Leading physical and materials-based hydrogen storage options are evaluated for their potential to meet the vehicular targets for gravimetric and volumetric capacity, cost, efficiency, durability and operability, fuel purity, and environmental health and safety. Our analyses show that hydrogen stored as a compressed gas at 350-700 bar in Type III or Type IV tanks cannot meet the near-term volumetric target of 28 g/L. The problems of dormancy and hydrogen loss with conventional liquid H-2 storage can be mitigated by deploying pressure-bearing insulated tanks. Alane (AlH3) is an attractive hydrogen carrier if it can be prepared and used as a slurry with >50%2solids loading and an appropriate volume-exchange tank is developed. Regenerating AlH3 is a major problem, however, since it is metastable and it cannot be directly formed by reacting the spent Al with H-2. We have evaluated two sorption-based hydrogen storage systems, one using AX-21, a high surface-area superactivated carbon, and the other using MOF-177, a metal-organic framework material. Releasing hydrogen by hydrolysis of sodium borohydride presents difficult chemical, thermal and water management issues, and regenerating NaBH4 by converting B-O bonds is energy intensive. We have evaluated the option of using organic liquid carriers, such as n-ethylcarbazole, which can be dehydrogenated thermolytically on-board a vehicle and rehydrogenated efficiently in a central plant by established methods and processes. While ammonia borane has a high hydrogen content, a solvent that keeps it in a liquid state needs to be found, and developing an AB regeneration scheme that is practical, economical and efficient remains a major challenge. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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