Patent Application
20120195823
This invention relates generally to hydrogen storage materials.
Hydrogen is a promising alternative energy carrier that can potentially facilitate the transition from fossil fuels to sources of clean energy due to its prominent advantages such as high energy density (142 MJ/kg)1, great variety of potential sources (e.g. water, biomass, organic matter), light weight, and low environmental impact (water is the sole combustion product). However, there remains a challenge to produce a material capable of simultaneously optimizing two conflicting criteria—absorbing hydrogen strongly enough to form a stable thermodynamic state, but weakly enough to release it on-demand with a small temperature rise. Many materials under development, including metal-organic frameworks,2 nanoporous polymers,3 and other carbon-based materials,4 physisorb only a small amount of hydrogen (typically 1-2 wt. %) at room temperature. Metal hydrides were traditionally thought to be unsuitable materials due to their high bond formation enthalpies (e.g. MgH2 has a ΔHf˜75 kJ/mol), thus requiring unacceptably high release temperatures5 resulting in low energy efficiency. However, recent theoretical calculations6,7 and metal-catalyzed thin film studies8 have shown that microstructuring of these materials can enhance the kinetics by decreasing diffusion path lengths for hydrogen and decreasing the required thickness of the poorly permeable hydride layer that forms during absorption.
The present invention provides for a composition capable of storing hydrogen from molecular hydrogen. The composition comprises a nanoparticle (NP) and a polymer, wherein the NP is essentially embedded in the polymer. The polymer is selectively permeable wherein the polymer is essentially not permeable to O2 and H2O. The composition is capable of absorbing and desorbing molecular hydrogen. Molecular hydrogen is H2.
The present invention also provides for a method of producing a composition capable of storing hydrogen from molecular hydrogen. In some embodiments of the invention, the method comprises the method described in Example 1 herein.
The present invention also provides for a method of storing hydrogen comprising: (a) providing the composition of the present invention, (b) contacting a molecular hydrogen with the composition, and (c) increasing the temperature of the composition such that the molecular hydrogen is separates from the composition.
The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings.
Before the present invention is described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.
Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited.
It must be noted that as used herein and in the appended claims, the singular forms “a”, “and”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a diacid” includes a plurality of such diacids, and so forth.
These and other objects, advantages, and features of the invention will become apparent to those persons skilled in the art upon reading the details of the invention as more fully described below.
The present invention provides for a composition capable of storing hydrogen from molecular hydrogen. The composition comprises a suitable metallic nanoparticle (NP), such a magnesium or titanium NP, and a polymer, wherein the NP is essentially embedded in the polymer. The polymer is selectively permeable wherein the polymer is essentially not permeable to O2 and H2O. The composition is capable of absorbing and desorbing molecular hydrogen. Molecular hydrogen is H2. In some embodiments, the NP content is equal to or greater than about 20 wt. %, 40 wt. %, 60 wt. %, or 80 wt. % of the composition.
In some embodiments of the invention, the NP is Mg NP and Mg is essentially a single-phase hexagonal Mg. In some embodiments of the invention, the NP is Ti NP. In some embodiments of the invention, the NP is a nanocrystal (NC), nanotube, nanorod, nanowire, or the like. In some embodiments of the invention, the Mg NP is a Mg NC.
In some embodiments of the invention, the size of the NP is from about 1 nm to about 10 nm in diameter for the longest or shortest linear dimension. In some embodiments of the invention, the size of the NP is from about 2.8 nm or 3 nm to about 7 nm in diameter for the longest or shortest linear dimension. In some embodiments, the size of the NP is from about 4 nm to about 6 nm in diameter for the longest or shortest linear dimension.
In some embodiments of the invention, the average size of the NP is from about 1 nm to about 10 nm in diameter for the longest or shortest linear dimension. In some embodiments of the invention, the average size of the NP is from about 2.8 nm or 3 nm to about 7 nm in diameter for the longest or shortest linear dimension. In some embodiments, the average size of the NP is from about 4 nm to about 6 nm in diameter for the longest or shortest linear dimension.
In some embodiments of the invention, when the NP is a nanotube, nanorod, or nanowire, the diameter of the cross-section of the NP is from about 1 nm to about 10 nm. In some embodiments of the invention, when the NP is a nanotube, nanorod, or nanowire, the diameter of the cross-section of the NP is from about 2.8 nm or 3 nm to about 7 nm. In some embodiments of the invention, when the NP is a nanotube, nanorod, or nanowire, the diameter of the cross-section of the NP is from about 4 nm to about 6 nm.
In some embodiments of the invention, when the NP is a nanotube, nanorod, or nanowire, the average diameter of the cross-section of the NP is from about 1 nm to about 10 nm. In some embodiments of the invention, when the NP is a nanotube, nanorod, or nanowire, the average diameter of the cross-section of the NP is from about 2.8 nm or 3 nm to about 7 nm. In some embodiments of the invention, when the NP is a nanotube, nanorod, or nanowire, the average diameter of the cross-section of the NP is from about 4 nm to about 6 nm.
In some embodiments of the invention, the longest or shortest linear dimension, or diameter of the cross-section, or average thereof, of the NP is equal to or less than 10 nm, 9 nm, 8 nm, 7 nm, 6 nm, or 5 nm.
In some embodiments of the invention, the NP can be Mg NP or Ti NP. Both Mg and Ti can be catalysts of the present invention.
The polymer is any suitable polymer that is essentially not permeable to both O2 and H2O. In some embodiments of the invention, the polymer has a ratio of permeability for H2 to permeability for O2 of equal to or higher than 5. In some embodiments of the invention, the polymer has a ratio of permeability for H2 to permeability for O2 of equal to or higher than 10. In some embodiments of the invention, the polymer has a ratio of permeability for H2 to permeability for O2 of equal to or higher than 20. In some embodiments of the invention, the polymer has a ratio of permeability for H2 to permeability for O2 of equal to or higher than 30. In some embodiments of the invention, the polymer has a ratio of permeability for H2 to permeability for O2 of equal to or higher than 40. In some embodiments, the polymer is at least sufficiently flexibility to be capable of withstanding volume expansion, such as up to about 33% or at least about 33%. The transition of Mg to MgH2 causes a volume expansion.
In some embodiments of the invention, the polymer is a polyacrylate, silicone, glass, or sulfonate. In some embodiments of the invention, the polymer is a poly(alkyl methacrylate). In some embodiments of the invention, the polymer is a poly(methyl methacrylate) (PMMA) or poly(methyl acrylate) (PMA).
In one embodiment of the invention, the composition comprises a Mg NP-PMMA or a Ti NP-PMMA composite.
In some embodiment of the invention, the composition is capable of absorbing equal to or more than about 4 wt. % of molecular hydrogen. In some embodiment of the invention, the composition is capable of absorbing equal to or more than about 5 wt. % of molecular hydrogen. In some embodiment of the invention, the composition is capable of absorbing equal to or more than about 6 wt. % of molecular hydrogen.
In some embodiment of the invention, the composition is capable of separating any or essentially all absorbed molecular hydrogen when the temperature of the composition is equal to or more than about 200° C.
The present invention also provides for a method of producing a composition capable of storing hydrogen from molecular hydrogen. In some embodiments, the method comprises: (a) providing a reaction solution comprising (i) an organometallic precursor, such as bis(cyclopentadienyl)magnesium (Cp2Mg) or bis(cyclopentadienyl)titanium, (ii) a reducing agent, such as lithium naphthalide, and (iii) a gas-selective polymer, such as poly(methyl methacrylate) (PMMA), polystyrene (PS), polyethylene (PE), or poly (lactic acid) (PLA), dissolved in a suitable organic solvent, such as tetrahydrofuran; and (b) growing metallic nanocrystals from the organometallic precursor. The gas-selective polymer is soluble and is selected for its hydrogen gas selectivity, such as a hydrogen gas selectivity equal to or better than about that of PMMA. The reducing agent is capable of reducing the organometallic precursor in the presence of a capping ligand. In some embodiments, the capping ligand is the gas-selective polymer. In some embodiments, the capping ligand is a compound different from the gas-selective polymer, and the reaction solution further comprises a capping ligand. In some embodiments, the growing step comprises a burst-nucleation and growth mechanism. The reaction mixture can performed at any suitable temperature, such as at about room temperature.
In some embodiments of the invention, the method comprises the method described in Example 1 herein.
The present invention also provides for a method of storing hydrogen comprising: (a) providing the composition of the present invention, (b) contacting a molecular hydrogen with the composition, and (c) increasing the temperature of the composition such that the molecular hydrogen is separates from the composition.
The present invention also provides for a composite of air-stable Mg NP and gas-selective polymer for hydrogen storage. Mg is one of the most promising inorganic materials for hydrogen storage. Specifically the corresponding hydride, MgH2, exhibits high hydrogen capacity (7.6 wt %). In the present invention, synthetic methodology for metallic Mg NP I solution phase is developed. Nanoscale metallic Mg has a high surface area, short diffusion lengths for hydrogen and reduced enthalpy barriers toward hydrogen molecules. The present invention also provides for a composite of air-stable Ti NP and gas-selective polymer for hydrogen storage. Further, composite materials which embed Mg NP, Mg NC, Ti Np, or Ti NC into a polymer, such as an organic polymer, with selective gas permeability. This provides a route to solve the predictable problems related to nanostructures such as poor cyclability and degradation of the sorption properties due to deformation and oxidation.
The composition can be used in any device that stores hydrogen and/or utilizes hydrogen for buoyancy or as a fuel. A device that utilizes hydrogen as fuel is a car that comprises an engine that runs on hydrogen.
The invention having been described, the following examples are offered to illustrate the subject invention by way of illustration, not by way of limitation.
Here, we report the synthesis of an air-stable composite material comprised of metallic Mg nanocrystals (NCs) in a gas-barrier polymer matrix that enables both the storage of a high density of hydrogen (up to 6 wt. % of Mg, 4 wt % for the composite) and rapid kinetics (loading in <30 mins. at 200° C.). Moreover, nanostructuring of the Mg provides rapid storage kinetics without heavy, expensive metal catalysts.
The Mg NCs/PMMA composites are prepared in an inert atmosphere. Bis(cyclopentadienyl)magnesium (Cp2Mg) (154 mg, 1.00 mol) is reduced in a solution of tetrahydrofuran (9 mL) containing lithium (9 mg, 12.9 mmol), naphthalene (120 mg, 9.4 mmol), and poly(methyl methacrylate) (60 mg), stirring under a nitrogen atmosphere overnight. The resultant product is isolated by centrifugation, washed with tetrahydrofuran, and dried under an inert atmosphere prior to performing all characterization and measurements. The hydrogenation experiments are performed on the composite samples after first annealing in a Helium environment for ≧24 hrs to remove solvent and unreacted monomer. The hydrogenation/desorption tests are performed at 200° C. and 35 and 0 bar of H2 respectively.
Poly(methyl methacrylate) (PMMA, MW=120 000, Aldrich), naphthalene (Aldrich), and 1-hexadecylamine (90%, technical grade, Aldrich) are dried completely under high vacuum, and tetrahydrofuran is freshly distilled immediately before use. High purity Li metal foils (Cyprus. USA) are used as received. Bulk Mg (44 μm) and MgO (50 nm) are purchased from Alfa Aesar and Aldrich, respectively. All synthetic procedures are performed in an inert atmosphere glove box.
Lithium naphthalide stock solution is prepared as described in H.-J. Liu, J. Yip, K.-S. Sia, Tetrahedron Lett. 38, 2253 (1997). Naphthalene (0.12 g, 9.4 mmol) is dissolved in 6 mL of tetrahydrofuran, followed by the addition of Li metal (9 mg, 12.9 mmol) at room temperature under inert atmosphere. This mixture is stirred until the lithium is completely dissolved. As lithium metal dissolves, the color of the solution changes from colorless to dark green as lithium napthalide forms. It is imperative to use this solution within 30 minutes of mixing.
Poly(methyl methacrylate) (60 mg) is mixed in tetrahydrofuran (3 mL) by stirring magnetically at room temperature under nitrogen atmosphere overnight. Bis(cyclopentadienyl)magnesium (CP2Mg) (154 mg, 1.00 mmol) is added and the solution is stirred until completely dissolved (30 minutes), producing a pale yellow mixture. Next, the 3 mL PMMA/CP2Mg solution is added to the 6 mL lithium naphthalide stock solution, becoming turbid within minutes to yield dark grey flocculate. The reaction mixture is magnetically stirred overnight. The resultant product is separated from the solution by centrifugation (9000 rpm, 20 min) and the resulting pellet is washed with THF (in the glovebox) and further centrifuged to remove residual PMMA. Samples are allowed to completely dry in the inert atmosphere glovebox before exposure to ambient atmosphere.
X-ray diffraction (XRD) patterns are obtained using a Bruker D8 Discover X-ray diffractometer with a general area detector diffraction system (GADDS) using Cu Kα radiation (λ=0.154 nm). The size and morphology of polymer matrix embedded Mg NCs are analyzed by transmission electron microscopy (Philips CM200 FEG, 200 kV accelerating voltage). Real time EELS is performed by a transmission electron aberration-corrected microscope (TEAM 0.5 80 kV accelerating voltage). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are performed under N2(g) at a scan rate of 5° C./min in TG/DSC instrument (Netzsch STA449 F3).
Mg NCs/PMMA composites (170 mg, stored in Ar) are loaded into a hydrogenation chamber (PCT Pro-2000, Hy-Energy) in an air environment. Prior to performing hydrogenation experiments, Mg NCs/PMMA composites are annealed at 200° C. in a He environment for 1 day to remove residual solvent and low density PMMA. Next, all gases are removed by heating samples under vacuum at 70° C. The sample is then cooled down to room temperature. The hydrogenation/desorption tests are performed at 200° C. and 35 and 0 bar of H2 respectively. The same experimental procedure is repeated exactly for bulk Mg (44 μm) except that it is handled in an Ar environment and never exposed to air.
There have been various efforts to synthesize nanosized magnesium, such as ballmilling,9 sonoelectrochemistry,10 gas-phase condensation,11 and infiltration of nanoporous carbon with molten magnesium.12 However, these approaches remain limited by inhomogeneous size distributions and high reactivity toward oxygen. Our synthesis for airstable alkaline earth metal NC/polymer composites consists of a one-pot reduction reaction of an organometallic Mg2+ precursor in the presence of a soluble organic polymer chosen for its hydrogen gas selectivity (
respectively,17 resulting in a H2/O2 permeability ratio of 42.9 at 35° C., far exceeding that of other commonly available polymers (for example, 1.03 at 35° C. and 8.57 at 25° C. for poly(dimethylsiloxane) and polycarbonate, respectively).16-18 Additionally, it is critical to have a mechanically flexible polymer, both to optimize volumetric storage capacity by eliminating “dead space” as well as providing an accommodating support for the large volume expansion (33% for the Mg to MgH2 transition) that metals undergo during absorption cycling.19
Transmission electron microscopy (TEM) micrographs showed that PMMA embedded Mg NCs have an approximately spherical morphology (
The hydrogen absorption capacity for Mg NCs/PMMA composites was measured in relation to a known reference material, bulk Mg (44 μm), using a Sieverts PCT-Pro at 35 bar H2 and 200° C. (
To verify that hydrogen uptake in this Mg NCs/PMMA composite is due to metalhydride formation and not polymer adsorption, time-resolved low-loss electron energy loss spectroscopy (EELS) was performed on MgH2 NCs/PMMA composites using a monochromated and aberration-corrected transmission electron microscope at 80 kV.23
Theoretical calculations indicate that Mg NPs can exhibit more favorable thermodynamics (i.e. lower enthalpies for hydride formation) than bulk Mg due to the destabilization of MgH2 formation.6,7 Additionally, it is known that the chemical reactivity of Mg NPs positively correlates to the large metal surface area and the short diffusion path of hydrogen.25 We postulate that nanostructuring of magnesium in the nanocomposites obviates the need for expensive heavy-metal catalysts by reducing the activation energy for absorption and release of hydrogen.11 To assess this, we have determined an activation energy (Ea) from analysis of the absorption and release of hydrogen at three different temperatures (
Hydrogen absorption in a crystalline solid can either occur due to isotropic diffusion and random nucleation, or preferential nucleation along certain favorable crystal axes. Further optimization of these materials requires a physical understanding of the mechanism of hydride formation in the nanocomposite, which can be obtained via modeling of the uptake kinetics. Here, experimental absorption data of the Mg NCs/PMMA composite materials was fitted with several basic empirical and theoretical kinetic models developed by Avrami,27 which enable characterization of the mechanism and dimensionality of MgH2 phase formation. Experimental data from the first 6 minutes of hydrogen absorption in the Mg NCs/PMMA composites was fit with the Johnson-Mehl-Avrami model (equation (1)):
[−ln(1−α)]1/n=kt (1)
where α is the hydrogenated fraction of Mg, k is the phase transformation constant, t is time, and n is the dimensionality of MgH2 growth. This model assumes a constant interface velocity of MgH2 formation.21
By using the dimensionality factor (n) in equation (1) as a fitting parameter, and solving for the best fit (R2) of the data, the dimensionality of the growth of the MgH2 phase was determined to be 1.1740 (R2=0.998). There exist numerous growth and nucleation scenarios consistent with a value of n=1, including nucleation and growth along 1-D dislocation lines and thickening of cylinders, needles, and plates.28 We posit that MgH2 growth in the individual Mg nanocrystals in the composite occurs nearly 1-dimensionally along columnar defects as they are exposed to H2 gas (
Phys. 7, 1103 (1939); M. Avrami, J. Chem. Phys. 9, 177 (1941)).
In summary, we have developed a new, simple method to synthesize air-stable crystalline Mg NCs/PMMA composites by encapsulation in a polymer with selective gas permeability, protecting the NCs from O2 and H2O. The Mg NCs/PMMA composites impressively showed no oxidation in HRTEM diffractograms after 2 weeks of air exposure. Rapid uptake (<30 mins at 200° C.) of hydrogen was achieved with a high capacity (˜6 wt. % in Mg, ˜4% overall) in the absence of heavy-metal catalysts, demonstrating a volumetric capacity (50 g/L) greater than that of compressed H2 gas. Theoretical modeling of the experimental data with a Johnson-Mehl-Avrami model indicates that hydrogenation of Mg NCs proceeds through 1-dimensional growth, which can occur along line defects in the Mg NCs, as observed via HRTEM. Generally, this approach of synthesizing nanosized air sensitive metal nanocrystals protected in a gas-selective polymer provides new opportunities in low-cost high capacity hydrogen storage media, batteries, and fuel cells.
While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/437,456, filed Jan. 28, 2011, which are hereby incorporated by reference in its entirety.
This invention was made with government support under Contract No. DE-AC02-05CH11231 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61437456 | Jan 2011 | US |
