Symmetric pseudocapacitors based on molybdenum disul ﬁ de (MoS 2 )-modi ﬁ ed carbon nanospheres: correlating physicochemistry and synergistic interaction on energy storage †

Molybdenum disul ﬁ de-modi ﬁ ed carbon nanospheres (MoS 2 /CNS) with two di ﬀ erent morphologies (spherical and ﬂ ower-like) have been synthesized using hydrothermal techniques and investigated as symmetric pseudocapacitors in an aqueous electrolyte. The physicochemical properties of these MoS 2 / CNS layered materials have been investigated using surface area analysis (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray di ﬀ raction (XRD), Raman, Fourier transform infrared (FTIR) spectroscopy, and advanced electrochemistry, including cyclic voltammetry (CV), galvanostatic cycling with potential limitation (GCPL), long-hour voltage-holding tests, and electrochemical impedance spectroscopy (EIS). The two di ﬀ erent MoS 2 /CNS layered materials exhibit unique di ﬀ erences in morphology, surface area, and structural parameters, which have been correlated with their electrochemical capacitive properties. The ﬂ ower-like morphology (f-MoS 2 /CNS) shows lattice expansion (XRD), large surface area (BET analysis), and small-sized nanostructures (corroborated by the larger FWHM of the Raman and XRD data). In contrast to the f-MoS 2 /CNS, the spherical morphology (s-MoS 2 /CNS) shows lattice contraction and small surface area with relatively large-sized nanostructures. The presence of CNS on the MoS 2 structure leads to slight softening of the characteristic Raman bands (E 12g and A 1g modes) with larger FWHM. MoS 2 and its CNS-based composites have been tested in symmetric electrochemical capacitors in an aqueous 1 M Na 2 SO 4 solution. CNS improves the conductivity of the MoS 2 and synergistically enhances the electrochemical capacitive properties of the materials, especially the f-MoS 2 /CNS-based symmetric cells (most notably, in terms of capacitance retention). The f-MoS 2 /CNS-based pseudocapacitor shows a maximum capacitance of 231 F g (cid:1) 1 , with high energy density 26 W h kg (cid:1) 1 and power density 6443 W kg (cid:1) 1 . For the s-MoS 2 /CNS-based pseudocapacitor, the equivalent values are 108 F g (cid:1) 1 , 7.4 W h kg (cid:1) 1 and 3700 W kg (cid:1) 1 . The high-performance of the f-MoS 2 /CNS is consistent with its physicochemical properties as determined by the spectroscopy and microscopy data. These ﬁ ndings have opened doors for further exploration of the synergistic e ﬀ ects between MoS 2 graphene-like sheets and CNS for energy storage.


Introduction
Pseudocapacitors are redox-based electrochemical capacitors (ECs).][3][4] Unlike batteries with high energy densities, ECs are characterized by their high power characteristics, which make them very attractive for several technologies and devices that require highpower applications (i.e., the ability to release energy pulses in a very short time, in a few seconds) such as in regenerative braking energy systems in vehicles and metro-rails, "stop-start" applications in modern cars, uninterrupted power supply (UPS), emergency doors in aircras, and escalators in buildings. 5,6ne of the emerging high-power supercapacitor electrode materials is molybdenum disulde (MoS 2 ), a member of the transition-metal dichalcogenides (TMDs).MoS 2 has found applications in electrochemical devices, hydrogen storage, catalysis, capacitors, solid lubricants, and intercalation hosts. 7,8oS 2 is a layered-structured material with close relationship to graphene, characterized by a sheet-like morphology.0][11] Due to its higher intrinsic fast ionic conductivity (than oxides) and higher theoretical capacity (than graphite), MoS 2 continues to attract a lot of attention, particularly in supercapacitors. 2,7,12Soon and Loh 13 have pointed out the use of MoS 2 as an electrode material for supercapacitors, and the results suggest that the supercapacitor performance of MoS 2 is comparable to that of carbon nanotube (CNT) array electrodes.In addition to double-layer capacitance, diffusion of the ions into the MoS 2 at slow scan rates obtains faradaic capacitance.Analogous to Ru in RuO 2 , the Mo central atom displays a range of oxidation states from +2 to +6.This plays an important role in enhancing charge storage capabilities. 14However, the electronic conductivity of MoS 2 is still lower compared to graphite and the specic capacitance of MoS 2 is still very limited when used alone for energy storage applications. 12,13,15As evident in several reports, there is the need to improve the capacitance of MoS 2 with conductive materials such as CNT, 12 polyaniline (PANI), 2 polypyrrole (PPy), 9 and reduced graphene (RGO). 11In a review by Márquez et al., 16 carbon nanospheres (CNS) were described as good candidates for catalytic and adsorption applications and their unclosed graphitic akes provide the necessary 'dangling bonds' that could enhance surface reactions.CNS has also been used to enhance the conductivity of the battery cathode material, LiFePO 4 . 17o the best of our knowledge, the supercapacitive properties of MoS 2 have only been investigated in half-cells (i.e., 3-electrode systems), which creates a huge knowledge gap on the true behavior of the electrode material when used in full-cells (2electrode systems).Moreover, there is no literature on the effect of CNS on the supercapacitance of MoS 2 .Pseudocapacitors usually suffer from poor electrical conductivity and irreversible redox-activity, thereby leading to gradual loss of capacitance.To tackle the abovementioned challenges, this study adopted two different synthesis protocols to prepare MoS 2 and MoS 2 /CNS composites with different physicochemistries (i.e., spherical and ower-like morphologies, structure, porous textures and electrochemistry) and then used them to fabricate symmetric pseudocapacitors.We clearly show that the pseudocapacitive properties of the MoS 2 /CNS (especially, in terms of cycling stability and electronic conductivity) are intrinsically linked to the presence of CNS.

Synthesis
Spherical MoS 2 .Spherical MoS 2 was synthesized using a well-established method by Huang et al., 18 as follows: 0.31 g Na 2 MoO 4 $2H 2 O were dissolved in 30 mL deionized water.Aer adjusting the pH value to 6.5 with 12 M HCl, 0.82 g L-cysteine was added and the mixture was diluted with water to 35 mL and then the solution was rapidly stirred for about an hour.Subsequently, the mixture was transferred into a 40 mL Teonlined stainless steel autoclave and heated at 180 C for 48 hours.The black MoS 2 precipitate was cooled naturally to room temperature, collected by centrifugation, washed with distilled water and absolute ethanol in three cycles and then dried in vacuum at 60 C for 24 hours.For simplicity, the obtained spherical material (as evident from SEM and TEM images) is abbreviated as s-MoS 2 .
Flower-like MoS 2 .Flower-like MoS 2 was synthesized as suggested by Wang et al. 19 1.21 g Na 2 MoO 4 $2H 2 O and 1.56 g thiourea powders were mixed together in 30 mL deionised water, and 0.14 g PEG-1000 were added.The resulting mixture was transferred to a Teon cup of capacity 40 mL and heated in a stainless steel autoclave at 180 C for 24 h.Aer cooling naturally to room temperature, the black MoS 2 precipitate was collected by centrifugation, washed with distilled water and absolute ethanol in three cycles, and then dried under vacuum at 60 C for 24 h. 19The obtained ower-like material (as evident from SEM and TEM images) is abbreviated herein as f-MoS 2 .
Carbon nanospheres (CNS).Carbon nanospheres (CNS) were obtained using the established method by Dlamini et al. 20 In brief, sucrose solution (0.3 M) was transferred into a 100 mL Teon-lined stainless steel autoclave and heated at 150 C for 5 hours.The reaction was then le to cool at room temperature.At this stage, the black precipitate that formed was puried by Soxhlet extraction, washed with ethanol and water and then dried at 80 C in an oven.
The CNS-modied spherical MoS 2 (abbreviated herein as s-MoS 2 /CNS) composite was prepared as follows.First, 0.028 g CNS were ultrasonically dispersed in 20 mL deionized water.Then, 0.30 g Na 2 MoO 4 $2H 2 O were added and ultrasonically dispersed for 30 min.Aer adjusting the pH value to 6.5 with 12 M HCl, 0.80 g L-cysteine was added.The resultant mixture was diluted with water to 30 mL and rapidly stirred for about 1 h.The mixture was then transferred into a 40 mL Teon cup and heated in a stainless steel autoclave at 180 C for 36 h.Upon completion, the product was cooled naturally to room temperature, the s-MoS 2 /CNS composite was collected by ltration, washed with distilled water and acetone several times, and nally dried in the oven at 80 C for 24 h.
The CNS-modied ower-like MoS 2 composite (abbreviated herein as f-MoS 2 /CNS) was prepared as follows.1.21 g Na 2 -MoO 4 $2H 2 O were added to the sonicated CNS in 20 mL deionised water and further sonicated for 30 min.Then, 1.56 g thiourea and 0.28 g PEG-1000 were added to the solution and the mixture diluted to 30 mL before heating to 180 C using a Teon-lined stainless steel autoclave for 36 h.Upon completion, the product was cooled naturally to room temperature, the f-MoS 2 /CNS composite was collected by ltration, washed with distilled water and acetone several times, and nally dried in oven at 80 C for 24 h.To homogenize the as-synthesized materials and further promote crystallization, the as-synthesized materials were annealed in a horizontal furnace with a quartz tube at 900 C under a ow of nitrogen at a rate of 100 mL min À1 for 4 h.

Characterization techniques
The XRD patterns of the as-prepared MoS 2 and MoS 2 /CNS nanopowders were obtained from a DMax/2500PC diffractometer using Co Ka radiation (K ¼ 1.5418 Å) at 40 kV, 100 mA and a 2q range of 10-90 .The FTIR spectra were obtained using a Bruker Tensor 27 FTIR spectrometer equipped with ZnSe crystal that absorbs strongly below 500 cm À1 .FTIR spectra were obtained in the range of 550-4000 cm À1 ; a single beam measurement as the background spectrum was acquired prior to running the actual sample.Raman measurements were carried out in air using a Horiba Jobin Yvon spectrometer equipped with an Olympus BX40 microscope attachment to focus the laser beam on a small selected area of the sample, a 30 mW green argon laser (l ¼ 514 nm) as the excitation source, and a 1800 lines per mm grating monochromator with liquid nitrogen-cooled CCD.The BET (Brunauer, Emmett and Teller) measurements were performed to determine the specic surface area and pore size of the nano-sheets using a Micromeritics TriStar II instrument.The SEM images were obtained using a JEOL-JSM 7500F scanning electron microscope operated at 2.0 kV and a FEI Nova Nanolab 600 SEM.TEM images were obtained from a FEI Tecnai T12 microscope operated at an acceleration voltage of 120 kV.Elemental composition was obtained using the Oxford INCA EDS soware on the SEM.

Fabrication of the symmetric pseudocapacitor and electrochemical measurements
Symmetric pseudocapacitive properties of the materials were investigated using Swagelok cells (MTI, Inc., USA).Nickel foam (Celmet: thickness ¼ 1.6 mm, surface area 7500 m 2 , cell size ¼ 0.5 mm, 48-52 cells per inch) was used as substrate and current collector in the fabrication of the symmetric pseudocapacitors.Before use, the nickel foam was thoroughly cleaned by sonicating in 1 M HCl solution for 30 min, washing with copious amount of distilled water, and nally drying under vacuum.A 1 M Na 2 SO 4 solution was used as the electrolyte, whereas a porous lter paper (Whatman®) served as the separator.The electrode materials were prepared by rst thoroughly mixing the active materials, either MoS 2 /CNS or MoS 2 , carbon black as the conducting agent, and polyvinylidene uoride (PVDF) as the binder (80 : 15 : 5 weight ratio) with a few drops of anhydrous N-methyl-2-pyrrolidone (NMP) using pestle and mortar, to produce a homogeneous paste.The resulting slurry was coated onto the nickel foam substrate with a spatula.The electrode was then dried at 60 C overnight in a vacuum oven.The mass of the active material on the nickel foam was between 5 and 20 mg.For complete impregnation, the assembled cells were le for 12 hours prior to testing.The measurements were carried out in 1 M Na 2 SO 4 aqueous electrolyte.All electrochemical measurements, cyclic voltammetry (CV), galvanostatic cycling with potential limitation (GCPL), and electrochemical impedance spectroscopy (EIS) were performed at room temperature using a computer-controlled multi-channel Potentiostat/Galvanostat Bio-Logic VMP3 work station driven by EC-Lab® v10.40 soware with Z-t tool for EIS data analysis.EIS measurements were carried out in the frequency ranging from 10 kHz to 10 mHz at the open circuit voltage, with AC voltage amplitude of 1.5 mV.
The specic capacitance (C sp ), maximum specic power density (P max ) and specic energy density (E sp ) were evaluated using the conventional eqn ( 1)-( 5): 21,22 where where i (A) is the applied current, DV (V)/Dt (s) is the slope of the discharge curve, m (g) is the total mass of the two electrodes, C (F) is the calculated capacitance, V (V) is the maximum voltage obtained during charge, and R ir is the internal resistance, which is determined from the voltage drop at the beginning of each discharge, whereas DV ir represents the voltage drop.

Material characterization
A one-pot hydrothermal route was used for the synthesis of MoS 2 , CNS and MoS 2 /CNS.Fig. 1 shows the SEM and TEM images of the CNS (Fig. 1c and d A closer examination of the TEM images of the CNS (Fig. 1d) and s-MoS 2 /CNS (Fig. 1f-h) clearly suggests a uniform dispersion and excellent integration of the MoS 2 with the CNS.The value of the d-spacing of the s-MoS 2 shown in the TEM image (Fig. 1i) is 0.62 nm, which is in agreement with literature. 23In this synthesis method, it seems that the CNS particles acted as substrates for nucleation, wherein MoO 4 À ions reacted with sulphur ions from L-cysteine to form the MoS 2 sheets on the CNS.The presence of CNS prevented MoS 2 stacking and thus the formation of a porous 3-D sphere-like architecture with highly dispersed wrinkled and uffy MoS 2 sheets on CNS particles.This unique structure is also apparent in the TEM images (Fig. 1f and h).In the s-MoS 2 /CNS composite, the MoS 2 nano-sheet edges are readily exposed and not entangled as in MoS 2 .This loose structure is desirable for superior charge storage.However, the MoS 2 sheets in the composite are not visible as layered; instead, they grew in various orientations and were intertwined around the spheres.An important consequence is the small accessible surface area coupled with the small micropores.This could potentially cause an obstruction for ion transportation and thus limit the capacitance.The elemental composition of as-synthesized s-MoS 2 , obtained from the EDS (see ESI, Similarly, the ower-like nanostructures (f-MoS 2 ) were obtained using a one-pot hydrothermal method.This method relies on sodium molybdate and thiourea to provide MoO 4 2À ions and sulphur atoms, respectively.Interestingly, the addition of small amounts of PEG in our synthesis protocol assisted in the efficient dispersion of the MoS 2 to generate the ower-like morphology.The starting precursor materials in the MoS 2 formation play a vital role, especially in the achieved morphology; a slight change, even in the reducing agent, can bring about enormous value.In this hydrothermal process, the reaction involves three steps: (a) the hydrolysis of sulfur precursor to form H 2 S, followed by (b) the reduction of Mo and (c) nally the formation of MoS 2 . 19

CS(NH
The SEM and TEM images of as-synthesized MoS 2 in Fig. 2 clearly show ower-like hierarchical 3-D structures.The f-MoS 2 sheets self-assemble into a highly porous structure (Fig. 2a-c  and e).The composite structure (Fig. 2d) also exhibits a clear ower-like morphology.The occurrence of this morphology may be associated with the presence of a surfactant (PEG-1000).Remarkably, aer coating CNS with MoS 2 sheets, the surface appears rough and wrinkled by the MoS 2 sheets (Fig. 2d, f and  g).The value of the d-spacing of the f-MoS 2 (Fig. 2h) is 0.61 nm, which is in agreement with literature. 23The EDS data (see ESI, Fig. S2 † MoS 2 has a hexagonal crystal system and layer-structured D 4 6h crystal system and P6 3 space group.Fig. 3 compares the XRD patterns of the CNS and its MoS 2 spherical and ower-like composites.There is no signicant difference between the MoS 2 and MoS 2 /CNS patterns, conrming good integration of the CNS with the MoS 2 structure.The diffraction peaks at 2q ¼ 15.8 , 37.9 and 41.5 are indexed to the hexagonal phase of MoS 2 (002), ( 100) and (201), respectively.For the spherical morphology (s-MoS 2 /CNS composite), the diffraction peaks are similar to the individual s-MoS 2 , meaning that CNS fully interacts with the MoS 2 and its presence does not interfere with the structure of the MoS 2 .For the spherical material, s-MoS 2 /CNS, the incorporation of CNS into the MoS 2 nanosheets decreases the intensity of the peaks, in particular the (002).The result indicates the formation of few layers of the MoS 2 in the composite; thus, CNS impedes the growth of the MoS 2 layer in a hexagonal array.Importantly, upon incorporation of the CNS, there is a slight shi in the diffraction lines of the MoS 2 to higher 2q (see Fig. 3a-c), which is an indication of a lattice contraction.However, for the ower-like morphology (f-MoS 2 / CNS), the incorporation of the CNS results in a slight shi to the lower 2q (see Fig. 3d-f), which is an indication of lattice expansion in the S-Mo-S interlayer spacing and formation of a few layers of MoS 2 .In addition, unlike the spherical composite (Fig. 3a-f), the presence of the CNS did not negatively impact the peak intensity of (002).In fact, the (002) peak became more intense and sharper, indicating a higher degree of crystallinity, comparable to that of the bulk MoS 2 .This nding may be due to strong integration between the MoS 2 and CNS arising from the synthesis protocol adopted in this study, which allowed for the This journal is © The Royal Society of Chemistry 2016 two materials to undergo chemical reaction rather than just physical mixing.Furthermore, the peak (103) is more apparent, with a new broad peak (006) of hexagonal MoS 2 at 46 (Fig. 3d  and e).
BET (ve-point analysis) was used to measure the specic surface area and porosity of the as-synthesized materials.As shown in Table 1, the s-MoS 2 has a specic surface area of 17.8 m 2 g À1 ; on the other hand, the specic surface area of the s-MoS 2 /CNS decreased to 9.17 m 2 g À1 .This dramatic reduction is explained by the hindrance to the growth of the MoS 2 , due to the presence of the CNS (also conrmed by the XRD patterns).In fact, the CNS makes the surface area of MoS 2 inaccessible.Moreover, the pore sizes of MoS 2 and MoS 2 /CNS were revealed to be 22.9 and 18.61 nm, respectively.The f-MoS 2 /CNS composite had a higher surface area with value of 61 m 2 g À1 compared to 25 m 2 g À1 recorded for the ower-like MoS 2 .This means that incorporation of CNS prevented the agglomeration of MoS 2 sheets (also suggested from the XRD data).Surprisingly, the BET data follow the XRD data, especially the (002) peak height.Considering that capacitance is a function of the surface area, one would expect the materials with higher surface area (such as the f-MoS 2 /CNS) to provide the best capacitance  value.Moreover, despite the fact that the pores do not directly contribute to the surface area, they provide accessible pathways for easy diffusion of ions and reversible charge storage.
Raman spectroscopy was used to provide more insight into the structure and topology of the as-synthesized MoS 2 -based nanocomposites.Fig. 4 shows the Raman spectra of as-synthesized MoS 2 nanosheets, CNS and MoS 2 /CNS nanocomposites.The CNS exhibited the signature D and G peaks of carbon-based materials at 1347 and 1590 cm À1 , respectively.As we expected, the MoS 2 /CNS composites also showed the D and G peaks close to the regions wherein they were observed for the CNS alone, conrming the successful integration of the CNS into the two MoS 2 -based composites.There was no detectable difference in the intensity ratios of the D to G band (I D : I G ) of the CNS (0.95), s-MoS 2 /CNS (0.94) and f-MoS 2 /CNS (0.94), which implies that the CNS essentially retained its pristine structure even aer integration with the MoS 2 .The characteristic Raman bands for bulk s-MoS 2 were observed at 375.69 and 402.49cm À1 due to E 1 2g and A 1g modes with full-widths at half maximum (FWHM) of 8.47 and 8.23 cm À1 , respectively.The E 1 2g mode describes the inlayer displacement of the Mo and S atoms, whereas the A 1g mode relates to the out-of-layer symmetric displacements of S atoms along the c axis. 24,25Interestingly, the incorporation of the CNS into the MoS 2 resulted in the slight soening of these two Raman bands compared to those of the bulk MoS 2 ; the spherical MoS 2 /CNS appeared at 375.07 cm À1 (E 1 2g ) and 400.84 cm À1  (A 1g ), while the ower-like morphology appeared at 374.91 cm À1 (E 1 2g ) and 401.78 cm À1 (A 1g ).Moreover, the increase in the FWHM values were more pronounced for the ower-like morphology than for the spherical morphology.
In the ower-like morphology, FWHM values are larger in the f-MoS 2 /CNS than in the bulk f-MoS 2 (cf.E 1 2g ¼ 13.55 vs. 9.29 cm À1 of the bulk f-MoS 2 or A 1g ¼ 10.88 vs. 8.72 cm À1 of the bulk f-MoS 2 ).The broadening of the Raman bands is related to phonon connement and also indicates that the lateral dimensions of these MoS 2 layers are in the nano-dimension. 25he larger FWHM for the f-MoS 2 /CNS compared to its s-MoS 2 / CNS counterpart is indicative of the smaller particle sizes and larger surface area, which corroborates the BET analysis.In a recent study by Lee et al., 26 the authors showed that the frequency difference between E 1 2g and A 1g modes could serve as a convenient and robust diagnosis of the layer thickness of MoS 2 samples.From our results in Table 2, the frequency difference (i.e., |E 1 2g À A 1g |) decreases as follows: bulk MoS 2 (ca.27 cm À1 ) > f-MoS 2 /CNS (ca.26 cm À1 ) ¼ s-MoS 2 /CNS (ca.26 cm À1 ).Thus, our results indicate that there is no signicant difference in the layer thicknesses of the two MoS 2 /CNS composites and the two synthesis methods we adopted in this study further conrm the change in the physicochemical properties of the MoS 2 complexes.Fig. 5 shows FT-IR spectra of the MoS 2 , CNS and MoS 2 /CNS nanocomposites.In MoS 2 , the peaks at 1583 and 1518 cm À1 are consistent with NH 2 in-group deformation (arising from the thiourea or L-cysteine used in the synthesis), and the peak at 1466 cm À1 is assigned to the N-C-N asymmetric stretching mode.The band at 1231 cm À1 is assigned to stretching vibrations of O-H bonds.The C-O-H stretching mode occurred at 1092 cm À1 .The C-S stretching vibrations are observed at 729 cm À1 .The weak peak around 600 cm À1 is assigned to Mo-S vibration.Similar peaks are observed for the MoS 2 /CNS nanocomposite with the additional peaks at 2892 cm À1 and 2971 cm À1 due to C]C symmetric and asymmetric stretching vibrations as explained elsewhere. 27Indeed, the presence of the functional groups on the CNS could have contributed to the efficient integration of the CNS with the MoS 2 (as observed in the TEM images).
Fig. 6 compares the XPS data of s-MoS 2 /CNS (Fig. 6a and b) and f-MoS 2 /CNS (Fig. 6c and d).XPS analysis reveals the predominant Mo 3d and S 2p characteristic peaks for both s-MoS 2 /CNS and f-MoS 2 /CNS.The two materials clearly show that the only elements present are Mo, S, adventitious C and O, and Na 'impurity' (Table 3).

Electrochemical properties
Fig. 7a shows the cyclic voltammetric (CV) evolutions of the individual CNS, s-MoS 2 and f-MoS 2 obtained from the 3-electrode conguration in 1 M Na 2 SO 4 at 5 mV s À1 .These results show that s-MoS 2 is more capacitive than the f-MoS 2 but, as it will be shown later, the capacitance retention of the s-MoS 2 is worse than that of the f-MoS 2 .From Fig. 7a, it is evident that CNS do not show any signicant current response.Fig. 7b compares the CVs of the composite materials, showing that the f-MoS 2 /CNS has better performance (high current response and slightly wider voltage window) compared to the s-MoS 2 /CNS.Fig. 7c and d exemplify typical GCPL curves of the various symmetric cells obtained in 1 M Na 2 SO 4 at 0.5 A g À1 .The performance of the s-MoS 2 is better than its composite, whereas the f-MoS 2 /CNS is better than its precursor f-MoS 2 .As will be shown later, the nanocomposite materials (both spherical and ower-like) show better capacitance retention and cycling stability than their MoS 2 materials.As observed in Fig. 7, the CV and GCPL curves of the composite materials, notably the owerlike materials, show quasi-rectangular shapes, which is a strong deviation from the ideal rectangular shape that was expected from the EDLC.Furthermore, the GCPL of the ower-like composite (Fig. 7d) shows a broad peak at around 0.3 V.These results clearly conrm the pseudocapacitive behavior of these MoS 2 -based composite materials.Fig. 8 shows the plots of specic capacitance versus current densities.With the exception of the s-MoS 2 /CNS-based cell, the capacitance values of all the cells decrease with increasing gravimetric currents.The capacitance of the s-MoS 2 /CNS-based cell remains essentially the same (cf.108 F g À1 at 0.1 A g À1 versus 94 F g À1 at 1 A g À1 , which is about 13% capacitance loss).This ability of the s-MoS 2 /CNS to maintain high capacitance even at high current density is indicative of high-power performance.
Table 4 compares the values of capacitance, maximum energy and power density of the symmetric cells; unfortunately, there is no related literature with which to compare our results, except for 3-electrode systems.The maximum specic capacitance for the ower-like composite f-MoS 2 /CNS electrodes is 231 F g À1 , with maximum energy density 26 W h kg À1 and power density 6443 W kg À1 .For the sphere-like morphologies, the equivalent values obtained were 108 F g À1 , 7.4 W h kg À1 and 3700 W kg À1 .The inferior performance of the s-MoS 2 /CNS electrodes can be explained by the inaccessible surface area for charge storage in the MoS 2 /CNS composite due to the presence  ) is more than twice that of MoS 2 (10 W h kg À1 ) alone.The high energy density is owed to the favorable porous nanostructure of the composite, in which MoS 2 sheets serve as active sites for redox reactions, coupled with CNS interaction with electrolyte and the synergistic effects of MoS 2 /CNS composites.
In theory, the specic capacitance of the MoS 2 may be estimated from the amount of power needed to carry out the electrolysis of 1 mol of active material (i.e., 1 F ¼ 96 485 C) and the molar mass of the material (MoS 2 ¼ 160.07 g mol À1 ) using the eqn (8): 31 where U is the voltage window and Q is electrical energy per 1 gram.Because our U ranges between 0.8 and 1 V, the theoretical pseudocapacitance is between 603 and 754 F g À1 .If we consider that the supercapacitance of a 3-electrode cell is about a quarter that of a 2-electrode system, our value for the f-MoS 2 /CNS based symmetric pseudocapacitor (231 F g À1 ) is in good agreement with theory.Voltage-holding cycling experiments, complemented with EIS experiments, were performed to provide insight into the cycling stability of the four symmetric cells.The zeroth hour cycling capacitance (brown data points in Fig. 9 and 10) for each of the four symmetric cells was found to be lower than that at the 10 th hour cycling, which suggests that MoS 2 -based cells require a signicant amount of time to equilibrate prior to recording data from voltage-holding tests.Thus, the best voltage-holding test data were obtained from the 10 th hour.Fig. 9 exemplies the typical voltage-holding test performed at 0.7 A g À1 for the spherical morphology.At the 10 th hour, the s-MoS 2 started with about 140 F g À1 and obtained 20 F g À1 at the end of the 50 th hour (Fig. 9a), which is about 86% loss of capacitance.The composite s-MoS 2 /CNS started with about 90 F g À1 , but maintained 55 F g À1 at the end of the 50 th hour (Fig. 9c), which is about 39% loss of capacitance.The tted EIS data for both the s-MoS 2 (Fig. 9b) and its composite form (s-MoS 2 /CNS, Fig. 9d) obtained before and aer the 50 hour long-cycling experiments are summarized in Table 5.For the ower-like materials (Fig. 10), the capacitance of the f-MoS 2 at the 10 th hour was about 24 F g À1 and obtained 14 F g À1 at the end of the 50 th hour (Fig. 10a), which is about 42% loss of capacitance.The composite f-MoS 2 /CNS started with about 90 F g À1 , but maintained 84 F g À1 at the end of the 50 th hour (Fig. 10c), which is a mere 6% loss of capacity.The tted EIS data for both the f-MoS 2 (Fig. 10b) and its composite form (f-MoS 2 /CNS, Fig. 10d), obtained before and aer the 50 hour long-cycling experiments are summarized in Table 6.
From the results of the cycling performances of the spherical (Fig. 9) and ower-like (Fig. 10) materials, the following important ndings should be emphasized.First, the zeroth hour cycling capacitance for each of the cells was found to be lower than that at the 10 th hour cycling, which suggests that MoS 2 -based cells or related layered materials require a signicant amount of time to equilibrate prior to recording data from voltage-holding tests.Second, the cycling stability for MoS 2 is very poor, but can be greatly improved by integrating it with conductive carbon materials such as the CNS.Third, the cycling stability of the cells from the MoS 2 or its carbon composite is strictly dependent on its morphology; the ower-like morphology shows enhanced electrochemistry compared to the spherical morphology.
Finally, to obtain some insight into the capacitive properties of the cells, we were able to satisfactorily t the raw EIS data with the electrical equivalent circuit (EEC).The EEC consists of Voigt RC elements (Fig. 9e), involving series resistance (R s ), charge-transfer resistance (R ct ) and constant-phase elements (CPE or Q).For the cell with the spherical morphology (Table 5), the R s values of the s-MoS 2 before and aer the 50 th hour were 0.23 and 0.49 U, respectively.For the s-MoS 2 /CNS, the R s values before and aer the 50 th hour were 0.33 and 1.14 U, respectively.The combined R ct values before and aer the 50 th hour were 16.26 and 17.28 U, respectively.For the s-MoS 2 /CNS it was 2.45 U before and 4.62 U aer the 50 th hour cycling.The result clearly proves that the presence of the CNS enhanced the conductivity of the s-MoS 2 -based cells.However, the data for the cells fabricated from the ower-like morphology (Table 6), the R s values of the f-MoS 2 before and aer the 50 th hour, were 0.
capacitance of the f-MoS 2 /CNS-based symmetric pseudocapacitor.The impedance of the CPE (Z CPE ) is related to the frequency-independent constant (Q) and radial frequency (w) according to the eqn (9): 32 where n (with values in the À1 # n # 1 range) is obtained from the slope of log Z versus log f.When n ¼ 0, 1, À1 or 0.5, the CPE describes a pure resistor, a pure capacitor, an inductor, or Warburg impedance (Z w ), respectively, due to the diffusion of the ions.For all the cells, the n values observed for these electrodes are generally greater than 0.5, which conrms the pseudocapacitive properties of the MoS 2 -based symmetric cells, corroborating the CV data in Fig. 7a and b.
The data for the Bode plots (see ESI, Fig. S3 †) summarised in Tables 5 and 6 show that the phase angle for each of the systems before and aer 50 hour cycling was greater than À75 , but lower than the À90 expected of an ideal EDLC system.The phase angle result is a further conrmation of the pseudocapacitive behaviour of these MoS 2 -based systems.In addition, the knee frequency (f o , f ¼ 45 ), which is the maximum frequency at which the dominant behaviour of the supercapacitor (power density), can be observed.The knee frequency relates to the rate or power capability of the supercapacitor; the higher the f o value is, the more rapidly such a supercapacitor can be charged and discharged.The f o values for the s-MoS 2 / CNS are 50 Hz (time constant ¼ 0.02 s) before cycling and 5 Hz (time constant ¼ 0.2 s) aer the 50 th hour voltage-holding testing.However, for the f-MoS 2 /CNS system, the f o value remained at 50 Hz before and aer the voltage-holding test, which is a further conrmation of the high electrochemical cycling stability of the f-MoS 2 /CNS system.
The energy-storage mechanism of MoS 2 in aqueous supercapacitors is well described in the literature; 29,31 it is the combined phenomena involving the transition from EDLC to Table 5 Cycling performance of the spherical MoS 2 and MoS 2 /CNS based symmetric pseudocapacitors in 1 M Na 2 SO 4 .EIS data before and after 50 h voltage-holding experiments were fitted with the Voigt equivalent circuit the pseudocapacitive process (faradaic/redox process) and increased active surface area due to possible exfoliation.First, there is the accumulation of ions at the double layer interface between the MoS 2 akes and the electrolyte.This is subsequently accompanied by a redox process: upon charging (reduction), the alkali metal ions in the electrolyte (Na + ) adsorb onto the surface and intercalate between the MoS 2 layers, followed by deintercalation upon discharging (oxidation), as shown in eqn (10).
The repeating intercalation-deintercalation process of the sodium ions over several cycles leads to partial exfoliation of the MoS 2 layers, resulting in an increased surface area and enhanced specic capacitance.The evidence for the possible partial exfoliation of the MoS 2 during the repeated intercalation-deintercalation process of electrolyte ions can be observed from the zeroth hour cycling capacitance (brown data points in Fig. 9 and 10 described above), which was lower than the 10 th hour cycling before stabilizing.

Conclusions
), conrming the spherical nature of the CNS in agreement with literature.The micrographs show the interconnected uniform amorphous CNS in the particle size diameter range of 100-200 nm.Fig.1compares the SEM and TEM images of the s-MoS 2 and s-MoS 2 /CNS.The SEM and TEM micrographs of the s-MoS 2 (Fig.1a and b) show the formation of a sphere-like morphology consisting of several agglomerated clusters of s-MoS 2 sheets.

Fig. 1
Fig. 1 SEM (a, c and e) and TEM (b, d, f, h and i) micrographs of s-MoS 2 (a and b), CNS (c and d), s-MoS 2 /CNS (e and f); magnified views of s-MoS 2 / CNS (g and h), and d-spacing of s-MoS 2 (i).
) depict strong Mo and S overlapping signals, the MoS 2 contains Na 1.3 atomic%, S 62.09 atomic% and Mo 36.61 atomic%, conrming the stoichiometry of the MoS 2 .

Fig. 3
Fig. 3 XRD patterns of MoS 2 , CNS and MoS 2 /CNS composites for spherical (a-c) and flower-like (d-f) products and their magnified views.
63 and 169.23, which are assigned to S 2p 3/2 , S 2p 1/2 , S 2 2À or S 2À and S 4+ , respectively), suggesting that the S atoms exist in two different chemical states.This nding is indicative of f-MoS 2 /CNS undergoing partial oxidation into MoS x O y at the edges and defect sites, which effectively leads to enhanced redox processes.The molybdenum spectra (Fig. 6a and c) show the expected Mo 3d 5/2 and Mo 3d 3/2 , including the Mo 6+ species (ca.236 eV) usually observed in partially oxidized MoS 2 complexes.In addition to the signicant changes in the S 2p of the f-MoS 2 /CNS, its surface atoms, with the exception of O and Na, are lower than observed for the s-MoS 2 /CNS.Considering that surface Na species are involved in the storage mechanism, one may conclude that Na will be more accessible to the f-MoS 2 /CNS in an aqueous solution for enhanced energy storage than the s-MoS 2 /CNS.

Fig. 6
Fig. 6 High resolution XPS analysis of s-MoS 2 /CNS (a and b) and f-MoS 2 /CNS (c and d).Mo 3d (a and c) and S 2p (b and d).
21 and 0.40 U, respectively.For the f-MoS 2 /CNS, the R s values before and aer the 50 th hour were 0.26 and 0.31 U, respectively.The combined R ct values for the f-MoS 2 before and aer the 50 th hour were 3.43 and 4.14 U, respectively.For the f-MoS 2 /CNS, it was 3.37 before and 5.09 U aer the 50 th hour cycling.Like the s-MoS 2 -based cells, the results for the f-MoS 2 -based cells suggest that the CNS component serves to decrease the internal resistance of the f-MoS 2 , thereby improving the conductivity and

Fig. 8
Fig. 8 GCPL results for specific capacitance at various current densities of spherical and flower-like MoS 2 and MoS 2 /CNS.

Fig. 9
Fig. 9 Typical GCPL plots at 0.7 A g À1 (a and c) and Nyquist plots (b and d) obtained before and after 50 hour voltage experiments for the spherical (a and b) MoS 2 and MoS 2 /CNS-based (c and d) symmetric pseudocapacitors.The electrical equivalent circuit used in fitting the Nyquist plots is shown in (e).

Fig. 10
Fig. 10 Typical GCPL plots at 1 A g À1 for f-MoS 2 (a) and 1.5 A g À1 for f-MoS 2 /CNS (c); Nyquist plots for f-MoS 2 (b) and f-MoS 2 /CNS (d) obtained before and after 50 hour voltage experiments.The electrical equivalent circuit used in fitting the Nyquist plots is shown in Fig. 9e.
Two variants of carbon nanosphere-modied molybdenum disulphide (MoS 2 /CNS) nanostructures were successfully synthesized by a simple one-pot hydrothermal method.The two different synthetic methods obtained two different morphologies: ower-like (f-MoS 2 /CNS) and spherical (s-MoS 2 /CNS) morphologies.The physical and chemical characterisations reveal that the two materials were properly integrated into the CNS surface.The addition of CNS impedes the growth of the s-MoS 2 crystals in the composite, but enhances the growth of the f-MoS 2 , particularly in the (002) plane of hexagonal MoS 2 .The f-MoS 2 /CNS shows lattice expansion and large surface area, whereas the s-MoS 2 /CNS shows lattice contraction and smaller surface area.The presence of CNS on the MoS 2 structure leads to slight soening of the characteristic Raman bands (E 1 2g and A 1g modes) with larger FWHM.The electrochemical capacitive behaviour of the MoS 2 -based materials was evaluated in symmetric cells.The electrochemical performance of the composites demonstrates that the MoS 2 /CNS composite from ower-like MoS 2 exhibits better capacitance, energy and power densities (231 F g À1 , 26 W h kg À1 and 6443 W kg À1 ) compared to the spherical morphology (108 F g À1 , 7.4 W h kg À1 and 3700 W kg À1 ).CNS play a vital role in improving the electrochemical properties of the MoS 2 -based electrode materials, especially with respect to improving the capacitance retention (i.e., stable electrochemical cycling).The excellent electrochemical performance of MoS 2 /CNS was accredited to the morphology of the composite and synergistic effects between MoS 2 sheets and CNS.These ndings show great promise for future studies of MoS 2 modied with other conducting carbon nanostructures for the development of high-performance electrochemical energy storage systems.

Table 1
Porous texture of the as-synthesized materials

Table 2
Comparison of Raman spectral data

Table 3
XPS data for the s-MoS 2 /CNS and f-MoS 2 /CNS samples of CNS, as shown by the Raman spectroscopic data.The maximum energy density achievable with f-MoS 2 /CNS (26 W h kg À1

Table 4
Comparison of the capacitive performance of various MoS 2 -based supercapacitors a