Patent Application
20190352190
This application claims priority from GB1701109.9 filed 23 Jan. 2017, the contents and elements of which are herein incorporated by reference for all purposes.
The invention relates to a method for the production of 1T-phase transition metal dichalcogenide few- and/or mono-layer nanosheets. The invention further relates to electrochemical capacitors comprising 1T-MoS2 and graphene composite electrodes.
Since the discovery of graphene, there has been a growing interest in other materials that can be produced as two-dimensional species. These atomically thin products typically exhibit interesting optical, electrical and mechanical properties.
One such class of materials is the so-called 2D transition metal dichalcogenides, of which molybdenum disulfide is perhaps the most famous and interesting example. MoS2 is stable as atomically thin sheets, and a single sheet of MoS2 consists of a plane of Mo atoms sandwiched between two layers of sulfur atoms in the form of S—Mo—S. The bond holding together the metal and sulfur atom is a strong covalent bond whilst each layer of MoS2 is held together loosely by van der Waals interactions. Other, analogous 2D transition metal dichalcogenides include tungsten disulfide and molybdenum diselenide and tungsten diselenide.
These transition metal dichalcogenides exhibit two possible crystal states, a naturally occurring trigonal prismatic phase (typically denoted by a 2H prefix as, for example, 2H-MoS2), and a non-naturally occurring tetrahedral phase (typically denoted by a 1T prefix as, for example, 1T-MoS2). Molybdenum disulfide in this latter form (i.e. 1T-MoS2) can be prepared by intercalation chemistry.1
In the case of molybdenum disulphide, the 2H phase is semiconducting with a direct bandgap of 1.9 eV for monolayer sheets.2 The 1T phase on the other hand is metallic, with a conductivity up to 107 times higher than the 2H phase.3
Consequently, metallic phase MoS2 nanosheets (and indeed those of other 2D transition metal dichalcogenides) are very attractive for a number of electrochemical applications including as a supercapacitor electrodes and as a catalyst for hydrogen evolution and dye-sensitized solar cells.3-7 For example, Acerce et al. recently demonstrated that the gravimetric capacitance of chemically exfoliated 1T-MoS2 was 20 times higher than the gravimetric capacitance of the 2H-MoS2,3 while the basal plane of 1T-MoS2 has been found to be the main active site for HER (hydrogen evolution reactions) resulting in a significant enhancement in the overall catalytic activity.5,8 By contrast, the basal plane of 2H-MoS2 is catalytically inert towards HER.
To date, only a few synthetic methods have been reported to produce 1T-MoS2. These are top-down (chemical exfoliation) and bottom-up (hydrothermal synthesis) approaches.7,9,10
The most common method for producing 1T-MoS2 nanosheets involves chemical organolithium intercalation between the layers of bulk MoS2.4,8-12 This process starts first by reacting butyllithium with MoS2 to produce LixMoS2.8,12 Then, the resulting Li intercalated bulk MoS2 is immersed in water and sonicated to exfoliate it, producing monolayer and/or few layer MoS2 nanosheets.11,13 The structural change from the 2H to 1T phase on lithium intercalation is attributed to the electron donation from lithium into the conduction band of MoS2.14,15
While this method is attractive in terms of its high yield and production of monolayer MoS2 sheets, it also has some major drawbacks. The experimental process requires a long lithiation time (2-3 days) under reflux (100° C.). Furthermore, organolithium compounds are highly sensitive to air and moisture; their pyrophoric nature means that the reaction must be performed under glove box conditions.
Another approach is “bottom-up”, using a hydrothermal process.7,16 According to reports, this method can produce stable 1T-MoS2 in large quantities with a high concentration of the 1T metallic phase. The product however is thicker MoS2 nanoplatelets and smaller flakes (˜100 nm lateral size).7 These may be less useful for electronic applications.
Recently, Zhang et al. reported an electrochemical lithiation method for the synthesis of 1T-MoS2.8,17a They prepared MoS2 electrode by mixing MoS2 powder with carbon black (10%) as conducting additive and polyvinylidene fluoride (PVDF) as a binder (10%), and then inserted it into a battery cell within an Ar-filled glove box using metallic lithium as a counter electrode.6,17b The experimental time scale was shorted from days (in chemical intercalation) to hours, but proved difficult to scale up, and separating the conductive additive contaminant is challenging. Potential unwanted side reactions were also observed and the conductive additive contaminates the product.17c
This method also proved to be advantageous in terms of controlling the degree of lithium insertion. Nonetheless, the use of metallic lithium means that the intercalation process must be carried out in a controlled inert atmosphere. The battery-type electrochemical cell setup also presents challenges in terms of scale up.17c
Rivolo et al. have also described work to generate mixed 1H-, 1T-MoS2, reduced graphene oxide aerogels for use as electrodes. The 1T phase is apparently obtained by an in situ co-synthesis of an MoS2 nanostructure together with the rGO interconnected 3D matrix.18
There remains a need in the art for efficient and scalable processes for the production of 1T-transition metal dichalcogenide (TMDC) nanosheets from their naturally occurring bulk 2H-allotropes.
The invention seeks to address this need by a two-step synthesis method comprising an electrochemical intercalation step followed by an exfoliation step. Using this method, high concentration few-layer 1T-MoS2 and other 1T-TMDCs can be produced from the corresponding bulk 2H-TMCDs in short (a few hours, or even less) timescales using air-stable reagents.
Accordingly, in a first aspect the invention may provide a method of producing 1T-transition metal dichalcogenide few-layer nanosheets and/or monolayer nanosheets, the method comprising:
Suitably, the electrolyte solvent is a solvent which forms a solid electrolyte interface (SEI), thereby reducing or preventing solvent co-intercalation. Without wishing to be bound by any particular theory, the inventors think that the formation of an SEI, which is permeable to lithium cations, enables a sufficient degree of lithium intercalation to effect the desired phase change while preventing solvent intercalation and/or exfoliation.
Suitably, the electrolyte solvent does not decompose to produce a gas. Alkyl carbonates and mixtures of alkyl carbonates can be used. Suitable solvents include propylene carbonate (PC), ethylene carbonate (EC) and dimethyl carbonate (DMC) and mixtures thereof. A preferred solvent is a mixture of dimethyl/ethylene carbonate (DMC/EC), preferably in about 1:1 v/v.
The lithium salt must be at least partially soluble in the electrolyte solvent. Suitable lithium salts include, but are not limited to, LiClO4 and LiPF6. For reasons of cost, the invention is exemplified herein using lithium perchlorate.
Advantageously, the counter electrode may be inert. This contrasts with the use of a lithium anode in the Zhang method.6 (It will be appreciated that, suitably, the counter electrode of the present invention is not lithium metal). As a consequence, the electrochemical cells used in the present invention can be set up on a bench top, without the need for a glove box, making commercial scale up and production feasible.
Suitable materials for use as a counter electrode are known in the art, and often include precious metals, mercury and carbon. The term precious metal is understood in the art and is used to refer to gold, silver, palladium and platinum. A carbon electrode may be graphitic. In some cases, the counter electrode is platinum, for example a platinum mesh.
Preferably, the potential difference of the cell is measure with reference a further electrode, termed a reference electrode, as is conventional in the art. Accordingly, in some cases the cell includes a reference electrode, which may be a silver electrode, for example a silver wire.
The method achieves both a phase change and exfoliation. The phase change is from the 2H to 1T geometry. Transition metal dichalcogenides which occur naturally in the 2H phase include molybdenum disulphide, tungsten disulphide, molybdenum diselenide and tungsten diselenide. The bulk 2H-TDMC may be MoS2, WS2, MoSe2 or WSe2, leading to 1T-MoS2, 1T-WS2, 1T-MoSe2 and 1T-WSe2 nanosheets, respectively.
Owing to the toxicity of selenium, MoS2 and WS2 are preferred.
In some cases, the invention primarily produces few-layer nanosheets which are at least 2 or 3 layers thick. For example, in some cases at least 50% by weight of the ‘few-layer nanosheets and/or monosheets’ produced is 3-10 layer nanosheets, for example 3-5 layer nanosheets. In some cases at least 75% by weight of the ‘few-layer nanosheets and/or monosheets’ produced is 3-10 layer nanosheets, for example 3-5 layer nanosheets.
In some cases, the transition metal dichalcogenide is MoS2. The inventors have observed that trilayer (3 layer) nanosheets are favoured for MoS2. In some cases, the transition metal dichalcogenide is MoS2 and at least 50% by weight, for example at least 75% by weight, of the 1T-transition metal dichalcogenide ‘few-layer nanosheets and/or monosheets’ is trilayer nanosheets. They have also observed that trilayer nanosheets are favoured for WS2.
The potential difference may be applied to the electrochemical cell for between 1 and 5 hours, preferably between 1 and 3 hours. In some cases, it is applied for about 2 hours. Advantageously, the process may be completed in about 1 hour.
In some cases, the negative electrode is a pellet of compressed 2H-transition metal dichalcogenide powder. The inventors have observed that use of these pellets leads to good yields, which may be attributed to the large surface area exposed for intercalation.
In some cases, the negative electrode comprises a 2H-transition metal dichalcogenide crystal. The inventors have found that the use of crystals leads to nanosheets having a large lateral size, which may be useful for some applications.
The material is contacted with a protic solvent to effect exfoliation. Preferably, the protic solvent is water. The resultant suspension may be sonicated. The method may further comprise a step of collecting the exfoliated material, for example by filtration of the suspension.
The first aspect of the invention provides exfoliated, phase-changed, material via a two-step process (intercalation then exfoliation).
In a second embodiment, the invention may provide a method for producing a lithium-intercalated bulk transition metal dichalcogenide, the method comprising an electrochemical intercalation step in an electrochemical cell, the cell comprising a negative electrode comprising a bulk 2H-transition metal dichalcogenide, a counter electrode, 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 the lithium-intercalated bulk transition metal dichalcogenide.
It will be appreciated that the options and preferences described for the first aspect apply to the second aspect.
The inventors have found that 1T-MoS2 (in this case characterised as about 60% 1T-phase concentration, and predominantly trilayer) can be combined with solution exfoliated graphene to produce supercapacitors having very desirable properties.
Accordingly, in a third aspect, the invention may provide an electrochemical capacitor comprising a composite electrode, the composite electrode comprising 1T-MoS2 nanosheets and graphene, wherein the 1T-MoS2 nanosheets is at least 50% by weight trilayer nanosheets.
It will be appreciated that the electrochemical capacitor may be termed a supercapacitor.
In some cases, the composite electrode comprises graphene and MoS2 nanosheets in a 1:1 weight ratio.
In some cases, the MoS2 nanosheets of the composite electrode are at least 50% 1T phase. In other words, the 1T phase concentration of the MoS2 nanosheets is at least 50%.
Suitably, the composite electrode is substantially free of MoO3.
In a fourth aspect, the invention may provide a method of producing a composite electrode for use in an electrochemical capacitor, the method comprising combining 1T-MoS2 nanosheets prepared by the method of the first aspect with graphene.
The 1T-MoS2 nanosheets and graphene may be combined to provide a dispersion. This may be filtered through a support, depositing the nanosheets to provide a membrane/film. The membrane/film is a composite electrode which may be layered in, for example, a coin cell. Other methods of forming composite electrodes are known in the art. For example, dispersions may be spin-coated or drop-cast.
Accordingly, in some cases, the method further comprises combining 1T-MoS2 nanosheets and graphene to provide a dispersion. The 1T-MoS2 and graphene may themselves be provided as dispersions, which may be mixed in a 1:1 concentration ratio. Dispersions may be provided in water, alcohol and mixtures thereof, preferably mixtures of water and isopropanol.
The invention produces mono- and/or few-layer sheets of material. In the context of the present invention, few-layer refers to 10 layers or fewer, for example, 9, 8, 7, 6, 5, 4, or 3 layers or fewer. This material is also referred to herein for brevity as nanosheets, although it will be appreciated that the number of layers (10 layers or fewer), rather than any particular absolute thickness, is intended. The term monolayer refers to material that is one layer thick.
Preferably, especially in the case of MoS2, the invention produces three layer sheets (so-called trilayers).
As used herein, the bulk is used to assist distinction between the 2D nanosheets of the invention, which are thin (10 layers or fewer) and their 3D (many layer) counterparts. For example, graphite is bulk material, while graphene is the corresponding 2D material. Bulk materials may be exfoliated to provide 2D nanosheets.
As explained herein, the term 1T-TDMC does not necessarily refer to material which is exclusively in the 1T crystal state. Indeed, to date no method has successfully produced exclusively 1T-MoS2. The skilled person will understand therefore that the prefix 1T denotes that the material has at least some 1T character.
As used herein, 1T-TDMC is used to denote a material having a least 10% by concentration 1T phase, for example, at least 20%, at least 30%, at least 40%, at least 50%. In the case of 1T-MoS2, the amount may be about 60%. Methods of determining the concentration of 1T phase are described herein. The presence of the 1T phase can also be determined observationally owing to an in-suspension colour difference for MoS2. Few-layer 2H-MoS2 suspended in water is yellow. The presence of 1T phase leads to a grey appearance, which can be determined visually. As described herein, 1T-MoS2 (˜60% 1T) produced according to the method of the invention produced to a very dark grey suspension.
The electrode comprises 2H-TDMC, which may be present, for example, as a powder which is compressed to form a pellet, or held together by a binder (for example, a polymer) or an electrode cohesion element such a mesh material. The electrode may also comprise crystalline 2H-TDMC, for example it may be large crystal. The use of crystals may result in laterally larger sheets.
It will be appreciated that the 2H-TDMC material of the electrode is a bulk (3D) material.
Suitably, the electrode does not comprise any conductive additive or other material. The electrode may be connected to the electrical circuit by a metal wire, which is preferably not inserted into the electrolyte.
Preferably the electrode is a pellet of compressed 2H-TDMC powder or is a crystal of 2H-TDMC. In some cases, the electrode is a pellet of compressed 2H-TDMC powder of less than 2.0 mm thickness, for example 1.0-1.5 mm thickness.
The electrode comprises a 2H-TDMC, which is a bulk (3D) material. During the method the TDMC undergoes a phase change and exfoliation to produce 2D TMDC having 1T phase (1T-TDMC). This 1T character is often referred to as “metallic” or “metallic phase”. Suitably, the 2H-TDMC is MoS2, WS2, MoSe2 or WSe2, preferably MoS2 or WS2, most MoS2.
The electrochemical intercalation step is suitably carried out at room temperature, although in some cases the electrolyte may be heated or cooled.
The skilled person will recognise that a range of suitable potential difference values may be applied, and these may be quantified with reference to a third (reference electrode). Accordingly, preferably the cell further comprises a reference electrode, which may be silver.
In some cases, the potential difference is kept substantially constant during the intercalation step. However, it will be appreciated that the resistance of the cell may change as intercalation occurs and/or the SEI layer forms on the electrode. According the current and/or voltage measured may change over the duration of the intercalation step.
In some cases, the applied potential difference is between −3.0 V and −5.0 V vs. Ag wire, preferably between −4.0 V and −5.0 V, most preferably about −4.5 V.
The duration of the intercalation step is suitably hours. Accordingly, in some embodiments it is less than 12 h, preferably less than 6 h, preferably less than 5 h, preferably less than 4 h, for example 3 h or less.
The duration of the intercalation step is suitably at least 30 minutes, for example at least 45 minutes, for example at least 60 minutes.
In some cases, the duration of the intercalation step is between 1 and 6 hours, for example, between 1 and 3 hours, for example between 1 and 2.5 hours, for example between one and 2 hours. In some cases, the duration is about 2 hours.
The following describes an optimised cell for MoS2. Cyclic voltammetry was used to determine the potential at which Li+ intercalation occurs into a MoS2 pellet and a cyclic voltammogram (CV) was recorded at the MoS2 electrode in 1.0 M LiClO4 in DMC/EC mixture. As the electrode potential was scanned from 0.5 V in a negative direction, a cathodic current started to flow around −3.0 V which increased gradually until −4.0 V. This process was due to the insertion of Li+ into the MoS2. In the return sweep, a broad anodic peak was seen due to the de-intercalation of Li+. Based on the CV result, −4.5 V was chosen for electrochemical insertion of Li+ into MoS2. In the scale of the reference electrode used, an applied potential between −4.0 and −5.0 V vs. Ag wire did not result in any notable decomposition reactions. Electrolysis was carried out at various time periods of 1 h, 2 h, 4 h and 6 h and the optimum electrolysis time was found to be 2 h. The optimum intercalation/exfoliation result was obtained when thinner (˜1.0-1.5 mm thickness) MoS2 pellets were used.
Also referred to in the literature as a solid-electrolyte interphase, this is a well-studied phenomenon in the field of lithium ion batteries. An SEI forms when appropriate organic solvents decompose on the negative electrode during the electrochemical reaction and form a solid layer called the solid electrolyte interphase which is electrically insulating yet provides significant ionic conductivity. In the present case, the inventors speculate that the formation of the SEI is important as it prevents exfoliation of the electrode, thereby permitting sufficient intercalation of the lithium ions to effect the 2H to 1T phase change.
The Li+ intercalated TMDC (Lix[TDMC]) electrode is contacted with a protic solvent.
It may be rinsed before exfoliation. For example, in some cases the intercalated electrode is rinsed with an aprotic solvent, suitably a polar aprotic solvent such as acetone.
Suitable protic solvents include water, alcohols and mixtures thereof. A preferred protic solvent is water, which is suitably deoxygenated and deionised. During this process, a significant amount of gas is evolved due to the reaction between the solvent and lithium which forms, for example lithium hydroxide and hydrogen gas between the layers of the TMDC, breaking the structure apart.
To assist nanosheet disassociation, the protic solvent may be sonicated, either during the contact or after the initial expansion. Sonication may be for about 30 minutes.
The invention will now be described with reference to the following figures, in which:
An electrochemical cell having a MoS2 pellet (Sigma, 99% with an average particle size of 6 μm) or a MoS2 natural crystal working electrode, a Pt mesh counter electrode and an Ag wire reference electrode was used. The electrolyte was 1 M LiClO4 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 MoS2 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, N2 gas was bubbled into the electrolyte for 30 min and during the electrochemical measurements an atmosphere of N2 was maintained above the electrolyte.
Electrochemical intercalation of Li+ 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 μm 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.
Raman spectroscopy provides rapid identification of the phase of MoS2 and the thickness of the MoS2 flake. The 2H and 1T phase can easily be distinguished since each phase has different symmetry structures.14
The thickness of the exfoliated samples was further characterised by AFM. Statistical analysis of 150 MoS2 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 μm can be obtained if the starting MoS2 material is a natural MoS2 crystal.
The majority (>95%) of the flake thicknesses displayed the same topographic height of 4.5 nm regardless of the source of MoS2 used for exfoliation (MoS2 powder or crystalline). Previous works showed that the measured flake thicknesses for solution/chemically exfoliated MoS2 deposited on Si/SiO2 varied between 1.1 and 1.9 nm for monolayer and ˜5 nm for trilayer nanosheets.11,25,26 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.11,20,24 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.7,13 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 MoS2 flakes.
The invention provides high concentration 1T phase products. As demonstrated by the prior art, complete phase transformation from 2H-MoS2 to 1T-MoS2 has never been realised using lithium intercalation and the resulting structures contain a portion of both 2H and 1T phases.11,15 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 MoS2 (
Tungsten disulfide was subjected to the same two-step process. Using Li+ 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 concentration of the 1T phase in exfoliated WS2 was 52%.
MoS2 powder (99%, ˜6 μm), lithium perchlorate (99.99%), anhydrous dimethyl carbonate (≥99%) and ethylene carbonate (99%) were purchased from Sigma-Aldrich® and used as received. MoS2 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 μm and polyvinylidene fluoride was obtained from Durapore® membrane filters (vvIp01300, 0.1 μm). Millipore water (18.2 MΩ) was obtained from a Milli-Q® water purification system. WS2 was obtained from Alfa Aesar®.
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 I/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/SiO2 wafer which was then dried at room temperature. For AFM analysis the TMDC product dispersion was spray coated onto a Si/SiO2 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 Kα 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).
2H-MoS2 was prepared by solution phase exfoliation. 1.0 g of MoS2 powder (Sigma, 99% with average particle size of 6 μm) dissolved in 100 mL of water/isopropanol mixture (1:1 v/v) was placed in a 250 mL round bottom flask.30 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 MoS2 was in the 2H-phase.
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.30 The resulting suspension was centrifuged at 6000 rpm twice for 30 min, and the supernatant collected for analysis.
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.
The method has been comprehensively exemplified for the production of MoS2. Production of 1T-WS2 has also been performed, demonstrating the applicability of the method to other 1T-TDMC products.
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 MoS2. Significantly, the flake thickness examination using atomic force microscopy (AFM) and Raman data analysis indicated that the majority of the MoS2 flakes were trilayer nanosheets. For WS2, formation of few (about 5 layers) WS2 with a 1T-phase with a concentration of 52% was observed.
The utility of the 1T-MoS2 has been demonstrated as an efficient electrocatalyst for the hydrogen evolution reaction (HER) and as an electrode material for high performing coin cell supercapacitors.
The capacitance of the produced 1T-MoS2 was tested in symmetrical coin cells using cyclic voltammetry and chronopotentiometry. Comparison 2H-MoS2 electrodes were also made.
MoS2 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 MoS2 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.31 A few drops of the desired electrolyte (deoxygenated 6.0 M KOH (aq) or 1.0 M tetraethylammonium tetrafluoroborate ([Tea][BF4]) 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.3,32
The CVs obtained using electrodes made from the 2H phase displayed a gravimetric capacitance of 6 F g−1 whereas the 1T phase displayed a gravimetric capacitance of 102 F g−1 at 10 mV s−1 (
The capacitance of 1T phase was found to be strongly dependent on the potential scan rate: at 5 mV s−1 the gravimetric capacitance was 114 F g−1 and this value decreased almost by half at 100 mV s−1 to 59 F g−1 (
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-MoS2 (
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 (
The synergistic effect was more evident when comparing the gravimetric capacitance of pure graphene (18 F g−1) and 1T-MoS2 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 (
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−3 at 10 mV s−1 and 458 F cm−3 at 100 mV s−1 were obtained. These values are higher than the volumetric capacitance achieved using the best performing carbon-based electrodes.33,34 Lin et al.33 reported 490 F cm−3 in Li2SO4 (aq) with N-doped mesoporous few-layer carbon at 2 mV s−1 and Yang et al.34 reported 250 F cm−3 using restacked graphene sheets. Acerece et al. reported 450 F cm−3 in KCl (aq) and 600 F cm−3 in H2SO4 (aq) at 10 mV s−1 using chemically exfoliated 1T-MoS2.3 Ghidiu et al. reported 900 F cm−3 in H2SO4 (aq) at 2 mV s−1 using conducting and hydrophilic Ti3C2 electrodes.35 While the gravimetric capacitance of the composite material exemplified herein is in the same range as in graphene-based electrodes,36,37 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 V at 1 A g−1 for 5,000 cycles exhibited excellent stability with a capacitance retention of over 92% (
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 (
Energy and power densities as well as gravimetric and volumetric capacitance obtained at various discharge currents in each electrolyte are shown in Table 1.
a ((Note that voltage used for energy and power density calculation changes according to applied discharge current because of ohmic drop)).
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−3 and 0.28 Wh cm−3 with corresponding power densities between 1 W cm−3 and 18 W cm−3 while in the PC electrolyte they were between 0.06-0.25 Wh cm−3 and 1.7-22 W cm−3. These values are much higher than the energy/power density obtained using carbon-based electrodes38-41 and within close agreement of chemically exfoliated 1T-MoS2 electrodes.3 This demonstrates that the 1T MoS2/graphene composite shows potential as an attractive electrode for portable supercapacitor devices.
The electrocatalytic activity of 1T-MoS2 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 H2SO4 (aq). The electrodes were prepared by drop-coating a suspension of 1T-MoS2 onto glassy carbon electrodes to a mass loading of 12 μg cm−2.
Hydrogen evolution reaction measurements were performed using a three-electrode cell consisting of a 3-mm-diameter glassy carbon (GC) disk working electrode, an AglAgCl reference electrode, and a Pt counter electrode (area of 1.2 cm2). Before use, GC working electrodes were polished with aqueous 0.3 μm alumina (Buehler, Lake Bluff, Ill.) slurries on felt polishing pads and rinsed with deionized water. GC electrodes were modified with 10 μL of the MoS2 dispersion (either 1T-MoS2 or 2H-MoS2) and then dried at room temperature in air. Cyclic voltammograms (CVs) were recorded in 1 M H2SO4 at 5 mV s−1 which was deoxygenated prior to use by bubbling with N2 for 30 min. A blanket of N2 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.).
Polarisation curves were compared at Pt, 2H-MoS2 and 1T-MoS2 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 (η) of ˜0.25 V and reasonable amount of current density (10 mA cm−2) only flowed when η 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-MoS2 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 η of 0.13 V with substantial improvement in electrocatalytic current density (for example, 50 mA cm−2 recorded at η=0.23 V). This observation correlates reasonably well with previously reported data.4,5
The measured Tafel slopes for Pt, 1T and 2H phase MoS2 were 33, 49, and 109 mV decade−1 respectively, and these values are consistent with previous studies.8,9 The proposed general models42,43 for HER suggest that the Tafel plot should have a slope of 118 mV decade−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−1 and 30 mV decade−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 η=0.25 V vs. RHE. The Nyquist plots were fitted to an equivalent circuit that contains solution resistance (RS), constant phase element (CPE) and a charge transfer resistance (RCT). The Nyquist plots show one semicircle due to RCT of HER. From the best fits of the data to the equivalent circuit, values for RCT at each electrocatalyst were obtained. For the 2H phase the RCT of HER was 320 Ω cm2, this value was decreased to 20 Ω cm2 at the 1T phase. The trend in the RCT values shows that charge transfer was significantly faster at the 1T-MoS2 surface than at the 2H-MoS2, which is consistent with the linear sweep voltammogram data.
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.
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.
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.
Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.
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.
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%.
A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1701109.9 | Jan 2017 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/051566 | 1/23/2018 | WO | 00 |
