Prior to 1996, most of the research on the storage of hydrogen in hydrides was devoted to intermetallic compounds or binary hydrides. The dilemma has always been that lightweight hydrides, such as MgH2, require a high temperature for hydrogen release, while materials that release hydrogen at low temperatures, such as FeTiH2, are too heavy for automotive applications. As a result, hydrogen storage has been a primary obstacle to the implementation of a hydrogen economy. To be considered acceptable by current DOE targets, any storage system must contain 6.0 wt% hydrogen or 2000 w-hr/kg and 1100 w-hr/L and be capable of reversible hydriding. Additional requirements include a cost of $5/kw-hr, a decomposition temperature of less than 80°C, a hydrogen release rate of 1.5 g/sec and a recharge time of less than 5 minutes. Currently no system meets all of these requirements.
Complex hydrides of aluminum have been considered 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 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 was rekindled.

The goal of this research was to investigate complex hydrides as hydrogen storage materials. It has been shown that the addition of titanium to sodium aluminum hydride (sodium alanate) not only improves the kinetics of hydrogen release, but also facilitates the reverse reaction. Unfortunately, although this compound contains adequate hydrogen to meet current guidelines, it decomposes to form sodium hydride, aluminum and hydrogen. As a result, one fourth of the total hydrogen cannot be recovered for use, leaving the practical hydrogen content lower than is required by current guidelines. Other complex hydrides contain larger amounts of hydrogen that would meet guidelines, even if they decompose via an analogous pathway. However, these compounds have not yet been shown to be reversible.

Studies involving complex hydrides of aluminum reported in the literature have been primarily restricted to sodium aluminum hydride, with a smaller amount of work reported involving lithium aluminum hydride. While these studies are important from a fundamental and mechanistic point of view, they do suffer serious limitations. One limitation is the fact that the reversible hydrogen capacity of sodium aluminum hydride falls short of the hydrogen content currently thought required for practical application.
The objective of this project was to determine the complex hydride that contains the highest percentage of hydrogen, has the best kinetics and is completely reversible. Because of the low reversible hydrogen content in the compounds that others were studying, this work purposely targeted complex hydrides that theoretically contain hydrogen at a higher level than does the sodium aluminum hydride.

Accomplishments

In this project, the synthesis of a number of alanates was investigated. Both wet chemical and mechano-chemical methods were studied. Wet chemical methods were used to attempt the synthesis of magnesium alanate, magnesium borohydride, and calcium borohydride. Solvent free methods were used for magnesium alanate, magnesium borohydride, a magnesium hydride/titanium hydride mixture, titanium alanate, and titanium borohydride synthesis attempts.

Synthetic methods for the preparation of contaminant-free Mg(AlH4)2 did not meet with success. Wet chemical methods resulted in material that was a solvent adduct and attempts to remove the solvent lead to decomposition of the hydride. Products from the solvent-free synthesis appeared to be largely unreacted starting materials.

Lithium alanate was studied in depth. It was observed that, in the DSC thermogram of the thermal decomposition process, the compound received from Lancaster Chemical exhibited an exotherm while material purchased from Aldrich Chemical exhibited the expected endotherm. Both characteristics have been previously reported in the literature but no explanations were given. Ball milling of a sample obtained from Lancaster resulted in the subsequent thermogram resembling that of an Aldrich sample.

It was determined that ball milling of an undoped sample of LiAlH4 decreased the temperature required for dehydrogenation and improved the kinetics of hydrogen release. It was hypothesized that the decrease in temperature described in previous literature reports is a result of ball milling rather than a specific form of the added catalyst. The addition of titanium in the +4 or +3 state or iron in the +3 state leads to a loss of storage capacity as the alanate ion reduces the titanium. The resulting salt further reduces storage capacity. Incorporation of elemental vanadium into LiAlH4 resulted in the same decrease in hydrogen release temperature but caused much less loss in hydrogen release. The addition of elemental iron, aluminum chloride, nickel, carbon black, elemental titanium, and titanium hydride, all result in a decrease in the temperature of hydrogen release without a significant decrease in the amount of hydrogen released. The decrease in hydrogen release temperatures caused by incorporation of these substances was not significantly different than that caused by ball milling alone. The catalytic effects of these substances on hydrogen absorption are yet to be determined.