WO2006003475A1

WO2006003475A1 - A gas storage material, a method of gas storage, a process for manufacturing gas storage material and applications thereof

Info

Publication number: WO2006003475A1
Application number: PCT/GB2005/050105
Authority: WO
Inventors: Denis Huguenin; Olivier Bordelanne
Original assignee: Iti Scotland Limited
Priority date: 2004-07-06
Filing date: 2005-07-06
Publication date: 2006-01-12
Also published as: CA2573240A1; FR2872716A1; EP1765491A1; FR2872716B1

Abstract

The invention provides a reversible hydrogen storage material, a method of reversible hydrogen-storage, a process for manufacturing a reversible hydrogen-storage material and applications thereof. In particular, the reversible hydrogen storage material comprises an active material and a catalyst material, wherein the active material comprises one or more metal oxides.

Description

A Gas Storage Material, a Method of Gas Storage, a Process for Manufacturing Gas Storage Material and Applications thereof.

The present invention relates to a reversible hydrogen storage material, a method of reversible hydrogen- storage, a process for manufacturing a reversible hydrogen- storage material and applications thereof.

The problems of global warming and climate change brought on by high levels of carbon dioxide emitted through the use of carbon-rich fuels, has forced the global energy system to shift towards new, cleaner and more sustainable alternatives. One such alternative is through the use of fuel cells.

A fuel cell is an energy conversion device that is capable of producing electricity from a gas such as hydrogen stored in storage media, and oxygen from the atmospheric air. The use of hydrogen in this way is extremely advantageous, not least because it is the most abundant element in the universe - over 95%, but also because this is a particularly low pollutant system; the fuel cell running wastes are principally water. Thus hydrogen is considered by many to be the fuel of the future.

Much work is currently being undertaken to utilize the fuel cell's potential high energy generation efficiency and fuel cells are being designed for a wide range of applications, such as supplying power to equip automotives, to replace traditional gasoline or diesel engines and supplying energy to computers or mobile phones.

However, fuel cell technology is not simple, and many problems have so far prevented their widespread usage. One key desirable requirement is for the fuel cell to be coupled with a hydrogen storage medium, in this way continuous power can be maintained for a sustained period giving the device being powered a high running autonomy. However, the running autonomy principally depends on the hydrogen storage media capacity. Several already known storage media are capable of storing hydrogen in different ways. Nevertheless, the already known hydrogen storage media present several disadvantages. It is well known that hydrogen, can be stored in tanks, but in order to transport a sufficient quantity of gas, it is necessary to store it under pressure. However, high pressure storage is not convenient because the storage tanks need to be constructed from heavy gauge material to be able to resist the pressure. In addition, in gaseous form, under pressure, the explosive character of hydrogen is enhanced. Thus, storing gaseous hydrogen under pressure is not only cumbersome but also dangerous.

Another method involves the storage of hydrogen in liquid form. We know that, when the pressure is 1 atm, hydrogen in liquid form can be obtained when the temperature is at -253⁰C. However, maintaining hydrogen at such a temperature is very costly in energy and it needs a cumbersome cooling system.

To resolve these problems, alternative chemical storage media have been devised; for example, it is possible to store hydrogen by creating a bond with a partial ionic character in a chemical storage compound such as a metal in the form of a metal hydride for example, magnesium hydride. In theory, these metal hydride chemical storage compounds are able to absorb a very high quantity of hydrogen, for example magnesium can store up to 7.65 weight % of hydrogen; making the volumetric storage capacity of hydrogen higher than in the previously described storage media. Also, based on theoretical thermodynamics, magnesium can be charged under standard conditions (room temperature and atmospheric pressure), however, the reaction proceeds too slowly for practical application due to slow diffusion of hydrogen within solid magnesium hydride, and this means that much more severe charging operating conditions are, in fact, needed: a temperature of 400⁰C and a pressure of 5 bars. In addition, the dissociation pressure for magnesium hydride is very low, i.e. below normal pressure (1 bar), and the desorption temperature is higher than 400⁰C, thus for desorption to operate at room temperature, the hydrogen partial pressure needs to be decreased to below 10^"4 bar. Clearly, such storage chemical compounds are impractical for everyday use.

Some of these difficulties have been overcome as reported in JP-56114801, which discloses that a small improvement in the hydrogen exchange kinetics can be achieved using magnesium-nickel hydride, and more recently in WO 2004/036664 which discloses the use of magnesium hydride stabilized into the fluorite structure. By stabilizing the magnesium hydride into this structure, a fast diffusion of hydride ions is reported, and is believed to be due to the presence of large empty octahedral sites in the fluorite structure through which the stored hydrogen species can move with high mobility.

However, even if volumic capacity and kinetic performances of these metal hydride storage chemical compounds are exceptional, several other technical and financial issues do not allow their industrial development for mass markets. Firstly, these compounds are expensive to produce, notably because they are made from expensive components using complex manufacturing procedures, and secondly, in the case of magnesium hydride, it is a flammable and corrosive solid, which makes large scale handling and transportation of this material complicated.

The present invention aims to resolve these problems by offering a reversible hydrogen- storage material and a method of storing hydrogen that have low manufacturing costs, that are safe to use and which are able to store hydrogen reversibly under conditions close to the normal temperature and pressure conditions. The present invention also aims to determine favourable methods of making a reversible hydrogen- storage material with the maximum hydrogen storage capacity and with the optimised hydrogen charge and discharge kinetics. The present invention further aims to highlight the use of such a reversible hydrogen storage material in a wide range of commercial applications.

In one embodiment the present invention provides a reversible hydrogen- storage material comprising: a) an active material, and b) a catalyst material, wherein the active material comprises one or more metal oxides having a conductivity of greater than 10^"10 Siemens.m^"1.

The fact that a metal oxide can be used to produce a reversible hydrogen-storage material is highly surprising. Firstly, unlike the metal hydride materials used previously, metal oxides are not capable of storing hydrogen. However, the applicant has surprisingly discovered that metal oxides can be made to be capable of storing hydrogen if they are used in the presence of a catalyst material; consequently, the presence of the catalyst material is critical to the present invention. Secondly, it is expected, from Chapter 7, by P.G. Dickens and A. Chippindale, in 'Proton Conductors', pplOl-121 Cambridge University Press - 1992, that H₂O would be discharged from hydrogen-containing metal oxides. However, this is not the case and the Applicant has observed discharge of diatomic hydrogen.

The Applicant has found that it is preferable that the active material comprises a metal oxide compound with high conduction properties and that it has a fluorite-type crystalline structure.

The conductivity (σ) of a solid is derived from the sum of two contributing factors: 1) the electronic conductivity, that is, the electron motion within the solid, and 2) the ionic conductivity of the solid, that is, the motion of the ions within the solid. Looking at the latter, ions move within a solid due to the presence of intrinsic defects such as holes or vacancies within the lattice structure, and also due to extrinsic defects such as impurities. The value of ionic conductivity is, therefore, a function of the degree of crystallinity of the solid and the amount of impurities weakly bonded to the solid, such as OH and CO. There are conventional methods that can be used to measure the conductivity of a solid, one of which is detailed in the Examples below. The applicant has found that active materials with a conductivity of higher than 10^"10 Siemens.m^"1 produce excellent reversible hydrogen- storage materials. Preferably, the conductivity is higher than 2xlO^"10 Siemens.m^" \ further preferably higher than 5xlO^"10 Siemens.m^"1 and particularly preferably higher than 1 OxIO^"10 Siemens.m^"1.

The fluorite-type crystalline structure can arise either because this is a natural crystalline form of the metal oxide, or because it can be stabilized in the fluorite form by introducing dopants. It is particularly preferred that the fluorite-type crystalline structure is stable at room temperature.

It is preferred that the catalyst material is deposited on the outer surface of the active material, for example as a substantially complete shell or as an incomplete shell, with the active material acting as a core material. Further, it is particularly advantageous that the catalyst material is deposited, either randomly or substantially evenly over the surface of the active material to provide one or more catalytically active sites on the active material. In this arrangement, the catalyst material has at least part of its surface exposed to the hydrogen being stored and the reversible hydrogen- storage material provides an interface between the catalyst material and the active material.

The metal oxide compounds used in the reversible hydrogen storage material of the present invention include one or more of ThO₂, TiO₂, CeO₂, ZrO₂, UO₂, TbO₂, PaO₂, HfO₂ and PrO₂ Preferred metal oxides include CeO₂, PrO₂ and ThO₂ Particularly preferred metal oxides are CeO₂ and PrO₂

The catalyst materials used in the reversible hydrogen- storage material of the present invention include any metal element that reacts with gaseous hydrogen; such metal elements are extremely well known to those skilled in the art. Preferred metal elements include one or more of Ir, Ni, Pd, Pt, Rh, Ru, Au, Ag, Fe and Cu, and particularly preferred metal elements include one or more of Ni, Cu, Pd and Pt.

The applicant has found that it is particularly advantageous when the reversible hydrogen- storage material is a powdered nano-composite material with a high surface area so as to favour the interaction between the storage material and the hydrogen being stored. There are two factors that determine overall storage capacity of the reversible hydrogen storage material. First, the amount of hydrogen stored depends on the density of the active material, and second, the storage capacity at the surface of the active material depends upon the surface area of the reversible hydrogen- storage material. In theory, the larger the surface area, the larger the surface storage capacity. However, taking practical considerations into account it is convenient if the surface area of the reversible hydrogen storage material is in the range 50m²/g to 400m²/g, preferably, 100m²/g to 300m²/g.

The amount of catalyst material used is chosen to allow good coverage by the catalyst material over the surface of the active material whilst maintaining the efficiency of hydrogen storage. A suitable ratio of the number of moles of the catalyst material (metal) / number of moles of the active material (in its oxide form) is in the range 1 - 0.05, and preferably in the range 0.5 - 0.1. The preferred reversible hydrogen- storage materials of the present invention comprise an active material selected from one or more of cerium dioxide and praseodymium dioxide, and a catalyst material selected from one or more of nickel metal and copper metal.

In another embodiment the invention also provides a method of reversible hydrogen- storage using a reversible hydrogen- storage material comprising: a) an active material, and b) a catalyst material, wherein the active material comprises one or more metal oxides having a conductivity of greater than 10^~10 Siemens.m^"1.

A suitable reversible hydrogen- storage material for use in this method of hydrogen storage is as described above.

The present invention further provides a process for manufacturing a reversible hydrogen- storage material such as the one previously described, characterized by the fact that it comprises an activation phase of the storage material.

Thus, in one embodiment the present invention provides a process for manufacturing the reversible hydrogen- storage material comprising:

a) a formation step during which an inactive precursor form of the reversible hydrogen- storage material is formed; and b) an activation step during which the inactive precursor form of the reversible hydrogen- storage material is activated, wherein the activation step comprises: i) a first phase during which the inactive precursor form of the reversible hydrogen- storage material is heated to an activation temperature under a reducing atmosphere; and ii) a second phase, during which the heated inactive precursor form of the reversible hydrogen- storage material is further heated to a discharging temperature under an atmosphere empty of hydrogen or oxidant species to form the reversible hydrogen- storage material. The formation step may be carried out using any one of the many methods well known to those skilled in the art, for example, in the field of reaction catalyst manufacture. Reaction catalysts are conventionally prepared by depositing active catalyst metal particles onto a chemical substrate with a high surface area. The most widely used preparative methods are performed in liquid media, for example, impregnation, equilibrium adsorption, coprecipitation, a sol gel procedure, an inverse micelle method, deposition precipitation and spray drying. Alternative methods include mechanical mixing using a mortar and pestle or ball mill and a hydrogen arc plasma method. However, the applicant has surprisingly found that when the active material and the catalyst material are combined together in such a way to promote intimate mixing of these components at the atomic level, a substantial increase is obtained in the ability of the resultant reversible hydrogen- storage material to store hydrogen. In particular, the Applicant has found that aqueous precipitation, also known as coprecipitation, is a convenient process for preparing the inactive precursor form of the reversible hydrogen- storage material used in the present invention. Such a process is described in some detail in WO 98/45212. Moreover, the applicant has found that conducting the coprecipitation process in a microreactor, is an excellent way to obtain the target material on a commercial scale.

Typically, coprecipitation produces hydrous hydroxides in water using acid-base neutralisations or ion exchange reactions. The starting materials are soluble metal salts, frequently including nitrates, carbonates and halides, which are dissolved together in distilled water, preferably at a total salt concentration of 10OgL^"1 to 20OgL^"1 ; the resulting solution is 'neutralised' by adding it to an aqueous alkali, for example an aqueous ammonium solution or a potassium hydroxide solution, to give a co-precipitate product which is then filtered and washed.

In the present invention, the coprecipitate product comprises a metal oxide active material core with a catalyst material in the form of a metal oxide / oxyhydroxide, deposited thereon. A preferred coprecipitate process uses cerium nitrate (Ce(NOs)₃-OH₂O) and nickel nitrate (Ni(NO₃)₂.6H₂O) as the starting materials.

When using a coprecipitation process as the formation step, or another process that also leads to the active material being in metal oxide form, the product of the formation step is preferably heat treated in an optional firing step. This is to clean the product by eliminating all the physisorbed and weakly chemisorbed species. The optional firing step should be carried out prior to the activation step.

The applicant has found that the firing temperature has a direct effect on the ability of the final reversible hydrogen- storage material to store hydrogen. If the firing temperature is too high then it is observed that the conductivity of the metal oxide reduces, and this consequently reduces hydrogen storage in the final reversible hydrogen- storage material. The maximum firing temperature can be defined as the highest temperature before which the conductivity of the metal oxide active material drops. A suitable firing temperature range is from 150⁰C to below 600⁰C, preferably from 200⁰C to 550⁰C, further preferably from 200⁰C to 500⁰C, and particularly preferably from 200⁰C to 450⁰C. Conveniently, the resulting product is very stable in air and may be stored at normal temperatures.

The purpose of the activation step is to 'activate' the inactive precursor form of the reversible hydrogen- storage material, that is to say to make it capable of reversibly absorbing or fixing a large quantity of hydrogen under temperature and pressure conditions close to normal conditions, i.e. around 25°C and around 1 bar. The activation step involves 2 separate phases. During a first phase, the inactive precursor form of the reversible hydrogen- storage material is heated to an activation temperature under a reducing atmosphere, for example under carbon monoxide, hydrogen or an atmosphere containing hydrogen diluted with an inert gas, at a heating rate of 10⁰CnUn^"1 . Then, the inactive precursor form of the hydrogen- storage material is held at the activation temperature, for several hours, for example from 1 to 24 hours, preferably at least 6 hours. The activation temperature, can be defined as the temperature sufficient to remove the oxygen atom of the metal oxide / oxyhydroxide catalyst material in the inactive precursor form of the reversible hydrogen- storage material. A suitable activation temperature to obtain a final reversible hydrogen- storage material with optimum storage capacity is up to 500⁰C, preferably in the range from 200⁰C to 500⁰C, particularly from 250⁰C to 450⁰C. The first phase of the activation step, is completed by maintaining the reducing atmosphere, and gradually reducing the temperature down to room temperature, around 25°C, for example at a cooling rate of 20⁰CnUn^"1. During a second phase of the activation step, the heated inactive precursor form of the reversible hydrogen- storage material obtained from the first phase is further heated to a discharging temperature under an atmosphere empty of hydrogen or oxidant species. The purpose of this second phase, is to ensure that the heated inactive precursor form of the reversible hydrogen- storage material obtained from the first phase is discharged of all hydrogenated species absorbed during the first phase, and to obtain a final reversible hydrogen-storage material that is ready to be charged with hydrogen.

Suitable examples of an atmosphere empty of hydrogen or oxidant species include an atmosphere of argon or helium or a vacuum. Preferably the heated inactive precursor form of the reversible hydrogen- storage material obtained from the first phase is heated at a heating rate of 10⁰CmIn^"1, up to a discharging temperature. A suitable discharging temperature is less than 600⁰C, preferably below 550⁰C, further preferably below 500⁰C, and particularly preferably below 450⁰C. The discharging temperature is maintained for a period of time sufficient to desorb the hydrogen bearing species that were absorbed during the first phase, for example from 1 hour to 24 hours. At the end of the second phase the resulting reversible hydrogen- storage material is cooled to room temperature at a cooling rate of 20⁰CnUn^"1, under an atmosphere empty of hydrogen or oxidant species.

In the method of the present invention, hydrogen is stored by the reversible hydrogen- storage material using a charging operation comprising heating the reversible hydrogen- storage material to a storage temperature under a dihydrogen containing atmosphere; preferably, the storage temperature is less than the activation temperature, and a suitable storage temperature lies between O⁰C and the activation temperature. Excellent hydrogen storage can be achieved when the reversible hydrogen- storage material of the present invention is charged using the above storage operation at atmospheric pressure, however, it is particularly advantageous that during the storage operation, the reversible hydrogen- storage material is subjected to a pressure in the range from 0.5 bar to 10 bar, preferably 0.5 to 5 bar. Notwithstanding this, even if the storage operation takes some time, the energy required for the storage operation is favourably very low.

In the method of the present invention, hydrogen is discharged from the reversible hydrogen storage material using a hydrogen discharging operation comprising heating the reversible hydrogen storage material charged with hydrogen to a discharging temperature, under vacuum or under an atmosphere empty of hydrogen species. A suitable discharging temperature is less than 600⁰C, preferably below 550⁰C, further preferably below 500⁰C and particularly preferably below 450⁰C.

The present invention also provides for the use of the reversible hydrogen- storage material described above, in a wide range of commercial applications; some of these are mentioned below. All of the applications described, below capitalise on the ability of the reversible hydrogen- storage material of the present invention to fix and absorb hydrogen. For example:

in the field of energy storage; the reversible hydrogen- storage material of the present invention can be used either to store hydrogen in tanks or by some other means, for example, to facilitate the use of hydrogen to power fuel cell devices and combustion engines; in the field of gas purification; the reversible hydrogen- storage material of the present invention can be used either as a trapping system to capture traces of hydrogen in a neutral gas such as argon, helium or nitrogen, or as a hydrogen filtering medium, such as a membrane, that can be used to extract hydrogen from a gaseous mixture. in the field of Ultra High Vacuum; the reversible hydrogen- storage material of the present invention can be used to trap any remaining hydrogen molecules to maintain the vacuum in sealed environments. in the field of safety device design; the reversible hydrogen- storage material of the present invention can be used in the design of hydrogen selective sensors. in the field of the reaction catalyst manufacture; the reversible hydrogen- storage material of the present invention can provide a 3D source of active hydrogen for hydrogenation reactions. For such a use, it is envisaged that the reversible hydrogen storage material may comprise a refractory oxide support such as alumina, zirconia, titania, silica, zinc oxide and zeolites. The invention will now be described with reference to the following examples.

As explained above, the process for manufacturing the reversible hydrogen- storage material of the present invention involves a formation step, an activation step, and optionally, a firing step. These steps will now be described in some detail.

The formation Step

The formation step is conveniently carried out using a co-precipitation method to produce an inactive precursor form of the reversible hydrogen- storage material of the present invention.

General Co-precipitation Method to form a Co-precipitate Containing An Active Material of Metal oxide and A Catalyst Material of metal oxide/oxyhydroxide.

First, catalyst metal nitrate and active material metal nitrate salts are dissolved in distilled water (150 ml) (total salt concentration catalyst material metal + active material metal is in the range 100 g.L ¹ to 200 g.L ¹). This solution is then added drop-wise to a basic solution of triethylamine (400 ml) under moderate stirring at 6O⁰C. Initially, a white/orange co-precipitate forms and this changes rapidly into a brown/olive co- precipitate that is kept at 6O⁰C under stirring for one hour. Following this, the solution is cooled to room temperature and maintained under moderate stirring overnight. The resulting brown precipitate is filtered using a sintered disk funnel with a water jet- aspirator, to give a solid that is washed, first with pure ethanol (800 ml) to remove traces of triethylamine and then with boiling distilled water (approx. 1.51) until the filtrate washing solution has a neutral pH, i.e. a pH of about 7.

The washed precipitated powder is then collected in a PTFE beaker and suspended in ethanol (200 ml). The obtained suspension is stirred in a sonicator for three hours and the stirring is maintained overnight without ultrasound. The aim of this step is to perfectly disperse the suspended precipitate in ethanol. After this overnight stirring, the brown/orange suspension is returned to the sonicator for one hour. The resulting suspension is then cooled using a lyophilization method, in order to get a solid with high surface area, by placing the PTFE beaker into a polystyrene box and pouring liquid nitrogen onto the suspension. The polystyrene box is then filled with liquid nitrogen (11) and closed for about twenty minutes. During this time, the suspension freezes to produce a solid lump.

Finally, this solid lump is dried, by placing it in a desiccator under vacuum at room temperature, using a liquid nitrogen trap to condense the ethanol. After two days and two nights under vacuum, a dried brown/orange powder is obtained of a co-precipitate containing metal oxide active material and metal oxide/oxyhydroxide catalyst material. The surface area of the powder is from 50m²/g to 400m²/g.

The general method outlined above was used to prepare a number of different co- precipitates containing metal oxide active material and metal oxide/oxyhydroxide catalyst material. The exact starting materials and the quantities used is detailed in Table 1 below.

Table 1

Firing Step

The inactive precursor of the reversible hydrogen- storage material produced in each of Examples 1-6 by the above formation step is in the form of a co-precipitate that contains an active material of metal oxide and a catalyst material of metal oxide/oxyhydroxide. These materials were then treated to a firing temperature of 150⁰C for 1 hour under vacuum to clean the product by eliminating all physisorbed and weakly chemisorbed species.

Effect of Firing Temperature on the Conductivity and hydrogen storage capacity of the Active Material

As described above, firing temperature has a marked effect on the ability if the final reversible hydrogen- storage material to store hydrogen. In addition, the applicants have observed that when the firing temperature is too high, the conductivity of the active material is reduced. It is clear, therefore, that the conductivity must be maintained above a certain level to preserve hydrogen storage capability. To show the effect of firing temperature on conductivity, a co-precipitate containing an active material of metal oxide and a catalyst material of metal oxide/oxyhydroxide was subjected to firing under air for 1 hour at different temperatures from 150⁰C to 950⁰C. The conductivity values were measured at 25°C under air and are reported in Table 2 below.

Table 2

As these results show, conductivity drops significantly as the firing temperature is increased from 450⁰C to 650⁰C.

Activation Step

First phase:

Each of the heated inactive precursor reversible hydrogen- storage materials obtained from the firing step above was further heated for 5 hours to an activation temperature of 250⁰C under an H₂/He atmosphere (5/95 weight %) and pressure of about 1.5 bar. Second phase:

The heated inactive precursor form of the reversible hydrogen- storage material obtained from the first phase in respect of the materials of Examples 2, 4 and 6 was further heated from room temperature to 600⁰C under an atmosphere empty of hydrogen or oxidant species and analysed using mass spectrometry to determine the amount of hydrogen emitted. The signal intensity results, given in Table 3, provide an indication of the amount of released hydrogen; the higher the current (signal intensity), the greater the amount of stored hydrogen being emitted.

Table 3

The reversible hydrogen- storage material obtained at the end of the second phase is ready to be charged with hydrogen under optimum storage conditions.

Measurement of Conductivity

A powdered sample of the material whose conductivity is being measured is pressed to form a disk pellet with a diameter equal to 13mm. Pressure in the 2.5 to 3.5 tonne range is applied and maintained for 15 minutes. The pellet is then placed into a conductivity cell (quartz tube) with 2 platinum electrodes connected to a complex impedance spectrometer analyser able to operate at frequencies between 0.1 Hz to 32Hz in order to measure resistances between lOmΩ to 100MΩ.

The applied alternative current has a maximum voltage amplitude of 300 mV.

The conductivity, σ, = e / (RS) where

S = surface area of the pellet m² e = pellet thickness m R = resistivity Ω

Optimisation of The Hydrogen Storage and Discharge Conditions for the Reversible Hydrogen-Storage Material of the present Invention.

The duration of the storage operation, and also the storage temperature have a marked effect on the amount of hydrogen stored; the amount of hydrogen increases as the duration of the storage operation increases, until a stable quantity of stored hydrogen, corresponding to the saturation level of the hydrogen storage material, is obtained.

The reversible hydrogen- storage material obtained in respect of Example 1 was subjected to a storage operation involving heating to a storage temperature of 250⁰C, under 1 bar of a dihydrogen diluted at 95% in a helium atmosphere for varying periods of time. The results are summarized below in Table 4.

Table 4

When the reversible hydrogen- storage material is saturated or "filled" with hydrogen, it is possible to discharge the hydrogen by submitting, again, the "filled" storage product to a discharge temperature and pressure conditions, during a desorbing or a discharging operation.

During the discharging operation, the "full" storage material is heated up to a discharging temperature of below 600⁰C under vacuum or an atmosphere empty of hydrogen. This heating induces desorption, in gas form, of the majority of the hydrogen adsorbed during the charging operation. Table 5 below presents results of discharging operations conducted on a storage material sample form Example 1, "filled" during a previous storage operation. In this example, previous storage operations have been conducted for 2 hours, under a 1 bar pressure of a diluted at 95% in helium hydrogen atmosphere. Each previous storage operation has been conducted at a different storage temperature, 5O⁰C, 15O⁰C and 25O⁰C.

Table 5

The reversible hydrogen- storage material of the present invention is able to desorb at least 0.7 g of molecular hydrogen per lOOg of material.

The reversible hydrogen- storage material of the present invention is able to undergo several charging/discharging cycles.

Claims

1. A reversible hydrogen- storage material comprising:

an active material, and a catalyst material.

and wherein active material comprises one or more metal oxides having a conductivity of greater than 10^~10 Siemens.m ¹.

2. A reversible hydrogen- storage material according to claim 1 wherein the one or more metal oxides have a conductivity of greater than 2xlO^~10 Siemens.m^"1.

3. A reversible hydrogen- storage material according to claims 1 or 2, wherein the active material has a fluorite structure.

4. A reversible hydrogen- storage material according to claims 1, 2 or 3 wherein the catalyst material is deposited on the outer surface of the active material.

5. A reversible hydrogen- storage material according to claim 4 wherein the catalyst material is deposited on the outer surface of the active material to provide one or more catalytically active sites on the active material.

6. A reversible hydrogen- storage material according to any preceding claim wherein the one or more metal oxides is selected from ThO₂, TiO₂, CeO₂, ZrO₂, UO₂,

TbO₂, PaO₂, HfO₂ and PrO₂

7. A reversible hydrogen- storage material according to any preceding claim wherein the one or more metal oxides is selected from CeO₂ and PrO₂

8. A reversible hydrogen- storage material according to any preceding claim wherein the catalyst material comprises one or more metals selected from Ir, Ni, Pd, Pt, Rh Ru, Au, Ag, Fe and Cu.

9. A reversible hydrogen- storage material according to any preceding claim wherein the catalyst material comprises one or more metals selected from Ni, Pt, Pd and Cu.

10. A reversible hydrogen- storage material according to any preceding claim wherein the catalyst material comprises nickel metal and/or copper metal.

11. A reversible hydrogen- storage material according to any preceding claim wherein the surface area of the reversible hydrogen- storage material is in the range 50m²/g to 400m²/g.

12. A reversible hydrogen- storage material according to any preceding claim wherein the ratio of the number of moles of the catalyst material (metal) / number of moles of the active material (in its oxide form) is in the range 1 - 0.05.

13. A reversible hydrogen- storage material according to claim wherein 12 wherein the ratio is in the range 0.5 - 0.1.

14. A method of reversible hydrogen- storage comprising the use of a reversible hydrogen- storage material, wherein the reversible hydrogen- storage material comprises: an active material, and a catalyst material

and wherein active material comprises one or more metal oxides having a conductivity of greater than 10^"10 Siemens.m ¹.

15. A method of reversible hydrogen- storage according to claim 14, wherein the one or more metal oxides have a conductivity of greater than 2xlO^"10 Siemens.m^"1.

16. A method according to claims 14 or 15 wherein the active material has a fluorite structure.

17. A method according to any of claims 14, 15 or 16 wherein the catalyst material is deposited on the outer surface of the active material.

18. A method according to claim 17 wherein the catalyst material is deposited on the outer surface of the active material to provide one or more catalytically active sites on the active material.

19. A method according to any of claims 14 tol8 wherein the one or more metal oxides is selected from ThO₂, TiO₂, CeO₂, ZrO₂, UO₂, TbO₂, PaO₂, HfO₂ and PrO₂

20. A method according to any of claims 13 to 19 wherein the one or more metal oxides is selected from CeO₂ and PrO₂

21. A method according to any of claims 13 to 18 wherein the catalyst material comprises one or more metals selected from Ir, Ni, Pd, Pt, Rh Ru, Au, Ag, Fe and Cu.

22. A method according to any of claims 13 to 21 wherein the catalyst material comprises one or more metals selected from Ni, Pt, Pd and Cu.

23. A method according to any of claims 13 to 22 wherein the catalyst material comprises nickel metal and/or copper metal.

24. A method according to any of claims 13 to 23 wherein the surface area of the reversible hydrogen- storage material is in the range 50m²/g to 400m²/g.

25. A method according to any of claims 13 to 24 wherein the ratio of the number of moles of the catalyst material (metal) / number of moles of the active material (in its oxide form) is in the range 1 - 0.05.

26. A method according to claim 25 wherein the ratio is in the range 0.5 - 0.1.

27. A process for manufacturing the reversible hydrogen- storage material of claims 1- 13, comprising the steps:

a formation step during which an inactive precursor form of the reversible hydrogen- storage material is formed; and an activation step during which the inactive precursor form of the reversible hydrogen- storage material is activated, wherein the activation step comprises: a first phase during which the inactive precursor form of the reversible hydrogen- storage material is heated to an activation temperature under a reducing atmosphere; and a second phase, during which the heated inactive precursor form of the reversible hydrogen- storage material obtained in the first phase is further heated to a discharging temperature under an atmosphere empty of hydrogen or oxidant species, to form the reversible hydrogen- storage material.

28. A process according to claim 27 further comprising a firing step during which the inactive precursor form of the reversible hydrogen- storage material obtained in step a), is heated to a firing temperature, wherein the firing temperature is from 150⁰C to below 600⁰C.

29. A process according to claim 28 wherein the temperature of the firing step is from 200⁰C to 550 ⁰C.

30. A process according to claims 27 or 28 wherein the activation temperature is up to 500 ⁰C.

31. A process according to claim 30 wherein the activation temperature is in the range from 200⁰C to 500⁰C.

32. A process according to claims 27 or 28 wherein the discharging temperature is below 600⁰C.

33. A process according to claims 27 or 28 wherein activation temperature during the first phase is held for 1 to 24 hours.

34. A process according to claims 27 or 28 wherein the discharging temperature during the second phase is held for 1 to 24 hours.

35. A process according to claims 27 or 28 wherein the formation step comprises the co-precipitation of cerium nitrate (Ce(NOs)₃-OH₂O) and nickel nitrate (Ni(NO₃)₂.6H₂O) to form an inactive precursor form of the reversible hydrogen- storage material.

36. A method of storing hydrogen using the reversible hydrogen- storage material according to claims 1 to 13 using a hydrogen charging operation, wherein the hydrogen charging operation comprises heating the reversible hydrogen- storage material to a storage temperature under a hydrogen atmosphere, and further wherein the storage temperature is up to 500⁰C.

37. A method of discharging hydrogen using the reversible hydrogen- storage material charged by the method of claim 36 in a hydrogen discharging operation, wherein the hydrogen discharging operation comprises heating the reversible hydrogen- storage material charged with hydrogen to a discharging temperature, wherein the discharging temperature is less than or equal to the activation temperature, and further wherein the discharging operation is conducted under vacuum or under an atmosphere empty of hydrogen species.

38. An inactive precursor form of the reversible hydrogen- storage material produced by the process according to any of claims 27, 28, 29 and 35.

39. An inactive precursor form of the reversible hydrogen- storage material formed during the process of any of claims 27, 28, 29 and 35, wherein the active material comprises one or more metal oxides and the catalyst material comprises one or more metal oxides/ oxyhydroxides.

40. An energy conversion device powered at least in part by hydrogen gas supplied by means comprising a reversible hydrogen- storage material according to claims 1 to 13.

41. An energy conversion device powered at least in part by hydrogen gas supplied by the hydrogen storage method according to claims 14 to 26.

42. An energy conversion device powered at least in part by hydrogen gas supplied by means comprising a reversible hydrogen- storage material manufactured by the process according to claims 27 to 35.

43. A hydrogen gas trap comprising a reversible hydrogen- storage material according to claims 1 to 13.

44. A hydrogen gas trap employing the reversible hydrogen- storage method according to claims 14 to 26.

45. A hydrogen gas trap comprising a reversible hydrogen- storage material manufactured by the process according to claims 27 to 35.

46. A hydrogen selective sensor comprising a reversible hydrogen-storage material according to claims 1 to 13.

47. A hydrogen selective sensor employing the reversible hydrogen- storage method according to claims 14 to 26.

48. A hydrogen selective sensor comprising a reversible hydrogen-storage material manufactured by the process according to claims 27 to 35.