Transition metal based borohydrides for hydrogen storage

Using \textit{ab-initio} studies based on the density-functional theory, we have calculated binding energies per hydrogen molecule for decomposition reactions of transition metal borohydrides \textit{MH}$_{x}B_{12}H_{12}$ to \textit{MB}$_{12}$ structures, where $M$ corresponds to Sc\textit{, Ti, or V}. Depending on the valence of the transition metal, $x$ can be 1$, 2$, or $3.$ Crystal structures considered for \textit{MB}$_{12}$ included both hypothetical and those found in the international crystallographic structural database. On the other hand, the crystal structure considered for \textit{MH}$_{x}B_{12}H_{12}$ belongs to C2/c (space group 15) structure as reported in a previous study [V. Ozolins \textit{et al.} JACS, \textbf{131,} 230 (2009)]. Among the structures investigated, Titanium-based metal borohydride structure has the lowest binding energy per hydrogen molecule relative to the cubic TiB$_{12}$ structure ($\sim $0.37 eV/H$_{2})$. Our finding should be contrasted with the binding energy/H$_{2}$ for simple metal based borohydrides ($e.g.,$ \textit{CaB}$_{12}H_{12}$ ), which has a value of $\sim $ 1.5 eV/H$_{2}$, suggesting that transition metals play a significant role in lowering the H$_{2}$ binding energy in borohydrides.