Electrochemical and electronic properties of flower-like MoS₂ nanostructures in aqueous and ionic liquid media

Abstract

In the present work, we report a facile strategy to synthesize uniform 3D flower-like MoS₂ nanostructures prepared by a one-pot hydrothermal method and investigate their supercapacitive behavior. A field emission scanning electron microscopy, atomic force microscopy and X-ray diffraction study reveals the formation of randomly stacked layers of MoS₂. The electrochemical properties of MoS₂ nanostructures were investigated using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectra (EIS) techniques in aqueous and ionic liquid media. The CV measurement shows that the as-synthesized MoS₂ electrode delivered a maximum capacitance of 218 F g⁻¹ at a scan rate of 5 mV s⁻¹ in aqueous medium. The GCD measurement shows a maximum specific capacitance of about 217.6 F g⁻¹ at a discharge current density of 0.1 A g⁻¹ in aqueous electrolyte. EIS with an appropriate electrical equivalent circuit was employed to understand the charge storage mechanism in the MoS₂ electrode. Cyclic stability tests in aqueous medium reveal a capacitance retention of about 76% after 1000 cycles. This study reveals that a nearly pure capacitive behavior is observed for aqueous electrolyte and a diffusive behavior is observed for the MoS₂ electrode in ionic liquid medium.

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Introduction

In the past few years supercapacitors have emerged as some of the most promising energy storage devices, where high power density is required.^1,2 Supercapacitors are widely employed as power sources in various applications such as hybrid electric vehicles, stand-by power systems, portable electronic equipment and digital telecommunication systems.^3–5 The field of supercapacitors has long been a topic of discussion among researchers for developing an advanced electrode material having high power and energy density at low cost.^6,7

A wide range of carbonaceous material, metal oxides, conducting polymers and metal chalcogenides are used in lithium ion batteries and supercapacitor applications.^8–12 Molybdenum sulfide (MoS₂), a metal chalcogenide, has attracted major attention in a variety of applications like hydrogen storage, solid lubricant lithium ion batteries and supercapacitors.^13–15 The reason for its success is due to its unique crystal structure and optical, electrical and physical properties. MoS₂ is composed of a layered structure like graphite and the layers are stacked together by weak van der Waals interactions. Due to the weak van der Waals interactions and high surface area, MoS₂ can be exfoliated to a sheet-like morphology which can exhibit better electrical and capacitive properties for supercapacitor applications.^16–19 Various synthetic routes have been developed for the synthesis of MoS₂ nanostructures and composites, such as gas phase reaction, magnetron sputtering, microwave irradiation, laser ablation and hydrothermal methods. Gao et al.²⁰ reported a facile one-pot synthesis of MoS₂ quantum dots/graphene/TiO₂ composites with highly enhanced photo-catalytic property. Sun et al.²¹ synthesized nanocomposites of C/MoS₂ which showed a capacitance of 210 F g⁻¹ at a current density of 1 A g⁻¹. Ma et al.¹⁹ reported polypyrrole/MoS₂ nanocomposites with a specific capacitance of 553.7 F g⁻¹ at 1 A g⁻¹. Recently, Huang and coworkers²² demonstrated polyaniline/2-dimensional graphene analog MoS₂ composites for supercapacitor application. Most of the studies discussed above have been reported for MoS₂-based composites. Reports on the applicability of MoS₂ nanostructures for supercapacitor application and supercapacitive behavior are still limited.

Electrochemical applications of MoS₂ electrode are still hindered by the limited understanding of the interface between the electrode material and aqueous and ionic liquid (IL) media. In the present work, we report a facile strategy to synthesize uniform 3D flower-like MoS₂ nanostructures prepared by a one-pot hydrothermal method and investigate their supercapacitive behavior. It is observed that the specific capacitance of MoS₂ electrode in IL for the entire range of scan rates is higher than that of the MoS₂ electrode in aqueous electrolyte due to the higher working voltage window. Moreover, the significant energy loss in aqueous medium not only reduces the capacitive performance but also degrades the electrode active material by generating much heat during the charge and discharge process.

Experimental section

Synthesis and characterization of MoS₂ nanostructures

Ammonium molybdate and thiourea used in this experiment were of research grade and used without further purification. The solutions were prepared using deionized water. The flower-like MoS₂ nanostructures were synthesized as follows: 0.23 g of ammonium molybdate and 0.3 g thiourea were dissolved in 20 ml deionized water under vigorous stirring for 2 hours. The solution was then transferred into a Teflon-lined stainless steel autoclave and kept for 22 hours at 220 °C. The autoclave was cooled down to room temperature naturally and the resulting black precipitates of MoS₂ were collected by filtration. The collected MoS₂ powder was washed and centrifuged with distilled water and ethanol several times to remove the residual reactants. The obtained MoS₂ powder was then dried at 60 °C for 12 hours. The morphological characterization of the as-synthesized MoS₂ powder was done using a Carl Zeiss field emission scanning electron microscope (FESEM). The elemental and structural characterization was conducted using energy dispersive X-ray analysis (EDAX) and X-ray diffraction (XRD).

Electrochemical studies of electrodes

The working electrode for the electrochemical measurement was fabricated by mixing the as-synthesized MoS₂, acetylene black, and polyvinylidene fluoride in a ratio of 80 : 10 : 10. A homogeneous paste was obtained by adding a small amount of N-methyl-2-pyrrolidone to the mixture and the paste was coated on a fluorine-doped glass substrate. The loading level of MoS₂ active material on the substrate is 5 mg cm⁻². The electrodes were then dried in air at 90 °C for 6 h to remove the solvent. Electrochemical measurements were performed with a CH Instruments 660D electrochemical workstation in 1 M KCl aqueous electrolyte and BMIMPF₆ ionic liquid solution with a three-electrode configuration at room temperature. Cyclic voltammetry (CV) measurement was carried out at different scan rates from 5 to 200 mV s⁻¹. A galvanostatic charge/discharge (GCD) test was also conducted at different current densities for the fabricated electrodes. Electrochemical impedance spectroscopy (EIS) measurements were conducted by applying an AC perturbation voltage of 10 mV amplitude in a frequency range from 10 mHz to 1 MHz.

Results and discussion

Morphological, elemental and structural analysis

Fig. 1A shows a low-magnification surface morphology of the as-synthesized MoS₂ nanostructures. The as-synthesized MoS₂ powder consists of several individual spherical shaped particles. The spherical growth of MoS₂ mainly depends upon the material used as a precursor. During the hydrothermal synthesis, ammonium molybdate releases MoO₄⁻ ions and ammonium ions whereas thiourea is used as a sulfur source. From the synthesized products, MoO₄⁻ ions react with sulfur ions to form MoS₂ and the interaction of residual ammonia prevents the stacking of MoS₂ nanostructures.²³ The high-magnification FESEM image shown in Fig. 1B clearly shows the self-assembled nanosheets where many interconnected nanosheets with lots of folds are seen. The average grain size of MoS₂ nanostructures was estimated from ImageJ software. The analysis reveals that the average radius of MoS₂ spheres is ∼300 to 350 nm.

Fig. 1 (A) Low- and (B) high-magnification FESEM images of MoS₂ nanoflowers, (C) AFM topological micrograph in 3D view and (D) EDAX analysis of synthesized MoS₂ nanostructures.

The roughness and surface morphology of MoS₂ thin film on FTO substrate were investigated using AFM in tapping mode. Fig. 1C shows the AFM image of MoS₂ thin film in 3D mode. The surface topographical image shows a good quality of the film with root mean square roughness approximately equal to 200 nm. High roughness and porous morphology of a supercapacitor electrode are ideally required for enhancing the electrochemical performance by providing fast ion/electron transfer, and sufficient contact between electrolyte and active material. Therefore an enhanced electrode performance and charge storage behavior could be expected for the as-synthesized MoS₂. The composition of MoS₂ thin film was analyzed by EDAX. Fig. 1D depicts the compositional EDAX image of mesoporous MoS₂ nanostructures. The EDAX analysis reveals the compositional contribution from Mo and S elements where, further, no extra elemental peak confirms the purity of as-synthesized samples.

The crystal structure and phase purity of the as-prepared MoS₂ nanostructures were characterized by XRD measurements. Fig. 2 shows the XRD pattern of MoS₂ nanostructure powder. The XRD profile of MoS₂ nanostructures shows diffraction peaks at 2θ values of 14.1°, 33.6°, 39.2°, 45.2°, 50.1°, 59.4° and 70.6° which are identified as the reflections from (002), (100), (103), (104), (105), (110) and (201) planes of hexagonal MoS₂.^24,25 The obtained diffraction peak are well matched with MoS₂ (JCPDS 37-1492) indicating that a pure MoS₂ is synthesized. The crystallite size of MoS₂ is obtained from the relation where β is the full width at half maximum, ε is the lattice strain and L is the crystallite size.²⁶ The reciprocal of the intercept on the axis gives information about the average crystallite size which happens to be ∼76 nm in the present study. This implies that the fundamental particles are of nano-dimensions. Moreover, their ordered organization under the reaction conditions leads to the two-dimensional (sheet-like structure) and finally three-dimensional hierarchical flower-like structure.

Fig. 2 XRD spectrum of MoS₂ nanostructure.

Electrochemical characterization

Cyclic voltammetry in aqueous medium. The electrochemical performance of supercapacitor electrodes was investigated using CV, GCD and EIS measurements. The active electrodes of flower-like MoS₂ were tested in a three-electrode configuration. Fig. 3A illustrates the CV performance of active electrode in 1 M KCl solution as a function of scan rate. It is observed that at a lower scan rate the CV curves exhibit a rectangular shape with a small reduction peak during the cathodic scan. The area under the CV curves of the MoS₂ electrode increases with an increase in scan rate indicating the capacitive property of the MoS₂ electrode. Moreover, the CV curves at higher scan rate exhibit a quasi-rectangular shape or resemble an ‘S’ shape suggesting that the charge storage is due to the pseudo-capacitance and double layer charge storage mechanism. The deviation in the rectangular shape and the occurrence of reduction peaks are also observed by other authors.^15,19 The holes injected from the electrolyte into the valence band of n-type MoS₂ during the cathodic polarization can diffuse into MoS₂ and recombine with electrons, giving rise to sharp reduction peaks. However, the corresponding anodic current is extremely small because only a few holes are available in n-type material.²⁷ The deviation in the rectangular shape is likely due to the effects from higher resistive leakage often seen in EDLC devices. Soon et al.²⁷ and Ramadoss et al.¹⁵ have explained the reduction of redox active species of Mo atoms as the origin of the reduction peak in CV and the charge storage mechanism is explained through two possible predictions. The first one is based on a non-Faradaic process where the adsorption of protons or cations on the MoS₂ nanoflower surface is given as

(MoS₂)_surface + C⁺ + e⁻ ↔ (MoS₂–C⁺)_surface

Fig. 3 Electrochemical characterization in a three-electrode cell. (A) Cyclic voltammograms of MoS₂ electrodes in aqueous medium at different scan rates, (B) variation of specific capacitance as a function of scan rate, (C) galvanostatic charge/discharge plots at different current densities and (D) Nyquist plot of MoS₂ electrode in aqueous medium at open circuit potential (the inset shows the high-frequency details).

On the other hand, the second process is based on the pseudo-capacitive behavior due to the Faradaic charge transfer process during which the electrolyte ions such as alkali metal cations or H⁺ ions may diffuse into the layer structure of MoS₂ (i.e. intercalation). The above reduction process can be expressed through the equation MoS₂ + C⁺(Na⁺, Li⁺ or H⁺) + e⁻ ↔ MoS–SC⁺. In the reverse scan the corresponding de-intercalation process gives rise to the feeble oxidation wave. The values of the specific capacitance were calculated from the CV curves using the general expression C_sp = ∫i dV/SAΔV F g⁻¹, where i is the net CV current, S is the scan rate, ΔV is the applied potential window through which the device is scanned and the integral of the equation provides the area of the CV curve.^28,29 Fig. 3B illustrates the variation of specific capacitance of the MoS₂ electrode in 1 M KCl with respect to the scan rate. It is observed that specific capacitance increases with a decrease in scan rate, which is due to the fact that at lower scan rate ions have sufficient time to diffuse into the interlayers of MoS₂ active layer, thereby providing more active sites for the charge transfer process to occur.² The highest value of C_s obtained from CV is about 218 F g⁻¹ at a scan rate of 5 mV s⁻¹. The obtained values of C_s for flower-like MoS₂ are comparable to the values obtained by Ramadoss et al.¹⁵ and Soon et al.²⁷ The authors have shown that C_s of the MoS₂ electrode in KCl electrolyte was higher than in Na₂SO₄ electrolyte which may be due to the small hydrated ionic radius and high ionic conductivity of K⁺ ions. Along with Ramadoss et al.,¹⁵ Soon et al.²⁷ also showed that MoS₂ nanostructures show pseudo-capacitance in a number of redox active systems. In this work also we have observed the characteristics of Faradaic charge transfer with electrolyte at higher scan rates.

Galvanostatic charge/discharge in aqueous medium. Fig. 3C depicts the GCD curves of the MoS₂ electrode at different current densities. It is observed that discharge time decreases and the iR potential drop increases with an increase in discharge current. The occurrence of the potential drop across the electrode–electrolyte interface (MoS₂–KCl) depends upon factors like internal resistance of the active material, solution resistance and contact resistance at the electrode–electrolyte interface.⁵ The specific capacitance of the MoS₂ electrode can also be calculated from the GCD cycles using the relation C_sp = iΔt/mΔV F g⁻¹, where Δt (s) is the discharge time and m (mg) is the mass of the electroactive material. The specific capacitance calculated at a current density of 0.1 A g⁻¹ is found to be 217.6 F g⁻¹. The obtained values of C_s are in good agreement with the CV curves. The cycling stability of the MoS₂ electrode was determined by CV measurement at a scan rate of 50 mV s⁻¹ for 1000 cycles. Fig. S1 (ESI†) shows the capacitance retention is ∼76% even after 1000 cycles which indicates a good cyclic stability of the electrode material.

Electrochemical impedance spectroscopy in aqueous medium. EIS has been done to further characterize the MoS₂ electrode. EIS of the MoS₂ electrode in KCl was conducted within the frequency range of 10 mHz to 1 MHz with 10 mV RMS voltage. Fig. 3D illustrates the complex Nyquist spectrum of the MoS₂ electrode at an open circuit potential (OCP) of 250 mV. The obtained complex spectrum consists of three distinguishable features, i.e. high-frequency semicircle associated with the double layer charging/discharging, mid-frequency spectrum related to the charge transport and ion diffusion, and low-frequency line parallel to the imaginary axis associated with the interfacial capacitance. To get a further insight into the charge transfer mechanism, the impedance spectrum is fitted with an electrical equivalent circuit. A reasonable equivalent circuit similar to Randle's circuit (shown in ESI†) is used to fit the experimental data. The element R_S corresponds to the series resistance due to the electronic and ionic contribution of the electrode–electrolyte interface. The resistor element R_ct is more likely due to the charge transfer barrier in the system³⁰ while the element R_i corresponds to the electrode–electrolyte interfacial resistance. The double layer capacitance (C_dl) and interfacial capacitance (C_i) are accounted for by the constant phase elements (CPE). CPE is frequently employed to account for surface inhomogeneity and porosity. In the present case this is mainly caused by the orderly arranged MoS₂ nanosheets. The obtained Nyquist spectra are fitted through the discussed circuit by using Autolab software. The fitting results of the equivalent circuit at OCP are described as, first, series resistance (R_S): by considering the high-frequency intercept of real impedance axis an approximate value of R_S of 21 Ω is obtained. The obtained value of R_S is comparable to the value of R_S obtained by Winchester et al.³¹ for liquid phase exfoliated two-dimensional layers of MoS₂ in KOH electrolyte. The second factor is R_ct, where the value of R_ct is 210 Ω. The obtained value of R_ct in our case is comparable with the value obtained by Winchester et al.³¹ and higher than that of Ramadoss et al.¹⁵ In general, a lower value of R_ct leads to the shortening of the ion diffusion path which reflects the higher charge discharge performance. The third factor is C_dl, where the value of C_dl was calculated using the relation C = P[f¹⁻ⁿ × S_rn(nπ/2)]. The value of n provides information of the electrode capacitance behavior where a value of n equal to 1 signifies an ideal capacitance. In the present study, the fitted value of n is approximately equal to 1. The fourth factor is w, which is obtained as 0.8 Ω s^−1/2. The presence of w is attributed to the contribution of pseudo-capacitance which is similar to the results obtained from the analysis of CV (in the context of Fig. 3B at high scan rate).

Cyclic voltammetry in IL medium. Electrochemical performance of the MoS₂ electrode was further investigated in a commonly used IL, i.e. BMIMPF₆, as an electrolyte. Fig. 4A shows the electrochemical characteristic of the MoS₂ electrode within the bias range of −1.8 to 1.5 V as a function of scan rate. As illustrated in Fig. 4A, the CV curves under different scan rates are less rectangular and symmetric than those of aqueous electrolytes. This may be likely due to the higher ionic resistance, often seen for IL-based EDLC devices. Fig. S2† summarizes the variation in the value of specific capacitance as a function of scan rate. The specific capacitance of the MoS₂ electrode in IL at the entire range of scan rates is higher than that of the MoS₂ electrode in aqueous electrolyte. We believe that the ease of ionic hopping in IL medium could improve the formation of electrical double layer and hence improve the specific capacitance in a supercapacitor. It is clear that the performance of the MoS₂ electrode in IL is better than that in the aqueous medium in terms of specific capacitance.

Fig. 4 Electrochemical characterization in a three-electrode cell. (A) Cyclic voltammograms of MoS₂ electrodes in ionic liquid medium at different scan rates and (B) Nyquist plot of MoS₂ electrode in ionic liquid at open circuit potential.

Electrochemical impedance spectroscopy in IL medium. Fig. 4B shows the complex Nyquist spectrum of MoS₂ in IL at OCP within the same frequency range and perturbation voltage as discussed in the context of aqueous electrolyte. The obtained complex Nyquist spectrum is fitted with the same electrical equivalent circuit shown in Fig. S3 (ESI†). As compared to the aqueous medium, the following changes are observed in the IL medium. First, R_S: a value of 342 Ω is obtained for MoS₂ in IL as compared to 21 Ω for aqueous electrolyte. In general R_S at the electrode–electrolyte interface involves two major aspects: (1) ionic, which is related to the intrinsic resistance of electrode and electrolyte in the interior of the porous MoS₂ electrode and (2) electronic, which is associated with the intrinsic resistance of current collector, contact resistance between current collector and MoS₂ active material, and intrinsic resistance of MoS₂.² A higher value of R_S in IL is ascribed to the first aspect because all the other mechanistic factors, i.e. current collector, active electrode and paradigm of cell, are the same for both test conditions. Second, the value of R_ct obtained in IL is 564 Ω and is comparable to the values obtained for MoS₂ in BMIMPF₆.³¹ Third factor, C_dl: the fitted value of n in IL is approximately equal to 0.82. A lower value of n in the case of IL could be correlated with the ion size and surface accessibility.³⁰ The ions in the IL are heavier than the ions of aqueous medium and therefore the surface accessibility of the MoS₂ electrode is restricted. Fourth, w: the obtained value of w in IL is equal to 0.03 Ω s^−1/2.

Ragone plot. The Ragone plots obtained from the discharge curves may also provide some important information.^32,33 The energy and power density of the MoS₂ electrode in aqueous and IL media were calculated from the discharge curve of CV according to the expressions respectively, where E and P represent the power density (W kg⁻¹) and specific energy density (W h kg⁻¹), respectively, and m represents the active mass of the electrode. The Ragone plots for the MoS₂ electrode calculated at 5, 10 and 20 mV s⁻¹ in both media are shown in Fig. 5. As observed, the energy density of MoS₂ in IL medium decreases from 603 to 194 W h kg⁻¹ with an increase in power density from 30 to 40 W kg⁻¹. However, MoS₂ in aqueous medium exhibits much lower energy density which may be attributed to the lower working cell voltage than that of IL.

Fig. 5 Ragone plots of MoS₂ electrodes in aqueous and ionic liquid media.

The energy density of MoS₂ in IL reaches a high value at a relatively low power and thus this configuration can be proposed to be an energy oriented device. Note that depending upon the potential window of the MoS₂ electrode in aqueous and IL media a significant energy loss is obtained for MoS₂ in aqueous medium. The energy loss at the electrode–electrolyte interface is defined as the difference between the electrical energy charged into the electrode and the electrical energy delivered by the active electrode. The electrical energy charged into the MoS₂ electrode in aqueous medium is 172 W h kg⁻¹ and the energy discharged is 60 W h kg⁻¹ while the energy charged and discharged is 271 W h kg⁻¹ and 166 W h kg⁻¹ for the MoS₂ electrode in IL medium. The electrical energy charged and discharged into the active electrode is calculated as 5 mV s⁻¹ from the CV curves obtained for the MoS₂ electrode in both media. The significant energy loss in aqueous medium not only reduces the capacitive performance but also degrades the electrode active material by generating much heat during the charge and discharge process. Accordingly, Joule heating will sharply increase the cell temperature to an unacceptable level.

Conclusions

A facile strategy to synthesize and investigate the supercapacitive behavior of uniform 3D flowerlike MoS₂ nanostructures prepared by a one-pot hydrothermal method is reported in the present article. The specific capacitance of the MoS₂ electrode in IL at the entire range of scan rates is higher than that of the MoS₂ electrode in aqueous electrolyte. It is observed that the energy density of the MoS₂ electrode in IL medium decreases from 603 to 194 W h kg⁻¹ with an increase in power density from 30 to 40 W kg⁻¹. However, MoS₂ in aqueous medium exhibits much lower energy density which may be attributed to the lower working cell voltage than that of IL. Also, the significant energy loss in aqueous medium not only reduces the capacitive performance but also degrades the electrode active material by generating much heat during the charge and discharge process.

Acknowledgements

The authors would like to acknowledge Ms Margi Jani for AFM and EDAX measurements. The authors gratefully acknowledge DST (project no. SR/S1/PC-44/2011 dated 04/07/2012) for financial assistance. One of the authors (K. P.) thanks the DST INSPIRE fellowship program for a junior research fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09282e

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