1T-phase transition metal dichalcogenide nanosheets

Abstract

A method for the production of 1T-transition metal dichalcogenide few-layer nanosheets and/or monolayer nanosheets comprising electrochemical intercalation of lithium ions into a negative electrode comprising a bulk 2H-transition metal dichalcogenide to provide an intercalated electrode, and an exfoliation step comprising contacting the intercalated electrode with a protic solvent to produce 1T-transition metal dichalcogenide few-layer nanosheets and/or monolayer nanosheets. An electrochemical capacitor comprising a composite electrode comprising 1T-MoS.sub.2 nanosheets and graphene, and a method of producing a composite electrode for use in an electrochemical capacitor.

Claims

1. An electrochemical capacitor comprising a composite electrode, the composite electrode comprising 1T-MoS.sub.2 nanosheets and graphene, wherein the 1T-MoS.sub.2 nanosheets is at least 50% by weight trilayer nanosheets and wherein the composite electrode comprises graphene and the MoS.sub.2 nanosheets of the composite electrode are at least 50% 1T phase.

2. The electrochemical capacitor of claim 1, wherein the composite electrode comprises graphene and MoS.sub.2 nanosheets in a 1:1 weight ratio.

3. The electrochemical capacitor of claim 1, wherein the composite electrode comprises graphene and MoS.sub.2 nanosheets in a 1:1 weight ratio.

4. A method of producing an electrochemical capacitor of claim 1, comprising: producing 1T-transition metal dichalcogenide few-layer nanosheets and/or monolayer nanosheets, the method comprising: (i) an electrochemical intercalation step in an electrochemical cell, the cell comprising a negative electrode comprising a bulk 2H-transition metal dichalcogenide, a counter electrode which is not lithium, and an electrolyte comprising a lithium salt in a solvent, wherein said solvent is capable of forming a solid electrolyte interface layer; wherein the electrochemical intercalation step applying a potential difference to the cell so as to intercalate lithium ions into the negative electrode to provide an intercalated electrode; then (ii) an exfoliation step comprising contacting the intercalated electrode with a protic solvent to produce 1T-transition metal dichalcogenide few-layer nanosheets and/or monolayer nanosheets, wherein the transition metal dichalcogenide is MoS.sub.2 which produces 1T-MoS.sub.2 nanosheets and the 1T-MoS.sub.2 nanosheets is at least 50% by weight trilayer nanosheets and wherein the MoS.sub.2 nanosheets of the composite electrode are at least 50% 1T phase; combining the 1T-MoS.sub.2 nanosheets with graphene to produce the composite electrode comprising graphene for use in the electrochemical capacitor; and producing the electrochemical capacitor comprising the composite electrode comprising graphene.

5. The method of claim 4, wherein the counter electrode comprises a precious metal.

6. The method of claim 4, wherein the counter electrode is platinum.

7. The method of claim 4, wherein the electrolyte comprises a solvent which is selected from dimethyl carbonate, ethylene carbonate, propylene carbonate, and mixtures thereof.

8. The method of claim 4, wherein the electrolyte is a lithium salt in a mixture of dimethyl carbonate and ethylene carbonate.

9. The method of claim 4, wherein the lithium salt is LiClO.sub.4.

10. The method of claim 4, wherein the potential difference is applied to the electrochemical cell for between 1 and 6 hours.

11. The method of claim 4, wherein negative electrode is a pellet of compressed 2H-MoS.sub.2 powder.

12. The method of claim 4, wherein the negative electrode comprises a 2H-MoS.sub.2 crystal.

Description

SUMMARY OF THE FIGURES

(1) The invention will now be described with reference to the following figures, in which:

(2) FIG. 1 shows (A) and (B) Raman spectra of electrochemically exfoliated MoS.sub.2 by intercalation of Li.sup.+. The electrochemical intercalation was performed at 4.5 V vs Ag wire from a solution containing 1.0 M LiClO.sub.4 in DMC/EC. The sample for Raman analysis was prepared by drop coating the dispersion of MoS.sub.2 on to Si/SiO.sub.2 wafer which was then dried in air at room temperature.

(3) FIG. 2 shows optical absorption spectra of exfoliated MoS.sub.2 dispersions in water and isopropanol mixture.

(4) FIG. 3 shows (A) High-resolution XP spectrum of bulk MoS.sub.2 in the Mo3d region, (B) High-resolution XP spectrum of bulk MoS.sub.2 in the S2p region, (C) High-resolution XP spectrum of electrochemically exfoliated MoS.sub.2 by intercalation of Li.sup.+ in the Mo3d region and (D) High-resolution XP spectrum of electrochemically exfoliated MoS.sub.2 by intercalation of Li.sup.+ in the S 2p region. All peak positions were charge-corrected by setting the binding energy of the C 1s signal to 284.5 eV. The samples for XPS were prepared by filtering the dispersions onto a PVDF membrane.

(5) FIG. 4 shows (A) Cyclic voltammograms recorded at 20 mV s.sup.1 in 6.0 M KOH (aq) using symmetrical coin cells constructed from indicated electrodes. The voltage was scanned between 0.0 V (initial potential) and 1.0 V, (B) Gravimetric capacitance as a function of scan rates at indicated electrodes, (C) Nyquist plots obtained using the coin cells at shown electrodes and the inset shows the Bode plot. The measurements were carried out at an amplitude of 5 mV in the frequency range of 100 mHz to 100 KHz at open circuit potential. (D) Charge-discharge curve obtained from indicated electrodes at 1.0 A g.sup.1, (E) Raman spectrum of composite membrane showing both the 1T-MoS.sub.2 peaks (enlarged in inset, J.sub.1, J.sub.2, J.sub.3, J.sub.2g and A.sub.1g) and graphene peaks (D, G and 2D bands), (F) Capacitance retention of 1T-MoS.sub.2/SEG (solution exfoliated graphene) electrodes after 5,000 cycles in 6.0 M KOH (aq).

(6) FIG. 5 shows (A) Cyclic voltammograms recorded at 100 mV s.sup.1 in 1.0 M [Tea][BF.sub.4] in acetonitrile and 1.0 M [Tea][BF.sub.4] in propylene carbonate using symmetrical coin cells constructed from 1T-MoS.sub.2/SEG electrodes. The voltage was scanned between 0.0 V (initial potential) and 2.7 V for acetonitrile and 0.0 V to 3.0 V for propylene carbonate, (B) galvanostatic charge-discharge curves obtained using 1.0 M [Tea][BF.sub.4] in propylene carbonate at indicated currents, (C) Volumetric capacitance as a function of scan rates and (D) Capacitance retention of 1T-MoS.sub.2/SEG electrodes after 5,000 cycles.

EXAMPLES

(7) Production of 1T-MoS.sub.2 Nanosheets

(8) An electrochemical cell having a MoS.sub.2 pellet (Sigma, 99% with an average particle size of 6 m) or a MoS.sub.2 natural crystal working electrode, a Pt mesh counter electrode and an Ag wire reference electrode was used. The electrolyte was 1 M LiClO.sub.4 in a mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC) in one to one volume ratio. The potential of an Ag wire was stable within a few mV for over 4 h. The MoS.sub.2 pellet (12 mm diameter) was made by hydraulic press of the powder (0.4-1.0 g) at 2.5 tons. Prior to performing electrolysis, N.sub.2 gas was bubbled into the electrolyte for 30 min and during the electrochemical measurements an atmosphere of N.sub.2 was maintained above the electrolyte.

(9) Electrochemical intercalation of Li.sup.+ was performed using chronoamperometry by applying a potential of 4.5 V vs Ag wire for 2 hr. The intercalated pellet was then rinsed with acetone and inserted immediately into deionised deoxygenated water for exfoliation and sonicated for 30 mins. The resulting black suspension was added to a separatory funnel and washed with n-hexane (50 mL) twice to extract any residual of organic impurities including the solid electrolyte interface. The concentrated black aqueous phase was collected and filtered over a PTFE membrane (0.2 m pore size) and washed with 1 L of deionised water to remove excess lithium in the form of LiOH. The resulting washed powder was re-dispersed in deionised water and sonicated for 20 min, and then centrifuged at 1500 rpm for 30 min to remove any non-exfoliated material as sediment yielding a highly stable dispersion.

(10) Characterisation of the 1T-MoS.sub.2 Nanosheet Product

(11) Raman spectroscopy provides rapid identification of the phase of MoS.sub.2 and the thickness of the MoS.sub.2 flake. The 2H and 1T phase can easily be distinguished since each phase has different symmetry structures. FIG. 1 shows the comparison between the Raman spectra of the raw MoS.sub.2 powder and after exfoliation by electrochemical intercalation of Li.sup.+. The Raman spectra were collected from various random spots and the average spectrum was reported. Two prominent Raman bands were observed in the bulk powder at 381.9 cm.sup.1 and 407.4 cm.sup.1 due to the in-plane vibration (E.sup.1.sub.2g) and the out-of-plane (A.sub.1g) vibration respectively. After exfoliation, a series of extra Raman signatures emerged at 155 cm.sup.1 (J.sub.1), 227 cm.sup.1 (J.sub.2) and 330.5 cm.sup.1 (J.sub.3) in addition to the E.sup.1.sub.2g (382.2 cm.sup.1) and A.sub.1g (405.3 cm.sup.1) (FIG. 1a). These bands are the characteristic features for the formation of the 1T phase. The formation of the 1T-phase in significant concentrations was also more evident by the major decrease in the intensity of the E.sup.1.sub.2g peak after exfoliation. Calandra suggested that J.sub.1 was due to the in-plane shearing mode of one side of the MoS.sub.2 chain relative to the other, J.sub.2 corresponds to the shifts of the S-atom layers with respect to the Mo atoms and J.sub.3 was due to the stretching of one side of the zig-zag chain relative to the other with a slightly out-of-plane component. The peak shape and position of E.sup.1.sub.2g and A.sub.1g was also changed after exfoliation (FIG. 1B). For example, the half width at half maximum of A.sub.1g increased from 4.5 in bulk MoS.sub.2 to 9.5 after exfoliation and the peak separation between E.sup.1.sub.2g and A.sub.1g decreased from 26 cm.sup.1 in bulk MoS.sub.2 to 23.1 cm.sup.1 in exfoliated samples. It has been reported in literature that that the Raman spectrum of mechanically exfoliated MoS.sub.2 crystals exhibits a peak separation of 20, 22 and 23 cm.sup.1 respectively for monolayer, bilayer and trilayer MoS.sub.2 nanosheets respectively. Based on this calibration data, the thickness of these exfoliated samples is therefore estimated to be trilayer.

(12) The thickness of the exfoliated samples was further characterised by AFM. Statistical analysis of 150 MoS.sub.2 flakes revealed that the majority of the lateral flake sizes vary between 300 nm and 500 nm. The inventors also observed that lateral sizes over 2 m can be obtained if the starting MoS.sub.2 material is a natural MoS.sub.2 crystal.

(13) The majority (>95%) of the flake thicknesses displayed the same topographic height of 4.5 nm regardless of the source of MoS.sub.2 used for exfoliation (MoS.sub.2 powder or crystalline). Previous works showed that the measured flake thicknesses for solution/chemically exfoliated MoS.sub.2 deposited on Si/SiO.sub.2 varied between 1.1 and 1.9 nm for monolayer and 5 nm for trilayer nanosheets. The deviations from their theoretical thickness (0.615 nm for monolayer) were attributed to the presence of adventitious adsorbates, trapped or adsorbed water molecules and flake-substrate equilibrium separation. In the case of 1T phase in particular, the excess negative charge formed due to the electron donation from lithium is stabilised by adsorption of water molecules on both sides of the nanosheets which then increases the overall thickness of the flake. Analysis of the product's Raman data in conjunction with AFM thickness measurement strongly suggests that the majority of the produced flakes are trilayer. Some few flakes showed a thickness of 1.5 nm, which are thought to be monolayer MoS.sub.2 flakes.

(14) FIG. 2 shows the UV-Visible absorption spectra of 1T-MoS.sub.2 and 2H-MoS.sub.2 dispersion (diluted in isopropanol). It is known that the 2H phase is semiconducting and therefore it exhibits characteristic excitonic peaks that are related to its band gap. As shown in FIG. 2, the dispersion obtained from the 2H phase showed two notable excitons at 604 and 667 nm that are associated with the direct-gap transition due to the energy split from valence band and spin-orbit coupling. After exfoliation, the two excitonic peaks (A and B) disappeared due to the phase transition from the semiconducting to metallic MoS.sub.2. The difference in optical properties was also evident when examining the colour of the respective phase dispersions: the 2H phase was dark yellow while the 1T-MoS.sub.2 was dark grey.

(15) The invention provides high concentration 1T phase products. As demonstrated by the prior art, complete phase transformation from 2H-MoS.sub.2 to 1T-MoS.sub.2 has never been realised using lithium intercalation and the resulting structures contain a portion of both 2H and 1T phases. XPS was used to quantify the fraction of each phase in the products of the invention by the deconvolution of the high resolution spectra of the Mo3d and S2p peaks of the bulk and exfoliated MoS.sub.2 (FIG. 3). The Mo 3d spectra of bulk 2H-MoS.sub.2 displayed a doublet with peaks at 229.2 and 232.4 eV that correspond to Mo.sup.4+3d.sub.5/2 and Mo.sup.4+3d.sub.3/2 respectively. After exfoliation, a new pair of peaks emerged, in addition to the known doublet peak of Mo.sup.4+ for 2H-MoS.sub.2, that are shifted to lower binding energies by 0.8 eV with respect to the 2H phase. Similarly, the S2p doublet also displayed a new pair of peaks at a lower binding energy of 161.5 eV (S2p.sub.3/2) and 162.7 eV (S2p.sub.1/2). These new pairs of peaks were due to the formation of the 1T phase. Deconvolution of the Mo 3d and S 2p regions indicates that the concentration 1T phase in the nanosheets is 60%, which is comparable to the concentration of the 1T phase using chemical exfoliation yields (typically 60 to 70% concentration).

(16) Production of 1T-WS.sub.2 Nanosheets

(17) Tungsten disulfide was subjected to the same two-step process. Using Li.sup.+ intercalation the desired 2H to 1T phase change was observed. Exfoliation was achieved through immersion of the intercalated product in water. Characterisation of the exfoliated product using UV-visible spectroscopy and X-ray photoelectron spectroscopy confirmed the formation the 1T-Phase. The concertation of the 1T phase in exfoliated WS.sub.2 was 52%.

(18) Materials and Reagents

(19) MoS.sub.2 powder (99%, 6 m), lithium perchlorate (99.99%), anhydrous dimethyl carbonate (99%) and ethylene carbonate (99%) were purchased from Sigma-Aldrich and used as received. MoS.sub.2 natural crystals were obtained from Manchester Nanomaterials Ltd. U.K. Polytetrafluroethylene was obtained from Omnipore membrane filters (JVWP01300) with a pore size of 0.2 m and polyvinylidene fluoride was obtained from Durapore membrane filters (vvlp01300, 0.1 m). Millipore water (18.2 M) was obtained from a Milli-Q water purification system. WS.sub.2 was obtained from Alfa Aesar.

(20) Characterisation Techniques

(21) Raman spectra were obtained using Renishaw inVia microscope with a 532 nm excitation laser operated at a low power of 0.274 mW with an 1800 l/mm grating and a 100 objective. The sample for Raman measurement was prepared by drop coating the dispersion of the TMDC product on to a Si/SiO.sub.2 wafer which was then dried at room temperature. For AFM analysis the TMDC product dispersion was spray coated onto a Si/SiO.sub.2 substrate which was dried in a vacuum oven at 40 C. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Axis Ultra DLD spectrometer with a monochromated Al K X-ray source (E=1486.6 eV, 10 mA emission), a hemispherical electron energy analyser and a multichannel plate and delay line detector (DLD). The samples for XPS were prepared by filtering the TMDC product dispersion on PVDF membrane. UV-visible spectroscopy measurements were taken using a model DH-2000-BAL (ocean optics).

(22) Production of 2H-MoS.sub.2 Nanosheets and Graphene for Comparison and Testing

(23) 2H-MoS.sub.2 was prepared by solution phase exfoliation. 1.0 g of MoS.sub.2 powder (Sigma, 99% with average particle size of 6 m) dissolved in 100 mL of water/isopropanol mixture (1:1 v/v) was placed in a 250 mL round bottom flask. The mixture was then sonicated in a water bath sonicator (Elmasonic P70H) which operates at 37 kHz and 40% amplitude for 12 h while cooling to maintain a stable temperature of 20 C. The resulting suspension was centrifuged at 6000 rpm twice for 30 min to remove any unexfoliated material. A stable dispersion of the supernatant was obtained and characterisation of the resulting suspension revealed that the exfoliated MoS.sub.2 was in the 2H-phase.

(24) In a similar way, 2.0 g of graphite powder (sigma) was dissolved in 100 mL of water/isopropanol mixture (1:1 v/v) and sonicated for 18 h at 37 kHz and 40% amplitude as previously method. The resulting suspension was centrifuged at 6000 rpm twice for 30 min, and the supernatant collected for analysis.

(25) Conclusions

(26) The invention therefore provides a simple and cost effective bench top experiment for the production of high concentration 1T-TMDCs. The method obviates the use of potentially explosive materials (such as metallic Li or organolithium compounds); by using lithium salts as the source of Li and an inert counter electrode, the experiment can be set up in ambient conditions without the need for a glove box.

(27) The method has been comprehensively exemplified for the production of MoS.sub.2. Production of 1T-WS.sub.2 has also been performed, demonstrating the applicability of the method to other 1T-TDMC products.

(28) Characterisation of the exfoliated products was performed using X-ray photoelectron spectroscopy, Raman spectroscopy and UV-visible absorption spectroscopy. This confirmed the formation of the 1T-phase with a concentration of 60% for MoS.sub.2. Significantly, the flake thickness examination using atomic force microscopy (AFM) and Raman data analysis indicated that the majority of the MoS.sub.2 flakes were trilayer nanosheets. For WS.sub.2, formation of few (about 5 layers) WS.sub.2 with a 1T-phase with a concentration of 52% was observed.

(29) The utility of the 1T-MoS.sub.2 has been demonstrated as an efficient electrocatalyst for the hydrogen evolution reaction (HER) and as an electrode material for high performing coin cell supercapacitors.

(30) An Electrode Material for a Supercapacitor

(31) The capacitance of the produced 1T-MoS.sub.2 was tested in symmetrical coin cells using cyclic voltammetry and chronopotentiometry. Comparison 2H-MoS.sub.2 electrodes were also made.

(32) MoS.sub.2 electrodes were prepared by filtering a known volume of the dispersions onto a pre-weighed PVDF filter using a syringe pump dispenser (New Era Pump Systems, Inc, NY). The MoS.sub.2 membranes were then dried at room temperature in air. Coin cell assembly was prepared in standard CR2032 coin cell hardware with symmetrical active materials. The cells were assembled by stacking two symmetrical membranes back-to-back with the active material contacting the current collector. A few drops of the desired electrolyte (deoxygenated 6.0 M KOH (aq) or 1.0 M tetraethylammonium tetrafluoroborate ([Tea][BF.sub.4]) in acetonitrile or propylene carbonate) was added to fill the electrode before the coin cell was sealed using a hydraulic crimping machine (MSK-160D). Coin cell assembly was carried out in an Ar-filled globe box for the case of organic electrolytes. Gravimetric capacitance, volumetric capacitance, energy and power density were calculated using the best practice methods established in literature.

(33) The CVs obtained using electrodes made from the 2H phase displayed a gravimetric capacitance of 6 F g.sup.1 whereas the 1T phase displayed a gravimetric capacitance of 102 F g.sup.1 at 10 mV s.sup.1 (FIG. 4a). These are consistent with previous reports.

(34) The capacitance of 1T phase was found to be strongly dependent on the potential scan rate: at 5 mV s.sup.1 the gravimetric capacitance was 114 F g.sup.1 and this value decreased almost by half at 100 mV s.sup.1 to 59 F g.sup.1 (FIG. 4b). Similarly, the capacitance obtained from the charge-discharge curve also showed an analogous trend to the CV data where the capacitance decreased from 118 F g.sup.1 at 0.5 A g.sup.1 to 50 F g.sup.1 at 5 A g.sup.1. The decrease in capacitance upon increasing scan rate (v) or discharge current might be associated with the effect of ion diffusion as well as with the resistivity of the electrode. In particular, when the charge storage mechanism involves faradic reaction, the effect of ion diffusion into the layered MoS.sub.2 structure becomes more pronounced. For example, the experimental time scale could be too short to intercalate ions into the layered structures at high v which then account for the low capacitance.

(35) The internal resistance of the device also contributes to the decrease in capacitance. The effect of resistance is more apparent when examining the charge-discharge curve. The obtained discharge curve showed a significant voltage drop (>0.3 V) for the cell that was constructed using 1T-MoS.sub.2 (FIG. 4d). Therefore, increasing the conductivity of the electrode, by adding highly conductive graphene, is expected to enhance the capacitance at higher discharge currents. The inventors fabricated a nanocomposite that consists of 1T-MoS.sub.2 and solution exfoliated graphene (SEG) to alleviate these problems. The composite was made by mixing 1T-MoS.sub.2 with SEG in a one to one concentration ratio. Representative Raman spectra showed the characteristic 1T-MoS.sub.2 peaks and graphene peaks, demonstrating that the two materials are uniformly mixed and distributed across the membrane (FIG. 4e). Moreover, the presence of graphene with the 1T-MoS.sub.2 in the composite did not induce a phase transition.

(36) The electrodes made from the composite materials displayed the typical capacitive behaviour with nearly rectangular shape. The charge-discharge curve also displayed a symmetrical linear shape (FIGS. 4a and 4d). Significantly, 1T-MoS.sub.2/graphene electrode showed an enhanced gravimetric capacitance with little loss as the discharge current increased. For example, the composite material displayed a capacitance of 147 F g.sup.1 at 10 mV s.sup.1 and 120 F g.sup.1 at 100 mV s.sup.1 (FIG. 4b). The enhanced capacitance in the composite material is probably due to the synergistic effect between 1T-MoS.sub.2 and graphene where graphene enhances the overall conductivity of the composite while also reducing the re-aggregation of the sheets.

(37) The synergistic effect was more evident when comparing the gravimetric capacitance of pure graphene (18 F g.sup.1) and 1T-MoS.sub.2 with that of the composite. EC impedance spectroscopy analysis was also carried out to further determine the electrochemical behaviour of each electrode. Nyquist and Bode plots were obtained at open circuit potential over the frequency range of 100 kHz to 10 m Hz (FIG. 4c). The Nyquist plot of the composite electrode displayed a nearly vertical curve with low serial resistance of 0.4 cm.sup.2 while the cell made from 1T-MoS.sub.2 electrode showed a serial resistance 1 cm.sup.2 and a transition from a linear looking feature to Warburg impedance at a frequency of 2.6 Hz. These observations, together with the fact that the Bode phase angle is close to 90 at the composite electrode, demonstrate that that the cell is behaving close to an ideal capacitor.

(38) High volumetric capacitance is attractive for portable electronics and vehicles so the volumetric capacitance of the cell constructed using this composite electrode was assessed. Volumetric capacitances of 560 F cm.sup.3 at 10 mV s.sup.1 and 458 F cm.sup.3 at 100 mV s.sup.1 were obtained. These values are higher than the volumetric capacitance achieved using the best performing carbon-based electrodes. Lin et al. reported 490 F cm.sup.3 in Li.sub.2SO.sub.4 (aq) with N-doped mesoporous few-layer carbon at 2 mV s.sup.1 and Yang et al. reported 250 F cm.sup.3 using restacked graphene sheets. Acerece et al. reported 450 F cm.sup.3 in KCl (aq) and 600 F cm.sup.3 in H.sub.2SO.sub.4 (aq) at 10 mV s.sup.1 using chemically exfoliated 1T-MoS.sub.2. Ghidiu et al. reported 900 F cm.sup.3 in H.sub.2SO.sub.4 (aq) at 2 mV s.sup.1 using conducting and hydrophilic Ti.sub.3C.sub.2 electrodes. While the gravimetric capacitance of the composite material exemplified herein is in the same range as in graphene-based electrodes, the volumetric capacitance of our composite material is twice that of most carbon-based electrodes. Galvanostatic charge-discharge cycling of the composite material between 0.0 V to 0.8 Vat 1 A g.sup.1 for 5,000 cycles exhibited excellent stability with a capacitance retention of over 92% (FIG. 4f). Moreover, the serial resistance of the device did not increase by any notable amount after cycling. The small loss in capacitance is an indication of no phase transition.

(39) Finally, the composite electrode was tested using non-aqueous electrolytes to take advantage of the wide electrochemical window. Acetonitrile (AN) and propylene carbonate (PC) with tetraethylammonium tetrafluoroborate electrolyte were tested. The AN-based electrolyte showed a potential window of 2.7 V whilst the PC-based electrolyte showed an enhanced potential window of 3.0 V (FIG. 5a). Furthermore, examination of the CV and charge-discharge curve showed the composite electrode produced responses that are expected for capacitive behaviour. The volumetric capacitance of each electrolyte as a function of v was measured (FIG. 5c). Volumetric capacitances of 292 F cm.sup.3 for AN and 247 F cm.sup.3 for PC were obtained at 10 mV s.sup.1 which are in close agreement with the ones obtained from charge-discharge at 0.5 A g.sup.1; 287 F cm.sup.3 for AN and 206 F cm.sup.3 for PC. Furthermore, the composite electrode showed excellent stability in PC electrolytes with more than 96% capacity retention after 5000 cycles while in AN the capacity retention was 90% (FIG. 5d).

(40) Energy and power densities as well as gravimetric and volumetric capacitance obtained at various discharge currents in each electrolyte are shown in Table 1.

(41) TABLE-US-00001 TABLE 1 Gravimetric capacitance (cg), volumetric capacitance, volumetric and gravimetric energy and power density obtained at different discharge current from 1.0M [Tea][BF4] in acetonitrile and 1.0M [Tea][BF4] in propylene carbonate. Current C.sub.g C.sub.v Ev .sup.a Pv .sup.a E.sub.g .sup.a P.sub.g .sup.a (A (F (F (Wh (W (Wh (W Electrolytes g.sup.1) g.sup.1) cm.sup.3) cm.sup.3) cm.sup.3) g.sup.1) g.sup.1) AN 0.5 75 287 0.282 4 0.07 1. 1 66 252 0.23 8 0.06 2 3 52 197 0.17 24 0.04 6 5 45 173 0.14 39 0.036 10 10 34 130 0.08 66 0.02 17.5 PC 0.5 48 211 0.25 7.4 0.057 1.7 1 46 174 0.2 9.5 0.05 2.5 3 38 146 0.15 24 0.04 6.3 5 32 123 0.1 53 0.026 14 10 28 105 0.056 85.5 0.015 22 .sup.a ((Note that voltage used for energy and power density calculation changes according to applied discharge current because of ohmic drop)).

(42) In general, the AN performed better than the PC electrolyte because of its low viscosity. In the AN electrolyte, depending on the magnitude of discharge current, the energy densities varied between 0.08 Wh cm.sup.3 and 0.28 Wh cm.sup.3 with corresponding power densities between 1 W cm.sup.3 and 18 W cm.sup.3 while in the PC electrolyte they were between 0.06-0.25 Wh cm.sup.3 and 1.7-22 W cm.sup.3. These values are much higher than the energy/power density obtained using carbon-based electrodes and within close agreement of chemically exfoliated 1T-MoS.sub.2 electrodes. This demonstrates that the 1T MoS.sub.2/graphene composite shows potential as an attractive electrode for portable supercapacitor devices.

(43) Hydrogen Evolution Reaction at 1T-MoS.sub.2

(44) The electrocatalytic activity of 1T-MoS.sub.2 produced according to the method of the invention towards HER was assessed using liner sweep voltammetry and electrochemical impedance spectroscopy using a three electrode configuration in deoxygenated 0.5 M H.sub.2SO.sub.4 (aq). The electrodes were prepared by drop-coating a suspension of 1T-MoS.sub.2 onto glassy carbon electrodes to a mass loading of 12 g cm.sup.2.

(45) Hydrogen evolution reaction measurements were performed using a three-electrode cell consisting of a 3-mm-diameter glassy carbon (GC) disk working electrode, an Ag|AgCl reference electrode, and a Pt counter electrode (area of 1.2 cm.sup.2). Before use, GC working electrodes were polished with aqueous 0.3 m alumina (Buehler, Lake Bluff, Ill.) slurries on felt polishing pads and rinsed with deionized water. GC electrodes were modified with 10 L of the MoS.sub.2 dispersion (either 1T-MoS.sub.2 or 2H-MoS.sub.2) and then dried at room temperature in air. Cyclic voltammograms (CVs) were recorded in 1 M H.sub.2SO.sub.4 at 5 mV s.sup.1 which was deoxygenated prior to use by bubbling with N.sub.2 for 30 min. A blanket of N.sub.2 was maintained above the electrolyte during measurements. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range from 100 kHz to 100 mHz at an oscillation amplitude of 5 mV and an applied potential of 0.45 V vs. Ag|AgCl. Impedance spectra were fitted to a Randles equivalent circuit model using Z-view software (Scribner Associates, Inc., Southern Pines, N.C.).

(46) Polarisation curves were compared at Pt, 2H-MoS.sub.2 and 1T-MoS.sub.2 electrocatalysts. In each case the current densities were normalized to the geometric area of each electrode and the data was corrected for iR (ohmic) drop. The 2H phase displayed an overpotential () of 0.25 V and reasonable amount of current density (10 mA cm.sup.2) only flowed when exceeded 0.35V. The low catalytic activity for HER is presumably due to the small surface area of the active edge sites as well as poor electrical transport between the active site and the basal plane. In particular, for 2D 2H-MoS.sub.2 nanosheets, the portion of the inert basal plane is significantly larger than its edge site this then led to the low HER electrocatalytic current. However, the 1T phase exhibited low of 0.13 V with substantial improvement in electrocatalytic current density (for example, 50 mA cm.sup.2 recorded at =0.23 V). This observation correlates reasonably well with previously reported data.

(47) The measured Tafel slopes for Pt, 1T and 2H phase MoS.sub.2 were 33, 49, and 109 mV decade.sup.1 respectively, and these values are consistent with previous studies. The proposed general models for HER suggest that the Tafel plot should have a slope of 118 mV decade.sup.1 at 25 C. if the adsorption of hydrogen atoms (the Volmer step) is the rate-determining step. If the Heyrovsky or Tafel steps is rate determining, the Tafel slope should be about 40 mV decade.sup.1 and 30 mV decade.sup.1, respectively. Therefore, our data suggest that the rate of the HER at the 1T phase electrocatalyst is controlled by the Volmer-Heyrovsky step while at the 2H-phase the Volmer step is the rate determining. The electrocatalytic activity of 1T and 2H phase were also further examined by electrochemical impendence spectroscopy at =0.25 V vs. RHE. The Nyquist plots were fitted to an equivalent circuit that contains solution resistance (R.sub.S), constant phase element (CPE) and a charge transfer resistance (R.sub.CT). The Nyquist plots show one semicircle due to R.sub.CT of HER. From the best fits of the data to the equivalent circuit, values for R.sub.CT at each electrocatalyst were obtained. For the 2H phase the R.sub.CT of HER was 320 cm.sup.2, this value was decreased to 20 cm.sup.2 at the 1T phase. The trend in the R.sub.CT values shows that charge transfer was significantly faster at the 1T-MoS.sub.2 surface than at the 2H-MoS.sub.2, which is consistent with the linear sweep voltammogram data.

(48) The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

(49) While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

(50) For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

(51) Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

(52) Throughout this specification, including the claims which follow, unless the context requires otherwise, the word comprise and include, and variations such as comprises, comprising, and including will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

(53) It must be noted that, as used in the specification and the appended claims, the singular forms a, an, and the include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent about, it will be understood that the particular value forms another embodiment. The term about in relation to a numerical value is optional and means for example+/10%.

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