Field of Invention

This invention relates to a composite material that is made from layers of a transition metal dichalcogenide that has intercalated zero-valent transition metal nanoparticles between layers of the transition metal dichalcogenide. The invention also relates to methods of making the composite material and its application as part of a membrane-electrode assembly to split water.

Background

The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

Hydrogen energy is an ideal, clean and highly efficient secondary energy resource to replace petroleum fuels in the future energy production. As an alternative to the traditional platinum catalysts used in hydrogen evolution reaction (HER), transition metal dichalcogenides (TMDCs) (i.e. two-dimensional (2D) molybdenum disulfide (MoS ₂ ) crystals) have recently attracted worldwide attention because of their low-cost, global availability and acid-stability. Although TMDC can catalyze hydrogen evolution in principle via its edges or defective sites, its activity is not competitive with respect to noble metals. MoS ₂ can exist as the thermodynamically stable 2H phase (space group, P63/mmc) or the metastable 1T phase (space group^Swl ). Bulk 2H-MoS ₂ is a semiconductor with an indirect band gap of ~1.2 eV, while the 1T-MoS ₂ phase is metallic.

Solution deposition of metal nanoparticles on exfoliated TMDC nanosheets has been used conventionally to form hybrid structures. However, their long-term operation is usually poor due to the gradual loss of electrochemical active area of metal nanoparticles by dissolution and aggregation or by the re-stacking of the TMDC nanosheets. Furthermore, the ability to make membrane-electrode assemblies (MEA) is critical for industrial operation and few recipes have been developed to make the powder samples or mm-sized films of HER catalysts to membrane-type assemblies. In addition, the complicated exfoliation and washing process used to form TMDC nanosheets also limits their bulk production and industrial application, as it makes the process difficult to scale and increases the cost of production significantly. For example, use of 2D MoS ₂ nanosheets as catalysts requires tedious exfoliation processes, which constrains mass production and industrial applications. Although, it is possible to coat metal nanoparticles on exfoliated 2H-MoS ₂ to enhance its catalytic activity, such nanoparticles rapidly corrode under an acidic HER environment and get leached, leading to the loss of activity. Restacked MoS ₂ nanosheets intercalated with transition metal ions can be synthesized from single layer MoS ₂ dispersions by an ion exchange method, in which the cation (M ²⁺ ) neutralizes the negative charge of the MoS ₂ layer and the material restacks with alternating layers of MoS ₂ and M(OH) ₂ . However, the chemical reduction of these metal ions requires the use of reducing agents and adds to the complexity of the process.

Most current research in this area are solely based on the electrochemical measurement of powder samples on a glassy carbon electrode where the long-term stability as well as scalability are not examined and no methods have been developed to bridge the technological gap. This is a major gap between lab research and industry.

Thus, there remains a need for improved materials for use as HER catalysts and methods of preparation thereof.

Summary of Invention

The current invention relates to a method of transforming 2D-chalcogenides into a ternary composite by a highly efficient intercalation of zero-valent metals to produce a highly active, ultrastable hydrogen evolution catalyst and membrane-electrode assembly for water splitting.

Aspects and embodiments of the invention are discussed in the numbered clauses below. 1. A method of forming a composite material, the method comprising the steps of:

(a) providing an alkali metal intercalated transition metal dichalcogenide, where the alkali metal intercalated transition metal dichalcogenide is formed by a plurality of 3- dimensionally stacked transition metal dichalcogenide layers and the alkali metal is intercalated between said layers; and

(b) reacting the alkali metal intercalated transition metal dichalcogenide with a transition metal ion precursor in the presence of an anhydrous organic solvent, to provide a composite material comprising zero-valent transition metal nanoparticles intercalated in between the 3-dimensionally stacked layers of the transition metal dichalcogenide.

2. The method according to Clause 1 , wherein the transition metal dichalcogenide has a generalised formula of MX ₂ , where M is a transition metal selected from group 4 to 10 of the period table and X is selected from the chalcogen group of the periodic table, optionally wherein X is selected from the group consisting of tellurium, or more particularly, selenium or sulfur. 3. The method according to Clause 2, wherein the transition metal dichalcogenide is selected from the group consisting of molybdenum disulfide (MoS ₂ ), tungsten disulfide (WS ₂ ), titanium disulfide (TiS ₂ ), tantalum sulfide (TaS ₂ ), vanadium disulfide (VS ₂ ), molybdenum diselenide (MoSe ₂ ), and tungsten diselenide (WSe ₂ ), optionally wherein the transition metal dichalcogenide is molybdenum disulfide (MoS ₂ ).

4. The method according to any one of the preceding clauses, wherein the transition metal ion precursor is a salt or acid of a transition metal ion selected from group 4 to 10 of the period table. 5. The method according to Clause 4, wherein the transition metal ion precursor is an organic or inorganic salt or acid and the transition metal ion is selected from platinum, iron, cobalt, nickel, copper, zinc, ruthenium, palladium, gold, silver, optionally wherein the transition metal ion precursor is selected from one or more of the group consisting of sodium hexachloroplatinate (Na ₂ PtCI ₆ ), gold chloride (AuCI ₃ ), ruthenium chloride (RuCI ₃ ), palladium chloride (PdCI ₂ ), silver nitrate (AgN0 ₃ ), iron chloride (FeCI ₃ ) and copper chloride (CuCI ₂ ), chloroauric acid (HAuCI ₄ ), chloroplatinic acid (H ₂ PtCI ₆ ), and solvates thereof (e.g. the transition metal ion precursor is Na ₂ PtCI ₆ ).

6. The method according to any one of the preceding clauses, wherein:

(a) the anhydrous organic solvent is selected from one or more of the group consisting of hexane, tetrahydrofuran (THF), Ν,Ν-dimethylformamide (DMF), N-methyl-2- pyrrolidone (NMP), and dimethyl sulfoxide (DMSO); and/or

(b) the reaction is conducted at a temperature of from 30 to 150°C for a period of from 4 hours to 10 days, optionally wherein the reaction is conducted at a temperature of from 40 to 00°C for a period of from 6 hours to 7 days. 7. The method according to any one of the preceding clauses, wherein the molar ratio of the alkali metal intercalated transition metal dichalcogenide to the transition metal ion precursor is from 1 :1 to 20:1. 8. The method according to any one of the preceding clauses, wherein the process further comprises a step of preparing the alkali metal intercalated transition metal dichalcogenide which step comprises providing a transition metal dichalcogenide and reacting it with an alkali metal or an alkali metal precursor in an anhydrous organic solvent, where the transition metal dichalcogenide is formed by a plurality of 3-dimensionally stacked transition metal dichalcogenide layers.

9. The method according to Clause 8, wherein:

(a) the reaction is conducted at room temperature to 80°C for a period of from 0.5 to 7 days; and/or

(b) the feeding molar ratio of alkali metal compound to the dichalcogenide is from

1 :1 to 10:1 ;

(c) the anhydrous organic solvent is selected from one or more of the group consisting of hexane, heptane, cyclohexane, dimethoxyethane (DME) and tetrahydrofuran (THF), optionally wherein the alkali metal concentration within the organic solvent is from 0.1 to 5 M; and/or

(d) the alkali metal or alkali metal precursor is selected from one or more of the group consisting of lithium, sodium, potassium, alkali metal hydrides and related compounds, organometallic compounds of alkali metals and complexes of alkali metals with ammonia, naphthalene and related complexes, optionally wherein the alkali metal or alkali metal precursor is selected from one or more of the group consisting of lithium, sodium, potassium, lithium hydride (LiH), lithium aluminum hydride (LiAIH ₄ ), sodium aluminum hydride (NaAlhk), methyllithium (L1CH3), n-butyllithium, and f-butyllithium.

10. A composite material comprising:

a transition metal dichalcogenide formed by a plurality of 3-dimensionally stacked transition metal dichalcogenide layers; and

zero-valent transition metal nanoparticles intercalated between the 3-dimensionally stacked layers of the transition metal dichalcogenide, wherein

the zero-valent transition metal nanoparticles are homogeneously distributed on the transition metal dichalcogenide. 11. The composite according to Clause 10, wherein the zero-valent transition metal nanoparticles:

(a) have a particle size of from 1.5 to 15.0 nm (e.g. from 2.0 to 10.0 nm); and/or

(b) are present in the amount of from 5 to 30 wt% (e.g. from 10 to 25 wt%) of the composite material; and/or

(c) are a transition metal selected from group 4 to 10 of the period table, optionally wherein the transition metal is selected from one or more of the group consisting of platinum, iron, cobalt, nickel, copper, zinc, ruthenium, palladium, gold, and silver (e.g. palladium, gold, ruthenium and, more particularly, platinum); and/or

(d) the zero-valent transition metal nanoparticles account for from 50 to 100% of the total amount of transition metal nanoparticles intercalated within the transition metal dichalcogenide.

12. The composite according to Clause 10 or Clause 11 , wherein the transition metal dichalcogenide has a generalised formula of MX ₂ , where M is a transition metal selected from group 4 to 10 of the period table and X is selected from the chalcogen group of the periodic table, optionally wherein X is selected from the group consisting of tellurium, or more particularly, selenium or sulfur. 13. The composite according to Clause 12, wherein the transition metal dichalcogenide is selected from the group consisting of molybdenum disulfide (MoS ₂ ), tungsten disulfide (WS ₂ ), titanium disulfide (TiS ₂ ), tantalum sulfide (TaS ₂ ), vanadium disulfide (VS ₂ ), molybdenum diselenide (MoSe ₂ ), and tungsten diselenide (WSe ₂ ), optionally wherein the transition metal dichalcogenide is molybdenum disulfide (MoS ₂ ).

14. A membrane-electrode assembly comprising a composite material according to any one of Clauses 10 to 13.

15. The membrane-electrode assembly, wherein the assembly comprises:

a first electrode and a second electrode that each comprise a substrate material and a composite material according to any one of Clauses 10 to 13 coated on the substrate material; and

a polymer electrolyte membrane, where

the polymer electrolyte membrane is sandwiched between the first electrode and the second electrode, optionally wherein the assembly further comprises a first and a second gas diffusion layer material overlaid onto the first and second electrodes, respectively. 16. The assembly according to Clause 5, wherein:

(a) the substrate material is a carbon paper, a carbon cloth or a gas diffusion layer material; and/or

(b) the composite material according to any one of Clauses 10 to 13 is provided in a total loading amount of from 50 pg/cm ² to 3 mg/cm ² on the substrate material (e.g. from

70 pg/cm ² to 2 mg/cm ² , such as 1 mg/cm ² ); and/or

(c) the first and second electrode further comprise Nafion™ coated on the substrate material, where the mass ratio of the composite material to Nafion™ is from 10:1 to 1 :1.

17. A process of manufacturing a membrane-electrode assembly comprising the steps of:

(a) providing a polymer electrolyte membrane and first and second electrode that each comprise a substrate material and a composite material according to any one of Clauses 10 to 13 coated on the substrate material; and

(b) sandwiching the polymer electrolyte membrane between the first electrode and second electrode to form an unsealed assembly and sealing the unsealed assembly to form a membrane electrode assembly. 18. The process of Clause 17, wherein:

(a) the substrate material is a carbon paper, a carbon cloth or a gas diffusion layer material; and/or

(b) the process further comprises adding a gas diffusion layer material onto each of the first and second electrodes before sealing the unsealed assembly; and/or

(c) the sealing process is a hot-pressing process, optionally wherein the hot- pressing process is conducted at a temperature of from 100 to 140°C for 1 to 10 minutes at a normalised pressure of from 50 to 250 kg/cm ² ; and/or

(d) the process further comprises applying a catalyst ink comprising a composite material according to any one of Clauses 10 to 13, Nafion™ solution and either N- methylpyrrolidine or a water and alcohol mixture onto the substrate material to form the first and second electrodes, optionally wherein:

(i) the concentration of the composite material within the catalyst ink is from 0.1 to 30 mg/mL; and/or

(ii) when present, the alcohol is one or more of the group selected from methanol, ethanol and isopropanol; and/or

(iii) the mass ratio of the composite material to Nafion™ is from 10:1 to

1 :1 ; and (iv) the concentration of the Nafion solution in N-methylpyrrolidine or the water and alcohol mixture is from 5 to 20 wt%; and/or

(v) the application of the catalyst ink is conducted by drop-coating, dip- coating, brush painting or spraying onto the substrate material; and/or

(e) the composite material according to any one of Clauses 10 to 13 is provided in a total loading amount of from 50 pg/cm ² to 3 mg/cm ² on the substrate material (e.g. from 70 pg/cm ² to 2 mg/cm ² , such as 1 mg/cm ² ).

19. A method of hydrogen evolution by water splitting, comprising use of a membrane- electrode assembly according to any one of Clauses 14 to 16.

20. The method of Clause 19, wherein the process is conducted under acidic conditions, optionally wherein the process is conducted in the presence of a mineral acid (e.g. H ₂ S0 ₄ , HCI or HCI0 ₄ ) that is present at a concentration of from 0.1 to 2.0 M.

Further aspects and embodiments of the invention are discussed in the lettered clauses hereinbelow.

A) A way of synthesizing a metal-intercalated layered composite where we used transition metal dichalcogenides (TMDC) as an exemplary host, and zero-valent metal nanoparticles as the intercalant; the composite can be applied as highly active, ultrastable hydrogen evolution catalysts for water splitting, TMDC includes a large class of layered compounds of the type MX ₂ where M is a transition metal and X is either tellurium or, more particularly, sulfur or selenium. The use of the layered host material is not restricted to TMDC, and can be any layered elements or compounds consisting of vertically stacked layers.

B) The method of clause A, wherein the highly active, ultrastable activity is achieved by the "sandwiching" of catalytically active metal nanoparticles in between the TMDC layers; the latter serves to protect the metal nanoparticles and also the synergetic interaction between the metal nanoparticle and MoS ₂ allows more effective catalysis at a lower metal loading than what was commonly achieved for metal nanoparticle-coated exfoliated MoS ₂ (or other 2D materials). C) The ternary structure of clause B is a 3D structure with layered stacking compared to that of exfoliated 2D TMDC nanosheets and/or composites. The ability to maintain intact layer stacking in such TMDCs is due to the avoidance of hydration-sonication in the direct intercalation process, which prevents exfoliation and random re-stacking in the solid state. D) The ternary structure of clause B is achieved by using anhydrous organic solvents to dissolve the metal ion precursors, this allows in situ reduction of the metal ion precursors at the molecular level and formation of uniform metal particles in between TMDC layers.

E) A way to intercalate zero-valent metal nanoparticles into TMDC layered structure by using alkali metal pre-intercalation and subsequent in-situ reduction of metal ion precursors in an organic solvent.

F) The TMDC in clause E has a generalized formula of MX ₂ where M is a transition metal of groups 4-10 and X is a chalcogen, for example, molybdenum disulfide (MoS ₂ ), tungsten disulfide (WS ₂ ), titanium disulfide (TiS ₂ ), tantalum sulfide (TaS ₂ ), vanadium disulfide (VS ₂ ), molybdenum diselenide (MoSe ₂ ) and tungsten diselenide (WSe ₂ ).

G) The method of clause E, wherein the TMDCs are treated with alkali metal compounds at room temperature to 80 °C for 0.5 to 7 days, the feeding molar ratio of alkali metal compound to the dichalcogenide is 1 :1 to 10:1 , the pre-intercalation is conducted in anhydrous hexane, heptane, cyclohexane, dimethoxyethane (DME) or tetrahydrofuran (THF) with an alkali metal concentration of 0.1 to 5 M.

H) The method of clause E, wherein the alkali metal compounds can include:

1 ) the solid or liquid form of lithium, sodium, potassium and their combinations;

2) alkali metal hydrides and related compounds, such as lithium hydride (LiH), lithium aluminum hydride (LiAIH ₄ ), sodium aluminum hydride (NaAIH ₄ );

3) organometallic compounds of alkali metals, such as methyl!ithium (LiCH ₃ ), n-butyllithium, f-butyllithium; and

4) ammonia, naphthalene or related complexes of alkali metals.

I) The method of clause E, wherein the in-situ reduction may use pre-intercalated TMDCs as reductant for the metal ion precursors. It may be conducted at elevated temperatures (e.g. from 40 to 100 °C) for 6 hours to 1 week. The feeding molar ratio of dichalcogenide to precursor may range from 1 :1 to 20:1. J) The method of clause E, wherein the metal ion precursors can comprise, for example, the organic and inorganic salts/acids of iron, cobalt, nickel, copper, zinc, ruthenium, palladium, silver, platinum and gold. In some preferred embodiments, the metal ion precursors are: i) sodium hexachloroplatinate (Na ₂ PtCI ₆ ), gold chloride (AuCI ₃ ), ruthenium chloride (RuCI ₃ ), palladium chloride (PdCI ₂ ), silver nitrate (AgN0 ₃ ), iron chloride

(FeCI ₃ ) and copper chloride (CuCI ₂ ); and

ii) chloroauric acid (HAuCI ₄ ), and chloroplatinic acid (H ₂ PtCI ₆ ).

K) The method of clause E, wherein the preferred organic solvents used in the in-situ reduction include hexane, tetrahydrofuran (THF), Ν,Ν-dimethylformamide (DMF), N-methyl- 2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO). These organic solvents are anhydrous, which can avoid the disruption of TMDCs layered structure by trace amount of water in solvents. L) A way to make membrane-electrode assemblies with TMDCs catalysts by casting "catalyst inks" onto carbon papers/cloths and then hot-pressing with commercial polymer electrolyte membrane, which can further improve cell performance as well as significantly reduce the sensitivity towards changes in the environment. M) The method of clause L, wherein the membrane-electrode assembly is composed of a polymer electrolyte membrane, two catalyst layers (anode/cathode), and/or two gas diffusion layers.

N) The method of clause L, wherein the carbon papers/cloths used for membrane electrode assemblies are modified with a microporous layer (MPL) as well as a hydrophobic treatment. The membrane is commercial Nafion™ 1 12, 1 15, 117, 212 or XL membrane with standard pre-treatment.

O) The method of clause L, wherein the standard pre-treatment of Nafion® membrane may be heated at 80 °C for 1 hour in 5 wt% H ₂ 0 ₂ solution, ultrapure water, 8 wt% H ₂ S0 ₄ solution and finally ultrapure water. After each treatment, the membrane may be thoroughly washed with ultrapure water to remove traces of H ₂ 0 ₂ or H ₂ S0 ₄ . The membrane may be soaked in ultrapure water before use.

P) The method of clause L, wherein the "catalyst inks" may be prepared by dispersing corresponding catalysts in any ratio of water/alcohol mixture with the aid of Nafion™ solution. The concentration of catalysts may range from 0.1 to 30 mg/mL. The alcohol can be methanol, ethanol or iso-propanol. The mass ratio of catalyst to Nafion® is 10:1 to 1 :1. The concentration of Nafion™ solution is 5 to 20 wt% in the water/alcohol mixture or N-methyl-2- pyrrolidone (NMP).

Q) The method of clause L, wherein the catalyst loading amount may range from 70 μg/cm ² to 2 mg/cm ² . The actual loading amount is determined by the mass difference before and after casting "catalyst inks". The deposition of catalyst can be done by drop-coating, dip- coating, brush painting or spraying the "catalyst ink" on carbon paper/cloth.

R) The method of clause L, wherein the condition of hot-pressing may be from 100 to 140 °C for 1 to 10 mins at a normalized pressure of 50 to 250 kg/cm ² .

S) The method of clause L, wherein the hydrogen evolution is conducted in an acidic condition. The concentration of acid is 0.1 to 2.0 M H ₂ S0 ₄ or equivalent concentration of HCI or HCI0 ₄ .

Drawings Figure 1. Preparation of metal nanoparticles/TMDC composites via the zero-valent intercalation strategy.

Figure 2. The morphology and chemical composition of metal intercalated MoS ₂ catalysts: (a) digital photo of Pt-MoS ₂ powders, SEM (b), TEM (c,d) images, XPS spectrum (e) and powder XRD pattern (f) of Pt-MoS ₂ composites.

Figure 3. (A) ToF-SIMS maps at the beginning (0 s), middle (300 s) and end (600 s) of the experiment. The false colours of blue and red corresponds to MoS ₂ ^~ (162 amu) and ΡΓ (195 amu). (B) ToF-SIMS side view and (C) depth profile of the same region (etching depth is roughly 400 nm, as calculated by the step of 300 nm Si0 ₂ /Si wafer). Scale bar, 20 pm.

Figure 4. Uniform distribution of intercalated metals on MoS ₂ . FESEM EDS mapping of (A) Pt-MoS ₂ , (B) Ru-MoS ₂ , (C) Pd-MoS ₂ and (D) Au-MoS ₂ . Scale bar: (A) 100 pm, (B) 200 pm, (C) 50 pm and (D) 100 pm.

Figure 5. Expanded morphology after n-BuLi and metal intercalation. FESEM images of (A) bulk MoS ₂ , (B) Li _x MoS ₂ , (C) Pt-MoS ₂ , (D) Ru-MoS ₂ , (E) Pd-MoS ₂ and (F) Au-MoS ₂ . The ordered layered structure of MoS ₂ was only slightly distorted after intercalation of noble metals.

Figure 6. TEM characterization of metal intercalated MoS ₂ : TEM images, SAED patterns and EDS spectra of (A) Pt-MoS ₂ , (B) Ru-MoS ₂ , (C) Pd-MoS ₂ and (D) Au-MoS ₂ . The SAED pattern of Li _x MoS ₂ (not shown here) also confirm the existence of 1T'-phase by the presence of the -5.6 A, 2 * 1 superstructure spots.

Figure 7. (a) GIXRD patterns of single crystal MoS ₂ and (b) Pt-MoS ₂ ; (c) Corresponding 1D spectra in the out-of-plane direction. (d,e) TEM images of the nanosheets exfoliated from bulk Pt-MoS ₂ . Corresponding (f) SAED and (g) EDS spectrum of the exfoliated Pt-MoS ₂ . Scale bar, d,e 50 nm; f 5 nm ^"1 .

Figure 8. (A) Dark field TEM image of Pt-MoS ₂ . (B) High resolution HAADF-STEM image and (C-E) EELS mapping showing the intercalation of Pt nanoparticles with an average size of ~ 2 nm in between MoS ₂ layers

Figure 9. Hydrogen evolution performance of metal intercalated MoS ₂ catalysts: (a) LSV curves and corresponding (b) Tafel plots; (c) long-term stability test at 50 mA/cm ² , and (d) cycling performance of Pt-MoS ₂ catalysts (Inset: TEM image after 10,000 cycles). All experiment were conducted in 0.5 M H ₂ S0 ₄ at 70 pg/crn ² loading.at room temperature.

Figure 10. (a) CO stripping and (b) Cu UPD of 40wt% Pt/C and Pt-MoS ₂ , showing the different accessibility of inner Pt nanoparticles. (c) The saturation rate of the Cu monolayer deposition upon holding at a fixed UPD potential, which indicates a much slower diffusion process in the case of Pt-MoS ₂ compared with Pt/C. (d) Schematics showing the anisotropic diffusion of CO gas and cupric ions into Pt-MoS ₂ .

Figure 11. Digital photos of a 25 cm ² membrane-electrode assemblies using metal intercalated MoS ₂ catalysts: (a) coating onto carbon paper, (b) coating onto carbon cloth, (c) combined with Nafion™ membrane and commercial 40% Pt/C gas diffusion electrode, all at a loading of 1 mg/cm ² .

Figure 12. Methanol oxidation performance of metal intercalated MoS ₂ catalysts. All experiments were conducted in 0.5 M H ₂ S0 ₄ + 1 M methanol at 300 pg/cm ² loading.at room temperature. Description

Disclosed herein is a processing strategy to enhance the application of TMDC in hydrogen production by zero-valent intercalation of metal nanoparticles into bulk TMDC layered structure to form a ternary composite that can be used to achieve excellent, and ultrastable catalytic performance as a hydrogen evolution catalyst in water splitting with very high efficiency. The identification of a processing strategy to intercalate TMDC with metal nanoparticles, which provides a composite material that allows for long-term operation with excellent catalytic activity for hydrogen production, is very important and advantages associated with the product and/or process disclosed herein include:

(1 ) the use of an in-situ reduction of metal ion precursors using an alkali-metal pre-intercalated transition metal dichalcogenide as a reductant (with only the addition of a solvent in addition to the reactants), which is scalable and avoids the time-consuming exfoliation process the exfoliation of 2D nanosheets to produce a high surface area. The bulk form of the material can be used, thus avoiding tedious washing and recovery processes that have to be used in exfoliation chemistry, and allows easy scalability for industrial application. It is also an easy-to-control and "green" method compared to the traditional hydrazine or sodium borohydride reduction methods. The pre-intercalated solids can be prepared in bulk amounts and stored for several months under inert atmosphere.;

(2) sandwich metal nanoparticles between two TMDC layers allows more effective catalysis at lower metal loading and minimizes the dissolution/aggregation of metal nanoparticles during long-term operation, while and significantly enhancing the activity of the catalyst (e.g. for hydrogen evolution). Without wishing to be bound by theory, it is believed that the synergetic combination of metal nanoparticles stabilized within TMDCs layered structure, is very effective in protecting susceptible metal nanoparticles from direct exposure to the corrosive environment, minimizes the dissolution/aggregation of metal nanoparticles during prolonged water splitting operations. The catalytic activity is also enhanced due to the strong metal-support interaction between metal nanoparticles and TMDCs, resulting in lower overall resistance; and

(3) the ability to fabricate membrane-electrode assemblies with TMDC catalysts to further improve cell performance as well as significantly reduce the sensitivity towards changes in the environment. Such a technique provides a much higher catalyst loading and thus enables a better control on water management to achieve high energy conversion rate and efficiency compared to the laboratory testing technique for TMDCs catalysts.

The preparation of metal intercalated TMDC composites is illustrated in Figure 1. Bulk materials such as MoS ₂ powders 100 were first intercalated with alkali metal compounds 150 (e.g., n-butyl lithium) for several days under inert conditions. The resulting pre-intercalated TMDC 200 was reacted with various metal ion precursors 250, which can be readily scaled up to kilogram scale to provide zero-valent transition metal nanoparticles intercalated within TMDC sheets 250. This strategy is fast, "green" and scalable when compared to the traditional exfoliation/deposition strategy. The latter usually requires multiple washing steps to obtain functionalized TMDC nanosheets, which can only be dispersed in good solvents at a very low concentration (e.g., 1 mg/mL in NMP or so), causing a huge difficulty in cost balance, storage and solvent waste. Meanwhile, the functionalized TMDC nanosheets will also return to their aggregation form once casting on electrode for hydrogen production purposes has been completed, reducing the stability and efficiency of the catalyst. In contrast, the zero-valent intercalation strategy avoids these issues because it uses the bulk material, but is demonstrated herein to be an effective catalyst for an extended period of time - even when compared to Pt/C electrodes. In a first step to the manufacture of the composite materials disclosed herein, there may be prepared an alkali metal intercalated transition metal dichalcogenide. An alkali metal intercalated transition metal dichalcogenide may be prepared by reacting a transition metal dichalcogenide with an alkali metal or an alkali metal precursor in an anhydrous organic solvent, where the transition metal dichalcogenide is a material that is formed by a plurality of 3-dimensionally stacked transition metal dichalcogenide layers.

When used herein, unless otherwise specified, the term "transition metal dichalcogenide" relates to the bulk form of such a material, which has a layered three-dimensional structure where layers of MX ₂ sheets are stacked on top of one another to form the bulk material. More particularly, the transition metal dichalcogenide may be represented by the generalised formula MX ₂ , where M is a transition metal selected from group 4 to 10 of the period table and X is selected from the chalcogen group of the periodic table. In particular examples, X may be selected from the group consisting of tellurium, or more particularly, selenium or sulfur.

Particular transition metal dichalcogenides that may be mentioned herein (both as starting material and in the composite material) include, but are not limited to, molybdenum disulfide (MoS ₂ ), tungsten disulfide (WS ₂ ), titanium disulfide (TiS ₂ ), tantalum sulfide (TaS ₂ ), vanadium disulfide (VS ₂ ), molybdenum diselenide (MoSe ₂ ), and tungsten diselenide (WSe ₂ ). For example the transition metal dichalcogenide may be titanium disulfide (TiS ₂ ) or, more particularly, molybdenum disulfide (MoS ₂ ). Suitable alkali metals or alkali metal precursors that may be used in this initial step include, but are not limited to lithium, sodium, potassium, alkali metal hydrides and related compounds, organometallic compounds of alkali metals and complexes of alkali metals with ammonia, naphthalene and related complexes. For example, the alkali metal or alkali metal precursor may be selected from one or more of the group consisting of lithium, sodium, potassium, lithium hydride (LiH), lithium aluminum hydride (LiAlhk), sodium aluminum hydride (NaAlh ), methyllithium (UCH3), n-butyllithium, and i-butyllithium.

When this initial reaction is conducted as part of the process, it may be conducted at a suitable temperature, such as from room temperature to 80°C for a suitable period of time, such as from 0.5 to 7 days, in a suitable solvent (e.g. in an anhydrous organic solvent, which may include, but is not limited to hexane, heptane, cyclohexane, dimethoxyethane (DME) and tetrahydrofuran (THF) and combinations thereof), and with the alkali metal/alkali metal precursor provided in a suitable concentration within the organic solvent, such as from 0.1 to 5 M. In certain embodiments, the feeding molar ratio of alkali metal/ alkali metal precursor to the transition metal dichalcogenide may be from 1 :1 to 10:1.

In the resulting intermediate product, the alkali metal may be lithium, sodium, or potassium (or combinations thereof).

The composite material of the current invention may be formed from the alkali metal intercalated transition metal dichalcogenides described hereinbefore by a process comprising the steps of:

(a) providing an alkali metal intercalated transition metal dichalcogenide, where the alkali metal intercalated transition metal dichalcogenide is formed by a plurality of 3- dimensionally stacked transition metal dichalcogenide layers and the alkali metal is intercalated between said layers; and

(b) reacting the alkali metal intercalated transition metal dichalcogenide with a transition metal ion precursor in the presence of an anhydrous organic solvent, to provide a composite material comprising zero-valent transition metal nanoparticles intercalated in between the 3-dimensionally stacked layers of the transition metal dichalcogenide.

It will be appreciated that the alkali metal intercalated transition metal dichalcogenide may be prepared just before use in the above process, prepared and then stored for a period of time before use, or may be bought from a commercial supplier. Transition metal ion precursors used herein to manufacture the composite material may be a salt or acid of a transition metal ion selected from group 4 to 10 of the period table. For example, the transition metal ion precursor may be an organic or inorganic salt or acid and the transition metal ion selected from platinum, iron, cobalt, nickel, copper, zinc, ruthenium, palladium, gold, silver. Suitable transition metal ion precursors that may be mentioned herein include, but are not limited to, sodium hexachloroplatinate (Na ₂ PtCI ₆ ), gold chloride (AuCI ₃ ), ruthenium chloride (RuCI ₃ ), palladium chloride (PdCI ₂ ), silver nitrate (AgN0 ₃ ), iron chloride (FeCI ₃ ) and copper chloride (CuCI ₂ ), chloroauric acid (HAuCI ₄ ), chloroplatinic acid (H ₂ PtCI ₆ ), solvates thereof and combinations thereof, (e.g. the transition metal ion precursor is Na ₂ PtCI ₆ ).

The composite material-forming reaction may be conducted at a suitable temperature, such as from 30 to 150°C (e.g. from 40 to 100°C) for a suitable period of time, such as from 4 hours to 10 days (e.g. from 6 hours to 7 days), in a suitable anhydrous organic solvent (e.g. which may include, but is not limited to hexane, tetrahydrofuran (THF), N,N- dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), and combinations thereof). In certain embodiments, the molar ratio of the alkali metal intercalated transition metal dichalcogenide to the transition metal ion precursor may be from 1 :1 to 20:1.

The processing strategy described herein allows a series of zero-valent metal nanoparticles, e.g., platinum, to be successfully intercalated into TMDC (such as MoS ₂ ) layered structure (ternary structure), achieving unprecedented stability and activity for hydrogen evolution. Such MoS ₂ catalysts with ~ 10 wt% platinum loading exhibits higher hydrogen evolution activity than the benchmark 40 wt% Pt C catalyst, with an overpotential down to 20 mV and a Tafel slope of ~20 mV/dec. Even at a current density up to 50 mA/cm ² , the overpotential can still reach to ~48 mV vs. RHE and is stable for 120,000 s (~ 33.3 hours) operation. This is among the best performance reported to date for TMDCs. A 25 cm ² membrane-electrode assemblies can be made by casting "catalyst inks" onto carbon papers/cloths and subsequent hot-pressing with Nafion™ membrane, thus showing the scalability of the processing methods disclosed herein.

Thus, there is also provided the composite material per se and a membrane-electrode assembly comprising said composite material.

The composite material comprises: a transition metal dichalcogenide formed by a plurality of 3-dimensionally stacked transition metal dichalcogenide layers; and

zero-valent transition metal nanoparticles intercalated between the 3-dimensionally stacked layers of the transition metal dichalcogenide, wherein

the zero-valent transition metal nanoparticles are homogeneously distributed on the transition metal dichalcogenide, optionally, the nanoparticles have a particle size of from 1.5 to 15.0 nm and are present in the amount of from 5 to 30 wt% of the composite material.

The morphology of transition metal-intercalated TMDCs was confirmed by the SEM and TEM images - for example, such as those shown in Figure 2. As depicted, the transition metal-intercalated TMDCs have a well-ordered layered structure with a lateral size of ~ 3 pm and an expanded layer spacing due to the intercalation of alkali metals and subsequent metal nanoparticles. It can be seen that the metal nanoparticles have a diameter of from 2 to 10 nm and that the nanoparticles are uniformly distributed on the TMDC flakes. The loading of metal nanoparticles ranges from 5 to 30 wt%, depending on the feeding ratio of TMDCs to metal ion precursors. Because of the semi-conducting nature of TMDCs, the intercalation of a zero-valent metal can improve the overall electrical conductivity and facilitate the diffusion of electrolyte in TMDCs. Moreover, in the example of Pt-MoS ₂ , the intercalated Pt nanoparticles are in their zero-valent state by XPS. The crystalline structure can be verified by powder XRD measurement, where Pt nanoparticles with the most thermal stable {111} plane and two other planes ({200}, {222}) can be observed. Two sets of MoS ₂ patterns in the range of 30 ~ 80° are also observed, which suggests the expanded layered spacing after zero-valent intercalation. The composite material comprise zero-valent transition metal nanoparticles. The term "zero- valent transition metal" refers to the metallic form of the transition metal in an uncharged state. The nanoparticles may have any suitable shape and may have a particle size of from 1.5 to 15.0 nm, or from 2.0 to 10.0 nm. The zero-valent transition metal nanoparticles may be present in an amount of from 5 to 30 wt% of the total weight of the composite material (e.g. from 10 to 25 wt%).

The zero-valent transition metal nanoparticles in the composite material may be formed from one or more transition metals selected from group 4 to 10 of the period table. For example, transition metals that may be mentioned herein include, but are not limited to, platinum, iron, cobalt, nickel, copper, zinc, ruthenium, palladium, gold, silver and combinations thereof (e.g. palladium, gold, ruthenium and, more particularly, platinum). It will be appreciated that not all of the transition metal nanoparticles within the composite material may be zero-valent, though some particles may contain transition metal atoms that may still be ionically charged. Nevertheless, the zero-valent transition metal nanoparticles may account for from 50 to 100% of the total amount of transition metal nanoparticles intercalated within the transition metal dichalcogenide. For example, the zero-valent transition metal nanoparticles may account for from 80 to 100% of the total amount of transition metal nanoparticles intercalated within the transition metal dichalcogenide.

The term "transition metal dichalcogenide" is as defined hereinbefore.

As mentioned hereinbefore, the composite material may be used to form the catalyst within a membrane-electrode assembly. As such, there is also provided a membrane-electrode assembly comprising a composite material as describe hereinbefore. The membrane-electrode assembly (MEA) may comprise:

a first electrode and a second electrode that each comprise a substrate material and a composite material as described herein coated on the substrate material; and

a polymer electrolyte membrane, where

the polymer electrolyte membrane is sandwiched between the first electrode and the second electrode.

The above describes a 3-Layer MEA, with is composed of a polymer electrolyte membrane (PEM), and two catalyst layers or electrode layers on either face of the membrane. It will be appreciated that a 5-Layer MEA may also be formed by the introduction of a first and a second gas diffusion layer (GDL) material overlaid onto the first and second electrode layers, respectively.

The PEM has two major functions in a fuel cell: to separate the fuel and the oxidant; and to transport protons across from the anode to the cathode to complete the redox reaction. The PEM must also provide strong mechanical, chemical and electrochemical stability in a harsh, chemical-rich environment over a range of operating conditions, while offering a long operating life, low reactant permeability, high proton conductivity and the ability to act as an electrical insulator. Suitable PEMs that may be mentioned herein include, but are not limited to Nafion™ 112, 115, 117, 212 or XL membranes, which may have undergone standard pre- treatments. Such standard pre-treatments may include (e.g. for Nafion™ membranes) heating the membrane at 80 °C for 1 hour in 5 wt% H ₂ 0 ₂ solution, ultrapure water, 8 wt% H ₂ S0 ₄ solution and finally ultrapure water. After each treatment, the membrane is thoroughly washed with uitrapure water to remove traces of H ₂ 0 ₂ or H ₂ S0 ₄ . The membrane may be soaked in uitrapure water before use in preparing the MEA.

When present as an additional component (see below), the GDLs sit outside the catalyst/electrode layers and facilitate transport of reactants into the catalyst/electrode layers, as well as removing the products. Each GDL is typically composed of a sheet of carbon paper or cloth in which carbon fibers are partially coated with polytetrafluoroethylene (PTFE). Gases diffuse rapidly through the pores of the GDL and these pores are kept open by the hydrophobic nature of the PTFE, which prevents excessive water build-up.

The substrate material in the MEA may be a carbon paper, a carbon cloth or a gas diffusion layer material. As noted above, GDLs may be formed using carbon cloths/papers and so may act as the substrate directly herein. For completeness, it is noted that further GDLs may be added over the GDLs comprising the composite material in the manner discussed above.

The composite material may be provided on the substrate material such that it provides a total loading amount of from 50 pg/cm ² to 3 mg/cm ² (e.g. from 70 pg/cm ² to 2 mg/cm ² , such as 1 mg/cm ² ). In certain embodiments, the first and second electrodes may further comprise Nafion™ coated on the substrate material, where the mass ratio of the composite material to Nafion™ is from 10:1 to 1 :1. It will be understood that the mass ratio mentioned here relates to the weight of Nafion™ alone.

It will be appreciated that the MEA will also comprise other conventional components to form a functional device that may be used to generate hydrogen. Such other components may include gaskets and bipolar plates, amongst others.

The membrane-electrode assembly may be manufactured by a process involving the following steps:

(a) providing a polymer electrolyte membrane and first and second electrode that each comprise a substrate material and a composite material as describe herein coated on the substrate material; and

(b) sandwiching the polymer electrolyte membrane between the first electrode and second electrode to form an unsealed assembly and sealing the unsealed assembly to form a membrane electrode assembly. As noted above, the product may further include a gas diffusion layer on the first and second electrodes. As such, the process may further involve adding a gas diffusion layer material onto each of the first and second electrodes before sealing the unsealed assembly. Any suitable sealing process may be used to form the MEA. A sealing process that may be mentioned herein is the hot-pressing process. Suitable conditions for the hot sealing process to form a MEA with the materials discussed above include sealing at a temperature of from 100 to 140°C for 1 to 10 minutes at a normalised pressure of from 50 to 250 kg/cm ² . It will be appreciated that the first and second electrodes are formed from a substrate material that may be a carbon paper, a carbon cloth or a gas diffusion coated with the composite material described herein. As such, the process may further include applying a catalyst ink comprising a composite material described herein, Nafion™ solution and either N-methylpyrrolidine or a water and alcohol mixture onto the substrate material to form the first and second electrodes.

The catalyst ink may be formed from a combination of the composite material disclosed herein, Nafion™ and a solvent. The solvent may be a water/alcohol mixture or N- methylpyrrolidine. When the solvent is a water/alcohol mixture, the alcohol may be one methanol, ethanol or isopropanol and the water/alcohol mixture may contain any suitable percentage of water:alcohol. The concentration of the Nafion™ solution in the solvent may be from 5 to 20 wt%, while the mass ratio of the composite material to Nafion™ is from 10:1 to 1 :1 (in this case the mass ratio is based on Nafion™ alone). The concentration of the composite material in the catalyst ink may be from 0.1 to 30 mg/mL.

The composite material may be provided in a total loading amount of from 50 g/cm ² to 3 mg/cm ² on the substrate material (e.g. from 70 pg/cm ² to 2 mg/cm ² , such as 1 mg/cm ² ). The catalyst ink may be applied to the substrate material by any suitable means, which include, but are not limited to, by drop-coating, dip-coating, brush painting or spraying.

It will be appreciated that the MEA described herein may be used in a process of hydrogen evolution by water-splitting. Thus, there is provided a process of hydrogen evolution by water splitting, comprising use of a membrane-electrode assembly described herein. Any suitable condition can be used in the process. Suitable conditions that may be mentioned herein include conducting the process using acidic conditions. For example, the process may be conducted in the presence of a mineral acid (e.g. H ₂ S0 ₄ , HCI or HCI0 ₄ ) present in a concentration of from 0.1 to 2.0 M. Additional uses of the MEA containing the composite material described herein include, when platinum or other noble metals are intercalated into the TMDC, its use in the methanol oxidation reaction (MOR; Zhai, C. et al. J. Power Sources 275, 483-488 (2015); Yuwen, L. ei al. Nanoscale 6, 5762-5769 (2014)) and oxygen reduction reaction (ORR; Wang, T. et al. ChemCatChem 6, 1877-1881 (2014)). These may form additional aspects and embodiments of the invention.

The composite material disclosed herein may be used in the following applications in addition to hydrogen evolution.

• When the composite material comprises platinum or other noble metals intercalated into the TMDC, the composite material may be used as a catalyst in carbon monoxide oxidation (Kown, S. ei al. Phys. Chem. Chem. Phys. 18, 13232-13238 (2016); Du, C. ei al. J. Mater. Chem. A 3, 231 13-23119 (2015)) and in an H ₂ S- containing solid oxide fuel cell (SOFC) (Xu, Z.-R. et al., J. Power Sources 188, 458-

462 (2009)), either in powder form or as part of a MEA. The inventors have already confirmed that powder samples can be used to catalyse carbon monoxide oxidation reactions.

• When the composite material comprises earth abundant transition metals (such as Cu, Ni, Fe etc) intercalated into the TMDC, the composite may be used as a catalyst for carbon dioxide (C0 ₂ ) reduction (Asadi, M. et al. Nature Commun. 5, 4470 (2014); Chan, K. et al. Chem Cat Chem 6, 1899-1905 (2014)). For C0 ₂ reduction reactions, the composite may preferably be provided in a MEA, though pre-measurements using powder samples may be necessary.

· When the composite material comprises Fe or Co intercalated into the TMDC, the composite may be used as a catalyst for hydrodesulfurization (HDS; Furimsky, E. Catal. Rev.-Sci. Eng. 22, 371 -400 (1980)). MoS ₂ is itself known as an industrial catalyst for HDS and it is believed that the composite materials (comprising Fe or Co as intercalated materials) disclosed herein, can significantly enhance the performance of MoS ₂ (Bouwens, S. M. A. M. ei al. J. Phys. Chem. 95, 123-134

(1991 )). It will be appreciated that the composite may be provided in powder form to act as a catalyst in this reaction, or attached to, or within, a suitable substrate.

These may form additional aspects and embodiments of the invention. In embodiments herein, the word "comprising" may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word "comprising" may also relate to the situation where only the components/features listed are intended to be present (e.g. the word "comprising" may be replaced by the phrases "consists of or "consists essentially of). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word "comprising" and synonyms thereof may be replaced by the phrase "consisting of or the phrase "consists essentially of or synonyms thereof and vice versa.

Examples

Equipment The following equipment was used. SEM/EDS (Jeol JSM-6701 F), XPS (AXIS UltraDLD, monochromatic Al K _a ), Electrochemistry (CHI 660E and Zahner Zennium with a three- electrodes cell), STEM & EELS (Nion UltraSTEM-100 with aberration-correction, 60 KV), TEM/EDS (FEI Titan, 80 kV), ToF-SIMS (ION-TOF SIMS ⁵ with Bi ⁺ and Cs ⁺ beams), XRD/GIXRD (Bruker D8 & GADDS).

General Methods

ToF-SIMS was performed on ION-TOF SIMS ⁵ with Bi ⁺ primary beam (25 keV with 80x80 urn 2 spot size) and Cs ⁺ secondary gun (2 keV, 70 nA with 230 x230 um2 analyse area). A prolonged n-BuLi pretreatment (1 week) and Pt precursor reaction (3 days) were adopted for single crystal MoS ₂ flake due to relatively slow intercalation. Pt-MoS ₂ flake was then peeled off using Scotch tape method and transferred onto 300 nm Si0 ₂ /Si substrates for mapping.

A Pt-MoS ₂ flake prepared from single-crystal MoS ₂ was used for GIXRD. GIXRD was performed on a Bruker GADDS diffractometer with an area detector under Cu Ka (1.5418 A) radiation (40 kV, 40 mA) at room temperature. The incident angle of primary beam to sample surface was moved from 0.5 to 4°, with a detection angle of 1.3 to 30°. Pt-MoS ₂ and single- crystal MoS ₂ flakes were directly placed on test holder for GIXRD measurements. Elemental analysis (EA) was conducted as follows: Mo, Li, Pt, Ru, Pd and Au content in the samples mentioned below were determined by inductively coupled plasma optical emission spectrometry (ICP-OES); S was determined by both CHNS analysis and anionic ion- exchange chromatography (IC) of S0 ^2" . At least 3 ~ 4 mg of samples were used for EA with parallel measurements of > 3 times to give a standard error of ~ 1 wt% and a detection limit of > 0.5 wt%. EA samples for ICP-OES and IC were firstly digested using aqua regia under microwave heating. EA samples for CHNS analysis were used in powder form for combustion in 0 ₂ . We also performed EDS and XPS for comparison. All results are in good agreement with the theoretical loading of metals (e.g., Pt, 10.9 wt% vs. 10.4 wt% from EA) on MoS ₂ . Preparation 1

Synthesis of lithium intercalated MoS?

Commercial MoS ₂ powders (< 2 pm in diameter) were dried in 80°C oven for 3 days prior to lithium intercalation. About 1.0 g dry MoS ₂ powders were dispersed in 20 ml. n-butyllithium solution (1.6 M in hexane) in an argon filled glove box. The dispersion was stirred at room temperature for 2 days. The resulting black powders were centrifuged at 3,000 rpm and washed with anhydrous hexane five times to remove unreacted n-butyllithium and other soluble impurities. The lithium-intercalated MoS ₂ powders were dried in glove box. Reference Example 1

Reduction Potential of lithium intercalated MoS?

The lithium-intercalated with 50 mg MoS ₂ powder of preparation 1 were combined in excess with metal ion solutions for 2 days at 80°C from the first two rows of the transition metals in the periodic table. Each metal ion solution used was coloured (with varying intensity and colour) and all were reduced to colourless solutions by the lithium-intercalated MoS ₂ powder. The metal ion solutions used were Na ₂ PtCI ₆ ^« 6H ₂ 0 (17.6 mg in THF), HAuCI ₄ ^« 4H ₂ 0 (15.8 mg in THF), PdCI ₂ (27.7 mg in NMP), RuCI ₃ *xH ₂ 0 (17.6 mg in NMP), Fe(OAc) ₂ (27.2 mg in NMP), CoCI ₂ (20.3 mg in NMP), NiCI ₂ (20.3 mg in NMP), CuCI ₂ (21.0 mg in NMP) and AgN0 ₃ (53.1 mg in NMP).

The metal ions reduced include, with their standard reduction potential indicated in the brackets: PtCI ₆ ^2~ (1.48 V, versus RHE), AuOf (0.93 V), Pd ²⁺ (0.92 V), Ag ⁺ (0.80 V), Ru ³⁺ (0.70 V), Cu ²⁺ (0.34 V), Ni ²⁺ (-0.25 V), Co ²⁺ (-0.28 V) and Fe ²⁺ (-0.44 V). The reduction was performed in a non-aqueous solution (anhydrous THF or NMP (N-methylpyrrolidine) depending on the solubility of precursors) to prevent the de-stabilization of the 1T'-phase. In this anhydrous environment, the high-valent state metal ions diffuse into the layers of 1T'- phase Li _x oS ₂ and are in-situ reduced to the zero-valent state. To compensate for the excess charges, Li ⁺ and Cl ^~ ions are completely leached from the MoS ₂ host after the ion exchange. It must be pointed out that bulk 2H-MoS ₂ and exfoliated MoS ₂ nanosheets cannot reduce PtCI ₆ ^2~ to Pt(0) as exfoliated MoS ₂ nanosheets only have a very weak reduction ability due to the rapid destruction of the 1T'-phase after hydration. This is demonstrated by comparing the reducing power of 50 mg of bulk MoS ₂ , with exfoliated MoS ₂ and Li _x MOS ₂ (the composite prepared in Preparation 1 ) on Na ₂ PtCI ₆ ^» 6H ₂ 0 (17.6 mg in THF) after heating for 2 days at 80°C. Without wishing to be bound by theory, it is speculated that the exfoliated MoS ₂ nanosheets only show weak reducing power due to loss of the 1T'-phase after water exfoliation.

Preparation 2

Synthesis of sodium intercalated TiS?

Commercial TiS ₂ powders were dried in 80°C oven for 3 days prior to sodium intercalation.

1.72 g Naphthalene was dissolved in 40 mL of anhydrous tetrahydrofuran (THF) with vigorous stirring. 0.6 g sodium was cut into thin pieces and added to the above solution. The dispersion was stirred at room temperature for 2 hours until all sodium had dissolved. After that, 1.0 g dry TiS ₂ was added to the mixture and reacted for 24 hours. The resulting black powders were centrifuged at 3,500 rpm and washed with anhydrous THF 5 times and with anhydrous hexane 3 times to remove any unreacted sodium/naphthalene and other soluble impurities. The sodium intercalated TiS ₂ powders were then dried in a glove box.

It is believed that the intercalation process proceeds

Example 1

Zero-valent intercalation of lithium intercalated MoS? with platinum salt

Sodium hexachloroplatinate hexahydrate (Na ₂ PtCI ₆ '6H ₂ 0; 33.7 mg) was dissolved in 20 mL anhydrous tetrahydrofuran in an argon filled glove box. About 100 mg of the lithium intercalated MoS ₂ from Preparation 1 was then added to the above solution, which resulted in the formation of many hydrogen bubbles. After the bubbling had reduced/stopped, the mixture was then sealed in a Teflon™-lined autoclave and kept at 60°C for 2 days. The product (Pt-MoS ₂ ) was thoroughly washed with THF or NMP (* 2, 12,000 rpm, 10 mins), and then with isopropanol, ethanol and water before being dried in a 60 °C oven.

The morphology of platinum-intercalated TMDCs is confirmed by the SEM and TEM images in Figure 2. As depicted, the composite materials have a well-ordered layered structure with a lateral size of ~ 3 \im and an expanded layer spacing due to the intercalation of alkali metals and subsequent metal nanoparticles. It is seen that metal nanoparticles have a diameter of from 2 - 10 nm and are uniformly distributed on the TMDC flakes. Moreover, in the example of Pt-MoS ₂ , the intercalated Pt nanoparticles are in their zero-valent state by XPS. The crystalline structure is verified by powder XRD measurement, where Pt nanoparticles with the most thermal stable {111} plane and two other planes ({200}, {222}) can be observed. We can also observe two sets of MoS ₂ patterns in the range of 30 ~ 80°, which suggests the expanded layered spacing after zero-valent intercalation. To assay for the spatial distribution of intercalated and reduced zero-valent Pt, time-of-flight secondary ion mass spectroscopy (ToF-SIMS) is used to acquire the 3D elemental distribution of Pt in MoS ₂ (see Liu, Y. et a/. Thermal oxidation of WSe ₂ nanosheets adhered on Si0 ₂ /Si substrates. Nano Lett. 15, 4979-4984 (2015)). As shown in Fig. 3, Pt nanoparticles can be clearly seen on MoS ₂ . At a longer sputtering time, both Pt ^" and MoS ₂ ^~ signals were observed in the composition map of the deeper layers, which proves the successful intercalation of Pt at deeper regions of MoS ₂ single crystal (~200 nm). From the sputtering profile (Fig 3a), Ρ species can be detected at depths up to 400 nm and beyond. The SIMS profiles of MoS ₂ ^" and Pt ^" signals in Fig. 3a suggests that the vertical distribution of Pt ^" is quite homogeneous. Consistent with the kinetics of the intercalation process, a diffusion gradient exists for the Pt nanometals when its lateral distribution on layers exfoliated from bulk Pt-MoS ₂ by the 'Scotch tape' method was analysed. A Pt signal can be detected by EDS mapping only at the edge of MoS ₂ after 12 h of intercalation, while the signal can be detected homogeneously across the flake after 3 days, as seen in the FESEM and EDS images in Figs. 4a and 5c. Pt-MoS ₂ has a well-ordered layered structure with an expanded layer spacing due to the intercalation of the metal nanoparticles. Its bulk morphology is markedly different from the restacked nanosheets generated from exfoliation strategy. The selected area electron diffraction (SAED) pattern of Pt-MoS ₂ is distorted from the typical hexagonal pattern of bulk 2H-MoS ₂ . The continuous ring with a measured d- spacing of 2.2 A as determined by Fourier transform can be assigned to the {1 11} planes of Pt, while the appearance of two new rings strongly suggests the intercalation of Pt species into MoS ₂ host structure (Fig. 6a). The change in interlayer spacing of MoS ₂ after intercalation by Pt was also verified by grazing incidence X-ray diffraction (GIXRD). As shown in Fig. 7, a broad peak at -2.46° (-3.6 nm) in 1 D and 2D GIXRD for the MoS ₂ |Pt|MoS ₂ sandwiched structure is shown, which is absent in the bulk sample. The d-spacing of MoS ₂ layer is calculated to be 0.63 nm from X-ray diffraction and the average size of Pt nanoparticles is found to be about 2 nm from HRTEM images in Fig. 8. Accordingly, Pt-MoS ₂ has a repeat unit of 2-3 layers of MoS ₂ and 1 layer of Pt nanoparticles. In addition, we also observe a peak shift of the MoS ₂ {002} from 14.10 to 14.34° together with an increase in FWHM after Pt intercalation, which can be ascribed to the stress on the MoS ₂ layers induced by Pt nanoparticles in-between. The existence of Pt nanoparticles in the inner layers of MoS ₂ is directly confirmed by the TEM images of exfoliated Pt-MoS ₂ . As shown in Fig. 7d and 7e, Pt nanoparticles with sizes ranging from 1.5 to 3.5 nm are homogeneously dispersed on the MoS ₂ flakes. The SAED pattern (Fig. 7f, as indicated by red arrow) and EDS spectrum (Fig. 7g) confirm the successful intercalation of Pt nanoparticles into the inner layers of MoS ₂ flakes. The cross- section HAADF-STEM images, as well as the EELS mapping in Fig. 8 also demonstrate the intercalation of Pt nanoparticles in between the MoS ₂ layers.

The loading of the platinum within the composite material was determined to be 10.4 wt% by elemental analysis.

Example 2

Zero-valent intercalation of lithium intercalated MoS? with other transition metal salts

Ru-MoS ₂ , Pd-MoS ₂ and Au-MoS ₂ catalysts were obtained using the same procedure outlined in Example 1 by using RuCI ₃ xH ₂ 0 (Ru 40 ~ 49%), PdCI ₂ (99%) and HAuCI ₄ -4H ₂ 0 (Au ~ 52%), respectively. For RuCI ₃ xH ₂ 0 and PdCI ₂ , anhydrous NMP instead of THF was used as the solvent to improve the solubility of the inorganic salts and so a temperature of 80°C was used for the reaction and for the subsequent drying step instead of 60°C. The intercalation strategy was also verified to be successful for a wide range of transition metals, such as Au, Ru and Pd as verified by SEM and EDX (Figs. 4b-d and Figs. 5d-f). The loading values for the ruthenium, palladium and gold within the composite material was determined to be 1 1.3 wt%, 24.5 wt% and 18.3 wt%, respectively, by elemental analysis. Example 3

Evaluation of catalytic performance for hydrogen evolution

The catalytic performance of the composite material of Example 1 was evaluated at room temperature in a three-electrode cell with a glassy carbon electrode as the working electrode, an Ag/AgCI electrode as the reference and a Pt wire as the counter electrode. A glassy carbon (GC) electrode for working electrode (3 mm diameter) was polished using 3 pm, 1 μιτι diamond and 0.05 μητι alumina slurries, followed by rinsing with ultrapure water, ethanol, acetone and ultrapure water. Finally, the GC electrode was dried under a continuous nitrogen stream.

The catalyst ink was prepared by dispersing 2.0 mg catalyst (the composite material of Example 1 ) in 2 ml_ 4:1 ethanol/water mixture with 5% Nafion™ solution (20 μΙ_) and sonicated for at least 2 hours. A quantity of 5 μΙ_ of the mixture was pipetted onto the glassy carbon electrode surface (70 pg/cm ² loading). The working electrode was then dried at room temperature in air for a few hours.

Linear sweep voltammetry (LSV) with a scan rate of 2 mV/s was recorded in a 0.5 M H ₂ S0 ₄ electrolyte at room temperature, on a CHI 660E electrochemical workstation. A continuous 10,000 potential cycling from -0.3 to 0.3 V (vs. Ag/AgCI) was performed at a scan rate of 100 mV/s. LSV curves were recorded every 2,000 cycles and after operation. Chronoamperometry was tested at 50 mA/cm ² for the designed period (120,000 s). LSV curves were recorded after operation. The above was also used for the materials of Example 2.

The metal intercalated TMDC composites of Examples 1 and 2 were employed as hydrogen evolution catalysts. As shown in Figure 9, metal intercalated TMDCs demonstrate superior hydrogen evolution activity compared to exfoliated TMDC nanosheets. Two common indicators were adopted to further assess the catalytic performance. The Tafel slope is the increase in overpotential required to elicit a magnitude rise in current density while the onset potential is the potential at which current density begins to fall steeply due to proton reduction. It is seen that the overpotential and Tafel slope of various metal-intercalated TMDCs are ~ 20 mV and 20 mV dec ^"1 , where both are comparable or even better than the benchmark 40 wt% Pt/C catalyst. More importantly, the cycling performance as well as the long-term stability of metal intercalated TMDCs in an acidic condition are assessed. In the case of Pt-MoS ₂ , no decay in the overpotential is found at 50 mA/cm ² even after 33.3 hours operation. The performance of Ru-MoS ₂ and AU-M0S ₂ is slightly improved due to the electro-chemical activation of metal nanoparticles while Pd-MoS ₂ undergoes an "increased-decreased" process because of the partial oxidation of Pd nanoparticles. However, the overpotential of commercial Pt/C is gradually increased from 65 to 175 mV after - 18 hours, which infers a fast degradation of Pt/C during operation. Meanwhile, the negligible difference in the polarization curves before and after 10,000 cycling test also demonstrates the metal intercalated TMDCs are of superior stability under long-term operation. The inset TEM image confirms the diameter of metal nanoparticles is almost the same to those without cycling test. To our best knowledge, these metal intercalated TMDCs demonstrate one of the best hydrogen evolution performance in term of catalytic activity and long-term stability, extremely surpassing the previous results. Such outstanding performance of metal intercalated TMDCs here is ascribed to not only the strong interaction between intercalated metal nanoparticles and the host TMDCs, but also to the protection of susceptible metal nanoparticles by zero valent intercalation. In a common case, catalyst particle agglomeration/dissolution, as well as carbon support corrosion are two major degradation mechanisms for commercial Pt/C catalyst. The intercalation of metal nanoparticles in TMDC layered structure can protect them from direct exposure to the corrosive environment, thus minimizing their dissolution/aggregation during long-term operation. Meanwhile, TMDCs have an inherently excellent mechanical resistance and stability in an acidic environment. All of above contribute to the excellent cycling and long-term stability of metal intercalated TMDC catalysts.

Example 4

Fabrication of a 25 cm ² membrane-electrode assembly with Pt-MoS? at 1 mg/cm ²

A catalyst in was prepared by adding 100 mg of Pt-MoS ₂ powders to a mixture of 4 ml_ ethanol and 1 mL of a 5 wt% Nafion™ solution, with 30 mins sonication.

A piece of Freudenberg H23C6 carbon paper was cut into 5 x 5 cm ² and then mounted onto a vacuum table with its front side on top. The "catalyst ink" prepared above was deposited onto this carbon paper using the brush painting method several times until the actual loading reached 1 mg/cm ² . The as-prepared electrode was dried at 80°C for 2 hours to totally remove organic solvents. After that, it was combined with a pre-treated Nafion™-117 membrane and a commercial Pt/C electrode (1 mg/cm ² loading on carbon paper), and then hot-pressed at 120°C for 5 mins at a normalized pressure of 60 kg/cm ² . Finally it was cooled down to room temperature and cut into the desired dimension (25 cm ² ).

As the core component of hydrogen/oxygen fuel cells, membrane-electrode assemblies (MEA) with a polymer electrolyte membrane, two catalyst layers (anode/cathode), and/or two gas diffusion layers can further improve the cell performance with high catalyst loading as well as significantly reducing the sensitivity towards changes in the environment. As shown in Figure 11 , we deposit the metal intercalated TMDC catalysts onto carbon papers and/or cloths at a loading of 1 mg/cm ² with the aid of commercial Nafion™ solution, and then hot- press them together with a polymer electrolyte membrane to make a 25 cm ² MEA. Compared to conventional methods of depositing on glassy carbon electrode, MEAs can provide much higher catalyst loading up to 4 mg/cm ² to reach DOE requirement for hydrogen production, as well as fast gas bubble release with the use of microporous layer and hydrophobic treatment (PTFE coating) on carbon papers/clothes. Moreover, water management is always a key issue to achieve high energy conversion rate and efficiency in the industrial application of hydrogen/oxygen fuel cell. By incorporating with humidifier and active transport systems, MEA techniques provide a better control to avoid extremes in humidity levels at both the low end (membrane dehydration) and the high end (water flooding) that can seriously affect cell performance. As a result, we consider this MEA fabrication technique as a major advance compared to the laboratory testing technique for TMDCs-based catalysts.

It is noted that the size of the water splitting membrane achieved above is significantly larger than what has been achieved with other TMDCs, which are often limited by the size of the exfoliated flakes and with difficulties in spin-coating a continuous film and this demonstrates the industrial applicability and potential of this composite material.

Example 5

Measurement of catalytic efficiency for surface- and embedded-Pt in the composite of Example 1

Stripping experiments were performed in a three-compartment with a carbon counter and a saturated calomel electrode (SCE) reference electrode at r. t. Carbon monoxide (CO) was bubbled through the 0.5 M H ₂ S0 ₄ electrolyte with the electrode held at 0.3 V for 600 s. The solution was then purged with N ₂ for an additional 900 s before a linear voltammetric scan was initiated from 0 to 1.1 V at 10 mV ^s"1 . Cu underpotential deposition (UPD) was carried out in 0.5 M H ₂ S0 ₄ and 1 mM CuS0 ₄ . After cleaning and transfer into solution containing dissolved cupric ions, the electrode was polarized at 0.275 V for 300 s. CV pattern was then performed at 10 mV ^{s 1} from 0.275 to 0.8 V. Charges obtained from CO stripping or Cu UPD were corrected for double layer capacity by subtracting the charge obtained for the same electrode under the same condition in N2 without cupric. Electrochemically active surface area (ECSA) was calculated with an empirical value of 0.7 ML for saturated CO coverage, 152 pC cm ^"2 for CO monolayer oxidation as well as 420 pC cm ^"2 for Cu monolayer deposition. LSV curve was recorded prior to any measurement to ensure the HER activity.

One reason for the extra stability of Pt-MoS ₂ demonstrated in Example 5 arises from the passivation effect enjoyed by the Pt nanoparticles sandwiched between the MoS ₂ layers. However, it was not known if the highly anisotropic diffusion needed to access the Pt in the inner layers would result in a lower catalytic efficiency. To evaluate the electrochemical activity of these MoS ₂ -sandwiched Pt catalysts, carbon monoxide (CO) voltammetric stripping and Cu underpotential deposition (UPD) (Herrero, E. er a/. Chem. Rev. 101 , 1897- 1930 (2001 )) experiments based on the material of Example 1 were performed. In stripping voltammetry, a saturated layer of CO is pre-adsorbed onto the Pt catalyst followed by its subsequent oxidation in voltammetric scans. In UPD, Cu metal is deposited above the Nernst potential to form sub-to-monolayer Cu films on Pt surface.

The presence of the CO stripping peak or UPD peak allows the electrochemical active surface area (ECSA) of the Pt nanoparticles to be assayed (Fig. 10a and 10b). In addition, kinetic information can be obtained by observing the saturation rate of the Cu monolayer deposition when implemented in the chronoamperometric mode. As presented in Figs. 10a and 10b, the benchmark Pt/C shows comparable ECSAs in CO stripping (0.62 cm ² ) and Cu UPD (0.58 cm ² ). The CO stripping peak at 0.81 V versus RHE arises primarily from its saturated adsorption on Pt {11 1}, while the two broad Cu UPD peaks correspond to two distinct redox reactions: an intermediate copper layer co-adsorbed with sulfate ions (Cu UPD1 ), as well as the 1 ^χ 1 monolayer deposition (Cu UPD2). However, the CO stripping peak is much weaker in the case of Pt-MoS ₂ . Due to the nanosized Pt particles in the MoS _2- confined layers, only one strong Cu UPD peak is observed in Pt-MoS ₂ , which is due to sulfate-coadsorbed copper layer. The ECSA as determined by CO stripping for Pt- MoS ₂ (0.16 cm ² ) is also much lower than that of Cu UPD (0.85 cm ² ), implying that the Pt sites in Pt-MoS ₂ are not accessible to CO adsorption, although these are accessible to cupric ions. The mass transport of Cu ions into the inner active surface of Pt-MoS ₂ can be electrically driven. However, neutral CO molecules rely entirely on mass diffusion and the anisotropic diffusion path involving through the open edges of the MoS ₂ layers becomes rate-limiting (Table 2). Sample Method Scan Rate Holding ECSA (cm-2)

(mV s-1) Time (s)

10 600 0.620

40 wt% Pt/C

CO Strip

0.160

Pt-MoS ₂

10 0 0.445

8 0.487

40 wt% Pt C

15 0.500

30 0.518

Cu UPD

60 0.532

120 0.545

300 0.584

8 0.487 Sample Method Scan Rate Holding ECSA (cm-2)

(mV s-1) Time (s)

10 0 0.461

15 0.662

30 0.710

60 0.762

120 0.827

Pt-MoS2 Cu UPD

300 0.853

600 0.970

900 1.021

50 300 0.851

Table 2

Diffusion kinetics can be used to distinguish exposed Pt and trapped Pt in the electrode structure, and this was investigated by holding the Cu UPD at a fixed voltage (275 mV versus RHE). Fig. 10c shows that the current signal is rapidly saturated for Pt/C, whereas it takes a longer time to reach saturation for Pt/MoS ₂ due to the slower diffusion to reach the sandwiched Pt nanoparticles. Such an intercalated structure is advantageous for electrocatalysis, because the inner surface is accessible to charged particles like protons and metal ions, while blocking poisoning by larger sized pollutants or neutral molecules as illustrated in Fig. 10d. Slow 0 ₂ or CO diffusion is beneficial for long-term operation due to reduced corrosion or poisoning. Thus, the bulk, layered structure of Pt-MoS ₂ imparts more stability than its randomly restacked counterparts, contributing to excellent activity and stability in HER.

Example 6

Zero-valent intercalation of sodium intercalated TiS? with copper salt

672 mg Copper chloride (CuCI ₂ ) was dissolved in 20 mL anhydrous N-methyl-2-pyrrolidone (NMP) in an argon filled glove box. About 50 mg of the sodium intercalated TiS ₂ prepared in Preparation 2 was then added to the above solution. The mixture was then sealed in a PTFE-lined autoclave and kept at 80°C for 7 days. The product (Cu-TiS ₂ ) was thoroughly washed with NMP until the supernatant became colourless. The product was then further washed with isopropanol and water before being dried in a 60°C oven.

Example 7

Evaluation of catalytic performance for methanol oxidation

The catalytic performance of the composite material of Example 1 was evaluated at room temperature in a three-electrode cell with a glassy carbon electrode as the working electrode, an Ag/AgCI electrode as the reference and a Pt wire as the counter electrode. A glassy carbon (GC) electrode for working electrode (3 mm diameter) was polished using 3 μιη, 1 μιη diamond and 0.05 μηι alumina slurries, followed by rinsing with ultrapure water, ethanol, acetone and ultrapure water. Finally, the GC electrode was dried under a continuous nitrogen stream.

The catalyst ink was prepared by dispersing 2.0 mg catalyst (the composite material of Example 1 ) in 2 mL 4:1 ethanol/water mixture with 5% Nafion™ solution (20 μΙ_) and sonicated for at least 2 hours. A quantity of 21.4 μΙ_ of the mixture was pipetted onto the glassy carbon electrode surface (300 μg/cm ² loading). The working electrode was then dried at room temperature in air for a few hours.

Linear sweep voltammetry (LSV) measurements in the potential range from -0.2 to 1.0 V (vs. Ag/AgCI) at a scan rate of 10 mV/s was recorded in a 0.5 M H ₂ S0 ₄ + 1 M methanol electrolyte at room temperature, on a CHI 660E electrochemical workstation. Cyclic voltammetry (CV) measurements in the same range were also recorded at 50 mV/s. Prior to test, the electrolyte solutions were deaerated by N ₂ bubbling for 30 min and the solutions were kept under a N ₂ atmosphere during the whole test period. MOR measurements in alkaline condition were conducted in 1 M KOH + 1 M methanol electrolyte with a potential range of -1.0 to 0.2 V (vs. Ag/AgCI). The above was also used for the materials of Example 2.

The metal intercalated TMDC composites of Examples 1 and 2 were employed as methanol oxidation catalysts. As shown in Figure 12, metal intercalated TMDCs demonstrate comparable methanol oxidation activity to commercial PtRu black, which is far superior to the exfoliated TMDC nanosheets. The LSV curves exhibit the typical profile for methanol oxidation between 0 and 0.6 V vs. RHE. The strong oxidation peak is attributed to the oxidative removal of adsorbed or dehydrogenated methanol fractions, such as carbon monoxide, formaldehyde on noble metal surface. We can observe another strong peak in the reverse scan in the CV patterns (not shown) as a result of the reoxidation of carbon monoxide or other absorbed species. The MOR activity of intercalated TMDCs follows theoretical prediction for noble metals, where Pt-MoS ₂ is the most active to MOR. The MOR stability of intercalated TMDCs was also examined by a continuous 1 ,000 CV cycles. The CV pattern remains nearly intact after stability test.