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Superior Capacitive Performance Enabled by Edge-oriented and Interlayerexpanded MoS2 Nanosheets Anchored on Reduced Graphene Oxide Sheets yajun JI, Qilin Wei, and Yugang Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05342 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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Industrial & Engineering Chemistry Research

Superior Capacitive Performance Enabled by Edge-oriented and Interlayer-expanded MoS2 Nanosheets Anchored on Reduced Graphene Oxide Sheets Yajun Ji,†,‡ Qilin Wei,† Yugang Sun*,† †Department of Chemistry, Temple University, 1901 North 13th Street, Philadelphia, Pennsylvania 19122, United States ‡Collage of science, University of Shanghai for Science and Technology, Jungong Road 334#, 200093Shanghai, People’s Republic of China

*Correspond author: [email protected]

Abstract

Performance of MoS2-based supercapacitors is severely restricted by the limited ionic intercalation in the MoS2 molecular interlayer gaps and the poor intrinsic electrical conductivity of MoS2. To tackle these challenges, high-density edge-oriented (EO) MoS2 nanosheets with an expanded interlayer spacing of 9.4 Å supported on reduced graphene oxide (rGO) sheets are synthesized with the assistance of microwave heating. Using the 1

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edge-oriented and interlayer-expanded (EO&IE) MoS2/rGO as active electrode materials of capacitors, the specific capacitance can reach 346.5 F∙g–1 at a scan rate of 1 mV∙s–1, which is higher than the values of most reported supercapacitors using various MoS2/graphene composites. The enhanced capacitance originates from the fast and easy intercalation of ions in the supported EO&IE MoS2 nanosheets, facilitated by high electrical conductivity of the rGO supporting sheets as well as the edge-oriented geometry and expanded interlayer spacing of the MoS2 nanosheets.

KEYWORDS: Molybdenum disulfide/graphene composite, edge-oriented MoS2 nanosheets with expanded interlayer spacing, microwave-assisted solvothermal synthesis, electrochemical energy storage, supercapacitor

1. Introduction Supercapacitors (SCs) have gained great research focuses due to their ultra-high power density, rapid charge/discharge rate, long cycle life, low cost of maintenance and reliable operation.1-5 High-capacitance electrode materials require high surface areas and good electrical conductivity,6-7 both of which benefits electrical double-layer capacitance8-9 and electrochemical pseudocapacitance.10 Therefore, nanostructured materials possessing

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adequate charge transport properties, suitable ionic diffusion kinetics, and diverse surface chemistries are promising for constructing high-performance supercapacitor electrodes. Electrically conductive carbon materials with high surface areas and suitable pore sizes have been widely used for capacitors relying on electrical double-layer capacitance, whereas electroactive conductive polymers or transition-metal oxides are mainly employed as electrode materials to achieve high electrochemical pseudocapacitance.11-12 Integrating carbon materials (such as rGO sheets) with two-dimensional (2D) nanostructured transition-metal dichalcogenides (TMDCs) (e.g., MoS2,13-14 WS2,15 TiS2,16 VS217 and TaS218), represents an efficient way to significantly increase capacitance of the hybrid nanocomposites. Among the TMDCs, nanostructured MoS2 composed of stacked 2D molecular S–Mo–S layers with numerous active sulfur sites has been intensively explored as promising supercapacitive,18-22 lithium/sodium ion electrode,23 biosensing materials24 and electrocatalytic materials.25 The major drawbacks of MoS2, including the low electrical conductivity and small interlayer spacing of stacked nanosheets, still limit its wide application in supercapacitors. For example, MoS2 nanosheets have a tendency to restack, leading to a significant reduction of the specific surface area of the electrochemically active MoS2 layers. Many researchers have managed to wrap MoS2 into porous conductive networks to enhance its electrochemical performance. The conductive network scaffolds of graphene nanosheet,13 carbon nanotube,26-27 carbon nanofiber,28 mesoporous carbon,29 graphite,30 and polyaniline31 are used as supports to 3



grow MoS2, increasing the electrical conductivity of the MoS2-based composites and the capacitive performance of the corresponding devices.32 However, the reported capacitances of MoS2 hybrids are still low. Therefore, design and synthesis of hybrid MoS2/graphene nanostructures with more accessible ionic transfer channels and high electrical conductivity represents the main challenge to achieve high capacitance.33-34 Herein, high-density EO&IE MoS2 nanosheets decorated on rGO sheets have been successfully synthesized via a microwave-assisted solvothermal route and explored for achieving superior performance in terms of specific capacitance. The edge-oriented geometry of the MoS2 nanosheets opens the maximum number of channels for ion intercalation. The expanded interlayer spacing in the MoS2 nanosheets significantly lowers the energy barrier for ion intercalation/transport and thus increases the ion intercalation capacity. The presence of highly electrical conductive rGO sheets dramatically reduces the electrical resistance between the MoS2 nanosheets and the charge collecting electrodes. The synergy between the unique features of the EO&IE MoS2 nanosheets and the rGO sheets, originated from the strong coupling via the intimate MoS2/rGO interfaces, results in superior electrochemical capacitance of the EO&IE MoS2/rGO hybrid structures using aqueous electrolytes of cations of the first group elements.

2 Experimental Section 4


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2.1 Synthesis of EO&IE MoS2/rGO and IE MoS2 The synthesis of EO&IE MoS2/rGO hybrid structures was adopted from our previous work.35 Briefly, graphene oxide (GO) sheets were firstly prepared by following a modified Hummers’ method.36 Ten mg of (NH4)2MoS4 (99.97%, Sigma-Aldrich) was then added to 6 ml of dimethylformamide (DMF, anhydrous, 99.8%, Sigma-Aldrich) containing 3 mg of GO followed by stirring for 15 min under ambient condition. The resulting solution was transferred to a 10-ml microwave reaction vessel, which was then heated up to 260°C at the fast ramp and the temperature was maintained for 2 h in a microwave reactor (Monowave 300, Anton Parr) operated under the sealed vessel mode. The reaction solution was cooled down to room temperature with pressurized nitrogen flow and the resulting black product was collected via centrifugation (8000 rpm, 10 min). The precipitate was washed with distilled water and absolute ethanol for at least four times. The sample was then dried at 60 °C in an oven for 4 h and the product was labeled as EO&IE MoS2/rGO.

The precursor of (NH4)2MoS4 was completely converted to

MoS2 after the synthesis reaction because the color of (NH4)2MoS4 completely disappeared. The corresponding mass of MoS2 was calculated as 6.15 mg.

Based on

our procedure, the molar ratio of C:O in the GO sheets was reasonably estimated as 2.5. 37 If GO was completely reduced to rGO, the mass of rGO was calculated as 2.14 mg. Therefore, the mass content of MoS2 in the as-synthesized EO&IE MoS2/rGO was estimated as 74.19%.

Pure MoS2 nanostructures were synthesized from the same 5



procedure except the absence of GO sheets in the reaction solution. The corresponding product is labeled as IE MoS2. 2.2 Synthesis of EO MoS2/rGO and MoS2 with normal interlayer spacing The EO&IE MoS2/rGO and IE MoS2 samples prepared in the previous step were annealed at 300 °C for 1 h followed by cooling down to room temperature naturally. The samples were protected by continuously flowing nitrogen gas at a rate of 0.5 ml/s. The thermal annealing shrunk the interlayer spacing in the MoS2 nanostructures to the value close to that of the bulk MoS2. The thermally annealed samples were labeled as EO MoS2/rGO(a) (corresponding to the EO&IE MoS2/rGO) and MoS2(a) (corresponding to the IE MoS2), respectively. 2.3 Characterization Morphologies of the synthesized samples were characterized with a scanning electron microscope (FEI Quanta 450 FEG microscope) at an accelerated voltage of 20 kV and high vacuum mode and a transmission electron microscope (JEOL JEM-1400 microscope). XRD patterns were recorded on a Bruker D8 X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm). The SEM microscope was equipped with an X-MaxN 50 spectrometer (Oxford Instruments) for energy dispersive X-ray spectroscopy. The samples were prepared by drop-casting appropriate amount of ethanol dispersion of the products on silicon wafers (for XRD measurement and SEM imaging) or carbon-coated copper grids (for TEM imaging), followed by drying in a fume hood at room temperature. 6


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2.4 Electrochemical tests The preparation of electrodes followed the procedure as described. Frist, 300 mg of the obtained powder of each sample was dispersed in 3 ml of ethanol followed by ultrasonication for 0.5 h, forming a well-dispersed ink. Second, 6 μl of the ink (equivalent to 600 μg of the as-synthesized MoS2-containing materials) was casted on the surface of a glassy carbon electrode (GCE, with a diameter of 3 mm) followed by drying under ambient condition, resulting in an electrode with an equivalent loading of 8.48 mg∙cm−2. All electrochemical measurements were conducted on a CH Instrument CHI604E electrochemical workstation in a three-electrode cell at room temperature. A GCE modified with the synthesized MoS2-containing materials was used as the working electrode. A Ag/AgCl electrode and a platinum wire were used as the reference electrode and counter electrode, respectively. Cyclic voltammograms of the obtained electrodes were collected in the potential range of 0.0-0.3 V with different scan rates from 1 mV∙s−1 to 50 mV∙s−1. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the same configuration at an overpotential of 0 V with the frequency of 105-0.1 Hz.

3 Results and Discussions The pure MoS2 synthesized from the microwave-assisted solvothermal method exhibits spheres of interlaced nanosheets (Figure S1A–B).38 The random orientations of individual MoS2 nanosheets in each flower significantly reduce the percentage of the 7



MoS2 nanosheets that can expose their edges to surrounding electrolyte for ion intercalation.38 The interlayer spacing in the MoS2 nanosheets is 9.4 Å, a value much larger than that in the bulk MoS2 (i.e., 6.15 Å).

Figure 1. SEM images of (A) the EO&IE MoS2/rGO hybrid structure and (C) the corresponding thermally annealed sample, i.e., MoS2/rGO(a). (B) XRD patterns of EO&IE MoS2/rGO (black curve) and MoS2/rGO(a) (red curve). The positions of reflection peaks of bulk MoS2 are also presented for reference (JPCDS No. 37-1492).

In contrast, the microwave-assisted synthesis in the presence of GO sheets results in the reduction of the GO sheets to rGO sheets, on which MoS2 nanosheets are formed to exhibit both EO and IE features.35 The corresponding scanning electron microscopy (SEM) images show that the MoS2 nanosheets uniformly distribute on the rGO sheets (Figure 1A, Figure S2). The nonwettability between the MoS2 and rGO nanosheets (i.e., hydrophilic MoS2 versus hydrophobic rGO) forces the overgrown MoS2 nanosheets to protrude out of the graphene surfaces, enabling the EO geometry and the full exposure of 8


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the edges of the MoS2 nanosheets. The weak imaging contrast of the structures in transmission electron microscopy (TEM) images (Figure S3A-B) confirms that both the MoS2 nanosheets and the rGO sheets are very thin and composed of only few layers. The X-ray diffraction (XRD) pattern (black curve, Figure 1B) of the MoS2/rGO hybrid structures exhibits two reflection peaks with diploid relationship in the low-angle region (i.e., 9.4o and 18.7o), originating from the (002) and (004) reflections of layer-structured MoS2 with an interlayer spacing of 9.4 Å. These results highlight that the MoS2 nanosheets on the rGO sheets exhibit both EO and IE features; the hybrid structures are labeled as EO&IE MoS2/rGO. Annealing the EO&IE MoS2/rGO at 300 oC under N2 atmosphere reduces the interlayer spacing in the MoS2 nanosheets, which is reflected in the shift of reflection peaks to higher angles in the corresponding XRD pattern (red, Figure 1B). The (002) reflection peak of the annealed sample centers at 13.5o, corresponding to an interlayer spacing of 6.6 Å. Thermal annealing removes the species in the interlayer gaps to reduce the interlayer spacing of the MoS2, leading to a shift of (002) peak to a higher angle. Due to the incomplete and non-uniform removal of the species in the interlayer gaps, the (002) reflection of the MoS2 becomes broader. The SEM image of the sample (Figure 1C) shows that the EO feature of the MoS2 nanosheets still remains after the thermal annealing. This annealed sample is labeled as EO MoS2/rGO(a) for easy description.

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Figure 2. Cyclic voltammetry curves of (A) EO&IE MoS2/rGO, (B) EO MoS2/rGO(a) in an aqueous HCl solution (1 M) at various scan rates.

Electrochemical behavior of the EO&IE MoS2/rGO has been investigated with cycling voltammetry (CV) using a three-electrode system in aqueous HCl electrolyte. The CV curves presented in Figure 2A are recorded by sweeping the electrode potential between 0.0 V and 0.3 V at various scan rates ranging from 1 mV∙s−1 to 50 mV∙s−1. The cathodic and anodic profiles of the CV curves are almost symmetric with quasi-rectangular shapes. The integrated area of each CV curve is larger than that of the pure IE MoS2 measured at the same conditions (Figure S4A), indicating that anchoring the EO&IE MoS2 nanosheets on the rGO sheets can enhance electrochemical capacitance. The CV curves become asymmetric and the integrated areas decrease when the annealed EO MoS2/rGO(a) sample is used as the active electrode material (Figure 2B), implying that the large interlayer spacing in the EO&IE MoS2 nanosheets benefits electrochemical activity.

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Similar changes are also observed for annealed pure IE MoS2 (Figure S4A versus Figure S4B). The CV curves of the EO&IE MoS2/rGO, IE MoS2, and their annealed samples exhibit the similar variations regardless of the type of cations (e.g., H+, Li+, Na+, K+) in the electrolytes. (Figure 2 and Figure S4-Figure S7). The similarity confirms the contributions of rGO sheets and expanded interlayer spacing of the MoS 2 nanosheets to electrochemical capacitance. It is worth pointing out that the CV profile of the EO&IE MoS2/rGO sample remains a symmetric quasi-rectangular shape even at potential scan rates as high as 50 mV∙s−1, indicating the cation intercalation and diffusion in the expanded interlayer gaps of the MoS2 nanosheets is fast and reversible. The specific capacitance values (Csp) of these samples are calculated from the CV curves using39 1

𝐶sp = 𝑆∙𝑣∙(V

b −Va

V

∫ b 𝐼d𝑉, ) V a

where S is the geometrical area of active material on the working electrode (with unit of cm2), v is the scan rate (with unite of V∙s–1), I is the discharge current (with unit of A), and Vb and Va are the high- and low-voltage (V with unit of V) limits of the CV curves.

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Figure 3. Specific capacitances of (A) EO&IE MoS2/rGO and (B) EO MoS2/rGO(a) calculated from CV curves recorded at varying potential scan rates with the use of different electrolytes (e.g., aqueous solutions of HCl, LiCl, NaCl, and KCl with a concentration of 1 M).

The Csp values of the EO&IE MoS2/rGO sample in different electrolytes and at different potential scan rates are plotted in Figure 3A. The electrochemical capacitance decreases with the increase of the potential scan rate. For example, the Csp value is 346.5 F∙g−1 at a scan rate of 1 mV∙s−1 while the Csp value decreases to 143.2 F∙g−1 at a scan rate of 50 mV∙s−1 when it is measured in HCl electrolyte. The Csp dramatically decreases after the EO&IE MoS2/rGO has been thermally annealed to reduce the interlayer spacing of the MoS2 nanosheets (Figure 3B). For instance, the Csp values of the EO MoS2/rGO(a) become 134.0 F∙g−1 at a scan rate of 1 mV∙s−1 and 67.8 F∙g−1 at a scan rate of 50 mV∙s−1 corresponding to 61% and 53% decrease compared to the EO&IE MoS 2/rGO sample, 12


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respectively. The Csp values of the pure IE MoS2 also significantly decreases from 153.1 F∙g−1 to 75.3 F∙g−1 (equivalent to 51% decrease) at a scan rate of 1 mV∙s−1 and from 58.2 F∙g−1 to 44.9 F∙g−1 (equivalent to 23% decrease) at a scan rate of 50 mV∙s−1 after the MoS2 sample has been thermally annealed (Figure S8). These differences are consistent with the electrochemical impedance spectroscopy (EIS) measurements (Figure S9). The comparisons further highlight the importance of tuning interlayer spacing on improving performance of MoS240 and confirm that narrowing the interlayer gaps in the MoS2 nanosheets is responsible for the reduced capacitance. Regardless of the potential scan rate and the electrode materials, using the HCl electrolyte always gives the highest capacitance, followed by KCl, NaCl, and LiCl electrolyte. Since the hydrated H+ (or H3O+) ions have a smaller size than the hydrated Li+, Na+, K+ ions, the hydrated H+ ions are more easily intercalated and accumulated in the interlayer gaps of the MoS2 nanosheets. The radii of the hydrated H+, Li+, Na+ and K+ ions are 0.28 nm, 0.395 nm, 0.358 nm, and 0.331 nm, respectively,41 following the order of H+(aq) < K+(aq) < Na+(aq) < Li+(aq). At a particular potential scan rate, the Csp values of the MoS2-containing electrodes exhibit an opposite order when the aqueous electrolytes of different cations are used (Figure 3 and Figure S8). The inverse dependence of the capacitance on the size of hydrated cations indicates that increasing the ratio of the interlayer spacing in the MoS2 naonsheets to the size of the hydrated cations favors the intercalation and accumulation of the cations in the electrode materials 13



and thus enhances the corresponding electrochemical capacitance. Therefore, the enlarged interlayer gaps in the synthesized EO&IE MoS2/rGO are beneficial for cation intercalation and accumulation to enhance its electrochemical capacitance.

Figure 4. Cycling stability of the EO&IE MoS2/rGO used as the active electrode material.

Using the EO&IE MoS2/rGO in HCl electrolyte achieves the highest capacitance of 346.5 F ∙ g−1, which is considerably higher than the values of most reported MoS2/graphene composites.42-43 For example, Thangappan et al. synthesized a MoS2-graphene nanosheet hybrid structure with a specific supercapacitance of 270 F∙g−1.43

Yang and co-workers grew MoS2 on graphene sheets and achieved a maximum 14


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capacitance of 320 F∙g−1.44 The electrochemical capacitance of the MoS2/graphene composites can benefit from the large surface area of their nanoscale structures. The synthesized EO&IE MoS2/rGO exhibits a specific surface area of 69.2 m2g1 (Figure S10), which is lower than those of the MoS2/graphene composites reported in literature.45,46 The smaller specific surface area of the EO&IE MoS2/rGO and larger electrochemical capacitance further confirm that the intercalation of cations significantly contributes to the supercapactive property. The stability of the electrode materials during electrochemical cycling is one crucial parameter for practical application of supercapacitors. The cycling performance has been investigated through continuous CV test at a potential scan rate of 5 mV∙s–1. As depicted in Figure 4, the EO&IE MoS2/rGO exhibits a semi–permanent cycle life with 92 % retention of its initial capacitance even after 1000 cycles.

4. Conclusion In summary, a superior specific capacitance of 346.5 F∙g–1 has been achieved by using EO&IE MoS2/rGO hybrid 2D nanosheets as active electrode material. The positive synergy between the electrochemically active MoS2 nanosheets and the highly electrical conductive rGO sheets alleviates the disadvantages of individual components, benefiting the improvement of electrochemical capacitance. In addition, the unique EO geometry of the EO&IE MoS2 nanosheets maximizes the edges to be exposed to electrolyte and thus 15



maximizes the open channels for ion intercalation. The expanded interlayer gaps lower the energy barrier for ion intercalation, improving the ion intercalation rate and accumulation capacity in the EO&IE MoS2 nanosheets. All these effects enable the superior capacitive performance of the EO&IE MoS2/rGO hybrid structures. The results shed a light on the importance of materials design, including novel structures and hybridization of components possessing different functionalities, on significantly improve energy storage performance.

Acknowledgement This work was supported by the startup and OVPR seed grant from Temple University. Partial characterizations were performed with the use of TMI (Temple Materials Institute) facilities. The authors appreciate Ian McKendry, Dr. Mykola Seredych, Prof. Yury Gogotsi and Prof. Michael Zdilla for their help on BET measurement.

Supporting Information SEM images, TEM images, CV curves and calculated Csp, EIS spectra, BET isothermal adsorption curve

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