Nanocrystalline Metal Hydrides for Hydrogen Storage

Abstract

Hydrogen is the ideal means of storage, transport and conversion of energy for 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 (0.0071 g/cm). 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 H-storage and, in addition, have a higher volumetric energy density [1]. Mg hydride has a high storage capacity by weight and is therefore favored for automotive applications. However, a potential drawback seems to be the high hydrogen desorption temperature of about 300 °C, as for most applications temperatures of about 200 °C are more suitable. Moreover, light metal hydrides have not been considered competitive because of their rather sluggish sorption kinetics. Filling a tank could take several hours. A breakthrough in hydride technology was achieved by preparing nanocrystalline hydrides using high energy ball milling [2]. These new materials show very fast aband desorption kinetics within few minutes, thus qualifying Mg-based hydrides for application. In this paper, we present recent results on the sorption behaviour of Mg and Mg-based alloys. The sorption kinetics of nanocrystalline Mg have been further enhanced by catalyst additions. Furthermore, different transition metals have been added to Mg to achieve a thermodynamic destabilization of the hydride, thus lowering the desorption temperatures.