Complex Hydrides for Hydrogen Storage

Complex hydrides, containing a minimum of 7.5 wt% hydrogen, are being investigated as hydrogen storage compounds for automotive use. As a new project, the work to date has largely involved refurbishment of equipment and acquisition of study materials. Initial experiments have confirmed that the instrumentation is functioning and that the data being obtained agree with that in the literature. Introduction Complex hydrides, sometimes referred to as classical chemical hydrides, previously were considered for hydrogen storage only in the context of releasing the hydrogen via hydrolysis. While hydrolysis increases the amount of hydrogen liberated, it also requires that water be carried as a part of the system and the products of the reaction include an aqueous solution that is not readily reversible. To be considered as a viable hydrogen storage medium, a material must be capable of being regenerated with a minimal energy penalty. It also must release the hydrogen at a temperature of less than 100C in order to be compatible with fuel cells and must have an installed hydrogen density of 6.5 wt%. Complex hydrides of aluminum are attractive as hydrogen storage compounds due to their large hydrogen content. Unfortunately, their application in this manner has been impractical as a result of the great difficulties in reversing the hydrogen release reaction. Since workers in several laboratories have reported the discovery of a number of catalysts that improve the reversing of the hydrogen release by NaAlH4, Na3AlH6, and LiAlH4, interest in the use of complex hydrides of aluminum as hydrogen storage media has been rekindled. Bogdanović and coworkers have recently reported a study of the catalytic effects of transition metal compounds on both the hydrogen release and uptake by sodium aluminum hydrides. In this report, it was revealed that the chlorides of many transition metals improved the hydrogen release and also provided reversibility to those reactions. This work also includes the study of combinations of these transition metal compounds for use as catalysts. It was found that titanium and iron function synergistically in their catalytic effect, particularly so with titanium in the form, Ti(OBu)4. Jensen, et al, found that the catalytic effect of titanium in the form Ti(OBu)4 depends on the method of its combination with the sodium aluminum hydride. These researchers reported that mechanical introduction of the titanium compound to the sodium aluminum hydride produced far superior results to those obtained by Bogdanović using solution methods. Not only were the reaction rates increased for hydrogen uptake and release, the reversible hydrogen content was 1 Proceedings of the 2002 U.S. DOE Hydrogen Program Review NREL/CP-610-32405