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Hydrogen StorageEnergy Resources DivisionThe future of hydrogen as an energy source is dependent upon the development of storage media with high volumetric and gravimetric capacities.
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Hydrogen storage has been identified as the bottleneck in the development of hydrogen-fueled vehicles. The conventional storage method, compressed H2 gas, requires a large tank volume and the possibility of a tank rupture poses a significant safety risk. Other methods involve condensing and cooling the H2 gas to 20 K (-252.8°C or -423.0°F) where it forms liquid H2.
Although this provides a significant volumetric improvement over compressed H2 gas, a considerable amount of energy is wasted on maintaining the low temperature required to keep the hydrogen in the liquid state. Therefore, liquid H2 is very expensive and impractical for most automotive applications. An alternative to these more traditional methods is to store the hydrogen in the solid state. This can be accomplished with adsorbents (e.g. carbon), where hydrogen is attached to the surface of a solid, or absorbents (e.g. metal hydrides), where hydrogen is inserted in between the atoms in a solid. A schematic diagram of solid-state hydrogen storage in a metal hydride is shown in the figure on the right.
The key requirements for any candidate hydrogen storage material in automotive applications are a high gravimetric and volumetric hydrogen densities, a release of hydrogen at moderate temperatures and pressures, and a low-cost method to recharge the material back to its original state. The US Department of Energy hydrogen storage system goals for the year 2010 are a 6.0 weight percent gravimetric capacity and a volumetric capacity of 0.045 kg/L. Conventional metal hydrides that can readily supply hydrogen at room temperature have storage capacities < 2 wt. % and cannot satisfy this need. However, a number of alternative metal hydrides being investigated at Brookhaven (e.g. AlH3) and the complex metal hydrides (e.g. LiAlH4) have appreciable gravimetric and volumetric hydrogen capacities and may be able to meet the DOE system targets.
Aluminum hydride or alane, AlH3, is potentially an attractive storage material due to the large amount of hydrogen that can be contained in a relatively small, light-weight package. AlH3 contains 10 % H by weight and has a theoretical H density of 148 g/L, which is more than double the density of liquid H2. Theoretically, based on thermodynamic considerations, AlH3 will decompose to H2 and Al at room temperature. However, due apparently to the presence of an oxide surface layer, early experiments on AlH3 synthesized by the DOW Chemical Company exhibited slow H2 evolution rates below 150° C. DOE researchers at Sandia-Livermore and Brookhaven National Laboratories have demonstrated that the addition of a dopant, LiH, introduced by ball milling, alters this surface barrier and lowers the decomposition temperature by 25-50° C. More recently, freshly synthesized, nanoscale AlH3 has been shown to decompose at less than 100° C without the need of a dopant or ball milling. In addition, the total H2 yield with the fresh material approaches the theoretical value of 10 wt. %.
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A total of seven different AlH3 phases are known to exist. Each phase has a unique arrangement of H and Al atoms and therefore exhibits different material properties. Changing the crystalline form from the α phase to the less-stable γ or β phases enhances the low temperature (60° C) H2 evolution rates. These compounds may be useful in low temperature, low power fuel cell applications. At higher temperatures (>100 C), the less-stable γ and β phases undergo a phase transformation to α-AlH3 prior to decomposition. The greater temperature stability and longer shelf life of α-AlH3 will likely make it the preferred polymorph for automotive fuel cell applications.
In order to fully utilize AlH3 as an on-board storage technology further improvement is still required. At present, there is no practical, low-cost method to regenerate the spent Al powder back into AlH3. In addition, the infrastructure implications of a solid-state hydride storage option that is not rechargeable on-board the vehicle have not been fully explored. This work is continuing through the DOE Metal Hydride Center of Excellence.
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The demonstration of reversible hydrogen cycling in Ti-doped sodium aluminum hydride has generated considerable interest in the complex metal hydrides. Since this discovery there have been a number of studies focused on improving the catalytic effects and understanding the role of the dopant in the alanates. However, the mechanism by which NaAlH4 is activated in the presence of a small amount of a transition metal is still not well understood.
At BNL, we are interested in the atomistic transport mechanisms of the reversible doped complex metal hydrides (e.g. sodium alanate, lithium borohydride) and have investigated these systems using x-ray absorption (XAS) spectroscopy and first-principles calculations. A series of XAS experiments on Ti-doped sodium alanate have shown that the Ti is present as a zero-valent species and is surround by 10 Al atoms at a distance of 2.8 A. The local structure of the Ti-Al cluster is similar to that of TiAl3. A plot of the Ti pair distribution function is shown in the figure on the right.
One of the principle questions we are trying to answer is "How do the Al atoms migrate the long distances required during the hydrogenation reaction?" One possible catalytic mechanism may be that the Ti, or Ti-Al clusters, lower the potential energy barrier to the formation of an alane species (e.g. AlH3). Although Al metal has a low affinity for hydrogen, when doped with Ti the surface characteristics change significantly. Our calculations show that the Al(001) containing two next-nearest-neighbor Ti atoms on/near the surface dissociates molecular hydrogen. The formation of a mobile alane species could explain the long-range transport of Al during hydrogenation. It is also interesting to note that preliminary absorption studies on the doped, destabilized borohydride have revealed a number of similarities with the alanates. Our results indicate that in the Ti-doped borohydrides the Ti is present as a divalent species with a local atomic structure similar to TiB2.
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The thermodynamics and structural properties of the hexahydride alanates with the general formula M2M'AlH6 (where M and M' are alkali metals (Li, Na, K,...)) were investigated in FY2005. Two new alanates were synthesized at BNL: K2LiAlH6 and K2NaAlH6. Both compounds reversibly absorb/desorb hydrogen without the need for a catalyst. Structural analyses using synchrotron x-ray diffraction show that these compounds favor the Fm-3m space group with the smaller ion (M') occupying an octahedral site and the larger ion (M) in a 12-fold coordinated site.
Thermodynamic measurements demonstrate that the partial substitution of the alkali metal can change the equilibrium pressures substantially. As an example, the substitution of one Li for Na in Na3AlH6 cryolite to form Na2LiAlH6 elpasolite increases the dissociation enthalpy by 6.5+-1.6 kJ/mol H2. This thermodynamic change lowers the plateau pressure by 30 bar at 518 K. Similar trends were observed in the potassium cryolite and elpasolite phases. This form of thermodynamic tuning may be applied to other, high capacity alanates (e.g. Mg(AlH4)2), which are currently hindered by reaction enthalpies that are largely unfavorable for PEM fuel cell applications.
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Nanoscale energy storage materials offer enhanced kinetics, material stability and gravimetric capacity, with respect to their bulk counterparts. Hydrogen-driven metallurgical reactions (HDMR) represent a novel method for synthesis of these nanomaterials. Nanoscale and nanocomposite electrodes for Li-ion batteries synthesized by HDMR demonstrate reversible lithium cycling at low temperature (298 C). The nanocomposite electrodes are composed of an electrochemically active species (Li-Sn, Li-Al-Sn and Li-Al-Si) imbedded within an inert Li2O matrix. These electrodes are prepared in the charged state and therefore do not suffer from the first cycle capacity loss that is characteristic of the intermetallic anodes. This novel synthesis technique may also be applied to the preparation of new hydrogen storage compounds.
For more information about hydrogen storage at Brookhaven National Laboratory please contact Jason Graetz at graetz@bnl.gov.
Last Modified: February 1, 2008
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