A Series of Mixed-Metal Borohydrides (original) (raw)

2009, Angewandte Chemie-international Edition

The transition towards a sustainable and reliable energy system capable of meeting the increasing energy demands is considered one of the greatest challenges in the 21st century. However, one of the major obstacles is that renewable energy sources are unevenly distributed both geographically and over time, and most countries need to integrate several different sources. Hydrogen is a potential, extremely interesting energy carrier system, [1] but a major challenge in a future "hydrogen economy" is the development of a safe, compact, robust, and efficient means of hydrogen storage, in particular for mobile applications. No single material has yet been identified that fulfills all the criteria for hydrogen storage, despite considerable research and technological efforts. Borohydride-based materials have recently received great attention owing to their high gravimetric hydrogen contents, but the utilization of this class of materials in real, practical technological applications is often hampered by unfavorable thermodynamic and kinetic properties. Much research has focused on improving the properties of known interesting hydrogen storage materials such as LiBH 4 , which possesses an extremely high hydrogen content (18.4 wt %) but unfortunately has a high enthalpy of decomposition (À67 kJ mol À1 ) and therefore a high decomposition temperature. Thus, researchers have tried to improve the thermodynamic properties by design of a reactive hydride composite 2 LiBH 4 /MgH 2 , which reduces the enthalpy of decomposition from À67 to À42 kJ mol À1 and which is still capable of reversibly storing 11.5 wt % H 2 . Recently, cation substitution in LiBH 4 has also been realized by, for example, the synthesis of LiK-(BH 4 ) 2 , which unfortunately possesses the same high thermodynamic stability as LiBH 4 and KBH 4 . The kinetic properties of alanates ([AlH 4 ] À ) and magnesium-based systems have been successfully improved by the exploitation of a variety of catalytic additives, but these materials appear less efficient than borohydride-based systems owing to their high chemical reactivity. Numerous other improvements of known materials have been explored, for example, incorporation of LiBH 4 into nanoporous scaffolds, but there has been no major breakthrough that allows the synthesis of ideal hydrogen storage materials with high hydrogen content, low hydrogen decomposition temperature (i.e. appropriate thermodynamic properties), and fast "refueling" of the material (i.e. good kinetic properties). Therefore, there is a great need for new types of compounds, such as ternary borohydrides involving a combination of very different elements, such as alkali metals and transition metals. Herein, we report the synthesis and detailed structural, physical, and chemical characterization of a new series of borohydride-based materials, LiZn 2 (BH 4 ) 5 , NaZn 2 (BH 4 ) 5 , and NaZn(BH 4 ) 3 . These materials have completely novel structures,