WO2022029408A1

WO2022029408A1 - Hydrogen generation

Info

Publication number: WO2022029408A1
Application number: PCT/GB2021/051946
Authority: WO
Inventors: Sotiris ALEXANDROU; Joseph Cook
Original assignee: Water Lane 6 SF Limited
Priority date: 2020-08-06
Filing date: 2021-07-28
Publication date: 2022-02-10
Also published as: GB2597789A; CN116323476A; GB202012274D0

Abstract

A pellet comprises a milled mixture of silicon 21 and dispersant, wherein the milled mixture 21 comprises particles with a mean diameter of at least 1 micron.

Description

Hydrogen Generation

This invention relates generally to a silicon composition for the generation of hydrogen. More specifically, although not exclusively, this invention relates to the generation of hydrogen by the reaction of silicon with water.

For years it has been the aim of many to reduce the world’s reliance on fossil fuels. This is, at least in part, driven by the deleterious effects that the production and combustion of fossil fuels has on the environment. Hydrogen has been shown to be a clean and renewable energy carrier with a high calorific value. This allows hydrogen to find application as a viable alternative to non-renewable energy sources such as fossil fuels that additionally produce harmful waste products. However, whilst hydrogen is energy rich compared to, say, petroleum on a per weight basis, it is comparatively poor on a per volume basis. Furthermore, the portability of hydrogen gas as a fuel is problematic, requiring the transportation of significant volumes under high pressure. These issues have combined to arrest progress of the ‘hydrogen economy’ as a viable alternative (and/or replacement) for the ‘fossil fuel economy’.

To seek to address the issues concerning hydrogen, alternative methods for storing and transporting hydrogen have been proposed. One such method entails the use of an energy carrier which can react to form hydrogen. It has been proposed that silicon can provide an energy carrier and that it can be utilised to produce hydrogen gas by reaction with water.

The hydrolysis of silicon is known to produce hydrogen in the following reaction:

Si + 2H₂O 2H₂ + SiO₂ (I)

It will be appreciated from reaction (I) that solid silicon reacts with water to generate gaseous hydrogen and silica (sand). Accordingly, the co-reactant is plentiful and the reaction products of the process are the usable hydrogen gas and silica, which is a benign solid co-product.

It will also be appreciated that the reaction between a solid and a liquid is typically rate limited by the surface area of solid that is available for contact with the liquid. Accordingly, in reaction (I) it is expected that the greater the surface area of silicon the higher the rate of reaction. It is also known that silicon reacts with air to form silica. The formation of silica on the surface of silicon ‘passivates’ the silicon thereby inhibiting the progress of reaction (I).

Whilst the prior art teaches to maximise the surface area of available silicon for reaction with water, it is known that fine powders can be difficult to handle, especially when it is required to use large amounts of reactant. Typically, fine powders are difficult to handle because of the build-up of electrostatic charges and/or the generation of dust when dispensing. However in reaction (I), both the rate of production of hydrogen, and the overall yield of hydrogen, are limited by the surface area of silicon that is available to react with water.

Our earlier patent application WO2019/158941 teaches wet milling silicon and a dispersant to form a silicon composition. The particles of the silicon composition have a mean diameter between 50 nm and 500 nm. The resulting silicon composition is then fabricated into pellets which are reacted with water to generate hydrogen. Whilst it might be expected that silicon powder would show a faster rate of reaction, we surprisingly demonstrated that silicon in pellet form can provide a comparable overall rate of reaction and yield to that exhibited by silicon in a powder form (with comparable masses of silicon).

We believe (although do not intend or wish to be bound by any theory) that wet milling allows powders to be generated with smaller particle sizes which therefore increases the available surface area. The wet milled powders were formed into pellets which is advantageous for handling reasons.

However, the process of wet milling silicon involves numerous steps, one of which requires addition of a solvent. The active milling time can be long and may use substantial amounts of energy. Further, not only does the inclusion of solvent in the milling process potentially pose handing issues due to the reactivity when exposed to air, an additional step is required to remove the solvent at the end of the process. In any case residual solvent may remain in the milled substance and hence in the so-formed pellet.

It is an object of the invention to simplify the manufacturing process and/or increase the yield or reaction rate of the reactants. Accordingly, a first aspect of the invention provides a pellet comprising a milled (e.g. dry milled), mixture of silicon and dispersant, wherein the silicon comprises silicon powder with a mean diameter of at least 1 micron. Preferably the mixture is solvent free.

The terms “dry milled” and “solvent-free” mixture in this context are intended to mean that the mixture of silicon and dispersant has been prepared using a solvent-free milling technique, i.e. a milling technique in which a solvent is not used or present. An example of a solvent-free milling technique includes dry milling the silicon and dispersant in a ball mill to produce smaller particles. Another example of a solvent-free milling technique includes jet milling, wherein the silicon and dispersant is contacted with a high speed jet of compressed air or inert gas to impact particles into one another.

Advantageously, solvent-free milling, e.g. dry milling, the silicon with dispersant, e.g. potassium hydroxide, to provide the composition for the pellet, uses significantly less energy than the wet milling procedures of the prior art. By milling the silicon powder such that it has a mean diameter of at least 1 micron, significant time and cost savings can be delivered.

Advantageously, we have found that yields and rates can be at least maintained, even when the silicon particles from which the pellets are formed are significantly larger than disclosed in the prior art.

The pellet may comprise silicon powder, or silicon powder comprising particles. The particles, e.g. the particles comprising silicon, may have a particle size distribution with a d value of between 3.0 to 10.0 microns, e.g. from any one of 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5 microns to any one of 10.0, 9.5, 9.0, 8.5, 8.0, 7.5, 7.0,

6.5, 6.0, 5.5, 5.0, 4.5, 4.0, 3.5 microns. The particles, e.g. the particles comprising silicon, may have a particle size distribution with a dso value of between 10 to 50 microns, or from 10 to 45 microns, or from 10 to 40 microns, or from 10 to 35 microns, or from 10.0 to 30.0 microns, e.g. from any one of 10.0, 10.5, 11.0, 11.5, 12.0, 12.5, 13.0, 13.5, 14.0, 14.5, 15.0,

15.5, 16.0, 16.5, 17.0, 17.5, 18.0, 18.5, 19,0, 19.5, 20.0, 20.5, 21.0, 21.5, 22.0, 22.5, 23.0,

23.5, 24.0, 24.5, 25.0, 25.5, 26.0, 26.5, 27.0, 27.5, 28.0, 29.5 to any one of 30.0, 29.5, 29.0,

28.5, 28.0, 27.5, 27.0, 26.5, 26.0, 25.5, 25.0, 24.5, 24.0, 23.5, 23.0, 22.5, 22.0, 21.5, 21.0,

20.5, 20.0, 19.5, 19.0, 18.5, 18.0, 17.5, 17.0, 16.5, 16.0, 15.5, 15.0, 14.5, 14.0, 13.5, 13.0,

12.5, 12.0, 11.5, 11.0, 10.5 microns. The particles, e.g. the particles comprising silicon, may have a particle size distribution with a dso value of between 12.0 to 25.0 microns. The particles, e.g. the particles comprising or consisting of silicon, may have a particle size distribution with a dgo value of between 25 to 70 microns, e.g. from any one of 25, 30, 35, 40, 45, 50, 55, 60, or 65 microns to any one of 70, 65, 60, 55, 50, 45, 40, 35, or 30 microns. The particle size, e.g. the silicon particle size, may be measured using laser diffraction. Conveniently particle size may be measured using a Malvern Mastersizer 3000 (Model).

It has been found that the use of solvent-free milling techniques produces larger particles when compared with particles produced from wet milling techniques. It was found that one batch of particles produced using wet milling had a d of 160 nm, dso of 310 nm, and a dgo of 640 nm.

Advantageously, solvent-free milling, e.g. dry milling or jet milling, the silicon and dispersant may generate an intimate mixture of the two.

The silicon in the pellet may comprise a uniform or monodisperse powder. The silicon may be homogeneous or have a substantially homogeneous morphology.

The pellet may comprise greater than or equal to 50, 55, 60, 65 or 70 w/w% silicon, i.e. the pellet may comprise from 50 to 100, 55 to 100, 60 to 100, 65 to 100, 70 to 100 or 75 to 100 w/w% silicon. The pellet may comprise 50, 51 , 52, 53, 54, 55, 56, 57, 58, 59, 60, 61 , 62, 63, 64, 65, 66, 67, 68, 69, 70, 71 , 72, 73, 74, 75, 76, 77, 78, 79, 80, 81 , 82, 83, 84, 85, 86, 87, 88, 89, 90, 91 , 92, 93, 94, 95, 96, 97, 98, 99 or 100 w/w% silicon. In embodiments, the pellet may comprise any one of 50, 51 , 52, 53, 54, 55, 56, 57, 58, 59, 60, 61 , 62, 63, 64, 65, 66, 67, 68, 69, 70, 71 , 72, 73, 74, 75, 76, 77, 78, 79, 80 to any one of 95, 94, 93, 92, 91 , 90, 89, 88, 87, 86, 85, 84, 83, 82, 81 , 80 w/w% silicon, e.g. 50 to 95, 55 to 95, 60 to 95, 65 to 95, 70 to 95 or 75 to 95, 50 to 90, 55 to 90, 60 to 90, 65 to 90, 70 to 90 or 75 to 90, 50 to 80, 55 to 80, 60 to 80, 65 to 80, 70 to 80, 75 to 80, 50 to 85, 55 to 85, 60 to 85, 65 to 85, 70 to 85 or 75 to 85 w/w% silicon. In an embodiment, the pellet comprises 80 w/w% silicon.

The pellet may further comprise a dispersant in less than or equal to 50, 45, 40, 35, 30, 20 or 25 w/w%. In embodiments the pellet comprises dispersant in the range of between any one of 50, 49, 48, 47, 46, 45, 44, 43, 42, 41 , 40, 39, 38, 37, 36, 35, 34, 33, 32, 31 , 30, 29, 28, 27, 26, 25, 24, 23, 22, 21 , 20, 19, 18, 17, 16, 15, 14, 13, 12, 11 , 10, 9, 8, 7, 6, 5, 4, 3, or 2 w/w% to any one of 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20,

21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 , 42, 43, 44,

45, 46, 47, 48, 49 or 50 w/w%. In embodiments the pellet comprises dispersant in the range 25 to 10 w/w%. In embodiments the pellet may comprise 50 to 15, 45 to 15, 40 to 15, 35 to 15, 30 to 15 or 25 to 15 w/w% dispersant. The pellet may comprise 50, 49, 48, 47, 46, 45,

44, 43, 42, 41 , 40, 39, 38, 37, 36, 35, 34, 33, 32, 31 , 30, 29, 28, 27, 26, 25, 24, 23, 22, 21 ,

20, 19, 18, 17, 16 or 15 w/w% dispersant. In an embodiment, the pellet comprises 20 w/w% dispersant.

Advantageously, by increasing the amount of dispersant (as compared, say, to our earlier prior art), the amount of silicon can be reduced. In an embodiment, doubling the amount of dispersant allows the silicon content to be reduced.

We have surprisingly found that decreasing the amount of silicon in a pellet does not lead to a reduction in yield of hydrogen. Coupled with the act of solvent-free milling (e.g. dry milling) the silicon, and hence having larger particle sizes, this is a surprising result.

The pellet may comprise more than one dispersant. The dispersant(s) may be any water- soluble ionic compound. In embodiments, the dispersant has a heat of solution in water of less than, i.e. more negative than, -20 kJ mol’¹. The dispersant may comprise a metal hydroxide, e.g. lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, and/or caesium hydroxide. Other salts may be used. The weight of the salt may also be important. For example, a lighter salt may have a higher relative heat of solution per unit mass and so may be beneficial from an overall system weight perspective. For example KOH has a heat of solution of -57.61 kJ mol’¹ and a MW of 56 g mol’¹, providing a heat of solution of -1.03 kJ g’¹, whereas NaOH has a heat of solution of -1.11 kJ g^-1 and LiOH a heat of solution of -0.98kJ g’¹. Although the heat of solution per mole is more negative for KOH than it is for NaOH and LiOH, the lower molecular weights of NaOH and LiOH result in comparable values for the heat of solution per gram. From an overall system weight perspective, it may therefore be beneficial to use a dispersant with a less negative heat of solution per mole, if the value per gram is more negative.

The dispersant may have a solubility in water of greater than 40 g /100 mL at 20°C, for example greater than 50, 60, 70 or 80 g / 100 mL at 20°C. It should be noted that some dispersants are sold in purities of less than 100%. For example, potassium hydroxide may be sold in a purity of 85%, the remainder being water and/or potassium carbonate. This is advantageous because cheaper grades can be used. In an embodiment a dispersant is used which is 85 w/w% pure dispersant. Therefore, by less than or equal to, say, 20 w/w% of a dispersant, we mean less than or equal to 20 w/w% of the pure dispersant, i.e. if 2 g of KOH as sold (85% purity) is used as part of a 10 g pellet, then the w/w% is equal to 17 w/w% of dispersant (rather than 20 w/w%) and if silicon makes up the remainder, the silicon will be present at 80 w/w%.

The density of the pellet may be between 0.5 g/cm³ and 2.2 g/cm³, for example, between 1 .0 g/cm³ to 1 .8 g/cm³, or between 1.0 g/cm³ to 1 .6 g/cm³, for example, between 1 .2 g/cm³ to 1.4 g/cm³. For example, the silicon powder may be compressed into pellets with a density of 1.0, 1.1 , 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1 , or 2.2 g/cm³. Preferably, the density of the pellet is between 1 .0 g/cm³ to 1 .6 g/cm³. The density of the pellet may be from 20 to 95 % of the theoretical value, for example from 30, 40, 50, 60 % theoretical to 90, 80 or 70 % theoretical.

We have found that given the same pelletising criteria, the density of the pellets according to the current invention is typically slightly higher than that made according to the prior art.

The mass of the pellet may be between 0.05 g and 20, 15, 10 or 5.0 g, for example, between 0.10 g and 18, 13, 8 or 4.0 g. In embodiments the pellet may be between 0.15 g and 3.0 g, or between 0.20 g and 2.0 g, or between 0.25 g and 1.0 g. In embodiments, the mass of the pellet is between 0.10 g and 0.50 g, for example, 0.10 g, 0.20 g, 0.30 g, 0.40 g, or 0.50 g. For example, the mass of the pellet may be 0.24 g. The volume of the pellet may be between 0.02 cm³ and 5.0 cm³, for example, between 0.05 cm³ and 3.0 cm³, or between 0.10 cm³ to 1.0 cm³, for example, between 0.15 cm³ and 0.24 cm³.

The pellet may be manufactured in a process comprising compressing a volume and/or mass of a silicon composition to create a pellet, the silicon composition preferably comprising silicon in the range 75 to 85 w/w% and a dispersant in in the range 25 to 15 w/w%.

The compressed volume of the silicon composition within the resulting pellet may be between 0.02 cm³ and 20, 15, 10 or 5.0 cm³. In embodiments the pellet may have a volume for example, between 0.05 cm³ and 3.0 cm³, or between 0.10 cm³ to 1.0 cm³, for example, between 0.15 cm³ and 0.24 cm³.

The mass of the silicon composition used to manufacture the pellet in the process may be between 0.05 g and 20, 15, 10 or 5.0 g, for example, between 0.10 g and 4.0 g, or between 0.15 g and 3 g, or between 0.20 g and 2.0 g, or between 0.25 g and 1.0 g. In embodiments, the mass of the silicon composition used to manufacture the pellet in the process is between 0.10 g and 0.50 g, for example, 0.10 g, 0.20 g, 0.30 g, 0.40 g, or 0.50 g. For example, the mass of the silicon composition used to manufacture the pellet may be between 0.20 g and 0.25 g, for example, 0.21 g, 0.22 g, 0.23 g, 0.24 g or 0.25 g.

The compression of a volume and/or mass of the silicon composition may comprise compression with a compressive force of between 10 kN to 30 kN, for example, 15 kN, or 20 kN, or 25 kN.

The density of the pellet of the process may be between 0.5 g/cm³ and 2.2 g/cm³, for example, between 1.0 g/cm³ to 1.8 g/cm³, or between 1.0 g/cm³ to 1.6 g/cm³, for example, between 1 .2 g/cm³ to 1.4 g/cm³. For example, the density of the pellet of the process may be 1.0, 1.1 , 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1 , or2.2 g/cm³. Preferably, the density of the pellet is between 1 .0 g/cm³ to 1 .6 g/cm³.

The process may be performed in an inert atmosphere, for example, argon and/or nitrogen.

The pellet may be any suitable geometry or shape. For example, the pellet may be spherical, cylindrical, toroidal, ellipsoidal, cuboid, or any other three-dimensional shape. In embodiments, the pellets are cylindrical. The pellet may have a length, width, and/or height, of between of between 2 mm and 20 mm, for example, between 5 mm and 15 mm, or between 8 mm and 12 mm, for example, 10 mm. In embodiments, the pellet is a cylinder with a diameter of approximately 8 mm. In an embodiment, a pellet has a diameter of less than 30mm, for example less than 25, 20, 15, 10 mm and a height of less than 15 mm, for example less than 12.5, 10, 7.5, 5 mm. In an embodiment the pellet has a diameter of 8.0 mm and a height of 4.5 mm.

Typically, the silicon in the pellet of the present invention is non-passivated. The term ‘passivated’ is defined as the formation of a non-reactive film or layer on the surface of the material. In WO2014/053799, it is described that silicon is normally unreactive towards water due to highly efficient passivation of the silicon surface by SiC>2 upon exposure to air or moisture; the SiC>2 layer formed can have a thickness of well below 1 nm. Such passivated silicon is not capable of reacting with water to produce hydrogen and/or is only capable or reacting with water once it is etched away to reveal non-passivated silicon.

The pellet of the invention may be coated or uncoated, wherein uncoated means that the pellet does not have a protective coating to inhibit contact of the surface of the pellet with oxygen in the air. By not providing a coating (i.e. by providing an uncoated pellet) the processing time to produce a pellet is reduced and/or the cost of producing a pellet is reduced.

A further aspect of the invention provides a method of forming a pellet, the method comprising:

(i) providing silicon and a dispersant;

(ii) solvent-free milling (e.g. dry milling) said silicon and dispersant to form a solvent-free (e.g. dry) milled material; and

(iii) pelletising the solvent-free (e.g. dry) milled material.

Advantageously, solvent-free milling the silicon and dispersant eliminates the need for solvent and its associated risks. Furthermore, eliminating solvent saves costs both because of the reduction in materials needed and removal of the time/energy required for the drying step of a wet milling protocol.

Providing silicon may involve providing silicon in greater than or equal to 50, 55, 60, 65 or 70 w/w% silicon, for example 50 to 100 w/w% silicon, 50 to 90 w/w%, 50 to 85 w/w% silicon, 55 to 100 w/w% silicon, 55 to 90 w/w%, 55 to 85 w/w% silicon, 60 to 100 w/w% silicon, 60 to 90 w/w%, 60 to 85 w/w% silicon, 65 to 100 w/w% silicon, 65 to 90 w/w%, 65 to 85 w/w% silicon, 70 to 100 w/w% silicon, 70 to 90 w/w%, 70 to 85 w/w% silicon, 75 to 100 w/w% silicon, 75 to 90 w/w%, or 75 to 85 w/w% silicon, preferably 80 w/w% silicon.

Providing dispersant may comprise providing less than or equal to 50, 45, 40, 35, 30 or 25 w/w% dispersant, for example 50 to 10 w/w% dispersant, 50 to 15 w/w% dispersant, 45 to 10 w/w% dispersant, 45 to 15 w/w% dispersant, 40 to 10 w/w% dispersant, 40 to 15 w/w% dispersant, 35 to 10 w/w% dispersant, 35 to 15 w/w% dispersant, 30 to 10 w/w% dispersant, 30 to 15 w/w% dispersant, 25 to 10 w/w% dispersant, or 25 to 15 w/w% dispersant, preferably 17 w/w% (pure) dispersant.

Step (i) may comprise providing a metal hydroxide as the dispersant, for example lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide and/or caesium hydroxide.

The method may further comprise a preliminary step of breaking the silicon into smaller pieces, to provide silicon for step (ii). In an embodiment the silicon, provided in step (i), may first be provided as silicon lumps. The silicon lumps may be broken down into smaller pieces or a powder, prior to solvent-free milling, for example dry milling, e.g. by using a pestle and mortar.

The solvent-free milling step may comprise dry milling. Dry milling the silicon and dispersant may involve ball milling, i.e. contacting the silicon and dispersant with grinding balls, for example stone or metal grinding balls, e.g. zirconium oxide balls.

The solvent-free milling step may comprise jet milling, i.e. contacting the silicon and dispersant with a high speed jet of compressed air or inert gas to impact particles into one another. The inert gas may comprise or consist of nitrogen or argon, for example.

The solvent-free milling step may comprise one or more of conical milling, pin milling, air classifier milling, hammer and screen milling, and/or continuous dry bead milling.

Ball milling may comprise providing a sealed container partially filled with grinding balls. The sealed container may be a milling jar or rotating milling chamber. Contacting the silicon and dispersant with the grinding balls may involve grinding the silicon and dispersant by friction and impact with the tumbling balls.

Whilst the prior art teaches to maximise the surface area of available silicon (for reaction with water), the invention teaches having a larger surface area than the prior art, i.e. the silicon powder having a mean diameter of greater than 1 micron. Solvent-free milling may comprise reducing the mean particle size of the silicon powder such that the dso of the powder is between 10.0 and 50.0 microns, e.g. from 10 to 40 microns, or from 10 to 30 microns.

Solvent-free milling may involve contacting the silicon and dispersant with balls for less than 20 minutes, for example less than 15 minutes, for example 10 minutes.

Solvent-free milling may be performed in a number of steps. For example, contacting the silicon and dispersant with grinding balls may comprise two milling steps. Each milling step may be between 2 and 8 minutes long, e.g. 5 minutes long.

Advantageously, solvent-free milling allows the milling/contact time to be reduced, which results in reduced production costs. Furthermore, the operator time is reduced, as there are fewer steps in the process.

In an embodiment, the method may further comprise cooling the silicon and dispersant between milling steps, e.g. between the first and second milling steps.

The method may further comprise providing an inert atmosphere, for example, argon and/or nitrogen.

Pelletising the solvent-free milled material may comprise compressing a volume and/or mass of the dry milled material. Pelletising the solvent-free milled material may comprise compressing the solvent-free milled material with a compressive force of between 10 kN to 30 kN, for example, 15 kN, or 20 kN, or 25 kN.

Advantageously, formation of a pellet overcomes the problems associated with handling silicon powders, i.e. the build-up of electrostatic charges and/or the generation of dust when dispensing.

Accordingly, a yet further aspect of the invention provides a method of reacting silicon and water or a method of generating hydrogen, the method comprising:

(i) providing a silicon pellet according to any of the aforementioned embodiments; and

(ii) contacting the pellet with water. The water of Step (ii) may be provided at any temperature of between 0 °C and 100 °C at standard pressure. Preferably, the temperature of the water is at ambient temperature, i.e. the water is not heated to a temperature above that of its source. The temperature of the water may be between 0 °C to 50 °C, for example, between 0 °C to 40 °C, or between 0 °C to 35 °C. For example, the temperature of the water may be provided at 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, 39 or 40 °C.

The water of Step (ii) may be provided to the silicon within the pellet in a range of ratios of between 1 : 100 moles of silicon to water to 1 : 1 moles of silicon to water, for example, 1 :50 moles, or 1 :25 moles, or 1 : 10 moles, or 1 :2 moles of silicon to water to 1 : 1 moles of silicon to water. In embodiments, the reactants are provided in a ratio of approximately 1 :3, 1 :6 or 1 :8 moles of silicon to water.

The water of Step (ii) may comprise sea water, potable or tap water, or water from a fresh water source, e.g. a fresh water lake, river or other body of water found in the environment.

Advantageously, contacting the pellet with water produces hydrogen by hydrolysis of the solvent-free milled silicon material comprised within the pellet.

Although it may seem counterintuitive to provide a pellet with a larger surface area and/or reduced size, as both the rate of production of hydrogen, and the overall yield of hydrogen, are thought to be limited by the surface area of silicon that is available to react with water. Surprisingly, the inventors have found that yields are not impaired by using pellets with a larger surface area. In fact, reaction rates and hydrogen yields may be improved.

Moreover, the inventors have surprisingly found that the solvent-free milled silicon pellets of the invention, with a lower silicon content, have achieved similar yields to those exhibited by the wet milled silicon pellets of the prior art, with a higher silicon content.

Notwithstanding, the shorter manufacturing timescale, the solvent-free milling process of the invention reduces risks associated with solvents and is advantageous from a system energy-balance perspective. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. For the avoidance of doubt, the terms “may”, “and/or”, “e.g.”, “for example” and any similar term as used herein should be interpreted as non-limiting such that any feature so-described need not be present. Indeed, any combination of optional features is expressly envisaged without departing from the scope of the invention, whether or not these are expressly claimed. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

To further exemplify the invention, reference is made to the following non-limiting Examples with reference to the accompanying drawings in which:

Figure 1 is a scanning electron microscope image of wet milled silicon powder according to the prior art;

Figure 2A and 2B are scanning electron microscope images of dry milled silicon powder;

Figure 3 is a graph showing the pressure and temperature change over time of the hydrolysis of pellets according to Example 2;

Figure 4 is a graph showing the pressure and temperature change over time of the hydrolysis of pellets according to Comparative Example 2;

Figure 5 is a graph showing the pressure and temperature change over time of the hydrolysis of pellets according to Example 3;

Figure 6 is a graph showing the pressure and temperature change over time of the hydrolysis of pellets according to Comparative Example 3;

Figure 7 is a graph showing the pressure and temperature change over time of the hydrolysis of pellets according to Example 4; and

Figure 8 is a graph showing the pressure and temperature change over time of the hydrolysis of pellets according to Comparative Example 4. In our earlier patent application WO2019/158941 , we disclose wet milling silicon and a dispersant to form a silicon powder. In an embodiment, the composition of the silicon powder comprises 90 w/w% silicon and 8.5 w/w% (pure) potassium hydroxide (10 w/w% KOH as sold). Referring to Figure 1 , there is shown a scanning electron microscope (SEM) image 1 which illustrates the resultant powder 11 having a submicron diameter. Typically, the mean diameter of the resultant powder is in the range 50 nm to 500 nm. A portion of the silicon powder was used to fabricate pellets. On reacting the pellets with deionised water, under an inert atmosphere, hydrogen yields of 66-67% were observed (see Examples 1 and 2).

In the Examples set out below, the following milling process was utilised.

Milling Process

Silicon pieces were crushed into a coarse powder by hand with a pestle and mortar. The resulting powder was milled with dispersant.

The milling process was performed using a ball mill (Retsch PM 100) and a zirconium oxide milling jar (125 mL capacity) with zirconium oxide balls under inert (non-oxidising) conditions. A maximum available speed of 650 rpm was used in the active milling steps. Dry milling (performed using 5 mm diameter balls) was performed comprising 10 minutes of active milling in two 5 minute steps, separated by a cooling period of 5 minutes (total time 15 minutes).

Example 1

Silicon pieces (10.4 g), CAS number 7440-21-3) were crushed into a coarse powder by hand with a pestle and mortar. The resulting powder was milled using the milling process (described above) with potassium hydroxide (2.6 g, 85% purity, CAS number 1310-58-3) to produce a silicon composition in 80 w/w% and 17 w/w% potassium hydroxide. The remaining 3 w/w% comprised impurities present in the potassium hydroxide as purchased.

Referring now to Figure 2A and 2B, there is shown scanning electron microscope (SEM) images 2A, 2B of the resultant silicon material. The composition comprises silicon particles 21 with a maximum transverse dimension of at least a micron (ca 12- 14 microns, e.g. 13.3 microns, in the image of Figure 2B). A portion of the resulting silicon composition (between 0.20 g and 0.30 g) was used to fabricate a pellet by compression using a tablet press (LFA Machines TDP0 manual tablet press) with a compressive force of approximately 20 kN. The resulting pellets had average dimensions of 8 mm in diameter and 3.9 mm in height, with an average mass of 0.27 g.

Example 2

A reactor was charged with 6 g of pellets (made according to Example 1). A background of hydrogen gas at a pressure of 120 kPa (1 .2 bar) absolute was provided within the reactor. After one minute, 20 mL of deionised water (T = 22 °C) was injected into the reactor.

The results of Example 2 are shown in Figure 3. The graph 3 shows the pressure change over time 31 and the temperature change over time 32 of the reaction of 6 g of pellets with 20 mL of water. The hydrogen generation was complete within two minutes of injection. The temperature increased to a peak of 151 °C. A final pressure of 377 kPa (3.77 bar) above starting pressure was recorded. The hydrogen yield was 53% (0.74 standard litres of hydrogen per gram of the pellets).

Comparative Example 2

A reactor was charged with 6 g of pellets (made according to Examples 1 and 2 of WO2019/158941). A background of hydrogen gas at a pressure of 120 kPa (1.2 bar) absolute was provided within the reactor. After one minute, 20 mL of deionised water (T = 24 °C) was injected into the reactor.

The results of Comparative Example 2 are shown in Figure 4. The graph 4 shows the pressure change over time 41 and the temperature change over time 42 of the reaction of 6 g of pellets with 20 mL of water. The hydrogen generation was complete within two minutes of injection. The temperature increased to a peak of 208 °C. A final pressure of 540 kPa (5.40 bar) above starting pressure was recorded. The hydrogen yield was 68% (1 .06 standard litres of hydrogen per gram of the pellets).

The initial increase in reaction is much faster with the pellets of the invention, and whilst they comprise a reduced silicon content (80 w/w% instead of 90 w/w%), the amount of (pure) potassium hydroxide has been doubled (from 8.5 w/w% in Comparative Example 2 to 17 w/w% in Example 2). The increase in potassium hydroxide appears to increase the initial reaction rate in the pellets of the invention. Although there is less silicon, and the silicon has a larger average particle size (compared to the Comparative Example) we believe that the initial rate increase indicates that a greater proportion of the silicon in the pellet is available to react.

Example 3

A reactor was charged with 6 g of pellets (made according to Example 1). A background of hydrogen gas at a pressure of 120 kPa (1 .2 bar) absolute was provided within the reactor. After one minute, 40 mL of deionised water (T = 25 °C) was injected into the reactor.

The results of Example 3 are shown in Figure 5. The graph 5 shows the pressure change over time 51 and the temperature change over time 52 of the reaction of 6 g of pellets with 40 mL of water. The hydrogen generation was complete within two minutes of injection. The temperature increased to a peak of 152 °C. A final pressure of 535 kPa (5.35 bar) above starting pressure was recorded. The hydrogen yield was 74% (1.03 standard litres of hydrogen per gram of the pellets).

Comparative Example 3

A reactor was charged with 6 g of pellets (made according to Examples 1 and 2 of WO2019/158941). A background of hydrogen gas at a pressure of 120 kPa (1.2 bar) absolute was provided within the reactor. After one minute, 40 mL of deionised water (T = 23 °C) was injected into the reactor.

The results of Comparative Example 3 are shown in Figure 6. The graph 6 shows the pressure change over time 61 and the temperature change over time 62 of the reaction of 6 g of pellets with 40 mL of water. The hydrogen generation was incomplete two hours after injection. The temperature increased to a peak of 30 °C. A pressure increase of 28 kPa (0.28 bar) above starting pressure was recorded after ten minutes. The hydrogen yield was 3.9% (0.055 standard litres of hydrogen per gram of the pellets) after ten minutes. A final pressure of 84 kPa (0.84 bar) above starting pressure was recorded after two hours. The hydrogen yield was 12% (0.16 standard litres of hydrogen per gram of the pellets) after two hours. Increasing the volume of water from 20 mL (Example 2) to 40 mL (Example 3) with the dry milled material resulted in an increase in hydrogen yield of 21 %. In contrast, increasing the volume of water from 20 mL (Comparative Example 2) to 40 mL (Comparative Example 3) with the wet milled material resulted in a decrease in hydrogen yield of 56% (after two hours in the case of Comparative Example 3).

It will be appreciated that the reaction between a solid and a liquid is rate limited by the surface area of solid that is available for contact with the liquid. Accordingly, when reacting silicon and water, it would be expected that the greater the surface area of silicon, the higher the rate of reaction.

Surprisingly, the larger silicon particles produced in the dry milling process are more efficient at producing hydrogen than the submicron particles of the prior art, produced in a wet milling process (e.g. a comparison of the results of Example 3 and Comparative Example 3) when there is an excess of water.

Whilst the wet milled silicon material produces a good hydrogen yield (68%) at relatively low water ratios (20 mL, Comparative Example 2), increasing the volume of water results in a temperature that is too low for favourable reaction, as the same amount of heat is released into a larger mass with a larger heat capacity. As such, the wet milled silicon material leads to a slower reaction on increasing the volume of water.

Whilst the dry milled silicon material generally requires more water to achieve the same percentage hydrogen yield as the wet milled silicon material, increasing the amount of catalyst (from 8.5 w/w% wet milling process to 17 w/w% dry milling process) means more heat is available in the early stages of the reaction. As a result, the reaction rate of the dry milled material is increased (in comparison to the wet milled material), the larger volume of water does not quench the reaction as it does with the wet milled material.

Example 4

A reactor was charged with 50 g of pellets (made according to Example 1) in an airtight cartridge with a breakable seal, with 417 mL of deionised water (T = 22°C) on top of the can. A background of hydrogen gas at a pressure of 120 kPa (1.2 bar) absolute was provided within the reactor. After one minute, the cartridge seal was broken, allowing the water to fill the can and contact the pellets.

The results of Example 4 are shown in Figure 7. The graph 7 shows the pressure change over time 71 and the temperature change over time 72 of the reaction of 50 g of pellets with 417 mL of water. The hydrogen generation was complete within two minutes of injection. The temperature between the can and vessel walls increased to a peak of 55 °C. A final pressure of 4310 kPa (43.1 bar) above starting pressure was recorded. The hydrogen yield was 63% (0.87 standard litres of hydrogen per gram of the pellets).

The temperature was measured by inserting a temperature probe outside of the reaction mixture, in a space (filled with water) between the pressure vessel and the wall of the cartridge (inside the vessel) containing the reaction mixture.

Comparative Example 4

A different reactor was charged with 40 g of pellets (made according to Examples 1 and 2 of WO2019/158941). A background of hydrogen gas at a pressure of 120 kPa (1.2 bar) absolute was provided within the reactor. After one minute, 200 mL of deionised water (T = 25 °C) was injected into the reactor.

The results of Comparative Example 4 are shown in Figure 8. The graph 8 shows the pressure change over time 81 and the temperature change over time 82 of the reaction of 40 g of pellets with 200 mL of water. The hydrogen generation was complete within three minutes of injection. The temperature inside the reaction mixture increased to a peak of 196 °C. A final pressure of 2530 kPa (25.3 bar) above starting pressure was recorded. The hydrogen yield was 64% (1 .00 standard litres of hydrogen per gram of the pellets).

The temperature was measured by inserting a temperature probe into the reaction mixture. The temperature probe enabled a higher temperature to be picked up than that of the dry milled material (Example 4), where the temperature probe was located outside of the reaction mixture. As the scale of the reaction was increased from 6 g (Comparative Examples 2 and 3) to 40 g (Comparative Example 4) of pellets (formed from wet milled material), it was found that the arrangement of the pellets was important to ensure good mixing of the reactants. In order to obtain a favourable yield, it was necessary to evenly distribute the pellets in the space available, for example by arranging the pellets in layers.

In contrast, on increasing the scale of the reaction from 6 g (Examples 2 and 3) to 50 g (Example 4) of pellets (formed from dry milled material), the reaction appears to be less sensitive to the arrangement of pellets. The result can be attributed to the increased amount of dispersant in the dry milling process (17% potassium hydroxide in comparison to 8.5% potassium hydroxide). The increased amount of dispersant ensures more heat is available in the early stages of the reaction, the energy of which helps to mix up the reactants.

A summary of the reaction of dry milled silicon powder of the invention and wet milled silicon of the prior art is shown in T able 1.

Table 1. A comparison of reacting dry and wet milled silicon with water.

Where: “Dry” means dry milled, “wet” means wet milled, AP is the pressure increase.

*After two hours.

^ANote that reaction vessels with different free volumes were used here.

Whilst the dry milled material typically requires a larger volume of water to achieve the same percentage yield as the wet milled material, the wet milled material, when reacted with water, is much more sensitive to pellet arrangement on a larger scale.

Further, the dry milling method uses significantly less energy at the lab scale than the wet milling protocol of our earlier patent application (WO2019/158941). The specific energy at 70% yield for the dry milled material is 1 .33 kWh/kg, compared to 1.50 kWh/kg for the wet milled material. Although the specific energy is lower for the dry milled material in comparison to the wet milled material, the manufacture and use of the dry milled material is overall more energy efficient than the wet milled material. Additionally, the active milling time is shorter which results in reduced production costs. Furthermore, the operator time is reduced, as there are fewer steps in the process. We would expect that the savings are magnified on scale-up.

Advantageously, the dry milling process leads to a safer material. The risk of fire when exposed to air is significantly reduced as the dry milled material is less reactive with air.

Advantageously, the elimination of solvent in the dry milling method, saves costs both because of the reduction in materials needed and removal of the time/energy required for the drying step.

It will be appreciated by those skilled in the art that several variations to the aforementioned embodiments are envisaged without departing from the scope of the invention.

It will also be appreciated by those skilled in the art that any number of combinations of the aforementioned features and/or those shown in the appended drawings provide clear advantages over the prior art and are therefore within the scope of the invention described herein.

Claims

1. A pellet comprising a milled mixture of silicon and dispersant, wherein the milled mixture comprises particles with a mean diameter of at least 1 micron.

2. A pellet according to Claim 1 , wherein the pellet comprises silicon powder with a medium or dso value of between 10 to 50 microns, say from 10 to 40 microns, or from 10 to 30 microns, say from 12.0 to 25.0 microns,

3. A pellet according to any preceding Claim, wherein the pellet comprises greater than or equal to 70 w/w% silicon, for example 75 to 100 w/w% silicon, or 75 to 90 w/w%.

4. A pellet according to Claim 3 wherein the pellet comprises 75 to 85 w/w% silicon, preferably 80 w/w% silicon.

5. A pellet according to any preceding Claim, wherein the pellet comprises a dispersant having a heat of solution of less than -20 KJ mol’¹.

6. A pellet according to any preceding Claim, wherein the solubility of the dispersant is greater than 40 g 1 100 mL in water (20 °C).

7. A pellet according to any preceding Claim, wherein the pellet comprises less than or equal to 25 w/w% dispersant, for example 25 to 10 w/w% dispersant.

8. A pellet according to Claim 7, wherein the pellet comprises 25 to 15 w/w% dispersant, preferably 20 w/w% dispersant.

9. A pellet according to any preceding Claim, wherein the dispersant comprises a metal hydroxide, e.g. lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide and/or caesium hydroxide.

10. A pellet according to any preceding Claim, wherein the pellet is in the form of a cylinder, toroid, ellipsoid, sphere, prism or cuboid.

11. A pellet according to any preceding Claim, wherein the silicon is non-passivated.

12. A method of forming a pellet, the method comprising: i. providing silicon and a dispersant; ii. solvent-free (e.g. dry) milling said silicon and dispersant to form a solvent- free (e.g. dry) milled material; and iii. pelletising the solvent-free (e.g. dry) milled material.

13. A method according to Claim 12, including a preliminary step of breaking the silicon into smaller pieces, to provide silicon for step (ii).

14. A method according to Claim 12 or 13, wherein step (i) comprises providing greater than or equal to 70 w/w% silicon, for example 75 to 100 w/w% silicon, or 75 to 90 w/w%, or 75 to 85 w/w% silicon, preferably 80 w/w% silicon.

15. A method according to any of Claims 12 to 14, wherein step (i) comprises providing less than or equal to 25 w/w% dispersant, for example 25 to 10 w/w% dispersant, or 25 to 15 w/w% dispersant, preferably 17 w/w% dispersant.

16. A method according to any of Claims 12 to 15, wherein step (i) comprises providing a metal hydroxide as the dispersant, for example lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide and/or caesium hydroxide.

17. A method according to any of Claims 12 to 16, wherein step (ii) comprises ball milling, i.e. contacting the silicon and dispersant with grinding balls, for example stone or metal grinding balls, e.g. zirconium oxide balls.

18. A method according to Claim 17, comprising contacting the silicon and dispersant with grinding balls for less than 20 minutes, for examples less than 15 minutes, for example 10 minutes.

19. A method according to Claim 17 or 18, comprising contacting the silicon and dispersant with grinding balls according to two milling steps, wherein each step may last between 2 and 8 minutes, e.g. 5 minutes. A method according to Claim 19, comprising cooling the silicon and dispersant between the first and second milling steps. A method according to any of Claims 12 to 20, further comprising providing an inert atmosphere. A method according to any of Claims 12 to 21 , wherein step (ii) comprises jet milling, i.e. wherein the silicon and dispersant is contacted with a high speed jet of compressed air or inert gas to impact particles into one another. . A method according to any of Claims 12 to 21 , wherein step (ii) comprises one or more of conical milling, pin milling, air classifier milling, hammer and screen milling, and/or continuous dry bead milling.