WO2019158940A1

WO2019158940A1 - Thermal management

Info

Publication number: WO2019158940A1
Application number: PCT/GB2019/050421
Authority: WO
Inventors: Gleb Ivanov; Joseph Cook; Sotiris ALEXANDROU
Original assignee: Silicon Fuel Limited
Priority date: 2018-02-16
Filing date: 2019-02-18
Publication date: 2019-08-22
Also published as: CN111699153B; GB2574178B; GB201802554D0; CN111699153A; GB2574178A

Abstract

A method of reacting silicon with water to produce hydrogen, the method comprises locating a silicon-containing composition in a reaction vessel, the reaction vessel comprising a reactant contacting surface, the reactant contacting surface comprising a thermally conductive first region having a thermal conductivity TC1 and a thermally insulative second region having a thermal conductivity TC2 and wherein TC1>TC2 and contacting the silicon- containing composition with water. There is also disclosed a vessel (1) for the reaction of silicon and water, the vessel (1) having reactant-contacting surface being formed of or comprising a first material (2, Figure 2) having a relatively high thermal conductivity (TC W/m.K) and comprising a second material (12) having a relatively low thermal conductivity 10 (TC2 W/m.K).

Description

THERMAL MANAGEMENT

This invention relates generally to thermal management of a chemical reaction. More specifically, although not exclusively, this invention relates to the thermal management of the reaction between silicon and water to produce hydrogen.

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 + 2H₂0 2H₂ + Si0₂ (I)

For example, the use of silicon to produce hydrogen is described in US20150266729A1 , wherein the development of a composition is described, the composition comprising non- passivated silicon, a dispersing agent, a dispersant, and a colloidal stabiliser for reaction with water to produce hydrogen.

It is useful to be able to control and tailor the rate of hydrogen production for specific applications. For example, it may be advantageous to produce a large amount of hydrogen in a short period of time, i.e. minutes rather than hours. It is also important to be able to generate a good yield of hydrogen from the silicon and water mixture, so that the transportation of silicon and water is not wasteful, and so that hydrogen may function as a fuel.

The reaction of silicon with water is highly exothermic (-339 kJ mol ¹ at RT). Controlling the temperature of the system as the reaction proceeds is crucial to ensure the reaction runs smoothly. If the temperature of the reaction is too high then the water reactant will be converted into steam. The generation of steam in the reaction is detrimental for several reasons. Firstly, the amount of water in the liquid phase of the system is reduced, which reduces the quantity of water available to react with the silicon, and therefore reduces the hydrogen yield. Secondly, the generation of steam further increases the pressure within the system leading to a high peak pressure value, which the reaction vessel has to be designed to withstand. Reaction vessels which are rated to higher pressures are, typically, heavier and/or more costly than those which are rated to lower pressures. Accordingly, higher reaction pressures require the use of heavier and/or more costly reaction vessels which is disadvantageous.

Accordingly, thermal management of the reaction has an impact on the reaction kinetics, i.e. the rate of reaction, and the thermodynamics, i.e. the activation energy that allows the reaction to initiate and proceed to completion.

It is therefore a first non-exclusive object of the invention to provide a reaction vessel that is able to manage the temperature of the reaction between silicon and water, in order to produce a sufficient yield of hydrogen at an appropriate rate of reaction.

Accordingly, a first aspect of the invention provides a method of reacting silicon with water to produce hydrogen, the method comprising:

(i) locating a silicon-containing composition in a reaction vessel, the reaction vessel comprising a reactant-contacting surface, the reactant-contacting surface comprising a relatively thermally conductive first region and a relatively thermally insulative second region;

(ii) contacting the silicon-containing composition with water.

A strategy to reduce the peak temperature of the reaction is to use a larger volume of water. Water has a high specific heat capacity and, as such, is able to absorb the heat generated as the reaction proceeds. However, if too much water is used, then the temperature of the reaction will not rise sufficiently for the rate of hydrogen production to be useful. Surprisingly, we have found that the temperature of the reaction can be adequately and appropriately controlled by the use of thermally conducting and thermally insulative regions of the reaction vessel.

The silicon-containing composition may be located on the thermally insulative second region. The water may be provided at a temperature of from 0 to 40 °C, say from 0 to 30°C. The water may be unheated.

It has been surprisingly found that the above method provides the optimum conditions for heat transfer during the reaction of silicon with water. The region of thermally conductive material removes heat from the reaction as it proceeds, which prevents the peak temperature and peak pressure from becoming too high. Concurrently, the region of thermally insulative material prevents removal of an excessive amount of heat from the reaction as it proceeds. This allows the temperature to increase to a sufficient value for hydrogen generation to occur at a useful rate.

A second aspect of the invention provides a reaction vessel for the reaction of silicon and water, the reaction vessel having a base and a peripheral wall extending therefrom, the reaction vessel having a reactant-contacting surface, the reactant-contacting surface being formed of or comprising a first material having a relatively high thermal conductivity (TC1 W/m.K) and comprising a second material having a relatively low thermal conductivity (TC2 W/m.K), the difference in thermal conductivities (TC1-TC2) being greater than 5 W/m.K, and wherein, in use, with water and silicon located within the reaction vessel the first material is located such that it is contacted by water.

Preferably the first material is located such that it is principally contacted by water, and not silicon. As silicon has a higher density than water, the silicon will typically rest on the base of the reaction vessel. Accordingly, locating the first material at a location away from, for example peripheral to, the base wall ensures that, in use, it is contacted by water and, at least in the main, not silicon.

A further aspect of the invention provides a reaction vessel comprising a reactant contacting surface, the reactant contacting surface comprising a thermally conductive first region and a thermally insulative second region, the first region exhibiting a thermal conductivity of above 5 W/m K or 10 W/m.K at 25 °C, and the second region exhibiting a thermal conductivity of below 5 W/m K at 25 °C, say below 4, 3, 2, 1 W/m.K at 25 °C, the difference between thermal conductivity of the first region and the second region being greater than 5 W/m K at 25 °C.

The first region may comprise a material that exhibits a thermal conductivity of greater than 7.5 W/m K at 25 °C, and preferably of greater than 8, 9 or 10 W/m K at 25 °C. The first region may comprise a metal, or a mixture of metals, or an alloy. In general, metals exhibit a thermal conductivity of between 5 to 450 W/m K at 25 °C. For example, stainless steel exhibits a thermal conductivity of between 12 to 45 W/m K at 25 °C depending on composition. In an embodiment, the thermally conductive region may comprise stainless- steel 316, which has a thermal conductivity of 16.3 W/m K at 100 °C.

The second region may comprise a material that exhibits a thermal conductivity of less than 4, 3 or 2 W/m K at 25 °C, and preferably of less than 1 W/m K at 25 °C. For example, the second region may be or may comprise a polymer, for example a thermoset polymer, e.g. PTFE (polytetrafluoroethylene). PTFE exhibits a thermal conductivity of 0.25 W/m K at 25 °C. Alternatively, the second region may be fabricated from a ceramic material, for example a glass ceramic such as that sold under the trade name Macor (RTM) by Corning Inc, New York, United States, and distributed by Precision Ceramics, Birmingham UK which is composed principally of 55% fluorophlogopite mica and 45% borosilicate glass (thermal conductivity of 1.5 W/m.K at 25 °C). Other ceramics that can be used include cordierite (3.0 W/m.K at 25 °C), mullite (3.5 W/m.K at 25 °C), steatite (2.9 W/m.K at 25 °C), zirconia (2.7 W/m.K at 25 °C). In embodiments, ceramic materials may be preferable due to their physical properties, for example melting point. Other materials with suitable thermal conductivities may also be deployed.

The difference between the thermal conductivity of the first region and the second region is greater than 2 W/m K at 25 °C, and is preferably greater than 10 W/m K at 25 °C.

A factor in selecting the material for the first region is its heat capacity. It is favourable to utilise a material with a relatively high heat capacity value per gram of material to ensure that a corresponding amount of heat can be absorbed for a relatively lower mass of heat sink. The first region may comprise a material that exhibits a specific heat capacity of greater than 0.1 J g^_1 C \ say greater than 0.2, 0.3, 0.4, J g^_1 °C^~ In an embodiment, the first, relatively thermally conductive, region may comprise stainless-steel 316, which has a specific heat capacity of 0.50 J g^_1 °C ¹.

The reactant-contacting surface may comprise a base and/or a side wall. The side wall may comprise a or the first, relatively thermally conductive, region. The base may comprise a or the second relatively thermally insulative second region. In use, the side wall may be, at least principally, contacted by the reactant water. The base may be contacted by both the main reactants, i.e. both silicon and water. For example, the silicon may be located on the base.

The reactant-contacting surface may define a reactant volume. The combined volume of reactants (water and silicon) may be such that the water contacts the first, relatively thermally conductive, region but the silicon does not.

The side wall extends from the base. The first, relatively thermally conductive, region may be adjacent the base or may be separated from the base. The first, relatively thermally conductive, region may be located and separated from the base by a first distance such that upon addition of the water to the silicon the water does not or only marginally contacts the first, relatively thermally conductive, region. As the reaction proceeds, and as hydrogen is generated, the effective volume of the water will increase, thereby raising the level of water within the vessel and bringing more of the water into contact with the first, relatively thermally conductive, region. For example, the thermally conductive region may be a rod which protrudes into a volume occupied by the reactants.

The base may comprise a portion of a liner that is located within the reaction vessel. The liner may comprise a polymer, for example, a thermoset polymer, e.g. PTFE (polytetrafluoroethylene). The liner may comprise any suitable shape, for example, the liner may be cylindrical. The liner may comprise a closed or sealed end, and/or an open end. The base wall may comprise the closed or sealed end of the liner. The open end may taper outwards, for example to define a funnel. The base may comprise the closed or sealed end of a reaction vessel liner, e.g. a ceramic, or plastic reaction vessel liner. In embodiments, the base may have a maximum transverse dimension (for example diameter) of between 50 to 110 mm in, for example, between 60 to 100 mm in maximum transverse dimension (e.g. diameter), or between 70 to 90 mm in maximum transverse dimension (e.g. diameter), for example, 80, or 83 mm in maximum transverse dimension (e.g. diameter).

The dimensions of the base and/or side wall are dependent on the scale of the reaction between silicon and water. The dimensions of the base wall and side wall must be sufficient to accommodate the reactant silicon and water. However, the dimensions of the base wall and/or side wall must not be so large as to prevent the water from contacting the reactant silicon, i.e. if the water fill level is too low.

The thermally conductive first region may be an insert. The insert may function as a heat sink, i.e. a component for transferring heat energy away from the reactants. The insert may be located within the reaction vessel, and/or located within a reaction vessel liner. The insert may be cylindrical, e.g. ring shaped. The insert may be a disc. The insert may be formed from metal, e.g. stainless steel.

The liner may comprise an integrally-formed thermally conductive first region and an integrally-formed thermally insulative second region

The ring-shaped insert may provide a volume in which at least a part of the reaction mix is contained. The cylindrical or ring-shaped insert may be formed from a thermally conductive material, e.g. stainless-steel. For a base having a maximum transverse diameter of between 50 and 1 10mm, the insert may be between 100 to 250 g in mass, for example, between 1 10 and 220 g in mass. Clearly, the mass of the insert will be determined by the dimensions of the vessel (and/or liner) and/or the amount of one or both of the reactants and/or the amount of hydrogen to be generated.

The reactant-contacting surface may be provided, in part, by an insert. The insert may provide the thermally-conductive first region. The insert may lie adjacent a wall of the reaction vessel. For example, the side wall may comprise the interior cylindrical wall of a ring-shaped insert, e.g. a stainless steel ring-shaped insert. In embodiments, the side wall may be between 5 to 60 mm in height, for example, 10 to 50 mm in height, or 15 to 40 mm in height, or 15 to 25 mm in height, or 20 mm in height.

In an embodiment, the base wall is 70 mm in width and the side wall is 20 mm in height. The physical parameters of the reaction vessel will be tailored to one or more of the amount of reactants, required rate of hydrogen production and/or yield of hydrogen.

Additionally or alternatively, the side wall may be formed from more than one component or part or insert. The side wall may be formed from several discrete components or parts or inserts. For example, the side wall may be formed from two or more disconnected regions of thermally conductive material, for example, metal, a mixture of metals, or one or more metal alloys, e.g. stainless steel.

Alternatively, the base may comprise the top surface of an insert, e.g. a disc-shaped insert for example a stainless-steel disc-shaped insert.

Alternatively, the side wall may comprise an upstanding wall of a liner for example, a reaction vessel liner, e.g. a ceramics or polymer or plastics reaction vessel liner. In an embodiment, the base wall is 83 mm in width and the side wall is 50 mm in height.

In an embodiment, the reaction vessel may be a pressure-rated reaction vessel, for example a metal, pressure-rated reaction vessel having a base and a peripheral wall, located on the base may be provided a relatively thermally-insulative body to provide the relatively thermally-insulative region to define a portion of the reactant-contacting surface.

The body may be provided as an insert, a liner, a coating, a stage. The body may be curved or undulatory, or flat and/or may have projections, rebates and so on. The composition, i.e. the silicon containing composition, may be provided in any form. For example, the composition may comprise silicon in a powdered form, or in a pelletised form. The composition may further comprise dispersants and/or dispersing agents, or any other additive. Preferably, the composition is provided in pellets. The silicon may be provided as a powder, e.g. a polydisperse powder, with particle sizes with a mean diameter and/or a D50 of between 50 nm and 500 nm, e.g. 100 to 400 nm, or 200 to 300 nm. The silicon powder may be compressed into pellets with a density of 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 1.7 g/cm³, for example, between 1.2 g/cm³ to 1.6 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.

The composition may further comprise a dispersant, for example, a metal hydroxide, e.g. lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide and/or caesium hydroxide.

The composition may comprise a colloidal stabiliser, a dispersant, and/or a dispersing agent in accordance with WO2014/053799, which is incorporated herein by reference.

The reaction vessel itself may be composed of any suitable material capable of withstanding the peak pressure generated by the reaction. For example, the reaction vessel may be composed of metal, e.g. stainless steel. It is advantageous for the reaction vessel to have a volume to allow the generation of high pressures of hydrogen (e.g. <1000 bar). The reaction vessel will be appropriately rated to withstand the pressure of hydrogen generated within the vessel. In embodiments, the vessels of the invention may be pressure rated to up to 100 MPa (1000 bar), for example up to 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, 30 MPa.

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.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of a reaction vessel liner;

Figure 2 is a disc-shaped heat sink, according to an embodiment of the invention; Figure 3A is a ring-shaped heat sink, according to an embodiment of the invention; Figure 3B is the ring-shaped heat sink of Figure 3A within the reaction vessel liner of Figure 1 , according to an embodiment of the invention;

Figure 4 is a graph showing the pressure change and temperature change over time of the reaction between silicon and water, according to Comparative Example 1 ; Figure 5 is a graph showing the pressure change and temperature change over time of the reaction between silicon and water, according to Example 1 of the invention; Figure 6 is a graph showing the pressure change and temperature change over time of the reaction between silicon and water, according to Example 2 of the invention; Figure 7 is a graph showing the pressure change and temperature change over time of the reaction between silicon and water, according to Example 3 of the invention; Figure 8 is a schematic representation of a reaction vessel of the invention

Figures 8A to 8C are schematic representations of various embodiments of reaction vessels of the invention; and

Figure 9 is a schematic representation of a further reaction vessel of the invention.

Referring first to Figure 1 , there is shown a reaction vessel liner 1. The reaction vessel liner 1 is a hollow receptacle comprising a frusto-conical top portion 10 and a cylindrical base portion 11. The cylindrical base portion 1 1 comprises a closed base 1 1 b and an upstanding peripheral wall 12.

The frusto-conical top portion 10 comprises a distal end 10a and a proximal end 10b. The distal end 10a comprises an opening 13 to the reaction vessel liner 1. The distal end 10a tapers towards the proximal end 10b to form the frusto-conical top portion 10. The cylindrical base portion 1 1 comprises a first end 1 1 a in communication with the proximal end 10b of the top portion 10. The frusto-conical top portion 10 comprises a largest external diameter d1 at the distal end 10a and a smallest internal diameter d2 at the proximal end 10b. The cylindrical base portion 11 comprises an external diameter d3, which is constant along the length of the base portion 11 , and an external cylinder base portion height hi .

The proximal end 10b of the frusto-conical top portion 10 is in communication with the first end 1 1 a of the cylindrical base portion 11 to form the reaction vessel liner 1. The upstanding peripheral wall 12 comprises a wall width w1. The reaction vessel liner 1 comprises a total height h2.

In this embodiment, the largest external diameter d1 is 120 mm, the smallest internal diameter d2 is 83 mm, the external diameter d3 is 93 mm, the wall width w1 is 5 mm, the external cylinder base portion height hi is 60 mm, the total height h2 is 120 mm, and the reaction vessel liner 1 is composed of PTFE (polytetrafluoroethylene).

The reaction vessel liner 1 is located within a reaction vessel (not shown), which is a pressure vessel. The reaction vessel (not shown) may be composed of metal, e.g. stainless- steel. In use, the reactants, i.e. the silicon composition and the water, are added to the reaction vessel liner 1 through the opening 13. The chemical reaction of silicon and water to produce hydrogen takes place within the cylindrical base portion 11 of the reaction vessel liner 1. The reactants contact the inner surface 12a at the base 1 1 b of the cylindrical base portion 1 1 of the reaction vessel liner 1.

Referring now to Figure 2, there is shown a disc-shaped heat sink 2, according to an embodiment of the invention. The disc-shaped heat sink 2 comprises a top surface 20, a circumferential surface 21 , and a bottom surface 22. The disc-shaped heat sink 2 is a solid disc in shape. The top surface 20 and the bottom surface 22 each form an uninterrupted flat surface.

The disc-shaped heat sink 2 comprises a diameter d4 and a height h3. In this embodiment, the diameter d4 is 82 mm and the height h3 is 3 mm. The disc-shaped heat sink 2 is composed of metal, e.g. stainless-steel, and has a total mass of 1 13 g.

In use, the disc-shaped heat sink 2 is placed inside the reaction vessel liner 1 before the reaction vessel (not shown) is charged with silicon and/or water. The disc-shaped heat sink 2 is located at the base 11 b so that the bottom surface 22 is in contact with the inner surface 12a of the upstanding peripheral wall 12 of the reaction vessel liner 1. In an embodiment, the smallest internal diameter d2 of the reaction vessel liner 1 is 83 mm, and the diameter d4 of the disc-shaped heat sink 2 is 82 mm, so that a relatively close fit of the disc-shaped heat sink 2 in the cylindrical base portion 11 of the reaction vessel liner 1 is achieved.

The reaction vessel liner 1 of the reaction vessel (not shown) is then charged with a silicon composition followed by water to initiate the reaction. The disc-shaped heat sink 2 functions to conduct heat away from the reaction as it proceeds to prevent overheating and/or thermal runaway.

Referring now to Figure 3A, there is shown a ring-shaped heat sink 3 according to an embodiment of the invention. The ring-shaped heat sink 3 comprises a top surface 30, a circumferential surface 31 , and a bottom surface 32. The ring-shaped heat sink 3 is a ring shape, comprising a hollow middle section 33 and an internal wall 34.

The top surface 30 of the ring-shaped heat sink 3 comprises a largest diameter d5 and a height h4. The hollow middle section 33 of the ring-shaped heat sink 3 comprises an internal diameter d6. In this embodiment, the largest diameter d5 is 82 mm, the internal diameter d6 is 70 mm, and the height h4 is 20 mm, the ring-shaped heat sink 3 is composed of stainless-steel, and has a total mass of 218 g.

In use, the ring-shaped heat sink 3 is placed inside the reaction vessel liner 1 before the reaction vessel (not shown) is charged with silicon and/or water.

Referring also to Figure 3B, there is shown the ring-shaped heat sink 3 located within the reaction vessel liner 1. The ring-shaped heat sink 3 is located on the inner surface 12a of the cylindrical base portion 11 of the reaction vessel liner 1. In this embodiment, a portion P of the inner surface 12a at the base 11 of the reaction vessel liner 1 is not covered or concealed by the ring-shaped heat sink 3. Instead, the portion P is exposed by the hollow middle section 33. The portion P has a diameter d7. In this embodiment, the portion P forms a base wall and the internal wall 34 of the ring-shaped heat sink 3 forms a side wall. In an embodiment, the portion P has a maximum transverse dimension (i.e. diameter) of 70 mm. In an embodiment, the smallest internal diameter d2 of the reaction vessel liner 1 is 83 mm, and the largest diameter d5 of the disc-shaped heat sink 2 is 82 mm, so that a relatively close fit of the ring-shaped heat sink 3 in the cylindrical base portion 11 of the reaction vessel liner 1 is achieved.

The reaction vessel liner 1 of the reaction vessel (not shown) is then charged with a silicon composition followed by water to initiate the reaction. The silicon composition and the water reactants are in contact with the portion P of the inner surface 12a of the reaction vessel liner 1. In contrast, the internal wall 34 of the ring-shaped heat sink 3 is substantially free from contact with the reactant silicon, i.e. the majority of the reactant silicon remains within the hollow middle section 33 of the ring-shaped heat sink 3 at the portion P. The ring-shaped heat sink 3 functions to conduct heat away from the reaction as it proceeds to prevent overheating and/or thermal runaway. Specifically, the thermal energy generated as the reaction proceeds is transferred to the ring-shaped heat sink 3 via the internal wall 34, which is in contact with the reactant water throughout the reaction.

Surprisingly, it has been found that the ring-shaped heat sink 3 comprises an optimal shape and/or geometry for thermal management of the reaction of silicon and water to produce hydrogen. Without wishing to be bound by theory, it is thought that the disc-shaped heat sink 2 provides a large surface area for thermal contact with the silicon composition as the reaction proceeds. This means that heat is transferred rapidly from the silicon composition and the water in the early stages of the reaction, which is disadvantageous to the rate of reaction because the local temperature of the silicon composition never reaches a sufficiently high temperature for hydrogen generation to occur at a useful rate.

In contrast, the ring-shaped heat sink 3 provides a geometry that allows a favourable initial rate of reaction. It is thought that the shape of the ring-shaped heat sink 3 prevents heat transfer from the silicon composition itself because the silicon composition is only in contact with the thermally insulative material at the base, i.e. the inner upstanding peripheral wall 12a at the base 1 1 b of the reaction vessel liner 1. The local temperature of the silicon composition is able to rise rapidly whilst allowing the optimum amount of heat to be transferred via the bulk reaction mixture, i.e. the water, as the reaction proceeds. This prevents overheating and/or thermal runaway but allows the reaction to proceed at a sufficient rate of hydrogen production, whilst keeping the peak temperature and/or pressure within acceptable limits. This is particularly surprising because, as shown in the Examples below, the ring-shaped heat sink 3 is approximately double the mass of the disc-shaped heat sink 2. Therefore, it would be expected that a larger mass of thermally conductive material would transfer a greater amount of heat away from the reaction, whereas the converse of this is observed.

To further exemplify the invention, reference is also made to the following non-limiting Examples:

Comparative Example 1

The process was performed in a stainless-steel pressure vessel with a 5 mm thick PTFE liner only. The stainless-steel pressure vessel was charged with pellets comprising 5.40 g of silicon (CAS number 7440-21-3, 99.95% purity). A background of hydrogen gas at a pressure of 1.2 bar absolute was provided. After one minute, 20 ml_ of deionised water was injected into the stainless-steel pressure vessel.

The results of Comparative Example 1 are shown in Figure 4. The graph 4 shows the pressure change over time 41 and the temperature change over time 42. Hydrogen generation was complete within two minutes of injection of the water. The temperature increased to a peak value of 208 °C within two minutes of injection. The pressure increased to a peak value of 10.7 bar above the starting pressure. The final pressure of 5.44 bar above starting pressure was recorded. The hydrogen yield was 68%.

Therefore, the hydrogen yield and the rate of release are both acceptable. However, the peak temperature and peak pressure values are high.

It is to be understood that the peak pressure relates to the maximum pressure value that the system must withstand. In contrast, the final pressure (once the system has cooled back to ambient temperature) relates to the amount of“useful” hydrogen generated. Clearly the reaction vessel has to be able to withstand the peak pressure generated in the reaction. The higher the peak pressure the more robust the reaction vessel has to be. In this case, robustness requires more material which adds to the weight of the reaction vessel. The extra mass will lead to increased costs and, for mobile systems the extra mass will be deleterious from an energy balance perspective (it takes more energy to move a heavier item than a light item). Example 1

The process was performed in a stainless-steel pressure vessel comprising a 5 mm thick PTFE liner and a 113 g stainless steel disc, which functions as a heat sink. The 113 g stainless-steel disc was placed inside the PTFE liner at the beginning of the process. The stainless-steel pressure vessel was charged with pellets comprising 5.40 g of silicon (CAS number 7440-21-3, 99.95% purity). The pellets were in direct contact with the stainless- steel disc. A background of hydrogen gas at a pressure of 1.2 bar absolute was provided. After one minute, 20 ml_ of deionised water was injected into the stainless-steel pressure vessel.

The results of Example 1 are shown in Figure 5. The graph 5 shows the pressure change over time 51 and the temperature change over time 52. The temperature increased to a peak value of 46 °C within two minutes of injection. A slow release of hydrogen was observed; the pressure reached 0.92 bar above starting pressure after ten minutes, and 2.27 bar above starting pressure after two hours. The experiment was stopped after two hours with a hydrogen yield of 28%.

Example 2

The process was performed in a stainless-steel pressure vessel comprising a 5 mm thick

PTFE liner and a 218 g stainless steel ring (OD 82 mm, ID 70 mm, height 20 mm), which functions as a heat sink. The 218 g stainless steel ring was placed inside the PTFE liner at the beginning of the process. The stainless-steel pressure vessel was charged with pellets comprising 5.40 g of silicon (CAS number 7440-21-3, 99.95% purity). The pellets were in direct contact with the PTFE liner. A background of hydrogen gas at a pressure of 1.2 bar absolute was provided. After one minute, 20 ml_ of deionised water was injected into the stainless-steel pressure vessel.

The results of Example 2 are shown in Figure 6. The graph 6 shows the pressure change over time 61 and the temperature change over time 62. The temperature increased to a peak value of 162 °C within two minutes of injection. The pressure increased to a peak value of 6.44 bar above the starting pressure. The final pressure of 5.47 bar above starting pressure was recorded. The hydrogen yield was 67%. Therefore, the hydrogen yield and the rate of release are both acceptable, and the peak temperature and peak pressure are also both acceptable. Clearly, because the reaction only generates a peak pressure of 644 kPa (6.44 bar) there is no need for the reaction vessel to be rated at higher pressures.

Example 3

The process was performed in a stainless-steel pressure vessel comprising a 5 mm thick PTFE liner and a 118 g stainless steel ring (OD 82 mm, ID 70 mm, height 10 mm), which functions as a heat sink. The 118 g stainless steel ring was placed inside the PTFE liner at the beginning of the process. The stainless-steel pressure vessel was charged with pellets comprising 5.40 g of silicon (CAS number 7440-21-3, 99.95% purity). The pellets were in direct contact with the PTFE liner. A background of hydrogen gas at a pressure of 1.2 bar absolute was provided. After one minute, 20 ml_ of deionised water was injected into the stainless-steel pressure vessel.

The results of Example 3 are shown in Figure 7. The graph 7 shows the pressure change over time 71 and the temperature change over time 72. The temperature increased to a peak value of 189 °C within two minutes of injection. The pressure increased to a peak value of 7.72 bar above the starting pressure. The final pressure of 5.73 bar above starting pressure was recorded. The hydrogen yield was 71 %.

The change in performance of the reaction system between Example 1 and Example 2 is surprising. As is demonstrated by a comparison between Example 1 and 2, the location of the thermally conductive region helps to determine how the reaction proceeds. Moreover, Example 3 shows that even greater performance (yield) can be achieved by tailoring the amount of thermally conductive material. Indeed, the actual mass of thermally conductive material is practically identical in Examples 1 and 3, however the reaction proceeded in very different ways.

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.

Referring now to Figure 8, there is a shown a reaction vessel 8 having an aspect ratio (H8/W8) of less than 1 , 1 or greater than 1 , preferably greater than 1. The reaction vessel 1 is preferably a cylinder (preferably having its principal axis oriented vertically) and has a conduit (not shown) for introduction of water R1 to the vessel 8. The reaction vessel 8 has a base 81 and a peripheral wall 82. As the water R1 enters the reaction vessel 8 the water R1 will fill the vessel 8 to a first fill level FL1. As the reaction proceeds, the water may adopt a second fill level FL2, typically due to the generation of bubbles of hydrogen and/or water vapour.

In accordance with the invention, the reactant-contacting surface has a first thermally conductive region and a second thermally insulative region.

With regards to Figure 8A (in which similar features have the suffix‘A’), the base 81 A and the peripheral wall 82A are thermally insulative. The vessel 8A is provided with an insert 83A formed of a thermally conductive material. In this case, the insert 83A is a torus to line the internal wall 82A of the vessel 8A. The insert 83A is located a distance DA from the base 81 A of the vessel 8A. The distance DA may be zero (i.e. the insert may contact the base 81 A) or may be larger than zero, in which case the distance DA may be chosen such that, as the reaction proceeds the water R1 contacts the insert 83A.

With regards to Figure 8B (in which similar features have the suffix Ί3’), the base 81 B and the peripheral wall 82B are thermally conductive. The vessel 8B is provided with an insert 83B formed of a thermally insulative material. In this case, the insert 83B is a disc to line the base wall 81 B of the vessel 8B. The insert 83B may have upstanding peripheral walls which extend above the base 81 B by a distance DB.

Referring to Figure 8C (in which similar features have the suffix‘C’), the base 81 C and the peripheral wall 82C are thermally insulative. The vessel 8C is provided with an insert 83C formed of a thermally conductive material. In this case, the insert 83C is a rod (although it may be any shaped body) which extends into the vessel 8C. The rod may be raised or lowered. Plural rods (or other shaped bodies) may be provided, each of which may be raiseable or lowerable. Together the walls of the vessel 8C and the insert 83C provide the reactant contacting surface. Alternatively, the wall 82C may be thermally conductive and the base 81 C thermally insulative.

Referring to Figure 9, there is shown a vessel 9 having a base 91 and a peripheral wall 92. The vessel is in the form of a cylinder oriented so that its principal axis is not vertical, and may be horizontal, as shown. The base 91 may be thermally insulative or may be provided with a coating, liner, insert or stage 93 which is thermally insulative. The walls 92 may be thermally conductive. In each of the embodiments of Figures 8, 8A to 8C, 9, the silicon composition will be provided on the base (or basal insert) and water introduced to effect a reaction between the silicon and the water, thereby to generate hydrogen. In each instance, the respective vessels may be provided with a hydrogen egress port. The vessels of the invention will be appropriately rated to withstand the pressure of hydrogen generated within the vessel. In embodiments, the vessels of the invention may be pressure rated to up to 100 MPa (1000 bar), for example up to 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, 30 MPa. The generated hydrogen may be used in a fuel cell, or as a fuel or fuel additive in a combustion engine, as a buoyancy aid or otherwise. For example, using the invention it is possible to continually supply hydrogen at a suitable pressure of e.g. 1.5 bar, which is a pressure at which it may be used by a device, for example a fuel cell. Alternatively, the hydrogen may be provided to an accumulator at a pressure of up to 1000 bar (e.g. at 700 or 350 bar or 10 bar) for use.

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 method of reacting silicon with water to produce hydrogen, the method comprising:

(i) locating a silicon-containing composition in a reaction vessel, the reaction vessel comprising a reactant contacting surface, the reactant contacting surface comprising a thermally conductive first region having a thermal conductivity TC1 and a thermally insulative second region having a thermal conductivity TC2 and wherein TC1>TC2;

(ii) contacting the silicon-containing composition with water.

2. A method according to Claim 1 , comprising locating the silicon-containing composition on the thermally insulative second region.

3. A method according to Claim 1 or 2, wherein the thermally conductive first region is substantially free from contact with the silicon-containing composition.

4. A method according to any preceding Claim, wherein the thermally conductive region comprises a material that exhibits a thermal conductivity (TC1) of greater than 5 W/rn- K at 25 °C, e.g. a thermal conductivity of greater than 10 W/rn- K at 25 °C.

5. A method according to any preceding Claim, wherein the thermally conductive region comprises a metal, or a mixture of metals, or an alloy.

6. A method according to any preceding Claim, wherein the thermally conductive region comprises stainless steel.

7. A method according to any preceding Claim, wherein the thermally insulative region comprises a material that exhibits a thermal conductivity (TC2) of less than 5 W/m-K at 25 °C, e.g. a thermal conductivity of less than 4, 3, 2 or 1 W/m-K at 25 °C.

8. A method according to any preceding Claim, wherein the thermally insulative region comprises a ceramics or polymer or plastics material.

9. A method according to any preceding Claim, wherein the difference between the thermal conductivity of the first region (TC1) and the second region (TC2) is greater than 5 W/m K at 25 °C, e.g. the difference is greater than 10 W/m K at 25 °C.

10. A reaction vessel for the reaction of silicon and water, the reaction vessel having a base and a peripheral wall, the reaction vessel having a reactant-contacting surface, the reactant-contacting surface being formed of or comprising a first material having a relatively high thermal conductivity (TC1 W/m.K) and comprising a second material having a relatively low thermal conductivity (TC2 W/m.K), the difference in thermal conductivities (TC1-TC2) being greater than 5 W/m.K, and wherein, in use, with water and silicon located within the reaction vessel the first material is located such that it is contacted by water.

11. A vessel according to Claim 10, wherein TC1 is above 10 W/m.K and TC2 is 5 W/m.K or less.

12. A reaction vessel comprising a reactant-contacting surface, the reactant-contacting surface comprising a thermally conductive first region and a thermally insulative second region, the first region exhibiting a thermal conductivity of above 10 W/m-K at 25 °C, and the second region exhibiting a thermal conductivity of below 5 W/m-K at 25 °C, the difference between thermal conductivity of the first region and the second region being greater than 5 W/m-K at 25 °C.

13. A vessel according to Claim 10, 11 or 12, wherein the or a base wall and the or a peripheral wall provide the reactant-contacting surface.

14. A vessel according to Claim 13, wherein the base wall comprises the thermally insulative second region.

15. A vessel according to Claim 10, 11 , 13 or 14, wherein the side wall comprises the thermally conductive first region.

16. A vessel according to any one of Claim 10, 11 , 13 to 15, wherein, in use, the side wall is substantially free from contact with the reactant silicon.

17. A vessel according to any of Claims 10, 11 , 13 to 16, wherein, in use, the base wall is contacted with both silicon and water.

18. A vessel according to any of Claims 10 to 17, wherein the first thermally conductive region comprises a metal, or a mixture of metals, or an alloy.

19. A vessel according to any of Claims 10 to 18, wherein the first thermally conductive region comprises stainless steel.

20. A vessel according to any of Claims 10 to 19, wherein the thermally insulative second region comprises a material that exhibits a thermal conductivity (TC2) of less than 5 W/rn- K at 25 °C, e.g. a thermal conductivity of less than 1 W/m-K at 25 °C.

21. A vessel according to any of Claims 10 to 20, wherein the thermally insulative region comprises a ceramic or polymer or plastics material.

22. A vessel according to any of Claims 10 to 21 , wherein the difference between the thermal conductivity of the first region and the second region is greater than 5, 6, 7, 8, 9, or 10 W/m K at 25 °C, e.g. the difference is greater than 10 W/m K at 25 °C.

23. A vessel according to any of Claims 10 to 22, wherein the reactant-contacting surface comprises an insert.

24. A vessel according to Claim 23, wherein the insert is a metal insert.

25. A vessel according to Claim 10 to 22, wherein the thermally insulative second region comprises an insert, coating, stage or liner.