Abstract:
The present invention refers to the ability of a metallic alloy to store hydrogen. Particularly, the present invention refers to the ability of an alloy, with hexagonal structure, to store, in reversible way, high amounts of hydrogen at temperatures and pressures that make an industrial applicability feasible. The present invention is applicable, e.g. for hydrogen storage—hydrogen fuel cells—with great applicability in the automobile industry.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention refers to a CuMg 2-y Li alloy (0≦x≦0.5 and 0≦y≦0.5), with a hexagonal structure, for hydrogen storage. Such property results from the ability of the alloy to absorb/release (in a reversible way) hydrogen at a temperature and pressure that make its industrial application feasible without reduction of the hydrogen storage capacity (between 3-6 wt % of hydrogen in the alloy) attained by the alloys commonly denominated high-temperature hydrides. 
       SUMMARY OF THE INVENTION 
       [0002]    It is an object of the present invention to provide a solution for hydrogen storage in a metallic alloy rendering possible the absorption/release of hydrogen at temperatures much lower than the alloys giving origin to high-temperature hydrides as, e.g. NiMg 2 . The present invention keeps an ability of hydrogen storage in hydride between 3-6 wt % (as in the case of high-temperature hydrides), enabling this way a broader industrial application. 
       BACKGROUND OF THE INVENTION 
       [0003]    The technology related to hydrogen storage is critical for a great variety of applications, being some of the most important the hydrogen cells, portable energy generators and hydrogen combustion engines. Such applications benefit substantially with the development of alloys with higher hydrogen storage capacity and which operate at temperatures and pressures that enable their applicability. 
         [0004]    Considerable attention has been given in the last years to the use of hydrogen as fuel. While oil reserves are running out quickly, hydrogen reserves remain virtually unlimited. Hydrogen can be produced from crude-oil, natural gas and other hydrocarbons or by water electrolysis. Furthermore, hydrogen can be produced without the use of fossil fuels, e.g. through water hydrolysis using nuclear or solar energy. Additionally, hydrogen is potentially a relatively low cost fuel. Hydrogen has the ability of supplying energy, attributed to the fuel weight unit, higher than any chemical fuel, and is a non-polluting source since its combustion product is the water. 
         [0005]    Being hydrogen a fuel with great potentialities, there are some difficulties regarding its use, especially when considering mobile application as in the case of fuel for cars. There is still no possibility of storing hydrogen in light deposits, meeting the required temperature and pressure conditions. Conventionally, hydrogen has been stored in high-pressure resistant deposits or stored as cryogenic liquid cooled to very low temperatures (T&lt;−253° C.). 
         [0006]    Certain metals and alloys allow the reversible storage of hydrogen. The storage of hydrogen as a solid hydride may supply higher volumetric densities of gas than storing the hydrogen in its liquid or gaseous states. 
         [0007]    Furthermore, hydrogen storage as solid hydride presents less problems concerning safety than the ones caused by the hydrogen storage in containers, in liquid or gaseous states. 
         [0008]    Hydrogen storage in metallic hydrides complies, in most of the cases, with a general scheme for absorbing/releasing hydrogen. At a given pressure and temperature:
   1. the ax phase (a metallic alloy for hydrogen storage) absorbs hydrogen. The continuous diffusion of hydrogen will be responsible for the transformation α         β(β being a metallic hydride), thereby giving occasion for the coexistence of α+β. While the two phases coexist, the isothermals (in the graphics of hydrogen pressure versus hydrogen concentration) exhibit a dwell, which length determines the quantity of H that may be stored in a reversible way.   2. α phase is completely transformed into β.   3. equilibrium reversing occurs with the release of hydrogen.   
 
         [0012]    The hydrogen storage as metallic hydride is a complex process involving several physicochemical processes and depending on several parameters. The metal surface has to be able of dissociating the hydrogen molecule and has to allow the hydrogen atoms to move easily to be, therefore, stored. The metals differ in their ability of dissociating hydrogen, being this ability dependent on the surface morphology, its structure and purity. 
         [0013]    The ideal material for hydrogen storage should therefore present a great storage capacity in relation to its weight, a release temperature/pressure of hydrogen favourable to practical applications, good reaction kinetics, good reversibility, resistance to pollutants, including those that the gaseous hydrogen may contain, and should be of relatively low cost. 
         [0014]    Low hydrogen release temperature reduces the amount of energy necessary for releasing the hydrogen. Furthermore, a relatively low releasing temperature of the stored hydrogen allows the efficient use of the exhaust heat from vehicles, machines, fuel cells or other similar equipment. 
         [0015]    The hydrogen storage comprises a great variety of systems of metallic materials such as Mg, Mg—Ni (see document US2005129566), Ti—Fe, Ti—Mn, Ti—Ni, Tr-Ni and Tr-Co (Tr being a rare-earth or a blend/alloy of rare-earths). However, these alloys do not present all the necessary properties for a universal commercial application. 
         [0016]    In order for the metallic alloy to function as a hydrogen deposit for the supply of hydrogen cells to be applied, e.g. in cars, it is not viable that the absorption/release process of hydrogen occurs at a high temperature. For the application in motor vehicles, the temperatures for the hydrogen release should be in the temperature range 0-100° C. and in the pressure range 1-10 bar. 
         [0017]    In the case of the NiMg 2  alloy, the absorption/release of hydrogen takes place at 555 K (282° C.), for 1 bar pressure, for a hydrogen storage capacity corresponding to 3.59 wt % (of the metallic hydride) (see article: L. Schlapbach and A. Züttel, Nature, vol. 414, p. 353-358, 2001). 
         [0018]    On the other hand, the necessity of an activation cycle is demonstrated. Studies show that the production of nanoparticles of the compound eliminates the necessity of the activation cycle and reduces slightly the temperature at which the absorption/release process takes place (see article: H. Shao, H. Xu, Y. Wang and X. Li, Nanotechnology, vol. 15, p. 269-274, 2004). 
         [0019]    The high temperature hydrides resulting from the absorption of hydrogen by metallic alloys, as e.g. NiMg, are the ones that absorb a higher percentage of hydrogen but present the inconvenient of only operating at temperatures much higher than the ones indicated for some industrial applications. 
         [0020]    The low temperature hydrides resulting from the absorption of hydrogen by metallic alloys, as e.g. LaNi 5 , although being applicable at low temperatures, are not very effective regarding the hydrogen storage capacity. In the case of LaNi 5 , for example, the mass of hydrogen in the hydride is 1.37 wt %, at 298 K (25° C.) and at 2 bar (see article: L. Schlapbach and A. Züttel, Nature, vol. 414, p. 353-358, 2001). 
       DESCRIPTION OF THE INVENTION 
       [0021]    The present invention refers to a CuMg 2-y Li alloy with 0≦x≦0.5 and 0≦y≦0.5, with a hexagonal structure, for hydrogen storage, which permits the absorption of 3-6 wt % of hydrogen and reversible desorption of 80-90 wt % of the absorbed hydrogen in a temperature range between room temperature and 423 K (150° C.) and under a pressure between 1-5 bar. 
         [0022]    The present invention is based on the fact that the association of Li to the CuMg 2  phase induces a phase transformation for CuMg 2-y Li x  with 0≦x≦0.5 and 0≦y≦0.5. The latter presents a hexagonal structure (P 6 2 22), contrary to the CuMg which presents an orthorhombic structure (Fddd). 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0023]      FIG. 1 : Phase diagram of the Ni—Mg system (from the database of the COST 507 action: Definition of thermochemical and thermophysical properties to provide a database for the development of new light alloys. Ed. by I. Ansara, A. T. Dinsdale, M. H. Rand, vol. 2, 1998). 
           [0024]      FIG. 2 : Phase diagram of the Cu—Mg system (from the database of the COST 507 action: Definition of thermochemical and thermophysical properties to provide a database for the development of new light alloys. Ed. by I. Ansara, A. T. Dinsdale, M. H. Rand, vol. 2, 1998). 
           [0025]      FIG. 3 : X-ray diffraction pattern of a sample with composition x(Cu)=0.355, x(Li)=0.067, x(Ng)=0.578. The not marked peaks refer to the CuMg 2-y Li x  phase with 0≦x≦0.5 and 0≦y≦0.5. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    As happens with NiMg 2 , the structure of the phase being submitted to hydrogenation is hexagonal (P6 2 22). 
         [0027]    Contrary to what was referred for the CuMg 2  phase (with orthorhombic structure-Fddd) which, in presence of hydrogen, decomposes irreversibly, the CuMg Li with 0≦x≦0.5 and 0≦y≦0.5, with hexagonal structure (P6 2 22), reacts reversibly in such a way that hydrogen is absorbed and desorbed. 
         [0028]    As can be analysed by comparing  FIGS. 1 and 2 , the phase diagrams of the Ni—Mg and Cu—Mg systems are very similar. In both, two phases that, at room temperature, are stoichiometric and present equivalent compositions (NiMg 2  and CuMg 2 , respectively), may be found. 
         [0029]    From the analysis of the two phase diagrams may be supposed that, if the NiMg 2  is an alloy with interesting properties for hydrogen storage, CuMg may also be, with the advantage of (as may be inferred from the observation of the phase diagram) probably enabling the absorption/release of hydrogen at a temperature much lower than NiMg 2 . However, studies (see article: L. Schlapbach and A. Züttel, Nature, vol. 414, p. 353-358, 2001) reveal that CuMg 2 , in the presence of hydrogen, is irreversibly decomposed into Cu 2 Mg+ hydride. This difference in the behaviour between the two alloys may be associated to the fact that its crystalline structures are different, as mentioned before. Contrary to what happens with the NiMg 2  and CuMg 2  phases, the CuMg 2-y Li x  phase, with 0≦x≦0.5 and 0≦y≦0.5, and the NiMg 2  phase have the same crystalline structure, as previously mentioned. 
         [0030]    From the study of the Cu—Li—Mg system, using experimental techniques as scanning electron microscopy with quantitative analysis, differential scanning calorimetry and powder x-ray diffraction (at room and high temperatures) resulted the observance of the existence of a ternary phase with stoichiometry more probably equal to CuMg 2-y Li x  with 0≦x≦0.5 and 0≦y≦0.5. 
         [0031]    Comparative studies showed that NiMg 2 (H,D) x  (x≈0.3) phase has exactly the same hexagonal structure (P6 2 22) and lattice parameters very similar to the CuMg 2-y Li x  phase, with 0≦x≦0.5 and 0≦y≦0.5. This latter phase is a solid solution of hydrogen in NiMg 2 . 
         [0032]    By analogy between crystalline structures, the master alloy CuMg 2-y Li x , with 0≦x≦0.5 and 0≦y≦0.5, was used for hydrogen storage instead of the CuMg alloy. 
         [0033]    Absorption/release assays of the hydrogen were carried out in the CuMg Li alloy, with 0≦x≦0.5 and 0≦y≦0.5, obtaining this way PCT curves (pressure curves versus hydrogen concentration, at a constant temperature). 
         [0034]    Samples with compositions similar to the CuMg Li compound with 0≦x≦0.5 and 0≦y≦0.5 were prepared from high purity Cu, Li and Mg, respectively Cu&gt;99.79%, Li&gt;99.8% and Mg&gt;99.96%. The melting of the alloys was carried out in a resistance electric furnace, using alumina crucibles, under flow of LiCl and LiF avoiding oxidation of the bath and Li losses during the elaboration of the alloy. 
         [0035]    Subsequently, some samples were milled to powder in a mechanical mill. 
         [0036]    Measurements of the bulk samples and of powders with diameters between 75 μm and 50 nm were taken. 
         [0037]    Samples were submitted to hydrogenation cycles where hydrogen at different pressures was inserted, registering measures of the incoming and outcoming pressures, at a given temperature, and calculating this way the hydrogen storage capacity of the alloy. 
         [0038]    The PCT curves (pressure-composition-temperature) were obtained for different grading, for different temperatures and for different compositions of the master alloy, next to the composition of the CuMg 2-y Li x  phase with 0≦x≦0.5 and 0≦y≦0.5. 
         [0039]    For some compositions of the master alloy, the presence of Cu 2 Mg (Laves-C15) or Cu 2 Mg (Laves-C15) and CuMg, apart from the CuMg 2-y Li x  phase with 0≦x≦0.5 and 0≦y≦0.5, was observed. 
         [0040]    From the performed assays may be perceived that the present invention encloses the following advantages in relation to the alloys already used for hydrogen storage:
       the operation temperature of the alloy (temperature at which occurs the absorption/release cycle of the hydrogen) is lower than 423 K (150° C.);   maintenance or slight increase of the hydrogen storage capacity of the alloy (3-6 wt %);   when milled do powder &lt;100 nm, the necessity of an activation cycle is eliminated.       
 
       Application Examples 
       [0044]    An alloy with ability for hydrogen storage was prepared, according to the present invention, and was tested for determining its ability for hydrogen storage. 
         [0045]    The Cu—Li—Mg alloy, with composition x (Cu)=0.312; x (Li)=0.066; x (Mg)=0.622, was prepared from high purity Cu, Li and Mg, respectively Cu&gt;99.79%, Li&gt;99.8% and Mg&gt;99.96%. The melting of the alloy was carried out in a resistance electric furnace, using alumina crucibles, under flow of LiCl and LiF avoiding oxidation of the bath and Li losses during the elaboration of the alloy. 
         [0046]    The composition of the sample was confirmed by atomic absorption spectrophotometry. 
         [0047]    The sample was milled to &lt;200#, using one part of the sample for obtaining the x-ray diffraction patterns. From the analysis was concluded that the sample was essentially composed of CuMg 2-y Li x  phase with 0≦x≦0.5 and 0≦y≦0.5, presenting traces of Cu 2 Mg and CuMg 2 . 
         [0048]    The remaining sample was submitted to hydrogenation cycles in order to obtain the PCT curves. 
         [0049]    The sample was heated at 300° C., under argon atmosphere, during one hour. Subsequently, hydrogen at 5 bar was introduced and the system was cooled to room temperature for activation. 
         [0050]    PCT curves were traced for temperatures between room temperature and 300° C. 
         [0051]    It was found that, at temperature of 100° C. and at a pressure of 1-2 bar, the release of hydrogen in the alloy corresponds to a hydrogen storage capacity between 3-4 wt % of the alloy. 
         [0052]    The description of the invention with an example of a preferred embodiment of the invention is not intended to limit the invention to its realization and its proceedings. On the contrary, the intention is sought to cover all alternatives, modifications and equivalencies that may be included in the spirit and scope of the described invention.