Metallic 1T-WS₂ nanoribbons as highly conductive electrodes for supercapacitors

Abstract

Layered tungsten disulfide (WS₂) has attracted great attention because of its high potential for electrochemical energy applications. However, the semiconducting nature of WS₂ with a 2H phase largely hinders its electrochemical performance due to poor electronic conductivity. In this study, we have successfully synthesized a metallic 1T-WS₂ nanoribbon with stable ammonia-ion intercalation as a highly conductive electrode for high-performance supercapacitors. The specific capacitance using the metallic 1T-WS₂ electrode exhibits significant enhancement upto the value of 2813 μF cm⁻². This value is 12 times higher compared to semiconducting 2H-WS₂. Moreover, the 1T-WS₂ electrode has good stability even under high current scans, which is ascribed to the stable ammonia-ion interaction. The correlation between the 1T-WS₂ structure and its electrochemical performance has also been discussed.

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1. Introduction

Due to their high power density, long life cycle and short charging time, supercapacitors or electrochemical capacitors have a significant role in the development of energy storage devices.^1,2 The supercapacitors have two storage mechanisms, i.e. pseudo-capacitance and electrochemical double-layer capacitance (EDLC).^3,4 Charge aggregates at the electrode–electrolyte interface in EDLC while pseudo capacitors undergo reversible redox reactions at the surface of their electrode materials.^5,6 The pseudo capacitance-based devices have high energy densities as compared to EDLCs. However, the phase changes within the electrode due to the faradic reaction restrict their power density and life time. In EDLC capacitors, different carbon-based materials such as graphene and carbon nanotubes (CNTs) have been explored to use as electrodes. In addition, two dimensional (2D) layered transition-metal dichalcogenides (TMDs) such as WS₂, MoS₂ and VS₂ showed good value during the past decades owing to their similarities to graphene and fullerene-like nanostructures.^7,8 In TMDs, layered tungsten disulfide (WS₂) displays exclusive structural, chemical, thermal, electronic, and optical characteristics.^9,10 The layered WS₂ are used in many applications such as solid lubricant,¹¹ optoelectronic,¹² catalytic,^13,14 and photothermal ablation.¹⁵ However, it has been forecasted tentatively that WS₂ layer has two different symmetries relying on the packaging of S atoms, i.e. 2H phase (trigonal prismatic D3h) and 1T phase (octahedral Oh), the alteration between two phases can be attained through a geometric rotation that involves a transversal displacement of one of the S planes. In fact, the two phases are fairly dissimilar in electronic structures and other characteristics, such as 1T-WS₂ is metallic while 2H-WS₂ is semiconducting in nature.^13,18 It has been predictive that the lamellar structure and features of 2D layered WS₂ will be advantageous for high power applications such as solid state batteries¹⁶ or supercapacitors.¹⁷ However, most of previous reports were related to semiconducting 2H-WS₂, and unfortunately its electrode utilization has been largely constrained due to the poor electronic conductivity. Therefore, tuning semiconducting 2H-phase to metallic 1T-phase is very efficient way for improving the electronic conductivity of WS₂ materials. For example, an alkali metal-exfoliated WS₂ samples with a mixture of 1T and 2H phase has been reported as a promising catalyst. Unlikely, most of such exfoliated WS₂ products were sensitive to atmosphere and exhibited less stability. Therefore, it is also highly desirable to develop 1T-WS₂ materials with stable metallic phase for greatly efficient electrode.

Recently, we have reported a new hydrothermal method to mass-prepare 1T-WS₂ with stable metallic via ammonia ions intercalations.¹⁹ In this work, we further used this material as electrochemical electrode for supercapacitor and attempted a comparative study of the supercapacitor behaviour with 1T-WS₂ and 2H-WS₂ electrode. As anticipated, the metallic 1T-WS₂ nanostructure can easily facilitate the electronic conductivity. Thus it largely improves the specific capacitance and the stability which present promising signs for this new 2D material to be utilized in energy storage devices.

2. Experimental

2.1 Hydrothermal synthesis of metallic 1T-WS₂ nanoribbons

Hydrothermal reaction was used to synthesize ammonium ion-intercalated WS₂ ultrathin layers in a sealed autoclave system. The detailed discussion has already been reported in our previous work.¹⁹ For comparison, the control 2H-WS₂ sample was obtained by annealing the fresh 1T-WS₂ samples at 300 °C for 3 hours under argon atmosphere, then naturally cool down to room temperature. Such heat treatment converts WS₂ nanoribbons from 1T phase to 2H-phase.¹⁹

2.2 Electrochemical measurements

Electrochemical properties of the samples were performed on a CHI660E electrochemical workstation (Shanghai, China) using three electrode. The glassy carbon electrode used as working electrodes, Pt foil was the counter electrode and Ag/AgCl (3 M KCl) was used as the reference electrode. The glassy carbon electrodes (GC) were prepared as, the samples (2.0 mg) was dispersed into a solution containing water (750 μL), isopropyl alcohol (250 μL) and Nafion solution (0.5 wt%, 30 μL), followed by ultrasonic treatment for 10 min. Then the resultant suspension (5 μL) was pipette onto the GC electrode and dried at room temperature.

The voltage range of cyclic voltammetry (CV) experiments was tested from −0.8 V to −0.2 V at different scan rates. The galvanostatic charge/discharge tests were performed from −0.8 V to −0.2 V at different current densities. Electrochemical impedance spectroscopy (EIS) measurements were carried out with amplitude of the signal of 10 mV and frequency in the range of 0.01–106 Hz. All electrochemical experiments were performed at room temperature in the electrolyte of 1 M H₂SO₄.

2.3 Materials characterizations

The prepared samples were characterized by field emission scanning electron microscopy (FE-SEM). Images were taken via a JEOL JSM-6700F SEM. JEM-2100F field emission electron microscopy (TEM) with an acceleration voltage of 200 kV. It was used to collect high-resolution TEM images. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were carried out in a Titan Cs-corrected Chemi-STEM (80 kV) via atomic resolution analytical microscope. Raman spectra were detected by a Renishaw RM3000 Micro-Raman system with a 514.5 nm Ar laser.

3. Results and discussion

Transmission electron microscope was used to investigate the surface and morphology of the synthesized WS₂ samples. A typical TEM image of samples in Fig. 1(a) revealed a ribbon-like structure with the width of 50 to 200 nm. The thickness is at range of 3–5 nm and the length is hundreds of nanometres. Fig. S1† shows typical SEM images of ammonium ion-intercalated WS₂ with ribbon-like morphology. For atomic structure investigations, we used HAADF imaging mode in an advanced aberration-corrected with high resolution scanning TEM facility (STEM) directly observed the 1T-WS₂ phase. From Fig. 1(b), it is seen that the material is present in unique zigzag chain superlattices with the nearest W–W distance of 2.7 Å. It indicates the presence of 1T-WS₂ nanoribbons. The detailed characterizations can be found in ESI,† which further confirms that the synthesized WS₂ materials have stable metallic 1T phase which is similar to our previous report.¹⁹

Fig. 1 (a) TEM image of 1T-WS₂ nanoribbons (b) a typical HADDF-STEM image of 1T-WS₂ shows clear atomic patterns.

Due to its high conductivity, we used the obtained 1T-WS₂ as electrode for supercapacitor. Fig. 2 shows the comparison of electrochemical performance for semiconducting 2H-WS₂ and metallic 1T-WS₂. It has been noticed from Fig. 2(a) that both 2H-WS₂ and 1T-WS₂ at 10 mV s⁻¹ scanning rate show near-rectangular shape of cyclic voltammograms, revealing the typical double-layer capacitor behaviour. As compare to the 2H-WS₂ electrode-based device, the area of the CV curve for 1T-WS₂ electrode significantly increases, which means it enhances the capacitance.

Fig. 2 Electrochemical performances: (a) cyclic voltammograms (CV) of supercapacitor cells having 1T-WS₂ and 2H-WS₂ electrodes, at a scan rate of 10 mV s⁻¹, (b) CV of 1T-WS₂ at different scanning rates of 10, 50, and 100 mV s⁻¹, (c) CV of 2H-WS₂ at different scanning rates of 10, 50, and 100 mV s⁻¹.

The enhancement of electrochemical performance suggests the effective utilization of electrode material by the electrolyte due to the metallic behaviour and the intercalated structure in 1T-WS₂ nanoribbons. Moreover, Fig. 2(b) and (c) present the CV curves at different scanning rate for 1T-WS₂ and 2H-WS₂. The absence of redox peaks in both cases confirms that there is no contribution of pseudocapacitor behaviour. Notably, the 1T-WS₂ electrode exhibited stable shape as the scan rate increased from 10 to 100 mV s⁻¹. This indicates its excellent rate of performance.

Fig. 3 shows the galvanostatic charge–discharge profiles of the supercapacitors with 1T-WS₂ and 2H-WS₂ electrodes. Following relation is used to calculate the specific capacitance:²⁰

where C represents the capacitance, I applied current, t discharge time and V represents the voltage while, A is the total electrode area. The thickness of the electrode was measured to be 200 nm. Accordingly, the calculated specific capacitance is 2813 μF cm⁻² for 1T-WS₂ and it is 223.3 μF cm⁻² for 2H-WS₂ at current density of 0.5 A m⁻². Notably, the capacitance value is enhanced more than 12 times by using metallic 1T-WS₂ electrode.

Fig. 3 Charge–discharge profiles at current density 0.5 A m⁻² and IR drop illustration.

The voltage drop (IR) observed in Fig. 3, implied that a very small intrinsic series resistance inside the electrodes and high conductivity which confirmed the metallic nature of the 1T-WS₂ nanoribbons. The initial voltage drop shows that the equivalent series resistance (ESR) is limited to the electron transfer and causes small voltage limitations.

Fig. 4(a) and (b) show the cycling performance of 1T-WS₂ and 2H-WS₂ electrodes for 2000 charge/discharge cycles. It is observed that the capacitance of 1T-WS₂ electrode was decreased rapidly in the first 300 cycles, and then shows high stability as compared to initial cycles. But in case of 2H-WS₂, the capacitance reduces continuously. It is also observed that 1T-WS₂ is more stable as compared to 2H-WS₂.

Fig. 4 Capacitance retention for 2000 cycles (a) 1T-WS₂ (b) 2H-WS₂ (c) Raman spectra of 1T-WS₂ before and after cycles.

To further understand the stability of 1T-WS₂ electrode, Raman spectroscopy was performed on the samples before and after electrochemical testing. Fig. 4(c) displays a typical Raman spectrum of 1T-WS₂ sample before and after 5000 charge/discharge cycles. The spectrum of 1T-WS₂ before cycling test shows rich peaks at low frequency region, which possesses enormous Raman features of 1T metallic phase.^19,21 It is observed that the spectra of 1T-WS₂ samples before and after electrochemical test are very similar. This strongly confirmed the high stability of metallic phase in the synthesized 1T-WS₂ samples during cycling.

Electrochemical impedance spectroscopy measurement was carried out with an ac bias voltage of 0.01 V with the frequency range from 0.01 to 106 Hz. Fig. 5 shows the Nyquist plot of the 1T-WS₂ and 2H-WS₂ samples. Semicircle region in high frequency region almost disappeared which shows the interfacial charge transfer resistance at the electrode and the electrolyte is very small due to high conductivity of the material.²² But straight line in low frequency region indicates the Warburg impedance. The interruptions during ions diffusion process in the sample is measured as Warburg impedance. The larger Warburg impedance shows the larger variations in ions diffusion path lengths and enhances the obstacle of ions movement. In low frequency region if the Warburg angle is more vertical than 45° then the electrode shows best performance for supercapacitor. The 1T-WS₂ electrode shows more vertical line as compared to 2H-WS₂ electrode. This observation indicates that 1T-WS₂ electrode provide less resistance to ions diffusion due to its metallic behaviour and the extended layer–layer space, which will be more efficient for supercapacitor.^23,24

Fig. 5 Comparison of Nyquist plots of 1T-WS₂ and 2H-WS₂ electrodes.

4. Conclusions

Stable metallic 1T-WS₂ nanoribbons were synthesized through hydrothermal processes for the study of supercapacitor. The metallic 1T-WS₂ nanoribbons with advantage of high conductivity were developed as electrode for high-performance supercapacitor. The electrochemical characterizations of 1T-WS₂ electrode supercapacitor have shown the specific capacitance of 2813 μF cm⁻², which is 12 times higher than that of semiconducting 2H-WS₂ electrode. The excellent capacitive performance of 1T-WS₂ is attributed to its high electrical conductivity and unique intercalation structure with extended layer space. This will facilitate the rapid transport of the electrolyte ions and subsequently will increase electrochemical utilization of the material. The 1T-WS₂ electrode provided technical feasibility for further improvement and scaled up the practical power source of intelligent electrical devices. Thus, the given sample accelerates the new 2D material that utilized in energy storage devices.

Acknowledgements

This work was partly supported by the 973 Program (No. 2014CB848900), NSFC (No. U1232131, U1532112, 11375198, 11574280), CUSF (WK2310000053), User with Potential from CAS Hefei Science Center (2015HSC-UP020) and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) Nankai University. L. S. thanks the recruitment program of global experts, the CAS Hundred Talent Program. We also thank the Hefei Synchrotron Radiation Facility (MCD and Catalysis, photoemission and Surface Science End stations, NSRL) for help in characterizations.

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Footnotes

† Electronic supplementary information (ESI) available: SEM, XRD and STEM data. See DOI: 10.1039/c6ra08975e

‡ The authors contributed equally to this work.

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