Nanocrystalline Mg-based hydrides for hydrogen storage
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Nanocrystalline Mg-based hydrides for hydrogen storage W. Oelerich, T. Klassen, and R. Bormann GKSS Research Center Geesthacht GmbH, Inst. for Materials Research Max-Planck-Strasse D-21502 Geesthacht, Germany
ABSTRACT
Hydrogen is the ideal means of energy storage for transportation and conversion of energy in a comprehensive clean-energy concept. However, appropriate storage facilities, both for stationary and for mobile applications, are complicated, because of the very low boiling point of hydrogen (20.4 K at 1 atm) and its low density in the gaseous state (90 g/m3). Furthermore, the storage of hydrogen in liquid or gaseous form imposes safety problems, in particular for mobile applications, e.g. the future zero-emission vehicle. Metal hydrides are a safe alternative for Hstorage and, in addition, have a high volumetric energy density that is about 60% higher than that of liquid hydrogen. Mg hydride has a high storage capacity by weight and is therefore favoured for automotive applications. However, so far light metal hydrides have not been considered competitive because of their rather sluggish sorption kinetics. Filling a tank could take several hours. Moreover, the hydrogen desorption temperature of about 300 °C is rather high for most applications. A breakthrough in hydrogen storage technology was achieved by preparing nanocrystalline hydrides using high-energy ball milling. These new materials show very fast aband desorption kinetics within few minutes, thus qualifying lightweight Mg-based hydrides for storage application. In this paper recent detailed results on the sorption behaviour of nanocrystalline Mg and Mg-based alloys are presented. In a following research effort the sorption kinetics of nanocrystalline Mg has been further enhanced by catalyst additions. Furthermore, different transition metals have been added to Mg to achieve a thermodynamic destabilisation of the hydride, thus lowering the desorption temperatures to about 230 °C. The newly developed materials are currently being tested in prototype storage tanks.
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INTRODUCTION
The use of hydrogen, e.g. as fuel for the zero-emission vehicle, may replace carbon based fuels and therefore involves the need of onboard hydrogen storage. Metal hydrides offer a safe alternative to storage in compressed or liquid form. In addition, metal hydrides have the highest storage capacity by volume. Magnesium hydride (MgH2) has also a high storage capacity by weight and is therefore favored for mobile applications. However, light metal hydrides have not been considered competitive because of their rather sluggish sorption kinetics. Filling a tank could take several hours. E.g., MgH2 needs to be heated up to more than 300 °C to obtain relevant sorption properties [1-4]. Therefore, many attempts have been made to qualify MgH2 for application by improving the absorption and desorption behavior of the material. Recently, a breakthrough in hydride technology was achieved by preparing nanocrystalline hydrides using high energy ball milling [5-12]. Nanocrystall
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