Development and Characterization of Novel Complex Hydrides Synthesized via Molten State Processing
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Development and Characterization of Novel Complex Hydrides Synthesized via Molten State Processing Ragaiy Zidan*, Kirk Shanahan, Donald Anton, Arthur Jurgensen, Jennifer Pittman
Savannah River National Laboratory Aiken, South Carolina 29808 USA Abstract: This study is aimed at developing novel hydrides for hydrogen storage through a new synthesis technique utilizing high hydrogen overpressure at elevated temperatures denoted as Molten State Processing, MSP. This synthesis technique holds the potential of fusing different known complex hydrides at elevated temperatures and pressures to form new complexes having different sorption and thermodynamic properties. Complex hydrides produced by this method (e.g. NaMgH3, K3AlH6 and Na2LiAlH6) were identified through structural determination and thermodynamic characterization. Introduction and Background Renewed interest in hydrogen as the ultimate clean fuel of the future has spawned major R&D efforts in the area of fuel cell and other hydrogen technology. One issue that still needs to be resolved before the hydrogen economy becomes realty is developing a safe and efficient hydrogen storage device. Despite decades of research and the discovery of new intermetallic hydrides none of these intermetallics proved suitable for widespread commercial implementation of hydrogen [1]. However, the recent activities in the area of alanate hydrides have created much excitement and stimulated extensive interest into the potential use of these alanates as practical hydrogen storage media [2-5]. It has been known for decades that complex hydride alanates such as lithium aluminum hydride and sodium aluminum hydride (LiAlH4 and NaAlH4) have high hydrogen capacity and favorable dehydriding thermodynamics. However, difficulties in rendering these complex hydrides reversible in the solid state under practical pressure and temperature conditions and the slow kinetics of their decomposition prevented their consideration as useful hydrogen storage materials in the past. Recently,
*Correspondence author
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Bogdanovic et al showed that sodium aluminum hydrides could be modified to reversibly store hydrogen [2-4], opening the door to new possibilities for a practical hydrogen storage material. However, despite extensive research into complex hydrides, these systems require additional research to reach a product that has a stable hydrogen capacity with cycling and enhanced kinetics compatible with application requirements. Further modification of complex hydride systems and the understanding of the formation and decomposition of these complex hydrides are needed. As was done with metal hydrides, attempts were made to alter their composition and form new stoichiometries of these complexes. For example, in the case of sodium aluminum hydride an elemental substitution using elements other than Na and Al was conducted and compositions such as Na2LiAlH6 have been reported [3-6]. The substitution of a sodium atom by a lithium atom and the transformation of the compound from Na3
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