WO2009056962A2

WO2009056962A2 - A system for effective storing and fuelling of hydrogen

Info

Publication number: WO2009056962A2
Application number: PCT/IB2008/002919
Authority: WO
Inventors: Phiroze Patel
Original assignee: Phiroze Patel
Priority date: 2007-11-01
Filing date: 2008-10-23
Publication date: 2009-05-07
Also published as: EP2217848A2; WO2009056962A3; WO2009057127A1

Abstract

A hydrogen storage system provides a pressurized storage tank 21 in a motor vehicle 10, the tank containing a heat exchanger 27 and an adsorbent material 22 comprising a soft mass of carbon micro-fibres in the form of carbon wool with a surface area of about 2250m2 /g - 4000m2 /g, having a metal, preferably palladium, dispersed over the surface of the adsorbent in nano-particulate form. The metal is preferably precipitated in ionic form from a solution, and may be reduced after precipitation to a metallic form by hydrogenation at an elevated temperature. A refuelling station 1 comprises a supply of hydrogen H at a first working temperature of about 36K - 77K and a first working pressure up to about 300 bar, which is supplied to the tank 21 via first releasable couplings 9, 24, and a supply of refrigerated helium gas He at about 20K which is simultaneously circulated via second releasable couplings 8, 24 through the heat exchanger 27 in the tank 21 to offset the heat of adsorption. An auxiliary tank 28 contains compressed hydrogen gas H at ambient temperature which is injected in controlled amounts into the storage tank 21 to offset the heat of desorption as the storage tank empties.

Description

A SYSTEM FOR EFFECTIVE STORING AND FUELLING OF HYDROGEN

This invention relates to technologies for the storage of hydrogen by adsorption, such as for use in fuelling motor vehicles.

It is known from US 7,036,324 to store hydrogen by adsorption at cryogenic temperatures in the range of about 4OK to 8OK onto a high surface area adsorbent such as activated carbon in a storage vessel at a pressure of about 10 bar to 30 bar.

DE 10 2005 023 036 Al teaches the adsorption of hydrogen onto a powdered carbon adsorbent in a pressurized tank cooled by liquid nitrogen to a temperature lying between the respective ebullition temperatures (at normal pressure) of liquid hydrogen and liquid nitrogen. The adsorbent may comprise carbon nanotubes.

US 5,653,951 teaches the adsorption of hydrogen onto an adsorbent comprising a carbon nanostructure, which may be treated with a metal such as Pd, Pt, Ni, Fe, Ru, Os, Co, Rh, Ir, La, or Mg in an amount from lwt% - 5 wt% based on the total weight of the nanostructure.

US 6,672,077 Bl discloses a storage system in which hydrogen is adsorbed by physisorption onto a nanostructured storage material cooled by liquid nitrogen.

US 4,716,736 teaches Metal- Assisted Cold Storage of hydrogen by physisorption at cold, but not cryogenic temperatures onto an activated carbon adsorbent having microcrystals of a Group VIII transition metal such as Ni, Pd, Pt, Co, Rh, Ir, Fe, Ru, or Os dispersed onto its surface; and references US 3,138,560, which discloses a process for depositing palladium onto a carbon catalyst.

The overall mass and volume of the on-board hydrogen storage system is a key impediment to the development of practical hydrogen fuelled vehicles. Adsorption of hydrogen offers advantages in safety, practicality and efficiency over competing technologies, which include storage in metal hydrides, as a liquid, or as a highly compressed gas. However, maximising the storage density (the overall volume of stored hydrogen at ambient pressure relative to the volume of the adsorbent) remains a key challenge. The object of the present invention is to increase the hydrogen storage density achievable by adsorption.

Accordingly in its various aspects the invention provides a system, a method and an apparatus as defined in the claims.

An illustrative embodiment will now be described, purely by way of example and without limitation to the scope of the claims, and with reference to the accompanying figures, in which:

Fig. 1 shows a hydrogen refuelling system comprising a hydrogen storage apparatus, and

Fig. 2 shows the hydrogen storage apparatus of Fig. 1.

Referring to the figures, a hydrogen refuelling system comprises a refuelling station 1 for supplying gaseous hydrogen H to a hydrogen storage apparatus 20 in a motor vehicle 10.

The storage apparatus 20 comprises a thermally insulated on-board storage vessel 21 adapted for pressurization to a first working pressure preferably from 1 bar to 300 bar and containing an adsorbent 22 as further described below. The storage vessel or cylinder 21 may be a tank made for example from stainless steel, titanium or metal or non-metal composite material with a thermally insulating jacket 23 as known in the art and comprising for example a high quality vacuum and reflective layers. In one embodiment, the jacket 23 comprises multiple layers of aluminized mylar or perlite, arranged in a hard vacuum.

The storage vessel 21 is provided with separate or combined inlet and outlet apparatus 24 comprising releasable couplings and valves as known in the art for filling the storage vessel with gaseous hydrogen H and for releasing gaseous hydrogen H from the storage vessel via a supply line 25 to the demand (for example, the fuel cell or engine that powers the vehicle). While the vehicle is in use, the release of gaseous hydrogen via the supply line 25 cools the adsorbent by the heat of desorption, offsetting the ambient heat load via the insulating jacket 23. A pressure relief valve 26 may be provided to release hydrogen from the vessel 21 when the vehicle is not in use, which relieves excess pressure within the vessel and also cools the adsorbent containing the remaining stored hydrogen by the heat of desorption.

The vessel 21 also includes a cooling apparatus which preferably comprises a heat exchanger 27, which may be a coil, an inner vessel, an outer jacket or any other structure as known in the art and adapted to contain a circulating coolant and thermally connected to the interior of the vessel 21. The heat exchanger 27 contains a coolant, the coolant comprising gaseous helium He for cooling the adsorbent to a first working temperature between about 36K and about 77K. The inlet and outlet apparatus 24 also includes detachable cryogenic couplings and valves as known in the art for connecting the heat exchanger 27 to an external circulating supply of refrigerated gaseous helium He.

Helium is particularly preferred for use in the cooling apparatus since it remains gaseous at very low temperatures, although other suitable gases or cooling technologies (e.g. liquid nitrogen) may be used in alternative embodiments or at other working temperature ranges. Optionally, a heating apparatus is provided for warming the adsorbent. The heating apparatus may comprise for example a heat exchanger in thermal contact with the adsorbent for carrying a flow of exhaust gas, such as from a fuel cell or engine of the vehicle, or a flow of heated fluid, such as from a cooling system of the vehicle.

In a particularly preferred embodiment, the heating apparatus comprises an auxiliary tank 28 which contains compressed hydrogen gas, which may be supplied via a filling valve 29, either when the vehicle is re-fuelled or, conveniently, from the supply line 25 or pressure relief valve 26. A warm hydrogen injection apparatus 30 is arranged to selectively inject controlled quantities of warm hydrogen gas (which is to say, hydrogen gas at a temperature higher than the first working temperature of the vessel 21) from the auxiliary tank 28 into the storage vessel 21 to warm the adsorbent, offsetting the heat of desorption so as to maintain the rate of hydrogen desorption when the vehicle is in use, particularly when the tank is nearly empty or during periods of high demand, e.g. when the engine of the vehicle is under heavy load. This allows substantially all of the stored hydrogen to be released from the vessel 21 as it empties. Conveniently, the auxiliary tank 28 contains compressed hydrogen gas at ambient temperature, and does not comprise an adsorbent.

The refuelling station 1 includes a supply of cooled hydrogen H. The hydrogen is drawn from a source 2, which may comprise a hydrogen generator, a cascade of pressurized cylinders, a liquefied hydrogen tank, or any other bulk storage or supply arrangement as known in the art.

The source 2 supplies gaseous hydrogen, if necessary via a compressor 3 which boosts the pressure to the first working pressure of the on-board storage vessel 21, to a thermally insulated intermediate storage vessel 4. The intermediate storage vessel 4 has a heat exchanger 5 which is connected to a circulating supply 6 of gaseous helium He cooled by a cryogenic refrigeration apparatus 7 to a temperature below the first working temperature. For example, the helium gas may be cooled to about 2OK. The hydrogen gas H is held within the intermediate storage vessel 4 (which need not contain an adsorbent) at the first working temperature and first working pressure so as to minimize the time required for refuelling.

The refuelling station also provides coolant connection apparatus 8 and hydrogen connection apparatus 9, comprising couplings and valves as known in the art, which are adapted for connection to corresponding couplings of the inlet and outlet apparatus 24 of the on-board storage vessel 21 so as to connect respectively the circulating supply of refrigerated gaseous helium to the on-board heat exchanger 27, and the supply of cooled hydrogen gas H to the on-board storage vessel 21. The helium and hydrogen couplings and valves 8, 9 may be separate, or may be integrated into a combined multiple coupling assembly, which may have separate flexible, thermally insulated hydrogen and helium supply lines or alternatively may have supply lines that are thermally connected, e.g. arranged one inside the other, and insulated by an external jacket so as to minimise the ambient heat load during transfer between the refuelling station and the vehicle.

In use, the couplings of the connection and inlet/outlet apparatus 8, 9, 24 are connected, and the supply of compressed gaseous hydrogen H cooled by the heat exchanger 5 is introduced into the storage vessel 21 until the storage vessel is pressurized to the first working pressure. Gaseous helium He cooled by the refrigeration apparatus 7 is simultaneously circulated through the heat exchanger 27 to offset the heat of adsorption, maintaining the adsorbent 22 at the first working temperature. The combination of pre-cooling of the hydrogen supply at the first working pressure and cooling of the adsorbent by a separate coolant circuit minimises refuelling time. In a development, the hydrogen supply may be held at a pressure higher than the first working pressure so that the adsorbent is further cooled by expansion of the hydrogen gas as it passes into the tank 21. The cooled hydrogen supply may also be circulated through the tank 21 so as to cool the adsorbent while part of the supply is adsorbed. In alternative embodiments, liquid or gaseous hydrogen may be supplied directly from the source 2 at a temperature at or below the first working temperature.

Preferably, the first working temperature lies in the range from about 36K to about 77K and the first working pressure lies in the range from about 1 bar to about 300 bar, more conveniently from about 1 bar to about 200 bar. In less preferred embodiments, higher or lower temperatures and/or higher pressures may be employed.

Preferably, the adsorbent 22 comprises activated carbon micro- fibres, on whose surface a metal is dispersed in finely divided, nano-particulate or nano-crystalline form in an amount of about lwt% - 5 wt% by weight of the adsorbent. In this specification, a micro-fibre is taken to be a filamentary element having a diameter or transverse dimension in the range from about 1 micron to about 100 microns, and a length of at least 10 times its diameter or transverse dimension.

In alternative embodiments the adsorbent may comprise carbon coated ceramic fibres, nano-structured carbon material, carbon powder, carbon granules, or other high surface area material as known in the art.

However, the applicant has found that carbon nano-structures can be problematic when used as adsorbents due to difficulties in handling and a propensity to coalesce into a solid mass, which may limit overall permeability, in addition to their high cost. It is hypothesised that this may represent a practical limit on the hydrogen storage density achievable using a nano-structured carbon adsorbent. Most preferably, the adsorbent is a carbon wool comprising a randomly oriented mass of activated carbon micro-fibres, for example, Kynol (TM) activated carbon micro-fibres, which are made from novoloid precursor fibres and commercially available from American Kynol, Inc., Pleasantville, NY, USA. The carbon wool may comprise micro-fibres having a diameter of about 8 microns to about 9 microns and a length of about lmm to about 5 mm, which may be spun into larger (macro scale) fibres, and is packed into the tank to form a soft, compact but highly permeable mass with a density of at least about lOOg/litre. The carbon wool is so called because it resembles natural wool, having similar mechanical properties in that it resists compaction, remaining soft and fluffy as the tank is pressurized so that its entire surface area (preferably in the range of at least about 2250 m²/g or more) is available for adsorption of the hydrogen gas.

It is known to produce activated carbon micro-fibres from a variety of alternative precursor materials including novoloid (cured phenol-aldehyde), polyacrylonitrile and petroleum pitch. The precursor material may be carbonized and activated by a process or a series of processes as known in the art, such as by pyrolysis followed by oxidation, to produce the adsorbent.

Advantageously, the adsorbent is oxidised by exposure to an atmosphere of CO2 at an elevated temperature, for example, about 800⁰C, to achieve a surface area of at least 2000m²/g, preferably at least 3000m²/g, most preferably at least 4000m²/g.

Surprisingly, applicant's experiments and calculations indicate that activated carbon micro-fibres in the form of carbon wool onto whose surface ionic palladium is precipitated in finely divided form as nano-particles or nano-crystals, can store as much as 300 kg hydrogen or more per cubic metre tank volume at a relatively modest pressure of not more than 300 bar, when cooled to a temperature in the range from 36K to 77K. This represents a substantial increase over the storage density hitherto achieved by adsorption. It is believed that the advantageous effect is due to a combination of adsorption and interstitial storage.

Preferably, a metal is dispersed over the adsorbent by precipitation from a solution in nano-crystalline or nano-particulate form.

The metal may be a transition metal.

The transition metal is preferably a Group VIIIB transition metal selected from the group consisting of Ru, Rh, Pd, Os, Ir, and Pt. More preferably, the Group VIIIB transition metal is selected from the group consisting of Ru, Rh, Pd, and Pt. Most preferably the metal is Pd.

In an alternative embodiment, the metal is selected from the group consisting of Na and K, more preferably K.

Preferably the metal is dispersed on the adsorbent in ionic form, and it is believed that the ionic form is more effective than the corresponding metallic form in assisting hydrogen adsorption onto the carbon adsorbent.

In a development, the carbon adsorbent carrying the ionic metal may then be exposed for a few hours to a dry, hot atmosphere of gaseous hydrogen at an elevated temperature of about 400⁰C to about 600⁰C, which reduces the ionic metal (e.g. ionic palladium) by hydrogenation to the metallic form (e.g. metallic palladium). This process of hydrogenation of the ionic metallic precipitate is also believed to enhance the hydrogen adsorption process although the mechanism is not fully understood.

In a particularly preferred embodiment, a solution of palladium chloride (PdCl₂) is applied to the carbon material and reacted with 20% hydrochloric acid, and then reduced with formaldehyde, precipitating crystalline nano-particles of ionic palladium onto the carbon adsorbent. This process may be carried out in a series of tanks.

Example 1

In a first experimental example, a cylindrical stainless steel storage vessel with a coiled tubular internal heat exchanger and an internal capacity (excluding the heat exchanger) of 1.55 litres, and surrounded with cryogenic insulation, was filled with 155g of carbon wool adsorbent comprising Kynol (TM) activated carbon micro-fibres which was packed into the storage vessel to surround the heat exchanger and compacted by hand to a density of lOOg/litre. The carbon wool was prepared by immersing it in a solution of palladium chloride (PdCl₂), and then reacting the solution with 20% hydrochloric acid, and then reducing it with formaldehyde, precipitating crystalline nano-particles of ionic palladium onto the carbon adsorbent in an amount of 5wt.% Pd with respect to the total mass of the carbon adsorbent to give the combined mass of 155g.

The storage vessel was refrigerated by circulating helium gas through the heat exchanger, with the temperature of the adsorbent being recorded by a temperature probe arranged inside the storage vessel.

Compressed hydrogen gas was then supplied directly to the storage vessel at room temperature from a cascade of pressurised, uninsulated cylinders, so that the hydrogen gas was cooled by contact with the adsorbent as it flowed into the storage vessel. The supplies of hydrogen gas and helium coolant were turned off once the adsorbent temperature had stabilised and the flow of hydrogen into the storage vessel had stopped, indicating that maximum hydrogen adsorption had been achieved, with the measured parameters at this time being indicated in Table 1 below as "starting parameters". It should be noted that, although for convenience in the experimental method the hydrogen was supplied at room temperature and cooled within the storage vessel, this is not expected to have any substantial effect on the starting parameters as against the preferred method in which the hydrogen gas is cooled before it is supplied to the storage vessel, although the latter method is expected to greatly reduce the time required for filling the storage vessel.

The hydrogen stored within the storage vessel was then discharged via a discharge valve through a mass flow meter, with the temperature of the adsorbent, the pressure within the storage vessel, and the flow rate of hydrogen gas discharged from the storage vessel being recorded at 2-second intervals. The measured parameters, sampled at 10-second intervals starting from the initiation of hydrogen discharge from the storage vessel, are shown in Table 1 as follows:

Table 1

A total of 54 grams of hydrogen gas was discharged from the 1.55 litre storage vessel during a period of 206 seconds, during which time the pressure in the storage vessel dropped by 97.0 bar and the temperature in the storage vessel fell from 11 IK to 80K as the hydrogen gas was desorbed from the carbon adsorbent. 54g in 1.55L equates to a hydrogen storage density of 34.8 kg/m storage vessel volume.

The applicant has extrapolated from these figures based on the formula: ((P₁ x V₁) / T₁)) = ((P₂ x V₂) / T₂)) , wherein P = drop in pressure in the storage vessel during the discharge cycle, T — initial temperature in the storage vessel, and V = storage vessel volume = (mass of stored hydrogen/density of stored hydrogen), wherein V is a constant.

The applicant believes that hydrogen storage density will increase linearly and positively with pressure and will also increase linearly and inversely with temperature. Hence for a starting temperature of 11 IK and a pressure drop of 200 bar (corresponding to an initial storage pressure of about 201 bar when the storage vessel is fully charged, and a final pressure after complete discharge of about 1 bar), the applicant calculates by extrapolation from the figures given in example 1 that the achievable hydrogen storage density will equate to (200/97) x 34.8 = 71.8 kg/m³. Similarly at a pressure drop of 97 bar, by reducing the storage temperature from 11 IK to 38K, the applicant calculates that the hydrogen storage density may be increased to (111/38) x 34.8 = 101.7 kg/m³. At a pressure drop of 200 bar and a storage temperature of 38K, the applicant calculates that the hydrogen storage density will equate to (200/97) x 101.7 = 209.7 kg/m³. At a pressure drop of 300 bar and a storage temperature of 38K, the applicant calculates that the hydrogen storage density will equate to (300/97) x 101.7 = 314.5 kg/m³. Example 2

The experiment of example 1 was repeated with a lower starting temperature, the results obtained being sampled as set out in Table 2.

Table 2

In the experiment of example 2, a total of 42.9 grams of hydrogen gas was discharged from the 1.55 litre storage vessel during an initial period of 194 seconds, during which time the pressure in the storage vessel dropped by 63.8 bar and the temperature in the storage vessel fell from 48K to 33K.

In practice, hydrogen at ambient temperature would be injected into the storage vessel from an auxiliary tank in order to offset the heat of desorption when the storage vessel temperature falls to about this level. Instead of this procedure, in order to determine the amount of hydrogen stored within the vessel, the hydrogen discharge valve was turned off and the vessel was allowed to warm up to 127K by heat transfer from the ambient air to release the remaining hydrogen from the adsorbent. The hydrogen discharge valve was then turned on again, and a further 36.4 grams of hydrogen gas was discharged over a further period of 138 seconds, during which time the pressure in the storage vessel dropped from 65.5 bar to 1.2 bar.

The storage vessel was then allowed to warm up further and H2 released and flow measured to determine that a further 6.0 grams of hydrogen gas remained inside.

The total amount of hydrogen stored in the 1.55 litre vessel was thus 42.9 + 36.4 + 6.0 = 85.3 grams, equating to a storage density of 55.0 kg/m³ storage vessel volume at a starting temperature of 48K and a total pressure drop of 64.8 bar.

Extrapolating from these figures in the same way as set out in the first example, the applicant calculates that a system employing an initial storage temperature of 48K and pressure of 201 bar (reducing to 1 bar after complete discharge) would provide a storage density of (200/64.8) x 55.0 = 169.8 kg hydrogen /m³ storage vessel volume. By reducing the storage temperature to 38K, the applicant calculates that a storage density of (48/38) x 169.8 = 214.5 kg hydrogen /m³ storage vessel volume could be obtained for the same pressure drop of 200 bar over the discharge cycle. At a storage temperature of 38K and a pressure drop of 300 bar (starting pressure of about 301 bar), the applicant calculates that a storage density of about (300/200) x 214.5 = 321.8 kg hydrogen /m³ storage vessel volume may be obtained.

Example 3

A third experiment was carried out generally as described above with reference to examples 1 and 2, but having a storage vessel with a volume of 8.33L and containing 833g of carbon adsorbent. The adsorbent was the same as that used for examples 1 and 2, except that it was prepared with only about lwt.% to 1.5wt.% Pd instead of 5wt% Pd. The gaseous helium coolant of examples 1 and 2 was substituted by liquid nitrogen coolant.

The starting pressure and temperature inside the storage vessel were 111.3 bar and 76K, reducing to 4.7 bar and 57K at the end of the discharge cycle. A total of 263 g of hydrogen gas was discharged from the storage vessel, equating to a storage density of 31.6 kg hydrogen / m³ storage vessel volume. By extrapolation as set out above, the applicant calculates that by increasing the initial storage pressure to 200 bar, a storage density of (195.3/106.6) x 31.6 = 57.9 kg / m³ could be achieved at a storage temperature of about 76K. It is believed that the higher storage density achieved in examples 1 and 2 relative to the storage temperature and pressure reflects in particular the higher percentage by weight of palladium present on the adsorbent.

In summary, a preferred embodiment provides a pressurized storage tank in a motor vehicle, the tank containing a heat exchanger and an adsorbent material comprising a soft mass of carbon micro-fibres in the form of carbon wool with a surface area of about 2250m²/g - 4000m²/g, having a metal, preferably palladium, dispersed over the surface of the adsorbent in nano-particulate form. The metal is preferably precipitated in ionic form from a solution, and may be reduced after precipitation to a metallic form by hydrogenation at an elevated temperature. A refuelling station comprises a supply of hydrogen at a first working temperature of about 36K - 77K and a first working pressure up to about 300 bar, which is supplied to the tank via first releasable couplings, and a supply of refrigerated helium gas at about 2OK which is simultaneously circulated via second releasable couplings through the heat exchanger in the tank to offset the heat of adsorption. An auxiliary tank contains compressed hydrogen gas at ambient temperature which is injected in controlled amounts into the storage tank to offset the heat of desorption as the storage tank empties.

In a development, the storage vessel and adsorbent may be cooled and filled with cold hydrogen as described above, at a first temperature of, for example, about 50K and a first pressure of, for example, up to about 125 bar; and then (after disconnection of the filling and cooling lines) warmed to a second working temperature higher than the first working temperature so that the internal pressure rises to a second working pressure higher than the first working pressure, the pressure relief valve then being adapted to vent hydrogen gas at the second working pressure. The second working temperature may be up to ambient temperature (i.e. up to about 300K), and the second working pressure may be, for example, from about 300 bar up to about 750 bar, the storage vessel being adapted to contain this pressure, e.g. comprising a composite, filament wound structure.

The second pressure may be maintained at not more than, for example, about 700 bar by the heat of desorption as the hydrogen gas is consumed by the fuel cell or engine of the vehicle. The warming may be accomplished in whole or in part by means of a heating apparatus as described above, e.g. by injection of hydrogen at ambient temperature from an auxiliary tank. The adsorbent may also be warmed by the ambient heat load on the storage vessel, and the thermal insulation may be less insulative than the more expensive super-insulation required to maintain the cryogenic storage temperature of the first embodiment. The hydrogen may be stored in the vessel at the second temperature and pressure for an extended period of time.

The invention may find uses in the storage of hydrogen, not only for use in motor vehicles but also in other static or mobile applications.

Claims

1. A hydrogen storage apparatus including:

a thermally insulated storage vessel containing an adsorbent,

the storage vessel being adapted for pressurization to a first working pressure;

inlet and outlet apparatus for filling the storage vessel with gaseous hydrogen and releasing gaseous hydrogen from the storage vessel;

and a cooling apparatus for cooling the adsorbent to a first working temperature;

characterised in that the adsorbent comprises activated carbon micro-fibres,

and a metal is dispersed on the adsorbent in finely divided form.

2. An apparatus according to claim 1, characterised in that the adsorbent is oxidised by exposure to CO2 at an elevated temperature.

3. An apparatus according to claim 1, characterised in that the metal is dispersed on the adsorbent in ionic form.

4. An apparatus according to claim 3, characterised in that the ionic metal is precipitated onto the adsorbent from a solution.

5. An apparatus according to claim 3, characterised in that the ionic metal dispersed on the adsorbent is reduced to a metallic form by exposing the adsorbent to hydrogen gas at an elevated temperature.

6. An apparatus according to claim 1, characterised in that the metal is selected from the group consisting of Ru, Rh, Pd, Os, Ir, and Pt.

7. An apparatus according to claim 6, characterised in that the metal is selected from the group consisting of Ru, Rh, Pd, and Pt.

8. An apparatus according to claim 7, characterised in that the metal is Pd.

9. An apparatus according to claim 1, characterised in that the metal is selected from the group consisting of Na and K.

10. An apparatus according to claim 1, characterised in that the first working temperature lies in the range from about 36K to about 77K.

11. An apparatus according to claim 1 , characterised in that the first working pressure lies in the range from about 1 bar to about 300 bar.

12. An apparatus according to claim 1, characterised in that the storage vessel is adapted for pressurization to a second working pressure up to about 750 bar.

13. An apparatus according to claim 1, characterised in that the cooling apparatus comprises a heat exchanger containing gaseous helium, the heat exchanger being adapted for connection to an external supply of refrigerated gaseous helium.

14. An apparatus according to claim 1, characterised in that the apparatus includes a heating apparatus for warming the adsorbent.

15. An apparatus according to claim 14, characterised in that the heating apparatus comprises an auxiliary tank containing gaseous hydrogen at a temperature higher than the first working temperature, and a warm hydrogen injection apparatus for injecting hydrogen from the auxiliary tank into the storage vessel.

16. A method of storing hydrogen on an adsorbent, comprising the steps of

i) dispersing a metal in finely divided form onto an adsorbent comprising activated carbon micro-fibres;

ii) arranging the adsorbent in a thermally insulated storage vessel adapted for pressurization to a first working pressure, the storage vessel including a heat exchanger;

iii) cooling a supply of hydrogen; and then

iv) introducing the cooled hydrogen into the storage vessel until the storage vessel is pressurized to the first working pressure, and simultaneously

v) circulating a coolant through the heat exchanger to maintain the adsorbent at a first working temperature.

17. A method according to claim 16, characterised in that the first working temperature lies in the range from about 36K to about 77K.

18. A method according to claim 16, characterised in that the first working pressure lies in the range from about 1 bar to about 300 bar.

19. A method according to claim 16, characterised in that the coolant is gaseous helium.

20. A method according to claim 16, characterised by the additional step of

vi) warming the adsorbent as hydrogen is released from the storage vessel.

21. A method according to claim 20, characterised in that step vi) comprises the step of injecting hydrogen at a temperature higher than the first working temperature into the storage vessel.

22. A method according to claim 16, the storage vessel being adapted for pressurization to a second working pressure higher than the first working pressure, characterised by the additional step of

vii) after steps (iv) and (v), warming the adsorbent to a second working temperature higher than the first working temperature, and storing the hydrogen in the vessel at the second working pressure corresponding to the second working temperature.

23. A method according to claim 22, characterised in that the second working pressure lies in the range from about 300 bar to about 750 bar.

24. A hydrogen refuelling system for a motor vehicle, comprising:

a thermally insulated storage vessel containing an adsorbent,

the storage vessel being arranged in the motor vehicle and adapted for pressurization to a first working pressure,

the storage vessel including inlet and outlet apparatus for filling the storage vessel with gaseous hydrogen and releasing gaseous hydrogen from the storage vessel; and a refuelling station remote from the motor vehicle,

the refuelling station including a supply of cooled hydrogen

and hydrogen connection apparatus for connecting the supply of hydrogen to the inlet apparatus of the storage vessel;

characterised in that the storage vessel includes a heat exchanger for cooling the adsorbent to a first working temperature between about 36K and about 77K,

the heat exchanger being adapted for connection to an external supply of refrigerated gaseous helium;

and in that the refuelling station includes a supply of gaseous helium,

refrigeration apparatus for cooling the gaseous helium,

and coolant connection apparatus for connecting the supply of refrigerated gaseous helium to the heat exchanger.

25. A system according to claim 24, characterised in that the adsorbent comprises activated carbon micro-fibres,

and a metal is dispersed over the adsorbent in finely divided form.

26. A system according to claim 24, characterised in that the first working pressure lies in the range from about 1 bar to about 300 bar.

27. A system according to claim 24, characterised in that the storage vessel is adapted for pressurization to a second working pressure higher than the first working pressure,

and the second working pressure lies in the range from about 300 bar to about 750 bar.

28. A system according to claim 24, characterised in that the refuelling station includes an intermediate storage vessel for cooling the supply of hydrogen,

the intermediate storage vessel having an intermediate heat exchanger,

the intermediate heat exchanger being connected to the supply of gaseous helium.