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Evaluation solid-state hydrogen storage systems, current status
Gepubliceerd door: Publicatie datum:
ECN Waterstof en Schoon Fossiel 29-4-2008
ECN publicatienummer: Publicatie type:
ECN-E--08-043 ECN rapport
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In this report to IEA annex XX task B, an overview is given of the current status of hydrogen storage materials in connection with on-board hydrogen storage applications. The storage methods and materials discussed here are: • High pressure H2 gas • Cryogenic H2 liquid • Metal glasses • Intermetallic compounds • Mg based hydrides • Complex hydrides, • Lithium nitrides • Borohydrides For each type of compound the specific hydrogen storage properties and limitations are discussed. The physisorption of hydrogen on large surface area structures (for example carbon-based) is not evaluated here due to the required cryogenic conditions. Furthermore, techniques to improve hydrogenation kinetics, thermodynamics and storage capacity are presented (ball-milling, addition of catalysts, destabilizing the hydride phase etc). For on-board hydrogen storage international organisations posed some criteria for efficient on-board hydrogen storage. The most important criteria posed by the FreedomCAR/DOE for mobile applications to be met in 2010 are; Useable energy density 1.5 kWh/L, storage weight percent 6 wt.% of H2, operating temperature -30/50?C, cycle life (> 1000 cycles), delivery pressure 2.5 bar, refuelling time < 3 min (5 kg H2). From a storage point of view, classical intermetallic compounds have a too low gravimetric hydrogen storage capacity for on-board applications. Mg is more promising as storage material, however slow kinetics and unpractical thermodynamics limits its practical use for on-board storage. During the last years the attention has shifted towards light weight metals like Li, Be, Na, Mg, B and Al and their compounds. Although the amounts of stored hydrogen are promising for certain compounds, additional research has to be done to improve kinetics, thermodynamics (ad/desorption temperature) and reversibility of the hydrogenation of these compounds. Other promising systems are the Li-N based ceramic systems and combinations of different types of hydrides to modify the overall hydrogenation enthalpies. Considering all these metals and their alloys, it is concluded that none of the today known compounds will satisfy all the targets simultaneously. Investigation of the current status of storage materials reveal that there are some very important properties which are more or less disregarded in research. Although it is generally accepted that catalysis plays a vital role in the feasibility of hydrogen storage in metallic compounds, their working mechanism is often unknown. Furthermore, most of the experiments on hydrogen storage compounds are performed under ideal laboratory conditions. This implies small scale experiments under idealized conditions (high purity hydrogen gas, clean conditions). Therefore it is necessary to investigate the mechanical stability, chemical stability, thermal conductivity and cycle life, because a possible application will operate under non-ideal conditions. Also the tolerance to impurities and contaminations, the adsorption/desorption kinetics under realistic conditions and heat and mass transfers in the material are properties that need to be considered. In this context it is important to realize that in many reports only the maximal storage capacity is reported and not the effective reversible hydrogen storage capacity, which is in general much lower. Also an inventory is given of the current activities and projects on material research and hydrogen storage in the Netherlands and abroad as well.

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