14 - Alanates as hydrogen storage materials

Abstract

Group I and II salts of alanates have recently received considerable attention as potential hydrogen storage materials. These materials are referred to as “complex hydrides,” although alanates contain anionic metal complexes. Complex hydrides were not considered as the candidates for application as rechargeable hydrogen carriers until recently. This situation was changed by B. Bogdanović and M. Schwickardi. Their pioneering studies demonstrated that upon doping with selected titanium compounds, the dehydriding of anionic aluminum hydrides could be kinetically enhanced and rendered reversible under moderate conditions in the solid state. This breakthrough has led to a worldwide effort to develop doped alanates as practical hydrogen storage materials. Any effort to develop complex hydrides as practical hydrogen storage materials requires knowledge of their atomic structure and the thermodynamics of their fundamental dehydrogenation and re-hydrogenation reaction chemistry. This chapter provides a summary of information and an overview of the progress that has been made towards the utilization of alanates as on-board hydrogen carriers. It focuses on the materials with high practical potential and excludes those with properties that clearly preclude practicality. One attractive feature of alanates is that lithium and sodium salts are readily available commercially. Magnesium alanate can be readily prepared with sodium alanate and magnesium hydride via a metathesis reaction. The mixed metal alanate, Na2LiAlH6, is prepared through ball milling of sodium hydride, lithium hydride, and sodium alanate. Potassium alanate can be prepared by the direct synthesis of potassium hydride and aluminum under high temperature and pressure.