Metal Hydrides for Hydrogen Storage
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Metal Hydrides for Hydrogen Storage Jason Graetz, James J Reilly, and James Wegrzyn Energy Sciences and Technology, Brookhaven National Laboratory, Upton, NY, 11973 ABSTRACT The emergence of a Hydrogen Economy will require the development of new media capable of safely storing hydrogen with high gravimetric and volumetric densities. Metal hydrides and complex metal hydrides, where hydrogen is chemically bonded to the metal atoms in the bulk, offer some hope of overcoming the challenges associated with hydrogen storage. Many of the more promising hydrogen materials are tailored to meet the unique demands of a low temperature automotive fuel cell and are therefore either entirely new (e.g. in structural or chemical composition) or in some new form (e.g. morphology, crystallite size, catalysts). This proceeding presents an overview of some of the challenges associated with metal hydride hydrogen storage and a few new approaches being investigated to address these challenges. INTRODUCTION The transition to a Hydrogen Economy will require significant technological advancements in proton exchange membrane (PEM) fuel cells, hydrogen production and infrastructure. However hydrogen storage may be the most challenging technological barrier to the advancement of hydrogen fuel cell technologies for portable applications. At the heart of the issue is the low volumetric density of compressed H2 gas. On a gravimetric scale one kilogram of H2 can replace about 2.8 kg of gasoline (1 gallon). On a volumetric scale 1 gallon of gasoline is equivalent to about 12 gallons of high pressure H2 gas (5000 psi). The volumetric capacity is improved significantly (up to ~10×) by storing hydrogen in the solid state. This can be accomplished with adsorbents (e.g. activated carbon), where molecular hydrogen attaches to a surface through physisorption or absorbents (e.g. metal hydrides) where hydrogen disassociates and reacts with the solid in a chemisorption process. The interaction of hydrogen with light elements, such as aluminum and boron, has become an important research area in the energy sciences. Table I. Overview of the three primary types of solid-state hydrogen storage in order of bonding strength showing the typical methods for hydrogen release and charging. Charging Method Media Type Bonding Strength Method of Hydrogen Release (∆Hf) Thermal decomposition On-board (low temperature Adsobents Weak + high pressure) (60 kJ/mol H2) temperatures (T > 200°C) regeneration)
This proceeding will briefly review a few of the reversible metal hydrides, some of the challenges associated with this approach and a few methods being explored to mitigate these problems. A less well-explored class of materials, the kinetically stabilized hydrides will be discussed as a possible alternative to the on-board reversible metal hydrides. Specific examples will be given for aluminum hydride (AlH3). A few details on system design and regeneration will be presented for the kinetically stabilized hydrides. Although there is clear potential with these
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