WO2006005892A1

WO2006005892A1 - Hydrogen storage materials

Info

Publication number: WO2006005892A1
Application number: PCT/GB2004/005130
Authority: WO
Inventors: Paul Alexander Anderson; Simon Richard Johnson; Peter Philip Edwards
Original assignee: The University Of Birmingham
Priority date: 2004-07-12
Filing date: 2004-12-07
Publication date: 2006-01-19
Also published as: GB0415561D0

Abstract

A method of making a hydrogen storage material, comprising the steps of: providing a metal M or first hydride of M, where metal M is selected from the group Li, Mg, Zn or Al and providing a second hydride comprising metal M' selected from the group Li, Na, K, Mg, Ca, Zn, Al; element X selected from the group B, N, Al wherein element X is different from metal M', and mixing said metal M or first hydride with said second hydride and heating in a substantially oxygen-free atmosphere.

Description

HYDROGEN STORAGE MATERIALS

TECHNICAL FBELD

This invention relates to novel hydride materials, a method of producing such materials and to a method of storing hydrogen using such, materials.

BACKGROUND ART

Hydrogen is widely regarded as the most promising alternative to carbon-based fuels: it can be produced from a variety of renewable resources through electrolysis of water (available xa virtually limitless amounts), and, when coupled with fuel cells, offers near-zero emission of pollutants and greenhouse gases. However, the development of hydrogen as a major energy carrier will require solutions to many scientific and technological challenges, including hydrogen storage methods. This has lead to a rapid increase in research spending in this field.

Conventional hydrogen storage solutions include, storage as liquid hydrogen or storage in compressed gas cylinders for which a substantial fraction of the energy (up to 30%) is employed in either liquefying or compressing the hydrogen. In addition, there are major safety concerns — gas boil-off and high-pressure operation (up to 700 bar) associated with these forms of hydrogen storage, that render them acceptable for trials but not for commercial use. It is widely accepted that the development of a viable solid-state storage material, for both mobile and stationary applications, would lead to a step-change in the transition to a hydrogen economy. A solution to the hydrogen storage problem would provide a means of energy storage that could be used with renewable energy sources (e.g. wind, wave and solar) that are by their very nature mtermittent. Furthermore, it is the key to the widespread introduction of non- polluting hydrogen-powered fuel cell vehicles.

Currently, solid-state storage employs metal hydrides (e.g. LaNi₅) that have excellent volumetric storage densities — higher than for both compressed gas cylinders and liquid hydrogen — but which have poor gravimetric storage densities (< ca 2.5 weight%; 1.37 wt % for LaNi₅), thereby precluding their use for mobile storage applications (e.g. in hydrogen fuel-cell vehicles), for which a capacity of 5-6 wt % is regarded as a minimum requirement.

Magnesium reacts reversibly with hydrogen at around 300⁰C to produce magnesium hydride, MgH₂. Magnesium is a prime candidate for a solid state storage medium, with a theoretical reversible hydrogen uptake value of 7.6 wt %, a value that exceeds the US Department of Energy target of 6 wt % for use as a complete onboard storage system. However, the absorption and desorption kinetics need to be accelerated, before Mg or Mg-based materials can be used to form the basis of a practical hydrogen storage system. It has been shown by a number of research groups that the hydrogen sorption kinetics of Mg and Mg-based alloys can be improved to usable rates if the material is prepared with a nanocrystalline microstructure and an increased volume of grain boundary material, which acts as a preferred pathway for hydrogen diffusion. The established process for achieving such a nanostructure is by High Velocity Ball Milling (HVBM), where the microcrystalline Mg-based powder is mechanically ground (typically for 5—80 hours depending on the milling conditions) in an atmosphere of either hydrogen or an inert gas. However, the cost of using the energy intensive HVBM technique for batch-milling large quantities of powder for long periods of time, make it highly unlikely that it could be used to provide an economic storage material on the scale required to supply, for example, the domestic vehicle market, and this has necessitated the search for alternative, less expensive alternatives.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved method of making a hydrogen storage material; in particular a method that avoids high velocity ball milling. A further object of the invention is to provide a method of storing hydrogen. A yet further object is to provide a new hydride material suitable for cyclic hydrogen storage.

The present invention derives from the discovery of a means of producing, through simple heat treatment, activated hydride materials that have similar hydrogen absorption/desorption kinetics to those prepared by grinding Mg-based powder using HVBM.

In accordance with one aspect of the invention there is provided a method of making a hydrogen storage material, comprising the steps of: (a) providing a metal M or first hydride of formula MH_a, where metal M is selected from the group Li, Mg, Zn or Al and where a is greater than 0 and less than or equal to 3; (b) providing a second hydride of formula M¹XbH₀ comprising metal M' selected from the group Li, Na, K, Mg, Ca, Zn, Al; element X selected from the group B, N, Al wherein element X is different from the metal M', where b is less than or equal to 3, c is greater than 0 but less than or equal to 12, and where M and M¹ are different metals if b is 0; (c) mixing said metal M or first hydride with said second hydride and heating in a substantially oxygen-free atmosphere.

Step (c) may comprise heating to a temperature in the range 50 to 600 °C, preferably to a temperature in the range 150 to 450 °C, more preferably to a temperature in the range 250 to 400 ⁰C, and most preferably to a temperature in the range 270 to 330 °C. Step (c) may comprise heating for a period of between 0.1 and 72 hours; preferably for a period between 1 and 24 hours, and more preferably between 6 and 12 hours.

Preferably, the method comprises a further step (d) wherein the material produced by step (c) is cyclically dehydrogenating and hydrogenating several times in order to maximise the degree of adsorption obtained during the cyclic step.

Preferably the metal M' is Li. Preferably the second hydride is a borohydride. Preferably, step (c) is conducted in an atmosphere comprising hydrogen and/or nitrogen. Preferably the atomic ratio M'/M is greater than 0 and less than 0.5; more preferably the atomic ratio M'/M is greater than 0 and less than 0.25.

The composition of the present invention is particularly well suited for cyclic storage of hydrogen, in use the composition cycling between a hydrogenated state and a dehydrogenated state. The dehydrogenated state is produced when hydrogen is liberated from the hydrogen storage composition in its hydrogenated form. The term dehydrogenated does not necessarily imply complete removal of hydrogen, but rather may correspond to partial removal. Likewise, hydrogenation refers to addition of hydrogen to the dehydrogenated composition, by the formation of hydrides and the like.

In another aspect of the invention there is provided a hydrogen storage material having a hydrogenated state from which hydrogen may be desorbed and a dehydrogenated state wherein hydrogen may be absorbed to produce said hydrogenated state, said hydrogenated state and dehydrogenated state being cyclically reversible; wherein said hydrogenated state comprises: a non-mechanically alloyed composition with: (a) a first metal M selected from the group Li, Mg, Zn or Al; (b) a second metal M' selected from the group Li, Na, K, Mg, Ca₅ Zn or Al; element X selected from the group B, N, Al; wherein the formula of the composition is M M'_e X_f Hg _; where the first metal and the second metal are different metals, the element X and metal M' are different elements; e is less than 1 ; e is greater than 0 if f is 0; f is less than 3; and g is greater than 0 but less than 12.

Preferably the element M' is Li. Preferably, e is greater than 0 and up to 0.25. Preferably, f is greater than 0 and up to 3 and more preferably f is greater than 0 and up to 0.75.

The hydrogen storage material of the invention may be prepared by mixing substantially dehydrogenated first and second hydrides and heating in a vacuum. Consequently, metal M may normally be used in place of first hydride MH_a in step (a) of the method. It follows that the values of a, b, c in Claim 1 and e, f and g in Claim 10 can be non-integers.

ha another aspect of the invention there is provided a method of storing hydrogen using a hydrogen storage material according to the invention wherein hydrogen is absorbed when the hydrogen storage material is in a fully or partly dehydrogenated state and wherein hydrogen is subsequently desorbed by lowering the pressure and/or raising the temperature of the hydrogen storage material. Preferably, the hydrogen storage material is cyclically hydrogenated and dehydrogenated. DESCRIPTION OF DRAWINGS

Figure 1 illustrates the apparatus described in Example 1 where the reactants are heated under vacuum; Figure 2 illustrates the apparatus described in Example 2 where the reactants are heated in a gas atmosphere;

Figure 3 shows morphological SEM (Secondary) images OfMgH₂ and MgH₂ / LiBH₄ both before cycling of the material on an Intelligent Gravimetric Analyser (IGA) and after 6 cycles of hydrogen desorption / absorption Figure 4 is an X-ray diffraction pattern for MgH₂ (not milled), MgH₂ milled for 10 hours and activated material prepared by heating under vacuum according to the invention in all cases after 6 cycles of hydrogen desorption / absorption; Figure 5 is an X-ray diffraction pattern for MgH₂ (not milled), MgH₂ milled for 10 hours and activated material prepared by heating in an Argon atmosphere according to the invention in all cases after 6 cycles of hydrogen desorption / absorption;

Figure 6 is an X-ray diffraction pattern for MgH₂ (not milled), MgH₂ milled for 10 hours and activated material prepared by heating in a hydrogen/nitrogen atmosphere in all cases after 6 cycles of hydrogen desorption / absorption ; Figure 7 shows ¹¹B MAS NMR spectra of for LiBH₄, MgH₂ and activated material prepared by heating under vacuum according to the invention before and after 6 cycles of hydrogen desorption / absorption;

Figure 8 shows hydrogen absorption kinetics data for MgH₂ (not milled), MgH₂ milled for 10 hours and activated material prepared by heating under vacuum according to the invention in all cases after 6 cycles of hydrogen desorption / absorption;

Figure 9 shows hydrogen absorption kinetics data for MgH₂ (not milled), material activated prepared by heating under vacuum according to the invention; and material activated by heating in an argon atmosphere in all cases after 6 cycles of hydrogen desorption / absorption; Figure 10 shows hydrogen absorption kinetics data for MgH₂ (not milled), activated material prepared by heating under vacuum according to the invention; and material activated by heating in a nitrogen/hydrogen atmosphere in all cases after 6 cycles of hydrogen desorption / absorption; Figure 11 shows hydrogen adsorption kinetics data measured at 300 °C and 10 mbar for material prepared by heating in a nitrogen/hydrogem atmosphere according to the invention;

Figure 12 shows hydrogen absorption as a function of temperature measured at 10 mbar for material prepared by heating in a nitrogen/hydrogen atmosphere according to the invention;

Figure 13 shows hydrogen desorption kinetic data measured at 300 °C and 10 mbar hydrogen pressure for a range of Mg-based materials after 6 cycles of hydrogen desorption / absorption; and Figure 14 shows hydrogen desorption kinetic data measured at 300 °C and 1 bar after 6 cycles of hydrogen desorption / absorption; and

Figure 15 shows hydrogen desorption kinetic data measured at 300 ⁰C and 1 bar for material prepared by heating MgH₂ with Mg B₂.

DESCRIPTION OF PREFERRED EMBODIMENTS

It has surprisingly been found that heat treatment of simple metal hydrides or metal powders mixed with certain simple or complex metal hydrides results hi a hydrogen storage composition that exhibits substantial improvement in sorption and desorption properties compared with un-milled metal hydrides and thereby avoids the need for High Velocity Ball Milling. No special effort has to be taken to ensure a homogeneous mixture of reactants prior to heating. During the heating step one of the reactants typically melts, but there is generally little swelling of the reactants.

The following Examples describe four experiments where hydrogen storage compositions according to the invention were prepared.

EXAMPLE 1 - Heating in a Static Vacuum (See Figure 1)

MgH₂ (0.3 g) and LiBH₄ (0.025g) were weighed out in an argon atmosphere glove box (O₂ content: 6 pip) and transferred to a quartz tube 10. A Young's tap 12 was attached to the quartz tube via a Cajon flexible fitting 14 and the whole assembly was removed from the glove box. The assembly was attached to a vacuum line, consisting of a rotary pump 16 and turbo pump 18, and the Young's tap was opened allowing the assembly to be evacuated. The assembly was evacuated to a pressure of 10^"9 bar. The Young's tap was then closed and the quartz tube was sealed via a gas torch. The sealed tube, containing the MgH₂ and LiBH₄ was then placed in a furnace and heated to 300 °C for 12 hours, after which the ensemble was taken into the glove box. While this Example used a very low pressure of 10^~9 bar to prepare the material under vacuum, this merely corresponds to the performance of available laboratory machines and much lower degrees of vacuum may be used.

EXAMPLE 2 - Heating in an Argon Atmosphere (see Figure 2)

MgH₂ (0.3g) and LiBH₄ (0.025g) were weighed out in an argon atmosphere glove box (O₂ content: 6 ppmv) and transferred to a quartz tube 10. A Young's tap modified to enable gas flow 12 was attached to the quartz tube via a Cajon fitting 14 and the whole assembly was removed from the glove box. The assembly was attached to a vacuum line, consisting of a rotary pump 16 and turbo pump 18, and a gas cylinder was attached at inlet 20 of the gas flow enabled Young's tap.

The assembly was first evacuated to a pressure of 10^"9 bar, after which the Young's tap was closed and the gas taps opened, allowing the argon to flow through the assembly at a pressure of 1 bar.

The tube containing the MgH₂ and LiBH₄ was then placed in a furnace and heated to 300 °C for 12 hrs; after which the assembly was taken into the glove box. The gas flow enabled Young's tap was then swapped with a 'normal' Young's tap and the whole assembly was removed from the glove box. The assembly was reattached to the vacuum line and evacuated to a pressure of 10^"9 bar before being sealed via a gas torch. This final step is required in order to protect it from degradation.

EXAMPLE 3 - Mg metal and LiBH₄ in a Static Vacuum (See Figure 1) This example followed the method of Example 1 except that 0.3g of Mg was used instead of 0.3g OfMgH₂.

EXAMPLE 4 - Heating in a Nitrogen Atmosphere (See Figure 2)

TMs example followed the method of Example 2, except that after the assembly was first evacuated to a pressure of 10^'9 bar, the Young's tap was closed and the gas taps opened, Nitrogen was then allowed to flow through the assembly at a pressure of 1 bar.

Use of hydrogen/nitrogen mixtures simplifies activation of the initial product by conveniently allowing this material to be cyclically hydrogenated and dehydrogenated several times (typically 5 or 6 tunes) in order to achieve peak absorption/desorption performance.

EXAMPLE 5 - MgH₂ and MgB₂ in a Static Vacuum (See Figure 1)

This example followed the method of Example 1 except that 0.026g OfMgB₂ was used instead of 0.026g Of LiBH₄.

EXAMPLE 6 - MgH₂ and LiNH₂ in a Static Vacuum (See Figure 2)

This example followed the method of Example 4 except that 0.026g OfLiNH₂ was used instead of 0.025g OfLiBH₄, a gas mixture of 90% nitrogen and 10% hydrogen was used instead of nitrogen, and the material was heated to 375 °C instead of 300 ⁰C.

¹¹B MAS NMR Characterization

Figure 7 shows ¹¹B MAS NMR spectra of an increased surface area material

(synthesised under vacuum, before and after cycling 6 times on the IGA), together with corresponding spectra for LiBH₄ and MgH₂. After cycling 6 times boron is present in a form that is clearly chemically different from that in LiBH₄ and that before cycling.

Morphological Investigation

Figure 3 shows morphological SEM (Secondary) images Of MgH₂ and MgH₂ / LiBH₄ both before cycling of the material on an Intelligent Gravimetric Analyser (IGA) and after 6 cycles of hydrogen desorption / absorption.

Before treatment, each grain was a solid particulate OfMgH₂ with a relatively smooth surface. After treatment with LiBH₄ followed by cycling, the particles are all characterised by a porous.

Crystallographic Investigation

A powder X — Ray diffraction (XRD) trace of an increased surface area material (Synthesised in a static vacuum, cycled six times on the IGA), together with corresponding traces for MgH₂ and ball milled MgH₂ is shown in Figure 4. The high- energy impact of the balls in tiie process leads to a significant reduction in crystallite size. Thus, for the ball-milled MgH₂ material before IGA cycling (not shown) all the peaks are broad indicating a small crystallite size (number); while after cycling (see Figure 4) the peaks do not show any significant broadening compared with the un- milled MgH₂. Importantly, the increased surface area does not result in any significant broadening of the powder XRD pattern for MgH₂ vacuum treated witfi LiBH₄, but each of the diffraction lines exhibited a shoulder at low angle, indicating the presence of at least two isostructural compounds.

A powder XRD trace of an increased surface area material (synthesised in an argon atmosphere, cycled six times on the IGA), together with corresponding traces for MgH₂ and ball milled MgH₂ is shown in Figure 5. The extra peaks present in the pattern of the increased surface area material are due to the presence of Mg metal. As with the previous material, the increased surface area structure formed by the reaction Of LiBH₄ with MgH₂ does not result in significant broadening of the powder XRD pattern, and in this case no shoulder on the diffraction lines was observed. A powder XRD trace of the increased surface area material (Synthesised in a H₂ / N₂, 10 % / 90 % atmosphere, cycled 6 times on the IGA), together with corresponding traces for MgH2 and ball milled MgH₂ is shown in Figure 6. As with the previous materials, the increased surface area structure formed by the reaction of LiBH₄ with MgH₂ does not result in any significant broadening of the powder XRD pattern. Again no shoulder on the diffraction lines was observed.

Use of the Increased Surface Area Materials for Hydrogen Uptake

The hydrogen absorption curves OfMgH₂ (not milled and milled for 10 hrs), and MgH₂ / LiBH₄ (heated in static vacuum) at 300 °C and 10 bar of H₂ are shown in Figure 8; while Figure 9 shows similar data for MgH₂ / LiBH₄ heated in an argon atmosphere, and Figure 10 for MgH₂ / LiBH₄ heated in a H₂ / N₂ atmosphere.

Figure 8 shows that the absorption of hydrogen in MgH₂ is very slow; at 300 °C and 10 bar of H₂ it takes two hours to reach maximum absorption. In contrast, the absorption Of MgH₂ / LiBH₄ (heated in a static vacuum) at the same temperature and pressure occurs within 20 minutes to a value of 6 wt %. This is very similar to the hydrogen uptake data of ball milled MgH₂.

Figure 9 shows that MgH₂ / LiBH₄ (heated in an argon atmosphere) at the same temperature and pressure reaches maximum absorption of hydrogen within 40 minutes. However it has higher uptake value, 6.6 wt %.

Figure 10 shows that MgH₂ / LiBH₄ (heated in a H₂ / N₂ atmosphere) at the same temperature and pressure reaches maximum hydrogen absorption within 20 minutes to a value of 6 wt %.

Figure 11 shows that MgH₂ / LiNH₂ (heated in a H₂ / N₂ atmosphere) absorbs hydrogen at 300 °C at the very low pressure of 10 mbar. Figure 12 shows that MgH₂ / LiNH₂ (heated in a H₂ /N₂ atmosphere) absorbs hydrogen at 10 mbar, at temperatures as low as 100 °C.

The method of the invention is expected to result in considerable cost savings over the alternative of using a HVBM-based method. The option of preparing the active material using metal powder (for example; magnesium powder) promises to further reduce preparation costs. As the method does not necessarily depend upon producing a nano crystalline product further improvement in properties may be possible. The process may also be applied to Mg-Ni based alloys suitable for electrochemical hydrogen storage in rechargeable batteries.

Claims

1. A method of making a hydrogen storage material, comprising the steps of:

(a) providing a metal M or first hydride of formula MH_a5 where metal M is selected from the group Li, Mg, Zn or Al and where a is greater than 0 and less than or equal to 3;

(b) providing a second hydride of formula M¹XbH₀ comprising metal M' selected from the group Li, Na₅ K, Mg, Ca, Zn, Al; element X selected from the group B₅ N, Al wherein element X is different from metal M', and where b is less than or equal to 3, and c is greater than 0 but less than or equal to 12, and where M and M¹ are different metals if b is 0;

(c) mixing said metal M or first hydride with said second hydride and heating in a substantially oxygen-free atmosphere.

2. A method according to Claim 1 comprising the further step (d) wherein the material produced by step (c) is cyclically dehydrogenating and hydrogenating several times in order to maximise the degree of adsorption obtained during the cyclic step.

3. A method accotding to Claims 1 or 2 wherein M' is Li.

4. A method according to any preceding claim wherein step (c) is conducted in an atmosphere comprising hydrogen and/or nitrogen.

5. A method according to any preceding claim wherein the mixture is heated to a temperature in the range 150 to 450 ⁰C.

6. A method according to any preceding claim wherein the mixture is heated for a period of between 0.1 and 72 hours.

7. A method according to any preceding claim wherein the second hydride comprises a borohydride.

8. A method according to any preceding claim wherein the atomic ratio of MVM is greater than 0 and less than 0.5

9. A method according to Claim 8 wherein the atomic ratio of M'/M is greater than 0 and less than 0.25.

10. A hydrogen storage material having a hydro genated state from which hydrogen may be desorbed and a dehydrogenated state wherein hydrogen may be absorbed to produce said hydrogenated state, said hydrogenated state and dehydrogenated state being cyclically reversible; wherein said hydrogenated state comprises:

a non-mechanically alloyed composition with:

(a) a first metal M selected from the group Li, Mg, Zn or Al; (b) a second metal M' selected from the group Li, Na, K, Mg, Ca, Zn or Al; element X selected from the group B, N, Al;

Wherein the formula of the composition is M M'_e X_f H_{g ;} where the first metal and the second metal are different metals, the element X and metal M' are different elements; e is less than 1 ; e is greater than 0 if f is 0; fis less than 3; and g is greater than 0 but less than 12.

11. A hydrogen storage material according to Claim 10 wherein M' is Li.

12. A hydrogen storage material according to Claim 10 or 11 wherein e is greater than 0 and up to 0.25.

13. A hydrogen storage material according to Claim 10 or 12 wherein f is greater than 0 and up to 0.75 or greater than 0 and up to 0.25.

14. A method of storing hydrogen using a hydrogen storage material according to any of Claims 10 to 13 wherein hydrogen is absorbed when the hydrogen storage material is fully or partly dehydrogenated state and wherein hydrogen is subsequently desorbed by lowering the pressure and/or raising the temperature of the hydrogen storage material.

15. A method according to Claim 14 wherein the hydrogen storage material is cyclically hydrogenated and dehydrogenated.