyrrolidine” using a magnetic stirrer. The stirring process continued until a transparent solution was formed, and then, the mixture of BM-MoS2 (24 mg) and carbon black (3 mg) was added to this solution and stirred for 12 hours. Through this process, we obtained a thick slurry paste spread on the area of one centimeter square of the current collector (graphite sheet) with a mass loading of 1.39 mg of the active material and dried it in a vacuum oven for 12 hours at 80°. The ingredient’s mass ratio was 8 : 1:1 in the slurry. The same method was adopted to prepare the BL-MoS2 working electrode. KCl (1 M) was used as an electrolyte to test the fabricated electrode at room temperature using a conventional three electrodes setup. The BL-MoS2 and BM-MoS2, Ag/AgCl electrodes, and spiral platinum wire were used as working, reference, and counter electrodes, respectively. At different scan rates of 10 mV/s to 200 mV/s, the CV recorded in the voltage window of −0.7 V to 0.7 V. GCD profiles were carried out at different current densities 1, 1.5, 2, 2.5, and 3 A/g. Further, the EIS analysis was carried out in the frequency range of 10 MHz to 100 kHz. Using equation (1), the specific capacitance was calculated from the GCD curve. The detailed discussion about how the specific capacitance was calculated is given in Table 1 of supplementary file.(1)Cp=I×tm×∆V,where Cp, I, t, m, and ΔV represent their usual meanings, i.e., the specific capacitance (F/g), discharge current (A), time duration of discharging (s), mass deposited on the electrode (g), and potential window (V), respectively.


## 3. Results and Discussion

### 3.1. XRD Analysis

Figure1(a) shows the XRD spectra of both BL-MoS2 and BM-MoS2. The XRD spectra were observed from 10° to 80°. The diffraction peak observed at 14.2°, 32.6°, 39.5°, 44.2°, 49.8°, and 58.3°, relating to the (002), (100), (103), (006), (105), and (110) planes [9], respectively, is as shown in Figure 1(a). From this, we confirm that BL-MoS2 and BM-MoS2 both have crystalline natures. The BL-MoS2 has a very sharp and highly intense peak at 14.38°, which belongs to the 2H structure of MoS2. After ball milling (12 h), the processed material is BM- MoS2 that has the same sharp peak but with reduced intensity, which shows the sign of layered MoS2 [10] formation. It is important to note that the presence of the same peak after exfoliation can be connected to a certain level of layer restacking during the ball-milling process. The average crystallite size (D) of the BL-MoS2 and BM-MoS2 were calculated using the Scherer formula in equation (2). The average crystallite size of BL-MoS2 and BM MoS2 was 36 nm and 30 nm, respectively.(2)D=kλβcosθ,where λ is the wavelength of Cu-Kα radiation, k (0.9) is the structure factor, β is the FWHM, and θ is the diffraction angle.Figure 1
(a) XRD pattern of BL-MoS2 and BM-MoS2. (b) N2adsorption-desorption isotherms of BL-MoS2 and BM-MoS2. (c) Pore size distribution. (d) FTIR spectra of BL-MoS2 and BM-MoS2 in the range of 400 to 4000 cm−1.
(a)(b)(c)(d)


### 3.2. BET Analysis

The surface area was determined using the Brunauer–Emmet–Teller (BET) method for N2adsorption-desorption isotherms. Figure 1(b) displays the N2adsorption-desorption isotherms plots for BL-MoS2 and BM-MoS2. As already discussed, BL-MoS2 exfoliated into numerous layers, size of the particles, and thickness decreases, which affects the surface area of the MoS2 [11, 12]. The higher surface area has a more active site, which can also increase the interaction of electrode and electrolyte; as a result, we have higher electrochemical performance [8]. Both samples exhibit type IV isotherms, and the N2adsorption-desorption isotherms illustrate that the volume of gas trapped was higher in the case of BM-MoS2 than the BL-MoS2. Therefore, because the MoS2 layer is restacked by ball milling, BM-MoS2 has a higher specific surface area (22.593 m2/g) than BL-MoS2 (11.239 m2/g). Figure 1(c) represents both samples’ pore size distribution. BM-MoS2 has a total pore volume of 0.544 cc/g determined by the density function theory (DFT) method for a radius smaller than 173.3 nm, and BL-MoS2 has a total pore volume of 0.482 cc/g for a radius less than 183 nm calculated at a relative pressure of 0.9944.


### 3.3. FTIR Analysis

The FTIR spectra of BL-MoS2 and BM-MoS2 obtained in the IR region between 400 cm−1 and 4000 cm−1 are shown in Figure 1(d). The spectra of MoS2 include an absorption peak at 596 cm−1, which corresponds to the presence of the Mo-S bond, and an additional peak at 903 cm−1, caused by the S-S bonds in MoS2. The absorption bands at 1125 cm−1 and 1634 cm−1 are attributable to the stretching vibrations of the Mo-O bond and the hydroxyl group [13], respectively. The prominent peaks at about 3421 cm−1 are caused by the stretching vibration of OH groups present due to moisture in the sample, while narrow peaks at 2917 cm−1 are related to CH2 [14].


### 3.4. FESEM Analysis

FESEM is used to characterize the morphologies and nanostructures shown in Figures2(a)–2(d) at various scales. The crumpled BL-MoS2 indicates the creation of a few-layer MoS2 nanosheet with a flower-like structure. The ball milling method results in the exfoliation of the BL-MoS2 Figure 2(a) into nanosheets. The difference can also be seen in the FESEM pictures, as shown in Figures 2(b)–2(d). The BL-MoS2, after the ball milling process, exhibits a flower-like nanosheet shape with an average thickness of 40 nm of nanosheets in the FESEM images. The energy dispersive X-ray (EDX) analysis shown in Figures 2(e) and 2(f) was performed to check the purity of both samples and to find out if any alterations were caused during the ball milling process. The EDX spectra of BL-MoS2 are shown in Figure 2(e), and it confirms the purity of bulk material as it can be seen by the as-expected characterized peaks of Mo and S as major elements. We also get small content of carbon and oxygen. After the milling process, the purity of the sample (BM-MoS2) remains intact, as shown in Figure 2(f), in which no additional signal was observed.Figure 2
FESEM images of the pristine bulk and ball milling MoS2. (a) BL-MoS2 100 nm scale. (b) BM-MoS2 at 1 m scale (c and d) BM-MoS2 at 100 nm scale. (e) EDX spectrum of BL-MoS2. (f) EDX spectrum BM-MoS2.
(a)(b)(c)(d)(e)(f)


### 3.5. Electrochemical Analysis

The capacitive behavior of the material was initially tested using CV. Figure3(a) depicts the bare graphite sheet CV curves, BL-MoS2 and BM-MoS2. BM-MoS2 has a higher current response, resulting in a larger area of the voltammogram than pristine BL-MoS2 and bare graphite sheet, indicating that it has a higher specific capacitance. The performance of CV for BL-MoS2- and BM-MoS2-coated graphite sheets at scan speeds ranging from 10 to 200 mV/s is depicted in Figures 3(b) and 3(c), respectively. Meanwhile, the integral area of all CV curves grows with increasing scan speeds in both cases without distortion in the CV curve. Moreover, this indicates the sample’s strong rate capabilities and capacitance nature. It is also evident that a minor departure from the perfect EDLC profiles in CV curves shows pseudocapacitance behavior [15], which might be caused by ion (K+) intercalation and deintercalation inside the layers of MoS2 and electrolyte ion (K+) adsorption on the surface [9]. The ionic conductivity of electrolytes also plays an essential role in energy storage [16] and KCl matches this condition. Also, KCl has neutral nature and benefits such as reduced corrosion, lower costs, and environmental friendliness.Figure 3
(a) Comparative CV curves of bare graphite sheet, BL-MoS2, and BM-MoS2 at scan rate 100 mV/s. (b) CV plot of BL-MoS2. (c) CV plot of BM-MoS2. (d) GCD profile of BL-MoS2 at different current densities. (e) GCD profile BM-MoS2 at different current densities. (f) EIS spectra of BL-MoS2 and BM-MoS2 with an electrochemical circuit fit. (g) Cyclability of BM-MoS2 at 100 mV/s up to 5000 cycles. (h) Capacitance retention of BM-MoS2 at 10 A/g for 5000 cycles.
(a)(b)(c)(d)(e)(f)(g)(h)Because BM-MoS2 has a higher surface area than BL-MoS2, the specific capacitance rises [17]. The ball milling procedure to the bulk MoS2 results in a large surface area, allowing ions to occupy more space and enriching the specific capacitance of the BL-MoS2. The GCD investigation was performed to evaluate the prepared electrode’s capacity for charge storage. Both BL-MoS2 and BM-MoS2 have asymmetric GCD curves with a slightly irregular triangle shape, exposing the material’s pseudocapacitive nature and electric double-layer capacitance. Also, Dunn’s method was used for exploring the capacitive contribution and surface kinetics of the material (Figure 1(a) of supplementary file). We observed that the Faradaic process contributed as the main mechanism for charge storage in BL-MoS2 electrode (Figure 1(b) of supplementary file). If the capacitance involved only ideal EDLC, the charging-discharging curve has a perfect triangular shape. The GCD curve of BL-MoS2 and BM-MoS2 at different current densities are shown in Figures 3(d) and 3(e). The BL-MoS2 and BM-MoS2 exhibit a specific capacitance of 96 F/g and 114 F/g, respectively, at a current density of 1 A/g. This study exposed that the BM-MoS2 electrode would have greater capacitance than BL-MoS2 because the increase in interaction area with the electrolyte ions due to large surface area [16] provides more ion diffusion channels than BL-MoS2. The prepared electrode was subjected to EIS for further analysis. EIS includes valuable information about the material’s capacitive behavior, ion transportation kinetics, and various internal resistances of the electrode [18]. The EIS analysis was carried out in the frequency range of 10 MHz to 100 kHz, as shown in Figure 3(f), along with the circuit fit diagram. Both BL-MoS2 and BM-MoS2 have a small circular arc in the high-frequency region and a vertically aligned line with a high slope in the lower-frequency region. Using electrochemical circuit fit, we have found the material’s resistance of charge transfers (Rct) from the radius of the semicircle in the high-frequency region [9]. The charge transfer resistance was 4.5 Ω and 5.3 Ω for BM-MoS2 and BL-MoS2, respectively. The lower Rct indicates the enhanced diffusivity of electrolyte ions on the surface [19] of the processed material, which happens because of the large surface area and increased active sites of the BM-MoS2. The BM-MoS2 shows excellent cyclability and reversibility even after 5000 cycles as shown in Figure 3(g). The capacitance retention of the prepared electrode of BM-MoS2 was also tested at a high current density of 10 A/g for 5000 cycles, shown in Figure 3(h), and it was found that after the 5000 cycles, the retention of capacitance was 91.7%, and this shows the high efficiency of the BM-MOS2 electrode.


## 3.1. XRD Analysis

Figure1(a) shows the XRD spectra of both BL-MoS2 and BM-MoS2. The XRD spectra were observed from 10° to 80°. The diffraction peak observed at 14.2°, 32.6°, 39.5°, 44.2°, 49.8°, and 58.3°, relating to the (002), (100), (103), (006), (105), and (110) planes [9], respectively, is as shown in Figure 1(a). From this, we confirm that BL-MoS2 and BM-MoS2 both have crystalline natures. The BL-MoS2 has a very sharp and highly intense peak at 14.38°, which belongs to the 2H structure of MoS2. After ball milling (12 h), the processed material is BM- MoS2 that has the same sharp peak but with reduced intensity, which shows the sign of layered MoS2 [10] formation. It is important to note that the presence of the same peak after exfoliation can be connected to a certain level of layer restacking during the ball-milling process. The average crystallite size (D) of the BL-MoS2 and BM-MoS2 were calculated using the Scherer formula in equation (2). The average crystallite size of BL-MoS2 and BM MoS2 was 36 nm and 30 nm, respectively.(2)D=kλβcosθ,where λ is the wavelength of Cu-Kα radiation, k (0.9) is the structure factor, β is the FWHM, and θ is the diffraction angle.Figure 1
(a) XRD pattern of BL-MoS2 and BM-MoS2. (b) N2adsorption-desorption isotherms of BL-MoS2 and BM-MoS2. (c) Pore size distribution. (d) FTIR spectra of BL-MoS2 and BM-MoS2 in the range of 400 to 4000 cm−1.
(a)(b)(c)(d)


## 3.2. BET Analysis

The surface area was determined using the Brunauer–Emmet–Teller (BET) method for N2adsorption-desorption isotherms. Figure 1(b) displays the N2adsorption-desorption isotherms plots for BL-MoS2 and BM-MoS2. As already discussed, BL-MoS2 exfoliated into numerous layers, size of the particles, and thickness decreases, which affects the surface area of the MoS2 [11, 12]. The higher surface area has a more active site, which can also increase the interaction of electrode and electrolyte; as a result, we have higher electrochemical performance [8]. Both samples exhibit type IV isotherms, and the N2adsorption-desorption isotherms illustrate that the volume of gas trapped was higher in the case of BM-MoS2 than the BL-MoS2. Therefore, because the MoS2 layer is restacked by ball milling, BM-MoS2 has a higher specific surface area (22.593 m2/g) than BL-MoS2 (11.239 m2/g). Figure 1(c) represents both samples’ pore size distribution. BM-MoS2 has a total pore volume of 0.544 cc/g determined by the density function theory (DFT) method for a radius smaller than 173.3 nm, and BL-MoS2 has a total pore volume of 0.482 cc/g for a radius less than 183 nm calculated at a relative pressure of 0.9944.


## 3.3. FTIR Analysis

The FTIR spectra of BL-MoS2 and BM-MoS2 obtained in the IR region between 400 cm−1 and 4000 cm−1 are shown in Figure 1(d). The spectra of MoS2 include an absorption peak at 596 cm−1, which corresponds to the presence of the Mo-S bond, and an additional peak at 903 cm−1, caused by the S-S bonds in MoS2. The absorption bands at 1125 cm−1 and 1634 cm−1 are attributable to the stretching vibrations of the Mo-O bond and the hydroxyl group [13], respectively. The prominent peaks at about 3421 cm−1 are caused by the stretching vibration of OH groups present due to moisture in the sample, while narrow peaks at 2917 cm−1 are related to CH2 [14].


## 3.4. FESEM Analysis

FESEM is used to characterize the morphologies and nanostructures shown in Figures2(a)–2(d) at various scales. The crumpled BL-MoS2 indicates the creation of a few-layer MoS2 nanosheet with a flower-like structure. The ball milling method results in the exfoliation of the BL-MoS2 Figure 2(a) into nanosheets. The difference can also be seen in the FESEM pictures, as shown in Figures 2(b)–2(d). The BL-MoS2, after the ball milling process, exhibits a flower-like nanosheet shape with an average thickness of 40 nm of nanosheets in the FESEM images. The energy dispersive X-ray (EDX) analysis shown in Figures 2(e) and 2(f) was performed to check the purity of both samples and to find out if any alterations were caused during the ball milling process. The EDX spectra of BL-MoS2 are shown in Figure 2(e), and it confirms the purity of bulk material as it can be seen by the as-expected characterized peaks of Mo and S as major elements. We also get small content of carbon and oxygen. After the milling process, the purity of the sample (BM-MoS2) remains intact, as shown in Figure 2(f), in which no additional signal was observed.Figure 2
FESEM images of the pristine bulk and ball milling MoS2. (a) BL-MoS2 100 nm scale. (b) BM-MoS2 at 1 m scale (c and d) BM-MoS2 at 100 nm scale. (e) EDX spectrum of BL-MoS2. (f) EDX spectrum BM-MoS2.
(a)(b)(c)(d)(e)(f)


## 3.5. Electrochemical Analysis

The capacitive behavior of the material was initially tested using CV. Figure3(a) depicts the bare graphite sheet CV curves, BL-MoS2 and BM-MoS2. BM-MoS2 has a higher current response, resulting in a larger area of the voltammogram than pristine BL-MoS2 and bare graphite sheet, indicating that it has a higher specific capacitance. The performance of CV for BL-MoS2- and BM-MoS2-coated graphite sheets at scan speeds ranging from 10 to 200 mV/s is depicted in Figures 3(b) and 3(c), respectively. Meanwhile, the integral area of all CV curves grows with increasing scan speeds in both cases without distortion in the CV curve. Moreover, this indicates the sample’s strong rate capabilities and capacitance nature. It is also evident that a minor departure from the perfect EDLC profiles in CV curves shows pseudocapacitance behavior [15], which might be caused by ion (K+) intercalation and deintercalation inside the layers of MoS2 and electrolyte ion (K+) adsorption on the surface [9]. The ionic conductivity of electrolytes also plays an essential role in energy storage [16] and KCl matches this condition. Also, KCl has neutral nature and benefits such as reduced corrosion, lower costs, and environmental friendliness.Figure 3
(a) Comparative CV curves of bare graphite sheet, BL-MoS2, and BM-MoS2 at scan rate 100 mV/s. (b) CV plot of BL-MoS2. (c) CV plot of BM-MoS2. (d) GCD profile of BL-MoS2 at different current densities. (e) GCD profile BM-MoS2 at different current densities. (f) EIS spectra of BL-MoS2 and BM-MoS2 with an electrochemical circuit fit. (g) Cyclability of BM-MoS2 at 100 mV/s up to 5000 cycles. (h) Capacitance retention of BM-MoS2 at 10 A/g for 5000 cycles.
(a)(b)(c)(d)(e)(f)(g)(h)Because BM-MoS2 has a higher surface area than BL-MoS2, the specific capacitance rises [17]. The ball milling procedure to the bulk MoS2 results in a large surface area, allowing ions to occupy more space and enriching the specific capacitance of the BL-MoS2. The GCD investigation was performed to evaluate the prepared electrode’s capacity for charge storage. Both BL-MoS2 and BM-MoS2 have asymmetric GCD curves with a slightly irregular triangle shape, exposing the material’s pseudocapacitive nature and electric double-layer capacitance. Also, Dunn’s method was used for exploring the capacitive contribution and surface kinetics of the material (Figure 1(a) of supplementary file). We observed that the Faradaic process contributed as the main mechanism for charge storage in BL-MoS2 electrode (Figure 1(b) of supplementary file). If the capacitance involved only ideal EDLC, the charging-discharging curve has a perfect triangular shape. The GCD curve of BL-MoS2 and BM-MoS2 at different current densities are shown in Figures 3(d) and 3(e). The BL-MoS2 and BM-MoS2 exhibit a specific capacitance of 96 F/g and 114 F/g, respectively, at a current density of 1 A/g. This study exposed that the BM-MoS2 electrode would have greater capacitance than BL-MoS2 because the increase in interaction area with the electrolyte ions due to large surface area [16] provides more ion diffusion channels than BL-MoS2. The prepared electrode was subjected to EIS for further analysis. EIS includes valuable information about the material’s capacitive behavior, ion transportation kinetics, and various internal resistances of the electrode [18]. The EIS analysis was carried out in the frequency range of 10 MHz to 100 kHz, as shown in Figure 3(f), along with the circuit fit diagram. Both BL-MoS2 and BM-MoS2 have a small circular arc in the high-frequency region and a vertically aligned line with a high slope in the lower-frequency region. Using electrochemical circuit fit, we have found the material’s resistance of charge transfers (Rct) from the radius of the semicircle in the high-frequency region [9]. The charge transfer resistance was 4.5 Ω and 5.3 Ω for BM-MoS2 and BL-MoS2, respectively. The lower Rct indicates the enhanced diffusivity of electrolyte ions on the surface [19] of the processed material, which happens because of the large surface area and increased active sites of the BM-MoS2. The BM-MoS2 shows excellent cyclability and reversibility even after 5000 cycles as shown in Figure 3(g). The capacitance retention of the prepared electrode of BM-MoS2 was also tested at a high current density of 10 A/g for 5000 cycles, shown in Figure 3(h), and it was found that after the 5000 cycles, the retention of capacitance was 91.7%, and this shows the high efficiency of the BM-MOS2 electrode.


## 4. Conclusion

We have two MoS2 samples, one being bulk MoS2 (BL-MoS2), while the other has undergone ball milling processing (BM-MoS2). Both samples were characterized using XRD, which helps to determine the material’s crystallinity. From BET plots, the active surface area of BL-MoS2 and BM-MoS2 was determined to be 11.239 m2/g and 22.593 m2/g, with a total pore volume of 0.544 cc/g and 0.482 cc/g, respectively. BM-MoS2 results in a slightly higher specific surface area. At a current density of 1 A/g, the constructed electrode using BM-MoS2 has a specific capacitance of 114 F/g, while BL-MoS2 has 96 F/g. MoS2 treated by ball milling has a higher active surface area for electrolyte ion intercalation, which is beneficial for increasing specific capacitance with 91.7% capacitance retention after 5000 cycles. As a result, the suggested approach would open the way for developing high-efficiency supercapacitors using a cost-effective, environment-friendly, scalable, and sustainable approach containing MoS2 as an active material imposed with a ball milling.



---

*Source: 1288623-2022-12-31.xml*