CN116323476A

CN116323476A - Hydrogen generation

Info

Publication number: CN116323476A
Application number: CN202180068322.8A
Authority: CN
Inventors: S·亚历桑德罗; J·库克
Original assignee: Waterway 6 Ltd
Current assignee: Waterway 6 Ltd
Priority date: 2020-08-06
Filing date: 2021-07-28
Publication date: 2023-06-23
Also published as: GB202012274D0; GB2597789A; WO2022029408A1

Abstract

A pellet comprising a mixture of ground silicon 21 and a dispersant, wherein the ground mixture 21 comprises particles having an average diameter of at least 1 micron.

Description

Hydrogen generation

The present invention relates generally to silicon compositions for the production of hydrogen. More particularly, although not exclusively, the invention relates to the production of hydrogen by the reaction of silicon with water.

For many years, reducing the world's dependence on fossil fuels has been the goal of many people. This is driven, at least in part, by the detrimental effects of fossil fuel manufacture and combustion on the environment. Hydrogen has proven to be a clean and renewable energy carrier with a high heating value. This allows hydrogen to be a viable alternative to non-renewable energy sources such as fossil fuels that otherwise produce hazardous waste. However, while hydrogen is energy rich per unit weight compared to, for example, petroleum, it is relatively lean per unit volume. Furthermore, the portability of hydrogen as a fuel is problematic, requiring significant amounts of transportation at high pressures. These problems, in combination, have hindered the development of "hydrogen economy" as a viable alternative (and/or surrogate) to "fossil fuel economy".

In an effort to address the problems associated with hydrogen, alternative methods for storing and transporting hydrogen have been proposed. One such method requires the use of an energy carrier that reacts to form hydrogen. It has been proposed that silicon can provide an energy carrier and that it can be used to produce hydrogen by reaction with water.

Hydrolysis of silicon is known to produce hydrogen according to the following reaction:

Si+2H ₂ O→2H ₂ +SiO ₂ (I)

it is understood from reaction (I) that solid silicon reacts with water to produce gaseous hydrogen and silica (sand). Thus, the co-reactants are abundant and the reaction products of the process are available hydrogen and silica, which are mild solid co-products.

It should also be appreciated that the reaction between a solid and a liquid is generally limited by the rate at which the surface area of the solid is available for contact with the liquid. Thus, in reaction (I), the larger the surface area of silicon is expected, the higher the reaction rate. It is also known that silicon reacts with air to form silicon dioxide. The formation of silicon dioxide on the silicon surface "passivates" the silicon, thereby inhibiting the progress of reaction (I).

While the prior art teaches maximizing the surface area of silicon available for reaction with water, it is known that fine powders can be difficult to handle, especially when large amounts of reactants are required. Generally, fine powders are difficult to handle due to the accumulation of electrostatic charges and/or the generation of dust during dispersion. However, in reaction (I), both the rate of hydrogen production and the overall yield of hydrogen are limited by the surface area of silicon that can react with water.

Our earlier patent application WO2019/158941 teaches wet milling of silicon and a dispersant to form a silicon composition. The particles of the silicon composition have an average diameter of 50nm to 500 nm. The resulting silicon composition is then formed into pellets, which react with water to produce hydrogen. While it is expected that silicon powder will exhibit a faster reaction rate, we surprisingly demonstrate that silicon in pellet form can provide comparable overall reaction rates and yields to that exhibited by silicon in powder form (with comparable silicon mass).

It is believed (although not intended to be bound by or to be bound by any theory) that wet milling allows for the production of powders having smaller particle sizes, which thus increases the available surface area. Wet milling of the powder into pellets is advantageous for handling reasons.

However, the wet grinding process of silicon involves a number of steps, one of which requires the addition of a solvent. The effective grinding time can be long and a large amount of energy can be used. Furthermore, not only does the inclusion of solvent during milling potentially cause handling problems due to reactivity upon exposure to air, but an additional step is required to remove the solvent at the end of the process. In any case, residual solvent may remain in the ground material and thus in the pellets so formed.

The aim of the invention is to simplify the production process and/or to increase the yield or reaction rate of the reactants.

Accordingly, in a first aspect the invention provides a granulate comprising a mixture of ground (e.g. dry ground) silicon and a dispersant, wherein the silicon comprises a silicon powder having an average diameter of at least 1 micron. Preferably, the mixture is free of solvent.

The terms "dry milled" and "solvent-free" mixtures are intended herein to mean that the solvent-free milling technique has been used, i.e., milling techniques in which no solvent is used or is present, to prepare a mixture of silicon and dispersant. Examples of solvent-free milling techniques include dry milling silicon and dispersant in a ball mill to produce smaller particles. Another example of a solvent-free milling technique includes jet milling, in which silicon and a dispersant are contacted with a high-velocity jet of compressed air or an inert gas to cause the particles to impinge upon one another.

Advantageously, solvent-free milling, such as dry milling of silicon with a dispersant, such as potassium hydroxide, to provide a composition for the pellets uses significantly less energy than prior art wet milling procedures. Significant time and cost savings can be achieved by grinding the silicon powder so that it has an average diameter of at least 1 micron.

Advantageously, we have found that even when the silicon particles forming the pellets are significantly larger than those disclosed in the prior art, at least the yield and rate can be maintained.

The pellets may comprise silicon powder or silicon powder comprising particles. Particles, such as silicon-containing particles, may have d ₁₀ A value of 3.0 to 10.0 microns, such as a particle size distribution from any 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 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. Particles, such as silicon-containing particles, may have d ₅₀ A value of 10 to 50 microns, or 10 to 45 microns, or 10 to 40 microns, or 10 to 35 microns, or 10.0 to 30.0 microns, such as any of particle sizes from 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, 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, 21.5, 20.0, 19.5, 19.0, 16.0, 14.5, 16.5, 13.0, 13.5, 16.5, 13.5, and 10.5 microns. Particles, such as silicon-containing particles, may have d ₅₀ Particle size distribution with a value of 12.0 to 25.0 microns. Particles, e.g. comprising or consisting of silicon, may have d ₉₀ A particle size distribution having a value of from 25 to 70 microns, such as from any of 25, 30, 35, 40, 45, 50, 55, 60 or 65 microns to any of 70, 65, 60, 55, 50, 45, 40, 35 or 30 microns. Particle size, such as silicon particle size, may be measured using laser diffraction. Conveniently, particle size can be measured using Malvern Mastersizer 3000 (model).

It has been found that the use of solvent-free milling techniques produces larger particles when compared to particles produced by wet milling techniquesAnd (3) particles. It was found that a batch of particles produced using wet milling had a d of 160nm ₁₀ D at 310nm ₅₀ And d of 640nm ₉₀ 。

Advantageously, solvent-free milling, such as dry milling or jet milling, silicon and dispersant, can produce an intimate mixture of the two.

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

The pellets may comprise greater than or equal to 50, 55, 60, 65, or 70 wt/wt% silicon, i.e., the pellets may comprise 50 to 100, 55 to 100, 60 to 100, 65 to 100, 70 to 100, or 75 to 100 wt/wt% 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 weight/weight percent silicon. In embodiments, the pellet may comprise any 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 wt/wt% silicon to any of 95, 94, 93, 92, 91, 90, 89, 88, 87, 86, 85, 84, 83, 82, 81, 80 wt/wt% silicon, such as 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 wt/wt% silicon. In one embodiment, the pellet comprises 80 wt/wt% silicon.

The pellets may further comprise less than or equal to 50, 45, 40, 35, 30, 20, or 25 weight/weight% of a dispersant. In embodiments, the pellet comprises a dispersant in the range of any 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 wt/wt% to any 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 wt/wt%. In embodiments, the pellets comprise a dispersant in the range of 25 to 10 wt/wt%. In embodiments, the pellets may comprise 50 to 15, 45 to 15, 40 to 15, 35 to 15, 30 to 15, or 25 to 15 weight/weight% of the 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 wt/wt% dispersant. In one embodiment, the pellet comprises 20 wt/wt% dispersant.

Advantageously, by increasing the amount of dispersant (as compared to our earlier prior art), the amount of silicon can be reduced. In one embodiment, doubling the amount of dispersant allows for a reduction in the silicon content.

We have surprisingly found that reducing the amount of silicon in the pellets does not result in a reduction in hydrogen yield. This is a surprising result in combination with the action of solvent-free milling (e.g., dry milling) of silicon, and thus with a larger particle size.

The pellets may contain 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-20 kJ mol ^-1 I.e. more negative than-20 kJ mol ^-1 . The dispersant may comprise a metal hydroxide such as lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide and/or cesium hydroxide. Other salts may be used. The weight of the salt may also be important. For example, lighter salts may have a higher relative heat of solution per unit mass and thus may be beneficial from an overall system weight perspective. For example, KOH has a heat of solution of-57.61 kJ mol ^-1 MW 56g mol ^-1 Provides a heat of solution of-1.03 kJ g ^-1 While the heat of solution of NaOH was-1.11 kJ g ^-1 And LiOHHeat of solution of-0.98 kJ g ^-1 . Although the heat of solution is more negative per mole of KOH than for NaOH and LiOH, the lower molecular weights of NaOH and LiOH result in comparable values per gram of heat of solution. From the point of view of the weight of the whole system, it may therefore be advantageous to use dispersants with a less negative heat of solution per mole if the value per gram is more negative.

The solubility of the dispersant in water may be greater than 40g/100mL at 20 ℃, for example greater than 50, 60, 70 or 80g/100mL at 20 ℃.

It should be noted that some dispersants are sold in less than 100% purity. For example, potassium hydroxide may be sold in 85% purity, with the remainder being water and/or potassium carbonate. This is advantageous because cheaper grades can be used. In one embodiment, a dispersant of 85 wt/wt% pure dispersant is used. Thus, by less than or equal to, say, 20 wt/wt% dispersant we mean less than or equal to 20 wt/wt% pure dispersant, i.e., if 2g of KOH sold (85% purity) is used as part of a 10g pellet, then wt/wt% is equal to 17 wt/wt% dispersant (instead of 20 wt/wt% dispersant), and if silicon constitutes the remainder, then silicon will be present at 80 wt/wt%.

The density of the pellets may be 0.5g/cm ³ To 2.2g/cm ³ For example, 1.0g/cm ³ To 1.8g/cm ³ Or 1.0g/cm ³ To 1.6g/cm ³ For example 1.2g/cm ³ To 1.4g/cm ³ . For example, the silicon powder may be compressed to 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 2.2g/cm ³ Is a pellet of (a). Preferably, the pellet has a density of 1.0g/cm ³ To 1.6g/cm ³ . The density of the pellets may be 20 to 95% of theoretical, for example 30, 40, 50, 60% of theoretical to 90, 80 or 70% of theoretical.

We have found that given the same pelletisation criteria, the density of the pellets according to the invention is generally slightly higher than that of pellets prepared according to the prior art.

The mass of the pellets may be 0.05g to 20, 15, 10 or5.0g, for example 0.10g to 18, 13, 8 or 4.0g. In embodiments, the pellet may be 0.15g to 3.0g, or 0.20g to 2.0g, or 0.25g to 1.0g. In embodiments, the pellet has a mass of 0.10g to 0.50g, such as 0.10g, 0.20g, 0.30g, 0.40g, or 0.50g. For example, the mass of the pellets may be 0.24g. The volume of the pellets may be 0.02cm ³ To 5.0cm ³ For example 0.05cm ³ To 3.0cm ³ Or 0.10cm ³ To 1.0cm ³ For example, 0.15cm ³ To 0.24cm ³ 。

The pellets may be made by a process that includes compressing a volume and/or mass of a silicon composition, preferably comprising silicon in the range of 75 to 85 weight/weight% and dispersant in the range of 25 to 15 weight/weight%, to produce the pellets.

The compressed volume of the silicon composition in the resulting pellets may be 0.02cm ³ To 20, 15, 10 or 5.0cm ³ . In embodiments, the volume of the pellet may be, for example, 0.05cm ³ To 3.0cm ³ Or 0.10cm ³ To 1.0cm ³ For example 0.15cm ³ To 0.24cm ³ 。

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

Compression of a volume and/or mass of a silicon composition may include compression with a compression force of 10kN to 30kN, such as 15kN or 20kN or 25 kN.

The pellets of the process may have a density of 0.5g/cm ³ To 2.2g/cm ³ For example 1.0g/cm ³ To 1.8g/cm ³ Or 1.0g/cm ³ To 1.6g/cm ³ For example 1.2g/cm ³ To 1.4g/cm ³ . For example, the pellets of the process may have 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 2.2g/cm ³ . Preferably, the pellet has a density of 1.0g/cm ³ To 1.6g/cm ³ 。

The process may be carried out under an inert atmosphere, for example argon and/or nitrogen.

The pellets may be of any suitable geometry or shape. For example, the pellets may be spherical, cylindrical, toroidal, oval, rectangular, or any other three-dimensional shape. In embodiments, the pellet is cylindrical. The length, width and/or height of the pellets may be from 2mm to 20mm, for example from 5mm to 15mm, or from 8mm to 12mm, for example 10mm. In embodiments, the pellet is a cylinder having a diameter of about 8 mm. In one embodiment, the pellets have a diameter of less than 30mm, such as less than 25, 20, 15, 10mm, and a height of less than 15mm, such as less than 12.5, 10, 7.5, 5mm. In one embodiment, the pellet has a diameter of 8.0mm and a height of 4.5mm.

In general, the silicon in the pellets of the present invention is non-passivated. The term "passivation" is defined as the formation of a non-reactive film or layer on the surface of a material. In WO2014/053799 it is described that silicon is generally not reactive to water, due to SiO when exposed to air or moisture ₂ High-efficiency passivation of the silicon surface; siO formed ₂ The thickness of the layer may be well below 1nm. Such passivated silicon cannot react with water to produce hydrogen and/or can only react with water once it has been etched away to expose the unpassivated silicon.

The pellets of the present invention may be coated or uncoated, where uncoated means that the pellets do not have a protective coating to inhibit the surface of the pellets from contacting oxygen in the air. By not providing a coating (i.e., by providing uncoated pellets), processing time for manufacturing the pellets is reduced and/or costs for manufacturing the pellets are reduced.

Another 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) the silicon and dispersant to form a solvent-free (e.g., dry) milled material; and

(iii) The solvent-free (e.g., dry) milled material is granulated.

Advantageously, solvent-free milling of silicon and dispersant eliminates the need for solvents and their associated risks. Furthermore, eliminating solvents saves costs due to both the reduction of required materials and the removal of time/energy required for the drying step of the wet milling scheme.

Providing silicon may include providing greater than or equal to 50, 55, 60, 65, or 70 wt/wt% silicon, such as 50 to 100 wt/wt% silicon, 50 to 90 wt/wt% silicon, 50 to 85 wt/wt% silicon, 55 to 100 wt/wt% silicon, 55 to 90 wt/wt% silicon, 55 to 85 wt/wt% silicon, 60 to 100 wt/wt% silicon, 60 to 90 wt/wt% silicon, 60 to 85 wt/wt% silicon, 65 to 100 wt/wt% silicon, 65 to 90 wt/wt% silicon, 65 to 85 wt/wt% silicon, 70 to 100 wt/wt% silicon, 70 to 90 wt/wt% silicon, 70 to 85 wt/wt% silicon, 75 to 100 wt/wt% silicon, 75 to 90 wt/wt% silicon, or 75 to 85 wt/wt% silicon, preferably 80 wt/wt% silicon.

Providing a dispersant may include providing less than or equal to 50, 45, 40, 35, 30, or 25 wt/wt% of a dispersant, such as 50 to 10 wt/wt% of a dispersant, 50 to 15 wt/wt% of a dispersant, 45 to 10 wt/wt% of a dispersant, 45 to 15 wt/wt% of a dispersant, 40 to 10 wt/wt% of a dispersant, 40 to 15 wt/wt% of a dispersant, 35 to 10 wt/wt% of a dispersant, 35 to 15 wt/wt% of a dispersant, 30 to 10 wt/wt% of a dispersant, 30 to 15 wt/wt% of a dispersant, 25 to 10 wt/wt% of a dispersant, or 25 to 15 wt/wt% of a dispersant, preferably 17 wt/wt% (pure) of a dispersant.

Step (i) may include providing a metal hydroxide, such as lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, and/or cesium hydroxide, as a dispersant.

The method may further comprise a preliminary step of breaking the silicon into smaller pieces to provide silicon for step (ii). In one embodiment, the silicon provided in step (i) may be provided first in the form of a silicon block. The silicon chunks may be broken into smaller pieces or powders prior to solvent-free grinding, such as by dry grinding using a pestle and mortar, for example.

The solvent-free milling step may include dry milling. Dry milling the silicon and dispersant may include ball milling, i.e., contacting the silicon and dispersant with milling balls, such as stone or metal milling balls, such as zirconia balls.

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

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

Ball milling may include providing a sealed container partially filled with grinding balls. The sealed container may be a milling jar or a rotary milling chamber. Contacting the silicon and the dispersant with the grinding balls may include grinding the silicon and the dispersant by friction and impact with the rolling balls.

While the prior art teaches maximizing the surface area of available silicon (for reaction with water), the present invention teaches that it has a larger surface area than the prior art, i.e., silicon powder has an average diameter of greater than 1 micron.

Solvent-free milling may include reducing the average particle size of the silicon powder such that d of the powder ₅₀ From 10.0 to 50.0 microns, such as from 10 to 40 microns, or from 10 to 30 microns.

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

Solvent-free milling can be performed in multiple steps. For example, contacting the silicon and dispersant with the grinding balls may include two grinding steps. Each milling step may be 2 to 8 minutes long, such as 5 minutes long.

Advantageously, solvent-free milling allows for reduced milling/contact time, which results in reduced manufacturing costs. Furthermore, since there are fewer steps in the process, the operating time is reduced.

In one embodiment, the method may further comprise cooling the silicon and the dispersant between the milling steps, such as between the first milling step and the second milling step.

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

Granulating the solvent-free abrasive material may include compressing a volume and/or mass of dry abrasive material. Granulating the solvent-free abrasive material may include compressing the solvent-free abrasive material with a compression force of 10kN to 30kN, such as 15kN, or 20kN, or 25 kN.

Advantageously, the formation of granules overcomes the problems associated with handling silicon powder, namely the accumulation of static charges and/or the generation of dust upon dispersion.

Accordingly, a further aspect of the present invention provides a process for reacting silicon with water or a process for generating hydrogen, the process comprising:

(i) Providing a silicon pellet according to any one of the preceding embodiments; and

(ii) The pellets are contacted with water.

The water of step (ii) may be provided at any temperature from 0 ℃ to 100 ℃ at standard pressure. Preferably, the temperature of the water is ambient, i.e. the water is not heated above the temperature from which it originated. The temperature of the water may be between 0 ℃ and 50 ℃, for example between 0 ℃ and 40 ℃, or between 0 ℃ and 35 ℃. For example, the temperature of the water may be provided as 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 ℃.

The water of step (ii) may be provided to the silicon in the pellets in a ratio range of from 1:100 molar silicon to 1:1 molar silicon to water, for example 1:50 molar, or 1:25 molar, or 1:10 molar, or 1:2 molar silicon to 1:1 molar silicon to water ratio. In embodiments, the reactants are provided in a ratio of about 1:3, 1:6, or 1:8 molar silicon to water.

The water of step (ii) may comprise seawater, potable or tap water, or water from a fresh water source such as a freshwater lake, river or other body of water found in the environment.

Advantageously, contacting the pellets with water produces hydrogen by hydrolysis of the solvent-free ground silicon material contained within the pellets.

Although providing pellets with larger surface areas and/or reduced sizes seems counterintuitive, because both the rate of hydrogen production and the overall yield of hydrogen are considered limited by the surface area of silicon available for reaction with water. Surprisingly, the inventors have found that the use of pellets with a larger surface area does not impair the yield. In fact, the reaction rate and hydrogen yield can be improved.

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

The solvent-free milling method of the present invention reduces the risks associated with solvents despite the shorter manufacturing time scale and is advantageous from a system energy balance point of view.

Within the scope of the present 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 carried out independently or in any combination. That is, features of all embodiments and/or any embodiments may be combined in any manner and/or combination unless such features are incompatible. For the avoidance of doubt, the terms "may", "and/or", "such as", "for example" and any like terms used herein should be interpreted as non-limiting such that no such described feature is required to be present. Indeed, any combination of the optional features, whether or not explicitly claimed, may be clearly contemplated without departing from the scope of the present invention. The applicant reserves the right to alter any originally presented claim or to present any new claim accordingly, including modifying any originally presented claim to depend upon and/or incorporate any feature of any other claim, although not initially claimed in this manner.

For further illustration of the invention, reference is made to the following non-limiting examples and to the accompanying drawings in which:

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

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

FIG. 3 is a graph showing the pressure and temperature of hydrolysis of pellets according to example 2 over time;

FIG. 4 is a graph showing the pressure and temperature of hydrolysis of pellets according to comparative example 2 over time;

FIG. 5 is a graph showing the pressure and temperature of hydrolysis of pellets according to example 3 over time;

FIG. 6 is a graph showing the pressure and temperature of hydrolysis of pellets according to comparative example 3 over time;

FIG. 7 is a graph showing the pressure and temperature of hydrolysis of pellets according to example 4 over time; and

fig. 8 is a graph showing the pressure and temperature of hydrolysis of pellets according to comparative example 4 as a function of time.

In our earlier patent application WO2019/158941 we disclose wet milling of silicon and a dispersant to form a silicon powder. In one embodiment, the composition of the silicon powder comprises 90 wt/wt% silicon and 8.5 wt/wt% (pure) potassium hydroxide (sold as 10 wt/wt% KOH). Referring to fig. 1, a Scanning Electron Microscope (SEM) image 1 is shown illustrating the resulting powder 11 having submicron diameters. Generally, the average diameter of the resulting powder is in the range of 50nm to 500 nm. A portion of the silicon powder is used to make the pellets. Under an inert atmosphere, a hydrogen yield of 66-67% was observed when the pellets were reacted with deionized water (see examples 1 and 2).

In the examples set forth below, the following grinding methods were used.

Grinding method

The wafer was crushed by hand with a pestle and mortar to a coarse powder. The resulting powder was milled with a dispersant.

The milling process was carried out under inert (non-oxidizing) conditions using a ball mill (Retsch PM 100) and a zirconia milling tank (capacity 125 mL) with zirconia balls. A maximum usable speed of 650rpm was used in the effective grinding step. Dry grinding (using balls of 5mm diameter) was performed including effective grinding for 10 minutes (total time 15 minutes) in two 5-minute steps with a cooling time interval of 5 minutes.

Example 1

Silicon wafer (10.4 g, CAS number 7440-21-3) was crushed by hand with a pestle and mortar to a coarse powder. The resulting powder was milled with potassium hydroxide (2.6 g,85% purity, CAS number 1310-58-3) using the milling method (described above) to produce a silicon composition containing 80 wt/wt% silicon and 17 wt/wt% potassium hydroxide. The remaining 3 wt./wt.% contains impurities present in the potassium hydroxide at the time of purchase.

Referring now to fig. 2A and 2B, scanning Electron Microscope (SEM)

images

2A, 2B of the resulting silicon material are shown. The composition comprises silicon particles 21 having a maximum lateral dimension of at least one micron (about 12-14 microns, such as 13.3 microns in the image of fig. 2B).

A portion of the resulting silicon composition (0.20 g to 0.30 g) was used to make pellets by compression using a tablet press (LFA Machines TDP0 manual tablet press) at a pressure of about 20 kN. The average size of the pellets obtained was 8mm in diameter and 3.9mm in height, and the average mass was 0.27g.

Example 2

The reactor was charged with 6g of pellets (prepared according to example 1). A hydrogen background with an absolute pressure of 120kPa (1.2 bar) was provided in the reactor. After one minute, 20mL deionized water (t=22℃) was injected into the reactor.

The results of example 2 are shown in fig. 3. Graph 3 shows the pressure 31 and temperature 32 over time for a 6g pellet reaction with 20mL water. Hydrogen production was completed within two minutes of injection. The temperature rose to a peak of 151 ℃. The final pressure recorded was 377kPa (3.77 bar) higher than the starting pressure. The hydrogen yield was 53% (0.74 standard liter of hydrogen per gram of pellets).

Comparative example 2

The reactor was charged with 6g of pellets (prepared according to examples 1 and 2 of WO 2019/158941). A hydrogen background with an absolute pressure of 120kPa (1.2 bar) was provided in the reactor. After one minute, 20mL deionized water (t=24℃) was injected into the reactor.

The results of comparative example 2 are shown in fig. 4. Graph 4 shows the pressure versus time 41 and temperature versus time 42 for a 6g pellet reacted with 20mL water. Hydrogen production was completed within two minutes of injection. The temperature rises to a peak of 208 ℃. The final pressure recorded was 540kPa (5.40 bar) higher than the initial pressure. The hydrogen yield was 68% (1.06 standard liters of hydrogen per gram of pellets).

With the pellets of the present invention, the initial increase in reaction was much faster and when they contained a reduced silicon content (80 wt/wt% instead of 90 wt/wt%), the amount of (pure) potassium hydroxide doubled (from 8.5 wt/wt% in comparative example 2 to 17 wt/wt% in example 2). The increase in potassium hydroxide appears to increase the initial reaction rate of the pellets of the present 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 larger proportion of silicon in the pellet is available for reaction.

Example 3

The reactor was charged with 6g of pellets (prepared according to example 1). A hydrogen background with an absolute pressure of 120kPa (1.2 bar) was provided in the reactor. After one minute, 40mL deionized water (t=25℃) was injected into the reactor.

The results of example 3 are shown in fig. 5. Graph 5 shows the pressure 51 and temperature 52 over time for a 6g pellet reaction with 40mL water. Hydrogen production was completed within two minutes of injection. The temperature rises to a peak of 152 ℃. The final pressure recorded was 535kPa (5.35 bar) higher than the initial pressure. The hydrogen yield was 74% (1.03 standard liters of hydrogen per gram of pellets).

Comparative example 3

The reactor was charged with 6g of pellets (prepared according to examples 1 and 2 of WO 2019/158941). A hydrogen background with an absolute pressure of 120kPa (1.2 bar) was provided in the reactor. After one minute, 40mL deionized water (t=23℃) was injected into the reactor.

The results of comparative example 3 are shown in fig. 6. Graph 6 shows the pressure 61 and temperature 62 over time for a 6g pellet reaction with 40mL water. Hydrogen production was not completed two hours after injection. The temperature rises to a peak of 30 ℃. The pressure recorded after ten minutes was 28kPa (0.28 bar) higher than the starting pressure. The hydrogen yield after ten minutes was 3.9% (0.055 standard liters of hydrogen per gram of pellet). The final pressure recorded after two hours was 84kPa (0.84 bar) higher than the starting pressure. The hydrogen yield after two hours was 12% (0.16 standard liters of hydrogen per gram of pellets).

Increasing the volume of water from 20mL (example 2) to 40mL (example 3) with dry milling material resulted in a 21% increase in hydrogen yield. In contrast, increasing the volume of water from 20mL (comparative example 2) to 40mL (comparative example 3) with wet milled material resulted in a 56% decrease in hydrogen yield (in the case of comparative example 3 after two hours).

It should be appreciated that the rate of reaction between a solid and a liquid is limited by the surface area of the solid available for contact with the liquid. Thus, when silicon reacts with water, the greater the surface area of the silicon, the higher the reaction rate is expected.

Surprisingly, the larger silicon particles produced in the dry milling process were more efficient in producing hydrogen when there was excess water than the prior art submicron particles produced in the wet milling process (comparison of the results of example 3 and comparative example 3).

Although wet milled silicon materials produced good hydrogen yields (68%) at a relatively low water ratio (20 mL, comparative example 2), increasing the water volume resulted in a temperature that was too low for an advantageous reaction, as the same amount of heat was released into a larger mass with a larger heat capacity. Thus, wet grinding silicon materials results in a slower reaction when the water volume is increased.

While dry milled silicon materials generally require more water to achieve the same percentage of hydrogen yield as wet milled silicon materials, increasing the amount of catalyst (from 8.5 wt/wt% wet milling process to 17 wt/wt% dry milling process) means that more heat is available at the early stages of the reaction. As a result, the reaction rate of the dry milled material increases (as compared to the wet milled material), and the greater volume of water does not quench the reaction as does the wet milled material.

Example 4

The reactor was charged with 50g of pellets (prepared according to example 1) in an airtight cartridge with a frangible seal, and 417mL of deionized water (t=22 ℃) was charged on top of the tank. A hydrogen background with an absolute pressure of 120kPa (1.2 bar) was provided in the reactor. After one minute, the cartridge seal was broken, allowing water to fill the canister and contact the pellets.

The results of example 4 are shown in fig. 7. Graph 7 shows the pressure 71 and temperature 72 over time for 50g of pellets reacted with 417mL of water. Hydrogen production was completed within two minutes of injection. The temperature between the tank and the vessel wall rises to a peak of 55 ℃. The final pressure recorded was 4310kPa (43.1 bar) higher than the initial pressure. The hydrogen yield was 63% (0.87 standard liters of hydrogen per gram of pellets).

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

Comparative example 4

The different reactors were charged with 40g of pellets (prepared according to examples 1 and 2 of WO 2019/158941). A hydrogen background with an absolute pressure of 120kPa (1.2 bar) was provided in the reactor. After one minute, 200mL deionized water (t=25℃) was injected into the reactor.

The results of comparative example 4 are shown in fig. 8. Graph 8 shows the pressure change 81 and temperature change 82 over time for a 40g pellet reacted with 200mL water. Hydrogen production was completed within three minutes of injection. The temperature inside the reaction mixture rose to a peak of 196 ℃. The final pressure recorded was 2530kPa (25.3 bar) higher than the starting pressure. The hydrogen yield was 64% (1.00 standard liters of hydrogen per gram of pellets).

The temperature was measured by inserting a temperature probe into the reaction mixture. The temperature probe makes it possible to acquire a higher temperature than the dry milled material (example 4), wherein the temperature probe is located outside the reaction mixture.

As the reaction scale increased from 6g (comparative examples 2 and 3) to 40g (comparative example 4) of pellets (formed of wet milled material), the alignment of the pellets was found to be important to ensure good mixing of the reactants. In order to obtain a favourable yield, the pellets must be uniformly distributed in the available space, for example by arranging the pellets into layers.

In contrast, when the reaction scale was increased from 6g (examples 2 and 3) to 50g (example 4) of pellets (formed from dry ground material), the reaction appeared to be less sensitive to the alignment of the pellets. The results can be attributed to the increased amount of dispersant in the dry milling process (17% potassium hydroxide compared to 8.5% potassium hydroxide). The increased amount of dispersant ensures that more heat is available at the early stages of the reaction, the energy of which aids in mixing the reactants.

An overview of the reactions of the dry milled silicon powder of the present invention and the wet milled silicon of the prior art is shown in table 1.

TABLE 1 comparison of reactions of Dry ground silicon and Wet ground silicon with Water

Wherein: "dry" means dry milled, "wet" means wet milled, Δp being the pressure increase.

* After two hours.

Note that reaction vessels with different free volumes are used here.

While dry milled materials generally require a greater volume of water to achieve the same percentage yield as wet milled materials, wet milled materials are much more susceptible to larger scale pellet alignment when reacted with water.

Furthermore, the dry milling process uses significantly less energy on a laboratory scale than the wet milling scheme of our earlier patent application (WO 2019/158941). The specific energy of the dry grinding material at 70% yield was 1.33kWh/kg compared to the specific energy of the wet grinding material of 1.50 kWh/kg. Although the specific energy of dry-milled materials is lower than wet-milled materials, the manufacture and use of dry-milled materials is generally more energy efficient than wet-milled materials. In addition, the effective grinding time is shorter, which results in reduced manufacturing costs. Furthermore, since there are fewer steps in the process, the operating time is reduced. We will expect the savings to be amplified when scaling up.

Advantageously, the dry milling process results in a safer material. Because dry abrasive materials are less reactive with air, the risk of ignition when exposed to air is significantly reduced.

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

Those skilled in the art will appreciate that several variations of the foregoing embodiments are contemplated without departing from the scope of the invention.

Those skilled in the art will also appreciate that any number of combinations of the foregoing features and/or those shown in the drawings provide significant advantages over the prior art and are therefore within the scope of the invention described herein.

Claims

1. A pellet comprising a mixture of ground silicon and a dispersant, wherein the ground mixture comprises particles having an average diameter of at least 1 micron.

2. The pellet of claim 1, wherein the pellet comprises a median value or d ₅₀ Silicon powder having a value of 10 to 50 microns, such as 10 to 40 microns, or 10 to 30 microns, such as 12.0 to 25.0 microns.

3. The pellet of any preceding claim, wherein the pellet comprises greater than or equal to 70 wt/wt% silicon, such as 75 to 100 wt/wt% silicon, or 75 to 90 wt/wt% silicon.

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

5. The pellet of any preceding claim, wherein the pellet comprises a heat of solution of less than-20 KJ mol ^-1 Is a dispersant of (a).

6. The pellet of any preceding claim, wherein the dispersant has a solubility in water (20 ℃) of greater than 40g/100mL.

7. The pellet of any preceding claim, wherein the pellet comprises less than or equal to 25 wt/wt% dispersant, such as 25 to 10 wt/wt% dispersant.

8. The pellet of claim 7, wherein the pellet comprises 25 to 15 wt/wt% of dispersant, preferably 20 wt/wt% of dispersant.

9. The pellet of any preceding claim, wherein the dispersant comprises a metal hydroxide, such as lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide and/or cesium hydroxide.

10. The pellet of any preceding claim, wherein the pellet is in the form of a cylinder, ring, ellipsoid, sphere, prism or cuboid.

11. The pellet of 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;

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

granulating the solvent-free (e.g., dry) milled material.

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

14. The method according to claim 12 or 13, wherein step (i) comprises providing greater than or equal to 70 wt/wt% silicon, such as 75 to 100 wt/wt% silicon, or 75 to 90 wt/wt% silicon, or 75 to 85 wt/wt% silicon, preferably 80 wt/wt% silicon.

15. The method according to any one of claims 12 to 14, wherein step (i) comprises providing less than or equal to 25 wt/wt% of dispersant, such as 25 to 10 wt/wt% of dispersant, or 25 to 15 wt/wt% of dispersant, preferably 17 wt/wt% of dispersant.

16. A method according to any one of claims 12 to 15, wherein step (i) comprises providing a metal hydroxide, such as lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide and/or cesium hydroxide, as the dispersing agent.

17. A method according to any one of claims 12 to 16, wherein step (ii) comprises ball milling, i.e. contacting the silicon and dispersant with grinding balls, such as stone or metal grinding balls, such as zirconia balls.

18. The method of claim 17, comprising contacting the silicon and dispersant with a grinding ball for less than 20 minutes, such as less than 15 minutes, such as 10 minutes.

19. A method according to claim 17 or 18, comprising contacting the silicon and dispersant with grinding balls according to two grinding steps, wherein each step may last from 2 to 8 minutes, such as 5 minutes.

20. The method of claim 19, comprising cooling the silicon and dispersant between a first grinding step and a second grinding step.

21. The method of any one of claims 12 to 20, further comprising providing an inert atmosphere.

22. A method according to any one of claims 12 to 21, wherein step (ii) comprises jet milling, i.e. wherein the silicon and dispersant are contacted with a high velocity jet of compressed air or inert gas to cause the particles to impinge upon each other.

23. The method of any one of claims 12 to 21, wherein step (ii) comprises one or more of cone milling, pin milling, air classifier milling, hammer milling and screen milling and/or continuous dry bead milling.