Complex Hydrides for Energy Storage, Conversion, and Utilization

Functional materials are the key enabling factor in the development of clean energy technologies. Materials of particular interest, which are reviewed herein, are a class of hydrogenous compound having the general formula of M(XH_n )_m , where M is usually a metal cation and X can be Al, B, C, N, O, transition metal (TM), or a mixture of them, which sets up an iono-covalent or covalent bonding with H. M(XH_n )_m is generally termed as a complex hydride by the hydrogen storage community. The rich chemistry between H and B/C/N/O/Al/TM allows complex hydrides of diverse composition and electronic configuration, and thus tunable physical and chemical properties, for applications in hydrogen storage, thermal energy storage, ion conduction in electrochemical devices, and catalysis in fuel processing. The recent progress is reviewed here and strategic approaches for the design and optimization of complex hydrides for the abovementioned applications are highlighted.

Initial steps for the thermal decomposition of alkaline-earth metal amidoboranes: a cluster approximation

A DFT study of thermal decomposition mechanisms of [M(NH₂BH₃)₂]₄ clusters with M = Mg, Ca, and Sr is presented. Multi-step reaction pathways leading to elimination of the first H₂ molecule are explored at the M06/TZVP level of theory. For all studied M, the clusters adopt similar structures and exhibit similar transformations along the reaction pathways. Their activation energies decrease in the order Mg < Ca ≤ Sr. Four metal atoms in the cluster form a rigid planar construction that is found to be nearly unchanged during all transformations. Cleavage of the B-H bond in the environment of alkaline-earth metal atoms leads to the "capture" of the released H atom by neighboring metal atoms with the formation of a M₃H moiety. While the activation energies for the cleavage of H^δ- can be as low as 14.3, 22.6 and 23.3 kcal mol^-1 for M = Mg, Ca and Sr, respectively, barriers for the subsequent cleavage of H^δ+via destruction of the M₃H moiety are about twice larger.