Patent Grant
7608239
This invention relates to a process for reversible storage of the hydrogen using new materials that are potentially advantageous for the storage of hydrogen.
Within the scope of research of new energy systems, the development of processes for storage and transport of hydrogen seems to be very important. Compounds based on metal elements or metalloid-nitrogen elements were examined.
The publication of Leng et al. J. Phys. Chem. B 2004, 108, 8763-8765 examines the hydrogen storage properties of a 3Mg(NH2)2:8LiH mixture. Document US 2003/0129126 Al also essentially relates to lithium-based materials that can be used for storing hydrogen. However, the excessive stability of lithium hydride makes these materials less favorable thermodynamically in the reversible storage of hydrogen.
Document WO 2005/005310 A describes a composition that can be used for storing hydrogen comprising a hydrogenated state and a dehydrogenated state. In the hydrogenated state, this composition consists of an amide and a hydride. In the dehydrogenated state, the composition comprises an imide, therefore an already partially hydrogenated compound.
This invention relates to a process for reversible storage of hydrogen using new materials that are potentially advantageous for the storage of hydrogen (theoretically more than 5% by mass) under the following conditions, defined by the pressure-temperature isothermal plateau:
These new materials comprise a balanced system that is formed between a material that consists of magnesium elements and nitrogen elements and the corresponding hydride; they are more particularly of the type:
Mg3N2Mg(NH2)2.
The role of nitrogen, by forming the nitride phase of magnesium in equilibrium with the magnesium amide Mg(NH2)2, is to improve the thermodynamic properties of the simple magnesium hydride. The Mg(NH2)2 structure is known.
In the storage process according to the invention, the materials that consist of magnesium elements and nitrogen elements are brought into contact with gaseous hydrogen and thus lead to the formation of (a) corresponding hydride compound(s) (hydrogen absorption). By slightly increasing the temperature or slightly reducing the hydrogen pressure, the formed hydride restores the hydrogen (desorption). It is therefore a reversible storage process.
Many useful properties of a solid material can be derived directly from determining its chemical cohesion energy. This cohesion energy is intrinsically based on the chemical composition, the local atomic structure of the material, its electronic properties, and all the physical properties that are derived therefrom. Quantum physics and more specifically the density functional theory (whose abbreviation DFT is obtained from the English “Density Functional Theory”) provide a reliable base for the quantitative prediction of structural, electronic and thermodynamic properties of an atomic, molecular or crystalline structure before any attempt at synthesis of the laboratory material (see: W. Kohn, L. J. Sham, Phys. Rev. A 140, 1133 (1965)). In particular, the formalism of the DFT, as it is implemented in many current quantum software applications, such as:
In the search for new materials for the storage of hydrogen, the experimenters need to rely on the knowledge and a methodology of the chemistry of the solid. On the basis of thermodynamic concepts such as the formation enthalpy, the relative stabilities of the structures of materials can be quantified based on temperature and pressure conditions. The modem techniques of quantum calculation such as the DFT offer the advantage of relying on a minimal knowledge of empirical data for determining these same thermodynamic properties. Thanks to the knowledge of basic constants of physics, these techniques, thus often called “ab initio,” therefore make it possible to predict the energy stability and the physico-chemical properties of a crystalline structure defined by its composition and its crystallographic mesh, independently of any experimental approach. Moreover, these techniques make it possible to eliminate experimental uncertainties on the structure of a material.
The use of intermetallic hydrides as materials for storing hydrogen is based on the following chemical balance:
where M represents the stable metallic phase being transformed into the stoichiometric hydride phase MHn.
This hydride phase has a theoretical mass storage capacity that is equal to nMH/(nMH+MM)×100%, where MH is the molar mass of atomic hydrogen and MM is that of metal.
The thermodynamic characteristics of transformation (1) are described by a pressure- temperature isotherm. When the two hydride and metal phases co-exist, the isotherm has a plateau. Temperature T and equilibrium pressure Peq of the plateau are determined by the Van't Hoff equation:
where:
ΔHhyd (or ΔShyd) represents the enthalpy variation (or the entropy variation) of transformation (1);
R=8.314510 J.mol−1.K−1 is the molar constant of the ideal gases, and
P0=1 bar is the standard pressure (or 0.1 MPa).
This approach can be generalized for hydrides of metal alloys, ABx, in the following way:
where A and B are two metal elements and x is the atomic ratio B/A in the alloy.
It is commonly recognized that the primary contribution according to the terms of entropic variation ΔShyd is the loss of entropy of the hydrogen molecule that passes from the gas phase in an adsorbed state into the solid state of the final hydride. The value of ΔS hyd is known for being close to 130 J. K−1.mol−1 of H2, regardless of the hydride (see: “Hydrogen-Storage Materials for Mobile Applications,”L. Schlapbach, A. Zuittel, Nature 414 (2001) 353-358 reference [2]; and “Hydrogen Storage Properties of Mg Ultrafine Particles Prepared by Hydrogen Plasma-Metal Reaction,” H. Shao, Y. Wang, H. Xu, X. Li, Materials Science Engineering B 110 (2004) 221-226, reference [3]). Below, we retained this value. According to equation (2), also valid for reaction (3), the logarithm of the pressure at equilibrium, Peq, varies linearly with the opposite of temperature T. The slope of the linear relationship is determined by ΔHhyd. In the following examples, we will show, thanks to the Van't Hoff diagrams, the variations of the logarithm of Peq based on 1/T (more specifically 1000/T for reasons of providing units). Such diagrams make it possible to identify potentially advantageous materials for storing hydrogen in a targeted range of Peq and T
Consequently, the prediction (by a reliable theoretical approach) is of major interest for the knowledge of temperature and pressure conditions in which the metal or alloy is transformed into hydride. Since ΔHhyd is in general exothermic (for the stable hydrides), the slope is negative. The value of ΔHhyd closely depends on the stability of the hydride relative to the metallic phase or to the alloy: the more thermodynamically stable the hydride, the more reaction (1) or (3) is exothermic.
The formation enthalpy of the hydride, ΔHhyd , can be expressed based on the variation of internal energy during hydrogenation, ΔEhyd:
ΔEhyd=EAB
where E represents the internal energy of the hydride phases, metal and the hydrogen molecule in gaseous phase. The internal energy of a material is linked to interactions between the atomic centers that constitute the material and the electrons. This energy is also often called electronic energy and is directly connected to the cohesion energy of the material. The expression of ΔHhyd based on ΔEhyd is as follows:
ΔHhyd=ΔEhyd+PΔV+ΔZPE+TΔcp (5)
where
Δcp represents the calorific capacity variation between the hydride phase and the metal phase,
ΔZPE is the energy variation at the zero point between the hydride phase and the metal phase, and
ΔV is the variation of molar volume between the hydride phase and the metal phase.
The modern techniques for quantum simulation make it possible to calculate systematically the values of EAB
For the new structures (unknown or not totally resolved experimentally), the same standard description will be adopted in this invention.
For any structure (known or new), the process of rigorous simulation is adopted so as to determine the so-called basic state of the structure, i.e., the stable state of the structure. In this basic state, the values of EAB
For the applications of storage of on-board hydrogen, a temperature at an equilibrium close to 300 K (1000/T # 3,3 K−1) is generally sought for a pressure that is close to 1 atm (about 0.1 MPa). Due to equation (2), this corresponds to a value of ΔHhyd that is close to -39 kJ per mol of hydrogen. For this invention, and because of the precision of the simulation approach defined above, we will designate materials that are potentially advantageous for storing hydrogen, all the materials whose isothermal plateau verifies the following conditions:
270<T<370K(or2.7<1000/T<3.7K−1)
and
1<Peq<10atm(or about 0.1MPa<Peq<about 10MPa). (6)
The target window that materializes this domain will be shown in all the Van't Hoff diagrams in the following examples.
According to the invention, the material that consists of magnesium elements and nitrogen elements can also comprise, in a proportion of less than 5% by weight, at least one transition metal of groups 3 to 12 of the periodic table selected from among, for example, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Pd.
The material of the invention can come in solid form or in dispersed form, obtained by, for example, grinding.
The invention is also directed to a process for storing hydrogen using the materials according to the invention. The process is applied to, for example, the storage of on-board, stationary or portable hydrogen.
Example 1 is provided by way of comparison, and Example 2 illustrates the invention.
The diagram of
The crystallographic structures that are used are those of hydride and metal phases that are stable under conditions that are close to those set forth above in (6). They are recorded in Table 1.
The result of
For example, the case of magnesium hydride, which is used as a reference to the following, reveals that ΔHhyd(MgH2) is equal to −75.0 kJ per mol of H2 (see references [2] and [3] above). The calculation provides a very close value, on the order of −70.2 kJ per mol of H2. The equilibrium temperature at atmospheric pressure is 575 K experimentally (see reference [2]), which is much too high to be able to be used.
Another structure identified as potentially advantageous relates to the magnesium amide phase, Mg(NH2)2, in equilibrium with the magnesium nitride. These two phases are identified in the crystallographic databases (see Table 2 below).
The equilibrium that is used during the storage of hydrogen is as follows:
¼Mg3N2+H2→¼Mg(NH2)2+½MgH2
The calculation of the value of ΔEhyd according to the process described above results in a value of −31.9 kJ per mol of hydrogen, which is advantageous. for the storage of hydrogen under advantageous conditions (see Equations 6), as the Van't Hoff diagram of
It is also important to note the intrinsic high mass storage capacity of this material that is around 7.3%, which confirms the great advantage of this system for the targeted application.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.
The entire disclosures of all applications, patents and publications, cited herein and of corresponding French application No. 05/01,230, filed Feb. 7, 2005 are incorporated by reference herein.
The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.
From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05 01230 | Feb 2005 | FR | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 6672077 | Bradley et al. | Jan 2004 | B1 |
| 6967012 | Meisner et al. | Nov 2005 | B2 |
| 20030013605 | Klassen et al. | Jan 2003 | A1 |
| 20030129126 | Chen et al. | Jul 2003 | A1 |
| Number | Date | Country |
|---|---|---|
| 35 35 378 | Apr 1987 | DE |
| 0 360 203 | Mar 1990 | EP |
| WO 2005005310 | Jan 2005 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20060193767 A1 | Aug 2006 | US |
