Patent Application
20100021377
This invention relates generally to hydrogen storage materials and more specifically relates to the synthesizing, recharging, reprocessing and chemical doping of hydrogen storage materials using supercritical fluids.
Hydrogen storage materials or media (HSMs) are a class of chemical compounds containing hydrogen in a chemically or physically bound form. There is a particular current interest in HSMs for hydrogen storage applications and in particular, for hydrogen-powered vehicles for use in a ‘hydrogen economy’. This use requires an on-board source of hydrogen fuel. Hydrogen storage for transportation must operate within minimum volume and weight specifications, supply enough hydrogen for sufficient distance, charge/recharge near room temperature, and provide hydrogen at rates fast enough for fuel cell locomotion of automotive vehicles. Therefore, in order to create a useful on-board source of hydrogen fuel, an efficient method of storing the hydrogen is required.
Despite optimism over the last three decades, a hydrogen economy remains a utopian vision. The United States Department of Energy (US DOE) Basic Science group recently summarized the fundamental scientific challenges that must be met before a hydrogen economy becomes viable. In Basic Research Needs For The Hydrogen Economy, US DOE Report, May 2003, the following design criteria were identified for a viable HSM:
Virtually all HSMs used in prior art technologies have been known for several decades, and yet none of them meet all five of the criteria listed above. For example, a number of alloys such as FeTi, Mg2Ni and LaNi5 satisfy criteria (2)-(5) but fail on criterion (1), containing only a few wt % hydrogen when fully loaded. Li3BeH7 reversibly stores 8.7% hydrogen by weight, but is highly toxic, thereby failing on criterion (5). Materials such as LiBH4 and NaBH4 react rapidly with water (hydrolysis) to release large amounts of hydrogen, but this process is chemically irreversible. Many other materials satisfy criteria (1), (2), (4) and (5), but not criterion (3).
Accordingly, this invention relates to the use of a supercritical fluid in a process of synthesizing an inorganic hydrogen storage material wherein a supercritical fluid is used as a reaction medium is disclosed.
In accordance with another aspect of this invention, the invention provides a process of recharging and reprocessing discharged inorganic hydrogen storage materials wherein a supercritical fluid is used as a reaction medium is disclosed.
In accordance with another aspect of this invention, the invention provides a process of synthesizing an inorganic hydrogen storage material comprising the steps of: providing a mixture of the substrates of an inorganic hydrogen storage material, hydrogen and a fluid medium; bringing the fluid medium to its supercritical phase; and recovering the hydrogen storage material from the supercritical fluid.
In accordance with another aspect of this invention, the invention provides a process of recharging and reprocessing discharged inorganic hydrogen storage materials comprising steps of: providing a mixture of the discharged inorganic hydrogen storage material, hydrogen and a fluid medium; subjecting the mixture to a pressure and temperature sufficient to bring the fluid medium to its supercritical phase; recovering the hydrogen storage material from the supercritical fluid.
The present invention involves the use of supercritical fluids (SCFs) for the synthesis, recharging, reprocessing and chemical doping of HSMs. SCFs can be used either as a neat material, or in combination with small amounts of conventional solvents, to effect dissolution or suspension of the discharged HSM and its subsequent rehydrogenation.
Processes for synthesizing HSMs are known in the prior art. As a general example of such a prior art process, the complex hexahydride Na3AlH6 can be prepared by the high-pressure, high-temperature processes described in Equations 1 and 2[1]:
In one embodiment of the invention, the heptane and toluene in Eqs. 1 and 2 are replaced with supercritical ethane (Tc 32° C.; pc 49 bar) or propane (Tc 97° C.; pc 43 bar) which allows H2 to become totally miscible with the organic solvent (the solvent is heptane in Eq. 1 and toluene in Eq. 2), and permits the reaction to occur at much lower temperatures (<120° C.) and hydrogen pressures (<50 bar) than for the reactions of Eq. 1 and 2.
This invention includes a process for bringing two reactants into contact and more particularly, H2 and a hydrogen storage material. H2 is generally very insoluble in organic solvents, such as heptane and toluene, requiring very high temperatures and hydrogen pressures to attain a significant concentration of dissolved H2. Changing the reaction medium to a supercritical phase allows the H2 to become totally miscible with the reaction medium, creating more favorable kinetics for the reaction. The heptane and toluene for Equs. 1 and 2 set out above could be brought to their supercritical phase. Similarly, solvents used in hydrogenation reactions of the same general type as in Eqs. 1 and 2 set out above can be used in their supercritical phase. Preferably, the supercritical fluid should have a Tc that is higher than about room temperature and lower than about 120° C. Supercritical fluids with a Tc above 120° C. can still be useful if the solvent properties are favourable to the reaction. For example, dimethyl ether, Me2O has a rather high Tc of 127° C., but its polar nature and ability to coordinate to metal cations can make it the SCR of choice in certain instances. The Tc for heptane and toluene is 267 and 320° C., respectively. Any hydride that is stable above 150° C. would not be useful as an HSM, as it would require very high temperatures and unfavourable kinetics to discharge the hydrogen, therefore, not satisfying criteria (2) and (3) set out above. On the other hand, a hydride HSM with a discharge temperature equal to or lower than about room temperature would be more stable in its dehydrogenated state and therefore of no use as an HSM. This type of HSM would discharge too easily and would be very difficult to maintain charged. Preferred criteria for selection of an HSM according to the invention include a temperature window of SCR supercriticality and HSM reversibility that meets criteria (2) and (3) above, subject to compatible solvent-solute properties of the SCF and the HSM.
In another embodiment of the invention, supercritical carbon dioxide (scCO2) doped with small amounts of an ether solvent such as tetrahydrofuran (THF) or diethyl ether (Et2O) dissolves/suspends a discharged HSM and exposes it efficiently for reaction with H2. In a variation of this embodiment, a pure ether can also be used for this process. For example, dimethyl ether, Me2O, becomes supercritical above 127° C. and 54 bar pressure. (Trifluoromethyl)methyl ether, CF3OMe (Tc=134° C.) and methoxyethane, MeOEt (Tc=165° C.) can also be used as SCF media for this rehydrogenation.
The complete miscibility of H2 with the SCF medium under the conditions of supercritical behavior according to the present invention ensures a fast and quantitative reaction to return the discharged HSM to its hydrogen-charged state. According to another embodiment of the invention the gas-solid kinetic issues that currently make it difficult to rehydrogenate NaAlH4 are overcome through the use of a supercritical fluid as the reaction medium to synthesize and allow efficient reaction of the spent material with high concentrations of hydrogen (rehydrogenation of NaAlH4 using prior art methods requires very severe conditions 200-400° C. and 100-400 bar H2). In another embodiment of the invention, a dehydrogenated HSM is introduced into a pressure vessel or a SCF reactor, and pressurized at room temperature with H2 (10-40 bar) and CO2 (60-80 bar). The vessel is then heated to an appropriate temperature (usually 60-100° C.), to ensure that the mixture enters its supercritical phase. The mixture is stirred to ensure homogeneous temperature and composition throughout. The course of the reaction may be monitored spectroscopically by means of a window built into the body of the vessel, or by insertion of a fiber optic sensor through one of the walls. A small amount of a co-solvent, such as tetrahydrofuran (THF) or diethyl ether (Et2O), may be added along with the dehydrogenated HSM when the vessel is loaded to assist dissolution of the dehydrogenated HSM and its efficient distribution throughout the SCF during the course of the reaction.
A further advantage afforded by this invention is the improved material properties achievable by rapid expansion of a supercritical solution (RESS)[2]. This produces a fine, dry powder with a large surface area and a narrow distribution of particle sizes—all desirable properties for a reproducible HSM. For example, RESS has found utility in the pharmaceutical industry for ensuring reliable drug delivery[3]. A second major drawback in recharging HSMs using heterogeneous gas-solid methods is the degradation and sintering of the solid during thermal cycling. Dissolution and reconstitution in an SCF medium each time it is recharged affords greater sample quality and reliability, and can be incorporated into a recharging technology much more easily than high-pressure hydrogenation. RESS can also be used as a method for introducing small amounts of a transition-metal catalyst into samples of an HSM from solution, rather than via mechanochemical methods[4].
In addition to NaAlH4, other metal hydrides and their complexes, or mixtures thereof known to be useful as HSMs, can be synthesized, recharged and reprocessed using the methods of the invention.
KAlH4, Mg(AlH4)2 and Ca(AlH4)2 can also be synthesised, recharged and reprocessed using the methods of the invention.
In another embodiment of this invention, two distinct HSMs can be co-dissolved and co-precipitated from a SCF, thereby producing a novel HSM with improved properties. For example, NaAlH4 and KAlH4 can be formed into a solid solution. Although KAlH4 contains a heavier alkali metal cation, and consequently has a lower wt % hydrogen than its Na congener, it decomposes smoothly and reversibly without the assistance of a transition metal catalyst. Thus, a 2:1 mixture of the sodium and potassium alanates deposited by RESS from a supercritical solution will produce Na2KAlH6, (6.8 wt % H), with superior H2 adsorption/desorption characteristics to pure NaAlH4.
In a further embodiment of this invention, an expanded solvent or near-critical fluid may be used as the reaction medium in place of a SCF. Expanded solvents are conventional solvents that have been pressurized with a gas such as CO2, to pressures below pc[5]. A typical example of an expanded solvent is acetonitrile (CH3CN) pressurized with around 50 bar CO2, to give a CH3CN/CO2 mixture of approximate composition 1:2. The solubility of a permanent gas like H2 is typically two orders of magnitude greater in such an expanded solvent than it is in the conventional solvent; at the same time, the high proportion of solvent present ensures that the medium is able to solvate the substrate effectively. Near-critical fluids are fluids close to but below either Tc or pc; there are dramatic changes in the density, dielectric constant and solvent power as the critical point is approached. For the purposes of this invention the fluid will become less dense and its capacity to dissolve H2 will increase rapidly in the near-critical regime.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2,529,427 | Dec 2005 | CA | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA2005/001908 | 12/16/2005 | WO | 00 | 10/5/2009 |
| Number | Date | Country | |
|---|---|---|---|
| 60636549 | Dec 2004 | US |
